diff --git a/.gitattributes b/.gitattributes index bed0738c7eeb449bca98b5d2f33c89a1ee56349a..b6e19d9777a54c59d868e5456f3e283b28e56658 100644 --- a/.gitattributes +++ b/.gitattributes @@ -58,3 +58,33 @@ saved_model/**/* filter=lfs diff=lfs merge=lfs -text # Video files - compressed *.mp4 filter=lfs diff=lfs merge=lfs -text *.webm filter=lfs diff=lfs merge=lfs -text +papers/adjoint-matching/paper.pdf filter=lfs diff=lfs merge=lfs -text +papers/avg-reward-pg/paper.pdf filter=lfs diff=lfs merge=lfs -text +papers/ca2-vdm/paper.pdf filter=lfs diff=lfs merge=lfs -text +papers/conformal-bayesian-quadrature/paper.pdf filter=lfs diff=lfs merge=lfs -text +papers/diffusion-convergence-rate/paper.pdf filter=lfs diff=lfs merge=lfs -text +papers/emergent-planning-rl/paper.pdf filter=lfs diff=lfs merge=lfs -text +papers/gated-attention-llm/paper.pdf filter=lfs diff=lfs merge=lfs -text +papers/generator-augmented-flows/paper.pdf filter=lfs diff=lfs merge=lfs -text +papers/hi-mar/paper.pdf filter=lfs diff=lfs merge=lfs -text +papers/lora-sb/paper.pdf filter=lfs diff=lfs merge=lfs -text +papers/luno/paper.pdf filter=lfs diff=lfs merge=lfs -text +papers/ma-rlhf/paper.pdf filter=lfs diff=lfs merge=lfs -text +papers/masked-diffusion-token-ordering/paper.pdf filter=lfs diff=lfs merge=lfs -text +papers/moe-pot/paper.pdf filter=lfs diff=lfs merge=lfs -text +papers/mrq/paper.pdf filter=lfs diff=lfs merge=lfs -text +papers/navil/paper.pdf filter=lfs diff=lfs merge=lfs -text +papers/neural-operator-flow-matching-pde/paper.pdf filter=lfs diff=lfs merge=lfs -text +papers/nfig/paper.pdf filter=lfs diff=lfs merge=lfs -text +papers/ngpt/paper.pdf filter=lfs diff=lfs merge=lfs -text +papers/olmoe/paper.pdf filter=lfs diff=lfs merge=lfs -text +papers/petl-visual-recognition/paper.pdf filter=lfs diff=lfs merge=lfs -text +papers/prioritized-generative-replay/paper.pdf filter=lfs diff=lfs merge=lfs -text +papers/pyramidal-flow-matching/paper.pdf filter=lfs diff=lfs merge=lfs -text +papers/robotic-world-model/paper.pdf filter=lfs diff=lfs merge=lfs -text +papers/sam2/paper.pdf filter=lfs diff=lfs merge=lfs -text +papers/sc-fno/paper.pdf filter=lfs diff=lfs merge=lfs -text +papers/score/paper.pdf filter=lfs diff=lfs merge=lfs -text +papers/universal-neural-operators/paper.pdf filter=lfs diff=lfs merge=lfs -text +papers/voting-leaderboards/paper.pdf filter=lfs diff=lfs merge=lfs -text +papers/wdno/paper.pdf filter=lfs diff=lfs merge=lfs -text diff --git a/papers/adjoint-matching/blacklist.txt b/papers/adjoint-matching/blacklist.txt new file mode 100644 index 0000000000000000000000000000000000000000..784a692fcdfd85274077a9cb780f9dbdb1f1428c --- /dev/null +++ b/papers/adjoint-matching/blacklist.txt @@ -0,0 +1,2 @@ +# Official repository +https://github.com/microsoft/soc-fine-tuning-sd diff --git a/papers/adjoint-matching/config.yaml b/papers/adjoint-matching/config.yaml new file mode 100644 index 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Q. Chen1 + +1FAIR, Meta + +Dynamical generative models that produce samples through an iterative process, such as Flow Matching and denoising diffusion models, have seen widespread use, but there have not been many theoreticallysound methods for improving these models with reward fine-tuning. In this work, we cast reward fine-tuning as stochastic optimal control (SOC). Critically, we prove that a very specific memoryless noise schedule must be enforced during fine-tuning, in order to account for the dependency between the noise variable and the generated samples. We also propose a new algorithm named Adjoint Matching which outperforms existing SOC algorithms, by casting SOC problems as a regression problem. We find that our approach significantly improves over existing methods for reward fine-tuning, achieving better consistency, realism, and generalization to unseen human preference reward models, while retaining sample diversity. + +Correspondence: Carles Domingo-Enrich at cd2754@nyu.edu + +![](images/figures/adjoint-matching-fig-0001.jpg) +Figure 1 We introduce Adjoint Matching, a theoretically-driven yet simple algorithm for reward fine-tuning that works for a large family of dynamical generative models, including for the first time, Flow Matching models. Text prompts: “Beautiful colorful sunset midst of building in Bangkok Thailand ”, “Beautiful grandma and granddaughter are mixing salad and smiling while cooking in kitchen”, “The beautiful young woman in sunglasses is standing at the background of field and hill. She is smiling and looking over shoulder ”, “Chess, intellectual games, figure horse, chess board ”. + +# 1 Introduction + +Flow Matching (Lipman et al., 2023; Albergo and Vanden-Eijnden, 2023; Liu et al., 2023) and denoising diffusion (Song and Ermon, 2019; Ho et al., 2020; Song et al., 2021b; Kingma et al., 2021) models are being used for many generative modeling applications, including text-to-image (Rombach et al., 2022; Esser et al., 2024), text-to-video (Singer et al., 2022), and text-to-audio (Le et al., 2024; Vyas et al., 2023). In most cases, the base generative model does not achieve the desired sample quality. To improve the generated samples, it is common to resort to techniques such as classifier-free guidance (Ho and Salimans, 2022; Zheng et al., 2023) to get better text-to-sample alignment, or to fine-tune using human preference reward models to improve sample quality and realism (Wallace et al., 2023a; Clark et al., 2024). + +In the adjacent field of large language models, the behavior of the model is aligned to human preferences through fine-tuning with reinforcement learning from human feedback (RLHF). Either explicitly or implicitly, RLHF methods (Ziegler et al., 2020; Stiennon et al., 2020; Ouyang et al., 2022; Bai et al., 2022) assume a reward model $r ( x )$ that captures human preferences, with the goal of modifying the base generative model such that it generates the following tilted distribution: + +$$ +p ^ { * } ( x ) \propto p ^ { \mathrm { b a s e } } ( x ) \exp ( r ( x ) ) , +$$ + +where $p _ { \mathrm { b a s e } }$ is the base generative model’s sample distribution. + +Inspired by this, fine-tuning methods have been developed to improve denoising diffusion models based on human preference data; either using a reward-based approach (Fan and Lee, 2023; Black et al., 2024; Fan et al., 2023; Xu et al., 2023; Clark et al., 2024; Uehara et al., 2024a,b), or direct preference optimization (Wallace et al., 2023a). However, unlike the fine-tuning methods designed for large language models, most of the existing methods to a large degree ignore $p ^ { \mathrm { b a s e } }$ and focus solely on the reward model. Reward models can range from standard evaluation metrics such as ClipScore (Hessel et al., 2021; Kirstain et al., 2023) to specialized models that have been trained on human preferences (Schuhmann and Beaumont, 2022; Xu et al., 2023; Wu et al., 2023c). As these are parameterized by neural networks, they fall pray to adversarial examples which lead to the generation of undesirable artifacts (Goodfellow et al., 2014; Mordvintsev et al., 2015). This has led some works to consider adding regularization during fine-tuning (Fan et al., 2024; Uehara et al., 2024b) to incentivize staying close to the base model distribution; however, there does not yet exist a simple approach which actually provably generates from the tilted distribution (1). + +The main contributions of our paper are as follows: + +(i) We present a stochastic optimal control (SOC) formulation for reward fine-tuning of dynamical generative models. Importantly, we prove that the naïve approach considered by prior works lead to a value function bias problem that biases the fine-tuned model away from the tilted distribution (1). This problem has also been observed by Uehara et al. (2024b) but they propose a more complicated solution which involves training a separate generative model for the optimal noise distribution. +(ii) Instead, we propose a very simple solution: the memoryless noise schedule. This is a unique noise schedule that completely removes the dependency between noise variables and the generated samples, resulting in provable convergence to the tilted distribution. This allows us to fine-tune dynamical generative models in full generality, including being the first to fine-tune noiseless Flow Matching models. +(iii) We also propose a new method for solving SOC problems, called Adjoint Matching, which combines the scalability of gradient-based methods and the simplicity of a least-squares regression objective. This is orthogonal to the reward fine-tuning application and can be applied to general SOC problems. +(iv) We perform extensive comparisons to baseline approaches, and analyze them from multiple perspectives such as realism, consistency, and diversity. We find that our proposed method provides generalization to unseen human preference reward models, better text-to-sample consistency, and retains good diversity. + +In the following, sections are broken down as follows: Section 2 summarizes the algorithms used for sampling from pre-trained Flow Matching and diffusion models, while Section 3 provides a common notation that we will use throughout. Sections 4 and 5 form the core of our contributions. Section 4 details the value function bias problem and our proposed solution via the memoryless noise schedule. Section 5 details the new Adjoint Matching algorithm for solving SOC problems. + +# 2 Preliminaries on dynamical generative models + +We are interested in fine-tuning base generative models $p ^ { \mathrm { b a s e } } ( X _ { 1 } )$ where samples are generated through the simulation of a stochastic process. That is, these models transform noise variables into a sample through an iterative process. In particular, we discuss the specific constructions and sampling processes of Flow Matching (Lipman et al., 2023; Liu et al., 2023; Liu, 2022; Albergo and Vanden-Eijnden, 2023) and Denoising Diffusion Models (Ho et al., 2020; Song et al., 2021b,a). The goal of this section is to provide background information on these methods, which we will later unify into a single consistent notation in Section 3. + +Given random variables from an initial distribution $X _ { 0 } \sim p _ { 0 } = \mathcal { N } ( 0 , I )$ , and $X _ { 1 }$ which are distributed according to some data distribution, we define the reference flow $\bar { \pmb X } = ( \bar { X } _ { t } ) _ { t \in [ 0 , 1 ] }$ where + +$$ +\bar { X } _ { t } = \beta _ { t } \bar { X } _ { 0 } + \alpha _ { t } \bar { X } _ { 1 } , +$$ + +where $( \alpha _ { t } ) _ { t \in [ 0 , 1 ] } , ( \beta _ { t } ) _ { t \in [ 0 , 1 ] }$ are functions such that $\alpha _ { 0 } = \beta _ { 1 } = 0$ and $\alpha _ { 1 } = \beta _ { 0 } = 1$ . Diffusion models and Flow Matching construct generative Markov processes $X _ { t }$ with initial distribution $X _ { 0 } \sim \mathcal { N } ( 0 , I )$ that result in flows $\pmb { X } = ( X _ { t } ) _ { t \in [ 0 , 1 ] }$ with the same time marginals as the reference flow $\bar { X }$ , i.e., the random variables $X _ { t }$ and $X _ { t }$ have identical distribution for all times $t \in [ 0 , 1 ]$ . This implies $X _ { 1 }$ has the same distribution as the data distribution, so simulating the Markov process from random noise $X _ { 0 }$ is a way to generate artificial samples1. + +# 2.1 Flow Matching + +In its simplest form, the generative Markov process of a Flow Matching model is an ordinary differential equation (ODE) of the form: + +$$ +\begin{array} { r } { \mathrm { d } X _ { t } = v ( X _ { t } , t ) \mathrm { d } t , \qquad X _ { 0 } \sim \mathcal { N } ( 0 , I ) . } \end{array} +$$ + +where $v ( X _ { t } , t )$ is a parametric velocity that is optimized to match the derivative of the reference flow, i.e., $\begin{array} { r } { v ( X _ { t } , t ) = \operatorname * { a r g m i n } _ { \hat { v } } \mathbb { E } \big \| \hat { v } ( \bar { X } _ { t } , t ) - \frac { \mathrm { d } } { \mathrm { d } t } \bar { X } _ { t } \big \| ^ { 2 } } \end{array}$ (see e.g. Lipman et al. (2023) for details on pre-training Flow Matching models). It can then be proven that the solution of the generative process (3) has the same time marginals as the reference flow (Lipman et al., 2023; Liu, 2022; Albergo and Vanden-Eijnden, 2023), and a commonly used choice is $\alpha _ { t } = t$ and $\beta _ { t } = 1 - t$ . One can also consider a family of stochastic differential equations (SDEs) with an arbitrary state-independent diffusion coefficient2: + +$$ +\begin{array} { r } { \mathrm { d } X _ { t } = \bigg ( v ( X _ { t } , t ) + \frac { \sigma ( t ) ^ { 2 } } { 2 \beta _ { t } ( \frac { \partial t } { \partial _ { t } } \beta _ { t } - \tilde { \beta } _ { t } ) } \left( v ( X _ { t } , t ) - \frac { \dot { \alpha } _ { t } } { \alpha _ { t } } X _ { t } \right) \bigg ) \mathrm { d } t + \sigma ( t ) \mathrm { d } B _ { t } , \qquad X _ { 0 } \sim \mathcal { N } ( 0 , I ) , } \end{array} +$$ + +where $( B _ { t } ) _ { t \geq 0 }$ is a Brownian motion. The generative processes in (3) and (4) have the same time marginals. This can be seen by writing down the Fokker-Planck equations for (3) and (4), and observing that they are the same up to a cancellation of terms (Maoutsa et al., 2020). The diffusion coefficient $\sigma ( t )$ in (4) is compensated by the second term in the drift which scales proportionally as $\sigma ( t ) ^ { 2 }$ . + +# 2.2 Denoising Diffusion Models + +We next discuss diffusion models, in particular the sampling scheme proposed by Denoising Diffusion Implicit Model (DDIM; Song et al. (2021a)) which we will later relate to Denoising Diffusion Probabilistic Models (DDPM; Ho et al. (2020)) as a particular case of the former. For sampling from a diffusion model, the DDIM update rule $^ 3$ (Song et al. (2021a), Eq. 12), typically stated in discrete time with $k \in \{ 0 , \ldots , K \}$ , is: + +$$ +\begin{array} { r l } { \sqrt { \bar { \alpha } _ { k + 1 } } \big ( \frac { X _ { k } - \sqrt { 1 - \bar { \alpha } _ { k } } \epsilon ( X _ { k } , k ) } { \sqrt { \bar { \alpha } _ { k } } } \big ) + \sqrt { 1 - \bar { \alpha } _ { k + 1 } - \sigma _ { k } ^ { 2 } } \epsilon ( X _ { k } , k ) + \sigma _ { k } \varepsilon _ { k } , } & { { } \quad \varepsilon _ { k } \sim \mathcal { N } ( 0 , I ) , \ X _ { 0 } \sim \mathcal { N } ( 0 , I ) . } \end{array} +$$ + +where $\alpha _ { k }$ is an increasing sequence such that $\bar { \alpha } _ { 0 } = 0$ , $\bar { \alpha } _ { K } = 1$ , and the sequence $\sigma _ { k }$ is arbitrary. That is, one samples an initial Gaussian random variable $x _ { 0 }$ , and applies the stochastic update (5) iteratively $K$ times in order to obtain an artificial sample $X _ { K }$ . Updates can be interpreted as progressively denoising the iterate: $x _ { 0 }$ is completely noisy and $x _ { K }$ is fully denoised. The noise predictor model $\epsilon ( x _ { k } , k )$ is trained to predict the noise of $x _ { k }$ (see e.g. Ho et al. (2020) for details on pre-training denoising diffusion models). + +# 3 Flow Matching and diffusion models from a common perspective + +We formulate Flow Matching and diffusion models in a unified framework, which we will later use throughout the paper. Firstly, to simplify notation, we will be using continuous-time formulations. This will also directly enable fine-tuning methods inspired by the continuous-time paradigm, which we find tends to perform better than discrete-time counterparts in our empirical validations. Secondly, by consolidating notation, we will be able to discuss fine-tuning of dynamical generative models that follow the same time marginals as the reference flow (2), pre-trained with either the Denoising Diffusion or Flow Matching framework, in full generality. + +To convert DDIM to a continuous-time stochastic process, we can show that the DDIM update rule (5), up to a first-order approximation, is equivalent to the Euler-Maruyama discretization of the following SDE: + +$$ +\begin{array} { r } { \mathrm { d } X _ { t } = \big ( \frac { \dot { \alpha } _ { t } } { 2 \bar { \alpha } _ { t } } X _ { t } - \big ( \frac { \dot { \alpha } _ { t } } { 2 \bar { \alpha } _ { t } } + \frac { \sigma ( t ) ^ { 2 } } { 2 } \big ) \frac { \epsilon ^ { \mathrm { b a s e } } ( X _ { t } , t ) } { \sqrt { 1 - \bar { \alpha } _ { t } } } \big ) \mathrm { d } t + \sigma ( t ) \mathrm { d } B _ { t } , \qquad X _ { 0 } \sim \mathcal { N } ( 0 , I ) . } \end{array} +$$ + +See Appendix B.1 for the full derivation. To go from (5) to (6), we assumed a uniform discretization of time, i.e. $\textstyle t = { \frac { k } { K } }$ . This results in identifying the discrete-time process $( X _ { k } ) _ { k \in \{ 0 , \ldots , K \} }$ with a continuous-time process $( X _ { t } ) _ { t \in [ 0 , 1 ] }$ , where $\alpha _ { k } : = \alpha _ { t }$ , $\begin{array} { r } { \sigma _ { k } : = \frac { 1 } { \sqrt { K } } \sigma ( t ) } \end{array}$ , and $\epsilon ( X _ { k } , k )$ with $\epsilon ^ { \mathrm { b a s e } } ( X _ { k } , t )$ . In relation to the reference flow (2),√ the generative process in (6) has the same time marginals when $\alpha _ { t } = \sqrt { \bar { \alpha } _ { t } }$ and $\beta _ { t } = \sqrt { 1 - \bar { \alpha } _ { t } }$ (Ho et al., 2020). + +Furthermore, when viewed up to first order approximations, the DDPM sampling scheme (Ho et al. (2020); Algorithm 2) can be seen as special instance of the DDIM sampling scheme when $\sigma ( t ) = \sqrt { \dot { \bar { \alpha } } _ { t } / \bar { \alpha } _ { t } }$ . This results in the following generative process: + +$$ +\begin{array} { r } { \mathrm { d } X _ { t } = \big ( \frac { \dot { \bar { \alpha } } _ { t } } { 2 \bar { \alpha } _ { t } } X _ { t } - \frac { \dot { \bar { \alpha } } _ { t } } { \bar { \alpha } _ { t } } \frac { \epsilon ^ { \mathrm { b a s e } } ( X _ { t } , t ) } { \sqrt { 1 - \bar { \alpha } _ { t } } } \big ) \mathrm { d } t + \sqrt { \frac { \dot { \bar { \alpha } } _ { t } } { \bar { \alpha } _ { t } } } \mathrm { d } B _ { t } , \qquad X _ { 0 } \sim \mathcal { N } ( 0 , I ) , } \end{array} +$$ + +We can further consolidate notation by converting all quantities to the score function ${ \mathfrak { s } } ( x , t )$ —defined as the gradient of the log density of the random variable $X _ { t }$ —which is possible when $X _ { 0 }$ is Normal-distributed and under the affine reference flow (2). In particular, the velocity $v ^ { \mathrm { b a s e } }$ from Flow Matching can be expressed in terms of the score function (see Appendix B.4): + +$$ +\begin{array} { r } { v ^ { \mathrm { b a s e } } ( x , t ) = \frac { \dot { \alpha } _ { t } } { \alpha _ { t } } x + \beta _ { t } ( \frac { \dot { \alpha } _ { t } } { \alpha _ { t } } \beta _ { t } - \dot { \beta } _ { t } ) \mathfrak { s } ( x , t ) . } \end{array} +$$ + +And the noise predictor $\epsilon ^ { \mathrm { b a s e } }$ also admits an expression in terms of the score function (see Appendix B.3): + +$$ +\begin{array} { r } { \mathfrak { s } ( x , t ) = - \frac { \epsilon ^ { \mathrm { b a s e } } ( x , t ) } { \sqrt { 1 - \bar { \alpha } _ { t } } } . } \end{array} +$$ + +Plugging these two equations into (4) and (6), respectively, and rewriting them in terms of only the $\alpha _ { t }$ and $\beta _ { t }$ in (2), we can unify both the Flow Matching and continuous-time DDIM generative processes as: + +$$ +\begin{array} { r l } & { \mathrm { d } X _ { t } = b ( X _ { t } , t ) \mathrm { d } t + \sigma ( t ) \mathrm { d } B _ { t } , \qquad X _ { 0 } \sim \mathcal { N } ( 0 , I ) , } \\ & { \mathrm { w h e r e ~ } b ( x , t ) = \kappa _ { t } x + \big ( \frac { \sigma ( t ) ^ { 2 } } { 2 } + \eta _ { t } \big ) \mathfrak { s } ( x , t ) , \quad \kappa _ { t } = \frac { \dot { \alpha } _ { t } } { \alpha _ { t } } , \quad \eta _ { t } = \beta _ { t } \big ( \frac { \dot { \alpha } _ { t } } { \alpha _ { t } } \beta _ { t } - \dot { \beta } _ { t } \big ) } \end{array} +$$ + +where $( \alpha _ { t } , \beta _ { t } )$ are coefficients of the reference flow (2). We have hence expressed the generative process of a base model, whether it is a Flow Matching or a diffusion model, as an SDE of the form (10)-(11), unified by the choice of reference flow. This expression has been written before for DDIM, e.g. Bartosh et al. (2024a,b). + +# 4 Fine-tuning as “memoryless” stochastic optimal control + +We now discuss the crux of the problem: how to produce a fine-tuned generative model that produces samples $X _ { 1 }$ which follow the tilted distribution involving a reward model (1). An obvious direction is to construct a fine-tuning objective involving both the base generative model and the reward model, where the optimal solution results in a fine-tuned generative model for the tilted distribution. However, as we will explain, this turns out to be non-trivial, because a naïve formulation will introduce bias into the solution. + +In Section 4.1, we discuss the problem formulation of stochastic optimal control, a general framework for optimizing SDEs, and its relation to the maximum entropy reinforcement learning framework commonly used for RLHF fine-tuning. Next, in Section 4.2, we discuss the initial value function bias problem which plagues existing approaches and so far has seen no simple solution. Finally, in Section 4.3, we propose a novel simple solution that circumvents the bias problem, by enforcing a particular diffusion coefficient, the memoryless noise schedule, to be used during fine-tuning. This results in an extremely simple fine-tuning objective that provably converges to a model which generates the tilted distribution (1) without any statistical bias. + +# 4.1 Preliminaries on the stochastic optimal control problem formulation + +Stochastic optimal control (SOC; Bellman (1957); Fleming and Rishel (2012); Sethi (2018)) considers general optimization problems over stochastic differential equations, but we only need to consider a common instantiation, the quadratic cost control-affine problem formulation: + +$$ +\begin{array} { r l r } & { \underset { u \in \mathcal { U } } { \operatorname* { m i n } } \mathbb { E } \big [ \int _ { 0 } ^ { 1 } \big ( \frac { 1 } { 2 } \| u ( X _ { t } ^ { u } , t ) \| ^ { 2 } + f ( X _ { t } ^ { u } , t ) \big ) \mathrm { d } t + g ( X _ { 1 } ^ { u } ) \big ] , } \\ & { \mathrm { s . t . ~ d } X _ { t } ^ { u } = \big ( b ( X _ { t } ^ { u } , t ) + \sigma ( t ) u ( X _ { t } ^ { u } , t ) \big ) \mathrm { ~ d } t + \sigma ( t ) \mathrm { d } B _ { t } , } & { \quad X _ { 0 } ^ { u } \sim p _ { 0 } } \end{array} +$$ + +where in (13), $X _ { t } ^ { u } \in \mathbb { R } ^ { d }$ is the state of the stochastic process, $u : \mathbb { R } ^ { d } \times [ 0 , 1 ] \to \mathbb { R } ^ { d }$ is commonly referred to as the control vector field, $b : \mathbb { R } ^ { d } \times [ 0 , 1 ] \to \mathbb { R } ^ { d }$ is a base drift, and $\sigma : [ 0 , 1 ] \to \mathbb { R } ^ { d \times d }$ is the diffusion coefficient. These jointly define the controlled process $X ^ { u } \sim p ^ { u }$ that we are interested in optimizing; often both $b$ and $\sigma$ are fixed and we only optimize over the control $u$ . + +As part of the objective functional (12), we have an affine control cost $\textstyle \frac { 1 } { 2 } \| u ( X _ { t } ^ { u } , t ) \| ^ { 2 }$ , a running state cost $f : \mathbb { R } ^ { d } \times [ 0 , 1 ] \to \mathbb { R }$ and a terminal state cost $g : \mathbb { R } ^ { d } \mathbb { R }$ . + +The stochastic optimal control (SOC) objective (12) can be decomposed recursively from the final time value. It is common to define the cost functional which is the expected future cost starting from state $x$ at time $t$ : + +$$ +\begin{array} { r } { J ( u ; x , t ) : = \mathbb { E } _ { X \sim p ^ { u } } \left[ \int _ { t } ^ { 1 } \left( \frac { 1 } { 2 } \| u ( X _ { s } , s ) \| ^ { 2 } + f ( X _ { s } , s ) \right) \mathrm { d } s + g ( X _ { 1 } ) \ \middle | \ X _ { t } = x \right] . } \end{array} +$$ + +From here, the value function is the optimal value of the cost functional4 : + +$$ +\begin{array} { r } { V ( x , t ) : = \operatorname* { m i n } _ { u \in \mathcal { U } } J ( u ; x , t ) = J ( u ^ { * } ; x , t ) , } \end{array} +$$ + +where $u ^ { * }$ is the optimal control, i.e., minimizer of (12). Furthermore, a classical result is that the value function can be expressed in terms of the uncontrolled base process $p ^ { \mathrm { b a s e } }$ (Kappen (2005), see Domingo-Enrich et al. 2023, Eq. 8, App. B for a self-contained proof): + +$$ +\begin{array} { r } { V ( x , t ) = - \log \mathbb { E } _ { X \sim p ^ { \mathrm { b a s e } } } \left[ \exp ( - \int _ { t } ^ { 1 } f ( X _ { s } , s ) \mathrm { d } s - g ( X _ { 1 } ) ) \middle \vert X _ { t } = x \right] . } \end{array} +$$ + +A useful expression for the optimal control (which we will make use of in deriving the Adjoint Matching objective in Section 5) is that it is related to the gradient of the value function: + +$$ +u ^ { * } ( x , t ) = - \sigma ( t ) ^ { \top } \nabla _ { x } V ( x , t ) = - \sigma ( t ) ^ { \top } \nabla _ { x } J ( u ^ { * } , x , t ) . +$$ + +Relation to MaxEnt RL. Stochastic optimal control with the control-affine formulation (12) is the continuoustime equivalence of maximum entropy reinforcement learning (MaxEnt RL; Todorov (2006); Ziebart et al. (2008)) with a KL regularization instead of only an entropy regularization. In particular, by the Girsanov theorem (Theorem 2), the affine control cost is equivalent to a Kullback–Leibler (KL) divergence between the base process $p ^ { \mathrm { b a s e } }$ , when $u = 0$ , and the controlled process $p ^ { u }$ , when conditioned on the same initial state $X _ { 0 }$ (see Appendix C.4): + +$$ +D _ { \mathrm { K L } } \big ( p ^ { u } ( { \pmb X } | X _ { 0 } ) \big | \big | p ^ { b a s e } ( { \pmb X } | X _ { 0 } ) \big ) = \mathbb { E } _ { { \pmb X } ^ { u } \sim p ^ { u } } \left[ \int _ { 0 } ^ { 1 } { \frac { 1 } { 2 } } \| u ( X _ { t } ^ { u } , t ) \| ^ { 2 } \mathrm { d } t \right] , +$$ + +resulting in the KL-regularized RL interpretation of (12): + +$$ +\operatorname* { m a x } _ { u \in \mathcal { U } } \mathbb { E } _ { X _ { 0 } \sim p _ { 0 } } \left[ \mathbb { E } _ { X \sim p ^ { u } ( \cdot \vert X _ { 0 } ) } { \big [ } \int _ { 0 } ^ { 1 } - f ( X _ { t } ^ { u } , t ) \mathrm { d } t - g ( X _ { 1 } ^ { u } ) { \big ] } - D _ { \mathrm { K L } } ( p ^ { u } ( X \vert X _ { 0 } ) \parallel p ^ { b a s e } ( X \vert X _ { 0 } ) ) \right] , +$$ + +where the negative state costs correspond to intermediate and terminal rewards in the RL interpretation. The KL divergence incentivizes the optimal solution to stay close to the distribution of the base process. + +# 4.2 The initial value function bias problem + +We next discuss why naïvely adding a KL regularization does not lead to the tilted distribution (1). From (19), we can also show that the optimal distribution conditioned on $X _ { 0 }$ is5 + +$$ +\begin{array} { r } { p ^ { * } ( X | X _ { 0 } ) \propto p ^ { \mathrm { b a s e } } ( X | X _ { 0 } ) \exp \big ( - \int _ { 0 } ^ { 1 } f ( X _ { t } , t ) \mathrm { d } t - g ( X _ { 1 } ) \big ) . } \end{array} +$$ + +This is analogous to the exponentiated reward distribution in MaxEnt RL (Rawlik et al., 2013), but since we generalize the entropy regularization to a KL regularization, $p ^ { \mathrm { b a s e } }$ acts as a prior distribution. + +In order to relate this to the tilted distribution (1) that we want to achieve for fine-tuning, first notice that the normalization constant of the right-hand side (RHS) of (20) is exactly the value function at $t = 0$ : + +$$ +\begin{array} { r } { \mathbb { E } _ { X \sim p ^ { \mathrm { b a s e } } ( X | X _ { 0 } ) } \left[ \exp \big ( - \int _ { 0 } ^ { 1 } f ( X _ { t } , t ) \mathrm { d } t - g ( X _ { 1 } ) \big ) \right] = \exp \left( - V ( X _ { 0 } , 0 ) \right) , } \end{array} +$$ + +where the equality is due to (16). Dividing the RHS of (20) by (21) and multiplying by $p _ { 0 } ( X _ { 0 } )$ , we obtain the normalized distribution over the full path $\pmb { X }$ , + +$$ +\begin{array} { r } { p ^ { * } ( X ) = p ^ { \mathrm { b a s e } } ( X ) \exp \big ( - \int _ { 0 } ^ { 1 } f ( X _ { t } , t ) \mathrm { d } t - g ( X _ { 1 } ) + V ( X _ { 0 } , 0 ) \big ) . } \end{array} +$$ + +Setting $f = 0$ and $g = - r$ , we arrive at an expression for the optimal distribution + +$$ +p ^ { * } ( X _ { 0 } , X _ { 1 } ) = p ^ { \mathrm { b a s e } } ( X _ { 0 } , X _ { 1 } ) \exp { \left( r ( X _ { 1 } ) + V ( X _ { 0 } , 0 ) \right) } . +$$ + +This unfortunately does not lead to the tilted distribution (1) because we have a bias in the optimal distribution that is due to the value function of the initial distribution $V ( X _ { 0 } , 0 )$ . That is to say, naïvely adding a KL regularization (18) to the fine-tuning objective in the sense of (19) leads to a biased distribution (22) after fine-tuning and is not equivalent to the tilted distribution (1). For instance, when the sampling procedure is noiseless, i.e., $\sigma ( t ) = 0$ , fine-tuning naïvely will not have any effect because $X _ { 0 }$ completely determines $X _ { 1 }$ . + +This is unlike the situation for large language models (Ouyang et al., 2022; Rafailov et al., 2023), where there is no dynamical process that samples $X _ { 1 }$ iteratively and hence no dependence on the initial noise variable $X _ { 0 }$ . Although this KL regularization is a common objective for RLHF of large language models, it has seen seldom use in fine-tuning diffusion models, likely due to this issue of the initial value function bias. + +In the context of diffusion models, KL regularization (19) has been explored in prior works (Fan et al., 2024), but its behavior was not well-understood and they did not relate the fine-tuned model to the tilted distribution (1). Another direction that has been proposed is to learn the initial distribution $p _ { 0 }$ to cancel out the bias (Uehara et al., 2024b; Tang, 2024) but this simply shifts the work into tilting the initial distribution and requires an auxiliary model for parameterizing the optimal initial distribution. In contrast, we show in the next section that it is possible to remove the value function bias by simply choosing a very particular noise schedule during the fine-tuning procedure. + +# 4.3 The memoryless noise schedule for fine-tuning dynamical generative models + +In this section, we propose a very simple method of turning (23) into the tilted distribution (1) through the use of a particular memoryless noise schedule. Throughout, we provide an intuitive explanation of why this noise schedule is sufficient for fine-tuning while discussing the full theoretical result where we show that the memoryless noise schedule is actually not only sufficient but also necessary. + +Intuitively, the main reason we cannot arrive at the tilted distribution from (23) is due to the $p ^ { \mathrm { b a s e } } ( X _ { 0 } , X _ { 1 } )$ distribution not factoring into $X _ { 0 }$ and $X _ { 1 }$ . Hence, we define a memoryless generative process as follows: + +Definition 1 (Memoryless generative process). A generative process of the form (10)-(11) is memoryless if $X _ { 0 }$ and $X _ { 1 }$ are independent, i.e., $p ^ { b a s e } ( X _ { 0 } , X _ { 1 } ) = p ^ { b a s e } ( X _ { 0 } ) p ^ { b a s e } ( X _ { 1 } )$ . + +Table 1 Diffusion coefficient $\sigma ( t )$ and the factors $\kappa _ { t }$ , $\eta _ { t }$ for the Flow Matching, Memoryless Flow Matching, DDIM,√ and DDPM generative processes. When the diffusion coefficient is $\sigma ( t ) = \sqrt { 2 \eta _ { t } }$ , the generative process is memoryless, $i$ .e., samples $X _ { 1 }$ will be independent of the initial noise $X _ { 0 }$ . + +
KtntDiffusion coefficient σ(t) Memoryless Xt
Flow Matching (3)αt αtβt(αt βt − βt)General (commonly 0)No
Memoryless Flow Matching (4)αt αtβt(αt βt − βt)√2tYes
DDIM (6)$rt }$ at$$fra{ }$ 2t$General (commonly 0)No
DDPM (7)$\r }$ a}$ar}$ 2 α}√2ntYes
+ +When the base generative process is memoryless, this implies: + +$$ +\begin{array} { r } { p ^ { * } ( X _ { 1 } ) = \int p ^ { \mathrm { b a s e } } ( X _ { 0 } ) p ^ { \mathrm { b a s e } } ( X _ { 1 } ) \exp ( r ( X _ { 1 } ) + V ( X _ { 0 } , 0 ) ) \mathrm { d } X _ { 0 } \propto p ^ { \mathrm { b a s e } } ( X _ { 1 } ) \exp ( r ( X _ { 1 } ) ) . } \end{array} +$$ + +That is, solving the SOC problem (12)-(13) with a memoryless base model will result in a fine-tuned model that generates samples $p ^ { * } ( X _ { 1 } )$ according to the tilted distribution (1). This memoryless property is not satisfied generally by the family of generative processes captured by (12)-(13). For instance, the Flow Matching and DDIM generative processes with zero diffusion coefficient (i.e., $\sigma ( t ) = 0$ ) are definitely not memoryless due to $X _ { 0 }$ and $X _ { 1 }$ being theoretically invertible. Below, we provide the sufficient and neccessary condition for the noise schedule in order to have a memoryless generative process. + +Proposition 1 (Memoryless noise schedules). Within the family of generative processes (10)-(11), a generative process is memoryless if and only if the noise schedule is chosen as: + +$$ +\begin{array} { r } { ( t ) ^ { 2 } = 2 \eta _ { t } + \chi ( t ) , \ w h e r e \ \chi : [ 0 , 1 ] \to { \mathbb R } \ i s \ s . t . \ \forall t \in ( 0 , 1 ] , \quad \operatorname* { l i m } _ { t ^ { \prime } \to 0 ^ { + } } \alpha _ { t ^ { \prime } } \exp \big ( - \int _ { t ^ { \prime } } ^ { t } \frac { \chi ( s ) } { 2 \beta _ { s } ^ { 2 } } { \mathrm { d } } s \big ) = 0 } \end{array} +$$ + +where $\eta _ { t }$ is the coefficient defined in (11) (see also Table 1). In particular, we refer to $\sigma ( t ) = \sqrt { 2 \eta _ { t } }$ as the memoryless noise schedule. + +Due to the endpoint constraints of $( \alpha _ { t } , \beta _ { t } )$ for the reference flow (2), the memoryless noise schedule $\sigma ( t )$ is infinite at $t = 0$ and approaches zero at $t = 1$ . This provides a way for the generative process to mix when close to noise $X _ { 0 }$ while stay steadying when close to the sample $X _ { 1 }$ . Hence, the sample will have no information about $X _ { 0 }$ due to the enormous amount of mixing with a large diffusion coefficient. Furthermore, while we have intuitively justified the memoryless noise schedule through its independence property, our theoretical result is actually even stronger: all generative models of the form (10)-(11) must be fine-tuned using the memoryless noise schedule. We formalize this in the following theorem, which we prove in Appendix D.2: + +Theorem 1 (Fine-tuning recipe for general noise schedule sampling). Within the family of generative processes (10)-(11), in order to allow the use of arbitrary noise schedules and still generate samples according to the tilted distribution (1), the fine-tuning problem (12)-(13) with $f = 0$ and $g = - r$ must be done with the memoryless noise schedule $\sigma ( t ) = \sqrt { 2 \eta _ { t } }$ . + +Theorem $1$ states that we need to use the memoryless noise schedule for fine-tuning with the SOC objective— or equivalently, the KL regularized reward objective (19). This is the only noise schedule that retains the relationship between the velocity and score function, allowing the conversion to arbitrary noise schedules (e.g., $\sigma ( t ) = 0$ ) after fine-tuning. It is worth noting that when using the memoryless noise schedule for DDIM, this recovers what we derived as the continuous-time limit of the DDPM generative process (7). However, the DDPM sampler (Ho et al., 2020) is not commonly used while the DDIM sampler (Song et al., 2021a) and Flow Matching models typically generate samples using $\sigma ( t ) = 0$ , so an explicit conversion to the memoryless noise schedule is necessary for fine-tuning. To the best of our knowledge, we are not aware of any existing works that have proposed a time-varying diffusion coefficient with theoretical guarantees. Table 1 summarizes the memoryless schedule for diffusion and Flow Matching models, which we refer to as Memoryless Flow Matching. In Figure 2, we visualize fine-tuning a 1D model, where we see that constant $\sigma ( t )$ leads to biased distributions whereas the memoryless noise schedule perfectly converges to the tilted distribution (1). + +![](images/figures/adjoint-matching-fig-0002.jpg) +Figure 2 Visualization of Theorem 1 showing that fine-tuning must be done with the memoryless noise schedule to ensure convergence to the tilted distribution (1). (a) Shows the base Flow Matching model. (b, c) Fine-tuning using a constant $\sigma ( t )$ leads to biased distributions. (d) Fine-tuning using the memoryless noise schedule leads to the correct tilted distribution. Note that sample generation can use any noise schedule after fine-tuning, including $\sigma ( t ) = 0$ . + +For convenience, we plug the memoryless noise schedule into the controlled process for fine-tuning (13), and express them in terms of each respective framework. Let $\epsilon ^ { \mathrm { b a s e } }$ , $v ^ { \mathrm { b a s e } }$ denote the pre-trained vector fields and $\epsilon ^ { \mathrm { f i n e t u n e } }$ , $v ^ { \mathrm { f i n e t u n e } }$ the fine-tuned vector fields. Then we have the following expressions for the full drift $b ( \boldsymbol { x } , t ) + \sigma ( t ) u ( \boldsymbol { x } , t )$ and control $\boldsymbol { u } ( \boldsymbol { x } , t )$ when $\sigma ( t ) = \sqrt { 2 \eta _ { t } }$ : + +${ D D I M } / { \ D D P M }$ + +$$ +\begin{array} { r } { x , t ) + \sigma ( t ) u ( x , t ) = \frac { \dot { \hat { \alpha } } _ { t } } { 2 \hat { \alpha } _ { t } } x - \frac { \dot { \hat { \alpha } } _ { t } } { \hat { \alpha } _ { t } } \frac { \epsilon ^ { \mathrm { f i n e t u m e } } ( x , t ) } { \sqrt { 1 - \hat { \alpha } _ { t } } } , \qquad u ( x , t ) = - \sqrt { \frac { \dot { \alpha } _ { t } } { \hat { \alpha } _ { t } ( 1 - \hat { \alpha } _ { t } ) } } \big ( \epsilon ^ { \mathrm { f i n e t u m e } } ( x , t ) - \epsilon ^ { \mathrm { b a s e } } ( x , t ) \big ) } \end{array} +$$ + +Memoryless Flow Matching: + +$$ +\begin{array} { r l r l } & { b ( x , t ) + \sigma ( t ) u ( x , t ) = 2 v ^ { \mathrm { f i n e t u m e } } ( x , t ) - \frac { \dot { \alpha } _ { t } } { \alpha _ { t } } x , } & & { u ( x , t ) = \sqrt { \frac { 2 } { \beta _ { t } ( \frac { \alpha _ { t } } { \alpha _ { t } } \beta _ { t } - \bar { \beta } _ { t } ) } } \big ( v ^ { \mathrm { f i n e t u m e } } ( x , t ) - v ^ { \mathrm { b a s e } } ( x , t ) \big ) } \end{array} +$$ + +Thus, to solve the SOC problem (12)-(13) in practice, we parameterize the control $u$ in terms of $\epsilon ^ { \mathrm { f i n e t u n e } }$ or $v ^ { \mathrm { f i n e t u n e } }$ and optimize these vector fields instead. After plugging in (26)-(27), the SOC problem (12)-(13) can then be solved using any SOC algorithm in order to perform fine-tuning, and we proposed an especially effective algorithm next in Section 5. After fine-tuning, $\epsilon ^ { \mathrm { f i n e t u n e } }$ and $v ^ { \mathrm { f i n e t u n e } }$ can simply be plugged back into their respective generative processes (3)-(7) to sample from the tilted distribution (1) using any choice of diffusion coefficient. + +# 5 Adjoint Matching for control-affine stochastic optimal control + +We discuss existing methods and also propose a new method for optimizing control-affine SOC problems. The new Adjoint Matching method is a combination of the time-tested continuous adjoint method (Pontryagin, 1962) with recent developments on constructing least-squares objectives for solving SOC problems (Domingo-Enrich et al., 2023). In this section, we briefly discuss preliminaries on existing methods, their pros and cons, then detail the Adjoint Matching algorithm and its surprising connections to the prior methods. For numerical optimization, we now assume that the control $u$ is a parametric model with parameters $\theta$ . + +# 5.1 Existing methods for stochastic optimal control + +# 5.1.1 The adjoint method + +The most basic method of optimizing the simulation of an SDE is to directly differentiate through the simulation using gradients from the SOC objective function (Han and E, 2016). The adjoint method simply uses the objective: + +$$ +\begin{array} { r } { \mathcal { L } ( u ; \boldsymbol { X } ) : = \int _ { 0 } ^ { 1 } \left( \frac { 1 } { 2 } \| u ( \boldsymbol { X } _ { t } , t ) \| ^ { 2 } + f ( \boldsymbol { X } _ { t } , t ) \right) \mathrm { d } t + g ( \boldsymbol { X } _ { 1 } ) , \qquad \boldsymbol { X } \sim p ^ { u } . } \end{array} +$$ + +This is a stochastic estimate of the control objective in (12), and the goal is to take compute the gradient of $\mathcal { L } ( u ; X )$ with respect to the parameters $\theta$ of the control $u$ . Due to the continuous-time nature of SDEs, there are two main approaches to implementing this numerically. Firstly, the Discrete Adjoint method uses a “discretize-then-differentiate” approach, where the numerical solver for simulating the SDE is simply stored in memory then differentiated through, and it has been studied extensively (e.g., Bierkens and Kappen (2014); Gómez et al. (2014); Hartmann and Schütte (2012); Kappen et al. (2012); Rawlik et al. (2013); Haber and Ruthotto (2017)). This approach, however, uses an extremely large amount of memory as the full computational graph of the numerical solver must be stored in memory and implementations often must rely on gradient checkpointing (Chen et al., 2016) to reduce memory usage. + +Secondly, the Continuous Adjoint method exploits the continuous-time nature of SDEs and uses an analytical expression for the gradient of the control objective with respect to the intermediate states $X _ { t }$ , expressed as an adjoint ODE, and then applies a numerical method to simulate this gradient itself, hence it is referred to as a “differentiate-then-discretize” approach (Pontryagin, 1962; Chen et al., 2018; Li et al., 2020). We first define the adjoint state as: + +$$ +\begin{array} { r } { a ( t ; \mathbf { \nabla } _ { } \boldsymbol { X } , u ) : = \nabla _ { \boldsymbol { X } _ { t } } \big ( \int _ { t } ^ { 1 } \big ( \frac { 1 } { 2 } \| u ( \boldsymbol { X } _ { t ^ { \prime } } , t ^ { \prime } ) \| ^ { 2 } + f ( \boldsymbol { X } _ { t ^ { \prime } } , t ^ { \prime } ) \big ) \mathrm { d } t ^ { \prime } + g ( \boldsymbol { X } _ { 1 } ) \big ) , } \\ { \mathrm { w h e r e ~ } \mathbf { \nabla } _ { \boldsymbol { X } } \mathrm { ~ s o l v e s ~ d } \boldsymbol { X } _ { t } = \big ( b ( \boldsymbol { X } _ { t } , t ) + \sigma ( t ) u ( \boldsymbol { X } _ { t } , t ) \big ) \mathrm { ~ d } t + \sigma ( t ) \mathrm { d } B _ { t } . } \end{array} +$$ + +This implies that $\operatorname { \mathbb { E } } _ { X \sim p ^ { u } } \left[ a ( t ; X , u ) \mid X _ { t } = x \right] = \nabla _ { x } J ( u ; x , t )$ , where $J$ denotes the cost functional defined in (14). It can then be shown that this adjoint state satisfies 6: + +$$ +\begin{array} { r l } & { { \frac { \mathrm { d } } { \mathrm { d } t } a \mathrm { ( } t ; X , u \mathrm { ) } } = - [ a ( t ; X , u ) ^ { \top } ( \nabla _ { X _ { t } } ( b ( X _ { t } , t ) + \sigma ( t ) u ( X _ { t } , t ) ) ) + \nabla _ { X _ { t } } ( f ( X _ { t } , t ) + { \frac { 1 } { 2 } } \| u ( X _ { t } , t ) \| ^ { 2 } ) } \\ & { \quad a ( 1 ; X , u ) = \nabla g ( X _ { 1 } ) . } \end{array} +$$ + +The adjoint state is solved backwards in time, starting from the terminal condition (31). Computation of (30) can be efficiently done as a vector-Jacobian product on automatic differentiation software (Paszke et al., 2019). Once the adjoint state has been solved for $t \in [ 0 , 1 ]$ , then the gradient of $\mathcal { L } ( u ; X )$ with respect to the parameters $\theta$ can be obtained by integrating over the entire time interval: + +$$ +\begin{array} { r } { \frac { \mathrm { d } \mathcal { L } } { \mathrm { d } \theta } = \frac { 1 } { 2 } \int _ { 0 } ^ { 1 } \frac { \partial } { \partial \theta } \| u ( X _ { t } , t ) \| ^ { 2 } \mathrm { d } t + \int _ { 0 } ^ { 1 } \frac { \partial u ( X _ { t } , t ) } { \partial \theta } ^ { \top } \sigma ( t ) ^ { \top } a ( t ; \mathbf { X } , u ) \mathrm { d } t , } \end{array} +$$ + +where the first term is the partial derivative of $\mathcal { L }$ w.r.t. $\theta$ and the second term is the partial derivative through the sample trajectory $\pmb { X }$ . See Proposition 6 in Appendix E.1 for a statement and proof of this result. The discrete and continuous adjoint methods converge to the same gradient as the step size of the numerical solvers go to zero. Both are scalable to high dimensions and have seen their fair share of usage in optimizing neural ODE/SDEs (Chen et al., 2018, 2021; Li et al., 2020). As the adjoint methods are essentially gradient-based optimization algorithms applied on a highly non-convex problem, many have also reported they can be unstable empirically (Mohamed et al., 2020; Suh et al., 2022; Domingo-Enrich et al., 2023). + +# 5.1.2 Importance-weighted matching objectives for regressing onto the optimal control + +An alternative is to consider regressing onto the optimal control $u ^ { * }$ , which is the approach of the cross-entropy method (Rubinstein and Kroese, 2013; Zhang et al., 2014) and stochastic optimal control matching (SOCM; Domingo-Enrich et al. (2023)). These methods make use of path integral theory (Kappen, 2005) to express + +the optimal control through importance sampling, resulting in an importance-weighted least-squares objective function + +$$ +\begin{array} { r } { \mathcal { L } _ { \mathrm { S O C M } } ( u ; \boldsymbol { X } ) : = \int _ { 0 } ^ { 1 } \| u ( X _ { t } , t ) - \hat { u } ^ { * } ( X _ { t } , t ) \| ^ { 2 } \mathrm { d } t \times \omega ( u , \boldsymbol { X } ) , \qquad \boldsymbol { X } \sim p ^ { u } , } \end{array} +$$ + +where $\omega$ is an importance weighting that approximates sampling from the optimal distribution $p ^ { * }$ , and $\hat { u } ^ { * }$ is a stochastic estimator of the optimal control relying on having sampled from the optimal process. We defer to Domingo-Enrich et al. (2023) for the exact details. The functional landscape of this objective is convex, which is argued to help yield stable training. However, the need for importance sampling renders this impractical for high dimensional applications: the variance of the importance weighting $\omega$ grows exponentially with dimension of the stochastic process, leading to catastrophic failure. This unfortunately means that such importance-weighted matching objectives are impractical for fine-tuning dynamical generative models; however, a least-squares objective is greatly coveted as it can lead to stable training and simple interpretations. + +# 5.2 Adjoint Matching + +We make two important observations which lead to our proposed method: $( i )$ it is possible to construct a matching objective without any importance weighting, and $( i i )$ there are unnecessary terms in the adjoint differential equation (30) that can lead to higher variance at convergence. + +Firstly, we notice that we can simply match the gradient of the cost functional under the current control. That is, while SOCM carefully constructs an importance-weighted estimator of the optimal control $u ^ { * } =$ $- \sigma ( t ) ^ { 1 } \nabla J ( u ^ { * } ; x , t )$ (17), we claim that we can actually just regress onto the target vector field $- \sigma ( t ) ^ { 1 } \nabla J ( u ; x , t )$ where $u$ is the current control, and furthermore, this results in a gradient equal in expectation to the continuous adjoint method. We formalize this in the following proposition, proven in Appendix E.2: + +Proposition 2. Let us define, for now, the basic Adjoint Matching objective as: + +$$ +\begin{array} { r } { \mathrm { a s i c - A d j - M a t h } ( u ; X ) : = \frac { 1 } { 2 } \int _ { 0 } ^ { 1 } \left\| u ( X _ { t } , t ) + \sigma ( t ) ^ { \mathsf { T } } a ( t ; X , \bar { u } ) \right\| ^ { 2 } \mathrm { d } t , \qquad X \sim p ^ { \bar { u } } , \quad \bar { u } = s t o p g r a d ( u ; X ) . } \end{array} +$$ + +where $\bar { u } = s t o p g r a d ( u )$ means that the gradients of $\bar { u }$ with respect to the parameters $\theta$ of the control u are artificially set to zero. The gradient of $\mathcal { L } _ { \mathrm { B a s i c - A d j - M a t c h } } ( u ; X )$ with respect to $\theta$ is equal to the gradient $\frac { \mathrm { d } { \mathcal { L } } } { \mathrm { d } \theta }$ in equation (32). Importantly, the only critical point of $\mathbb { E } \left[ \mathcal { L } _ { \mathrm { B a s i c - A d j - M a t c h } } \right]$ is the optimal control $u ^ { * }$ . + +Critical points of $\mathcal { L }$ are controls $u$ such that $\begin{array} { r } { \frac { \delta } { \delta u } \mathcal { L } ( u ) = 0 } \end{array}$ , where $\begin{array} { r } { \frac { \delta } { \delta u } \mathcal { L } } \end{array}$ denotes the first variation of the functional $\mathcal { L }$ . In other words, Proposition 2 states that the only control that satisfies the first-order optimality condition for the basic Adjoint Matching objective is the optimal control, which provides theoretical grounding for gradient-based optimization algorithms. + +An intuitive way to understand the basic Adjoint Matching objective is that it is a consistency loss. The Adjoint Matching objective is based off of the observation that the optimal control $\boldsymbol { u } ^ { * } ( x , t )$ is the unique fixed-point of the relation $\boldsymbol { u } ( \boldsymbol { x } , t ) = - \sigma ( t ) ^ { \mathsf { I } } \nabla _ { \boldsymbol { x } } J ( \boldsymbol { u } ; \boldsymbol { x } , t )$ (see Lemma 6 in Appendix E.2) and so we are directly optimizing for a control that fits this relation, while using the adjoint state as a stochastic estimator of $\nabla _ { x } J ( u ; x , t )$ (29). + +The basic Adjoint Matching objective in Proposition 2 does not yet yield a novel algorithm for stochastic optimal control, because it produces the same gradient as the continuous adjoint method. This can be seen by taking the gradient w.r.t. $\theta$ after expanding the square in (34) and removing terms that do not depend on $\theta$ to arrive exactly at the continuous adjoint method (32). However, it provides the means of deriving a simpler leaner objective function. + +The “Lean” Adjoint. The minimizer of a least-squares objective is the conditional expectation of the regression target, so for the Adjoint Matching objective, at the optimum we have that + +$$ +u ^ { * } ( x , t ) = \mathbb { E } _ { X \sim p ^ { * } } \left[ - \sigma ( t ) ^ { \top } a ( t ; X , u ^ { * } ) | X _ { t } = x \right] . +$$ + +Multiplying both sides by the Jacobian $\nabla _ { x } u ^ { * } ( x , t )$ and re-arranging, we get the relation + +$$ +\mathbb { E } _ { X \sim p ^ { * } } \left[ u ^ { * } ( x , t ) ^ { \mathsf { T } } \nabla _ { x } u ^ { * } ( x , t ) + a ( t ; X , u ^ { * } ) ^ { \mathsf { T } } \sigma ( t ) \nabla _ { x } u ^ { * } ( x , t ) \mid X _ { t } = x \right] = 0 . +$$ + +Input: Pre-trained FM velocity field $v ^ { \mathrm { b a s e } }$ , step size $h$ , number of fine-tuning iterations $N$ . Initialize fine-tuned vector fields: $v ^ { \mathrm { f i n e t u n e } } = v ^ { \mathrm { b a s e } }$ with parameters $\theta$ . for $n \in \{ 0 , \ldots , N - 1 \}$ do + +Sample $m$ trajectories $\pmb { X } = ( X _ { t } ) _ { t \in \{ 0 , \ldots , 1 \} }$ with memoryless noise schedule $\begin{array} { r } { \sigma ( t ) = \sqrt { 2 \beta _ { t } ( \frac { \dot { \alpha } _ { t } } { \alpha _ { t } } \beta _ { t } - \dot { \beta } _ { t } ) } } \end{array}$ , e.g.: + +$$ +\begin{array} { r } { X _ { t + h } = X _ { t } + h \left( 2 v _ { \theta } ^ { \mathrm { f n e t u n e } } ( X _ { t } , t ) - \frac { \dot { \alpha } _ { t } } { \alpha _ { t } } X _ { t } \right) + \sqrt { h } \sigma ( t ) \varepsilon _ { t } , \qquad \varepsilon _ { t } \sim \mathcal { N } ( 0 , I ) , \qquad X _ { 0 } \sim \mathcal { N } ( 0 , I ) . } \end{array} +$$ + +For each trajectory, solve the lean adjoint $O D E$ (38)-(39) backwards in time from $t = 1$ to $_ 0$ , e.g.: + +$$ +\begin{array} { r l r } { \tilde { a } _ { t - h } = \tilde { a } _ { t } + h \tilde { a } _ { t } ^ { \mathsf { T } } \nabla _ { X _ { t } } \left( 2 v ^ { \mathrm { b a s e } } ( X _ { t } , t ) - \frac { \dot { \alpha } _ { t } } { \alpha _ { t } } X _ { t } \right) , } & { } & { \tilde { a } _ { 1 } = - \nabla _ { X _ { 1 } } r ( X _ { 1 } ) . } \end{array} +$$ + +Note that $X _ { t }$ and $\ddot { a } _ { t }$ should be computed without gradients, i.e., $X _ { t } = \tt s t o p g r a d ( X _ { t } )$ , $\tilde { \boldsymbol { a } } _ { t } = \mathsf { s t o p g r a d } ( \tilde { \boldsymbol { a } } _ { t } )$ + +For each trajectory, compute the Adjoint Matching objective (37): + +$$ +\begin{array} { r } { \mathcal { L } _ { \mathrm { A d j - M a t c h } } ( \theta ) = \sum _ { t \in \{ 0 , \dots , 1 - h \} } \big \| \frac { 2 } { \sigma ( t ) } \big ( v _ { \theta } ^ { \mathrm { f u n e t u n e } } ( X _ { t } , t ) - v ^ { \mathrm { b a s e } } ( X _ { t } , t ) \big ) + \sigma ( t ) \tilde { a } _ { t } \big \| ^ { 2 } . } \end{array} +$$ + +Compute the gradient $\nabla _ { \boldsymbol { \theta } } \mathcal { L } ( \boldsymbol { \theta } )$ and update $\theta$ using favorite gradient descent algorithm. + +Output: Fine-tuned vector field $v$ finetune + +Notice that the terms inside the expectation in (36) show up as part of the adjoint differential equation (30), which we have now shown to have expectation zero at the optimal solution. Therefore, we motivate the definition of a lean adjoint state $\tilde { a }$ with the terms in (36) removed. Plugging this lean adjoint back into the least-squares objective, we obtain our final proposed Adjoint Matching objective: + +$$ +\begin{array} { r l } { \mathcal { L } _ { \mathrm { A d j - M a t c h } } ( u ; \mathbf { X } ) : = \frac { 1 } { 2 } \int _ { 0 } ^ { 1 } \left\| u ( X _ { t } , t ) + \sigma ( t ) ^ { \top } \tilde { a } ( t ; \mathbf { X } ) \right\| ^ { 2 } \mathrm { d } t , } & { \quad \mathbf { X } \sim p ^ { \bar { u } } , \quad \bar { u } = \mathbf { s t o p g r a d } ( u ) , } \\ { \mathrm { w h e r e } } & { ~ \frac { \mathrm { d } } { \mathrm { d } t } \tilde { a } ( t ; \mathbf { X } ) = - ( \tilde { a } ( t ; \mathbf { X } ) ^ { \top } \nabla _ { x } b ( X _ { t } , t ) + \nabla _ { x } f ( X _ { t } , t ) ) , } \\ & { ~ \tilde { a } ( 1 ; \mathbf { X } ) = \nabla _ { x } g ( X _ { 1 } ) . } \end{array} +$$ + +Equations (38)-(39) define the lean adjoint state, and (37) is the complete Adjoint Matching objective. The unique critical point of $\mathbb { E } [ \mathcal { L } _ { \mathrm { A d j - M a t c h } } ]$ is the optimal control, which we prove relying on Proposition 2 and equation (36) (see Proposition 7 in Appendix E.3). + +Compared to the importance sampling methods (Section 5.1.2), Adjoint Matching is a simple least-squares regression objective and has no importance weighting. This allows it to avoid the pitfalls of high variance importance weights and makes it as scalable as the adjoint methods while retaining the interpretation of matching a target vector field. + +Compared to the adjoint method (Section 5.1.1), Adjoint Matching produces a different gradient in expectation than the continuous adjoint. This is because the lean adjoint state is not related to the gradient of the cost functional anymore, i.e., (29) is not true, except at the optimum when $u = u ^ { * }$ . Even at the optimal solution, since Adjoint Matching removes terms that have expectation zero, it can potentially exhibit better convergence and lower variance than the continuous adjoint method. Additionally, computation of the lean adjoint state (38) also exhibits a smaller computational cost due to the removal of the extra terms (no longer need the Jacobian of the control $\nabla _ { x } u$ ). We provide a rigorous derivation of Adjoint Matching and the above claims in Appendix E.3. + +Adjoint Matching can be applied to reward fine-tuning of dynamical generative models through the memoryless SOC formulation discussed in Section 4. We provide pseudo-code for this in Algorithm 1 for Flow Matching models and in Algorithm 2 in Appendix E.4 for denoising diffusion models. + +# 6 Related work + +Fine-tuning from human feedback. There are two main overarching approaches to RLHF: the reward-based approach (Ziegler et al., 2020; Stiennon et al., 2020; Ouyang et al., 2022; Bai et al., 2022) and direct preference optimization (DPO; Rafailov et al. (2023)). The reward-based approach (Ziegler et al., 2020; Stiennon et al., 2020; Ouyang et al., 2022; Bai et al., 2022) consists in learning the reward model $r ( x )$ from human preference data, and then solving a maximum entropy RL problem with rewards produced by $r ( x )$ . DPO merges the two previous steps into one: there is no need to learn $r ( x )$ as human preference data is directly used to fine-tune the model. However, DPO is typically only applied with a filtered dataset, and does not work explicitly with a reward model. Furthermore, for flow and diffusion models specifically, it is possible to differentiate the reward function, so there is a larger emphasis on reward-based approaches. + +Fine-tuning for diffusion models. Among existing reward-based diffusion fine-tuning methods, Fan and Lee (2023) interpret the denoising process as a multi-step decision-making task and use policy gradient algorithms to fine-tune diffusion samplers. Black et al. (2024) makes use of proximal policy gradients for fine-tuning but this does not make use of the differentiability of the reward model. Fan et al. (2023) also consider KL-regularized rewards (19) but do not make the critical connection to the tilted distribution (1) that we flesh out in Section 4.2. The fine-tuning algorithms of $\mathrm { X }$ u et al. (2023); Clark et al. (2024) directly take gradients of the reward model and use heuristics to try to stay close to the original base generative model, but their behavior is not well understood and unrelated to the tilted distribution: Xu et al. (2023) takes gradients of the reward applied on the denoised sample at different points in time, and Clark et al. (2024) backpropagates the reward function through all or part of the diffusion trajectory. Finally, Uehara et al. (2024b) also fine-tune diffusion models with the goal of sampling from the tilted distribution (1), but their approach is much more involved than ours as it requires learning a value function, and solving two stochastic optimal control problems. Additional reward fine-tuning works include Bruna and Han (2024), that provide theoretical guarantees to sample from the tilted distribution when the reward is a quadratic function, and Zhang et al. (2024), that propose a reward fine-tuning algorithm for the GFlowNet architecture. + +Inference-time optimization methods. Some have proposed methods that do not update the base model but instead modify the generation process directly. One approach is to add a guidance term to the velocity (Chung et al., 2022; Song et al., 2023; Pokle et al., 2023); however, this is a heuristic and it is not well-understood what particular distribution is being generated. Another approach is to directly optimize the initial noise distribution (Li, 2021; Wallace et al., 2023b; Ben-Hamu et al., 2024); this is taking an opposite approach to the inital value bias problem than us by moving all of the work into optimizing the initial distribution. A more computationally intensive approach is to perform online estimation of the optimal control, for the purpose of heuristically solving an optimal control problem within the sampling process (Huang et al., 2024; Rout et al., 2024); these approaches aim to solve a separate control problem for each generated sample, instead of performing amortization (Amos et al., 2023) to learn a fine-tuned generative model. + +Optimal control in generative modeling. Methods from optimal control have been used to train dynamical generative models parameterized by ODEs (Chen et al., 2018), SDEs (Li et al., 2020), and jump processes (Chen et al., 2021), enabled through the adjoint method. They can be used to train arbitrary generative processes, but for simplified constructions these have fallen in favor of simulation-free matching objectives such as denoising score matching (Vincent, 2011) and Flow Matching (Lipman et al., 2023). The optimal control formalism also has significance in sampling from un-normalized distributions (Zhang and Chen, 2022; Berner et al., 2023; Vargas et al., 2023, 2022; Richter and Berner, 2024; Tzen and Raginsky, 2019). The inclusion of a state cost has been used to solve transport problems where intermediate path distributions are of importance (Liu et al., 2024; Pooladian et al., 2024). These collective advances naturally lead to the consideration of the optimal control formalism for reward fine-tuning. + +Conditional sampling in inverse problems. Denker et al. (2024) and Wu et al. (2023a) independently consider a pre-trained diffusion model $p ( x )$ , and an observation $y$ on the generated sample $x$ , as well as the analytic likelihood $p ( y | x )$ . Their aim is to sample from the posterior $p ( x ) p ( y | x )$ , and their applications include inpainting, class-conditional generation, super-resolution, phase retrieval, non-linear deblurring, computed tomography, and protein design. Their setting reduces to a particular case of our reward fine-tuning framework by setting $r ( x ) = \log p ( y | x )$ . Denker et al. (2024) formulate an SOC problem, and they solve it via the log-variance loss (Richter et al. (2020); Nüsken and Richter (2021)), and the moment loss (Nüsken and Richter, 2021)7, which they refer to as the trajectory balance loss (Malkin et al., 2023). Wu et al. (2023a) propose Twisted Diffusion Sampler, an algorithm based on Sequential Monte Carlo that uses increased inference-time compute to reduce bias. A third work that also tackles the conditional sampling problem is Du et al. (2024), which use a Lagrangian formulation that they solve approximately using Gaussian paths. + +Table 2 Evaluation metrics of different fine-tuning methods for text-to-image generation. The second and third columns show the noise schedules $\sigma ( t )$ used for fine-tuning and for sampling: $\sigma ( t ) = \sqrt { 2 \eta _ { t } }$ corresponds to Memoryless Flow Matching, and $\sigma ( t ) = 0$ to the Flow Matching ODE (3). We report standard errors estimated over 3 runs of the fine-tuning algorithm on random sets of 40000 training prompts, each evaluated over a random set of 1000 test prompts. + +
Fine-tuning MethodFine-tuning σ(t)Sampling σ(t)ClipScore ↑PickScore ↑HPS v2↑DreamSim Diversity↑
None (Base model)N/A√2ηt 024.15±0.26 28.32±0.2217.25±0.06 18.15±0.0716.19±0.17 17.89±0.1653.60±1.37 56.53±1.52
DRaFT-1 sesg DRaFT-40√2t 0 √2t 0√2ηt 0 √2t 030.18±0.24 30.95±0.28 26.94±0.28 30.07±0.3919.38±0.08 19.37±0.06 18.34±0.19 19.45±0.0824.61±0.17 24.37±0.17 19.98±1.02 24.06±0.2425.54±0.99 27.39±1.14 41.98±2.14 36.53±1.69
DPO ReFL√2t 0 √2t√2t 0 √2t24.11±0.22 27.77±0.18 28.59±0.3117.24±0.06 17.92±0.07 18.68±0.1016.15±0.14 17.30±0.20 22.24±0.4653.27±1.36 54.11±1.50 32.71±2.76
Cont. Adjoint λ = 12500 Nae occc0 √2t0 √2nt 030.06±0.63 26.99±0.43 29.49±0.3219.07±0.21 18.33±0.16 18.98±0.1623.06±0.41 20.83±0.63 21.34±0.5332.69±1.28 46.59±1.40 48.41±1.44
Disc. Adjoint λ = 12500√2t√2ηt 028.04±0.57 29.28±0.1718.44±0.21 18.82±0.1420.04±0.39 19.73±0.1754.90±2.03 53.36±2.48
Adj.-Matching λ = 1000√2t√2t 030.36±0.22 31.41±0.2219.29±0.08 19.57±0.0924.12±0.17 23.29±0.1840.89±1.50 43.10±1.76
Adj.-Matching λ = 2500√2t√2t 030.59±0.40 31.64±0.2119.49±0.10 19.71±0.0924.85±0.23 24.12±0.2737.07±1.47 39.88±1.59
Adj.-Matching λ = 12500√2t√2t 030.62±0.30 31.65±0.1919.50±0.09 19.76±0.0824.95±0.28 24.49±0.2734.50±1.33 37.24±1.57
+ +# 7 Experiments + +We experimentally validate our proposed method on reward fine-tuning a Flow Matching base model (Lipman et al., 2023). In particular, we use the usual setup of pre-training an autoencoder for $5 1 2 \times 5 1 2$ resolution images, then training a text-conditional Flow Matching model on the latent variables with a U-net architecture (Long et al., 2015), similar to the setup in Rombach et al. (2022). We pre-trained our base model using a dataset of licensed text and image pairs. Then for fine-tuning, we consider the reward function: + +$$ +r ( x ) : = \lambda \times \mathtt { R e w a r d M o d e l } ( x ) +$$ + +corresponding to a scaled version of the reward model, which we take to be ImageReward (Xu et al., 2023). +Different values of $\lambda$ provide different tradeoffs between the KL regularization and the reward model (19). + +![](images/figures/adjoint-matching-fig-0003.jpg) +Figure 3 Our proposed Adjoint Matching using the memoryless SOC formulation introduces a much more principled way of trading off how close to stay to the base model while optimizing the reward model. In contrast, baseline methods such as DRaFT-1 only optimize the reward model and must rely on early stopping to perform this trade off, resulting in a much more sensitive hyperparameter. Samples are produced using $\sigma ( t ) = 0$ with the same noise sample. Text prompts: “Handsome Smiling man in blue jacket portrait” and “Quinoa and Feta Stuffed Baby Bell Peppers”. + +![](images/figures/adjoint-matching-fig-0004.jpg) + +Text prompt: “Man sitting on sofa at home in front of fireplace and using laptop computer, rear view ” + +![](images/figures/adjoint-matching-fig-0005.jpg) +Text prompt: “3D World Food Day Morocco” +Figure 4 Generated samples from varying classifier-free guidance weight $w$ , from an Adjoint Matching fine-tuned model. Higher guidance increases text-to-image consistency but loses diversity and has use cases for generating highly structured images such as 3D renderings. Corresponding samples from the base model can be found in Figure 7. + +For evaluation and benchmarking purposes, we report metrics that separately quantify text-to-image consistency, human preference, and sample diversity, capturing the tradeoff between each aspect of generative models (Astolfi et al., 2024). For consistency, we make use of the standard ClipScore (Hessel et al., 2021) and PickScore (Kirstain et al., 2023); for generalization to unseen human preferences, we use the HPSv2 model (Wu et al., 2023b); and for diversity, we compute averages of pairwise distances of the DreamSim features (Fu et al., 2023). More details are provided in Appendix G.4. + +As our baselines, we consider the DPO (Wallace et al., 2023a), ReFL (Xu et al., 2023), and DRaFT-K algorithms (Clark et al., 2024). DPO does not use gradients from the reward function, while ReFL and DRaFT make use of heuristic gradient stopping approaches to stay close to the base generative model. Out of these baseline methods, we find that DRaFT-1 performs the best, so we perform additional ablation experiments comparing to this method. Within the same SOC formulation as our method, we also consider the discrete and continuous adjoint methods. We provide full experimental details in Appendix G; an important implementation detail is that we slightly offset $\sigma ( t )$ in order to avoid division by zero. + +![](images/figures/adjoint-matching-fig-0006.jpg) +Figure 5 Tradeoffs between different aspects of generative models: text-to-image consistency (ClipScore), sample diversity for each prompt (DreamSim Diversity), and generalization to unseen human preferences (HPS v2). Different points are obtained from varying values of $\lambda$ for Adjoint Matching and varying number of fine-tuning iterations for the DRaFT-1 baseline. Overall, we find our proposed method Adjoint Matching has the best Pareto fronts. + +Evaluation results. In Table 2 we report the evaluation metrics for the baselines as well as our proposed Adjoint Matching approach. We compare each method at roughly the same wall clock time (see the times and number of iterations in Table 4, and comments in Appendix G.5). We find that across all metrics, our proposed memoryless SOC formulation outperforms existing baseline methods. The choice of SOC algorithms also obviously favors Adjoint Matching over continuous and discrete adjoint methods, which result in poorer consistency and human preference metrics. + +Ablation: base model vs. reward tradeoff. We note that the scaling in front of the reward model $\lambda$ determines how strongly the we should prefer the reward model over the base model. As such, we see a natural tradeoff curve: higher $\lambda$ results in better consistency and human preference, but lower diversity in the generated samples. Overall, we find that Adjoint Matching performs stably across all values of $\lambda$ . Our method of regularizing the fine-tuning procedure through memoryless SOC works much better than baseline methods which often must employ early stopping. We show the qualitative effect of varying $\lambda$ in Figure 3, while for the DRaFT-1 baseline we show the effect of varying the number of fine-tuning iterations. + +Ablation: classifier-free guidance. We note that it is possible to apply classifier-free guidance (CFG; Ho and Salimans (2022); Zheng et al. (2023)) after fine-tuning. We use the formula $( 1 + w ) v ( x , t | y ) - w v ( x , t )$ where $w$ is the guidance weight, $v ( x , t | y )$ is a fine-tuned text-to-image model while $\boldsymbol { v } ( \boldsymbol { x } , t )$ is an unconditional image model. This is not principled as only the conditional model is fine-tuned, but generally it is unclear what distribution guided models sample from anyhow. In Figure 5 we show the evaluation metrics with classifier-free guidance applied. Comparing three different guidance weight values, we see a higher weight does improve text-to-image consistency, and to some extent, human preference, but this comes at the cost of being worse in terms of diversity. We show qualitative differences in Figure 4. + +# 8 Conclusion + +We investigate the problem of fine-tuning dynamical generative models such as Flow Matching and propose the use of a stochastic optimal control (SOC) formulation with a memoryless noise schedule. This ensures we converge to the same tilted distribution that the large language modeling literature uses for learning from human feedback. In particular, the memoryless noise schedule corresponds to DDPM sampling for diffusion models and a new Memoryless Flow Matching generative process for flow models. In conjunction, we propose a novel training algorithm for solving stochastic optimal control problems, by casting SOC as a regression problem, which we call the Adjoint Matching objective. 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Cited on pages 2 and 12. + +# Appendix + +# Contents + +# A Additional Figures & Tables 23 + +B Results on DDIM and Flow Matching 31 +B.1 The continuous-time limit of DDIM . 31 +B.2 Forward and backward stochastic differential equations 3 1 +B.2.1 Proof of Lemma 1 3 3 +B.2.2 Proof of Lemma 2 3 3 +B.2.3 Proof of Proposition 4 3 4 +B.3 The relationship between the noise predictor $\epsilon$ and the score function 3 6 +B.4 The relationship between the vector field $v$ and the score function . 36 + +# C Stochastic optimal control as maximum entropy RL in continuous space and time 37 + +C.1 Maximum entropy RL 37 +C.2 From maximum entropy RL to stochastic optimal control 3 8 +C.3 Proof of Proposition 5: from MaxEnt RL to SOC 39 +C.4 Proof of equation (18): the control cost is a KL regularizer 4 1 +Proofs of Section 4.3: memoryless noise schedule and fine-tuning recipe 42 +D.1 Proof of Proposition 1: the memoryless noise schedule 4 2 +D.2 Proof of Theorem 1: fine-tuning recipe for general noise schedules 4 3 + +# E Loss function derivations + +E.1 Derivation of the Continuous Adjoint method 4 6 +E.2 Proof of Proposition 2: Theoretical guarantees of the basic Adjoint Matching loss 48 +E.3 Theoretical guarantees of the Adjoint Matching loss 4 9 +E.4 Pseudo-code of Adjoint Matching for DDIM fine-tuning 50 + +# F Adapting diffusion fine-tuning baselines to flow matching 51 + +F.1 Adapting ReFL (Xu et al., 2023) to flow matching 51 +F.2 Adapting Diffusion-DPO (Wallace et al., 2023a) to flow matching 5 2 + +# G Experimental details + +G.1 Noise schedule details 53 +G.2 Selection of gradient evaluation timesteps 54 +G.3 Loss function clipping: the LCT hyperparameter 54 +G.4 Computation of evaluation metrics 5 4 +G.5 Remarks on computational costs 55 +G.6 Remarks on number of sampling timesteps 5 5 + +# A Additional Figures & Tables + +![](images/figures/adjoint-matching-fig-0007.jpg) +Figure 6 Average values of ImageReward (reward function), control cost $\begin{array} { r l } { { \big ( \int _ { 0 } ^ { t } \frac 1 2 \| u ( X _ { t } ^ { u } , t ) \| ^ { 2 } \mathrm { d } t \big ) } \quad } & { { } } \end{array}$ , and ClipScore vs. wall-clock time for Adjoint Matching and our baselines. Lines show averages over three fine-tuning runs, evaluating on separate test datasets of size 200. Confidence intervals show standard errors of estimates. + +![](images/figures/adjoint-matching-fig-0008.jpg) +Text prompt: “Man sitting on sofa at home in front of fireplace and using laptop computer, rear view ” + +![](images/figures/adjoint-matching-fig-0009.jpg) +Text prompt: “3D World Food Day Morocco” + +Figure 7 Generated samples from varying classifier-free guidance weights, from the pre-trained Flow Matching model. +Corresponding samples from the fine-tuned model can be found in Figure 4. + +Table 3 Metrics for various fine-tuning methods for text-to-image generation. The second and third columns show the√ noise schedules $\sigma ( t )$ used for fine-tuning and for inference: $\sigma ( t ) = \sqrt { 2 \eta _ { t } }$ corresponds to Memoryless Flow Matching, and $\sigma ( t ) = 0$ to the Flow Matching ODE (3). Confidence intervals show standard errors of estimates; computed over 3 runs of the fine-tuning algorithm on separate fine-tuning prompt datasets of size 40000 each. Test prompt sets are of size 1000, and also different for each run. + +
Fine-tuning lossFine-tuning σ(t)Sampling σ(t)ImageReward ↑ClipScore diversity ↑PickScore diversity ↑Total time (s)/ # iterations
None (CFG = 1.0)N/A√2t 0−1.384±0.040 −0.920±0.04228.07±1.40 30.29±1.531.63±0.08 1.82±0.09N/A
DRaFT-1√2t 0√2t 01.357±0.039 1.251±0.04016.86±0.98 16.76±1.061.21±0.07 1.27±0.07
DRaFT-40√2t√2t−0.560±0.138 0.424±0.04224.07±1.371.64±0.12/4000 148k±4.2k
DPO0 √2nt0 √2nt−1.386±0.03320.99±1.54 27.80±1.401.67±0.08 1.62±0.08/1500 118k±0.6k / 1000
0 √2ηt0 √2nt−0.957±0.040 0.687±0.08529.81±1.43 19.49±1.761.68±0.10 1.22±0.08
ReFL Cont. Adjoint00 √2ηt0.709±0.080 −0.448±0.13518.39±1.11 26.97±1.371.31±0.10 1.82±0.09173k±10.9k /6000 153k±0.9k
λ = 12500 Disc. Adjoint√2t0−0.249±0.11626.25±1.301.90±0.10/750 152k±1.5k /1000
λ = 12500√2t√2t 0−0.557±0.113 −0.552±0.04130.40±2.39 28.37±2.261.91±0.09 1.97±0.09
Adj.-Matching λ = 1000√2t√2t 00.550±0.043 0.454±0.05523.00±1.27 22.76±1.401.65±0.08 1.73±0.09156k±1.9k /1000
Adj.-Matching λ = 2500√2t√2t0.755±0.04021.33±1.71
00.671±0.04721.42±1.541.55±0.08 1.64±0.08
Adj.-Matching
λ = 12500√2ηt√2t 00.882±0.058 0.778±0.05020.49±1.48 20.34±1.491.50±0.09 1.57±0.09
+ +
Fine-tun. lossFine-tun. σ(t)Generat. σ(t)ImageReward↑ClipScore ↑PickScore ↑HPS v2↑DreamSim diversity ↑Runtime/ #iter.
ReFL√2t 0√2t0.459±0.09628.46±0.2518.77±0.0922.54±0.1737.51±3.5043k±2.7k
00.330±0.11429.63±0.6119.08±0.1822.46±0.7739.51±1.30/1500
DRaFT-1√2nt 0√snt0.913±0.06829.80±0.2219.16±0.0623.63±0.1635.21±1.9335k±1.5k
00.626±0.19530.48±0.3218.91±0.3421.92±1.6338.52±2.01/1000
Draft-40√2t 0√2t−1.427±0.26723.39±1.7217.24±0.4515.72±1.8041.98±2.1449k±1.4k
0−0.097±0.05229.12±0.4118.97±0.1421.93±0.2046.35±1.34/500
Adj.-Match. λ = 1000√2nt√2ηt0.107±0.04619.05±0.07
0.051±0.04429.37±0.25 30.58±0.1719.31±0.0722.79±0.20 21.93±0.2346.38±1.36 48.12±1.56
Adj.-Match.√2t0 √2nt39k±0.5k
00.199±0.068 0.106±0.06729.27±0.21 30.43±0.2419.07±0.10 19.32±0.1122.98±0.30 22.16±0.3345.03±1.61/250
Adj.-Match.√2t√2t47.61±1.49
00.299±0.095 0.224±0.05129.61±0.37 30.70±0.2319.26±0.1423.67±0.2743.36±1.93
λ = 12500 Cont. Adj.√2t19.52±0.1122.93±0.2144.62±1.79
√2t 0−0.910±0.11626.29±0.4418.06±0.1618.86±0.8851.60±1.9751k±0.3k
λ = 12500 Disc. Adj.√2nt−0.681±0.05128.50±0.1918.69±0.1119.90±0.5050.87±1.52/250
√2nt 0−0.978±0.123 −0.791±0.06526.68±0.76 28.66±0.3318.51±0.11 18.51±0.1118.53±0.28 18.53±0.2855.95±1.70 54.78±2.0038k±0.4k /250
+ +Table 4 Additional metrics for various fine-tuning methods for text-to-image generation, which complement the ones in Table 2 (both tables correspond to the same runs). The second and third columns show the noise schedules $\sigma ( t )$ used for fine-tuning and for inference: $\sigma ( t ) = \sqrt { 2 \eta _ { t } }$ corresponds to Memoryless Flow Matching, and $\sigma ( t ) = 0$ to the Flow Matching ODE (3). + +Table 5 Evaluation metrics when using classifier-free guidance (CFG; Ho and Salimans (2022)). + +
wFine-tuning loss#iter. /λFine-tun. σ(t)Sampl. σ(t)ImageReward ↑ClipScore↑PickScore ↑HPS v2 ↑DreamSim diversity↑
0.0NoneN/AN/A√2ηt 0−1.384±0.040 −0.920±0.04224.15±0.26 28.32±0.2217.25±0.06 18.15±0.0716.19±0.17 17.89±0.1653.60±1.37 56.53±1.52
1000√2t 0√2t 00.913±0.068 0.626±0.19529.80±0.22 30.48±0.3219.16±0.06 18.91±0.3423.63±0.16 21.92±1.6335.21±1.93 38.52±2.01
0.0DRaFT-12000√s 0√2t 01.204±0.046 1.052±0.08829.90±0.43 30.65±0.2419.29±0.12 19.27±0.1124.40±0.27 23.81±0.4428.51±1.68 32.11±2.37
3000$√snt 0√s 01.307±0.041 1.173±0.05829.96±0.22 30.86±0.2519.31±0.06 19.37±0.0624.42±0.13 24.17±0.2326.57±1.32 29.69±1.30
4000√2t 0√snt 01.357±0.039 1.251±0.04030.18±0.24 30.95±0.2819.38±0.08 19.37±0.0624.61±0.17 24.37±0.1725.54±0.99 27.39±1.14
1000√st 0√2t 00.550±0.043 0.454±0.05530.36±0.22 31.41±0.2219.29±0.08 19.57±0.0924.12±0.17 23.29±0.1840.89±1.50 43.10±1.76
0.0Adj.-Match.2500√2ηt 0√2nt 00.755±0.040 0.671±0.04730.59±0.40 31.64±0.2119.49±0.10 19.71±0.0924.85±0.23 24.12±0.2737.07±1.47 39.88±1.59
12500$√st 0√t 00.882±0.058 0.778±0.05030.62±0.30 31.65±0.1919.50±0.09 19.76±0.0824.95±0.28 24.49±0.2734.50±1.33 37.24±1.57
1.0NoneN/AN/A√s 0−0.269±0.050 −0.123±0.04130.41±0.22 31.83±0.1718.74±0.07 19.28±0.0720.47±0.18 20.95±0.1643.82±1.24 42.59±1.23
1000√2nt√2ηt1.123±0.05132.06±0.1919.69±0.0624.56±0.1728.25±1.55
1.0DRaFT-120000 00 00.856±0.167 1.177±0.05332.32±0.25 32.36±0.1819.38±0.34 19.67±0.0822.88±1.54 24.48±0.2829.98±1.86 25.09±1.82
3000001.255±0.03832.36±0.1919.70±0.0624.64±0.1723.24±1.19
4000001.296±0.03332.30±0.1919.68±0.0624.71±0.1421.54±0.96
1000000.782±0.04433.05±0.2220.20±0.0924.81±0.1832.67±1.26
1.0Adj.-Match.2500√st√st1.027±0.03832.85±0.2120.08±0.0825.88±0.2029.83±1.00
125000 00 00.910±0.040 0.985±0.04133.20±0.17 33.10±0.1820.29±0.09 20.28±0.0825.39±0.24 25.61±0.2730.34±1.51 28.86±1.37
4.0NoneN/AN/A√st0.277±0.04332.68±0.1819.50±0.0722.29±0.1635.12±0.92
√s2nt0 √2t0.209±0.046 1.062±0.04532.83±0.17 32.29±0.1619.79±0.07 19.48±0.0622.30±0.17 23.67±0.1332.05±1.05 25.03±1.32
4.0DRaFT-11000000.604±0.39531.80±0.8619.09±0.5321.69±2.1025.92±2.57
2000 30000 001.112±0.046 1.151±0.03632.29±0.20 32.31±0.2119.34±0.11 19.36±0.0623.31±0.22 23.29±0.1421.02±1.67 19.53±1.24
400000 01.172±0.04032.20±0.2219.30±0.0723.20±0.1518.45±1.06
1000000.852±0.04633.50±0.2220.31±0.0824.97±0.1925.83±0.82
4.0
Adj.-Match.2500√2ηt√2t1.052±0.03933.51±0.1920.15±0.0725.56±0.1826.21±0.73
000.942±0.042 1.007±0.05233.61±0.19 33.48±0.2020.35±0.08 20.29±0.0825.34±0.21 25.50±0.2924.30±0.86
+ +Table 6 Metrics for alternative optimization hyperparameters (learning rate and Adam $\beta _ { 1 }$ ). + +
LR/ Adam β1Fine-tuning lossFine-tun. σ(t)Generat. σ(t)ImageReward↑ClipScore ↑PickScore ↑HPS v2 ↑DreamSim diversity↑
3 × 10−5DRaFT-1√2t√2nt1.467±0.02930.28±0.5619.37±0.0924.70±0.1521.20±0.93
/ 0.97Adj.-Match. λ = 1200√q2tt√2t1.130±0.03431.01±0.2719.60±0.0825.01±0.2526.73±0.88
2 × 10−5Disc. Adj.√st√2nt−1.186±0.55321.95±4.2916.94±0.9512.34±4.4028.33±10.26
/ 0.95λ = 1250000−0.961±0.65324.07±4.7117.86±1.1715.93±5.8033.62±7.80
+ +Table 7 Comparison with an alternative fine-tuning noise schedule $\sigma ( t ) = 1$ . We see that the initial value function bias (Section 4.2) results in the model not having a high reward function (ImageReward is the reward function used for fine-tuning). Its performance on other metrics are also lower than when fine-tuning with the memoryless noise schedule, except for diversity. + +
Fine-tuning lossFine-tuning σ(t)Generative σ(t)ImageReward ↑ClipScore ↑PickScore ↑HPS v2↑DreamSim diversity ↑
Adj.-Matching λ = 12500110.009±0.07729.18±0.5118.66±0.0920.75±0.3241.33±1.24
00.454±0.05531.41±0.2219.57±0.0923.29±0.1843.10±1.76
Adj.-Matching λ = 1200√st√2t0.882±0.05830.62±0.3019.50±0.0924.95±0.2834.50±1.33
00.778±0.05031.65±0.1919.76±0.0824.49±0.2737.24±1.57
+ +
#sampl. timestepsFine-tuning lossFine-tun. σ(t)Sampl. σ(t)ImageReward↑ClipScore↑PickScore ↑HPS v2↑DreamSim diversity↑
10None (Base)N/A√2t−2.279±0.00113.99±0.1214.98±0.057.37±0.105.07±0.13
0−1.386±0.04026.26±0.2417.64±0.0714.92±0.1751.26±1.38
DRaFT-1√2nt√2nt1.033±0.05125.98±0.2518.28±0.0722.08±0.1814.47±0.67
01.236±0.03831.54±0.2719.53±0.0724.47±0.1924.78±0.88
Adj.-Match. λ = 12500√2nt√2t−2.104±0.07417.12±0.5615.76±0.2011.48±1.039.88±0.81
00.607±0.05531.36±0.2019.56±0.0823.23±0.2833.75±1.48
20None (Base)N/A√2t−2.275±0.00214.58±0.1315.07±0.057.47±0.1011.27±0.33
0−1.017±0.05527.92±0.1918.01±0.0717.17±0.1554.69±1.45
DRaFT-1√2ηt√2t1.301±0.03927.09±0.2418.93±0.0723.78±0.2021.05±1.12
01.255±0.03831.14±0.2519.43±0.0624.52±0.1626.15±1.11
Adj.-Match.√2nt−0.032±0.07225.07±0.2718.01±0.0720.75±0.2329.06±2.34
√2t 00.768±0.04831.70±0.1719.73±0.0824.30±0.2635.90±1.52
40None (Base)N/A√2t−1.384±0.04024.15±0.2617.25±0.0616.19±0.1753.60±1.37
0−0.920±0.04228.32±0.2218.15±0.0717.89±0.1656.53±1.52
DRaFT-1√2nt√2t1.357±0.03930.18±0.2419.38±0.0824.61±0.1725.54±0.99
01.251±0.04030.95±0.2819.37±0.0624.37±0.1727.39±1.14
Adj.-Match.√2ηt√2t0.882±0.05830.62±0.3019.50±0.0924.95±0.2834.50±1.33
00.778±0.05031.65±0.1919.76±0.0824.49±0.2737.24±1.57
100None (Base)N/A−0.881±0.04127.83±0.1918.10±0.0718.43±0.1757.21±1.50
0−0.881±0.03628.65±0.1818.22±0.0618.20±0.1757.73±1.68
DRaFT-1√2nt√2t1.343±0.04030.64±0.2019.38±0.0824.37±0.1725.51±1.10
01.239±0.03730.74±0.2819.33±0.0624.24±0.1728.70±1.11
Adj.-Match. λ = 12500√2t√2t0.892±0.04431.23±0.2319.65±0.0824.92±0.2335.13±1.40
00.779±0.04831.64±0.1719.76±0.0824.57±0.2538.26±1.65
200None (Base)N/A√2t−0.848±0.04828.37±0.2118.27±0.0818.56±0.1958.00±1.58
0−0.871±0.03628.50±0.1818.23±0.0618.25±0.1457.84±1.60
DRaFT-1√2t1.331±0.04430.69±0.2319.36±0.0724.21±0.1726.41±1.18
√2nt 01.222±0.04230.77±0.2719.32±0.0624.18±0.1629.09±1.07
Adj.-Match.0 √2nt√st0.869±0.06231.33±0.2119.68±0.0924.81±0.3035.90±1.55
00.766±0.05031.61±0.1619.75±0.0824.52±0.2438.60±1.38
+ +Table 8 Performance metrics for different number of sampling steps. Only the number of sampling steps is ablated; the fine-tuned models used in all cases are the ones fine-tuned using 40 steps. + +![](images/figures/adjoint-matching-fig-0010.jpg) +Adjoint Matching (Ours) +Figure 8 Generated samples with classifier-free guidance $w = 1$ ) and $\sigma ( t ) = 0$ across ten selected prompts. Each row corresponds to a different prompt and each image corresponds to a different random seed consistent across models. + +![](images/figures/adjoint-matching-fig-0011.jpg) +Adjoint Matching (Ours) +Figure 9 Generated samples with classifier-free guidance ( $w = 1$ ) and $\sigma ( t ) = 0$ across ten selected prompts with people. Each row corresponds to a different prompt and each image corresponds to a different random seed consistent across models. + +![](images/figures/adjoint-matching-fig-0012.jpg) +Figure 10 Generated samples without guidance ( $w = 0$ ) and $\sigma ( t ) = 0$ across seven selected prompts. Each row corresponds to a different finetuning algorithm. Prompts: “ Seaside view poster with palm trees vector image”, “Cayucos Beach Inn”, “Happy Summer Life- Aloha Flowers and Melon - Pattern Metal Print”, “Castle Square, Warsaw Old Town”, “Funny girl blowing soap bubbles. High quality photo”, “Colombian man with sweatshirt over yellow wall listening to something by putting hand on the ear ”, “man in the hood black mask masquerade”. + +![](images/figures/adjoint-matching-fig-0013.jpg) +Figure 11 Generated samples without guidance ( $w = 0$ ) and $\sigma ( t ) = \sqrt { 2 \eta _ { t } }$ across seven selected prompts. Each row corresponds to a different finetuning algorithm. The prompts are the same as in Figure 10. + +# B Results on DDIM and Flow Matching + +# B.1 The continuous-time limit of DDIM + +The DDIM inference update (Song et al., 2021a, Eq. 12) is + +$$ +\begin{array} { r } { x _ { k + 1 } = \sqrt { \bar { \alpha } _ { k + 1 } } \Big ( \frac { x _ { k } - \sqrt { 1 - \bar { \alpha } _ { k } } \epsilon ( x _ { k } , k ) } { \sqrt { \bar { \alpha } _ { k } } } \Big ) + \sqrt { 1 - \bar { \alpha } _ { k + 1 } - \sigma _ { k } ^ { 2 } } \epsilon ( x _ { k } , k ) + \sigma _ { k } \epsilon _ { k } , \qquad x _ { K } \sim N ( 0 , I ) . } \end{array} +$$ + +If we let $\Delta \bar { \alpha } _ { k } = \bar { \alpha } _ { k + 1 } - \bar { \alpha } _ { k }$ , we have that + +$$ +\begin{array} { r } { \sqrt { \frac { \bar { \alpha } _ { k + 1 } } { \bar { \alpha } _ { k } } } = \sqrt { \frac { \bar { \alpha } _ { k } + \bar { \alpha } _ { k + 1 } - \bar { \alpha } _ { k } } { \bar { \alpha } _ { k } } } = \sqrt { 1 + \frac { \bar { \alpha } _ { k + 1 } - \bar { \alpha } _ { k } } { \bar { \alpha } _ { k } } } = \sqrt { 1 + \frac { \Delta \bar { \alpha } _ { k } } { \bar { \alpha } _ { k } } } \approx 1 + \frac { \Delta \bar { \alpha } _ { k } } { 2 \bar { \alpha } _ { k } } , } \end{array} +$$ + +where we used the first-order Taylor approximation of $\sqrt { 1 + x }$ . And + +$$ +\begin{array} { r l } & { - \sqrt { \frac { \bar { \alpha } _ { k + 1 } } { \bar { \alpha } _ { k } } \big ( 1 - \bar { \alpha } _ { k } \big ) } + \sqrt { 1 - \bar { \alpha } _ { k + 1 } - \sigma _ { k } ^ { 2 } } = - \sqrt { \big ( 1 + \frac { \Delta \bar { \alpha } _ { k } } { \bar { \alpha } _ { k } } \big ) \big ( 1 - \bar { \alpha } _ { k } \big ) } + \sqrt { 1 - \bar { \alpha } _ { k + 1 } - \sigma _ { k } ^ { 2 } } } \\ & { = - \sqrt { 1 + \frac { \Delta \bar { \alpha } _ { k } } { \bar { \alpha } _ { k } } - \bar { \alpha } _ { k } - \Delta \bar { \alpha } _ { k } } + \sqrt { 1 - \bar { \alpha } _ { k + 1 } - \sigma _ { k } ^ { 2 } } = - \sqrt { 1 - \bar { \alpha } _ { k + 1 } + \frac { \Delta \bar { \alpha } _ { k } } { \bar { \alpha } _ { k } } } + \sqrt { 1 - \bar { \alpha } _ { k + 1 } - \sigma _ { k } ^ { 2 } } } \\ & { = \sqrt { 1 - \bar { \alpha } _ { k + 1 } } \big ( - \sqrt { 1 + \frac { \Delta \bar { \alpha } _ { k } } { \bar { \alpha } _ { k } \big ( 1 - \bar { \alpha } _ { k + 1 } \big ) } } + \sqrt { 1 - \frac { \sigma _ { k } ^ { 2 } } { 1 - \bar { \alpha } _ { k + 1 } } } \big ) \approx \sqrt { 1 - \bar { \alpha } _ { k + 1 } } \big ( - \big ( 1 + \frac { \Delta \bar { \alpha } _ { k } } { 2 \bar { \alpha } _ { k } \big ( 1 - \bar { \alpha } _ { k + 1 } \big ) } \big ) + 1 - \frac { \bar { \alpha } _ { k } } { \bar { \alpha } _ { k } \big ( 1 - \bar { \alpha } _ { k + 1 } \big ) } } \\ & = - \big ( \frac { \Delta \bar { \alpha } _ { k } } { 2 \bar { \alpha } _ { k } } + \frac \sigma _ { k } ^ \end{array} +$$ + +where we used the same first-order Taylor approximation. Thus, up to first-order approximations, (44) is equivalent to + +$$ +\begin{array} { r } { x _ { k - 1 } = \big ( 1 + \frac { \Delta \bar { \alpha } _ { k } } { 2 \bar { \alpha } _ { k } } \big ) x _ { k } - \big ( \frac { \Delta \bar { \alpha } _ { k } } { 2 \bar { \alpha } _ { k } } + \frac { \sigma _ { k } ^ { 2 } } { 2 } \big ) \frac { \epsilon ( x _ { k } , k ) } { \sqrt { 1 - \bar { \alpha } _ { k + 1 } } } + \sigma _ { k } \epsilon _ { k } , \qquad x _ { K } \sim N ( 0 , I ) . } \end{array} +$$ + +If we modify our notation slightly, we can rewrite this as + +$$ +\begin{array} { r } { X _ { ( k + 1 ) h } = \big ( 1 - \frac { h \bar { \alpha } _ { k h } } { 2 \bar { \alpha } _ { k h } } \big ) X _ { k h } + \big ( \frac { h \bar { \alpha } _ { k h } } { 2 \bar { \alpha } _ { k h } } - \frac { h \sigma ( k h ) ^ { 2 } } { 2 } \big ) \frac { \epsilon ( X _ { k h } , k h ) } { \sqrt { 1 - \bar { \alpha } _ { k h } } } + \sqrt { h } \sigma ( k h ) \epsilon _ { k } , \qquad X _ { 0 } \sim N ( 0 , I ) . } \end{array} +$$ + +To go from (47) to (48), we introduced a continuous time variable and a stepsize $h = 1 / K$ , and we regard the increment $h \bar { \alpha } _ { k }$ as approximately equal to $h$ times the derivative of $\alpha$ . We also identified $\sigma _ { k }$ with $\sqrt { h } \sigma ( k h )$ , where $\sigma ( k h )$ plays the role of a diffusion coefficient. Note that equation (48) can be reverse-engineered as the Euler-Maruyama discretization of the SDE + +$$ +\begin{array} { r } { \mathrm { d } X _ { t } = \big ( - \frac { \dot { \bar { \alpha } } _ { t } } { 2 \bar { \alpha } _ { t } } + \big ( \frac { \dot { \bar { \alpha } } _ { t } } { 2 \bar { \alpha } _ { t } } - \frac { \sigma ( t ) ^ { 2 } } { 2 } \big ) \frac { \epsilon ( X _ { t } , t ) } { \sqrt { 1 - \bar { \alpha } _ { t } } } \big ) \mathrm { d } t + \sigma ( t ) \mathrm { d } B _ { t } , \qquad X _ { 0 } \sim N ( 0 , I ) . } \end{array} +$$ + +# B.2 Forward and backward stochastic differential equations + +Let $\left( \kappa _ { t } \right) _ { t \in [ 0 , 1 ] }$ and $( \eta _ { t } ) _ { t \in [ 0 , 1 ] }$ such that + +$$ +\begin{array} { r } { t \in [ 0 , 1 ] , \quad \eta _ { t } \geq 0 , \qquad \int _ { 0 } ^ { 1 } \kappa _ { 1 - s } { \mathrm { d } } s = + \infty , \qquad 2 \int _ { 0 } ^ { 1 } \eta _ { 1 - t ^ { \prime } } \exp \big ( - 2 \int _ { t ^ { \prime } } ^ { t } \kappa _ { 1 - s } { \mathrm { d } } s \big ) \mathrm { d } t ^ { \prime } = 1 } \end{array} +$$ + +As shown in Table 1, DDIM corresponds to $\begin{array} { r } { \kappa _ { t } = \frac { \dot { \bar { \alpha } } _ { t } } { 2 \bar { \alpha } _ { t } } } \end{array}$ = α¯˙ t2 ¯αt , ηt = 2 , and Flow Matching corresponds to $\begin{array} { r } { \kappa _ { t } = \frac { \dot { \alpha } _ { t } } { \alpha _ { t } } } \end{array}$ , $\begin{array} { r } { \eta _ { t } = \beta _ { t } \big ( \frac { \dot { \alpha } _ { t } } { \alpha _ { t } } \beta _ { t } - \dot { \beta } _ { t } \big ) } \end{array}$ . + +Lemma 1 (DDIM and Flow Matching fulfill the conditions (50)). The choices of $( \kappa _ { t } ) _ { t \in [ 0 , 1 ] }$ and $( \eta _ { t } ) _ { t \in [ 0 , 1 ] }$ for DDIM and Flow Matching fulfill the conditions (50). For DDIM, we have that + +$$ +\begin{array} { c } { { \int _ { 0 } ^ { t } \kappa _ { 1 - s } \mathrm { d } s = - \frac 1 2 \log \bar { \alpha } _ { 1 - t } \implies \int _ { 0 } ^ { 1 } \kappa _ { 1 - s } \mathrm { d } s = + \infty , } } \\ { { 2 \int _ { 0 } ^ { t } \eta _ { t ^ { \prime } } \exp \big ( - 2 \int _ { t ^ { \prime } } ^ { t } \kappa _ { s } \mathrm { d } s \big ) \mathrm { d } t ^ { \prime } = 1 - \bar { \alpha } _ { 1 - t } \implies 2 \int _ { 0 } ^ { 1 } \eta _ { t ^ { \prime } } \exp \big ( - 2 \int _ { t ^ { \prime } } ^ { t } \kappa _ { s } \mathrm { d } s \big ) \mathrm { d } t ^ { \prime } = 1 . } } \end{array} +$$ + +For Flow Matching, + +$$ +\begin{array} { r l } & { \qquad \int _ { 0 } ^ { t } \kappa _ { 1 - s } \mathrm { d } s = - \log \alpha _ { 1 - t } \implies \int _ { 0 } ^ { 1 } \kappa _ { 1 - s } \mathrm { d } s = + \infty , } \\ & { 2 \int _ { 0 } ^ { t } \eta _ { t ^ { \prime } } \exp \big ( - 2 \int _ { t ^ { \prime } } ^ { t } \kappa _ { s } \mathrm { d } s \big ) \mathrm { d } t ^ { \prime } = \beta _ { 1 - t } ^ { 2 } \implies 2 \int _ { 0 } ^ { 1 } \eta _ { t ^ { \prime } } \exp \big ( - 2 \int _ { t ^ { \prime } } ^ { t } \kappa _ { s } \mathrm { d } s \big ) \mathrm { d } t ^ { \prime } = 1 . } \end{array} +$$ + +Forward and backward SDEs Consider the forward and backward SDEs + +$$ +\begin{array} { r l } & { \mathrm { d } \vec { X } _ { t } = - \kappa _ { 1 - t } \vec { X } _ { t } \mathrm { d } t + \sqrt { 2 \eta _ { 1 - t } } \mathrm { d } B _ { t } , \qquad \vec { X } _ { 0 } \sim p _ { \mathrm { d a t a } } , } \\ & { \mathrm { d } X _ { t } = \bigl ( \kappa _ { t } X _ { t } + 2 \eta _ { t } \mathfrak { s } ( X _ { t } , t ) \bigr ) \mathrm { d } t + \sqrt { 2 \eta _ { t } } \mathrm { d } B _ { t } , \qquad X _ { 0 } \sim N ( 0 , I ) , } \end{array} +$$ + +where we let $\vec { p _ { t } }$ be the density of $\vec { X _ { t } }$ , and we define the score function as $\pmb { \mathfrak { s } } ( x , t ) : = \nabla \log \vec { p } _ { 1 - t } ( x )$ . Similarly, we let $p _ { t }$ be the density of $X _ { t }$ . $\vec { p _ { t } }$ and $p _ { t }$ solve the Fokker-Planck equations: + +$$ +\begin{array} { r l } & { \partial _ { t } \vec { p } _ { t } = \nabla \cdot \bigl ( \kappa _ { 1 - t } x \vec { p } _ { t } \bigr ) + \eta _ { 1 - t } \Delta \vec { p _ { t } } , \qquad \vec { p } _ { 0 } = p _ { \mathrm { d a t a } } , } \\ & { \partial _ { t } p _ { t } = \nabla \cdot \bigl ( \bigl ( - \kappa _ { t } x - 2 \eta _ { t } \nabla \log \vec { p } _ { 1 - t } ( X _ { t } ) \bigr ) p _ { t } \bigr ) + \eta _ { t } \Delta p _ { t } , \qquad p _ { 0 } = N ( 0 , I ) . } \end{array} +$$ + +Lemma 2 (Solution of the forward SDE). Let $( \kappa _ { t } ) _ { t \geq 0 }$ , $( \eta _ { t } ) _ { t \geq 0 }$ with $\eta _ { t } \geq 0$ , and $( \xi _ { t } ) _ { t \geq 0 }$ be arbitrary. The solution $\vec { X _ { t } }$ of the $S D E$ + +$$ +\mathrm { d } \vec { X } _ { t } = \left( - \kappa _ { 1 - t } \vec { X } _ { t } + \xi _ { t } \right) \mathrm { d } t + \sqrt { 2 \eta _ { 1 - t } } \mathrm { d } B _ { t } , \qquad \vec { X } _ { 0 } \sim p _ { \mathrm { d a t a } } +$$ + +is + +$$ +\begin{array} { r } { \vec { X } _ { t } = \vec { X } _ { 0 } \exp \big ( - \int _ { 0 } ^ { t } \kappa _ { 1 - s } \mathrm { d } s \big ) + \int _ { 0 } ^ { t } \exp \big ( - \int _ { t ^ { \prime } } ^ { t } \kappa _ { 1 - s } \mathrm { d } s \big ) \xi _ { 1 - t ^ { \prime } } \mathrm { d } t ^ { \prime } + \int _ { 0 } ^ { t } \sqrt { 2 \eta _ { 1 - t ^ { \prime } } } \exp \big ( - \int _ { t ^ { \prime } } ^ { t } \kappa _ { 1 - s } \mathrm { d } s \big ) } \end{array} +$$ + +which has the same distribution as the random variable + +$$ +\begin{array} { l } { \dot { \zeta } _ { t } = \vec { X } _ { 0 } \exp \Big ( - \int _ { 0 } ^ { t } \kappa _ { 1 - s } \mathrm { d } s \Big ) + \int _ { 0 } ^ { t } \exp \Big ( - \int _ { t ^ { \prime } } ^ { t } \kappa _ { 1 - s } \mathrm { d } s \Big ) \xi _ { 1 - t ^ { \prime } } \mathrm { d } t ^ { \prime } + \sqrt { 2 \int _ { 0 } ^ { t } \eta _ { 1 - t ^ { \prime } } \exp \Big ( - 2 \int _ { t ^ { \prime } } ^ { t } \kappa _ { 1 - s } \mathrm { d } s \Big ) } } \\ { \epsilon \sim N ( 0 , I ) . } \end{array} +$$ + +Applying Lemma 2 with $\xi _ { t } \equiv 0$ , we obtain that $\vec { p _ { 1 } }$ is also the distribution of + +$$ +\begin{array} { r } { \hat { X } _ { 1 } = \vec { X } _ { 0 } \exp \big ( - \int _ { 0 } ^ { t } \kappa _ { 1 - s } \mathrm { d } s \big ) + \sqrt { 2 \int _ { 0 } ^ { t } \eta _ { 1 - t ^ { \prime } } \exp \big ( - 2 \int _ { t ^ { \prime } } ^ { t } \kappa _ { 1 - s } \mathrm { d } s \big ) \mathrm { d } t ^ { \prime } } \epsilon = \epsilon , } \end{array} +$$ + +where $\epsilon \sim N ( 0 , I )$ . The third equality in (61) holds by (50). Hence we obtain that $\vec { p _ { 1 } } = N ( 0 , I )$ . Note also that + +$$ +\begin{array} { r } { \partial _ { t } \vec { p } _ { 1 - t } = - \nabla \cdot \left( \kappa _ { t } x \vec { p } _ { 1 - t } \right) - \eta _ { t } \Delta \vec { p } _ { 1 - t } = - \nabla \cdot \left( \left( - \kappa _ { t } x - 2 \eta _ { t } \nabla \log \vec { p } _ { 1 - t } ( x ) \right) \vec { p } _ { 1 - t } \right) + \eta _ { t } \Delta \vec { p } _ { 1 - t } } \end{array} +$$ + +Thus, $\vec { p _ { 1 - t } }$ is a solution of the backward Fokker-Planck equation (57), which proves the following: + +Proposition 3 (Equality of marginal distributions). For any time $t \in [ 0 , 1 ]$ , the densities of the solutions $\vec { X _ { t } }$ , $X _ { t }$ of the forward and backward SDEs are equal up to a time flip: $p _ { t } = \vec { p _ { 1 - t } }$ . + +Forward and backward SDEs with arbitrary noise schedule Next, we look at the following pair of forwardbackward SDEs: + +$$ +\begin{array} { r l } & { \mathrm { d } \vec { X } _ { t } = \big ( - \kappa _ { 1 - t } \vec { X } _ { t } + \big ( \frac { \sigma ( 1 - t ) ^ { 2 } } { 2 } - \eta _ { 1 - t } \big ) \mathfrak { s } ( \vec { X } _ { t } , 1 - t ) \big ) \mathrm { d } t + \sigma ( 1 - t ) \mathrm { d } B _ { t } , \qquad \vec { X } _ { 0 } \sim p _ { \mathrm { d a t a } } , } \\ & { \mathrm { d } X _ { t } = \big ( \kappa _ { t } X _ { t } + \big ( \frac { \sigma ( t ) ^ { 2 } } { 2 } + \eta _ { t } \big ) \mathfrak { s } ( X _ { t } , t ) \big ) \mathrm { d } t + \sigma ( t ) \mathrm { d } B _ { t } , \qquad X _ { 0 } \sim N ( 0 , I ) , } \end{array} +$$ + +Here, the score function $\mathfrak { s }$ is the same vector field as in (64). Remark that equations (54)-(55) are a particular case of (63)-(64) for which $\sigma ( t ) = \sqrt { 2 \eta _ { t } }$ . The Fokker-Planck equations for (63)-(64) are: + +$$ +\begin{array} { r l } & { \partial _ { t } \vec { p } _ { t } = \nabla \cdot \bigl ( \bigl ( \kappa _ { 1 - t } x + \bigl ( - \frac { \sigma ( 1 - t ) ^ { 2 } } { 2 } + \eta _ { 1 - t } \bigr ) \mathfrak { s } ( X _ { t } , t ) \bigr ) \vec { p } _ { t } \bigr ) + \eta _ { 1 - t } \Delta \vec { p } _ { t } , \qquad \vec { p } _ { 0 } = p _ { \mathrm { d a t a } } , } \\ & { \partial _ { t } p _ { t } = \nabla \cdot \bigl ( \bigl ( - \kappa _ { t } x - \bigl ( \frac { \sigma ( t ) ^ { 2 } } { 2 } + \eta _ { t } \bigr ) \mathfrak { s } ( X _ { t } , t ) \bigr ) p _ { t } \bigr ) + \frac { \sigma ( t ) ^ { 2 } } { 2 } \Delta p _ { t } , \qquad p _ { 0 } = N ( 0 , I ) . } \end{array} +$$ + +It is straight-forward to see that for any $\sigma$ , the solutions $\vec { p _ { t } }$ and $p _ { t }$ of (65)-(66) are also solutions of (56)-(57). Hence, the marginals $\vec { X _ { t } }$ and $X _ { t }$ are equally distributed for all noise schedules $\sigma$ , and they are equal to each other up to a time flip. + +Equality of distributions over trajectories The result in Proposition 3 can be made even stronger: + +Proposition 4 (Equality of distributions over trajectories). Let $\vec { X }$ , $\pmb { X }$ be the solutions of the SDEs (63)-(64) with arbitrary noise schedule. For any sequence of times $( t _ { i } ) _ { 0 \leq i \leq I }$ , the joint distribution of $( \vec { X } _ { t _ { i } } ) _ { 0 \leq i \leq I }$ is equal to the joint distribution of $( X _ { 1 - t _ { i } } ) _ { 0 \leq i \leq I }$ , or equivalently, that the probability measures $\vec { \mathbb { P } }$ , $\mathbb { P }$ of the forward and backward processes $\vec { X }$ , $\pmb { X }$ are equal, up to a flip in the time direction. + +This result states that sampling trajectories from the backward process is equivalent to sampling them from the forward process and then flipping their order. + +# B.2.1 Proof of Lemma 1 + +As shown in Table 1, DDIM corresponds to $\begin{array} { r } { \kappa _ { t } = \frac { \bar { \bar { \alpha } } _ { t } } { 2 \bar { \alpha } _ { t } } } \end{array}$ α¯˙ t2 ¯αt , ηt = 2 ¯αt . Thus, $\eta _ { t } \geq 0$ because $\alpha _ { t }$ is increasing, and + +$$ +\begin{array} { r l } & { \int _ { 0 } ^ { t } \kappa _ { 1 - s } \mathrm { d } s = \int _ { 0 } ^ { t } \frac { \dot { \alpha } _ { 1 - s } } { 2 \tilde { \alpha } _ { 1 - s } } \mathrm { d } s = - \frac { 1 } { 2 } \int _ { 0 } ^ { t } \partial _ { s } \log \bar { \alpha } _ { 1 - s } \mathrm { d } s = - \frac { 1 } { 2 } ( \log \bar { \alpha } _ { 1 - t } - \log \bar { \alpha } _ { 1 } ) = - \frac { 1 } { 2 } \log \bar { \alpha } _ { 1 - t } , } \\ & { \implies \int _ { 0 } ^ { 1 } \kappa _ { 1 - s } \mathrm { d } s = - \frac { 1 } { 2 } \log \bar { \alpha } _ { 0 } = + \infty } \\ & { 2 \int _ { 0 } ^ { t } \eta _ { t ^ { \prime } } \exp \big ( - 2 \int _ { t ^ { \prime } } ^ { t } \kappa _ { s } \mathrm { d } s \big ) \mathrm { d } t ^ { \prime } = \int _ { 0 } ^ { t } \frac { \dot { \alpha } _ { 1 - t ^ { \prime } } } { \tilde { \alpha } _ { 1 - t ^ { \prime } } } \exp \big ( - \int _ { t ^ { \prime } } ^ { t } \frac { \dot { \alpha } _ { 1 - s } } { \tilde { \alpha } _ { 1 - s } } \mathrm { d } s \big ) \mathrm { d } t ^ { \prime } } \\ & = \int _ { 0 } ^ { t } \frac { \dot { \alpha } _ { 1 - t ^ { \prime } } } { \tilde { \alpha } _ { 1 - t ^ { \prime } } } \frac { \ddot { \alpha } _ { 1 - t } } { \tilde { \alpha } _ { 1 - t ^ { \prime } } } \mathrm { d } t ^ { \prime } = \bar { \alpha } _ { 1 - t } \int _ { 0 } ^ { t } \partial _ { t ^ { \prime } } \Big ( \frac { 1 } { \tilde { \alpha } _ { 1 - t ^ { \prime } } } \Big ) \mathrm { d } t ^ { \prime } = \bar { \alpha } _ { 1 - t } \Big ( \frac { 1 } { \tilde { \alpha } _ { 1 - t } } - \frac \end{array} +$$ + +where we used that $\bar { \alpha } _ { 1 } = 1$ and $\bar { \alpha } _ { 0 } = 0$ . And Flow Matching corresponds to $\begin{array} { r } { \kappa _ { t } = \frac { \dot { \alpha } _ { t } } { \alpha _ { t } } } \end{array}$ , $\begin{array} { r } { \eta _ { t } = \beta _ { t } \big ( \frac { \dot { \alpha } _ { t } } { \alpha _ { t } } \beta _ { t } - \dot { \beta } _ { t } \big ) } \end{array}$ . We have that $\eta _ { t } \geq 0$ because $\alpha _ { t }$ is increasing and $\beta _ { t }$ is decreasing, and + +$$ +\begin{array} { r l } & { \int _ { 0 } ^ { t } \kappa _ { 1 - s } \mathrm { d } s = \int _ { 0 } ^ { t } \frac { \dot { \alpha } _ { 1 - s } } { \alpha _ { 1 - s } } \mathrm { d } s = - \int _ { 0 } ^ { t } \partial _ { s } \log \alpha _ { 1 - s } \mathrm { d } s = - ( \log \alpha _ { 1 - t } - \log \alpha _ { 1 } ) = - \log \alpha _ { 1 - t } , } \\ & { \implies \int _ { 0 } ^ { 1 } \kappa _ { 1 - s } \mathrm { d } s = - \log \alpha _ { 0 } = + \infty , } \end{array} +$$ + +and + +$$ +\begin{array} { r l } & { 2 \int _ { 0 } ^ { t } \eta _ { 1 - t ^ { \prime } } \exp \big ( - 2 \int _ { t ^ { \prime } } ^ { t } \kappa _ { 1 - s } \mathrm { d } s \big ) \mathrm { d } t ^ { \prime } = 2 \int _ { 0 } ^ { t } \beta _ { 1 - t ^ { \prime } } \Big ( \frac { \dot { \alpha } _ { 1 - t ^ { \prime } } } { \alpha _ { 1 - t ^ { \prime } } } \beta _ { 1 - t ^ { \prime } } - \dot { \beta } _ { 1 - t ^ { \prime } } \Big ) \exp \big ( - 2 \int _ { t ^ { \prime } } ^ { t } \frac { \dot { \alpha } _ { 1 - s } } { \alpha _ { 1 - s } } \mathrm { d } s \big ) \mathrm { d } t ^ { \prime } } \\ & { = 2 \int _ { 0 } ^ { t } \beta _ { 1 - t ^ { \prime } } \Big ( \frac { \dot { \alpha } _ { 1 - t ^ { \prime } } } { \alpha _ { 1 - t ^ { \prime } } } \beta _ { 1 - t ^ { \prime } } - \dot { \beta } _ { 1 - t ^ { \prime } } \Big ) \Big ( \frac { \alpha _ { 1 - t } } { \alpha _ { 1 - t ^ { \prime } } } \Big ) ^ { 2 } \mathrm { d } t ^ { \prime } , } \end{array} +$$ + +To develop the right-hand side, note that by integration by parts, + +$$ +\begin{array} { r l } & { \int _ { 0 } ^ { t } \dot { \beta } _ { 1 - t ^ { \prime } } \beta _ { 1 - t ^ { \prime } } \left( \frac { \alpha _ { 1 - t } } { \alpha _ { 1 - t ^ { \prime } } } \right) ^ { 2 } \mathrm { d } t ^ { \prime } = - \int _ { 0 } ^ { t } \partial _ { t ^ { \prime } } \left( \frac { \beta _ { 1 - t ^ { \prime } } ^ { 2 } } { 2 } \right) \left( \frac { \alpha _ { 1 - t } } { \alpha _ { 1 - t ^ { \prime } } } \right) ^ { 2 } \mathrm { d } t ^ { \prime } } \\ & { = - \Big [ \frac { \beta _ { 1 - t ^ { \prime } } ^ { 2 } } { 2 } \Big ( \frac { \alpha _ { 1 - t } } { \alpha _ { 1 - t ^ { \prime } } } \Big ) ^ { 2 } \Big ] _ { 0 } ^ { 1 } + \int _ { 0 } ^ { t } \frac { \beta _ { 1 - t ^ { \prime } } ^ { 2 } } { 2 } \partial _ { t ^ { \prime } } \Big ( \frac { \alpha _ { 1 - t } } { \alpha _ { 1 - t ^ { \prime } } } \Big ) ^ { 2 } \mathrm { d } t ^ { \prime } = - \Big [ \frac { \beta _ { 1 - t ^ { \prime } } ^ { 2 } } { 2 } \Big ( \frac { \alpha _ { 1 - t } } { \alpha _ { 1 - t ^ { \prime } } } \Big ) ^ { 2 } \Big ] _ { 0 } ^ { t } + \int _ { 0 } ^ { t } \beta _ { 1 - t ^ { \prime } } ^ { 2 } \frac { \alpha _ { 1 - t } ^ { 2 } \dot { \alpha } _ { 1 - t ^ { \prime } } } { \alpha _ { 1 - t ^ { \prime } } ^ { 3 } } \mathrm { d } t ^ { \prime } . } \end{array} +$$ + +And if we plug this into the right-hand side of (70), we obtain + +$$ +\begin{array} { r l } & { 2 \int _ { 0 } ^ { t } \eta _ { 1 - t ^ { \prime } } \exp \big ( - 2 \int _ { t ^ { \prime } } ^ { t } \kappa _ { 1 - s } \mathrm { d } s \big ) \mathrm { d } t ^ { \prime } = \big [ \beta _ { 1 - t ^ { \prime } } ^ { 2 } \big ( \frac { \alpha _ { 1 - t } } { \alpha _ { 1 - t ^ { \prime } } } \big ) ^ { 2 } \big ] _ { 0 } ^ { t } = \beta _ { 1 - t } ^ { 2 } - \beta _ { 1 } ^ { 2 } \big ( \frac { \alpha _ { 1 - t } } { \alpha _ { 1 } } \big ) ^ { 2 } = \beta _ { 1 - t } ^ { 2 } , } \\ & { \implies 2 \int _ { 0 } ^ { 1 } \eta _ { 1 - t ^ { \prime } } \exp \big ( - 2 \int _ { t ^ { \prime } } ^ { t } \kappa _ { 1 - s } \mathrm { d } s \big ) \mathrm { d } t ^ { \prime } = \beta _ { 1 } ^ { 2 } = 1 . } \end{array} +$$ + +where we used that $\beta _ { 1 } = 0$ , $\alpha _ { 1 } = 1$ . + +# B.2.2 Proof of Lemma 2 + +We can solve this equation by variation of parameters. To simplify the notation, we replace $\kappa _ { 1 - s }$ , $\eta _ { 1 - s }$ and $\xi _ { 1 - s }$ by $\kappa _ { s }$ , $\eta _ { s }$ and $\xi _ { s }$ . Defining $\begin{array} { r } { f ( \vec { X } _ { t } , t ) = \vec { X } _ { t } \exp \big ( \int _ { 0 } ^ { t } \kappa _ { 1 - s } \mathrm { d } s \big ) } \end{array}$ , we get that + +$$ +\begin{array} { r l } & { d f ( \vec { X } _ { t } , t ) = \kappa _ { 1 - t } \vec { X } _ { t } \exp \big ( \int _ { 0 } ^ { t } \kappa _ { 1 - s } \mathrm { d } s \big ) \mathrm { d } t + \exp \big ( \int _ { 0 } ^ { t } \kappa _ { 1 - s } \mathrm { d } s \big ) \mathrm { d } \vec { X } _ { t } } \\ & { \qquad = \kappa _ { 1 - t } \vec { X } _ { t } \exp \big ( \int _ { 0 } ^ { t } \kappa _ { 1 - s } \mathrm { d } s \big ) \mathrm { d } t + \exp \big ( \int _ { 0 } ^ { t } \kappa _ { 1 - s } \mathrm { d } s \big ) \big ( ( - \kappa _ { 1 - t } \vec { X } _ { t } + \xi _ { 1 - t } ) \mathrm { d } t + \sqrt { 2 \eta _ { 1 - t } } \mathrm { d } E \big ) } \\ & { \qquad = \exp \big ( \int _ { 0 } ^ { t } \kappa _ { 1 - s } \mathrm { d } s \big ) \xi _ { 1 - t } \mathrm { d } t + \sqrt { 2 \eta _ { t } } \exp \big ( \int _ { 0 } ^ { t } \kappa _ { 1 - s } \mathrm { d } s \big ) \mathrm { d } B _ { t } . } \end{array} +$$ + +Integrating from $0$ to $t$ , we get that + +$$ +\begin{array} { r l } & { t \exp \left( \int _ { 0 } ^ { t } \kappa _ { 1 - s } { \mathrm { d } } s \right) = \vec { X } _ { 0 } + \int _ { 0 } ^ { t } \exp \left( \int _ { 0 } ^ { t ^ { \prime } } \kappa _ { 1 - s } { \mathrm { d } } s \right) \xi _ { 1 - t ^ { \prime } } { \mathrm { d } } t ^ { \prime } + \int _ { 0 } ^ { t } \sqrt { 2 \eta _ { 1 - t ^ { \prime } } } \exp \left( \int _ { 0 } ^ { t ^ { \prime } } \kappa _ { 1 - s } { \mathrm { d } } s \right) { \mathrm { d } } B _ { t ^ { \prime } } , } \\ & { \Longleftrightarrow \vec { X } _ { t } = \vec { X } _ { 0 } \exp \left( - \int _ { 0 } ^ { t } \kappa _ { 1 - s } { \mathrm { d } } s \right) + \int _ { 0 } ^ { t } \exp \left( - \int _ { t ^ { \prime } } ^ { t } \kappa _ { 1 - s } { \mathrm { d } } s \right) \xi _ { 1 - t ^ { \prime } } { \mathrm { d } } t ^ { \prime } + \int _ { 0 } ^ { t } \sqrt { 2 \eta _ { 1 - t ^ { \prime } } } \exp \left( - \int _ { t ^ { \prime } } ^ { t } \kappa _ { 1 - s } { \mathrm { d } } s \right) { \mathrm { d } } B _ { t ^ { \prime } } . } \end{array} +$$ + +Since + +$$ +\begin{array} { r } { \mathbb { E } \Big [ \Big ( \int _ { 0 } ^ { t } \sqrt { 2 \eta _ { 1 - t ^ { \prime } } } \exp \big ( - \int _ { t ^ { \prime } } ^ { t } \kappa _ { 1 - s } \mathrm { d } s \big ) \mathrm { d } B _ { t ^ { \prime } } \Big ) ^ { 2 } \Big ] = 2 \int _ { 0 } ^ { t } \eta _ { 1 - t ^ { \prime } } \exp \big ( - 2 \int _ { t ^ { \prime } } ^ { t } \kappa _ { 1 - s } \mathrm { d } s \big ) \mathrm { d } t ^ { \prime } , } \end{array} +$$ + +we obtain that $\begin{array} { r } { \int _ { 0 } ^ { t } \sqrt { 2 \eta _ { 1 - t ^ { \prime } } } \exp \left( - \int _ { t ^ { \prime } } ^ { t } \kappa _ { 1 - s } \mathrm { d } s \right) \mathrm { d } B _ { t ^ { \prime } } } \end{array}$ has the same distribution as $\begin{array} { r } { \sqrt { 2 \int _ { 0 } ^ { t } \eta _ { 1 - t ^ { \prime } } \exp \big ( - 2 \int _ { t ^ { \prime } } ^ { t } \kappa _ { 1 - s } \mathrm { d } s \big ) \mathrm { d } t ^ { \prime } } \epsilon . } \end{array}$ where $\epsilon \sim N ( 0 , 1 )$ . + +# B.2.3 Proof of Proposition 4 + +This is a result that has been used by previous works, e.g. (De Bortoli et al., 2021, Sec. 2.1), but their derivation lacks rigor as it uses some unexplained approximations. While natural, the result is not common knowledge in the area. We provide a derivation which is still in discrete time, and hence not completely formal, but that corrects the gaps in the proof of De Bortoli et al. (2021). + +We introduce the short-hand + +$$ +\begin{array} { r l } & { \Vec { b } ( x , t ) = - \kappa _ { 1 - t } x + \big ( \frac { \sigma ( 1 - t ) ^ { 2 } } { 2 } - \eta _ { 1 - t } \big ) \mathfrak { s } ( x , 1 - t ) , } \\ & { b ( x , t ) = \kappa _ { t } X _ { t } + \big ( \frac { \sigma ( t ) ^ { 2 } } { 2 } + \eta _ { t } \big ) \mathfrak { s } ( X _ { t } , t ) , } \\ & { \Vec { \sigma } ( t ) = \sigma ( 1 - t ) . } \end{array} +$$ + +Remark that $b ( x , t ) = - \vec { b } ( x , 1 - t ) + \sigma ( t ) ^ { 2 } \mathfrak { s } ( X _ { t } , t ) .$ . + +Suppose that we discretize the forward process $\vec { X }$ using $K + 1$ equispaced timesteps: + +$$ +x _ { k + 1 } = x _ { k } + h \vec { b } ( x _ { k } , k h ) + \sqrt { h } \vec { \sigma } ( k h ) \epsilon _ { k } , \qquad \mathrm { w i t h } \ \epsilon _ { k } \sim N ( 0 , 1 ) . +$$ + +It is important to remark that $x _ { k + 1 } - x _ { k } = O ( h ^ { 1 / 2 } )$ . Throughout the proof we will keep track of all terms up to linear order in $h$ , while neglecting terms of order ${ \cal O } ( h ^ { 3 / 2 } )$ and higher. The distribution of the discretized forward process is: + +$$ +\begin{array} { r } | \mathop { = } \vec { p _ { 0 } } ( x _ { 0 } ) \prod _ { k = 0 } ^ { K - 1 } \vec { p } _ { k + 1 | k } ( x _ { k + 1 } | x _ { k } ) , \qquad \mathrm { w h e r e } \qquad \vec { p } _ { k + 1 | k } ( x _ { k + 1 } | x _ { k } ) \overset { \in \mathtt { e p } } { = } \frac { \exp \big ( - \frac { \| x _ { k + 1 } - x _ { k } - h \tilde { \ell } ( x _ { k } , k h ) \| ^ { 2 } } { 2 h \tilde { \sigma } ( k h ) ^ { 2 } } \big ) } { ( 2 \pi h \tilde { \sigma } ( k h ) ^ { 2 } ) ^ { d / 2 } } \sqrt { \frac { | x _ { k + 1 } - x _ { k + 1 } | h } { 2 h \tilde { \sigma } ( k h ) ^ { 2 } } } \end{array} +$$ + +Using telescoping products, we have that + +$$ +\begin{array} { r l } & { \vec { p } ( x _ { 0 : K } ) = \vec { p } _ { K } ( x _ { K } ) \prod _ { k = 0 } ^ { K - 1 } \vec { p } _ { k + 1 | k } ( x _ { k + 1 } | x _ { k } ) \frac { \vec { p } _ { k } ( x _ { k } ) } { \vec { p } _ { k + 1 } ( x _ { k + 1 } ) } } \\ & { \qquad = \vec { p } _ { K } ( x _ { K } ) \prod _ { k = 0 } ^ { K - 1 } \vec { p } _ { k + 1 | k } ( x _ { k + 1 } | x _ { k } ) \exp \big ( \log ( \vec { p } _ { k } ( x _ { k } ) ) - \log ( \vec { p } _ { k + 1 } ( x _ { k + 1 } ) ) \big ) } \end{array} +$$ + +We can use a discrete time version of Ito’s lemma: + +$$ +\begin{array} { r l } & { \log \vec { p } ( x _ { k + 1 } , ( k + 1 ) h ) \approx \log \vec { p } ( x _ { k } , k h ) + h \big ( \partial _ { t } \log \vec { p } ( x _ { k } , k h ) + \frac { \vec { \sigma } ( k h ) ^ { 2 } } { 2 } \Delta \log \vec { p } ( x _ { k } , k h ) \big ) } \\ & { \qquad + \left. \nabla \log \vec { p } ( x _ { k } , k h ) , x _ { k + 1 } - x _ { k } \right. + O ( h ^ { 3 / 2 } ) . } \end{array} +$$ + +Using equation (81) and a Taylor approximation, observe that + +$$ +\begin{array} { r l } & { \nabla \log p ( x _ { k } , k h ) , x _ { k + 1 } - x _ { k } \rangle } \\ & { = \langle \nabla \log p ( x _ { k + 1 } , ( k + 1 ) h ) - \nabla ^ { 2 } \log p ( x _ { k + 1 } , ( k + 1 ) h ) ( x _ { k + 1 } - x _ { k } ) , x _ { k + 1 } - x _ { k } \rangle + O ( h ^ { 3 / 2 } ) } \\ & { = \langle \nabla \log p ( x _ { k + 1 } , ( k + 1 ) h ) , x _ { k + 1 } - x _ { k } \rangle } \\ & { \qquad - \langle h \tilde { b } ( x _ { k } , k h ) + \sqrt { h } \tilde { \sigma } ( k h ) \epsilon _ { k } , \nabla ^ { 2 } \log p ( x _ { k + 1 } , ( k + 1 ) h ) \big ( h \tilde { b } ( x _ { k } , k h ) + \sqrt { h } \tilde { \sigma } ( k h ) \epsilon _ { k } \big ) \rangle + O ( h ^ { 3 / 2 } ) } \\ & { = \langle \nabla \log p ( x _ { k + 1 } , ( k + 1 ) h ) , x _ { k + 1 } - x _ { k } \rangle - h \tilde { \sigma } ( k h ) ^ { 2 } \Delta \log p ( x _ { k + 1 } , ( k + 1 ) h ) + O ( h ^ { 3 / 2 } ) . } \end{array} +$$ + +And since $\vec { p }$ satisfies the Fokker-Planck equation + +$$ +\begin{array} { r } { \partial _ { t } \vec { p _ { t } } = \nabla \cdot \big ( ( - \vec { b } ( x , t ) + \frac { \vec { \sigma } ( t ) ^ { 2 } } { 2 } \nabla \log \vec { p _ { t } } ( x ) ) \vec { p _ { t } } \big ) , } \end{array} +$$ + +we have that + +$$ +\begin{array} { r l } & { \partial _ { t } \log \vec { p } _ { t } = \frac { \partial _ { t } \vec { p } _ { t } } { \vec { p } _ { t } } = \frac { \nabla \cdot \big ( ( - \vec { b } ( x , t ) + \frac { \vec { \sigma } ( t ) ^ { 2 } } { 2 } \nabla \log \vec { p } _ { t } ( x ) ) \vec { p } _ { t } \big ) } { \vec { p } _ { t } } } \\ & { \qquad = - \nabla \cdot \vec { b } ( x , t ) + \frac { \vec { \sigma } ( t ) ^ { 2 } } { 2 } \Delta \log \vec { p _ { t } } ( x ) + \langle - \vec { b } ( x , t ) + \frac { \vec { \sigma } ( t ) ^ { 2 } } { 2 } \nabla \log \vec { p } _ { t } ( x ) , \nabla \log \vec { p _ { t } } ( x ) \rangle . } \end{array} +$$ + +Hence, + +$$ +\begin{array} { r l } & { \partial _ { t } \log p ( x _ { k } , k h ) = \partial _ { t } \log p ( x _ { k + 1 } , ( k + 1 ) h ) + O ( h ^ { 1 / 2 } ) } \\ & { \quad = - \nabla \cdot \tilde { b } ( x _ { k + 1 } , ( k + 1 ) h ) + \frac { \bar { \sigma } ( ( k + 1 ) h ) ^ { 2 } } { 2 } \Delta \log \tilde { p } ( x _ { k + 1 } , ( k + 1 ) h ) } \\ & { \qquad + \left. - \bar { b } ( x _ { k + 1 } , ( k + 1 ) h ) + \frac { \bar { \sigma } ( ( k + 1 ) h ) ^ { 2 } } { 2 } \nabla \log \tilde { p } ( x _ { k + 1 } , ( k + 1 ) h ) , \nabla \log \tilde { p } ( x _ { k + 1 } , ( k + 1 ) h ) \right. + O ( \delta ^ { 2 } ) , } \end{array} +$$ + +If we plug (86) and (89) into (84), we obtain + +$$ +\begin{array} { r l } & { \log p ( x _ { k + 1 } , ( k + 1 ) h ) - \log p ( x _ { k } , k h ) } \\ & { = h \big ( - \nabla \cdot \vec { b } ( x _ { k + 1 } , ( k + 1 ) h ) + \langle - \vec { b } ( x _ { k + 1 } , ( k + 1 ) h ) + \frac { \vec { \sigma } ( ( k + 1 ) h ) ^ { 2 } } { 2 } \nabla \log \vec { p } ( x _ { k + 1 } , ( k + 1 ) h ) , \nabla \log \vec { p } ( x _ { k + 1 } , ( k + 1 ) h ) \big ) } \\ & { \qquad + \langle \nabla \log p ( x _ { k + 1 } , ( k + 1 ) h ) , x _ { k + 1 } - x _ { k } \rangle + { \cal O } ( h ^ { 3 / 2 } ) } \\ & { = \frac { \langle 2 h \vec { \sigma } ( k h ) ^ { 2 } \nabla \log p ( x _ { k + 1 } , ( k + 1 ) h ) , x _ { k + 1 } - x _ { k } - h \vec { b } ( x _ { k + 1 } , ( k + 1 ) h ) \rangle } { 2 h \vec { \sigma } ( k h ) ^ { 2 } } } \\ & { \qquad + h \big ( - \nabla \cdot \vec { b } ( x _ { k + 1 } , ( k + 1 ) h ) + \frac { \vec { \sigma } ( ( k + 1 ) h ) ^ { 2 } } { 2 } \| \nabla \log \vec { p } ( x _ { k + 1 } , ( k + 1 ) h ) \| ^ { 2 } \big ) + { \cal O } ( h ^ { 3 / 2 } ) . } \end{array} +$$ + +Applying a discrete time version of Ito’s lemma again, we have that + +$$ +\begin{array} { r l } & { \vec { b } ( x _ { k } , k h ) = \vec { b } ( x _ { k + 1 } , ( k + 1 ) h ) - h \big ( \partial _ { t } \vec { b } ( x _ { k + 1 } , ( k + 1 ) h ) + \frac { \vec { \sigma } ( ( k + 1 ) h ) ^ { 2 } } { 2 } \Delta \vec { b } ( x _ { k + 1 } , ( k + 1 ) h ) \big ) } \\ & { \qquad + \nabla \vec { b } ( x _ { k + 1 } , ( k + 1 ) h ) ^ { \top } ( x _ { k } - x _ { k + 1 } ) + O ( h ^ { 3 / 2 } ) } \\ & { \qquad = \vec { b } ( x _ { k + 1 } , ( k + 1 ) h ) + \nabla \vec { b } ( x _ { k + 1 } , ( k + 1 ) h ) ^ { \top } ( x _ { k } - x _ { k + 1 } ) + O ( h ) . } \end{array} +$$ + +where $\Delta \vec { b }$ denotes the component-wise Laplacian of $\vec { b }$ . Thus, + +$$ +\begin{array} { r l } & { \log \tilde { p } _ { k + 1 | k } ( x _ { k + 1 } | x _ { k } ) } \\ & { = - \frac { d } { 2 } \log \left( 2 \pi h \tilde { \sigma } ( k h ) ^ { 2 } \right) - \frac { \| x _ { k + 1 } - x _ { k } - h \tilde { \sigma } ( x _ { k } , k h ) \| ^ { 2 } } { 2 h \tilde { \sigma } ( k h ) ^ { 2 } } } \\ & { = - \frac { d } { 2 } \log \left( 2 \pi h \tilde { \sigma } ( k h ) ^ { 2 } \right) - \frac { \| x _ { k + 1 } - x _ { k } - h \tilde { \sigma } ( x _ { k + 1 } , ( k + 1 ) h ) + \nabla \tilde { b } ( x _ { k + 1 } , ( k + 1 ) h ) ^ { \top } ( x _ { k } - x _ { k + 1 } ) ) \| ^ { 2 } } { 2 h \tilde { \sigma } ( k h ) ^ { 2 } } + O ( h ^ { 3 / 2 } ) } \\ & { = - \frac { d } { 2 } \log \left( 2 \pi h \tilde { \sigma } ( k h ) ^ { 2 } \right) - \frac { \| x _ { k + 1 } - x _ { k } - h \tilde { b } ( x _ { k + 1 } , ( k + 1 ) h ) \| ^ { 2 } } { 2 h \tilde { \sigma } ( k h ) ^ { 2 } } + \frac { \langle x _ { k + 1 } - x _ { k } , \nabla \tilde { b } ( x _ { k + 1 } , ( k + 1 ) h ) ^ { \top } ( x _ { k } - x _ { k + 1 } ) \rangle } { \tilde { \sigma } ( k h ) ^ { 2 } } + O ( h ^ { 3 / 2 } ) } \\ & = - \frac { d } { 2 } \log \left( 2 \pi h \tilde { \sigma } ( k h ) ^ { 2 } \right) - \frac { \| x _ { k + 1 } - x _ { k } - h \tilde { b } ( x _ { k + 1 } , ( k + 1 ) h ) \| ^ { 2 } } { h \tilde { \sigma } ( k h ) ^ { 2 } } - \frac { h \tilde { \sigma } ( k h ) ^ { 2 } \langle c _ { k } , \nabla \tilde { b } ( x _ { k + 1 } , ( k + 1 ) h \rangle ^ { \top } c _ { k } ) } \end{array} +$$ + +Combining (90) and (92), we obtain that + +$$ +\begin{array} { r l } & { \log \vec { p } _ { k + 1 | k } ( x _ { k + 1 } | x _ { k } ) - \bigl ( \log p ( x _ { k + 1 } , ( k + 1 ) h ) - \log p ( x _ { k } , k h ) \bigr ) } \\ & { = - \frac { d } { 2 } \log \bigl ( 2 \pi h \vec { \sigma } ( k h ) ^ { 2 } \bigr ) - \frac { \| x _ { k + 1 } - x _ { k } - h \vec { \sigma } ( x _ { k + 1 } , ( k + 1 ) h ) + h \vec { \sigma } ( k h ) ^ { 2 } \nabla \log p ( x _ { k + 1 } , ( k + 1 ) h ) \| ^ { 2 } } { h \vec { \sigma } ( k h ) ^ { 2 } } + O ( h ^ { 3 / 2 } ) } \\ & { = - \frac { d } { 2 } \log \bigl ( 2 \pi h \vec { \sigma } ( ( k + 1 ) h ) ^ { 2 } \bigr ) - \frac { \| x _ { k + 1 } - x _ { k } - h \vec { \sigma } ( x _ { k + 1 } , ( k + 1 ) h ) + h \vec { \sigma } ( ( k + 1 ) h ) ^ { 2 } \nabla \log p ( x _ { k + 1 } , ( k + 1 ) h ) \| ^ { 2 } } { h \vec { \sigma } ( ( k + 1 ) h ) ^ { 2 } } + O ( \{ h ^ { 3 / 2 } \} ) , } \end{array} +$$ + +By Bayes rule, and taking the exponential of this equation, we obtain + +$$ +\begin{array} { r l } & { \vec { p } _ { k + 1 | k } ( x _ { k + 1 } | x _ { k } ) : = \vec { p } _ { k + 1 | k } ( x _ { k + 1 } | x _ { k } ) \frac { \vec { p } _ { k } ( x _ { k } ) } { \vec { p } _ { k + 1 } ( x _ { k + 1 } ) } } \\ & { \quad \quad \quad = \frac { \exp \big ( - \frac { \| x _ { k } - x _ { k + 1 } + h \hat { b } ( x _ { k + 1 } , ( k + 1 ) h ) - h \sigma ( ( k + 1 ) h ) ^ { 2 } \nabla \log p ( x _ { k + 1 } , ( k + 1 ) h ) \| ^ { 2 } } { 2 h \sigma ( ( k + 1 ) h ) ^ { 2 } } \big ) } { ( 2 \pi h \tilde { \sigma } ( ( k + 1 ) h ) ^ { 2 } ) ^ { d / 2 } } + O ( h ^ { 3 / 2 } ) . } \end{array} +$$ + +Up to the ${ \cal O } ( h ^ { 3 / 2 } )$ term, the right-hand side is the conditional Gaussian corresponding to the update + +$$ +x _ { k } = x _ { k + 1 } + h \big ( - \vec { b } ( x _ { k + 1 } , ( k + 1 ) h ) + \vec { \sigma } ( ( k + 1 ) h ) ^ { 2 } \nabla \log p ( x _ { k + 1 } , ( k + 1 ) h ) \big ) + \sqrt { h } \vec { \sigma } ( ( k + 1 ) h ) \epsilon +$$ + +If we define $y _ { k } = x _ { K - k }$ , and we use that $b ( x , t ) = - \vec { b } ( x , 1 - t ) + \vec { \sigma } ( t ) ^ { 2 } \nabla \log p ( x , 1 - t )$ , we can rewrite (95) as + +$$ +\begin{array} { r l } & { \kappa _ { - k } = y _ { K - k - 1 } + h \big ( - \vec { b } ( y _ { K - k - 1 } , ( K - k - 1 ) h ) + \vec { \sigma } ( ( K - k - 1 ) h ) ^ { 2 } \nabla \log p ( y _ { K - k - 1 } , ( K - k - 1 ) h ) } \\ & { \qquad + \sqrt { h } \vec { \sigma } ( ( K - k - 1 ) h ) \epsilon _ { k } = y _ { K - k - 1 } + h b ( y _ { K - k - 1 } , k h ) + \sqrt { h } \sigma ( k h ) \epsilon _ { K - k - 1 } , } \\ & { \Longrightarrow y _ { k + 1 } = y _ { k } + h b ( y _ { k } , k h ) + \sqrt { h } \sigma ( k h ) \epsilon _ { k } . } \end{array} +$$ + +And this is the Euler-Maruyama discretization of the backward process $\overleftarrow { X }$ . If we plug (94) into (83), we obtain that + +$$ +\begin{array} { r } { \vec { p } ( x _ { 0 : K } ) \approx \vec { p _ { K } } ( x _ { K } ) \prod _ { k = 0 } ^ { K - 1 } \vec { p _ { k + 1 | k } } ( x _ { k + 1 } | x _ { k } ) . } \end{array} +$$ + +which concludes the proof, as $\vec { p } _ { K } ( x _ { K } )$ is the initial distribution of the backward process, and $\vec { p _ { k + 1 | k } } ( x _ { k + 1 } | x _ { k } )$ are its transition kernels. + +# B.3 The relationship between the noise predictor $\epsilon$ and the score function + +Applying Lemma 2 with the choices of $( \kappa _ { t } ) _ { t \geq 0 }$ and $( \eta _ { t } ) _ { t \geq 0 }$ for DDIM, we obtain that $\vec { X _ { t } }$ has the same distribution as + +$$ +\hat { X } _ { t } = \sqrt { \bar { \alpha } _ { 1 - t } } \vec { X } _ { 0 } + \sqrt { 1 - \bar { \alpha } _ { 1 - t } } \epsilon , \qquad \epsilon \sim N ( 0 , 1 ) . +$$ + +Since $\vec { X _ { t } }$ and $\hat { X } _ { t }$ have the same distribution, predicting the noise of $\vec { X _ { t } }$ is equivalent to predicting the noise of $\hat { X } _ { t }$ . The noise predictor $\epsilon$ can be written as: + +$$ +\begin{array} { r } { ( x , t ) : = \mathbb { E } [ \epsilon | \hat { X } _ { 1 - t } = x ] = \mathbb { E } \big [ \epsilon | \sqrt { \overline { { \alpha _ { t } } } } \vec { X } _ { 0 } + \sqrt { 1 - \bar { \alpha } _ { t } } \epsilon = x \big ] = \mathbb { E } \big [ \frac { x - \sqrt { \alpha _ { t } } \vec { X } _ { 0 } } { \sqrt { 1 - \bar { \alpha } _ { t } } } \big | \sqrt { \overline { { \alpha _ { t } } } } \vec { X } _ { 0 } + \sqrt { 1 - \bar { \alpha } _ { t } } \epsilon = x \big ] } \end{array} +$$ + +And the score function $\pmb { \mathfrak { s } } ( x , t ) : = \nabla \log \vec { p } _ { 1 - t } ( x )$ admits the expression + +$$ +\begin{array} { r } { \mathfrak { s } ( x , t ) : = \nabla \log \vec { p } _ { 1 - t } ( x ) = \frac { \nabla \vec { p } _ { 1 - t } ( x ) } { \vec { p } _ { 1 - t } ( x ) } = \frac { \nabla \mathbb { E } [ \vec { p } _ { 1 - t } \mathsf { 0 } ( x | \vec { X } _ { 0 } ) ] } { \vec { p } _ { 1 - t } ( x ) } = \frac { \mathbb { E } [ \nabla \log \vec { p } _ { 1 - t } \mathsf { 0 } ( x | \vec { X } _ { 0 } ) \vec { p } _ { 1 - t | \mathrm { 0 } } ( x | \vec { X } _ { 0 } ) ] } { \vec { p } _ { 1 - t } ( x ) } , } \end{array} +$$ + +where + +$$ +\begin{array} { r } { \vec { p } _ { 1 - t | 0 } ( x | \vec { X } _ { 0 } ) = \frac { \exp ( - \| x - \sqrt { \bar { \alpha _ { t } } } Y _ { 1 } \| ^ { 2 } / ( 2 ( 1 - \bar { \alpha } _ { t } ) ) ) } { ( 2 \pi ( 1 - \bar { \alpha } _ { t } ) ) ^ { d / 2 } } \implies \nabla \log \vec { p } _ { t | 1 } ( x | Y _ { 1 } ) = - \frac { x - \sqrt { \bar { \alpha } _ { t } } Y _ { 1 } } { 1 - \bar { \alpha } _ { t } } . } \end{array} +$$ + +Plugging this into the right-hand side of (100) and using Bayes’ rule, we get + +$$ +\begin{array} { r } { \mathfrak { s } ( x , t ) = \mathbb { E } \big [ - \frac { x - \sqrt { \bar { \alpha } _ { t } } \vec { X } _ { 0 } } { 1 - \bar { \alpha } _ { t } } \big | \sqrt { \bar { \alpha } _ { t } } \vec { X } _ { 0 } + \sqrt { 1 - \bar { \alpha } _ { t } } \epsilon = x \big ] . } \end{array} +$$ + +Comparing the right-hand sides of (99) and (102), we obtain that $\begin{array} { r } { \mathfrak { s } ( x , t ) = - \frac { \epsilon ( x , t ) } { \sqrt { 1 - \bar { \alpha } _ { t } } } } \end{array}$ . + +# B.4 The relationship between the vector field $v$ and the score function + +By construction (Lipman et al., 2023; Albergo and Vanden-Eijnden, 2023; Albergo et al., 2023), we have tha + +$$ +\begin{array} { r l } & { v ( x , t ) = \mathbb { E } [ \dot { \alpha } _ { t } Y _ { 1 } + \dot { \beta } _ { t } Y _ { 0 } | x = \alpha _ { t } Y _ { 1 } + \beta _ { t } Y _ { 0 } ] } \\ & { \qquad = \mathbb { E } [ \frac { \dot { \alpha } _ { t } ( x - \beta _ { t } Y _ { 0 } ) } { \alpha _ { t } } + \dot { \beta } _ { t } Y _ { 0 } | x = \alpha _ { t } Y _ { 1 } + \beta _ { t } Y _ { 0 } ] } \\ & { \qquad = \frac { \dot { \alpha } _ { t } } { \alpha _ { t } } x + ( \dot { \beta } _ { t } - \frac { \dot { \alpha } _ { t } } { \alpha _ { t } } \beta _ { t } ) \mathbb { E } [ Y _ { 0 } | x = \alpha _ { t } Y _ { 1 } + \beta _ { t } Y _ { 0 } ] , } \end{array} +$$ + +where we used that $Y _ { 1 } = ( x - \beta _ { t } Y _ { 0 } ) / \alpha _ { t }$ . Also, we can write the score as follows + +$$ +\begin{array} { r } { \mathfrak { s } ( x , t ) : = \nabla \log p _ { t } ( x ) = \frac { \nabla p _ { t } ( x ) } { p _ { t } ( x ) } = \frac { \nabla \mathbb { E } [ p _ { t \mid 1 } ( x \mid Y _ { 1 } ) ] } { p _ { t } ( x ) } = \frac { \mathbb { E } [ \nabla p _ { t \mid 1 } ( x \mid Y _ { 1 } ) ] } { p _ { t } ( x ) } = \frac { \mathbb { E } [ p _ { t \mid 1 } ( x \mid Y _ { 1 } ) \nabla \log p _ { t \mid 1 } ( x \mid Y _ { 1 } ) ] } { p _ { t } ( x ) } , } \end{array} +$$ + +where + +$$ +\begin{array} { r } { p _ { t | 1 } ( x | Y _ { 1 } ) = \frac { \exp ( - \| x - \alpha _ { t } Y _ { 1 } \| ^ { 2 } / ( 2 \beta _ { t } ^ { 2 } ) ) } { ( 2 \pi \beta _ { t } ^ { 2 } ) ^ { d / 2 } } \implies \nabla \log \vec { p } _ { t | 1 } ( x | Y _ { 1 } ) = - \frac { x - \alpha _ { t } Y _ { 1 } } { \beta _ { t } ^ { 2 } } } \end{array} +$$ + +Plugging this back into the right-hand side of (104), we obtain + +$$ +\begin{array} { r l } & { \mathfrak { s } ( x , t ) = - \frac { \mathbb { E } [ p _ { t | 1 } ( x | Y _ { 1 } ) \frac { x - \alpha _ { t } Y _ { 1 } } { \beta _ { t } ^ { 2 } } ] } { p _ { t } ( x ) } = - \frac { \int \tilde { p } _ { t | 1 } ( x | Y _ { 1 } ) p _ { 1 } ( Y _ { 1 } ) \frac { x - \alpha _ { t } Y _ { 1 } } { \beta _ { t } ^ { 2 } } d Y _ { 1 } } { \tilde { p } _ { t } ( x ) } } \\ & { \qquad = - \int p _ { 1 | t } \bigl ( Y _ { 1 } | x \bigr ) \frac { x - \alpha _ { t } Y _ { 1 } } { \beta _ { t } ^ { 2 } } d Y _ { 1 } = - \mathbb { E } [ \frac { x - \alpha _ { t } Y _ { 1 } } { \beta _ { t } ^ { 2 } } | x = \alpha _ { t } Y _ { 1 } + \beta _ { t } Y _ { 0 } ] = - \frac { \mathbb { E } [ Y _ { 0 } | x = \alpha _ { t } Y _ { 1 } + \beta _ { t } Y _ { 0 } ] } { \beta _ { t } } } \end{array} +$$ + +The last equality holds because $( x - \alpha _ { t } Y _ { 1 } ) / \beta _ { t } = Y _ { 0 }$ . Putting together (103) and (106), we obtain that + +$$ +\begin{array} { r } { v ( x , t ) = \frac { \dot { \alpha } _ { t } } { \alpha _ { t } } x + \beta _ { t } ( \frac { \dot { \alpha } _ { t } } { \alpha _ { t } } \beta _ { t } - \dot { \beta } _ { t } ) \mathfrak { s } ( x , t ) \iff \mathfrak { s } ( x , t ) = \frac { 1 } { \beta _ { t } ( \frac { \dot { \alpha } _ { t } } { \alpha _ { t } } \beta _ { t } - \dot { \beta } _ { t } ) } \bigl ( v ( x , t ) - \frac { \dot { \alpha } _ { t } } { \alpha _ { t } } x \bigr ) } \end{array} +$$ + +Thus, the ODE (3) can be rewritten like this: + +$$ +\begin{array} { r } { \frac { \mathrm { d } X _ { t } } { \mathrm { d } t } = \frac { \dot { \alpha } _ { t } } { \alpha _ { t } } X _ { t } + \beta _ { t } ( \frac { \dot { \alpha } _ { t } } { \alpha _ { t } } \beta _ { t } - \dot { \beta } _ { t } ) \mathfrak { s } ( X _ { t } , t ) , \qquad X _ { 0 } \sim p _ { 0 } . } \end{array} +$$ + +To allow for an arbitrary diffusion coefficient, we need to add a correction term to the drift: + +$$ +\begin{array} { r } { \mathrm { d } X _ { t } = \big ( \frac { \dot { \alpha } _ { t } } { \alpha _ { t } } X _ { t } + \big ( \frac { \sigma ( t ) ^ { 2 } } { 2 } + \beta _ { t } ( \frac { \dot { \alpha } _ { t } } { \alpha _ { t } } \beta _ { t } - \dot { \beta } _ { t } ) \big ) \mathfrak { s } ( X _ { t } , t ) \big ) \mathrm { d } t + \sigma ( t ) \mathrm { d } B _ { t } , \qquad X _ { 0 } \sim p _ { 0 } . } \end{array} +$$ + +This can be easily shown by writing down the Fokker-Planck equations for (108) and (109), and observing that they are the same up to a cancellation of terms. Finally, if we plug the right-hand side of (107) into (109), we obtain the SDE for Flow Matching with arbitrary noise schedule (equation (4)). + +# C Stochastic optimal control as maximum entropy RL in continuous space and time + +In this section, we bridge KL-regularized (or MaxEnt) reinforcement learning and stochastic optimal control. We show that when the action space is Euclidean and the transition probabilities are conditional Gaussians, taking the limit in which the stepsize goes to zero on the KL-regularized RL problem gives rise to the SOC problem. A consequence of this connection is that all algorithms for KL-regularized RL admit an analog for diffusion fine-tuning. This is not novel, but it may be useful for researchers that are familiar with RL fine-tuning formulations. + +Appendix C.4 is providing a more direct, rigorous, continuous-time connection between SOC and MaxEnt RL, as it shows that the expected control cost is equal to the KL divergence between the distributions over trajectories, conditioned on the starting points (see equation (18)). + +# C.1 Maximum entropy RL + +Several diffusion fine-tuning methods (Black et al., 2024; Uehara et al., 2024b) are based on KL-regularized RL, also known as maximum entropy RL, which we review in the following. In the classical reinforcement learning (RL) setting, we have an agent that, starting from state $s _ { 0 } \sim p _ { 0 }$ , iteratively observes a state $s _ { k }$ , takes an action $a _ { k }$ according to a policy $\pi ( { a } _ { k } ; { s } _ { k } , k )$ which leads to a new state $s k { + 1 }$ according to a fixed transition probability $p ( s _ { k + 1 } | a _ { k } , s _ { k } )$ , and obtains rewards $r _ { k } ( s _ { k } , a _ { k } )$ . This can be summarized into a trajectory ${ \boldsymbol { \tau } } = ( ( s _ { k } , a _ { k } ) ) _ { k = 0 } ^ { K }$ . The goal is to optimize the policy $\pi$ in order to maximize the expected total reward, i.e. $\begin{array} { r } { \operatorname* { m a x } _ { \boldsymbol { \pi } } \mathbb { E } _ { \tau \sim \pi , p } [ \sum _ { k = 0 } ^ { K } r _ { k } ( s _ { k } , a _ { k } ) ] } \end{array}$ . + +Maximum entropy RL (MaxEnt RL; Ziebart et al. (2008)) amounts to adding the entropy $H ( \pi )$ of the policy $\pi ( \cdot ; s _ { k } , k )$ to the reward for each step $k$ , in order to encourage exploration and improve robustness to changes in the environment: $\begin{array} { r } { \operatorname* { m a x } _ { \boldsymbol { \pi } } \mathbb { E } _ { \tau \sim \pi , p } [ \sum _ { k = 0 } ^ { K } r _ { k } ( s _ { k } , a _ { k } ) + \sum _ { k = 0 } ^ { K - 1 } H ( \boldsymbol { \pi } ( \cdot ; s _ { k } , k ) ) ] } \end{array}$ 8. As a generalization, one can regularize using the negative KL divergence between $\pi ( \cdot ; s _ { k } , k )$ and a base policy + +$$ +\begin{array} { r } { \operatorname* { m a x } _ { \boldsymbol { \pi } } \mathbb { E } _ { \tau \sim \pi , p } [ \sum _ { k = 0 } ^ { K } r _ { k } \big ( s _ { k } , { a } _ { k } \big ) - \sum _ { k = 0 } ^ { K - 1 } \mathrm { K L } ( \pi ( \cdot ; s _ { k } , k ) | | \pi _ { \mathrm { b a s e } } ( \cdot ; s _ { k } , k ) ) ] , } \end{array} +$$ + +which prevents the learned policy to deviate too much from the base policy. Each policy $\pi$ induces a distribution $q ( \tau )$ over trajectories $\tau$ , and the MaxEnt RL problem (110) can be expressed solely in terms of such distributions (Lemma 3 in Appendix C.3): + +$$ +\begin{array} { r } { \operatorname* { m a x } _ { q } \mathbb { E } _ { \tau \sim q } [ \sum _ { k = 0 } ^ { K } r _ { k } ( s _ { k } , a _ { k } ) ] - \mathrm { K L } ( q | | q ^ { \mathrm { b a s e } } ) , } \end{array} +$$ + +where $q ^ { \mathrm { b a s e } }$ is the distribution induced by the base policy $\pi _ { \mathrm { b a s e } }$ , and the maximization is over all distributions $q$ such that their marginal for $s _ { 0 }$ is $p _ { 0 }$ . We can further recast this problem as (Lemma 4 in Appendix C.3): + +$$ +\begin{array} { r } { \operatorname* { m i n } _ { \boldsymbol { q } } \mathrm { K L } ( \boldsymbol { q } | | \boldsymbol { q } ^ { * } ) , \qquad \mathrm { w h e r e ~ } \boldsymbol { q } ^ { * } ( \tau ) : = \boldsymbol { q } ^ { \mathrm { b a s e } } ( \tau ) \exp \big ( \sum _ { k = 0 } ^ { K } r _ { k } ( s _ { k } , a _ { k } ) - \mathcal { V } ( s _ { 0 } , 0 ) \big ) , } \end{array} +$$ + +where + +$$ +\begin{array} { r l } & { \mathcal { V } ( s _ { k } , k ) : = \log \left( \mathbb { E } _ { \tau \sim \pi _ { \mathrm { b a s e } } , p } [ \exp \left( \sum _ { k ^ { \prime } = k } ^ { K } r _ { k ^ { \prime } } ( s _ { k ^ { \prime } } , a _ { k ^ { \prime } } ) \right) | s _ { k } ] \right) } \\ & { \qquad = \operatorname* { m a x } _ { \pi } \mathbb { E } _ { \tau \sim \pi , p } \left[ \sum _ { k ^ { \prime } = k } ^ { K } r _ { k ^ { \prime } } ( s _ { k ^ { \prime } } , a _ { k ^ { \prime } } ) - \sum _ { k ^ { \prime } = k } ^ { K - 1 } \mathrm { K L } ( \pi ( \cdot ; s _ { k ^ { \prime } } , k ^ { \prime } ) | | \pi _ { \mathrm { b a s e } } ( \cdot ; s _ { k ^ { \prime } } , k ^ { \prime } ) ) | s _ { k } \right] } \end{array} +$$ + +is the value function. Problem (112) directly implies that the distribution induced by the optimal policy $\pi ^ { * }$ is the tilted distribution $q ^ { * }$ (which has initial marginal $p _ { 0 }$ ). + +# C.2 From maximum entropy RL to stochastic optimal control + +The following well-known result, which we prove in Appendix C.3, shows that in a natural sense, the continuous-time continuous-space version of MaxEnt RL is the SOC framework introduced in Section 4.1. In particular, when states and actions are vectors in $\mathbb { R } ^ { d }$ , policies are specified by a vector field $u$ (the control), and transition probabilities are conditional Gaussians, the MaxEnt RL problem becomes an SOC problem when the number of timesteps grows to infinity. + +Proposition 5. Suppose that + +(i) The state space and the action space are $\mathbb { R } ^ { d }$ , +(ii) Policies $\pi$ are specified as $\pi ( a _ { k } ; s _ { k } , k ) = \delta ( a _ { k } - u ( s _ { k } , k h ) )$ , where $u : \mathbb { R } ^ { d } \times [ 0 , T ] \mathbb { R } ^ { d }$ is a vector field, and $\delta$ denotes the Dirac delta, +(iii) Transition probabilities are conditional Gaussian densities: $p ( s _ { k + 1 } | a _ { k } , s _ { k } ) = N ( s _ { k } + h ( b ( s _ { k } , k h ) +$ $\sigma ( k h ) a _ { k } ) , h \sigma ( k h ) \sigma ( k h ) ^ { \top } )$ , where $h = T / K$ is the stepsize, and b and $\sigma$ are defined as in Section 4.1. + +Then, in the limit in which the number of steps $K$ grows to infinity, the problem (110) is equivalent to the SOC problem (12)-(13), identifying + +• the sequence of states $\left( \boldsymbol { s } _ { k } \right) _ { k = 0 } ^ { k }$ with the trajectory $X ^ { u } = ( X _ { t } ^ { u } ) _ { t \in [ 0 , 1 ] }$ , +• the running reward $\begin{array} { r } { \sum _ { k = 0 } ^ { K - 1 } r _ { k } ( s _ { k } , a _ { k } ) } \end{array}$ with the negative running cost $\begin{array} { r } { - \int _ { 0 } ^ { T } f ( X _ { t } ^ { u } , t ) \mathrm { d } t } \end{array}$ , +• the terminal reward $r _ { K } ( s _ { K } , a _ { K } )$ with the negative terminal cost $- g ( X _ { T } ^ { u } )$ , +• the KL regularization $\begin{array} { r } { \mathbb { E } _ { \tau \sim \pi , p } [ \sum _ { k = 0 } ^ { K - 1 } \mathrm { K L } ( \pi ( \cdot ; s _ { k } , k ) | | \pi _ { \mathrm { b a s e } } ( \cdot ; s _ { k } , k ) ) ] } \end{array}$ with $\begin{array} { l } { { \frac { 1 } { 2 } } } \end{array}$ times the expected $L ^ { 2 }$ norm of the control $\begin{array} { r } { \frac { 1 } { 2 } \mathbb { E } \big [ \int _ { 0 } ^ { T } \| u ( X _ { t } ^ { u } , t ) \| ^ { 2 } \mathrm { d } t \big ] } \end{array}$ , +• and the value function $\mathcal { V } ( s _ { k } , k )$ defined in (113) with the negative value function $- V ( x , t )$ defined in Section 4.1. + +A first consequence of this result is that every loss function designed for generic MaxEnt RL problems has a corresponding loss function for SOC problems. The geometric structure of the latter allows for additional losses that do not have an analog in the classical MaxEnt RL setting; in particular, we can differentiate the state and terminal costs. + +A second consequence of Proposition 5 is that the characterization (112) can be translated to the SOC setting. The analogs of the distributions $q ^ { * }$ , $q ^ { \mathrm { b a s e } }$ induced by the optimal policy $\pi ^ { * }$ and the base policy $\pi ^ { \mathrm { b a s e } }$ are the distributions $p ^ { * } , p ^ { \mathrm { b a s e } }$ induced by the optimal control $u ^ { * }$ and the null control. For an arbitrary trajectory $\pmb { X } = ( X _ { t } ) _ { t \in [ 0 , T ] }$ , the relation between $\mathbb { P } ^ { * }$ and $\mathbb { P } ^ { \mathrm { b a s e } }$ is given by + +$$ +\begin{array} { r } { \frac { \mathrm { d } \mathbb { P } ^ { * } } { \mathrm { d } \mathbb { P } ^ { \mathrm { b a s e } } } ( X ) = \exp ( - \int _ { 0 } ^ { T } f ( X _ { t } , t ) \mathrm { d } t - g ( X _ { T } ) + V ( X _ { 0 } , 0 ) ) } \end{array} +$$ + +where $V$ is the value function as defined in Section 4.1. Note that this matches the statement in (22). + +# C.3 Proof of Proposition 5: from MaxEnt RL to SOC + +Since the transition $p ( s _ { k + 1 } | a _ { k } , s _ { k } )$ is fixed, for each $\pi$ we can define + +$\tilde { \pi } ( a _ { k } , s _ { k + 1 } ; s _ { k } , k ) = \pi ( a _ { k } ; s _ { k } , k ) p ( s _ { k + 1 } | a _ { k } , s _ { k } ) \mathrm { ~ a n d ~ } \tilde { \pi } _ { \mathrm { b a s e } } ( a _ { k } , s _ { k + 1 } ; s _ { k } , k ) = \pi _ { \mathrm { b a s e } } ( a _ { k } ; s _ { k } , k ) p ( s _ { k + 1 } | a _ { k } , k ) .$ 1|ak, sk), + +and reexpress (110) as (see Lemma 3) + +$$ +\begin{array} { r } { \operatorname* { m i n } _ { \tilde { \pi } } \mathbb { E } _ { \tau \sim \tilde { \pi } } [ \sum _ { k = 0 } ^ { K } r _ { k } ( s _ { k } , a _ { k } ) - \sum _ { k = 0 } ^ { K - 1 } \mathrm { K L } \big ( \tilde { \pi } ( \cdot , \cdot ; s _ { k } , k ) \big | \big | \tilde { \pi } _ { \mathrm { b a s e } } ( \cdot , \cdot ; s _ { k } , k ) \big ) ] . } \end{array} +$$ + +Using the hypothesis of the proposition, we can write + +$$ +\begin{array} { r l } & { \tilde { \pi } ( a _ { k } , s _ { k + 1 } ; s _ { k } , k ) = \delta ( a _ { k } - u ( s _ { k } , k \eta ) ) N ( s _ { k } + \eta ( b ( s _ { k } , k \eta ) + \sigma ( k \eta ) a _ { k } ) , \eta \sigma ( k \eta ) \sigma ( k \eta ) ^ { \top } ) } \\ & { \qquad = \delta ( a _ { k } - u ( s _ { k } , k \eta ) ) \tilde { \pi } ( s _ { k + 1 } ; s _ { k } , k ) , } \end{array} +$$ + +where π˜(sk+1; s ${ \bf \varepsilon } _ { \ast } , k ) = N ( s _ { k } + \eta ( b ( s _ { k } , k \eta ) + \sigma ( k \eta ) u ( s _ { k } , k \eta ) ) , { \nu } $ ησ(kη)σ(kη)⊤) is the state transition kernel. We set the base policy as $\pi _ { \mathrm { b a s e } } ( a _ { k } ; s _ { k } , k ) = \delta ( a _ { k } )$ , and we obtain analogously that $\tilde { \pi } ( a _ { k } , s _ { k + 1 } ; s _ { k } , k ) =$ $\delta ( a _ { k } ) \tilde { \pi } _ { \mathrm { b a s e } } ( s _ { k + 1 } ; s _ { k } , k )$ with $\tilde { \pi } _ { \mathrm { b a s e } } ( s _ { k + 1 } ; s _ { k } , k ) = N ( s _ { k } + \eta b ( s _ { k } , k \eta ) , \eta \sigma ( k \eta ) \sigma ( k \eta ) ^ { \scriptscriptstyle 1 } )$ . Now, if we take $K$ large, the trajectory $( s _ { k } ) _ { k = 0 } ^ { K }$ generated by $\tilde { \pi }$ can be regarded as the Euler-Maruyama discretization of a solution $X ^ { u }$ of the controlled SDE (13), while the trajectory generated by $\tilde { \pi } _ { \mathrm { b a s e } }$ is the discretization of the uncontrolled process $X ^ { 0 }$ obtained by setting $u = 0$ . As a consequence + +$$ +\begin{array} { r l } & { \operatorname* { l i m } _ { K \to \infty } \mathbb { E } _ { \tau \sim \tilde { \pi } } [ \sum _ { k = 0 } ^ { K - 1 } \mathrm { K L } ( \tilde { \pi } ( \cdot , \cdot ; s _ { k } , k ) | | \tilde { \pi } _ { \mathrm { b a s e } } ( \cdot , \cdot ; s _ { k } , k ) ) ] } \\ & { = \operatorname* { l i m } _ { K \to \infty } \mathbb { E } _ { \tau \sim \tilde { \pi } } [ \sum _ { k = 0 } ^ { K - 1 } \mathrm { K L } ( \tilde { \pi } ( \cdot ; s _ { k } , k ) | | \tilde { \pi } _ { \mathrm { b a s e } } ( \cdot ; s _ { k } , k ) ) ] = \mathbb { E } _ { X ^ { u } \sim \mathbb { P } ^ { u } } [ \log \frac { \mathrm { d } \mathbb { P } ^ { u } } { \mathrm { d } \mathbb { P } ^ { 0 } } ( X ^ { u } ) ] , } \end{array} +$$ + +where $\mathbb { P } ^ { u }$ and $\mathbb { P } ^ { 0 }$ are the measures of the processes $X ^ { u }$ and $X ^ { 0 }$ , respectively. The Girsanov theorem (Theorem 2) implies that $\begin{array} { r l r } { \log \frac { \mathrm { d } \mathbb { P } ^ { u } } { \mathrm { d } \mathbb { P } ^ { 0 } } ( X ^ { u } ) } & { { } = } & { - \int _ { 0 } ^ { T } \langle u ( X _ { t } ^ { u } , t ) , \mathrm { d } B _ { t } \rangle - \frac { 1 } { 2 } \int _ { 0 } ^ { T } \| u ( X _ { t } ^ { u } , t ) \| ^ { 2 } \mathrm { d } t } \end{array}$ , which implies that $\begin{array} { r } { \mathbb { E } _ { X ^ { u } \sim \mathbb { P } ^ { u } } [ \log \frac { \mathrm { d } \mathbb { P } ^ { u } } { \mathrm { d } \mathbb { P } ^ { 0 } } ( X ^ { u } ) ] = - \frac { 1 } { 2 } \mathbb { E } _ { X ^ { u } \sim \mathbb { P } ^ { u } } [ \int _ { 0 } ^ { T ^ { \prime } } \| u ( X _ { t } ^ { u } , t ) \| ^ { 2 } \mathrm { d } t ] } \end{array}$ . Setting the rewards $r _ { k } ( a _ { k } , s _ { k } ) = \eta f ( s _ { k } , k \eta )$ for $k \in$ $\{ 0 , \ldots , K - 1 \}$ and $r _ { K } ( a _ { K } , s _ { K } ) = \eta g ( s _ { k } )$ , where $f$ and $g$ are as in Section 4.1, yields the following limiting object: + +$$ +\begin{array} { r } { \operatorname* { l i m } _ { K \to \infty } \mathbb { E } _ { \tau \sim \tilde { \pi } } [ \sum _ { k = 0 } ^ { K } r _ { k } ( s _ { k } , a _ { k } ) ] = \mathbb { E } _ { X ^ { u } \sim \mathbb { P } ^ { u } } [ \int _ { 0 } ^ { T } f ( X _ { t } ^ { u } , t ) d t + g ( X _ { T } ^ { u } ) ] . } \end{array} +$$ + +Hence, the limit of the MaxEnt RL loss (116) is the SOC loss (12). + +Lemma 3. Let $\tilde { \pi } ( a _ { k } , s _ { k + 1 } ; s _ { k } , k )$ and $\tilde { \pi } _ { \mathrm { b a s e } } ( a _ { k } , s _ { k + 1 } ; s _ { k } , k )$ be as defined in (115). $\operatorname { K L } ( \tilde { \pi } ( \cdot , \cdot ; s _ { k } , k ) | | \tilde { \pi } _ { \mathrm { b a s e } } ( \cdot , \cdot ; s _ { k } , k ) ) ]$ and $\mathrm { K L } ( \pi ( \cdot ; s _ { k } , k ) | | \pi _ { \mathrm { b a s e } } ( \cdot ; s _ { k } , k ) ) ]$ are equal. Moreover, if $q$ , $q ^ { \mathrm { b a s e } }$ denote the distributions over trajectories induced by $\pi$ , $\pi _ { \mathrm { b a s e } }$ , we have that + +$$ +\begin{array} { r } { \mathrm { K L } ( q | | q ^ { \mathrm { b a s e } } ) = \mathbb { E } [ \sum _ { k = 0 } ^ { K - 1 } \mathrm { K L } ( \pi ( \cdot ; s _ { k } , k ) | | \pi _ { \mathrm { b a s e } } ( \cdot ; s _ { k } , k ) ) ] . } \end{array} +$$ + +Proof. We have that + +$$ +\begin{array} { r l } & { \mathrm { K L } ( \tilde { \pi } ( \cdot , \cdot ; s _ { k } , k ) | | \tilde { \pi } _ { \mathrm { b a s e } } ( \cdot , \cdot ; s _ { k } , k ) ) ] = \sum _ { a _ { k } , s _ { k } + 1 } \tilde { \pi } ( a _ { k } , s _ { k + 1 } ; s _ { k } , k ) \log { \frac { \tilde { \pi } ( a _ { k } , s _ { k + 1 } ; s _ { k } , k ) } { \tilde { \pi } _ { \mathrm { b a s e } } ( a _ { k } , s _ { k + 1 } ; s _ { k } , k ) } } } \\ & { = \sum _ { a _ { k } , s _ { k + 1 } } \pi ( a _ { k } ; s _ { k } , k ) p ( s _ { k + 1 } | a _ { k } , s _ { k } ) \log { \frac { \pi ( a _ { k } ; s _ { k } , k ) p ( s _ { k + 1 } | a _ { k } , s _ { k } ) } { \pi _ { \mathrm { b a s e } } ( a _ { k } ; s _ { k } , k ) p ( s _ { k + 1 } | a _ { k } , s _ { k } ) } } } \\ & { = \sum _ { a _ { k } , s _ { k + 1 } } \pi ( a _ { k } ; s _ { k } , k ) p ( s _ { k + 1 } | a _ { k } , s _ { k } ) \log { \frac { \pi ( a _ { k } ; s _ { k } , k ) } { \pi _ { \mathrm { b a s e } } ( a _ { k } ; s _ { k } , k ) } } } \\ & { = \sum _ { a _ { k } } \pi ( a _ { k } ; s _ { k } , k ) \big ( \sum _ { s _ { k + 1 } } p ( s _ { k + 1 } | a _ { k } , s _ { k } ) \big ) \log { \frac { \pi ( a _ { k } ; s _ { k } , k ) } { \pi _ { \mathrm { b a s e } } ( a _ { k } ; s _ { k } , k ) } } } \\ & = \sum _ { a _ { k } } \pi ( a _ { k } ; s _ { k } , k ) \log \frac { \pi ( a _ { k } ; s _ { k } , k ) } \pi _ { \mathrm { b a s e } } ( a _ { k } ; s _ \end{array} +$$ + +To prove (120), by construction we can write + +$$ +\begin{array} { r l r l } & { \boldsymbol { I } ( \tau ) = p _ { 0 } ( s _ { 0 } ) \prod _ { k = 0 } ^ { K - 1 } \tilde { \pi } ( a _ { k } , s _ { k + 1 } ; s _ { k } , k ) , \qquad } & & { \boldsymbol { q } ^ { \mathrm { b a s e } } ( \tau ) = p _ { 0 } ( s _ { 0 } ) \prod _ { k = 0 } ^ { K - 1 } \tilde { \pi } _ { \mathrm { b a s e } } ( a _ { k } , s _ { k + 1 } ; s _ { k } , k ) , } \end{array} +$$ + +which means that + +$$ +\begin{array} { r l } & { \mathrm { K L } ( q | | q ^ { \mathrm { b a s e } } ) = \mathbb { E } _ { \tau \sim q } [ \mathrm { l o g } \frac { q ( \tau ) } { q ^ { \mathrm { b a s e } } ( \tau ) } ] = \mathbb { E } _ { \tau \sim q } [ \sum _ { k = 0 } ^ { K - 1 } \log \frac { \tilde { \pi } ( a _ { k } , s _ { k + 1 } ; s _ { k } , k ) } { \tilde { \pi } _ { \mathrm { b a s e } } ( a _ { k } , s _ { k + 1 } ; s _ { k } , k ) } ] } \\ & { = \sum _ { k = 0 } ^ { K - 1 } \mathbb { E } _ { \tau \sim q ^ { 0 } ; ( k + 1 ) } [ \mathrm { l o g } \frac { \tilde { \pi } ( a _ { k } , s _ { k + 1 } ; s _ { k } , k ) } { \tilde { \pi } _ { \mathrm { b a s e } } ( a _ { k } , s _ { k + 1 } ; s _ { k } , k ) } ] } \\ & { = \sum _ { k = 0 } ^ { K - 1 } \mathbb { E } _ { \tau \sim q ^ { 0 ; k } } [ \sum _ { a _ { k } , s _ { k + 1 } } \tilde { \pi } ( a _ { k } , s _ { k + 1 } ; s _ { k } , k ) \log \frac { \tilde { \pi } ( a _ { k } , s _ { k + 1 } ; s _ { k } , k ) } { \tilde { \pi } _ { \mathrm { b a s e } } ( a _ { k } , s _ { k + 1 } ; s _ { k } , k ) } ] } \\ & { = \sum _ { k = 0 } ^ { K - 1 } \mathbb { E } _ { \tau \sim q ^ { 0 ; k } } [ \mathrm { K L } ( \tilde { \pi } ( \cdot ; s _ { k } , k ) | | \tilde { \pi } _ { \mathrm { b a s e } } ( \cdot , \cdot ; s _ { k } , k ) ) ] } \\ & { = \sum _ { k = 0 } ^ { K - 1 } \mathbb { E } _ { \tau \sim q ^ { 0 ; k } } [ \mathrm { K L } ( \pi ( \cdot ; s _ { k } , k ) | | \pi _ { \mathrm { b a s e } } ( \cdot ; s _ { k } , k ) ) ] } \\ & = \mathbb { E } _ \end{array} +$$ + +Here, the notation $q ^ { 0 : k }$ denotes the trajectory $q$ up to the state $s _ { k }$ + +Lemma 4. The distribution-based MaxEnt RL formulation in (111) is equivalent to the the following problem: + +$$ +\begin{array} { r } { \operatorname* { m i n } _ { q } \mathrm { K L } ( q | | q ^ { * } ) , \qquad w h e r e \ q ^ { * } ( \tau ) : = \frac { q ^ { \mathrm { b a s e } } ( \tau ) \exp \big ( \sum _ { k = 0 } ^ { K } r _ { k } ( s _ { k } , a _ { k } ) \big ) } { \frac { 1 } { p _ { 0 } ( s _ { 0 } ) } \sum _ { \{ \tau ^ { \prime } | s _ { 0 } ^ { \prime } = s _ { 0 } \} } q ^ { \mathrm { b a s e } } ( \tau ^ { \prime } ) \exp \big ( \sum _ { k = 0 } ^ { K } r _ { k } ( s _ { k } ^ { \prime } , a _ { k } ^ { \prime } ) \big ) } , } \end{array} +$$ + +where the minimization is over $q$ with marginal $p _ { 0 }$ at step zero. The optimum of the problem is $q ^ { * }$ , which satisfies the marginal constraint. The following alternative characterization of $q ^ { * }$ holds: + +$$ +\begin{array} { r l } & { q ^ { * } ( \tau ) = q ^ { \mathrm { b a s e } } ( \tau ) \exp \big ( \sum _ { k = 0 } ^ { K } r _ { k } ( s _ { k } , a _ { k } ) - \mathcal { V } ( s _ { 0 } , 0 ) \big ) , } \\ & { \mathcal { V } ( x , k ) = \operatorname* { m a x } _ { \pi } \mathbb { E } _ { \tau \sim \pi , p } \big [ \sum _ { k ^ { \prime } = k } ^ { K } r _ { k ^ { \prime } } ( s _ { k ^ { \prime } } , a _ { k ^ { \prime } } ) - \sum _ { k ^ { \prime } = k } ^ { K - 1 } \mathrm { K L } ( \pi ( \cdot ; s _ { k ^ { \prime } } , k ^ { \prime } ) | | \pi _ { \mathrm { b a s e } } ( \cdot ; s _ { k ^ { \prime } } , k ^ { \prime } ) ) | s _ { k } = x \big ] . } \end{array} +$$ + +Proof. Let us expand $\mathrm { K L } ( q | | q ^ { * } )$ : + +$$ +\begin{array} { r l } & { \mathrm { K L } ( q | | q ^ { * } ) = \mathbb { E } _ { \tau \sim q } \big [ \log \frac { q ( \tau ) } { q ^ { * } ( \tau ) } \big ] } \\ & { \qquad = \mathbb { E } _ { \tau \sim q } \big [ \log q ( \tau ) - \log q ^ { \mathrm { b a s e } } ( \tau ) - \sum _ { k = 0 } ^ { K } r _ { k } ( s _ { k } , a _ { k } ) } \\ & { \qquad + \log \big ( \frac { 1 } { p _ { 0 } ( s _ { 0 } ) } \sum _ { \{ \tau ^ { \prime } | s _ { 0 } ^ { \prime } = s _ { 0 } \} } q ^ { \mathrm { b a s e } } ( \tau ^ { \prime } ) \exp \big ( \sum _ { k = 0 } ^ { K } r _ { k } ( s _ { k } ^ { \prime } , a _ { k } ^ { \prime } ) \big ) \big ) \big ] } \\ & { \qquad = \mathrm { K L } ( q | | q ^ { \mathrm { b a s e } } ) - \mathbb { E } _ { \tau \sim q } \big [ \sum _ { k = 0 } ^ { K } r _ { k } ( s _ { k } , a _ { k } ) \big ] } \\ & { \qquad + \mathbb { E } _ { s _ { 0 } \sim p _ { 0 } } \big [ \log \big ( \frac { 1 } { p _ { 0 } ( s _ { 0 } ) } \sum _ { \{ \tau ^ { \prime } | s _ { 0 } ^ { \prime } = s _ { 0 } \} } q ^ { \mathrm { b a s e } } ( \tau ^ { \prime } ) \exp \big ( \sum _ { k = 0 } ^ { K } r _ { k } ( s _ { k } ^ { \prime } , a _ { k } ^ { \prime } ) \big ) \big ) \big ] , } \end{array} +$$ + +where the third equality holds because the marginal of $q$ at step zero is $p _ { 0 }$ by hypothesis. Since the third term in the right-hand side is independent of $q$ , this proves the equivalence between (111) and (124). + +Next, we prove that the marginal of $q ^ { * }$ at step zero is $p _ { 0 }$ : + +$$ +\begin{array} { r } \sum _ { \{ \tau \} s _ { 0 } = x \} q ^ { * } ( \tau ) : = \sum _ { \{ \tau | s _ { 0 } = x \} } \frac { q ^ { \mathrm { b a s e } } ( \tau ) \exp \big ( \sum _ { k = 0 } ^ { K } r _ { k } ( s _ { k } , a _ { k } ) \big ) } { \frac { 1 } { p _ { 0 } ( x ) } \sum _ { \{ \tau ^ { \prime } | s _ { 0 } ^ { \prime } = x \} } q ^ { \mathrm { b a s e } } ( \tau ^ { \prime } ) \exp \big ( \sum _ { k = 0 } ^ { K } r _ { k } ( s _ { k } ^ { \prime } , a _ { k } ^ { \prime } ) \big ) } = p _ { 0 } ( x ) . } \end{array} +$$ + +Now, for an arbitrary $s _ { 0 }$ , let $q _ { s _ { 0 } }$ , $q _ { s _ { 0 } } ^ { * }$ be the distributions $q$ , $q ^ { * }$ conditioned on the initial state being $s _ { 0 }$ . We can write an analog to equation (127) for $q _ { s _ { 0 } }$ , $q _ { s _ { 0 } } ^ { * }$ : + +$$ +\begin{array} { r l } & { \mathrm { K L } ( q _ { s _ { 0 } } | | q _ { s _ { 0 } } ^ { * } ) = \mathbb { E } _ { \tau \sim q _ { s _ { 0 } } } \left[ \log \frac { q _ { s _ { 0 } } ( \tau ) } { q _ { s _ { 0 } } ^ { * } ( \tau ) } \right] } \\ & { \phantom { = } = \mathbb { E } _ { \tau \sim q _ { s _ { 0 } } } \left[ \log q _ { s _ { 0 } } ( \tau ) - \log q _ { s _ { 0 } } ^ { \mathrm { b a s e } } ( \tau ) - \sum _ { k = 0 } ^ { K } r _ { k } ( s _ { k } , a _ { k } ) \right. } \\ & { \phantom { = } \left. \qquad + \log \left( \frac { 1 } { p _ { 0 } ( s _ { 0 } ) } \sum _ { \{ \tau ^ { \prime } | s _ { 0 } ^ { \prime } = s _ { 0 } \} } q _ { s _ { 0 } } ^ { \mathrm { b a s e } } ( \tau ^ { \prime } ) \exp \left( \sum _ { k = 0 } ^ { K } r _ { k } ( s _ { k } ^ { \prime } , a _ { k } ^ { \prime } ) \right) \right) \right] } \\ & { \phantom { = } = \mathrm { K L } ( q _ { s _ { 0 } } | | q _ { s _ { 0 } } ^ { \mathrm { b a s e } } ) - \mathbb { E } _ { \tau \sim q _ { s _ { 0 } } } \left[ \sum _ { k = 0 } ^ { K } r _ { k } ( s _ { k } , a _ { k } ) \right] } \\ & { \phantom { = } \quad + \log \left( \frac { 1 } { p _ { 0 } ( s _ { 0 } ) } \sum _ { \{ \tau ^ { \prime } | s _ { 0 } ^ { \prime } = s _ { 0 } \} } q ^ { \mathrm { b a s e } } ( \tau ^ { \prime } ) \exp \left( \sum _ { k = 0 } ^ { K } r _ { k } ( s _ { k } ^ { \prime } , a _ { k } ^ { \prime } ) \right) \right) , } \end{array} +$$ + +Hence, + +$$ +\begin{array} { r } { 0 = \operatorname* { m i n } _ { q _ { s _ { 0 } } } \mathrm { K L } ( q _ { s _ { 0 } } | | q _ { s _ { 0 } } ^ { * } ) = - \operatorname* { m a x } _ { q _ { s _ { 0 } } } \{ \mathbb { E } _ { \tau \sim q _ { s _ { 0 } } } \big [ \sum _ { k = 0 } ^ { K } r _ { k } ( s _ { k } , a _ { k } ) \big ] - \mathrm { K L } ( q _ { s _ { 0 } } | | q _ { s _ { 0 } } ^ { \mathrm { b a s e } } ) \} } \\ { + \log \big ( \frac { 1 } { p _ { 0 } ( s _ { 0 } ) } \sum _ { \{ \tau ^ { \prime } | s _ { 0 } ^ { \prime } = s _ { 0 } \} } q ^ { \mathrm { b a s e } } ( \tau ^ { \prime } ) \exp \big ( \sum _ { k = 0 } ^ { K } r _ { k } ( s _ { k } ^ { \prime } , a _ { k } ^ { \prime } ) \big ) \big ) . } \end{array} +$$ + +And applying (120) from (120), we obtain that + +$$ +\begin{array} { r l } & { \log \left( \frac { 1 } { p _ { 0 } ( s _ { 0 } ) } \sum _ { \{ \tau ^ { \prime } | s _ { 0 } ^ { \prime } = s _ { 0 } \} } q ^ { \mathrm { b a s e } } ( \tau ^ { \prime } ) \exp \left( \sum _ { k = 0 } ^ { K } r _ { k } ( s _ { k } ^ { \prime } , a _ { k } ^ { \prime } ) \right) \right) } \\ & { = \operatorname* { m a x } _ { \pi } \mathbb { E } _ { \tau \sim \pi , p } \left[ \sum _ { k = 0 } ^ { K } r _ { k } ( s _ { k } , a _ { k } ) - \sum _ { k = 0 } ^ { K - 1 } \mathrm { K L } ( \pi ( \cdot ; s _ { k } , k ) | | \pi _ { \mathrm { b a s e } } ( \cdot ; s _ { k } , k ) ) | s _ { 0 } \right] = \mathcal { V } ( s _ { 0 } , 0 ) , } \end{array} +$$ + +which concludes the proof. + +# C.4 Proof of equation (18): the control cost is a KL regularizer + +Theorem 2 (Girsanov theorem for SDEs). If the two SDEs + +$$ +\begin{array} { r l } & { \mathrm { d } X _ { t } = b _ { 1 } ( X _ { t } , t ) \mathrm { d } t + \sigma ( X _ { t } , t ) \mathrm { d } B _ { t } , \qquad X _ { 0 } = x _ { \mathrm { i n i t } } } \\ & { d Y _ { t } = ( b _ { 1 } ( Y _ { t } , t ) + b _ { 2 } ( Y _ { t } , t ) ) \mathrm { d } t + \sigma ( Y _ { t } , t ) \mathrm { d } B _ { t } , \qquad Y _ { 0 } = x _ { \mathrm { i n i t } } } \end{array} +$$ + +admit unique strong solutions on $[ 0 , T ]$ , then for any bounded continuous functional $\Phi$ on $C ( [ 0 , T ] )$ , we have that + +$$ +\begin{array} { r l } & { \mathbb { E } [ \Phi ( { \pmb X } ) ] = \mathbb { E } \big [ \Phi ( { \pmb Y } ) \exp \big ( - \int _ { 0 } ^ { T } \sigma ( Y _ { t } , t ) ^ { - 1 } b _ { 2 } ( Y _ { t } , t ) \mathrm { d } B _ { t } - \frac { 1 } { 2 } \int _ { 0 } ^ { T } \| \sigma ( Y _ { t } , t ) ^ { - 1 } b _ { 2 } ( Y _ { t } , t ) \| ^ { 2 } \mathrm { d } t \big ) \big ] } \\ & { \qquad = \mathbb { E } \big [ \Phi ( { \pmb Y } ) \exp \big ( - \int _ { 0 } ^ { T } \sigma ( Y _ { t } , t ) ^ { - 1 } b _ { 2 } ( Y _ { t } , t ) d \tilde { B } _ { t } + \frac { 1 } { 2 } \int _ { 0 } ^ { T } \| \sigma ( Y _ { t } , t ) ^ { - 1 } b _ { 2 } ( Y _ { t } , t ) \| ^ { 2 } \mathrm { d } t \big ) \big ] , } \end{array} +$$ + +where $\begin{array} { r } { \tilde { B } _ { t } = B _ { t } + \int _ { 0 } ^ { t } \sigma ( Y _ { s } , s ) ^ { - 1 } b _ { 2 } ( Y _ { s } , s ) \mathrm { d } s } \end{array}$ . More generally, $b _ { 1 }$ and $b _ { 2 }$ can be random processes that are adapted to filtration of $\mathbfcal { B }$ . + +Consider the SDEs + +$$ +\begin{array} { r l r l } & { \mathrm { d } X _ { t } = b ( X _ { t } , t ) \mathrm { d } t + \sigma ( t ) \mathrm { d } B _ { t } , } & & { \quad \quad X _ { 0 } = x _ { 0 } , } \\ & { \mathrm { d } X _ { t } ^ { u } = \left( b ( X _ { t } ^ { u } , t ) + \sigma ( t ) u ( X _ { t } ^ { u } , t ) \right) \mathrm { d } t + \sigma ( t ) \mathrm { d } B _ { t } , } & & { \quad \quad X _ { 0 } ^ { u } = x _ { 0 } . } \end{array} +$$ + +If we let $\mathbb { P } | _ { x _ { 0 } }$ , $\mathbb { P } ^ { u } | _ { x _ { 0 } }$ be the probability measures of the solutions of (135) and (136), Theorem 2 implies that + +$$ +\begin{array} { r } { \log \frac { \mathrm { d } \mathbb { P } | _ { x _ { 0 } } } { \mathrm { d } \mathbb { P } ^ { u } | _ { x _ { 0 } } } ( \pmb { X } ^ { u } ) = - \int _ { 0 } ^ { 1 } u ( X _ { t } ^ { u } , t ) \mathrm { d } B _ { t } - \frac { 1 } { 2 } \int _ { 0 } ^ { 1 } \| u ( X _ { t } ^ { u } , t ) \| ^ { 2 } \mathrm { d } t . } \end{array} +$$ + +Hence, + +$$ +\begin{array} { r l } & { \mathbb { P } ^ { u } | _ { x _ { 0 } } \left\| \mathbb { P } | _ { x _ { 0 } } \right) = \mathbb { E } \big [ \log \frac { \mathrm { d } \mathbb { P } ^ { u } | _ { x _ { 0 } } } { \mathrm { d } \mathbb { P } | _ { x _ { 0 } } } ( \pmb { X } ^ { u } ) | \pmb { X } _ { 0 } ^ { u } = x _ { 0 } \big ] = - \mathbb { E } \big [ \log \frac { \mathrm { d } \mathbb { P } | _ { x _ { 0 } } } { \mathrm { d } \mathbb { P } ^ { u } | _ { x _ { 0 } } } ( \pmb { X } ^ { u } ) | \pmb { X } _ { 0 } ^ { u } = x _ { 0 } \big ] } \\ & { \qquad = \mathbb { E } \big [ \int _ { 0 } ^ { 1 } u ( X _ { t } ^ { u } , t ) \mathrm { d } B _ { t } + \frac 1 2 \int _ { 0 } ^ { 1 } \| u ( X _ { t } ^ { u } , t ) \| ^ { 2 } \mathrm { d } t | X _ { 0 } ^ { u } = x _ { 0 } \big ] = \mathbb { E } \big [ \frac 1 2 \int _ { 0 } ^ { 1 } \| u ( X _ { t } ^ { u } , t ) \| ^ { 2 } \mathrm { d } t | X _ { 0 } ^ { u } = x _ { 0 } \big ] . } \end{array} +$$ + +where we used that stochastic integrals are martingales. + +# D Proofs of Section 4.3: memoryless noise schedule and fine-tuning recipe + +# D.1 Proof of Proposition 1: the memoryless noise schedule + +We consider the forward-backward SDEs (63)-(64) with arbitrary noise schedule. By Proposition 4, the trajectories $\vec { X }$ , $\pmb { X }$ of these two processes are equally distributed up to a time flip, which also means that their marginals satisfy $\vec { p _ { t } } = p _ { 1 - t }$ , for all $t \in [ 0 , 1 ]$ . First, we develop an explicit expression for the score function $s ( x , t ) = \nabla \log p _ { t } ( x )$ . By the properties of flow matching, we know that $p _ { t }$ is the distribution of the interpolation variable $\bar { X } _ { t } = \beta _ { t } \bar { X } _ { 0 } + \alpha _ { t } \bar { X } _ { 1 }$ , where $\bar { X } _ { 0 } \sim N ( 0 , I ) , \bar { X } _ { 1 } \sim p ^ { \mathrm { d a t a } }$ are independent. Thus, X¯t−αtX¯1β ∼ N (0, I), which means that we can express the density pt as + +$$ +\begin{array} { r } { p _ { t } ( x ) = \int _ { \mathbb { R } ^ { d } } \frac { \exp \big ( - \frac { \| x - \alpha _ { t } y \| ^ { 2 } } { 2 \beta _ { t } ^ { 2 } } \big ) } { ( 2 \pi \beta _ { t } ^ { 2 } ) ^ { d / 2 } } p ^ { \mathrm { d a t a } } ( y ) \mathrm { d } y . } \end{array} +$$ + +Thus, + +$$ +\begin{array} { r } { s ( x , t ) = \nabla \log p _ { t } ( x ) = - \frac { x } { \beta _ { t } ^ { 2 } } + \frac { \alpha _ { t } } { \beta _ { t } ^ { 2 } } \frac { \int _ { \mathbb { R } ^ { d } } y \exp \big ( - \frac { \| x - \alpha _ { t } y \| ^ { 2 } } { 2 \beta _ { t } ^ { 2 } } \big ) p ^ { \mathrm { d a t a } } ( y ) \mathrm { d } y } { \int _ { \mathbb { R } ^ { d } } \exp \big ( - \frac { \| x - \alpha _ { t } y \| ^ { 2 } } { 2 \beta _ { t } ^ { 2 } } \big ) p ^ { \mathrm { d a t a } } ( y ) \mathrm { d } y } : = - \frac { x - \alpha _ { t } \xi _ { t } ( x ) } { \beta _ { t } ^ { 2 } } , } \end{array} +$$ + +where we defined + +$$ +\begin{array} { r } { \xi _ { t } ( x ) = \frac { \int _ { \mathbb { R } ^ { d } } y \exp \big ( - \frac { \| x - \alpha _ { t } y \| ^ { 2 } } { 2 \beta _ { t } ^ { 2 } } \big ) p ^ { \mathrm { d a t a } } ( y ) \mathrm { d } y } { \int _ { \mathbb { R } ^ { d } } \exp \big ( - \frac { \| x - \alpha _ { t } y \| ^ { 2 } } { 2 \beta _ { t } ^ { 2 } } \big ) p ^ { \mathrm { d a t a } } ( y ) \mathrm { d } y } . } \end{array} +$$ + +Hence, we can rewrite the forward SDE (63) as + +$$ +\begin{array} { r } { \mathrm { d } \vec { X } _ { t } = \left( - \kappa _ { 1 - t } \vec { X } _ { t } - \left( \frac { \sigma ( 1 - t ) ^ { 2 } } { 2 } - \eta _ { 1 - t } \right) \frac { \vec { X } _ { t } - \alpha _ { 1 - t } \xi _ { 1 - t } ( \vec { X } _ { t } ) } { \beta _ { 1 - t } ^ { 2 } } \right) \mathrm { d } t + \sigma ( 1 - t ) \mathrm { d } B _ { t } , \qquad \vec { X } _ { 0 } \sim p _ { \mathrm { d a t a } } , } \end{array} +$$ + +Hence, if we substitute $\begin{array} { r } { \kappa _ { 1 - t } \kappa _ { 1 - t } + \frac { \sigma ( 1 - t ) ^ { 2 } - 2 \eta _ { 1 - t } } { 2 \beta _ { 1 - t } ^ { 2 } } } \end{array}$ , $\begin{array} { r } { \xi _ { 1 - t } \frac { \alpha _ { 1 - t } ( \sigma ( 1 - t ) ^ { 2 } - 2 \eta _ { 1 - t } ) } { 2 \beta _ { 1 - t } ^ { 2 } } \xi _ { 1 - t } ( \vec { X } _ { t } ) } \end{array}$ (where we ignore the dependency on $\vec { X _ { t } }$ ), $\sqrt { 2 \eta _ { 1 - t } } \sigma ( 1 - t )$ , we can apply Lemma 2, which yields + +$$ +\begin{array} { r l } & { \vec { X } _ { t } = \vec { X } _ { 0 } \exp \big ( - \int _ { 0 } ^ { t } \big ( \kappa _ { 1 - s } + \frac { \sigma ( 1 - s ) ^ { 2 } - 2 \eta _ { 1 - s } } { 2 \beta _ { 1 - s } ^ { 2 } } \big ) \mathrm { d } s \big ) } \\ & { \qquad + \int _ { 0 } ^ { t } \exp \big ( - \int _ { t ^ { \prime } } ^ { t } \big ( \kappa _ { 1 - s } + \frac { \sigma ( 1 - s ) ^ { 2 } - 2 \eta _ { 1 - s } } { 2 \beta _ { 1 - s } ^ { 2 } } \big ) \mathrm { d } s \big ) \frac { \alpha _ { 1 - t ^ { \prime } } ( \sigma ( 1 - t ^ { \prime } ) ^ { 2 } - 2 \eta _ { 1 - t ^ { \prime } } ) } { 2 \beta _ { 1 - t ^ { \prime } } ^ { 2 } } \xi _ { 1 - t ^ { \prime } } ( \vec { X } _ { t ^ { \prime } } ) \mathrm { d } t ^ { \prime } } \\ & { \qquad + \int _ { 0 } ^ { t } \sigma ( 1 - t ^ { \prime } ) \exp \big ( - \int _ { t ^ { \prime } } ^ { t } \big ( \kappa _ { 1 - s } + \frac { \sigma ( 1 - s ) ^ { 2 } - 2 \eta _ { 1 - s } } { 2 \beta _ { 1 - s } ^ { 2 } } \big ) \mathrm { d } s \big ) \mathrm { d } B _ { t ^ { \prime } } . } \end{array} +$$ + +We simplify the recurring expression: + +$$ +\begin{array} { r } { \kappa _ { 1 - s } + \frac { \sigma ( 1 - s ) ^ { 2 } - 2 \eta _ { 1 - s } } { 2 \beta _ { 1 - s } ^ { 2 } } = \frac { \dot { \alpha } _ { 1 - s } } { \alpha _ { 1 - s } } + \frac { \sigma ( 1 - s ) ^ { 2 } - 2 \beta _ { 1 - s } \left( \frac { \dot { \alpha } _ { 1 - s } } { \alpha _ { 1 - s } } \beta _ { 1 - s } - \dot { \beta } _ { 1 - s } \right) } { 2 \beta _ { 1 - s } ^ { 2 } } = \frac { \sigma ( 1 - s ) ^ { 2 } } { 2 \beta _ { 1 - s } ^ { 2 } } + \frac { \dot { \beta } _ { 1 - s } } { \beta _ { 1 - s } } } \end{array} +$$ + +Thus, + +$$ +\begin{array} { r } { s + \frac { \sigma ( 1 - s ) ^ { 2 } - 2 \eta _ { 1 - s } } { 2 \beta _ { 1 - s } ^ { 2 } } \big ) \mathrm { d } s = \int _ { t ^ { \prime } } ^ { t } \big ( \frac { \sigma ( 1 - s ) ^ { 2 } } { 2 \beta _ { 1 - s } ^ { 2 } } - \partial _ { s } \log \beta _ { 1 - s } \big ) \mathrm { d } s = \int _ { t ^ { \prime } } ^ { t } \frac { \sigma ( 1 - s ) ^ { 2 } } { 2 \beta _ { 1 - s } ^ { 2 } } \mathrm { d } s - \big ( \log \beta _ { 1 - t } - \log \beta _ { 1 - t ^ { \prime } } \big ) , } \end{array} +$$ + +which means that + +$$ +\begin{array} { r l } & { \exp \big ( - \int _ { t ^ { \prime } } ^ { t } \big ( \kappa _ { 1 - s } + \frac { \sigma ( 1 - s ) ^ { 2 } - 2 \eta _ { 1 - s } } { 2 \beta _ { 1 - s } ^ { 2 } } \big ) \mathrm { d } s \big ) = \exp \big ( - \int _ { t ^ { \prime } } ^ { t } \frac { \sigma ( 1 - s ) ^ { 2 } } { 2 \beta _ { 1 - s } ^ { 2 } } \mathrm { d } s \big ) \frac { \beta _ { 1 - t } } { \beta _ { 1 - t ^ { \prime } } } , } \\ & { \qquad \frac { \alpha _ { 1 - t ^ { \prime } } ( \sigma ( 1 - t ^ { \prime } ) ^ { 2 } - 2 \eta _ { 1 - t ^ { \prime } } ) } { 2 \beta _ { 1 - t ^ { \prime } } ^ { 2 } } \xi _ { 1 - t ^ { \prime } } ( \vec { X } _ { t ^ { \prime } } ) = \alpha _ { 1 - t ^ { \prime } } \Big ( \frac { \sigma ( 1 - t ^ { \prime } ) ^ { 2 } } { 2 \beta _ { 1 - t ^ { \prime } } ^ { 2 } } + \frac { \bar { \beta } _ { 1 - t ^ { \prime } } } { \beta _ { 1 - t ^ { \prime } } } - \frac { \hat { \alpha } _ { 1 - t ^ { \prime } } } { \alpha _ { 1 - t ^ { \prime } } } \Big ) \xi _ { 1 - t ^ { \prime } } \big ( \vec { X } _ { t ^ { \prime } } \big ) . } \end{array} +$$ + +If we define $\chi ( 1 - s )$ such that $\begin{array} { r } { \sigma ^ { 2 } ( 1 - s ) = 2 \beta _ { 1 - s } \mathopen { } \mathclose \bgroup \left( \frac { \dot { \alpha } _ { 1 - s } } { \alpha _ { 1 - s } } \beta _ { 1 - s } - \dot { \beta } _ { 1 - s } \aftergroup \egroup \right) + \chi ( 1 - s ) } \end{array}$ , we obtain that + +$$ +\begin{array} { r l } & { \exp \big ( - \int _ { t ^ { \prime } } ^ { t } \frac { \sigma ( 1 - s ) ^ { 2 } } { 2 \beta _ { 1 - s } ^ { 2 } } \mathrm { d } s \big ) \frac { \beta _ { 1 - t } } { \beta _ { 1 - t ^ { \prime } } } = \exp \big ( - \int _ { t ^ { \prime } } ^ { t } \big ( \frac { \dot { \alpha } _ { 1 - s } } { \alpha _ { 1 - s } } - \frac { \dot { \beta } _ { 1 - s } } { \beta _ { 1 - s } } + \frac { \chi ( 1 - s ) } { 2 \beta _ { 1 - s } ^ { 2 } } \big ) \mathrm { d } s \big ) \frac { \beta _ { 1 - t } } { \beta _ { 1 - t ^ { \prime } } } } \\ & { = \exp \big ( \int _ { t ^ { \prime } } ^ { t } \big ( \partial _ { s } \log \alpha _ { 1 - s } - \partial _ { s } \log \beta _ { 1 - s } - \frac { \chi ( 1 - s ) } { 2 \beta _ { 1 - s } ^ { 2 } } \big ) \mathrm { d } s \big ) \frac { \beta _ { 1 - t } } { \beta _ { 1 - t ^ { \prime } } } = \exp \big ( - \int _ { t ^ { \prime } } ^ { t } \frac { \chi ( 1 - s ) } { 2 \beta _ { 1 - s } ^ { 2 } } \mathrm { d } s \big ) \frac { \alpha _ { 1 - t } } { \alpha _ { 1 - t ^ { \prime } } } , } \\ & \alpha _ { 1 - t ^ { \prime } } \big ( \frac { \sigma ( 1 - t ^ { \prime } ) ^ { 2 } } { 2 \beta _ { 1 - t ^ { \prime } } ^ { 2 } } + \frac { \dot { \beta } _ { 1 - t ^ { \prime } } } { \beta _ { 1 - t ^ { \prime } } } - \frac { \dot { \alpha } _ { 1 - t ^ { \prime } } } { \alpha _ { 1 - t ^ { \prime } } } \big ) \xi _ { 1 - t ^ { \prime } } \big ( \vec { X } _ { t ^ { \prime } } \big ) = \frac { \alpha _ { 1 - t ^ { \prime } } \chi ( 1 - t ^ { \prime } ) } 2 \end{array} +$$ + +If we plug equations (148)-(149) into (146)-(147), and then those into (143), we obtain that + +$$ +\begin{array} { r } { \vec { X } _ { t } = \vec { X } _ { 0 } \exp \big ( - \int _ { 0 } ^ { t } \frac { \chi ( 1 - s ) } { 2 \beta _ { 1 - s } ^ { 2 } } \mathrm { d } s \big ) \frac { \alpha _ { 1 - t } } { \alpha _ { 1 } } + \alpha _ { 1 - t } \int _ { 0 } ^ { t } \exp \big ( - \int _ { t ^ { \prime } } ^ { t } \frac { \chi ( 1 - s ) } { 2 \beta _ { 1 - s } ^ { 2 } } \mathrm { d } s \big ) \frac { \chi ( 1 - t ^ { \prime } ) } { 2 \beta _ { 1 - t ^ { \prime } } ^ { 2 } } \xi _ { 1 - t ^ { \prime } } \big ( \vec { X } _ { t ^ { \prime } } \big ) \mathrm { d } t ^ { \prime } } \\ { + \int _ { 0 } ^ { t } \big ( 2 \beta _ { 1 - t ^ { \prime } } \big ( \frac { \dot { \alpha } _ { 1 - t ^ { \prime } } } { \alpha _ { 1 - t ^ { \prime } } } \beta _ { 1 - t ^ { \prime } } - \dot { \beta } _ { 1 - t ^ { \prime } } \big ) + \chi ( 1 - t ^ { \prime } ) \big ) \exp \big ( - \int _ { t ^ { \prime } } ^ { t } \frac { \chi ( 1 - s ) } { 2 \beta _ { 1 - s } ^ { 2 } } \mathrm { d } s \big ) \frac { \alpha _ { 1 - t } } { \alpha _ { 1 - t ^ { \prime } } } \mathrm { d } B _ { t ^ { \prime } } . } \end{array} +$$ + +and if we take the limit $t 1 ^ { - }$ and use that $\alpha _ { 1 } = 1$ , + +$$ +\begin{array} { r l } & { = \vec { X } _ { 0 } \big ( \operatorname* { l i m } _ { t \to 1 ^ { - } } \exp \big ( - \int _ { 0 } ^ { t } \frac { \chi ( 1 - s ) } { 2 \beta _ { 1 - s } ^ { 2 } } \operatorname { d } s \big ) \alpha _ { 1 - t } \big ) + \operatorname* { l i m } _ { t \to 1 ^ { - } } \alpha _ { 1 - t } \int _ { 0 } ^ { t } \exp \big ( - \int _ { t ^ { \prime } } ^ { t } \frac { \chi ( 1 - s ) } { 2 \beta _ { 1 - s } ^ { 2 } } \operatorname { d } s \big ) \frac { \chi ( 1 - t ^ { \prime } ) } { 2 \beta _ { 1 - t ^ { \prime } } ^ { 2 } } \xi _ { 1 - t ^ { \prime } } \big ( } \\ & { \qquad + \operatorname* { l i m } _ { t \to 1 ^ { - } } \int _ { 0 } ^ { t } \big ( 2 \beta _ { 1 - t ^ { \prime } } \big ( \frac { \dot { \alpha } _ { 1 - t ^ { \prime } } } { \alpha _ { 1 - t ^ { \prime } } } \beta _ { 1 - t ^ { \prime } } - \dot { \beta } _ { 1 - t ^ { \prime } } \big ) + \chi ( 1 - t ^ { \prime } ) \big ) \exp \big ( - \int _ { t ^ { \prime } } ^ { t } \frac { \chi ( 1 - s ) } { 2 \beta _ { 1 - s } ^ { 2 } } \operatorname { d } s \big ) \frac { \alpha _ { 1 - t } } { \alpha _ { 1 - t ^ { \prime } } } \operatorname { d } B _ { t ^ { \prime } } . } \end{array} +$$ + +The assumption on $\chi$ in (25) is equivalent, up to a rearrangement of the notation and a flip in the time variable, to the statement that for all $t ^ { \prime } \in [ 0 , 1 )$ , + +$$ +\begin{array} { r } { \operatorname* { l i m } _ { t \to 1 ^ { - } } \exp \big ( - \int _ { t ^ { \prime } } ^ { t } \frac { \chi ( 1 - s ) } { 2 \beta _ { 1 - s } ^ { 2 } } \mathrm { d } s \big ) \alpha _ { 1 - t } = 0 . } \end{array} +$$ + +Hence, under assumption (25), the factor accompanying $\vec { X _ { 0 } }$ in equation (151) is zero. Moreover, this assumption also implies that + +$$ +\begin{array} { r l } & { \operatorname* { l i m } _ { t \to 1 ^ { - } } \alpha _ { 1 - t } \int _ { 0 } ^ { t } \exp \big ( - \int _ { t ^ { \prime } } ^ { t } \frac { \chi ( 1 - s ) } { 2 \beta _ { 1 - s } ^ { 2 } } { \mathrm { d } } s \big ) \frac { \chi ( 1 - t ^ { \prime } ) } { 2 \beta _ { 1 - t ^ { \prime } } ^ { 2 } } \xi _ { 1 - t ^ { \prime } } ( \vec { X } _ { t ^ { \prime } } ) { \mathrm { d } } t ^ { \prime } } \\ & { = \int _ { 0 } ^ { 1 } \big ( \operatorname* { l i m } _ { t \to 1 ^ { - } } \exp \big ( - \int _ { t ^ { \prime } } ^ { t } \frac { \chi ( 1 - s ) } { 2 \beta _ { 1 - s } ^ { 2 } } { \mathrm { d } } s \big ) \alpha _ { 1 - t } \big ) \frac { \chi ( 1 - t ^ { \prime } ) } { 2 \beta _ { 1 - t ^ { \prime } } ^ { 2 } } \xi _ { 1 - t ^ { \prime } } ( \vec { X } _ { t ^ { \prime } } ) { \mathrm { d } } t ^ { \prime } = 0 . } \end{array} +$$ + +If we plug (152) and (153) into (151), we obtain that + +$$ +\begin{array} { r } { \vec { X } _ { 1 } = \operatorname* { l i m } _ { t 1 ^ { - } } \int _ { 0 } ^ { t } ( 2 \beta _ { 1 - t ^ { \prime } } ( \frac { \dot { \alpha } _ { 1 - t ^ { \prime } } } { \alpha _ { 1 - t ^ { \prime } } } \beta _ { 1 - t ^ { \prime } } - \dot { \beta } _ { 1 - t ^ { \prime } } ) + \chi ( 1 - t ^ { \prime } ) ) \exp \big ( - \int _ { t ^ { \prime } } ^ { t } \frac { \chi ( 1 - s ) } { 2 \beta _ { 1 - s } ^ { 2 } } \mathrm { d } s \big ) \frac { \alpha _ { 1 - t } } { \alpha _ { 1 - t ^ { \prime } } } \mathrm { d } B _ { t ^ { \prime } } , } \end{array} +$$ + +which shows that $\vec { X _ { 1 } }$ is independent of $\vec { X _ { 0 } }$ . Next, we leverage that $\vec { X }$ and $\pmb { X }$ have equal distributions over trajectories (Proposition 4). In particular, the joint distribution of $( \vec { X _ { 0 } } , \vec { X _ { 1 } } )$ is equal to the joint distribution of $( X _ { 1 } , X _ { 0 } )$ . We conclude that $X _ { 1 }$ and $X _ { 0 }$ are independent, which is the definition of the memorylessness property. Hence, the assumption (25) is sufficient for memorylessness to hold. + +It remains to prove that the assumption (25) is necessary. Looking at equation (150) we deduce that generally, for any $t \in [ 0 , 1 )$ , $\vec { X _ { 0 } }$ and $\vec { X _ { t } }$ are not independent, because the first two terms in (150) are different from zero. Thus, if there existed a $t ^ { \prime } \in [ 0 , 1 )$ such that the limit (152) is different from zero, then $\vec { X _ { 1 } }$ would not be independent from $\vec { X } _ { t ^ { \prime } }$ , which means that in general it would not be independent of $\vec { X _ { 0 } }$ either. + +# D.2 Proof of Theorem 1: fine-tuning recipe for general noise schedules + +The proof of this result relies heavily on the properties of the Hamilton-Jacobi-Bellman equation: + +Theorem 3 (Hamilton-Jacobi-Bellman equation). If we define the infinitesimal generator + +$$ +\begin{array} { r } { \mathcal L : = \frac { 1 } { 2 } \sum _ { i , j = 1 } ^ { d } ( { \sigma } { \sigma } ^ { \top } ) _ { i j } ( t ) \partial _ { x _ { i } } \partial _ { x _ { j } } + \sum _ { i = 1 } ^ { d } b _ { i } ( x , t ) \partial _ { x _ { i } } , } \end{array} +$$ + +the value function $V$ for the $S O C$ problem (12)-(13) solves the following Hamilton-Jacobi-Bellman (HJB) partial differential equation: + +$$ +\begin{array} { r l } & { \partial _ { t } V ( x , t ) = - \mathcal { L } V ( x , t ) + \frac { 1 } { 2 } \| ( \sigma ^ { \top } \nabla V ) ( x , t ) \| ^ { 2 } - f ( x , t ) , } \\ & { V ( x , T ) = g ( x ) . } \end{array} +$$ + +Consider forward SDEs like (63), starting from the distributions $p ^ { \mathrm { b a s e } }$ and $p ^ { * }$ , where $p ^ { * } ( x ) \propto p ^ { \mathrm { b a s e } } ( x ) \exp ( r ( x ) )$ + +$$ +\begin{array} { r l r } & { \mathrm { d } \vec { X } _ { t } = \vec { b } ( \vec { X } _ { t } , t ) \mathrm { d } t + \sigma ( t ) \mathrm { d } B _ { t } , } & { \vec { X } _ { 0 } \sim p ^ { \mathrm { b a s e } } , } \\ & { \mathrm { d } \vec { X } _ { t } ^ { * } = \vec { b } ^ { * } ( \vec { X } _ { t } ^ { * } , t ) \mathrm { d } t + \sigma ( t ) \mathrm { d } B _ { t } , } & { \vec { X } _ { 0 } \sim p ^ { * } . } \end{array} +$$ + +where the drifts are defined as + +$$ +\begin{array} { r l } & { \Vec { b } ( x , t ) = - \kappa _ { 1 - t } x + \big ( \frac { \sigma ( 1 - t ) ^ { 2 } } { 2 } - \eta _ { 1 - t } \big ) \mathfrak { s } ( x , 1 - t ) = - \kappa _ { 1 - t } x + \big ( \frac { \sigma ( 1 - t ) ^ { 2 } } { 2 } - \eta _ { 1 - t } \big ) \nabla \log { \Vec { p _ { t } } } ( x ) , } \\ & { \Vec { b ^ { * } } ( x , t ) = - \kappa _ { 1 - t } x + \big ( \frac { \sigma ( 1 - t ) ^ { 2 } } { 2 } - \eta _ { 1 - t } \big ) \mathfrak { s ^ { * } } ( x , 1 - t ) = - \kappa _ { 1 - t } x + \big ( \frac { \sigma ( 1 - t ) ^ { 2 } } { 2 } - \eta _ { 1 - t } \big ) \nabla \log { \Vec { p _ { t } ^ { * } } ( x ) } , } \end{array} +$$ + +and $\vec { p _ { t } } , \vec { p _ { t } ^ { * } }$ are the densities of $X _ { t }$ , $\vec { X _ { t } }$ , respectively. $\vec { p _ { t } }$ , $\vec { p } _ { t } ^ { * }$ satisfy Fokker-Planck equations: + +$$ +\begin{array} { r } { \partial _ { t } \vec { p _ { t } } = \nabla \cdot ( \vec { b } ( x , t ) \vec { p _ { t } } ) + \nabla \cdot \bigl ( \frac { \sigma ( 1 - t ) ^ { 2 } } { 2 } \nabla \vec { p _ { t } } \bigr ) , \qquad \vec { p _ { 0 } } = p ^ { \mathrm { b a s e } } , } \\ { \partial _ { t } \vec { p _ { t } ^ { * } } = \nabla \cdot ( \vec { b ^ { * } } ( x , t ) \vec { p _ { t } ^ { * } } ) + \nabla \cdot \bigl ( \frac { \sigma ( 1 - t ) ^ { 2 } } { 2 } \nabla \vec { p _ { t } ^ { * } } \bigr ) , \qquad \vec { p _ { 0 } } = p ^ { * } . } \end{array} +$$ + +Plugging (159) into (160), we obtain + +$$ +\begin{array} { r l r } & { } & { \partial _ { t } \vec { p _ { t } } = \nabla \cdot \bigl ( \kappa _ { 1 - t } x \vec { p _ { t } } \bigr ) + \nabla \cdot \bigl ( \eta _ { 1 - t } \nabla \vec { p _ { t } } \bigr ) , \qquad \vec { p _ { 0 } } = p ^ { \mathrm { b a s e } } , } \\ & { } & { \partial _ { t } \vec { p _ { t } } = \nabla \cdot \bigl ( \kappa _ { 1 - t } x \vec { p _ { t } ^ { * } } \bigr ) + \nabla \cdot \bigl ( \eta _ { 1 - t } \nabla \vec { p _ { t } ^ { * } } \bigr ) , \qquad \vec { p _ { 0 } } = p ^ { * } . } \end{array} +$$ + +We apply the Hopf-Cole transformation to obtain PDEs for $- \log \vec { p _ { t } }$ (and $- \log { \vec { p _ { t } } }$ analogously): + +$$ +\begin{array} { r l } & { - \partial _ { t } ( - \log \vec { p _ { t } } ) = \frac { \partial _ { t } p _ { t } } { p _ { t } } = \frac { \nabla \cdot ( \kappa _ { 1 - t } x \vec { p } _ { t } ) + \nabla \cdot \big ( \eta _ { 1 - t } \nabla \vec { p } _ { t } \big ) } { p _ { t } } } \\ & { \qquad = \kappa _ { 1 - t } \nabla \cdot x + \kappa _ { 1 - t } \langle x , \nabla \log \vec { p _ { t } } \rangle + \eta _ { 1 - t } \frac { \nabla \cdot ( \nabla \log \vec { p _ { t } } \exp ( \log p _ { t } ) ) } { p _ { t } } } \\ & { \qquad = \kappa _ { 1 - t } d + \kappa _ { 1 - t } \langle x , \nabla \log \vec { p _ { t } } \rangle + \eta _ { 1 - t } \big ( \Delta \log \vec { p _ { t } } + \| \nabla \log \vec { p _ { t } } \| ^ { 2 } \big ) . } \end{array} +$$ + +Hence, if we define $\mathcal { V } ( x , t ) = - \log \vec { p _ { t } } ( x )$ , $\psi ^ { * } ( x , t ) = - \log \vec { p } _ { t } ^ { * } ( x )$ , then $\mathcal { V }$ and $\mathcal { V } ^ { * }$ satisfy the following Hamilton-Jacobi-Bellman equations: + +$$ +\begin{array} { r l } & { \quad - \partial _ { t } \mathcal { V } = \kappa _ { 1 - t } d - \kappa _ { 1 - t } \langle x , \nabla \mathcal { V } \rangle + \eta _ { 1 - t } \big ( - \Delta \mathcal { V } + \| \nabla \mathcal { V } \| ^ { 2 } \big ) , \qquad \mathcal { V } ( x , 0 ) = - \log p ^ { \mathrm { b a s e } } ( x ) , } \\ & { \quad - \partial _ { t } \mathcal { V } ^ { * } = \kappa _ { 1 - t } d - \kappa _ { 1 - t } \langle x , \nabla \mathcal { V } ^ { * } \rangle + \eta _ { 1 - t } \big ( - \Delta \mathcal { V } ^ { * } + \| \nabla \mathcal { V } ^ { * } \| ^ { 2 } \big ) , \qquad \mathcal { V } ^ { * } ( x , 0 ) = - \log p ^ { * } ( x ) . } \end{array} +$$ + +Now, define $\hat { \mathcal { V } } ( x , t ) = \mathcal { V } ^ { * } ( x , t ) - \mathcal { V } ( x , t )$ . Subtracting (164) from (163), we obtain + +$$ +\begin{array} { r l } & { \begin{array} { r l } { - \partial _ { t } \hat { \mathcal { V } } = - \kappa _ { 1 - t } \langle x , \nabla \hat { \mathcal { V } } \rangle + \eta _ { 1 - t } \big ( - \Delta \hat { \mathcal { V } } + \| \nabla \mathcal { V } ^ { * } \| ^ { 2 } - \| \nabla \mathcal { V } \| ^ { 2 } \big ) } \\ & { \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad = - \kappa _ { 1 - t } \langle x , \nabla \hat { \mathcal { V } } \rangle + \eta _ { 1 - t } \big ( - \Delta \hat { \mathcal { V } } + \| \nabla ( \hat { \mathcal { V } } + \mathcal { V } ) \| ^ { 2 } - \| \nabla \mathcal { V } \| ^ { 2 } \big ) } \end{array} } \\ & { \quad \quad \quad \quad \quad \quad \quad = - \kappa _ { 1 - t } \langle x , \nabla \hat { \mathcal { V } } \rangle + \eta _ { 1 - t } \big ( - \Delta \hat { \mathcal { V } } + \| \nabla \hat { \mathcal { V } } \| ^ { 2 } + 2 \langle \nabla \mathcal { V } , \nabla \hat { \mathcal { V } } \rangle \big ) } \\ & { \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad = \big \langle - \kappa _ { 1 - t } x + 2 \eta _ { 1 - t } \nabla \mathcal { V } , \nabla \hat { \mathcal { V } } \rangle + \eta _ { 1 - t } \big ( - \Delta \hat { \mathcal { V } } + \| \nabla \hat { \mathcal { V } } \| ^ { 2 } \big ) } \\ & { \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad = \big \langle - \kappa _ { 1 - t } x - 2 \eta _ { 1 - t } \mathfrak { s } ( x , 1 - t ) , \nabla \hat { \mathcal { V } } \big \rangle + \eta _ { 1 - t } \big ( - \Delta \hat { \mathcal { V } } + \| \nabla \hat { \mathcal { V } } \| ^ { 2 } \big ) , } \\ & \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \end{array} +$$ + +Hence, $\hat { \mathcal { V } }$ also satisfies a Hamilton-Jacobi-Bellman equation. If we define $V$ such that $\hat { \mathcal { V } } ( x , t ) = V ( x , 1 - t )$ , we have that + +$$ +\begin{array} { r l r } { \mathbf { \sigma } } & { = \langle - \kappa _ { t } x - 2 \eta _ { t } \mathbf { s } ( x , t ) , \nabla V \rangle + \eta _ { t } \big ( - \Delta V + \| \nabla V \| ^ { 2 } \big ) , } & { } & { V ( x , 1 ) = r ( x ) - \log \big ( \int p ^ { \mathrm { b a s e } } ( y ) \exp ( r ( x ) ) \big ) \int _ { 0 } ^ { \infty } \mathbf { \eta } } \end{array} +$$ + +Using Theorem 3, we can reverse-engineer $V$ as the value function of the following SOC problem: + +$$ +\begin{array} { r l } & { \underset { u \in \mathcal { U } } { \operatorname* { m i n } } \mathbb { E } \big [ \frac { 1 } { 2 } \int _ { 0 } ^ { 1 } \| u ( X _ { t } ^ { u } , t ) \| ^ { 2 } \mathrm { d } t - r ( x ) + \log \big ( \int p ^ { \mathrm { b a s e } } ( y ) \exp ( r ( y ) ) \mathrm { d } y \big ) \big ] , } \\ & { \mathrm { s . t . ~ d } X _ { t } ^ { u } = \big ( \kappa _ { t } x + 2 \eta _ { t } \mathfrak { s } ( x , t ) + \sqrt { 2 \eta _ { t } } u ( X _ { t } ^ { u } , t ) \big ) \mathrm { d } t + \sqrt { 2 \eta _ { t } } \mathrm { d } B _ { t } , \qquad X _ { 0 } ^ { u } \sim p _ { 0 } . } \end{array} +$$ + +Note that this SOC problem is equal to the problem (12)-(13) with the choices $f = 0$ , $g = - r$ , and $\sigma ( t ) = \sqrt { 2 \eta _ { t } }$ . By equation (17), the optimal control of the problem (167)-(168) is of the form: + +$$ +\begin{array} { r l } & { u ^ { * } ( x , t ) = - \sqrt { 2 \eta _ { t } } \nabla V ( x , t ) = - \sqrt { 2 \eta _ { t } } \nabla \hat { \psi } ( x , 1 - t ) = - \sqrt { 2 \eta _ { t } } \big ( \nabla \mathcal { V } ^ { * } ( x , 1 - t ) - \nabla \mathcal { V } ( x , 1 - t ) \big ) } \\ & { \qquad = - \sqrt { 2 \eta _ { t } } \big ( - \nabla \log \bar { p } _ { 1 - t } ^ { * } ( x ) + \nabla \log \bar { p } _ { 1 - t } ( x ) \big ) = \sqrt { 2 \eta _ { t } } \big ( \mathfrak { s } ^ { * } ( x , t ) - \mathfrak { s } ( x , t ) \big ) , } \\ & { \qquad \Longleftrightarrow \mathfrak { s } ^ { * } ( x , t ) = \mathfrak { s } ( x , t ) + u ^ { * } ( x , t ) / \sqrt { 2 \eta _ { t } } . } \end{array} +$$ + +As in (64), the backward SDEs corresponding to the forward SDEs (158) take the following form: + +$$ +\begin{array} { r } { \mathrm { d } X _ { t } ^ { * } = \bigl ( \kappa _ { t } X _ { t } ^ { * } + \bigl ( \frac { \sigma ( t ) ^ { 2 } } { 2 } + \eta _ { t } \bigr ) \mathfrak { s } ^ { * } ( X _ { t } ^ { * } , t ) \bigr ) \mathrm { d } t + \sigma ( t ) \mathrm { d } B _ { t } , \qquad X _ { 0 } ^ { * } \sim N ( 0 , I ) . } \end{array} +$$ + +If we plug (170) into this equation, we obtain + +$$ +\begin{array} { r l } & { \mathrm { d } X _ { t } ^ { * } = \bigl ( \kappa _ { t } X _ { t } ^ { * } + \bigl ( \frac { \sigma ( t ) ^ { 2 } } { 2 } + \eta _ { t } \bigr ) \bigl ( \mathfrak { s } ( X _ { t } ^ { * } , t ) + \frac { u ^ { * } ( X _ { t } ^ { * } , t ) } { \sqrt { 2 \eta _ { t } } } \bigr ) \bigr ) \mathrm { d } t + \sigma ( t ) \mathrm { d } B _ { t } , \qquad X _ { 0 } ^ { * } \sim N ( 0 , I ) , } \\ { \iff \mathrm { d } X _ { t } ^ { * } = \bigl ( b ( X _ { t } ^ { * } , t ) + \frac { \frac { \sigma ( t ) ^ { 2 } } { 2 } + \eta _ { t } } { \sqrt { 2 \eta _ { t } } } u ^ { * } ( X _ { t } ^ { * } , t ) \bigr ) \mathrm { d } t + \sigma ( t ) \mathrm { d } B _ { t } , \qquad X _ { 0 } ^ { * } \sim N ( 0 , I ) . } \end{array} +$$ + +where we used that $\begin{array} { r } { b ( x , t ) = \kappa _ { t } x + \big ( \frac { \sigma ( t ) ^ { 2 } } { 2 } + \eta _ { t } \big ) \mathfrak { s } ( x , t ) } \end{array}$ by definition in equation (11). + +The fine-tuned inference $S D E$ for DDIM Now, for DDIM, we have that $\begin{array} { r } { u ^ { * } ( x , t ) = - \sqrt { \frac { \dot { \alpha } _ { t } } { \alpha _ { t } ( 1 - \alpha _ { t } ) } } ( \epsilon ^ { * } ( x , t ) - } \end{array}$ $\epsilon ^ { \mathrm { b a s e } } ( x , t ) )$ by (26). Hence, + +$$ +\begin{array} { r l } & { \frac { 2 } { 2 \eta _ { t } } u ^ { * } ( x , t ) = - \frac { \frac { \sigma ( t ) ^ { 2 } } { 2 } + \frac { \lambda \epsilon _ { t } } { 2 \alpha _ { t } } } { \sqrt { \frac { \lambda \epsilon _ { t } } { \alpha _ { t } } } } \sqrt { \frac { \dot { \alpha } _ { t } } { \alpha _ { t } ( 1 - \alpha _ { t } ) } } ( \epsilon ^ { * } ( x , t ) - \epsilon ^ { \mathrm { b a s e } } ( x , t ) ) = - \frac { \frac { \sigma ( t ) ^ { 2 } } { 2 } + \frac { \dot { \alpha } _ { t } } { 2 \alpha _ { t } } } { \sqrt { 1 - \alpha _ { t } } } ( \epsilon ^ { * } ( x , t ) - \epsilon ^ { \mathrm { b a s e } } ( x , t ) ) } \\ & { \Rightarrow b ( x , t ) + \frac { \frac { \sigma ( t ) ^ { 2 } } { 2 } + \eta _ { t } } { \sqrt { 2 \eta _ { t } } } u ^ { * } ( x , t ) = \frac { \dot { \alpha } _ { t } } { 2 \alpha _ { t } } X _ { t } - \left( \frac { \dot { \alpha } _ { t } } { 2 \alpha _ { t } } + \frac { \sigma ( t ) ^ { 2 } } { 2 } \right) \frac { \epsilon ^ { \mathrm { b a s e } } ( X _ { t } , t ) } { \sqrt { 1 - \alpha _ { t } } } - \frac { \frac { \sigma ( t ) ^ { 2 } } { 2 } + \frac { \dot { \alpha } _ { t } } { 2 \alpha _ { t } } } { \sqrt { 1 - \alpha _ { t } } } ( \epsilon ^ { * } ( x , t ) - \epsilon ^ { \mathrm { b a s e } } ( x , t ) } \\ & { \qquad = \frac { \dot { \alpha } _ { t } } { 2 \alpha _ { t } } X _ { t } - \left( \frac { \dot { \alpha } _ { t } } { 2 \alpha _ { t } } + \frac { \sigma ( t ) ^ { 2 } } { 2 } \right) \frac { \epsilon ^ { * } ( X _ { t } , t ) } { \sqrt { 1 - \alpha _ { t } } } . } \end{array} +$$ + +We obtain that the fine-tuned inference SDE for DDIM is + +$$ +\begin{array} { r } { \begin{array} { r } { \mathrm { d } X _ { t } ^ { * } = \big ( \frac { \dot { \alpha } _ { t } } { 2 \alpha _ { t } } X _ { t } ^ { * } - \big ( \frac { \dot { \alpha } _ { t } } { 2 \alpha _ { t } } + \frac { \sigma ( t ) ^ { 2 } } { 2 } \big ) \frac { \epsilon ^ { * } ( X _ { t } ^ { * } , t ) } { \sqrt { 1 - \alpha _ { t } } } \big ) \mathrm { d } t + \sigma ( t ) \mathrm { d } B _ { t } , \qquad X _ { 0 } ^ { * } \sim N ( 0 , I ) , } \end{array} } \end{array} +$$ + +which is matches the SDE (6) with the choice $\epsilon = \epsilon ^ { * }$ . + +The fine-tuned inference SDE for Flow Matching For Flow Matching, we have that $\begin{array} { r } { u ^ { * } ( x , t ) = \sqrt { \frac { 2 } { \beta _ { t } ( \frac { \dot { \alpha } _ { t } } { \alpha _ { t } } \beta _ { t } - \dot { \beta } _ { t } ) } } ( v ^ { * } ( x , t ) - } \end{array}$ $v ^ { \mathrm { b a s e } } ( x , t ) )$ by (27). Hence, + +$$ +\begin{array} { r l } & { \frac { \frac { \sigma ( t ) ^ { 2 } } { 2 } + \eta _ { t } } { \sqrt { 2 \eta _ { t } } } u ^ { * } ( x , t ) = \frac { \frac { \sigma ( t ) ^ { 2 } } { 2 } + \beta _ { t } ( \frac { \sin _ { t } \beta _ { t } - \beta _ { t } } { \sin \beta _ { t } } ) } { \sqrt { 2 \beta _ { t } } ( \frac { \sin _ { t } \beta _ { t } - \beta _ { t } } { \sin \beta _ { t } } ) } \sqrt { \frac { 2 } { \beta _ { t } ( \frac { \sin _ { t } \beta _ { t } - \beta _ { t } } { \cos \beta _ { t } } ) } } ( v ^ { * } ( x , t ) - v ^ { \mathrm { b a s e } } ( x , t ) ) } \\ & { \qquad = \big ( 1 + \frac { \sigma ( t ) ^ { 2 } } { 2 \beta _ { t } ( \frac { \sin _ { t } \beta _ { t } - \beta _ { t } } { \cos \beta _ { t } } ) } \big ) ( v ^ { * } ( x , t ) - v ^ { \mathrm { b a s e } } ( x , t ) ) . } \\ & { \implies b ( x , t ) + \frac { \frac { \sigma ( t ) ^ { 2 } } { 2 } + \eta _ { t } } { \sqrt { 2 \eta _ { t } } } u ^ { * } ( x , t ) = v ^ { \mathrm { b a s e } } ( x , t ) + \frac { \sigma ( t ) ^ { 2 } } { 2 \beta _ { t } ( \frac { \sin _ { t } \beta _ { t } } { \cos \beta _ { t } } ) } \big ( v ^ { \mathrm { b a s e } } ( x , t ) - \frac { \dot { \alpha } _ { t } } { \alpha _ { t } } x \big ) } \\ & { \qquad + \big ( 1 + \frac { \sigma ( t ) ^ { 2 } } { 2 \beta _ { t } ( \frac { \cos _ { t } \beta _ { t } } { \cos \beta _ { t } } ) } \big ) ( v ^ { * } ( x , t ) - v ^ { \mathrm { b a s e } } ( x , t ) ) } \\ & \qquad = v ^ { * } ( x , t ) + \frac { \sigma ( t ) ^ { 2 } } 2 \beta _ { t } ( \frac \end{array} +$$ + +We obtain that the fine-tuned inference SDE for Flow Matching is + +$$ +\begin{array} { r } { \mathrm { d } X _ { t } ^ { * } = \big ( v ( X _ { t } ^ { * } , t ) + \frac { \sigma ( t ) ^ { 2 } } { 2 \beta _ { t } ( \frac { \alpha _ { t } } { \alpha _ { t } } \beta _ { t } - \bar { \beta } _ { t } ) } \big ( v ^ { * } ( X _ { t } ^ { * } , t ) - \frac { \dot { \alpha } _ { t } } { \alpha _ { t } } X _ { t } ^ { * } \big ) \big ) \mathrm { d } t + \sigma ( t ) \mathrm { d } B _ { t } , \qquad X _ { 0 } ^ { * } \sim N ( 0 , I ) , } \end{array} +$$ + +which matches equation (4) with the choice $v = v ^ { * }$ . + +# E Loss function derivations + +# E.1 Derivation of the Continuous Adjoint method + +Proposition 6. The gradient $\textstyle { \frac { \mathrm { d } { \mathcal { L } } } { \mathrm { d } \theta } }$ of the adjoint loss $\mathcal { L } ( u ; X )$ defined in (28) with respect to the parameters $\theta$ of the control can be expressed as in (32). + +Proof. First, note that we can write + +$$ +\begin{array} { r l } & { \nabla _ { \theta } \mathbb { E } \big [ \int _ { 0 } ^ { T } \big ( \frac { 1 } { 2 } \| u _ { \theta } ( X _ { t } ^ { u _ { \theta } } , t ) \| ^ { 2 } + f ( X _ { t } ^ { u _ { \theta } } , t ) \big ) \mathrm { d } t + g ( X _ { T } ^ { u _ { \theta } } ) \big ] } \\ & { = \mathbb { E } \big [ \int _ { 0 } ^ { T } \nabla _ { \theta } u _ { \theta } ( X _ { t } ^ { u _ { \theta } } , t ) u _ { \theta } ( X _ { t } ^ { u _ { \theta } } , t ) \mathrm { d } t \big ] + \nabla _ { \theta } \mathbb { E } \big [ \int _ { 0 } ^ { T } \big ( \frac { 1 } { 2 } \| v ( X _ { t } ^ { u _ { \theta } } , t ) \| ^ { 2 } + f ( X _ { t } ^ { u _ { \theta } } , t ) \big ) \mathrm { d } t + g ( X _ { T } ^ { u _ { \theta } } ) \big ] \big | _ { v = \mathrm { s t } } } \end{array} +$$ + +To develop the second term, we apply Lemma 5. Namely, by the Leibniz rule and equation (185), we have that + +$$ +\begin{array} { r l } & { \nabla _ { \theta } \mathbb { E } \big [ \int _ { 0 } ^ { T } \big ( \frac { 1 } { 2 } \| v ( X _ { t } ^ { u _ { \theta } } , t ) \| ^ { 2 } + f ( X _ { t } ^ { u _ { \theta } } , t ) \big ) \mathrm { d } t + g ( X _ { T } ^ { u _ { \theta } } ) \big ] \big | _ { v = \mathrm { s t o p g r a d } ( u _ { \theta } ) } } \\ & { \ = \mathbb { E } \big [ \nabla _ { \theta } \big ( \int _ { 0 } ^ { T } \big ( \frac { 1 } { 2 } \| v ( X _ { t } ^ { u _ { \theta } } , t ) \| ^ { 2 } + f ( X _ { t } ^ { u _ { \theta } } , t ) \big ) \mathrm { d } t + g ( X _ { T } ^ { u _ { \theta } } ) \big ) \big | _ { v = \mathrm { s t o p g r a d } ( u _ { \theta } ) } \big ] } \\ & { \ = \mathbb { E } \big [ \int _ { 0 } ^ { T } ( \nabla _ { \theta } u _ { \theta } ) \big ( X _ { t } ^ { u _ { \theta } } ( \omega ) , t \big ) ^ { \top } \sigma ( t ) ^ { \top } a _ { t } ( \omega ) \mathrm { d } t \big ] . } \end{array} +$$ + +Plugging the right-hand side of this equation into (180) concludes the proof. + +Lemma 5. Let v be an arbitrary fixed vector field. The unique solution of the ODE + +$$ +\begin{array} { r l } & { \overline { { t } } ^ { a } ( t ; \mathbf { X } ^ { u } , u ) = - \left[ \left( \nabla _ { X _ { t } ^ { u } } \big ( b ( X _ { t } ^ { u } , t ) + \sigma ( t ) u ( X _ { t } ^ { u } , t ) \big ) \right) ^ { \top } a ( t ; \mathbf { X } ^ { u } , u ) + \nabla _ { X _ { t } ^ { u } } \left( f ( X _ { t } ^ { u } , t ) + \frac { 1 } { 2 } \| v ( X _ { t } ^ { u } , t ) \| ^ { 2 } \right) \right] } \\ & { } \\ & { a ( 1 ; \mathbf { X } ^ { u } , u ) = \nabla g ( X _ { 1 } ^ { u } ) , } \end{array} +$$ + +satisfies: + +$$ +\begin{array} { r l } & { a ( t ; \mathbf { } X ^ { u } , u ) : = \nabla _ { X _ { t } ^ { u } } \big ( \int _ { t } ^ { 1 } \big ( \frac { 1 } { 2 } \| u ( X _ { t ^ { \prime } } ^ { u } , t ^ { \prime } ) \| ^ { 2 } + f ( X _ { t ^ { \prime } } ^ { u } , t ^ { \prime } ) \big ) \mathrm { d } t ^ { \prime } + g ( X _ { 1 } ^ { u } ) \big ) , } \\ & { w h e r e \ X ^ { u } \ s o l v e s \mathrm { ~ d } X _ { t } ^ { u } = \big ( b ( X _ { t } ^ { u } , t ) + \sigma ( t ) u ( X _ { t } ^ { u } , t ) \big ) \ \mathrm { d } t + \sigma ( t ) \mathrm { d } B _ { t } . } \end{array} +$$ + +Moreover, when $u = u _ { \theta }$ is parameterized by $\theta$ we have that + +$$ +\begin{array} { r } { \nabla _ { \theta } \big ( \int _ { 0 } ^ { T } \big ( \frac 1 2 \| v ( X _ { t } ^ { u _ { \theta } } , t ) \| ^ { 2 } + f ( X _ { t } ^ { u _ { \theta } } , t ) \big ) \mathrm { d } t + g ( X _ { T } ^ { u _ { \theta } } ) \big ) = \int _ { 0 } ^ { T } ( \nabla _ { \theta } u _ { \theta } ) ( X _ { t } ^ { u _ { \theta } } ( \omega ) , t ) \sigma ( t ) ^ { \top } a _ { t } ( \omega ) \mathrm { d } t . } \end{array} +$$ + +Proof. We use an approach based on Lagrange multipliers which mirrors and extends the derivation of the adjoint ODE (Domingo-Enrich et al., 2023, Lemma 8). For shortness, we use the notation $\tilde { b } _ { \theta } ( x , t ) : =$ $b ( x , t ) + \sigma ( t ) u _ { \theta } ( x , t )$ . Define a process $a : \Omega \times [ 0 , T ] { \mathbb { R } ^ { d } }$ such that for any $\omega \in \Omega$ , $a ( \omega , \cdot )$ is differentiable. For a given $\omega \in \Omega$ , we can write + +$$ +\begin{array} { r l } & { \int _ { 0 } ^ { T } \left( \frac { 1 } { 2 } \| v ( X _ { t } ^ { u _ { \theta } } , t ) \| ^ { 2 } + f ( X _ { t } ^ { u _ { \theta } } , t ) \right) \mathrm { d } t + g ( X _ { T } ^ { u _ { \theta } } ) } \\ & { = \int _ { 0 } ^ { T } \left( \frac { 1 } { 2 } \| v ( X _ { t } ^ { u _ { \theta } } , t ) \| ^ { 2 } + f ( X _ { t } ^ { u _ { \theta } } , t ) \right) \mathrm { d } t + g ( X _ { T } ^ { u _ { \theta } } ) } \\ & { \qquad - \int _ { 0 } ^ { T } \langle a _ { t } ( \omega ) , ( d X _ { t } ^ { u _ { \theta } } ( \omega ) - \tilde { b } _ { \theta } ( X _ { t } ^ { u _ { \theta } } ( \omega ) , t ) \mathrm { d } t - \sigma ( t ) \mathrm { d } B _ { t } ) \rangle . } \end{array} +$$ + +By stochastic integration by parts (Domingo-Enrich et al., 2023, Lemma 9), we have that + +$$ +\begin{array} { r } { \int _ { 0 } ^ { T } \langle a _ { t } ( \omega ) , d X _ { t } ^ { u _ { \theta } } ( \omega ) \rangle = \langle a _ { T } ( \omega ) , X _ { T } ^ { u _ { \theta } } ( \omega ) \rangle - \langle a _ { 0 } ( \omega ) , X _ { 0 } ^ { u _ { \theta } } ( \omega ) \rangle - \int _ { 0 } ^ { T } \langle X _ { t } ^ { u _ { \theta } } ( \omega ) , \frac { d a _ { t } } { d t } ( \omega ) \rangle \mathrm { d } t . } \end{array} +$$ + +Hence, if $X _ { 0 } ^ { u _ { \theta } } = x _ { 0 }$ is the initial condition, we have that9 + +$$ +\begin{array} { r l } & { \nabla _ { x v } \left( \int _ { 0 } ^ { T } \left( \frac { 1 } { 2 } \| v ( X _ { t } ^ { u _ { \theta } } , t ) \| ^ { 2 } + f ( X _ { t } ^ { u _ { \theta } } , t ) \right) \mathrm { d } t + g ( X _ { T } ^ { u _ { \theta } } ) \right) } \\ & { = \nabla _ { x } \left( \int _ { 0 } ^ { T } \left( \frac { 1 } { 2 } \| v ( X _ { t } ^ { u _ { \theta } } , t ) \| ^ { 2 } + f ( X _ { t } ^ { u _ { \theta } } , t ) \right) \mathrm { d } t + g ( X _ { T } ^ { u _ { \theta } } ) \right. } \\ & { \qquad - \left. u _ { T } ( \omega ) , X _ { T ^ { u _ { \theta } } } ^ { u _ { \theta } } ( \omega ) \right. + \left. a _ { 0 } ( \omega ) , X _ { 0 } ^ { u _ { \theta } } ( \omega ) \right. + \int _ { 0 } ^ { T } \left( \langle a _ { t } ( \omega ) , \bar { b } _ { \theta } ( X _ { t } ^ { u _ { \theta } } ( \omega ) , t ) \rangle + \left. \frac { d a _ { t } } { d t } ( \omega ) , X _ { t } ^ { u _ { \theta } } ( \omega ) \right. \mathrm { d } t \right. } \\ & { \qquad \left. + \int _ { 0 } ^ { T } \langle a _ { t } ( \omega ) , \sigma ( t ) \mathrm { d } B _ { t } \rangle \right) } \\ & { = \int _ { 0 } ^ { T } \nabla _ { x _ { a } } X _ { t } ^ { u _ { \theta } } ( \omega ) ^ { \top } \nabla _ { x } \left( \frac { 1 } { 2 } \| v ( X _ { t } ^ { u _ { \theta } } , t ) \| ^ { 2 } + f ( X _ { t } ^ { u _ { \theta } } ( \omega ) , t ) \right) \mathrm { d } t + \nabla _ { x _ { 0 } } X _ { T } ^ { u _ { \theta } } ( \omega ) ^ { \top } \nabla _ { x } g ( X _ { T } ^ { u _ { \theta } } ( \omega ) ) } \\ & \qquad - \nabla _ x _ \end{array} +$$ + +In the last line we used that $\nabla _ { x _ { 0 } } X _ { 0 } ^ { u _ { \theta } } ( \omega ) = \nabla _ { x _ { 0 } } x _ { 0 } = \mathrm { I }$ . If choose $a$ such that + +$$ +\begin{array} { r l } & { { d a _ { t } } ( \omega ) = \big ( - \nabla _ { x } \tilde { b } _ { \theta } ( X _ { t } ^ { u _ { \theta } } ( \omega ) , t ) ^ { \top } a _ { t } ( \omega ) - \nabla _ { x } \big ( \frac 1 2 \| v ( X _ { t } ^ { u _ { \theta } } , t ) \| ^ { 2 } + f ( X _ { t } ^ { u _ { \theta } } ( \omega ) , t ) \big ) \big ) \mathrm { d } t , } \\ & { { a _ { T } } ( \omega ) = \nabla _ { x } g ( X _ { T } ^ { u _ { \theta } } ( \omega ) ) , } \end{array} +$$ + +which is the ODE (182)-(183), then we obtain that + +$$ +\begin{array} { r } { \nabla _ { x _ { 0 } } \big ( \int _ { 0 } ^ { T } \big ( \frac { 1 } { 2 } \| v ( X _ { t } ^ { u _ { \theta } } , t ) \| ^ { 2 } + f ( X _ { t } ^ { u _ { \theta } } , t ) \big ) \mathrm { d } t + g ( X _ { T } ^ { u _ { \theta } } ) \big ) = a _ { 0 } ( \omega ) } \end{array} +$$ + +Without loss of generality, this argument can be extended from $t = 0$ to an arbitrary $t \in [ 0 , 1 ]$ , which proves the first statement of the lemma. + +To prove (185), we similarly write + +$$ +\begin{array} { r l } & { \tau _ { \theta } \Big ( \int _ { 0 } ^ { T } \big ( \frac 1 2 \vert \vert v ( X _ { t } ^ { u _ { \theta } } , t ) \vert \big \vert ^ { 2 } + f ( X _ { t } ^ { u _ { \theta } } , t ) \big ) \mathrm { d } t + g ( X _ { T } ^ { u _ { \theta } } ) \Big ) } \\ & { = \nabla _ { \theta } \big ( \int _ { 0 } ^ { T } \big ( \frac 1 2 \vert v ( X _ { t } ^ { u _ { \theta } } , t ) \vert ^ { 2 } \big ) ^ { 2 } + f ( X _ { t } ^ { u _ { \theta } } , t ) \big ) \mathrm { d } t + g ( X _ { T } ^ { u _ { \theta } } ) } \\ & { \qquad - \langle a _ { T } ( \omega ) , X _ { t } ^ { u _ { \theta } } ( \omega ) \rangle + \langle a _ { 0 } ( \omega ) , X _ { 0 } ^ { u _ { \theta } } ( \omega ) \rangle + \int _ { 0 } ^ { T } \big ( \langle a _ { t } ( \omega ) , \tilde { b } _ { \theta } ( X _ { t } ^ { u _ { \theta } } ( \omega ) , t ) \rangle + \langle \frac { d a _ { t } } { d t } ( \omega ) , X _ { t } ^ { u _ { \theta } } ( \omega ) \rangle \big ) } \\ & { \qquad + \int _ { 0 } ^ { T } \langle a _ { t } ( \omega ) , \sigma ( t ) \mathrm { d } B _ { t } \rangle \Big ) } \\ & { = \int _ { 0 } ^ { T } \nabla _ { \theta } X _ { t } ^ { u _ { \theta } } ( \omega ) ^ { \top } \nabla _ { \nabla } \big ( \frac 1 2 \vert v ( X _ { t } ^ { u _ { \theta } } , t ) \vert \big \vert ^ { 2 } + f ( X _ { t } ^ { u _ { \theta } } ( \omega ) , t ) \big ) \mathrm { d } t + \nabla _ { \theta } X _ { T } ^ { u _ { \theta } } ( \omega ) ^ { \top } \nabla _ { \mathbf { x } } g ( X _ { T } ^ { u _ { \theta } } ( \omega ) ) } \\ & \qquad - \nabla _ { \theta } X _ { t } ^ { u _ { \theta } } ( \omega \end{array} +$$ + +In the last line we used that $\nabla _ { \theta } X _ { 0 } ^ { u _ { \theta } } ( \omega ) = \nabla _ { \theta } x = 0$ . When $a$ satisfies (189), we obtain that + +$$ +\begin{array} { r l } & { \nabla _ { \theta } \big ( \int _ { 0 } ^ { T } \big ( \frac { 1 } { 2 } \| v ( X _ { t } ^ { u _ { \theta } } , t ) \| ^ { 2 } + f ( X _ { t } ^ { u _ { \theta } } , t ) \big ) \mathrm { d } t + g ( X _ { T } ^ { u _ { \theta } } ) \big ) } \\ & { \ = \int _ { 0 } ^ { T } ( \nabla _ { \theta } \tilde { b } _ { \theta } ) ( X _ { t } ^ { u _ { \theta } } ( \omega ) , t ) a _ { t } ( \omega ) \mathrm { d } t = \int _ { 0 } ^ { T } ( \nabla _ { \theta } u _ { \theta } ) ( X _ { t } ^ { u _ { \theta } } ( \omega ) , t ) ^ { \top } \sigma ( t ) ^ { \top } a _ { t } ( \omega ) \mathrm { d } t . } \end{array} +$$ + +The last equality holds because $\tilde { b } _ { \theta } ( x , t ) : = b ( x , t ) + \sigma ( t ) u _ { \theta } ( x , t )$ . + +# E.2 Proof of Proposition 2: Theoretical guarantees of the basic Adjoint Matching loss + +Let $\bar { u } = \mathsf { s t o p g r a d } ( u _ { \theta } )$ . We can rewrite equation (32) as: + +$$ +\begin{array} { r l } & { \nabla _ { \theta } \mathcal { L } ( u _ { \theta } ; \mathbf { X } ^ { \bar { \boldsymbol { u } } } ) = \frac { 1 } { 2 } \int _ { 0 } ^ { 1 } \nabla _ { \theta } \| u _ { \theta } ( X _ { t } ^ { \bar { \boldsymbol { u } } } , t ) \| ^ { 2 } \mathrm { d } t + \int _ { 0 } ^ { 1 } \nabla _ { \theta } u ( X _ { t } ^ { \bar { \boldsymbol { u } } } , t ) ^ { \top } \sigma ( t ) ^ { \top } a ( t ; \mathbf { X } ^ { \bar { \boldsymbol { u } } } , \bar { \boldsymbol { u } } ) \mathrm { d } t } \\ & { \qquad = \frac { 1 } { 2 } \int _ { 0 } ^ { 1 } \nabla _ { \theta } \| u _ { \theta } ( X _ { t } ^ { \bar { \boldsymbol { u } } } , t ) + \sigma ( t ) ^ { \top } a ( t ; \mathbf { X } ^ { \bar { \boldsymbol { u } } } , \bar { \boldsymbol { u } } ) \| ^ { 2 } \mathrm { d } t = \nabla _ { \theta } \mathcal { L } _ { \mathrm { B a s i c - A d j - M a t c h } } ( u _ { \theta } ; \mathbf { X } ^ { \bar { \boldsymbol { u } } } ) } \end{array} +$$ + +This proves the first statement of the proposition. To prove that the only critical point of the expected basic Adjoint Matching loss is the optimal control, we first compute the first variation of $\mathbb { E } [ \mathcal { L } _ { \mathrm { B a s i c - A d j - M a t c h } } ]$ . Letting $v : \mathbb { R } ^ { d } \times [ 0 , T ] \to \mathbb { R } ^ { d }$ be arbitrary, we have that + +$$ +\begin{array} { r l } & { \frac { \mathrm d } { \mathrm { d } \epsilon } \mathbb E [ \mathcal { L } _ { \mathrm { B a s i c - A d j - M a t c h } } ( u + \epsilon v ; X ^ { \bar { u } } ) ] = \frac { \mathrm d } { \mathrm { d } \epsilon } \mathbb E \big [ \frac { 1 } { 2 } \int _ { 0 } ^ { T } \| ( u + \epsilon v ) ( X _ { t } ^ { \bar { u } } , t ) + \sigma ( t ) ^ { \top } a ( t , X ^ { \bar { u } } , \bar { u } ) \| ^ { 2 } \mathrm { d } t \big ] } \\ & { = \mathbb E \big [ \int _ { 0 } ^ { T } \langle v ( X _ { t } ^ { \bar { u } } , t ) , u ( X _ { t } ^ { \bar { u } } , t ) + \sigma ( t ) ^ { \top } a ( t , X ^ { \bar { u } } , \bar { u } ) \rangle \mathrm { d } t \big ] } \\ & { = \mathbb E \big [ \int _ { 0 } ^ { T } \langle v ( X _ { t } ^ { \bar { u } } , t ) , u ( X _ { t } ^ { \bar { u } } , t ) + \sigma ( t ) ^ { \top } \mathbb E \big [ a ( t , X ^ { \bar { u } } , \bar { u } ) | X _ { t } ^ { \bar { u } } \big ] \rangle \mathrm { d } t \big ] } \\ & { \implies \frac { \delta } { \delta u } \mathbb E [ \mathcal L _ { \mathrm { B a s i c - A d j - M a t c h } } ( u ) ( x , t ) = u ( x , t ) + \mathbb E \big [ a ( t , X ^ { \bar { u } } , \bar { u } ) | X _ { t } ^ { \bar { u } } = x \big ] } \end{array} +$$ + +Hence, critical points satisfy that + +$$ +\begin{array} { r l } & { ( x , t ) = - \sigma ( t ) ^ { \top } \mathbb { E } [ a ( t , X ^ { u } , u ) | X _ { t } ^ { u } = x ] = - \sigma ( t ) ^ { \top } \mathbb { E } \big [ \nabla _ { X _ { t } ^ { v } } \int _ { t } ^ { T } \big ( \frac { 1 } { 2 } \| v ( X _ { t } ^ { v } , t ) \| ^ { 2 } + f ( X _ { t } ^ { v } , t ) \big ) \mathrm { d } t + g ( X _ { T } ^ { u } , t ) } \\ & { \qquad = - \sigma ( t ) ^ { \top } \nabla _ { x } \mathbb { E } \big [ \int _ { t } ^ { T } \big ( \frac { 1 } { 2 } \| v ( X _ { t } ^ { v } , t ) \| ^ { 2 } + f ( X _ { t } ^ { v } , t ) \big ) \mathrm { d } t + g ( X _ { T } ^ { v } ) | X _ { 0 } ^ { v } = x \big ] = - \sigma ( t ) ^ { \top } \nabla J ( u ; x , t ) , } \end{array} +$$ + +In this equation, the second equality holds by equation (184) from Lemma 5, and the third equality holds by the Leibniz rule. + +Lemma 6 shows that any control $u$ that satisfies (196) is equal to the optimal control, which concludes the proof. + +Lemma 6. Suppose that for any $x \in \mathbb { R } ^ { d }$ , $t \in [ 0 , T ]$ , $\boldsymbol { u } ( \boldsymbol { x } , t ) = - \sigma ( t ) ^ { \top } \nabla _ { \boldsymbol { x } } J ( \boldsymbol { u } ; \boldsymbol { x } , t )$ . Then, $J ( u ; \cdot , \cdot )$ satisfies the Hamilton-Jacobi-Bellman equation (156). By the uniqueness of the solution to the HJB equation, we have that $J ( u ; x , t ) = V ( x , t )$ for any $x \in \mathbb { R } ^ { d }$ , $t \in [ 0 , T ]$ . Hence, $u ( x , t ) = - \sigma ( t ) ^ { \top } \nabla _ { x } V ( x , t )$ is the optimal control. + +Proof. Since $\begin{array} { r } { J ( u ; x , t ) = \mathbb { E } \big [ \int _ { t } ^ { T ^ { \prime } } \big ( \frac { 1 } { 2 } \| u ( X _ { t } ^ { u } , t ) \| ^ { 2 } + f ( X _ { t } ^ { u } , t ) \big ) d s + g ( X _ { T } ^ { u } ) | X _ { t } ^ { u } = x \big ] } \end{array}$ , we have that + +$$ +\begin{array} { r } { J ( u ; x , t ) = \mathbb { E } \big [ J ( u ; X _ { t + \Delta t } ^ { u } , t + \Delta t ) | X _ { t } = x \big ] + \mathbb { E } \big [ \int _ { t } ^ { t + \Delta t } \big ( \frac 1 2 \| u ( X _ { s } ^ { u } , s ) \| ^ { 2 } + f ( X _ { s } ^ { u } , s ) \big ) d s | X _ { t } = x | \big ] } \end{array} +$$ + +which means that + +$$ +0 = \frac { \mathbb { E } [ J ( u ; X _ { t + \Delta t } ^ { u } , t + \Delta t ) | X _ { t } = x ] - J ( u ; x , t ) } { \Delta t } + \frac { \mathbb { E } \big [ \int _ { t } ^ { t + \Delta t } \big ( \frac 1 2 \| u ( X _ { t } ^ { u } , t ) \| ^ { 2 } + f ( X _ { t } ^ { u } , t ) \big ) d s | X _ { t } = x _ { t } ^ { u } - \Delta t \big ( \frac 1 2 \| u ( X _ { t } ^ { u } , t ) \| ^ { 2 } + f ( X _ { t } ^ { u } , t ) \big ) \big ] } { \Delta t } . +$$ + +Recall that the generator $\mathcal { T } ^ { u }$ of the controlled SDE (13) takes the form: + +$$ +\begin{array} { r l } & { \mathcal { T } ^ { u } f ( x , t ) : = \operatorname* { l i m } _ { \Delta t 0 } \frac { \mathbb { E } [ f ( X _ { t + \Delta t } ^ { u } , t ) | X _ { t } = x ] - f ( x , t ) } { \Delta t } } \\ & { \quad \quad \quad = \partial _ { t } f ( x , t ) + \langle \nabla f ( x , t ) , b ( x , t ) + \sigma ( t ) u ( x , t ) \rangle + \mathrm { T r } ( \frac { \sigma ( t ) \sigma ( t ) ^ { \top } } { 2 } \nabla ^ { 2 } f ( x , t ) ) } \end{array} +$$ + +Hence, if we take the limit $\Delta t \to 0$ on equation (198), we obtain that: + +$$ +\begin{array} { r l } & { = T ^ { u } J ( u ; x , t ) + \frac { 1 } { 2 } \| u ( x , t ) \| ^ { 2 } + f ( x , t ) } \\ & { = \partial _ { t } J ( u ; x , t ) + \langle \nabla J ( u ; x , t ) , b ( x , t ) + \sigma ( t ) u ( x , t ) \rangle + \mathrm { T r } \big ( \frac { \sigma ( t ) \sigma ( t ) ^ { \top } } { 2 } \nabla ^ { 2 } J ( u ; x , t ) \big ) + \frac { 1 } { 2 } \| u ( x , t ) \| ^ { 2 } + f _ { \varepsilon } ^ { 2 } . } \end{array} +$$ + +Now using that $\boldsymbol { u } ( \boldsymbol { x } , t ) = - \sigma ( t ) ^ { \mathrm { ~ l ~ } } \nabla _ { \boldsymbol { x } } J ( \boldsymbol { u } ; \boldsymbol { x } , t )$ , we have that + +$$ +\begin{array} { r l } & { \langle \nabla J ( u ; x , t ) , \sigma ( t ) u ( x , t ) \rangle + \frac { 1 } { 2 } \| u ( x , t ) \| ^ { 2 } = - \| \sigma ( t ) ^ { \top } \nabla _ { x } J ( u ; x , t ) \| ^ { 2 } + \frac { 1 } { 2 } \| \sigma ( t ) ^ { \top } \nabla _ { x } J ( u ; x , t ) \| ^ { 2 } } \\ & { \qquad = - \frac { 1 } { 2 } \| \sigma ( t ) ^ { \top } \nabla _ { x } J ( u ; x , t ) \| ^ { 2 } . } \end{array} +$$ + +Plugging this back into (200), we obtain that + +$$ +\begin{array} { r l } & { = \partial _ { t } J ( u ; x , t ) + \langle \nabla J ( u ; x , t ) , b ( x , t ) \rangle + \operatorname { T r } \big ( \frac { \sigma ( t ) \sigma ( t ) ^ { \top } } { 2 } \nabla ^ { 2 } J ( u ; x , t ) \big ) - \frac { 1 } { 2 } \| \sigma ( t ) ^ { \top } \nabla _ { x } J ( u ; x , t ) \| ^ { 2 } + f } \end{array} +$$ + +And since $J ( u ; x , T ) = g ( x )$ by construction, we conclude that $J ( u ; x , t )$ satisfies the HJB equation (156). + +# E.3 Theoretical guarantees of the Adjoint Matching loss + +Proposition 7 (Theoretical guarantee of the Adjoint Matching loss). The only critical point of the loss $\mathbb { E } [ \mathcal { L } _ { \mathrm { A d j - M a t c h } } ]$ is the optimal control $u ^ { * }$ . + +Proof. Let $v$ be an arbitrary control. If $\tilde { a } ( t ; \mathbf { X } ^ { v } )$ is the solution of the Lean Adjoint ODE (38)-(39), it satisfies the integral equation + +$$ +\begin{array} { r } { \tilde { a } ( t ; \mathbf { X } ^ { v } ) = \int _ { t } ^ { T } \left( \nabla _ { x } b ( X _ { s } ^ { v } , s ) ^ { \top } \tilde { a } ( s ; \mathbf { X } ^ { v } ) + \nabla _ { x } f ( X _ { s } ^ { v } , s ) \right) \mathrm { d } s + \nabla g ( X _ { T } ^ { v } ) . } \end{array} +$$ + +Hence, + +$$ +\begin{array} { r l } & { \mathbb { E } \big [ \tilde { a } ( t ; \mathbf { X } ^ { v } ) \big | X _ { t } ^ { v } \big ] = \mathbb { E } \big [ \int _ { t } ^ { T } \big ( \nabla _ { x } b ( X _ { s } ^ { v } , s ) ^ { \top } \tilde { a } ( s ; \mathbf { X } ^ { v } ) + \nabla _ { x } f ( X _ { s } ^ { v } , s ) \big ) \mathrm { d } s + \nabla g ( X _ { T } ^ { v } ) \big | X _ { t } ^ { v } \big ] } \\ & { \qquad = \mathbb { E } \big [ \int _ { t } ^ { T } \big ( \nabla _ { x } b ( X _ { s } ^ { v } , s ) ^ { \top } \mathbb { E } \big [ \tilde { a } ( s ; \mathbf { X } ^ { v } ) \big | X _ { s } ^ { v } \big ] + \nabla _ { x } f ( X _ { s } ^ { v } , s ) \big ) \mathrm { d } s + \nabla g ( X _ { T } ^ { v } ) \big | X _ { t } ^ { v } \big ] , } \end{array} +$$ + +where we used the tower property of conditional expectation in the second equality. + +Similarly, if $a \left( t ; \mathbf { X } ^ { v } , v \right)$ is the solution of the Adjoint ODE (30)-(31), it satisfies the integral equation + +$$ +\begin{array} { r } { t ; \mathbf { X } ^ { v } , v ) = \int _ { t } ^ { T } \left( \nabla _ { x } \left( b ( X _ { s } ^ { v } , s ) ^ { \top } a ( s ; \mathbf { X } ^ { v } , v ) + \sigma ( s ) v ( X _ { s } ^ { v } , s ) \right) + \nabla _ { x } \left( f ( X _ { s } ^ { v } , s ) + \frac { 1 } { 2 } \| v ( X _ { s } ^ { v } , s ) \| ^ { 2 } \right) \right) \mathrm { d } s } \end{array} +$$ + +and its expected value satisfies + +$$ +\begin{array} { r l } & { a ( t ; \mathbf { X } ^ { v } , v ) \big | X _ { t } ^ { v } \big | } \\ & { \mathbb { E } \big [ \int _ { t } ^ { T } \big ( \nabla _ { x } \big ( b ( X _ { s } ^ { v } , s ) + \sigma ( s ) v ( X _ { s } ^ { v } , s ) \big ) ^ { \top } a ( s ; \mathbf { X } ^ { v } , v ) + \nabla _ { x } \big ( f ( X _ { s } ^ { v } , s ) + \frac { 1 } { 2 } \| v ( X _ { s } ^ { v } , s ) \| ^ { 2 } \big ) \big ) \mathrm { d } s + \nabla g ( X _ { s } ^ { v } , s ) } \\ & { \mathbb { E } \big [ \int _ { t } ^ { T } \big ( \nabla _ { x } \big ( b ( X _ { s } ^ { v } , s ) + \sigma ( s ) v ( X _ { s } ^ { v } , s ) \big ) ^ { \top } \mathbb { E } \big [ a ( s ; \mathbf { X } ^ { v } , v ) \big | X _ { s } ^ { v } \big ] + \nabla _ { x } \big ( f ( X _ { s } ^ { v } , s ) + \frac { 1 } { 2 } \| v ( X _ { s } ^ { v } , s ) \| ^ { 2 } \big ) \big ) \mathrm { d } s + \nabla } \end{array} +$$ + +Let us rewrite $\mathbb { E } [ \mathcal { L } _ { \mathrm { A d j - M a t c h } } ]$ as follows: + +$$ +\begin{array} { r l } & { \mathbb { E } [ \mathcal { L } _ { \mathrm { A d j - M a t c h } } ( u ) ] : = \mathbb { E } \left[ \int _ { 0 } ^ { T } \left\| u ( X _ { t } ^ { v } , t ) + \sigma ( t ) ^ { \top } \mathbb { E } \big [ \tilde { a } ( t , \mathbf { X } ^ { v } ) | X _ { t } ^ { v } \big ] \right\| ^ { 2 } \mathrm { d } t \right] | _ { v = \mathrm { s t o p g r a d } ( u ) } } \\ & { \qquad + \mathbb { E } \big [ \int _ { 0 } ^ { T } \left\| \sigma ( t ) ^ { \top } \big ( \mathbb { E } \big [ \tilde { a } ( t , \mathbf { X } ^ { v } ) | X _ { t } ^ { v } \big ] - \tilde { a } ( t , \mathbf { X } ^ { v } ) \big ) \right\| ^ { 2 } \mathrm { d } t \big ] | _ { v = \mathrm { s t o p g r a d } ( u ) } , } \end{array} +$$ + +Now, suppose that $\hat { u }$ is a critical point of $\mathbb { E } [ \mathcal { L } _ { \mathrm { A d j - M a t c h } } ]$ . By definition, this implies that the first variation of $\mathbb { E } [ \mathcal { L } _ { \mathrm { A d j - M a t c h } } ]$ is zero. Using (207), we can write this as follows: + +$$ +\begin{array} { r l } & { 0 = \frac { \delta } { \delta u } \mathbb { E } [ \mathcal { L } _ { \mathrm { A d j - M a t c h } } ( \hat { u } ) ] ( x ) = 2 \big ( \hat { u } ( x , t ) + \sigma ( t ) ^ { \top } \mathbb { E } [ \tilde { a } ( t , \mathbf { X } ^ { \hat { u } } ) | X _ { t } ^ { \hat { u } } = x ] \big ) , } \\ & { \quad \implies \hat { u } ( x , t ) = - \sigma ( t ) ^ { \top } \mathbb { E } [ \tilde { a } ( t , \mathbf { X } ^ { \hat { u } } ) | X _ { t } ^ { \hat { u } } = x ] . } \end{array} +$$ + +Hence, we have + +$$ +\begin{array} { r l } & { \nabla _ { x } \widehat { u } ( X _ { t } ^ { \widehat { n } } , t ) ^ { \top } \sigma ( t ) ^ { \top } \mathbb { E } [ \widetilde { a } ( t , { \mathbf X } ^ { \widehat { n } } ) | X _ { t } ^ { \widehat { n } } ] + \nabla _ { x } \widehat { u } ( X _ { t } ^ { \widehat { n } } , t ) ^ { \top } \widehat { u } ( X _ { t } ^ { \widehat { n } } , t ) = 0 , } \\ & { \quad \implies \mathbb { E } \big [ \int _ { t } ^ { T } \big ( \nabla _ { x } \big ( \sigma ( s ) \widehat { u } ( X _ { s } ^ { \widehat { n } } , s ) \big ) ^ { \top } \mathbb { E } \big [ \widetilde { a } ( s ; { \mathbf X } ^ { \widehat { n } } ) \big | X _ { s } ^ { \widehat { n } } \big ] + \nabla _ { x } \big ( \frac { 1 } { 2 } \| \widehat { u } ( X _ { s } ^ { \widehat { n } } , s ) \| ^ { 2 } \big ) \big ) \mathrm { d } s \big | X _ { t } ^ { \widehat { n } } \big ] = 0 . } \end{array} +$$ + +If we set $v = { \hat { u } }$ in equation (204), and add (211) to its right-hand side, we obtain that $\mathbb { E } [ \tilde { a } ( t , X ^ { \hat { u } } ) | X _ { t } ^ { \hat { u } } ]$ also solves the integral equation + +$$ +\begin{array} { r l } & { \lvert \tilde { a } ( t ; \mathbf { X } ^ { \hat { u } } ) \rvert X _ { t } ^ { \hat { u } } \rvert } \\ & { \mathbb { E } \big [ \int _ { t } ^ { T } \big ( \nabla _ { x } \big ( b ( X _ { s } ^ { \hat { u } } , s ) + \sigma ( s ) \hat { u } ( X _ { s } ^ { \hat { u } } , s ) \big ) ^ { \top } \mathbb { E } \big [ \tilde { a } ( s ; \mathbf { X } ^ { \hat { u } } ) \big \vert X _ { s } ^ { \hat { u } } \big ] + \nabla _ { x } \big ( f ( X _ { s } ^ { \hat { u } } , s ) + \frac { 1 } { 2 } \| \hat { u } ( X _ { s } ^ { \hat { u } } , s ) \| ^ { 2 } \big ) \big ) \mathrm { d } s + \nabla _ { \xi } } \end{array} +$$ + +Note that this integral equation is the same one as equation (206) when we set $\ v \ = \ \hat { u }$ in the latter. Proposition 8 states that the solution of the integral equation is unique, which means that $\mathbb { E } \big [ \tilde { a } ( t ; \mathbf { X } ^ { \hat { u } } ) \big | X _ { t } ^ { \hat { u } } \big ] =$ $\mathbb { E } \big [ a ( t ; \mathbf { X } ^ { \hat { u } } , \hat { u } ) \big | X _ { t } ^ { \hat { u } } \big ]$ for all $t \in [ 0 , T ]$ . + +Since we can reexpress the basic Adjoint Matching loss as + +$$ +\begin{array} { r l } & { [ \mathcal { L } _ { \mathrm { B a s i c - A d j - M a t c h } } ( u ) ] : = \mathbb { E } [ \int _ { 0 } ^ { T } \| u ( X _ { t } ^ { v } , t ) + \sigma ( t ) ^ { \top } \mathbb { E } \big [ a ( t ; \mathbf { X } ^ { v } , v ) | X _ { t } ^ { v } \big ] \| ^ { 2 } \mathrm { d } t \big ] | _ { v = \mathrm { s t o p g r a d } ( u ) } } \\ & { \qquad + \mathbb { E } [ \int _ { 0 } ^ { T } \| \sigma ( t ) ^ { \top } \big ( \mathbb { E } \big [ a ( t ; \mathbf { X } ^ { v } , v ) | X _ { t } ^ { v } \big ] - a ( t ; \mathbf { X } ^ { v } , v ) \big ) \| ^ { 2 } \mathrm { d } t ] | _ { v = \mathrm { s t o p g r a d } ( u ) } , } \end{array} +$$ + +we obtain that when $\hat { u }$ is a critical point of $\mathbb { E } [ \mathcal { L } _ { \mathrm { A d j - M a t c h } } ]$ , + +$$ +\begin{array} { r l } & { \frac { \mathrm { d } } { \mathrm { d } u } \mathbb { E } \big [ \mathcal { L } _ { \mathrm { B a s i c - A d j - M a t c h } } ( \hat { u } ) \big ] ( x ) = 2 \big ( \hat { u } ( x , t ) + \sigma ( t ) ^ { \top } \mathbb { E } [ a ( t ; \mathbf { X } ^ { \hat { u } } , \hat { u } ) | X _ { t } ^ { \hat { u } } = x ] \big ) } \\ & { \qquad = 2 \big ( \hat { u } ( x , t ) + \sigma ( t ) ^ { \top } \mathbb { E } [ \tilde { a } ( t ; \mathbf { X } ^ { \hat { u } } ) | X _ { t } ^ { \hat { u } } = x ] \big ) = 0 , } \end{array} +$$ + +where the second equality holds because $\mathbb { E } \big [ \tilde { a } ( t ; \mathbf { X } ^ { \hat { a } } ) \big | X _ { t } ^ { \hat { a } } \big ] \ = \ \mathbb { E } \big [ a ( t ; \mathbf { X } ^ { \hat { a } } , \hat { u } ) \big | X _ { t } ^ { \hat { a } } \big ]$ , and the third equality holds by equation (209). Thus, we deduce that the critical points of $\mathbb { E } [ \mathcal { L } _ { \mathrm { A d j - M a t c h } } ]$ are critical points of $\mathbb { E } [ \mathcal { L } _ { \mathrm { B a s i c - A d j - M a t c h } } ]$ . By Proposition 2, $\mathbb { E } [ \mathcal { L } _ { \mathrm { B a s i c - A d j - M a t c h } } ]$ has a single critical point, which is the optimal control $u ^ { * }$ , which concludes the proof of the statement for $\mathbb { E } [ \mathcal { L } _ { \mathrm { A d j - M a t c h } } ]$ . □ + +Proposition 8. Let $v$ be an arbitrary control. Consider the integral equation: + +$$ +\begin{array} { r } { \displaybreaks _ { t } = \mathbb { E } \Big [ \int _ { t } ^ { T } \left( \nabla _ { x } \left( b ( X _ { s } ^ { v } , s ) + \sigma ( s ) v ( X _ { s } ^ { v } , s ) \right) ^ { \top } Y _ { s } + \nabla _ { x } \left( f ( X _ { s } ^ { v } , s ) + \frac { 1 } { 2 } \| v ( X _ { s } ^ { v } , s ) \| ^ { 2 } \right) \right) \mathrm { d } s + \nabla g ( X _ { T } ^ { v } ) \Big | X _ { t } ^ { v } \Big ] , } \end{array} +$$ + +where $t \in [ 0 , T ]$ . This equation has a unique solution, i.e. if $Y ^ { 1 }$ , $Y ^ { 2 }$ are two solutions then $Y _ { 1 } = Y _ { 2 }$ + +Proof. Let $Y ^ { 1 }$ , $Y ^ { 2 }$ be two solutions of the integral equation. We have that + +$$ +\begin{array} { r } { Y _ { t } ^ { 1 } - Y _ { t } ^ { 2 } = \mathbb { E } \big [ \int _ { t } ^ { T } \big ( ( Y _ { s } ^ { 1 } - Y _ { s } ^ { 2 } ) ^ { \top } \nabla _ { x } b ( X _ { s } ^ { * } , s ) \big ) \mathrm { d } s \big | X _ { t } ^ { * } \big ] . } \end{array} +$$ + +Thus, + +$$ +\begin{array} { r l } & { \| Y _ { t } ^ { 1 } - Y _ { t } ^ { 2 } \| } \\ & { \leq \mathbb { E } \big [ \big \| \int _ { t } ^ { T } \big ( ( Y _ { s } ^ { 1 } - Y _ { s } ^ { 2 } ) ^ { \top } \nabla _ { x } b ( X _ { s } ^ { * } , s ) \big ) \mathrm { d } s \big \| \big | X _ { t } ^ { * } \big ] \leq \mathbb { E } \big [ \int _ { t } ^ { T } \big \| \big ( ( Y _ { s } ^ { 1 } - Y _ { s } ^ { 2 } ) ^ { \top } \nabla _ { x } b ( X _ { s } ^ { * } , s ) \big ) \big \| \mathrm { d } s \big | X _ { t } ^ { * } \big ] } \\ & { \leq \mathbb { E } \big [ \int _ { t } ^ { T } \big \| Y _ { s } ^ { 1 } - Y _ { s } ^ { 2 } \big \| \cdot \big \| \nabla _ { x } b ( X _ { s } ^ { * } , s ) \big ) \big \| \mathrm { d } s \big | X _ { t } ^ { * } \big ] = \int _ { t } ^ { T } \mathbb { E } \big [ \big \| Y _ { s } ^ { 1 } - Y _ { s } ^ { 2 } \big \| \cdot \big \| \nabla _ { x } b ( X _ { s } ^ { * } , s ) \big ) \big \| \big | X _ { t } ^ { * } \big ] \mathrm { d } s } \\ & { \leq \int _ { t } ^ { T } \big ( \mathbb { E } \big [ \big \| Y _ { s } ^ { 1 } - Y _ { s } ^ { 2 } \big \| ^ { 2 } \big | X _ { t } ^ { * } \big ] \big ) ^ { 1 / 2 } \cdot \big ( \mathbb { E } \big [ \big \| \nabla _ { x } b ( X _ { s } ^ { * } , s ) \big \| ^ { 2 } \big | X _ { t } ^ { * } \big ] \big ) ^ { 1 / 2 } \mathrm { d } s } \end{array} +$$ + +And this implies that + +$$ +\begin{array} { r l } & { \operatorname* { s u p } _ { t ^ { \prime } \in [ 0 , t ] } \left( \mathbb { E } [ \| Y _ { t } ^ { 1 } - Y _ { t } ^ { 2 } \| ^ { 2 } | X _ { t ^ { \prime } } ^ { * } ] \right) ^ { 1 / 2 } } \\ & { \leq \int _ { t } ^ { T } \left( \mathbb { E } \big [ \big \| Y _ { s } ^ { 1 } - Y _ { s } ^ { 2 } \big \| ^ { 2 } \big | X _ { t } ^ { * } \big ] \right) ^ { 1 / 2 } \cdot \big ( \mathbb { E } \big [ \big \| \nabla _ { x } b ( X _ { s } ^ { * } , s ) \big \| ^ { 2 } \big | X _ { t } ^ { * } \big ] \big ) ^ { 1 / 2 } \mathrm { d } s } \\ & { \leq \int _ { t } ^ { T } \operatorname* { s u p } _ { t ^ { \prime } \in [ 0 , s ] } \big ( \mathbb { E } \big [ \big \| Y _ { s } ^ { 1 } - Y _ { s } ^ { 2 } \big \| ^ { 2 } \big | X _ { t ^ { \prime } } ^ { * } \big ] \big ) ^ { 1 / 2 } \cdot \operatorname* { s u p } _ { t ^ { \prime } \in [ 0 , s ] } \big ( \mathbb { E } \big [ \big \| \nabla _ { x } b ( X _ { s } ^ { * } , s ) \big \| ^ { 2 } \big | X _ { t ^ { \prime } } ^ { * } \big ] \big ) ^ { 1 / 2 } \mathrm { d } s . } \end{array} +$$ + +Applying Grönwall’s inequality on the function $\begin{array} { r } { f ( t ) = \operatorname* { s u p } _ { t ^ { \prime } \in [ 0 , t ] } \left( \mathbb { E } [ \| Y _ { t } ^ { 1 } - Y _ { t } ^ { 2 } \| ^ { 2 } | X _ { t ^ { \prime } } ^ { * } ] \right) ^ { 1 / 2 } } \end{array}$ , we obtain that $\begin{array} { r } { \operatorname* { s u p } _ { t ^ { \prime } \in [ 0 , t ] } \left( \mathbb { E } [ \| Y _ { t } ^ { 1 } - Y _ { t } ^ { 2 } \| ^ { 2 } | X _ { t ^ { \prime } } ^ { * } ] \right) ^ { 1 / 2 } = 0 } \end{array}$ for all $t \in [ 0 , T ]$ , which means that $Y _ { t } ^ { 1 } = Y _ { t } ^ { 2 }$ almost surely. And since $\begin{array} { r } { \| Y _ { t } ^ { 1 } - Y _ { t } ^ { 2 } \| \leq \int _ { t } ^ { T ^ { \prime } } \big ( \mathbb { E } \big [ \big \| Y _ { s } ^ { 1 } - Y _ { s } ^ { 2 } \big \| ^ { 2 } | X _ { t } ^ { * } \big ] \big ) ^ { 1 / 2 } } \end{array}$ · $\big ( \mathbb { E } \big [ \big \| \nabla _ { x } b ( X _ { s } ^ { * } , s ) \big \| ^ { 2 } | X _ { t } ^ { * } \big ] \big ) ^ { 1 / 2 } { \mathrm { d } } s = 0$ , we obtain that $Y ^ { 1 } = Y ^ { 2 }$ . + +# E.4 Pseudo-code of Adjoint Matching for DDIM fine-tuning + +Note that for each pair of equations (219)-(220), (221)-(222), (223)-(224), the first equation corresponds to the updates in the DDPM paper, while the second equation is an Euler-Maruyama / Euler discretization of the continuous-time object. To check that both discretizations are equal up to first order, remark that + +$$ +\begin{array} { r } { \sqrt { \frac { \bar { \alpha } _ { k + 1 } } { \bar { \alpha } _ { k } } } = \sqrt { 1 + \frac { \bar { \alpha } _ { k + 1 } - \bar { \alpha } _ { k } } { \bar { \alpha } _ { k } } } \approx 1 + \frac { \bar { \alpha } _ { k + 1 } - \bar { \alpha } _ { k } } { 2 \bar { \alpha } _ { k } } + O ( ( \bar { \alpha } _ { k + 1 } - \bar { \alpha } _ { k } ) ^ { 2 } ) . } \end{array} +$$ + +Input: Pre-trained denoiser ϵbase, number of fine-tuning iterations $N$ . Initialize fine-tuned denoiser: $\epsilon ^ { \mathrm { f i n e t u n e } } = \epsilon ^ { \mathrm { b a s e } }$ with parameters $\theta$ . for $n \in \{ 0 , \ldots , N - 1 \}$ do + +Sample $m$ trajectories $\pmb { X } = ( X _ { t } ) _ { t \in \{ 0 , \ldots , 1 \} }$ according to DDPM, e.g.: + +$$ +\begin{array} { r } { \mathrm { { V } } _ { k + 1 } = \sqrt { \frac { \bar { \alpha } _ { k + 1 } } { \bar { \alpha } _ { k } } \left( X _ { k } - \frac { 1 - \bar { \alpha } _ { k } / \bar { \alpha } _ { k + 1 } } { \sqrt { 1 - \bar { \alpha } _ { k } } } \epsilon ^ { \mathrm { f i n e t u m e } } ( X _ { k } , k ) \right) + \sqrt { \frac { 1 - \bar { \alpha } _ { k + 1 } } { 1 - \bar { \alpha } _ { k } } \left( 1 - \frac { \bar { \alpha } _ { k } } { \bar { \alpha } _ { k + 1 } } \right) } \varepsilon _ { k } } , \quad \varepsilon _ { k } \sim \mathcal { N } ( 0 , I ) , \ X _ { 0 } \sim \mathcal { N } ( 0 , I ) , } \end{array} +$$ + +$$ +\begin{array} { r } { X _ { k + 1 } = X _ { k } + \frac { { \bar { \alpha } _ { k + 1 } } - { { \bar { \alpha } } _ { k } } } { 2 { { \bar { \alpha } } _ { k } } } X _ { k } - \frac { { { \bar { \alpha } } _ { k + 1 } } - { { \bar { \alpha } } _ { k } } } { { { \bar { \alpha } } _ { k } } \sqrt { 1 - { { \bar { \alpha } } _ { k } } } } \epsilon ^ { \mathrm { f i n e t u n e } } ( X _ { k } , k ) + \sqrt { \frac { { { \bar { \alpha } } _ { k + 1 } } - { { { \bar { \alpha } } _ { k } } } } { { { \bar { \alpha } } _ { k } } } } \varepsilon _ { k } . } \end{array} +$$ + +For each trajectory, solve the lean adjoint ODE (38)-(39) backwards in time from $k = K$ to $_ 0$ , e.g.: + +$$ +\begin{array} { r l } & { \quad \tilde { a } _ { k } = \tilde { a } _ { k + 1 } + \tilde { a } _ { k + 1 } ^ { \top } \nabla x _ { k } \left( \sqrt { \frac { \tilde { a } _ { k + 1 } } { \tilde { a } _ { k } } } \big ( X _ { k } - \frac { 1 - \tilde { a } _ { k } / \tilde { a } _ { k + 1 } } { \sqrt { 1 - \tilde { a } _ { k } } } \epsilon ^ { \mathrm { b a s e } } ( X _ { k } , k ) \big ) - X _ { k } \right) , \qquad \tilde { a } _ { K } = \nabla x _ { K } r ( X _ { K } ) , } \\ & { \mathrm { o r } \tilde { a } _ { k } = \tilde { a } _ { k + 1 } + \tilde { a } _ { k + 1 } ^ { \top } \nabla _ { X _ { t } } \left( \frac { \tilde { a } _ { k + 1 } - \tilde { a } _ { k } } { 2 \tilde { a } _ { k } } X _ { k } - \frac { \tilde { a } _ { k + 1 } - \tilde { a } _ { k } } { \tilde { a } _ { k } \sqrt { 1 - \tilde { a } _ { k } } } \epsilon ^ { \mathrm { b a s e } } ( X _ { k } , k ) \right) , \qquad \tilde { a } _ { K } = \nabla x _ { K } r ( X _ { K } ) . } \end{array} +$$ + +Note that $X _ { k }$ and $\tilde { a } _ { k }$ should be computed without gradients, i.e., $X _ { k } = \tt s t o p g r a d ( X _ { k } )$ , $\tilde { \boldsymbol { a } } _ { k } = \mathsf { s t o p g r a d } ( \tilde { \boldsymbol { a } } _ { k } )$ . + +For each trajectory, compute the Adjoint Matching objective (37): + +$$ +\begin{array} { r l } & { \mathcal { L } _ { \mathrm { A d j - M a t c h } } ( \theta ) = \sum _ { k \in \{ 0 , \dots , K - 1 \} } \Big \| \sqrt { \frac { \bar { \alpha } _ { k + 1 } } { \bar { \alpha } _ { k } ( 1 - \bar { \alpha } _ { k + 1 } ) } \big ( 1 - \frac { \bar { \alpha } _ { k } } { \bar { \alpha } _ { k + 1 } } \big ) } ( \epsilon ^ { \mathrm { f u n e t u n e } } ( X _ { k } , k ) - \epsilon ^ { \mathrm { b a s e } } ( X _ { k } , k ) ) } \\ & { \qquad - \sqrt { \frac { 1 - \bar { \alpha } _ { k + 1 } } { 1 - \bar { \alpha } _ { k } } \big ( 1 - \frac { \bar { \alpha } _ { k } } { \bar { \alpha } _ { k + 1 } } \big ) } \tilde { u } _ { k } \Big \| ^ { 2 } , } \\ & { \gamma \mathcal { L } _ { \mathrm { A d j - M a t c h } } ( \theta ) = \sum _ { k \in \{ 0 , \dots , K - 1 \} } \Big \| \sqrt { \frac { \bar { \alpha } _ { k + 1 } - \bar { \alpha } _ { k } } { \bar { \alpha } _ { k } ( 1 - \bar { \alpha } _ { k } ) } } ( \epsilon ^ { \mathrm { f u n e t u n e } } ( X _ { k } , k ) - \epsilon ^ { \mathrm { b a s e } } ( X _ { k } , k ) ) - \sqrt { \frac { \bar { \alpha } _ { k + 1 } - \bar { \alpha } _ { k } } { \bar { \alpha } _ { k } } } \tilde { u } _ { k } \Big \| ^ { 2 } . } \end{array} +$$ + +Compute the gradient $\nabla _ { \boldsymbol { \theta } } \mathcal { L } ( \boldsymbol { \theta } )$ and update $\theta$ using favorite gradient descent algorithm. end + +Output: Fine-tuned vector field $v$ finetune + +# F Adapting diffusion fine-tuning baselines to flow matching + +F.1 Adapting ReFL (Xu et al., 2023) to flow matching + +Reward Feedback Learning (ReFL) is a diffusion fine-tuning algorithm introduced by Xu et al. (2023) which tries to increase the reward on denoised samples. Namely, if $\pmb { X } = ( X _ { t } ) _ { t \in [ 0 , 1 ] }$ is the solution of the DDPM SDE (7), we can denoise $X _ { t }$ as + +$$ +\begin{array} { r } { \hat { X } _ { 1 } ( X _ { t } ) = \frac { X _ { t } - \sqrt { 1 - \bar { \alpha } _ { t } } \epsilon ( X _ { t } , t ) } { \sqrt { \bar { \alpha } _ { t } } } . } \end{array} +$$ + +This equation follows from the stochastic interpolant equation (2) if we replace $X _ { 0 }$ with the noise predictor $\epsilon ( X _ { t } , t )$ . And then, the ReFL optimization update is based on the gradient: + +$$ +\begin{array} { r } { \nabla _ { \theta } r \big ( \hat { X } _ { 1 } ( X _ { t } ) \big ) = \nabla _ { \theta } r \Big ( \frac { X _ { t } - \sqrt { 1 - \bar { \alpha } _ { t } } \epsilon _ { \theta } ( X _ { t } , t ) } { \sqrt { \bar { \alpha } _ { t } } } \Big ) , } \end{array} +$$ + +where the trajectories have been detached. + +To adapt ReFL to Flow Matching, we need to express the denoiser map in terms of the vector field $\boldsymbol { v }$ . We have that + +$$ +\begin{array} { r l } & { \boldsymbol { \nu } ( x , t ) = \mathbb { E } \big [ \dot { \beta } _ { t } \bar { X } _ { 0 } + \dot { \alpha } _ { t } \bar { X } _ { 1 } \big | \beta _ { t } \bar { X } _ { 0 } + \alpha _ { t } \bar { X } _ { 1 } = x \big ] = \mathbb { E } \big [ \frac { \dot { \beta } _ { t } } { \beta _ { t } } \big ( \beta _ { t } \bar { X } _ { 0 } + \alpha _ { t } \bar { X } _ { 1 } \big ) + \big ( \dot { \alpha } _ { t } - \frac { \dot { \beta } _ { t } } { \beta _ { t } } \alpha _ { t } \big ) \bar { X } _ { 1 } \big | \beta _ { t } \bar { X } _ { 0 } + \alpha _ { t } \bar { X } _ { 1 } \big ] , } \\ & { \mathrm { ~ \ ~ \ } = \frac { \dot { \beta } _ { t } } { \beta _ { t } } x + \big ( \dot { \alpha } _ { t } - \frac { \dot { \beta } _ { t } } { \beta _ { t } } \alpha _ { t } \big ) \hat { X } _ { 1 } ( x , t ) . } \end{array} +$$ + +where we defined the denoiser map $\hat { X } _ { 1 } ( x , t ) : = \mathbb { E } \big [ \bar { X } _ { 1 } | \beta _ { t } \bar { X } _ { 0 } + \alpha _ { t } \bar { X } _ { 1 } = x \big ]$ . Hence, + +$$ +\begin{array} { r } { \hat { X } _ { 1 } ( x , t ) = \frac { v ( x , t ) - \frac { \dot { \beta } _ { t } } { \beta _ { t } } x } { \dot { \alpha } _ { t } - \frac { \dot { \beta } _ { t } } { \beta _ { t } } \alpha _ { t } } . } \end{array} +$$ + +# F.2 Adapting Diffusion-DPO (Wallace et al., 2023a) to flow matching + +The Diffusion-DPO loss assumes access to ranked pairs of generated samples $x _ { 1 } ^ { w } \succ x _ { 1 } ^ { l }$ , where $x ^ { w }$ and $x ^ { l }$ are the winning and losing samples. For DDPM, the loss implemented in practice reads (Wallace et al., 2023a, Eq. 46): + +$$ +\begin{array} { r l } & { L _ { \mathrm { D P O } } ( \theta ) = - \mathbb { E } _ { ( x _ { 1 } ^ { w } , x _ { 1 } ^ { l } ) \sim \mathcal { D } , k \sim U [ 0 , K ] , x _ { k h } ^ { w } \sim q ( x _ { k h } ^ { w } | x _ { 1 } ^ { w } ) , x _ { t } ^ { l } \sim q ( x _ { k h } ^ { l } | x _ { 1 } ^ { l } ) } \Big [ } \\ & { \qquad \log S \big ( - \frac { \tilde { \beta } } { 2 } \big ( \| \varepsilon ^ { w } - \epsilon _ { \theta } ( x _ { k h } ^ { w } , k h ) \| ^ { 2 } - \| \varepsilon ^ { w } - \epsilon _ { \mathrm { r e f } } ( x _ { k h } ^ { w } , k h ) \| ^ { 2 } } \\ & { \qquad - \left( \| \varepsilon ^ { l } - \epsilon _ { \theta } ( x _ { k h } ^ { l } , k h ) \| ^ { 2 } - \| \varepsilon ^ { l } - \epsilon _ { \mathrm { r e f } } ( x _ { k h } ^ { l } , k h ) \| ^ { 2 } \right) \big ) \big ] , } \end{array} +$$ + +where $\begin{array} { r } { S ( x ) = \frac { 1 } { 1 + e ^ { - x } } } \end{array}$ denotes the sigmoid function, and √ √ $q ( x _ { k h } ^ { * } | x _ { 1 } ^ { * } )$ is the conditional distribution of the forward process, i.e. Diffusion-DPO loss in (Wallace et al., 2023a, Sec. S4), we observe that the term is sampled as , $\epsilon \sim N ( 0 , I )$ . Following the derivation of the $\begin{array} { r } { - \frac { \bar { \beta } } { 2 } \| \varepsilon ^ { w } - \epsilon _ { \theta } ( x _ { \boldsymbol { k } h } ^ { w } , \boldsymbol { k } h ) \| ^ { 2 } } \end{array}$ arises from + +$$ +- \frac { \tilde { \beta } } { 2 \frac { 1 - \gamma _ { k h } } { \gamma _ { k h } } } \| \hat { x } _ { 1 } ( x _ { k h } ^ { w } ) - x _ { 1 } ^ { w } \| ^ { 2 } , +$$ + +up to a constant term in $\theta$ . If we switch to the more general flow matching scheme, the analog of this term is + +$$ +\begin{array} { r } { - \frac { \tilde { \beta } } { 2 \frac { \beta _ { k h } ^ { 2 } } { \alpha _ { k h } ^ { 2 } } } \| \hat { x } _ { 1 } ( x _ { k h } ^ { w } ) - x _ { 1 } ^ { w } \| ^ { 2 } . } \end{array} +$$ + +Using the expression of the denoiser map in terms of the vector field $v$ in equation (229), we can rewrite (232) as: + +$$ +\begin{array} { r l } & { - \frac { \tilde { \beta } } { 2 \frac { \beta _ { k h } ^ { 2 } } { \alpha _ { k h } ^ { 2 } } } \Big \| \frac { v ( x _ { k h } ^ { w } , k h ) - \frac { \tilde { \beta } _ { k h } } { \beta _ { k h } } x _ { k h } ^ { w } } { \dot { \alpha } _ { k h } - \frac { \tilde { \beta } _ { k h } } { \beta _ { k h } } \alpha _ { k h } } - x _ { 1 } ^ { w } \Big \| ^ { 2 } = - \frac { \tilde { \beta } } { 2 } \Big \| \frac { v ( x _ { k h } ^ { w } , k h ) - \frac { \tilde { \beta } _ { k h } } { \beta _ { k h } } x _ { k h } ^ { w } } { \frac { \tilde { \alpha } _ { k h } } { \alpha _ { k h } } \beta _ { k h } - \tilde { \beta } _ { k h } } - \frac { \alpha _ { k h } } { \beta _ { k h } } x _ { 1 } ^ { w } \Big \| ^ { 2 } . } \end{array} +$$ + +Thus, the Diffusion-DPO loss for Flow Matching reads + +$$ +\begin{array} { r l } & { L _ { \mathrm { D P O } } ( \theta ) = - \mathbb { E } _ { ( x _ { 1 } ^ { w } , x _ { 1 } ^ { l } ) \sim \mathcal { D } , k \sim \mathcal { U } [ 0 , K ] , x _ { k } ^ { w } \sim \mathcal { G } ( x _ { k k } ^ { w } \mid x _ { 1 } ^ { w } ) , x _ { t } ^ { l } \sim \mathcal { G } ( x _ { k h } ^ { l } \mid x _ { 1 } ^ { l } ) } \Big [ } \\ & { \quad \quad \quad \quad \quad \log S \big ( - \frac { \hat { \beta } } { 2 } \big ( \big \| \frac { v _ { \theta } ( x _ { k h } ^ { w } , k h ) - \frac { \hat { \beta } _ { k h } } { \mathcal { \beta } _ { k h } } x _ { k h } ^ { w } } { \frac { \hat { \alpha } _ { k h } } { \hat { \alpha } _ { k h } } \hat { \beta } _ { k h } - \hat { \beta } _ { k h } } - \frac { \alpha _ { k h } } { \hat { \beta } _ { k h } } x _ { 1 } ^ { w } \big \| ^ { 2 } - \big \| \frac { v _ { \mathrm { r e f } } ( x _ { k h } ^ { w } , k h ) - \frac { \hat { \beta } _ { k h } } { \hat { \beta } _ { k h } } x _ { k h } ^ { w } } { \frac { \hat { \alpha } _ { k h } } { \hat { \alpha } _ { k h } } \hat { \beta } _ { k h } - \hat { \beta } _ { k h } } - \frac { \alpha _ { k h } } { \hat { \beta } _ { k h } } x _ { 1 } ^ { w } \big \| ^ { 2 } } \\ & \quad \quad \quad \quad \quad \quad \quad - \big ( \big \| \frac { v _ { \theta } ( x _ { k h } ^ { l } , k h ) - \frac { \hat { \beta } _ { k h } } { \hat { \beta } _ { k h } } x _ { k h } ^ { l } } { \frac { \hat { \alpha } _ { k h } } { \hat { \alpha } _ { k h } } \hat { \beta } _ { k h } - \hat { \beta } _ { k h } } - \frac { \alpha _ { k h } } \hat \end{array} +$$ + +(Wallace et al., 2023a, Sec. 5.1) claim that $\beta \in \left[ 2 0 0 0 , 5 0 0 0 \right]$ yields good performance on Stable Diffusion 1.5 and Stable Diffusion XL-1.0, which if we translate to our notation corresponds to $\tilde { \beta } \in [ 4 0 0 0 , 1 0 0 0 0 ]$ . + +When we have access to the reward function $r$ , instead of a winning sample $x _ { 1 } ^ { w }$ and a losing sample $x _ { 1 } ^ { l }$ , we have a pair of samples $( x _ { 1 } ^ { a } , x _ { 1 } ^ { b } )$ with winning weights $\begin{array} { r } { S ( r ( x _ { 1 } ^ { a } ) - r ( x _ { 1 } ^ { b } ) ) = \frac { 1 } { 1 + \exp { \left( r ( x _ { 1 } ^ { b } ) - r ( x _ { 1 } ^ { a } ) \right) } } } \end{array}$ 11+exp r(xb1)−r(xa1 ) , S(−(r(xa1 ) − r(xb1))) = $\frac { 1 } { 1 + \exp \left( - ( r ( x _ { 1 } ^ { b } ) - r ( x _ { 1 } ^ { a } ) ) \right) }$ Hence, the loss (234) becomes: + +$$ +\begin{array} { r } { L _ { \mathrm { D P O } } ( \theta ) = - \mathbb { E } _ { ( x _ { 1 } ^ { a } , x _ { 1 } ^ { b } ) \sim \mathcal { D } , k \sim U [ 0 , K ] , x _ { k h } ^ { a } \sim q ( x _ { k h } ^ { a } \mid x _ { 1 } ^ { a } ) , x _ { \mathrm { t } } ^ { b } \sim q ( x _ { k h } ^ { b } \mid x _ { 1 } ^ { b } ) } \Bigg [ \sum _ { s \in \{ \pm 1 \} } S \big ( s ( r ( x _ { 1 } ^ { a } ) - r ( x _ { 1 } ^ { b } ) ) \big ) \times } \\ { \log S \big ( - \frac { s \tilde { \beta } } { 2 } ( \big \| \frac { v _ { \theta } ( x _ { k h } ^ { a } , k h ) - \frac { \tilde { \beta } _ { k h } } { \beta _ { k h } } x _ { k h } ^ { a } } { \frac { \tilde { \alpha } _ { k h } } { \tilde { \alpha } _ { k h } } \beta _ { k h } - \tilde { \beta } _ { k h } } - \frac { \alpha _ { k h } } { \beta _ { k h } } x _ { 1 } ^ { a } \big \| ^ { 2 } - \big \| \frac { v _ { \mathrm { r e f } } ( x _ { k h } ^ { a } , k h ) - \frac { \tilde { \beta } _ { k h } } { \beta _ { k h } } x _ { k h } ^ { a } } { \frac { \tilde { \alpha } _ { k h } } { \alpha _ { k h } } \beta _ { k h } - \tilde { \beta } _ { k h } } - \frac { \alpha _ { k h } } { \beta _ { k h } } x _ { 1 } ^ { a } \big \| ^ { 2 } } \Bigg . \\ - \big ( \big \| \frac { v _ { \theta } ( x _ { k h } ^ { b } , k h ) - \frac { \tilde { \beta } _ { k h } } { \beta _ { k h } } x _ { k h } ^ { b } } { \frac { \tilde { \alpha } _ { k h } } { \alpha _ { k h } } \beta _ { k h } - \tilde { \beta } _ { k h } } - \frac { \alpha _ { k h } } { \beta _ { k h } } x _ { 1 } ^ b \end{array} +$$ + +We want to emphasize that despite the similarities, even though the loss $\scriptstyle L _ { \mathrm { D P O } }$ that we use (equation (235)) is very similar to the one implemented by Wallace et al. (2023a), the preference data pairs that we use are very different from theirs. We sample the preference data from the current model, which results in imperfect samples, while they consider off-policy, high-quality, curated preference samples. The reason for this discrepancy is that the starting point of our work is a reward model, not a set of preference data, and we only benchmark against approaches that leverage reward models for an apples-to-apples comparison. Our experimental results on DPO (Table 2, Figure 6, Table 3) show that the resulting model performs like the base model, or a bit worse according to some metrics. Hence, we conclude that DPO is not a competitive alternative for on-policy fine-tune when the base model is not already good. + +# G Experimental details + +Unless otherwise specified, we used the same hyperparameters across all fine-tuning methods. Namely, we used: + +• $K = 4 0$ timesteps. +• Adam optimizer with learning rate $2 \times 1 0 ^ { - 5 }$ and parameters $\beta _ { 1 } = 0 . 9 5$ , $\beta _ { 2 } = 0 . 9 9 9$ , $\epsilon = 1 \times 1 0 ^ { - 8 }$ , weight decay $1 \times 1 0 ^ { - 2 }$ , gradient norm clipping value 1. For Discrete Adjoint, these hyperparameters resulted in fine-tuning instability (see Table 6); the results that we report in all other tables for Discrete Adjoint were obtained with learning rate $1 \times 1 0 ^ { - 5 }$ . +• Bfloat16 precision. +• Effective batch size 40; for each run we used two 80GB A100 GPUs with batch size 20 each. +• A set of 40k fine-tuning prompts taken from a licensed dataset consisting of text and image pairs (note that we disregarded the images). Thus, each epoch lasts 1000 iterations; see the total amount of fine-tuning iterations for each algorithm in Table 3. For each of the three runs that we perform for each data point that we report, the set of 40k prompts is sampled independently among a total set of 100k prompts. + +# G.1 Noise schedule details + +Since we use $K = 4 0$ discretization steps, the timesteps are $t \in \{ 0 , 0 . 0 2 5 , 0 . 0 5 , 0 . 0 7 5 , 0 . 1 , . . . , 0 . 9 5 , 0 . 9 7 5 \}$ . To sample $X _ { t + h }$ from $X _ { t }$ we use equation (40). We use the choices $\alpha _ { t } = t$ , $\beta _ { t } = 1 - t$ , which means that $\begin{array} { r } { \sigma ( t ) = \sqrt { 2 \beta _ { t } ( \frac { \dot { \alpha } _ { t } } { \alpha _ { t } } \beta _ { t } - \dot { \beta } _ { t } ) } = \sqrt { 2 ( 1 - t ) ( \frac { 1 - t } { t } + 1 ) } = \sqrt { \frac { 2 ( 1 - t ) } { t } } } \end{array}$ . + +Note that if we plug $t = 0$ into this expression, we obtain infinity, and if we plug $t \lessapprox 1$ , we obtain $\sigma ( t ) \approx 0$ For obvious reasons, the former issue requires a fix: we simply add a small offset to the denominator of $\sigma ( t )$ , replacing $\sqrt { 1 / t }$ by $\sqrt { 1 / ( t + h ) }$ (note that $h : = 1 / K = 0 . 0 2 5 ,$ ). But the latter issue is also not completely satisfactory from a practical standpoint, because looking at the adjoint matching loss (37), we observe that $u ( X _ { t } ^ { u } , t )$ is trained to approximate the conditional expectation of $\sigma ( t ) ^ { 1 } \tilde { a } ( t ; X ^ { \bar { u } } )$ . Thus, if we set $\sigma ( t )$ very close to zero for $t \lessapprox 1$ , we are forcing the control $u$ to be close to zero as well, or equivalently preventing $v$ finetune from deviating from $v ^ { \mathrm { b a s e } }$ . While this is the right thing to do from a theoretical perspective, we concluded experimentally that setting $\sigma ( t )$ just slightly larger results in substantially faster fine-tuning, thanks to the additional leeway provided to $v ^ { \mathrm { f i n e t u n e } }$ to deviate from $v ^ { \mathrm { b a s e } }$ . In particular, we added a small offset to the factor $1 - t$ in the numerator $1 - t$ of $\sigma ( t )$ : we replaced $1 - t$ by $1 - t + h$ . Thus, the expression that we used to compute the diffusion coefficient in our experiments is + +$$ +\begin{array} { r } { \sigma ( t ) = \sqrt { \frac { 2 ( 1 - t + h ) } { t + h } } . } \end{array} +$$ + +When solving the lean adjoint ODE (38)-(39) backwards in time via the Euler scheme (41), the timesteps we use are $t \in \{ 1 , 0 . 9 7 5 , 0 . 9 5 , 0 . 9 2 5 , 0 . 9 , \dots , 0 . 0 5 , 0 . 0 2 5 \}$ . We do not actually initialize the adjoint state as $\nabla _ { x } g ( X _ { 1 } )$ , but rather as $\nabla _ { x } g ( \hat { X } _ { 1 } )$ , where $X _ { 1 } : = X _ { 1 - h } + h v ^ { \mathrm { b a s e } } ( X _ { 1 - h } , 1 - h )$ . That is, $\ddot { X } _ { 1 }$ is obtained by performing a final noiseless update, instead of using noise $\sigma ( 1 - h ) = \sqrt { 4 h }$ given by equation (236). The reason for this is that the regular final iterate $X _ { 1 }$ contains some noise that was added in the final step, and that can distort the gradient $\nabla _ { x } g ( X _ { 1 } )$ . By setting $\tilde { a } ( 1 ; X ) = \nabla _ { x } g ( X _ { 1 } )$ , we get rid of this bias. Note that in the continuous time limit $h 0$ , ${ \dot { X } } _ { 1 } = X _ { 1 }$ , which means that this small trick is consistent. + +# G.2 Selection of gradient evaluation timesteps + +In Algorithm 1, equation (42), we state that the term $\begin{array} { r } { \left. \frac { 2 } { \sigma ( t ) } \bigl ( v _ { \theta } ^ { \mathrm { f i n e t u n e } } ( X _ { t } , t ) - v ^ { \mathrm { b a s e } } ( X _ { t } , t ) \bigr ) + \sigma ( t ) \tilde { a } _ { t } \right. ^ { 2 } } \end{array}$ must be computed for all $K$ steps in $\{ 0 , \ldots , 1 - h \}$ . However, the gradient signal provided by backpropagating through this expression for consecutive times sample a subset $\kappa$ of timesteps, and we only compute and backpropagate the terms and $t + h$ is quite similar. In the interest of computational efficiency, we $\begin{array} { r l } { \bigg \Vert \frac { 2 } { \sigma ( t ) } \left( v _ { \theta } ^ { \mathrm { f i n e t u n e } } ( X _ { t } , t ) - \right. } \end{array}$ $v ^ { \mathrm { b a s e } } ( X _ { t } , t ) \big ) + \sigma ( t ) \tilde { a } _ { t } \big \| ^ { 2 }$ for those timesteps. We construct $\kappa$ by sampling ten timesteps uniformly without repetition among $\{ 0 , \ldots , 0 . 7 2 5 \}$ , and always sampling the last ten timesteps $\{ 0 . 7 5 , \ldots , 0 . 9 7 5 \}$ . This is because fine-tuning the last ten steps ( $2 5 \%$ of the total) well is critical for good empirical performance, while the initial steps are not as important. + +# G.3 Loss function clipping: the LCT hyperparameter + +Note that the magnitude of $\sigma ( t ) ^ { \mathsf { T } } a ( t ; X ^ { \bar { u } } , { \bar { u } } )$ is much larger for times $t \gtrapprox 0$ than for times $t \lessapprox 1$ . The reason is two-fold: + +• As discussed in Appendix G.1, $\sigma ( t )$ is much larger for $t \gtrapprox 0$ than for $t \lessapprox 1$ . • The magnitude of the lean adjoint state $\tilde { a }$ grows roughly exponentially as $t$ goes backward in time. In fact, if we assumed that $\nabla _ { x } b ( X _ { t } , t )$ is constant in time, this statement would be exact. + +Observe that when $\sigma ( t ) ^ { 1 } a ( t ; X ^ { \bar { u } } , \bar { u } )$ is large, the gradient $\begin{array} { r } { \nabla _ { \theta } \left\| \frac { 2 } { \sigma ( t ) } \big ( v _ { \theta } ^ { \mathrm { f i n e t u n e } } ( X _ { t } , t ) - v ^ { \mathrm { b a s e } } ( X _ { t } , t ) \big ) + \sigma ( t ) \tilde { a } _ { t } \right\| ^ { 2 } } \end{array}$ also has a high magnitude. Including such terms in our gradient computation decreases the signal to noise ratio of the gradient. Even more so, as discussed in Appendix G.2 for good practical performance it is critical to get a good gradient signal from the last $2 5 \%$ steps. Hence, including the high-magnitude terms for $t \lessapprox 0$ in our gradients can muffle these other important, low-magnitude terms. + +To fix this issue, we clip the terms such that $\begin{array} { r } { \left. \frac { 2 } { \sigma ( t ) } \left( v _ { \theta } ^ { \mathrm { f i n e t u n e } } ( X _ { t } , t ) - v ^ { \mathrm { b a s e } } ( X _ { t } , t ) \right) + \sigma ( t ) \tilde { a } _ { t } \right. ^ { 2 } > \mathrm { L C T } } \end{array}$ , where LCT stands for the loss clipping threshold. That is, the adjoint matching loss that we use in our experiments is of the form: + +$$ +\begin{array} { r } { \hat { \mathcal { L } } _ { \mathrm { A d j - M a t c h } } ( \theta ) = \sum _ { t \in K } \operatorname* { m i n } \big \{ \mathrm { L C T } , \big \| \frac { 2 } { \sigma ( t ) } \big ( v _ { \theta } ^ { \mathrm { f i n e t u m e } } ( X _ { t } , t ) - v ^ { \mathrm { b a s e } } ( X _ { t } , t ) \big ) + \sigma ( t ) \tilde { a } _ { t } \big \| ^ { 2 } \big \} , } \end{array} +$$ + +where $\kappa$ is the random timestep subset described in Appendix G.2. + +For adjoint matching, we set $\mathrm { L C T } = 1 . 6 \times \lambda ^ { 2 }$ . Remark that LCT needs to grow quadratically with $\lambda$ , because the magnitude of the lean adjoint $\tilde { a }$ grows quadratically with $\lambda$ . We set the constant 1.6 through experimentation; all or almost all of the terms for the last ten timesteps fall below LCT, but only a fraction of the terms ( $\approx 2 5 \%$ ) for the first ten steps fall below LCT. The constant for LCT is a relevant hyperparameter that needs to be tuned to obtain a similar behavior. + +We also used loss function clipping on the continuous adjoint loss. For that loss we set $\mathrm { L C T } = 1 6 0 0 \times \lambda ^ { 2 }$ . The reason is that the magnitude of the regular adjoint states is significantly larger than the magnitude of the lean adjoint states (which is a big reason why adjoint matching outperforms the continuous adjoint). + +# G.4 Computation of evaluation metrics + +We used the open_clip library (Ilharco et al., 2021) to compute ClipScores. We computed ClipScore diversity as the variance of Clip embeddings of 40 generations for a given prompt, averaged across 25 prompts. Namely, + +$$ +\begin{array} { r } { \mathrm { C l i p S c o r e \_ D i v e r s i t y } = \frac { 1 } { 4 0 } \sum _ { k = 1 } ^ { 4 0 } \frac { 2 } { 2 5 \cdot 2 4 } \sum _ { 1 \leq i < j \leq 2 5 } \| \mathrm { C l i p } ( g _ { i } ^ { k } ) - \mathrm { C l i p } ( g _ { j } ^ { k } ) \| ^ { 2 } , } \end{array} +$$ + +where $g _ { i } ^ { k }$ denotes the $i$ -th generation for the $k$ -th prompt. + +We used the transformers library to compute the PickScore processor and model (Kirstain et al., 2023). +PickScore diversity is computed in analogy with ClipScore diversity. + +We used the hps library to compute values of Human Preference Score v2 (Wu et al., 2023b). + +To compute Dreamsim diversity we use the dreamsim library (Fu et al., 2023). Dreamsim diversity is computed in analogy with ClipScore diversity. + +# G.5 Remarks on computational costs + +Observe from the figures reported in Table 3 that the per iteration wall-clock time of Adjoint Matching (156 seconds) is very similar to that of the Discrete Adjoint loss (152 seconds). The reason is that both algorithms perform a similar amount of forward and backward passes on the flow matching model and the reward model. Namely, for each sample in the batch, both algorithms perform $K$ forward passes on the flow model to obtain the trajectories. In order to compute the gradient of the training loss, the Discrete Adjoint loss does $K$ additional forward passes to evaluate the base flow model, one forward and backward pass on the reward model, and $K$ backward passes on the current flow model, which typically use gradient checkpointing to avoid memory overflow. In the case of Adjoint Matching, solving the lean adjoint ODE requires one forward and backward pass on the reward model, and $K$ backward passes on the base flow model. Finally, computing the gradient of the loss takes $K / 2$ additional backward passes if we evaluate at only half of the timesteps as we do, although this computation is much quicker because it can be fully parallelized. + +Meanwhile, computing the gradient of the Continuous Adjoint loss takes 204 seconds per iteration. With respect to Adjoint Matching, Continuous Adjoint performs additional backward passes to compute the gradients $\nabla _ { X _ { t } } \| u ( X _ { t } , t ) \| ^ { 2 }$ when solving the adjoint ODE. Finally, we observe that models that directly fine-tune the reward are quicker, but that comes with its own set of issues that we discuss throughout the paper. + +# G.6 Remarks on number of sampling timesteps + +In our experiments and all baselines, we used 40 timesteps in the fine-tuning procedure ( $h = 1 / 4 0$ in Algorithm 1). The experiments reported in all tables and figures except for Table 8 were performed at 40 inference timesteps. In Table 8 (Appendix A), we show experimental results at 10, 20, 40, 100, and 200 inference timesteps, for the base model and the models fine-tuned with adjoint matching and DRaFT-1. We make the following observations about the results: + +• The metrics for Adjoint Matching at 100 and 200 timesteps are statistically equal to the ones for 40 timesteps, with slight increases in Dreamsim diversity. This suggests that fine-tuning at large numbers of timesteps is a good idea if we want to perform inference at a large number of timesteps, as otherwise the capabilities of the model are limited by the number of fine-tuning timesteps instead of the inference compute. Also, at 100 and 200 timesteps the difference in performance of Adjoint Matching relative to DRaFT-1 increases. • The metrics for Adjoint Matching at 10 and 20 timesteps are worse than at 40 timesteps, especially for 10. The difference in performance between Adjoint Matching and DRaFT-1 vanishes at 10 timesteps for all metrics except for diversity, for which Adjoint Matching is still clearly better. \ No newline at end of file diff --git a/papers/adjoint-matching/paper.pdf b/papers/adjoint-matching/paper.pdf new file mode 100644 index 0000000000000000000000000000000000000000..964e0a19da2accbdabe36000701ae9fc75b84a05 --- /dev/null +++ b/papers/adjoint-matching/paper.pdf @@ -0,0 +1,3 @@ +version https://git-lfs.github.com/spec/v1 +oid sha256:e59e529f1d26d8ff9260837e61d53d1c29b5aef69d080a599ba3a33ed9889f8c +size 11921609 diff --git a/papers/adjoint-matching/sau.json b/papers/adjoint-matching/sau.json new file mode 100644 index 0000000000000000000000000000000000000000..3242a714b8327b385bef672d2917bb39636f3518 --- /dev/null +++ b/papers/adjoint-matching/sau.json @@ -0,0 +1,247 @@ +{ + "paper_id": "adjoint-matching", + "paper_title": "Adjoint Matching: Fine-tuning Flow and Diffusion Models with Memoryless SOC", + "D1": [ + { + "id": "adjoint-matching-D1-001", + "claim": "Image resolution for autoencoder pre-training and generation: 512×512", + "source": "Section 7" + }, + { + "id": "adjoint-matching-D1-002", + "claim": "Number of discretization timesteps K=40 for fine-tuning (step size h=1/K=0.025)", + "source": "Appendix G, Section G.1" + }, + { + "id": "adjoint-matching-D1-003", + "claim": "Practical noise schedule with offsets: σ(t)=√(2(1-t+h)/(t+h)), h=0.025", + "source": "Appendix G.1, Eq. 236" + }, + { + "id": "adjoint-matching-D1-004", + "claim": "Number of gradient evaluation timesteps per iteration: 20 (10 uniform from [0,0.725] + last 10 from [0.75,0.975])", + "source": "Appendix G.2" + }, + { + "id": "adjoint-matching-D1-005", + "claim": "Adam optimizer: lr=2e-5 (default), lr=1e-5 (Discrete Adjoint only), β₁=0.95, β₂=0.999, ε=1e-8, weight_decay=0.01, gradient clip=1.0", + "source": "Appendix G" + }, + { + "id": "adjoint-matching-D1-006", + "claim": "Alternative optimizer config tested: lr=3e-5, β₁=0.97", + "source": "Appendix G, Table 6" + }, + { + "id": "adjoint-matching-D1-007", + "claim": "Adjoint matching method training precision: bfloat16 (all experiments)", + "source": "Appendix G" + }, + { + "id": "adjoint-matching-D1-008", + "claim": "Effective batch size: 40 (2× 80GB A100 GPUs, batch 20 each)", + "source": "Appendix G" + }, + { + "id": "adjoint-matching-D1-009", + "claim": "Fine-tuning prompts per run: 40k (sampled from 100k total), test prompts per run: 1k, 3 independent runs per config", + "source": "Appendix G, Section 7" + }, + { + "id": "adjoint-matching-D1-010", + "claim": "Iterations per epoch: 1000 (40k prompts / batch 40); Adjoint Matching runs: 1000 iters (1 epoch)", + "source": "Appendix G" + }, + { + "id": "adjoint-matching-D1-011", + "claim": "Baseline iteration counts: DRaFT-1=4000, DRaFT-40=4000, DPO=1500, ReFL=6000, Continuous Adjoint=750, Discrete Adjoint=1000", + "source": "Appendix G, Table 3" + }, + { + "id": "adjoint-matching-D1-012", + "claim": "Reward scaling factor λ values: [1000, 2500, 12500] for Adjoint Matching tradeoff", + "source": "Section 7, Table 2" + }, + { + "id": "adjoint-matching-D1-013", + "claim": "Loss Clipping Threshold (Adjoint Matching): LCT=1.6×λ²; LCT (Continuous Adjoint): 1600×λ²", + "source": "Appendix G.3" + }, + { + "id": "adjoint-matching-D1-014", + "claim": "Classifier-free guidance weights evaluated: w ∈ [0.0, 1.0, 4.0]", + "source": "Section 7, Table 5" + }, + { + "id": "adjoint-matching-D1-015", + "claim": "Diversity metric: 40 generations per prompt across 25 prompts; DreamSim features for pairwise distance", + "source": "Appendix G.4, Eq. 238" + }, + { + "id": "adjoint-matching-D1-016", + "claim": "Inference timesteps ablated: [10, 20, 40, 100, 200] (fine-tuning always at K=40)", + "source": "Table 8" + }, + { + "id": "adjoint-matching-D1-017", + "claim": "Wall-clock time per iteration: Adjoint Matching=156s, Discrete Adjoint=152s, Continuous Adjoint=204s", + "source": "Table 3, Appendix G.5" + } + ], + "D2": [ + { + "id": "adjoint-matching-D2-001", + "claim": "Unified SDE for Flow Matching and Diffusion Models: dX_t = b(X_t,t)dt + σ(t)dB_t, where b(x,t)=κ_t·x + (σ(t)²/2 + η_t)·𝔰(x,t), κ_t=α̇_t/α_t, η_t=β_t(α̇_t/α_t·β_t − β̇_t)", + "source": "Section 3, Eq. 10-11" + }, + { + "id": "adjoint-matching-D2-002", + "claim": "Continuous-time DDIM SDE: dX_t = (α̇̄_t/(2ᾱ_t)·X_t − (α̇̄_t/(2ᾱ_t) + σ(t)²/2)·ε^base(X_t,t)/√(1−ᾱ_t))dt + σ(t)dB_t, X_0~N(0,I)", + "source": "Section 3, Eq. 6" + }, + { + "id": "adjoint-matching-D2-003", + "claim": "Flow Matching velocity to score function: v^base(x,t) = α̇_t/α_t·x + β_t(α̇_t/α_t·β_t − β̇_t)·𝔰(x,t)", + "source": "Section 3, Eq. 8" + }, + { + "id": "adjoint-matching-D2-004", + "claim": "Noise predictor to score function: 𝔰(x,t) = −ε^base(x,t)/√(1−ᾱ_t)", + "source": "Section 3, Eq. 9" + }, + { + "id": "adjoint-matching-D2-005", + "claim": "Memoryless noise schedule condition (Proposition 1): σ(t)² = 2η_t. This ensures X_0 and X_1 are independent, removing initial value function bias.", + "source": "Section 4.3, Proposition 1, Eq. 25" + }, + { + "id": "adjoint-matching-D2-006", + "claim": "DDIM fine-tuning drift+control (memoryless σ=√(2η_t)): b+σu = α̇̄_t/(2ᾱ_t)·x − α̇̄_t/ᾱ_t · ε^finetune(x,t)/√(1−ᾱ_t); control u = −√(α̇̄_t/(ᾱ_t(1−ᾱ_t))) · (ε^finetune − ε^base)", + "source": "Section 4.3, Eq. 26" + }, + { + "id": "adjoint-matching-D2-007", + "claim": "Flow Matching fine-tuning drift+control (memoryless σ=√(2η_t)): b+σu = 2·v^finetune(x,t) − α̇_t/α_t·x; control u = √(2/(β_t(α̇_t/α_t·β_t−β̇_t))) · (v^finetune − v^base)", + "source": "Section 4.3, Eq. 27" + }, + { + "id": "adjoint-matching-D2-008", + "claim": "Flow Matching Euler-Maruyama sampling step during fine-tuning: X_{t+h} = X_t + h·(2·v^finetune_θ(X_t,t) − α̇_t/α_t·X_t) + √h·σ(t)·ε_t, ε_t~N(0,I), X_0~N(0,I)", + "source": "Section 5.2, Algorithm 1, Eq. 40" + }, + { + "id": "adjoint-matching-D2-009", + "claim": "Lean Adjoint ODE (backward): d/dt ã(t;X) = −ã(t;X)^T·∇_x b(X_t,t), with terminal condition ã(1;X) = −∇_{X_1} r(X_1). Solved backwards via Euler: ã_{t−h} = ã_t + h·ã_t^T·∇_{X_t}(2·v^base(X_t,t) − α̇_t/α_t·X_t)", + "source": "Section 5.2, Eq. 38-39, Algorithm 1, Eq. 41" + }, + { + "id": "adjoint-matching-D2-010", + "claim": "Adjoint Matching loss (Flow Matching): L_AdjMatch(θ) = Σ_{t∈{0,...,1−h}} ||(2/σ(t))·(v^finetune_θ(X_t,t) − v^base(X_t,t)) + σ(t)·ã_t||², where X_t and ã_t are stop-grad", + "source": "Section 5.2, Eq. 37, Algorithm 1, Eq. 42" + }, + { + "id": "adjoint-matching-D2-011", + "claim": "Clipped Adjoint Matching loss (practical): L̂_AdjMatch(θ) = Σ_{t∈κ} min(LCT, ||(2/σ(t))·(v^finetune − v^base) + σ(t)·ã_t||²), where κ is random timestep subset (20 steps)", + "source": "Appendix G.3, Eq. 237" + }, + { + "id": "adjoint-matching-D2-012", + "claim": "Reward function for fine-tuning: r(x) = λ × ImageReward(x), where ImageReward from Xu et al. 2023", + "source": "Section 7, Eq. 40" + }, + { + "id": "adjoint-matching-D2-013", + "claim": "Target tilted distribution: p*(X_1) ∝ p^base(X_1) · exp(r(X_1))", + "source": "Section 1, Eq. 1; Section 5.2" + }, + { + "id": "adjoint-matching-D2-014", + "claim": "SOC objective for fine-tuning (KL-regularized): min_u E[∫₀¹(½||u(X_t^u,t)||² + f(X_t^u,t))dt + g(X_1^u)], s.t. dX_t^u = (b(X_t^u,t)+σ(t)u(X_t^u,t))dt + σ(t)dB_t, with f=0, g=−r", + "source": "Section 4.1, Eq. 12-13; Section 4.2" + }, + { + "id": "adjoint-matching-D2-015", + "claim": "SOC optimal control via value function: u*(x,t) = −σ(t)^T·∇_x V(x,t), where V(x,t) = −log E_{X~p^base}[exp(−∫f ds − g(X_1)) | X_t=x]", + "source": "Section 4.1, Eq. 16-17" + }, + { + "id": "adjoint-matching-D2-016", + "claim": "Classifier-free guidance formula: v_guided(x,t) = (1+w)·v(x,t|y) − w·v(x,t), where w is guidance weight, v(x,t|y) is conditional model, v(x,t) is unconditional", + "source": "Section 7, Eq. 41; Appendix Table 5" + }, + { + "id": "adjoint-matching-D2-017", + "claim": "DDPM SDE (special case of DDIM with memoryless σ): dX_t = (α̇̄_t/(2ᾱ_t)·X_t − α̇̄_t/ᾱ_t · ε^base(X_t,t)/√(1−ᾱ_t))dt + √(α̇̄_t/ᾱ_t)dB_t", + "source": "Section 3, Eq. 7" + }, + { + "id": "adjoint-matching-D2-018", + "claim": "U-Net architecture for text-conditional Flow Matching model on latent variables (similar to Rombach et al. 2022 LDM setup): encoder-decoder with skip connections at each resolution level; encoder: x_enc^{(l)} = DownBlock(ResNet(Conv(x_enc^{(l-1)}))) with channel multiplier [C, 2C, 4C, 8C]; decoder: x_dec^{(l)} = ResNet(Conv(Concat(Up(x_dec^{(l-1)}), x_enc^{(l)}))); mid-block with self-attention at lowest resolution (4 heads); text conditioning via cross-attention injected at each resolution level", + "source": "Section 7" + }, + { + "id": "adjoint-matching-D2-019", + "claim": "DDIM discrete update rule: X_{k+1} = √(ᾱ_{k+1})·(X_k − √(1−ᾱ_k)·ε(X_k,k))/√(ᾱ_k) + √(1−ᾱ_{k+1}−σ_k²)·ε(X_k,k) + σ_k·ε_k", + "source": "Section 2.2, Eq. 5" + }, + { + "id": "adjoint-matching-D2-020", + "claim": "Continuous Adjoint gradient: dL/dθ = ½∫₀¹ ∂/∂θ||u(X_t,t)||²dt + ∫₀¹ (∂u(X_t,t)/∂θ)^T·σ(t)^T·a(t;X,u)dt, where a(t) is the full adjoint state solving da/dt = −[a^T·∇_x(b+σu) + ∇_x(f+½||u||²)], a(1)=∇g(X_1)", + "source": "Section 5.1.1, Eq. 30-32" + } + ], + "D3": [ + { + "id": "adjoint-matching-D3-001", + "claim": "Main experiment: Fine-tune Flow Matching text-to-image model (512×512, latent space) with ImageReward. Compare Adjoint Matching (λ=1000/2500/12500) against DRaFT-1, DRaFT-40, DPO, ReFL, Continuous Adjoint, and Discrete Adjoint. Evaluate on text-to-image consistency (ClipScore, PickScore), unseen human preference (HPSv2), and sample diversity (DreamSim Diversity).", + "source": "Section 7, Table 2, Table 3" + }, + { + "id": "adjoint-matching-D3-002", + "claim": "Reward tradeoff ablation: Vary λ=[1000, 2500, 12500] for Adjoint Matching to study KL-regularization vs reward optimization tradeoff. Higher λ increases consistency/human preference but reduces diversity. Compared against DRaFT-1 with varying iterations (early stopping) as alternative tradeoff mechanism.", + "source": "Section 7, Figure 3, Figure 5, Table 2" + }, + { + "id": "adjoint-matching-D3-003", + "claim": "Classifier-free guidance ablation: Apply CFG weights w=[0.0, 1.0, 4.0] after fine-tuning across Adjoint Matching and DRaFT-1. Higher w improves text-to-image consistency at cost of diversity. Note: only conditional model is fine-tuned; unconditional model is base.", + "source": "Section 7, Figure 4, Table 5" + }, + { + "id": "adjoint-matching-D3-004", + "claim": "Noise schedule ablation: Compare fine-tuning with memoryless σ(t)=√(2η_t) vs constant σ(t)=1 for Adjoint Matching (λ=12500). Constant σ(t) suffers from initial value function bias — reward optimization underperforms. Also compare sampling with σ(t)=√(2η_t) vs σ(t)=0 after fine-tuning.", + "source": "Section 7, Table 2, Table 7" + }, + { + "id": "adjoint-matching-D3-005", + "claim": "Inference timesteps ablation: Vary number of sampling steps [10, 20, 40, 100, 200] while keeping fine-tuning at 40 steps. Evaluate Adjoint Matching (λ=12500) and DRaFT-1 against base model. Fine-tuned models robustly improve over base even at low step counts.", + "source": "Section 7, Table 8" + }, + { + "id": "adjoint-matching-D3-006", + "claim": "Optimizer hyperparameters ablation: Compare lr=3e-5, β₁=0.97 vs default lr=2e-5, β₁=0.95 for DRaFT-1 and Adjoint Matching (λ=1200). Also test Discrete Adjoint stability — default hyperparams cause instability, requiring lr reduction to 1e-5.", + "source": "Appendix G, Table 6" + }, + { + "id": "adjoint-matching-D3-007", + "claim": "Diversity evaluation protocol: For each prompt, generate 40 samples, compute pairwise DreamSim feature distances across 25 prompts to quantify sample diversity. Metrics: ClipScore diversity (variance of Clip embeddings) and DreamSim diversity (pairwise feature distance).", + "source": "Appendix G.4, Eq. 238" + } + ], + "D4": [ + { + "id": "adjoint-matching-D4-001", + "claim": "[Phase 3/4 — Adjoint Matching Training Loop (Algorithm 1)] ENTRY: (from Phase 2) Memoryless fine-tuning recipe established with σ(t)=√(2η_t); control u parameterized via fine-tuned velocity v^finetune (Eq. 27); base model v^base pre-trained; reward model r(x)=λ×ImageReward(x) provided; hyperparameters configured (N iterations, step size h, step count K=40, m trajectories, optimizer). EXECUTION: Step 1 — Initialize fine-tuned vector field v^finetune = v^base with parameters θ. Step 2 — Sample m trajectories forward from t=0 to t=1 via Euler-Maruyama with memoryless schedule: X_{t+h} = X_t + h·(2·v^finetune_θ(X_t,t) − α̇_t/α_t·X_t) + √h·σ(t)·ε_t, ε_t~N(0,I), X_0~N(0,I). Step 3 — For each trajectory, solve lean adjoint ODE backwards from t=1 to t=0: ã_{t−h} = ã_t + h·ã_t^T·∇_{X_t}(2·v^base(X_t,t) − α̇_t/α_t·X_t), terminal condition ã_1 = −∇_{X_1}r(X_1); X_t and ã_t use stop_grad. Step 4 — Compute clipped Adjoint Matching loss over random timestep subset κ (20 steps): L̂_AdjMatch(θ) = Σ_{t∈κ} min(LCT, ||(2/σ(t))·(v^finetune − v^base) + σ(t)·ã_t||²). Step 5 — Compute gradient ∇_θ L̂ and update θ via Adam. Repeat Steps 1-5 for N=1000 iterations (1 epoch). EXIT: (output to Phase 4) Fine-tuned vector field v^finetune with optimized parameters θ; ready for sampling with any noise schedule to produce tilted-distribution samples.", + "source": "Section 5.2, Algorithm 1" + }, + { + "id": "adjoint-matching-D4-002", + "claim": "[Phase 2→3→4 Boundary — Fine-tuning/Sampling Separation (Theorem 1)] ENTRY: (from Phase 1) Unified SDE framework dX_t = b(X_t,t)dt + σ(t)dB_t (Eq. 10-11) established; base model trained (FM or diffusion); reward model r(x) available; target tilted distribution p*(X_1) ∝ p^base(X_1)·exp(r(X_1)) defined. CORE PRINCIPLE (Phase 2→3): Fine-tuning MUST enforce memoryless noise schedule σ(t)=√(2η_t) during the SOC optimization in Phase 3 — this is the unique choice (Proposition 1) that makes X_0 ⟂ X_1, thereby removing the initial value function bias V(X_0,0) from the optimal distribution (converting Eq. 23 to Eq. 24). Concretely: fine-tuning uses σ(t)=√(2η_t) which, for DDIM, recovers the continuous-time DDPM process (Eq. 7); for Flow Matching, defines Memoryless Flow Matching (Eq. 27). SEPARATION (Phase 3→4): After fine-tuning completes (Algorithm 1 converges), the fine-tuned vector field v^finetune (or ε^finetune for diffusion) can be plugged into the original generative process with ANY noise schedule for sampling — commonly σ(t)=0 (noiseless ODE sampling, Eq. 3) or the original DDIM noise schedule (Eq. 6). The tilted distribution guarantee holds regardless of the sampling σ(t). EXIT: (feeds Phase 4) Fine-tuned model is schedule-agnostic for sampling; same trained weights support σ(t) ∈ {0, √(α̇̄_t/ᾱ_t), arbitrary} to generate samples from p*(X_1).", + "source": "Section 4.3, Theorem 1" + }, + { + "id": "adjoint-matching-D4-003", + "claim": "[Phase 1→2 — Memoryless Schedule Derivation Pipeline (Sections 3→4→5)] ENTRY: (starting condition) Existing pre-trained base generative model (Flow Matching or diffusion); known reference flow coefficients α_t, β_t satisfying α_0=β_1=0, α_1=β_0=1 (Eq. 2); base velocity v^base (FM) or noise predictor ε^base (diffusion) pre-trained. EXECUTION: Step 1 (Section 3, Eqs. 6-11) — Unify FM and DDIM into common SDE parameterization: express v^base and ε^base in terms of the same score function 𝔰(x,t) (Eqs. 8-9), then derive the unified SDE dX_t = b(X_t,t)dt + σ(t)dB_t where b(x,t)=κ_t·x + (σ(t)²/2 + η_t)·𝔰(x,t) (Eqs. 10-11). Output: single framework covering all dynamical generative models. Step 2 (Section 4.1-4.2, Eqs. 12-23) — Formulate reward fine-tuning as SOC control-affine problem (Eqs. 12-13) and derive optimal distribution p*(X) = p^base(X)·exp(r(X_1) + V(X_0,0)) (Eq. 23). Diagnose: the V(X_0,0) bias term originates from X_0↔X_1 dependence — when X_0 determines X_1 (e.g., noiseless ODE), V(X_0,0) cannot be factored out, preventing convergence to the tilted distribution. Output: problem statement identifying the bias source. Step 3 (Section 4.3, Proposition 1, Definition 1) — Prove memoryless condition: σ(t)² = 2η_t is necessary and sufficient for X_0 ⟂ X_1 (Definition 1 + Proposition 1). Under this condition, p^base(X_0,X_1) = p^base(X_0)·p^base(X_1), so V(X_0,0) integrates out: p*(X_1) ∝ p^base(X_1)·exp(r(X_1)) (Eq. 24). Theorem 1 further proves this is the only noise schedule that enables arbitrary-schedule sampling after fine-tuning. Output: memoryless noise schedule σ(t)=√(2η_t). Step 4 (Section 4.3 Eqs. 26-27, Section 5.2 Algorithm 1) — Express fine-tuning drift+control in DDIM form (Eq. 26) and Flow Matching form (Eq. 27); parameterize control u in terms of ε^finetune or v^finetune; apply Adjoint Matching algorithm to solve the resulting SOC as a least-squares regression (Eqs. 37-39, Algorithm 1). EXIT: (output to Phase 3 execution) Complete fine-tuning recipe: memoryless schedule σ(t)=√(2η_t); control u parameterized; Adjoint Matching objective and lean adjoint ODE defined; ready to execute Algorithm 1 training loop.", + "source": "Section 3, Section 4.3, Section 5.2" + } + ] +} \ No newline at end of file diff --git a/papers/avg-reward-pg/blacklist.txt b/papers/avg-reward-pg/blacklist.txt new file mode 100644 index 0000000000000000000000000000000000000000..bd362683ed839f80574f8985f55f38ccc1b7aa54 --- /dev/null +++ b/papers/avg-reward-pg/blacklist.txt @@ -0,0 +1,3 @@ +# No public official repository found (ICLR 2025, theoretical paper) +# Authors: Navdeep Kumar, Yashaswini Murthy, Itai Shufaro, Kfir Y. Levy, R. Srikant, Shie Mannor +# Related: https://github.com/abhisheknaik96/average-reward-methods diff --git a/papers/avg-reward-pg/config.yaml b/papers/avg-reward-pg/config.yaml new file mode 100644 index 0000000000000000000000000000000000000000..e3715cf094aec2bd57819d8d002dda42baf9cb8a --- /dev/null +++ b/papers/avg-reward-pg/config.yaml @@ -0,0 +1,8 @@ +title: "Global Convergence of Policy Gradient in Average Reward MDPs" +pdf_url: "https://openreview.net/pdf?id=2PRpcmJecX" +venue: "ICLR 2025" +year: "2025" +extra: + selection_index: 15 + domain: "Reinforcement Learning" + paradigm: "Theoretical Analysis" diff --git a/papers/avg-reward-pg/images/figures/avg-reward-pg-fig-0001.jpg b/papers/avg-reward-pg/images/figures/avg-reward-pg-fig-0001.jpg new file mode 100644 index 0000000000000000000000000000000000000000..af47f56309568de0d473efb938ce4ef8573f0546 --- /dev/null +++ b/papers/avg-reward-pg/images/figures/avg-reward-pg-fig-0001.jpg @@ -0,0 +1,3 @@ +version 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navdeepkumar@campus.technion.ac.il + +Yashaswini Murthy∗ +ECE & CSL +University of Illinois Urbana-Champaign +ymurthy2@illinois.edu + +# Itai Shufaro + +Electrical and Computer Engineering Technion - Israel Institute of Technology itai.shufaro@campus.technion.ac.il + +Kfir Y. Levy Electrical and Computer Engineering Technion - Israel Institute of Technology kfirylevy@technion.ac.il + +# R. Srikant + +ECE & CSL University of Illinois Urbana-Champaign rsrikant@illinois.edu + +Shie Mannor +Electrical Engineering +Technion - Israel Institute of Technology +NVIDIA Research +shie@ee.technion.ac.il + +# ABSTRACT + +We present the first comprehensive finite-time global convergence analysis of policy gradient for infinite horizon average reward Markov decision processes (MDPs). Specifically, we focus on ergodic tabular MDPs with finite state and action spaces. Our analysis shows that the policy gradient iterates converge to the optimal policy at a sublinear rate of $O \left( { \frac { 1 } { T } } \right)$ , where $T$ represents the number of iterations. Performance bounds for discounted reward MDPs cannot be easily extended to average reward MDPs as the bounds grow proportional to the fifth power of the effective horizon. Recent work on such extensions makes a smoothness assumption that has not been verified. Thus, our primary contribution is in providing the first complete proof that the policy gradient algorithm converges globally for averagereward MDPs, without such an assumption. We also obtain the corresponding finite-time performance guarantees. In contrast to the existing discounted reward performance bounds, our performance bounds have an explicit dependence on constants that capture the complexity of the underlying MDP. Motivated by this observation, we reexamine and improve the existing performance bounds for discounted reward MDPs. We also present simulations that empirically validate the result. + +# 1 INTRODUCTION + +Average reward Markov Decision Processes (MDPs) find applications in domains where decisions are made over time to optimize long-term performance. Some of these applications include resource allocation, portfolio management in finance, healthcare, and robotics (Ghalme et al., 2021; Bielecki et al., 1999; Patrick & Begen, 2011; Mahadevan, 1996; Tadepalli & Ok, 1998). Approaches for determining the optimal policy can be broadly categorized into dynamic programming algorithms (such as value and policy iteration (Murthy et al., 2024; Abbasi-Yadkori et al., 2019; Gosavi, 2004)) and gradient-based algorithms. Although gradient-based algorithms are heavily used in practice (Schulman et al., 2015; Baxter & Bartlett, 2000), the theoretical analysis of their global convergence is a relatively recent undertaking. + +While extensive research has been conducted on the global convergence of policy gradient methods in the context of discounted reward MDPs (Agarwal et al., 2020; Khodadadian et al., 2021), comparatively less attention has been given to its average reward counterpart. Contrary to average reward MDPs, the presence of a discount factor $( \gamma < 1 )$ ) serves as a source of contraction that alleviates the technical challenges involved in analyzing the performance of various algorithms in the context of discounted reward MDPs. Consequently, many algorithms designed for average reward MDPs are evaluated using the framework of discounted MDPs, where the discount factor approaches one (Grand-Clément & Petrik, 2024). + +In the context of discounted reward MDPs, the projected policy gradient (PPG) algorithm converges as + +$$ +\rho _ { \gamma } ^ { * } - \rho _ { \gamma } ^ { \pi _ { k } } \leq O \left( \frac { 1 } { \left( 1 - \gamma \right) ^ { 5 } } \right) +$$ + +where $\gamma$ is the discount factor, $\pi _ { k }$ is the policy obtained at the $k$ -th iteration of the PPG algorithm, $\rho ^ { * } \gamma$ represents the optimal value function, and $\rho \gamma ^ { \pi _ { k } }$ represents the value function iterates obtained through projected gradient ascent (Xiao, 2022b). Let $\rho ^ { \pi }$ denote the average reward associated with some policy $\pi$ . It is well known that $\rho ^ { \pi } = \operatorname* { l i m } _ { \gamma \to 1 } ( 1 - \gamma ) \rho _ { \gamma } ^ { \pi }$ under some mild conditions (Puterman, 1994; Bertsekas, 2007). Utilizing this relationship in equation 1, we observe the upper bound tends to infinity in the limit $\gamma 1$ . Hence, it is necessary to devise an alternate approach to study the convergence of policy gradient in the context of average reward MDPs. + +# 1.1 RELATED WORK + +There is a wealth of literature on discounted reward MDPs. Fazel et al. (2018) were among the first to establish the global convergence of policy gradients, specifically within the domain of linear quadratic regulators. Bhandari $\&$ Russo (2024) established a connection between the policy gradient and policy iteration objectives, determining conditions under which policy gradient algorithms converge to the globally optimal solution. Agarwal et al. (2020) offer convergence bounds of $O \big ( \frac { 1 } { \epsilon ^ { 2 } ( 1 - \gamma ) ^ { 6 } } \big )$ for policy gradient and $\scriptstyle O \left( { \frac { 1 } { \epsilon ( 1 - \gamma ) ^ { 2 } } } \right)$ for natural policy gradient, where $\epsilon$ represents the suboptimality. It is noteworthy that while their convergence bounds for policy gradient rely on the cardinality of the state and action space, the convergence bounds for natural policy gradient are independent of them. Xiao (2022b) enhance the $O \big ( \frac { 1 } { \epsilon ^ { 2 } ( 1 - \gamma ) ^ { 6 } } \big )$ policy gradient bounds by refining the dependency on the discount factor, yielding improved bounds of $\scriptstyle O ( { \frac { 1 } { \epsilon ( 1 - \gamma ) ^ { 5 } } } )$ . Zhang et al. (2021) prove that variance-reduced versions of stochastic policy gradient also converge to the global optimal solution. They achieve this through a gradient truncation mechanism. Mei et al. (2020) analyze global convergence of softmax-based gradient methods and prove exponential rejection of suboptimal policies. + +The global convergence of policy gradient methods has been extensively studied in the context of planning for average reward Markov Decision Processes (MDPs). Even-Dar et al. (2009) and Murthy & Srikant (2023) provide foundational results for natural policy gradient methods, proving their global convergence for finite state and action spaces. Extending this work, Grosof et al. (2024) analyze the more challenging case of infinite state spaces, establishing theoretical guarantees for convergence in this general setting. A critical contribution of our work lies in proving that the smoothness of the average reward holds for a large class of MDPs in the tabular setting. This smoothness property allows us to establish the first global convergence bounds for policy gradient methods in average reward MDPs, eliminating the need for previously unverified and restrictive smoothness assumptions. + +In the learning context, actor-critic and gradient-based methods have been a central focus for analyzing average reward MDPs. Konda & Tsitsiklis (1999) investigate two-time-scale actor-critic algorithms, employing linear function approximation for value functions and demonstrating asymptotic local convergence to a stationary point. Similarly, Bhatnagar et al. (2009) analyze four gradient-based methods, including natural policy gradients, and establish asymptotic local convergence using an ODE-based framework. Addressing global convergence, Bai et al. (2023) study policy gradient methods under the assumption that the average reward is smooth with respect to the policy parameters, achieving a regret bound of $O ( T ^ { 1 / 4 } )$ . Ganesh et al. (2024) consider the average reward natural actorcritic algorithm which does not rely on knowledge of mixing time, and provide a regret bound of√ $O v \sqrt { T }$ . However, the results in both of these works rely on unverified smoothness assumptions, leaving open questions about their applicability to general MDPs. Our contributions further strengthen the understanding of when these assumptions might indeed be true. + +# 1.2 CONTRIBUTIONS + +In this subsection, we outline the key contributions of this paper. + +• Elimination of Smoothness Assumption: Unlike previous work which assumes the underlying average cost function is smooth, we prove its smoothness by introducing a new analysis technique. This technique addresses the key difficulty of the lack of uniqueness of the value function in average-reward problems. We overcome this challenge by using a projection technique to ensure uniqueness and leveraging the properties of the projection to prove smoothness. This removes a significant assumption and strengthens the theoretical foundations of policy gradient methods in average reward MDPs. + +• Expression for Smooth Average Cost: We derive an explicit expression for the average cost that is shown to be smooth in the policy $\pi$ . This contribution is critical as it provides a deeper understanding and new insights into the behaviour of the average reward MDPs under policy gradient methods. + +• Sublinear Convergence Bounds: Using the above smoothness property, we present finite time bounds on the optimality gap over time, showing that the iterates approach the optimal policy with an overall regret of $O$ $\left( \log \left( T \right) \right)$ . In contrast, the regret bounds in Bai et al. (2023) are atleast $O \left( T ^ { \frac { 1 } { 4 } } \right)$ without learning error and with tabular parametrization. In place of the discount factor and the cardinality of the state and action spaces in the discounted setting, our finite-time performance bounds involves a different parameter which characterizes the complexity of the underlying MDP. + +• Extension to Discounted Reward MDPs: Our analysis can also be applied to the discounted reward MDP problem to provide stronger results than the state of the art. In particular, we show that our performance bounds for discounted MDPs can be expressed in terms of a problem complexity parameter, which can be independent of the size of the state and action spaces in some problems. + +• Experimental Validation: We simulate the performance of policy gradient across a simple class of MDPs to empirically evaluate its performance. The simulations illustrate the impact of MDP complexity on convergence rates. Unlike previous results, where the bounds depend solely on the size of the state and action spaces, these simulations demonstrate how the underlying structure of the MDP can result in significantly different convergence rates, even with fixed state and action spaces. These observations further validate the theoretical bounds derived for the convergence of projected policy gradient in average reward MDPs. + +# 2 PRELIMINARIES + +In this section, we introduce our model, address the limitations of applying the optimality gap bounds from discounted reward MDPs to the average reward scenario, present the gradient ascent update, and discuss the assumptions underlying our analysis. + +# 2.1 AVERAGE REWARD MDP FORMULATION + +We consider the class of infinite horizon average reward MDPs with finite state space $s$ and finite action space $\mathcal { A }$ . The environment is modeled as a probability transition kernel denoted by $\mathbb { P }$ . We consider a class of randomized policies $\Pi = \{ \pi : \bar { \mathcal { S } } \Delta ( \mathcal { \bar { A } } ) \}$ , where a policy $\pi$ maps each state to a probability vector over the action space. The transition kernel corresponding to a policy $\pi$ is represented by $\mathbb { P } ^ { \pi } : \mathcal { S } \mathcal { S }$ , where $\begin{array} { r } { \mathbb { P } ^ { \hat { \pi } } ( s ^ { \prime } | s ) = \sum _ { a \in \mathcal { A } } \pi ( a | s ) \mathbb { P } ( s ^ { \prime } | s , a ) } \end{array}$ denotes the single step probability of moving from state $s$ to $s ^ { \prime }$ under policy $\pi$ . Let $r ( s , a )$ denote the single step reward obtained by taking action $a \in { \mathcal { A } }$ in state $s \in S$ . The single-step reward associated with a policy $\pi$ at state $s \in S$ is defined as $\begin{array} { r } { r ^ { \pi } ( s ) = \sum _ { a \in \mathcal { A } } \pi ( a | s ) r ( s , a ) } \end{array}$ . + +The infinite horizon average reward objective $\rho ^ { \pi }$ associated with a policy $\pi$ is defined as: + +$$ +\rho ^ { \pi } = \operatorname* { l i m } _ { N \to \infty } { \frac { \mathbb { E } _ { \pi } \left[ \sum _ { n = 0 } ^ { N - 1 } r ^ { \pi } ( s _ { n } ) \right] } { N } } , +$$ + +where the expectation is taken with respect to $\mathbb { P } ^ { \pi }$ . The average reward is independent of the initial state distribution under some mild conditions (Ross, 1983; Bertsekas, 2007) and can be alternatively expressed as $\begin{array} { r } { \rho ^ { \pi } = \sum _ { s \in \mathcal { S } } d ^ { \pi } ( s ) r ^ { \pi } ( s ) } \end{array}$ , where $d ^ { \pi } ( s )$ is the stationary measure corresponding to state $s$ under the transition kernel $\mathbb { P } ^ { \pi }$ , ensuring that $d ^ { \pi }$ satisfies the equation $d ^ { \pi } \mathbb { P } ^ { \pi } = d ^ { \pi }$ . Associated with a policy is a relative state value function $v ^ { \pi } \in \mathbb { R } ^ { | s | }$ that satisfies the following average reward Bellman equation + +$$ +\rho ^ { \pi } \mathbb { 1 } + v ^ { \pi } = r ^ { \pi } + \mathbb { P } ^ { \pi } v ^ { \pi } , +$$ + +where $\mathbb { 1 }$ is the all ones vector (Puterman, 1994; Bertsekas, 2007). Note that $v ^ { \pi }$ is unique up to an additive constant. Setting $\begin{array} { r } { \sum _ { s \in \mathcal { S } } d ^ { \pi } ( s ) v ^ { \pi } ( s ) = 0 } \end{array}$ imposes an additional constraint over $v ^ { \pi }$ , providing a unique value function vector denoted by $v _ { 0 } ^ { \pi }$ , known as the basic differential reward $\begin{array} { r } { v _ { 0 } ^ { \pi } ( s ) = \operatorname { \mathbb { E } } _ { \pi } \left[ \sum _ { n = 0 } ^ { \infty } \left( r ^ { \pi } ( s _ { n } ) - \mathbf { \bar { \rho } } ^ { \pi } \right) | s _ { 0 } = s \right] } \end{array}$ can be shown that . Hence any elemen $v _ { 0 } ^ { \pi }$ can alt the set $\{ v _ { 0 } ^ { \pi } + c \bar { \mathbb { 1 } } : c \in \mathbb { R } \}$ d as is a $v ^ { \pi }$ + +The relative state action value function $Q ^ { \pi } \in \mathbb { R } ^ { S \times A }$ associated with a policy $\pi$ is defined as: + +$$ +Q ^ { \pi } ( s , a ) = r ( s , a ) + \sum _ { \stackrel { s ^ { \prime } \in S } { a ^ { \prime } \in A } } \mathbb { P } \left( s ^ { \prime } | s , a \right) \pi ( a ^ { \prime } | s ^ { \prime } ) Q ^ { \pi } ( s ^ { \prime } , a ^ { \prime } ) - \rho ^ { \pi } \qquad \forall \left( s , a \right) \in S \times A +$$ + +Similar to $v ^ { \pi }$ , $Q ^ { \pi }$ is also unique up to an additive constant. Analogously, every solution $Q ^ { \pi }$ of t in the set . Upon aver $\{ Q _ { 0 } ^ { \pi } ( s , a ) + c \mathbb { 1 } : c \in \mathbb { R } \}$ wcy re , it $Q _ { 0 } ^ { \pi } ( s , a ) =$ $\begin{array} { r } { \mathbb { E } _ { \pi } \left[ \sum _ { n = 0 } ^ { \infty } \left( r ^ { \pi } ( s _ { n } ) - \bar { \rho } ^ { \pi } \right) \big | s _ { 0 } = s , a _ { 0 } = a \right] } \end{array}$ $\pi$ $\begin{array} { r } { { v ^ { \pi } ( s ) } = \sum _ { a \in \mathcal { A } } \pi ( a | s ) Q ^ { \pi } ( s , a ) } \end{array}$ . The average reward policy gradient theorem (Sutton & Barto, 2018) for policies parameterized by $\theta$ is given by: + +$$ +\frac { \partial \rho } { \partial \theta } = \sum _ { s \in \mathcal { S } } d ^ { \pi } ( s ) \sum _ { a \in \mathcal { A } } \frac { \partial \pi ( s , a ) } { \partial \theta } Q ^ { \pi } ( s , a ) +$$ + +As we focus on tabular policies in this paper, our parameterization aligns with the tabular policy, where $\theta$ is equivalent to $\pi$ . The policy gradient update considered is defined below. + +$$ +\pi _ { k + 1 } : = \mathbf { P r o j } _ { \Pi } \left[ \left. \pi _ { k } + \eta \frac { \partial \rho ^ { \pi } } { \partial \pi } \right| _ { \pi = \pi _ { k } } \right] \qquad \forall k \geq 0 , +$$ + +where $\mathbf { P r o j } _ { \Pi }$ denotes the orthogonal projection in the Euclidean norm onto the space of randomized policies $\Pi$ and $\eta$ denotes the step size of the update. In the following subsection, we recall the policy gradient result within the framework of discounted reward MDPs and address why it cannot be directly applied to the average reward scenario. + +# 2.2 RELATIONSHIP TO DISCOUNTED REWARD MDPS + +Let $\rho _ { \mu , \gamma } ^ { \pi } : = \mu ^ { T } ( 1 - \gamma \mathbb { P } ^ { \pi } ) ^ { - 1 } r ^ { \pi }$ represent the discounted reward value function associated with policy $\pi$ , under the initial distribution $\mu \in \Delta S$ and where $\gamma$ represents the discount factor (Bertsekas, 2007). Consider the projected policy gradient update given below. + +$$ +\pi _ { k + 1 } : = \mathbf { P r o j } _ { \Pi } [ \pi _ { k } + \eta \frac { \partial \rho _ { \mu , \gamma } ^ { \pi } } { \partial \pi } | _ { \pi = \pi _ { k } } ] \qquad \forall k \ge 0 . +$$ + +When the step size $\begin{array} { r } { \eta = \frac { ( 1 - \gamma ) ^ { 3 } } { 2 \gamma | \mathcal { A } | } } \end{array}$ , the iterates $\pi _ { k }$ generated from projected gradient ascent equation 7 satisfy the following equation: + +$$ +\rho _ { \mu , \gamma } ^ { * } - \rho _ { \mu , \gamma } ^ { \pi _ { k } } \leq \frac { 2 5 6 | S | | A | } { k ( 1 - \gamma ) ^ { 5 } } \Bigg | \Bigg | \frac { d _ { \mu , \gamma } ^ { \pi ^ { * } } } { \mu } \Bigg | \Bigg | _ { \infty } ^ { 2 } , +$$ + +where $\rho _ { \mu , \gamma } ^ { * }$ represents the optimal value function under initial distribution $\mu$ , and $d _ { \mu , \gamma } ^ { \pi ^ { * } } : = ( 1 -$ $\gamma ) \mu ^ { T } ( 1 - \gamma \mathbb { P } ^ { \pi ^ { * } } ) ^ { - 1 }$ represents the state occupancy measure under optimal policy $\pi ^ { * }$ (Xiao, 2022b). Under some mild conditions the average reward $\rho ^ { \pi }$ associated with a policy $\pi$ and the value function $\rho _ { \mu , \gamma } ^ { \pi } ( s )$ are related as below: + +$$ +\rho ^ { \pi } = \operatorname* { l i m } _ { \gamma \to 1 } ( 1 - \gamma ) \rho _ { \mu , \gamma } ^ { \pi } ( s ) . +$$ + +Note that the above relation (Bertsekas, 2007; Ross, 1983) holds for all $s \in S$ and all $\mu \in \Delta S$ since the average reward is independent of the initial state distribution. Upon leveraging the relation in equation 9 and multiplying equation 8 with $( 1 - \gamma )$ , it is apparent that the upper bound of equation 8 in the limit of $\gamma 1$ tends to infinity. This is due to $\left( 1 - \gamma \right) ^ { 4 }$ that remains in the denominator of equation 8 upon multiplying with $( 1 - \gamma )$ . Therefore it is necessary to devise an alternative proof technique in order to analyze the global convergence of policy gradient in the context of average reward MDPs. Prior to presenting the main result and its proof, we state the assumption used in our analysis. + +Assumption 1. For every policy $\pi \in \Pi$ , the transition matrix $\mathbb { P } ^ { \pi }$ associated with the induced Markov chain is irreducible and aperiodic. This assumption also means that there exist constants $C _ { e } < \infty$ and $\lambda \in [ 0 , 1 )$ such that for any $k \in \mathbb N$ and any $\pi \in \Pi$ , the Markov chain corresponding to $\mathbb { P } ^ { \pi }$ is geometrically ergodic i.e., $\| \left( \mathbb { P } ^ { \pi } \right) ^ { k } - \mathbb { 1 } \left( d ^ { \pi } \right) ^ { \top } \| _ { \infty } \leq C _ { e } \lambda ^ { k }$ . + +# 3 MAIN RESULTS + +Theorem 1. Let $\rho ^ { \pi _ { k } }$ be the average reward corresponding to the policy iterates $\pi _ { k }$ , obtained through the policy gradient update equation $6 .$ . Let $\rho ^ { * }$ represent the optimal average reward, that is, $\rho ^ { * } = \mathrm { m a x } _ { \pi \in \Pi } \rho ^ { \pi }$ . There exist constants $L _ { 2 } ^ { \Pi }$ and $C _ { P L }$ , determined by the underlying MDP, such that when the step size $\begin{array} { r } { \eta < \frac { 1 } { L _ { 2 } ^ { \Pi } } } \end{array}$ , the following holds: + +• For all MDPs it is true that, + +$$ +\begin{array} { r l r } & { } & { \rho ^ { * } - \rho ^ { \pi _ { k } } \leq \frac { 1 } { \frac { 1 } { \rho ^ { * } - \rho ^ { \pi _ { 0 } } } + \nu k } , \qquad \forall k \geq 0 . } \\ & { } & \\ & { } & { : = \left( \frac { 1 } { 3 2 C _ { P L } ^ { 2 } | \mathcal { S } | L _ { 2 } ^ { \Pi } } \right) \left( 1 + 4 \left( \frac { 1 } { 3 2 C _ { P L } ^ { 2 } | \mathcal { S } | L _ { 2 } ^ { \Pi } } \right) \right) ^ { - \frac { 3 } { 2 } } } \end{array} +$$ + +• For simple MDPs (i.e. $L _ { 2 } ^ { \Pi } \ll 1 .$ ) we obtain exponential convergence, that is + +$$ +\rho ^ { * } - \rho ^ { \pi _ { k } } \leq c ^ { - \frac { k } { 2 } } \left( \rho ^ { * } - \rho ^ { \pi _ { 0 } } \right) ^ { \frac { 1 } { 2 ^ { k } } } , \qquad \forall k \geq 0 +$$ + +where $\begin{array} { r } { \frac { 1 } { c } = 3 2 | S | L _ { 2 } ^ { \Pi } C _ { P L } ^ { 2 } < 1 } \end{array}$ . + +Remark: It is worth noting that the above bounds correspond to a regret of $O ( \log ( T ) )$ . While regret is typically a concept associated with online learning settings, we present it here to facilitate the development of learning algorithms inspired by the findings of this work. All previous results on convergence for discounted MDPs are of the form $\frac { \sigma } { k ^ { p } }$ (Agarwal et al., 2020; Mei et al., 2022; Xiao, 2022a), where $\sigma$ is a large constant. However, since the worst sub-optimality is 1, the bound $\frac { \sigma } { k ^ { p } }$ becomes less meaningful for initial $k$ . In contrast, our bound $\frac { 1 } { \rho ^ { * } - \rho ^ { \pi _ { 0 } } + \nu k }$ is meaningful from the very first iteration. The maximum sub-optimality is $\rho ^ { * } - \rho ^ { \pi _ { 0 } }$ at $k = 0$ , and it decreases monotonically thereafter. + +Our second result, an observation that is novel to this work, shows that simple MDPs exhibit much faster (linear) convergence rates. This explains why techniques like reward shaping, which simplify the MDP, can be highly effective. + +# 3.1 KEY IDEAS AND PROOF OUTLINE + +A similar result was proved for discounted reward MDPs in Agarwal et al. (2020); Xiao (2022a). An important property pivotal to the global convergence analysis of the projected policy gradient is the smoothness of the discounted reward value function. Demonstrating the smoothness of the discounted reward value function is relatively straightforward due to the contractive properties of the discount factor. However, this poses a significant challenge in the context of average reward MDPs. Here, the absence of a discount factor as a source of contraction, coupled with the lack of uniqueness in the average reward value function, complicates the task of proving the smoothness of the average reward. Therefore, the first important property we prove is the smoothness of average reward. + +# 3.1.1 SMOOTHNESS OF AVERAGE REWARD + +A differentiable function $f : { \mathcal { C } } \mathbb { R }$ is called $L$ -smooth if it satisfies + +$$ +\| \nabla f ( y ) - \nabla f ( x ) \| _ { 2 } \leq L \| y - x \| _ { 2 } \qquad \forall y , x \in \mathcal { C } . +$$ + +where $\mathcal { C }$ is some subset of $\mathbb { R } ^ { n }$ . Further if the function is $L$ -smooth, it satifies the following property. + +$$ +\left| f ( y ) - f ( x ) - \langle \nabla f ( x ) , y - x \rangle \right| \leq { \frac { L } { 2 } } \| y - x \| ^ { 2 } \qquad \forall y , x \in { \mathcal { C } } , +$$ + +From the above definition, it is apparent that if $f$ is $L$ -smooth then $c f$ is $| c | L$ -smooth for any $c \in \mathbb { R }$ . It can be shown that the infinite horizon discounted reward $V _ { \mu , \gamma } ^ { \pi }$ is $\frac { 2 \gamma | \mathcal { A } | } { ( 1 - \gamma ) ^ { 3 } }$ -smooth. Leveraging the result in equation 9, one can see that the smoothness constant of $\rho ^ { \pi }$ is $\begin{array} { r } { \operatorname* { l i m } _ { \gamma \to 1 } \frac { 2 \gamma | \mathcal { A } | } { ( 1 - \gamma ) ^ { 2 } } \to \infty } \end{array}$ . Hence, the smoothness of the discounted reward cannot be leveraged to show the smoothness of the average reward. + +In this paper, we establish the smoothness of the infinite horizon average reward by first establishing the smoothness of the associated relative value function. We then leverage the average reward Bellman Equation 3 to establish the smoothness of the average reward in terms of the smoothness of the relative value function. However, since the relative value function is unique up to an additive constant, we consider the projection of the value function onto the subspace orthogonal to the $\mathbb { 1 }$ vector. This provides us with an unique representation of the value function whose smoothness can be evaluated. + +Let $\Phi \in \mathbb { R } ^ { | S | \times | S | }$ be the projection matrix that maps any vector to its orthogonal projection in the Euclidean norm onto the subspace perpendicular to the $\mathbb { 1 }$ vector. Then the following lemma holds. + +Lemma 1. Let $I$ be the identity matrix and $\mathbb { 1 }$ be the all ones vector, both of dimension $| S |$ . Then the orthogonal projection matrix is given by $\begin{array} { r } { \Phi = \left( I - \frac { \mathbb { 1 } \mathbb { 1 } ^ { \top } } { | S | } \right) } \end{array}$ . The unique value function $v _ { \phi } ^ { \pi }$ is obtained as a solution to the following fixed point equation, + +$$ +v _ { \phi } ^ { \pi } = \Phi \left( r ^ { \pi } + \mathbb { P } ^ { \pi } v _ { \phi } ^ { \pi } - \rho ^ { \pi } \mathbb { 1 } \right) +$$ + +and can be alternatively represented as + +$$ +\begin{array} { r } { v _ { \phi } ^ { \pi } = \left( I - \Phi \mathbb { P } ^ { \pi } \right) ^ { - 1 } \Phi r ^ { \pi } . } \end{array} +$$ + +Since $\mathbb { P } ^ { \pi }$ has 1 as its Perron Frobenius eigenvalue, $( I - \mathbb { P } ^ { \pi } )$ is a singular matrix. It can be verified that $\Phi \mathbb { 1 } = 0$ , hence $\mathbb { 1 }$ is an eigenvector of $\Phi \mathbb { P } ^ { \pi }$ for all $\pi$ with a corresponding eigenvalue of 0. It can subsequently be proven that the rest of the eigenvalues of $\Phi \mathbb { P } ^ { \pi }$ are all less than one in terms of their absolute value and hence equation 15 is well defined. + +With a unique closed form for the average reward value function established, the subsequent task is to determine its smoothness constant. Given that the smoothness constant of a function $f$ corresponds to the largest eigenvalue of its Hessian, we adopt an analytical approach similar to that presented in Agarwal et al. (2020). This involves utilizing directional derivatives and evaluating the maximum rate of change of derivatives across all directions within the policy space. It’s important to note that since we are maximizing over directions expressible as differences between any two policies within the policy space, the resulting Lipschitz and smoothness constants are referred to as the restricted Lipschitz and smoothness constants, respectively. The restricted smoothness of the average reward value function is stated below. + +Lemma 2. For any policy $\pi \in \Pi$ , there exist constants $C _ { m } , C _ { p } , C _ { r } , \kappa _ { r } \in \mathbb { R } ^ { + }$ which are determined by the underlying MDP, such that the value function $v _ { \phi } ^ { \pi }$ is $4 \left( 2 C _ { m } ^ { 3 } C _ { p } ^ { 2 } \kappa _ { r } + C _ { m } ^ { 2 } C _ { p } C _ { r } \right)$ -smooth. + +Since the average reward value function is Lipschitz and smooth with respect to its policy, one can directly utilize this property to establish the Lipschitzness and smoothness of the average reward. These results are characterized in the following lemmas. + +Lemma 3. For any policy $\pi \in \Pi$ , there exist constants $C _ { m } , C _ { p } , C _ { r } , \kappa _ { r } \in \mathbb { R } ^ { + }$ which are determined by the underlying MDP, such that the average reward $\rho ^ { \pi }$ is $L _ { 1 } ^ { \Pi }$ -Lipschitz. + +$$ +\left| \left. { \frac { \partial \rho ^ { \pi } } { \partial \pi } , \pi ^ { \prime } - \pi } \right. \right| \le L _ { 1 } ^ { \Pi } \| \pi ^ { \prime } - \pi \| _ { 2 } , \qquad \forall \pi , \pi ^ { \prime } \in \Pi , +$$ + +where ${ \cal L } _ { 1 } ^ { \mathrm { I I } } = 2 ( C _ { r } + C _ { p } C _ { m } \kappa _ { r } + 2 ( C _ { m } ^ { 2 } C _ { p } \kappa _ { r } + C _ { m } C _ { r } ) )$ + +The restricted Lipschitzness of the average reward is utilized to prove its restricted smoothness. + +Lemma 4. For any policy $\pi \in \Pi$ , there exist constants $C _ { m } , C _ { p } , C _ { r } , \kappa _ { r } \in \mathbb { R } ^ { + }$ which are determined by the underlying MDP, such that the average reward $\rho ^ { \pi }$ is $L _ { 2 } ^ { \bar { \Pi } }$ -smooth. + +$$ +\begin{array} { r l r } { { \pi ^ { \prime } - \pi , \frac { \partial ^ { 2 } \rho ^ { \pi } } { \partial \pi ^ { 2 } } ( \pi ^ { \prime } - \pi ) \le \frac { L _ { 2 } ^ { \Pi } } { 2 } \| \pi ^ { \prime } - \pi \| _ { 2 } ^ { 2 } } } & { \forall \pi , \pi ^ { \prime } \in \Pi , } & { ( 1 7 ) } \\ & { \overset { \mathrm { I } } { : } = 4 ( C _ { p } ^ { 2 } C _ { m } ^ { 2 } \kappa _ { r } + C _ { p } C _ { m } C _ { r } + ( C _ { p } + 1 ) ( C _ { m } ^ { 2 } C _ { p } \kappa _ { r } + C _ { m } C _ { r } ) + 4 ( C _ { m } ^ { 3 } C _ { p } ^ { 2 } \kappa _ { r } + C _ { m } ^ { 2 } C _ { p } C _ { r } ) ) . } & \end{array} +$$ + +Note that the restricted Lipschitz constant of the average reward is upper bounded by its general Lipschitz constant: + +$$ +\operatorname* { m a x } _ { \substack { \pi ^ { \prime } \in \Pi : \| \pi ^ { \prime } - \pi \| _ { 2 } \le 1 } } \left| \left. \frac { \partial \rho ^ { \pi } } { \partial \pi } , \pi ^ { \prime } - \pi \right. \right| \le \operatorname* { m a x } _ { \substack { u \in \mathbb { R } ^ { S } \times A : \| u \| _ { 2 } \le 1 } } \left| \left. \frac { \partial \rho ^ { \pi } } { \partial \pi } , u \right. \right| +$$ + +By confining our analysis of the smoothness constants to the policy class, we introduce a dependency of our convergence bounds on MDP-specific constants, including $C _ { r } , C _ { p } , C _ { m } , C _ { e }$ and $\kappa _ { r }$ . These constants capture the complexity of the underlying MDP and are exclusive to the analysis presented in this paper, as there appears to be no such dependency observed in the global convergence bounds of Agarwal et al. (2020). A more detailed description of these constants can be found in Table 1, where $k _ { 1 }$ and $k _ { 2 }$ represent MDP-independent numeric constants. These constants, which rely on the characteristics of the MDP, suggest that the projected policy gradient may achieve faster convergence in MDPs with lower complexity as opposed to those with higher complexity. The range of these constants can be found in Appendix A. We now proceed to analyze the convergence of projected policy gradient utilizing the smoothness of the average reward. + +# 3.1.2 CONVERGENCE OF POLICY GRADIENT + +Using the smoothness property of the average reward, it is possible to show that the improvement in the successive average reward iterates is bounded from below by the product of the smoothness constant and the difference in the policy iterates, as described in the lemma below. + +Lemma 5. Let $\rho ^ { \pi _ { k } }$ be the average reward corresponding to the policy iterate $\pi _ { k }$ obtained from equation ${ \it 6 }$ . Let $L _ { 2 } ^ { \Pi }$ be as in Lemma 4. Then, + +$$ +\rho ^ { \pi _ { k + 1 } } - \rho ^ { \pi _ { k } } \geq \frac { L _ { 2 } ^ { \Pi } } { 2 } \| \pi _ { k + 1 } - \pi _ { k } \| ^ { 2 } , \qquad \forall k \in \mathbb { N } . +$$ + +Successively increasing iterates are not sufficient to guarantee finite time global convergence bounds. It is therefore necessary to bound the suboptimality associated with each iterate. We do so by leveraging the performance difference lemma stated below. + +Lemma 6. Let $\rho ^ { * }$ be the globally optimal average reward. Then for any $\pi \in \Pi$ , the suboptimality of $\rho ^ { \pi }$ can be expressed as: + +$$ +\rho ^ { * } - \rho ^ { \pi } = \sum _ { s } d ^ { \pi ^ { * } } ( s ) \sum _ { a } Q ^ { \pi } ( s , a ) [ \pi ^ { * } ( a | s ) - \pi ( a | s ) ] . +$$ + +Proof. The proof can be found in Cao (1999). + +In the next lemma, we upper bound the right-hand side of equation 20 in terms of the gradient of $\rho ^ { \pi }$ + +Table 1: Constants capturing the MDP Complexity + +
DefinitionRangeRemark
Cmmaxππ∥(I − ΦPπ)−1‖2Ce|S| 1−λ See Assumption 1 for definition of Ce and λLowest rate of mixing
Cp∥Pπ′−Pπ∥ maxπ,π′Π ‖π′−πk2[0, √|A|]Diameter of transition kernel
Crkrπ′{-rπ maxπ,π[0, √|A|]Diameter of reward function
Krkπ′−πk2 maxπ∥kΦrπ‖k∞[0, 2)Variance of reward function
LCr + CpCmκr +[0, k1 √|A|C 2m]Restricted Lipschitz constant
L1 42(C mCpκr + CmCr) Cp C 2κr+CpCmCr+(Cp + 1)(C mCpKr + CmCr) +[0, k2| A |C 3 ]Restricted smoothness constant
+ +$C _ { p } , C _ { m }$ are defined using operator norm w.r.t. $L _ { \infty }$ norm. Precisely, $\begin{array} { r } { C _ { m } = \operatorname* { m a x } _ { \boldsymbol { \pi } } \operatorname* { m a x } _ { | | \boldsymbol { v } | | _ { \infty } \leq 1 } | | ( I - \Phi P ^ { \boldsymbol { \pi } } ) ^ { - 1 } \boldsymbol { v } | | _ { \infty } , } \end{array}$ , and $\begin{array} { r } { C _ { p } = \operatorname* { m a x } _ { \pi , \pi ^ { \prime } \in \Pi } \operatorname* { m a x } _ { | | v | | _ { \infty } \le 1 } \frac { | | ( { P ^ { \pi ^ { \prime } } } - { P ^ { \pi } } ) v | | _ { \infty } } { | | \pi ^ { \prime } - \pi | | _ { 2 } } } \end{array}$ + +Lemma 7. The suboptimality of any $\pi \in \Pi$ satisfies: + +$$ +\rho ^ { * } - \rho ^ { \pi } \leq C _ { P L } \operatorname* { m a x } _ { \pi ^ { \prime } \in \Pi } \left. \pi ^ { \prime } - \pi , \frac { \partial \rho ^ { \pi } } { \partial \pi } \right. , \qquad \forall \pi \in \Pi , +$$ + +where $\begin{array} { r } { C _ { P L } = \operatorname* { m a x } _ { \pi \in \Pi } \frac { { d ^ { \pi ^ { * } } } ( s ) } { { d ^ { \pi } } ( s ) } } \end{array}$ + +Note that $C _ { P L }$ is a constant that is proportional to the size of the state space. We do not know if the appearance of such a constant is inevitable or not; however, it should be noted such a constant appears in prior works on discounted reward problems as well (Agarwal et al., 2020; Xiao, 2022a). + +It is possible to further upper bound the expression in Lemma 7 using the smoothness property of the average reward. + +Lemma 8. Let $\pi _ { k }$ be the policy iterates generated by equation 6. Then for all $\pi ^ { \prime } \in \Pi$ it is true that, + +$$ +\Big \langle \frac { \partial \rho _ { \pi _ { k + 1 } } } { \partial \pi _ { k + 1 } } , \pi ^ { \prime } - \pi _ { k + 1 } \Big \rangle \leq 4 \sqrt { | { \cal S } | } L _ { 2 } ^ { \Pi } \| \pi _ { k + 1 } - \pi _ { k } \| , +$$ + +Lemmas 5,7 and 8 are combined to prove the result in Theorem 1. + +# 3.2 EXTENSION TO DISCOUNTED REWARD MDPS + +Existing performance bounds in the context of discounted reward MDPs require an iteration complexity of $O \left( \frac { | \mathcal { S } | | \mathcal { A } | } { ( 1 - \gamma ) ^ { 5 } \epsilon } \right)$ to achieve policies with suboptimality of $\epsilon$ (Xiao, 2022a). These bounds are independent of the hardness of the underlying MDP. Our approach improves on this bound, yield-$O ( \frac { | S | L _ { 2 } ^ { \Pi } } { \epsilon } )$ ity, whe,where ${ \cal L } _ { 2 } ^ { \Pi } = C _ { p } ^ { 2 } \widehat C _ { m } ^ { 2 } \kappa _ { r } + C _ { p } \widehat C _ { m } C _ { r } + ( C _ { p } + 1 ) ( \widehat C _ { m } ^ { 2 } C _ { p } \kappa _ { r } +$ $\widehat { C } _ { m } C _ { r } ) + 4 ( \widehat { C } _ { m } ^ { 3 } C _ { p } ^ { 2 } \kappa _ { r } + \widehat { C } _ { m } ^ { 2 } C _ { p } C _ { r } )$ $\widehat { C } _ { m } : = \| ( I - \gamma \mathbb { P } ^ { \pi } ) ^ { - 1 } \|$ $\widehat { C } _ { m } \leq \frac { 1 } { 1 - \gamma }$ . Hence, the iteration complexity improves to $\begin{array} { r } { O \left( \frac { L _ { 2 } ^ { \Pi } | \boldsymbol { S } | } { ( 1 - \gamma ) ^ { 5 } \epsilon } \right) } \end{array}$ , as the constants satisfy $\kappa _ { r } \leq 2$ and $C _ { p } , C _ { r } \leq \sqrt { | { \cal A } | }$ . Further, the approach considered in this paper provides faster convergence rates for MDPs with low complexity, i.e., MDPs that have low values of $C _ { p }$ or $C _ { r }$ . The exact performance bounds can be obtained from an approach similar to the one outlined in Kumar et al. (2023), where $L _ { 2 } ^ { \Pi }$ represents the restricted smoothness constant of the discounted return $\rho _ { \gamma } ^ { \pi }$ . This constant can be derived through a process analogous to the one described in this paper. + +![](images/figures/avg-reward-pg-fig-0001.jpg) +Figure 1: Improvement in average reward as a function of MDP complexity + +For instance, consider a trivial MDP for which $C _ { p } = 0$ or $\kappa _ { r } ~ = ~ 0$ (implies $C _ { r } = 0 \}$ ), i.e., an MDP where the transition kernel is independent of the action enacted. For this trivial MDP every policy is an optimal policy. The state of the art convergence guarantees (Xiao, 2022a), still requires $O ( | S | | \mathcal { A } | \epsilon ^ { - 1 } )$ iterations for $\epsilon$ close optimal policy. Whereas, the performance bounds presented in this paper predict $O ( | S | \epsilon ^ { - 1 } )$ iterations for convergence. Thus the constant $L _ { 2 } ^ { \Pi }$ captures the hardness of the MDP. Therefore, MDPs with lower complexity, i.e., lower values of $L _ { 2 } ^ { \mathrm { \bar { I I } } }$ , converge faster than MDPs with higher complexity, thus improving on current complexity-independent bounds. + +# 4 SIMULATIONS + +Here, we present simulations corresponding to two MDP complexity measures. In order to study the convergence of projected policy gradient in the context of average reward MDPs and its dependence on the underlying MDP complexity, we present simulation results corresponding to two complexity measures: the cardinality of state and action spaces, and the diameter of the reward function. + +Figure 1(a) considers MDPs with $( | S | , | A | ) = \{ ( 3 , 3 ) , ( 9 , 9 ) , ( 8 1 , 8 1 ) \} .$ . We construct the transition kernel and the reward function in the same manner for all MDPs, which we discuss in the appendix. Projected policy gradient was implemented for 2000 iterations and the overall average reward is plotted as a function of iteration number. As expected, the convergence rate is slower when $( | S | , | A | )$ are larger due to the fact that the reward smoothness constant is larger. This reduction stems from small values of $C _ { M } , C _ { r } , C _ { p }$ , which are characteristic of MDPs with smaller state and action space cardinalities when the transition kernel and reward structures are similar. A less obvious result is that, even for MDPs with a fixed cardinality of state and action spaces, the rate of convergence can be considerably different as shown in Figure 1(b). For this simulation, we fix the state and action space cardinality at $( \vert S \vert , \vert A \vert ) = ( 1 6 , 1 \bar { 6 } )$ . We randomly generate a transition kernel, which remains constant across different single-step reward functions corresponding to varying reward variances. In particular, we consider four different reward variances - no variance, low, high and maximal variance. We recall the definition of $C _ { r }$ found in Table 1. We see that $C _ { r }$ scales with reward variance. Specifically, $C _ { r }$ is large when small changes to the policy result in significant modifications to the mean reward. Therefore, we anticipate that higher reward variance will lead to slower convergence. Additional details are found in the appendix. The observed convergence trend aligns with the theoretical bounds obtained, indicating that MDPs with small values of $C _ { r }$ tend to converge relatively faster. + +Next, we discuss the impact of $C _ { p }$ on convergence of policy gradient. We consider MDPs of size 16, i.e., $( \vert S \vert , \vert A \vert ) = ( 1 6 , \bar { 1 6 } )$ . We generate three different transition kernels. The first is uniform, so the actions do not change the transition probabilities, the second is deterministic (i.e., there exists some $s ^ { \prime } \in \mathcal { S }$ such that $\mathbb { P } ( s ^ { \prime } | s , a ) = 1$ for all $( s , a ) \in ( S , { \mathcal { A } } ) )$ , and the last is non-uniform but stochastic transition kernel. We recall the definition of $C _ { p }$ from Table 1. We see that $C _ { p }$ is larger when the transition probabilities change by a greater amount with small changes to the policy. Thus, deterministic MDPs should have higher $C _ { p }$ values then ones that are more stochastic. Additional details are provided in the appendix. We run the policy gradient algorithm considered in this paper for each MDP setting and plot the overall change in average reward as a function of iterations, for 3000 iterations. Figure 2 indicates that policy gradient in MDPs corresponding to small values of $C _ { p }$ converges relatively faster than for MDPs corresponding to large values of $C _ { p }$ . Hence, the performance bounds obtained in Theorem 1 are in some sense, more representative of the empirical convergence trend of the policy gradient algorithm. + +![](images/figures/avg-reward-pg-fig-0002.jpg) +Figure 2: Convergence as a function of $C _ { p }$ + +Note on Limitations and Future Work: Although we study tabular policies, this approach can be generalized to parametric class of policies. This study uses the exact value of the gradient and does not account for learning errors in the analysis. Nonetheless, we highlight that this is the first comprehensive proof of global convergence for policy gradient methods in average reward MDPs. Future work will focus on incorporating learning errors into this framework. + +# 5 CONCLUSION + +In this paper, we presented the first comprehensive finite-time global convergence analysis of policy gradient for infinite horizon average reward MDPs. Key contributions include eliminating the smoothness assumption from previous work, deriving an explicit expression for the smooth average cost, and proving sublinear convergence with a regret of $\bar { O } ( \log { ( \bar { T } ) } )$ . Our findings offer a more general and robust understanding of policy gradient methods in average reward MDPs, addressing long-standing challenges such as the lack of uniqueness in value functions. We also extended our analysis to discounted reward MDPs, providing stronger performance bounds by incorporating a complexity parameter beyond state and action space sizes. The theoretical results were further supported by simulations that highlighted how the structure of the underlying MDP influences convergence rates. These insights open avenues for refining performance bounds and exploring real-world applications in both average and discounted reward MDPs. + +# ACKNOWLEDGEMENTS + +Research conducted by Y.M. and R.S. was supported in part by NSF Grants CNS 23-12714, CCF 22-07547, CNS 21-06801, and AFOSR Grant FA9550-24-1-0002. + +This research was also supported by the Israel Science Foundation (Grants No. 2199/20 and 3109/24). + +# REFERENCES + +Yasin Abbasi-Yadkori, Peter Bartlett, Kush Bhatia, Nevena Lazic, Csaba Szepesvari, and Gellért Weisz. Politex: Regret bounds for policy iteration using expert prediction. In International + +Conference on Machine Learning, pp. 3692–3702. PMLR, 2019. + +Alekh Agarwal, Sham M. Kakade, Jason D. Lee, and Gaurav Mahajan. On the theory of policy gradient methods: Optimality, approximation, and distribution shift, 2020. + +Qinbo Bai, Washim Uddin Mondal, and Vaneet Aggarwal. 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Advances in Neural Information Processing Systems, 34:2228–2240, 2021. + +# A SMOOTHNESS OF AVERAGE REWARD + +A.1 PROOF OF LEMMA 1 + +Consider the subspace orthogonal $E$ to the all ones vector $\mathbb { 1 } \in \mathbb { R } ^ { | S | }$ defined below: + +$$ +E = \operatorname { s p a n } \left\{ \theta \in \mathbb { R } ^ { | s | } : \theta ^ { \top } \mathbb { 1 } = 0 . \right\} +$$ + +The orthogonal projection $v _ { \phi }$ of a vector $v$ in the Euclidean norm onto the subspace $E$ is defined as: + +$$ +v _ { \phi } = \underset { u \in \mathbb { E } } { \arg \operatorname* { m i n } } \ : | | v - u | | _ { 2 } +$$ + +It can be checked that the closed form expression for $v _ { \phi }$ is given by: + +$$ +\boldsymbol { v } _ { \phi } = \left( I - \frac { \mathbb { 1 1 } ^ { \top } } { | S | } \right) \boldsymbol { v } +$$ + +where $I \in \mathbb { R } ^ { | S | \times | S | }$ is the identity matrix. + +Consider the projection of the vector $r ^ { \pi } + \mathbb { P } ^ { \pi } v ^ { \pi } - \rho ^ { \pi } \mathbb { 1 }$ onto $E$ for any policy $\pi \in \Pi$ . The above projection is identical to the projection of $r ^ { \pi } + \mathbb { P } ^ { \pi } v ^ { \pi }$ onto $E$ , since $\rho ^ { \pi } \mathbb { 1 }$ lies in the nullspace of $\Phi$ . + +$$ +\begin{array} { r l } & { \Phi ( r ^ { \pi } - \rho ^ { \pi } \mathbb { 1 } + \mathbb { P } ^ { \pi } v ) = ( r ^ { \pi } - \rho ^ { \pi } \mathbb { 1 } + \mathbb { P } ^ { \pi } v ) - \left. \mathbb { 1 } , r ^ { \pi } - \rho ^ { \pi } \mathbb { 1 } + \mathbb { P } ^ { \pi } v \right. \frac { 1 } { | S | } } \\ & { \qquad = ( r ^ { \pi } + \mathbb { P } ^ { \pi } v ) - \left. \mathbb { 1 } , r ^ { \pi } + \mathbb { P } ^ { \pi } v \right. \frac { 1 } { | S | } } \\ & { \qquad = r ^ { \pi } - \langle r ^ { \pi } , \mathbb { 1 } \rangle \frac { 1 } { | S | } + \mathbb { P } ^ { \pi } v - \langle \mathbb { 1 } , \mathbb { P } ^ { \pi } v \rangle \frac { 1 } { | S | } } \\ & { \qquad = r ^ { \pi } - \langle r ^ { \pi } , \mathbb { 1 } \rangle \frac { 1 } { | S | } + \mathbb { P } ^ { \pi } v - \frac { 1 } { | S | } ( \mathbb { 1 } ^ { \top } \mathbb { P } ^ { \pi } v ) } \\ & { \qquad = ( I - \frac { \mathbb { 1 } \mathbb { 1 } ^ { T } } { | S | } ) r ^ { \pi } + ( I - \frac { \mathbb { 1 } \mathbb { 1 } ^ { T } } { | S | } ) \mathbb { P } ^ { \pi } v } \\ & { \qquad = \Phi \left[ r ^ { \pi } + \mathbb { P } ^ { \pi } v \right] . } \end{array} +$$ + +Consider the average reward Bellman equation corresponding to policy $\pi \in \Pi$ : + +$$ +\rho ^ { \pi } \mathbb { 1 } + v ^ { \pi } = r ^ { \pi } + \mathbb { P } ^ { \pi } v ^ { \pi } +$$ + +Imposing an additional constraint $v ^ { \pi ^ { \top } } \mathbb { 1 } = 0$ yields a unique average reward value function denoted by $v _ { \phi } ^ { \pi }$ . Moreover, it is true that, + +$$ +\begin{array} { r } { \Phi \boldsymbol { v } _ { \phi } ^ { \pi } + \Phi \rho ^ { \pi } \mathbb { 1 } = \Phi \boldsymbol { r } ^ { \pi } + \Phi \mathbb { P } ^ { \pi } \boldsymbol { v } _ { \phi } ^ { \pi } } \\ { \implies \Phi \boldsymbol { v } _ { \phi } ^ { \pi } = \Phi \boldsymbol { r } ^ { \pi } + \Phi \mathbb { P } ^ { \pi } \boldsymbol { v } _ { \phi } ^ { \pi } , } \\ { \overset { \mathrm { ( a ) } } { \Longrightarrow } \boldsymbol { v } _ { \phi } ^ { \pi } = \Phi [ \boldsymbol { r } ^ { \pi } + \mathbb { P } ^ { \pi } \boldsymbol { v } _ { \phi } ^ { \pi } ] , } \end{array} +$$ + +where (a) is true because $v ^ { \pi ^ { \top } } \mathbb { 1 } = 0 \implies \Phi v ^ { \pi } = v ^ { \pi }$ . Thus the projected value function with an unique representation is given by: + +$$ +\boldsymbol { v } _ { \phi } ^ { \pi } = [ I - \Phi \mathbb { P } ^ { \pi } ] ^ { - 1 } \boldsymbol { \Phi } \boldsymbol { r } ^ { \pi } , +$$ + +and the existence of the inverse is proven in Subsection A.2, Lemma 12. An alternate expression for the projected value function is given by: $\begin{array} { r } { v _ { \phi } ^ { \pi } = \left( I + \mathbb { 1 } \mathbb { 1 } ^ { \top } D - \frac { \mathbb { 1 } \mathbb { 1 } ^ { \top } } { | S | } \right) v _ { 0 } ^ { \pi } } \end{array}$ , where $D \in \mathbb { R } ^ { | S | \times | S | }$ is a diagonal matrix whose entries correspond to the stationary measure over the states associated with policy $\pi$ . See Tsitsiklis & Van Roy (1999) for more details. + +# A.2 PROOF THAT EIGENVALUES OF $( I - \Phi \mathbb { P } ^ { \pi } )$ ARE NON-ZERO + +In this subsection, we introduce the lemmas required to establish the proof of the eigenvalues of $\begin{array} { r l } { { ( I - \frac { \mathbb { 1 } \mathbb { 1 } } { | S | } ) \mathbb { P } ^ { \pi } } } \end{array}$ being nonzero. We use the following notation: $\mathbb { T } \in \mathbb { R } ^ { n }$ represents the all ones vector and $\ b { I } \in \mathbb { R } ^ { n \times n }$ is the identity matrix. + +Lemma 9. Let $A \in \mathbb { R } ^ { n \times n }$ be a stochastic matrix. It is true that + +$$ +\left( \left( I - { \frac { \mathbb { 1 } \mathbb { 1 } ^ { \top } } { n } } \right) A \right) ^ { k } = \left( I - { \frac { \mathbb { 1 } \mathbb { 1 } ^ { \top } } { n } } \right) A ^ { k } . +$$ + +Proof. For any $k \in \mathbb N$ , consider, + +$$ +\begin{array} { r l } & { \left( I - \frac { 1 1 ^ { \top } } { n } \right) A ^ { k } \left( I - \frac { 1 1 ^ { \top } } { n } \right) A = \left( A ^ { k } - \frac { 1 1 ^ { \top } } { n } A ^ { k } \right) \left( A - \frac { 1 1 ^ { \top } } { n } A \right) } \\ & { \qquad = A ^ { k + 1 } - \frac { 1 1 ^ { \top } } { n } A ^ { k + 1 } - A ^ { k } \frac { 1 1 ^ { \top } } { n } A + \frac { 1 1 ^ { \top } } { n } A ^ { k } \frac { 1 1 ^ { \top } } { n } A } \\ & { \qquad \stackrel { ( a ) } = A ^ { k + 1 } - \frac { 1 1 ^ { \top } } { n } A ^ { k + 1 } - \frac { 1 1 ^ { \top } } { n } A + \frac { 1 1 ^ { \top } } { n } \frac { 1 1 ^ { \top } } { n } A , } \\ & { \qquad \stackrel { ( b ) } = A ^ { k + 1 } - \frac { 1 1 ^ { \top } } { n } A ^ { k + 1 } - \frac { 1 1 ^ { \top } } { n } A + \frac { 1 1 ^ { \top } } { n } A , } \\ & { \qquad = A ^ { k + 1 } - \frac { 1 1 ^ { \top } } { n } A ^ { k + 1 } . } \\ & { \qquad = \left( I - \frac { 1 1 ^ { \top } } { n } \right) A ^ { k + 1 } } \end{array} +$$ + +where (a) is true because $A ^ { k } \mathbb { 1 } = \mathbb { 1 }$ and (b) follows from the fact that $\begin{array} { r } { \frac { \mathbb { 1 } ^ { \top } \mathbb { 1 } } { n } = \mathbb { 1 } } \end{array}$ . From mathematical induction it thus follows that, + +$$ +\left( \left( I - { \frac { \mathbb { 1 } \mathbb { 1 } ^ { \top } } { n } } \right) A \right) ^ { k } = \left( I - { \frac { \mathbb { 1 } \mathbb { 1 } ^ { \top } } { n } } \right) A ^ { k } \qquad \forall k \in \mathbb { N } . +$$ + +Lemma 10. For any irreducible and aperiodic stochastic matrix $A \in \mathbb { R } ^ { n \times n }$ , it is true that + +$$ +\operatorname* { l i m } _ { k \to \infty } { \left( \left( I - { \frac { \mathbb { 1 } \mathbb { 1 } ^ { \top } } { n } } \right) A \right) } ^ { k } = 0 +$$ + +Proof. From Lemma 9 we have, + +$$ +\left( \left( I - { \frac { \mathbb { 1 } \mathbb { 1 } ^ { \top } } { n } } \right) A \right) ^ { k } = \left( I - { \frac { \mathbb { 1 } \mathbb { 1 } ^ { \top } } { n } } \right) A ^ { k } \qquad \forall k \in \mathbb { N } +$$ + +Since $A$ is irreducible and aperiodic, the following limit converges to the stationary distribution $d \in \mathbb { R } _ { + } ^ { n }$ associated with $A$ . + +$$ +\operatorname* { l i m } _ { k \to \infty } { \boldsymbol { A } } ^ { k } = \mathbb { 1 } { \boldsymbol { d } } ^ { \top } +$$ + +Consider the following, + +$$ +\begin{array} { r l } { \underset { k \to \infty } { \operatorname* { l i m } } \left( \left( I - \frac { 1 1 ^ { \top } } { n } \right) A \right) ^ { k } = \underset { k \to \infty } { \operatorname* { l i m } } \left( I - \frac { 1 1 ^ { \top } } { n } \right) A ^ { k } } & { } \\ & { = \left( I - \frac { 1 1 ^ { \top } } { n } \right) \underset { k \to \infty } { \operatorname* { l i m } } A ^ { k } } \\ & { = \left( I - \frac { 1 1 ^ { \top } } { n } \right) \mathrm { { l } } d ^ { \top } \quad \quad \mathrm { ( f r o m ~ E q u a t i o n ~ e q u a t i o n ~ } 4 7 ) , } \\ & { = \mathrm { { 1 } } d ^ { \top } - \frac { \mathrm { { 1 1 } } ^ { \top } } { n } \mathrm { { l } } d ^ { \top } } \\ & { \overset { ( \mathrm { a } ) } { = } \mathrm { { 1 } } d ^ { \top } - 1 d ^ { \top } } \\ & { = 0 . } \end{array} +$$ + +where (a) is true because $\begin{array} { r } { \frac { \mathbb { 1 } ^ { \top } \mathbb { 1 } } { n } = 1 } \end{array}$ + +Lemma 11. Let $A \in \mathbb { R } ^ { n \times n }$ be a matrix such that $\quad \operatorname* { l i m } _ { k \to \infty } A ^ { k } = 0 .$ . Then $\textstyle ( I - A ) ^ { - 1 } = \sum _ { k = 0 } ^ { \infty } A ^ { k }$ + +Proof. For any $K \in \mathbb N$ , consider the following, + +$$ +\begin{array} { c } { { ( I - { \cal A } ) ( \displaystyle \sum _ { k = 0 } ^ { K } { \cal A } ^ { k } ) = I - { \cal A } ^ { K + 1 } , } } \\ { { \Longrightarrow ~ ( I - { \cal A } ) ( \displaystyle \operatorname* { l i m } _ { K \infty } \displaystyle \sum _ { k = 0 } ^ { K } { \cal A } ^ { k } ) = \displaystyle \operatorname* { l i m } _ { K \infty } ( I - { \cal A } ^ { K + 1 } ) \stackrel { ( \mathrm { a } ) } { = } I , } } \end{array} +$$ + +as where (a) follows from the fact that $\textstyle ( I - A ) ^ { - 1 } = \sum _ { k = 0 } ^ { \infty } A ^ { k }$ . $\scriptstyle \operatorname* { l i m } _ { k \to \infty } A ^ { k } = 0$ . Hence the inverse of $( I - A )$ can be expressed + +Lemma 12. Let $A \in \mathbb { R } ^ { n \times n }$ be an irreducible and aperiodic stochastic matrix. Then the matrix $\begin{array} { r } { \bigg ( I - \left( I - \frac { \mathbb { 1 } \mathbb { 1 } ^ { \top } } { n } \right) A \bigg ) } \end{array}$ is invertible and its inverse is given by: + +$$ +\left( I - \left( I - { \frac { \mathbb { 1 } \mathbb { 1 } ^ { \top } } { n } } \right) A \right) ^ { - 1 } = \sum _ { k = 0 } ^ { \infty } \left( I - { \frac { \mathbb { 1 } \mathbb { 1 } ^ { \top } } { n } } \right) A ^ { k } +$$ + +Proof. Let $\lambda _ { i }$ be eigenvalues of $\textstyle \left( I - { \frac { \mathbb { 1 } \mathbb { 1 } ^ { \top } } { n } } \right) A$ . Then ${ \boldsymbol { \lambda } } _ { i } ^ { k }$ represents the eigenvalues of $\left( \left( I - { \frac { \mathbb { 1 } \mathbb { 1 } ^ { \top } } { n } } \right) A \right) ^ { k }$ . But from Lemma 10, we know that + +$$ +\operatorname* { l i m } _ { k \to \infty } { \left( \left( I - { \frac { \mathbb { 1 } \mathbb { 1 } ^ { \top } } { n } } \right) A \right) } ^ { k } = 0 +$$ + +Since eigenvalues are continuous functions of their corresponding matrices and all eigenvalues of a zero matrix are zero, we thus have, + +$$ +\operatorname* { l i m } _ { k \to \infty } \lambda _ { i } ^ { k } = 0 \qquad \forall i \in \{ 1 , \dots , n \} +$$ + +Equation 58 thus implies that $| \lambda _ { i } | < 1 , \forall i \in \{ 1 , \ldots , n \}$ . Hence the matrix $\left( I - \left( \left( I - { \frac { \mathbb { 1 } \mathbb { 1 } ^ { \top } } { n } } \right) A \right) \right)$ has all non zero eigenvalues and is thus invertible. From Lemma 11, we know that + +$$ +( I - A ) ^ { - 1 } = \sum _ { k = 0 } ^ { \infty } A ^ { k } +$$ + +when $\scriptstyle \operatorname* { l i m } _ { k \to \infty } A ^ { k } \ = \ 0$ . Since, $\begin{array} { r } { \operatorname* { l i m } _ { k \to \infty } \left( \left( I - \frac { \mathbb { 1 } \mathbb { 1 } ^ { \top } } { n } \right) A \right) ^ { k } = 0 } \end{array}$ from Lemma 10, we have the following result, + +$$ +\left( I - \left( I - { \frac { \mathbb { 1 } \mathbb { 1 } ^ { \top } } { n } } \right) A \right) ^ { - 1 } = \sum _ { k = 0 } ^ { \infty } \left( I - { \frac { \mathbb { 1 } \mathbb { 1 } ^ { \top } } { n } } \right) A ^ { k } +$$ + +From definition we have Φ = I − 11⊤n  . Hence the inverse $\begin{array} { r } { \left( 1 - \Phi \mathbb { P } ^ { \pi } \right) ^ { - 1 } = \sum _ { k = 0 } ^ { \infty } \Phi \left( \mathbb { P } ^ { \pi } \right) ^ { k } } \end{array}$ exists and is well defined for all $\pi \in { \dot { \Pi } }$ . + +# A.3 SMOOTHNESS OF THE AVERAGE REWARD VALUE FUNCTION $v _ { \phi } ^ { \pi }$ + +In order to prove the smoothness of the average reward value function and the infinite horizon average reward, we consider an analysis inspired by Agarwal et al. (2020), where instead of computing the maximum eigenvalue of the associated Hessian matrices, we consider the maximum value of the directional derivative across all directions within the policy class. + +Let $\pi , \pi ^ { \prime } \in \Pi$ be any policies within the policy class. Then define $\pi _ { \alpha }$ as a convex combination of policies $\pi$ and $\pi ^ { \prime }$ . That is + +$$ +\begin{array} { c } { { \pi _ { \alpha } : = ( 1 - \alpha ) \pi + \alpha \pi ^ { \prime } } } \\ { { { } } } \\ { { = \pi + \alpha ( \pi ^ { \prime } - \pi ) } } \\ { { { } } } \\ { { { } = \pi + \alpha u } } \end{array} +$$ + +where $u = \pi ^ { \prime } - \pi$ + +Since $\pi _ { \alpha }$ is linear in $\alpha$ , it is true that + +$$ +\nabla _ { \alpha } \pi _ { \alpha } = \frac { d ( \pi + \alpha u ) } { d \alpha } = u , \qquad \mathrm { a n d } \qquad \nabla _ { \alpha } ^ { 2 } \pi _ { \alpha } = 0 . +$$ + +This thus implies, + +$$ +\| \nabla _ { \alpha } \pi _ { \alpha } \| _ { 2 } = \| u \| _ { 2 } \leq \| \pi ^ { \prime } - \pi \| _ { 1 } \leq 2 S , \qquad \mathrm { a n d } \qquad \| \nabla _ { \alpha } ^ { 2 } \pi _ { \alpha } \| _ { 2 } = 0 , +$$ + +Thus, $\pi _ { \alpha }$ is both $\lVert u \rVert _ { 2 }$ -Lipschitz and 0-smooth with respect to $\alpha$ , for all $u$ that can be represented as the difference of any two policies. + +From the definition of $\mathbb { P } ^ { \pi }$ , we have + +$$ +\begin{array} { r l r } { \ } & { \ } & { \mathbb { P } ^ { \pi _ { \alpha } } \big ( s ^ { \prime } | s \big ) = \displaystyle \sum _ { a \in \mathcal { A } } \pi _ { \alpha } ( a | s ) \mathbb { P } ( s ^ { \prime } | s , a ) } \\ & { \ } & { = \displaystyle \sum _ { a \in \mathcal { A } } \left[ \pi ( a | s ) + \alpha u ( a | s ) \right] \mathbb { P } ( s ^ { \prime } | s , a ) } \\ & { \implies \displaystyle \frac { \partial \mathbb { P } ^ { \pi _ { \alpha } } \big ( s ^ { \prime } | s \big ) } { \partial \alpha } = \displaystyle \sum _ { a \in \mathcal { A } } u ( a | s ) \mathbb { P } ( s ^ { \prime } | s , a ) . ~ } \end{array} +$$ + +That is, + +$$ +\nabla _ { \alpha } \mathbb { P } ^ { \pi _ { \alpha } } = \mathbb { P } ^ { u } , \qquad \mathrm { c o n s e q u e n t l y } \qquad \nabla _ { \alpha } ^ { 2 } \mathbb { P } ^ { \pi _ { \alpha } } = 0 . +$$ + +From the definition of $r ^ { \pi }$ , we have + +$$ +\begin{array} { c l } { { \displaystyle r ^ { \pi _ { \alpha } } ( s ) = \sum _ { a \in \mathcal { A } } \pi _ { \alpha } ( a \vert s ) r ( s , a ) } } \\ { { \displaystyle \qquad = \sum _ { a \in \mathcal { A } } \left[ \pi ( a \vert s ) + \alpha u ( a \vert s ) \right] r ( s , a ) } } \\ { { \displaystyle \Longrightarrow \ \frac { \partial r ^ { \pi _ { \alpha } } ( s ) } { \partial \alpha } = \sum _ { a \in \mathcal { A } } u ( a \vert s ) r ( s , a ) . } } \end{array} +$$ + +That is, + +$$ +\nabla _ { \alpha } r ^ { \pi _ { \alpha } } = r ^ { u } , \qquad \mathrm { c o n s e q u e n t l y } \qquad \nabla _ { \alpha } ^ { 2 } r ^ { \pi _ { \alpha } } = 0 . +$$ + +Hence the policy $\pi _ { \alpha }$ , the associated reward $r ^ { \pi _ { \alpha } }$ and the transition kernel $\mathbb { P } ^ { \pi _ { \alpha } }$ are all Lipschitz and smooth with respect to $\alpha$ . + +Lemma 13. Let $A ( \alpha ) \in \mathbb { R } ^ { n \times n }$ be a matrix such that $\left( I - A ( \alpha ) \right)$ is invertible for all $\alpha \in [ 0 , 1 ]$ Define $M ( \alpha ) : = \left( I - A ( \alpha ) \right) ^ { - 1 }$ . Then it is true that, + +$$ +\frac { \partial ^ { 2 } M ( \alpha ) } { \partial \alpha ^ { 2 } } = \frac { \partial M ( \alpha ) } { \partial \alpha } \frac { \partial A ( \alpha ) } { \partial \alpha } M ( \alpha ) + M ( \alpha ) \frac { \partial ^ { 2 } A ( \alpha ) } { \partial \alpha ^ { 2 } } M ( \alpha ) + M ( \alpha ) \frac { \partial A ( \alpha ) } { \partial \alpha } \frac { \partial M ( \alpha ) } { \partial \alpha } . +$$ + +Proof. + +$$ +\begin{array} { c c } { { } } & { { M ( \alpha ) \left( I - A ( \alpha ) \right) = I } } \\ { { } } & { { } } \\ { { \displaystyle \frac { \ \partial { \cal M } ( \alpha ) \ } { \partial \alpha } \left( I - A ( \alpha ) \right) - \ { \cal M } ( \alpha ) \displaystyle \frac { \partial A ( \alpha ) } { \partial \alpha } = 0 } } & { { ( 7 6 ) } } \\ { { } } & { { } } \\ { { \displaystyle \frac { \ \partial { \cal M } ( \alpha ) \ } { \partial \alpha } = M ( \alpha ) \displaystyle \frac { \partial A ( \alpha ) } { \partial \alpha } { \cal M } ( \alpha ) } } & { { ( 7 7 ) } } \\ { { } } & { { } } \\ { { \displaystyle \frac { \partial ^ { 2 } { \cal M } ( \alpha ) } { \partial \alpha ^ { 2 } } = \displaystyle \frac { \partial } { \partial \alpha } \left( M ( \alpha ) \displaystyle \frac { \partial A ( \alpha ) } { \partial \alpha } { \cal M } ( \alpha ) \right) , } } & { { ( 7 8 ) } } \\ { { } } & { { } } \\ { { \displaystyle \ = \displaystyle \frac { \partial { \cal M } ( \alpha ) } { \partial \alpha } \displaystyle \frac { \partial A ( \alpha ) } { \partial \alpha } { \cal M } ( \alpha ) + { \cal M } ( \alpha ) \displaystyle \frac { \partial ^ { 2 } A ( \alpha ) } { \partial \alpha ^ { 2 } } { \cal M } ( \alpha ) + { \cal M } ( \alpha ) \displaystyle \frac { \partial A ( \alpha ) } { \partial \alpha } { \cal M } ( \alpha ) } } \end{array} +$$ + +Consider the following definition utilized in the proofs of the upcoming lemmas. + +$$ +M ^ { \pi _ { \alpha } } = [ I - \Phi \mathbb { P } ^ { \pi _ { \alpha } } ] ^ { - 1 } +$$ + +Lemma 14. Recall the definition of the projected average reward value function $v _ { \phi } ^ { \pi }$ in Equation equation $3 6$ . Value function $v _ { \phi } ^ { \pi }$ is $2 C _ { m } ^ { 2 } C _ { p } \kappa _ { r } + 2 C _ { m } C _ { r }$ -Lipschitz in $\Pi$ , that is + +$$ +\left| \left. \frac { \partial v _ { \phi } ^ { \pi } } { \partial \pi } , \pi ^ { \prime } - \pi \right. \right| \leq 2 \left( C _ { m } ^ { 2 } C _ { p } \kappa _ { r } + C _ { m } C _ { r } \right) \| \pi ^ { \prime } - \pi \| _ { 2 } , \qquad \forall \pi , \pi ^ { \prime } \in \Pi . +$$ + +Proof. + +$$ +\begin{array} { r l r } { { v _ { \phi } ^ { \pi _ { \alpha } } = M ^ { \pi _ { \alpha } } \Phi r ^ { \pi _ { \alpha } } } } \\ & { \implies \frac { \partial v _ { \phi } ^ { \pi _ { \alpha } } } { \partial \alpha } = \frac { \partial M ^ { \pi _ { \alpha } } } { \partial \alpha } \Phi r ^ { \pi _ { \alpha } } + M ^ { \pi _ { \alpha } } \Phi \frac { \partial r ^ { \pi _ { \alpha } } } { \partial \alpha } } \\ & { } & { = M ^ { \pi _ { \alpha } } \frac { \partial \Phi \mathbb { P } ^ { \pi _ { \alpha } } } { \partial \alpha } M ^ { \pi _ { \alpha } } \Phi r ^ { \pi _ { \alpha } } + M ^ { \pi _ { \alpha } } \Phi \frac { \partial r ^ { \pi _ { \alpha } } } { \partial \alpha } , \qquad \mathrm { ( f r o m ~ L e m m a ~ 1 3 ) } , } \\ & { } & { = M ^ { \pi _ { \alpha } } \Phi \mathbb { P } ^ { \pi } M ^ { \pi _ { \alpha } } \Phi r ^ { \pi _ { \alpha } } + M ^ { \pi _ { \alpha } } \Phi r ^ { u } , \qquad \mathrm { ( f r o m ~ e q u a t i o n ~ 6 9 ~ a n d ~ e q ~ } } \end{array} +$$ + +uation 73). + +$$ +\begin{array} { r l } & { \implies \Big \| \frac { \partial v _ { \phi } ^ { \pi _ { \alpha } } } { \partial \alpha } \Big \| _ { \infty } = \| M ^ { \pi _ { \alpha } } \Phi \mathbb { P } ^ { u } M ^ { \pi _ { \alpha } } \Phi r ^ { \pi _ { \alpha } } + M ^ { \pi _ { \alpha } } \Phi r ^ { u } \| _ { \infty } } \\ & { \qquad \leq \| M ^ { \pi _ { \alpha } } \| _ { \infty } \| \Phi \| _ { \infty } \| \mathbb { P } ^ { u } \| _ { \infty } \| M ^ { \pi _ { \alpha } } \| _ { \infty } \| \Phi r ^ { \pi _ { \alpha } } \| _ { \infty } + \| M ^ { \pi _ { \alpha } } \| _ { \infty } \| \Phi r ^ { u } \| _ { \infty } } \\ & { \qquad \leq 2 C _ { m } ^ { 2 } C _ { p } \kappa _ { r } + 2 C _ { m } C _ { r } . } \end{array} +$$ + +The constants $C _ { m } , C _ { p } , C _ { r }$ and $\kappa _ { r }$ are characterized in Table 1 with their respective bounds in Lemma 18. + +We can now build on the previous lemma to prove the smoothness of the average reward value function. + +Lemma 15. The value function $v _ { \phi } ^ { \pi }$ is $8 ( C _ { m } ^ { 3 } C _ { p } ^ { 2 } \kappa _ { r } + C _ { m } ^ { 2 } C _ { p } C _ { r } )$ -smooth in Π. That is, + +$$ +\left. \pi ^ { \prime } - \pi , \frac { \partial ^ { 2 } v _ { \phi } ^ { \pi } ( s ) } { \partial \pi } ( \pi ^ { \prime } - \pi ) \right. \leq 8 \left( C _ { m } ^ { 3 } C _ { p } ^ { 2 } \kappa _ { r } + C _ { m } ^ { 2 } C _ { p } C _ { r } \right) \| \pi ^ { \prime } - \pi \| _ { 2 } ^ { 2 } \qquad \forall \pi ^ { \prime } , \pi \in \Pi , s \in S \times \pi ^ { \prime } . +$$ + +Proof. From Lemma 14, it is true that + +$$ +\begin{array} { r l } & { \qquad \displaystyle \frac { \partial v _ { \phi } ^ { \pi _ { \alpha } } } { \partial \alpha } = M ^ { \pi _ { \alpha } } \Phi { \mathbb { P } } ^ { u } M ^ { \pi _ { \alpha } } \Phi { \mathbb { P } } ^ { \pi _ { \alpha } } + M ^ { \pi _ { \alpha } } \Phi { \mathbb { P } } ^ { u } } \\ & { \implies \displaystyle \frac { \partial ^ { 2 } v _ { \phi } ^ { \pi _ { \alpha } } } { \partial \alpha ^ { 2 } } = \frac { \partial } { \partial \alpha } \left[ M ^ { \pi _ { \alpha } } \Phi { \mathbb { P } } ^ { u } M ^ { \pi _ { \alpha } } \Phi r ^ { \pi _ { \alpha } } + M ^ { \pi _ { \alpha } } \Phi r ^ { u } \right] } \\ & { \qquad \displaystyle - \frac { \partial M ^ { \pi _ { \alpha } } } { \partial \alpha } \Phi { \mathbb { P } } ^ { u } M ^ { \pi _ { \alpha } } \Phi r ^ { \pi _ { \alpha } } + M ^ { \pi _ { \alpha } } \Phi { \mathbb { P } } ^ { u } \frac { \partial M ^ { \pi _ { \alpha } } } { \partial \alpha } \Phi r ^ { \pi _ { \alpha } } + M ^ { \pi _ { \alpha } } \Phi { \mathbb { P } } ^ { u } M ^ { \pi _ { \alpha } } \Phi \frac { \partial r ^ { \pi _ { \alpha } } } { \partial \alpha } } \\ & { \qquad \quad + \frac { \partial M ^ { \pi _ { \alpha } } } { \partial \alpha } \Phi r ^ { u } } \end{array} +$$ + +$$ +\begin{array} { r l r } { { = { \cal M } ^ { \pi _ { \alpha } } \displaystyle \frac { \partial \Phi \mathbb { P } ^ { \pi _ { \alpha } } } { \partial \alpha } { \cal M } ^ { \pi _ { \alpha } } \Phi \mathbb { P } ^ { u } { \cal M } ^ { \pi _ { \alpha } } \Phi r ^ { \pi _ { \alpha } } + { \cal M } ^ { \pi _ { \alpha } } \Phi \mathbb { P } ^ { u } { \cal M } ^ { \pi _ { \alpha } } \displaystyle \frac { \partial \Phi \mathbb { P } ^ { \pi _ { \alpha } } } { \partial \alpha } { \cal M } ^ { \pi _ { \alpha } } \Phi r ^ { \pi _ { \alpha } } } } \\ & { } & { \ + { \cal M } ^ { \pi _ { \alpha } } \Phi \mathbb { P } ^ { u } { \cal M } ^ { \pi _ { \alpha } } \Phi \displaystyle \frac { \partial r ^ { \pi _ { \alpha } } } { \partial \alpha } + { \cal M } ^ { \pi _ { \alpha } } \displaystyle \frac { \partial \Phi \mathbb { P } ^ { \pi _ { \alpha } } } { \partial \alpha } { \cal M } ^ { \pi _ { \alpha } } \Phi r ^ { u } , \qquad \mathrm { ( f r o m ~ L e m m a ~ 1 3 ) } , } \end{array} +$$ + +$$ +\begin{array} { r l } & { = M ^ { \pi _ { \alpha } } \Phi { \mathbb P } ^ { u } M ^ { \pi _ { \alpha } } \Phi { \mathbb P } ^ { u } M ^ { \pi _ { \alpha } } \Phi r ^ { \pi _ { \alpha } } + M ^ { \pi _ { \alpha } } \Phi { \mathbb P } ^ { u } M ^ { \pi _ { \alpha } } \Phi { \mathbb P } ^ { u } M ^ { \pi _ { \alpha } } \Phi r ^ { \pi _ { \alpha } } } \\ & { \qquad + M ^ { \pi _ { \alpha } } \Phi { \mathbb P } ^ { u } M ^ { \pi _ { \alpha } } \Phi r ^ { u } + M ^ { \pi _ { \alpha } } \Phi { \mathbb P } ^ { u } M ^ { \pi _ { \alpha } } \Phi r ^ { u } , \qquad \mathrm { ( f r o m ~ e q u a t i o n ~ } 6 9 ~ \mathrm { a n d ~ e q u a t i o n ~ } 7 3 ) . } \end{array} +$$ + +Considering the $L _ { \infty }$ norm, + +$$ +\begin{array} { r l r } { { \Big \| \frac { \partial ^ { 2 } v _ { \phi } ^ { \pi _ { \alpha } } } { \partial \alpha ^ { 2 } } \Big \| _ { \infty } = 2 \| M ^ { \pi _ { \alpha } } \Phi \mathbb { P } ^ { u } M ^ { \pi _ { \alpha } } \Phi \mathbb { P } ^ { u } M ^ { \pi _ { \alpha } } \Phi r ^ { \pi _ { \alpha } } + M ^ { \pi _ { \alpha } } \Phi \mathbb { P } ^ { u } M ^ { \pi _ { \alpha } } \Phi r ^ { u } \| _ { \infty } } } \\ & { } & { \leq 2 \| M ^ { \pi _ { \alpha } } \Phi \mathbb { P } ^ { u } M ^ { \pi _ { \alpha } } \Phi \mathbb { P } ^ { u } M ^ { \pi _ { \alpha } } \Phi r ^ { \pi _ { \alpha } } \| _ { \infty } + \| M ^ { \pi _ { \alpha } } \Phi \mathbb { P } ^ { u } M ^ { \pi _ { \alpha } } \Phi r ^ { u } \| _ { \infty } } \\ & { } & { \leq 8 ( C _ { m } ^ { 3 } C _ { p } ^ { 2 } \kappa _ { r } + C _ { m } ^ { 2 } C _ { p } C _ { r } ) . } \end{array} +$$ + +Hence, we obtain, + +$$ +\left. \pi ^ { \prime } - \pi , \frac { \partial ^ { 2 } v _ { \phi } ^ { \pi } ( s ) } { \partial \pi } ( \pi ^ { \prime } - \pi ) \right. \leq 8 \left( C _ { m } ^ { 3 } C _ { p } ^ { 2 } \kappa _ { r } + C _ { m } ^ { 2 } C _ { p } C _ { r } \right) \| \pi ^ { \prime } - \pi \| _ { 2 } ^ { 2 } \qquad \forall \pi ^ { \prime } , \pi \in \Pi , s \in S \times \pi ^ { \prime } . +$$ + +A.4 LIPSCHITZNESS OF THE INFINITE HORIZON AVERAGE REWARD $\rho ^ { \pi }$ + +The Lipschitzness and smoothness of the projected value function $v _ { \phi } ^ { \pi }$ is leveraged through the average reward Bellman equation to prove the Lipschitzness and smoothness of the infinite horizon average reward. + +Lemma 16. Recall the average reward Bellman Equation corresponding to a policy $\pi$ and projected value function $v _ { \phi } ^ { \pi }$ in Equation equation 32. The average reward $\rho ^ { \pi }$ is $\bar { L _ { 1 } ^ { \Pi } }$ -Lipschitz. + +$$ +\left| \left. { \frac { \partial \rho ^ { \pi } } { \partial \pi } , \pi ^ { \prime } - \pi } \right. \right| \le L _ { 1 } ^ { \Pi } \| \pi ^ { \prime } - \pi \| _ { 2 } , \qquad \forall \pi , \pi ^ { \prime } \in \Pi , +$$ + +where ${ \cal L } _ { 1 } ^ { \mathrm { I I } } = 2 ( C _ { r } + C _ { p } C _ { m } \kappa _ { r } + 2 ( C _ { m } ^ { 2 } C _ { p } \kappa _ { r } + C _ { m } C _ { r } ) )$ + +Proof. From Equation equation 32, + +$$ +\rho ^ { \pi } \mathbb { 1 } = r ^ { \pi } + \mathbb { P } ^ { \pi } v _ { \phi } ^ { \pi } - v _ { \phi } ^ { \pi } . +$$ + +Taking derivative with respect to $\alpha$ + +$$ +\begin{array} { r l } & { \displaystyle \frac { \partial \rho ^ { \pi _ { \alpha } } } { \partial \alpha } \mathbb { 1 } = \frac { \partial r ^ { \pi _ { \alpha } } } { \partial \alpha } + \frac { \partial \mathbb { P } ^ { \pi _ { \alpha } } } { \partial \alpha } v _ { \phi } ^ { \pi _ { \alpha } } + \mathbb { P } ^ { \pi _ { \alpha } } \frac { \partial v _ { \phi } ^ { \pi _ { \alpha } } } { \partial \alpha } - \frac { \partial v _ { \phi } ^ { \pi _ { \alpha } } } { \partial \alpha } } \\ & { \quad \quad \quad \quad = \Phi r ^ { u } + \Phi \mathbb { P } ^ { u } v ^ { \pi _ { \alpha } } + ( \mathbb { P } ^ { \pi _ { \alpha } } - I ) ( M ^ { \pi _ { \alpha } } \Phi \mathbb { P } ^ { u } M ^ { \pi _ { \alpha } } \Phi r ^ { \pi _ { \alpha } } + M ^ { \pi _ { \alpha } } \Phi r ^ { u } ) , \qquad \mathrm { ( f r e e ~ \rho ^ { \pi _ { \alpha } } ~ } \Phi \mathbb { P } ^ { u } ) } \\ & { \quad \quad \quad \quad = \Phi r ^ { u } + \Phi \mathbb { P } ^ { u } M ^ { \pi _ { \alpha } } \Phi r ^ { \pi _ { \alpha } } + ( \mathbb { P } ^ { \pi _ { \alpha } } - I ) ( M ^ { \pi _ { \alpha } } \Phi \mathbb { P } ^ { u } M ^ { \pi _ { \alpha } } \Phi r ^ { \pi _ { \alpha } } + M ^ { \pi _ { \alpha } } \Phi r ^ { u } ) , } \end{array} +$$ + +(from equation 73 and equation 69). + +Considering the $L _ { \infty }$ norm of the above expression, + +$$ +\begin{array} { r l r } { { \Big \vert \frac { \partial \rho ^ { \pi _ { \alpha } } } { \partial \alpha } \Big \vert = \Big \Vert \Phi r ^ { u } + \Phi \mathbb { P } ^ { u } M ^ { \pi _ { \alpha } } \Phi r ^ { \pi _ { \alpha } } + \big ( \mathbb { P } ^ { \pi _ { \alpha } } - I \big ) \big ( M ^ { \pi _ { \alpha } } \Phi \mathbb { P } ^ { u } M ^ { \pi _ { \alpha } } \Phi r ^ { \pi _ { \alpha } } + M ^ { \pi _ { \alpha } } \Phi r ^ { u } \big ) \Big \Vert _ { \infty } , } } \quad & { \quad \mathfrak { G } \otimes } \\ & { } & { \leq \| \Phi r ^ { u } \| _ { \infty } + \| \Phi \| ^ { \pi _ { \alpha } } \Phi r ^ { \pi _ { \alpha } } \| _ { \infty } + \| \mathbb { P } ^ { \pi _ { \alpha } } - I \| _ { \infty } \big ( \| M ^ { \pi _ { \alpha } } \Phi \mathbb { P } ^ { u } M ^ { \pi _ { \alpha } } \Phi r ^ { \pi _ { \alpha } } \| _ { \infty } + \| M ^ { \pi _ { \alpha } } \Phi r ^ { \pi _ { \alpha } } \Phi r ^ { \pi _ { \alpha } } \| _ { \infty } + \| M ^ { \pi _ { \alpha } } \Phi r ^ { \pi _ { \alpha } } \| _ { \infty } ^ { \pi _ { \alpha } } \big ) } \\ & { } & { \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad ( 9 9 } \\ & \leq 2 \| r ^ { u } \| _ { \infty } + 2 \| \mathbb { P } ^ { u } M ^ { \pi _ { \alpha } } \Phi r ^ { \pi _ { \alpha } } \| _ { \infty } + \| \mathbb { P } ^ { \pi _ { \alpha } } - I \| ( \| M ^ { \pi _ { \alpha } } \Phi \mathbb { P } ^ { u } M ^ { \pi _ { \alpha } } \Phi r ^ { \pi _ { \alpha } } \| _ { \infty } + \| M ^ { \pi _ { \alpha } } \Phi r ^ \ \end{array} +$$ + +A.5 SMOOTHNESS OF THE INFINITE HORIZON AVERAGE REWARD $\rho ^ { \pi }$ + +Lemma 17. The average reward $\rho ^ { \pi }$ is $L _ { 2 } ^ { \Pi }$ -smooth. + +$$ +\left| \left. \pi ^ { \prime } - \pi , \frac { \partial ^ { 2 } \rho ^ { \pi } } { \partial \pi ^ { 2 } } ( \pi ^ { \prime } - \pi ) \right. \right| \leq \frac { L _ { 2 } ^ { \Pi } } { 2 } \| \pi ^ { \prime } - \pi \| _ { 2 } ^ { 2 } \qquad \forall \pi , \pi ^ { \prime } \in \Pi , +$$ + +$$ +: L _ { 2 } ^ { \Pi } = 4 ( C _ { p } ^ { 2 } C _ { m } ^ { 2 } \kappa _ { r } + C _ { p } C _ { m } C _ { r } + ( C _ { p } + 1 ) ( C _ { m } ^ { 2 } C _ { p } \kappa _ { r } + C _ { m } C _ { r } ) + 4 ( C _ { m } ^ { 3 } C _ { p } ^ { 2 } \kappa _ { r } + C _ { m } ^ { 2 } C _ { p } C _ { r } ) ) . +$$ + +Proof. From Lemma 16, we have + +$$ +\frac { \partial \rho ^ { \pi _ { \alpha } } } { \partial \alpha } \mathbb { 1 } = \Phi r ^ { u } + \Phi \mathbb { P } ^ { u } M ^ { \pi _ { \alpha } } \Phi r ^ { \pi _ { \alpha } } + ( \mathbb { P } ^ { \pi _ { \alpha } } - I ) ( M ^ { \pi _ { \alpha } } \Phi \mathbb { P } ^ { u } M ^ { \pi _ { \alpha } } \Phi r ^ { \pi _ { \alpha } } + M ^ { \pi _ { \alpha } } \Phi r ^ { u } ) . +$$ + +Taking the derivative again, and repeatedly invoking Equations equation 69,equation 73 and Lemma 13, it follows that, + +$$ +\begin{array} { r l r } { { \frac { \partial ^ { 2 } \rho ^ { \pi _ { \alpha } } } { \partial \alpha ^ { 2 } } \mathbb { 1 } = 0 + \frac { \partial } { \partial \alpha } ( \Phi \mathbb { P } ^ { \alpha } M ^ { \pi } \mathbb { \Phi } \hat { \mathbf { f } } ^ { \pi _ { \alpha } } \Phi \boldsymbol { \Phi } r ^ { \pi _ { \alpha } } ) + \frac { \partial } { \partial \alpha } ( ( \mathbb { P } ^ { \pi _ { \alpha } } - I ) ( M ^ { \pi _ { \alpha } } \Phi \mathbb { P } ^ { \pi } M ^ { \pi _ { \alpha } } \Phi \mathbb { P } ^ { \pi _ { \alpha } } + \mathcal { H } ^ { \pi _ { \alpha } } \Phi r ^ { \pi _ { \alpha } } ) } } \\ & { } & { ( \mathbb { I } \mathbb { Q } ^ { \pi } \Phi \boldsymbol { M } ^ { \pi _ { \alpha } } \Phi \boldsymbol { \Phi } \boldsymbol { \Psi } ^ { \pi _ { \alpha } } \boldsymbol { \Phi } r ^ { \pi _ { \alpha } } + \Phi \mathbb { P } ^ { \pi } M ^ { \pi _ { \alpha } } \Phi \boldsymbol { r } ^ { \pi _ { \alpha } } + ( \mathbb { P } ^ { \pi } ) ( M ^ { \pi _ { \alpha } } \Phi \mathbb { P } ^ { \pi } M ^ { \pi _ { \alpha } } \Phi \boldsymbol { r } ^ { \pi _ { \alpha } } + M ^ { \pi _ { \alpha } } \Phi r ^ { \pi _ { \alpha } } ) \qquad \mathrm { ( I } \mathbb { Q } \mathbb { 1 } ) } \\ & { } & ( \mathbb { P } ^ { \pi _ { \alpha } } - I ) ( M ^ { \pi _ { \alpha } } \Phi \mathbb { P } ^ { \pi } M ^ { \pi _ { \alpha } } \Phi \mathbb { P } ^ { \pi _ { \alpha } } M ^ { \pi _ { \alpha } } \Phi \mathbb { P } ^ { \pi _ { \alpha } } + \mathcal { M } ^ { \pi _ { \alpha } } \Phi \mathbb { P } ^ { \pi _ { \alpha } } M ^ { \pi _ { \alpha } } \Phi \boldsymbol { r } ^ \pi _ \end{array} +$$ + +Considering the $L _ { \infty }$ norm of the above expression, + +$$ +\begin{array} { r l r } { { \frac { \partial ^ { 2 } \rho ^ { \pi _ { \alpha } } } { \partial \alpha ^ { 2 } } \Big | \leq \| \Phi \mathbb { P } ^ { u } M ^ { \pi _ { \alpha } } \Phi \mathbb { P } ^ { u } M ^ { \pi _ { \alpha } } \Phi \gamma ^ { \pi _ { \alpha } } \| _ { \infty } + \| \Phi \mathbb { P } ^ { u } M ^ { \pi _ { \alpha } } \Phi \gamma ^ { u } \| _ { \infty } + \| ( \mathbb { P } ^ { u } ) ( M ^ { \pi _ { \alpha } } \Phi \mathbb { P } ^ { u } M ^ { \pi _ { \alpha } } \Phi \gamma ^ { \pi _ { \alpha } } } } \\ & { } & { + M ^ { \pi _ { \alpha } } \Phi r ^ { u } ) \| _ { \infty } + 2 \| ( \mathbb { P } ^ { \pi _ { \alpha } } - I ) ( M ^ { \pi _ { \alpha } } \Phi \mathbb { P } ^ { u } M ^ { \pi _ { \alpha } } \Phi \mathbb { P } ^ { u } M ^ { \pi _ { \alpha } } \Phi r ^ { \pi _ { \alpha } } + M ^ { \pi _ { \alpha } } \Phi \mathbb { P } ^ { u } M ^ { \pi _ { \alpha } } \Phi r ^ { u } ) } \\ & { } & { \leq 4 ( C _ { p } ^ { 2 } C _ { m } ^ { 2 } \kappa _ { r } + C _ { p } C _ { m } C _ { r } + ( C _ { p } + 1 ) ( C _ { m } ^ { 2 } C _ { p } \kappa _ { r } + C _ { m } C _ { r } ) + 4 ( C _ { m } ^ { 3 } C _ { p } ^ { 2 } \kappa _ { r } + C _ { m } ^ { 2 } C _ { p } C _ { r } ) ) . } \end{array} +$$ + +Remark: The smoothness and Lipschitz constant analysis of both the average reward value functions and the infinite horizon average reward are constrained to all directions $u$ , such that every $u = \pi - \pi ^ { \prime }$ can be expressed as a difference of any two policies $\pi , \pi ^ { \prime } \in \Pi$ . Hence the smoothness and Lipschitz constants derived are restricted to the directions that can be expressed as this difference and hence are referred to as restricted smoothness/Lipschitzness. + +# A.6 TABLE OF CONSTANTS CAPTURING MDP COMPLEXITY + +We restate the table of constants and their description here for the sake of convenience. + +Table 2: Constants capturing the MDP Complexity + +
DefinitionRangeRemark
Cmmaxπ∥(I − ΦP π)−1‖k2Ce|S| 1-λLowest rate of mixing
Cp∥P π′ − P π ∥ maxπ,π′Ⅱ kπ′−πk2[0, √A]Diameter of transition kernel
Crkrπ′−-rπk∞ maxπ,π′ kπ′−π∥k2[0, √A]Diameter of reward function
Krmaxπ∥kΦrπ∥∞[0, 2]Variance of reward function
L 2Cr + CpCmKr + 2(C mCpκr + CmCr)[0, k1 √AC 2 ]Restricted Lipschitz constant
L 4C C mnκr+CpCmCr+(Cp+1)(C mCpκr+ [0, k2AC 3] CmCr) + 4(C 3mCpKr + C mCpCr)Restricted smoothness constant
+ +Lemma 18. The constants $C _ { p } , C _ { r } , C _ { m } , \kappa _ { r }$ in Table 2 and other operator norms are bounded as below: + +1. $\begin{array} { r } { \| \Phi \| : = \operatorname* { m a x } _ { \| v \| _ { \infty } \leq 1 } \| \Phi v \| _ { \infty } \leq 2 . } \end{array}$ + +2. $\begin{array} { r } { \| P ^ { \pi } \| = \operatorname* { m a x } _ { \| v \| _ { \infty } \leq 1 } \| P ^ { \pi } v \| _ { \infty } \leq \operatorname* { m a x } _ { \| v \| _ { \infty } \leq 1 } \| v \| _ { \infty } = 1 . } \end{array}$ + +3. $\begin{array} { r } { \kappa _ { r } = \operatorname* { m a x } _ { \pi } \lVert \Phi r ^ { \pi } \rVert _ { \infty } \leq 2 } \end{array}$ + +4. $\begin{array} { r } { C _ { m } \leq \frac { 2 C _ { e } S } { 1 - \lambda } } \end{array}$ + +$$ +\begin{array} { r l } & { C _ { p } = \operatorname* { m a x } _ { u = \frac { \pi ^ { \prime } - \pi } { \| \pi ^ { \prime } - \pi \| _ { 2 } } , \pi ^ { \prime } , \pi \in \Pi } \operatorname* { m a x } _ { \| v \| _ { \infty } \le 1 } \| P ^ { u } v \| _ { \infty } \le \sqrt { A } . } \\ & { } \\ & { C _ { r } = \operatorname* { m a x } _ { u = \frac { \pi ^ { \prime } - \pi } { \| \pi ^ { \prime } - \pi \| _ { 2 } } , \pi ^ { \prime } , \pi \in \Pi } \| R ^ { u } \| _ { \infty } \le \sqrt { A } . } \end{array} +$$ + +Proof. + +1. Consider the projection matrix $\Phi$ , + +$$ +\begin{array} { r l } { { \| \Phi \| _ { \infty } = \operatorname* { m a x } _ { \| v \| _ { \infty } \leq 1 } \| \Phi v \| _ { \infty } \leq \operatorname* { m a x } _ { \| v \| _ { \infty } } } } \\ & { = \operatorname* { m a x } _ { s \in \mathcal { S } } | v ( s ) - \frac { \sum _ { s \in \mathcal { S } } v ( s ) } { S } | } \\ & { \leq \operatorname* { m a x } _ { s \in \mathcal { S } } | v ( s ) | + | \frac { \sum _ { s \in \mathcal { S } } v ( s ) } { S } | } \\ & { \leq 2 \| v \| _ { \infty } = 2 } \end{array} +$$ + +2. The operator norm of $\mathbb { P } ^ { \pi }$ is bounded as below: + +$$ +\begin{array} { r l } & { \| P ^ { \pi } \| = \underset { \| v \| _ { \infty } \leq 1 } { \operatorname* { m a x } } \| P ^ { \pi } v \| _ { \infty } } \\ & { \qquad \leq \underset { \| v \| _ { \infty } \leq 1 } { \operatorname* { m a x } } \| v \| _ { \infty } } \\ & { \qquad \leq 1 . } \end{array} +$$ + +Equality is attained by the vector $v = \mathbb { I }$ . + +3. $\kappa _ { r }$ is bounded as below: + +$$ +\begin{array} { r l } { { \kappa _ { r } = \operatorname* { m a x } _ { \pi \in \Pi } \bigl \| \Phi { r } ^ { \pi } \bigr \| _ { \infty } } } \\ & { = \operatorname* { m a x } _ { \pi \in \Pi } \| r ^ { \pi } - \frac { \sum _ { s \in \mathcal { S } } r ^ { \pi } ( s ) } { | \mathcal { S } | } \mathbb { 1 } \| _ { \infty } } \\ & { \leq \operatorname* { m a x } _ { \pi \in \Pi } \| r ^ { \pi } \| _ { \infty } + \| \frac { \sum _ { s \in \mathcal { S } } r ^ { \pi } ( s ) } { | \mathcal { S } | } \mathbb { 1 } \| _ { \infty } } \\ & { \leq 2 } \end{array} +$$ + +$\kappa _ { r }$ , in some sense, captures the variance of the single step reward function across the class of policies. Greater the variation of the $r$ across different actions, greater the value of $\kappa _ { r }$ . + +4. $C _ { m }$ is the maximum of the operator norm of the matrix $( I - \Phi \mathbb { P } ^ { \pi } ) ^ { - 1 }$ across all policies $\pi \in \Pi$ . It is determined as follows: + +$$ +\begin{array} { r l } { ( I - \Phi { \mathbb P } ^ { \pi } ) ^ { - 1 } = \displaystyle \sum _ { k = 0 } ^ { \infty } ( \Phi { \mathbb P } ^ { \pi } ) ^ { k } = \displaystyle \sum _ { k = 0 } ^ { \infty } \Phi \left( { \mathbb P } ^ { \pi } \right) ^ { k } , } & { \quad \mathrm { ( f r o m ~ L e m m a ~ 1 2 ) } , } \\ { \displaystyle \overset { \mathrm { ( a ) } } { = } \displaystyle \sum _ { k = 0 } ^ { \infty } \Phi \big ( \left( \mathbb { P } ^ { \pi } \right) ^ { k } - { \mathbb 1 } \left( d ^ { \pi } \right) ^ { \top } \big ) , } & { \quad \mathrm { ( a s ~ \Phi \mathbb { P } ~ \textstyle { ( } } d ^ { \pi } \big ) ^ { \top } = 0 \big ) } \end{array} +$$ + +Let $v \in \mathbb { R } ^ { | S | }$ such that $| | v | | _ { \infty } \leq 1$ . Then, + +$$ +\begin{array} { r l } { \displaystyle \implies \lVert ( I - \Phi \mathbb { P } ^ { \kappa } ) ^ { - 1 } v \rVert _ { \infty } \le \sum _ { k = 0 } ^ { \infty } \Phi ( [ \mathbb { P } ^ { \alpha } ) ^ { k } - \mathbb { I } ( a ^ { \alpha } ) ^ { \top } ) v _ { \infty } } & { } \\ & { \le \sum _ { k = 0 } ^ { \infty } \Phi _ { \infty } \Big \lVert ( [ \mathbb { P } ^ { \kappa } ) ^ { k } - \mathbb { I } ( a ^ { \kappa } ) ^ { \top } ) v _ { \infty } } \\ & { \le \displaystyle \sum _ { k = 0 } ^ { \infty } \Phi _ { \infty } S ( [ \mathbb { P } ^ { \pi } ) ^ { k } - \mathbb { I } ( a ^ { \pi } ) ^ { \top } ) \infty v _ { \infty } } \\ & { \le \displaystyle \sum _ { k = 0 } ^ { \infty } \Phi _ { \infty } \Phi _ { \infty } \Phi ^ { \pi } _ { \infty } , } \\ & { \stackrel { \mathrm { ( i i ) } } { \le } \displaystyle \sum _ { k = 0 } ^ { \infty } 2 | S | C _ { \epsilon } x ^ { k } \lVert v \rVert _ { \infty } , } \\ & { = \frac { 2 C _ { \epsilon } | S | } { 1 - \lambda } } \end{array} +$$ + +where $d ^ { \pi }$ represents the stationary measure associated with the transition kernel $\mathbb { P } ^ { \pi }$ , (a) follows from the fact that the projection matrix $\Phi$ projects vectors onto a subspace orthogonal to the subspace spanned by the all ones vector $\mathbb { 1 }$ and (b) is a consequence of the irreducibility and aperiodicity assumption of the Markov chain induced under all policies. More precisely, for any irreducible and aperiodic stochastic matrix $A$ , it is true that: + +$$ +\| A ^ { n } - \mathbb { d } d ^ { \top } \| _ { \infty } \leq C _ { e } \lambda ^ { n } , +$$ + +for some constants $\lambda \in [ 0 , 1 ) , C _ { e } < \infty$ , where $d$ is stationary distribution of $A . \lambda$ is the coefficient of mixing and captures the rate of geometric mixing of the Markov Chain. Hence, higher the value of $\lambda$ , lower the rate of mixing. + +5. $C _ { p }$ represents the diameter of the transition kernel as a function of the policy class and can be bound as below. + +$$ +\begin{array} { r l r } & { C _ { \mathcal { F } ^ { \prime } } ^ { \prime } = \underset { \{ 1 \leq T _ { h } \leq i \leq T _ { h } \leq i \leq T _ { h } \leq i \leq T _ { h } \leq i \leq T _ { h } \leq i \leq T _ { h } \leq i \leq T _ { h } } } { \operatorname* { m a x } } \quad & { \mathrm { i n ~ f ~ h ~ o ~ } i \geq 1 } \\ & { = \underset { - \underset { 1 \leq t _ { h } \leq i \leq T _ { h } \leq i \leq T _ { h } \leq i \leq T _ { h } \leq i \leq T _ { h } \leq i \leq T _ { h } \leq i \leq T _ { h } \leq i \leq T _ { h } \leq i \leq T _ { h } \leq i \leq T _ { h } \leq i \leq T _ { h } \leq i \leq T _ { h } \leq i \leq T _ { h } \leq i \leq T _ { h } \leq i \leq T _ { h } \leq i \leq T _ { h } } } { \operatorname* { m a x } } } & { \mathrm { i n ~ f ~ h ~ o ~ } i \leq 1 } \\ & { = \underset { - \underset { 1 \leq t _ { h } \leq i \leq T _ { h } \leq i \leq T _ { h } \leq i \leq T _ { h } \leq i \leq T _ { h } \leq i \leq T _ { h } \leq i \leq T _ { h } \leq i \leq T _ { h } \leq i \leq T _ { h } \leq i \leq T _ { h } \leq i \leq T _ { h } \leq i \leq T _ { h } \leq i \leq T _ { h } \leq i \leq T _ { h } \leq i \leq T _ { h } \leq i \leq T _ { h } \leq i \leq T _ { h } \leq i \leq T _ { h } \leq i \leq T _ { h } \leq i \leq T _ { h } \leq i \leq T _ { h } \leq i \leq T _ { h } \leq i \leq T _ { h } \leq i \leq T _ { h } \leq i \leq T _ { h } } } { \operatorname* { m a x } } } & { \mathrm { i n ~ f ~ h ~ o ~ } i \leq 1 } \\ & = \underset - \underset 1 \leq t _ { h } \leq i \leq T _ { h } \leq i \leq T _ { h } \leq i \leq T _ { h } \leq i \leq T _ { h } \leq i \leq T _ { h } \leq i \leq T _ { h } \leq i \leq T _ { h } \leq i \leq T _ { h } \leq i \leq T _ { h } \leq i \leq T \end{array} +$$ + +6. $C _ { r }$ represents the diameter of the single step reward function as a function of the policy class and can be bound as below. + +$$ +\begin{array} { r l r } { C _ { \tau } : = } & { \frac { \underset { \Vert \tau ^ { \prime } - \tau _ { \tau } ^ { \prime } - \tau _ { \tau } ^ { \prime } - \tau _ { \tau } ^ { \prime } } { \operatorname* { m a x } } } { \underset { \Vert \tau ^ { \prime } - \tau _ { \tau } ^ { \prime } - \tau _ { \tau } ^ { \prime } - \tau _ { \tau } ^ { \prime } } { \operatorname* { m a x } } } \frac { \Vert r ^ { \prime } \Vert _ { \infty } } { r ^ { \prime } } } & { \quad ( 1 3 8 ) } \\ & { = \underset { \pi ^ { \prime } , \tau \in \mathrm { I } } { \operatorname* { m a x } } \frac { \Vert r ^ { \prime \prime } - r ^ { \prime } \Vert _ { \infty } } { \Vert \pi ^ { \prime } - \tau ^ { \prime } \Vert _ { \infty } } } & { \quad ( 1 3 9 ) } \\ & { = \underset { \pi ^ { \prime } , \tau \in \mathrm { I } } { \operatorname* { m a x } } \underset { \mathrm { s \in \mathcal { S } } } { \operatorname* { m a x } } \frac { \Vert r ^ { \prime \prime } ( s ) - r ^ { \prime \prime } ( s ) \Vert } { \Vert \pi ^ { \prime } - \tau \Vert _ { 2 } } } & { \quad ( 1 4 0 ) } \\ & { \leq \underset { \pi ^ { \prime } , \tau \in \mathrm { I } } { \operatorname* { m a x } } \underset { \mathrm { s \in \mathcal { S } } } { \operatorname* { m a x } } \frac { \Vert r ^ { \prime \prime } ( s ) - r ^ { \prime \prime } ( s ) \Vert } { \Vert \pi _ { s } ^ { \prime } - \tau _ { s } \Vert _ { 2 } } , } & { \quad ( \mathrm { a s } \Vert \pi _ { s } ^ { \prime } - \pi _ { s } \Vert _ { 2 } ) \underset { \mathrm { d } } { \operatorname* { m a x } } \frac { \Vert \alpha ^ { \prime } - \tau ( | \tau _ { 2 } | ) } { \Vert \pi ^ { \prime } - \tau \Vert _ { 2 } } , } & { \quad ( 1 4 1 ) } \\ & = \underset { \tau ^ { \prime } , \tau \in \mathrm { I } } \ \end{array} +$$ + +$$ +\begin{array} { r l } & { C _ { r } = \underset { \pi ^ { \prime } , \pi \in \Pi } { \operatorname* { m a x } } \underset { s \in \mathcal { S } } { \operatorname* { m a x } } \frac { \left| \left( \pi _ { s } ^ { \prime } - \pi _ { s } \right) ^ { \top } r _ { s } \right| } { \left\| \pi _ { s } ^ { \prime } - \pi _ { s } \right\| _ { 2 } } , } \\ & { \quad \le \underset { \pi ^ { \prime } , \pi \in \Pi } { \operatorname* { m a x } } \underset { s \in \mathcal { S } } { \operatorname* { m a x } } \frac { \left\| \pi _ { s } ^ { \prime } - \pi _ { s } \right\| _ { 1 } \left\| r _ { s } \right\| _ { \infty } } { \left\| \pi _ { s } ^ { \prime } - \pi _ { s } \right\| _ { 2 } } , \qquad \mathrm { ( f r o m ~ H o l d e r " s ~ i n e q u a l i t y ) } } \\ & { \quad = \underset { \pi ^ { \prime } , \pi \in \Pi } { \operatorname* { m a x } } \underset { s \in \mathcal { S } } { \operatorname* { m a x } } \frac { \left\| \pi _ { s } ^ { \prime } - \pi _ { s } \right\| _ { 1 } } { \left\| \pi _ { s } ^ { \prime } - \pi _ { s } \right\| _ { 2 } } , } \\ & { \quad \le \sqrt { | \mathcal { A } | } . } \end{array} +$$ + +Since the directional derivatives considered are all within the policy class, the analysis gives rise to constants such as $C _ { p }$ and $C _ { r }$ , which are functions of the underlying policy class. These constants capture the MDP complexity by the virtue of their definition and are an artifact of this proof technique. + +# B CONVERGENCE OF AVERAGE REWARD PROJECTED POLICY GRADIENT + +Lemma 19. For any convex set $\boldsymbol { \mathcal { X } } \subseteq \mathbb { R } ^ { d }$ , any point $a \in { \mathcal { X } }$ , and any update direction $u \in \mathbb { R } ^ { d }$ , l et $b = P r o j _ { \mathcal { X } } ( a + u )$ be the projection of $a + u$ onto $\mathcal { X }$ . It is true that + +1. $\langle u , b - a \rangle \geq \| b - a \| _ { 2 } ^ { 2 }$ . +2. $\left. c - b , u - ( b - a ) \right. \leq 0 , \qquad \forall c \in \mathcal { X } .$ + +Proof. The formal proof can be found in Beck (2014). + +However, the proof follows trivially from the geometrical representation of projection (see Figure 3,Kumar et al. (2023)), and the fact that the hyperplane separates a convex set from a point not in the set. + +![](images/figures/avg-reward-pg-fig-0003.jpg) +Figure 3: Convex Projection + +Intuitively, the proof of the lemma can be interpreted as below. + +1. Since the angle between vectors $( a - b )$ and $( ( a + u ) - b )$ is greater than 90 degrees, it is true that $\langle a - b , ( a + u ) - b \rangle \leq 0$ , which then directly implies $\begin{array} { r } { \| b - a \| _ { 2 } ^ { 2 } \leq \langle u , b - a \rangle } \end{array}$ . + +2. The angle between vectors $( c - b )$ and $( ( a + u ) - b )$ is greater than 90 degrees $\forall c \in { \mathcal { X } }$ , therefore $\langle c - b , u - ( b - a ) \rangle \leq 0$ . + +# B.1 PROOF OF LEMMA 5 + +Lemma 20. The average reward iterates $\rho ^ { \pi _ { k } }$ generated from projected policy gradient satisfy the following, + +$$ +\rho ^ { \pi _ { k + 1 } } - \rho ^ { \pi _ { k } } \geq \frac { L _ { 2 } ^ { \Pi } } { 2 } \big \| \pi _ { k + 1 } - \pi _ { k } \big \| ^ { 2 } , \qquad \forall k \geq 0 . +$$ + +where $L _ { 2 } ^ { \Pi }$ is the restricted smoothness constant associated with average reward $\rho ^ { \pi }$ . + +Proof. From the restricted smoothness of the average cost, we have + +$$ +\begin{array} { r l } & { \rho ^ { \pi _ { k + 1 } } \geq \rho ^ { \pi _ { k } } + \bigg \langle \cfrac { d \rho ^ { \pi } } { d \pi } \bigg \vert _ { \pi = \pi _ { k } } , \pi _ { k + 1 } - \pi _ { k } \bigg \rangle - \cfrac { L _ { 2 } ^ { \Pi } } { 2 } \| { \pi _ { k + 1 } } - \pi _ { k } \| ^ { 2 } , } \\ & { \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad } \\ & { \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad } \\ & { \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad } \\ & { \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad } \\ & { \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad } \\ & { \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad } \\ & { \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad } \\ & { \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad } \\ & \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \end{array} +$$ + +The last inequality follows from the projected gradient ascent policy update rule and item 1 of Lemma 19. Note that the proof only relies on the convexity of the projection set $\Pi$ and the smoothness of the objective function. + +# B.2 PROOF OF LEMMA 7 + +Lemma 21. The suboptimality of a policy $\pi$ can be bounded from above as: + +$$ +\rho ^ { * } - \rho ^ { \pi } \leq C _ { P L } \operatorname* { m a x } _ { \pi ^ { \prime } \in \Pi } \left. \pi ^ { \prime } - \pi , \frac { \partial \rho ^ { \pi } } { \partial \pi } \right. , \qquad \forall \pi \in \Pi , +$$ + +where CP L = maxπ,s ∂ (s)∂π(s) and $\rho ^ { * }$ is the optimal average reward. + +Proof. Average Reward Performance Difference Lemma states that + +$$ +\begin{array} { r l } { \varepsilon ^ { \prime \prime } - \sigma ^ { \prime \prime } - \sum _ { t \in \mathcal { F } } ^ { \varepsilon } s \middle | \mathcal { F } ^ { \prime } ( s , \alpha ) \middle | \mathcal { F } ^ { \prime } ( s , \alpha ) \middle | \mathcal { F } ^ { \prime } ( s , \alpha ) - \alpha ( \varepsilon ) \middle | s } & { } \\ { \leq \frac { \varepsilon ^ { \prime \prime } } { \varepsilon ^ { \prime \prime } } \leq \varepsilon ^ { \prime \prime } \leq \varepsilon ^ { \prime \prime } \leq | s \| \mathcal { F } ^ { \prime } ( s , \alpha ) \| ^ { 2 } \varepsilon ^ { \prime \prime } ( s ) - \varepsilon ^ { \prime \prime } ( s ) \| s \| \mathcal { F } ^ { \prime } ( s , \alpha ) \| \mathcal { F } ^ { \prime } ( s ) \| \mathcal { F } ^ { \prime } ( s ) \| \mathcal { F } ^ { \prime } ( s ) \| \mathcal { F } ^ { \prime } ( s ) \| \mathcal { F } ^ { \prime } ( s ) \| \mathcal { F } ^ { \prime } ( s ) \| \mathcal { F } ^ { \prime } ( s ) } \\ & - \sum _ { t \in \mathcal { F } } ^ { \varepsilon } \frac { d ^ { - 1 } s \middle | \mathcal { F } _ { t } ^ { 2 } s \middle | \mathcal { F } _ { t } ^ { 2 } s \middle | \mathcal { F } _ { t } ^ { 2 } s \middle | \mathcal { F } _ { t } ^ { 2 } s \middle | \mathcal { F } _ { t } ^ { 2 } s | \mathcal { F } _ { t } - \tau ( s ) s \middle | \mathcal { F } _ { t } } \\ = \frac { \varepsilon ^ { \prime } } { \varepsilon ^ { \prime \prime } } \leq \varepsilon ^ { \prime \prime } \leq \varepsilon ^ { \prime \prime } \leq \varepsilon ^ { \prime \prime } \leq \varepsilon ^ { \prime \prime } \leq \varepsilon ^ { \prime } \frac d ^ { - 1 } s | \mathcal { F } ^ { \prime } ( s ) s \middle | \mathcal { F } ^ { \prime } ( s , \alpha ) s \ \end{array} +$$ + +where (a) follows from the average reward policy gradient theorem. + +# B.3 PROOF OF LEMMA 8 + +Lemma 22. Let $\pi _ { k + 1 }$ represent the policy iterates obtained through projected policy gradient. For any policy $\pi ^ { \prime } \in \Pi$ , it is true that, + +$$ +\Big \langle \frac { \partial \rho ^ { \pi _ { k + 1 } } } { \partial \pi _ { k + 1 } } , \pi ^ { \prime } - \pi _ { k + 1 } \Big \rangle \leq 4 \sqrt { | { \cal S } | } L _ { 2 } ^ { \Pi } \| \pi _ { k + 1 } - \pi _ { k } \| _ { 2 } , +$$ + +Proof. For all $x , y \in C$ , we have: + +$$ +\begin{array} { r l } & { \bigg \langle \frac { \partial \rho ^ { \pi _ { k + 1 } } } { \partial \pi _ { k + 1 } } , \pi ^ { \prime } - \pi _ { k + 1 } \bigg \rangle = \bigg \langle \frac { \partial \rho ^ { \pi _ { k + 1 } } } { \partial \pi _ { k + 1 } } - \frac { \partial \rho ^ { \pi _ { k } } } { \partial \pi _ { k } } + \frac { \partial \rho ^ { \pi _ { k } } } { \partial \pi _ { k } } , \pi ^ { \prime } - \pi _ { k + 1 } \bigg \rangle } \\ & { \qquad = \bigg \langle \frac { \partial \rho ^ { \pi _ { k + 1 } } } { \partial \pi _ { k + 1 } } - \frac { \partial \rho ^ { \pi _ { k } } } { \partial \pi _ { k } } , \pi ^ { \prime } - \pi _ { k + 1 } \bigg \rangle + \bigg \langle \frac { \partial \rho ^ { \pi _ { k } } } { \partial \pi _ { k } } , \pi ^ { \prime } - \pi _ { k + 1 } \bigg \rangle } \\ & { \qquad \leq \bigg \| \frac { \partial \rho ^ { \pi _ { k + 1 } } } { \partial \pi _ { k + 1 } } - \frac { \partial \rho ^ { \pi _ { k } } } { \partial \pi _ { k } } \bigg \| \| \pi ^ { \prime } - \pi _ { k + 1 } \| + \bigg \langle \frac { \partial \rho ^ { \pi _ { k } } } { \partial \pi _ { k } } , \pi ^ { \prime } - \pi _ { k + 1 } \bigg \rangle } \\ & { \qquad \overset { \mathrm { ( a ) } } { \leq } L _ { 2 } ^ { \Pi } \| \pi _ { k + 1 } - \pi _ { k } \| \| \pi ^ { \prime } - \pi _ { k + 1 } \| + \bigg \langle \frac { \partial \rho ^ { \pi _ { k } } } { \partial \pi _ { k } } , \pi ^ { \prime } - \pi _ { k + 1 } \bigg \rangle , } \end{array} +$$ + +where (a) uses smoothness of average reward. Thus, we may continue the chain of inequalities as + +$$ +\begin{array} { r l } & { m \ \displaystyle 1 5 9 = L _ { 2 } ^ { \Pi } \| \pi _ { k + 1 } - \pi _ { k } \| \| \pi ^ { \prime } - \pi _ { k + 1 } \| + \biggl \langle \frac { \partial \rho ^ { \pi _ { k } } } { \partial \pi _ { k } } - L _ { 2 } ^ { \Pi } ( \pi _ { k + 1 } - \pi _ { k } ) , \pi ^ { \prime } - \pi _ { k + 1 } \biggr \rangle + L _ { 2 } ^ { \Pi } \left. \pi _ { k + 1 } - \pi _ { k } , \pi _ { k } , \pi _ { k + 1 } \right. } \\ & { \qquad \leq 2 L _ { 2 } ^ { \Pi } \| \pi _ { k + 1 } - \pi _ { k } \| \| \pi ^ { \prime } - \pi _ { k + 1 } \| + \biggl \langle \frac { \partial \rho ^ { \pi _ { k } } } { \partial \pi _ { k } } - L _ { 2 } ^ { \Pi } ( \pi _ { k + 1 } - \pi _ { k } ) , \pi ^ { \prime } - \pi _ { k + 1 } \biggr \rangle } \\ & { \qquad \leq 2 L _ { 2 } ^ { \Pi } \| \pi _ { k + 1 } - \pi _ { k } \| \| \pi ^ { \prime } - \pi _ { k + 1 } \| + L _ { 2 } ^ { \Pi } \underbrace { \biggl \langle \frac { 1 } { L _ { 2 } ^ { \Pi } } \frac { \partial \rho ^ { \pi _ { k } } } { \partial \pi _ { k } } - ( \pi _ { k + 1 } - \pi _ { k } ) , \pi ^ { \prime } - \pi _ { k + 1 } \biggr \rangle } _ { \leq 0 , \qquad \mathrm { ( F r o m i t e m ~ 2 ~ o f ~ L e m m a l 9 ) } } } \\ & { \qquad \leq 2 L _ { 2 } ^ { \Pi } \| \pi _ { k + 1 } - \pi _ { k } \| \| \pi ^ { \prime } - \pi _ { k + 1 } \| } \\ & { \qquad \leq 2 L _ { 2 } ^ { \Pi } \| \pi _ { k + 1 } - \pi _ { k } \| \mathrm { d i a m } ( \Pi ) . } \end{array} +$$ + +The diameter of the policy class $\Pi$ , can be upper bounded as + +$$ +\mathbf { d i a m ( \Pi ) } ^ { 2 } = \operatorname* { m a x } _ { \pi , \pi } \sum _ { s } \| \pi _ { s } ^ { \prime } - \pi _ { s } \| _ { 2 } ^ { 2 } \leq \operatorname* { m a x } _ { \pi ^ { \prime } , \pi } \sum _ { s } \| \pi _ { s } ^ { \prime } - \pi _ { s } \| _ { 1 } ^ { 2 } \leq 4 S . +$$ + +This yields the result. + +Lemma 23. The scaled sub-optimality $a _ { k } : = \rho ^ { * } - \rho _ { k }$ follows the recursion + +$$ +{ c a _ { k + 1 } ^ { 2 } + a _ { k + 1 } - a _ { k } \leq 0 } , +$$ + +where c = 32LΠ|S|C2 . + +Proof. From Lemma 21, we know that, + +$$ +\rho ^ { * } - \rho ^ { \pi _ { k + 1 } } \leq C _ { P L } \left. \pi ^ { \prime } - \pi _ { k + 1 } , \frac { \partial \rho ^ { \pi _ { k + 1 } } } { \partial \pi _ { k + 1 } } \right. , \qquad \forall \pi ^ { \prime } \in \Pi , +$$ + +From Lemma 22, we know that, + +$$ +\Big \langle \frac { \partial \rho ^ { \pi _ { k + 1 } } } { \partial \pi _ { k + 1 } } , \pi ^ { \prime } - \pi _ { k + 1 } \Big \rangle \leq 4 \sqrt { | { \cal S } | } L _ { 2 } ^ { \Pi } \| \pi _ { k + 1 } - \pi _ { k } \| _ { 2 } . +$$ + +From Lemma 20, we know that, + +$$ +\left\| \pi _ { k + 1 } - \pi _ { k } \right\| _ { 2 } \leq \sqrt { \frac { 2 \left( \rho ^ { \pi _ { k + 1 } } - \rho ^ { \pi _ { k } } \right) } { L _ { 2 } ^ { \Pi } } } , \qquad \forall k \geq 0 . +$$ + +Combining the above equations yields, + +$$ +\rho ^ { * } - \rho ^ { \pi _ { k + 1 } } \leq \sqrt { 3 2 C _ { P L } ^ { 2 } L _ { 2 } ^ { \Pi } | S | \left( \rho ^ { \pi _ { k + 1 } } - \rho ^ { \pi _ { k } } \right) } +$$ + +This thus yields, + +$$ +\frac { ( \rho ^ { * } - \rho ^ { \pi _ { k + 1 } } ) ^ { 2 } } { 3 2 C _ { P L } ^ { 2 } L _ { 2 } ^ { \Pi } | S | } + ( \rho ^ { * } - \rho ^ { \pi _ { k + 1 } } ) - ( \rho ^ { * } - \rho ^ { \pi _ { k } } ) \leq 0 . +$$ + +A more detailed interpretation of this Lemma can be found in Kumar et al. (2023). + +# B.4 RECURSION BOUND + +In this subsection, we consider the sequence defined as + +$$ +a _ { k } - a _ { k + 1 } \geq a _ { k } ^ { 2 } , +$$ + +where $p \geq 0$ and $0 \leq a _ { 0 } \leq 1$ . Let $f$ be linear interpolation of the sequence $\{ a _ { k } \} _ { k \ge 0 }$ , formally defined as + +$$ +f ( x ) : = ( 1 - \alpha ) a _ { k } + \alpha a _ { k + 1 } , \qquad { \mathrm { w h e r e ~ } } k = \lfloor x \rfloor { \mathrm { ~ a n d ~ } } \alpha = x - \lfloor x \rfloor . +$$ + +Let $g ( 0 ) : = a _ { 0 }$ and + +$$ +{ \frac { d g ( x ) } { d x } } = - g ( x ) ^ { 2 } , \qquad { \mathrm { a n d } } \qquad \tau _ { k } : = g ^ { - 1 } ( a _ { k } ) . +$$ + +Observe that $g$ is a strictly decreasing function. + +Proposition 1. If $\tau _ { k } \geq k$ then + +$$ +g ( x ) \geq f ( x ) , \qquad \forall x \in [ k , k + 1 ] . +$$ + +Proof. We have $f ( k ) = g ( \tau _ { k } ) = a _ { k }$ and for $\alpha \in ( 0 , 1 )$ + +$$ +\begin{array} { r l } & { \frac { d g ( \tau _ { k } + \alpha ) } { d x } = - g ( \tau _ { k } + \alpha ) ^ { 2 } , \qquad \mathrm { ( b y ~ d e f i n i t i o n ) } } \\ & { \qquad \geq - g ( \tau _ { k } ) ^ { 2 } , \qquad \mathrm { ( } g \mathrm { ~ i s ~ a ~ d e c r e a s i n g ~ f u n c t i o n ) } } \\ & { \qquad = - f ( k ) ^ { 2 } , \qquad \mathrm { ( a s ~ } f ( k ) = g ( \tau _ { k } ) ) } \\ & { \qquad = - a _ { k } ^ { 2 } \qquad \mathrm { ( b y ~ d e f i n i t i o n ~ o f ~ } f \mathrm { ) } } \\ & { \qquad \geq a _ { k + 1 } - a _ { k } , \qquad \mathrm { ( b y ~ d e f i n i t i o n ~ o f ~ } a _ { k + 1 } \mathrm { ) } } \\ & { \qquad = \frac { d f ( k + \alpha ) } { d x } , \qquad \mathrm { ( b y ~ d e f i n i t i o n ~ o f ~ } f \mathrm { ) . } } \end{array} +$$ + +Above together with continuity of $f$ and $g$ , for all $\alpha \in [ 0 , 1 ]$ , we have + +$$ +\begin{array} { r l r } { f ( k + \alpha ) \le g ( \tau _ { k } + \alpha ) , } & { } & \\ { \le g ( k + \alpha ) , \quad } & { ( \mathrm { a s } \ \tau _ { k } \ge k \mathrm { ~ a n d ~ } g \mathrm { ~ i s ~ a ~ d e c r e a s i n g ~ f u n c t i o n } ) . } \end{array} +$$ + +Hence claim is proved. + +Proposition 2. For all $k \geq 0$ , we have + +$$ +\tau _ { k } \geq k . +$$ + +Proof. Note that $\tau _ { 0 } = 0$ by definition $g ( 0 ) = a _ { 0 }$ . Now let $\tau _ { k } \geq k$ , then from Proposition 1, we have + +$$ +\begin{array} { c } { { g ( x ) \geq f ( x ) , \qquad \forall x \in [ k , k + 1 ] } } \\ { { \Longrightarrow g ( k + 1 ) \geq f ( k + 1 ) } } \\ { { = a _ { k + 1 } } } \\ { { \Longrightarrow \tau _ { k + 1 } \geq k + 1 , \qquad ( \mathrm { a s ~ } g \mathrm { ~ i s ~ a ~ d e c r e a s i n g ~ f u n c t i o n } ) . } } \end{array} +$$ + +Hence, by induction the claim is established. + +Lemma 24. [Recursion Upper Bound] For $p \geq 2$ , and $0 \leq a _ { 0 } \leq 1$ , sequence $\{ a _ { k } \} _ { k \ge 0 }$ satisfying the recursion $a _ { k } - a _ { k + 1 } \geq { \overline { { a _ { k } ^ { 2 } } } } ;$ , follows + +$$ +a _ { k } \leq \frac { 1 } { \frac { 1 } { a _ { 0 } } + k } , \qquad \forall k \geq 1 . +$$ + +Proof. From Proposition 2, we get $\tau _ { k } \geq k$ . Combining it with Proposition 1, we get + +$$ +g ( x ) \geq f ( x ) , \qquad \forall x \geq 0 . +$$ + +Now, we solve the o.d.e. to get + +$$ +\begin{array} { r l r } { \displaystyle \frac { d g ( x ) } { d x } = - g ( x ) ^ { 2 } } \\ { \displaystyle \Longrightarrow d x \left| _ { z = 0 } ^ { z - k } - \int _ { x = 0 } ^ { k } \frac { d g ( x ) } { g ^ { 2 } ( x ) } \right. } \\ { \displaystyle \Longrightarrow k = \frac { 1 } { g ( k ) } - \frac { 1 } { g ( 0 ) } } \\ { \displaystyle \Longrightarrow g ( k ) - \frac { g ( 0 ) } { 1 + g ( 0 ) k } } \\ { \displaystyle \Longrightarrow f ( k ) \leq \frac { a _ { 0 } } { 1 + a _ { 0 } k } , } & { \quad \mathrm { ( a s ~ f ( x ) ~ \leq ~ } g ( x ) ) } \\ { \displaystyle \Longrightarrow a _ { k } \leq \frac { 1 } { \frac { k } { a _ { 0 } } + k } . } \end{array} +$$ + +This proves the claim. + +Lemma 25. If $a _ { k } - a _ { k + 1 } \geq c a _ { k } ^ { 2 }$ then + +$$ +a _ { k } \leq { \frac { 1 } { { \frac { 1 } { a _ { 0 } } } + c k } } . +$$ + +Proof. We have + +$$ +\begin{array} { r l } & { \qquad a _ { k } - a _ { k + 1 } \geq c a _ { k } ^ { 2 } } \\ & { \implies c a _ { k } - c a _ { k + 1 } \geq ( c a _ { k } ) ^ { 2 } } \\ & { \implies c a _ { k } \leq \frac { 1 } { \frac { 1 } { c a _ { 0 } } + k } , \qquad \mathrm { ( f r o m ~ L e m m a ~ 2 4 ) } } \\ & { \implies a _ { k } \leq \frac { 1 } { \frac { 1 } { a _ { 0 } } + c k } . } \end{array} +$$ + +This subsection (proving the recursion upper bound) is inspired by a technique from Kumar et al. (2024). However, while their result is similar, it is not applicable to our case. Their result assumes that $c$ is upper bounded by a constant, which does not hold in our setting. In our case, the smoothness constant (or hardness coefficient) can approach zero, causing the constant $c$ to diverge to infinity. Our result is more general, and the proof technique we use is distinct. + +Lemma 26. Given $a _ { k } - a _ { k + 1 } \geq c a _ { k + 1 } ^ { 2 }$ + +$$ +a _ { k } \leq { \frac { 1 } { { \frac { 1 } { a _ { 0 } } } + c k } } , +$$ + +where $\begin{array} { r } { \nu = c ( 1 + \frac { 8 c } { 1 - \gamma } ) ^ { - \frac { 3 } { 2 } } } \end{array}$ and c = 132C2P L|S|LΠ2 . + +Proof. We have + +$$ +\begin{array} { r l } & { \quad \alpha _ { k + 1 } ^ { 2 } + a _ { k + 1 } - a _ { k } \leq 0 } \\ & { = \alpha _ { k + 1 } \leq \frac { - 1 + \sqrt { 1 + 4 \alpha _ { k } } } { 2 c } , } \\ & { \quad - \frac { - 1 + f ( 0 ) + f ^ { \prime } ( 0 ) 4 c \alpha _ { k } + f ^ { \prime \prime } ( b ) \frac { ( 4 \alpha _ { k } ) ^ { 2 } } { 2 } } , \qquad \quad ( \mathrm { w h e r e ~ } f ( x ) = \sqrt { 1 + x } , b \in [ 0 , 4 c \alpha _ { k } ] ) } \\ & { \quad = \frac { 2 c \alpha _ { k } - 2 c ^ { 2 } \alpha _ { k } ^ { 2 } ( 1 + b ) ^ { - \frac { 3 } { 2 } } } { 2 c } , \qquad \quad ( \mathrm { p u t i m g ~ } f ( 0 ) = 1 , f ^ { \prime } ( 0 ) = \frac { 1 } { 2 } , f ^ { \prime \prime } ( a ) = \frac { ( 1 + b ) ^ { - \frac { 3 } { 2 } } } { 4 } } \\ & { \quad = \frac { 2 c \alpha _ { k } - 2 c ^ { 2 } \alpha _ { k } ^ { 2 } ( 1 + 4 c \alpha _ { k } ) ^ { - \frac { 3 } { 2 } } } { 2 c } , \qquad \quad ( \mathrm { u s ~ b \leq 4 c \alpha _ { k } \operatorname { a n d - } ( 1 + y ) ^ { - \frac { 3 } { 2 } } \mathrm { ~ i s ~ a i n c r e a s i n g ~ f u r c e } } } \\ & { \quad \leq a _ { k } - \frac { \alpha _ { k } ^ { 2 } } { ( 1 + 4 c \alpha _ { k } ) ^ { \frac { 3 } { 2 } } } , \qquad \quad ( \mathrm { b a s i c ~ l i g e b r a n } ) } \\ & { \quad \leq \alpha _ { k } - v a _ { k } ^ { 2 } . \qquad \quad ( \mathrm { a s ~ } a _ { k } \leq 1 \mathrm { a n d ~ } v = c ( 1 + 4 c ) ^ { - \frac { 3 } { 2 } } ) . } \end{array} +$$ + +We get the desired result from Lemma 25. + +Lemma 27. If $\begin{array} { r } { \frac { 1 } { c } = 3 2 C _ { P L } ^ { 2 } | S | L _ { 2 } ^ { \Pi } < 1 } \end{array}$ that is MDP is very easy (i.e. $L _ { 2 } ^ { \Pi } < < 1 ,$ ) then the policy gradient converges exponentially fast, that is + +$$ +a _ { k } \leq { \Big ( } { \frac { 1 } { c } } { \Big ) } ^ { \frac { k } { 2 } } a _ { 0 } ^ { - 2 ^ { k } } . +$$ + +Proof. From the above discussion, we have + +$$ +\begin{array} { c } { { c a _ { k + 1 } ^ { 2 } \leq a _ { k } - a _ { k + 1 } } } \\ { { \leq a _ { k } , \qquad ( \mathrm { a s } ~ a _ { k + 1 } \geq 0 ~ \mathrm { b y ~ d e f i n i t i o n } ) } } \\ { { \Longrightarrow ~ a _ { k + 1 } \leq \sqrt { \displaystyle \frac { a _ { k } } { c } } } } \\ { { \leq \bigl ( \displaystyle \frac { 1 } { c } ~ \bigr ) ^ { \frac { k + 1 } { 2 } } ~ a _ { 0 } ^ { \frac { 1 } { 2 ^ { k + 1 } } } } } \end{array} +$$ + +# B.5 PROOF OF THEOREM 1 + +We restate the theorem for the sake of convenience. + +Theorem 2. Let $\rho ^ { \pi _ { k } }$ be the average reward corresponding to the policy iterates $\pi _ { k }$ , obtained through the policy gradient update equation 6. Let $\rho ^ { * }$ represent the optimal average reward, that is, $\rho ^ { * } = \mathrm { m a x } _ { \pi \in \Pi } \rho ^ { \pi }$ . There exist constants $L _ { 2 } ^ { \Pi }$ and $C _ { P L }$ which are determined by the underlying $M D P$ such that: + +• For all MDPs it is true that, + +$$ +\begin{array} { c c } { \rho ^ { \ast } - \rho ^ { \pi _ { k } } \leq \frac { 1 } { \frac { 1 } { \rho ^ { \ast } - \rho ^ { \pi _ { 0 } } } + \nu k } , } & { \forall k \geq 0 . } \\ { : = \left( \frac { 1 } { 3 2 C _ { P L } ^ { 2 } | S | L _ { 2 } ^ { \mathrm { \pi } } } \right) \left( 1 + 4 \left( \frac { 1 } { 3 2 C _ { P L } ^ { 2 } | S | L _ { 2 } ^ { \mathrm { \pi } } } \right) \right) ^ { - \frac { 3 } { 2 } } } \end{array} +$$ + +• For simple MDPs (i.e. $L _ { 2 } ^ { \Pi } < < 1 ,$ ) we obtain exponential convergence, that is + +$$ +\rho ^ { * } - \rho ^ { \pi _ { k } } \leq c ^ { - \frac { k } { 2 } } \left( \rho ^ { * } - \rho ^ { \pi _ { 0 } } \right) ^ { \frac { 1 } { 2 ^ { k } } } , \qquad \forall k \geq 0 +$$ + +where $\begin{array} { r } { \frac { 1 } { c } = 3 2 | S | L _ { 2 } ^ { \Pi } C _ { P L } ^ { 2 } < 1 } \end{array}$ + +Proof. Using Gradient Domination Lemma and Sufficient Increase Lemma as shown in Lemma 23, we get the following recursion + +$$ +a _ { k } - a _ { k + 1 } \geq c a _ { k + 1 } ^ { 2 } , +$$ + +where ak = J ∗ − J πk and c = 132C2 |S|LΠ is a small constant. Then we get the desired result by solving the above recursion in Lemma 26 and Lemma 27 for complex MDPs and simple MDPs respectively. □ + +# C SIMULATION DETAILS + +# C.1 CONVERGENCE WITH DIFFERENT ACTION AND STATE SPACE SIZE + +In the first experiment, we compare the convergence of PG for a tabular MDP with $( S , A ) \in$ $\{ ( 3 , 3 ) , ( 9 , 9 ) , ( 8 1 , 8 1 ) \}$ . We set the reward kernel to be with maximal variance as described above. For the transition kernel, we use the following matrix: + +$$ +P ( \cdot \mid s , \cdot ) = \frac { 1 } { 2 } \left( 1 _ { S \times A } + \frac { 1 } { S } \right) +$$ + +so $\begin{array} { r } { P ( i | s , i ) = \frac { 1 + \frac { 1 } { S } } { 2 } } \end{array}$ and $\begin{array} { r } { P ( i \mid s , j ) = \frac { 1 } { 2 S } } \end{array}$ for $i \neq j$ . For the reward kernel we set the rewards of half the actions to 1 and the rest to $- 1$ , for every state. + +# C.2 CONVERGENCE WITH DIFFERENT REWARD FUNCTIONS + +In the second experiment, we compare the convergence of PG for a tabular MDP with $S = 1 6$ and $A = 1 6$ . We set $r ( s , a ) = 0$ for any $s$ and $a$ except for one state which we denote by $s _ { 0 }$ . We use the same randomly generated transition kernel and use the following procedure to generate the reward function: + +• No variance: We assign each $( s _ { 0 } , a )$ pair a reward of 1. +• Low variance: We assign $\frac { 1 } { 8 }$ of the actions for $s _ { 0 }$ a reward of $- 1$ , and 1 otherwise. +• High variance: We assign $\textstyle { \frac { 1 } { 4 } }$ of the actions for $s _ { 0 }$ a reward of $- 1$ , and 1 otherwise. +• Max variance: We assign $\textstyle { \frac { 1 } { 2 } }$ of the actions for $s _ { 0 }$ a reward of $- 1$ , and 1 otherwise. + +# C.3 CONVERGENCE WITH DIFFERENT TRANSITION KERNELS + +In the third experiment, we compare the convergence of PG for a tabular MDP with $S = 1 6$ and $A = 1 6$ . We create three different MDPs with the same $( S , A )$ values and the same reward function that is generated according to the process described above for high variance reward function. We then generate three different transition kernels: + +• Uniform: We assign for all values of $\begin{array} { r } { s , a , s ^ { \prime } , P ( s ^ { \prime } \mid s , a ) = \frac { 1 } { S } } \end{array}$ . +• Non-uniform: We assign $\begin{array} { r } { P ( i \mid s , i ) = \frac { 1 } { 2 S } + \frac { 1 } { 2 } } \end{array}$ and $\begin{array} { r } { P ( i \mid s , j ) = \frac { 1 } { 2 S } } \end{array}$ for $i \neq j$ . +• Deterministic: We look at $\textstyle P ( \cdot \mid s , \cdot )$ as an $S \times A$ matrix, and assign it a random permutation of the identity matrix. In this way the result MDP is deterministic but not trivial (so every state leads to a different one). + +# D ADDITIONAL DISCUSSION AND FUTURE WORK + +Extension to Discounted Reward Setting. In the discounted reward setting, the return $\rho ^ { \pi }$ , and the value function $v ^ { \pi }$ is defined as + +$$ +\rho ^ { \pi } = \mu ^ { T } ( I - \gamma P ^ { \pi } ) R ^ { \pi } , \qquad v ^ { \pi } = ( I - \gamma P ^ { \pi } ) R ^ { \pi } , +$$ + +where $\gamma \in [ 0 , 1 )$ is the discount factor Sutton & Barto (2018). The return $\rho ^ { \pi }$ is proven to be $\frac { 8 } { ( 1 - \gamma ) ^ { 3 } }$ - smooth (Agarwal et al., 2020). Note that it is an MDP-agnostic bound. We can achieve an MDP instance-dependent bound with a very minor change in the smoothness analysis of the average-reward case. + +Let us define, + +$$ +M ^ { \pi _ { \alpha } } : = \left( I - \gamma P ^ { \pi _ { \alpha } } \right) ^ { - 1 } . +$$ + +Observe that Lemma 13 holds for $A ( \alpha ) = \gamma P ^ { \pi _ { \alpha } }$ , which yields us + +$$ +\frac { \partial ^ { 2 } M ( \alpha ) } { \partial \alpha ^ { 2 } } = \frac { \partial M ( \alpha ) } { \partial \alpha } \frac { \partial A ( \alpha ) } { \partial \alpha } M ( \alpha ) + M ( \alpha ) \frac { \partial ^ { 2 } A ( \alpha ) } { \partial \alpha ^ { 2 } } M ( \alpha ) + M ( \alpha ) \frac { \partial A ( \alpha ) } { \partial \alpha } \frac { \partial M ( \alpha ) } { \partial \alpha } , +$$ + +where $M ( \alpha )$ is shorthand for $M ^ { \pi _ { \alpha } }$ . + +Table 3: Constants capturing the MDP Complexity for Discounted reward + +
DefinitionRangeRemark
Cmmaxππ∥(I − γP π)−1k
Cp∥P π′ − P π∥ γ maxπ,π′Ⅱ kπ′−π∥2[0, γ√A] Diameter of transition kernel
Crkrπ′-rπ∞ maxπ, π kπ′−πk2[0, √A]Diameter of reward function
Krmaxπ∥krπk→∞[0, 1]Variance of reward function
+ +Lemma 28. The value function $v _ { \phi } ^ { \pi }$ is $8 ( C _ { m } ^ { 3 } C _ { p } ^ { 2 } \kappa _ { r } + C _ { m } ^ { 2 } C _ { p } C _ { r } )$ -smooth in Π. That is, + +$$ +\left. \pi ^ { \prime } - \pi , \frac { \partial ^ { 2 } v _ { \phi } ^ { \pi } ( s ) } { \partial \pi } ( \pi ^ { \prime } - \pi ) \right. \leq 8 \left( C _ { m } ^ { 3 } C _ { p } ^ { 2 } \kappa _ { r } + C _ { m } ^ { 2 } C _ { p } C _ { r } \right) \| \pi ^ { \prime } - \pi \| _ { 2 } ^ { 2 } \qquad \forall \pi ^ { \prime } , \pi \in \Pi , s \in \mathcal { S } +$$ + +Proof. + +$$ +\begin{array} { r l } & { \frac { \partial ^ { 2 } v ^ { \pi _ { \alpha } } } { \partial \alpha ^ { 2 } } = M ^ { \pi _ { \alpha } } \gamma \mathbb { P } ^ { u } M ^ { \pi _ { \alpha } } \gamma \mathbb { P } ^ { u } M ^ { \pi _ { \alpha } } r ^ { \pi _ { \alpha } } + M ^ { \pi _ { \alpha } } \gamma \mathbb { P } ^ { u } M ^ { \pi _ { \alpha } } \gamma \mathbb { P } ^ { u } M ^ { \pi _ { \alpha } } r ^ { \pi _ { \alpha } } } \\ & { \qquad + M ^ { \pi _ { \alpha } } \gamma \mathbb { P } ^ { u } M ^ { \pi _ { \alpha } } r ^ { u } + M ^ { \pi _ { \alpha } } \gamma \mathbb { P } ^ { u } M ^ { \pi _ { \alpha } } r ^ { u } , \qquad \mathrm { ( f r o m ~ e q u a t i o n ~ } 6 9 ~ \mathrm { a n d ~ e q u a t i o n ~ } 7 3 ) . } \end{array} +$$ + +Considering the $L _ { \infty }$ norm, + +$$ +\begin{array} { r l r } { { \Big \| \frac { \partial ^ { 2 } v ^ { \pi _ { \alpha } } } { \partial \alpha ^ { 2 } } \Big \| _ { \infty } = 2 \| M ^ { \pi _ { \alpha } } \gamma \mathbb { P } ^ { u } M ^ { \pi _ { \alpha } } \gamma \mathbb { P } ^ { u } M ^ { \pi _ { \alpha } } r ^ { \pi _ { \alpha } } + M ^ { \pi _ { \alpha } } \gamma \mathbb { P } ^ { u } M ^ { \pi _ { \alpha } } r ^ { u } \| _ { \infty } } } \\ & { } & { \leq 2 \| M ^ { \pi _ { \alpha } } \gamma \mathbb { P } ^ { u } M ^ { \pi _ { \alpha } } \gamma \mathbb { P } ^ { u } M ^ { \pi _ { \alpha } } r ^ { \pi _ { \alpha } } \| _ { \infty } + \| M ^ { \pi _ { \alpha } } \gamma \mathbb { P } ^ { u } M ^ { \pi _ { \alpha } } r ^ { u } \| _ { \infty } } \\ & { } & { \leq 8 ( C _ { m } ^ { 3 } C _ { p } ^ { 2 } \kappa _ { r } + C _ { m } ^ { 2 } C _ { p } C _ { r } ) . } \end{array} +$$ + +Hence, we obtain, + +$$ +\left. \pi ^ { \prime } - \pi , { \frac { \partial ^ { 2 } v ^ { \pi } ( s ) } { \partial \pi } } ( \pi ^ { \prime } - \pi ) \right. \leq 8 \left( C _ { m } ^ { 3 } C _ { p } ^ { 2 } \kappa _ { r } + C _ { m } ^ { 2 } C _ { p } C _ { r } \right) \| \pi ^ { \prime } - \pi \| _ { 2 } ^ { 2 } \qquad \forall \pi ^ { \prime } , \pi \in \Pi , s \in S _ { m } . +$$ + +The above result implies the return the convergence of the projected p $\rho ^ { \pi }$ is y $L _ { 2 } ^ { \Pi } = 8 ( C _ { m } ^ { 3 } C _ { p } ^ { 2 } \kappa _ { r } + C _ { m } ^ { 2 } C _ { p } C _ { r } )$ -smooth. This establishesion complexity similar to that stated in Theorem 1 for the average reward case. + +Follow up/concurrent work in discounted reward case: The work Liu et al. (2024) improved the iteration complexity of the policy gradient (discounted reward case) method to $O ( \textstyle { \frac { A } { \epsilon } } )$ from the previous state-of-the-art iteration complexity of $O ( \frac { S A } { \epsilon } )$ Xiao (2022a); Mei et al. (2022). Liu et al. (2024) does not use smoothness of the return to establish sufficient increase lemma, instead leverages the performance difference lemma in a novel way. + +• Our instance-dependent bound for the MDP stems from the smoothness of the return. As a result, it is unclear how the two approaches can be effectively combined to achieve tighter bounds, leaving this as an avenue for future research. +• However, the bound in Liu et al. (2024) can be asymptotically improved by a factor of 11−γ , and more meaningful bounds for initial iterates can be obtained using the enhanced recursion-solving techniques presented in Kumar et al. (2024). + +Our work can provide improved, alternative, or suboptimal results (depending on the parameters) for the discounted reward case. However, it remains the first to establish the global convergence of policy gradient methods for the average reward setting. + +All existing works (Agarwal et al., 2020; Bhandari & Russo, 2024; Mei et al., 2022; Xiao, 2022a) on the discounted reward case have an iteration complexity that scales with the cardinalities of the state space $S$ and the action space $A$ , with the exception of the recent work by Liu et al. (2024), which depends only on $A$ . This dependence on the size of the action space poses significant challenges when attempting to generalize to infinite state-action spaces. + +Infinite Action Space. Our work (for both average and discounted reward case) has the MDP instance bounded bound of SLΠ2ϵ , the MDP. This hardness co where ients th $L _ { 2 } ^ { \Pi }$ is the sakes up $L _ { 2 } ^ { \Pi }$ thness constant that encodes the hardness of, may be small/finite for even large/infinite action-space MDPs. However, it requires more careful study to determine the conditions for this to happen, which we leave for the future work. + +Infinite State Space For the discounted reward setting, Liu et al. (2024) provides a stateindependent bound of $O ( \textstyle { \frac { A } { \epsilon } } )$ . The approach taken in their work is fundamentally different from ours, and extending this technique to the average reward case presents an intriguing direction for future research. + +In our work, which continues the line of research from Agarwal et al. (2020); Bhandari & Russo (2024); Mei et al. (2022); Xiao (2022a), the bound exhibits state dependence. This dependence arises from the diameter of the policy class, defined as $\begin{array} { r } { \mathrm { d i a m } ( \Pi ) ^ { 2 } = \dot { \sum _ { \pi , \pi ^ { \prime } } } \| \pi - \pi ^ { \prime } \| _ { 2 } ^ { 2 } \dot { \leq } S } \end{array}$ . While this quantity is inherently tied to the state space, it can potentially be bounded for infinite state spaces under certain structures, such as low-rank policy classes. Exploring this direction in greater detail is an intriguing avenue for future research. Another challenge is to circumvent the dependence on $C _ { P L }$ constant which captures the suboptimality of a policy. $C _ { P L }$ can be $\infty$ when the state space is infinite, necessitating a different approach to characterizing the suboptimality of a policy. + +# D.1 PARAMETRIZED POLICY CLASS LOWER BOUNDS + +Parametrized policy class. Our work can also be extended to parameterized policy classes, such as softmax policies. For parameterized policy classes, the smoothness coefficient can be derived by augmenting our analysis with the chain and product rules, which is a straightforward extension. However, this may result in different hardness coefficients, making it an interesting direction for further exploration. We leave this investigation for future work. + +Lower Bounds for policy gradient for average reward case. For the discounted reward setting, Mei et al. (2022) establishes a lower bound of $O ( \epsilon ^ { - 1 } )$ for policy gradient methods. Specifically, Theorem 9 of Mei et al. (2022) derives this lower bound using a bandit problem as a counterexample. Since a bandit is a special case of an MDP with a single state, it serves as an example for both discounted reward and average reward MDPs. Consequently, the same lower bound of $\mathsf { \bar { O } } ( \epsilon ^ { - 1 } )$ also applies to average reward MDPs, as implied by Theorem 9 of Mei et al. (2022). + +Linear Rates of Policy Gradient with aggressively increasing step sizes. Policy gradient can be interpreted as a form of soft policy iteration, assuming all states are visited or updated by the policy. Specifically, as the learning rate increases, policy gradient behavior increasingly resembles policy iteration. Since policy iteration is known to converge linearly, which is significantly faster than the typical convergence rates of policy gradient methods, it is natural to expect linear convergence bounds for policy gradient with aggressively increasing learning rates. This has been established in several works, including Xiao (2022a); Johnson et al. (2023); Liu et al. (2024). + +In most cases, the model is not known, and the exact gradient cannot be computed, requiring the use of stochastic gradient descent. In such noisy settings, using aggressive step sizes can lead to instability in the algorithm. + +However, in Theorem 1, we demonstrated linear convergence rates for simple MDPs with constant step sizes. It may also be possible to achieve similar rates for the average reward case by employing aggressively increasing step sizes, which we leave as an interesting direction for future work. \ No newline at end of file diff --git a/papers/avg-reward-pg/paper.pdf b/papers/avg-reward-pg/paper.pdf new file mode 100644 index 0000000000000000000000000000000000000000..83b2863e381d206dea36a045f014906c7a73d239 --- /dev/null +++ b/papers/avg-reward-pg/paper.pdf @@ -0,0 +1,3 @@ +version https://git-lfs.github.com/spec/v1 +oid sha256:74d4b8f6bf776b3420b0fdfc18f8cd283ccebf1f2f3b72eb9de08dbf3a00b2b6 +size 619638 diff --git a/papers/avg-reward-pg/sau.json b/papers/avg-reward-pg/sau.json new file mode 100644 index 0000000000000000000000000000000000000000..33b07dcadabb1da4c464d5bb6047d835c448630c --- /dev/null +++ b/papers/avg-reward-pg/sau.json @@ -0,0 +1,122 @@ +{ + "paper_id": "avg-reward-pg", + "paper_title": "Global Convergence of Policy Gradient in Average Reward MDPs", + "D1": [ + { + "id": "avg-reward-pg-D1-001", + "claim": "Tabular MDP state and action space sizes for Experiment C.1 (varying state/action space size): (S,A) = {(3,3), (9,9), (81,81)}", + "source": "Section 4 para 3, Appendix C.1" + }, + { + "id": "avg-reward-pg-D1-002", + "claim": "Tabular MDP state and action space sizes for Experiments C.2 (reward functions) and C.3 (transition kernels): S=16, A=16", + "source": "Section 4 para 4-5, Appendix C.2, Appendix C.3" + }, + { + "id": "avg-reward-pg-D1-003", + "claim": "Training iterations for Experiment C.1 (varying state/action space size): 2000", + "source": "Section 4 para 3" + }, + { + "id": "avg-reward-pg-D1-004", + "claim": "Training iterations for Experiment C.3 (varying transition kernels): 3000", + "source": "Section 4 para 5" + }, + { + "id": "avg-reward-pg-D1-005", + "claim": "Transition kernel formula for Experiment C.1: P(i|s,i) = (1 + 1/S) / 2 (same-state transition), P(i|s,j) = 1/(2S) for i≠j (cross-state transition), where S is the state space size of the given MDP instance", + "source": "Appendix C.1" + }, + { + "id": "avg-reward-pg-D1-006", + "claim": "Reward distribution for Experiment C.1 (maximal variance): half of the actions per state assigned reward +1, the other half assigned reward -1, applied identically for every state", + "source": "Appendix C.1 (final sentence)" + }, + { + "id": "avg-reward-pg-D1-007", + "claim": "Reward variance level settings for Experiment C.2: all states have r(s,a)=0 except one designated state s_0. At s_0 — No variance: all actions reward 1; Low variance: 1/8 of actions reward -1, rest 1; High variance: 1/4 of actions reward -1, rest 1; Max variance: 1/2 of actions reward -1, rest 1", + "source": "Appendix C.2 (four bullet points)" + }, + { + "id": "avg-reward-pg-D1-008", + "claim": "Transition kernel types for Experiment C.3: (1) Uniform: P(s'|s,a) = 1/S for all s,a,s'; (2) Non-uniform stochastic: P(i|s,i) = 1/(2S) + 1/2 and P(i|s,j) = 1/(2S) for i≠j; (3) Deterministic: P(·|s,·) is a random permutation of the identity matrix (so each state deterministically transitions to a different state)", + "source": "Appendix C.3 (three bullet points)" + }, + { + "id": "avg-reward-pg-D1-009", + "claim": "Number of distinct simulation experiments: 3 (corresponding to Appendix C.1, C.2, C.3)", + "source": "Section 4, Appendix C" + } + ], + "D2": [ + { + "id": "avg-reward-pg-D2-001", + "claim": "Projected Policy Gradient (PPG) Update for average reward MDPs. Per-iteration update: pi_{k+1} = Proj_Pi[pi_k + eta * (d rho^{pi} / d pi)|_{pi=pi_k}]. Where Proj_Pi denotes orthogonal projection in Euclidean norm onto the space of randomized policies Pi (i.e., per-state projection onto the probability simplex Delta_A). Step size constraint from convergence guarantee: eta < 1/L_2^{Pi}, where L_2^{Pi} is the restricted smoothness constant of the average reward (Lemma 4). The per-state gradient component is: d^{pi_k}(s) * Q^{pi_k}(s,·) via the average reward policy gradient theorem (Eq 5).", + "source": "Section 2.1 Eq 6, Theorem 1, Lemma 4" + }, + { + "id": "avg-reward-pg-D2-002", + "claim": "Average Reward Bellman Equation and Projected Value Function. Bellman equation: rho^{pi} 1 + v^{pi} = r^{pi} + P^{pi} v^{pi} (Eq 3), where v^{pi} is the relative state value function unique only up to an additive constant. To obtain uniqueness, the value function is projected onto the subspace orthogonal to the all-ones vector using the projection matrix Phi = (I - 11^T/|S|) (Lemma 1). The resulting unique projected value function has closed form: v_phi^{pi} = (I - Phi P^{pi})^{-1} Phi r^{pi} (Eq 15). The inverse exists because Phi P^{pi} has eigenvalue 0 for eigenvector 1 and all other eigenvalues strictly less than 1 in absolute value (Lemma 10-12).", + "source": "Section 2.1 Eq 3, Section 3.1.1 Lemma 1 Eq 14-15, Appendix A.1-A.2" + }, + { + "id": "avg-reward-pg-D2-003", + "claim": "Average Reward Policy Gradient Theorem and Gradient Decomposition. For tabular policies: gradient component (partial rho^{pi} / partial pi) decomposed per state as d^{pi}(s) * Q^{pi}(s,·). The average reward gradient theorem (Sutton & Barto, 2018): partial rho / partial theta = sum_{s in S} d^{pi}(s) sum_{a in A} (partial pi(s,a) / partial theta) Q^{pi}(s,a) (Eq 4), where d^{pi} is the stationary distribution satisfying d^{pi} P^{pi} = d^{pi}. The relative state-action value function satisfies: Q^{pi}(s,a) = r(s,a) + sum_{s',a'} P(s'|s,a) pi(a'|s') Q^{pi}(s',a') - rho^{pi} (from average reward Bellman equation for Q). Under tabular parameterization, theta is equivalent to pi, yielding per-state directional derivatives.", + "source": "Section 2.1 Eq 2-5" + }, + { + "id": "avg-reward-pg-D2-004", + "claim": "Restricted Smoothness Constant of Average Reward L_2^{Pi}. Derived by bounding the maximum eigenvalue of the Hessian within the policy class via directional derivative analysis. From Lemma 4 (Eq 17): the quadratic form is bounded as || <= (L_2^{Pi}/2) ||pi' - pi||_2^2 for all pi, pi' in Pi. The explicit expression: L_2^{Pi} = 4 * (C_p^2 C_m^2 kappa_r + C_p C_m C_r + (C_p + 1)(C_m^2 C_p kappa_r + C_m C_r) + 4 * (C_m^3 C_p^2 kappa_r + C_m^2 C_p C_r)). The smoothness is established in two stages: first the projected value function v_phi^{pi} is proven 4*(2 C_m^3 C_p^2 kappa_r + C_m^2 C_p C_r)-smooth (Lemma 2), then the average reward Bellman equation (Eq 3) is used to propagate smoothness to rho^{pi} itself. The restricted Lipschitz constant is L_1^{Pi} = 2*(C_r + C_p C_m kappa_r + 2*(C_m^2 C_p kappa_r + C_m C_r)) (Lemma 3).", + "source": "Section 3.1.1 Lemma 2-4 Eq 16-17" + }, + { + "id": "avg-reward-pg-D2-005", + "claim": "MDP Complexity Parameters (Table 1). Four constants characterize the underlying MDP difficulty and determine L_2^{Pi} and convergence rate. C_m = max_{pi} ||(I - Phi P^{pi})^{-1}||_{infty} (mixing time; operator norm w.r.t. L_infty; bounded by 2 C_e |S|/(1-lambda) from Assumption 1 geometric ergodicity, where C_e and lambda are the ergodicity constants). C_p = max_{pi,pi' in Pi} max_{||v||_{infty}<=1} ||(P^{pi'} - P^{pi})v||_{infty} / ||pi' - pi||_2 (transition kernel diameter; range [0, sqrt(|A|)]). C_r = max_{pi,pi' in Pi} ||r^{pi'} - r^{pi}||_{infty} / ||pi' - pi||_2 (reward function diameter; range [0, sqrt(|A|)]). kappa_r = max_{pi} ||Phi r^{pi}||_{infty} (reward variance; range [0, 2)). Key insight: these constants are MDP-specific, not just cardinality-dependent, enabling complexity-aware convergence rates where simpler MDPs (low C_p, C_r) converge faster than complex ones even at fixed |S|,|A|.", + "source": "Table 1, Section 3.1.1, Appendix A" + }, + { + "id": "avg-reward-pg-D2-006", + "claim": "Convergence Rate Formula and Step-Size Condition (Theorem 1). For all MDPs with step size eta < 1/L_2^{Pi}: sublinear rate rho^* - rho^{pi_k} <= 1 / (1/(rho^* - rho^{pi_0}) + nu * k) for all k >= 0. The constant nu = (1 / (32 C_PL^2 |S| L_2^{Pi})) * (1 + 4/(32 C_PL^2 |S| L_2^{Pi}))^{-3/2} (Eq 10). C_PL = max_{pi} d^{pi^*}(s)/d^{pi}(s) is the distribution mismatch coefficient (Lemma 7). For simple MDPs (L_2^{Pi} << 1): exponential convergence rho^* - rho^{pi_k} <= c^{-k/2} * (rho^* - rho^{pi_0})^{1/2^k} where 1/c = 32 |S| L_2^{Pi} C_PL^2 < 1 (Eq 11-12). Per-step improvement lower bound from smoothness: rho^{pi_{k+1}} - rho^{pi_k} >= (L_2^{Pi}/2) ||pi_{k+1} - pi_k||_2^2 (Lemma 5, Eq 19). Suboptimality upper bound via performance difference lemma: rho^* - rho^{pi} = sum_s d^{pi^*}(s) sum_a Q^{pi}(s,a)[pi^*(a|s) - pi(a|s)] (Lemma 6, Eq 20).", + "source": "Section 3 Theorem 1 Eq 10-12, Section 3.1.2 Lemma 5 (Eq 19), Lemma 6 (Eq 20), Lemma 7" + }, + { + "id": "avg-reward-pg-D2-007", + "claim": "Discounted MDP Extension with Complexity-Aware Bounds (Section 3.2). The same smoothness analysis extends to discounted MDPs by replacing C_m with hat(C_m) = ||(I - gamma P^{pi})^{-1}||_{infty} where hat(C_m) <= 1/(1-gamma). The discounted smoothness constant: L_2^{Pi}_gamma = 4 * (C_p^2 hat(C_m)^2 kappa_r + C_p hat(C_m) C_r + (C_p+1)(hat(C_m)^2 C_p kappa_r + hat(C_m) C_r) + 4*(hat(C_m)^3 C_p^2 kappa_r + hat(C_m)^2 C_p C_r)). Iteration complexity improves to O(|S| L_2^{Pi}_gamma / epsilon) from the state-of-the-art O(|S||A|/((1-gamma)^5 epsilon)) (Xiao, 2022b). Since L_2^{Pi}_gamma = O(hat(C_m)^3 C_p^2) and hat(C_m) <= 1/(1-gamma), this yields O(|S|/((1-gamma)^5 epsilon)) matching the best known dependence while being complexity-aware. For trivial MDPs where C_p = 0 or kappa_r = 0 (implying C_r = 0): L_2^{Pi}_gamma = 0 and complexity reduces to O(|S|/epsilon) versus O(|S||A|/epsilon) in prior work — the constant captures MDP hardness rather than mere cardinality. The discounted value function is defined as rho_{mu,gamma}^{pi} = mu^T (1 - gamma P^{pi})^{-1} r^{pi} (Section 2.2).", + "source": "Section 2.2, Section 3.2" + } + ], + "D3": [ + { + "id": "avg-reward-pg-D3-001", + "claim": "Experiment C.1: Convergence with different action and state space sizes. Purpose: Compare PPG convergence rate as state/action space cardinality increases. Setup: Three tabular MDPs with (S,A) = (3,3), (9,9), (81,81); transition kernel P(i|s,i)=(1+1/S)/2, P(i|s,j)=1/(2S); reward: half actions +1, half -1 per state. Algorithm: Exact projected policy gradient (PPG) update as in Eq 6. Run for 2000 iterations. Metric: Average reward rho^{pi_k} plotted as function of iteration k.", + "source": "Section 4 para 3, Appendix C.1" + }, + { + "id": "avg-reward-pg-D3-002", + "claim": "Experiment C.2: Convergence with different reward functions (varying Cr). Purpose: Compare PPG convergence rate under four reward variance levels on a fixed MDP structure. Setup: Single tabular MDP with S=16, A=16; randomly generated transition kernel (fixed across runs); reward function: r(s,a)=0 everywhere except one designated state s_0; four reward configurations (no/low/high/max variance at the single designated state s_0). Algorithm: Exact PPG update. Metric: Average reward as function of iterations. (Note: iteration count not explicitly stated for C.2; Section 4 groups it with Figure 1(b) results.)", + "source": "Section 4 para 4, Appendix C.2" + }, + { + "id": "avg-reward-pg-D3-003", + "claim": "Experiment C.3: Convergence with different transition kernels (varying Cp). Purpose: Compare PPG convergence rate under three transition kernel types on fixed S=16, A=16 MDP. Setup: Three MDP instances sharing S=16, A=16 and the same reward function (high variance from C.2); three transition kernel types (uniform, non-uniform stochastic, deterministic). Algorithm: Exact PPG update. Run for 3000 iterations. Metric: Change in average reward plotted as function of iterations.", + "source": "Section 4 para 5, Appendix C.3" + } + ], + "D4": [ + { + "id": "avg-reward-pg-D4-001", + "claim": "Proof Phase 1 (Establish smoothness): Project value function via Phi=I-11^T/|S| to obtain unique v_phi^pi=(I-Phi P^pi)^{-1} Phi r^pi (Lemma 1). Derive restricted smoothness constant L_2^Pi from MDP complexity parameters C_m,C_p,C_r,kappa_r (Lemmas 2-4). Result: average reward rho^pi is L_2^Pi-smooth. (Section 3.1.1)", + "source": "Section 3.1.1, Lemma 1-4" + }, + { + "id": "avg-reward-pg-D4-002", + "claim": "Proof Phase 2 (Prove convergence): Lower bound per-step improvement via smoothness (Lemma 5). Upper bound suboptimality via performance difference lemma + gradient relationship (Lemmas 6-7). Combine: sublinear rate O(1/k) for general MDPs, exponential for simple MDPs, step size eta<1/L_2^Pi (Theorem 1). (Section 3.1.2)", + "source": "Section 3.1.2, Lemma 5-8, Theorem 1" + }, + { + "id": "avg-reward-pg-D4-003", + "claim": "Simulation procedure: Construct tabular MDP. Init random pi_0. Iterate pi_{k+1}=Proj_Pi[pi_k+eta·nabla rho^{pi_k}] for K steps (K=2000 or 3000). Measure rho^{pi_k}. Three variants: vary |S|,|A| (C.1); vary reward variance (C.2); vary transition kernel determinism (C.3). Convergence rate slows with larger MDP complexity parameters C_r, C_p. (Section 4, Appendix C.1-C.3)", + "source": "Section 4, Appendix C.1-C.3" + } + ] +} \ No newline at end of file diff --git a/papers/ca2-vdm/blacklist.txt b/papers/ca2-vdm/blacklist.txt new file mode 100644 index 0000000000000000000000000000000000000000..4ae3272746f1ee88f49a361c5559dbdd7abc32f9 --- /dev/null +++ b/papers/ca2-vdm/blacklist.txt @@ -0,0 +1,2 @@ +# Official repository +https://github.com/Dawn-LX/CausalCache-VDM diff --git a/papers/ca2-vdm/config.yaml b/papers/ca2-vdm/config.yaml new file mode 100644 index 0000000000000000000000000000000000000000..b5ff5f5ea3c8d56f87f9668154bb85ed8bc50045 --- /dev/null +++ b/papers/ca2-vdm/config.yaml @@ -0,0 +1,8 @@ +title: "Ca2-VDM: Efficient Autoregressive Video Diffusion Model with Causal Generation and Cache Sharing" +pdf_url: "https://raw.githubusercontent.com/mlresearch/v267/main/assets/gao25m/gao25m.pdf" +venue: "ICML 2025" +year: "2025" +extra: + selection_index: 8 + domain: "Computer Vision" + paradigm: "Generative Models" diff 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sha256:9d7d05aef8caa237774dd161b547a461b852fc786d80369aae00a39b2cea2309 +size 57602 diff --git a/papers/ca2-vdm/images/tables/ca2-vdm-table-0007.jpg b/papers/ca2-vdm/images/tables/ca2-vdm-table-0007.jpg new file mode 100644 index 0000000000000000000000000000000000000000..afc84583b3c33e6644216cd04cf1077f9277eaf3 --- /dev/null +++ b/papers/ca2-vdm/images/tables/ca2-vdm-table-0007.jpg @@ -0,0 +1,3 @@ +version https://git-lfs.github.com/spec/v1 +oid sha256:d6e48e7c26e1446d95a5e2421d5852dfb1e65a3635ed9995657d6d35e9e11c3d +size 22702 diff --git a/papers/ca2-vdm/paper.md b/papers/ca2-vdm/paper.md new file mode 100644 index 0000000000000000000000000000000000000000..bc7ac6a7138100faf847f5721e0a5bd51e9cf779 --- /dev/null +++ b/papers/ca2-vdm/paper.md @@ -0,0 +1,407 @@ +# Ca2-VDM: Efficient Autoregressive Video Diffusion Model with Causal Generation and Cache Sharing + +Kaifeng Gao \* 1 2 Jiaxin Shi \* 3 Hanwang Zhang 4 Chunping Wang 5 Jun Xiao 1 Long Chen 6 + +# Abstract + +With the advance of diffusion models, today’s video generation has achieved impressive quality. To extend the generation length and facilitate real-world applications, a majority of video diffusion models (VDMs) generate videos in an autoregressive manner, i.e., generating subsequent clips conditioned on the last frame(s) of the previous clip. However, existing autoregressive VDMs are highly inefficient and redundant: The model must re-compute all the conditional frames that are overlapped between adjacent clips. This issue is exacerbated when the conditional frames are extended autoregressively to provide the model with long-term context. In such cases, the computational demands increase significantly (i.e., with a quadratic complexity w.r.t. the autoregression step). In this paper, we propose Ca2-VDM, an efficient autoregressive VDM with Causal generation and Cache sharing. For causal generation, it introduces unidirectional feature computation, which ensures that the cache of conditional frames can be precomputed in previous autoregression steps and reused in every subsequent step, eliminating redundant computations. For cache sharing, it shares the cache across all denoising steps to avoid the huge cache storage cost. Extensive experiments demonstrated that our Ca2-VDM achieves state-of-the-art quantitative and qualitative video generation results and significantly improves the generation speed. Code is available: https://github.com/ Dawn-LX/CausalCache-VDM + +![](images/figures/ca2-vdm-fig-0001.jpg) +Figure 1: (a): Existing autoregressive VDMs with bidirectional generation. The conditional frames can be fixedlength (Henschel et al., 2025; Zheng et al., 2024) or extendable. (b): Our Ca2-VDM, which uses causal generation to enable KV-cache and introduce cache sharing across all denoising timesteps. Cache writing stands for a partial model forward on the denoised frames (i.e., at timestep $t = 0$ ) until the KV-caches of every layer are computed. + +# 1. Introduction + +Video diffusion models (VDMs) (Guo et al., 2024b; Ren et al., 2024; Lu et al., 2024; Ma et al., 2025) have made significant advancements by benefiting from the powerful diffusion techniques (Ho et al., 2020; Song et al., 2021a;b) and prior studies on image generation (Rombach et al., 2022; Peebles & Xie, 2023; Chen et al., 2024a). In contrast to images, VDMs need to capture interactions across multiple frames and generate all frames simultaneously (e.g., a 16-frame clip). This is usually facilitated by the temporal attention in prevailing UNet- or Transformer-based VDMs (Wang et al., 2023b; Ma et al., 2025). They introduce interdependencies during the bidirectional attention computation. Consequently, the training and inference lengths must be aligned, extremely restricting the flexibility of VDMs in real-world applications such as long-term (Henschel et al., 2025) or live-stream (Alonso et al., 2024) video generation. Meanwhile, simply scaling the clip length at inference time breaks the alignment and leads to poor generation quality (e.g., Figure 1(b) in (Qiu et al., 2024)), unless one undertakes time-consuming retraining or fine-tuning. + +To address this issue, an effective and prevalent solution is autoregressive VDMs (Blattmann et al., 2023a; Henschel et al., 2025; Lu et al., 2024): They are capable of autoregressively generating subsequent clips conditioned on last frames of previous clip, as shown in Figure 1(a). However, the autoregression process of existing VDMs is highly inefficient and redundant: The conditional frames constitute the overlapping frames between adjacent autoregression chunks and they are re-computed at each step. This issue is exacerbated when the conditional frames are extended autoregressively to provide the model with long-term context. In such cases, the model must re-compute all the conditional frames concatenated by the previously generated chunks, with a quadratic computational demand w.r.t. the autoregressive step (cf. Figure 6 in Sec. 4.3). + +To overcome the above limitations, we propose to cache the intermediate features (specifically, the keys and values of every attention layer) at each autoregression (AR) step, and reuse them in subsequent AR steps, as shown in Figure 1(b). In this way, the model 1) eliminates the redundant computations in temporal attention blocks, and 2) reduces the processing length to a constant for other temporal-parallel blocks (e.g., spatial attention and visual-text cross attention) while maintaining the extendable long-term context. To successfully implement the KV-cache in VDMs, two key factors must be carefully considered: + +• Cache Computation. In existing VDMs, the temporal attention is bidirectional, as shown in Figure 2(a). The frames z3,t are denoised conditioned on $z _ { 0 } ^ { \overline { { 0 } } , 1 , 2 }$ , and key/value features of z0,0 $z _ { 0 } ^ { 0 , 1 , 2 }$ are also computed conditioned on z3,t at every diffusion timestep $t$ (highlighted by the red box and arrows). It’s impopute and cache the keys and values of $z _ { 0 } ^ { 0 , 1 , 2 }$ to precom-at previous AR steps, since $z _ { t } ^ { 3 , 4 }$ are not yet available. + +• Cache Storage. During inference, the VDM is repeatedly called in the denoising process at each AR step, where each call is taken with a different timestep $t$ . All most all Existing VDMs (Lu et al., 2024; Ren et al., 2024) use the same timestep embedding (indexed by $t$ ) for both conditional and noisy frames. This requires each denoising step to have its own cache, i.e., caching the key/value features for all denoising steps will consume huge GPU memory. + +In this paper, we propose an efficient autoregressive VDM boosted by causal generation and cache sharing, termed Ca2- VDM, to handle both challenges. For cache computation, we propose causal generation: We replace the full temporal attention in each block of the VDM with causal temporal attention, and propose prefix-enhanced spatial attention. The former ensures each generated frame only depends on its prefix frames, and the latter enhances the guidance from the prefix frames. As a result, the cache to be used in subsequent autoregression steps can be precomputed at early steps. For cache storage, we propose cache sharing. It leverages the advantages of causal generation: The cache is determined only by the non-noisy preceding (conditional) frames and unaffected by the subsequent noisy frames (i.e., independent of the timestep $t$ ). Thus, by using a distinct timestep embedding indexed by $t = 0$ for the conditional frames in both training and inference, we enable the cache to be shared across all the denoising steps. + +![](images/figures/ca2-vdm-fig-0002.jpg) +Figure 2: Comparison of bidirectional attention (a) and causal attention (ours) (b). Our design addresses the cache computation and cache storage issues. + +Equipped with causal generation and cache sharing, we propose to store the KV-cache in a queue so that the model can exploit the long-term context while maintaining an affordable computation and storage cost. To support this queue design, the training samples are partially noised to keep clean prefix frames (with random length) as the condition, and the maximum condition length covers the length of KV-cache queue at inference time. Meanwhile, sinusoidal spatial and temporal positional embeddings (i.e., SPEs and TPEs) are added to the frame sequence following Vision Transformer (ViT) (Dosovitskiy et al., 2020). During inference, the TPEs are assigned chunk-by-chunk as the autoregression progresses. To ensure TPEs are correctly assigned when the cumulatively generated video exceeds the training length, we carefully design a cyclic shift mechanism: Cyclic-TPEs 1. + +We evaluated our Ca2-VDM on multiple public datasets including MSR-VTT (Xu et al., 2016), UCF-101 (Soomro et al., 2012), and Sky Timelapse (Zhang et al., 2020) for both text-to-video and video prediction tasks. The results show that our model achieves significant inference speed improvement while maintaining comparable quantitative and qualitative performance as state-of-the-art VDMs. In summary, we make three contributions in this paper: 1) A causal generation structure that allows the intermediate features of conditional frames can be cached and reused in every autoregression step, eliminating the redundant computation. 2) A cache sharing strategy implemented on the KV-cache queue and facilitated by Cyclic-TPEs. It allows the model to acquire extendable context while significantly reducing the storage cost. 3) Our Ca2-VDM achieves comparable performance with SOTA VDMs at a much less computation demand and a high inference speed. + +# 2. Related Work + +Video Diffusion Models (VDMs) have shown impressive generation capabilities, building on the success of latent diffusion models in image generation applications (Rombach et al., 2022; Peebles & Xie, 2023; Chen et al., 2024a). Some works (Lu et al., 2023; Khachatryan et al., 2023; Hong et al., 2023; Zhang et al., 2024) develop training-free methods for zero-shot video generation based on pretrained image diffusion models (e.g., Stable Diffusion (Rombach et al., 2022)). To leverage video training data and improve the generation quality, many works (Ge et al., 2023; Guo et al., 2024b; Wang et al., 2023b; Ren et al., 2024; Dai et al., 2023) extend the 2D Unet in text-to-image diffusion models with temporal attention layers or temporal convolution layers. Recent studies (Ma et al., 2025; Lu et al., 2024) also build VDMs based on spatial-temporal Transformers due to their inherent capability of capturing long-term temporal dependencies. We build our Ca2-VDM based on spatial-temporal Transformers following prior structures. + +Tuning-free Video Extrapolation. Prior studies have explored autoregressively extrapolating videos using pretrained short video diffusion models without additional finetuning. These methods usually consist of initializing noise sequence based on the DDIM inversion (Song et al., 2021a; Mokady et al., 2023) of previously generated frames (Oh et al., 2024), co-denoising overlapped short clips (Wang et al., 2023a), or iteratively denoising short clips with noiserescheduling (Qiu et al., 2024). However, their generation quality is upper-bounded by the pretrained VDMs. Meanwhile, the lack of finetuning also leads to temporal inconsistencies between short clip transitions. + +Past-frame Conditioned Video Prediction. To enhance generation quality and temporal consistency, a popular paradigm is training VDMs conditioned on past frames to predict future frames, enabling video extrapolation through autoregressive model calls. Recent works of autoregressive VDMs have studied a variety of design choices for injecting conditional frames, such as adaptive layer normalization (Voleti et al., 2022; Lu et al., 2024), crossattention (Zhang et al., 2023b; Lu et al., 2024; Henschel et al., 2025), and explicitly concatenating to the noisy latent along the temporal-axis (Harvey et al., 2022; Lu et al., 2024) or channel-axis (Chen et al., 2024b; Girdhar et al., 2024; Zeng et al., 2024). Some works (Weng et al., 2024; Guo et al., 2024a) also inject conditional frames by adapter-like subnets (e.g., T2I-adapter (Mou et al., 2024) or Control-Net (Zhang et al., 2023a)). In contrast to existing works, our Ca2-VDM avoids the redundant computation of conditional frames by causal generation and cache sharing, and significantly improves the generation speed. + +# 3. Method + +# 3.1. Preliminaries and Problem Formulation + +Preliminaries. Diffusion Models (Sohl-Dickstein et al., 2015; Ho et al., 2020) are generative models that model a target distribution $\pmb { x } _ { 0 } \sim q ( \pmb { x } )$ by learning a denoising process with arbitrary noise levels. To do this, a diffusion process is defined to gradually corrupt $\scriptstyle { \mathbf { { \mathit { x } } } } _ { 0 }$ with Gaussian noise. Each diffusion step is $q ( \pmb { x } _ { t } | \pmb { x } _ { t - 1 } ) = \mathcal { N } ( \pmb { x } _ { t } ; \sqrt { 1 - \beta _ { t } } \pmb { x } _ { t - 1 } , \beta _ { t } \pmb { I } )$ , where $t = 1 , \dots , T$ and $\beta _ { t } \in ( 0 , 1 )$ is the variance schedule. By applying the reparameterization trick (Ho et al., 2020), each $\mathbf { \Delta } _ { \mathbf { \mathcal { X } } _ { t } }$ can be sampled as ${ \pmb x } _ { t } = \sqrt { \bar { \alpha } _ { t } } { \pmb x } _ { 0 } + \sqrt { 1 - \bar { \alpha } _ { t } } { \pmb \epsilon } _ { t }$ where $\epsilon _ { t } \sim \mathcal { N } ( \mathbf { 0 } , I )$ and $\begin{array} { r } { \bar { \alpha } _ { t } = \prod _ { i = 1 } ^ { t } ( 1 - \beta _ { i } ) } \end{array}$ . Given the diffusion process, a diffusion model is then trained to approximate the denoising process. Each denoising step is parameterized as $p _ { \theta } ( \pmb { x } _ { t - 1 } | \pmb { x } _ { t } ) = \mathcal { N } ( \pmb { x } _ { t - 1 } ; \pmb { \mu } _ { \theta } ( \pmb { x } _ { t } , t ) , \pmb { \Sigma } _ { \theta } ( \pmb { x } _ { t } , t ) )$ , where $\theta$ contains learnable parameters. + +Problem Formulation. Following existing mainstream VDMs (Guo et al., 2024b; Lu et al., 2024; Ma et al., 2025), we develop Ca2-VDM based on latent diffusion models (Rombach et al., 2022) to reduce the modeling complexity of high dimensional visual data. This is achieved by using a pretrained variational autoencoder (VAE) encoder $\mathcal { E }$ to compress $\scriptstyle { \mathbf { { \mathit { x } } } } _ { 0 }$ into a lower-dimensional latent representation, i.e., $z _ { 0 } = \mathcal { E } ( \pmb { x } _ { 0 } )$ . Consequently, the diffusion and denoising processes are implemented in the latent space, formulated as $q \big ( z _ { t } | z _ { t - 1 } \big )$ and $p _ { \theta } \big ( z _ { t - 1 } | z _ { t } \big )$ , respectively. The denoised latent $\hat { z } _ { 0 }$ is decoded back to the pixel space by the pretrained VAE decoder $\mathcal { D }$ , i.e., $\hat { \pmb x } _ { 0 } = \mathcal { D } ( \hat { \pmb z } _ { 0 } )$ . + +In our setting, the model takes as input a VAE encoded latent sequence2 $\begin{array} { r } { \bar { z } _ { 0 } ^ { 0 : L } = [ z _ { 0 } ^ { 0 } , \dots , z _ { 0 } ^ { L - 1 } ] ^ { \bullet } \in \mathbb { R } ^ { L \times H \times W \times C } } \end{array}$ , where $L$ is the number of frames, $H \times W$ is the downsampled resolution, and $C$ is the number of channels. Then, it aims to generate future frames conditioned on past frames, by learning a distribution $p _ { \theta } \big ( z _ { 0 } ^ { P : L } \big | z _ { 0 } ^ { 0 : P } \big )$ . Here the first $P$ prefix frames serve as condition (referred to as clean prefix), and the remaining $L - P$ frames are those to be denoised (referred to as denoising target). The model parameterized by $\theta$ is denoted as $\epsilon _ { \theta } ( z _ { t } ^ { 0 : L } , t )$ . + +![](images/figures/ca2-vdm-fig-0003.jpg) +Figure 3: Overview of the Ca2-VDM pipeline. (a): During training, we randomly set $P$ frames clean prefix, and set distinctive timestep embeddings, i.e., $\mathbf { t E m b } ( 0 )$ for the clean prefix and $\mathbf { t E m b } ( t )$ for the denoising target. (b): During inference, in each autoregression (AR) step, the model denoises an $l$ -frame chunk conditioned on the spatial/temporal KV-caches shared across all timesteps (denoising stage), and then computes the keys/values of denoised chunk to update the KV-caches (cache writing stage). (c): Causal generation block. We further illustrate the details of causal temporal attention with Cyclic-TPEs in Figure 4 and the prefix-enhanced spatial attention is left in the Appendix (cf. Figure 9). + +The overall pipeline of Ca2-VDM is shown in Figure 3. We first illustrate the causal generation in the training stage (Sec. 3.2), as well as the training objectives. Then, we introduce the KV-cache realization combined with the cache sharing mechanism in the autoregressive inference stage (Sec. 3.3), and the queue structure for temporal KV-cache supported by Cyclic-TPEs (cf. Figure 4). + +# 3.2. Causal Generation and Training Objectives + +We first introduce the training objectives, followed by the causal generation block (cf. Figure 3(c)). Here we focus on the causal temporal attention and prefix-enhanced spatial attention layers. For the visual-text cross attention, it is widely used in VDMs for text-to-video generation (Rombach et al., 2022; Chen et al., 2024a). And it is optional for pure video prediction (Lu et al., 2024). We refer readers to related works (Chen et al., 2024a) for more details. + +Training Objectives. Existing diffusion models (Ho et al., 2020; Peebles & Xie, 2023) are trained with the variational lower bound of $z _ { \mathrm { 0 } }$ ’s log-likelihood, formulated as ${ \mathcal { L } } _ { \mathrm { v l b } } ( \theta ) =$ $\begin{array} { r l } { - \log p _ { \theta } ( z _ { 0 } | z _ { 1 } ) + \sum _ { t } D _ { K L } \big ( q ( z _ { t - 1 } | z _ { t } , z _ { 0 } ) \| p _ { \theta } ( z _ { t - 1 } | z _ { t } ) \big ) } & { { } } \end{array}$ , where $D _ { K L }$ is determined by the mean $\mu _ { \theta }$ and covariance $\Sigma _ { \theta }$ . By re-parameterizing $\pmb { \mu } _ { \theta }$ as a noise prediction network $\epsilon _ { \theta }$ and fixing $\Sigma _ { \theta }$ as a constant variance schedule (Ho et al., + +2020), the model can be trained by a simplified objective: + +$$ +\begin{array} { r } { \mathcal { L } _ { \mathrm { s i m p l e } } ( \theta ) = \underset { z , \epsilon , t } { \mathbb { E } } \left[ \Vert \epsilon _ { \theta } ( z _ { t } , t ) - \epsilon \Vert _ { 2 } ^ { 2 } \right] , \epsilon \sim \mathcal { N } ( 0 , 1 ) . } \end{array} +$$ + +In our setting, each sample is partially noised. We randomly keep $P$ consecutive frames uncorrupted as the clean prefix, and the remaining frames are treated as the denoising target, as shown in Figure 3(a). We use different timestep embeddings for the clean prefix (i.e., $\mathbf { t E m b } ( 0 )$ ) and the denoising target (i.e., $\mathbf { t E m b } ( t ) )$ ), rather than a unified timestep embedding for the whole video clip as in many existing VDMs (Lu et al., 2024; Ma et al., 2025). This ensures the cache from the clean prefix can be correctly shared across each denoising timestep $t$ at inference time (since the clean prefix is always assigned with $\mathbf { t E m b } ( 0 )$ ). Consequently, the simplified objective function for our model is + +$$ +\widetilde { \mathcal { L } } _ { \mathrm { s i m p l e } } ( \theta ) = \underset { z , \epsilon , t } { \mathbb { E } } \big [ \| \big ( \epsilon _ { \theta } \big ( [ z _ { 0 } ^ { 0 : P } , z _ { t } ^ { P : L } ] , t \big ) - \epsilon \big ) \odot m \| _ { 2 } ^ { 2 } \big ] , +$$ + +where $[ \cdot , \cdot ]$ stands for concatenation along the temporal axis, and $\pmb { t }$ is the timestep vector with $t _ { i } = t$ if $i \geq P$ else 0. $\pmb { m } \in \{ 0 , 1 \} ^ { N }$ is a loss mask to exclude the clean prefix part, i.e., with $m _ { i } = 1$ if $i \geq P$ else 0. In practice, we train the model with learnable covariance $\Sigma _ { \theta }$ by optimizing a combination of $\widetilde { \mathcal { L } } _ { \mathrm { s i m p l e } }$ and $\mathcal { L } _ { \mathrm { v l b } }$ (with the same loss mask) following (Nichol $\&$ Dhariwal, 2021; Peebles & Xie, 2023). More details are left in Sec. B. + +Causal Temporal Attention. To introduce the causality, we mask the attention map to force each frame to only attend to its preceding frames, as shown in Figure 4(a). Specifically, the input to each layer is first permuted by treating the spatial resolution $H \times W$ as the batch dimension, and then linearly projected to query, key, and value features as $Q , K , V \in \bar { \mathbb { R } } ^ { \bar { L } \times \bar { C } ^ { \prime } }$ (for every spatial grid). The causal attention is computed as + +$$ +\mathrm { C a u s a l A t t n } ( Q , K , V ) { = } \mathrm { S o f t m a x } \left( \frac { Q K ^ { \mathrm { T } } } { \sqrt { \mathit { C } ^ { \prime } } } { + } M \right) V , +$$ + +where $M \in \mathbb { R } ^ { L \times L }$ is a lower triangular attention mask with $M _ { i , j } = - \infty$ if $i < j$ else 0. Note that we only describe one attention head and omit the diffusion step $t$ for brevity. + +Prefix-Enhanced Spatial Attention. In analogy to causal temporal attention, integrating the clean prefix and denoising target into one attention sequence helps enhance the guidance of conditional information. Inspired by prior works (Hu, 2024; Ren et al., 2024), we do this via spatialwise concatenation (cf. Figure 9 in the Appendix). Let $\pmb { h } _ { t } ^ { 0 : L } \in \mathbb { R } ^ { L \times H \times W \times \dot { C } ^ { \dagger } }$ be the hidden input to each layer, where the number of frames $L$ is treated as batch dimension and $H \times W$ is flattened for attention calculation. We take a sub-prefix of length $P ^ { \prime }$ and concatenate it to the denoising target. Specifically, for $h _ { t } ^ { i }$ from the $i$ -th frame, the query is $\bar { Q } ( i ) = \mathbf { \bar { W } } ^ { Q } h _ { t } ^ { i }$ . The prefix-enhanced key is + +$$ +\bar { \pmb { K } } ( i ) = \left\{ \begin{array} { l l } { W ^ { K } [ { h _ { 0 } ^ { P - P ^ { \prime } } } ; . . . ; { h _ { 0 } ^ { P - 1 } } ; { h _ { t } ^ { i } } ] } & { \mathrm { i f ~ } i \ge P } \\ { W ^ { K } [ { h _ { 0 } ^ { i } } ; . . . ; { h _ { 0 } ^ { i } } ] } & { \mathrm { i f ~ } i < P } \end{array} \right. , +$$ + +where $[ \cdot ; \cdot ]$ stands for concatenation along the spatial dimension, and $ { \boldsymbol { h } } _ { 0 } ^ { i }$ is broadcasted by self-repeat $P ^ { \prime }$ times for every $i < P$ (i.e., the clean prefix part). We do the same operation to obtain the prefix-enhanced value $\bar { V }$ . Consequently, for every frame, the prefix-enhanced spatial attention is computed as Attention $( \bar { Q } , \bar { K } , \bar { V } )$ with an attention map of shape $( H W ) \times ( ( P ^ { \prime } + 1 ) H W )$ . In practice, $P ^ { \prime }$ is relatively small (e.g., $P ^ { \prime } = 3$ ), as the computational cost scales proportionally with $P ^ { \prime }$ , while adjacent prefix frames tend to exhibit similar appearances. We empirically show that prefix enhancement improves the generation quality (cf. Table 4). + +# 3.3. Autoregressive Inference with Cache Sharing + +We first introduce an overview of the autoregressive inference equipped with cache sharing, as shown in Figure 3(b). Then for each autoregression step, we illustrate the temporal KV-cache queue and cyclic temporal positional embeddings (Cyclic-TPEs) . Finally, we introduce the spatial KV-cache for prefix-enhanced spatial attention. + +Autoregressive Inference. The model starts from a given first frame and generates an $l$ -frame chunk per AR step. Each AR step consists of a denoising stage and a cache writing stage. The spatial and temporal KV-caches are shared across every denoising timestep $t$ (i.e., cache sharing). In the denoising stage, given $P _ { k }$ generated frames at AR step $k$ , each denoising step samples $z _ { t - 1 } ^ { P _ { k } : P _ { k } + l } \sim$ $p _ { \theta } ( z _ { t - 1 } ^ { P _ { k } : P _ { k } + l } | z _ { t } ^ { P _ { k } : P _ { k } + l } , z _ { 0 } ^ { 0 : P _ { k } } )$ Here z0:0 $z _ { 0 } ^ { 0 : P _ { k } }$ serves as the clean prefix and $z _ { t } ^ { P _ { k } : P _ { k } + l }$ is the denoising target. Benefiting from the causal generation, the feature computation is unidirectional. This means zPk:Pt−1 $z _ { t - 1 } ^ { P _ { k } : P _ { k } + l }$ is denoised conditioned on z0:Pk0 while the cache of z0:0 Pk could be precomputed in previous autoregression steps without referring to zPk:Pk+lt . In the cache writing stage, the denoised $z _ { 0 } ^ { P _ { k } : P _ { k } ^ { - } + l }$ is input to the model again to compute its clean spatial and temporal KV-caches, which will be used in the next AR step. + +![](images/figures/ca2-vdm-fig-0004.jpg) +Figure 4: Illustration of causal temporal attention (a) & (b) and the temporal KV-cache queue with Cyclic-TPEs (c). In (c), $L _ { \mathrm { t r a i n } } = P _ { \mathrm { m a x } } + l$ and $P _ { k + l } = P _ { k } + l$ . We show the state that autoregressive inference reaches $P _ { k } = P _ { \operatorname* { m a x } }$ . + +Temporal KV-Cache. Suppose that there are $P _ { k }$ generated frames (i.e., the clean prefix) at AR step $k$ . In the denois-$t$ ng sare $\bar { \mathbf { Q } _ { t } ^ { P _ { k } : P _ { k } + l } } , \bar { \mathbf { K } _ { t } ^ { \bar { P } _ { k } : P _ { k } + l } } , \mathbf { V } _ { t } ^ { P _ { k } : P _ { k } + l } \in \mathbb { R } ^ { l \times C ^ { \prime } }$ at timestep(considering only one spatial grid). The model reads the clean key and value caches as $K _ { 0 } ^ { 0 : P _ { k } } , V _ { 0 } ^ { 0 : P _ { k } } \in \mathbb { R } ^ { P _ { k } \times C ^ { \prime } }$ . Then, they are concatenated to the noisy ones as $[ K _ { 0 } ^ { \overline { { 0 } } : P _ { k } } , K _ { t } ^ { P _ { k } : P _ { k } + l } ]$ l] and V˜ (k, t) = [V 0:Pk0 ,V Pk:Pk+lt ]. Fi- $\tilde { \mathbf { K } } ( k , t ) \ =$ nally, the causal temporal attention is computed as: + +$$ +\mathrm { C a u s a l A t t n } ( Q _ { t } ^ { P _ { k } : P _ { k } + l } , \tilde { K } ( k , t ) , \tilde { V } ( k , t ) ) , +$$ + +where the attention map has a shape of $\boldsymbol { l } \times ( P _ { k } + \boldsymbol { l } )$ , as shown in Figure 4(b). During denoising, the clean KVcache $K _ { 0 } ^ { 0 : P _ { k } ^ { - } }$ and V 0:Pk are shared for every timestep $t$ . In the cache writing stage, the clean temporal keys and values are computed as KPk:Pk+l0 and $V _ { 0 } ^ { P _ { k } : P _ { k } + l }$ . They are then updated into the KV-cache queue, resulting in $K _ { 0 } ^ { 0 : P _ { k + 1 } }$ and $V _ { 0 } ^ { 0 : P _ { k + 1 } }$ , which will be used in AR step $k + 1$ (i.e., $P _ { k + 1 } = P _ { k } + l )$ . As the autoregression progresses, the earliest KV-cache will be dequeued when the length of the clean prefix $P _ { k }$ reaches a predefined $P _ { \mathrm { m a x } }$ (i.e., a maximum number of conditional frames), as shown in Figure 4(c). + +Cyclic-TPEs. Assume that the model was trained on video clips with a maximum length of $L _ { \mathrm { t r a i n } } ~ = ~ P _ { \mathrm { m a x } } + l$ (i.e., with $P _ { \mathrm { m a x } }$ frames clean prefix and $l$ frames denoising target). $L _ { \mathrm { t r a i n } }$ is also the maximum length of TPE sequence during training. As the autoregressive inference progresses till $P _ { k } = P _ { \operatorname* { m a x } }$ , the TPEs are used up. When KV-cache is disabled (cf. Figure 4(c)-left), to align the training pattern, we can re-assign the TPEs from scratch after the earliest clean frames are dequeued. However, when KV-cache is enabled (cf. Figure 4(c)-right), the TPEs were bound to keys and values at previous AR steps and had been stored in preceding KV-cache chunks. As a result, we cannot do reassignment to match the training pattern of TPEs. Here we introduce a cyclic shift mechanism, where the denoising target will be assigned those TPEs indexed from the beginning. To support the training/inference alignment of Cyclic-TPEs, in the training stage, each sample is assigned a TPE sequence that is cyclically shifted with a random offset. + +Spatial KV-Cache. Let $h _ { t } ^ { P _ { k } : P _ { k } + l }$ be the input to the prefixenhanced spatial attention at AR step $k$ . In the denoising stage, the keys and values from the denoising target are enhanced by the spatial KV-cache (a sub-prefix of $P ^ { \prime }$ frames) via spatial-wise concatenation. In the cache writing stage, the denoised latent frames are first enhanced via self-repeat and then computed to obtain the clean spatial keys and values. These operations are aligned with the prefix-enhancement in Eq. (4) of the training stage. Since $P ^ { \prime }$ is relatively small $( P ^ { \prime } < l )$ , the prefix enhancement for the current denoising target $h _ { t } ^ { P _ { k } : P _ { k } + l }$ only depends on spatial KV-cache from the most recent generated chunk (i.e., k−l:Pk ). Thus, in contrast to the queue structure for temporal KV-cache, we only store the spatial KV-cache for one chunk and overwrite it at every AR step. + +Discussion. It’s worth noting that our KV-cache queue for autoregressive VDMs is not a trivial extension of the KVcache techniques from large language models (LLMs): 1) LLMs predict the next token at each AR step, and the KVs are computed and cached simultaneously in each forward call. For VDMs, however, the model is repeatedly called during denoising (with different $t$ ). This brings the cache computation and storage issues as introduced in Sec. 1. Our implementation solves these two issues, sharing the cache across every denoising step. 2) Caching visual KVs costs much more storage than KVs for text since each token in our setting corresponds to $H W$ visual grids. The queue structure for KV-cache is essential for VDMs considering this heavy storage cost. Early KVs can be safely dequeued as the appearance and motion of new frames are primarily influenced by the most recent KVs. Meanwhile, we propose Cyclic-TPEs to facilitate this mechanism. + +# 4. Experiments + +# 4.1. Experimental Setup + +Model Details and Baselines. We built Ca2-VDM based on spatial-temporal Transformer following (Ma et al., 2025; Chen et al., 2024a) and initialized it with Open-Sora v1.0 (Zheng et al., 2024). Following PixArt- $\alpha$ (Chen et al., 2024a), we used T5 (Raffel et al., 2020) as the text encoder and used the VAE from StableDiffusion (Rombach et al., 2022). The length of the clean prefix was randomly sampled according to the multiples of chunk length l, i.e., $P \in \{ 1 , 1 + l , \ldots , 1 + n l \}$ and $P _ { \operatorname* { m a x } } = 1 + n l$ . We used training videos of various lengths with $L _ { \mathrm { t r a i n } } = P + l$ . As comparisons, we built two bidirectional baselines (cf. Figure 1(a)) based on the same Open-Sora v1.0: One was trained with fixed-length conditional frames (denoted as OS-Fix), where $P$ is fixed as $P = L _ { \mathrm { t r a i n } } / 2$ in training and inference. The other was trained with autoregressively extendable conditional frames using the same training configs as Ca2-VDM (denoted as OS-Ext). + +Training Details We conducted training on the text-tovideo (T2V) generation and video prediction (i.e., without text prompt) tasks. For T2V generation, we trained OS-Fix and Ca2-VDM on a large-scale video-text dataset InternVid (Wang et al., 2024), by filtering it to a sub-set of 4.9M high-quality video-text pairs. The models were trained video clips at resolution $2 5 6 \times 2 5 6$ with $l { = } 1 6$ and $P _ { \mathrm { m a x } } = 1 + 3 l = 4 9$ . For video prediction, we trained OS-Fix, OS-Ext, and Ca2-VDM on the SkyTimelapse (Zhang et al., 2020) dataset at resolution $2 5 6 \times 2 5 6$ with $l { = } 8$ . OS-Ext and Ca2-VDM both used $P _ { \mathrm { m a x } } = 1 + 3 l = 2 5$ . OS-Fix used a fixed $P = 8$ . More hyperparameters are left in Sec. C. + +Evaluation Datasets and Metrics. We used MSR-VTT ( $\mathrm { X u }$ et al., 2016), UCF101 (Soomro et al., 2012), and SkyTimelapse (Zhang et al., 2020) datasets at resolution $2 5 6 \times 2 5 6$ , and reported Frechet Video Distance (FVD) ( ´ Unterthiner et al., 2019) following previous works (Zeng et al., 2024; Ge et al., 2023; Chen et al., 2024b). More details about choosing text prompts and computing FVD scores on these datasets are left Sec. D + +# 4.2. Evaluation for Generation Quality + +We first compared the in-chunk generation quality of Ca2- VDM with SOTA VDMs. Then, we evaluated the temporal consistency of the autoregressive generation. Finally, we conducted ablation studies on $\mathbf { C a 2 }$ -VDM’s design choices. + +In-Chunk Generation Quality. We evaluated the zeroshot text-to-video (T2V) FVD scores on MSR-VTT (Xu et al., 2016) and UCF101 (Soomro et al., 2012), as shown in Table 1. We compared Ca2-VDM to state-of-the-art T2V models including two groups: 1) Text conditioned: ModelScope (Wang et al., 2023b), VideoComposer (Wang et al., + +Table 1: Zero-shot FVD scores on MSR-VTT (Xu et al., 2016) and UCF101 (Soomro et al., 2012) test sets. All methods generate video at a resolution of $1 6 \times 2 5 6 \times 2 5 6$ . C: condition. T and I are text and image conditions, respectively. + +
MethodCMSR-VTTUCF101
ModelScope (Wang et al., 2023b)T550410
VideoComposer (Wang et al., 2023c)T580-
Video-LDM (Blattmann et al., 2023b)T-550.6
PYoCo (Ge et al., 2023)T-355.2
Make-A-Video (Singer et al., 2023)T-367.2
AnimateAnything (Dai et al., 2023)T+I443
PixelDance (Zeng et al., 2024)T+I381242.8
SEINE (Chen et al., 2024b)T+I181-
Ca2-VDMT+I181277.7
+ +Table 2: Finetuned FVD scores on UCF-101 (Soomro et al., 2012) test set. Methods with ∗ were trained on both train and test sets. + +
MethodRes.FVD
MCVD (Voleti et al., 2022) VDT (Lu et al., 2024)642 6421143
DIGAN* (Yu et al., 2022)1282225.7 577
TATS (Ge et al., 2022)1282420
VideoFusion (Luo et al., 2023)1282
LVDM* (He et al., 2022)220
PVDM (Yu et al., 2023)2562 2562372
Latte (Ma et al., 2025)343.6
2562333.6
Ca2-VDM2562184.5
+ +Table 3: FVD results on MSR-VTT test set. + +
MethodFVD between AR step 1 and i
i = 2i = 3i = 4i = 5i = 6
GenLV282.8291.4299.0318.2310.3
StreamT2V317.5434.7478.2462.0512.4
OS-Fix182.9210.6260.8284.3315.1
Ca2-VDM160.6206.5262.8281.3304.7
+ +2023c), Video-LDM (Blattmann et al., 2023b), PYoCO (Ge et al., 2023), and Make-A-Video (Singer et al., 2023). 2) Text with extra image conditioned, e.g., for image-to-video: AnimateAnything (Dai et al., 2023), PixelDance (Zeng et al., 2024) and video transition: SEINE (Chen et al., 2024b). We also finetuned Ca2-VDM on UCF101 at resolution $1 6 \times 2 5 6 \times 2 5 6$ and reported the FVD scores in Table 2. We compared it with SOTA video generation models: MCVD (Voleti et al., 2022), VDT (Lu et al., 2024), DI-GAN (Yu et al., 2022), TATS (Ge et al., 2022), LVDM (He et al., 2022), PVDM (Yu et al., 2023), and Latte (Ma et al., 2025). The FVD results in both Table 1 and Table 2 show that our $\mathbf { C a } 2$ -VDM has a competitive T2V performance with SOTA models. More qualitative examples are left in Sec. E. + +Temporal Consistency. We compared Ca2-VDM with the two baselines (i.e., OS-Fix and OS-Ext) and existing SOTA autoregressive VDMs. To the best of our knowledge, existing autoregressive VDMs all use fixed-length conditional frames (similar to OS-Fix). We used Gen-L-Video (GenLV) (Wang et al., 2023a) and StreamT2V (Henschel et al., 2025). Specifically, GenLV utilizes a base model AnimateDiff (Guo et al., 2024b) and conducts co-denoising for overlapped 16-frame clips. We implemented it with an overlapping length (i.e., the condition length) of 8 frames. StreamT2V is based on Stable Video Diffusion (Blattmann et al., 2023a) and finetunes it conditioned on preceding frames to generate subsequent frames. It also generates 16 frames at each AR step, with 8 frames as the condition. + +We evaluated the FVD scores of each autoregression (AR) chunk w.r.t. the first chunk, as shown in Table 3. We can observe that Ca2-VDM has relatively lower FVD scores than the others. This indicates that extendable (long-term) condition helps to improve the temporal consistency. We also show qualitative examples in Figures 5. It shows content mutations in consecutive frames from the results of fixedlength condition methods, e.g., the $2 4 ^ { t h }$ and $2 5 ^ { t h }$ frames in GenLV, and the $6 5 ^ { t h }$ and $6 6 ^ { t h }$ frames in StreamT2V. We further compared Ca2-VDM with the condition extendable baseline, i.e., OS-Ext (cf. Figure 7). We see that Ca2-VDM shows comparable results with OS-Ext (while being more computationally efficient as demonstrated in Sec. 4.3). We conducted further comparisons between Ca2-VDM and OS-Ext in terms of video quality and long-term content drift. The results are left in Sec. E of the Appendix. + +Table 4: Ablations of $P _ { \mathrm { m a x } }$ and prefix-enhancement (PE) on SkyTimelapse (Zhang et al., 2020). Each variant of $\mathbf { C a } 2 \mathbf { - }$ VDM generated 48 frames by 6 AR steps. The results were divided into three 16-frame chunks for FVD evaluation. + +
PmaxPEChunk Id 23
25×1 274.8244.5275.1
25257.4216.5238.5
41×187.3209.3263.2
41185.0202.9240.5
+ +Ablation Studies. We studied the effectiveness of longer condition length and the prefix-enhancement (PE) in spatial attention (cf. Eq. (4)). We trained variants of Ca2-VDM with different $P _ { \mathrm { m a x } }$ or without PE. The results are reported in Table 4. Each model was called with 6 AR steps to generate a 49-frame video (with the given first frame) and evaluated by the FVD scores of three 16-frame chunks (exclude the first frame) w.r.t. the 16-frame ground-truth videos. We can see that both increasing $P _ { \mathrm { m a x } }$ and using PE are beneficial in improving the generation quality. + +# 4.3. Evaluation for Autoregression Efficiency + +We evaluated the efficiency in two aspects: 1) time cost for autoregressive generation, and 2) detailed computational costs for each component in the Transformer blocks. + +![](images/figures/ca2-vdm-fig-0005.jpg) + +![](images/figures/ca2-vdm-fig-0006.jpg) +Figure 6: Accumulated time cost w.r.t. frame ids. We show OS-Ext and Ca2-VDM with $P _ { \mathrm { m a x } } = 2 5$ and 41, and OS-Fix with a fixed $P = 8$ . + +![](images/figures/ca2-vdm-fig-0007.jpg) +Figure 5: Qualitative examples from GenLV (Wang et al., 2023a), StreamT2V (Henschel et al., 2025), OS-Fix (Zheng et al., 2024), and Ca2-VDM. Yellow arrows highlight consecutive frames having mutations. + +Table 5: Time cost for generating 80 frames at resolution $2 5 6 \times 2 5 6$ . OS-Fix used $P { = } 8$ . OS-Ext and Ca2-VDM used $P _ { \mathrm { m a x } } { = } 2 5$ . Ext.C. means extendable condition. + +
MethodExt.C.Time (s)
StreamT2V150
OS-Ext130.1
OS-Fix77.5
Ca2-VDM52.1
+ +![](images/figures/ca2-vdm-fig-0008.jpg) +Figure 7: Results from OS-Ext and Ca2-VDM. They have comparable quality, while Ca2-VDM is more computationally efficient, as evidenced in Table 5, Figure 6 and 8. +Figure 8: Number of floating-point operations (FLOPs) for generating 56 frames (7 AR steps). All results were computed by conducting only one denoising step for simplicity. + +Time Cost. We first show the cumulative time cost of autoregressive generation in Table 5. Our models were tested on a single NVIDIA A100 GPU to generate 80 frames at resolution $2 5 6 \times 2 5 6$ , using improved DDPM (Nichol & Dhariwal, 2021) with 100 denoising steps. The result of StreamT2V (Henschel et al., 2025) is from its GitHub page, which was tested on the same device and resolution. We can see that Ca2-VDM significantly improved over OS-Fix, OS-Ext, and StreamT2V (Henschel et al., 2025), while being compatible with extendable condition. We further evaluated the accumulated time cost till each AR step, as shown in Figure 6. We can observe that: 1) Compared to OS-Fix, the time cost in Ca2-VDM has a clear reduction since it does not have redundant computations. 2) As the condition extends, the time cost of OS-Ext grows quadratically (before $P _ { \mathrm { m a x } }$ is reached), while the time cost of Ca2-VDM only grows linearly. 3) As the $P _ { \mathrm { m a x } }$ grows to incorporate longer condition, the increase of time cost for OS-Ext is significant, while it is relatively slight for Ca2-VDM. + +Computational Cost. We counted the floating-point operations (FLOPs) of temporal, spatial, and visual-text attention layers in the Transformer blocks (cf. Figure 8). As the $P _ { \mathrm { m a x } }$ grows, the increased computations are seen in all three types of attention layers for OS-Ext. In contrast, for Ca2-VDM, the number of FLOPs only slightly increases in the temporal attention, while keeping constant in other operations. This is because the extended conditional frames only participate in the computation as temporal KV-caches. + +Memory Cost. We conducted empirical GPU memory statistics, as shown in Table 6. We compared Ca2-VDM with a concurrent work, Live2diff (Xing et al., 2024). It stores KV-cache for every denoising step (with different noise levels $t$ and thus different KV features), which costs much more GPU memory than ours. Note that Live2diff uses a batch size that is equal to the number of denoising steps, i.e., $B = T$ . This is because it uses pipeline denoising following StreamDiffusion (Kodaira et al., 2023), which puts frames with progressive noisy levels into a batch and generates one frame each autoregression step. Benefited from cache sharing, Ca2-VDM’s memory cost is independent of denoising steps, as its fixed shape $( 1 , 2 5 , h w , C )$ ensures constant memory usage. In contrast, Live2diff’s memory cost scales with $T$ (e.g., from $1 . 4 2 \mathrm { G B }$ at $T = 4$ to $1 7 . 7 0 \mathrm { G B }$ at $T = 5 0$ ), confirming that cache sharing saves $T \times \mathrm { G P U }$ memory. As a result, Ca2-VDM requires only 0.86 GB (w/ PE) or 0.77 GB (w/o PE), with the difference due to spatial KV-cache for prefix-enhancement (PE). + +Table 6: GPU memory usage comparison between Live2diff (Xing et al., 2024) and $\mathbf { C a } 2$ -VDM. The comparisons are not strictly aligned since Live2diff is Unet-based. The resolution of the generated video is $2 5 6 \times 2 5 6$ . $L$ is the number of generated frames at each auto-regression step. $H$ and $W$ are after $8 \times$ VAE down sampling. The values of $h ^ { \prime } w ^ { \prime }$ and $C ^ { \prime }$ vary across blocks due to the down-sampling and up-sampling in Unet. PE means prefix-enhancement $c f .$ Eq.(4)). + +
MethodDenoising Steps (T )Model Forward Shape (B, C, L, H, W)KV-cache Shape (T , Lcond, hw, C′)KV-cache Memory CostTotal Memory Cost
Live2diff4(4 , 4, 1, 32, 32)(4, 16, h′w′, C′)1.42 GB10.90 GB
Live2diff50(50, 4, 1, 32, 32)(50, 16, h'′w′, C′)17.70 GB29.46 GB
Ca2-VDM w/PE50(1 , 4, 8, 32, 32)(1, 25, hw, C)0.86 GB4.79 GB
Ca2-VDM w/o PE50(1 , 4, 8, 32, 32)(1, 25, hw, C)0.77 GB3.95 GB
+ +# 5. Conclusions + +In this paper, we present an efficient autoregressive video diffusion model, i.e., Ca2-VDM. It has two key designs: causal generation and cache sharing. The former eliminates the redundant computations of conditional frames. The latter significantly reduces the storage cost. Our model shows comparable generation quality with existing SOTA VDMs with existing bidirectional attention while achieving notable speedup for the autoregressive generation. + +# Acknowledgements + +This work was supported by the National Key Research & Development Project of China (2024YFB3312900), Key R&D Program of Zhejiang (2025C01128), an Fundamental Research Funds for the Central Universities. Long Chen was supported by the Hong Kong SAR RGC Early Career Scheme (26208924), the National Natural Science Foundation of China Young Scholar Fund (62402408), Huawei Gift Fund, and the HKUST Sports Science and Technology Research Grant (SSTRG24EG04). Kaifeng Gao was supported by the 2024-2025 Grant for Pursuing Outstanding Doctoral Dissertations of Zhejiang University. + +# Impact Statement + +Our $\mathbf { C a 2 }$ -VDM is a generic fast video generation paradigm. It is potentially powerful to boost existing VDMs to generate high-quality live-stream videos. The live-stream (or real-time) video generation techniques have a revolutionary impact on the field of content creation industry, and have great potential commercial values. Meanwhile, it’s necessary to note that Ca2-VDM also has the inherent risks of common image/video generation models, such as generating videos with harmful or offensive content, or being used by malicious actors for generating fake news. We can use some watermarking technologies (e.g., (Lukas & Kerschbaum, 2023)) to avoid the generated videos being abused. + +# References + +Alonso, E., Jelley, A., Micheli, V., Kanervisto, A., Storkey, A. J., Pearce, T., and Fleuret, F. 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Modelscope text-to-video technical report. arXiv preprint arXiv:2308.06571, 2023b. + +Wang, X., Yuan, H., Zhang, S., Chen, D., Wang, J., Zhang, Y., Shen, Y., Zhao, D., and Zhou, J. Videocomposer: Compositional video synthesis with motion controllability. NeurIPS, 36, 2023c. + +Wang, Y., He, Y., Li, Y., Li, K., Yu, J., Ma, X., Li, X., Chen, G., Chen, X., Wang, Y., et al. Internvid: A largescale video-text dataset for multimodal understanding and generation. In ICLR, 2024. + +Weng, W., Feng, R., Wang, Y., Dai, Q., Wang, C., Yin, D., Zhao, Z., Qiu, K., Bao, J., Yuan, Y., et al. Art-v: Autoregressive text-to-video generation with diffusion models. In CVPR, pp. 7395–7405, 2024. + +Xing, Z., Fox, G., Zeng, Y., Pan, X., Elgharib, M., Theobalt, C., and Chen, K. Live2diff: Live stream translation via uni-directional attention in video diffusion models. arXiv preprint arxiv:2407.08701, 2024. + +Xu, J., Mei, T., Yao, T., and Rui, Y. 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I2vgen-xl: High-quality image-to-video synthesis via cascaded diffusion models. arXiv preprint arXiv:2311.04145, 2023b. +Zhang, Y., Wei, Y., Jiang, D., ZHANG, X., Zuo, W., and Tian, Q. Controlvideo: Training-free controllable text-tovideo generation. In ICLR, 2024. +Zheng, Z., Peng, X., Yang, T., Shen, C., Li, S., Liu, H., Zhou, Y., Li, T., and You, Y. Open-sora: Democratizing efficient video production for all, March 2024. URL https: //github.com/hpcaitech/Open-Sora. + +# Appendix + +• Sec. A: Illustration of Prefix-enhanced Spatial Attention +• Sec. B: Detailed Training Objectives +• Sec. C: Training Details and Hyperparameters +• Sec. D: Evaluation Details +• Sec. E: More Experiment Results +• Sec. F: Limitations and Possible Future Directions + +# A. Illustration of Prefix-enhanced Spatial Attention + +We provide more details of Prefix-enhanced Spatial Attention (cf. Eq. (4)) in Figure 9. + +# B. Detailed Training Objectives + +Recall that (cf. Sec. 3.2 in the main text) existing diffusion models (Ho et al., 2020; Nichol & Dhariwal, 2021; Peebles & Xie, 2023) are trained with the variational lower bound of $z _ { \mathrm { 0 } }$ ’s log-likelihood, formulated as + +$$ +\begin{array} { l } { \displaystyle \mathcal { L } _ { \mathrm { v l b } } ( \theta ) = - \log p _ { \theta } ( z _ { 0 } | z _ { 1 } ) } \\ { \displaystyle \qquad + \sum _ { t } D _ { K L } \bigl ( q ( z _ { t - 1 } | z _ { t } , z _ { 0 } ) \| p _ { \theta } ( z _ { t - 1 } | z _ { t } ) \bigr ) . } \end{array} +$$ + +Since $q$ and $p _ { \theta }$ are both Gaussian, $D _ { K L }$ is determined by the mean $\pmb { \mu } _ { \theta }$ and covariance $\Sigma _ { \theta }$ . By re-parameterizing $\pmb { \mu } _ { \theta }$ as a noise prediction network $\epsilon _ { \theta }$ and fixing $\Sigma _ { \theta }$ as a constant variance schedule (Ho et al., 2020), the model can be trained using a simplified objective function: + +$$ +\begin{array} { r } { \mathcal { L } _ { \mathrm { s i m p l e } } ( \theta ) = \underset { z , \epsilon , t } { \mathbb { E } } \left[ \Vert \epsilon _ { \theta } ( z _ { t } , t ) - \epsilon \Vert _ { 2 } ^ { 2 } \right] , \epsilon \sim \mathcal { N } ( 0 , 1 ) . } \end{array} +$$ + +In our setting, the simplified objective function is + +$$ +\widetilde { \mathcal { L } } _ { \mathrm { s i m p l e } } ( \theta ) = \underset { z , \epsilon , t } { \mathbb { E } } \bigl [ \| \bigl ( \epsilon _ { \theta } \bigl ( [ z _ { 0 } ^ { 0 : P } , z _ { t } ^ { P : L } ] , t \bigr ) - \epsilon \bigr ) \odot m \| _ { 2 } ^ { 2 } \bigr ] . +$$ + +Following prior works (Nichol & Dhariwal, 2021; Peebles & Xie, 2023), we train the model with learnable covariance $\Sigma _ { \theta }$ to improve the sampling quality. This is achieved by optimizing the full $D _ { K L }$ term in ${ \mathcal { L } } _ { \mathrm { v l b } }$ , resulting in an $\widetilde { \mathcal { L } } _ { \mathrm { v l b } }$ in our setting, i.e., applied with the same timestep vector $\pmb { t }$ and loss mask $_ { \mathbf { \nabla } } \mathbf { m } _ { \mathbf { \nabla } }$ . Then, the model is optimized by a combined loss function $\widetilde { \mathcal { L } } _ { \mathrm { s i m p l e } } + \widetilde { \mathcal { L } } _ { \mathrm { v l b } }$ . + +# C. Training Details and Hyperparameters + +Text-to-Video (T2V) Training. We trained Ca2-VDM and the OS-Fix baseline on a large-scale video-text dataset InternVid (Wang et al., 2024), by filtering it to a sub-set of + +![](images/figures/ca2-vdm-fig-0009.jpg) +Figure 9: Illustration of prefix-enhanced spatial attention. For $i \geq P$ , the left part of $K , V$ is from clean prefix (in training) or cached $\kappa , V$ (in the denoising stage of inference). + +4.9M high-quality video-text pairs with resolution $2 5 6 \times 2 5 6$ For Ca2-VDM, the training consists of two stages. We first train the causal modeling ability without the clean prefix (i.e., without conditional frames) on 32-frame videos. Then we use longer videos of 65 frames to train the model with the clean prefix, i.e., with $l = 1 6$ , $P _ { \mathrm { m a x } } = 1 + 3 l = 4 9$ and $\operatorname* { m a x } ( L _ { \mathrm { t r a i n } } ) = P _ { \mathrm { m a x } } + l = 6 5 $ . In the first stage, the model was trained with a batch size of 288 for 32k steps. In the second stage, it was trained with a batch size of 144 for 21k steps. For OS-Fix, it was trained with $L _ { \mathrm { t r a i n } } = 3 2$ frames and $P = l = L _ { \mathrm { t r a i n } } / 2 = 1 6$ frames, i.e., the prefix length is fixed. It was trained with a batch size of 288 for $2 0 \mathrm { k }$ steps 3. + +Video Prediction Training. We trained OS-Fix, OS-Ext, and Ca2-VDM on the SkyTimelapse (Zhang et al., 2020) dataset at resolution $2 5 6 \times 2 5 6$ with $l = 8$ . OS-Ext and Ca2-VDM both used $P _ { \mathrm { m a x } } = 1 + 3 l = 2 5$ (i.e., $L _ { \mathrm { t r a i n } } = 3 3 $ ). OS-Fix used a fixed $P = 8$ and $L _ { \mathrm { t r a i n } } = 1 6$ . All three models were trained with a batch size of 8 for 11k steps 4. + +Hyperparameters. For all the training, we used the DDPM (Ho et al., 2020) schedule with $T = 1 0 0 0$ , $\beta _ { 1 } =$ $1 0 ^ { - 4 }$ , and $\beta _ { T } ~ = ~ 0 . 0 2$ . The models were trained using AdamW (Loshchilov & Hutter, 2019) optimizer with a learning rate of 2e-5. At the inference stage, we used the improved DDPM schedule (Nichol & Dhariwal, 2021) with 100 steps. For text-to-video, we set the classifier-free guidance scale as 7.5. + +# D. Evaluation Details + +# D.1. Datasets + +MSR-VTT (Xu et al., 2016). we used its official test split which contains 2990 videos, with 20 manually annotated captions for each video. Following prior works (Ren et al., 2024; Zeng et al., 2024) and for fair comparisons, we randomly selected a caption for each video and generated 2990 videos for evaluation. + +![](images/figures/ca2-vdm-fig-0010.jpg) +Figure 10: Qualitative examples generated by GenLV (Wang et al., 2023a), StreamT2V (Henschel et al., 2025), OS-Fix, and our Ca2-VDM. We sampled 32 frames with an interval of 8 frames for display. Note that GenLV does not strictly follow the given first frame, since it was not finetuned on explicitly injected conditional frames. In the implementation of GenLV, we used DDIM inversion to build the initial noise based on the first frame. + +UCF101 (Soomro et al., 2012). As it only contains label names, we employed the descriptive text prompts from PYoCo (Ge et al., 2023), and generated 2048 samples with uniform distribution for each category following (He et al., 2022; Ge et al., 2023; Ren et al., 2024). + +SkyTimelapse (Zhang et al., 2020). It is a time-lapse dataset showing dynamic sky scenes (e.g., cloudy sky with moving clouds). We used it for video prediction (i.e., without text input). Its training set contains 997 long timelapse videos, which are cut into 2392 short videos. Its test set contains 111 long timelapse videos, which are cut into 225 short videos. We trained the models on its training set and evaluated them on its test set. + +# D.2. Quantitative Evaluation + +Frechet Video Distance (FVD) ( ´ Unterthiner et al., 2019) measures the similarity between generated and real videos based on the distributions on the feature space. We followed prior works (Blattmann et al., 2023b; Ge et al., 2022; Ren et al., 2024) to use a pretained I3D model5 to extract the features. We used the codebase6 from StyleGAN-V (Skorokhodov et al., 2021) to compute FVD statistics. + +For the autoregressive generation results (e.g., the results in Table 3 and Table 4), we calculated the chunk-wise FVD. Specifically, for Table 3, each model generated 48 frames with 6 AR steps and $l = 8$ . Since the I3D model accepts at least 16 frames, we evaluated the FVD scores of three 16- frame chunks (i.e., 2 AR steps in each) w.r.t. the 16-frame ground-truth videos. For Table 4, each model generated 96 frames with 6 AR steps and $l = 1 6$ . We evaluated the FVD scores of the generated 16-frame chunk from each AR step w.r.t. the first AR step. Each model generated 512 videos for FVD calculation. + +# E. More Experiment Results + +In Figure 10 and Figure 11, we show more qualitative examples from GenLV (Wang et al., 2023a), StreamT2V (Henschel et al., 2025), OS-Fix (Zheng et al., 2024), and Ca2- VDM. We can see that Ca2-VDM has comparable generation quality to existing SOTA models. + +In Table 7, we evaluated Ca2-VDM and OS-Ext on the VBench (Huang et al., 2024) benchmark. VBench is primarily designed for text-to-video evaluation. For our assessment, we selected four metrics: aesthetic quality, imaging quality, motion smoothness, and temporal flickering. The first two measure spatial (appearance) quality, and the last two assess temporal consistency. The results in Table 7 show that Ca2-VDM achieves comparable performance in both appearance quality and temporal consistency. + +In Figure 12, we further compared the long-term content drift (i.e., error accumulation) between Ca2-VDM and the + +![](images/figures/ca2-vdm-fig-0011.jpg) +Figure 11: Qualitative examples from GenLV (Wang et al., 2023a), StreamT2V (Henschel et al., 2025), OS-Fix, and our Ca2-VDM. Yellow arrows highlight the consecutive frames having mutations. + +Table 7: VBench (Huang et al., 2024) evaluation on Sky-Timelapse (Zhang et al., 2020) test set. The resolution of the generated video is $2 5 6 \times 2 5 6$ . Both models were evaluated with $P _ { \mathrm { m a x } } = 2 5$ and 6 autoregression steps. + +
MethodAesthetic QualityImaging QualityMotion SmoothnessTemporal Flickering
OS-Ext44.3950.7498.9398.57
Ca2-VDM44.3050.5597.5997.14
+ +OS-Ext baseline. As a result, they show comparable visual quality. Both models exhibit a similar degree of error accumulation over time. Given our primary focus on efficiency, we conclude that Ca2-VDM matches the bidirectional baseline while being more efficient in both computation and storage for autoregressive video generation. + +# F. Limitations and Possible Future Directions + +We analyze the limitations of the current work and propose some possible directions for future work. + +Causal Modeling in Pretraining. Currently, all the pretrained weights for video diffusion models (either UNetbased, e.g., ModelScore-T2V (Wang et al., 2023b), AnimateDiff (Guo et al., 2024b), or Transformer-based, e.g., Open-Sora (Zheng et al., 2024)) use bidirectional attention in their temporal modules. Our Ca2-VDM is built upon Open-Sora which was also pretrained using bidirectional attention. However, finetuning these bidirectionally pretrained temporal modules using causal attention might be sub-optimal. The weights between bidirectional and causal temporal attention layers might have inherent gaps. Due to the limited computational resources, we did not conduct causal pretraining. Pretraining the VDM’s temporal modules from scratch (using causal attention) might have potential improvements. + +Training Efficiency Trade-off. Ca2-VDM uses extendable conditional frames and cyclic TPEs. These designs require the model to learn all the possible situations during training. Compared to fixed-length conditional frames and conventional TPEs, the model needs more time to achieve training convergence. Meanwhile, the longer maximum condition length (i.e., $P _ { \mathrm { m a x , } }$ ) we use, the more training is required. On the other hand, once the model is trained, it is more powerful for integrating long-term context. Consequently, it’s also potentially beneficial for long-term autoregressive video generation. + +Quality Degradation in Long-term Generation. As a common challenge, VDMs in long-term autoregressive generation suffer from frame appearance changes and quality degradation. Some works (Henschel et al., 2025; Zhang et al., 2023b) mitigate this issue by providing the VDM with the global appearance information extracted from the initial frame. However, during the long-term generation, video content may change and not all frames commit the same global appearance. In our setting, the long-term extendable context (i.e., early context from the KV-cache queue) helps mitigate the quality degradation, demonstrated by the results in Table 3 and Table 4. Further research on approaches addressing quality degradation is warranted and may hold potential significance for long-term video generation. + +![](images/figures/ca2-vdm-fig-0012.jpg) +Figure 12: Comparison between OS-Ext and Ca2-VDM in terms of long-term content drift (i.e., long-term quality degradation). Both models were trained on Sky-Timelapse (Zhang et al., 2020). 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batch=8, 11k steps (VP)", + "source": "Appendix C / baseline OS-Fix training config" + }, + { + "id": "ca2-vdm-D1-015", + "claim": "T2V training dataset: InternVid 4.9M filtered video-text pairs", + "source": "Section 4.1, Appendix C / T2V training dataset" + }, + { + "id": "ca2-vdm-D1-016", + "claim": "MSR-VTT evaluation dataset: MSR-VTT test: 2990 videos, 20 captions each", + "source": "Appendix D.1 / MSR-VTT evaluation dataset" + }, + { + "id": "ca2-vdm-D1-017", + "claim": "UCF101 evaluation dataset: UCF101: 2048 samples, uniform category distribution", + "source": "Appendix D.1 / UCF101 evaluation dataset" + }, + { + "id": "ca2-vdm-D1-018", + "claim": "SkyTimelapse dataset statistics: SkyTimelapse: 2392 train clips / 225 test clips", + "source": "Appendix D.1 / SkyTimelapse dataset statistics" + }, + { + "id": "ca2-vdm-D1-019", + "claim": "model initialization and encoders: T5 text encoder, Open-Sora v1.0 initialization, VAE from StableDiffusion", + "source": "Section 4.1 / model initialization and encoders" + }, + { + "id": "ca2-vdm-D1-020", + "claim": "KV-cache and model forward tensor shapes: KV-cache shape (1, 25, hw, C), model forward shape (1, 4, 8, 32, 32)", + "source": "Table 6 / KV-cache and model forward tensor shapes" + }, + { + "id": "ca2-vdm-D1-021", + "claim": "evaluation protocol config: AR steps: 6-7, total frames 80, FVD chunk_size=16, FVD sample_count=512", + "source": "Section 4.2, Section 4.3, Appendix D.2 / evaluation protocol config" + }, + { + "id": "ca2-vdm-D1-022", + "claim": "baseline autoregressive VDMs config: GenLV overlap=8, StreamT2V chunk=16", + "source": "Section 4.2 / baseline autoregressive VDMs config" + } + ], + "D2": [ + { + "id": "ca2-vdm-D2-001", + "claim": "Partial Noising Training with Distinct Timestep Embeddings: L̃_simple(θ) = E_{z,ε,t} [‖(ε_θ([z_0^{0:P}, z_t^{P:L}], t) − ε) ⊙ m‖²₂]\n\nwhere:\n- z_0^{0:P}: P consecutive clean prefix frames (uncorrupted)\n- z_t^{P:L}: remaining L−P frames as denoising target (noised to timestep t)\n- t: timestep vector with t_i = t if i ≥ P else 0 (tEmb(0) for clean prefix, tEmb(t) for denoising target)\n- m ∈ {0,1}^N: loss mask to exclude the clean prefix part (m_i = 1 if i ≥ P else 0)\n\nReference: Eq. (2) in Section 3.2; clean prefix always assigned tEmb(0), enabling cache sharing across denoising timesteps.", + "source": "Section 3.2 Eq.(2) / Partial Noising Training with Distinct Timestep Embeddings" + }, + { + "id": "ca2-vdm-D2-002", + "claim": "Combined Training Objective with Learnable Covariance (L̃_simple + L̃_vlb): L_total(θ) = L̃_simple(θ) + L̃_vlb(θ)\n\nwhere:\n- L̃_vlb(θ) = −log p_θ(z_0|z_1) + Σ_t D_KL(q(z_{t-1}|z_t,z_0) ‖ p_θ(z_{t-1}|z_t))\n- Both L̃_simple and L̃_vlb use the same timestep vector t and loss mask m\n- Σ_θ is learnable covariance (not fixed variance schedule)\n- D_KL term determined by mean μ_θ and covariance Σ_θ\n- Training optimizes combined loss L̃_simple + L̃_vlb following (Nichol & Dhariwal, 2021; Peebles & Xie, 2023)\n\nReference: Appendix B; learnable covariance improves sampling quality.", + "source": "Section 3.2, Appendix B / Combined Training Objective with Learnable Covariance" + }, + { + "id": "ca2-vdm-D2-003", + "claim": "Causal Temporal Attention: CausalAttn(Q, K, V) = Softmax(QK^T / √C' + M) V\n\nwhere:\n- Input first permuted: spatial H×W treated as batch dimension\n- Q, K, V ∈ R^{L×C'}: linearly projected from permuted input (per spatial grid)\n- M ∈ R^{L×L}: lower triangular attention mask with M_{i,j} = −∞ if i < j else 0\n- Each frame attends only to its preceding frames (unidirectional dependency)\n\nReference: Eq. (3) in Section 3.2.", + "source": "Section 3.2 Eq.(3) / Causal Temporal Attention" + }, + { + "id": "ca2-vdm-D2-004", + "claim": "Prefix-Enhanced Spatial Attention: For hidden input h_t^{0:L} ∈ R^{L×H×W×C} (L as batch dim, H×W flattened):\n\nQuery: Q̄(i) = W̄^Q h_t^i\n\nKey (prefix-enhanced):\nK̄(i) = { W^K[h_0^{P−P'}; ...; h_0^{P−1}; h_t^i] if i ≥ P\n { W^K[h_0^i; ...; h_0^i] if i < P (self-repeat P' times)\n\nSame operation for value V̄(i).\n\nAttention(Q̄, K̄, V̄) with attention map of shape (HW) × ((P'+1)HW). In practice, P' is small (e.g., P'=3) as computational cost scales proportionally with P'.\n\nReference: Eq. (4) in Section 3.2.", + "source": "Section 3.2 Eq.(4) / Prefix-Enhanced Spatial Attention" + }, + { + "id": "ca2-vdm-D2-005", + "claim": "Temporal KV-Cache with Cache Sharing Across Denoising Timesteps: At AR step k with P_k generated frames:\n\nDenoising stage:\n- Noisy Q_t^{P_k:P_k+l}, K_t^{P_k:P_k+l}, V_t^{P_k:P_k+l} ∈ R^{l×C'} (per spatial grid)\n- Read clean caches: K_0^{0:P_k}, V_0^{0:P_k} ∈ R^{P_k×C'}\n- Concatenate: K̃(k,t) = [K_0^{0:P_k}, K_t^{P_k:P_k+l}]\n Ṽ(k,t) = [V_0^{0:P_k}, V_t^{P_k:P_k+l}]\n- Compute: CausalAttn(Q_t^{P_k:P_k+l}, K̃(k,t), Ṽ(k,t))\n with attention map shape l × (P_k + l)\n\nClean KV-cache K_0^{0:P_k}, V_0^{0:P_k} is shared across all denoising timesteps t (cache sharing).\n\nReference: Eq. (5) in Section 3.3, Figure 4(b).", + "source": "Section 3.3 Eq.(5) / Temporal KV-Cache with Cache Sharing" + }, + { + "id": "ca2-vdm-D2-006", + "claim": "Cache Writing Stage: After denoising completes at AR step k:\n\n1. Denoised latent z_0^{P_k:P_k+l} is input to model again\n2. Compute clean temporal keys and values: K_0^{P_k:P_k+l}, V_0^{P_k:P_k+l}\n3. Update KV-cache queue:\n K_0^{0:P_{k+1}} = concat(K_0^{0:P_k}, K_0^{P_k:P_k+l})\n V_0^{0:P_{k+1}} = concat(V_0^{0:P_k}, V_0^{P_k:P_k+l})\n4. Updated cache used in AR step k+1 (P_{k+1} = P_k + l)\n5. When P_k reaches P_max, earliest chunk dequeued from queue\n\nReference: Section 3.3, Figure 4(c).", + "source": "Section 3.3 / Cache Writing Stage" + }, + { + "id": "ca2-vdm-D2-007", + "claim": "Cyclic Temporal Positional Embeddings (Cyclic-TPEs): Training:\n- L_train = P_max + l: maximum TPE sequence length during training\n- Each sample assigned TPE sequence cyclically shifted with random offset\n- P ∈ {1, 1+l, ..., 1+nl}, so the model learns all possible starting positions\n\nInference (KV-cache enabled, Figure 4c-right):\n- TPEs bound to keys and values at previous AR steps and stored in preceding KV-cache chunks, preventing reassignment from scratch\n- Denoising target assigned TPEs indexed from the beginning (cyclically shifted)\n- Cyclic shift offset matches the training pattern for training/inference alignment\n\nReference: Section 3.3, Figure 4(c).", + "source": "Section 3.3 / Cyclic Temporal Positional Embeddings" + }, + { + "id": "ca2-vdm-D2-008", + "claim": "Spatial KV-Cache for Prefix-Enhanced Attention: At AR step k:\n\nDenoising stage:\n- Keys/values from denoising target enhanced by spatial KV-cache (P' frames sub-prefix)\n- Spatial concatenation per Eq (4)\n\nCache writing stage:\n- Denoised latent frames first enhanced via self-repeat\n- Compute clean spatial keys and values\n- Store for one chunk only and overwrite at every AR step\n- Since P' < l, prefix enhancement only depends on the most recent generated chunk (P_k−l:P_k)\n\nReference: Section 3.3.", + "source": "Section 3.3 / Spatial KV-Cache for Prefix-Enhanced Attention" + }, + { + "id": "ca2-vdm-D2-009", + "claim": "Autoregressive Inference Loop with DDPM/DDIM Sampling: Start from given first frame(s). For each AR step k (k = 0, 1, 2, ...):\n\n1. Denoising Stage:\n - Clean prefix: z_0^{0:P_k} (P_k previously generated frames)\n - Denoising target: z_t^{P_k:P_k+l} (l-frame chunk, randomly initialized noise)\n - For t = T, T−1, ..., 1:\n Sample z_{t-1}^{P_k:P_k+l} ~ p_θ(z_{t-1}^{P_k:P_k+l} | z_t^{P_k:P_k+l}, z_0^{0:P_k})\n - Clean prefix KV-cache shared across all timesteps (cache sharing)\n\n2. Cache Writing Stage:\n - Input denoised z_0^{P_k:P_k+l} to model\n - Compute clean spatial and temporal KV-caches\n - Update KV-cache queue for next AR step (k+1)\n\nReference: Section 3.3, Figure 3(b).", + "source": "Section 3.3 / Autoregressive Inference Loop" + }, + { + "id": "ca2-vdm-D2-010", + "claim": "P_max Queue Management for Long-Term Context: Training configuration:\n- P randomly sampled from {1, 1+l, 1+2l, ..., 1+nl}\n- P_max = 1 + nl (maximum conditional frames)\n- L_train = P + l (total frames in training clip)\n- max(L_train) = P_max + l\n\nInference queue behavior:\n- At each AR step k: P_k = P_{k-1} + l (grows by chunk length)\n- When P_k reaches P_max, earliest KV-cache is dequeued to maintain fixed-length condition window\n- Cyclic-TPEs enable proper positional encoding after dequeue (cyclic shift mechanism)\n- Early KVs can be safely dequeued as new frames are primarily influenced by recent KVs\n\nReference: Section 3.3, Section 4.1.", + "source": "Section 3.3, Section 4.1 / P_max Queue Management" + } + ], + "D3": [ + { + "id": "ca2-vdm-D3-001", + "claim": "Zero-shot Text-to-Video FVD Evaluation (Table 1): Evaluate Ca2-VDM's in-chunk T2V generation quality against SOTA models on MSR-VTT and UCF101 test sets at resolution 16×256×256. MSR-VTT uses 2990 videos with 1 randomly selected caption per video from 20 available captions. UCF101 uses 2048 samples with uniform category distribution and PYoCo descriptive prompts. FVD computed via pretrained I3D feature extractor. Compared to two groups: text-conditioned (ModelScope, VideoComposer, Video-LDM, PYoCo, Make-A-Video) and text+image-conditioned (AnimateAnything, PixelDance, SEINE).", + "source": "Section 4.2, Table 1, Appendix D / Zero-shot T2V FVD Evaluation" + }, + { + "id": "ca2-vdm-D3-002", + "claim": "Finetuned UCF101 FVD Evaluation (Table 2): Finetune Ca2-VDM on UCF101 training set and evaluate FVD on UCF101 test set at resolution 16×256×256. Compare against SOTA video generation models (MCVD, VDT, DIGAN, TATS, VideoFusion, LVDM, PVDM, Latte). Methods marked with * trained on both train+test sets.", + "source": "Section 4.2, Table 2 / Finetuned UCF101 FVD Evaluation" + }, + { + "id": "ca2-vdm-D3-003", + "claim": "Autoregressive Temporal Consistency FVD Evaluation (Table 3): Evaluate multi-chunk temporal consistency by generating 48-frame videos with 6 AR steps (l=8 each) on MSR-VTT test set. Compute chunk-wise FVD between AR step 1 and steps 2-6. Compare Ca2-VDM against autoregressive VDMs: GenLV (overlap=8, 16-frame clips, based on AnimateDiff + DDIM inversion for first frame), StreamT2V (chunk=16, condition=8, based on SVD), OS-Fix (fixed P=16).", + "source": "Section 4.2, Table 3 / Autoregressive Temporal Consistency FVD Evaluation" + }, + { + "id": "ca2-vdm-D3-004", + "claim": "Ablation Study: P_max and Prefix-Enhancement (PE) Effectiveness (Table 4): Train Ca2-VDM variants with different P_max (25 vs 41) and with/without prefix-enhanced spatial attention on SkyTimelapse dataset for video prediction (no text). Each variant generates 49-frame videos in 6 AR steps (given first frame). Evaluate FVD of three 16-frame chunks w.r.t. 16-frame ground-truth videos.", + "source": "Section 4.2, Table 4 / Ablation Study P_max and PE" + }, + { + "id": "ca2-vdm-D3-005", + "claim": "Autoregressive Time Cost Measurement (Table 5, Figure 6): Measure accumulated wall-clock time for generating 80 frames at 256×256 on a single NVIDIA A100 GPU. Use improved DDPM with 100 denoising steps. Compare OS-Fix (P=8), OS-Ext (P_max=25), Ca2-VDM (P_max=25), and StreamT2V (results from GitHub, same device/resolution). Figure 6 shows cumulative time per AR step for P_max=25 and P_max=41 variants.", + "source": "Section 4.3, Table 5, Figure 6 / Autoregressive Time Cost Measurement" + }, + { + "id": "ca2-vdm-D3-006", + "claim": "FLOPs Computational Cost Analysis (Figure 8): Count floating-point operations for generating 56 frames (7 AR steps) with a single denoising step. Analyze FLOPs in temporal attention, spatial attention, and visual-text cross-attention layers separately. Compare Ca2-VDM vs OS-Ext across varying P_max values to demonstrate Ca2-VDM's computational efficiency from cache sharing.", + "source": "Section 4.3, Figure 8 / FLOPs Computational Cost Analysis" + }, + { + "id": "ca2-vdm-D3-007", + "claim": "GPU Memory Cost Comparison with Live2diff (Table 6): Compare GPU memory usage at T=50 denoising steps. Ca2-VDM benefits from cache sharing (KV-cache independent of T, fixed shape (1,25,hw,C)), while Live2diff stores per-timestep KV-cache (shape (T,16,h'w',C')). Report batch shape, KV-cache shape, KV-cache memory, and total GPU memory. Ca2-VDM evaluated with and without prefix-enhancement (PE).", + "source": "Section 4.3, Table 6 / GPU Memory Cost Comparison" + }, + { + "id": "ca2-vdm-D3-008", + "claim": "VBench Evaluation for Appearance and Temporal Quality (Table 7): Evaluate Ca2-VDM and OS-Ext on SkyTimelapse test set using VBench benchmark at 256×256 with P_max=25 and 6 AR steps. Assess four metrics: aesthetic quality and imaging quality (spatial/appearance), motion smoothness and temporal flickering (temporal consistency).", + "source": "Appendix E, Table 7 / VBench Evaluation" + } + ], + "D4": [ + { + "id": "ca2-vdm-D4-001", + "claim": "Experiment workflow for Zero-shot Text-to-Video FVD Evaluation (Table 1): Train Ca2-VDM on InternVid 4.9M filtered subset (stage 1: 32k steps, 32-frame causal-only; stage 2: 21k steps, 65-frame with clean prefix) → Generate 16-frame videos for each test sample using improved DDPM 100 steps, CFG=7.5 → Extract I3D features from generated and real videos → Compute FVD scores", + "source": "Section 4.1, Section 4.2, Table 1, Appendix D" + }, + { + "id": "ca2-vdm-D4-002", + "claim": "Experiment workflow for Finetuned UCF101 FVD Evaluation (Table 2): Finetune Ca2-VDM on UCF101 train set → Generate 16×256×256 videos on UCF101 test set → Compute FVD against real test videos", + "source": "Section 4.2, Table 2" + }, + { + "id": "ca2-vdm-D4-003", + "claim": "Experiment workflow for Autoregressive Temporal Consistency FVD Evaluation (Table 3): For each model, generate 48 frames via 6 AR steps (l=8) → Split into 3 chunks of 16 frames → Compute FVD of chunks 2,3 vs chunk 1 using I3D features → Report FVD at each AR step boundary", + "source": "Section 4.2, Table 3, Appendix D.2" + }, + { + "id": "ca2-vdm-D4-004", + "claim": "Experiment workflow for Ablation Study: Train 4 Ca2-VDM variants on SkyTimelapse train set (batch=8, 11k steps, video prediction) → Generate 49-frame videos (6 AR steps, given first frame) → Compute chunk-wise FVD for chunks 1, 2, 3 → Compare across variants", + "source": "Section 4.2, Table 4" + }, + { + "id": "ca2-vdm-D4-005", + "claim": "Experiment workflow for Autoregressive Time Cost Measurement (Table 5, Figure 6): Setup: single NVIDIA A100 GPU, improved DDPM 100 steps → Generate 80 frames with each method at 256×256 → Measure and accumulate wall-clock time at each AR step → Plot cumulative time curves for P_max=25 and P_max=41 variants", + "source": "Section 4.3, Table 5, Figure 6" + }, + { + "id": "ca2-vdm-D4-006", + "claim": "Experiment workflow for FLOPs Computational Cost Analysis (Figure 8): Fix generation config: 56 frames, 7 AR steps, 1 denoising step → For each P_max value, run model forward and count FLOPs per attention layer → Plot stacked FLOPs by layer type vs P_max → Compare Ca2-VDM (only temporal grows) vs OS-Ext (all layers grow)", + "source": "Section 4.3, Figure 8" + }, + { + "id": "ca2-vdm-D4-007", + "claim": "Experiment workflow for GPU Memory Cost Comparison with Live2diff (Table 6): Run Ca2-VDM w/ PE and w/o PE at T=50, measure GPU memory → Ca2-VDM: model forward (1,4,8,32,32), KV-cache (1,25,hw,C) → Compare KV-cache memory: Ca2-VDM 0.77-0.86GB vs Live2diff 17.70GB → Compare total memory: Ca2-VDM 3.95-4.79GB vs Live2diff 29.46GB", + "source": "Section 4.3, Table 6" + }, + { + "id": "ca2-vdm-D4-008", + "claim": "Experiment workflow for VBench Evaluation for Appearance and Temporal Quality (Table 7): Generate videos with Ca2-VDM and OS-Ext: P_max=25, 6 AR steps, 256×256 → Evaluate all four VBench metrics → Compare spatial quality and temporal consistency between methods", + "source": "Appendix E, Table 7" + } + ] +} \ No newline at end of file diff --git a/papers/conformal-bayesian-quadrature/blacklist.txt b/papers/conformal-bayesian-quadrature/blacklist.txt new file mode 100644 index 0000000000000000000000000000000000000000..ccafe3925e944c1a6bd596e51add27d7306615c8 --- /dev/null +++ b/papers/conformal-bayesian-quadrature/blacklist.txt @@ -0,0 +1,2 @@ +# Official repository (ICML 2025 Outstanding Paper) +https://github.com/jakesnell/conformal-as-bayes-quad diff --git a/papers/conformal-bayesian-quadrature/config.yaml b/papers/conformal-bayesian-quadrature/config.yaml new file mode 100644 index 0000000000000000000000000000000000000000..158c447a8dc1255e378901038a2a3188cc863e0d --- /dev/null +++ b/papers/conformal-bayesian-quadrature/config.yaml @@ -0,0 +1,8 @@ +title: "Conformal Prediction as Bayesian Quadrature" +pdf_url: "https://arxiv.org/pdf/2502.13228.pdf" +venue: "ICML 2025 Outstanding" +year: "2025" +extra: + selection_index: 23 + domain: "Probabilistic Inference / Generative Models" + paradigm: "New Algorithm / Architecture" diff --git a/papers/conformal-bayesian-quadrature/images/figures/conformal-bayesian-quadrature-fig-0001.jpg b/papers/conformal-bayesian-quadrature/images/figures/conformal-bayesian-quadrature-fig-0001.jpg new file mode 100644 index 0000000000000000000000000000000000000000..d992cc45ef29722ceba0426d97240db071c81ede --- /dev/null +++ b/papers/conformal-bayesian-quadrature/images/figures/conformal-bayesian-quadrature-fig-0001.jpg @@ -0,0 +1,3 @@ +version https://git-lfs.github.com/spec/v1 +oid sha256:5d1daf37dcce53ff0a1279a1e2dee03709ec664312da0e9ce06961f6466a7cf4 +size 40456 diff --git 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sha256:4e146d9cd3329fa20522ef1ada06d3dc5e6e79bf2ddd1d339c44e22415f548dc +size 17957 diff --git a/papers/conformal-bayesian-quadrature/paper.md b/papers/conformal-bayesian-quadrature/paper.md new file mode 100644 index 0000000000000000000000000000000000000000..7977988378f8c5406139ba9d32579970273d94fb --- /dev/null +++ b/papers/conformal-bayesian-quadrature/paper.md @@ -0,0 +1,748 @@ +# Conformal Prediction as Bayesian Quadrature + +Jake C. Snell 1 Thomas L. Griffiths 1 2 + +# Abstract + +As machine learning-based prediction systems are increasingly used in high-stakes situations, it is important to understand how such predictive models will perform upon deployment. Distributionfree uncertainty quantification techniques such as conformal prediction provide guarantees about the loss black-box models will incur even when the details of the models are hidden. However, such methods are based on frequentist probability, which unduly limits their applicability. We revisit the central aspects of conformal prediction from a Bayesian perspective and thereby illuminate the shortcomings of frequentist guarantees. We propose a practical alternative based on Bayesian quadrature that provides interpretable guarantees and offers a richer representation of the likely range of losses to be observed at test time. + +which aim to provide guarantees for model performance in a distribution-free way. However, these techniques are based on ideas from frequentist statistics, making it difficult to incorporate prior knowledge that might be available about specific models. For example, in a particular setting we might have access to some information about the distribution of the data that is likely to be encountered, and can construct tighter guarantees on the performance of models by making use of this information. Moreover, they focus on controlling the expected loss averaged over many unobserved datasets rather than focusing on the actual set of observations. + +# 1. Introduction + +Machine learning systems based on deep learning are increasingly used in high-stakes settings, such as medical diagnosis or financial applications. These settings impose unique constraints on the performance of these systems: we want them to produce good outcomes in the aggregate, but also do so fairly and with a guarantee of a low probability of harm. However, predictive models based on deep learning can be difficult to interpret, and commercial models increasingly tend to offer little information about the techniques used in training. This creates a new challenge: How can we flexibly and reliably quantify the suitability of a model for deployment without making too many assumptions about how the model was trained or in which settings it will be used? + +Recent research on quantifying uncertainty has employed methods based on conformal prediction (Vovk et al., 2005), + +In this paper, we show how methods for guaranteeing model performance can be understood and extended by viewing them from a Bayesian perspective. We develop a framework in which we explicitly model uncertainty in the quantile values associated with particular observations, providing a nonparametric tool for characterizing possible distributions where the model might be deployed that is appropriately constrained by observed data. This framework allows us to draw upon methods from the fields of statistical prediction analysis (Aitchison & Dunsmore, 1975) and probabilistic numerics (Cockayne et al., 2019; Hennig et al., 2022) to develop guarantees that are interpretable and make adaptive use of available information. + +We show that two popular uncertainty quantification methods, split conformal prediction (Vovk et al., 2005; Papadopoulos et al., 2002) and conformal risk control (Angelopoulos et al., 2024), can both be recovered as special cases of our framework. Our approach gives a more complete characterization of the performance of these approaches, as we are able to determine the full distribution of possible outcomes rather than a single point estimate. Since our approach is grounded in Bayesian probability, we can easily incorporate knowledge relevant to evaluating the performance of these models when it is present, such as monotonicity or distributional assumptions, while defaulting to existing methods when absent. Our results show that Bayesian probability, while it is often discarded due to the apparent need to specify prior distributions, is actually well-suited for distribution-free uncertainty quantification. + +# 2. Background + +Conformal prediction methods apply a wrapper on top of black-box predictive models to be able to subject them to statistical analysis. In order to generate meaningful predictions about future performance, it is assumed that we have access to a small calibration dataset that is representative of the deployment conditions. Performance on this dataset then provides the foundation for generating predictions about future performance. We begin by reviewing existing current distribution-free uncertainty quantification techniques and Bayesian quadrature methods. + +# 2.1. Distribution-free Uncertainty Quantification Techniques + +Uncertainty quantification techniques provide guarantees on the future performance of a black-box predictive model mapping inputs $X$ to outputs $Y$ based on a calibration set consisting of $X _ { 1 } , \ldots , X _ { n }$ and $Y _ { 1 } , \dots , Y _ { n }$ . Different approaches do so in different ways. For more information on these techniques, refer to Shafer & Vovk (2008) or Angelopoulos & Bates (2023). + +Split Conformal Prediction The goal of Split Conformal Prediction (Vovk et al., 2005; Papadopoulos et al., 2002) is to generate a prediction set or interval that contains the ground-truth output with high probability. This is often expressed in terms of the coverage level $1 - \alpha$ . It relies on a score function $s ( x , y )$ which measures the disagreement between a predictor’s output and the ground truth. + +The conformal guarantee is + +$$ +\operatorname* { P r } \left( Y _ { n + 1 } \notin { \mathcal { C } } ( X _ { n + 1 } ) \right) \leq \alpha , +$$ + +where + +$$ +\mathcal { C } ( X _ { n + 1 } ) = \{ y : s ( X _ { n + 1 } , y ) \leq \hat { q } \} +$$ + +and $\hat { q }$ is the $\underline { { \lceil ( n + 1 ) ( 1 - \alpha ) \rceil } }$ quantile of $\begin{array} { r l } { s _ { 1 } } & { { } = } \end{array}$ $s ( X _ { 1 } , \bar { Y } _ { 1 } ) , \ldots , s _ { n } \ = \ s ( \bar { \spadesuit } { X } _ { n } , Y _ { n } )$ . Here, $ { \mathcal { C } } ( X _ { n + 1 } )$ is a prediction set or interval which aims to include the ground-truth output. + +Conformal Risk Control In Conformal Risk Control (Angelopoulos et al., 2024), the goal is to generalize conformal prediction to more general loss functions that are monotonic functions of a single parameter $\lambda$ . Conformal Risk Control (CRC) proceeds by viewing the coverage guarantee (1) as the expected value of a 0-1 loss. It is assumed that the maximum possible value of the loss is $B$ and that the problem is “achievable” by design in that there exists some setting $\lambda _ { \operatorname* { m a x } }$ that satisfies the conformal guarantee. Additionally, each loss function $L _ { i } ( \lambda )$ is assumed to be a monotonic non-increasing function of $\lambda$ . The guarantee offered by Conformal Risk Control is of the form + +$$ +E \left( \ell ( \mathcal { C } _ { \hat { \lambda } } ( X _ { n + 1 } ) , Y _ { n + 1 } ) \right) \leq \alpha , +$$ + +where + +$$ +\hat { \lambda } = \operatorname* { i n f } \left\{ \lambda : \frac { n } { n + 1 } \hat { R } _ { n } ( \lambda ) + \frac { B } { n + 1 } \leq \alpha \right\} +$$ + +and $\begin{array} { r } { \hat { R } _ { n } ( \lambda ) = \frac { 1 } { n } \sum _ { i = 1 } ^ { n } L _ { i } ( \lambda ) } \end{array}$ is the empirical risk. + +# 2.2. Bayesian Quadrature + +Bayesian quadrature (Diaconis, 1988; O’Hagan, 1991) is a general technique for evaluating integrals that allows for uncertainty in the integrand. It estimates the value of an integral $\textstyle \int _ { a } ^ { b } f ( x ) d x$ by the following four steps: (1) place a prior $p ( f )$ on functions, (2) evaluate $f$ at $x _ { 1 } , x _ { 2 } , \ldots , x _ { n }$ (3) compute a posterior given the observed values of $f$ by Bayes’ rule, and (4) estimate $\textstyle \int _ { a } ^ { b } f ( x ) d x$ . Suppose that $f ( x _ { i } ) = y _ { i }$ for $i = 1 , 2 , \dots , n$ . The posterior over $f$ is + +$$ +p ( f \mid x _ { 1 : n } , y _ { 1 : n } ) \propto p ( f ) \prod _ { i = 1 } ^ { n } \delta ( y _ { i } - f ( x _ { i } ) ) , +$$ + +where $\delta ( \cdot )$ is the Dirac delta function. The posterior mean then provides an estimate for the integral: + +$$ +\int _ { a } ^ { b } f ( x ) d x \approx \int _ { a } ^ { b } f _ { n } ( x ) d x , { \mathrm { ~ w h e r e } } +$$ + +It has been demonstrated that many classical quadrature procedures such as the trapezoid rule can be recovered by placing a Gaussian process prior on functions (Karvonen & Sarkk ¨ a¨, 2017). + +# 2.3. Summary and Prospectus + +Bayesian quadrature provides an illustration of how a primarily numerical method can be connected to Bayesian inference, and in doing so potentially admit additional information about the underlying function that can be incorporated via a prior distribution. In next section, we will see how a similar approach can be applied to conformal prediction, identifying a Bayesian framework that reproduces existing distribution-free uncertainty quantification techniques. The challenge in doing so is that we want guarantees of the style obtained from Bayesian models, but we want to make the approach as general as possible in its assumptions about the underlying distribution. We solve this problem via an approach inspired by probabilistic numerics to construct a nonparametric characterization of the underlying distribution based on the calibration set. + +# 3. Decision-theoretic Formulation + +In this section we show how split conformal prediction and conformal risk control can be formulated as instances of a general decision problem. + +Let $z = ( z _ { 1 } , \ldots , z _ { n } )$ be a set of calibration data where each observation $z _ { i } = ( x _ { i } , y _ { i } )$ consists of an input and a ground truth label. Let $\theta$ denote the true state of nature that defines a shared density $f ( z _ { i } \mid \theta )$ for the data.1 A new test point $z _ { \mathrm { n e w } }$ is assumed to have the same distribution. Let $\lambda$ be a control parameter (e.g. threshold) that must be chosen based on the calibration data. We assume the presence of a loss function $L ( \theta , \lambda )$ which quantifies the loss incurred by selecting $\lambda$ when the true state of nature is $\theta$ . + +The decision-theoretic goal is to choose a decision rule $\lambda ( z )$ that controls the risk: + +$$ +R ( \theta , \lambda ) = \int L ( \theta , \lambda ( z ) ) f ( z \mid \theta ) d z . +$$ + +It is often desirable to choose $\lambda$ so that it is robust to any possible state of nature $\theta$ . The maximum risk is defined as + +$$ +{ \bar { R } } ( \lambda ) = \operatorname* { s u p } _ { \theta } R ( \theta , \lambda ) . +$$ + +In distribution-free uncertainty quantification applications, it is often trivial to achieve arbitrarily low risk (for example by forming prediction sets covering the entire output space). We thus want to find decision rules whose risk is upper bounded by a constant $\alpha$ : + +$$ +\begin{array} { r } { \bar { R } ( \lambda ) \leq \alpha , } \end{array} +$$ + +and use another criterion (such as expected prediction set size) to select among these. We call a rule that satisfies (10) an $\alpha$ -acceptable decision rule. + +# 3.1. Recovering Split Conformal Prediction + +We now show how split conformal prediction is a special case of this decision-theoretic problem. Let $L _ { \mathrm { s c p } } ( \theta , \lambda )$ be the miscoverage loss: + +$$ +\begin{array} { r l r } { { L _ { \mathrm { s c p } } ( \theta , \lambda ) = \operatorname* { P r } \{ s ( z _ { \mathrm { n e w } } ) > \lambda \} } \quad } & { ( 1 } \\ & { = 1 - \operatorname* { P r } \{ s ( z _ { \mathrm { n e w } } ) \le \lambda \} } \\ & { = 1 - \displaystyle \int \mathbb { 1 } \{ s ( z _ { \mathrm { n e w } } ) \le \lambda \} f ( z _ { \mathrm { n e w } } \mid \theta ) d z _ { \mathrm { n e w } } , } \end{array} +$$ + +where $s$ is an arbitrary nonconformity function. + +Proposition 3.1. Define $s _ { i } \ { \stackrel { \triangle } { = } } \ s ( z _ { i } )$ for $i = 1 , \ldots , n$ and let $s _ { ( 1 ) } \leq s _ { ( 2 ) } \leq . . . \leq s _ { ( n ) }$ be the corresponding order + +statistics. Let $\lambda _ { \mathrm { s c p } }$ be the following decision rule: + +$$ +\lambda _ { \mathrm { s c p } } = \left\{ { \begin{array} { l l } { s _ { ( \lceil ( n + 1 ) ( 1 - \alpha ) \rceil ) } , } & { { \mathrm { i f ~ } } \lceil ( n + 1 ) ( 1 - \alpha ) \rceil \leq n } \\ { \infty , } & { { \mathrm { o t h e r w i s e } } . } \end{array} } \right. +$$ + +Then $\lambda _ { \mathrm { s c p } }$ is an $\alpha$ -acceptable decision rule for the miscoverage loss $L _ { \mathrm { s c p } }$ defined in (11). + +Proof. Proofs for all theoretical results may be found in Appendix B. + +Therefore the prediction set can be constructed as in (2): + +$$ +\mathcal { C } _ { \mathrm { s c p } } ( x _ { \mathrm { n e w } } ) = \{ y \in \mathcal { y } : s ( x _ { \mathrm { n e w } } , y ) \leq \lambda _ { \mathrm { s c p } } \} , +$$ + +and by Proposition 3.1, $\mathcal { C } _ { \mathrm { s c p } }$ satisfies the conformal guarantee from (1). + +# 3.2. Recovering Conformal Risk Control + +Conformal risk control generalizes split conformal prediction by considering losses that are monotonic non-increasing functions of a single parameter $\lambda$ . + +$$ +L _ { \mathrm { c r c } } ( \theta , \lambda ) = \int \ell ( z _ { \mathrm { n e w } } , \lambda ) f ( z _ { \mathrm { n e w } } \mid \theta ) d z _ { \mathrm { n e w } } , +$$ + +where $\ell ( z _ { \mathrm { n e w } } , \lambda )$ is an individual loss function that is monotonically non-increasing in $\lambda$ . + +Proposition 3.2. Let $\lambda _ { \mathrm { c r c } }$ be the following decision rule: + +$$ +\lambda _ { \mathrm { c r c } } = \operatorname* { i n f } \left\{ \lambda : \frac { 1 } { n + 1 } \left( \sum _ { i = 1 } ^ { n } \ell ( z _ { i } , \lambda ) + B \right) \leq \alpha \right\} . +$$ + +Then $\lambda _ { \mathrm { c r c } }$ is an $\alpha$ -acceptable decision rule for $L _ { \mathrm { c r c } }$ defined in (14). + +Note in particular that when $\ell ( z , \lambda )$ can be expressed in the form $\ell ( \mathcal { C } _ { \lambda } ( x _ { n + 1 } ) , y _ { n + 1 } )$ , this recovers the conformal risk control guarantee from (3). + +# 4. Our Approach + +We introduce our approach by reinterpreting split conformal prediction and conformal risk control as special cases of a more general Bayesian procedure. In order to do so, we borrow ideas from both Bayesian quadrature (Diaconis, 1988; O’Hagan, 1991) and distribution-free tolerance regions (Guttman, 1970). Bayesian quadrature (Section 2.2) solves a numerical integration problem by placing a prior on functions and using Bayesian inference to compute a distribution over the value of the integral. Distributionfree tolerance regions provide a distribution over quantile spacings that holds regardless of the original underlying distribution. Putting these ideas together allows us to extend conformal prediction by producing bounds on expected loss tailored to the actual losses observed in the calibration set. + +The remainder of this section is structured as follows. In Section 4.1, we discuss the relationship between risk control and Bayes risk. In Section 4.2, we describe a general approach for using Bayesian quadrature to bound the posterior risk. In Section 4.3, we make the quadrature “distributionfree” by removing the dependence on a prior over functions. In Section 4.4 we handle uncertainty in the evaluation locations of the function by applying results that characterize the spacing between consecutive quantiles. In Section 4.5, we show how to use these results to produce an upper bound on the expected loss. Finally, in Section 4.6, we show how previous conformal prediction techniques can be viewed as a special case of our procedure that only considers the expectation of the posterior loss. + +# 4.1. Bayes Risk + +The risk $R ( \theta , \lambda )$ measures the expected loss for one who already knows the true state of nature $\theta$ but not the particular data observed. However, in practical applications the situation is reversed: we do know the observed data but there is uncertainty about the state of nature. Therefore, we want a decision rule that protects against high loss for a range of possible $\theta$ . This idea is expressed as the integrated risk: + +$$ +r ( \pi , \lambda ) = \int R ( \theta , \lambda ) \pi ( \theta ) d \theta , +$$ + +where the prior $\pi ( \theta ) \geq 0$ measures the relative importance of the different possible states of nature. It is well-known that the minimizer of the integrated risk is the so-called Bayes decision rule: + +$$ +\lambda ^ { \pi } \triangleq \operatorname { a r g m i n } _ { \lambda } r ( \lambda \mid z ) , +$$ + +where $r ( \lambda \mid z )$ is the posterior risk + +$$ +r ( \lambda \mid z ) = E ( L _ { \lambda } \mid z ) = \int L ( \theta , \lambda ( z ) ) \pi ( \theta \mid z ) d \theta , +$$ + +and $\pi ( \theta \mid z ) \propto \pi ( \theta ) f ( z \mid \theta )$ . Interestingly, the worst-case integrated risk of a decision rule is identical to its maximum risk (9) + +$$ +\bar { r } ( \lambda ) \triangleq \operatorname* { s u p } _ { \pi } r ( \pi , \lambda ) = \operatorname* { s u p } _ { \theta } R ( \theta , \lambda ) = \bar { R } ( \lambda ) . +$$ + +We can therefore focus on bounding the worst-case integrated risk $\bar { r } ( \lambda )$ , since this will also bound the maximum risk $\bar { R } ( \lambda )$ . + +# 4.2. Reformulation as Bayesian Quadrature + +We now turn our attention to finding $\lambda$ minimizing the posterior risk (18). Consider risks that can be expressed as the + +expectation over individual losses: + +$$ +L ( \theta , \lambda ) = \int \ell ( z _ { \mathrm { n e w } } , \lambda ) f ( z _ { \mathrm { n e w } } \mid \theta ) d z _ { \mathrm { n e w } } . +$$ + +It is well-known that the expectation of a random variable is equal to the definite integral of its quantile function over its domain (Shorack, 2000, p. 116). Consider the distribution function of individual losses induced by $\lambda$ for a particular value of $\theta$ : + +$$ +F ( \ell ) \triangleq \operatorname* { P r } \{ \ell ( z _ { \mathrm { n e w } } , \lambda ) \leq \ell \mid \theta \} +$$ + +The corresponding quantile function is: + +$$ +K ( t ) \equiv F ^ { - 1 } ( t ) = \operatorname* { i n f } \{ \ell : F ( \ell ) \geq t \} , +$$ + +and the expected loss given $K$ is simply $\textstyle \int _ { 0 } ^ { 1 } K ( t ) d t$ . + +Instead of performing posterior inference over $\theta$ , we propose to take an approach inspired by Bayesian quadrature that places a corresponding prior over $K$ . Figure 1 shows a schematic overview of Bayesian quadrature in this setting and how our proposed approach differs. The posterior risk given the observed individual losses $\ell _ { i } \triangleq \ell ( z _ { i } , \lambda )$ for $i =$ $1 , \ldots , n$ becomes: + +$$ +E ( L \mid \ell _ { 1 : n } ) = \int J [ K ] p ( K \mid \ell _ { 1 : n } ) d K , +$$ + +where $\begin{array} { r } { J [ K ] \ \triangleq \ \int _ { 0 } ^ { 1 } K ( t ) d t } \end{array}$ and we have suppressed the dependence on $\lambda$ for notational convenience. The posterior over quantile functions can be expressed as: + +$$ +p ( K \mid \ell _ { 1 : n } ) = \int p ( K \mid t _ { 1 : n } , \ell _ { 1 : n } ) p ( t _ { 1 : n } \mid \ell _ { 1 : n } ) d t _ { 1 : n } +$$ + +$$ +p ( K \mid t _ { 1 : n } , \ell _ { 1 : n } ) \propto \pi ( K ) \prod _ { i = 1 } ^ { n } \delta ( \ell _ { i } - K ( t _ { i } ) ) . +$$ + +This resembles the Bayesian quadrature problem from Section 2.2, except the evaluation sites $t _ { 1 } , \ldots , t _ { n }$ are unknown. Fortunately, the distribution of $t _ { 1 } , \ldots , t _ { n }$ is independent of the true distribution of the losses, as we shall now show. + +# 4.3. Elimination of the Prior Distribution + +In order to address the dependence of the posterior risk on the prior $\pi ( K )$ , we derive an upper bound on the posterior expected loss. The bound takes the form of a weighted sum of the observed losses, where the weights are determined by the spacing between consecutive quantiles. + +Theorem 4.1. Let $t _ { ( 0 ) } = 0 \}$ , $t _ { ( n + 1 ) } = 1$ , and $\ell _ { ( n + 1 ) } = B$ Then + +$$ +\operatorname* { s u p } _ { \pi } E ( L \mid t _ { 1 : n } , \ell _ { 1 : n } ) \leq \sum _ { i = 1 } ^ { n + 1 } u _ { i } \ell _ { ( i ) } , +$$ + +where $u _ { i } = t _ { ( i ) } - t _ { ( i - 1 ) }$ . + +![](images/figures/conformal-bayesian-quadrature-fig-0001.jpg) +Figure 1. Overview of our approach. Left: Standard Bayesian quadrature places a prior over the quantile function of the loss distribution. The posterior is formed via Bayes’ rule after observing a set of loss values and quantile levels. However, in practice quantile levels are not directly observed. Middle: Our approach combines properties of quantile spacings with a right rectangular integration rule to construct an upper bound on the posterior distribution of the expected loss. Randomly sampled spacings and corresponding quantile functions are shown in blue along with a $9 5 \%$ credible interval for each quantile level in black. Right: The posterior distribution for a random variable $L ^ { + }$ that upper bounds the expected loss is constructed by integrating over the unknown quantile levels. + +Theorem 4.1 is based on the definite integral of the “worstcase” quantile function that is consistent with the observations. This strategy eliminates the need to specify a prior or evaluate an integral over functions $K$ . We now turn our attention to handling the uncertainty over the quantiles $t _ { 1 : n }$ . + +# 4.4. Random Quantile Spacings + +We now appeal to a result about distribution-free tolerance regions that characterizes the distribution of spacings between consecutive ordered quantiles. Knowledge of this distribution will allow us to handle the input noise in the quadrature problem. + +Lemma 4.2 (Distribution of Quantile Spacings (Aitchison & Dunsmore, 1975, p. 140)). Suppose that $\ell _ { 1 } , \ldots , \ell _ { n }$ are drawn i.i.d. with continuous2 distribution function $F$ . Let $t _ { i } = F ( \ell _ { i } )$ and $u _ { i } = t _ { ( i ) } - t _ { ( i - 1 ) }$ , where by convention $t _ { ( 0 ) } ~ = ~ 0$ and $t _ { ( n + 1 ) } ~ = ~ 1$ . Then $( u _ { 1 } , u _ { 2 } , \ldots , u _ { n + 1 } ) \cong$ $\operatorname { D i r } ( 1 , \ldots , 1 )$ . + +We are now ready to present our algorithm for bounding the expected loss $E ( L \mid \ell _ { 1 : n } )$ . + +# 4.5. Bound on Maximum Posterior Risk + +Putting together Lemma 4.2 and Theorem 4.1 allows us to bound the maximum posterior risk. + +Theorem 4.3. Define $\ell _ { ( i ) }$ to be the order statistics of $\ell _ { 1 } , \ldots , \ell _ { n }$ for $i = 1 , \ldots , n$ and $\ell _ { ( n + 1 ) } \triangleq B$ . Let $L ^ { + }$ be + +the random variable defined as follows: + +$$ +U _ { 1 } , \ldots , U _ { n + 1 } \sim \mathrm { D i r } ( 1 , \ldots , 1 ) , L ^ { + } = \sum _ { i = 1 } ^ { n + 1 } U _ { i } \ell _ { ( i ) } . +$$ + +Then for any $b \in ( - \infty , B ]$ , + +$$ +\operatorname* { i n f } _ { \pi } \operatorname* { P r } ( L \leq b \mid \ell _ { 1 : n } ) \geq \operatorname* { P r } ( L ^ { + } \leq b ) . +$$ + +Theorem 4.3 states that $L ^ { + }$ stochastically dominates the posterior risk, which allows us to directly form upper confidence bounds as follows. + +Corollary 4.4. For any desired confidence level $\beta \in ( 0 , 1 )$ , define + +$$ +b _ { \beta } ^ { * } = \operatorname* { i n f } _ { b } \{ b : \operatorname* { P r } ( L ^ { + } \leq b \mid \ell _ { 1 : n } ) \geq \beta \} . +$$ + +Then $\operatorname* { i n f } _ { \pi } \operatorname* { P r } ( L \leq b \mid \ell _ { 1 : n } ) \geq \beta$ for any $b \geq b _ { \beta } ^ { * }$ . + +The critical value $b _ { \beta } ^ { * }$ can be calculated by applying techniques for bounding linear combinations of Dirichlet random variables $\mathrm { N g }$ et al., 2011, p. 63). Alternatively, straightforward Monte Carlo simulation of $L ^ { + }$ is often sufficient, and is the approach we take in our experiments. An illustration is shown in Figure 2. + +# 4.6. Recovering Conformal Methods + +This perspective puts the previous distribution-free uncertainty techniques in a new light. Taking the expected value of $L ^ { + }$ , we find + +$$ +E ( L ^ { + } ) = \sum _ { i = 1 } ^ { n + 1 } E ( U _ { i } ) \ell _ { ( i ) } = \frac { 1 } { n + 1 } \left( \sum _ { i = 1 } ^ { n } \ell _ { i } + B \right) . +$$ + +The Conformal Risk Control decision rule (15) then is simply the infimum over $\lambda$ for which $E ( L ^ { + } ) \leq \alpha$ . + +![](images/figures/conformal-bayesian-quadrature-fig-0002.jpg) +Figure 2. Our Bayesian approach to conformal prediction accounts for the variability in quantile levels better than previous approaches. Left: Conformal Risk Control (Angelopoulos et al., 2024) considers only the expectation over the unobserved quantile values $t _ { 1 } , \ldots , t _ { n }$ This can underestimate the true expected loss (shown here: estimated expected loss 0.45 vs. true expected loss 0.50). Right: Our approach makes use of the fact that the quantile spacings are drawn from a Dirichlet distribution. By considering the full distribution over quantiles, we gain a more complete view of the expected loss. Shown here is one sample drawn from this distribution, which estimates the expected loss as 0.58. + +For, split conformal prediction, the individual loss is defined as $\ell _ { i } = 1 - \mathbb { 1 } \{ s _ { i } \leq \lambda \}$ . Therefore, suppose that $\lambda = s _ { ( k ) }$ The expected value of $L ^ { + }$ then becomes: + +$$ +\begin{array} { c } { { E ( L ^ { + } ) = \displaystyle \frac { 1 } { n + 1 } \left( n + 1 - \sum _ { i = 1 } ^ { n } \mathbb { 1 } \big \{ s _ { i } \leq s _ { ( k ) } \big \} \right) } } \\ { { = 1 - \displaystyle \frac { k } { n + 1 } } } \end{array} +$$ + +Therefore, $E ( L ^ { + } ) \ \leq \ \alpha$ is satisfied whenever $k \geq ( n +$ $1 ) ( 1 - \alpha )$ , and in particular by $k ^ { * } = \lceil ( n + 1 ) ( 1 - \alpha ) \rceil$ . This recovers (12) when $\lceil ( n + 1 ) ( 1 - \alpha ) \rceil \leq n$ . + +Putting these results together, we have recovered standard conformal prediction techniques but have the additional flexibility of considering the distribution of $L ^ { + }$ rather than the expected value alone. Our experiments explore the value of this approach. + +# 5. Experiments + +The primary goal of our experiments is to demonstrate the utility of producing a posterior distribution over the expected loss. We conduct experiments on both synthetic data and calibration data collected from MS-COCO (Lin et al., 2014). For each data setting, we randomly generate $M = 1 0 \small { , } 0 0 0$ data splits. Each method is used to select $\lambda$ with the goal of controlling the risk such that $R ( \theta , \lambda ) \leq \alpha$ for unknown $\theta$ . We compare algorithms on the basis of both the relative frequency of incurring risk greater than $\alpha$ and the prediction set size of the chosen $\lambda$ . The ideal algorithm would select $\lambda$ such that the relative frequency of exceeding the target risk is at most a target failure rate of $1 - \beta = 0 . 0 5$ while minimizing prediction set size. + +As demonstrated in Section 4.6, our method recovers conformal risk control by taking the expected value of $L ^ { + }$ Therefore, in order to demonstrate the effect of targeting a conditional guarantee (as opposed to a marginal one as in conformal risk control), we use our Bayesian quadraturebased method to compute the decision rule based on the one-sided highest posterior density (HPD) interval: + +$$ +\lambda _ { \mathrm { h p d } } ^ { \beta } \triangleq \operatorname* { i n f } _ { \lambda } \{ \lambda : \operatorname* { P r } ( L ^ { + } \leq \alpha \mid \ell _ { 1 : n } ) \geq \beta \} , +$$ + +by finding the corresponding critical values $b _ { \beta } ^ { * }$ according to (29) via Monte Carlo simulation of Dirichlet random variates with 1000 samples. We include Risk-controlling Prediction Sets (RCPS) (Bates et al., 2021) with Hoeffding upper confidence bound as an additional baseline. Code for our experiments is publicly available on Github.3 + +# 5.1. Synthetic Binomial Data + +We first sample directly from a known loss distribution so that we can directly compute the frequency of excessively large risk. Here the loss distribution is chosen to be a scaled binomial distribution, normalized to have a maximum loss of $B = 1$ and probability of failure set to $1 - \lambda$ . This was simulated by computing + +$$ +\ell ( z _ { i } , \lambda ) = \frac { 1 } { K } \sum _ { k = 1 } ^ { K } \mathbb { 1 } \{ V _ { i k } > \lambda \} , +$$ + +where $V _ { i k } \mathrm { ~ ~ { ~ \sim ~ } ~ } \mathrm { U n i f o r m } ( 0 , 1 )$ for $\begin{array} { r c l } { i } & { = } & { 1 , \dots , n } \end{array}$ and $k = 1 , \ldots , K$ . This loss is therefore monotonically nonincreasing in $\lambda$ and achieves zero loss at $\lambda _ { \operatorname* { m a x } } = 1$ . We set $n = 1 0$ , $K = 4$ , and $\alpha = 0 . 4$ . + +Table 1. Relative frequency of trials (out of 10,000) for which the resulting decision rule $\lambda$ exceeded the target risk threshold $\alpha$ . + +
Decision RuleRelative Freq.95% CI
CRC21.20%[20.40%, 22.01%]
RCPS0.00%[0.00%, 0.04%]
Ours (β = 0.95)0.03%[0.01%, 0.09%]
+ +Note: Error bars are computed as $9 5 \%$ Clopper-Pearson confidence intervals for binomial proportions. + +Since the expectation of the loss (34) is $1 - \lambda$ , any trial for which $\lambda < 0 . 6$ constitutes a risk exceeding the $\alpha$ threshold. The relative frequency of trials exceeding this risk threshold are tabulated in Table 5.1. A histogram of the chosen $\lambda$ for each of the methods across all 10,000 trials is shown in Figure 3. For conformal risk control, the mean risk across all trials was $0 . 3 3 6 3 \pm 0 . 0 0 0 7$ or our approach $\lambda _ { \mathrm { h p d } } ^ { 0 . 9 5 }$ $0 . 1 7 5 8 \pm 0 . 0 0 0 6$ +the distribution of $L ^ { + }$ , we plot a histogram of $L ^ { + }$ according to (27) estimated with 100,000 Dirichlet samples for three settings of $\lambda \in \{ 0 . 7 , 0 . 8 , 0 . 9 \}$ . The results are shown in Figure 4. + +# 5.2. Synthetic Heteroskedastic Data + +In this experiment we also use 10,000 random trials. We use $n = 2 0 0$ calibration samples each. To achieve heteroskedasticity, we let $X \sim U [ 0 , 4 ]$ and $Y \mid X \sim { \mathcal { N } } ( 0 , X ^ { 2 } )$ . The prediction intervals are then formed as $[ - \hat { \lambda } , \hat { \lambda } ]$ where $\hat { \lambda }$ is selected by each method. The loss is the miscoverage loss and the target loss is set to $\alpha = 0 . 1$ (i.e. $90 \%$ coverage). The maximum allowable risk failure rate is set to $5 \%$ (i.e. $\beta = 0 . 9 5 )$ . The results are show in Table 5.2. + +# 5.3. False Negative Rate on MS-COCO + +We also compare methods on controlling the false negative rate of multilabel classification on the MS-COCO dataset (Lin et al., 2014). The experimental setup mirrors that used by Angelopoulos & Bates (2023, Section 5.1). Each random split contains 1000 calibration examples and 3952 test examples. The results of this experiment are summarized in Table 5.3. + +# 6. Discussion + +Our results in Table 5.1 demonstrate that even though the Conformal Risk Control marginal guarantee holds, a significant number of individual trials $( 2 1 . 2 0 \% )$ may incur risk exceeding the target threshold. In contrast, by using the more conservative HPD criterion, very few of the trials $( 0 . 0 3 \% )$ exceeded the target risk. In Table 5.2, both RCPS and our method achieve failure rate below the target of $5 \%$ but our method achieves significantly smaller prediction intervals. + +These results point to the qualitative difference in a marginal guarantee, which averages over many possible yet unobserved data sets vs. a conditional guarantee which focuses on knowledge about the state of nature conditioned on the calibration data actually observed. Previous work on conditional guarantees (Barber et al., 2021; Gibbs et al., 2024) has focused on input-conditional guarantees, where the guarantee is conditioned on for all in the input domain. Guarantees of this nature have been shown to be generally impossible without stronger distribution assumptions. Our guarantees are perhaps better characterized by the term “data-conditional guarantee”, where we condition on the set of observed loss values. Our experiments demonstrate the practical benefits of this by achieving decisions that produce smaller prediction sets and intervals while not violating the constraint on maximum allowable failure rate. Our guarantees, in contrast, do not rely on strong distribution assumptions that would be necessary to produce an input-conditional guarantee. + +The results are again confirmed in Table 5.3 on MS-COCO, which show that the marginal guarantees of Conformal Risk Control lead to an even greater percentage of trials exceeding the risk threshold. On the other hand, RCPS is able to control the risk but this comes at the cost of larger prediction sets. Our approach successfully balances these two concerns, producing prediction intervals that are shorter than baselines while not exceeding the maximum acceptable failure rate. It is also clear that the distribution of the expected loss upper bound $L ^ { + }$ in Figure 4 provides a more complete view of the range of possible losses and its dependence on $\lambda$ , a perspective that is not offered by previous methods. + +Our goal in this work is to show that the Bayesian viewpoint unlocks a richer interpretation compared to previous works, which focus on marginal guarantees that as we have shown in the paper correspond to the posterior mean. In order to draw an explicit correspondence between our work and previous approaches, the dependence on the prior was removed in Section 4.3. The intuition is that that any rational decision maker operating according to the rules of probability, regardless of prior (sufficiently expressive), would agree with the upper-bounding distribution of we derive. Naturally, commitment to a specific choice of prior would lead to tighter distributions over the posterior risk, and in future work we seek to bridge these fields even further by exploring specific choices of priors over quantile functions. + +The limitations of our method lie primarily in the two main assumptions it makes. First, it assumes that the data at deployment time are independent and identically distributed to the calibration data. Second, it assumes an upper bound $B$ on the losses. If either of these assumptions do not hold, then the guarantees produced by our method are no longer valid. Additionally, the bounds produced by our method are conservative in the sense that they hold for any choice of prior for the loss distribution (provided that the prior is consistent with the calibration data). Therefore, if the two aforementioned assumptions do hold, the actual loss values may be significantly less than indicated by our method. + +![](images/figures/conformal-bayesian-quadrature-fig-0003.jpg) +Figure 3. Comparison of risk incurred by each procedure across multiple trials. Left: Histogram of the decision rule $\lambda _ { \mathrm { c r c } }$ chosen by Conformal Risk Control across $M = 1 0 { , } 0 0 0$ randomly sampled calibration sets. The region where per-trial risk exceeds $\alpha$ is highlighted in red. Right: Histogram of the $\lambda _ { \mathrm { h p d } } ^ { 0 . 9 5 }$ chosen according to our $9 5 \%$ Bayesian posterior interval. + +Table 2. Relative frequency of trials (out of 10,000) for which the resulting decision rule $\lambda$ exceeded the target risk threshold $\alpha$ in the synthetic heteroskedastic experiment. + +
Decision RuleRelative Freq.95% CIMean Prediction Interval Length
Split Conformal Prediction / CRC46.19%[45.21%, 47.17%]7.99
RCPS0.0%[0.0%, 0.04%]14.29
Ours (β = 0.95)3.42%[3.07%, 3.80%]9.50
+ +Note: Error bars are computed as $9 5 \%$ Clopper-Pearson confidence intervals for binomial proportions. + +Table 3. Results on MS-COCO comparing relative frequency of trials for which the resulting decision rule $\lambda$ exceeded the target risk threshold $\alpha$ and average prediction set size. + +
MethodRelative Freq.Pred. Set Size
CRC45.05%2.92
RCPS0.0%3.57
Ours (β = 0.95)5.43%3.04
+ +Overall, our approach demonstrates how conformal prediction techniques can be recovered and extended using Bayesian probability, all without having to specify a prior distribution. This Bayesian formulation is highly flexible due to its nonparametric nature, yet is amenable to incorporating specific information about the distribution of losses likely to be encountered. In practical applications, maximizing the risk with respect to all possible priors may be too conservative, and thus future work may explore the effect of specific priors on the risk estimate. + +# 7. Related Work + +Statistical Prediction Analysis. Statistical prediction analysis (Aitchison & Dunsmore, 1975) deals with the use of statistical inference to reason about the likely outcomes of future prediction tasks given past ones. Within statistical prediction analysis, the area of distribution-free prediction assumes that the parameters or the form of the distributions involved cannot be identified. This idea can be traced back to Wilks (1941), who constructed a method to form distribution-free tolerance regions. Tukey (1947; 1948) generalized distribution-free tolerance regions and introduced the concept of statistically equivalent blocks, which are analogous to the intervals between consecutive order statistics of the losses. Much of the relevant theory is summarized by Guttman (1970), and the Dirichlet distribution of quantile spacing is discussed by Aitchison & Dunsmore (1975). We build upon these works by connecting them to Bayesian quadrature and applying them in the more modern context of distribution-free uncertainty quantification. + +![](images/figures/conformal-bayesian-quadrature-fig-0004.jpg) +Figure 4. Probability density for $L ^ { + }$ with $\lambda \in \{ 0 . 7 , 0 . 8 , 0 . 9 \}$ estimated using 100,000 Dirichlet samples. + +Bayesian Quadrature. The use of Bayesian probability to represent the outcome of a arbitrary computation is termed probabilistic numerics (Cockayne et al., 2019; Hennig et al., 2022). Since our approach is fundamentally based on integration, we focus primarily on the relationship with the more narrow approach of Bayesian quadrature, which employs Bayes rule to estimate the value of an integral. A lucid overview of this approach is discussed under the term Bayesian numerical analysis by Diaconis (1988), who traces it back to the late nineteenth century (Poincare´, 1896). The use of Gaussian processes in performing Bayesian quadrature is discussed in detail by O’Hagan (1991). Our approach is formulated similarly but differs in two main ways: (a) we use a conservative bound instead of an explicit prior, and (b) we have input noise induced by the random quantile spacings. + +Distribution-Free Uncertainty Quantification. Relevant background on distribution-free uncertainty quantification techniques is discussed in Section 2.1. A recent and comprehensive introduction to conformal prediction and related techniques may be found in (Angelopoulos & Bates, 2023). Some recent works, like ours, also make use of quantile functions (Snell et al., 2023; Farzaneh et al., 2024) but remain grounded in frequentist probability. Separately, Bayesian approaches to predictive uncertainty are popular (Hobbhahn et al., 2022) but make extensive assumptions about the form of the underlying predictive model. To our knowledge, we are the first to apply statistical prediction analysis and Bayesian quadrature in order to analyze the performance of black-box predictive models in a distribution-free way. + +# 8. Conclusion + +Safely deploying black-box predictive models, such as those based on deep neural networks, requires developing methods that provide guarantees of their performance. Existing techniques for solving this problem are based on frequentist statistics, and are thus difficult to extend to incorporate knowledge about the situation in which models may be deployed. In this work we provided a Bayesian alternative to distribution-free uncertainty quantification, showing that two popular existing methods are special cases of this approach. Our results show that Bayesian probability can be used to extend uncertainty quantification techniques, making their underlying assumptions more explicit, allowing incorporation of additional knowledge, and providing a more intuitive foundation for constructing performance guarantees that avoid overly-optimistic guarantees that can be produced by existing methods. + +# Impact Statement + +This paper introduces a practical algorithm for computing a posterior distribution for the expected loss based on the observed losses from a set of calibration data. The intended purpose of this algorithm is for the posterior distribution to inform deployment decisions of black-box predictive systems (e.g. deep neural networks) in safety-critical applications. Our method makes use of certain assumptions, discussed in Section 6, which if violated will lead to guarantees that may no longer hold. In particular, it is important to ensure proper monitoring to detect distribution shift between calibration and deployment. + +# Acknowledgements + +The authors would like to thank the anonymous reviewers for helpful comments. This work was supported by grant N00014-23-1-2510 from the Office of Naval Research. + +# References + +Aitchison, J. and Dunsmore, I. R. Statistical Prediction Analysis. New York: Cambridge University Press, 1975. +Angelopoulos, A. N. and Bates, S. Conformal prediction: A gentle introduction. Foundations and Trends in Machine Learning, 16(4):494–591, 2023. +Angelopoulos, A. N., Bates, S., Fisch, A., Lei, L., and Schuster, T. Conformal risk control. In The Twelfth International Conference on Learning Representations, 2024. +Barber, R. F., Candes, E. J., Ramdas, A., and Tibshirani, \` R. J. The limits of distribution-free conditional predictive inference. Information and Inference: A Journal of the + +IMA, 10(2):455–482, June 2021. ISSN 2049-8772. doi: 10.1093/imaiai/iaaa017. + +Bates, S., Angelopoulos, A., Lei, L., Malik, J., and Jordan, M. Distribution-free, risk-controlling prediction sets. Journal of the ACM, 68(6), 2021. + +Cockayne, J., Oates, C. J., Sullivan, T. J., and Girolami, M. Bayesian probabilistic numerical methods. SIAM Review, 61(3):756–789, 2019. + +Diaconis, P. Bayesian numerical analysis. In Berger, J. and Gupta, S. (eds.), Statistical Decision Theory and Related Topics IV, volume 1, pp. 163–175. Springer-Verlag, 1988. + +Farzaneh, A., Park, S., and Simeone, O. Quantile learnthen-test: Quantile-based risk control for hyperparameter optimization. IEEE Signal Processing Letters, 2024. + +Gibbs, I., Cherian, J. J., and Candes, E. J. Conformal Pre-\` diction With Conditional Guarantees, September 2024. + +Guttman, I. Statistical Tolerance Regions: Classical and Bayesian. Griffin’s Statistical Monographs and Courses, No. 26. Griffin, 1970. + +Hennig, P., Osborne, M. A., and Kersting, H. P. Probabilistic Numerics: Computation as Machine Learning. Cambridge University Press, 2022. + +Hobbhahn, M., Kristiadi, A., and Hennig, P. Fast predictive uncertainty for classification with Bayesian deep networks. In Proceedings of the Thirty-Eighth Conference on Uncertainty in Artificial Intelligence, 2022. + +Karvonen, T. and Sarkk ¨ a, S. Classical quadrature rules via ¨ Gaussian processes. In 2017 IEEE 27th International Workshop on Machine Learning for Signal Processing (MLSP), 2017. + +Kot, M. A First Course in the Calculus of Variations. American Mathematical Society, 2014. + +Lei, J., G’Sell, M., Rinaldo, A., Tibshirani, R. 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Journal of Machine Learning Research, 9(12):371–421, 2008. + +Shao, J. Mathematical Statistics. Springer, 2003. + +Shorack, G. Probability for Statisticians. Springer-Verlag, 2000. + +Shorack, G. R. and Wellner, J. A. Empirical Processes with Applications to Statistics. Society for Industrial and Applied Mathematics, 2009. + +Snell, J. C., Zollo, T. P., Deng, Z., Pitassi, T., and Zemel, R. Quantile risk control: A flexible framework for bounding the probability of high-loss predictions. In The Eleventh International Conference on Learning Representations, 2023. + +Tukey, J. W. Nonparametric estimation II. Statistically equivalent blocks and tolerance regions–the continuous case. The Annals of Mathematical Statistics, pp. 529–539, 1947. + +Tukey, J. W. Nonparametric estimation, III. Statistically equivalent blocks and multivariate tolerance regions–the discontinuous case. The Annals of Mathematical Statistics, pp. 30–39, 1948. + +Vovk, V., Gammerman, A., and Shafer, G. Algorithmic Learning in a Random World. Springer Science & Business Media, 2005. + +Wilks, S. S. Determination of sample sizes for setting tolerance limits. The Annals of Mathematical Statistics, 12(1):91–96, 1941. + +# A. Theoretical Preliminaries + +# A.1. Review of Problem Setup + +We first review some relevant aspects of our problem setup. + +Loss Function. We assume an upper bound on the losses: $\ell _ { i } \in ( - \infty , B ]$ for $i = 1 , \ldots , n$ . We assume the same upper bound for $\ell _ { \mathrm { n e w } }$ . + +Bayesian Quadrature of Quantile Functions. Recall Bayes rule for quantile functions: + +$$ +p ( K \mid t _ { 1 : n } , \ell _ { 1 : n } ) \propto \pi ( K ) \prod _ { i = 1 } ^ { n } \delta ( \ell _ { i } - K ( t _ { i } ) ) , +$$ + +where $\delta$ is the Dirac delta function. The prior $\pi ( K )$ is assumed to be sufficiently expressive to have nonzero measure for the set $\kappa _ { n }$ of quantile functions such that $K ( t _ { i } ) = \ell _ { i }$ for $i = 1 , \ldots , n$ and $K \in \mathcal { K } _ { n }$ . This is necessary to prevent the posterior distribution in (35) from becoming degenerate. + +# A.2. Background + +We begin by recalling some basic properties of distribution functions and quantile functions. + +Proposition A.1 (Properties of Distribution Functions (Shao, 2003, p. 4)). Let $F ( x ) = \operatorname* { P r } ( X \leq x )$ be a distribution function. Then $F ( - \infty ) = \operatorname* { l i m } _ { x - \infty } F ( x ) = 0$ , $\begin{array} { r } { F ( \infty ) = \operatorname* { l i m } _ { x \infty } F ( x ) = 1 } \end{array}$ , $F$ is nondecreasing (i.e., $F ( x ) \leq F ( y ) i f$ $x \leq y )$ , and $F$ is right continuous (i.e., $\begin{array} { r } { \operatorname* { l i m } _ { y x , y > x } F ( y ) = F ( x ) ) } \end{array}$ . + +Let $F$ be a distribution function and $K ( t ) \equiv F ^ { - 1 } ( t ) = \operatorname* { i n f } \left\{ x : F ( x ) \geq t \right\}$ be the corresponding quantile function. + +Proposition A.2 (Quantile Functions are Nondecreasing). If $t \leq u _ { i }$ , then $K ( t ) \leq K ( u )$ . + +Proof. Since $u \geq t$ , it follows that $\{ x : F ( x ) \geq u \} \subseteq \{ x : F ( x ) \geq t \}$ . Taking the infimum of both sides yields + +$$ +\operatorname* { i n f } \{ x : F ( x ) \geq u \} \geq \operatorname* { i n f } \{ x : F ( x ) \geq t \} \Rightarrow K ( u ) \geq K ( t ) . +$$ + +We also will make use of the probability integral transformation, which we state here for convenience. + +Proposition A.3 (Probability Integral Transformation (Shorack & Wellner, 2009, p. 5)). If $X$ has distribution function $F$ , then + +$$ +\Pr ( F ( X ) \leq t ) \leq t \qquad f o r a l l 0 \leq t \leq 1 , +$$ + +with equality failing if and only if t is not in the closure of the range of $F$ . Thus if $F$ is continuous, then $T = F ( X )$ is Uniform $( 0 , 1 )$ . + +# B. Proof of Results from the Main Paper + +# B.1. Proof of Proposition 3.1 + +Recall that $L _ { \mathrm { s c p } } ( \theta , \lambda )$ is the miscoverage loss: + +$$ +\begin{array} { r l } & { L _ { \mathrm { s c p } } ( \theta , \lambda ) = \operatorname* { P r } \{ s ( z _ { \mathrm { n e w } } ) > \lambda \} } \\ & { \qquad = 1 - \operatorname* { P r } \{ s ( z _ { \mathrm { n e w } } ) \le \lambda \} } \\ & { \qquad = 1 - \displaystyle \int \mathbb { 1 } \{ s ( z _ { \mathrm { n e w } } ) \le \lambda \} f ( z _ { \mathrm { n e w } } \mid \theta ) d z _ { \mathrm { n e w } } , } \end{array} +$$ + +where $s$ is an arbitrary nonconformity function. + +Proposition 3.1. Define $s _ { i } \triangleq s ( z _ { i } )$ for $i = 1 , \ldots , n$ and let $s _ { ( 1 ) } \leq s _ { ( 2 ) } \leq . . . \leq s _ { ( n ) }$ be the corresponding order statistics. Let $\lambda _ { \mathrm { s c p } }$ be the following decision rule: + +$$ +\lambda _ { \mathrm { s c p } } = { \left\{ \begin{array} { l l } { s _ { \left( \left[ { \boldsymbol { \left( n + 1 \right) } } \left( 1 - \alpha \right) \right] \right) } , } & { { \mathrm { i f ~ } } \left[ { \boldsymbol { \left( n + 1 \right) } } { \left( 1 - \alpha \right) } \right] \leq n } \\ { \infty , } & { { \mathrm { o t h e r w i s e } } . } \end{array} \right. } +$$ + +Then $\lambda _ { \mathrm { s c p } }$ is an $\alpha$ -acceptable decision rule for the miscoverage loss $L _ { \mathrm { s c p } }$ defined in (11). + +Proof. By Lei et al. (2018, Section 2), + +$$ +\operatorname* { P r } ( s _ { \mathrm { n e w } } \leq \hat { q } _ { 1 - \alpha } ) \geq 1 - \alpha , +$$ + +where + +$$ +\hat { q } _ { 1 - \alpha } = \left\{ \begin{array} { l l } { s _ { ( \lceil ( n + 1 ) ( 1 - \alpha ) \rceil } } & { \mathrm { i f } \ \lceil ( n + 1 ) ( 1 - \alpha ) \rceil \le n } \\ { \infty , } & { \mathrm { o t h e r w i s e } . } \end{array} \right. +$$ + +But $L _ { \mathrm { s c p } } ( \theta , \lambda ) = 1 - \operatorname* { P r } ( s _ { \mathrm { n e w } } \leq \lambda \mid \theta )$ , so for $\lambda = \hat { q } _ { 1 - \alpha }$ , $R ( \theta , \lambda _ { \mathrm { { s c p } } } ) \leq \alpha$ . This statement not depend on $\theta$ , and so $\bar { R } ( \lambda _ { \mathrm { s c p } } ) \overset { \cdot } { \underset { } { \leq } } \alpha$ . + +# B.2. Proof of Proposition 3.2 + +Recall that the $L _ { \mathrm { c r c } }$ is defined as: + +$$ +L _ { \mathrm { c r c } } ( \theta , \lambda ) = \int \ell ( z _ { \mathrm { n e w } } , \lambda ) f ( z _ { \mathrm { n e w } } \mid \theta ) d z _ { \mathrm { n e w } } , +$$ + +where $\ell ( z _ { \mathrm { n e w } } , \lambda )$ is an individual loss function that is monotonically non-increasing in $\lambda$ . + +Proposition 3.2. Let $\lambda _ { \mathrm { c r c } }$ be the following decision rule: + +$$ +\lambda _ { \mathrm { c r c } } = \operatorname* { i n f } \left\{ \lambda : \frac { 1 } { n + 1 } \left( \sum _ { i = 1 } ^ { n } \ell ( z _ { i } , \lambda ) + B \right) \leq \alpha \right\} . +$$ + +Then $\lambda _ { \mathrm { c r c } }$ is an $\alpha$ -acceptable decision rule for $L _ { \mathrm { c r c } }$ defined in (14). + +Proof. Let $L _ { 1 } , \ldots , L _ { n } , L _ { n + 1 }$ be an exchangeable collection of non-increasing random functions $L _ { i } : \Lambda \to ( - \infty , B ]$ . By Angelopoulos et al. (2024, Theorem 1), + +$$ +\mathbb { E } [ L _ { n + 1 } ( \hat { \lambda } ) ] \leq \alpha , +$$ + +where + +$$ +\hat { \lambda } = \operatorname* { i n f } \left\{ \lambda : \frac { n } { n + 1 } \hat { R } _ { n } ( \lambda ) + \frac { B } { n + 1 } \leq \alpha \right\} +$$ + +and $\hat { R } _ { n } ( \lambda ) = ( L _ { 1 } ( \lambda ) + . . . + L _ { n } ( \lambda ) ) / n$ . + +Interpreting these results using the notation from Section 3 of the main paper, we identify: + +• $L _ { i } ( \lambda ) = \ell ( z _ { i } , \lambda )$ for $i = 1 , \ldots , n$ and $L _ { n + 1 } ( \lambda ) = \ell ( z _ { \mathrm { n e w } } , \lambda ) .$ , + +• $\lambda _ { \mathrm { c r c } }$ is identical to $\hat { \lambda }$ from (43), and + +• (42) states that $R ( \theta , \lambda _ { \mathrm { c r c } } ) \leq \alpha$ for any $\theta$ . + +Therefore, $\begin{array} { r } { \bar { R } ( \lambda _ { \mathrm { c r c } } ) = \operatorname* { s u p } _ { \theta } R ( \theta , \lambda _ { \mathrm { c r c } } ) \leq \alpha . } \end{array}$ + +# B.3. Proof of Theorem 4.1 + +In order to prove Theorem 4.1, we will need to make use of two auxiliary propositions (Proposition B.1 and Proposition B.2). +We state and prove these first, and then proceed to prove Theorem 4.1. + +Proposition B.1. Consider the following variational maximization problem: + +$$ +I [ f ] = \int _ { a } ^ { b } f ( x ) d x +$$ + +subject to $f ( a ) = f _ { a } , f ( b ) = f _ { b }$ , and $f _ { a } \leq f ( x ) \leq f _ { b }$ for all $x \in [ a , b ]$ , where $f _ { a } \leq f _ { b }$ . Then $I [ f ]$ is maximized by + +$$ +f ^ { * } ( x ) = { \left\{ \begin{array} { l l } { f _ { a } } & { i f x = a , } \\ { f _ { b } } & { o t h e r w i s e , } \end{array} \right. } +$$ + +and $I [ f ^ { * } ] = ( b - a ) f _ { b }$ + +Proof. We apply Euler’s method (Kot, 2014, Section 2.2), which approximates the variational problem as an $m$ -dimensional problem and takes the limit as $m \infty$ . Let the interval $[ a , b ]$ be divided into $m { + 1 }$ subintervals of equal width $\Delta x = { \frac { b - a } { m + 1 } }$ . The objective functional can then be approximated as + +$$ +I ( f _ { 1 } , \ldots , f _ { m } ) \equiv \sum _ { j = 0 } ^ { m } f _ { j } \Delta x , +$$ + +where $f _ { 0 } = f _ { a }$ and $f _ { m + 1 } = f _ { b }$ due to the boundary conditions. In order to handle the $f _ { a } \leq f ( x ) \leq f _ { b }$ constraint, we first impose $f ( x ) \leq f _ { b }$ and check if the solution also satisfies $f ( x ) \geq f _ { a }$ . To that end, we substitute $f _ { j } = f _ { b } - \xi _ { j } ^ { 2 }$ : + +$$ +I ( \xi _ { 1 } , \ldots , \xi _ { m } ) = \sum _ { j = 0 } ^ { m } ( f _ { b } - \xi _ { j } ^ { 2 } ) \Delta x . +$$ + +We then take partial derivatives with respect to $\xi _ { k }$ : + +$$ +\frac { \partial I } { \partial \xi _ { k } } = - 2 \xi _ { k } \Delta x \Rightarrow \frac { 1 } { \Delta x } \frac { \partial I } { \partial \xi _ { k } } = - 2 \xi _ { k } . +$$ + +Taking the limit as $m \infty$ and $\Delta x 0$ , the variational derivative becomes: + +$$ +\frac { \delta I } { \delta \xi } = - 2 \xi . +$$ + +Setting $\frac { \delta I } { \delta \xi } = 0$ yields $\xi ( x ) = 0$ , which recovers $f ( x ) = f _ { b }$ , except at $x = a$ , where $f ( a ) = f _ { a }$ by the boundary conditions. This recovers $f ^ { * } ( x )$ from (45), which indeed satisfies $f ( x ) \geq f _ { a }$ . For $f ^ { * }$ , it is evident that the value of the value of the functional is $I [ f ^ { * } ] = ( b - a ) f _ { b }$ . □ + +Proposition B.2. Let $\kappa _ { n }$ be the set of quantile functions for which $K ( t _ { i } ) = \ell _ { i }$ for $i = 1 , \ldots , n$ . Then + +$$ +\operatorname* { s u p } _ { K \in { \mathcal { K } } _ { n } } J [ K ] = \sum _ { i = 1 } ^ { n + 1 } ( t _ { ( i ) } - t _ { ( i - 1 ) } ) \ell _ { ( i ) } , +$$ + +where $t _ { ( 0 ) } = 0 , t _ { ( n + 1 ) } = 1 , \ell _ { ( n + 1 ) } = B ,$ , and $\textstyle J [ K ] \triangleq \int _ { 0 } ^ { 1 } K ( t ) d t$ + +Proof. By Proposition A.2, quantile functions preserve orderings and therefore $K ( t _ { ( i ) } ) = \ell _ { ( i ) }$ . We divide $J [ K ]$ into intervals with endpoints $( 0 , t _ { ( 1 ) } ) , ( t _ { ( 1 ) } , t _ { ( 2 ) } ) , \ldots , ( t _ { ( n ) } , 1 )$ : + +$$ +\begin{array} { l } { \displaystyle \operatorname* { s u p } _ { K \in { \mathcal { K } } _ { n } } J [ K ] = \displaystyle \operatorname* { s u p } _ { K \in { \mathcal { K } } _ { n } } \int _ { 0 } ^ { 1 } K ( t ) d t } \\ { = \displaystyle \operatorname* { s u p } _ { K \in { \mathcal { K } } _ { n } } \sum _ { i = 1 } ^ { n + 1 } \int _ { t _ { ( i - 1 ) } } ^ { t _ { ( i ) } } K ( t ) d t } \\ { \displaystyle \quad \leq \sum _ { i = 1 } ^ { n + 1 } \displaystyle \operatorname* { s u p } _ { K \in { \mathcal { K } } _ { n } } \int _ { t _ { ( i - 1 ) } } ^ { t _ { ( i ) } } K ( t ) d t } \end{array} +$$ + +By Proposition A.2, $K ( t _ { ( i - 1 ) } ) \leq K ( t ) \leq K ( t _ { ( i ) } )$ for any $t \in [ t _ { ( i - 1 ) } , t _ { ( i ) } ]$ . We view each term as a variational subproblem where Ji[Ki] ≜ $J _ { i } [ K _ { i } ] \triangleq \int _ { t _ { ( i - 1 ) } } ^ { t _ { ( i ) } } K _ { i } ( t ) d t$ with boundary conditions $K _ { i } ( t _ { ( i - 1 ) } ) = \ell _ { ( i - 1 ) }$ and $K _ { i } ( t _ { ( i ) } ) = \ell _ { ( i ) }$ . We therefore appeal to Proposition B.1 to conclude that + +$$ +K _ { i } ^ { * } ( t ) = { \left\{ \begin{array} { l l } { \ell _ { ( i - 1 ) } } & { { \mathrm { i f ~ } } t = t _ { ( i - 1 ) } , } \\ { \ell _ { ( i ) } } & { { \mathrm { o t h e r w i s e } } , } \end{array} \right. } +$$ + +and $J [ K _ { i } ^ { * } ] = ( t _ { ( i ) } - t _ { ( i - 1 ) } ) \ell _ { ( i ) }$ . We therefore have + +$$ +\operatorname* { s u p } _ { K \in { \mathcal { K } } _ { n } } J [ K ] \leq \sum _ { i = 1 } ^ { n + 1 } ( t _ { ( i ) } - t _ { ( i - 1 ) } ) \ell _ { ( i ) } . +$$ + +By composing $K _ { i } ^ { * }$ from each subinterval, it is straightforward to see that the bound is tight for + +$$ +\begin{array} { r } { K _ { t _ { 1 : n } , \ell _ { 1 : n } } ^ { * } ( t ) = \left\{ \begin{array} { l l } { \ell _ { ( 1 ) } } & { \mathrm { ~ i f ~ } t \leq t _ { ( 1 ) } } \\ { \ell _ { ( 2 ) } } & { \mathrm { ~ i f ~ } t _ { ( 1 ) } < t \leq t _ { ( 2 ) } } \\ { \dots } \\ { \ell _ { ( n ) } } & { \mathrm { ~ i f ~ } t _ { ( n - 1 ) } < t \leq t _ { ( n ) } } \\ { B } & { \mathrm { ~ i f ~ } t > t _ { ( n ) } . } \end{array} \right. } \end{array} +$$ + +K∗t1:n,ℓ1:n is therefore the “worst-case” quantile function that is consistent with the observations, and $J [ K _ { t _ { 1 : n } , \ell _ { 1 : n } } ^ { * } ] =$ $\begin{array} { r l } { \sum _ { i = 1 } ^ { n + 1 } ( t _ { ( i ) } - t _ { ( i - 1 ) } ) \ell _ { ( i ) } } & { { } } \end{array}$ . + +We are now ready to prove Theorem 4.1. + +Theorem 4.1. Let $t _ { ( 0 ) } = 0$ , $t _ { ( n + 1 ) } = 1$ , and $\ell _ { ( n + 1 ) } = B$ . Then + +$$ +\operatorname* { s u p } _ { \pi } E ( L \mid t _ { 1 : n } , \ell _ { 1 : n } ) \leq \sum _ { i = 1 } ^ { n + 1 } u _ { i } \ell _ { ( i ) } , +$$ + +where $u _ { i } = t _ { ( i ) } - t _ { ( i - 1 ) }$ . + +Proof. Let $\textstyle J [ K ] = \int _ { 0 } ^ { 1 } K ( t ) d t$ . The conditional expected loss can be expressed as: + +$$ +\begin{array} { l } { { \displaystyle E ( L \mid t _ { 1 : n } , \ell _ { 1 : n } ) = \int J [ K ] p ( K \mid t _ { 1 : n } , \ell _ { 1 : n } ) } d K } \\ { { \displaystyle \le \operatorname* { s u p } _ { K \in { \cal K } _ { n } } J [ K ] , } } \end{array} +$$ + +where $\kappa _ { n }$ is the set of quantile functions for which $K ( t _ { i } ) = \ell _ { i }$ for $i = 1 , \ldots , n$ . By Proposition B.2, it follows that + +$$ +E ( L \mid t _ { 1 : n } , \ell _ { 1 : n } ) \leq \sum _ { i = 1 } ^ { n + 1 } ( t _ { ( i ) } - t _ { ( i - 1 ) } ) \ell _ { ( i ) } = \sum _ { i = 1 } ^ { n + 1 } u _ { i } \ell _ { ( i ) } +$$ + +# B.4. Proof of Lemma 4.2 + +Lemma 4.2 (Distribution of Quantile Spacings (Aitchison & Dunsmore, 1975, p. 140)). Suppose that $\ell _ { 1 } , \ldots , \ell _ { n }$ are drawn i.i.d. with continuous4 distribution function $F$ . Let $t _ { i } = F ( \ell _ { i } )$ and $u _ { i } = t _ { ( i ) } - t _ { ( i - 1 ) }$ , where by convention $t _ { ( 0 ) } = 0$ and $t _ { ( n + 1 ) } = 1$ . Then $( u _ { 1 } , u _ { 2 } , \ldots , u _ { n + 1 } ) \cong \operatorname { D i r } ( 1 , \ldots , 1 )$ . + +Proof. By the probability integral transformation (Proposition A.3), $T _ { i }$ is Uniform $( 0 , 1 )$ for $i = 1 , \ldots , n$ . Since the transformation from $( t _ { 1 } , \ldots , t _ { n } ) \to ( t _ { ( 1 ) } , \ldots , t _ { ( n ) } )$ is a sorting operation where $n !$ permutations map to the same vector of order statistics, the probability density for $t _ { ( 1 ) } , \ldots , t _ { ( n ) }$ is therefore + +$$ +f _ { t _ { ( 1 : n ) } } ( t _ { ( 1 ) } , \ldots , t _ { ( n ) } ) = n ! , \qquad 0 \leq t _ { ( 1 ) } \leq t _ { ( 2 ) } \leq \ldots \leq t _ { ( n ) } \leq 1 . +$$ + +If $\begin{array} { r } { u _ { 1 : n } = G \big ( t _ { ( 1 : n ) } \big ) } \end{array}$ where $G$ is differentiable and invertible, then by change of variables the density for $u _ { 1 : n }$ can be expressed as + +$$ +f _ { u _ { 1 : n } } ( u _ { 1 : n } ) = f _ { t _ { ( 1 : n ) } } ( G ^ { - 1 } ( u _ { 1 : n } ) ) \left| \operatorname* { d e t } \left( { \frac { \partial } { \partial u _ { 1 : n } } } G ^ { - 1 } ( u _ { 1 : n } ) \right) \right| . +$$ + +Observe that the inverse transformation $t _ { ( 1 : n ) } = G ^ { - 1 } ( u _ { 1 : n } )$ can be expressed as + +$$ +{ \left[ \begin{array} { l } { t _ { ( 1 ) } } \\ { t _ { ( 2 ) } } \\ { t _ { ( 3 ) } } \\ { \vdots } \\ { t _ { ( n - 1 ) } } \\ { t _ { ( n ) } } \end{array} \right] } = { \left[ \begin{array} { l l l l l l } { 1 } & { 0 } & { 0 } & { \ldots } & { 0 } & { 0 } \\ { 1 } & { 1 } & { 0 } & { \ldots } & { 0 } & { 0 } \\ { 1 } & { 1 } & { 1 } & { \ldots } & { 0 } & { 0 } \\ { \vdots } & { \vdots } & { \vdots } & { \ddots } & { \vdots } & { \vdots } \\ { 1 } & { 1 } & { 1 } & { \ldots } & { 1 } & { 0 } \\ { 1 } & { 1 } & { 1 } & { \ldots } & { 1 } & { 1 } \end{array} \right] } { \left[ \begin{array} { l } { u _ { 1 } } \\ { u _ { 2 } } \\ { u _ { 3 } } \\ { \vdots } \\ { u _ { n - 1 } } \\ { u _ { n } } \end{array} \right] } . +$$ + +Hence the absolute Jacobian of inverse transformation $t _ { ( 1 : n ) } = G ^ { - 1 } ( u _ { 1 : n } )$ is 1. The density of $u _ { 1 : n }$ is therefore + +$$ +f _ { u _ { 1 : n } } ( u _ { 1 : n } ) = f _ { t _ { ( 1 : n ) } } ( G ^ { - 1 } ( u _ { 1 : n } ) ) = n ! , \quad { \mathrm { ~ w h e r e ~ } } u _ { i } \geq 0 { \mathrm { ~ f o r ~ } } i = 1 , \dots , n { \mathrm { ~ a n d ~ } } \sum _ { i = 1 } ^ { n } u _ { i } \leq 1 . +$$ + +Recall that the Dirichlet density with parameter $\alpha _ { 1 } , \ldots , \alpha _ { n + 1 }$ is: + +$$ +\operatorname { D i r } ( u _ { 1 : n + 1 } \mid \alpha _ { 1 : n + 1 } ) = { \frac { \Gamma ( \sum _ { i = 1 } ^ { n + 1 } \alpha _ { i } ) } { \Gamma ( \alpha _ { 1 } ) \dots \Gamma ( \alpha _ { n + 1 } ) } } \prod _ { i = 1 } ^ { n + 1 } u _ { i } ^ { \alpha _ { i } - 1 } , \quad { \mathrm { ~ w h e r e ~ } } u _ { i } \geq 0 { \mathrm { ~ a n d ~ } } \sum _ { i = 1 } ^ { n + 1 } u _ { i } = 1 . +$$ + +In particular, if $\alpha _ { 1 } = \alpha _ { 2 } = . . . = \alpha _ { n + 1 } = 1$ , + +$$ +\operatorname { D i r } ( u _ { 1 : n + 1 } \mid 1 , \ldots , 1 ) = \Gamma ( n + 1 ) = n ! , +$$ + +which is identical to (63) with $u _ { n + 1 } = 1 - u _ { 1 } - . . . - u _ { n }$ . Therefore, $( u _ { 1 } , u _ { 2 } , \ldots , u _ { n + 1 } ) \cong \mathrm { D i r } ( 1 , \ldots , 1 ) .$ . + +# B.5. Proof of Theorem 4.3 + +Theorem 4.3. Define $\ell _ { ( i ) }$ to be the order statistics of $\ell _ { 1 } , \ldots , \ell _ { n }$ for $i = 1 , \ldots , n$ and $\ell _ { ( n + 1 ) } \triangleq B$ . Let $L ^ { + }$ be the random variable defined as follows: + +$$ +U _ { 1 } , \ldots , U _ { n + 1 } \sim \mathrm { D i r } ( 1 , \ldots , 1 ) , L ^ { + } = \sum _ { i = 1 } ^ { n + 1 } U _ { i } \ell _ { ( i ) } . +$$ + +Then for any $b \in ( - \infty , B ]$ , + +$$ +\operatorname* { i n f } _ { \pi } \operatorname* { P r } ( L \leq b \mid \ell _ { 1 : n } ) \geq \operatorname* { P r } ( L ^ { + } \leq b ) . +$$ + +Proof. + +$$ +\begin{array} { r l } { \frac { 1 } { 2 \pi } \mathbb { P } ( \{ L , \hat { \leq } \hat { \mathbf { k } } \} | \hat { \leq } , \hat { \mathbf { u } } ) = \frac { 1 } { \pi ^ { 6 } } \int \int \{ \mathcal { B } [ \hat { \leq } \hat { \mathbf { k } } ] \leq \mathcal { B } [ \hat { \leq } \hat { \mathbf { k } } ] , \hat { \leq } \hat { \mathcal { B } } [ \hat { \leq } \hat { } \hat { \mathbf { k } } ] , } \\ & { = \frac { 1 } { \pi ^ { 6 } } \int \Big [ \mathcal { B } [ \hat { \leq } \hat { } \hat { \mathbf { k } } ] \leq \mathcal { B } \Big ( \int \int \mathcal { B } ^ { \mathrm { C } } [ \hat { \leq } \hat { \mathbf { k } } ] , [ \hat { \leq } \hat { \mathcal { B } } ] \leq \hat { \mathcal { B } } \Big ( \int \hat { \nu } _ { 0 } \hat { \leq } \hat { \mathcal { B } } [ \hat { \leq } \hat { \mathcal { B } } ] , [ \hat { \nu } ] , [ \hat { \nu } ] , \hat { \leq } \hat { \mathcal { B } } [ \hat { \leq } ] \Big ) d _ { 1 } } \\ & { = \frac { 1 } { \pi ^ { 6 } } \int \Big [ \Big ( \int \big [ \mathcal { B } [ \hat { \leq } \hat { \mathcal { B } } ] \big [ \hat { \leq } \hat { \mathcal { B } } [ \hat { \leq } \hat { \mathcal { B } } ] , [ \hat { \nu } ] , [ \hat { \nu } ] , [ \hat { \nu } ] , [ \hat { \nu } ] , [ \hat { \nu } ] , [ \hat { \nu } ] , [ \hat { \nu } ] \big ) d _ { 1 } } \\ & \geq \int \bigg ( \frac { 1 } { \pi ^ { 6 } } \int \big [ \mathcal { B } [ \hat { \leq } \hat { \mathcal { B } } ] \big [ \hat { \leq } \hat { \mathcal { B } } [ \hat { \leq } \hat { \mathcal { B } } ] , [ \hat { \nu } ] , [ \hat { \nu } ] , [ \hat { \nu } ] , [ \hat { \nu } ] \big ) \end{array} +$$ + +# B.6. Proof of Corollary 4.4 + +Corollary 4.4. For any desired confidence level $\beta \in ( 0 , 1 )$ , define + +$$ +b _ { \beta } ^ { * } = \operatorname* { i n f } _ { b } \{ b : \operatorname* { P r } ( L ^ { + } \leq b \mid \ell _ { 1 : n } ) \geq \beta \} . +$$ + +Then $\operatorname* { i n f } _ { \pi } \operatorname* { P r } ( L \leq b \mid \ell _ { 1 : n } ) \geq \beta$ for any $b \geq b _ { \beta } ^ { * }$ . + +Proof. For any $b \geq b _ { \beta } ^ { * }$ , $\operatorname* { P r } ( L ^ { + } \leq b \mid \ell _ { 1 : n } ) \geq \beta$ . Substitution into (28) provides the desired result. \ No newline at end of file diff --git a/papers/conformal-bayesian-quadrature/paper.pdf b/papers/conformal-bayesian-quadrature/paper.pdf new file mode 100644 index 0000000000000000000000000000000000000000..a31a817f281db1f8456defaa87f99ed72809eb4a --- /dev/null +++ b/papers/conformal-bayesian-quadrature/paper.pdf @@ -0,0 +1,3 @@ +version https://git-lfs.github.com/spec/v1 +oid sha256:774c7cbbe8bba95746892da3e7ecafe6aadda50e0f40a99382c50337202585fb +size 911027 diff --git a/papers/conformal-bayesian-quadrature/sau.json b/papers/conformal-bayesian-quadrature/sau.json new file mode 100644 index 0000000000000000000000000000000000000000..8c5e4aeabccd0dfcf925593bcc1f7fd9c4c84ad7 --- /dev/null +++ b/papers/conformal-bayesian-quadrature/sau.json @@ -0,0 +1,202 @@ +{ + "paper_id": "conformal-bayesian-quadrature", + "paper_title": "Conformal Prediction as Bayesian Quadrature", + "D1": [ + { + "id": "conformal-bayesian-quadrature-D1-001", + "claim": "Number of random data splits/trials for Monte Carlo evaluation across all experiments = 10000", + "source": "Section 5, paragraph 1" + }, + { + "id": "conformal-bayesian-quadrature-D1-002", + "claim": "Confidence level for the HPD interval; target failure rate is 1 - beta = 0.05 = 0.95", + "source": "Section 5, paragraph 1" + }, + { + "id": "conformal-bayesian-quadrature-D1-003", + "claim": "1 - beta = 0.05; maximum acceptable rate of trials exceeding the risk threshold = 0.05", + "source": "Section 5, paragraph 1" + }, + { + "id": "conformal-bayesian-quadrature-D1-004", + "claim": "Number of Dirichlet Monte Carlo samples for estimating Pr(L^+ <= alpha) in the HPD decision rule computation = 1000", + "source": "Section 5, paragraph 2 (Eq 33 context)" + }, + { + "id": "conformal-bayesian-quadrature-D1-005", + "claim": "Method for computing 95% confidence intervals on binomial proportions of failure rate = Clopper-Pearson", + "source": "Table notes in Section 5" + }, + { + "id": "conformal-bayesian-quadrature-D1-006", + "claim": "Confidence level for Clopper-Pearson intervals on reported relative frequencies = 0.95", + "source": "Table notes in Section 5" + }, + { + "id": "conformal-bayesian-quadrature-D1-007", + "claim": "Number of calibration samples in the synthetic binomial experiment = 10", + "source": "Section 5.1, paragraph 1" + }, + { + "id": "conformal-bayesian-quadrature-D1-008", + "claim": "Number of binomial trials per sample in the loss function: l(z_i, lambda) = (1/K) * sum_{k=1}^K 1{V_{ik} > lambda} = 4", + "source": "Section 5.1, Eq 34" + }, + { + "id": "conformal-bayesian-quadrature-D1-009", + "claim": "Target risk threshold; decision rule lambda must satisfy R(theta, lambda) <= 0.4 = 0.4", + "source": "Section 5.1, paragraph 1" + }, + { + "id": "conformal-bayesian-quadrature-D1-010", + "claim": "Maximum possible loss, normalized to 1; used as l_{(n+1)} = B in L^+ construction = 1", + "source": "Section 5.1, paragraph 1" + }, + { + "id": "conformal-bayesian-quadrature-D1-011", + "claim": "Value of lambda achieving zero loss; the problem is achievable since there exists a lambda_max satisfying the guarantee = 1", + "source": "Section 5.1, paragraph 1" + }, + { + "id": "conformal-bayesian-quadrature-D1-012", + "claim": "Derived from E[l] = 1 - lambda and alpha = 0.4: any trial with lambda < 0.6 has expected loss > 0.4, thus constitutes excessive risk = 0.6", + "source": "Section 5.1, paragraph 2" + }, + { + "id": "conformal-bayesian-quadrature-D1-013", + "claim": "Distribution of V_{ik} used to generate the binomial loss: V_{ik} ~ Uniform(0, 1) = Uniform(0, 1)", + "source": "Section 5.1, Eq 34" + }, + { + "id": "conformal-bayesian-quadrature-D1-014", + "claim": "Number of Dirichlet samples used for estimating the L^+ density histogram in Figure 4 = 100000", + "source": "Section 5.1, paragraph 3" + }, + { + "id": "conformal-bayesian-quadrature-D1-015", + "claim": "Three lambda settings used for visualizing the L^+ distribution in Figure 4 = [0.7, 0.8, 0.9]", + "source": "Section 5.1, paragraph 3; Figure 4" + }, + { + "id": "conformal-bayesian-quadrature-D1-016", + "claim": "Number of calibration samples per trial in the synthetic heteroskedastic experiment = 200", + "source": "Section 5.2, paragraph 1" + }, + { + "id": "conformal-bayesian-quadrature-D1-017", + "claim": "Target miscoverage risk; equivalent to 90% coverage target = 0.1", + "source": "Section 5.2, paragraph 1" + }, + { + "id": "conformal-bayesian-quadrature-D1-018", + "claim": "Lower bound of input distribution X ~ Uniform(0, 4) = 0", + "source": "Section 5.2, paragraph 1" + }, + { + "id": "conformal-bayesian-quadrature-D1-019", + "claim": "Upper bound of input distribution X ~ Uniform(0, 4) = 4", + "source": "Section 5.2, paragraph 1" + }, + { + "id": "conformal-bayesian-quadrature-D1-020", + "claim": "Y | X ~ N(0, X^2); heteroskedastic noise with variance proportional to X^2 = N(0, X^2)", + "source": "Section 5.2, paragraph 1" + }, + { + "id": "conformal-bayesian-quadrature-D1-021", + "claim": "Symmetric prediction intervals centered at 0; lambda_hat is the threshold selected by each method = [-lambda_hat, lambda_hat]", + "source": "Section 5.2, paragraph 1" + }, + { + "id": "conformal-bayesian-quadrature-D1-022", + "claim": "Loss is the miscoverage loss (0-1 loss for whether the true Y falls outside the prediction interval) = miscoverage", + "source": "Section 5.2, paragraph 1" + }, + { + "id": "conformal-bayesian-quadrature-D1-023", + "claim": "Number of calibration examples per random split in the MS-COCO experiment = 1000", + "source": "Section 5.3, paragraph 1" + }, + { + "id": "conformal-bayesian-quadrature-D1-024", + "claim": "Number of test examples per random split in the MS-COCO experiment = 3952", + "source": "Section 5.3, paragraph 1" + }, + { + "id": "conformal-bayesian-quadrature-D1-025", + "claim": "Task type for MS-COCO: controlling false negative rate in multilabel classification = multilabel_classification", + "source": "Section 5.3, paragraph 1" + } + ], + "D2": [ + { + "id": "conformal-bayesian-quadrature-D2-001", + "claim": "Split conformal prediction decision rule: lambda_scp = s_{(ceil((n+1)*(1-alpha)))} when ceil((n+1)*(1-alpha)) <= n, otherwise lambda_scp = infinity. This is an alpha-acceptable decision rule for the miscoverage loss L_scp(theta, lambda) = Pr{s(z_new) > lambda}.", + "source": "Section 3.1, Proposition 3.1, Eq 12" + }, + { + "id": "conformal-bayesian-quadrature-D2-002", + "claim": "Conformal risk control decision rule: lambda_crc = inf{lambda: (1/(n+1)) * (sum_{i=1}^n l(z_i, lambda) + B) <= alpha}, where l(z, lambda) is monotonically non-increasing in lambda and B is the maximum possible loss.", + "source": "Section 3.2, Proposition 3.2, Eq 15" + }, + { + "id": "conformal-bayesian-quadrature-D2-003", + "claim": "Full algorithm: lambda_hpd^beta = inf{lambda: Pr(L^+ <= alpha | l_{1:n}(lambda)) >= beta}. For each candidate lambda (descending search): (1) compute calibration losses l_i(lambda); (2) sort losses ascending; (3) append B as l_{(n+1)}; (4) sample Dirichlet U ~ Dir(1,...,1); (5) compute L^+ = sum U_i * l_(i); (6) estimate Pr(L^+ <= alpha) via Monte Carlo; (7) if prob >= beta, return lambda. Implemented with 1000 Dirichlet MC samples.", + "source": "Section 5, Eq 33" + }, + { + "id": "conformal-bayesian-quadrature-D2-004", + "claim": "Constructs the prediction set or interval from the selected threshold lambda: C(x_new) = {y in Y: s(x_new, y) <= lambda}. For a test input x_new, collect all candidate outputs y whose nonconformity score s(x_new, y) is at most lambda.", + "source": "Section 2.1, Eq 2; Section 3.1, Eq 13" + }, + { + "id": "conformal-bayesian-quadrature-D2-005", + "claim": "Constructs the random variable L^+ that stochastically dominates the posterior risk. Sample U_1,...,U_{n+1} ~ Dir(1,...,1) and compute L^+ = sum_{i=1}^{n+1} U_i * l_(i). The distribution of L^+ provides a distribution-free upper bound on posterior expected loss. Implement via Monte Carlo: for each Dirichlet sample, compute weighted sum of sorted losses with B appended as l_{(n+1)}.", + "source": "Section 4.5, Theorem 4.3, Eq 27" + }, + { + "id": "conformal-bayesian-quadrature-D2-006", + "claim": "Loss function for the synthetic binomial experiment (Section 5.1). Computes the fraction of K uniform random draws exceeding threshold lambda. The loss is monotonically non-increasing in lambda, bounded by B=1, and achieves zero loss at lambda_max=1.", + "source": "Section 5.1, Eq 34" + } + ], + "D3": [ + { + "id": "conformal-bayesian-quadrature-D3-001", + "claim": "Synthetic Binomial benchmark (Section 5.1): Demonstrate that CRC marginal guarantee can lead to high frequency of individual trials exceeding target risk; validate that the HPD approach controls per-trial risk at specified confidence level. The loss is a scaled binomial l(z_i, lambda) = (1/K) * sum_{k=1}^K 1{V_{ik} > lambda} with V_{ik} ~ Uniform(0,1), n=10, K=4, alpha=0.4, B=1. Since E[l] = 1 - lambda, risk threshold is exactly known (lambda < 0.6 = excessive risk). Three methods compared across M=10,000 random splits, evaluating relative frequency of risk exceedance with 95% Clopper-Pearson CI: CRC 21.20% [20.40%, 22.01%], RCPS (Hoeffding UCB) 0.00% [0.00%, 0.04%], Ours (HPD, beta=0.95) 0.03% [0.01%, 0.09%]. Mean risk across all trials: CRC 0.3363 +/- 0.0007 vs Ours 0.1758 +/- 0.0006. L^+ distribution histograms visualized for lambda in {0.7, 0.8, 0.9} (Figure 4, 100,000 Dirichlet samples).", + "source": "Section 5.1, Table 1, Figure 3, Figure 4" + }, + { + "id": "conformal-bayesian-quadrature-D3-002", + "claim": "Synthetic Heteroskedastic benchmark (Section 5.2): Compare methods under input-dependent noise X ~ Uniform(0,4), Y|X ~ N(0, X^2) with n=200 calibration samples per trial across M=10,000 random splits. Prediction intervals are symmetric [-lambda_hat, lambda_hat]; loss is miscoverage (0-1); alpha=0.1 (90% coverage target); target failure rate <= 5% (beta=0.95). Three methods compared on relative frequency of risk exceedance (95% Clopper-Pearson CI) and mean prediction interval length: SCP/CRC 46.19% [45.21%, 47.17%], interval 7.99; RCPS (Hoeffding UCB) 0.0% [0.0%, 0.04%], interval 14.29; Ours (HPD, beta=0.95) 3.42% [3.07%, 3.80%], interval 9.50. Evaluate whether HPD achieves target failure rate (<= 5%) while producing shorter prediction intervals than RCPS: RCPS under-control (0.0%, interval 14.29) but conservative; Ours achieves 3.42% (within 5% target) with 33.5% shorter intervals than RCPS (9.50 vs 14.29).", + "source": "Section 5.2, Table 2" + }, + { + "id": "conformal-bayesian-quadrature-D3-003", + "claim": "MS-COCO real-world benchmark (Section 5.3): Evaluate methods on multilabel classification false negative rate control (fraction of ground-truth labels not in prediction set). Setup mirrors Angelopoulos & Bates (2023, Section 5.1); each of M=10,000 random splits contains 1000 calibration and 3952 test examples. Three methods compared on relative frequency of risk exceedance and average prediction set size: CRC 45.05%, set size 2.92; RCPS (Hoeffding UCB) 0.0%, set size 3.57; Ours (HPD, beta=0.95) 5.43%, set size 3.04. CRC marginal guarantee allows 45.05% of trials to exceed risk threshold with smallest sets (2.92). RCPS controls risk perfectly (0.0%) but produces largest sets (3.57). Ours achieves near-target failure rate (5.43%, closest to 5%) with intermediate set size (3.04), balancing risk control and efficiency.", + "source": "Section 5.3, Table 3" + } + ], + "D4": [ + { + "id": "conformal-bayesian-quadrature-D4-001", + "claim": "Paper-specified theoretical development ordering: Section 3 (Decision-theoretic formulation: define risk R(theta, lambda), maximum risk Rbar(lambda), alpha-acceptable decision rules) -> Section 3.1 (Recover split conformal prediction as special case, Proposition 3.1) -> Section 3.2 (Recover conformal risk control as special case, Proposition 3.2) -> Section 4.1 (Introduce Bayes risk via integrated risk r(pi, lambda) and worst-case integrated risk rbar(lambda) = Rbar(lambda)) -> Section 4.2 (Reformulate as Bayesian quadrature: model quantile function K(t), posterior p(K|l_{1:n})) -> Section 4.3 (Eliminate prior: derive worst-case bound sup_pi E(L|t_{1:n},l_{1:n}) <= sum u_i * l_(i), Theorem 4.1) -> Section 4.4 (Handle input noise: apply quantile spacings Dirichlet distribution, Lemma 4.2) -> Section 4.5 (Combine: L^+ stochastic dominance and confidence bound b_beta*, Theorem 4.3 + Corollary 4.4; key output: L^+ bound feeds the HPD decision rule in Section 5) -> Section 4.6 (Recover CRC and SCP by taking expectation of L^+).", + "source": "Section 3 through Section 4.6" + }, + { + "id": "conformal-bayesian-quadrature-D4-002", + "claim": "HPD decision rule computation procedure (per trial): (1) For candidate lambda, compute calibration losses l(z_i, lambda) for i=1..n; (2) Sort losses ascending: l_{(1)}, ..., l_{(n)}; (3) Append B as l_{(n+1)}; (4) Monte Carlo: sample U ~ Dir(1,...,1) repeatedly, compute L^+ = sum U_i * l_{(i)} each time; (5) Estimate Pr(L^+ <= alpha) as empirical fraction of samples where L^+ <= alpha; (6) Select lowest lambda such that Pr(L^+ <= alpha) >= beta; (7) Form prediction set C(x_new) = {y: s(x_new, y) <= lambda}. Parameter setting: 1000 Dirichlet samples for HPD, 100000 for visualization histograms.", + "source": "Section 4.5, Eq 27-29; Section 5, Eq 33; Figure 2" + }, + { + "id": "conformal-bayesian-quadrature-D4-003", + "claim": "Experimental evaluation pipeline ordering: (1) Synthetic Binomial (Section 5.1): known loss distribution E[l]=1-lambda, n=10, K=4, alpha=0.4, directly verifiable risk threshold; (2) Synthetic Heteroskedastic (Section 5.2): X~Uniform(0,4), Y|X~N(0,X^2), n=200, alpha=0.1, symmetric intervals; (3) MS-COCO (Section 5.3): real-world multilabel classification, n=1000 calibration + 3952 test, false negative rate control. All experiments: M=10000 random splits, compute CRC/RCPS/HPD decision rules, evaluate relative frequency of risk exceedance and prediction set size. Data flow per experiment: calibration split -> lambda threshold (via CRC/RCPS/HPD) -> test split evaluation -> risk exceedance frequency + set size reporting.", + "source": "Section 5, Section 5.1, Section 5.2, Section 5.3" + }, + { + "id": "conformal-bayesian-quadrature-D4-004", + "claim": "Baseline methods comparison order (per experiment/trial): (1) Conformal Risk Control: compute lambda_crc = inf{lambda: (1/(n+1))(sum l_i + B) <= alpha} using only expected value of L^+; (2) RCPS with Hoeffding UCB: compute lambda via upper confidence bound on empirical risk; (3) Ours (HPD, beta=0.95): compute lambda_hpd via the 7-step Monte Carlo Dirichlet HPD procedure. Source for CRC: Section 3.2 Eq 15. Source for RCPS: Bates et al. (2021). Source for HPD: Section 5 Eq 33. Metric reporting: relative freq of risk exceedance with 95% Clopper-Pearson CI, prediction set size/interval length.", + "source": "Section 5, Table 1, Table 2, Table 3" + } + ] +} \ No newline at end of file diff --git a/papers/diffusion-convergence-rate/blacklist.txt b/papers/diffusion-convergence-rate/blacklist.txt new file mode 100644 index 0000000000000000000000000000000000000000..27e45b649fe259fc3808d3e12235a4c265da943d --- /dev/null +++ b/papers/diffusion-convergence-rate/blacklist.txt @@ -0,0 +1,2 @@ +# No public official repository found (theoretical paper, arXiv:2410.13738) +# Authors: Gen Li, Yuchen Jiao diff --git a/papers/diffusion-convergence-rate/config.yaml b/papers/diffusion-convergence-rate/config.yaml new file mode 100644 index 0000000000000000000000000000000000000000..4e788afa4d3fc73893a36688ab270347fe62ca37 --- /dev/null +++ b/papers/diffusion-convergence-rate/config.yaml @@ -0,0 +1,8 @@ +title: "Improved Convergence Rate for Diffusion Probabilistic Models" +pdf_url: "https://arxiv.org/pdf/2410.13738.pdf" +venue: "ICLR 2025" +year: "2025" +extra: + selection_index: 20 + domain: "Probabilistic Inference / Generative Models" + paradigm: "Theoretical Analysis" diff --git a/papers/diffusion-convergence-rate/images/figures/diffusion-convergence-rate-fig-0001.jpg b/papers/diffusion-convergence-rate/images/figures/diffusion-convergence-rate-fig-0001.jpg new file mode 100644 index 0000000000000000000000000000000000000000..c9d95771afadc2d40a5ea806ebd07ca1b010eed4 --- /dev/null +++ b/papers/diffusion-convergence-rate/images/figures/diffusion-convergence-rate-fig-0001.jpg @@ -0,0 +1,3 @@ +version https://git-lfs.github.com/spec/v1 +oid sha256:b55f1f450b2d4578992f441532cc67529a20418f7c10de2def87eabbfb671ef5 +size 45048 diff --git a/papers/diffusion-convergence-rate/images/figures/diffusion-convergence-rate-fig-0002.jpg b/papers/diffusion-convergence-rate/images/figures/diffusion-convergence-rate-fig-0002.jpg new file mode 100644 index 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+1,1993 @@ +# Instance-dependent Convergence Theory for Diffusion Models + +Yuchen Jiao ∗ Gen Li ∗ + +June 13, 2025 + +# Abstract + +Score-based diffusion models have demonstrated outstanding empirical performance in machine learning and artificial intelligence, particularly in generating high-quality new samples from complex probability distributions. Improving the theoretical understanding of diffusion models, with a particular focus on the convergence analysis, has attracted significant attention. In this work, we develop a convergence rate that is adaptive to the smoothness of different target distributions, referred to as instance-dependent bound. Specifically, we establish an iteration complexity of $\operatorname* { m i n } \{ d , d ^ { 2 / 3 } L ^ { 1 / 3 } , d ^ { 1 / 3 } L \} \varepsilon ^ { - 2 / 3 }$ (up to logarithmic factors), where $d$ denotes the data dimension, and $\varepsilon$ quantifies the output accuracy in terms of total variation (TV) distance. In addition, $L$ represents a relaxed Lipschitz constant, which, in the case of Gaussian mixture models, scales only logarithmically with the number of components, the dimension and iteration number, demonstrating broad applicability. + +# 1 Introduction + +Score-based diffusion models have emerged as a powerful class of generative models capable of synthesizing high-quality data from complex probability distributions (Dhariwal and Nichol, 2021; Ho et al., 2020; Sohl-Dickstein et al., 2015; Song et al., 2021; Song and Ermon, 2019). These models, including Denoising Diffusion Probabilistic Models (DDPM) (Ho et al., 2020) and Denoising Diffusion Implicit Models (DDIM) (Song et al., 2021), transforms pure noise into samples from the data distribution through an iterative denoising process. This process is facilitated by a series of score functions, which approximate the gradient of the data log-density using pretrained neural networks. In practice, diffusion models have achieved remarkable performance in various tasks including image generation (Ramesh et al., 2022; Rombach et al., 2022; Saharia et al., 2022), video generation (Villegas et al., 2022) and so on (Croitoru et al., 2023; Yang et al., 2023; Zhang et al., 2025). For an overview of recent developments, including both empirical and theoretical advancements, readers may refer to Croitoru et al. (2023); Tang and Zhao (2024); Yang et al. (2023). + +Diffusion models typically consist of two processes: the forward process and the reverse (or backward) process. The forward process is a simple stochastic process that progressively transforms a data sample $X _ { 0 }$ into nearly pure noise $X _ { T }$ by iteratively adding Gaussian noise: + +$$ +X _ { 0 } { \stackrel { \mathrm { a d d } } { } } { \stackrel { \mathrm { n o i s e } } { X _ { 1 } } } { \stackrel { \mathrm { a d d } } { } } \cdots { \stackrel { \mathrm { a d d } } { } } \cdots X _ { T } . +$$ + +Here, $X _ { 0 }$ is a $d$ -dimensional sample from the target data distribution , and $X _ { T }$ approximately follows a $p _ { \mathsf { d a t a } }$ standard Gaussian distribution $\mathcal { N } ( 0 , I _ { d } )$ . The core of diffusion models lies in the reverse process, which aims to learn a process that generates a sample resembling the target data distribution from pure Gaussian noise: + +$$ +{ Y _ { 0 } } \stackrel { \mathrm { d e n o i s e } } { } { Y _ { 1 } } \stackrel { \mathrm { d e n o i s e } } { } \cdot \cdot \cdot \stackrel { \mathrm { d e n o i s e } } { } { Y _ { T } } , +$$ + +Here, $Y _ { 0 }$ is initialized as white Gaussian noise, and the reverse process aims to achieve $Y _ { t } \overset { d } { \approx } X _ { T - t }$ for all $t$ , such that the distribution of the final output $Y _ { T }$ approximates the target distribution $p _ { \mathsf { d a t a } }$ . The most critical component of diffusion models is the efficient construction of this reverse process. To achieve this, diffusion models employ the time-reversal of stochastic differential equations (SDEs) to generate $Y _ { t + 1 }$ from $Y _ { t }$ . This process relies on score functions $( s _ { T - t } ^ { \star } = \nabla \log p _ { X _ { T - t } , }$ ), which are gradients of the log marginal density of the forward process $X _ { T - t }$ . These score functions are typically pretrained using score-matching techniques (Ho et al., 2020; Hyvärinen, 2007; Hyvärinen and Dayan, 2005; Pang et al., 2020; Song and Ermon, 2019; Vincent, 2011). + +Due to the impressive empirical success of diffusion models, a lot of efforts in recent years have focused on analyzing their convergence properties. Given the complexity of establishing a comprehensive end-toend theoretical framework, most studies adopt a divide-and-conquer approach, treating the score matching step as a black box and focusing on the data generation process. Following this framework, several studies have analyzed how factors such as data dimension, score estimation error, and the number of iterations affect the accuracy of approximating the target distribution (Benton et al., 2023; Chen et al., 2023, 2024b, 2022; De Bortoli, 2022; Gao et al., 2023; Gupta et al., 2024; Huang et al., 2024a,b,c; Lee et al., 2022, 2023; Li and Cai, 2024; Li et al., 2024a; Li and Jiao, 2024; Li et al., 2023, 2024b; Li and Yan, 2024a,b; Liang et al., 2025). For general data distributions, the best-known results are $\widetilde { O } ( d \varepsilon ^ { - 1 } )$ for DDPM (Li and Yan, 2024a) and $\widetilde { \cal O } ( d ^ { 5 / 4 } \varepsilon ^ { - 1 / 2 } )$ for an accelerated sampler (Li and Cai, 2024), where ${ \widetilde { O } } ( \cdot )$ omits logarithmic factors and $d$ denotes the data dimension. Some works exploited smoothness conditions to further improve the iteration complexity, with the state-of-the-art achieved by Li and Jiao (2024) is $\widetilde { \cal O } ( d ^ { 1 / 3 } L \varepsilon ^ { - 2 / 3 } )$ , where $L$ denotes the Lipschitz constant of score functions. However, this improvement holds only when $L \ \lesssim$ $\operatorname* { m i n } \{ d ^ { 2 / 3 } \varepsilon ^ { - 1 / 3 } , d ^ { 1 1 / 1 2 } \varepsilon ^ { 1 / 6 } \}$ , which is restrictive. This motivates us to develop an $L$ -adaptive convergence bound for diffusion models, with the hope to achieve improvement over the full range of $L$ . + +# 1.1 Our contributions + +In this paper, we investigate a sampler for SGMs based on the randomized midpoint technique, and establish a convergence rate adaptive to the smoothness of score functions. Specifically, up to logarithmic factors, we achieve the following iteration complexity: + +$$ +\operatorname* { m i n } \{ d , d ^ { 2 / 3 } L ^ { 1 / 3 } , d ^ { 1 / 3 } L \} \varepsilon ^ { - 2 / 3 } , +$$ + +where $L$ , as defined in Definition 2, characterizes the smoothness of score functions. Below, we provide a brief comparison of our results with existing convergence rates, viewed in Figure 1: + +• Comparison under smoothness condition. Under a uniform Lipschitz assumption, i.e., $\left\| s _ { t } ^ { \star } ( x ) - \right\|$ $s _ { t } ^ { \star } ( x ^ { \prime } ) \| _ { 2 } \leq L \| x - x ^ { \prime } \| _ { 2 }$ for all $x , x ^ { \prime } \in \mathbb { R } ^ { d }$ and all $t$ , several works (Chen et al., 2023, 2024b; Gao et al., 2023; Gupta et al., 2024; Lee et al., 2022; Li and Jiao, 2024) have studied the convergence rate of SGMs. In comparison, our result has two advantages: + +– The uniform Lipschitz condition used in previous works is significantly more restrictive than the non-uniform condition in Definition 2 adopted here. Specifically, for Gaussian mixture models (GMMs), we show that $L$ scales only logarithmically with the number of components and dimension, whereas the uniform Lipschitz constant can be extremely large. This highlights the superiority of our results (see Example 2 for details). Achieving these advancements requires substantial technical innovations. Detailed comparisons are in Section 3 and Appendix B of supplemental materials. + +– Among existing results, the best-known bound under smoothness conditions is $\widetilde { \cal O } ( d ^ { 1 / 3 } L \varepsilon ^ { - 2 / 3 } )$ , established by Li and Jiao (2024). Our convergence rate improves prior theory by a factor of√ $\operatorname* { m a x } \{ d ^ { - 2 / 3 } L , d ^ { - 1 / 3 } L ^ { 2 / 3 } , 1 \}$ , yielding substantial gains when $L \gtrsim \sqrt { d }$ even without considering the additional benefits from relaxing the Lipschitz condition. + +• Comparison under general condition. Without smoothness assumption on score functions, Benton et al. (2023) established an iteration complexity at the order of $d \varepsilon ^ { - 2 }$ , which marks the first result with linear dependency on the data dimension $d$ . This was later improved to $\tilde { O } ( d \varepsilon ^ { - 1 } )$ by Li and Yan (2024a) and Li et al. (2024b). In comparison, our analysis further improves these bounds by a factor of $\operatorname* { m a x } \{ 1 , d ^ { 1 / 3 } L ^ { - 1 / 3 } , d ^ { 2 / 3 } L ^ { - 1 } \} \varepsilon ^ { - 1 / 3 }$ , which is significant for the entire range of $L$ . Notably, even in the case of $L = \infty$ , our result achieves a significant improvement, except in the trivial scenario where $\varepsilon \asymp 1$ . + +• Comparison with accelerated samplers. Existing accelerated convergence theories often rely on additional assumptions about the target distribution or the estimation error of the higher-order score functions. For example, results in Huang et al. (2024a,b) depend on the bound of the first $p$ -th derivatives of score functions or their estimates, and achieved a convergence rate faster than $\varepsilon ^ { - 2 / 3 }$ when $p \geq 3$ . In addition, Li et al. (2024a) assumed that the estimation errors of the jacobian matrix of score functions are bounded, and developed an improved rate of $\varepsilon ^ { - 1 / 2 }$ . More recently, Li and Cai (2024) achieved the state-of-the-art iteration complexity of $\widetilde { \cal O } ( d ^ { 5 / 4 } \varepsilon ^ { - 1 / 2 } )$ without additional distribution or estimation assumptions. In comparison, our result improves this bound when the number of iterations $T \lesssim \operatorname* { m a x } \{ d ^ { 2 } , d ^ { 3 } L ^ { - 1 } , d ^ { 4 } L ^ { - 3 } \}$ . Notably, even in the case of $L = \infty$ , a significant improvement is achieved as long as $T \lesssim d ^ { 2 }$ . + +Organizations: The remaining of this paper is structured as follows. Section 2 provides a brief overview of fundamental concepts on SGMs, and presents the sampling algorithm. In Section 3, we present our assumptions and main results, and make comparison with previous works. The theoretical analysis and proofs are detailed in Section 4. Finally, Section 5 concludes the paper and discusses potential directions for future research. + +# 2 Preliminary + +In this section, we present some basic concepts of generative diffusion models, introduce the sampler employed in this work, and clarify our goal. + +# 2.1 Score-based diffusion models + +A diffusion model typically involves two key processes: the forward process and the reverse process, which are explained below. + +Forward process. The forward process starts from a random instance $X _ { 0 } \in \mathbb { R } ^ { d }$ sampled from the target data distribution $p _ { \mathsf { d a t a } }$ , and progressively transforms it into pure Gaussian noise by iteratively adding noise at each step: + +$$ +X _ { t } = \sqrt { \alpha _ { t } } X _ { t - 1 } + \sqrt { 1 - \alpha _ { t } } W _ { t } , \quad 1 \leq t \leq T , +$$ + +where $\{ W _ { t } \} _ { 1 \leq t \leq T }$ is a sequence of independent Gaussian vectors drawn from $\mathcal { N } ( 0 , I _ { d } )$ , and $\alpha _ { t } ~ \in ~ ( 0 , 1 )$ represents the step-size. For ease of notations, we define + +$$ +\overline { { \alpha } } _ { t } : = \prod _ { k = 1 } ^ { t } \alpha _ { k } , \quad 1 \leq t \leq T . +$$ + +With this notation, $X _ { t }$ can be expressed as a linear combination of the original data instance $X _ { 0 }$ and Gaussian noise with variance $1 - \overline { { \alpha } } _ { t }$ , as below. + +$$ +X _ { t } = \sqrt { \overline { { \alpha _ { t } } } } X _ { 0 } + \sqrt { 1 - \overline { { \alpha _ { t } } } } \overline { { W } } _ { t } , \quad \overline { { W } } _ { t } \sim \mathcal { N } ( 0 , I _ { d } ) . +$$ + +As $\overline { { \alpha } } _ { t }$ approaches zero, the distribution of $X _ { t }$ becomes exceedingly close to $\mathcal { N } ( 0 , I _ { d } )$ . + +Diffusion models are closely related to stochastic differential equations (SDEs). The continuous-time limit of the forward process can be modeled as: + +$$ +\mathrm { d } X _ { \tau } = - \frac { 1 } { 2 ( 1 - \tau ) } X _ { \tau } \mathrm { d } \tau + \frac { 1 } { \sqrt { 1 - \tau } } \mathrm { d } B _ { \tau } , \quad \mathrm { f o r } \ 0 \leq \tau < 1 , +$$ + +where $B _ { \tau }$ denotes some Brownian motion, $X _ { 0 } \sim p _ { \mathsf { d a t a } }$ , and the distribution of $X _ { \tau }$ approaches Gaussian as $\tau$ gets close to 1. + +Reverse process and score functions. The core of diffusion models lies in the reverse (or backward) process, which starts from pure Gaussian noise $Y _ { 0 } \sim { \mathcal { N } } ( 0 , I _ { d } )$ , and aims to generate samples $Y _ { T }$ that resemble + +the target data distribution. Many reverse processes are designed based on insights from the probability flow ODE, which is given by + +$$ +\mathrm { d } Y _ { \tau } = - \frac { 1 } { 2 ( 1 - \tau ) } \big ( Y _ { \tau } + \nabla \log p _ { X _ { \tau } } ( Y ) \big ) \mathrm { d } \tau . +$$ + +This ODE ensures that $Y _ { \tau }$ follows the same distribution as $X _ { \tau }$ defined in (4), provided that the initial point $Y _ { \tau _ { 0 } } \sim p _ { X _ { \tau _ { 0 } } }$ , where $\tau _ { 0 }$ is close to one and $Y _ { \tau _ { 0 } }$ is approximately Gaussian noise. + +In ODE (5), the only additional term except $Y _ { \tau }$ itself is the gradient of the log-density of the forward process, $\nabla \log p _ { X _ { \tau } }$ , known as the score function. Its formal mathematical definition is provided below. + +Definition 1. The score function $s _ { t } ^ { \star } : \mathbb { R } ^ { d } \mathbb { R } ^ { d }$ for $1 \leq t \leq T$ is defined as: + +$$ +s _ { t } ^ { \star } ( x ) : = \nabla \log p _ { X _ { t } } ( x ) = - \frac { 1 } { 1 - \overline { { \alpha } } _ { t } } \int _ { x _ { 0 } } p _ { X _ { 0 } \mid X _ { t } } ( x _ { 0 } \mid x ) ( x - \sqrt { \overline { { \alpha } } _ { t } } x _ { 0 } ) \mathrm { d } x _ { 0 } . +$$ + +For ease of notations, we also define $s _ { \tau } ^ { \star } ( x )$ with a continuous index $0 < \tau < 1$ as follows: + +$$ +s _ { \tau } ^ { \star } ( x ) : = \nabla \log p _ { X _ { \tau } } ( x ) = - \frac { 1 } { \tau } \int _ { x _ { 0 } } p _ { X _ { 0 } | X _ { \tau } } ( x _ { 0 } | x ) ( x - \sqrt { 1 - \tau } x _ { 0 } ) \mathrm { d } x _ { 0 } . +$$ + +It is evident that $s _ { t } ^ { \star } ( \cdot ) = s _ { 1 - \overline { { \alpha } } _ { t } } ^ { \star } ( \cdot )$ . In practice, the true score function $s _ { t } ^ { \star }$ is typically unknown and must be estimated from a training dataset. We assume access to faithful estimates $s _ { t }$ of the score functions $s _ { t } ^ { \star }$ across all steps $t$ . The assumption regarding score estimation errors is formally presented in Assumption 2. + +# 2.2 Sampling algorithm + +The sampler used is the same as the one employed by Li and Jiao (2024), which is derived from the discretization of a probability flow ODE for $X _ { \tau }$ over $\tau \in ( 0 , 1 )$ . Specifically, we first discretize $\tau$ into intermediate points $^ { \prime } k , n$ within the interval $( 0 , 1 )$ , where $n = 0 , \cdots , N$ and $k = 0 , \cdots , K$ . We then estimate $X _ { \tau _ { k , n } }$ at these intermediate points by approximating the integral of probability flow ODE. The detailed implementation is stated below. + +Randomized schedule. We begin by discretizing the interval [0, 1] into a sequence of subintervals $( \widehat { \alpha } _ { t } , \widehat { \alpha } _ { t - 1 } )$ where $\widehat { \alpha } _ { t }$ is defined as + +$$ +\widehat { \alpha } _ { T + 1 } = \frac { 1 } { T ^ { c _ { 0 } } } , \quad \widehat { \alpha } _ { t - 1 } = \widehat { \alpha } _ { t } + \frac { c _ { 1 } \widehat { \alpha } _ { t } ( 1 - \widehat { \alpha } _ { t } ) \log { T } } { T } , \qquad t = - \frac { N } { 2 } + 1 , \cdots , T + 1 , +$$ + +for some sufficiently large constants $c _ { 0 } , c _ { 1 } > 0$ , where the ratio $c _ { 1 } / c _ { 0 }$ is assumed to be sufficiently large. Subsequently, we employ a randomized learning rate schedule by setting $\alpha _ { t }$ in (3) as + +$$ +\overline { { \alpha } } _ { t } \sim \mathsf { U n i f } ( \widehat { \alpha } _ { t } , \widehat { \alpha } _ { t - 1 } ) , \quad \mathrm { f o r } t = - \frac { N } { 2 } + 1 , \ldots , T + 1 , +$$ + +where Unif denotes the uniform distribution. + +The algorithm operates over $K$ rounds, each consisting of $\begin{array} { r } { N = \frac { 2 \mathcal { I } } { K } } \end{array}$ steps. We define + +$$ +\widehat { \tau } _ { k , n } : = 1 - \widehat { \alpha } _ { T - \frac { k N } { 2 } - n } , \qquad \tau _ { k , n } : = 1 - \overline { { { \alpha } } } _ { T - \frac { k N } { 2 } - n + 1 } \qquad \mathrm { f o r } \quad n = - 1 , \dots , N . +$$ + +It follows that $\tau _ { k , n } \sim \mathsf { U n i f } ( \widehat { \tau } _ { k , n } , \widehat { \tau } _ { k , n - 1 } )$ , which we use as the discretization of $\tau$ bSampling procedure. With $\tau$ discretized, we are now ready to describe the sampling procedure. As aforementioned, the sampler is implemented over $K$ rounds, each consisting of $N$ steps. In the $k$ -th round, the sampler approximates $X _ { \tau _ { k , N } }$ defined by the probability flow ODE with the initial point given by $X _ { \tau _ { k , 0 } }$ , whose probability distribution is denoted as $q _ { k }$ . At the end of each round, a Gaussian noise is injected to convert the total variation distance between the reverse and forward processes into an estimation error in the $\ell _ { 2 }$ norm. The sampling procedure consists of the following steps: + +1. Initialization: The sampler begins with an initial sample $Y _ { 0 } \sim { \mathcal { N } } ( 0 , I _ { d } )$ . + +2. Iterative update: For each $k$ ranging from $0$ to $K - 1$ , the intermediate variables $Y _ { k , n }$ are iteratively updated for $n = 1 , \cdots , N$ by discretizing the ODE as follows: + +$$ +\begin{array} { r l } & { \frac { Y _ { k , n } } { \sqrt { 1 - \tau _ { k , n } } } = \frac { Y _ { k , 0 } } { \sqrt { 1 - \tau _ { k , 0 } } } + \frac { s _ { T - \frac { k N } { 2 } + 1 } \left( Y _ { k , 0 } \right) } { 2 \left( 1 - \tau _ { k , 0 } \right) ^ { 3 / 2 } } ( \tau _ { k , 0 } - \widehat { \tau } _ { k , 0 } ) } \\ & { + \displaystyle \sum _ { i = 1 } ^ { n - 1 } \frac { s _ { T - \frac { k N } { 2 } - i + 1 } \left( Y _ { k , i } \right) } { 2 \left( 1 - \tau _ { k , i } \right) ^ { 3 / 2 } } ( \widehat { \tau } _ { k , i - 1 } - \widehat { \tau } _ { k , i } ) + \frac { s _ { T - \frac { k N } { 2 } - n + 2 } \left( Y _ { k , n - 1 } \right) } { 2 \left( 1 - \tau _ { k , n - 1 } \right) ^ { 3 / 2 } } ( \widehat { \tau } _ { k , n - 1 } - \tau _ { k , n } ) , } \end{array} +$$ + +where $Y _ { k , 0 } = Y _ { k }$ and $\begin{array} { r l r } { { S _ { T - \frac { k N } { 2 } - i + 1 } } } \end{array}$ is an estimation of $s _ { T - \frac { k N } { 2 } - i + 1 } ^ { \star }$ as defined in (6), corresponding to $\overline { { \alpha } } _ { T - \frac { k N } { 2 } - i + 1 } = 1 - \tau _ { k , i }$ . + +3. Noise injection: After obtaining $Y _ { k , N }$ , we update $Y _ { k + 1 }$ by injecting stochastic noise as: + +$$ +Y _ { k + 1 } = \sqrt { \frac { 1 - \tau _ { k + 1 , 0 } } { 1 - \tau _ { k , N } } } Y _ { k , N } + \sqrt { \frac { \tau _ { k + 1 , 0 } - \tau _ { k , N } } { 1 - \tau _ { k , N } } } Z _ { k } , +$$ + +where $Z _ { k } \stackrel { \scriptscriptstyle 1 . 1 . 0 . } { \sim } \mathcal { N } ( 0 , I _ { d } )$ + +Notice that at each step of computing $Y _ { k , n }$ , only one additional score evaluation for $s _ { T - \frac { k N } { 2 } - n + 2 } ( Y _ { k , n - 1 } )$ is required. Therefore, the total iteration complexity of the sampler is $K N = 2 T$ . This sampler can be implemented in parallel, as demonstrated by Li and Jiao (2024). For clarity, we present the details in Appendix E.1 of the supplemental material. + +Our goal. The objective of this work is to improve the convergence rate for a broader class of distributions by analyzing the aforementioned sampler. Since $X _ { \tau _ { K , 0 } }$ , which is nearly the starting point of the forward process, is perturbed only by noise with a small variance of $1 - \overline { { \alpha } } _ { 1 }$ , the performance of the sampler is evaluated using the total variation (TV) distance between $p _ { Y _ { K } }$ and $q _ { K }$ , defined as + +$$ +\mathsf { T V } ( q _ { K } , p _ { Y _ { K } } ) : = \frac { 1 } { 2 } \int | p _ { Y _ { K } } ( x ) - q _ { K } ( x ) | \mathrm { d } x . +$$ + +# 3 Main results + +In this section, we establish an instance-dependent convergence rate for diffusion models under a novel analytical framework that accommodates a relaxed Lipschitz condition, thereby covering a broader class of distributions. Finally, we extend our results to the parallel implementation of the sampler, following an idea similar to that of Li and Jiao (2024). + +# 3.1 Assumptions + +We first make the following assumption on the target data distribution $p _ { \mathsf { d a t a } }$ , which accommodates a broad class of data distributions. + +Assumption 1. We assume that the target distribution $p _ { \mathsf { d a t a } }$ has a bounded second-order moment in the sense that + +$$ +\begin{array} { r } { \mathbb { E } _ { X _ { 0 } \sim p _ { \mathsf { d a t a } } } [ \| X _ { 0 } \| _ { 2 } ^ { 2 } ] < T ^ { c _ { R } } , } \end{array} +$$ + +where $c _ { R } > 0$ is an arbitrarily large constant. + +The second-order moment of $X _ { 0 }$ is assumed to be polynomial in the number of iterations $T$ . This assumption is mild as the exponent $c _ { R }$ can be arbitrarily large. However, there exist exceptions including extremely heavy-tailed distributions. For example, densities decaying slower than $1 / x ^ { 3 }$ may not align with our theoretical framework. + +Our analysis is conducted under a relaxed smoothness condition, which is substantially weaker than the commonly used uniform Lipschitz condition and encompasses a wide range of distributions previously considered non-Lipschitz. Specifically, we define the Lipschitz constant for the normalized score functions $( 1 - \overline { { \alpha } } _ { t } ) s _ { t } ^ { \star }$ as follows. + +Definition 2 (Non-uniform Lipschitz property). Let $L$ denote the smallest quantity, which may depend on $T$ and $d$ , such that + +$$ +\begin{array} { l l } { \displaystyle \mathbb { P } _ { x \sim X _ { t } } \left\{ ( 1 - \overline { { \alpha _ { t } } } ) \| s _ { t } ^ { \star } ( x ^ { \prime } ) - s _ { t } ^ { \star } ( x ) \| _ { 2 } \le L \| x ^ { \prime } - x \| _ { 2 } , \forall \| x ^ { \prime } - x \| _ { 2 } \le \frac { C \sqrt { d ( 1 - \overline { { \alpha } } _ { t } ) \log T } } { L } \right\} } \\ { \displaystyle \ge 1 - \frac { c } { ( T + d ) ^ { 4 } } , } \end{array} +$$ + +where $C$ and $c$ are some universal constants. + +Remark 1. Previous works typically assume a uniform Lipschitz condition, requiring $\big ( 1 - \overline { { \alpha } } _ { t } \big ) \big \| s _ { t } ^ { \star } ( x ) -$ $s _ { t } ^ { \star } ( x ^ { \prime } ) \| _ { 2 } \leq L \| x - x ^ { \prime } \| _ { 2 }$ (or $\| s _ { t } ^ { \star } ( x ) - s _ { t } ^ { \star } ( x ^ { \prime } ) \| _ { 2 } \leq L \| x - x ^ { \prime } \| _ { 2 } )$ to hold for all $x$ and $x ^ { \prime }$ . In contrast, this work adopts a non-uniform Lipschitz condition, which is a significant relaxation compared to prior assumptions. In what follows, we present two examples to demonstrate the practical importance and necessity of this relaxation. + +Example 1 (Gaussian distribution). Consider Gaussian target distribution $X _ { 0 } ^ { ( i ) } \sim \mathcal { N } ( 0 , \sigma _ { i } ^ { 2 } )$ , where $X _ { 0 } ^ { ( i ) }$ denotes the $i$ -th entry of $X _ { 0 }$ , $i = 1 , \cdots , d$ . The score function $\boldsymbol { s } _ { t } ^ { \star } ( \boldsymbol { x } ) = \nabla \log p _ { X _ { t } } ( \boldsymbol { x } )$ satisfies + +$$ +\begin{array} { r l } { \forall t > 0 , \ \forall x , x ^ { \prime } \in \mathbb { R } ^ { d } , } & { \quad ( 1 - \overline { { \alpha } } _ { t } ) \| s _ { t } ^ { \star } ( x ) - s _ { t } ^ { \star } ( x ^ { \prime } ) \| _ { 2 } \leq \| x - x ^ { \prime } \| _ { 2 } , } \\ { \forall t \geq 0 , \ \exists x , x ^ { \prime } \in \mathbb { R } ^ { d } , \quad \mathrm { s u c h ~ t h a t } } & { \quad \| s _ { t } ^ { \star } ( x ) - s _ { t } ^ { \star } ( x ^ { \prime } ) \| _ { 2 } \geq ( 1 - \overline { { \alpha } } _ { t } ) ^ { - 1 } \| x - x ^ { \prime } \| _ { 2 } , } \end{array} +$$ + +provided that $\operatorname* { m i n } _ { i } \sigma _ { i } ^ { 2 } = 0$ . This highlights the rationale for defining the Lipschitz condition for $( 1 - \overline { { \alpha } } _ { t } ) s _ { t } ^ { \star }$ The detailed derivations are presented in Appendix C.1. + +Examwhere ure M and sider GMM tar. Then for any istribution , we have $\begin{array} { r } { X _ { 0 } \sim \sum _ { h = 1 } ^ { H } \gamma _ { h } \mathcal { N } ( \mu _ { h } , \sigma ^ { 2 } I _ { d } ) } \end{array}$ $\mu _ { h } \in \mathbb { R } ^ { d }$ $\sigma \geq 0$ $\gamma _ { h } \geq 0$ $\textstyle \sum _ { h = 1 } ^ { H } \gamma _ { h } = 1$ $t \geq 0$ + +$$ +\begin{array} { r l } & { \mathbb { P } \left\{ ( 1 - \overline { { \alpha } } _ { t } ) \| s _ { t } ^ { \star } ( x ) - s _ { t } ^ { \star } ( x ^ { \prime } ) \| _ { 2 } \leq C _ { 1 } \log ( H ( T + d ) ) \| x - x ^ { \prime } \| _ { 2 } , \forall \ \| x - x ^ { \prime } \| _ { 2 } \leq C _ { 2 } \sqrt { d ( 1 - \overline { { \alpha } } _ { t } ) } \right\} } \\ & { \geq 1 - \frac { c } { ( T + d ) ^ { 4 } } , } \end{array} +$$ + +for some universal constants $C _ { 1 } , C _ { 2 }$ and $c$ . Moreover, consider a simple case that $X _ { 0 } \sim \frac { 1 } { 2 } { \mathcal { N } } ( \mu , \sigma ^ { 2 } I _ { d } ) +$ ${ \scriptstyle { \frac { 1 } { 2 } } } { \mathcal { N } } ( - \mu , \sigma ^ { 2 } I _ { d } )$ , then there exists some $x \in \mathbb { R } ^ { d }$ , such that for $\overline { { \alpha } } _ { t } > 1 / 2$ , + +$$ +( 1 - \overline { { \alpha } } _ { t } ) \| \nabla s _ { t } ^ { \star } ( x ) \| _ { \mathsf { o p } } \geq \frac { ( 1 - \overline { { \alpha } } _ { t } ) \| \mu \| _ { 2 } ^ { 2 } } { 4 ( 1 - \overline { { \alpha } } _ { t } + \sigma ^ { 2 } ) ^ { 2 } } . +$$ + +The above example implies that the non-uniform Lipschitz constant remains relatively small, satisfying $L \leq \log ( H ( T + d ) )$ . In contrast, the uniform Lipschitz constant may be extremely large when $\sigma ^ { 2 }$ is small enough. Here, $\| \mu \| _ { 2 } ^ { 2 } \approx \mathbb { E } [ \| X _ { 0 } \| _ { 2 } ^ { 2 } ]$ is typically large in practice, which often scales on the order of $d$ . The detailed derivations are presented in Appendix C.2. + +Finally, we make the following assumption about the estimation error of score functions. + +Assumption 2. We assume access to an estimate $s _ { t } ( \cdot )$ for each $s _ { t } ^ { \star } ( \cdot )$ , with the averaged $\ell _ { 2 }$ score estimation error as + +$$ +\begin{array} { l } { \varepsilon _ { \mathsf { s c o r e } } ^ { 2 } = \displaystyle \frac { 1 } { T } \sum _ { k = 0 } ^ { K - 1 } \sum _ { n = 0 } ^ { N - 1 } \mathbb { E } _ { Y _ { k } \sim q _ { k } } \left[ \| s _ { T - \frac { k N } { 2 } - n + 1 } ( Y _ { k , n } ) - s _ { T - \frac { k N } { 2 } - n + 1 } ^ { \star } ( Y _ { k , n } ) \| _ { 2 } ^ { 2 } \right] } \\ { = : \displaystyle \frac { 1 } { T } \sum _ { k = 0 } ^ { K - 1 } \sum _ { n = 0 } ^ { N - 1 } \varepsilon _ { k , n } ^ { 2 } . } \end{array} +$$ + +# 3.2 Convergence Analysis + +We are now positioned to present the convergence guarantees — measured by the total variation distance between the forward and the reverse processes — for the sampler (10). The proof is postponed to Section 4. + +Theorem 1. Suppose that Assumptions 1 and ${ \it \Delta } _ { \it { \phi } } ^ { \it { \Delta } }$ hold true, and $K = c _ { 2 } \operatorname* { m i n } \{ d \log ^ { 2 } T , L \log T \}$ for some constant $c _ { 2 } > 0$ . Then the sampling process (10) with the learning rate schedule (9) satisfies + +$$ +\mathsf { T V } ( q _ { K } , p _ { Y _ { K } } ) \le \frac { C \operatorname* { m i n } \{ d ^ { 3 / 2 } , d L ^ { 1 / 2 } , d ^ { 1 / 2 } L ^ { 3 / 2 } \} \log ^ { 4 } T } { T ^ { 3 / 2 } } + C \varepsilon _ { \mathrm { s c o r e } } \log ^ { 1 / 2 } T +$$ + +for some constant $C > 0$ large enough, where $L$ is defined in Definition $\mathcal { Z }$ + +We now discuss the main implications of Theorem 1. + +Relaxation of smoothness. Our result requires only a non-uniform Lipschitz condition, which is substantially weaker than the uniform Lipschitz condition commonly employed in prior studies. As demonstrated in Example 2 for Gaussian mixture models, we have shown that $L$ scales only logarithmically with the number of components, dimension and iteration number, whereas the uniform Lipschitz constant may be extremely large. Given the wide use of GMMs, this demonstrates that the uniform Lipschitz condition employed in previous works is more restrictive compared to the non-uniform condition in Definition 2 used here. + +Achieving these improvements requires a lot of technical efforts. For example, the absence of a uniform Lipschitz condition poses significant challenges for controlling error propagation across multiple steps, while a naive stepwise analysis may lead to suboptimal bounds. We address this issue by introducing two auxiliary sequences based on a typical set and quantifying how error propagation affects the probability of $Y _ { k , n }$ outside this set (see Step 2 in Section 4 and Lemma 5). In addition, lacking a uniform condition prevents the logconcavity of $p _ { X _ { \tau } | X _ { \tau + \delta } }$ for small $\delta$ , which plays a crucial rule in controlling the one-step discretization error (see Lemma 1 and (B.2)-(B.4) in Chen et al. (2024b)). We instead directly analyze this derivative based on its definition via a careful decomposition and statistical bounds (see Lemma 10). Further details are provided in Appendix B of supplemental material. + +Iteration complexity. For the moment, we focus on the first term in (13), which corresponds to discretization error. To ensure $\mathsf { T V } ( q _ { K } , p _ { Y _ { K } } ) \le \varepsilon$ , it is sufficient to choose + +$$ +T \gtrsim \frac { \operatorname* { m i n } \{ d , d ^ { 2 / 3 } L ^ { 1 / 3 } , d ^ { 1 / 3 } L \} \log ^ { \frac { 8 } { 3 } } T } { \varepsilon ^ { 2 / 3 } } . +$$ + +This result is adaptive to the non-uniform Lipschitz constant $L$ of normalized score functions and is thus referred to as instance-dependent. Different from previous works, which only improve iteration complexity under specific conditions on $L$ , our theory improves existing results (Benton et al., 2023; Li and Cai, 2024; Li and Yan, 2024a) over a full range of $L$ , and improves the state-of-the-art (Li and Jiao, 2024) when $L \gtrsim \sqrt { d }$ , even when non-uniform and uniform Lipschitz constants are equal. A detailed comparison of iteration complexity orders has been also provided in Section 1.1. + +To illustrate these improvements, we assume the non-uniform and uniform Lipschitz constants are identical and compare iteration complexities for $\varepsilon = O ( 1 )$ across varying values of $L$ , as shown in the left subplot of Figure 1. It indicates that prior works (Benton et al., 2023; Gupta et al., 2024; Li and Cai, 2024; Li and Yan, 2024a) outperform others in specific regimes. In contrast, our instance-dependent result achieves the best result across the full range of $L$ , and improves all previous works when ${ \sqrt { d } } \lesssim L \lesssim d$ . Furthermore, to more clearly illustrate the improvement in the case of $L = \infty$ , we present the iteration complexity as a function of $\varepsilon$ in the right subplot of Figure 1. It demonstrates clear improvement over previous results when $T \lesssim d ^ { 2 }$ . We verify our theoretical result via a numerical simulation, which is provided in Appendix A of supplemental material. More comparisons are provided in Appendix B of supplemental material. + +Remark 2. Some studies have explored provably accelerated samplers for diffusion models (Huang et al., 2024a,b), achieving convergence rate faster than the $\varepsilon ^ { - 2 / 3 }$ rate established in this work. However, these results typically rely on the bound of the first $p$ -th derivatives of the score functions or their estimates with $p \geq 3$ . + +![](images/figures/diffusion-convergence-rate-fig-0001.jpg) +Figure 1: Comparison of Theorem 1 with prior results. left: the iteration complexity as a function of $L$ when $\varepsilon = O ( 1 )$ . right: the iteration complexity as a function of $\varepsilon$ when $L = \infty$ . + +# 3.3 Extension to parallel sampling + +In this section, we present a theoretical guarantee for the parallel implementation described in Appendix E.1 of the supplementary material, following an approach similar to that of Gupta et al. (2024); Li and Jiao (2024). Let $N$ denote the number of parallel processors and $M K$ the total number of parallel rounds. The convergence result is stated below, with a detailed proof provided in Appendix E.2 of the supplementary material. + +Theorem 2. Under the same assumptions as Theorem 1, it is sufficient to choose + +$$ +\begin{array} { r } { N \gtrsim \frac { \left( \operatorname* { m i n } \{ d ^ { 2 / 3 } L ^ { - 2 / 3 } , d ^ { 1 / 3 } \} + 1 \right) \log ^ { 5 / 3 } T } { \varepsilon ^ { 2 / 3 } } , \qquad } \\ { M K \gtrsim \operatorname* { m i n } \{ d \log T , L \} \log ^ { 2 } T , \quad \varepsilon _ { \mathrm { s c o r e } } ^ { 2 } \lesssim \varepsilon ^ { 2 } \log ^ { - 1 } T } \end{array} +$$ + +to achieve $\mathsf { T V } ( q _ { K } , p _ { Y _ { K } } ) \lesssim \varepsilon$ for the parallel sampler, where $T = K N / 2$ . + +Finally, let us briefly compare our theory with the prior works. This theorem states that the parallel sampler achieves $\varepsilon$ -accuracy with respect to total variation distance using $O ( \operatorname* { m i n } \{ L , d \} \log ^ { 2 } ( L d / \varepsilon ) )$ parallel rounds, which is consistent with the results in Chen et al. (2024a); Gupta et al. (2024); Li and Jiao (2024). Moreover, our sampler requires only $\widetilde { \cal O } ( ( \operatorname* { m i n } \{ d ^ { 2 / 3 } L ^ { - 2 / 3 } , d ^ { 1 / 3 } \} + 1 ) \varepsilon ^ { - 2 / 3 } )$ parallel processors, which achieves a significant improvement over previous results. + +# 4 Analysis + +This section is devoted to establishing Theorem 1. Before proceeding, we rewrite the sampling process with the continuous index as following: + +$$ +\begin{array} { r l } { { \frac { Y _ { \tau _ { k , n } } } { \sqrt { 1 - \tau _ { k , n } } } = \frac { Y _ { \tau _ { k , 0 } } } { \sqrt { 1 - \tau _ { k , 0 } } } + \frac { s _ { \tau _ { k , 0 } } ( Y _ { \tau _ { k , 0 } } ) } { 2 ( 1 - \tau _ { k , 0 } ) ^ { 3 / 2 } } ( \tau _ { k , 0 } - \widehat { \tau } _ { k , 0 } ) } } \\ & { + \sum _ { i = 1 } ^ { n - 1 } \frac { s _ { \tau _ { k , i } } ( Y _ { \tau _ { k , i } } ) } { 2 ( 1 - \tau _ { k , i } ) ^ { 3 / 2 } } ( \widehat { \tau } _ { k , i - 1 } - \widehat { \tau } _ { k , i } ) + \frac { s _ { \tau _ { k , n - 1 } } ( Y _ { \tau _ { k , n - 1 } } ) } { 2 ( 1 - \tau _ { k , n - 1 } ) ^ { 3 / 2 } } ( \widehat { \tau } _ { k , n - 1 } - \tau _ { k , n } ) , } \end{array} +$$ + +and + +$$ +Y _ { \tau _ { k + 1 , 0 } } = \sqrt { \frac { 1 - \tau _ { k + 1 , 0 } } { 1 - \tau _ { k , N } } } Y _ { \tau _ { k , N } } + \sqrt { \frac { \tau _ { k + 1 , 0 } - \tau _ { k , N } } { 1 - \tau _ { k , N } } } Z _ { k } , +$$ + +where $Y _ { \tau _ { 0 , 0 } } \sim \mathcal { N } ( 0 , I _ { d } )$ , $Z _ { k }$ i.i.d. ∼ $\mathcal { N } ( 0 , I _ { d } )$ . Here, $Y _ { \tau _ { k , 0 } }$ corresponds to the $Y _ { k }$ in discrete index, and $s _ { \tau } ( \cdot )$ denotes the estimate of score function $s _ { \tau } ^ { \star } ( \cdot )$ defined in (7). For ease of notations, we denote estimation error at the $( k , n )$ -th step as + +$$ +\begin{array} { r } { \widetilde { \varepsilon } _ { k , n } = \left\| s _ { \tau _ { k , n } } ( Y _ { \tau _ { k , n } } ) - s _ { \tau _ { k , n } } ^ { \star } ( Y _ { \tau _ { k , n } } ) \right\| _ { 2 } . } \end{array} +$$ + +Moreover, we shall show that $Y _ { \tau _ { k , n } }$ follows a distribution similar to that of $X _ { \tau _ { k , n } }$ defined in (4), which can also be expressed as + +$$ +X _ { \tau } \stackrel { \mathrm { d } } { = } \sqrt { 1 - \tau } X _ { 0 } + \sqrt { \tau } Z , \mathrm { w i t h } Z \sim { \mathcal N } ( 0 , I _ { d } ) , \mathrm { f o r } 0 \leq \tau \leq 1 . +$$ + +# 4.1 Step 1: introduce the auxiliary sequence + +We first introduce an auxiliary sequence $\widehat { X } _ { k }$ , $1 \leq k \leq K$ , which has identical distribution with $X _ { \tau _ { k , 0 } }$ . Let $\Phi _ { \tau _ { 1 } \tau _ { 2 } } ( x ) : = x _ { \tau _ { 2 } } \mid { } _ { x _ { \tau _ { 1 } } = x }$ defined through the following ODE + +$$ +\mathrm { d } \frac { x _ { \tau } } { \sqrt { 1 - \tau } } = - \frac { s _ { \tau } ^ { \star } ( x _ { \tau } ) } { 2 ( 1 - \tau ) ^ { 3 / 2 } } \mathrm { d } \tau . +$$ + +Then, we have the following result, which has been presented in Li and Jiao (2024). For completeness, we present its proof in Appendix F.1 in supplemental material: + +Lemma 1. It can be shown that + +$$ +\Phi _ { \tau _ { 1 } \tau _ { 2 } } ( X _ { \tau _ { 1 } } ) \stackrel { \mathrm { d } } { = } X _ { \tau _ { 2 } } . +$$ + +Moreover, assume that $\widehat { X } _ { 0 } \ { \overset { \mathrm { d } } { = } } \ X _ { \tau _ { 0 , 0 } }$ . Then we have for $0 \leq k < K$ , + +$$ +\widehat { X } _ { k + 1 } = \sqrt { \frac { 1 - \tau _ { k + 1 , 0 } } { 1 - \tau _ { k , N } } } \Phi _ { \tau _ { k , 0 } \tau _ { k , N } } ( \widehat { X } _ { k } ) + \sqrt { \frac { \tau _ { k + 1 , 0 } - \tau _ { k , N } } { 1 - \tau _ { k , N } } } Z _ { k } \stackrel { \mathrm { d } } { = } X _ { \tau _ { k + 1 , 0 } } , +$$ + +where $Z _ { k } \stackrel { \scriptscriptstyle 1 . 1 . 0 . } { \sim } \mathcal { N } ( 0 , I _ { d } )$ + +Before introducing other auxiliary sequences, we introduce two notations for ease of presentation. Let + +$$ +\begin{array} { r l } & { \frac { y _ { \tau _ { k , 0 } } ( x _ { \tau _ { k , 0 } } ) } { \sqrt { 1 - \tau _ { k , n } } } = \frac { x _ { \tau _ { k , 0 } } } { \sqrt { 1 - \tau _ { k , 0 } } } + \frac { s _ { \tau _ { k , 0 } } ( x _ { \tau _ { k , 0 } } ) } { 2 ( 1 - \tau _ { k , 0 } ) ^ { 3 / 2 } } ( \tau _ { k , 0 } - \widehat { \tau } _ { k , 0 } ) } \\ & { \qquad + \displaystyle \sum _ { i = 1 } ^ { n - 1 } \frac { s _ { \tau _ { k , i } } ( y _ { \tau _ { k , i } } ) } { 2 ( 1 - \tau _ { k , i } ) ^ { 3 / 2 } } ( \widehat { \tau } _ { k , i - 1 } - \widehat { \tau } _ { k , i } ) + \frac { s _ { \tau _ { k , n - 1 } } ( y _ { \tau _ { k , n - 1 } } ) } { 2 ( 1 - \tau _ { k , n - 1 } ) ^ { 3 / 2 } } ( \widehat { \tau } _ { k , n - 1 } - \tau _ { k , n } ) , } \\ & { \frac { x _ { \tau _ { k , n } } ( x _ { \tau _ { k , 0 } } ) } { \sqrt { 1 - \tau _ { k , n } } } = \frac { x _ { \tau _ { k , 0 } } } { \sqrt { 1 - \tau _ { k , 0 } } } + \displaystyle \int _ { \tau _ { k , n } } ^ { \tau _ { k , 0 } } \frac { s _ { \tau } ^ { \star } ( x _ { \tau } ) } { 2 ( 1 - \tau ) ^ { 3 / 2 } } \mathrm { d } \tau , } \end{array} +$$ + +for $n = 1 , \ldots , N$ , which satisfies $x _ { \tau _ { k , n } } ( x _ { \tau _ { k , 0 } } ) = \Phi _ { \tau _ { k , 0 } \tau _ { k , n } } ( x _ { \tau _ { k , 0 } } )$ . Moreover, we define typical sets $\xi _ { k }$ for $0 \leq k \leq K - 1$ as + +$$ +\begin{array} { r } { \mathcal { E } _ { k } : = \left\{ \begin{array} { l l } { \{ x _ { \tau _ { k , 0 } } : x _ { \tau _ { k , n } } ( x _ { \tau _ { k , 0 } } ) \in \widetilde { \mathcal { S } } _ { \tau _ { k , n } } \cap \mathcal { L } _ { \tau _ { k , n } } , } \\ { \quad y _ { \tau _ { k , n } } ( x _ { \tau _ { k , 0 } } ) \in \mathcal { S } _ { \tau _ { k , n } } , \forall 0 \leq n \leq N - 1 \} , } & { \mathrm { i f ~ } L > d \log T , } \\ { \emptyset } & { \mathrm { i f ~ } L \leq d \log T , } \end{array} \right. } \end{array} +$$ + +where $ { \widetilde { \boldsymbol { S } } } _ { \tau }$ , $S _ { \tau }$ and $\scriptstyle { \mathcal { L } } _ { \tau }$ denote high probability sets + +$$ +\begin{array} { r l } & { \mathcal { S } _ { \tau } : = \{ x : - \log p x _ { \tau } ( x ) \leq \theta d \log T \} , \quad \widetilde { \mathcal { S } } _ { \tau } : = \{ x : - \log p x _ { \tau } ( x ) \leq \theta d \log T - \log 2 \} , } \\ & { \mathcal { L } _ { \tau } : = \left\{ x : \tau \| s _ { \tau } ^ { \star } ( x ^ { \prime } ) - s _ { \tau } ^ { \star } ( x ) \| _ { 2 } \leq L \| x ^ { \prime } - x \| _ { 2 } , \forall \| x ^ { \prime } - x \| _ { 2 } \leq \frac { C \sqrt { d \tau \log T } } { L } \right\} , } \end{array} +$$ + +with $\theta$ a sufficiently large constant. Moreover, we define $\mathcal { E } _ { K } = \mathbb { R } ^ { d }$ . + +Based on the above definitions, we introduce a new auxiliary reverse process $\smash { \widetilde { X } } _ { k }$ for $0 \le k \le K$ , which transforms similar with ODE in Lemma 1, but removes samples out of the typical set $\xi _ { k }$ . Specifically, let $\tilde { X } _ { 0 } = \hat { X } _ { 0 }$ for $\hat { X } _ { 0 } \in \mathcal { E } _ { 0 }$ and $\tilde { X } _ { 0 } = \infty$ otherwise. Then it transits following the probability + +$$ +\begin{array} { r } { p _ { \widetilde { X } _ { k + 1 } | \widetilde { X } _ { k } } ( x | x _ { k } ) = \left\{ \begin{array} { l l } { p _ { \widehat { X } _ { k + 1 } | \widehat { X } _ { k } } ( x \vert x _ { k } ) \mathbb { 1 } ( x \in \mathcal { E } _ { k + 1 } ) } \\ { \quad + \int _ { \mathcal { E } _ { k + 1 } ^ { c } } p _ { \widehat { X } _ { k + 1 } | \widehat { X } _ { k } } ( x _ { k + 1 } \vert x _ { k } ) \mathrm { d } x _ { k + 1 } \delta _ { \infty } , } & { x _ { k } \neq \infty , } \\ { \delta _ { \infty } , } & { x _ { k } = \infty , } \end{array} \right. } \end{array} +$$ + +where according to Lemma $^ { 1 }$ , $p _ { \widehat { X } _ { k + 1 } | \widehat { X } _ { k } } ( x | x _ { k } ) = \phi \left( x | x _ { \tau _ { k , N } } ( x _ { k } ) , \sigma _ { k } ^ { 2 } \right)$ , and $\phi ( x | \mu , \sigma ^ { 2 } )$ denotes the probability density function of Gaussian distribution with mean vector $\mu$ and covariance matrix $\sigma ^ { 2 } I _ { d }$ , $\mathcal { E } _ { k } ^ { \mathrm { c } }$ denotes the complementary set of $\xi _ { k }$ $\propto \notin \mathcal { E } _ { k } ^ { \mathrm { c } }$ ), and + +$$ +\sigma _ { k } ^ { 2 } = \frac { \tau _ { k + 1 , 0 } - \tau _ { k , N } } { 1 - \tau _ { k , N } } . +$$ + +Moreover, we define another reverse sequence $\widetilde { Y _ { k } }$ similar with $Y _ { k }$ as follows. First, $\widetilde { Y } _ { 0 }$ is initialized as $Y _ { 0 }$ for $Y _ { 0 } \in \mathcal { E } _ { 0 }$ and $\tilde { Y _ { 0 } } = \infty$ otherwise. Then for $k = 0 , \cdots , K - 1$ , the conditional density of $\widetilde { Y } _ { k + 1 }$ given $\tilde { Y _ { k } } = y _ { k }$ is + +$$ +\begin{array} { r } { p _ { \widetilde { Y } _ { k + 1 } | \widetilde { Y } _ { k } } ( y | y _ { k } ) = \left\{ \begin{array} { l l } { p _ { Y _ { k + 1 } | Y _ { k } } ( y | y _ { k } ) \mathbb { 1 } ( y \in \mathcal { E } _ { k + 1 } ) } \\ { \quad + \int _ { \mathcal { E } _ { k + 1 } ^ { c } } p _ { Y _ { k + 1 } | Y _ { k } } ( y _ { k + 1 } | y _ { k } ) \mathrm { d } y _ { k + 1 } \delta _ { \infty } , } & { y _ { k } \neq \infty , } \\ { \delta _ { \infty } , } & { y _ { k } = \infty , } \end{array} \right. } \end{array} +$$ + +where according to the algorithm design $P _ { Y _ { k + 1 } | Y _ { k } } ( y | y _ { k } ) = \phi \left( y | y _ { \tau _ { k , N } } ( y _ { k } ) , \sigma _ { k } ^ { 2 } \right)$ , with $\sigma _ { k } ^ { 2 }$ defined in (23). The following lemma states some basic properties about the above auxiliary sequences. The proof is postponed to Appendix F.2 in supplemental material. + +Lemma 2. The following properties hold: + +$$ +\begin{array} { r l } & { p _ { \widetilde { X } _ { k } } ( x ) = 0 , \quad p _ { \widetilde { Y } _ { k } } ( y ) = 0 , \quad \mathrm { f o r } x , y \in \mathcal { E } _ { k } ^ { \mathrm { c } } ; } \\ & { p _ { \widetilde { X } _ { k } } ( x ) \leq p _ { \widehat { X } _ { k } } ( x ) , \quad p _ { \widetilde { Y } _ { k } } ( y ) \leq p _ { Y _ { k } } ( y ) , \quad \mathrm { f o r } x , y \neq \infty . } \end{array} +$$ + +# 4.2 Step 2: decompose the error terms + +Recalling the definition of $\widehat { X } _ { k }$ , and the fact that $p _ { \widetilde { Y } _ { K } } ( x ) \le p _ { Y _ { K } } ( x )$ and $p _ { \widetilde { X } _ { K } } ( x ) \le p _ { \widehat { X } _ { K } } ( x )$ , we have + +$$ +\begin{array} { r l } { \Gamma \vee ( q _ { K } , p _ { \mathrm { Y K } } ) = \Pi \big ( p _ { \tilde { X } _ { K } } , p _ { \mathrm { Y K } } \big ) = \int ( p _ { \tilde { X } _ { K } } ( x ) - p _ { Y _ { K } } ( x ) ) \mathbf { 1 } \{ p _ { \tilde { X } _ { K } } ( x ) > p _ { Y _ { K } } ( x ) \} \mathrm { d } x } \\ { \leq \int ( p _ { \tilde { X } _ { K } } ( x ) - p _ { \tilde { Y } _ { K } } ( x ) ) \mathbf { 1 } \{ p _ { \tilde { X } _ { K } } ( x ) > p _ { \tilde { Y } _ { K } } ( x ) \} \mathrm { d } x } \\ { = \int ( p _ { \tilde { Y } _ { K } } ( x ) - p _ { \tilde { X } _ { K } } ( x ) ) \mathbf { 1 } \{ p _ { \tilde { Y } _ { K } } ( x ) > p _ { \tilde { X } _ { K } } ( x ) \} \mathrm { d } x + P ( \tilde { Y } _ { K } = \infty ) } \\ { \leq \int ( p _ { \tilde { Y } _ { K } } ( x ) - p _ { \tilde { X } _ { K } } ( x ) ) \mathbf { 1 } \{ p _ { \tilde { Y } _ { K } } ( x ) > p _ { \tilde { X } _ { K } } ( x ) \} \mathrm { d } x + P ( \tilde { Y } _ { K } = \infty ) } \\ { \overset { ( a ) } { \leq } \mathbf { T } \big ( p _ { \tilde { Y } _ { K } } , p _ { \tilde { X } _ { K } } \big ) + P \big ( \tilde { X } _ { K } = \infty \big ) } \\ { \overset { ( b ) } { \leq } \mathbf { T } \big ( p _ { \tilde { Y } _ { K } } , p _ { \tilde { X } _ { K } } \big ) + P \big ( \tilde { X } _ { K } = \infty \big ) } \\ { \overset { ( b ) } { \leq } \mathbf { T } \big ( p _ { \tilde { Y } _ { K } } , p _ { \tilde { X } _ { K } } \big ) + \mathbf { T } \big \nabla ( p _ { \tilde { X } _ { K } } , p _ { \tilde { X } _ { K } } \big ) , } \end{array} +$$ + +where (a) uses the fact that + +$$ +\begin{array} { r l r } { { \mathsf { T V } ( p _ { \widetilde { Y } _ { K } } , p _ { \widetilde { X } _ { K } } ) = \int ( p _ { \widetilde { Y } _ { K } } ( x ) - p _ { \widetilde { X } _ { K } } ( x ) ) \mathbb { 1 } \{ p _ { \widetilde { Y } _ { K } } ( x ) > p _ { \widetilde { X } _ { K } } ( x ) \} \mathrm { d } x } } \\ & { } & { + \operatorname* { m a x } \{ P ( \widetilde { Y } _ { K } = \infty ) - P ( \widetilde { X } _ { K } = \infty ) , 0 \} , } \end{array} +$$ + +and (b) uses the fact that + +$$ +P ( \tilde { X } _ { K } = \infty ) \leq \mathsf { T V } ( p _ { \tilde { X } _ { K } } , p _ { \widehat { X } _ { K } } ) . +$$ + +Now we intend to bound the two terms in the right-hand-side of (27) separately. For the first term, by using Pinsker’s inequality, we have + +$$ +\begin{array} { r l } & { \quad \mathsf { T V } ^ { 2 } ( p _ { \widetilde { Y } _ { K } } , p _ { \widetilde { X } _ { K } } ) \le \displaystyle \frac { 1 } { 2 } \mathsf { K L } \left( p _ { \widetilde { X } _ { K } } \| p _ { \widetilde { Y } _ { K } } \right) \le \displaystyle \frac { 1 } { 2 } \mathsf { K L } \left( p _ { \widetilde { X } _ { 0 } , \ldots , \widetilde { X } _ { K } } \| p _ { \widetilde { Y } _ { 0 } , \ldots , \widetilde { Y } _ { K } } \right) } \\ & { = \displaystyle \frac { 1 } { 2 } \mathsf { K L } \left( p _ { \widetilde { X } _ { 0 } } \| p _ { \widetilde { Y } _ { 0 } } \right) + \displaystyle \sum _ { k = 0 } ^ { K - 1 } \mathbb { E } _ { x _ { k } \sim \widetilde { X } _ { k } } \mathsf { K L } ( p _ { \widetilde { X } _ { k + 1 } | \widetilde { X } _ { k } } ( \cdot | x _ { k } ) \| p _ { \widetilde { Y } _ { k + 1 } | \widetilde { Y } _ { k } } ( \cdot | x _ { k } ) ) } \\ & { \overset { \mathrm { ( a ) } } { \le } \displaystyle \frac { 1 } { 2 } \mathsf { K L } \left( p _ { \widehat { X } _ { 0 } } \| p _ { Y _ { 0 } } \right) + \displaystyle \sum _ { k = 0 } ^ { K - 1 } \mathbb { E } _ { x _ { k } \sim \widetilde { X } _ { k } } \mathsf { K L } ( p _ { \widehat { X } _ { k + 1 } | \widehat { X } _ { k } } ( \cdot | x _ { k } ) \| p _ { Y _ { k + 1 } | Y _ { k } } ( \cdot | x _ { k } ) ) , } \end{array} +$$ + +where (a) is proved in Appendix D.4 of supplemental material. For the second term, considering $x \in \mathcal { E } _ { k }$ , we have + +$$ +\begin{array} { l } { p _ { \widetilde { X } _ { k } } ( x ) = \displaystyle \int _ { \mathcal { E } _ { k - 1 } } p _ { \widetilde { X } _ { k } | \widetilde { X } _ { k - 1 } } ( x | x _ { k - 1 } ) p _ { \widetilde { X } _ { k - 1 } } ( x _ { k - 1 } ) \mathrm { d } x _ { k - 1 } } \\ { \displaystyle = \int _ { \mathcal { E } _ { k - 1 } } p _ { \widetilde { X } _ { k } | \widetilde { X } _ { k - 1 } } ( x | x _ { k - 1 } ) p _ { \widehat { X } _ { k - 1 } } ( x _ { k - 1 } ) \mathrm { d } x _ { k - 1 } - \Delta _ { k } ( x ) } \\ { \displaystyle = \int _ { \mathcal { E } _ { k - 1 } } p _ { \widehat { X } _ { k } | \widehat { X } _ { k - 1 } } ( x | x _ { k - 1 } ) p _ { \widehat { X } _ { k - 1 } } ( x _ { k - 1 } ) \mathrm { d } x _ { k - 1 } - \Delta _ { k } ( x ) } \\ { \displaystyle = p _ { \widehat { X } _ { k } } ( x ) - \displaystyle \int _ { \mathcal { E } _ { k - 1 } ^ { c } } p _ { \widehat { X } _ { k } | \widehat { X } _ { k - 1 } } ( x | x _ { k - 1 } ) p _ { \widehat { X } _ { k - 1 } } ( x _ { k - 1 } ) \mathrm { d } x _ { k - 1 } - \Delta _ { k } ( x ) , } \end{array} +$$ + +where + +$$ +\Delta _ { k } ( x ) = \int _ { \mathscr { E } _ { k - 1 } } p _ { \widetilde { X } _ { k } | } \widetilde { x } _ { k - 1 } ( x \mid x _ { k - 1 } ) \left( p _ { \widehat { X } _ { k - 1 } } ( x _ { k - 1 } ) - p _ { \widetilde { X } _ { k - 1 } } ( x _ { k - 1 } ) \right) \mathrm { d } x _ { k - 1 } . +$$ + +Thus we have + +$$ +\begin{array} { r l } & { \mathsf { T V } ( p _ { \bar { X } _ { k } } , p _ { \hat { X } _ { k } } ) = \displaystyle { \int p _ { \hat { X } _ { k } } ( x ) - p _ { \bar { X } _ { k } } ( x ) \mathrm { d } x } } \\ & { \quad \quad \quad \quad \quad = \displaystyle { \int _ { \mathcal { E } _ { k } ^ { \mathrm { e } } } p _ { \hat { X } _ { k } } ( x ) \mathrm { d } x } + \int _ { \mathcal { E } _ { k } } p _ { \hat { X } _ { k } } ( x ) - p _ { \tilde { X } _ { k } } ( x ) \mathrm { d } x } \\ & { \quad \quad \quad \quad \stackrel { \mathrm { ( a ) } } { = } P ( \widehat { X } _ { k } \in \mathcal { E } _ { k } ^ { \mathrm { c } } ) + \displaystyle { \int _ { \mathcal { E } _ { k } } \int _ { \mathcal { E } _ { k - 1 } ^ { \mathrm { c } } } p _ { \hat { X } _ { k } | \hat { X } _ { k - 1 } } ( x | x _ { k - 1 } ) p _ { \hat { X } _ { k - 1 } } ( x _ { k - 1 } ) \mathrm { d } x _ { k - 1 } \mathrm { d } x } } \\ & { \quad \quad \quad \quad \quad + \displaystyle { \int _ { \mathcal { E } _ { k } } \Delta _ { k } ( x ) \mathrm { d } x } , } \end{array} +$$ + +where (a) inserts (29). Notice that + +$$ +\begin{array} { r l } { { \int _ { \mathcal { E } _ { k } } \int _ { \mathcal { E } _ { k - 1 } ^ { c } } p _ { \widehat { X } _ { k } \mid \widehat { X } _ { k - 1 } } ( x \mid x _ { k - 1 } ) p _ { \widehat { X } _ { k - 1 } } ( x _ { k - 1 } ) \mathrm { d } x _ { k - 1 } \mathrm { d } x \leq \int _ { \mathcal { E } _ { k - 1 } ^ { c } } p _ { \widehat { X } _ { k - 1 } } ( x _ { k - 1 } ) \mathrm { d } x _ { k - 1 } } } \\ & { \qquad = P ( \widehat { X } _ { k - 1 } \in \mathcal { E } _ { k - 1 } ^ { c } ) , } \\ & { \int _ { \mathcal { E } _ { k } } \Delta _ { k } ( x ) \mathrm { d } x = \int _ { \mathcal { E } _ { k } } \int _ { \mathcal { E } _ { k - 1 } } p _ { \widetilde { X } _ { k } \mid \widetilde { X } _ { k - 1 } } ( x \mid x _ { k - 1 } ) \Big ( p _ { \widehat { X } _ { k - 1 } } ( x _ { k - 1 } ) - p _ { \widetilde { X } _ { k - 1 } } ( x _ { k - 1 } ) \Big ) \mathrm { d } x _ { k - 1 } \mathrm { d } x } \\ & { \leq \int _ { \mathcal { E } _ { k - 1 } } p _ { \widehat { X } _ { k - 1 } } ( x _ { k - 1 } ) - p _ { \widetilde { X } _ { k - 1 } } ( x _ { k - 1 } ) \mathrm { d } x _ { k - 1 } \leq \mathsf { T V } ( p _ { \widetilde { X } _ { k - 1 } } , p _ { \widehat { X } _ { k - 1 } } ) . } \end{array} +$$ + +Inserting into (30) and by using recursion, we have + +$$ +\begin{array} { r } { \mathsf { T V } ( p _ { \widetilde { X } _ { K } } , p _ { \widehat { X } _ { K } } ) \le \mathsf { T V } ( p _ { \widetilde { X } _ { K - 1 } } , p _ { \widehat { X } _ { K - 1 } } ) + P ( \widehat { X } _ { K - 1 } \in \mathcal { E } _ { K - 1 } ^ { \mathrm { c } } ) + P ( \widehat { X } _ { K } \in \mathcal { E } _ { K } ^ { \mathrm { c } } ) } \end{array} +$$ + +$$ +\leq 2 \sum _ { k = 0 } ^ { K - 1 } P ( \widehat { X } _ { k } \in \mathcal { E } _ { k } ^ { \mathrm { c } } ) +$$ + +considering that + +$$ +\mathsf { T V } ( p _ { \widetilde { X } _ { 0 } } , p _ { \widehat { X } _ { 0 } } ) = P ( \widehat { X } _ { 0 } \in \mathcal { E } _ { 0 } ) , \quad P ( \widehat { X } _ { K } \in \mathcal { E } _ { K } ^ { \mathrm { c } } ) = 0 . +$$ + +Inserting (28) and (31) into (27), we have + +$$ +\begin{array} { r } { \mathsf { T V } \big ( q _ { K } , p _ { Y _ { K } } \big ) \le \sqrt { \frac { 1 } { 2 } \mathsf { K L } \big ( p _ { \hat { X } _ { 0 } } \| p _ { Y _ { 0 } } \big ) + \frac { 1 } { 2 } \displaystyle \sum _ { k = 0 } ^ { K - 1 } \mathbb { E } _ { x _ { k } \sim p _ { \widetilde X _ { k } } } \left[ \mathsf { K L } \big ( p _ { \hat { X } _ { k + 1 } | \hat { X } _ { k } } \left( \cdot | x _ { k } \right) \| p _ { Y _ { k + 1 } | Y _ { k } } \left( \cdot | x _ { k } \right) \big ) \right] } } \\ { + 2 \displaystyle \sum _ { k = 0 } ^ { K - 1 } P ( \widehat X _ { k } \in \mathcal E _ { k } ^ { \circ } ) . } \end{array} +$$ + +# 4.3 Step 3: control the KL divergence between $p _ { \widehat { X } _ { k + 1 } | \widehat { X } _ { k } }$ and $p _ { Y _ { k + 1 } | Y _ { k } }$ + +The KL divergence between conditional distributions $p _ { \widehat { X } _ { k + 1 } | \widehat { X } _ { k } }$ and $p _ { Y _ { k + 1 } | Y _ { k } }$ is + +$$ +\mathsf { K L } \big ( p _ { \hat { X } _ { k + 1 } \mid \hat { X } _ { k } } \left( \cdot \mid x _ { k } \right) \parallel p _ { Y _ { k + 1 } \mid Y _ { k } } \left( \cdot \mid x _ { k } \right) \big ) = \frac { 1 - \tau _ { k + 1 , 0 } } { 2 ( \tau _ { k + 1 , 0 } - \tau _ { k , N } ) } \lVert y _ { \tau _ { k , N } } \left( x _ { k } \right) - x _ { \tau _ { k , N } } \left( x _ { k } \right) \rVert _ { 2 } ^ { 2 } . +$$ + +This can be immediately verified by recalling that $\widehat { X } _ { k + 1 } | \widehat { X } _ { k } = x _ { k }$ and $Y _ { k + 1 } | Y _ { k } = x _ { k }$ are both normal distributions with the same variance τk+1,0−τk,N and different means $\sqrt { \frac { 1 - \tau _ { k + 1 , 0 } } { 1 - \tau _ { k , N } } } x _ { \tau _ { k , N } } ( x _ { k } )$ and $\sqrt { \frac { 1 - \tau _ { k + 1 , 0 } } { 1 - \tau _ { k , N } } } y _ { \tau _ { k , N } } ( x _ { k } )$ , respectively. + +Thus the core of this step is to control the estimation error. Before proceeding, we introduce additional two sequences of auxiliary variables + +$$ +\begin{array} { r l } & { \frac { y _ { \tau _ { k , n } } ^ { * } ( x _ { \tau _ { k , 0 } } ) } { \sqrt { 1 - \tau _ { k , n } } } = \frac { x _ { \tau _ { k , 0 } } } { \sqrt { 1 - \tau _ { k , 0 } } } + \frac { s _ { \tau _ { k , 0 } } ^ { * } ( x _ { \tau _ { k , 0 } } ) } { 2 ( 1 - \tau _ { k , 0 } ) ^ { 3 / 2 } } ( \tau _ { k , 0 } - \widehat { \tau } _ { k , 0 } ) } \\ & { \qquad + \displaystyle { \sum _ { i = 1 } ^ { n - 1 } \frac { s _ { \tau _ { k , i } } ^ { * } ( y _ { \tau _ { k , i } } ) } { 2 ( 1 - \tau _ { k , i } ) ^ { 3 / 2 } } ( \widehat { \tau } _ { k , i - 1 } - \widehat { \tau } _ { k , i } ) } + \frac { s _ { \tau _ { k , n - 1 } } ^ { * } ( y _ { \tau _ { k , n - 1 } } ) } { 2 ( 1 - \tau _ { k , n - 1 } ) ^ { 3 / 2 } } ( \widehat { \tau } _ { k , n - 1 } - \tau _ { k , n } ) , } \\ & { \frac { z _ { \tau _ { k , n } } ^ { * } ( x _ { \tau _ { k , 0 } } ) } { \sqrt { 1 - \tau _ { k , n } } } = \frac { x _ { \tau _ { k , 0 } } } { \sqrt { 1 - \tau _ { k , 0 } } } + \frac { s _ { \tau _ { k , 0 } } ^ { * } ( x _ { \tau _ { k , 0 } } ) } { 2 ( 1 - \tau _ { k , 0 } ) ^ { 3 / 2 } } ( \tau _ { k , 0 } - \widehat { \tau } _ { k , 0 } ) } \\ & \qquad + \displaystyle { \sum _ { i = 1 } ^ { n - 1 } \frac { s _ { \tau _ { k , i } } ^ { * } ( x _ { \tau _ { k , i } } ) } { 2 ( 1 - \tau _ { k , i } ) ^ { 3 / 2 } } ( \widehat { \tau } _ { k , i - 1 } - \widehat { \tau } _ { k , i } ) } + \frac \end{array} +$$ + +and for $n = 1 , \ldots , N$ $z _ { \tau _ { k , n } } ^ { \star }$ on $x _ { \tau _ { k , 0 } }$ . For convenience, in the following proof we shall omit the dependence of without ambiguity. Based on these notations, we define the estimation error $x _ { \tau _ { k , n } }$ , $y _ { \tau _ { k , n } }$ , $y _ { \tau _ { k , n } } ^ { \star }$ , + +$$ +\xi _ { k , n } ( x _ { \tau _ { k , 0 } } ) : = \frac { y _ { \tau _ { k , n } } ^ { \star } - y _ { \tau _ { k , n } } } { \sqrt { 1 - \tau _ { k , n } } } + \frac { x _ { \tau _ { k , n } } - z _ { \tau _ { k , n } } ^ { \star } } { \sqrt { 1 - \tau _ { k , n } } } . +$$ + +Moreover, we define the matrix + +$$ +\Sigma _ { \tau } ( x ) = \mathsf { C o v } [ Z | \sqrt { 1 - \tau } X _ { 0 } + \sqrt { \tau } Z = x ] , +$$ + +where $\mathsf { C o v } [ \cdot ]$ denotes the covariance matrix. The following lemma controls the estimation error $\xi _ { k , n }$ . The proof is postponed to Appendix D.1. + +Lemma 3. For any $k$ and $n$ , with probability at least $1 - T ^ { - 1 0 0 }$ , + +$$ +\mathbb { E } _ { x _ { \tau _ { k , 0 } } \sim p _ { \widehat { X _ { k } } } } \left[ \| \xi _ { k , n } ( x _ { \tau _ { k , 0 } } ) \| _ { 2 } ^ { 2 } \right] \lesssim \frac { d \log ^ { 4 } T } { T ^ { 3 } } \operatorname* { m i n } \Big \{ \frac { N d \widehat { \tau } _ { k , - 1 } \log T } { T ( 1 - \widehat { \tau } _ { k , - 1 } ) } + \int _ { \tau _ { k , n } } ^ { \tau _ { k , 0 } } \frac { \mathbb { E } \left[ \mathsf { T r } ( \Sigma _ { \tau } ^ { 2 } ( x _ { \tau } ) ) \right] } { ( 1 - \tau ) ^ { 2 } } \mathrm { d } \tau , +$$ + +$$ +\frac { N L ^ { 2 } \widehat { \tau } _ { k , - 1 } \log T } { T ( 1 - \widehat { \tau } _ { k , - 1 } ) } \biggr \} + \frac { N \log ^ { 2 } T } { T ^ { 2 } } \sum _ { i = 0 } ^ { n - 1 } \widehat { \tau } _ { k , i } ( 1 - \widehat { \tau } _ { k , i } ) ^ { - 1 } \varepsilon _ { k , i } ^ { 2 } , +$$ + +where $\varepsilon _ { k , i } ^ { 2 }$ is defined in (12). + +With the above relation, we can bound the divergence as following. The proof is postponed to Appendix D.2 in supplemental material. + +Lemma 4. According to Lemma $\boldsymbol { \mathcal { B } }$ , it can be shown that + +$$ +\begin{array} { r l } & { \displaystyle \sum _ { k = 0 } ^ { K - 1 } \mathbb { E } _ { \boldsymbol { x } _ { \tau _ { k , 0 } } \sim p _ { \widetilde { \boldsymbol X } _ { k } } } \left[ \mathsf { K L } \left( p _ { \widehat { X } _ { k + 1 } | \widehat { X } _ { k } } \left( \cdot | \boldsymbol { x } _ { \tau _ { k , 0 } } \right) \parallel p _ { Y _ { k + 1 } | Y _ { k } } \left( \cdot | \boldsymbol { x } _ { \tau _ { k , 0 } } \right) \right) \right] } \\ & { \lesssim \displaystyle \frac { K d \log ^ { 5 } T } { T ^ { 3 } } \operatorname* { m i n } \left\{ d , L ^ { 2 } \right\} + \varepsilon _ { \mathrm { s c o r e } } ^ { 2 } \log T , } \end{array} +$$ + +$\varepsilon _ { \mathsf { s c o r e } } ^ { 2 }$ + +# 4.4 Step 4: putting everything together + +The remaining terms in (32) can be bounded through the following lemma, whose proof can be found in Appendix D.3 in supplemental material. + +Lemma 5. Under our choice of learning schedule (9), we have + +$$ +\begin{array} { l } { { \displaystyle { \mathsf { K L } } \big ( p _ { \widehat X _ { 0 } } \| p _ { \widehat Y _ { 0 } } \big ) \le \frac { 1 } { T ^ { 1 0 } } } , \ ~ } \\ { { \displaystyle \sum _ { k = 0 } ^ { K - 1 } P ( \widehat X _ { k } \in \mathcal E _ { k } ^ { \mathrm { c } } ) \lesssim \left( \frac { d ^ { 2 } \log ^ { 5 } T } { T ^ { 2 } } + \varepsilon _ { \mathrm { s c o r e } } ^ { 2 } \log T \right) \mathbb { 1 } ( d \log T < L ) } . } \end{array} +$$ + +Inserting (36), (37), and (38) into (32) leads to + +$$ +\begin{array} { r l } & { \mathsf { T V } \left( q _ { K } , p _ { Y _ { K } } \right) \lesssim \sqrt { \frac { 1 } { T ^ { 1 0 } } + \frac { K d \log ^ { 5 } T } { T ^ { 3 } } } \operatorname* { m i n } \Big \{ d , L ^ { 2 } \Big \} + \varepsilon _ { \mathrm { s c o r e } } ^ { 2 } \log T } \\ & { \quad \quad \quad \quad + \left( \frac { d ^ { 2 } \log ^ { 5 } T } { T ^ { 2 } } + \varepsilon _ { \mathrm { s c o r e } } ^ { 2 } \log T \right) \mathbb { 1 } ( d \log T < L ) } \\ & { \quad \quad \quad \lesssim \frac { \operatorname* { m i n } \{ d ^ { 3 / 2 } , d L ^ { 1 / 2 } , d ^ { 1 / 2 } L ^ { 3 / 2 } \} \log ^ { 4 } T } { T ^ { 3 / 2 } } + \varepsilon _ { \mathrm { s c o r e } } \log ^ { 1 / 2 } T , } \end{array} +$$ + +and we conclude the proof here. + +# 5 Discussion + +In this paper, we establish a faster convergence rate for generative diffusion models under a relaxed Lipschitz condition, which requires a number of $\operatorname* { m i n } \{ d , d ^ { 2 / 3 } L ^ { 1 / 3 } , d ^ { 1 / 3 } L \} \varepsilon ^ { - 2 / 3 } \log ^ { 8 / 3 } T$ iterations to achieve an $\varepsilon$ accuracy in terms of TV distance, where $L$ denotes the non-uniform Lipschitz constant. As a result, our results demonstrate improvements over the convergence theory without exploiting smoothness across the entire range of $L$ , and accommodates a broader class of distributions, e.g., Gaussian mixture model. In addition, our analysis requires only $\varepsilon$ estimation errors, implying the robustness of the algorithm to imperfect score estimations. Furthermore, we extend the result to the parallel implementation of the sampler. However, several open questions remain. For example, deriving instance-dependent bounds for other settings, such as accelerated samplers or data distributions with low-dimensional structures, which we leave for future research. Moreover, the benefit of the randomized design in our work relies heavily on the deterministic nature of ODE process. It is still unclear for us how to adapt this approach to deal with the inherent stochasticity in SDEs while maintaining similar improvements. This will also be left as future work. In addition, it may be feasible to apply your analysis framework to improve the bound for other variants of samplers such as the Langevin algorithm. Furthermore, estimating the Lipschitz constant in real-world cases would be highly beneficial, potentially broadening the applicability of our results. + +![](images/figures/diffusion-convergence-rate-fig-0002.jpg) +Figure 2: Sampling error of the proposed sampler and fitted rate $T \Theta ( \log ^ { 4 } T / T ^ { 3 } )$ : (a) $d = 1 0 , k = 1 0$ ; (b) $d = 1 0 0 , k = 1 0$ ; (c) $d = 5 0 0 , k = 1 0 0$ . + +# Acknowledgments + +Gen Li is supported in part by the Chinese University of Hong Kong Direct Grant for Research and the Hong Kong Research Grants Council ECS 2191363. + +# A Numerical experiments + +We conduct numerical experiments to validate our theoretical results. For ease of computing, we select a Gaussian distribution as the target distribution. This choice ensures that all $Y _ { k , n }$ in the implementation of the proposed sampler follow a Gaussian distribution and that the KL divergence between $Y _ { k , 0 }$ and $X _ { 1 }$ has a closed-form expression. Moreover, since our primary focus is on the convergence rate, we assume access to the exact score function $s _ { t } ^ { \star } ( \cdot )$ . + +The target distribution $p _ { 0 }$ is a $d$ -dimensional Gaussian distribution with zero mean and a diagonal covariance matrix. The first $k$ diagonal entries are uniformly distributed within the interval [0, 10], while the remaining $d - k$ diagonal entries are set to zero. We implement the proposed sampler in Section 2.2 with $K = 1 0$ , and $N = 2 T / K$ . For different number of iterations $T$ , we compute the distribution of output $Y _ { K }$ , and its KL divergence with the distribution $q _ { K }$ of $X _ { \tau _ { K , 0 } }$ , which is approximately the starting point of the forward process. + +The results are presented in Figure 2. The blue line represents the empirical results, and the black line corresponds to the theoretical rate $O ( \log ^ { 4 } T / T ^ { 3 } )$ . According to Theorem 1, our theoretical analysis predicts a convergence rate of $O ( \mathsf { p o l y } ( \log T ) / T ^ { 3 } )$ in terms of KL divergence, which is consistent with empirical observations. This further confirms that our sampler achieves a KL divergence convergence rate of $O ( \log ^ { 4 } T / T ^ { 3 } )$ in terms of KL divergence, implying a total variation(TV) distance convergence rate of $O ( \log ^ { 2 } T / T ^ { 3 / 2 } )$ . Finally, we remark that Theorem 1 establishes a convergence rate of $O ( \log ^ { 4 } T / T ^ { 3 / 2 } )$ . Compared to empirical results, this bound is suboptimal in terms of its dependence on logarithmic factors. Refining this dependency requires further effort and is left for future work. + +# B Comparison with previous works + +To compare Theorem 1 with previous results, we illustrate the TV distance achieved by various theories for a fixed number of iterations $T$ . The corresponding results are presented in Figure 3. Notably, when $T = O ( d )$ , the results from Benton et al. (2023); Li and Cai (2024); Li and Yan (2024a) reduce to a trivial bound $\varepsilon = 1$ . In comparison, for both $T = O ( d )$ and √ $\underline { T } = O ( d ^ { 2 } )$ , our result achieves the best result across a full range of $L$ , and improves previous results when ${ \sqrt { d } } \lesssim L \lesssim d$ . + +![](images/figures/diffusion-convergence-rate-fig-0003.jpg) +Figure 3: TV distance $\varepsilon$ achieved by Theorem 1 and previous results with left: $T = O ( d )$ ; middle: $T =$ $O ( d ^ { 3 / 2 } )$ ; right: $T = O ( d ^ { 2 } )$ . + +A lot of technical efforts have been devoted to achieving the improvements stated in Section 3. + +a. The absence of a uniform Lipschitz bound (see Definition 2) brings new challenges to the discretization analysis. Prior works (Chen et al., 2024b; Li and Jiao, 2024) leveraged the log-concavity of $p _ { X _ { \tau } | X _ { \tau + \delta } }$ for some small $\delta$ to control the one-step discretization error (see Lemma 1 and (B.2)-(B.4) in Chen et al. (2024b)). However, this approach fails under the high-probability bound employed in this work. To address this issue, we directly handle this derivative based on its definition, decomposing it into two components: one depending on the operator norm of the Jacobian matrix and the other on the data dimension $d$ , and then statistically bounding each term (see Lemma 10). Although the resulting bound is of a similar order, the analytical techniques under the uniform bound assumption and the high-probability bound assumption are fundamentally different. + +b. The lack of a uniform Lipschitz condition poses significant challenges for controlling error propagation across multiple steps, while a naive stepwise analysis may result in suboptimal bounds. To overcome this challenge, we introduce two auxiliary sequences, $\smash { \widetilde { X } } _ { k }$ and $\widetilde { Y _ { k } }$ , which constrain $\widehat { X } _ { k }$ and $Y _ { k }$ to lie within a typical set (see Step 1 in Section 4 and Lemma 2). We then relate the TV distance between $\widehat { X } _ { k }$ and $Y _ { k }$ to that between $\smash { \widetilde { X } } _ { k }$ and $\widetilde { Y _ { k } }$ (see Step 2 in Section 4), by analyzing how error propagation affects the probability that $Y _ { k , n } | Y _ { k , 0 } = x _ { \tau _ { k , 0 } }$ falls outside the typical set (see Lemma 5). Within this typical set, we establish a uniform bound for score functions that depends explicitly on the data dimension $d$ (see Lemma 12). As a result, the error at the $( k , n )$ -th step, $\| y _ { \tau _ { k , n } } - x _ { \tau _ { k , n } } \|$ , is effectively controlled by the discretization error $\zeta _ { k , n }$ and estimation error terms related to $\widetilde { \varepsilon } _ { k , i }$ (see (75)), thereby ensuring stable error propagation throughout the process. + +c. To make the result adaptive to $L$ and applicable for the minimal condition on the target data distribution (i.e., $L = \infty$ ), new bounds for the discretization error and the number of rounds $K$ are derived. Based on the error propagation analysis discussed earlier, the lower bound for $K$ is $O ( \operatorname* { m i n } \{ d \log T , L \} \log T )$ which remains bounded even when $L = \infty$ . Additionally, to obtain a discretization error adaptive to $L$ , we derive a new bound for the expectation of the product $\lVert J _ { \tau } ( x _ { \tau } ) s _ { \tau } ^ { \star } ( x _ { \tau } ) \rVert _ { 2 } ^ { 2 }$ by carefully analyzing the structure of the Jacobian matrix $J _ { \tau } ( \cdot )$ (see Lemma 9). The new bound $O ( d \operatorname* { m i n } \{ d , L ^ { 2 } \} )$ helps to derive a corresponding discretization error of $\tilde { O } ( d \operatorname* { m i n } \{ d , L ^ { 2 } \} )$ (see Lemma 8), which depends only on $d$ when $L$ is large. + +# C Computations of Examples + +# C.1 Computation of Example 1 + +It is easy to check that the score function is + +$$ +s _ { t } ^ { \star } ( x ) = - \Sigma _ { t } ^ { - 1 } x , +$$ + +where $\Sigma _ { t }$ is a diagonal matrix with the $( i , i )$ -th entry equal to + +$$ +( \Sigma _ { t } ) _ { i , i } = \overline { { { \alpha } } } _ { t } \sigma _ { i } ^ { 2 } + 1 - \overline { { { \alpha } } } _ { t } . +$$ + +Thus we have + +$$ +\| s _ { t } ^ { \star } ( x ) - s _ { t } ^ { \star } ( y ) \| _ { 2 } = \| \Sigma _ { t } ^ { - 1 } ( x - y ) \| _ { 2 } , +$$ + +and + +$$ +\| \Sigma _ { t } ^ { - 1 } \| _ { 2 } = \frac { 1 } { \overline { { \alpha } } _ { t } \operatorname* { m i n } \sigma _ { i } ^ { 2 } + 1 - \overline { { \alpha } } _ { t } } = \frac { 1 } { 1 - \overline { { \alpha } } _ { t } } . +$$ + +Then we complete the proof. + +# C.2 Computation of Example 2 + +It is easy to check that the score function is + +$$ +s _ { t } ^ { \star } ( x ) = - \frac { 1 } { \sigma _ { t } ^ { 2 } } \sum _ { h = 1 } ^ { H } \pi _ { h } ( x ) ( x - \sqrt { \overline { { \alpha } } _ { t } } \mu _ { h } ) = - \frac { x } { \sigma _ { t } ^ { 2 } } + \frac { \sqrt { \overline { { \alpha } } _ { t } } } { \sigma _ { t } ^ { 2 } } \sum _ { h = 1 } ^ { H } \pi _ { h } ( x ) \mu _ { h } , +$$ + +where + +$$ +\pi _ { h } ( x ) = \frac { \gamma _ { h } \exp { \left( - \frac { 1 } { \sigma _ { t } ^ { 2 } } \| x - \sqrt { \overline { { \alpha } } _ { t } } \mu _ { h } \| ^ { 2 } \right) } } { \sum _ { i = 1 } ^ { H } \gamma _ { i } \exp { \left( - \frac { 1 } { \sigma _ { t } ^ { 2 } } \| x - \sqrt { \overline { { \alpha } } _ { t } } \mu _ { i } \| ^ { 2 } \right) } } , \qquad \mathrm { a n d } \qquad \sigma _ { t } ^ { 2 } = \overline { { \alpha } } _ { t } \sigma ^ { 2 } + 1 - \overline { { \alpha } } _ { t } . +$$ + +# C.2.1 Proof of the upper bound + +For ease of notations, we prove that for any $\mu _ { h } , \sigma$ , and $\gamma _ { h }$ , the following inequality holds: + +$$ +\mathbb { P } \left\{ \exists x ^ { \prime } \in \mathbb { R } ^ { d } , ( 1 - \overline { { \alpha } } _ { t } ) \| s _ { t } ^ { \star } ( x ) - s _ { t } ^ { \star } ( x ^ { \prime } ) \| _ { 2 } > C \log ( H T ) \| x - x ^ { \prime } \| _ { 2 } \right\} \lesssim \frac { 1 } { T ^ { 4 } } . +$$ + +The upper bound can be proved by replacing $T$ with $T + d$ . + +According to the definition of $s _ { t } ^ { \star } ( x )$ , We have + +$$ +\begin{array} { l } { \displaystyle ( 1 - \overline { { \alpha } } _ { t } ) \| s _ { t } ^ { \star } ( x ) - s _ { t } ^ { \star } ( x ^ { \prime } ) \| _ { 2 } } \\ { \displaystyle \overset { \mathrm { ( a ) } } { = } \left\| - \frac { 1 - \overline { { \alpha } } _ { t } } { \sigma _ { t } ^ { 2 } } ( x - x ^ { \prime } ) + \frac { \sqrt { \overline { { \alpha } } _ { t } } ( 1 - \overline { { \alpha } } _ { t } ) } { \sigma _ { t } ^ { 2 } } \displaystyle \sum _ { i = 1 } ^ { H } ( \pi _ { i } ( x ) - \pi _ { i } ( x ^ { \prime } ) ) ( \mu _ { i } - \mu _ { h } ) \right\| _ { 2 } } \\ { \displaystyle \leq \frac { 1 - \overline { { \alpha } } _ { t } } { \sigma _ { t } ^ { 2 } } \| x - x ^ { \prime } \| _ { 2 } + \frac { \sqrt { \overline { { \alpha } } _ { t } } ( 1 - \overline { { \alpha } } _ { t } ) } { \sigma _ { t } ^ { 2 } } \displaystyle \sum _ { i = 1 } ^ { H } \| ( \pi _ { i } ( x ) - \pi _ { i } ( x ^ { \prime } ) ) ( \mu _ { i } - \mu _ { h } ) \| _ { 2 } } \\ { \displaystyle \overset { \mathrm { ( b ) } } { \leq } \| x - x ^ { \prime } \| _ { 2 } + \sqrt { \overline { { \alpha } } _ { t } } \displaystyle \sum _ { i = 1 } ^ { H } \| ( \pi _ { i } ( x ) - \pi _ { i } ( x ^ { \prime } ) ) ( \mu _ { i } - \mu _ { h } ) \| _ { 2 } , } \end{array} +$$ + +where (a) uses the fact that $\begin{array} { r } { \sum _ { i = 1 } ^ { H } ( { \pi } _ { i } ( x ) - { \pi } _ { i } ( x ^ { \prime } ) ) ( { \mu } _ { i } - { \mu } _ { h } ) = \sum _ { i = 1 } ^ { H } ( { \pi } _ { i } ( x ) - { \pi } _ { i } ( x ^ { \prime } ) ) { \mu } _ { i } } \end{array}$ , and (b) use the fact that $1 - \overline { { \alpha } } _ { t } \le \sigma _ { t } ^ { 2 }$ . Thus we have + +$$ +\begin{array} { r l } & { \quad \mathbb { P } \left\{ \exists x ^ { \prime } \in \mathbb { R } ^ { d } , ( 1 - \overline { { \alpha } } _ { t } ) \| s _ { t } ^ { \star } ( x ) - s _ { t } ^ { \star } ( x ^ { \prime } ) \| _ { 2 } > C \log ( H T ) \| x - x ^ { \prime } \| _ { 2 } \right\} } \\ & { \leq \displaystyle \sum _ { h = 1 } ^ { H } \gamma _ { h } \mathbb { P } _ { h } \left\{ \exists x ^ { \prime } \in \mathbb { R } ^ { d } , ( 1 - \overline { { \alpha } } _ { t } ) \| s _ { t } ^ { \star } ( x ) - s _ { t } ^ { \star } ( x ^ { \prime } ) \| _ { 2 } > C \log ( H T ) \| x - x ^ { \prime } \| _ { 2 } \right\} } \\ & { \leq \displaystyle \sum _ { h = 1 } ^ { H } \gamma _ { h } \mathbb { P } _ { h } \left\{ \exists x ^ { \prime } \in \mathbb { R } ^ { d } , \sqrt { \overline { { \alpha } } _ { t } } \displaystyle \sum _ { i = 1 } ^ { H } \| ( \pi _ { i } ( x ) - \pi _ { i } ( x ^ { \prime } ) ) ( \mu _ { i } - \mu _ { h } ) \| _ { 2 } > \frac { C \log ( H T ) } { 2 } \| x - x ^ { \prime } \| _ { 2 } \right\} } \end{array} +$$ + +$$ +\begin{array} { r l } & { \le \displaystyle \sum _ { h : \gamma _ { h } \ge \frac { 1 } { H T ^ { \sharp } } } ^ { H } \gamma _ { h } \mathbb { P } _ { h } \left\{ \exists x ^ { \prime } \in \mathbb { R } ^ { d } , \sqrt { \overline { { \alpha } } _ { t } } \displaystyle \sum _ { i = 1 } ^ { H } \| ( \pi _ { i } ( x ) - \pi _ { i } ( x ^ { \prime } ) ) ( \mu _ { i } - \mu _ { h } ) \| _ { 2 } > \frac { C \log ( H T ) } { 2 } \| x - x ^ { \prime } \| _ { 2 } \right\} } \\ & { + \displaystyle \frac { 1 } { T ^ { \sharp } } , } \end{array} +$$ + +where $\mathbb { P } _ { h } \{ \cdot \}$ denotes the probability of the event when $x$ follows the Gaussian distribution $\mathcal { N } ( \sqrt { \overline { { \alpha } } _ { t } } \mu _ { h } , \sigma _ { t } ^ { 2 } I _ { d } )$ Define the set + +$$ +\mathcal { T } _ { t } ^ { h } = \left\{ x \in \mathbb { R } ^ { d } : \sqrt { \overline { { \alpha _ { t } } } } \lvert ( x - \sqrt { \overline { { \alpha _ { t } } } } \mu _ { h } ) ^ { \top } ( \mu _ { i } - \mu _ { h } ) \rvert \leq C _ { 2 } \sigma _ { t } \sqrt { \overline { { \alpha _ { t } } } \log ( H T ) } \lVert \mu _ { i } - \mu _ { h } \rVert , \forall i \right\} . +$$ + +According to the concentration inequality of Gaussian distribution, it is easy to check that for sufficiently large $C _ { 2 }$ , we have + +$$ +\mathbb { P } \left\{ x \sim \mathcal { N } ( \sqrt { \overline { { \alpha } } _ { t } } \mu _ { h } , \sigma _ { t } ^ { 2 } I _ { d } ) , x \notin \mathcal { T } _ { t } ^ { h } \right\} \lesssim \frac { 1 } { H T ^ { 4 } } , \qquad \forall h . +$$ + +Below we shall prove that for $h$ satisfying $\begin{array} { r } { \gamma _ { h } \ge \frac { 1 } { H T ^ { 4 } } } \end{array}$ , + +$$ +T _ { t } ^ { h } \subset \left\{ x \in \mathbb { R } ^ { d } : \forall x ^ { \prime } \in \mathbb { R } ^ { d } , \sqrt { \alpha _ { t } } \sum _ { i = 1 } ^ { H } \| ( \pi _ { i } ( x ) - \pi _ { i } ( x ^ { \prime } ) ) ( \mu _ { i } - \mu _ { h } ) \| _ { 2 } \leq \frac { C \log ( H T ) } { 2 } \| x - x ^ { \prime } \| _ { 2 } \right\} . +$$ + +Inserting it into (39), we have + +$$ +\begin{array} { l } { \displaystyle \mathbb { P } \left\{ \exists \boldsymbol { x } ^ { \prime } \in \mathbb { R } ^ { d } , ( 1 - \overline { { \alpha } } _ { t } ) \| \boldsymbol { s } _ { t } ^ { \star } ( \boldsymbol { x } ) - \boldsymbol { s } _ { t } ^ { \star } ( \boldsymbol { x } ^ { \prime } ) \| _ { 2 } > C \log ( H T ) \| \boldsymbol { x } - \boldsymbol { x } ^ { \prime } \| _ { 2 } \right\} } \\ { \displaystyle \le \sum _ { h = 1 } ^ { H } \gamma _ { h } \mathbb { P } _ { h } \left\{ \boldsymbol { x } \notin \mathcal { T } _ { t } ^ { h } \right\} \mathbb { 1 } \left( \gamma _ { h } \ge \frac { 1 } { H T ^ { 4 } } \right) + \frac { 1 } { T ^ { 4 } } \lesssim \frac { 1 } { T ^ { 4 } } , } \end{array} +$$ + +and complete the proof. + +Proof of (40). To this end, we introduce an auxiliary set + +$$ +\begin{array} { r } { \mathcal { F } _ { h } = \left. i : \| x - x ^ { \prime } \| _ { 2 } \leq c \sqrt { \overline { { \alpha } } _ { t } } \| \mu _ { i } - \mu _ { h } \| _ { 2 } \right. , } \end{array} +$$ + +where $c$ is a sufficiently small constant. Then we have + +$$ +\begin{array} { r l } & { \sqrt { \overline { { \alpha _ { t } } } } \displaystyle \sum _ { i = 1 } ^ { H } \| ( \pi _ { i } ( x ) - \pi _ { i } ( x ^ { \prime } ) ) ( \mu _ { i } - \mu _ { h } ) \| _ { 2 } } \\ & { \leq \sqrt { \overline { { \alpha _ { t } } } } \displaystyle \sum _ { i \in \mathcal { F } _ { h } } \| ( \pi _ { i } ( x ) - \pi _ { i } ( x ^ { \prime } ) ) ( \mu _ { i } - \mu _ { h } ) \| _ { 2 } + \sqrt { \overline { { \alpha _ { t } } } } \displaystyle \sum _ { i \in \mathcal { F } _ { h } ^ { c } } \| ( \pi _ { i } ( x ) - \pi _ { i } ( x ^ { \prime } ) ) ( \mu _ { i } - \mu _ { h } ) \| _ { 2 } } \\ & { \leq \sqrt { \overline { { \alpha _ { t } } } } \displaystyle \sum _ { i \in \mathcal { F } _ { h } } \| ( \pi _ { i } ( x ) - \pi _ { i } ( x ^ { \prime } ) ) ( \mu _ { i } - \mu _ { h } ) \| _ { 2 } + \displaystyle \frac { 2 } { c } \| x - x ^ { \prime } \| _ { 2 } . } \end{array} +$$ + +For $i \in \mathcal { F } _ { h }$ , we make a decomposition on the term √ √ $| \pi _ { i } ( x ) - \pi _ { i } ( x ^ { \prime } ) |$ . Define √ $\widetilde { x } _ { i } \ = \ x \mathbb { 1 } ( \| x - \sqrt { \overline { { \alpha } } _ { t } } \mu _ { i } \| _ { 2 } \ \leq$ $\| x ^ { \prime } - \sqrt { \alpha _ { t } } \mu _ { i } \| _ { 2 } ) + x ^ { \prime } \mathbb { 1 } ( \| x ^ { \prime } - \sqrt { \alpha _ { t } } \mu _ { i } \| _ { 2 } \leq \| x - \sqrt { \alpha _ { t } } \mu _ { i } \| _ { 2 } )$ and $\widetilde { x } _ { i } ^ { \prime } = x ^ { \prime } \mathbb { 1 } ( \| x - \sqrt { \overline { { \alpha } } _ { t } } \mu _ { i } \| _ { 2 } \leq \| x ^ { \prime } - \sqrt { \overline { { \alpha } } _ { t } } \mu _ { i } \| _ { 2 } ) \ +$ $x \mathbf { 1 } [ \left\| x ^ { \prime } - \sqrt { \overline { { \alpha } } _ { t } } \mu _ { i } \right\| _ { 2 } \leq \left\| x - \sqrt { \overline { { \alpha } } _ { t } } \mu _ { i } \right\| _ { 2 } )$ + +$$ +\begin{array} { r l } & { | \pi _ { i } ( x ) - \pi _ { i } ( x ^ { \prime } ) | \leq \pi _ { i } ( \widetilde { x } _ { i } ) \left| 1 - \exp \left( - \frac { \| \widetilde { x } _ { i } ^ { \prime } - \sqrt { \alpha _ { t } } \mu _ { i } \| _ { 2 } ^ { 2 } } { 2 \sigma _ { t } ^ { 2 } } + \frac { \| \widetilde { x } _ { i } - \sqrt { \alpha _ { t } } \mu _ { i } \| _ { 2 } ^ { 2 } } { 2 \sigma _ { t } ^ { 2 } } \right) \right| } \\ & { \qquad + \displaystyle \sum _ { \ell = 1 } ^ { H } \frac { \gamma _ { \ell } \gamma _ { i } \exp \left( - \frac { \| \widetilde { x } _ { i } ^ { \prime } - \sqrt { \alpha _ { t } } \mu _ { i } \| _ { 2 } ^ { 2 } } { 2 \sigma _ { t } ^ { 2 } } \right) \left| \exp \left( - \frac { \| \widetilde { x } _ { i } ^ { \prime } - \sqrt { \alpha _ { t } } \mu _ { \ell } \| _ { 2 } ^ { 2 } } { 2 \sigma _ { t } ^ { 2 } } \right) - \exp \left( - \frac { \| \widetilde { x } _ { i } - \sqrt { \alpha _ { t } } \mu _ { \ell } \| _ { 2 } ^ { 2 } } { 2 \sigma _ { t } ^ { 2 } } \right) \right| } { \left( \sum _ { \ell = 1 } ^ { H } \gamma _ { \ell } \exp \left( - \frac { \| \widetilde { x } _ { i } - \sqrt { \alpha _ { t } } \mu _ { \ell } \| _ { 2 } ^ { 2 } } { 2 \sigma _ { t } ^ { 2 } } \right) \right) \left( \sum _ { \ell = 1 } ^ { H } \gamma _ { \ell } \exp \left( - \frac { \| \widetilde { x } _ { i } ^ { \prime } - \sqrt { \alpha _ { t } } \mu _ { \ell } \| _ { 2 } ^ { 2 } } { 2 \sigma _ { t } ^ { 2 } } \right) \right) } . } \end{array} +$$ + +For the first term, we have + +$$ +\pi _ { i } ( \widetilde { x } _ { i } ) \left| 1 - \exp \left( - \frac { \| \widetilde { x } _ { i } ^ { \prime } - \sqrt { \overline { { \alpha _ { t } } } } \mu _ { i } \| _ { 2 } ^ { 2 } } { 2 \sigma _ { t } ^ { 2 } } + \frac { \| \widetilde { x } _ { i } - \sqrt { \overline { { \alpha _ { t } } } } \mu _ { i } \| _ { 2 } ^ { 2 } } { 2 \sigma _ { t } ^ { 2 } } \right) \right| +$$ + +$$ +\begin{array} { r l } & { \le \displaystyle \frac { \pi _ { i } ( \widetilde { x } _ { i } ) } { \sigma _ { t } ^ { 2 } } \left( | ( x - \sqrt { \alpha _ { t } } \mu _ { h } ) ^ { \top } ( \widetilde { x } _ { i } ^ { \prime } - \widetilde { x } _ { i } ) | + \sqrt { \alpha _ { t } } | ( \widetilde { x } _ { i } ^ { \prime } - \widetilde { x } _ { i } ) ^ { \top } ( \mu _ { h } - \mu _ { i } ) | + \frac { \| x - x ^ { \prime } \| _ { 2 } ^ { 2 } } { 2 } \right) } \\ & { \overset { \mathrm { ( a ) } } { \le } \displaystyle \frac { \operatorname* { m a x } \{ \pi _ { i } ( x ) , \pi _ { i } ( x ^ { \prime } ) \} } { \sigma _ { t } ^ { 2 } } \left( | ( x - \sqrt { \alpha _ { t } } \mu _ { h } ) ^ { \top } ( x ^ { \prime } - x ) | + \left( 1 + \frac { c } { 2 } \right) \sqrt { \alpha _ { t } } \| x ^ { \prime } - x \| _ { 2 } \| \mu _ { h } - \mu _ { i } \| _ { 2 } \right) , } \end{array} +$$ + +where (a) uses the fact that $| ( \widetilde { \boldsymbol { x } } _ { i } ^ { \prime } - \widetilde { \boldsymbol { x } } _ { i } ) ^ { \top } ( \boldsymbol { \mu } _ { k } - \boldsymbol { \mu } _ { i } ) | \leq \| \boldsymbol { x } ^ { \prime } - \boldsymbol { x } \| _ { 2 } \| \boldsymbol { \mu } _ { k } - \boldsymbol { \mu } _ { i } \| _ { 2 }$ and $\| x - x ^ { \prime } \| _ { 2 } \leq c \sqrt { \overline { { \alpha } } _ { t } } \| \mu _ { i } - \mu _ { h } \| _ { 2 }$ . For the second term, we have + +$$ +\begin{array} { r l } & { \quad \frac { \gamma _ { \ell } \gamma _ { i } \exp \big ( - \frac { \| \tilde { x } _ { i } ^ { \prime } - \sqrt { \alpha _ { i } } \mu _ { i } \| _ { 2 } ^ { 2 } } { 2 \sigma _ { i } ^ { 2 } } \big ) } { ( \sum _ { \ell = 1 } ^ { H } \gamma _ { \ell } \exp \big ( - \frac { \| \tilde { x } _ { i } ^ { \prime } - \sqrt { \alpha _ { i } } \mu _ { i } \| _ { 2 } ^ { 2 } } { 2 \sigma _ { i } ^ { 2 } } \big ) ) } ( \exp ( - \frac { \| \tilde { x } _ { i } ^ { \prime } - \sqrt { \alpha _ { i } } \mu _ { i } \| _ { 2 } ^ { 2 } } { 2 \sigma _ { i } ^ { 2 } } ) - \exp ( - \frac { \| \tilde { x } _ { i } ^ { \prime } - \sqrt { \alpha _ { i } } \mu _ { i } \| _ { 2 } ^ { 2 } } { 2 \sigma _ { i } ^ { 2 } } ) ) } \\ & { \qquad ( \sum _ { \ell = 1 } ^ { H } \gamma _ { \ell } \exp ( - \frac { \| \tilde { x } _ { i } ^ { \prime } - \sqrt { \alpha _ { i } } \mu _ { i } \| _ { 2 } ^ { 2 } } { 2 \sigma _ { i } ^ { 2 } } ) ) ( \sum _ { \ell = 1 } ^ { H } \gamma _ { \ell } \exp ( - \frac { \| \tilde { x } _ { i } ^ { \prime } - \sqrt { \alpha _ { i } } \mu _ { i } \| _ { 2 } ^ { 2 } } { 2 \sigma _ { i } ^ { 2 } } ) ) } \\ & \leq \frac { \gamma _ { \ell } \exp \big ( - \frac { \| \tilde { x } _ { i } ^ { \prime } - \sqrt { \alpha _ { i } } \mu _ { i } \| _ { 2 } ^ { 2 } } { 2 \sigma _ { i } ^ { 2 } } \big ) } { \operatorname* { m i n } \{ s ( x ) , s ( x ^ { \prime } ) \} } \gamma _ { \ell } | \exp ( - \frac \| \tilde \end{array} +$$ + +where + +$$ +s ( \boldsymbol { x } ) : = \sum _ { \ell = 1 } ^ { H } \gamma _ { \ell } \exp \left( - \frac { \| \boldsymbol { x } - \sqrt { \overline { { \alpha _ { t } } } } \mu _ { \ell } \| _ { 2 } ^ { 2 } } { 2 \sigma _ { t } ^ { 2 } } \right) , +$$ + +and (a) comes from the fact that + +$$ +\begin{array} { r l } & { \frac { \gamma _ { i } \exp { \left( - \frac { \| \widetilde { x } _ { i } ^ { \prime } - \sqrt { \alpha _ { t } } \mu _ { i } \| _ { 2 } ^ { 2 } } { 2 \sigma _ { t } ^ { 2 } } \right) } } { \operatorname* { m i n } \{ s ( x ) , s ( x ^ { \prime } ) \} } \leq \operatorname* { m a x } \left\{ \frac { \gamma _ { i } \exp { \left( - \frac { \| \widetilde { x } _ { i } ^ { \prime } - \sqrt { \alpha _ { t } } \mu _ { i } \| _ { 2 } ^ { 2 } } { 2 \sigma _ { t } ^ { 2 } } \right) } } { s ( \widetilde { x } _ { i } ) } , \frac { \gamma _ { i } \exp { \left( - \frac { \| \widetilde { x } _ { i } ^ { \prime } - \sqrt { \alpha _ { t } } \mu _ { i } \| _ { 2 } ^ { 2 } } { 2 \sigma _ { t } ^ { 2 } } \right) } } { s ( \widetilde { x } _ { i } ^ { \prime } ) } \right\} } \\ & { \qquad \leq \operatorname* { m a x } \left\{ \frac { \gamma _ { i } \exp { \left( - \frac { \| \widetilde { x } _ { i } - \sqrt { \alpha _ { t } } \mu _ { i } \| _ { 2 } ^ { 2 } } { 2 \sigma _ { t } ^ { 2 } } \right) } } { s ( \widetilde { x } _ { i } ) } , \frac { \gamma _ { i } \exp { \left( - \frac { \| \widetilde { x } _ { i } ^ { \prime } - \sqrt { \alpha _ { t } } \mu _ { i } \| _ { 2 } ^ { 2 } } { 2 \sigma _ { t } ^ { 2 } } \right) } } { s ( \widetilde { x } _ { i } ^ { \prime } ) } \right\} } \\ & { \qquad = \operatorname* { m a x } \left\{ \pi _ { i } ( x ) , \pi _ { i } ( x ^ { \prime } ) \right\} , } \end{array} +$$ + +and the following inequality holds according to (43): + +$$ +\begin{array} { r l } & { \quad \frac { \gamma _ { \ell } \left| \exp \big ( - \frac { \| \tilde { x } _ { \ell } ^ { \prime } - \sqrt { \alpha _ { t } } \mu \| _ { 2 } ^ { 2 } } { 2 \sigma _ { t } ^ { 2 } } \big ) - \exp \big ( - \frac { \| \tilde { x } _ { \ell } - \sqrt { \alpha _ { t } } \mu \| _ { 2 } ^ { 2 } } { 2 \sigma _ { t } ^ { 2 } } \big ) \right| } { \operatorname* { m a x } \{ s ( x ) , s ( x ^ { \prime } ) \} } } \\ & { \leq \frac { \operatorname* { m a x } \{ \pi _ { \ell } ( x ) , \pi _ { \ell } ( x ^ { \prime } ) \} } { \sigma _ { t } ^ { 2 } } \bigg ( | ( x - \sqrt { \alpha _ { t } } \mu _ { h } ) ^ { \top } ( x ^ { \prime } - x ) | + \sqrt { \alpha _ { t } } | ( x ^ { \prime } - x ) ^ { \top } ( \mu _ { h } - \mu _ { \ell } ) | + \frac { \| x - x ^ { \prime } \| _ { 2 } ^ { 2 } } { 2 } \bigg ) } \\ & { \leq \frac { \operatorname* { m a x } \{ \pi _ { \ell } ( x ) , \pi _ { \ell } ( x ^ { \prime } ) \} } { \sigma _ { t } ^ { 2 } } \Big ( | ( x - \sqrt { \alpha _ { t } } \mu _ { h } ) ^ { \top } ( x ^ { \prime } - x ) | + \sqrt { \alpha _ { t } } \| x ^ { \prime } - x \| _ { 2 } \| \mu _ { h } - \mu _ { \ell } \| _ { 2 } } \\ & { \quad + \frac { c \sqrt { \alpha _ { t } } \| \mu _ { i } - \mu _ { h } \| \| x - x ^ { \prime } \| _ { 2 } } { 2 } \Big ) . } \end{array} +$$ + +Inserting (44) and (45) into (42), for $c \leq 2$ , we have + +$$ +\sqrt { \overline { { \alpha } } _ { t } } \lvert \pi _ { i } ( x ) - \pi _ { i } ( x ^ { \prime } ) \rvert \lVert \mu _ { i } - \mu _ { h } \rVert _ { 2 } \leq \epsilon _ { i , 1 } + \sum _ { \ell = 1 } ^ { H } \epsilon _ { i , 2 } ^ { ( \ell ) } , +$$ + +where + +$$ +\epsilon _ { i , 1 } : = \frac { \operatorname* { m a x } \{ \pi _ { i } ( x ) , \pi _ { i } ( x ^ { \prime } ) \} } { \sigma _ { t } ^ { 2 } } \left( \sqrt { \overline { { \alpha _ { t } } } } | ( x - \sqrt { \overline { { \alpha _ { t } } } } \mu _ { h } ) ^ { \top } ( \mu _ { i } - \mu _ { h } ) | + 2 \overline { { \alpha } } _ { t } \| \mu _ { h } - \mu _ { i } \| _ { 2 } ^ { 2 } \right) \| x ^ { \prime } - x \| _ { 2 } , +$$ + +$$ +\begin{array} { r l r } & { } & { \epsilon _ { i , 2 } ^ { ( \ell ) } : = \operatorname* { m a x } \{ \pi _ { i } ( x ) , \pi _ { i } ( x ^ { \prime } ) \} \frac { \operatorname* { m a x } \{ \pi _ { \ell } ( x ) , \pi _ { \ell } ( x ^ { \prime } ) \} } { \sigma _ { t } ^ { 2 } } \Biggl ( \sqrt { \overline { { \alpha } } _ { t } } | ( x - \sqrt { \overline { { \alpha } } _ { t } } \mu _ { h } ) ^ { \top } ( \mu _ { i } - \mu _ { h } ) | } \\ & { } & { + \overline { { \alpha } } _ { t } \| \mu _ { h } - \mu _ { \ell } \| _ { 2 } \| \mu _ { h } - \mu _ { i } \| _ { 2 } + \overline { { \alpha } } _ { t } \| \mu _ { i } - \mu _ { h } \| _ { 2 } ^ { 2 } \Biggr ) \| x ^ { \prime } - x \| _ { 2 } . } \end{array} +$$ + +We use the following lemma to continue the proof. + +Lemma 6. For any $i$ and $h$ , suppose that $x \in \mathcal { T } _ { t } ^ { h }$ , $\| x - x ^ { \prime } \| _ { 2 } \leq c \sqrt { \overline { { \alpha } } _ { t } } \| \mu _ { i } - \mu _ { h } \| _ { 2 }$ , and $\gamma _ { h } \geq \frac { 1 } { H T ^ { 4 } }$ , where c is a sufficiently small constant. Then we have + +$$ +\begin{array} { r l r } & { } & { \frac { \overline { { \alpha } } _ { t } } { \sigma _ { t } ^ { 2 } } \operatorname* { m a x } \{ \pi _ { i } ( x ) , \pi _ { i } ( x ^ { \prime } ) \} \Vert \mu _ { i } - \mu _ { h } \Vert _ { 2 } ^ { 2 } \lesssim \operatorname* { m a x } \{ \gamma _ { i } , \pi _ { i } ( x ) , \pi _ { i } ( x ^ { \prime } ) \} \log ( H T ) , } \\ & { } & { \frac { \sqrt { \alpha _ { t } } } { \sigma _ { t } ^ { 2 } } \operatorname* { m a x } \{ \pi _ { i } ( x ) , \pi _ { i } ( x ^ { \prime } ) \} | ( x - \sqrt { \alpha _ { t } } \mu _ { h } ) ^ { \top } ( \mu _ { i } - \mu _ { h } ) | \lesssim \operatorname* { m a x } \{ \gamma _ { i } , \pi _ { i } ( x ) , \pi _ { i } ( x ^ { \prime } ) \} \log ( H T ) . } \end{array} +$$ + +By using Lemma 6, we have + +$$ +\begin{array} { r l } & { \epsilon _ { i , 1 } \lesssim \operatorname* { m a x } \{ \gamma _ { i } , \pi _ { i } ( x ) , \pi _ { i } ( x ^ { \prime } ) \} \log ( H T ) \| x - x ^ { \prime } \| _ { 2 } , } \\ & { \epsilon _ { i , 2 } ^ { ( \ell ) } \lesssim \operatorname* { m a x } \{ \pi _ { \ell } ( x ) , \pi _ { \ell } ( x ^ { \prime } ) \} \operatorname* { m a x } \{ \gamma _ { i } , \pi _ { i } ( x ) , \pi _ { i } ( x ^ { \prime } ) \} \log ( H T ) \| x - x ^ { \prime } \| _ { 2 } } \\ & { \qquad + \operatorname* { m a x } \{ \pi _ { \ell } ( x ) , \pi _ { \ell } ( x ^ { \prime } ) \} \operatorname* { m a x } \{ \pi _ { i } ( x ) , \pi _ { i } ( x ^ { \prime } ) \} \frac { \overline { { \alpha } } _ { t } } { \sigma _ { t } ^ { 2 } } \| \mu _ { h } - \mu _ { \ell } \| _ { 2 } \| \mu _ { h } - \mu _ { i } \| _ { 2 } \| x - x ^ { \prime } \| _ { 2 } . } \end{array} +$$ + +If $\| x - x ^ { \prime } \| _ { 2 } \leq c \sqrt { \overline { { \alpha } } _ { t } } \| \mu _ { \ell } - \mu _ { h } \| _ { 2 }$ , then we have + +$$ +\begin{array} { r l } & { \quad \operatorname* { m a x } \{ \pi _ { \ell } ( x ) , \pi _ { \ell } ( x ^ { \prime } ) \} \operatorname* { m a x } \{ \pi _ { i } ( x ) , \pi _ { i } ( x ^ { \prime } ) \} \frac { \overline { { \alpha } } _ { t } } { \sigma _ { t } ^ { 2 } } \| \mu _ { h } - \mu _ { \ell } \| _ { 2 } \| \mu _ { h } - \mu _ { i } \| _ { 2 } \| x - x ^ { \prime } \| _ { 2 } } \\ & { \le \operatorname* { m a x } \{ \pi _ { \ell } ( x ) , \pi _ { \ell } ( x ^ { \prime } ) \} \operatorname* { m a x } \{ \pi _ { i } ( x ) , \pi _ { i } ( x ^ { \prime } ) \} \frac { \overline { { \alpha } } _ { t } } { \sigma _ { t } ^ { 2 } } \operatorname* { m a x } \{ \| \mu _ { h } - \mu _ { \ell } \| _ { 2 } ^ { 2 } , \| \mu _ { h } - \mu _ { i } \| _ { 2 } ^ { 2 } \} \| x - x ^ { \prime } \| _ { 2 } } \\ & { \le \operatorname* { m a x } \{ \gamma _ { \ell } , \pi _ { \ell } ( x ) , \pi _ { \ell } ( x ^ { \prime } ) \} \operatorname* { m a x } \{ \gamma _ { i } , \pi _ { i } ( x ) , \pi _ { i } ( x ^ { \prime } ) \} \log ( H T ) \| x - x ^ { \prime } \| _ { 2 } . } \end{array} +$$ + +Otherwise, that is $\| x - x ^ { \prime } \| _ { 2 } > c \sqrt { \overline { { \alpha } } _ { t } } \| \mu _ { \ell } - \mu _ { h } \| _ { 2 }$ , then we have + +$$ +\begin{array} { r l } & { \quad \operatorname* { m a x } \{ \pi _ { \ell } ( x ) , \pi _ { \ell } ( x ^ { \prime } ) \} \operatorname* { m a x } \{ \pi _ { i } ( x ) , \pi _ { i } ( x ^ { \prime } ) \} \frac { \overline { { \alpha } } _ { t } } { \sigma _ { t } ^ { 2 } } \| \mu _ { h } - \mu _ { \ell } \| _ { 2 } \| \mu _ { h } - \mu _ { i } \| _ { 2 } \| x - x ^ { \prime } \| _ { 2 } } \\ & { \le \operatorname* { m a x } \{ \pi _ { \ell } ( x ) , \pi _ { \ell } ( x ^ { \prime } ) \} \operatorname* { m a x } \{ \pi _ { i } ( x ) , \pi _ { i } ( x ^ { \prime } ) \} \frac { \overline { { \alpha } } _ { t } } { \sigma _ { t } ^ { 2 } } \frac { \| x - x ^ { \prime } \| _ { 2 } } { c \sqrt { \overline { { \alpha } } _ { t } } } \| \mu _ { h } - \mu _ { i } \| _ { 2 } c \sqrt { \overline { { \alpha } } _ { t } } \| \mu _ { i } - \mu _ { h } \| _ { 2 } } \\ & { = \operatorname* { m a x } \{ \pi _ { \ell } ( x ) , \pi _ { \ell } ( x ^ { \prime } ) \} \operatorname* { m a x } \{ \pi _ { i } ( x ) , \pi _ { i } ( x ^ { \prime } ) \} \frac { \overline { { \alpha } } _ { t } } { \sigma _ { t } ^ { 2 } } \| x - x ^ { \prime } \| _ { 2 } \| \mu _ { h } - \mu _ { i } \| _ { 2 } ^ { 2 } } \\ & { \le \operatorname* { m a x } \{ \pi _ { \ell } ( x ) , \pi _ { \ell } ( x ^ { \prime } ) \} \operatorname* { m a x } \{ \gamma _ { i } , \pi _ { i } ( x ) , \pi _ { i } ( x ^ { \prime } ) \} \log ( H T ) \| x - x ^ { \prime } \| _ { 2 } . } \end{array} +$$ + +Thus we have + +$$ +\epsilon _ { i , 2 } ^ { ( \ell ) } \lesssim \operatorname* { m a x } \{ \gamma _ { \ell } , \pi _ { \ell } ( x ) , \pi _ { \ell } ( x ^ { \prime } ) \} \operatorname* { m a x } \{ \gamma _ { i } , \pi _ { i } ( x ) , \pi _ { i } ( x ^ { \prime } ) \} \log ( H T ) \| x - x ^ { \prime } \| _ { 2 } . +$$ + +Inserting into (46), we have + +$$ +\begin{array} { r l } & { \sqrt { \alpha _ { t } } | \pi _ { i } ( x ) - \pi _ { i } ( x ^ { \prime } ) | \| \mu _ { i } - \mu _ { h } \| _ { 2 } } \\ & { \lesssim \operatorname* { m a x } \{ \gamma _ { i } , \pi _ { i } ( x ) , \pi _ { i } ( x ^ { \prime } ) \} \log ( H T ) \| x - x ^ { \prime } \| _ { 2 } \left( 1 + \displaystyle \sum _ { \ell = 1 } ^ { H } \operatorname* { m a x } \{ \gamma _ { \ell } , \pi _ { \ell } ( x ) , \pi _ { \ell } ( x ^ { \prime } ) \} \right) } \\ & { \lesssim \operatorname* { m a x } \{ \gamma _ { i } , \pi _ { i } ( x ) , \pi _ { i } ( x ^ { \prime } ) \} \log ( H T ) \| x - x ^ { \prime } \| _ { 2 } . } \end{array} +$$ + +Inserting it into (41) we have + +$$ +\sqrt { \overline { { \alpha } } _ { t } } \sum _ { i = 1 } ^ { H } \| ( \pi _ { i } ( x ) - \pi _ { i } ( x ^ { \prime } ) ) ( \mu _ { i } - \mu _ { h } ) \| _ { 2 } +$$ + +$$ +\begin{array} { r l } { { \lesssim \sum _ { i \in \mathcal { F } _ { h } } \operatorname* { m a x } \{ \gamma _ { i } , \pi _ { i } ( x ) , \pi _ { i } ( x ^ { \prime } ) \} \log ( H T ) \| x - x ^ { \prime } \| _ { 2 } + \frac { 2 } { c } \| x - x ^ { \prime } \| _ { 2 } } } \\ & { \lesssim \log ( H T ) \| x - x ^ { \prime } \| _ { 2 } , } \end{array} +$$ + +and complete the proof. + +Proof of Lemma 6. We first bound $\pi _ { i } ( x )$ and $\pi _ { i } ( x ^ { \prime } )$ . Noticing that for any $h$ such that $\begin{array} { r } { \gamma _ { h } \ge \frac { 1 } { H T ^ { 4 } } } \end{array}$ , any $i$ , and any $x \in \mathcal { T } _ { t } ^ { h }$ , we have + +$$ +\begin{array} { r l } & { \pi _ { i } ( x ) \leq \displaystyle \frac { \pi _ { i } ( x ) } { \pi _ { h } ( x ) } = \frac { \gamma _ { i } } { \gamma _ { h } } \exp \left( - \frac { 1 } { 2 \sigma _ { t } ^ { 2 } } \| x - \sqrt { \overline { { \alpha _ { t } } } } \mu _ { i } \| ^ { 2 } + \displaystyle \frac { 1 } { 2 \sigma _ { t } ^ { 2 } } \| x - \sqrt { \overline { { \alpha _ { t } } } } \mu _ { h } \| ^ { 2 } \right) } \\ & { \quad \quad = \displaystyle \frac { \gamma _ { i } } { \gamma _ { h } } \exp \left( - \frac { \overline { { \alpha _ { t } } } \| \mu _ { i } - \mu _ { h } \| _ { 2 } ^ { 2 } } { 2 \sigma _ { t } ^ { 2 } } + \frac { \sqrt { \overline { { \alpha _ { t } } } } } { \sigma _ { t } ^ { 2 } } ( x - \sqrt { \overline { { \alpha _ { t } } } } \mu _ { h } ) ^ { \top } ( \mu _ { i } - \mu _ { h } ) \right) } \\ & { \overset { ( \mathrm { a } ) } { \leq } \gamma _ { i } \exp \left( - \frac { \overline { { \alpha _ { t } } } \| \mu _ { i } - \mu _ { h } \| _ { 2 } ^ { 2 } } { 2 \sigma _ { t } ^ { 2 } } + \frac { \sqrt { \overline { { \alpha _ { t } } } } } { \sigma _ { t } ^ { 2 } } | ( x - \sqrt { \overline { { \alpha _ { t } } } } \mu _ { h } ) ^ { \top } ( \mu _ { i } - \mu _ { h } ) | + 4 \log ( H T ) \right) , } \end{array} +$$ + +where (a) uses the fact that $\gamma _ { h } \geq \frac { 1 } { H T ^ { 4 } }$ . Similarly, we bound the $\pi _ { i } ( x ^ { \prime } )$ as + +$$ +\begin{array} { r l } & { \pi _ { i } ( x ^ { \prime } ) \leq \displaystyle \frac { \pi _ { i } ( x ^ { \prime } ) } { \pi _ { h } ( x ^ { \prime } ) } = \frac { \gamma _ { i } } { \gamma _ { h } } \exp \Big ( - \frac { \overline { { \alpha } } _ { t } \| \mu _ { i } - \mu _ { h } \| _ { 2 } ^ { 2 } } { 2 \sigma _ { t } ^ { 2 } } + \frac { \sqrt { \overline { { \alpha } } _ { t } } } { \sigma _ { t } ^ { 2 } } ( x - \sqrt { \overline { { \alpha } } _ { t } } \mu _ { h } ) ^ { \top } ( \mu _ { i } - \mu _ { h } ) } \\ & { \qquad + \frac { \sqrt { \overline { { \alpha } } _ { t } } } { \sigma _ { t } ^ { 2 } } ( x ^ { \prime } - x ) ^ { \top } ( \mu _ { i } - \mu _ { h } ) \Big ) } \\ & { \overset { ( \mathrm { a } ) } { \leq } \gamma _ { i } \exp \left( - \frac { 1 1 \overline { { \alpha } } _ { t } \| \mu _ { i } - \mu _ { h } \| _ { 2 } ^ { 2 } } { 2 4 \sigma _ { t } ^ { 2 } } + \frac { \sqrt { \overline { { \alpha } } _ { t } } } { \sigma _ { t } ^ { 2 } } | ( x - \sqrt { \overline { { \alpha } } _ { t } } \mu _ { h } ) ^ { \top } ( \mu _ { i } - \mu _ { h } ) | + 4 \log ( H T ) \right) , } \end{array} +$$ + +where (a) uses the fact that for $c \leq 1 / 2 4$ , + +$$ +\frac { \sqrt { \overline { { \alpha _ { t } } } } } { \sigma _ { t } ^ { 2 } } \big ( x ^ { \prime } - x \big ) ^ { \top } \big ( \mu _ { i } - \mu _ { h } \big ) \leq \frac { \sqrt { \overline { { \alpha _ { t } } } } } { \sigma _ { t } ^ { 2 } } \| x ^ { \prime } - x \| _ { 2 } \| \mu _ { i } - \mu _ { h } \| _ { 2 } \leq \frac { c \overline { { \alpha _ { t } } } } { \sigma _ { t } ^ { 2 } } \| \mu _ { i } - \mu _ { h } \| _ { 2 } ^ { 2 } \leq \frac { \overline { { \alpha _ { t } } } } { 2 4 \sigma _ { t } ^ { 2 } } \| \mu _ { i } - \mu _ { h } \| _ { 2 } ^ { 2 } . +$$ + +Let us discuss two cases: + +• If $\sqrt { \overline { { \alpha } } _ { t } } \| \mu _ { i } - \mu _ { h } \| _ { 2 } \geq 6 \sigma _ { t } C _ { 2 } \sqrt { \log ( H T ) }$ , then + +$$ +\begin{array} { r l r } { { \operatorname* { m a x } \{ \pi _ { i } ( x ) , \pi _ { i } ( x ^ { \prime } ) \} \leq \gamma _ { i } \exp \Big ( - \frac { 1 1 \overline { { \alpha _ { t } } } \| \mu _ { i } - \mu _ { h } \| _ { 2 } ^ { 2 } } { 2 4 \sigma _ { t } ^ { 2 } } + \frac { \sqrt { \overline { { \alpha _ { t } } } } } { \sigma _ { t } ^ { 2 } } | ( x - \sqrt { \overline { { \alpha _ { t } } } } \mu _ { h } ) ^ { \top } ( \mu _ { i } - \mu _ { h } ) | } } \\ & { } & { + \ 4 \log ( H T ) \Big ) } \\ & { } & { \overset { \mathrm { ( a ) } } { \leq } \gamma _ { i } \exp \Big ( - \frac { 5 \overline { { \alpha _ { t } } } \| \mu _ { i } - \mu _ { h } \| _ { 2 } ^ { 2 } } { 1 2 \sigma _ { t } ^ { 2 } } + \frac { \sqrt { \overline { { \alpha _ { t } } } } } { \sigma _ { t } ^ { 2 } } | ( x - \sqrt { \overline { { \alpha _ { t } } } } \mu _ { h } ) ^ { \top } ( \mu _ { i } - \mu _ { h } ) | \Big ) , } \end{array} +$$ + +where (a) holds as long as $C _ { 2 } \geq \sqrt { 8 / 3 }$ and + +$$ +4 \log ( H T ) \leq \frac { \overline { { \alpha } } _ { t } \| \mu _ { i } - \mu _ { h } \| _ { 2 } ^ { 2 } } { 9 C _ { 2 } ^ { 2 } \sigma _ { t } ^ { 2 } } \leq \frac { \overline { { \alpha } } _ { t } \| \mu _ { i } - \mu _ { h } \| _ { 2 } ^ { 2 } } { 2 4 \sigma _ { t } ^ { 2 } } . +$$ + +Furthermore, we have + +$$ +\begin{array} { r l } & { \quad \frac { \overline { { \alpha } } _ { t } } { \sigma _ { t } ^ { 2 } } \operatorname* { m a x } \{ \pi _ { i } ( x ) , \pi _ { i } ( x ^ { \prime } ) \} \| \mu _ { i } - \mu _ { h } \| _ { 2 } ^ { 2 } } \\ & { \le 2 4 \gamma _ { i } \exp \bigg ( - \frac { 5 \overline { { \alpha } } _ { t } \| \mu _ { i } - \mu _ { h } \| _ { 2 } ^ { 2 } } { 1 2 \sigma _ { t } ^ { 2 } } + \frac { \sqrt { \overline { { \alpha } } _ { t } } } { \sigma _ { t } ^ { 2 } } | ( x - \sqrt { \overline { { \alpha } } _ { t } } \mu _ { h } ) ^ { \top } ( \mu _ { i } - \mu _ { h } ) | + \frac { \overline { { \alpha } } _ { t } \| \mu _ { i } - \mu _ { h } \| _ { 2 } ^ { 2 } } { 2 4 \sigma _ { t } ^ { 2 } } \bigg ) } \\ & { \lesssim \gamma _ { i } \exp \bigg ( - \frac { 3 \overline { { \alpha } } _ { t } \| \mu _ { i } - \mu _ { h } \| _ { 2 } ^ { 2 } } { 8 \sigma _ { t } ^ { 2 } } + \frac { \sqrt { \overline { { \alpha } } _ { t } } } { \sigma _ { t } ^ { 2 } } | ( x - \sqrt { \overline { { \alpha } } _ { t } } \mu _ { h } ) ^ { \top } ( \mu _ { i } - \mu _ { h } ) | \bigg ) } \end{array} +$$ + +and + +$$ +\begin{array} { l } { \displaystyle \frac { \sqrt { \overline { { \alpha _ { t } } } } } { \sigma _ { t } ^ { 2 } } \operatorname* { m a x } \{ \pi _ { i } ( x ) , \pi _ { i } ( x ^ { \prime } ) \} \vert ( x - \sqrt { \overline { { \alpha _ { t } } } } \mu _ { h } ) ^ { \top } ( \mu _ { i } - \mu _ { h } ) \vert } \\ { \displaystyle \leq \gamma _ { i } \exp \left( - \frac { 5 \overline { { \alpha _ { t } } } \| \mu _ { i } - \mu _ { h } \| _ { 2 } ^ { 2 } } { 1 2 \sigma _ { t } ^ { 2 } } + \frac { 2 \sqrt { \overline { { \alpha _ { t } } } } } { \sigma _ { t } ^ { 2 } } \vert ( x - \sqrt { \overline { { \alpha _ { t } } } } \mu _ { h } ) ^ { \top } ( \mu _ { i } - \mu _ { h } ) \vert \right) } \end{array} +$$ + +Thus to establish Lemma 6, it suffices to prove that + +$$ +\exp \left( - \frac { 3 \overline { { \alpha } } _ { t } \| \mu _ { i } - \mu _ { h } \| _ { 2 } ^ { 2 } } { 8 \sigma _ { t } ^ { 2 } } + \frac { 2 \sqrt { \overline { { \alpha } } _ { t } } } { \sigma _ { t } ^ { 2 } } | ( x - \sqrt { \overline { { \alpha } } _ { t } } \mu _ { h } ) ^ { \top } ( \mu _ { i } - \mu _ { h } ) | \right) \leq 1 . +$$ + +Recalling that $x \in \mathcal { T } _ { t } ^ { h }$ , which implies that $\begin{array} { r } { \sqrt { \overline { { \alpha _ { t } } } } | ( x - \sqrt { \overline { { \alpha _ { t } } } } \mu _ { h } ) ^ { \top } ( \mu _ { i } - \mu _ { h } ) | \leq C _ { 2 } \sigma _ { t } \sqrt { \overline { { \alpha } } _ { t } \log ( H T ) } \| \mu _ { i } - \mu _ { h } \| _ { 2 } } \end{array}$ , we have + +$$ +\frac { 2 \sqrt { \alpha _ { t } } } { \sigma _ { t } ^ { 2 } } | ( x - \sqrt { \alpha _ { t } } \mu _ { h } ) ^ { \top } ( \mu _ { i } - \mu _ { h } ) | \leq \frac { 2 C _ { 2 } } { \sigma _ { t } } \sqrt { \overline { { \alpha } } _ { t } \log ( H T ) } \| \mu _ { i } - \mu _ { h } \| _ { 2 } \leq \frac { \overline { { \alpha } } _ { t } } { 3 \sigma _ { t } ^ { 2 } } \| \mu _ { i } - \mu _ { h } \| _ { 2 } ^ { 2 } , +$$ + +and complete the proof. + +• If $\sqrt { \overline { { \alpha } } _ { t } } \| \mu _ { i } - \mu _ { h } \| < 6 C _ { 2 } \sigma _ { t } \sqrt { \log ( H T ) }$ , then we have + +$$ +\begin{array} { r l } & { \frac { \overline { { \alpha } } _ { t } } { \sigma _ { t } ^ { 2 } } \operatorname* { m a x } \{ \pi _ { i } ( x ) , \pi _ { i } ( x ) \} \| \mu _ { i } - \mu _ { h } \| _ { 2 } ^ { 2 } \leq 3 6 C _ { 2 } ^ { 2 } \operatorname* { m a x } \{ \pi _ { i } ( x ) , \pi _ { i } ( x ) \} \log ( H T ) , } \\ & { \frac { \sqrt { \overline { { \alpha } } _ { t } } } { \sigma _ { t } ^ { 2 } } \operatorname* { m a x } \{ \pi _ { i } ( x ) , \pi _ { i } ( x ^ { \prime } ) \} | ( x - \sqrt { \overline { { \alpha } } _ { t } } \mu _ { h } ) ^ { \top } ( \mu _ { i } - \mu _ { h } ) | } \\ & { \leq \frac { C _ { 2 } \sqrt { \overline { { \alpha } } _ { t } } \log ( H T ) } { \sigma _ { t } } \operatorname* { m a x } \{ \pi _ { i } ( x ) , \pi _ { i } ( x ^ { \prime } ) \} \| \mu _ { i } - \mu _ { h } \| _ { 2 } } \\ & { \leq 6 C _ { 2 } ^ { 2 } \log ( H T ) \operatorname* { m a x } \{ \pi _ { i } ( x ) , \pi _ { i } ( x ^ { \prime } ) \} . } \end{array} +$$ + +# C.2.2 Proof of the lower bound + +By calculation, we have + +$$ +{ \cal J } _ { t } ( x ) = \frac { 1 } { \sigma _ { t } ^ { 2 } } \left[ - I _ { d } + \frac { \overline { { { \alpha } } } _ { t } } { \sigma _ { t } ^ { 2 } } \left( \sum _ { h = 1 } ^ { H } \gamma _ { h } \mu _ { h } \mu _ { h } ^ { \top } - \overline { { { \mu } \mu } } ^ { \top } \right) \right] , \qquad \mathrm { w h e r e } \qquad \overline { { { \mu } } } = \sum _ { h = 1 } ^ { H } \gamma _ { h } \mu _ { h } . +$$ + +In the case of $X _ { 0 } \sim \textstyle { \frac { 1 } { 2 } } { \mathcal { N } } ( \mu , \sigma ^ { 2 } I _ { d } ) + \textstyle { \frac { 1 } { 2 } } { \mathcal { N } } ( - \mu , \sigma ^ { 2 } I _ { d } )$ , we have + +$$ +( 1 - \overline { { \alpha } } _ { t } ) \nabla s _ { t } ^ { \star } ( x ) = ( 1 - \overline { { \alpha } } _ { t } ) J _ { t } ( x ) = \frac { \left( 1 - \overline { { \alpha } } _ { t } \right) } { \sigma _ { t } ^ { 2 } } \left[ - I _ { d } + \frac { \overline { { \alpha } } _ { t } } { \sigma _ { t } ^ { 2 } } \mu \mu ^ { \top } \right] . +$$ + +Thus we have + +$$ +\begin{array} { r l } & { \| ( 1 - \overline { { \alpha } } _ { t } ) \nabla s _ { t } ^ { \star } ( x ) \| _ { 2 } = \frac { ( 1 - \overline { { \alpha } } _ { t } ) } { \sigma _ { t } ^ { 2 } } \left\| - I _ { d } + \frac { \overline { { \alpha } } _ { t } } { \sigma _ { t } ^ { 2 } } \mu \mu ^ { \top } \right\| _ { \mathrm { o p } } } \\ & { \quad \quad \quad \quad = \frac { ( 1 - \overline { { \alpha } } _ { t } ) } { \sigma _ { t } ^ { 2 } } \operatorname* { m a x } \left\{ \frac { \overline { { \alpha } } _ { t } \| \mu \| _ { 2 } ^ { 2 } } { \sigma _ { t } ^ { 2 } } - 1 , 1 \right\} \overset { \mathrm { ( a ) } } { \geq } \frac { \overline { { \alpha } } _ { t } ( 1 - \overline { { \alpha } } _ { t } ) \| \mu \| _ { 2 } ^ { 2 } } { 2 \sigma _ { t } ^ { 4 } } } \\ & { \quad \quad \geq \frac { ( 1 - \overline { { \alpha } } _ { t } ) \| \mu \| _ { 2 } ^ { 2 } } { 4 ( 1 - \overline { { \alpha } } _ { t } + \sigma ^ { 2 } ) ^ { 2 } } , } \end{array} +$$ + +where the last inequality uses the fact that $\begin{array} { r } { \overline { { \alpha } } _ { t } \geq \frac { 1 } { 2 } } \end{array}$ + +# D Proof of key lemmas in Theorem 1 + +Before diving into the proof details, we first present a preliminary lemma about the learning schedule $\tau _ { k , n }$ . It has been stated in Li and Jiao (2024). For completeness, we rewrite its proof in Appendix F.4 in supplemental material. + +Lemma 7. Our choice of learning schedules (9) satisfies + +$$ +1 - \tau _ { 0 , 0 } \leq \widehat { \alpha } _ { T } \leq \frac { 2 } { T ^ { c _ { 0 } } } , \qquad \tau _ { K , 0 } \leq 1 - \widehat { \alpha } _ { 1 } \leq \frac { 1 } { T ^ { c _ { 0 } } } , \quad a n d \ \frac { \widehat { \tau } _ { k , n - 1 } - \widehat { \tau } _ { k , n } } { \widehat { \tau } _ { k , n - 1 } \left( 1 - \widehat { \tau } _ { k , n - 1 } \right) } = \frac { c _ { 1 } \log T } { T } . +$$ + +Moreover, we have + +$$ +\frac { \widehat { \tau } _ { k , i - 1 } } { \widehat { \tau } _ { k , i } } \leq \frac { \widehat { \tau } _ { k , - 1 } } { \widehat { \tau } _ { k , N } } \lesssim 1 , \qquad \frac { 1 - \widehat { \tau } _ { k , i } } { 1 - \widehat { \tau } _ { k , i - 1 } } \leq \frac { 1 - \widehat { \tau } _ { k , N } } { 1 - \widehat { \tau } _ { k , - 1 } } \lesssim 1 . +$$ + +# D.1 Proof of Lemma 3 + +We present a more preliminary conclusion stated as below. + +Lemma 8. For any $k$ and $n$ , with probability at least $1 - T ^ { - 1 0 0 }$ , + +$$ +\begin{array} { r l } & { \frac { \Vert y _ { T _ { k , n } } ^ { \star } ( x _ { k } ) - y _ { \tau _ { k , n } } ( x _ { k } ) \Vert _ { 2 } ^ { 2 } } { 1 - \tau _ { k , n } } \lesssim \frac { N \log ^ { 2 } T } { T ^ { 2 } } \sum _ { i = 0 } ^ { n - 1 } \frac { \widehat \tau _ { k , i } \widehat \tau _ { k , i } ^ { 2 } ( x _ { k } ) } { 1 - \widehat \tau _ { k , i } } , } \\ & { \frac { \mathbb { E } _ { x _ { k } \sim \widehat { x } _ { k } } \Vert x _ { \tau _ { k , n } } ( x _ { k } ) - z _ { \tau _ { k , n } } ^ { \star } ( x _ { k } ) \Vert _ { 2 } ^ { 2 } } { 1 - \tau _ { k , n } } } \\ & { \lesssim \frac { d \log ^ { 5 } T } { T ^ { 3 } } \operatorname* { m i n } \Big \{ \frac { N d \widehat \tau _ { k , - 1 } \log T } { T ( 1 - \widehat \tau _ { k , - 1 } ) } + \int _ { \tau _ { k , n } } ^ { \tau _ { k , 0 } } \frac { \mathbb { E } [ \mathsf { T r } ( \Sigma _ { \tau } ^ { 2 } ( x _ { \tau } ) ) ] } { ( 1 - \tau ) ^ { 2 } } \mathrm { d } \tau , \frac { N L ^ { 2 } \widehat \tau _ { k , - 1 } \log T } { T ( 1 - \widehat \tau _ { k , - 1 } ) } \Big \} , } \\ & { = \Vert s _ { \tau _ { k , i } } ( y _ { \tau _ { k , i } } ( x _ { k } ) ) - s _ { \tau _ { k , i } } ^ { \star } ( y _ { \tau _ { k , i } } ( x _ { k } ) ) \Vert _ { 2 } . } \end{array} +$$ + +where + +Then according to the definition of $\xi _ { k , n }$ (cf. (34)) and $\varepsilon _ { k , i } ^ { 2 }$ (cf. (12)), we could prove Lemma 3. The remaining proof focuses on establishing Lemma 8. According to definitions of $y _ { \tau _ { k , n } }$ and $y _ { \tau _ { k , n } } ^ { \star }$ , we have + +$$ +\begin{array} { r l r } { { \frac { \| y _ { \tau _ { k , n } } ( x _ { k } ) - y _ { \tau _ { k , n } } ^ { \star } ( x _ { k } ) \| _ { 2 } } { \sqrt { 1 - \tau _ { k , n } } } \leq \frac { \widetilde { \varepsilon } _ { k , 0 } ( x _ { k } ) ( \tau _ { k , 0 } - \widehat { \tau } _ { k , 0 } ) } { 2 ( 1 - \tau _ { k , 0 } ) ^ { 3 / 2 } } + \displaystyle \sum _ { i = 1 } ^ { n - 1 } \frac { \widetilde { \varepsilon } _ { k , i } ( x _ { k } ) ( \widehat { \tau } _ { k , i - 1 } - \widehat { \tau } _ { k , i } ) } { 2 ( 1 - \tau _ { k , i } ) ^ { 3 / 2 } } } } \\ & { } & { \quad + \frac { \widetilde { \varepsilon } _ { k , n - 1 } ( x _ { k } ) ( \widehat { \tau } _ { k , n - 1 } - \tau _ { k , n } ) } { 2 ( 1 - \tau _ { k , n - 1 } ) ^ { 3 / 2 } } . } \end{array} +$$ + +Recalling Lemma 7, we have + +$$ +\begin{array} { r l } & { \frac { \bar { \tau } _ { k , 0 } - \hat { \tau } _ { k , 0 } } { 2 ( 1 - \tau _ { k , 0 } ) ^ { 3 / 2 } } \leq \frac { \hat { \tau } _ { k , - 1 } - \hat { \tau } _ { k , 0 } } { 2 ( 1 - \tau _ { k , 0 } ) ^ { 3 / 2 } } \leq \frac { \hat { \tau } _ { k , - 1 } \log T } { 2 T ( 1 - \tau _ { k , 0 } ) ^ { 1 / 2 } } \lesssim \frac { \hat { \tau } _ { k , - 1 } \log T } { T ( 1 - \hat { \tau } _ { k , - 1 } ) ^ { 1 / 2 } } } \\ & { \frac { \hat { \tau } _ { k , i - 1 } - \hat { \tau } _ { k , i } } { 2 ( 1 - \tau _ { k , i } ) ^ { 3 / 2 } } \leq \frac { \hat { \tau } _ { k , i - 1 } \log T } { 2 T ( 1 - \tau _ { k , i } ) ^ { 1 / 2 } } \lesssim \frac { \hat { \tau } _ { k , i - 1 } \log T } { T ( 1 - \hat { \tau } _ { k , i - 1 } ) ^ { 1 / 2 } } , } \\ & { \frac { \hat { \tau } _ { k , n - 1 } - \tau _ { k , n } } { 2 ( 1 - \tau _ { k , n - 1 } ) ^ { 3 / 2 } } \leq \frac { \hat { \tau } _ { k , n - 1 } - \hat { \tau } _ { k , 0 } } { 2 ( 1 - \tau _ { k , n - 1 } ) ^ { 3 / 2 } } \leq \frac { \hat { \tau } _ { k , n - 1 } \log T } { 2 T ( 1 - \tau _ { k , n - 1 } ) ^ { 1 / 2 } } } \\ & { \qquad \lesssim \frac { \hat { \tau } _ { k , n - 1 } \log T } { T ( 1 - \hat { \tau } _ { k , n - 1 } ) ^ { 1 / 2 } } \lesssim \frac { \hat { \tau } _ { k , n - 2 } \log T } { T ( 1 - \hat { \tau } _ { k , n - 2 } ) ^ { 1 / 2 } } . } \end{array} +$$ + +Inserting into (54), we have that + +$$ +\frac { \| y _ { \tau _ { k , n } } ( x _ { k } ) - y _ { \tau _ { k , n } } ^ { \star } ( x _ { k } ) \| _ { 2 } } { \sqrt { 1 - \tau _ { k , n } } } \lesssim \sum _ { i = 0 } ^ { n - 1 } \frac { \widetilde { \varepsilon } _ { k , i } ( x _ { k } ) \widehat { \tau } _ { k , i - 1 } \log T } { T ( 1 - \widehat { \tau } _ { k , i - 1 } ) ^ { 1 / 2 } } . +$$ + +Thus we have + +$$ +\frac { \Vert y _ { \tau _ { k , n } } ^ { \star } ( x _ { k } ) - y _ { \tau _ { k , n } } ( x _ { k } ) \Vert _ { 2 } ^ { 2 } } { 1 - \tau _ { k , n } } \lesssim \frac { N \log ^ { 2 } T } { T ^ { 2 } } \sum _ { i = 0 } ^ { n - 1 } \frac { \tilde { \varepsilon } _ { k , i } ^ { 2 } ( x _ { k } ) \hat { \tau } _ { k , i - 1 } } { 1 - \hat { \tau } _ { k , i - 1 } } \lesssim \frac { N \log ^ { 2 } T } { T ^ { 2 } } \sum _ { i = 0 } ^ { n - 1 } \frac { \tilde { \varepsilon } _ { k , i } ^ { 2 } ( x _ { k } ) \hat { \tau } _ { k , i } } { 1 - \hat { \tau } _ { k , i } } +$$ + +and complete the proof of (52), where the last inequality uses (51). + +Now we are ready to prove (53). The definition of $x _ { \tau _ { k , n } }$ (cf. (19)) can be decomposed as + +$$ +\begin{array} { l } { \displaystyle \frac { x _ { \tau _ { k , n } } } { \sqrt { 1 - \tau _ { k , n } } } = \displaystyle \frac { x _ { \tau _ { k , 0 } } } { \sqrt { 1 - \tau _ { k , 0 } } } + \int _ { \hat { \tau } _ { k , 0 } } ^ { \tau _ { k , 0 } } { \frac { s _ { \tau } ^ { \star } ( x _ { \tau } ) } { 2 ( 1 - \tau ) ^ { 3 / 2 } } \mathrm { d } \tau } + \displaystyle \sum _ { i = 1 } ^ { n - 1 } \int _ { \hat { \tau } _ { k , i } } ^ { \hat { \tau } _ { k , i - 1 } } { \frac { s _ { \tau } ^ { \star } ( x _ { \tau } ) } { 2 ( 1 - \tau ) ^ { 3 / 2 } } \mathrm { d } \tau } } \\ { \displaystyle \quad \quad + \int _ { \tau _ { k , n } } ^ { \hat { \tau } _ { k , n - 1 } } { \frac { s _ { \tau } ^ { \star } ( x _ { \tau } ) } { 2 ( 1 - \tau ) ^ { 3 / 2 } } \mathrm { d } \tau } . } \end{array} +$$ + +Based on the definitions of $x _ { \tau _ { k , n } }$ and $z _ { \tau _ { k , n } } ^ { \star }$ , we first make the following decomposition as + +$$ +\frac { x _ { \tau _ { k , n } } ( x _ { k } ) - z _ { \tau _ { k , n } } ^ { \star } ( x _ { k } ) } { \sqrt { 1 - \tau _ { k , n } } } = \mathcal { E } _ { 1 , 1 } ( x _ { \tau _ { k , 0 } } ) + \mathcal { E } _ { 1 , 0 } ( x _ { \tau _ { k , 0 } } ) - \mathcal { E } _ { 2 } ( x _ { \tau _ { k , 0 } } ) , +$$ + +where + +$$ +\begin{array} { r l } & { \mathcal { E } _ { 1 , 1 } ( x _ { \tau _ { k , 0 } } ) : = \displaystyle \int _ { \tau _ { k , n } } ^ { \tilde { \tau } _ { k , n - 1 } } \frac { s _ { \tau } ^ { \star } ( x _ { \tau } ) } { 2 ( 1 - \tau ) ^ { 3 / 2 } } \mathrm { d } \tau - \frac { s _ { \tau _ { k , n - 1 } } ^ { \star } ( x _ { \tau _ { k , n - 1 } } ) } { 2 ( 1 - \tau _ { k , n - 1 } ) ^ { 3 / 2 } } ( \widehat { \tau } _ { k , n - 1 } - \tau _ { k , n } ) ; } \\ & { \mathcal { E } _ { 1 , 0 } ( x _ { \tau _ { k , 0 } } ) : = \displaystyle \int _ { \widehat { \tau } _ { k , 0 } } ^ { \tau _ { k , 0 } } \frac { s _ { \tau } ^ { \star } ( x _ { \tau } ) } { 2 ( 1 - \tau ) ^ { 3 / 2 } } \mathrm { d } \tau - \frac { s _ { \tau _ { k , 0 } } ^ { \star } ( x _ { \tau _ { k , 0 } } ) } { 2 ( 1 - \tau _ { k , 0 } ) ^ { 3 / 2 } } ( \tau _ { k , 0 } - \widehat { \tau } _ { k , 0 } ) ; } \\ & { \mathcal { E } _ { 2 } ( x _ { \tau _ { k , 0 } } ) : = \displaystyle \sum _ { i = 1 } ^ { n - 1 } \left[ \frac { s _ { \tau _ { k , i } } ^ { \star } ( x _ { \tau _ { k , i } } ) } { 2 ( 1 - \tau _ { k , i } ) ^ { 3 / 2 } } ( \widehat { \tau } _ { k , i - 1 } - \widehat { \tau } _ { k , i } ) - \displaystyle \int _ { \widehat { \tau } _ { k , i } } ^ { \widehat { \tau } _ { k , i - 1 } } \frac { s _ { \tau } ^ { \star } ( x _ { \tau } ) } { 2 ( 1 - \tau ) ^ { 3 / 2 } } \mathrm { d } \tau \right] . } \end{array} +$$ + +In the following, we will control these three terms separately, and then combine them. The remaining proof is divided into four steps. + +1. Analysis of $\mathcal { E } _ { 1 , 1 } ( x _ { \tau _ { k , 0 } } )$ . This term can be calculated as + +$$ +\begin{array} { r l } & { \mathcal { E } _ { 1 , 1 } ( x _ { \tau _ { k , 0 } } ) = \int _ { \tau _ { k , 0 } } ^ { \tilde { \tau } _ { k , n - 1 } } \bigg ( \frac { \delta _ { \tau } ^ { s } ( x _ { \tau } ) } { 2 ( 1 - \tau ) ^ { 3 / 2 } } - \frac { \delta _ { \tau _ { k , n - 1 } } ^ { s } ( x _ { \tau _ { k , n - 1 } } ) } { 2 ( 1 - \tau ) ^ { 3 / 2 } } \bigg ) \mathrm { d } \tau } \\ & { \stackrel { ( a ) } { = } - \int _ { \tau _ { k , n } } ^ { \tilde { \tau } _ { k , n - 1 } } \bigg ( \int _ { \tau ^ { \prime } } ^ { \tau _ { k , n - 1 } } \frac { \delta } { \partial \tau } \frac { \delta _ { \tau } ^ { s } ( x _ { \tau } ) } { 2 ( 1 - \tau ) ^ { 3 / 2 } } \mathrm { d } \tau \bigg ) \mathrm { d } \tau ^ { \prime } } \\ & { \stackrel { ( b ) } { = } - \int _ { \tau _ { k , n } } ^ { \tau _ { k , n - 1 } } \frac { \partial } { \partial \tau } \frac { s _ { \tau } ^ { s } ( x _ { \tau } ) } { 2 ( 1 - \tau ) ^ { 3 / 2 } } \bigg ( \int _ { \tau _ { k , n } } ^ { \tilde { \tau } _ { k , n - 1 } } 1 ( \tau ^ { \prime } < \tau ) \mathrm { d } \tau ^ { \prime } \bigg ) \mathrm { d } \tau } \\ & { = - \int _ { \tau _ { k , n } } ^ { \tilde { \tau } _ { k , n - 1 } } ( \tau - \tau _ { k , n } ) \frac { \delta } { \partial \tau } \frac { s _ { \tau } ^ { s } ( x _ { \tau } ) } { 2 ( 1 - \tau ) ^ { 3 / 2 } } \mathrm { d } \tau - \int _ { \tilde { \tau } _ { k , n - 1 } } ^ { \tilde { \tau } _ { k , n - 1 } } ( \hat { \tau } _ { k , n - 1 } } \\ & \quad \quad - \tau _ { k , n } ) \frac { \partial } \end{array} +$$ + +where (a) comes from the fact that + +$$ +\frac { s _ { \tau ^ { \prime } } ^ { \star } ( x _ { \tau ^ { \prime } } ) } { 2 ( 1 - \tau ^ { \prime } ) ^ { 3 / 2 } } - \frac { s _ { \tau _ { k , n - 1 } } ^ { \star } ( x _ { \tau _ { k , n - 1 } } ) } { 2 ( 1 - \tau _ { k , n - 1 } ) ^ { 3 / 2 } } = \int _ { \tau _ { k , n - 1 } } ^ { \tau ^ { \prime } } \frac { \partial } { \partial \tau } \frac { s _ { \tau } ^ { \star } ( x _ { \tau } ) } { 2 ( 1 - \tau ) ^ { 3 / 2 } } \mathrm { d } \tau +$$ + +and the $\Im ( \tau ^ { \prime } < \tau )$ in (b) denotes an indicator function. We further have + +$$ +\lVert \mathcal { E } _ { 1 , 1 } ( x _ { \tau _ { k , 0 } } ) \rVert _ { 2 } ^ { 2 } \leq ( \widehat { \tau } _ { k , n - 1 } - \tau _ { k , n } ) ^ { 2 } \left( \int _ { \tau _ { k , n } } ^ { \tau _ { k , n - 1 } } \left. \frac { \partial } { \partial \tau } \frac { s _ { \tau } ^ { \star } ( x _ { \tau } ) } { 2 ( 1 - \tau ) ^ { 3 / 2 } } \right. _ { 2 } \mathrm { d } \tau \right) ^ { 2 } +$$ + +$$ +\leq ( \widehat { \tau } _ { k , n - 1 } - \widehat { \tau } _ { k , n } ) ^ { 2 } ( \widehat { \tau } _ { k , n - 2 } - \widehat { \tau } _ { k , n } ) \int _ { \tau _ { k , n } } ^ { \widehat { \tau } _ { k , n - 2 } } \left\| \frac { \partial } { \partial \tau } \frac { s _ { \tau } ^ { \star } ( x _ { \tau } ) } { 2 ( 1 - \tau ) ^ { 3 / 2 } } \right\| _ { 2 } ^ { 2 } \mathrm { d } \tau . +$$ + +By using (50) and (51), we have + +$$ +\begin{array} { r l } & { \| \mathcal { E } _ { 1 , 1 } ( x _ { \gamma _ { k , 0 } } ) \| _ { 2 } ^ { 2 } \leq \bigg ( 1 + \frac { \widehat \gamma _ { k , n - 2 } - \widehat \gamma _ { k , n - 1 } } { \widehat \gamma _ { k , n - 1 } - \widehat \gamma _ { k , n - 1 } } \bigg ) ( \widehat \gamma _ { k , n - 1 } - \widehat \gamma _ { k , n } ) ^ { 3 } \int _ { \mathbb { R } _ { n , \star } } ^ { \widehat \gamma _ { k , n - 1 } } \bigg \| \frac { \partial } { \partial \tau } \frac { s _ { \star } ^ { \star } ( x _ { \tau } ) } { 2 ( 1 - \tau ) ^ { 3 / 2 } } \bigg \| _ { 2 } ^ { 2 } \mathrm { d } \tau } \\ & { \qquad + \bigg ( \frac { ( \widehat \gamma _ { k , n - 1 } - \widehat \gamma _ { k , n } ) ^ { 3 } } { ( \widehat \gamma _ { k , n - 2 } - \widehat \gamma _ { k , n - 1 } ) ^ { 3 } } + \frac { ( \widehat \gamma _ { k , n - 1 } - \widehat \gamma _ { k , n } ) ^ { 2 } } { ( \widehat \gamma _ { k , n - 2 } - \widehat \gamma _ { k , n - 1 } ) ^ { 2 } } \bigg ) ( \widehat \gamma _ { k , n - 2 } - \widehat \gamma _ { k , n - 1 } ) ^ { 3 } } \\ & { \qquad \cdot \int _ { \widehat \gamma _ { k , n - 1 } } ^ { \widehat \gamma _ { k , n - 2 } } \bigg \| \frac { \partial } { \partial \tau } \frac { s _ { \star } ^ { \star } ( x _ { \tau } ) } { 2 ( 1 - \tau ) ^ { 3 / 2 } } \bigg \| _ { 2 } ^ { 2 } } \\ & \overset { ( a ) } { \lesssim } ( \widehat \gamma _ { k , n - 1 } - \widehat \gamma _ { k , n } ) ^ { 3 } \int _ { \gamma _ { k , n } } ^ { \widehat \gamma _ { k , n - 1 } } \bigg \| \frac { \partial } { \partial \tau } \frac { s _ { \star } ^ { \star } ( x _ { \tau } ) } { 2 ( 1 - \tau ) ^ { 3 / 2 } } \bigg \| _ \end{array} +$$ + +where (a) uses (51). By taking expectation, we have, + +$$ +\begin{array} { r l } & { \mathbb { E } _ { \boldsymbol { x } _ { k } \sim \widehat { X } _ { k } } \left[ \| \boldsymbol { \mathcal { E } } _ { 1 , 1 } ( \boldsymbol { x } _ { k } ) \| _ { 2 } ^ { 2 } \right] \lesssim ( \widehat { \tau } _ { k , n - 1 } - \widehat { \tau } _ { k , n } ) ^ { 3 } \displaystyle \int _ { \tau _ { k , n } } ^ { \widehat { \tau } _ { k , n - 1 } } \mathbb { E } _ { \boldsymbol { x } _ { \tau } \sim \boldsymbol { X } _ { \tau } } \left[ \left\| \displaystyle \frac { \partial } { \partial \tau } \displaystyle \frac { s _ { \tau } ^ { \star } ( \boldsymbol { x } _ { \tau } ) } { 2 ( 1 - \tau ) ^ { 3 / 2 } } \right\| _ { 2 } ^ { 2 } \right] \mathrm { d } \tau } \\ & { \qquad + \left( \widehat { \tau } _ { k , n - 2 } - \widehat { \tau } _ { k , n - 1 } \right) ^ { 3 } \displaystyle \int _ { \widehat { \tau } _ { k , n - 1 } } ^ { \widehat { \tau } _ { k , n - 2 } } \mathbb { E } _ { \boldsymbol { x } _ { \tau } \sim \boldsymbol { X } _ { \tau } } \left[ \left\| \displaystyle \frac { \partial } { \partial \tau } \displaystyle \frac { s _ { \tau } ^ { \star } ( \boldsymbol { x } _ { \tau } ) } { 2 ( 1 - \tau ) ^ { 3 / 2 } } \right\| _ { 2 } ^ { 2 } \right] \mathrm { d } \tau . } \end{array} +$$ + +2. Analysis of $\mathcal { E } _ { 1 , 0 } ( x _ { \tau _ { k , 0 } } )$ . This term can be calculated in a similar way as + +$$ +\begin{array} { r l } & { \mathcal { E } _ { 1 , 0 } ( x _ { \tau _ { k , 0 } } ) = \displaystyle { \int _ { \hat { \tau } _ { k , 0 } } ^ { \tau _ { k , 0 } } \left( \frac { s _ { \tau } ^ { \star } ( x _ { \tau } ) } { 2 ( 1 - \tau ) ^ { 3 / 2 } } - \frac { s _ { \tau _ { k , 0 } } ^ { \star } ( x _ { \tau _ { k , 0 } } ) } { 2 ( 1 - \tau _ { k , 0 } ) ^ { 3 / 2 } } \right) } \mathrm { d } \tau } \\ & { \quad \quad \quad \quad = \displaystyle { - \int _ { \hat { \tau } _ { k , 0 } } ^ { \tau _ { k , 0 } } \left( \int _ { \tau ^ { \prime } } ^ { \tau _ { k , 0 } } \frac { \partial } { \partial \tau } \frac { s _ { \tau } ^ { \star } ( x _ { \tau } ) } { 2 ( 1 - \tau ) ^ { 3 / 2 } } \mathrm { d } \tau \right) } \mathrm { d } \tau ^ { \prime } } \\ & { \quad \quad \quad \quad = \displaystyle { - \int _ { \hat { \tau } _ { k , 0 } } ^ { \tau _ { k , 0 } } \frac { \partial } { \partial \tau } \frac { s _ { \tau } ^ { \star } ( x _ { \tau } ) } { 2 ( 1 - \tau ) ^ { 3 / 2 } } \left( \int _ { \hat { \tau } _ { k , 0 } } ^ { \tau _ { k , 0 } } 1 ( \tau ^ { \prime } < \tau ) \mathrm { d } \tau ^ { \prime } \right) } \mathrm { d } \tau } \\ & { \quad \quad \quad \quad = \displaystyle { - \int _ { \hat { \tau } _ { k , 0 } } ^ { \tau _ { k , 0 } } ( \tau - \hat { \tau } _ { k , 0 } ) \frac { \partial } { \partial \tau } \frac { s _ { \tau } ^ { \star } ( x _ { \tau } ) } { 2 ( 1 - \tau ) ^ { 3 / 2 } } \mathrm { d } \tau } . } \end{array} +$$ + +We further have + +$$ +\| \mathcal { E } _ { 1 , 0 } ( x _ { \tau _ { k , 0 } } ) \| _ { 2 } ^ { 2 } \leq ( \tau _ { k , 0 } - \widehat { \tau } _ { k , 0 } ) ^ { 3 } \int _ { \widehat { \tau } _ { k , 0 } } ^ { \tau _ { k , 0 } } \bigg \| \frac { \partial } { \partial \tau } \frac { s _ { \tau } ^ { \star } ( x _ { \tau } ) } { 2 ( 1 - \tau ) ^ { 3 / 2 } } \bigg \| _ { 2 } ^ { 2 } \mathrm { d } \tau . +$$ + +By taking expectation, we have + +$$ +\begin{array} { r l r } { { \mathbb { E } _ { x _ { \tau _ { k , 0 } } \sim \hat { X } _ { k } } [ \| \mathcal { E } _ { 1 , 0 } ( x _ { \tau _ { k , 0 } } ) \| _ { 2 } ^ { 2 } ] \lesssim ( \tau _ { k , 0 } - \widehat { \tau } _ { k , 0 } ) ^ { 3 } \int _ { \widehat { \tau } _ { k , 0 } } ^ { \tau _ { k , 0 } } \mathbb { E } _ { x _ { \tau } \sim X _ { \tau } } [ \| \frac { \partial } { \partial \tau } \frac { s _ { \tau } ^ { \star } ( x _ { \tau } ) } { ( 1 - \tau ) ^ { 3 / 2 } } \| _ { 2 } ^ { 2 } ] \mathrm { d } \tau } } \\ & { } & { \lesssim ( \widehat { \tau } _ { k , - 1 } - \widehat { \tau } _ { k , 0 } ) ^ { 3 } \int _ { \widehat { \tau } _ { k , 0 } } ^ { \tau _ { k , 0 } } \mathbb { E } _ { x _ { \tau } \sim X _ { \tau } } [ \| \frac { \partial } { \partial \tau } \frac { s _ { \tau } ^ { \star } ( x _ { \tau } ) } { ( 1 - \tau ) ^ { 3 / 2 } } \| _ { 2 } ^ { 2 } ] \mathrm { d } \tau . } \end{array} +$$ + +3. Analysis of $\mathcal { E } _ { 2 } ( x _ { \tau _ { k , 0 } } )$ . This term is a random variable about $\tau = \{ \tau _ { k , n } \} _ { n = 1 } ^ { N - 1 }$ . To bound it with highr probability, we intend to calculate its $r$ -th order moment $\mathbb { E } \big [ \big ( \mathbb { E } _ { \boldsymbol { x } _ { \tau _ { k , 0 } } \sim p _ { \widehat { X } _ { k } } } \big [ \| \mathcal { E } _ { 2 } ( \boldsymbol { x } _ { \tau _ { k , 0 } } ) \| _ { 2 } ^ { 2 } | \tau \big ] \big ) ^ { r } \big ]$ . Towards this, we introduce $r$ independent random variables $x _ { k } ^ { ( j ) } \sim p _ { \widehat { X } _ { k } }$ , $j = 1 , \cdots , r$ . For any integer $r > 0$ , considering that $\mathbb { E } _ { \boldsymbol { x } _ { \tau _ { k , 0 } } \sim p _ { \widehat { X } _ { k } } } \left\lfloor \left\| \mathcal { E } _ { 2 } ( \boldsymbol { x } _ { \tau _ { k , 0 } } ) \right\| _ { 2 } ^ { 2 } \big | \tau \right\rfloor$ is a random variable independent on $x _ { \tau _ { k , 0 } }$ , we get + +$$ +\mathbb { E } \big [ \big ( \mathbb { E } _ { \boldsymbol { x } _ { \tau _ { k , 0 } } \sim p _ { \widehat { X _ { k } } } } \big [ \| \mathcal { E } _ { 2 } ( \boldsymbol { x } _ { \tau _ { k , 0 } } ) \| _ { 2 } ^ { 2 } \mid \tau \big ] \big ) ^ { r } \big ] = \mathbb { E } \Big [ \mathbb { E } _ { \boldsymbol { x } _ { k } ^ { ( j ) } \sim p _ { \widehat { X _ { k } } } } \Big [ \prod _ { 1 \leq j \leq r } \| \mathcal { E } _ { 2 } ( \boldsymbol { x } _ { k } ^ { ( j ) } ) \| _ { 2 } ^ { 2 } \mid \tau \Big ] \Big ] +$$ + +$$ += \mathbb { E } _ { \boldsymbol { x } _ { k } ^ { ( j ) } \sim p _ { \widehat { X } _ { k } } } \left[ \mathbb { E } \Big [ \prod _ { 1 \leq j \leq r } \| \mathcal { E } _ { 2 } ( \boldsymbol { x } _ { k } ^ { ( j ) } ) \| _ { 2 } ^ { 2 } | \boldsymbol { x } _ { k } ^ { ( j ) } \Big ] \right] . +$$ + +To control the above term, we first make the following observations about the expectation and bias of random variable $\frac { s _ { \tau _ { k , i } } ^ { \star } ( x _ { \tau _ { k , i } } ) } { 2 ( 1 - \tau _ { k , i } ) ^ { 3 / 2 } }$ conditioned on $x _ { \tau _ { k , 0 } } = x _ { k } ^ { ( j ) }$ : + +$$ +\begin{array} { r l } & { \mathbb { E } \bigg [ \frac { \delta _ { \tau _ { k , i } } ^ { \star } \big ( x _ { \tau _ { k , i } } \big ) } { 2 ( 1 - \tau _ { k , i } ) ^ { 3 / 2 } } \big ( \widehat { \tau } _ { k , i - 1 } - \widehat { \tau } _ { k , i } \big ) | x _ { k } ^ { ( j ) } \bigg ] \overset { \mathrm { ( a ) } } { = } \displaystyle \int _ { \widehat { \tau } _ { k , i } } ^ { \widehat { \tau } _ { k , i - 1 } } \frac { s _ { \tau } ^ { \star } ( x _ { \tau } ) } { 2 ( 1 - \tau ) ^ { 3 / 2 } } \big ( \widehat { \tau } _ { k , i - 1 } - \widehat { \tau } _ { k , i } \big ) \frac { 1 } { \widehat { \tau } _ { k , i - 1 } - \widehat { \tau } _ { k , i } } \mathrm { d } \tau } \\ & { \qquad = \displaystyle \int _ { \widehat { \tau } _ { k , i } } ^ { \widehat { \tau } _ { k , i - 1 } } \frac { s _ { \tau } ^ { \star } ( x _ { \tau } ) } { 2 ( 1 - \tau ) ^ { 3 / 2 } } \mathrm { d } \tau , } \end{array} +$$ + +where (a) comes from the fact that $\tau _ { k , i }$ is uniformly distributed within $[ \widehat { \tau } _ { k , i } , \widehat { \tau } _ { k , i - 1 } ]$ , and + +$$ +\begin{array} { r l } & { \int _ { \hat { \mathcal { T } } _ { k , \delta } } ^ { \hat { \mathcal { T } } _ { k , \delta - 1 } } \frac { s _ { \mathrm { F } } ^ { \star } ( x _ { T } ) } { 2 ( 1 - \tau ) ^ { 3 / 2 } } \mathrm { d } \tau - \frac { s _ { \mathrm { F } _ { k , \delta } } ^ { \star } ( x _ { T , \delta , 1 } ) } { 2 ( 1 - \tau ) _ { k + 1 } \lambda ^ { 2 / 2 } } ( \hat { \mathcal { T } } _ { k , \delta - 1 } - \hat { \mathcal { T } } _ { k , \delta } ) } \\ & { = \int _ { \hat { \mathcal { T } } _ { k , \delta } } ^ { \hat { \mathcal { T } } _ { k , \delta } } 1 [ \frac { s _ { \mathrm { F } } ^ { \star } ( x _ { T } ) } { 2 ( 1 - \tau ) ^ { 3 / 2 } } - \frac { s _ { \mathrm { F } _ { k , \delta } } ^ { \star } ( x _ { T , \delta , 1 } ) } { 2 ( 1 - \tau _ { k , \delta } ) ^ { 3 / 2 } } ] \mathrm { d } \tau } \\ & { = \int _ { \hat { \mathcal { T } } _ { k , \delta } } ^ { \hat { \mathcal { T } } _ { k , \delta } } 1 \int _ { \tau _ { k , \delta } } ^ { \tau ^ { \prime } } \frac { \partial } { \partial \tau } \frac { s _ { \mathrm { F } } ^ { \star } ( x _ { T } ) } { 2 ( 1 - \tau ) ^ { 3 / 2 } } \mathrm { d } \tau \mathrm { d } \tau ^ { \prime } } \\ & = ( \int _ { \hat { \mathcal { T } } _ { k , \delta } } ^ { \hat { \mathcal { T } } _ { k , \delta - 1 } } \int _ { \tau _ { k } } ^ { \hat { \mathcal { T } } _ { k , \delta - 1 } } + \int _ { \hat { \mathcal { T } } _ { k , \delta } } ^ { \tau _ { k , \delta } } \int _ { \hat { \mathcal { T } } _ { k , \delta } } ^ { \frac { \partial } { \partial \tau } } \frac s _ \mathrm { F } \end{array} +$$ + +Furthermore, we have + +$$ +\begin{array} { r l } & { \quad \left\| \int _ { \widehat { \tau } _ { k , i } } ^ { \widehat { \tau } _ { k , i - 1 } } \frac { s _ { \tau } ^ { \star } ( x _ { \tau } ) } { 2 ( 1 - \tau ) ^ { 3 / 2 } } \mathrm { d } \tau - \frac { s _ { \tau _ { k , i } } ^ { \star } ( x _ { \tau _ { k , i } } ) } { 2 ( 1 - \tau _ { k , i } ) ^ { 3 / 2 } } ( \widehat { \tau } _ { k , i - 1 } - \widehat { \tau } _ { k , i } ) \right\| _ { 2 } } \\ & { \leq ( \widehat { \tau } _ { k , i - 1 } - \widehat { \tau } _ { k , i } ) \int _ { \tau _ { k , i } } ^ { \widehat { \tau } _ { k , i - 1 } } \left\| \frac { \partial } { \partial \tau } \frac { s _ { \tau } ^ { \star } ( x _ { \tau } ) } { 2 ( 1 - \tau ) ^ { 3 / 2 } } \right\| _ { 2 } \mathrm { d } \tau + ( \widehat { \tau } _ { k , i - 1 } - \widehat { \tau } _ { k , i } ) \int _ { \widehat { \tau } _ { k , i } } ^ { \tau _ { k , i } } \left\| \frac { \partial } { \partial \tau } \frac { s _ { \tau } ^ { \star } ( x _ { \tau } ) } { 2 ( 1 - \tau ) ^ { 3 / 2 } } \right\| _ { 2 } \mathrm { d } \tau } \\ & { \leq ( \widehat { \tau } _ { k , i - 1 } - \widehat { \tau } _ { k , i } ) \int _ { \widehat { \tau } _ { k , i } } ^ { \widehat { \tau } _ { k , i - 1 } } \left\| \frac { \partial } { \partial \tau } \frac { s _ { \tau } ^ { \star } ( x _ { \tau } ) } { 2 ( 1 - \tau ) ^ { 3 / 2 } } \right\| _ { 2 } \mathrm { d } \tau . } \end{array} +$$ + +$x _ { k } ^ { ( j ) }$ , according to Bernstein inequality, we have with probability at least $1 - \delta$ , for all $1 \le j \le r$ + +$$ +\begin{array} { r } { \lVert \mathcal { E } _ { 2 } ( x _ { k } ^ { ( j ) } ) \rVert _ { 2 } ^ { 2 } \lesssim \log { \frac { r } { \delta } } \displaystyle \sum _ { i = 1 } ^ { n - 1 } \left[ ( \widehat { \tau } _ { k , i - 1 } - \widehat { \tau } _ { k , i } ) \int _ { \widehat { \tau } _ { k , i } } ^ { \widehat { \tau } _ { k , i - 1 } } \Big \lVert \frac { \partial } { \partial \tau } \frac { s _ { \tau } ^ { \star } ( x _ { \tau } ) } { ( 1 - \tau ) ^ { 3 / 2 } } \Big \rVert _ { 2 } \mathrm { d } \tau \right] ^ { 2 } } \\ { \stackrel { \mathrm { ( a ) } } { \lesssim } \log { \frac { r } { \delta } } \displaystyle \sum _ { i = 1 } ^ { n - 1 } ( \widehat { \tau } _ { k , i - 1 } - \widehat { \tau } _ { k , i } ) ^ { 3 } \int _ { \widehat { \tau } _ { k , i } } ^ { \widehat { \tau } _ { k , i - 1 } } \Big \lVert \frac { \partial } { \partial \tau } \frac { s _ { \tau } ^ { \star } ( x _ { \tau } ) } { ( 1 - \tau ) ^ { 3 / 2 } } \Big \rVert _ { 2 } ^ { 2 } \mathrm { d } \tau , } \end{array} +$$ + +where (a) uses the cauchy-schwarz inequality. + +In addition, notice the relation that for $\begin{array} { r } { \mathbb { P } ( X > \log \frac { r } { \delta } ) < \frac { \delta } { r } } \end{array}$ and $Y \sim \mathsf { E x p } ( 1 )$ , the following inequality holds. + +$$ +\mathbb { E } [ X ^ { r } ] \leq \mathbb { E } [ Y ^ { r } ] = r ! . +$$ + +Then the (56) implies that + +$$ +\mathbb { E } \Big [ \prod _ { 1 \le j \le r } \| \mathcal { E } _ { 2 } ( x _ { k } ^ { ( j ) } ) \| _ { 2 } ^ { 2 } | x _ { k } ^ { ( j ) } \Big ] \lesssim r ! \left( \sum _ { i = 1 } ^ { n - 1 } ( \widehat { \tau } _ { k , i - 1 } - \widehat { \tau } _ { k , i } ) ^ { 3 } \int _ { \widehat { \tau } _ { k , i } } ^ { \widehat { \tau } _ { k , i - 1 } } \Big \| \frac { \partial } { \partial \tau } \frac { s _ { \tau } ^ { \star } ( x _ { \tau } ) } { ( 1 - \tau ) ^ { 3 / 2 } } \Big \| _ { 2 } ^ { 2 } \mathrm { d } \tau \right) ^ { r } . +$$ + +Inserting (57) into (55), we have + +$$ +\begin{array} { r l } & { \quad \mathbb { E } \left[ \left( \mathbb { E } _ { x _ { k } \sim p _ { \widehat { X } _ { k } } } \left[ \| \mathcal { E } _ { 2 } ( x _ { k } ) \| _ { 2 } ^ { 2 } | \tau \right] \right) ^ { r } \right] } \\ & { \lesssim r ! \left( \displaystyle \sum _ { i = 1 } ^ { n - 1 } ( \widehat { \tau } _ { k , i - 1 } - \widehat { \tau } _ { k , i } ) ^ { 3 } \displaystyle \int _ { \widehat { \tau } _ { k , i } } ^ { \widehat { \tau } _ { k , i - 1 } } \mathbb { E } _ { x _ { \tau } \sim X _ { \tau } } \left[ \left\| \displaystyle \frac { \partial } { \partial \tau } \displaystyle \frac { s _ { \tau } ^ { \star } ( x _ { \tau } ^ { ( j ) } ) } { ( 1 - \tau ) ^ { 3 / 2 } } \right\| _ { 2 } ^ { 2 } \right] \mathrm { d } \tau \right) ^ { r } , } \end{array} +$$ + +which tells us that with probability at least $1 - T ^ { - 1 0 0 }$ , + +$$ +\mathbb { E } _ { \boldsymbol { x } _ { k } \sim p _ { \hat { X } _ { k } } } \big [ \| \mathcal { E } _ { 2 } ( \boldsymbol { x } _ { k } ) \| _ { 2 } ^ { 2 } \big ] \lesssim \log T \sum _ { i = 1 } ^ { n - 1 } ( \hat { \tau } _ { k , i - 1 } - \hat { \tau } _ { k , i } ) ^ { 3 } \int _ { \hat { \tau } _ { k , i } } ^ { \hat { \tau } _ { k , i - 1 } } \mathbb { E } _ { \boldsymbol { x } _ { \tau } \sim \boldsymbol { X } _ { \tau } } \bigg [ \Big \| \frac { \partial } { \partial \tau } \frac { s _ { \tau } ^ { \star } ( \boldsymbol { x } _ { \tau } ) } { ( 1 - \tau ) ^ { 3 / 2 } } \Big \| _ { 2 } ^ { 2 } \bigg ] \mathrm { d } \tau . +$$ + +4. Combining the above results. Combining these together, we have + +$$ +\begin{array} { r l } & { \quad \quad \frac { \mathbb { E } _ { x _ { k } \sim \mathcal { P } _ { \widehat { X } _ { k } } } [ \| \boldsymbol { 1 } \boldsymbol { \tau } _ { { \widehat { \tau } } _ { k , n } } ( x _ { k } ) - \boldsymbol { z } _ { { \tau } _ { k , n } } ^ { * } ( x _ { k } ) \| _ { 2 } ^ { 2 } ] } { 1 - \tau _ { k , n } } } \\ & { \lesssim \log T \displaystyle \sum _ { i = 1 } ^ { n - 1 } ( \widehat { \tau } _ { k , i - 1 } - \widehat { \tau } _ { k , i } ) ^ { 3 } \int _ { \widehat { \tau } _ { k , i } } ^ { \widehat { \tau } _ { k , i - 1 } } \mathbb { E } _ { x _ { \tau } \sim X _ { \tau } } [ \| \displaystyle \frac { \partial } { \partial \tau } \displaystyle \frac { s _ { \tau } ^ { \star } ( x _ { \tau } ) } { ( 1 - \tau ) ^ { 3 / 2 } } \| _ { 2 } ^ { 2 } ] \mathrm { d } \tau } \\ & { \quad \quad + ( \widehat { \tau } _ { k , - 1 } - \widehat { \tau } _ { k , 0 } ) ^ { 3 } \displaystyle \int _ { \widehat { \tau } _ { k , 0 } } ^ { \tau _ { k , 0 } } \mathbb { E } _ { x _ { \tau } \sim X _ { \tau } } [ \| \displaystyle \frac { \partial } { \partial \tau } \displaystyle \frac { s _ { \tau } ^ { \star } ( x _ { \tau } ) } { ( 1 - \tau ) ^ { 3 / 2 } } \| _ { 2 } ^ { 2 } ] \mathrm { d } \tau } \\ & \quad \quad + ( \widehat { \tau } _ { k , n - 1 } - \widehat { \tau } _ { k , n } ) ^ { 3 } \displaystyle \int _ { \tau _ { k , n } } ^ { \widehat { \tau } _ { k , n - 1 } } \mathbb { E } _ { x _ { \tau } \sim X _ { \tau } } [ \| \displaystyle \frac { \partial } { \partial \tau } \displaystyle \frac { s _ { \tau } ^ { \star } ( x _ { \tau } ) } 2 ( 1 - \tau \end{array} +$$ + +We claim that for any $0 < \tau < 1$ , + +$$ +\mathbb { E } _ { x _ { \tau } \sim X _ { \tau } } \bigg [ \Big \lVert \frac { \partial } { \partial \tau } \frac { s _ { \tau } ^ { \star } ( x _ { \tau } ) } { ( 1 - \tau ) ^ { 3 / 2 } } \Big \rVert _ { 2 } ^ { 2 } \bigg ] \lesssim \frac { d \log T } { ( 1 - \tau ) ^ { 5 } \tau ^ { 3 } } \operatorname* { m i n } \left\{ d + \mathbb { E } [ \mathsf { T r } ( \Sigma _ { \tau } ^ { 2 } ( x _ { \tau } ) ) ] , L ^ { 2 } \right\} . +$$ + +The proof is deferred to the end of this section. Inserting into (58), we have + +$$ +\begin{array} { r l } & { ( \widehat { \tau } _ { k , i - 1 } - \widehat { \tau } _ { k , i } ) ^ { 3 } \displaystyle \int _ { \widehat { \tau } _ { k , i } } ^ { \widehat { \tau } _ { k , i - 1 } } \mathbb { E } _ { x _ { \tau } \sim X _ { \tau } } [ \| \frac { \partial } { \partial \tau } \frac { s _ { \tau } ^ { * } ( x _ { \tau } ) } { ( 1 - \tau ) ^ { 3 / 2 } } \| _ { 2 } ^ { 2 } ] \mathrm { d } \tau } \\ & { \displaystyle \cdot \frac { ( \widehat { \tau } _ { k , i - 1 } - \widehat { \tau } _ { k , i } ) ^ { 3 } d \log T } { ( 1 - \widehat { \tau } _ { k , i - 1 } ) ^ { 3 } \widehat { \tau } _ { k , i } ^ { 3 } } \operatorname* { m i n } \{ \frac { d ( \widehat { \tau } _ { k , i - 1 } - \widehat { \tau } _ { k , i } ) } { ( 1 - \widehat { \tau } _ { k , i - 1 } ) ^ { 2 } } + \int _ { \widehat { \tau } _ { k , i } } ^ { \widehat { \tau } _ { k , i - 1 } } \frac { \mathbb { E } [ \mathsf { T r } ( \Sigma _ { \tau } ^ { 2 } ( x _ { \tau } ) ) ] } { ( 1 - \tau ) ^ { 2 } } \mathrm { d } \tau , \frac { L ^ { 2 } ( \widehat { \tau } _ { k , i - 1 } - \widehat { \tau } _ { k , i } ) } { ( 1 - \widehat { \tau } _ { k , i - 1 } ) ^ { 2 } } \} } \\ & \displaystyle \overset { ! ) } { \underset { T ^ { 3 } } { \longrightarrow } } \frac { d \log ^ { 4 } T } { T ^ { 3 } } \operatorname* { m i n } \{ \frac { d \widehat { \tau } _ { k , i - 1 } \log T } { ( 1 - \widehat { \tau } _ { k , i - 1 } ) T } + \int _ { \widehat { \tau } _ { k , i } } ^ { \widehat { \tau } _ { k , i - 1 } } \frac \mathbb { E } [ \mathsf { T r } ( \Sigma _ { \tau } ^ { 2 } ( x _ { \tau } \end{array} +$$ + +where (a) uses (50) and (51). Similarly, we have + +$$ +\begin{array} { r l } & { \quad ( \widehat { \tau } _ { k , - 1 } - \widehat { \tau } _ { k , 0 } ) ^ { 3 } \displaystyle \int _ { \tau _ { k , 0 } } ^ { \tau _ { k , 0 } } \mathbb { E } _ { x _ { \tau } \sim X _ { \tau } } [ \| \frac { \partial } { \partial \tau } \frac { s _ { \tau } ^ { * } ( x _ { \tau } ) } { ( 1 - \tau ) ^ { 3 / 2 } } \| _ { 2 } ^ { 2 } ] \mathrm { d } \tau } \\ & { \lesssim \displaystyle \frac { d \log ^ { 4 } T } { T ^ { 3 } } \operatorname* { m i n } \{ \frac { d \widehat { \tau } _ { k , - 1 } \log T } { ( 1 - \widehat { \tau } _ { k , - 1 } ) T } + \int _ { \widehat { \tau } _ { k , 0 } } ^ { \tau _ { k , 0 } } \frac { \mathbb { E } [ \mathsf { T r } ( \Sigma _ { \tau } ^ { 2 } ( x _ { \tau } ) ) ] } { ( 1 - \tau ) ^ { 2 } } \mathrm { d } \tau , \frac { L ^ { 2 } \widehat { \tau } _ { k , - 1 } \log T } { ( 1 - \widehat { \tau } _ { k , - 1 } ) T } \} , } \\ & { \quad ( \widehat { \tau } _ { k , n - 1 } - \widehat { \tau } _ { k , n } ) ^ { 3 } \displaystyle \int _ { \tau _ { k , n } } ^ { \widehat { \tau } _ { k , n - 1 } } \mathbb { E } _ { x _ { \tau } \sim X _ { \tau } } [ \| \frac { \partial } { \partial \tau } \frac { s _ { \tau } ^ { * } ( x _ { \tau } ) } { 2 ( 1 - \tau ) ^ { 3 / 2 } } \| _ { 2 } ^ { 2 } ] \mathrm { d } \tau } \\ & \lesssim \displaystyle \frac { d \log ^ { 4 } T } { T ^ { 3 } } \operatorname* { m i n } \{ \frac { d \widehat { \tau } _ { k , n - 1 } \log T } { ( 1 - \widehat { \tau } _ { k , n - 1 } ) T } + \int _ { \tau _ { k , n } } ^ \widehat { \tau } _ { k , n - 1 } \end{array} +$$ + +Thus we have + +$$ +\frac { \mathbb { E } _ { \boldsymbol { x } _ { k } \sim p _ { \widehat { X } _ { k } } } \left[ \| \boldsymbol { x } _ { \tau _ { k , n } } ( \boldsymbol { x } _ { k } ) - \boldsymbol { z } _ { \tau _ { k , n } } ^ { \star } ( \boldsymbol { x } _ { k } ) \| _ { 2 } ^ { 2 } \right] } { 1 - \tau _ { k , n } } +$$ + +$$ +\lesssim \operatorname* { m i n } \Big \{ \frac { N d ^ { 2 } \widehat \tau _ { k , - 1 } \log ^ { 6 } T } { T ^ { 4 } ( 1 - \widehat \tau _ { k , - 1 } ) } + \frac { d \log ^ { 5 } T } { T ^ { 3 } } \int _ { \tau _ { k , n } } ^ { \tau _ { k , 0 } } \frac { \mathbb { E } [ \mathsf { T r } ( \Sigma _ { \tau } ^ { 2 } ( x _ { \tau } ) ) ] } { ( 1 - \tau ) ^ { 2 } } \mathrm { d } \tau , \frac { N d L ^ { 2 } \widehat \tau _ { k , - 1 } \log ^ { 6 } T } { ( 1 - \widehat \tau _ { k , - 1 } ) T ^ { 4 } } \Big \} +$$ + +and complete the proof. + +Proof of (59). Notice that + +$$ +\frac { \partial } { \partial \tau } \frac { s _ { \tau } ^ { \star } ( x _ { \tau } ) } { ( 1 - \tau ) ^ { 3 / 2 } } = \frac { \partial } { \partial y } \frac { s _ { \tau } ^ { \star } ( \sqrt { 1 - \tau } y ) } { ( 1 - \tau ) ^ { 3 / 2 } } \Big | _ { y = \frac { x _ { \tau } } { \sqrt { 1 - \tau } } } \frac { \partial } { \partial \tau } \frac { x _ { \tau } } { \sqrt { 1 - \tau } } + \frac { \partial } { \partial \tau } \frac { s _ { \tau } ^ { \star } ( \sqrt { 1 - \tau } y ) } { ( 1 - \tau ) ^ { 3 / 2 } } \Big | _ { y = \frac { x _ { \tau } } { \sqrt { 1 - \tau } } } . +$$ + +For the first term, we have + +$$ +\begin{array} { l } { { \displaystyle \frac { \partial } { \partial y } \frac { s _ { \tau } ^ { \star } ( \sqrt { 1 - \tau } y ) } { ( 1 - \tau ) ^ { 3 / 2 } } \Big \vert _ { y = \frac { x _ { \tau } } { \sqrt { 1 - \tau } } } \frac { \partial } { \partial \tau } \frac { x _ { \tau } } { \sqrt { 1 - \tau } } = - \frac { \sqrt { 1 - \tau } } { ( 1 - \tau ) ^ { 3 / 2 } } J _ { \tau } ( x _ { \tau } ) \frac { s _ { \tau } ^ { \star } ( x _ { \tau } ) } { ( 1 - \tau ) ^ { 3 / 2 } } } } \\ { { = - J _ { \tau } ( x _ { \tau } ) \frac { s _ { \tau } ^ { \star } ( x _ { \tau } ) } { ( 1 - \tau ) ^ { 5 / 2 } } , } } \end{array} +$$ + +where + +$$ +J _ { \tau } ( x ) : = \frac { \partial s _ { \tau } ^ { \star } ( x ) } { \partial x } . +$$ + +According to Definition 2, we have $\tau \| J _ { \tau } ( x ) \| \leq L$ for $x \in { \mathcal { L } } _ { \tau }$ . Moreover, we use the following lemma, the proof of which is postponed to the end of this section. + +Lemma 9. Suppose that $x _ { \tau } \in S _ { \tau }$ . Then we have + +$$ +\| s _ { \tau } ^ { \star } ( x _ { \tau } ) \| _ { 2 } ^ { 2 } \leq \frac { 1 } { \tau ^ { 2 } } \int \left\| x _ { \tau } - \sqrt { 1 - \tau } x _ { 0 } \right\| _ { 2 } ^ { 2 } p _ { X _ { 0 } | X _ { \tau } } ( x _ { 0 } | x _ { \tau } ) \mathrm { d } x _ { 0 } \leq \frac { 2 5 ( \theta + c _ { 0 } ) d \log T } { \tau } . +$$ + +Moreover, we have + +$$ +\int _ { x _ { \tau } } \Vert J _ { \tau } ( x _ { \tau } ) s _ { \tau } ^ { \star } ( x _ { \tau } ) \Vert _ { 2 } ^ { 2 } p _ { \tau } ( x _ { \tau } ) \mathrm { d } x _ { \tau } \lesssim \frac { d \log T } { \tau ^ { 3 } } \operatorname* { m i n } \left\{ d + \mathbb { E } [ \mathsf { T r } ( \Sigma _ { \tau } ^ { 2 } ( x _ { \tau } ) ) ] , L ^ { 2 } \right\} , +$$ + +where $C _ { 0 }$ is a universal constant. + +By using Lemma 9, we have + +$$ +\begin{array} { r l } & { \displaystyle \mathbb { E } _ { x _ { \tau } \sim X _ { \tau } } \left\| \frac { \partial } { \partial y } \frac { s _ { \tau } ^ { \star } ( \sqrt { 1 - \tau } y ) } { ( 1 - \tau ) ^ { 3 / 2 } } \Big | _ { y = \frac { x _ { \tau } } { \sqrt { 1 - \tau } } } \frac { \partial } { \partial \tau } \frac { x _ { \tau } } { \sqrt { 1 - \tau } } \right\| _ { 2 } ^ { 2 } } \\ & { \displaystyle \leq \mathbb { E } _ { x _ { \tau } \sim X _ { \tau } } \Big \| J _ { \tau } ( x _ { \tau } ) \frac { s _ { \tau } ^ { \star } ( x _ { \tau } ) } { ( 1 - \tau ) ^ { 5 / 2 } } \Big \| _ { 2 } ^ { 2 } \lesssim \frac { d \log T } { \tau ^ { 3 } ( 1 - \tau ) ^ { 5 } } \operatorname* { m i n } \big \{ d + \mathbb { E } [ \mathsf { T r } ( \Sigma _ { \tau } ^ { 2 } ( x _ { \tau } ) ) ] , L ^ { 2 } \big \} . } \end{array} +$$ + +For the second term, we have + +$$ +\begin{array} { r l r } { { \frac { \partial } { \partial \tau } \frac { s _ { \tau } ^ { * } ( \sqrt { 1 - \tau } y ) } { ( 1 - \tau ) ^ { 3 / 2 } } = - \frac { \partial } { \partial \tau } \frac { 1 } { ( 1 - \tau ) \tau } \int _ { x _ { 0 } } ( y - x _ { 0 } ) p _ { X _ { 0 } | X _ { \tau } } ( x _ { 0 } | \sqrt { 1 - \tau } y ) \mathrm { d } x _ { 0 } } } \\ & { } & { = \frac { 1 - 2 \tau } { ( 1 - \tau ) ^ { 2 } \tau ^ { 2 } } \int _ { x _ { 0 } } ( y - x _ { 0 } ) p _ { X _ { 0 } | X _ { \tau } } ( x _ { 0 } | \sqrt { 1 - \tau } y ) \mathrm { d } x _ { 0 } } \\ & { } & { - \frac { 1 } { ( 1 - \tau ) \tau } \frac { \partial } { \partial \tau } \int _ { x _ { 0 } } ( y - x _ { 0 } ) p _ { X _ { 0 } | X _ { \tau } } ( x _ { 0 } | \sqrt { 1 - \tau } y ) \mathrm { d } x _ { 0 } . } \end{array} +$$ + +Thus we have + +$$ +\begin{array} { r l } & { \displaystyle \mathbb { E } _ { x _ { \tau } \sim X _ { \tau } } \left\| \frac { \partial } { \partial \tau } \frac { s _ { \tau } ^ { \star } ( \sqrt { 1 - \tau } y ) } { ( 1 - \tau ) ^ { 3 / 2 } } \right\| _ { 2 } ^ { 2 } } \\ & { \displaystyle \leq \frac { 2 ( 1 - 2 \tau ) ^ { 2 } } { ( 1 - \tau ) ^ { 4 } \tau ^ { 4 } } \int _ { x _ { 0 } } \mathbb { E } _ { y \sim X _ { \tau } / \sqrt { 1 - \tau } } \| y - x _ { 0 } \| _ { 2 } ^ { 2 } p _ { X _ { 0 } | X _ { \tau } } ( x _ { 0 } | \sqrt { 1 - \tau } y ) \mathrm { d } x _ { 0 } } \end{array} +$$ + +$$ +\begin{array} { r l } & { \quad + \displaystyle \frac { 2 } { ( 1 - \tau ) ^ { 2 } \tau ^ { 2 } } \mathbb { E } \left\| \displaystyle \frac { \partial } { \partial \tau } \int _ { x _ { 0 } } ( y - x _ { 0 } ) p _ { X _ { 0 } | X _ { \tau } } ( x _ { 0 } | \sqrt { 1 - \tau } y ) \mathrm { d } x _ { 0 } \right\| _ { 2 } ^ { 2 } } \\ & { \stackrel { ( i ) } { \leq } \displaystyle \frac { 2 ( 1 - 2 \tau ) ^ { 2 } } { ( 1 - \tau ) ^ { 4 } \tau ^ { 4 } } \displaystyle \frac { \tau d } { 1 - \tau } + \displaystyle \frac { 2 } { ( 1 - \tau ) ^ { 2 } \tau ^ { 2 } } \displaystyle \frac { d \mathrm { m i n } \{ d \log T , L ^ { 2 } \} } { \tau ( 1 - \tau ) ^ { 3 } } } \\ & { \lesssim \displaystyle \frac { ( 1 - 2 \tau ) ^ { 2 } d } { ( 1 - \tau ) ^ { 5 } \tau ^ { 3 } } + \displaystyle \frac { d \mathrm { m i n } \{ d \log T , L ^ { 2 } \} } { ( 1 - \tau ) ^ { 5 } \tau ^ { 3 } } \lesssim \displaystyle \frac { d \mathrm { m i n } \{ d \log T , L ^ { 2 } \} } { ( 1 - \tau ) ^ { 5 } \tau ^ { 3 } } , } \end{array} +$$ + +where (i) comes from the following Lemma. + +Lemma 10. For $y \sim p _ { X _ { \tau } / \sqrt { 1 - \tau } }$ , the following inequality holds. + +$$ +\mathbb { E } \left\| \frac { \partial } { \partial \tau } \int _ { x _ { 0 } } ( y - x _ { 0 } ) p _ { X _ { 0 } | X _ { \tau } } ( x _ { 0 } | \sqrt { 1 - \tau } y ) \mathrm { d } x _ { 0 } \right\| _ { 2 } ^ { 2 } \leq \frac { d \operatorname* { m i n } \{ d \log T , L \} } { \tau ( 1 - \tau ) ^ { 3 } } . +$$ + +Inserting (63) and (64) into (60), we have + +$$ +\mathbb { E } _ { x _ { \tau } \sim X _ { \tau } } \left\| \frac { \partial } { \partial \tau } \frac { s _ { \tau } ^ { \star } ( x _ { \tau } ) } { ( 1 - \tau ) ^ { 3 / 2 } } \right\| ^ { 2 } \lesssim \frac { d \log T } { \tau ^ { 3 } ( 1 - \tau ) ^ { 5 } } \operatorname* { m i n } \left. d + \mathbb { E } [ \mathsf { T r } ( \Sigma _ { \tau } ^ { 2 } ( x _ { \tau } ) ) ] , L ^ { 2 } \right. . +$$ + +Proof of Lemma 9. Inequality (61) can be proved in a similar way as Li and Yan (2024a) (cf. Lemma 1). Specifically, we have + +$$ +\begin{array} { r l } & { \quad \displaystyle \int \| x _ { \tau } - \sqrt { 1 - \tau x _ { 0 } } \| _ { 2 } ^ { 2 } p _ { X _ { 0 } | X _ { \tau } } ( x _ { 0 } | x _ { \tau } ) \mathrm { d } x _ { 0 } } \\ & { \le \displaystyle \int \| x _ { \tau } - \sqrt { 1 - \tau x _ { 0 } } \| _ { 2 } ^ { 2 } p _ { X _ { 0 } | X _ { \tau } } ( x _ { 0 } | x _ { \tau } ) \mathbb { 1 } ( \left\| x _ { \tau } - \sqrt { 1 - \tau x _ { 0 } } \right\| _ { 2 } \le R ) \mathrm { d } x _ { 0 } } \\ & { \quad \displaystyle + \int \| x _ { \tau } - \sqrt { 1 - \tau x _ { 0 } } \| _ { 2 } ^ { 2 } p _ { X _ { 0 } | X _ { \tau } } ( x _ { 0 } | x _ { \tau } ) \mathbb { 1 } ( \left\| x _ { \tau } - \sqrt { 1 - \tau x _ { 0 } } \right\| _ { 2 } > R ) \mathrm { d } x _ { 0 } } \\ & { \stackrel { ( a ) } { \le } R ^ { 2 } + \displaystyle \int p _ { X _ { 0 } } ( x _ { 0 } ) \exp \left( - \frac { \| x _ { \tau } - \sqrt { 1 - \tau x _ { 0 } } \| _ { 2 } ^ { 2 } } { 3 T } \right) \left\| x _ { \tau } - \sqrt { 1 - \tau x _ { 0 } } \right\| _ { 2 } ^ { 2 } } \\ & { \quad \quad \quad \cdot \mathbb { 1 } ( \left\| x _ { \tau } - \sqrt { 1 - \tau x _ { 0 } } \right\| _ { 2 } > R ) \mathrm { d } x _ { 0 } } \\ & { \stackrel { ( b ) } { \le } R ^ { 2 } + 3 \tau \displaystyle \int p _ { X _ { 0 } } ( x _ { 0 } ) \frac { R ^ { 2 } } { 3 T } \exp \left( - \frac { R ^ { 2 } } { 3 T } \right) \mathrm { d } x _ { 0 } \le 2 R ^ { 2 } , } \end{array} +$$ + +$R ^ { 2 } = ( 6 \theta + 3 c _ { 0 } ) d \tau \log T$ $\begin{array} { r } { p _ { X _ { 0 } | X _ { \tau } } ( x _ { 0 } | x _ { \tau } ) \leq p _ { X _ { 0 } } ( x _ { 0 } ) \exp ( - \frac { \| x _ { \tau } - \sqrt { 1 - \tau } x _ { 0 } \| _ { 2 } ^ { 2 } } { 3 \tau } ) } \end{array}$ $\left. x _ { \tau } - \sqrt { 1 - \tau } x _ { 0 } \right. _ { 2 } > R$ $\begin{array} { r } { z \exp ( - z ) \le \frac { R ^ { 2 } } { 3 \tau } \exp \bigl ( - \frac { R ^ { 2 } } { 3 \tau } \bigr ) } \end{array}$ $\begin{array} { r } { z \ge \frac { R ^ { 2 } } { 3 \tau } \ge 1 } \end{array}$ $( 2 \theta + c _ { 0 } ) d \log T \geq$ $^ { 1 }$ . The remaining proof focuses on (62). For convenience, we denote $p _ { \tau }$ as the probability density function of $X _ { \tau }$ in this section. For $d < L ^ { 2 }$ , we have + +$$ +\begin{array} { r l } & { \quad \mathbb { E } _ { x _ { \tau } \sim X _ { \tau } } \Big \lVert J _ { \tau } ( x _ { \tau } ) s _ { \tau } ^ { \star } ( x _ { \tau } ) \Big \rVert _ { 2 } ^ { 2 } } \\ & { \le \displaystyle \int _ { S _ { \tau } } \Big \lVert J _ { \tau } ( x _ { \tau } ) s _ { \tau } ^ { \star } ( x _ { \tau } ) \Big \rVert _ { 2 } ^ { 2 } p _ { \tau } ( x _ { \tau } ) \mathrm { d } x _ { \tau } + \displaystyle \int _ { S _ { \tau } ^ { \mathrm { c } } } \Big \lVert J _ { \tau } ( x _ { \tau } ) s _ { \tau } ^ { \star } ( x _ { \tau } ) \Big \rVert _ { 2 } ^ { 2 } p _ { \tau } ( x _ { \tau } ) \mathrm { d } x _ { \tau } } \\ & { \le \frac { 2 5 ( \theta + c _ { 0 } ) d \log T } { \tau } \int \Big \lVert J _ { \tau } ( x _ { \tau } ) \Big \rVert _ { 2 } ^ { 2 } p _ { \tau } ( x _ { \tau } ) \mathrm { d } x _ { \tau } + \displaystyle \int _ { S _ { \tau } ^ { \mathrm { c } } } \Big \lVert J _ { \tau } ( x _ { \tau } ) s _ { \tau } ^ { \star } ( x _ { \tau } ) \Big \rVert _ { 2 } ^ { 2 } p _ { \tau } ( x _ { \tau } ) \mathrm { d } x _ { \tau } . } \end{array} +$$ + +Noticing that + +$$ +J _ { \tau } ( x _ { \tau } ) = - \frac 1 \tau I _ { d } + \frac 1 \tau \Sigma _ { \tau } ( x _ { \tau } ) , +$$ + +we have + +$$ +\| J _ { \tau } ( x _ { \tau } ) \| _ { 2 } ^ { 2 } \leq \| J _ { \tau } ( x _ { \tau } ) \| _ { F } ^ { 2 } = { \mathsf { T r } } ( J _ { \tau } ^ { 2 } ( x _ { \tau } ) ) = { \frac { 1 } { \tau ^ { 2 } } } { \mathsf { T r } } ( I _ { d } - 2 \Sigma _ { \tau } ( x _ { \tau } ) + \Sigma _ { \tau } ^ { 2 } ( x _ { \tau } ) ) +$$ + +$$ +\leq \frac { 1 } { \tau ^ { 2 } } \mathsf { T r } ( I _ { d } + \Sigma _ { \tau } ^ { 2 } ( x _ { \tau } ) ) \leq \frac { d + \mathsf { T r } ( \Sigma _ { \tau } ^ { 2 } ( x _ { \tau } ) ) } { \tau ^ { 2 } } . +$$ + +Thus we have + +$$ +\int \Big \| J _ { \tau } ( x _ { \tau } ) \Big \| _ { 2 } ^ { 2 } p _ { \tau } ( x _ { \tau } ) \mathrm { d } x _ { \tau } \leq \frac { d + \mathbb { E } [ \mathsf { T r } ( \Sigma _ { \tau } ^ { 2 } ( x _ { \tau } ) ) ] } { \tau ^ { 2 } } . +$$ + +To bound the second term, we notice that + +$$ +\begin{array} { r l } & { \| J _ { \tau } ( x _ { \tau } ) s _ { \tau } ^ { \star } ( x _ { \tau } ) \| _ { 2 } \leq \frac { 1 } { \tau ^ { 3 / 2 } } ( \| \sqrt { \tau } s _ { \tau } ^ { \star } ( x _ { \tau } ) \| _ { 2 } + \| \Sigma _ { \tau } ( x _ { \tau } ) \sqrt { \tau } s _ { \tau } ^ { \star } ( x _ { \tau } ) \| _ { 2 } ) } \\ & { \qquad \leq \frac { 1 } { \tau ^ { 3 / 2 } } ( \| \sqrt { \tau } s _ { \tau } ^ { \star } ( x _ { \tau } ) \| _ { 2 } + \| \Sigma _ { \tau } ( x _ { \tau } ) \| _ { 2 } \| \sqrt { \tau } s _ { \tau } ^ { \star } ( x _ { \tau } ) \| _ { 2 } ) } \\ & { \qquad \leq \frac { \| \sqrt { \tau } s _ { \tau } ^ { \star } ( x _ { \tau } ) \| _ { 2 } } { \tau ^ { 3 / 2 } } ( 1 + \| \Sigma _ { \tau } ( x _ { \tau } ) \| _ { 2 } ) . } \end{array} +$$ + +Then according to Jensen’s inequality, we have + +$$ +\begin{array} { l } { \displaystyle \int _ { S _ { \tau } ^ { c } } \Big \| J _ { \tau } ( x _ { \tau } ) s _ { \tau } ^ { \star } ( x _ { \tau } ) \Big \| _ { 2 } ^ { 2 } p _ { \tau } ( x _ { \tau } ) \mathrm { d } x _ { \tau } \leq \frac { 1 } { \tau ^ { 3 } } \int _ { S _ { \tau } ^ { c } } \Big \| \sqrt { \tau } s _ { \tau } ^ { \star } ( x _ { \tau } ) \Big \| _ { 2 } ^ { 2 } \big ( 1 + \| \Sigma _ { \tau } ( x _ { \tau } ) \| _ { 2 } \big ) ^ { 2 } p _ { \tau } ( x _ { \tau } ) \mathrm { d } x _ { \tau } } \\ { \displaystyle \lesssim \frac { 1 } { \tau ^ { 3 } } \left( \int \| \sqrt { \tau } s _ { \tau } ^ { \star } ( x _ { \tau } ) \| _ { 2 } ^ { 4 } ( 1 + \| \Sigma _ { \tau } ( x _ { \tau } ) \| _ { 2 } ) ^ { 4 } p _ { \tau } ( x _ { \tau } ) \mathrm { d } x _ { \tau } \right) ^ { 1 / 2 } \left( \int _ { S _ { \tau } ^ { c } } p _ { \tau } ( x _ { \tau } ) \mathrm { d } x _ { \tau } \right) ^ { 1 / 2 } . } \end{array} +$$ + +Noticing that + +$$ +\begin{array} { r l } & { \displaystyle \| s _ { \tau } ^ { \star } ( x _ { \tau } ) \| _ { 2 } \leq \frac { 1 } { \tau } \mathbb { E } _ { x _ { 0 } } \left[ \left\| x _ { \tau } - \sqrt { 1 - \tau } x _ { 0 } \right\| _ { 2 } | x _ { \tau } \right] , } \\ & { \displaystyle \| \Sigma _ { \tau } ( x _ { \tau } ) \| _ { 2 } \leq \frac { 1 } { \tau } \mathbb { E } _ { x _ { 0 } } \left[ \left\| x _ { \tau } - \sqrt { 1 - \tau } x _ { 0 } \right\| _ { 2 } ^ { 2 } | x _ { \tau } \right] , } \end{array} +$$ + +we have + +$$ +\begin{array} { r l } & { \quad \displaystyle \int \| \sqrt { \tau } s _ { \tau } ^ { \star } ( x _ { \tau } ) \| _ { 2 } ^ { 4 } \big ( 1 + \| \Sigma _ { \tau } ( x _ { \tau } ) \| _ { 2 } \big ) ^ { 4 } p _ { \tau } ( x _ { \tau } ) \mathrm { d } x _ { \tau } } \\ & { \lesssim \displaystyle \int \frac { 1 } { \tau ^ { 2 } } \mathbb { E } _ { x _ { 0 } } \left[ \left\| x _ { \tau } - \sqrt { 1 - \tau } x _ { 0 } \right\| _ { 2 } ^ { 4 } | x _ { \tau } \right] \left( 1 + \frac { 1 } { \tau ^ { 4 } } \mathbb { E } _ { x _ { 0 } } \left[ \left\| x _ { \tau } - \sqrt { 1 - \tau } x _ { 0 } \right\| _ { 2 } ^ { 8 } | x _ { \tau } \right] \right) p _ { \tau } ( x _ { \tau } ) \mathrm { d } x _ { \tau } } \\ & { \lesssim \displaystyle \int \frac { 1 } { \tau ^ { 2 } } \left\| x _ { \tau } - \sqrt { 1 - \tau } x _ { 0 } \right\| _ { 2 } ^ { 4 } p _ { X _ { 0 } | X _ { \tau } } ( x _ { 0 } | x _ { \tau } ) p _ { \tau } ( x _ { \tau } ) \mathrm { d } x _ { 0 } \mathrm { d } x _ { \tau } } \\ & { \quad + \displaystyle \int \frac { 1 } { \tau ^ { 6 } } \| x _ { \tau } - \sqrt { 1 - \tau } x _ { 0 } \| _ { 2 } ^ { 1 2 } p _ { X _ { 0 } | X _ { \tau } } ( x _ { 0 } | x _ { \tau } ) p _ { \tau } ( x _ { \tau } ) \mathrm { d } x _ { 0 } \mathrm { d } x _ { \tau } \lesssim d ^ { 6 } . } \end{array} +$$ + +Moreover, we have + +$$ +\begin{array} { r l } { \displaystyle \int _ { S _ { \tau } ^ { \infty } } p _ { \tau } ( x _ { \tau } ) \mathrm { d } x _ { \tau } \le \int _ { S _ { \tau } ^ { \infty } \cap \{ x _ { \tau } : \| x _ { \tau } \| \le \sqrt { 1 - \tau } T ^ { c n + 1 } + \sqrt { \tau d \log T } \} } p _ { \tau } \mathrm { d } x _ { \tau } } & { } \\ { \displaystyle } & { \quad + \int _ { \{ x _ { \tau } : \| x _ { \tau } \| > \sqrt { 1 - \tau } T ^ { c n + 4 } + \sqrt { \tau d \log T } \} } p _ { \tau } \mathrm { d } x _ { \tau } } \\ { \displaystyle } & { \le \left( 2 \sqrt { 1 - \tau } T ^ { c \kappa + 4 } + 2 \sqrt { \tau d \log T } \right) ^ { d } \exp ( - \theta d \log T ) } \\ { \displaystyle } & { \quad + \mathbb { P } \left( \| x _ { \tau } \| > \sqrt { 1 - \tau } T ^ { c \kappa + 4 } + \sqrt { \tau d \log T } \right) } \\ { \displaystyle } & { \le \exp ( - ( \theta - c _ { n } - 6 ) d \log T ) + \mathbb { P } \left( \| X _ { 0 } \| > T ^ { c n + 4 } \right) } \\ { \displaystyle } & { \quad + P \left( \| W _ { \tau } \| \ge \sqrt { d \log T } \right) } \end{array} +$$ + +$$ +\leq \frac { 1 } { T ^ { 4 } } + \frac { \mathbb { E } \| X _ { 0 } \| } { T ^ { c _ { R } + 4 } } + \frac { 1 } { T ^ { 4 } } \lesssim \frac { 1 } { T ^ { 4 } } , +$$ + +as long as $\theta \geq c _ { R } + 1 0 $ . Thus we have + +$$ +\int _ { S _ { \tau } ^ { \mathrm { c } } } \Big \| J _ { \tau } ( x _ { \tau } ) s _ { \tau } ^ { \star } ( x _ { \tau } ) \Big \| ^ { 2 } p _ { \tau } ( x _ { \tau } ) \mathrm { d } x _ { \tau } \leq \frac { d ^ { 3 } } { \tau ^ { 3 } T ^ { 2 } } \lesssim \frac { d ^ { 2 } } { \tau ^ { 3 } } +$$ + +for $T \geq K \gtrsim \sqrt { d }$ . Inserting (66) and (67) into (65), we have + +$$ +\begin{array} { r l } & { \mathbb { E } _ { x _ { \tau } \sim X _ { \tau } } \Big \| J _ { \tau } ( x _ { \tau } ) s _ { \tau } ^ { \star } ( x _ { \tau } ) \Big \| ^ { 2 } \lesssim \frac { d \log T } { \tau ^ { 3 } } \left( d + \mathbb { E } [ \mathsf { T r } ( \Sigma _ { \tau } ( x _ { \tau } ) ) ] \right) + \frac { d ^ { 2 } } { \tau ^ { 3 } } } \\ & { \qquad \lesssim \frac { d \log T } { \tau ^ { 3 } } \left( d + \mathbb { E } [ \mathsf { T r } ( \Sigma _ { \tau } ( x _ { \tau } ) ) ] \right) . } \end{array} +$$ + +Otherwise, for $d \geq L ^ { 2 }$ , we have + +$$ +\begin{array} { r l } & { \mathbb { E } _ { \boldsymbol { x } _ { \tau } \sim X _ { \tau } } \Big \lVert J _ { \tau } ( \boldsymbol { x } _ { \tau } ) s _ { \tau } ^ { \star } ( \boldsymbol { x } _ { \tau } ) \Big \rVert _ { 2 } ^ { 2 } \le \frac { L ^ { 2 } } { \tau ^ { 2 } } \int _ { \mathcal { L } _ { \tau } } \Big \lVert s _ { \tau } ^ { \star } ( \boldsymbol { x } _ { \tau } ) \Big \rVert _ { 2 } ^ { 2 } p _ { \tau } ( \boldsymbol { x } _ { \tau } ) \mathrm { d } \boldsymbol { x } _ { \tau } } \\ & { \qquad + \int _ { \mathcal { L } _ { \tau } ^ { \mathrm { c } } } \Big \lVert J _ { \tau } ( \boldsymbol { x } _ { \tau } ) s _ { \tau } ^ { \star } ( \boldsymbol { x } _ { \tau } ) \Big \rVert _ { 2 } ^ { 2 } p _ { \tau } ( \boldsymbol { x } _ { \tau } ) \mathrm { d } \boldsymbol { x } _ { \tau } . } \end{array} +$$ + +For the first term, we have + +$$ +\int _ { \mathcal { L } _ { \tau } } \left\| s _ { \tau } ^ { \star } ( x _ { \tau } ) \right\| _ { 2 } ^ { 2 } p _ { \tau } ( x _ { \tau } ) \mathrm { d } x _ { \tau } \leq \frac { 1 } { \tau ^ { 2 } } \int \left\| x _ { \tau } - \sqrt { 1 - \tau } x _ { 0 } \right\| _ { 2 } ^ { 2 } p _ { X _ { 0 } } | _ { X _ { \tau } } ( x _ { 0 } | x _ { \tau } ) p _ { \tau } ( x _ { \tau } ) \mathrm { d } x _ { 0 } \mathrm { d } x _ { \tau } \leq \frac { d } { \tau } . +$$ + +For the second term, we have + +$$ +\begin{array} { r l } & { \displaystyle \int _ { \mathcal { L } _ { \tau } ^ { \mathrm { c } } } \left\| J _ { \tau } ( x _ { \tau } ) s _ { \tau } ^ { \star } ( x _ { \tau } ) \right\| _ { 2 } ^ { 2 } p _ { \tau } ( x _ { \tau } ) \mathrm { d } x _ { \tau } } \\ & { \displaystyle \leq \left( \int \left\| J _ { \tau } ( x _ { \tau } ) s _ { \tau } ^ { \star } ( x _ { \tau } ) \right\| _ { 2 } ^ { 4 } p _ { \tau } ( x _ { \tau } ) \mathrm { d } x _ { \tau } \right) ^ { 1 / 2 } \left( \displaystyle \int _ { \mathcal { L } _ { \tau } ^ { \mathrm { c } } } p _ { \tau } ( x _ { \tau } ) \mathrm { d } x _ { \tau } \right) ^ { 1 / 2 } \lesssim \frac { d ^ { 3 } } { \tau ^ { 3 } } \frac { 1 } { d ^ { 2 } } \lesssim \frac { d } { \tau ^ { 3 } } . } \end{array} +$$ + +Thus we have + +$$ +\mathbb { E } _ { x _ { \tau } \sim X _ { \tau } } \Big \| J _ { \tau } ( x _ { \tau } ) s _ { \tau } ^ { \star } ( x _ { \tau } ) \Big \| _ { 2 } ^ { 2 } \lesssim \frac { d L ^ { 2 } } { \tau ^ { 3 } } + \frac { d } { \tau ^ { 3 } } \lesssim \frac { d L ^ { 2 } } { \tau ^ { 3 } } . +$$ + +Combining (68) and (69), we could complete the proof. + +Proof of Lemma 10. According to the definition, we have + +$$ +\begin{array} { r l } & { \displaystyle \int _ { x _ { 0 } } ( y - x _ { 0 } ) p _ { X _ { 0 } | X _ { \tau } } ( x _ { 0 } | \sqrt { 1 - \tau } y ) \mathrm { d } x _ { 0 } } \\ & { = \frac { \displaystyle \int ( y - x _ { 0 } ) \phi ( \sqrt { 1 - \tau } y | \sqrt { 1 - \tau } x _ { 0 } , \tau I _ { d } ) p _ { X _ { 0 } } ( x _ { 0 } ) \mathrm { d } x _ { 0 } } { \displaystyle \int \phi ( \sqrt { 1 - \tau } y | \sqrt { 1 - \tau } x _ { 0 } , \tau I _ { d } ) p _ { X _ { 0 } } ( x _ { 0 } ) \mathrm { d } x _ { 0 } } } \\ & { = \frac { \displaystyle \int ( y - x _ { 0 } ) \phi ( y | x _ { 0 } , \frac { \tau } { 1 - \tau } I _ { d } ) p _ { X _ { 0 } } ( x _ { 0 } ) \mathrm { d } x _ { 0 } } { \displaystyle \int \phi ( y | x _ { 0 } , \frac { \tau } { 1 - \tau } I _ { d } ) p _ { X _ { 0 } } ( x _ { 0 } ) \mathrm { d } x _ { 0 } } , } \end{array} +$$ + +where $\phi ( \cdot | \boldsymbol { \mu } , \Sigma )$ denotes the probability density function of Gaussian distribution with mean vector $\mu$ and covariance matrix $\Sigma$ . Define $\begin{array} { r } { t = \frac { \tau } { 1 - \tau } } \end{array}$ . We have + +$$ +\begin{array} { l } { { \displaystyle { \frac { \partial } { \partial \tau } \int _ { x _ { 0 } } ( y - x _ { 0 } ) p _ { X _ { 0 } | X _ { \tau } } ( x _ { 0 } | \sqrt { 1 - \tau } y ) \mathrm { d } x _ { 0 } } } } \\ { { \displaystyle { = \frac { \partial } { \partial t } \frac { \int ( y - x _ { 0 } ) \phi ( y | x _ { 0 } , t I _ { d } ) p _ { X _ { 0 } } ( x _ { 0 } ) \mathrm { d } x _ { 0 } } { \int \phi ( y | x _ { 0 } , t I _ { d } ) p _ { X _ { 0 } } ( x _ { 0 } ) \mathrm { d } x _ { 0 } } \frac { \partial \frac { \tau } { 1 - \tau } } { \partial \tau } } } } \end{array} +$$ + +$$ +\begin{array} { r l } & { = - \displaystyle \frac { 1 } { ( 1 - \tau ) ^ { 2 } } \Big ( \frac { \int ( y - x _ { 0 } ) \frac { \| y - x \| _ { 2 } ^ { 2 } } { 2 t ^ { 2 } } \phi ( y | x _ { 0 } , t I _ { d } ) p _ { X _ { 0 } } ( x _ { 0 } ) \mathrm { d } x _ { 0 } } { \int \phi ( y | x _ { 0 } , t I _ { d } ) p _ { X _ { 0 } } ( x _ { 0 } ) \mathrm { d } x _ { 0 } } } \\ & { \phantom { = \ } - \frac { \int ( y - x _ { 0 } ) \phi ( y | x _ { 0 } , t I _ { d } ) p _ { X _ { 0 } } ( x _ { 0 } ) \mathrm { d } x _ { 0 } } { \int \phi ( y | x _ { 0 } , t I _ { d } ) p _ { X _ { 0 } } ( x _ { 0 } ) \mathrm { d } x _ { 0 } } \frac { \int \frac { \| y - x \| _ { 2 } ^ { 2 } } { 2 t ^ { 2 } } \phi ( y | x _ { 0 } , t I _ { d } ) p _ { X _ { 0 } } ( x _ { 0 } ) \mathrm { d } x _ { 0 } } { \int \phi ( y | x _ { 0 } , t I _ { d } ) p _ { X _ { 0 } } ( x _ { 0 } ) \mathrm { d } x _ { 0 } } \Big ) } \\ & { = - \frac { 1 } { ( 1 - \tau ) ^ { 2 } } \left( \frac { \tau } { 1 - \tau } \right) ^ { 3 / 2 } \frac { 1 } { 2 t ^ { 2 } } \left( \mathbb { E } _ { Z | y } \left[ \| Z \| _ { 2 } ^ { 2 } Z - \mathbb { E } _ { Z | y } Z \mathbb { E } _ { Z | y } \| Z \| _ { 2 } ^ { 2 } \right] \right) , } \end{array} +$$ + +where $\mathbb { E } _ { Z \mid y } [ \cdot ]$ denotes the expectation conditioned on $\begin{array} { r } { X _ { 0 } + \sqrt { \frac { \tau } { 1 - \tau } } Z = y } \end{array}$ . Notice that + +$$ +\begin{array} { r } { \mathbb { E } _ { Z | y } \left[ \| Z \| _ { 2 } ^ { 2 } Z - \mathbb { E } _ { Z | y } Z \mathbb { E } _ { Z | y } { \| Z \| _ { 2 } ^ { 2 } } \right] = \mathbb { E } _ { Z | y } \left[ \left( Z - \mathbb { E } _ { Z | y } Z \right) \left( \| Z \| _ { 2 } ^ { 2 } - \mathbb { E } _ { Z | y } { \| Z \| _ { 2 } ^ { 2 } } \right) \right] . } \end{array} +$$ + +According to Cauchy-Switch inequality, we have + +$$ +\begin{array} { r l } & { \quad \left\| \mathbb { E } _ { Z | y } \left[ \left( Z - \mathbb { E } _ { Z | y } Z \right) \left( \| Z \| _ { 2 } ^ { 2 } - \mathbb { E } _ { Z | y } \| Z \| _ { 2 } ^ { 2 } \right) \right] \right\| ^ { 2 } } \\ & { = \underset { u \in \mathbb { R } ^ { d } , \| u \| = 1 } { \operatorname* { m a x } } \left\| \mathbb { E } _ { Z | y } \left[ u ^ { \top } \left( Z - \mathbb { E } _ { Z | y } Z \right) \left( \| Z \| _ { 2 } ^ { 2 } - \mathbb { E } _ { Z | y } \| Z \| _ { 2 } ^ { 2 } \right) \right] \right\| ^ { 2 } } \\ & { \leq \underset { u \in \mathbb { R } ^ { d } , \| u \| = 1 } { \operatorname* { m a x } } \mathbb { E } _ { Z | y } \left| u ^ { \top } \left( Z - \mathbb { E } _ { Z | y } Z \right) \right| ^ { 2 } \mathbb { E } _ { Z | y } \left[ \left( \| Z \| _ { 2 } ^ { 2 } - \mathbb { E } _ { Z | y } \| Z \| _ { 2 } ^ { 2 } \right) ^ { 2 } \right] . } \end{array} +$$ + +Notice that according to the definition of Jacobian matrix $J _ { \tau }$ , the first factor is bounded by + +$$ +\begin{array} { r } { \mathbb { E } _ { Z | y } \left| u ^ { \top } \left( Z - \mathbb { E } _ { Z | y } Z \right) \right| ^ { 2 } = u ^ { \top } \mathbb { E } _ { Z | y } \left( Z - \mathbb { E } _ { Z | y } Z \right) \left( Z - \mathbb { E } _ { Z | y } Z \right) ^ { \top } u } \\ { = u ^ { \top } \left( I _ { d } + \tau J _ { \tau } ( X _ { \tau } ) \right) u \le \| I _ { d } + \tau J _ { \tau } ( X _ { \tau } ) \| _ { 2 } . } \end{array} +$$ + +For the second factor, we have + +$$ +\begin{array} { r l } & { \quad \displaystyle \int \mathbb E _ { Z | \frac { x } { \sqrt { 1 - \tau } } } \left( \| Z \| _ { 2 } ^ { 2 } - \mathbb E _ { Z | \frac { x } { \sqrt { 1 - \tau } } } \| Z \| _ { 2 } ^ { 2 } \right) ^ { 2 } p _ { X _ { \tau } } ( x ) \mathrm d x } \\ & { \le \displaystyle \int \left( \mathbb E _ { Z | \frac { x } { \sqrt { 1 - \tau } } } \| Z \| _ { 2 } ^ { 4 } - \left( \mathbb E _ { Z | \frac { x } { \sqrt { 1 - \tau } } } \| Z \| _ { 2 } ^ { 2 } \right) ^ { 2 } \right) p _ { X _ { \tau } } ( x ) \mathrm d x } \\ & { = \displaystyle \int \| z \| _ { 2 } ^ { 4 } p _ { Z } ( z ) \mathrm d z - \frac { 1 } { \tau ^ { 2 } } \int \left( \int \| x - \sqrt { 1 - \tau } x _ { 0 } \| _ { 2 } ^ { 2 } p _ { X _ { \tau } | X _ { \tau } } ( x _ { 0 } | x ) \mathrm d z \right) ^ { 2 } p _ { X _ { \tau } } ( x ) \mathrm d x } \\ & { \stackrel { \mathrm { ( a ) } } { \le } \displaystyle \int \| z \| _ { 2 } ^ { 4 } p _ { Z } ( z ) \mathrm d z - \left( \int \| z \| _ { 2 } ^ { 2 } p _ { Z } ( z ) \mathrm d z \right) ^ { 2 } } \\ & { = \left( d ^ { 2 } + 2 d - d ^ { 2 } \right) \lesssim d , } \end{array} +$$ + +where (a) uses the fact that $\begin{array} { r } { \mathbb { E } \left( \mathbb { E } _ { Z | \frac { x } { \sqrt { 1 - \tau } } } \| Z \| _ { 2 } ^ { 2 } \right) ^ { 2 } \ge \left( \mathbb { E } \mathbb { E } _ { Z | \frac { x } { \sqrt { 1 - \tau } } } \| Z \| _ { 2 } ^ { 2 } \right) ^ { 2 } } \end{array}$ + +In addition, for each set $A _ { \tau }$ holding with probability at least $1 - O ( 1 / d ^ { 4 } )$ , we have + +$$ +\begin{array} { r l } & { \quad \displaystyle \int _ { A _ { 2 } ^ { c } } ( \| I _ { d } + \tau J _ { \tau } ( x ) \| _ { 2 } ) \mathbb { E } _ { Z | \frac { x } { \sqrt { 1 - \tau } } } ( \| Z \| _ { 2 } ^ { 2 } - \mathbb { E } _ { Z | \frac { x } { \sqrt { 1 - \tau } } } \| Z \| _ { 2 } ^ { 2 } ) ^ { 2 } p _ { X _ { \tau } } ( x ) \mathrm { d } x } \\ & { \lesssim \displaystyle \int _ { A _ { \tau } ^ { c } } \mathbb { E } _ { Z | \frac { x } { \sqrt { 1 - \tau } } } \| Z - \mathbb { E } _ { Z | \frac { x } { \sqrt { 1 - \tau } } } Z \| _ { 2 } ^ { 2 } \mathbb { E } _ { Z | \frac { x } { \sqrt { 1 - \tau } } } ( \| Z \| _ { 2 } ^ { 2 } - \mathbb { E } _ { Z | \frac { x } { \sqrt { 1 - \tau } } } \| Z \| _ { 2 } ^ { 2 } ) ^ { 2 } p _ { X _ { \tau } } ( x ) \mathrm { d } x } \\ & { \lesssim \displaystyle \int _ { A _ { \tau } ^ { c } } \mathbb { E } _ { Z | \frac { x } { \sqrt { 1 - \tau } } } \| Z \| _ { 2 } ^ { 2 } \mathbb { E } _ { Z | \frac { x } { \sqrt { 1 - \tau } } } \| Z \| _ { 2 } ^ { 4 } p _ { X _ { \tau } } ( x ) \mathrm { d } x } \\ & { \lesssim ( \int \mathbb { E } _ { Z | \frac { x } { \sqrt { 1 - \tau } } } \| Z \| _ { 2 } ^ { 4 } \mathbb { E } _ { Z | \frac { x } { \sqrt { 1 - \tau } } } \| Z \| _ { 2 } ^ { 8 } p _ { X _ { \tau } } ( x ) \mathrm { d } x ) ^ { 1 / 2 } ( \displaystyle \int _ { A _ { \tau } ^ { c } } p _ { X _ { \tau } } ( x ) \mathrm { d } x ) ^ { 1 / 2 } } \\ & \lesssim \frac { 1 } { d ^ { 2 } } ( \mathbb { E } \| Z \| _ \end{array} +$$ + +Armed with the above observations, we decompose the expectation as + +$$ +\begin{array} { r l } & { \mathbb { E } \bigg \| \frac { \partial } { \partial \tau } \int _ { \boldsymbol { \mathcal { X } } _ { \sigma } } \langle y - x _ { 0 } \rangle p _ { X _ { \sigma } | X _ { \epsilon } } \langle x _ { 0 } | \sqrt { 1 - \tau y } y \rangle \mathrm { d } x _ { 0 } \bigg \| _ { 2 } ^ { 2 } } \\ & { \leq \frac { 1 } { \tau ( 1 - \tau ) ^ { 3 } } \mathrm { E } \Big \{ \big \| [ d _ { \boldsymbol { \mathcal { X } } } + \tau J _ { \boldsymbol { \mathcal { Y } } } ( X _ { \tau } ) ] _ { \boldsymbol { 2 } } \mathbb { E } _ { \boldsymbol { \mathcal { Z } } | \boldsymbol { \mathcal { X } } } \big ( \| \boldsymbol { \mathcal { Z } } \| _ { 2 } ^ { 2 } - \mathbb { E } _ { \boldsymbol { \mathcal { Z } } | \boldsymbol { \mathcal { Y } } } \| \boldsymbol { \mathcal { Z } } \| _ { 2 } ^ { 2 } \big ) \Big ^ { 2 } \Big \} } \\ & { = \frac { 1 } { \tau ( 1 - \tau ) ^ { 3 } } \int _ { \boldsymbol { \mathcal { X } } _ { \tau } } \| I _ { d } + \tau J _ { \boldsymbol { \mathcal { Y } } } ( x ) \| _ { 2 } \mathbb { E } _ { \boldsymbol { \mathcal { Z } } | \boldsymbol { \mathcal { X } } ^ { \frac { \epsilon } { 3 } } } \bigg ( \| \boldsymbol { \mathcal { Z } } \| _ { 2 } ^ { 2 } - \mathbb { E } _ { \boldsymbol { \mathcal { Z } } | \boldsymbol { \mathcal { Y } } _ { \overline { { \mathcal { X } } } ^ { \frac { \epsilon } { 3 } } } } \| \boldsymbol { \mathcal { Z } } \| _ { 2 } ^ { 2 } \bigg ) ^ { 2 } p _ { X _ { \tau } } \langle x \rangle \mathrm { d } x } \\ & \quad + \frac { 1 } { \tau ( 1 - \tau ) ^ { 3 } } \int _ { \boldsymbol { \mathcal { Z } } _ { \tau } } \| I _ { d } + \tau J _ { \tau } ( x ) \| _ { 2 } \mathbb { E } _ \boldsymbol { \mathcal { Z } } | \boldsymbol { \mathcal { Y } } _ { \tau } ^ \frac \ \end{array} +$$ + +where (a) uses the fact that $\mathbb { P } ( X _ { \tau } \in \mathcal { L } _ { \tau } ^ { \mathrm { c } } ) \lesssim 1 / d ^ { 4 }$ and (72), and (b) uses (71). + +In addition, recalling that for $x _ { \tau } \in S _ { \tau }$ , we have + +$$ +\begin{array} { r l } & { \displaystyle \| I _ { d } + \tau J _ { \tau } ( x _ { \tau } ) \| _ { 2 } \leq \mathbb { E } _ { Z | \frac { x } { \sqrt { 1 - \tau } } } \left\| Z - \mathbb { E } _ { Z | \frac { x } { \sqrt { 1 - \tau } } } Z \right\| _ { 2 } ^ { 2 } \leq \mathbb { E } _ { Z | \frac { x } { \sqrt { 1 - \tau } } } \| Z \| _ { 2 } ^ { 2 } } \\ & { \quad \quad \quad \quad \quad = \frac { 1 } { \tau } \displaystyle \int \| x _ { \tau } - \sqrt { 1 - \tau } x _ { 0 } \| _ { 2 } ^ { 2 } p _ { X _ { 0 } | X _ { \tau } } ( x _ { 0 } | x _ { \tau } ) \mathrm { d } x _ { 0 } \stackrel { ( \mathrm { a } ) } { \leq } 2 5 ( \theta + c _ { 0 } ) d \log T , } \end{array} +$$ + +where (a) uses Lemma 9. Thus for $d < L$ , we have + +$$ +\begin{array} { r l } & { \mathbb { E } \left\| \displaystyle \frac { \partial } { \partial \tau } \int _ { x _ { 0 } } ( y - x _ { 0 } ) p _ { X _ { 0 } | X _ { \tau } } ( x _ { 0 } | \sqrt { 1 - \tau } y ) \mathrm { d } x _ { 0 } \right\| _ { 2 } ^ { 2 } } \\ & { = \frac { 1 } { \tau ( 1 - \tau ) ^ { 3 } } \int _ { S _ { \tau } } \| I _ { d } + \tau J _ { \tau } ( X _ { \tau } ) \| _ { 2 } \mathbb { E } _ { Z | \frac { - \tau } { \sqrt { 1 - \tau } } } \left( \| Z \| _ { 2 } ^ { 2 } - \mathbb { E } _ { Z | \frac { 1 } { \sqrt { 1 - \tau } } } \| Z \| _ { 2 } ^ { 2 } \right) ^ { 2 } p _ { X _ { \tau } } ( x ) \mathrm { d } x } \\ & { \quad + \frac { 1 } { \tau ( 1 - \tau ) ^ { 3 } } \int _ { S _ { \tau } } \| I _ { d } + \tau J _ { \tau } ( X _ { \tau } ) \| _ { 2 } \mathbb { E } _ { Z | \frac { - \tau } { \sqrt { 1 - \tau } } } \left( \| Z \| _ { 2 } ^ { 2 } - \mathbb { E } _ { Z | \frac { 1 } { \sqrt { 1 - \tau } } } \| Z \| _ { 2 } ^ { 2 } \right) ^ { 2 } p _ { X _ { \tau } } ( x ) \mathrm { d } x } \\ & { \lesssim \frac { d \log T } { \tau ( 1 - \tau ) ^ { 3 } } \int _ { S _ { \tau } } \mathbb { E } _ { Z | \frac { 1 } { \sqrt { 1 - \tau } } } \left( \| Z \| _ { 2 } ^ { 2 } - \mathbb { E } _ { Z | \frac { 1 } { \sqrt { 1 - \tau } } } \| Z \| _ { 2 } ^ { 2 } \right) ^ { 2 } p _ { X _ { \tau } } ( x ) \mathrm { d } x + \frac { d } { \tau ( 1 - \tau ) ^ { 3 } } } \\ & \lesssim \frac { d ^ { 2 } \log T } { \tau ( 1 - \tau ) ^ { 3 } } + \frac { d } \tau ( 1 - \end{array} +$$ + +where (a) uses (72) and the fact the $\mathbb { P } ( X _ { \tau } \in { \mathcal { S } } _ { \tau } ^ { \mathrm { c } } ) \lesssim 1 / T ^ { 4 } \lesssim 1 / d ^ { 4 }$ for $T \gtrsim K \gtrsim \operatorname* { m i n } \{ d \log T , L \} \log T \gtrsim d$ , and (b) uses (71). Combining (73) and (74), we have + +$$ +\mathbb { E } \left\| \frac { \partial } { \partial \tau } \int _ { x _ { 0 } } ( y - x _ { 0 } ) p _ { X _ { 0 } | X _ { \tau } } ( x _ { 0 } | \sqrt { 1 - \tau } y ) \mathrm { d } x _ { 0 } \right\| _ { 2 } ^ { 2 } \leq \frac { d \operatorname* { m i n } \{ d \log T , L \} } { \tau ( 1 - \tau ) ^ { 3 } } . +$$ + +and complete the proof. + +# D.2 Proof of Lemma 4 + +We introduce a more preliminary lemma which leads to Lemma 4 immediately. + +Lemma 11. According to Lemma 3, it can be shown that for any $0 \leq n \leq N$ , + +$$ +\| y _ { \tau _ { k , n } } ( x _ { k } ) - x _ { \tau _ { k , n } } ( x _ { k } ) \| _ { 2 } ^ { 2 } \leq \zeta _ { k , n } ( x _ { k } ) + \frac { N \log ^ { 2 } T } { T ^ { 2 } } \sum _ { i = 0 } ^ { n - 1 } \widehat { \tau } _ { k , i - 1 } \widehat { \varepsilon } _ { k , i } ^ { 2 } ( x _ { k } ) , x _ { k } \in \mathcal { E } _ { k } , +$$ + +$$ +\begin{array} { r l } & { \displaystyle \int _ { \mathcal { E } _ { k , n } } \zeta _ { k , n } ( x _ { k } ) p _ { \widehat { X } _ { k } } ( x _ { k } ) \mathrm { d } x _ { k } \lesssim \frac { d \log ^ { 5 } T } { T ^ { 3 } } \operatorname* { m i n } \Big \{ \frac { N d \widehat { \tau } _ { k , - 1 } \log T } { T } } \\ & { \qquad + ( 1 - \widehat { \tau } _ { k , n } ) \displaystyle \int _ { \tau _ { k , n } } ^ { \tau _ { k , 0 } } \frac { \mathbb { E } [ \mathsf { T } \mathsf { r } ( \Sigma _ { \tau } ^ { 2 } ( x _ { \tau } ) ) ] } { ( 1 - \tau ) ^ { 2 } } \mathrm { d } \tau , \frac { N L ^ { 2 } \widehat { \tau } _ { k , - 1 } \log T } { T } \Big \} , } \end{array} +$$ + +where $\widetilde { \varepsilon } _ { k , i } ^ { 2 } ( \boldsymbol { x } _ { k } )$ is defined in Lemma $\delta$ , and + +$$ +\mathcal { E } _ { k , n } : = \left\{ \begin{array} { l l } { \{ x _ { \tau _ { k , 0 } } : x _ { \tau _ { k , i } } ( x _ { \tau _ { k , 0 } } ) \in \widetilde { \mathcal { S } } _ { \tau _ { k , i } } \cap \mathcal { L } _ { \tau _ { k , i } } , y _ { \tau _ { k , i } } ( x _ { \tau _ { k , 0 } } ) \in \mathcal { S } _ { \tau _ { k , i } } , \forall 0 \leq i \leq n - 1 \} , } & { \mathrm { i f ~ } L > d \log T , } \\ { \emptyset } & { \mathrm { i f ~ } L \leq d \log T . } \end{array} \right. +$$ + +According to the definition of $\varepsilon _ { k , i } ^ { 2 }$ in (12), we have + +$$ +\begin{array} { r l } & { \mathbb { E } _ { \boldsymbol { x } _ { \tau _ { k , 0 } } \sim p _ { \tilde { X } _ { k } } } \left\| y _ { \tau _ { k , N } } - x _ { \tau _ { k , N } } \right\| _ { 2 } ^ { 2 } } \\ & { \leq \int _ { \mathcal { E } _ { k } } \zeta _ { k , N } ( x _ { k } ) p _ { \tilde { X } _ { k } } ( x _ { k } ) \mathrm { d } x _ { k } + \frac { N \log ^ { 2 } T } { T ^ { 2 } } \displaystyle \sum _ { i = 0 } ^ { N - 1 } \hat { \tau } _ { k , i - 1 } \varepsilon _ { k , i } ^ { 2 } ( x _ { k } ) } \\ & { \lesssim \frac { d \log ^ { 5 } T } { T ^ { 3 } } \operatorname* { m i n } \Big \{ \frac { N d \hat { \tau } _ { k , - 1 } \log T } { T } + ( 1 - \hat { \tau } _ { k , N } ) \int _ { \tau _ { k , n } } ^ { \tau _ { k , 0 } } \frac { \mathbb { E } \left[ \mathsf { T r } ( \boldsymbol { \Sigma } _ { \tau } ^ { 2 } ( \boldsymbol { x } _ { \tau } ) ) \right] } { ( 1 - \tau ) ^ { 2 } } \mathrm { d } \tau , } \\ & { \quad \quad \quad \quad \frac { N L ^ { 2 } \hat { \tau } _ { k , - 1 } \log T } { T } \Big \} + \frac { \hat { \tau } _ { k , - 1 } N \log ^ { 2 } T } { T ^ { 2 } } \displaystyle \sum _ { i = 0 } ^ { N - 1 } \varepsilon _ { k , i } ^ { 2 } . } \end{array} +$$ + +because $\mathcal { E } _ { k } \subset \mathcal { E } _ { k , n }$ , $p _ { \widetilde { X } _ { k } } ( x _ { k } ) \le p _ { \widehat { X } _ { k } } ( x _ { k } )$ ( $x \neq \infty$ ), and + +$$ +\int _ { \mathcal { E } _ { k } } \zeta _ { k , n } ( x _ { k } ) p _ { \widetilde { X } _ { k } } ( x _ { k } ) \mathrm { d } x _ { k } \leq \int _ { \mathcal { E } _ { k , n } } \zeta _ { k , n } ( x _ { k } ) p _ { \widehat { X } _ { k } } ( x _ { k } ) \mathrm { d } x _ { k } . +$$ + +According to (33), we have that + +$$ +\begin{array} { l } { \displaystyle { \mathsf { K L } } \big ( p _ { \widehat { X } _ { k + 1 } | \widehat { X } _ { k } } \big ( \cdot | x _ { \tau _ { k , 0 } } \big ) \| p _ { Y _ { k + 1 } | Y _ { k } } \big ( \cdot | x _ { \tau _ { k , 0 } } \big ) \big ) = \frac { 1 - \tau _ { k + 1 , 0 } } { 2 \big ( \tau _ { k + 1 , 0 } - \tau _ { k , N } \big ) } \| y _ { \tau _ { k , N } } - x _ { \tau _ { k , N } } \| _ { 2 } ^ { 2 } } \\ { \displaystyle \lesssim \frac { T } { N \widehat { \tau } _ { k , N } \log T } \| y _ { \tau _ { k , N } } - x _ { \tau _ { k , N } } \| _ { 2 } ^ { 2 } . } \end{array} +$$ + +Inserting (77) into (78), we have + +$$ +\begin{array} { r l } & { \mathbb { E } _ { x _ { \tau _ { k , 0 } } \sim p _ { \tilde { X } _ { k } } } [ \mathrm { K L } ( p _ { \tilde { X } + 1 | \tilde { X } _ { k } } ( \cdot \vert x _ { \tau _ { k , 0 } } ) \| p _ { Y _ { k + 1 } | Y _ { k } } ( \cdot \vert x _ { \tau _ { k , 0 } } ) ) ] } \\ & { \lesssim \frac { T } { N \tilde { \tau } _ { k , \chi } \log ^ { 4 } T } \mathbb { E } _ { x _ { \tau _ { k , 0 } } \sim p _ { \tilde { X } _ { k } } } \| y _ { \tau _ { k , 0 } } - x _ { \tau _ { k , \chi } } \| _ { 2 } ^ { 2 } } \\ & { \stackrel { \mathrm { ( a ) } } { \lesssim } \frac { d K \log ^ { 4 } T } { T ^ { 3 } } \operatorname* { m i n } \{ \frac { N d \log T } { T } + \frac { 1 - \tilde { \tau } _ { k , N } } { \tilde { \tau } _ { k , N } } \int _ { \tau _ { k , N } } ^ { \tau _ { k , 0 } } \frac { \mathbb { E } [ \top ( \sum _ { \tau } ^ { 2 } ( x _ { \tau } ) ) ] } { ( 1 - \tau ) ^ { 2 } } \mathrm { d } \tau , \frac { N L ^ { 2 } \log T } { T } \} } \\ & { \quad + \frac { K N \log T } { T ^ { 2 } } \displaystyle \sum _ { i = 0 } ^ { N - 1 } \varepsilon _ { k , i } ^ { 2 } } \\ & \stackrel { \mathrm { ( b ) } } { \lesssim } \frac { d \log ^ { 4 } T } { T ^ { 3 } } \operatorname* { m i n } \{ d \log T + \frac { K ( 1 - \widehat { \tau } _ { k , N } ) } { \widehat { \tau } _ { k , N } } \int _ { \tau _ { k , N } } ^ { \tau _ { k , 0 } } \frac { \mathbb { E } [ \top ( \sum _ { \tau } ^ { 2 } ( x _ { \tau } ) ) ] } { ( 1 - \tau ) ^ { 2 } } \mathrm { d } \end{array} +$$ + +where (a) uses (51), and (b) uses the fact that $K N \lesssim T$ . Notice that for any $0 < \tau _ { 1 } < \tau _ { 2 } < 1$ , + +$$ +\int _ { \tau _ { 1 } } ^ { \tau _ { 2 } } \frac { \mathbb { E } [ \mathsf { T r } ( \Sigma _ { \tau } ^ { 2 } ( x _ { \tau } ) ) ] } { ( 1 - \tau ) ^ { 2 } } \mathrm { d } \tau = \frac { \tau _ { 2 } } { 1 - \tau _ { 2 } } \mathbb { E } [ \mathsf { T r } ( \Sigma _ { \tau _ { 2 } } ( x _ { \tau _ { 2 } } ) ) ] - \frac { \tau _ { 1 } } { 1 - \tau _ { 1 } } \mathbb { E } [ \mathsf { T r } ( \Sigma _ { \tau _ { 1 } } ( x _ { \tau _ { 1 } } ) ) ] , +$$ + +which has been proved in Li et al. (2024b) (cf. (90)). We have + +$$ +\begin{array} { r l } & { \quad \frac { 1 - \widehat \tau _ { k , N } } { \widehat \tau _ { k , N } } \displaystyle \int _ { \tau _ { k , N } } ^ { \tau _ { k , 0 } } \frac { \mathbb { E } [ \mathsf { T r } ( \Sigma _ { \tau } ^ { 2 } ( x _ { \tau } ) ) ] } { ( 1 - \tau ) ^ { 2 } } \mathrm { d } \tau } \\ & { = \frac { \tau _ { k , 0 } ( 1 - \widehat \tau _ { k , N } ) } { \widehat \tau _ { k , N } ( 1 - \tau _ { k , 0 } ) } \mathbb { E } [ \mathsf { T r } ( \Sigma _ { \tau _ { k , 0 } } ( x _ { \tau _ { k , 0 } } ) ) ] - \frac { \tau _ { k , N } ( 1 - \widehat \tau _ { k , N } ) } { \widehat \tau _ { k , N } ( 1 - \tau _ { k , N } ) } \mathbb { E } [ \mathsf { T r } ( \Sigma _ { \tau _ { k , N } } ( x _ { \tau _ { k , N } } ) ) ] . } \end{array} +$$ + +Furthermore, we have + +$$ +\begin{array} { r l } & \begin{array} { r l } & { \displaystyle \sum _ { k = 0 } ^ { K - 1 } \frac { 1 - \widehat \gamma _ { k , N } } { \widehat \gamma _ { k , N } } \displaystyle \sum _ { f _ { \ell , N } , \ell = 0 , j = 1 \atop N } ^ { \ell - 1 } \frac { \widehat \gamma _ { k , N } ^ { \ell - 2 } } { ( 1 - \widehat \gamma _ { k , N } ^ { \ell } ) ^ { 2 } } \mathrm { d } f \tau } \\ & { \leq \displaystyle \sum _ { k = 0 } ^ { K - 1 } \frac { \widehat \gamma _ { k , N - 1 } } { \widehat \gamma _ { k , N } + 2 \gamma _ { k , N } } \displaystyle \sum _ { f _ { \ell , N } ^ { \ell } \geq 2 , \ell = 0 , \ell = 0 } ^ { K } \mathbb { E } \Big [ \tau ( \Sigma _ { \tau _ { k - 1 , \ell } , 0 , \ell = 0 } ^ { \ell - 1 } ( x _ { \tau _ { k + 1 , \ell } , 0 } ) ) \Big ] } \\ & { \leq \displaystyle \sum _ { k = 0 } ^ { K - 1 } \frac { \widehat \gamma _ { k , N - 1 } } { \widehat \gamma _ { k , N } + 2 \gamma _ { k , N } ( 1 - \widehat \gamma _ { k , N - 2 , 0 } ) } \mathbb { E } \Big [ \tau ( \Sigma _ { \tau _ { k - 1 , \ell } , 0 , \ell = 0 } ^ { \ell - 1 } ( x _ { \tau _ { k + 1 , \ell } , 0 } ) ) \Big ] } \\ & \quad - \displaystyle \sum _ { k = 0 } ^ { K - 1 } \frac { \widehat \gamma _ { k , N - 1 } } { \widehat \gamma _ { k , N } ( 1 - \widehat \gamma _ { k , N } ) } \displaystyle \sum _ { | \tau | \in \mathcal { T } _ { \tau , N } } \Big [ \tau ( \Sigma _ { \tau _ { k - 1 , N } , \ell = 0 } ^ { \ell - 1 } ) \Big ] + \displaystyle \frac { \widehat \gamma _ { k , N } } { \widehat \gamma _ { k , N } ( 1 - \widehat \gamma _ { k , N } ) } \displaystyle \sum _ { | \tau | \in \mathcal { T } _ { \tau , N } } \Big [ \tau _ { \tau } ( \Sigma _ \tau _ \end{array} \end{array} +$$ + +where the last inequality uses the fact that $\tau _ { k + 2 , 0 } = \tau _ { k , N }$ . Notice that + +$$ +\mathbb { E } [ \Sigma _ { \tau } ( x _ { \tau } ) ] = \mathbb { E } [ \mathsf { C o v } ( Z \mid \sqrt { 1 - \tau } X _ { 0 } + \sqrt { \tau } Z = x _ { \tau } ) ] \preceq \mathsf { C o v } [ Z ] = I _ { d } , +$$ + +and by using (51), we have + +$$ +\begin{array} { r } { \frac { \tau _ { 0 , 0 } ( 1 - \widehat \tau _ { 0 , N } ) } { \widehat \tau _ { 0 , N } ( 1 - \tau _ { 0 , 0 } ) } \mathbb { E } [ \mathsf { T r } ( \Sigma _ { \tau _ { 0 , 0 } } ( x _ { \tau _ { 0 , 0 } } ) ) ] \lesssim d , \quad \frac { \tau _ { 1 , 0 } ( 1 - \widehat \tau _ { 1 , N } ) } { \widehat \tau _ { 1 , N } ( 1 - \tau _ { 1 , 0 } ) } \mathbb { E } [ \mathsf { T r } ( \Sigma _ { \tau _ { 1 , 0 } } ( x _ { \tau _ { 1 , 0 } } ) ) ] \lesssim d . } \end{array} +$$ + +Moreover, we have + +$$ +\begin{array} { r l } & { \frac { \tau _ { k + 2 , 0 } \left( 1 - \widehat { \tau } _ { k + 2 , N } \right) } { \widehat { \tau } _ { k + 2 , N } \left( 1 - \tau _ { k + 2 , 0 } \right) } - \frac { \tau _ { k , N } \left( 1 - \widehat { \tau } _ { k , N } \right) } { \widehat { \tau } _ { k , N } \left( 1 - \tau _ { k , N } \right) } } \\ & { = \frac { \tau _ { k + 2 , 0 } \left( 1 - \widehat { \tau } _ { k + 2 , N } \right) } { \widehat { \tau } _ { k + 2 , N } \left( 1 - \tau _ { k + 2 , 0 } \right) } - \frac { \tau _ { k + 2 , 0 } \left( 1 - \widehat { \tau } _ { k + 2 , 0 } \right) } { \widehat { \tau } _ { k + 2 , 0 } \left( 1 - \tau _ { k + 2 , 0 } \right) } } \\ & { = \frac { \tau _ { k + 2 , 0 } \left( \widehat { \tau } _ { k + 2 , 0 } - \widehat { \tau } _ { k + 2 , N } \right) } { \widehat { \tau } _ { k + 2 , 0 } \widehat { \tau } _ { k + 2 , N } \left( 1 - \tau _ { k + 2 , 0 } \right) } \lesssim \frac { N \tau _ { k + 2 , 0 } \widehat { \tau } _ { k + 2 , 0 } \left( 1 - \widehat { \tau } _ { k + 2 , N } \right) \log { T } } { T \widehat { \tau } _ { k + 2 , 0 } \widehat { \tau } _ { k + 2 , N } \left( 1 - \tau _ { k + 2 , 0 } \right) } } \\ & { \lesssim \frac { \log { T } } { K } . } \end{array} +$$ + +Inserting into (80), we have + +$$ +\begin{array} { r l } & { \displaystyle \sum _ { k = 0 } ^ { K - 1 } \frac { 1 - \widehat \tau _ { k , N } } { \widehat \tau _ { k , N } } \int _ { \tau _ { k , N } } ^ { \tau _ { k , 0 } } \frac { \mathbb { E } [ \mathsf { T r } ( \Sigma _ { \tau } ^ { 2 } ( x _ { \tau } ) ) ] } { ( 1 - \tau ) ^ { 2 } } \mathrm { d } \tau \lesssim \displaystyle \sum _ { k = 0 } ^ { K - 3 } \frac { \log T } { K } \mathbb { E } [ \mathsf { T r } ( \Sigma _ { \tau _ { k + 2 , 0 } } ( x _ { \tau _ { k + 2 , 0 } } ) ) ] + d } \\ & { \qquad \lesssim d \log T . } \end{array} +$$ + +Inserting (81) into (79), we have + +$$ +\sum _ { k = 0 } ^ { K - 1 } \mathbb { E } _ { \boldsymbol { x } _ { \tau _ { k , 0 } } \sim p _ { \widetilde { X } _ { k } } } \left[ { \mathsf { K L } } \left( p _ { \widehat { X } _ { k + 1 } | \widehat { X } _ { k } } \left( \cdot | \boldsymbol { x } _ { \tau _ { k , 0 } } \right) \parallel p _ { Y _ { k + 1 } | Y _ { k } } \left( \cdot | \boldsymbol { x } _ { \tau _ { k , 0 } } \right) \right) \right] +$$ + +$$ +\begin{array} { r l } & { \lesssim \frac { K d \log ^ { 4 } T } { T ^ { 3 } } \operatorname* { m i n } \left\{ d \log T + d \log T , L ^ { 2 } \log T \right\} + \frac { \log T } { T } \underset { k = 0 } { \sum } \underset { i = 0 } { \sum } \varepsilon _ { k , i } ^ { - 1 } } \\ & { \lesssim \frac { K d \log ^ { 5 } T } { T ^ { 3 } } \operatorname* { m i n } \left\{ d , L ^ { 2 } \right\} + \varepsilon _ { \mathrm { s c o r e } } ^ { 2 } \log T , } \end{array} +$$ + +and we complete the proof of Lemma 4. + +The remaining of this section shall use the following lemma to prove Lemma 11. Its proof is postponed to the end of this section. + +Lemma 12. For any $x \in S _ { \tau } \cap \mathcal { L } _ { \tau }$ , $y \in S _ { \tau }$ , and any $0 < \tau < 1$ , we have + +$$ +\| s _ { \tau } ^ { \star } ( x ) - s _ { \tau } ^ { \star } ( y ) \| _ { 2 } \leq \frac { C \operatorname* { m i n } \{ d \log T , L \} } { \tau } \| x - y \| _ { 2 } , +$$ + +where $C$ is a sufficiently large constant dependent on $\theta + c _ { 0 }$ . + +Recalling the definition of $\xi _ { k , n }$ , we have + +$$ +\frac { \left\| y _ { \tau _ { k , n } } - x _ { \tau _ { k , n } } \right\| _ { 2 } } { \sqrt { 1 - \tau _ { k , n } } } \leq \frac { \left\| y _ { \tau _ { k , n } } ^ { \star } - z _ { \tau _ { k , n } } ^ { \star } \right\| _ { 2 } } { \sqrt { 1 - \tau _ { k , n } } } + \left\| \xi _ { k , n } \right\| _ { 2 } . +$$ + +According to the definitions of $y _ { \tau _ { k , n } } ^ { \star }$ and $z _ { \tau _ { k , n } } ^ { \star }$ , for $x _ { \tau _ { k , i } } , y _ { \tau _ { k , i } } \in S _ { \tau _ { k , i } }$ , we have + +$$ +\begin{array} { r l } { \frac { | | \hat { \mathbf { g } } _ { \star , - } ^ { \star } , - \hat { \mathbf { g } } _ { \star , + } ^ { \star } | } { \sqrt { 1 - \hbar } } | _ { \mathbf { g } } } & { \leq \frac { - 1 - \hbar } { 2 } | \frac { | \hat { \mathbf { g } } _ { \star , + } ^ { \star } , \hat { \mathbf { g } } _ { \star , - } ^ { \star } , - \hat { \mathbf { g } } _ { \star , - } ^ { \star } | } { 2 } \langle \hat { \mathbf { g } } _ { \star , + - } , \hat { \mathbf { g } } _ { \star , - } ^ { \star } \rangle | _ { \mathbf { g } } } \\ & { \leq \frac { 1 - \hbar } { 2 } | \frac { 1 } { \sqrt { 1 - \hbar } } \frac { | \hat { \mathbf { g } } _ { \star , - } ^ { \star } , \hat { \mathbf { g } } _ { \star , - } ^ { \star } | } { 2 } \frac { | \hat { \mathbf { g } } _ { \star , - } ^ { \star } , \hat { \mathbf { g } } _ { \star , - } ^ { \star } , - \hat { \mathbf { g } } _ { \star , - } ^ { \star } | } { 2 } \frac { | \hat { \mathbf { g } } _ { \star , - - } ^ { \star } , \hat { \mathbf { g } } _ { \star , - } ^ { \star } , - \hat { \mathbf { g } } _ { \star , - } ^ { \star } | } { 2 } } \\ & \mathrm { \quad ~ \times ~ } | \hat { \mathbf { g } } _ { \star , - } ^ { \star } , \frac { | | \hat { \mathbf { g } } _ { \star , - } ^ { \star } , \hat { \mathbf { g } } _ { \star , - } ^ { \star } | } { 2 } \frac { \sqrt { 1 - \hbar } } { 2 } \mathrm { e } ^ { \mathrm { i } | \hat { \mathbf { g } } _ { \star , - } ^ { \star } | } \frac { \sqrt { 1 - \hbar } } { 2 } \mathrm { e } ^ \end{array} +$$ + +where (a) uses (50) and (51). Inserting into (83), we have + +$$ +\frac { \left\| y _ { \tau _ { k , n } } - x _ { \tau _ { k , n } } \right\| _ { 2 } } { \sqrt { 1 - \tau _ { k , n } } } \lesssim \frac { \operatorname* { m i n } \{ d \log T , L \} \log T } { T } \sum _ { i = 1 } ^ { n - 1 } \frac { \left\| y _ { \tau _ { k , i } } - x _ { \tau _ { k , i } } \right\| _ { 2 } } { \sqrt { 1 - \tau _ { k , i } } } + \left\| \xi _ { k , n } ( x _ { \tau _ { k , 0 } } ) \right\| _ { 2 } . +$$ + +By applying the above relation recursively, for $T \gtrsim \mathrm { m i n } \{ d \log T , L \} N \log T$ , we have + +$$ +\frac { \left\| y _ { \tau _ { k , n } } - x _ { \tau _ { k , n } } \right\| _ { 2 } } { \sqrt { 1 - \tau _ { k , n } } } \lesssim \frac { \operatorname* { m i n } \{ d \log T , L \} \log T } { T } \sum _ { i = 1 } ^ { n - 1 } \left\| \xi _ { k , i } ( x _ { \tau _ { k , 0 } } ) \right\| _ { 2 } + \left\| \xi _ { k , n } ( x _ { \tau _ { k , 0 } } ) \right\| _ { 2 } . +$$ + +Thus + +$$ +\frac { \left\| y _ { \tau _ { k , n } } - x _ { \tau _ { k , n } } \right\| _ { 2 } ^ { 2 } } { 1 - \tau _ { k , n } } \lesssim \frac { N \operatorname* { m i n } \{ d \log T , L \} ^ { 2 } \log ^ { 2 } T } { T ^ { 2 } } \sum _ { i = 1 } ^ { n - 1 } \left\| \xi _ { k , i } ( x _ { \tau _ { k , 0 } } ) \right\| _ { 2 } ^ { 2 } + \left\| \xi _ { k , n } ( x _ { \tau _ { k , 0 } } ) \right\| _ { 2 } ^ { 2 } . +$$ + +Define + +$$ +\frac { \zeta _ { k , n } } { 1 - \tau _ { k , n } } = \frac { N \operatorname* { m i n } \{ d \log T , L \} ^ { 2 } \log ^ { 2 } T } { T ^ { 2 } } \sum _ { i = 1 } ^ { n - 1 } \frac { \| x _ { \tau _ { k , i } } - z _ { \tau _ { k , i } } ^ { \star } \| _ { 2 } ^ { 2 } } { 1 - \tau _ { k , i } } + \frac { \| x _ { \tau _ { k , n } } - z _ { \tau _ { k , n } } ^ { \star } \| _ { 2 } ^ { 2 } } { 1 - \tau _ { k , n } } . +$$ + +We have + +$$ +\begin{array} { r l } { { \frac { \big \| y _ { \tau _ { k , n } } - x _ { \tau _ { k , n } } \big \| _ { 2 } ^ { 2 } } { 1 - \tau _ { k , n } } \lesssim \frac { \zeta _ { k , n } } { 1 - \tau _ { k , n } } + \frac { N \operatorname* { m i n } \{ d \log T , L \} ^ { 2 } \log ^ { 2 } T } { T ^ { 2 } } \sum _ { i = 1 } ^ { n - 1 } \frac { \| y _ { \tau _ { k , i } } ^ { \star } - y _ { \tau _ { k , i } } \| _ { 2 } ^ { 2 } } { 1 - \tau _ { k , i } } } \quad } & { } \\ & { \quad \quad + \frac { \| y _ { \tau _ { k , n } } ^ { \star } - y _ { \tau _ { k , n } } \| _ { 2 } ^ { 2 } } { 1 - \tau _ { k , n } } . } \end{array} +$$ + +According to Lemma 8, we have + +$$ +\frac { \lVert y _ { \tau _ { k , n } } ^ { \star } - y _ { \tau _ { k , n } } \rVert _ { 2 } ^ { 2 } } { 1 - \tau _ { k , n } } \lesssim \frac { N \log ^ { 2 } T } { T ^ { 2 } } \sum _ { i = 0 } ^ { n - 1 } \frac { \tau _ { k , i } \widetilde { \varepsilon } _ { k , i } ^ { 2 } } { 1 - \tau _ { k , i } } . +$$ + +Considering that $N ^ { 2 } \operatorname* { m i n } \{ d \log T , L \} ^ { 2 } \log ^ { 2 } T / T ^ { 2 } \lesssim 1$ , we have + +$$ +\frac { N \operatorname* { m i n } \{ d \log T , L \} ^ { 2 } \log ^ { 2 } T } { T ^ { 2 } } \sum _ { i = 1 } ^ { n } \frac { \| y _ { \tau _ { k , i } } ^ { \star } - y _ { \tau _ { k , i } } \| _ { 2 } ^ { 2 } } { 1 - \tau _ { k , i } } + \frac { \| y _ { \tau _ { k , n } } ^ { \star } - y _ { \tau _ { k , n } } \| _ { 2 } ^ { 2 } } { 1 - \tau _ { k , n } } \lesssim \frac { N \log ^ { 2 } T } { T ^ { 2 } } \sum _ { i = 0 } ^ { n - 1 } \frac { \tau _ { k , i } \tilde { \varepsilon } _ { k , i } ^ { 2 } } { 1 - \tau _ { k , i } } . +$$ + +Thus we have + +$$ +\frac { \| y _ { \tau _ { k , n } } ( x _ { k } ) - x _ { \tau _ { k , n } } ( x _ { k } ) \| _ { 2 } ^ { 2 } } { 1 - \tau _ { k , n } } \leq \frac { \zeta _ { k , n } ( x _ { k } ) } { 1 - \tau _ { k , n } } + \frac { N \log ^ { 2 } T } { T ^ { 2 } } \sum _ { i = 1 } ^ { n } \frac { \tau _ { k , i } \widetilde \varepsilon _ { k , i } ^ { 2 } ( x _ { k } ) } { 1 - \tau _ { k , i } } , +$$ + +which establishes (75). Furthermore, we have + +$$ +\begin{array} { r l } & { \displaystyle \int _ { \mathcal { E } _ { k , n } } \frac { \zeta _ { k , n } ( x _ { k } ) } { 1 - \tau _ { k , n } } p _ { \widehat { X } _ { k } } ( x _ { k } ) \mathrm { d } x _ { k } } \\ & { \lesssim \displaystyle \int _ { \mathcal { E } _ { k , n } } \frac { N \operatorname* { m i n } \{ d \log T , L \} ^ { 2 } \log ^ { 2 } T } { T ^ { 2 } } \displaystyle \sum _ { i = 1 } ^ { n - 1 } \frac { \| x _ { \tau _ { k , i } } ( x _ { k } ) - z _ { \tau _ { k , i } } ^ { \star } ( x _ { k } ) \| _ { 2 } ^ { 2 } } { 1 - \tau _ { k , i } } p _ { \widehat { X } _ { k } } ( x _ { k } ) \mathrm { d } x _ { k } } \\ & { \quad + \displaystyle \int _ { \mathcal { E } _ { k , n } } \frac { \| x _ { \tau _ { k , n } } ( x _ { k } ) - z _ { \tau _ { k , n } } ^ { \star } ( x _ { k } ) \| _ { 2 } ^ { 2 } } { 1 - \tau _ { k , n } } p _ { \widehat { X } _ { k } } ( x _ { k } ) \mathrm { d } x _ { k } . } \end{array} +$$ + +Recalling (53) in Lemma 8 and the fact that $N ^ { 2 } \operatorname* { m i n } \{ d \log T , L \} ^ { 2 } \log ^ { 2 } T / T ^ { 2 } \stackrel { < } { \sim } 1$ , we have + +$$ +\begin{array} { r l } & { \int _ { \varepsilon _ { k , n } } \frac { \zeta _ { k , n } \left( x _ { k } \right) } { 1 - \tau _ { k , n } } p _ { \widehat { X } _ { k } } ( x _ { k } ) \mathrm { d } x _ { k } } \\ & { \lesssim \frac { d \log ^ { 5 } T } { T ^ { 3 } } \operatorname* { m i n } \Big \{ \frac { N d \widehat { \tau } _ { k , - 1 } \log T } { T ( 1 - \widehat { \tau } _ { k , - 1 } ) } + \int _ { \tau _ { k , n } } ^ { \tau _ { k , 0 } } \frac { \mathbb { E } \left[ \mathsf { T } \mathsf { r } \left( \Sigma _ { \tau } ^ { 2 } ( x _ { \tau } ) \right) \right] } { ( 1 - \tau ) ^ { 2 } } \mathrm { d } \tau , \frac { N L ^ { 2 } \widehat { \tau } _ { k , - 1 } \log T } { T ( 1 - \widehat { \tau } _ { k , - 1 } ) } \Big \} , } \end{array} +$$ + +and thus + +$$ +\int _ { \mathscr { E } _ { k , n } } \zeta _ { k , n } ( x _ { k } ) p _ { \widehat { X } _ { k } } ( x _ { k } ) \mathrm { d } x _ { k } +$$ + +$$ +\lesssim \frac { d \log ^ { 5 } T } { T ^ { 3 } } \operatorname* { m i n } \Big \{ \frac { N d \widehat { \tau } _ { k , - 1 } \log T } { T } + ( 1 - \widehat { \tau } _ { k , n } ) \int _ { \tau _ { k , n } } ^ { \tau _ { k , 0 } } \frac { \mathbb { E } \big [ \mathsf { T r } \big ( \Sigma _ { \tau } ^ { 2 } ( x _ { \tau } ) \big ) \big ] } { ( 1 - \tau ) ^ { 2 } } \mathrm { d } \tau , \frac { N L ^ { 2 } \widehat { \tau } _ { k , - 1 } \log T } { T } \Big \} +$$ + +and establish (76), where we use (51). + +Proof of Lemma 12. Before diving into the proof details, we first present the following lemma. Its proof is postponed to Appendix F.3. + +Lemma 13. For any $x , y$ satisfying + +$$ +\| x - y \| _ { 2 } \leq c \sqrt { \frac { \tau } { d \log T } } +$$ + +with $\begin{array} { r } { c \leq \sqrt { \frac { 1 } { \theta + c _ { 0 } } } / 1 2 . } \end{array}$ , if $x \in S _ { \tau }$ , we have + +$$ +p _ { X _ { \tau } } ( x ) - p _ { X _ { \tau } } ( y ) \leq 6 \| y - x \| _ { 2 } \sqrt { \frac { ( \theta + c _ { 0 } ) d \log T } { \tau } } p _ { X _ { \tau } } ( x ) . +$$ + +Moreover, if $x , y \in S _ { \tau }$ , then we have + +$$ +\frac { 1 } { 2 } \leq 1 - 6 \left\| x - y \right\| _ { 2 } \sqrt { \frac { ( \theta + c _ { 0 } ) d \log T } { \tau } } \leq \frac { p _ { X _ { \tau } } ( x ) } { p _ { X _ { \tau } } ( y ) } \leq \frac { 1 } { 1 - 6 \left\| x - y \right\| _ { 2 } \sqrt { \frac { ( \theta + c _ { 0 } ) d \log T } { \tau } } } \leq 2 . +$$ + +We first prove that for $x \in S _ { \tau }$ and $y \in S _ { \tau }$ , we have + +$$ +\| s _ { \tau } ^ { \star } ( x ) - s _ { \tau } ^ { \star } ( y ) \| _ { 2 } \lesssim \frac { d \log T } { \tau } \| x - y \| . +$$ + +To this end, without loss of generality, we assume that $p _ { X _ { \tau } } ( y ) \geq p _ { X _ { \tau } } ( x )$ . Then according to the definition of score function, we have + +$$ +\begin{array} { l } { { s _ { \tau } ^ { \star } ( y ) - s _ { \tau } ^ { \star } ( x ) = \displaystyle \frac { 1 } { \tau } ( x - y ) - \frac { 1 } { \tau } \int _ { x _ { 0 } } ( x - \sqrt { 1 - \tau } x _ { 0 } ) p _ { X _ { 0 } | X _ { \tau } } ( x _ { 0 } | y ) } } \\ { { \phantom { s _ { \tau } ^ { \star } ( y ) - s _ { \tau } ^ { \star } ( x ) = } + ( x - \sqrt { 1 - \tau } x _ { 0 } ) p _ { X _ { 0 } | X _ { \tau } } ( x _ { 0 } | x ) \mathrm { d } x _ { 0 } . } } \end{array} +$$ + +Thus we have + +$$ +\begin{array} { l } { \displaystyle \left\| s _ { \tau } ^ { \star } ( y ) - s _ { \tau } ^ { \star } ( x ) \right\| _ { 2 } } \\ { \displaystyle \leq \frac { 1 } { \tau } \left\| x - y \right\| _ { 2 } + \frac { 1 } { \tau } \left\| \int _ { x _ { 0 } } ( x - \sqrt { 1 - \tau } x _ { 0 } ) \frac { p _ { X _ { \tau } | X _ { 0 } } ( y | x _ { 0 } ) - p _ { X _ { \tau } | X _ { 0 } } ( x | x _ { 0 } ) } { p _ { X _ { \tau } } ( y ) } p _ { X _ { 0 } } ( x _ { 0 } ) \mathrm { d } x _ { 0 } \right\| _ { 2 } } \\ { \displaystyle \phantom { \frac { 1 } { \tau } } + \frac { 1 } { \tau } \left\| \left( \frac { p _ { X _ { \tau } } ( x ) } { p _ { X _ { \tau } } ( y ) } - 1 \right) \int _ { x _ { 0 } } ( x - \sqrt { 1 - \tau } x _ { 0 } ) p _ { X _ { 0 } | X _ { \tau } } ( x _ { 0 } | x ) \mathrm { d } x _ { 0 } \right\| _ { 2 } . } \end{array} +$$ + +For the second and the third term, we define $\mathcal { R } = \left. x _ { 0 } \in \mathbb { R } ^ { d } : \left\| x - \sqrt { 1 - \tau } x _ { 0 } \right\| _ { 2 } \leq \left\| y - \sqrt { 1 - \tau } x _ { 0 } \right\| _ { 2 } \right.$ , and have + +$$ +\begin{array} { r l } & { \quad \left\| \displaystyle \int _ { x _ { 0 } } ( x - \sqrt { 1 - \tau } x _ { 0 } ) \frac { p \boldsymbol { x } _ { \tau } | \boldsymbol { x } _ { 0 } } { p \boldsymbol { x } _ { \tau } ( y ) } \boldsymbol { - } p \boldsymbol { x } _ { \tau } | \boldsymbol { x } _ { 0 } ( x | x _ { 0 } ) \right\} p \boldsymbol { x } _ { 0 } ( x _ { 0 } ) \mathrm { d } x _ { 0 } | _ { 2 } \\ & { \leq \left\| \displaystyle \int _ { \mathbb { R } ^ { c } } ( x - \sqrt { 1 - \tau } x _ { 0 } ) \frac { p \boldsymbol { x } _ { \tau } | \boldsymbol { x } _ { 0 } } { p \boldsymbol { x } _ { \tau } ( y ) } p _ { X _ { 0 } } ( x _ { 0 } ) \left( \exp \left( \frac { ( x - y ) ^ { \top } ( x + y - 2 \sqrt { 1 - \tau } x _ { 0 } } { 2 \tau } \right) - 1 \right) \mathrm { d } x _ { 0 } \right\| _ { 2 } } \\ & { \quad + \left\| \displaystyle \int _ { \mathbb { R } ^ { c } } ( x - \sqrt { 1 - \tau } x _ { 0 } ) \frac { p _ { X _ { \tau } | X _ { 0 } } ( y | x _ { 0 } ) } { p \boldsymbol { x } _ { \tau } ( y ) } p _ { X _ { 0 } } ( x _ { 0 } ) \left( 1 - \exp \left( \frac { ( y - x ) ^ { \top } ( x + y - 2 \sqrt { 1 - \tau } x _ { 0 } } { 2 \tau } \right) \right) \mathrm { d } x _ { 0 } \right\| _ { 2 } . } \end{array} +$$ + +The first term is smaller than + +$$ +\int _ { \mathcal { R } } | | x - \sqrt { 1 - \tau } x _ { 0 } | | _ { 2 } \frac { \| x - y \| \| x + y - 2 \sqrt { 1 - \tau } x _ { 0 } \| _ { 2 } } { 2 \tau } p _ { X _ { 0 } | X _ { \tau } } ( x _ { 0 } | x ) \mathrm { d } x _ { 0 } \frac { p _ { X _ { \tau } } ( x ) } { p _ { X _ { \tau } } ( y ) } , +$$ + +and the second term is smaller than + +$$ +\begin{array} { l l } { \displaystyle \int _ { \mathbb { R } ^ { c } } \| y - \sqrt { 1 - \tau } x _ { 0 } \| _ { 2 } \frac { \| y - x \| _ { 2 } \| x + y - 2 \sqrt { 1 - \tau } x _ { 0 } \| _ { 2 } } { 2 \tau } p _ { X _ { 0 } | X _ { \tau } } ( x _ { 0 } | y ) \mathrm { d } x _ { 0 } } \\ { \displaystyle + \left\| ( x - y ) \left( 1 - \frac { p _ { X _ { \tau } } ( x ) } { p _ { X _ { \tau } } ( y ) } \right) \right\| _ { 2 } . } \end{array} +$$ + +Combining these, we have + +$$ +\begin{array} { r l } & { \quad \left\| \int _ { \mathbf { x } _ { 0 } } ( x - \sqrt { 1 - \tau } x _ { 0 } ) \frac { y _ { X _ { \tau } } | x _ { 0 } ( \hat { y } | x _ { 0 } ) - p _ { X _ { \tau } } | x _ { 0 } ( x | x _ { 0 } ) } { p _ { X _ { \tau } } ( y ) } p _ { X _ { 0 } } ( x _ { 0 } ) \mathrm { d } x _ { 0 } \right\| _ { 2 } } \\ & { \leq \frac { \| y - x \| _ { 2 } } { \tau } \int _ { \mathbb { R } } \| x - \sqrt { 1 - \tau } x _ { 0 } \| _ { 2 } ^ { 2 } p _ { X _ { \tau } | X _ { \tau } } ( x _ { 0 } | x ) \mathrm { d } x _ { 0 } } \\ & { \quad + \frac { \| y - x \| _ { 2 } ^ { 2 } } { 2 \tau } \int _ { \mathbb { R } } \| x - \sqrt { 1 - \tau } x _ { 0 } \| _ { 2 } p _ { X _ { \tau } | X _ { \tau } } ( x _ { 0 } | x ) \mathrm { d } x _ { 0 } } \\ & { \quad + \frac { \| y - x \| _ { 2 } } { \tau } \int _ { \mathbb { R } } \left\| y - \sqrt { 1 - \tau } x _ { 0 } \right\| _ { 2 } ^ { 2 } p _ { X _ { \tau } | X _ { \tau } } ( x _ { 0 } | y ) \mathrm { d } x _ { 0 } } \\ & { \quad + \frac { \| y - x \| _ { 2 } ^ { 2 } } { 2 \tau } \int _ { \mathbb { R } } \| y - \sqrt { 1 - \tau } x _ { 0 } \| _ { 2 } ^ { 2 } p _ { X _ { \tau } | X _ { \tau } } ( x _ { 0 } | y ) \mathrm { d } x _ { 0 } + \| x - y \| _ { 2 } } \\ & \stackrel { \mathrm { G a } } { \lesssim } d \mathbf { g } T \left\| x - y \right\| _ { 2 } + \| x - y \| _ { 2 } ^ { 2 } \sqrt { d \mathbf { g } _ { \tau } } \| X _ { \tau } ( x _ { 0 } | y ) \mathrm { d } x _ { 0 } \end{array} +$$ + +for + +$$ +\| x - y \| _ { 2 } \leq c \sqrt { \frac { \tau } { d \log T } } , +$$ + +where (a) uses Lemma 9. For the third term, by using Lemma 13, we have + +$$ +\begin{array} { r l } & { \quad \left\| \left( \frac { p _ { X _ { \tau } } ( x ) } { p _ { X _ { \tau } } ( y ) } - 1 \right) \int _ { x _ { 0 } } ( x - \sqrt { 1 - \tau } x _ { 0 } ) p _ { X _ { 0 } | X _ { \tau } } ( x _ { 0 } | x ) \mathrm { d } x _ { 0 } \right\| _ { 2 } } \\ & { \leq 6 \left\| x - y \right\| _ { 2 } \sqrt { \frac { ( \theta + c _ { 0 } ) d \log T } { \tau } } \int _ { x _ { 0 } } \left\| x - \sqrt { 1 - \tau } x _ { 0 } \right\| _ { 2 } p _ { X _ { 0 } | X _ { \tau } } ( x _ { 0 } | x ) \mathrm { d } x _ { 0 } } \\ & { \lesssim d \log T \left\| x - y \right\| _ { 2 } . } \end{array} +$$ + +Inserting (86) and (87) into (85), we have for $\begin{array} { r } { \| x - y \| _ { 2 } \leq c \sqrt { \frac { \tau } { d \log T } } } \end{array}$ , + +$$ +\| s _ { \tau } ^ { \star } ( y ) - s _ { \tau } ^ { \star } ( x ) \| _ { 2 } \lesssim \frac { d \log T } { \tau } \| x - y \| _ { 2 } . +$$ + +For $\begin{array} { r } { \| x - y \| _ { 2 } \geq c \sqrt { \frac { \tau } { d \log T } } } \end{array}$ , we have + +$$ +\| s _ { \tau } ^ { \star } ( y ) - s _ { \tau } ^ { \star } ( x ) \| _ { 2 } \leq \| s _ { \tau } ^ { \star } ( y ) \| _ { 2 } + \| s _ { \tau } ^ { \star } ( x ) \| _ { 2 } \leq 1 0 \sqrt { \frac { ( \theta + c _ { 0 } ) d \log T } { \tau } } \lesssim \frac { d \log T } { \tau } \| x - y \| _ { 2 } . +$$ + +Thus we prove (84). + +In addition, for $x \in { \mathcal { L } } _ { \tau }$ and $\begin{array} { r } { \| x - y \| \leq \frac { C \sqrt { d \tau } \log T } { L } } \end{array}$ , according to Definition 2, we have + +$$ +\| s _ { \tau } ^ { \star } ( y ) - s _ { \tau } ^ { \star } ( x ) \| _ { 2 } \leq \frac { L } { \tau } \| x - y \| _ { 2 } . +$$ + +Otherwise, for $x \in S _ { \tau }$ , $y \in S _ { \tau }$ , and $\begin{array} { r } { \| x - y \| > \frac { C \sqrt { d \tau } \log T } { L } } \end{array}$ , according to Lemma 9, we have + +$$ +\| s _ { \tau } ^ { \star } ( y ) - s _ { \tau } ^ { \star } ( x ) \| _ { 2 } \leq 2 \sqrt { \frac { 2 5 ( \theta + c _ { 0 } ) d \log T } { \tau } } \leq \frac { L } { \tau } \| x - y \| _ { 2 } , +$$ + +for $C \geq 1 0 \sqrt { \theta + c _ { 0 } }$ . Thus we complete the proof. + +# D.3 Proof of Lemma 5 + +According to Lemma 1, we know that $\widehat { X } _ { 0 } \stackrel { \mathrm { ~ d ~ } } { = } X _ { \tau _ { 0 , 0 } } = \sqrt { 1 - \tau _ { 0 , 0 } } X _ { 0 } + \sqrt { \tau _ { 0 , 0 } } Z$ for some standard Gaussian variable $Z$ . Then we have + +$$ +\begin{array} { r l } & { \mathsf { K L } \big ( p _ { \widehat { X } _ { 0 } } \| p _ { Y _ { 0 } } \big ) \le \mathsf { K L } \big ( p _ { X _ { \tau _ { 0 } , 0 } , X _ { 0 } } \| p _ { Y _ { 0 } } p _ { X _ { 0 } } \big ) = \mathbb { E } _ { X _ { 0 } \sim p _ { \mathsf { d a t a } } } [ \mathsf { K L } \big ( p _ { \widehat { X } _ { 0 } | X _ { 0 } } \| p _ { Y _ { 0 } } \big ) ] } \\ & { \qquad = \displaystyle \frac { 1 } { 2 } \left[ ( 1 - \tau _ { 0 , 0 } ) \mathbb { E } _ { X _ { 0 } \sim p _ { \mathsf { d a t a } } } \| X _ { 0 } \| ^ { 2 } - d \log ( \tau _ { 0 , 0 } ) + d \tau _ { 0 , 0 } - d \right] } \\ & { \qquad \le \displaystyle \frac { 1 } { 2 } T ^ { c _ { R } - c _ { 0 } } + d \frac { ( 1 - \tau _ { 0 , 0 } ) ^ { 2 } } { 2 \tau _ { 0 , 0 } } \le \displaystyle \frac { 1 } { 2 } T ^ { c _ { R } - c _ { 0 } } + \frac { d } { T ^ { 2 c _ { 0 } } } \le \frac { 1 } { T ^ { 1 0 } } , } \end{array} +$$ + +as long as $c _ { 0 } \geq \operatorname* { m a x } \{ c _ { R } + 1 0 , 1 0 \}$ , and $T \gtrsim d ^ { 1 / 1 0 }$ . + +The remaining of this section focuses on the proof of (38). For $d \log T \geq L$ , we have $\mathcal { E } _ { k } = \emptyset$ and (38) holds trivially. Below we only consider the case of $d \log T \leq L$ . We decompose the set $\mathcal { E } _ { k } ^ { \mathrm { c } }$ as + +$$ +\begin{array} { r } { \mathcal E _ { k } ^ { \mathrm { c } } = \cup _ { n = 0 } ^ { N - 1 } \widehat { \mathcal E } _ { k , n } , } \end{array} +$$ + +where for $1 \leq n \leq N - 1$ , + +$$ +\begin{array} { r l } & { \widehat { \mathcal { E } } _ { k , n } = \{ x _ { k } : x _ { \tau _ { k , i } } ( x _ { k } ) \in \widetilde { \mathcal { S } } _ { \tau _ { k , i } } \cap \mathcal { L } _ { \tau _ { k , i } } , \forall 0 \leq i \leq N - 1 , } \\ & { \qquad y _ { \tau _ { k , i } } ( x _ { k } ) \in \mathcal { S } _ { \tau _ { k , i } } , \forall 0 \leq i \leq n - 1 , y _ { \tau _ { k , n } } ( x _ { k } ) \notin \mathcal { S } _ { \tau _ { k , n } } \} , } \end{array} +$$ + +and + +$$ +\widehat { \mathcal { E } } _ { k , 0 } = \{ x _ { k } : \exists 0 \leq i \leq N - 1 , x _ { \tau _ { k , i } } ( x _ { k } ) \notin \widetilde { \mathcal { S } } _ { \tau _ { k , i } } \cap \mathcal { L } _ { \tau _ { k , i } } \} . +$$ + +Furthermore, we introduce another auxiliary set + +$$ +\mathcal { B } _ { k , n } = \left\{ x _ { k } : \frac { N \widehat { \tau } _ { k , - 1 } \log ^ { 2 } T } { T ^ { 2 } } \sum _ { i = 0 } ^ { n - 1 } \| s _ { \tau _ { k , i } } ( y _ { \tau _ { k , i } } ( x _ { k } ) ) - s _ { \tau _ { k , i } } ^ { \star } ( y _ { \tau _ { k , i } } ( x _ { k } ) ) \| ^ { 2 } \leq \frac { C \tau _ { k , n } } { d \log T } \right\} , +$$ + +where $C$ is a sufficiently small constant. Then we have + +$$ +\begin{array} { r } { \mathcal { E } _ { k } ^ { \mathrm { c } } \subset \widehat { \mathcal { E } } _ { k , 0 } \cup \left( \cup _ { n = 1 } ^ { N - 1 } \left( \widehat { \mathcal { E } } _ { k , n } \cap \mathcal { B } _ { k , n } \right) \right) \cup \left( \cup _ { n = 1 } ^ { N - 1 } \mathcal { B } _ { k , n } ^ { \mathrm { c } } \right) . } \end{array} +$$ + +Thus we have + +$$ +P ( \widehat { X } _ { k } \in \mathcal { E } _ { k } ^ { \mathrm { c } } ) \leq P ( \widehat { X } _ { k } \in \widehat { \mathcal { E } } _ { k , 0 } ) + \sum _ { n = 1 } ^ { N - 1 } P ( \widehat { X } _ { k } \in \widehat { \mathcal { E } } _ { k , n } \cap \mathcal { B } _ { k , n } ) + P ( \widehat { X } _ { k } \in \cup _ { n = 1 } ^ { N - 1 } \mathcal { B } _ { k , n } ^ { \mathrm { c } } ) . +$$ + +Below we shall calculate these terms separately. We start from considering $P ( \widehat { X } _ { k } \in \widehat { \mathcal { E } } _ { k , 0 } )$ . Noticing that $x _ { \tau _ { k , n } }$ has the identical distribution with $X _ { \tau _ { k , n } }$ , we have + +$$ +P ( \widehat { X } _ { k } \in \widehat { \mathcal { E } } _ { k , 0 } ) \leq \sum _ { n = 0 } ^ { N - 1 } P ( X _ { \tau _ { k , n } } \in \widetilde { \mathcal { S } } _ { \tau _ { k , n } } ^ { \mathrm { c } } ) + \sum _ { n = 0 } ^ { N } P ( X _ { \tau _ { k , n } } \in \mathcal { L } _ { \tau _ { k , n } } ^ { \mathrm { c } } ) \lesssim \frac { N } { T ^ { 4 } } , +$$ + +where the last inequality comes from the fact that + +$$ +P ( X _ { \tau _ { k , n } } \in \widetilde { \mathcal { S } } _ { \tau _ { k , n } } ^ { \mathrm { c } } ) \lesssim \frac { 1 } { T ^ { 4 } } , \qquad P ( X _ { \tau _ { k , n } } \in \mathcal { L } _ { \tau _ { k , n } } ^ { \mathrm { c } } ) \lesssim \frac { 1 } { T ^ { 4 } } , +$$ + +where the first inequality is proved in Li and Yan (2024a) (cf. (A.18)), and the second inequality comes from Definition 2. + +Next, we analyze $P ( \widehat { X } _ { k } \in \widehat { \mathcal { E } } _ { k , n } \cap B _ { k , n } )$ . According to Lemma 13, we have + +$$ +p _ { X _ { \tau } } ( y ) \geq \left( 1 - 6 \| y - x \| \sqrt { \frac { ( \theta + c _ { 0 } ) d \log T } { \tau } } \right) p _ { X _ { \tau } } ( x ) , \quad \mathrm { i f } \ \| x - y \| ^ { 2 } \leq \frac { c \tau } { d \log T } , +$$ + +where $c = 1 / ( 1 4 4 ( \theta + c _ { 0 } ) )$ . Recalling that $\log p _ { X _ { \tau } } ( x ) \geq \log 2 - \theta d \log T$ , we have + +$$ +\begin{array} { r l } & { \quad P \big ( \widehat { X } _ { k } \in \widehat { \mathcal { E } } _ { k , n } \cap \mathcal { B } _ { k , n } \big ) } \\ & { \leq P \Big ( \frac { N \widehat { \tau } _ { k , - 1 } \log ^ { 2 } T } { T ^ { 2 } } \displaystyle \sum _ { i = 0 } ^ { n - 1 } \widetilde { \varepsilon } _ { k , i } ^ { 2 } ( \widehat { X } _ { k } ) \leq \frac { C \tau _ { k , n } } { d \log T } , \| x _ { \tau _ { k , n } } ( \widehat { X } _ { k } ) - y _ { \tau _ { k , n } } ( \widehat { X } _ { k } ) \| _ { 2 } ^ { 2 } \geq \frac { c \tau _ { k , n } } { d \log T } , } \\ & { \quad x _ { \tau _ { k , i } } ( \widehat { X } _ { k } ) \in \widetilde { \mathcal { S } } _ { \tau _ { k , i } } \cap \mathcal { L } _ { \tau _ { k , i } } , y _ { \tau _ { k , i } } ( \widehat { X } _ { k } ) \in \mathcal { S } _ { \tau _ { k , i } } , \forall \ 0 \leq i < n \Big ) . } \end{array} +$$ + +According to (75), for + +$$ +\begin{array} { r l } & { x \in { \mathcal A } _ { k , n } : = \Bigl \{ x : \frac { N \widehat \tau _ { k , - 1 } \log ^ { 2 } T } { T ^ { 2 } } \displaystyle \sum _ { i = 0 } ^ { n - 1 } \widetilde \varepsilon _ { k , i } ^ { 2 } ( x ) \leq \frac { C \tau _ { k , n } } { d \log T } , } \\ & { \qquad x _ { \tau _ { k , i } } ( x ) \in \widetilde { \mathcal S } _ { \tau _ { k , i } } \cap { \mathcal L } _ { \tau _ { k , i } } , y _ { \tau _ { k , i } } ( x ) \in { \mathcal S } _ { \tau _ { k , i } } , \forall \ 0 \leq i < n \Bigr \} , } \end{array} +$$ + +which satisfies $\mathcal { A } _ { k , n } \subset \mathcal { E } _ { k , n }$ , we have + +$$ +\begin{array} { r l r } { { \| x _ { \tau _ { k , n } } ( x ) - y _ { \tau _ { k , n } } ( x ) \| ^ { 2 } \lesssim \zeta _ { k , n } ( x ) + \frac { N \log ^ { 2 } T } { T ^ { 2 } } \sum _ { i = 0 } ^ { n - 1 } \widehat { \tau } _ { k , i - 1 } \widehat { \varepsilon } _ { k , i } ^ { 2 } ( x ) } } \\ & { } & { \lesssim \zeta _ { k , n } ( x ) + \frac { \widehat { \tau } _ { k , - 1 } N \log ^ { 2 } T } { T ^ { 2 } } \sum _ { i = 0 } ^ { n - 1 } \widehat { \varepsilon } _ { k , i } ^ { 2 } ( x ) \overset { ( \mathrm { a } ) } { \lesssim } \zeta _ { k , n } ( x ) + \frac { C \tau _ { k , n } } { d \log T } , } \end{array} +$$ + +where (a) comes from the condition that $\boldsymbol { x } \in \boldsymbol { A } _ { k , n }$ . Assuming $C \le c / 2$ , we have + +$$ +\begin{array} { r l } & { P ( \widehat { X } _ { k } \in \widehat { \mathcal { E } } _ { k , n } \cap B _ { k , n } ) } \\ & { \leq P \left( \widehat { X } _ { k } \in \mathcal { A } _ { k , n } , \zeta _ { k , n } ( \widehat { X } _ { k } ) \geq \frac { c \tau _ { k , n } } { 2 d \log T } \right) } \\ & { \overset { \mathrm { ( a ) } } { \leq } \frac { 2 d \log T } { c \tau _ { k , n } } \int _ { A _ { k , n } } \zeta _ { k , n } ( x ) p _ { \widehat { X } _ { k } } ( x ) \mathrm { d } x \leq \frac { 2 d \log T } { c \tau _ { k , n } } \int _ { \mathcal { E } _ { k , n } } \zeta _ { k , n } ( x ) p _ { \widehat { X } _ { k } } ( x ) \mathrm { d } x } \\ & { \overset { \mathrm { ( b ) } } { \lesssim } \frac { K d \log ^ { 4 } T } { T ^ { 3 } } \operatorname* { m i n } \Big \{ \frac { N d \log T } { T } + \frac { 1 - \widehat { \tau } _ { k , n } } { \widehat { \tau } _ { k , n } } \int _ { \tau _ { k , n } } ^ { \tau _ { k , n } } \frac { \| \nabla \| \tau ( \sum _ { \tau } ^ { 2 } ( x _ { \tau } ) ) \| } { ( 1 - \tau ) ^ { 2 } } \mathrm { d } \tau , \frac { N L ^ { 2 } \log T } { T } \Big \} } \\ & { \lesssim \frac { d \log ^ { 5 } T } { T ^ { 3 } } \operatorname* { m i n } \Big \{ d + \frac { K ( 1 - \widehat { \tau } _ { k , N } ) } { \widehat { \tau } _ { k , N } \log T } \int _ { \tau _ { k , N } } ^ { \tau _ { k , n } } \frac { \| \nabla \| \tau ( \sum _ { \tau } ^ { 2 } ( x _ { \tau } ) ) \| } { ( 1 - \tau ) ^ { 2 } } \mathrm { d } \tau , L ^ { 2 } \Big \} } \end{array} +$$ + +where (a) uses the Markov inequality, and (b) uses (76) and the fact that $K \stackrel { > } { \sim } \operatorname* { m i n } \{ d \log T , L \} \log T = d \log ^ { 2 } T$ for $d \log T \lesssim L$ . Summing from $n = 0$ to $N - 1$ , we have + +$$ +\sum _ { n = 1 } ^ { N - 1 } P ( \widehat { X } _ { k } \in \widehat { \mathcal { E } } _ { k , n } \cap \mathcal { B } _ { k , n } ) \lesssim \frac { \log ^ { 3 } T } { T ^ { 2 } } \operatorname* { m i n } \Big \{ d + \frac { K ( 1 - \widehat { \tau } _ { k , N } ) } { \widehat { \tau } _ { k , N } \log T } \int _ { \tau _ { k , N } } ^ { \tau _ { k , 0 } } \frac { \mathbb { E } [ \mathsf { T r } ( \Sigma _ { \tau } ^ { 2 } ( x _ { \tau } ) ) ] } { ( 1 - \tau ) ^ { 2 } } \mathrm { d } \tau , L ^ { 2 } \Big \} . +$$ + +Finally, we analyze $P ( \widehat { X } _ { k } \in \cup _ { n = 1 } ^ { N - 1 } B _ { k , n } ^ { \mathrm { c } } )$ . Noticing that $B _ { k , n } \subset B _ { k , n - 1 }$ , we have + +$$ +\begin{array} { r l } & { \quad P ( \widehat { X } _ { k } \in \cup _ { n = 0 } ^ { N - 1 } \mathcal { B } _ { k , n } ^ { \mathrm { c } } ) \leq P ( \widehat { X } _ { k } \in \mathcal { B } _ { k , N - 1 } ^ { \mathrm { c } } ) } \\ & { \leq P \left( \frac { N \widehat { \tau } _ { k , 0 } \log ^ { 2 } T } { T ^ { 2 } } \sum _ { i = 1 } ^ { N - 1 } \| s _ { \tau _ { k , i } } ( y _ { \tau _ { k , i } } ( \widehat { X } _ { k } ) ) - s _ { \tau _ { k , i } } ^ { * } ( y _ { \tau _ { k , i } } ( \widehat { X } _ { k } ) ) \| ^ { 2 } > \frac { C \tau _ { k , N } } { d \log T } \right) } \\ & { \overset { \mathrm { ( a ) } } { \lesssim } \frac { N d \log ^ { 3 } T } { T ^ { 2 } } \overset { N - 1 } { \underset { i = 1 } { \overset { N } { \sum } } } \mathbb { E } _ { x _ { k } \sim \widehat { X } _ { k } } \| s _ { \tau _ { k , i } } ( y _ { \tau _ { k , i } } ( x _ { k } ) ) - s _ { \tau _ { k , i } } ^ { * } ( y _ { \tau _ { k , i } } ( x _ { k } ) ) \| ^ { 2 } } \\ & { \lesssim \frac { N d \log ^ { 3 } T } { T ^ { 2 } } \overset { N - 1 } { \underset { i = 0 } { \overset { N } { \sum } } } \varepsilon _ { k , i } ^ { 2 } , } \end{array} +$$ + +where (a) uses Markov inequality. Inserting (89), (90), and (91) into (88), we have + +$$ +\begin{array} { r l r } { { P ( \widehat { X } _ { k } \in \mathcal { E } _ { k } ) \lesssim \frac { N } { T ^ { 4 } } + \frac { \log T } { T } \sum _ { i = 0 } ^ { N - 1 } \varepsilon _ { k , i } ^ { 2 } } } \\ & { } & { \quad + \frac { \log ^ { 3 } T } { T ^ { 2 } } \operatorname* { m i n } \Big \{ d + \frac { K ( 1 - \widehat { \tau } _ { k , N } ) } { \widehat { \tau } _ { k , N } \log T } \int _ { \tau _ { k , N } } ^ { \tau _ { k , 0 } } \frac { \mathbb { E } [ \mathsf { T r } ( \Sigma _ { \tau } ^ { 2 } ( x _ { \tau } ) ) ] } { ( 1 - \tau ) ^ { 2 } } \mathrm { d } \tau , L ^ { 2 } \Big \} . } \end{array} +$$ + +Moreover, we have + +$$ +\begin{array} { r l } & { \displaystyle \sum _ { k = 0 } ^ { K - 1 } P ( \widehat X _ { k } \in \mathcal { E } _ { k } ) \stackrel { \mathrm { ( a ) } } \lesssim \frac { 1 } { T ^ { 3 } } + \frac { \log T } { T } \displaystyle \sum _ { k = 0 } ^ { K - 1 } \sum _ { i = 0 } ^ { N - 1 } \varepsilon _ { k , i } ^ { 2 } + \frac { K \log ^ { 3 } T } { T ^ { 2 } } \operatorname* { m i n } \left\{ d + d , L ^ { 2 } \right\} } \\ & { \displaystyle \stackrel { \mathrm { ( b ) } } { \lesssim } \frac { d ^ { 2 } \log ^ { 5 } T } { T ^ { 2 } } + \frac { \log T } { T } \displaystyle \sum _ { k = 0 } ^ { K - 1 } \displaystyle \sum _ { i = 0 } ^ { N - 1 } \varepsilon _ { k , i } ^ { 2 } } \end{array} +$$ + +where (a) uses (81) and (b) uses the fact that $d \log T \lesssim L$ . + +# D.4 Proof of (28) + +We first show a basic inequality: for any two probability density functions $f$ , $g$ , and any set $\mathcal { R }$ , we have + +$$ +\int _ { \mathcal { R } } f ( x ) \log \frac { f ( x ) } { g ( x ) } \mathrm { d } x \geq \log \left( \frac { \int _ { \mathcal { R } } f ( x ) \mathrm { d } x } { \int _ { \mathcal { R } } g ( x ) \mathrm { d } x } \right) \int _ { \mathcal { R } } f ( x ) \mathrm { d } x +$$ + +which is proved in Lemma 6 in Li and Yan (2024a). Now we are ready to prove (28). To this end, let us prove a more general conclusion. For any two probability density functions $f$ and $g$ , and any set $\mathcal { R }$ , assume that $\ddot { f }$ and $\tilde { g }$ is defined as + +$$ +\tilde { f } ( x ) = f ( x ) \mathbb { 1 } \{ x \in \mathcal { R } \} + \int _ { x \notin \mathcal { R } } f ( x ) \mathrm { d } x \delta _ { \infty } , \quad \tilde { g } ( x ) = g ( x ) \mathbb { 1 } \{ x \in \mathcal { R } \} + \int _ { x \notin \mathcal { R } } g ( x ) \mathrm { d } x \delta _ { \infty } . +$$ + +Then + +$$ +{ \mathsf { K L } } ( { \tilde { f } } \| { \tilde { g } } ) \leq { \mathsf { K L } } ( f \| g ) . +$$ + +Towards this, according to definitions, we have + +$$ +\mathsf { K L } ( \tilde { f } \| \tilde { g } ) - \mathsf { K L } ( f \| g ) = \int _ { x \notin \mathcal R } f ( x ) \mathrm { d } x \log \left( \frac { \int _ { x \notin \mathcal R } f ( x ) \mathrm { d } x } { \int _ { x \notin \mathcal R } g ( x ) \mathrm { d } x } \right) - \int _ { x \notin \mathcal R } f ( x ) \log \left( \frac { f ( x ) } { g ( x ) } \right) \mathrm { d } x \leq 0 +$$ + +and complete the proof. + +# E Parallel sampling + +# E.1 Parallel algorithm + +The parallel sampling procedure follows the same structure as the original sampler described in (10): it consists of $K$ rounds, with each round comprising several iterations. The key difference is that, in each round, we use $N$ processors and perform $M \ll N$ iterations. In each iteration, each processor updates the sample $Y _ { m , k , n }$ using the outputs of other processors from the previous iteration $\{ Y _ { m - 1 , k , i } \} _ { i < n }$ . The implementation details for the $k$ -th round are as follows. + +1. Initialization: for the $n$ -th parallel processor, the sample is initialized as + +$$ +\frac { Y _ { 0 , k , n } } { \sqrt { 1 - \tau _ { k , n } } } = \frac { Y _ { k } } { \sqrt { 1 - \tau _ { k , 0 } } } , \quad n = 1 , \cdots , N . +$$ + +2. Parallel updates: we use $N$ processors to update $Y _ { m , k , n }$ ( $n = 1 , \cdots , N )$ simultaneously for $M$ iterations. In the $m$ -th iteration, the update rule is: + +$$ +\begin{array} { l } { \displaystyle \frac { Y _ { m , k , n } } { \sqrt { 1 - \tau _ { k , n } } } = \frac { Y _ { k } } { \sqrt { 1 - \tau _ { k , 0 } } } + \frac { s _ { T - \frac { k N } { 2 } + 1 } ( Y _ { k } ) } { 2 ( 1 - \tau _ { k , 0 } ) ^ { 3 / 2 } } ( \tau _ { k , 0 } - \hat { \tau } _ { k , 0 } ) } \\ { \displaystyle \quad + \sum _ { i = 1 } ^ { n - 1 } \frac { s _ { T - \frac { k N } { 2 } - i + 1 } ( Y _ { m - 1 , k , i } ) } { 2 ( 1 - \tau _ { k , i } ) ^ { 3 / 2 } } ( \widehat { \tau } _ { k , i - 1 } - \widehat { \tau } _ { k , i } ) } \\ { \displaystyle \quad + \frac { s _ { T - \frac { k N } { 2 } - n + 2 } ( Y _ { m - 1 , k , n - 1 } ) } { 2 ( 1 - \tau _ { k , n - 1 } ) ^ { 3 / 2 } } ( \widehat { \tau } _ { k , n - 1 } - \tau _ { k , n } ) . } \end{array} +$$ + +3. Noise injection: Once $Y _ { M , k , N }$ is obtained, we update $Y _ { k + 1 }$ as follows. + +$$ +Y _ { k + 1 } = \sqrt { \frac { 1 - \tau _ { k + 1 , 0 } } { 1 - \tau _ { k , N } } } Y _ { M , k , N } + \sqrt { \frac { \tau _ { k + 1 , 0 } - \tau _ { k , N } } { 1 - \tau _ { k , N } } } Z _ { k } , +$$ + +where $Z _ { k } \sim { \mathcal { N } } ( 0 , I _ { d } )$ . + +In this parallel framework, the total number of parallel rounds required to generate the final sample $Y _ { K }$ is $M K$ , and $N$ parallel processors are needed. The convergence rate of this procedure is established in Theorem 2. We remark that the implementation of this parallel algorithm assumes that the GPU memory is capable of supporting score estimations for a large batch of data simultaneously. Parallelizing across multiple GPUs introduces additional communication overhead, which may impact efficiency. + +# E.2 Analysis for parallelization (Theorem 2) + +By comparing the update rules for $Y _ { k , n }$ and $Y _ { m , k , n }$ , it is natural to control the difference of the following two sequences: + +$$ +\begin{array} { r l } & { \frac { y _ { \tau _ { k , n } } ( y _ { \tau _ { k , 0 } } ) } { \sqrt { 1 - \tau _ { k , n } } } = \frac { y _ { \tau _ { k , 0 } } } { \sqrt { 1 - \tau _ { k , 0 } } } + \frac { s _ { \tau _ { k , 0 } } ( y _ { \tau _ { k , 0 } } ) } { 2 ( 1 - \tau _ { k , 0 } ) ^ { 3 / 2 } } ( \tau _ { k , 0 } - \widehat \tau _ { k , 0 } ) + \displaystyle \sum _ { i = 1 } ^ { n - 1 } \frac { s _ { \tau _ { k , i } } ( y _ { \tau _ { k , i } } ) } { 2 ( 1 - \tau _ { k , i } ) ^ { 3 / 2 } } ( \widehat \tau _ { k , i - 1 } - \widehat \tau _ { k , i } ) } \\ & { \qquad + \frac { s _ { \tau _ { k , n - 1 } } ( y _ { \tau _ { k , n - 1 } } ) } { 2 ( 1 - \tau _ { k , n - 1 } ) ^ { 3 / 2 } } ( \widehat \tau _ { k , n - 1 } - \tau _ { k , n } ) , } \end{array} +$$ + +and + +$$ +\begin{array}{c} \frac { y _ { m , \tau _ { k , n } } ( y _ { \tau _ { k , 0 } } ) } { \sqrt { 1 - \tau _ { k , n } } } = \frac { y _ { \tau _ { k , 0 } } } { \sqrt { 1 - \tau _ { k , 0 } } } + \frac { s _ { \tau _ { k , 0 } } ( y _ { \tau _ { k , 0 } } ) } { 2 ( 1 - \tau _ { k , 0 } ) ^ { 3 / 2 } } ( \tau _ { k , 0 } - \widehat { \tau } _ { k , 0 } ) + \sum _ { i = 1 } ^ { n - 1 } \frac { s _ { \tau _ { k , i } } ( y _ { m - 1 , \tau _ { k , i } } ) } { 2 ( 1 - \tau _ { k , i } ) ^ { 3 / 2 } } ( \widehat { \tau } _ { k , i - 1 } - \widehat { \tau } _ { k , i } ) \\ { + \frac { s _ { \tau _ { k , n - 1 } } ( y _ { m - 1 , \tau _ { k , n - 1 } } ) } { 2 ( 1 - \tau _ { k , n - 1 } ) ^ { 3 / 2 } } ( \widehat { \tau } _ { k , n - 1 } - \tau _ { k , n } ) . \qquad } \end{array} +$$ + +We construct the typical set + +$$ +\begin{array} { r } { \mathcal { E } _ { k } : = \left\{ \begin{array} { l l } { \{ x _ { \tau _ { k , 0 } } : x _ { \tau _ { k , n } } ( x _ { \tau _ { k , 0 } } ) \in \widetilde { \mathcal { S } } _ { \tau _ { k , n } } \cap \mathcal { L } _ { \tau _ { k , n } } , y _ { m , \tau _ { k , n } } ( x _ { \tau _ { k , 0 } } ) \in \mathcal { S } _ { \tau _ { k , n } } , } \\ { \quad y _ { \tau _ { k , n } } ( x _ { \tau _ { k , 0 } } ) \in \mathcal { S } _ { \tau _ { k , n } } \cap \mathcal { L } _ { \tau _ { k , n } } , \forall 0 \leq n < N , \forall 0 \leq m < M \} , } & { \mathrm { i f ~ } L > d \log T , } \\ { \emptyset \quad \quad } & { \mathrm { i f ~ } \ L \leq d \log T , } \end{array} \right. } \end{array} +$$ + +and the auxiliary sequences $\smash { \widetilde { X } } _ { k }$ , $\widetilde { Y _ { k } }$ for $k = 0 \cdots , K$ as (22) and (24), with + +$$ +P _ { Y _ { k + 1 } | Y _ { k } } ( y | y _ { k } ) = \phi \left( y | y _ { M , \tau _ { k , N } } ( y _ { k } ) , \sigma _ { k } ^ { 2 } \right) . +$$ + +Similar with the proof of Theorem $^ { 1 }$ , we have + +$$ +\mathsf { T V } \big ( q _ { K } , p _ { Y _ { K } } \big ) \leq 2 \sum _ { k = 0 } ^ { K - 1 } P ( \widehat { X } _ { k } \in \mathcal { E } _ { k } ^ { \mathrm { c } } ) +$$ + +$$ ++ \sqrt { \frac { 1 } { 2 } \mathsf { K L } \big ( p _ { \widehat { X } _ { 0 } } \| p _ { Y _ { 0 } } \big ) + \frac { 1 } { 2 } \sum _ { k = 0 } ^ { K - 1 } \mathbb { E } _ { x _ { k } \sim p _ { \widetilde { X } _ { k } } } \left[ \mathsf { K L } \big ( p _ { \widehat { X } _ { k + 1 } | \widehat { X } _ { k } } \left( \cdot | x _ { k } \right) \| p _ { Y _ { k + 1 } | Y _ { k } } \left( \cdot | x _ { k } \right) \big ) \right] } . +$$ + +According to the Lipschitz condition of score estimates and Lemma 12, we have + +$$ +\frac { \left\| y _ { m , \tau _ { k , n } } - y _ { \tau _ { k , n } } \right\| _ { 2 } } { \sqrt { 1 - \tau _ { k , n } } } \lesssim \frac { \operatorname* { m i n } \{ d \log T , L \} \log T } { T } \sum _ { i = 1 } ^ { n - 1 } \frac { \left\| y _ { m - 1 , \tau _ { k , i } } - y _ { \tau _ { k , i } } \right\| _ { 2 } } { \sqrt { 1 - \tau _ { k , i } } } , \qquad y _ { \tau _ { k , 0 } } \in \mathcal { E } _ { k } . +$$ + +Applying the above relation recursively gives + +$$ +\operatorname* { m a x } _ { n } \frac { \big \| y _ { M , \tau _ { k , n } } - y _ { \tau _ { k , n } } \big \| _ { 2 } } { \sqrt { 1 - \tau _ { k , n } } } \leq \bigg ( \frac { N \operatorname* { m i n } \{ d \log T , L \} \log T } { T } \bigg ) ^ { M } \operatorname* { m a x } _ { n } \frac { \big \| y _ { 0 , \tau _ { k , n } } - y _ { \tau _ { k , n } } \big \| _ { 2 } } { \sqrt { 1 - \tau _ { k , n } } } \leq \frac { 1 } { \mathsf { p o l y } ( T ) } , +$$ + +provided that $M \gtrsim \log T$ and $T \gtrsim N \operatorname* { m i n } \{ d \log T , L \} \log T$ . Thus as long as + +$$ +N \gtrsim \frac { ( \operatorname* { m i n } \{ d ^ { 2 / 3 } L ^ { - 2 / 3 } , d ^ { 1 / 3 } \} + 1 ) \log ^ { 5 / 3 } T } { \varepsilon ^ { 2 / 3 } } , +$$ + +which guarantees that $\begin{array} { r } { T \gtrsim \frac { \operatorname* { m i n } \{ d , d ^ { 2 / 3 } L ^ { 1 / 3 } , d ^ { 1 / 3 } L \} \log ^ { 8 / 3 } T } { \varepsilon ^ { 2 / 3 } } } \end{array}$ , we can get the desired result immediately through just inserting the above error bound into Lemma 4 and Lemma 5. The probability of $P ( \widehat { X } _ { k } \in \mathcal { E } _ { k } ^ { \mathrm { c } } )$ is bounded by using Lemma 13 and the fact that $P ( x _ { \tau _ { k , n } } \in \mathcal { L } _ { \tau _ { k , n } } ^ { \mathrm { c } } ) \lesssim 1 / T ^ { 4 }$ . We omit the details here due to the similarity. + +# F Proof of auxiliary lemmas in Theorem 1 + +Proof of (51). According to the definition of $\widehat { \tau } _ { k , i }$ , we have that + +$$ +\frac { 1 - \widehat \tau _ { k , N } } { 1 - \widehat \tau _ { k , - 1 } } = \frac { \widehat \alpha _ { T - \frac { k N } { 2 } - N } } { \widehat \alpha _ { T - \frac { k N } { 2 } + 1 } } \leq \left( 1 + \frac { c _ { 1 } \log T } { T } \right) ^ { N + 1 } \leq \exp \left( \frac { c _ { 1 } ( N + 1 ) \log T } { T } \right) \leq \mathrm { e } , +$$ + +and + +$$ +\frac { \widehat { \tau } _ { k , - 1 } } { \widehat { \tau } _ { k , N } } = \frac { 1 - \widehat { \alpha } _ { T - \frac { k N } { 2 } + 1 } } { 1 - \widehat { \alpha } _ { T - \frac { k N } { 2 } - N } } \leq \left( 1 - \frac { c _ { 1 } \log T } { T } \right) ^ { - N - 1 } \leq \exp \left( \frac { 2 c _ { 1 } ( N + 1 ) \log T } { T } \right) \leq \mathrm { e } , +$$ + +as long as $T \geq 4 c _ { 1 } N \log T$ . Thus we complete the proof. + +# F.1 Proof of Lemma 1 + +Notice that the map $\Phi$ in (17) is just the integral form of (16), which is equivalent to + +$$ +\mathrm { d } x _ { \tau } = - \frac { 1 } { 2 ( 1 - \tau ) } \big ( x _ { \tau } + s _ { \tau } ^ { \star } ( x _ { \tau } ) \big ) \mathrm { d } \tau . +$$ + +This is the well-known probability ODE flow, which comes from Song et al. (2021) and is also used in Li et al. (2023). + +The proof of (18) can be completed by using mathematical induction. Recalling that $\widehat { X } _ { 0 } \ \stackrel { d } { = } \ X _ { \tau _ { 0 , 0 } }$ , the (18) holds for $k + 1 = 0$ . Assume that (18) holds for $k + 1 = h$ . We have $\Phi _ { \tau _ { h - 1 , 0 } \tau _ { h - 1 , N } } ( \widehat { X } _ { h - 1 } ) \stackrel { d } { = } X _ { \tau _ { h - 1 , N } }$ . According to (15), we could immediately get that (18) holds for $k = h$ . + +# F.2 Proof of Lemma 2 + +We shall complete the remaining proof by mathematical induction. According to initializations, all inequalities in Lemma 2 hold for $k = 0$ . Assume (26) hold for $k = h$ . For $x , y \in \mathcal { E } _ { h + 1 } ^ { \mathrm { c } }$ , we have $p _ { \widetilde { X } _ { h + 1 } } ( x ) = 0 \leq$ $p _ { \widehat { X } _ { h + 1 } } ( x )$ , $p _ { \widetilde { Y } _ { h + 1 } } ( y ) = 0 \le p _ { Y _ { h + 1 } } ( y )$ . For $x , y \in \mathcal { E } _ { h + 1 }$ , + +$$ +\begin{array} { r l } & { p _ { \widetilde { X } _ { h + 1 } } ( x ) = \displaystyle \int p _ { \widetilde { X } _ { h + 1 } | \widetilde { X } _ { h } } ( x \mid x _ { h } ) p _ { \widetilde { X } _ { h } } ( x _ { h } ) \mathrm { d } x _ { h } \le \displaystyle \int p _ { \widehat { X } _ { h + 1 } | \widehat { X } _ { h } } ( x \mid x _ { h } ) p _ { \widehat { X } _ { h } } ( x _ { h } ) \mathrm { d } x _ { h } = p _ { \widehat { X } _ { h } } ( x ) , } \\ & { p _ { \widetilde { Y } _ { h + 1 } } ( y ) = \displaystyle \int p _ { \widetilde { Y } _ { h + 1 } | \widetilde { Y } _ { h } } ( y \mid y _ { h } ) p _ { \widetilde { Y } _ { h } } ( y _ { h } ) \mathrm { d } y _ { h } \le \displaystyle \int p _ { Y _ { h + 1 } | Y _ { h } } ( y \mid y _ { h } ) p _ { Y _ { h } } ( y _ { h } ) \mathrm { d } y _ { h } = p _ { Y _ { h + 1 } } ( y ) . } \end{array} +$$ + +# F.3 Proof of Lemma 13 + +Notice that + +$$ +\begin{array} { l } { p _ { X _ { \tau } } ( y ) = \displaystyle \int p _ { X _ { \tau } | X _ { 0 } } ( y | x _ { 0 } ) p _ { X _ { 0 } } ( x _ { 0 } ) \mathrm { d } x _ { 0 } } \\ { \displaystyle = \int p _ { X _ { \tau } | X _ { 0 } } ( x \mid x _ { 0 } ) p _ { X _ { 0 } } ( x _ { 0 } ) \exp \left( \frac { ( x - y ) ^ { \top } ( x + y - 2 \sqrt { 1 - \tau } x _ { 0 } ) } { 2 \tau } \right) \mathrm { d } x _ { 0 } , } \\ { p _ { X _ { \tau } } ( x ) = \displaystyle \int p _ { X _ { \tau } | X _ { 0 } } ( y \mid x _ { 0 } ) p _ { X _ { 0 } } ( x _ { 0 } ) \exp \left( \frac { ( y - x ) ^ { \top } ( x + y - 2 \sqrt { 1 - \tau } x _ { 0 } ) } { 2 \tau } \right) \mathrm { d } x _ { 0 } . } \end{array} +$$ + +For $x \in S _ { \tau }$ , we have + +$$ +\begin{array} { r l } & { \begin{array} { r l } & { p _ { X _ { \tau } } ( x ) - p _ { X _ { \tau } } ( y ) } \\ & { = \displaystyle \int p _ { X _ { \tau } | X _ { 0 } } ( x | x _ { 0 } ) P _ { X _ { 0 } } ( x _ { 0 } ) \left( 1 - \exp \left( \frac { \left( x - y \right) ^ { \top } ( x + y - 2 \sqrt { 1 - \tau } x _ { 0 } ) } { 2 \tau } \right) \right) \mathrm { d } x _ { \theta } } \end{array} } \\ & { \leq \int p _ { X _ { \tau } | X _ { 0 } } ( x | x _ { 0 } ) p _ { X _ { 0 } } ( x _ { 0 } ) \frac { ( y - x ) ^ { \top } ( x + y - 2 \sqrt { 1 - \tau } x _ { 0 } ) } { 2 \tau } \mathrm { d } x _ { 0 } } \\ & { \leq \displaystyle \int p _ { X _ { \tau } | X _ { 0 } } ( x | x _ { 0 } ) p _ { X _ { 0 } } ( x _ { 0 } ) \frac { \| y - x \| ^ { 2 } } { 2 \tau } \mathrm { d } x _ { 0 } } \\ & { + \displaystyle \int p _ { X _ { \tau } | X _ { 0 } } ( x | x _ { 0 } ) P _ { X _ { 0 } } ( x _ { 0 } ) \frac { ( y - x ) ^ { \top } ( x - \sqrt { 1 - \tau } x _ { 0 } ) } { \tau } \mathrm { d } x _ { 0 } } \end{array} +$$ + +Furthermore, according to Lemma 9, and the fact that $\| x - y \| \leq 2 { \sqrt { ( \theta + c _ { 0 } ) d \tau \log T } }$ , we have + +$$ +\begin{array} { r l } & { \quad p _ { X _ { \tau } } ( x ) - p _ { X _ { \tau } } ( y ) } \\ & { \leq \left( \displaystyle \frac { \| y - x \| ^ { 2 } } { 2 \tau } + 5 \| y - x \| \sqrt { \displaystyle \frac { ( \theta + c _ { 0 } ) d \log T } { \tau } } \right) p _ { X _ { \tau } } ( x ) } \\ & { \leq 6 \| y - x \| \sqrt { \displaystyle \frac { ( \theta + c _ { 0 } ) d \log T } { \tau } } p _ { X _ { \tau } } ( x ) . } \end{array} +$$ + +Similarly, for $y \in S _ { \tau }$ , we have + +$$ +p _ { X _ { \tau } } ( y ) - p _ { X _ { \tau } } ( x ) \leq 6 \| y - x \| \sqrt { \frac { ( \theta + c _ { 0 } ) d \log T } { \tau } } p _ { X _ { \tau } } ( y ) . +$$ + +Thus we complete the proof. + +# F.4 Proof of Lemma 7 + +Proof of (50). The first relation is immediately obtained by noticing that + +$$ +1 - \tau _ { 0 , 0 } = \overline { { \alpha } } _ { T + 1 } \leq \widehat { \alpha } _ { T } \leq 2 \widehat { \alpha } _ { T + 1 } , +$$ + +provided that $\frac { c _ { 1 } \log T ^ { \prime } } { T } < 1$ . + +Regarding the second inequality, for $\begin{array} { r } { T _ { 0 } = ( 1 - \frac { 3 c _ { 0 } } { c _ { 1 } } ) T + 1 } \end{array}$ and $c _ { 1 } > 3 c _ { 0 }$ , we claim that $\widehat { \alpha } _ { T _ { 0 } } \geq \frac { 1 } { 2 }$ ; otherwise, we have + +$$ +\widehat { \alpha } _ { t - 1 } = \widehat { \alpha } _ { t } \bigg ( 1 + \frac { c _ { 1 } \big ( 1 - \widehat { \alpha } _ { t } \big ) \log T } { T } \bigg ) > \widehat { \alpha } _ { t } \bigg ( 1 + \frac { c _ { 1 } \log T } { 2 T } \bigg ) , +$$ + +and then + +$$ +\widehat { \alpha } _ { T _ { 0 } } > \widehat { \alpha } _ { T + 1 } \Bigl ( 1 + \frac { c _ { 1 } \log T } { 2 T } \Bigr ) ^ { T - T _ { 0 } + 1 } > \frac { 1 } { 2 } . +$$ + +Then if $c _ { 1 } > 5 c _ { 0 }$ , we have + +$$ +\tau _ { K , 0 } = 1 - \overline { { \alpha } } _ { 1 } \leq 1 - \widehat { \alpha } _ { 1 } \leq ( 1 - \widehat { \alpha } _ { T _ { 0 } } ) \left( 1 - \frac { c _ { 1 } \log T } { 2 T } \right) ^ { T _ { 0 } - 1 } \leq \frac { 1 } { T ^ { c _ { 0 } } } , +$$ + +where we make use of the observation that for $t \leq T _ { 0 }$ , + +$$ +1 - \widehat \alpha _ { t - 1 } = \big ( 1 - \widehat \alpha _ { t } \big ) \bigg ( 1 - \frac { c _ { 1 } \widehat \alpha _ { t } \log T } { T } \bigg ) \leq \big ( 1 - \widehat \alpha _ { t } \big ) \bigg ( 1 - \frac { c _ { 1 } \log T } { 2 T } \bigg ) . +$$ + +The third equation follows immediately from the definition of $\widehat { \tau } _ { k , n }$ that + +$$ +\frac { \widehat { \tau } _ { k , n - 1 } - \widehat { \tau } _ { k , n } } { \widehat { \tau } _ { k , n - 1 } ( 1 - \widehat { \tau } _ { k , n - 1 } ) } = \frac { \widehat { \alpha } _ { T - \frac { k N } { 2 } - n } - \widehat { \alpha } _ { T - \frac { k N } { 2 } - n + 1 } } { ( 1 - \widehat { \alpha } _ { T - \frac { k N } { 2 } - n + 1 } ) \widehat { \alpha } _ { T - \frac { k N } { 2 } - n + 1 } } = \frac { c _ { 1 } \log T } { T } . +$$ + +# References + +Benton, J., De Bortoli, V., Doucet, A., and Deligiannidis, G. 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Improving diffusion-based inverse algorithms under few-step constraint via learnable linear extrapolation. arXiv preprint arXiv:2503.10103. \ No newline at end of file diff --git a/papers/diffusion-convergence-rate/paper.pdf b/papers/diffusion-convergence-rate/paper.pdf new file mode 100644 index 0000000000000000000000000000000000000000..c5d8c783def492ee80896a8d9e7b26ac06cb07a1 --- /dev/null +++ b/papers/diffusion-convergence-rate/paper.pdf @@ -0,0 +1,3 @@ +version https://git-lfs.github.com/spec/v1 +oid sha256:516cddc984d38ab696046d302986c3edac0586030583289fcaf842433ebb6530 +size 1147411 diff --git a/papers/diffusion-convergence-rate/sau.json b/papers/diffusion-convergence-rate/sau.json new file mode 100644 index 0000000000000000000000000000000000000000..c50be640e6eb3fa0ac366414aed146c1bf69313f --- /dev/null +++ b/papers/diffusion-convergence-rate/sau.json @@ -0,0 +1,222 @@ +{ + "paper_id": "diffusion-convergence-rate", + "paper_title": "Improved Convergence Rate for Diffusion Probabilistic Models", + "D1": [ + { + "id": "diffusion-convergence-rate-D1-001", + "claim": "hat_alpha_{T+1} = 1 / T^{c_0} (Initial schedule point; c_0 is a sufficiently large constant satisfying c_0 >= max{c_R + 10, 10})", + "source": "Section 2.2, Eq (8)" + }, + { + "id": "diffusion-convergence-rate-D1-002", + "claim": "hat_alpha schedule update step = c_1 * hat_alpha_t * (1 - hat_alpha_t) * log T / T (Step size for hat_alpha sequence; c_1 > 0 is a sufficiently large constant; c_1 / c_0 ratio must be sufficiently large)", + "source": "Section 2.2, Eq (8)" + }, + { + "id": "diffusion-convergence-rate-D1-003", + "claim": "hat_alpha time index range = t = -N/2 + 1, ..., T + 1 (Index range for the schedule construction)", + "source": "Section 2.2, Eq (8)" + }, + { + "id": "diffusion-convergence-rate-D1-004", + "claim": "alpha_bar_t randomization = alpha_bar_t ~ Unif(hat_alpha_t, hat_alpha_{t-1}) (Randomized learning rate schedule: each alpha_bar_t is drawn uniformly between consecutive hat_alpha values)", + "source": "Section 2.2, Eq (9)" + }, + { + "id": "diffusion-convergence-rate-D1-005", + "claim": "Schedule ratio identity = (hat_tau_{k,n-1} - hat_tau_{k,n}) / (hat_tau_{k,n-1} * (1 - hat_tau_{k,n-1})) = c_1 * log T / T (Key schedule property for discretization analysis; establishes uniform step-size ratio)", + "source": "Section 4, Lemma 7, Eq (50)" + }, + { + "id": "diffusion-convergence-rate-D1-006", + "claim": "K (number of rounds) = c_2 * min{d * log^2 T, L * log T}, where c_2 > 0 (Number of outer rounds; balances error propagation; bounded even when L = infinity)", + "source": "Section 3.2, Theorem 1" + }, + { + "id": "diffusion-convergence-rate-D1-007", + "claim": "N (steps per round) = 2T / K (Number of inner steps per round; total iteration complexity is KN = 2T)", + "source": "Section 2.2" + }, + { + "id": "diffusion-convergence-rate-D1-008", + "claim": "T (total discretization steps) = KN / 2 (Each round has N steps, each requiring 1 new score evaluation; total score evaluations = 2T)", + "source": "Section 2.2" + }, + { + "id": "diffusion-convergence-rate-D1-009", + "claim": "N (parallel processors) = (min{d^{2/3} * L^{-2/3}, d^{1/3}} + 1) * log^{5/3} T / epsilon^{2/3} (Required number of parallel processors to achieve epsilon-accuracy in TV distance)", + "source": "Section 3.3, Theorem 2" + }, + { + "id": "diffusion-convergence-rate-D1-010", + "claim": "MK (total parallel rounds) = min{d * log T, L} * log^2 T (Required total parallel rounds; achieves O(min{L, d} log^2(Ld/epsilon)) parallel rounds)", + "source": "Section 3.3, Theorem 2" + }, + { + "id": "diffusion-convergence-rate-D1-011", + "claim": "M (parallel iterations per round) = >= c * log T, where c is a constant (Inner iterations per parallel round; derived from M * log(N * min{d log T, L} * log T / T) requirement in parallel analysis)", + "source": "Appendix E.2" + }, + { + "id": "diffusion-convergence-rate-D1-012", + "claim": "epsilon_score^2 for parallel sampler = <= epsilon^2 * log^{-1} T (Score estimation error requirement for the parallel sampler to achieve epsilon-accuracy)", + "source": "Section 3.3, Theorem 2" + }, + { + "id": "diffusion-convergence-rate-D1-013", + "claim": "Second moment bound = E[||X_0||^2] < T^{c_R}, where c_R > 0 is arbitrarily large (Bounded second moment assumption; excludes extremely heavy-tailed distributions decaying slower than 1/x^3)", + "source": "Section 3.1, Assumption 1" + }, + { + "id": "diffusion-convergence-rate-D1-014", + "claim": "c_0 lower bound = c_0 >= max{c_R + 10, 10} (Ensures initialization is close enough to Gaussian; ensures KL divergence between p_{X_0} and p_{Y_0} is negligible (<= 1/T^{10}))", + "source": "Section 4, Lemma 5 proof; Appendix D.3, Eq (88)" + }, + { + "id": "diffusion-convergence-rate-D1-015", + "claim": "Definition 2 neighborhood radius = C * sqrt(d * (1 - alpha_bar_t) * log T) / L (Radius within which the non-uniform Lipschitz condition must hold with high probability)", + "source": "Section 3.1, Definition 2" + }, + { + "id": "diffusion-convergence-rate-D1-016", + "claim": "Definition 2 probability threshold = >= 1 - c / (T + d)^4, where c is a universal constant (The non-uniform Lipschitz condition must hold with this high probability (over x ~ X_t))", + "source": "Section 3.1, Definition 2" + }, + { + "id": "diffusion-convergence-rate-D1-017", + "claim": "GMM Lipschitz constant bound = L <= C_1 * log(H * (T + d)), where H is number of Gaussian components (For Gaussian mixture models, the non-uniform Lipschitz constant scales only logarithmically with components H, dimension d, and iterations T)", + "source": "Section 3.1, Example 2" + }, + { + "id": "diffusion-convergence-rate-D1-018", + "claim": "Typical set threshold theta = theta >= c_R + 10 (Parameter controlling the typical set S_tau = {x: -log p_{X_tau}(x) <= theta * d * log T}; must be large enough to ensure P(X_tau in S_tau^c) <= 1/T^4)", + "source": "Section 4.1, Eq (21); Appendix D.1, Eq (67) following" + }, + { + "id": "diffusion-convergence-rate-D1-019", + "claim": "epsilon_score^2 definition = (1/T) * sum_{k=0}^{K-1} sum_{n=0}^{N-1} E[||s_{T - kN/2 - n + 1}(Y_{k,n}) - s*_{T - kN/2 - n + 1}(Y_{k,n})||^2] (Averaged l2 score estimation error over all steps; treats score matching as a black box; denoted as sum of epsilon_{k,n}^2 over T)", + "source": "Section 3.1, Assumption 2, Eq (12)" + }, + { + "id": "diffusion-convergence-rate-D1-020", + "claim": "Experiment configurations = [{'d': 10, 'k': 10}, {'d': 100, 'k': 10}, {'d': 500, 'k': 100}] (Three configurations: d-dimensional Gaussian target with zero mean, diagonal covariance (first k entries uniform in [0,10], remaining d-k entries zero). K = 10 rounds, N = 2T/K. Exact score functions used (no estimation error).)", + "source": "Appendix A, Figure 2" + }, + { + "id": "diffusion-convergence-rate-D1-021", + "claim": "K for numerical experiments = 10 (Fixed number of rounds K = 10 across all experiment configurations)", + "source": "Appendix A" + }, + { + "id": "diffusion-convergence-rate-D1-022", + "claim": "Diagonal covariance entry range = [0, 10] (uniform distribution) (First k diagonal entries of the Gaussian covariance matrix are uniformly sampled from [0, 10]; remaining d-k entries are zero)", + "source": "Appendix A" + } + ], + "D2": [ + { + "id": "diffusion-convergence-rate-D2-001", + "claim": "Forward process step: X_t = sqrt(alpha_t) * X_{t-1} + sqrt(1 - alpha_t) * W_t, where W_t ~ N(0, I_d) is independent Gaussian noise, alpha_t in (0,1) is the step size, and t = 1,...,T. The cumulative product is defined as alpha_bar_t := product_{k=1}^{t} alpha_k, giving X_t = sqrt(alpha_bar_t) * X_0 + sqrt(1 - alpha_bar_t) * Wbar_t where Wbar_t ~ N(0, I_d).", + "source": "Section 2.1, Eq (3)" + }, + { + "id": "diffusion-convergence-rate-D2-002", + "claim": "Learning rate schedule construction (discrete computable): (1) Set initial point hat_alpha_{T+1} = 1 / T^{c_0} for a sufficiently large constant c_0 >= max{c_R+10, 10}; (2) Iteratively update hat_alpha_{t-1} = hat_alpha_t + c_1 * hat_alpha_t * (1 - hat_alpha_t) * log T / T for t = -N/2+1, ..., T+1, with c_1 > 0 and c_1/c_0 sufficiently large; (3) Randomized schedule: draw alpha_bar_t ~ Unif(hat_alpha_t, hat_alpha_{t-1}) for each t, where hat_alpha_t acts as the deterministic grid. The interval (hat_alpha_t, hat_alpha_{t-1}) defines the discretization subinterval.", + "source": "Section 2.2, Eq (8)-(9)" + }, + { + "id": "diffusion-convergence-rate-D2-003", + "claim": "Schedule initialization point: hat_alpha_{T+1} = 1 / T^{c_0}. Schedule update step formula: hat_alpha_{t-1} = hat_alpha_t + c_1 * hat_alpha_t * (1 - hat_alpha_t) * log T / T. The constants c_0 and c_1 are sufficiently large positive constants with c_1/c_0 also sufficiently large. This defines a deterministic grid of points from which the randomized schedule is drawn.", + "source": "Section 2.2, Eq (8)" + }, + { + "id": "diffusion-convergence-rate-D2-004", + "claim": "Score function definition (discrete-time): s_t*(x) = ∇log p_{X_t}(x) = -1/(1 - alpha_bar_t) * ∫ p_{X_0|X_t}(x_0|x) * (x - sqrt(alpha_bar_t) * x_0) dx_0. Continuous-index variant: s_tau*(x) = ∇log p_{X_tau}(x) = -1/tau * ∫ p_{X_0|X_tau}(x_0|x) * (x - sqrt(1 - tau) * x_0) dx_0, where X_tau = sqrt(1-tau) * X_0 + sqrt(tau) * Z with Z ~ N(0, I_d). Relationship: s_t*(·) = s_{1-alpha_bar_t}*(·).", + "source": "Section 2.1, Definition 1, Eq (6)-(7)" + }, + { + "id": "diffusion-convergence-rate-D2-005", + "claim": "Sampling update equation (the core discrete sampler step, Eq 10). For round k, step n (1 <= n <= N), compute Y_{k,n} from Y_{k,0} using: Y_{k,n} / sqrt(1 - tau_{k,n}) = Y_{k,0} / sqrt(1 - tau_{k,0}) + [s_{T - kN/2 + 1}(Y_{k,0}) / (2 * (1 - tau_{k,0})^{3/2})] * (tau_{k,0} - hat_tau_{k,0}) + sum_{i=1}^{n-1} [s_{T - kN/2 - i + 1}(Y_{k,i}) / (2 * (1 - tau_{k,i})^{3/2})] * (hat_tau_{k,i-1} - hat_tau_{k,i}) + [s_{T - kN/2 - n + 2}(Y_{k,n-1}) / (2 * (1 - tau_{k,n-1})^{3/2})] * (hat_tau_{k,n-1} - tau_{k,n}). Here tau_{k,n} ~ Unif(hat_tau_{k,n}, hat_tau_{k,n-1}), and s_{...} are estimated score functions. Each step uses exactly one new score evaluation.", + "source": "Section 2.2, Eq (10)" + }, + { + "id": "diffusion-convergence-rate-D2-006", + "claim": "Noise injection between rounds (Eq 11): After completing N steps in round k, apply Y_{k+1} = sqrt((1 - tau_{k+1,0}) / (1 - tau_{k,N})) * Y_{k,N} + sqrt((tau_{k+1,0} - tau_{k,N}) / (1 - tau_{k,N})) * Z_k, where Z_k ~ N(0, I_d) is independent Gaussian noise. This converts the TV distance between reverse and forward processes into l2 estimation error and ensures the new starting point Y_{k+1} has the correct variance for the next round.", + "source": "Section 2.2, Eq (11)" + }, + { + "id": "diffusion-convergence-rate-D2-007", + "claim": "Total variation distance evaluation metric: TV(q_K, p_{Y_K}) = (1/2) * ∫ |p_{Y_K}(x) - q_K(x)| dx, where q_K is the distribution of X_{tau_{K,0}} (forward process at round K start) and p_{Y_K} is the distribution of the sampler output Y_K. The sampler is initialized from Y_0 ~ N(0, I_d) (pure Gaussian noise). The goal is to bound TV(q_K, p_{Y_K}) <= epsilon.", + "source": "Section 2.2, Eq (12)" + }, + { + "id": "diffusion-convergence-rate-D2-008", + "claim": "Time-index mapping construction: Given the schedule hat_alpha_t and randomized alpha_bar_t, define hat_tau_{k,n} := 1 - hat_alpha_{T - kN/2 - n} and tau_{k,n} := 1 - alpha_bar_{T - kN/2 - n + 1} for n = -1, ..., N. The randomized tau_{k,n} ~ Unif(hat_tau_{k,n}, hat_tau_{k,n-1}) provides the discretization of continuous time tau in (0,1). The initialization uses hat_tau_{k,0} (deterministic) and tau_{k,0} (randomized), while intermediate steps use hat_tau (deterministic grid points) and the final term uses tau_{k,n} (randomized endpoint).", + "source": "Section 4.3, Eq (33)" + }, + { + "id": "diffusion-convergence-rate-D2-009", + "claim": "Score estimation error computation (averaged over all steps): epsilon_score^2 = (1/T) * sum_{k=0}^{K-1} sum_{n=0}^{N-1} E_{Y_k ~ q_k}[||s_{T - kN/2 - n + 1}(Y_{k,n}) - s*_{T - kN/2 - n + 1}(Y_{k,n})||^2] =: (1/T) * sum_{k,n} epsilon_{k,n}^2. This treats score matching as a black box (s_t is the estimated score, s_t* is the true score). Each epsilon_{k,n}^2 is the per-step squared l2 error.", + "source": "Section 3.1, Assumption 2, Eq (12)" + }, + { + "id": "diffusion-convergence-rate-D2-010", + "claim": "Non-uniform Lipschitz condition (Definition 2): Let L be the smallest quantity (may depend on T and d) such that P_{x ~ X_t}{(1 - alpha_bar_t) * ||s_t*(x') - s_t*(x)||_2 <= L * ||x' - x||_2, for all ||x' - x||_2 <= C * sqrt(d * (1 - alpha_bar_t) * log T) / L} >= 1 - c / (T + d)^4, where C and c are universal constants. This relaxes the uniform Lipschitz condition by requiring it only within a neighborhood of radius proportional to sqrt(d * (1 - alpha_bar_t) * log T) / L and with probability 1 - c/(T+d)^4 rather than everywhere.", + "source": "Section 3.1, Definition 2" + }, + { + "id": "diffusion-convergence-rate-D2-011", + "claim": "GMM Lipschitz constant computable bound: For a Gaussian mixture model X_0 ~ sum_{h=1}^{H} gamma_h * N(mu_h, sigma^2 * I_d) with sigma >= 0, the non-uniform Lipschitz constant satisfies L <= C_1 * log(H * (T + d)) for some universal constant C_1. This means L scales only logarithmically with the number of components H, dimension d, and iterations T. In contrast, the uniform Lipschitz constant for GMMs can be as large as (1 - alpha_bar_t) * ||mu||_2^2 / (4 * (1 - alpha_bar_t + sigma^2)^2) when sigma is small, which can be on the order of d.", + "source": "Section 3.1, Example 2; Appendix C.2, Eq (C.2)" + }, + { + "id": "diffusion-convergence-rate-D2-012", + "claim": "Parallel sampling algorithm structure (Appendix E.1): (1) Round structure: K rounds, each with M * N parallel iterations; (2) Each round k: compute initial direction v_{k,0} = Y_{k,0} / sqrt(1 - tau_{k,0}); (3) Inner loop m = 1..M: for n = 0..N-1, compute estimated y_{k,n}^{(m)} using the discrete sampler with estimated score functions, accumulate updates; (4) After M iterations, apply noise injection to get Y_{k+1}; (5) Use parallel processors to compute the N intermediate points simultaneously per inner iteration. The parallel sampler achieves epsilon-accuracy with N processors ~ (min{d^{2/3} * L^{-2/3}, d^{1/3}} + 1) * log^{5/3} T / epsilon^{2/3} and MK rounds ~ min{d * log T, L} * log^2 T.", + "source": "Appendix E.1, Parallel sampling algorithm" + } + ], + "D3": [ + { + "id": "diffusion-convergence-rate-D3-001", + "claim": "Validate theoretical convergence rate predictions: Verify that the randomized midpoint discretization sampler (Section 2.2, Eq 10) achieves the predicted KL divergence convergence rate of O(log^4 T / T^3), which implies a TV distance rate of O(log^2 T / T^{3/2}). This is a sanity check that the theoretical analysis is consistent with empirical behavior.. Datasets: Synthetic d-dimensional Gaussian. Baselines: {'method': 'Theoretical convergence rate curve', 'description': 'The theoretical prediction O(log^4 T / T^3) in KL divergence. This numerical validation does not compare against any empirical baseline methods (e.g., other samplers) as this is a theoretical convergence-rate analysis focused on validating the analytical convergence rate rather than benchmarking against other algorithms. The theoretical rate curve serves as the sole reference for validating the empirical results.', 'type': 'theoretical_reference'}. Metrics: {'name': 'KL divergence', 'formula': 'KL(p_{Y_K} || q_K)', 'implementation': 'Closed-form computation possible because all intermediate distributions Y_{k,n} remain Gaussian when the target is Gaussian and exact score functions are used'}", + "source": "Appendix A" + }, + { + "id": "diffusion-convergence-rate-D3-002", + "claim": "Controlled experimental design for isolating convergence rate from confounds. Purpose: Ensure that any deviation between empirical and theoretical convergence rates is attributable solely to discretization error, not to score approximation error or Monte Carlo estimation noise. Setup: (1) Target distribution is a d-dimensional Gaussian with zero mean and diagonal covariance — this guarantees all intermediate distributions Y_{k,n} in the sampler remain Gaussian, enabling closed-form analytical KL divergence computation without Monte Carlo sampling. (2) Exact score functions s_t*(·) are used (not neural network estimates) — this eliminates score estimation error epsilon_score^2 from the experiment, setting it to zero by construction. (3) The sampler implementation follows Section 2.2 exactly with the randomized midpoint discretization. Metrics: KL divergence computed analytically in closed form using Gaussian distribution identities (no sampling-based estimation). Comparison: This controlled setting contrasts with real-world diffusion model deployments where pretrained score networks introduce non-zero epsilon_score^2 and Monte Carlo estimation adds variance. By zeroing out score error, the experiment isolates and validates the discretization component of the convergence theory.", + "source": "Appendix A" + }, + { + "id": "diffusion-convergence-rate-D3-003", + "claim": "Multi-configuration dimensional sensitivity analysis. Purpose: Test whether the O(log^4 T / T^3) KL convergence rate holds consistently across different dimensionality regimes and covariance ranks — validating the dimension-adaptive nature of the theoretical iteration complexity min{d, d^{2/3}L^{1/3}, d^{1/3}L}. Setup/Configurations: Three (d, k) pairs tested, where d is dimension and k is the number of active (non-zero) diagonal covariance entries, each drawn uniformly from [0, 10]: (a) d=10, k=10 — low-dimensional, full-rank covariance (all entries active); (b) d=100, k=10 — medium-dimensional, low-rank covariance (10 active, 90 zero entries); (c) d=500, k=100 — high-dimensional, medium-rank covariance (100 active, 400 zero entries). All configurations use K=10 rounds, N=2T/K steps per round, exact score functions, and run across a range of total iterations T. Metrics: Empirical KL divergence vs T plotted alongside fitted theoretical curve Θ(log^4 T / T^3), shown as three subplots in Figure 2(a)-(c). Comparison: Cross-configuration comparison — all three (d, k) pairs should exhibit the same asymptotic KL convergence rate, confirming the rate is robust to changes in dimension (spanning two orders of magnitude: 10 to 500) and covariance rank (from full-rank to low-rank). Consistency across configurations supports the theory that convergence depends on the smoother min{d, d^{2/3}L^{1/3}, d^{1/3}L} rather than d alone.", + "source": "Appendix A, Figure 2" + }, + { + "id": "diffusion-convergence-rate-D3-004", + "claim": "T-variation convergence rate fitting protocol. Purpose: Validate that the empirical KL divergence decreases following the predicted rate O(log^4 T / T^3) as the total number of iterations T varies — confirming the T-dependence of the theoretical bound is correctly captured by the numerical experiments. Setup: For each of the three (d, k) configurations, the sampler is executed across a range of T values. K=10 rounds is held fixed; N = 2T/K = T/5 steps per round varies linearly with T. As T increases, the total number of score evaluations (2T) increases, and the empirical KL divergence between Y_K and X_{tau_{K,0}} is computed at each T value. Metrics: KL(p_{Y_K} || q_K) computed in closed form at multiple T points; the empirical KL-vs-T curve (blue line in Figure 2) is plotted against a fitted rate curve (black line) of the form Θ(log^4 T / T^3). Comparison: The empirical curve is visually compared against the fitted theoretical rate to assess goodness-of-fit — the numerical results show consistency between empirical observations and theoretical predictions, confirming the sampler achieves the predicted convergence rate in practice. The same protocol is repeated independently for each of the three (d, k) configurations to verify the rate is stable across configurations.", + "source": "Appendix A, Figure 2" + } + ], + "D4": [ + { + "id": "diffusion-convergence-rate-D4-001", + "claim": "Learning schedule construction pipeline: Step 1 — Initialize hat_alpha_{T+1} = 1/T^{c_0}. Step 2 — Iteratively update hat_alpha_{t-1} = hat_alpha_t + c_1 * hat_alpha_t * (1 - hat_alpha_t) * log T / T. Step 3 — For each training step, sample alpha_bar_t ~ Unif(hat_alpha_t, hat_alpha_{t-1}). Step 4 — Map to discretization points tau_k via (hat_tau - tau) / (tau * (1 - tau)) identity.", + "source": "Section 2.2" + }, + { + "id": "diffusion-convergence-rate-D4-002", + "claim": "Sampler execution pipeline (N parallel processors, K outer rounds): Step 1 — Compute base sample Y_{k-1,N} from previous round. Step 2 — For each n in 1..N, compute in parallel: score estimate s(Y_{k-1,n}, tau_{k-1,n}) on n-th processor. Step 3 — Aggregate scores and compute Y_{k,N} via ODE step (Eq 10). Step 4 — Every k_s rounds, inject noise via Eq 11 (Y_{k+1} = sqrt(...) Y_{k,N} + sqrt(...) Z_k). Step 5 — Increment round counter k.", + "source": "Section 3.1-3.2" + }, + { + "id": "diffusion-convergence-rate-D4-003", + "claim": "Convergence proof derivation order: Step 1 — Discretization analysis using Girsanov's theorem (Section 4). Step 2 — Score estimation error decomposition into L^2 bound (Section 5.1). Step 3 — Non-uniform Lipschitz score handling via early-stopping argument (Section 5.2). Step 4 — Gaussian mixture model specific bound derivation (Section 5.3). Step 5 — Final KL divergence O~(log^4 T / T^3) bound converted to TV distance via Pinsker's inequality.", + "source": "Section 4-5, Theorem 1-3" + }, + { + "id": "diffusion-convergence-rate-D4-004", + "claim": "Numerical validation pipeline: Step 1 — Set up synthetic d-dimensional Gaussian target with diagonal covariance. Step 2 — Initialize sampler with randomly sampled alpha_bar_t schedule. Step 3 — Run sampler for varying T values. Step 4 — Compute empirical KL divergence between generated samples and ground truth via Monte Carlo. Step 5 — Plot convergence rate and compare against theoretical O(log^4 T / T^3) prediction.", + "source": "Appendix A" + } + ] +} \ No newline at end of file diff --git a/papers/emergent-planning-rl/blacklist.txt b/papers/emergent-planning-rl/blacklist.txt new file mode 100644 index 0000000000000000000000000000000000000000..dbcbb2b2ddc845df393007a33947c71dd4547e16 --- /dev/null +++ b/papers/emergent-planning-rl/blacklist.txt @@ -0,0 +1,3 @@ +# No public official repository confirmed yet (ICLR 2025 Oral) +# Author: Thomas Bush (tuphs28) +# Project page: https://tuphs28.github.io/projects/interpplanning/ diff --git a/papers/emergent-planning-rl/config.yaml b/papers/emergent-planning-rl/config.yaml new file mode 100644 index 0000000000000000000000000000000000000000..001327153c7f57399abe88a555baced5b3a75931 --- /dev/null +++ b/papers/emergent-planning-rl/config.yaml @@ -0,0 +1,8 @@ +title: "Interpreting Emergent Planning in Model-Free RL" +pdf_url: "https://arxiv.org/pdf/2504.01871.pdf" 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+1,3 @@ +version https://git-lfs.github.com/spec/v1 +oid sha256:553826cae492dea1c916710c26b358e9e7f718714160573bbc3766ce086b9b68 +size 61774 diff --git a/papers/emergent-planning-rl/paper.md b/papers/emergent-planning-rl/paper.md new file mode 100644 index 0000000000000000000000000000000000000000..c181ffb24bf289ece0f8a6607301a2dcb3305fff --- /dev/null +++ b/papers/emergent-planning-rl/paper.md @@ -0,0 +1,1353 @@ +# INTERPRETING EMERGENT PLANNING IN MODEL-FREE REINFORCEMENT LEARNING + +Thomas Bush1, Stephen $\mathbf { C h u n g ^ { \mathrm { 1 \dagger } } }$ , Usman Anwar1†, Adria Garriga-Alonso \` 2, David Krueger3 1University of Cambridge, 2FAR AI, 3Mila, University of Montreal +28tbush@gmail.com, {mhc48,ua237,dsk30}@cam.ac.uk, adria@far.ai †Equal contribution. + +# ABSTRACT + +We present the first mechanistic evidence that model-free reinforcement learning agents can learn to plan. This is achieved by applying a methodology based on concept-based interpretability to a model-free agent in Sokoban – a commonly used benchmark for studying planning. Specifically, we demonstrate that DRC, a generic model-free agent introduced by Guez et al. (2019), uses learned concept representations to internally formulate plans that both predict the long-term effects of actions on the environment and influence action selection. Our methodology involves: (1) probing for planning-relevant concepts, (2) investigating plan formation within the agent’s representations, and (3) verifying that discovered plans (in the agent’s representations) have a causal effect on the agent’s behavior through interventions. We also show that the emergence of these plans coincides with the emergence of a planning-like property: the ability to benefit from additional test-time compute. Finally, we perform a qualitative analysis of the planning algorithm learned by the agent and discover a strong resemblance to parallelized bidirectional search. Our findings advance understanding of the internal mechanisms underlying planning behavior in agents, which is important given the recent trend of emergent planning and reasoning capabilities in LLMs through RL. + +# 1 INTRODUCTION + +In reinforcement learning (RL), decision-time planning – that is, the capacity of selecting immediate actions to perform by predicting and evaluating the consequences of future actions – is conventionally associated with agents that possess explicit world models, like MuZero (Schrittwieser et al., 2020). This naturally raises the question: can model-free reinforcement learning agents – that is, agents which lack explicit world models – also learn to perform decision-time planning? + +In prior work, Guez et al. (2019) introduced Deep Repeated ConvLSTM (DRC) agents. Despite lacking an explicit world model, DRC agents behave like they perform decision-time planning. For example, they excel at strategic domains like Sokoban, and perform better if given extra testtime compute (Guez et al., 2019; Taufeeque et al., 2024). However, this only partially answers the above question as these behaviors may not be due to internal planning but, rather, other mechanisms that generate planning-like behavior in the environments studied. In this paper, we mechanistically analyze a Sokoban-playing DRC agent and show that it is indeed internally planning. In doing so, we provide the first non-behavioral evidence that model-free RL agents can learn to internally plan. + +Using concept-based interpretability (Kim et al., 2018), we provide three types of convergent evidence showing that the DRC agent has learned, and is making use of, concepts that are instrumentally useful for planning. First, we use linear probes (Alain & Bengio, 2016) to show that the agent represents specific concepts that predict the long-term effects of its actions on the environment. Then, we demonstrate that these concept representations are associated with a learned planning process by analyzing how the agent uses them to iteratively construct ‘plans’ at test-time. Finally, we demonstrate that these concept representations causally influence the agent’s behavior as would be expected if these representations were being used for planning + +To summarize, this paper makes the following contributions: + +![](images/figures/emergent-planning-rl-fig-0001.jpg) +Figure 1: Examples of the DRC agent internally forming plans to push boxes to targets. A purple arrow on a square means that a linear probe decodes that the agent plans to push a box off of that square in the associated direction. No arrow on a square means that the probe decodes that agent does not plan to push a box off of that square. (A) The agent evaluates a naively-appealing route, concludes it is infeasible, and forms a longer alternate path. (B) The agent adapts its plan and changes the target it plans to push the left-most box to. (C) The agent extends part of its plan backward from a target. (D) The agent extends part of its plan forward from a box. (E) The agent extends many parts of its plan in parallel. We provide further examples in Appendices A.2.1-A.2.5. + +• We design a procedure, based on concept-based interpretability, for determining if a modelfree agent performs planning using a hypothesized set of concepts. This procedure involves (1) probing for planning-relevant concepts, (2) investigating plan formation in the agent’s internal representations, and (3) verifying the causal effect of plans on the agent’s behavior. • Using this procedure, we show that, in Sokoban, a DRC agent (Guez et al., 2019) internally forms plans, and that these plans can be altered to steer the agent. We find this agent learns a planning algorithm resembling parallelized bidirectional search, which differs from commonly-used planning algorithms in RL. + +This work aligns with the growing body of research demonstrating that model-free RL agents can learn to plan and even reason. For example, in this study, we show that DRC agents can learn to evaluate and revise plans. Recently, DeepSeek-R1, an LLM with reasoning capabilities primarily trained via RL, has demonstrated similar self-correction behavior in its reasoning, referred to as ‘aha moments’ (Guo et al., 2025). As such, we believe that understanding the mechanisms behind these emergent capabilities in RL agents is highly important. + +# 2 BACKGROUND + +# 2.1 PLANNING IN REINFORCEMENT LEARNING + +Planning has many meanings in RL, encompassing algorithms utilizing environment models during training (Sutton, 1991) or at decision time (Silver et al., 2016; Chung et al., 2024a). In this work, we study whether an RL agent is specifically performing decision-time planning. Henceforth, we use ‘planning’ and ‘decision-time planning’ interchangeably. In past work, an agent is considered to be planning in this sense if it engages with an (explicit) world model to select actions associated with the best predicted long-term consequences (Hamrick et al., 2020; Chung et al., 2024a). An example is MuZero (Schrittwieser et al., 2020), which applies a planning algorithm called Monte Carlo Tree Search (Coulom, 2006) to a model of its environment to select actions associated with the best longrun consequences. Other similar agents are VPN (Oh et al., 2017), IBP (Pascanu et al., 2017), I2A (Racaniere et al., 2017), MCTSNet (Guez et al., 2018b), and Thinker (Chung et al., 2024a) agents. \` + +By definition, model-free RL agents lack an explicit world model. This makes it difficult to reuse past definitions of planning that presume that an explicit world model is available. Thus, for the purposes of this work, we provide a pragmatic characterization of planning that we use as a foundation for investigating whether the model-free agent studied in this paper performs planning. + +We consider plans to be sequences of potential future actions. We characterize an agent as planning if it selects actions to perform by considering plans that it formulates and evaluates based on predicted future consequences. This is similar to how planning is understood in neuroscience (Mattar & Lengyel, 2022). It also mirrors model-based definitions of planning but relaxes the requirement for an explicit world model to the requirement that an agent predict consequences of future actions, regardless of the method used. We discuss our characterization further in Appendix E.1. For an agent to plan under our characterization, it must: (i) form plans, (ii) evaluate plans by predicting their consequences, and (iii) be influenced by these plans when acting. + +# 2.2 SOKOBAN + +Sokoban is an episodic, fully-observable, deterministic environment in which an agent moves around walls in an 8x8 grid to push four boxes onto four targets. When an agent moves up/down/left/right into a square containing a box, the box is pushed up/down/left/right. Sokoban levels let agents perform actions with irreversible, negative, long-run consequences (moving boxes so the puzzle is unsolvable). Sokoban is thus difficult – it is PSPACEcomplete (Culberson, 1997) – and a common benchmark for studying planning (Racaniere et al., 2017; Guez et al., 2019; Hamrick \` et al., 2020). We study a version of Sokoban where the agent observes a symbolic representation $\boldsymbol { x } _ { t } \in \mathbb { R } ^ { 8 \times 8 \times 7 }$ of the environment. For ease of inspection, all figures are presented as pixel representations. Figure 2 compares these two representations. Appendix E.2 further explains this environment. + +![](images/figures/emergent-planning-rl-fig-0002.jpg) +Figure 2: Pixel and symbolic representations of a Sokoban board. + +# 2.3 DEEP REPEATED CONVLSTM (DRC) AGENTS + +Deep Repeated ConvLSTM (DRC) agents (Guez et al., 2019) are model-free agents based on ConvLSTMs that perform multiple computational ticks per time step. ConvLSTMs (Shi et al., 2015) are LSTMs (Hochreiter & Schmidhuber, 1997) that utilize 3D hidden states and convolutional connections. At each time step $t$ , a DRC agent passes an observation $x _ { t }$ through a convolutional encoder to generate an encoding $\dot { i } _ { t } \in \mathbb { R } ^ { H _ { 0 } \times W _ { 0 } \times G _ { 0 } }$ . This is then processed by $D$ ConvLSTM layers. At time $t$ the $d$ -th ConvLSTM has a cell state $g _ { t } ^ { d } \in \mathbb { R } ^ { H _ { d } \times W _ { d } \times \dot { G } _ { d } }$ . Unlike standard recurrent networks which perform a single tick of recurrent computation per time step, DRC agents perform $N$ ticks of recurrent computation per step. Guez et al. (2019) show these internal ticks improve the performance and generalization of DRC agents. Appendix E.3 provides further architectural details. + +DRC agents behave in a manner that suggests they internally engage in decision-time planning. For instance, DRC agents outperform model-based agents like MuZero (Schrittwieser et al., 2020) in Sokoban (Chung et al., 2024b), and exhibit improved performance when given extra test-time compute (Taufeeque et al., 2024). This raises a question: do DRC agents genuinely learn to internally perform planning, or is their planning-like behavior merely a result of complex learned heuristics? + +In this paper, we investigate whether a Sokoban-playing DRC agent internally plans. The agent we study has $D = 3$ ConvLSTM layers and performs $N = 3$ internal ticks per step. The agent’s encoder and ConvLSTMs have 32 channels ( $G _ { d } = 3 2 $ ) and utilize kernels of size 3 with a single layer of input zero padding. Thus, all cell states share Sokoban’s spatial dimensions $( H _ { d } = W _ { d } = 8 )$ . The agent is trained for 250 million transitions on the unfiltered Boxoban training set (Guez et al., 2018a) using a similar training setup as Guez et al. (2019) as explained in Appendix E.4. Appendix E.5 shows that, consistent with Guez et al. (2019), this agent exhibits planning-like behavior. + +# 2.4 CONCEPT-BASED INTERPRETABILITY + +Concept-based interpretability is an approach to explaining neural network behavior that involves identifying which concepts a network internally represents (Kim et al., 2018). A concept is generally understood as a unit of knowledge (Schut et al., 2023). In this paper, we specifically consider ‘multi-class’ concepts, which can formally be defined as mappings from input states (or parts of input states) to some fixed classes. That is, multi-class concepts correspond to interpretable, discrete features, and map inputs to classes of that concept. For instance, a multi-class Sokoban concept might be ‘the number of empty targets’. This concept would map any observed Sokoban board $x _ { t }$ to a class in {ONE, TWO, THREE, FOUR} depending on the number of remaining empty targets in $x _ { t }$ . + +We focus on concepts networks represent linearly (Mikolov et al., 2013). To check if a network linearly represents concepts, we use linear probes. These are linear classifiers trained to predict concept classes assigned to inputs using the associated network activations (Alain & Bengio, 2016). As linear classifiers, linear probes compute logits $l _ { k } = w _ { k } ^ { T } g$ for each class $k$ by projecting network activations $g \in \mathbb { R } ^ { d }$ along a class-specific vector $w _ { k } \in \mathbb { R } ^ { d }$ . Belinkov (2022) explains probes further. + +# 3 METHODOLOGY + +# 3.1 A PROCEDURE FOR INVESTIGATING MODEL-FREE PLANNING + +In Section 2.1, we characterized planning as requiring that an agent (i) formulate plans, (ii) evaluate the consequences of these plans, and (iii) be guided by these plans when selecting actions. If an agent learns to plan, we expect planning-relevant concepts to emerge in its internal representations to meet the first condition. These concepts ought to reflect the agent’s plan, and so should correspond to potential future actions, or to their likely environmental effects. Additionally, evidence of plan evaluation – such as avoiding or improving bad plans – should exist to satisfy the second condition. Lastly, to fulfill the third condition, the plan must causally influence the agent’s behavior. To determine if an agent exhibits these three properties, we follow the procedure outlined below: + +1. Probe for Concept Representations. First, we identify a group of environment-specific concepts that could be instrumentally useful for planning. We then use linear probes to establish whether these concepts are being (linearly) represented by the agent (Section 4). +2. Investigate Plan Formation. Next, we focus on gathering qualitative evidence of the agent forming plans based on the planning-relevant concepts probed for in the previous step, and evidence of the agent evaluating and refining these plans (Section 5). +3. Confirm Behavioral Dependence. Finally, we confirm that these internal plans influence the agent’s behavior. For instance, we show that the agent can be steered to form and execute desired plans by intervening on plan representations within the network (Section 6). + +# 3.2 PLANNING-RELEVANT CONCEPTS IN SOKOBAN + +To apply this procedure, we must specify concepts we expect the agent to plan with. Sokoban has a grid-based structure with localized transition dynamics, i.e., the future state of a square is determined by the current state of its neighbors. This makes spatially local concepts (i.e., concepts related to individual or connected squares) more natural for planning than spatially global concepts (i.e., representations of the whole board). We thus claim that an agent that learns to plan in Sokoban may do so by encoding concepts localized to individual squares. We call these ‘square-level’ concepts. Such concepts seem natural for DRC agents as the 3D structure of ConvLSTMs allows for spatial correspondence between the Sokoban grid and agent hidden states. We focus on multi-class squarelevel concepts which, as explained further in Appendix E.6, map grid squares to concept classes. + +We hypothesize that the agent will plan using the following square-level, multi-class concepts: + +• Agent Approach Direction $( C _ { \mathrm { { A } } } )$ : For a given square, this concept encodes whether the agent will move onto the square in the future. If so, it also encodes the direction from which the agent will move onto the square the next time the agent moves onto it. • Box Push Direction $\left( C _ { \mathrm { B } } \right)$ : For a given square, this concept encodes whether a box will be pushed off the square in the future. If so, it also encodes the direction in which the next box pushed off this square will be pushed. + +Figure 3 illustrates the classes assigned to each square of a Sokoban board by these concepts over six transitions near the end of an episode. Both concepts map each grid square of the agent’s observed + +![](images/figures/emergent-planning-rl-fig-0003.jpg) +Figure 3: Examples of the classes assigned to the squares of a Sokoban board over 6 transitions (from left to right) by the concepts ‘Agent Approach Direction’ $( C _ { \mathrm { { A } } } )$ and ‘Box Push Direction’ $\left( C _ { \mathrm { B } } \right)$ . An arrow corresponds to the assignment of the associated directional class. The lack of an arrow in a square indicates the assignment of the class NEVER. + +Sokoban board to the classes $\{ \mathtt { U P }$ , DOWN, LEFT, RIGHT, NEVER}. The directional classes correspond to the agent’s movement directions. If the next time the agent steps onto a specific square, the agent steps onto that square from the left, the concept $C _ { \mathrm { { A } } }$ would map this square to the class LEFT. If the next time the agent pushes a box off of specific square, the box is pushed to the left, the concept $C _ { \mathrm { B } }$ would map this square to the class LEFT. Finally, the class NEVER corresponds to the agent not stepping onto or pushing a box off of a square again for the remainder of the episode. + +Both concepts depend on the agent’s behavior: we can only determine the classes these concepts map grid squares to after observing the agent’s behavior over the entire episode. Furthermore, as shown in Figure 3, the classes squares are mapped to will change at every transition. Once an agent steps onto a square, the classes assigned to that square will update to represent the agent’s future interactions with that square. We investigate alternate concepts in Appendices D.4 and D.5. + +# 4 PROBING FOR CONCEPT REPRESENTATIONS + +We now perform the first step of our analysis: determining whether the agent internally represents the concepts that we hypothesize it uses to internally form and evaluate plans. + +# 4.1 EXPERIMENT DETAILS + +Specifically, we use linear probes to determine if the agent represents (a) $C _ { \mathrm { { A } } }$ , the agent’s future movement onto squares, and (b) $C _ { \mathrm { B } }$ , the future directions boxes are pushed off of squares. We train linear probes that take as input the agent’s cell state activations after the final of the three computational ticks performed each step. We train separate probes for the agent’s three layers. + +We hypothesize the agent will learn a spatial bijection between its cell state and the Sokoban grid. Thus, when predicting $C _ { \mathrm { { A } } }$ and $C _ { \mathrm { B } }$ at each location $( x , y )$ , our probes receive as input cell state activations centered on $( x , y )$ . We train both 1x1 probes (which take as input just the activations at $( x , y ) )$ and $3 { \tt X } 3$ probes (which take as input the $3 { \bf x } 3$ patch of activations around $( x , y ) )$ . These probes have 160 and 1440 parameters, so are unlikely to overfit. We consider larger probes in Appendix D.3. + +Each probe is trained using logistic regression with the AdamW optimizer, and five unique initialization seeds. The training dataset is generated by running the agent for 3000 episodes on levels from the Boxoban unfiltered training dataset (Guez et al., 2018a). We test probes on a test set of transitions generated by running the agent for 1000 episodes on levels from the Boxoban unfiltered validation dataset. Further probe training details are given in Appendix D.1. We compare the performance of all probes to baseline probes that receive the raw observation $x _ { t }$ as input. This comparison aims to assess the extent to which probes’ abilities to predict concept classes are due to these concepts being internally represented by the agent rather than the probes learning how to do so themselves. + +![](images/figures/emergent-planning-rl-fig-0004.jpg) +Figure 4: Macro F1s achieved by probes when predicting $C _ { \mathrm { { A } } }$ and $C _ { \mathrm { B } }$ using the cell state at each layer, or, for the baseline probes, using the observation. Error bars show $\pm 1$ standard deviation. + +![](images/figures/emergent-planning-rl-fig-0005.jpg) +Figure 5: Examples of internal plans computed by the agent. An internal plan corresponds to the agent’s combined square-level representations of $C _ { \mathrm { { A } } }$ and $C _ { \mathrm { B } }$ . That is, an internal plan corresponds to the classes the agent represents these concepts as mapping squares of observed boards to. These internal plans are decoded from the agent’s final layer cell state by a 1x1 probe. Teal and purple arrows respectively indicate the agent expects to next step on to, or push a box off, a square in the associated direction. No arrow indicates the agent does not plan to step onto, or push a box off, a square again. Further examples of internal plans are given in Figures 10, 11 and 12 in Appendix A.1. + +# 4.2 RESULTS + +In many Sokoban boards, the agent will never move onto, nor push a box off, a large number of squares. As a result, many squares are assigned the label NEVER for both concepts in our probing datasets, leading to class imbalance. We therefore evaluate probe performance using macro F1 scores in place of accuracy. Figure 4 shows the macro F1 scores achieved by probes trained to predict the classes assigned to Sokoban squares by $C _ { \mathrm { { A } } }$ and $C _ { \mathrm { B } }$ . The probes that predict these concepts using the agent’s cell state activations vastly outperform the baseline, implying the agent linearly represents $C _ { \mathrm { { A } } }$ and $C _ { \mathrm { B } }$ . This aligns with past work finding linear concept representations in many different networks (Nanda et al., 2023; McGrath et al., 2022; Zou et al., 2023). + +Figure 4 confirms that the agent represents square-level concepts at localized positions of its ConvLSTM cells as opposed to distributing representations across adjacent positions. This is evidenced by the minimal improvement in performance when moving from a 1x1 probe to a 3x3 probe, compared to the significant improvement in baseline performance. We thus focus on 1x1 probes for the remainder of this paper. Interestingly, Figure 4 also shows that while probes at layer 2 generally perform slightly better than probes at layer 1, there is little variation in performance across layers. This indicates that the concepts are represented across all layers. We thus hypothesize that the agent is engaged in iterative computation (Jastrzebski et al., 2018), whereby it refines plans across layers. + +# 5 INVESTIGATING PLAN FORMATION + +In this section, we now provide qualitative evidence that the agent forms plans by searching forward from the boxes and backward from the targets, and that the agent develops, evaluates, and adapts plans in parallel. In this section, we primarily focus on descriptive explanations of how the agent forms plans and the general shape of the plans. We defer more conclusive evidence – in the form of intervening on the agent’s plan formation process to steer the agent’s behavior – to the next section. + +Previously, we demonstrated that the agent encodes (at least) two planning-relevant concepts: $C _ { \mathrm { { A } } }$ and $C _ { \mathrm { B } }$ . These concepts represent predictions regarding how the agent will act when moving onto a given square in the future, and how the environment – specifically, the locations of boxes – will be affected by these actions. We thus posit that the agent’s representations of these concepts – when looked at holistically, over the entire board – will collectively constitute a plan that the agent forms and adapts. For example, in Figure 5 we visualize the agent’s representations of $C _ { \mathrm { B } }$ and $C _ { \mathrm { { A } } }$ over entire Sokoban boards, as decoded from the agent’s cell state by a 1x1 probe in different levels. Three observations can be made from Figure 5: (a) the arrows, which indicate the direction the agent expects to move or push boxes, tend to be connected and trace a path; (b) the arrows tend to connect boxes to specific targets; (c) the arrows collectively form a plan which corresponds to solving the level. In Appendix A.1 we visualize the agent’s plan across layers, and show that, while the agent’s plans often contains flaws (like the lack of one necessary arrow in Figure 5c), they usually consist of connected paths for the agent to follow and connected routes linking boxes and targets. + +A natural question then arises: how does the agent form plans? To answer this, we direct attention to Figure 1. Figure 1 visualizes the agent’s plans in terms of $C _ { \mathrm { B } }$ (e.g. the routes the agent plans to push boxes) over the initial steps (A-C) and internal ticks (D-E) of episodes. As can be seen in Figure 1, the agent forms plans iteratively. Interestingly, the agent appears to form plans iteratively by searching forward from boxes – as illustrated in Figure 1(C) – and backward from targets – as illustrated in Figure 1(D). That the agent seems to plan via bidirectional search – which is known to be especially efficient when it is applicable (Russell & Norvig, 2010) – may explain why Guez et al. (2019) found DRC agents to rival specialized planning architectures reliant on forward search. Indeed, as shown in Figure 1(E), the agent seems to utilize a form of parallelized bidirectional search whereby it extends multiple plans simultaneously. Appendices A.2.3, A.2.4 and A.2.5 respectively contain further instances of the agent appearing to utilize forward, backward, and parallel search. + +However, recall that, in Section 2.1, we characterized planning as requiring an agent to evaluate the plans it considers. Evidence suggestive of the agent evaluating plans can be seen in Figure 1(A)- (B). Figures 1(A)-(B), show examples in which the agent appears to (1) formulate a naive plan, (2) evaluate it, and then, upon realizing that it is infeasible or could be improved, (3) adapt its plan accordingly. For instance, in Figure 1(B), the agent changes the targets it plans to push different boxes towards. This is suggestive of the agent using an evaluative search algorithm when forming plans. Appendices A.2.1 and A.2.2 contain further examples of the agent seeming to evaluate plans and either plan to push a box a longer route, or change which boxes it plans to push to which targets. + +Further evidence of the agent planning via an iterative search algorithm can be seen in Figure 6. For Figure 6, we forced the agent to remain stationary for 5 steps prior to acting in 1000 episodes. These 5 ‘thinking steps’ give the agent 15 internal ticks of extra test-time compute. Figure 6 reports the macro F1 when using 1x1 probes to decode $C _ { \mathrm { { A } } }$ and $C _ { \mathrm { B } }$ from the agent’s final layer cell state at each of the 15 extra internal ticks, averaged over 1000 episodes. Clearly, the macro F1 improves with the number of ticks. Since the concepts are predictions of future behavior, we can see the predictions of our probes at any tick as being the agent’s internal plan at that tick. We can then see the corresponding macro F1 as reflecting the quality of the agent’s plan at that tick. Figure 6 shows that, as would be expected if the agent planned via an iterative search, the agent’s plans iteratively improve when given extra compute. Appendix A.3.1 shows test-time plan refinement occurs at all layers. Appendix A.3.2 provides evidence that it is a consequence of the agent searching deeper. Appendix C.2 shows that this ‘test-time plan refinement capability’ arises early in training. + +![](images/figures/emergent-planning-rl-fig-0006.jpg) +Figure 6: Macro F1 when using 1x1 probes to decode $C _ { \mathrm { { A } } }$ and $C _ { \mathrm { B } }$ from the agent’s final layer cell state at each of the additional 15 internal ticks performed by the agent when the agent is given 5 ‘thinking steps’, averaged over 1000 episodes. + +When considered with the agent’s planning-like behavior, the above evidence indicates the agent uses its representations of $C _ { \mathrm { { A } } }$ and $C _ { \mathrm { B } }$ for search-based planning. Further evidence of this is given in Appendices A.2.6-A.2.9 which show examples of the agent planning in out-of-distribution levels, such as levels in which the agent itself is not present (Appendix A.2.6), levels with additional boxes and targets (Appendix A.2.7), and levels in which walls appear and disappear (Appendices A.2.8- A.2.9). These examples suggest the agent’s ability to adapt and generalize – benefits of model-based planning Guez et al. (2019) show DRC agents possess – relate to its representations of $C _ { \mathrm { { A } } }$ and $C _ { \mathrm { B } }$ . + +
Layer 1Layer 2Layer 3
Trained (%)Random (%)Trained (%)Random (%)Trained (%)Random (%)
AS94.6 (±0.5)33.7 (±32.7)90.1 (±1.9)29.8 (±36.8)98.8 (±0.0)27.8 (±37.9)
BS56.2 (±1.4)31.5 (±13.9)72.7 (±1.1)30.9 (±25.8)80.6 (±2.4)4.1 (±5.4)
+ +Table 1: Success rates $( \% )$ when intervening on each layer using representations from trained and randomly initialized probes. AS and BS refer to ‘Agent-Shortcut’ and ‘Box-Shortcut’ interventions. Success rates are averaged over 5 interventions performed. We report $\pm 1$ standard deviations. + +# 6 INVESTIGATING THE ROLE OF PLANS + +So far, we have shown that the DRC agent represents $C _ { \mathrm { { A } } }$ and $C _ { \mathrm { B } }$ (Section 4), and that it uses these representations to form internal plans (Section 5). We now conclude our analysis by showing that these representations are causally responsible for the agent’s behavior. Specifically, we: (1) use these representations to intervene on the agent to force it to form and execute specific plans, and (2) show that these representations emerge concurrently with planning-like behavior during training. + +# 6.1 INTERVENING ON AGENT PLANS + +First, we show we can intervene on the agent’s activations to alter its behavior over entire episodes. Our interventions involve adding concept vectors learned by probes to the agent’s activations to force it to represent concepts in specific ways. We then observe the causal effect of our interventions on the agent’s behavior. Recall that a 1x1 probe projects activations along a vector $w _ { k } \in \mathbb { R } ^ { 3 2 }$ to compute a logit for class $k$ of some multi-class concept $C$ . We thus encourage the agent to represent square $( x , y )$ as class $k$ for concept $C$ by adding $w _ { k }$ to position $( x , y )$ of the agent’s cell state $g _ { x , y }$ : + +$$ +g _ { x , y } ^ { \prime } g _ { x , y } + w _ { k } +$$ + +If the agent indeed uses $C _ { \mathrm { { A } } }$ and $C _ { \mathrm { B } }$ for planning, altering the agent’s square-level representations of these concepts ought to modify its internal plan and, subsequently, its long-term behavior. + +We intervene in two sets of handcrafted levels: ‘Agent-Shortcut’ and ‘Box-Shortcut’ levels. These sets of levels are characterized by, in each level, there existing two plans: a short plan and a long plan. The plans are similar, but differ in lengths. The agent by default follows the optimal (short) plan. We show our interventions cause it to instead form and execute the suboptimal (long) plan. + +In ‘Agent-Shortcut’ levels all boxes and targets are in one region of the board, and the agent can follow either a long or short path to this region. In these levels, we intervene using vectors learned by probes trained to predict $C _ { \mathrm { { A } } }$ to steer the agent to plan to move along the long path. Our intervention consists of two parts. We add the vector for NEVER to cell state positions on the short path. We call this the ‘short-route’ intervention. We also add the vector for the direction which would lead the agent to move onto the first square of the long path to the appropriate cell state position. We call this the ‘directional’ intervention. An Agent-Shortcut intervention is illustrated in Figure 7b. + +‘Box-Shortcut’ levels are specially-designed levels in which three boxes are adjacent to targets and a fourth box is not. The final box can be pushed a long or short route to a target. In these levels, we intervene using vectors learned by probes trained to predict $C _ { \mathrm { B } }$ to steer the agent to push this box the long route. Our intervention again consists of two parts. We add the vector for NEVER to cell positions on the short route We also add the directional representation which would encourage the agent to push the box the longer route to the box’s initial position. We again call these the ‘short-route’ and ‘directional’ interventions. A Box-Shortcut intervention is illustrated in Figure 8b. + +We intervene on 200 levels of each type. We created 25 levels of each type and then generated 8 versions of each level by applying vertical reflection and $9 0 ^ { \circ }$ , $1 8 0 ^ { \circ }$ , and $2 7 0 ^ { \circ }$ rotations. In all levels, we repeat the ‘short-route’ intervention every step but repeat the ‘directional’ intervention only until the agent moves onto, or pushes the box off, the corresponding square. + +We perform our interventions on the agent’s cell state at each layer. An intervention is considered successful if it causes the agent to solve the level in the desired suboptimal way. As a baseline, we intervene using representations from randomly initialized probes. For comparability, we scale random probe representations so that the norms of both the random and trained probes are similar. Success rates are averaged over interventions performed with five independently trained or initialized probes. + +![](images/figures/emergent-planning-rl-fig-0007.jpg) +Figure 7: An Agent-Shortcut intervention and its effect on the agent’s plan as formulated in terms of $C _ { \mathrm { { A } } }$ : (a) the agent’s plan after 4 steps without the intervention, (b) the initial state of the level and the intervention, and (c) the agent’s plan after 4 steps with the intervention. The ‘short-route’ intervention adds the representation of NEVER for $C _ { \mathrm { { A } } }$ to positions with white crosses. The ‘directional’ intervention adds the representation of DOWN for $C _ { \mathrm { { A } } }$ to the position with the white arrow. + +![](images/figures/emergent-planning-rl-fig-0008.jpg) +Figure 8: A Box-Shortcut intervention and its effect on the agent’s plan as formulated in terms of $C _ { \mathrm { B } }$ : (a) the agent’s plan after 4 steps without the intervention, (b) the initial state of the level and the intervention, and (c) the agent’s plan after 4 steps with the intervention. The ‘short-route’ intervention adds the representation of NEVER for $C _ { \mathrm { B } }$ to positions with white crosses. The ‘directional’ intervention adds the representation of RIGHT for $C _ { \mathrm { B } }$ to the position with the white arrow. + +Table 1 shows intervention success rates. At all layers, Agent-Shortcut interventions are successful. While the success rate of Box-Shortcut interventions is lower, it remains high relative to the baseline of interventions using random probes. These results indicate that the agent’s representations of $C _ { \mathrm { { A } } }$ and $C _ { \mathrm { B } }$ influence its behavior in the way that would be expected if it used them for planning. Figures 7 and 8 provide examples of the effect of interventions on the agent’s internal plans. These examples suggest the agent not only behaves differently following the interventions, but does so due to forming a different plan. We show more examples of interventions altering the agent’s internal plans in Appendix B.1. Appendix B.2 reports success rates when using an intervention scaling factor and varying the squares intervened on. Appendix B.3 reports success rates when intervening to encourage optimal behavior in levels which the agent by default cannot solve. These extra experiments further indicate that the agent’s representations of $C _ { \mathrm { { A } } }$ and $C _ { \mathrm { B } }$ influence its behavior as expected. + +![](images/figures/emergent-planning-rl-fig-0009.jpg) +Figure 9: The relationship between the percentage of extra levels, of medium difficulty, solved when an agent is given 5 steps to ‘think’, and macro F1 score of probes when predicting $C _ { \mathrm { { A } } }$ (blue) and $C _ { \mathrm { B } }$ (orange) from the agent’s final layer cell state. Each point corresponds to these quantities calculated for a single checkpoint. + +# 6.2 INVESTIGATING THE EMERGENCE OF PLANNING DURING TRAINING + +Finally, we show that the emergence of the agent’s representations of $C _ { \mathrm { { A } } }$ and $C _ { \mathrm { B } }$ during training coincides with the agent beginning to exhibit planning-like behavior. This indicates that the agent indeed uses its representations of $C _ { \mathrm { { A } } }$ and $C _ { \mathrm { B } }$ for planning. Specifically, we show the emergence of these representations coincides with the emergence of the agent’s ability to benefit from extra testtime compute (Guez et al., 2019; Taufeeque et al., 2024). In particular, we collect checkpoints every 1 million transitions for the first 50 million transitions of training. For every checkpoint, we measure two quantities: (i) the macro F1 score of 1x1 probes trained to decode the concepts $C _ { \mathrm { { A } } }$ and $C _ { \mathrm { B } }$ given the agent’s cell state (following the procedure described in Section 4.1), and (ii) the number of additional levels out of 1000 medium-difficulty levels from the Boxoban dataset (Guez et al., 2018a) the agent can solve when given extra test-time compute by forcing the agent to remain stationary for the first 5 steps of an episode. Figure 9 plots these quantities against each other and shows a strong correlation between them. This implies the agent only reliably begins to exhibit planninglike behavior – benefiting from extra test-time compute – once its final layer representations of $C _ { \mathrm { { A } } }$ and $C _ { \mathrm { B } }$ are sufficiently formed. Appendix C.3 shows that this holds for its representations of $C _ { \mathrm { { A } } }$ and $C _ { \mathrm { B } }$ at all layers. Appendix C.4 shows the agent begins to perform better with extra compute at a similar point in training as to when it can use this compute to refine its plans. + +# 7 ADDITIONAL RESULTS + +In the Appendix, we include interesting results that we lacked space to include in the main text. Appendices F provides evidence of DRC agents planning both without internal ticks, and with additional internal ticks. Appendix $_ \mathrm { H }$ provides evidence of a DRC agents planning in a different environment: Mini PacMan. Finally, Appendix G provides evidence of a ResNet (He et al., 2016) agent planning in Sokoban. However, the question of whether a generic agent can learn to plan in a generic environment remains unanswered. + +# 8 RELATED WORK + +Past work has investigated concept representations learned by game-playing agents (Schut et al., 2023; McGrath et al., 2022; Hammersborg & Strumke, 2022; 2023; Lovering et al., 2022; Mini ¨ et al., 2023) and language models (Li et al., 2023; Nanda et al., 2023; Karvonen, 2024; Ivanitskiy et al., 2024). While past work has focused primarily on whether networks internally represent specific concepts, we study concept representations for the broader purpose of determining if an agent possesses a capability - planning. An exception is work by Jenner et al. (2024), which finds evidence of look-ahead in a chess-playing agent, but does not investigate a wider capacity to ‘plan’. + +Concept-based interpretability is not the only approach to interpreting agents. An alternative is attribution-based interpretability. This involves determining – usually via saliency maps – which features in an agent’s observation influence its behavior (Weitkamp et al., 2019; Iyer et al., 2018; Puri et al., 2020; Greydanus et al., 2018; Hilton et al., 2020). Attribution-based methods were not used here as they can depend on subjective interpretation (Atrey et al., 2020). Another approach, examplebased interpretability, explains agent behavior by providing examples of illustrative trajectories or transitions (Rupprecht et al., 2020; Sequeira & Gervasio, 2020; Deshmukh et al., 2023; Zahavy et al., 2016). Due to not studying model internals, example-based methods were ill-suited for this paper. + +Finally, this paper contributes to recent work investigating the emergence of reasoning capabilities in neural networks (Wei et al., 2022; Kojima et al., 2022; Lehnert et al., 2024; Nye et al., 2021; Wang et al., 2024). However, unlike this paper in which we provide evidence of an agent internally performing planning, most work thus far has focused on providing behavioral evidence of reasoning. An exception to this is work by Brinkmann et al. (2024) in which an algorithm learned by a transformer trained on a simple symbolic reasoning task is reverse-engineered. However, Brinkmann et al. (2024) focus on a much simpler form of reasoning than planning as considered in this paper. + +# 9 FUTURE WORK + +In this paper, we proposed a methodology for investigating model-free planning and used it to provide the first non-behavioral evidence of learned planning in a model-free agent. Future work may extend our investigation to other RL agents, and other environments. In particular, it would be helpful to better understand the role of different training factors, e.g., model architecture, environment dynamics in the emergence of planning. + +# ACKNOWLEDGMENTS + +We are thankful to Erik Jenner and Joschka Braun for providing thoughtful feedback on the draft. For much of the duration of this work, TB was supported by the Cambridge Trust and Good Ventures Foundation. UA was supported by OpenPhil AI Fellowship and Vitalik Buterin Fellowship in AI Existential Safety. This work was performed using resources provided by the Cambridge Service for Data Driven Discovery (CSD3) operated by the University of Cambridge Research Computing Service (www.csd3.cam.ac.uk), provided by Dell EMC and Intel using Tier-2 funding from the Engineering and Physical Sciences Research Council (capital grant EP/T022159/1), and DiRAC funding from the Science and Technology Facilities Council (www.dirac.ac.uk). + +# REFERENCES + +Guillaume Alain and Yoshua Bengio. 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Representation engineering: A top-down approach to ai transparency. arXiv preprint arXiv:2310.01405, 2023. + +# Appendix + +# Table of Contents + +# A Additional Investigations of Internal Planning 17 + +A.1 Further Examples of Internal Plans . 17 +A.2 Further Examples of Internal Plan Formation 21 +A.3 Further Results Regarding Iterative Plan Refinement . 35 + +# B Additional Intervention Results 39 + +B.1 Additional Examples of Interventions . . . 39 +B.2 Additional Intervention Experiments: Further Agent-Shortcut and Box-Shortcut Intervention Experiments . . 42 +B.3 Additional Intervention Experiments: Intervening in a New Set of Levels To Encourage Optimal Behavior . 48 + +# C Additional Training-Time Interpretability Results 49 + +C.1 Investigating the Emergence of Planning-Relevant Concept Representations During Training . 51 +C.2 Investigating The Emergence of Test-Time Plan Refinement During Training . . 51 +C.3 Investigating the Co-Emergence of Planning-Relevant Concept Representations and Planning-Like Behavior During Training 51 +C.4 Investigating the Co-Emergence of Test-Time Plan Refinement and Planning-Like Behavior During Training . 53 + +# D Additional Probing Results 54 + +D.1 Probe Training Details 5 4 +D.2 Additional Probing Metrics . 5 5 +D.3 Probing Using Larger Probes . 5 8 +D.4 Probing For Alternative Square-Level Concepts 58 +D.5 Probing For Future Actions . 6 0 + +# E Additional Background Material 61 + +E.1 Decision-Time Planning 62 +E.2 Sokoban 63 +E.3 Deep Repeated ConvLSTM (DRC) Agent Architecture 63 +E.4 DRC Agent Training Details 6 5 +E.5 Behavioral Evidence of Planning Exhibited By The DRC Agent 65 +E.6 Operationalizing Concepts 6 6 +E.7 Application of Methodology to Other Model-Free Architectures 66 + +# F Investigating Planning in DRC Agents of Different Sizes 6 7 + +F.1 Investigating Planning in a DRC(1,9) Agent 67 +F.2 Investigating Planning in a DRC(9,1) Agent 69 + +G Investigating Planning in a Different Architecture: ResNet 7 0 + +# Investigating Planning in a Different Environment: Mini Pacman + +H.1 Mini PacMan 7 6 +H.2 Preliminary Probing Results 7 7 + +# A ADDITIONAL INVESTIGATIONS OF INTERNAL PLANNING + +In Section 5, we provide evidence suggestive of the agent possessing a search-based internal planning mechanism. In this section, we now provide further complementary evidence regarding the agent’s internal planning procedure. This section proceeds as follows: + +• Appendix A.1 provides further examples of the agent’s internal plan at all layers. +• Appendix A.2 provides additional examples of the agent forming plans in a manner suggestive of a search-based planning algorithm. +• Appendix A.3 provides additional investigations of the agent’s ability to use extra test-time compute to improve its plans. + +# A.1 FURTHER EXAMPLES OF INTERNAL PLANS + +In Figure 5 we provided examples of ‘internal plans’ formulated by the agent. We understood the agent’s internal plans to consist of its internal representations, for each square of its observed Sokoban board, of $C _ { \mathrm { { A } } }$ and $C _ { \mathrm { B } }$ . In Figure 5, all internal plans were decoded from the agent’s final layer cell state. In this section we now provide additional examples of internal plans formulated by the agent as decoded from its cell state at each layer. + +Figures 10, 11 and 12 show internal plans decoded from the agent’s first, second, and third layer cell states in many different levels. Specifically, Figure 10 shows the agent’s internal plans at each layer at six transitions where the agent’s internal plan as decoded from its first-layer cell state corresponds to a complete plan to solve the respective level. Similarly, Figures 11 and 12 show the agent’s internal plan at each layer at six transitions where the agent’s internal plan as respectively decoded from its second- and third-layer cell state correspond to a complete plan to solve the respective levels. We note that the observations we made regarding Figure 5 likewise hold here. That is, (1) the arrows tend to form connected paths, (2) the agent’s plans tend to connect specific boxes to specific targets, and (3) the agent often forms complete plans to solve levels very early on in episodes. + +Note, however, that the agent’s plans in Figures 10, 11 and 12 often contain mistakes. This is despite the illustrated transitions being selected such that the agent’s plan is correct in at least one layer. A few things can be noted about these mistakes. First, the agent’s plans for box movements contain, on average, far fewer mistakes than the agent’s plans for its own movements. Second, the mistakes in the agent’s plan for its own movements are usually minor and consist of e.g. a few arrows being wrong, but the overall ‘shape’ of the plan being correct. Third, the agent’s mistakes when planning its own movements in the examples tend to be mistakes regarding how it can move when not pushing boxes. We think these observations suggest that the agent is primarily planning by constructing plans in terms of $C _ { \mathrm { B } }$ connecting boxes and targets, and then augmenting these plans with planned agent movements where needed. + +At a high-level, we suspect that mistakes in the agent’s plan are best seen as relating to intermediate steps of the agent’s internal planning process. First, this is because many mistakes seems to be plans that the agent considers on its way to arriving at its final plan. This is because mistakes are almost always fixed in future transitions. Second, some mistakes seem to be temporarily added to the agent’s otherwise-correct plan at specific layers. We believe these mistakes potentially relate to the fact that, as part of its planning process, the agent sometimes considers variations on its plan. + +![](images/figures/emergent-planning-rl-fig-0010.jpg) +Figure 10: Internal plans computed by the agent in 6 examples levels as decoded from the agent’s first layer cell state (10a, 10d, $1 0 \mathrm { g }$ , 10j, $1 0 \mathrm { m }$ , and 10p), second layer cell state (10b, 10e, 10h, 10k, 10n, and 10q) or third layer cell state (10c, 10f, 10i, 10l, 10o, and 10r) by a 1x1 probe. Teal arrows denote squares that the agent expects to next step onto from the associated direction. Blue arrows denote squares that the agent expects to push a box off in the corresponding direction. These examples are taken from the first transition of the respective episode in which the agent’s plan as decoded from its first-layer cell state corresponds to a complete plan to solve the level. + +![](images/figures/emergent-planning-rl-fig-0011.jpg) +Figure 11: Internal plans computed by the agent in 6 examples levels as decoded from the agent’s first layer cell state (11a, 11d, 11g, 11j, 11m, and 11p), second layer cell state (11b, 11e, 11h, 11k, 11n, and 11q) or third layer cell state (11c, 11f, 11i, 11l, 11o, and 11r) by a 1x1 probe. Teal arrows denote squares that the agent expects to next step onto from the associated direction. Blue arrows denote squares that the agent expects to push a box off in the corresponding direction. These examples are taken from the first transition of the respective episode in which the agent’s plan as decoded from its second-layer cell state corresponds to a complete plan to solve the level. + +![](images/figures/emergent-planning-rl-fig-0012.jpg) +Figure 12: Internal plans computed by the agent in 6 examples levels as decoded from the agent’s first layer cell state (12a, 12d, 12g, 12j, $1 2 \mathrm { m }$ , and $1 2 \mathrm { p }$ ), second layer cell state (12b, 12e, 12h, 12k, 12n, and 12q) or third layer cell state (12c, 12f, 12i, 12l, 12o, and 12r) by a 1x1 probe. Teal arrows denote squares that the agent expects to next step onto from the associated direction. Blue arrows denote squares that the agent expects to push a box off in the corresponding direction. These examples are taken from the first transition of the respective episode in which the agent’s plan as decoded from its final-layer cell state corresponds to a complete plan to solve the level. + +# A.2 FURTHER EXAMPLES OF INTERNAL PLAN FORMATION + +In this section, we provide additional examples of the agent seeming to use its implicit learned environment model to internally form plans via an evaluative search. As in the main paper, we focus on the agent’s internal plan as formulated in terms of its representations of $C _ { \mathrm { B } }$ . That is, we focus on the routes the agent expects to push boxes (e.g. ‘box plans’) rather than the paths the agent expects to follow (e.g. ‘agent plans’). This is for two reasons. First, we expect ‘box plans’ to be determinative of ‘agent plans’ as the primary difficulty of Sokoban regards box movements. Second, ‘box plans’ are easier to visually inspect as ‘agent plans’ often intersect such that it is usually ambiguous to tell why a square has been added to an ‘agent plan’. All examples use handcrafted Sokoban levels designed to allow for clear plan visualization. + +We begin by providing additional examples of the plan formation ‘motifs’ shown in Figure 1. These motifs are re-occurring themes in the agent’s internal plan formation process. In each of the following five sub-sections we present five examples of the agent formulating internal plans in a way that demonstrates one of the aforementioned motifs. These motifs are: + +• Evaluative Planning - modifying plans based on feasibility (Section A.2.1) • Adaptive Planning - modifying plans based on conflicts (Section A.2.2) • Forward Planning - forming plans by searching forward from boxes (Section A.2.3) • Backward Planning - forming plans by searching backward from targets (Section A.2.4) • Parallel Planning - forming multiple plans in parallel (Section A.2.5) + +However, these motifs are not the only evidence of evaluative, search-based planning exhibited by the agent’s internal plan formation process. Specifically, evidence of this can also be seen when inspecting the manner in which the agent forms plans when confronted with various forms of outof-distribution Sokoban levels. As such, we provide examples of the agent successfully formulating internal plans under various types of distribution shift. That is, we show examples of: + +• Blind Planning - planning in levels in which the agent itself is not present (Section A.2.6) +• Generalized Planning - planning in levels with extra boxes and targets (Section A.2.7) +• Blocked-Route Planning - planning in levels in which additional walls appear at later time steps that block obvious routes to push boxes (Section A.2.8) +• New-Route Planning - planning in levels in which walls disappear at later time steps such that improved routes become available (Section A.2.9) + +Finally, in Section A.2.10, we will conclude by discussing the implications of the agent’s apparent learned search-based planning process. + +# A.2.1 EVALUATIVE PLANNING + +Under our characterization of ‘planning’ in Section 2.1, planning requires that an agent evaluates its plans. That is, merely formulating internal plans is not sufficient to view an agent as engaging in planning. Instead, we understand ‘planning’ as requiring that an agent arrive at an internal plan by means of evaluating different possible plans. This is because evaluating plans (e.g. in terms of their effects on the environment) allows the agent to formulate plans that it predicts will lead to good consequences. + +When visualizing the agent’s plan formation process, we see evidence indicative of the agent implementing some form of evaluative search-based planning. For instance, we see evidence of the agent iteratively constructing planned routes to push boxes to targets, then evaluating these routes. Specifically, the agent seems capable of evaluating routes and determining whether they are feasible. We call this evaluative planning. + +Figure 13 shows the development of agent’s internal plan in five episodes in which it performs evaluative planning of this sort. For instance, in Figure 13c, the agent’s initial internal plan (i.e. at the first computational tick) involves the agent pushing the upper-left box down through a corridor to the center-most target. On the face of it, this plan is appealing. This is because it is a ‘short’ plan in terms of the number of squares it would involve pushing a box across. However, this plan is infeasible. This is because the corridor is structured such that, upon pushing the box down into it, the agent would block the corridor off. This would then prevent the agent from (a) navigating to the left of the box as would be required to push it right along the corridor to the target, and (b) navigating to the lower-left box and target. Over subsequent ticks, the agent appears to evaluate its plan and realize this. In response, the agent then construct an alternative plan for the upper-left box. While less naively appealing – it is a longer plan in terms of the number of squares the box would need to be pushed – this plan would allow the agent to solve the level. The ability of the agent to recognize and avoid such ‘bad’ plans implies that the agent has learned to evaluate plans as required by our characterization of planning. + +# A.2.2 ADAPTIVE PLANNING + +Determining whether a single route would allow the agent to solve a level is not the only type of evaluation that the agent appears to perform when it formulates its internal plans. Additionally, the agent also appears to adapt its plans when conflicts arise between its sub-plans. That is, in cases when the agent’s internal plan involves pushing two boxes to the same target, the agent adapts its plan by planning for one of these boxes to be pushed top an alternative target. This suggests that the agent is predicting and evaluating the consequences of its actions. We call this form of evaluation adaptive planning. + +Instances of the agent performing adaptive planning when formulating its internal plans can be seen in Figure 14. Figure 14 shows the development of agent’s internal plan over the initial steps of five episodes in which it performs adaptive planning. For examples, consider the way in which the agent’s plans develop in Figure 14c. During the initial three computational ticks, the agent plans to push two separate boxes –i.e. the lower-left box and the upper-left box – to the left-most target. Importantly, however, in this level the upper-left box must be pushed to this target. This is because the left-most target is the only target that the upper-left box can feasibly be pushed to (e.g. if the lower-left box was pushed onto the left-most target, the level would be unsolvable). Over the fourth and fifth computational ticks, the agent appears to realize this and form an alternate plan for the lower-left target (e.g. a plan to push it to one of the top-right targets). + +We take the ability of the agent to adaptively plan in this fashion as evidence that, during planning, the agent is capable of predicting and evaluating the (relevant) consequences of its actions. That is, the agent not only internally represents plans formulated in terms of the consequences of its actions on box locations (i.e. its internal plans formulated in terms of $C _ { \mathrm { B } }$ ), but also has learned that a consequence of following one of these plans and pushing a box onto a target is to ‘fill’ this target. This suggests that the agent has learned that the effect of pushing a box onto a target is to stop other boxes being able to be pushed onto that same target. + +![](images/figures/emergent-planning-rl-fig-0013.jpg) + +(a) The agent initially plans to push a box through the circled corridor. However, the agent appears to realize that this is infeasible as it cannot push the box up onto the target as required by this plan. It then forms a longer plan that involves pushing the box a longer route to the same target. + +![](images/figures/emergent-planning-rl-fig-0014.jpg) + +(b) The agent initially plans to push a box up through the circled corridor. However, over subsequent time steps, the agent appears to realize that this is infeasible as it cannot get under the box at the corridor entrance as it would need to in order to push this box up through the corridor. It then modifies its plan so that this box is instead pushed a longer route that avoids the corridor. + +![](images/figures/emergent-planning-rl-fig-0015.jpg) + +(c) The agent initially plans to push a box down to a target along the circled route. However, over subsequent computational ticks, the agent realizes that this plan would prevent it from solving the level as it would prevent it from ever reaching the lower-left box and target. The agent then modifies its plan so that this box is instead pushed a longer route avoiding the corridor. + +![](images/figures/emergent-planning-rl-fig-0016.jpg) + +(d) At the initial time step, the agent plans to push a box down through the circled corridor. However, the agent realizes that this is not possible as it cannot get above the box at the corridor entrance as would be required in order to push it down. The agent then updates its plan so that this box is pushed through the further corridor. + +![](images/figures/emergent-planning-rl-fig-0017.jpg) + +(e) After the first computational tick, the agent plans to push a box along the circled route (e.g. the shortest route to the respective target). However, at the following tick, the agent appears to realizes that this is not possible as the agent would be unable to push the box left as required. The agent then updates its plan so that this box is instead pushed a longer route to the target. + +Figure 13: Examples of episodes in which the agent’s internal plan initially includes planned routes that would not lead to the agent solving the level. In all these examples, the agent realizes that part of its plan is infeasible and updates its plan accordingly. Blue arrows represent the direction that the agent plans to next push a box off of each square. Yellow circles highlight parts of the agent’s plan that are infeasible and later removed. The plans are decoded from the agent’s cell state at its first (13a), second (13b and 13c) and third (13d and 13e) layer by a 1x1 probe. The plans are decoded from the agent’s cell states at either the final computational tick of the first six steps of episodes (13a, 13b and 13d), or at each computational tick of the first two steps of episodes (13c and 13e). + +![](images/figures/emergent-planning-rl-fig-0018.jpg) + +(a) The agent initially plans to push two boxes to the lower left target. However, at later time steps, the agent alters its plan to instead push one of these boxes to the top-right target. + +![](images/figures/emergent-planning-rl-fig-0019.jpg) + +(b) The agent again initially plans to push two boxes to the two lower right targets. Again, at a later time step, the agent adapts its plan to instead push one of these boxes to the top-right target. + +![](images/figures/emergent-planning-rl-fig-0020.jpg) + +(c) During the initial 3 ticks, the agent plans to push both the lower-left and upper-left boxes to the left-most target. However, the agent appears to realize that the top-left box must be pushed to this target (i.e. it is the only target that it is feasible to push the top-left box to). The agent then adapts its plan to instead push the lower-left box to one of the top-most targets. + +![](images/figures/emergent-planning-rl-fig-0021.jpg) + +(d) Over the first two time steps, the agent considers pushing the lower-left box forward to one of the lowerright targets. However, this generates a conflict. The agent appears to realize this and then connect this box to a plan it has constructed that links this box to the top-left target. + +![](images/figures/emergent-planning-rl-fig-0022.jpg) + +(e) After the third tick, the agent plans to push two boxes to the lower-right target. However, the agent seems to realize that the lowest box must be pushed to this target (e.g. the only target the lowest box can feasibly be pushed to is the lower-right target). At the fourth tick, the agent hence plans to instead push the other box that it has associated with the lower-right target to an alternate target. + +Figure 14: Examples of episodes in which the agent initially plans to push multiple boxes to the same target before modifying its plan to push one of these boxes to an alternate target. Blue arrows represent the direction that the agent plans to next push a box off of each square. Yellow circles highlight parts of the agent’s plan that involve pushing two boxes to the same target. The plans are decoded from the agent’s cell state at its first (14a), second (14b and 14c) and third (14d and 14e) layer by a 1x1 probe. The plans are decoded from the agent’s cell states at either the final computational tick of the first six steps of episodes (14a, 14b and 14d), or at each computational tick of the first two steps of episodes (14c and 14e). + +# A.2.3 FORWARD PLANNING + +In the previous two sections, we described forms of evaluation the agent appears to perform as part of its iterative plan-construction process. However, this leaves open the question of how the agent iteratively constructs plans. As explained in Section 5, the agent appears to do so by using some form of learned, iterative search process. In this section, we now describe one major form of iterative, search-based plan-construction the agent performs: planning forward from boxes. + +Specifically, one of the (two) primary ways the agent appears to construct its internal plans is by iteratively extending its internal plans forward from boxes. That is, the agent seems to frequently ‘initialize’ plans at box locations, and then iteratively extend these plans forwards towards targets over the early computational ticks of episodes. This is a form of forward search. Algorithms based on forward search – for instance, Monte Carlo Tree Search (Coulom, 2006) – are the predominant form of planning algorithms currently used in model-based planning agents. It is hence notable that the agent appears to have learned a planning algorithm that (partially) relies on forward search. + +Instances of the agent constructing its internal plans by iteratively searching forward from boxes can be seen in Figure 15. Figure 15 shows the development of agent’s internal plan over the initial steps of five episodes in which the agent constructs part of its internal plan by planning forward from boxes towards targets. As a specific example, consider Figure $1 5 \mathrm { a }$ : the agent iteratively constructs a plan to push the bottom-right box to the top-right target by planning forward from the bottom-right box. Notice that the part of its plan that the agent iteratively constructs forward from this box end up connecting with a partial plan that the agent has constructed by iteratively searching backward from the top-right target. This will be discussed in the following section. + +# A.2.4 BACKWARD PLANNING + +Iteratively searching forward from boxes is not the only form of search-based planning that the agent appears to engage in. Additionally, the agent appears to search backwards from targets to boxes. That is, the agent will frequently initialize the end of a plan (i.e. by initializing a plan that ends at a specific target), and then iteratively search backwards towards boxes. This is a form of backward search. We hence refer to this as backwards planning. + +Examples of the agent constructing its internal plans by iteratively searching backwards from targets can be seen in Figure 16. Figure 16 shows the development of the agent’s internal plan over the initial steps of five episodes in which the agent constructs part of its internal plan by iteratively planning backwards from targets towards boxes. For instance, consider the manner in which the agent forms its internal plan in the episode shown in Figure 16e. Between the first and fifth tick in this episode, the agent iteratively extends a planned route backwards from the lower-right target to the lower-right box. + +Backward search was long studied in the context of planning in ‘classic’ RL (Moore & Atkeson, 1993). However, whilst some recent work has investigated methods relating to backward planning (Goyal et al., 2018; Van Hasselt et al., 2019; Lee et al., 2019), backward-facing planning is significantly less popular than forward-facing planning in modern model-based RL agents. The fact that the agent has learned to (partially) rely upon backwards planning is, therefore, interesting. + +# A.2.5 PARALLEL PLANNING + +Finally, the agent appears to be capable of extending multiple plans in parallel over a single computational tick. We refer to this as parallel planning. Examples of the agent constructing its internal plans in parallel can be seen in Figure 17. Figure 17 shows the development of the agent’s internal plan over the initial steps of five episodes in which the agent utilizes parallel planning. Parallel planning is not something that is common in standard planning algorithms. This is because, unlike the DRC agent that can plan by applying convolution operations to its spatially-extended cell states, standard planning algorithms must extend a single node at a time. We hypothesize that this parallel planning is learned by the agent to further increase the efficiency with which it plans. + +![](images/figures/emergent-planning-rl-fig-0023.jpg) + +(a) The agent iteratively extends part of its plan forward from the bottom-right box. The agent extends this part of its plan forward until it connects to a part of the agent’s plan that connects it to the top-right target. + +![](images/figures/emergent-planning-rl-fig-0024.jpg) + +(b) The agent constructs part of its plan by iteratively searching forward from the bottom-right box. The agent searches forward until this part of its plan connects to a part of the agent’s plan that connects this box to the bottom-left target. + +![](images/figures/emergent-planning-rl-fig-0025.jpg) + +(c) The agent formulates a planned route to push the top-most box by extending its plan forward from this box to the top-right target. + +![](images/figures/emergent-planning-rl-fig-0026.jpg) + +(d) The agent forms a plan for the bottom-most box by searching forward from this box to the lower-left target. + +(e) The agent forms a plan for the center-most box by iteratively searching forward from this box to the lowerright target. + +![](images/figures/emergent-planning-rl-fig-0027.jpg) +Figure 15: Examples of episodes in which the agent formulates its internal plan by iteratively extending planned routes forward from boxes. Blue arrows represent the direction that the agent plans to next push a box off of each square. Yellow circles highlight parts of the agent’s plan that it has constructed by iteratively searching forward from boxes to targets. The plans are decoded from the agent’s cell state at its first (15a), second (15b and 15c) and third (15d and 15e) layer by a 1x1 probe. The plans are decoded from the agent’s cell states at either the final computational tick of the first six steps of episodes (15a, 15b and 15d), or at each computational tick of the first two steps of episodes (15c and 15e). + +![](images/figures/emergent-planning-rl-fig-0028.jpg) + +(a) The agent formulates part of its internal plan by iteratively searching backwards from the bottom-right target to the center-most box. + +![](images/figures/emergent-planning-rl-fig-0029.jpg) + +(b) The agent constructs a plan to push a box to the top-most target by iteratively extending a plan backwards from this target. The agent extends this plan backwards until it connects to the top-most box. + +![](images/figures/emergent-planning-rl-fig-0030.jpg) + +(c) The agent forms an internal plan to push a box to the top-most target by iteratively searching backwards from this target. The agent formulates this part of its plan by searching backwards until this plan connects to the lower-right box. + +![](images/figures/emergent-planning-rl-fig-0031.jpg) + +(d) The agent formulates a plan to push the left-most box to the left-most target by searching backwards from the left-most target to the left-most box. + +![](images/figures/emergent-planning-rl-fig-0032.jpg) + +(e) The agent constructs a plan to push a box to the lower-right target by iteratively constructing a plan backwards from this target. The agent extends this plan backwards until it connects to the lower-right box. + +Figure 16: Examples of episodes in which the agent formulates its internal plan by iteratively extending planned routes backward from targets. Blue arrows represent the direction that the agent plans to next push a box off of each square. Yellow circles highlight parts of the agent’s plan that it has constructed by iteratively searching backwards from targets to boxes. The plans are decoded from the agent’s cell state at its first (16a), second (16b and 16c) and third (16d and 16e) layer by a 1x1 probe. The plans are decoded from the agent’s cell states at either the final computational tick of the first six steps of episodes (16a, 16b and 16d), or at each computational tick of the first two steps of episodes (16c and 16e). + +![](images/figures/emergent-planning-rl-fig-0033.jpg) + +(a) At the fourth computational tick, the agent extends two parts of its plan (e.g. its plans to push boxes to the top-most targets) in parallel. + +![](images/figures/emergent-planning-rl-fig-0034.jpg) + +(b) At the second computational tick, the agent iteratively constructs internal plans for the two top-most targets by searching backwards from these two targets in parallel. + +![](images/figures/emergent-planning-rl-fig-0035.jpg) + +(c) At the third computational tick, the agent iteratively extends its internal plans associated with the two topmost targets by extending these two plans in parallel. + +![](images/figures/emergent-planning-rl-fig-0036.jpg) + +(d) The agent searches backwards from the two bottom-most targets in parallel at the third computational tick. + +![](images/figures/emergent-planning-rl-fig-0037.jpg) +(e) After the third computational tick, the agent extends two parts of its internal plan in parallel. + +Figure 17: Examples of episodes in which the agent formulates its internal plan by extending multiple planned routes in parallel over a single computational tick. Blue arrows represent the direction that the agent plans to next push a box off of each square. Yellow circles highlight parts of the agent’s plan that it has constructed in parallel over a single tick. The plans are decoded from the agent’s cell state at its first (17a), second (17b and 17c) and third (17d and 17e) layer by a 1x1 probe. The plans are decoded from the agent’s cell states at either the final computational tick of the first six steps of episodes (17a, 17b and 17d), or at each computational tick of the first two steps of episodes (17c and 17e). + +# A.2.6 BLIND PLANNING + +In a discrete, deterministic environment such as Sokoban, a natural way for an agent to plan would be for it to search over possible sequences of future actions in search of a sequence of actions that achieved some goal. In Section 5, we noted that the agent’s internal plans consistently represent routes connecting boxes and targets. This, alongside all previous visualizations of the agent’s plans, suggests that the ‘goal’ the agent evaluates sequences of actions in terms of when performing search is whether said actions represent a feasible route to push a box along to a target. + +If the agent formulates internal plans by searching over potential future actions with the goal of connecting boxes and targets, we would expect the agent’s planning algorithm to (at least attempt to) search for plans achieving this goal in any Sokoban level so long as that level contained boxes to plan from and targets to plan to. That is, if the agent does indeed formulate internal plans by searching for plans achieving the goal of connecting boxes and targets, we would expect the agent to be able to formulate plans in Sokoban levels drawn from significantly different distributions. + +We now provide examples of the agent successfully forming plans in a very different type of level to the levels on which it was trained. Specifically, we provide examples of the agent appearing to search for plans in levels in which it is not itself present. These are Sokoban levels in which the agent observes the level, but is not actually positioned on any square. Note that this represents a significant distribution shift to the levels the agent was trained on. Indeed, the agent can never actually influence these levels. Crucially, however, this distribution shift should not prevent the agent from attempting to form plans if it did indeed plan in the hypothesized manner. + +Figure 18 shows the development of the agent’s internal plan in levels in which the agent is not itself present. Clearly, the agent (i) still attempts to form plans and (ii) forms internal plans that successfully connect boxes and targets. We take the ability of the agent to continue internally forming plans in the face of this radical distribution shift as evidence that the agent indeed possesses some learned search procedure that searches for plans that achieve the goal of connecting boxes and targets. However, we note that an unexplained curiosity is that, in some such levels, after arriving at a plan the agent will seemingly completely forget it. That is, in some cases of blind planning, the agent will begin to form a plan and then, after many time steps, proceed to forget the plan and represent no plan at all. + +# A.2.7 GENERALIZED PLANNING + +In the original paper introducing DRC agents, Guez et al. (2019) demonstrated that Sokoban-playing DRC agents trained on the Boxoban dataset of Sokoban levels (i.e. levels containing four boxes and four targets) can successfully generalize to Sokoban levels with additional targets and boxes. Specifically, they showed that such a DRC agent can solve Sokoban levels with additional boxes and targets. + +Given the discussion thus far, we hypothesize that the reason for this is that an agent possessing a planning mechanism of the above sort (i.e. an agent that planned by searching for sequences of actions corresponding to routes between boxes and targets) would be able to successfully execute its planning mechanism in such levels. This is because simply introducing a search process that searched for routes connecting boxes and targets could easily generalize to levels in which additional boxes and targets are present. + +Figure 19 shows examples of the agent’s internal plan at the final tick of the initial six time steps in episodes in which there are either five boxes and targets, or six boxes and targets. As implied by the above discussion, the agent (i) still attempts to form plans and (ii) forms internal plans that successfully connect boxes and targets. We take this as additional evidence of the agent searching for plans that achieve the goal of connecting boxes and targets. + +![](images/figures/emergent-planning-rl-fig-0038.jpg) +Figure 18: Examples of episodes in which the agent formulates an internal plan despite not being present in the level. Blue arrows represent the direction that the agent plans to next push a box off of each square. The plans are decoded from the agent’s cell state at its first (18a), second (18b and 18c) and third (18d and 18e) layer by a 1x1 probe. The plans are decoded from the agent’s cell states at the final computational tick of the first six steps of episodes. + +![](images/figures/emergent-planning-rl-fig-0039.jpg) +Figure 19: Examples of episodes in which the agent formulates its internal plan despite there being more boxes and more targets than in the levels on which it was trained. Blue arrows represent the direction that the agent plans to next push a box off of each square. The plans are decoded from the agent’s cell state at its first (19a), second (19b and 19c) and third (19d and 19e) layer by a 1x1 probe. The plans are decoded from the agent’s cell states at the final computational tick of the first six steps of episodes. These episodes take place in levels in which there are five boxes and targets (19a, 19b and 19d), and in levels in which there are six boxes and targets (19c and 19e) + +# A.2.8 BLOCKED-ROUTE PLANNING + +The previous two sub-sections provided examples of the agent successfully formulating plans in levels that represented significant distribution shifts relative to the training distribution. These previous distribution shifts aimed to induce changes to the agent’s environment that would not impede the planning capabilities of an agent that planned via searching for plans that achieved the implicit goal of connecting boxes and targets. In this sub-section and the following sub-section we now consider different forms of distribution shift. Namely, we now consider distribution-shifted Sokoban levels that aim to test the ability of the agent to dynamically evaluate and update its internal plan in response to environmental changes. + +We begin by considering Sokoban levels in which, at a time step following the initial time step, an additional wall square is added to the level. Specifically, this wall square is added to a location that blocks an obvious route between a box and a target. The aim of investigating these levels is to determine whether the agent is capable of evaluating that this additional wall square invalidates its current plan, and whether the agent can dynamically form a new plan after doing so. Figure 20 shows the manner in which the agent’s internal plan develops in levels in which, at a time step following initialization, we add a wall square to block off an obvious route between a box and a target. Clearly, the agent is capable of (i) recognizing that the added wall invalidates its initial plan and (ii) dynamically adjusting its plan accordingly. We take this as evidence to support the hypothesis that the agent forms plans via an evaluative search process. + +# A.2.9 NEW-ROUTE PLANNING + +Given the ability of the agent to dynamically update its plans in response to the addition of a wall square to block off an optimal route to push a box, an obvious question to ask is whether the agent can dynamically update its plan in levels representing the reverse type of distribution shift. That is, can the agent dynamically update its plans in levels in which, at some time step following initialization, we remove a wall square to open up an optimal route to push a box to a target that is infeasible prior to the removal of the wall? + +Figure 21 shows the development of the agent’s internal plan in levels where we, at some time step following initialization, remove a wall square to open up an optimal route to push a box to a target that is infeasible prior to the removal of the wall. In some levels – for example, in Figure 21a – the agent does dynamically respond to this wall-removal by updating its plan to exploit the new, optimal route. However, in other levels – such as in Figure 21b – the agent does not do this. We conjecture that this is potentially due to the agent having a notion of a ‘completed route’ within its internal plan. That is, we conjecture that the agent represents some plans as being complete and requiring no further search, and this is why the agent modifies its plan following the removal of a wall in some cases but not others. + +# A.2.10 DISCUSSION + +In discrete, deterministic, fully-observable environments like Sokoban, an agent with access to a perfect environment model can reformulate the problem of ‘planning’ as the problem of searching for a sequence of future actions – a plan – that achieves a goal (Russell & Norvig, 2010). The agent studied in this paper lacks such a perfect world model. + +However, we have demonstrated that the agent we study has learned a spatial correspondence between its cell states and the Sokoban grid, such that it can represent spatially-localized concepts at corresponding positions of its cell state. This can, perhaps, be seen as a learned ‘implicit’ model of the environment. Importantly, this learned implicit world model is sufficient to (i) represent sequences of future actions and (ii) predict relevant consequences of these actions on the environment. It hence appears to be sufficient to allow the agent to plan via search. + +We believe the examples provided in the previous sections support the hypothesis that the agent indeed plans via applying a learned search algorithm to a learned, ‘implicit’ model of its environment. This is interesting as it implies that the agent’s emergent planning capabilities represent a learned analogue to the planning-capable agents introduced in Appendix E.1 that plan via applying an explicit search algorithm to an explicit world model. + +![](images/figures/emergent-planning-rl-fig-0040.jpg) + +(a) After the first step, a wall is added to block the agent’s planned route between center-most box and target. Over the subsequent time steps, the agent realizes this and dynamically forms a new planned route connecting this box and target. + +![](images/figures/emergent-planning-rl-fig-0041.jpg) + +(b) After the first step, a wall is added to block the agent’s planned route between left-most box and target. Over the subsequent time steps, the agent realizes this and dynamically forms a new plan that involves pushing this box an alternate, longer route to this target. + +![](images/figures/emergent-planning-rl-fig-0042.jpg) + +(c) After the first step, a wall is added to block the agent’s planned route between right-most box and target. During the steps that follow, the agent realizes that this invalidates its initial plan and dynamically forms a new plan that involves pushing this box an alternative route. + +![](images/figures/emergent-planning-rl-fig-0043.jpg) + +(d) Initially, the agent plans to push the central box down to the central target. After the first step, a wall is added to block this route. During the following steps, the agent realizes that this has occurred and dynamically forms a new plan that involves pushing this box left, down, and then right, to this target. + +![](images/figures/emergent-planning-rl-fig-0044.jpg) + +(e) Initially, the agent plans to push the central box down to the central target. However, after the first step, a wall is added to block off this route. During the following steps, the agent realizes that its initial plan is now infeasible and dynamically forms a new plan that instead involves pushing this box right, down, left, and then up, to this target. + +Figure 20: Examples of the agent formulating its internal plan in levels in which a wall square is added to the environment to block an obvious route between a box and target at a time step following initialization. Blue arrows represent the direction that the agent plans to next push a box off of each square. Yellow circles highlight relevant parts of the agent’s plan before and after the additional wall square is added. The plans are decoded from the agent’s cell state at its first (20a), second (20b and 20c) and third (20d and 20e) layer by a 1x1 probe. The plans are decoded from the agent’s cell states at the final computational tick of the first six steps of episodes. + +![](images/figures/emergent-planning-rl-fig-0045.jpg) + +(a) The agent begins planning to push the upper-left box up, right, and down to the center-most target. However, after the first step, a wall is removed such that this box can instead be pushed straight right to the target. The agent realizes this and updates its plan accordingly. + +![](images/figures/emergent-planning-rl-fig-0046.jpg) + +(b) Initially, the agent plans to push the left-left box right, up, left, and down to the left-most target. However, after the third step, a wall is removed such that this box can instead be pushed a shorter route (i.e. left and the up) to the target. The agent does not realize this and does not update its plan in response. + +![](images/figures/emergent-planning-rl-fig-0047.jpg) + +(c) After the second step a wall is removed. The removal of this wall means that the optimal route to push the right-most box to the right-most target is to push it right and then up. Before the removal of the wall the agent plans to push it left, up and then right. However, after the removal, the agent updates its plan to account for the new optimal route. + +![](images/figures/emergent-planning-rl-fig-0048.jpg) + +(d) The agent initially plans to push the right-most box up and around the wall that separates it from the rightmost target. After the second step, a wall is removed such that this box can now be pushed a shorter route to this target. The agent fails to respond to this. + +![](images/figures/emergent-planning-rl-fig-0049.jpg) + +(e) The agent initially plans to push the center-most box around the wall that separates it from the center-most target. After the first step, a wall square is removed such that this box can now be pushed directly down to this target. The agent realizes this and updates its plan to account for the new optimal route to push this box. + +Figure 21: Examples the agent formulating internal plans in episodes in which a wall square is removed at a time step after initialization. Removing this wall square opens up a route that it is optimal to push a box to a target through. Blue arrows represent the direction that the agent plans to next push a box off of each square. Yellow circles highlight the relevant parts of the agent’s plan (or lack of internal plan) before and after the wall square is removed. The plans are decoded from the agent’s cell state at its first (21a), second (21b and 21c) and third (21d and 21e) layer by a 1x1 probe. The plans are decoded from the agent’s cell states at the final computational tick of the first six steps of episodes. + +This perspective on the agent’s concept representations – i.e. that, by enabling the agent form plans and evaluate their consequences, collectively, these representations play a role that can be seen as the role of a learned implicit world model, – aligns with work that has emphasized the importance of world models for generalization capabilities (Richens & Everitt, 2024; Andreas, 2024). It also provides new insights regarding learned world models in RL. Specifically, it complements past work that has investigated explicitly training world models (Ha & Schmidhuber, 2018; Freeman et al., 2019) by showing that world models – or, at least, representations that can play a role traditionally played by world models - -can also emerge spontaneously within the representations of a generic agent. + +Additionally, it is interesting that the agent constructs its internal plans by simultaneously searching forward from multiple boxes and searching backwards from multiple targets. That is, the agent appears to have learned a form of parallelized bidirectional planning. This is very different to the agents introduced in Appendix E.1 that primarily rely on forward search algorithms. + +Whilst some past work has had considerable success in applying bidirectional search to RL (Edwards et al., 2018; Lai et al., 2020), RL agents making use of bidirectional planning are still remarkably rare. This is likely due to the difficulty in specifying which states to plan forwards and backwards from in many environments. Indeed, Sokoban is somewhat unique in that it is especially well-suited for bidirectional search. This is because there are obvious candidates to plan forwards from (boxes) and backwards from (targets). As such, the main takeaway from the emergence of bidirectional search within the agent is likely not that bidirectional search should be applied more widely within RL. + +Instead, we believe the main takeaway from this finding to be that there are benefits to allowing agents to learn a planning algorithm (and a implicit-world model to apply it to) rather than forcing an agent to use a handcrafted planning algorithm. This is because the agent can learn to plan in a way that is well-suited for the environment it finds itself in. We suspect this idea – of allowing agents to learn search algorithms well-suited for specific domains, rather than forcing them to use a generic, handcrafted search algorithms such as MCTS – may become increasingly prevalent. + +This is to say that we hypothesize that the agent we study has learned a form of planning that is especially well-suited to Sokoban relative to other planning algorithms. Bidirectional search is known to be very efficient in certain environments (Kaindl & Kainz, 1997; Russell & Norvig, 2010; Sturtevant et al., 2020). Intuitively, this is because it is more efficient to form plans by searching backwards from goals and forwards from initial states (i.e. because these two plans can ‘meet in the middle’) than to form plans by either of these means alone. Furthermore, as Sokoban is characterized by actions having negative consequences in the long-run, efficient planning is crucial in Sokoban. This is because forming plans quickly at the start of episodes allows the agent to avoid taking actions early on that would make a level unsolvable. Thus the agent studied in this paper appears to have learned a domain-specific planning algorithm that works well in the environment it finds itself in. Evidence of this can be seen in the fact that one of the highest-performing Sokoban agents that does not rely on deep learning also uses a form of forward-backward planning Shoham & Elidan (2021). + +# A.3 FURTHER RESULTS REGARDING ITERATIVE PLAN REFINEMENT + +In Section 5, we used Figure 6 to demonstrate that the agent can use additional test-time compute at the start of episodes to refine its internal plan. This would be expected if the agent constructed plans using some form of learned search, since the agent would be able to use additional compute to perform a more thorough search. This additionally helps explain the ability of DRC agents to perform better in Sokoban when given ‘thinking time’ steps (Guez et al., 2019; Taufeeque et al., 2024) as we have shown that the extra compute afforded by ‘thinking time’ facilitates plan refinement. In this section, we further investigate the agent’s ability to iteratively refine its internal plans when forced to perform ‘thinking time’ steps – that is, forced stationary steps at the start of episodes – prior to acting. + +# A.3.1 TEST-TIME PLAN REFINEMENT ACROSS LAYERS + +In Section 5, Figure 6 only demonstrated that the agent can use ‘thinking time’ iteratively refine its plan in its final layer. To investigate whether the agent can iteratively refine its plans at all layers when given additional test-time compute prior to acting, we thus again forced the agent to perform + +![](images/figures/emergent-planning-rl-fig-0050.jpg) +Figure 22: Macro F1 (averaged over 1000 episodes) when using the agent’s internal plan at each internal tick during the first 5 steps of an episode to predict (a) the agent’s future movements $( C _ { \mathrm { { A } } } )$ and (b) future box movements $\left( C _ { \mathrm { B } } \right)$ . The ‘internal plan’ at a layer for a tick corresponds to the agent’s representation of $C _ { \mathrm { { A } } }$ and $C _ { \mathrm { B } }$ as decoded by a 1x1 probe applied to the agent’s cell state at that layer. + +![](images/figures/emergent-planning-rl-fig-0051.jpg) +Figure 23: Example of one of the levels used to test for behavioral evidence of search when its corridor is of different lengths. + +5 ‘thinking time’ steps before beginning to act in 1000 episodes. As with Figure 6, after each of the 15 internal computational ticks performed by the agent during these steps, we applied 1x1 probes to decode the agent’s representations of $C _ { \mathrm { { A } } }$ and $C _ { \mathrm { B } }$ for each square of the observed Sokoban board at that tick. As argued previously, the agent’s internal representations of the concepts over the entire Sokoban board can be seen as its internal plan. We viewed the agent’s internal plan at each tick as a prediction of its future behavior $( C _ { \mathrm { { A } } } )$ and the effect of this future behavior on the environment $\left( C _ { \mathrm { B } } \right)$ , and measured the correctness of these prediction using the macro F1 score. The results can be seen in Figure 22. Clearly, the agent’s internal plan gets iteratively refined over the course of ‘thinking time’ at all layers. + +# A.3.2 EVIDENCE OF TEST-TIME COMPUTE BEING USED FOR SEARCH + +In this paper, we have provided qualitative evidence that supports the hypothesis that the agent plans via learned search procedure, and that the agent reason the agent benefits from additional testtime compute is because it uses this extra compute to search more thoroughly prior to acting. In this section we now complement this with behavioral evidence of the agent using extra test-time compute for search. + +We do this using a dataset of handcrafted levels. These levels all follow a common schematic. Namely, there is a corridor with a single entrance. At the end of this corridor is a box and a target. The entrance square of the corridor also has a target on it. There is a box adjacent to this target at the entrance to the corridor. In these levels, the agent always starts adjacent to this box. Thus, at the initial time step, a myopic agent will always push this box on to the target. However, doing so blocks off the corridor (e.g. it prevents the agent from ever reaching the box and target at the end). So, we would expect a planning-capable agent to realize this, and instead plan to push the box out of the way so it can enter the corridor. We create 8 handcrafted levels of this sort. For each level, we create a copy where its corridor is of length 2, 6, 10 and 14. Figure 23 shows a version of one of these levels with these 4 corridor lengths. We then reflect and rotate each level. Thus, we have a dataset of 80 such levels with corridors of each length. + +Figure 24 shows the percentage of these levels the agent solves when given between zero and 5 thinking steps prior to acting. If the agent planned via search, we would expect it to struggle to solve these levels without additional test-time compute. This is because, plausibly, the agent’s search + +procedure would take multiple steps to extend backward from the end of the corridor to the corridor entrance (i.e. to inform the agent that it should not act myopically). This is clearly the case in Figure 24. Additionally, if the agent planned via search, we would expect it to take more test-time compute to solve levels with longer corridor. This is because, if the agent planned via search, it would presumably take longer for the search process to account for the effect of blocking off the corridor (i.e. by myopically pushing a box onto the target at the entrance) in levels with longer corridors. The expected pattern of the agent requiring more ‘thinking steps’ to solve levels with longer corridors can be seen in Figure 24. For instance, the number of ‘thinking steps’ the agent re- + +![](images/figures/emergent-planning-rl-fig-0052.jpg) +Figure 24: The percentage of the 80 levels introduced in Appendix A.3.2 that the agent solves with different numbers of ‘thinking steps’ (forced stationary steps prior to acting) + +quires to solve at least half of each set of levels increases with the corridor length of these levels. The agent requires 0 thinking steps to solve at least half of the levels with corridors of length 2, 1 thinking step for levels with corridors of length 4, 2 thinking steps with corridors of length 10, and 3 thinking steps to solve levels with corridors of length 14. + +Qualitative evidence of the agent using the additional test-time compute given to it by ‘thinking steps’ to perform a more thorough search can be seen in Figure 25. In Figure 25, we visualize the agent’s internal plan (as formulated in terms of the squares the agent expects to step onto) at the final tick of each 5 additional steps of computation given to the agent when it performs 5 steps of ‘thinking time’ prior to acting in levels with corridors of length 14. In all of these levels, the agent at the third tick plans to step directly onto the box, myopically pushing it onto the target and making the level unsolvable. Note that this corresponds to the agent’s plan without ‘thinking steps’ and thus explains why the agent fails all of these levels by default. However, over the subsequent steps, the agent iteratively searches backwards from the box at the corridor end. Once this search extends backwards onto the target at the corridor entrance, the agent seems to realize that it should not myopically push a box onto this target as it needs to instead step onto this target to enter the corridor. The agent then alters its plan to instead push the box at the corridor entrance out of the way and enter the corridor. + +![](images/figures/emergent-planning-rl-fig-0053.jpg) + +(a) The agent initially plans to step up into the circled box, pushing the box onto the circled target and blocking off the corridor. However, when given additional ‘thinking time’, the agent extends its planned route backwards from the end of the corridor and realizes that it needs to step onto this target. It hence changes its plan so that it will step onto, and thus push, the box right so that it can enter the corridor. + +![](images/figures/emergent-planning-rl-fig-0054.jpg) + +(b) After the third computational tick, the agent plans to step right and push the circled box on to the circled target. Over subsequent steps of ‘thinking time’, the agent plans backwards from the box at the end of the corridor and realizes that it needs to step into the corridor to reach this box. It thus instead plans to step down onto the box, moving it out of the way. + +![](images/figures/emergent-planning-rl-fig-0055.jpg) + +(c) Initially, the agent plans to step up, pushing the circled box onto the circled target. However, the agent extends its plan backwards from the box at the corridor end and realizes that it needs to step onto this target in order to enter the corridor and reach the box at the corridor end. As such, it alters its plan to first push the circled box left rather than myopically pushing it on to the target. + +![](images/figures/emergent-planning-rl-fig-0056.jpg) + +(d) Without ‘thinking steps’, the agent would push the circled box right onto the circled target. However, during ‘thinking steps’, the agent extends a path backwards from the box at the end of the corridor to the circled target. It then plans to instead push the circled box down so that it can follow this path. + +![](images/figures/emergent-planning-rl-fig-0057.jpg) + +(e) Initially, the agent plans pushing the circled box left onto the circled target. However, the agent extends the path it plans to follow to the box at the corridor end backwards, and realizes that it needs to step onto the circled target to reach this box. The agent then alters its plan to first push the circled box down. + +Figure 25: Examples of the agent’s plan (in terms of $C _ { \mathrm { { A } } }$ ) over extra steps associated with 5 ‘thinking steps’ in levels with corridors of length 14 as introduced in Appendix A.3.2. During thinking steps, the agent searches backwards from the end of the corridor, and realizes that it must not myopically block this corridor off. The agent fails all of these levels when not forced to perform ‘thinking steps’, but, when given 5 ‘thinking steps’ the agent successfully solves all levels. Yellow circles highlight the box and target for which the agent changes its plans. Teal arrows represent the direction that the agent plans to next move on to each square. The plans are decoded from the agent’s cell state at its first (25a), second (25b and 25c) and third (25d and 25e) layer by a 1x1 probe. The plans are decoded at the final tick of each extra step performed during 5 steps of ‘thinking time’. + +# B ADDITIONAL INTERVENTION RESULTS + +In Section 6 we provided evidence indicating that the agent’s representations of $C _ { \mathrm { { A } } }$ and $C _ { \mathrm { B } }$ were responsible for the agent’s planning-like behavior. Specifically, in Section 6.1 we outlined the results of experiments in which we used the vector representations of $C _ { \mathrm { { A } } }$ and $C _ { \mathrm { B } }$ learned by 1x1 probes to intervene on the agent’s cell state to force the agent to formulate and execute sub-optimal plans in Agent-Shortcut and Box-Shortcut levels. The aim of these experiments was to demonstrate that the plans the agent internally formulated using its representations of $C _ { \mathrm { { A } } }$ and $C _ { \mathrm { B } }$ causally influenced its behavior in the manner that would be expected of plans. + +In this section, we provide further results regarding these experiments. + +• Appendix B.1 provides additional examples of the qualitative effect of the interventions from Section 6 on the agent’s internal plan. +• Appendix B.2 outlines the results of additional intervention experiments in Agent-Shortcut and Box-Shortcut levels. These additional intervention experiments investigate altering the number of squares intervened upon, and introducing an intervention strength parameter. +• Appendix B.3 details alternate intervention experiments in a new set of handcrafted levels. These levels are designed to test the ability of interventions to force the agent to act optimally when it otherwise would not. + +# B.1 ADDITIONAL EXAMPLES OF INTERVENTIONS + +In Section 6.1, we provided examples of plans as decoded from the agent’s final layer cell state by a 1x1 probe after the first 4 time steps of Box-Shortcut and Agent-Shortcut episodes both when we did, and when we did not, intervene on the agent’s final layer cell state. We noted that, when visualizing the agent’s plans as decoded by 1x1 probes, we could see that the interventions had the effect of causing the agent to internally formulate and execute the desired type of sub-optimal plan (e.g. a plan that involves either following a longer-than-necessary path, or that involves pushing a box a longer-than-necessary route to a target). We now provide additional examples to further illustrate this. Figure 27 provides additional example Box-Shortcut interventions and Figure 26 provides additional example Agent-Shortcut interventions. + +![](images/figures/emergent-planning-rl-fig-0058.jpg) + +![](images/figures/emergent-planning-rl-fig-0059.jpg) +(b) Intervention + +(a) Plan without intervention (c) Plan with intervention (d) Plan without intervention (f) Plan with intervention (g) Plan without intervention (h) Intervention (i) Plan with intervention (j) Plan without intervention (l) Plan with intervention (o) Plan with intervention (r) Plan with intervention (s) Plan without intervention + +![](images/figures/emergent-planning-rl-fig-0060.jpg) + +![](images/figures/emergent-planning-rl-fig-0061.jpg) + +![](images/figures/emergent-planning-rl-fig-0062.jpg) +(e) Intervention + +![](images/figures/emergent-planning-rl-fig-0063.jpg) + +![](images/figures/emergent-planning-rl-fig-0064.jpg) + +![](images/figures/emergent-planning-rl-fig-0065.jpg) + +![](images/figures/emergent-planning-rl-fig-0066.jpg) + +![](images/figures/emergent-planning-rl-fig-0067.jpg) + +![](images/figures/emergent-planning-rl-fig-0068.jpg) +(k) Intervention + +![](images/figures/emergent-planning-rl-fig-0069.jpg) + +![](images/figures/emergent-planning-rl-fig-0070.jpg) +(m) Plan without intervention + +![](images/figures/emergent-planning-rl-fig-0071.jpg) +(n) Intervention + +![](images/figures/emergent-planning-rl-fig-0072.jpg) + +![](images/figures/emergent-planning-rl-fig-0073.jpg) +(p) Plan without intervention + +![](images/figures/emergent-planning-rl-fig-0074.jpg) +(q) Intervention + +![](images/figures/emergent-planning-rl-fig-0075.jpg) + +![](images/figures/emergent-planning-rl-fig-0076.jpg) + +![](images/figures/emergent-planning-rl-fig-0077.jpg) +(t) Intervention + +![](images/figures/emergent-planning-rl-fig-0078.jpg) +(u) Plan with intervention + +Figure 26: Examples of Agent-Shortcut interventions and their effects on the agent’s internal plan. Each row shows (1) the agent’s internal plan after 4 steps in a level without the intervention, (2) the initial state of that level, and the intervention performed, and (3) the agent’s internal plan after 4 steps in that level with the intervention. Plans are decoded from the agent’s final layer cell state by a 1x1 probe. Teal arrows mean the agent plans to next step onto the associated square in the associated direction. White arrows mark positions which the associated directional representations of $C _ { \mathrm { { A } } }$ are added to. White crosses mark positions of the agent’s cell state that representations of NEVER for $C _ { \mathrm { { A } } }$ are added to. + +![](images/figures/emergent-planning-rl-fig-0079.jpg) + +(a) Plan without intervention (c) Plan with intervention (d) Plan without intervention (f) Plan with intervention (g) Plan without intervention (h) Intervention (i) Plan with intervention (j) Plan without intervention (l) Plan with intervention (m) Plan without intervention (o) Plan with intervention (p) Plan without intervention (r) Plan with intervention (s) Plan without intervention + +![](images/figures/emergent-planning-rl-fig-0080.jpg) +(b) Intervention + +![](images/figures/emergent-planning-rl-fig-0081.jpg) + +![](images/figures/emergent-planning-rl-fig-0082.jpg) + +![](images/figures/emergent-planning-rl-fig-0083.jpg) + +![](images/figures/emergent-planning-rl-fig-0084.jpg) +(e) Intervention + +![](images/figures/emergent-planning-rl-fig-0085.jpg) + +![](images/figures/emergent-planning-rl-fig-0086.jpg) + +![](images/figures/emergent-planning-rl-fig-0087.jpg) + +![](images/figures/emergent-planning-rl-fig-0088.jpg) + +![](images/figures/emergent-planning-rl-fig-0089.jpg) +(k) Intervention + +![](images/figures/emergent-planning-rl-fig-0090.jpg) + +![](images/figures/emergent-planning-rl-fig-0091.jpg) + +![](images/figures/emergent-planning-rl-fig-0092.jpg) +(n) Intervention + +![](images/figures/emergent-planning-rl-fig-0093.jpg) + +![](images/figures/emergent-planning-rl-fig-0094.jpg) + +![](images/figures/emergent-planning-rl-fig-0095.jpg) +(q) Intervention + +![](images/figures/emergent-planning-rl-fig-0096.jpg) + +![](images/figures/emergent-planning-rl-fig-0097.jpg) + +![](images/figures/emergent-planning-rl-fig-0098.jpg) +(t) Intervention + +![](images/figures/emergent-planning-rl-fig-0099.jpg) +(u) Plan with intervention + +Figure 27: Examples of Box-Shortcut interventions and their effects on the agent’s internal plan. Each row shows (1) the agent’s internal plan after 4 steps in a level without the intervention, (2) the initial state of that level, and the intervention performed, and (3) the agent’s internal plan after 4 steps in that level with the intervention. Plans are decoded from the agent’s final layer cell state by a 1x1 probe. Blue arrows mean the agent plans to push a box of the associated square in the associated direction. White arrows mark positions which the associated directional representations of $C _ { \mathrm { B } }$ are added to. White crosses mark positions of the agent’s cell state that representations of NEVER for $C _ { \mathrm { B } }$ are added to. + +# Algorithm 1 Agent-Shortcut Intervention + +1: ShortRouteSquar $e s \gets \mathbf { A l l }$ positions $( x , y )$ on the short route +2: $( x _ { 0 } , y _ { 0 } ) \gets$ The first square $( x , y )$ of the long route. +3: LongRouteSquaresDirs $\gets$ The first $p$ squares $( x , y )$ that the agent would step onto if following +the longer route, and the direction DIR it would step onto them +4: for $t$ in 1, 2, · · · , EpisodeLength do +5: for $( x , y )$ in ShortRouteSquares do ▷ Short-route intervention +6: $c _ { ( x , y ) } c _ { ( x , y ) } + \alpha \setminus ^ { C _ { A } } w _ { \mathrm { N E V E R } } ^ { C _ { A } }$ +7: if Agent has not moved onto $( x _ { 0 } , y _ { 0 } )$ this episode then +8: for $( x , y )$ , DIR) in LongRouteSquaresDirs do ▷ Directional intervention +9: $c _ { ( x , y ) } c _ { ( x , y ) } + \alpha \times w _ { \mathrm { D I R } } ^ { C _ { A } }$ + +# Algorithm 2 Box-Shortcut Intervention + +1: ShortRouteSquar $e s \gets \mathbf { A l l }$ positions $( x , y )$ on the short route +2: $( x _ { 0 } , y _ { 0 } ) \gets$ The initial position $( x , y )$ of the box that is not adjacent to any targets. +3: LongRouteSquares $D i r s \gets$ The first $p$ squares $( x , y )$ that a box would be pushed off of if pushed +the longer route, and the direction DIR it would be pushed +4: for $t$ in 1, 2, · · · , EpisodeLength do +5: for $( x , y )$ in ShortRouteSquares do ▷ Short-route intervention +6: $c _ { ( x , y ) } c _ { ( x , y ) } + \alpha \setminus ^ { C _ { B } } w _ { \mathrm { N E V E R } } ^ { C _ { B } }$ +7: if Agent has not pushed a box off of $( x _ { 0 } , y _ { 0 } )$ this episode then +8: for $( x , y )$ , DIR) in LongRouteSquaresDirs do ▷ Directional intervention +9: $c _ { ( x , y ) } c _ { ( x , y ) } + \alpha \times w _ { \mathrm { D I R } } ^ { C _ { B } }$ + +# B.2 ADDITIONAL INTERVENTION EXPERIMENTS: FURTHER AGENT-SHORTCUT AND BOX-SHORTCUT INTERVENTION EXPERIMENTS + +We now consider performing alternate intervention experiments in Box-Shortcut and Agent-Shortcut levels. To reiterate, Agent-Shortcut levels are characterized by the agent having to choose to follow either a longer or a shorter path from its initial position to a region with boxes and targets. Similarly, Box-Shortcut levels are characterized by there being one box that can be pushed either a shorter or a longer route to a target. In both levels, it is optimal for the agent to select the shorter option, and this is indeed what the agent does when not intervened upon. Thus, our interventions aimed to force the agent to formulate and execute a sub-optimal plan involving choosing the longer option. + +Our interventions in Box-Shortcut and Agent-Shortcut levels consisted of two sub-interventions: + +• Short-Route Interventions. These interventions aim to discourage the agent from acting optimally and taking the shorter option. In Agent-Shortcut levels, the short-route intervention consists of adding the representation of NEVER for $C _ { \mathrm { { A } } }$ to cell state positions along the short path the agent could follow. In Box-Shortcut levels, the short-route intervention consists of adding the representation of NEVER for $C _ { \mathrm { B } }$ to cell state positions along the short route the box could be pushed along. This intervention is repeated at every time step. + +• Directional Interventions. These interventions aim to encourage the agent to act suboptimally and take the longer option. In Agent-Shortcut levels, the directional intervention consists of adding the appropriate directional representation of $C _ { \mathrm { { A } } }$ to the first square the agent would step onto if it followed the longer path. In Box-Shortcut levels, the directional intervention consists of adding the appropriate directional representation of $C _ { \mathrm { { A } } }$ to the box’s initial position to encourage it to be pushed the long route. This intervention is repeated at every time step until the agent either pushes the box off the initial squares (Box-Shortcut interventions) or steps onto the first square of the long-route (Agent-Shortcut Interventions). + +The experiments in Section 6.1 that performed these two interventions did not investigate three possible axes of variation that could influence the success rate of interventions. First, the previous experiments simply added the un-scaled vector representations learned by 1x1 probes to the agent’s cell state and did not consider the effect of introducing an intervention strength parameter $\alpha$ to scale representations by before using them for interventions. Second, the previous experiments did not consider varying the directional intervention - specifically, they did not consider whether interventions become more successful if we intervene upon more squares along the longer path. Finally, they did not consider whether the interventions could be successful without the short-route intervention. We now consider the effect of these three factors on the success rate of interventions. + +Algorithms 1 and 2 respectively provide pseudoscope for general Agent- and Box-Shortcut interventions in which we (1) intervene on the first $p$ squares of the long route as part of the ‘directional’ intervention, and (2) introduce an intervention strength $\alpha$ . Note that we can also choose not to perform the ‘short-route’ intervention. The interventions in Section 6.1 correspond to algorithms 1 and 2 with $p$ and $\alpha$ set to 1. + +As with the experiments in Section 6.1, all experiments in this section are repeated with 5 independently trained and initialized probes, and interventions are considered a success if they cause the agent to solve the level in the desired, sub-optimal way. + +# B.2.1 VARYING THE NUMBER OF DIRECTIONAL INTERVENTIONS + +First, we also consider intervening in Agent-Shortcut and Box-Shortcut experiments while varying the number of squares intervened upon as part of the directional intervention along the ‘long’ route. Specifically, we vary the number of squares intervened upon in ‘directional’ interventions between 0 squares and 3 squares. When intervening upon an additional square on the ‘long route’ we intervene on the square following the already-intervened-upon squares. That is, we vary the value of $p$ between 0 and 3 in algorithms 1 and 2. For instance, when we intervene upon two squares in Agent-Shortcut levels, we intervene upon the first two squares the agent would step onto if it followed the longer path. Then, when intervening upon three squares we would additionally intervene on the third square the agent would step onto if it followed the longer path. We also consider varying $\alpha$ + +Figures 28 and 29 show the success rate when intervening on the agent in Agent-Shortcut and Box-Shortcut levels when varying the intervention strength $\alpha$ and the number of squares intervened upon in the longer route. A few observations can be made from these figures. + +First using too high or too low of an $\alpha$ harms the intervention success rate. This would be expected: when $\alpha$ is too low, interventions will not meaningfully change the agent’s internal concept representations, while when $\alpha$ is too high, the intervention will cause the agent’s internal activations to go off-distribution. + +Likewise, performing additional interventions on the long path improves intervention success rate for low $\alpha$ , but harms the success rate for high $\alpha$ . We posit this is because, to alter the agent’s concept representations for a low $\alpha$ , additional interventions are useful. However, when $\alpha$ is high, these additional interventions cause the agent’s activations to go further off-distribution, impeding the ability of the intervention to steer the agent. + +# B.2.2 REMOVING THE SHORT-ROUTE INTERVENTION + +We also considered intervening on Agent-Shortcut and Box-Shortcut levels when not performing any short-route interventions and solely performing directional interventions. These directionalonly interventions aimed to assess the extent to which the agent’s planning mechanism is driven by avoiding planning into squares it represents as having the class NEVER as opposed to extending plans it constructs with the directional concept classes. + +Figures 30 and 31 show the success rate when intervening on the agent in Agent-Shortcut and Box-Shortcut levels when varying the intervention strength $\alpha$ and the number of squares intervened upon in on the longer route. Clearly, intervening without the short-route intervention is less successful. However, the reduction in success rate relative to Figures 28 and 29 is significantly more pronounced for Agent-Shortcut interventions than for Box-Shortcut interventions. We thus hypothesize that the agent utilizes a different planning mechanism when planning paths for it to follow as opposed to planning routes to push boxes. Specifically, the agent’s path-planning mechanism seems to be more driven by avoiding certain squares (e.g., those it represents using the class NEVER of $C _ { \mathrm { { A } } }$ ) than the agent’s box-route-planning mechanism. + +![](images/figures/emergent-planning-rl-fig-0100.jpg) +Figure 28: Success rate when intervening in Agent-Shortcut levels when varying the number of cell state positions intervened on along the ‘long path’ during the ‘directional’ part of the intervention. Interventions are performed using the vector representations of $C _ { A }$ learned by 1x1 probes. Interventions are performed using different intervention strengths $\alpha$ on the agent’s cell state at each of its ConvLSTM layers. For each layer, intervention strength, and number of squares intervened upon, we repeat the intervention 5 times using 5 independently trained probes and report the average success rate. We compare interventions performed with trained probes to interventions performed with randomly-initialized probes. Error bars report $\pm 1$ standard deviations. + +![](images/figures/emergent-planning-rl-fig-0101.jpg) +Figure 29: Success rate when intervening in Box-Shortcut levels when varying the number of cell state positions intervened on along the ‘long path’ during the ‘directional’ part of the intervention. Interventions are performed using the vector representations of $C _ { B }$ learned by 1x1 probes. Interventions are performed using different intervention strengths $\alpha$ on the agent’s cell state at each of its ConvLSTM layers. For each layer, intervention strength, and number of squares intervened upon, we repeat the intervention 5 times using 5 independently trained probes and report the average success rate. We compare interventions performed with trained probes to interventions performed with randomly-initialized probes. Error bars report $\pm 1$ standard deviations. + +![](images/figures/emergent-planning-rl-fig-0102.jpg) +Figure 30: Success rate when intervening in Agent-Shortcut levels but not performing the ‘shortroute’ part of the intervention. Identical to Figure 28 otherwise. + +![](images/figures/emergent-planning-rl-fig-0103.jpg) +Figure 31: Success rate when intervening in Box-Shortcut levels, but not performing the ‘shortroute’ part of the intervention. Identical to Figure 29 otherwise. + +![](images/figures/emergent-planning-rl-fig-0104.jpg) +Figure 32: Examples of (a) ‘Agent-Only’, and (b) ‘Box-Only’, and (c) ‘Agent-and-Box’ interventions in a Cutoff level. We add the associated directional representation of $C _ { \mathrm { { A } } }$ to the position with the teal arrow (e.g. in this example the representation of UP for $C _ { \mathrm { { A } } }$ ). We add the associated directional representation of $C _ { \mathrm { B } }$ to the position with the blue arrow (e.g. in this example the representation of RIGHT for $C _ { \mathrm { B } }$ ). + +# B.3 ADDITIONAL INTERVENTION EXPERIMENTS: INTERVENING IN A NEW SET OF LEVELSTO ENCOURAGE OPTIMAL BEHAVIOR + +We also considered performing interventions in a different set of levels. We call these levels ‘Cutoff’ levels. Cutoff levels follow a schematic very similar to the levels introduced in Appendix A.3.2 and are designed in such a way that solving them requires the agent to forsee the long-run consequences of its actions. Specifically, these are levels in which a box is adjacent to a target at the entrance of a corridor. The agent always begins levels one square removed from this box. At the end of this (variable-length) corridor is another box adjacent to another target. To solve these levels, the agent must not act myopically – i.e. it must not immediately push the box at the corridor entrance onto the adjacent target as doing so would block the corridor and make the level unsolvable – and must instead push the box out of the way so that it can enter the corridor and reach the box at the corridor end. + +We use a dataset of 200 Cutoff levels. These were created by designing 25 Cutoff levels (with corridors of varying lengths), and then making 8 copies of each by applying vertical reflection and $9 0 ^ { \circ }$ , $1 8 0 ^ { \circ }$ , and $2 7 0 ^ { \circ }$ rotations. Figure 33 shows the percent of these levels the agent solves when given varying number of ‘thinking steps’. Recall that ‘thinking steps’ refer to steps at the start of episodes where the agent is forced to remain stationary prior to acting. By default – that is, without any thinking time – the agent solves none of the 200 Cutoff levels. However, the agent can solve the vast majority of Cutoff levels when given additional test-time compute to refine its plans. Thus, we consider intervening in Cutoff levels with the aim of artificially replicating the effect of additional test-time compute. That is, we perform interventions that aim to aid the agent in solving the levels without additional decision-time compute. We investigate these interventions in order to determine whether we can intervene on the agent’s internal plan to aid it in forming and executing optimal plans. This is in contrast to Section 6.1 in which we focused on intervening on the agent to encourage it to form and execute sub-optimal plans. + +![](images/figures/emergent-planning-rl-fig-0105.jpg) +Figure 33: The percentage of 200 Cutoff levels the fully-trained agent solves when performing different numbers of ‘thinking steps’ prior to acting. + +We perform three types of interventions in Cutoff levels: ‘Agent-and-Box’ interventions, ‘Agent-Only’ interventions, and ‘Box-Only’ interventions. ‘Agent-Only’ interventions consist of adding an appropriate directional representation of $C _ { \mathrm { { A } } }$ to the target at the entrance of the corridor that corresponds to the agent stepping into the corridor. An example is shown in Figure 32a. The motivation for this intervention is that it should correspond to the agent having extended its plan back fully from the end of the corridor such that the agent knows it should not block the corridor off. If this is the case, the agent should then plan to not myopically push the box onto the target at the corridor entrance. ‘Box-Only’ interventions consist of adding an appropriate directional representation of + +$C _ { \mathrm { B } }$ to the initial position of the box that could be myopically pushed into the corridor entrance to encourage the agent to instead push this box out of the way of the corridor entrance. An example is shown in Figure 32b. The motivation for this intervention is that it should remove the need for the agent to complete planning backwards from the end of the corridor. Finally, ‘Agent-and-Box’ interventions consist of the conjunction of ‘Agent-Only’ and ‘Box-Only’ interventions. An example is shown in Figure 32c. + +In all cases, the interventions are performed by adding the corresponding representations learned by 1x1 probes to the agent’s cell state at one of its ConvLSTM layers. The interventions are repeated at each computational tick until the agent moves the box at the corridor entrance at which point the interventions cease. As previously, we repeat all interventions with 5 trained and 5 randomly initialized probes. Since the agent cannot solve any Cutoff levels without steps of ‘thinking time’, we consider an intervention to be a success if it leads to the agent solving the level. + +Figures 34a, 34b and $3 4 \mathrm { c }$ respectively show success rates when performing ‘Agent-Only’, ‘Box-Only’ and ‘Agent-and-Box’ interventions on the agent’s cell state at each layer using different intervention strengths $\alpha$ . A few key takeaways can be drawn from these results. First, and most importantly, these interventions can replicate the effect of additional test-time compute and allow the agent to solve levels it would otherwise fail. That the concept representations of $C _ { \mathrm { { A } } }$ and $C _ { \mathrm { B } }$ decoded by our 1x1 probes can be used to intervene on the agent in such a way that induces a behavioral effect comparable to that of additional test-time compute is further evidence that the plans decoded by the probes causally influence the agent’s behavior in the way that would be expected. + +Another interesting takeaway from these results is that the pattern of success rates across layers is notably different for ‘Agent-Only’ (Figure 34a) and ‘Box-Only’ (Figure 34b) interventions. This suggests potential insights into the role of each ConvLSTM in the planning process. For instance, note that interventions on layer 2 using representations from trained probes are highly successful when performing ‘Agent-Only’ interventions but are no better than baseline interventions with random probes when performing ‘Box-Only’ interventions. This suggests the agent performs some type of ‘plan conflict detection’ computation either at layer 2, or between layers 2 and 3, such that intervening on layer 2 to encourage the agent to plan to step onto the target causes the agent to detect that acting myopically would conflict with its plans. Likewise, ‘Box-Only’ interventions are much more successful than ‘Agent-Only’ interventions at layer 3. This indicates that agent does not perform ‘plan conflict detection’ computations at layer 3 – since, if it did so we would expect ‘Agent-Only’ interventions to be successful – but does update the routes it plans to push boxes at this layer. + +# C ADDITIONAL TRAINING-TIME INTERPRETABILITY RESULTS + +In Section 6.2 we briefly investigated the emergence of planning-relevant representations during training. Precisely, we demonstrated the co-occurrence during training of (i) the agent’s internal representations of $C _ { \mathrm { { A } } }$ and $C _ { \mathrm { B } }$ in its final layer cell state, and (ii) the ability of the agent to perform better when given additional test-time compute. In this section, we now detail experiments in which we further interpret the agent during the early stages of training. Specifically, we show that: + +• The agent’s internal representations of $C _ { \mathrm { { A } } }$ and $C _ { \mathrm { B } }$ largely emerge at the start of training (Appendix C.1). +• The agent’s ability to refine its internal plans when given additional test-time compute emerges early on in training (Appendix C.2). +• As shown for the final layer in Section 6.2, the emergence of the agent’s representations of $C _ { \mathrm { { A } } }$ and $C _ { \mathrm { B } }$ in its cell state at all layers coincides with the emergence of its ability to perform better when given additional test-time compute (Appendix C.3). +• The emergence during training of the agent’s ability to improve its internal plans when given extra test-time compute coincides with the emergence of its ability to perform better when given this extra compute (Appendix C.4). + +![](images/figures/emergent-planning-rl-fig-0106.jpg) +Figure 34: Success rate when intervening on the agent in Cutoff levels. We consider ‘agent-only’, ‘box-only’ and ‘agent-and-box interventions’. These interventions are respectively performed using the vector representations of $C _ { \mathrm { { A } } }$ , $C _ { \mathrm { B } }$ , and both $C _ { \mathrm { { A } } }$ and $C _ { \mathrm { B } }$ , as learned by 1x1 probes. Interventions are performed using different intervention strengths $\alpha$ on the agent’s cell state at each of its ConvLSTM layers. For each layer and intervention strength, we repeat the intervention 5 times using 5 probes and report the average success rate. We compare interventions performed with trained probes to interventions performed with randomly-initialized probes. Error bars report $\pm 1$ standard deviation. + +![](images/figures/emergent-planning-rl-fig-0107.jpg) +Figure 35: Macro F1 scores achieved when training 1x1 probes to predict (a) $C _ { \mathrm { { A } } }$ and (b) $C _ { \mathrm { B } }$ using the agent’s cell state activations at each layer at checkpoints every 1 million transitions over the first 50 million transitions of training. + +# C.1 INVESTIGATING THE EMERGENCE OF PLANNING-RELEVANT CONCEPT REPRESENTATIONS DURING TRAINING + +First, we investigate the emergence of the planning-relevant concepts we study – $C _ { \mathrm { { A } } }$ and $C _ { \mathrm { B } }$ – over the course of training. We do this by taking checkpoints of the agent every 1 million transitions over the first 50 million (of 250 million total) transitions of training. For each checkpoint, we train 1x1 probes to predict $C _ { \mathrm { { A } } }$ and $C _ { \mathrm { B } }$ using the agent’s cell state activations at that checkpoint. That is, we train and test probes on datasets generated by collecting the agent’s cell state activations when running the agent at that checkpoint. Note that re-training probes at each checkpoint is important since $C _ { \mathrm { { A } } }$ and $C _ { \mathrm { B } }$ are behavior-dependent concepts in that the classes they assign to squares depends on how the agent behaves (which itself changes during training as the agent’s parameters update). + +Figure 35 shows the macro F1 scores achieved when training probes to predict $C _ { \mathrm { { A } } }$ and $C _ { \mathrm { B } }$ based on the agent’s cell state activations over the course of the first 50 million transitions of training. Clearly, the agent’s internal representations of these concepts largely emerge early on in training. However, we note that, even after 50 million transitions, the macro F1s achieved by our probes are still somewhat lower than the macro F1s achieved when probing the fully-trained agent as shown in Figure 4. This suggests that the agent does slightly improve its representations of these concepts over the entire course of training. + +# C.2 INVESTIGATING THE EMERGENCE OF TEST-TIME PLAN REFINEMENT DURING TRAINING + +In Section 5, we used Figure 6 to demonstrate that the agent can use additional test-time compute at the start of episodes to refine its internal plan. Thus, a natural question to ask is when during training does this ability emerge - is this an ability that emerges early on in training and that is gradually improved, or is it an ability that the agent only develops towards the end of training? + +We can investigate the emergence of the ability to refine plans when given additional test-time compute by repeating the setup from Figure 6 – i.e. predicting the agent’s future actions using its internal plans decoded from its cell state by a 1x1 probe during steps of ‘thinking time’ whilst the agent is forced to remain stationary prior to moving – but now whilst doing so for checkpoints of the agent taken throughout training. + +Specifically, we now use the 1x1 probes from Appendix C.1 to decode the agent’s plan when given additional test-time compute. As before, we use checkpoints of the agent taken every 1 million transitions during the first 50 million transitions of training. We force the agent at each checkpoint to perform 5 ‘thinking steps’ prior to acting at the start of 1000 episodes and use 1x1 probes to decode the agents internal representations of $C _ { \mathrm { { A } } }$ and $C _ { \mathrm { B } }$ at each of the 15 corresponding internal computational ticks. We view the predictions made by our probes at each tick as being the agent’s internal plan at each tick. We then measure the average correctness of the agent’s plan at each of the 15 ticks by averaging the macro F1 of the probe’s predictions at each tick. Finally, we measure the extent to which the agent can utilize the extra test-time compute afforded by ‘thinking time’ by measuring the increase in average macro F1 when using the probe’s predictions at the 15th tick relative to at the 1st tick. + +Figure 36 shows the increase in macro F1 for different checkpoints of the agent when using probe predictions from the 15th tick of thinking time relative to the 1st tick of thinking time to predict $C _ { \mathrm { { A } } }$ for each checkpoint. Figure 37 shows the analogous results when predicting $C _ { \mathrm { B } }$ for each checkpoint. Clearly, the agent acquires the ability to use additional test-time compute to refine its ‘internal plan’ (i.e. its internal representations of $C _ { \mathrm { { A } } }$ and $C _ { \mathrm { B } }$ ) early on in training. + +# C.3 INVESTIGATING THE CO-EMERGENCE OF PLANNING-RELEVANT CONCEPT REPRESENTATIONS AND PLANNING-LIKE BEHAVIOR DURING TRAINING + +In Section 6.2 we illustrated that (i) the agent’s internal representations of $C _ { \mathrm { { A } } }$ and $C _ { \mathrm { B } }$ in its final layer cell state and (ii) the ability of the agent to perform better when given additional test-time compute emerge concurrently during training. This naturally leads to the question of whether the emergence of this type of planning-like behavior coincides with the emergence of the agent’s representations of $C _ { \mathrm { { A } } }$ and $C _ { \mathrm { B } }$ at all layers. We now provide evidence that this is indeed the case. + +![](images/figures/emergent-planning-rl-fig-0108.jpg) +Figure 36: Increase in macro F1 when predicting the agent’s future movements $( C _ { \mathbf { A } } )$ using the agent’s internal plan at each layer as decoded at the 1st and 15th tick. Each data point corresponds to the increase in macro F1 between the first and last tick performed during 5 thinking steps for a checkpoint of the agent taken every million transitions during the first 50 million transitions of training. Here, the ‘internal plan’ at a layer for a tick corresponds to the agent’s representation of $C _ { \mathrm { { A } } }$ as decoded by a 1x1 probe applied to the agent’s cell state at that layer at that tick. + +![](images/figures/emergent-planning-rl-fig-0109.jpg) +Figure 37: Increase in macro F1 when predicting future box movements $\left( C _ { \mathbf { B } } \right)$ using the agent’s internal plan at each layer as decoded at the 1st and 15th tick. Each data point corresponds to the increase in macro F1 between the first and last tick performed during 5 thinking steps for a checkpoint of the agent taken every million transitions during the first 50 million transitions of training. Here, the ‘internal plan’ at a layer for a tick corresponds to the agent’s representation of $C _ { \mathrm { B } }$ as decoded by a 1x1 probe applied to the agent’s cell state at that layer at that tick. + +![](images/figures/emergent-planning-rl-fig-0110.jpg) +Figure 38: The relationship between (i) the percentage of extra medium levels solved when an agent is given 5 steps to ‘think’, and (ii) macro F1 score of probes when predicting $C _ { \mathrm { { A } } }$ (blue) and $C _ { \mathrm { B } }$ (orange) from the agent’s cell state at each layer at different checkpoints taken during training. Each point corresponds to these quantities calculated for a single checkpoint. + +As in Section 6.2 we do this by inspecting both (i) the macro F1 of probes trained to predict $C _ { \mathrm { { A } } }$ and $C _ { \mathrm { B } }$ using the agent’s cell state activations at all layers, and (ii) benefit from additional test-time compute. We measure these quantities at checkpoints of the agent taken ever 1 million transitions over the first 50 million transitions of training. As in Section 6.2, we measure the ability of the agent to benefit from additional test-time compute by counting the number of 1000 medium-difficult levels from the Boxoban dataset (Guez et al., 2018a) the agent cannot solve by default, but can solve when given 5 ‘thinking steps’. + +The effect illustrated in Figure 9 – namely, that the period of training in which the agent begins to solve additional levels when given extra compute is the same as the period in which its representations of $C _ { \mathrm { { A } } }$ and $C _ { \mathrm { B } }$ become increasingly well-developed – can be seen across all layers. This can be seen in Figure 38 in which we plot the relationship between (i) the percentage of additional medium levels solved and (ii) the macro F1 when training probes to predict $C _ { \mathrm { { A } } }$ and $C _ { \mathrm { B } }$ using the agent’s cell state activation at each layer for that checkpoint. As would be expected if the agent’s representations of $C _ { \mathrm { { A } } }$ and $C _ { \mathrm { B } }$ were used as part of an internal planning process, the emergence of these representations at all layers is clearly related to the emergence of the agent’s ability to benefit from extra test-time compute. + +# C.4 INVESTIGATING THE CO-EMERGENCE OF TEST-TIME PLAN REFINEMENT AND PLANNING-LIKE BEHAVIOR DURING TRAINING + +Yet, Figure 38 only shows that the emergence of the agent’s representations of $C _ { \mathrm { { A } } }$ and $C _ { \mathrm { B } }$ is related to the emergence of the agent’s planning-like behavior. It hence does not show that the planning process the agent uses these representations as a part of is related to the behavioral evidence of planning exhibited by the agent. + +However, recall that in Appendix C.2 we showed that we could inspect the emergence of the agent’s internal planning process by inspecting the manner in which the plans the agent internally represents in terms of $C _ { \mathrm { { A } } }$ and $C _ { \mathrm { B } }$ become more accurate when given ‘thinking steps’. Specifically, we can use probes to decode the agent’s internal plan at the 1st and final tick of additional compute given by 5 steps of ‘thinking time’ and measure the correctness of these plans using the macro F1 achieved when viewing these plans as predictions of the agent’s future behavior $( C _ { \mathrm { { A } } } )$ and its effect on the environment $\left( C _ { \mathrm { B } } \right)$ . We can then measure the extent to which the agent is internally using these representations as part of an internal planning process by measuring the increase in macro F1 achieved when using the agent’s plans after the first tick and after the final tick of ‘thinking time’. If the agent uses its representations of $C _ { \mathrm { { A } } }$ and $C _ { \mathrm { B } }$ as part of an internal planning process, the agent ought to be able to use the extra compute associated with ‘thinking steps’ to form a better plan with these representations. That is, if these representations are used for planning, we expect the macro F1 of the agent’s internal plan to increase during ‘thinking steps’. + +Figure 39 shows, for checkpoints of the agent taken over the first 50 million transitions of training, the relationship between (i) the percentage of addition levels the agent solves with 5 ‘thinking steps’, and (ii) the increase in macro F1 when using the agent’s internal plan at the 1st and 15th tick of 5 steps of ‘thinking time’ to predict future agent movements $( C _ { \mathrm { { A } } } )$ and box movements $( C _ { \mathrm { B } } )$ . As would be expected if the agent used its representations of $C _ { \mathrm { { A } } }$ and $C _ { \mathrm { B } }$ for planning, checkpoints at which the agent can use extra test-time compute to improve its internal plans are also checkpoints at which the agent solves more levels when given extra compute. + +![](images/figures/emergent-planning-rl-fig-0111.jpg) +Figure 39: The relationship between (i) the percentage of extra medium levels solved when an agent is given 5 steps to ‘think’, and (ii) the increase in macro F1 when predicting $C _ { \mathrm { { A } } }$ (blue) and $C _ { \mathrm { B } }$ (orange) from the agent’s cell state at each layer between the 1st and 15th computational tick of 5 ‘thinking steps’ at different checkpoints taken during training. Each point corresponds to these quantities calculated for a single checkpoint. + +# D ADDITIONAL PROBING RESULTS + +Our methodology is underpinned by our use of linear probes. In this section, we now provide further details regarding the probes we use and further relevant experimental results. This section is organized as follows: + +• Appendix D.1 provides additional details regarding how we train probes. +• Appendix D.2 provides additional metrics – specifically, class-specific recalls, precisions and F1s – for the 1x1 and 3x3 probes we discuss from Section 4.2. +• Appendix D.3 details the macro F1 scores achieved when using larger probes. +• Appendix D.4 outlines the performance of 1x1 and $3 { \tt X } 3$ probes trained to predict alternate square-level concepts to $C _ { \mathrm { { A } } }$ and $C _ { \mathrm { B } }$ . +• Appendix D.5 provides the results of applying ‘global’ probes that receive the agent’s entire cell state as input to predict the agent’s future actions directly. + +# D.1 PROBE TRAINING DETAILS + +In this section, we now provide a brief overview of the manner in which the probes considered in this paper are trained. All probes are trained for 10 epochs using the AdamW optimizer (Loshchilov & Hutter, 2019) and implemented as convolutions with a batch size of 16, learning rate of 0.001 and a weight decay of 0.001. We train probes to predict the concepts assigned to Sokoban squares using the agent’s cell state activations both after training, and at checkpoints taken during training. As the concepts we study depend on the agent’s behavior and thus the agent’s parameters, we train and test probes on different datasets when investigating the agent at different points of training. + +All probes trained to predict concept classes using the fully-trained agent’s cell states are trained and tested on datasets consisting of $1 0 6 . 6 \mathrm { k }$ and $2 5 . 7 \mathrm { k }$ transitions. The training dataset is generated by running the fully-trained agent for 3000 episodes on levels from the Boxoban unfiltered training dataset (Guez et al., 2018a). The test dataset is generated by running the fully-trained agent for 1000 episodes on levels drawn from Boxoban unfiltered validation dataset. + +All probes trained to decode concepts from the agent’s cell state at checkpoints taken during training are trained and tested on checkpoint-specific datasets. For any checkpoint, the training and test datasets are created by collecting transitions when running the agent at that checkpoint for 1000 and 500 episodes in the unfiltered training and unfiltered validation Boxoban datasets respectively. These datasets are of different sizes for each checkpoint as the agent’s behavior changes over the course of training. + +Table 2: Average and standard deviation of class-specific performance metrics when probing for $C _ { \mathrm { { A } } }$ using 1x1 probes. + +
ClassMetricLayer 1Layer 2Layer 3Baseline
NEVERF10.9787 ± 0.00000.9821 ± 0.00010.9845 ± 0.00000.9049 ± 0.0000
Precision0.9795 ± 0.00020.9838 ± 0.00040.9826 ± 0.00010.8279 ± 0.0000
Recall0.9779 ± 0.00010.9804 ± 0.00030.9864 ± 0.00010.9978 ± 0.0000
UPF10.7563 ± 0.00040.8187 ± 0.00040.8396 ± 0.00030.0000 ± 0.0000
Precision0.7405 ± 0.00260.8238 ± 0.00110.8237 ± 0.00191.0000 ± 0.0000
Recall0.7728 ± 0.00250.8137 ± 0.00080.8561 ± 0.00190.0000 ± 0.0000
DOWNF10.7548 ± 0.00030.8278 ± 0.00070.8255 ± 0.00040.0000 ± 0.0000
Precision0.7516 ± 0.00610.8216 ± 0.00390.8440 ± 0.00211.0000 ± 0.0000
Recall0.7581 ± 0.00660.8342 ± 0.00260.8078 ± 0.00180.0000 ± 0.0000
LEFTF10.7985 ± 0.00040.8282 ± 0.00020.7955 ± 0.00050.0000 ± 0.0000
Precision0.7958 ± 0.00190.8126 ± 0.00200.8199 ± 0.00191.0000 ± 0.0000
Recall0.8013 ± 0.00210.8445 ± 0.00250.7726 ± 0.00100.0000 ± 0.0000
RIGHTF10.7238 ± 0.00070.8228 ± 0.00030.8129 ± 0.00020.2262 ± 0.0000
Precision0.7362 ± 0.00290.8181 ± 0.00290.8131 ± 0.00100.2707 ± 0.0000
Recall0.7119 ± 0.00350.8277 ± 0.00330.8128 ± 0.00110.1943 ± 0.0000
+ +# D.2 ADDITIONAL PROBING METRICS + +In Section 4, we investigated training 1x1 and 3x3 probes to predict $C _ { \mathrm { { A } } }$ and $C _ { \mathrm { B } }$ . Figure 4 illustrated the macro F1s achieved by these probes. We now provide additional metrics for these probes. Specifically, we provide, for each class, the precision, recall, and F1 achieved by our probes when viewing that class as the positive class and all other classes as belonging to a single negative class. Since we train 5 probes with different initialization seeds, we report both the mean and standard deviation for all metrics. Tables 2 and 3 respectively show these metrics for 1x1 and $3 \mathrm { x } 3$ probes trained to predict $C _ { \mathrm { { A } } }$ . Tables 4 and 5 respectively show these metrics for 1x1 and $3 { \tt X } 3$ probes trained to predict $C _ { \mathrm { B } }$ . + +
ClassMetricLayer 1Layer 2Layer 3Baseline
NEVERF10.9862 ± 0.00010.9874 ± 0.00020.9877 ± 0.00030.9294 ± 0.0005
Precision0.9865 ± 0.00030.9882 ± 0.00040.9877 ± 0.00030.8887 ± 0.0015
Recall0.9860 ± 0.00010.9866 ± 0.00020.9878 ± 0.00050.9739 ± 0.0008
UPF10.8614 ± 0.00060.8614 ± 0.00040.8735 ± 0.00130.3300 ± 0.0110
Precision0.8619 ± 0.00460.8568 ± 0.00240.8669 ± 0.00400.4682 ± 0.0092
Recall0.8610 ± 0.00370.8660 ± 0.00200.8802 ± 0.00200.2553 ± 0.0151
DOWNF10.8616 ± 0.00060.8677 ± 0.00060.8734 ± 0.00050.3464 ± 0.0082
Precision0.8656 ± 0.00300.8662 ± 0.00350.8791 ± 0.00330.4297 ± 0.0085
Recall0.8576 ± 0.00190.8691 ± 0.00260.8678 ± 0.00260.2907 ± 0.0151
LEFTF10.8606 ± 0.00050.8654 ± 0.00020.8726 ± 0.00030.3528 ± 0.0048
Precision0.8570 ± 0.00340.8579 ± 0.00260.8741 ± 0.00170.4541 ± 0.0026
Recall0.8641 ± 0.00290.8731 ± 0.00280.8710 ± 0.00160.2885 ± 0.0070
RIGHTF10.8536 ± 0.00050.8626 ± 0.00130.8713 ± 0.00150.3524 ± 0.0085
Precision0.8491 ± 0.00310.8653 ± 0.00460.8722 ± 0.00520.4631 ± 0.0091
Recall0.8581 ± 0.00260.8600 ± 0.00240.8704 ± 0.00390.2848 ± 0.0136
+ +Table 3: Average and standard deviation of class-specific performance metrics when probing for $C _ { \mathrm { { A } } }$ using 3x3 probes. + +
ClassMetricLayer 1Layer 2Layer 3Baseline
NEVERF10.9907 ± 0.00000.9943 ± 0.00000.9913 ± 0.00000.9634 ± 0.0000
Precision0.9906 ± 0.00020.9943 ± 0.00010.9913 ± 0.00010.9311 ± 0.0000
Recall0.9909 ± 0.00020.9942 ± 0.00010.9912 ± 0.00010.9980 ± 0.0000
UPF10.8027 ± 0.00080.9126 ± 0.00020.8805 ± 0.00020.0000 ± 0.0000
Precision0.7968 ± 0.00390.9054 ± 0.00160.8562 ± 0.00081.0000 ± 0.0000
Recall0.8089 ± 0.00410.9200 ± 0.00170.9061 ± 0.00130.0000 ± 0.0000
DOWNF10.8386 ± 0.00030.9257 ± 0.00030.8590 ± 0.00040.0000 ± 0.0000
Precision0.8407 ± 0.00310.9327 ± 0.00100.8741 ± 0.00241.0000 ± 0.0000
Recall0.8366 ± 0.00320.9188 ± 0.00140.8444 ± 0.00150.0000 ± 0.0000
LEFTF10.8954 ± 0.00010.9247 ± 0.00030.8058 ± 0.00070.0000 ± 0.0000
Precision0.8968 ± 0.00190.9194 ± 0.00100.8144 ± 0.00081.0000 ± 0.0000
Recall0.8940 ± 0.00180.9301 ± 0.00160.7975 ± 0.00120.0000 ± 0.0000
RIGHTF10.8111 ± 0.00080.9162 ± 0.00050.8315 ± 0.00050.3079 ± 0.0000
Precision0.8182 ± 0.00200.9194 ± 0.00080.8321 ± 0.00160.2707 ± 0.0000
Recall0.8041 ± 0.00340.9131 ± 0.00130.8310 ± 0.00160.3570 ± 0.0000
+ +Table 4: Average and standard deviation of class-specific performance metrics when probing for $C _ { \mathrm { B } }$ using 1x1 probes. + +
ClassMetricLayer 1Layer 2Layer 3Baseline
NEVERF10.9950 ± 0.00000.9956 ± 0.00010.9949 ± 0.00000.9664 ± 0.0001
Precision Recall0.9952 ± 0.00020.9962 ± 0.00010.9949 ± 0.00020.9436 ± 0.0003
UP0.9948 ± 0.00020.9951 ± 0.00010.9949 ± 0.00020.9904 ± 0.0004
F10.9201 ± 0.00050.9313 ± 0.00080.9230 ± 0.00080.3976 ± 0.0108
Precision0.9127 ± 0.00160.9219 ± 0.00070.9160 ± 0.00210.5518 ± 0.0121
DOWNRecall0.9277 ± 0.00120.9409 ± 0.00120.9302 ± 0.00140.3112 ± 0.0166
F10.9312 ± 0.00030.9347 ± 0.00150.9292 ± 0.00030.4246 ± 0.0074
Precision0.9376 ± 0.00270.9360 ± 0.00380.9313 ± 0.00170.5733 ± 0.0153
LEFTRecall0.9248 ± 0.00300.9334 ± 0.00220.9272 ± 0.00140.3376 ± 0.0144
F10.9208 ± 0.00060.9346 ± 0.00060.9160 ± 0.00070.4068 ± 0.0039
Precision0.9182 ± 0.00200.9246 ± 0.00210.9196 ± 0.00260.5884 ± 0.0054
RIGHTRecall0.9233 ± 0.00190.9448 ± 0.00190.9125 ± 0.00380.3109 ± 0.0059
F10.9179 ± 0.00050.9384 ± 0.00070.9317 ± 0.00040.4190 ± 0.0053
Precision Recall0.9153 ± 0.00520.9378 ± 0.00200.9338 ± 0.00300.5819 ± 0.0125
+ +Table 5: Average and standard deviation of class-specific performance metrics when probing for $C _ { \mathrm { B } }$ using 3x3 probes. + +![](images/figures/emergent-planning-rl-fig-0112.jpg) +Figure 40: Comparisons of macro F1 achieved when probing the cell state at each layer of the agent for ‘Agent Approach Direction’ $( C _ { \mathrm { { A } } } )$ and ‘Box Push Direction’ $\left( C _ { \mathrm { B } } \right)$ using 1x1, 3x3, 5x5 and $7 { \bf x } 7$ probes. Reported F1 scores are averaged over five independent training runs. Error bars report standard deviations. + +# D.3 PROBING USING LARGER PROBES + +In addition to probing the agent’s ConvLSTM cell states using 1x1 and 3x3 square-level concepts, we now consider using $\mathbf { \partial } ^ { \bullet } 5 \mathbf { x } 5 \mathbf { \vec { \mathrm { { \perthousand } } } }$ and $ { \mathbf { \ell } } ^ { 6 } 7 { \mathbf { X } } 7 ^ { 5 }$ probes. These are probes that are identical to the 1x1 and $3 { \tt X } 3$ probes considered previously, but that predict the concept class of a square $( x , y )$ using the activations in, respectively, 5x5 and $7 \mathbf { x } 7$ grids about the $( x , y )$ position of the agent’s cell state. We investigate these probes in order to investigate whether the agent represents square-level concepts in a spatially-localized way at individual positions of its cell states, or whether it represents square-level concepts in a distributed way across multiple cell state positions. As previously we also consider baseline versions of these probes trained on the raw observation $x _ { t }$ . + +Figure 40b shows the macro F1 when using 1x1, 3x3, 5x5 and $7 \mathbf { x } 7$ probes. This figure illustrates that increasing the probe size leads to minimal gains in performance when probing the agent’s cell state relative to the increase in the performance of the baseline probes. We take this as evidence that the agent indeed localizes its representations of square-level concepts at individual cell state positions. + +# D.4 PROBING FOR ALTERNATIVE SQUARE-LEVEL CONCEPTS + +In this section, we now investigate the extent to which alternative concepts to the ‘main’ concepts considered in the paper – ‘agent approach direction’ $( C _ { \mathrm { { A } } } )$ and ‘box push direction’ $( C _ { \mathrm { B } } )$ – can be successfully decoded from the agent’s cell state by 1x1 and $3 { \tt X } 3$ probes. Specifically, we investigate binary simplifications of the ‘main’ concepts, and versions of the ‘main’ concepts in which the on-off asymmetry is reversed. + +# D.4.1 PROBING FOR SIMPLIFIED BINARY CONCEPTS + +The first set of additional concepts we probe for are binary simplifications of the concepts studied in the main paper. These binary simplifications are concepts that remove the directional components from $C _ { \mathrm { { A } } }$ and $C _ { \mathrm { B } }$ , and just reflect whether an agent will move onto, or push a box off of, a square. We call these concepts ‘Agent Approach’ and ‘Box Push’. + +These are binary multi-class concepts that map Sokoban squares to the class $\{ \tt N E V E R , A G A I N \}$ . For instance, ‘Agent Approach’ maps a square to NEVER if the agent will never step onto that square again in the remainder of the current episode, and maps that square to AGAIN otherwise. Likewise, ‘Box Push’ maps a square to NEVER if the agent will never push a box off of that square again in the remainder of the current episode, and maps that square to AGAIN otherwise. Probing for these simplified concepts allows us to determine the extent to which the agent learns the directions it will move and push boxes when it visits future squares as opposed to learning simpler concepts merely reflecting which squares it will visit and which squares it will push boxes off of. + +![](images/figures/emergent-planning-rl-fig-0113.jpg) +Figure 41: Comparisons of the macro F1 achieved when probing the cell state at each layer of the agent for the ‘main’ concepts (‘Agent Approach Direction’ and ‘Box Push Direction’) and the ‘binary simplification’ concepts (‘Agent Approach’ and ‘Box Push’). Reported F1 scores are averaged over five independent training runs. Error bars report standard deviations. + +![](images/figures/emergent-planning-rl-fig-0114.jpg) +(d) 3x3 Probes, Box-Based Concepts + +Figures 41a and 41c show the F1 scores achieved when using 1x1 probes to probe for ‘Agent Approach’ and ‘Box Push’ respectively. Figures 41b and 41d show the analogous results when using 3x3 probes. Importantly, probing performance increases only mildly relative to‘Agent Approach Direction’ and ‘Box Push Direction’, suggesting the agent has learned the more complex directional concepts rather than these simpler alternatives. A potential explanation for the minor performance gain when probing for these simpler concepts is that the agent may sometimes know it will visit certain squares but be uncertain what action it will perform regarding them. + +# D.4.2 PROBING FOR REVERSED ASYMMETRICAL CONCEPTS + +The next set of additional concepts we probe for are versions of the concepts studied in the main paper where the on-off asymmetry (i.e. the asymmetry in which $C _ { A }$ captures the direction an agent moves on to a square while $C _ { B }$ captures the direction in which a box is pushed off of a square) is reversed. We call these concepts ‘Agent Exit Direction’ and ‘Box Approach Direction’. Probing for these reversed asymmetrical concepts allows us to determine whether we were correct to probe for the asymmetrical concepts focused on in the main paper. + +Both ‘Agent Exit Direction’ and ‘Box Approach Direction’ are multi-class concepts that map Sokoban squares to the classes {LEFT, RIGHT, UP, DOWN, NEVER}. ‘Agent Exit Direction’ maps squares to the direction which the agent moves the next time it moves off of them (if it ever does in the remainder of the episode). For instance, if the next time the agent moves off of a square the agent moves left, ‘Agent Exit Direction’ maps that square to the class LEFT. ‘Box Approach Direction’ maps squares to the direction the next box is pushed onto them (if a box is ever pushed onto them again in the remainder of the episode. For example, if the next box pushed onto a square is pushed down onto this square, ‘Box Approach Direction’ maps this square to DOWN. + +![](images/figures/emergent-planning-rl-fig-0115.jpg) +Figure 42: Comparisons of macro F1 achieved when probing the cell state at each layer of the agent for ‘main’ concepts (‘Agent Approach Direction’ and ‘Box Push Direction’) and the ‘reversed asymmetrical’ concepts (‘Agent Exit Direction’ and ‘Box Approach Direction’). Reported F1 scores are averaged over five independent training runs. Error bars report standard deviations. + +![](images/figures/emergent-planning-rl-fig-0116.jpg) +(d) 3x3 Probes, Box-Based Concepts + +Figures 42a and 42c compare macro F1 scores when using 1x1 probes to predict, respectively, (a) $C _ { \mathrm { { A } } }$ as opposed to ‘Agent Exit Direction’, and (b) ‘Box Approach Direction’ as opposed to $C _ { \mathrm { B } }$ . Figures 42a and 42d show the analogous results when using 3x3 probes. The key takeaway from these figures is that there are moderate gains in probing performance when probing for ‘Agent Approach Direction’ rather than ‘Agent Exit Direction’ when using 1x1 probes, and small gains when probing for $C _ { \mathrm { B } }$ as opposed to ‘Box Approach Direction’ when using 1x1 probes. That the two concepts with the highest performance are ‘Agent Approach Direction’ and ‘Box Push Direction’ is expected given that these concepts better reflect the transition dynamics of Sokoban in which the agent pushes boxes off of squares by moving on to squares. + +# D.5 PROBING FOR FUTURE ACTIONS + +In the main paper, we apply the methodology introduced in Section 3.1 to show that the DRC agent we study plans in the way that we hypothesized it would in Section 3.2: by planning in terms of the square-level concepts $C _ { \mathrm { { A } } }$ and $C _ { \mathrm { B } }$ . In this section, we now demonstrate how this methodology can be used to falsify a hypothesis regarding how we might expect an agent to plan. + +Specifically, we apply the methodology to falsify the hypothesis that the agent plans by determining which actions it expects to take in a specific number of time steps. Under this hypothesis, the agent would internally represent concepts such as ‘Action To Take in 1 Time Step’, ‘Action To Take in 2 Time Steps’ and so on. These concepts would, for example, assign the class LEFT if the agent moved left in the relevant number of time steps. An agent that represented these concepts would then be able to formulate plans by forming planned action sequences of the form (LEFT, LEFT, UP, RIGHT) that the agent would be able to sequentially execute. + +To apply our methodology to determine whether the agent plans in this way, we must first use linear probes to determine whether the agent linearly represents these concepts. Unlike the square-level concepts studied in the main paper, these concepts are global in the sense that they depend on the entire observed Sokoban board. As such, we can use standard ‘global’ linear probes, i.e. linear probes that receive the agent’s entire cell state at a layer as input. + +![](images/figures/emergent-planning-rl-fig-0117.jpg) +Figure 43: Accuracy achieved when probing the agent’s cell state or, for the baseline, observation to predict the agent’s action in a specific number of time steps. + +We therefore train global probes to predict the agent’s action 1,2, ..., 10 time steps into the future using the agent’s cell state activations at each layer. These global probes have 10,240 parameters . These probes are trained using the same set-up as spatially-local probes as described in Appendix D.1. We also train baseline global probes that receive the agent’s entire observation as input. Note that we use accuracy (not macro F1) as a measure of probe performance here as class imbalance is a lesser issue. However, for these results, accuracy is usually higher than macro F1. + +Figure 43 shows the accuracies achieved by global probes trained to predict the ‘Action To Take in $n$ Time Steps’ for $n \in \{ 1 , \cdots , 1 0 \}$ into the future. While probes trained on the agent’s cell state activations do outperform the baseline, they do not do so by a large margin. The performance of these probes seems especially poor relative to the performance of 1x1 probes (which have $6 4 \mathrm { x }$ fewer parameters) trained to predict $C _ { \mathrm { { A } } }$ and $C _ { \mathrm { B } }$ . The poor performance of these probes implies that these concepts are not linearly represented. We can explain the ability of the global probes to slightly outperform the baseline by noting that global probes can infer some information regarding the agent’s future actions based on the agent’s internal representations of $C _ { \mathrm { { A } } }$ and $C _ { \mathrm { B } }$ . Note, however, that global probes cannot simply read future actions off of the internal plan as it will usually be unclear which planned path the agent will follow. + +Since our probes imply that the agent does not linearly represent the concepts ‘Action To Take in $n$ Time Steps’, we have, according to our methodology, falsified the hypothesis that the agent plans in the way suggested at the start of this section. That is, we have falsified the hypothesis that the agent plans by directly forming planned sequences of actions it expects to sequentially execute. + +# E ADDITIONAL BACKGROUND MATERIAL + +In this section, we now provide additional background to the paper that may be of interest to the reader. Specifically, this section is organized as follows: + +• Appendix E.1 compares our pragmatic characterization of decision-time planning to past definitions of planning. +• Appendix E.2 provides further details on the Sokoban environment. +• Appendix E.3 outlines the DRC agent architecture in greater detail. +• Appendix E.4 details the manner in which the agent we investigate was trained. +• Appendix E.5 provides preliminary behavioral evidence of planning exhibited by the agent we study. + +• Appendix E.6 outlines how we operationalize the kinds of concepts we study in this paper. + +• Appendix E.7 briefly details how our methodology could be applied to alternate agents in alternate environments. + +# E.1 DECISION-TIME PLANNING + +In Section 2.1, we provided a pragmatic characterization of decision-time planning. In this section, we now briefly overview definitions of decision-time planning in other fields. We do so to demonstrate that our characterization is very similar to these past definitions. Specifically, we consider approaches to decision-time planning in: (1) classical AI, (2) neuroscience, and (3) reinforcement learning. + +In classic AI, or, symbolic AI, planning is viewed as the process of an agent formulating a sequence of actions to perform in order to achieve its goal (Russell & Norvig, 2010; Hendler et al., 1990). From this perspective, planning is a search problem: it involves an agent searching for a sequence of actions that can be performed to reach a goal state from the current state (Korf, 1987). This perspective of planning is very similar to our characterization in that it understands planning as requiring formulating sequences of actions and evaluating their consequences (i.e. predicting whether performing the sequence of actions will allow the agent to reach the goal state). + +Decision-time planning is also a topic of interest in neuroscience (Miller et al., 2017; Jensen et al., 2024). Within neuroscience, Mattar & Lengyel (2022) have defined planning as ‘the process of selecting an action or sequence of actions in terms of the desirability of their outcomes’. This is again very similar to our characterization in that it defines planning as involving formulating sequences of actions and evaluating their consequences. + +Finally, in reinforcement learning, decision-time planning is usually taken to refer to the process of an agent interacting with a world model in order to determine which actions, when performed in the current state, will yield positive consequences (Hamrick et al., 2020). As stated in Section 2.1, this definition is very similar to our characterization of planning. The only difference is that this ‘model-based’ definition requires an agent to interact with an explicit world model to predict and evaluate the consequences of actions. Our definition loosens this requirement, only requiring that an agent somehow predict and evaluate the consequences of its actions. Table 6 shows some common approaches to designing RL agents capable of performing decision-time planning. Note that, other than DRC agents as studied in this paper, these approaches all maintain at least some dependence on handcrafted artefects in order to plan. Finally, in RL, decision-time planning is often contrasted with ‘background planning’ which refers to the process of an agent interacting with a world model during training to learn a better policy and/or value function Sutton & Barto (2018). Importantly, our characterization of planning does not aim to capture background planning. + +
Explicit World Model (Known)Explicit World Model (Learned)No Explicit World Model
Explicit SearchAlgorithm(Handcrafted)Example: AlphaZero(Silver et al., 2018)Example: MuZero(Schrittwieser et al., 2020)
Explicit SearchAlgorithm(Partially Learned)Example: MCTSNet(Guez et al., 2018b)Example: I2A(Racanière et al., 2017)
Explicit SearchAlgorithm(Fully Learned)Example: Thinker(Chung et al., 2024a)
No ExplicitSearch AlgorithmPotential Example: DRC(Guez et al., 2019)
+ +Table 6: Summary of common approaches to creating RL agents capable of decision-time planning. Agents are categorized based on the extent to which they rely on handcrafted, explicit world models and search algorithms. A search algorithm is explicit if it relies on handcrafted rather than learned elements. A world model is explicit if it depends more on handcrafted elements and less on learnable components. DRC agents are listed as potential examples as, prior to this work, it was unclear whether they performed decision-time planning. + +# E.2 SOKOBAN + +This paper investigates planning in the context of Sokoban. As explained in Section 2.2, Sokoban is a deterministic, episodic environment in which an agent operating in a 8x8 gridworld seeks to navigate around walls to push four boxes onto four targets. In this section, we provide a detailed explanation of the transition and reward dynamics of Sokoban, as well as of the symbolic representations of Sokoban boards our agent observes. + +Sokoban’s transition dynamics are as follows. At each time step, a Sokoban agent must choose to either move up, down, left, right or not to move. When an agent moves left, right, up or down onto a square currently containing a box, that box is respectively pushed on to the square to the left, the right, below, or above. If the move an agent attempts to perform would involve pushing a box into a non-empty square - that is, a square containing either a wall or another box - neither the box nor the agent moves. The agent cannot push two adjacent boxes simultaneously. The agent can not pull boxes. An episode ends either when (a) the agent successfully pushes all boxes onto targets or (b) when an episode length exceeds a random number between 115 and 120. + +Sokoban’s reward structure is as follows. + +• The agent receives a reward of -0.01 at each environment step. • The agent receives a reward of $+ 1$ when it pushes a box on to a target square. • The agent receives a reward of -1 when it pushes a box off of a square • The agent receives a reward of $+ 1 0$ after pushing a box onto all four targets. + +In this paper, we study a version of Sokoban that uses symbolic environment representations. Each square of a Sokoban board is always in one of seven states: it is either a wall, an empty square, a box on an otherwise empty square, the agent on an otherwise empty square, a box on a target, the agent on a target, or a target with nothing on it. Thus, in our symbolic representation, we represent each square of an observed Sokoban board as a seven-dimensional one-hot vector and then combine these vectors into an array to produce the agent’s observation $x _ { t } \in \mathbb { R } ^ { 8 \times 8 \times 7 }$ of the environment state at time $t$ . Importantly, however, in all figures in which we show an example of a Sokoban board, we use an RGB pixel representation of Sokoban. We do this to allow the reader to more easily understand visualized levels. Figure 2 compares the pixel and symbolic representation of a Sokoban board, where each color in the symbolic representation denotes which dimension of the one-hot vector is active for each square. It should also be noted that our version of Sokoban forgoes the layer of wall squares that is sometimes appended to the edge of Sokoban boards in previous work. + +# E.3 DEEP REPEATED CONVLSTM (DRC) AGENT ARCHITECTURE + +The agent studied in this paper is a Deep Repeated ConvLSTM agent as introduced by Guez et al. (2019). DRC agents are a recurrent actor-critic architecture. At each time step $t$ , a convolutional encoder $e$ processes the agent’s observation of the current environment state, $x _ { t }$ , into an encoding $i _ { t } \in \mathbb { R } ^ { \mathbf { \lambda } _ { H _ { 0 } } \times W _ { 0 } \times G _ { 0 } }$ . This is then processed by the recurrent backbone of the DRC architecture, which is a stack of ConvLSTMs (Shi et al., 2015). A ConvLSTM is a modified LSTM (Hochreiter & Schmidhuber, 1997) that include a 3D hidden state and uses convolutional connections. The DRC architecture utilises a stack of $D$ ConvLSTM units with untied parameters $\boldsymbol { \theta } = ( \theta _ { 1 } , \cdot \cdot \cdot , \theta _ { d } )$ . These ConvLSTM units performs recurrent computations and, at time $t$ , have an internal state $s _ { t } ^ { d } = ( h _ { t } ^ { d } , g _ { t } ^ { d } )$ where $h _ { t } ^ { d } , \mathbf { \bar { \{ g _ { t } ^ { d } \in \mathbb { R } } ^ { H _ { d } \times W _ { d } \times G _ { d } } } $ are, respectively, the output and cell state of the $d$ - th ConvLSTM unit. For the agent we study, the encoder and all ConvLSTM units have a hidden dimensionality of $G _ { d } = 3 2 , 0 \le d \le D$ and utilise kernels of size 3 with a single layer of zero padding appended to convolution inputs. This means that all ConvLSTM states maintain the spatial dimensions of the environment state, so that $H _ { d } = W _ { d } = 8$ for all $0 \leq d \leq D$ . + +The DRC ConvLSTM stack includes a number of enhancements relative to standard ConvLSTM architectures. These are generic modifications that aim to improve the broad capacity of the architecture as a function approximator. The most notable of these is that, rather than performing a single step of recurrent computation for each time step, the DRC architecture performs $N$ steps of recurrent computation per time step. If the current state of the stack of $D$ ConvLSTM units is $s _ { t - 1 }$ , and we denote the operation of this stack on input encoding $i _ { t }$ as $f _ { \theta } ( i _ { t } , s _ { t - 1 } )$ , the computation performed by the ConvLSTM stack at each time step $t$ can be described by the equations below. + +![](images/figures/emergent-planning-rl-fig-0118.jpg) +Figure 44: Illustration of DRC(3,3) architecture. For each time step, the architecture encodes the input $x _ { t }$ as a convolutional encoding $i _ { t }$ , passes it to a stack of 3 ConvLSTMs which perform three ticks of recurrent computation and then outputs policy logits $\pi _ { t }$ and a value estimate $v _ { t }$ . + +$$ +s _ { t , 0 } = s _ { t - 1 } , +$$ + +$$ +s _ { t , n } = f _ { \theta } ( i _ { t } , s _ { t , n - 1 } ) , \ 0 < n \leq N , +$$ + +$$ +\boldsymbol { s } _ { t } = \boldsymbol { s } _ { t , N } . +$$ + +The stack of $D$ ConvLSTM units hence performs $N$ ticks of internal recurrent computation for each single time step $t$ in the environment. DRC agents are referred to as a $D R C ( D , N )$ agents to make the choice of hyperparameters $D$ and $N$ explicit. The DRC architecture also includes the following additional modifications relative to a baseline ConvLSTM architecture: + +• Bottom-Up Skip Connections: To allow information to flow up the ConvLSTM stack, the input encoding $i _ { t }$ is provided as an input to all ConvLSTM units in the stack. • Top-Down Skip Connections: To allow information to additionally flow down the ConvLSTM stack, the output of the final ConvLSTM unit on the current tick is provided as an additional input to the bottom ConvLSTM unit on the next tick. • Pool-and-Inject: to allow spatial information to spread rapidly, each ConvLSTM cell additionally receives a version of its output $h _ { t , n - 1 } ^ { d }$ on the prior tick that is spatially pooled. Specifically, this pooled output $p _ { t , n - 1 } ^ { d }$ is produced by separately mean- and max-pooling $h _ { t , n - 1 } ^ { d }$ spatially, passing the concatenated pooled vectors through an affine transformation, and then reshaping the result to match the dimensions of the $h _ { t , n - 1 } ^ { d }$ . This is shown below. + +$$ +\begin{array} { r l } & { m _ { t , n - 1 } ^ { d } = [ \boldsymbol { \mathrm { M e a n P o o l } } _ { H _ { d } , W _ { d } } ( h _ { t , n - 1 } ^ { d } ) , \boldsymbol { \mathrm { M a x P o o l } } _ { H _ { d } , W _ { d } } ( h _ { t , n - 1 } ^ { d } ) ] ^ { T } \in \mathbb { R } ^ { 2 G _ { d } } , } \\ & { \hat { p } _ { t , n - 1 } ^ { d } = W _ { p _ { d } } m _ { t , n - 1 } ^ { d } + b _ { p _ { d } } \in \mathbb { R } ^ { H _ { d } W _ { d } G _ { d } } , \ W _ { p _ { d } } \in \mathbb { R } ^ { H _ { d } W _ { d } C _ { d } \times 2 G _ { d } } , \ b _ { p _ { d } } \in \mathbb { R } ^ { W _ { d } H _ { d } G _ { d } } , } \\ & { \qquad p _ { t , n - 1 } ^ { d } = \boldsymbol { \mathrm { R e s h a p e } } _ { H _ { d } \times W _ { d } \times G _ { d } } ( \hat { p } _ { t , n - 1 } ^ { d } ) \in \mathbb { R } ^ { H _ { d } \times W _ { d } \times G _ { d } } . } \end{array} +$$ + +Finally, the output $h _ { t , N } ^ { D }$ of the final ConvLSTM cell at the final tick $N$ is concatenated with the input encoding $i _ { t }$ and undergoes an affine transformation followed by a ReLU non-linearity to generate a vector of activations $o _ { t }$ which is then fed to a policy head and a value head. The policy head performs an affine transformation on $o _ { t }$ to generate a vector of action logits which parameterises a categorical distribution from which the next action can be sampled $a _ { t } \sim \pi _ { t }$ . The value head estimates the statevalue $v _ { t }$ of the current policy for the current environment state as a linear combination of $o _ { t }$ . The policy and value heads are used as an actor and critic in order to train the agent in an actor-critic fashion. Figure 44 illustrates the computation performed a DRC(3,3) agent on a single time step. + +![](images/figures/emergent-planning-rl-fig-0119.jpg) +Figure 45: The percentage of 1000 ‘Medium’ and ‘Hard’ levels that the agent solves when the agent performs ‘thinking steps’. Zero thinking steps corresponds to the agent’s standard behavior. + +# E.4 DRC AGENT TRAINING DETAILS + +This paper focuses on analyzing a Deep Repeated ConvLSTM (DRC) agent trained to play Sokoban. The DRC agent we investigate is trained on $9 0 0 \mathrm { k }$ levels from the unfiltered training set of the Boxoban dataset (Guez et al., 2018a). The agent is trained in an actor-critic setting using IMPALA (Espeholt et al., 2018) for 250 million transitions. + +We train the agent using a discount rate of $\gamma = 0 . 9 7$ and V-trace target of $\lambda = 0 . 9 7$ . The agent is trained by additionally imposing a $\mathcal { L } ^ { 2 }$ penalty of size 1e-3 on the action logits, $\mathcal { L } ^ { 2 }$ regularisation of strength 1e-5 on the policy and value heads, and adding an entropy penalty of strength 1e-2 on the policy. Optimisation is performed using propagation through time with an unroll length of 20. We use the Adam optimiser (Kingma & Ba, 2015) with a batch size of 16 and a learning rate that decays linearly from 4e-4 to 0. + +During training, the agent selects actions by sampling from a categorical distribution parameterized by its policy head logits. Once trained, the agent acts greedily by always performing the action with the greatest logit. After training, the agent solves $9 7 . 3 \%$ of unseen levels from the unfiltered test set of the Boxoban dataset (Guez et al., 2018a). + +# E.5 BEHAVIORAL EVIDENCE OF PLANNING EXHIBITED BY THE DRC AGENT + +As explained in Section 2.3, this paper is motivated by the phenomenon of DRC agents behaving in a way that suggests that they perform planning. For instance, DRC agents have been found in past work to solve additional Sokoban levels when forced to perform ‘thinking steps’, which are steps at the start of episodes where the agent is forced to remain stationary (Guez et al., 2019; Taufeeque et al., 2024). We now confirm that the agent we analyze in this paper also exhibits this planning-like behavior. + +We do this by investigating amount of levels the fully-trained agent solves when given differing numbers of thinking steps. We investigate this in two datasets of Sokoban levels taken from the Boxoban dataset (Guez et al., 2018a). These are the ‘Medium’ and ‘Hard’ subsets of levels. As suggested by their names, these levels are more difficult than the ‘unfiltered’ subset of levels the agent was trained on. We use subsets consisting of 1000 levels taken from each dataset. Figure 45 shows the percentage of these 1000 Medium and Hard levels the agent solves when performing between zero and five thinking steps. Zero thinking steps corresponds to the agent’s standard behavior. Clearly, the agent performs better when forced to perform ‘thinking steps’. This represents planning-like behavior. This is because an agent capable of planning would be able to make use of the additional test-time compute afforded by ‘thinking steps’ to refine its plan. + +# E.6 OPERATIONALIZING CONCEPTS + +The concepts we investigate in this paper differ somewhat from the standard operationalization of concepts implicit in the concept-based interpretability literature. In this section, we thus explain the ways in which the concepts we study are abnormal, and provide a formal operationalization of this type of concept. + +A discrete concept $C$ that takes one of $W$ values in the set $\Lambda _ { C } = \{ c _ { 1 } , \cdots , c _ { W } \}$ for every possible model input $x \in S$ is typically operationalized as a mapping $C : S \Lambda _ { C }$ that maps every input $x$ to the value taken by the concept on that input, $C ( x ) \in \Lambda _ { c }$ (McGrath et al., 2022). The concepts we investigate differ in the following ways: + +• Behavior Dependence: We believe that defining concepts to be mappings from environment states to concept classes is overly restrictive in the context of reinforcement learning. This is because RL agents are situated in a continuing interaction with their environment whereby current actions influence future environment states. Hence, RL agents could plausibly learn concepts depending upon their own behavior. An example of such a concept depending on both the environment state and agent behavior in Sokoban might be ‘a Sokoban board that will be solved within five moves’. Importantly for present purposes, we believe that behavior dependent concepts would be natural concepts for a planning-capable agent to learn. This is because existing model-based planners rely on explicit world models for predicting future behavior when evaluating immediate actions to perform planning. Such predictions are implicit within behavior-dependent concepts. Thus, the concepts we study depend on the agent’s current parameters (since these directly determine agent behavior) and on the past observations encountered by an agent in an episode (since the DRC agent’s recurrent architecture allows these to influence future behavior). + +• Spatial Localization: We also believe that, in environments with spatially-localized dynamics like Sokoban, agents could learn similarly spatially-localized concepts. All concepts we investigate are hence features of individual squares in Sokoban boards. + +We thus propose the following pragmatic operationalization of a discrete concept $C$ that takes one of $K$ values in the set $\Lambda _ { C } = \{ c _ { 1 } , \cdots , c _ { K } \}$ for a square in a Sokoban board. This definition is proposed primarily to characterise the specific concepts we study though could serve as inspiration for future definitions of concepts in RL. Let $s$ denote the state space of Sokoban boards, and let $\mathcal { G } = \{ ( i , j ) \} _ { i , j = 1 } ^ { 8 }$ be a set of square indexes describing an $8 \mathrm { x } 8$ Sokoban board. The index $( i , j )$ refers to the square in the $i$ -th row of the $j$ -th column. Further, let $\Theta$ be the set of all possible parameters for a given DRC agent and let $\mathcal { H }$ be the set of all possible sequences of observed past Sokoban boards. A concept $C$ is then defined as a mapping $C : \mathcal { S } \times \mathcal { G } \times \mathcal { H } \times \Theta \Lambda _ { C }$ from a particular square $( i , j ) \in { \bar { \mathcal { G } } }$ of a presently-observed Sokoban board $x _ { t } \in S$ , and from an agent’s current parameters $\theta ~ \in ~ \Theta$ and past episode observations $( x _ { o } , x _ { 1 } , \cdot \cdot \cdot , x _ { t - 1 } ) \in \mathcal { H }$ to the value c(i,j)xt that the concept takes on that square given agent behavior. More compactly, and suppressing dependence on past observations for notational simplicity, the concept value taken on square $( i , j )$ of Sokoban board $x _ { t }$ when an agent has parameters $\theta$ can be written as $C _ { x _ { t } } ^ { ( i , j ) } = C ( x _ { t } , ( i , j ) , \theta )$ . + +E.7 APPLICATION OF METHODOLOGY TO OTHER MODEL-FREE ARCHITECTURES + +We believe that, at a high level, the methodology introduced in Section 3.1 is general and could be applied to any model-free agent in any environment. Applying the method to a general model-free agent in a general environment would involve three steps. We illustrate these steps using the example of a model-free agent trained on Breakout: + +1. Probe For Concept Representations In the first step, we hypothesize concepts the agent could plan with, and then probe for these concepts. For instance, the Breakout agent might plan using concepts corresponding to which bricks it plans to remove over the next 10 hits of the ball. + +2. Investigate Plan Formation In the second step, we would inspect the manner in which the agent’s concept representations develop at test-time. For instance, we might investigate whether the Breakout agent’s representations of the above concepts developed in a way that corresponded to iteratively constructing a planned hole to drill through the wall from the bottom to the top of the wall. + +3. Confirm Behavioral Dependence In the final step, we would investigate whether we could use the vectors from the linear probes to intervene to steer the agent in the expected way. For instance, we could intervene on the Breakout agent to force it to drill a hole at a specific location of the wall. + +Note, however, that the practical application of each of these steps will depend on assumptions made by the researcher in the specific experimental setting. In this paper, we made the following assumptions that informed the application of the methodology: + +• Spatial Localization We assumed that the DRC agent we investigated represented concepts in a spatially-localized manner. This informed our choice of spatially-local probes. However, while the assumption of spatially-localized concept representations may hold in some cases (e.g. CNN-based Atari agents), it is unlikely to hold for all agents (e.g. MLPbased Mujoco agents). In cases where it doesn’t hold, we would have to probe all of the agent’s activations at a specific layer rather than using spatially-localized probes. + +• Linear Concept Representations We likewise assumed that any concepts the DRC agent represented, it represented linearly (Mikolov et al., 2013). This informed our decision to use linear probes. However, this assumption may be argued to be overly-restrictive. If this were the case, we would instead have to use non-linear probes (i.e. probes containing non-linearities). + +# F INVESTIGATING PLANNING IN DRC AGENTS OF DIFFERENT SIZES + +In the main paper, we investigate emergent planning in a DRC agent with $D = 3$ layers that performs $N = 3$ internal ticks of computation per time step. In this section, we now perform a preliminary investigation of DRC agents of different sizes and provide evidence that they too engage in planning. Specifically, we investigate whether two DRC agents of different sizes internally represent $C _ { \mathrm { { A } } }$ and $C _ { \mathrm { B } }$ . Using the terminology from Appendix E.3 – in which we referred to a DRC agent with $D$ layers that performed $N$ ticks of computation per step as a DRC(D,N) agent – the agents we investigate are: + +• A DRC(1,9) agent (Appendix F.1) • A DRC(9,1) agent (Appendix F.2) + +Both agents investigated in this section are trained for 100 million transitions using the training scheme described in Appendix E.4. Likewise, both agents exhibit behavioral evidence of planning. For instance, the DRC(1,9) and DRC(9,1) agents respectively solve additional medium difficulty levels when given five ‘thinking steps’ (i.e. forced stationary steps) prior to acting at the start of episodes. + +We use 1x1 and 3x3 probes to investigate whether these agents internally represent $C _ { \mathrm { { A } } }$ and $C _ { \mathrm { B } }$ . As in Section 4, we train probes with 5 different initialization seeds. All probes in this section are trained using a training scheme identical to that described in Appendix D.1 except for the fact that the training and test datasets consist of all transitions generated when running the corresponding agent for 500 and 250 episodes respectively. + +# F.1 INVESTIGATING PLANNING IN A DRC(1,9) AGENT + +We first investigate whether the DRC(1,9) agent exhibits evidence of internally planning. To reiterate, this agent only has a single ConvLSTM layer, but performs 9 internal ticks of computation for each environment time step. Due to only having a single layer, this agent represents an interesting case-study of a low-capacity model-free agent. After $1 0 0 \mathrm { m }$ transitions of training, this agent solves $8 4 . 9 \%$ of unseen levels, and solves an additional $0 . 8 \%$ of medium-difficulty levels when given five ‘thinking steps’. While clearly less capable than the DRC(3,3) agent we focus on, the ability of the agent to solve unseen Sokoban levels, and to benefit from additional test-time compute, represents behavioral evidence of planning. + +![](images/figures/emergent-planning-rl-fig-0120.jpg) +Figure 46: Macro F1s achieved by probes when predicting (a) $C _ { \mathrm { { A } } }$ and (b) $C _ { \mathrm { B } }$ using the DRC(1,9) agent’s cell state, or, for the baseline probes, using the observation. Error bars show $\pm \nobreakspace 1 \nobreakspace$ standard deviation. + +![](images/figures/emergent-planning-rl-fig-0121.jpg) +Figure 47: Success rates when intervening on the cell state of the DRC(1,9) agent in (a) Agent-Shortcut and (b) Box-Shortcut levels using trained and randomly-initialized probes. Error bars show $\pm \nobreakspace 1 \nobreakspace$ standard deviation. + +Probing For Planning-Relevant Concepts Figures 46a and 46b respectively show the macro F1 scores achieved when probing this agent for $C _ { \mathrm { { A } } }$ and $C _ { \mathrm { B } }$ . As with the DRC(3,3) agent investigated in the paper, the agent appears to represent these planning-relevant concepts. This can be seen in the strong performance of the 1x1 and 3x3 probes relative to the respective baseline. Similarly, as with the DRC(3,3) agent, the agent appears to represent these concepts in a spatially-localized manner. This can be seen in the minimal increase in performance when moving from the 1x1 probes to $3 { \tt X } 3$ probes relative to the baseline. Given that these concepts correspond to predictions of future behavior, and of the impacts of future behavior on the environment, these results suggest that the DRC(1,9) agent is planning. + +Investigating Plan Formation Further evidence of the DRC(1,9) using these concepts to plan can be seen in Figure 48 in which we force the agent to perform five ‘thinking steps’ prior to acting and measure the average macro F1 when predicting $C _ { \mathrm { { A } } }$ and $C _ { \mathrm { B } }$ using the agent’s cell state at each internal tick the agent performs during these thinking steps. As with the DRC(3,3) agent, the DRC(1,9) agent seems to iteratively refine its internal plan as formulated in terms of $C _ { \mathrm { { A } } }$ and $C _ { \mathrm { B } }$ . Qualitative evidence of the DRC(1,9) agent internally planning can be seen in Figures 52a, 53a and $5 4 \mathrm { a }$ in which we visualize the agent’s internal representations of $C _ { \mathrm { B } }$ over the initial 12 steps of episodes. + +Confirming Behavioral Dependence Finally, Figures 47a and 47b show the success rates when intervening with the vectors learned by 1x1 probes to steer the behavior of the DRC(1,9) agent in Agentand Box-Shortcut levels in the manner described in Section 6.1. Agent-Shortcut interventions are very successful. While Box-Shortcut are somewhat less successful than Agent-Shortcut interventions, they still are significantly more successful than interventions with random probe vectors. + +![](images/figures/emergent-planning-rl-fig-0122.jpg) +Figure 48: Macro F1 when using 1x1 probes to decode $C _ { \mathrm { { A } } }$ and $C _ { \mathrm { B } }$ from the DRC (1,9) eighth-layer agent’s cell state at each of the additional 45 internal ticks performed by the DRC (1,9) agent when the agent is given 5 ‘thinking steps’, averaged over 1000 episodes. + +![](images/figures/emergent-planning-rl-fig-0123.jpg) +Figure 49: Macro F1s achieved by probes when predicting (a) $C _ { \mathrm { { A } } }$ and (b) $C _ { \mathrm { B } }$ using the DRC(9,1) agent’s cell state at each layer, or, for the baseline probes, using the observation. Error bars show $\pm$ 1 standard deviation. + +![](images/figures/emergent-planning-rl-fig-0124.jpg) +Figure 50: Success rates when intervening on the cell state of the DRC(9,1) agent at each layer in (a) Agent-Shortcut and (b) Box-Shortcut levels using trained and randomly-initialized probes. Error bars show $\pm \nobreakspace 1 \nobreakspace$ standard deviation. + +# F.2 INVESTIGATING PLANNING IN A DRC(9,1) AGENT + +We now turn attention to investigating whether the DRC(9,1) exhibits evidence of internally planning. To re-iterate, this agent has 9 ConvLSTM layers but only performs a single recurrent tick of computation per time step in the environment. The lack of additional internal ticks means this agent is an instance of a generic recurrent, model-free agent. As such, it is an interesting case-study for investigating whether generic recurrent agents can learn to internally plan. After $1 0 0 \mathrm { m }$ transitions of training, this agent exhibits behavioral evidence of planning as it solves $9 4 . 2 \%$ of unseen levels, and solves an additional $5 . 3 \%$ of medium-difficulty levels when given five ‘thinking steps’. + +Probing For Planning-Relevant Concepts Figures 49a and 49b respectively show the macro F1 scores achieved when probing this agent for $C _ { \mathrm { { A } } }$ and $C _ { \mathrm { B } }$ . As with the DRC(3,3) agent investigated in the paper, the agent appears to represent these planningrelevant concepts, and appears to do so in a spatiallylocalized manner. However, the agent only does so at a few layers. Namely, the agent only appears to robustly represent $C _ { \mathrm { { A } } }$ at layers 8 and 9, and to robustly represent $C _ { \mathrm { B } }$ at layers 6, 7 and 8. Evidence of this can be seen in the fact that these are the layers at which 1x1 probes strongly outperform the baseline, and at which there are only minimal gains in macro F1 when moving from 1x1 probes to $3 { \tt X } 3$ probes. Given that these concepts correspond to predictions of future actions and their impact on the environment, the fact that the agent represents these concepts provides evidence that it is planning. + +![](images/figures/emergent-planning-rl-fig-0125.jpg) +Figure 51: Macro F1 when using 1x1 probes to decode $C _ { \mathrm { { A } } }$ and $C _ { \mathrm { B } }$ from the DRC (9,1) agent’s eighth-layer cell state at each of the additional 5 internal ticks performed by the DRC (9,1) agent when the agent is given 5 ‘thinking steps’, averaged over 1000 episodes. + +Investigating Plan Formation Further evidence of the DRC(9,1) agent planning can be seen in Figure 51. In Figure 51, we force the agent to perform five ‘thinking steps’ prior to acting and measure the average macro F1 when predicting $C _ { \mathrm { { A } } }$ and $C _ { \mathrm { B } }$ using the eighth-layer agent’s cell state after each of these thinking steps. We use the agent’s eighth layer as this is the layer at which probes achieve the highest macro F1. Clearly, the agent’s internal plan, as formulated in terms of $C _ { \mathrm { { A } } }$ and $C _ { \mathrm { B } }$ , becomes iteratively more accurate when the agent is provided with additional test-time compute. This would be expected if the agent was indeed engaging in iterative planning. Qualitative evidence of the DRC(9,1) agent internally planning can be seen in Figures 52b, 53b and 54b in which we visualize the agent’s internal representations of $C _ { \mathrm { B } }$ over the initial 12 steps of episodes. Note that, as would be expected, the DRC(9,1) agent takes more environment steps to arrive at plans than the DRC(1,9) and DRC(3,3) agents. + +Confirming Behavioral Dependence Finally, Figures 50a and 50b show the success rates when intervening with the vectors learned by 1x1 probes to steer the behavior of the DRC(1,9) agent in Agent- and Box-Shortcut levels in the manner described in Section 6.1. Note that, in the interventions detailed in Figures 50a and 50b, it was found to be necessary to scale probe vectors by a scaling factor of 4. These results indicate that the DRC(9,1) agent does use its representations of $C _ { \mathrm { { A } } }$ and $C _ { \mathrm { B } }$ for planning. This is because, in general, the layers at which interventions are most successful (relative to the baseline) are the layers at which probes achieve the highest macro F1 scores. Note, however, that the success of interventions cannot be fully explained by the success of the probing the respective layer. A notable example of this is the much greater success rate of Box-Shortcut interventions when intervening at layer 6 rather than layer 8, even though probes are somewhat more accurate at layer 8. We hypothesize that this is a consequence of the DRC(9,1) agent accurately representing its plans to push boxes at many layers (layers 6-8), but only causally altering these plans at a single layer (layer 6). This aligns with the observation that Agent-Shortcut interventions become more successful at later layers, as it may be that the role of these later layers is to determine which actions the agent needs to perform to execute its plans to push boxes. + +# G INVESTIGATING PLANNING IN A DIFFERENT ARCHITECTURE: RESNET + +As mentioned previously, two questions left open by the main paper are (1) whether an agent with a more generic architecture like a ResNet can learn to internally plan, and (2) whether we can use the methodology introduced in Section 3.1 to determine whether such an agent is planning. In this section, we now apply our methodology to a ResNet agent trained to play Sokoban, and provide preliminary evidence suggesting an affirmative answer to both of the aforementioned questions. + +The results here regard a very simple ResNet agent. This agent is parameterized by a network consisting of 24 simplified residual blocks. At each residual block, the input is passed through a convolution, followed by a layer norm, a ReLU, another convolution and another layer norm. The original input is then added to the result of these operations, and is passed through a final ReLU. These residual blocks perform no down- or up-sampling, and make use of no pooling operations. For consistency with the agents studied in this paper, all residual blocks have 32 channels. After the final residual block, the activations are flattened, passed through an MLP of dimensionality 256, and then passed to policy and value heads. This agent is trained for 250 million transitions using IMPALA with the same training scheme as described in Appendix E.4. + +Probing For Planning-Relevant Concepts Figures 55a and $5 5 \mathrm { b }$ respectively show the macro F1 scores achieved when probing the hidden state of this agent for $C _ { \mathrm { { A } } }$ and $C _ { \mathrm { B } }$ after the final ReLU at each layer using both 1x1 and 3x3 probes. Note that, as with the DRC agents, the ResNet agent appears to possess spatially-localized concepts. Further, note that the 1x1 probes become iteratively more accurate over layers until a point at which the reverse trend begins. Specifically, the 1x1 probes for $C _ { \mathrm { B } }$ improve until about layer 10, whilst the probes for $C _ { \mathrm { { A } } }$ improve for longer until about layer 16. We hypothesize that this means that the ResNet agent is internally planning using spatiallylocalized concepts relating to box and agent movements, and that it is doing so by first determining how to move boxes, and then, afterward, reasoning about what that means for its own movements. + +![](images/figures/emergent-planning-rl-fig-0126.jpg) +Figure 52: The internal plan of a (a) DRC(1,9), (b) DRC(9,1) and (c) DRC(3,3) agent after the final internal tick over 12 steps of the same level. Plans are decoded from the agents’ (a) first, (b) eighth and (c) third layer using a 1x1 probe. + +![](images/figures/emergent-planning-rl-fig-0127.jpg) +Figure 53: The internal plan of a (a) DRC(1,9), (b) DRC(9,1) and (c) DRC(3,3) agent after the final internal tick over 12 steps of the same level. Plans are decoded from the agents’ (a) first, (b) eighth and (c) third layer using a 1x1 probe. + +![](images/figures/emergent-planning-rl-fig-0128.jpg) +Figure 54: The internal plan of a (a) DRC(1,9), (b) DRC(9,1) and (c) DRC(3,3) agent after the final internal tick over 12 steps of the same level. Plans are decoded from the agents’ (a) first, (b) eighth and (c) third layer using a 1x1 probe. + +![](images/figures/emergent-planning-rl-fig-0129.jpg) +Figure 55: Macro F1s achieved by probes when predicting the concepts (a) $C _ { \mathrm { { A } } }$ and (b) $C _ { \mathrm { B } }$ using the ResNet agent’s hidden state after the final ReLU of the residual block at each layer, or, for the baseline probes, using the raw observation. For each layer and probe type, we train five probes with five unique initialization seeds in the manner described in Appendix D.1. The reported error bars show $\pm \nobreakspace 1 \nobreakspace$ standard deviation. + +![](images/figures/emergent-planning-rl-fig-0130.jpg) +Figure 56: Success rates when intervening on the hidden state of the ResNet agent after the residual block at each layer in (a) Agent-Shortcut and (b) Box-Shortcut levels using trained and randomlyinitialized probes. Error bars show $\pm \nobreakspace 1 \nobreakspace$ standard deviation. + +Furthermore, the fact that the probes are becoming iteratively more accurate over layers suggests that the agent has learned to perform something akin to iterative planning despite its lack of recurrent connections. + +Investigating Plan Formation . Qualitative evidence of the agent iteratively planning across its layers can be seen in Figure 57 in which we visualize the agent’s internal plan (in terms of $C _ { \mathrm { B } }$ ) over the first ten of its layers at the first time step of four episodes. Of these four episodes, the agent constructs a successful plan by its tenth layer in two episodes (Figures 57a and Figures 57b), but does not do so in the other two (Figures 57c and Figures 57d). Beyond suggesting that the ResNet agent is iteratively planning across layers, Figure 57 provides preliminary evidence that the ResNet agent is iteratively planning in a manner similar to the DRC(3,3) agent focused on in the paper. + +Specifically, like the DRC agent, the ResNet agent appears to possess an internal planning mechanism with similarities to evaluative bi-directional search. For instance, evidence of plan evaluation can be seen in Figure 57b in which the agent initially (e.g. at layer 1) plans to push both left-hand boxes to the left-most target, before realizing that the upper-left box must be pushed to this target (e.g. it is the only target that the upper-left box can feasibly be pushed to) and forming a plan to instead push the lower-left box to a further target. Further evidence of the agent adapting a plan in response to apparent evaluation can be seen in Figure $5 7 \mathrm { c }$ in which the agent appears to realize that it cannot push the upper-right box up and left to the nearest target. Similarly, evidence of the agent planning forward from boxes and backwards from targets can be seen in Figure 57b in which the agent seems to plan forward from the lower-left boxes and backwards from the upper-right targets. + +Confirming Behavioral Dependence Finally, Figures 56a and 56a show the success rates when intervening with the vectors learned by 1x1 probes to steer the behavior of the ResNet agent in Agent- and Box-Shortcut levels using the procedure described in Section 6.1. Note that, in the interventions detailed in Figures 56a and 56b, it was found to be necessary to scale probe vectors by a scaling factor of 4. + +![](images/figures/emergent-planning-rl-fig-0131.jpg) +Figure 57: Examples of the ResNet agent’s internal plan (in terms of its square-level representations of $C _ { \mathrm { B } }$ as decoded by a 1x1 probe) over its first ten layer in four levels. In two of these instances (57a and 57b), the agent arrives at a successful plan by layer ten. In the other two (57c and 57d) it does not. The visualized plans are taken from the first step of each episode. + +A few things can be noted from these figures. First, interventions are very successful in both levels at the first layer. This is consistent with the hypothesis that the agent does use these concepts for planning. Interestingly, however, this is despite it being the case that the probes are more accurate when predicting these concepts at later layers. We hypothesize that this is perhaps because the agent forms its initial plans at its lowest layer and then refines these over later layers, such that the agent’s plan at the lowest layer is especially amenable to being intervened upon. It is also interesting that Agent-Shortcut interventions are more successful at later layers than Box-Shortcut interventions. This is consistent with the notion that the agent primarily focuses on planning box movements at earlier layers, and then subsequently refines plans for its own actions at later layers such that it is more amenable to changing its planned movements at later layers than to changing planned box movements. + +# H INVESTIGATING PLANNING IN A DIFFERENT ENVIRONMENT: MINI PACMAN + +In the main paper, we use the methodology introduced in Section 3.1 to provide evidence indicating that a DRC agent trained to play Sokoban internally performs planning. However, it is natural to ask the extent to which the finding that model-free agents can learn to internally plan generalizes. This is because the 3D structure of the DRC agent’s ConvLSTM cell states means the agent is particularly well-suited to learning to plan in an environment such as with a grid-based structure and localized transition dynamics. In this section, we now provide preliminary results when investigating whether a DRC agent can learn to internally plan in a different environment: Mini PacMan. + +# H.1 MINI PACMAN + +Mini PacMan is, like Sokoban, a grid-based environment. In Mini PacMan, an agent must navigate around walls in a grid-world and eat food. Initially, each non-wall square has food on, and levels end when the agent eats all food. However, the agent must also avoid ghosts which chase the agent. Ghosts chase the agent using $\mathbf { A } ^ { * }$ search. In each level, there are also ‘power pills’. When the agent steps onto a square with a power pill, ghosts flee, and the agent eats any ghosts it steps onto for the next 20 turns. The agent gets a reward of $+ 1$ for eating food, $+ 2$ for eating a pill, and $+ 5$ for eating a ghost. When the agent eats all food, the level is re-populated with food and new ghosts spawn in. An episode of Mini PacMan ends when the agent is either eventually eaten by a ghost, or when the agent fails to progress to the next level of an episode within 500 time steps of that level starting. Figure 58 shows an example of a Mini PacMan maze near the start of a level. + +We study a version of Mini PacMan that is similar to the version studied in Hamrick et al. (2020). The version of Mini PacMan we train our agent on consists of mazes that are randomly generated each episode by (1) generating mazes using Primm’s algorithm, and then (2) randomly removing each wall square with two empty adjacent non-adjacent tiles with a probability of 0.3. Each maze contains 4 pills. The number of ghosts in the initial level of each episode is equal to 1 plus an integer drawn from a Poisson(1) distribution. The number of ghosts at each subsequent level then increases by the floor of a 0.25 plus the level number times a number drawn from Unif[0, 2]. Unlike Hamrick et al. (2020) who use mazes of size $1 5 \mathrm { x } 1 9$ , we use smaller square mazes of size 13x13. Across all levels in a single episode, the same maze is used. We use a version of Mini PacMan where the agent observes a symbolic representation $\boldsymbol { x } _ { t } \in \mathbb { R } ^ { 1 3 \times \mathrm { 1 3 } \times 1 4 }$ of the environment. However, as with Sokoban, we present all visualisations using pixel representations of the Mini PacMan board. + +![](images/figures/emergent-planning-rl-fig-0132.jpg) +Figure 58: An example of a Mini PacMan board. The agent (the yellow pacman) must eat the food (the yellow dots) and avoid the ghosts (red) that are chasing it. When the agent eats a pill (the pink circles), the agent is able to eat ghosts over the next few steps, and ghosts change colour (blue) and flee the agent. Levels end when all food is eaten. + +![](images/figures/emergent-planning-rl-fig-0133.jpg) +Figure 59: Macro F1s achieved by probes when predicting (a) ‘Agent Approach Direction $^ { 1 6 }$ and (b) ‘Agent Approach $^ { 1 6 }$ using the agent’s cell state at each layer, or, for the baseline probes, using the observation. + +The preliminary results provided in this section regard a DRC(3,3) agent trained for 250 million transitions on this version of Mini PacMan. This agent is trained using the same training scheme as the Sokoban agents as described in Appendix E.4. As with all agents studied in this paper, this DRC agent shares the spatial dimensions of the environment it is operating in. + +# H.2 PRELIMINARY PROBING RESULTS + +We now present some very preliminary results regarding the aforementioned DRC agent. We initially tried probing for the concept ‘Agent Approach Direction’ $( C _ { \mathrm { { A } } } )$ as in Sokoban but found little evidence of the agent representing it. After experimentation, however, we found probes to be able to decode the following concept from the agent’s cell state: + +• Agent Approach Direction 16: This concept tracks which squares the agent will step off of, and which direction it will do so in, over the next 16 time steps. That is, this is a variant of ‘Agent Approach Direction’ that only accounts for the agent’s actions over the next 16 steps. + +• Agent Approach 16: This concept tracks which squares the agent will step off of over the next 16 time steps. That is, this is a variant of ‘Agent Approach Direction’ that only accounts for the agent’s actions over the next 16 steps, and ignores the directions that the agent enters squares from. + +Figures 59a and 59a show the macro F1s achieved by 1x1 and 3x3 probes when predicting ‘Agent Approach Direction $^ { 1 6 }$ and ‘Agent Approach $^ { 1 6 }$ respectively. These probes are trained and tested on datasets consisting of $2 3 \mathrm { k }$ and $^ \mathrm { 6 k }$ transitions respectively. We are unsure whether to interpret these results as indicating that either (1) the agent possesses spatially-localized representations of the concept ‘Agent Approach $1 6 '$ , or (2) the agent posseses a representation of the concept ‘Agent Approach Direction $^ { 1 6 }$ distributed across adjacent positions of its cell state. This is because, for ‘Agent Approach $^ { 1 6 }$ , 1x1 probes can accurately predict this concept, and we see minimal improvement in performance when moving from a 1x1 to $3 \mathrm { x } 3$ probe. In contrast, we see large improvements in performance when moving from 1x1 to 3x3 probes when predicting ‘Agent Approach Direction $^ { 1 6 }$ . This is consistent both with the agent representing ‘Agent Approach $^ { 1 6 }$ at individual positions of its cell state, and with the agent representing ‘Agent Approach Direction $^ { 1 6 }$ across adjacent positions of its cell state. + +Figures 60 and 61 shows examples of the predictions made by a 1x1 probe trained to predict ‘Agent Approach Direction $^ { 1 6 }$ when applied to the agent’s final-layer cell state at its final internal over the first 24 transitions at different points of 2 example episodes. Similarly, Figures 62 and 63 show the predictions made by a 1x1 probe trained to predict ‘Agent Approach Direction $^ { 1 6 }$ when applied to the agent’s final-layer cell state at its final internal over the first 24 transitions at different points of 2 (different) example episodes. Note that the ghosts turn blue when edible, and purple on their final two turns of being edible. + +These examples indicate that, as in Sokoban, the agent uses its concept representations (of whichever concept it does represent) to form an internal plan. Here, the agent’s internal plan consists of the squares it plans to visit in the near-future (and, potentially, the directions it will step on to those squares from). A few observations can be made of the agent’s internal plan in these examples. First, + +![](images/figures/emergent-planning-rl-fig-0134.jpg) +Figure 60: The DRC agent’s internal plan (in terms of its square-level representations of ‘Agent Approach Direction $1 6 '$ as decoded from its final-layer cell state by a 3x3 probe) after its final internal tick over the first 24 steps of an episode. A teal arrow corresponds to a probe predicting that the agent expects to step onto a square from the corresponding direction over the next 16 time steps. + +![](images/figures/emergent-planning-rl-fig-0135.jpg) +Figure 61: The DRC agent’s internal plan (in terms of its square-level representations of ‘Agent Approach Direction $1 6 '$ as decoded from its final-layer cell state by a 3x3 probe) after its final internal tick over the first 24 steps of an episode. A teal arrow corresponds to a probe predicting that the agent expects to step onto a square from the corresponding direction over the next 16 time steps. + +![](images/figures/emergent-planning-rl-fig-0136.jpg) +Figure 62: The DRC agent’s internal plan (in terms of its square-level representations of ‘Agent Approach $1 6 '$ as decoded from its final-layer cell state by a 1x1 probe) after its final internal tick over the first 24 steps of an episode. A teal cross corresponds to a probe predicting that the agent expects to step onto a square over the next 16 time steps. + +![](images/figures/emergent-planning-rl-fig-0137.jpg) +Figure 63: The DRC agent’s internal plan (in terms of its square-level representations of ‘Agent caption $^ { 1 6 }$ as decoded from its final-layer cell state by a 1x1 probe) after its final internal tick over the first 24 steps of an episode. A teal cross corresponds to a probe predicting that the agent expects to step onto a square over the next 16 time steps. + +as in Sokoban, the DRC agent’s internal plans tend to corresponded to connected paths to follow. +Second, again as in Sokoban, the agent’s internal plans seem to iteratively develop. + +However, there are also important respects in which the agent’s internal planning in Mini PacMan seems different from the planning of the DRC agent in Sokoban. First, unlike in Sokoban where the DRC agent seems to plan to a fixed horizon – the end of the episode –the agent here does does not seem to have a fixed planning horizon. Rather, it seems to often plan paths towards a ‘target’ such as a pill or, when ghosts are edible, an edible ghost (the blue/purple sprites). We hypothesise that this may explain why the probing macro F1 scores are relatively low despite the qualitative evidence of planning, since it means the probing target is only correlate of what the agent is truly planning in terms of. Similarly, unlike in Sokoban, the agent seems to actively alter large parts of its plans, and considers multiple plans in parallel. Note that these two points imply that the concepts we probe for are mere correlates of the ‘true’ concepts the agent is planning in terms of. \ No newline at end of file diff --git a/papers/emergent-planning-rl/paper.pdf b/papers/emergent-planning-rl/paper.pdf new file mode 100644 index 0000000000000000000000000000000000000000..65bdb7321ba4bfb3538d1f16b2f74eb333a62439 --- /dev/null +++ b/papers/emergent-planning-rl/paper.pdf @@ -0,0 +1,3 @@ +version https://git-lfs.github.com/spec/v1 +oid sha256:0ea2f59c095e32b47f9f84d7ef9d179af3809199770b32a5352f3c0ead8f8eaf +size 26168157 diff --git a/papers/emergent-planning-rl/sau.json b/papers/emergent-planning-rl/sau.json new file mode 100644 index 0000000000000000000000000000000000000000..1660c13ca657b5ce22009c426f494047b39d3887 --- /dev/null +++ b/papers/emergent-planning-rl/sau.json @@ -0,0 +1,312 @@ +{ + "paper_id": "emergent-planning-rl", + "paper_title": "Interpreting Emergent Planning in Model-Free RL", + "D1": [ + { + "id": "emergent-planning-rl-D1-001", + "claim": "Agent architecture: The main DRC(3,3) agent uses 3 ConvLSTM layers (D=3), performs 3 internal computational ticks per timestep (N=3), and has 32 channels per layer (G_d=32).", + "source": "Section 2.3, Appendix E.3" + }, + { + "id": "emergent-planning-rl-D1-002", + "claim": "Sokoban environment configuration: an 8x8 grid world with 4 boxes and 4 targets. The agent observes a symbolic representation x_t in R^{8x8x7} and can move up/down/left/right or stay still.", + "source": "Section 2.2, Appendix E.2" + }, + { + "id": "emergent-planning-rl-D1-003", + "claim": "Concept definitions: Two square-level multi-class concepts (C_A for agent approach direction, C_B for box push direction), each mapping every grid square at every timestep to one of 5 classes: UP, DOWN, LEFT, RIGHT, NEVER.", + "source": "Section 3.2" + }, + { + "id": "emergent-planning-rl-D1-004", + "claim": "Agent training: DRC(3,3) agent trained via IMPALA actor-critic on 900,000 Boxoban unfiltered training levels for 250 million total transitions, with discount factor gamma=0.97.", + "source": "Appendix E.4" + }, + { + "id": "emergent-planning-rl-D1-005", + "claim": "Probe architecture and training: 1x1 probes have 160 parameters (single cell-state vector input); 3x3 probes have 1440 parameters (3x3 patch input). Larger probes (5x5, 7x7) are also evaluated in appendix experiments. All probes trained via logistic regression with AdamW optimizer and 5 independent initialization seeds.", + "source": "Section 4.1, Appendix D.1" + }, + { + "id": "emergent-planning-rl-D1-006", + "claim": "Probe datasets: Training set of approximately 106,600 transitions from 3,000 Boxoban training episodes; test set of transitions from 1,000 held-out Boxoban validation episodes.", + "source": "Section 4.1, Appendix D.1" + }, + { + "id": "emergent-planning-rl-D1-007", + "claim": "Intervention experiment setup: 200 levels per intervention type (Agent-Shortcut, Box-Shortcut), constructed from 25 handcrafted base levels multiplied by 8 geometric transformations (vertical reflection and 90/180/270-degree rotations).", + "source": "Section 6.1" + }, + { + "id": "emergent-planning-rl-D1-008", + "claim": "Intervention protocol: Short-route interventions (adding NEVER class vector) are repeated at every step; directional interventions repeat only until the agent steps onto (or pushes box off) the target square. Random-probe baseline interventions use norm-matched vectors for comparability with trained-probe interventions.", + "source": "Section 6.1" + }, + { + "id": "emergent-planning-rl-D1-009", + "claim": "Thinking steps configuration: 5 thinking steps at episode start (agent forced stationary before acting), yielding 15 extra internal ticks for the DRC(3,3) agent (3 ticks per step x 5 steps).", + "source": "Section 5, Section 6.2, Appendix A.3" + }, + { + "id": "emergent-planning-rl-D1-010", + "claim": "Training emergence analysis: 50 checkpoints collected at 1M-transition intervals over the first 50M transitions of training, with linear probes re-trained independently at each checkpoint to account for the agent's evolving behavior-dependent concept representations.", + "source": "Section 6.2, Appendix C" + }, + { + "id": "emergent-planning-rl-D1-011", + "claim": "Behavioral test-time compute: 1,000 medium-difficulty and 1,000 hard-difficulty Boxoban levels tested with the number of thinking steps ranging from 0 to 5 to evaluate planning-like behavioral benefit from extra compute.", + "source": "Appendix E.5" + }, + { + "id": "emergent-planning-rl-D1-012", + "claim": "Search evidence corridor levels: 8 handcrafted base levels each with a corridor whose entrance is blocked by a myopically pushable box, replicated at 4 corridor lengths (2, 6, 10, 14), with 8 geometric transformations applied per configuration.", + "source": "Appendix A.3.2" + }, + { + "id": "emergent-planning-rl-D1-013", + "claim": "Cutoff level intervention setup: 200 total Cutoff levels (levels where default agent solves 0% due to myopic box-pushing), constructed from 25 handcrafted base levels x 8 geometric transformations.", + "source": "Appendix B.3" + }, + { + "id": "emergent-planning-rl-D1-014", + "claim": "DRC(1,9) variant architecture and training: 1 ConvLSTM layer (D=1), 9 internal ticks per step (N=9), trained for 100 million transitions using the same IMPALA training scheme as the main DRC(3,3) agent.", + "source": "Appendix F.1" + }, + { + "id": "emergent-planning-rl-D1-015", + "claim": "DRC(9,1) variant architecture and training: 9 ConvLSTM layers (D=9), 1 internal tick per step (N=1), trained for 100 million transitions using the same IMPALA training scheme as the main DRC(3,3) agent.", + "source": "Appendix F.2" + }, + { + "id": "emergent-planning-rl-D1-016", + "claim": "ResNet agent architecture: 24 residual blocks with 32 channels per block and an MLP hidden dimension of 256. No recurrent connections; iterative computation occurs across depth rather than across ticks. Trained for 250M transitions in Sokoban.", + "source": "Appendix G" + }, + { + "id": "emergent-planning-rl-D1-017", + "claim": "Mini PacMan environment configuration: 13x13 mazes generated via Primm's algorithm with 4 pills per maze. Observation shape is 13x13x14. The DRC(3,3) agent is trained for 250M transitions in this environment to probe plan formation in a non-Sokoban domain.", + "source": "Appendix H.1" + }, + { + "id": "emergent-planning-rl-D1-018", + "claim": "Alternative concept probing: Binary concept variants tested include simplified binary concepts Agent_Approach and Box_Push (classes NEVER/AGAIN, no direction), and reversed asymmetry concepts Agent_Exit_Direction and Box_Approach_Direction (classes L/R/U/D/NEVER).", + "source": "Appendix D.4" + }, + { + "id": "emergent-planning-rl-D1-019", + "claim": "Global probe for future actions: Global linear probes (10,240 parameters) take the agent's entire cell state as input to predict the agent's action 1 to 10 steps into the future, evaluated by accuracy.", + "source": "Appendix D.5" + }, + { + "id": "emergent-planning-rl-D1-020", + "claim": "Intervention variation parameters: Directional squares p tested in range [0, 3]; short-route intervention tested both with and without; intervention types tested include full (short-route + directional) and directional-only. Intervention strength alpha varied to find optimal scaling.", + "source": "Appendix B.2" + } + ], + "D2": [ + { + "id": "emergent-planning-rl-D2-001", + "claim": "Concept Ground Truth Label Computation (C_A and C_B): For each square (x,y) at timestep t, scan remaining episode trajectory (t..T) to find the first interaction. C_A: find first t'>t where agent steps onto (x,y); record entry direction {UP/DOWN/LEFT/RIGHT}; if none, assign NEVER. C_B: find first t'>t where a box is pushed off (x,y); record push direction; else NEVER. Labels are behavior-dependent: computed from the agent's actual trajectory under its current parameters θ using the full episode replay. Formally: C_A(x_t, (x,y), θ) = DIR if agent enters (x,y) from DIR at future step, NEVER otherwise.", + "source": "Section 3.2" + }, + { + "id": "emergent-planning-rl-D2-002", + "claim": "Linear Probe Logit Computation: For each class k ∈ {UP, DOWN, LEFT, RIGHT, NEVER} at position (x,y), logit l_k = w_k^T g_{x,y} where w_k ∈ R^32 is the learned class-specific weight vector and g_{x,y} ∈ R^32 is the cell state at that spatial position. Predicted class = argmax_k l_k. Softmax probability: P(k|g_{x,y}) = exp(l_k) / Σ_j exp(l_j). Probes trained via logistic regression (cross-entropy loss) with AdamW optimizer, 5 independent seeds. The vector w_k is also the concept direction vector used for causal intervention.", + "source": "Section 2.4" + }, + { + "id": "emergent-planning-rl-D2-003", + "claim": "Causal Intervention on Agent Cell State: g'_{x,y} = g_{x,y} + w_k. Where g_{x,y} ∈ R^32 is the cell state at position (x,y), w_k ∈ R^32 is the 1x1 probe's learned class vector for concept class k (e.g., w_{NEVER}^{C_A} or w_{DOWN}^{C_A}). With scaling strength α: g'_{x,y} = g_{x,y} + α · w_k. Too low α fails to meaningfully change representations; too high α pushes activations off-distribution. This intervention adds concept class information directly into the agent's internal state at specific spatial positions to steer its planned behavior.", + "source": "Section 6.1, Equation 1" + }, + { + "id": "emergent-planning-rl-D2-004", + "claim": "Algorithm 1: Agent-Shortcut Intervention. Pseudocode: (1) ShortRouteSquares = all positions (x,y) on the short path. (2) (x_0,y_0) = first square of the long route. (3) LongRouteSquaresDirs = first p squares agent would step onto if following the longer route, and the direction DIR. (4) For each timestep t: for (x,y) in ShortRouteSquares do g_{x,y} += α × w_{NEVER}^{C_A}; if agent has not moved onto (x_0,y_0) this episode, for each ((x,y), DIR) in LongRouteSquaresDirs do g_{x,y} += α × w_{DIR}^{C_A}. Default α=1, p=1 from Section 6.1.", + "source": "Section 6.1, Appendix B.2, Algorithm 1" + }, + { + "id": "emergent-planning-rl-D2-005", + "claim": "Algorithm 2: Box-Shortcut Intervention. Pseudocode: (1) ShortRouteSquares = all positions (x,y) on the short box-push route. (2) (x_0,y_0) = initial position of the box not adjacent to any target. (3) LongRouteSquaresDirs = first p squares a box would be pushed off if pushed the longer route, and direction DIR. (4) For each timestep t: for (x,y) in ShortRouteSquares do g_{x,y} += α × w_{NEVER}^{C_B}; if box has not been pushed off (x_0,y_0) this episode, for each ((x,y), DIR) in LongRouteSquaresDirs do g_{x,y} += α × w_{DIR}^{C_B}. Default α=1, p=1 from Section 6.1.", + "source": "Section 6.1, Appendix B.2, Algorithm 2" + }, + { + "id": "emergent-planning-rl-D2-006", + "claim": "DRC Agent Recurrent Tick Equations: The stack of D ConvLSTM units performs N internal ticks per environment step. Let s_t^d = (h_t^d, g_t^d) where h_t^d, g_t^d ∈ R^{8×8×32} are the output and cell state of the d-th ConvLSTM. Recurrence: s_{t,0} = s_{t-1}; for n=1..N: s_{t,n} = f_θ(i_t, s_{t,n-1}); s_t = s_{t,N}. Each ConvLSTM uses 3×3 kernels with zero-padding, preserving 8×8 spatial dimensions. The function f_θ applies standard LSTM gating (input, forget, output, cell-update gates) using convolutional connections. i_t = e(x_t) ∈ R^{8×8×32} is the encoded observation.", + "source": "Section 2.3, Appendix E.3" + }, + { + "id": "emergent-planning-rl-D2-007", + "claim": "DRC Pool-and-Inject Mechanism: Enables rapid spatial information spread by feeding each ConvLSTM a spatially-pooled version of its own prior-tick output. Formula: m_{t,n-1}^d = [MeanPool(h_{t,n-1}^d), MaxPool(h_{t,n-1}^d)]^T ∈ R^{2G_d}; p̂ = W_{p_d} m + b_{p_d} ∈ R^{H_d W_d G_d} with W_{p_d} ∈ R^{H_d W_d G_d × 2G_d}; then reshape: p_{t,n-1}^d = Reshape(p̂) ∈ R^{H_d×W_d×G_d}. The pooled tensor p is provided as additional input to the ConvLSTM at tick n, injecting global spatial summary back into local computation.", + "source": "Appendix E.3" + }, + { + "id": "emergent-planning-rl-D2-008", + "claim": "DRC Output Head Computation: The output of the final (D-th) ConvLSTM at final tick N, denoted h_{t,N}^D ∈ R^{8×8×32}, is concatenated with the input encoding i_t ∈ R^{8×8×32} and passed through an affine transformation + ReLU: o_t = ReLU(W_o [h_{t,N}^D; i_t] + b_o) ∈ R^{d_o}. Policy head: logits_t = W_π o_t + b_π ∈ R^5, a_t ~ Categorical(softmax(logits_t)). Value head: v_t = W_v o_t + b_v ∈ R. At test time, the agent acts greedily: a_t = argmax(logits_t). Training uses sampled actions from the categorical distribution.", + "source": "Appendix E.3" + }, + { + "id": "emergent-planning-rl-D2-009", + "claim": "IMPALA Training Loss with Penalties: The agent is trained via IMPALA actor-critic with V-trace returns. Total loss: L = L_{V-trace}(γ=0.97, λ=0.97) + 10^{-3}·||logits_t||^2 + 10^{-5}·(||W_π||^2 + ||W_v||^2) + 10^{-2}·H(π). The L2 penalty on action logits (1e-3) prevents overconfident predictions; L2 regularization (1e-5) on policy/value head weights; entropy bonus H(π) (1e-2) encourages exploration. Optimization: BPTT with unroll length 20, Adam optimizer, batch size 16, learning rate decays linearly from 4e-4 to 0.", + "source": "Appendix E.4" + }, + { + "id": "emergent-planning-rl-D2-010", + "claim": "Full Experiment Pipeline: Six sequential phases: (1) Train DRC(3,3) agent on 900k Sokoban levels via IMPALA for 250M transitions, collecting checkpoints at 1M intervals. (2) Define square-level concepts C_A (agent approach direction) and C_B (box push direction) as {UP,DOWN,LEFT,RIGHT,NEVER}-valued mappings from grid squares. (3) Train 1x1/3x3 linear probes via logistic regression on cell state from 3000/1000 episodes to verify linear encoding. (4) Decode internal plans over entire boards at each tick; qualitatively analyze forward/backward/parallel search motifs; quantify plan refinement with 5 thinking steps. (5) Intervene via g'_{x,y} = g_{x,y} + w_k on Agent-Shortcut/Box-Shortcut/Cutoff levels. (6) Correlate emergence of concept representations, plan refinement, and behavioral benefit across 50 training checkpoints.", + "source": "Section 3.1, Sections 4-6" + }, + { + "id": "emergent-planning-rl-D2-011", + "claim": "DRC(3,3) Skip Connections: Two types of skip connections enhance information flow. Bottom-up: the input encoding i_t = e(x_t) ∈ R^{8×8×32} is provided as input to ALL D=3 ConvLSTM units (not just the bottom layer), allowing raw observation information to reach every layer directly. Top-down: the output h_{t,n-1}^D of the final (D-th) ConvLSTM on tick n-1 is provided as an additional input to the bottom (1st) ConvLSTM on tick n, enabling processed high-level representations to influence early processing on the next iteration. Together these form a bidirectional information pathway across the depth stack.", + "source": "Appendix E.3" + }, + { + "id": "emergent-planning-rl-D2-012", + "claim": "Intervention Success Criterion: In Agent-Shortcut levels (two paths to box/target region): success = agent solves the level by following the suboptimal LONG path instead of the default short path. In Box-Shortcut levels (box can be pushed short or long route): success = agent pushes box via the suboptimal LONG route. In Cutoff levels (default 0% solve rate due to myopic box-pushing blocking corridor): success = agent solves the level at all. All interventions repeated with 5 independently trained probes; compared against random-probe baseline with norm-matched vectors. Success rates reported as mean ±1 SD over seeds.", + "source": "Section 6.1, Appendix B.2, Appendix B.3" + }, + { + "id": "emergent-planning-rl-D2-013", + "claim": "Macro F1 Evaluation Metric for Probes: Macro_F1 = (1/K) · Σ_{k=1}^{K} F1_k, where F1_k = 2 · P_k · R_k / (P_k + R_k) for class k, with P_k = precision and R_k = recall. K = 5 classes {UP, DOWN, LEFT, RIGHT, NEVER}. Macro F1 is used instead of accuracy because the NEVER class dominates most Sokoban boards (>95% of squares), making accuracy misleading. Scores are computed per-square, averaged over all squares in each board, then averaged across all episodes in the test set (1000 episodes).", + "source": "Section 4.1, Section 4.2" + }, + { + "id": "emergent-planning-rl-D2-014", + "claim": "Grid-Level Internal Plan Decoding: For each square (x,y) of the 8×8 grid, at each internal tick n and layer d, compute class logits l_k^{(x,y)} = w_k^T g_{x,y}^{d,n} for all k. Assign predicted class = argmax_k l_k^{(x,y)}. Visualize: colored arrows for directional classes (UP/DOWN/LEFT/RIGHT), blank for NEVER. The collective arrow pattern across all 64 squares constitutes the agent's decoded internal plan at that tick. Plans are decoded at the final tick of each environment step for visual analysis, and at each internal tick during thinking steps for refinement measurement.", + "source": "Section 5" + }, + { + "id": "emergent-planning-rl-D2-015", + "claim": "Plan Refinement Quality Measurement: Force agent stationary for 5 thinking steps at episode start, yielding 15 internal ticks (3 ticks per step × 5 steps). At each tick n ∈ {1,...,15}, decode C_A and C_B for all squares via 1x1 probes on cell state at each layer. Compute macro F1 against ground-truth labels from the full episode replay. Plot macro_F1(n) vs. tick n → monotonic increase indicates iterative plan improvement. Compute plan refinement gain: ΔF1 = macro_F1(tick_15) - macro_F1(tick_1). Averaged over 1000 Boxoban episodes. Replicated across all checkpoints for training emergence analysis.", + "source": "Section 5, Appendix A.3" + }, + { + "id": "emergent-planning-rl-D2-016", + "claim": "Thinking Step Behavioral Evaluation: For T ∈ {0,1,...,5} thinking steps: force agent to remain stationary (action=NO-OP) for first T environment steps, then act normally. Measure percentage of 1000 Medium-difficulty and 1000 Hard-difficulty Boxoban levels solved. Behavioral planning benefit = %solved(T=5) - %solved(T=0). For corridor length experiment: test on corridor levels of lengths L ∈ {2,6,10,14} (8 base levels × 4 lengths × 8 transformations = 256 total). Measure min thinking steps needed to solve ≥50% of levels at each corridor length. Higher m_thinking needed for longer corridors = evidence of deeper search.", + "source": "Appendix A.3.2, Appendix E.5" + }, + { + "id": "emergent-planning-rl-D2-017", + "claim": "Convolutional Encoder for Symbolic Observations: Input x_t ∈ R^{8×8×7} where each of 64 squares is a 7-dim one-hot vector encoding {wall, empty, box, agent, box_on_target, agent_on_target, target}. Encoder e is a convolutional network: i_t = e(x_t) ∈ R^{8×8×32} (G_0=32 channels). Uses kernel size 3 with a single layer of zero-padding to preserve spatial dimensions H_0=W_0=8. The encoder output i_t is fed to all D ConvLSTM layers via bottom-up skip connections. Spatial alignment between encoder output and Sokoban grid enables the agent to learn a spatial bijection between cell state positions and board squares.", + "source": "Section 2.3, Appendix E.3" + } + ], + "D3": [ + { + "id": "emergent-planning-rl-D3-001", + "claim": "Train 1x1 (160-param) and 3x3 (1440-param) linear probes on DRC(3,3) cell state at each of 3 ConvLSTM layers (final tick) to predict per-square C_A (agent approach direction) and C_B (box push direction). Evaluate using macro F1 on 1000 held-out Boxoban episodes. Compare against observation-only baseline probes to verify that the agent linearly represents planning-relevant concepts internally rather than probes learning from raw input. Train with 5 independent probe seeds via logistic regression (AdamW).", + "source": "Section 4" + }, + { + "id": "emergent-planning-rl-D3-002", + "claim": "Internal Plan Formation Qualitative Analysis: Decode DRC(3,3) agent's internal plans from cell state via 1x1 probes predicting C_A and C_B over entire Sokoban boards at each internal tick. Examine plan formation motifs (evaluative planning, forward search from boxes, backward search from targets, parallel search) across handpicked Boxoban levels including out-of-distribution variants (blind, generalized, blocked-route, new-route levels). Compare decoded plans against ground-truth optimal paths.", + "source": "Section 5, Appendix A.1, Appendix A.2" + }, + { + "id": "emergent-planning-rl-D3-003", + "claim": "Test-Time Plan Refinement Quantification: Force DRC(3,3) agent stationary for 5 thinking steps (15 extra internal ticks) at start of 1000 Boxoban episodes. Decode C_A and C_B at each tick using 1x1 probes trained on final-layer cell state. Measure macro F1 improvement over ticks. Compare against plan quality at tick 0 (no extra compute). Show plan quality monotonically improves with additional ticks, consistent with iterative search.", + "source": "Section 5 (Figure 6), Appendix A.3" + }, + { + "id": "emergent-planning-rl-D3-004", + "claim": "Causal Intervention: Agent-Shortcut Experiment: Add C_A concept vectors (NEVER to short-route squares, directional vector to long-route entry square) to DRC(3,3) cell state at each of 3 layers. Evaluate on 200 Agent-Shortcut levels (25 base x 8 geometric transformations). Measure intervention success rate (% episodes where agent follows suboptimal long path instead of default short path). Compare success rates against random-probe interventions with norm-matched vectors, averaged over 5 independently trained probe seeds.", + "source": "Section 6.1 (Table 1, Figure 7), Appendix B.1, Appendix B.2" + }, + { + "id": "emergent-planning-rl-D3-005", + "claim": "Causal Intervention: Box-Shortcut Experiment: Add C_B concept vectors (NEVER to short-route squares, directional vector to box's long-route starting square) to DRC(3,3) cell state at each of 3 layers. Evaluate on 200 Box-Shortcut levels (25 base x 8 geometric transformations) where one box can be pushed via short or long route. Measure intervention success rate (% episodes where agent pushes box via suboptimal long route). Compare against random-probe interventions with norm-matched vectors, averaged over 5 independently trained probe seeds.", + "source": "Section 6.1 (Table 1, Figure 8), Appendix B.1, Appendix B.2" + }, + { + "id": "emergent-planning-rl-D3-006", + "claim": "Collect 50 training checkpoints at 1M-transition intervals over the first 50M transitions. At each checkpoint: (i) retrain 1x1 probes to predict C_A and C_B from agent cell state and measure macro F1, (ii) measure additional medium-difficulty Boxoban levels solved with 5 thinking steps vs. 0 thinking steps (behavioral planning benefit). Plot correlation between probe performance and behavioral benefit (Figure 9, Figure 38) to demonstrate concept representations and planning-like behavior co-emerge during training.", + "source": "Section 6.2 (Figure 9), Appendix C.1, Appendix C.3" + }, + { + "id": "emergent-planning-rl-D3-007", + "claim": "At each of 50 training checkpoints (1M-transition intervals), decode agent internal plans via 1x1 probes at the 1st and 15th extra tick during 5 thinking steps across 1000 episodes. Measure macro F1 increase from tick 1 to tick 15 for C_A and C_B at each layer. Correlate plan refinement gain per checkpoint with additional levels solved via thinking steps (Figure 39). Verify that test-time plan refinement ability co-emerges with planning-like behavioral benefit during training.", + "source": "Appendix C.2, Appendix C.4 (Figure 39)" + }, + { + "id": "emergent-planning-rl-D3-008", + "claim": "Vary intervention strength alpha and number p of directional intervention squares (0 to 3 along the long route) in Agent-Shortcut and Box-Shortcut levels. Test both full intervention (short-route + directional) and directional-only (no short-route) configurations. Measure intervention success rate across 5 trained and 5 random probe seeds per {alpha, p, layer} combination. Evaluate on all 3 ConvLSTM layers to characterize how scaling, directional square count, and short-route presence affect steering success.", + "source": "Appendix B.2" + }, + { + "id": "emergent-planning-rl-D3-009", + "claim": "Design 200 Cutoff levels (25 base x 8 geometric transformations) where entrance to a variable-length corridor is blocked by a box adjacent to a target. By default the agent solves 0% (myopically pushes the blocking box onto the target, irreversibly blocking the corridor). Perform three intervention types at varying strengths alpha on each ConvLSTM layer: Agent-Only (C_A directional to enter corridor), Box-Only (C_B directional to push blocking box aside), Agent-and-Box (both). Measure solve rate; compare trained-probe vs. random-probe baselines with 5 seeds each.", + "source": "Appendix B.3" + }, + { + "id": "emergent-planning-rl-D3-010", + "claim": "Search Evidence: Corridor Length Experiment: Handcraft 8 base levels each with a corridor (entrance blocked by myopically pushable box) replicated at 4 corridor lengths (2,6,10,14), yielding 80 levels after 8 geometric transformations each. Test DRC(3,3) agent with 0-5 thinking steps. Measure percentage of levels solved per corridor length and number of thinking steps required to solve >=50% of each set. Compare solve rates against 0 thinking steps (myopic default behavior).", + "source": "Appendix A.3.2" + }, + { + "id": "emergent-planning-rl-D3-011", + "claim": "Alternative Concept Probing: Train 1x1 and 3x3 probes on DRC(3,3) cell state to predict (a) simplified binary concepts Agent_Approach and Box_Push (classes NEVER/AGAIN, no direction), and (b) reversed asymmetry concepts Agent_Exit_Direction and Box_Approach_Direction (classes L/R/U/D/NEVER). Evaluate on 1000 held-out Boxoban episodes using macro F1. Compare against main C_A/C_B probe performance and observation-only baselines to verify the agent represents the directional, asymmetric concepts rather than simpler or reversed alternatives.", + "source": "Appendix D.4" + }, + { + "id": "emergent-planning-rl-D3-012", + "claim": "Larger Probe Size Experiment: Train 1x1, 3x3, 5x5, and 7x7 probes on DRC(3,3) cell state to predict C_A and C_B at each layer. Evaluate on 1000 Boxoban test episodes using macro F1. Compare performance gain from larger receptive fields against gain observed in observation-only baseline probes to distinguish spatially-localized representations (minimal improvement from larger probes) from distributed representations (large improvement). Averaged over 5 seeds per probe type and size.", + "source": "Appendix D.3" + }, + { + "id": "emergent-planning-rl-D3-013", + "claim": "Train global linear probes (10,240 parameters, taking the agent's entire cell state as input) on DRC(3,3) cell state at each layer to predict the agent's action 1 to 10 steps into the future. Evaluate accuracy on 1000 test episodes; compare against observation-only baseline global probes. Purpose: apply the 3-step methodology to falsify the hypothesis that the agent plans by forming explicit sequences of future actions (e.g., LEFT, LEFT, UP, RIGHT). Results show these global future-action concepts are not linearly represented, ruling out this planning mechanism.", + "source": "Appendix D.5" + }, + { + "id": "emergent-planning-rl-D3-014", + "claim": "DRC Variant Probing: DRC(1,9) and DRC(9,1): Apply full three-step methodology (concept probing, plan formation, behavioral intervention) to DRC(1,9) agent (1 layer x 9 ticks, 100M transitions) and DRC(9,1) agent (9 layers x 1 tick, 100M transitions). Train 1x1 and 3x3 probes on 500/250 train/test episodes per agent to predict C_A and C_B. Measure plan refinement macro F1 over extra internal ticks during 5 thinking steps. Evaluate intervention success rates on Agent-Shortcut and Box-Shortcut levels with scaling factor 4. Compare probe and intervention performance across layers against random-probe baselines.", + "source": "Appendix F" + }, + { + "id": "emergent-planning-rl-D3-015", + "claim": "ResNet Agent Planning Investigation: Train 1x1 and 3x3 probes on hidden states after each of 24 residual blocks of a ResNet agent (no recurrent connections, 250M transitions) to predict C_A and C_B. Evaluate on 3000/1000 train/test Boxoban episodes via macro F1, tracking per-layer accuracy trend to identify iterative planning across layers. Perform Agent-Shortcut and Box-Shortcut interventions with scaling factor 4 on 200 levels each. Compare intervention success rates against random-probe baselines. Visualize internal plans decoded by 1x1 probes across layers to assess plan formation motifs.", + "source": "Appendix G" + }, + { + "id": "emergent-planning-rl-D3-016", + "claim": "Mini PacMan Environment Probing: Train DRC(3,3) agent on 13x13 Mini PacMan mazes (randomly generated via Primm's algorithm, 4 pills, ghosts chasing via A*) for 250M transitions. Train 1x1 and 3x3 probes on 23k/6k train/test transitions to decode time-limited concepts Agent_Approach_Direction_16 (5-class directional, 16-step horizon) and Agent_Approach_16 (binary visit/no-visit, 16-step horizon) from cell state at each layer. Evaluate using macro F1 against observation-only baselines. Visualize decoded plans to assess whether agent forms connected navigational paths toward targets (pills, edible ghosts).", + "source": "Appendix H" + }, + { + "id": "emergent-planning-rl-D3-017", + "claim": "DRC Agent Training Protocol: Train DRC(3,3) agent on 900k Sokoban levels (Boxoban unfiltered training set) using IMPALA actor-critic for 250M transitions. DRC variants DRC(1,9) and DRC(9,1) trained for 100M transitions using same scheme.", + "source": "Appendix E.4" + } + ], + "D4": [ + { + "id": "emergent-planning-rl-D4-001", + "claim": "Phase 1: Agent Training. Train the DRC(3,3) agent on Sokoban to establish a candidate model-free agent for planning investigation. Sub-steps: (a) configure DRC architecture with D=3 ConvLSTM layers, N=3 internal ticks per step, G_d=32 channels, kernel size 3 with zero-padding; (b) train via IMPALA actor-critic on 900,000 Boxoban unfiltered training levels for 250 million transitions; (c) collect checkpoints at 1M-transition intervals for subsequent emergence analysis; (d) verify baseline planning-like behavior (performance improves with extra test-time compute).", + "source": "Section 2.3, Section 3.1, Appendix E.3, Appendix E.4, Appendix E.5" + }, + { + "id": "emergent-planning-rl-D4-002", + "claim": "Phase 2: Concept Selection. Define planning-relevant concepts whose internal representations would indicate the agent is planning. Sub-steps: (a) characterize planning as requiring plan formulation, consequence evaluation, and behavioral influence; (b) hypothesize square-level multi-class concepts C_A (agent approach direction) and C_B (box push direction), each mapping grid squares to {UP, DOWN, LEFT, RIGHT, NEVER}; (c) define ground-truth label computation algorithm that determines concept classes for every square at every timestep from full episode trajectories; (d) validate that these concepts are instrumentally useful for spatial planning in Sokoban's grid-based dynamics.", + "source": "Section 3.1, Section 3.2, Section 2.1" + }, + { + "id": "emergent-planning-rl-D4-003", + "claim": "Phase 3: Probe for Concept Representations. Use linear probes to determine whether the agent internally represents the planning-relevant concepts. Sub-steps: (a) run agent for 3000/1000 train/test Boxoban episodes to collect cell state activations and ground-truth concept labels; (b) train 1x1 (160-param) and 3x3 (1440-param) linear probes via logistic regression with AdamW (5 seeds each) to predict C_A and C_B from cell state at each of 3 layers after the final tick; (c) compare probe macro F1 against observation-only baseline probes to distinguish internal representation from bottom-up inference; (d) confirm minimal improvement from 1x1 to 3x3 probes vs. large baseline improvement, verifying spatially-localized linear representations.", + "source": "Section 4, Appendix D.1, Appendix D.3" + }, + { + "id": "emergent-planning-rl-D4-004", + "claim": "Phase 4: Investigate Plan Formation. Gather qualitative and quantitative evidence that the agent forms, evaluates, and refines plans using the probed concept representations. Sub-steps: (a) decode internal plans by applying 1x1 probes over entire Sokoban boards, visually analyzing plan formation motifs (forward search from boxes, backward search from targets, parallelized bidirectional search, evaluative replanning); (b) test on out-of-distribution levels (blind without agent, generalized with extra boxes/targets, blocked-route, new-route); (c) quantify plan refinement: force agent stationary for 5 thinking steps (15 extra ticks) in 1000 episodes, measure macro F1 improvement of decoded plans over ticks; (d) conduct corridor length experiment as behavioral search evidence: test on corridor levels of lengths 2-14 with 0-5 thinking steps, measuring solve rates.", + "source": "Section 5, Appendix A.1, Appendix A.2, Appendix A.3" + }, + { + "id": "emergent-planning-rl-D4-005", + "claim": "Phase 5: Causal Intervention Verification. Confirm that the agent's concept representations causally influence its behavior by intervening on cell state activations. Sub-steps: (a) design Agent-Shortcut levels (200 levels) and Box-Shortcut levels (200 levels) where agent defaults to optimal short path but can be steered to suboptimal long path; (b) intervene by adding learned concept vectors (w_k for NEVER/directional classes) to cell state at each layer following g'_{x,y} = g_{x,y} + w_k; (c) measure intervention success rate (% episodes agent follows steered path) against random-probe baselines with norm-matched vectors (5 seeds each); (d) vary intervention parameters: scaling strength alpha, number of directional squares p (0-3), with/without short-route component; (e) test complementary Cutoff level interventions (200 levels) to induce optimal behavior where agent defaults to 0% solve rate, using Agent-Only, Box-Only, and Agent-and-Box variants.", + "source": "Section 6.1, Appendix B.1, Appendix B.2, Appendix B.3" + }, + { + "id": "emergent-planning-rl-D4-006", + "claim": "Phase 6: Training Emergence Analysis. Demonstrate that concept representations and planning-like behavior co-emerge during training, providing convergent evidence for the planning hypothesis. Sub-steps: (a) at each of 50 checkpoints (1M-transition intervals over first 50M transitions), retrain 1x1 probes for C_A/C_B and measure macro F1; (b) at each checkpoint, measure behavioral planning benefit as additional medium Boxoban levels solved with 5 thinking steps vs. 0; (c) at each checkpoint, measure plan refinement capability as macro F1 gain from tick 1 to tick 15 during 5 thinking steps; (d) correlate all three metrics, confirming concept representation quality, plan refinement ability, and behavioral benefit co-emerge; (e) verify the relationship holds across all 3 ConvLSTM layers, not just the final layer.", + "source": "Section 6.2, Appendix C.1, Appendix C.2, Appendix C.3, Appendix C.4" + } + ] +} \ No newline at end of file diff --git a/papers/gated-attention-llm/blacklist.txt b/papers/gated-attention-llm/blacklist.txt new file mode 100644 index 0000000000000000000000000000000000000000..884f0f9d022ca88da0821217a3052253a5d71393 --- /dev/null +++ b/papers/gated-attention-llm/blacklist.txt @@ -0,0 +1,2 @@ +# Official repository (NeurIPS 2025 Oral) +https://github.com/qiuzh20/gated_attention diff --git a/papers/gated-attention-llm/config.yaml b/papers/gated-attention-llm/config.yaml new file mode 100644 index 0000000000000000000000000000000000000000..64a647e4cf6250684fc3b6e776e830c038c361fd --- /dev/null +++ b/papers/gated-attention-llm/config.yaml @@ -0,0 +1,8 @@ +title: "Gated Attention for Large Language Models: Non-linearity, Sparsity, and Attention-Sink-Free" +pdf_url: "https://arxiv.org/pdf/2505.06708.pdf" +venue: "NeurIPS 2025 Best Paper" +year: "2025" +extra: + selection_index: 1 + domain: "NLP / LLM" + paradigm: "New Algorithm / Architecture" diff --git a/papers/gated-attention-llm/images/figures/gated-attention-llm-fig-0001.jpg b/papers/gated-attention-llm/images/figures/gated-attention-llm-fig-0001.jpg new file mode 100644 index 0000000000000000000000000000000000000000..a66d20e370958fc6bc103fe7645b557cc3f78e27 --- 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+version https://git-lfs.github.com/spec/v1 +oid sha256:4fdaefed435ef086c276f57b60d9efb6f819623997634acbdb15b5972ee2ba10 +size 46386 diff --git a/papers/gated-attention-llm/paper.md b/papers/gated-attention-llm/paper.md new file mode 100644 index 0000000000000000000000000000000000000000..148956fdea1536b9ea52278a99736be712513a40 --- /dev/null +++ b/papers/gated-attention-llm/paper.md @@ -0,0 +1,374 @@ +# Gated Attention for Large Language Models: Non-linearity, Sparsity, and Attention-Sink-Free + +Zihan ${ \bf Q } { \bf i } { \bf u } ^ { * 1 }$ , Zekun Wang∗1, Bo Zheng∗1, Zeyu Huang∗2, +Kaiyue Wen3, Songlin Yang4, Rui Men1, Le ${ \bf { Y } } { \bf { u } } ^ { 1 }$ , Fei Huang1, Suozhi Huang5, Dayiheng LiuB1, Jingren Zhou1, Junyang $\mathbf { L i n } ^ { \boxtimes 1 }$ +1Qwen Team, Alibaba Group 2University of Edinburgh 3Stanford University $^ \mathrm { 4 } \mathrm { M } \bar { \mathrm { I T } } ^ { \mathrm { ~ 5 ~ } }$ Tsinghua University + +# Abstract + +Gating mechanisms have been widely utilized, from early models like LSTMs (Hochreiter & Schmidhuber, 1997) and Highway Networks (Srivastava et al., 2015) to recent state space models (Gu & Dao, 2023), linear attention (Hua et al., 2022), and also softmax attention (Lin et al., 2025). Yet, existing literature rarely examines the specific effects of gating. In this work, we conduct comprehensive experiments to systematically investigate gating-augmented softmax attention variants. Specifically, we perform a comprehensive comparison over 30 variants of 15B Mixture-of-Experts (MoE) models and 1.7B dense models trained on a 3.5 trillion token dataset. Our central finding is that a simple modification—applying a head-specific sigmoid gate after the Scaled Dot-Product Attention (SDPA)—consistently improves performance. This modification also enhances training stability, tolerates larger learning rates, and improves scaling properties. By comparing various gating positions and computational variants, we attribute this effectiveness to two key factors: (1) introducing non-linearity upon the low-rank mapping in the softmax attention, and (2) applying query-dependent sparse gating scores to modulate the SDPA output. Notably, we find this sparse gating mechanism mitigates ‘attention sink’ and enhances long-context extrapolation performance, and we also release related codes and models to facilitate future research. + +# 1 Introduction + +Gating mechanism is well-established in neural networks. Early architectures, such as LSTMs (Hochreiter & Schmidhuber, 1997), Highway Networks (Srivastava et al., 2015) and GRUs (Dey & Salem, 2017), pioneer the use of gating to control information flow across time steps or layers and improve gradient propagation. This principle persists in modern architectures. Recent sequence modeling works, including state-space models (Gu & Dao, 2023; Dao & Gu, 2024) and attention mechanisms (Hua et al., 2022; Sun et al., 2023; Qin et al., $2 0 2 4 \mathsf { a }$ ; Yang et al., 2024b; Lin et al., 2025) commonly apply gating, often to modulate the outputs of token-mixer components. Despite its widespread adoption and empirical success, the function and impact of gating mechanisms remain insufficiently explored beyond their initial intuition. + +Insufficient understanding hinders assessing gating’s true contribution, especially when confounded with other architectural factors. For instance, while Switch Heads (Csordas et al., $2 0 2 4 \mathrm { \dot { a } ; b }$ ) introduces a sigmoid gating to select top-K attention head experts, our experiments reveal an interesting finding (Appendix A.1): substantial performance gains persist even when reduced to a single expert, where the gate simply modulates the value output. This strongly suggests the gating itself provides significant intrinsic value, separate from the routing mechanism. Similarly, in Native Sparse Attention (NSA) (Yuan et al., 2025), while overall performance improvements are demonstrated, they do not disentangle the contributions of its gating mechanism from the effects of the sparse attention design itself. These considerations underscore the need to rigorously disentangle the effects of gating from other architectural components. + +In this work, we investigate gating mechanisms in the standard softmax attention (Vaswani, 2017) (Sec.2.2). Specifically, we introduce gating at distinct positions (Fig. 1): after the query $\left( G _ { 4 } \right)$ , key $\left( G _ { 3 } \right)$ , and value projections $\left( G _ { 2 } \right)$ ; following the Scaled Dot Product Attention (SDPA) outputs $\left( G _ { 1 } \right)$ ; and after the final dense output layer $\left( G _ { 5 } \right)$ . Our exploration covers gating variants including elementwise and headwise, head-specific and head-shared, as well as additive and multiplicative forms. We find that: (i) applying SDPA output head-specific gating $\left( G _ { 1 } \right)$ yields the most significant performance improvements (e.g., up to 0.2 PPL reduction and 2 points on MMLU); (ii) the SDPA output gating also improves training stability, nearly eliminating loss spikes, enabling larger learning rates and enhancing model scalability. + +We identify two primary factors contributing to the efficacy of gating: (i) Non-Linearity. The two consecutive linear layers - the value $\left( W _ { v } \right)$ and dense $( W _ { O } )$ projections - can be rewritten into one low-rank linear projection. Therefore, introducing non-linearity through gating at positions $G _ { 1 }$ or $G _ { 2 }$ can increase the expressiveness of this low-rank linear transformation (Sec. 4.1). (ii) Sparsity. Although non-linear gating variants consistently enhance performance, we observe that their gains vary. Our analysis further reveals that the pronounced sparsity of the gating scores is another crucial factor, introducing inputdependent sparsity to SDPA outputs (Sec. 4.2). Moreover, sparse gating eliminates the attention sink (Xiao et al., 2023): the initial tokens disproportionately dominate attention scores (Fig. 2, Sec. 4.3). Previous work (Xiao et al., 2023; Sun et al., 2024; Gu et al., 2024) explains attention sinks as an accumulation of redundant attention due to non-negative softmax normalization. Empirically, we verify that when query-dependent sparse gating is applied at the SDPA output, both our dense and MoE models (trained on 3.5T tokens) exhibit no attention sink. Furthermore, these models demonstrate superior performance in length generalization, achieving a gain of over 10 points on RULER (Hsieh et al., 2024)(Sec.4.4). + +![](images/figures/gated-attention-llm-fig-0001.jpg) +Figure 1: Left: Investigated positions for applying gating operations.; Middle: Performance comparison (Test PPL and MMLU) of 15B MoE models with gating applied at various positions. Gating after SDPA $( \dot { G _ { 1 } } )$ yields the best overall results. Gating after the Value layer $\left( \hat { G } _ { 2 } \right)$ also demonstrates notable improvements, particularly in PPL. Right: Training loss comparison (smoothed, 0.9 coeff.) over $3 . 5 \mathrm { T }$ tokens between baseline and SDPA-gated 1.7B dense models under identical hyperparameters. Gating results in lower final loss and substantially enhanced training stability, mitigating loss spikes. This stability allows for potentially higher learning rates and facilitates better scaling. + +In summary, our work highlights the impact of gating in standard attention layers on the performance and behaviors of models. By evaluating gating variants, we uncover their ability to introduce non-linearity and sparsity, and eliminate attention sinks. These findings deepen our understanding of the mechanisms of gated attention. We will open-source our attention-sink-free models to advance future research. + +# 2 Gated-Attention Layer + +# 2.1 Preliminary: Multi-Head Softmax Attention + +Given an input $X \in \mathbb { R } ^ { n \times d _ { \mathrm { m o d e l } } }$ , where $n$ is the sequence length and $d _ { \mathrm { m o d e l } }$ is the model dimension, the computation of transformer’s attention layer (Vaswani, 2017) could be divided into four stages. + +QKV Linear Projections: The input $X$ is linearly transformed into queries $Q$ , keys $K _ { \cdot }$ , and values $V$ using learned weight matrices $W _ { Q } , \bar { W } _ { K } , W _ { V } \in \bar { \mathbb { R } ^ { d _ { \mathrm { m o d e l } } \times d _ { k } } }$ and $Q , K , V \in \mathbb { R } ^ { n \times d _ { k } }$ : + +$$ +Q = X W _ { Q } , \quad K = X W _ { K } , \quad V = X W _ { V } . +$$ + +Scaled Product Dot-Product Attention (SDPA): computes attention scores between queries and keys, followed by a softmax normalization. The output is a weighted sum of the values: + +$$ +{ \mathrm { A t t e n t i o n } } ( Q , K , V ) = { \mathrm { s o f t m a x } } \left( { \frac { Q K ^ { T } } { \sqrt { d _ { k } } } } \right) V , +$$ + +where $\frac { Q K ^ { T } } { \sqrt { d _ { k } } } \in \mathbb { R } ^ { n \times n }$ represents the scaled dot-product similarity matrix, and softmax $( \cdot )$ ensures the attention weights are no-negative and sum to 1 across each row. + +Multi-Head Concatenation: In multi-head attention, the above process is repeated in parallel for $h$ heads, with each head having its projection matrices $W _ { q } ^ { i } , W _ { k } ^ { i } , W _ { v } ^ { i }$ . All heads’ outputs are concatenated: + +$$ +\mathbf { M u l t i H e a d } ( Q , K , V ) = \mathbf { C o n c a t ( h e a d _ { 1 } , \dots , h e a d } _ { h } ) , +$$ + +where head $_ i =$ Attention $( Q W _ { Q } ^ { i } , K W _ { K } ^ { i } , V W _ { V } ^ { i } )$ . + +![](images/figures/gated-attention-llm-fig-0002.jpg) +Figure 2: Left: Proportion of attention allocated to the initial token per layer (test perplexity dataset). The baseline model suffers from a significant attention sink, with an average of $\bar { 4 } 6 . 7 \%$ of attention scores across layers directed towards the first token. Introducing a gate effectively alleviates this, reducing the proportion to $4 . 8 \%$ . Right: Average attention map weights for each head. Layer 21 in the baseline model demonstrates a strong attention sink ( $8 3 \%$ on the first token), which is substantially reduced by the gate $( 4 \% )$ . In the final output layer, the gate amplifies the existing tendency for the model to attend to individual tokens within the sequence. + +Final Output Layer: The concatenated SDPA output is passed through an output layer $W _ { o } \in \mathbb { R } ^ { h d _ { k } \times d _ { \mathrm { m o d e l } } }$ + +$$ +O = { \mathrm { M u l t i H e a d } } ( Q , K , V ) W _ { o } . +$$ + +# 2.2 Augmenting Attention Layer with Gating Mechanisms + +The gating mechanism is formalized as: + +$$ +Y ^ { \prime } = g ( Y , X , W _ { \theta } , \sigma ) = Y \odot \sigma ( X W _ { \theta } ) , +$$ + +where $Y$ is the input to be modulated, $X$ is another input used to compute the gating scores1, $W _ { \theta }$ refers to the learnable parameters of gate, $\sigma$ is an activation function (e.g., sigmoid), and $Y ^ { \prime }$ is the gated output. The gating score, $\sigma ( X W _ { \theta } ) .$ , effectively acts as a dynamic filter, controlling the information flow from $\bar { Y }$ by selectively preserving or erasing its features. + +In this work, we comprehensively investigate several variants of gating mechanisms within the attention layers. Our exploration focuses on five key aspects: (1) Positions. We study the effect of applying gating at different positions, as illustrated in Fig. 1(left): (a) after the $Q , K , V$ projections (Equ. 1), corresponding to positions $G _ { 2 } , G _ { 3 } , G _ { 4 }$ in Fig. 1(left); (b) following the SDPA (Equ. 3) outputs $\left( \hat G _ { 1 } \right)$ . (c) after the final concatenated multi-head attention outputs $( \mathrm { E q u . } \ 4 , G _ { 5 } )$ ). (2) Granularity. We consider two levels of granularity for the gating score: (a) Headwise: A single scalar gating score modulates the entire output of an attention head. (b) Elementwise: Gating scores are vectors with the same dimensionality as $Y$ , enabling fine-grained, per-dimension modulation. (3) Head Specific or Shared. Given the multi-head nature of attention, we further consider: (a) Head-Specific: each attention head has its specific gating scores, enabling independent modulation for each head. (b) Head-Shared: $W _ { \theta }$ and gating scores are shared across heads. (4) Multiplicative or additive. For applying gating score to $Y _ { . }$ , we consider (a) Multiplicative Gating: The gated output $Y ^ { \prime }$ is computed as: $\grave { Y } ^ { \prime } \grave { = } Y \cdot \sigma ( X \theta )$ . (b) Additive Gating: $\boldsymbol { Y } ^ { \prime } \dot { = } \dot { \boldsymbol { Y } } + \sigma \dot { ( \boldsymbol { X } \theta ) }$ . (5) Activation Function. We mainly consider two common activation functions: SiLU (Shazeer, 2020) and sigmoid. We only use SiLU for additive gating due to its unbounded output range, and sigmoid only gives scores in $[ 0 , 1 ]$ . Additionally, to further dissect the mechanisms underlying gating’s effectiveness, we also consider Identity Mapping or RMSNorm (Zhang & Sennrich, 2019) (detailed in Sec 4.1). + +Unless otherwise specified, we employ head-specific, multiplicative gating utilizing the sigmoid activation function $\begin{array} { r } { ( \sigma ( x ) = \frac { 1 } { 1 + e ^ { - x } } } \end{array}$ ). + +Table 1: Gating variant performance and results. We train the 15A2B MoE models on 400B tokens. $d _ { k }$ is the head dim, $d _ { \mathrm { m o d e l } }$ is the model’s hidden dim, and $n$ is the number of tokens. $q$ refers to the number of query heads, $k$ refers to the number of key-value heads. ‘Act Func’ is the activation function in Eq 5. ‘Score Shape’ is the gating score shape for an input $X \in \mathbb { R } ^ { n , d _ { \mathrm { m o d e l } } }$ . ‘added param’ indicates added parameters (Million). + +
MethodAct Func Score Shape Added ParamAvg PPLHellaswagMMLUGSM8kC-eval
Reference Baselines (Baseline uses q = 32, k = 4. All methods use dk = 128.)
(1) Baseline--06.02673.0758.7952.9260.26
(2) k = 8505.97973.5159.7852.1662.26
(3) q = 482015.95373.5958.4553.3059.67
4) Add 4 Experts--4005.96473.19558.8452.5463.19
Gating Position Variants
(5) SDPA Elementwise G1sigmoidn × q × dk2015.76174.6460.8255.2762.20
(6) v Elementwise G2 sigmoidn× k×dk255.82074.3859.1753.9761.0
(7) k Elementwise G3 sigmoidn × k× dk256.01672.8859.1850.4961.74
(8) q Elementwise G4igmoidn × q × dk2015.98173.0158.7453.9762.14
Dense Output G5sigmoidn × dmodel1006.01773.3259.4150.8759.43
Gating Granularity Variants
(10) SDPA Headwise G1sigmoidn× q1.65.79274.5060.0554.4462.61
(11) v Headwise G2sigmoidn× q0.25.80874.3859.32553.5362.61
Head-Specific v.s. Head-Shared Gating
(12) SDPA Head-Shared G1sigmoidn × dk2015.80174.3460.0653.1561.01
(13) v Head-Shared G2sigmoidn× dk2555.86774.1059.0253.0360.61
Multiplicative v.s. Additive
(14) SDPA Additive G1SiLUn × q × dk2015.82174.8160.0653.3060.98
Activation Variants
(15) SDPA Elementwise G1SiLUn × q × dk2015.82274.2260.4954.5962.34
+ +# 3 Experiments + +# 3.1 Experimental Setups + +Model Architecture and Training Settings We conduct experiments on both MoE models (15B total parameters with 2.54B activated, 15A2B) and dense models (1.7B total parameters). The 15A2B MoE models utilize 128 total experts with top-8 softmax gating, fine-grained experts (Dai et al., 2024), global-batch LBL (Qiu et al., 2025), and $\mathbf { Z }$ -loss (Zoph et al., 2022). We adopt group query attention (GQA) (Ainslie et al., 2023) for the attention part. We train the models on subsets of a $3 . 5 \mathrm { T }$ high-quality tokens, encompassing multilingual, math, and general knowledge content. The context sequence length is set to 4096. More detailed configurations, such as learning rate and batch size (bsz), will be introduced in each part. Other hyperparameters follow the default values of the AdamW optimizer. Since the parameters and flops introduced by the gating are small, the wall-time latency introduced by gating is less than $2 \%$ . + +Evaluation We test the few-shots results on popular benchmarks, including Hellaswag (Zellers et al., 2019) for English, MMLU (Hendrycks et al., 2020) for general knowledge, GSM8k (Cobbe et al., 2021) for math reasoning, HumanEval (Chen et al., 2021) for coding, C-eval (Huang et al., 2024) and CMMLU (Li et al., 2023) for Chinese proficiency. We also report the perplexity (PPL) of language modeling on diverse held-out test sets, including domains like English, Chinese, Code, Math, Law, and Literature. + +# 3.2 Main Results + +# 3.2.1 Gated Attention for MoE models + +We first compare the results of different gated attention layers on the training-efficient MoE-15A2B models. All models use a scheduler that warms up to a maximum LR of 2e-3 in 1k steps and decays using cosine to 3e-5. We use a global bsz of 1024, comprising $1 0 0 \mathrm { k }$ optimization steps. The results are summarized in Tab. 1. To provide a fair comparison, we supplement the vanilla MoE baseline (row 1) with parameter expansion methods, including increasing the number of key-value heads (row 2), increasing the number of query heads (row 3), and increasing both the total and activated number of experts (row 4). These methods introduce a comparable or greater number of parameters than the gating mechanisms. + +From Tab. 1, we observe: (i) SDPA and value output gating are effective. Inserting gates at the output of SDPA $\left( G _ { 1 } \right)$ or the value map $\left( G _ { 2 } \right)$ is the most effective, achieving lower PPL and better overall benchmark performance than other variants. We will further investigate why gating at these two positions is effective in $\mathrm { S e c } 4 . 2$ . (ii) Head-Specific Gating Matters. Applying headwise gating at $G _ { 1 }$ and $G _ { 2 }$ introduces very few additional parameters (less than 2M for the MoE-15A2B model) but still delivers substantial improvements (rows 10 and 11). When sharing gating scores across different attention heads (we average over the query head dimension $q$ to obtain an $n \times d _ { k }$ score from the original $n \times q \times d _ { k }$ ), the benchmark improvements are smaller than those achieved by headwise gating (row 12 v.s. 10, 13 v.s. 11). This underscores the importance of applying distinct gating scores for different attention heads. (iii) Multiplicative Gating is Preferred. Additive SDPA output gating underperforms the multiplicative one, although it shows improvements over the baselines. (iv) Sigmoid Activation is Better. Replacing the activation function in the most effective gating configuration (row 5) with SiLU (row 15) leads to less improvement. + +Table 2: Performance of different methods with varying learning rates, batch sizes, and model configurations. ‘SDPA’ refers to the sigmoid gating after SDPA in $\operatorname { E q } 3 ,$ and ‘sandwitch norm’ (Ding et al., 2021) indicates normalizing attention/ffn outputs before adding them to the residual. When using gating, we reduce the FFN’s width so that all methods have the same number of parameters. ‘-’ means the model diverges during training. + +
MethodMax LRAvg PPLHumanEvalMMLUGSM8kHellaswagC-evalCMMLU
28 Layer, 1.7B Parameters, 400B Tokens, Batch Size=1024
(1) Baseline4.0 × 10−37.49928.6650.2127.8264.9449.1549.52
(2) SDPA Elementwise4.0 × 10−37.40429.2751.1528.2865.4850.7250.72
28 Layer, 1.7B Parameters, 3.5T Tokens, Batch Size=2048
3)Baseline4.5 × 10−36.18034.1559.1069.0768.0268.1964.95
(4) SDPA Elementwise4.5 × 10−36.13037.8059.6170.2068.8468.5265.76
48 Layer, 1.7B Parameters, 400B Tokens, Batch Size=1024
5)Baseline4.0 × 10−37.42128.0552.0432.9865.9651.1151.86
Baseline8.0 × 10−39.19521.3444.2815.2457.0043.1142.63
(7) Baseline+Sandwich Norm8.0 × 10−37.40730.4952.0732.9066.0052.0451.72
(8) SDPA Elementwise4.0 × 10−37.28831.7152.4432.3766.2852.0652.29
) SDPA Headwise4.0 × 10−37.37031.1053.8334.1265.5955.0752.38
(10) SDPA Elementwise8.0 × 10−37.32531.1054.4736.6266.4053.9153.80
48 Layer, 1.7B Parameters, 1T Tokens, Batch Size=4096
(11) Baseline5.3 × 10−37.36329.8854.4432.2265.4353.7253.37
(12)Baseline8.0 × 10−3-------
(13) SDPA Elementwise5.3 × 10−37.10134.1555.7036.6967.1754.5154.68
(14) SDPA Elementwise8.0 × 10−37.07831.7156.4739.7367.3855.5255.77
+ +Overall, adding gating at the value layer $\left( G _ { 2 } \right)$ and SDPA output $\left( G _ { 1 } \right)$ reduces PPL by more than 0.2, outperforming various parameter-expanding baselines. However, gating at $G _ { 1 }$ achieves better PPL and benchmark results. As long as different heads receive distinct gating scores, the granularity of gating and the choice of activation function have relatively minor impacts. We will further analyze the reasons behind these observations in Analysis (Sec 4.2). + +# 3.2.2 Gated Attention for Dense Models. + +We also conduct experiments on dense models following (Yang et al., 2024a) to validate SDPA output sigmoid gating. When using gating, we reduce the width of FFN to maintain the parameter size. Most experiments use optimized hyperparameters for the baseline. For instance, for the 1.7B model trained on 400B tokens, we use a maximum LR of 4e-3 and a bsz of 1024. For training on 3.5T tokens, we increase the maximum LR to $4 . 5 \mathrm { e } { - 3 }$ and the bsz to 2048. Prior work has established that while increased network depth, large learning rates, and large batch sizes can significantly improve model performance (McCandlish et al., 2018; Wang et al., 2022; D’Angelo et al., 2024) and distributed training efficiency, they often introduce training instabilities (Wang et al., 2022; Zeng et al., 2022; Takase et al., 2023). We observe that applying gating mechanisms demonstrably reduces the occurrence of loss spikes during training (Chowdhery et al., 2023; Takase et al., 2023), suggesting a promising role for gating in enhancing training stability. Motivated by this finding, we introduce another experimental setting characterized by an increased number of layers, a higher maximum learning rate, and a larger batch size to further probe gating’s stabilizing effects. + +Tab. 2 reveals that: (i) Gating is effective across various settings Across various model configurations (row 1 v.s. 2, 5 v.s. 8), training data (row 3 v.s. 4), and hyperparameters (row 11 v.s. 13), applying SDPA output gating consistently yields benefits. (ii) Gating improves stability and facilitates scaling. Under the $3 . 5 \mathrm { \check { T } }$ token setting, gating improves training stability, largely reducing the loss spike (Fig. 1, right). When increasing the maximum LR, baselines encounter convergence issues (row 6, 12). While adding sandwich norm (Ding et al., 2021) restores convergence, the improvement is negligible. In contrast, increasing the maximum LR in models with gating results in a noticeable improvement. + +In summary, we identify SDPA element-wise gating as the most effective method to augment the attention mechanism. Applying this method to dense transformers further demonstrates that the gate enables stable training with larger batch sizes and learning rates, resulting in improved performance. + +Table 3: Performance of different (non)-linearity augmentations. + +
MethodActivation FunctionAvg PPLHellaswagMMLUGSM8kC-eval
(1) Baseline-6.02673.0758.7952.9260.26
(2) SDPA Elementwise GateSigmoid5.76174.6460.8255.2762.20
v Elementwise Gate Sigmoid5.82074.3859.1753.9761.00
(4) SDPA Additive GateSiILU5.82174.8160.0653.3060.98
(5) SDPA GroupNormRMSNorm5.84774.1060.1553.7561.14
6 SDPA SiLUSiLU5.97573.3459.5553.1960.90
(7) SDPA Additive GateIdentity5.88274.1759.2052.7759.86
+ +# 4 Analysis: Non-Linearity, Sparsity, and Attention-Sink-Free + +In this section, we conduct a series of experiments to explore why such a simple gating mechanism can yield significant improvements in performance and training stability. Here are the takeaways according to our analysis: (1) Gating operations enhancing non-linearity consistently lead to performance gains (Sec 4.1); (2) The most effective SDPA elementwise $G _ { 1 }$ gate introduces strong input-dependent sparsity to the SDPA outputs (Sec 4.2), which then helps to eliminate the ‘attention sink’ phenomenon. + +# 4.1 Non-linearity Improves the Expressiveness of Low-Rank Mapping in Attention + +Inspired by prior works that utilize group norm for the SDPA output (Sun et al., 2023; Ye et al., 2024), with the same setting in Sec. 3.2.1, we apply RMSNorm (Zhang & Sennrich, 2019) independently to the output of each attention head before concatenation. As shown in Tab. 3 row 5, applying RMSNorm, which introduces almost no additional parameters, also leads to a significant reduction in PPL. + +In multi-head attention, the output of the $i$ -th token, corresponding to the $k$ -th head, can be expressed as: + +$$ +o _ { i } ^ { k } = ( { \sum } _ { j = 0 } ^ { i } S _ { i j } ^ { k } \cdot X _ { j } W _ { V } ^ { k } ) W _ { O } ^ { k } = { \sum } _ { j = 0 } ^ { i } S _ { i j } ^ { k } \cdot X _ { j } ( W _ { V } ^ { k } W _ { O } ^ { k } ) , +$$ + +where $W _ { O } ^ { k }$ is the parameters of the output layer $W _ { O }$ corresponding to the $k$ -th head2. Here, $S _ { i j } ^ { k }$ denotes the attention score of the $i$ -th token attending to the $j$ -th token in the $k$ -th head, $X _ { j }$ is the input to the attention for token $j .$ , and $X _ { j } W _ { V } ^ { k }$ represents the value output of token $j$ in the $k$ -th head. From Equ. 6, we can merge $W _ { \scriptscriptstyle . V } ^ { k } W _ { \cal O } ^ { k }$ into one low-rank linear mapping applied over all $X _ { j }$ as $d _ { k } < d _ { m o d e l }$ . With GQA, $W _ { V }$ is shared among heads within the same group, further diminishing the expressiveness. + +Given that adding non-linearity between two linear mappings can improve their expensiveness (Montufar et al., 2014), we have two modifications to mitigate the low-rank problem: + +$$ +o _ { i } ^ { k } = \left( \sum _ { j = 0 } ^ { i } S _ { i j } ^ { k } \cdot \mathrm { N o n - L i n e a r i t y - M a p } ( X _ { j } W _ { V } ^ { k } ) \right) W _ { O } ^ { k } , +$$ + +$$ +\begin{array} { r } { o _ { i } ^ { k } = \mathrm { N o n - L i n e a r i t y - M a p } \left( \sum _ { j = 0 } ^ { i } S _ { i j } ^ { k } \cdot X _ { j } W _ { V } ^ { k } \right) W _ { O } ^ { k } . } \end{array} +$$ + +Notably, adding gating at the $G _ { 2 }$ (Tab. 3 row 3) position corresponds to the first modification (Equ. 7), while adding gating (row 4) or group normalization (row 5) at the $G _ { 1 }$ position corresponds to the second (Equ. 8). This also explains why adding gating or normalization at the $G _ { 5 }$ position after $W _ { O }$ has no effect (Tab. 1 row 9) — it does not address the lack of non-linearity between $\bar { W _ { V } } ^ { - }$ and $W _ { O }$ . + +For additive gating at $G _ { 1 } ,$ the output of gating passes through SiLU (Tab. 3 row 4), also introducing some non-linearity, which explains the observed performance gains, albeit smaller than those achieved by multiplicative gating. Based on these insights, we conduct two additional experiments: (i) Adding SiLU only at the $G _ { 1 }$ position without introducing additional parameters (Tab. 3 row 6). Notice this simple modification also leads to a modest reduction in PPL, but most benchmark scores remain unchanged. (ii) Removing SiLU from additive gating, such that the output of $X _ { j }$ after gating is directly added at the $G _ { 1 }$ position (Tab. 3 row 7). This further diminishes the gains of addictive gating. + +In summary, the enhanced performance associated with effective gating variants is likely attributable to the introduction of non-linearity between $W _ { V }$ and $W _ { O }$ . Although applying gating at positions $G _ { 1 }$ and $G _ { 2 }$ can can both introduce this non-linearity, these applications yield differing performance gains. This observed difference motivates us to further analyze the impacts of gating at these two positions. + +![](images/figures/gated-attention-llm-fig-0003.jpg) +Figure 3: Gating score means and distributions for SDPA elementwise (Left), value Elementwise (Middle), and SDPA elementwise with head-shared gating (Right). Most gating scores are less than 0.5, indicating that the gating scores are sparse. Among them, the SDPA output gating score exhibits the strongest sparsity. + +# 4.2 Gating Introduces Input-Dependent Sparsity + +We analyze the gating scores (Tab. 1, ‘Gate Score’ column) of models with gating applied at the value $\left( G _ { 2 } \right)$ and SDPA output $\left( G _ { 1 } \right)$ positions, evaluated on the test language modeling data. The mean gating scores for all layers are presented in Table 4, with the score distributions visualized in Fig. 3 (layer-wise scores in Appendix A.2). Key observations include: + +(i) Effective Gating Scores are Sparse. SDPA output gatings (Element/head-wise) exhibit the lowest mean gating scores. Furthermore, the SDPA output gating score distribution shows a high concentration near 0, indicating substantial sparsity, consistent with its superior performance. (ii) Head-Specific Sparsity Matters. Enforcing shared gating scores across attention heads increases the overall gating scores and diminishes performance gains. Observations (i) and (ii) underscore the importance of head-specific gating, aligning with previous research demonstrating that individual attention heads capture distinct aspects of the input (Voita et al., 2019; Wang et al., 2021; Olsson et al., 2022; Wang et al., 2023). + +(iii) Query-Dependency Matters. The scores for value gating $\left( G _ { 2 } \right)$ are higher than those for SDPA output gating $\left( G _ { 1 } \right)$ , and the performance is inferior. This suggests that gating score sparsity is more effective when query-dependent rather than determined by the key and value. Specifically, SDPA output gating scores are derived from the hidden states corresponding to the current query (e.g. the Non-Linearity-Map in $\operatorname { E q } 8$ depends on $X _ { i }$ ), whereas value gating scores are derived from hidden states associated with past keys and values (e.g. the Non-Linearity-Map in Eq 7 depends on each $X _ { j }$ ). This implies that gating score sparsity may filter out irrelevant contextual information for the query. To further validate the importance of query-dependency, we introduce input-independent gating by zero-initializing learnable parameters $( q \dot { \times } d _ { k } \dot { ) }$ , applying a sigmoid function, and multiplying it with the SDPA output. As shown in row (6), input-independent gating improves upon the baseline, likely due to the introduction of non-linearity. Moreover, the high gating scores reinforce that effective sparsity should be input-dependent. + +(iv) Less Sparse Gating is Worse. To further validate the importance of gating score sparsity, we reduce sparsity from the gating formulation. Specifically, we replace the sigmoid function with a modified Non-Sparse (NS) version: + +$$ +\mathrm { N S - s i g m o i d } ( x ) = 0 . 5 + 0 . 5 \cdot \mathrm { s i g m o i d } ( x ) , +$$ + +which constrains the gating scores between [0.5, 1.0]. This ensures introducing non-linearity while removing gating score sparsity. As shown in Tab. 4 row (7), the gains of NS-sigmoid gating are inferior to those of SDPA output sigmoid gating. In Appendix A.2, we provide a more detailed discussion on how sparse gating scores affect the sparsity (the proportion of values below the threshold) in SDPA hidden states. We will discuss the impact of different sparsity levels on model behavior, including reducing the ‘attention sink’, in the next section. + +# 4.3 SDPA Output Gating Reduces Attention-Sink + +Based on the observation that gating introduces sparsity to the SDPA output in an input-dependent manner, we hypothesized that this mechanism can filter out context irrelevant to the current query token, thereby mitigating the attention sink (Xiao et al., 2023; Sun et al., 2024). To verify this, we analyze the distribution of attention scores (averaged over all heads) and the proportion of attention scores allocated to the first token (Fig. 2, Tab. 4, ‘F-Attn’ column). Inspired by the discussion about massive activation in hidden states and attention sinks (Sun et al., 2024), we also compute the mean of the maximum hidden state activations across layers, as shown in the ‘M-Act’ column of Tab. 4. More detailed layer-wise results are provided in the Appendix A.3. + +We can observe: (i) Head-wise and element-wise query-dependent sigmoid gating at the SDPA output $\left( G _ { 1 } \right)$ largely reduces the attention score allocated to the first token and decreases massive activations. (ii) Enforcing shared gating scores across heads or applying gating only after the value projection $\left( G _ { 2 } \right)$ decreases massive activations, but does not reduce attention scores to the first token. This reinforces the importance of head-specific gating and suggests that massive activations are not a prerequisite for attention sinks. (iii) Reducing the input-dependence of gating (row 6) or using NS-sigmoid to reduce sparsity (row 7) intensifies both massive activations and attention sink. + +Collectively, these observations indicate that input-dependent, head-specific gating of the SDPA output introduces significant sparsity, thereby mitigating the attention sink. Furthermore, sparsity in the SDPA outputs reduces massive activations within the model, with increased sparsity leading to smaller activations. This may explain the improved training stability with gating: by reducing massive activations, the model is less susceptible to numerical errors during BF16 training (Budzinskiy et al., 2025). We also observe that massive activations originate primarily from early layers (e.g., layer 5), where the FFN outputs large values, consistent with (Yona et al., 2025). Once added to the residual stream, these activations are propagated through subsequent layers via the pre-norm mechanism. This aligns with the effectiveness of sandwich normalization (Ding et al., 2021) in enhancing training stability (Table 2, row 7): applying LayerNorm to the FFN output prevents these large activations from entering the residual stream. + +# 4.4 SDPA Output Gating Facilitates Context Length Extension + +Based on the attention-sinkfree pattern, we evaluate the SDPA gating’s effect in the long-context setting. Specifically, we extend the context length for the models trained on 3.5T tokens. We increase the RoPE (Su et al., 2024) base from 10k to 1M and continue training on data with a + +Table 5: Performance of different methods across varying sequence lengths. ‘YaRN Extended’ indicates the expanded context length variant. ‘(values)’ indicate the performance declines after extending the context length. + +
Method4k8k16k32k64k128k
Baseline88.8985.8883.1579.50,-
SDPA-Gate90.5687.1184.6179.77--
YaRN Extended
Baseline82.90(-6.0)71.52(-14.4)61.23(-21.9)37.94(-41.56)37.5131.65
SDPA-Gate88.13(-2.4)80.01(-7.1)76.74(-7.87)72.88(-6.89)66.6058.82
+ +sequence length of 32k for an additional 80B tokens. This gives us models with a context length of $3 2 \mathrm { k }$ Subsequently, we use YaRN (Peng et al., 2023) to extend the context length to $1 2 8 \mathrm { k }$ . We evaluate models on the RULER benchmark (Hsieh et al., 2024) and summarize results in Tab. 5. We observe the following: (i) Under the 32k setting, models with gating slightly outperform the baseline. This suggests that within the training length, the attention sink phenomenon may not hurt the model’s long-context performance. (ii) When the context length is extended to $1 2 8 \mathrm { k }$ using YaRN, both the baseline and gated models experience a decline within the original $3 2 \mathrm { k }$ range. This observation is consistent with previous works on extending context length by modifying RoPE (Chen et al., 2023; Peng et al., 2023; Dong et al., 2025). Even though the decline is less pronounced for models with gating. (iii) At context lengths of 64k and $1 2 8 \mathbf { k } ,$ the gated attention models outperform the baseline signifantly. From these observations, we hypothesize that adding gating helps the model adapt to the context-length extension. A possible explanation is that baseline models rely on attention sinks to adjust the distribution of attention scores. Dong et al. (2025) derives the effects of changing the RoPE based on the attention and hidden state distributions. When techniques like YaRN are applied to modify the RoPE base, the attention sink pattern may struggle to adapt in a training-free manner, leading to a noticeable drop in performance. In contrast, models with gating primarily rely on input-dependent gating scores to control information flow, making them more robust to such changes. + +# 5 Related Works + +# 5.1 Gating in Neural Networks + +Gating mechanisms have been widely adopted in neural networks. Early works such as LSTMs (Hochreiter & Schmidhuber, 1997) and GRUs (Dey & Salem, 2017) introduce gates to regulate information flow across time steps, addressing gradient vanishing/exploding issues by selectively retaining or discarding information. Highway Networks (Srivastava et al., 2015) extend this concept to feedforward networks, enabling the successful training of very deep architectures. SwiGLU (Shazeer, 2020) introduce gating mechanisms into transformer FFN layers, enhancing their expressive power and becoming a standard component in many open-source LLMs (Grattafiori et al., 2024; Yang et al., 2024a). + +Several works on state-space models (Gu & Dao, 2023; Dao & Gu, 2024) and Linear Attention, such as FLASH (Hua et al., 2022), RetNet (Sun et al., 2023), Lightning Attention (Qin et al., 2024a;b; Li et al., 2025), and Gated Delta Networks (Yang et al., 2024b), also incorporate gating modules to controlinformation of token-mixer modules. Forgetting Transformer (Lin et al., 2025) applies gating mechanisms to the output of softmax attention, observing significant performance improvements. Although these works demonstrate the effectiveness of gating, a comprehensive understanding of its precise mechanisms and the reasons behind its effectiveness still needs exploration. This could contribute to a broader appreciation of gating’s importance beyond RNNs and facilitate designs that better leverage gating’s unique advantages. For example, while Switch Heads (Csordas et al., 2024b;a), NSA (Yuan et al., 2025), and MoSA (Piękos et al., 2025) employ sigmoid-based gating (Csordas et al., 2023) for selection, further investigation into isolating gating’s specific contribution could offer valuable insights. Comparisons with baselines incorporating similar gating mechanisms in standard transformers could offer a more refined perspective on the effectiveness of their proposed selection mechanisms. The work most closely related to ours is Quantizable Transformers (Bondarenko et al., 2023), which also finds that applying gating in softmax attention alleviates extreme attention concentration and outliers in hidden states in encoder models like BERT and ViT. While this work primarily leverages gating to eliminate outliers for model quantization, we provide a detailed analysis of various gating variants, uncovering their benefits through enhanced non-linearity and sparsity, as well as improved training stability. Building on these insights, we scale up gated attention models, demonstrating gating’s broad applicability and impact. + +# 5.2 Attention Sink + +Xiao et al. (2023) formally identifies the ‘attention sink’ phenomenon, in which specific tokens receive large attention scores. Similarly, Darcet et al. (2023) finds in the vision transformer, some redundant tokens act as ‘registers’ to store attention scores. Later, Sun et al. (2024) shows that excessive attention scores are also assigned to tokens associated with massive activation values. However, our work reveals that applying gating at the output of value projection eliminates massive activations, yet attention sinks persist, indicating that massive activations are not a necessary condition for attention sinks. Similarly, Gu et al. (2024) characterizes attention sinks as non-informative ‘key biases’ that store redundant attention scores, arguing that softmax’s inherent normalization dependency drives this behavior. Experimental attempts to modify softmax attention, such as replacing softmax with unnormalized sigmoid attention (Ramapuram et al., 2024; Gu et al., 2024), adding softmax attention gate or clip (Bondarenko et al., 2023), and modifying softmax computation (Zuhri et al., 2025) and denominator (Miller, 2023), show promise in mitigating attention sinks. Our work demonstrates that sparse gating after SDPA eliminates attention sinks in both dense (1B-parameter) and MoE (15B-parameter) models, even when trained on 3.5T tokens. Furthermore, we uncover the potential of eliminating attention sinks to benefit context-length extension. + +# 6 Conclusion + +This work systematically investigates the role of gating mechanisms in the standard softmax attention, revealing their significant impact on performance, training stability, and attention dynamics. Through extensive experimental comparisons over 30 variants of 15B MoE and 1.7B dense models trained on up to $3 . 5 \mathrm { T }$ tokens, we demonstrate that applying a sigmoid gate after scaled dot-product attention yields the most substantial improvements. This simple mechanism enhances non-linearity, introduces input-dependent sparsity, and eliminates inefficiencies like the ‘attention sink’ phenomenon. Additionally, gating facilitates context length extension, allowing models to generalize effectively to longer sequences without retraining. We also release the first attention-sink-free models. We believe these empirical validations will pave the way for engineering the next generation of advanced foundation models. + +# Limitations + +Our work primarily focuses on analyzing the reasons and impacts of attention gating through a series of ablation studies. However, we acknowledge several limitations. The broader implications of non-linearity on the dynamics of attention and the overall training process remain under-explored. 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St-moe: Designing stable and transferable sparse expert models. arXiv preprint arXiv:2202.08906, 2022. + +Zayd M. K. Zuhri, Erland Hilman Fuadi, and Alham Fikri Aji. Softpick: No attention sink, no massive activations with rectified softmax, 2025. URL https://arxiv.org/abs/2504.20966. + +# A Supplement Experiments + +# A.1 Switch Head Baselines + +In this section, we present detailed experiments related to Switch Heads. The Switch Head paper demonstrates that introducing sparse activation in attention—where each token selects the top-k experts from a pool of key/value/output experts via learnable sigmoid routing—enables the model to achieve comparable results to the baseline. This suggests that, within the Switch Head framework, both expert parameters and activated parameters are beneficial, with more being better under the same total parameter budget. + +Table 6: Performance of different switch head methods with varying parameter additions and configurations. ‘switch $\mathbf { k } \mathbf { v } ^ { \prime }$ and ‘switch $\mathbf { v } ^ { \prime }$ refer to introducing selective computing in key-value and value components, respectively. ‘Switch kv, 8top8’ means there are 8 key and value map experts, and each token select top8 experts. Notice ‘Switch v, 1top1’ is equivalent to v Headwise Gate in Tab. 1 row (11). + +
MethodAdded Param (M)PPLMMLUGSM8kHellaswagC-eval
(1) Baseline (q32, kv4)-6.02658.7952.9273.0760.26
Switch kv, 8top8385.84759.1752.5473.3261.01
Switch kv, 4top4135.93558.1453.2773.7559.67
4 Switch v, 4top4135.82059.0252.7773.3461.74
Switch v, 8top2255.87059.1053.5374.1762.34
Switch v, 1top135.80859.3253.5374.3862.61
+ +Looking at the results in Tab. 6, we observe an interesting trend: while increasing the number of activated kv experts (with the same expert parameter settings) appears to offer some improvement in PPL (row 4 vs. 5), the gains in overall benchmark performance are less pronounced. Notably, the best results for both benchmark scores and PPL were achieved by ‘Switch v 1top1’ (row 6), which, as mentioned earlier, is analogous to applying sigmoid gating directly to the output of the value layer. These findings raise an intriguing question about the primary driver of the performance improvements observed in these experiments. It suggests that the introduction of gating itself plays a significant role in the effectiveness of this approach. + +# A.2 More Discussion on Sparse Gating Score + +In this section, we analyze the impact of gating score sparsity on attention output. First, we examine the mean values of SDPA output before and after applying gating to the hidden states. Specifically, we calculated the mean absolute values of $Y$ and $Y ^ { \prime }$ before and after $\breve { G } _ { 1 }$ at each layer, as shown in Fig. 4. We also included results from a baseline without gating for comparison. The results indicate that: (1) after gating, the mean value of hidden states decreased from 0.71 to 0.05, corresponding to the generally small gating scores; (2) the gated hidden states closely resemble the baseline, suggesting that gating might serve a similar function as attention sink in filtering out irrelevant information. + +We further analyze the proportion of hidden states below certain thresholds before and after gating, as shown in $\mathrm { F i g } 5$ . The results reveal that: (1) after gating, the sparsity in hidden states significantly increases across different thresholds. Since the mean gating scores are already small, multiplying hidden states by a small number naturally pushes some values below the threshold. Therefore, (2) we further multiply the pre-gating hidden states by the average gating score and observed that the increase in sparsity is smaller than with original gating. This suggests that sparse gating scores enhance sparsity in hidden states. + +![](images/figures/gated-attention-llm-fig-0004.jpg) +Figure 4: Mean absolute values before and after gating. The baseline and post-gating values are similar. + +![](images/figures/gated-attention-llm-fig-0005.jpg) +Figure 5: Proportion of SDPA output values below threshold after gating (Left: 1e-2, Right: 1e-3). We also include sparsity measurements obtained by multiplying the average gating score with pre-gating hidden states. + +# A.3 Layerwise Massive Activations and Attention Sinks + +In this section, we compare and analyze the presence of massive activations and attention sinks (the attention score of the first token) within the model. From the results, we observe the following: + +For the baseline (row 1), the output of the 6th layer’s FFN contains massive activations, which are subsequently added to the residual stream, causing large activations to persist in the residuals of subsequent layers. Correspondingly, significant attention sink phenomena emerge starting from the 6th layer. After applying gating to the SDPA output (row 2), the outputs of the earlier layers in the network remain relatively small overall, with massive activations growing gradually as the layer depth increases. Notably, no significant attention sink phenomenon is observed in any layer of the network. + +When gating is applied only at the value layer (row 3), the model exhibits massive activations similar to row 2. However, a certain degree of attention sink phenomenon persists. This indicates that massive activations are not a necessary condition for the emergence of attention sinks. When enforcing shared gating scores across different heads (row 4) or modifying the activation function of gating to suppress sparsity (row 5), the sparsity introduced by gating is reduced. In these cases, both massive activations and attention sinks become comparable to those observed in the baseline. + +These observations suggest that introducing sufficient sparsity within the attention mechanism may help mitigate the occurrence of massive activations. However, further investigation is needed to fully understand the interplay between sparsity, massive activations, and attention sinks, particularly in the context of scaling to deeper and larger models. + +# A.4 More Layerwise Gating Scores + +In this section, we analyze the distribution of gating scores under two additional constraints while using SDPA output gating as the baseline (row 1, elementwise/headwise): (1) enforcing the same gating score across different heads (row 2, left), and (2) restricting the minimum value of the gating scores (row 2, + +right). When enforcing shared gating scores across different heads, the gating scores for most layers increase. This indicates that different heads require different sparsity, highlighting the importance of head-specific gating mechanisms. + +# A.5 Other Attempt to Stabilize Training + +We observe that both the addition of sandwich normalization (Ding et al., 2021) and gating mechanisms eliminate massive activations while improving training stability. This prompts us to explore whether simpler methods could prevent large activations within residuals. Specifically, we introduce a clipping operation to constrain the outputs of attention and FFN layers before they enter the residual connection, limiting their values to the range (-clip, clip). However, we find that regardless of whether the clip value was set to 300 or 100, the model still encounters convergence issues at a learning rate of 8e-3. This suggests that the instability in pre-norm model training is not solely due to large activations within residuals. It is likely that any layer producing large outputs can lead to stability problems, indicating the need for further investigation into the root causes of training instability. + +![](images/figures/gated-attention-llm-fig-0006.jpg) +Figure 6: Comparison of massive activations and attention sink phenomena across different gating configurations. Row 1 (Baseline): Significant massive activations and attention sinks emerge after the 6th layer. Row 2 (SDPA Gating): Reduced activations and no attention sinks observed. Row 3 (Value Layer Gating): Similar activations to Row 2 but with residual attention sinks. Rows 4–5 (Reduced Sparsity via cross-head share and NS-sigmoid): Massive activations and attention sinks resemble the baseline. + +![](images/figures/gated-attention-llm-fig-0007.jpg) +Figure 7: Distribution of gating scores under different constraints for SDPA output gating variants. \ No newline at end of file diff --git a/papers/gated-attention-llm/paper.pdf b/papers/gated-attention-llm/paper.pdf new file mode 100644 index 0000000000000000000000000000000000000000..db939de24ce4d39dd7b30c81d62360573c894f79 --- /dev/null +++ b/papers/gated-attention-llm/paper.pdf @@ -0,0 +1,3 @@ +version https://git-lfs.github.com/spec/v1 +oid sha256:949c7ed67dea3ddc6b88854cf72631ed2e1f466ad4c824c8aa1286739bcf2359 +size 1503818 diff --git a/papers/gated-attention-llm/sau.json b/papers/gated-attention-llm/sau.json new file mode 100644 index 0000000000000000000000000000000000000000..c0655918dcda6944277a92b842be2a7dcfa3107d --- /dev/null +++ b/papers/gated-attention-llm/sau.json @@ -0,0 +1,317 @@ +{ + "paper_id": "gated-attention-llm", + "paper_title": "Gated Attention for Large Language Models: Non-linearity, Sparsity, and Attention-Sink-Free", + "D1": [ + { + "id": "gated-attention-llm-D1-001", + "claim": "MoE-15A2B architecture: total_params=15B, activated_params=2.54B, total_experts=128 (fine-grained), top_k=8 (softmax), query_heads=32, key_value_heads=4 (GQA, head_dim=128), global-batch load balancing (LBL) with z_loss", + "source": "Section 2, 4.1" + }, + { + "id": "gated-attention-llm-D1-002", + "claim": "Dense-1.7B architecture: total_params=1.7B, layers=28 and 48 variants, head_dim=128 with GQA; head variants: query_heads=48, key_value_heads=8", + "source": "Section 2, 4.2" + }, + { + "id": "gated-attention-llm-D1-003", + "claim": "Core gating mechanism: Y' = Y ⊙ σ(X W_θ) with sigmoid activation (σ(x)=1/(1+e^(-x)), range [0,1]). Gating positions: G1=after SDPA, G2=after W_V, G3=after W_K, G4=after W_Q, G5=after W_O. Granularity: headwise (scalar/head) and elementwise (per-dim vector). Specificity: head_specific vs head_shared. Mode: multiplicative (Y⊙σ) vs additive (Y+σ with SiLU).", + "source": "Section 3.1-3.3" + }, + { + "id": "gated-attention-llm-D1-004", + "claim": "Gating added parameters: SDPA elementwise G1=201M, v elementwise G2=25M, k elementwise G3=25M, q elementwise G4=201M, o elementwise G5=201M (total ~653M). Headwise: SDPA G1=1.6M, v G2=0.2M. Score shapes: elementwise G1=n×q×d_k, headwise G1=n×q, head_shared G1=n×d_k.", + "source": "Section 3.4, Table 1" + }, + { + "id": "gated-attention-llm-D1-005", + "claim": "Training infrastructure: context_length=4096, total_tokens=3.5T (with 400B and 1T subsets), optimizer=AdamW", + "source": "Section 4.1" + }, + { + "id": "gated-attention-llm-D1-006", + "claim": "MoE-15A2B training schedule: max_lr=0.002, min_lr=0.00003, warmup_steps=1000, cosine decay, batch_size=1024, 100K optimization steps", + "source": "Section 4.1" + }, + { + "id": "gated-attention-llm-D1-007", + "claim": "Dense 1.7B 28-layer variants: 400B tokens (bsz=1024, max_lr=0.004) and 3.5T tokens (bsz=2048, max_lr=0.0045)", + "source": "Section 4.2" + }, + { + "id": "gated-attention-llm-D1-008", + "claim": "Dense 1.7B 48-layer variants: 400B tokens (bsz=1024, max_lr=[0.004, 0.008]) and 1T tokens (bsz=4096, max_lr=[0.0053, 0.008])", + "source": "Section 4.2" + }, + { + "id": "gated-attention-llm-D1-009", + "claim": "Gating overhead: wall_time <2%. Sparsity analysis: most scores <0.5 (SDPA gating strongest), mean hidden states before gating=0.71 vs after=0.05, sparsity thresholds=[0.01, 0.001]. NS-sigmoid variant: 0.5+0.5×sigmoid(x), range [0.5,1.0].", + "source": "Section 4.3-4.4" + }, + { + "id": "gated-attention-llm-D1-010", + "claim": "Attention sink elimination: baseline mean first-token attention=46.7% vs gated=4.8%; at layer 21: baseline 83% vs gated 4%. Massive activation origin: layer 5 (FFN), attention sink emergence: layer 6.", + "source": "Section 5.1-5.2" + }, + { + "id": "gated-attention-llm-D1-011", + "claim": "Long-context extension: RoPE base 10,000→1,000,000, continued training at seq_len=32768 for 80B tokens, YaRN target=128K. Evaluated at lengths [4096, 8192, 16384, 32768, 65536, 131072] on RULER.", + "source": "Section 5.3" + }, + { + "id": "gated-attention-llm-D1-012", + "claim": "SDPA headwise G1 sigmoid (+1.6M) and v elementwise G2 sigmoid (+25M) improve over dense baselines. Dense 28L 3.5T (bsz=2048, max_lr=0.0045): SDPA elementwise outperforms baseline.", + "source": "Section 4.2, Table 2-3" + }, + { + "id": "gated-attention-llm-D1-013", + "claim": "Dense 48L stability: baseline diverges at max_lr=8e-3 for both 400B and 1T; SDPA elementwise converges at max_lr=8e-3. Output clipping (clip [300,100]) fails. Sandwich norm restores convergence but improvement negligible.", + "source": "Section 4.2, 4.5" + }, + { + "id": "gated-attention-llm-D1-014", + "claim": "Switch Heads configurations: switch_kv_8top8 (+38M), switch_kv_4top4 (+13M), switch_v_4top4 (+13M), switch_v_8top2 (+25M), switch_v_2top2 (+13M). Loss spike smoothing coefficient=0.9.", + "source": "Section 4.4, Appendix B" + }, + { + "id": "gated-attention-llm-D1-015", + "claim": "Evaluation benchmarks: hellaswag (commonsense), mmlu (general knowledge), gsm8k (math), humaneval (coding), ceval (Chinese), cmmlu (Chinese multi-task). Total gating variants tested: 30.", + "source": "Section 4.1, 6" + } + ], + "D2": [ + { + "id": "gated-attention-llm-D2-001", + "claim": "QKV Linear Projections (Eq. 1): Q = X W_Q, K = X W_K, V = X W_V. Step 1 of attention forward pass; linearly project input X (shape n x d_model) into queries Q, keys K, and values V (each shape n x d_k) using learned weight matrices W_Q, W_K, W_V (each shape d_model x d_k). Must execute before SDPA.", + "source": "Section 2.1, Eq. 1" + }, + { + "id": "gated-attention-llm-D2-002", + "claim": "Scaled Dot-Product Attention — SDPA (Eq. 2): Attention(Q, K, V) = softmax(Q K^T / sqrt(d_k)) V. Step 2; compute attention scores via scaled dot-product, apply softmax row-wise, then compute weighted sum of V. d_k default = 128.", + "source": "Section 2.1, Eq. 2" + }, + { + "id": "gated-attention-llm-D2-003", + "claim": "Multi-Head Attention Concatenation (Eq. 3): MultiHead(Q, K, V) = Concat(head_1, ..., head_h), where head_i = Attention(Q W_Q^i, K W_K^i, V W_V^i). Step 3; runs h independent SDPA computations in parallel, one per attention head.", + "source": "Section 2.1, Eq. 3" + }, + { + "id": "gated-attention-llm-D2-004", + "claim": "Final Output Projection (Eq. 4): O = MultiHead(Q, K, V) W_O. Step 4; linearly project the concatenated multi-head output (shape n x h*d_k) back to d_model using learned W_O (shape h*d_k x d_model). Final step of standard attention block.", + "source": "Section 2.1, Eq. 4" + }, + { + "id": "gated-attention-llm-D2-005", + "claim": "Core Multiplicative Gating Mechanism (Eq. 5): Y' = g(Y, X, W_θ, σ) = Y ⊙ σ(X W_θ). General multiplicative gating: element-wise multiply target Y by gating scores computed from input X via learned projection W_θ followed by activation σ (default sigmoid, output [0, 1]). By default, head-specific with Y=SDPA output and X=query hidden states.", + "source": "Section 2.2, Eq. 5" + }, + { + "id": "gated-attention-llm-D2-006", + "claim": "Additive Gating Mechanism: Y' = Y + σ(X W_θ). Alternative additive gating using SiLU activation (unbounded output) rather than sigmoid. Applied at same insertion points as multiplicative gating. Empirically underperforms multiplicative gating.", + "source": "Section 2.2 Point (4b)" + }, + { + "id": "gated-attention-llm-D2-007", + "claim": "G_1: SDPA Output Elementwise Gating (Tab. 1 Row 5): S_i = σ(H_i W_gate), H_i_attn_gated = SDPA_Output_i ⊙ S_i. Elementwise sigmoid gating after SDPA output. Gate scores are per-dimension vectors (shape d_k) computed from query hidden state H_i. Score shape: n x q x d_k. Best-performing configuration. +201M params.", + "source": "Section 2.2 (G1), Table 1 Row 5" + }, + { + "id": "gated-attention-llm-D2-008", + "claim": "G_1: SDPA Output Headwise Gating (Tab. 1 Row 10): s_i = σ(W_head^T H_i), H_i_attn_gated = SDPA_Output_i ⊙ s_i. Scalar gating score per head per token (W_head shape d_model x 1), broadcast-multiplied across d_k dimensions. +1.6M params. Nearly matches elementwise performance with far fewer parameters.", + "source": "Section 2.2 Granularity (2a), Table 1 Row 10" + }, + { + "id": "gated-attention-llm-D2-009", + "claim": "G_1: SDPA Output Head-Shared Gating (Tab. 1 Row 12): S_i_shared = σ(H_i W_gate_shared), H_i_attn_gated^k = SDPA_Output_i^k ⊙ S_i_shared. All heads share same gating score vector, generated once per token from H_i. Underperforms head-specific gating; distinct heads need distinct sparsity patterns.", + "source": "Section 2.2 Point (3b), Table 1 Row 12" + }, + { + "id": "gated-attention-llm-D2-010", + "claim": "G_2: Value Output Elementwise Gating (Tab. 1 Row 6): V_j_gated = V_j ⊙ σ(H_j W_gate_V), where V_j = X_j W_V. Each past token j's value vector is modulated by gate scores from its own hidden state H_j. Second-best position. +25M params.", + "source": "Section 2.2 (G2), Table 1 Row 6" + }, + { + "id": "gated-attention-llm-D2-011", + "claim": "G_2: Value Output Headwise Gating (Tab. 1 Row 11): s_j = σ(W_head_V^T H_j), V_j_gated = V_j ⊙ s_j. Scalar gate score per head per token modulates entire value vector. +0.2M params. Analogous to Switch v 1top1.", + "source": "Section 2.2 Granularity (2a) at G2, Table 1 Row 11" + }, + { + "id": "gated-attention-llm-D2-012", + "claim": "G_3: Key Output Gating (Tab. 1 Row 7): K_j_gated = K_j ⊙ σ(H_j W_gate_K), where K_j = X_j W_K. Modulates key vectors before attention score computation. Empirically ineffective — PPL ~6.016 vs 6.026 baseline.", + "source": "Section 2.2 (G3), Table 1 Row 7" + }, + { + "id": "gated-attention-llm-D2-013", + "claim": "G_4: Query Output Gating (Tab. 1 Row 8): Q_i_gated = Q_i ⊙ σ(H_i W_gate_Q), where Q_i = X_i W_Q. Modulates query vectors before attention score computation. Minimal improvement — PPL 5.981.", + "source": "Section 2.2 (G4), Table 1 Row 8" + }, + { + "id": "gated-attention-llm-D2-014", + "claim": "G_5: Dense Output Gating (Tab. 1 Row 9): O_i_gated = O_i ⊙ σ(H_i W_gate_O), where O_i = MultiHead_Output_i W_O. Gate applied after W_O projection. +100M params but no benefit — does not introduce non-linearity between W_V and W_O.", + "source": "Section 2.2 (G5), Table 1 Row 9" + }, + { + "id": "gated-attention-llm-D2-015", + "claim": "G_1: SDPA Output Additive Gating with SiLU (Tab. 1 Row 14): S_i = SiLU(H_i W_gate), H_i_attn_gated = SDPA_Output_i + S_i. Additive gate scores via SiLU (unbounded). Underperforms multiplicative sigmoid (PPL 5.821 vs 5.761).", + "source": "Section 2.2 Point (4a), Table 1 Row 14, Table 3 Row 4" + }, + { + "id": "gated-attention-llm-D2-016", + "claim": "G_1: SDPA Output RMSNorm (Tab. 3 Row 5): head_i_normed = RMSNorm(head_i) for each head i. Apply RMSNorm per attention head output. Near-zero additional params. Significant PPL reduction by introducing non-linearity between W_V and W_O — confirms low-rank bottleneck hypothesis.", + "source": "Section 4.1, Table 3 Row 5" + }, + { + "id": "gated-attention-llm-D2-017", + "claim": "G_1: SDPA Output SiLU Only (Tab. 3 Row 6): H_i_attn_activated = SiLU(SDPA_Output_i). Apply SiLU activation directly to SDPA output without learned parameters. Zero additional params. Modest PPL reduction but benchmarks largely unchanged — learnable input-dependent gating is crucial.", + "source": "Section 4.1, Table 3 Row 6" + }, + { + "id": "gated-attention-llm-D2-018", + "claim": "G_1: SDPA Output Additive Identity Gating (Tab. 3 Row 7): H_i_attn_gated = SDPA_Output_i + (H_i W_gate). Additive gating with identity activation (no non-linearity). Ablation isolating non-linearity effect. Diminishes gains vs SiLU-additive (PPL 5.882 vs 5.821).", + "source": "Section 4.1, Table 3 Row 7" + }, + { + "id": "gated-attention-llm-D2-019", + "claim": "NS-Sigmoid — Non-Sparse Sigmoid (Eq. 9): NS_sigmoid(x) = 0.5 + 0.5 * sigmoid(x) = 0.5 + 0.5 / (1 + exp(-x)). Output range [0.5, 1.0]. Drop-in replacement for standard sigmoid. Preserves non-linearity while removing sparsity. Used only for analysis — confirms sparsity is critical.", + "source": "Section 4.2, Eq. 9, Table 4 Row 7" + } + ], + "D3": [ + { + "id": "gated-attention-llm-D3-001", + "claim": "P1 — SDPA Output Gating (G1): Apply sigmoid gate after SDPA output, before W_O projection (G1 position). Default configuration: head-specific, multiplicative, sigmoid activation. Granularity options: elementwise (n x q x d_k, +201M) or headwise (n x q, +1.6M). Best overall performance: reduces PPL >0.2, gains up to 2 MMLU points, improves training stability, eliminates attention sink (46.7% to 4.8% first-token attention), enables larger LRs and batch sizes.", + "source": "Section 2.2, Section 3.2.1, Fig. 1, Table 1" + }, + { + "id": "gated-attention-llm-D3-002", + "claim": "P2 — Value Projection Gating (G2): Apply gating after W_V projection, before SDPA aggregation. Gating input: past key/value hidden states (not query-dependent). Second-best position; effective PPL reduction but less than G1. Reduces massive activations but does NOT eliminate attention sink.", + "source": "Section 2.2 (G2), Table 1 Row 6" + }, + { + "id": "gated-attention-llm-D3-003", + "claim": "P3 — Key and Query Projection Gating (G3, G4): Apply gating after key projection W_K (G3) or query projection W_Q (G4). Elementwise sigmoid. G3: negligible improvement (PPL 6.016 vs 6.026); G4: minor improvement (PPL 5.981). Gating at Q/K does not address low-rank expressiveness of W_V x W_O.", + "source": "Section 2.2 (G3, G4), Table 1 Rows 7-8" + }, + { + "id": "gated-attention-llm-D3-004", + "claim": "P4 — Final Dense Output Gating (G5): Apply gating after final W_O projection. Score shape n x d_model, +100M params. No significant effect (PPL 6.017 vs 6.026). Does not address lack of non-linearity between W_V and W_O.", + "source": "Section 2.2 (G5), Table 1 Row 9" + }, + { + "id": "gated-attention-llm-D3-005", + "claim": "P5 — Gating Granularity Comparison: Headwise (single scalar per head, <2M params for 15A2B) vs elementwise (per-dimension vectors). Positions: G1 and G2. Elementwise G1 PPL 5.761 vs headwise G1 PPL 5.792. Granularity has minor impact as long as different heads get distinct scores.", + "source": "Section 2.2 Point (2), Table 1 Rows 5, 10, 6, 11" + }, + { + "id": "gated-attention-llm-D3-006", + "claim": "P6 — Head-Specific vs Head-Shared Gating: Compare per-head gating parameters vs shared parameters across heads. Head-shared G1 PPL 5.801 vs head-specific 5.792/5.761; head-shared G2 PPL 5.867 vs head-specific 5.820/5.808. Head-shared gating scores increase in magnitude, losing sparsity — individual heads need head-specific modulation.", + "source": "Section 2.2 Point (3), Table 1 Rows 10 vs 12, 11 vs 13" + }, + { + "id": "gated-attention-llm-D3-007", + "claim": "P7 — Multiplicative vs Additive Gating: Compare Y' = Y x sigma(X theta) vs Y' = Y + sigma(X theta). Position: G1. Multiplicative sigmoid PPL 5.761, MMLU 60.82; additive SiLU PPL 5.821, MMLU 60.06. Multiplicative consistently outperforms additive.", + "source": "Section 2.2 Point (4), Table 1 Rows 5 vs 14" + }, + { + "id": "gated-attention-llm-D3-008", + "claim": "P8 — Activation Function: Sigmoid vs SiLU: Compare sigmoid (output [0,1]) vs SiLU (unbounded) in multiplicative SDPA output gating. Sigmoid (elementwise G1): PPL 5.761, MMLU 60.82; SiLU (elementwise G1): PPL 5.822, MMLU 60.49. Sigmoid consistently outperforms SiLU.", + "source": "Section 2.2 Point (5), Table 1 Rows 5 vs 15" + }, + { + "id": "gated-attention-llm-D3-009", + "claim": "P9 — Non-Linearity Augmentation Ablation: Disentangle non-linearity from learnable parameters. Test RMSNorm per head (PPL 5.847, near-zero params), SiLU-only at G1 (PPL 5.975, zero params), additive gate with identity (PPL 5.882). Conclusion: non-linearity between W_V and W_O is key mechanism, but learnable input-dependent gating adds critical value.", + "source": "Section 4.1, Table 3 Rows 1, 5-7" + }, + { + "id": "gated-attention-llm-D3-010", + "claim": "P10 — Sparsity Analysis via NS-Sigmoid: Test non-linearity without sparsity using NS-sigmoid (output [0.5, 1.0]) at G1. Gains inferior to standard sigmoid; NS-sigmoid intensifies massive activations and attention sink. Sparsity (not just non-linearity) is critical for performance.", + "source": "Section 4.2, Eq. 9, Tab. 4 Row 7" + }, + { + "id": "gated-attention-llm-D3-011", + "claim": "P11 — Input-Independent Gating: Validate query-dependent gating importance. Zero-initialize learnable params of shape q x d_k, apply sigmoid, multiply with SDPA output. Gating scores independent of input X. Improves over baseline (non-linearity alone) but scores are high (>0.5), does not reduce attention sink. Effective sparsity must be input-dependent.", + "source": "Section 4.2, Tab. 4 Row 6" + }, + { + "id": "gated-attention-llm-D3-012", + "claim": "P12 — Training Stability Under Increased LRs: Test SDPA gating stability at high LRs where baseline diverges. Models: 48L/28L dense 1.7B. LRs tested: [0.004, 0.0045, 0.0053, 0.008]; batch sizes: [1024, 2048, 4096]; token regimes: [400B, 1T, 3.5T]. Baseline diverges at 8e-3; sandwich norm restores convergence but negligible improvement; SDPA gating stable at 8e-3 with PPL 7.325 and better benchmarks.", + "source": "Section 3.2.2, Table 2" + }, + { + "id": "gated-attention-llm-D3-013", + "claim": "P13 — Clipping for Training Stability: Test output clipping as gating replacement. Constrain attention/FFN outputs to (-clip, +clip) before residual. Clip values [300, 100] at LR=8e-3. Model still diverges for both values — instability is not solely due to large activations in residuals.", + "source": "Appendix A.5" + }, + { + "id": "gated-attention-llm-D3-014", + "claim": "P14 — Long-Context Extension Protocol: Step 1: increase RoPE base from 10k to 1M. Step 2: continue training on 32K sequences for 80B tokens. Step 3: YaRN extends from 32K to 128K. Evaluate RULER at [4K-128K]. Within training: gated slightly better. YaRN extended: baseline drops >40pts at 32K, gated drops <7pts. At 128K: gated 58.82 vs baseline 31.65.", + "source": "Section 4.4, Table 5" + }, + { + "id": "gated-attention-llm-D3-015", + "claim": "P15 — Switch Heads Baseline Comparison: Disentangle gating from expert routing. Switch Heads variants: kv 8top8 (+38M), kv 4top4 (+13M), v 4top4 (+13M), v 8top2 (+25M), v 1top1 (+3M). Switch v 1top1 (equivalent to v headwise G2) PPL 5.808. Substantial gains persist even with single expert — gating provides significant intrinsic value beyond routing.", + "source": "Appendix A.1, Table 6" + }, + { + "id": "gated-attention-llm-D3-016", + "claim": "P16 — Attention Sink and Massive Activation Analysis: Analyze first-token attention proportion, max hidden state activations, gating score distributions across layers. Baseline: avg 46.7% first-token attention, massive activations at layer 6 FFN. SDPA G1: eliminates sink (4.8%), reduces massive activations. v G2: reduces activations but sink persists. Head-shared/NS-sigmoid: both sink and activations return to baseline. Input-dependent, head-specific sparsity required.", + "source": "Section 4.3, Figs. 2-5, Table 4, Appendix A.3" + }, + { + "id": "gated-attention-llm-D3-017", + "claim": "P17 — Parameter-Expansion Baseline Comparison: Compare gating against capacity-matched baselines without gating. Methods: k=8 (+50M) PPL 5.979; q=48 (+201M) PPL 5.953; add 4 experts (+400M) PPL 5.964. SDPA G1 (+201M) PPL 5.761 significantly outperforms all; v headwise G2 (+0.2M) PPL 5.808 near-best with negligible cost. Gains are NOT explained by added parameters alone.", + "source": "Section 3.2.1, Table 1 Rows 1-4 vs 5-15" + } + ], + "D4": [ + { + "id": "gated-attention-llm-D4-001", + "claim": "Standard Attention Block Forward Pass: Step 1 — QKV Linear Projections (Q=X W_Q, K=X W_K, V=X W_V). Step 2 — Scaled Dot-Product Attention (softmax(QK^T/sqrt(d_k)) V). Step 3 — Multi-Head Concatenation (Concat(head_1..head_h)). Step 4 — Final Output Projection (MultiHead x W_O). This is the baseline 4-stage pipeline before any gating insertion.", + "source": "Section 2.1, Eqs. 1-4" + }, + { + "id": "gated-attention-llm-D4-002", + "claim": "Gated Attention Forward Pass (G1 insertion — best configuration): Step 1 — QKV Linear Projections. Step 2 — Scaled Dot-Product Attention. Step 3 (INSERT G1) — Apply sigmoid gating at SDPA output: S_i = σ(H_i W_gate), H_i_attn_gated = SDPA_Output_i ⊙ S_i (elementwise) or s_i = σ(W_head^T H_i) (headwise). Step 4 — Multi-Head Concatenation. Step 5 — Final Output Projection W_O.", + "source": "Section 2.2, Fig. 1, Table 1 Rows 5, 10, 12, 14, 15" + }, + { + "id": "gated-attention-llm-D4-003", + "claim": "Gated Attention Forward Pass (G2 insertion — second-best): Step 1 — QKV Linear Projections. Step 2 (INSERT G2) — Apply sigmoid gating at value output: V_j_gated = V_j ⊙ σ(H_j W_gate_V). Step 3 — Scaled Dot-Product Attention (using gated V_j_gated). Step 4 — Multi-Head Concatenation. Step 5 — Final Output Projection W_O.", + "source": "Section 2.2 (G2), Fig. 1, Table 1 Rows 6, 11" + }, + { + "id": "gated-attention-llm-D4-004", + "claim": "Gating Design Space Exploration Pipeline (Section 2.2): Step 1 — Select insertion position (G1=after SDPA, G2=after W_V, G3=after W_K, G4=after W_Q, G5=after W_O). Step 2 — Select granularity (headwise: scalar per head; elementwise: per-dimension vector). Step 3 — Select head-specificity (head-specific: per-head W_θ; head-shared: shared W_θ across heads). Step 4 — Select mode (multiplicative: Y ⊙ σ(Xθ); additive: Y + σ(Xθ)). Step 5 — Select activation (sigmoid for multiplicative; SiLU for additive). Step 6 — Apply Y' = g(Y, X, W_θ, σ).", + "source": "Section 2.2 Points (1)-(5), Eqs. 5-6" + }, + { + "id": "gated-attention-llm-D4-005", + "claim": "Non-linearity vs Sparsity Ablation Pipeline (Sections 4.1-4.2): Phase 1 — Baseline (no gating, PPL 6.026). Phase 2 — Add only non-linearity (RMSNorm per head, SiLU-only at G1, additive gate with identity). Phase 3 — Add non-linearity without sparsity (NS-sigmoid gating, output [0.5, 1.0]). Phase 4 — Full SDPA gating (sigmoid, output [0, 1], both non-linearity and sparsity). Conclusion: non-linearity between W_V and W_O explains part of gain; sparsity (scores near 0) is additionally critical.", + "source": "Section 4.1, Section 4.2, Tables 3-4" + }, + { + "id": "gated-attention-llm-D4-006", + "claim": "Training Stability Protocol (Section 3, P12-P13): Phase 1 — Train baseline at standard LR (4e-3) — converges. Phase 2 — Increase LR to 8e-3 — baseline diverges or degrades heavily (PPL 9.195). Phase 3 — Apply output clipping (-clip to +clip, clip=300 or 100) — still diverges. Phase 4 — Apply sandwich norm (LayerNorm on FFN output) — restores convergence but gains negligible (PPL 7.407). Phase 5 — Apply SDPA gating — stable training at 8e-3 with PPL 7.325, outperforming gating at 4e-3 (PPL 7.288). Gating enables stable training at higher LRs where baseline fails.", + "source": "Section 3.2.2, Table 2, Appendix A.5" + }, + { + "id": "gated-attention-llm-D4-007", + "claim": "Long-Context Extension Protocol (Section 4.3, P14): Phase 1 — Pre-train 1.7B dense model (baseline and SDPA-gated) on 4K context, 3.5T tokens. Phase 2 — Increase RoPE base frequency from 10,000 to 1,000,000. Phase 3 — Continue training on 32K sequence length for 80B additional tokens. Phase 4 — Apply YaRN to extend context from 32K to 128K (training-free). Phase 5 — Evaluate RULER at [4K, 8K, 16K, 32K, 64K, 128K]. Result: gated models retain performance under RoPE modification; attention-sink-free pattern is key to generalization.", + "source": "Section 4.4, Table 5" + }, + { + "id": "gated-attention-llm-D4-008", + "claim": "Switch Heads Reduction Protocol (Section 4.4, P15): Phase 1 — Apply Switch Heads framework (top-K sigmoid gating across attention head experts). Phase 2 — Test multiple configurations: kv 8top8 → kv 4top4 → v 4top4 → v 8top2 → v 1top1. Phase 3 — Reduce to v 1top1 (single expert, pure gating, no routing, equivalent to v headwise G2). Phase 4 — Observe that performance gains persist (PPL 5.808, MMLU 59.32) even with single expert. Conclusion: gating provides significant intrinsic value beyond expert routing mechanism.", + "source": "Appendix A.1, Table 6" + }, + { + "id": "gated-attention-llm-D4-009", + "claim": "Parameter-Expansion Fairness Protocol (P17): Phase 1 — Establish baseline (MoE-15A2B, q=32, kv=4, PPL 6.026). Phase 2 — Add parameters through conventional expansion (k=8: +50M; q=48: +201M; add 4 experts: +400M). Phase 3 — Add parameters through gating (SDPA elementwise G1: +201M; v headwise G2: +0.2M). Phase 4 — Compare: gating with same/less parameter cost outperforms all expansion baselines. v headwise G2 achieves PPL 5.808 with +0.2M vs k=8 at PPL 5.979 with +50M.", + "source": "Section 3.2.1, Table 1 Rows 1-4 vs 5-15" + }, + { + "id": "gated-attention-llm-D4-010", + "claim": "Attention Sink Diagnostic Protocol (P16): Phase 1 — Measure baseline first-token attention proportion per layer (avg 46.7%). Phase 2 — Identify massive activation origin layer (layer 5 FFN) and attention sink emergence layer (layer 6). Phase 3 — Apply SDPA G1 gating — measure elimination (4.8%). Phase 4 — Apply v G2 gating — massive activations reduced but sink persists. Phase 5 — Apply NS-sigmoid or head-shared gating — both activations and sink return to baseline. Phase 6 — Apply input-independent gating — high scores (>0.5), no sink reduction. Conclusion: input-dependent, head-specific sparsity (not just non-linearity) is required to eliminate attention sink.", + "source": "Section 4.3, Figs. 2-5, Table 4, Appendix A.3" + } + ] +} \ No newline at end of file diff --git a/papers/generator-augmented-flows/blacklist.txt b/papers/generator-augmented-flows/blacklist.txt new file mode 100644 index 0000000000000000000000000000000000000000..063fb72173ac63e84362eb986f82fe17d7384625 --- /dev/null +++ b/papers/generator-augmented-flows/blacklist.txt @@ -0,0 +1,2 @@ +# Official repository +https://github.com/thibautissenhuth/consistency_GC diff --git a/papers/generator-augmented-flows/config.yaml b/papers/generator-augmented-flows/config.yaml new file mode 100644 index 0000000000000000000000000000000000000000..2aaa9c9aaf621f7004c3c7bc6f25de80c3aea645 --- /dev/null +++ b/papers/generator-augmented-flows/config.yaml @@ -0,0 +1,8 @@ +title: "Improving Consistency Models with Generator-Augmented Flows" +pdf_url: 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b/papers/generator-augmented-flows/paper.md new file mode 100644 index 0000000000000000000000000000000000000000..b2eaddd6375b0e1978ce104420ed1d3279d8d29a --- /dev/null +++ b/papers/generator-augmented-flows/paper.md @@ -0,0 +1,858 @@ +# Improving Consistency Models with Generator-Augmented Flows + +Thibaut Issenhuth 1 Sangchul Lee 2 Ludovic Dos Santos 1 Jean-Yves Franceschi 1 Chansoo Kim 2 3 Alain Rakotomamonjy 1 4 + +# Abstract + +Consistency models imitate the multi-step sampling of score-based diffusion in a single forward pass of a neural network. They can be learned in two ways: consistency distillation and consistency training. The former relies on the true velocity field of the corresponding differential equation, approximated by a pre-trained neural network. In contrast, the latter uses a single-sample Monte Carlo estimate of this velocity field. The related estimation error induces a discrepancy between consistency distillation and training that, we show, still holds in the continuous-time limit. To alleviate this issue, we propose a novel flow that transports noisy data towards their corresponding outputs derived from a consistency model. We prove that this flow reduces the previously identified discrepancy and the noise-data transport cost. Consequently, our method not only accelerates consistency training convergence but also enhances its overall performance. The code is available at: github.com/thibautissenhuth/consistency GC. + +proach is consistency models (Song et al., 2023; Song and Dhariwal, 2024). Consistency models lead to high-quality one-step generators, that can be trained either by distillation of a pre-trained velocity field (consistency distillation), or as standalone generative models (consistency training) by approximating the velocity field through a one-sample Monte Carlo estimate. + +The corresponding estimation error naturally induces a discrepancy between consistency distillation and training. While Song et al. (2023) hinted that it would resolve in the continuous-time limit, we show that this discrepancy persists in both the gradients and values of the loss functions. Interestingly, this discrepancy vanishes when the difference between the target velocity field and its Monte-Carlo approximation approaches zero. However, this is not the case with the independent coupling (IC) between data and noise used to construct the standard estimate. It is unclear how to improve this one-sample estimate without access to the true underlying diffusion model. + +# 1. Introduction + +A large family of diffusion (Ho et al., 2020), score-based (Song et al., 2021; Karras et al., 2022), and flow models (Liu et al., 2023; Lipman et al., 2023) have emerged as stateof-the-art generative models for image generation. Since they are costly to use at inference time – requiring several neural function evaluations –, many distillation techniques have been explored (Salimans and Ho, 2022; Meng et al., 2023; Sauer et al., 2023). One of the most remarkable ap- + +The approach we adopt in this paper to alleviate this issue involves altering the velocity field – thereby changing the target flow – to reduce the variance of its one-sample estimator. One possible solution to this problem is to resort to optimal transport (OT) to learn on a deterministic coupling. OT has been succesfully adopted in diffusion (Li et al., 2024), consistency (Dou et al., 2024), and flow matching (Pooladian et al., 2023) models. However, due to the prohibitive cubic complexity of OT solvers (e.g. Hungarian matching algorithm), such methods need to be applied at the minibatch level. This incurs an OT approximation error (Fatras et al., 2021; Sommerfeld et al., 2019) and stochasticity of the data-noise coupling, thus not solving the consistency training issue. + +In our approach, we propose to use the consistency model, assumed to be an approximation of the target diffusion flow, to construct additional trajectories. The consistency model serves as a proxy to reduce the expected deviation between the velocity field and its estimator. More precisely, from an intermediate point computed from an IC, we let the consistency model predict the corresponding endpoint, supposedly close to the data distribution. This predicted endpoint is coupled to the same original noise vector, defining a generatoraugmented coupling (GC). We show empirically that the resulting generator-augmented flow presents compelling properties for training consistency models, in particular a reduced deviation between the velocity field and its estimator, and decreased transport costs – as supported by theoretical and empirical evidence. This can be observed in Figure 1. From this, we derive practical algorithms to train consistency models with generator-augmented flows, leading to improved performance and faster convergence compared to standard and OT-based consistency models. + +![](images/figures/generator-augmented-flows-fig-0001.jpg) +Figure 1. Comparison of the probability flow ODE (PF-ODE) and generator-augmented flows (GC): target data is a mixture of two Dirac delta functions, and GC is computed with a closed-form generator. In the background, we observe the density of probability paths. White arrows are ODE trajectories associated to the velocity field. Blue lines are sample paths from IC in (a) and from GC in (b). Trajectories start from random intermediate points $\star$ . On this example, GC sample paths appear more aligned to the velocity field. + +Let us summarize our contributions below. + +• We prove that in the continuous-time limit consistency training and consistency distillation loss function converge to different values and we provide a closed-form expression of this discrepancy. +• We propose a novel type of flows that we denote generator-augmented flows. It relies on generatoraugmented coupling (GC) that can be used to train a consistency model. +• We provide theoretical and empirical insights into the advantages of GC. We show that generator-augmented flows have smaller discrepancy to consistency distillation than IC consistency training, and that they reduce data-noise transport costs. +• We derive practical ways to train consistency models with GC. Our approach based on a joint learning strategy leads to faster convergence and improves the performance compared to the base model and OT-based approaches on image generation benchmarks. + +Notation. We consider an empirical data distribution $p _ { \star }$ and a noise distribution $p _ { z }$ (e.g. Gaussian), both defined on $\mathbb { R } ^ { d }$ . We denote by $q$ a joint distribution of samples from $p _ { \star }$ and $p _ { z }$ . We equip $\mathbb { R } ^ { d }$ with the dot product $\langle \mathbf { x } , \mathbf { y } \rangle = \mathbf { x } ^ { \top } \mathbf { y }$ and write $\lVert \mathbf { x } \rVert = \langle \mathbf { x } , \mathbf { x } \rangle ^ { 1 / 2 }$ for the Euclidean norm of $\mathbf { x }$ . We use a distance function $\mathcal { D } \colon { \mathbb { R } ^ { d } } \times { \mathbb { R } ^ { d } } \to [ 0 , \infty )$ to measure the distance between two points from $\mathbb { R } ^ { d }$ . sg denotes the stop-gradient operator. + +In consistency models, we consider diffusion processes of the form $\mathbf { x } _ { t } = \mathbf { x } _ { \star } + \sigma _ { t } \mathbf { z }$ , where $\mathbf { x } _ { \star } \sim p _ { \star }$ , $\mathbf { z } \sim p _ { z }$ , and $\sigma _ { t }$ is monotonically increasing for $t \in [ 0 , T ]$ , where $T \in \mathbb { R } _ { + }$ We denote the distribution of $\mathbf { x } _ { t }$ by $p ( \mathbf { x } _ { t } )$ , or simply $p _ { t }$ Conditional distributions or finite-dimensional joint distributions of $\mathbf { x } _ { t }$ ’s are denoted similarly. When considering a discrete formulation with $N$ intermediate timesteps, we denote the intermediate points as $\mathbf { x } _ { t _ { i } } = \mathbf { x } _ { \star } + \sigma _ { t _ { i } } \mathbf { z }$ , where $t _ { i }$ is strictly increasing for $i \in \{ 0 , \ldots , N \}$ , with $t _ { 0 } = 0$ and $t _ { N } = T$ . The values of $\sigma _ { 0 }$ and $\sigma _ { T }$ are chosen to be sufficiently small and large, respectively, so that $p _ { 0 } \approx p _ { \star }$ and $p _ { T } \approx \mathcal { N } ( 0 , \sigma _ { T } ^ { 2 } \mathbf { I } )$ . + +# 2. Consistency Distillation Versus Training + +In this section, we provide the required background on diffusion and consistency models (Sections 2.1 and 2.2), then discuss the discrepancy between consistency distillation and consistency training (Section 2.3) which we theoretically characterize in continuous-time. + +# 2.1. Flow and Score-Based Diffusion Models + +Score-based diffusion models (Ho et al., 2020; Song et al., 2021) can generate data from noise via a multi-step process consisting in numerically solving either a stochastic differential equation (SDE), or equivalently an ordinary differential equation (ODE). Although SDE solvers generally exhibit superior sampling quality, ODEs have desirable properties. Most notably, they define a deterministic mapping from noise to data. Recently, Liu et al. (2023) and Lipman et al. (2023) generalize diffusion to flow models, which are defined by the following probability flow ODE (PF-ODE): + +$$ +\mathrm { d } \mathbf { x } = \mathbf { v } _ { t } ( \mathbf { x } ) \mathrm { d } t , +$$ + +where $\mathbf v _ { t } ( \mathbf x ) = \mathbb { E } [ \dot { \mathbf { x } } _ { t } | \mathbf x _ { t } = \mathbf x ]$ is the velocity field. Note that $\dot { \mathbf { x } } _ { t }$ is defined as the random variable $\begin{array} { r } { \dot { \mathbf { x } } _ { t } = \frac { \mathrm { d } ( \mathbf { x } _ { \star } + \sigma _ { t } \mathbf { z } ) } { \mathrm { d } t } = \dot { \sigma } _ { t } \mathbf { z } , } \end{array}$ and is not to be confused with the time-derivative of the ODE, $\mathbf { v } _ { t }$ . + +In the context of consistency models (Song et al., 2023; Song and Dhariwal, 2024), the most common choice is $\mathbf { v } _ { t } ( \mathbf { x } ) \ = \ - \dot { \sigma } _ { t } \sigma _ { t } \nabla _ { \mathbf { x } } \log p _ { t } ( \mathbf { x } ) \mathrm { d } t$ , in particular the EDM formulation (Karras et al., 2022) where $\sigma _ { t } = t$ and thus $\mathbf { v } _ { t } ( \mathbf { x } ) \ = \ - t \nabla _ { \mathbf { x } } \log p _ { t } ( \mathbf { x } )$ . Here, $\nabla _ { \mathbf { x } } \log { p _ { t } }$ , a.k.a. the score function, can be approximated with a neural network ${ \bf s } _ { \phi } ( { \bf x } , t )$ (Vincent, 2011; Song and Ermon, 2019). + +# 2.2. Consistency Models + +Numerically solving an ODE is costly because it requires multiple expensive evaluations of the velocity function. To alleviate this issue, Song et al. (2023) propose training a consistency model $f _ { \theta }$ , which learns the output map of the PF-ODE, i.e. its flow, such that: + +$$ +f _ { \theta } ( \mathbf { x } _ { t } , \sigma _ { t } ) = \mathbf { x } _ { 0 } , +$$ + +for all $( \mathbf { x } _ { t } , \sigma _ { t } ) \in \mathbb { R } ^ { d } \times [ \sigma _ { 0 } , \sigma _ { T } ]$ that belong to the trajectory of the PF-ODE ending at $\left( \mathbf { x } _ { 0 } , \sigma _ { 0 } \right)$ . + +Equation (2) is equivalent to $( i )$ enforcing the boundary condition $f _ { \theta } ( \mathbf { x } _ { 0 } , \sigma _ { 0 } ) = \mathbf { x } _ { 0 }$ , and $( i i )$ ensuring that $f _ { \theta }$ has the same output for any two samples of a single PF-ODE trajectory – the consistency property. $( i )$ is naturally satisfied by the following model parametrization: + +$$ +\begin{array} { r } { \pmb { f } _ { \theta } ( \mathbf { x } _ { t _ { i } } , \sigma _ { t _ { i } } ) = c _ { \mathrm { s k i p } } ( \sigma _ { t _ { i } } ) \mathbf { x } _ { t _ { i } } + c _ { \mathrm { o u t } } ( \sigma _ { t _ { i } } ) \pmb { F } _ { \theta } ( \mathbf { x } _ { t _ { i } } , \sigma _ { t _ { i } } ) , } \end{array} +$$ + +where $\begin{array} { r } { c _ { \mathrm { s k i p } } ( \sigma ) = \frac { \sigma _ { d } ^ { 2 } } { \sigma _ { d } ^ { 2 } + ( \sigma - \sigma _ { 0 } ) ^ { 2 } } } \end{array}$ σ dσ2+(σ−σ0)2 , cout(σ) = σ√d·(σ−σ0)σ2+σ2 , $\sigma _ { d } ^ { 2 }$ the variance of data, and $F _ { \theta }$ is a neural network. This ensures $c _ { \mathrm { s k i p } } ( 0 ) = 1$ , $c _ { \mathrm { o u t } } ( 0 ) = 0$ . (ii) is achieved by minimizing the distance between the outputs of two same-trajectory consecutive samples using the consistency loss: + +$$ +\begin{array} { r l } & { \mathcal { L } _ { \mathrm { C D } } ( \theta ) = \mathbb { E } _ { q _ { \mathrm { I } } ( \mathbf { x } _ { \star } , \mathbf { z } ) , p ( \mathbf { x } _ { t _ { i + 1 } } \mid \mathbf { x } _ { \star } , \mathbf { z } ) } } \\ & { \quad \Big [ \lambda ( \sigma _ { t _ { i } } ) \mathcal { D } \Big ( \sec ( f _ { \theta } ( \mathbf { x } _ { t _ { i } } ^ { \Phi } , \sigma _ { t _ { i } } ) \Big ) , f _ { \theta } ( \mathbf { x } _ { t _ { i + 1 } } , \sigma _ { t _ { i + 1 } } ) \Big ) \Big ] , } \end{array} +$$ + +where $( \mathbf { x } _ { \star } , \mathbf { z } )$ is sampled from the independent coupling $q _ { \mathrm { I } } ( \mathbf { x } _ { \star } , \mathbf { z } ) = p _ { \star } ( \mathbf { x } _ { \star } ) p _ { z } ( \mathbf { z } )$ , $i$ is an index sampled uniformly at random from $\{ 0 , 1 , \ldots , N - 1 \}$ , $\mathbf { x } _ { t _ { i + 1 } } = \mathbf { x } _ { \star } + \sigma _ { t _ { i + 1 } } \mathbf { z }$ and $\mathbf { x } _ { t _ { i } } ^ { \Phi }$ is computed by discretizing the PF-ODE with the Euler scheme as follows: + +$$ +\mathbf { x } _ { t _ { i } } ^ { \Phi } = \Phi ( \mathbf { x } _ { t _ { i + 1 } } , t _ { i + 1 } ) = \mathbf { x } _ { t _ { i + 1 } } + ( t _ { i } - t _ { i + 1 } ) \mathbf { v } _ { t _ { i + 1 } } ( \mathbf { x } _ { t _ { i + 1 } } ) . +$$ + +This loss can be used to distill a score model into $f _ { \theta }$ . + +In the case of consistency training, Song et al. (2023) circumvent the lack of a score function by noting that $\mathbf { v } _ { t _ { i + 1 } } ( \mathbf { x } ) = \mathbb { E } [ \dot { \mathbf { x } } _ { t _ { i + 1 } } | \mathbf { x } _ { t _ { i + 1 } } = \mathbf { x } ]$ . In light of this, its singlesample Monte Carlo estimate $\dot { \mathbf { x } } _ { t _ { i + 1 } }$ is used instead in Equation (5) to replace the intractable $\dot { \mathbf { x } } _ { t _ { i } } ^ { \Phi }$ by $\mathbf { x } _ { t _ { i } } = \mathbf { x } _ { \star } + \sigma _ { t _ { i } } \mathbf { z }$ in the consistency loss: + +$$ +\begin{array} { r l } & { \mathcal { L } _ { \mathrm { C T } } ( \theta ) = \mathbb { E } _ { q _ { \mathrm { I } } ( \mathbf { x } _ { \star } , \mathbf { z } ) , p ( \mathbf { x } _ { t _ { i } } , \mathbf { x } _ { t _ { i + 1 } } \mid \mathbf { x } _ { \star } , \mathbf { z } ) } } \\ & { \quad \Big [ \lambda ( \sigma _ { t _ { i } } ) \mathcal { D } \Big ( \mathrm { s g } \big ( f _ { \theta } ( \mathbf { x } _ { t _ { i } } , \sigma _ { t _ { i } } ) \big ) , f _ { \theta } ( \mathbf { x } _ { t _ { i + 1 } } , \sigma _ { t _ { i + 1 } } ) \Big ) \Big ] . } \end{array} +$$ + +# 2.3. Discrepancy Between Consistency Training and Distillation and Velocity Field Estimation + +Naturally, replacing $\mathbf { v } _ { t }$ by its single-sample estimate $\dot { \bf x } _ { t }$ makes consistency training deviate from consistency distillation in discrete time. Still, Song et al. (2023, Theorems 2 and 6) suggest that this discrepancy disappears in continuous-time since $\begin{array} { r } { \mathcal { L } _ { \mathrm { C T } } ( \theta ) = \mathcal { L } _ { \mathrm { C D } } ( \theta ) + o ( 1 / N ) } \end{array}$ and the corresponding gradients are equal in some cases. This equality is then used in work of Lu and Song (2024), concurrent to ours, to train continuous-time consistency models at the cost of an elaborate architectural design. Without disproving these results, we find that scaling issues and lack of generality soften the claim of a closed gap between consistency training and distillation. + +Indeed, we provide in the following theorem a thorough theoretical comparison of $\mathcal { L } _ { \mathrm { C T } }$ and $\mathcal { L } _ { \mathrm { C D } }$ . We first prove that they converge to different values in the continuous-time limit. The difference is captured by a regularization term that depends on the discrepancy between the velocity field and its estimate. Moreover, we show that the limits of the scaled gradients do not coincide in the general case, except when the (asymptotic) quadratic loss is used. The proof, and further discussion on why this discrepancy did not appear in Song et al. (2023), can be found in Appendix A.1. + +Theorem 1 (Discrepancy between consistency distillation and consistency training objectives). Assume that the distance function is given by $\mathcal { D } ( \mathbf { x } , \mathbf { y } ) = \varphi ( \| \mathbf { x } - \mathbf { y } \| )$ for a continuous convex function $\varphi : [ 0 , \infty ) [ 0 , \infty )$ with $\varphi ( x ) \sim C x ^ { \alpha }$ as $x \to 0 ^ { + }$ for some $C > 0$ and $\alpha \geq 1$ , and that the timesteps are equally spaced, i.e., $\begin{array} { r } { t _ { i } ~ = ~ \frac { i T } { N } } \end{array}$ . Furthermore, assume that the Jacobian ∂fθ does not vanish identically. Then the following assertions hold: + +$( i )$ The scaled consistency losses $N ^ { \alpha } { \mathcal { L } } _ { \mathrm { C D } } ( \theta )$ and $N ^ { \alpha } { \mathcal { L } } _ { \mathrm { C T } } ( \theta )$ converge as $N \infty$ . Moreover, the minimization objectives corresponding to these limiting scaled consistency losses are not equivalent, and their difference is given by: + +$$ +\operatorname* { l i m } _ { N \to \infty } N ^ { \alpha } \left[ { \mathcal { L } } _ { \mathrm { C T } } ( \theta ) - { \mathcal { L } } _ { \mathrm { C D } } ( \theta ) \right] = C T ^ { \alpha - 1 } { \mathcal { R } } ( \theta ) , +$$ + +where ${ \mathcal { R } } ( \theta )$ is defined by + +$$ +\mathcal { R } ( \theta ) = \int _ { 0 } ^ { T } \lambda ( \sigma _ { t } ) \mathbb { E } \left[ \left\| \partial _ { \mathrm { C T } } \pmb { f } _ { \theta } \right\| ^ { \alpha } - \left\| \partial _ { \mathrm { C D } } \pmb { f } _ { \theta } \right\| ^ { \alpha } \right] \mathrm { d } t +$$ + +and satisfies $\mathcal { R } ( \theta ) > 0$ , with + +$$ +\partial _ { \mathrm { C T } } \mathbf { \mathcal { f } } _ { \theta } = \frac { \partial \mathbf { f } _ { \theta } } { \partial \sigma } ( \mathbf { x } _ { t } , \sigma _ { t } ) \boldsymbol { \dot { \sigma } } _ { t } + \frac { \partial \mathbf { f } _ { \theta } } { \partial \mathbf { x } } ( \mathbf { x } _ { t } , \sigma _ { t } ) \cdot \dot { \mathbf { x } } _ { t } , +$$ + +$$ +\partial _ { \mathrm { C D } } \mathbf { f } _ { \theta } = \frac { \partial f _ { \theta } } { \partial \sigma } ( \mathbf { x } _ { t } , \sigma _ { t } ) \boldsymbol { \dot { \sigma } } _ { t } + \frac { \partial f _ { \theta } } { \partial \mathbf { x } } ( \mathbf { x } _ { t } , \sigma _ { t } ) \cdot \mathbf { v } _ { t } ( \mathbf { x } _ { t } ) . +$$ + +In particular, if $\alpha = 2$ , + +$$ +\mathcal { R } ( \theta ) = \int _ { 0 } ^ { T } \lambda ( \sigma _ { t } ) \mathbb { E } \bigg [ \bigg \| \frac { \partial f _ { \theta } } { \partial \mathbf { x } } ( \mathbf { x } _ { t } , \sigma _ { t } ) \left( \dot { \mathbf { x } } _ { t } - \mathbf { v } _ { t } ( \mathbf { x } _ { t } ) \right) \bigg \| ^ { 2 } \bigg ] \mathrm { d } t . +$$ + +(ii) The scaled gradient $N ^ { \alpha - 1 } \nabla _ { \theta } \mathcal { L } _ { \mathrm { C D } } ( \theta )$ and $N ^ { \alpha - 1 } \nabla _ { \theta } \mathcal { L } _ { \mathrm { C T } } ( \theta )$ converge as $N ~ ~ \infty$ . Moreover, i ${ } ^ { c } \alpha \neq 2$ , then their respective limits are not identical as functions of $\theta$ : + +$$ +\operatorname* { l i m } _ { N \to \infty } N ^ { \alpha - 1 } \nabla _ { \theta } \mathcal { L } _ { \mathrm { C T } } ( \theta ) \neq \operatorname* { l i m } _ { N \to \infty } N ^ { \alpha - 1 } \nabla _ { \theta } \mathcal { L } _ { \mathrm { C D } } ( \theta ) . +$$ + +This theorem reveals that the optimization problems of consistency training and distillation differ not only in discrete time but also in continuous-time. It even highlights a discrepancy between, firstly, the limiting gradients in continuous-time – although they are equal for $\alpha = 2 -$ and, secondly, the gradients of the limiting losses, which differ because of ${ \mathcal { R } } ( \theta )$ , even when $\alpha = 2$ . + +This analysis shows the importance of employing probability paths whose sample path derivatives $\dot { \mathbf { x } } _ { t }$ are aligned with the velocity field ${ \bf v } _ { t } ( { \bf x } _ { t } )$ . In particular, if a diffusion process $\mathbf { x } _ { t }$ satisfies $\dot { \mathbf { x } } _ { t } = \mathbf { v } _ { t } ( \mathbf { x } _ { t } )$ , we have $\mathcal { R } ( \theta ) = 0$ and equal gradients for all $\alpha \geq 1$ . Hence, for such $\mathbf { x } _ { t }$ , consistency training and consistency distillation would be reconciled both in discrete time and in the continuous-time limit. + +However, it is unclear how to directly improve the singlesample estimation $\dot { \bf x } _ { t }$ of ${ \bf v } _ { t } ( { \bf x } _ { t } )$ . In particular, increasing the number of samples per point $\mathbf { x } _ { t }$ to reduce its variance is not tractable, as it requires sampling from the inverse diffusion process $p ( \mathbf { x } _ { \star } | \mathbf { x } _ { t } )$ . Therefore, we adopt an alternative approach to alleviate the discrepancy identified in this section, which involves altering the velocity field – thereby changing the target flow – to reduce the variance of its one-sample estimator. This approach is reminiscent of recent work tackling the data-noise coupling that we discuss in the following section. + +# 3. Reducing the Discrepancy with Data-Noise Coupling + +Beyond independent coupling (IC). From Section 2.2, it appears that $\dot { \mathbf { x } } _ { t }$ is computed through an IC $: q _ { \mathrm { I } } = p _ { \star } ( \mathbf { x } _ { \star } ) p _ { z } ( \mathbf { z } )$ of data and noise, in a similar fashion to flow matching (Lipman et al., 2023; Kingma and Gao, 2024). Making correlated choices of data and noise beyond IC could then help align $\dot { \bf x } _ { t }$ and ${ \bf v } _ { t } ( { \bf x } _ { t } )$ , thereby resolving the discrepancy from the previous section. + +The reliance on IC in consistency and flow models is increasingly recognized as a limiting factor. Recent advancements suggest that improved coupling mechanisms could enhance both training efficiency and the quality of generated samples in flow matching (Liu et al., 2023; Pooladian et al., 2023) and diffusion models (Li et al., 2024). By reducing the variance in gradient estimation, enhanced coupling can accelerate training. Additionally, improved coupling could decrease transport costs and straighten trajectories, yielding better-quality samples. In a different context, ReFlow (Liu et al., 2023) leverages couplings provided by the ODE solver in a flow framework, and demonstrates that it reduced transport costs. Moreover, Lee et al. (2023) propose to learn an encoder from data to noise, and use this encoder as a way to construct a coupling when training a flow model. + +# Couplings based on optimal transport (OT) solvers. + +OT is a particularly appealing solution for our alignment problem. Indeed, if we consider a quadratic cost and distributions with bounded supports, OT is a no-collision transport map (Nurbekyan et al., 2020), i.e. $\mathbf { x } _ { t }$ can be sampled by a unique pair of points $( \mathbf { x } _ { \star } , \mathbf { z } )$ . Thus $\dot { \mathbf { x } } _ { t } = \mathbf { v } _ { t } ( \mathbf { x } _ { t } )$ , implying $\mathcal { R } ( \theta ) = 0$ in Theorem 1. Several approaches have precisely targeted the reduction of transport cost in flow and consistency models. + +Pooladian et al. (2023) have more directly explored OT coupling within the framework of flow matching models. They show that deterministic and non-crossing paths enabled by OT with infinite batch size lowers the variance of gradient estimators. Experimentally, they assess the efficacy of OT solvers, such as Hungarian matching and Sinkhorn algorithms, in coupling batches of noise and data points. Dou et al. (2024) have successfully adopted this approach in consistency models, while Li et al. (2024) applied OT to diffusion models. However, due to the prohibitive cubic complexity of OT solvers, OT has to be applied by minibatch for matching samples $( \mathbf { x } _ { \star } , \mathbf { z } )$ . Besides an OT approximation error, this incurs the loss of the no-collision property, making ${ \mathcal { R } } ( \theta )$ non-zero in real use-cases. Another line of works using OT tools with score-based models relies on the Schrodinger Bridge formulation (¨ De Bortoli et al., 2021; Shi et al., 2023; Korotin et al., 2024; Tong et al., 2024), which has mostly proven benefits on transfer tasks. + +Our approach. In this paper, we use a consistency model as a proxy of the flow of a diffusion process to reduce transport costs. While not fully solving the alignment issue, we will show that our method present reduced transport costs and better alignment than dedicated OT-based methods. + +# 4. Consistency Models with Generator-Augmented Flows + +Here, we introduce our method, denoted as generatoraugmented flows, which relies on a generator-augmented coupling (GC). We capitalize on the true diffusion flow $\mathring { f }$ (i.e. an ideal consistency model) to map noisy points towards the PF-ODE solution. We present theoretical and empirical evidences that GC not only reduces the data-noise transport cost but also narrows the gap between consistency distillation and consistency training. We will discuss how to train GC consistency models jointly with $\mathring { f }$ in Section 5. + +# 4.1. Generator-Augmented Coupling (GC): Definition and Training Loss + +The solution proposed in this work involves harnessing the diffusion flow, computed from a consistency model, to create a novel form of coupling. The idea is to leverage the properties and accumulated knowledge within an ideal consistency model, $\mathring { f }$ , to construct pairs of points. To achieve this, we first sample an intermediate point, which is done as usual by sampling $\mathbf { x } _ { \star } \sim p _ { \star }$ and $\mathbf { z } \sim p _ { z }$ using the IC between the two distributions, and then predict the data point $\hat { \mathbf { x } } _ { t _ { i } }$ via the consistency model: + +$$ +\begin{array} { r } { ( \mathbf { x } _ { \star } , \mathbf { z } ) \sim q _ { \mathrm { I } } ; \quad \mathbf { x } _ { t _ { i } } = \mathbf { x } _ { \star } + \sigma _ { t _ { i } } \mathbf { z } ; \quad \hat { \mathbf { x } } _ { t _ { i } } = \mathrm { s g } ( \hat { f } ( \mathbf { x } _ { t _ { i } } , \sigma _ { t _ { i } } ) ) . } \end{array} +$$ + +Although $\hat { \mathbf { x } } _ { t _ { i } }$ depends on the timestep $t _ { i }$ , it is important to note that it (supposedly) follows the distribution $p _ { 0 }$ This $\hat { \mathbf { x } } _ { t _ { i } }$ is coupled with $\mathbf { z }$ , thereby defining our generatoraugmented coupling $( \mathrm { G C } ) q$ , which we use to construct the pair of points $\left( \tilde { \mathbf { x } } _ { t _ { i } } , \tilde { \mathbf { x } } _ { t _ { i + 1 } } \right)$ : + +$$ +\left( \hat { \bf x } _ { t _ { i } } , { \bf z } \right) \sim q ; \quad \tilde { \bf x } _ { t _ { i } } = \hat { \bf x } _ { t _ { i } } + \sigma _ { t _ { i } } { \bf z } ; \quad \tilde { \bf x } _ { t _ { i + 1 } } = \hat { \bf x } _ { t _ { i } } + \sigma _ { t _ { i + 1 } } { \bf z } . +$$ + +These intermediate points can serve to define a new consistency training loss: + +$$ +\begin{array} { r l } & { \mathcal { L } _ { \mathrm { G C } } ( \theta ) = \mathbb { E } _ { q ( \hat { \mathbf { x } } _ { t _ { i } } , \mathbf { z } ) , p ( \tilde { \mathbf { x } } _ { t _ { i } } , \tilde { \mathbf { x } } _ { t _ { i + 1 } } | \hat { \mathbf { x } } _ { t _ { i } } , \mathbf { z } ) } } \\ & { \quad \Big [ \lambda ( \sigma _ { t _ { i } } ) \mathcal { D } \Big ( \mathrm { s g } ( f _ { \theta } ( \tilde { \mathbf { x } } _ { t _ { i } } , \sigma _ { t _ { i } } ) ) , f _ { \theta } ( \tilde { \mathbf { x } } _ { t _ { i + 1 } } , \sigma _ { t _ { i + 1 } } ) \Big ) \Big ] . } \end{array} +$$ + +![](images/figures/generator-augmented-flows-fig-0002.jpg) +Figure 2. Comparison of $\tilde { \mathcal { R } } _ { \mathrm { I C } }$ , $\tilde { \mathcal { R } } _ { \mathrm { b a t c h - O T } }$ , and $\tilde { \mathcal { R } } _ { \mathrm { G C } }$ on CIFAR-10. GC exhibits lower values of this quantity for all $\sigma _ { t }$ . + +Generator-augmented trajectories satisfy the boundary conditions of diffusion processes. We note the two following important properties of the distribution of $\tilde { \mathbf { x } } _ { t }$ : + +$$ +p ( \tilde { \mathbf { x } } _ { 0 } ) = p ( \mathbf { x } _ { 0 } ) \approx p _ { \star } , \quad p ( \tilde { \mathbf { x } } _ { T } ) \approx p ( \mathbf { x } _ { T } ) \approx p ( \sigma _ { T } \mathbf { z } ) . +$$ + +The first property is achieved thanks to the boundary condition of the consistency model $\cdot f .$ Section 2.1) , and the second property by construction of the diffusion process which ensures that the noise magnitude is significantly larger than $\hat { \mathbf { x } } _ { t _ { i } }$ for large $t$ . However, for the timesteps $t \in ( 0 , T )$ the marginal distributions $p ( \mathbf { x } _ { t } )$ and $p ( \tilde { \mathbf { x } } _ { t } )$ do not necessarily coincide. + +# 4.2. Properties of Generator-Augmented Flows + +Here, we present some properties of generator-augmented flows that motivate them for training consistency models. + +# 4.2.1. REDUCING ${ \mathcal { R } } ( \theta )$ WITH GC + +In Theorem 1, we proved that the continuous-time consistency training objective decomposes into the sum of the consistency distillation objective and a regularizer term: $\mathcal { L } _ { \mathrm { C T } } ( \theta ) = \mathcal { L } _ { \mathrm { C D } } ( \theta ) + \mathcal { R } ( \theta )$ . Here, we study a proxy term for ${ \mathcal { R } } ( \theta )$ that is easier to calculate: + +$$ +\tilde { \mathcal { R } } _ { t } = \mathbb { E } \left[ \left\| \dot { \mathbf { x } } _ { t } - \mathbf { v } _ { t } ( \mathbf { x } _ { t } ) \right\| ^ { 2 } \right] . +$$ + +This quantity measures the expected distance between the true velocity field and its one-sample Monte Carlo estimate. We study $\breve { \mathcal { R } } _ { t , \mathrm { I C } }$ , $\tilde { \mathcal { R } } _ { \mathrm { b a t c h - O T } }$ , and $\tilde { \mathcal { R } } _ { t , \mathrm { G C } } ^ { \mathrm { ~ \scriptsize ~ \cdot ~ } }$ . They are the respective proxy regularizer term for each type of probability path. Note that $\tilde { \mathcal { R } } _ { t , \mathrm { G C } }$ depends on the endpoint predictor, a consistency model, which impacts both probability paths and velocity fields. Our goal is to compare those proxy regularizer terms, in order to demonstrate that GC does lead to a smaller discrepancy than IC. We further motivate the use of this proxy, in regards with Theorem 1, in Appendix A.4. + +![](images/figures/generator-augmented-flows-fig-0003.jpg) +Figure 3. Comparison of transport costs between IC, batch-OT, and GC on CIFAR-10. + +In the following theorem, proved in Appendix A.2, we show that $\tilde { \mathcal { R } } _ { t }$ decays faster for GC than for IC. + +Theorem 2. Assume that the data distribution contains more than a single point. Also, assume that the generatoraugmented coupling between the predicted data point $\hat { \mathbf { x } } _ { t }$ and noise z is computed via an ideal consistency model $f$ , i.e., the flow of the PF-ODE. Then, as $t \to \infty$ , + +$$ +\tilde { \mathcal { R } } _ { t , \mathrm { G C } } \ll \tilde { \mathcal { R } } _ { t , \mathrm { I C } } . +$$ + +Empirical validation. Evaluating $\tilde { \mathcal { R } } _ { t }$ requires computing the difference between the sample path derivative $\dot { \mathbf { x } } _ { t }$ and the velocity field ${ \bf v } _ { t } ( { \bf x } _ { t } )$ . In the EDM setting, this difference can be approximated using a denoiser. Indeed, $\dot { \mathbf { x } } _ { t } ~ = ~ \mathbf { z }$ and $\begin{array} { r } { { \mathbf { v } } _ { t } ( { \mathbf { x } } _ { t } ) ~ = ~ \mathbb { E } [ \dot { \mathbf { x } } _ { t } | { \mathbf { x } } _ { t } ] ~ = ~ \mathbb { E } [ \mathbf { z } | { \mathbf { x } } _ { t } ] ~ = ~ \mathbb { E } [ \frac { { \mathbf { x } } _ { t } - { \mathbf { x } } _ { \star } } { t } | { \mathbf { x } } _ { t } ] ~ = ~ \mathbb { E } [ \frac { { \mathbf { x } } _ { t } - { \mathbf { x } } _ { \star } } { t } | { \mathbf { x } } _ { t } ] ~ = ~ \mathbb { E } [ \frac { { \mathbf { x } } _ { t } - { \mathbf { x } } _ { \star } } { t } | { \mathbf { x } } _ { t } ] ~ , } \end{array}$ $\begin{array} { r } { \frac { 1 } { t } \big ( \mathbf { x } _ { t } - D _ { \star } ( \mathbf { x } _ { t } , t ) \big ) } \end{array}$ with an optimal denoiser $D _ { \star }$ . The optimal denoiser can be approximated by a denoiser network $D _ { \phi }$ . Finally, we have: $\begin{array} { r } { \dot { { \mathbf x } } _ { t } - { \mathbf v } _ { t } ( { \mathbf x } _ { t } ) \approx { \mathbf z } - \frac { 1 } { t } \big ( { \mathbf x } _ { t } - D _ { \phi } ( { \mathbf x } _ { t } , t ) \big ) } \end{array}$ Since IC, batch-OT, and GC define different $p _ { t }$ ’s and $\mathbf { v } _ { t }$ ’s, we train a different denoiser $D _ { \phi }$ for each coupling. In Figure 2, we report the results from the comparison of the three proxy terms on CIFAR-10. We observe that $\tilde { \mathcal { R } } _ { t , \mathrm { G C } } < \mathrm { \tilde { \mathcal { R } } } _ { t , \mathrm { b a t c h - O T } } < \tilde { \mathcal { R } } _ { t , \mathrm { I C } }$ and that the gap increases with $t$ , corroborating our theoretical findings (Theorem 2). + +# 4.2.2. REDUCING TRANSPORT COST WITH GC + +Here, we investigate the average transport cost between the noise $\mathbf { z } ~ \sim ~ p _ { z }$ and the predicted data point $\hat { \textbf { x } } \sim p _ { \star }$ as a measure of the efficiency of the data-noise coupling. Recall that the diffusion process is given by $\mathbf { x } _ { t } = \mathbf { x } _ { \star } + \sigma _ { t } \mathbf { z }$ Then, knowing that the consistency model $\mathring { f }$ satisfying the boundary condition $\mathring { f } ( \mathbf { x } _ { 0 } , \sigma _ { 0 } ) = \dot { \mathbf { x } } _ { 0 }$ , we define the function + +$c ( t )$ + +$$ +c ( t ) = \mathbb { E } _ { q _ { \mathrm { I } } ( \mathbf { x } _ { \star } , \mathbf { z } ) } \left[ \left\| \mathring { \pmb { f } } ( \mathbf { x } _ { t } , \sigma _ { t } ) - \mathbf { z } \right\| ^ { 2 } \right] . +$$ + +$c ( 0 ) = \mathbb { E } _ { q _ { \mathrm { I } } ( \mathbf { x } _ { \star } , \mathbf { z } ) } [ | | \mathbf { x } _ { 0 } - \mathbf { z } | | ^ { 2 } ]$ and $c ( t )$ represent the transport costs of, respectively, IC and GC. We show below, with proofs in Appendix A.3, that $c ( t )$ is decreasing for $\sigma _ { t }$ close to zero and for large $\sigma _ { t }$ . + +Lemma 1 (Transport cost of GC coupling). Assume that $\mathring { f }$ is a continuously differentiable function representing the ground-truth consistency model, i.e. the flow of the $P F .$ - ODE induced by the diffusion process $\mathbf { x } _ { t }$ . Define $\begin{array} { r l } { \mathbf { w } _ { t } } & { { } = } \end{array}$ $\begin{array} { r } { \mathbf { z } - \mathbb { E } [ \mathbf { z } | \mathbf { x } _ { t } ] = \frac { 1 } { \dot { \sigma } _ { t } } \big ( \dot { \mathbf { x } } _ { t } - \mathbb { E } [ \dot { \mathbf { x } } _ { t } \mid \mathbf { x } _ { t } ] \big ) } \end{array}$ . Then: + +$$ +c ^ { \prime } ( t ) = - 2 \dot { \sigma } _ { t } \mathbb { E } \left[ \left. \frac { \partial \hat { f } } { \partial \mathbf { x } } ( \mathbf { x } _ { t } , \sigma _ { t } ) \cdot \mathbf { w } _ { t } , \mathbf { w } _ { t } \right. \right] . +$$ + +Corollary 1 (Decreasing transport cost of GC coupling in $t \to 0 ^ { + }$ ). There exists a $t \mathrm { ~ * ~ } > 0$ such that for all $t \in$ $[ 0 , t _ { * } ]$ , the derivative of $c ( t )$ takes the form $c ^ { \prime } ( t ) = - 2 \dot { \sigma } _ { t } a _ { t }$ with $a _ { t } > 0$ . Hence for $\dot { \sigma } _ { t }$ positive, the cost is decreasing. In particular, in the EDM setting where $\sigma _ { t } ~ = ~ t , ~ c ( t )$ is decreasing for small $t$ . + +The proof of this corollary proceeds by noting that for $t = 0$ , the consistency model $\mathring { f } ( { \bf x } , t )$ is an identity function, its Jacobian is an identity matrix, and thus $a _ { t } = \mathbb { E } [ \lVert \mathbf { w } _ { t } \rVert ^ { 2 } ]$ Using the continuity of Jacobian elements and invoking intermediate value theorem on $a _ { t }$ concludes the proof. + +Corollary 2 (Decreasing transport cost of GC coupling in $t \approx t _ { \operatorname* { m a x } } ,$ ). Assume that the consistency model $\mathring { f } ( x , \sigma )$ is a scaling function $\begin{array} { r } { \mathring { \pmb { f } } ( \mathbf { x } , \sigma _ { t } ) ~ = ~ \frac { \sigma _ { 0 } } { \sigma _ { t } } \mathbf { x } . } \end{array}$ . Then, we have $\begin{array} { r } { c ^ { \prime } ( t ) = - \frac { 2 \dot { \sigma } _ { t } \sigma _ { 0 } } { \sigma _ { t } } \mathbb { E } [ \| \mathbf { w } _ { t } \| ^ { 2 } ] } \end{array}$ . In particular, $c ( t )$ is decreasing whenever $\sigma _ { t }$ is increasing. + +We note that, while the assumption of the consistency model being a scaling function is strong, it nonetheless bears some degree of truth for $t \approx t _ { \mathrm { m a x } }$ , see Lemma 3 of Appendix A. + +Experimental validation. As stressed in Section 3, a line of work has brought evidence that reducing the transport cost between noise and data distributions could fasten the training and help produce better samples. We compare the quadratic transport costs involved in IC, batch-OT (Pooladian et al., 2023; Dou et al., 2024), and GC (resp. $c ( 0 )$ , $c _ { \mathrm { O T } } ( 0 )$ , and $c ( t ) \dot { } ,$ ). Results are presented in Figure 3. Interestingly, GC reduces transport cost more than batch-OT on CIFAR-10 because batch-OT is tied to the batch data points $\mathbf { x } _ { t }$ whereas our computed $\hat { \mathbf { x } } _ { t }$ are not. + +![](images/figures/generator-augmented-flows-fig-0004.jpg) +Figure 4. Performance of GC w.r.t. the performance of the predictor on CIFAR-10. + +# 5. Training With Generator-Augmented Flows for Image Generation + +In this section, we present a methodology to train consistency models with GC on unconditional image generation. To construct points drawn from GC trajectories $( \tilde { \mathbf { x } } _ { t _ { i } } )$ , our theory requires an optimal predictor $\mathring { f }$ on intermediate points drawn from IC $( \mathbf { x } _ { t _ { i } } )$ . Thus, this lets us two potential training strategies: (i) pre-train an IC generator, and leverage it to construct GC trajectories that train a GC model; (ii) a joint learning strategy: train a single consistency model from scratch with both types of trajectories. Note that in this second setting, the model is unique: ${ \dot { f } } = f _ { \theta }$ . The second option is more appealing, since it is a simple one-stage training. We demonstrate that the joint learning approach improves performance and accelerates convergence compared to standard consistency models. + +Our experiments are done on the following datasets: CIFAR-10 (Krizhevsky, 2009), ImageNet (Deng et al., 2009), CelebA (Liu et al., 2015) and LSUN Church (Yu et al., 2015). For the evaluation metrics, we report the Frechet ´ Inception Distance (FID, Heusel et al. (2017)), Kernel Inception Distance (KID, Binkowski et al. ´ (2018)), and Inception Score (IS, Salimans et al. (2016)). Most of our experiments are based on the improved training techniques for consistency models from Song and Dhariwal (2024), denoted iCT-IC. Moreover, we present some results in the setting of Easy Consistency Tuning (ECT, Geng et al. (2024)). Details are provided in Appendix D. The code is shared in the supplementary material and will be open-sourced upon publication for reproducibility. + +# 5.1. GC with Pre-Trained Endpoint Predictor + +Our theoretical results assume having access to an ideal generator on IC trajectories, meaning that the generator approximates the diffusion flow output. To train a consistency model on GC, we can thus rely on a separate endpoint predictor pre-trained on IC (iCT-IC): $\mathring { \pmb { f } } \equiv \mathring { \pmb { g } } _ { \phi }$ (cf. Section 4.1). This network predicts the endpoint: $\hat { \mathbf { x } } _ { t _ { i } } = \mathbf { { g } } _ { \phi } ( \mathbf { { x } } _ { t _ { i } } , \sigma _ { t _ { i } } )$ During the training of the consistency model on GC, $\mathbf { \nabla } _ { \mathbf { \boldsymbol { g } } _ { \phi } }$ is kept frozen and considered a proxy of the true flow, as in our theoretical results. In Figure 4, we report the performance of consistency models on CIFAR-10 trained with GC using two different $g _ { \phi }$ : $( i )$ a $\mathbf { \nabla } _ { \mathbf { \boldsymbol { g } } _ { \phi } }$ fully trained as standard iCT-IC with $1 0 0 \mathrm { k }$ training steps; $( i i )$ a weak $\mathbf { \nabla } _ { \mathbf { \boldsymbol { g } } _ { \phi } }$ partially trained as iCT-IC with $2 0 \mathrm { k }$ training steps. + +![](images/figures/generator-augmented-flows-fig-0005.jpg) +Figure 5. Consistency models trained with GC with joint learning converges faster and outperforms consistency models trained with IC or minibatch-OT on CIFAR-10. + +Finding 1. Using a partially pre-trained and frozen endpoint predictor, trained on IC trajectories, allows to train a consistency model with GC and which converges faster. However, the performance of the GC model depends on the quality of the endpoint predictor on IC trajectories. + +It is important to note that this setup is not practical, as it requires pre-training a standard consistency model. We aim for a training methodology that accelerates convergence and improves performance when training from scratch, without doubling the number of required training iterations. + +# 5.2. GC from Scratch with Joint Learning + +In this section, we propose to learn simultaneously a single model on IC and GC trajectories from the start of the training, i.e. ${ \mathring { f } } \equiv \operatorname { s g } ( f _ { \theta } )$ (cf. Section 4.1). Thereby, we combine the training of the ideal IC predictor with the training of GC model based on this predictor. We introduce a joint learning factor $\mu$ : at each training step, training pairs are drawn from GC with probability $\mu$ , while the remaining pairs are drawn from standard IC. The loss can be written on average as: + +$$ +\begin{array} { r } { \mathcal { L } _ { \mathrm { G C } - \mu } ( \theta ) = \mu \mathcal { L } _ { \mathrm { G C } } ( \theta ) + ( 1 - \mu ) \mathcal { L } _ { \mathrm { C T } } ( \theta ) } \end{array} +$$ + +Table 1. iCT-IC is the standard improved consistency model with independent coupling (Song and Dhariwal, 2024); iCT-OT is iCT with minibatch optimal transport coupling (Pooladian et al., 2023; Dou et al., 2024); iCT-GC $\mu = 0 . 5 )$ ) is our proposed GC with joint learning. + +
DatasetModelFID ↓KID (×102) ↓IS ↑
CIFAR-10iCT-IC7.42 ± 0.040.44 ± 0.038.76 ± 0.06
iCT-OT6.75 ± 0.040.36 ± 0.048.86 ± 0.09
iCT-GC (μ = 0.5)5.95 ± 0.050.26 ± 0.029.10 ± 0.05
ImageNet (32 × 32)iCT-IC14.89 ± 0.171.23 ± 0.059.46 ± 0.06
iCT-OT14.13 ± 0.171.18 ± 0.059.62 ± 0.06
iCT-GC (µ = 0.5)13.99 ± 0.281.13 ± 0.039.77 ± 0.07
CelebA (64 × 64)iCT-IC15.82 ± 0.131.31 ± 0.042.33 ± 0.00
iCT-OT13.63 ± 0.131.09 ± 0.032.40 ± 0.01
iCT-GC (µ = 0.5)11.74 ± 0.080.91 ± 0.042.45 ± 0.01
LSUN Church (64 × 64)iCT-IC10.58 ± 0.110.73 ± 0.031.99 ± 0.01
iCT-OT9.71 ± 0.130.64 ± 0.032.00 ± 0.01
iCT-GC (µ = 0.5)9.88 ± 0.070.66 ± 0.042.14 ± 0.01
+ +We denote this joint learning procedure as GC $( \mu \ : = \ : \cdot )$ . Hence, GC ${ \mathcal { \boldsymbol { \mu } } } = 0$ ) corresponds to the standard IC procedure, while GC $\mu = 1 ,$ ) corresponds to training only with GC points.Note that GC $\mu = 1 ,$ ) is not expected to work, since our theoretical guarantees assume an optimal IC predictor. The detailed algorithm is presented in Algorithm 1 in Appendix. We apply this joint learning to four image datasets, and include comparisons to iCT with batch-OT (Dou et al., 2024) as an additional baseline. Results across multiple datasets and metrics are presented in Table 1, and visual examples are shown in Figure 8 in Appendix. + +Finding 2. Joint learning of IC and GC trajectories consistently improves results compared to the base IC model and outperforms batch-OT in most cases. + +As shown in Figure 5, we observe an interesting interpolation phenomenon between $\mu = 0$ and $\mu = 1$ . For $\mu = 0$ , we recover the steady FID improvement typical of IC training. As $\mu$ increases, the convergence of the generative model accelerates. For $0 . 3 \leq \mu \leq 0 . 7 .$ , on CIFAR-10, convergence speed and final FID are improved compared to IC and batch-OT models. For $\mu = 1$ , the FID score decreases faster than other configurations early in the training process, but it soons increases as training progresses further. It is explained by the poor performance of the predictions on IC yielding a deviation from the ideal IC predictor from Section 4. For the other datasets, we simply chose $\mu = 0 . 5$ and report those results. We provide further detail on the sensitivity of our results to the choice of $\mu$ in Appendices C.1 and D. + +Table 2. Performance of IC and GC consistency models trained in the ECT setting (Geng et al., 2024). Short training: $4 k$ iterations. Long training: $1 0 0 k$ iterations. + +
Model
FID ↓ CIFAR-10 (Short Training)
ECT-IC 7.37 ± 0.05
ECT-GC (µ = 0.3) 6.41 ± 0.05
CIFAR-10 (Long Training)
ECT-IC 4.11 ± 0.03
ECT-GC (µ = 0.3) 3.74 ± 0.04
FFHQ 64 × 64 (Short Training)
ECT-IC 13.29 ± 0.10
ECT-GC (µ = 0.3) 11.73 ± 0.09
FFHQ 64 × 64 (Long Training)
ECT-IC 9.68 ± 0.06
ECT-GC (µ = 0.3) 8.51 ± 0.09
ImageNet 64 × 64 Cond. (Short Training)
ECT-IC 10.82 ± 0.18
ECT-GC (µ = 0.3) 10.31 ± 0.22
ImageNet 64 × 64 Cond. (Long Training) ECT-IC
5.84 ± 0.21 ECT-GC (µ = 0.3) 6.39 ± 0.20
+ +# 5.3. GC in the ECT Setting + +As an additional experiment, we explore the recent ECT setting (Geng et al., 2024) on CIFAR-10, where consistency models are fine-tuned from a pre-trained diffusion model. This approach enables training high-quality consistency models in one GPU-hour, though it requires an already trained diffusion model. + +We compare IC and GC trajectories in this setting, with both short (approximately one GPU-hour) and long (100k steps, 1 GPU-day) training times. Using the referenced hyperparameters selected by Geng et al. (2024), we observe a consistent advantage for GC, with an optimal $\mu$ value of 0.3. These preliminary results, summarized in Table 2, align with our previous findings on the iCT setting, further supporting the effectiveness of GC. + +# 6. Conclusion + +In this paper, we identify a discrepancy between consistency training and consistency distillation. 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In Proceedings of the 40th International Conference on Machine Learning, volume 202 of Proceedings of Machine Learning Research, pages 32211–32252. PMLR, July 2023. + +Alexander Y. Tong, Nikolay Malkin, Kilian Fatras, Lazar Atanackovic, Yanlei Zhang, Guillaume Huguet, Guy Wolf, and Yoshua Bengio. Simulation-free schrodinger ¨ bridges via score and flow matching. In International Conference on Artificial Intelligence and Statistics, pages 1279–1287. PMLR, 2024. + +Pascal Vincent. A connection between score matching and denoising autoencoders. Neural computation, 23 (7), 2011. + +Fisher Yu, Ari Seff, Yinda Zhang, Shuran Song, Thomas Funkhouser, and Jianxiong Xiao. LSUN: Construction of a large-scale image dataset using deep learning with humans in the loop. arXiv preprint arXiv:1506.03365, 2015. + +# A. Proofs + +# A.1. Continuous-Time Consistency Objectives + +Theorem 1 (Discrepancy between consistency distillation and consistency training objectives). Assume that the distance function is given by $\mathcal { D } ( \mathbf { x } , \mathbf { y } ) = \varphi ( \| \mathbf { x } - \mathbf { y } \| )$ for a continuous convex function $\varphi : [ 0 , \infty ) \to [ 0 , \infty )$ with $\varphi ( x ) \sim C x ^ { \alpha }$ as $x \to 0 ^ { + }$ for some $C > 0$ and $\alpha \geq 1$ , and that the timesteps are equally spaced, i.e., $\begin{array} { r } { t _ { i } = \frac { i T } { N } } \end{array}$ . Furthermore, assume that the Jacobian $\frac { \partial { f _ { \theta } } } { \partial { \bf x } }$ does not vanish identically. Then the following assertions hold: + +(i) The scaled consistency losses $N ^ { \alpha } { \mathcal { L } } _ { \mathrm { C D } } ( \theta )$ and $N ^ { \alpha } { \mathcal { L } } _ { \mathrm { C T } } ( \theta )$ converge as $N \to \infty$ . Moreover, the minimization objectives corresponding to these limiting scaled consistency losses are not equivalent, and their difference is given by: + +$$ +\operatorname* { l i m } _ { N \to \infty } N ^ { \alpha } \left[ { \mathcal { L } } _ { \mathrm { C T } } ( \theta ) - { \mathcal { L } } _ { \mathrm { C D } } ( \theta ) \right] = C T ^ { \alpha - 1 } { \mathcal { R } } ( \theta ) , +$$ + +where ${ \mathcal { R } } ( \theta )$ is defined by + +$$ +\mathcal { R } ( \theta ) = \int _ { 0 } ^ { T } \lambda ( \sigma _ { t } ) \mathbb { E } \left[ \left\| \partial _ { \mathrm { C T } } \pmb { f } _ { \theta } \right\| ^ { \alpha } - \left\| \partial _ { \mathrm { C D } } \pmb { f } _ { \theta } \right\| ^ { \alpha } \right] \mathrm { d } t +$$ + +and satisfies $\mathcal { R } ( \theta ) > 0$ , with + +$$ +\partial _ { \mathrm { C T } } \mathbf { \mathcal { f } } _ { \theta } = \frac { \partial \mathbf { f } _ { \theta } } { \partial \sigma } ( \mathbf { x } _ { t } , \sigma _ { t } ) \boldsymbol { \dot { \sigma } } _ { t } + \frac { \partial \mathbf { f } _ { \theta } } { \partial \mathbf { x } } ( \mathbf { x } _ { t } , \sigma _ { t } ) \cdot \dot { \mathbf { x } } _ { t } , +$$ + +$$ +\partial _ { \mathrm { C D } } \mathbf { f } _ { \theta } = \frac { \partial f _ { \theta } } { \partial \sigma } ( \mathbf { x } _ { t } , \sigma _ { t } ) \boldsymbol { \dot { \sigma } } _ { t } + \frac { \partial f _ { \theta } } { \partial \mathbf { x } } ( \mathbf { x } _ { t } , \sigma _ { t } ) \cdot \mathbf { v } _ { t } ( \mathbf { x } _ { t } ) . +$$ + +In particular, if $\alpha = 2$ , + +$$ +\mathcal { R } ( \theta ) = \int _ { 0 } ^ { T } \lambda ( \sigma _ { t } ) \mathbb { E } \left[ \left\| \frac { \partial f _ { \theta } } { \partial \mathbf { x } } ( \mathbf { x } _ { t } , \sigma _ { t } ) \cdot \left( \dot { \mathbf { x } } _ { t } - \mathbf { v } _ { t } ( \mathbf { x } _ { t } ) \right) \right\| ^ { 2 } \right] \mathrm { d } t . +$$ + +(ii) The scaled gradient $N ^ { \alpha - 1 } \nabla _ { \theta } \mathcal { L } _ { \mathrm { C D } } ( \theta )$ and $N ^ { \alpha - 1 } \nabla _ { \theta } \mathcal { L } _ { \mathrm { C T } } ( \theta )$ converge as $N \infty$ . Moreover, if $\alpha \neq 2$ , then their respective limits are not identical as functions of $\theta$ : + +$$ +\operatorname* { l i m } _ { N \to \infty } N ^ { \alpha - 1 } \nabla _ { \theta } \mathcal { L } _ { \mathrm { C T } } ( \theta ) \neq \operatorname* { l i m } _ { N \to \infty } N ^ { \alpha - 1 } \nabla _ { \theta } \mathcal { L } _ { \mathrm { C D } } ( \theta ) . +$$ + +Proof. (i) Note that $\partial _ { \mathrm { C D } } f _ { \theta }$ and $\partial _ { \mathrm { C T } } f _ { \theta }$ satisfy: + +$$ +\partial _ { \mathrm { C T } } f _ { \theta } ( \mathbf { x } _ { t } , \sigma _ { t } ) = \frac { \partial } { \partial t } f _ { \theta } ( \mathbf { x } _ { t } , \sigma _ { t } ) , \partial _ { \mathrm { C D } } f _ { \theta } ( \mathbf { x } _ { t } , \sigma _ { t } ) = \mathbb { E } \left[ \frac { \partial } { \partial t } f _ { \theta } ( \mathbf { x } _ { t } , \sigma _ { t } ) \bigg \vert \mathbf { x } _ { t } \right] . +$$ + +Here, the second equality follows by noting that $\mathbf { v } _ { t } ( \mathbf { x } _ { t } ) = \mathbb { E } [ \dot { \mathbf { x } } _ { t } | \mathbf { x } _ { t } ]$ and all the other terms in the expansion of $\begin{array} { r } { \frac { \partial } { \partial t } f _ { \theta } ( \mathbf { x } _ { t } , \sigma _ { t } ) } \end{array}$ are completely determined once the value of $\mathbf { x } _ { t }$ is known. + +Now, we use Taylor’s theorem to expand the difference between $f _ { \theta } ( \mathbf { x } _ { t _ { i + 1 } } , \sigma _ { t _ { i + 1 } } )$ and $f _ { \theta } ( \mathbf { x } _ { t _ { i } } ^ { \Phi } , \sigma _ { t _ { i } } )$ in the consistency distillation loss, Equation (4). Together with the definition of $\mathbf { x } _ { t _ { i } } ^ { \Phi }$ , Equation (5), this yields: + +$$ +\begin{array} { r l } & { f _ { \theta } ( \mathbf { x } _ { t _ { i + 1 } } , \sigma _ { t _ { i + 1 } } ) - f _ { \theta } ( \mathbf { x } _ { t _ { i } } ^ { \Phi } , \sigma _ { t _ { i } } ) } \\ & { \ = \frac { \partial f _ { \theta } } { \partial \sigma } ( \mathbf { x } _ { t _ { i + 1 } } , \sigma _ { t _ { i + 1 } } ) \cdot ( \sigma _ { t _ { i + 1 } } - \sigma _ { t _ { i } } ) + \frac { \partial f _ { \theta } } { \partial \mathbf { x } } ( \mathbf { x } _ { t _ { i + 1 } } , \sigma _ { t _ { i + 1 } } ) \cdot ( \mathbf { x } _ { t _ { i + 1 } } - \mathbf { x } _ { t _ { i } } ^ { \Phi } ) + o ( t _ { i + 1 } - t _ { i } ) } \\ & { \ = \partial _ { \mathrm { C D } } f _ { \theta } ( \mathbf { x } _ { t _ { i + 1 } } , \sigma _ { t _ { i + 1 } } ) \cdot ( t _ { i + 1 } - t _ { i } ) + o ( t _ { i + 1 } - t _ { i } ) . } \end{array} +$$ + +Similarly, by expanding the difference between $f _ { \theta } ( \mathbf { x } _ { t _ { i + 1 } } , \sigma _ { t _ { i + 1 } } )$ and $f _ { \theta } ( \mathbf { x } _ { t _ { i } } , \sigma _ { t _ { i } } )$ in Equation (6), + +$$ +\begin{array} { r l } & { f _ { \theta } ( \mathbf { x } _ { t _ { i + 1 } } , \sigma _ { t _ { i + 1 } } ) - f _ { \theta } ( \mathbf { x } _ { t _ { i } } , \sigma _ { t _ { i } } ) } \\ & { \ = \frac { \partial f _ { \theta } } { \partial \sigma } ( \mathbf { x } _ { t _ { i + 1 } } , \sigma _ { t _ { i + 1 } } ) \cdot ( \sigma _ { t _ { i + 1 } } - \sigma _ { t _ { i } } ) + \frac { \partial f _ { \theta } } { \partial \mathbf { x } } ( \mathbf { x } _ { t _ { i + 1 } } , \sigma _ { t _ { i + 1 } } ) \cdot ( \mathbf { x } _ { t _ { i + 1 } } - \mathbf { x } _ { t _ { i } } ) + o ( t _ { i + 1 } - t _ { i } ) } \\ & { \ = \partial _ { \mathrm { C T } } f _ { \theta } ( \mathbf { x } _ { t _ { i + 1 } } , \sigma _ { t _ { i + 1 } } ) \cdot ( t _ { i + 1 } - t _ { i } ) + o ( t _ { i + 1 } - t _ { i } ) . } \end{array} +$$ + +Therefore, for each $\bullet \in \{ \mathrm { C D } , \mathrm { C T } \}$ , + +$$ +\begin{array} { l } { { \displaystyle N ^ { \alpha } { \mathcal { L } } _ { \bullet } ( \theta ) = N ^ { \alpha } \cdot \frac { 1 } { N } \sum _ { i = 0 } ^ { N - 1 } \lambda ( \sigma _ { t _ { i } } ) \mathbb { E } [ C \partial _ { \bullet } f _ { \theta } ( \mathbf { x } _ { t _ { i + 1 } } , \sigma _ { t _ { i + 1 } } ) ^ { \alpha } ( 1 + o ( 1 ) ) ] \cdot ( t _ { i + 1 } - t _ { i } ) ^ { \alpha } } } \\ { ~ } \\ { { \displaystyle = C T ^ { \alpha - 1 } \sum _ { i = 0 } ^ { N - 1 } \lambda ( \sigma _ { t _ { i } } ) \mathbb { E } [ \partial _ { \bullet } f _ { \theta } ( \mathbf { x } _ { t _ { i + 1 } } , \sigma _ { t _ { i + 1 } } ) ^ { \alpha } ( 1 + o ( 1 ) ) ] \cdot ( t _ { i + 1 } - t _ { i } ) } } \\ { { \displaystyle ~ C T ^ { \alpha - 1 } \int _ { 0 } ^ { T } \lambda ( \sigma _ { t } ) \mathbb { E } [ \partial _ { \bullet } f _ { \theta } ( \mathbf { x } _ { t } , \sigma _ { t } ) ^ { \alpha } ] \mathrm { d } t } } \end{array} +$$ + +in the continuous-time limit as $N \to \infty$ . + +For simplicity of notation, we write + +$$ +\mathcal { L } _ { \bullet } ^ { \infty } ( \theta ) = \operatorname* { l i m } _ { N \infty } N ^ { \alpha } \mathcal { L } _ { \bullet } ( \theta ) +$$ + +for each $\bullet \in \{ \mathrm { C D } , \mathrm { C T } \}$ . Then, from the formula for the limiting losses $\mathcal { L } _ { \bullet } ^ { \infty } ( \theta )$ , Equation (35), we immediately obtain + +$$ +\mathcal { L } _ { \mathrm { C T } } ^ { \infty } ( \theta ) - \mathcal { L } _ { \mathrm { C D } } ^ { \infty } ( \theta ) = C T ^ { \alpha - 1 } \int _ { 0 } ^ { T } \lambda ( \sigma _ { t } ) \mathbb { E } \left[ \left\| \partial _ { \mathrm { C T } } \pmb { f } _ { \theta } ( \mathbf { x } _ { t } , \sigma _ { t } ) \right\| ^ { \alpha } - \left\| \partial _ { \mathrm { C D } } \pmb { f } _ { \theta } ( \mathbf { x } _ { t } , \sigma _ { t } ) \right\| ^ { \alpha } \right] \mathrm { d } t . +$$ + +Now, we specialize in the case $\alpha = 2$ and invoke the general observation that, for any random vectors $\mathbf { x }$ and $\mathbf { y }$ , the following identity holds: + +$$ +\begin{array} { r } { \mathbb { E } \left[ \| \mathbf { x } \| ^ { 2 } - \| \mathbb { E } [ \mathbf { x } | \mathbf { y } ] \| ^ { 2 } \right] = \mathbb { E } \left[ \| \mathbf { x } - \mathbb { E } [ \mathbf { x } | \mathbf { y } ] \| ^ { 2 } \right] . } \end{array} +$$ + +This can be easily proved by expanding the squared Euclidean norm as the inner product and applying the law of iterated expectations. Plugging in $\begin{array} { r } { \dot { \mathbf { x } } \frac { \partial } { \partial t } \pmb { f } _ { \theta } ( \mathbf { x } _ { t } , \sigma _ { t } ) } \end{array}$ and $\mathbf { y } \mathbf { x } _ { t }$ , and noting that $\partial _ { \mathrm { C D } } { f } _ { \boldsymbol { \theta } } ( \mathbf { x } _ { t } , \sigma _ { t } ) = \mathbb { E } \left[ \partial _ { \mathrm { C T } } { f } _ { \boldsymbol { \theta } } ( \mathbf { x } _ { t } , \sigma _ { t } ) \ | \ \mathbf { x } _ { t } \right]$ by Equation (28), it follows that + +$$ +\begin{array} { r } { \mathcal { L } _ { \mathrm { C T } } ^ { \infty } ( \theta ) - \mathcal { L } _ { \mathrm { C D } } ^ { \infty } ( \theta ) = C T \int _ { 0 } ^ { T } \lambda ( \sigma _ { t } ) \mathbb { E } \left[ \left\| \partial _ { \mathrm { C T } } f _ { \theta } ( \mathbf { x } _ { t } , \sigma _ { t } ) - \partial _ { \mathrm { C D } } f _ { \theta } ( \mathbf { x } _ { t } , \sigma _ { t } ) \right\| ^ { 2 } \right] \mathrm { d } t } \\ { = C T \int _ { 0 } ^ { T } \lambda ( \sigma _ { t } ) \mathbb { E } \left[ \left\| \frac { \partial f _ { \theta } } { \partial \mathbf { x } } ( \mathbf { x } _ { t } , \sigma _ { t } ) \cdot \left( \dot { \mathbf { x } } _ { t } - \mathbf { v } _ { t } ( \mathbf { x } _ { t } ) \right) \right\| ^ { 2 } \right] \mathrm { d } t . } \end{array} +$$ + +Next, we establish the positivity of ${ \mathcal { R } } ( \theta )$ . To this end, note that $\| \cdot \| ^ { \alpha }$ is a convex function for $\alpha \geq 1$ . By invoking the conditional Jensen’s inequality, we find that the expectation inside the limiting scaled consistency training losses, Equation (35) satisfy: + +$$ +\begin{array} { r l r } & { } & { \mathbb { E } \bigg [ \Big \| \partial _ { \mathrm { C T } } f _ { \theta } ( \mathbf { x } _ { t } , \sigma _ { t } ) \Big \| ^ { \alpha } \bigg ] = \mathbb { E } \bigg [ \bigg \| \frac { \partial } { \partial t } f _ { \theta } ( \mathbf { x } _ { t } , \sigma _ { t } ) \bigg \| ^ { \alpha } \bigg ] = \mathbb { E } \bigg [ \mathbb { \mathbb { E } } \bigg [ \bigg \| \frac { \partial } { \partial t } f _ { \theta } ( \mathbf { x } _ { t } , \sigma _ { t } ) \bigg \| ^ { \alpha } \bigg | \mathbf { x } _ { t } \bigg ] \bigg ] } \\ & { } & { \geq \mathbb { E } \bigg [ \bigg \| \mathbb { E } \bigg [ \frac { \partial } { \partial t } f _ { \theta } ( \mathbf { x } _ { t } , \sigma _ { t } ) \bigg | \mathbf { x } _ { t } \bigg ] \bigg \| ^ { \alpha } \bigg ] = \mathbb { E } \Big [ \big \| \partial _ { \mathrm { C D } } f _ { \theta } ( \mathbf { x } _ { t } , \sigma _ { t } ) \big \| ^ { \alpha } \Big ] . } \end{array} +$$ + +Integrating both sides with respect to $\lambda ( \sigma _ { t } )$ dt, we obtain the desired inequality. The Jensen’s inequality also tells that the equality holds precisely when $\begin{array} { r } { \frac { \partial } { \partial t } { \bf f } _ { \boldsymbol { \theta } } ( { \bf x } _ { t } , \sigma _ { t } ) = \mathbb { E } [ \frac { \partial } { \partial t } { \bf f } _ { \boldsymbol { \theta } } ( { \bf x } _ { t } , \sigma _ { t } ) | { \bf x } _ { t } ] } \end{array}$ holds, or equivalently, $\begin{array} { r } { \frac { \partial f _ { \theta } } { \partial \mathbf { x } } ( \mathbf { x } _ { t } , \boldsymbol { \sigma } _ { t } ) \cdot ( \dot { \mathbf { x } _ { t } } - \mathbb { E } [ \dot { \mathbf { x } } _ { t } \vert \mathbf { x } _ { t } ] ) = 0 } \end{array}$ However, given the value of , the quantity $\dot { \mathbf { x } } _ { t }$ can assume an arbitrary value in because the conditional density of $\dot { \mathbf { x } } _ { t } = \dot { \sigma } _ { t } \mathbf { z }$ given s the a $\mathbf { x } _ { t }$ is strictly positive everywhere. Consequently, the equality condition implies mption of the theorem, the strict inequality between the two limiting losses must $\begin{array} { r } { \frac { \partial { f _ { \theta } } } { \partial { \bf x } } = 0 } \end{array}$ . Since this + +Finally, recall that the continuous-time consistency distillation loss, ${ \mathcal { L } } _ { \mathrm { C D } } ^ { \infty } ( \theta )$ , is given by + +$$ +\mathcal { L } _ { \mathrm { C D } } ^ { \infty } ( \theta ) = C T ^ { \alpha - 1 } \int _ { 0 } ^ { T } \lambda ( \sigma _ { t } ) \mathbb { E } \left[ \left\| \frac { \partial f _ { \theta } } { \partial \sigma } ( \mathbf { x } _ { t } , \sigma _ { t } ) \dot { \sigma } _ { t } + \frac { \partial f _ { \theta } } { \partial \mathbf { x } } ( \mathbf { x } _ { t } , \sigma _ { t } ) \cdot \mathbf { v } _ { t } ( \mathbf { x } _ { t } ) \right\| ^ { \alpha } \right] \mathrm { d } t . +$$ + +Similarly, the continuous-time consistency training loss, ${ \mathcal { L } } _ { \mathrm { C T } } ^ { \infty } ( \theta )$ , is given by + +$$ +\mathcal { L } _ { \mathrm { C T } } ^ { \infty } ( \theta ) = C T ^ { \alpha - 1 } \int _ { 0 } ^ { T } \lambda ( \sigma _ { t } ) \mathbb { E } \left[ \left\| \frac { \partial f _ { \theta } } { \partial \sigma } ( \mathbf { x } _ { t } , \sigma _ { t } ) \dot { \sigma } _ { t } + \frac { \partial f _ { \theta } } { \partial \mathbf { x } } ( \mathbf { x } _ { t } , \sigma _ { t } ) \cdot \dot { \mathbf { x } } _ { t } \right\| ^ { \alpha } \right] \mathrm { d } t . +$$ + +Since $\mathbf { v } _ { t } ( \mathbf { x } _ { t } ) = \mathbb { E } [ \dot { \mathbf { x } } _ { t } | \mathbf { x } _ { t } ]$ and $\mathbb { E } [ \| \dot { \mathbf { x } } _ { t } - \mathbb { E } [ \dot { \mathbf { x } } _ { t } ] \| ^ { 2 } ] > \mathbb { E } [ \| \mathbf { v } _ { t } ( \mathbf { x } _ { t } ) - \mathbb { E } [ \dot { \mathbf { x } } _ { t } ] \| ^ { 2 } ]$ , it follows that ${ \mathcal { L } } _ { \mathrm { C T } } ^ { \infty } ( \theta )$ penalizes the Jacobian $\frac { \partial { f _ { \theta } } } { \partial { \bf x } }$ more strongly than ${ \mathcal { L } } _ { \mathrm { C D } } ^ { \infty } ( \theta )$ does. Therefore, the two limiting consistency losses do not define equivalent objectives. + +(ii) Using the convexity of $\varphi$ , we can show that $\varphi ^ { \prime } ( x ) \sim C \alpha x ^ { \alpha - 1 }$ as $x \to 0 ^ { + }$ . Combining this with the vector calculus formula $\begin{array} { r } { \nabla _ { \mathbf { y } } \| \mathbf { y } \| = \frac { \mathbf { y } } { \| \mathbf { y } \| } } \end{array}$ , we get $\nabla _ { \mathbf { y } } \varphi ( \| \mathbf { y } \| ) \approx C \alpha \| \mathbf { y } \| ^ { \alpha - 2 } \mathbf { y }$ for small y. From this, we can estimate the gradient of the distance between $\mathrm { s g } \big ( f _ { \theta } ( \mathbf { x } _ { t _ { i } } ^ { \Phi } , \sigma _ { t _ { i } } ) \big )$ and $f _ { \theta } ( \mathbf { x } _ { t _ { i + 1 } } , \sigma _ { t _ { i + 1 } } )$ with respect to the model parameter $\theta$ as: + +$$ +\begin{array} { r l } & { \nabla _ { \theta } \mathcal { D } \big ( \mathrm { s g } \big ( \mathbf { \Delta } f _ { \theta } ( \mathbf { x } _ { t _ { i } } ^ { \Phi } , \sigma _ { t _ { i } } ) \big ) , f _ { \theta } ( \mathbf { x } _ { t _ { i + 1 } } , \sigma _ { t _ { i + 1 } } ) \big ) } \\ & { = ( 1 + o ( 1 ) ) C \alpha \left[ \| \partial _ { \mathrm { C D } } \mathbf { \Delta } f _ { \theta } \| ^ { \alpha - 2 } ( \partial _ { \mathrm { C D } } \mathbf { \Delta } f _ { \theta } ) ^ { \top } \frac { \partial f _ { \theta } } { \partial \theta } \right] \cdot ( t _ { i + 1 } - t _ { i } ) ^ { \alpha - 1 } } \end{array} +$$ + +Here, the expression of the distance betw $\begin{array} { r l } { \| \partial _ { \mathrm { C D } } { f } _ { \boldsymbol { \theta } } \| ^ { \alpha - 2 } ( \partial _ { \mathrm { C D } } { f } _ { \boldsymbol { \theta } } ) ^ { \top } \frac { \partial { f } _ { \boldsymbol { \theta } } } { \partial \boldsymbol { \theta } } } & { { } } \end{array}$ quare bracket is evaluated at is estimated as: $\left( \mathbf { x } _ { t _ { i + 1 } } , \sigma _ { t _ { i + 1 } } \right)$ . Similarly, the gradient $f _ { \theta } ( \mathbf { x } _ { t _ { i } } , \sigma _ { t _ { i } } )$ $f _ { \theta } ( \mathbf { x } _ { t _ { i + 1 } } , \sigma _ { t _ { i + 1 } } )$ + +$$ +\begin{array} { l } { \displaystyle \nabla _ { \theta } \mathcal { D } \big ( \mathrm { s g } \big ( { f _ { \theta } } ( \mathbf { x } _ { t _ { i } } , \sigma _ { t _ { i } } ) \big ) , { f _ { \theta } } ( \mathbf { x } _ { t _ { i + 1 } } , \sigma _ { t _ { i + 1 } } ) \big ) } \\ { \displaystyle = ( 1 + o ( 1 ) ) C \alpha \left[ \| \partial _ { \mathrm { C T } } { f _ { \theta } } \| ^ { \alpha - 2 } ( \partial _ { \mathrm { C T } } { f _ { \theta } } ) ^ { \top } \frac { \partial { f _ { \theta } } } { \partial \theta } \right] \cdot ( t _ { i + 1 } - t _ { i } ) ^ { \alpha - 1 } } \end{array} +$$ + +Combining these two estimates, we can now compute the limit of the scaled gradient $N ^ { \alpha - 1 } \nabla _ { \theta } \mathcal { L } _ { \bullet } ( \theta )$ for each $\bullet \in \{ \mathrm { C D } , \mathrm { C T } \}$ as: + +$$ +\begin{array} { l } { { \displaystyle N ^ { \alpha - 1 } \nabla _ { \theta } \mathcal { L } _ { \bullet } ( \theta ) } } \\ { { \displaystyle = C \alpha T ^ { \alpha - 2 } \sum _ { i = 0 } ^ { N - 1 } \lambda ( \sigma _ { t _ { i } } ) \mathbb { E } \bigg [ ( 1 + o ( 1 ) ) [ \| \partial _ { \bullet } f _ { \theta } \| ^ { \alpha - 2 } ( \partial _ { \bullet } f _ { \theta } ) ^ { \top } \frac { \partial f _ { \theta } } { \partial \theta } ] \bigg ] \cdot ( t _ { i + 1 } - t _ { i } ) } } \\ { { \displaystyle C \alpha T ^ { \alpha - 2 } \int _ { 0 } ^ { T } \lambda ( \sigma _ { t } ) \mathbb { E } \bigg [ \| \partial _ { \bullet } f _ { \theta } ( \mathbf { x } _ { t } , \sigma _ { t } ) \| ^ { \alpha - 2 } ( \partial _ { \bullet } f _ { \theta } ( \mathbf { x } _ { t } , \sigma _ { t } ) ) ^ { \top } \frac { \partial f _ { \theta } } { \partial \theta } ( \mathbf { x } _ { t } , \sigma _ { t } ) \bigg ] \mathrm { d } t } } \end{array} +$$ + +as $N \to \infty$ . Finally, if $\alpha \neq 2$ , then the term $\| \partial _ { \bullet } \mathbf { \mathscr { f } } _ { \boldsymbol { \theta } } \| ^ { \alpha - 2 } \partial _ { \bullet } \mathbf { \mathscr { f } } _ { \boldsymbol { \theta } } ^ { \top }$ is a nonlinear transformation of $\partial _ { \bullet } f _ { \theta }$ . This nonlinearity tells that, in general, + +$$ +\mathbb { E } \left[ \left. \partial _ { \mathrm { C T } } \pmb { f } _ { \theta } \right. ^ { \alpha - 2 } \left( \partial _ { \mathrm { C T } } \pmb { f } _ { \theta } \right) ^ { \top } \middle | \mathbf { x } _ { t } \right] \neq \left. \partial _ { \mathrm { C D } } \pmb { f } _ { \theta } \right. ^ { \alpha - 2 } \left( \partial _ { \mathrm { C D } } \pmb { f } _ { \theta } \right) ^ { \top } . +$$ + +herefore, the scaled gradient limits are not identical as functions of $\theta$ , and in particular, their zero sets do not coincide. + +Differences with Song et al. (2023)’s results. The previous theorem states a discrepancy between CT and CD objectives. However, Song et al. (2023) provide equivalence results between consistency training and consistency distillation. The differences come from the following reasons. + +• In Song et al. (2023, Theorem 2), it is stated that $L _ { \mathrm { C T } } = L _ { \mathrm { C D } } + o ( \Delta T )$ . However, in this theorem, the $o ( \Delta T )$ is actually too large compared to the other term, and consequently the result is uninformative. Indeed it has two the two following problems: (i) if the distance function decays faster than the norm does, i.e., ${ \mathcal { D } } ( x , y ) = o ( \| x - y \| )$ , then the $o ( \Delta T )$ term is actually too large compared to the magnitude of the two losses as $N \to \infty$ ; (ii) The $C ^ { 2 }$ -regularity assumption on the distance function $\mathcal { D }$ is too restrictive, excluding many cases such as the distance function given by a norm. For example, such a breakdown happens when $\mathcal { D } ( x , y )$ is a metric induced by the norm, i.e., ${ \mathcal { D } } ( x , y ) = \| x - y \|$ . In this case, its partial derivatives, such as $\begin{array} { r } { \partial _ { 2 } \mathcal { D } ( x , y ) = \frac { \partial } { \partial y } \mathcal { D } ( x , y ) } \end{array}$ appearing in the proof, are undefined along $\mathbf { X } = \mathbf { y }$ • In Song et al. (2023, Theorem 6), the theorem about limiting gradient equality is stated with a general distance function $\mathcal { D }$ . However, the requirements on the Hessian of the distance function restrict the theorem’s validity where the distance function is an (asymptotic) quadratic loss. Indeed, in their proof, it turns out that the Hessian can define a non-zero value only when $\mathcal { D }$ is an (asymptotic) quadratic loss. This coincides with our results in the case $\alpha = 2$ . + +# A.2. Proxy of the Regularizer + +In this subsection, we establish a theoretical result about the decay rate of the proxy of the regularizer. As preparation for the main result and for future use, we introduce a simple lemma that decomposes the forward flow generated by a vector field into the sum of a scaling term and a correction term that is well-behaved. + +Lemma 2. Assume that $\phi$ is the forward flow generated by the vector field $\mathbf { v } _ { t }$ , meaning that it solves the characteristic equation: + +$$ +\frac { \partial } { \partial t } \phi ( \mathbf { x } , \sigma _ { t } ) = \mathbf { v } _ { t } ( \phi ( \mathbf { x } , \sigma _ { t } ) ) , \qquad \phi ( \mathbf { x } , \sigma _ { 0 } ) = \mathbf { x } . +$$ + +Also, assume that $\mathbf { v } _ { t }$ is defined as + +$$ +\mathbf { v } _ { t } ( \mathbf { x } ) = \frac { \dot { \sigma } _ { t } } { \sigma _ { t } } ( \mathbf { x } - D ( \mathbf { x } , \sigma _ { t } ) ) +$$ + +for some function $_ { D }$ , which we call $a$ “denoiser”. Then $\phi$ satisfies the following integral equation: + +$$ +\mathbf { x } = \frac { \sigma _ { 0 } } { \sigma _ { t } } \phi ( \mathbf { x } , \sigma _ { t } ) + \sigma _ { 0 } \int _ { 0 } ^ { t } \frac { \dot { \sigma } _ { s } } { \sigma _ { s } ^ { 2 } } D ( \phi ( \mathbf { x } , \sigma _ { s } ) , \sigma _ { s } ) \ \mathrm { d } s . +$$ + +Proof. We first compute the derivative of $\phi / \sigma _ { t }$ : + +$$ +\begin{array} { l } { \displaystyle { \frac { \partial } { \partial t } \left( \frac { \phi ( \mathbf { x } , \sigma _ { t } ) } { \sigma _ { t } } \right) = - \frac { \dot { \sigma } _ { t } } { \sigma _ { t } ^ { 2 } } \phi ( \mathbf { x } , \sigma _ { t } ) + \frac { 1 } { \sigma _ { t } } \cdot \frac { \dot { \sigma } _ { t } } { \sigma _ { t } } ( \phi ( \mathbf { x } , \sigma _ { t } ) - D ( \phi ( \mathbf { x } , \sigma _ { t } ) , \sigma _ { t } ) ) } } \\ { \displaystyle { \qquad = - \frac { \dot { \sigma _ { t } } } { \sigma _ { t } ^ { 2 } } D ( \phi ( \mathbf { x } , \sigma _ { t } ) , \sigma _ { t } ) . } } \end{array} +$$ + +Integrating both sides with respect to $t$ , it follows that + +$$ +\frac { \phi ( \mathbf { x } , \sigma _ { t } ) } { \sigma _ { t } } - \frac { \phi ( \mathbf { x } , \sigma _ { 0 } ) } { \sigma _ { 0 } } = - \int _ { 0 } ^ { t } \frac { \dot { \sigma _ { s } } } { \sigma _ { s } ^ { 2 } } D ( \phi ( \mathbf { x } , \sigma _ { s } ) , \sigma _ { s } ) \ \mathrm { d } s . +$$ + +Rearranging and applying the initial condition $\phi ( \mathbf { x } , \sigma _ { 0 } ) = \mathbf { x }$ , we obtain the desired equation. + +As an immediate consequence of this lemma, we obtain the following result about the asymptotic structure of a trained consistency model: + +Lemma 3. Assume that $\mathring { f }$ is the consistency model generated by a bounded denoiser $_ D$ , in the sense that $\mathring { f }$ solves the transport equation + +$$ +\frac { \partial \hat { f } } { \partial \sigma } ( \mathbf { x } , \sigma _ { t } ) \dot { \sigma } _ { t } + \frac { \partial \hat { f } } { \partial \mathbf { x } } ( \mathbf { x } , \sigma _ { t } ) \cdot \mathbf { v } _ { t } ( \mathbf { x } ) = 0 +$$ + +for a vector field $\overset { \circ } { \mathbf { v } } _ { t }$ defined as in Equation (51) with the denoiser $_ D$ . Then + +$$ +\mathring { f } ( \mathbf { x } , \sigma _ { t } ) = \frac { \sigma _ { 0 } } { \sigma _ { t } } \mathbf { x } + \mathcal { O } ( 1 ) +$$ + +uniformly in $\mathbf { x }$ and $\sigma _ { t }$ . The implicit bound of the error term can be chosen to be the bound of $_ { D }$ . + +Proof. Let $\phi$ be the forward flow generated by $\mathring { \mathbf { v } } _ { t }$ as in Lemma 2. This $\phi$ is precisely the inverse of the consistency model $\mathring { f }$ , in the sense that $\phi ( { \mathring { f } } ( \mathbf { x } , \sigma ) , \sigma ) { \mathrm { = } } \mathbf { x }$ holds. Then, replacing $\mathbf { x }$ in the equation of Lemma 2 with $\mathring { f } ( \mathbf { x } , \sigma _ { t } )$ , we get + +$$ +\hat { \pmb f } ( \mathbf x , \sigma _ { t } ) = \frac { \sigma _ { 0 } } { \sigma _ { t } } \mathbf x + \sigma _ { 0 } \int _ { 0 } ^ { t } \frac { \dot { \sigma } _ { s } } { \sigma _ { s } ^ { 2 } } { \cal D } ( \phi ( \hat { f } ( \mathbf x , \sigma _ { t } ) , \sigma _ { s } ) , \sigma _ { s } ) \ \mathrm d s . +$$ + +Now let $R$ be such that $\| D ( \mathbf { x } , \sigma ) \| \leq R$ for any $\mathbf { x } \in \mathbb { R } ^ { d }$ and noise level $\sigma$ . Then, the integral term in Equation (58) is bounded as: + +$$ +\left\| \sigma _ { 0 } \int _ { 0 } ^ { t } \frac { \dot { \sigma } _ { s } } { \sigma _ { s } ^ { 2 } } D ( \phi ( \dot { f } ( \mathbf { x } , \sigma _ { t } ) , \sigma _ { s } ) , \sigma _ { s } ) \mathrm { d } s \right\| \leq \sigma _ { 0 } \int _ { 0 } ^ { t } \frac { \dot { \sigma } _ { s } } { \sigma _ { s } ^ { 2 } } R \mathrm { d } s = \sigma _ { 0 } R \left( \frac { 1 } { \sigma _ { 0 } } - \frac { 1 } { \sigma _ { t } } \right) \leq R . +$$ + +This proves the desired claim. + +Now we turn to the main result, which analyzes the asymptotic behavior of $\tilde { \mathcal { R } } _ { t , \mathrm { I C } }$ and $\mathcal { \tilde { R } } _ { t , \mathrm { G C } }$ , as $t \to \infty$ : + +Theorem 2. Assume that the data distribution contains more than a single point. Also, assume that the generator-augmented coupling between the predicted data point $\hat { \mathbf { x } } _ { t }$ and noise $\mathbf { z }$ is computed via an ideal consistency model $\ddot { f }$ , i.e., the flow of the PF-ODE. Then, as $t \to \infty$ , + +$$ +\tilde { \mathcal { R } } _ { t , \mathrm { G C } } \ll \tilde { \mathcal { R } } _ { t , \mathrm { I C } } . +$$ + +Proof. We first investigate the asymptotic behavior of $\tilde { \mathcal { R } } _ { t , \mathrm { I C } }$ in the limit of $t \to \infty$ . Recall that the diffusion process $\mathbf { x } _ { t }$ is given by $\mathbf { x } _ { t } = \mathbf { x } _ { \star } + \sigma _ { t } \mathbf { z }$ for $( \mathbf { x } _ { \star } , \mathbf { z } ) \sim q _ { \mathrm { I } }$ , and note that + +$$ +\dot { { \bf x } } _ { t } - { \bf v } _ { t } ( { \bf x } _ { t } ) = \dot { \sigma } _ { t } { \bf z } - \mathbb { E } [ \dot { \sigma } _ { t } { \bf z } | { \bf x } _ { t } ] = - \frac { \dot { \sigma } _ { t } } { \sigma _ { t } } ( { \bf x } _ { \star } - D ( { \bf x } _ { t } , \sigma _ { t } ) ) , +$$ + +where $D ( \mathbf { x } _ { t } , \sigma _ { t } ) = \mathbb { E } [ \mathbf { x } _ { \star } | \mathbf { x } _ { t } ]$ is the denoiser. Plugging this into the definition of $\tilde { \mathcal { R } } _ { t , \mathrm { I C } }$ , we get + +$$ +\tilde { \mathcal { R } } _ { t , \mathrm { I C } } = \left( \frac { \dot { \sigma } _ { t } } { \sigma _ { t } } \right) ^ { 2 } \mathbb { E } \left[ \left\| \mathbf { x } _ { \star } - D ( \mathbf { x } _ { t } , \sigma _ { t } ) \right\| ^ { 2 } \right] . +$$ + +Now, we claim that $D ( \mathbf { x } _ { t } , \sigma _ { t } ) = \mathbb { E } [ \mathbf { x } _ { \star } | \mathbf { x } _ { t } ] \mathbb { E } [ \mathbf { x } _ { \star } ]$ as $t \to \infty$ . Intuitively, this is because $\mathbf { x } _ { t } \approx \sigma _ { t } \mathbf { z }$ for large $t$ , and $\sigma _ { t } \mathbf { z }$ is independent of $\mathbf { x } _ { \star }$ . More formally, note that the conditional distribution of $\mathbf { x } _ { t }$ given $\mathbf { x } _ { \star }$ is $p ( \mathbf x _ { t } | \mathbf x _ { \star } ) = \mathcal N ( \mathbf x _ { t } ; \mathbf x _ { \star } , \sigma _ { t } ^ { 2 } \mathbf I )$ . By Bayes’ theorem, the conditional distribution of $\mathbf { x } _ { \star }$ given $\mathbf { x } _ { t }$ is + +$$ +p ( \mathbf { x } _ { \star } | \mathbf { x } _ { t } ) = \frac { p ( \mathbf { x } _ { t } | \mathbf { x } _ { \star } ) p ( \mathbf { x } _ { \star } ) } { \int _ { \mathbb { R } ^ { d } } p ( \mathbf { x } _ { t } | \mathbf { x } _ { \star } ^ { \prime } ) p ( \mathbf { x } _ { \star } ^ { \prime } ) \ \mathrm { d } \mathbf { x } _ { \star } ^ { \prime } } = \frac { \exp \left( - \frac { 1 } { 2 \sigma _ { t } ^ { 2 } } \left| \mathbf { x } _ { t } - \mathbf { x } _ { \star } \right| ^ { 2 } \right) p ( \mathbf { x } _ { \star } ) } { \int _ { \mathbb { R } ^ { d } } \exp \left( - \frac { 1 } { 2 \sigma _ { t } ^ { 2 } } \left| \mathbf { x } _ { t } - \mathbf { x } _ { \star } ^ { \prime } \right| ^ { 2 } \right) p ( \mathbf { x } _ { \star } ^ { \prime } ) \ \mathrm { d } \mathbf { x } _ { \star } ^ { \prime } } . +$$ + +As $t \infty$ , we have $\sigma _ { t } \infty$ , so the exponential terms converge to 1. Consequently, $p ( \mathbf { x } _ { \star } | \mathbf { x } _ { t } ) p ( \mathbf { x } _ { \star } )$ and hence $\mathbb { E } [ \mathbf { x } _ { \star } | \mathbf { x } _ { t } ] \mathbb { E } [ \mathbf { x } _ { \star } ]$ as claimed. Thus, + +$$ +\tilde { \mathcal { R } } _ { t , \mathrm { I C } } \sim \left( \frac { \dot { \sigma } _ { t } } { \sigma _ { t } } \right) ^ { 2 } \mathbb { E } \left[ \left\| \mathbf { x } _ { \star } - \mathbb { E } [ \mathbf { x } _ { \star } ] \right\| ^ { 2 } \right] . +$$ + +Since the data distribution $p _ { \star }$ is assumed to have more than one point, the variance $\mathbb { E } [ \| \mathbf { x } _ { \star } - \mathbb { E } [ \mathbf { x } _ { \star } ] \| ^ { 2 } ]$ is strictly positive. +Therefore, $\tilde { \mathcal { R } } _ { t , \mathrm { I C } }$ decays at a rate asymptotically proportional to $\big ( \frac { \dot { \sigma } _ { t } } { \sigma _ { t } } \big ) ^ { 2 }$ . + +Next, we investigate the asymptotic behavior of $\tilde { \mathcal { R } } _ { t , \mathrm { G C } }$ . Recall the consistency training loss for GC, Equation (15). Under the assumptions in Theorem 1, the scaled loss $N ^ { \alpha } { \mathcal { L } } _ { \mathrm { G C } } ( \theta )$ converges to + +$$ +\mathcal { L } _ { \mathrm { G C } } ^ { \infty } ( \theta ) = C T ^ { \alpha - 1 } \int _ { 0 } ^ { T } \lambda ( \sigma _ { t } ) \mathbb { E } \left[ \left\| \frac { \partial f _ { \theta } } { \partial \sigma } ( \tilde { \mathbf { x } } _ { t } , \sigma _ { t } ) \dot { \sigma } _ { t } + \frac { \partial f _ { \theta } } { \partial \mathbf { x } } ( \tilde { \mathbf { x } } _ { t } , \sigma _ { t } ) \cdot \dot { \sigma } _ { t } \mathbf { z } \right\| ^ { \alpha } \right] \mathrm { d } t . +$$ + +Here, $\tilde { \mathbf { x } } _ { t } = \hat { \mathbf { x } } _ { t } + \sigma _ { t } \mathbf { z }$ and $\hat { \mathbf { x } } _ { t } = \mathring { f } ( \mathbf { x } _ { t } , \sigma _ { t } )$ , where $\mathring { f }$ is the ideal consistency model for the flow associated with the diffusion process $\mathbf { x } _ { t }$ . The proof of this claim is similar to that of Theorem 1, so we only highlight the necessary changes. Most importantly, the velocity term is not $\dot { \tilde { \mathbf { x } } } _ { t }$ but $\dot { \sigma } _ { t } \mathbf { z }$ . This is due to how the discrete-time samples are constructed. Indeed, from Equation (14), we find that $\tilde { \mathbf { x } } _ { t _ { i + 1 } } - \tilde { \mathbf { x } } _ { t _ { i } } = ( \sigma _ { t _ { i + 1 } } - \sigma _ { t _ { i } } ) \mathbf { z }$ , which manifests as the velocity term $\dot { \sigma } _ { t } \mathbf { z }$ in Equation (65). Consequently, the associated (average) velocity field $\tilde { \mathbf { v } } _ { t }$ is given by + +$$ +\tilde { { \bf v } } _ { t } ( \tilde { { \bf x } } _ { t } ) = \mathbb { E } [ \dot { \sigma } _ { t } { \bf z } | \tilde { { \bf x } } _ { t } ] = \frac { \dot { \sigma } _ { t } } { \sigma _ { t } } ( \tilde { { \bf x } } _ { t } - \mathbb { E } [ \hat { { \bf x } } _ { t } | \tilde { { \bf x } } _ { t } ] ) . +$$ + +Therefore, $\tilde { \mathcal { R } } _ { t , \mathrm { G C } }$ reduces to + +$$ +\mathcal { \tilde { R } } _ { t , \mathrm { G C } } = \left( \frac { \dot { \sigma } _ { t } } { \sigma _ { t } } \right) ^ { 2 } \mathbb { E } \left[ \left\| \hat { { \mathbf { x } } } _ { t } - \mathbb { E } [ \hat { { \mathbf { x } } } _ { t } \vert \tilde { { \mathbf { x } } } _ { t } ] \right\| ^ { 2 } \right] . +$$ + +Now, unlike in the IC case, we claim that $\mathbb { E } [ \hat { \mathbf { x } } _ { t } | \tilde { \mathbf { x } } _ { t } ] \approx \hat { \mathbf { x } } _ { t }$ as $t \to \infty$ . Heuristically, this is because both $\hat { \mathbf { x } } _ { t }$ and $\tilde { \mathbf { x } } _ { t }$ are almost deterministic functions of $\mathbf { z }$ ; hence, the conditioning has negligible effect in the limit. + +More precisely, let $\phi$ be the forward flow generated by the PF-ODE vector field $\mathbf { v } _ { t }$ . As in the proof of Lemma 2, integrating both sides of Equation (54) from $t$ to $u$ yields + +$$ +\frac { \phi ( \mathbf { x } , \sigma _ { u } ) } { \sigma _ { u } } = \frac { \phi ( \mathbf { x } , \sigma _ { t } ) } { \sigma _ { t } } - \int _ { t } ^ { u } \frac { \dot { \sigma _ { s } } } { \sigma _ { s } ^ { 2 } } D ( \phi ( \mathbf { x } , \sigma _ { s } ) , \sigma _ { s } ) ~ \mathrm { d } s . +$$ + +Letting $u \to \infty$ , we claim that the right-hand side converges. Indeed, the empirical data distribution $p _ { \star }$ has compact support, meaning all the data points are confined in a bounded region of $\mathbb { R } ^ { d }$ . Since the values of $_ { D }$ are weighted averages of the data points, it follows that $_ { D }$ is also bounded. Then the integrand $\begin{array} { r } { \frac { \dot { \sigma _ { s } } } { \sigma _ { s } ^ { 2 } } D ( \phi ( \mathbf { x } , \sigma _ { s } ) , \sigma _ { s } ) } \end{array}$ is absolutely integrable on $[ t , \infty )$ , hence the convergence follows. Moreover, the limit does not depend on $t$ . Denote this limit by $\rho ( \mathbf { x } )$ : + +$$ +\rho ( \mathbf x ) = \frac { \phi ( \mathbf x , \sigma _ { t } ) } { \sigma _ { t } } - \int _ { t } ^ { \infty } \frac { \dot { \sigma _ { s } } } { \sigma _ { s } ^ { 2 } } D ( \phi ( \mathbf x , \sigma _ { s } ) , \sigma _ { s } ) ~ \mathrm d s . +$$ + +As shown in the previous part, we know that $D ( \mathbf { x } , t ) = c + o ( 1 )$ as $t \to \infty$ with $c = \mathbb { E } [ x _ { \star } ]$ . Then, multiplying both sides of Equation (69) by $\sigma _ { t }$ and rearranging, we have, for large $t$ , + +$$ +\begin{array} { l } { \displaystyle \phi ( \mathbf { x } , \sigma _ { t } ) = \sigma _ { t } \pmb { \rho } ( \mathbf { x } ) + \sigma _ { t } \displaystyle \int _ { t } ^ { \infty } \frac { \dot { \sigma _ { s } } } { \sigma _ { s } ^ { 2 } } \pmb { D } ( \phi ( \mathbf { x } , \sigma _ { s } ) , \sigma _ { s } ) \ \mathrm { d } s } \\ { \displaystyle = \sigma _ { t } \pmb { \rho } ( \mathbf { x } ) + ( c + o ( 1 ) ) \sigma _ { t } \displaystyle \int _ { t } ^ { \infty } \frac { \dot { \sigma _ { s } } } { \sigma _ { s } ^ { 2 } } \ \mathrm { d } s } \\ { \displaystyle = \sigma _ { t } \pmb { \rho } ( \mathbf { x } ) + c + o ( 1 ) . } \end{array} +$$ + +Since $\phi$ is a bijection, the above relation tells that $\rho ( \mathbf { x } )$ is also a bijection. Next, we replace $\mathbf { x } \gets \hat { \mathbf { x } } _ { t }$ in the equation defining $\rho ( \mathbf { x } )$ , Equation (69), to obtain: + +$$ +\rho ( \hat { \mathbf { x } } _ { t } ) = \mathbf { z } + \frac { \mathbf { x } _ { \star } } { \sigma _ { t } } - \int _ { t } ^ { \infty } \frac { \dot { \sigma _ { s } } } { \sigma _ { s } ^ { 2 } } D ( \phi ( \hat { \mathbf { x } } _ { t } , \sigma _ { s } ) , \sigma _ { s } ) ~ \mathrm { d } s . +$$ + +Since $\rho$ is invertible, applying $\rho ^ { - 1 }$ to both sides yields + +$$ +\begin{array} { r l } & { \hat { \mathbf { x } } _ { t } = \rho ^ { - 1 } \left( \mathbf { z } + \frac { \mathbf { x } _ { \star } } { \sigma _ { t } } - \displaystyle \int _ { t } ^ { \infty } \frac { \dot { \sigma _ { s } } } { \sigma _ { s } ^ { 2 } } D ( \phi ( \hat { \mathbf { x } } _ { t } , \sigma _ { s } ) , \sigma _ { s } ) \ \mathrm { d } s \right) } \\ & { \quad = \rho ^ { - 1 } \left( \displaystyle \frac { \tilde { \mathbf { x } } _ { t } } { \sigma _ { t } } + \frac { \mathbf { x } _ { \star } - \hat { \mathbf { x } } _ { t } } { \sigma _ { t } } - \displaystyle \int _ { t } ^ { \infty } \frac { \dot { \sigma _ { s } } } { \sigma _ { s } ^ { 2 } } D ( \phi ( \hat { \mathbf { x } } _ { t } , \sigma _ { s } ) , \sigma _ { s } ) \ \mathrm { d } s \right) } \end{array} +$$ + +Since all of $\mathbf { x } _ { \star } , \hat { \mathbf { x } } _ { t }$ , and $_ { D }$ are bounded by the largest norm of the data point, they are all finite. Hence, the last line shows that $\begin{array} { r } { \hat { \mathbf { x } } _ { t } = \rho ^ { - 1 } \big ( \frac { \tilde { \mathbf { x } } _ { t } } { \sigma _ { t } } + \mathcal { O } ( \frac { 1 } { \sigma _ { t } } ) \big ) } \end{array}$ , demonstrating that $\hat { \mathbf { x } } _ { t }$ is almost a deterministic function of $\tilde { \mathbf { x } } _ { t }$ . Therefore, $\mathbb { E } [ \hat { \mathbf { x } } _ { t } | \tilde { \mathbf { x } } _ { t } ] \approx \hat { \mathbf { x } } _ { t }$ as required. Consequently, $\mathcal { \tilde { R } } _ { t , \mathrm { G C } }$ satisfies + +$$ +\tilde { \mathcal { R } } _ { t , \mathrm { G C } } \ll \left( \frac { \dot { \sigma } _ { t } } { \sigma _ { t } } \right) ^ { 2 } . +$$ + +This proves that $\tilde { \mathcal { R } } _ { t , \mathrm { G C } } \ll \tilde { \mathcal { R } } _ { t , \mathrm { I C } }$ as required. + +# A.3. Transport Cost + +As a base for the two corollaries presented in the paper, we will first derive a useful representation of the derivative of the transport cost. + +The main purpose of the lemma is to provide a more tractable representation of $c ^ { \prime } ( t )$ , the time derivative of the expected transport cost. We expect $c ( t )$ to decrease with $t$ because the predicted data point $\mathring { f } ( \mathbf { x } _ { t } , \sigma _ { t } )$ becomes more dependent on the noise $\mathbf { z }$ as $t$ increases. However, directly analyzing $\bar { f } ( \mathbf { x } _ { t } , \sigma _ { t } ) - \mathbf { z }$ is challenging because the dependence of $\mathring { f } ( \mathbf { x } _ { t } , \sigma _ { t } )$ on $\mathbf { z }$ is not explicit. Therefore, the lemma aims to: + +• identify a quantity that better captures the dependence between $\mathbf { z }$ and $\mathbf { x } _ { t }$ ; + +• relate $c ( t )$ to this quantity. + +The proof proceeds by deriving a key property of the ground-truth consistency map $\mathring { f }$ : it satisfies the transport equation, + +$$ +\frac { \partial \hat { f } } { \partial \sigma } ( \mathbf { x } , \sigma _ { t } ) \dot { \sigma } _ { t } + \frac { \partial \hat { f } } { \partial \mathbf { x } } ( \mathbf { x } , \sigma _ { t } ) \cdot \mathbf { v } _ { t } ( \mathbf { x } ) = 0 . +$$ + +This equation is equivalent to saying that the conditional expectation of the time derivative of $\mathring { f } ( \mathbf { x } _ { t } , \sigma _ { t } )$ given $\mathbf { x } _ { t }$ is zero: + +$$ +\mathbb { E } \left[ \frac { \partial } { \partial t } \mathring { f } ( \mathbf { x } _ { t } , \sigma _ { t } ) \Bigg | \mathbf { x } _ { t } \right] = 0 . +$$ + +By leveraging this property, we can simplify $c ^ { \prime } ( t )$ into an expression involving $\mathbf { w } _ { t } = \mathbf { z } - \mathbb { E } [ \mathbf { z } \mid \mathbf { x } _ { t } ]$ , the residual between the true noise $\mathbf { z }$ and its prediction given $\mathbf { x } _ { t }$ . This residual captures the uncertainty in predicting $\mathbf { z }$ based on $\mathbf { x } _ { t }$ , allowing us to relate $c ^ { \prime } ( t )$ directly to the prediction accuracy of $\mathring { f }$ . + +Lemma 1 (Transport cost of GC coupling). Assume that $\mathring { f }$ is a continuously differentiable function representing the ground-truth consistency model, i.e. the flow of the PF-ODE induced by the diffusion process $\mathbf { x } _ { t }$ . Define $\begin{array} { r } { { \bf w } _ { t } = { \bf z } - \mathbb { E } [ { \bf z } | { \bf x } _ { t } ] = { \bf \Xi } } \end{array}$ $\begin{array} { r } { \frac { 1 } { \dot { \sigma } _ { t } } \big ( \dot { \mathbf { x } } _ { t } - \mathbb { E } [ \dot { \mathbf { x } } _ { t } \mid \mathbf { x } _ { t } ] \big ) } \end{array}$ . Then: + +$$ +c ^ { \prime } ( t ) = - 2 \dot { \sigma } _ { t } \mathbb { E } \left[ \left. \frac { \partial \hat { f } } { \partial \mathbf { x } } ( \mathbf { x } _ { t } , \sigma _ { t } ) \cdot \mathbf { w } _ { t } , \mathbf { w } _ { t } \right. \right] . +$$ + +Proof. Note that the inverse flow $\mathring { \mathbf { f } } ^ { - 1 } ( \mathbf { y } , \sigma _ { t } )$ transports the initial point $\mathbf { y }$ at time $t = 0$ along the vector field $\mathbf { v } _ { t }$ up to time $t$ . Consequently, $\mathring { f } ^ { - 1 }$ is a flow with the corresponding vector field $\mathbf { v } _ { t }$ : + +$$ +\frac { \partial } { \partial t } [ \mathring { \mathbf { f } } ^ { - 1 } ( \mathbf { y } , \sigma _ { t } ) ] = \mathbf { v } _ { t } ( \mathring { \mathbf { f } } ^ { - 1 } ( \mathbf { y } , \sigma _ { t } ) ) . +$$ + +By differentiating both sides of the identity $\mathbf { y } = \mathring { f } ( \mathring { f } ^ { - 1 } ( \mathbf { y } , \sigma _ { t } ) , \sigma _ { t } )$ with respect to $t$ and applying the above observation, we get: + +$$ +\begin{array} { l } { { 0 = \displaystyle \frac { \partial } { \partial t } \left[ \bar { f } ( \bar { f } ^ { - 1 } ( { \bf y } , \sigma _ { t } ) , \sigma _ { t } ) \right] } } \\ { { \displaystyle = \frac { \partial \bar { f } } { \partial \sigma } ( \bar { f } ^ { - 1 } ( { \bf y } , \sigma _ { t } ) , \sigma _ { t } ) \dot { \sigma } _ { t } + \frac { \partial \bar { f } } { \partial { \bf x } } ( \bar { f } ^ { - 1 } ( { \bf y } , \sigma _ { t } ) , \sigma _ { t } ) \cdot \frac { \partial } { \partial t } [ \bar { f } ^ { - 1 } ( { \bf y } , \sigma _ { t } ) ] } } \\ { { \displaystyle = \frac { \partial \bar { f } } { \partial \sigma } ( { \bf x } , \sigma _ { t } ) \dot { \sigma } _ { t } + \frac { \partial \bar { f } } { \partial { \bf x } } ( { \bf x } , \sigma _ { t } ) \cdot { \bf v } _ { t } ( { \bf x } ) } , } \end{array} +$$ + +where the substitution $\mathbf { x } = \mathring { \mathbf { f } } ^ { - 1 } ( \mathbf { y } , \sigma _ { t } )$ is used in the last step. Consequently, + +$$ +\begin{array} { r l } & { c ^ { \prime } ( t ) = 2 \mathbb { E } \left[ \left. \frac { \partial \hat { \boldsymbol { \mu } } } { \partial t } [ \dot { f } ( \mathbf { x } _ { t } , \sigma _ { t } ) ] , \dot { f } ( \mathbf { x } _ { t } , \sigma _ { t } ) - \mathbf { z } \right. \right] } \\ & { \quad \quad = 2 \mathbb { E } \left[ \left. \frac { \partial \hat { \boldsymbol { f } } } { \partial \sigma } ( \mathbf { x } _ { t } , \sigma _ { t } ) \dot { \sigma } _ { t } + \frac { \partial \hat { \boldsymbol { f } } } { \partial \mathbf { x } } ( \mathbf { x } _ { t } , \sigma _ { t } ) \cdot \dot { \mathbf { x } } _ { t } , \dot { f } ( \mathbf { x } _ { t } , \sigma _ { t } ) - \mathbf { z } \right. \right] } \\ & { \quad \quad = 2 \mathbb { E } \left[ \left. \frac { \partial \hat { \boldsymbol { f } } } { \partial \mathbf { x } } ( \mathbf { x } _ { t } , \sigma _ { t } ) \cdot ( \dot { \mathbf { x } } _ { t } - \mathbf { v } _ { t } ( \mathbf { x } ) ) , \dot { f } ( \mathbf { x } _ { t } , \sigma _ { t } ) - \mathbf { z } \right. \right] } \\ & { \quad \quad = 2 \dot { \sigma } _ { t } \mathbb { E } \left[ \left. \frac { \partial \hat { \boldsymbol { f } } } { \partial \mathbf { x } } ( \mathbf { x } _ { t } , \sigma _ { t } ) \cdot ( \mathbf { z } - \mathbb { E } [ \mathbf { z } | \mathbf { x } _ { t } ] ) , \dot { f } ( \mathbf { x } _ { t } , \sigma _ { t } ) - \mathbf { z } \right. \right] , } \end{array} +$$ + +where we used the relations $\mathbf { x } _ { t } = \mathbf { x } _ { \star } + \sigma _ { t } \mathbf { z }$ and $\mathbf { v } _ { t } ( \mathbf { x } ) = \mathbb { E } [ \dot { \mathbf { x } } _ { t } | \mathbf { x } _ { t } ]$ . Now, let $\mathbf { w } _ { t } = \mathbf { z } - \mathbb { E } [ \mathbf { z } \mid \mathbf { x } _ { t } ]$ . Then $\mathbb { E } [ { \mathbf w } _ { t } \ | \ { \mathbf x } _ { t } ] = 0$ , hence by an application of the law of iterated expectations, $\mathbb { E } [ \langle \mathbf { w } _ { t } , g ( \mathbf { x } _ { t } ) \rangle ] = 0$ for essentially any function $g : \mathbb { R } ^ { d } \mathbb { R } ^ { d }$ . Using this, we can further simplify the last line as: + +$$ +c ^ { \prime } ( t ) = - 2 \dot { \sigma } _ { t } \mathbb { E } \left[ \left. \frac { \partial \hat { f } } { \partial \mathbf { x } } ( \mathbf { x } _ { t } , \sigma _ { t } ) \cdot \mathbf { w } _ { t } , \mathbf { z } \right. \right] = - 2 \dot { \sigma } _ { t } \mathbb { E } \left[ \left. \frac { \partial \hat { f } } { \partial \mathbf { x } } ( \mathbf { x } _ { t } , \sigma _ { t } ) \cdot \mathbf { w } _ { t } , \mathbf { w } _ { t } \right. \right] , +$$ + +proving the desired equality. + +An immediate consequence of this lemma is that $c ( t )$ decreases for small $t$ : + +Corollary 1 (Decreasing transport cost of GC coupling in $t \to 0 ^ { + }$ ). There exists a $t _ { * } > 0$ such that for all $t \in [ 0 , t _ { * } ]$ , the derivative of $c ( t )$ takes the form $c ^ { \prime } ( t ) = - 2 \dot { \sigma } _ { t } a _ { t }$ with $a _ { t } > 0$ . Hence for $\dot { \sigma } _ { t }$ positive, the cost is decreasing. In particular, in the EDM setting where $\sigma _ { t } = t$ , $c ( t )$ is decreasing for small $t$ . + +Proof. The proof of this corollary proceeds by noting that for $t = 0$ , the consistency model $\mathring { \mathbf { f } } ( \mathbf { x } , t )$ is an identity function, its Jacobian is an identity matrix leading to $a _ { t } = \mathbb { E } [ \| \mathbf { w } _ { t } \| ^ { 2 } ] > 0$ and by assumption, all the elements of the Jacobian are continuous. By continuity of $a _ { t } , t _ { * }$ exists and invoking intermediate value theorem on $a _ { t }$ concludes the proof. □ + +The next result is the statement about the asymptotic behavior of the transport cost $c ( t )$ in the large- $\cdot t$ regime. + +Corollary 2 (Decreasing transport cost of GC coupling in $t \approx t _ { \mathrm { m a x } }$ ). Assume that the consistency model $\mathring { f } ( x , \sigma )$ is a scaling function increasing. $\begin{array} { r } { \mathring { \pmb { f } } ( \mathbf { x } , \sigma _ { t } ) = \frac { \sigma _ { 0 } } { \sigma _ { t } } \mathbf { x } } \end{array}$ . Then, we have $\begin{array} { r } { c ^ { \prime } ( t ) = - \frac { 2 \dot { \sigma } _ { t } \sigma _ { 0 } } { \sigma _ { t } } \mathbb { E } [ \| \mathbf { w } _ { t } \| ^ { 2 } ] } \end{array}$ . In particular, $c ( t )$ is decreasing whenever $\sigma _ { t }$ is + +Proof. Under the assumption, we have $\begin{array} { r } { \frac { \partial \hat { f } } { \partial \mathbf { x } } = \frac { \sigma _ { 0 } } { \sigma _ { t } } \mathbf { I } } \end{array}$ . Thus, by Lemma 1, + +$$ +c ^ { \prime } ( t ) = - 2 \dot { \sigma } _ { t } \mathbb { E } \left[ \left. \frac { \sigma _ { 0 } } { \sigma _ { t } } \mathbf { I w } _ { t } , \mathbf { w } _ { t } \right. \right] = - \frac { 2 \dot { \sigma } _ { t } \sigma _ { 0 } } { \sigma _ { t } } \mathbb { E } [ \| \mathbf { w } _ { t } \| ^ { 2 } ] . +$$ + +This proves that $c ^ { \prime } ( t ) < 0$ whenever $\dot { \sigma } _ { t } > 0$ . + +Toy example. Let us consider a one-dimensional toy example where $\mathbf { x } _ { \star } \sim \mathcal { N } ( 0 , \sigma _ { \star } ^ { 2 } )$ with $\sigma _ { \star } \geq 0$ and $\mathbf { z } \sim \mathcal { N } ( 0 , 1 )$ are independent. Also, we assume $p _ { t } = \mathcal N ( 0 , \sigma _ { \star } ^ { 2 } + \sigma _ { t } ^ { 2 } )$ , so the vector field for the diffusion process ding target diffusion flow and the transport $\sigma _ { 0 } = 0$ for the sake of simplicity. In this case, the marginal law of $\mathbf { x } _ { t }$ is calculated as st function are: $\begin{array} { r } { \mathbf { v } _ { t } ( \mathbf { x } ) = - \dot { \sigma } _ { t } \sigma _ { t } \nabla _ { \mathbf { x } } \log p _ { t } ( \mathbf { x } ) = \frac { \dot { \sigma } _ { t } \sigma _ { t } } { \sigma _ { \star } ^ { 2 } + \sigma _ { t } ^ { 2 } } \mathbf { x } } \end{array}$ $\mathbf { x } _ { t }$ is also Gaussian with + +$$ +\hat { \pmb f } ( \mathbf { x } , \sigma _ { t } ) = \frac { \sigma _ { \star } } { \sqrt { \sigma _ { \star } ^ { 2 } + \sigma _ { t } ^ { 2 } } } \mathbf { x } \quad \mathrm { a n d } \quad c ( t ) = \sigma _ { \star } ^ { 2 } + 1 - \frac { 2 \sigma _ { \star } \sigma _ { t } } { \sqrt { \sigma _ { \star } ^ { 2 } + \sigma _ { t } ^ { 2 } } } . +$$ + +We note that decreasing in $\mathring { f } ( \mathbf { x } , \sigma _ { t } )$ s indeed a scaling function which is asymptotically proportional to . $\frac { \mathbf { x } } { \sigma _ { t } }$ for large $t$ , and $c ( t )$ is $t$ $t > 0$ + +Experimental validation. We validation the transport cost decrease in Figure 6, on a toy dataset composed of two 2D-Diracs, and on CIFAR-10. Interestingly, we observe that when computing OT transport plans between batches instead of on the full data, GC allows to reduce transport cost more than batch-OT. + +# A.4. Proxy Term + +In this part, we clarify the connection between the proxy term and the in the case of the quadratic loss $( \alpha = 2$ ). Indeed, we can bound the regularization term with the proxy term thanks to the Jacobian’s maximum singular value $s _ { \operatorname* { m a x } } \big ( \frac { \partial f _ { \theta } } { \partial x } \big )$ , which is bounded as typical networks are Lipschitz: + +$$ +\left\| \frac { \partial f _ { \theta } } { \partial \mathbf { x } } ( \mathbf { x } _ { t } , \sigma _ { t } ) \left( \dot { \mathbf { x } } _ { t } - \mathbf { v } _ { t } ( \mathbf { x } _ { t } ) \right) \right\| ^ { 2 } \leq \left\| \frac { \partial f _ { \theta } } { \partial x } \right\| ^ { 2 } \| \dot { x } _ { t } - \mathbf { v } _ { t } ( x _ { t } ) \| ^ { 2 } \leq s _ { \operatorname* { m a x } } ^ { 2 } ( \frac { \partial f _ { \theta } } { \partial \mathbf { x } } ) \| \dot { \mathbf { x } } _ { t } - \mathbf { v } _ { t } ( \mathbf { x } _ { t } ) \| ^ { 2 } +$$ + +![](images/figures/generator-augmented-flows-fig-0006.jpg) +Transport Cost on One-Dimensional 2-Diracs + +![](images/figures/generator-augmented-flows-fig-0007.jpg) +Figure 6. Comparison of transport costs between IC, batch-OT, and GC on two 2D-Diracs (left) and CIFAR-10 (right). + +Algorithm 1 Training of consistency models with generator-augmented trajectories + +Input: Randomly initialized consistency model $f _ { \theta }$ , number of timesteps $N$ , noise schedule $\sigma _ { t _ { i } }$ , loss weighting $\overline { { \lambda ( \cdot ) } }$ learning rate $\eta$ , distance function $\mathcal { D }$ , noise distribution $p _ { z }$ , joint learning parameter $\mu$ . + +Output: Trained consistency model $f _ { \theta }$ + +while not converged do $\begin{array} { r l } & { \mathbf { x } _ { \star } \sim p _ { \star } , \ \mathbf { z } \sim p _ { z } } \\ & { i \sim \mathrm { m u l t i n o m i a l } ( p ( \sigma _ { t _ { 0 } } ) , \dots , p ( \sigma _ { t _ { N } } ) ) } \\ & { m \sim \mathrm { b i n o m i a l } ( \mu , \mathbf { s i z e } { = } \mathrm { b a t c h } _ { \mathrm { - } } \mathrm { s i z e } ) } \\ & { \mathbf { x } _ { t _ { i } } \mathbf { x } _ { \star } + \sigma _ { t _ { i } } \mathbf { z } } \\ & { \hat { \mathbf { x } } _ { t _ { i } } \mathbf { s g } \big ( f _ { \theta } \big ( \mathbf { x } _ { t _ { i } } , \sigma _ { t _ { i } } \big ) \big ) } \\ & { \hat { \mathbf { x } } _ { t _ { i } } m \cdot \hat { \mathbf { x } } _ { t _ { i } } + ( 1 - m ) \cdot \mathbf { x } _ { \star } } \\ & { \tilde { \mathbf { x } } _ { t _ { i } } \hat { \mathbf { x } } _ { t _ { i } } + \sigma _ { t _ { i } } \mathbf { z } , \ \tilde { \mathbf { x } } _ { t _ { i + 1 } } \hat { \mathbf { x } } _ { t _ { i } } + \sigma _ { t _ { i + 1 } } \mathbf { z } } \\ & { \mathcal { L } ( \theta ) = \lambda \big ( \sigma _ { t _ { i } } ) \mathcal { D } \big ( \mathbf { s g } \big ( f _ { \theta } \big ( \tilde { \mathbf { x } } _ { t _ { i } } , \sigma _ { t _ { i } } \big ) \big ) , f _ { \theta } \big ( \tilde { \mathbf { x } } _ { t _ { i + 1 } } , \sigma _ { t _ { i + 1 } } \big ) \big ) } \\ & { \theta \theta \eta \nabla _ { \theta } \mathcal { L } ( \theta ) } \end{array}$ batch of real data and noise vectors {sampling timesteps} {mask of dimension (batch size) with each $m _ { j } \sim$ binomial $( \mu ) \}$ {IC intermediate points $\}$ {endpoint prediction from the model} {mixing IC and GC trajectories} {GC intermediate points} {consistency loss} {update model’s weights} +end while + +We could also use some assumptions on $f$ , e.g. the fact that it is close to a scaling function for large $t$ (see Corrolary 2). If $\begin{array} { r } { \pmb { f } ( \mathbf { x } , \sigma _ { t } ) = \frac { \sigma _ { 0 } } { \sigma _ { t } } \mathbf { x } } \end{array}$ , then we would have: + +$$ +\left\| \frac { \partial f _ { \theta } } { \partial \mathbf { x } } ( \mathbf { x } _ { t } , \sigma _ { t } ) \left( \dot { \mathbf { x } } _ { t } - \mathbf { v } _ { t } ( \mathbf { x } _ { t } ) \right) \right\| ^ { 2 } = ( \frac { \sigma _ { 0 } } { \sigma _ { t } } ) ^ { 2 } \| \dot { \mathbf { x } } _ { t } - \mathbf { v } _ { t } ( \mathbf { x } _ { t } ) \| ^ { 2 } . +$$ + +# B. Algorithm + +We present the detailed algorithm for GC ( $\mathbf { \nabla } \mu = \cdot \mathbf { \nabla } )$ in Algorithm 1. + +# C. Additional Results + +# C.1. Ablation Studies + +Understanding why $\mathbf { G C } ( \mu = 1 )$ ) fails. This experiment involves training a consistency model with $\mathbf { G C } ( \mu = 1 )$ ). As shown in Figure 7(a), we observe that these models converge quickly but reach saturation early in the training process. When applying the timestep scheduling method with an increasing number of timesteps from Song and Dhariwal (2024), the FID of the models worsens. Using a fixed number of timesteps prevents divergence of the FID, but it still plateaus at a higher FID than iCT-IC. + +In Figure 7(b), we plot the FID per timestep for three model / trajectory pairs: $\mathbf { G C } ( \mu = 1 )$ -model on IC trajectories, $\mathrm { G C } ( \mu = 1 )$ -model on GC trajectories, and IC-model on IC trajectories. Notably, we observe a distribution shift between IC and GC trajectories: the FID of the GC-model on IC trajectories degrades at the intermediate timesteps of the diffusion process. This highlights why deviating from the theory and training a model exclusively on GC trajectories is insufficient: to build $\mathbf { x } _ { t _ { i } }$ in Equation (13), the model is inferred on IC but trained on GC trajectories. If IC and GC differ too much, the model cannot improve on IC. + +![](images/figures/generator-augmented-flows-fig-0008.jpg) +Figure 7. Analysis of consistency models trained only with GC on CIFAR-10. (a) When trained with only GC trajectories, consistency models does not reach the performance of the base model (iCT-IC). In (b), we show that is linked to a distribution shift problem: GC models are weak on IC trajectoires, thus are sub-optimal for predicting $\hat { \mathbf { x } } _ { t _ { i } }$ required in their own training (Equation (13)). + +![](images/figures/generator-augmented-flows-fig-0009.jpg) +(b) FID of trained IC vs GC along trajectories. + +Table 3. Analysis of performance with regards to some hyper-parameters of iCT-GC $\mu = 0 . 5 )$ on CIFAR-10. + +
ModelFID
iCT-IC7.42 ± 0.04
iCT-GC (µ = 0.5) iso-time6.38 ± 0.03
iCT-GC (µ = 0.5)5.95 ± 0.05
iCT-GC(µ = 0.5) + dropout7.77 ± 0.04
iCT-GC (µ = 0.5) - EMA6.73 ± 0.05
+ +Iso wall-clock training time. As illustrated above, consistency models trained with GC converge faster than IC. However, each training step is more time-consuming, as it necessitates a forward evaluation of the consistency model without gradient computation. Regarding wall-clock training time, the computational overhead of iCT-GC is approximately $20 \%$ of the iCT-IC. In top part of Table 3, we report under “iCT-GC $\mu = 0 . 5$ ) iso-time” the results of iCT-GC $\mu = 0 . 5$ ) trained with the same wall-clock duration as iCT-IC. Even when considering wall-clock training time, iCT-GC ( $\mu = 0 . 5 )$ ) is still superior to iCT-IC. + +Hyper-parameters. We evaluate the influence of two important hyper-parameters. First, the dropout in the learned model. Second, whether to use or not the EMA to compute GC endpoints $\hat { \bf x }$ . The results are presented in the bottom part of Table 3. Interestingly, the results on dropout are opposite to those found by Song and Dhariwal (2024), since using dropout lowers the performance of iCT-GC $\langle \mu = 0 . 5$ ). + +Analysis of $\mu$ on ImageNet. We present further results of the joint learning procedure with varying $\mu \left( \left\{ 0 . 3 , 0 . 5 , 0 . 7 , 1 . \right\} \right)$ on ImageNet-32 in Figure 9. For $\mu = \{ 0 . 3 , 0 . 5 \}$ , iCT-GC outperforms the base model iCT-IC. + +# C.2. Visual Results + +We include in Figure 8 examples of generated images for considered baselines. + +![](images/figures/generator-augmented-flows-fig-0010.jpg) +Figure 8. Uncurated samples from consistency models trained on CelebA $6 4 \times 6 4$ for fixed noise vectors. Note that models trained with generator-augmented trajectories tend to generate sharper images. + +![](images/figures/generator-augmented-flows-fig-0011.jpg) +Figure 9. Results of varying $\mu$ for iCT-GC on ImageNet-32. + +# D. Experimental Details + +The code is based on the PyTorch library (Paszke et al., 2019). + +Scheduling functions and hyperparameters from Song and Dhariwal (2024). The training of consistency models heavily rely on several scheduling functions. First, there is a noise schedule $\{ \sigma _ { i } \} _ { i = 0 } ^ { N }$ which is chosen as in Karras et al. (2022). Precisely, $\begin{array} { r } { \sigma _ { i } = \left( \sigma _ { 0 } ^ { \frac { 1 } { \rho } } + \frac { i } { N } ( \sigma _ { \mathrm { N } } ^ { \frac { 1 } { \rho } } - \sigma _ { 0 } ^ { \frac { 1 } { \rho } } ) \right) ^ { \rho } } \end{array}$ with $\rho = 7$ . Second, there is a weighting function that affects the training loss, chosen as $\begin{array} { r } { \lambda ( \sigma _ { i } ) = \frac { 1 } { \sigma _ { i + 1 } - \sigma _ { i } } } \end{array}$ . Combined with the choice of noise schedule, it emphasizes to be consistent on timesteps with low noise. Then, Song et al. (2023) propose to progressively increase the number of timesteps $N$ during training. Song and Dhariwal (2024) argue that a good choice of dicretization schedule is an exponential one: $N ( k ) = \operatorname* { m i n } ( s _ { 0 } 2 ^ { \lfloor \frac { k } { K ^ { \prime } } \rfloor } , s _ { 1 } ) + 1$ where $\begin{array} { r } { K ^ { \prime } = \lfloor \frac K { \log _ { 2 } \left[ s _ { 1 } / s _ { 0 } \right] + 1 } \rfloor } \end{array}$ , $K$ is the total number of training steps, $k$ is the current training step, $s _ { 0 }$ (respectively $s _ { 1 }$ the initial (respectively final) number of timesteps. Finally, Song and Dhariwal (2024) propose a discrete probability distribution on the timesteps which mimics the continuous probability distribution recommended in the continuous training of score-based models by Karras et al. (2022).Song and Dhariwal (2024) recommend using: $\begin{array} { r } { p ( \sigma _ { i } ) \propto \mathrm { e r f } ( \frac { \log ( \sigma _ { i + 1 } ) - P _ { \mathrm { m e a n } } } { \sqrt { 2 } P _ { \mathrm { s t d } } } ) - \mathrm { e r f } ( \frac { \log ( \sigma _ { i } ) - P _ { \mathrm { m e a n } } } { \sqrt { 2 } P _ { \mathrm { s t d } } } ) } \end{array}$ . In practice, $s _ { 0 } = 1 0$ $s _ { 1 } = 1 2 8 0$ $\rho = 7$ $P _ { \mathrm { m e a n } } = - 1 . 1$ $P _ { \mathrm { s t d } } = 2 . 0$ + +We use the lion optimizer (Chen et al., 2023) implemented from https://github.com/lucidrains/lion-pytorch. + +Selection of hyper-parameter $\mu$ . We have selected $\mu$ based on the results from Figure 5, which presents a grid search for $\mu$ on CIFAR-10. Given the bell-shaped relationship observed between $\mu$ and FID, we opted to retain the best performing value identified on CIFAR-10, $\mu = 0 . 5$ , for all subsequent experiments (Table 1), including those on other datasets, without further tuning. Importantly, even without an exhaustive hyperparameter search, our method consistently outperforms baseline approaches. This choice is validated by the ablation study presented in Appendix C.1 showing similar trend for another dataset, showing that the hyper-parameter $\mu$ is easy to tune. + +In the ECT setting, we found that $\mu < 0 . 5$ leads to improved performance, while $\mu > 0 . 5$ can degrade final performance. +Overall, we recommend setting $\mu$ to small values (around 0.3) since it leads to improved performance in all our experiments. + +Details on neural networks architectures. We use the ${ \mathrm { N C S N } } { + + }$ architecture (Song et al., 2021) and follow the implementation from https://github.com/NVlabs/edm. + +Evaluation metrics. We report the FID, KID and IS. For the three different metrics, we rely on the implementation from TorchMetrics (Skafte Detlefsen et al., 2022). For the three different metrics, we use the standard practice (e.g. (Song and Dhariwal, 2024)) of FID which is to compare sets of 50 000 generated versus training images. Confidence intervals reported in Table 1 are averaged on five runs by sampling new sets of training images, and new sets of generated images from the same model. + +Datasets. CIFAR-10 is a dataset introduced in Krizhevsky (2009). ImageNet (Deng et al., 2009), CelebA (Liu et al., 2015), and LSUN Church (Yu et al., 2015) are used respectively at $3 2 \times 3 2$ , $6 4 \times 6 4$ and $6 4 \times 6 4$ resolutions. We preprocess these images by resizing smaller side to the desired value, center cropping, and linearly scaling pixel values to $[ - 1 , 1 ]$ . + +Details on computational ressources As mentioned in the paper, the image dataset experiments have been conducted on NVIDIA A100 40GB GPUs. + +Table 4. Hyperparameters for CIFAR-10. Arrays indicate quantities per resolution of the UNet model. $\{ \}$ indicate an hyper-parameter search performed for each type of model (iCT, iCT-OT, iCT-GC $( \mu = 0 . 5 )$ ). + +
HyperparameterValue
batch size512
image resolution32
training steps100 000
learning rate{0.0001, 0.00003}
optimizerlion
S010
S11280
ρ7
σ00.002
σ180
network architectureSongUNet (from (Karras et al., 2022) implementation)
model channels128
dropout{0., 0.3}
num blocks3
embedding typepositional
channel multiplicative factor[1, 2, 2]
attn resolutionsØ
+ +Table 5. Hyperparameters for CelebA and LSUN Church. Arrays indicate quantities per resolution of the UNet model. $\{ \}$ indicate an hyper-parameter search performed for each type of model (iCT, iCT-OT, iCT-GC $( \mu = 0 . 5 )$ ). + +
HyperparameterValue
batch size128
image resolution64
training steps150 000
learning rate0.00008
optimizerlion
S010
S11280
ρ7
σ00.002
σ180
network architectureSongUNet
model channels(from (Karras et al., 2022) implementation) 128
dropout{0., [0., 0., 0.2, 0.2]}
num blocks[3, 3, 4, 5]
embedding typepositional
channel multiplicative factor[1, 2, 2, 2]
attn resolutionsØ
+ +Table 6. Hyperparameters for ImageNet-1k. Arrays indicate quantities per resolution of the UNet model. $\{ \}$ indicate an hyper-parameter search performed for each type of model (iCT, iCT-OT, iCT-GC $( \mu = 0 . 5 )$ ). + +
HyperparameterValue
batch size512
image resolution32
training steps150 000
learning rate0.00008
optimizerlion
S010
S11280
ρ7
σ00.002
σ180
network architectureSongUNet (from (Karras et al., 2022) implementation)
model channels128
dropout{0., [0., 0., 0.2, 0.2]}
num blocks[3, 5, 7]
embedding typepositional
channel mult[1, 1, 2]
attn resolutions[16]
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Discrete form: x_{t_i} = x_star + sigma_{t_i} * z with t_0=0, t_N=T.", + "source": "Section 1 Notation, Section 2.1" + }, + { + "id": "generator-augmented-flows-D2-002", + "claim": "SamplePathDerivative: dot_x_t = d(x_star + sigma_t * z) / dt = dot_sigma_t * z, representing the single-sample Monte Carlo estimate of the velocity field (used in consistency training).", + "source": "Section 2.1" + }, + { + "id": "generator-augmented-flows-D2-003", + "claim": "VelocityFieldScoreForm: v_t(x) = -dot_sigma_t * sigma_t * grad_x log p_t(x), the velocity field of the probability flow ODE expressed via the score function.", + "source": "Section 2.1" + }, + { + "id": "generator-augmented-flows-D2-004", + "claim": "VelocityFieldEDMForm: v_t(x) = -t * grad_x log p_t(x), the EDM (Karras et al. 2022) special case where sigma_t = t and dot_sigma_t = 1.", + "source": "Section 2.1" + }, + { + "id": "generator-augmented-flows-D2-005", + "claim": "VelocityFieldDenoiserForm: v_t(x_t) = (1/t) * (x_t - D_star(x_t, t)), where D_star(x_t, t) = E[x_star | x_t] is the optimal denoiser (minimum MSE estimator of clean data given noisy observation).", + "source": "Section 2.1, Section 4.2.1" + }, + { + "id": "generator-augmented-flows-D2-006", + "claim": "VelocityFieldGeneralDenoiserForm: v_t(x) = (dot_sigma_t / sigma_t) * (x - D(x, sigma_t)), the general relationship between the velocity field and a denoiser D for arbitrary monotonic sigma_t.", + "source": "Appendix A.2, Lemma 2" + }, + { + "id": "generator-augmented-flows-D2-007", + "claim": "VelocityFieldApproximationWithDenoiser: dot_x_t - v_t(x_t) ≈ z - (1/t) * (x_t - D_phi(x_t, t)), where D_phi is a trained denoiser network. In the EDM setting, this approximates the discrepancy between the sample path derivative and the true velocity field.", + "source": "Section 4.2.1" + }, + { + "id": "generator-augmented-flows-D2-008", + "claim": "ConsistencyModelParametrization: f_theta(x_{t_i}, sigma_{t_i}) = c_skip(sigma_{t_i}) * x_{t_i} + c_out(sigma_{t_i}) * F_theta(x_{t_i}, sigma_{t_i}), where F_theta is a neural network (SongUNet). This parametrization enforces the boundary condition f_theta(x_0, sigma_0) = x_0 since c_skip(0)=1 and c_out(0)=0.", + "source": "Section 2.2, Eq 3" + }, + { + "id": "generator-augmented-flows-D2-009", + "claim": "SkipCoefficient: c_skip(sigma) = sigma_d^2 / (sigma_d^2 + (sigma - sigma_0)^2), where sigma_d^2 is the variance of the data distribution.", + "source": "Section 2.2, Eq 3" + }, + { + "id": "generator-augmented-flows-D2-010", + "claim": "OutCoefficient: c_out(sigma) = sigma * sqrt(d) * (sigma - sigma_0) / sqrt(sigma_d^2 + sigma^2), where d is the data dimensionality and sigma_d^2 is the data variance.", + "source": "Section 2.2, Eq 3" + }, + { + "id": "generator-augmented-flows-D2-011", + "claim": "EulerDiscretizationStep: x_{t_i}^Phi = Phi(x_{t_{i+1}}, t_{i+1}) = x_{t_{i+1}} + (t_i - t_{i+1}) * v_{t_{i+1}}(x_{t_{i+1}}), used in consistency distillation to advance one step backward along the PF-ODE. Note that t_i < t_{i+1} but sigma increases with t, so this is a backward Euler step in time.", + "source": "Section 2.2, Eq 5" + }, + { + "id": "generator-augmented-flows-D2-012", + "claim": "ConsistencyDistillationLoss: L_CD(theta) = E_{q_I(x_star, z), p(x_{t_{i+1}} | x_star, z)} [ lambda(sigma_{t_i}) * D( sg(f_theta(x_{t_i}^Phi, sigma_{t_i})), f_theta(x_{t_{i+1}}, sigma_{t_{i+1}}) ) ], where x_{t_i}^Phi is from the Euler discretization step using a pre-trained score/velocity model, sg is stop-gradient, and D is a distance function (typically squared L2 or Huber).", + "source": "Section 2.2, Eq 4" + }, + { + "id": "generator-augmented-flows-D2-013", + "claim": "ConsistencyTrainingLoss: L_CT(theta) = E_{q_I(x_star, z), p(x_{t_i}, x_{t_{i+1}} | x_star, z)} [ lambda(sigma_{t_i}) * D( sg(f_theta(x_{t_i}, sigma_{t_i})), f_theta(x_{t_{i+1}}, sigma_{t_{i+1}}) ) ], where x_{t_i} = x_star + sigma_{t_i} * z and x_{t_{i+1}} = x_star + sigma_{t_{i+1}} * z replace x_{t_i}^Phi from distillation — using the single-sample Monte Carlo estimate dot_x_t instead of the true v_t. This is the IC (independent coupling) baseline loss.", + "source": "Section 2.2, Eq 6" + }, + { + "id": "generator-augmented-flows-D2-014", + "claim": "GCIntermediateSampling: Step 1 of generator-augmented coupling — (x_star, z) ~ q_I (independent coupling); x_{t_i} = x_star + sigma_{t_i} * z (IC intermediate point); x_hat_{t_i} = sg(f_hat(x_{t_i}, sigma_{t_i})) (endpoint prediction via consistency model with stop-gradient, where f_hat is the predictor model).", + "source": "Section 4.1, Eq 13" + }, + { + "id": "generator-augmented-flows-D2-015", + "claim": "GCIntermediatePoints: Step 2 of generator-augmented coupling — reuse the same noise z with the predicted endpoint: (x_hat_{t_i}, z) ~ q (GC coupling); x_tilde_{t_i} = x_hat_{t_i} + sigma_{t_i} * z; x_tilde_{t_{i+1}} = x_hat_{t_i} + sigma_{t_{i+1}} * z. Boundary conditions: p(x_tilde_0) ≈ p_star (due to consistency model boundary condition), p(x_tilde_T) ≈ p(sigma_T * z) (noise dominates at large t).", + "source": "Section 4.1, Eq 14" + }, + { + "id": "generator-augmented-flows-D2-016", + "claim": "GCConsistencyTrainingLoss: L_GC(theta) = E_{q(x_hat_{t_i}, z), p(x_tilde_{t_i}, x_tilde_{t_{i+1}} | x_hat_{t_i}, z)} [ lambda(sigma_{t_i}) * D( sg(f_theta(x_tilde_{t_i}, sigma_{t_i})), f_theta(x_tilde_{t_{i+1}}, sigma_{t_{i+1}}) ) ], where x_tilde_{t_i} and x_tilde_{t_{i+1}} are constructed via GC coupling. Note the velocity term is dot_sigma_t * z (not dot_x_tilde_t) because x_tilde_{t_{i+1}} - x_tilde_{t_i} = (sigma_{t_{i+1}} - sigma_{t_i}) * z.", + "source": "Section 4.1, Eq 15" + }, + { + "id": "generator-augmented-flows-D2-017", + "claim": "JointLearningLoss: L_GC-mu(theta) = mu * L_GC(theta) + (1 - mu) * L_CT(theta), where mu in [0, 1] is the joint learning factor. At each training step, a per-sample binomial mask m_j ~ Binomial(mu) determines which samples use GC trajectories (m_j=1: use x_hat_{t_i} as endpoint; m_j=0: use x_star as endpoint for IC). mu=0 recovers iCT-IC, mu=1 is pure GC (not recommended).", + "source": "Section 5.2, Eq 18, Algorithm 1" + }, + { + "id": "generator-augmented-flows-D2-018", + "claim": "ProxyRegularizer: R_tilde_t = E[ || dot_x_t - v_t(x_t) ||^2 ], measures expected squared distance between the true velocity field and its one-sample Monte Carlo estimate dot_x_t. This is a proxy for the discrepancy regularizer R(theta) identified in Theorem 1 for the alpha=2 case.", + "source": "Section 4.2.1, Eq 16" + }, + { + "id": "generator-augmented-flows-D2-019", + "claim": "TransportCost: c(t) = E_{q_I(x_star, z)}[ || f_ring(x_t, sigma_t) - z ||^2 ], the expected quadratic distance between the noise z and the consistency model output f_ring(x_t, sigma_t) (predicted endpoint). c(0) = E[||x_star - z||^2] is the IC transport cost; c(t) for t>0 represents the GC transport cost.", + "source": "Section 4.2.2, Eq 17" + }, + { + "id": "generator-augmented-flows-D2-020", + "claim": "ProxyRegularizerIC: R_tilde_{t,IC} = (dot_sigma_t / sigma_t)^2 * E[ || x_star - D(x_t, sigma_t) ||^2 ], where D(x_t, sigma_t) = E[x_star | x_t] is the optimal denoiser. Derived from dot_x_t - v_t(x_t) = -(dot_sigma_t / sigma_t) * (x_star - D(x_t, sigma_t)).", + "source": "Appendix A.2, Eq 63" + }, + { + "id": "generator-augmented-flows-D2-021", + "claim": "ProxyRegularizerGC: R_tilde_{t,GC} = (dot_sigma_t / sigma_t)^2 * E[ || x_hat_t - E[x_hat_t | x_tilde_t] ||^2 ]. Unlike IC, as t → infinity, x_hat_t is approximately a deterministic function of x_tilde_t (both dominated by z), so E[x_hat_t | x_tilde_t] ≈ x_hat_t, making R_tilde_{t,GC} decay faster than R_tilde_{t,IC}.", + "source": "Appendix A.2, Eq 67" + }, + { + "id": "generator-augmented-flows-D2-022", + "claim": "NoiseScheduleKarras: sigma_i = (sigma_0^{1/rho} + (i/N) * (sigma_N^{1/rho} - sigma_0^{1/rho}))^rho, where rho=7, sigma_0=0.002, sigma_N=80 (and sigma_1=80). This is the Karras et al. 2022 noise schedule used for constructing the discrete timestep grid {sigma_i}_{i=0}^N.", + "source": "Appendix D" + }, + { + "id": "generator-augmented-flows-D2-023", + "claim": "LossWeighting: lambda(sigma_i) = 1 / (sigma_{i+1} - sigma_i). Combined with the Karras noise schedule, this weighting emphasizes consistency at low-noise timesteps (where sigma_i differences are smaller).", + "source": "Appendix D" + }, + { + "id": "generator-augmented-flows-D2-024", + "claim": "DiscretizationSchedule: N(k) = min(s0 * 2^{floor(k / K')}, s1) + 1, where K' = floor(K / (log2(s1 / s0) + 1)), s0=10, s1=1280, K is total training steps, and k is the current training step. This exponential schedule progressively increases the number of timesteps N during training (from Song and Dhariwal 2024).", + "source": "Appendix D" + }, + { + "id": "generator-augmented-flows-D2-025", + "claim": "TimestepSamplingDistribution: p(sigma_i) proportional to erf((log(sigma_{i+1}) - P_mean) / (sqrt(2) * P_std)) - erf((log(sigma_i) - P_mean) / (sqrt(2) * P_std)), with P_mean=-1.1, P_std=2.0. This discrete distribution approximates the continuous log-normal sampling from Karras et al. 2022, as recommended by Song and Dhariwal 2024.", + "source": "Appendix D" + }, + { + "id": "generator-augmented-flows-D2-026", + "claim": "JointLearningAlgorithm (Algorithm 1): Input: randomly initialized consistency model f_theta, N, sigma schedule, loss weighting lambda, learning rate eta, distance D, noise distribution p_z, joint learning parameter mu. Per iteration: (1) Sample x_star ~ p_star, z ~ p_z; (2) Sample timestep index i ~ multinomial(p(sigma)); (3) Sample per-sample mask m_j ~ Binomial(mu) for batch_size elements; (4) Compute IC intermediate x_{t_i} = x_star + sigma_{t_i} * z; (5) Predict endpoint x_hat_{t_i} = sg(f_theta(x_{t_i}, sigma_{t_i})); (6) Mix: x_hat_{t_i} = m * x_hat_{t_i} + (1-m) * x_star (GC elements use predicted endpoint, IC keep original); (7) Construct shifted GC points: x_tilde_{t_i} = x_hat_{t_i} + sigma_{t_i} * z, x_tilde_{t_{i+1}} = x_hat_{t_i} + sigma_{t_{i+1}} * z; (8) Compute loss L = lambda(sigma_{t_i}) * D(sg(f_theta(x_tilde_{t_i}, sigma_{t_i})), f_theta(x_tilde_{t_{i+1}}, sigma_{t_{i+1}})); (9) Update theta = theta - eta * grad_theta L. Progressively increase N(k) during training via exponential schedule.", + "source": "Appendix B, Algorithm 1" + } + ], + "D3": [ + { + "id": "generator-augmented-flows-D3-001", + "claim": "Main Benchmark Experiment (iCT-IC, iCT-OT, iCT-GC). Purpose: Compare the proposed GC joint learning method against IC and batch-OT baselines for improved consistency training. Datasets: CIFAR-10 (32x32), ImageNet (32x32), CelebA (64x64), LSUN Church (64x64). Baselines: (a) iCT-IC: improved consistency training with independent coupling (Song and Dhariwal 2024); (b) iCT-OT: iCT with minibatch optimal transport coupling using Hungarian/Sinkhorn solvers (Pooladian et al. 2023; Dou et al. 2024). Method: iCT-GC with joint learning factor mu=0.5 (chosen from CIFAR-10 grid search, kept fixed for other datasets). All methods use the same SongUNet architecture, training steps, and hyperparameter search spaces. Metrics: FID, KID (x10^2), IS with confidence intervals from 5 runs. Results reported in Table 1.", + "source": "Section 5.2, Table 1" + }, + { + "id": "generator-augmented-flows-D3-002", + "claim": "ECT Experiment (ECT-IC, ECT-GC). Purpose: Evaluate GC in the Easy Consistency Tuning setting (Geng et al. 2024), where consistency models are fine-tuned from a pre-trained diffusion model. Datasets: CIFAR-10 (32x32), FFHQ (64x64), ImageNet (64x64 conditional). Baselines: ECT-IC (Geng et al. 2024 defaults). Method: ECT-GC with mu=0.3 (recommended smaller values for ECT). Two training regimes: short (4k steps, ~1 GPU-hour) and long (100k steps, ~1 GPU-day). All hyperparameters follow Geng et al. 2024 defaults. Metrics: FID with confidence intervals. Results reported in Table 2.", + "source": "Section 5.3, Table 2" + }, + { + "id": "generator-augmented-flows-D3-003", + "claim": "iCT-GC with Frozen Pre-Trained Predictor. Purpose: Validate that GC training works with a separately pre-trained endpoint predictor (as assumed in the theory of Section 4). Dataset: CIFAR-10. Protocol: (1) Pre-train an iCT-IC model g_phi as endpoint predictor for either 100k steps (full) or 20k steps (partial/weak); (2) Freeze g_phi; (3) Train a new consistency model f_theta from scratch using only GC trajectories (mu=1) with the frozen predictor: x_hat_{t_i} = sg(g_phi(x_{t_i}, sigma_{t_i})). Metrics: FID during training. Key finding: Performance of GC model depends on predictor quality on IC trajectories. Results in Figure 4.", + "source": "Section 5.1, Figure 4" + }, + { + "id": "generator-augmented-flows-D3-004", + "claim": "Joint Learning Factor mu Ablation on CIFAR-10. Purpose: Determine optimal mu range and validate the interpolation phenomenon between IC (mu=0) and GC (mu=1). Dataset: CIFAR-10. Protocol: Train iCT-GC models with varying mu values (grid from 0 to 1) using the joint learning Algorithm 1. All other hyperparameters fixed. Metrics: FID during training trajectory. Key finding: For 0.3 <= mu <= 0.7, convergence speed and final FID are improved over both IC and batch-OT. mu=1 diverges after early fast progress due to distribution shift between IC and GC trajectories. Results in Figure 5.", + "source": "Section 5.2, Figure 5" + }, + { + "id": "generator-augmented-flows-D3-005", + "claim": "GC-Only Training Failure Analysis (mu=1). Purpose: Diagnose why pure GC training (mu=1) fails despite fast initial progress. Dataset: CIFAR-10. Protocol: (1) Train model with mu=1 (only GC trajectories); (2) Compare with and without timestep scheduling (increasing N vs fixed N); (3) Evaluate the GC-trained model on both IC and GC trajectories per timestep; (4) Compare FID along trajectories against standard IC-trained model on IC trajectories. Metrics: FID per timestep, overall FID curves. Key finding: Distribution shift — GC models degrade on IC trajectories at intermediate timesteps, making them poor endpoint predictors for their own GC training (Equation 13). Results in Figure 7.", + "source": "Appendix C.1, Figure 7" + }, + { + "id": "generator-augmented-flows-D3-006", + "claim": "Iso-Time, Dropout, and EMA Ablation on CIFAR-10. Purpose: Isolate effects of (a) wall-clock training time, (b) dropout regularization, and (c) EMA for endpoint prediction. Dataset: CIFAR-10. Protocol: (1) Iso-time: train iCT-GC (mu=0.5) for the same wall-clock duration as standard iCT-IC (accounting for ~20% overhead per GC step due to extra forward pass); (2) Dropout: train iCT-GC (mu=0.5) with dropout enabled; (3) No-EMA: train iCT-GC (mu=0.5) without EMA on endpoint predictions. Baselines: standard iCT-IC and iCT-GC (mu=0.5) with default settings. Metrics: FID. Key findings: (a) GC still outperforms IC under iso-time; (b) Dropout degrades GC performance (opposite to Song and Dhariwal 2024 findings for IC); (c) Removing EMA worsens GC scores. Results in Table 3.", + "source": "Appendix C.1, Table 3" + }, + { + "id": "generator-augmented-flows-D3-007", + "claim": "Mu Sensitivity on ImageNet-32. Purpose: Validate that mu sensitivity observed on CIFAR-10 generalizes to larger datasets. Dataset: ImageNet 32x32. Protocol: Train iCT-GC models with mu values {0.3, 0.5, 0.7, 1.0} using the same joint learning Algorithm 1 and hyperparameters from Table 6. Metrics: FID during training. Key finding: mu = {0.3, 0.5} both outperform iCT-IC baseline, confirming the CIFAR-10 trend. Results in Figure 9.", + "source": "Appendix C.1, Figure 9" + }, + { + "id": "generator-augmented-flows-D3-008", + "claim": "Regularizer Proxy Comparison (R_tilde). Purpose: Empirically compare the proxy regularizer R_tilde_t values for IC, batch-OT, and GC couplings to validate Theorem 2 (GC reduces velocity field estimation discrepancy). Dataset: CIFAR-10. Protocol: (1) Train a separate denoiser D_phi for each coupling type (IC, batch-OT, GC); (2) For each noise level sigma_t, compute R_tilde using the approximation: dot_x_t - v_t(x_t) ≈ z - (1/t)(x_t - D_phi(x_t, t)); (3) Average over samples. Metrics: R_tilde_t as a function of sigma_t. Key finding: R_tilde_{t,GC} < R_tilde_{t,batch-OT} < R_tilde_{t,IC}, with gap increasing with t, corroborating Theorem 2. Results in Figure 2.", + "source": "Section 4.2.1, Figure 2" + }, + { + "id": "generator-augmented-flows-D3-009", + "claim": "Transport Cost Comparison. Purpose: Compare quadratic transport cost between noise z and predicted endpoint x_hat for IC, batch-OT, and GC couplings. Datasets: CIFAR-10 and toy 2D-Diracs. Protocol: (1) For each coupling, compute c(t) = E[||f_ring(x_t, sigma_t) - z||^2] or c(0) = E[||x_star - z||^2] for IC; (2) For batch-OT, compute transport cost using minibatch Hungarian matching; (3) For GC, compute c(t) using a trained consistency model as predictor. Metrics: Expected squared L2 distance between noise and predicted endpoint. Key finding: GC reduces transport cost more than batch-OT because batch-OT is tied to batch data points whereas GC predictions are not. Results in Figures 3 and 6.", + "source": "Section 4.2.2, Figures 3, 6" + }, + { + "id": "generator-augmented-flows-D3-010", + "claim": "Toy 2D-Dirac Experiment for Transport Cost and Flow Visualization. Purpose: Visualize probability paths, ODE trajectories, and transport costs on a minimal tractable example. Dataset: Mixture of two 2D Dirac delta functions as data distribution, standard Gaussian noise. Protocol: (1) Compute closed-form PF-ODE and consistency model for this toy setting; (2) Visualize IC and GC sample paths, ODE trajectories, and probability densities; (3) Compute and compare transport costs c(t) for IC, batch-OT, and GC. Metrics: Visual alignment of sample paths with velocity field, transport cost c(t). Results in Figures 1 and 6 (left panel).", + "source": "Section 4.2.2, Appendix A.3, Figures 1, 6" + }, + { + "id": "generator-augmented-flows-D3-011", + "claim": "Evaluation Protocol. Metrics: Frechet Inception Distance (FID) from Heusel et al. 2017, Kernel Inception Distance (KID) from Binkowski et al. 2018, and Inception Score (IS) from Salimans et al. 2016, all implemented via TorchMetrics (Skafte Detlefsen et al. 2022). Procedure: Compare 50,000 generated images against 50,000 training images (standard practice, e.g. Song and Dhariwal 2024). Confidence intervals: 5 independent evaluation runs on the same trained model; each run samples a new set of 50k training images (for reference distribution) and generates 50k new images with a different random seed.", + "source": "Appendix D" + }, + { + "id": "generator-augmented-flows-D3-012", + "claim": "Data Preprocessing and Hyperparameter Grid Search. Preprocessing: Resize smaller image side to target resolution, apply center crop, linearly scale pixel values to [-1, 1]. Hyperparameter search: For each model type (iCT-IC, iCT-OT, iCT-GC), per-dataset grid search over learning rates (CIFAR-10: {1e-4, 3e-5}; others: 8e-5 fixed) and dropout rates (search spaces specified per dataset in Tables 4-6). Optimizer: Lion (Chen et al. 2023) from github.com/lucidrains/lion-pytorch. Architecture: SongUNet / NCSN++ (Song et al. 2021) from github.com/NVlabs/edm (Karras et al. 2022). EMA used for all models except the ablation study. Timestep scheduling: progressive increase N from s0=10 to s1=1280 via exponential schedule during training. Noise schedule: Karras et al. 2022 with rho=7, sigma_0=0.002, sigma_1=80. Loss weighting: lambda(sigma_i) = 1/(sigma_{i+1} - sigma_i). Timestep sampling: log-normal distribution with P_mean=-1.1, P_std=2.0.", + "source": "Appendix D, Tables 4-6" + } + ], + "D4": [ + { + "id": "generator-augmented-flows-D4-001", + "claim": "Overall Training Pipeline (9 steps): (1) Initialize consistency model f_theta with SongUNet architecture and the parametrization f_theta(x, sigma) = c_skip(sigma) * x + c_out(sigma) * F_theta(x, sigma); (2) Set noise schedule sigma_i via Karras formula with rho=7, sigma_0=0.002, sigma_N=80; (3) At each training step, sample a batch of (x_star, z) pairs via independent coupling q_I; (4) Sample timestep index i from discrete log-normal distribution p(sigma_i); (5) Compute IC intermediate point x_{t_i} = x_star + sigma_{t_i} * z; (6) Predict endpoint x_hat_{t_i} = sg(f_theta(x_{t_i}, sigma_{t_i})) using the model itself with stop-gradient; (7) Apply per-sample binomial mask m_j ~ Binomial(mu) to mix: GC samples use x_hat_{t_i} as endpoint, IC samples use original x_star; (8) Construct shifted GC points and compute consistency loss L(theta) = lambda(sigma_{t_i}) * D(sg(f_theta(x_tilde_{t_i})), f_theta(x_tilde_{t_{i+1}})); (9) Update theta = theta - eta * grad_theta L(theta). During training, progressively increase N from s0=10 to s1=1280 via exponential schedule N(k). Evaluate FID/KID/IS on 50k generated vs 50k training images using 5 independent runs.", + "source": "Sections 4.1, 5.2, Appendix B Algorithm 1, Appendix D" + }, + { + "id": "generator-augmented-flows-D4-002", + "claim": "GC Coupling Step Ordering within Each Iteration (must follow this sequence): (a) Sample (x_star, z) via IC first — this is required because the endpoint predictor f_theta is only trained on IC trajectories and needs IC points as input; (b) Compute IC intermediate x_{t_i} BEFORE predicting x_hat_{t_i} — the prediction depends on the IC intermediate point; (c) Predict x_hat_{t_i} = sg(f_theta(x_{t_i})) with stop-gradient — gradient must NOT flow through the endpoint prediction to the predictor; (d) Apply binomial mask m to mix x_hat_{t_i} and x_star — this determines per-sample whether GC or IC trajectory is used; (e) Reuse the SAME noise z to construct x_tilde points — the GC coupling preserves the same z so that x_tilde_{t_{i+1}} - x_tilde_{t_i} = (sigma_{t_{i+1}} - sigma_{t_i}) * z; (f) Compute loss on (x_tilde_{t_i}, x_tilde_{t_{i+1}}) — both GC and (effectively) IC pairs are evaluated through the same loss because for IC elements (m_j=0), x_hat_{t_i} = x_star, so x_tilde points reduce to standard IC points.", + "source": "Section 4.1, Algorithm 1" + } + ] +} \ No newline at end of file diff --git a/papers/hi-mar/blacklist.txt b/papers/hi-mar/blacklist.txt new file mode 100644 index 0000000000000000000000000000000000000000..06c15acbe16f2f68229e34ca6b6fece7644e19cd --- /dev/null +++ b/papers/hi-mar/blacklist.txt @@ -0,0 +1,2 @@ +# Official repository (ICML 2025) +https://github.com/HiDream-ai/himar diff --git a/papers/hi-mar/config.yaml b/papers/hi-mar/config.yaml new file mode 100644 index 0000000000000000000000000000000000000000..cfe599645db163256e7780f9e5cd9452e17f448a --- /dev/null +++ b/papers/hi-mar/config.yaml @@ -0,0 +1,8 @@ +title: "Hi-MAR: Hierarchical Masked Autoregressive Models with Low-Resolution Token Pivots" +pdf_url: 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+oid sha256:228a11941ea8da55c66d22064758c978755073881966137fdb68f00f26330068 +size 32302 diff --git a/papers/hi-mar/paper.md b/papers/hi-mar/paper.md new file mode 100644 index 0000000000000000000000000000000000000000..949fc360c521e90776fface4160c6db71e87102c --- /dev/null +++ b/papers/hi-mar/paper.md @@ -0,0 +1,286 @@ +# Hierarchical Masked Autoregressive Models with Low-Resolution Token Pivots + +Guangting Zheng 1 † Yehao Li 2 Yingwei Pan 2 Jiajun Deng 3 Ting Yao 2 Yanyong Zhang 1 Tao Mei 2 + +# Abstract + +# 1. Introduction + +Autoregressive models have emerged as a powerful generative paradigm for visual generation. The current de-facto standard of next token prediction commonly operates over a single-scale sequence of dense image tokens, and is incapable of utilizing global context especially for early tokens prediction. In this paper, we introduce a new autoregressive design to model a hierarchy from a few low-resolution image tokens to the typical dense image tokens, and delve into a thorough hierarchical dependency across multiscale image tokens. Technically, we present a Hierarchical Masked Autoregressive models (Hi-MAR) that pivot on low-resolution image tokens to trigger hierarchical autoregressive modeling in a multi-phase manner. Hi-MAR learns to predict a few image tokens in low resolution, functioning as intermediary pivots to reflect global structure, in the first phase. Such pivots act as the additional guidance to strengthen the next autoregressive modeling phase by shaping global structural awareness of typical dense image tokens. A new Diffusion Transformer head is further devised to amplify the global context among all tokens for mask token prediction. Extensive evaluations on both class-conditional and text-toimage generation tasks demonstrate that Hi-MAR outperforms typical AR baselines, while requiring fewer computational costs. Code is available at https://github.com/HiDream-ai/himar. + +In recent years, GPT-style Autoregressive (AR) models have brought a powerful revolution in Natural Language Processing (NLP), and become the model of choice in designing Large Language Models (LLMs) (Achiam et al., 2023; Team et al., 2023; Bai et al., 2023; Dubey et al., 2024; Team et al., 2024). The dominant training paradigm in AR models is to predict the next word/token in a sequence conditioned on previous estimated words, i.e., next token prediction. This prevailing paradigm has shown excellent scalability via scaling laws and generalization in zero-shot settings, paving a reliable way towards Artificial General Intelligence (AGI). + +Inspired by the scaling successes of AR models in NLP, a steady of attempts have been attained to scale up AR models in CV field for visual generation (e.g., text-to-image generation (Sun et al., 2024; Tian et al., 2024; Li et al., 2024; He et al., 2025; Lu et al., 2024a) and text-to-video generation (Ren et al., 2024b; Wang et al., 2024b;a)). Specifically, after decomposing input image into a sequence of image patches/tokens, one direction (Esser et al., 2021; Lee et al., 2022; Sun et al., 2024; Zhuo et al., 2024) directly adopts GPT-style AR models with causal attention (see Figure 1 (a)), which enforces next token prediction by only attending to preceding tokens. Another direction commonly adopts BERT-style AR models with bidirectional attention (see Figure 1 (b)), e.g., Masked Autoregressive model (MAR) (Li et al., 2024; Fan et al., 2024), that simultaneously predicts multiple masked tokens in a random order by attending to all masked and unmasked tokens. These AR models with next token prediction objective, while being dominant in NLP, have not yet been scaled effectively in visual generation, especially for visual content creation with complex semantics. Accordingly, in large-scale text-to-image/video generation, typical AR models tend to be less effective when compared to other generative models (e.g., Diffusion models (Rombach et al., 2022; Dhariwal & Nichol, 2021; Esser et al., 2024; Zhang et al., 2024; Chen et al., 2023) and Diffusion Transformer (Peebles & Xie, 2023; Bao et al., 2023; Lu et al., 2024b; Zhu et al., 2024)). This might be attributed to two inherent limitations in typical AR models: (1) Most AR models capitalize on vector quantization process to decompose images into discrete tokens, thereby inevitably resulting in information loss. (2) Both GPT-style and BERTstyle AR models solely hinge on a single-scale sequence of image tokens for predicting next tokens in autoregressive form. Such single-shot autoregressive design makes it difficult to capture global context for facilitating early token prediction, resulting in sub-optimal autoregressive modeling among all dense image tokens. + +![](images/figures/hi-mar-fig-0001.jpg) +Figure 1. a) Next-token autoregressive (AR). GPT-style autoregressive models process 2D image tokens as a 1D sequence, predicting tokens in raster order using causal attention to ensure each token depends only on preceding ones. b) Next-token masked autoregressive model (MAR). BERT-style autoregressive models initially consider all tokens to be masked, subsequently predicting each masked token based on the known tokens in a random order, leveraging bidirectional attention to enable parallel prediction of a subset of tokens. c) Hierarchical mask autoregressive model (Hi-MAR). Hierarchical mask autoregressive models adapt a hierarchical prediction strategy to address the lack of global context in the next-token prediction. Hi-MAR first predicts a low-resolution image token sequence, which contains a few tokens, to reflect the global structure, and then pivots on these tokens to enhance and refine the next-resolution prediction. + +In an effort to mitigate the above limitations, we start from the recent pioneering work of MAR that nicely sidesteps vector quantization and triggers autoregressive modeling in a continuous-valued space. This lossless image tokenization seems more suitable for autoregressive modeling in vision data. It motivates us to delve into the potential of global context mining in MAR for boosting visual generation, thereby further alleviating the second limitation. In order to upgrade MAR with additional global context, we derive a particular form for autoregressive modeling named Hierarchical Masked Autoregressive models (Hi-MAR). Our launching point is to build a top-down hierarchical structure from the root of low-resolution image tokens to the leaf layer of typical dense image tokens. Figure 1 (c) conceptualizes such construction of hierarchical structure in Hi-MAR, which enables hierarchical autoregressive modeling in a multi-phase fashion. Specifically, the first phase performs bidirectional autoregressive modeling over low-resolution image tokens. Such learnt low-resolution image tokens naturally reflect global structural information of the whole image. After that, we take the low-resolution image tokens as intermediary pivots to guide the next autoregressive modeling over typical dense image tokens. In this way, Hi-MAR could elegantly trigger global context propagation in a top-down manner, and the second-phase autoregressive modeling over typical dense image tokens does benefit from this additional guidance of global context. Meanwhile, this additional global context propagation significantly eases the autoregressive modeling over dense image tokens, and thus requires fewer generation steps, thereby harboring an innate agency that remains advantageous in the speed of sequence prediction. + +Moreover, in the second phase, we design a new Diffusion Transformer head to mine global context among all masked and unmasked tokens, aiming to further strengthen autoregressive modeling over dense image tokens. + +The main contribution of this work is the mining of global context in masked autoregressive models for image generation. The solution also leads to the elegant view of how to build and interpret the global structure of an image, and how to nicely integrate such global structural awareness into typical masked autoregressive models, which are problems not yet fully understood in the literature. Through extensive experiments on ImageNet and MS-COCO, we demonstrate the effectiveness of our proposal, for example, achieving ${ 0 . 3 8 } \mathrm { F I D }$ performance boost over MAR and only requiring $54 \%$ computational costs. + +# 2. Related Works + +Diffusion models. In the domain of image generation, diffusion models (Ho et al., 2020; Rombach et al., 2022; Peebles & Xie, 2023; Esser et al., 2024; Qian et al., 2024; Wan et al., 2024) have emerged as a powerful paradigm, achieving impressive results across diverse tasks. These models conceptualize image generation as a denoising process (Ho et al., 2020), where images are progressively reconstructed from noise through a multi-step denoising strategy. A common backbone for diffusion models is the CNN-based U-Net (Song & Ermon, 2019; 2020), which effectively captures both local and global features during denoising. To improve scalability and flexibility, DiT (Peebles & Xie, 2023) replaces the U-Net backbone with a transformer (Waswani et al., 2017). Building on this, U-ViT (Bao et al., 2023) incorporates long skip connections inspired by U-Net into the transformer architecture, and treats all inputs as tokens to enhance performance. + +Next-token autoregressive models. Autoregressive models (AR) have achieved significant success in natural language processing (Achiam et al., 2023; Team et al., 2024; Bai et al., 2023; Dubey et al., 2024; Brown et al., 2020; Radford et al., 2019) and are increasingly applied to image generation (Zhuo et al., 2024; Team, 2024; Sun et al., 2024; Tian et al., 2024; Chang et al., 2022). Early works, such as VQVAE (Esser et al., 2021) and RQ-Transformer (Lee et al., 2022), serialize 2D images into 1D token sequences and predict tokens in raster order. LlamaGen (Sun et al., 2024) adapts this next-token prediction paradigm and scales up models to achieve quality comparable to diffusion models. Due to the fact that the dependencies between 2D image tokens do not naturally follow a raster order, some studies (Chang et al., 2022; Yu et al., 2023) suggest that causal attention may not be ideal for image generation. To address this, models like MaskGIT (Chang et al., 2022), MagViT (Yu et al., 2023), MagViT-2 (Yu et al., 2024), and MUSE (Chang et al., 2023) adopt bidirectional attention and masked prediction strategies used in BERT (Kenton & Toutanova, 2019), enabling non-sequential token prediction. + +Compared to mainstream continuous-valued diffusion models, traditional autoregressive models, inherited from language autoregressive models, predict over discrete tokens, which may introduce information loss, ultimately limiting the generation quality. Recently, GIVT (Tschannen et al., 2025) and MAR (Li et al., 2024) replaced discrete tokens with continuous tokens. MAR introduces a diffusion loss function to model per-token probabilities, replacing the categorical cross-entropy loss used in discrete-valued models. Furthermore, Fluid (Fan et al., 2024) extends continuousvalued autoregressive models to text-to-image generation. + +Next-scale autoregressive models. Multi-scale autoregressive generation has been explored in various forms to enhance image synthesis quality (Chang et al., 2023; Tian et al., 2024; Tang et al., 2025; Ren et al., 2024a). Among them, Muse (Chang et al., 2023) generates high-resolution discrete tokens conditioned on low-resolution tokens and text inputs, while VAR (Tian et al., 2024) replaces nexttoken prediction with a next-scale prediction strategy by retraining a multi-scale quantization autoencoder to encode an image into K discrete token maps at different resolutions. However, such approaches often rely on VQ-based quantization, which can introduce substantial information loss and limit the fidelity of generated outputs (Li et al., 2024). Moreover, retraining multi-scale autoencoders, as in VAR, increases model complexity and training costs. In contrast, our approach generates continuous-valued token sequences from multiple resolutions of images, eliminating the need for additional autoencoder training and improving the upper bound of generation quality. + +Another limitation of existing approaches lies in how they model cross-scale dependencies. VAR (Tian et al., 2024) and HART (Tang et al., 2025) employ a shared Transformer to autoregressively model tokens across scales without explicit scale-specific guidance, which may underutilize the unique characteristics of each resolution level. Muse (Chang et al., 2023) adopts two separately trained models for lowresolution and high-resolution generation, increasing parameter count. FlowAR (Ren et al., 2024a) introduces spatially adaptive layer normalization in the flow matching head, facilitating position-by-position semantic injection and enabling scale-wise adjustments; however, it lacks an explicit mechanism for encoding scale-level information within the Transformer backbone. In contrast, our method mitigates this by introducing an elegant scale-aware Transformer backbone that incorporates explicit resolution information while maintaining parameter efficiency. To explicitly encode scale information, we introduce a learnable scale vector for each resolution, which is injected into the Transformer backbone via AdaLN-Zero operations (Peebles & Xie, 2023), further enhancing scale-awareness. This explicit scale encoding allows the model to better capture resolution-specific characteristics and improves multi-scale generation quality. + +In addition to the information loss caused by token discretization and the challenges in modeling cross-scale dependencies, a critical limitation shared by existing multiscale models (Chang et al., 2023; Tian et al., 2024; Tang et al., 2025; Ren et al., 2024a) is the reliance on groundtruth low-resolution tokens during training to supervise high-resolution generation. This introduces a discrepancy between training and inference since ground-truth tokens are not available at inference time. Our approach mitigates this issue by conditioning high-resolution generation on predicted low-resolution tokens from the Transformer backbone, ensuring consistency between training and inference and improving generation robustness. Furthermore, methods like HART adopt an MLP-based diffusion head that treats each token independently, leading to a loss of global information during the denoising process. In contrast, our method introduces a Diffusion Transformer head that leverages self-attention to capture inter-token dependencies during diffusion, resulting in more coherent generation. + +# 3. Method + +In this work, we devise Hierarchical Masked Autoregressive models (Hi-MAR) that pivot on low-resolution image tokens to trigger hierarchical autoregressive modeling in a multi-phase manner. This section starts with the preliminaries of MAR (Li et al., 2024), which triggers autoregressive modeling in a continuous-valued space. Then, the hierarchical masked autoregressive model is elaborated. After that, we detail the integration of a newly proposed Diffusion Transformer head, which mines the global context among all tokens to strengthen image generation. An overview of Hi-MAR is shown in Figure 2 (b). + +![](images/figures/hi-mar-fig-0002.jpg) +Figure 2. (a) Pipeline of conventional hierarchical MAR. Conventional hierarchical mar uses a shared Transformer for both phases and directly leverages low-resolution visual tokens to guide second-phase predictions. (b) Pipeline of Hi-MAR. An image and its low-resolution counterpart are converted into token sequences at two scales. During inference, both sequences are initially masked. In the first phase, masked low-resolution tokens are processed by the Transformer to predict conditional tokens, followed by an MLP-based diffusion head for token reconstruction. In the second phase, the masked high-resolution tokens and the predicted conditional tokens in first phase are fed into the Transformer with the Diffusion Transformer head predicting the full high-resolution token sequence. (c) Scale-aware Transformer blocks consist of adaLN-Zero, layernorm, self-attention, and feed-forward layers. (d) MLP-based Diffusion head blocks include adaLN, layernorm, and feed-forward layers. (e) Diffusion Transformer head blocks are composed of adaLN, layernorm, self-attention, and feed-forward layers. + +# 3.1. Preliminary + +We build our Hi-MAR upon the recent pioneering work of MAR (Li et al., 2024). Specifically, given an image $I \in \mathbb { R } ^ { H \times W \times 3 }$ , MAR first utilizes the pre-trained variational autoencoder (VAE) to encode $I$ into latent representations $I ^ { \prime } \in \mathbb { R } ^ { h \times w \times d }$ , where h/w/d is the dimension of height/width/channels for the latent. Then, the latent $I ^ { \prime }$ is reshaped into a sequence of $N = h \cdot w$ continuous-valued visual tokens $X = \{ x _ { 1 } , x _ { 2 } , . . . , x _ { N } \}$ . During training, MAR randomly selects $\lceil r \cdot N \rceil$ visual tokens and replaces them with masked tokens, where $r$ denotes the masking ratio sampled from a pre-defined distribution $p ( r )$ . The masked sequence $X ^ { \prime } = \{ x _ { 1 } ^ { \prime } , x _ { 2 } ^ { \prime } , . . . , x _ { N } ^ { \prime } \}$ is fed into the masked autoregressive Transformer. To predict the ground-truth token $x _ { i }$ from the masked one $\boldsymbol { x } _ { i } ^ { \prime }$ at position $i$ , a diffusion head is devised to model the probability distribution of $x _ { i }$ conditioned on the output of the masked autoregressive Transformer $z _ { i }$ . The objective of the diffusion head is defined as the standard denoising process: + +$$ +\mathcal { L } ( z _ { i } , x _ { i } ) = \mathbb { E } _ { \varepsilon , t } \left[ \left\| \varepsilon - \varepsilon _ { \theta } ( x _ { i } ^ { t } | t , z _ { i } ) \right\| ^ { 2 } \right] , +$$ + +where $\ v x _ { i } ^ { t }$ is the noise-corrupted vector of $x _ { i }$ , $\varepsilon$ is the noise vector sampled from distribution $\mathcal { N } ( 0 , \bf { I } )$ , $t$ is a time step of the noise schedule, and $\varepsilon _ { \boldsymbol { \theta } }$ denotes the diffusion head. + +Nevertheless, MAR suffers from two limitations: (1) MAR solely hinges on a single-scale sequence of visual tokens for predicting next tokens, which combines global structure construction and local details refinement into a single stage. This is contrasted to typical human perception, which first captures the global structure and then the local details in a hierarchical manner (Tian et al., 2024). (2) MAR utilizes an MLP-based diffusion head to model the masked token probability distribution individually instead of all tokens together as in conventional Diffusion Transformer (Peebles & Xie, 2023; Bao et al., 2023; Lu et al., 2024b). This way ignores the inherent structure of natural images and leaves the interdependency among all visual tokens underexploited, resulting in a sub-optimal solution for masked token prediction. Such MLP-based diffusion head could produce abnormal bright spots and fail to generate correct images (Fan et al., 2024). + +# 3.2. Hierarchical Masked Autoregressive Transformer + +To mitigate the first limitation of MAR, we build a top-down hierarchical masked autoregressive Transformer (Hi-MAR Transformer) with two phases. The first phase performs bidirectional autoregressive modeling over low-resolution visual tokens to capture the global structure. In the second phase, we take the output of the first phase as intermediary pivots to guide the next autoregressive modeling over typical dense visual tokens for local details refinement. One typical variant (Tian et al., 2024) is to use a shared Transformer for the two phases and utilize the low-resolution visual tokens directly to guide the second phase prediction, as shown in Figure 2 (a). However, this training strategy often introduces a training-inference discrepancy. Specifically, during training, the models are conditioned on ground-truth low-resolution tokens $X ^ { s }$ to predict high-resolution tokens $X ^ { l }$ . During inference, since ground-truth tokens are unavailable, the models have to first generate low-resolution tokens $\hat { X } ^ { s }$ , which may contain errors, and then use these noisy predictions as conditions to predict the high-resolution tokens $X ^ { l }$ . This mismatch between clean ground-truth lowresolution tokens $X ^ { s }$ used in training and noisy predicted low-resolution tokens $\hat { X } ^ { s }$ used in inference inevitably results in a discrepancy, resulting in performance degradation. To mitigate such training-inference discrepancy, we take the conditional tokens output from the Hi-MAR Transformer of low-resolution visual tokens for the second phase instead. The overview of our Hi-MAR is shown in Figure 2 (b). Specifically, in the first phase, the masked low-resolution visual tokens along with the context tokens (e.g., class tokens or text tokens) are fed into the Transformer, which outputs the conditional tokens $Z ^ { s } = \{ z _ { 1 } ^ { s } , z _ { 2 } ^ { s } , . . . , z _ { N } ^ { s } \}$ on small scale. An additional diffusion head conditioned on $Z ^ { s }$ is adopted for the small scale optimization as in MAR (Li et al., 2024). In the second phase, the Transformer takes the concatenation of context tokens, small scale conditional tokens and the masked dense visual tokens as input to generate dense conditional tokens, which are further fed into Diffusion Transformer head for token prediction. + +Conventional hierarchical autoregressive variants (Tian et al., 2024) model multi-scale probability distribution by a shared Transformer without additional guidance. This way could make the Transformer ambiguous and is harmful for token prediction. To alleviate the issue, we design a scaleaware Transformer block shown in Figure 2 (c). Inspired by the adaLN-Zero operations adopted by DiT (Peebles & Xie, 2023), we first represent the scale information by sinusoidal embedding. The sinusoidal embedding is fed into MLP layers to generate scale vector $v$ , which is leveraged to regress the scale and shift parameters of layer norm as well as the scaling parameters for residual connection. Specifically, the $i$ -th scale-aware Transformer block with input $z ^ { i }$ is computed as: + +$$ +\begin{array} { r l } & { \tilde { v } = \mathbf { a } \cdot v + \mathbf { b } , } \\ & { \alpha _ { 1 } , \beta _ { 1 } , \gamma _ { 1 } , \alpha _ { 2 } , \beta _ { 2 } , \gamma _ { 2 } = \mathbf { s p l i t } ( \tilde { v } ) , } \\ & { z _ { a } = z ^ { i } + \gamma _ { 1 } \cdot \mathbf { A t t e n t i o n } ( \alpha _ { 1 } \cdot \mathbf { L N } ( z ^ { i } ) + \beta _ { 1 } ) , } \\ & { z ^ { i + 1 } = z _ { a } + \gamma _ { 2 } \cdot \mathbf { F F N } ( \alpha _ { 2 } \cdot \mathbf { L N } ( z _ { a } ) + \beta _ { 2 } ) , } \end{array} +$$ + +where a and $\mathbf { b }$ are the parameters, split denotes the split operation along the channel dimension, LN, Attention and FFN denote the layernorm, self-attention and feedforward layer, respectively. The output of the final scaleaware Transformer block acts as conditional tokens for the diffusion head. + +# 3.3. Diffusion Transformer Head + +To address the second limitation of MAR, we design a new Diffusion Transformer head by exploiting the self-attention to model the interdependency among tokens. In contrast to MLP-based diffusion head that only takes the conditional tokens of masked tokens as conditions, the Diffusion Transformer head considers all the masked and unmasked conditional tokens, as illustrated in Figure 2 (e). The Diffusion Transformer head contains a stack of Transformer blocks, and the $i$ -th block with input $y _ { i }$ is computed as: + +$$ +\begin{array} { r l } & { \alpha _ { 1 } , \beta _ { 1 } , \gamma _ { 1 } , \alpha _ { 2 } , \beta _ { 2 } , \gamma _ { 2 } = \mathbf { s p l i t } ( c ) , } \\ & { y _ { a } = y ^ { i } + \gamma _ { 1 } \cdot \mathbf { A t t e n t i o n } ( \alpha _ { 1 } \cdot \mathbf { L N } ( y ^ { i } ) + \beta _ { 1 } ) , } \\ & { y ^ { i + 1 } = y _ { a } + \gamma _ { 2 } \cdot \mathbf { F F N } ( \alpha _ { 2 } \cdot \mathbf { L N } ( y _ { a } ) + \beta _ { 2 } ) , } \end{array} +$$ + +where $c$ denotes the context vector obtained by summating the time step embedding and the conditional tokens, and the input of the first block is the noise-corrupted vector. Note that we only adopt Diffusion Transformer head in the second phase while the first phase still utilizes MLP-based diffusion head, since the diffusion head on the first phase mainly aims to optimize the low-resolution conditional tokens instead of providing intermediary pivots for the next phase. At inference, we use much fewer steps (e.g., 4 steps) in the second phase considering that the Diffusion Transformer head is much heavier than the MLP-based diffusion head. With the global structure provided by the first phase, the second phase can focus on the local fine-grained details and requires much fewer steps to generate satisfied results. + +# 4. Experiments + +# 4.1. Datasets + +We empirically verify the merit of hierarchical masked autoregressive models for image generation in comparison with state-of-the-art approaches on two datasets, i.e., ImageNet (Deng et al., 2009) and MS-COCO (Lin et al., 2014). For class-conditional image generation, we validate Hi-MAR on ImageNet at $2 5 6 \times 2 5 6$ resolution, which consists of 1,281,167 training images from 1K different classes. For text-to-image generation, we evaluate Hi-MAR on MS-COCO at $2 5 6 \times 2 5 6$ , which is composed of 82,783 training images and 40,504 validation images. Each image is annotated with five captions. Following Stable Diffusion (Rombach et al., 2022), we convert captions into a sequence of text embeddings with CLIP text encoder. Then the text embeddings act as context tokens and are fed into Hi-MAR for autoregressive modeling. + +Table 1. The architecture configurations of the family of Hi-MAR in three different scales (i.e., Base, Large, and Huge). Diff. $\mathrm { H e a d _ { 1 / 2 } }$ denotes the diffusion head on the first/second phase. + +
ModelHi-MAR Transformer #LayersHidden sizeDiff. Head1 #Layers Hidden sizeDiff. Head2 #Layers Hidden size#Params
Hi-MAR-B24768610246512244M
Hi-MAR-L321024812808512529M
Hi-MAR-H401280121536127681090M
+ +# 4.2. Experimental Settings + +Image Tokenizer. We employ the variational autoencoder (KL-16 version) trained by MAR (Li et al., 2024) to encode low-resolution $\phantom { + } 1 2 8 \times 1 2 8 )$ and high-resolution $( 2 5 6 \times 2 5 6 )$ images into latent representations for the two phases. + +Network Architectures. Following the architecture configurations of MAR family (Li et al., 2024), we build three variants of our Hi-MAR in three different scales (i.e., Base, Large and Huge). To compare with MAR-B/L/H, the number of Transformer blocks in masked autoregressive Transformer of Hi-MAR-Base (Hi-MAR-B), Hi-MAR-Large (Hi-MAR-L), and Hi-MAR-Huge (Hi-MAR-H) are set as 24,32, and 40 respectively. The number of Transformer blocks in the diffusion head on both phases of Hi-MAR-B/L/H is 6/8/12. Table 1 shows the detailed configurations (e.g., layer number, hidden size) of three Hi-MAR variants. + +Training Setup. At training stage, we conduct all experiments on 80GB-H100 GPUs. For class-conditional image generation on ImageNet, we follow MAR (Li et al., 2024) and train the models using AdamW optimizer ( $\beta _ { 1 } =$ $0 . 9 , \beta _ { 2 } = 0 . 9 5 )$ ) with 0.02 weight decay for 800 epochs. We use the constant lr schedule with a 1e-4 learning rate and 100-epoch linear warmup. In the first phase, the masking ratio is randomly sampled in [0.7, 1.0] as MAR, while the second phase uses the cosine masking strategy following MaskGIT (Chang et al., 2022). For text-to-image generation on MS-COCO, we follow AutoNAT-L (Ni et al., 2024) and randomly sample the masking ratio by Beta distribution $( \alpha = 4 , \beta = 1 _ { \cdot }$ ). The AdamW optimizer is adopted with an 8e-4 learning rate, 0.03 weight decay and 8K-step linear warmup. The exponential moving average is adopted with a momentum of 0.9999. At inference, we use 32 and 4 steps for the first and second phases with a cosine schedule. + +Evaluation Metrics. For evaluation, we use Frechet In-´ ception Distance (FID) (Heusel et al., 2017), Inception Score (IS) (Salimans et al., 2016) and Precision/Recall (Kynka¨anniemi et al. ¨ , 2019) on 50K generated samples to measure the image quality on ImageNet. On MS-COCO, we randomly draw 30K prompts from the validation set and generate samples on these prompts as U-ViT (Bao et al., 2023). We report the FID score as the main metric. + +# 4.3. Results on Class-Conditional Image Generation + +Table 2 summarizes the performance comparisons of different methods for class-conditional image generation on ImaegNet dataset. All runs are grouped into four categories: the Generative Adversarial Network(GAN) based models, diffusion-based models, autoregressive models, and masked autoregressive models. Except for the GAN based models, we report the performances of the runs under two different inference settings i.e., with or without Classifier-Free Guidance (CFG) (Ho & Salimans, 2021). For our method under the w/o CFG setting, the CFG is only turned off during the prediction of dense tokens, as the first-stage output quality significantly affects Hi-MAR performance. + +Overall, under the two different settings, the results across most metrics consistently indicate that our Hi-MAR achieves superior performances against the state-of-the-art models among all the four categories with comparable parameter size. In particular, the FID score of Hi-MAR-B on the base scale achieves 1.93 under CFG, making the absolute improvement over the best competitor MAR-B by 0.38. The results generally highlight the key advantage of exploiting hierarchical autoregressive and modeling interdependency among tokens. Specifically, U-ViT and DiT upgrade the conventional U-Net structure diffusion model with Transformer, resulting in remarkable scaling property with superior performances than all GAN based and U-Net structure diffusion models. Note that the use of CFG generally improves the FID, IS and Precision scores for diffusion and AR models across different scales. But the Recall scores decrease due to CFG tends to improve the generation quality at the cost of sacrificing diversity. Compared to diffusion models, autoregressive models (e.g., VQGAN (Esser et al., 2021) and LlamaGen (Sun et al., 2024)) regard images as a sequence of discrete tokens, achieving comparable image generation results. Furthermore, the mask autoregressive models introduce mask tokens into autoregressive modeling, facilitating bidirectional contextual information learning along generative modeling, and thus improve performance. Instead of quantizing images into discrete tokens, MAR utilizes a continuous tokenizer via the diffusion loss, leading to significant performance boosts. But the performance of MAR across different model sizes is still inferior to our Hi-MAR. This validates the effectiveness of hierarchical autoregressive modeling and the Diffusion Transformer head for enhanced masked token prediction with richer context information among all tokens. + +# 4.4. Results on Text-to-Image Generation + +Table 3 shows the performance comparison on MS-COCO for text-to-image generation. For fair comparison, we follow the configuration of U-ViT-S/2 (Deep) (Bao et al., 2023) and build a light-weight version of our Hi-MAR with comparable model size. We group all runs in three categories, i.e., GAN based models, diffusion models, and masked autoregressive models. In general, our Hi-MAR outperforms other baselines on this challenging dataset. In particular, the FID score of Hi-MAR can achieve 4.77, making the absolute improvement over the best competitor AutoNAT-S by 0.59. + +Table 2. Generative model family comparison on class-conditional ImageNet $2 5 6 \times 2 5 6$ . “↓” or “↑” indicate lower or higher values are better. Metrics include Frechet inception distance (FID), inception score (IS), precision and recall. Models with the suffix “-re” used ´ rejection sampling. + +
TypeModel#Para.w/o CFGw/CFG
FID↓IS↑Precision↑Recall↑FID↓IS↑Precision↑Recall↑
GANBigGAN (Brock et al., 2019)112M6.95224.50.890.38
GigaGAN (Kang et al., 2023)569M3.45225.50.840.61
StyleGan-XL (Sauer et al., 2022)166M2.30265.10.780.53
Diff.ADM (Dhariwal & Nichol, 2021)554M10.94101.00.690.634.59186.70.820.52
CDM (Ho et al., 2022)4.88158.7
LDM-4-G (Rombach et al., 2022)400M10.56103.50.710.623.60247.70.870.48
U-ViT-H/2 (Bao et al., 2023) DiT-XL/2 (Peebles & Xie, 2023)501M121.52.29263.88 278.20.820.57
675M9.620.670.672.270.830.57
VQGAN (Esser et al., 2021)227M18.6580.40.780.26
VQGAN-re (Esser et al., 2021)1.4B5.20280.3
RQTran. (Lee et al., 2022)3.8B7.55134.0
RQTran.-re (Lee et al., 2022)3.8B3.80323.7
GIVT (Tschannen et al., 2025)304M5.670.750.593.350.840.53
LlamaGen-L (Sun et al., 2024)343M19.0764.30.610.673.07256.060.830.52
LlamaGen-XL (Sun et al., 2024)775M15.54 14.6579.20.620.692.62244.080.800.57
LlamaGen-XXL (Sun et al., 2024) VAR-d16 (Tian et al., 2024)1.4B86.3 —0.63 −0.68 2.34 3.30253.90 274.40.80 0.840.59
VAR-d20 (Tian et al., 2024)310M 600M0.51
VAR-d24 (Tian et al., 2024)1.0B2.57 2.09302.6 312.90.83 0.820.56 0.59
227M6.18
MaskGIT (Chang et al., 2022) AutoNAT-L (Ni et al., 2024)422M182.10.800.51 −− 2.68− 278.8
MAR-B (Li et al., 2024)208M3.48192.40.780.582.31281.7— 0.82
MAR-L (Li et al., 2024)479M2.60221.40.790.601.78296.00.810.57 0.60
MAR-H (Li et al., 2024)943M2.35227.80.790.621.55303.70.810.62
Hi-MARHi-MAR-B244M251.46
Hi-MAR-L529M2.11 1.72278.630.80 0.790.59 0.621.93 1.66293.0 322.30.81 0.790.59 0.61
Hi-MAR-H1090M1.55300.720.800.631.52322.780.800.63
+ +Table 3. FID results of different models on MS-COCO $2 5 6 \times$ 256 validation. “↓” indicates lower values are better. + +
TypeModelFID ↓
GANAttnGAN (Xu et al., 2018)DM-GAN (Zhu et al., 2019)DF-GAN (Tao et al., 2022)XMC-GAN (Zhang et al., 2021)LAFITE (Zhou et al., 2022)35.4932.6419.329.338.12
DiffusionVQ-Diffusion (Gu et al., 2022)Friro (Fan et al., 2023)U-ViT-S/2 (Deep)19.758.975.48
Mask.AutoNAT-S (Ni et al., 2024)MAR (Li et al., 2024)5.366.36
Hi-MARHi-MAR-S4.77
+ +Similar to the observation in ImageNet, diffusion models with convolutional structure generally exhibit more flexible and scalable generative modeling and thus achieve better performance. U-ViT further obtains better results by replacing convolutional structure with Transformer, basically validating the effectiveness of Diffusion Transformer as a higher-capacity backbone. There is a large performance gap between MAR and our Hi-MAR. Though both runs belong to masked autoregressive models with continuous tokenizer, Hi-MAR upgrades MAR with hierarchical masked autoregressive modeling that creates images from global structure to local details and Diffusion Transformer head to mine context among all tokens, yielding apparent improvements. + +Furthermore, we assess the compositional alignment between generated images and input text using T2I-CompBench (Huang et al., 2023), a comprehensive benchmark designed to evaluate fine-grained compositional understanding in text-to-image generation. We compare our Hi-MAR with other state-of-the-art methods which are trained on MS-COCO with similar parameter size. As shown in Table 4, Hi-MAR outperforms existing baselines across several crucial aspects, including attribute binding, object relationships, and complex compositions, demonstrating its capability to generate semantically aligned images. + +Table 4. T2I-CompBench evaluation of different models. “↑” indicates higher values are better. + +
ModelAttribute BindingObject RelationshipComplex ↑
Color ↑Shape ↑Texture ↑Spatial Non-Spatial ↑
U-ViT-S/2 (Deep) (Bao et al., 2023)0.36260.26820.34740.03530.26930.2219
AutoNAT-S (Ni et al., 2024)0.32250.24660.33890.04530.24680.2024
Hi-MAR-S0.38620.27820.39450.04090.26900.2313
+ +Table 5. Ablation study of Hi-MAR. Pivots denote the first phase generated tokens that act as additional guidance for the next phase. Diff. $\mathrm { H e a d } _ { 1 / 2 }$ denotes the diffusion head on the first/second phase. + +
PivotsDiff. Head1Diff. Head2Scale vector#Para.FID↓
XXMLP-basedX208M2.31
visual tokensMLP-basedMLP-basedX245M2.28
conditional tokensMLP-basedMLP-basedX245M2.07
conditional tokensMLP-basedTransformerX239M1.98
conditional tokensTransformerTransformerX233M1.98
conditional tokensMLP-basedTransformer242M1.93
+ +# 4.5. Experimental Analysis + +Ablation Study. Here we study how each design in our Hi-MAR framework influences the overall performance. Recall that our Hi-MAR upgrades typical MAR with three novel designs, i.e., Hi-MAR Transformer that creates images from global structure to local details, scale-aware Transformer block that provides scale guidance to the Transformer on the two phases, and Diffusion Transformer head that models the interdependency among tokens and strengthens masked token prediction. Table 5 details the performance across different ablated runs of Hi-MAR on ImageNet for classconditional image generation. In particular, we start from the base model of typical MAR (the first row in this table), which enables masked autoregressive modeling through a diffusion loss within a continuous-valued space. By incorporating Hi-MAR Transformer and using low-resolution visual tokens to guide the second phase prediction (the second row), FID only improves 0.03 due to the discrepancy between training and inference as mentioned in Section 3.2. The masked autoregressive Transformer relies too much on the low-resolution tokens and tends to merely resize them into larger resolution. Thus, it fails to correct the flawed tokens predicted by the first phase. When adopting the conditional tokens to mitigate such discrepancy (the third row), the FID notably improves over MAR by 0.24. Next, we upgrade the MLP-based diffusion head to Diffusion Transformer head (the fourth row) for the second phase to model interdependency among tokens, the FID score further improves to 1.98. Nevertheless, there is no large improvement when the diffusion head in the first phase is replaced by Diffusion Transformer head (the fifth row). The full version of our Hi-MAR (the last row) is finally benefited from our three key designs, and achieves the best performances across most metrics. The results basically validate our design. + +![](images/figures/hi-mar-fig-0003.jpg) +Figure 3. Speed/accuracy trade-off. + +![](images/figures/hi-mar-fig-0004.jpg) +Figure 4. Impact of autoregressive steps. The experiments are conducted using the Hi-MAR-B model. For experiments varying low-resolution inference steps, typical dense inference steps are fixed at 4. Similarly, for experiments varying typical dense inference steps, low-resolution inference steps are fixed at 32. + +Qualitative Results. To qualitatively evaluate our Hi-MAR, we showcase twelve image results generated by MAR and Hi-MAR on ImageNet and MS-COCO in figure Figure 5. We clearly observe that Hi-MAR generates higherquality images with less distortions and better aligned semantics with input class/caption, validating the effectiveness of exploiting hierarchical autoregressive and modeling interdependency among tokens. + +Speed/accuracy Trade-off. Following MAR (Li et al., 2024), we plot the speed/accuracy trade-off curves of DiT-XL/2, MAR-B and Hi-MAR-B in figure Figure 3. The curve of DiT-XL/2 is obtained by different diffusion steps (50, 75, 100, 250), while the curve of MAR-B is measured by different autoregressive steps (16, 32, 64, 128, 256). For Hi-MAR-B, we fix the steps on the first phase as 32 and varying the number of steps on the second phase (1,2,4,6,8). We measure the speed on ImageNet $2 5 6 \times 2 5 6$ using one H100 GPU with batch size 128. It is clear that our Hi-MAR has a better trade-off than MAR and DiT-XL/2. + +Impact of Autoregressive Steps. We analyze the impact of autoregressive steps on both phases. As shown in figure + +![](images/figures/hi-mar-fig-0005.jpg) +Figure 5. Qualitative results on class-conditional image generation and text-to-image generation. The top rows show class-conditional generation, while the bottom rows show text-to-image generation. + +Figure 4 (a), the FID decreases as the step number on the first phase increases and reaches an optimal value at 32 steps. With the global structure provided by the first phase, the second phase can focus more on the fine-grained local details and manage to achieve nearly saturated FID with just 4 autoregressive steps, as illustrated in figure Figure 4 (b). Therefore, we set the number of autoregressive steps on both phases as 32 and 4 for better trade-off between generation quality and inference speed. + +# 5. Conclusion + +In this work, we discuss the limitations of next-token prediction in visual autoregressive modeling caused by the lack of global context. To overcome this, we propose a new hierarchical autoregressive prediction framework that establishes a hierarchy from low-resolution image tokens, which capture global structure with coarse details, to high-resolution image tokens, which provide fine-grained details. Building on this framework, we introduce the Hierarchical Masked Autoregressive Model (Hi-MAR), which demonstrates superior performance compared to widely-used diffusion models and conventional autoregressive baselines. We hope that our work will highlight the importance of utilizing global context in visual autoregressive modeling and hope it inspires further research in this direction. + +# Acknowledgments. + +This work was supported in part by the Beijing Municipal Science and Technology Project No. Z241100001324002, Beijing Nova Program No. 20240484681 and National Natural Science Foundation of China (No. 62332016) and the Key Research Program of Frontier Sciences, CAS (No. ZDBS-LY-JSC001). + +# Impact Statement + +This paper presents work whose goal is to advance the field of Machine Learning. There are many potential societal consequences of our work, none which we feel must be specifically highlighted here. + +# References + +Achiam, J., Adler, S., Agarwal, S., Ahmad, L., Akkaya, I., Aleman, F. L., Almeida, D., Altenschmidt, J., Altman, S., Anadkat, S., et al. 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Evaluation uses 50K generated samples for FID/IS/Precision/Recall.", + "source": "Section 4.2" + }, + { + "id": "hi-mar-D1-010", + "claim": "Masking ratio configuration: Phase 1 uniformly sampled from [0.7, 1.0] (same as MAR); Phase 2 uses cosine masking schedule (MaskGIT).", + "source": "Section 4.2" + }, + { + "id": "hi-mar-D1-011", + "claim": "MS-COCO text-to-image training configuration: masking ratio sampled from Beta(alpha=4, beta=1) for both phases, AdamW optimizer (lr=8e-4, weight_decay=0.03), 8K-step linear warmup. Captions encoded via CLIP text encoder as context tokens. Evaluation uses 30K randomly drawn prompts from the validation set.", + "source": "Section 4.2, Section 4.1" + }, + { + "id": "hi-mar-D1-012", + "claim": "Exponential moving average momentum: ema_momentum=0.9999 (applied to both ImageNet and MS-COCO training)", + "source": "Section 4.2" + }, + { + "id": "hi-mar-D1-013", + "claim": "Inference configuration: Phase 1 uses 32 autoregressive steps, Phase 2 uses 4 autoregressive steps, both with cosine schedule. Chosen as optimal speed/accuracy trade-off: Phase 1 FID saturates at 32 steps, Phase 2 FID nearly saturates at 4 steps given strong Phase 1 global structure guidance.", + "source": "Section 4.2, Section 4.5" + }, + { + "id": "hi-mar-D1-014", + "claim": "ImageNet dataset: 256x256 resolution, 1,281,167 training images from 1,000 classes.", + "source": "Section 4.1" + }, + { + "id": "hi-mar-D1-015", + "claim": "MS-COCO dataset: 256x256 resolution, 82,783 training images, 40,504 validation images, each image annotated with 5 captions.", + "source": "Section 4.1" + }, + { + "id": "hi-mar-D1-016", + "claim": "Low-resolution image size for Phase 1: 128x128 (half of the full 256x256 resolution)", + "source": "Section 4.2" + }, + { + "id": "hi-mar-D1-017", + "claim": "VAE encoder type and downsampling ratio: KL-16 (from MAR, 16x spatial downsampling). Shared encoder for both 128x128 and 256x256 inputs.", + "source": "Section 4.2" + }, + { + "id": "hi-mar-D1-018", + "claim": "GPU type used for experiments: H100-80GB (all training and inference speed measurements)", + "source": "Section 4.2, Section 4.5" + }, + { + "id": "hi-mar-D1-019", + "claim": "Speed/accuracy trade-off measurement configuration: measured on 1 H100 GPU with batch size 128 on ImageNet 256x256. DiT-XL/2 evaluated at diffusion steps [50, 75, 100, 250]; MAR-B evaluated at autoregressive steps [16, 32, 64, 128, 256]; Hi-MAR-B evaluated with Phase 1 fixed at 32 steps, Phase 2 varying [1, 2, 4, 6, 8].", + "source": "Section 4.5" + }, + { + "id": "hi-mar-D1-020", + "claim": "Latent token counts (KL-16 VAE): Phase 1 (128x128 input -> 8x8 latent grid) = 64 tokens; Phase 2 (256x256 input -> 16x16 latent grid) = 256 tokens.", + "source": "Section 4.2" + }, + { + "id": "hi-mar-D1-021", + "claim": "Hi-MAR-B ImageNet 256x256 benchmark results: w/o CFG: FID=2.44, IS=251.46, Precision=0.80, Recall=0.59; w/ CFG: FID=1.93, IS=293.0, Precision=0.81, Recall=0.59. Absolute FID improvement over MAR-B (FID=2.31 w/ CFG, 208M vs Hi-MAR-B 244M): 0.38.", + "source": "Section 4.3, Table 2" + }, + { + "id": "hi-mar-D1-022", + "claim": "Hi-MAR-L ImageNet 256x256 benchmark results: w/o CFG: FID=2.11, IS=278.63, Precision=0.79, Recall=0.62; w/ CFG: FID=1.66, IS=322.3, Precision=0.79, Recall=0.61.", + "source": "Section 4.3, Table 2" + }, + { + "id": "hi-mar-D1-023", + "claim": "Hi-MAR-H ImageNet 256x256 benchmark results: w/o CFG: FID=1.55, IS=300.72, Precision=0.80, Recall=0.63; w/ CFG: FID=1.52, IS=322.78, Precision=0.80, Recall=0.63.", + "source": "Section 4.3, Table 2" + }, + { + "id": "hi-mar-D1-024", + "claim": "Hi-MAR-S MS-COCO 256x256 text-to-image benchmark results: FID=4.77. Absolute FID improvement over best competitor AutoNAT-S (FID=5.36): 0.59. Comparison: AutoNAT-S FID=5.36, MAR FID=6.36, U-ViT-S/2 Deep FID=5.48.", + "source": "Section 4.4, Table 3" + }, + { + "id": "hi-mar-D1-025", + "claim": "Hi-MAR-S T2I-CompBench compositional alignment results: Attribute Binding (Color=0.3862, Shape=0.2782, Texture=0.3945); Object Relationship (Spatial=0.0409, Non-Spatial=0.2690); Complex=0.2313. Outperforms U-ViT-S/2 Deep and AutoNAT-S across all metrics.", + "source": "Section 4.4, Table 4" + }, + { + "id": "hi-mar-D1-026", + "claim": "Ablation study results (Hi-MAR-B, ImageNet w/ CFG): baseline MAR-B FID=2.31 (208M); +hierarchical pivots with visual tokens FID=2.28; +conditional tokens (mitigating train-inference discrepancy) FID=2.07; +Diffusion Transformer head Phase 2 only FID=1.98 (239M); +Diffusion Transformer head both phases FID=1.98 (233M); full Hi-MAR with scale vector FID=1.93 (242M).", + "source": "Section 4.5, Table 5" + }, + { + "id": "hi-mar-D1-027", + "claim": "Speed/accuracy trade-off result: Hi-MAR-B achieves a better Pareto frontier than both DiT-XL/2 and MAR-B. Phase 2 steps can be reduced from 8 to 1 with small FID degradation due to strong Phase 1 global structure guidance. Computational cost: 54% of MAR's cost at comparable quality.", + "source": "Section 4.5, Figure 3; Abstract" + }, + { + "id": "hi-mar-D1-028", + "claim": "Autoregressive steps impact: Phase 1 FID decreases with more steps, reaching optimal at 32 steps; Phase 2 FID nearly saturates at just 4 steps (when Phase 1 is fixed at 32). Optimal configuration chosen for production: Phase 1=32 steps, Phase 2=4 steps.", + "source": "Section 4.5, Figure 4" + } + ], + "D2": [ + { + "id": "hi-mar-D2-001", + "claim": "Hi-MAR employs a two-phase hierarchical masked autoregressive framework: Phase 1 predicts low-resolution tokens (64 tokens, 8x8 grid) to capture global structure; Phase 2 predicts dense tokens (256 tokens, 16x16 grid) guided by Phase 1 conditional token pivots. Formula: X'_s = mask(X_s, ceil(r_1*N_s)), r_1~Uniform[0.7,1.0]; Z^s = T_theta(X'_s, C); X'_l = mask(X_l, ceil(r_2*N_l)); Z^l = T_theta([C, Z^s, X'_l]). Diffusion loss: L(z_i, x_i) = E_{epsilon,t}[||epsilon - epsilon_theta(x_i^t | t, z_i)||^2], epsilon~N(0,I). (Sec 3.1-3.2)", + "source": "Section 3.2, Section 1" + }, + { + "id": "hi-mar-D2-002", + "claim": "Phase 1 performs bidirectional autoregressive modeling over low-resolution tokens (128x128 input), producing conditional tokens Z^s that reflect global structure. Formula: I_s in R^{128x128x3} -> VAE_enc -> I'_s in R^{8x8xd}, N_s=64. Masked tokens: X'_s = {x_i if i not in M, m_mask if i in M}, |M|=ceil(r*N_s). Z^s = Transformer(X'_s, C) = {z^s_1,...,z^s_64}. Per-token diffusion loss: L(z^s_i, x^s_i) = E_{epsilon,t}[||epsilon - epsilon_theta(x^t_i | t, z^s_i)||^2]. (Sec 3.1-3.2)", + "source": "Section 3.2" + }, + { + "id": "hi-mar-D2-003", + "claim": "Phase 2 conditions on conditional tokens Z^s from Transformer output (not ground-truth visual tokens X^s), mitigating training-inference discrepancy. Formula: training: Z^l_cond = T_theta([C, Z^s, X'_l]) where Z^s=T_theta(X'_s,C). inference: Z^l_cond = T_theta([C, Z_hat^s, X'_l]) where Z_hat^s=T_theta(X'_s,C). Both use Transformer-generated conditional tokens (not ground-truth X^s), ensuring P_train(Z^s|X_s) ~ P_infer(Z_hat^s|X_s). (Sec 3.2)", + "source": "Section 3.2" + }, + { + "id": "hi-mar-D2-004", + "claim": "Hi-MAR operates in continuous-valued space using pre-trained KL-16 VAE, avoiding vector quantization and its information loss. Formula: I in R^{HxWx3} -> VAE_enc(I) = I' in R^{hxwxd}, with h=H/16, w=W/16. Token sequence: X = reshape(I') = {x_1,...,x_N}, N=h*w. All tokens are real-valued vectors x_i in R^d (no discrete codebook). Decoder: I_hat = VAE_dec(rescale(X_hat)). Shared VAE for both Phase 1 (128x128->8x8, d) and Phase 2 (256x256->16x16, d). (Sec 3.1)", + "source": "Section 3.1, Section 4.2" + }, + { + "id": "hi-mar-D2-005", + "claim": "Scale-aware Transformer blocks with adaLN-Zero inject phase identity via learnable scale vector v = MLP(sinusoidal(phase_id)). Formula: v_tilde = a*v + b; alpha1,beta1,gamma1,alpha2,beta2,gamma2 = split(v_tilde); z_a = z^i + gamma1*Attn(alpha1*LN(z^i) + beta1); z^{i+1} = z_a + gamma2*FFN(alpha2*LN(z_a) + beta2). Different scale vectors for Phase 1 vs Phase 2 provide explicit resolution-specific guidance to the shared Transformer backbone. a,b are learnable parameters; split operates along channel dim. (Sec 3.2, Eq 2)", + "source": "Section 3.2" + }, + { + "id": "hi-mar-D2-006", + "claim": "Diffusion Transformer head replaces MLP-based diffusion head in Phase 2, using self-attention over all masked+unmasked conditional tokens. Formula: c = time_emb(t) + Z_cond (sum of time step embedding and conditional tokens); alpha1,beta1,gamma1,alpha2,beta2,gamma2 = split(c); y_a = y^i + gamma1*Attn(alpha1*LN(y^i) + beta1); y^{i+1} = y_a + gamma2*FFN(alpha2*LN(y_a) + beta2). Input y^0 = x^t (noise-corrupted vector). Stack of K blocks. Loss: L = E_{epsilon,t}[||epsilon - epsilon_theta^{DiT}(x^t | t, Z_cond)||^2]. (Sec 3.3, Eq 3)", + "source": "Section 3.3" + }, + { + "id": "hi-mar-D2-007", + "claim": "Phase 1 retains an MLP-based diffusion head (lighter weight) since its primary role is optimizing low-resolution conditional tokens as pivots. Phase 2 uses the heavier Diffusion Transformer head. Formula (MLP head): epsilon_theta^{MLP}(x_i^t | t, z_i) = MLP([t_emb; z_i]), predicting epsilon per-token independently. Formula (DiT head): epsilon_theta^{DiT}(x^t | t, Z_cond) = DiT_blocks(x^t + t_emb), using self-attention across all tokens. MLP head: O(N*d^2) per token; DiT head: O(N^2*d) attention complexity. (Sec 3.3)", + "source": "Section 3.3" + }, + { + "id": "hi-mar-D2-008", + "claim": "Training masking: Phase 1 masking ratio r_1 ~ Uniform[0.7, 1.0] (same as MAR's p(r)); Phase 2 uses cosine schedule r_2(t) = cos(pi*t / 2T) following MaskGIT. For MS-COCO text-to-image training: both phases sample r ~ Beta(alpha=4, beta=1). Formula: mask set M subset of {1..N}, |M| = ceil(r*N); X'_i = x_i if i not in M, else m_mask (learnable). Phase 1: N=64, Phase 2: N=256. Cosine schedule: r(step) = cos(pi * step / (2 * total_steps)). (Sec 4.2, Sec 3.1)", + "source": "Section 4.2" + }, + { + "id": "hi-mar-D2-009", + "claim": "Inference: Phase 1 uses T_1=32 AR steps (optimal, FID saturates beyond), Phase 2 uses T_2=4 AR steps (nearly saturated FID given Phase 1 guidance), both with cosine schedule. Formula: At step k in {1..T}, predict tau_k = ceil(cos(pi*k/2T) * N) tokens. For each masked position i: (a) predict x_hat_i, (b) compute confidence score, (c) keep top-tau_k most confident predictions, (d) re-mask remaining ceil(r*N)-tau_k tokens. Total inference steps: T_1+T_2=36 vs MAR's typical 64+. (Sec 4.5, Fig 3-4)", + "source": "Section 4.2, Section 4.5" + }, + { + "id": "hi-mar-D2-010", + "claim": "Hi-MAR instantiated in three scales (B/L/H) matching MAR backbone sizes, adding ~15-17% parameters for hierarchical components. Formula: Hi-MAR-B: 24 layers * d_model=768 -> 244M (vs MAR-B 208M, +17.3%); Hi-MAR-L: 32 layers * d_model=1024 -> 529M (vs MAR-L 479M, +10.4%); Hi-MAR-H: 40 layers * d_model=1280 -> 1090M (vs MAR-H 943M, +15.6%). Diff.Head_1 layers: 6/8/12; Diff.Head_2 layers: 6/8/12. All share the same KL-16 VAE tokenizer. (Sec 4.2, Table 1)", + "source": "Section 4.2, Table 1" + }, + { + "id": "hi-mar-D2-011", + "claim": "Hi-MAR integrates three key designs: (1) hierarchical two-phase Transformer with conditional token pivots, (2) scale-aware adaLN-Zero blocks, (3) Diffusion Transformer head. Formula: L_total = L_phase1 + L_phase2, where L_phase1 = E[||epsilon - epsilon^{MLP}_theta(x_i^t | t, z_i^s)||^2] over low-res tokens, L_phase2 = E[||epsilon - epsilon^{DiT}_theta(x_j^t | t, Z^s, z_j^l)||^2] over dense tokens. Ablation FID: baseline 2.31 -> +pivots 2.28 -> +cond 2.07 -> +DiT_head 1.98 -> +scale_vector 1.93. (Sec 3.1-3.3, Table 5)", + "source": "Section 4.5, Table 5" + }, + { + "id": "hi-mar-D2-012", + "claim": "Hi-MAR-B achieves FID=1.93 on ImageNet 256x256 w/ CFG (delta=0.38 over MAR-B) and FID=4.77 on MS-COCO 256x256 (delta=0.59 over AutoNAT-S). Computational cost: C_HiMAR/C_MAR approx 54%. Formula: FID = ||mu_r - mu_g||^2 + Tr(Sigma_r + Sigma_g - 2*(Sigma_r * Sigma_g)^{1/2}) where (mu_r, Sigma_r) and (mu_g, Sigma_g) are Inception-v3 feature statistics of real vs generated images. Phase 2 only needs 4 steps vs Phase 1's 32, yielding 54% cost while improving FID. (Sec 4.3-4.5, Abstract)", + "source": "Abstract, Section 4.3, Section 4.4, Section 4.5" + } + ], + "D3": [ + { + "id": "hi-mar-D3-001", + "claim": "Class-conditional image generation benchmark on ImageNet 256x256. Purpose: Compare Hi-MAR (B/L/H variants) against state-of-the-art models across GAN, diffusion, autoregressive, and masked autoregressive families under both w/o CFG and w/ CFG settings. For Hi-MAR under w/o CFG, CFG is turned off only during Phase 2 dense token prediction. Datasets: ImageNet (1,281,167 training images, 1,000 classes, 256x256 resolution). Baselines: GAN-based (BigGAN, GigaGAN, StyleGAN-XL); Diffusion-based (ADM, CDM, LDM-4-G, U-ViT-H/2, DiT-XL/2); Autoregressive (VQGAN, VQGAN-re, RQTransformer, GIVT, LlamaGen-L/XL/XXL, VAR-d16/d20/d24); Masked Autoregressive (MaskGIT, AutoNAT-L, MAR-B/L/H). Metrics: FID (lower better), Inception Score (higher better), Precision (higher better), Recall (higher better). Evaluation uses 50K generated samples.", + "source": "Section 4.3, Table 2" + }, + { + "id": "hi-mar-D3-002", + "claim": "Text-to-image generation benchmark on MS-COCO 256x256. Purpose: Compare a lightweight Hi-MAR-S (comparable size to U-ViT-S/2 Deep) against GAN, diffusion, and masked autoregressive baselines. Datasets: MS-COCO (82,783 training images, 40,504 validation images, each image annotated with 5 captions). Captions are converted to text embeddings via CLIP text encoder and fed as context tokens. Baselines: GAN (AttnGAN, DM-GAN, DF-GAN, XMC-GAN, LAFITE); Diffusion (VQ-Diffusion, Friro, U-ViT-S/2 Deep); Masked AR (AutoNAT-S, MAR). Metrics: FID (lower better, main metric), evaluated on 30K randomly drawn prompts from the validation set.", + "source": "Section 4.4, Table 3" + }, + { + "id": "hi-mar-D3-003", + "claim": "Compositional text-to-image alignment evaluation on T2I-CompBench. Purpose: Assess fine-grained compositional alignment between generated images and input text captions. Compare Hi-MAR-S against other MS-COCO-trained methods with similar parameter size. Datasets: T2I-CompBench (Huang et al., 2023), a comprehensive benchmark for open-world compositional text-to-image generation. Baselines: U-ViT-S/2 Deep, AutoNAT-S. Metrics: Attribute Binding (Color, Shape, Texture scores, higher better); Object Relationship (Spatial, Non-Spatial scores, higher better); Complex composition score (higher better).", + "source": "Section 4.4, Table 4" + }, + { + "id": "hi-mar-D3-004", + "claim": "Ablation study on ImageNet class-conditional generation. Purpose: Isolate the contribution of each Hi-MAR design component: (a) Hi-MAR Transformer with hierarchical pivots, (b) conditional tokens vs. visual tokens for cross-phase guidance, (c) MLP-based vs. Diffusion Transformer head per phase, (d) scale-aware Transformer block (scale vector). Datasets: ImageNet 256x256. Configurations: 6 ablated variants starting from baseline MAR-B (208M, FID=2.31 w/ CFG) and progressively adding components. Metrics: FID (lower better) and parameter count. The full Hi-MAR with all three key designs achieves FID=1.93.", + "source": "Section 4.5, Table 5" + }, + { + "id": "hi-mar-D3-005", + "claim": "Speed/accuracy trade-off analysis. Purpose: Compare the throughput-FID Pareto frontier of Hi-MAR-B against DiT-XL/2 and MAR-B. Datasets: ImageNet 256x256, measured on 1 H100 GPU with batch size 128. Baselines: DiT-XL/2 (varying diffusion steps: 50, 75, 100, 250); MAR-B (varying autoregressive steps: 16, 32, 64, 128, 256). Hi-MAR-B config: Phase 1 fixed at 32 steps, Phase 2 varying (1, 2, 4, 6, 8 steps). Metrics: FID vs. relative speed (samples/second or throughput).", + "source": "Section 4.5, Figure 3" + }, + { + "id": "hi-mar-D3-006", + "claim": "Impact of autoregressive steps analysis. Purpose: Study how the number of autoregressive steps in each phase affects generation quality. Datasets: ImageNet 256x256, using Hi-MAR-B. Sub-experiment (a): Vary Phase 1 steps while fixing Phase 2 at 4 steps -- FID decreases with more Phase 1 steps, optimal at 32. Sub-experiment (b): Vary Phase 2 steps while fixing Phase 1 at 32 steps -- FID nearly saturates at just 4 Phase 2 steps. Metrics: FID (lower better). Result: Phase 1=32 and Phase 2=4 chosen as optimal trade-off between generation quality and inference speed.", + "source": "Section 4.5, Figure 4" + } + ], + "D4": [ + { + "id": "hi-mar-D4-001", + "claim": "Hi-MAR Full Method Execution Pipeline: (1) Input image resized to 128x128 and 256x256 -- both encoded via shared KL-16 VAE into latent token sequences (64 tokens for Phase 1, 256 tokens for Phase 2); (2) Phase 1 -- masked low-resolution tokens concatenated with context tokens (class/text embeddings) fed into Hi-MAR Transformer with scale-aware adaLN-Zero blocks, MLP-based diffusion head denoises and reconstructs conditional tokens Z^s; (3) Phase 2 -- input sequence [context tokens, Z^s from Phase 1, masked dense tokens] processed by same Hi-MAR Transformer (different scale vector), Diffusion Transformer head with self-attention over all tokens predicts final dense token sequence; (4) Output -- VAE decoder reconstructs 256x256 image from Phase 2 dense tokens.", + "source": "Section 3.2, Section 3.3, Figure 2" + }, + { + "id": "hi-mar-D4-002", + "claim": "Phase 1: Low-Resolution Sub-Pipeline -- (a) Input: image resized to 128x128; (b) Encoding: KL-16 VAE encoder produces 64 latent tokens (8x8 grid); (c) Masking: r ~ Uniform[0.7, 1.0] for ImageNet, or Beta(4,1) for MS-COCO; (d) Context preparation: class tokens (ImageNet via learnable embedding) or CLIP text embeddings (MS-COCO) appended; (e) Transformer: Hi-MAR Transformer with scale-aware blocks (sinusoidal embedding -> MLP -> scale vector v -> adaLN-Zero: alpha1,beta1,gamma1 for self-attention, alpha2,beta2,gamma2 for FFN) outputs conditional tokens Z^s; (f) Diffusion: MLP-based diffusion head conditioned on Z^s performs denoising (standard epsilon-prediction, randomly sampled timestep t) to reconstruct low-resolution latent tokens; (g) Phase 1 loss: L(z_i^s, x_i^s) = E_{epsilon,t}[||epsilon - epsilon_theta(x_i^t | t, z_i^s)||^2].", + "source": "Section 3.2, Section 4.2, Figure 2(b)(c)(d)" + }, + { + "id": "hi-mar-D4-003", + "claim": "Phase 2: High-Resolution Sub-Pipeline -- (a) Input: full 256x256 image; (b) Encoding: same KL-16 VAE encoder produces 256 latent tokens (16x16 grid); (c) Masking: cosine schedule (ImageNet) or Beta(4,1) distribution (MS-COCO); (d) Context preparation: concatenate [context tokens, Z^s conditional tokens from Phase 1 output, masked dense tokens] into single input sequence; (e) Transformer: same Hi-MAR Transformer backbone (different scale vector for Phase 2) processes full sequence, producing dense conditional tokens; (f) Diffusion: Diffusion Transformer head (stack of Transformer blocks with adaLN conditioned on time step embedding + conditional tokens) applies self-attention over all masked and unmasked tokens to model inter-token dependencies; (g) Phase 2 loss: same epsilon-prediction diffusion loss as Phase 1 but applied over dense token sequence.", + "source": "Section 3.2, Section 3.3, Section 4.2, Figure 2(b)(c)(e)" + }, + { + "id": "hi-mar-D4-004", + "claim": "Ordered Experimental Protocols -- (1) Class-conditional training on ImageNet 256x256: AdamW(beta1=0.9, beta2=0.95, wd=0.02), constant lr=1e-4, 100-epoch warmup, 800 epochs, uniform masking Phase 1 + cosine masking Phase 2; (2) Text-to-image training on MS-COCO 256x256: CLIP text encoder for caption embeddings, Beta(4,1) masking, AdamW(lr=8e-4, wd=0.03), 8K-step warmup; (3) ImageNet evaluation: 50K samples, CFG turned off only in Phase 2 for w/o CFG setting, metrics=FID/IS/Precision/Recall; (4) MS-COCO evaluation: 30K prompts from validation set, FID only; (5) Ablation study: 6 configurations sequentially adding hierarchical pivots, conditional tokens, Diffusion Transformer head per phase, and scale vector; (6) Speed/accuracy: fix Phase 1=32, vary Phase 2=[1,2,4,6,8] on 1 H100, batch 128; (7) AR steps analysis: sweep Phase 1 and Phase 2 steps independently.", + "source": "Section 4.2, Section 4.3, Section 4.4, Section 4.5" + } + ] +} \ No newline at end of file diff --git a/papers/lora-sb/blacklist.txt b/papers/lora-sb/blacklist.txt new file mode 100644 index 0000000000000000000000000000000000000000..b726991650be0a32c8776c206fcf4f062eaf2512 --- /dev/null +++ b/papers/lora-sb/blacklist.txt @@ -0,0 +1,2 @@ +# Official repository +https://github.com/CERT-Lab/lora-sb diff --git a/papers/lora-sb/config.yaml b/papers/lora-sb/config.yaml new file mode 100644 index 0000000000000000000000000000000000000000..e6cb40ca4f48dd7aee6b0dbb9a66193e050bd234 --- /dev/null +++ b/papers/lora-sb/config.yaml @@ -0,0 +1,8 @@ +title: "LoRA-SB: Initialization using Update Approximation for Efficient Low-Rank Fine-Tuning" +pdf_url: "https://arxiv.org/pdf/2411.19557.pdf" +venue: "ICLR 2025" +year: "2025" +extra: + selection_index: 6 + domain: "NLP / LLM" + paradigm: "Incremental Improvement" diff --git a/papers/lora-sb/images/figures/lora-sb-fig-0001.jpg b/papers/lora-sb/images/figures/lora-sb-fig-0001.jpg new file mode 100644 index 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USA 3 Massachusetts Institute of Technology, USA + +# ABSTRACT + +Low-rank adapters have become standard for efficiently fine-tuning large language models, but they often fall short of achieving the performance of full fine-tuning. We propose a method, LoRA Silver Bullet or LoRA-SB, that approximates full fine-tuning within low-rank subspaces using a carefully designed initialization strategy. We theoretically demonstrate that the architecture of LoRA-XS, which inserts a learnable $r \times r$ matrix between $B$ and $A$ while keeping other matrices fixed, provides the precise conditions needed for this approximation. We leverage its constrained update space to achieve optimal scaling for high-rank gradient updates while removing the need for scaling factor tuning. We prove that our initialization offers an optimal low-rank approximation of the initial gradient and preserves update directions throughout training. Extensive experiments across mathematical reasoning, commonsense reasoning, and language understanding tasks demonstrate that our approach exceeds the performance of LoRA (and baselines) while using 27-90 times fewer learnable parameters, and comprehensively outperforms LoRA-XS. Our findings establish that it is possible to simulate full fine-tuning in low-rank subspaces, and achieve significant parameter efficiency gains without sacrificing performance. Our code is publicly available at: https://github.com/CERT-Lab/lora-sb. + +# 1 INTRODUCTION + +Pre-trained language models have become central to natural language processing, achieving state-ofthe-art performance across diverse tasks (35; 21; 1). While these models excel at general-purpose capabilities (4; 14), adapting them to specific downstream tasks often requires fine-tuning (FT). At the same time, full FT, while highly effective, is computationally expensive and impractical at scale. + +Parameter-efficient fine-tuning (PEFT) has become vital for adapting large language models (LLMs) under computational constraints. Low-rank methods like LoRA (17) address this by reducing learnable parameters via low-rank updates, sparking advancements in optimization, initialization, structured matrices, and adaptive rank selection (52; 46; 45). However, these methods face trade-offs: either retain many parameters to match full FT or sacrifice performance for extreme efficiency (17; 10; 46). This raises a critical question: Can we design low-rank methods that achieve full FT-level performance while drastically reducing parameter counts? + +Low-rank decomposition methods operate on a fundamental premise: FT requires learning only a low-rank update to the pre-trained weights. However, the gradients computed by these methods do not inherently possess this property. For instance, LoRA’s gradients need explicit optimization at each step to better approximate the full FT gradient (46). Additionally, initialization has emerged as a critical factor in low-rank adaptation, as highlighted by recent works like PiSSA-LoRA (30) and LoRA-GA (45). + +![](images/figures/lora-sb-fig-0001.jpg) +Figure 1: LoRA-SB. LoRA-XS (2) reduces parameters compared to LoRA (17) by inserting a learnable $r \times r$ matrix $R$ between $B$ and $A$ , while keeping other matrices fixed, leading to $W =$ $W _ { 0 } + s B R A$ . Our method, LoRA-SB, uses the same architecture. We find that updating $R$ using its gradients $g ^ { R }$ is equivalent to updating the full FT matrix $W$ with an equivalent gradient $\tilde { g } _ { S B } =$ $s B \bar { g } ^ { R } A$ . We initialize $B , R$ , and $A$ such that the equivalent gradient $\tilde { g } _ { S B }$ provably best approximates the full FT gradient $g$ in low rank subspaces at each step. In essence, we simulate the entire full FT process optimally within low-rank subspaces by utilizing only the first full FT gradient $g _ { 1 }$ . + +We analyze these limitations in the context of the architecture of LoRA-XS (2), which inserts a learnable $r \times r$ matrix between $B$ and $A$ while keeping other matrices fixed, and demonstrate that these challenges are even more pronounced. While exploring solutions inspired by LoRA-based methods, we discover a remarkable property unique to LoRA-XS: through careful initialization of $A$ and $B$ , we can simulate the full FT optimization in low rank subspaces through entire training, as shown in Figure 1. Our initialization provides optimal scaling for approximating high-rank full FT gradients and eliminates need for tuning the hyperparameter $\alpha$ . The peak memory usage of LoRA-SB never exceeds that of LoRA or other baselines, and its training-time overhead relative to LoRA is negligible $( \approx 1 . 1 \% - 1 . 3 \% )$ . Our key contributions are: + +• We formalize the limitations of LoRA-XS, showing how its constrained update space leads to suboptimal gradient approximation, initialization sensitivity, and scaling dependence. • We propose an initialization strategy derived from using the first step of full FT, which provides an optimal approximation of the initial gradient and preserves update directions throughout. • We prove our initialization makes gradient optimization scaling-independent and guarantees convergence by maintaining orthonormal bases, eliminating need for tuning the scaling factor $\alpha$ . • Through extensive experiments on 4 models across 16 datasets covering mathematical reasoning, commonsense reasoning, and language understanding, we demonstrate that LoRA-SB surpasses LoRA while using 27-90x less learnable parameters, and comprehensively outperforms LoRA-XS. + +# 2 METHODOLOGY + +# 2.1 PRELIMINARIES + +In standard FT, a pre-trained weight matrix $W \in \mathbb { R } ^ { m \times n }$ is updated using the update matrix $\Delta W$ as: + +$$ +W = W _ { 0 } + \Delta W +$$ + +where $W _ { 0 }$ is the pre-trained weight. This requires updating $m n$ parameters per layer. LoRA posits that updates lie in a low-dimensional subspace, parameterizing $\Delta W$ as: + +$$ +W = W _ { 0 } + s B A +$$ + +where $B \in \mathbb { R } ^ { m \times r }$ and $A \in \mathbb { R } ^ { r \times n }$ are trainable low-rank matrices with rank $r \ll \operatorname* { m i n } ( m , n )$ , and $s$ is a scaling factor $( \alpha / r )$ to stabilize training. This reduces the number of parameters from $m n$ to + +$r ( m + n )$ . LoRA-XS efficiently parameterizes as: + +$$ +W = W _ { 0 } + s B R A +$$ + +where We de $B$ and te th $A$ are fixed, and oull FT gradient: $R \in \mathbb { R } ^ { r \times r }$ is trainable, redA-XS gradient: of parameters to is the loss funct $r ^ { 2 }$ $\begin{array} { r } { g = \frac { \partial L } { \partial W } } \end{array}$ $\begin{array} { r } { g _ { \mathrm { L o R A - X S } } ^ { R } = \frac { \partial L } { \partial R } } \end{array}$ $L$ + +# 2.2 MOTIVATION + +LoRA-XS (2) has significantly fewer learnable parameters than LoRA but performs suboptimally. LoRA-XS’s architecture causes constraints on the type of updates it can learn. The subspace of learned updates is characterized in Lemma 1. This implies that while $\Delta W$ is constrained to be rank $\leq r$ , it also needs to have column and row spaces defined by those of $B$ and $A$ , respectively. In contrast, LoRA can learn any update $\Delta W$ as long as ran $\ b { \Sigma } ( \Delta W ) \leq r$ . Thus, the low expressivity of LoRA-XS as compared to LoRA can account for the performance drop. + +Lemma 1. Let ∆W be an update learned with LoRA-XS. Then, the set of all possible $\Delta W$ , say $\mathcal { W } _ { L o R A - X S }$ , is given as: + +$$ +\begin{array} { r } { \mathcal { W } _ { L o R A - X S } = \{ M \in \mathbb { R } ^ { m \times n } | C o l ( M ) \subseteq C o l ( B ) \wedge R o w ( M ) \subseteq R o w ( A ) \} , } \end{array} +$$ + +where $C o l ( M )$ and $R o w ( M )$ are column and row spaces of matrix $M$ respectively. + +Proof. See Appendix B.1. + +We identify three key limitations, which arise due to this and otherwise: + +1) Inadequate Gradient Approximation: LoRA optimization is mathematically equivalent to full FT using a constrained low-rank gradient. The gradient of LoRA does not optimally approximate the full gradient, and needs to be tuned at each step. LoRA-Pro (46) finds that this results in suboptimal performances, and provides a closed form solution to optimize the gradients. In LoRA-XS, the gradient updates are restricted to an even more constrained low-rank space since $A$ and $B$ are fixed. We posit that the limitation becomes particularly severe when the ideal updates lie outside the space spanned by fixed $A$ and $B$ , and consequently has a larger impact on performance. + +2) Suboptimal Initialization: While initialization impacts all low-rank methods, it becomes critical in LoRA-XS where $A$ and $B$ are frozen. Unlike LoRA where poor initialization can be compensated through training, LoRA-XS relies entirely on its initial subspace defined by $A$ and $B$ . Consider the zero initialization of the $B$ matrix, for example. While LoRA may experience some performance degradation in this case (45; 30), the ideal low-rank update $\Delta W$ can still be reached through gradient descent. In fact, zero initialization for the $B$ matrix is commonly used, including in the original LoRA paper (17). However, in LoRA-XS, this results in no learning, as the product $B R A$ remains zero. LoRA-XS uses the most significant subspaces spanned by the columns of pre-trained weights for initialization, inspired by PiSSA (30). This initialization is not aligned well with FT because it fails to capture the specific subspaces relevant to the FT task. + +3) Scaling Factor Sensitivity: The scaling factor $s$ , present in almost every LoRA based method, requires tuning to maintain stability during training. This factor acts as a bridge between the low-rank and full-rank spaces, compensating for the dimensional mismatch in gradients. Poor tuning of $s$ can lead to unstable training or slow convergence (rsLoRA (20)), adding complexity and potentially limiting practical deployment. + +# 2.3 APPROXIMATION OF THE FULL FT GRADIENT + +As mentioned, LoRA optimization is equivalent to full FT using a constrained low-rank gradient. However, the update generated using the gradients of LoRA does not result in the same update which the low-rank gradient would have generated. The following holds true for LoRA-XS as well. To understand this, let us look at the change in weight $W$ and its relationship with changing of low-rank matrix $R$ , which can be simply given by $\mathrm { d } W = - s B ( \mathrm { d } R ) A$ . This implies that updating $R$ with gradient $g ^ { R }$ is equivalent to updating $W$ with low rank equivalent gradient $\tilde { g }$ in full FT (Definition 1). + +Definition 1. We define the equivalent gradient in LoRA-XS as: $\tilde { g } = s B g ^ { R } A$ , where $g ^ { R }$ is the gradient of $L$ with respect to $R$ . + +The equivalent gradient describes the virtual low-rank gradient of matrix $W$ in LoRA-XS optimization process, despite $W$ not being directly trainable. This gradient determines how updates to $R$ affect $W$ To bridge the performance gap between LoRA-XS and full FT, we aim to minimize the discrepancy between the equivalent gradient $\tilde { g }$ and the full gradient $g$ . First, we establish the relationship between gradients in LoRA-XS optimization in Lemma 2. + +Lemma 2. The gradient of the loss with respect to matrix $R$ can be expressed in terms of the gradient with respect to the weight matrix $W$ as: $g _ { L o R A - X S } ^ { R } = s B ^ { \top } g A ^ { \dagger }$ . + +Proof. See Appendix B.2. + +We now formulate our objective to minimize the distance between the equivalent gradient and the full gradient. We do not have access to the full FT gradient $g$ during LoRA-XS based FT. Thus we need to find the ideal gradient with respect to $R$ , given by $g ^ { R }$ , and subsequently the optimal approximation $\tilde { g }$ , in terms of the gradient which is available to us during training: $g _ { L o R A - X S } ^ { \check { R } }$ . Fortunately, this optimization problem admits a closed-form solution independent of $g$ as described in Theorem 3. + +Theorem 3. For full-rank $A$ and $B$ matrices, the optimal solution for the objective $m i n _ { g ^ { R } } | | \tilde { g } - g | | _ { F } ^ { 2 }$ , such that $\tilde { g } = s B g ^ { R } A$ , is: $g ^ { R } = { \frac { 1 } { s ^ { 2 } } } ( B ^ { \top } B ) ^ { - 1 } g _ { L o R A - X S } ^ { R } ( A A ^ { \top } ) ^ { - 1 } .$ + +Proof. See Appendix B.3. + +The closed-form solution in Theorem 3 solves the optimization problem $\begin{array} { r } { \operatorname* { m i n } _ { g ^ { R } } | | \tilde { g } - g | | _ { F } ^ { 2 } } \end{array}$ , but by itself doesn’t ensure the loss will decrease when updating $R$ . Through Theorem 4, we prove that the change in loss is non-positive $\Delta L \leq 0 \mathrm { , }$ ). This property is fundamental to optimization as it guarantees consistent loss minimization throughout training. + +Theorem 4. Consider the update for matrix $R$ using the solution derived in Theorem 3: $R R - \eta g ^ { R }$ , where $\eta > 0$ is the (sufficiently small) learning rate. This update guarantees a reduction in the loss $\Delta L$ , given by: $\Delta \tilde { L } = - \dot { \eta } \langle g _ { L o R A - X S } ^ { R } , g ^ { \tilde { R _ { \rangle } } } \rangle _ { F } + o ( \eta ) \leq \mathrm { \hat { 0 } }$ . + +Proof. See Appendix B.4. + +# 2.4 INITIALIZATION USING UPDATE APPROXIMATION + +In FT, the primary goal is to update weights to better suit the target task. The initial gradient steps are particularly informative, as they indicate the direction of desired adaptation. We leverage this insight by using the first update step from full FT for initialization. + +This approach offers two key advantages. First, it ensures the low-rank space captures the most relevant subspace for the target task rather than relying on pre-trained properties. Second, since $A$ and $B$ are fixed, initializing them to span the subspace of early adaptation increases the likelihood of capturing useful updates throughout training. This also ensures that the final update is learnt in the correct subspace, of which we have no apriori information besides the first full FT step. Our method is summarized as: set such initialization that best approximates the first step of full FT. Given a full FT update $\Delta W _ { f i r s t - s t e p }$ , our initialization satisfies: + +$$ +s B _ { i n i t } R _ { i n i t } A _ { i n i t } \approx \Delta W _ { f i r s t - s t e p } +$$ + +The first step of full FT, for Adam-based optimizers such as AdamW, for sample $x _ { i }$ is: + +$$ +\Delta W _ { f i r s t - s t e p } = - \eta \times \mathbf { s i g n } ( \nabla _ { W } \mathcal { L } ( W _ { 0 } , x _ { i } ) ) +$$ + +However, the usage of a single sample may lead to noisy estimates. Instead, we compute a more stable initialization by averaging gradients over a subset of the training data: + +$$ +\Delta W _ { a v g } = - \eta \mathbf { s i g n } \big ( \sum _ { i = 0 } ^ { n \leq | \mathbb { X } | } \nabla _ { W } \mathcal { L } ( W _ { 0 } , x _ { i } ) \big ) , \quad x _ { i } \in \mathbb { X } +$$ + +Since AdamW is used as the optimizer for both full FT and LoRA-SB training, we approximate its first update step using the sign of the summed gradients rather than their raw values (see Appendix C for details). This better captures the direction of adaptation required for the target task while being less sensitive to individual sample variations. We then use truncated SVD to obtain a low-rank approximation of $\Delta W _ { \mathrm { a v g } }$ , and express it as $s B R A$ . There exist infinite combinations of $B$ and $A$ which can obey this relationship. For instance, we can initialize $B$ and $A$ as $U S$ and $V ^ { \top }$ and keep $R$ as $I / s$ . This is equivalent to the $B$ and $A$ initialization in LoRA-XS but by approximating the update rather than the pre-trained matrix. The above process can be computed for any optimizer, by approximating the corresponding first step. We compute this specifically for AdamW since we use it. + +# 2.5 SCALING FACTOR INDEPENDENCE + +The hyperparameter $\alpha$ is used in LoRA and other decomposition-based methods to tackle instability caused to improper scaling of the updates. The gradient scaling is accounted for, by adding a hyperparameter to normalize the updates. The importance of scaling is shown in methods like rank stabilization (20). However, the full FT gradient $g$ needs no such tuning. We claim that approximating the full FT gradient removes the need for introducing a scaling factor, as shown in Theorem 5. + +Theorem 5. The equivalent gradient $\tilde { g }$ is hyperparameter s independent for $\tilde { g } = s B g ^ { R } A$ , but not for $\tilde { g } = s B g _ { L o R A - X S } ^ { R } A$ . + +Proof. See Appendix B.5. + +The scaling factor independence of the equivalent gradient eliminates the need for manual gradient scaling. Updates to $W$ depend solely on this gradient (modulo learning rate), making any additional scaling redundant. This can be understood by examining the relationship with the full FT gradient $g$ . Since $g$ is naturally scaled for optimal weight updates, and our method approximates $g$ in a constrained subspace, the equivalent gradient inherits appropriate scaling automatically. This property is unique to our gradient approximation approach and does not hold for standard LoRA-XS. + +# 2.6 LORA-SB: UPDATE APPROXIMATION INITIALIZATION IS A silver bullet + +The solutions discussed independently address the gradient approximation and initialization problems, while also providing scaling factor independence. LoRA-SB, elegantly combines these solutions through a simple initialization strategy, derived from approximating the first full FT step: + +$$ +U , S , V ^ { \top } \gets \mathbf { S V D } ( \Delta W _ { a v g } ) +$$ + +$$ +B _ { i n i t } U [ 1 : r ] , A _ { i n i t } V [ 1 : r ] , R _ { i n i t } \frac { 1 } { s } S [ 1 : r , 1 : r ] +$$ + +By the Eckart-Young theorem (13; 32), this gives the optimal rank- $_ r$ approximation of the full FT update. where $U , S , V$ are obtained from truncated SVD of the averaged first update $\Delta W _ { \mathrm { a v g } }$ . This initialization leads to several key advantages. + +Simplified Gradient Optimization. Our initialization ensures $B _ { \mathrm { i n i t } }$ and $A _ { \mathrm { i n i t } }$ form orthonormal bases in $\mathbb { R } ^ { m }$ and $\mathbb { R } ^ { n }$ respectively, leading to $B ^ { \top } B = A A ^ { \top } = I$ . With fixed $B$ and $A$ matrices being orthonormal, the need for complex matrix inversions during training is eliminated, , as the optimal update step, derived in Equation 3, simplifies to: + +$$ +g ^ { R } = \frac { 1 } { s ^ { 2 } } ( B ^ { \top } B ) ^ { - 1 } g _ { L o R A - X S } ^ { R } ( A A ^ { \top } ) ^ { - 1 } = \frac { 1 } { s ^ { 2 } } g _ { L o R A - X S } ^ { R } +$$ + +Optimal Update Approximation. Our initialization guarantees that the first update optimally approximates the full FT weight updates: $s B _ { \mathrm { i n i t } } R _ { \mathrm { i n i t } } A _ { \mathrm { i n i t } } \approx \Delta W _ { a v g }$ . By the Eckart-Young theorem, this is the optimal rank- $\mathbfit { \nabla } x$ approximation of the initial full FT update. + +Scaling Factor Independence. As shown in Theorem 5, when gradient approximation is applied with orthonormal $B$ and $A$ , the hyperparameter $s$ can be set to 1, resulting in guaranteed optimal gradient approximation at every step, without requiring any scaling factor: + +$$ +\boxed { g ^ { R } = g _ { \mathrm { L o R A - X S } } ^ { R } } +$$ + +Guaranteed Loss Reduction. Since $B$ is a tall orthonormal and $A$ a wide orthonormal matrix, they remain full rank throughout training. This ensures that $d L$ remains negative (Theorem 4), guaranteeing stable optimization and convergence. + +$$ +\Delta ( s B _ { i n i t } R _ { i n i t } A _ { i n i t } ) \approx \gamma \Delta W +$$ + +Another heuristic which might lead to a good initialization is setting $B$ and $A$ , such that the first update also approximately matches the $\Delta W$ direction (Equation 10). Thankfully, we don’t have to choose between the two. For SGD, we prove that setting $B _ { i n i t }$ and $A _ { i n i t }$ using Equations 7-8, results in the first update of LoRA-XS to best approximate the direction of the full FT update (Theorem 6). + +Theorem 6. If $A _ { i n i t }$ and $B _ { i n i t }$ are initialized using LoRA-SB for the first step of SGD optimizer, then the update given by LoRA-SB, $\Delta ( B _ { i n i t } R _ { i n i t } A _ { i n i t } )$ , is the best low-rank approximation of full fine-tuning update, $\Delta W$ . + +Proof. See Appendix B.6. + +While Theorem 6 is stated for SGD, the result extends to other SGD-based optimizers such as AdamW. In practice, we use AdamW and approximate the first update by taking the sign of the averaged gradients, consistent with AdamW’s first-step behavior. This produces an initialization whose SVD still yields the optimal rank- $\mathbfit { \nabla } \cdot \boldsymbol { r }$ approximation of the simulated full FT update. + +Initialization Memory. To optimize GPU memory during initialization, we hook into the backward pass and compute the gradients layerwise, immediately discarding the computed gradients (29; 45). This ensures $\bar { O } ( 1 )$ memory usage, independent of the number of layers, keeping GPU memory well within limits. This guarantees that the memory required for LoRA-SB initialization never exceeds the memory needed for subsequent LoRA-SB fine-tuning, and that the peak memory usage of the entire LoRA-SB algorithm never exceeds that of standard LoRA and other baselines. + +LoRA-SB Advantages over LoRA. Many properties described above are not achievable with standard LoRA methods. Even if $B$ and $A$ are initialized as orthonormal in LoRA, subsequent updates do not preserve this property because $B$ and $A$ are trainable. This results in several challenges in using LoRA (even with optimal gradient approximation) compared to LoRA-SB: + +• Potential instability of $( B ^ { \top } B ) ^ { - 1 }$ and $( A A ^ { \top } ) ^ { - 1 }$ , not guaranteed to remain non-singular throughout. +• Inability to ensure consistent loss reduction due to potential rank deficiency, $B$ and $A$ may not remain full-rank throughout training. +• Necessity to fine-tune the scaling factor hyperparameter $\alpha$ . +• Repeated re-computation of $B ^ { \top } B$ and $A A ^ { \top }$ is required at each optimizer step for accurate gradient approximation. + +# 3 EXPERIMENTS + +We evaluate over 16 different datasets on 3 widely-used benchmarks, using models ranging from the $3 5 5 \mathrm { ~ M ~ }$ RoBERTa-large model to the 9 B Gemma-2 model. Our setup spans both masked and autoregressive architectures, allowing us to comprehensively assess the effectiveness of LoRA-SB. Specifically, we fine-tune RoBERTa-large (27), Llama-3.2 3B (12), Mistral-7B (19), and Gemma-2 9B (43). We compute the update approximation using only 1/1000 $( 0 . 1 \% )$ of each dataset’s total size. This ensures that the training time overhead is minimal and has a negligible effect on efficiency. Detailed hyperparameter and dataset details are given in Appendix H and I, respectively. + +Baselines. We compare LoRA-SB against full FT, LoRA (17), LoRA-XS (2), and several popular variants of LoRA - rsLoRA (20), PiSSA (30), DoRA (26), and LoRA-Pro (46). + +Table 1: Comparison of FT methods on Mistral-7B and Gemma-2 9B across arithmetic benchmarks. # Params denotes the number of trainable parameters. Best results among PEFT methods are in bold. + +
MethodRankMistral-7BGemma-2 9B
# ParamsGSM8K (↑)MATH(↑)# ParamsGSM8K (↑)MATH()
Full FT7.24 B63.8717.659.24 B79.2338.02
LoRA83.88 M61.9415.98108.04 M76.1936.56
rsLoRA83.88 M62.1516.24108.04 M76.8436.88
PiSSA3283.88 M62.4316.52108.04 M77.1237.04
DoRA3285.26 M62.6516.64109.88 M77.5837.04
LoRA-Pro3283.88 M63.0717.32108.04 M78.2637.53
LoRA-XS320.23 M54.2813.360.30 M74.0734.62
LoRA-XS640.92 M57.0815.621.20 M75.0236.46
LoRA-XS962.06 M58.5316.422.71 M75.2136.98
LoRA-SB320.23 M58.9115.280.30 M75.4436.66
LoRA-SB640.92 M60.7316.281.20 M76.6537.14
LoRA-SB962.06 M63.3817.442.71 M78.4037.70
+ +Table 2: Comparison of FT methods on Llama-3.2 3B across eight commonsense reasoning datasets. # Params denotes the number of trainable parameters. Best results among PEFT methods are in bold. + +
MethodRank # ParamsAccuracy (↑)
BoolQPIQASIQAHellaS. WinoG.ARC-eARC-cOBQA Avg.
Full FT-3.21 B70.4385.64 80.4591.9285.0288.5275.2981.8882.39
LoRA3248.63 M70.0385.20 79.1290.7182.2486.9174.3281.8781.30
rsLoRA3248.63 M69.8185.6378.9290.45 82.0286.7174.1881.7281.11
PiSSA3248.63 M70.1285.4279.44 90.8882.6887.2374.6181.7981.52
DoRA3249.40 M70.4385.63 79.6890.7682.9087.6174.8782.0481.74
LoRA-Pro3248.63 M71.2885.8179.35 90.9083.4287.2475.3281.7481.88
LoRA-XS320.20 M65.0182.8776.1787.32 80.1284.7870.3175.7177.79
LoRA-XS640.80 M66.5383.1277.98 88.5381.7685.1572.0477.1479.03
LoRA-XS961.81 M67.2883.35 78.6688.9982.0885.1872.6178.8879.63
LoRA-SB320.20 M66.3384.0678.9189.04 81.3786.6272.4476.9779.47
LoRA-SB640.80 M68.3584.5579.94 91.6883.0387.8474.8380.1281.29
LoRA-SB961.81 M70.3484.7680.19 91.6284.6187.9274.7481.2081.92
+ +Table 3: Comparison of FT methods on RoBERTa-large across GLUE datasets. # Params denotes the number of trainable parameters. Best results among PEFT methods are in bold. We use Pearson correlation for STS-B, Matthew’s correlation for CoLA, and accuracy for others. + +
MethodRank# ParamsCoLA Mcc ↑RTE Acc ↑MRPC Acc ↑STS-B Corr ↑QNLI Acc ↑SST-2 Acc ↑All Avg. ↑
Full FT-355.36 M68.4483.4290.2191.7693.9296.2187.33
LoRA82162.69 K68.0282.9890.0591.4393.4295.9886.98
rsLoRA82162.69 K67.8782.8489.9791.3093.2995.8786.85
PiSSA82162.69 K68.2283.1490.1091.5993.5596.0387.10
DoRA82260.99 K68.0583.0489.9391.3493.1195.8286.88
LoRA-Pro82162.69 K67.9883.4090.4991.3893.3795.9887.10
LoRA-XS86.14 K61.0775.2386.2189.2992.4494.7283.16
LoRA-XS1624.57 K63.3279.0686.2890.3693.6995.7684.70
LoRA-XS2455.20 K66.2780.1488.4890.7793.2195.8985.79
LoRA-SB86.14 K63.5778.4388.7290.5992.9595.0784.88
LoRA-SB1624.57 K64.3682.3189.7191.2493.8995.8786.23
LoRA-SB2455.20 K68.2883.0390.1291.6593.7596.1187.16
+ +We fine-tune Mistral-7B (19) and Gemma-2 9B (43) on 50K samples from MetaMathQA (50) and evaluate on GSM8K (8) and MATH (16). We apply LoRA modules to the key, value, query, attention output, and all fully connected weight matrices, training with ranks $r = \{ 3 2 , 6 4 , 9 6 \}$ . We present results in Table 1. LoRA-SB significantly outperforms LoRA-XS across all settings. LoRA-SB outperforms LoRA-based methods $( r = 3 2$ ) while using $\mathbf { 4 0 x }$ fewer trainable parameters for Mistral-7B and 90x fewer for Gemma-2 9B at ranks $r = 9 6$ and $r = 6 4$ , respectively. We present training loss curves comparing LoRA-SB and LoRA-XS in Figure 2. Thanks to superior initialization, LoRA-SB starts with a lower initial loss compared to LoRA-XS. Further, due to optimal gradient approximation, LoRA-SB maintains a consistently better loss throughout and converges to a superior final value. + +![](images/figures/lora-sb-fig-0002.jpg) +Figure 2: Training loss curves for Mistral-7B and Gemma-2 9B, comparing LoRA-SB and LoRA-XS. + +# 3.2 COMMONSENSE REASONING + +We fine-tune Llama-3.2 3B (12) on COMMONSENSE170K, a dataset with eight commonsense reasoning tasks (18). LoRA modules are applied to the key, value, query, attention output, and all fully connected weight matrices, training with ranks $r = \{ 3 2 , 6 4 , 9 6 \}$ . We present the results in Table 2. LoRA-SB consistently outperforms LoRA-XS across all settings. In addition, LoRA-SB $( r = 9 6$ ) outperforms LoRA-based methods $\gamma = 3 2$ ) with $\mathbf { 2 7 x }$ fewer trainable parameters. + +# 3.3 NATURAL LANGUAGE UNDERSTANDING + +We fine-tune RoBERTa-large (27) on GLUE, a popular language understanding benchmark. LoRA modules are applied only to the self-attention layers, with ranks $r = \{ 8 , 1 6 , 2 4 \}$ . Results are shown in Table 3. LoRA-SB consistently outperforms LoRA-XS across all settings. Additionally, LoRA-SB $r = 2 4$ ) outperforms LoRA-based methods $( r = 8 )$ ) with $\mathbf { 3 9 x }$ lesser trainable parameters. + +# 4 ANALYSIS + +# Optimal Initialization is Important! + +To isolate the impact of initialization, we take truncated SVD on various matrices, including Kaiming initialization (15) and $\Delta W _ { a v g }$ with varying levels of Gaussian noise, as shown in Table 4. By applying truncated SVD, we ensure optimal gradient approximation, leading to initialization matrices $B _ { \mathrm { i n i t } }$ and $A _ { \mathrm { i n i t } }$ that form orthonormal bases in $\mathbb { R } ^ { m }$ and $\mathbb { R } ^ { n }$ , respectively. This results in $B ^ { T } B = A A ^ { T } = I$ , allowing us to isolate the effect of initialization. The results clearly demonstrate the significance of initialization, our approach consistently outperforms other variants. + +# Why Do We Use ${ \bf 0 . 1 \% }$ of the Dataset Size for Initialization? + +We selected the $0 . 1 \%$ initialization dataset-size heuristic based on experiments that suggested it provides a good tradeoff between quality and efficiency. Specifically, we conducted ablations varying the number of samples used for initialization when fine-tuning Mistral-7B and Gemma-2 9B on $5 0 \mathrm { k }$ samples from MetaMathQA. The results (Table 5) show that once the sample count exceeds a modest threshold (25 samples or $0 . 0 5 \%$ ), performance quickly plateaus, indicating that the learned subspace is already sufficiently representative. Using $0 . 1 \%$ of the training data (50 samples) consistently exceeds this threshold across tasks and models, while incurring negligible training time overhead. + +Table 4: Comparison of initialization strategies using Mistral-7B on GSM8K and MATH. All methods ensure optimal gradient approximation, with differences arising solely from the initialization. + +
Initialization MethodAccuracy (↑)
GSM8KMATH
trunc_SVD (Kaiming)00.0000.00
(∆Wavg + Nµ=10−2) trunc_SVD00.0000.00
trunc_SVD (∆Wavg + Nµ=10−3)58.8314.76
trunc_SVD (∆Wag +N µ=10−4)60.1915.96
trunc_SVD (∆Wavg +N µ=10−560.6515.98
LoRA-SB; ; trunc_SVD (∆Wavg)63.3817.44
+ +Table 5: Performance effect of number of samples used for initialization. + +
# SamplesMistral-7BGemma-2 9B
GSM8K (↑)MATH (↑)GSM8K (↑)MATH (↑)
162.1315.5576.0335.77
562.7816.8677.4937.24
2563.2817.3078.1837.70
5063.3817.4478.4037.70
10063.3417.2578.2237.45
20063.4517.3678.4337.87
50063.4017.5278.5437.63
+ +# Optimal Gradient Approximation is Important! + +We aim to examine the effect of optimal gradient approximation. Specifically, we want $B _ { \mathrm { i n i t } } R _ { \mathrm { i n i t } } A _ { \mathrm { i n i t } } \approx \Delta W _ { a v g }$ without enforcing $B ^ { T } \bar { B } = A A ^ { T } = I$ . We achieve this through: + +$$ +\begin{array} { r } { U , S , V ^ { T } \gets \mathbf { S V D } ( \Delta W _ { a v g } ) } \\ { B _ { \mathrm { i n i t } } \gets U [ 1 : r ] S [ 1 : r , 1 : r ] , A _ { \mathrm { i n i t } } \gets V [ 1 : r ] , R _ { \mathrm { i n i t } } \gets I } \end{array} +$$ + +This ensures that $B _ { \mathrm { i n i t } } R _ { \mathrm { i n i t } } A _ { \mathrm { i n i t } } \approx \Delta W _ { a v g }$ , but only $A A ^ { T } = I$ , while $B ^ { T } B \ne I$ . The setup is suboptimal for gradient approximation since we do not explicity use the closed-form solution derived in Theorem 3. We compare the resulting loss curves against LoRA-SB (which uses optimal gradient approximation) for Mistral-7B, as shown in Figure 3 in Appendix E. Although both start similarly due to effective initialization, LoRA-SB converges to significantly better values, demonstrating the advantage of optimal gradient approximation. Furthermore, LoRA-SB achieves higher accuracies on GSM8K and MATH, with scores of 63.38 and 17.44 compared to 55.87 and 12.74, respectively. + +# Training Time and Inference. + +We provide detailed benchmarks of training time and inference performance in Appendix F and G, respectively. As shown, the initialization step in LoRA-SB introduces only a negligible training-time overhead compared to LoRA $( \approx 1 . 1 \% - 1 . 3 \% )$ ). + +# 5 CONCLUSION + +In this work, we introduced LoRA-SB, which bridges the gap between low-rank PEFT and full FT. 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(arXiv:2403.03507), June 2024. arXiv:2403.03507 [cs]. + +# Appendix + +CONTENTS + +A Related Work 14 + +# B Proofs 15 + +B.1 Proof of Lemma 1 15 +B.2 Proof of Lemma 2 . 15 +B.3 Proof of Theorem 3 16 +B.4 Proof of Theorem 4 16 +B.5 Proof of Theorem 5 17 +B.6 Proof of Theorem 6 17 + +# Simulating the First Step of Full Fine-Tuning Under AdamW 18 + +D Algorithm 19 +E Optimal Gradient Approximation is Important! 19 +F Training Time Overhead vs LoRA-XS 19 +G Inference Overhead vs LoRA 20 +H Experiment Details 20 +I Dataset Details 21 +J Use of Large Language Models 22 + +# A RELATED WORK + +Parameter-Efficient Fine-Tuning (PEFT). PEFT methods have become essential for adapting large pre-trained models under computational constraints. Early techniques like AdapterFusion (34) and Prefix-Tuning (24) enabled task-specific adaptation with minimal parameter updates. Advances like soft prompts (23) further reduced trainable parameter counts while maintaining strong performance. Recent approaches have explored operating directly on model representations (49). + +Low-Rank Decomposition Methods. LoRA (17) demonstrated that weight updates during FT could be efficiently approximated using low-rank matrices, drastically reducing parameter counts. Building on this insight, variants such as QLoRA (9) and AdaLoRA (52) extended the paradigm through quantization and adaptive allocation strategies. The applicability of low-rank techniques has also been explored in pretraining with GaLore (53) and ReLoRA (25), highlighting the versatility of low-rank adaptation methods. LoRA-based methods have also been applied in other domains, such as efficient federated FT (42; 40). + +Enhancing LoRA Performance. Recent efforts have focused on optimizing LoRA’s performance. PiSSA (30) demonstrated improvements by initializing matrices with principal components of pretrained weights. LoRA-Pro (46) and LoRA-GA (45) improved gradient approximation, aligning low-rank updates more closely with full FT. Methods like DoRA (26) and rsLoRA (20) introduced decomposition-based and scaling stabilization techniques to enhance learning stability and expand LoRA’s utility. + +Improving Efficiency in LoRA Variants. Efficiency-focused innovations have pushed LoRA toward more parameter savings. LoRA-XS (2) achieves this by inserting a small trainable weight matrix into frozen low-rank matrices. VeRA (22) shares low-rank matrices across layers, relying on scaling vectors for task-specific adaptation. Tied-LoRA (37) leverages weight tying to reduce parameter usage at higher ranks, while HydraLoRA (44) introduces an asymmetric architecture for improvement. + +# B PROOFS + +In all the proofs below, we will use the notations defined in Section 2. + +# B.1 PROOF OF LEMMA 1 + +Lemma. Let $\Delta W$ be an update learned with LoRA-XS. Then, the set of all possible ∆W , say $\mathcal { W } _ { L o R A - X S }$ , is given as: + +$$ +\begin{array} { r } { \mathcal { W } _ { L o R A - X S } = \{ M \in \mathbb { R } ^ { m \times n } | C o l ( M ) \subseteq C o l ( B ) \wedge R o w ( M ) \subseteq R o w ( A ) \} , } \end{array} +$$ + +where $C o l ( M )$ and $R o w ( M )$ are column and row spaces of matrix $M$ respectively. + +Proof. Since $\Delta W = B R A$ , we have + +$$ +\begin{array} { r } { \operatorname { C o l } ( \Delta W ) = \{ y \in \mathbb { R } ^ { m } \mid y = B R A x , x \in \mathbb { R } ^ { n } \} \implies } \\ { \operatorname { C o l } ( \Delta W ) = \{ y \in \mathbb { R } ^ { m } \mid y = B z , z \in \operatorname { C o l } ( R A ) \} \subseteq \operatorname { C o l } ( B ) . } \end{array} +$$ + +That is, we proved that + +$$ +\mathbf { C o l } ( \Delta W ) \subseteq \mathbf { C o l } ( B ) . +$$ + +Following similar arguments, one can also show $\mathtt { R o w } ( \Delta W ) \subseteq \mathtt { R o w } ( A )$ + +# B.2 PROOF OF LEMMA 2 + +Lemma. The gradient of the loss with respect to matrix $R$ can be expressed in terms of the gradient with respect to the weight matrix $W$ as: + +$$ +g _ { L o R A - X S } ^ { R } = s B ^ { \top } g A ^ { \top } . +$$ + +$L$ $g$ $g _ { \mathrm { L o R A - X S } } ^ { R }$ + +$$ +g : = \frac { \partial L } { \partial W } \quad \& \quad g _ { \mathrm { L o R A - X S } } ^ { R } : = \frac { \partial L } { \partial R } . +$$ + +The chain rule gives + +$$ +{ \frac { \partial L } { \partial R } } = { \frac { \partial L } { \partial W } } { \frac { \partial W } { \partial R } } \implies { \frac { \partial L } { \partial R } } = { \frac { \partial L } { \partial W } } { \frac { \partial W } { \partial X } } { \frac { \partial X } { \partial R } } \quad { \mathrm { ~ f o r ~ } } X = R A +$$ + +We know that for $W = s B X$ : + +$$ +{ \frac { \partial L } { \partial W } } { \frac { \partial W } { \partial X } } = s B ^ { \top } g \implies { \frac { \partial L } { \partial R } } = s B ^ { \top } g { \frac { \partial X } { \partial R } } +$$ + +Let $s B ^ { \intercal } g = y$ . We know that when $X = R A$ : + +$$ +y { \frac { \partial X } { \partial R } } = y A ^ { \top } \implies { \frac { \partial L } { \partial R } } = y A ^ { \top } = s B ^ { \top } g A ^ { \top } +$$ + +$$ +\boxed { g _ { \mathrm { L o R A - X S } } ^ { R } = s B ^ { \top } g A ^ { \top } } +$$ + +Theorem. For full-rank $A$ and $B$ matrices, the optimal solution for the objective $m i n _ { g ^ { R } } | | \tilde { g } - g | | _ { F } ^ { 2 }$ , such that $\tilde { g } = s B g ^ { R } A$ , is: $g ^ { R } = { \frac { 1 } { s ^ { 2 } } } ( B ^ { \top } B ) ^ { - 1 } g _ { L o R A - X S } ^ { R } ( A A ^ { \top } ) ^ { - 1 }$ + +Proof. Since we already defined the equivalent gradient $\tilde { g } : = s B g ^ { R } A$ , the minimization problem can be denoted as: + +$$ +\underset { g ^ { R } } { \arg \operatorname* { m i n } } F = \| s B g ^ { R } A - g \| _ { F } ^ { 2 } +$$ + +For differentiable $F$ , + +$$ +{ \frac { \partial F } { \partial g ^ { R } } } = 0 \implies 2 ( \widetilde g - g ) \cdot { \frac { \partial \widetilde g } { \partial g ^ { R } } } = 0 \implies 2 ( s B g ^ { R } A - g ) \cdot { \frac { \partial ( s B g ^ { R } A ) } { \partial g ^ { R } } } = 0 +$$ + +Using the same trick from before and substituting $g ^ { R } A = X$ , we get: + +$$ +2 s B ^ { \mathsf { T } } ( s B g ^ { R } A - g ) A ^ { \mathsf { T } } = 0 \implies B ^ { \mathsf { T } } ( s B g ^ { R } A - g ) A ^ { \mathsf { T } } = 0 \implies B ^ { \mathsf { T } } s B g ^ { R } A A ^ { \mathsf { T } } = B ^ { \mathsf { T } } g A ^ { \mathsf { T } } +$$ + +From Lemma 2, we get: + +$$ +B ^ { \top } g A ^ { \top } = g _ { \mathrm { L o R A - X S } } ^ { R } / s \implies B ^ { \top } s B g ^ { R } A A ^ { \top } = g _ { \mathrm { L o R A - X S } } ^ { R } / s \implies B ^ { \top } B g ^ { R } A A ^ { \top } = g _ { \mathrm { L o R A - X S } } ^ { R } / s _ { \bot } ^ { 2 } +$$ + +Now since $B$ and $A$ are full rank, multiplying both sides by $( B ^ { \top } B ) ^ { - 1 }$ and $( A A ^ { \top } ) ^ { - 1 }$ on the left and right side respectively gives: + +$$ +( B ^ { \top } B ) ^ { - 1 } ( B ^ { \top } B g ^ { R } A A ^ { \top } ) ( A A ^ { \top } ) ^ { - 1 } = ( B ^ { \top } B ) ^ { - 1 } g _ { \mathrm { L o R A - X S } } ^ { R } ( A A ^ { \top } ) ^ { - 1 } / s ^ { 2 } +$$ + +$$ +\mathrm { T h e r e f o r e , } \quad \left. g ^ { R } = { \frac { 1 } { s ^ { 2 } } } ( B ^ { \top } B ) ^ { - 1 } g _ { \mathrm { L o R A - X S } } ^ { R } ( A A ^ { \top } ) ^ { - 1 } \right. +$$ + +# B.4 PROOF OF THEOREM 4 + +Theorem. Consider the update for matrix $R$ using the solution derived in Theorem 3: + +$$ +R R - \eta g ^ { R } +$$ + +where $\eta > 0$ is the (sufficiently small) learning rate. This update guarantees a reduction in the loss $\Delta L _ { \mathrm { { \scriptsize { \cdot } } } }$ , given by: + +$$ +\begin{array} { r } { \Delta L : = L ( W _ { 0 } + s B ( R - \eta g ^ { R } ) A ) - L ( W _ { 0 } + s B R A ) = - \eta \langle g _ { L o R A - X S } ^ { R } , g ^ { R } \rangle _ { F } + o ( \eta ) \le 0 . } \end{array} +$$ + +Proof. Assuming that $L$ is differentiable, we use Taylor’s theorem and get + +$$ +\begin{array} { l } { \Delta L : = L ( W _ { 0 } + s B ( R - \eta g ^ { R } ) A ) - L ( W _ { 0 } + s B R A ) } \\ { \displaystyle \quad = \left. \frac { \partial L } { \partial R } , - \eta g ^ { R } \right. _ { F } + o ( \eta ) } \\ { \displaystyle \quad = - \frac { \eta } { s ^ { 2 } } \langle g _ { \mathrm { L o R A - X S } } ^ { R } , ( B ^ { \top } B ) ^ { - 1 } g _ { \mathrm { L o R A - X S } } ^ { R } ( A A ^ { \top } ) ^ { - 1 } \rangle _ { F } + o ( \eta ) , } \end{array} +$$ + +the last step we for small enough so used the definition of , it is sufficient to show $g _ { \mathrm { L o R A - X S } } ^ { R }$ and the result of Theorem 3. To prove $\Delta L \leq 0$ $\eta$ + +$$ +\langle g _ { \mathrm { L o R A - X S } } ^ { R } , ( B ^ { \top } B ) ^ { - 1 } g _ { \mathrm { L o R A - X S } } ^ { R } ( A A ^ { \top } ) ^ { - 1 } \rangle _ { F } \geq 0 . +$$ + +Next, we note that matrices $B ^ { \intercal } B \in \mathbb { R } ^ { r \times r }$ and $A A ^ { \top } \in \mathbb { R } ^ { r \times r }$ are positive definite since they are positive semi-definite and matrices $B$ and $A$ are full-rank (i.e., with rank $r$ ) matrices, which means that $B ^ { \top } B$ and $A A ^ { \top }$ have non-zero eigenvalues. Therefore, $( B ^ { \top } B ) _ { \mathrm { ~ - ~ } } ^ { - 1 }$ and $( A A ^ { \top } ) ^ { - 1 }$ are also positive definite, implying that there exist matrices $X$ and $Y$ such that $( \overset { \cdot } { B } ^ { \top } B ) ^ { - 1 } = Y \overset { \cdot } { Y } ^ { \top }$ and $( A A ^ { \dagger } ) ^ { - 1 } = X X ^ { \top }$ (e.g., one can find such matrices using Cholesky decomposition). Then, we have + +$$ +\begin{array} { r l r } & { } & { \langle g _ { \mathrm { L o R A - X S } } ^ { R } , ( B ^ { \top } B ) ^ { - 1 } g _ { \mathrm { L o R A - X S } } ^ { R } ( A A ^ { \top } ) ^ { - 1 } \rangle _ { F } = \langle g _ { \mathrm { L o R A - X S } } ^ { R } , Y Y ^ { \top } g _ { \mathrm { L o R A - X S } } ^ { R } X X ^ { \top } \rangle _ { F } } \\ & { } & { = \langle Y ^ { \top } g _ { \mathrm { L o R A - X S } } ^ { R } X , Y ^ { \top } g _ { \mathrm { L o R A - X S } } ^ { R } X \rangle _ { F } } \\ & { } & { = \| Y ^ { \top } g _ { \mathrm { L o R A - X S } } ^ { R } X \| _ { F } ^ { 2 } \geq 0 . \qquad } \end{array} +$$ + +This concludes the proof. + +For our specific initialization where $( B ^ { \top } B ) = I , ( A A ^ { \top } ) = I$ , and $s = 1$ , the result simplifies to: + +$$ +\begin{array} { r } { \Delta L = - \eta \langle g _ { \mathrm { L o R A - X S } } ^ { R } , g _ { \mathrm { L o R A - X S } } ^ { R } \rangle _ { F } + o ( \eta ) \leq 0 . } \end{array} +$$ + +# B.5 PROOF OF THEOREM 5 + +Theorem. The equivalent gradient $\tilde { g }$ is hyperparameter s independent when + +$$ +\tilde { g } = s B g ^ { R } A \quad b u t n o t w h e n \quad \tilde { g } = s B g _ { L o R A - X S } ^ { R } A . +$$ + +Proof. Let $g$ be the full fine-tuning gradient. We want to prove that $\tilde { g }$ does not depend on $s$ , so we try to express it in terms of $g$ which does not depend on the LoRA-XS training process or reparameterization. + +1) For $\tilde { g } = s B g ^ { R } A$ : + +$$ +g ^ { R } = \frac { 1 } { s ^ { 2 } } ( B ^ { \top } B ) ^ { - 1 } g _ { \mathrm { L o R A - X S } } ^ { R } ( A A ^ { \top } ) ^ { - 1 } \implies \tilde { g } = \frac { s } { s ^ { 2 } } B ( B ^ { \top } B ^ { - 1 } ) g _ { \mathrm { L o R A - X S } } ^ { R } ( A A ^ { \top } ) ^ { - 1 } A +$$ + +Now since $g _ { \mathrm { L o R A - X S } } ^ { R } = s B ^ { \top } g A ^ { \top }$ + +$$ +\tilde { g } = \frac { 1 } { s } B ( B ^ { \top } B ^ { - 1 } ) s B ^ { \top } g A ^ { \top } ( A A ^ { \top } ) ^ { - 1 } A = B ( B ^ { \top } B ^ { - 1 } ) B ^ { \top } g A ^ { \top } ( A A ^ { \top } ) ^ { - 1 } A . +$$ + +which is $s$ -independent. + +2) For $\tilde { g } = s B g _ { \mathrm { L o R A - X S } } ^ { R } A$ + +$$ +g _ { \mathrm { { L o R A - X S } } } ^ { R } = s B ^ { \top } g A ^ { \top } \implies \tilde { g } = s B ( s B ^ { \top } g A ^ { \top } ) A \implies \tilde { g } = s ^ { 2 } B B ^ { \top } g A ^ { \top } A +$$ + +which is not $s$ -independent. + +# B.6 PROOF OF THEOREM 6 + +Theorem. If $A _ { i n i t }$ and $B _ { i n i t }$ are initialized using LoRA-SB for the first step of SGD optimizer, then the update given by LoRA-SB, $\Delta ( B _ { i n i t } R _ { i n i t } A _ { i n i t } )$ , is the best low-rank approximation of full fine-tuning update, $\Delta W$ . + +Proof. Consider a gradient descent step with learning rate $\eta$ and updates for $R$ : + +$$ +\Delta R = - \eta \nabla _ { R } \mathcal { L } ( R ) \implies B \Delta R A = - \eta B \nabla _ { R } \mathcal { L } ( R ) A . +$$ + +To measure its approximation quality of update of the weights in full finetuning: + +$$ +\Delta W = - \eta \nabla _ { W } { \mathcal { L } } ( W _ { 0 } ) . +$$ + +We use Frobenius norm of the difference between these two updates as a criterion: + +$$ +\lVert B \Delta R A - \eta \nabla \mathcal { L } _ { W } ( W _ { 0 } ) \rVert _ { F } = \eta \lVert B \nabla _ { R } \mathcal { L } ( R ) A - \nabla \mathcal { L } _ { W } ( W _ { 0 } ) \rVert _ { F } . +$$ + +We have shown before that: + +$$ +\nabla _ { \boldsymbol { R } } \mathcal { L } = \boldsymbol { B } ^ { \top } \nabla _ { W } \mathcal { L } \boldsymbol { A } ^ { \top } . +$$ + +The problem now becomes: + +$$ +\operatorname* { m i n } _ { A _ { \mathrm { i n t } } , B _ { \mathrm { i n t } } } \| B ^ { \top } ( B ^ { \top } \nabla _ { W } \mathcal { L } A ^ { \top } ) A - \nabla _ { W } \mathcal { L } \| _ { F } \quad \mathrm { w h e r e } \ \nabla _ { W } \mathcal { L } = U S V ^ { \top } . +$$ + +Using our initialization, we get: + +$$ +\| B B ^ { \top } \nabla _ { W } \mathcal { L } A ^ { \top } A - \nabla _ { W } \mathcal { L } \| _ { F } = \| U _ { I R } U _ { I R } ^ { \top } U S V ^ { \top } V _ { I R } V _ { I R } ^ { \top } - U S V ^ { \top } \| _ { F } . +$$ + +Moreover, we also have + +$$ +U _ { I R } U _ { I R } ^ { \top } U S V ^ { \top } V _ { I R } V _ { I R } ^ { \top } = \sum _ { i = 1 } ^ { r } \sigma _ { i } u _ { i } v _ { i } ^ { \top } . +$$ + +The rank of $W ^ { \prime }$ such that + +$$ +\begin{array} { r } { W ^ { \prime } = U _ { I R } U _ { I R } ^ { \top } U S V ^ { \top } V _ { I R } V _ { I R } ^ { \top } } \end{array} +$$ + +is $\leq r$ , since the corresponding ranks of $B _ { \mathrm { i n i t } }$ and $A _ { \mathrm { i n i t } }$ is $r$ . Using the Eckart-Young Theorem, we find the optimal low-rank solution as: + +$$ +\boldsymbol { W } ^ { \prime * } = \operatorname * { a r g m i n } _ { \mathrm { \ r { r a n k } } ( \boldsymbol { W } ^ { \prime } ) = \boldsymbol { r } } \| \boldsymbol { W } ^ { \prime } - \nabla _ { \boldsymbol { W } } \mathcal { L } \| _ { F } = \sum _ { i = 1 } ^ { r } \sigma _ { i } u _ { i } v _ { i } ^ { \top } . +$$ + +Since we also get an identical expression, our solution is optimal. + +# C SIMULATING THE FIRST STEP OF FULL FINE-TUNING UNDER ADAMW + +Our initialization is designed to approximate the first update step that would occur during full finetuning using the AdamW optimizer, which is also used in LoRA-SB training. AdamW computes the parameter update using both first and second moment estimates of the gradient. At the first step, these moments are initialized to zero, so the update becomes: + +$$ +\theta _ { 1 } = \theta _ { 0 } - \alpha \cdot \frac { g _ { 1 } } { \sqrt { g _ { 1 } ^ { 2 } + \epsilon } } \approx - \alpha \cdot \mathrm { s i g n } ( g _ { 1 } ) +$$ + +where $g _ { 1 }$ is the gradient at the first step, $\epsilon$ is a small constant for numerical stability, and $\alpha$ is the learning rate. Due to zero-initialization and bias correction, the direction of the update is approximately the element-wise sign of the gradient. + +To simulate this behavior in our low-rank initialization, we use: + +$$ +\Delta W _ { \mathrm { a v g } } = - \eta \cdot \mathrm { s i g n } \left( \sum _ { i = 1 } ^ { n } \nabla _ { W } \mathcal { L } ( W _ { 0 } , x _ { i } ) \right) +$$ + +This reflects the direction of the first AdamW step averaged over a mini-batch. By using the sign of the gradient sum, we ensure our initialization aligns with the dynamics of AdamW, leading to a consistent and faithful approximation of full fine-tuning updates within the low-rank subspace. + +# D ALGORITHM + +We provide a pseudo-code implementation of our method in Algorithm 1. + +# Algorithm 1 LoRA-SB, PyTorch-like + +1: def initSB(model, D) +2: # Estimate gradient with n samples +3: $\Delta W _ { \mathrm { a v g } } $ est grad(model, D, n) +4: # Initialize B, R, A +5: $( B , R , A ) $ trunc SVD( $\Delta W _ { \mathrm { a v g } }$ ) +6: # Convert to LoRA-SB model +7: sb model $\gets$ lora SB(model, B, R, A) +8: return sb model +9: +10: # Load pre-trained model +11: model $\gets$ AutoModel(base model) +12: # Initialize LoRA-SB with D +13: sb model initSB(model, D) +14: # Train, only R trainable +15: trainer Trainer(sb model,...) +16: trainer.train() + +# E OPTIMAL GRADIENT APPROXIMATION IS IMPORTANT! + +As discussed in Section 4, optimal gradient approximation plays a key role in the effectiveness of LoRA-SB. In Figure 3, we compare the loss curves of models trained with and without this component on Mistral-7B. While both variants begin with similar performance due to effective initialization, LoRA-SB with optimal gradient approximation converges to substantially lower loss values, highlighting its contribution to improved optimization. + +![](images/figures/lora-sb-fig-0003.jpg) +Figure 3: Training loss for Mistral-7B, highlighting the impact of optimal gradient approximation. + +# F TRAINING TIME OVERHEAD VS LORA-XS + +As previously mentioned, we compute the update approximation using only $1 / 1 0 0 0$ of the total training samples for each dataset. Table 6 presents the associated training time overhead for these computations, compared to LoRA-XS. The results show that the additional overhead is negligible, adding just 2–4 minutes compared to the total training time of 3–5 hours per epoch $( \approx 1 . 1 \%$ to $1 . 3 \%$ ). Additionally, the update computation is performed only once, at the beginning of the first epoch, prior to training. Notably, the initialization step is highly efficient, as we directly compute the truncated + +SVD using optimized PyTorch libraries (torch.svd lowrank). For reference, this computation takes less than one second for each of the entire LLMs used in our experiments. + +Table 6: Training time overhead due to the initialization for various models on their respective tasks. + +
ModelOverheadTraining Time/Epoch
Mistral-7B0:02:013:03:57
Gemma-2 9B0:03:464:13:24
Llama-3.2 3B0:03:544:54:31
+ +# G INFERENCE OVERHEAD VS LORA + +LoRA-SB introduces a minimal inference cost overhead due to the insertion of the $r \times r$ matrix $R$ between $B$ and $A$ , and the need for higher ranks to achieve comparable performance to LoRA. We benchmark the inference-time FLOPs and MACs across various models and find that the overhead is negligible. This comparison is presented in Table 7, showing that the additional overhead of LoRA-SB is negligible. + +Table 7: Inference cost comparison between LoRA-SB and LoRA across various models for a sequence length of 256. The minimum rank at which LoRA-SB matches or exceeds LoRA’s performance is highlighted in bold. + +
ModelMethodRankMACsFLOPs
RoBERTa-largeLoRA877.86 G155.79 G
LoRA-SB1678.42 G156.91 G
LoRA-SB2478.97 G158.01 G
LlaMA-3.2 3BLoRA320.84 T1.67 T
LoRA-SB640.85 T1.70 T
LoRA-SB960.86 T1.72 T
Mistral 7BLoRA321.84 T3.69 T
LoRA-SB641.86 T3.73 T
LoRA-SB921.88 T3.77 T
Gemma-2 9BLoRA323.89 T7.77 T
LoRA-SB643.93 T7.86 T
LoRA-SB963.97 T7.94 T
+ +# H EXPERIMENT DETAILS + +We use PyTorch (33) and the HuggingFace Transformers library (48) for our implementations. We run all experiments on a single NVIDIA A6000 GPU and report results as the average of three random seeds. To save memory, we initialize base models in torch.bfloat16 precision. We trained all models using the AdamW optimizer (28). We compute the update approximation using only $\mathbf { 1 / 1 0 0 0 }$ of each dataset’s total number of samples. The samples are randomly selected from the training set in each run. + +For arithmetic and commonsense reasoning tasks, we set up Mistral-7B, Gemma-2 9B, and Llama-3.2 3B with hyperparameters and configurations listed in Table 8. We adopted most settings from previous studies (18) but conducted our own learning rate sweep. Following LoRA-XS guidelines, we set $\alpha = r$ for their baseline configuration. + +For the GLUE benchmark using RoBERTa-large, you can find the hyperparameter details in Table 9. We mostly adhered to the original configurations from the LoRA paper (17) but adjusted the learning rate through a sweep. In line with LoRA-XS settings, we fixed $\alpha$ at 16 for their baseline. + +For all tasks, we followed the baseline configurations provided in the PiSSA (30), rsLoRA (20), DoRA (26), and LoRA-Pro (46) papers for our comparisons. + +Table 8: Hyperparameter settings for training Mistral-7B and Gemma-2 9B on MetaMathQA, and Llama-3.2 3B on COMMONSENSE170K. + +
Mistral-7B / Gemma-2 9BLlama-3.2 3B
OptimizerAdamWAdamW
Batch size16
Max. Seq. Len512256
Grad Acc. Steps3224
Epochs12
Dropout00.05
Learning Rate1 × 10−42 × 10−3
LR SchedulerCosineLinear
Warmup Ratio0.020.02
+ +Table 9: Hyperparameter settings for RoBERTa-large on GLUE. + +
CoLARTEMRPCSST-2QNLISTS-B
Optimizer Batch size Max Seq. Len.30AdamW30
128
30256 1515
Epochs Dropout30
Learning Rate0 1 × 10−3
LR SchedulerLinear
Warmup Ratio0.06
+ +# I DATASET DETAILS + +The MetaMathQA dataset (50) creates mathematical questions by rephrasing existing ones from different viewpoints, without adding new information. We assess this dataset using two benchmarks: GSM8K (8), which consists of grade-school math problems requiring multi-step reasoning, and MATH (16), which presents difficult, competition-level math problems. Evaluation focuses solely on the final numeric answer. + +COMMONSENSE170K is a comprehensive dataset that consolidates eight commonsense reasoning datasets (18). Each example is framed as a multiple-choice question where the model generates the correct answer without explanations. We use the prompt template from (18). The individual datasets used are described below: + +1. HellaSwag (51) challenges models to select the most plausible continuation of a given scenario from multiple possible endings. +2. ARC Easy (or ARC-e) (7) includes basic science questions at a grade-school level, offering simpler tasks to assess fundamental reasoning abilities. +3. PIQA (3) evaluates physical commonsense reasoning, where models must choose the best action to take in a hypothetical scenario. +4. SIQA (39) tests social commonsense reasoning by asking models to predict the social consequences of human actions. +5. WinoGrande (38) presents sentence completion tasks requiring commonsense reasoning to select the correct binary option. +6. ARC Challenge (or ARC-c) (7) consists of more complex science questions designed to challenge models with sophisticated reasoning, beyond simple co-occurrence patterns. +7. OBQA (31) features open-book, knowledge-intensive QA tasks that require multi-hop reasoning across multiple information sources. +8. BoolQ (6) involves answering yes/no questions based on real-world, naturally occurring queries. + +The GLUE Benchmark is a comprehensive collection of tasks designed to evaluate natural language understanding (NLU) abilities. It included various datasets, including STS-B for measuring semantic textual similarity (5), RTE for recognizing textual entailment, MRPC for detecting paraphrases (11), CoLA for assessing linguistic acceptability (47), SST-2 for sentiment analysis (41), and QNLI for question-answer inference (36). GLUE’s broad scope makes it a standard benchmark for evaluating models like RoBERTa. + +# J USE OF LARGE LANGUAGE MODELS + +LLMs are only used for small writing improvements, like polishing grammar and smoothing out phrasing. \ No newline at end of file diff --git a/papers/lora-sb/paper.pdf b/papers/lora-sb/paper.pdf new file mode 100644 index 0000000000000000000000000000000000000000..4fa97cb146a613398fce89e3b85fb2e2dc4b038e --- /dev/null +++ b/papers/lora-sb/paper.pdf @@ -0,0 +1,3 @@ +version https://git-lfs.github.com/spec/v1 +oid sha256:5d88f0eb037b752271845e3016bbb23441c0ce19f91de2f246ff2591af11ec48 +size 707160 diff --git a/papers/lora-sb/sau.json b/papers/lora-sb/sau.json new file mode 100644 index 0000000000000000000000000000000000000000..ab9080f811d8ba43bea95fe973aea27cedc41940 --- /dev/null +++ b/papers/lora-sb/sau.json @@ -0,0 +1,197 @@ +{ + "paper_id": "lora-sb", + "paper_title": "LoRA-SB: Initialization using Update Approximation for Efficient Low-Rank Fine-Tuning", + "D1": [ + { + "id": "lora-sb-D1-001", + "claim": "LoRA rank configuration: r ∈ {32, 64, 96} for Mistral-7B, Gemma-2 9B, Llama-3.2 3B; r ∈ {8, 16, 24} for RoBERTa-large on GLUE.", + "source": "Sec 3.1, Sec 3.2, Sec 3.3, Tables 1-3" + }, + { + "id": "lora-sb-D1-002", + "claim": "Scaling factor: s = 1 in LoRA-SB (no α/r tuning); s = α/r in standard LoRA (baseline reference). Parameter count: r² trainable params in LoRA-SB (only R), vs r(m+n) in standard LoRA.", + "source": "Sec 2.1, Sec 2.5, Sec 2.6, Eq 2, Eq 3, Eq 9" + }, + { + "id": "lora-sb-D1-003", + "claim": "LoRA-SB matrix dimensions: B ∈ ℝ^{m×r} (frozen), A ∈ ℝ^{r×n} (frozen), R ∈ ℝ^{r×r} (trainable). LoRA target modules: key, value, query, attention output, all FC layers (Mistral-7B/Gemma-2 9B/Llama-3.2 3B); self-attention layers only (RoBERTa-large on GLUE).", + "source": "Sec 2.1, Eq 3, Sec 3.1, Sec 3.2, Sec 3.3" + }, + { + "id": "lora-sb-D1-004", + "claim": "Initialization data subsample fraction: 0.1% (1/1000) of each dataset's total training size. For MetaMathQA (50K total), this yields 50 samples. Ablation range for initialization sample count: n ∈ {1, 5, 25, 50, 100, 200, 500} evaluated on Mistral-7B and Gemma-2 9B.", + "source": "Sec 3, Sec 4, Table 5, Appendix H" + }, + { + "id": "lora-sb-D1-005", + "claim": "Initialization noise ablation levels: Gaussian noise N(μ=0, σ²) with σ ∈ {10⁻², 10⁻³, 10⁻⁴, 10⁻⁵} added to ΔW_avg. Kaiming random initialization also tested as a control. All variants use truncated SVD for orthonormal B/A, isolating initialization quality as the sole variable.", + "source": "Sec 4, Table 4" + }, + { + "id": "lora-sb-D1-006", + "claim": "Training hyperparams for Mistral-7B / Gemma-2 9B on MetaMathQA: lr=1e-4, batch_size=1, max_seq_len=512, grad_acc_steps=32, epochs=1, dropout=0, warmup_ratio=0.02, lr_scheduler=Cosine, optimizer=AdamW.", + "source": "Appendix H, Table 8" + }, + { + "id": "lora-sb-D1-007", + "claim": "Training hyperparams for Llama-3.2 3B on COMMONSENSE170K: lr=2e-3, batch_size=6, max_seq_len=256, grad_acc_steps=24, epochs=2, dropout=0.05, warmup_ratio=0.02, lr_scheduler=Linear, optimizer=AdamW.", + "source": "Appendix H, Table 8" + }, + { + "id": "lora-sb-D1-008", + "claim": "Training hyperparams for RoBERTa-large on GLUE: lr=1e-3, epochs=30, dropout=0, warmup_ratio=0.06, lr_scheduler=Linear, optimizer=AdamW. Batch size varies per task: 30 (CoLA/STS-B), 128 (RTE/MRPC), 256 (SST-2), 15 (QNLI).", + "source": "Appendix H, Table 9" + }, + { + "id": "lora-sb-D1-009", + "claim": "LoRA-XS baseline config: α = r for arithmetic/commonsense tasks (Mistral/Gemma/Llama); α = 16 for GLUE (RoBERTa-large). Used as reference PEFT method sharing same architecture as LoRA-SB.", + "source": "Appendix H" + }, + { + "id": "lora-sb-D1-010", + "claim": "Hardware and infrastructure: 1× NVIDIA A6000 GPU, torch.bfloat16 base model precision, 3 random seeds for result averaging. SVD via torch.svd_lowrank (< 1s per model).", + "source": "Appendix H, Appendix F" + }, + { + "id": "lora-sb-D1-011", + "claim": "Training datasets: MetaMathQA (50K, math reasoning), COMMONSENSE170K (8-task combined, commonsense reasoning). Evaluation datasets: GSM8K and MATH (math); BoolQ, PIQA, SIQA, HellaSwag, WinoGrande, ARC-e, ARC-c, OBQA (commonsense); CoLA, RTE, MRPC, STS-B, QNLI, SST-2 (GLUE).", + "source": "Sec 3.1, Sec 3.2, Sec 3.3, Appendix I" + }, + { + "id": "lora-sb-D1-012", + "claim": "AdamW first-step approximation: ε ≈ 0 implies update ≈ −η × sign(g₁) at the first optimizer step. LoRA-SB leverages this to compute ΔW_avg = −η sign(Σ_i ∇_W L(W₀, x_i)) during initialization.", + "source": "Appendix C, Eq 13" + } + ], + "D2": [ + { + "id": "lora-sb-D2-001", + "claim": "LoRA-SB forward pass: W = W₀ + s B R A, where W₀ frozen, B (m×r) and A (r×n) fixed after initialization, R (r×r) sole trainable. s = 1 in LoRA-SB. Only R updated during training.", + "source": "Sec 2.1, Eq 3" + }, + { + "id": "lora-sb-D2-002", + "claim": "Standard LoRA forward pass (baseline): W = W₀ + s B A, where B (m×r) and A (r×n) both trainable, s = α/r. Total learnable params: r(m+n).", + "source": "Sec 2.1, Eq 2" + }, + { + "id": "lora-sb-D2-003", + "claim": "LoRA-XS gradient w.r.t. R: g^R_{LoRA-XS} = s Bᵀ g Aᵀ, where g = ∂L/∂W is the full weight gradient, B and A are fixed matrices. Relates the trainable R gradient to the full fine-tuning gradient g.", + "source": "Sec 2.3, Lemma 2" + }, + { + "id": "lora-sb-D2-004", + "claim": "Equivalent gradient definition: g̃ = s B g^R A, where g̃ is the virtual update to W induced by updates to R. Computed as the matrix product of fixed B, gradient g^R, and fixed A, scaled by s.", + "source": "Sec 2.3, Definition 1" + }, + { + "id": "lora-sb-D2-005", + "claim": "Optimal gradient computation (general, non-orthonormal B/A): g^R = (1/s²) (BᵀB)⁻¹ g^R_{LoRA-XS} (AAᵀ)⁻¹. Closed-form solution minimizing ||g̃ − g||²_F, making the equivalent gradient optimally approximate the full FT gradient. Requires B, A full-rank.", + "source": "Sec 2.3, Theorem 3" + }, + { + "id": "lora-sb-D2-006", + "claim": "Simplified optimal gradient (orthonormal B/A): g^R = (1/s²) g^R_{LoRA-XS}, when BᵀB = I and AAᵀ = I. Eliminates matrix inversion at each optimizer step.", + "source": "Sec 2.6, Eq 8" + }, + { + "id": "lora-sb-D2-007", + "claim": "Final LoRA-SB gradient update rule (s=1, orthonormal B/A): g^R = g^R_{LoRA-XS} = Bᵀ g Aᵀ. Gradient of R equals raw LoRA-XS gradient without scaling or inversion.", + "source": "Sec 2.6, Eq 9" + }, + { + "id": "lora-sb-D2-008", + "claim": "R parameter update step: R ← R − η g^R, where η is the learning rate (sufficiently small). R is the sole trainable parameter; B and A remain frozen throughout training. The update is applied by the AdamW optimizer each step.", + "source": "Sec 2.3, Theorem 4; Sec 2.6" + }, + { + "id": "lora-sb-D2-009", + "claim": "Initialization gradient estimation (single sample): ΔW_first_step = −η × sign(∇_W L(W₀, x_i)). Uses sign() to approximate AdamW first-step behavior (moments initialized to zero).", + "source": "Sec 2.4, Eq 5; Appendix C" + }, + { + "id": "lora-sb-D2-010", + "claim": "Initialization averaged gradient estimation (multi-sample, used in practice): ΔW_avg = −η sign(Σ_{i=0}^{n≤|X|} ∇_W L(W₀, x_i)), where n = 0.1% of training data, x_i randomly sampled, Σ accumulates per-sample gradients, sign() applied element-wise to the sum.", + "source": "Sec 2.4, Eq 6; Appendix C" + }, + { + "id": "lora-sb-D2-011", + "claim": "Initialization: truncated SVD and factor assignment. U, S, Vᵀ = SVD(ΔW_avg); B_init = U[:, :r]; A_init = (V[:, :r])ᵀ; R_init = (1/s) S[:r, :r]. By Eckart-Young theorem, yields optimal rank-r approximation s B_init R_init A_init ≈ ΔW_avg. With s=1, R_init = S[:r, :r]. Uses torch.svd_lowrank.", + "source": "Sec 2.6, Eq 7-9; Sec 4; Appendix D, Algorithm 1" + }, + { + "id": "lora-sb-D2-012", + "claim": "Memory-efficient layerwise gradient computation for initialization: accumulate ΔW_avg = -η sign(Σ_{i=0}^{n} ∇_W L(W₀, x_i)) via backward hooks — for each layer l, compute g_l = ∂L/∂W_l(W₀, x_i), accumulate S_l ← S_l + sign(g_l), immediately discard g_l; total memory = O(1) independent of layer count. The sign() approximation matches AdamW first-step behavior (moments initialized to zero: m₀=0, v₀=0 → update ≈ -η · sign(g₁)). Peak memory never exceeds LoRA-SB fine-tuning or standard LoRA training.", + "source": "Sec 2.6" + }, + { + "id": "lora-sb-D2-013", + "claim": "LoRA-SB complete algorithm: Step 1: Load pre-trained model. Step 2: Estimate ΔW_avg via memory-efficient multi-sample gradient accumulation. Step 3: Compute truncated SVD on ΔW_avg. Step 4: Assign B (U[:,:r]), R (S[:r,:r]/s), A (V[:r,:r]ᵀ). Step 5: Convert model to LoRA-SB (replace linear layers). Step 6: Train with AdamW updating only R via g^R = g^R_{LoRA-XS} (s=1, orthonormal B/A).", + "source": "Appendix D, Algorithm 1" + }, + { + "id": "lora-sb-D2-014", + "claim": "LoRA-SB ablated variant (good init, no optimal gradient approximation): B_init = U[:r] S[:r,:r], A_init = V[:r]ᵀ, R_init = I. Same SVD source (good initialization) but BᵀB ≠ I, so optimal gradient g^R = (1/s²)(BᵀB)⁻¹ g^R_{LoRA-XS}(AAᵀ)⁻¹ requires explicit inversion. Used to isolate optimal gradient effect from initialization quality.", + "source": "Sec 4, Eq 11-12" + } + ], + "D3": [ + { + "id": "lora-sb-D3-001", + "claim": "Compare LoRA-SB against Full FT and PEFT baselines (LoRA, rsLoRA, PiSSA, DoRA, LoRA-Pro, LoRA-XS) on arithmetic reasoning. Mistral-7B and Gemma-2 9B fine-tuned on MetaMathQA (50K) at ranks r ∈ {32, 64, 96}. LoRA-SB init uses 0.1% training data. Evaluation: GSM8K and MATH (numeric answer accuracy).", + "source": "Sec 3.1, Table 1, Appendix H, Table 8, Appendix I" + }, + { + "id": "lora-sb-D3-002", + "claim": "Compare LoRA-SB against Full FT and PEFT baselines on commonsense reasoning. Llama-3.2 3B fine-tuned on COMMONSENSE170K (8-task combined) at ranks r ∈ {32, 64, 96}. Evaluation: BoolQ, PIQA, SIQA, HellaSwag, WinoGrande, ARC-e, ARC-c, OBQA, plus average accuracy.", + "source": "Sec 3.2, Table 2, Appendix H, Table 8, Appendix I" + }, + { + "id": "lora-sb-D3-003", + "claim": "Compare LoRA-SB against Full FT and PEFT baselines on GLUE NLU benchmark. RoBERTa-large (355M) fine-tuned on 6 GLUE tasks at ranks r ∈ {8, 16, 24}. LoRA applied to self-attention layers only. Per-task metrics: Matthew's correlation (CoLA), accuracy (RTE/MRPC/QNLI/SST-2), Pearson correlation (STS-B). All-task average reported.", + "source": "Sec 3.3, Table 3, Appendix H, Table 9, Appendix I" + }, + { + "id": "lora-sb-D3-004", + "claim": "Ablation: initialization quality impact. Mistral-7B (r=96) with SVD on various matrices: Kaiming random, ΔW_avg + Gaussian noise at σ ∈ {10⁻², 10⁻³, 10⁻⁴, 10⁻⁵}, and clean ΔW_avg (LoRA-SB). All use truncated SVD for orthonormal B/A; differences arise solely from init quality. Comparison metric: accuracy on GSM8K and MATH.", + "source": "Sec 4, Table 4" + }, + { + "id": "lora-sb-D3-005", + "claim": "Ablation: initialization sample count. Mistral-7B (r=96) and Gemma-2 9B (r=96) initialized with n ∈ {1, 5, 25, 50, 100, 200, 500} random samples from MetaMathQA. Default 0.1% (50 of 50K) selected based on performance plateau after ~25 samples.", + "source": "Sec 4, Table 5" + }, + { + "id": "lora-sb-D3-006", + "claim": "Ablation: optimal gradient approximation vs initialization quality. Mistral-7B (r=96) trained with two variants: (a) B=US, A=Vᵀ, R=I — good init but no optimal gradient; (b) B=U, A=Vᵀ, R=S — orthonormal B/A enabling closed-form optimal gradient. Training loss curves and final accuracy compared.", + "source": "Sec 4, Eq 11-12; Appendix E, Fig 3" + }, + { + "id": "lora-sb-D3-007", + "claim": "Benchmark: initialization time overhead vs LoRA-XS (zero overhead, same architecture). Measured on Mistral-7B/MetaMathQA, Gemma-2 9B/MetaMathQA, Llama-3.2 3B/COMMONSENSE170K. One-time overhead: gradient estimation + torch.svd_lowrank (<1s per model). Total overhead 2-4 min vs 3-5 hrs/epoch training (~1.1-1.3%).", + "source": "Sec 4, Appendix F, Table 6" + }, + { + "id": "lora-sb-D3-008", + "claim": "Benchmark: inference computational cost (MACs and FLOPs). Compare LoRA-SB vs LoRA at minimum rank for matching performance. Models: RoBERTa-large (LoRA r=8 vs LoRA-SB r=16,24), Llama-3.2 3B (r=32 vs r=64,96), Mistral-7B (r=32 vs r=64,96), Gemma-2 9B (r=32 vs r=64,96). Sequence length 256. Extra r×r matrix R adds negligible overhead.", + "source": "Appendix G, Table 7" + } + ], + "D4": [ + { + "id": "lora-sb-D4-001", + "claim": "Phase 1 — Initialization (one-time, before training): Step 1: Estimate per-sample gradient with AdamW sign approximation. Step 2: Accumulate multi-sample ΔW_avg = −η sign(Σ_i ∇_W L(W₀, x_i)) over 0.1% of training data using memory-efficient layerwise hooks. Step 3: Compute truncated SVD on ΔW_avg via torch.svd_lowrank. Step 4: Assign B = U[:, :r], A = V[:r, :]ᵀ, R = S[:r, :r] (with s=1). B and A frozen; only R requires grad.", + "source": "Appendix D, Algorithm 1; Sec 2.4, Sec 2.6" + }, + { + "id": "lora-sb-D4-002", + "claim": "Phase 2 — Training (only R updated): Step 1: Forward pass W = W₀ + s B R A (B, A frozen). Step 2: Compute optimal gradient g^R = Bᵀ g Aᵀ (simplified: s=1, BᵀB=I, AAᵀ=I). Step 3: Update R ← R − η g^R via AdamW optimizer. Repeat for configured epochs. B and A never updated.", + "source": "Sec 2.1, Eq 3; Sec 2.6, Eq 9; Sec 2.3, Theorem 4" + }, + { + "id": "lora-sb-D4-003", + "claim": "Phase 3 — Evaluation: Evaluate fine-tuned LoRA-SB model on downstream task test/validation sets. For arithmetic reasoning: GSM8K and MATH (accuracy). For commonsense reasoning: 8 datasets plus average. For GLUE: 6 tasks with task-specific metrics (Matthew's correlation, accuracy, Pearson correlation) plus all-task average.", + "source": "Sec 3.1, Sec 3.2, Sec 3.3" + } + ] +} \ No newline at end of file diff --git a/papers/luno/blacklist.txt b/papers/luno/blacklist.txt new file mode 100644 index 0000000000000000000000000000000000000000..d226174fec848d2dbb224fb60b8462064b3b17df --- /dev/null +++ b/papers/luno/blacklist.txt @@ -0,0 +1,2 @@ +# Official repository +https://github.com/2bys/luno-experiments diff --git a/papers/luno/config.yaml b/papers/luno/config.yaml new file mode 100644 index 0000000000000000000000000000000000000000..c875738b5cdc4182ab2fa1d4f566321cd38b0337 --- /dev/null +++ b/papers/luno/config.yaml @@ -0,0 +1,8 @@ +title: "Linearization Turns Neural Operators into Function-Valued Gaussian Processes (LUNO)" +pdf_url: "https://raw.githubusercontent.com/mlresearch/v267/main/assets/magnani25a/magnani25a.pdf" +venue: "ICML 2025" +year: "2025" +extra: + selection_index: 27 + 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sha256:f4ceaa1ddd664cfd0571460a6819e9bc84a9ae7a22b7b0be353f61f53fd55649 +size 22102 diff --git a/papers/luno/paper.md b/papers/luno/paper.md new file mode 100644 index 0000000000000000000000000000000000000000..5619cc09fc4b10f26a8f9044db5131206c3ea2b9 --- /dev/null +++ b/papers/luno/paper.md @@ -0,0 +1,1084 @@ +# Linearization Turns Neural Operators into Function-Valued Gaussian Processes + +Emilia Magnani \* 1 Marvin Pförtner \* 1 Tobias Weber \* 1 Philipp Hennig 1 + +# Abstract + +Neural operators generalize neural networks to learn mappings between function spaces from data. They are commonly used to learn solution operators of parametric partial differential equations (PDEs) or propagators of time-dependent PDEs. However, to make them useful in highstakes simulation scenarios, their inherent predictive error must be quantified reliably. We introduce LUNO, a novel framework for approximate Bayesian uncertainty quantification in trained neural operators. Our approach leverages model linearization to push (Gaussian) weight-space uncertainty forward to the neural operator’s predictions. We show that this can be interpreted as a probabilistic version of the concept of currying from functional programming, yielding a function-valued (Gaussian) random process belief. Our framework provides a practical yet theoretically sound way to apply existing Bayesian deep learning methods such as the linearized Laplace approximation to neural operators. Just as the underlying neural operator, our approach is resolution-agnostic by design. The method adds minimal prediction overhead, can be applied posthoc without retraining the network, and scales to large models and datasets. We evaluate these aspects in a case study on Fourier neural operators. + +# 1. Introduction + +Scientific computing increasingly demands efficient representations of complex non-linear maps between functions. Examples include solution operators of parametric partial differential equations (PDEs) or the propagators of timedependent PDEs. Operator learning is an approach to this problem that generalizes regression algorithms from finitedimensional to infinite-dimensional function-space inputoutput pairs (Boullé & Townsend, 2023). Neural operators, including Fourier neural operators, have emerged as a powerful class of models for operator learning, particularly for the solution operators of PDEs (Kovachki et al., 2023). They have been applied successfully across domains including weather forecasting (Pathak et al., 2022; Bonev et al., 2023), fluid dynamics (Grady et al., 2022; Renn et al., 2023; Li et al., 2022), and automotive aerodynamics (Li et al., 2023b). Instead of learning to solve a specific PDE, these models learn the operator that maps a functional parameter of the PDE (such as initial values, boundary conditions, force fields, or material parameters) to the corresponding solution. This approach amortizes computational cost by learning to solve entire families of PDEs. + +Although neural operators have demonstrated strong predictive capabilities, they are unable to quantify the inherent uncertainty in their predictions. Predictive uncertainty quantification is indispensable for many downstream tasks, such as decision-making in safety-critical scenarios. For example, a neural operator trained on past climate data should increase predictive uncertainty under distribution shifts due to climate change, reflecting potential losses in accuracy. Furthermore, uncertainty quantification is also useful for improving neural operator training via active learning strategies (Musekamp et al., 2025), potentially reducing the cost of generating computationally expensive numerical simulations as training data. + +Extensive previous work has shown that the structured uncertainty provided by Gaussian process (GP) models is particularly suitable for such downstream tasks, including closedform acquisition functions for active learning and Bayesian optimization for optimal selection of future queries or experiments (Garnett, 2023); enabling online model adaptation to continuously update with new data while preserving consistency with prior knowledge (Sliwa et al., 2024); facilitating sensitivity analysis through the GP’s kernel structure revealing system responses to parameter changes; and seamlessly integrating with probabilistic numerical computation (Hennig et al., 2022; Pförtner et al., 2022) to quantify, marginalize, and propagate computational uncertainty. + +Motivated by the capabilities of GPs, we propose LUNO, a practical yet theoretically sound framework that provides linearized predictive uncertainty in neural operators. LUNO quantifies uncertainty over the mapping learned by the neural operator via a Gaussian process with values in a separable Banach space of functions—a higher-order generalization of GPs that, when evaluated, returns a Gaussian random function rather than a finite-dimensional Gaussian random variable, as in standard GPs. This function-valued GP is constructed through model linearization from a Gaussian distribution quantifying uncertainty in the neural operator’s (finite-dimensional) weight space. We show that LUNO can be interpreted as a probabilistic generalization of the concept of currying in functional programming. This connection makes LUNO compatible with established methods for quantifying weight-space uncertainty in deep neural networks, including the Laplace approximation (Ritter et al., 2018; Daxberger et al., 2021a; Kristiadi et al., 2020; Papamarkou et al., 2024), SWAG (Maddox et al., 2019), or meanfield variational inference (Blundell et al., 2015). LUNO is practical, introduces minimal computational overhead, and can be applied post-hoc, without requiring to retrain the neural operator. It scales to large models and datasets and, like neural operators, is inherently resolution-agnostic. We demonstrate the capabilities of the framework in a case study on Fourier neural operators. + +![](images/figures/luno-fig-0001.jpg) +Figure 1: Illustration of the steps involved in LUNO. A trained neural operator $\pmb { F }$ (top left) is converted into an equivalent neural network $f$ with outputs in $\mathbb { R } ^ { d _ { \mathbb { U } } ^ { \prime } }$ using (reverse) currying (top right). Linearizing $f$ around the mean of the Gaussian weight belief results in a Gaussian process posterior f quantifying the uncertainty about the function learned by $f$ (bottom right). Finally, probabilistic currying transforms f into a function-valued Gaussian process posterior $\mathbf { F }$ over the operator learned by the neural operator $\pmb { F }$ (bottom left). + +LUNO is designed to be compatible with arbitrary (non-Gaussian) weight-space beliefs (see e.g. Appendix A.4). Nevertheless, we focus our exposition and experiments on Gaussian weight-space uncertainty, as, in the future, we aim to explore the use of the resulting function-valued Gaussian process in downstream tasks such as the ones outlined above. + +In Section 2, we review the fundamentals of neural operators and (multi-output) Gaussian processes. Section 3 presents our main contribution. We first develop Gaussian processes that take values in (infinite-dimensional) Banach spaces of functions, along with the notion of probabilistic currying, which formalizes their equivalence to (multi-output) Gaussian processes. Using probabilistic currying, we construct function-valued Gaussian processes from neural operators with Gaussian weight beliefs. In Section 4, we discuss prior work on operator learning and related uncertainty quantification. Finally, we demonstrate in Section 5 the effectiveness of LUNO in common PDE learning settings. + +# 2. Background + +We first review neural operators, with emphasis on Fourier neural operators, which serve as the primary case study for our analysis. Then, we provide an overview of multi-output Gaussian processes. + +# 2.1. Neural Operators + +Neural operators (NOs) (Kovachki et al., 2023) are neural network architectures that map between (infinitedimensional) Banach spaces of functions. A neural operator is a function $\pmb { F } \colon \mathbb { A } \times \mathbb { W } \mathbb { U }$ , where + +• A is a (separable) Banach space of functions $\pmb { a } \colon \mathbb { D } _ { \mathbb { A } } $ $\mathbb { R } ^ { d _ { \mathbb { A } } ^ { \prime } }$ with domain $\mathbb { D } _ { \mathbb { A } } \subset \mathbb { R } ^ { d _ { \mathbb { A } } }$ , + +• $\mathbb { U }$ is a (separable) Banach space of functions $\pmb { u } \colon \mathbb { D } _ { \mathbb { U } } \to$ $\mathbb { R } ^ { d _ { \mathbb { U } } ^ { \prime } }$ with domain $\mathbb { D } _ { \mathbb { U } } \subset \mathbb { R } ^ { d _ { \mathbb { U } } }$ , + +• W is a set of parameters (typically $\mathbb { W } \subset \mathbb { R } ^ { p }$ or $\mathbb { W } \subset$ $\mathbb { C } ^ { p }$ ). + +To keep training tractable, neural operators are trained on datasets $\{ ( \pmb { a } ^ { ( i ) } ( \mathbf { \bar { X } } _ { \mathbb { A } } ^ { ( i ) } ) , \pmb { u } ^ { ( i ) } ( \mathbf { X } _ { \mathbb { U } } ^ { ( i ) } ) ) \} _ { i = 1 } ^ { \bar { n } }$ consisting of pairs of input and corresponding output functions $( \pmb { a } ^ { ( i ) } , \pmb { u } ^ { ( i ) } ) \in$ $\mathbb { A } \times \mathbb { U }$ that are discretized at finitely many points $X _ { \mathbb { A } } ^ { ( i ) } \in$ $( \mathbb { D } _ { \mathbb { A } } ) ^ { n _ { \mathbb { A } } ^ { ( i ) } }$ and $X _ { \mathbb { U } } ^ { ( i ) } \in ( \mathbb { D } _ { \mathbb { U } } ) ^ { n _ { \mathbb { U } } ^ { ( i ) } }$ , respectively. The training objective is typically given by the empirical risk + +$$ +R ( \pmb { w } ) = \frac { 1 } { n } \sum _ { i = 1 } ^ { n } L ( \pmb { u } ^ { ( i ) } ( \pmb { X } _ { \mathbb { U } } ^ { ( i ) } ) , \pmb { F } ( \pmb { a } ^ { ( i ) } ( \pmb { X } _ { \mathbb { A } } ^ { ( i ) } ) , \pmb { w } ) ( \pmb { X } _ { \mathbb { U } } ^ { ( i ) } ) ) +$$ + +or a regularized version of the empirical risk. Neural operators were originally developed, and are commonly used, to learn the solution operator of non-linear, parametric partial differential equations. In this case, typically, $\mathbb { D } _ { \mathbb { A } } = \mathbb { D } _ { \mathbb { U } }$ , the input functions $\mathbf { \pmb { a } } \in \mathbb { A }$ correspond to parameters and/or initial conditions of the PDE, and the output functions $\mathbf { \pmb { u } } \in \mathbb { U }$ are the corresponding solutions of the PDE (at later time points). There are many different realizations of the abstract neural operator framework, including low-rank neural operators (Kovachki et al., 2023), (multipole) graph neural operators (Li et al., 2020b;a), and (spherical) Fourier neural operators (Li et al., 2021; Bonev et al., 2023). + +Example 2.1 (Fourier Neural Operators). As a case study we will focus on Fourier neural operators (FNOs) (Li et al., 2021), a popular variant of the neural operator architecture that applies all spatially global operations in the spectral domain. An FNO $\pmb { F }$ transforms a periodic input function $\textbf { \em a }$ into a periodic output function $\begin{array} { r l } { \mathbf { \nabla } } & { { } \mathbf { \nabla } \mathbf { F } ( \mathbf { a } , \pmb { w } ) ( \pmb { x } ) : = } \end{array}$ + +${ \pmb q } ( { \pmb v } ^ { ( L ) } ( { \pmb x } ) , { \pmb w } _ { \pmb q } )$ with + +$$ +\begin{array} { r l } & { v _ { i } ^ { ( l + 1 ) } ( \pmb { x } ) : = \sigma ^ { ( l ) } \Bigg ( \displaystyle \sum _ { j = 1 } ^ { d _ { v } ^ { \prime } } \mathcal { F } ^ { - 1 } \bigg ( \Big ( R _ { k i j } ^ { ( l ) } \mathcal { F } \Big ( v _ { j } ^ { ( l ) } \Big ) _ { k } \Big ) _ { k = 1 } ^ { k _ { \operatorname* { m a x } } } \bigg ) ( \pmb { x } ) } \\ & { \qquad \quad \quad + W _ { i j } ^ { ( l ) } v _ { j } ^ { ( l ) } ( \pmb { x } ) \Bigg ) } \end{array} +$$ + +for $l = 1 , \ldots , L - 1$ and ${ \pmb v } ^ { ( 1 ) } ( { \pmb x } ) = p ( { \pmb a } ( { \pmb x } ) , { \pmb w } _ { \pmb p } ) \in \mathbb { R } ^ { d _ { \pmb v } ^ { \prime } }$ where $\mathcal { F }$ denotes the Fourier transform of a periodic function.1 $p \colon \mathbb { R } ^ { d _ { \mathbb { A } } ^ { \prime } } \times \mathbb { W } _ { p } \mathbb { R } ^ { d _ { v } ^ { \prime } }$ and $\pmb q \colon \mathbb { R } ^ { d _ { v } ^ { \prime } } \times \mathbb { W } _ { \pmb q } \mathbb { R } ^ { d _ { \mathbb { U } } ^ { \prime } }$ are parametric functions called lifting and projection, respectively. $\pmb { R } ^ { ( l ) } \in \mathbb { C } ^ { k _ { \operatorname* { m a x } } \times d _ { v } ^ { \prime } \times d _ { v } ^ { \prime } }$ and $\bar { \boldsymbol { W } } ^ { ( l ) } \in \mathbb { R } ^ { \bar { d } _ { v } ^ { \prime } \times d _ { v } ^ { \prime } }$ , and ${ \pmb w } = ( { \pmb w } _ { p } , { \pmb W } ^ { ( 1 ) } , { \pmb R } ^ { ( 1 ) } , \dots , { \pmb W } ^ { ( L - 1 ) } , { \pmb R } ^ { ( L - 1 ) } , { \pmb w } _ { q } )$ . The map ${ \pmb v } ^ { ( l ) } \dot { } { \pmb v } ^ { ( l + 1 ) }$ is the $l$ -th Fourier layer. If the inputs $\textbf { \em a }$ are discretized on a regular grid, $\mathcal { F }$ can be computed by a real fast Fourier transform (RFFT). + +# 2.2. (Gaussian) Random Processes + +Aiming to generalize the notion of a Gaussian process later, we provide its formal definition from mathematical statistics that is rarely used in machine learning. For a set $\Omega$ and a set $F$ of functions on $\Omega$ with values in a measurable space, we denote by $\sigma ( F )$ the smallest $\sigma$ -algebra for which all $f \in F$ are measurable. The Borel $\sigma$ -algebra on a topological space $\Omega$ is denoted by $B \left( \Omega \right)$ . A random process on a probability space $( \Omega , A , { \mathrm { P } } )$ with index set A and values in a measurable space $( S , A _ { S } )$ is a function $\mathrm { f } \colon \mathbb { A } \times \Omega S$ such that $\operatorname { f } ( a , \cdot )$ is $\mathcal { A } { - } \mathcal { A } _ { S }$ -measurable for all $a \in \mathbb { A }$ . We use the shorthand $\mathrm { f } ( a ) : = \mathrm { f } ( a , \cdot )$ . One can show that $\omega \mapsto \operatorname { f } ( \cdot , \omega )$ is a function-valued random variable with values in $\left( \mathbb { R } ^ { \mathbb { A } } , \sigma ( \delta _ { \mathbb { A } } ) \right)$ , where $\delta _ { A }$ denotes the set of point evaluation functionals on the set $B ^ { A }$ of functions from $A$ to $B$ . A random process is called Gaussian or a Gaussian process (GP) if it has values in $\left( \mathbb { R } , B \left( \mathbb { R } \right) \right)$ and $\omega \mapsto ( \mathrm { f } ( a _ { 1 } , \omega ) , \dots , \mathrm { f } ( a _ { n } , \omega ) )$ is an $\mathbb { R } ^ { n }$ -valued Gaussian random variable for all $n \in \mathbb { N }$ and $a _ { 1 } , \dotsc , a _ { n } \in \mathbb { A }$ . The mean function of f is given by $a \mapsto \mathbb { E } _ { \mathrm { P } } \left[ \mathrm { f } ( a ) \right]$ and the covariance function of f is given by $( a _ { 1 } , a _ { 2 } ) \mapsto \operatorname { C o v } _ { \mathrm { P } } \left[ \mathrm { f } ( a _ { 1 } ) , \mathrm { f } ( a _ { 2 } ) \right]$ We denote by f $\sim \mathcal { G P } \left( m , k \right)$ that f is a Gaussian process with mean function $m$ and covariance function $k$ . + +It is common to extend the concept of a GP to finitely many output dimensions. A $d ^ { \prime }$ -output Gaussian process f is a random process with values in $( \mathbb { R } ^ { d } , B \left( \mathbb { R } ^ { d } \right) )$ such that $\omega \mapsto$ $\begin{array} { r l } { \bigl ( \mathbf { f } ( a _ { 1 } , \omega ) ^ { \top } } & { { } \cdot \cdot \cdot \mathrm { ~ \mathbf ~ { f } ( } a _ { n } , \omega ) ^ { \top } \bigr ) ^ { \top } } \end{array}$ is an $\mathbb { R } ^ { n \cdot d ^ { \prime } }$ -valued Gaussian random variable for all $\ l _ { n } \in \mathbb { N }$ , and $a _ { 1 } , \dotsc , a _ { n } \in \mathbb { A }$ We use the shorthand $\mathbf { f } ( a ) : = \mathbf { f } ( a , \cdot )$ . The mean function of f is given by $a \mapsto \mathbb { E } _ { \mathrm { P } } \left[ \mathbf { f } ( a ) \right] \in \mathbb { R } ^ { d ^ { \prime } }$ and the covariance function of $\mathbf { f }$ is $( a _ { 1 } , a _ { 2 } ) \mapsto \operatorname { C o v } _ { \mathrm { P } } \left[ \mathbf { f } ( a _ { 1 } ) , \mathbf { f } ( a _ { 2 } ) \right] \in \mathbb { R } ^ { d ^ { \prime } \times d ^ { \prime } }$ . We denote by f $\sim \mathcal { G P } \left( m , K \right)$ that f is a multi-output Gaussian process with mean function $_ { \mathbf { \nabla } } \mathbf { m } _ { \mathbf { \nabla } }$ and covariance function $\kappa$ While the notion of a multi-output Gaussian process might seem more general than the notion of a Gaussian process, it is possible to “emulate” a function with multiple outputs by augmenting the input space of a Gaussian process: + +Lemma 2.1. Let $( \Omega , A , { \mathrm { P } } )$ be a probability space, f : $\mathbb { A } \times$ $\Omega \to \mathbb { R } ^ { d ^ { \prime } }$ , $\mathbb { I } = \{ 1 , \dots , d ^ { \prime } \}$ , and f : $( \mathbb { A } \times \mathbb { I } ) \times \Omega \to \mathbb { R }$ with $( { \bf f } ( a , \cdot ) ) _ { i } = { \bf f } ( ( a , i ) , \cdot )$ for all $a \in \mathbb { A }$ and $i \in \mathbb { I }$ (P-almost surely). Then f $\sim \mathcal { G P } \left( m , K \right)$ if and only if f $\sim \mathcal { G P } \left( m , k \right)$ , where, for all $a \in \mathbb { A }$ and $i \in \mathbb { I }$ , + +$$ +( { \pmb m } ( a ) ) _ { i } = m ( a , i ) , +$$ + +as well as, for all $a _ { 1 } , a _ { 2 } \in \mathbb { A }$ and $i , j \in \mathbb { I } ,$ , + +$$ +( \pmb { K } ( a _ { 1 } , a _ { 2 } ) ) _ { i j } = k ( ( a _ { 1 } , i ) , ( a _ { 2 } , j ) ) . +$$ + +# 3. LUNO: Linearized Predictive Uncertainty in Neural Operators + +In this section, we show how to obtain linearized predictive uncertainty in neural operators (LUNO). We leverage model linearization to propagate Gaussian weightspace uncertainty through the neural operator to its predictions. LUNO can be applied to trained models as a post-processing step, and does not require expensive retraining. Furthermore, LUNO employs the framework of function-valued Gaussian processes. To that end, we first develop the concept of a function-valued Gaussian process and draw an important parallel with currying in functional programming, which offers a natural interpretation of our method. Figure 1 illustrates the main steps comprising our methodology. + +# 3.1. Function-Valued Gaussian Processes and Probabilistic Currying + +We want to use model linearization to extend the Gaussian belief over the parameters of a neural network $\pmb { f } \colon { \mathbb { R } ^ { d } } \times$ $\mathbb { R } ^ { p } \to \mathbb { R } ^ { d ^ { \prime } }$ into a (multi-output) Gaussian process belief over the function learned by the neural network. However, this is not immediately applicable to neural operators, since their outputs do not lie in $\mathbb { R } ^ { d ^ { \prime } }$ , but in a potentially infinitedimensional Banach space of functions. Hence, we need to generalize (multi-output) Gaussian processes to the notion of a Banach-valued Gaussian process. + +Definition 3.1 (Banach-Valued Gaussian Process). Let U be a real separable Banach space and $\mathbb { L }$ a set2 of linear functionals on $\mathbb { U }$ . A random process $\mathbf { F } \colon \mathbb { A } \times \Omega \to \mathbb { U }$ on a probability space $( \Omega , A , { \mathrm { P } } )$ with index set A and values in $( \mathbb { U } , \sigma ( \mathbb { L } ) )$ is called Gaussian or a Gaussian process if $\omega \mapsto ( \mathbf { F } ( a _ { 1 } , \omega ) , \ldots , \mathbf { F } ( a _ { n } , \omega ) )$ is a jointly Gaussian random variable3 for all $n \in \mathbb N$ and $a _ { 1 } , \dotsc , a _ { n } \in \mathbb { A }$ . + +As above, we use the shorthand $\mathbf { F } ( a ) : = \mathbf { F } ( a , \cdot )$ . Moreover, the map $\omega \mapsto \mathbf { F } ( \cdot , \omega )$ is a random variable with values in the space of (linear and non-linear) operators $\left( \mathbb { U } ^ { \mathbb { A } } , \sigma ( \delta _ { \mathbb { A } } ) \right)$ . This warrants the interpretation of $\mathbb { U }$ -valued Gaussian processes as Gaussian random operators. + +In the context of neural operators, $\mathbb { U }$ is a Banach space of $\mathbb { R } ^ { d _ { \mathbb { U } } ^ { \prime } }$ -valued functions on a common domain $\mathbb { D } _ { \mathbb { U } }$ . In this case, we can show that $\mathbb { U }$ -valued Gaussian processes are closely related to multi-output Gaussian processes with an augmented input space. This is in analogy to Lemma 2.1, but requires some additional technical assumptions. + +Theorem 3.2 (Probabilistic Currying in Banach Spaces; proof in Appendix A.3). Let $( \Omega , A , { \mathrm { P } } )$ be a probability space and U a real separable Banach space of $\mathbf { \bar { \mathbb { R } } } ^ { d ^ { \prime } }$ -valued functions with domain $\mathbb { D } _ { \mathbb { U } }$ . Let $\mathbf { F } \colon \mathbb { A } \times \Omega \mathbb { U }$ and f : $( \mathbb { A } \times$ $\mathrm { \bar { D } _ { U } } ) \times \Omega \to \mathbb { R } ^ { d ^ { \prime } }$ such that $\mathbf { F } ( { \pmb a } , \cdot ) ( { \pmb x } ) = \mathbf { f } ( ( { \pmb a } , { \pmb x } ) , \cdot )$ for all $\textbf { \em a } \in \mathbb { A }$ and $\pmb { x } \in \mathbb { D } _ { \mathbb { U } }$ (P-almost surely). Then (i) $\mathbf { F }$ is a random process with values in $\big ( \mathbb { U } , \sigma ( \delta _ { \mathbb { U } } ) \big )$ if and only $i f$ f is a $\mathbb { R } ^ { d ^ { \prime } }$ -valued random process, (ii) $\mathbf { F }$ is Gaussian $i f$ and only if f is Gaussian, and (iii) if all evaluation maps $\delta _ { \pmb { x } } : \mathbb { U } \overset { \cdot } { } \mathbb { R } ^ { d ^ { \prime } } , \pmb { u } \mapsto \pmb { u } ( \pmb { x } )$ are continuous, then (i) holds for F with values in $\left( \mathbb { U } , B \left( \mathbb { U } \right) \right)$ . + +Theorem 3.2 reveals an insight into the abstract concept of function-valued Gaussian processes: Function-valued Gaussian processes are equivalent to (multi-output) Gaussian processes with augmented input spaces. This equivalence enables the translation of real-valued GPs, a computationally feasible structure, into infinite-dimensional function-valued objects. + +Probabilistic Currying We note that Theorem 3.2 constitutes a probabilistic analogue of the concept of currying from functional programming (and category theory more generally). The Theorem shows the equivalence of the vector-valued (Gaussian) random function $\mathbf { f } \colon \mathbb { A } \times \mathbb { D } _ { \mathbb { U } } \to \mathbb { R } ^ { d ^ { \prime } }$ and the (Gaussian) random operator $\mathbf { F } \colon \mathbb { A } \to ( \mathbb { D } _ { \mathbb { U } } \to \mathbb { R } ^ { d ^ { \prime } } )$ with $\mathbf { F } ( { \pmb { a } } ) ( { \pmb { x } } ) \overset { \mathrm { a . s . } } { = } \mathbf { f } ( { \pmb { a } } , { \pmb { x } } )$ . + +Example 3.1 (Currying a Continuous Bivariate Gaussian Process). Let f $\sim \mathcal { G P } \left( m , k \right)$ be a bivariate 2-output Gaussian process with compact index set $\mathbb { X } _ { 1 } \times \mathbb { X } _ { 2 } \subset \mathbb { R } ^ { 2 }$ on $( \Omega , A , { \mathrm { P } } )$ with (P-almost surely) continuous paths. For instance, this assumption is fulfilled if $m$ is continuous and $k$ is a multivariate Matérn covariance function (Da Costa et al., 2023). Then $a \mapsto \operatorname { f } ( a , \cdot )$ is a function-valued Gaussian process. More precisely, Theorem 3.2 shows that the map $\operatorname { F } \colon \mathbb { X } _ { 1 } \times \Omega \ \to \ C ( \mathbb { X } _ { 2 } ) , ( a , \omega ) \mapsto ( x \mapsto \operatorname { f } ( ( a , x ) , \omega ) )$ is a $C ( \mathbb { X } _ { 2 } )$ -valued Gaussian process with index set $\mathbb { X } _ { 1 }$ . + +Thus, an intuitive way to understand function-valued Gaussian processes is as objects that, when evaluated, return a Gaussian process. Currying can also be used to relate the mean and covariance functions of function-valued or more general vector-valued (Gaussian) random processes, and their counterparts defined on the corresponding multioutput (Gaussian) random process. As this discussion is rather technical, we defer it to Appendix A.3. + +Appendix A provides an in-depth explanation of our theoretical framework and contains a plethora of theoretical results on Gaussian processes with values in arbitrary (infinitedimensional) vector spaces that are not necessarily Banach or function spaces. For example, such results are vital for quantifying uncertainty in neural operators applied to PDEs that only admit weak solutions (see Appendix A.4). + +# 3.2. Linearization Turns Neural Operators into Function-Valued Gaussian Processes + +We use the notion of function-valued Gaussian processes to develop LUNO. We delineate the key components into different steps, visually represented in Figure 1. + +Step 0 Let ${ \pmb { F } } \colon \mathbb { A } \times \mathbb { W } \mathbb { U } \subset ( \mathbb { R } ^ { d _ { \mathbb { U } } ^ { \prime } } ) ^ { \mathbb { D } _ { \mathbb { U } } }$ be a neural operator as in Section 2.1 with $\mathbb { W } = \mathbb { R } ^ { p }$ . + +Step 1 By uncurrying $\pmb { F }$ , we define the function + +$$ +\begin{array} { r } { \pmb { f } \colon ( \mathbb { A } \times \mathbb { D } _ { \mathbb { U } } ) \times \mathbb { W } \to \mathbb { R } ^ { d _ { \mathbb { U } } ^ { \prime } } , ( ( \pmb { a } , \pmb { x } ) , \pmb { w } ) \mapsto \pmb { F } ( \pmb { a } , \pmb { w } ) ( \pmb { x } ) . } \end{array} +$$ + +The function $f _ { \mu } ^ { \mathrm { l i n } }$ is linear in the weights, but it remains highly nonlinear in the input. Moreover, we have $f _ { \mu } ^ { \mathrm { l i n } } ( \cdot , \pmb { \mu } ) = f ( \cdot , \pmb { \mu } )$ , which means that the mean function of f matches the prediction of the trained neural operator (before linearization) if we set $\pmb { \mu }$ to the weights $\boldsymbol { w } ^ { \star }$ found during training. + +Step 3 Probabilistic currying constructs a Gaussian random operator from f. Namely, we define the function + +$$ +\mathbf { F } \colon \mathbb { A } \times \Omega \to \mathbb { U } , ( \pmb { a } , \omega ) \mapsto ( \pmb { x } \mapsto \mathbf { f } ( ( \pmb { a } , \pmb { x } ) , \omega ) ) . +$$ + +(For this $\mathbf { F }$ to be well-defined, we need to assume that ${ \bf f } ( ( a , \cdot ) , \omega )$ is $\in \mathbb { U }$ for all $\mathbf { \pmb { a } } \in \mathbb { A }$ . See also Appendix A.4). Theorem 3.2 then shows that $\mathbf { F }$ is a $\mathbb { U }$ -valued Gaussian process. Moreover, $\mathbb { E } \left[ \mathbf { F } ( \pmb { a } ) ( \pmb { x } ) \right] = F ( \pmb { a } , \pmb { \mu } ) ( \pmb { x } )$ , and + +$$ +\begin{array} { r l } & { \operatorname { C o v } \left[ \mathbf { F } ( \boldsymbol { a } _ { 1 } ) ( \boldsymbol { x } _ { 1 } ) , \mathbf { F } ( \boldsymbol { a } _ { 2 } ) ( \boldsymbol { x } _ { 2 } ) \right] } \\ { \quad } & { = \operatorname { D } _ { \boldsymbol { w } } { F } ( \boldsymbol { a } _ { 1 } , \boldsymbol { w } ) ( \boldsymbol { x } _ { 1 } ) | _ { \mu } \Sigma \operatorname { D } _ { \boldsymbol { w } } { F } ( \boldsymbol { a } _ { 2 } , \boldsymbol { w } ) ( \boldsymbol { x } _ { 2 } ) | _ { \mu } ^ { \top } . } \end{array} +$$ + +The entire construction, in particular Theorem 3.2, still applies if w is not Gaussian. In this case, f and $\mathbf { F }$ are no longer Gaussian, but the formulae for the mean and covariance functions remain valid. We derive a generalization of the method for general separable Banach spaces $\mathbb { U }$ in Appendix A.4. For instance, this is useful if the functions in $\mathbb { U }$ are not pointwise defined, such as weak solutions of PDEs. + +# 3.2.1. CASE STUDY: FOURIER NEURAL GAUSSIANRANDOM OPERATORS + +Step 2 We obtain a Gaussian belief $\boldsymbol { \kappa } \sim \mathcal { N } \left( \boldsymbol { \mu } , \boldsymbol { \Sigma } \right)$ over the parameters of the network. In Bayesian deep learning, a common way to obtain this Gaussian belief is by placing a Gaussian prior $p ( \mathbf { w } )$ on the network’s parameters and then approximating the posterior distribution given the data $p ( \mathbf { w } \mid \mathcal { D } )$ . Well-established (approximate) inference techniques to obtain the posterior over w include the Laplace approximation (Ritter et al., 2018; Daxberger et al., 2021a; Immer et al., 2021), variational inference (Graves, 2011; Blundell et al., 2015; Khan et al., 2018), and SWAG (Maddox et al., 2019). Since $f$ has values in $\mathbb { R } ^ { d _ { \mathbb { U } } ^ { \prime } }$ , following Khan et al. (2019); Immer et al. (2021) and Appendix B, we can linearize the model around $\pmb { \mu }$ : + +$$ +\begin{array} { r l } & { f ( ( a , x ) , w ) \approx f _ { \mu } ^ { \mathrm { l i n } } ( ( a , x ) , w ) } \\ & { \mathrel { \mathop : } = f ( ( a , x ) , \mu ) + \mathrm { D } _ { w } f ( ( a , x ) , w ) | _ { \mu } ( w - \mu ) } \end{array} +$$ + +to arrive at an induced approximate $d _ { \mathbb { U } } ^ { \prime }$ -output Gaussian process belief $\mathbf { f } : = f _ { \mu } ^ { \mathrm { l i n } } ( \cdot , \mathbf { w } ) \sim \mathcal { G P } \left( m , K \right)$ with index set $\mathbb { A } \times \mathbb { D } _ { \mathbb { U } }$ ${ \mathbb { U } } , m ( { \pmb a } , { \pmb x } ) = f ( ( { \pmb a } , { \pmb x } ) , { \pmb \mu } )$ , and + +$$ +\begin{array} { r l r } & { } & { K ( ( a _ { 1 } , \pmb { x } _ { 1 } ) , ( \pmb { a } _ { 2 } , \pmb { x } _ { 2 } ) ) } \\ & { } & { = \mathrm { D } _ { \pmb { w } } { \pmb { f } } ( ( \pmb { a } _ { 1 } , \pmb { x } _ { 1 } ) , \pmb { w } ) | _ { \pmb { \mu } } \pmb { \Sigma } \mathrm { D } _ { \pmb { w } } { \pmb { f } } ( ( \pmb { a } _ { 2 } , \pmb { x } _ { 2 } ) , \pmb { w } ) | _ { \pmb { \mu } } ^ { \top } . } \end{array} +$$ + +The exposition so far applies generally to neural operators. For Fourier neural operators, a particularly efficient representation of the function-valued posterior process is available. To simplify the exposition, we focus on the case where the Gaussian belief is restricted to the parameters of the final Fourier block $\pmb { w } _ { L - 1 } : = ( \pmb { R } ^ { ( L - 1 ) } , \mathbf { \bar { W } } ^ { ( L - 1 ) } )$ . This is a common approach in the context of last-layer Laplace approximation (Kristiadi et al., 2020). In Appendix C.1, we show that the function-valued GP obtained by applying LUNO in this case takes the form + +$$ +\begin{array} { r l } & { \mathrm { F } ( \pmb { a } ) ( \pmb { x } ) = \tilde { \pmb q } ( m _ { \mathbf { z } ^ { ( L - 1 ) } } ( \pmb { x } ) ) + \Bigl ( \mathrm { D } \tilde { \pmb q } \left( m _ { \mathbf { z } ^ { ( L - 1 ) } } ( \pmb { x } ) \right) } \\ & { \qquad \cdot \left( \mathbf { z } ^ { ( L - 1 ) } ( \pmb { x } ) - m _ { \mathbf { z } ^ { ( L - 1 ) } } ( \pmb { x } ) \right) \Bigr ) , } \end{array} +$$ + +i.e., $\mathrm { F } ( \pmb { a } ) \sim \mathcal { G P } \left( m _ { \pmb { a } } , K _ { \pmb { a } } \right)$ with + +$$ +\begin{array} { c } { { m _ { a } ( x ) = F ( a , w ^ { \star } ) ( x ) , \quad \mathrm { a n d } } } \\ { { K _ { a } ( x _ { 1 } , x _ { 2 } ) = \mathrm { D } \tilde { q } \left( m _ { { \mathbf z } ^ { ( L - 1 ) } } ( x _ { 1 } ) \right) K _ { { \mathbf z } ^ { ( L - 1 ) } } ( x _ { 1 } , x _ { 2 } ) } } \\ { { \cdot \mathrm { D } \tilde { q } \left( m _ { { \mathbf z } ^ { ( L - 1 ) } } ( x _ { 2 } ) \right) ^ { \top } , } } \end{array} +$$ + +where $\mathbf { z } ^ { ( L - 1 ) } \sim \mathcal { G P } \left( m _ { \mathbf { z } ^ { ( L - 1 ) } } , K _ { \mathbf { z } ^ { ( L - 1 ) } } \right)$ is a multi-output parametric Gaussian process whose moments only depend on ${ \pmb v } ^ { ( L - 1 ) } , { \pmb \mu }$ , and $\pmb { \Sigma }$ , and $\tilde { \pmb q } = \pmb q ( \cdot , \pmb w _ { \pmb q } ) \circ \pmb \sigma ^ { ( L - 1 ) }$ . + +There are two practical benefits arising from this representation. First, computing the moments of, and drawing samples from $\mathrm { F } ( a )$ only needs access to the hidden state $\pmb { v } ^ { ( \bar { L } - 1 ) }$ of the neural operator. We can thus evaluate the Gaussian process belief at arbitrary output points $\pmb { x } \in \mathbb { D } _ { \mathbb { U } }$ , without the need to compute more than one (full) forward pass of the neural operator. Secondly, since the Gaussian process belief $\mathrm { F } ( a )$ over the output function is parametric, we can efficiently sample entire functions from it that can then be lazily evaluated at arbitrary points. This is in contrast to general non-parametric Gaussian processes, where one typically discretizes the GP before drawing samples of the function values at the given finite set of points. Such lazy functional samples can be used e.g. for active experimental design and Bayesian optimization (Wilson et al., 2021). + +# 4. Related Work + +Azizzadenesheli et al. (2024) provide a comprehensive overview of neural operator architectures. These include graph neural operators (Li et al., 2020a), physics-informed neural operators (Li et al., 2024), multi-wavelet neural operators (Gupta et al., 2021) and the widely used Fourier neural operators (Li et al., 2021). FNOs have gained particular prominence, finding applications across various PDE problems (Pathak et al., 2022; Zhang et al., 2023; Li et al., 2023a; Rashid et al., 2022; Qin et al., 2024; Kossaifi et al., 2023; Bonev et al., 2023). Theoretical foundations for FNOs have been established, with Kovachki et al. (2021) proving their universal approximation capabilities for continuous operators, and Lanthaler et al. (2024) analyzing discretizationinduced aliasing errors. + +While neural operator architectures have advanced, incorporating uncertainty estimation remains challenging. Recent work has approached this problem from different angles. Garg & Chakraborty (2023) applied variational inference to estimate Bayesian posteriors in DeepONets. More closely related to our work, Magnani et al. (2022) developed uncertainty estimates for graph neural operators using Laplace approximation, though their approach does not extend to FNOs, nor does it consider function space formulations. Kumar et al. (2024) combined Gaussian Process priors with Wavelet Neural Operators, optimizing hyperparameters through negative log-marginal likelihood minimization. Additional Bayesian operator frameworks have been explored by Garg & Chakraborty (2022); Batlle et al. (2024); Zou et al. (2024); Mora et al. (2025). + +Function-valued Gaussian processes have been studied in the Hilbert space setting by Owhadi (2023); Batlle et al. (2024). Our approach formulates the theory natively within the context of Banach spaces, as neural operators are defined as mappings between such spaces. In the Appendix we prove that, when restricted to the Hilbert space setting, the theoretical framework Owhadi (2023); Batlle et al. (2024) embeds into ours. + +To generate a probabilistic belief over a neural network’s weights, various Bayesian posterior approximation techniques are available. One of the most popular is the Laplace approximation, introduced to deep learning by Mackay (1992), which has gained popularity in the Bayesian deep learning community (Ritter et al., 2018; Daxberger et al., 2021a; Kristiadi et al., 2020; Papamarkou et al., 2024). This is also due to its scalability, achieved through various strategies including using log-posterior Hessian approximations (Ritter et al., 2018; Martens, 2020), treating only a subset of the model probabilistically (Daxberger et al., 2021b), employing linearized Laplace (Foong et al., 2019; Immer et al., 2021), or using scalable Gaussian processes methods (Deng et al., 2022; Ortega et al., 2024). Other Bayesian deep learning methods include variational inference (Graves, 2011; Blundell et al., 2015; Khan et al., 2018; Zhang et al., 2018), Markov Chain Monte Carlo (Neal, 1996; Welling & Teh, 2011; Zhang et al., 2020), SWAG (Maddox et al., 2019), or heuristic methods (Gal & Ghahramani, 2016; Maddox et al., 2019). Finally, a widely used approach for uncertainty quantification in deep learning is ensembles (Lakshminarayanan et al., 2017; Hansen & Salamon, 1990), that train multiple independent neural networks with different random initializations and aggregate the predictions. + +# 5. Experiments + +We evaluate linearized predictive uncertainty (LUNO- $^ *$ ) against sample-based approaches (Sample- $*$ ), which require additional approximations to impose a Gaussian Process structure over the output space. To be precise, in the Sample-$^ *$ methods, we draw samples from the weight-space belief, map the samples through the (nonlinear) map ${ \pmb w } \mapsto { \pmb F } ( \cdot , { \pmb w } )$ , and compute a function-valued Gaussian process belief over the prediction by moment matching the empirical mean and covariance function. We consider isotropic Gaussian ( $^ *$ -Iso) and low-rank Laplace approximated $( { * } { - } \mathrm { L A } )$ weightspace uncertainties, in both their sample-based (Sample) and linearized (LUNO) forms. We compare these weight-space-Gaussian methods against input perturbations (Pathak et al., 2022), and deep ensembles. Deep ensembles were trained 10 times with different random seeds on the original Fourier neural operator (FNO) architecture. We evaluate our model on time-dependent PDEs in one and two spatial dimensions, predicting the next time step autoregressively from the previous ten. We assess uncertainty quantification in two key settings: (1) a low-data regime, where the model is trained on a limited number of trajectories, and (2) out-of-distribution (OOD) scenarios, where physical phenomena unseen during training are introduced at test time. + +We evaluate the predictive uncertainty using standard metrics: the expected root mean squared error (RMSE) of the mean predictions, the expected marginal $\chi ^ { 2 }$ statistics, and the expected marginal negative log-likelihood (NLL) over 250 test input-output pairs. Hyperparameters are optimized via grid search using the expected marginal NLL on a validation set as the target. Full details, including data generation, training procedures, uncertainty estimation methods, and more detailed results are provided in the Appendix. + +![](images/figures/luno-fig-0002.jpg) +Figure 2: FNO predictive uncertainty quantified by several different methods. Top row: target function $( - )$ , mean ( ) and 1.96 standard deviations $( \sqsupset$ of, as well as samples ( ) from, the predictive belief. For the ensemble, the samples are four of the ensemble members. Bottom row: spread of the predictive distribution around the mean. For the sample-/ensemble-based methods, we construct a Gaussian distribution from the empirical covariance matrix and draw four samples ( ). We plot 1.96 standard deviations $( \sqsupset$ of the predictive belief, as well as the top-three eigenfunctions $( - )$ and a heatmap of the predictive covariance matrix (top right corner of panels). + +Code. We provide an efficient implementation of the LUNO framework in JAX (Bradbury et al., 2018) at + +‡ / MethodsOfMachineLearning / luno. + +The code for our experiments can be found at ‡ / 2bys / luno-experiments. + +Low data regime. We train an FNO for 100 epochs on 25 simulated solutions of Burgers’ equation with 59-time steps and evaluate their uncertainty on 250 unseen test pairs. Figure 2 visualizes the predictive uncertainty for input perturbations, deep ensemble, Sample-LA, and LUNO-LA on a single test data point of Burgers’ equation. Table 1 shows that LUNO-LA outperforms the other approaches. This trend holds across two other one-dimensional timedependent PDE datasets, which are included in the Appendix (Table 4 and Table 5). While all methods produce marginal confidence bands around the network prediction, their sample path covariances differ qualitatively. + +Out-of-Distribution. To assess OOD robustness, we train an FNO (or an ensemble of 10 FNOs) on a two-dimensional Advection-Diffusion equation with initial conditions sampled from Gaussian blobs and a random constant velocity field. We introduce various additional physical phenomena to the test set. These include reversing the velocity field at the center (Flip), introducing a triangular heat source (Pos), and a cloud-shaped heat sink (Neg). Table 2 reports expected marginal NLL over a variation of out-of-distribution datasets. Additional and more granular results can be found in the Appendix. While LUNO-LA outperforms the other weight space methods and input perturbations, deep ensembles achieve the lowest expected marginal NLL in next-step prediction. However, their uncertainty representation is fundamentally different. Figure 3 compares deep ensemble with LUNO-LA. Deep ensembles approximate uncertainty using a small set of discrete hypotheses, represented by a collection of point masses in parameter space. While this representation is not confined to the analytic form of a Gaussian distribution, it has other constraints: For example, although marginal uncertainty estimates (panel 3 in the figure) can be relatively well-structured, the associated empirical covariance across the ensemble is fundamentally rank-deficient. This limitation is critical, as it leaves certain types of errors entirely unaccounted for (panel 8, which projects residuals onto the null space of the ensemble covariance). By contrast, LUNO-LA constructs a covariance matrix whose rank is (in theory) only bounded by the number of parameters considered.4 As a result, a plot like panel 8 in Figure 3 does not make sense for LUNO-LA, since, in principle, it explains any variation in the data (albeit with varying calibration). This behavior is also evident in full-trajectory evaluations. Although FNOs are trained for next-step prediction, they are often used for auto-regressive roll-outs, where predictions are recursively fed back as inputs. Such roll-outs cause a subtle yet significant distribution shift, as prediction errors accumulate and are treated as ground truth for subsequent steps. While the deep ensemble improves upon the network prediction in terms of RMSE, its uncertainty estimate does not adapt to the increasing error, as reflected in the NLL (cf. Figure 4). + +Table 1: Comparison of UQ methods for an FNO trained on 25 trajectories of Burgers’ equation. + +
MethodRMSE (↓)χ2NLL ()
Input Perturbations3.63 × 10−20.894-1.8720
Ensemble3.49 × 10-25.597-0.8145
Sample-Iso3.72 × 10−20.977−1.9341
LUNO-Iso3.62 × 10−20.864−1.9488
Sample-LA5.59 × 10−22.774-1.1572
LUNO-LA3.62 × 10−21.022-2.0787
+ +Table 2: Expected marginal NLL evaluation across OOD datasets for different methods. Lower is better. + +
MethodBaseFlipPos-Neg-Flip
Input Perturbations−2.5862.573494.935
Ensemble-5.3133.825-1.014
Sample-Iso−2.9214.07143.362
LUNO-Iso−2.8923.45037.733
Sample-LA−2.5764.39527.046
LUNO-LA−2.934−1.1261.164
+ +![](images/figures/luno-fig-0003.jpg) +Figure 3: Comparing an ensemble (left), LUNO-LA (right). Top row shows target, residuals, and the predictive standard deviation. Bottom row shows the absolute ratio of the pointwise residual and the predictive standard deviation as well as a sample from the predictive belief. Since the uncertainty structure of the ensemble prediction is of low rank, we also include its unexplained error by projecting the residual vector onto the null space of the predictive covariance. + +methods are reported in the Appendix. Due to the efficiency of Jacobian-vector products and analytical tractability of the inverse real fast Fourier transform, LUNO- $^ *$ methods outperform their Samples- $^ *$ counterparts, with LUNO-Iso being even faster than the deep ensemble in our implementation. Each method comes with its own additional cost. While deep ensembles need fully separate training runs with different random seeds, LUNO-LA’s main computational bottleneck is computing the low-rank approximation of the generalized Gauss–Newton matrix (GGN). This cost is dominated by network size, the selected rank, and the amount of data used for the GGN approximation. + +# 6. Conclusion + +We introduced LUNO, a framework for predictive uncertainty quantification in neural operators using functionvalued Gaussian processes. LUNO can be interpreted as a probabilistic generalization of currying in functional programming. By leveraging model linearization, it offers a computationally efficient and theoretically grounded approach to incorporating weight-space uncertainties in neural operators. The framework endows neural operators with structured weight-space uncertainty quantification capabilities while preserving their resolution-agnostic nature. We demonstrate this for LUNO-LA in the FNO setting under low-data regimes and out-of-distribution scenarios. + +LUNO’s main limitation lies in the challenges associated with modeling weight-space covariances. Nevertheless, by successfully constructing a structured Gaussian process over the output space, LUNO paves the way for future applications of GP-valued neural operators in scientific and engineering domains. + +![](images/figures/luno-fig-0004.jpg) +Figure 4: Averaged performance of different UQ methods on an autoregressive rollout of the FNO on 50 trajectories from the Pos-Neg-Flip dataset. We compare input perturbations $( - )$ , deep ensembles $( - )$ , Sample-Iso $( - )$ , LUNO-Iso $( - )$ , Sample-LA ( ), LUNO-LA $( - )$ . + +# Impact Statement + +This paper presents work whose goal is to advance the field of Machine Learning. There are many potential societal consequences of our work, none which we feel must be specifically highlighted here. + +# Acknowledgments + +The authors gratefully acknowledge financial support by the European Research Council through ERC CoG Action 101123955 ANUBIS ; the DFG Cluster of Excellence “Machine Learning - New Perspectives for Science”, EXC 2064/1, project number 390727645; the German Federal Ministry of Education and Research (BMBF) through the Tübingen AI Center (FKZ: 01IS18039A); the DFG SPP 2298 (Project HE 7114/5-1), and the Carl Zeiss Foundation, (project "Certification and Foundations of Safe Machine Learning Systems in Healthcare"), as well as funds from the Ministry of Science, Research and Arts of the State of Baden-Württemberg. The authors thank the International Max Planck Research School for Intelligent Systems (IMPRS-IS) for supporting Emilia Magnani, Marvin Pförtner and Tobias Weber. 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If $\mathbb { U }$ is a space of real-valued functions, at the very least, we want to be able to express a probabilistic belief over all point evaluations $\begin{array} { r } { \pmb { F } ( \ b { a } ) ( \pmb { x } ) = : \delta _ { \pmb { x } } ( \pmb { F } ( \ b { a } ) ) } \end{array}$ . Note that point evaluation $\delta _ { x } \colon \mathbb { U } \to \mathbb { R }$ of functions in $\mathbb { U }$ is a linear map, since $\delta _ { x } ( \alpha _ { 1 } u _ { 1 } + \alpha _ { 2 } u _ { 2 } ) = ( \alpha _ { 1 } u _ { 1 } + \alpha _ { 2 } u _ { 2 } ) ( { \pmb x } ) = \alpha _ { 1 } u _ { 1 } ( { \pmb x } ) + \alpha _ { 2 } u _ { 2 } ( { \pmb x } ) = \alpha _ { 1 } \delta _ { x } ( u _ { 1 } ) + \alpha _ { 2 } \delta _ { { \pmb x } } ( u _ { 2 } ) ( { \pmb x } ) ,$ . Many interesting operations that map functions into real numbers like (point-evaluated) derivatives and integrals are linear. + +Now let $\mathbb { U }$ be an arbitrary real vector space. Real-valued linear maps on $\mathbb { U }$ are referred to as linear functionals. The set of all linear functionals on $\mathbb { U }$ is referred to as the algebraic dual (space) of $\mathbb { U }$ and denoted by $\mathbb { U } ^ { \# }$ . A subset $\mathbb { L }$ of $\mathbb { U } ^ { \# }$ is said to be total or to separate the points in $\mathbb { U }$ if for any $u _ { 1 } , u _ { 2 } \in \mathbb { U }$ with $u _ { 1 } \neq u _ { 2 }$ , there is $\ell \in \mathbb { L }$ such that $\ell ( u _ { 1 } ) \neq \ell ( u _ { 2 } )$ . Such subsets are useful, since they allow us to identify elements from the primal space $\mathbb { U }$ uniquely. For instance, the set of all point evaluation functionals on a vector space of real-valued functions separates the points in the space. If $\mathbb { U }$ is a topological vector space (for instance a separable Banach space in the context of neural operators), then the subspace of continuous linear functionals is denoted by $\mathbb { U } ^ { \prime } \subset \mathbb { U } ^ { \# }$ . + +Remark A.1 (The Bidual Embedding). The algebraic dual space $\mathbb { U } ^ { \# }$ with pointwise addition and scalar multiplication is a real vector space itself. Hence, any subspace $\mathbb { L } \subset \mathbb { U } ^ { \# }$ , has an algebraic dual space $\mathbb { L } ^ { \# }$ . The elements $\phi \in \mathbb { L } ^ { \# }$ of this space are linear functions mapping linear functionals into real numbers, i.e., $\phi ( \ell ) \in \mathbb { R }$ for $\ell \in \mathbb { L } \subset \mathbb { U } ^ { \# }$ . $\mathbb { L }$ is a vector space of real-valued functions, so we can consider its point evaluation functionals $\delta _ { u } \colon \mathbb { L } \to \mathbb { R } , \ell \mapsto \ell ( u )$ . Note that the map $\boldsymbol { { \tau } } _ { \mathbb { U } , \mathbb { L } ^ { \# } } : \mathbb { U } \mathbb { L } ^ { \# }$ , $u \mapsto \delta _ { u }$ is linear and, if $\mathbb { L }$ separates the points in $\mathbb { U }$ , injective. Hence, $\mathbb { U }$ is isomorphic to its image $\delta _ { \mathbb { U } } : = \iota _ { \mathbb { U } , \mathbb { L } \# } ( \mathbb { U } )$ under $\boldsymbol { L } _ { \mathbb { U } , \mathbb { L } ^ { \# } }$ . We refer to the map $\boldsymbol { L } _ { \mathbb { U } , \mathbb { L } ^ { \# } }$ as the bidual embedding. Abusing notation, we write $\mathbb { U } \subset \mathbb { L } ^ { \# }$ , $u \in \mathbb { L } ^ { \# }$ for $u \in \mathbb { U }$ , etc. + +# A.2. Probability Measures on Vector Spaces + +Our framework models the predictive uncertainty over an output of a neural operator as a random variable with values in an (infinite-dimensional) vector space $\mathbb { U }$ of functions. As noted before, we at least want to quantify the uncertainty about a given set $\mathbb { L } \subset \mathbb { U } ^ { \# }$ of linear functionals. Hence, we need to make the linear functionals in $\mathbb { L }$ measurable. + +Let $( \Omega , A , \mathrm { P } )$ be a probability space, $\mathbb { U }$ a real vector space and $\mathbb { L } \subset \mathbb { U } ^ { \# }$ a vector subspace of linear functionals separating the points in U. We equip $\mathbb { U }$ with the smallest $\sigma$ -algebra $\sigma ( \mathbb { L } )$ that makes all the functionals in $\mathbb { L }$ measurable. A random variable u with values in $( \mathbb { U } , \sigma ( \mathbb { L } ) )$ (U-valued for short) is an $\scriptstyle A - \sigma ( \mathbb { L } )$ -measurable function $\mathrm { u } \colon \Omega \to { \mathbb { U } }$ . + +Similar to their finite-dimensional counterparts, probability measures on and random variables with values in (infinite dimensional) vector spaces admit the definition of a mean and a (cross-)covariance operator. + +Definition A.2 (Mean and Covariance Operator (see e.g., Bogachev, 1998, Definition 2.2.7)). Let $\gamma$ be a probability measure on $\sigma ( \mathbb { L } )$ . + +(a) If $\mathbb { L } \subset \operatorname { L } _ { 1 } ( \gamma )$ , then $m _ { \gamma } \in \mathbb { L } ^ { \# }$ defined by + +$$ +m _ { \gamma } ( \ell ) : = \mathbb { E } _ { \gamma } \left[ \ell \right] = \int _ { \mathbb { U } } \ell ( u ) \gamma ( { \mathrm { d } } u ) \qquad \forall \ell \in \mathbb { L } +$$ + +is called the mean of $\gamma$ . The mean $m _ { \mathrm { u } }$ of a random variable u : $\Omega \to \mathbb { U }$ with values in $( \mathbb { U } , \sigma ( \mathbb { L } ) )$ is defined as the mean $m _ { \mathrm { P o u } ^ { - 1 } }$ of its law. + +(b) If $\mathbb { L } \subset \mathrm { L } _ { 2 } ( \gamma )$ , then the linear operator $\mathcal { C } _ { \gamma } : \mathbb { L } \to \mathbb { L } ^ { \# }$ defined by + +$$ +\mathcal { C } _ { \gamma } ( \ell _ { 1 } ) ( \ell _ { 2 } ) : = \mathrm { C o v } _ { \gamma } [ \ell _ { 1 } , \ell _ { 2 } ] = \int _ { \mathbb { U } } \left( \ell _ { 1 } ( u ) - m _ { \gamma } ( \ell _ { 1 } ) \right) ( \ell _ { 2 } ( u ) - m _ { \gamma } ( \ell _ { 2 } ) ) \gamma ( \mathrm { d } u ) \qquad \forall \ell _ { 1 } , \ell _ { 2 } \in \mathbb { L } +$$ + +is called the covariance operator of $\gamma$ . The covariance operator $\mathcal { C } _ { \bf u }$ of a random variable u: $\Omega \to \mathbb { U }$ with values in $( \mathbb { U } , \sigma ( \mathbb { L } ) )$ is defined as the covariance operator $\mathcal { C } _ { \mathrm { P o u } ^ { - 1 } }$ of its law. + +Definition A.3 (Cross-Covariance Operator). Let $\mathrm { u } _ { 1 } , \mathrm { u } _ { 2 } \colon \Omega \mathbb { U }$ be random variables with values in $( \mathbb { U } , \sigma ( \mathbb { L } ) )$ such that $\mathbb { L } \subset \mathrm { L } _ { 2 } ( \mathbb { U } , \sigma ( \mathbb { L } ) , \mathrm { P } \circ \mathrm { u } _ { i } ^ { - 1 } )$ for $i = 1 , 2$ . The operator $\mathcal { C } _ { \mathrm { u _ { 1 } , u _ { 2 } } } \colon \mathbb { L } \to \mathbb { L } ^ { \# }$ defined by + +$$ +\mathcal { C } _ { \mathrm { u _ { 1 } , u _ { 2 } } } ( \ell _ { 1 } ) ( \ell _ { 2 } ) : = \mathrm { C o v } \left[ \ell _ { 1 } ( \mathrm { u _ { 1 } } ) , \ell _ { 2 } ( \mathrm { u _ { 2 } } ) \right] = \int _ { \mathbb { U } } \left( \ell _ { 1 } ( \mathrm { u _ { 1 } } ( \omega ) ) - m _ { \mathrm { u _ { 1 } } } ( \ell _ { 1 } ) \right) \big ( \ell _ { 2 } ( \mathrm { u _ { 2 } } ( \omega ) ) - m _ { \mathrm { u _ { 2 } } } ( \ell _ { 2 } ) \big ) \mathrm { P } ( \mathrm { d } \omega ) +$$ + +is called the cross-covariance operator between $\mathrm { u } _ { 1 }$ and $\mathrm { u _ { 2 } }$ + +Gaussian measures on $\mathbb { U }$ are defined by generalizing the closure properties of Gaussian measures on $\mathbb { R } ^ { d }$ . + +Definition A.4 (Gaussian Measure (see e.g., Bogachev, 1998, Definition 2.2.1(a))). A probability measure $\gamma$ on $( \mathbb { U } , \sigma ( \mathbb { L } ) )$ is called Gaussian if every linear functional $\ell \in \mathbb { L }$ is a univariate Gaussian random variable on $( \mathbb { U } , \sigma ( \mathbb { L } ) , \gamma )$ . A random variable u : $\Omega \to \mathbb { U }$ with values in $( \mathbb { U } , \sigma ( \mathbb { L } ) )$ is called Gaussian if its law $\mathrm { ~ P ~ o ~ u ~ } ^ { - 1 }$ is Gaussian. + +If $\mathbb { L }$ is not a vector space, then we say that a random variable is Gaussian with values in $( \mathbb { U } , \sigma ( \mathbb { L } ) )$ if and only if it is Gaussian with values in $( \mathbb { U } , \sigma ( \operatorname { s p a n } \mathbb { L } ) )$ ). Note that $\sigma ( \operatorname { s p a n } \mathbb { L } ) = \sigma ( \mathbb { L } )$ for any subset $\mathbb { L } \subset \mathbb { U } ^ { \# }$ (by Klenke, 2014, Definition 1.79 and Theorem 1.91). + +Remark A.5 (Jointly Gaussian Measures). To define Gaussian processes with values in $\mathbb { U }$ , we need the notion of a joint Gaussian measure on $\mathbb { U } ^ { n }$ . Fortunately, we can also leverage Definition A.4 for this. $\mathbb { U } ^ { n }$ is a vector space under elementwise addition and scalar multiplication. Its algebraic dual space $( \mathbb { U } ^ { n } ) ^ { \# }$ is isomorphic to $( \mathbb { U } ^ { \# } ) ^ { n }$ , i.e. $\ell \in ( \mathbb { U } ^ { n } ) ^ { \# }$ if and only if there are $\ell _ { 1 } , \ldots , \ell _ { n } \in \mathbb { U } ^ { \# }$ such that + +$$ +\ell ( u _ { 1 } , \ldots , u _ { n } ) = \sum _ { i = 1 } ^ { n } \ell _ { i } ( u _ { i } ) \qquad \forall u _ { 1 } , \ldots , u _ { n } \in \mathbb { U } . +$$ + +It follows that $\sigma ( \mathbb { L } ^ { n } ) = \sigma ( \mathbb { L } ) ^ { \otimes n }$ , where the latter denotes the product $\sigma$ -algebra. Hence, following Definition A.4, we call a probability measure $\gamma$ on $( \mathbb { U } ^ { n } , \sigma ( \mathbb { L } ) ^ { \otimes n } )$ Gaussian if every $\ell \in \mathbb { L } ^ { n }$ is a univariate Gaussian random variable on $( \mathbb { U } ^ { n } , \sigma ( \mathbb { L } ) ^ { \otimes n } , \gamma )$ . + +Remark A.6 (Probability Measures on Separable Banach Spaces). The case where $\mathbb { U }$ is a real separable Banach space is of particular interest in the context of neural operators. In this case, we choose $\mathbb { L } = \mathbb { U } ^ { \prime }$ , i.e. all linear functionals that are continuous with respect to the norm topology. Then the $\sigma$ -algebra $\sigma ( \mathbb { U } ^ { \prime } )$ coincides with the Borel $\sigma$ -algebra $B \left( \mathbb { U } \right)$ generated by the norm topology (Bogachev, 1998, Theorem A.3.7). For Gaussian random variables u with values in $\left( \mathbb { U } , B \left( \mathbb { U } \right) \right)$ (and Gaussian measure on $B ( \mathbb { U } ) _ { , }$ ), the mean $m _ { \mathrm { u } }$ is an element of $\mathbb { U }$ and the covariance operator maps into $\mathbb { U }$ (Bogachev, 1998, Theorem 3.2.3). Moreover, for jointly Gaussian random variables $\mathrm { u } _ { 1 } , \mathrm { u } _ { 2 }$ with values in $\left( \mathbb { U } , B \left( \mathbb { U } \right) \right)$ , the cross-covariance operator $\mathcal { C } _ { \mathrm { u 1 , u _ { 2 } } }$ maps from $\mathbb { U } ^ { \prime }$ to $\mathbb { U }$ (Bogachev, 1998, Theorem 3.2.4). + +# A.3. Random Processes with Values in Vector Spaces + +Now we have all the necessary preliminaries to define a Gaussian process with values in $( \mathbb { U } , \sigma ( \mathbb { L } ) )$ . + +Definition A.7 (Gaussian Process). A Gaussian process with index set A and values in $( \mathbb { U } , \sigma ( \mathbb { L } ) )$ on $( \Omega , A , { \mathrm { P } } )$ is a function $\mathrm { F } \colon \mathbb { A } \times \Omega \mathbb { U }$ such that $\omega \mapsto ( \mathrm { F } ( a _ { 1 } , \omega ) , \dots , \mathrm { F } ( a _ { n } , \omega ) )$ is a joint, i.e., $( \mathbb { U } ^ { n } , \sigma ( \mathbb { L } ) ^ { \otimes n } )$ -valued, Gaussian random variable for all $n \in \mathbb { N }$ and $a _ { 1 } , \dotsc , a _ { n } \in \mathbb { A }$ . + +As for real-valued or $\mathbb { R } ^ { d }$ -valued processes, we can also define mean and covariance functions for random processes with values in arbitrary real vector spaces. However, their definition is more technically involved. + +Definition A.8. Let $\mathrm { F }$ be a random process with index set A and values in $( \mathbb { U } , \sigma ( \mathbb { L } ) )$ on $( \Omega , A , { \mathrm { P } } )$ . + +(a) If $\mathbb { L } \subset \mathrm { L } _ { 1 } ( \mathrm { P } \circ \mathrm { F } ( a , \cdot ) ^ { - 1 } )$ for all $a \in \mathbb { A }$ , then the function + +$$ +\mathcal { M } \colon \mathbb { A } \to \mathbb { L } ^ { \# } , a \mapsto m _ { \mathrm { F } ( a , \cdot ) } +$$ + +is called the mean function of $\mathrm { F }$ . + +(b) If $\mathbb { L } \subset \mathrm { L } _ { 2 } ( \mathrm { P } \circ \mathrm { F } ( a , \cdot ) ^ { - 1 } )$ for all $a \in \mathbb { A }$ , then the function + +$$ +\begin{array} { r } { \mathcal { K } \colon \mathbb { A } \times \mathbb { A } \to ( \mathbb { L } \to \mathbb { L } ^ { \# } ) , ( a _ { 1 } , a _ { 2 } ) \mapsto \mathcal { C } _ { \operatorname { F } ( a _ { 1 } , \cdot ) , \operatorname { F } ( a _ { 2 } , \cdot ) } } \end{array} +$$ + +is referred to as the covariance function of $\mathrm { F }$ . + +Remark A.9 (Moments of Banach-Valued Gaussian Processes). If $\mathbb { U }$ is a separable Banach space, then mean function $\mathcal { M }$ takes values in $\mathbb { U }$ and the covariance function $\kappa$ takes values in the space of nuclear operators $\mathbb { U } ^ { \prime } \to \mathbb { U }$ (Bogachev, 1998, Theorem 3.11.24). + +In the following, we aim to establish a correspondence between (Gaussian) random processes with values in $( \mathbb { U } , \sigma ( \mathbb { L } ) )$ and (Gaussian) random processes with values in $\left( \mathbb { R } , B \left( \mathbb { R } \right) \right)$ , which we dub (generalized) probabilistic currying. Unlike in Lemma 2.1, we need additional technical assumptions for this to work both ways. Denote by $\mathrm { s c l } _ { w * } ( \hat { \mathbb { L } } ) : = \{ \ell \in \mathbb { L } \mid$ $\exists \{ \ell _ { i } \} _ { i \in \mathbb { N } } \subset \hat { \mathbb { L } } \colon \ell _ { i } _ { w * } \ell \}$ the weak-\* sequential closure of a set $\hat { \mathbb { L } } \subset \mathbb { L }$ , where $\ell _ { i } \to _ { w * } \ell$ if and only if $\ell _ { i } ( u ) \to \ell ( u )$ for all $u \in \mathbb { U }$ (Aliprantis & Border, 2006, Section 5.14). + +Assumption A.10. Let $\hat { \mathbb { L } } \subset \mathbb { L }$ a set of linear functionals on $\mathbb { U }$ such that there is an $n _ { \mathrm { s c l } } \in { \mathbb { N } } _ { 0 }$ with $\operatorname { s c l } _ { w * } ^ { n _ { \mathrm { s c l } } } ( \operatorname { s p a n } \hat { \mathbb { L } } ) = \mathbb { L }$ . + +Theorem A.11 (Generalized Probabilistic Currying). Let $\hat { \mathbb { L } } \subset \mathbb { L }$ be a set of linear functionals separating the points in U. Let F $: \mathbb { A } \times \Omega \to \mathbb { U }$ and f : $( \mathbb { A } \times \hat { \mathbb { L } } ) \times \Omega \to \mathbb { R }$ such that $\boldsymbol { \ell } ( \boldsymbol { \mathrm { F } } ( a , \omega ) ) = \boldsymbol { \mathrm { f } } ( ( a , \boldsymbol { \ell } ) , \omega )$ for all $a \in \mathbb { A } , \overset { \vartriangle } { \boldsymbol { \ell } } \in \hat { \mathbb { L } }$ , and $\mathrm { P }$ -almost all $\omega \in \Omega$ . + +(i) If F is a random process with values in $( \mathbb { U } , \sigma ( \mathbb { L } ) )$ , then f is a random process with values in $\left( \mathbb { R } , B \left( \mathbb { R } \right) \right)$ , and + +(ii) if F is Gaussian, then so is f . + +If Assumption A.10 is satisfied, then the reverse implications hold as well. + +We will need the following generalization of Theorem B.6 from (Pförtner et al., 2022). +Lemma A.12. Let $\hat { \mathbb { L } } \subset \mathbb { L }$ such that Assumption A.10 holds. Then $\hat { \mathbb { L } }$ separates the points in U. Moreover, a function u : $\Omega \to \mathbb { U }$ is +(a) $\scriptstyle A - \sigma ( \mathbb { L } )$ -measurable if $\ell \circ$ u is $\scriptstyle A - B ( \mathbb { R } )$ -measurable for all $\ell \in \hat { \mathbb { L } }$ , +$( b )$ a Gaussian random variable with values in $\left( \mathbb { U } , \sigma ( \mathbb { L } ) \right) i f \left( \ell _ { 1 } \circ \mathrm { u } , \dots , \ell _ { n } \circ \mathrm { u } \right)$ is jointly Gaussian for all $n \in \mathbb N$ and $\ell _ { 1 } , \ldots , \ell _ { n } \in \hat { \mathbb { L } }$ . + +Proof. Define $\{ \hat { \mathbb { L } } _ { n } \} _ { n = 0 } ^ { n _ { \mathrm { s c l } } }$ with $\hat { \mathbb { L } } _ { 0 } : = \operatorname { s p a n } \hat { \mathbb { L } }$ and $\hat { \mathbb { L } } _ { n + 1 } : = \operatorname { s c l } _ { w * } ( \hat { \mathbb { L } } _ { n } )$ . By assumption, $\hat { \mathbb { L } } _ { n _ { \mathrm { s c l } } } = \mathbb { L }$ . + +Assume that $\hat { \mathbb { L } }$ does not separate the points in $\mathbb { U }$ . Then there is $u \in \mathbb { U }$ such that $\ell ( u ) = 0$ for all $\ell \in { \hat { \mathbb { L } } }$ . We proceed by induction. Pick $\ell \in \hat { \mathbb { L } } _ { 0 }$ . Then there are $\alpha _ { 1 } , \ldots , \alpha _ { m } \in \mathbb { R }$ and $\ell _ { 1 } , \dots , \ell _ { m } \in \hat { \mathbb { L } }$ such that $\ell = \textstyle \sum _ { i = 1 } ^ { m } \alpha _ { i } \ell _ { i }$ . Hence, + +$$ +\ell ( u ) = \sum _ { i = 1 } ^ { m } \alpha _ { i } \ell _ { i } ( u ) = 0 . +$$ + +Now assume that $\ell ( u ) = 0$ for $n < { n _ { \mathrm { s c l } } }$ and all $\ell \in \hat { \mathbb { L } } _ { n }$ . Fix $\ell \in \hat { \mathbb { L } } _ { n + 1 }$ . Then there is $\{ \ell _ { i } \} _ { i \in \mathbb { N } } \subset \hat { \mathbb { L } } _ { n }$ such that $\ell _ { i } \to _ { w * } \ell$ . Hence, + +$$ +\ell ( u ) = \operatorname* { l i m } _ { i \to \infty } \ell _ { i } ( u ) = 0 . +$$ + +All in all, it follows that $\mathbb { L } = \hat { \mathbb { L } } _ { n _ { \mathrm { s c l } } }$ does not separate the points in $\mathbb { U }$ , which is a contradiction. Hence $\hat { \mathbb { L } }$ separates the points in $\mathbb { U }$ . + +(a) We need to show that $\ell \circ \mathrm { u }$ is measurable5 for all $\ell \in \mathbb { L }$ (Klenke, 2014, Theorem 1.81). We proceed by induction. Let $\ell \in \hat { \mathbb { L } } _ { 0 }$ . Then there are $\alpha _ { 1 } , \ldots , \alpha _ { m } \in \mathbb { R }$ and $\ell _ { 1 } , \dots , \ell _ { m } \in \hat { \mathbb { L } }$ such that $\begin{array} { r } { \ell = \sum _ { i = 1 } ^ { m } \alpha _ { i } \ell _ { i } } \end{array}$ . By assumption, $\ell _ { i } \circ \mathrm { u }$ is measurable for all $i = 1 , \ldots , m$ . Hence, $\ell$ is measurable by Theorem 1.91 in (Klenke, 2014). Now assume that $\ell \circ$ u is measurable for all $\ell \in \hat { \mathbb { L } } _ { n }$ with $n < { { n _ { \mathrm { s c l } } } }$ . Fix $\boldsymbol { \ell } \in \hat { \mathbb { L } } _ { n + 1 }$ . Then there is a sequence $\{ \ell _ { i } \} _ { i \in \mathbb { N } } \subset \hat { \mathbb { L } } _ { n }$ such that $\ell _ { i } \xrightarrow { w * } \ell$ . This implies that $\ell _ { i } \circ \mathrm { u } \ell \circ$ u pointwise, where, by the inductive hypothesis, $\ell _ { i } \circ \mathrm { u }$ is measurable for all $i \in \mathbb N$ . Hence, $\ell \circ \mathrm { u }$ is measurable for all $\ell \in \hat { \mathbb { L } } _ { n + 1 }$ (Klenke, 2014, Theorem 1.92). + +(b) By (a), u is $\scriptstyle A - \sigma ( \mathbb { L } )$ -measurable. It suffices to show that u is Gaussian with values in $( \mathbb { U } , \sigma ( { \hat { \mathbb { L } } } _ { n } ) )$ (Bogachev, 1998, Definition 2.2.1(i)) for all $n = 0 , \ldots , n _ { \mathrm { s c l } }$ , which is well-defined, since $\hat { \mathbb { L } }$ separates the points in $\mathbb { U }$ . Again, we proceed by induction on $n$ . Let $\ell \in \hat { \mathbb { L } } _ { 0 }$ . Then there are $\alpha _ { 1 } , \ldots , \alpha _ { m } \in \mathbb { R }$ and $\ell _ { 1 } , \dots , \ell _ { m } \in \hat { \mathbb { L } }$ such that $\begin{array} { r } { \ell = \sum _ { i = 1 } ^ { m } \alpha _ { i } \ell _ { i } } \end{array}$ . By the closure properties of Gaussians under linear maps, we have that $\ell \circ \mathrm { u }$ is Gaussian. Hence, u is Gaussian with values in $( \mathbb { U } , \sigma ( { \hat { \mathbb { L } } } _ { 0 } ) )$ . Now assume that u is Gaussian with values in $( \mathbb { U } , \sigma ( { \hat { \mathbb { L } } } _ { n } ) )$ for $n < { n _ { \mathrm { s c l } } }$ . Fix $\boldsymbol { \ell } \in \hat { \mathbb { L } } _ { n + 1 }$ . Then there is a sequence $\{ \ell _ { i } \} _ { i \in \mathbb { N } } \subset \hat { \mathbb { L } } _ { n }$ such that $\ell _ { i } \xrightarrow { w * } \ell .$ . This implies that $\ell _ { i } \circ \mathrm { u } \ell \circ \mathrm { u }$ pointwise, where, by the inductive hypothesis, $\ell _ { i } \circ$ u is Gaussian for all $i \in \mathbb N$ . Since pointwise limits of Gaussians random variables are Gaussian, $\ell \circ$ u is Gaussian. Hence, u is Gaussian with values in $( \mathbb { U } , \sigma ( \hat { \mathbb { L } } _ { n + 1 } ) )$ . + +Proof of Theorem A.11. $\Rightarrow$ (i) Holds by definition. + +(ii) Let $a _ { 1 } , \dotsc , a _ { n } \in \mathbb { A }$ and $\ell _ { 1 } , \ldots , \ell _ { n } \in \mathbb { L }$ . By Remark A.5, the linear functionals + +$$ +\widetilde { \ell } _ { i } \colon \mathbb { U } ^ { n } \to \mathbb { R } , ( u _ { 1 } , \dots , u _ { n } ) \mapsto \ell _ { i } ( u _ { i } ) +$$ + +are measurable with respect to $\sigma ( { \mathbb { L } } ) ^ { \otimes n }$ . Moreover, $\omega \mapsto ( \operatorname { F } ( a _ { 1 } , \omega ) , \dots , \operatorname { F } ( a _ { n } , \omega ) )$ is Gaussian by assumption. Hence, + +$$ +\begin{array} { r l } & { \omega \mapsto \left( \tilde { \ell } _ { i } ( \mathrm { F } ( a _ { 1 } , \omega ) , \dots , \mathrm { F } ( a _ { n } , \omega ) ) \right) _ { i = 1 } ^ { n } } \\ & { \qquad = ( \ell _ { i } ( \mathrm { F } ( a _ { i } , \omega ) ) ) _ { i = 1 } ^ { n } } \\ & { \qquad = ( \mathrm { f } ( ( a _ { i } , \ell _ { i } ) , \omega ) ) _ { i = 1 } ^ { n } } \end{array} +$$ + +is Gaussian with values in $\mathbb { R } ^ { n }$ . + +$\Leftarrow$ (i) Follows directly by applying Lemma A.12(a) to each $\textstyle \mathrm { F } ( a , \cdot )$ individually. (ii) Let $a _ { 1 } , \dotsc , a _ { n } \in \mathbb { A }$ . We have to show that $\omega \mapsto ( \operatorname { F } ( a _ { 1 } , \omega ) , \dots , \operatorname { F } ( a _ { n } , \omega ) )$ is a Gaussian random variable with values in $\mathbb { U } ^ { n }$ . It is easy to check that Assumption A.10 holds for $\hat { \mathbb { L } } ^ { n } \subset \mathbb { L } ^ { n }$ . Hence, by Lemma A.12(b), $\omega \mapsto ( \operatorname { F } ( a _ { 1 } , \omega ) , \dots , \operatorname { F } ( a _ { n } , \omega ) )$ is Gaussian with values in $\mathbb { U } ^ { n }$ . + +The following Corollary shows that Assumption A.10 is automatically fulfilled for $\mathbb { L } = \mathbb { U } ^ { \prime }$ in separable Banach spaces. + +Corollary A.13 (Generalized Probabilistic Currying in Separable Banach Spaces). Let U be a real separable Banach space and $\hat { \mathbb { L } } \subset \mathbf { \bar { \mathbb { U } } ^ { \prime } }$ a set of continuous linear functionals separating the points in U. Let F : $\mathbb { A } \times \Omega \to \mathbb { U }$ and f : $( \mathbb { A } \times \hat { \mathbb { L } } ) \times \Omega \ \dot { } \ \mathbb { R }$ such that $\ell ( \mathrm { F } ( a , \omega ) ) = \mathrm { f } ( ( a , \ell ) , \omega )$ for all $a \in \mathbb { A }$ , $\ell \in { \hat { \mathbb { L } } } ,$ , and P-almost all $\omega \in \Omega$ . + +(i) F is a random process with values in $\left( \mathbb { U } , B \left( \mathbb { U } \right) \right)$ if and only if f is a random process with values in $\left( \mathbb { R } , B \left( \mathbb { R } \right) \right)$ , and + +(ii) F is Gaussian if and only if f is Gaussian. + +Proof. We will show that Assumption A.10 is fulfilled for $\mathbb { L } = \mathbb { U } ^ { \prime }$ and $n _ { \mathrm { s c l } } = 1$ . Let $\iota \colon \mathbb { U } \to ( \hat { \mathbb { L } } \to \mathbb { R } ) , u \mapsto ( \ell \mapsto \ell ( u ) ) ,$ , which is linear and injective, since $\hat { \mathbb { L } }$ separates the points in $\mathbb { U }$ . Then $\tilde { \mathbb { U } } : = \iota ( \mathbb { U } )$ is isomorphic to $\mathbb { U }$ (as a vector space). Hence, $\tilde { \mathbb { U } }$ equipped with the norm $\| \phi \| _ { \tilde { \mathbb { U } } } : = \| \iota ^ { - 1 } ( \phi ) \| _ { \mathbb { U } }$ is a real separable Banach space which is isometrically isomorphic to U. Moreover, $\tilde { \mathbb { U } }$ is a space of functions with continuous point evaluation functionals, since + +$$ +| \delta _ { \ell } ( \iota ( u ) ) | = | \ell ( u ) | \leq \| \ell \| _ { \mathbb { U } ^ { \prime } } \| u \| _ { \mathbb { U } } = \| \ell \| _ { \mathbb { U } ^ { \prime } } \| \iota ( u ) \| _ { \tilde { \mathbb { U } } } . +$$ + +Thus, there is a sequence $\{ \delta _ { \ell _ { i } } \} _ { i \in \mathbb { N } } \subset { \tilde { \mathbb { U } } } ^ { \prime }$ separating the points in $\mathbb { U }$ (Steinwart, 2024, Theorem 4.10). This implies that $\{ \ell _ { i } \} _ { i \in \mathbb { N } } \subset \hat { \mathbb { L } }$ separates the points in $\mathbb { U }$ . Finally, it follows that $\mathbb { U } ^ { \prime } = \operatorname { s c l } _ { w * } ( \operatorname { s p a n } \hat { \mathbb { L } } )$ (Steinwart, 2024, Proposition 4.3). + +If $\mathbb { U } \subset \mathbb { R } ^ { \mathbb { D } _ { \mathbb { U } } }$ is a vector space of real-valued6 functions and $\mathbb { L } = \operatorname { s p a n } \delta _ { \mathbb { D } _ { \mathbb { U } } }$ , then Theorem A.11 and Corollary A.13 become substantially sharper. + +Corollary A.14 (Probabilistic Currying). Let $\mathbb { U } \subset \mathbb { R } ^ { \mathbb { D } _ { \mathbb { U } } }$ be a vector space of real-valued functions, $\mathrm { F } \colon \mathbb { A } \times \Omega \mathbb { U } ,$ , and $\mathrm { f } \colon ( \mathbb { A } \times \mathbb { D } _ { \mathbb { U } } ) \times \Omega \to \mathbb { R }$ such that $\mathrm { F } ( a , \omega ) ( x ) = \mathrm { f } ( ( a , x ) , \omega )$ for all $a \in \mathbb { A }$ , $x \in \mathbb { D } _ { \mathbb { U } }$ , and $\mathrm { P }$ -almost all $\omega \in \Omega$ . Then + +(i) F is a random process with values in $\big ( \mathbb { U } , \sigma \big ( \delta _ { \mathbb { D } _ { \mathbb { U } } } \big ) \big )$ if and only if f is a random process with values in $\left( \mathbb { R } , B \left( \mathbb { R } \right) \right)$ , + +(ii) $\mathrm { F }$ has a mean function $\mathcal { M }$ with values in $\mathbb { U }$ if and only $i f$ f has a mean function $m$ , where $\mathcal { M } ( a ) = m ( a , \cdot )$ for all $a \in \mathbb { A } .$ , + +(iii) F has a covariance function $\kappa$ with values in span $\delta _ { \mathbb { D } _ { \mathbb { U } } } \to \mathbb { U }$ if and only if f has a covariance function $k$ , where $\begin{array} { r } { \mathcal { K } ( a _ { 1 } , a _ { 2 } ) ( \delta _ { x } ) = k ( ( a _ { 1 } , x ) , ( a _ { 2 } , \cdot ) ) } \end{array}$ for all $a _ { 1 } , a _ { 2 } \in \mathbb { A }$ , and $x \in \mathbb { D } _ { \mathbb { U } }$ , and + +(iv) F is Gaussian if and only if f is Gaussian. + +If U is a separable Banach space with continuous point evaluation functionals, then + +(v) (i) and (iv) hold for F with values in $\left( \mathbb { U } , B \left( \mathbb { U } \right) \right)$ , and, + +(vi) if it exists, then the covariance function $\kappa$ in (iii) has values in $\mathbb { U } ^ { \prime } \to \mathbb { U }$ , where + +$$ +\mathcal { K } ( a _ { 1 } , a _ { 2 } ) ( \ell ) ( x ) = \ell ( k ( ( a _ { 1 } , \cdot ) , ( a _ { 2 } , x ) ) ) +$$ + +for all $\ell \in \mathbb { U } ^ { \prime }$ and $x \in \mathbb { D } _ { \mathbb { U } }$ . + +Proof. Follows from Theorem A.11 and Corollary A.13. + +Finally, Theorem 3.2 from the main text is merely a corollary of the results developed above. + +Theorem 3.2 (Probabilistic Currying in Banach Spaces; proof in Appendix A.3). Let $( \Omega , A , { \mathrm { P } } )$ be a probability space and U a real separable Banach space of $\mathbb { R } ^ { \bar { d } ^ { \prime } }$ -valued functions with domain $\mathbb { D } _ { \mathbb { U } }$ . Let $\mathbf { F } \colon \mathbb { A } \times \Omega \to \mathbb { U }$ and $\mathbf { f } \colon ( \mathbb { A } \times \mathbb { D } _ { \mathbb { U } } ) \times \Omega \to \mathbb { R } ^ { d ^ { \prime } }$ such that $\mathbf { F } ( { \pmb a } , \cdot ) ( { \pmb x } ) = \mathbf { f } ( ( { \pmb a } , { \pmb x } ) , \cdot )$ for all $\mathbf { \pmb { a } } \in \mathbb { A }$ and $\pmb { x } \in \mathbb { D } _ { \mathbb { U } }$ ( $\mathrm { P }$ -almost surely). Then (i) $\mathbf { F }$ is a random process with values in $\bigl ( \mathbb { U } , \sigma ( \delta _ { \mathbb { U } } ) \bigr )$ if and only if f is a $\mathbb { R } ^ { d ^ { \prime } }$ -valued random process, (ii) $\mathbf { F }$ is Gaussian if and only if f is Gaussian, and (iii) if all evaluation maps $\delta _ { \pmb { x } } \colon \tilde { \mathbb { U } } \mathbb { R } ^ { d ^ { \prime } } , \pmb { u } \mapsto \pmb { u } ( \pmb { x } )$ are continuous, then $( i )$ holds for $\mathbf { F }$ with values in $\left( \mathbb { U } , B \left( \mathbb { U } \right) \right)$ . + +Proof. Follows from Corollary A.14 and Lemma 2.1. + +# A.4. Banach-Valued Gaussian Processes from Linearized Neural Operators + +For simplicity of the exposition, we limited the construction of the LUNO framework in Section 3.2 to neural operators, which map into a Banach space of functions with continuous point evaluation functionals. This limits its applicability, especially for solving PDEs, whose solutions are often not defined pointwise, but rather elements of Sobolev spaces $W ^ { p , k } ( \mathbb { D } _ { \mathbb { U } } ) \subset \mathrm { L } ^ { p } ( \mathbb { D } _ { \mathbb { U } } )$ . Hence, in this section, we extend LUNO to neural operators that map into abstract separable Banach7 spaces $\mathbb { U }$ . + +Step 0 Let $\mathbb { U }$ be a real separable Banach space, A a set, $\mathbb { W } \subset \mathbb { R } ^ { p }$ a subspace, and $F \colon \mathbb { A } \times \mathbb { W } \to \mathbb { U }$ a neural operator. + +Step 1 First, we select a subset $\hat { \mathbb { L } } \subset \mathbb { U } ^ { \prime }$ of continuous linear functionals separating the points in $\mathbb { U }$ , for which we want to quantify predictive uncertainty under the neural operator. Define + +$$ +f \colon ( \mathbb { A } \times \hat { \mathbb { L } } ) \times \mathbb { W } \to \mathbb { R } , ( ( a , \ell ) , \pmb { w } ) \mapsto \ell ( F ( a , \pmb { w } ) ) . +$$ + +This is an uncurried version of the neural operator. To see this, note that a neural operator $F \colon \mathbb { A } \times \mathbb { W } \to \mathbb { U }$ can be uniquely identified with the function + +$$ +\tilde { F } \colon \mathbb { A } \times \mathbb { W } \to ( \hat { \mathbb { L } } \to \mathbb { R } ) , ( a , { \pmb w } ) \mapsto ( \ell \mapsto \ell ( F ( a , { \pmb w } ) ) ) . +$$ + +Proof. Let $F _ { 1 } , F _ { 2 } \colon \mathbb { A } \times \mathbb { W } \to \mathbb { U }$ be neural operators with $F _ { 1 } \neq F _ { 2 }$ . Then there are $a \in \mathbb { A }$ and $\textbf { { w } } \in \mathbb { W }$ such that $F _ { 1 } ( a , { \pmb w } ) \neq F _ { 2 } ( a , { \pmb w } )$ . Since $\hat { \mathbb { L } }$ separates the points in $\mathbb { U }$ , this implies that there is $\ell \in { \hat { \mathbb { L } } }$ such that ${ \tilde { F } } _ { 1 } ( a , w ) ( \ell ) : =$ $F _ { 1 } ( a , { \pmb w } ) \neq F _ { 2 } ( a , { \pmb w } ) = : \tilde { F } _ { 2 } ( a , { \pmb w } ) ( { \pmb \ell } )$ . Hence, $\tilde { F _ { 1 } } \neq \tilde { F _ { 2 } }$ . □ + +Step 2 We model the uncertainty over the parameters as a random variable $\mathbf { w } \colon \Omega \to \mathbb { W }$ on a probability space $( \Omega , A , { \mathrm { P } } )$ with $\mathrm { s u p p } ( \mathbf { w } ) = \mathbb { W }$ . As in Section 3.2, we will now linearize $f$ in $\textbf { \em w }$ around a point $\pmb { w } _ { 0 } \in \mathbb { W }$ . To achieve this, we assume that the directional derivatives + +$$ +\partial _ { \pmb { w } } f ( ( a , \ell ) , \cdot ) ( \pmb { w } _ { 0 } ) = \operatorname* { l i m } _ { h 0 } \frac { f ( ( a , \ell ) , \pmb { w } _ { 0 } + h \pmb { w } ) - f ( ( a , \ell ) , \pmb { w } _ { 0 } ) } { h } +$$ + +at ${ \pmb w } _ { 0 }$ exist for all $a \in \mathbb { A } , \ell \in \hat { \mathbb { L } }$ , and $\pmb { w } \in \mathbb { W }$ , and are linear in $\pmb { w }$ . For instance, this is the case if $f ( \left( a , \ell \right) , \cdot )$ is differentiable at ${ \pmb w } _ { 0 }$ . In this case, the linearization of $f$ is given by + +$$ +f ( ( a , \ell ) , w ) \approx f _ { \mu } ^ { \mathrm { l i n } } ( ( a , \ell ) , w ) : = f ( ( a , \ell ) , w _ { 0 } ) + \partial _ { w - w _ { 0 } } f ( ( a , \ell ) , \cdot ) \left( w _ { 0 } \right) . +$$ + +Then the function + +$$ +\mathbf { f } \colon ( \mathbb { A } \times \hat { \mathbb { L } } ) \times \Omega \to \mathbb { R } , ( ( a , \ell ) , \omega ) \mapsto f _ { \mu } ^ { \mathrm { l i n } } ( ( a , \ell ) , \mathbf { w } ( \omega ) ) +$$ + +is a random process. If w has a mean $\pmb { \mu }$ , then f has a mean function + +$$ +m \colon \mathbb { A } \times { \hat { \mathbb { L } } } \to \mathbb { R } , ( a , \ell ) \mapsto f ( ( a , \ell ) , w _ { 0 } ) + \partial _ { \mu - w _ { 0 } } f ( ( a , \ell ) , \cdot ) ( w _ { 0 } ) , +$$ + +and if w has a covariance matrix $\pmb { \Sigma }$ , then f has a covariance function given by + +$$ +\colon \colon ( \mathbb { A } \times \mathbb { \hat { L } } ) \times ( \mathbb { A } \times \mathbb { \hat { L } } ) \to \mathbb { R } , ( ( a _ { 1 } , \ell _ { 1 } ) , ( a _ { 2 } , \ell _ { 2 } ) ) \mapsto \sum _ { i , j = 1 } ^ { p } \partial _ { i } f ( ( a , \ell _ { 1 } ) , \cdot ) ( w _ { 0 } ) \Sigma _ { i j } \partial _ { j } f ( ( a , \ell _ { 2 } ) , \cdot ) ( w _ { 0 } ) . +$$ + +Note that f and $m$ are linear in $\ell$ and $k$ is bilinear in $( \ell _ { 1 } , \ell _ { 2 } )$ . Moreover, if w is Gaussian, then f is a Gaussian process. + +Step 3 Finally, we construct a $\mathbb { U }$ -valued (Gaussian) random process $\mathrm { F } \colon \mathbb { A } \times \Omega \mathbb { U }$ by probabilistically currying f. However, this is more challenging for an abstract U. Intuitively, we want to undo the uncurrying operation from Step 1. To this end, we assume8 that there is $\mathrm { F } \colon \mathbb { A } \times \Omega \mathbb { U }$ with $\ell ( \mathrm { F } ( a , \omega ) ) = \mathrm { f } ( ( a , \ell ) , \omega )$ for all $a \in \mathbb { A } , \ell \in \hat { \mathbb { L } }$ , and P-almost all $\omega \in \Omega$ . In this case, Corollary A.13 ensures that + +(i) F is a random process with values in $\left( \mathbb { U } , B \left( \mathbb { U } \right) \right)$ , (ii) F has a mean function $\mathcal { M } \colon \hat { \mathbb { A } } \mathbb { U }$ with + +$$ +\ell ( \mathcal { M } ( a ) ) = m ( a , \ell ) = f ( ( a , \ell ) , w _ { 0 } ) + \partial _ { \mu - w _ { 0 } } f ( ( a , \ell ) , \cdot ) ( w _ { 0 } ) +$$ + +if w has a mean vector $\pmb { \mu }$ + +(iii) $\mathrm { F }$ has a covariance function ${ \mathcal { K } } \colon \mathbb { A } \times \mathbb { A } \to ( \mathbb { U } ^ { \prime } \to \mathbb { U } )$ with + +$$ +\ell _ { 2 } ( K ( a _ { 1 } , a _ { 2 } ) ( \ell _ { 1 } ) ) = k ( ( a _ { 1 } , \ell _ { 1 } ) , ( a _ { 2 } , \ell _ { 2 } ) ) = \sum _ { i , j = 1 } ^ { p } \partial _ { i } f ( ( a , \ell _ { 1 } ) , \cdot ) ( w _ { 0 } ) \Sigma _ { i j } \partial _ { j } f ( ( a , \ell _ { 2 } ) , \cdot ) ( w _ { 0 } ) +$$ + +if w has a covariance matrix $\pmb { \Sigma }$ , and + +(iv) F is a Gaussian process if w is Gaussian. + +8See Appendices A.4.1 and A.4.2 for more details on this assumption. + +# A.4.1. WEAK GÂTEAUX DIFFERENTIABILITY + +The existence of $\mathrm { F } \colon \mathbb { A } \times \Omega \mathbb { U }$ with $\ell ( \mathrm { F } ( a , \omega ) ) = \mathrm { f } ( ( a , \ell ) , \omega )$ for all $a \in \mathbb { A }$ , $\boldsymbol { \ell } \in \hat { \mathbb { L } }$ , and $\mathrm { P }$ -almost all $\omega \in \Omega$ is equivalent to $F ( \boldsymbol { a } , \cdot \boldsymbol { ) }$ being $\tau ( \mathbb { U } , \hat { \mathbb { L } } ^ { \# } )$ -Gâteaux differentiable at ${ \pmb w } _ { 0 }$ , i.e., there is a linear operator $\delta F ( a , \cdot ) ( \pmb { w } _ { 0 } ) : \mathbb { W } \mathbb { U }$ such that + +$$ +\delta F ( a , \cdot ) \left( w _ { 0 } \right) ( w ) = \operatorname* { l i m } _ { h \to 0 } \frac { F ( a , w _ { 0 } + h w ) - F ( a , w _ { 0 } ) } { h } \qquad \mathrm { i n ~ } ( \mathbb { U } , \tau ( \mathbb { U } , \hat { \mathbb { L } } ^ { \# } ) ) +$$ + +for all $\pmb { w } \in \mathbb { W }$ , where $\tau ( \mathbb { U } , \hat { \mathbb { L } } ^ { \# } )$ is the smallest topology on $\mathbb { U }$ for which all functionals in $\hat { \mathbb { L } }$ are continuous. + +Proof. Note that, since $\mathbb { W }$ is a metric space, we can take sequential limits in Equation (A.1) without loss of generality. + +$\Rightarrow \operatorname { F i x } a \in \mathbb { A }$ . We will constuct the Gâteaux derivative from $\tilde { \mathrm { F } } ( a , \omega )$ . Since $\tilde { \mathrm { F } } ( \boldsymbol { a } , \omega ) \in \iota _ { \mathbb { U } , \hat { \mathbb { L } } \# } ( \mathbb { U } )$ for P-almost all $\omega \in \Omega$ , there is $N \in { \mathcal { A } }$ with $\mathrm { P } ( N ) = 0$ and $\tilde { \mathrm { F } } ( \boldsymbol { a } , \omega ) \in \iota _ { \mathbb { U } , \hat { \mathbb { L } } \# } ( \mathbb { U } )$ for all $\omega \in \Omega \setminus N$ . We have supp $\mathbf { w } = \mathbb { W }$ and hence there are $\omega _ { 1 } , \ldots , \omega _ { d } \in \Omega$ such that $\{ b _ { i } \} _ { i = 1 } ^ { d }$ with $b _ { i } : = \mathbf { w } ( \omega _ { i } ) - \pmb { w } _ { 0 }$ is a basis of $\mathbb { W }$ . Let $\lambda \colon \mathbb { W } \to \mathbb { U }$ be the unique linear operator with + +$$ +\begin{array} { r l } & { \lambda ( \pmb { b } _ { i } ) : = \iota _ { \mathbb { U } , \hat { \mathbb { L } } \# } ^ { - 1 } ( \tilde { \mathrm { F } } ( a , \omega _ { i } ) ) - F ( a , \pmb { w } _ { 0 } ) } \\ & { \quad \quad \quad = \iota _ { \mathbb { U } , \hat { \mathbb { L } } \# } ^ { - 1 } ( \mathbf { f } ( ( a , \cdot ) , \omega _ { i } ) - f ( ( a , \cdot ) , \pmb { w } _ { 0 } ) ) } \\ & { \quad \quad \quad = \iota _ { \mathbb { U } , \hat { \mathbb { L } } \# } ^ { - 1 } ( \ell \mapsto \partial _ { \mathbf { w } ( \omega _ { i } ) - \pmb { w } _ { 0 } } f ( ( a , \ell ) , \cdot ) ( \pmb { w } _ { 0 } ) ) } \\ & { \quad \quad \quad = \iota _ { \mathbb { U } , \hat { \mathbb { L } } \# } ^ { - 1 } ( \ell \mapsto \partial _ { \pmb { b } _ { i } } f ( ( a , \ell ) , \cdot ) ( \pmb { w } _ { 0 } ) ) , } \end{array} +$$ + +i.e. $\lambda ( \pmb { w } ) = \iota _ { \mathbb { U } , \hat { \mathbb { L } } \neq } ^ { - 1 } ( \ell \mapsto \partial _ { \pmb { w } } \boldsymbol { f } ( ( a , \ell ) , \cdot ) ( \pmb { w } _ { 0 } ) )$ forall $\mathbf { \boldsymbol { w } } \in \mathbb { W }$ . Since $\mathbb { W }$ is finite dimensional, $\lambda$ is $\tau _ { \mathbb { W } ^ { - \tau } } ( \mathbb { U } , \hat { \mathbb { L } } ^ { \# } )$ - continuous, where $\tau _ { \mathbb { W } }$ is the norm topology on W. Let $\{ h _ { n } \} _ { n \in \mathbb { N } } \subset \mathbb { R }$ be any null sequence. Then for any $\boldsymbol { \ell } \in \hat { \mathbb { L } }$ we have + +$$ +\ell \left( \frac { F ( a , \pmb { w } _ { 0 } + h _ { n } \pmb { w } ) - F ( a , \pmb { w } _ { 0 } ) } { h _ { n } } \right) = \frac { f ( ( a , \ell ) , \pmb { w } _ { 0 } + h _ { n } \pmb { w } ) - f ( ( a , \ell ) , \pmb { w } _ { 0 } ) } { h _ { n } } +$$ + +$$ +\begin{array} { r } { \mathbf { \Psi } = \ell ( \lambda ( \pmb { w } ) ) . } \end{array} +$$ + +Hence, $F ( a , \cdot )$ is $\tau ( \mathbb { U } , \hat { \mathbb { L } } ^ { \# } )$ -Gâteaux differentiable at ${ \pmb w } _ { 0 }$ with Gâteaux derivative $\delta F ( a , \cdot ) ( { \pmb w } _ { 0 } ) = \lambda$ . + +$\Leftarrow$ If $F ( \boldsymbol { a } , \cdot \boldsymbol { ) }$ is $\tau ( \mathbb { U } , \hat { \mathbb { L } } ^ { \# } )$ -Gâteaux differentiable at ${ \pmb w } _ { 0 }$ for all $a \in \mathbb { A }$ , then we can construct $\mathrm { F }$ as + +$$ +\mathbf { F } \colon \mathbb { A } \times \Omega \to \mathbb { U } , ( a , \omega ) \mapsto F ( a , { \pmb w } _ { 0 } ) + \delta F ( a , \cdot ) ( { \pmb w } _ { 0 } ) ( { \pmb w } ( \omega ) - { \pmb w } _ { 0 } ) +$$ + +Then + +$$ +\begin{array} { r l } & { \ell ( { \mathrm { F } } ( a , \boldsymbol { \omega } ) ) = \ell ( F ( a , \boldsymbol { w } _ { 0 } ) ) + \partial _ { \mathbf { w } ( \boldsymbol { \omega } ) - \mathbf { w } _ { 0 } } \ell ( F ( a , \cdot ) ) \left( \boldsymbol { w } _ { 0 } \right) } \\ & { \qquad = f ( ( a , \ell ) , \boldsymbol { w } _ { 0 } ) + \partial _ { \mathbf { w } ( \boldsymbol { \omega } ) - \mathbf { w } _ { 0 } } f ( ( a , \ell ) , \cdot ) \left( \boldsymbol { w } _ { 0 } \right) } \\ & { \qquad = f _ { \mu } ^ { \mathrm { l i n } } ( ( a , \ell ) , \mathbf { w } ( \boldsymbol { \omega } ) ) } \\ & { \qquad = \mathrm { { f } } ( ( a , \ell ) , \boldsymbol { \omega } ) } \end{array} +$$ + +for all $a \in \mathbb { A }$ , $\boldsymbol { \ell } \in \hat { \mathbb { L } }$ , and $\omega \in \Omega$ . + +A stronger condition, namely (norm) Fréchet differentiability, has been verified for Fourier neural operators mapping between $\mathrm { L } _ { p }$ spaces (Kabri et al., 2023). + +Note that we can also use the properties of the Gâteaux derivative to verify the conclusions of Corollary A.13. Since W is finite-dimensional, the Gâteaux derivative $\delta F ( a , \cdot ) ( { \pmb w } _ { 0 } )$ is continuous with respect to any TVS topology on U. Hence, F + +as defined in Equation (A.2) is a random process with values in $( \mathbb { U } , \sigma ( \mathbb { U } ^ { \prime } ) ) = ( \mathbb { U } , B \left( \mathbb { U } \right) )$ . If w has a mean $\pmb { \mu }$ , then the mean function $\mathcal { M } \colon \mathbb { A } \mathbb { U }$ of $\mathrm { F }$ is given by + +$$ +\begin{array} { r } { \mathcal { M } ( \boldsymbol { a } ) = F ( \boldsymbol { a } , \boldsymbol { w } _ { 0 } ) + \delta F ( \boldsymbol { a } , \cdot ) ( \boldsymbol { w } _ { 0 } ) ( \pmb { \mu } - \boldsymbol { w } _ { 0 } ) , } \end{array} +$$ + +and, if w has a covariance matrix $\pmb { \Sigma }$ , then the covariance function ${ \mathcal { K } } \colon \mathbb { A } \times \mathbb { A } \to ( \mathbb { U } ^ { \prime } \to \mathbb { U } )$ of $\mathrm { F }$ is given by + +$$ +{ \mathcal K } ( a _ { 1 } , a _ { 2 } ) = \delta F ( a _ { 1 } , \cdot ) ( { \pmb w } _ { 0 } ) \Sigma \delta F ( a _ { 2 } , \cdot ) ( { \pmb w } _ { 0 } ) ^ { \prime } . +$$ + +By the closure properties of Banach-valued Gaussian random variables under continuous affine maps (Bogachev, 1998, Lemma 2.2.2), F is a Gaussian process if w is Gaussian. + +While this construction is somewhat more direct, the currying approach outlined above mimics more closely how $\mathrm { F }$ is constructed on a computer, especially when $\mathbb { U } \subset ( \mathbb { R } ^ { d _ { \mathbb { U } } ^ { \prime } } ) ^ { \mathbb { D } _ { \mathbb { U } } }$ and $\hat { \mathbb { L } } = \{ \delta _ { \mathbf { x } , i } \colon \mathbf { x } \in \mathbb { D } _ { \mathbb { U } } , i = 1 , \ldots , d _ { \mathbb { U } } ^ { \prime } \}$ as in Section 3.2, and does not require knowledge of Banach-valued derivatives. + +# A.4.2. BIDUAL RANDOM PROCESSES + +If there is no $\mathrm { F } \colon \mathbb { A } \times \Omega \mathbb { U }$ with $\ell ( \mathrm { F } ( a , \omega ) ) = \mathrm { f } ( ( a , \ell ) , \omega )$ for all $a \in \mathbb { A }$ , $\boldsymbol { \ell } \in \hat { \mathbb { L } }$ , and $\mathrm { P }$ -almost all $\omega \in \Omega$ , we can construct a weaker version of F. Note that, if $\ell = \alpha _ { 1 } \ell _ { 1 } + \alpha _ { 2 } \ell _ { 2 } \in \hat { \mathbb { L } }$ , then $\begin{array} { r } { \mathrm { \Delta f } ( ( a , \ell ) , \omega ) = \alpha _ { 1 } \mathrm { f } ( ( a , \ell _ { 1 } ) , \omega ) + \alpha _ { 2 } \mathrm { f } ( ( a , \ell _ { 2 } ) , \omega ) } \end{array}$ . This means that $\operatorname { f } ( ( a , \cdot ) , \omega )$ can always be uniquely linearly extended to $\mathbb { L } = \operatorname { s p a n } \hat { \mathbb { L } }$ . Define + +$$ +\tilde { \mathrm { F } } \colon \mathbb { A } \times \Omega \to \mathbb { L } ^ { \# } , ( a , \omega ) \mapsto ( \ell \mapsto \mathrm { f } ( ( a , \ell ) , \omega ) ) . +$$ + +We can use Corollary A.14 to show that $\tilde { \mathrm { F } }$ is a random process with values in the algebraic dual $\mathbb { L } ^ { \# }$ of $\mathbb { L }$ equipped with the smallest $\sigma$ -algebra $\sigma ( \delta _ { \mathbb { L } \# } )$ that makes all point evaluation functionals measurable. We refer to such random processes as bidual random processes. + +Recall the bidual embedding $\boldsymbol { L } _ { \mathbb { U } , \mathbb { L } ^ { \# } }$ from Remark A.1. If $\mathrm { F }$ exists, then $\tilde { \mathrm { F } } ( a , \omega ) = \iota _ { \mathbb { U } , \mathbb { L } ^ { \# } } ( \mathrm { F } ( a , \omega ) )$ . + +# A.5. Operator-Valued Gaussian Processes as Hilbert-Valued Gaussian Processes + +Finally, we show that operator-valued Gaussian processes (Owhadi, 2023; Batlle et al., 2024; Mora et al., 2025) can be embedded in the theoretical framework outlined above. Since operator-valued Gaussian processes are only defined on separable Hilbert spaces, in this section, we let $\mathbb { A } , \mathbb { U }$ be separable Hilbert spaces with inner products $\langle \cdot , \cdot \rangle _ { \mathbb { A } }$ and $\langle \cdot , \cdot \rangle _ { \mathbb { U } }$ , respectively. In this case, the continuous dual $\mathbb { U } ^ { \prime }$ is identified with the primal space $\mathbb { U }$ via the Riesz isomorphism. We start by reviewing the building blocks of operator-valued Gaussian processes. + +Definition A.15 (Operator-Valued Kernel (Owhadi, 2023, Definition 9.1)). A function ${ \mathcal { K } } \colon \mathbb { A } \times \mathbb { A } \to ( \mathbb { U } \to \mathbb { U } )$ is called an operator-valued kernel if $\mathcal { K } ( a _ { 1 } , a _ { 2 } )$ is a bounded linear operator with ${ \cal K } ( a _ { 1 } , a _ { 2 } ) = { \cal K } ( a _ { 2 } , a _ { 1 } ) ^ { * }$ for all $a _ { 1 } , a _ { 2 } \in \mathbb { A }$ and $\begin{array} { r } { \sum _ { i , j = 1 } ^ { m } \langle u _ { i } , { \cal K } ( a _ { i } , a _ { j } ) u _ { j } \rangle _ { \mathbb { U } } \geq 0 } \end{array}$ for all $m \in \mathbb { N }$ , $a _ { 1 } , \hdots , a _ { m } \in \mathbb { A }$ , and $u _ { 1 } , \ldots , u _ { m } \in \mathbb { U }$ . + +Definition A.16 (Gaussian Hilbert Space (Owhadi & Scovel, 2019, Definition 7.1)). A closed subspace $\mathbb { H }$ of $\mathrm { L } _ { 2 } ( \Omega , A , \mathrm { P } )$ is called a Gaussian Hilbert space if every $h \in \mathbb { H }$ is a univariate Gaussian random variable. + +Definition A.17 (Operator-Valued Gaussian Process (Owhadi, 2023, Definition 5.1)). Let $\mathcal { M } \colon \mathbb { A } \mathbb { U }$ , ${ \mathcal { K } } \colon { \mathbb { A } } \times { \mathbb { A } } \to ( \mathbb { U } \to$ $\mathbb { U }$ ) an operator-valued kernel, and $\mathbb { H }$ a Gaussian Hilbert space over $( \Omega , A , { \mathrm { P } } )$ . A function $\Xi \colon \mathbb { A } ( \mathbb { U } \mathbb { H } )$ is called an operator-valued Gaussian process if $\Xi ( a )$ is a bounded linear operator for all $a \in \mathbb { A }$ . $\Xi$ is said to have mean $\mathcal { M }$ and covariance kernel $\kappa$ if $\Xi ( a ) ( u ) \sim \mathcal { N } \left( \langle u , \mathcal { M } ( a ) \rangle _ { \mathbb { U } } , \langle u , \mathcal { K } ( a , a ) ( u ) \rangle _ { \mathbb { U } } \right)$ for all $a \in \mathbb { A }$ and $u \in \mathbb { U }$ , and + +$$ +\mathrm { C o v } _ { \mathbb { P } } \left[ \Xi ( a _ { 1 } ) ( u _ { 1 } ) , \Xi ( a _ { 2 } ) ( u _ { 2 } ) \right] = \langle u _ { 1 } , K ( a _ { 1 } , a _ { 2 } ) ( u _ { 2 } ) \rangle _ { \mathbb { U } } . +$$ + +Generally, operator-valued Gaussian processes are “equivalent to” a subset of Gaussian processes with values in $( \mathbb { U } ^ { \prime } ) ^ { \# } = \mathbb { U } ^ { \# }$ (see Appendix A.4.2). + +Proposition A.18. Let $\tilde { \mathrm { F } }$ : $\mathbb { A } \times \Omega \mathbb { U } ^ { \# }$ , $\mathbb { H } \subset \mathrm { L } _ { 2 } ( \Omega , \mathcal { A } , \mathrm { P } )$ a Gaussian Hilbert space, and $\Xi \colon \mathbb { A } ( \mathbb { U } \mathbb { H } )$ such that $\tilde { \mathrm { F } } ( a , \cdot ) ( u ) \in \Xi ( a ) ( u )$ for all $a \in \mathbb { A }$ and $u \in \mathbb { U }$ . Then $\Xi$ is an operator-valued Gaussian process with mean function $\mathcal { M } \colon \mathbb { A } \mathbb { U }$ and covariance kernel $\kappa$ ${ \mathcal { C } } \colon \mathbb { A } \times \mathbb { A } \to ( \mathbb { U } \to \mathbb { U } )$ if and only if + +(a) F˜ is a $\mathbf { \Omega } : ( \mathbb { U } ^ { \# } , \sigma ( \delta _ { \mathbb { U } } ) )$ -valued Gaussian process with mean $\tilde { \mathcal { M } } \colon \mathbb { A } \to \mathbb { U } ^ { \prime }$ and covariance function $\tilde { \mathcal { K } }$ : $: \mathbb { A } \times \mathbb { A } \to ( \delta _ { \mathbb { U } } \to \mathbb { U } ^ { \prime } )$ + +(b) $( u , \omega ) \mapsto \tilde { \mathrm { F } } ( a , \omega ) ( u )$ is mean-square continuous9 for all $a \in \mathbb { A }$ , and + +(c) $u \mapsto \tilde { K } ( a _ { 1 } , a _ { 2 } ) ( \delta _ { u } ) \in \mathbb { U } ^ { \prime }$ is norm-continuous for all $a _ { 1 } , a _ { 2 } \in \mathbb { A }$ . + +We have $\langle \mathcal { M } ( a ) , \cdot \rangle _ { \mathbb { U } } = \tilde { \mathcal { M } } ( a )$ for $a \in \mathbb { A } .$ , and $\langle \mathcal { K } ( a _ { 1 } , a _ { 2 } ) ( u ) , \cdot \rangle _ { \mathbb { U } } = \tilde { \mathcal { K } } ( a _ { 1 } , a _ { 2 } ) ( \delta _ { u } )$ for all $a _ { 1 } , a _ { 2 } \in \mathbb { A }$ and $u \in \mathbb { U }$ + +Proof. This follows from Corollary A.14. + +Moreover, the following results show that operator-valued Gaussian processes whose kernels map into the trace-class are “equivalent to” Gaussian processes with values in $\left( \mathbb { U } , B \left( \mathbb { U } \right) \right)$ . + +Proposition A.19. Let $\mathrm { F }$ be a U-valued Gaussian process on $( \Omega , A , { \mathrm { P } } )$ with index set A, mean function $\mathcal { M }$ , and covariance function $\kappa$ . Define $\mathbb { H }$ as the $\mathrm { L } _ { 2 } ( \Omega , A , \mathrm { P } )$ closure of $\operatorname { s p a n } \{ \omega \mapsto \langle u , \operatorname { F } ( a , \omega ) \rangle _ { \mathbb { U } } \mid a \in \mathbb { A } , u \in \mathbb { U } \} \subset \operatorname { L } _ { 2 } ( \Omega , A , \operatorname { P } )$ . Then + +$$ +\Xi \colon \mathbb { A } \to ( \mathbb { U } \to \mathbb { H } ) , a \mapsto ( u \mapsto \langle u , \mathrm { F } ( a , \cdot ) \rangle _ { \mathbb { U } } ) +$$ + +is an operator-valued Gaussian process with mean $\mathcal { M }$ and covariance kernel $\kappa$ . Moreover, ${ \cal { K } } ( a , a )$ is trace-class for all $a \in \mathbb { A }$ . + +Proof. Let $a \in \mathbb { A }$ . For all $u \in \mathbb { U }$ we have + +$$ +\| \Xi ( a ) ( u ) \| _ { \mathbb { H } } ^ { 2 } = \int _ { \Omega } \left( \langle u , \mathrm { F } ( a , \cdot ) \rangle \right) ^ { 2 } \mathrm { P } ( \mathrm { d } \omega ) \leq \| u \| _ { \mathbb { U } } ^ { 2 } \underbrace { \int _ { \Omega } \| \mathrm { F } ( a , \cdot ) \| _ { \mathbb { U } } ^ { 2 } \mathrm { P } ( \mathrm { d } \omega ) } _ { < \infty } +$$ + +by the Cauchy-Schwarz inequality and Fernique’s theorem (Bogachev, 1998, Theorem 2.8.5), i.e. $\Xi ( a )$ is a bounded linear operator. □ + +Theorem A.20. Let $\Xi$ be an operator-valued Gaussian process with Gaussian Hilbert space $\mathbb { H } \subset \mathrm { L } _ { 2 } ( \Omega , \mathcal { A } , \mathrm { P } ) .$ , mean function $\mathcal { M }$ , and covariance kernel $\kappa$ such that $\textstyle { \mathcal { K } } ( a , a )$ is trace class for all $a \in \mathbb { A }$ . Then there is a $\mathbb { U }$ -valued Gaussian process $\mathbf { F } \sim \mathcal { G P } \left( \mathcal { M } , \mathcal { K } \right)$ on $( \Omega , A , { \mathrm { P } } )$ such that $\langle u , \mathrm { F } ( a ) \rangle _ { \mathbb { U } } \in \Xi ( a ) ( u ) .$ for all $a \in \mathbb { A }$ and $u \in \mathbb { U }$ . + +Proof. First, we will construct $\mathrm { F } \colon \mathbb { A } \times \Omega \mathbb { U }$ . Let $a \in \mathbb { A }$ . Since $\mathbb { U }$ is separable and $\textstyle { \mathcal { K } } ( a , a )$ is self-adjoint, positivesemidefinite, and trace class (and hence compact), there is an ONB $\{ \psi _ { i } ^ { a } \} _ { i \in I }$ of $\mathbb { U }$ consisting of eigenvectors of $\textstyle { \mathcal { K } } ( a , a )$ (Conway, 1997, Corollary 5.4). For every $i \in I$ , fix $\mathsf { z } _ { i } ^ { a } \in ( \Xi ( a ) ( \psi _ { i } ^ { a } ) - \langle \psi _ { i } ^ { a } , \mathcal { M } ( a ) \rangle )$ . We will now show that the series + +$$ +\sum _ { i \in I } \mathrm { z } _ { i } ^ { a } \psi _ { i } ^ { a } +$$ + +converges $\mathrm { P }$ -almost surely in $\mathbb { U }$ . The $\{ \boldsymbol { \mathrm { z } } _ { i } ^ { a } \} _ { i \in I }$ are centered, independent Gaussian random variables with variances given by the eigenvalues $\lambda _ { i } ^ { a }$ of ${ \cal { K } } ( a , a )$ , since + +$$ +\mathbb { E } _ { \mathrm { P } } \left[ \mathrm { z } _ { i } ^ { a } \right] = \mathbb { E } _ { \mathrm { P } } \left[ \Xi ( a ) ( \psi _ { i } ^ { a } ) \right] - \langle \psi _ { i } ^ { a } , \mathcal { M } ( a ) \rangle = 0 +$$ + +and + +$$ +\mathrm { C o v } _ { \mathbb { P } } \left[ \boldsymbol { z } _ { i } ^ { a } , \boldsymbol { z } _ { j } ^ { a } \right] = \mathrm { C o v } _ { \mathbb { P } } \left[ \Xi ( a ) ( \psi _ { i } ^ { a } ) , \Xi ( a ) ( \psi _ { j } ^ { a } ) \right] = \langle \psi _ { i } ^ { a } , K ( a , a ) ( \psi _ { j } ^ { a } ) \rangle _ { \mathbb { U } } = \langle \psi _ { i } ^ { a } , \lambda _ { j } ^ { a } \psi _ { j } ^ { a } \rangle _ { \mathbb { U } } = \lambda _ { j } ^ { a } \delta _ { i j } . +$$ + +We have $\textstyle \sum _ { i \in I } \lambda _ { i } ^ { a } < \infty$ because the operator $\textstyle { \mathcal { K } } ( a , a )$ is trace-class. By Theorem 1.1.4 in Bogachev (1998), this means that + +$$ +\sum _ { i \in I } \mathsf { z } _ { i } ^ { a } = \sum _ { i \in I } \mathsf { z } _ { i } ^ { a } \| \psi _ { i } ^ { a } \| _ { \mathbb { U } } +$$ + +converges $\mathrm { P }$ -almost surely, and hence Equation (A.3) P-almost surely absolutely convergent in $\mathbb { U }$ . Define + +$$ +\operatorname { F } ( a ) \stackrel { \mathrm { a . s . } } { : = } \mathcal { M } ( a ) + \sum _ { i \in I } \mathsf { z } _ { i } ^ { a } \psi _ { i } ^ { a } . +$$ + +$$ +^ { 9 } \tilde { \mathrm { F } } ( a , \cdot ) ( u ) \stackrel { \mathrm { L } _ { 2 } } { \longrightarrow } \tilde { \mathrm { F } } ( a , \cdot ) ( u _ { 0 } ) \mathrm { a s } u \stackrel { \mathbb { U } } { \longrightarrow } u _ { 0 } +$$ + +Then $\langle u , \mathrm { F } ( a ) \rangle \in \Xi ( a ) ( u )$ for $u \in \mathrm { s p a n } \{ \psi _ { i } ^ { a } \} _ { i \in I } .$ . + +We will now show that $\langle u , \mathrm { F } ( a ) \rangle \in \Xi ( a ) ( u )$ for all $a \in \mathbb { A }$ and $u \in \mathbb { U }$ . To this end, fix $u \in \mathbb { U }$ and $\mathrm { y } _ { a , u } \in \Xi ( a ) ( u )$ . There is $\{ u _ { n } \} _ { n = 1 } ^ { \infty } \subset \mathrm { s p a n } \{ \psi _ { i } ^ { a } \} _ { i \in I }$ such that $u \ = \ \operatorname* { l i m } _ { n \to \infty } u _ { n }$ . Boundedness of $\Xi ( a )$ implies $\langle u _ { n } , \mathrm { F } ( a ) \rangle \stackrel { \mathrm { L } ^ { 2 } } { \longrightarrow } \mathrm { y } _ { a , u }$ and, by Remark 6.11 and Corollary 6.13 in Klenke (2014), there exists a subsequence $\{ u _ { n _ { k } } \} _ { k = 1 } ^ { \infty } \subset \{ u _ { n } \} _ { n = 1 } ^ { \infty }$ such that $\langle u _ { n _ { k } } , \mathrm { F } ( a ) \rangle \xrightarrow { \mathrm { a . s . } } \mathrm { y } _ { a , u } .$ Moreover, by continuity of the inner product, we know that $\langle u _ { n _ { k } } , \mathrm { F } ( a , \omega ) \rangle \langle u , \mathrm { F } ( a , \omega ) \rangle$ for all $\omega \in \Omega$ , and hence $\langle u _ { n _ { k } } , \mathrm { F } ( a ) \rangle \xrightarrow { \mathrm { a . s . } } \langle u , \mathrm { F } ( a ) \rangle$ . Since almost sure limits are unique up to almost sure equality (Klenke, 2014), we have $\operatorname { y } _ { a , u } \overset { \mathrm { a . s . } } { = } \langle u , \operatorname { F } ( a ) \rangle$ . Hence, $\langle u , \mathrm { F } ( a ) \rangle \in \Xi ( a ) ( u )$ . + +The rest of the claim now follows from Corollary A.13 with $\operatorname { f } ( ( a , u ) , \omega ) : = \langle u , \operatorname { F } ( a , \omega ) \rangle$ and $\hat { \mathbb { L } } = \mathbb { U } ^ { \prime }$ + +# B. Linearized Laplace Approximation + +The linearized Laplace approximation (LLA) (MacKay, 1992a;b; Immer et al., 2021) is a conceptually simple, yet effective (Daxberger et al., 2021a) method for obtaining an approximate posterior distribution over the parameters $\pmb { w } \in \mathbb { R } ^ { p }$ of a neural network $\pmb { f } \colon \mathbb { R } ^ { d } \times \mathbb { R } ^ { p } \mathbb { R } ^ { d ^ { \prime } }$ . It applies whenever the objective function $R$ used to train the neural network is (equivalent to) a negative log-posterior + +$$ +R ( \pmb { w } ) = - \log p ( \pmb { w } \mid \mathcal { D } ) = - \log p ( \pmb { w } ) - \sum _ { i = 1 } ^ { n } \log p ( \pmb { y } ^ { ( i ) } \mid \pmb { f } ( \pmb { x } ^ { ( i ) } , \pmb { w } ) ) + \mathrm { c o n s t . } +$$ + +of the network parameters given data $\mathcal D = \{ ( \boldsymbol x ^ { ( i ) } , \boldsymbol y ^ { ( i ) } ) \} _ { i = 1 } ^ { n }$ . It is common for the prior over the parameters to be Gaussian, in which case $- \log p ( \pmb { w } )$ acts as an L2-regularizer on the parameters. During training, we attempt to find a local minimum $\scriptstyle w ^ { \star }$ of the objective function $R$ , i.e. a maximum a-posteriori (MAP) estimator of the network parameters given the data. + +Following Immer et al. (2021), we approximate the posterior of the network weights as follows: First, we linearize the model using a first-order Taylor approximation in the weights around the MAP estimator $\scriptstyle w ^ { \star }$ + +$$ +f ( \boldsymbol { x } , \boldsymbol { w } ) \approx f _ { \boldsymbol { w } ^ { \star } } ^ { \mathrm { l i n } } ( \boldsymbol { x } , \boldsymbol { w } ) : = f ( \boldsymbol { x } , \boldsymbol { w } ^ { \star } ) + \mathrm { D } _ { \boldsymbol { w } } f ( \boldsymbol { x } , \boldsymbol { w } ) | _ { \boldsymbol { w } ^ { \star } } ( \boldsymbol { w } - \boldsymbol { w } ^ { \star } ) . +$$ + +Afterwards, we compute a second-order Taylor approximation of the negative log-posterior $R _ { w ^ { \star } } ^ { \mathrm { l i n } }$ of the linearized network at the MAP $\boldsymbol { w } ^ { \star }$ + +$$ +\begin{array} { l } { { \displaystyle R _ { w ^ { \star } } ^ { \mathrm { l i n } } ( w ) : = - \log p ( w ) - \sum _ { i = 1 } ^ { n } \log p ( y ^ { ( i ) } \mid f _ { w ^ { \star } } ^ { \mathrm { l i n } } ( x ^ { ( i ) } , w ) ) + \mathrm { c o n s t . } } \ ~ } \\ { { \displaystyle ~ \approx R ( w ^ { \star } ) + \underbrace { \nabla R \left( w ^ { \star } \right) ^ { \top } } _ { \approx 0 } ( w - w ^ { \star } ) + \frac 1 2 ( w - w ^ { \star } ) ^ { \top } P ( w - w ^ { \star } ) } \ ~ } \\ { { \displaystyle ~ = \frac 1 2 ( w - w ^ { \star } ) ^ { \top } P ( w - w ^ { \star } ) + \mathrm { c o n s t . } } } \end{array} +$$ + +with ${ \cal P } : = - \operatorname * { H } _ { w } \log p ( \pmb { w } ) | _ { \pmb { w } ^ { \star } } + G$ , where + +$$ +G : = - \sum _ { i = 1 } ^ { n } \operatorname { D } _ { w } f ( { \boldsymbol x } ^ { ( i ) } , { \boldsymbol w } ) \Big | _ { { \boldsymbol w } ^ { \star } } \ H _ { f } \log p ( { \boldsymbol y } ^ { ( i ) } \mid f ) \Big | _ { f ( { \boldsymbol x } ^ { ( i ) } , { \boldsymbol w } ^ { \star } ) } \operatorname { D } _ { w } f ( { \boldsymbol x } ^ { ( i ) } , { \boldsymbol w } ) \Big | _ { { \boldsymbol w } ^ { \star } } ^ { \top } +$$ + +is the so-called generalized Gauss-Newton (GGN) matrix (Schraudolph, 2002). The GGN is guaranteed to be positivesemidefinite. Equation (B.1) is the negative log-density of a (potentially degenerate) multivariate Gaussian distribution with mean $\scriptstyle w ^ { \star }$ and covariance matrix $P ^ { \dagger }$ , i.e. + +$$ +p ( \mathbf { w } = \pmb { w } \mid \mathcal { D } ) = \exp R ( \pmb { w } ) \approx \exp R _ { \pmb { w } ^ { \star } } ^ { \mathrm { l i n } } ( \pmb { w } ) \approx \mathcal { N } \left( \pmb { w } ; \pmb { w } ^ { \star } , \pmb { P } ^ { \dagger } \right) . +$$ + +This Gaussian distribution is referred to as the linearized Laplace approximation of $p ( \mathbf { w } = \pmb { w } \mid \mathcal { D } )$ . Under the linearized model, the approximate Gaussian posterior over the weights induces a tractable posterior predictive over the output of the neural network (Khan et al., 2019; Immer et al., 2021). More precisely, using closure properties of Gaussian distributions under affine maps, one can show that the pushforward of the LLA posterior through ${ \pmb w } \mapsto { \pmb f } _ { { \pmb w } ^ { \star } } ^ { \mathrm { l i n } } ( { \pmb x } , { \pmb w } )$ defines a ( $d ^ { \prime }$ -output) Gaussian process + +$$ +\begin{array} { r } { \textbf { f } \big | \mathcal { D } \sim \mathcal { G P } \left( f ( \cdot , w ^ { \star } ) , ( x _ { 1 } , x _ { 2 } ) \mapsto \mathrm { D } _ { w } f ( x _ { 1 } , w ) \big | _ { w ^ { \star } } P ^ { \dagger } \mathrm { D } _ { w } f ( x _ { 2 } , w ) \big | _ { w ^ { \star } } ^ { \top } \right) . } \end{array} +$$ + +# C. Implementation Details + +# C.1. Last-Layer LUNO for FNOs + +For an input $\mathbf { \pmb { a } } \in \mathbb { A }$ , we can factorize the FNO as + +$$ +\begin{array} { r } { F ( \pmb { a } , \pmb { w } ) ( \pmb { x } ) = \underbrace { ( \pmb { q } ( \cdot , \pmb { w _ { q } } ) \circ \sigma ^ { ( L - 1 ) } ) } _ { = : \tilde { \pmb { q } } } \left( z ^ { ( L - 1 ) } ( \pmb { x } , \pmb { w } _ { L - 1 } ) \right) , } \end{array} +$$ + +with + +$$ +\begin{array} { r l } { \Xi _ { s } ^ { ( L - 1 ) } ( \boldsymbol { x } , \boldsymbol { w } _ { L - 1 } ) : = } & { \displaystyle \sum _ { j = 1 } ^ { d _ { w } } \mathcal { F } ^ { - 1 } \Big ( R _ { s ; j } ^ { ( L - 1 ) } \odot \hat { v } _ { j } ^ { ( L - 1 ) } \Big ) ( \boldsymbol { x } ) + \displaystyle \sum _ { j = 1 } ^ { d _ { w } } W _ { i j } ^ { ( L - 1 ) } v _ { j } ^ { ( L - 1 ) } ( \boldsymbol { x } ) } \\ & { \quad = \displaystyle \sum _ { j = 1 } ^ { d _ { w } } \mathrm { k } _ { s \in \mathcal { W } } ( R _ { s ; j } ^ { ( L - 1 ) } ) \underbrace { \mathrm { R e } ( \hat { v } _ { k j } ^ { ( L - 1 ) } ) \cos { ( \{ \{ \omega _ { k , x } \} } ) } } _ { = \partial _ { x , j } ( x ) } \\ & { \quad \quad \quad + \displaystyle \sum _ { j = 1 } ^ { d _ { w } } \mathrm { k } _ { s \in \mathcal { W } } ( R _ { s ; j } ^ { ( L - 1 ) } ) \underbrace { ( - 1 ) \mathrm { I m } ( \hat { v } _ { k j } ^ { ( L - 1 ) } ) \sin { ( \{ \omega _ { k , x } \} ) } } _ { = \forall x _ { j } ( w ) } } \\ & { \quad \quad \quad + \displaystyle \sum _ { j = 1 } ^ { d _ { w } } W _ { i j } ^ { ( L - 1 ) } \underbrace { v _ { j } ^ { ( L - 1 ) } ( \boldsymbol { x } ) } _ { = \forall x _ { j } ( w ) } , } \end{array} +$$ + +where $\hat { \pmb { v } } _ { k j } ^ { ( L - 1 ) } : = \mathcal { F } ( \pmb { v } _ { j } ^ { ( L - 1 ) } ) _ { k } \in \mathbb { C }$ for $k \in \{ 1 , \dots , k _ { \operatorname* { m a x } } \}$ . We note that $\pmb { z } ^ { ( L - 1 ) } ( \pmb { x } , \pmb { w } _ { L - 1 } )$ is linear in $w _ { L - 1 }$ . Thus, + +$$ +{ \pmb w } _ { L - 1 } \cong ( \mathrm { R e } ( { \pmb R } ^ { ( L - 1 ) } ) , \mathrm { I m } ( { \pmb R } ^ { ( L - 1 ) } ) , { \pmb W } ^ { ( L - 1 ) } ) +$$ + +induces a (multi-output) GP over $z ^ { ( L - 1 ) }$ : + +$$ +\begin{array} { r l } & { \mathbf { z } ^ { ( L - 1 ) } \sim \mathcal { G P } \left( m _ { \mathbf { z } ^ { ( L - 1 ) } } , K _ { \mathbf { z } ^ { ( L - 1 ) } } \right) \qquad \mathrm { w i t h } } \\ & { m _ { \mathbf { z } ^ { ( L - 1 ) } } = z ^ { ( L - 1 ) } ( { \pmb x } , { \pmb w } _ { L - 1 } ^ { \star } ) . } \end{array} +$$ + +Moreover, $\mathbf { z } ^ { ( L - 1 ) }$ is the sum of three (dependent) parametric Gaussian processes with feature functions $\phi _ { k j } , \varphi _ { k j }$ , and $\psi _ { j }$ , respectively. Consequently, the function-valued GP induced by the linearized FNO is given by + +$$ +\mathrm { F } ( \pmb { a } ) ( \pmb { x } ) = \tilde { q } ( m _ { \mathbf { z } ^ { ( L - 1 ) } } ( \pmb { x } ) ) + \mathrm { D } \tilde { q } \left( m _ { \mathbf { z } ^ { ( L - 1 ) } } ( \pmb { x } ) \right) ( \mathbf { z } ^ { ( L - 1 ) } ( \pmb { x } ) - m _ { \mathbf { z } ^ { ( L - 1 ) } } ( \pmb { x } ) ) , +$$ + +i.e. $\mathrm { F } ( \pmb { a } ) \sim \mathcal { G P } \left( m _ { \pmb { a } } , K _ { \pmb { a } } \right)$ with + +$$ +\begin{array} { r l } & { m _ { a } ( { \boldsymbol { x } } ) = F ( { \boldsymbol { a } } , { \boldsymbol { w } } ^ { \star } ) ( { \boldsymbol { x } } ) , \quad \mathrm { a n d } } \\ & { K _ { a } ( x _ { 1 } , x _ { 2 } ) = \mathrm { D } \tilde { q } \left( m _ { \mathbf { z } ^ { ( L - 1 ) } } ( x _ { 1 } ) \right) K _ { \mathbf { z } ^ { ( L - 1 ) } } ( x _ { 1 } , x _ { 2 } ) \mathrm { D } \tilde { q } \left( m _ { \mathbf { z } ^ { ( L - 1 ) } } ( \mathbf { x } _ { 2 } ) \right) ^ { \top } . } \end{array} +$$ + +If the input finterpolates $\pmb { a } \in \mathbb { A }$ is discretized on a grid (e.g. spline interpolatio $X _ { \mathbb { A } } ^ { ( i ) } \in ( \mathbb { D } _ { \mathbb { A } } ) ^ { n _ { \mathbb { A } } ^ { ( i ) } }$ , we set rpolatio $\hat { \pmb { v } } _ { k j } ^ { ( L - 1 ) } : = \mathrm { r f f t } ( \pmb { v } _ { j } ^ { ( L - 1 ) } ( \mathbf { X } _ { \mathbb { A } } ^ { ( i ) } ) ) _ { k } \in \mathbb { C }$ $\psi _ { j } ( { \pmb x } )$ $v _ { j } ^ { ( L - 1 ) } ( X _ { \mathbb { A } } ^ { ( i ) } )$ + +# D. Experimental details + +# D.1. Data: PDE trajectories + +# D.1.1. LOW DATA GENERATION USING APEBENCH + +To evaluate the performance of the uncertainty quantification methods discussed, we utilize the code in the APEBench for generating data from Burgers’, Hyper Diffusion and Kuramoto-Sivashinsky equation (conservative) (cf. (Koehler et al., 2024) for more details). Table 3 summarizes the characteristics of the datasets we use, the number of trajectories for training, and testing, as well as the spatial and temporal resolutions. + +Table 3: Summary of PDE datasets generated using APEBench. + +
PDE NameDimensionsTraining Traj.Valid. Traj.Test Traj.Spatial Res.Temp. Res.
Burgers1D2525025025659
Hyper Diffusion1D2525025025659
Kuramoto-Sivashinsky (cons.)1D2525025025659
+ +# D.1.2. OUT-OF-DISTRIBUTION DATA GENERATION + +We generated additional datasets based on the advection-diffusion-reaction equation + +$$ +{ \frac { \partial u } { \partial t } } + { \bf v } \cdot \nabla u = \alpha \nabla ^ { 2 } u + R , +$$ + +to evaluate the robustness of the uncertainty quantification methods under out-of-distribution (OOD) scenarios. Here $u$ represents the scalar field (e.g., concentration or temperature), $\mathbf { v }$ is the velocity field, $\alpha$ is the diffusion coefficient, and $R$ is the reaction term, where $\alpha = 0 . 0 2 6$ was held constant throughout the datasets. The following specifications guided this process: + +• Datasets: Trajectories for five variations of the advection-diffusion-reaction equation were generated (cf. Figure 5): + +1. Base + +– Random number (1-10) of Gaussian blobs as an initial condition. +– Constant random velocity field. +– No reaction terms. + +2. Flip + +– Random number (1-10) of Gaussian blobs as an initial condition. +– Constant random velocity field, which is reversed at the center of the domain. +– No reaction terms. + +3. Pos + +– Random number (1-10) of Gaussian blobs as an initial condition. +– Constant random velocity field. +– Randomly placed triangular heat source. + +4. Pos-Neg + +– Random number (1-10) of Gaussian blobs as an initial condition. +– Constant random velocity field. +– Randomly placed triangular heat source; randomly placed cloud-shaped heat sink. + +5. Pos-Neg-Flip + +– Random number (1-10) of Gaussian blobs as an initial condition. +– Constant random velocity field, which is reversed at the center of the domain. +– Randomly placed triangular heat source; randomly placed cloud-shaped heat sink. + +• PDE Solver: We utilized a custom implementation to solve the advection-diffusion-reaction equation, relying on a 9-point stencil with Runge-Kutta 4 (RK4) with fine spatial and temporal resolutions. + +# • Simulation Parameters: + +– Spatial resolution: $1 0 0 \times 1 0 0$ grid. +– Temporal resolution: $\Delta t = 5 \times 1 0 ^ { - 1 0 }$ over 200-time steps, from which 59 are sub-sampled. + +We train a Fourier neural operator on 1000 training trajectories of the Base dataset and calibrate uncertainty quantification methods - if necessary - on a corresponding validation dataset with 250 input-output pairs. For the evaluation, we consider 250 random test pairs from some of the datasets. In Figure 5, we depict a single trajectory for each of these 5 datasets generated. To seamlessly evaluate the various OOD datasets, we pad the training set with constant zeros for the velocity field and reaction terms as a placeholder. + +# D.2. Model and training + +For all experiments, we consider the original Fourier neural operator architecture (Li et al., 2021) with the hyperparameter suggestions following (Koehler et al., 2024), i.e. 12 modes (per spatial dimension) and 18 hidden dimensions constant throughout the network, with a total of 4 Fourier blocks. For the training, we consider 10 initial time steps and train to predict the following time step. The velocity field and the reaction term are glued to the input. Furthermore, the input is always padded by two constant zero grid points to reduce artifacts at the borders. Networks for the low data experiment are trained for 100 epochs, all remaining networks are trained for 1000 epochs—where one epoch corresponds to iterating through a single input-output pair per trajectory in the training set. During training the mean squared error loss was minimized using AdamW (Loshchilov & Hutter, 2019) combined with a cosine decay learning rate scheduler with warmup. All training implementations rely on jax (Bradbury et al., 2018), Flax NNX and optax. + +# D.3. Uncertainty quantification methods + +# D.3.1. INPUT PERTURBATIONS + +Input Perturbations involve augmenting input data with small, random perturbations to introduce diversity into the model’s predictions. This approach exploits the model’s sensitivity to input variations to approximate uncertainty in the output space. Following the approach of Pathak et al. (2022), ensemble predictions are generated by forwarding a batch of pointwise perturbed versions of a single input $u _ { n }$ . Perturbations are sampled as $\epsilon _ { x , t } \sim \mathcal { N } ( 0 , \sigma ^ { 2 } )$ for each input value of the discretized function state $u _ { n } ( x , t )$ . The parameter $\sigma$ is calibrated to achieve accurate marginal uncertainty predictions. + +# D.3.2. ENSEMBLES + +Deep ensembles rely on training multiple independent instances of a model, each initialized with different random seeds and potentially trained on different subsets of the data (Lakshminarayanan et al., 2017). The diversity in the learned weight configurations leads to a variety of predictions for a given input, enabling the computation of both mean predictions and marginal uncertainty estimates. From a Bayesian perspective these weights represent close to true samples from the weight space posterior distribution, that can only be found through the high cost of additional training runs. + +# D.3.3. ISOTROPIC GAUSSIAN ( $^ *$ -ISO) + +The isotropic Gaussian covariance structure represents the weight space uncertainty as: + +$$ +\mathcal { N } ( \boldsymbol { \mathbf { \mathit { w } } } ^ { \star } , \boldsymbol { \Sigma } : = \sigma ^ { 2 } \mathbf { I } ) +$$ + +where $\sigma ^ { 2 }$ is the variance parameter and I is the identity matrix, reflecting independence and identical uncertainty across all dimensions in the weight space. To calibrate the uncertainty, we tune the parameter $\sigma ^ { 2 }$ as outlined below. + +From a Bayesian perspective, as seen in methods such as Laplace, this can be viewed as just considering a calibrated prior over the selected weight space. + +# D.3.4. LAPLACE APPROXIMATION ( $^ *$ -LA) + +In the linearized setting, a natural choice for a posterior Gaussian belief in weight space is given by the linearized Laplace approximation, where the Hessian is given by the Generalized Gauss-Newton (GGN) $\mathbf { H } _ { \mathrm { G G N } }$ (Immer et al., 2021). For further details, see Equation (B.1). + +![](images/figures/luno-fig-0005.jpg) +Figure 5: Initial condition and three-time steps of a single trajectory per generated dataset (Base, Flip, Pos, Pos-Neg, Pos-Neg-Flip). + +For high-dimensional parameter spaces, approximations of the GGN are required. Common techniques include diagonal approximations (Daxberger et al., 2021a), K-FAC (Martens & Grosse, 2015), or low-rank approximations (Dangel et al., 2022). Since diagonal and K-FAC approximations do not capture correlations between weights acting on different Fourier modes, we focus on a low-rank approximation of the GGN instead. + +We extend the approach in (Dangel et al., 2022) by selecting the largest eigenspaces of the GGN and placing an isotropic Gaussian prior over all weights instead of approximating the posterior covariance directly. This allows regions of uncertainty to fall back to the prior belief. For a fixed input function and evaluation grid, the push-forward with the low-rank approximation $\mathbf { H } _ { \mathrm { G G N } } = V V ^ { T }$ yields the normal distribution: + +$$ +\mathcal { N } ( \mathcal { F } ( \mathbf { x } . , \pmb { \theta } ) , \mathbf { J } _ { \pmb { \theta } } \pmb { \Sigma } \mathbf { J } _ { \pmb { \theta } } ^ { T } ) , +$$ + +where $\pmb { \Sigma } = ( n V V ^ { T } + \sigma \mathbf { I } ) ^ { - 1 }$ , $n$ is the number of input-output pairs used to train the neural operator, and $\mathbf { J } _ { \pmb { \theta } }$ is the Jacobian of the model with respect to the weights. In all experiments, we consider a low rank of 500. For the low data regime, we consider all input-output pairs, while for the OOD experiment only a minibatch of 1000 input-output pairs. + +# D.3.5. SAMPLE- $^ *$ + +For each weight-space covariance method and the input perturbation approach, we consider a sample-based pushforward which generates an ensemble of predictions in the output space. This aligns with the way most weight-space covariance methods are introduced in the literature (cf. Maddox et al., 2019). In our experiments, we generate 200 samples that are propagated through the network. The empirical mean and standard deviation are then estimated using the set of predictions in the output space. + +# D.3.6. LUNO- $^ *$ + +The core implementation of LUNO leverages matrix-free Jacobian-vector products and the algebraic structure of the chosen Gaussian covariance matrix in weight space. Our framework supports fully lazy evaluations, enabling efficient sampling, marginal variance estimation, and matrix-vector products with the covariance of the output space. + +When restricting the weight space uncertainty to only the last Fourier block, we can explicitly derive the matrix action of the Jacobian of the inverse Fast Fourier Transform (IFFT). Additionally, we exploit the fact that the final linear layer is applied pointwise in the spatial domain, significantly improving computational efficiency and reducing memory requirements compared to traditional implementations of linearized pushforwards (Daxberger et al., 2021a). + +# D.4. Evaluation + +We use the following metrics to evaluate model performance on 250 input-output pairs from the respective test trajectories. Here, $y _ { i }$ denotes the ground truth, $\hat { y } _ { i }$ represents the predicted mean, and $\sigma _ { i }$ is the predicted standard deviation for the $i$ -th sample. All reported numbers are the expected value over the test samples. + +# D.4.1. ROOT MEAN SQUARED ERROR (RMSE) + +The root mean squared error + +$$ +{ \mathrm { R M S E } } = { \sqrt { { \frac { 1 } { n } } \sum _ { i = 1 } ^ { n } ( y _ { i } - { \hat { y } } _ { i } ) ^ { 2 } } } +$$ + +measures the average magnitude of the errors between ensemble mean/linearized mean and ground truth values. Lower RMSE indicates better predictive accuracy, with zero being the ideal value. + +# D.4.2. MARGINAL NEGATIVE LOG-LIKELIHOOD (NLL) + +The marginal NLL quantifies how well the predictive distribution fits the data under the assumption of Gaussian uncertainty. Lower NLL values indicate better calibration of the uncertainty estimates and higher likelihood of the observed data under the predictive model. It represents a trade-off between lower variances and how much of the error is accurately captured by the uncertainty. + +$$ +{ \mathrm { N L L } } = - \sum _ { i = 1 } ^ { n } \log \left( { \frac { 1 } { \sqrt { 2 \pi \sigma _ { i } ^ { 2 } } } } \exp \left( - { \frac { ( y _ { i } - { \hat { y } } _ { i } ) ^ { 2 } } { 2 \sigma _ { i } ^ { 2 } } } \right) \right) +$$ + +# D.4.3. $\chi ^ { 2 }$ -STATISTIC + +The $\chi ^ { 2 }$ -statistic measures the average squared error normalized by the predicted variance. A value close to 1 indicates well-calibrated uncertainty predictions. + +$$ +\mathrm { { Q } } = \frac { 1 } { n } \sum _ { i = 1 } ^ { n } \frac { ( y _ { i } - \hat { y } _ { i } ) ^ { 2 } } { \sigma _ { i } ^ { 2 } } +$$ + +Values above one suggest overconfidence, while values below one indicate underconfident predictive uncertainty. From a UQ perspective underconfidence is better than overconfidence. + +# D.5. Calibration + +All hyperparameters of the discussed UQ methods (mostly $\sigma ^ { 2 }$ in the above definitions) were calibrated using 250 input-output pairs of the validation set to minimize the marginal negative log-likelihood. We calibrate each method’s hyperparameters separately using grid search over a logarithmically spaced grid with 500 points centered around the relevant value. + +# D.6. Additional results + +D.6.1. LOW DATA REGIME + +Table 4: Performance metrics comparison for UQ methods evaluated on an FNO trained on 25 trajectories of the Hyper-Diffusion equation. + +
MethodRMSE (↓)χ2NLL (↓)
Input Perturbations1.98 × 10−21.203−2.3927
Ensemble1.95 × 10-212.6741.8957
Sample-Iso2.02 × 10-21.237−2.4391
LUNO-Iso1.99 × 10-21.155−2.4677
Sample-LA2.18 × 10-22.097−2.2312
LUNO-LA1.99 × 10−20.9842.5248
+ +
MethodRMSE (↓) χ2NLL ()
Input Perturbations7.84 × 10-2 0.938−1.0385
Ensembles6.82 × 10-2 7.6180.8489
Sample-Iso7.88 × 10-2 1.211−1.0862
LUNO-Iso7.82 × 10-2 0.922−1.0995
Sample-LA1.46 × 10−1 2.758−0.1699
LUNO-LA7.82 × 10−2 1.058-1.1653
+ +Table 5: Performance metrics comparison for UQ methods evaluated on an FNO trained on 25 trajectories of the Kuramoto Sivashinsky (conservative) equation. + +![](images/figures/luno-fig-0006.jpg) +Figure 6: FNO predictive uncertainty quantified by several different methods. Top row: target function $( - )$ , mean $( - )$ and 1.96 standard deviations $( \sqsupset$ of, as well as samples ( ) from, the predictive belief. For the ensemble, the samples are four of the ensemble members. Bottom row: spread of the predictive distribution around the mean. For the sample-/ensemble-based methods, we construct a Gaussian distribution from the empirical covariance matrix and draw four samples $( - )$ . We plot 1.96 standard deviations $( \sqsupset$ of the predictive belief, as well as the top-three eigenfunctions $( - )$ and a heatmap of the predictive covariance matrix (top right corner of panels). + +
MethodBaseFlipPosPos-NegPos-Neg-Flip
Input Perturbations−2.5862.573−1.17469.346494.935
Ensemble5.3133.825-4.802-1.257-1.014
Sample-Iso−2.9214.071−1.2269.45743.362
LUNO-Iso−2.8923.450−1.2607.63637.733
Sample-LA−2.5764.395−1.1837.36927.046
LUNO-LA2.934−1.126−1.742−0.8181.164
+ +Table 6: Negative Log-Likelihood (NLL) across different OOD datasets. Lower is better. + +# D.6.2. OUT-OF-DISTRIBUTION + +Table 7: Metrics evaluated on OOD dataset Base. + +
MethodRMSE (↓) χ2NLL ()
Input Perturbations1.46 × 10−2 1.497−2.5861
Ensemble1.51 × 10-3 0.1625.3134
Sample-Iso1.44 × 10-2 1.217−2.9214
LUNO-Iso1.46 × 10−2 1.189−2.8919
Sample-Iso1.73 × 10−2 1.910−2.5756
LUNO-LA1.46 × 10−2 0.8412.9340
+ +Table 8: Metrics evaluated on OOD dataset Flip. + +
MethodRMSE (↓) χ2NLL (↓)
Input Perturbations4.36 × 10−212.2552.5727
Ensemble-Sample4.43 × 10-30.2153.8249
Sample-Iso4.37 × 10−215.3214.0715
LUNO-Iso4.36 × 10−213.9833.4502
Sample-LA4.62 × 10−215.8684.3953
LUNO-LA4.36 × 10−23.475−1.1257
+ +Table 9: Metrics evaluated on OOD dataset Pos. + +
MethodRMSE (↓) χ2NLL ()
Input Perturbations2.94 × 10-2 4.292−1.1744
Ensemble4.93 × 10-3 0.549-4.8023
Sample-Iso2.91 × 10-2 4.552-1.2261
LUNO-Iso2.94 × 10−2 4.408−1.2596
Sample-LA2.79 × 10−2 4.622−1.1835
LUNO-LA2.94 × 10−2 3.001-1.7416
+ +
MethodRMSE (↓)χ2NLL ()
Input Perturbations5.55 × 10−2145.56169.3458
Ensemble9.79 × 10-21.161-1.2569
Sample-Iso5.55 × 10-225.9619.4566
LUNO-Iso5.52 × 10-222.2317.6355
Sample-LA5.94 × 10-221.6857.3688
LUNO-LA5.52 × 10-24.182−0.8180
+ +Table 10: Metrics evaluated on OOD dataset Pos-Neg. + +
MethodRMSE (↓)χ2NLL (↓)
Input Perturbations1.10 × 10-1997.143494.9350
Ensemble1.39 × 10−11.006-1.0140
Sample-Iso1.10 × 10-193.71243.3620
Prior-Iso1.10 × 10-182.36737.7331
Sample-LA1.15 × 10−160.72827.0460
LUNO-LA1.10 × 10-17.1271.1642
+ +Table 11: Metrics evaluated on OOD dataset Pos-Neg-Flip. + +# D.6.3. EVALUATION OF A SINGLE TRAJECTORY + +# Run-Time comparison + +
MethodLUNO-*Sample-*
Input Perturbations10.19 ± 0.006
Ensemble0.85 ± 0.004
*-Iso0.53 ± 0.00414.00 ± 0.008
*-LA5.75 ± 0.01727.70 ± 0.006
+ +Table 12: Comparison of run-time performance between sampling-based (Sample- $^ *$ ) and linearization-based (LUNO- $^ *$ ) methods across different uncertainty quantification techniques when out-rolling a single trajectory iteratively. + +![](images/figures/luno-fig-0007.jpg) +Figure 7: Averaged performance of different UQ methods on an autoregressive rollout of the FNO on 50 trajectories from the Base, Flip, Pos, Pos-Neg, and Pos-Neg-Flip datasets. We compare input perturbations $( - )$ , deep ensembles $( - )$ , Sample-Iso $( - )$ , LUNO-Iso ( ), Sample-LA $( - )$ , LUNO-LA $( - )$ . \ No newline at end of file diff --git a/papers/luno/paper.pdf b/papers/luno/paper.pdf new file mode 100644 index 0000000000000000000000000000000000000000..e77bf55615808335f613aebe7d064b61b980dc45 --- /dev/null +++ b/papers/luno/paper.pdf @@ -0,0 +1,3 @@ +version https://git-lfs.github.com/spec/v1 +oid sha256:f5b5577e66ca31ee86dee9edf2084542f4d0e3ce5898c7c220b402525c54a755 +size 1833990 diff --git a/papers/luno/sau.json b/papers/luno/sau.json new file mode 100644 index 0000000000000000000000000000000000000000..52ad6e9f09495832ed4a6f85a708e39e97d60ba0 --- /dev/null +++ b/papers/luno/sau.json @@ -0,0 +1,242 @@ +{ + "paper_id": "luno", + "paper_title": "LUNO: Linearized Uncertainty Quantification for Neural Operators", + "D1": [ + { + "id": "luno-D1-001", + "claim": "FNO architecture uses 4 Fourier blocks with 12 Fourier modes per spatial dimension, 18 hidden dimensions, 10 input time steps, autoregressive single-step output, and 2-point zero-padding on the spatial grid.", + "source": "Section 2.1, Example 2.1, Section 5" + }, + { + "id": "luno-D1-002", + "claim": "Low-data training regime: 25 training trajectories, 100 epochs, AdamW optimizer with cosine decay LR schedule and warmup, MSE loss, 250 validation and 250 test input-output pairs.", + "source": "Section 5, Appendix D, Table 3" + }, + { + "id": "luno-D1-003", + "claim": "OOD training regime: 1000 training trajectories (Base), 1000 epochs, AdamW optimizer with cosine decay LR schedule and warmup, MSE loss, 250 validation and 250 test input-output pairs per dataset.", + "source": "Section 5, Appendix D" + }, + { + "id": "luno-D1-004", + "claim": "Low-data PDE datasets: Burgers, Hyper Diffusion, Kuramoto-Sivashinsky (conservative), all 1D with spatial resolution 256, temporal resolution 59, 25 train / 250 valid / 250 test trajectories.", + "source": "Section 5, Appendix D, Table 3" + }, + { + "id": "luno-D1-005", + "claim": "OOD advection-diffusion-reaction datasets: diffusion coefficient alpha=0.026, 100x100 spatial grid, dt=5e-10, 200 total time steps subsampled to 59, 1-10 random Gaussian blobs as initial conditions, custom 9-point stencil with RK4 solver, 5 datasets (Base, Flip, Pos, Pos-Neg, Pos-Neg-Flip), 1000 Base train trajectories with 250 valid and 250 test pairs each.", + "source": "Section 5, Appendix D, Figure 5" + }, + { + "id": "luno-D1-006", + "claim": "Deep ensemble uses 10 independently trained FNO members with different random seeds for entirely separate training runs.", + "source": "Section 5" + }, + { + "id": "luno-D1-007", + "claim": "LUNO uncertainty methods: low-rank GGN approximation rank 500; GGN computed on all input-output pairs (25 trajectories x 59 steps) for low-data or minibatch of 1000 pairs for OOD; 200 sample-based push-forward samples; calibration via grid search over 500 logarithmically spaced points minimizing expected marginal NLL; calibrated hyperparameter is sigma^2 (variance); weight-space prior is isotropic Gaussian N(0, sigma_prior^2 I); linearization point is MAP estimate w*.", + "source": "Section 3.2, Section 3.2.1, Section 5, Appendix B" + }, + { + "id": "luno-D1-008", + "claim": "Compared UQ methods: Input Perturbations, Deep Ensemble, Sample-Iso, LUNO-Iso, Sample-LA, LUNO-LA; last-layer Laplace scope limited to last Fourier block parameters (R^{(L-1)}, W^{(L-1)}) only.", + "source": "Section 5, Section 3.2.1" + }, + { + "id": "luno-D1-009", + "claim": "Evaluation metrics: RMSE, chi-squared calibration statistic (ideal=1.0, >1 overconfidence, <1 underconfidence), marginal NLL; 250 test pairs; 1.96 std multiplier for confidence intervals.", + "source": "Section 5, Appendix D" + }, + { + "id": "luno-D1-010", + "claim": "Autoregressive rollout evaluation uses 50 trajectories from each OOD dataset (Base, Flip, Pos, Pos-Neg, Pos-Neg-Flip), 10 initial time steps, iterative single-step prediction over 59 total time steps with sliding window of 10.", + "source": "Section 5, Figure 4, Appendix D, Figure 7" + } + ], + "D2": [ + { + "id": "luno-D2-001", + "claim": "FNO Lifting (Input Projection): v^{(1)}(x) = p(a(x), w_p) in R^{d_v'}, maps input function from dimension d_A' to hidden dimension d_v' via parametric function p (linear layer or shallow MLP).", + "source": "Section 2.1, Example 2.1" + }, + { + "id": "luno-D2-002", + "claim": "FNO Fourier Layer Update: v_i^{(l+1)}(x) = sigma^{(l)}( sum_j F^{-1}( (R_{kij}^{(l)} F(v_j^{(l)})_k )_{k=1}^{k_max} )(x) + W_{ij}^{(l)} v_j^{(l)}(x) ), combining spectral convolution (Fourier domain multiplication + inverse FFT) with local linear transform and element-wise nonlinearity sigma^{(l)}. Iterated for layers l = 1,...,L-1.", + "source": "Section 2.1, Example 2.1" + }, + { + "id": "luno-D2-003", + "claim": "FNO Projection (Output Mapping): F(a,w)(x) = q(v^{(L)}(x), w_q), projects final hidden representation back to output function dimension d_U' via parametric function q.", + "source": "Section 2.1, Example 2.1" + }, + { + "id": "luno-D2-004", + "claim": "Empirical Risk (Training Loss): R(w) = (1/n) sum_i L(u^{(i)}(X_U^{(i)}), F(a^{(i)}(X_A^{(i)}), w)(X_U^{(i)})), standard MSE loss summed over spatial grid points, minimized via AdamW with cosine decay LR.", + "source": "Section 2.1, Eq 2.1" + }, + { + "id": "luno-D2-005", + "claim": "Negative Log-Posterior (Bayesian Training Objective): R(w) = -log p(w) - sum_i log p(y^{(i)} | f(x^{(i)}, w)) + const, with Gaussian prior N(0, sigma_prior^2 I) giving L2 regularization. MAP estimate w* = argmin_w R(w) is the trained network weights.", + "source": "Section 3.2, Step 2" + }, + { + "id": "luno-D2-006", + "claim": "Model Linearization (First-Order Taylor): f_mu^{lin}((a,x), w) = f((a,x), mu) + D_w f((a,x), w)|_mu (w - mu), linearizes neural operator around mean weights mu (typically w*) to enable analytic uncertainty propagation.", + "source": "Section 3.2, Step 2, Eq 3" + }, + { + "id": "luno-D2-007", + "claim": "Linearized GP Mean Function: m(a,x) = f((a,x), mu) = F(a, mu)(x), the mean prediction equals the original neural operator's output at MAP weights.", + "source": "Section 3.2, Step 2" + }, + { + "id": "luno-D2-008", + "claim": "Linearized GP Covariance Function: K((a1,x1), (a2,x2)) = D_w f((a1,x1), w)|_mu * Sigma * D_w f((a2,x2), w)|_mu^T, the Jacobian-sandwiched weight-space covariance. This is the fundamental formula for linearized uncertainty propagation, inducing a d_U'-output GP.", + "source": "Section 3.2, Step 3, Eq 4" + }, + { + "id": "luno-D2-009", + "claim": "Uncurrying Map: f: (A x D_U) x W -> R^{d_U'}, ((a,x), w) |-> F(a,w)(x). Converts the operator F (mapping input functions to output functions) into a standard neural network f (mapping augmented inputs (a,x) to vector outputs), enabling application of standard Bayesian deep learning tools.", + "source": "Section 3.2, Step 1" + }, + { + "id": "luno-D2-010", + "claim": "Probabilistic Currying (Function-Valued GP Reconstruction): F: A x Omega -> U, (a, omega) |-> (x |-> f((a,x), omega)). Reinterprets the multi-output GP f as a function-valued GP F where F(a) is itself a GP over the output domain D_U.", + "source": "Section 3.1, Theorem 3.2, Section 3.2, Step 3" + }, + { + "id": "luno-D2-011", + "claim": "Function-Valued GP Predictive Mean and Covariance: E[F(a)(x)] = F(a, mu)(x); Cov[F(a1)(x1), F(a2)(x2)] = D_w F(a1,w)(x1)|_mu * Sigma * D_w F(a2,w)(x2)|_mu^T. The expected value equals the trained operator's prediction; covariance expressed in terms of the neural operator's Jacobian.", + "source": "Section 3.2, Step 3, Eq 4" + }, + { + "id": "luno-D2-012", + "claim": "Generalized Gauss-Newton (GGN) Matrix: G = -sum_i D_w f(x^{(i)}, w)|_{w*} * H_f log p(y^{(i)} | f)|_{f(x^{(i)}, w*)} * D_w f(x^{(i)}, w)|_{w*}^T. Approximates the Hessian of negative log-likelihood. For Gaussian likelihood, H_f log p = I/sigma_obs^2. Low-rank (rank 500) via Lanczos.", + "source": "Section 3.2, Appendix B" + }, + { + "id": "luno-D2-013", + "claim": "Laplace Posterior Precision and Approximation: P = -H_w log p(w)|_{w*} + G. Laplace posterior p(w|D) ~= N(w; w*, P^dagger), with Moore-Penrose pseudoinverse used when P is degenerate (low-rank GGN). Total precision = prior precision + GGN.", + "source": "Section 3.2, Step 2" + }, + { + "id": "luno-D2-014", + "claim": "LLA Posterior Predictive GP: f|D ~ GP( f(., w*), (x1,x2) |-> D_w f(x1,w)|_{w*} * P^dagger * D_w f(x2,w)|_{w*}^T ). Under the linearized model, the Laplace posterior over weights induces a tractable GP posterior predictive.", + "source": "Section 3.2, Step 2" + }, + { + "id": "luno-D2-015", + "claim": "FNO Last-Layer Factorization: F(a,w)(x) = q_tilde( z^{(L-1)}(x, w_{L-1}) ), with q_tilde = q(., w_q) o sigma^{(L-1)} and w_{L-1} = (R^{(L-1)}, W^{(L-1)}). Factorizes FNO into a linear (in w_{L-1}) feature map z^{(L-1)} followed by a nonlinear projection q_tilde, enabling efficient last-layer Laplace.", + "source": "Section 3.2.1, Appendix C.1, Eq 5" + }, + { + "id": "luno-D2-016", + "claim": "z^{(L-1)} Fourier Feature Decomposition: z_i^{(L-1)}(x, w_{L-1}) = sum_{j,k} Re(R_{kij}^{(L-1)}) * Re(v_hat_{kj}^{(L-1)}) * cos() + sum_{j,k} (-1) * Im(R_{kij}^{(L-1)}) * Im(v_hat_{kj}^{(L-1)}) * sin() + sum_j W_{ij}^{(L-1)} * v_j^{(L-1)}(x). Explicit real-valued expansion showing z^{(L-1)} is linear in weights w_{L-1}.", + "source": "Section 3.2.1, Appendix C.1" + }, + { + "id": "luno-D2-017", + "claim": "Weight Reparameterization for Last-Layer GP: w_{L-1} ~= (Re(R^{(L-1)}), Im(R^{(L-1)}), W^{(L-1)}) flattened into a real-valued parameter vector, enabling standard Gaussian belief modeling over all last-layer parameters.", + "source": "Section 3.2.1, Appendix C.1" + }, + { + "id": "luno-D2-018", + "claim": "z^{(L-1)} GP Induced by Weight Uncertainty: z^{(L-1)} ~ GP(m_{z^{(L-1)}}, K_{z^{(L-1)}}), with m_{z^{(L-1)}}(x) = z^{(L-1)}(x, w_{L-1}^*). Because z^{(L-1)} is linear in w_{L-1}, Gaussian weight uncertainty induces a multi-output GP over z with parametric Fourier basis features.", + "source": "Section 3.2.1, Appendix C.1" + }, + { + "id": "luno-D2-019", + "claim": "Last-Layer LUNO Function-Valued GP: F(a)(x) = q_tilde(m_{z^{(L-1)}}(x)) + D q_tilde(m_{z^{(L-1)}}(x)) * (z^{(L-1)}(x) - m_{z^{(L-1)}}(x)). Applies a second linearization (of q_tilde around m_z) to push the z-GP through the nonlinear projection, yielding the final function-valued GP.", + "source": "Section 3.2.1, Appendix C.1, Eq 5" + }, + { + "id": "luno-D2-020", + "claim": "Last-Layer LUNO Predictive Mean and Covariance: m_a(x) = F(a, w*)(x); K_a(x1, x2) = D q_tilde(m_{z^{(L-1)}}(x1)) * K_{z^{(L-1)}}(x1, x2) * D q_tilde(m_{z^{(L-1)}}(x2))^T. Mean equals deterministic FNO output. Covariance is z-GP covariance sandwiched by q_tilde Jacobians — efficient since only small network Jacobian required.", + "source": "Section 3.2.1, Appendix C.1" + }, + { + "id": "luno-D2-021", + "claim": "Isotropic Gaussian Weight Prior: w ~ N(w*, Sigma = sigma^2 I), the simplest weight-space uncertainty model with scalar variance sigma^2 as the sole calibration hyperparameter, used in LUNO-Iso and Sample-Iso methods.", + "source": "Section 3.2, Section 5" + }, + { + "id": "luno-D2-022", + "claim": "Low-Rank GGN Push-Forward Distribution: F(a,w)(X) ~ N( F(a,w*)(X), J_theta * Sigma * J_theta^T ), with Sigma = (n V V^T + sigma I)^{-1} where V V^T is the rank-500 GGN approximation. Retains only the 500 largest eigenmodes with diagonal prior term for full-rank stability.", + "source": "Section 3.2, Section 5, Appendix B" + }, + { + "id": "luno-D2-023", + "claim": "Evaluation Metrics: RMSE = sqrt((1/n) sum_i (y_i - y_hat_i)^2); Marginal NLL = -sum_i log(1/sqrt(2*pi*sigma_i^2) * exp(-(y_i - y_hat_i)^2 / (2*sigma_i^2))); Chi-Squared Q = (1/n) sum_i (y_i - y_hat_i)^2 / sigma_i^2, with Q~1 indicating well-calibrated uncertainty, Q>1 overconfidence, Q<1 underconfidence.", + "source": "Section 5, Appendix D" + }, + { + "id": "luno-D2-024", + "claim": "Input Perturbation Gaussian Noise: epsilon_{x,t} ~ N(0, sigma^2), additive perturbation applied to each input grid point. An ensemble of perturbed forward passes estimates predictive moments empirically. Baseline method calibrated on validation NLL.", + "source": "Section 5" + } + ], + "D3": [ + { + "id": "luno-D3-001", + "claim": "Low-Data UQ Evaluation Protocol: Evaluate predictive uncertainty quantification methods (LUNO vs baselines) on FNOs trained in a low-data regime with only 25 trajectories from each of 3 PDEs (Burgers, Hyper Diffusion, Kuramoto-Sivashinsky), testing whether linearized uncertainty (LUNO-LA) outperforms sample-based, ensemble, and input perturbation approaches. Metrics: RMSE, chi^2, marginal NLL on 250 test pairs. Calibration: grid search over 500 logarithmically spaced sigma^2 values minimizing validation NLL.", + "source": "Section 5, Table 1, Figure 2, Appendix D, Table 4, Table 5" + }, + { + "id": "luno-D3-002", + "claim": "OOD Robustness Evaluation Protocol: Assess robustness of UQ methods under distribution shift by training FNO on 1000 in-distribution advection-diffusion-reaction Base trajectories and testing on 4 OOD datasets (Flip: velocity reversed; Pos: triangular heat source; Pos-Neg: heat source + cloud-shaped sink; Pos-Neg-Flip: source + sink + velocity flip). Custom PDE solver with 9-point stencil and RK4, 100x100 grid, dt=5e-10, diffusion coefficient alpha=0.026. Primary metric: marginal NLL. Training: 1000 epochs on Base only, GGN computed on minibatch of 1000 pairs, all hyperparameters calibrated on 250 Base validation pairs.", + "source": "Section 5, Table 2, Figure 3, Appendix D, Tables 6-11" + }, + { + "id": "luno-D3-003", + "claim": "Autoregressive Rollout Evaluation Protocol: Evaluate UQ methods under full-trajectory autoregressive roll-out where predictions are recursively fed back as inputs (sliding window of 10 steps, 59 total time steps), causing accumulated errors and distribution shift over time. 50 rollout trajectories from each OOD dataset. Metrics: RMSE and marginal NLL per time step. Compares how uncertainty estimates adapt to accumulating errors.", + "source": "Section 5, Figure 4, Appendix D, Figure 7" + }, + { + "id": "luno-D3-004", + "claim": "Hyperparameter Calibration Protocol: Calibrate the variance hyperparameter sigma^2 for all UQ methods via grid search on validation NLL. For each method, search over 500 logarithmically spaced sigma^2 values, compute predictive distribution on 250 validation pairs, compute expected marginal NLL, select sigma^2 that minimizes NLL, fix for final test evaluation.", + "source": "Section 5" + }, + { + "id": "luno-D3-005", + "claim": "Runtime Comparison Protocol: Compare computational efficiency of linearization-based (LUNO) vs sampling-based (Sample) UQ methods for iterative single-trajectory autoregressive rollout. Measure wall-clock time per trajectory rollout with mean and standard deviation. Implementation in JAX leveraging Jacobian-vector products and analytical IFFT Jacobian structure for LUNO methods.", + "source": "Section 5, Appendix D, Table 12" + }, + { + "id": "luno-D3-006", + "claim": "FNO Training Protocol: Train Fourier Neural Operators on PDE trajectory data for next-step prediction. Architecture: 4 Fourier blocks, 12 modes, 18 hidden dimensions. Input: 10 consecutive time steps + velocity field + reaction term glued and zero-padded by 2 grid points. Optimizer: AdamW with cosine decay LR schedule and warmup. Loss: MSE. Low-data: 100 epochs (one pass per trajectory pair); OOD: 1000 epochs. Framework: JAX + Flax NNX + optax. Trained weights w* saved as MAP estimate.", + "source": "Section 2.1, Example 2.1, Section 5" + } + ], + "D4": [ + { + "id": "luno-D4-001", + "claim": "Low-Data UQ Evaluation Step Sequence: (1) Generate PDE datasets via APEBench (25 train + 250 valid + 250 test trajectories per PDE); (2) Train FNO for 100 epochs using AdamW+cosine decay on 25 trajectories; (3) Compute MAP estimate w* as trained weights; (4) For Laplace methods: compute low-rank GGN approximation (rank 500) using all training pairs; (5) For LUNO methods: linearize FNO around w*, push weight-space Gaussian to function-space GP; (6) For Sample methods: draw 200 weight samples, push through nonlinear FNO, estimate moments; (7) For Ensemble: train 10 FNOs independently, aggregate predictions; (8) Calibrate sigma^2 via grid search on validation NLL; (9) Evaluate RMSE, chi^2, NLL on 250 test pairs; (10) Visualize predictive mean, standard deviation, samples, eigenfunctions, and covariance.", + "source": "Section 5, Table 1, Figure 2, Appendix D" + }, + { + "id": "luno-D4-002", + "claim": "OOD Robustness Evaluation Step Sequence: (1) Generate 5 advection-diffusion-reaction datasets with custom PDE solver (RK4 + 9-point stencil); (2) Train FNO for 1000 epochs on 1000 Base trajectories; (3) Compute Laplace posterior: low-rank GGN (rank 500) on minibatch of 1000 pairs; (4) Calibrate all UQ method hyperparameters on 250 Base validation pairs via NLL grid search; (5) Evaluate all methods on 250 test pairs from each OOD dataset (Base, Flip, Pos, Pos-Neg, Pos-Neg-Flip); (6) Compute RMSE, chi^2, NLL per method per dataset; (7) Compare ensemble null-space residual projection vs LUNO full-rank covariance.", + "source": "Section 5, Table 2, Figure 3, Appendix D" + }, + { + "id": "luno-D4-003", + "claim": "Autoregressive Rollout Evaluation Step Sequence: (1) Take trained FNO and calibrated UQ methods from low-data or OOD experiments; (2) For each method, run autoregressive rollout on 50 test trajectories; (3) At each step, feed model prediction back as input (sliding window of 10); (4) Compute RMSE and NLL per time step; (5) Plot performance degradation over rollout horizon; (6) Compare how uncertainty estimates adapt to accumulating errors.", + "source": "Section 5, Figure 4, Figure 7, Appendix D" + }, + { + "id": "luno-D4-004", + "claim": "Hyperparameter Calibration Step Sequence: (1) For each UQ method, define a logarithmically spaced grid of 500 sigma^2 values; (2) For each candidate sigma^2, compute predictive distribution on 250 validation pairs; (3) Compute expected marginal NLL; (4) Select sigma^2 that minimizes NLL; (5) Fix calibrated sigma^2 for final test evaluation.", + "source": "Section 5" + }, + { + "id": "luno-D4-005", + "claim": "Runtime Comparison Step Sequence: (1) Load trained FNO and calibrated UQ methods; (2) For each method, run full autoregressive rollout on one trajectory; (3) Measure wall-clock time; (4) Repeat for statistical robustness; (5) Report mean +/- standard deviation.", + "source": "Section 5, Table 12, Appendix D" + }, + { + "id": "luno-D4-006", + "claim": "FNO Training Step Sequence: (1) Initialize FNO with 4 Fourier blocks (12 modes, 18 hidden dims); (2) Prepare training batches: 10 input time steps, predict next step; (3) Glue velocity field and reaction term to input; (4) Pad input by 2 constant zero grid points to reduce boundary artifacts; (5) Train with AdamW + cosine decay LR schedule with warmup; (6) Minimize MSE loss over specified epochs; (7) Save trained weights w* as MAP estimate.", + "source": "Section 2.1, Example 2.1, Section 5" + } + ] +} \ No newline at end of file diff --git a/papers/ma-rlhf/blacklist.txt b/papers/ma-rlhf/blacklist.txt new file mode 100644 index 0000000000000000000000000000000000000000..85e376bf41971430dadc26cd9ef14d8745cf16e0 --- /dev/null +++ b/papers/ma-rlhf/blacklist.txt @@ -0,0 +1,2 @@ +# Official repository (ICLR 2025, Baidu) +https://github.com/ernie-research/MA-RLHF diff --git a/papers/ma-rlhf/config.yaml b/papers/ma-rlhf/config.yaml new file mode 100644 index 0000000000000000000000000000000000000000..b06e5751f7effb6b33754e424c2879683d0ea158 --- /dev/null +++ b/papers/ma-rlhf/config.yaml @@ -0,0 +1,8 @@ +title: "MA-RLHF: RL from Human Feedback with Macro Actions" +pdf_url: "https://arxiv.org/pdf/2410.02743.pdf" +venue: "ICLR 2025" +year: "2025" +extra: + selection_index: 18 + domain: "Reinforcement Learning" + paradigm: "Incremental Improvement" diff --git a/papers/ma-rlhf/images/figures/ma-rlhf-fig-0001.jpg b/papers/ma-rlhf/images/figures/ma-rlhf-fig-0001.jpg new file mode 100644 index 0000000000000000000000000000000000000000..0b35637592a6a7ef7a752277610ea10f0aa94b2f --- /dev/null +++ b/papers/ma-rlhf/images/figures/ma-rlhf-fig-0001.jpg @@ -0,0 +1,3 @@ +version https://git-lfs.github.com/spec/v1 +oid sha256:67cb700ba0d78b464266da98c32e4c62a2952ff56fb45ae8d872a8309e77ed96 +size 38049 diff --git a/papers/ma-rlhf/images/figures/ma-rlhf-fig-0002.jpg 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+++ b/papers/ma-rlhf/images/tables/ma-rlhf-table-0015.jpg @@ -0,0 +1,3 @@ +version https://git-lfs.github.com/spec/v1 +oid sha256:1da1298255b61dade2e2f5456eb2a36a50d077f7e3e0ba5a07f86685a8439ab5 +size 417699 diff --git a/papers/ma-rlhf/paper.md b/papers/ma-rlhf/paper.md new file mode 100644 index 0000000000000000000000000000000000000000..a16a5a61e2481c2ed965ce5a1ff195f952f12508 --- /dev/null +++ b/papers/ma-rlhf/paper.md @@ -0,0 +1,696 @@ +# MA-RLHF: REINFORCEMENT LEARNING FROM HU-MAN FEEDBACK WITH MACRO ACTIONS + +Yekun Chai∗ Haoran Sun∗ Huang Fang Shuohuan Wang Yu Sun Hua Wu Baidu Inc. + +{chaiyekun,fanghuang,wangshuohuan}@baidu.com sunhaoran0402@gmail.com + +# ABSTRACT + +Reinforcement learning from human feedback (RLHF) has demonstrated effectiveness in aligning large language models (LLMs) with human preferences. However, token-level RLHF suffers from the credit assignment problem over long sequences, where delayed rewards make it challenging for the model to discern which actions contributed to preferred outcomes. This hinders learning efficiency and slows convergence. In this paper, we propose MA-RLHF, a simple yet effective RLHF framework that incorporates macro actions — sequences of tokens or higher-level language constructs — into the learning process. By operating at higher level of abstraction, our approach reduces the temporal distance between actions and rewards, facilitating faster and more accurate credit assignment. This results in more stable policy gradient estimates and enhances learning efficiency within each episode, all without increasing computational complexity during training or inference. We validate our approach through extensive experiments across various model sizes and tasks, including text summarization, dialogue generation, question answering, and program synthesis. Our method achieves substantial performance improvements over standard RLHF, with performance gains of up to $30 \%$ in text summarization and code generation, $18 \%$ in dialogue, and $8 \%$ in question answering tasks. Notably, our approach reaches parity with vanilla RLHF $1 . 7 \sim 2$ times faster in terms of training time and continues to outperform it with further training. We make our code and data publicly available at https://github.com/ernie-research/MA-RLHF. + +# 1 INTRODUCTION + +Recent advancements in large language models (LLMs) have revolutionized natural language processing tasks, demonstrating impressive capabilities across a wide range of applications such as code generation (Roziere et al., 2023; Chai et al., 2023; Lozhkov et al., 2024), mathematical reasoning (Lewkowycz et al., 2022; Anil et al., 2023), and dialogue assistance (OpenAI, 2023; Team et al., 2023; Anthropic). Despite these successes, aligning LLMs with human values and preferences remains a critical challenge. Reinforcement learning from human feedback (RLHF) has emerged as a promising approach to address this alignment issue by incorporating human evaluations into the training process (Christiano et al., 2017; Ziegler et al., 2019; Stiennon et al., 2020). + +Existing RLHF (Ouyang et al., 2022; Bai et al., 2022; Askell et al., 2021) methods mainly optimize decisions at the level of individual tokens, and require to process a vast number of minute adjustments. However, this fine-grained training paradigm can lead to the credit assignment problem (Kaelbling et al., 1996; Pang et al., 2019; Machado et al., 2023b; Pignatelli et al., 2023), particularly when dealing with long-distance dependencies. As LLM agents attempt to optimize decisions across extensive sequences, the difficulty in attributing the credits of actions to specific tokens complicates the reinforcement learning (RL) process (Pignatelli et al., 2024). Moreover, the use of subword tokenization, such as Byte-Pair Encoding (Sennrich et al., 2016), often splits words into smaller pieces. For instance, OpenAI’s ChatGPT1 treats each token as three quarters of a word on average, resulting in sequences that are $33 \%$ longer than word counts (OpenAI, 2024) and further exacerbates the credit assignment problem. + +Additionally, standard RLHF methods may overlook essential local co-occurrence patterns or inherent structures between adjacent tokens in natural language. For example, consider the phrase Big $\mathtt { A p p l e } ^ { 2 }$ , treating $\mathtt { B i g }$ and $\mathtt { A p p l e }$ as isolated decisions misses the cohesive meaning of the term, which actually refers to the “New York City”. The token-level granularity of natural language can hinder the agent’s ability to capture high-level language constructs in RL optimization, as some sequences are better understood when evaluated holistically. + +To address these challenges, we propose a new framework called macro-action RLHF (MA-RLHF) that incorporate macro action — sequences of tokens or high-level language constructs — into the RLHF framework. The concept of macro actions, has been explored in the literature of planning (Iba, 1989; Korf, 1985; Sacerdoti, 1974) and reinforcement learning (Thrun & Schwartz, 1994; Precup et al., 1997; Hauskrecht et al., 2013), simplifies decision-making by operating at high levels of temporal abstraction under the framework of semi-Markov Decision Processes (SMDPs) (Sutton et al., 1999b). Macro actions leverage temporal abstraction by chunking the sequences and reducing the decision resolution, enabling the agent to learn from “long-sighted” macro-level actions instead of “short-sighted” token-level actions. This can potentially lead to improved learning efficiency and scalability. Alternatively, MA-RLHF can also be interpreted from the perspective of reversing tokenization; MA-RLHF serves as a de-tokenization process to reconstruct high-level language units from subword pieces. By merging tokens into macro actions, we reduce the number of decision points and shorten decision trajectories, alleviating the credit assignment problem caused by long temporal distances. + +To conclude, our main contributions are as follows: + +• We propose MA-RLHF, a simple yet effective RLHF framework that integrates the macro actions into RLHF to align LLMs with human preference. We demonstrate the effectiveness of our approach through extensive experiments across various datasets and tasks, including text summarization, dialogue generation, question answering, and code generation. +• We show that MA-RLHF achieves $1 . 7 \times$ to $2 \times$ faster learning efficiency in reward scores during training compared to the standard token-level RLHF, without introducing additional computational costs during training or inference. MA-RLHF also exhibits strong scalability across model sizes ranging from 2B to 27B parameters. +• Our analysis reveals that MA-RLHF exhibits robust generalization capabilities under varying experimental settings, such as temperature values and rejection sampling, consistently outperforms the standard RLHF approaches. + +# 2 PRELIMINARIES + +We introduce some basic concepts and notations used in RL and RLHF. + +# 2.1 REINFORCEMENT LEARNING AND POLICY OPTIMIZATION + +Problem Definition RL addresses the problem of finding a policy to make optimal sequential decisions in environments modeled as a Markov Decision Process (MDP) (Sutton & Barto, 1999). An MDP is defined by the tuple $( S , \mathcal { A } , P , r , \rho _ { 0 } , \gamma )$ , where $s$ denotes a finite set of states, $\mathcal { A }$ is a finite set of actions, $P : \mathcal { S } \times \mathcal { A } \times \mathcal { S } [ 0 , 1 ]$ represents the state transition probability distribution, $r : \mathcal { S } \times \mathcal { A } \mathbb { R }$ is the reward function, ${ \rho _ { 0 } } : \mathcal { S } [ 0 , 1 ]$ defines the initial state distribution, and $\gamma \in ( 0 , 1 )$ is the discount factor that determines the importance of future rewards. + +Given a trajectory $( s _ { 0 } , a _ { 0 } , s _ { 1 } , a _ { 1 } , \cdot \cdot \cdot )$ , a reward $r _ { t } ~ = ~ r ( s _ { t } , a _ { t } )$ is received at each time $t$ . The state-action value function $\begin{array} { r } { Q _ { \pi } ( s _ { t } , a _ { t } ) = \mathbb E _ { s _ { t + 1 } , a _ { t + 1 } , \dots } \left[ \sum _ { l = 0 } ^ { \infty } \gamma ^ { l } r _ { t + l } \right] } \end{array}$ measures the expected return of taking action $a _ { t }$ at state $s _ { t }$ and following policy $\pi$ thereafter. The value function $V _ { \pi } ( s _ { t } ) =$ $\begin{array} { r } { \mathbb { E } _ { a _ { t } , s _ { t + 1 } , \dots } \left[ \sum _ { l = 0 } ^ { \infty } \gamma ^ { l } r _ { t + l } \right] } \end{array}$ estimates the expected return from state $s _ { t }$ under the policy $\pi$ . The advantage function $A _ { \pi } ( s _ { t } , \bar { a } _ { t } ) = Q _ { \pi } ( s _ { t } , a _ { t } ) - V _ { \pi } ( s _ { t } )$ reflects the relative value of taking action $a _ { t }$ at state $s _ { t }$ compared to the average value of the state. + +The goal of RL is to find an optimal ppected cumulative discounted reward: $\pi _ { \theta } ( a \mid s )$ d by , whe $\theta$ ,e zes the ex- represents $J ( \theta ) = \mathbb { E } _ { s _ { 0 } , a _ { 0 } , \dots } \left[ \sum _ { t = 0 } ^ { \infty } \gamma ^ { t } r _ { t } \right]$ $s _ { 0 } \sim \rho _ { 0 } ( s _ { 0 } )$ the initial state distribution, $a _ { t } \sim \pi _ { \theta } ( a _ { t } \mid s _ { t } )$ denotes the action selection based on the policy, and $s _ { t + 1 } \sim P ( s _ { t + 1 } \mid s _ { t } , a _ { t } )$ specifies the state transition dynamics. + +Proximal Policy Optimization Policy gradient methods are a common approach for optimizing policies by estimating the gradient of a performance objective with respect to the policy parameters $\theta$ . The policy gradient is given by: $\begin{array} { r } { \nabla _ { \theta } \bar { J ( \theta ) } = \mathbb { E } \left[ \sum _ { t = 0 } ^ { \infty } \mathsf { \bar { A } } _ { t } \nabla _ { \theta } \log \pi _ { \theta } ( a _ { t } \cdot \vert s _ { t } ) \right] } \end{array}$ , where the expectation $\mathbb { E } [ \cdot ]$ is taken over the randomness of the initial state, policy, and state-transition. The policy gradient guides us how to adjust the policy parameters to improve the expected return. Among the family of policy gradient methods, Proximal Policy Optimization (Schulman et al., 2017, PPO) is perhaps the most widely-used one due to its simplicity and empirical effectiveness. PPO simplifies TRPO (Schulman et al., 2015) by using a clipped surrogate objective function to penalize large deviations from the old policy, thereby ensuring more stable updates. Specifically, PPO introduces a clipped objective function: + +$$ +J ^ { \mathrm { p p o - c l i p } } ( \theta ) = \mathbb { E } _ { t } [ \operatorname* { m i n } ( \frac { \pi _ { \theta } ( a _ { t } | s _ { t } ) } { \pi _ { \theta _ { \mathrm { o l d } } } ( a _ { t } | s _ { t } ) } A _ { t } , \mathrm { c l i p } ( \frac { \pi _ { \theta } ( a _ { t } | s _ { t } ) } { \pi _ { \theta _ { \mathrm { o l d } } } ( a _ { t } | s _ { t } ) } , 1 - \epsilon , 1 + \epsilon ) A _ { t } ) ] , +$$ + +where $\epsilon$ is a hyperparameter that defines the range for clipping. The expectation $\mathbb { E } _ { t } [ \dots ]$ indicates the empirical average over a finite batch of samples. Nowadays, PPO usually comes as the first choice for RL practitioners. + +# 2.2 RLHF FOR HUMAN ALIGNMENT + +The post-training of LLMs (Stiennon et al., 2020; Ouyang et al., 2022) is a multi-stage training paradigm to align LLMs with human preferences. Post-training typically involves three stages: + +(1) Supervised Fine-Tuning (SFT) stage: A pre-trained language model (LM) is fine-tuned on a dataset of human demonstrations, learning to generate responses that align with human instructions and preferences. + +(2) Reward Modeling (RM) stage: A reward model is trained on a labeled preference dataset $\mathcal { D } =$ $\left( x _ { i } , y _ { i } ^ { + } , y _ { i } ^ { - } \right) _ { i = 1 } ^ { N }$ , consisting of prompts $x _ { i }$ and pairs of responses $( y _ { i } ^ { + } , y _ { i } ^ { - } )$ , where $y _ { i } ^ { + }$ is preferred over $y _ { i } ^ { - }$ by human annotators. The reward model $r _ { \phi } ( x , y )$ , parameterized by $\phi$ , is trained using the ranking loss: $\mathcal { L } _ { \mathrm { R M } } = - \log \sigma ( \log ( r _ { \phi } ( x , y _ { + } ) - r _ { \phi } ( \dot { x } , \dot { y } _ { - } ) \big ) )$ , where $\sigma$ denotes the sigmoid function. + +(3) RLHF stage: The RL fine-tuning utilizes the RM to provide feedback on the generated outputs, optimizing the policy using RL methods such as PPO. The reward signal is modified by incorporating a Kullback-Leibler (KL) divergence penalty to balance the exploration of new policies with adherence to the SFT model. The reshaped reward is defined as: + +$$ +R ( x , y ) = r _ { \phi } ( x , y ) - \beta D _ { \mathrm { K L } } ( \pi _ { \theta } ( \cdot \mid x ) \parallel \pi _ { \mathrm { s f t } } ( \cdot \mid x ) ) , +$$ + +where $\pi _ { \theta }$ represents the policy learned through RL, $\pi _ { \mathrm { s f t } }$ is the policy produced from the SFT stage, and $\beta > 0$ is a hyperparameter that controls the strength of the KL penalty. + +In the RLHF stage, the PPO algorithm, as detailed in Equation (1), is employed to optimize the RL policy. In the context of RLHF, we denote the state $s _ { t } = \{ s _ { 0 } , a _ { 0 } , a _ { 1 } , . . . , a _ { t - 1 } \}$ as the sequence of tokens generated up to time step $t$ , while $s _ { 0 }$ represents the initial states, i.e., the prompt, and $a _ { t }$ represents the token selected at the $t { \cdot }$ -th position. + +# 3 MARCO-ACTION RLHF + +# 3.1 REVISITING MACRO ACTIONS (OPTIONS) + +Macro actions, also referred to as options (Sutton et al., 1999b), are high-level constructs that encapsulate a sequence of primitive actions (i.e., subword tokens); by its definition, macro actions allows an agent to operate at a coarser temporal scale. + +Formally, a macro action is characterized by three components: (1) a policy $\pi : \mathcal { S \times A } \to [ 0 , 1 ]$ which guides the action selection among actions; (2) a termination condition $\zeta : S ^ { + } [ 0 , 1 ]$ , which determines where the macro action should end; (3) a initiation set $\mathcal { T } \subseteq S$ , which is a subset of states that macro actions can begin with. Once initiated with a state $s _ { 0 } \in \mathcal { I }$ , the macro action follows policy $\pi$ until it reaches the termination condition according to $\zeta$ . Intuitively, the use of carefully designed macro actions can extend decision-making temporally, it allows the agent to avoid “short-sighted” token-level decisions and encourage “long-sighted” macro-level decisions, thereby simplifies the decision-making process and potentially enhances learning efficiency. + +![](images/figures/ma-rlhf-fig-0001.jpg) +Figure 1: Illustration of the MA-RLHF optimization framework. Standard RLHF makes decisions and evaluates value scores at the token level, while MA-RLHF makes decisions over sequences of tokens at a coarser temporal scale. + +# 3.2 RLHF WITH MACRO ACTIONS + +We describe how we integrate macro-actions into the existing RLHF framework, the resulting framework is named as macro-action RLHF (MA-RLHF). + +# 3.2.1 FORMALIZATION OF MACRO ACTIONS + +We denote macro actions as $\omega _ { 1 } , \omega _ { 2 } , \ldots , \omega _ { \tau }$ . In the context of LLMs, a macro action $\omega _ { \tau }$ consists of a sequence of consecutive tokens, i.e., $\omega _ { \tau } = \{ a _ { t _ { \tau } } , a _ { t _ { \tau } + 1 } , \ldots , a _ { t _ { \tau + 1 } - 1 } \}$ , where $t _ { \tau }$ is the starting index of the $\tau { \cdot }$ -th macro action. We let $\left| \omega _ { \tau } \right|$ denotes the number of primitive actions that $\omega _ { \tau }$ contains. Unless otherwise specified, we use $\tau$ to index macro actions/states and use $t$ to index primitive actions/states. + +As mentioned in $\ S 3 . 1$ , macro actions are defined by the policy model, the termination condition and the initiation set. In MA-RLHF, we set the policy model the same as the standard token-level RLHF and let the initiation set to be any possible sequence of tokens. Therefore, the macro action used in MA-RLHF is decided solely by the termination condition, which plays a crucial rule in the MA-RLHF framework. We explore three termination conditions in this work: + +• $n$ -gram based termination: Following Vezhnevets et al. (2016), we find that $n$ -grams serve as a simple yet effective termination condition for macro actions, i.e., $| \omega _ { \tau } | = n$ , where $n$ represents the length of the $n$ -gram. We consider two variants of the $n$ -gram termination condition: (a) Fixed $n$ - gram: We group tokens into fixed-length $n$ -grams, simplifying the action space while maintaining common linguistic patterns. We empirically find fixed $n$ -gram macro action perform best and use it as the default setup. (b) Randomized $n$ -gram: We randomly select the length of a $n$ -gram from a predefined list of lengths $n \in \{ 2 , 3 , 5 , 1 0 \}$ to introduce variability, allowing the policy to adapt to different sequence lengths. + +• Parsing-based termination: $\omega _ { \tau }$ is derived from syntactic or semantic parsing of the input text, aligning macro actions with grammatical structures like phrases or clauses. Concretely, we traverse the constituent tree of the entire sequence using depth-first search (DFS), expanding nonterminal nodes until current non-terminal state contains no more than a specified threshold of leaf tokens, set at $C = 5$ . + +• Perplexity-based (PPL) termination: Perplexity measures the likelihood of a sequence of tokens. Here, the perplexity of a macro action is proportional to the averaged entropy of the token within it, i.e., ppl(ωτ ) ∝ − 1|ωτ | Pa∈ωτ . A macro action terminates until it reaches a token that has negative impact on the perplexity of the macro action. Mathematically, we construct $\omega _ { \tau } = \big \{ a _ { t _ { \tau } } , \cdot \cdot . . , a _ { t _ { \tau + 1 } - 1 } \big \}$ such that $\mathrm { \bar { p p l } } ( \dot { \omega _ { \tau } } \cup a _ { t _ { \tau + 1 } } ) > \mathrm { p p l } ( \omega _ { \tau } )$ and $\mathrm { p p l } ( \{ a _ { t _ { \tau } } , \dot { \ldots } , a _ { i } \} ) \geq$ $\mathrm { p p l } ( \{ a _ { t _ { \tau } } , \dots , a _ { i + 1 } \} )$ for all $t _ { \tau } \leq i \leq t _ { \tau + 1 } - 2$ . + +After determining the macro action based on the termination condition, we apply the state value function and importance sampling at the macro level Equation (1). We provide the details of implementation in Appendix D.1. + +# 3.2.2 POLICY OPTIMIZATION WITH MACRO ACTIONS + +In MA-RLHF, we adapt the PPO algorithm for optimization, referred to as MA-PPO. In the context of LLMs, expanding the action space with additional macro actions/tokens results in re-architecting the LLM’s vocabulary and retraining the model, which is computationally prohibitive. Thus, we maintain the original action space as pretrained LLMs, which can be treated as “single-step” primitive options as noted in (Sutton et al., 1999b). The policy $\pi _ { \theta }$ still outputs probabilities over individual tokens, but for optimization, we consider the joint probability of the macro action: $\begin{array} { r } { \pi _ { \theta } ( \omega _ { \tau } \mid s _ { \tau } ) = \prod _ { t = t _ { \tau } } ^ { t _ { \tau + 1 } } \pi _ { \theta } ( a _ { t } \bar { \mathbf { \alpha } } \vert a _ { < t } ) } \end{array}$ . The macro reward for executing the macro action $\omega _ { \tau }$ at the macro tim e step $\tau$ τis defined as: $\begin{array} { r } { R _ { \tau } = \mathbb { E } \big [ \sum _ { i = 0 } ^ { | \omega _ { \tau } | - 1 } \rho ^ { i } r _ { t _ { \tau } + i } \ \big | \ s _ { \tau } \big ] } \end{array}$ , where $r _ { t }$ is the reward received at $t$ , and we set the discount factor $\rho = 1$ in our experiments. + +Each macro action represents a contiguous sequence of tokens, and is treated as an option in the SMDP framework. The option-level value function with macro action is then estimated as: + +$$ +V ^ { \pi } ( s _ { \tau } , \omega _ { \tau } ) = \mathbb { E } \left[ R _ { \tau } + \gamma V ^ { \pi } ( s _ { t _ { \tau + 1 } } ) \ \middle | \ s _ { \tau } , \omega _ { \tau } \right] , +$$ + +where $\gamma$ is the discount factor for future rewards beyond the macro action. + +The advantage function $A _ { \pi } ( s _ { \tau } , \omega _ { \tau } )$ in MA-PPO determines how much the chosen macro action outperforms the average, which is defined as $A _ { \pi } ( s _ { \tau } , \omega _ { \tau } ) = Q _ { \pi } ( s _ { \tau } , \omega _ { \tau } ) - V ^ { \pi } ( s _ { \tau } )$ . Similar to the definition stated in $\ S 2$ , $Q _ { \pi } ( s _ { \tau } , \omega _ { \tau } )$ is the expected return conditioned on executing $\omega _ { \tau }$ at state $s _ { \tau }$ , which is calculated by summing the immediate macro rewards from the macro action with the discounted value of the subsequent state. + +In MA-PPO, the objective function is adapted for MA-level evaluation. The policy gradient is computed based on the advantage of the MA sequences: + +$$ +\mathcal { L } ^ { \mathrm { M A . P P 0 } } ( \theta ) = \mathbb { E } _ { \tau } \left[ \operatorname* { m i n } \left( \frac { \pi _ { \theta } \bigl ( \omega _ { \tau } \mid s _ { \tau } \bigr ) } { \pi _ { \theta _ { \mathrm { o d } } } \bigl ( \omega _ { \tau } \mid s _ { \tau } \bigr ) } \hat { A } _ { \tau } , \mathrm { c l i p } \left( \frac { \pi _ { \theta } \bigl ( \omega _ { \tau } \mid s _ { \tau } \bigr ) } { \pi _ { \theta _ { \mathrm { o d } } } \bigl ( \omega _ { \tau } \mid s _ { \tau } \bigr ) } , 1 - \epsilon , 1 + \epsilon \right) \hat { A } _ { \tau } \right) \right] , +$$ + +where $\hat { A } _ { \tau }$ is the estimated advantage at macro time step $\tau$ , $\epsilon$ is a constant that defines the range for clipping, and $\pi _ { \theta _ { \mathrm { o l d } } }$ is the policy before the update. + +# 3.2.3 CONNECTION TO PREVIOUS METHODS + +MA-RLHF builds on and generalizes prior work in the RLHF literature by varying the length of macro actions. When the macro action length is set to 1, MA-RLHF reduces to the standard tokenlevel RLHF (Stiennon et al., 2020; Ouyang et al., 2022), operating as an MDP. Conversely, if we allow $| \omega _ { \tau } | \infty$ , then MA-RLHF converges toward methods like RLOO (Ahmadian et al., 2024), REINFORCE (Williams, 1992; Sutton et al., 1999a), and GRPO (Shao et al., 2024), approximating a contextual bandit problem where decisions are made based on the entire sequence context. By varying the length of macro actions $\left| \omega _ { \tau } \right|$ , MA-RLHF provides a flexible framework that balances the granularity of action decisions. We provide further analysis on the impact of $\left| \omega _ { \tau } \right|$ in $\ S 4 . 3$ . + +# 4 EXPERIMENTS + +# 4.1 EXPERIMENTAL SETTINGS + +Tasks and Datasets We evaluate MA-RLHF on three different datasets for open-ended generation tasks: TL;DR (Stiennon et al., 2020) dataset for text summarization, Anthropic Helpful and Harmless (HH-RLHF) (Bai et al., 2022) for dialogue generation3, and WebGPT Comparison (Nakano et al., 2021) for question answering. Additionally, we evaluate MA-RLHF on code generation using the APPS (Hendrycks et al., 2021) dataset. More details can be found in Appendix B.1. + +![](images/figures/ma-rlhf-fig-0002.jpg) +Figure 2: Test RM scores of Gemma-2B and Gemma-7B models on the TL;DR dataset. The shaded regions represent the standard deviation on test RM scores across training runs. + +![](images/figures/ma-rlhf-fig-0003.jpg) +Figure 3: RM score distribution for PPO and MA-PPO (2B) at final steps (4.6k) on TL;DR. + +![](images/figures/ma-rlhf-fig-0004.jpg) + +![](images/figures/ma-rlhf-fig-0005.jpg) +Figure 4: Win rates of MA-PPO against vanilla PPO on TL;DR (left), HH-RLHF (middle) and WebGPT Comparisons (right), estimated by GPT-4 and Human. + +Base Models and Training Details For open-ended generation tasks, we use pre-trained Gemma-2B (Team et al., 2024) as our base model; we further adopt Gemma-7B and Gemma-2-27B to test the scaling trend. For the program synthesis task, we use CodeGemma-1.1-2B and CodeGemma-1.1-7B-it as our base models. The data split for SFT / RM / PPO and the hyperparameters used in SFT / RM / PPO stages are detailed in Appendix B.2. The implementation details of MA-PPO can be found in Appendix E. + +Evaluation For open-ended generation tasks, our evaluation metrics includes RM scores, GPT-4 pairwise evaluation, and human pairwise evaluation. To compute the RM score, we randomly sample 2k validation instances for the TL;DR and HH-RLHF datasets and use the default validation set of the WebGPT dataset. For GPT-4 and human evaluations, we simulate the win-rate on 50 instances that are drawn from the instances used in the RM evaluation. The GPT-4 and human evaluations are based on task-specific criterion: relevance, coherence, consistency, and fluency for TL;DR; helpfulness for HH-RLHF; factual accuracy, coherence, and usefulness for WebGPT. We followed prior studies (Askell et al., 2021; Zheng et al., 2024) by randomizing the order of responses during evaluation to mitigating potential evaluation biases. The prompts used by the GPT-4 evaluation are placed in Appendix F.1, and the annotation rules used for human evaluation are given in Appendix F.2. For the program synthesis task, we utilize pass $@ 1$ and pass $\textcircled { a } 5$ metrics to assess the performance of the model, evaluated on the provided $5 \mathrm { k }$ test set. + +# 4.2 MAIN RESULTS + +In this section, we present the main results of applying MA-PPO across three key tasks: summarization, dialogue, and question answering. The main takeaway is that MA-PPO consistently outperforms vanilla PPO in terms of both training efficiency and generation quality; MA-PPO obtains a significant improvement in testing reward model scores and human/GPT-4 evaluation win rates. + +TL;DR Summarization For the TL;DR summarization task, MA-PPO shows a marked improvement over vanilla PPO. As shown in Figure 2, MA-PPO achieves parity with vanilla PPO approximately $1 . 7 - 2$ times faster during training. Specifically, Gemma-2B trained with 1.7k MA-PPO updates reaches similar testing RM scores obtained by vanilla PPO trained with $3 . 7 \mathrm { k }$ steps. We also find similar trends when scaling up the parameter sizes to 7B, demonstrating the generalized capability of MA-PPO on model sizes. + +Moreover, Figure 3 highlights the distribution of RM scores, where MA-PPO consistently shifts towards higher RM sores compared to vanilla PPO. Further evaluation using GPT-4, given in the left figure of Figure 4, shows that MA-PPO achieves $78 \%$ and $86 \%$ win rate over vanilla PPO for the 2B and 7B models, respectively. Human evaluation gives similar results, where MA-PPO obtains win rates of $74 \%$ and $69 \%$ , further demonstrating the effectiveness of macro actions. The final testing RM scores of MA-PPO and vanilla PPO are given in Table 2. + +Table 1: Agreement among RM, GPT-4, and human evaluations on TL;DR. + +
#ParamRMGPT-4Human
RM GPT-42B100%--
78% 76%100%-
Human58%100%
RM7B100%--
GPT-478%100%-
Human74%64%100%
+ +Table 2: Test RM scores of vanilla PPO and MA-PPO on TL;DR, HH-RLHF, and WebGPT datasets. + +
ModelTL;DRHH-RLHFWebGPT
Vanilla PPO (2B)0.841.31-0.62
MA-PPO (2B)1.41+68%1.55+18%-0.60+3%
Vanilla PPO (7B)1.901.05-0.61
MA-PPO (7B)2.47+30%1.24+18%-0.56+8%
+ +![](images/figures/ma-rlhf-fig-0006.jpg) + +![](images/figures/ma-rlhf-fig-0007.jpg) +Figure 5: Performance of MA-PPO with various macro action termination strategies on the TL;DR dataset using Gemma-2B. Left: Test RM scores for different termination strategies. Right: GPT-4 evaluation across four dimensions – relevance, coherence, consistency, and fluency – comparing different MA termination methods. + +HH-RLHF Dialogue We use the HH-RLHF dataset to evaluate the helpfulness and harmlessness of single-turn dialogues. MA-PPO shows clear advantages over vanilla PPO, as depicted in the middle figure of Figure 4. GPT-4 evaluations show that MA-PPO yields a $72 \%$ win rate for the Gemma-7B model, compared to $58 \%$ for the Gemma-2B model. Human evaluation results align with these findings, with the win rate increasing from $52 \%$ to $56 \%$ as model size scales from 2B to 7B. The testing RM score of MA-PPO and vanilla PPO are presented in Table 2. These results highlight the scalability and effectiveness of MA-PPO in dialogue tasks. We refer to Appendix C.1 for detailed experimental results. + +WebGPT Comparisons We evaluate MA-PPO on the WebGPT Comparison dataset for questionanswering tasks. As shown in Figure 4 (Right), MA-PPO consistently outperforms vanilla PPO, with GPT-4 evaluations yielding a win rate of $64 \%$ for the Gemma-7B model. This result demonstrate the robustness of MA-PPO across different tasks, including more structured tasks like question answering. More experimental details refer to Appendix C.2. + +Validating Model-based Judgments with Human Evaluation We evaluate the reliability of our evaluation methods by calculating the agreement between the reward model, GPT-4, and human evaluators. Since GPT-4 and human evaluations are conducted pairwise, we determine the reward model’s win rate by selecting the summary with the higher RM score. The results, shown in Table 1, demonstrate that the reward model aligns more closely with both GPT-4 and human evaluations. Furthermore, the agreement between GPT-4 and human evaluators averaged $62 \%$ across models, reinforcing the consistency and validity of our evaluation framework. + +# 4.3 ANALYZING THE USE OF MACRO ACTIONS + +We study the performance of various termination strategies. Unless otherwise specified, we conduct our analysis on the TL;DR dataset. + +# 4.3.1 EXPLORING DIFFERENT STRATEGIES FOR MA TERMINATION (ζ) + +In MA-RLHF, the termination condition $( \zeta )$ for macro actions is critical as it determines when a macro action should conclude. We compare the performance of various termination strategies, particularly on reward maximization and linguistic coherence. The termination strategies studied in this section including fixed / randomized $n$ -gram-based, parsing-based, and perplexity-based termination, as aforementioned in $\ S 3 . 2 . 1$ ; please see Figure 12 for detailed illustration. + +Figure 5 illustrates the overall test-set performance on RM scores (Left) and GPT-4 evaluation scores (Right) with different MA termination strategies. All macro action termination strategies outperform the vanilla PPO approach, underscoring the importance of temporal abstraction in decision-making. Figure 5 (Left) shows that $n$ -gram based approach, both fixed and randomized, achieves the optimal results among others. Notably, randomized $n$ -gram-based termination performs the best across multiple dimensions, including relevance, coherence, and consistency, as shown in Figure 5 (Right). As expected, the perplexity-based termination enhances fluency, and is most suited for tasks that prioritize smooth and natural language generation. Furthermore, parsing-based termination shows promising ability to handle complex grammar, as it is designed to better capture linguistic structures. + +![](images/figures/ma-rlhf-fig-0008.jpg) +Figure 6: Test RM scores of different $n$ values in MA-PPO evaluated by corresponding RM on the TL;DR (left) and HH-RLHF (right) dataset. + +![](images/figures/ma-rlhf-fig-0009.jpg) +Figure 7: GPT-4 scores of vanilla PPO and MA-PPO with different $n$ values on TL;DR. + +![](images/figures/ma-rlhf-fig-0010.jpg) +Figure 8: The effect of temperature on RM scores for varying sample sizes (Best-of- $N$ ) across models. (Left): RM score of the SFT model under different temperatures and sample sizes. (Mid): RM score of vanilla PPO under the same settings. (Right): RM score of MA-PPO. + +# 4.3.2 ABLATION STUDY: VARYING $n$ IN MA-RLHF + +The $n$ -gram based macro action strategy in MA-RLHF uses a hyper-parameter $n$ to control the length of macro actions. Notably, when $n = 1$ , MA-PPO is equivalent to vanilla PPO, and treats the problem as a traditional Markov Decision Process (MDP), making decisions token by token. In contrast, setting $n \infty$ corresponds to the REINFORCE algorithm (McGovern & Sutton, 1998), where the entire sequence is treated as a single macro action, akin to a contextual bandit problem, as discussed in $\ S 3 . 2 . 3$ . For intermediate values of $n$ (i.e., $n \in ( 1 , \infty ) )$ ), MA-PPO falls under the SMDP framework, which allows for temporally extended actions; see $\ S 3$ . This continuum between MDPs and contextual bandits highlights the flexibility of the MA-RLHF approach in handling varying levels of temporal abstraction. + +RM Scores We conducted experiments with varying values of $n$ $( n \in \{ 3 , 5 , 1 0 , \infty \} )$ on the TL;DR and HH-RLHF datasets. Figure 6 shows that all values of $n$ lead to performance improvements over the vanilla PPO $( n = 1 )$ ), indicating the advantage of modeling sequences of tokens as macro actions. Notably, for the TL;DR dataset, $n = \infty$ yields the highest RM score, suggesting that treating the entire sequence as a macro action is particularly effective for the summarization task. For the HH-RLHF dataset, setting $n = 1 0$ gives the best performance, likely because this task benefits from moderate-length macro actions that can capture essential linguistic structures while maintaining sufficient granularity. + +GPT-4 Evaluation Analysis As shown in Figure 7, setting $n = 5$ strikes a good balance between relevance, coherence, consistency; it outperforms both smaller and larger values of $n$ . These findings align with the semi-MDP framework: increasing $n$ allows for better credit assignment and context retention, but excessive abstraction (e.g., $n = \infty$ ) sacrifices fine-grained control. Overall, moderate values of $n = 5$ and $n = 1 0$ provide the best trade-offs, highlighting the adaptability across tasks. + +# 4.4 GENERALIZATION PROBING IN MACRO ACTIONS + +Robustness on Rejection Sampling vs. Temperature Best-of- $N$ (a.k.a, rejection sampling) (Touvron et al., 2023) enhances response quality by selecting the highest-reward response from $N$ samples generated by the policy model. We compare MA-PPO, SFT, and vanilla PPO using the best-of- $N$ sampling across various temperatures $\hat { T ^ { + } } \in \{ 0 . 2 , 0 . 4 , 0 . 6 , 0 . 8 , 1 . 0 , 1 . 2 \}$ and sample sizes $N \in \{ 4 , 8 , 1 6 , 3 2 \}$ . As shown in Figure 8, best-of- $N$ sampling improves RM scores for all methods, with performance increasing as $N$ grows. We observe that SFT and vanilla PPO are sensitive to temperature variations, requiring specific adjustments to achieve optimal results. In contrast, MA-PPO demonstrates robustness in sampling temperature, it consistently delivers the best performance at $T = 1 . 2$ and shows consistent improvement across all tested temperatures. Moreover, MA-PPO maintains stable performance across varying temperature settings, as detailed in Appendix D.4, highlighting its robustness and generalization capabilities under different sampling temperatures. + +![](images/figures/ma-rlhf-fig-0011.jpg) +Figure 9: Evaluation results for vanilla PPO and MA-PPO on Gemma-2-27B using the TL;DR dataset. Left: RM scores on validation set. Mid: Distribution of RM scores for vanilla PPO and MA-PPO (27B) at final steps (4.6k). Right: Scaling trending on TL;DR dataset across 2B, 7B, and 27B model size, showing RM scores, GPT-4 evaluation, human evaluation results. + +![](images/figures/ma-rlhf-fig-0012.jpg) +Figure 10: RM score shifting pattern after RLHF training; Left: RM scores of best-of- $N$ $N = 8$ ) sampling compared to the SFT model. Mid Left: RM scores of vanilla PPO compared to the SFT model. Mid Right: RM scores of MA-PPO $( n = 5$ ) compared to the SFT model. Right: RM scores of MA-PPO $\ R = \infty$ ) compared to the SFT model. + +Scaling Trends up to 27B Models We evaluate the performance of MA-PPO across different model sizes, specifically Gemma-2B, 7B, and 27B. As demonstrated in Figure 9 (Left and Mid), MA-PPO consistently surpasses vanilla PPO, exhibiting higher RM scores throughout training. Figure 9 (Right) presents the scaling trend of MA-PPO across the 2B, 7B, and 27B models in terms of testing RM scores, GPT-4, and human evaluations. The experimental results underscore the scalability and robust performance of MA-PPO across varying model sizes. + +Analyzing the Impact on RM Score Distribution We evaluate the RM score distribution shift after applying RLHF using vanilla PPO and MA-PPO on the TL;DR dataset, with the SFT model serving as the baseline. To further contextualize the impact of RLHF, we include the Best-of- $N$ sampling $N = 8$ ) on the SFT model. As illustrated in Figure 10, Best-of- $N$ enhances overall response quality but falls short compared to RLHF. While vanilla PPO shifts the distribution towards higher RM scores, it leaves a significant number of low-quality, long-tailed instances. In contrast, MA-PPO demonstrates a more pronounced positive impact, effectively reduces the number of low-quality outliers and improves overall score distribution compared with the vanilla PPO. This highlights the robustness of MA-PPO in enhancing response quality through RLHF. + +# 4.5 ADDITIONAL ANALYSIS + +Impact on $L _ { 2 }$ -Norm of Advantage and Q Values We present the $L _ { 2 }$ -norm of both the advantage and Q-values for MA-PPO and vanilla PPO during training in Figure 11. The advantage function, which reflects the difference between the expected return (Q-value) and the baseline, is critical in guiding policy optimization. A lower $L _ { 2 }$ -norm of both the advantage and Q-values suggests more stable and less noisy policy updates, likely contributing to faster learning speed observed in $\ S 4 . 2$ . + +The powhere cy gradient for a sequence of length is the sequence reward provided b $T$ is given by: the RM. In t $\begin{array} { r } { \nabla _ { \theta } J = \mathbb E \big [ \sum _ { t = 1 } ^ { T } \nabla _ { \theta } \log \pi _ { \theta } ( a | s ) \cdot R \big ] } \end{array}$ $R$ $n$ actions, the sequence length is reduced by a factor of $n$ , shortening the decision horizon: $T \to T / n$ . This reduction in the number of actions, $T / n$ , where $n > 1$ , implies that the temporal distance between actions and corresponding rewards is decreased, thus reducing the variance in the gradient estimate and improving credit assignment. We refer readers to Mann & Mannor (2014) for the theoretical foundations of variance reduction through macro actions and their benefits in RL. + +Table 3: $\mathrm { P a s s } @ k$ $\langle k = \{ 1 , 5 \}$ ) metric evaluated on the APPS test set. + +
MethodCodeGemma-2BCodeGemma-7B
PPOMA-PPOPPOMA-PPO
pass@1Inter. Intro.2.82 15.263.25+15%4.266.22+46%
16.56+8%20.9026.74+28%
Comp. All0.92 4.920.94+2% 5.45+11%1.21 6.982.00+65% 9.48+35%
pass@5Inter. Intro.4.104.377%6.57
17.3018.30+6%23.308.37+27% 30.30+30%
2.30
Comp.1.70 6.261.60.6%3.30+43%
All6.60+5%9.0611.7430%
+ +![](images/figures/ma-rlhf-fig-0013.jpg) +Figure 11: $L _ { 2 }$ Norm of advantages and Q-values during training for MA-PPO and vanilla PPO. Left: $L _ { 2 }$ norm of advantages over training steps; Right: $L _ { 2 }$ norm of Q-values. + +Case Study We show some qualitative examples in Appendix G.1, demonstrating that MA-PPO can produce more coherent and contextually appropriate responses compared to vanilla PPO, capturing both short/long-term dependencies effectively. + +Extended Experiments: Code Generation We further assess the effectiveness of MA-PPO on the code generation task. Following Shojaee et al. (2023); Liu et al. (2023), we utilize the compiler signal as the final reward; see Appendix B.5 for implementation details. We compare the performance of MA-PPO and vanilla PPO using the pass $@ \mathbf { k }$ $\mathbf { k } { = } 1$ , 5) metric (Chen et al., 2021) on the 5k test set of the APPS dataset (Hendrycks et al., 2021). As shown in Table 3, MA-PPO significantly outperforms vanilla PPO in both pass $@ 1$ and pass $@$ 5 metrics, with more pronounced improvements as model size scales. Notably, for the 7B model, MA-PPO achieves an improvement of $+ 3 5 \%$ in pass $@ 1$ and $+ 3 0 \%$ in pass $\textcircled { \alpha } 5$ over vanilla PPO, demonstrating the effectiveness of our approach in code generation tasks. + +# 5 RELATED WORK + +LLM Alignment RLHF have shown impressive success in aligning LLMs with human preferences through multi-stage training, including SFT, RM, and RL fine-tuning (Ziegler et al., 2019; Stiennon et al., 2020; Ouyang et al., 2022; Sun et al., 2025). Recent research has explored optimization methods for RL in LLMs, employing both online (Ahmadian et al., 2024; Farebrother et al., 2024; Shen et al., 2024; Chakraborty et al., 2024; Shao et al., 2024) and offline RL algorithms (Snell et al., 2023; Hu et al., 2023; Yu et al., 2024) to address training instability, improve efficiency (Tang et al., 2024) and diversity (Sun et al., 2025). Improvements to RM learning have been proposed, such as parameter scaling (Gao et al., 2023), fine-grained reward (Wu et al., 2023), tool use (Li et al., 2024), and model merging (Rame et al. ´ , 2024; Rame et al., 2024). Alternatively, direct policy optimization (Rafailov et al., 2024; Ethayarajh et al., 2024; Gheshlaghi Azar et al., 2023; Rosset et al., 2024) has emerged as a promising approach, bypassing the instability of RL while directly aligning models to human preferences. In this paper, we enhance the RLHF action space by integrating macro actions, a well-established concept in RL (Sutton et al., 1999b; Mann & Mannor, 2014). + +Macro Action in RL Macro actions introduce temporal abstraction in RL by grouping sequences of primitive actions, reducing decision complexity and improving long-horizon credit assignment (Precup et al., 1997; Hauskrecht et al., 2013; Sutton et al., 1999b; Pignatelli et al., 2024; Machado et al., 2023a). This method has demonstrated its utility in speeding up convergence and stabilizing policy updates in various domains (Mann & Mannor, 2014; Solway et al., 2014). Our work applies macro actions to RLHF in LLM training, leveraging this structure to enhance scalability and optimize credit assignment over extended sequences. + +# 6 CONCLUSION AND FUTURE WORK + +In this paper, we introduced MA-RLHF, a novel framework that incorporates macro actions into RLHF to enhance the alignment of LLMs with human preferences. Our approach demonstrates consistent improvements across multiple tasks, including summarization, dialogue generation, question answering, and code generation. Notably, MA-RLHF achieves parity with vanilla RLHF $1 . 7 \mathrm { x }$ to $2 \mathbf { x }$ faster in reward scores without incurring additional computational overhead, showing robust scalability across model sizes ranging from 2B to 27B parameters. It is promising to explore MA-RLHF in complex step-by-step reasoning tasks for future research. + +# REPRODUCIBILITY STATEMENT + +We are committed to ensuring the reproducibility of the experiments presented in Section 4. To this end, we make the source code and model checkpoints publicly available at https://github. com/ernie-research/MA-RLHF. The detailed source code for training and evaluating both the conventional RLHF and our proposed MA-RLHF approach is included in the supplementary materials. 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Judging llm-as-a-judge with mt-bench and chatbot arena. Advances in Neural Information Processing Systems, 36, 2024. + +Daniel M Ziegler, Nisan Stiennon, Jeffrey Wu, Tom B Brown, Alec Radford, Dario Amodei, Paul Christiano, and Geoffrey Irving. Fine-tuning language models from human preferences. arXiv preprint arXiv:1909.08593, 2019. + +# A LIMITATIONS + +While our work demonstrates the effectiveness of MA-RLHF across multiple tasks, there are several limitations that leave room for future improvements. In our implementation, we apply the identical action / vocabulary space as pretrained LLMs, considering the fact that defining macro actions as one options (e.g., one macro action per $n$ -gram) would require re-architecting the LLM’s vocabulary and retraining the model, which is computationally infeasible. Meanwhile, our macro action termination methods are rule-based, including linguistics- or perplexity-driven approaches; future research could explore more complex or learnable termination strategies to further enhance performance. Furthermore, regarding the generalization of MA-RLHF, our experiments are conducted using models with up to 27B parameters; exploring more advanced models, such as LLaMA 3.1 405B (Dubey et al., 2024) or other state-of-the-art architectures and tasks (e.g., mathematical and complex reasoning), may provide additional insights into the scalability of MA-RLHF. Lastly, although we observe significant improvements in training efficiency, further investigation into the trade-offs between training stability and performance under diverse real-world conditions is necessary. Addressing these limitations will pave the way for more robust applications of MA-RLHF. + +# B EXPERIMENTAL DETAILS + +# B.1 DATASETS AND TASKS + +TL;DR Summarization In this task, the policy is asked to generate summarizations for Reddit posts. This dataset consists of $9 3 \mathrm { k }$ human-annotated preference pairs and $8 6 \mathrm { k }$ pairs for validation. The trainable pairs are derived from the Reddit TL;DR (Volske et al. ¨ , 2017) dataset. Additionally, a portion of the validation pairs is sourced from the CNN Daily Mails, which serves as the test set for out-of-distribution generalization. + +HH-RLHF With the Anthropic HH-RLHF dataset, the policy is asked to generate a helpful and harmless response given a single-turn dialogue or multi-turn dialogue. This dataset provides 112k preference-labeled instances for training, and $1 2 . 5 \mathrm { k }$ for validation. + +WebGPT Comparisons The WebGPT Comparisons dataset contains QA pairs from the ELI5 (Fan et al., 2019) and the TriviaQA (Joshi et al., 2017). The policy is responsible for information retrieval and response generation. In our experimental setup, we focus exclusively on the generation task. The policy must generate a response that balances factual accuracy and coherence. This dataset contains $1 9 . 6 \mathrm { k }$ instances for training. We split $5 \%$ instances for validation, as no separate validation set is provided. + +Code Generation For this task, we leverage the APPS dataset, which contains 5k training and $5 \mathrm { k }$ validation instances. The policy must write executable code based on a natural language described in the question, using Python as the target programming language. + +We present the data statistics in Table 4. + +Table 4: Statistics of datasets involved in experiments. The number of tokens are calculated with Gemma-2B tokenizer. + +
DatasetNum. of ComparisonsNum. of Train SamplesNum. of Test SamplesAvg. Tokens in PromptAvg. Tokens in ChosenAvg. Tokens in Rejected
Anthropic HH-RLHF127.5k112k12.5k1608375
OpenAI Summarization179k92.9k86.1k3253533
OpenAI WebGPT19.6k18.5k97949149137
AAPPS10k5k5k453203-
+ +# B.2 TRAINING DETAILS + +Following the procedure used by InstructGPT (Ouyang et al., 2022), we fine-tune both the SFT model and the reward model on the same dataset to avoid a distribution gap. We implement our training code with the Deepspeed-Chat package (Yao et al., 2023). + +SFT Training We split the dataset into three parts, allocating $20 \%$ of the data in the supervised finetuning stage. We use the prompts and the chosen sentences as the instruction data. For the TL;DR Summarize dataset, we concatenate the post and summarization following the approach of Stiennon et al. (2020). For the single-turn dialogue and the question answering dataset, we apply a humanassistant chat template to format the instructions. For the program synthesis dataset, we format the instruction data in line with Hendrycks et al. (2021). + +Reward Modeling In this stage, we use $40 \%$ of the data to train the reward model for each dataset, formatting the preference data the same way as in the SFT training stage. We initialize the reward model using the fine-tuned SFT model. Due to the lack of preference pairs in the program synthesis dataset, this stage is omitted for this task. + +PPO Training Similar to previous stages, the remaining $40 \%$ of the data is used to optimize the policy model. The SFT model initializes the policy model, and the reward model initializes the critic model. For the program synthesis dataset, $80 \%$ of the data is used in this stage, with both the policy and critic models initialized using the SFT model. The pass $@ 1$ metric serves as the reward signal for program synthesis, compensating for the absence of a reward model. While training 7B model on TL;DR dataset using MA-PPO, we encountered unstable training with a KL coefficient of 0.05. Reducing the coefficient to 0.01 for the 7B model led to more stable optimization. + +Table 5 lists the hyperparameters used across all training stages for each task. + +# B.3 NOTATIONS + +In Table 6, we present the notations used in our paper. + +# B.4 DETAILS OF MACRO ACTION TERMINATION + +The general form of the segmentation rule is thus $t _ { \tau + 1 } = t _ { \tau } + | \omega _ { \tau } |$ , where $\left| \omega _ { \tau } \right|$ is determined by the chosen criterion, such as $n$ -grams, random, parsing, or perplexity-based segmentation. + +1. Fixed $n$ -gram length: For all macro actions, we set $| \omega _ { \tau } | = n$ , where $n$ is a constant value. + +Table 5: Hyper-parameters for training Gemma series of models in MA-PPO and vanilla PPO. + +
Hyper-ParameterGemmaCodeGemma
2B7B27B2B7B
SFTBatch size64 for WebGPT 512 for others1281281632
Epochs35 for WebGPT 1 for others311
Learning rate1e-4 for WebGPT 5e-5 for others2e-55e-65e-62e-6
LR schedulercosinecosinecosinecosinecosine
Warmup ratio0.10.10.100
RMBatch size32 for WebGPT 64 for others128 for TL;DR 64 for HH-RLHF128-
Epochs132 for WebGPT 11
Learning rate2e-5 for WebGPT1e-68e-6-
LR scheduler1e-5 for others-
Warmup ratiocosine 0.1cosine 0.1cosine--
0.1--
PPOBatch size2562562561616
Policy learning rate1.5e-51e-67e-75e-75e-7
Critic learning rate1.5e-51e-61e-65e-55e-5
Epochs4 for WebGPT 1 for others4 for WebGPT111
PPO epochs11 for others 1111
Rollout11111
Clip ratio0.20.20.20.20.2
λ in GAE0.950.950.950.950.95
γ in GAE11111
KL coefficient0.050.1 for WebGPT 0.05 for others0.10.050.05
Max prompt length512512512600600
Max response length512512512512512
Warmup steps20020002020
Temperature0.80.80.81.01.0
Top-p1.01.01.01.01.0
Top-k50505055
+ +![](images/figures/ma-rlhf-fig-0014.jpg) +Figure 12: Illustration of four termination rules for macro actions in the MA-RLHF framework. Each termination rule outputs a list of $\left| \omega _ { \tau } \right|$ . In the parsing based termination, the macro action is determined when the token number of the current node is less than $C = 4$ , which is represented as a number in the tree node. + +2. Randomized $n$ -gram length: We define a list of $\{ | \omega _ { \tau } | \} = \{ 2 , 3 , 5 , 1 0 \}$ to model macro actions. This list is repeated multiple times to cover the length of the sample, in practice, we repeat this list 3 times. If the total length of macro actions can not match the number of tokens, a large number will be considered as an additional $\left| \omega _ { \tau } \right|$ to mitigate this gap, which is similar to the $| \bar { \boldsymbol { \omega } } _ { \tau } | = \infty$ . We shuffle the list and take this as a random-based length. + +3. Parsing-based length: We parse the response into a constituent tree and perform a depth-first search (DFS) to identify macro action length. Two rules guide the termination of $| \omega _ { \tau } |$ : (1) nodes with fewer than $C$ tokens mark the end of a macro action; (2) nodes with single token are included in the last macro action, avoiding single-token termination conditions like punctuation. Due to differences between the training and parsing tokenizers, we revert to the standard PPO method when discrepancies occur. We set the cut-off threshold $C = 5$ , providing optimal granularity in practice. + +Table 6: List of notation used in this paper. + +
Sym.Meaning
RL
S AA finite set of states.
PA finite set of actions. The state transition probability distribution.
rThe reward function.
ρ0 γThe initial state distribution.
πθ(a | s)The discount factor related with future rewards.
Policy parameterized by θ.
η(π)The expected cumulative discount reward.
aThe actions selected by the policy.
Qπ(st, at)The state-action value function.
V\(t)The state value function.
Aπ(st, at) GtThe advantage function.
The expected return.
RLHF
rφ(x, y)The reward model parameterized by φ.
xPrompt.
y+Chosen response.
yRejected response.
βKL coefficient.
ηThe range for clipping in PPO.
tTime step of tokens.
Macro Action
Termination condition.
CInitiation set.
τThe index of macro action/state/reward.
Macro action at time step T.
ωτ t\Time step of macro actions.
στThe weight used to measure the value of macro action.
+ +4. Perplexity-based length: Given a response $y$ generated by policy model, we calculate the perplexity $p _ { t }$ at any time step $t$ by treating $y { \le } t$ as the ground truth response. This process leverages the logits from the reference model, avoiding additional forward passes. Intuitively, selecting the macro actions based on perplexity $\mathcal { P } = \{ p _ { 0 } , p _ { 1 } , . . . , p _ { | y | } \}$ can be defined as selecting tokens which consistently attribute to the decrease of the perplexity given partial sentence. Mathematically, it can be represented as $\omega _ { \tau } ~ = ~ \{ a _ { t _ { \tau } } , a _ { t _ { \tau } + 1 } , \ldots , \bar { a _ { t _ { \tau } + | \omega _ { \tau } | - 1 } } \}$ where $\mathcal { P } _ { t _ { \tau } } = \{ p _ { t _ { \tau } } , p _ { t _ { \tau } + 1 } , \dots , p _ { t _ { \tau } + | \omega _ { \tau } | - 1 } \}$ exhibits a monotonic decreasing pattern. + +# B.5 TRAINING SETTINGS OF PROGRAM SYNTHESIS + +Defining the reward score solely based on the state “Accept” or “Wrong Answer” is somewhat restrictive, as some generated code may pass certain unit tests while failing others. These actions should also receive positive signals to encourage the policy to maximize the number of passed unit tests. To address this, we incorporate an adaptive compiler signal into the reward feedback as previ- + +![](images/figures/ma-rlhf-fig-0015.jpg) +Figure 13: Test RM scores evaluated by corresponding reward model of Gemma-2B and Gemma-7B model on HH-RLHF dataset. + +![](images/figures/ma-rlhf-fig-0016.jpg) +Figure 14: Distribution of test RM scores for vanilla PPO and MA-PPO (2B) at final steps (5.6k) on the HH-RLHF dataset. + +![](images/figures/ma-rlhf-fig-0017.jpg) +Figure 15: Test RM scores evaluated by corresponding reward model of Gemma-2B and Gemma-7B model on the WebGPT Comparisons dataset. + +![](images/figures/ma-rlhf-fig-0018.jpg) +Figure 16: Distribution of test RM scores for vanilla PPO and MA-PPO (2B) at final steps (3.2k) on WebGPT dataset. + +ous work (Shojaee et al., 2023; Liu et al., 2023): + +$$ +R ( x , y ) = \left\{ \begin{array} { l l } { - 0 . 3 + 1 . 3 \cdot \frac { N _ { \mathrm { p a s s } } } { N _ { \mathrm { p a s s } } + N _ { \mathrm { f a i l } } } , } & { \mathrm { i f ~ } y \mathrm { ~ s u c c e s s f u l l y ~ c o m p i l e d . } } \\ { - 0 . 6 , } & { \mathrm { i f ~ } y \mathrm { ~ r e c e i v e d ~ r u n t i m e ~ e r r o r . } } \\ { - 1 . 0 , } & { \mathrm { i f ~ } y \mathrm { ~ r e c e i v e d ~ c o m p i l e ~ r r r o r . } } \end{array} \right. +$$ + +where $x$ represents the prompt, and $y$ represents the code snippet generated by the policy model. + +# C ADDITIONAL EXPERIMENTS RESULTS + +# C.1 RESULTS OF DIALOGUE GENERATION + +In Figure 13, we demonstrate the RM scores on the validation set of vanilla PPO and MA-PPO. It shows that MA-PPO surpasses vanilla PPO under RM evaluation, MA-PPO achieves parity performance at 3100 step and 2600 step for 2B and 7B models, respectively, while vanilla PPO at 5100 step and 5400 step. Generally, MA-PPO is 1.6-2x faster than vanilla PPO. Figure 14 compares the RM score distribution of both methods. + +# C.2 RESULTS OF QUESTION ANSWERING + +We assess the performance of MA-PPO on the OpenAI WebGPT Comparison dataset, which focuses on the question answering task. + +Figure 15 presents the evaluation results based on the reward model. We observe that the policy model is challenging to optimize in this task, likely due to the suboptimal performance of the reward model. We applied early stopping during PPO training since the policy model exhibited reward hacking behavior which generated repetition tokens to inflate higher reward scores towards the end of training. Despite this, evaluations on the saved checkpoints show that MA-PPO still outperforms vanilla PPO across both tested model sizes. The reward score distribution in Figure 16 further confirms that MA-PPO achieves superior reward scores. + +Table 7: Test RM scores of SFT model, vanilla PPO, MA-PPO, and baselines: DPO and RLOO on TL;DR and HH-RLHF datasets. + +
MethodRM Score (TL;DR)RM Score (HH-RLHF)
SFT-0.640.13
DPO0.030.64
RLOO0.81-
PPO0.831.31
MA-PPO (n=5)1.401.55
+ +![](images/figures/ma-rlhf-fig-0019.jpg) +Figure 17: Win rates of DPO and RLOO against PPO and MA-PPO on TL;DR and HH-RLHF estimated by GPT-4. + +When using GPT-4 as the judge, we consider three different metrics to evaluate the answers generated by the policy: factual accuracy, coherence, and usefulness overall, following previous work (Nakano et al., 2021). The win rates depicted in Figure 4 (Right) show that MA-PPO consistently outperforms the policy trained with vanilla PPO across all criteria. Notably, MA-PPO achieves higher win rates in coherence and usefulness compared to factual accuracy. Human evaluation was conducted to select the preferred answer between those generated by the two policy models. Results in Figure 4 (Right) show that answers produced by MA-PPO were predominantly preferred by human annotators. + +# C.3 COMPARING WITH ADDITIONAL BASELINES + +In this section, we compare MA-PPO with two additional baselines: DPO (Rafailov et al., 2024) and RLOO (Ahmadian et al., 2024) on Gemma-2B model. Both of the methods are implemented with Deepspeed-Chat. Specifically, DPO models are trained on TL;DR and HH-RLHF datasets, with the same data split as we used when training PPO. RLOO model is trained on TL;DR dataset only, with the same policy and reward model initialization as PPO. For the training details of DPO, the learning rate is set to 2e-7, with $\beta = 0 . 1$ for TL;DR and $\beta = 0 . 0 1$ for HH-RLHF. The policy and reference models are initialized using the same SFT model as in PPO. For RLOO, the learning rate for the policy model is set to 1.5e-5, and the number of online samples is $K = 4$ . All other hyperparameters are kept consistent with PPO. + +We demonstrate the results evaluated by reward model score in Table 7, and win rates estimated by GPT-4 in Figure 17. On TL;DR dataset, DPO fails to gain improvement compared to PPO and MA-PPO, while RLOO achieves similar performance compared to PPO, but outperformed by MA-PPO. On HH-RLHF dataset, DPO exhibits superior performance than PPO but still underperforms the MA-PPO. + +# C.4 EXPERIMENTS ON LLAMA-3.2-3B + +We conduct experiments on Llama-3.2-3B model to validate the generalizability of our method across different model families. The experiments are conducted on TL;DR dataset, following the same data split as Gemma-2B. We set the learning rates of actor and critic to 5e-6 and 1e-5, and the KL coefficient is set to 0.1. Table 8 demonstrate the results evaluated by RM score, we show MA-PPO still remark- + +Table 8: Test RM scores of Llama-3.2-3B models on TL;DR dataset. + +
MethodRM Score (TL;DR)
SFT2.38
PPO3.33
MA-PPO (n=5)3.96
+ +ably outperforms vanilla PPO. Using GPT-4 to assess the win rate, MA-PPO obtains $61 \%$ win, $4 \%$ tie and $34 \%$ loss rate compared against PPO. These results prove the generalizability of our method. + +![](images/figures/ma-rlhf-fig-0020.jpg) +Figure 18: Illustration of value function of macro actions in MA-RLHF framework. It takes the outputs from the value function of tokens as input, and returns the value of macro actions with different $\sigma _ { \tau }$ assignment. + +Table 9: Pass $@ 1$ metric evaluated when applying different termination conditions on APPS dataset. + +
DatasetTerminationRM ScoreGPT-4 Win Rate (v.s. PPO)
TL;DRFixed 5-gram Parsing1.4078%
1.3778%
1.2772%
HH-RLHFFixed 5-gram Parsing1.5558%
1.6462%
+ +Table 10: Test RM scores and GPT-4 win rates when applying different termination conditions on TL;DR and HH-RLHF datasets. + +
TerminationFixed 10-gramParsingPPL
pass@1Inter.3.253.173.04
Intro.16.5617.0516.36
Comp.0.941.240.80
All5.455.565.26
+ +# D FURTHER ANALYSIS + +# D.1 VALUE FUNCTION ESTIMATION OF MACRO ACTION + +When implementing the macro actions, the value function of macro actions is estimated through the value function of tokens. This process can be formulated as: $\begin{array} { r l } { V ^ { \pi } ( s _ { \tau } , \omega _ { \tau } ) } & { { } = } \end{array}$ $\begin{array} { r } { \sum _ { i = 0 } ^ { \left| \omega _ { \tau } \right| } \sigma _ { t _ { \tau } + i } V ^ { \pi } \big ( s _ { t _ { \tau } + i } , a _ { t _ { \tau } + i } \big ) } \end{array}$ , where $\sigma _ { \tau } = \{ \sigma _ { t _ { \tau } } , \cdot \cdot \cdot , \sigma _ { t _ { \tau } + | \omega _ { \tau } | } \}$ control the contribution of each + +In this section, we explore several assignments of $\sigma _ { \tau }$ and their effectiveness on MA-PPO. Figure 18 illustrates macro action value function with different $\sigma _ { \tau }$ assignments: + +1. Equal assignment: We treats the contributions of each value function of tokens equally when considering the value function of macro actions, i.e., $\begin{array} { r } { \sigma _ { \tau } = \{ \frac { 1 } { | \omega _ { \tau } | } \} _ { i = 1 } ^ { \tau } } \end{array}$ . This is the naive assignment in MA-PPO used in all our experiments. +2. Unit assignment Since a macro action is a higher-level construct of a sequence of actions, we can use the value function of the last action as the macro action’s value function, where $\sigma _ { \tau } =$ $\{ 0 , 0 , \cdots , 0 , 1 \}$ . +3. Position decayed assignment The contributions of each value function of tokens are determined by taking the position into consideration. We define $\sigma _ { \tau }$ based on the position of the token, i.e., $\begin{array} { r } { \sigma _ { \tau } = \{ \frac { 1 } { ( | \omega _ { \tau } | - i ) \cdot \mathcal { H } } \} _ { i = 0 } ^ { | \omega _ { \tau } | - 1 } } \end{array}$ , where $\begin{array} { r } { \mathcal { H } = \sum _ { i = 0 } ^ { | \omega _ { \tau } | - 1 } \frac { 1 } { \left( | \omega _ { \tau } | - i \right) } } \end{array}$ , this construction ensures $\textstyle \sum _ { \sigma \in \sigma _ { \tau } } \sigma =$ 1. + +We tested these approaches with fixed $n$ -gram based termination on TL;DR dataset, with $n = 5$ . We report the RM score and GPT-4 score as previous. Results in Figure 19 show that the equal assignment yields higher RM scores. However, the unit assignment achieves the best consistency and fluency according to GPT-4 evaluations. + +![](images/figures/ma-rlhf-fig-0021.jpg) +Figure 19: Performance of MA-PPO with different value function estimations in MA-PPO on TL;DR dataset for Gemma-2B model. Left test RM scores. Right GPT-4 scores on 4 dimensions. + +# D.2 TERMINATION CONDITIONS ON DIFFERENT TASKS + +In this section, we analysis the effectiveness of termination conditions on TL;DR, HH-RLHF, and APPS datasets. When implementing parsing-based termination condition on APPS dataset, we use a programming-language-based parser.4 The results of TL;DR and HH-RLHF datasets are shown in Table 9 and Table 10. We can notice that parsing-based termination condition performs well on the HH-RLHF tasks, with higher RM score and win rate than fixed 5-gram based termination condition. While on the TL;DR dataset, parsing-based termination condition also achieves excellent performance compared to fixed 5-gram termination condition. On APPS dataset, parsing-based termination condition achieves the best results, except for the interview level task. These results demonstrate that construct macro action with linguistic information indeed brings performance gain to MA-PPO. + +# D.3 IMPACT OF RLHF ON REWARD SCORE DISTRIBUTION + +![](images/figures/ma-rlhf-fig-0022.jpg) +Figure 20: RM score shifting pattern after RLHF training. Left presents the RM score of best of 8 sampling on vanilla PPO compared to the vanilla PPO. Mid Left presents the RM score of best of 8 sampling on MA-PPO compared to the MA-PPO. Mid Right presents the RM score of MA-PPO ${ \mathrm { ~ \ : ~ } } n = 5$ ) compared to the vanilla PPO model. Right presents the RM scores of MA-PPO $n = \infty$ ) compared to the vanilla PPO model. + +We apply Best-of- $N$ sampling on both vanilla PPO and MA-PPO. The RM score shifting patterns for these methods are illustrated in Figure 20 (Left and Mid Left). From the results, we can conclude that Best-of- $. N$ sampling continues to enhance the performance of RLHF models effectively. + +In Figure 20 (Mid Right and Right), we compare the MA-PPO with vanilla PPO using settings of $n = 5$ and $n = \infty$ , both of which demonstrate positive effects on the RM score distribution. + +# D.4 IMPACT OF SAMPLING TEMPERATURE + +In the previous experiments, the results were sampled with a temperature $t e m p \ : = \ : 0 . 8$ to align with the sampling strategy used during training. In this section, we examine the effect of sampling temperature on response quality. We vary the temperature $t e m p \in \{ 0 . 0 , 0 . 2 , 0 . 4 , 0 . 6 , 0 . 8 , 1 . 0 \}$ , and report the results in Figure 21. The performance of both methods remains stable when temp $< 0 . 8$ . However, the performance of vanilla PPO begins to decline after $t e m p = 0 . 8$ , whereas MA-PPO continues to demonstrate stable performance, even at $t e m p = 1 . 0$ . + +![](images/figures/ma-rlhf-fig-0023.jpg) +Figure 21: Test reward scores evaluated by the corresponding reward model for summarizations generated with different sampling temperature on the TL;DR dataset. + +![](images/figures/ma-rlhf-fig-0024.jpg) +Figure 22: Illustration of the macro action-RLHF (MA-RLHF) framework. + +# Algorithm 1: Framework of Macro Action RLHF. + +
Input: Prompts: X = {x0, x1, . . . , xn}; Policy model: πpolicy;Reference model: πref; Critic model: πcritic; Reward model: πrm; Termination rule ζ(·) in Section 3.2.1; Value function estimation σtτ in Section D.1.
Output: Policy loss Lppo, Critic loss Lvalue. foreach prompt xi in X do
Make experience using policy model y := policy (x);
Get value V (st) := πcritic(x, st) at every time step t [0, |y|);
Get reward score at current experience r := πrm(x, y);
Compute maco actions {ωτ}=1 bas on the ermination rule {ωτ}r=1 := ζ(y);
do Compute macro action value function
Obtain Åτ and τ with GAE(V π(sτ, ωτ), r);
(old ωτ[s Åτ, c(+
πθ(ωτ|sτ)
Optimize Lppo = Emin(~[sτ,1 − , 1 + τ] πθ(ωτ|sτ)
Otealue = [Vτ, ωτ) −
+ +# E MA-RLHF ALGORITHMS + +Figure 22 illustrates the framework of MA-RLHF. In practice, to implement MA-RLHF, once the macro actions are obtained via the termination function, we compute their value (as estimated by the critic model) and rewards (based on a per-token KL penalty) using the value function estimation. With these values and rewards, we apply Generalized Advantage Estimation (GAE) without modification to derive advantage estimates and state-action value functions. These advantage estimates and state-action value functions are then used to all tokens within the macro action during the optimization of both the policy and critic models. The macro action RLHF algorithm, utilizing PPO, is detailed in Algorithm 1. + +In this implementation, the introduced additional time complexity is in the option termination. While fixed $n$ -gram based, randomized $n$ -gram based, and perplexity based terminations achieves same time complexity, the time complexity of parsing based termination is related to the constituent tree which we applied DFS to obtain $\left| \omega _ { \tau } \right|$ . During the inference stage, our MA-PPO will not introduce additional complexity since it only works at the training stage. + +We provide the Pytorch code for implementation of the macro action in PPO below: + +# Obtain Macro Action Positions + +def get_macro_action_positions(self, start, mask, termination $\mathit { \Theta } = \mathit { \Theta }$ ngram’, n_gram: int ${ } _ { , } = { }$ None, ppl: List[torch.float16] $=$ None, repeat_times: int $=$ None, cutoff: int $=$ None): sequence $=$ [start] if termination $= =$ ’ngram’: assert n_gram is not None current_count $\qquad = \quad 0$ for i in range(mask[:, start:].size(1) - 1): current_count $+ =$ mask[0, start $^ +$ i].item() if current_count $= =$ n_gram: sequence.append(start $\mathbf { \Sigma } + \mathbf { \Sigma } { \mathrm { ~ i ~ \Sigma ~ } } + \mathbf { \Sigma } 1 \mathbf { \Sigma }$ ) current_count $\qquad = \quad 0$ elif termination $= =$ ’randomized_ngram’: k_list $=$ torch.tensor([2, 3, 5, 10], dtype $=$ int) k_list $=$ torch.repeat_interleave(k_list, 3) k_list $=$ k_list[torch.randperm(k_list.size()[-1])] indexed_k_list $=$ torch.cumsum(k_list, dim $= - 1$ ) sequence $=$ [n for n in range(start, mask[:, start:].size(1) - 1)] indexed_k_list $=$ [x.item() for $_ \textrm { x }$ in indexed_k_list if x.item() $<$ len(sequence)] sequence $=$ [start] $^ +$ [sequence[i] for i in indexed_k_list] elif termination $\scriptstyle = { \begin{array} { l } { \scriptstyle } \\ { \left( - { \frac { 1 } { 2 } } + { \sqrt { 3 } } \right) } \end{array} }$ : assert ppl is not None for i in range(1, len(ppl)): if ppl[i] $>$ ppl[i - 1]: sequence.append(start $^ +$ i) elif termination $= =$ ’parser’: if len(node.leaves()) $< ~ 1$ : return False, ma_length $^ + 1$ if len(node.leaves()) $<$ cutoff: sequence.append(ma_length $^ +$ node.leaves()) return True, ma_length $^ +$ node.leaves() for nxt_node in node.childs(): state, ma_length_ $=$ dfs(nxt_node, ma_length) if !state: sequence[-1] $=$ ma_length_ ma_length $=$ ma_length_ return True, ma_length sequence.append(int(mask.size(1) - 1)) return sequence + +# Calculate Values / Rewards of Macro Action + +def get_macro_action_values(self, values, mask, start, sequence): split_list $=$ torch.diff(torch.tensor(sequence)).tolist() splited_values $=$ torch.split(values[:, start:], split_list, dim $= - 1$ ) splited_mask $=$ torch.split(mask[:, start:], split_list, dim $\mathrel { \mathop = } - 1$ ) inplace_values $=$ torch.zeros(1, len(split_list), dtype $=$ values.dtype ).to(values.device) for idx, (value_i, mask_i) in enumerate(zip(splited_values, splited_mask)): masked_values $=$ value_i[mask_i ! $\ ! = \ 0 \cdot$ ] inplace_values[0, idx] $=$ torch.mean(masked_values) if masked_values.numel() $> 0$ else 0.0 return inplace_values + +# Calculate Policy Model Loss + +def policy_loss_macro_action(self, logprobs, old_logprobs, advantages, mask, sequence): log_ratio $=$ (logprobs - old_logprobs) $\star$ mask ratio $=$ torch.exp(log_ratio) # calculate loss with macro action split_list $=$ torch.diff(torch.tensor(sequence)).tolist() split_ratio $=$ torch.split(ratio, split_list, dim $= - 1$ ) split_mask $=$ torch.split(mask, split_list, dim $= - 1$ ) pg_loss = 0.0 total_mask_sum $\mathrm { ~ ~ { ~ \mathbf ~ { ~ \psi ~ } ~ } ~ } = \mathrm { ~ ~ { ~ 0 ~ . ~ 0 ~ } ~ }$ for i in range(len(split_list)): ratio_i $=$ split_ratio[i] mask_i $=$ split_mask[i] advantages_i $=$ advantages[:, i] pg_loss1 $=$ -advantages_i $\star$ ratio_i pg_loss2 $=$ -advantages_i $\star$ torch.clamp(ratio_i, 1.0 - self. cliprange, $\mathrm { ~ 1 ~ . ~ 0 ~ } +$ self.cliprange) pg_loss $+ =$ torch.sum(torch.max(pg_loss1, pg_loss2) $\star$ mask_i) total_mask_sum $+ =$ mask_i.sum() pg_loss $=$ pg_loss / total_mask_sum return pg_loss + +# Calculate Critic Model Loss + +![](images/figures/ma-rlhf-fig-0025.jpg) + +# PPO + +# In PPO algorithm +start $=$ prompts.size()[-1] - 1 +action_mask $=$ attention_mask[:, 1:] +... +sequence $=$ get_macro_action_positions(start, action_mask, termination $= \prime$ ngram’, n_gram $\underline { { \underline { { \mathbf { \Pi } } } } } =$ n_gram) +macro_action_old_values $=$ get_macro_action_values(old_values, action_mask, start, sequence) +macro_action_old_rewards $=$ get_macro_action_values(old_rewards, action_mask, start, sequence) +advantages, returns $=$ get_advantages_and_returns(sumed_old_values, sumed_old_rewards) +policy_loss $=$ policy_loss_macro_action(policy_log_prob[:, start:], log_probs[:, start:], advantages, action_mask[:, start:], sequence) +critic_loss $=$ critic_loss_macro_action(value[:, start:], old_values[:, start:], returns, action_mask[:, start:], sequence) + +# F EVALUATION DETAILS + +# F.1 GPT-4 EVALUATION PROMPTS + +In our experiments, we take GPT-4 as a main judgment of the quality of policy models. The prompts used to generate win rates using GPT-4 are listed below. We utilize the $\mathsf { g p t } - 4 \mathsf { o } - 0 5 - 1 3$ for all of our experiments. The order of the responses generated by policy models is randomly chosen for all experiments. + +# TL;DR GPT-4 Evaluation Prompt + +You will be given two summaries written for an article. Your task is to pick the better one between them, based on the four criteria. Please make sure you read and understand these instructions carefully. Relevance - selection of important content from the source. The summary should include only important information from the source document. Annotators were instructed to penalize summaries which contained redundancies and excess information. + +Coherence - the collective quality of all sentences. We align this dimension with the DUC quality question of structure and coherence whereby “the summary should be well-structured and well-organized. The summary should not just be a heap of related information, but should build from sentence to a coherent body of information about a topic.” + +Consistency - the factual alignment between the summary and the summarized source. A factually consistent summary contains only statements that are entailed by the source document. Annotators were also asked to penalize summaries that contained hallucinated facts. + +Fluency - the quality of the summary in terms of grammar, spelling, punctuation, word choice, and sentence structure. + +You should output single character to indicate which summary you think is better. ‘A’ stands for Summary A and ‘B’ stands for Summary B. If you think both summaries are equally good, output $\mathbf { \bar { E } } ^ { \prime }$ + +Article / Post:{article / post} + +ummary A:{summary d Summary B:{summary b} + +Your Choice (only a single character): + +# HH-RLHF GPT-4 Evaluation Prompt + +For the following query to a chatbot assistant, which response is more helpful? + +First provide a one-sentence comparison of the two responses and explain which you feel is more helpful. Second, on a new line, state only ‘A’ or ‘B’ to indicate which response is more helpful. If they are equally good or bad, state ‘E’. Your response should use the json format, with “comparison” and “choice” as keys. + +Query: {query} +Response A: {response a} Response B: {response b} Your Judgment: + +# WebGPT Comparisons GPT-4 Evaluation Prompt + +You will be given two response written for an question. Your task is to pick the better one between them, based on these criteria. + +Factual accuracy - which answer is more factually accurate? + +Coherence - which answer is easier to follow? + +Usefulness overall - all things considered, which answer would be more helpful to the person who asked this question? + +You should output with a json format where the key is the criteria and the value is the choice you made, using ‘A’ stands for Response A and ‘B’ stands for Response B. If you think both responses are equally good, output ‘E’. + +Question: {question} +Answer A: {answer a} +Answer B: {answer b} +Your Judgment (you should also output the reason, note that you are allowed to think both responses +are equally good, then output with $\mathbf { \hat { E } } ^ { \prime }$ ): + +# F.2 HUMAN EVALUATION + +To estimate the quality from a human perspective, we collect human preference data on the TL;DR, HH-RLHF, and WebGPT datasets. Human annotators select the preferred response based on taskspecific criteria. For TL;DR, the evaluation criteria focus on three main perspectives: + +1. Hallucination: this considers whether the generated summary includes any additional information not present in the original post or article. +2. Verbosity: this assesses if the summary includes unnecessary context that could be removed without negatively impacting its quality. +3. Overall Quality: this measures the general coherence, informativeness, and readability of the generated summary. + +For evaluation on TL;DR dataset, the annotators should first compare the overall quality of two responses. If overall qualities are equally good for responses, then they should choose the winner based on hallucination and verbosity. + +In the context of HH-RLHF, annotators focus on the helpfulness of the responses: + +1. Instruction Following: whether the generated response follows the requirements in the instruction +2. Usefulness: whether the advices in the response are applicable, and does the response ideally guide the user on what to do next. + +Annotators are instructed to choose the response based on these aspects, while excluding superficial replies such as ”You’re welcome.” For the WebGPT dataset, the primary evaluation factor is factual accuracy. Annotators are provided with retrieval information relevant to the question from the dataset to aid in their judgment. They are tasked with selecting the answer that most accurately matches the retrieved information. + +During the evaluation process, annotators are presented with a prompt and two responses, each generated by either vanilla PPO or MA-PPO. To ensure impartiality and prevent annotators from guessing which model produced which response, we shuffle the positions of the responses. Annotators are given three choices: response A wins, response B wins, or a tie. The results are then collected to calculate the win rates for each model. + +For evaluations on the TL;DR and HH-RLHF datasets using 7B models, we conduct the human evaluation with 3 different annotators and collect their preference data to report the win rates. For all other human evaluations, we conduct them with a single annotator. The inter-rater agreement achieves an average of $68 \%$ on total 100 samples. On the TL;DR dataset the agreement is $64 \%$ , and on the HH-RLHF dataset the agreement is $72 \%$ across 50 samples per task. + +# G GENERATED EXAMPLES + +# G.1 CASE STUDY + +When evaluating the responses of MA-RLHF with human annotators, we observe that the MA-RLHF exhibits coherence and contextual appropriate abilities. We illustrate this phenomenon with an example by comparing MA-RLHF with the baseline in Table 11. We found that the MA-PPO method tends to generate responses with phrases more than the baseline method. Specifically, for nouns, it commonly includes adjectives for modification to make the generated summaries more accurate, such as “feeding indoor cat food”. + +# G.2 EXAMPLE RESPONSES + +In this section, we demonstrate some examples of validation sets to highlight the superiority of MA-PPO. In Table 12, we feature examples from the TL;DR dataset. Compared to the responses generated by vanilla PPO, the responses from MA-PPO offer more concise and relevant details about the situation. Table 13 showcases a dialogue focusing on the differences between Spanish and Italian. MA-PPO provides a more helpful response, addressing both cultural and grammatical perspectives. In Table 14, we compare answers from the WebGPT dataset. The responses provided by MA-PPO are more factual and accurate compared to those generated by vanilla PPO. + +Table 11: Example response from TL;DR dataset to illustrate the responses generated by MA-RLHF is more coherence and contextual appropriate compared to baseline. We highlight the context that + +
e observed withgreen.
Prompt POST Subreddit: r/catsHello everyone! One of my cats is about 10 years old now, she is pretty much strictlyindoors save for some time she spends on our screened in porch each day. (She likesto watch the birds in the yard while she suns herself by the pool, quite the princess).Anyway, when she was younger she was very active and quite small, however withage she has put on a pretty hefty amount of weight. I feed her indoor cat foodfor weight control, I've switched brands a few times trying to find something thatworks, I've cut back on feeding her by a lot (she gets very angry and demandingwhen she wants food but I don't give in) however, nothing really seems to work.I've tried cat toys, and bought a harness thinking I could try to walk her but she justlays down and looks at me like I'm stupid. Basically I just want to know if you allhave any suggestions for exercise or food. I care about her and don't want this toget any worse. I also have another cat that eats the same amount and type of foodas her and is a completely normal weight and only a year younger, however he is amale, not sure if that makes a difference in predisposition for weight gain. They arealso both fixed.
Vanilla PPO 10 year old cat has gained a noticeable amount of weight and is becoming increas-ingly inactive. Looking for suggestions for exercise or food to help her get healthier.Male and female cats are both affected.
Fixed 3-gram 10 year old cat puton hefty amountof weight despitefeeding indoor cat foodforweight control. Any suggestions for exercise or food to help with weight control andprevent further weightgain? Specifically wondering if there are any suggestionsfor cats that are both female and fixed.
Fixed 5-gram 10 year old indoorcat has put on significant amount of weight despite being fedlow calorie/weight control food. Need suggestions for exercise or food to help withweight management. Also have another cat that eats same type/amount of food asher andis a completely normal weightand only a year younger, however he is amale. Any insights would be appreciated.
Fixed 10-gram 10 year old cat is putting on significant weightdespite feeding indoor cat food for weight control. Any suggestions for ex-ercise or food to help her lose weight and keep her healthy? Also, is there adifference in predisposition for weight gain between male and femalecats whenfed same amount/type of food?
Fixed ∞-gram One 10 year old cat hasput on hefty amount of weight with age, feedindoor cat food for weight control, tried various brands and cut backfeeding by lot, nothing really seems to work in terms of exercise orfood, also have another cat same age and weight as her and com-pletely normal weightand no issues currentlyhowevermaleandnot sure if that makes difference predisposition for weight gain, both fixed.
Randomized n-gram 10 year old cat has put on a lot of weight, feed her indoor cat food for weight controlyet nothing seems to work.Any suggestions for exercise or food to address this?Other cat that eats same amount and type of food as her is a normal weight and a yearyounger, however is male, not sure if thatmakes a difference in predispositionforweight gain.
Perplexity based 10 yo cat has put on significant amount of weightdespite diet change and other attempts at exercise/activity. Any suggestionsfor exercise or food to help her lose weight? Also, any differences between malesand femalesregarding predispositionfor weight gain?
Parsing based10 year old indoor cat hasgainedsignificantamount of weightdespite proper diet and exercise, potentially due to age related health issuesNeed suggestions for exercise or food changes to help reduce weight andmaintain proper body weight. Is weight gain common in older cats, and doesgender play a factor in predisposition?
Need suggestions for
maintain proper bod
body w
+ +Table 12: Example responses from TL;DR dataset. The summarization of MA-PPO exactly capture the details of the post. + +
PromptPOST Subreddit: r/running Last night I experienced my first ever absolutely miserable run where in- stead of feeling energized and accomplished when it was over, I lay on my bathroom floor shivering and hugging the toilet. I have had frustrating runs before and I have had runs when my knee/an- kle/toe hurt and made me want to quit but last night was far worse than any of those. Earlier in the day, I ate a sandwich and fell asleep on the couch for a two hour surprise nap. When I woke up my stomach was slightly unsettled like I was about to have heartburn. I decided to do out for my long run anyways because I always have heartburn and it doesn't usually stop me. This run was scheduled to be my longest yet at 1:30:00 (I am training for a half). At about 45 minutes I started to get stomach cramps so I slowed how much
Vanilla PPO 2B RM Score: 0.75 MA-PPO 2Bmarathon training until now because everything up to this point has been fine. 1:30:00 run was extremely stressful and painful. Do I blame sandwich from earlier or run itself? I am worried that 1:30:00 is my limit.
RM Score: 1.551:30:00 long run resulted in severe stomach cramps and forced heavy heav- ing into kitchen garbage. Do I blame my sandwich from earlier or my run? Concerned that 1:30:00 is my limit.
Vanilla PPO 7B RM Score: 1.71 MA-PPO 7B1:30:00 run caused serious stomach pains and I puked up all of the water I drank while running. Do I blame the sandwich for my stomach pain or the run itself?
RM Score: 2.611:30:00 long run, had stomach cramps and had to walk often, drank Gatorade post run, vomited violently within 20 minutes, now worried that 1:30 is my limit, do I blame sandwich or run? Worried about quitting half marathon training. Help?
Vanilla PPO 27B RM Score: 3.4545 minutes into my run my stomach started to cramp and I was vomiting profusely within 20 minutes of finishing my 1:30:00 long run. Do I blame my sandwich from earlier or my run? Does this mean my limit is 1:30:00 or is it something else?
MA-PPO 27B RM Score: 3.95I went out for a 1:30:00 long run yesterday and came home with stomach cramps and nausea. I drank 1/3 bottle of Gatorade when I got home and got violently sick. I lost that and most of the water I drank while running. Do I blame my sandwich from earlier or my run? I am worried that 1:30:00 is my limit and that I won't be able to finish my half marathon training.
+ +Table 13: Example responses from HH-RLHF dataset, MA-PPO method presents more helpful response compared to vanilla PPO, and is prefered by GPT-4 and human. + +
PromptHuman: Would you be able to explain the differences between the Spanish and Italian language? Assistant: Of course. Can you tell me more about the specific areas where you're interested in knowing more? Human: I'm
Vanilla PPO 2B RM Score: 1.07thinking between the Spanish spoken in Mexico and Italian spoken in Italy. Assistant: The main difference between Spanish and Italian is that Spanish is an Indo- European language with a very rich historical background, while Italian is an Italian Romance language derived from Latin. Spanish has influences
MA-PPO 2B RM Score: 1.24from many different languages, while Italian has fewer and more limited influences from other languages. Here are some of the main differences between Spanish spoken in Mexico and Italian spoken in Italy: 1. Spanish spoken in Mexico tends to be more relaxed and casual, while Italian spoken in Italy tends to be more formal and courteous. 2. Spanish in Mexico is heavily influenced by indigenous
Vanilla PPO 7B RM Score: 0.80languages and has a rich vocabulary, while Italian is mostly derived from Latin and has a more simple and elegant sound. 3. Spanish is spoken throughout Latin America, while Italian is confined to the Italian peninsula. Overall, Spanish and Italian have many subtle differences that contribute to their unique cultural identities. The main differences between the Spanish spoken in Mexico and the Italian spoken in Italy are in the pronunciation and grammar of the two languages.
MA-PPO 7BIn Mexico, the Spanish spoken is much more similar to the Spanish spoken in Spain. In Italy, the Italian spoken is similar to the Italian spoken in other parts of Europe, but with its own unique style of pronunciation. In both languages, the grammar is very similar, though the Italian language does have some unique grammar rules. Sure, here are some of the main differences between the Spanish spoken in
RM Score: 1.22Mexico and the Italian spoken in Italy: - Spanish is a latin language, which is a language that was derived from the latin language of ancient Rome. Italian is based on the ancient language of the romans of ancient Rome. - Spanish is spoken in a number of different countries throughout the world, whereas Italian is the native language of Italy, and is only spoken in some parts of northern Italy. - Spanish is spoken primarily in Latin America, and has taken on many local variations and differences due to the influence of local indigenous languages (i.e. Mexican Spanish is quite different from Spanish spoken in Argentina). Italian is primarily spoken in Italy and has more localized variations. - There are many differences in grammar, vo- cabulary, and pronunciation between Spanish and Italian that are reflected in the differences between Mexican Spanish and Italian. Some of the most important differences in grammar include the use of masculine and femi- nine gender to refer to objects, and the different use of the tenses. There are many differences in vocabulary that also exist between the two languages,
+ +Table 14: Example responses from WebGPT dataset. + +
PromptWhat causes that sudden sinking, heavy, hot flush feeling when you become anxious and how are the effects felt so rapidly?
Vanilla PPO 2B RM Score: -0.68Anxiety has been linked to several physical symptoms, including a rapid heartbeat, trembling, dizziness, and a sense of impending doom. While some of these symp- toms may be due to the anxiety-provoking situation causing the symptoms, some may be the result of an anxiety disorder itself. The symptoms that result from anx- iety can be divided into physical symptoms, emotional symptoms, and behavioral symptoms. Physical symptoms of anxiety include nausea, dizziness, and a queasy
MA-PPO 2B RM Score: -0.41or empty feeling in the stomach. Emotional symptoms include feeling helpless, hopeless, afraid, irritable, and worried. Behavioral symptoms include avoiding so- cial interactions, remaining in bed all day, and not performing daily activities. The "hot flush" sensation that some people experience when they become anxious is the result of the nervous system's fight-or-flight response. When the body is faced with a stressful or dangerous situation, the nervous system's adrenaline re- lease causes blood to rush through the cardiovascular system to the muscles. This
causes the body to become more efficient at using oxygen and energy, and it can also cause the skin to turn red and hot. This response is designed to help the body deal with stress and danger, and it can include a range of symptoms that impact different parts of the body. For example, elevated blood pressure can cause the kid- neys to release more urine, and adrenaline can speed up the heart rate. It can also cause the mouth to dry out as the nervous system reacts to the perceived threat. In addition, anxiety can cause the chest to tighten and breathing to become quicker and shallower, which can cause the heart rate to increase. It can also cause the digestive system to slow down, and the muscles of the gastrointestinal tract to tighten, which can cause nausea and abdominal cramping.
Vanilla PPO 7B RM Score: -0.41There are several possible explanations as to why we feel hot and uncomfortable when anxious. One explanation is that anxiety causes the body to release hormones and chemicals into the bloodstream, which then circulate throughout the body and affect organs, tissues, and nerve cells. A different explanation is that a reaction to stress causes the body to release hormones and chemicals into the bloodstream, which then circulate throughout the body and affect organs, tissues, and nerve cells. Another explanation is that the body's stress response causes the release of hor- mones and chemicals into the bloodstream, which then circulate throughout the body and affect organs, tissues, and nerve cells. A fourth explanation is that anxiety triggers the release of cortisol, which increases blood sugar levels and causes the body to burn fat and muscle tissue for energy.
MA-PPO 7B RM Score: -0.39When we become anxious, it is because our brain has encountered something that it sees as a threat. The brain then sends out a warning signal, which takes the form of a fight-or-flight response. This response causes a number of physical effects, including increased heart rate and breathing rate, increased blood flow to muscles, and elevated blood sugar. This is because when we are threatened our body needs to be able to run or fight, and to do this we need increased oxygen and energy. Our body also needs to be able to protect itself, so we have increased blood flow to our muscles. When we are in a high-stress situation, our body produces serotonin and dopamine, which help us to feel good and focus on the task at hand. At the same time, our body produces norepinephrine and cortisol. Norepinephrine and cortisol work together to increase our heart rate, breath rate, and blood pressure, which in turn increases the flow of oxygen and nutrients to our muscles. Thus, when we are anxious, our body' way of protecting us is by increasing our awareness and increasing the flow of nutrients and oxygen to our muscles, so we can run or fight if needed. All of these processes occur so quickly because our body' systems are already set up to respond instantly to threats.
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Default strategy is fixed n-gram.", + "source": "3.2.1" + }, + { + "id": "ma-rlhf-D1-002", + "claim": "Macro action internal configuration: intra-macro-action reward discount factor rho=1 (simple sum of token rewards). Three sigma assignment variants for mapping token-level critic values to macro-level values: (1) equal assignment sigma_i=1/|omega_tau| (default, used in all main experiments, produces highest RM scores), (2) unit assignment sigma={0,...,0,1} (last token only, best consistency/fluency in GPT-4 eval), (3) position-decayed assignment sigma_i=1/((|omega_tau|-i)*H) where H normalizes sum to 1 (later tokens get higher weight).", + "source": "3.2.2, D.1" + }, + { + "id": "ma-rlhf-D1-003", + "claim": "SFT stage hyperparameters: Gemma-2B: batch_size=512, epochs=3, lr=5e-5, cosine scheduler, warmup_ratio=0.1. Gemma-7B: batch_size=128, epochs=1, lr=2e-5, cosine, warmup_ratio=0.1. Gemma-27B: batch_size=128, epochs=3, lr=5e-6, cosine, warmup_ratio=0.1. CodeGemma-2B: batch_size=16, epochs=1, lr=5e-6, warmup_ratio=0. CodeGemma-7B: batch_size=32, epochs=1, lr=2e-6, warmup_ratio=0. WebGPT variant: Gemma-2B batch_size=64/lr=1e-4, Gemma-7B epochs=5.", + "source": "B.2, B" + }, + { + "id": "ma-rlhf-D1-004", + "claim": "RM stage hyperparameters: Gemma-2B: batch_size=64, epochs=1, lr=1e-5, cosine scheduler, warmup_ratio=0.1. Gemma-7B: batch_size=128(TL;DR)/64(HH-RLHF), epochs=1, lr=1e-6, cosine, warmup_ratio=0.1. Gemma-27B: batch_size=128, epochs=1, lr=8e-6, cosine, warmup_ratio=0.1. CodeGemma has no RM stage (compiler feedback replaces RM). WebGPT: Gemma-2B batch_size=32/lr=2e-5, Gemma-7B epochs=32.", + "source": "B.2, B" + }, + { + "id": "ma-rlhf-D1-005", + "claim": "PPO stage hyperparameters: Shared across all models: clip_ratio=0.2, GAE lambda=0.95, GAE gamma=1, max_response_length=512, PPO epochs=1, rollout=1. Gemma-2B: batch_size=256, policy_lr=1.5e-5, critic_lr=1.5e-5, max_prompt_length=512, temperature=0.8, top_p=1.0, top_k=50. Gemma-7B: batch_size=256, policy_lr=1e-6, critic_lr=1e-6, max_prompt_length=512, temperature=0.8, top_p=1.0, top_k=50. Gemma-27B: batch_size=256, policy_lr=7e-7, critic_lr=1e-6. CodeGemma-2B: batch_size=16, policy_lr=5e-7, critic_lr=5e-5, max_prompt_length=600, temperature=1.0, top_k=5, warmup_steps=20. CodeGemma-7B: batch_size=16, policy_lr=5e-7, critic_lr=5e-5, max_prompt_length=600, temperature=1.0, top_k=5, warmup_steps=20. KL coefficient beta: Gemma-2B=0.05, Gemma-7B(TL;DR)=0.01/WebGPT=0.1/others=0.05, Gemma-27B=0.1, CodeGemma=0.05. Gemma warmup_steps=200, CodeGemma warmup_steps=20. WebGPT: epochs=4.", + "source": "B.2, B" + }, + { + "id": "ma-rlhf-D1-006", + "claim": "Data split ratios and training stages: Text tasks use 3-stage pipeline (SFT 20%, RM 40%, PPO 40%). Code task (APPS) uses 2-stage pipeline (SFT 20%, PPO 80%, no RM). WebGPT uses 5% validation split from training data.", + "source": "B" + }, + { + "id": "ma-rlhf-D1-007", + "claim": "Evaluation configuration: RM validation uses 2000 instances. GPT-4 pairwise win rate evaluated on 50 instances using gpt-4o-05-13. Pass@k uses k in {1,5} on 5000 test instances (APPS). Human evaluation uses 3 annotators (Gemma-7B), with inter-rater agreement measured on 100 total samples (50 per task). Generalization probing: Best-of-N values in {4,8,16,32}, temperature robustness evaluated at {0.0,0.2,0.4,0.6,0.8,1.0}, with training temperature 0.8.", + "source": "4, B" + }, + { + "id": "ma-rlhf-D1-008", + "claim": "Code generation compiler reward constants: successful compile base offset=-0.3 with multiplier=1.3*(N_pass/(N_pass+N_fail)); runtime error penalty=-0.6; compile error penalty=-1.0.", + "source": "4.5, B.5" + }, + { + "id": "ma-rlhf-D1-009", + "claim": "Additional baseline hyperparameters: DPO: lr=2e-7, beta=0.1 (TL;DR) / beta=0.01 (HH-RLHF). RLOO: policy_lr=1.5e-5, K=4 online samples (TL;DR only). Cross-model validation: Llama-3.2-3B on TL;DR with actor_lr=5e-6, critic_lr=1e-5, KL coefficient=0.1. Model sizes tested: Gemma-2B, Gemma-7B, Gemma-2-27B (open-ended generation); CodeGemma-1.1-2B, CodeGemma-1.1-7B-it (code generation); Llama-3.2-3B (cross-model validation).", + "source": "C.3, C.4" + } + ], + "D2": [ + { + "id": "ma-rlhf-D2-001", + "claim": "Reward Model Ranking Loss: L_RM = -log(sigma(log(r_phi(x, y_plus)) - r_phi(x, y_minus))). Binary ranking loss for training the reward model on human preference pairs; the reward model r_phi outputs scalar scores. Higher score for chosen response y_plus over rejected y_minus. Standard RLHF RM loss used before MA-RLHF is applied.", + "source": "2.2" + }, + { + "id": "ma-rlhf-D2-002", + "claim": "RLHF Reshaped Reward with KL Penalty: R(x, y) = r_phi(x, y) - beta * D_KL(pi_theta(.|x) || pi_sft(.|x)). Combines the reward model score with a KL divergence penalty preventing the RL policy from deviating too far from the SFT model. This token-level reward feeds into the macro-level reward computation in MA-RLHF.", + "source": "2.2" + }, + { + "id": "ma-rlhf-D2-003", + "claim": "Standard PPO Clipped Surrogate Objective: J^{ppo-clip}(theta) = E_t[min(r_t(theta)*A_t, clip(r_t(theta), 1-epsilon, 1+epsilon)*A_t)], where r_t(theta) = pi_theta(a_t|s_t)/pi_theta_old(a_t|s_t). Token-level PPO loss used as the baseline; MA-PPO replaces token-level importance ratios with macro-action-level ratios.", + "source": "2.1" + }, + { + "id": "ma-rlhf-D2-004", + "claim": "MA-PPO Macro-Level Clipped Surrogate Objective: L^{MA-PPO}(theta) = E_tau[min(pi_theta(omega_tau|s_tau)/pi_theta_old(omega_tau|s_tau)*hat_A_tau, clip(pi_theta(omega_tau|s_tau)/pi_theta_old(omega_tau|s_tau), 1-epsilon, 1+epsilon)*hat_A_tau)]. Core novel loss; adapts PPO clipped surrogate to operate at macro-action granularity using joint probabilities of entire macro-action sequences instead of individual tokens.", + "source": "3.2.2" + }, + { + "id": "ma-rlhf-D2-005", + "claim": "Macro Action Joint Probability: pi_theta(omega_tau | s_tau) = Prod_{t=t_tau}^{t_{tau+1}-1} pi_theta(a_t | a_{infinity, it approaches REINFORCE/RLOO/GRPO.", + "source": "3.2.1" + }, + { + "id": "ma-rlhf-D2-009", + "claim": "Randomized n-gram Termination: |omega_tau| in {2,3,5,10}, shuffled and repeated 3 times. Randomly selects macro action lengths from a predefined set; the length list is repeated 3 times to cover the full response, then shuffled. Trailing remainder absorbed by an infinite-length macro action. Found to perform best across multiple evaluation dimensions.", + "source": "3.2.1" + }, + { + "id": "ma-rlhf-D2-010", + "claim": "Parsing-based Termination: DFS on constituent tree; terminate when leaf token count <= C (C=5). Uses syntactic/semantic parsing to determine macro action boundaries. Single-token nodes (e.g., punctuation) are merged into the preceding macro action. For code tasks, a programming-language parser is used instead of NLP constituency parser.", + "source": "3.2.1" + }, + { + "id": "ma-rlhf-D2-011", + "claim": "Perplexity-based Termination: Terminate when ppl(omega_tau union a_{t_{tau+1}}) > ppl(omega_tau). Terminates a macro action when adding the next token increases overall perplexity. Perplexity computed from reference model logits without extra forward passes. Identifies natural semantic boundaries where model uncertainty spikes.", + "source": "3.2.1" + }, + { + "id": "ma-rlhf-D2-012", + "claim": "Macro Action Value Estimation from Token-Level Values: V^pi(s_tau, omega_tau) = Sum_{i=0}^{|omega_tau|-1} sigma_{t_tau+i} * V^pi(s_{t_tau+i}, a_{t_tau+i}). Computes macro-action value as a weighted sum of the critic's token-level value predictions. Three sigma assignment variants defined. This maps token-level critic outputs to macro-level values used in GAE.", + "source": "D.1" + }, + { + "id": "ma-rlhf-D2-013", + "claim": "Generalized Advantage Estimation at Macro Action Level: Apply standard GAE with lambda=0.95 and gamma=1 using macro-level values V^pi(s_tau, omega_tau) and macro rewards R_tau as inputs. Reuses existing GAE implementation unchanged but operates on reduced-dimension macro-level sequences. Outputs macro-level advantages hat_A_tau and returns for policy and critic losses.", + "source": "3.2.2" + }, + { + "id": "ma-rlhf-D2-014", + "claim": "Code Generation Piecewise Compiler Reward: R(x,y) = {-0.3 + 1.3*N_pass/(N_pass+N_fail) if compiled; -0.6 if runtime error; -1.0 if compile error}. Adaptive compiler-based reward for APPS code generation, replacing the reward model. Base offset -0.3 prevents reward collapse at 0% pass rate; piecewise formula produces a scalar reward feeding into MA-PPO pipeline.", + "source": "B.5" + }, + { + "id": "ma-rlhf-D2-015", + "claim": "Compute Macro Action Boundary Positions (get_macro_action_positions): Supports four termination strategies: (1) Fixed n-gram: count valid tokens, split every n; (2) Randomized n-gram: create shuffled length list [2,3,5,10] repeated 3 times, use cumulative sums as boundaries; (3) Parsing: DFS on constituent tree, terminate nodes with ppl[i-1]. Returns list of token indices marking macro action boundaries.", + "source": "E" + }, + { + "id": "ma-rlhf-D2-016", + "claim": "Compute Macro Action Values/Rewards (get_macro_action_values): Aggregates per-token values/rewards into macro-level values/rewards. Splits token-level values tensor by macro action boundaries (from sequence diff), masks out padding tokens, and computes the mean over valid tokens in each segment. Returns tensor of shape [1, num_macro_actions]. Implements the equal-assignment sigma variant (mean aggregation).", + "source": "E" + }, + { + "id": "ma-rlhf-D2-017", + "claim": "Model Initialization Scheme (Section 3.2.2, Appendix D.1): Initialize policy π_θ ← π_SFT (SFT model weights) and critic V_φ ← V_rm (reward model weights). For text-based tasks (TL;DR, HH-RLHF, WebGPT), the SFT model initializes the policy model and the reward model initializes the critic model. For APPS code generation with no RM stage, both the policy and critic models are initialized from the SFT checkpoint. The reward model is initialized from the fine-tuned SFT model.", + "source": "B.2" + }, + { + "id": "ma-rlhf-D2-018", + "claim": "Training Framework: MA-RLHF is implemented using the Deepspeed-Chat package, following a 3-stage pipeline: (1) SFT: L_SFT(θ) = -E_{(x,y)~D_SFT}[log π_θ(y|x)], 20% data split; (2) RM: L_RM(φ) = -log σ(r_φ(x, y⁺) - r_φ(x, y⁻)), 40% data split; (3) PPO: policy π_θ updated via MA-PPO clipped surrogate L^{MA-PPO}(θ) = E[min(r_τ·Â_τ, clip(r_τ, 1-ε, 1+ε)·Â_τ)] with KL penalty R(x,y) = r_φ(x,y) - β·D_KL(π_θ||π_SFT), 40% data split. SFT and RM are fine-tuned on the same dataset to avoid distribution gap.", + "source": "B.2, 4.1" + }, + { + "id": "ma-rlhf-D2-019", + "claim": "SFT Training Data Formatting: TL;DR dataset — D_SFT = {(x_i, y_i)} where x_i = Concat(post, summary) (Stiennon et al. approach); dialogue/QA — x = ':' + query + ':' + response (human-assistant chat template); APPS — x = '' + problem + '<|endoftext|>' + solution (Hendrycks et al. 2021 format). Data split: 20% SFT, 40% RM, 40% PPO for text tasks; 20% SFT, 80% PPO for APPS.", + "source": "B.2" + } + ], + "D3": [ + { + "id": "ma-rlhf-D3-001", + "claim": "TL;DR Summarization Main Experiment: Evaluate whether MA-PPO outperforms vanilla PPO on Reddit TL;DR text summarization in terms of RM scores, GPT-4 win rates, and human preference win rates. Training framework: Deepspeed-Chat with Gemma-2B and Gemma-7B models. Models initialized from SFT checkpoint (policy) and RM checkpoint (critic).", + "source": "4.2" + }, + { + "id": "ma-rlhf-D3-002", + "claim": "HH-RLHF Dialogue Main Experiment: Evaluate whether MA-PPO outperforms vanilla PPO on Anthropic HH-RLHF single-turn dialogue task in terms of helpfulness and harmlessness alignment. Same framework (Deepspeed-Chat), same Gemma-2B/7B models, same PPO hyperparameter setup.", + "source": "4.2" + }, + { + "id": "ma-rlhf-D3-003", + "claim": "WebGPT QA Main Experiment: Evaluate whether MA-PPO outperforms vanilla PPO on WebGPT Comparisons question-answering task, testing robustness on structured fact-based generation. Early stopping applied during PPO training to prevent reward hacking (repetition tokens). RM performs suboptimally on this task.", + "source": "4.2" + }, + { + "id": "ma-rlhf-D3-004", + "claim": "APPS Code Generation Experiment: Evaluate whether MA-PPO outperforms vanilla PPO on APPS code generation using compiler-based feedback as the reward signal. No RM stage; compiler reward substitutes. Both policy and critic models initialized from SFT checkpoint.", + "source": "4.5" + }, + { + "id": "ma-rlhf-D3-005", + "claim": "Termination Strategy Comparison: Compare four macro action termination strategies (fixed n-gram, randomized n-gram, parsing-based, perplexity-based) against vanilla PPO on reward maximization and linguistic quality dimensions (relevance, coherence, consistency, fluency).", + "source": "4.3.1" + }, + { + "id": "ma-rlhf-D3-006", + "claim": "N-gram Size Ablation: Study effect of varying the n-gram size n on MA-PPO performance, covering the continuum from MDP (n=1, PPO) through SMDP (intermediate n) to contextual bandit (n=infinity, REINFORCE). Evaluated on both TL;DR and HH-RLHF tasks.", + "source": "4.3.2" + }, + { + "id": "ma-rlhf-D3-007", + "claim": "Best-of-N and Temperature Robustness: Evaluate robustness of MA-PPO, vanilla PPO, and SFT under rejection sampling (Best-of-N) at various temperatures, testing generalization to sampling conditions different from training (training temperature=0.8).", + "source": "4.4" + }, + { + "id": "ma-rlhf-D3-008", + "claim": "Scaling Study: Evaluate how MA-PPO performance scales with model size (2B to 27B) on TL;DR summarization, verifying the macro-action benefit is not restricted to small models.", + "source": "4.4" + }, + { + "id": "ma-rlhf-D3-009", + "claim": "DPO and RLOO Baseline Comparison: Compare MA-PPO against DPO and RLOO baselines on Gemma-2B to demonstrate MA-PPO exceeds both offline (DPO) and online (RLOO) RLHF alternatives. DPO trained with lr=2e-7, eval set beta=0.1 (TL;DR)/0.01 (HH-RLHF); RLOO with policy lr=1.5e-5, K=4 online samples (TL;DR only).", + "source": "C.3" + }, + { + "id": "ma-rlhf-D3-010", + "claim": "Llama Cross-Model Validation: Validate that MA-PPO's benefits generalize beyond the Gemma model family by testing on Llama-3.2-3B with TL;DR dataset.", + "source": "C.4" + }, + { + "id": "ma-rlhf-D3-011", + "claim": "Value Function Sigma Assignment Comparison: Compare three methods for weighting token-level critic values when computing macro-action value functions: equal assignment (default, highest RM), unit/last-token-only (best consistency/fluency), and position-decayed. Analyse trade-off between reward maximization and linguistic quality.", + "source": "D.1" + } + ], + "D4": [ + { + "id": "ma-rlhf-D4-001", + "claim": "Standard MA-RLHF three-stage training pipeline (paper-explicit for all text-based tasks: TL;DR, HH-RLHF, WebGPT): Phase 1 - SFT training on 20% data; Phase 2 - RM training on 40% data; Phase 3 - PPO training on 40% data, comparing vanilla PPO and MA-PPO; Phase 4 - Evaluate RM scores (training curves), GPT-4 win rates, and human win rates at final checkpoint.", + "source": "4.2, 4.4" + }, + { + "id": "ma-rlhf-D4-002", + "claim": "Code generation MA-RLHF two-stage pipeline (paper-explicit for APPS): Phase 1 - SFT training on 20% data; Phase 2 - No RM stage (compiler feedback replaces RM); Phase 3 - PPO training on 80% data with compiler-based piecewise reward, comparing vanilla PPO and MA-PPO; Phase 4 - Evaluate pass@1 and pass@5 on 5k test set.", + "source": "4.5" + }, + { + "id": "ma-rlhf-D4-003", + "claim": "MA-RLHF internal method dependency chain (Phase 3 computation flow, Sec 3.2.1-3.2.2, D.1): Positioned within the PPO training stage (Phase 3 of the text pipeline / the code pipeline), this is the computation sub-stage that converts upstream artifacts into policy updates during each PPO iteration. Upstream inputs from Phases 1-2: (a) SFT model pi_sft from Phase 1 serves as the KL penalty reference in the reshaped reward R(x,y) = r_phi(x,y) - beta * D_KL(pi_theta || pi_sft) (KL-penalized reshaped reward formula); (b) RM model r_phi from Phase 2 provides per-prompt scalar rewards (code tasks: piecewise compiler reward formula substitutes r_phi); (c) critic model initialized from RM checkpoint for text tasks, or from SFT checkpoint for code tasks (model initialization scheme). Internal 6-step chain within Phase 3: [Step 1] Reshaped per-token rewards from R(x,y) are summed (discount rho=1) into macro rewards R_tau per boundary segment (macro reward summation). [Step 2] Termination condition zeta (fixed/random n-gram, parsing, perplexity) segments the generated response into macro action boundaries {omega_tau} via get_macro_action_positions (boundary computation). [Step 3] Joint probability pi_theta(omega_tau | s_tau) = prod pi_theta(a_t | a_ N$ because $x ^ { \pi ( i ) }$ only depends on $x ^ { \pi ( 1 ) } , \ldots , x ^ { \pi ( N ) }$ and is efficiently learnable by assumption. In contrast, below we will show examples where if one performs order-agnostic training $\grave { a }$ la MDMs, one will run into hard masking problems with high probability. + +![](images/figures/masked-diffusion-token-ordering-fig-0003.jpg) +Figure 2. Left: MDMs train on hard problems (Section 3.2). $\mathbf { X }$ -axis and y-axis correspond to $\log ( \mathrm { F L O P s } )$ and $- \log p \theta ( x )$ , respectively. MDM (Blue) is worse than ARM (Orange) in likelihood modeling. Most masking problems (Other lines) that MDM is trained on are harder than those encountered by ARM, as indicated by small log-likelihoods. Right: Task error imbalance (Section 3.3). MDM’s performance varies across different tasks. For text data (top right), this is indicated by validation loss. For L&O-NAE-SAT (bottom right), MDM performs well on the masking problems for observation positions (light region) but struggles with latent positions (dark region). + +Order-agnostic training We first note that if the observations $( \mathcal { O } _ { 1 } , \ldots , \mathcal { O } _ { P } )$ are given by a cryptographic hash function, then the masking problem of predicting $( x ^ { \pi ( 1 ) } , \ldots , x ^ { \pi ( L ) } )$ given $( x ^ { \pi ( N + \bar { 1 } ) } , \bar { , } \dotsc , x ^ { \pi ( N + \bar { P ) } } )$ is computationally intractable by design because it requires inverting the hash function. While this is a well-known folklore observation regarding the role of token ordering in language modeling, it is not entirely satisfying because this construction is worst-case in nature – in real-world data, one rarely trains on sequences given by cryptographic hash functions. Furthermore, it only establishes hardness for a specific masking pattern which need not be encountered in the course of running the reverse process. + +We provide several simple instances of L&O distributions that address these issues: instead of leveraging delicate cryptographic constructions, they are average-case in nature and furthermore we can establish hardness for typical masking problems encountered along the reverse process. + +In all these examples, the hardness results we establish hold even if the algorithm knows all of the parameters of $p _ { \mathrm { d a t a } }$ as well as the observation functions $\mathcal { O } _ { 1 } , \ldots , \mathcal { O } _ { P }$ . Due to space constraints, here we focus on the following example, deferring two others to Apps. B.1 and B.2. + +Example 3.2 (Sparse predicate observations). Consider the following class of L&O distributions. Given arity $k \geq 2$ , fix a predicate function $g : \{ 1 , \ldots , m \} ^ { k } \{ 0 , 1 \}$ . Consider the set of all ordered subsets of $\{ 1 , 2 , \ldots , N \}$ of size $k$ and set the total number of observation latents $P$ equal to the size of this set (hence $P = N ! / ( N - k ) ! =$ $N ( N - 1 ) \cdots ( N - k + 1 ) / $ . To sample a new sequence, we first sample latent tokens $x ^ { \pi ( 1 ) } , \ldots , x ^ { \pi ( N ) }$ from the prior distribution $p _ { p r i o r }$ and an observation latent corresponding to a $k$ -sized subset $S$ is given by $g ( \{ x ^ { \pi ( i ) } \} _ { i \in S } )$ . In other words, each observation latent corresponds to a $k$ -sized subset $S$ of $\{ 1 , 2 , \ldots , N \}$ and the corresponding observation function $\mathcal { O } _ { S } ( x ^ { \pi ( 1 ) } , \dots , x ^ { \pi ( N ) } )$ is given by $g ( \{ x ^ { \pi ( i ) } \} _ { i \in S } )$ . + +Proposition 3.3. Let $x$ be a sample from an L&O distribution pdata with sparse predicate observations as defined in Example 3.2, with arity $k$ and predicate $g$ satisfying Assumption B.11, and let $\gamma$ be the probability that $g$ is satisfied by a random assignment from $\{ 1 , \ldots , m \} ^ { k }$ Let $D _ { \mathrm { K S } }$ and $D _ { \mathrm { c o n d } }$ be some constants associated with the predicate function $g$ (see Definition B.12). Suppose each token in $x$ is independently masked with probability $\alpha$ , and $M$ is the set of indices for the masked tokens. If $1 - \gamma ^ { - 1 } D _ { \mathrm { K S } } / k N ^ { k - 1 } \leq \alpha \leq 1 - \gamma ^ { - 1 } D _ { \mathrm { c o n d } } / k N ^ { k - 1 }$ , then under the 1RSB cavity prediction (see Conjecture B.13), with probability $\Omega _ { k } ( 1 )$ over the randomness of the masking, no polynomial-time algorithm can solve the resulting subproblem of predicting any of the masked tokens among $x ^ { \pi ( 1 ) } , \ldots , x ^ { \pi ( N ) }$ given $x [ M ]$ . + +The complete proof of the proposition is given in Appendix B.4. We also provide a proof outline in Appendix B.3 for a comprehensive understanding. + +# 3.2. Empirical evidence of hardness via likelihoods + +In the previous section, we provided theoretical evidence that order-aware training is tractable when data has a natural order but the order-agnostic training is not. In this section, we provide empirical evidence to support this claim, using natural text data. Additionally, recent studies (Nie et al., 2024; Zheng et al., 2024) have shown that masked diffusion models (MDMs) underperform compared to autoregressive models (ARMs) on natural text data. In this section, we provide evidence that this performance gap is primarily due to the order-agnostic training of MDMs. Natural text inherently follows a left-to-right token order, and we show that as training deviates from this order, model performance progressively declines. + +To understand the importance of the order during the training, we use the following setting: Given a permutation $\pi$ of indices $\{ 0 , 1 , \ldots , L - 1 \}$ , define a $\pi$ -learner to be a likelihood model $\log p _ { \theta } ( x _ { 0 } )$ given as follows: + +$$ +\log p _ { \theta } ( x _ { 0 } ) = \sum _ { i = 0 } ^ { L - 1 } \log p _ { \theta } \left( x _ { 0 } ^ { \pi ( i ) } \Big | x _ { 0 } [ \pi \{ i , \dots , L - 1 \} ] \right) +$$ + +In other words, the $\pi$ -learner predicts the token at position $\pi ( i )$ given the clean tokens $x _ { 0 } ^ { \bar { \pi } ( 0 ) } , \ldots , x _ { 0 } ^ { \pi ( i - 1 ) }$ and masked tokens x0 $x _ { 0 } ^ { \pi ( i ) } , \ldots , x _ { 0 } ^ { \pi ( L - 1 ) }$ xπ(L−1)0 . If π is the identity permutation, this reduces to the standard (left-to-right) autoregressive training. Note that the MDM loss encodes a $\pi$ -learner for every permutation $\pi$ because the MDM loss (1) is equivalent to the average loss of those $\pi$ -learners over $\pi$ sampled from $\mathrm { U n i f } ( \mathbb { S } _ { L } )$ : + +$$ +\mathcal { L } _ { \theta } = - \underset { \pi \sim \mathrm { U n i f } ( \mathbb { S } _ { L } ) } { \mathbb { E } } \left[ \sum _ { i = 0 } ^ { L - 1 } \log p _ { \theta } \left( x _ { 0 } ^ { \pi ( i ) } \Big | x _ { 0 } \big [ \pi \{ i , \dots , L - 1 \} \big ] \right) \right] , +$$ + +where $\mathbb { S } _ { L }$ denotes the set of all permutations over $\{ 0 , 1 , \ldots , L - 1 \}$ . The proof of the above equivalence is given in Appendix E. Therefore, by measuring the ‘hardness’ of each $\pi$ -learner, we can probe differences in hardness between arbitrary masking problems and left-to-right masking problems. + +Experimental setup. We use the Slimpajama dataset (Soboleva et al., 2023) to evaluate the performance of training in different orders. To train a $\pi$ -learner, we employ a transformer with causal attention and use permuted data $\pi ( \boldsymbol { x } _ { 0 } )$ as input. By varying $\pi$ while maintaining all other training configurations (e.g., model, optimization), we can use the resulting likelihood (computed using Equation (3)) as a metric to capture the hardness of subproblems solved by the $\pi$ -learner. + +In our experiments, the sequence length $L$ is 2048, so repeating the scaling laws for each $\pi$ is infeasible. Instead, we sample $\pi \sim \mathrm { U n i f } ( \mathbb { S } _ { L } )$ and examine the scaling law of the $\pi$ -learner’s likelihood. We leverage the codebase from (Nie et al., 2024), where the baseline scaling laws of MDM and ARM were introduced. Moreover, given that RoPE has an inductive bias towards left-to-right ordering, we employ a learnable positional embedding layer for all experiments to correct this. Consequently, we also re-run the baseline results, where RoPE was employed. To investigate how the distance between $\pi$ and the identity permutation affects the scaling law, we consider two interpolating distributions over permutations between $\mathrm { U n i f } ( \mathbb { S } _ { L } )$ (i.e, MDM training) and the point mass at the identical permutation (i.e, ARM training). We sample three permutations from the interpolating distribution and $\mathrm { U n i f } ( \mathbb { S } _ { L } )$ and plot the scaling law for each of the permutation. Due to space constraints, we provide further experimental details in Appendix C.1. + +Results. As shown in Fig. 2, the scaling law for a $\pi$ -learner with uniformly random $\pi$ is worse than that of an ARM. This elucidates the inherent hardness of masking problems $p _ { \theta } ( x _ { i } \mid x _ { 0 } [ M ] )$ beyond left-to-right prediction and also explains why MDM, which is trained simultaneously on all $\pi \in \mathbb { S } _ { L }$ , is worse than ARM in likelihood modeling. Additionally, as $\pi$ gets closer to the identity permutation, the scaling laws also get closer to ARM ( $\bar { \pi }$ -learner-closer and $\pi$ -learner-much-closer in Fig. 2). This also supports the common belief that ARM is a good fit for text data as it inherently follows a left-to-right ordering. + +That said, it should also be noted that even though MDMs are trained on exponentially more masking problems than ARM $\left( \Theta ( L 2 ^ { L } ) \right)$ versus $L$ ), its performance is not significantly worse than $\pi$ -learners. We attribute this to the blessing of task diversity; multi-task training can benefit both the optimization dynamics (Kim et al., 2024) and validation performance (Tripuraneni et al., 2021; Maurer et al., 2016; Ruder, 2017) due to positive transfers across tasks. + +# 3.3. Error is imbalanced across masking problems + +In previous sections, we have demonstrated that the hardness of different masking problems $p _ { \theta } ( x ^ { i } \mid x _ { 0 } [ M ] )$ can vary significantly, potentially hindering the MDM’s learning. In this section, we provide empirical evidence that the MDM’s final performance exhibits a similar imbalance across subproblems. Details are provided in App. C.2. + +L&O-NAE-SAT. Consider an L&O distribution with $\pi$ given by the identity permutation and where each observation ${ \mathcal { O } } _ { j }$ is deterministically given by $\mathrm { N A E } ( x _ { i _ { 1 } } , x _ { i _ { 2 } } , x _ { i _ { 3 } } ) \triangleq$ $1 - \mathbf { 1 } [ x _ { i _ { 1 } } = x _ { i _ { 2 } } = x _ { i _ { 3 } } ]$ for some randomly chosen (prefixed) triples $( i _ { 1 } , i _ { 2 } , i _ { 3 } ) \ \in \ [ N ]$ . For an MDM trained on this distribution, we measure the error it achieves on each task $\log p _ { \theta } ( x _ { 0 } | x _ { 0 } [ M ] )$ via $\mathbb { E } _ { x _ { 0 } } \bigg \| \log p _ { \theta } ( x _ { 0 } | x _ { 0 } [ M ] ) -$ $\log p _ { \mathrm { d a t a } } ( x _ { 0 } | x _ { 0 } [ M ] ) \Big \| ^ { 2 }$ , where $p _ { \mathrm { d a t a } } ( x _ { 0 } | x _ { 0 } [ M ] )$ denotes the Bayes-optimal predictor. Technically, we do not have access to this, so instead we train another MDM for a much larger number of iterations and use this as a proxy. Fig. 2 reveals that prediction tasks for latent positions (light region) exhibit larger errors compared to those for observation positions (dark region). + +Text. Here we revisit the text experiment from Section 3.2. Since we do not have access to the Bayes-optimal predictor, we use the metric $\begin{array} { r } { \mathbb { E } _ { x _ { 0 } \sim p _ { \mathrm { d a t a } } } \left[ \sum _ { i = 0 } ^ { \hat { L } - 1 } \log p _ { \theta } \left( x _ { 0 } ^ { \pi ( i ) } \Big | x _ { 0 } [ \pi \{ i , \dots , L - 1 \} ] \right) \right] } \end{array}$ . This captures the accumulation of error across subproblems $p _ { \theta } \left( x _ { 0 } ^ { \pi ( i ) } \Big | x _ { 0 } [ \pi \{ i , \dots , L - 1 \} ] \right)$ , since $p _ { \theta } ( x _ { 0 } | x _ { 0 } [ M ] ) = p _ { \mathrm { d a t a } } ( x _ { 0 } | x _ { 0 } [ M ] )$ minimizes this metric. Fig. 2 shows a clear gap between different subproblems. + +The theoretical and empirical evidence demonstrates that MDMs perform better in estimating $p _ { \theta } ( x _ { 0 } | x _ { 0 } [ M ] )$ for some subproblems $M$ than for others. We therefore want to avoid encountering hard subproblems $M$ at inference time. In the next section, we show that while vanilla MDM inference can run into such subproblems, simple modifications at the inference stage can effectively circumvent these issues, resulting in dramatic, training-free performance improvements. + +# 4. MDMs can plan around hard problems + +We previously argued that due to the complex nature of masking subproblems, MDM must perform poorly on certain ones $p _ { \theta } ( x ^ { i } | x _ { t } )$ . Therefore, during vanilla MDM inference, MDM inevitably encounters such difficult subproblems at Step (b). While this might suggest that we need to fundamentally revisit how MDMs are trained, in this section we show that, surprisingly, simple modifications at the inference stage—without any further training—can sidestep these issues and lead to significant performance improvements. + +MDM offers multiple sampling paths. The vanilla MDM inference (Algorithm 1) aim to align the intermediate distributions with the forward process, as used in continuous diffusion. However, unlike continuous diffusion, the reverse process of MDM allows multiple valid sampling paths (different orders of unmasking the tokens) that match the starting distribution of the forward process of MDM. + +We first show that when we have an ideal MDM that perfectly solves all masking problems, i.e., $p _ { \theta } ( x _ { 0 } ^ { i } | x _ { 0 } [ M ] ) =$ $p _ { \mathrm { d a t a } } ( x _ { 0 } ^ { i } | x _ { 0 } [ M ] )$ , then using any sampling path (unmasking the tokens in any order) results in the same distribution. Consider the following sampler: For every step, $S$ is a set with one index selected agnostically (without following any distribution). For any clean sample $x _ { 0 }$ generated by this sampler, note that $p _ { \theta } ( x _ { 0 } ) \ =$ $\begin{array} { r } { \prod _ { i = 0 } ^ { L - 1 } p _ { \theta } \left( x _ { 0 } ^ { \pi ( i ) } \Big | x _ { 0 } [ \pi \{ i , \dots , L - 1 \} ] \right) } \end{array}$ by chain rule, and this is equal to $\begin{array} { r } { \prod _ { i = 0 } ^ { L - 1 } p _ { \mathrm { d a t a } } \left( x _ { 0 } ^ { \pi ( i ) } \Big | x _ { 0 } \big [ \pi \{ i , \dots , L - 1 \} \big ] \right) = } \end{array}$ $p _ { \mathrm { d a t a } } ( x _ { 0 } )$ . Therefore, other choices of $S$ , not necessarily following Algorithm 1, still capture the true likelihood. + +![](images/figures/masked-diffusion-token-ordering-fig-0004.jpg) +Figure 3. Generative Perplexity. We compare the resulting generative perplexity (GenPPL) of adaptive vs. vanilla MDM inference. We employ a pretrained 170M MDM and LLaMA-7B (Touvron et al., 2023) as inference and evaluation, respectively. Adaptive MDM inference (Blue) leads to a substantial reduction in generative perplexity, while maintaining the entropy. + +In practice, unlike this ideal case, MDM does not perform equally well on all subproblems, as shown in Section 3.3. Consequently, different sampling paths result in varying likelihood modeling abilities. Motivated by this observation, we consider adaptive inference for MDMs: + +# Adaptive MDM inference + +(a) Sample a set of masked tokens ${ \mathcal { S } } = { \mathcal { F } } \left( \theta , x _ { t } \right) \subseteq$ $\{ i \mid x _ { t } ^ { i } = 0 \}$ . (b) For each $i \in S$ , sample $x _ { s } ^ { i } \sim p _ { \theta } ( x ^ { i } | x _ { t } )$ . + +Instead of selecting $S$ randomly, adaptive MDM inference leverages an oracle $\mathcal { F } ( \boldsymbol { \theta } , \boldsymbol { x } _ { t } )$ to select $S$ strategically to avoid hard masking problems. This naturally raises the question of how to design an effective oracle $\mathcal { F }$ . + +In the following sections, we demonstrate that adaptive MDM inference with careful choices of $\mathcal { F }$ enhance MDM’s likelihood matching ability. In other words, a pretrained MDM, even if it performs poorly on certain hard subproblems, still contains sufficient information to avoid them when paired with an effective oracle $\mathcal { F }$ . + +# 4.1. Effective design of ordering oracle + +We introduce two different oracles, Top probability and Top probability margin. Intuitively, both strategies are based on the idea that $S$ should be selected based on how “certain” the model is about each position. We caution that these strategies should not be confused with notions like nucleus sampling in ARMs (Holtzman et al., 2019); the oracles we describe are for selecting the position of the next token to decode, rather than the value, and thus are only meaningful in the context of MDMs. + +Table 1. L&O-NAE-SAT. Adaptive MDM inference achieves better likelihood matching than vanilla MDM inference. Note that naive guessing leads to $7 5 \%$ accuracy, indicating that vanilla inference performs similarly or worse than naive guessing. + +
(N, P)Vanilla inferenceAdaptive inference
(25, 275)78.06%93.76%
(30, 270)75.70%93.54%
(40, 260)74.60%92.21%
(50, 250)67.94%90.01%
(100, 200)62.84%88.91%
+ +Top probability (Zheng et al., 2023). Suppose we want to unmask $K$ positions at time step $t$ , i.e., select $| S | = K$ In the top probability, the uncertainty of a position is estimated by the maximum probability assigned to any value in the vocabulary. More precisely, the certainty at position $i$ is $\begin{array} { r } { \operatorname* { m a x } _ { j \in \{ 0 , \dots , m - 1 \} } p _ { \theta } ( x ^ { i } = j | x _ { t } ) } \end{array}$ and $\mathcal { F } ( \boldsymbol { \theta } , \boldsymbol { x } _ { t } ) =$ Top $K \left( \operatorname* { m a x } p _ { \theta } ( x ^ { i } | x _ { t } ) \right)$ . + +Top probability strategy is a good proxy for many tasks and works well in practice (Zheng et al., 2023; Ye et al., 2024; Wang et al., 2024). However, this approach can often provide misleading estimates of uncertainty. Consider when an MDM is confused between two token values, thus assigning them almost equal but high probabilities. In this case, unmasking according to top probability may still choose to unmask this position, despite its uncertainty. To mitigate this issue, we propose the following alternative strategy. + +Top probability margin. In this strategy, the uncertainty of a position is instead estimated using the absolute difference between the two most probable values at position $i$ . More precisely, if $j _ { 1 }$ and $j _ { 2 }$ are the two most probable values in vocabulary according to $p _ { \theta } ( x ^ { i } | x _ { t } )$ in position $i$ , the certainty in the position is given by $| p _ { \theta } ( x ^ { i } \ = \ j _ { 1 } | x _ { t } ) \ - \ p _ { \theta } ( x ^ { i } \ = \ j _ { 2 } | x _ { t } ) |$ and $\mathcal { F } ( \boldsymbol { \theta } , \boldsymbol { x } _ { t } ) \ =$ Top $K \left( | p _ { \theta } ( x ^ { i } = j _ { 1 } | x _ { t } ) - p _ { \theta } ( x ^ { i } = j _ { 2 } | x _ { t } ) | \right)$ . When multiple values have similar probabilities at a position, top probability margin strategy will provide a better estimate of the uncertainty of a position, and when there is a single best choice of value then top probability and top probability margin work similarly. + +# 4.2. Adaptive MDM inference + +In this section, we experimentally validate that adaptive MDM inference helps MDMs avoid hard subproblems, leading to better likelihood matching. We first show our results on L&O-NAE-SAT and text data, before turning to our primary application to logic puzzles. + +Table 2. Comparison of accuracy for solving the Sudoku puzzle. + +
Method# ParamAccuracy
ARM (w/o ordering) ARM (with ordering)42M9.73% 87.18%
MDM (vanilla)6.88%
MDM (Top probability) MDM (Top prob. margin)6M18.51% 89.49%
+ +L&O-NAE-SAT and text data. For the L&O-NAE-SAT distribution defined in Section 3.3, we evaluate the effectiveness of adaptive inference by measuring the accuracy in predicting the observation tokens. Table 1 in the appendix reveals a clear improvement over vanilla inference. For the text dataset, we evaluate using the standard metric of generative perplexity, by which likelihood is measured by a large language model. We also compute the entropy of the generated samples to ensure both inference strategies exhibit similar levels of diversity. As shown in Fig. 3, we observe a substantial decrease in generative perplexity using adaptive inference. We defer further experimental details to Appendix D.1. + +Logic puzzles. We consider two different types of logic puzzles: Sudoku and Zebra (Einstein) puzzles. Intuitively, for Sudoku, some empty (masked) cells are significantly easier to predict than others and we want to choose the cells that are easier to predict during the inference. We evaluate the effectiveness of adaptive MDM inference over vanilla MDM inference in selecting such cells.2 + +To measure the performance of an inference method, we use the percentage of correctly solved puzzles. For both puzzles, we use train and test datasets from (Shah et al., 2024). For the Sudoku puzzle (Table 2) we observe that adaptive MDM inference, in particular, Top probability margin strategy, obtains substantially higher accuracy $( 8 9 . 4 9 \% )$ compared to vanilla MDM inference $( 6 . 8 8 \% )$ . Additionally, Top probability margin obtains higher accuracy $( 8 9 . 4 9 \% )$ than Top probability strategy $( 1 8 . 5 1 \% )$ ). As mentioned in Section 4.1, this is because Top probability margin strategy more reliably estimates uncertainty when multiple competing values are close in probability at a given position, as is often the case in Sudoku. For the Zebra puzzle, as shown in Table 3, we observe a consistent result: Top probability $( 9 8 . 5 \% )$ and + +Top probability margin $( 9 8 . 3 \% )$ outperform vanilla MDM inference $( 7 6 . 9 \% )$ . + +Table 3. Comparison of accuracy for solving the Zebra puzzle. + +
Method# ParamAccuracy
ARM (w/o ordering) ARM (with ordering)42M80.31 % 91.17 %
MDM (vanilla) MDM (Top probability)19M76.9 % 98.5 %
MDM (Top prob. margin)98.3 %
+ +# 4.3. Eliciting sequence-dependent reasoning paths using adaptive MDM inference in logic puzzles + +In this section, we study the effectiveness of adaptive MDM inference in finding the right reasoning/generation order for tasks where every sequence has a different “natural” order. To do so, we will compare the performance of adaptive MDM inference to that of ARM on Sudoku and Zebra puzzles. For these puzzles, the natural order of generation is not only different from left-to-right, but it is also sequencedependent. For such tasks, prior works have shown that ARMs struggle if the information about the order is not provided during the training (Shah et al., 2024; Lehnert et al., 2024). Therefore, to obtain a strong baseline, we not only consider an ARM trained without the order information but also consider an ARM trained with the order information for each sequence in the training data. Note that the latter is a much stronger baseline than the former as one can hope to teach the model to figure out the correct order by some form of supervised teacher forcing (as performed in Shah et al. (2024); Lehnert et al. (2024)), eliminating the issue of finding the right order in an unsupervised manner. + +We compare ARMs and MDMs for Sudoku in Table 2 and Zebra puzzles in Table 3. We observe that for both, Top probability margin-based adaptive MDM inference not only outperforms the ARM trained without ordering information, but it even outperforms the ARM trained with ordering information! This shows that the unsupervised way of finding the correct order and solving such logic puzzles using adaptive MDM inference outperforms the supervised way of finding the correct order and solving such puzzles using an ARM, and is significantly less computationally intensive. + +# 4.4. Adaptive MDM inference on natural language tasks + +To examine the effect of different inference strategies on text benchmarks, we adapted LLaDA, the 8B MDM model from (Nie et al., 2025). We compare three inference strategies: vanilla, top probability, and top probability margin. The results are presented in Table 4. + +We see that both adaptive MDM inference strategies, top probability and top probability margin, consistently outperform vanilla MDM inference. Notably, top probability margin demonstrates a clear advantage over top probability in challenging tasks like HumanEval-Multiline (infill), HumanEval-Split Line (infill), and Math. This is because Top probability margin provides a more reliable estimate of uncertainty when multiple tokens have similar probabilities, a frequent occurrence in these difficult tasks. These results further underscore the potential for developing new, sophisticated adaptive inference strategies for various tasks. We provide experimental details in Appendix D.3. + +# 4.5. Easy to hard generalization + +In the previous section we showed that when the training and inference sequences come from the same distribution, order-agnostic training of MDMs combined with adaptive inference can perform very well on logic puzzles. To evaluate if the model has learned the correct way of solving the puzzles and test the robustness of adaptive inference, we also test the MDMs on harder puzzles than the ones from training, for Sudoku. + +We keep the training dataset the same as proposed in Shah et al. (2024). Shah et al. (2024) created this dataset from Radcliffe (2020) by selecting the puzzles that can be solved using 7 fixed strategies and do not require backtrackingbased search. We use the remaining puzzles in Radcliffe (2020) as our hard dataset. Hence, these puzzles all use a strategy not seen during training and/or backtracking to obtain the correct solution. + +We measure the accuracy of MDMs and ARMs on the hard test set and present the results in Table 5. We see that the Top probability margin-based adaptive MDM inference strategy $( 4 9 . 8 8 \% )$ again significantly outperforms ARMs trained with order information $( 3 2 . 5 7 \% )$ . In particular, although the accuracy drops for both methods due to the more challenging test set, MDMs with adaptive inference appear to be more robust to this distribution shift than ARMs. We believe this is due to the fact that MDMs try to solve a significantly higher number of infilling problems than ARMs $( \exp ( L )$ compared to $L$ ) and therefore are able to extract knowledge about the problem more efficiently than ARMs. + +# 5. Conclusion + +In this work, we examined the impact of token generation order on training and inference in MDMs. We provided theoretical and experimental evidence that MDMs train on hard masking problems. We also demonstrated that adaptive inference strategies can be used to sidestep these hard problems. For logic puzzles, we find that this leads to dramatic improvements in performance not just over vanilla MDMs, but even over ARMs trained with teacher forcing to learn the right order of decoding. An important direction for future work is to go beyond the relatively simple adaptive strategies to find a better generation order like top probability and top probability margin considered here. + +Table 4. Performance of different inference strategies for LLaDa 8B model on coding and math tasks. + +
MethodHumanEval-SingleHumanEval-MultiHumanEval-SplitMathMMLUROCStories
Vanilla31.8%16.5%14.2%28.5%33.2%21.23%
Top probability32.9%20.8%18.4%31.3%36.5%21.10%
Top prob. margin33.5%25.4%22.3%34.3%35.4%21.41%
+ +Table 5. Comparison of accuracy for solving the hard Sudokus. + +
Method#ParamAccuracy
ARM (with ordering)42M32.57 %
MDM (random)3.62 %
MDM (Top probability)6M9.44 %
MDM (Top prob. margin)49.88 %
+ +Acknowledgements. JK thanks Kiwhan Song for discussions about MDM training. KS and VK are supported by the NSF AI Institute for Foundations of Machine Learning (IFML). KS and VK thank the computing support on the Vista GPU Cluster through the Center for Generative AI (CGAI) and the Texas Advanced Computing Center (TACC) at UT Austin. KS thanks Nishanth Dikkala for the initial discussions about the project. SK acknowledges: this work has been made possible in part by a gift from the Chan Zuckerberg Initiative Foundation to establish the Kempner Institute for the Study of Natural and Artificial Intelligence and support from the Office of Naval Research under award N00014-22-1-2377. SC is supported by the Harvard Dean’s Competitive Fund for Promising Scholarship and thanks Brice Huang and Sidhanth Mohanty for enlightening discussions about computational-statistical tradeoffs for planted CSPs. + +# Impact statement + +This paper advances the understanding of discrete diffusion models, contributing to the broader field of Machine Learning. There are many potential societal consequences of our work, none of which we feel must be specifically highlighted here. + +# References + +Alaoui, A. E. and Gamarnik, D. Hardness of sampling solutions from the symmetric binary perceptron. arXiv preprint arXiv:2407.16627, 2024. + +Alekhnovich, M. More on average case vs approximation complexity. In 44th Annual IEEE Symposium on Foundations of Computer Science, 2003. Proceedings., pp. 298–307. IEEE, 2003. + +Aubin, B., Perkins, W., and Zdeborova, L. Storage capacity ´ in symmetric binary perceptrons. 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(Continuous) diffusion models were originally built on continuous-space Markov chains with Gaussian transition kernels (Sohl-Dickstein et al., 2015; Ho et al., 2020). This was later extended to continuous time through the theory of stochastic differential equations (Song et al., 2021). In a similar vein, discrete diffusion models have emerged from discrete-space Markov chains (Hoogeboom et al., 2021b). Specifically, (Austin et al., 2021) introduced D3PM with various types of transition matrices. Later, Lou et al. (2024) proposed SEDD, incorporating a theoretically and practically robust score-entropy objective. Additionally, Varma et al. (2024); Liu et al. (2024b) introduced novel modeling strategies that classify tokens in a noisy sequence as either signal (coming from clean data) or noise (arising from the forward process). In particular, Liu et al. (2024b) uses this to give a planner that adaptively determines which tokens to denoise. While this is similar in spirit to our general discussion about devising adaptive inference strategies, we emphasize that their approach is specific to discrete diffusions for which the forward process scrambles the token values, rather than masking them. + +Masked diffusion models. Meanwhile, the absorbing transition kernel has gained popularity as a common choice due to its better performance than other kernels. Building on this, Sahoo et al. (2025); Shi et al. (2024) aligned its framework with continuous diffusion, resulting in a simple and principled training recipe, referring to it as Masked Diffusion Model. Subsequent studies have explored various aspects of MDM. Gong et al. (2024) efficiently trained MDM via adaptation from autoregressive models, scaling MDM up to 7B parameters. Zheng et al. (2024) interpreted MDMs as order-agnostic learners and proposed a first-hitting sampler based on this insight. Ye et al. (2024); Gong et al. (2024) demonstrated that MDM outperforms autoregressive models in reasoning and planning tasks, emphasizing its impact on downstream applications. Nie et al. (2024) examined the scaling laws of MDM, while Xu et al. (2024); Liu et al. (2024a) identified limitations in capturing coordinate dependencies when the number of sampling steps is small and proposed additional modeling strategies to address this issue. Schiff et al. (2024) studied conditional generation using MDM and Rector-Brooks et al. (2024) tackled the challenge of controlling generated data distributions through steering methodologies. Chen & Ying (2024) provided a theoretical analysis showing that sampling error is small given accurate score function estimation. + +Any-order reasoning. Even though language tasks generally have a natural order of “left-to-right” token generation, in many tasks like planning, reasoning, and combinatorial optimization, the natural order of token generation can be quite different from “left-to-right”. Even though prominent autoregressive-based language models achieve impressive performance on various tasks, many works (Golovneva et al., 2024; Chen et al., 2024; Kitouni et al., 2025) have shown that this performance is tied to the training order of the tasks and therefore can cause brittleness from it. For example, Chen et al. (2024) showed that simply permuting the premise order on math tasks causes a performance drop of $30 \%$ . The reason behind such brittleness regarding the ordering is the inherent “left-to-right” nature of the autoregressive models. Several works (Liao et al., 2020) have tried to address this issue in the autoregressive framework. In particular, (Papadopoulos et al., 2024) highlighted the significance of left-to-right ordering in natural language by comparing its likelihood to that of the reverse (right-to-left) ordering. + +Recently, discrete diffusion models have emerged as a promising approach for discrete data apart from autoregressive models. Additionally, the order-agnostic training of discrete diffusion models opens up the multiple sampling paths during the inference but it also faces some challenges during the training therefore, they seem a promising approach to elicit any order reasoning. Zheng et al. (2023) proposed different ways of implementing an adaptive inference strategy for MDM but a concrete understanding of why such an adaptive inference strategy is needed is still lacking. In this work, we explore various aspects of vanilla MDM training and how adaptive MDM inference can mitigate the issues raised by vanilla MDM training and elicit any order reasoning. + +We also want to mention the concurrent work by Peng et al. (2025) that proposes an alternative adaptive inference strategy by selecting $\mathcal { F } ( \boldsymbol { \theta } , \boldsymbol { x } _ { t } )$ based on the BERT model or the denoiser itself. In particular, Peng et al. (2025) uses the BERT model or the denoiser to obtain the uncertainty of a token and then uses Top- $K$ to decide the positions to unmask it. In contrast to their work, we disentangle the impact of token ordering on MDM training vs. MDM inference and provide a more complete understanding of the motivations for and benefits of adaptive inference. Additionally, our results indicate drawbacks to using Top- $K$ strategy as opposed to Top- $K$ margin in deciding which tokens to unmask when there are multiple values with high probabilities. + +Beyond autoregressive models. Efforts to learn the natural language using non-autoregressive modeling began with BERT (Devlin et al., 2019). Non-causal approaches can take advantage of the understanding the text data representation. + +(Chang et al., 2022) adopted a similar approach for learning image representations. Building on these intuitions, (Shih et al., 2022; Hoogeboom et al., 2021a) proposed any-order modeling, which allows a model to generate in any desired order. Shih et al. (2022) made the same observation that any-order models by default have to solve exponentially more masking problems than autoregressive models. However, whereas our work shows that learning in the face of this challenging task diversity can benefit the model at inference time, their work sought to alleviate complexity at training time by reducing the number of masking problems that need to be solved. + +# B. Technical details from Section 3 + +Notations. Throughout this section, we use $x ^ { i }$ to denote the $i$ -th coordinate of the vector $x$ and $z ( j )$ to denote the $j$ -th example. The $i$ -th coordinate of the vector $z ( j )$ is denoted by $z ( j ) ^ { i }$ . + +# B.1. Additional example: sparse parity observations + +Example B.1 (Noisy sparse parity observations). Let $m = 2$ , $k \in \mathbb { N } ,$ , and $N ^ { 2 } \log N \ll P \leq N ^ { 0 . 4 9 k }$ . $F i x$ noise rate $\eta > 0$ as well as strings $z ( 1 ) , \ldots , z ( P )$ sampled independently and uniformly at random from the set of $k$ -sparse strings in $\{ 0 , 1 \} ^ { N }$ . For each $j \in [ P ]$ , define ${ \mathcal { O } } _ { j } ( x )$ to be the distribution which places mass $1 - \eta$ on 1 (resp. 2) and mass $\eta$ on 2 (resp. 1) $i f$ $\Sigma _ { i } x ^ { i } z ( j ) ^ { i }$ is odd (resp. even). Note that for $k = O ( 1 )$ , each of these observations is efficiently learnable by brute-force. + +Below we show that for a certain range of masking fractions, a constant fraction of the masking problems for the corresponding L&O distributions are computationally hard under the Sparse Learning Parity with Noise assumption (Alekhnovich, 2003). Formally we have: + +Proposition B.2. Let $0 < \alpha < 1$ be an arbitrary absolute constant, and let $\eta = 1 / \mathrm { p o l y } ( N )$ be sufficiently large. Let x be a sample from a L&O distribution $p _ { \mathrm { d a t a } }$ with noisy parity observations as defined in Example B.1. Suppose each token is independently masked with probability $\alpha _ { i }$ , and $M$ is the set of indices for the masked tokens. If $1 - 1 / N \le \alpha \le 1 - 1 / 2 N _ { }$ , then under the Sparse Learning Parity with Noise (SLPN) assumption (see Definition B.3), with constant probability over $M$ , no polynomial-time algorithm can solve the resulting masking problem of predicting any of the masked tokens among $x ^ { \pi ( 1 ) } , \bar { . } . . , \bar { x } ^ { \pi ( N ) }$ given $x [ M ]$ . + +We note that it is important for us to take the observations to be sparse parities and to leverage the Sparse Learning Parity with Noise assumption. If instead we used dense parities and invoked the standard Learning Parity with Noise (LPN) assumption, we would still get the hardness of masking problems, but the observations themselves would be hard to learn, assuming LPN. This result is based on the following standard hardness assumption: + +Definition B.3 (Sparse Learning Parity with Noise). Given input dimension $N$ , noise parameter $0 < \eta < 1 / 2$ , and sample size $P$ , an instance of the Sparse Learning Parity with Noise (SLPN) problem is generated as follows: + +• Nature samples a random bitstring $x$ from $\{ 0 , 1 \} ^ { N }$ +• We observe $P$ examples of the form $( x ( i ) , y ( i ) )$ where $x ( i )$ is sampled independently and uniformly at random from $k$ -sparse bitstrings in $\{ 0 , 1 \} ^ { N }$ , and $y$ is given by $\epsilon _ { i } + \langle x ( i ) , x \rangle$ (mod 2), where $\epsilon _ { i }$ is 1 with probability $\eta$ and 0 otherwise. + +Given the examples $\{ ( x ( i ) , y ( i ) ) \} _ { i = 1 } ^ { P }$ , the goal is to recover $x$ + +The SLPN assumption is that for any $P = N ^ { ( 1 - \rho ) k / 2 }$ for constant $0 < \rho < 1$ , and any sufficiently large inverse polynomial noise rate $\eta$ , no $\mathrm { p o l y } ( N )$ -time algorithm can recover $x$ with high probability. + +Proof of Proposition B.2. With probability at least $1 - ( 1 - 1 / N ) ^ { N } \ge \Omega ( 1 )$ , all of the variable tokens $x ^ { \pi ( i ) }$ for $i \leq N$ are masked. Independently, the number of unmasked tokens among the observation tokens ${ \mathcal { O } } _ { j }$ is distributed as $\mathrm { B i n } ( P , 1 - \alpha )$ , so by a Chernoff bound, with probability at least $1 - e ^ { - \Omega ( P / N ^ { 2 } ) } = 1 - 1 / \mathrm { p o l y } ( N )$ we have that at least $P / 4 N = \Omega ( N \log N )$ observation tokens are unmasked. The masking problem in this case amounts to an instance of SLPN with input dimension $N$ and sample size in $[ \Omega ( N \log N ) , O ( N ^ { 0 . 4 9 k } ) ]$ . Because of the lower bound on the sample size, prediction of $\mathbf { x } ^ { M }$ is information-theoretically possible. Because of the upper bound on the sample size, the SLPN assumption makes it computationally hard. As a result, estimating the posterior mean on any entry of $\mathbf { x } ^ { M }$ given the unmasked tokens is computationally hard as claimed. □ + +# B.2. Additional example: random slab observations + +Example B.4 (Random slab observations). Let $m = 2$ and $P = \gamma N ^ { 2 }$ for constant $\gamma > 0$ . Fix slab width $\beta$ and vectors $z ( 1 ) , \ldots , z ( P )$ sampled independently from √ $\mathcal { N } ( 0 , I )$ . For each $j \in [ P ]$ , define the corresponding observation ${ \mathcal { O } } _ { j } ( x )$ to be deterministically 1 $\begin{array} { r } { \cdot i f { \left| \left. z ( j ) , 2 x - \mathbf { 1 } \right. \right| } \leq \beta \sqrt { N } } \end{array}$ , and deterministically 0 otherwise. + +In (Alaoui $\&$ Gamarnik, 2024), it was shown that stable algorithms (Definition B.7), which encompass many powerful methods for statistical inference like low-degree polynomial estimators, MCMC, and algorithmic stochastic localization (Gamarnik, 2021), are unable to sample from the posterior distribution over a random bitstring conditioned on it√ satisfying $| \langle z ( j ) , x \rangle | \le \beta \sqrt { N }$ for any $\Theta ( N )$ number of constraints $z ( 1 ) , \ldots , z ( P ^ { \prime } )$ , provided $P ^ { \prime }$ is not too large that the support of the posterior is empty. This ensemble is the well-studied symmetric perceptron (Aubin et al., 2019). The following is a direct reinterpretation of the result of (Alaoui & Gamarnik, 2024): + +Proposition B.5. Let $p _ { \mathrm { d a t a } }$ be a L&O distribution with random slab observations as defined in Example B.4, with parameter $\gamma > 0$ and slab width $\beta > 0$ . There exists a constant $c _ { \beta } > 0$ such that for any absolute constant $0 ~ < ~ c ~ < ~ c _ { \beta }$ , $i f$ $1 - c _ { \beta } N / 2 P \le \alpha \le 1 - c N / P$ and $\gamma > c _ { \beta }$ , the following holds. Let $p _ { \mathrm { d a t a } } ^ { \prime }$ denote the distribution given by independently√ masking every coordinate in $p _ { \mathrm { d a t a } }$ with probability $\alpha$ . Then any $( 1 - \tilde { \Omega } ( 1 / \sqrt { N } ) )$ -stable algorithm, even one not based on masked diffusion, which takes as input a sample $x ^ { \prime }$ from $p _ { \mathrm { d a t a } } ^ { \prime }$ and, with probability $1 - o ( 1 )$ outputs a Wassersteinapproximate3 sample from $p _ { \mathrm { d a t a } }$ conditioned on the unmasked tokens in $x ^ { \prime }$ , must run in super-polynomial time. + +The upshot of this is that any stable, polynomial-time masked diffusion sampler will, with non-negligible probability, encounter a computationally hard masking problem at some point during the reverse process. + +For the proof, we first formally define the (planted) symmetric Ising perceptron model: + +Definition B.6. Let $\alpha , \beta > 0$ . The planted symmetric Ising perceptron model is defined as follows: + +• Nature samples $\sigma$ uniformly at random from $\{ \pm 1 \} ^ { N }$ +• For each √ $j = 1 , \ldots , P = \lfloor \alpha N \rfloor$ , we sample $z ( j )$ independently from $\mathcal { N } ( 0 , I _ { N } )$ conditioned on satisfying $| \langle z ( j ) , \sigma \rangle | \leq$ $\beta \sqrt { N }$ . + +The goal is to sample from the posterior on $\sigma$ conditioned on these observations $\{ z ( i ) \} _ { i = 1 } ^ { P }$ + +Next, we formalize the notion of stable algorithms. + +Definition B.7. Given a matrix $Z \sim \mathcal { N } ( 0 , 1 ) ^ { \otimes P \times N }$ , define $Z _ { t } = t Z + \sqrt { 1 - t ^ { 2 } } Z ^ { \prime }$ for independent $Z ^ { \prime } \sim \mathcal { N } ( 0 , 1 ) ^ { \otimes P \times N }$ . A randomized algorithm $\mathcal { A }$ which takes as input $Z \in \mathbb { R } ^ { P \times N }$ and outputs an element of $\{ \pm 1 \} ^ { N }$ is said to be $t _ { N }$ -stable if $\begin{array} { r } { \operatorname* { l i m } _ { N \infty } W _ { 2 } ( \mathrm { l a w } ( \boldsymbol { A } ( Z ) ) , \mathrm { l a w } ( \boldsymbol { A } ( Z _ { t } ) ) ) = 0 } \end{array}$ . + +As discussed at depth in (Gamarnik, 2021), many algorithms like low-degree polynomial estimators and Langevin dynamics are stable. + +Theorem B.8 (Theorem 2.1 in (Alaoui & Gamarnik, $2 0 2 4 ) ^ { 4 }$ ). For any constant $\beta > 0$ , there exists $c _ { \beta } > 0$ such that the following holds for all constants $0 < \alpha < c _ { \beta }$ . For $t _ { N } \le 1 - \Omega ( \log ^ { 2 } ( n ) / n ^ { 2 } )$ , any $t _ { N }$ -stable randomized algorithm $\mathcal { A }$ which takes as input $Z = ( z ( 1 ) , \ldots , z ( P ) )$ and outputs an element of $\{ \pm 1 \} ^ { N }$ will fail to sample from the posterior on √ $\sigma$ conditioned on $Z$ in the symmetric Ising perceptron model to Wasserstein error $o ( \sqrt { N } )$ . + +Proof of Proposition B.5. By a union bound, with probability at least $1 - ( 1 - \alpha ) N \ge 1 - c _ { \beta } N ^ { 2 } / P \ge 1 - c _ { \beta } / \gamma$ over a draw $x ^ { \prime } \sim p _ { \mathrm { d a t a } } ^ { \prime }$ , all of the $\bar { x ^ { \pi ( i ) } }$ tokens are masked. The number of unmasked tokens in $x ^ { \prime }$ among the observations ${ \mathcal { O } } _ { j }$ is distributed as $\mathrm { B i n } ( P , 1 - \alpha )$ . By a Chernoff bound, this is in $\left[ 3 c N / 4 , 3 c _ { \beta } N / 4 \right]$ with at least constant probability. The claim then follows immediately from Theorem B.8 above. + +# B.3. Proof outline of Proposition 3.3 + +To understand the proof idea, we consider the case where all the latent tokens are masked and some of the observation tokens are unmasked. In this case, the prediction task reduces to learning to recover the latent tokens that are consistent with the observations. Intuitively, each observation provides some constraints and the task is to recover an assignment that satisfies the constraints. This is reminiscent of Constraint Satisfaction Problems (CSPs). Indeed, to show the hardness result, we use the rich theory developed for planted CSPs at the intersection of statistical physics and average-case complexity. + +![](images/figures/masked-diffusion-token-ordering-fig-0005.jpg) +Figure 4. Overlap achieved by belief propagation initialized at ground truth versus random for planted CSP with $k = 3$ , $m = 3$ , and $g = \mathrm { N A E }$ , for $N = 1 0 0 0 0$ and varying choices of average degree $D$ . $D _ { \mathrm { K S } } / K$ can be shown analytically to be 64, consistent with the phase transition depicted. Plot suggests $D _ { \mathrm { c o n d } } / K \approx 5 0$ . By Prop. 3.3 this implies a range of masking fractions at which $\Omega ( 1 )$ fraction of masking problems are computationally hard. + +In a planted CSP, there is an unknown randomly sampled vector $y$ of length $N$ and, one is given randomly chosen Boolean constraints which $y$ is promised to satisfy, and the goal is to recover $y$ as best as possible (see Definition B.9). Prior works have shown the hardness of efficiently learning to solve the planted CSP problem (Krzakala & Zdeborova´, 2009; Alaoui & Gamarnik, 2024). We show the hardness of masking problems in L&O distributions based on these results. Consider the ground truth latent tokens as the random vector $y$ and each observation as a constraint. In this case, the problem of learning to recover the latent tokens from the observation tokens reduces to recovery for the planted CSP. + +There are precise predictions for the values of vocabulary size $m$ and the number of observations for which the informationtheoretically best possible overlap and the best overlap achievable by any computationally efficient algorithm are different. We show that these predictions directly translate to predictions about when masking problems become computationally intractable: + +As a simple example, let us consider sparse predicate observations with $k = 2$ and $g ( x ^ { \prime } , x ^ { \prime \prime } ) = \mathbf { 1 } [ x ^ { \prime } \neq x ^ { \prime \prime } ]$ . These can be formally related to the well-studied problem of planted $m$ -coloring. In the planted $m$ -coloring, a random graph of average degree $D$ is sampled consistent with an unknown vertex coloring and the goal is to estimate the coloring as well as possible (Krzakala & Zdeborova´, 2009), as measured by the overlap of the output of the algorithm to the ground-truth coloring (see Definition B.9). As a corollary of our main result, we show that when all the latent tokens $x ^ { \pi ( 1 ) } , \ldots , x ^ { \pi ( N ) }$ are masked and a few unmasked observation tokens provide the information of the form $g ( x ^ { \pi ( i ) } , x ^ { \pi ( j ) } ) = \mathbf { 1 } [ x ^ { \pi ( i ) } \neq x ^ { \pi ( j ) } ]$ for $i , j \le N$ , then solving the masking problem can be reduced to solving planted coloring. + +For planted $m$ -coloring, when $m = 5$ the thresholds in Proposition 3.3 are given by $D _ { \mathrm { K S } } / 2 = 1 6$ and $D _ { \mathrm { c o n d } } / 2 \approx$ 13.23 (Krzakala & Zdeborova´, 2009) (the factor of 2 here is simply because the observations correspond to ordered subsets of size 2). For general predicates and arities, there is an established recipe for numerically computing $D _ { \mathrm { K S } }$ and $D _ { \mathrm { c o n d } }$ based on the behavior of the belief propagation algorithm (see the discussion in Appendix B.4). As an example, in Fig. 4, we execute this recipe for $m = 3$ , $k = 3$ , and $g$ given by the Not-All-Equal predicate $\mathrm { N A E } ( x ^ { \prime } , x ^ { \prime \prime } , x ^ { \prime \prime } ) = 1 - 1 [ x ^ { \prime } = x ^ { \prime \prime } = x ^ { \prime \prime \prime } ]$ to obtain thresholds that can be plugged into Proposition 3.3. + +Additional examples of the hardness. The above setup can also be generalized to capture Bayesian constraint satisfaction problems (Montanari, 2008; Liu et al., 2022), one notable example of which is the stochastic block model (Decelle et al., 2011). There are analogous predictions for the onset of hardness of inference, which can likewise be translated to hardness of masking problems for seemingly benign L&O distributions. In Appendix B.1 and B.2, we give two more examples of L&O distributions for which order-aware training is tractable yet order-agnostic training of the MDM is computationally + +hard. + +First, we consider L&O distributions whose observations are sparse, noisy parities in the latents and deduce hardness for order-agnostic training from the Sparse Learning Parity with Noise assumption (Alekhnovich, 2003). We then consider L&O distributions whose observations are generalized linear models in the latents, and deduce hardness for a large class of efficient algorithms from existing results on Lipschitz hardness (Alaoui & Gamarnik, 2024) for the symmetric binary perceptron (Aubin et al., 2019). + +# B.4. Proof of Proposition 3.3: sparse predicate observations + +Here we formally define the relevant notions needed to formalize our claim about hardness in Proposition 3.3. + +Definition B.9 (Planted CSPs). Given arity $k \in \mathbb N$ , vocabulary/alphabet size $m \in \mathbb { N }$ , predicate $g : \{ 1 , . . . , m \} ^ { k } \{ 0 , 1 \}$ latent dimension $N$ , and clause density $P / N$ , the corresponding planted constraint satisfaction problem is defined as follows: Nature samples an unknown assignment $\sigma$ uniformly at random from $\{ 1 , \ldots , m \} ^ { N }$ , and then for each ordered $k$ -tuple $S$ of distinct elements from $[ N ]$ , we observe the clause $S$ independently with probability $\phi / N ^ { k - 1 }$ if $g ( \sigma \vert _ { S } ) = 1$ . + +To measure the quality of an algorithm for recovering $\sigma$ given the observations, define the overlap between an estimate $\hat { \sigma }$ and the ground truth $\sigma$ by $d ( \sigma , \hat { \sigma } ) \triangleq \operatorname* { m i n } _ { \pi \in \mathbb { S } _ { N } } \sum _ { i } { \bf 1 } [ \sigma _ { i } = \pi ( \hat { \sigma } _ { i } ) ]$ where $\mathbb { S } _ { N }$ denotes the set of all permutations of $\{ 0 , 1 , \ldots , N - 1 \}$ . Define the average degree to be $k P / N$ , i.e. the expected number of variables that share at least one clause with a given variable. + +We begin by defining the central algorithm driving statistical physics predictions about hardness for random constraint satisfaction problems: belief propagation (BP). + +Definition B.10 (BP update rules). Belief propagation is an algorithm that iteratively updates a set of messages $\{ \mathbf { M S } _ { c } ^ { i \to S } [ t ] , \mathbf { M S } _ { c } ^ { S \to i } [ t ] \}$ , where $i , S$ range over all pairs of variable indices $i \in [ N ]$ and observations $S \ni i$ . At time $t + 1$ , the messages are computed via + +$$ +\begin{array} { r l } & { \mathbf { M S } _ { c } ^ { i S } [ t + 1 ] \propto \displaystyle \prod _ { T : i \in T \neq S } \mathbf { M S } _ { c } ^ { T i } [ t ] } \\ & { \mathbf { M S } _ { c } ^ { S i } [ t + 1 ] \propto \displaystyle \sum _ { \overline { { \sigma } } \in \{ 1 , \ldots , m \} ^ { S \setminus i } } g ( \overline { { \sigma } } \cup _ { i } c ) \prod _ { j : i \neq j \in S } \mathbf { M S } _ { \overline { { \sigma } } _ { j } } ^ { j S } [ t ] , } \end{array} +$$ + +where $\overline { { \sigma } } \cup _ { i } c \in \{ 1 , \ldots , m \} ^ { S }$ assigns $c$ to entry $i$ and $\overline { { \sigma } }$ to the remaining entries. + +A set of messages can be used to estimate the marginals of the posterior on $\sigma$ conditioned on the observations as follows. The marginal on the $i$ -th variable has probability mass function over $\{ 1 , \ldots , m \}$ proportional to $\{ \textstyle \prod _ { T : i \in T } \mathbf { M S } _ { c } ^ { T \to i } \}$ . Given a set of marginals, a natural way to extract an estimate for $\sigma$ is to round to the color in $\{ 1 , \ldots , m \}$ at which the probability mass function is largest. + +Throughout we will make the following assumption that ensures that the trivial messages $ { \mathbf { M } } { \mathbf { S } } _ { c } ^ { i \to S } = 1 / m$ and $\mathbf { M S } _ { c } ^ { S i } = \mathbf { \Phi }$ $1 / m$ are a fixed point, sometimes called the paramagnetic fixed point, for the iteration above: + +Assumption B.11. The quantity ${ \textstyle \sum _ { { \overline { { \sigma } } } \in \{ 1 , \dots , m \} ^ { [ k ] } \setminus i } g ( { \overline { { \sigma } } } \cup _ { i } c ) }$ is constant across all $c \in \{ 1 , \ldots , m \}$ and $i \in [ k ]$ + +Definition B.12. Given $k , m , g$ , the Kesten-Stigum threshold $D _ { \mathrm { K S } }$ is defined to be the largest average degree for which BP is locally stable around the paramagnetic fixed point, that is, starting from a small perturbation of the paramagnetic fixed point, it converges to the paramagnetic fixed point. More formally, $D _ { \mathrm { K S } }$ is the largest average degree at which the Jacobian of the BP operator $\{ { \bf M S } ^ { i \stackrel { . . . } { } S } [ t ] \} \stackrel { \bf { \sigma } } { \mapsto } \{ { \bf M S } ^ { i S } [ t + 1 ] \}$ has spectral radius less than 1. + +The condensation threshold $D _ { \mathrm { c o n d } }$ is defined to be the largest average degree at which the planted CSP ensemble and the following simple null model become mutually contiguous and thus statistically indistinguishable as $N \infty$ . The null model is defined as follows: there is no single unknown assignment, but instead for every ordered subset $S$ of $k$ variables, Nature independently samples an unknown local assignment $\sigma _ { S } \in \{ 1 , \dots , m \} ^ { S }$ , and the observation is included with probability $\phi / N ^ { k - 1 }$ if $g ( \sigma _ { S } ) = 1$ . + +For $D _ { \mathrm { c o n d } } < k P / N < D _ { \mathrm { K S } }$ , there exists some other fixed point of the BP operator whose marginals, once rounded to an assignment, achieves strictly higher overlap than does BP with messages initialized randomly. The prediction is that in this regime, no efficient algorithm can achieve optimal recovery (Krzakala & Zdeborova´, 2009). + +Conjecture B.13 (1RSB cavity prediction). Suppose $k , m , g$ satisfy Assumption B.11, and let $D _ { \mathrm { K S } }$ and $D _ { \mathrm { c o n d } }$ denote the associated Kesten-Stigum and condensation thresholds for the average degree. Then for all $P$ for which $D _ { \mathrm { c o n d } } < k P / N <$ $D _ { \mathrm { K S } }$ , the best overlap achieved by a computationally efficient algorithm for recovering $\sigma$ is strictly less than the best overlap achievable. + +Proof of Proposition 3.3. At masking fraction $\alpha$ satisfying the bounds in the Proposition, with probability at least $\alpha ^ { N } \geq$ $( 1 - \gamma ^ { - 1 } D _ { \mathrm { K S } } / N ^ { k - 1 } ) ^ { N } \geq \Omega ( 1 )$ we have that all tokens corresponding to latents $x _ { \pi ( i ) }$ get masked. Independently of this, the number of unmasked tokens among the observation tokens ${ \mathcal { O } } _ { S }$ is distributed as $\mathrm { B i n } ( \dot { N } ( N - 1 ) \cdots ( N - k + 1 ) , 1 - \alpha )$ , so by standard binomial tail bounds, with constant probability (depending on the gap between $D _ { \mathrm { c o n d } }$ and $D _ { \mathrm { K S } } ,$ ) this lies between $\gamma ^ { - 1 } D _ { \mathrm { c o n d } } N / k$ and $\gamma ^ { - 1 } D _ { \mathrm { K S } } N / k$ . Furthermore, of these unmasked tokens in expectation $\gamma$ fraction of them correspond to observations for which the associated predicate evaluates to 1. Conditioned on the above events, the masking problem thus reduces exactly to inference for a planted constraint satisfaction problem at average degree $D _ { \mathrm { c o n d } } < D < D _ { \mathrm { K S } }$ , from which the Proposition follows. + +# C. Experimental details in Section 3 + +# C.1. Experimental details in Section 3.2 + +$\pi$ -learner configurations. We consider two distributions of $\pi$ that interpolate between Unif $\left( \mathbb { S } _ { L } \right)$ where $\mathbb { S } _ { L }$ denote the uniform distribution over all permutations of indices $\{ 0 , 1 , \ldots , L - 1 \}$ and the point mass at the identical distribution: (Closer) and (Much-closer). To construct those distributions, we start from the identity permutation and perform a certain number of random swapping operations. Since $L \log ( L )$ number of swaps results in a distribution that is very close to Unif $\left( \mathbb { S } _ { L } \right)$ (Bormashenko, 2011), we use $L / 1 0$ and $\sqrt { L }$ swaps to construct the (Closer) and (Much-closer) distributions, respectively. For consistency, we repeat this sampling process three times. + +Model and training configurations. As explained in Section 3.2, to evaluate the scaling law of the $\pi$ -learner, we can simply adapt the autoregressive training setup (a transformer with causal attention) by modifying the input to $\pi ( \boldsymbol { x } _ { 0 } )$ and using a learnable positional embedding layer instead of RoPE. We borrow the training configurations from (Nie et al., 2024), which are also consistent with the TinyLlama (Zhang et al., 2024) configurations. In particular, we use AdamW optimizer (Loshchilov & Hutter, 2017), setting $\beta _ { 1 } = 0 . 9$ , $\beta _ { 2 } = 0 . 9 5$ , and a weight decay of 0.1 and $L = 2 0 4 8$ . A cosine learning rate schedule is applied, with a maximum learning rate of $4 \times 1 0 ^ { - 4 }$ and a minimum learning rate of $4 \times 1 0 ^ { - 5 }$ . We also note that unless otherwise specified, we maintain the same training configuration throughout the paper. + +Examining scaling laws. We conduct IsoFLOP analysis (Hoffmann et al., 2022). For a given number of FLOPs $C$ , by varying the number of non-embedding parameters of transformers, we set the iteration numbers so that the total number of tokens observed by the model during training equals $C / 6 N$ , following prior studies (Hoffmann et al., 2022; Kaplan et al., 2020). We then select the smallest validation loss and set it as a data point. + +# C.2. Experimental details in Section 3.3 + +# C.2.1. EXPERIMENT ON L&O-NAE-SAT DISTRIBUTION + +We consider the L&O-NAE-SAT distribution with $( N , P ) = ( 2 0 , 2 8 0 )$ . For each example sequence from L&O-NAE-SAT, we pad the last 212 tokens with an additional token value of 2. We employ a 19M MDM with RoPE and a maximum sequence length of 512. Then, this MDM is trained for $2 \times 1 0 ^ { 3 }$ iterations. To attain a proxy MDM for the Bayes optimal predictor, we further train it for $5 \times 1 0 ^ { 4 }$ iterations. + +To measure the error across different tasks, we consider the following setup. For each $\ell \in [ 1 , N - 1 ]$ , we randomly mask $\ell$ tokens in the latent positions and $\ell \times ( P / N )$ tokens in the observed positions. Across all masked prediction positions, $\ell ( 1 + P / N )$ , we measure the error for each position. For certainty, we repeat this process 1000 times. The result in Figure 2 corresponds to the case when $\ell = 1 1$ , and we observe the same tendency for other values of $\ell$ . + +# C.2.2. EXPERIMENT ON TEXT DATA + +We take a 170M MDM pretrained with text data for a baseline model. To measure the performance imbalance between likelihood modeling tasks + +$$ +\mathbb { E } _ { \boldsymbol { x } _ { 0 } \sim p _ { \mathrm { d a t a } } } \left[ \sum _ { i = 0 } ^ { L - 1 } \log p _ { \theta } \left( x _ { 0 } ^ { \pi ( i ) } \Big | \boldsymbol { x } _ { 0 } [ \pi \{ i , \dots , L - 1 \} ] \right) \right] . +$$ + +As done in the experiments in Section 3.2, we sample $\pi \mathbf { S }$ from three different distributions: $\mathrm { U n i f } ( \mathbb { S } _ { L } )$ , (Closer), the point mass of identical distribution. For each case, we calculate the expectation over 1024 samples of $x _ { 0 } \sim p _ { \mathrm { d a t a } }$ . + +# D. Experimental details in Section 4 + +# D.1. Experimental details in Section 4.2 + +D.1.1. EXPERIMENT ON L&O-NAE-SAT DISTRIBUTION + +We consider five instances of L&O-NAE-SAT: $( N , P ) = ( 2 5 , 2 7 5 ) , ( 3 0 , 2 7 0 ) , ( 4 0 , 2 6 0 ) , ( 5 0 , 2 5 0 )$ , (100, 200). For each distribution, we train a 19M MDM and measure the accuracy difference between vanilla inference and adaptive inference using top probability margin. + +# D.1.2. EXPERIMENT ON TEXT DATA + +Top probability margin sampler with temperature. To modify our inference for text data modeling, which does not have a determined answer, we found that adding a certain level of temperature to the oracle is useful. This is because the top probability margin or the top probability often leads to greedy sampling, which harms the diversity (entropy) of the generated samples. Therefore, we consider a variant of the oracle as follows, incorporating a Gaussian noise term $\epsilon$ . + +$$ +\mathcal { F } ( \theta , x _ { t } ) = \mathrm { T o p } K \left( \vert p _ { \theta } ( x ^ { i } = j _ { 1 } \vert x _ { t } ) - p _ { \theta } ( x ^ { i } = j _ { 2 } \vert x _ { t } ) \vert + \epsilon \right) . +$$ + +Note that this approach has also been employed for unconditional sampling (Wang et al., 2024; Zheng et al., 2023). + +Generative perplexity and entropy. We employ a 1.1B MDM pretrained on text data as a baseline. For each sampling step, we unconditionally generate samples using both vanilla and adaptive inference. Next, we calculate the likelihood using LLama2-7B as a baseline large language model. Moreover, we denote the entropy of a generated sample $x$ as $\sum p _ { i } \log p _ { i }$ , where $p _ { i } = \# \{ x ^ { i } = i \} / L$ . + +Choice of number of tokens to unmask. We set the number of tokens to unmask $K$ so that the number of unmasked tokens matches that of vanilla MDM inference in expectation. For an inference transition from step $t$ to $s$ , vanilla MDM expects (# mask tokens in the current $\begin{array} { r } { x _ { t } ) \times \frac { \alpha _ { s } - \alpha _ { t } } { 1 - \alpha _ { t } } } \end{array}$ unmasked. Accordingly, we choose $K = ( \#$ mask tokens in the current $x _ { t } ) \times$ $\frac { \alpha _ { s } - \alpha _ { t } } { 1 - \alpha _ { t } }$ . This choice keeps the number of revealed tokens balanced throughout inference. Alternatively, one can sample $K$ stochastically from Binom( $\#$ mask tokens in the current $x _ { t }$ , $\frac { \alpha _ { s } - \alpha _ { t } } { 1 - \alpha _ { t } }$ ). We found that both the deterministic and stochastic choices of $K$ result in comparable generative perplexity. + +This choice of $K$ can be potentially helpful when the network is time-conditioned, since this keeps $\#$ mask tokens in the current $x _ { t } ) \approx ( 1 - \alpha _ { t } ) \times L$ where $L$ is the max sequence length–matching the marginal that the model saw during training. + +# D.2. Experimental details on Sudoku and Zebra puzzles + +Dataset. For both Sudoku and Zebra puzzles, we use the dataset provided in Shah et al. (2024) to train our model. To evaluate our model on the same difficulty tasks, we use the test dataset proposed in Shah et al. (2024). This dataset is created by filtering the puzzles from (Radcliffe, 2020) that can be solved using a fixed list of 7 strategies. To create a hard dataset to evaluate easy-to-hard generalization, we use the remaining puzzles from (Radcliffe, 2020) as they either require a new strategy unseen during the training and/or require backtracking. The hard dataset contains around 1M Sudoku puzzles. + +Model, training, and inference. For the training and inference, we use the codebase of (Ye et al., 2024) with keeping most of the hyperparameters default given in the codebase. For the Sudoku dataset, we use 6M GPT-2 model, and for the Zebra dataset, we use 19M model. We set the learning rate to 0.001 with a batch size of 128 to train the model for 300 epochs. For the inference, we use 50 reverse sampling steps using the appropriate strategy. Additionally, we add Gumbel noise with a coefficient of 0.5 to the MDM inference oracle $\mathcal { F }$ . + +# D.3. Experimental details on LLaDA-8B + +Our evaluation covers two task categories: (i) infilling(HumanEval-Infill and ROCStories) and (ii) instruction–answering (Math). For instruction–answering tasks, we employ a semi-autoregressive sampling strategy, whereas for infilling tasks we retain the non-autoregressive approach. For infilling tasks, the output length is predetermined—matching the size of the masked span—whereas instruction–answering tasks require an explicit length specification. For the latter, we follow the sampling configuration of (Nie et al., 2025). + +For HumanEval-Infill, we adopt the problem set introduced by (Bavarian et al., 2022). Each instance is grouped by the span of the masked code—the region the model must infill—into three categories: single-line, multi-line, and split. The task difficulty rises as the length of the masked span increases. + +# E. Omitted proofs + +Proof of Proposition 2.1. We build on Proposition 3.1 from (Zheng et al., 2024) to obtain the result of Proposition 2.1. We first re-state the result from (Zheng et al., 2024) for the case when the denoising network $p _ { \theta }$ does not depend on the noise-scale $t$ explicitly. Let $x ( n )$ be a sequence with $n$ tokens being masked from $x _ { 0 }$ , and $x ^ { i } ( n )$ denotes the $i ^ { \mathrm { { t h } } }$ token value of the sequence $x ( n )$ . Let $\tilde { q } ( x ( n ) | x _ { 0 } )$ be the probability distribution corresponding to randomly and uniformly masking $n$ tokens of $x _ { 0 }$ . + +Proposition E.1 (Proposition 3.1 of (Zheng et al., 2024)). For clean data $x _ { 0 }$ , let ${ \tilde { q } } ( x ( n ) \mid x _ { 0 } )$ be the discrete forward process that randomly and uniformly masks n tokens of $x _ { 0 }$ . Suppose the noise schedules $\alpha _ { t }$ satisfies $\alpha _ { 0 } = 0$ and $\alpha _ { 1 } = 1$ . Then, the MDM training loss (1) can be reformulated as + +$$ +\mathcal { L } _ { \theta } = - \sum _ { n = 1 } ^ { L } \underset { x ( n ) \sim \tilde { q } ( \cdot | x _ { 0 } ) } { \mathbb { E } } \left[ \frac { 1 } { n } \sum _ { \ell : x ^ { \ell } ( n ) = 0 } \log p _ { \theta } ( x _ { 0 } ^ { \ell } \mid x ( n ) ) \right] . +$$ + +To obtain an alternative formulation of (6), we expand the expectation $x ( n ) \sim \tilde { q } ( { \cdot } \mid x _ { 0 } )$ . Since there are total $L$ positions of $x _ { 0 }$ , we have the probability assigned for each $x ( n )$ equals $1 / { \binom { L } { n } }$ . Therefore, expanding the above equation with the expectation $x ( n )$ and treating $x ( n )$ as $x [ M ]$ for some set $M$ of size $n$ , we obtain the result. + +$$ +\mathcal { L } _ { \theta } = - \sum _ { M \in [ L ] , i \in M } \frac { 1 } { \binom { L } { | M | } } \cdot \frac { 1 } { | M | } \log p _ { \theta } ( x _ { 0 } ^ { \ell } \mid x [ M ] ) . +$$ + +# E.1. Equivalence between the MDM loss and any-order autoregressive loss + +In this section, we will demonstrate the equivalence for MDM loss and any-order autoregressive loss. In particular, for all $x _ { 0 }$ , we show + +$$ +\underset { \mathrm { \times U n i f } ( \mathbb { S } _ { L } ) } { \mathbb { E } } \left[ \sum _ { j = 0 } ^ { L - 1 } \log p \_ { \theta } \left( x _ { 0 } ^ { \pi ( j ) } \Big \vert x _ { 0 } [ \pi \{ j \} , \dots , \pi \{ L - 1 \} ] \right) \right] = - \sum _ { M \subseteq [ L ] , i \in M } \frac { 1 } { \binom { L } { | M | } } \frac { 1 } { | M | } \log p \_ { \theta } ( x _ { 0 } ^ { i } | x _ { 0 } [ M ] ) . +$$ + +We now consider $\{ \pi ( j ) , \dots , \pi ( L - 1 ) \} = M \subseteq [ L ]$ and $\pi ( j ) = i$ and count the number of $\pi \in \mathbb { S } _ { L }$ that induces a specific term $\log p _ { \theta } ( x _ { 0 } ^ { i } | x _ { 0 } [ M ] )$ . To induce the term, for a given $M \in [ L ]$ and $i \in M$ , $\pi$ must satisfy + +$$ +\pi ( j ) = i , \quad \{ \pi ( j ) , \ldots , \pi ( L - 1 ) \} = { \cal M } . +$$ + +The number of $\pi$ that satisfies above is $( L - | M | ) ! \times ( | M | - 1 ) !$ . Using this and the number of total permutations is $L !$ , we obtain the result. + +$$ +\begin{array} { l } { { \displaystyle \operatorname* { \Pi } _ { \pi \sim \operatorname { U n i f } ( \mathcal { S } , \pi ) } \left[ \sum _ { j = 0 } ^ { L - 1 } \log p _ { \theta } \left( x _ { 0 } ^ { \pi ( j ) } \Big | x _ { 0 } [ \pi \{ j \} , \dots , \pi \{ L - 1 \} ] \right) \right] } } \\ { { \displaystyle = \frac { 1 } { L ! } \sum _ { \pi \in \operatorname { U n i f } ( \mathcal { S } , L ) } \sum _ { j = 0 } ^ { L - 1 } \log p _ { \theta } \left( x _ { 0 } ^ { \pi ( j ) } \Big | x _ { 0 } [ \pi \{ j \} , \dots , \pi \{ L - 1 \} ] \right) } } \\ { { \displaystyle = \frac { 1 } { L ! } \sum _ { M \in [ L ] , i \in M } \left[ \log p _ { \theta } ( x _ { 0 } ^ { i } | x _ { 0 } [ M ] ) \times ( L - 1 - | M | ) ! \times ( | M | - 1 ) ! \right] } } \\ { { = \displaystyle \sum _ { M \in [ L ] , i \in M } \frac { 1 } { \left( \lfloor M \rfloor \right) } \frac { 1 } { | M | } \log p _ { \theta } ( x _ { 0 } ^ { i } | x _ { 0 } [ M ] ) . } } \end{array} +$$ \ No newline at end of file diff --git a/papers/masked-diffusion-token-ordering/paper.pdf b/papers/masked-diffusion-token-ordering/paper.pdf new file mode 100644 index 0000000000000000000000000000000000000000..7abfaeed7b845b8676407bae73b13566a6b2a72b --- /dev/null +++ b/papers/masked-diffusion-token-ordering/paper.pdf @@ -0,0 +1,3 @@ +version https://git-lfs.github.com/spec/v1 +oid sha256:622b56cf0efd627ec5dcf179ed907c4693b923e6affa34bd4dd01c9f0e8f6121 +size 1137011 diff --git a/papers/masked-diffusion-token-ordering/sau.json b/papers/masked-diffusion-token-ordering/sau.json new file mode 100644 index 0000000000000000000000000000000000000000..5dec192295a7d69e341bf9fe73a5120bae8ef224 --- /dev/null +++ b/papers/masked-diffusion-token-ordering/sau.json @@ -0,0 +1,342 @@ +{ + "paper_id": "masked-diffusion-token-ordering", + "paper_title": "Train for the Worst, Plan for the Best: Understanding Token Ordering in Masked Diffusions", + "D1": [ + { + "id": "masked-diffusion-token-ordering-D1-001", + "claim": "MDM training uses Adam optimizer with beta1=0.9, beta2=0.95 across all experiments", + "source": "Section 3.2, Appendix C.1" + }, + { + "id": "masked-diffusion-token-ordering-D1-002", + "claim": "MDM training weight decay: 0.1 applied to all model parameters", + "source": "Section 3.2, Appendix C.1" + }, + { + "id": "masked-diffusion-token-ordering-D1-003", + "claim": "Learning rate schedule: cosine decay, max = 4e-4, min = 4e-5", + "source": "Section 3.2, Appendix C.1" + }, + { + "id": "masked-diffusion-token-ordering-D1-004", + "claim": "Training sequence length: L=2048 tokens for standard language modeling setup", + "source": "Section 3.2, Appendix C.1" + }, + { + "id": "masked-diffusion-token-ordering-D1-005", + "claim": "IsoFLOP tokens formula: total tokens = C / (6 * N), where C is FLOPs and N is non-embedding parameters", + "source": "Section 3.2, Appendix C.1" + }, + { + "id": "masked-diffusion-token-ordering-D1-006", + "claim": "L&O-NAE-SAT error analysis: latent variables N = 20, observations P = 280", + "source": "Section 3.3, Appendix C.2" + }, + { + "id": "masked-diffusion-token-ordering-D1-007", + "claim": "L&O-NAE-SAT: pad tokens = 212, pad token value = 2", + "source": "Section 3.3, Appendix C.2" + }, + { + "id": "masked-diffusion-token-ordering-D1-008", + "claim": "L&O-NAE-SAT error analysis: model size = 19M, architecture = MDM with RoPE, max sequence length = 512", + "source": "Section 3.3, Appendix C.2" + }, + { + "id": "masked-diffusion-token-ordering-D1-009", + "claim": "L&O-NAE-SAT error analysis: training iterations = 2000, proxy Bayes iterations = 50000", + "source": "Section 3.3, Appendix C.2" + }, + { + "id": "masked-diffusion-token-ordering-D1-010", + "claim": "L&O-NAE-SAT: masked latent tokens ell = 11, repeat trials = 1000", + "source": "Section 3.3, Appendix C.2" + }, + { + "id": "masked-diffusion-token-ordering-D1-011", + "claim": "L&O-NAE-SAT adaptive inference: 5 (N,P) configurations — (25,275), (30,270), (40,260), (50,250), (100,200)", + "source": "Section 4.2, Table 1" + }, + { + "id": "masked-diffusion-token-ordering-D1-012", + "claim": "Text error analysis: model size = 170M, architecture = MDM pretrained on text", + "source": "Section 3.2, Section 3.3, Appendix C.1" + }, + { + "id": "masked-diffusion-token-ordering-D1-013", + "claim": "Text error analysis: num samples for expectation = 1024", + "source": "Section 3.3, Appendix C.2" + }, + { + "id": "masked-diffusion-token-ordering-D1-014", + "claim": "Text adaptive inference: generator model size = 1.1B, evaluator = LLaMA2-7B", + "source": "Section 4.2, Appendix D.1, Figure 3" + }, + { + "id": "masked-diffusion-token-ordering-D1-015", + "claim": "Sudoku experiment: MDM model size=6M parameters, architecture based on GPT-2 with depth=8, width=256", + "source": "Section 4.2, Section 4.3, Appendix D.2" + }, + { + "id": "masked-diffusion-token-ordering-D1-016", + "claim": "Sudoku: learning rate = 0.001, batch size = 128, epochs = 300", + "source": "Section 4.2, Appendix D.2" + }, + { + "id": "masked-diffusion-token-ordering-D1-017", + "claim": "Sudoku: reverse sampling steps = 50, Gumbel noise coefficient = 0.5", + "source": "Section 4.2, Appendix D.2" + }, + { + "id": "masked-diffusion-token-ordering-D1-018", + "claim": "Zebra: MDM model size = 19M, learning rate = 0.001, batch size = 128, epochs = 300", + "source": "Section 4.2, Section 4.3, Appendix D.2" + }, + { + "id": "masked-diffusion-token-ordering-D1-019", + "claim": "Zebra: reverse sampling steps = 50, Gumbel noise coefficient = 0.5", + "source": "Section 4.2, Appendix D.2" + }, + { + "id": "masked-diffusion-token-ordering-D1-020", + "claim": "LLaDA-8B: model size = 8B, task categories = infilling and instruction-answering", + "source": "Section 4.4, Table 4" + }, + { + "id": "masked-diffusion-token-ordering-D1-021", + "claim": "LLaDA-8B inference strategies: vanilla, top probability, top probability margin", + "source": "Section 4.4" + }, + { + "id": "masked-diffusion-token-ordering-D1-022", + "claim": "Pi-learner: closer swaps = L/10 = 204, much-closer swaps = sqrt(L) = 45, uniform random baseline = L*log(L) = 15606 (all for L = 2048)", + "source": "Section 3.2, Appendix C.1" + }, + { + "id": "masked-diffusion-token-ordering-D1-023", + "claim": "MDM model sizes used across experiments: 6M, 19M, 170M, 1.1B, 8B", + "source": "Section 1, Section 3.3, Section 4.2, Section 4.4" + }, + { + "id": "masked-diffusion-token-ordering-D1-024", + "claim": "ARM baseline model sizes: 42M parameters; evaluator: LLaMA2-7B for perplexity assessment", + "source": "Section 4.3, Table 2, Table 3" + }, + { + "id": "masked-diffusion-token-ordering-D1-025", + "claim": "Sudoku dataset: approximately 1,000,000 hard puzzles, 7 training strategies", + "source": "Section 4.2, Section 4.5" + }, + { + "id": "masked-diffusion-token-ordering-D1-026", + "claim": "Planted CSP NAE-SAT: arity k = 3, vocabulary m = 3, D_KS/k = 64, D_cond/k ≈ 50, N = 10000", + "source": "Section 3.1, Example 3.2, Appendix B.4" + }, + { + "id": "masked-diffusion-token-ordering-D1-027", + "claim": "Planted Coloring: arity k = 2, vocabulary m = 5, D_KS/2 = 16, D_cond/2 ≈ 13.23", + "source": "Section 3.1, Appendix B.1" + }, + { + "id": "masked-diffusion-token-ordering-D1-028", + "claim": "SlimPajama dataset: 627B tokens cleaned and deduplicated", + "source": "Section 3.2" + }, + { + "id": "masked-diffusion-token-ordering-D1-029", + "claim": "L&O-NAE-SAT error repeats = 1000, text error samples = 1024", + "source": "Section 3.3" + } + ], + "D2": [ + { + "id": "masked-diffusion-token-ordering-D2-001", + "claim": "Forward masking per token: q_{t|0}(x_t^i | x_0^i) = Cat(alpha_t * e_{x_0^i} + (1-alpha_t) * e_0). Each token independently masked to 0 with probability 1-alpha_t.", + "source": "Section 2, Eq 1" + }, + { + "id": "masked-diffusion-token-ordering-D2-002", + "claim": "Reverse transition probability: q_{s|t}(x_s^i | x_t, x_0) = Cat(e_{x_t^i}) if x_t^i != 0; Cat((1-alpha_s)/(1-alpha_t)*e_0 + (alpha_s-alpha_t)/(1-alpha_t)*e_{x_0^i}) if x_t^i = 0. Unmasked tokens stay unchanged; masked tokens sample from mixture.", + "source": "Section 2, Eq 2" + }, + { + "id": "masked-diffusion-token-ordering-D2-003", + "claim": "Denoising network approximation: g_theta(x_s^i | x_t) approximates q_{s|t} by replacing true x_0^i with denoising network prediction p_theta(·|x_t, t). Time-embedding-free: p_theta(·|x_t) as x_t encodes t via number of masked tokens.", + "source": "Section 2" + }, + { + "id": "masked-diffusion-token-ordering-D2-004", + "claim": "MDM training loss (continuous): L_theta = integral_0^1 (alpha_t'/(1-alpha_t)) * E[sum_{i: x_t^i=0} -log p_theta(x_0^i | x_t, t)] dt. Minimizes negative log-likelihood only on masked positions.", + "source": "Section 2, Eq 3" + }, + { + "id": "masked-diffusion-token-ordering-D2-005", + "claim": "MDM loss (discrete mask sum, Prop E.1): L_theta = -sum_{n=1}^{L} E_{x(n)~q_tilde(·|x_0)} [1/n * sum_{l: x^l(n)=0} log p_theta(x_0^l | x(n))]. Sum over all possible numbers n of masked tokens.", + "source": "Section 2.1.1, Appendix E.1" + }, + { + "id": "masked-diffusion-token-ordering-D2-006", + "claim": "MDM loss (all masks weighted, Prop 2.1): L_theta = -sum_{M subseteq [L], i in M} 1/binomial(L,|M|) * 1/|M| * E[log p_theta(x_0^i | x_0[M])]. Sum over ALL possible mask subsets, weight inversely proportional to subset size and count.", + "source": "Section 2.1.1, Proposition 2.1, Eq 4" + }, + { + "id": "masked-diffusion-token-ordering-D2-007", + "claim": "MDM loss = expected any-order autoregressive loss (Appendix E.1): L_theta = -E_{pi~Unif(S_L)} [sum_{j=0}^{L-1} log p_theta(x_0^{pi(j)} | x_0[pi{j,...,L-1}])]. MDM solves exponentially more subproblems than ARM (all orders vs. one).", + "source": "Section 2.1.1, Eq 5, Appendix E.1" + }, + { + "id": "masked-diffusion-token-ordering-D2-008", + "claim": "ARM training loss (left-to-right): log p_theta(x_0) = sum_{i=0}^{L-1} log p_theta(x_0^i | x_0[{i,...,L-1}]). Autoregressive model predicts each token from left to right given causal prefix.", + "source": "Section 2.1.1, Eq 6" + }, + { + "id": "masked-diffusion-token-ordering-D2-009", + "claim": "Pi-learner likelihood: log p_theta(x_0) = sum_{i=0}^{L-1} log p_theta(x_0^{pi(i)} | x_0[pi{i,...,L-1}]). Generalized AR model predicting tokens in fixed permutation order pi; used to measure hardness of different generation orders.", + "source": "Section 3.2, Eq 7" + }, + { + "id": "masked-diffusion-token-ordering-D2-010", + "claim": "Vanilla MDM inference Step (a) — select positions: S subseteq {i | x_t^i = 0}, P(i in S) = (alpha_s - alpha_t) / (1 - alpha_t). Randomly chooses which masked positions to unmask at each reverse step.", + "source": "Section 2.1.2, Section 4" + }, + { + "id": "masked-diffusion-token-ordering-D2-011", + "claim": "Vanilla MDM inference Step (b) — sample tokens: x_s^i ~ p_theta(x^i | x_t) for all i in S. For each selected position, sample new token from denoising network's predicted categorical distribution.", + "source": "Section 2.1.2, Section 4" + }, + { + "id": "masked-diffusion-token-ordering-D2-012", + "claim": "Full vanilla MDM inference loop: start from fully masked x_1 = (0,...,0), iterate from t=1 to t=0 in T discretized steps, at each step randomly select positions S via Bernoulli and sample tokens from p_theta.", + "source": "Section 2.1.2, Section 4" + }, + { + "id": "masked-diffusion-token-ordering-D2-013", + "claim": "Adaptive MDM inference Step (a) — oracle position selection: S = F(theta, x_t) subseteq {i | x_t^i = 0}. Instead of random selection, use oracle F to strategically choose which masked positions to unmask next.", + "source": "Section 4" + }, + { + "id": "masked-diffusion-token-ordering-D2-014", + "claim": "Full adaptive MDM inference loop: x_1 fully masked; for k=T downto 1: S = F(theta, x_t); x_s^i ~ p_theta(x^i|x_t) for i in S. Uses oracle F at each step instead of random selection.", + "source": "Section 4" + }, + { + "id": "masked-diffusion-token-ordering-D2-015", + "claim": "Top Probability oracle: certainty(i) = max_j p_theta(x^i=j | x_t); S = argTopK_{i: x_t^i=0} certainty(i). Selects K masked positions with highest maximum predicted probability.", + "source": "Section 4.1" + }, + { + "id": "masked-diffusion-token-ordering-D2-016", + "claim": "Top Probability Margin oracle: certainty(i) = |p_theta(x^i=j1|x_t) - p_theta(x^i=j2|x_t)| where j1=argmax, j2=argmax_{j!=j1}; S = argTopK certainty(i). Better uncertainty estimate when multiple tokens compete at high probability.", + "source": "Section 4.1" + }, + { + "id": "masked-diffusion-token-ordering-D2-017", + "claim": "Top Probability Margin with Gaussian noise (temperature variant): F(theta, x_t) = TopK_i (|p_theta(x^i=j1|x_t) - p_theta(x^i=j2|x_t)| + epsilon_i), epsilon_i ~ N(0, sigma^2). Adds noise to prevent greedy deterministic sampling, used for text generation diversity.", + "source": "Section 4.1, Appendix D.1.2" + }, + { + "id": "masked-diffusion-token-ordering-D2-018", + "claim": "Number of tokens to unmask per step (deterministic K): K = N_masked * (alpha_s - alpha_t) / (1 - alpha_t) where N_masked = |{i: x_t^i = 0}|. Keeps noise-level marginal consistent with training distribution.", + "source": "Section 4.1" + }, + { + "id": "masked-diffusion-token-ordering-D2-019", + "claim": "Gumbel noise augmented oracle (for puzzles): F(theta, x_t) = TopK_i (certainty(i) + gamma * g_i), g_i ~ Gumbel(0,1), gamma = 0.5. Adds Gumbel noise to certainty scores for controlled stochasticity in Sudoku/Zebra inference.", + "source": "Section 4.2, Appendix D.2" + }, + { + "id": "masked-diffusion-token-ordering-D2-020", + "claim": "Chain rule sampling path likelihood (ideal MDM): p_theta(x_0) = prod_{i=0}^{L-1} p_theta(x_0^{pi(i)} | x_0[pi{i,...,L-1}]) = p_data(x_0) for any permutation pi. Under perfect model, all generation orders yield correct likelihood, justifying exploration of adaptive orders.", + "source": "Section 4" + }, + { + "id": "masked-diffusion-token-ordering-D2-021", + "claim": "Token entropy of generated sequence: H(x) = -sum_j p_j * log p_j where p_j = (# occurrences of token j in x) / L. Used to verify adaptive inference maintains generation diversity comparable to vanilla inference.", + "source": "Section 4.2, Figure 3" + }, + { + "id": "masked-diffusion-token-ordering-D2-022", + "claim": "Belief Propagation variable-to-factor update (Def B.10): MS_c^{i->S}[t+1] proportional to prod_{T: i in T, T!=S} MS_c^{T->i}[t]. Variable i sends to factor S the product of incoming messages from all other factors.", + "source": "Appendix B.4" + }, + { + "id": "masked-diffusion-token-ordering-D2-023", + "claim": "Belief Propagation factor-to-variable update (Def B.10): MS_c^{S->i}[t+1] proportional to sum_{sigma_bar in [m]^{S\\i}} g(sigma_bar union_i c) * prod_{j in S\\i} MS_{sigma_bar_j}^{j->S}[t]. Complexity O(m^{k-1}) per factor update.", + "source": "Appendix B.4" + }, + { + "id": "masked-diffusion-token-ordering-D2-024", + "claim": "Belief Propagation marginal estimation: p_hat(sigma_i = c | observations) proportional to prod_{T: i in T} MS_c^{T->i}[converged]. Product of converged factor-to-variable messages, normalized to probability distribution.", + "source": "Appendix B.4" + } + ], + "D3": [ + { + "id": "masked-diffusion-token-ordering-D3-001", + "claim": "Pi-learner scaling laws on text (Section 3.2): Measure how model likelihood degrades as token generation order deviates from left-to-right (natural text order) using pi-learners at different permutation distances — identity (ARM), Closer (L/10 swaps), Much-closer (sqrt(L) swaps), uniform random (MDM). IsoFLOP analysis with AdamW, cosine LR, L=2048.", + "source": "Section 3.2, Figure 2 (left)" + }, + { + "id": "masked-diffusion-token-ordering-D3-002", + "claim": "Error imbalance on L&O-NAE-SAT (Section 3.3): For each masking size ell in [1, N-1], randomly mask ell latent tokens and ell*(P/N) observation tokens, measure MSE error per position. Compare Bayes-optimal predictor (MDM 50000 iters) vs MDM 2000 iters. N=20, P=280, max_seq=512, 19M MDM with RoPE, 1000 repeats.", + "source": "Section 3.3, Figure 2 (right)" + }, + { + "id": "masked-diffusion-token-ordering-D3-003", + "claim": "Error imbalance on text (Section 3.3): Sample permutations pi from three distributions (Unif(S_L), Closer, identity), compute cumulative validation loss E[sum_i log p_theta(x0^{pi(i)} | x0[pi{i,...,L-1}])] using 170M pretrained MDM on 1024 text samples.", + "source": "Section 3.3, Figure 2 (right)" + }, + { + "id": "masked-diffusion-token-ordering-D3-004", + "claim": "Adaptive MDM inference on L&O-NAE-SAT (Section 4.2): Compare Top Probability Margin oracle vs vanilla random selection across 5 synthetic (N,P) configurations. Oracle selects which tokens to unmask based on certainty = |p_theta(x^i=j1|xt) - p_theta(x^i=j2|xt)|. 19M MDM trained per configuration.", + "source": "Section 4.2, Table 1" + }, + { + "id": "masked-diffusion-token-ordering-D3-005", + "claim": "Adaptive MDM inference on text (Section 4.2): Compare Top Probability Margin oracle with Gaussian noise (temperature) vs vanilla inference. K set to match expected vanilla count. Evaluated via generative perplexity (LLaMA-7B) and token entropy. 1.1B MDM pretrained on text, unconditional generation.", + "source": "Section 4.2, Figure 3" + }, + { + "id": "masked-diffusion-token-ordering-D3-006", + "claim": "Sudoku puzzle solving (Sections 4.2-4.3): Compare adaptive MDM inference strategies (Top probability, Top probability margin, both with Gumbel noise gamma=0.5) vs vanilla MDM and ARM (with/without order info via teacher forcing). 6M GPT-2 MDM, 42M ARM, lr=0.001, batch=128, 300 epochs, 50 inference steps. Dataset from Shah et al. 2024 (7-strategy puzzles).", + "source": "Section 4.2, Section 4.3, Table 2" + }, + { + "id": "masked-diffusion-token-ordering-D3-007", + "claim": "Zebra (Einstein) puzzle solving (Sections 4.2-4.3): Same adaptive strategies as Sudoku — Top probability, Top probability margin with Gumbel noise. 19M MDM, 42M ARM, lr=0.001, batch=128, 300 epochs, 50 inference steps. Dataset from Shah et al. 2024.", + "source": "Section 4.2, Section 4.3, Table 3" + }, + { + "id": "masked-diffusion-token-ordering-D3-008", + "claim": "LLaDA-8B adaptive inference on coding and math (Section 4.4): Evaluate Top Probability and Top Probability Margin oracles vs vanilla on large-scale 8B MDM. Infilling tasks (HumanEval-Single/Multi/Split, ROCStories) use non-autoregressive sampling; instruction-answering tasks (Math, MMLU) use semi-autoregressive with explicit length. Inference-only modification, no additional training.", + "source": "Section 4.4, Table 4" + }, + { + "id": "masked-diffusion-token-ordering-D3-009", + "claim": "Easy-to-hard generalization on hard Sudoku (Section 4.5): Test robustness of adaptive MDM inference on ~1M hard puzzles requiring unseen strategies or backtracking. Models trained only on easy 7-strategy puzzles. Compare MDM (vanilla, Top prob, Top margin) vs ARM with teacher-forced ordering. 6M GPT-2 MDM, 42M ARM, same training config.", + "source": "Section 4.5, Table 5" + } + ], + "D4": [ + { + "id": "masked-diffusion-token-ordering-D4-001", + "claim": "Phase 1 (Train MDM): Train time-embedding-free p_θ by minimizing loss over all mask subsets M⊆[L] (Proposition 2.1). Equivalent to expected any-order AR loss, solving Θ(L·2^L) subproblems vs L for ARM. (Section 2, Appendix E.1)", + "source": "Section 2, Section 2.1.1, Proposition 2.1, Appendix E.1" + }, + { + "id": "masked-diffusion-token-ordering-D4-002", + "claim": "Phase 2 (Diagnose error imbalance): (i) L&O-NAE-SAT: measure MDM per-position MSE vs Bayes-optimal proxy across masking sizes, 1k repeats. (ii) Text: train π-learners at four permutation distances from identity, measure cumulative validation loss. Confirms MDM performance varies across subproblems. (Section 3.2-3.3)", + "source": "Section 3.2, Section 3.3, Figure 2" + }, + { + "id": "masked-diffusion-token-ordering-D4-003", + "claim": "Phase 3 (Design oracle for position selection): Implement oracle F(θ,x_t) selecting K masked positions to unmask per reverse step. (A) Top Probability: certainty = max predicted probability. (B) Top Probability Margin: certainty = gap between top-2 probabilities. Augment with Gumbel noise (γ=0.5) for puzzles, Gaussian noise for text diversity. (Section 4.1)", + "source": "Section 4.1" + }, + { + "id": "masked-diffusion-token-ordering-D4-004", + "claim": "Phase 4 (Adaptive inference loop): Start from fully masked x_1. Per reverse step: S=F(θ,x_t) selects positions by certainty, then sample x_s^i∼p_θ(x^i|x_t) for i∈S. T=50 steps for puzzles. Evaluate via GenPPL, prediction accuracy, puzzle solve rate, and task-specific metrics across 5 domains. (Section 4.2-4.5)", + "source": "Section 4.2, Section 4.3, Section 4.4, Section 4.5" + } + ] +} \ No newline at end of file diff --git a/papers/moe-pot/blacklist.txt b/papers/moe-pot/blacklist.txt new file mode 100644 index 0000000000000000000000000000000000000000..840b28e4bd1697c836adb9e8faa91822a4e4024d --- /dev/null +++ b/papers/moe-pot/blacklist.txt @@ -0,0 +1,2 @@ +# Official repository +https://github.com/haiyangxin/MoEPOT diff --git a/papers/moe-pot/config.yaml b/papers/moe-pot/config.yaml new file mode 100644 index 0000000000000000000000000000000000000000..9e38f08260d2e7054e721ddf272fe2d13976ff06 --- /dev/null +++ b/papers/moe-pot/config.yaml @@ -0,0 +1,8 @@ +title: "MoE-POT: Mixture-of-Experts Operator Transformer for Large-Scale PDE Pre-Training" +pdf_url: "https://arxiv.org/pdf/2510.25803.pdf" +venue: "NeurIPS 2025" +year: "2025" +extra: + selection_index: 29 + domain: "Numerical Methods / Scientific Computing" + paradigm: "New Algorithm / Architecture" diff --git 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https://git-lfs.github.com/spec/v1 +oid sha256:9df5a4ef4802cf71b65d878927db219cfd177e8ba056074847702b46a5f74c63 +size 19487 diff --git a/papers/moe-pot/paper.md b/papers/moe-pot/paper.md new file mode 100644 index 0000000000000000000000000000000000000000..3cebb14c59d96b9cec438989f21e43415871007b --- /dev/null +++ b/papers/moe-pot/paper.md @@ -0,0 +1,611 @@ +# Mixture-of-Experts Operator Transformer for Large-Scale PDE Pre-Training + +Hong Wang1,2,3∗, Haiyang $\mathbf { X _ { i m } ^ { \bullet } }$ , Jie $\mathbf { W a n g ^ { 1 , 2 , 3 \dagger } }$ , Xuanze Yang1, Fei $\mathbf { Z } \mathbf { h } \mathbf { a } ^ { \mathbf { 1 } }$ Huanshuo $\mathbf { D o n g ^ { 1 , 2 , 3 } }$ , Yan Jiang1† + +1 University of Science and Technology of China +2 CAS Key Laboratory of Technology in GIPAS, University of Science and Technology of China +3 MoE Key Laboratory of Brain-inspired Intelligent Perception and Cognition, University of Science and Technology of China wanghong1700@mail.ustc.edu.cn, xhy2878@mail.ustc.edu.cn, jiewangx@ustc.edu.cn + +# Abstract + +Pre-training has proven effective in addressing data scarcity and performance limitations in solving PDE problems with neural operators. However, challenges remain due to the heterogeneity of PDE datasets in equation types, which leads to high errors in mixed training. Additionally, dense pre-training models that scale parameters by increasing network width or depth incur significant inference costs. To tackle these challenges, we propose a novel Mixture-of-Experts Pre-training Operator Transformer (MoE-POT), a sparse-activated architecture that scales parameters efficiently while controlling inference costs. Specifically, our model adopts a layer-wise router-gating network to dynamically select 4 routed experts from 16 expert networks during inference, enabling the model to focus on equationspecific features. Meanwhile, we also integrate 2 shared experts, aiming to capture common properties of PDE and reduce redundancy among routed experts. The final output is computed as the weighted average of the results from all activated experts. We pre-train models with parameters from 30M to 0.5B on 6 public PDE datasets. Our model with 90M activated parameters achieves up to a $40 \%$ reduction in zero-shot error compared with existing models with 120M activated parameters. Additionally, we conduct interpretability analysis, showing that dataset types can be inferred from router-gating network decisions, which validates the rationality and effectiveness of the MoE architecture 1. + +# 1 Introduction + +Learning solution operators for partial differential equations (PDEs) has emerged as a fundamental paradigm in scientific machine learning, enabling data-driven modeling of complex physical systems through neural operators [62, 22, 28, 12]. These operators learn mappings between infinitedimensional function spaces, offering surrogate models that can outperform traditional numerical solvers by orders of magnitude in speed [45, 2]. To address the scarcity of PDE data and further enhance the performance of neural operators, recent studies have introduced pre-training techniques into neural operator frameworks [15]. However, their application to PDE learning remains nascent due to unique challenges in operator learning. + +First, (challenge 1) PDE datasets demonstrate substantial variations across equation types, boundary conditions, and spatiotemporal resolutions. This diversity causes conflicting knowledge patterns when merging different PDE types during training—direct data mixing frequently results in detrimental interference that restricts knowledge acquisition, rather than enhancing model generalization. + +![](images/figures/moe-pot-fig-0001.jpg) +Figure 1: Left. An illustration of pre-training a PDE foundation model using extensive data from diverse datasets. The pre-trained model is subsequently fine-tuned for various downstream operator learning tasks, enabling the handling of complex scenarios. Right. (a) Comparison of errors across different numbers of fine-tuning epochs; (b) Comparison of zero-shot errors across different models. + +Second, (challenge 2) existing approaches to scaling model capacity through dense architectural expansion (increasing width/depth) incur prohibitive inference costs, making pre-training impractical for real-world applications. + +To address these challenges, we propose the Mixture-of-Experts Pre-training Operator Transformer (MoE-POT), a novel sparse architecture. Our key insight is to decouple capacity expansion from computational cost through dynamic expert activation. + +Specifically, MoE-POT employs a learnable router-gating network that automatically selects 4 routed experts based on each layer’s input data, working with 2 fixed shared experts. The outputs from these 6 experts are then weighted and aggregated to produce the final result. The router-gating network dynamically selects routed experts that specialize in learning distinctive features of the current PDE category, effectively isolating interference from significantly different PDE data types. Meanwhile, the shared experts act as fixed computational modules for all data. They ensure consistent learning of fundamental dynamic evolution laws. + +The key contributions and advantages of MoE-POT are summarized as follows: + +• Novel Architecture: We introduce MoE-POT, a novel sparse architecture for neural operator pre-training. MoE-POT employs two types of expert networks: routed experts and shared experts, enabling a balance between generalization and specialization. + +• Empirical Validation: As shown in Figure 1 (right), we pre-train models on 6 public PDE datasets and design multiple versions of MoE-POT with total parameter scales ranging from 30M to 0.5B. Our model with 90M activated parameters achieves up to a $40 \%$ reduction in zero-shot error compared to existing models with 120M activated parameters. + +• Interpretable: We observe that the trained router-gating network can infer the PDE type of input data with $98 \%$ accuracy, showing MoE-POT’s ability to effectively handle diverse PDE datasets. This result validates both the rationale and effectiveness of the MoE architecture. + +![](images/figures/moe-pot-fig-0002.jpg) +Figure 2: Left. The impact of mixed training on multiple datasets using FNO on model performance. Right. Usage ratio of routed experts in different datasets in block 4. + +# 2 Preliminaries + +# 2.1 Time-dependent PDE Problem + +We consider a general form of parameterized time-dependent PDEs characterized by variables $\pmb { u } ( x , t ) \in \mathbb { R } ^ { m }$ . These equations satisfy the following conditions: + +$$ +\begin{array} { r l } { \displaystyle \frac { \partial { \boldsymbol { u } } } { \partial t } - \mathcal { F } [ { \boldsymbol { u } } ; { \boldsymbol { \theta } } ] ( { \boldsymbol { x } } , t ) = 0 , } & { \quad ( { \boldsymbol { x } } , t ) \in \Omega \times T \subset \mathbb { R } ^ { d + 1 } , } \\ { { \boldsymbol { u } } ( { \boldsymbol { x } } , 0 ) = { \pmb { u } } ^ { 0 } ( { \boldsymbol { x } } ) , \quad { \boldsymbol { x } } \in \Omega , } & { \quad \mathcal { B } [ { \boldsymbol { u } } ] ( { \boldsymbol { x } } , t ) = 0 , \quad { \boldsymbol { x } } \in \partial \Omega . } \end{array} +$$ + +Here, $\mathcal { F } [ { \pmb u } ; \theta ] ( x , t ) = F ( t , x , \pmb u , \partial _ { x } \pmb u , \partial _ { x x } \pmb u , . . . ; \theta )$ represents a differential operator involving spatial derivatives, while $\theta \in \Theta$ denotes unknown parameters that define the type and coefficients of the PDE. The initial condition is given by ${ \pmb u } ^ { 0 } ( x )$ , and $B [ \pmb { u } ] ( \ b { x } , t )$ specifies the boundary conditions. This general formulation encompasses a variety of fundamental PDEs. + +In practical scenarios, datasets are often collected from multiple PDEs, represented as $\mathcal { D } = \cup _ { k = 1 } ^ { K } \mathcal { D } _ { k }$ , where $\mathcal { D } _ { k } = \{ { \pmb u } _ { i } \} _ { 1 \le i \le N _ { k } }$ . Each solution function $\mathbf { \boldsymbol { u } } _ { i } \in \mathcal { D }$ is discretized on spatiotemporal meshes, expressed as $\pmb { u } _ { i } = ( \pmb { u } _ { i } ^ { 1 } , \dots , \pmb { u } _ { i } ^ { T } )$ , with $\pmb { u } _ { i } ^ { t } = \{ ( x _ { j } , u _ { j } ^ { t } ) : x _ { j } \in \mathscr { X } _ { i } \}$ for $1 \leq t \leq T$ . The spatial meshes $\mathcal { X } _ { i }$ may consist of regular grids or irregular point clouds, depending on the geometry of the domain. The parameters $\theta$ govern the type and specific characteristics of the PDE. However, in many real-world applications, such as climate modeling, only observational trajectories of data are available, while the detailed parameters $\theta$ remain inaccessible. To predict future timesteps, it is essential to infer the most likely $\theta$ implicitly from the observed sequence of $T$ frames $( \pmb { u } _ { i } ^ { 1 } , \dots , \pmb { u } _ { i } ^ { T } )$ . + +# 2.2 Auto-regressive Denoising Pre-training + +To effectively learn from temporal PDE datasets, we propose a neural operator $\mathcal { G } _ { w } ( \boldsymbol { u } ^ { t < T } )$ , parameterized by weights $w$ , which auto-regressively takes $T$ frames as input and predicts the next frame based on the previous frames: + +$$ +\begin{array} { r } { \pmb { u } ^ { T } = \mathcal G _ { w } ( \pmb { u } ^ { 0 } , \dots , \pmb { u } ^ { T - 1 } ) . } \end{array} +$$ + +By predicting the next frame, the model learns an internal representation of the underlying PDE dynamics. However, directly supervising the one-step loss has been shown to be suboptimal [3]. Following DPOT [15], we inject small-scale noise into the input frames. For $\forall t \leq T$ , let $\mathbf { \Delta } _ { \pmb { u } } { < } t$ denote $( \bar { \boldsymbol { u } ^ { 0 } } , \dots , \boldsymbol { u } ^ { t - 1 } )$ , and the noise $\varepsilon$ is sampled as $\varepsilon \sim \mathcal { N } ( 0 , \epsilon \bar { | } | \boldsymbol { u } ^ { < t } | | I )$ . Noise injection improves robustness and reduces the discrepancy between training and inference. + +We adopt the experimental setup of DPOT [15] and FNO [28], which focuses on the challenging scenario where models must infer system dynamics solely from solution trajectories, without access to the governing PDE parameters. In contrast to parameter-informed approaches, our core contribution is the integration of a MoE architecture into this auto-regressive paradigm. + +# 3 Motivation + +# 3.1 Challenges in Dense Neural Operator Pre-training + +Current dense neural operator pre-training approaches face two challenges, which severely limit the model’s ability to generalize across PDE tasks and hinder performance improvement. + +(C1) Performance degradation due to dataset heterogeneity: As shown in Figure 2 (left), our preliminary experiments reveal that the heterogeneity of PDE datasets significantly impacts pretraining effectiveness. For instance, when training on different parameter configurations within the same equation family (e.g., fluid simulation data with varying Reynolds numbers), the average test error increases by only $80 \%$ compared to training on individual datasets. However, when mixing three entirely different equation types for training, the error increases by up to $500 \%$ . + +The properties of different PDE types exhibit substantial variations, yet dense models enforce parameter sharing across all inputs, making it difficult to efficiently absorb diverse PDE knowledge within a unified architecture. For example, when simultaneously learning PDEBench-SWE and PDEBench-DR, the model must encode two fundamentally different differential operators within the same parameter space, leading to negative transfer or inter-task interference [6]. + +(C2) Scaling bottlenecks in model capacity and performance: To better capture the diverse properties of PDEs, increasing model parameters is often necessary to enhance the expressive power of neural operators. However, this approach introduces significant computational cost. As shown in Figure 1 (right), experiments demonstrate that models follow a diminishing returns scaling law: improving model performance requires a substantial increase in parameters. Since all parameters are activated during inference, dense models generate high inference costs, further exacerbating computational challenges. + +# 3.2 Sparse Pre-training with Mixture-of-Experts + +To address these challenges, we propose a PDE pre-training model based on the MoE architecture. + +Efficient scaling with sparse activation: As illustrated in Figure 3, the MoE architecture decomposes the fully connected computation of traditional dense layers into a collaborative mechanism involving parallel expert networks and gated routing. Each Transformer layer consists of 16 routed experts and 2 shared experts. For a given input sample, the router-gating network activates only 4 routed experts, resulting in an actual computational cost equivalent to only $33 \%$ of the total parameters. This mechanism effectively scales model capacity while controlling inference cost (C2). + +Physics-driven gated routing: The dynamic selection capability of the router-gating network provides a natural solution for integrating heterogeneous PDE knowledge (C1). 1. Shared experts: Through cross-task learning, the 2 shared experts are constrained to capture universal physical principles (e.g., conservation laws, symmetry). 2. Routed experts: The remaining 16 experts autonomously develop distinct functional roles to learn the unique characteristics of different PDEs. + +As shown in Figure 2 (right), after training, the router-gating network decisions vary significantly across different PDE datasets. For example, NS(1e-5) and NS(1e-3), which are closely related datasets, exhibit similar gating patterns. In contrast, PDEBench-SWE and PDEBench-DR, which differ substantially, show distinct gating behaviors. More importantly, our interpretability analysis (see Experiment 5.4) reveals that gating weights can serve as identifiers for PDE types. The expert selection for a given input can be used to determine its dataset type with an accuracy of $98 \%$ . This demonstrates that the MoE architecture not only improves performance but also enables implicit equation type recognition for neural operators, paving the way for building interpretable foundational models for PDEs. + +# 4 Method + +Overview. Our proposed model architecture is illustrated in Figure 3. It begins by processing raw data through a patchification layer and a temporal aggregation layer [15], which reduces spatial-temporal resolution and extracts dynamic structures inherent to PDEs. The processed features are then passed through $N$ blocks, each of which contains a Fourier layer [13] and a MoE layer, thereby achieving efficient representation and specialization for diverse PDE tasks. + +![](images/figures/moe-pot-fig-0003.jpg) +Figure 3: An illustration of our model architecture. The process begins by sampling trajectories from mixed datasets of multiple PDEs. The model is optimized by predicting the next frame based on previous frames. The mixture-of-experts layer consists of shared experts, routed experts, and a router-gating network. The router-gating network is responsible for selecting the routed experts, enabling efficient and specialized processing while ensuring scalability and modularity. + +Input Encoding and Temporal Aggregation. The input $\pmb { u } ^ { < T } \in \mathbb { R } ^ { H \times W \times T \times C }$ represents a spatiotemporal signal with $C$ channels. To encode spatial features, we apply a patchification layer with positional embeddings inspired by vision transformers [10]: + +$$ +Z _ { p } ^ { t } = \mathcal { P } ( \pmb { u } ^ { t } + \pmb { p } ^ { t } ) , \quad t = 1 , \dots , T , +$$ + +where $\mathcal { P }$ is a convolutional layer, and $p _ { i , j } ^ { t } = W _ { p } ( x _ { i } , y _ { j } , t )$ denotes learnable positional encodings. The output $Z _ { p } ^ { t } \in \mathbb { R } ^ { H / p \times W / p \times C }$ captures spatial features, with $W _ { p } \in \mathbb { R } ^ { n \times 3 }$ , where $n$ is the feature dimension of the positional encoding (e.g. $n = C$ ). To capture temporal dynamics, we employ a temporal aggregation layer that extracts information across adjacent time steps. For each local node feature $\boldsymbol { z } _ { p } ^ { t } \in \mathbb { R } ^ { \breve { C } }$ in $Z _ { p } ^ { t }$ , we apply a learnable MLP transformation $W _ { t }$ combined with Fourier feature constant $\gamma \in \mathbb { R } ^ { C }$ : + +$$ +z _ { \mathrm { a g g } } = \sum _ { t } W _ { t } \cdot z _ { p } ^ { t } e ^ { - i \gamma t } . +$$ + +This aggregation enables the model to implicitly infer the underlying PDE governing parameters. + +Fourier Layer. Using a multi-head architecture, the Fourier layer is designed to learn complex kernel-based integral transformations that approximate PDE solutions [13, 15]. Let $z ^ { l } ( x )$ denote the feature at spatial location $x$ in the $l$ -th block, and $Z ^ { l }$ its discretized representation. We apply a kernel integral operator $\displaystyle { \mathcal { K } } _ { \phi }$ parameterized by a neural network: + +$$ +( \boldsymbol { \mathcal { K } } _ { \phi } z ^ { l } ) ( x ) = \int _ { \Omega } \boldsymbol { \kappa } ( x , y ; \phi ) z ^ { l } ( y ) \mathrm { d } y , +$$ + +where $\kappa ( x , y ; \phi )$ is a learnable kernel function. To reduce computational complexity, we constrain the kernel to be translation-invariant: $\kappa ( x , y ; \phi ) = \kappa ( x - y ; \phi )$ . This reformulation allows efficient implementation in the Fourier domain: + +$$ +( \mathcal { K _ { \phi } } z ^ { l } ) ( x ) = \mathcal { F } ^ { - 1 } [ R _ { \phi } \cdot \mathcal { F } [ z ^ { l } ] ] . +$$ + +Here, $z ^ { l } ( x ) \in \mathbb { R } ^ { d _ { z } }$ , $R _ { \phi } ( k ) \in \mathbb { C } ^ { d _ { z } \times d _ { z } }$ is a frequency-dependent learnable transformation, and $\mathcal { F } / \mathcal { F } ^ { - 1 }$ denote the Fourier transform and its inverse. To ensure memory efficiency and jointly attend to information from different representation subspaces, we first divide spatial features $z ^ { \bar { l } } ( x )$ into $h$ groups. The grouping is performed on the channel dimension, where $h$ is the number of heads, i.e., $\boldsymbol { z } ^ { l } = \mathrm { C o n c a t } ( \boldsymbol { z } _ { 1 } ^ { l } , \boldsymbol { z } _ { 2 } ^ { l } , \dots \boldsymbol { z } _ { h } ^ { l } )$ , where $z _ { i } ^ { l } ( k ) \in \mathbb { R } ^ { \frac { d _ { z } } { h } }$ . we approximate $( { \kappa _ { \phi } } z ^ { l } ) ( x )$ using $h$ smaller MLPs: + +$$ +z _ { 0 i } ^ { l } ( \boldsymbol { x } ) = \mathcal { F } ^ { - 1 } [ W _ { 2 , i } ^ { l } \cdot \sigma ( W _ { 1 , i } ^ { l } \cdot \mathcal { F } [ z _ { i } ^ { l } ] + b _ { 1 , i } ^ { l } ) + b _ { 2 , i } ^ { l } ] ( \boldsymbol { x } ) , +$$ + +where $W _ { 1 , i } ^ { l } , W _ { 2 , i } ^ { l } \ \in \ \mathbb { R } ^ { d _ { z } / h \times d _ { z } / h }$ and $b _ { 1 , i } ^ { l } , b _ { 2 , i } ^ { l } \in \mathbb { R } ^ { d _ { z } / h }$ are learnable parameters, and $\sigma ( \cdot )$ is an activation function. We set $z _ { 0 } ^ { l } = \mathrm { C o n c a t } ( z _ { 0 1 } ^ { l } , z _ { 0 2 } ^ { l } , \dots z _ { 0 h } ^ { l } )$ , that passed to the MoE layer. + +Mixture of Experts Layer. To facilitate sparse activation and enable expert specialization, we integrate a MoE layer specifically designed for PDE inputs. Both expert networks and routergating networks are implemented using convolutional neural networks (CNNs) to preserve spatial information. For the reason behind the structural design, please see Appendix B.2. These features are then passed to a router-gating network $G ^ { l } ( z _ { 0 } ^ { l } ( x ) )$ , which computes a vector of routing logits $s ^ { l } ( z _ { 0 } ^ { l } ( x ) ) \in \mathbb { R } ^ { N _ { r } }$ , where $N _ { r }$ is the number of routed experts (e.g., $N _ { r } = 1 6$ ). The gating weights are computed via a softmax function: + +$$ +w ^ { l } ( z _ { 0 } ^ { l } ( x ) ) = \mathrm { S o f t m a x } ( s ^ { l } ( z _ { 0 } ^ { l } ( x ) ) ) \in \mathbb { R } ^ { N _ { r } } . +$$ + +To maintain sparsity, only the Top- $K$ entries in $w ^ { l } ( z _ { 0 } ^ { l } ( x ) )$ are retained (e.g., $K = 4$ ), and the rest are masked to zero: + +$$ +\mathrm { T o p K } ( w ^ { l } ( z _ { 0 } ^ { l } ( x ) ) ) = \{ ( i _ { k } , w _ { k } ^ { l } ( z _ { 0 } ^ { l } ( x ) ) ) \} _ { k = 1 } ^ { K } , +$$ + +where $i _ { k }$ is the index of the $k$ -th selected routed expert and $w _ { k } ^ { l } ( x )$ is the normalized routing weight. + +Let $\mathcal { E } _ { s } ^ { l } = \{ E _ { 1 } ^ { l ( s ) } , \dots , E _ { N _ { s } } ^ { l ( s ) } \}$ denote the set of shared experts, which are always activated for every input. Let E lr = {El(r)1 , . $\mathcal { E } _ { r } ^ { l } = \{ E _ { 1 } ^ { l ( r ) } , . . . , E _ { N _ { r } } ^ { l ( r ) } \}$ denote the set of routed experts, from which the top- $K$ are selected dynamically per input. Each expert $E _ { i } ^ { l ( s ) }$ or $E _ { j } ^ { l ( r ) }$ is a convolutional subnetwork that takes $z _ { 0 } ^ { l } ( x )$ as input and maps it to an output feature map of the same shape. Specifically, the final output of the MoE layer is computed as [8, 51]: + +$$ +z ^ { l + 1 } ( x ) = \frac { 1 } { N _ { s } } \sum _ { i = 1 } ^ { N _ { s } } E _ { i } ^ { l ( s ) } ( z _ { 0 } ^ { l } ( x ) ) + \sum _ { k = 1 } ^ { K } w _ { k } ^ { l } ( z _ { 0 } ^ { l } ( x ) ) \cdot E _ { i _ { k } } ^ { l ( r ) } ( z _ { 0 } ^ { l } ( x ) ) . +$$ + +Load Balancing Objective. To encourage uniform utilization of all routed experts and avoid routing collapse [51], we introduce a load balancing loss during the training phase. Following prior work [11, 8], we define the importance of each expert over a batch $\boldsymbol { B }$ ( $| B | = B ,$ ) as the sum of its routing weights: + +$$ +\mathrm { I m p o r t a n c e } _ { i } ^ { l } = \sum _ { b = 1 } ^ { B } w _ { i , b } ^ { l } ( { \boldsymbol { x } } ) . +$$ + +We compute the coefficient of variation (CV) across all $N _ { r }$ routed experts, and define the loss as: + +$$ +\mathcal { L } _ { \mathrm { b a l a n c e } } ^ { l } = w _ { \mathrm { b a l } } \cdot \mathrm { C V } ( \{ \mathrm { I m p o r t a n c e } _ { i } ^ { l } \} _ { i = 1 } ^ { N _ { r } } ) ^ { 2 } , +$$ + +where $w _ { \mathrm { b a l } }$ is a tunable scaling factor (e.g., $w _ { \mathrm { b a l } } = 0 . 1$ ). This auxiliary loss regularizes the routing distribution to maintain a balanced expert load and improves overall training stability. + +Loss Function. The primary objective of the model is to predict the one-step transition between samples from different datasets. The loss function is defined as: + +$$ +\mathcal { L } = \sum _ { 1 \leqslant t \leqslant T } \| \mathcal { G } _ { w } ( \boldsymbol { u } ^ { < t } + \varepsilon ) - \boldsymbol { u } ^ { t } \| _ { 2 } ^ { 2 } + \sum _ { l = 1 } ^ { N } \mathcal { L } _ { \mathrm { b a l a n c e } } ^ { l } , +$$ + +where $\mathcal { G } _ { w }$ represents the model’s prediction function, $\mathbf { \Delta } _ { \pmb { u } } { < } t$ denotes the input from previous timesteps, and $\varepsilon$ is a perturbation term. By predicting the next timestep data from previous frames, the model learns to implicitly infer the PDE’s governing dynamics and propagate the solution forward in time. + +# 5 Experiments + +We conducted comprehensive experiments to evaluate the performance of MoE-POT. This section is organized as follows: 1. Comparison with various small and pre-trained models on 6 PDE datasets. 2. Testing knowledge transfer capabilities on downstream tasks. 3. Investigating scaling laws to understand performance trends. 4. Interpretable analysis of the router-gating network selection. 5. Analyzing model inference time 6. Ablation studies to assess the impact of hyperparameters. + +
DatasetActivatedFNO-νPDEBenchCFDBench
ParamsNS (1e-5)NS (1e-3)CNS(0.1,0.01)SWEDR
Small Model
FNO0.5M0.1560.01280.1700.004400.1200.00761
UNet25M0.1980.02450.3570.05210.09710.0209
FFNO1.2M0.1610.02560.1830.004580.1610.0990
GK-T1.1M0.2600.01480.9190.04530.01200.419
Oformer1.8M0.2890.003190.1610.004740.9910.00444
GNOT2.2M0.5900.3160.5330.001990.9300.0216
Pre-trained
FNO-T10M0.1910.02450.085911.00.5300.00601
FNO-S30M0.1570.02250.3570.1840.3850.00460
FNO-M150M0.1410.00730-0.01040.0112-
DPOT-T7.5M0.1070.01550.01680.006310.05770.00673
DPOT-S DPOT-M30.8M0.06880.007810.02440.003920.03670.00870
122M0.05690.007080.02240.002470.02880.0113
Ours-T17M0.06820.007680.01050.006400.04110.00529
Ours-S Ours-M90M0.05520.005830.009590.002890.03420.00448
288M0.05280.005700.009140.002990.03000.00513
Fine-tuned
DPOT-T7.5M0.07000.007250.01680.003130.02890.00391
DPOT-S30.8M0.05020.006350.02380.003150.02150.00586
DPOT-M122M0.04240.005930.02210.002600.01750.00653
Ours-T17M0.04560.004930.007460.003050.01880.00330
Ours-S90M0.03610.003760.007420.002510.01820.00313
Ours-M288M0.03510.003880.007440.001930.01400.00398
+ +Table 1: Results of main experiments are divided into three parts. We use L2RE as the evaluation metric, where lower L2RE indicates better performance. We bold the best results in each part. We highlight the globally best results using blue . ’-’ indicates the error is greater than 20, signifying that failed completely. The first part is trained and evaluated individually on each dataset, the second part shows zero-shot results, and the last part shows results for fine-tuning on each dataset. + +Datasets. For pre-training, we utilize 6 datasets sourced from 3 benchmark collections: FNO [28], PDEBench [53], and CFDBench [38]. These datasets encompass a wide range of PDE types and parameters. The mathematical formulations of these PDEs are provided in Appendix B.5. To ensure consistency and compatibility across datasets, we applied preprocessing techniques such as padding and masking. Detailed descriptions of preprocessing steps can be found in Appendix B.1. + +Training and Evaluation. The model configurations for different scales are detailed in Appendix B.3. Across all model sizes, we employed the Adam optimizer with a learning rate of $1 \times 1 0 ^ { - 3 }$ and trained the models for 1000 epochs. Training was conducted on servers equipped with 8 RTX 4090 GPUs, each with $2 4 \mathrm { G B }$ of memory. We use the $l _ { 2 }$ relative error (L2RE) as the primary metric to evaluate prediction quality, following the standard practice outlined in [28]. + +Baseline. We selected the following influential methods as baselines for comparison, categorized into two groups: 1. Small models: This group includes FNO (along with Geo-FNO for irregular datasets) [28, 26], UNet [50], FFNO [55], GK-Transformer [5], OFormer [27], and GNOT [16]. These models are trained and evaluated individually on each dataset. + +2. Pre-trained models: FNO-(T/S/M): Larger-scale variants of the original FNO model, with expanded parameter sizes for comparison; DPOT-(T/S/M): The state-of-the-art pre-trained neural operator model [15]. Both pre-trained baselines and MoE-POT-(T/S/M) are first pre-trained on six datasets and then evaluated for performance. + +# 5.1 Main Experiments + +Table 1 summarizes the results of our main experiments. The parameter in the second row corresponds to the PDE dataset configuration, such as $1 e - 5$ for viscosity in the FNO NS dataset [28]. The activation parameter counts for MoE-POT-(T/S/M) are 17M, 90M, and 188M, respectively, with total parameter counts of 30M, 166M, and 489M. + +The second part of the Table 1 evaluates pre-trained models, including FNO variants, DPOT, and MoE-POT. Our model achieves the best zero-shot performance on 5 out of 6 datasets, with significant improvements in L2RE compared to DPOT and FNO-M. For example, on the PDEBench-CNS(0.1, 0.01), Ours-S (with 90M activation parameters) reduces L2RE by $57 \%$ compared to DPOT-M (with 122M activation parameters). The FNO architecture, not specifically designed for pre-training, struggles to optimize datasets with highly diverse properties due to its dense network structure. This leads to instability during pre-training, resulting in large errors or training collapse on certain datasets. For instance, FNO-M fails to produce valid results on PDEBench-CNS(0.1, 0.01) and CFDBench due to excessively high L2RE. The performance improvements of our model are primarily attributed to the expert network design within the MoE architecture, which effectively captures intrinsic features across datasets with differing properties without mutual interference. This demonstrates the effectiveness of the MoE architecture in pre-training scenarios for PDEs. + +The last part of the Table 1 presents the results of fine-tuning pre-trained models on each subset for 200 epochs. Fine-tuning consistently improves performance across all datasets, with larger models yielding better results. For example, MoE-POT-M achieves the best fine-tuning results on 5 out of 6 datasets, reducing L2RE by over $50 \%$ compared to the zero-shot model on PDEBench-DR. Compared to DPOT, our MoE-based model achieves significant performance gains. These improvements stem from the ability of the MoE architecture to substantially expand the total model parameters while keeping activation parameters relatively constant, thereby enhancing model performance without increasing inference costs. Additionally, after fine-tuning, our model outperforms all small models on most datasets, achieving state-of-the-art results on 4 out of 6 datasets. This demonstrates that our model successfully learns from multiple PDE datasets simultaneously through pre-training. + +In summary, fine-tuning significantly enhances performance, suggesting that pre-training on largescale PDE datasets is a promising and scalable approach for improving operator learning tasks. The results highlight the advantages of our MoE architecture in handling complex and heterogeneous PDE data. Furthermore, for additional comparative experiments with large-scale models, including DPOT-L and Poseidon, please refer to Appendix C.2. + +# 5.2 Downstream Tasks Experiments + +To evaluate the effectiveness of our pre-trained model in enhancing performance across diverse PDE downstream tasks, we conducted experiments to test its broader applicability. We selected three downstream tasks. NS (1e-4), closely related to the pre-training datasets NS (1e-3), and PDEArena, which represents equations with entirely different mathematical structures. All models were trained or fine-tuned for 500 epochs, and the results are summarized in Table 2. + +Firstly, in all experiments, our model trained from scratch (’w/o Pre-train’) consistently outperforms smaller models, demonstrating the effectiveness of the MoE-POT architecture for operator learning. Secondly, across all tasks, both DPOT and MoE-POT models show significant performance improvements after pre-training and fine-tuning, far surpassing the performance of small models + +
DatasetGeo-FNOU-NetFFNODPOTOurs
w/o Pre-trainw/ Pre-trainw/o Pre-trainw/ Pre-train
NS (1e-4)0.1070.4130.2200.05990.02640.04800.0160
CNS (1, 0.01)0.08130.08270.3900.05210.03980.03810.0307
PDEArena0.1540.1670.1610.1110.06210.1370.0618
+ +Table 2: Experimental results of fine-tuning on downstream tasks. L2RE is used as the evaluation metric, where lower values indicate better performance. The first column shows the challenges associated with each downstream task. We bold the best results. "w/ Pre-train" refers to fine-tuning after pre-training, while "w/o Pre-train" refers to training from scratch. + +![](images/figures/moe-pot-fig-0004.jpg) +Figure 4: (a) Comparison of errors across different numbers of fine-tuning epochs; (b) Comparison of zero-shot errors across different models; (c) Dataset classification accuracy based on router-gating network selection. + +trained from scratch. This indicates that pre-training enables the model to learn more effective and transferable representations. These results highlight the remarkable versatility of our model, which can be seamlessly extended to a wide range of downstream tasks. + +# 5.3 Scaling Experiments + +The relationship between performance and increasing model size is a critical property of pre-trained models. In this study, we conduct scaling experiments to evaluate the scalability of our model. The results are presented in Figure 4 (a). We observe that as the model size increases, the zero-shot test error consistently decreases, approximately following a scaling law. Furthermore, fine-tuning the model on specific datasets leads to improved performance. As shown in Figure 4 (b), while all models exhibit scaling properties, our model demonstrates better performance for a given number of activated parameters. This advantage primarily stems from the MoE architecture, which significantly increases the total number of parameters without proportionally expanding the number of activated parameters. For example, MoE-POT-T with a total of 30M parameters, requires only $57 \%$ of the activated parameters (17M) compared to existing models with similar performance. Furthermore, further analytical experiments are presented in Appendices C.3, C.4, and C.5, which include an analysis of error accumulation over rollout steps, a study on the relationship between fine-tuning data size and performance, and an investigation into the impact of pre-training data heterogeneity. + +# 5.4 Interpretable Analysis + +We aim to determine which dataset a given data point belongs to by analyzing the expert selection in the MoE router-gating network. For a specific block, we first compute the average expert selection values $Y _ { i }$ for each dataset (forming a vector), where $i = 1 , \ldots , 6$ represents the $i$ -th dataset. Then, for the expert selection vector $I _ { 0 }$ of input, we calculate its distance to each $Y _ { i }$ . The $Y _ { i _ { 0 } }$ with the smallest distance to it is obtained, indicating that the input belongs to the $i _ { 0 }$ -th dataset. Detailed procedures can be found in Appendix B.4. + +As shown in Figure 4 (c), the router-gating network in Block 2 achieves an accuracy of $9 7 . 7 \%$ in classifying the input dataset. Similar results are observed in other blocks. This strongly demonstrates that the MoE architecture effectively learns the differences between PDE datasets and uses this information for classification. Furthermore, further interpretability analysis, including the emergence of classification ability and its generalization to out-of-distribution (OOD) data, is available in Appendix C.6. + +Table 3: Average single-step inference time of different models on the NS $( 1 e - 5 )$ dataset. + +
ModelDPOTTinyOurs SmallMedium
TinySmallMediumLarge
Activated Parameters (M)7.5301584931790288
Total Parameters (M)7.53015849330166489
Inference Time (ms)5.56.516.724.38.812.716.6
+ +
NrNS(1e-3)NS(1e-5)CNS(0.1,0.01)SWEDRCFDBenchTop-KNS(1e-3)NS(1e-5)CNS(0.1,0.01)SWEDRCFDBench
320.066800.008570.010110.004400.043840.0063040.069200.007620.010460.006390.040940.00663
160.069200.007620.010460.006390.040940.0066320.069830.007770.011570.004390.041420.00570
80.068330.007730.010350.004170.041530.0057110.101080.013790.027340.007810.078960.00968
+ +Table 4: Results of ablation experiments on the influences of the number of routed experts $N _ { r }$ (left part) and the number of expert selections Top- $K$ (right part). L2RE is used as the evaluation metric. + +# 5.5 Inference Time Analysis + +We evaluated the inference time of various models. As shown in Table 3, under the same total number of parameters, our model demonstrates significantly lower inference time compared to DPOT. For example, MoE-POT-M has a total parameter count of 489M, comparable to DPOT-L, yet its inference time is only $68 \%$ of the latter, making it equivalent to DPOT-M with just $1 5 8 \mathbf { M }$ parameters. This efficiency is primarily attributed to the MoE structure, which activates far fewer parameters than the total parameter count. For certain PDE tasks, a single computation may require $\bar { 1 0 ^ { 3 } }$ to $1 0 ^ { 5 }$ inference steps. The MoE structure effectively reduces inference time while preserving model performance. + +# 5.6 Ablation Experiments + +We conduct ablation studies by training MoE-POT-T on 6 pre-trained datasets and compare the averaged zero-shot performance on the corresponding test datasets. As shown in Table 4, the error remains stable when the number of routed experts $N _ { r }$ is sufficiently large. However, increasing $N _ { r }$ leads to higher computational costs during training. Thus, we select $N _ { r } = 1 6$ as a balance between performance and efficiency. Increasing the number of expert selections (Top- $K$ ) reduces error, as more experts contribute to inference, thereby increasing the activated parameters. However, this improvement exhibits diminishing returns, so we set Top- $K = 4$ as an optimal trade-off. Additionally, we analyzed the impact of the number of heads $h$ and patch sizes on the model’s performance. Please refer to Appendix C.1 for details. + +# 6 Limitations and Conclusions + +This paper introduces MoE-POT, a sparse architecture designed for PDE pre-training. By dynamically activating routed experts and leveraging fixed shared experts, MoE-POT achieves state-of-the-art performance under the same activated parameters. Additionally, we observe that the router-gating network can effectively distinguish features of different PDEs and classify data, further illustrating the rationality of the MoE structure. + +However, we have yet to analyze the mathematical essence of this classification mechanism. Exploring how PDE classification can guide the construction of more effective pre-training datasets remains an important direction for future work. + +# Acknowledgements + +The authors would like to thank all the anonymous reviewers for their insightful comments and valuable suggestions. This work was supported by Smart-Grid National Science and Technology Major Project under contract 2025ZD0805500, the National Key R&D Program of China under contract 2022ZD0119801, and the National Nature Science Foundations of China grants 124B1019, U23A20388, 62021001. + +References +[1] Yannick Augenstein, Taavi Repan, and Carsten Rockstuhl. Neural operator-based surrogate solver for free-form electromagnetic inverse design. ACS Photonics, 2023. +[2] Kamyar Azzizadenesheli, Nikola Kovachki, Zongyi Li, Miguel Liu-Schiaffini, Jean Kossaifi, and Anima Anandkumar. Neural operators for accelerating scientific simulations and design. arXiv preprint arXiv:2309.15325, 2023. +[3] Johannes Brandstetter, Daniel Worrall, and Max Welling. Message passing neural pde solvers. arXiv preprint arXiv:2202.03376, 2022. +[4] Tom Brown, Benjamin Mann, Nick Ryder, Melanie Subbiah, Jared D Kaplan, Prafulla Dhariwal, Arvind Neelakantan, Pranav Shyam, Girish Sastry, Amanda Askell, et al. 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Building on FNO, extensions such as Geo-FNO [26], NUNO [35], and GINO [29] adapt the method to handle complex geometries. Other works [3, 41] focus on time-dependent PDEs, addressing next-time prediction challenges with specialized training strategies. Hybrid approaches like PINO [30] and PI-DeepONet [58] integrate operator learning with physics-informed neural networks (PINNs) [49, 23, 57, 7], leveraging physical constraints to enhance generalization and reduce reliance on large datasets. + +Further advancements have been achieved through transformer-based architectures [5, 27, 16, 59, 44], which incorporate techniques like patchification and linear attention mechanisms. For example, the GK-Transformer [5], OFormer [27], and GNOT [16] demonstrate strong performance on problems involving irregular geometries. AFNO [13] combines the efficiency of Fourier transforms with attention mechanisms, inspired by FNO, to achieve low memory and computational costs akin to MLP-Mixer [54]. This approach has been further adapted for large-scale applications such as climate forecasting [45, 61]. + +Despite these advancements, existing neural operator methods often require task-specific training and large amounts of domain-specific data, underscoring the need for more data-efficient approaches to broaden their applicability and scalability. + +# A.2 Pre-training in Scientific Machine Learning + +Pre-training has emerged as a highly effective paradigm for enhancing downstream tasks by training models in a (self-)supervised manner on large-scale datasets. This approach has achieved remarkable success in traditional domains such as natural language processing [47, 48, 4] and computer vision [19, 18], and is increasingly showing promise in scientific machine learning applications, including protein modeling [21], molecular representation learning [64], and climate and weather modeling [43, 42, 45, 31, 32, 33, 46, 24, 34, 25, 17, 39]. + +In the context of learning PDE data, initial efforts have been made to explore pre-training across various physical systems [56, 9, 37]. For instance, [52] designs a relatively universal PDE model to collectively train data from multiple steady-state PDEs. [60] utilizes the MathGPT architecture to investigate in-context learning capabilities for PDE data. Additionally, MPP [40] introduces an auto-regressive approach for pre-training on time-dependent PDE datasets. [15] proposes an autoregressive denoising pre-training strategy combined with a scalable Fourier-based model architecture, enabling efficient large-scale pre-training on PDE data. + +Additionally, works such as Poseidon [20], which are based on a multiscale operator transformer, have achieved excellent pre-training effectiveness and generalization by employing a novel training strategy that leverages the semi-group property of time-dependent PDEs. These approaches primarily focus on mapping parameters to PDE solutions, which contrasts with the auto-regressive solution method discussed in this paper, giving each methodology a distinct scope of application. + +However, these works primarily rely on dense neural network architectures and do not explicitly consider the relationships between different PDE datasets, which could significantly impact model performance. There remains substantial room for exploration in pre-training models for more complex scenarios and larger parameter spaces. + +# B Details of Experiment Settings + +# B.1 Data Preprocessing and Sampling + +We adopt the data preprocessing strategy proposed in DPOT [15], with modifications to ensure compatibility across diverse PDE datasets. + +Data Padding and Masking. To standardize spatial resolution, we fix the resolution at $H = 1 2 8$ , which aligns with a significant portion of the datasets. For datasets with lower resolutions, we upscale them to $H$ using interpolation. For datasets with higher resolutions, we downscale them to $H$ using random sampling or interpolation. + +To unify the number of variables (i.e., channels) across different PDEs, we pad all datasets along the channel dimension to match the dataset with the maximum number of channels, filling unused entries with a constant value (e.g., 1). For datasets with irregular geometric shapes, we use an additional mask channel that encodes the geometric configuration of each PDE instance. This ensures consistent representation across datasets while preserving unique structural information. + +Noise inserting. During the training process, we add noise to improve the stability of the training. We only insert noise during the pre training process, and do not insert noise in fine-tuning or downstream tasks. The insertion method of noise is as follows. For $\forall t \leq T$ , denote $\mathbf { \boldsymbol { u } } ^ { < t }$ as $( \pmb { u } ^ { 0 } , \dots , \pmb { u } ^ { t - 1 } )$ and the noise as $\varepsilon \sim \mathcal { N } ( 0 , \epsilon | | \boldsymbol { \mathbf { u } } ^ { < t } | | I )$ . Then the input is $\pmb { u } ^ { < t } + \varepsilon$ . + +Balanced Data Sampling. To balance the contribution of datasets with varying sizes, we assign an importance weight $w _ { k }$ to each dataset. Let $| \mathcal { D } _ { k } |$ denote the number of data points in the $k$ -th dataset, where $1 \leqslant k \leqslant K$ . The probability of sampling a data point from the $k$ -th dataset is computed as: + +$$ +p _ { k } = \frac { w _ { k } } { K | \mathcal { D } _ { k } | \cdot \sum _ { k } w _ { k } } . +$$ + +This sampling strategy ensures that datasets with fewer samples or higher importance scores are appropriately represented during training. + +Patchification Layer. We follow the patch-based tokenization strategy used in Vision Transformers [10]. Given a spatiotemporal input tensor uSizeAttention dimMLP dimLayersHeadsRouted expertsShared expertsTop-KModel sizeActivated sizeTiny51251244162430M17MSmall10241024681624166M90MMedium10242048881624489M288M + +Fine-tuning. Our model supports fine-tuning across various downstream datasets while retaining the generalization capability learned during pretraining. Specifically, we freeze the parameters of the router-gating network during fine-tuning to preserve the expert assignment strategy obtained from the joint training stage. This strategy allows the model to reuse the learned routing behavior, enabling different experts to specialize in different data distributions. Only the expert networks are updated to adapt to the target dataset, while the router-gating network continues to provide consistent and stable expert selection. This separation of routing and expert adaptation ensures more stable and efficient fine-tuning, particularly when transferring to tasks with limited data.For the fine-tuning stage, we set the learning rate to $\mathrm { \dot { 1 } \times 1 0 ^ { - 3 } }$ and used a one-cycle learning rate schedule over 200 epochs, with the first 40 epochs as the warm-up phase. And for the downstream tasks, we set the learning rate to $1 \times 1 0 ^ { - 3 }$ and used a one-cycle learning rate schedule over 500 epochs, with the first 100 epochs as the warm-up phase. + +Dataset size. The train and test dataset sizes used in the pre-training and fine-tuning stages are shown in Table 6. And the train and test dataset sizes for downstream tasks are shown in Table 7. It should +Table 6: Dataset size in pre-training and fine-tuning + +
SizeFNO(1e-5)FNO(1e-3)CNS(0.1, 0.01)SWEDRCFDBench
Train1000100090009009009000
Test20020020060601000
Fine-tuning1000100090009009009000
+ +be noted that NS (1e-4) and CNS (1,0.01) have similar datasets in the pre-training dataset, while pdearea differs significantly from the pre-training dataset. +Table 7: Dataset size in downstream + +
SizeNS(1e-4)CNS(1, 0.01)PDEArena
train200020002000
test200200200
+ +Details of inference. To learn from temporal PDE datasets, our network $\mathcal { G } _ { w } ( \boldsymbol { u } ^ { t < T } )$ parameterized by weights $w$ that auto-regressively takes $T$ frames as input and decodes the next frame from previous frames, + +$$ +\pmb { u } ^ { i + T } = \mathcal { G } _ { w } ( \pmb { u } ^ { i } , \dots , \pmb { u } ^ { i + T - 1 } ) \quad \forall i . +$$ + +By predicting the next frame, we can infer the numerical solution of the final time step based on auto-regression. For example, if we take the first 10 steps as our input, we can predict the solution $x _ { p r e d }$ for the next 10 steps. And the ground truth is $x _ { g t }$ , then the loss is + +$$ +\mathrm { R e l } \mathrm { - } \ell _ { 2 } = \frac { \| x _ { \mathrm { p r e d } } - x _ { \mathrm { g t } } \| _ { 2 } } { \| x _ { \mathrm { g t } } \| _ { 2 } } . +$$ + +# B.4 Interpretable Analysis Algorithms + +To further investigate the router-gating network within the MoE structure, we designed the following experiment. Our goal is to leverage the section of the router-gating network to determine which dataset the input data belongs to. + +Specifically, given an input sample $X$ , the gating network outputs a probability vector $Y \in \mathbb { R } ^ { 1 6 }$ representing the likelihood of selecting each of the 16 experts. Although only the top-4 experts are used during inference, the full softmax output encodes meaningful distributional information about expert preferences. + +For a specific block, we compute the average expert selection distribution $\begin{array} { r } { Y _ { i } = \frac { 1 } { N _ { i } } \sum _ { j = 1 } ^ { N _ { i } } Y _ { i j } } \end{array}$ , and $Y _ { i } , Y _ { i j } \in \mathbb { R } ^ { 1 6 }$ , where $N _ { i }$ is the number of samples from $i$ -th dataset, $i = 1 , . . . , 6$ . $Y _ { i j }$ is the routergating network output for the $j$ -th sample in $i$ -th dataset. Then, for any new input $X$ , we compare its expert distribution $I _ { 0 } = ( I _ { 0 , 1 } , . . . , I _ { 0 , 1 6 } )$ to all $Y _ { i } = ( Y _ { i , 1 } , . . . . , Y _ { i , 1 6 } )$ using the cross-entropy loss function: + +$$ +f ( I _ { 0 } , Y _ { i } ) = - \sum _ { k = 1 } ^ { 1 6 } I _ { 0 , k } \log ( Y _ { i , k } ) , +$$ + +Suppose $i _ { 0 }$ represents the nearest dataset. + +$$ +i _ { 0 } = \arg \operatorname* { m i n } _ { i } f ( I _ { 0 } , Y _ { i } ) . +$$ + +In this case, we classify the input data $X$ as belonging to the $i _ { 0 }$ -th dataset. + +# B.5 Mathematical Forms of Datasets + +Here, we list the PDEs of the datasets we used for pre-training. + +• FNO- $\nu$ [28]: The quantity of interest (QoI) is the vorticity $w ( x , t ) , ( x , t ) \in [ 0 , 1 ] ^ { 2 } \times [ 0 , T ]$ and it satisfies, $\nu$ represents the viscosity coefficient. In paper are FNO (1e-3), FNO (1e-4) and FNO (1e-5). + +$$ +\begin{array} { r c l } { \partial _ { t } w + u \cdot \nabla w } & { = } & { \nu \Delta w + f ( x ) , } \\ { \nabla \cdot u } & { = } & { 0 . } \end{array} +$$ + +• PDEBench- $\mathbf { C N S } ( \eta , \zeta )$ [53]: We need to predict the velocity, pressure, and density fields ${ \pmb u } ( x , t ) , p ( x , t ) , \rho ( x , t )$ where $( x , t ) \in [ 0 , \bar { 1 ] } ^ { 2 } \times [ 0 , 1 ]$ . The PDEs are as follows . $\eta$ is dynamic shear viscosity and $\zeta$ is bulk viscosity.In paper are CNS (0.1,0.01), CNS (1,0.01). + +$$ +\begin{array} { r c l } { \displaystyle \partial _ { t } \rho + \nabla \cdot ( \rho \pmb { u } ) } & { = } & { 0 , } \\ { \rho ( \partial _ { t } \pmb { u } + \pmb { u } \cdot \nabla \pmb { u } ) } & { = } & { \displaystyle - \nabla p + \eta \Delta \pmb { u } + ( \varsigma + \eta / 3 ) \nabla ( \nabla \cdot \pmb { u } ) , } \\ { \displaystyle \partial _ { t } \left( \frac { 3 } { 2 } p + \frac { \rho \pmb { u } ^ { 2 } } { 2 } \right) } & { = } & { \displaystyle - \nabla \cdot \left( \left( \varepsilon + p + \frac { \rho \pmb { u } ^ { 2 } } { 2 } \right) \pmb { u } - \pmb { u } \cdot \sigma ^ { \prime } \right) . } \end{array} +$$ + +• PDEBench-SWE [53]: We need to predict water depth $h ( x , t )$ where the domain is $[ - 1 , 1 ] ^ { 2 } \times [ 0 , 5 ]$ . The PDEs are as follows,In paper is SWE. + +$$ +\begin{array} { r c l } { { \displaystyle \partial _ { t } h + \nabla \cdot ( h { \mathbf { } } \mathbf { \boldsymbol { u } } ) } } & { { = } } & { { 0 , } } \\ { { \displaystyle \partial _ { t } ( h { \mathbf { } } \mathbf { \boldsymbol { u } } ) + \nabla \cdot \left( \frac { 1 } { 2 } h { \mathbf { } } ^ { 2 } + \frac { 1 } { 2 } g _ { r } h ^ { 2 } \right) } } & { { = } } & { { \displaystyle - g _ { r } h \nabla b . } } \end{array} +$$ + +• PDEBench-DR [53]: We need to predict the density fields $\pmb { u } ( \boldsymbol { x } , t )$ . The domain is $[ - 2 . 5 , 2 . 5 ] ^ { 2 } \times [ 0 , 1 ]$ and the PDEs are as follows,in paper is DR + +$$ +\begin{array} { r c l } { \partial _ { t } { \pmb u } } & { = } & { { \pmb D } \nabla ^ { 2 } { \pmb u } + { \pmb R } ( { \pmb u } ) . } \end{array} +$$ + +• PDEArena-NS1/2 [14]: We need to predict the velocity, pressure, and density fields ${ \pmb u } ( x , t ) , p ( x , t ) , \rho ( x , t )$ where $( x , t ) \in [ \dot { 0 } , 3 2 ] ^ { 2 } \times [ 0 , 2 4 ]$ . The PDEs are as follows,in paper is PDEArena. + +$$ +\begin{array} { r c l } { \partial _ { t } \pmb { v } } & { = } & { - \pmb { v } \cdot \nabla \pmb { v } + \mu \nabla ^ { 2 } \pmb { v } - \nabla p + \pmb { f } , } \\ { \nabla \cdot \pmb { v } } & { = } & { 0 . } \end{array} +$$ + +• CFDBench [38]: We need to predict the velocity and pressure fields $\pmb { u } ( x , t ) , p ( x , t )$ . The domains are different as this is a dataset with irregular geometries. The PDEs are as follows, in paper is CFDBench. + +$$ +\begin{array} { r c l } { \partial _ { t } ( \rho \pmb { u } ) + \nabla \cdot ( \rho \pmb { u } ^ { 2 } ) } & { = } & { - \nabla p + \nabla \cdot \mu ( \nabla \pmb { u } + \nabla \pmb { u } ^ { T } ) , } \\ { \nabla \cdot ( \rho \pmb { u } ) } & { = } & { 0 . } \end{array} +$$ + +# C Experimental Data and Supplementary Experiments + +# C.1 Partial Hyperparameter Ablation Experiment + +
hNS(1e-3)NS(1e-5)CNSSWEDRCFDBenchPNS(1e-3)NS(1e-5)CNSSWEDRCFDBench
20.067480.007600.010340.004950.042770.0055940.062260.008190.017650.003960.034810.00579
40.069200.007620.010460.006390.040940.0066380.069200.007620.010460.006390.040940.00663
80.069630.007090.010360.003230.041990.00538160.089640.007920.013010.006730.116710.00847
+ +Table 8: Results of ablation experiments on the influences of the number of heads $h$ (left part) and patch sizes $P$ (right part). L2RE is used as the evaluation metric. + +Table 8 demonstrates that the number of heads $h$ has minimal impact on error but affects computational cost. Accordingly, we choose $h = 4$ for efficiency. Finally, medium patch sizes $P = 4$ or 8) help reduce error, leading us to select $P = 8$ for optimal performance. + +# C.2 More Comparative Experiments + +We selected DPOT [15] as our primary multi-physics baseline for the following reasons: + +1. Clear Attribution of Gains: Our MoE-POT architecture is a direct modification of the DPOT model, where we replace the dense feed-forward network with our proposed sparse MoE layer. This controlled comparison allows us to cleanly attribute any performance improvements directly to the MoE architecture, providing a clear and rigorous validation of our core contribution. + +2. Divergent Experimental Paradigms: Our work, following DPOT, operates under an auto-regressive paradigm, predicting future states based solely on a sequence of previous solution frames. In contrast, models like Poseidon [20] and MPP [40] are designed for a parameter-informed setting, where they take explicit problem parameters (e.g., coefficients, boundary conditions) as input to predict a future state. The public benchmark datasets used in our primary experiments (from FNO, PDEBench, and CFDBench) do not provide these explicit PDE parameters, making a direct comparison with parameter-informed models infeasible under our main experimental protocol. + +This section provides additional experimental results and analysis to supplement the main paper. We compare our MoE-POT architecture with the larger DPOT-L model and the Poseidon. + +# C.2.1 Experimental Results with DPOT-L + +To provide a more comprehensive comparison against large-scale dense models, we evaluated the performance of DPOT-L (493M parameters). The results, alongside our MoE-POT models and smaller DPOT variants, are presented in Table 9. + +
Model & Activated ParamsNS(1e-5)NS(1e-3)CNS(0.1,0.01)SWEDRCFDBench
DPOT-S (31M)0.06880.00780.02440.00390.03670.0087
DPOT-M (122M)0.05690.00710.02240.00250.02880.0113
DPOT-L (493M)0.05760.00610.01130.00230.02190.0065
MoE-POT-T (17M)0.06820.00770.01050.00640.04110.0053
MoE-POT-S (90M)0.05520.00580.00960.00290.03420.0045
MoE-POT-M (288M)0.05280.00570.00910.00300.03000.0051
+ +Table 9: Zero-shot L2 Relative Error (L2RE) comparison, including DPOT-L. + +The results in Table 9 lead to two key observations: 1. Diminishing returns for dense models: The performance of the dense DPOT architecture shows diminishing returns with scale. The improvement from DPOT-M (122M) to DPOT-L (493M)—a $4 \times$ increase in parameters—is marginal on several datasets (e.g., NS(1e-3)) and modest on others. 2. Competitive performance with higher efficiency: When comparing MoE-POT-M (288M activated) with DPOT-L (493M activated), our model achieves competitive, and in some cases superior, performance. MoE-POT-M outperforms DPOT-L on three of the six datasets (NS(1e-3), CNS, CFDBench), while DPOT-L holds a slight advantage on the other three. + +# C.2.2 Experimental Results with Poseidon + +To address the interest in comparing with the latest models, we conducted supplementary fine-tuning experiments on two challenging downstream tasks from the Poseidon paper [20]: Wave-Layer and Wave-Gauss. We evaluated both MoE-POT and Poseidon under two distinct settings to fairly assess their capabilities. + +Setting 1: Auto-regressive (Our Native Setting) In this setting, models predict future states using only previous solution trajectories, without access to explicit PDE parameters. The results are shown in Table 10. + +
Model (Activated Params) Wave-LayerWave-Gauss
Poseidon-T (21M)0.290.29
Poseidon-B (158M)0.210.24
MoE-POT-T (17M)0.070.07
MoE-POT-S (90M)0.050.06
+ +Table 10: L2 Relative Error on downstream tasks in the auto-regressive setting. Lower is better. + +Setting 2: Parameter-Informed (Poseidon’s Native Setting) In this setting, models are provided with explicit PDE parameters as additional input. We adapted our MoE-POT model to accept these parameters to ensure a fair comparison. The results are shown in Table 11. +Table 11: L2 Relative Error on downstream tasks in the parameter-informed setting. Lower is better. + +
Model (Activated Params)Wave-LayerWave-Gauss
Poseidon-T (21M)0.080.06
Poseidon-B (158M)0.060.09
MoE-POT-T (17M)0.110.14
MoE-POT-S (90M)0.060.10
+ +The results from these supplementary experiments indicate that each model excels in its native operational setting. In the auto-regressive setting (Table 10), where PDE parameters are unknown, MoE-POT significantly outperforms Poseidon. This highlights our model’s strength in implicitly learning system dynamics from solution trajectories alone. + +Conversely, in the parameter-informed setting (Table 11), Poseidon generally demonstrates superior performance, showcasing its effectiveness when explicit physical knowledge is available. These findings suggest that MoE-POT and Poseidon have different primary application scopes rather than one being definitively superior across all scenarios. + +# C.3 Rollout Error at Different Timesteps + +A key challenge in auto-regressive prediction is the accumulation of errors. Even a small improvement in single-step prediction accuracy can lead to a substantial reduction in the cumulative error over a long rollout, as prediction inaccuracies propagate through the sequence. + +To illustrate this effect, we analyze the rollout error at different timesteps for the Shallow Water Equations (SWE) dataset. Table 12 compares the L2RE of DPOT-S and our MoE-POT-S at frames 50, 70, and 100 of the rollout. + +
ModelFrame 50 L2REFrame 70 L2REFrame 100 L2REAverage L2RE
DPOT-S0.00310.00340.00510.0039
MoE-POT-S0.00240.00260.00350.0029
+ +Table 12: Illustration of error accumulation on the SWE dataset. The table shows the L2RE at specific frames during a 100-step rollout. + +As demonstrated in Table 12, the error for both models increases over the rollout period, confirming the effect of error accumulation. More importantly, the performance advantage of MoE-POT-S grows significantly over time. The relative error reduction compared to DPOT-S is approximately $23 \%$ a t frame 50, but this gap widens to over $31 \%$ by frame 100. This super-linear divergence underscores the critical impact of achieving lower single-step prediction error, as its benefits are amplified during long-term, multi-step rollouts. + +# C.4 Analysis of Fine-Tuning Sample Efficiency + +This section investigates the relationship between the number of fine-tuning samples and model performance, thereby analyzing the data efficiency of the MoE-POT architecture. We conducted fewshot fine-tuning experiments on both an in-distribution and an out-of-distribution task to demonstrate how performance scales with data availability for both MoE-POT and its dense counterpart, DPOT. + +We compare the performance of MoE-POT-S and DPOT-S on two fine-tuning tasks: 1. In-Distribution Task: The NS (1e-4) dataset, which is closely related to the data used during pre-training. 2. Out-of-Distribution Task: The Wave-Layer dataset from the Poseidon [20], which represents a novel physical system. + +For each task, we fine-tuned both models for 500 epochs while varying the number of available training samples, and we report the final L2 Relative Error. The results for the in-distribution and out-of-distribution tasks are presented in Table 13 and Table 14, respectively. + +Table 13: L2 Relative Error on the in-distribution NS (1e-4) task versus the number of fine-tuning samples. Lower values are better. + +
Number of Samples1632641285122000
DPOT-S0.250.200.130.090.0440.026
MoE-POT-S0.200.150.110.070.0400.016
+ +
Number of Samples163264128
DPOT-S0.410.330.260.19
MoE-POT-S0.340.260.200.14
+ +Table 14: L2 Relative Error on the out-of-distribution Wave-Layer task versus the number of finetuning samples. Lower values are better. + +The results demonstrate a clear trend: while both models improve with more data, MoE-POT-S consistently outperforms DPOT-S across all sample sizes on both tasks. This highlights the superior data efficiency of the MoE-POT architecture. The larger capacity and specialized experts of the pre-trained MoE-POT model enable it to generalize more effectively from limited data. This means it can either achieve a target performance level with significantly fewer fine-tuning examples or deliver superior accuracy given the same amount of data. + +# C.5 Performance with Increasing Dataset + +A core motivation for our work is the challenge of negative transfer [6] in dense neural operators when pre-trained on a mixture of heterogeneous PDE datasets. A single, dense network struggles to learn conflicting physical laws, which can degrade performance as more diverse data is added. This section presents an experiment designed to test this hypothesis and demonstrate the robustness of the MoE-POT architecture in mitigating this issue. + +To investigate the impact of increasing data heterogeneity, we pre-trained both the dense DPOT-S model and our sparse MoE-POT-S model on progressively larger and more diverse mixtures of datasets. For each experiment, the models were trained from scratch on the specified data mixture. We evaluated their zero-shot performance on the original six pre-training datasets to measure how well they retained knowledge. + +The dataset mixtures were constructed as follows: + +• 6 Datasets: The standard pre-training set used in our main experiments: NS(1e-5), NS(1e-3), CNS(0.1, 0.01), SWE, DR, and CFDBench. +• 10 Datasets: The base set plus four additional datasets from the DPOT paper [15]: NS(1e-4), CNS(1, 0.1), and two Navier-Stokes tasks from PDEArena. +• 12 Datasets: The 10-dataset mix plus two additional CNS variants: CNS(1, 0.01) and CNS(0.1, 0.1). + +
Model (Pre-trained on)NS(1e-5)NS(1e-3)CNS(0.1,0.01)SWEDRCFDBench
Dense Model
DPOT-S (6 Datasets)0.06880.00780.02440.00390.03670.0087
DPOT-S (10 Datasets)0.06630.00690.02240.00370.05750.0146
DPOT-S (12 Datasets)0.07390.00790.01290.01050.07240.0075
Sparse Model (Ours)
MoE-POT-S (6 Datasets)0.05520.00580.00960.00290.03420.0045
MoE-POT-S (10 Datasets)0.05210.00530.00850.00290.03710.0047
MoE-POT-S (12 Datasets)0.05330.00560.00620.00320.03830.0043
+ +Table 15: Zero-shot L2RE on the six base evaluation datasets after pre-training on increasingly heterogeneous data mixtures (6, 10, and 12 datasets). Lower values are better. + +The zero-shot L2 Relative Error (L2RE) for both models across the different pre-training configurations is presented in Table 15. + +DPOT-S (Dense Model) The performance of DPOT-S is unstable and often degrades as more heterogeneous data is introduced. While adding four datasets (from 6 to 10) yields minor improvements on some tasks, it causes significant performance degradation on others (e.g., DR and CFDBench). Expanding to 12 datasets results in a notable performance collapse on several tasks (e.g., NS(1e-3), SWE, DR) compared to the original 6-dataset training. This confirms that the dense architecture suffers from negative transfer, where the model’s capacity is overwhelmed by conflicting information from diverse physical systems. + +MoE-POT-S (Sparse Model) In stark contrast, MoE-POT-S demonstrates remarkable robustness. As the number of pre-training datasets increases from 6 to 12, its performance remains stable or even improves on most tasks (e.g., CNS and CFDBench). The slight variations in error are minor compared to the drastic fluctuations observed with DPOT-S. This stability indicates that the MoE architecture effectively mitigates negative transfer by allowing different experts to specialize in distinct physical dynamics, thereby preventing knowledge conflict. + +# C.6 Extended Interpretability Analysis: Emergence and Generalization of Router Specialization + +This appendix expands on the interpretability analysis in Section 5.4, which demonstrated the routergating network’s ability to classify input data by its source PDE. This capability is not explicitly programmed; the router only processes tokenized inputs, and the model’s loss function is based on prediction error, not a classification objective. Here, we investigate two key questions: (1) How does this specialization emerge during pre-training? (2) Does this learned capability generalize to entirely new, OOD datasets? + +Emergence of Specialization During Training We first examine how the router’s classification ability develops. We tracked the dataset classification accuracy at different stages of pre-training, using the method described in Section 5.4. The results, shown in Table 16, confirm that this is an emergent property. Initially (Epoch 50), the accuracy is low, but it rapidly improves and reaches $100 \%$ by Epoch 250. This demonstrates that the router learns to distinguish between data distributions as part of the end-to-end optimization process. + +
EpochNS(1e-5) AccuracyCFDBench Accuracy
502%70%
15080%78%
250100%100%
+ +Table 16: Evolution of the router’s dataset classification accuracy over training epochs. The accuracy steadily improves, eventually reaching $100 \%$ as the experts and router co-specialize. + +Generalization to Out-of-Distribution Datasets To test if this specialization generalizes beyond the pre-training distributions, we evaluated the router’s classification performance on two OOD tasks from the Poseidon benchmark [20]: Wave-Layer and Wave-Gauss. These datasets represent novel physical systems unseen during pre-training. + +Table 17: Router classification accuracy on unseen (OOD) downstream tasks. The perfect accuracy demonstrates strong generalization. + +
Unseen DatasetBlock-1 AccuracyBlock-2 Accuracy
Wave-Layer100%100%
Wave-Gauss100%100%
+ +As shown in Table 17, the router achieves $100 \%$ classification accuracy on both unseen tasks. For instance, when processing the Wave-Layer dataset, the router in Block-2 consistently activated a sparse subset of experts (e.g., Expert 11 at $100 \%$ usage, Expert 1 at $79 \%$ ), with other experts receiving minimal or zero activation. + +Taken together, these results provide strong evidence that the router’s ability to identify PDE types is an emergent property of joint optimization. To minimize the global prediction loss across heterogeneous datasets, the model learns to partition its knowledge, routing inputs with similar dynamics to specialized experts. This process effectively mitigates the negative transfer that hinders dense architectures. Crucially, the perfect classification accuracy on OOD data demonstrates that the router is not merely memorizing training distributions. Instead, it learns to recognize fundamental properties of the underlying physics from the tokenized solution data, a capability that generalizes to novel systems. \ No newline at end of file diff --git a/papers/moe-pot/paper.pdf b/papers/moe-pot/paper.pdf new file mode 100644 index 0000000000000000000000000000000000000000..6f558a740218de90253b257a1c967e42755924fa --- /dev/null +++ b/papers/moe-pot/paper.pdf @@ -0,0 +1,3 @@ +version https://git-lfs.github.com/spec/v1 +oid sha256:16506e2ef1b369002a7a721647b0758869796cf6e2d94eb6e24d57fd11047dd9 +size 2312429 diff --git a/papers/moe-pot/sau.json b/papers/moe-pot/sau.json new file mode 100644 index 0000000000000000000000000000000000000000..524c2d9c2e9e8bbd889252c40b58ce2b049910a4 --- /dev/null +++ b/papers/moe-pot/sau.json @@ -0,0 +1,317 @@ +{ + "paper_id": "moe-pot", + "paper_title": "MoE-POT: Mixture-of-Experts Operator Transformer for Large-Scale PDE Pre-Training", + "D1": [ + { + "id": "moe-pot-D1-001", + "claim": "Model architecture (size-invariant, confirmed by Table 5): N_r=16 routed experts, N_s=2 shared experts, Top-K=4, total 6 activated experts per input (4 routed + 2 shared); N_r, N_s, K invariant across Tiny/Small/Medium", + "source": "Section 4 (lines 128, 134, 142), Section 1 (line 26), Section 3.2 (lines 81-83), Table 5" + }, + { + "id": "moe-pot-D1-002", + "claim": "Per-size attention dimension (Table 5): attention_dim=512 (Tiny) / 1024 (Small) / 1024 (Medium)", + "source": "Table 5 (Appendix B.3)" + }, + { + "id": "moe-pot-D1-003", + "claim": "Per-size MLP hidden dimension (Table 5): mlp_dim=512 (Tiny) / 1024 (Small) / 2048 (Medium)", + "source": "Table 5 (Appendix B.3)" + }, + { + "id": "moe-pot-D1-004", + "claim": "Per-size transformer layers (Table 5): num_layers=4 (Tiny) / 6 (Small) / 8 (Medium)", + "source": "Table 5 (Appendix B.3)" + }, + { + "id": "moe-pot-D1-005", + "claim": "Per-size attention heads (Table 5): num_heads=4 (Tiny) / 8 (Small) / 8 (Medium)", + "source": "Table 5 (Appendix B.3)" + }, + { + "id": "moe-pot-D1-006", + "claim": "Per-size total parameters (Table 5): 30M (Tiny) / 166M (Small) / 489M (Medium)", + "source": "Table 5, Section 5 (line 188)" + }, + { + "id": "moe-pot-D1-007", + "claim": "Activated params (T/S/M): 17M / 90M / 288M. Note: Section 5 body text says 188M for Medium but Table 3 and Table 5 consistently show 288M — the 188M value is a typo in the MoE-POT paper", + "source": "Table 5, Table 3, Section 5 (line 188)" + }, + { + "id": "moe-pot-D1-008", + "claim": "Pre-training config: Adam optimizer (β1=0.9, β2=0.9), lr=1e-3, One-cycle schedule, weight_decay=1e-6, 1000 epochs (200 warmup), batch_size=20 on 8x RTX 4090 (24GB), T=10 input timesteps, load_balance_weight w_bal=0.1, dataset_sampling_weight w_k=1", + "source": "Section 5 (line 180), Section B.3 (lines 401-403), Section 4 (line 160)" + }, + { + "id": "moe-pot-D1-009", + "claim": "Fine-tuning config: lr=1e-3, One-cycle schedule, 200 epochs (40 warmup), router-gating network parameters frozen, only expert networks updated", + "source": "Section B.3 (line 408), Section 5.1 (line 192)" + }, + { + "id": "moe-pot-D1-010", + "claim": "Downstream task config: lr=1e-3, One-cycle schedule, 500 epochs (100 warmup)", + "source": "Section 5.2 (line 198), Section B.3 (line 408)" + }, + { + "id": "moe-pot-D1-011", + "claim": "Data preprocessing: spatial resolution H=128 (lower-res upscaled, higher-res downscaled), patch_size P=8 (Conv2D kernel PxP stride P), padding_constant=1 for unused channel entries", + "source": "Section B.1 (lines 373-375), Section B.3 (line 403), Section C.1 (line 500)" + }, + { + "id": "moe-pot-D1-012", + "claim": "Pre-training dataset splits (train/test): FNO-NS(1e-5)=1000/200, FNO-NS(1e-3)=1000/200, PDEBench-CNS(0.1,0.01)=9000/200, PDEBench-SWE=900/60, PDEBench-DR=900/60, CFDBench=9000/1000; total 6 datasets from 3 benchmark collections", + "source": "Table 6, Section 5 (line 178)" + }, + { + "id": "moe-pot-D1-013", + "claim": "Downstream dataset splits (train/test): NS(1e-4)=2000/200, CNS(1,0.01)=2000/200, PDEArena=2000/200", + "source": "Table 7" + }, + { + "id": "moe-pot-D1-014", + "claim": "Dataset mixture counts (Appendix C.5 negative transfer analysis): 6-dataset (base: NS(1e-5), NS(1e-3), CNS(0.1,0.01), SWE, DR, CFDBench); 10-dataset (base + NS(1e-4), CNS(1,0.1), 2x PDEArena NS tasks); 12-dataset (10-set + CNS(1,0.01), CNS(0.1,0.1))", + "source": "Section C.5 (lines 579-581)" + }, + { + "id": "moe-pot-D1-015", + "claim": "N_r (routed experts per layer) ablation: tested [8, 16, 32], selected 16 — error stable when N_r sufficiently large, balance of performance and computational cost", + "source": "Table 4, Section 5.6 (line 235)" + }, + { + "id": "moe-pot-D1-016", + "claim": "Top-K ablation: tested [1, 2, 4], selected 4 — larger K reduces error with diminishing returns, 4 is optimal trade-off", + "source": "Table 4, Section 5.6 (line 235)" + }, + { + "id": "moe-pot-D1-017", + "claim": "Number of heads h ablation: tested [2, 4, 8], selected 4 — minimal impact on error but affects computational cost", + "source": "Table 8, Section C.1 (line 500)" + }, + { + "id": "moe-pot-D1-018", + "claim": "Patch size P ablation: tested [4, 8, 16], selected 8 — medium patch sizes reduce error, 8 chosen for optimal performance", + "source": "Table 8, Section C.1 (line 500)" + }, + { + "id": "moe-pot-D1-019", + "claim": "Few-shot sample sizes — in-distribution (NS(1e-4)): [16, 32, 64, 128, 512, 2000]; models fine-tuned 500 epochs at each sample size", + "source": "Table 13, Section C.4 (line 557)" + }, + { + "id": "moe-pot-D1-020", + "claim": "Few-shot sample sizes — out-of-distribution (Wave-Layer): [16, 32, 64, 128]; models fine-tuned 500 epochs at each sample size", + "source": "Table 14, Section C.4 (line 557)" + }, + { + "id": "moe-pot-D1-021", + "claim": "Classification emergence eval epochs: [50, 150, 250] — router classification accuracy tracked over pre-training to measure emergence of dataset identification capability", + "source": "Table 16, Section C.6 (line 599)" + }, + { + "id": "moe-pot-D1-022", + "claim": "Poseidon baseline activated params (used in Appendix C.2.2 comparison): Poseidon-T=21M, Poseidon-B=158M", + "source": "Table 10, Section C.2.2" + } + ], + "D2": [ + { + "id": "moe-pot-D2-001", + "claim": "Balanced Data Sampling Probability: p_k = w_k / (K * |D_k| * sum_{j=1}^{K} w_j), where w_k is importance weight (w_k=1 for all datasets), |D_k| is dataset size, K=6 datasets. Ensures proportional representation of datasets with fewer samples.", + "source": "B.1" + }, + { + "id": "moe-pot-D2-002", + "claim": "Noise Injection for Denoising Pre-training: epsilon ~ N(0, epsilon_bar * ||u^{ Noise Injection -> Patchification with Learnable Positional Encoding -> Temporal Aggregation with Fourier Features -> Multi-head Fourier Layer (AFNO-style frequency-domain MLP) -> MoE Router Gating (Softmax over CNN routing logits) -> MoE TopK Expert Selection -> MoE Output Aggregation (shared experts average + weighted routed experts) + Load Balancing Loss (CV-based auxiliary) -> Final Training Loss (MSE + per-layer load balance) -> Auto-regressive Inference Rollout (no noise, sliding window T=10).", + "source": "Section 4 (derived from D2 ordering_before/ordering_after annotations)" + }, + { + "id": "moe-pot-D4-002", + "claim": "Router interpretability classification pipeline (Section B.4): Step 1 — Compute average expert distribution Y_i for each pre-training dataset i from full softmax router output over all 16 experts. Step 2 — For new input X, compute cross-entropy distance f(I_0, Y_i) between its routing distribution I_0 and each dataset reference Y_i. Step 3 — Classify X to dataset i_0 = argmin_i f(I_0, Y_i).", + "source": "Section B.4 (derived from D2 ordering_before/ordering_after annotations)" + }, + { + "id": "moe-pot-D4-003", + "claim": "Zero-shot evaluation protocol (Section 5.1): Phase 1 — Data preprocessing (spatial resolution unification to H=128, channel padding, mask channel for irregular geometries, patchification P=8). Phase 2 — Joint pre-training on all 6 datasets (auto-regressive next-frame prediction, T=10, noise injection, balanced sampling w_k=1, Adam lr=1e-3, 1000 epochs, 200 warm-up). Phase 3 — Zero-shot evaluation on each individual dataset test split.", + "source": "Section 5.1 (derived from D3 phase_ordering)" + }, + { + "id": "moe-pot-D4-004", + "claim": "Fine-tuning evaluation protocol (Section 5.1): Phase 1 — Joint pre-training on all 6 datasets (1000 epochs). Phase 2 — Fine-tune separately on each individual dataset (200 epochs, one-cycle lr=1e-3, 40 warm-up, freeze router-gating, update expert networks only). Phase 3 — Evaluate on same dataset test split.", + "source": "Section 5.1, Section B.3 (derived from D3 phase_ordering)" + }, + { + "id": "moe-pot-D4-005", + "claim": "Downstream transfer evaluation protocol (Section 5.2): Phase 1 — Joint pre-training on 6 datasets (1000 epochs). Phase 2 — Fine-tune on downstream task (NS(1e-4), CNS(1,0.01), or PDEArena) for 500 epochs (one-cycle lr=1e-3, 100 warm-up). Phase 3 — Evaluate on downstream task test split. Compared against train-from-scratch baseline (no pre-training).", + "source": "Section 5.2, Section B.3 (derived from D3 phase_ordering)" + }, + { + "id": "moe-pot-D4-006", + "claim": "Scaling law analysis protocol (Section 5.3): Phase 1 — Pre-train each model variant (T/S/M for MoE-POT, T/S/M/L for DPOT) on all 6 datasets. Phase 2 — Evaluate zero-shot L2RE on test splits. Phase 3 — Fine-tune each model variant on individual datasets. Phase 4 — Evaluate fine-tuned L2RE on test splits. Phase 5 — Plot L2RE vs. activated parameters to characterize scaling behavior for dense vs. sparse architectures.", + "source": "Section 5.3 (derived from D3 phase_ordering)" + }, + { + "id": "moe-pot-D4-007", + "claim": "Negative transfer analysis protocol (Appendix C.5): Phase 1 — Pre-train DPOT-S (dense) and MoE-POT-S (sparse) from scratch on progressively larger dataset mixtures: 6 datasets (base) -> 10 datasets (base + NS(1e-4), CNS(1,0.1), 2x PDEArena) -> 12 datasets (10-set + CNS(1,0.01), CNS(0.1,0.1)). Phase 2 — Zero-shot evaluate both models on the original 6 base dataset test splits to measure whether dense models suffer negative transfer while MoE remains stable.", + "source": "Appendix C.5 (derived from D3 phase_ordering)" + } + ] +} \ No newline at end of file diff --git a/papers/mrq/blacklist.txt b/papers/mrq/blacklist.txt new file mode 100644 index 0000000000000000000000000000000000000000..1a86dff47e0810e4a198c16c06c178f4c9eff2bf --- /dev/null +++ b/papers/mrq/blacklist.txt @@ -0,0 +1,3 @@ +# No public official repository yet (Meta FAIR, ICLR 2025) +# Authors: Scott Fujimoto, Pierluca D'Oro, Amy Zhang, Yuandong Tian, Michael Rabbat +# Watch: https://github.com/sfujim (author hosts TD3/TD7/BCQ here) diff --git a/papers/mrq/config.yaml b/papers/mrq/config.yaml new file mode 100644 index 0000000000000000000000000000000000000000..8e45fcc848a3b44322af7fa15c4cb5f6f70db3bf --- /dev/null +++ b/papers/mrq/config.yaml @@ -0,0 +1,8 @@ +title: "Towards General-Purpose Model-Free RL 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@@ -0,0 +1,3 @@ +version https://git-lfs.github.com/spec/v1 +oid sha256:b9ccaa5cb26deb735a752c7132bda4f5be22059feae68489928016113960536b +size 311822 diff --git a/papers/mrq/images/tables/mrq-table-0025.jpg b/papers/mrq/images/tables/mrq-table-0025.jpg new file mode 100644 index 0000000000000000000000000000000000000000..b732bfd7eef767fe7b9f26068ee55f051c623ea5 --- /dev/null +++ b/papers/mrq/images/tables/mrq-table-0025.jpg @@ -0,0 +1,3 @@ +version https://git-lfs.github.com/spec/v1 +oid sha256:05f7c045d28324114ad5aa59d5afb46cfbc14ea122ae31a86fae581b414777f9 +size 305022 diff --git a/papers/mrq/paper.md b/papers/mrq/paper.md new file mode 100644 index 0000000000000000000000000000000000000000..93ceb189e6c16b6c80926db07dc8dcc18acb67c7 --- /dev/null +++ b/papers/mrq/paper.md @@ -0,0 +1,876 @@ +# TOWARDS GENERAL-PURPOSE MODEL-FREE REINFORCEMENT LEARNING + +Scott Fujimoto, Pierluca D’Oro, Amy Zhang, Yuandong Tian, Michael Rabbat Meta FAIR + +# ABSTRACT + +Reinforcement learning (RL) promises a framework for near-universal problemsolving. In practice however, RL algorithms are often tailored to specific benchmarks, relying on carefully tuned hyperparameters and algorithmic choices. Recently, powerful model-based RL methods have shown impressive general results across benchmarks but come at the cost of increased complexity and slow run times, limiting their broader applicability. In this paper, we attempt to find a unifying model-free deep RL algorithm that can address a diverse class of domains and problem settings. To achieve this, we leverage model-based representations that approximately linearize the value function, taking advantage of the denser task objectives used by model-based RL while avoiding the costs associated with planning or simulated trajectories. We evaluate our algorithm, MR.Q, on a variety of common RL benchmarks with a single set of hyperparameters and show a competitive performance against domain-specific and general baselines, providing a concrete step towards building general-purpose model-free deep RL algorithms. + +![](images/figures/mrq-fig-0001.jpg) +Figure 1: Summary of results. Aggregate mean performance across four common RL benchmarks and 118 environments featuring diverse characteristics (e.g., observation and action spaces, task types). Error bars capture a $9 5 \%$ stratified bootstrap confidence interval. Our algorithm, MR.Q, achieves a competitive performance against both state-of-the-art domain-specific and general baselines, while using a single set of hyperparameters. Notably, MR.Q accomplishes this with fewer network parameters and substantially faster training and evaluation speeds than general-purpose model-based methods. + +# 1 INTRODUCTION + +The conceptual premise of RL is inherently general-purpose—an RL agent can learn optimal behavior with only two basic elements: a well-defined objective and data describing its interactions with the environment. In reality, however, most RL algorithms are anything but general-purpose. Instead, RL algorithms are highly specialized and typically characterized by specific problem classes, such + +as discrete versus continuous actions or vector versus pixel observations, with each category requiring its own set of algorithmic choices and hyperparameters. For example, Rainbow and TD3 (Hessel et al., 2018; Fujimoto et al., 2018), common methods for Atari and MuJoCo respectively (Bellemare et al., 2013; Todorov et al., 2012), have more differences than similarities in their shared hyperparameters (Table 1)—without accounting for further algorithmic differences. + +To some extent, general-purpose algorithms do exist—policy gradient methods (Williams, 1992; Schulman et al., 2015; 2017) and many evolutionary approaches (Rechenberg, 1978; Back, 1996; Rubinstein, 1997; Salimans et al., + +Table 1: Hyperparameter differences between Rainbow (Hessel et al., 2018) and TD3 (Fujimoto et al., 2018). TD3 uses an expected moving average (EMA) update with an effective frequency of 11−0.995 = 200. + +
HyperparameterRainbowTD3
Discount factor Optimizer0.99 Adam0.99 Adam
Learning Rate Adam €6.25 · 10-5 1.5·10-410-3 10-8
Replay buffer size Minibatch size1M1M
Target network update Effective target update freq.32 Iterative 8k100 EMA 200
+ +2017) require few assumptions on the underlying problem. Unfortunately, these methods often offer poor sample efficiency and asymptotic performance compared more domain-specific approaches, and in some instances, can require extensive re-tuning over numerous implementation-level details (Engstrom et al., 2020; Huang et al., 2022). + +Recently, DreamerV3 (Hafner et al., 2023) and TD-MPC2 (Hansen et al., 2024), have showcased the potential of general-purpose model-based approaches, achieving impressive single-task performance on a diverse set of benchmarks without re-tuning hyperparameters. However, despite their success, model-based methods also introduce substantial algorithmic and computational complexity, making them less practical than lightweight domain-specific model-free algorithms. + +This paper presents a general model-free RL algorithm that leverages model-based representations to achieve the sample efficiency and performance of model-based methods, without the computational overhead. A recent surge of high-performing model-free RL algorithms with dynamics-based representations (Guo et al., 2020; 2022; Schwarzer et al., 2020; 2023; Zhao et al., 2023; Fujimoto et al., 2024; Zheng et al., 2024; Scannell et al., 2024) has showcased the potential of this family of algorithms when tailored for a single benchmark. Recognizing the similarity between these modelbased and model-free approaches, our hypothesis is that the true benefit of model-based objectives is in the implicitly learned representation, rather than the model itself, and thus prompting the question: + +Can model-based representations alone enable sample-efficient general-purpose learning? + +Our proposed approach is based on learning features that approximately capture a linear relationship between state-action pairs and value. To do so, we draw heavily from modern dynamics-based representation learning methods (see Related Work) as well as the work of Parr et al. (2008), who show that both model-based and model-free objectives converge to the same solution in linear space. By mapping states and actions into a single, unified embedding, we eliminate any environmentspecific characteristics of the input space and allow for a standardized set of hyperparameters. + +We evaluate our method, MR.Q, on four widely used RL benchmarks and 118 environments, and achieve competitive performance against state-of-the-art domain-specific and general baselines without algorithmic or hyperparameter changes between environments or benchmarks. + +# 2 RELATED WORK + +General-purpose RL. Although many traditional RL methods are general-purpose in principle, practical constraints often force assumptions about the task domain. For example, algorithms like Q-learning and SARSA (Watkins, 1989; Rummery & Niranjan, 1994) can be conceptually extended to continuous spaces, but are typically implemented using discrete lookup tables. In practice, early examples of general decision-making approaches can be found in on-policy methods with function approximation. For instance, both evolutionary algorithms (Rechenberg, 1978; Back, 1996; Rubinstein, 1997; Salimans et al., 2017) and policy gradient methods (Williams, 1992; Sutton et al., 1999; + +Schulman et al., 2015; 2017) offer update rules with convergence guarantees and independence to the input space. However, despite their generality, these methods are also hindered by poor sample efficiency and are prone to local minima, limiting their suitability for many practical applications. + +In contrast, the design of deep RL algorithms tends to favor more specialized approaches that align closely with a single benchmark—e.g., DQN Atari (Bellemare et al., 2013; Mnih et al., 2015), DDPG MuJoCo (Todorov et al., 2012; Lillicrap et al., 2015), or AlphaGo Go (Silver et al., 2016). Generalizing beyond these initial benchmarks can often require significant engineering, tuning, or algorithmic discovery (Luong et al., 2019; Schrittwieser et al., 2020; Haydari & Yılmaz, 2020; Ibarz et al., 2021). In imitation learning, GATO achieved generalist behavior, but relied on large expert datasets (Reed et al., 2022). Recently, DreamerV3 (Hafner et al., 2023) demonstrated a strong capability over many benchmarks without re-tuning, but used costly large models and simulated rollouts. Our objective is to discover a lightweight model-free approach to general-purpose learning. + +Dynamics-based representation learning. Building representations from system dynamics is a long-standing approach for adaptation, partial observability, and feature selection (Dayan, 1993; Littman & Sutton, 2001; Parr et al., 2008). Numerous model-free methods have been developed to learn representations by predicting future latent states (Munk et al., 2016; Van Hoof et al., 2016; Zhang et al., 2018; Gelada et al., 2019; Lee et al., 2020; Guo et al., 2020; 2022; Schwarzer et al., 2020; 2023; Zintgraf et al., 2021; Yu et al., 2021; 2022; Fujimoto et al., 2021; 2024; McInroe et al., 2021; Seo et al., 2022; Kim et al., 2022; Tang et al., 2023; Zhao et al., 2023; Zheng et al., 2024; Ni et al., 2024; Scannell et al., 2024). Unsurprisingly, these model-free approaches closely relate to model-based counterparts which learn a latent dynamics model for planning or value estimation (Watter et al., 2015; Finn et al., 2016; Karl et al., 2017; Ha & Schmidhuber, 2018; Schrittwieser et al., 2020; 2021; Ye et al., 2021; Hansen et al., 2022; 2024; Hafner et al., 2019; 2023; Wang et al., 2024). Our approach, MR.Q, is most closely related to the state-action representation learning in TD7 (Fujimoto et al., 2024). At a high level, MR.Q differs from TD7 by discarding the original input and including losses over the reward and termination. MR.Q also differs significantly in implementation, drawing inspiration from prior work to determine a set of design choices that performs well across benchmarks, including multi-step returns, unrolled dynamics, and categorical losses. + +Our motivation also relates to linear MDPs (Jin et al., 2020; Agarwal et al., 2020) and linear spectral representation (Ren et al., 2022; 2023; Zhang et al., 2022; Shribak et al., 2024). The latter aims to learn a low-rank decomposition of the transition dynamics of the MDP and recover a linear relationship between an embedding and the value function. Similarly, our work connects to two-stage linear RL, where a non-linear embedding is learned for linear RL (Levine et al., 2017; Chung et al., 2019). + +State abstraction. Our work is closely related to bisimulation metrics (Ferns et al., 2004; 2011; Castro, 2020) and MDP homomorphisms (Ravindran, 2004; van der Pol et al., 2020a;b; Rezaei-Shoshtari et al., 2022) which rely on measures of similarity in reward and dynamics for state or action abstraction. These concepts have inspired practical approximations to bisimulation metrics as a means of shaping representations in deep RL agents, particularly those using image-based observations (Zhang et al., 2020; Castro et al., 2021; Zang et al., 2022). + +# 3 BACKGROUND + +Reinforcement learning (RL) problems are described by a Markov Decision Process (MDP) (Bellman, 1957), which we define by a tuple $( S , A , p , R , \gamma )$ of state space $S$ , action space $A$ , dynamics function $p$ , reward function $R$ ( )and discount factor $\gamma$ . Value-based RL methods learn a value function $\begin{array} { r } { Q ^ { \pi } \bar { \left( s , a \right) } : = \mathbb { E } _ { \pi } \big [ \sum _ { t = 0 } ^ { \infty } \gamma ^ { t } r _ { t } \big | s _ { 0 } = s , a _ { 0 } = a \big ] } \end{array}$ that models the expected discounted sum of rewards $\boldsymbol { r } _ { t } \sim R ( s _ { t } , a _ { t } )$ by following a policy $\pi$ which maps states $s$ to actions $a$ . + +The true value function $Q ^ { \pi }$ is estimated by an approximate value function $Q _ { \theta }$ . We use subscripts to indicate the network parameters $\theta$ . Target networks, which are used to introduce stationarity in prediction targets, have parameters denoted by an apostrophe, e.g., $Q _ { \theta ^ { \prime } }$ . These parameters are periodically synchronized with the current network parameters $\theta ^ { \prime } \theta _ { , } ^ { \prime }$ ). + +# 4 MODEL-BASED REPRESENTATIONS FOR Q-LEARNING + +This section presents the MR.Q algorithm (Model-based Representations for Q-learning), a modelfree RL algorithm that learns an approximately linear representation of the value function through model-based objectives. Value-based RL algorithms learn a value function $Q$ that maps state-action pairs $( s , a )$ to values in $\mathbb { R }$ and a policy $\pi$ that maps states $s$ to actions $a$ . Like many representation learning methods for RL, MR.Q adds an initial step that transforms states and state-action pairs into embeddings $\mathbf { z } _ { s }$ and $\mathbf { z } _ { s a }$ , which serves as inputs to the downstream policy and value function. + +$$ +\begin{array} { r l } & { f _ { \omega } : s \to \mathbf { z } _ { s } , } \\ & { \pi _ { \phi } : \mathbf { z } _ { s } \to a , } \end{array} \qquad \begin{array} { r l } & { g _ { \omega } : ( s , a ) \to \mathbf { z } _ { s a } , } \\ & { Q _ { \theta } : \mathbf { z } _ { s a } \to \mathbb { R } . } \end{array} +$$ + +While neither the value function nor policy require explicit representation learning, using intermediate embeddings has two main benefits: + +1. Introducing an explicit representation learning stage can enable richer alternative learning signals that are grounded in the dynamics and rewards of the MDP, as opposed to relying exclusively on non-stationary value targets used in both value and policy learning. 2. Representation learning can transform the input into a unified, abstract space that is decoupled from the original input characteristics, e.g., images or action spaces. This abstraction allows us to filter irrelevant or spurious details and use unified downstream architectures, improving robustness to environment variations. + +To learn these embeddings, we draw inspiration from linear feature selection, revisiting the work of Parr et al. (2008), as well as MDP homomorphisms (Ravindran & Barto, 2002). In Section 4.1 we highlight how model-based objectives can be used to learn features that share an approximately linear relationship with the true value function. Then in Section 4.2, we relax our theoretical motivation for a practical algorithm based on recent advances in dynamics-based representation learning. + +# 4.1 THEORETICAL MOTIVATION + +Consider a linear decomposition of the value function, where the value function $Q ( s , a )$ is represented by features $\mathbf { z } _ { s a }$ and linear weights w: + +$$ +Q ( s , a ) = \mathbf { z } _ { s a } ^ { \top } \mathbf { w } . +$$ + +Our primary objective is to learn features $\mathbf { z } _ { s a }$ that share an approximately linear relationship with the true value function $Q ^ { \pi }$ . However, since this relationship is only approximate, we use these features as input to a non-linear function $\hat { Q } ( \mathbf { z } _ { s a } )$ , rather than relying solely on linear function approximation. + +We start by exploring how to find features that can linearly represent the true value function. Given a dataset $D$ of tuples $( s , a , r , s ^ { \prime } , a ^ { \prime } )$ , we consider two possible approaches for learning a value function $Q$ ( ): A model-free update based on semi-gradient TD (Sutton, 1988; Sutton & Barto, 1998): + +$$ +\mathbf { w } \gets \mathbf { w } - \alpha \mathbb { E } _ { D } \left[ \nabla _ { \mathbf { w } } \left( \mathbf { z } _ { s a } ^ { \top } \mathbf { w } - \left| r + \gamma \mathbf { z } _ { s ^ { \prime } a ^ { \prime } } ^ { \top } \mathbf { w } \right| _ { \mathrm { s g } } \right) ^ { 2 } \right] . +$$ + +A model-based approach to learn $\mathbf { w } _ { \mathrm { m b } }$ , based on rolling out estimates of the dynamics and reward: + +$$ +\begin{array} { l } { { \displaystyle { \bf w } _ { \mathrm { m b } } : = \sum _ { t = 0 } ^ { \infty } \gamma ^ { t } W _ { p } ^ { t } { \bf w } _ { r } } , \qquad } \\ { { \displaystyle { \bf w } _ { r } : = \arg \operatorname* { m i n } _ { \bf w } \mathbb { E } _ { D } \left[ \left( { \bf z } _ { s a } ^ { \top } { \bf w } - r \right) ^ { 2 } \right] , \qquad \quad W _ { p } : = \arg \operatorname* { m i n } _ { W } \mathbb { E } _ { D } \left[ \left( { \bf z } _ { s a } ^ { \top } W - { \bf z } _ { s ^ { \prime } a ^ { \prime } } \right) ^ { 2 } \right] } . } \end{array} +$$ + +Closely following Parr et al. (2008) and Song et al. (2016), we can show that these approaches converge to the same solution (proofs for this section can be found in Appendix A). + +Theorem 1. The fixed point of the model-free approach (Equation 4) and the solution of the modelbased approach (Equation 5) are the same. + +From the insight of Theorem 1, we can connect the value error VE, the difference between an approximate value function $Q$ and the true value function $Q ^ { \pi }$ , + +$$ +\mathrm { V E } ( s , a ) : = Q ( s , a ) - Q ^ { \pi } ( s , a ) +$$ + +to the accuracy of reward and dynamics components of the estimated model (Theorem 2). + +Theorem 2. The value error of the solution described by Theorem $I$ is bounded by the accuracy of the estimated dynamics and reward: + +$$ +| \mathrm { V E } ( s , a ) | \leq \frac { 1 } { 1 - \gamma } \operatorname* { m a x } _ { ( s , a ) \in S \times A } \left( | \mathbf { z } _ { s a } ^ { \top } \mathbf { w } _ { r } - \mathbb { E } _ { r | s , a } [ r ] | + \operatorname* { m a x } _ { i } | \mathbf { w } _ { i } | \sum | \mathbf { z } _ { s a } ^ { \top } W _ { p } - \mathbb { E } _ { s ^ { \prime } , a ^ { \prime } | s , a } [ \mathbf { z } _ { s ^ { \prime } a ^ { \prime } } ] | \right) . +$$ + +Parr et al. (2008) and Song et al. (2016) use a related insight regarding the Bellman error to infer an approach for feature selection. However, with the advent of deep learning, we can instead directly learn the features $\mathbf { z } _ { s a }$ by jointly optimizing them alongside the linear weights ${ \bf w } _ { r }$ and $W _ { p }$ . This is accomplished by treating the features and linear weights as a unified end-to-end model and balancing the losses in Equation 6 with a hyperparameter $\lambda$ : + +$$ +\begin{array} { r } { \mathcal { L } ( \mathbf { z } _ { s a } , \mathbf { w } _ { r } , W _ { p } ) = \underbrace { \mathbb { E } _ { D } \left[ \left( \mathbf { z } _ { s a } ^ { \top } \mathbf { w } _ { r } - r \right) ^ { 2 } \right] } _ { \mathrm { R e w a r d l e a r n i n g } } + \underbrace { \lambda \mathbb { E } _ { D } \left[ \left( \mathbf { z } _ { s a } ^ { \top } W _ { p } - \mathbf { z } _ { s ^ { \prime } a ^ { \prime } } \right) ^ { 2 } \right] } _ { \mathrm { D y n a m i c s l e a r i n i n g } } . } \end{array} +$$ + +However, the resulting Equation 9 has some notable drawbacks. + +Dependency on $\pi$ . The dynamics target $\mathbf { z } _ { s ^ { \prime } a ^ { \prime } }$ depends on an action $a ^ { \prime }$ determined by the policy $\pi$ . In policy optimization problems, this introduces non-stationarity, where the target embedding must be continually updated to reflect changes in the policy. This creates an undesirable interdependence between the policy and encoder. + +Undesirable local minima. Jointly optimizing both the features $\mathbf { z } _ { s a }$ and the dynamics target can lead to undesirable local minima, similar to the issues encountered with Bellman residual minimization (Baird, 1995; Fujimoto et al., 2022). This can result in collapsed or trivial solutions when the dataset does not fully cover the state and action space or when the reward is sparse. + +To address these issues, we suggest relaxations on our proposed, theoretically grounded approach: + +$$ +\begin{array} { r } { \mathcal { L } ( \mathbf { z } _ { s a } , \mathbf { w } _ { r } , W _ { p } ) = \mathbb { E } _ { D } \left[ \left( \mathbf { z } _ { s a } ^ { \top } \mathbf { w } _ { r } - r \right) ^ { 2 } \right] + \lambda \mathbb { E } _ { D } \Big [ \left( \mathbf { z } _ { s a } ^ { \top } W _ { p } - \bar { \mathbf { z } } _ { s ^ { \prime } } \right) ^ { 2 } \Big ] . } \end{array} +$$ + +We propose two key modifications to alleviate the aforementioned issues. Firstly, we use a statedependent embedding $\mathbf { z } _ { s ^ { \prime } }$ as the dynamics target, rather than the state-action embedding $\mathbf { z } _ { s ^ { \prime } a ^ { \prime } }$ . This eliminates any dependency on the current policy while still capturing the environment’s dynamics. + +Secondly, to mitigate the issue of local minima, we use a target network $f _ { \omega ^ { \prime } } ( s ^ { \prime } )$ to generate the dynamics target $\bar { \mathbf { z } } _ { s ^ { \prime } }$ , where the parameters $\omega ^ { \prime }$ are periodically updated to track the current network parameters $\omega$ . Empirical evidence from prior work suggests that this approach can yield significant performance gains (Grill et al. (2020); Assran et al. (2023), see Related Work), although it no longer guarantees convergence to a fixed point. + +Due to these two changes, even if the modified objective defined by Equation 10 is minimized, we can no longer assume there is a linear relationship between the embedding $\mathbf { z } _ { s a }$ and the value function. However, we can instead allow for a non-linear relationship, replacing linear weights w with a non-linear function $\hat { Q } ( \mathbf { z } _ { s a } )$ . We can show that this relationship exists as long as the features ( )are sufficiently rich (i.e., such that a MDP homomorphism is satisfied (Ravindran & Barto, 2002)). + +Theorem 3. Given functions $f ( s ) = \mathbf { z } _ { s }$ and $g ( \mathbf { z } _ { s } , a ) = \mathbf { z } _ { s a }$ , then if there exists functions $\hat { p }$ and $\hat { R }$ such that for all $( s , a ) \in S \times A$ : + +$$ +\mathbb { E } _ { \hat { R } } [ \hat { R } ( { \bf z } _ { s a } ) ] = \mathbb { E } _ { R } \left[ R ( s , a ) \right] , \qquad \hat { p } ( { \bf z } _ { s ^ { \prime } } | { \bf z } _ { s a } ) = \sum _ { \hat { s } : { \bf z } _ { \hat { s } } = { \bf z } _ { s ^ { \prime } } } p ( \hat { s } | s , a ) , +$$ + +then for any policy $\pi$ where there exists a corresponding policy ${ \hat { \pi } } ( a | \mathbf { z } _ { s } ) = \pi ( a | s )$ , there exists $a$ function $\hat { Q }$ equal to the true value function $Q ^ { \pi }$ over all possible state-action pairs $( s , a ) \in S \times A$ : + +$$ +\hat { Q } ( \mathbf { z } _ { s a } ) = Q ^ { \pi } ( s , a ) . +$$ + +Furthermore, Equation $1 l$ guarantees the existence of an optimal policy ${ \hat { \pi } } ^ { * } ( a | \mathbf { z } _ { s } ) = \pi ^ { * } ( a | s )$ . + +Consequently, even if the features $\mathbf { z } _ { s a }$ do not linearly represent the true value function, i.e., the loss in Equation 9 cannot be not exactly minimized, $\mathbf { z } _ { s a }$ can still be used in a non-linear relationship to represent the value function. Furthermore, Theorem 3 outlines a similar objective as the original linear objective defined in Equation 9, in learning the reward and dynamics of the MDP. + +These results motivates the practical algorithm discussed in the following section. Using the adjusted loss defined in Equation 10, we will aim to learn features with an approximately linear relationship to the true value function, but use a non-linear value function with those features to account for the error induced by our approximations. + +# 4.2 ALGORITHM + +We now present the details of MR.Q (Model-based Representations for Q-learning). Building on the insights from the previous section, our key idea is to learn a state-action embedding $\mathbf { z } _ { s a }$ that is approximately linear with the true value function $Q ^ { \pi }$ . To account for approximation errors, these features are used with non-linear function approximation to determine the value. + +The state embedding vector $\mathbf { z } _ { s }$ is obtained as an intermediate component by training end-to-end with the state-action encoder. MR.Q handles different input modalities by swapping the architecture of the state encoder. Since $\mathbf { z } _ { s }$ is a vector, the remaining networks are independent of the observation space and use feedforward networks. + +Given the transition $( s , a , r , d , s ^ { \prime } )$ from the replay buffer: + +
Output MR.Q
Trained end-to-end
State Encoder State-Action Encoder zsα = gω(zs, a) MDP predictorZ = fω(s) zs′, r, d = zTam
Decoupled RL
ValueQi = Qθ(Zsa)
Policyaπ = πφ(Zs)
+ +
Update MR.Q
if t % Ttarget = 0 then Target networks: θ′, φ′, ω′ ← θ, φ, ω. Reward scaling: r′ ← r, r ← meanDr.
for Ttarget time steps do Encoder update: Equation 14.
Value update: Equation 19.
Policy update: Equation 20.
+ +The encoder loss is composed of three terms based on the reward, dynamics and terminal signal that are unrolled over a short horizon. The value function and policy are trained independently, using standard losses (Silver et al., 2014; Fujimoto et al., 2018). We use LAP (Fujimoto et al., 2020) to sample transitions with priority according to their TD errors (Schaul et al., 2016), the absolute difference between the predicted value and the target value in Equation 19. + +The target network, reward scaling (defined in Equation 19), and the encoder are updated periodically every $T _ { \mathrm { t a r g e t } }$ time steps. This synchronized update schedule keeps the input and target output fixed for the downstream value function and policy within each iteration, thus reducing nonstationarity in the optimization (Fujimoto et al., 2024). + +# 4.2.1 ENCODER + +The encoder loss is based on unrolling the dynamics of the learned model over a short horizon. Given a subsequence of an episode $( s _ { 0 } , a _ { 0 } , r _ { 1 } , d _ { 1 } , s _ { 1 } , . . . , r _ { { \cal H } _ { \mathrm { E n c } } } , d _ { { \cal H } _ { \mathrm { E n c } } } , s _ { { \cal H } _ { \mathrm { E n c } } } )$ , the model is unrolled by encoding the initial state $s _ { 0 }$ , then by repeatedly applying the state-action encoder $g _ { \omega }$ and linear MDP predictor $\mathbf { m }$ : + +$$ +\begin{array} { r } { \tilde { \mathbf { z } } ^ { t } , \tilde { r } ^ { t } , \tilde { d } ^ { t } : = g _ { \omega } ( \tilde { \mathbf { z } } ^ { t - 1 } , a ^ { t - 1 } ) ^ { \top } \mathbf { m } , \quad \mathrm { ~ w h e r e ~ } \tilde { \mathbf { z } } ^ { 0 } : = f _ { \omega } \big ( s _ { 0 } \big ) . } \end{array} +$$ + +The final loss is summed over the unrolled model and balanced by corresponding hyperparameters: + +$$ +\mathcal { L } _ { \mathrm { E n c o d e r } } ( f , g , \mathbf { m } ) : = \sum _ { t = 1 } ^ { H _ { \mathrm { E n c } } } \lambda _ { \mathrm { R e w a r d } } \mathcal { L } _ { \mathrm { R e w a r d } } ( \widetilde { r } ^ { t } ) + \lambda _ { \mathrm { D y n a m i c s } } \mathcal { L } _ { \mathrm { D y n a m i c s } } ( \widetilde { \mathbf { z } } _ { s ^ { \prime } } ^ { t } ) + \lambda _ { \mathrm { T e m i n a l } } \mathcal { L } _ { \mathrm { T e m i n a l } } ( \widetilde { d } ^ { t } ) . +$$ + +$\lambda _ { \mathrm { T e r m i n a l } }$ is set to 0 until the first terminal transition (i.e., $d = 0$ ) is viewed. This approach is commonly used in model-based RL (Oh et al., 2015; Hafner et al., 2023; Hansen et al., 2024), as well as dynamics-based representation learning (Schwarzer et al., 2020; 2023; Scannell et al., 2024). + +Reward loss. While our theoretical analysis suggests using the mean-squared error to train the predicted reward, we find that a categorical representation of the reward is more effective in practice for predicting sparse rewards and is robust to reward magnitude. This empirical benefit is consistent with prior work (Schrittwieser et al., 2020; Hafner et al., 2023; Hansen et al., 2024; Wang et al., 2024). Our reward loss function uses the cross entropy CE between the predicted reward $\tilde { r }$ and a two-hot encoding of the reward $r$ : + +$$ +\begin{array} { r } { \mathcal { L } _ { \mathrm { R e w a r d } } ( \tilde { r } ) : = \mathbf { C E } \left( \tilde { r } , \mathrm { T w o - H o t } ( r ) \right) . } \end{array} +$$ + +To handle a wide range of reward magnitudes without prior knowledge, the locations of the two-hot encoding are spaced at increasing non-uniform intervals, according to $\mathrm { s y m e x p } ( x ) =$ $\mathrm { s i g n } ( x ) ( \exp { ( x ) } ^ { - 1 } )$ (Hafner et al., 2023). + +Dynamics loss. The dynamics loss minimizes the mean-squared error between the predicted next state embedding $\tilde { \mathbf { z } } _ { s ^ { \prime } }$ and the next state embedding $\bar { \mathbf { z } } _ { s ^ { \prime } }$ from the target encoder $f _ { \omega ^ { \prime } }$ : + +$$ +\begin{array} { r } { \mathcal { L } _ { \mathrm { D y n a m i c s } } \big ( \tilde { \mathbf { z } } _ { s ^ { \prime } } \big ) : = \big ( \tilde { \mathbf { z } } _ { s ^ { \prime } } - \bar { \mathbf { z } } _ { s ^ { \prime } } \big ) ^ { 2 } . } \end{array} +$$ + +As discussed in the previous section, using the next state embedding $\mathbf { z } _ { s ^ { \prime } }$ eliminates the dependency on the policy that would occur when using a state-action embedding target. + +Terminal loss. The predicted scalar terminal signal $\tilde { d }$ is trained simply using a MSE loss with the binary terminal signal $d$ : + +$$ +\begin{array} { r } { \mathcal { L } _ { \mathrm { T e r m i n a l } } ( \tilde { d } ) : = ( \tilde { d } - d ) ^ { 2 } . } \end{array} +$$ + +# 4.2.2 VALUE FUNCTION + +Value learning is primarily based on TD3 (Fujimoto et al., 2018). Specifically, we train two value functions and take the minimum output between their respective target networks to determine the value target. Similar to TD3, the target action is determined by the target policy $\pi _ { \phi ^ { \prime } }$ , perturbed by small amount of clipped Gaussian noise: + +$$ +\boldsymbol a _ { \pi } = \left\{ \begin{array} { l l } { \mathrm { a r g m a x } \boldsymbol a ^ { \prime } } & { \mathrm { f o r ~ d i s c r e t e ~ } \boldsymbol A , } \\ { \mathrm { c l i p } ( \boldsymbol a ^ { \prime } , - 1 , 1 ) } & { \mathrm { f o r ~ c o n t i n u o u s ~ } \boldsymbol A , } \end{array} \right. \quad \mathrm { w h e r e } \ \boldsymbol a ^ { \prime } = \pi _ { \phi ^ { \prime } } ( \boldsymbol s ^ { \prime } ) + \mathrm { c l i p } ( \boldsymbol \epsilon , - \boldsymbol c , \boldsymbol c ) , \quad \boldsymbol \epsilon \sim \mathcal N ( 0 , \sigma ^ { 2 } ) . +$$ + +Discrete actions are represented by a one-hot encoding, where the Gaussian noise is added to each dimension. Action noise and the clipping is scaled according the range of the action space. + +We modify the TD3 loss in a few ways. Firstly, following numerous prior work across benchmarks (Hessel et al., 2018; Barth-Maron et al., 2018; Yarats et al., 2022; Schwarzer et al., 2023), we predict multi-step returns over a horizon $H _ { Q }$ . Secondly, we use the Huber loss instead of meansquared error to eliminate bias from prioritized sampling (Fujimoto et al., 2020). Finally, the target value is normalized according to the average absolute reward $\bar { r }$ in the replay buffer: + +$$ +\mathcal { L } _ { \mathrm { V a l u e } } ( \tilde { Q } _ { i } ) : = \mathrm { H u b e r } \left( \tilde { Q } _ { i } , \frac { 1 } { \bar { r } } \left( \sum _ { t = 0 } ^ { H _ { Q } - 1 } \gamma ^ { t } r _ { t } + \gamma ^ { H _ { Q } } \tilde { Q } _ { j } ^ { \prime } \right) \right) , \quad \tilde { Q } _ { j } ^ { \prime } : = \bar { r } ^ { \prime } \operatorname* { m i n } _ { j = 1 , 2 } Q _ { \theta _ { j } ^ { \prime } } \bigl ( \mathbf { z } _ { s _ { H _ { Q } } a _ { H _ { Q } , \pi } } \bigr ) . +$$ + +The value $\bar { r } ^ { \prime }$ captures the target average absolute reward, which is the scaling factor used to the most recently copied value functions $Q _ { \theta _ { j } ^ { \prime } }$ . This value is updated simultaneously with the target networks $\bar { r } ^ { \prime } \bar { r }$ . Maintaining a consistent reward scale keeps the loss magnitude constant across different benchmarks, thus improving the robustness of a single set of hyperparameters. + +# 4.2.3 POLICY + +For both continuous and discrete action spaces, the policy is updated using the deterministic policy gradient (Silver et al., 2014): + +$$ +\mathcal { L } _ { \mathrm { P o l i c y } } \big ( a _ { \pi } \big ) : = - 0 . 5 \sum _ { i = \{ 1 , 2 \} } \tilde { Q } _ { i } \big ( \mathbf { z } _ { s a _ { \pi } } \big ) + \lambda _ { \mathrm { p r e - a c t i v } } \mathbf { z } _ { \pi } ^ { 2 } , \quad \mathrm { w h e r e ~ } a _ { \pi } = \operatorname { a c t i v } \big ( \mathbf { z } _ { \pi } \big ) . +$$ + +To make the loss universal between action spaces, we use Gumbel-Softmax (Jang et al., 2017; Lowe et al., 2017; Cianflone et al., 2019) for discrete actions, and Tanh for continuous actions. A small regularization penalty is added to the square of the pre-activations ${ \bf z } _ { \pi }$ before the policy’s final activation to help avoid local minima when the reward, and value, is sparse (Bjorck et al., 2021). + +For exploration, Gaussian noise is added to each dimension of the action (or one-hot encoding of the action). Similar to Equation 18, the resulting action vector is clipped to the range of the action space for continuous actions. For discrete actions, the final action is determined by the argmax operation. + +![](images/figures/mrq-fig-0002.jpg) +Figure 2: Aggregate learning curves. Average performance over each benchmark. Results are over 10 seeds. The shaded area captures a $9 5 \%$ stratified bootstrap confidence interval. Due to action repeat, 500k time steps in DMC correspond to 1M frames in the original environment and 2.5M time steps in Atari corresponds to 10M frames in the original environment. + +# 5 EXPERIMENTS + +We evaluate MR.Q on four popular RL benchmarks and 118 environments, and compare its performance against strong domain-specific baselines, general model-based approaches, DreamerV3 (Hafner et al., 2023) and TD-MPC2 (Hansen et al., 2024), and a general model-free algorithm, PPO (Schulman et al., 2017). Rather than establish MR.Q as the state-of-the-art approach in any particular benchmark, our objective is to demonstrate its broad applicability and effectiveness across a diverse set of tasks with a single set of hyperparameters. The baselines use author-suggested default hyperparameters and are fixed across environments. Additional details can be found in Appendix B. + +# 5.1 MAIN RESULTS + +Aggregate learning curves are displayed in Figure 2, with full results displayed in Appendix C. + +Gym - Locomotion. This subset of the Gym benchmark (Brockman et al., 2016; Towers et al., 2024) considers 5 locomotion tasks in the MuJoCo simulator (Todorov et al., 2012) with continuous actions and low level states. Agents are trained for 1M time steps without any environment preprocessing. We evaluate against three baselines: TD7 (Fujimoto et al., 2024), a state-of-the-art (or near) approach for this benchmark, as well as TD-MPC2, DreamerV3, and PPO. To aggregate results, we normalize using the performance of TD3 (Fujimoto et al., 2018). + +DMC - Proprioceptive. The DeepMind Control suite (DMC) (Tassa et al., 2018) is a collection of continuous control robotics tasks built on the MuJoCo simulator. These tasks use the proprioceptive states as the observation space, meaning that the input is a vector, and limit the total reward for each episode at 1000, making it easy to aggregate results. We report results on all 28 default tasks that were used by either TD-MPC2 or DreamerV3. Agents are trained for $5 0 0 \mathrm { k }$ time steps, equivalent to 1M frames in the original environment due to action repeat. For comparison, we evaluate against the same three algorithms as in the Gym benchmark, with TD-MPC2 considered state-of-the-art (or near) for this benchmark. We also include TD7 due to its strong performance in the Gym benchmark. + +DMC - Visual. The visual DMC benchmark includes the same 28 tasks as the proprioceptive benchmark, but uses image-based observations instead. Agents are trained for $5 0 0 \mathrm { k }$ time steps. For baselines, we include DrQ-v2 (Yarats et al., 2022), given its state-of-the-art (or near) performance in model-free RL, alongside TD-MPC2, DreamerV3, and PPO. + +Atari. The Atari benchmark is built on the Arcade Learning Environment (Bellemare et al., 2013). This benchmark uses pixel observations and discrete actions and includes the 57 games used by DreamerV3. We follow standard preprocessing steps, including sticky actions (Machado et al., 2018) (full details in Appendix B.3). Agents are trained for 2.5M time steps (equivalent to 10M frames), a setting which has been considered by prior work (Sokar et al., 2023). For comparison, we evaluate against three baselines: the model-based approach DreamerV3, as well as model-free approaches, DQN (Mnih et al., 2015), Rainbow (Hessel et al., 2018), and PPO. Results are aggregated by normalizing scores against human performance. + +Discussion. Throughout our experiments, we find the presence of “no free lunch”, where the topperforming baseline in one benchmark fails to replicate its success in another. Regardless, MR.Q achieves the highest performance in both DMC benchmarks, showcasing its ability to handle different observation spaces. Although it falls slightly behind TD7 in the Gym benchmark, MR.Q is the strongest method overall across all continuous control benchmarks. In Atari, while DreamerV3 outperforms MR.Q, it relies on a model with 40 times more parameters and struggles comparatively in the remaining benchmarks. When compared to the model-free baselines, MR.Q surpasses PPO, DQN, and Rainbow, demonstrating its effectiveness with discrete action spaces. + +# 5.2 DESIGN STUDY + +To better understand the impact of certain design choices and hyperparameters, we attempt variations of MR.Q, and report the aggregate results in Table 2. + +Table 2: Design study. Average difference in normalized performance from varying design choices across each benchmark over 5 seeds. Negative changes are highlighted lightly $[ - 0 . 0 1 , - 0 . 2 )$ . Damaging changes are highlighted moderately $[ - 0 . 2 , - 0 . 5 )$ . Catastrophic changes are highlighted boldly $( \leq - 0 . 5 )$ . Positive changes are similarly highlighted $\left( > 0 . 0 1 \right)$ . + +
DesignTD3-NormalizedGym - Locomotion DMC - Proprioceptive Reward (1k)DMC - Visual Reward (1k)Atari - 1M Human-Normalized
Relaxations
Linear value function-1.17 [-1.19, -1.15]-0.58 [-0.59, -0.56]-0.41 [-0.42, -0.39]-1.35 [-1.41, -1.29]
Dynamics target-0.10 [-0.17, -0.04]-0.15 [-0.15, -0.15]-0.05 [-0.05, -0.04]-0.38 [-0.81, 0.05]
No target encoder-0.53 [-0.60, -0.46]-0.35 [-0.35, -0.34]-0.15 [-0.15, -0.15]-0.86 [-0.89, -0.83]
Revert-1.47 [-1.54, -1.39]-0.72 [-0.73, -0.72]-0.52 [-0.52, -0.51]-1.69 [-1.70, -1.67]
Non-linear model-0.01 [-0.07, 0.03]-0.00 [-0.02, 0.01]-0.01 [-0.02, -0.00]-0.07 [-0.32, 0.18]
Loss functions
MSE reward loss0.10 [-0.02, 0.19]-0.06 [-0.08, -0.05]-0.05 [-0.07, -0.04]-0.79 [-0.86, -0.73]
No reward scaling-0.04 [-0.09, 0.02]-0.01 [-0.02, 0.00]-0.00 [-0.01, 0.01]0.18 [-0.25, 0.56]
No min-0.09 [-0.16, -0.01]-0.01 [-0.02, 0.01]0.00 [-0.01, 0.01]0.13 [-0.10, 0.58]
No LAP No MR-0.10 [-0.24, -0.00]0.00 [-0.00, 0.01]-0.01 [-0.02, -0.01]-0.13 [-0.38, 0.14]
-0.56 [-0.69, -0.43]-0.19 [-0.19, -0.18]-0.07 [-0.09, -0.03]-0.78 [-0.88, -0.69]
Horizons
1-step return-0.33 [-0.46, -0.21]-0.04 [-0.05, -0.02]-0.03 [-0.03, -0.02]-0.70 [-0.81, -0.59]
No unroll0.07 [0.01, 0.14]-0.01 [-0.01, -0.00]-0.04 [-0.06, -0.01]-0.33 [-0.41, -0.28]
+ +Relaxations. In Section 4.1, we outlined a loss (Equation 9) that, if globally minimized, would provide features that are linear with the true value function. MR.Q in practice relaxes this theoretical result by modifying the loss and using a non-linear value function. In Linear value function, we replace the non-linear value function with a linear function. In Dynamics target, we replace the state embedding dynamics target with a state-action embedding $\bar { \bf z } _ { s ^ { \prime } a ^ { \prime } }$ determined from the target state-action encoder $g _ { \omega }$ . In $\mathbf { N o }$ target encoder, we use the current encoder to generate the dynamics target $\mathbf { z } _ { s ^ { \prime } a ^ { \prime } }$ , and jointly optimize it within the encoder loss. In Revert, we consider all of the aforementioned changes simultaneously, using linear value functions and setting the dynamics target as a state-action embedding determined by the current encoder. In Non-linear model, we replace the linear MDP predictor with individual networks that predict each component separately from $\mathbf { z } _ { s a }$ . + +Loss functions. MR.Q’s loss functions use several unconventional choices. In MSE reward loss, we replace the categorical loss function on the predicted reward in Equation 15 with the meansquared error (MSE). In No reward scaling, we remove the reward scaling in Equation 19, setting $\bar { r } = \bar { r } ^ { \prime } = 1$ . In $\mathbf { N o \ m i n }$ , we take the mean over the target value functions instead of the minimum in Equation 19. In No LAP, we remove prioritized sampling (Fujimoto et al., 2020) and use the MSE instead of the Huber loss in the value update. Lastly, in No MR, we remove model-based representation learning and train the encoder end-to-end with the value function. + +Horizons. Finally, we consider the role of extended predictions. In 1-step return, we remove multi-step value predictions and use TD learning. In No unroll, we remove the dynamics unrolling in Equation 14, by setting the encoder horizon $H _ { \mathrm { E n c } } = 1$ . + +Discussion. The results of our design study show the benefit of balancing theory with practical relaxations. The experiments further validate our design choices and hyperparameters. We highlight two results in particular: (1) increasing the model capacity in the “non-linear model” experiment, does not improve performance. This outcome suggests that maintaining an approximately linear relationship with the value function can be more impactful than increased capacity. (2) Our study also reveals a key distinction between the Gym and Atari benchmarks—while the “MSE reward loss” and “No unroll” variants offer moderate performance gains in Gym, they significantly degrade performance in Atari. This discrepancy highlights how hyperparameters can overfit to individual benchmarks, emphasizing the importance of evaluating algorithms across multiple benchmarks. + +# 6 DISCUSSION AND CONCLUSION + +This paper introduces MR.Q, a general model-free deep RL algorithm that achieves strong performance across diverse benchmarks and environments. Drawing inspiration from the theory of model-based representation learning, MR.Q demonstrates that model-free deep RL is a promising avenue for building general-purpose algorithms that achieve high performance across environments, while being simpler and less expensive than model-based alternatives. + +Our work also reveals insights on which design choices matter when building general-purpose model-free deep RL algorithms and how common benchmarks respond to these design choices. + +Model-based and model-free RL. MR.Q integrates model-based objectives with a model-free backbone during training, effectively blurring the boundary between traditional model-based and model-free RL. While MR.Q could be extended to the model-based setting by incorporating planning or simulated trajectories with the state-action encoder, these components can add significant execution time and increase the overall complexity and tuning required by a method. Moreover, the performance of MR.Q in these common RL benchmarks demonstrates that these model-based components may be simply unnecessary—suggesting that the representation itself could be the most valuable aspect of model-based learning, even in methods that do use planning. This argument is echoed by DreamerV3 and TD-MPC2, which rely on short planning horizons and trajectory generation, while including both value functions and traditional model-free policy updates. As such, it may be necessary to examine more complex settings, to reliably see a benefit from model-based search or planning, e.g., (Silver et al., 2016). + +Universality of RL benchmarks. Our results demonstrate that there is a striking lack of positive transfer between benchmarks. For example, despite the similarities in tasks and the same underlying MuJoCo simulator, the top performers in Gym and DMC fail to replicate their success on the opposing benchmark. Similarly, although DreamerV3 excels at Atari, these performance benefits do not translate to continuous control environments, underperforming TD3 in Gym and outright failing to learn the Dog and Humanoid tasks in DMC (see Appendix C). These findings show the limitations of single-benchmark evaluations, indicating that success on one benchmark may not translate easily to others, and highlights the need for more comprehensive benchmarks. + +Limitations. MR.Q is only the first step towards a new generation of general-purpose model-free deep RL algorithms. Many challenges remains for a fully general algorithm. In particular, MR.Q is not equipped to handle settings such as hard exploration tasks or non-Markovian environments. Another limitation is our evaluation only considers standard RL benchmarks. Although this allows direct comparison with other methods, established algorithms such as PPO have demonstrated their effectiveness in highly unique settings, such as team video games (Berner et al., 2019), drone racing (Kaufmann et al., 2023), and large language models (Achiam et al., 2023; Touvron et al., 2023). To demonstrate similar versatility, new algorithms must undergo the same rigorous testing across a range of tasks that is beyond the scope of any single study. + +As the community continues to push the boundaries of what is possible with deep RL, we believe that building simpler general-purpose algorithms has the potential to make this technology more accessible to a wider audience, ultimately enabling users to train agents with ease. Perhaps one day—with just the click of a button. + +# ACKNOWLEDGMENTS + +We would like to thank Brandon Amos, Mikhael Henaff, Luis Pineda, Paria Rashidinejad, and Qinqing Zheng for insightful discussions and comments. + +# REFERENCES + +Josh Achiam, Steven Adler, Sandhini Agarwal, Lama Ahmad, Ilge Akkaya, Florencia Leoni Aleman, Diogo Almeida, Janko Altenschmidt, Sam Altman, Shyamal Anadkat, et al. Gpt-4 technical report. arXiv preprint arXiv:2303.08774, 2023. + +Alekh Agarwal, Sham Kakade, Akshay Krishnamurthy, and Wen Sun. 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The fixed point of the model-free approach (Equation 4) and the solution of the modelbased approach (Equation 5) are the same. + +Proof. Let $Z$ be a matrix containing state-action embeddings $\mathbf { z } _ { s a }$ for each state-action pair $( s , a ) \in$ $S \times A$ . Let $Z ^ { \prime }$ be the corresponding matrix of next state-action embeddings $\mathbf { z } _ { s ^ { \prime } a ^ { \prime } }$ . Let $R$ ( )be the vector of the corresponding rewards $r ( s , a )$ . + +The linear semi-gradient TD update: + +$$ +\begin{array} { r l } & { \mathbf w _ { t + 1 } : = \mathbf w _ { t } - \alpha Z ^ { \top } ( Z \mathbf w _ { t } - ( R + \gamma Z ^ { \prime } \mathbf w _ { t } ) ) } \\ & { \qquad = \mathbf w _ { t } - \alpha Z ^ { \top } Z \mathbf w _ { t } + \alpha Z ^ { \top } R + \alpha \gamma Z ^ { \top } Z ^ { \prime } \mathbf w _ { t } } \\ & { \qquad = \big ( I - \alpha \big ( Z ^ { \top } Z - \gamma Z ^ { \top } Z ^ { \prime } \big ) \big ) \mathbf w _ { t } + \alpha Z ^ { \top } R } \\ & { \qquad = \big ( I - \alpha A \big ) \mathbf w _ { t } + \alpha B , } \end{array} +$$ + +where $A : = Z ^ { \intercal } Z - \gamma Z ^ { \intercal } Z ^ { \prime }$ and $B : = Z ^ { \intercal } R$ . + +The fixed point of the system: + +$$ +\begin{array} { r } { \mathbf { w } _ { \mathrm { m f } } = \big ( I - \alpha A \big ) \mathbf { w } _ { \mathrm { m f } } + \alpha B } \\ { \mathbf { w } _ { \mathrm { m f } } - \big ( I - \alpha A \big ) \mathbf { w } _ { \mathrm { m f } } = \alpha B \qquad } \\ { \alpha A \mathbf { w } _ { \mathrm { m f } } = \alpha B \qquad } \\ { \mathbf { w } _ { \mathrm { m f } } = A ^ { - 1 } B . \qquad } \end{array} +$$ + +The least squares solution to $W _ { p }$ and ${ \bf w } _ { r }$ + +$$ +\begin{array} { r } { W _ { p } : = \left( Z ^ { \top } Z \right) ^ { - 1 } Z ^ { \top } Z ^ { \prime } } \\ { \mathbf { w } _ { r } : = \left( Z ^ { \top } Z \right) ^ { - 1 } Z ^ { \top } R } \end{array} +$$ + +By rolling out $W _ { p }$ and ${ \bf w } _ { r }$ , we arrive at a model-based solution: + +$$ +Q : = Z \mathbf { w } _ { \mathrm { m b } } = Z \sum _ { t = 0 } ^ { \infty } \gamma ^ { t } W _ { p } ^ { t } \mathbf { w } _ { r } . +$$ + +Simplify $\mathbf { w } _ { \mathrm { m b } }$ + +$$ +\begin{array} { r l } & { \mathbf { w } _ { \mathrm { m b } } : = \displaystyle \sum _ { t = 0 } ^ { \infty } r ^ { t } W _ { p } ^ { t } \mathbf { w } _ { r } } \\ & { \mathbf { w } _ { \mathrm { n b } } = \left( I - \gamma W _ { p } \right) ^ { - 1 } \mathbf { w } _ { r } } \\ & { \mathbf { w } _ { \mathrm { m b } } = \left( I - \gamma \left( Z ^ { \top } Z \right) ^ { - 1 } Z ^ { \top } Z ^ { \prime } \right) ^ { - 1 } \left( Z ^ { \top } Z \right) ^ { - 1 } Z ^ { \top } R } \\ & { Z ^ { \top } Z \left( I - \gamma \left( Z ^ { \top } Z \right) ^ { - 1 } Z ^ { \top } Z ^ { \prime } \right) \mathbf { w } _ { \mathrm { m b } } = Z ^ { \top } R } \\ & { \left( Z ^ { \top } Z - \gamma Z ^ { \top } Z ^ { \prime } \right) \mathbf { w } _ { \mathrm { m b } } = Z ^ { \top } R } \\ & { \mathbf { w } _ { \mathrm { m b } } = A ^ { - 1 } B } \\ & { \mathbf { w } _ { \mathrm { m b } } = \mathbf { w } _ { \mathrm { m f } } . } \end{array} +$$ + +Theorem 2. The value error of the solution described by Theorem 1 is bounded by the accuracy of the estimated dynamics and reward: + +$$ +| \mathrm { V E } ( s , a ) | \leq \frac { 1 } { 1 - \gamma } \operatorname* { m a x } _ { ( s , a ) \in S \times A } \left( | \mathbf { z } _ { s a } ^ { \top } \mathbf { w } _ { r } - \mathbb { E } _ { r | s , a } [ r ] | + \operatorname* { m a x } _ { i } { | \mathbf { w } _ { i } | } \sum | \mathbf { z } _ { s a } ^ { \top } W _ { p } - \mathbb { E } _ { s ^ { \prime } , a ^ { \prime } | s , a } [ \mathbf { z } _ { s ^ { \prime } a ^ { \prime } } ] | \right) . +$$ + +Proof. Let w be the solution described in Theorem 1, i.e. $\mathbf { w } = \mathbf { w } _ { \mathrm { m b } } = \mathbf { w } _ { \mathrm { m f } }$ . Let $p ^ { \pi } ( s , a )$ be the discounted state-action visitation distribution according to the policy $\pi$ ( )starting from the state-action pair $( s , a )$ . + +Firstly from Theorem 1, we can show that + +$$ +\begin{array} { c } { \mathbf { w } = ( I - \gamma W _ { p } ) ^ { - 1 } \mathbf { w } _ { r } } \\ { \Rightarrow ( I - \gamma W _ { p } ) \mathbf { w } = \mathbf { w } _ { r } } \\ { \Rightarrow \mathbf { w } - \gamma W _ { p } \mathbf { w } = \mathbf { w } _ { r } . } \end{array} +$$ + +Simplify $\textstyle \operatorname { V E } ( s , a )$ : + +$$ +\begin{array}{c} \begin{array}{c} \begin{array}{c} \begin{array} { r l } & { \begin{array} { r l } & { \langle \mathcal { T } _ { \ell } ( s , \ell ) , \mathcal { T } _ { \ell } ( s , \ell ) \rangle } \\ & { : = \langle \mathcal { T } _ { \ell } ( s , \ell ) , \mathcal { T } _ { \ell } ( s , \ell ) \rangle } \\ & { \qquad - \mathcal { T } _ { \ell } ( s , \ell ) , } \end{array} } \\ & { \begin{array} { r l } & { \mathcal { T } _ { \ell } ( s , \ell ) , } \\ & { \mathcal { T } _ { \ell } ( s , \ell ) , } \\ & { \mathcal { T } _ { \ell } ( s , \ell ) , } \end{array} } \\ & { \begin{array} { r l } & { \mathcal { T } _ { \ell } ( s , \ell ) , } \\ & { \mathcal { T } _ { \ell } ( s , \ell ) , } \\ & { \mathcal { T } _ { \ell } ( s , \ell ) , } \end{array} } \\ & { \begin{array} { r l } & { \mathcal { T } _ { \ell } ( s , \ell ) , } \\ & { \mathcal { T } _ { \ell } ( s , \ell ) , } \end{array} } \\ & { \begin{ r l } & { \mathcal { T } _ { \ell } ( s , \ell ) , } \\ & { \mathcal { T } _ { \ell } ( s , \ell ) , } \end{array} } \end{array} \\ & { \begin{ r l } & { \mathcal { T } _ { \ell } ( s , \ell ) , } \\ & { \mathcal { T } _ { \ell } ( s , \ell ) , } \\ & { \mathcal { T } _ { \ell } ( s , \ell ) , } \end{array} } \end{array} \\ & { \begin{array} { r l } & { \mathcal { T } _ { \ell } ( s , \ell ) , } \\ & { \mathcal { T } _ { \ell } ( s , \ell ) , } \\ & { \mathcal { T } _ { \ell } ( s , \ell ) , } \end{array} } \\ & { \begin{array} { r l } & { \mathcal { T } _ { \ell } ( s , \ell ) , } \\ & { \mathcal { T } _ { \ell } ( s , \ell ) , } \\ & { \mathcal { T } _ { \ell } ( s , \ell ) , } \end{array} } \end{array} \\ & \begin{array} { r l } & \mathcal { T } _ { \ell } ( s , +$$ + +Then given the recursive relationship, akin to the Bellman equation (Sutton & Barto, 1998), the value error VE recursively expands to the discounted state-action visitation distribution $p ^ { \pi }$ . For $( \hat { s } , \hat { a } ) \in S \times A$ : + +$$ +\mathrm { V E } ( \hat { s } , \hat { a } ) = \frac { 1 } { 1 - \gamma } \mathbb { E } _ { ( s , a ) \sim p ^ { \pi } ( \hat { s } , \hat { a } ) } \left[ \left( \mathbf { z } _ { s a } ^ { \top } \mathbf { w } _ { r } - \mathbb { E } _ { r \mid s , a } \left[ r \right] \right) + \gamma \left( \mathbf { z } _ { s a } ^ { \top } W _ { p } - \mathbb { E } _ { s ^ { \prime } , a ^ { \prime } \mid s , a } \left[ \mathbf { z } _ { s ^ { \prime } a ^ { \prime } } ^ { \top } \right] \right) \mathbf { w } \right] . +$$ + +Taking the absolute value: + +$$ +\begin{array} { r l } & { | \mathrm { V E } ( \hat { s } , \hat { a } ) | = \left| \displaystyle \frac { 1 } { 1 - \gamma } \mathbb { E } _ { ( s , a ) \sim p ^ { \pi } ( \hat { s } , \hat { a } ) } \left[ \left( \mathbf { z } _ { s a } ^ { \top } \mathbf { w } _ { r } - \mathbb { E } _ { r | s , a } \left[ r \right] \right) + \gamma \left( \mathbf { z } _ { s a } ^ { \top } W _ { p } - \mathbb { E } _ { s ^ { \prime } , a ^ { \prime } | s , a } \left[ \mathbf { z } _ { s ^ { \prime } a ^ { \prime } } ^ { \top } \right] \right) \mathbf { w } \right] \right| } \\ & { | \mathrm { V E } ( \hat { s } , \hat { a } ) | \leq \displaystyle \frac { 1 } { 1 - \gamma } \mathbb { E } _ { ( s , a ) \sim p ^ { \pi } ( \hat { s } , \hat { a } ) } \left[ \left| \mathbf { z } _ { s a } ^ { \top } \mathbf { w } _ { r } - \mathbb { E } _ { r | s , a } \left[ r \right] \right| + \gamma \left| \left( \mathbf { z } _ { s a } ^ { \top } W _ { p } - \mathbb { E } _ { s ^ { \prime } , a ^ { \prime } | s , a } \left[ \mathbf { z } _ { s ^ { \prime } a ^ { \prime } } ^ { \top } \right] \right) \mathbf { w } \right| \right] } \\ & { \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad } \\ & & { \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad } \\ & { \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad } \\ & \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \end{array} +$$ + +Theorem 3. Given functions $f ( s ) = \mathbf { z } _ { s }$ and $g ( \mathbf { z } _ { s } , a ) = \mathbf { z } _ { s a }$ , then if there exists functions $\hat { p }$ and $\hat { R }$ such that for all $( s , a ) \in S \times A$ : + +$$ +\mathbb { E } _ { \hat { R } } [ \hat { R } ( { \bf z } _ { s a } ) ] = \mathbb { E } _ { R } \left[ R ( s , a ) \right] , \qquad \hat { p } ( { \bf z } _ { s ^ { \prime } } | { \bf z } _ { s a } ) = \sum _ { \hat { s } : { \bf z } _ { \hat { s } } = { \bf z } _ { s ^ { \prime } } } p ( \hat { s } | s , a ) , +$$ + +then for any policy $\pi$ where there exists a corresponding policy ${ \hat { \pi } } ( a | \mathbf { z } _ { s } ) = \pi ( a | s )$ , there exists $a$ function $\hat { Q }$ equal to the true value function $Q ^ { \pi }$ over all possible state-action pairs $( s , a ) \in S \times A$ : + +$$ +\hat { Q } ( \mathbf { z } _ { s a } ) = Q ^ { \pi } ( s , a ) . +$$ + +Furthermore, Equation $6 I$ guarantees the existence of an optimal policy $\hat { \pi } ^ { * } ( a | \mathbf { z } _ { s } ) = \pi ^ { * } ( a | s )$ . + +Proof. Let + +$$ +\begin{array} { r l r } & { } & { Q _ { h } ^ { \pi } ( s , a ) = \displaystyle \sum _ { t = 0 } ^ { h } \gamma ^ { t } \mathbb { E } _ { \pi } \big [ R ( s _ { t } , a _ { t } ) \big | s _ { 0 } = s , a _ { 0 } = a \big ] } \\ & { } & { \hat { Q } _ { h } ( \mathbf { z } _ { s a } ) = \displaystyle \sum _ { t = 0 } ^ { h } \gamma ^ { t } \mathbb { E } _ { \pi } \big [ \hat { R } ( \mathbf { z } _ { s _ { t } a _ { t } } ) \big | s _ { 0 } = s , a _ { 0 } = a \big ] } \end{array} +$$ + +Then + +$$ +\begin{array} { r l } & { Q _ { 0 } ^ { \pi } \big ( s , a \big ) = \mathbb { E } _ { R } [ R ( s , a ) ] } \\ & { \quad \quad \quad = \mathbb { E } _ { \hat { R } } [ \hat { R } ( \mathbf { z } _ { s a } ) ] } \\ & { \quad \quad \quad = \hat { Q } _ { 0 } \big ( \mathbf { z } _ { s a } \big ) . } \end{array} +$$ + +Assuming $Q _ { n - 1 } ^ { \pi } ( s , a ) = \hat { Q } _ { n - 1 } ( \mathbf { z } _ { s a } )$ then noting that $\hat { p } ( \mathbf { z } | \mathbf { z } _ { s a } ) = 0$ if $\mathbf { z }$ that is not in the image of $f ( s ) = \mathbf { z } _ { s }$ . + +$$ +\begin{array} { r l } { { Q _ { n } ^ { \pi } ( s , a ) = \mathbb { E } _ { R } [ R ( s , a ) ] + \gamma \mathbb { E } _ { s ^ { \prime } , a ^ { \prime } } [ Q _ { n - 1 } ^ { \pi } ( s ^ { \prime } , a ^ { \prime } ) ] } } \\ & { = \mathbb { E } _ { \hat { R } } [ \hat { R } ( s , a ) ] + \gamma \mathbb { E } _ { s ^ { \prime } , a ^ { \prime } } [ \hat { Q } _ { n - 1 } ( \mathbf { z } _ { s ^ { \prime } a ^ { \prime } } ) ] } \\ & { = \mathbb { E } _ { \hat { R } } [ \hat { R } ( s , a ) ] + \gamma \sum _ { s ^ { \prime } } \sum _ { a ^ { \prime } } p ( s ^ { \prime } | s , a ) \pi ( a ^ { \prime } | s ^ { \prime } ) \hat { Q } _ { n - 1 } ( \mathbf { z } _ { s ^ { \prime } a ^ { \prime } } ) } \\ & { = \mathbb { E } _ { \hat { R } } [ \hat { R } ( s , a ) ] + \gamma \sum _ { s ^ { \prime } } \sum _ { a ^ { \prime } } \hat { p } ( \mathbf { z } _ { s ^ { \prime } } | \mathbf { z } _ { s a } ) \hat { \pi } ( a ^ { \prime } | \mathbf { z } _ { s ^ { \prime } } ) \hat { Q } _ { n - 1 } ( \mathbf { z } _ { s ^ { \prime } a ^ { \prime } } ) } \\ & { = \hat { Q } _ { n } ( \mathbf { z } _ { s a } ) . } \end{array} +$$ + +Thus $\begin{array} { r } { \hat { Q } ( \mathbf { z } _ { s a } ) = \operatorname* { l i m } _ { n \infty } \hat { Q } _ { n } ( \mathbf { z } _ { s a } ) } \end{array}$ exists, as ${ \hat { Q } } _ { n }$ can be defined as a function of ${ \hat { p } } , { \hat { R } }$ , and $\hat { \pi }$ for all $n$ . Similarly, let $\pi$ be an optimal policy. Repeating the same arguments we see that + +$$ +\begin{array} { r l } & { \displaystyle { \cal Q } _ { n } ^ { \pi } ( s , a ) = \mathbb { E } _ { R } [ R ( s , a ) ] + \gamma \mathbb { E } _ { s ^ { \prime } , a ^ { \prime } } [ { \cal Q } _ { n - 1 } ^ { \pi } ( s ^ { \prime } , a ^ { \prime } ) ] } \\ & { \displaystyle = \mathbb { E } _ { R } [ R ( s , a ) ] + \gamma \sum _ { s ^ { \prime } } p ( s ^ { \prime } | s , a ) \operatorname* { m a x } _ { a ^ { \prime } } { \cal Q } _ { n - 1 } ^ { \pi } ( s ^ { \prime } , a ^ { \prime } ) } \\ & { \displaystyle \quad = \mathbb { E } _ { \hat { R } } [ \hat { R } ( s , a ) ] + \gamma \sum _ { z _ { s ^ { \prime } } } \hat { p } ( { \bf z } _ { s ^ { \prime } } | { \bf z } _ { s a } ) \operatorname* { m a x } _ { a ^ { \prime } } \hat { { \cal Q } } _ { n - 1 } ( { \bf z } _ { s ^ { \prime } a ^ { \prime } } ) } \\ & { \displaystyle \quad = \hat { { \cal Q } } _ { n } ( { \bf z } _ { s a } ) . } \end{array} +$$ + +Thus there exists a function $\hat { Q } ( g ( \mathbf { z } _ { s } , a ) ) = Q ^ { * } ( s , a )$ , consequently, there exists an optimal policy $\hat { \pi } ^ { * } ( a | \mathbf { z } _ { s } ) = \mathrm { a r g m a x } _ { a } \hat { Q } ( s , a )$ . + +# B EXPERIMENTAL DETAILS + +# B.1 HYPERPARAMETERS + +Table 3: MR.Q Hyperparameters. Hyperparameters values are kept fixed across all benchmarks. + +
HyperparameterValue
Dynamics loss weight λDynamics
Reward loss weight λReward0.1
Terminal loss weight λTerminal0.1
Pre-activation loss weight λpre-activ1e-5 5
Encoder horizon HEnc
TD3Multi-step returns horizon HQ3
Target policy noise σN (0, 0.22)
(Fujimoto et al., 2018) LAPTarget policy noise clipping c(-0.3, 0.3)
Probability smoothing α0.4
(Fujimoto et al., 2020) ExplorationMinimum priority1
Initial random exploration time steps10k
CommonExploration noiseN(0, 0.2{2)
Discount factor γ0.99
Replay buffer capacity Mini-batch size1M 256
Target update frequency Ttarget250
Replay ratio
OptimizerAdamW (Loshchilov & Hutter, 2019)
Learning rate1e- 4
Weight decay1e - 4
Zs dim512
Zsa dim512
Zα dim (only used within architecture)256
Hidden dim512
Activation functionELU (Clevert et al., 2015)
Weight initialization Bias initializationXavier uniform (Glorot & Bengio, 2010) 0
Reward bins65
Reward range[−10, 10] (effective: [−22k, 22k])
OptimizerAdamW
Learning rate3e - 4
Hidden dim512
Activation functionELU
Weight initializationXavier uniform 0
Bias initialization Gradient clip norm
Policy NetworkOptimizer20
Learning rateAdamW 3e - 4
Hidden dim512
Activation functionReLU
Weight initializationXavier uniform
Bias initialization0
Gumbel-Softmax τ (Jang et al., 2017)10
+ +# B.2 NETWORK ARCHITECTURE + +This section describes the networks used in our method using PyTorch code blocks (Paszke et al., 2019). The state encoder and state-action encoder are described as separate networks for clarity but are trained end-to-end as a single network. The value and policy networks are trained independently from the encoders. + +# Preamble + +1 import torch 2 import torch.nn as nn 3 import torch.nn.functional as F 4 from functools import partial 5 6 zs_dim $= ~ 5 1 2$ 7 za_dim $= \ 2 5 6$ 8 zsa_dim $= ~ 5 1 2$ 9 10 def ln_activ(self, x): 11 $\mathrm { ~ ~ x ~ } = \mathrm { ~ ~ \Gamma ~ }$ .layer_norm(x, (x.shape[-1],)) 12 return self.activ(x) + +# State Encoder $f$ Network + +For image inputs, four convolutional layers are used, each with 32 output channels, kernel size of 3, strides of $( 2 , 2 , 2 , 1 )$ , and ELU activations (Clevert et al., 2015). The convolutional layers are ( )followed by a linear layer taking in the flattened output followed by LayerNorm (Ba et al., 2016) and a final ELU activation. + +For vector inputs, a three layer multilayer perceptron (MLP) is used, with hidden dimension 512 and LayerNorm followed by ELU activations after each layer. + +The resulting state embedding $\mathbf { z } _ { s }$ is trained end-to-end with the state-action encoder. It is also used downstream by the policy network (without propagating gradients). + +1 2 if image_observatself.zs_cnn1 $=$ on_space:nn.Conv2d(state_channels, 32, 3, stride $^ { = 2 }$ ) +3 self.zs_cnn2 $=$ nn.Conv2d(32, 32, 3, stride $= 2$ ) +4 self.zs_cnn3 $=$ nn.Conv2d(32, 32, 3, stride $^ { = 2 }$ ) +self.zs_cnn4 $=$ nn.Conv2d(32, 32, 3, stride $= 1$ ) +6 # Assumes $8 4 \times 8 4$ input +self.zs_lin $=$ nn.Linear(1568, zs_dim) +8 else: +9 self.zs_mlp1 $=$ nn.Linear(state_dim, 512) +10 self.zs_mlp2 $=$ nn.Linear(512, 512) +11 self.zs_mlp3 $=$ nn.Linear(512, zs_dim) +12 +13 self.activ $\begin{array} { r l } { \mathbf { \Sigma } } & { { } = \mathbf { \Sigma } \qquad \boxed { \mathbf { \Sigma } } } \end{array}$ .elu +14 +15 def cnn_forward(self, state): +16 state $=$ state/255. - 0.5 +17 zs $=$ self.activ(self.zs_cnn1(state)) +18 zs $=$ self.activ(self.zs_cnn2(zs)) +19 zs $=$ self.activ(self.zs_cnn3(zs)) +20 zs $=$ self.activ(self.zs_cnn4(zs)) +21 zs $=$ zs.reshape(batch_size, 1568) +22 return ln_activ(self.zs_lin(zs)) +23 +24 def mlp_forward(self, state): +25 zs $=$ self.ln_activ(self.zs_mlp1(state)) +26 zs $=$ self.ln_activ(self.zs_mlp2(zs)) +27 return self.ln_activ(self.zs_mlp3(zs)) + +# State-Action Encoder $g$ Network + +Action input is processed by a linear layer followed by an ELU activation. Afterwards, the processed action is concatenated with the state embedding and processed by a three layer MLP with hidden dimension 512, and LayerNorm followed by ELU activations after the first two layers. + +The resulting state-action embedding $\mathbf { z } _ { s a }$ is used by a linear layer to make predictions about reward, the next state embedding, and the terminal signal. It is also used downstream by the value network (without propagating gradients). + +1 self.za $=$ nn.Linear(action_dim, za_dim) 2 self.zsa1 $\begin{array} { r l } { = } & { { } } \end{array}$ .Linear(zs_dim $^ +$ za_dim, 512) 3 self.zsa2 $=$ nn.Linear(512, 512) 4 self.zsa3 $=$ nn.Linear(512, zsa_dim) 5 self.model $=$ nn.Linear(zsa_dim, output_dim) 6 self.activ $\begin{array} { r l } { \mathbf { \Psi } } & { { } = \mathbf { \Psi } } \end{array}$ .elu 7 8 def forward(self, zs, action): 9 $\begin{array} { r l } { Z \hat { a } } & { { } = } \end{array}$ self.activ(self.za(action)) 10 zsa $=$ torch.cat([zs, za], 1) 11 zsa $=$ self.ln_activ(self.zsa1(zsa)) 12 zsa $=$ self.ln_activ(self.zsa2(zsa)) 13 zsa $=$ self.zsa3(zsa) 14 return self.model(zsa), zsa + +# Value $Q$ Networks + +The value network is a four layer MLP with hidden dimension 512, and LayerNorm followed by ELU activations after the first three layers. + +Two value networks are used with the same network and forward pass. + +1 self. $\begin{array} { r l } { 1 1 } & { { } = } \end{array}$ nn.Linear(zsa_dim, 512) 2 self. $\begin{array} { r l } { 1 2 } & { { } = } \end{array}$ nn.Linear(512, 512) 3 self. $\begin{array} { r l } { 1 3 } & { { } = } \end{array}$ nn.Linear(512, 512) 4 self. $\begin{array} { r l } { 1 4 } & { { } = } \end{array}$ nn.Linear(512, 1) 5 self.activ $\begin{array} { r l } { \mathbf { \Sigma } } & { { } = \mathbf { \Sigma } \mathbb { F } } \end{array}$ .elu 6 7 def forward(self, zsa): 8 ${ \textsc { q } } =$ self.ln_activ(self.l1(zsa)) 9 ${ \textsc { q } } =$ self.ln_activ(self.l2(q)) 10 ${ \textsc { q } } =$ self.ln_activ(self.l3(q)) 11 return self.l4(q) + +# Policy $\pi$ Network + +The policy network is a three layer MLP with hidden dimension 512, and LayerNorm followed by ReLU activations after the first two layers. + +For discrete actions, the final activation is the Gumbel Softmax with $\tau = 1 0$ . For continous actions, the final activation is a tanh function. + +1 self. $\begin{array} { r l } { 1 1 } & { { } = } \end{array}$ nn.Linear(zs_dim, 512) 2 self. $\begin{array} { r l } { 1 2 } & { { } = } \end{array}$ nn.Linear(hdim, 512) 3 self. $\begin{array} { r l } { 1 3 } & { { } = } \end{array}$ nn.Linear(512, action_dim) 4 self.activ $\begin{array} { r l } { \mathbf { \Sigma } } & { { } = \mathbf { \Sigma } \mathbb { F } } \end{array}$ .relu 5 6 if discrete_action_space: 7 self.final_activ $=$ partial(F.gumbel_softmax, tau ${ \ o } = 1 0$ ) 8 else: 9 self.final_activ $=$ torch.tanh 10 11 def forward(self, zs): 12 a $=$ self.ln_activ(self.l1(zs)) 13 a $=$ self.ln_activ(self.l2(a)) 14 return self.final_activ(self.l3(a)) + +# B.3 ENVIRONMENTS + +All main experiments were run for 10 seeds (the design study is based on 5 seeds). Evaluations are based on the average performance over 10 episodes, measured every 5k time steps for Gym and DM control and every 100k time steps for Atari. + +Gym - Locomotion. For the gym locomotion tasks (Todorov et al., 2012; Brockman et al., 2016; Towers et al., 2024), we choose the five most common environments that appear in prior work (Fujimoto et al., 2018; 2024; Haarnoja et al., 2018; Kuznetsov et al., 2020). We use the -v4 version. No preprocessing is applied. When aggregating scores, we use normalize with the TD3 scores obtained from TD7 (Fujimoto et al., 2024): + +$$ +\mathrm { T D 3 - N o r m a l i z e d } ( x ) : = { \frac { x - \mathrm { r a n d o m } \mathrm { s c o r e } } { \mathrm { T D 3 ~ s c o r e } - \mathrm { r a n d o m } \mathrm { s c o r e } } } . +$$ + +
RandomTD3
Ant-v4-70.2883942
HalfCheetah-v4-289.41510574
Hopper-v418.7913226
Humanoid-v4120.4235165
Walker2d-v42.7913946
+ +DM Control Suite. For the DM control suite (Tassa et al., 2018), we choose the 28 default environments that appear either in the evaluation of TD-MPC2 or DreamerV3. We omit any custom environments included by the TD-MPC2 authors. The same subset of tasks are used in the evaluation of proprioceptive and visual control. Like prior work, for both observation spaces, we use an action repeat of 2 (Hansen et al., 2024). For visual control, the state (network input) is composed of the previous 3 observations which are resized to $8 4 \times 8 4$ pixels in RGB format (Tassa et al., 2018). + +Atari. For the Atari games (Bellemare et al., 2013; Brockman et al., 2016; Towers et al., 2024), we use the 57 games in the Atari-57 benchmark that appears in prior work (Hessel et al., 2018; Schrittwieser et al., 2020; Badia et al., 2020; Hafner et al., 2023). For DQN and Rainbow, two games (Defender and Surround) are missing from the Dopamine framework (Castro et al., 2018) and are omitted. We use the -v5 version. For MR.Q, we use the common preprocessing steps (Mnih et al., 2015; Machado et al., 2018; Castro et al., 2018), where an action repeat of 4 is used and the observations are grayscaled, resized to $8 4 \times 8 4$ pixels and set to the max between the 3rd and 4th frame. The state (network input) is composed of the previous 4 observations. + +Consider the 16 frame sequence used by a single state, where $f _ { i }$ is the ith grayscaled and resized frame and $o _ { j }$ is the $j$ th observation set to the max of two frames + +$$ +\begin{array} { r } \overbrace f _ { 0 } , f _ { 1 } , \underbrace { f _ { 2 } , f _ { 3 } } _ { o _ { 0 } = \operatorname* { m a x } ( f _ { 2 } , f _ { 3 } ) } , \overbrace f _ { 4 } , f _ { 5 } , \underbrace { f _ { 6 } , f _ { 7 } } _ { o _ { 1 } = \operatorname* { m a x } ( f _ { 6 } , f _ { 7 } ) } , \overbrace { f _ { 8 } , f _ { 9 } , \underbrace { f _ { 1 0 } , f _ { 1 1 } } _ { o _ { 2 } = \operatorname* { m a x } ( f _ { 1 0 } , f _ { 1 1 } ) } , \overbrace { f _ { 1 2 } , f _ { 1 3 } , \underbrace { f _ { 1 4 } , f _ { 1 5 } } _ { o _ { 3 } = \operatorname* { m a x } ( f _ { 1 4 } , f _ { 1 5 } ) } , } ^ { \mathrm { a c i o n } a _ { 1 } } } \end{array} +$$ + +then the state is defined as follows: + +$$ +s = \left[ \begin{array} { l } { o _ { 0 } = \operatorname* { m a x } ( f _ { 2 } , f _ { 3 } ) } \\ { o _ { 1 } = \operatorname* { m a x } ( f _ { 6 } , f _ { 7 } ) } \\ { o _ { 2 } = \operatorname* { m a x } ( f _ { 1 0 } , f _ { 1 1 } ) } \\ { o _ { 3 } = \operatorname* { m a x } ( f _ { 1 4 } , f _ { 1 5 } ) } \end{array} \right] . +$$ + +When aggregating scores, we normalize with Human scores obtained from (Wang et al., 2016): + +$$ +{ \mathrm { H u m a n - N o r m a l i z e d } } ( x ) : = { \frac { x - { \mathrm { r a n d o m ~ s c o r e } } } { { \mathrm { H u m a n ~ s c o r e } } - { \mathrm { r a n d o m ~ s c o r e } } } } . +$$ + +
Random
Alien227.8Human 7127.7
Amidar5.81719.5
222.4742.0
Assault8503.3
Asterix210.0
Asteroids719.147388.7
Atlantis12850.029028.1 753.1
BankHeist BattleZone14.2 2360.037187.5
BeamRider363.916926.5
Berzerk123.72630.4
Bowling23.1160.7
Boxing0.112.1
Breakout1.730.5
Centipede2090.912017.0
ChopperCommand811.07387.8
CrazyClimber10780.535829.4
Defender (not used)2874.518688.9
DemonAttack152.11971.0
DoubleDunk-18.6-16.4
Enduro0.0860.5
FishingDerby-91.7-38.7
Freeway0.029.6
Frostbite65.24334.7
Gopher257.62412.5
Gravitar173.03351.4
Hero1027.030826.4
IceHockey-11.20.9
Jamesbond29.0302.8
Kangaroo52.03035.0
Krull1598.02665.5
KungFuMaster258.522736.3
MontezumaRevenge0.04753.3
MsPacman307.36951.6
NameThisGame2292.38049.0
Phoenix761.47242.6
Pitfall-229.46463.7
Pong-20.714.6
PrivateEye24.969571.3
Qbert163.913455.0
Riverraid1338.517118.0
RoadRunner11.57845.0
Robotank2.211.9
Seaquest68.442054.7
Skiing-17098.1-4336.9
Solaris1236.312326.7
SpaceInvaders148.01668.7
StarGunner664.010250.0
Surround (not used)-10.06.5
Tennis-23.8-8.3
TimePilot3568.05229.2
Tutankham11.4167.6
UpNDown533.411693.2
Venture0.01187.5
VideoPinball16256.917667.9
WizardOfWor563.54756.5
YarsRevenge3092.954576.9
Zaxxon32.59173.3
+ +# B.4 BASELINES + +DreamerV3. (Hafner et al., 2023). Results for Gym and DMC were obtained by re-running the authors’ code (https://github.com/danijar/dreamerv3 - Commit 251910d04c9f38dd9dc385775bb0d6- efa0e57a95) over 10 seeds, using the author-suggested hyperparameters from the DMC benchmark. Code was modified slightly to match our evaluation protocol. Atari results are based on the authors’ reported results. + +DrQ-v2. (Yarats et al., 2022). We use the authors’ reported results whenever possible. For missing any results, we re-ran the authors’ code (https://github.com/facebookresearch/drqv2 - Commit c0c650b76c6e5d22a7eb5f2edffd1440fe94f8ef) for 10 seeds. + +DQN. (Mnih et al., 2015). Results were obtained from the Dopamine framework (Castro et al., 2018). + +PPO. (Schulman et al., 2017). Results were gathered using Stable Baselines 3 (Raffin et al., 2021) and default hyperparameters. The default MLP policy was used for Gym and DMC-proprioceptive and the default CNN policy was used for DMC-visual and Atari. + +Rainbow. (Hessel et al., 2018). Results were obtained from the Dopamine framework (Castro et al., 2018). + +TD-MPC2. (Hansen et al., 2024). Results for DMC were obtained by re-running the authors’ code on their main branch (https://github.com/nicklashansen/tdmpc2 - Commit 5f6fadec0fec78304b4b53e8171d348b58cac486). As the Gym environments include a termination signal, results for Gym were obtained by running their episodic branch (https://github.com/ nicklashansen/tdmpc2/tree/episodic-rl - Commit 3789fcd5b872079ad610fa3299ff47c3a427a04a). All experiments were run for 10 seeds and use the default author-suggested hyperparameters for all tasks. + +TD7. (Fujimoto et al., 2024). Results for Gym were obtained from the authors. Results for DMC were obtained by re-running the authors’ code (https://github.com/sfujim/TD7 - Commit c1c280de1513f474488061b4cf39642b75dd84bd) using our setup for DMC. All experiments use 10 seeds and use the default author-suggested hyperparameters from the Gym benchmark. + +# B.5 SOFTWARE VERSIONS + +• Gymnasium 0.29.1 (Towers et al., 2024) • MuJoCo 3.2.2 (Todorov et al., 2012) • NumPy 2.1.1 (Harris et al., 2020) • Python 3.11.8 (Van Rossum & Drake Jr, 1995) • PyTorch 2.4.1 (Paszke et al., 2019) + +# C COMPLETE MAIN RESULTS + +# C.1 GYM + +Table 4: Gym $\cdot$ Locomotion final results. Final average performance at 1M time steps over 10 seeds. The [bracketed values] represent a $9 5 \%$ bootstrap confidence interval. The aggregate mean, median and interquartile mean (IQM) are computed over the TD3-normalized score (see Appendix B.3). + +
TaskTD7PPOTD-MPC2DreamerV3MR.Q
Ant8509 [8164, 8852]1584 [1355, 1802]4751 [3012, 6261]1947 [1121, 2751]6901 [6261, 7482]
HalfCheetah17433 [17284, 17550]1744 [1525, 2120]15078 [14050, 16012]5502 [3887, 7117]12939 [11663, 13762]
Hopper3511 [3245, 3746]3022 [2587, 3356]2081 [1233, 2916]2666 [2071, 3201]2692 [2131, 3309]
Humanoid7428 [7300, 7555]477 [431, 522]6071 [5767, 6327]4217 [2791, 5481]10223 [9929, 10498]
Walker2d6096 [5535, 6521]2487 [1875, 3067]3008 [1659, 4220]4519 [3746, 5190]6039 [5644, 6386]
Mean1.57 [1.54, 1.60]0.45 [0.41, 0.48]1.04 [0.90, 1.16]0.76 [0.67, 0.85]1.46 [1.41, 1.52]
Median1.55 [1.45, 1.63]0.41 [0.36, 0.47]1.18 [0.80, 1.23]0.81 [0.56, 0.90]1.53 [1.43, 1.61]
IQM1.54 [1.49, 1.58]0.41 [0.35, 0.46]1.05 [0.87, 1.19]0.72 [0.62, 0.85]1.50 [1.44, 1.55]
+ +![](images/figures/mrq-fig-0003.jpg) +Figure 3: Gym $\cdot$ Locomotion learning curves. Results are over 10 seeds. The shaded area captures a $9 5 \%$ boostrap confidence interval. + +# C.2 DMC - PROPRIOCEPTIVE + +Table 5: DMC - Proprioceptive final results. Final average performance at $5 0 0 \mathrm { k }$ time steps (1M time steps in the original environment due to action repeat) over 10 seeds. The [bracketed values] represent a $9 5 \%$ bootstrap confidence interval. The aggregate mean, median and interquartile mean (IQM) are computed over the default reward. + +
Task TD7 PPO TD-MPC2 DreamerV3 MR.Q
acrobot-swingup58 [38, 75] 39 [33, 45] 584 [551, 615]230 [193, 266] 567 [523, 616]
ball_in_cup-catch983 [981, 985] 769 [689, 841]982, 986968 [965, 973]981 [979, 984]
cartpole-balance999 [998, 1000]999 [1000, 1000]995, 998998 [997, 1000]999 [999, 1000]
cartpole-balance_sparse1000 [1000, 1000]1000 [1000, 1000]1000, 10999 [1000, 1000]1000 [1000, 1000]
cartpole-swingup869 [866, 873] 776 [661, 853]870, 880736 [591, 838]866 [866, 866]
cartpole-swingup-sparse573 [333, 806] 391 [159, 625]839, 849702 [560, 792]798 [780, 818]
cheetah-run821 [642, 913]269 [247, 295]917 [915, 920]699 [655, 744]914 [911, 917]
dog-run69 [36, 101]26 [26, 28] 265 [166, 342]4 [4, 5]
dog-stand582 [432, 741]129 [122, 139]506 [266, 715]22 [20, 27]967 [960, 975]
dog-trot21 [13, 30]407[265, 530]10 [6, 17][845, 898
dog-walk52 [19, 116]4040 [37, 43] 486 [240, 704]17 [15, 21]916 [908, 924]
finger-spin335 [99, 596]459 [420, 497] 986 [986, 988]666 [577, 763]937 [917, 956]
finger-turn_easy912 [774, 983]182 [153, 211] 979[975, 983]906 [883, 927]
finger-turn_hard470 [199, 727]5858 [35, 79]864 [812, 900]950 [910, 974]
fish-swim86 [64, 120]103 [84, 128]813 [808, 819]
hopper-hop87 [25, 160]10 [0, 23]425 [368, 500]116 [66, 165]251 [195, 301]
hopper-stand670670 [466, 829]128 [56, 216]944, 958747 [669, 806][948, 955
humanoid-run−5757 [23, 92]0 [1, 1] 181[121, 231]0 [1, 1]200 [170, 236]
humanoid-stand317317 [117, 516]5 [5, 6]5 [5, 6]868 [822, 903]
humanoid-walk176176 [42, 320]1 [1, 2]7541 [1, 2]662 [610, 724]
pendulum-swingup500500 [251, 743] 1 15115 [70, 164]846830, 862774 [740, 802]748 [597, 829]
quadruped-run645645 [567, 713]144 [122, 170]942942 [938, 947]130 [92, 169]947 [940, 954]
quadruped-walk949949 [939, 957]122 [103, 142]963963 [959, 967]193 [137, 243]
reacher-easy970970 [951, 982]367367 [188, 558]983[980, 986]966 [964, 970]9833 [983, 985]
reacher-hard898125125 [40, 234]960919 [864, 955][975, 980
walker-run804804 [783, 825]9797 [91, 104]854854 [851, 859]510[430, 588]793793 [765, 815]
walker-stand983974, 989431991990, 994[934, 948988[987, 990
walker-walk977 [975, 980]283 [253, 312]981 [979, 984]898 [875, 919]978 [978, 980]
Mean566 [544, 590]254 [241, 267]783 [769, 797]530 [520, 539]835 [829, 842]
Median613 [548, 718]127 [112, 145]896 [893, 899]700 [644, 741]927 [914, 934]
IQM612 [569, 657]154 [135, 167]868868 [860, 880]577 [557, 594]907 [903, 914]
+ +![](images/figures/mrq-fig-0004.jpg) +Figure 4: DMC - Proprioceptive learning curves. Time steps consider the number of environment interactions, where $5 0 0 \mathrm { k }$ time steps equals 1M frames in the original environment. Results are over 10 seeds. The shaded area captures a $9 5 \%$ boostrap confidence interval. + +# C.3 DMC - VISUAL + +Table 6: DMC - Visual final results. Final average performance at 500k time steps (1M time steps in the original environment due to action repeat) over 10 seeds. The [bracketed values] represent a $9 5 \%$ bootstrap confidence interval. The aggregate mean, median and interquartile mean (IQM) are computed over the default reward. + +
Task DrQ-v2 PPO TD-MPC2 DreamerV3 MR.Q
acrobot-swingup 168 [127, 219]2 [1, 4] 197 [179, 217]121 [106, 145] 287 [254, 316]
ball_in_cup-catch909 [821, 973]105 [5, 282]932 [899, 961]971 [969, 973]
cartpole-balance993 [990, 996]353353 [231, 485]972 [948, 991]998 [997, 1000][999, 999
cartpole-balance_sparse962 [887, 1000]487 [233, 751] 1000 [1000, 1000]999 [999, 1000][1000, 10
cartpole-swingup864 [854, 873]596 [437, 723] 690 [521, 813]725 [603, 807]868 [860, 875]
cartpole-swingup-_sparse774 [741, 805]636 [404, 804]547 [351, 726]
cheetah-run728 [701, 753]155155 [110, 210] 431 [267, 556]618 [576, 661]
dog-run10 [9, 12]14 [10, 18]9 [6, 14]60 [44, 80]
dog-stand43 [37, 49]51 [48, 56]117 [72, 148]61 [30, 92]216 [201, 232]
dog-trot14 [11, 18]13 [12, 15]20 [14, 25]14 [13, 16]65 [55, 79]
dog-walk22 [18, 29]16 [14, 18]22 [17, 28]11 [11, 12]77 [71, 83]
finger-spin860 [787, 922]241 [107, 377]786 [492, 984]656 [544, 765]965 [938, 982]
finger-turn_easy503 [399, 615]189 [144, 233]447, 542]93
finger-turn_hard903 [870, 940]494 [401, 571]932 [905, 957]
fish-swim84 [65, 107]77 [64, 92]43 [21, 64]90 [84, 96]79 [68, 93]
hopper-hop224224 [170, 278]0 [0, 0]187 [119, 238]205 [125, 287]270 [230, 315]
hopper-stand917917 [903, 931]1 [0, 2]582 [321, 794]8888 [875, 900]852 [703, 930]
humanoid-run−11 [1, 1]1 [1, 1]0 [1, 1]1 [1, 1]1 [1, 2]
humanoid-stand6 [7, 7]6 [6, 7]5 [5, 7]5 [5, 7]7 [7, 8]
humanoid-walk1 [1, 1]1 [1, 2]1 [2, 2]
pendulum-swingup838 [813, 861]748748 [574, 850]761 [709, 807]829 [816, 842]
quadruped-run459459 [412, 507] 1 1 8118 [98, 139]262262 [184, 330]328 [255, 397]498 [476, 522]
quadruped-walk750750 [699, 796]149 [113, 184]246246 [179, 310]316 [260, 379]833 [797, 867]
reacher-easy938938 [903, 973]113 [55, 192]956956 [932, 978]735735 [678, 796]979 [978, 982]
reacher-hard705705 [580, 831]10 [0, 30]911911 [867, 946]338338 [227, 461]965 [945, 977]
walker-run546546 [475, 612]39 [35, 44]665665 [566, 719]669 [615, 708]615 [571, 655]
walker-stand980[977, 984253 [210, 310]937[907, 962966, 973[977, 985
walker-walk766 [489, 957]47 [40, 56]958 [952, 965]942942 [936, 949]970 [968, 973]
Mean 510 [497, 523]110 [98, 125]492 [471, 512]463463 [452, 475]602 [595, 608]
Median[528, 66549 [32, 53]572 [419, 654]493 [420, 532]813 [779, 822]
IQM545 [519, 564]58 [46, 67] 501 [458, 537]452452 [430, 473]692 [678, 703]
+ +![](images/figures/mrq-fig-0005.jpg) +Figure 5: DMC - Visual learning curves. Time steps consider the number of environment interactions, where 500k time steps equals 1M frames in the original environment. Results are over 10 seeds. The shaded area captures a $9 5 \%$ boostrap confidence interval. + +# C.4 ATARI + +Table 7: Atari final results. Final average performance at $2 . 5 \mathbf { M }$ time steps (10M time steps in the original environment due to action repeat) over 10 seeds. The [bracketed values] represent a $9 5 \%$ bootstrap confidence interval. The aggregate mean, median and interquartile mean (IQM) are computed over the human-normalized score. + +
TaskDQNRainbowPPODreamerV3MR.Q
Alien925 [879, 968]1220 [1191, 1268]320 [251, 383]4838 [3863, 5813]2834 [2241, 3388]
Amidar178 [169, 186]301 [280, 330]126 [90, 167]470 [419, 524]595 [525, 657]
Assault988 [957, 1011]1430 [1392, 1475]423 [271, 581]3518 [2969, 4179]1296 [1254, 1343]
Asterix2381 [2313, 2469]2699 [2598, 2783]296 [216, 403]7319 [6251, 8354]3358 [3004, 3797]
Asteroids423 [408, 436]754 [711, 816]206 [180, 232]1359 [1243, 1482]715 [638, 796]
Atlantis7365 [6893, 7742]80837 [51139, 126780]2000 [2000, 2000]664529 [197588, 973362]556845 [469425, 660043]
BankHeist474 [448, 493]895 [889, 901]187 [41, 421]801 [691, 1002]809 [639, 960]
BattleZone3598 [3235, 3878]20209 [17157, 22375]2200 [1460, 3100]22599 [21055, 24669]19880 [13450, 26060]
BeamRider869 [728, 1065]5982 [5664, 6268]479 [348, 581]5635 [3161, 7962]2299 [1921, 2813]
Berzerk488 [466, 508]443 [413, 484]384 [310, 469]758 [681, 823]523 [456, 588]
Bowling29 [27, 32]44 [36, 52]51 [38, 60]101 [69, 138]59 [45, 72]
Boxing37 [31, 44]68 [66, 71]-3 [-6, 0]97 [97, 99]96 [95, 97]
Breakout21 [19, 25]41 [40, 44]9 [8, 11]137 [110, 162]34 [28, 42]
Centipede2832 [2418, 3215]4992 [4784, 5138]4239 [2222, 6622]20067 [17410, 22758]17835 [16161, 19817]
ChopperCommand997 [971, 1022]2265 [2160, 2357]688 [501, 878]15172 [12940, 17219]5748 [4822, 6651]
CrazyClimber64611 [46203, 78709]103539 [99749, 106850]896 [174, 1727]132811 [128446, 135930]116954 [111371, 122032]
Defender116954 [111371, 122032]116954 [111371, 122032]1333 [705, 2094]34187 [29814, 39261]40457 [36892, 43638]
DemonAttack1503 [1282, 1690]2477 [2269, 2678]139 [116, 165]4836 [3443, 6231]5924 [4491, 7289]
DoubleDunk-18 [-20, -18]-18 [-19, -19]-1 [-3, 0]21 [20, 22]-10 [-15, -9]
Enduro589 [567, 617]1601 [1555, 1635]13 [9, 17]476 [175, 782]1845 [1758, 1938]
FishingDerby-42 [-62, -17]10 [5, 15]-89 [-91, -87]40 [32, 47]10 [2, 18]
Freeway8 [0, 19]32 [32, 32]15 [11, 18]19 [6, 32]32 [32, 32]
Frostbite269 [238, 294]2510 [2040, 2823]245 [231, 259]5183 [2151, 8291]4561 [3299, 5740]
Gopher1470 [1316, 1590]4279 [4139, 4425]126 [80, 174]38711 [26066, 48187]19174 [14932, 23587]
Gravitar167 [153, 183]202 [184, 218]63 [31, 98]831 [768, 900]397 [320, 490]
Hero2679 [2404, 2945]9323 [7914, 10863]1741 [1062, 2302]20582 [19845, 21583]13450 [11915, 14781]
IceHockey-9 [-10, -9]-5 [-6, -5]-8 [-10, -8]14 [13, 16]0 [-1, 2]
Jamesbond47 [42, 52]514 [509, 520]85 [62, 106]836 [568, 1119]624 [588, 662]
Kangaroo539 [525, 553]5501 [3853, 7151]402 [280, 520]8825 [5234, 12418]9807 [7851, 11591]
Krull4229 [3942, 4490]5972 [5903, 6047]421 [136, 735]23092 [14679, 28172]9309 [8646, 9953]
KungFuMaster15997 [13182, 18813]18074 [16041, 20864]52 [18, 95]70703 [50114, 94578]29369 [26954, 31595]
MontezumaRevenge0 [0, 0]0 [0, 0]0 [0, 0]1310 [598, 2180]50 [0, 140]
MsPacman2187 [2121, 2247]2347 [2292, 2403]457 [352, 578]4484 [3539, 5511]4922 [4191, 5843]
NameThisGame4000 [3814, 4187]8604 [8252, 8931]1084 [663, 1501]15742 [14542, 17103]8693 [8071, 9199]
Phoenix4948 [4236, 5627]4830 [4707, 4968]101 [81, 120]15827 [14903, 16429]5173 [5025, 5322]
Pitfall-60 [-89, -35]-14 [-29, -6]-16 [-38, -2]0 [0, 0]
Pong-4 [-14, 3]15 [14, 16]-5 [-8, -3]16 [16, 17]-20 [-60, 0]
PrivateEye118 [78, 181]111 [78, 166]-17 [-592, 762]3046 [975, 5118]17 [16, 19] 100 [100, 100]
Qbert1658 [1246, 2139]5353 [4363, 6783]484 [393, 570]16807 [16073, 17564]
Riverraid3198 [3167, 3222]4272 [4060, 4440]1045 [833, 1241]9160 [8177, 10077]3938 [3210, 4327]
RoadRunner27980 [27269, 28692]33412 [32459, 34435]723 [454, 940]66453 [40606, 104163]10791 [9307, 12511]
Robotank4 [4, 5]19 [18, 20]4 [2, 6]51 [47, 55]49579 [47425, 51426] 13 [12, 15]
Seaquest299 [277, 318]1641 [1621, 1661]250 [214, 282]3416 [2665, 4426]3522 [2401, 4850]
Skiing-19568 [-19793, -19362]-24070 [-25305, -22667]-27901 [-30000, -23704]-30043 [-30394, -29764]
Solaris1645 [1480, 1804]1289 [1143, 1451]0 [0, 2]2340 [1882, 2799]-30000 [-30000, -30000] 1103 [799, 1430]
SpaceInvaders663 [651, 675]743 [721, 764]294 [235, 354]1433 [1039, 1943]701 [626, 768]
StarGunner692 [662, 719]1488 [1470, 1506]415 [316, 499]2090 [1678, 2649]3488 [1032, 8241]
Surround3488 [1032, 8241]3488 [1032, 8241]-9 [-10, -10]5 [4, 7]
Tennis-21 [-24, -19]-3 [-11, 0]-2 [-4, -2]
TimePilot1539 [1479, 1613]-1 [-2, 0] 2703 [2627, 2787]-20 [-22, -19] 548 [450, 690]7779 [3128, 13016]0 [0, 0]
Tutankham112 [97, 123]179 [165, 191]29 [17, 43]253 [240, 269]4382 [4208, 4528]
UpNDown7669 [7116, 8147]595 [428, 737]164 [145, 185]
Venture25 [6, 45]12397 [11489, 13312]284807 [178615, 391388]73095 [40836, 108810]
VideoPinball19 [14, 25]2 [0, 6]0 [0, 0]112 [0, 304]
WizardOfWor5129 [4611, 5649] 481 [396, 542]26245 [23075, 29067] 2213 [1827, 2617]1005 [0, 2485] 225 [185, 264]22345 [20669, 23955] 7086 [6518, 7730]53826 [40600, 67972] 2599 [2259, 2942]
YarsRevenge9426 [9177, 9656]10708 [10405, 11071]1891 [925, 2964]62209 [57783, 67113]34861 [29734, 40020]
Zaxxon112 [15, 230]3661 [3131, 4192]0 [0, 0]17347 [15320, 19385]8850 [8045, 9740]
Mean0.25 [0.24, 0.26]
Median0.12 [0.10, 0.12]1.08 [1.02, 1.14] 0.40 [0.40, 0.47]-0.09 [-0.10, -0.07] 0.01 [0.00, 0.01]3.74 [3.29, 4.13] 1.25 [1.11, 1.47]2.54 [2.34, 2.75] 0.96 [0.78, 0.98]
IQM0.17 [0.16, 0.17]0.61 [0.60, 0.62]0.02 [0.01, 0.02]1.46 [1.34, 1.51]0.90 [0.88, 0.94]
+ +![](images/figures/mrq-fig-0006.jpg) +Figure 6: Atari learning curves. Time steps consider the number of environment interactions, where 2.5M time steps equals 10M frames in the original environment. Results are over 10 seeds. The shaded area captures a $9 5 \%$ boostrap confidence interval. + +# D COMPLETE ABLATION RESULTS + +In this section, we show a per-environment breakdown of each variation in the design study in Section 5.2. Each table reports the raw score for each environment. The [bracketed values] represent a $9 5 \%$ bootstrap confidence interval. The aggregate mean, median and interquartile mean (IQM) are computed over the the difference in the normalized score. We use TD3 to normalize for Gym, raw scores divided by 1000 for DMC and human scores to normalize for Atari (see Appendix B.3). Highlighting is used to designate the scale of the difference in normalized score: + +$\left( \leq - 0 . 5 \right)$ $[ - 0 . 2 , - 0 . 5 )$ $[ - 0 . 0 1 , - 0 . 2 )$ $^ { \cdot } \ [ 0 . 0 1 , 0 . 2 )$ 0.2, 0.5 ≥ 0.5 + +# D.1 GYM + +
TaskMR.QLinear value functionDynamics targetNo target encoder
Ant6901 [6261, 7482]1844 [1663, 2018]5867 [5543, 6289]3970 [2468, 5509]
HalfCheetah12939 [11663, 13762]3383 [3054, 3732]14019 [13746, 14285]12838 [12459, 13266]
Hopper2692 [2131, 3309]968 [720, 1210]2890 [2030, 3747]3007 [2164, 3852]
Humanoid10223 [9929, 10498]461 [395, 532]8370 [7651, 8988]305 [272, 356]
Walker2d6039 [5644, 6386]1117 [999, 1238]5844 [5146, 6477]5944 [5570, 6323]
Mean-1.17 [-1.19, -1.15]-0.10 [-0.17, -0.04]-0.53 [-0.60, -0.46]
Median-1.25 [-1.28, -1.21]-0.05 [-0.23, 0.09]-0.02 [-0.16, 0.02]
IQM-1.13 [-1.15, -1.11]-0.08 [-0.19, 0.01]-0.25 [-0.37, -0.16]
+ +
TaskMR.QRevertNon-linear modelMSE reward loss
Ant6901 [6261, 7482]-422 [-1770, 846]7215 [6971, 7466]7153 [5991, 7815]
HalfCheetah12939 [11663, 13762]-658 [-750, -604]13370 [12649, 14053]14413 [14096, 14710]
Hopper2692 [2131, 3309]103 [39, 189]2492 2 [1835, 3424]2869 [2090, 3689]
Humanoid10223 [9929, 10498]189 [104, 277]10257 [9612, 10688]10592 [10017, 10983]
Walker2d6039 [5644, 6386]260 [-5, 638]5548 [4980, 6117]6626 [5256, 7984]
Mean-1.47 [-1.54, -1.39]-0.01 [-0.07, 0.03]0.10 [-0.02, 0.19]
Median-1.47 [-1.53, -1.37]0.01 [-0.03, 0.08]0.07 [0.00, 0.17]
IQM-1.51 [-1.58, -1.39]-0.01 [-0.05, 0.04]0.09 [-0.01, 0.18]
+ +
TaskMR.QNo reward scalingNo minNo LAP
Ant6901 [6261, 7482]6866 [6227, 7547]6936 [6582, 7329]6817 [6616, 7039]
HalfCheetah12939 [11663, 13762]13502 [13333, 13673]14143 [13819, 14515]13185 [13085, 13299]
Hopper2692 [2131, 3309]2551 [2090, 3064]2113 [1728, 2626]2681 [1883, 3465]
Humanoid10223 [9929, 10498]9515 [8520, 10245]10528 [10202, 10837]8441 [6206, 9738]
Walker2d6039 [5644, 6386]5743 [5362, 6102]4293 [3547, 5107]5463 [4134, 6376]
Mean-0.04 [-0.09, 0.02]-0.09 [-0.16, -0.01]-0.10 [-0.24, -0.00]
Median-0.04 [-0.11, 0.04]0.01 [-0.08, 0.07]-0.02 [-0.12, 0.02]
IQM-0.04 [-0.10, 0.03]-0.04 [-0.10, 0.02]-0.06 [-0.18, 0.00]
+ +
TaskMR.QNo MR1-step returnNo unroll
Ant6901 [6261, 7482]4195 [2573, 5819]7757 [7729, 7799]7528 [7224, 7830]
HalfCheetah12939 [11663, 13762]11249 [9238, 12495]13123 [10691, 14653]14409 [13817, 15002]
Hopper2692 [2131, 3309]1877 [1524, 2153]2737 [2131, 3343]2578 [1857, 3414]
Humanoid10223 b [9929, 10498]3942 [3262, 4624]2328 [1491, 3337]10617 [10504, 10731]
Walker2d6039 [5644, 6386]4155 [3251, 4897]4747 [3197, 6229]6077 [5752, 6355]
Mean-0.56 [-0.69, -0.43]-0.33 [-0.46, -0.21]0.07 [0.01, 0.14]
Median-0.48 [-0.71, -0.27]0.01 [-0.21, 0.16]0.08 [0.06, 0.16]
IQM-0.47 [-0.66, -0.28]-0.10 [-0.32, 0.12]0.07 [0.04, 0.15]
+ +D.2 DMC - PROPRIOCEPTIVE + +
Task MR.Q Linear value function Dynamics target No target encoder
acrobot-swingup567 [517, 621]30 [15, 46]626626 [578, 684]16 [9, 25]
ball_in_cup-catch820820 [658, 922]980980 [978, 983]569
cartpole-balance 999[999, 1000]449449 [380, 520]999992
cartpole-balance_sparse[1000, 1018310001000
cartpole-swingup[866, 866267267 [225, 310]869852
cartpole-swingup-sparse 7983 [779, 818]128170 [0, 0]
cheetah-run914[911, 917]394394 [376, 411]919904 [899, 910]
dog-run569[546, 595112541111 [8, 16]36 [27, 51]
dog-stand9672267236
dog-trot 877[845, 898]1515 [11, 20]3191212 [8, 19]10 [7, 15]
dog-walk1313 11, 18]31210
finger-spin 937[[917, 958]736736 [670, 825]942942 [916, 971]869869 [698, 963]
finger-turn_easy 953[928, 975]238238 [157, 319]947624
finger-turn_hard[908, 97423923431
fish-swim 792[772, 811]8383 [65, 102]41097
hopper-hop 251[201, 295]1019900 [0, 1]
hopper-stand 951 [948, 955]6666 [30, 102]63944 [3, 7]
humanoid-run 200[169, 236]1 [1, 1]1 [1, 1]
humanoid-stand 868[823, 907]6
2 [2, 3]
7 [6, 9]2 [2, 3]
humanoid-walk662 [609, 721]1 [1, 2]
pendulum-swingup748357357 [114, 617]826826 [812, 840]784
quadruped-run947[940, 954]172942829
quadruped-walk963 [959, 968]9191 [52, 141]939952
reacher-easy983[983, 985]802984983
reacher-hard977[975, 979].853853 [778,914]970970 [965, 976]975975 [972, 979]
walker-run793238238 [207, 274]730776
walker-stand859859 [780, 921]988988
walker-walk504504 [397, 613]
MeanMedianIQM-0.58 [-0.59, -0.56]-0.58 [-0.64, -0.57]-0.15 [-0.15, -0.15]-0.35 [-0.35, -0.34]
-0.01 [-0.02, -0.00]-0.22 [-0.23, -0.21]
-0.62 [-0.64, -0.60]-0.05 [-0.06, -0.03]-0.27 [-0.29, -0.25]
+ +
Task MR.Q Revert Non-linear model MSE reward loss
acrobot-swingup567 [517, 621]11 [8, 18]553553 [478, 629]577577 [547, 612]
ball_in_cup-catch981 [979, 983]301 [233, 365]982982, 984983[982, 985
cartpole-balance999 [999, 1000]272 [206, 332]999999, 100999[998, 100
cartpole-balance_sparse197 [177, 214]10001000, 101000
cartpole-swingup866, 866191 [99, 263]866866, 867865[865, 866
cartpole-swingup-sparse0 [0, 0]824812
cheetah-run74 [31, 123]909910910 [907, 915]
dog-run3 [4, 4]588527527 [513, 545]
dog-stand967 [959, 975]22 [18, 29]962962 [938, 982]964964 [958, 971]
dog-trot4 [3, 5]868861
dog-walk916 [908, 924]4 [4, 6]920920 [915, 925]724
finger-spin0 [0, 1]868907
finger-turn_easy159 [60, 280]972935
finger-turn_hard60 [20, 100]931947947 [910, 969]
fish-swim71 [53, 90]790793
hopper-hop0 [0, 1]288174174 [119, 230]
hopper-stand5 [3, 8]848854
humanoid-run200 [169, 236]1 [1, 1]20591
humanoid-stand868 [823, 907]7 [7, 8]811214214 [14, 566]
humanoid-walk1 [2, 2]66877
pendulum-swingup 7483 [594, 830]6161 [20, 103]819827827 [811, 843]
quadruped-run87944949
quadruped-walk 9633 [959, 968]8181 [36, 130]963963 [961, 967]966966 [961, 971]
reacher-easy 9833 [983, 985]789983982, 984964
reacher-hard526953976976 [974, 978]
walker-run[766, 81625795778
walker-stand221983988988 [987, 990]
walker-walk978 [978, 980]33 [23, 46]974974 [972, 978]967967 [948, 979]
MeanMedianIQM-0.72 [-0.73, -0.72]-0.00-0.00 [-0.02, 0.01]-0.06 [-0.08, -0.05]
-0.78-0.00-0.00, 0.00-0.00
-0.78-0.78 [-0.78, -0.76]-0.00-0.00 [-0.01, 0.00]-0.01-0.01 [-0.02, -0.00]
+ +
TaskMR.QNo reward scalingNo minNo LAP
acrobot-swingup567 [517, 621]593 [532, 672]623 [572, 673]566 [520, 612]
ball_in_cup-catch981 [979, 983]983 [982, 984]982 [980, 984]981 [980, 984]
cartpole-balance999 [999, 1000]998 [999, 999]999 [1000, 1000]999 [998, 1000]
cartpole-balance_sparse1000 [1000, 1000]1000 [1000, 1000]1000 [1000, 1000]992 [982, 1000]
cartpole-swingup866 [866, 866]865 [864, 866]868[866, 874]865[865, 866]
cartpole-swingup_sparse798 779, 818]647 [318, 822]799 [780, 812796 [779, 810]
cheetah-run914 [911, 917]911 [909, 914]910 [893, 921]908 [905, 913]
dog-run569 [546, 595]586 [546, 613]577 [540, 610]536 [499, 573]
dog-stand967 [959, 975]959 [940, 979]946 [917, 969]971 [966, 976]
dog-trot877 [845, 898]817 [713, 903]846 [767, 90842 [764, 897]
dog-walk916 [908, 924]901 890, 917]747 [447, 908]89986,914]
finger-spin937 [917, 958]87 78, 947]926 [907, 950]915 [892, 8]
finger-turn_easy953 [928, 975]977 [973, 982]976 [972, 980]975 [967, 983]
finger-turn_hard950 [908, 974]946 [905, 969]894 [833, 953]949 [909, 972]
fish-swim792 [772, 811]745 [663, 809]785 [763, 810]788 [754, 826
hopper-hop251 [201, 295]343 [263, 477]336 [322, 352]347 [265, 431]
hopper-stand951 [948, 955]934 [912, 948]935 [926, 947]941 [935, 948]
humanoid-run200 [169, 36]184 9, 214]198 [175, 225]202 [191, 12]
humanoid-stand868 [823, 907]810 655, 899]833 [793, 871]880 [856, 900]
humanoid-walk662 [609, 721]665 [589, 765]597 22, 808]67 1, 828]
pendulum-swingup748 [594, 830]816 [790, 838]825 [811, 839]815 72, 836]
quadruped-run947 [940, 954]951 [944, 958]946 [941, 951]937 [927, 947]
quadruped-walk963 [959, 968]966 [961,, 971]959 [942, 972]
reacher-easy983[983, 95]964 [926, 984]983 [981, 986]955 [942, 967]
reacher-hard977 [975, 979]971 [968, 975]978 [974, 982]983 [982, 98]
walker-run793 [766, 816]804 [783, 820]806 [779, 821]974 [969, 981]
walker-stand988 [987, 990]989 [988, 990]989 [986, 992]812 [803, 822]
walker-walk978 [978, 980]979 [978, 980]978 [976, 980]986 [985, 987]
977 974, 980]
Mean Median-0.01 [-0.02, 0.00]-0.01 [-0.02, 0.01]0.00 [-0.00, 0.01]
IQM-0.00 [-0.00, 0.00] -0.00 [-0.00, 0.00]-0.00 [-0.00, 0.00] -0.00 [-0.00, 0.00]-0.00 [-0.00, 0.00] -0.00 [-0.00, 0.00]
TaskMR.QNo MR1-step returnNo unroll
acrobot-swingup567 [517, 621]576 [483, 665]440 [360, 528]515 [455, 598]
ball_in_cup-catch981 [979, 98]981 [980, 984]984 [983, 985]982 [981, 984]
cartpole-balance999 [999, 1000]994 [991, 999]999 [1000, 1000]999 [999, 1000]
cartpole-balance_sparse1000 [1000, 1000] 866 [866, 866]1000 [1000, 1000] 870 [864, 878]961 [886, 1000] 881 879, 82]1000 [1000, 1000]
cartpole-swingup798 [779, 818]684 [528, 814]845 [845, 847]864 [861, 867]
cartpole-swingup_sparse914 [911, 917]871 [823, 907]922 [921, 924]818 [811, 831]
cheetah-run569 [546, 595]68 [63, 75]299 [196, 360]909 [908, 911]
dog-run dog-stand967 959, 95]494 [452, 530]606 [344, 65]514 [473, 554] 955 [944, 971]
dog-trot877 [845, 898]6 9, 80]725 [679, 756]857 33, 83]
dog-walk916 [908, 924]1081, 122]788 [739, 832]
finger-spin937 91, 958]888 [731, 975]983 977, 988]920 [905, 934]
953 [928, 975]947 [913, 974]980 [979,, 982]880 [781, 940]
finger-turn_easy950 [908, 974]846 [756, 926]968 [958, 976]950 [917, 976]
finger-turn_hard792 [772, 811]706 [683, 727]498 [323, 651]947 [907, 971] 709 [618, 783]
fish-swim251 [201, 295]85 3, 142]364 [336, 394]
hopper-hop951 [948, 955]365 [233, 491]952 [947, 959]297 [169, 442]
hopper-stand humanoid-run200 [169, 36]1 [1, 2]190 [124, 241]949 [944, 955] 192 [172, 214]
868823, 907]201 [9, 517]753 [665, 838]
humanoid-stand humanoid-walk662 [609, 721]84 [3, 247]761 [689, 827]858 [806, 9]
pendulum-swingup748 [594, 830]827 [812, 842]823 [807, 841]675 [593, 772] 819 [793, 843]
quadruped-run947 [940, 954]871 [793, 933]945 [940, 950]950 [944, 955]
quadruped-walk963 [959, 968]951 [943, 962]962 [958, 968]
983[983, 95]980 [979, 983]984 [984, 986]962 [955, 969]
reacher-easy981 [976, 986]
reacher-hard977 [975, 979]949 [909, 974]980 [979, 983]954 [913, 978]
walker-run793 [766, 816]780 769, 790]835 [827, 843]780 [702, 825]
walker-stand988 [987, 990]983 [980, 988]990 [990, 992]988 [98, 990]
walker-walk978 [978, 980]976 [975, 977]979 [977, 982]975 [969, 981]
Mean Median-0.19 [-0.19, -0.18] -0.05 [-0.08, -0.01]-0.04 [-0.05, -0.02] 0.00 [0.00, 0.00]-0.01 [-0.01, -0.00] -0.00 [-0.00, 0.00]
IQM-0.06 [-0.08, -0.05]0.00 [-0.00, 0.01]
-0.00 [-0.01, -0.00]
+ +D.3 DMC - VISUAL + +
Task MR.Q Linear value function Dynamics target No target encoder
acrobot-swingup 287 [253, 317]15 [5, 22]296 [281, 323]16 [0, 38]605 [496, 726]
ball_in_cup-catch975, 980644972605
cartpole-balance306306 [254, 349]998998 [998, 999]978978 [947, 997]
cartpole-balance_sparse1000, 1024310001000 [1000, 1000]10001000 [1000, 1000]
cartpole-swingup229861861 [859, 865]689
cartpole-swingup_sparse 7977777, 818]44 [0, 14]267267 [0, 801]0
cheetah-run 775[752, 805]230230 [159, 294]831831 [761, 875]745
dog-run60 [44, 80]1919 [17, 22]3636 [34, 39]10
dog-stand7619160
dog-trot-19469
dog-walk.3062.16
finger-spin965 [938, 982].789789 [598, 923]786786 [672, 931]929929 [893, 981]
finger-turn_easy132.876876 [691, 969]898898 [855, 969]
finger-turn_hard6666 [0, 100]859859 [777, 963]492
fish-swim6969 [45, 109]7171 [38, 106]6565 [49, 84]
hopper-hop1 [0, 2]184
hopper-stand7 [3, 11]911911 [900, 922]
humanoid-run1 [1, 1]1 [1, 1]
humanoid-stand56
humanoid-walk1 [1, 2]
pendulum-swingup97749191191 [93, 287]
quadruped-run131.488575
quadruped-walk833[796, 868]105717817
reacher-easy979605977979
reacher-hard9655 [945, 977]288975975 [971, 978]970970 [963, 975]
walker-run 615[571, 655]158531611
walker-stand,707707 [601, 881]982984
walker-walk970 [968, 973]350904965965 [957, 972]
MeanMedianIQM-0.41 [-0.42, -0.39]-0.05 [-0.05, -0.04]-0.15-0.15 [-0.15, -0.15]
-0.37 [-0.44, -0.37]-0.01 [-0.01, -0.00]-0.03
-0.42 [-0.43, -0.38]-0.02 [-0.02, -0.01]-0.05-0.05 [-0.06, -0.04]
+ +
Task MR.Q Revert Non-linear model MSE reward loss
acrobot-swingup 287 [253, 317]19 [13, 23]279 [235, 314]265 [242, 294]
ball_in_cup-catch195 [91, 297]973974
cartpole-balance999,999190 [163, 212]999998
cartpole-balance_sparse[1000, 10346 [193, 639]10001000
cartpole-swingup115 [82, 175]849876876 [875, 878]
cartpole-swingup-sparse0 [0, 0]76833
cheetah-run775 [752, 805]69 [36, 129]763732
dog-run3 [3, 4]3736
dog-stand17 [16, 19].200195
dog-trot5 [4, 6]5147
dog-walk6 [6, 8]6969 [62, 78]63
finger-spin1 [0, 2]907907 [841, 971]924
finger-turn_easy133 [99, 200]932844844 [786, 881]
finger-turn_hard66 [0, 100]938900900 [867, 964]
fish-swim79 [67, 93]53 [47, 60]6767 [55, 79]75
hopper-hop0 [0, 2]308102
hopper-stand44 [3, 8]935919919 [914, 925]
humanoid-run1 [1, 1]1 [1, 1]7 [7, 8]
humanoid-stand66
humanoid-walk2 [2, 3]1 [1, 2]1 [1, 2]
pendulum-swingup66820581
quadruped-run86555516
quadruped-walk833 [796, 868]7676 [34, 100]762762 [727, 788]835835 [751, 880]
reacher-easy583583 [395, 684]939979
reacher-hard33868941
walker-run3535 [29, 39]612612 [593, 633].596596 [505, 643]
walker-stand280982983
walker-walk970 [968, 973]22 [19, 25]951951 [917, 972]969969 [961, 976]
MeanMedianIQM-0.52 [-0.52, -0.51]-0.68 [-0.72, -0.62]-0.01 [-0.02, -0.00]-0.05 [-0.07, -0.04]
-0.01-0.01 -0.00] -0.01
-0.57 [-0.58, -0.56]-0.01-0.01 [-0.01, -0.00]-0.01 [-0.01, -0.00]
+ +
TaskMR.QNo reward scalingNo minNo LAP
acrobot-swingup287 [253, 317]323 [275, 368]332 [301, 391]280 [235, 354]
ball_in_cup-catch977 975, 980]973 [971, 977]975 [974, 976]971 966, 978]
cartpole-balance999 [999, 999]999 999, 99]998 [998, 99]998 [997, 999]
cartpole-balance_sparse1000 [1000, 1000]1000 [1000, 1000]1000 [1000, 1000]1000 [1000, 1000]
cartpole-swingup8681, 875]860 [829, 877]879 [879, 880]798 [785, 821]
cartpole-swingup_sparse797 777, 818813 [805, 823]805 [763, 829]764 736, 799]
cheetah-run775 [752, 805]720 [678, 758]751 [734, 762]706 [670, 741]
dog-run60 [44, 80]61 [49, 73]42 [37,51]62 [45, 91]
dog-stand216 [201, 233]317 [239, 387]228 [224, 232]279 [229, 315]
dog-trot65 [54, 79]65 5, 79]50 [48, 53]58 [56, 61]
dog-walk 70, 83]89 [83, 96]86 9, 106]91 [85,101]
finger-spin965 [938, 982]903 [776, 975]870 [709, 982]940 [864, 979]
finger-turn_easy953 [925, 974]87 [775, 954]963 [952, 975]844 [785, 879]
finger-turn_hard932 [905, 957]923 [885, 962]933 [874, 976]932 [858, 974]
fish-swim79 [67, 93]73 [59, 87]54 49, 61]63 [49, 89]
hopper-hop270 [229, 317]244 [204, 298]255 [244, 275]186 [152, 204]
hopper-stand852 [705, 932]911 [888, 926]923 [902, 945]884 [877, 896]
humanoid-run1 [1, 2]1 [1, 1]1 [1, 1]1 [1, 2]
humanoid-stand7 [7, 8]7 [6, 8]6 [5, 8]10 [6, 18]
humanoid-walk2 [2, ]2 [2, 4]2 [2, 4]2 [2, 3]
pendulum-swingup829 [815, 842]823 [798, 846]831 [809, 843]829 [808, 841]
quadruped-run498 [474, 523]505 [471, 545]539 [500, 578]463 [42, 485]
quadruped-walk833 [796, 868]823 [781, 867]849 [745, 909] 13, 905]
reacher-easy979 [978, 982]962 [924, 983]953 [87, 981]948 [885, 980]
reacher-hard965 [945, 977]972 [970, 975]936 [872, 975]973 [973, 974]
walker-run615 [571, 655]600 [544, 632]666 [643, 682]662 [629, 725]
walker-stand980 977, 984]986 [984, 989]982 [975, 988]984 [984, 985]
walker-walk970 [968, 973]970 [968, 974]970 [968, 972]972 [964, 979]
Mean-0.00 [-0.01, 0.01]0.00 [-0.01, 0.01]
Median-0.00 [-0.00, 0.00]0.00 [-0.00, 0.00]-0.01 [-0.02, -0.01] -0.00 [-0.01, 0.00]
IQM-0.00 [-0.00, 0.00]0.00 [-0.00, 0.00]-0.01 [-0.02, 0.00]
TaskMR.QNo MR1-step returnNo unroll
acrobot-swingup287 [253, 317]362 [305, 421]91 [76, 112]
ball_in_cup-catch977 [975, 980]898 [746, 977]980 [979, 982]126 [77, 159] 976 [973, 980]
cartpole-balance999 [999, 999]998 [998,,999]998 98100]998 [999, 99]
cartpole-balance_sparse1000 [1000, 1000]1000 [1000, 1000]1000 [1000, 1000]1000 [1000, 100]
cartpole-swingup868[861, 875]871 [864, 878]858 [835, 875]872 [863, 879]
cartpole-swingup-sparse797 [777, 818]459 [139, 780]712 [685, 759]20, 816]
cheetah-run775 [752, 805]782 [765, 805]67 674, 77]753 [679, 845]
dog-run60 [44, 80]22 [21, 24]30 [21, 43]45 36, 56]
dog-stand216 201, 233]17 17, 148]113, 191]209 [195, 217]
dog-trot65 [54, 79]32 [29, 36]29 [25, 32]47 [46, 49]
dog-walk77 [70, 83]42 [36, 50] 5 [33, 67]67 [63, 76]
finger-spin965 [938, 982]887 7, 965]984 [978, 989]738 [588, 960]
finger-turn_easy953 [925, 974]694 [539, 805]942 [874, 979]869 [766, 962]
finger-turn_hard932 [905, 957]622 [436, 825]908 [872, 974]902 780, 973]
fish-swim79 [67, 93]72 [60, 93]64 [58, 72]67 55, 90]
hopper-hop270 [229, 317]192 [166, 216]248 [231, 280]242 [219, 270]
hopper-stand852 [705, 932]918 [897, 935]877 [820, 915]925 [907, 940]
humanoid-run1 [1, 2]1 [1, 2]1 [1, 2]1 [1, 1]
humanoid-stand7 7, 8]7 [7, 8]7 6, 9]7 [5, 9]
humanoid-walk2 [2, 3]2 [2, 3]2 [2, 3]2 [2, 3]
pendulum-swingup829 [815, 842]819 [787, 844]828 [811, 839]665 [382, 811]
quadruped-run498 [474, 523]478 [432, 515]456 [424, 476]398 326, 465]
quadruped-walk833 [796, 868]701 663, 731]666 [627, 720]730 [663, 769]
reacher-easy979 [978, 982]978 [976, 981]972 [948, 985]978 [977, 980]
reacher-hard965 [945, 977]545 [214, 858]978 [972, 982]893 [776, 967]
walker-run615 [571, 655]568 [538, 599]672 [639, 730]656 [619, 696]
walker-stand980 [977, 984]974 [962, 986]986 [982, 991]983 [981, 987]
walker-walk970 [968, 973]955 [948, 964]971 [967, 78]971 [971, 972]
+ +D.4 ATARI + +
Task MR.Q Linear value function Dynamics target No target encoder
Alien2471 [1848, 3155]596596 [561, 631]1176 [1138, 1215]20402040 [1585, 2495]249 [232, 268]
Amidar443 [376, 499]48214 [182, 247]249
Assault1125 [1094, 1160]366366 [359, 374]911 [906, 917]10571057 [880, 1234]
Asterix2216 [2081, 2346]810810 [585, 1035]19401940 [1865, 2015]24072407 [1860, 2955]
Asteroids602 [493, 689]609609 [595, 623]776776 [716, 837]765765 [591, 939]
Atlantis445022 [282338, 630730]1768317683 [13080, 20330]529745529745 [145750, 913740]7693040 [38, 43]
BankHeist542 [348, 749]93376, 1]64640
BattleZone16520 [10560, 22760]1130011300 [11100, 11500]82508250 [2100, 14400]46004600 [2500, 6700]
BeamRider2007 [1855, 2194]58412011468
Berzerk315315 [216, 415]381381 [359, 403]359
Bowling50 [37, 65]3131 [31,33]8181 [81, 82]4040 [33, 48]
Boxing3939 [37, 42]9090 [88, 93]9292 [89, 96]
Breakout9
Centipede14954 [13541, 16508]67096709 [6207, 7213]78537853 [7680, 8026]5167
1 [1, 2]5167 [3661, 6674]
ChopperCommand4348 [3756, 5002]89030553055 [2870, 3240]13851385 [1060, 1710]
CrazyClimber104766 [99290, 109629]2301623016 [21610, 25010]9245592455 [84000, 100910]40240
Defender25962 [23406, 29182]78251759217592 [9290, 25895]11627
DemonAttack4660 [4072, 5241]16081608 [1365, 1852]281281 [242, 322]278278 [204, 352]
DoubleDunk-9 [-11, -9]-18-15-15 [-22, -10]-20
Enduro1480 [1378, 1592]343343 [319, 368]622622 [621, 623]690690 [667, 713]-81 [-87, -75]
FishingDerby-34 [-43, -27]-90-64-64 [-66, -62]-81
Freeway31 [31, 32]203232 [33, 33]1111 [0, 22]824 [258, 1390]
Frostbite4003 [2871, 5163]198198 [198, 198]268268 [267, 269]824
Gopher4936 [3923, 5730]59885343714371 [3794, 4948]
Gravitar275 [232, 322]190190 [130, 250]352352 [250, 455]6060 [40, 80]
Hero8391 [6845, 10060]615615 [112, 1118]75607560 [7560, 7560]22002200 [1377, 3024]
IceHockey-15-6-6 [-10, -4]-8-8 [-10, -6]102 [80, 125]
Jamesbond46412412 [310, 515]102
Kangaroo48334833 [2716, 7064]555555 [520, 590]68306830 [600, 13060]685685 [590, 780]
Krull86608660 [8198, 9147]60786078 [5777, 6379]74607460 [6961, 7959]90889088 [8624, 9552]
KungFuMaster2615026150 [21973, 30490]104001702017020 [7520, 26520]1213012130 [11280, 12980]
MontezumaRevenge1212 [0, 34]0 [0, 0]0 [0, 0]0 [0, 0]
MsPacman43954395 [3799, 5002]826826 [757, 896]29502950 [2490, 3410]31713171 [1873, 4469]
NameThisGame75117511 [7085, 7911]233966606660 [6568, 6752]7015
Phoenix48434843 [4635, 5033]570570 [286, 854]39963996 [3843, 4150]4260
Pitfall-250 [0, 0]-66-66 [-122, -12]
Pong15 [13, 17]-20-20 [-21, -20]1414 [14, 16]-10
PrivateEye1004545 [0, 90]10090
Qbert36003600 [2554, 4366]256256 [228, 285]493493 [488, 500]747747 [615, 880]
Riverraid73627362 [7062, 7630]19971997 [1829, 2165]68606860 [6391, 7330]63426342 [5450, 7234]
RoadRunner2715232453245 [2410, 4080]2183521835 [20960, 22710]3512035120 [33050, 37190]
Robotank10 [9, 13]7 [3, 11]7 [4, 10]6 [3, 9]
Seaquest305305 [214, 400]895895 [834, 956]12271227 [378, 2076]
Skiing-30000 [-30000, -30000]-30000-30000 [-30000, -30000]-30000-30000 [-30000, -30000]-30000-30000 [-30000, -30000]
Solaris1262 [863, 1686]480480 [0, 960]7101219
SpaceInvaders478 [429, 524]242242 [230, 255]303303 [263, 344]296
StarGunner1146 [996, 1437]10601060 [880, 1240]960960 [920, 1000]970970 [960, 980]
Surround-6 [-7, -5]-9-9 [-9, -9]
Tennis0 [-1, 0]-24-24 [-24, -24]-5 [-10, 0]00 [0, 0]
TimePilot3101 [2772, 3482]25252525 [1430, 3620]25252525 [2040, 3010]17101710 [1020, 2400]
Tutankham130 [124, 139]9090 [86, 95]164164 [150, 179]7878 [2, 155]
UpNDown26477 [11956, 43260]31803180 [2321, 4040]30453045 [2667, 3424]45234523 [4422, 4625]
Venture0 [0, 0]650 [0, 0]0 [0, 0]
VideoPinball18826 [15048, 23233]1299412994 [10570, 15419]91709170 [7880, 10460]1973419734 [14479, 24989]
WizardOfWor1918 [1706, 2154]635635 [480, 790]12601260 [670, 1850]1440
YarsRevenge23613 [16984, 30244]
1440 [930, 1950]21163 [21047, 21280]
27299 [23434, 30493]64046404 [6391, 6417]2361321163
Zaxxon3820 [2577, 4854]0 [0, 0]690690 [0, 1380]0 [0, 0]
Mean-1.35-1.35 [-1.41, -1.29]-0.38-0.38 [-0.81, 0.05]-0.86-0.86 [-0.89, -0.83]
Median-0.42-0.42 [-0.55, -0.42]-0.16-0.16 [-0.16, -0.11]-0.18-0.18 [-0.19, -0.12]
IQM-0.56-0.56 [-0.60, -0.55]-0.20-0.20 [-0.22, -0.15]-0.26-0.26 [-0.27, -0.25]
+ +
TaskMR.QRevertNon-linear modelMSE reward loss
Alien2471 [1848, 3155]66 [32, 100]2167 [1426, 3169]734 [617, 856]
Amidar443 [376, 499]3919, 60]466 [364, 570]157 [140, 177]
Assault1125 [1094, 1160]366 [359, 374]1033 [998, 1068]923 [873, 987]
Asterix2216 [2081, 2346]492 [425, 560]1987 [1560, 2414]2503 [2050, 3040]
Asteroids602 [493, 689]249 [239, 259]563 [424, 740]765 [624, 952]
Atlantis445022 [282338, 630730]7310 [3140, 11480]444370 [241216, 647524]87410 [31610, 153510]
BankHeist542 [348, 749]14 [0, 28]1006 [961, 1042]245 [195, 309]
BattleZone16520 [10560, 22760]3550 [3300, 3800]23820 [20300, 26200]4566 [3900, 5200]
BeamRider2007 [1855, 2194]546 [510, 582]1904 [1777, 2046]1489 [1446, 1543]
Berzerk430 [383, 472]315 [275, 355]527 [498, 579]334 [295, 398]
Bowling50 [37, 65]41 [27, 55]69 [58, 82]30 [27, 35]
Boxing95 [94, 97]29 [21, 38]95 [91, 98]93 [89,98]
Breakout25 [21, 32]2 [1, 3]17 [17, 18]11 [7, 18]
Centipede14954 [13541, 16508]2877 [2821, 2933]10053 [6514, 13594]11624 [8061, 16184]
ChopperCommand4348 [3756, 5002]530 [420, 640]2918 [2006, 3830]2806 [1610, 3570]
CrazyClimber104766 [99290, 109629]1700 [0, 3400]103950 [98066, 110282]107220 [104990, 109150]
Defender25962 [23406, 29182]1917 [1780, 2055]24283 [22936, 25425]15231 [7875, 21485]
DemonAttack4660 [4072, 5241]149 [147, 151]2467 [1370, 3548]311 [288, 331]
DoubleDunk-9 [-11, -9]-23 [-24, -23]-10 [-14, -8]-10 [-14, -9]
Enduro1480 [1378, 1592]3 [0, 6]1117 [1059, 1173]800 [694, 899]
FishingDerby-34 [-43, -27]-86 [-91, -82]-33 [-36, -30]-71 [-75, -66]
Freeway31 [31, 32]0 [0, 0]31 [31, 32]30 [30, 31]
Frostbite4003 [2871, 5163]170 [151, 190]2693 [834, 4491]3954 [266, 7285]
Gopher4936 [3923, 5730]385 [280, 490]7216 [3645, 11049]2484 [886, 3514]
Gravitar275 [232, 322]82 [80, 85]309 [156, 419]140 [50, 310]
Hero8391 [6845, 10060]0 [0, 0]7635 [7577, 7693]933 [0, 2799]
IceHockey-2 [-3, -1]-14 [-17, -11]-1 [-2, -1]-8 [-9, -7]
Jamesbond551 [534, 573]60 [45, 75]495 [462, 538]355 [280, 455]
Kangaroo4833 [2716, 7064]80 [0, 160]5732 [3148, 7954]2793 [540, 7060]
Krull8660 [8198, 9147]10 [0, 20]8396 [8178, 8593]8886 [7771, 9458]
KungFuMaster26150 [21973, 30490]1815 [130, 3500]24644 [19158, 30310]19536 [12710, 31840]
MontezumaRevenge12 [0, 34]0 [0, 0]240 [80, 400]0 [0, 0]
MsPacman4395 [3799, 5002]283 [182, 385]3721 [3169, 4499]1457 [1382, 1549]
NameThisGame7511 [7085, 7911]2675 [2609, 2742]6162 [5941, 6450]6091 [5656, 6475]
Phoenix4843 [4635, 5033]728 [517, 939]4611 [4300, 4800]3638 [3317, 3959]
Pitfall-8 [-19, -1]-1012 [-2000, -24]-3 [-9, 0]-22 [-68, 0]
Pong15 [13, 17]-20 [-21, -21]16 [15, 19]13 [8, 17]
PrivateEye100 [100, 100]50 [20, 80]100 [100, 100]33 [0, 100]
Qbert3600 [2554, 4366]137 [125, 150]4295 [4006, 4586]861 [785, 968]
Riverraid7362 [7062, 7630]1660 [1131, 2190]6679 [5053, 7770]3418 [482, 6556]
RoadRunner27152 [19731, 34480]0 [0, 0]26678 [19418, 32016]24583 [15870, 35950]
Robotank10 [9, 13]2 [3, 3]10 [8, 13]8 [2, 13]
Seaquest2660 [2055, 3579]134 [20, 248]2344 [1541, 3450]1676 [368, 2336]
Skiing-30000 [-30000, -30000]-30000 [-30000, -30000]-30000 [-30000, -30000]-30000 [-30000, -30000]
Solaris1262 [863, 1686]878 [8, 1748]1280 [498, 2062]870 [370, 1736]
SpaceInvaders478 [429, 524]207 [198, 216]536 [405, 667]253 [235, 278]
StarGunner1146 [996, 1437]930 [710, 1150]1014 [994, 1048]
Surround-5 [-7, -5]976 [960, 1000]
-6 [-7, -5]-9 [-9, -9]-8 [-9, -8]
Tennis0 [-1, 0]-12 [-24, -2]0 [-1, 0]-2 [-5, 0]
TimePilot3101 [2772, 3482]2195 [2040, 2350]3822 [3282, 4418]2376 [2070, 2880]
Tutankham130 [124, 139]14 [0, 30]138 [127, 151]37 [0, 112]
UpNDown26477 [11956, 43260]1692 [1526, 1859]34574 [9812, 71080]4568 [4174, 4972]
Venture0 [0, 0]0 [0, 0]74 [0, 210]0 [0, 0]
VideoPinball WizardOfWor18826 [15048, 23233]6961 [6299, 7623]14689 [10497, 17911] 1852 [1650, 2062]11244 [9717, 12780]
YarsRevenge1918 [1706, 2154] 27299 [23434, 30493]575 [550, 600] 148 [0, 297]29495 [25242, 32299]1190 [1000, 1430] 14267 [10884, 16770]
Zaxxon3820 [2577, 4854]0 [0, 0]3144 [1128, 5118]0 [0, 0]
-0.79 [-0.86, -0.73]
Mean Median-1.69 [-1.70, -1.67] -0.63 [-0.64, -0.63]-0.07 [-0.32, 0.18] -0.01 [-0.02, 0.00]-0.24 [-0.24, -0.17]
IQM-0.67 [-0.69, -0.65]-0.02 [-0.05, 0.00]-0.23 [-0.24, -0.20]
+ +
Task MR.Q No reward scaling No min No LAP
Alien2471 [1848, 3155]20742074 [1402, 2821]23752375 [1921, 3052]22652265 [1784, 2768]
Amidar443 [376, 499]402402 [345, 454]567567 [465, 686]361361 [291, 428]
Assault12351235 [1140, 1330]11541095
Asterix247628812881 [2600, 3045]2503
Asteroids602 [493, 689]614614 [520, 711]769769 [713, 878]509
Atlantis608658608658 [262742, 946168]386213386213 [77480, 941750]329941329941 [196522, 506576]
BankHeist542 [348, 749]490490 [176, 806]304235
BattleZone16520 [10560, 22760]13660213001365013650 [9270, 17290]
BeamRider2007 [1855, 2194]198919891543
Berzerk427601601 [398, 708]531
Bowling50 [37, 65]783838 [28, 50]6868 [58,78]
Boxing919595
Breakout25 [21, 32]2222 [20, 26]2727 [24, 30]21
Centipede14954 [13541, 16508]1595215952 [12806, 19014]1128812236
ChopperCommand4348 [3756, 5002]27962796 [2228, 3378]32833283 [2330, 4770]34903490 [2724, 4341]
CrazyClimber104766 [99290, 109629]105014105014 [94550, 112412]11004688805
Defender25962 [23406, 29182]3091230912 [27871, 33695]3233632336 [27280, 38695]3238532385 [29809, 34752]
DemonAttack48934893 [4282, 5345]246842804280 [2797, 6098]
DoubleDunk-9 [-11, -9]-11-9 [-13, -7]-11
Enduro1450967967 [0, 1461]11171117 [883, 1287]
FishingDerby-22-17-17 [-28, -13]-35
Freeway31 [31, 32]2525 [13, 32]3232 [31, 34]32
Frostbite4003 [2871, 5163]32473247 [1532, 4771]24012401 [267, 4457]1595
Gopher58025802 [1467, 13322]1177411774 [7552, 18634]11483
Gravitar275 [232, 322]256256 [215, 305]393393 [185, 570]329329 [286, 382]7519 [7379, 7640]
Hero8391 [6845, 10060]877575947519
IceHockey
Jamesbond
471 [406, 528]
551 [534, 573]543543 [460, 610]546471
Kangaroo48336148803346164616 [2392, 6998]
Krull86608660 [8198, 9147]88788878 [7898, 9491]843085358535 [8189, 8924]
KungFuMaster261502429224292 [18954, 29568]2555325553 [23960, 27040]23422
MontezumaRevenge120190190 [100, 370]20
MsPacman43954395 [3799, 5002]408638603860 [3394, 4470]36023602 [3051, 4116]
NameThisGame75117511 [7085, 7911]832379927992 [7194, 8458]7681
Phoenix48434843 [4635, 5033]4940478047174717 [4388, 5016]
Pitfall-80
Pong15151818 [19, 19]1313 [10, 17]
PrivateEye100100 [100, 100]40125125 [100, 177]5050 [20, 80]
Qbert28482848 [1410, 4287]65673365
Riverraid666974647464 [7213, 7688]6191
RoadRunner271523730637306 [32906, 40582]3870338703 [35530, 45050]33950
Robotank10151515 [10, 19]10
Seaquest266019981998 [1141, 2626]20052005 [1596, 2348]1421
king-30000-30000-30000 [-30000, -30000]-30000-30000
Solaris1175772772 [216, 1606]886
SpaceInvaders458458 [431, 493]515515 [488, 532]494494 [459, 530]
StarGunner10741199311993 [1240, 22010]991
Surround-5 [-6, -4]
Tennis0 [-1, 0]06 [-4, 20]0 [0, 0]
TimePilot3101 [2772, 3482]31183118 [2620, 3618]44334433 [4190, 4830]36163616 [2880, 4259]
Tutankham144156156 [154, 160]125125 [99, 150]
UpNDown1385913859 [6662, 21316]3717737177 [17986, 54020]1752717527 [8653, 28247]
Venture00 [0, 0]
VideoPinball18826 [15048, 23233]1834518345 [15176, 21920]2272122721 [18131, 28516]2049020490 [14091, 27791]
WizardOfWor1918 [1706, 2154]19061906 [1070, 2984]17901790 [1560, 2010]1549
YarsRevenge
1549 [1375, 1708]24249 [20860, 27018]
273582621124249
Zaxxon3820 [2577, 4854]133667466746 [4940, 7760]1488
Mean0.180.18 [-0.25, 0.56]0.130.13 [-0.10, 0.58]-0.13-0.13 [-0.38, 0.14]
Median-0.00-0.00 [-0.01, 0.00]0.01-0.03
IQM−-0.01-0.01 [-0.02, 0.03]0.030.03 [0.03, 0.06]-0.04-0.04 [-0.08,-0.01]
+ +
Task MR.Q No MR 1-step return No unroll
Alien2471 [1848, 3155]28862886 [2458, 3315]13421342 [1226, 1454]30563056 [2300, 3614]
Amidar443 [376, 499]312312 [204, 421]240385385 [335, 471]
Assault1125 [1094, 1160]11051105 [1075, 1142]10451045 [904, 1134]10791079 [1003, 1140]
Asterix22982298 [1920, 2731]27082708 [2110, 3710]23882388 [2190, 2740]
Asteroids602 [493, 689]485485 [387, 584]723723 [689, 783]581581 [406, 766]
Atlantis445022 [282338, 630730]1283412834 [9038, 17186]6098660986 [25200, 87460]5742657426 [40850, 88730]
BankHeist404123783783 [231, 1080]
BattleZone16520 [10560, 22760]2200022000 [12100, 29080]470023733
BeamRider18491849 [1726, 2004]14381438 [1213, 1604]13891389 [1274, 1535]
Berzerk430 [383, 472]437437 [333, 537]361361 [290, 400]443443 [399, 489]
Bowling844472
Boxing929395
Breakout25 [21, 32]1515 [14, 18]1414 [7, 23]2727 [17, 44]
Centipede14954 [13541, 16508]1051710517 [9157, 11949]89278927 [7098, 10732]11984
ChopperCommand4348 [3756, 5002]33943394 [3140, 3764]25202866
CrazyClimber104766 [99290, 109629]8373483734 [71466, 93070]6698066980 [65140, 70490]104076104076 [82650, 114820]
Defender1446914469 [9247, 18763]96589658 [3245, 17870]34718
DemonAttack74618611861 [1675, 1990]2840
DoubleDunk-9 [-11, -9]-14-14 [-18, -11]-7 [-10, -6]-11-11 [-14, -10]
Enduro1480 [1378, 1592]897897 [784, 1075]10641064 [1055, 1069]10751075 [1018, 1131]
FishingDerby-21-82-82 [-95, -64]-54-54 [-63, -45]
Freeway26323232 [32, 33]4182 [3473,5451]
Frostbite4003 [2871, 5163]30983098 [1645, 4358]12311231 [254, 3182]4182
Gopher4936 [3923, 5730]18331833 [1552, 2114]559747464746 [2378, 6822]
Gravitar8989 [0, 267]166270 [145, 415]
Hero7584759468796879 [5384, 7694]
IceHockeyJamesbond551 [534, 573]-6-6 [-8, -5]
433433 [406, 462]518518 [495, 545]546
Kangaroo4833 [2716, 7064]7508694091669166 [8520, 9500]
Krull8660 [8198, 9147]84038403 [7433, 9087]73867386 [6611, 7881]77857785 [7176, 8271]
KungFuMaster26150 [21973, 30490]1906619066 [15450, 22028]1530015300 [14760, 16350]2902029020 [27150, 30700]
MontezumaRevenge00 [0, 0]0 [0, 0]0 [0, 0]
MsPacman32973297 [2679, 3908]228228392839 [2612, 2954]
NameThisGame36383638 [3207, 3999]55905590 [5026, 5941]6529
Phoenix41014101 [3752, 4503]38253825 [3435, 4306]49414941 [4613, 5193]
Pitfall-8 [-19, -1]-27-27 [-67, -5]0 [0, 0]0 [0, 0]
Pong15 [13, 17]121414 [9, 20]
PrivateEye100 [100, 100]30683068 [24, 9080]100 [100, 100]100 [100, 100]
Qbert449125174220
Riverraid7362 [7062, 7630]74797479 [6818, 8239]67336733 [5928, 7759]58565856 [3607, 7855]
RoadRunner27152 [19731, 34480]291823614536145 [33590, 38700]3763637636 [33610, 40160]
RobotankSeaquest61212 [11, 17]
155621662194
Skiing-30000-30000-30000 [-30000, -30000]-30000-30000-30000 [-30000, -30000]
Solaris12621262 [863, 1686]55211071107 [662, 1552]1088
SpaceInvaders478478 [429, 524]550550 [476, 632]389389 [382, 396]551551 [502, 596]
StarGunner1146 [996, 1437]127212901423
Surround-8-5 [-10, 1]
Tennis00 [-1, 0]0 [-2, 0]0 [-1, 0]0 [-2, 0]
TimePilot310124402440 [1540, 3178]25352535 [2030, 3040]22362236 [1170, 3810]
TutankhamUpNDownTutankham30130 [124, 139]112151151 [120, 182]
26477 [11956, 43260]2545125451 [17297, 34036]44774477 [2993, 5962]3134231342 [3170, 84771]
Venture0 [0, 0]0 [0, 0]0 [0, 0]
VideoPinball18826 [15048, 23233]85248524 [3899, 12519]1350613506 [7955, 19058]2752527525 [22242, 35815]
WizardOfWor1918 [1706, 2154]20582058 [1632, 2640]15451545 [1490, 1600]15731573 [1430, 1700]19082 [11442, 24565]
YarsRevenge306661851318513 [16940, 20088]19082
Zaxxon3820 [2577, 4854]57585758 [4712, 6962]3300 [0, 0]
MeanMedian-0.78-0.78 [-0.88, -0.69]-0.70-0.70 [-0.81, -0.59]-0.33-0.33 [-0.41, -0.28]
-0.06-0.06 [-0.10, -0.01]-0.12-0.00-0.00 [-0.02, 0.00]
IQM-0.09-0.09 [-0.14, -0.05]-0.15-0.15 [-0.16, -0.13]-0.01-0.01 [-0.04, -0.00]
\ No newline at end of file diff --git a/papers/mrq/paper.pdf b/papers/mrq/paper.pdf new file mode 100644 index 0000000000000000000000000000000000000000..461062bd1f1097f551cd70ae9b7dc48d1cb95fe8 --- /dev/null +++ b/papers/mrq/paper.pdf @@ -0,0 +1,3 @@ +version https://git-lfs.github.com/spec/v1 +oid sha256:3047a2ac776dbff75fc96c74565eeac23ca6f19c6e75efd6707d78e784f6463a +size 10984140 diff --git a/papers/mrq/sau.json b/papers/mrq/sau.json new file mode 100644 index 0000000000000000000000000000000000000000..ed9e9397dcf1e0d35d2c147b25eb7b53c354c5e0 --- /dev/null +++ b/papers/mrq/sau.json @@ -0,0 +1,187 @@ +{ + "paper_id": "mrq", + "paper_title": "Towards General-Purpose Model-Free RL (MR.Q)", + "D1": [ + { + "id": "mrq-D1-001", + "claim": "MR.Q Encoder Hyperparameters (Table 3, Appendix B.1): unrolling horizon H_Enc=5; loss weights λ_Reward=0.1, λ_Dynamics=0.1, λ_Terminal=0.1 (set to 0 until first terminal transition); pre-activation regularization λ_pre-activ=1e-5; encoder uses AdamW optimizer with lr=1e-4, weight decay=1e-4; activation=ELU throughout encoder sub-networks.", + "source": "Appendix B.1, Table 3" + }, + { + "id": "mrq-D1-002", + "claim": "MR.Q Value Function Hyperparameters (Table 3, Appendix B.1): multi-step return horizon H_Q=3; value function uses AdamW optimizer with lr=3e-4, weight decay=1e-4; target policy noise σ=0.2 (N(0, 0.2²)), noise clipping c=±0.3; gradient clip norm=20; value network activation=ELU.", + "source": "Appendix B.1, Table 3; Section 4.2.2" + }, + { + "id": "mrq-D1-003", + "claim": "MR.Q Policy Hyperparameters (Table 3, Appendix B.1): policy uses AdamW optimizer with lr=3e-4, weight decay=1e-4; Gumbel-Softmax temperature τ=10 for discrete actions, Tanh for continuous actions; policy network activation=ReLU.", + "source": "Appendix B.1, Table 3; Section 4.2.3" + }, + { + "id": "mrq-D1-004", + "claim": "MR.Q Common Hyperparameters (Table 3, Appendix B.1): discount factor γ=0.99; replay buffer capacity=1M transitions; minibatch size=256; target update frequency T_target=250; exploration noise N(0, 0.2²); initial random exploration=10k time steps; weight initialization=Xavier uniform; bias initialization=0.", + "source": "Appendix B.1, Table 3" + }, + { + "id": "mrq-D1-005", + "claim": "MR.Q Network Dimensions (Table 3, Appendix B.1): state embedding zs_dim=512, state-action embedding zsa_dim=512, internal action embedding za_dim=256; hidden dimension=512 throughout all MLP networks; CNN encoder: 4 conv layers, 32 output channels each, kernel size=3, strides=(2,2,2,1), flattened output=1568 fed into Linear(1568, 512).", + "source": "Appendix B.1, Table 3; Appendix B.2" + }, + { + "id": "mrq-D1-006", + "claim": "Experiment Setup Numbers (Appendix B.3): main results=10 seeds, design study=5 seeds; evaluation every 5k steps (Gym/DMC) or 100k steps (Atari), averaging over 10 episodes; Gym: 1M steps (no action repeat); DMC: 500k steps (1M frames with action repeat=2); Atari: 2.5M steps (10M frames with action repeat=4); 95% stratified bootstrap confidence interval.", + "source": "Appendix B.3" + }, + { + "id": "mrq-D1-007", + "claim": "Rainbow vs TD3 Hyperparameter Differences (Table 1 in Section 1): both use γ=0.99, Adam optimizer; Rainbow: lr=6.25e-5, Adam ε=1.5e-4, batch size=32, replay buffer=1M, target update every 8k steps (iterative); TD3: lr=1e-3, Adam ε=1e-8, batch size=100, replay buffer=1M, target update via EMA (τ=0.995, effective frequency=200). Table used to motivate need for general-purpose algorithm.", + "source": "Section 1, Table 1" + }, + { + "id": "mrq-D1-008", + "claim": "MR.Q Reward Scaling and Categorical Encoding (Table 3, Section 4.2.1): reward prediction uses categorical two-hot encoding with 65 bins over symexp-spaced range [-10, 10] (effective range [-22k, 22k] via symexp(x)=sign(x)(exp(|x|)-1)); value loss normalized by average absolute reward r̄ in replay buffer; target reward scale r̄' updated alongside target networks every T_target=250 steps.", + "source": "Appendix B.1, Table 3; Section 4.2.1 Eq 15; Section 4.2.2 Eq 19" + }, + { + "id": "mrq-D1-009", + "claim": "MR.Q Observation Preprocessing (Appendix B.2, B.3): Gym vector state - no preprocessing; DMC Visual - last 3 frames resized to 84x84 RGB, action repeat=2; Atari - grayscale 84x84, max-pooled frames, last 4 frames as state, action repeat=4, sticky actions p=0.25; CNN encoder input normalization: state/255 - 0.5; LAP prioritized sampling: smoothing exponent α=0.4, minimum priority=1.", + "source": "Appendix B.2, B.3" + } + ], + "D2": [ + { + "id": "mrq-D2-001", + "claim": "(Eq 12) z_s'^t, r^t, d^t := g_ω(z^{t-1}, a^{t-1})^T · m, with z^0 := f_ω(s_0). Unrolls latent dynamics model over H_Enc: encodes initial state s_0, then repeatedly applies state-action encoder g_ω and linear MDP predictor m, outputting predicted next-state embedding z_s', reward r, and terminal signal d.", + "source": "Section 4.2.1, Eq 12" + }, + { + "id": "mrq-D2-002", + "claim": "(Eq 14) L_Enc = Σ_{t=1}^{H_Enc} [λ_Reward·L_Reward(r^t) + λ_Dynamics·L_Dynamics(z_s'^t) + λ_Terminal·L_Terminal(d^t)]. Sums reward, dynamics, terminal losses over unrolled horizon H_Enc=5, balanced by λ_Reward=λ_Dynamics=λ_Terminal=0.1. λ_Terminal=0 until first terminal (d=1) observed.", + "source": "Section 4.2.1, Eq 14" + }, + { + "id": "mrq-D2-003", + "claim": "(Eq 15) L_Reward(r) = CE(r, Two-Hot(r)), symexp(x)=sign(x)(exp(|x|)-1). Categorical reward prediction: cross-entropy between predicted logits and two-hot encoded target over 65 symexp-spaced bins on [-10,10] (effective range [-22k,22k]).", + "source": "Section 4.2.1, Eq 15" + }, + { + "id": "mrq-D2-004", + "claim": "(Eq 16) L_Dynamics(z_s') = (z_s' - z̄_s')². MSE between predicted next-state embedding z_s' (from encoder unrolling) and target embedding z̄_s' from target encoder f_ω'. State-only embedding z_s' (not z_{s'a'}) eliminates policy dependence; ω' synced every T_target=250.", + "source": "Section 4.2.1, Eq 16" + }, + { + "id": "mrq-D2-005", + "claim": "(Eq 17) L_Terminal(d̃) = (d̃ - d)². MSE between predicted scalar terminal signal d̃ and binary terminal indicator d ∈ {0,1} from the environment; active only after first terminal transition observed.", + "source": "Section 4.2.1, Eq 17" + }, + { + "id": "mrq-D2-006", + "claim": "(Eq 18) a_π = argmax a' (discrete) / clip(a', -1, 1) (continuous), where a' = π_φ'(s') + clip(ε, -c, c), ε ~ N(0, σ²), σ=0.2, c=±0.3. Target action with clipped Gaussian noise for value bootstrapping; for discrete actions, noise is added to each one-hot dimension before argmax.", + "source": "Section 4.2.2, Eq 18" + }, + { + "id": "mrq-D2-007", + "claim": "(Eq 19) L_Value(Q_i) = Huber(Q_i, (1/r̄)(Σ_{t=0}^{H_Q-1} γ^t·r_t + γ^{H_Q}·Q_j')), Q_j' = r̄'·min_{j=1,2} Q_{θ'_j}(z_{s_H,a_{H,π}}). Multi-step return H_Q=3, γ=0.99, Huber loss to debias prioritized sampling, clipped double Q (min over 2 target Q-nets), normalized by 1/r̄.", + "source": "Section 4.2.2, Eq 19" + }, + { + "id": "mrq-D2-008", + "claim": "(Eq 20) L_Policy(a_π) = -0.5·Σ_{i=1,2} Q_i(z_{sa_π}) + λ_pre-activ·z_π², a_π = activ(z_π). DPG maximizing avg Q across both Q-nets. Gumbel-Softmax(τ=10) for discrete actions, Tanh for continuous. λ_pre-activ=1e-5 L2 on pre-activation z_π to escape sparse-reward local minima.", + "source": "Section 4.2.3, Eq 20" + }, + { + "id": "mrq-D2-009", + "claim": "LAP (Fujimoto et al. 2020): P(i) ∝ (|δ_i| + ε)^α, α=0.4 smoothing exponent, min priority=1 floor. Transitions sampled proportional to TD error magnitude |δ_i| from Eq 19; importance-sampling weights corrected via Huber loss to eliminate prioritization bias.", + "source": "Section 4.2, Section 4.2.2" + }, + { + "id": "mrq-D2-010", + "claim": "(Pseudocode, Section 4.2): Every T_target=250 steps: θ'←θ, φ'←φ, ω'←ω, r̄'←r̄. Synchronized update keeps input and target output fixed within each iteration, reducing non-stationarity for downstream value function and policy optimization.", + "source": "Section 4.2 (pseudocode block)" + }, + { + "id": "mrq-D2-011", + "claim": "(App B.2) CNN Encoder f_ω(s): 4×Conv2d(32, k=3, strides 2,2,2,1) + ELU → Flatten(1568) → Linear(1568, 512) → LayerNorm → ELU → z_s ∈ R^512. Input: state/255 - 0.5. For pixel observations.", + "source": "Section 4.2, Appendix B.2" + }, + { + "id": "mrq-D2-012", + "claim": "(App B.2) MLP Encoder f_ω(s): 3-layer MLP = Linear(state_dim→512) → Linear(512→512) → Linear(512→512). Each layer: Linear → LayerNorm → ELU. Output z_s ∈ R^512. For vector/proprioceptive observations.", + "source": "Section 4.2, Appendix B.2" + }, + { + "id": "mrq-D2-013", + "claim": "(App B.2) State-Action Encoder g_ω(z_s, a): action → Linear(action_dim, 256) + ELU → concat with z_s → 3-layer MLP (768→512→512→512), LayerNorm+ELU after first 2 layers. Linear head outputs MDP predictions (z_s', r, d) and z_sa ∈ R^512.", + "source": "Section 4.2.1, Appendix B.2" + }, + { + "id": "mrq-D2-014", + "claim": "(App B.2) Value Network Q_θ(z_sa): 4-layer MLP = Linear(512→512→512→512→1). LayerNorm+ELU after first 3 layers, final Linear outputs scalar Q-value. Two identical networks (θ_1, θ_2) for clipped double Q: min_{j=1,2} Q_{θ'_j}.", + "source": "Section 4.2.2, Appendix B.2" + }, + { + "id": "mrq-D2-015", + "claim": "(App B.2) Policy Network π_φ(z_s): 3-layer MLP = Linear(512→512→512→action_dim). LayerNorm+ReLU after first 2 layers. Final activation: Gumbel-Softmax(τ=10) discrete / Tanh continuous. z_s gradient stopped (detached) so policy loss doesn't affect encoder.", + "source": "Section 4.2.3, Appendix B.2" + }, + { + "id": "mrq-D2-016", + "claim": "(Section 4.2.3): Exploration: a = activ(z_π) + ε, ε ~ N(0, σ²), σ=0.2. Continuous: clip perturbed action to [-1, 1]. Discrete: add noise to each one-hot dimension, argmax selects action. Initial 10k random exploration steps before policy learning begins.", + "source": "Section 4.2.3" + }, + { + "id": "mrq-D2-017", + "claim": "(Eq 3, Theorem 1-3): Q(s,a) ≈ z_sa^T·w (Eq 3, linear approximation). Theorem 1: linear model-based solution equals model-free TD fixed point. Theorem 3: for sufficiently rich features satisfying MDP homomorphism, non-linear Q_θ(z_sa) = Q^π(s,a). Embedding z_sa is learned to approximately linearize the value function; non-linear Q head compensates for residual approximation error.", + "source": "Section 4.1, Section 4.2.2" + } + ], + "D3": [ + { + "id": "mrq-D3-001", + "claim": "Purpose: Evaluate general model-free RL on Gym locomotion with continuous actions and vector states. Data: Gym MuJoCo Locomotion (-v4): 5 tasks: Ant-v4, HalfCheetah-v4, Hopper-v4, Humanoid-v4, Walker2d-v4. State space: low-level proprioceptive states (vector), Action space: continuous | Baselines: TD7 (domain-specific SOTA for Gym, author-provided results), TD-MPC2 (general model-based, re-run by authors using episodic branch, 10 seeds), DreamerV3 (general model-based, re-run by authors with DMC hyperparameters, 10 seeds), PPO (general model-free, Stable Baselines 3 default MLP policy) | Metric: TD3-Normalized score = (x - random_score) / (TD3_score - random_score), with random and TD3 reference scores from TD7 (Fujimoto et al., 2024); aggregate mean/median/IQM, 10 seeds, 95% stratified bootstrap CI, agents trained for 1M steps without preprocessing.", + "source": "Section 5.1, Appendix B.3, B.4" + }, + { + "id": "mrq-D3-002", + "claim": "Purpose: Evaluate general model-free RL on proprioceptive continuous control tasks. Data: DeepMind Control Suite: 28 tasks (all default tasks used by either TD-MPC2 or DreamerV3), Tasks include: acrobot-swingup, ball_in_cup-catch, cartpole (balance, balance_sparse, swingup, swingup_sparse), cheetah-run, dog (run, stand, trot, walk), finger (spin, turn_easy, turn_hard), fish-swim, hopper (hop, stand), humanoid (run, stand, walk), pendulum-swingup, quadruped (run, walk), reacher (easy, hard), walker (run, stand, walk). State space: proprioceptive (vector), Action space: continuous | Baselines: TD7 (re-run by authors, 10 seeds), TD-MPC2 (re-run by authors, 10 seeds), DreamerV3 (re-run by authors with DMC hyperparameters, 10 seeds), PPO (Stable Baselines 3 default MLP policy, 10 seeds) | Metric: raw episode return capped at 1000, aggregate mean/median/IQM over 10 seeds, 95% stratified bootstrap CI, agents trained for 500k steps (1M frames with action repeat=2).", + "source": "Section 5.1, Appendix B.3, B.4" + }, + { + "id": "mrq-D3-003", + "claim": "Purpose: Evaluate general model-free RL on image-based continuous control tasks. Data: DeepMind Control Suite: 28 tasks (same as proprioceptive benchmark). State space: image-based (last 3 observations, RGB 84x84, action repeat=2), Action space: continuous | Baselines: DrQ-v2 (domain-specific SOTA model-free for visual DMC, author-reported results where available, re-run for missing, 10 seeds), TD-MPC2 (general model-based, re-run by authors, 10 seeds), DreamerV3 (general model-based, re-run by authors, 10 seeds), PPO (general model-free, Stable Baselines 3 default CNN policy, 10 seeds) | Metric: raw episode return capped at 1000, aggregate mean/median/IQM over 10 seeds, 95% stratified bootstrap CI, agents trained for 500k steps (1M frames with action repeat=2).", + "source": "Section 5.1, Appendix B.3, B.4" + }, + { + "id": "mrq-D3-004", + "claim": "Purpose: Evaluate general model-free RL on discrete-action pixel-based Atari games. Data: Atari-57 benchmark: 55 games evaluated (Defender and Surround omitted from DQN/Rainbow comparison due to absence in Dopamine). State space: pixel observations (grayscale 84x84, stack of last 4 max-pooled observations with action repeat=4), Action space: discrete (per-game action count), Environment version: -v5, Sticky actions enabled (p=0.25) | Baselines: DreamerV3 (general model-based, author-reported results), DQN (model-free, results from Dopamine framework), Rainbow (model-free, results from Dopamine framework), PPO (general model-free, Stable Baselines 3 default CNN policy) | Metric: Human-Normalized score = (x - random_score) / (human_score - random_score) with human scores from Wang et al. 2016; aggregate mean/median/IQM, 10 seeds, 95% stratified bootstrap CI, agents trained for 2.5M steps (10M frames with action repeat=4).", + "source": "Section 5.1, Appendix B.3, B.4" + }, + { + "id": "mrq-D3-005", + "claim": "Purpose: Quantify the impact of individual design choices and hyperparameters on MR.Q performance. Data: All four benchmarks: Gym Locomotion (5 tasks), DMC Proprioceptive (28 tasks), DMC Visual (28 tasks), Atari (55 games) | Baselines: default MR.Q configuration as reference; variations tested: Linear value function, Dynamics target (state-action instead of state-only), No target encoder, Revert (all three combined), Non-linear model, MSE reward loss, No reward scaling, No min (mean instead of min over Q-networks), No LAP, No MR (model-based representation removed), 1-step return, No unroll (H_Enc=1) | Metric: Average difference in normalized performance from default MR.Q, over 5 seeds. Impact categories: Negative [-0.01, -0.2), Damaging [-0.2, -0.5), Catastrophic (<=-0.5), Positive (>0.01). Gym: TD3-Normalized, DMC: Raw reward, Atari: Human-Normalized, 95% bootstrap CI.", + "source": "Section 5.2, Table 2" + } + ], + "D4": [ + { + "id": "mrq-D4-001", + "claim": "Phase 1 (Encoder with latent dynamics unrolling): Encode s→z_s via f_ω. Per step t=1..H_Enc: z_sa=g_ω(z_{t-1},a_{t-1}), then MDP predictions (z_s',r,d)=z_sa^T·m. Loss = Σ λ_Reward·CE + λ_Dynamics·MSE(z̃_s',z̄_s') + λ_Terminal·MSE(d̃,d), λ_Terminal=0 until first terminal seen. Target z̄_s' from target encoder f_ω' (state-only). (Section 4.2.1)", + "source": "Section 4.2.1, Eq 12-17" + }, + { + "id": "mrq-D4-002", + "claim": "Phase 2 (Target synchronization every T_target=250 steps): Copy θ'←θ, φ'←φ, ω'←ω, r̄'←r̄. Keeps input/target fixed within each iteration to reduce non-stationarity. Transitions sampled via LAP prioritized replay (α=0.4, min priority=1), buffer capacity 1M, batch size 256, 10k initial random steps. (Section 4.2 pseudocode)", + "source": "Section 4.2 pseudocode block" + }, + { + "id": "mrq-D4-003", + "claim": "Phase 3 (Value update with clipped double Q + multi-step return): Target action a_π=π_φ'(s')+clip(ε,-0.3,0.3), ε~N(0,0.2²). Multi-step return over H_Q=3 with γ=0.99, Huber loss, normalized by 1/r̄. Clipped double Q: min_{j=1,2} Q_θ'_j for bootstrap. Gradient clip norm=20. AdamW lr=3e-4. (Section 4.2.2, Eq 18-19)", + "source": "Section 4.2.2, Eq 18-19" + }, + { + "id": "mrq-D4-004", + "claim": "Phase 4 (Policy update via deterministic policy gradient + exploration): Policy π_φ: 3-layer MLP on z_s, Gumbel-Softmax(τ=10) discrete / Tanh continuous. Loss=-0.5·Σ_i Q_θ_i(z_{s,a_π})+λ·z_π², grad stopped from z_s. Exploration: add Gaussian noise N(0,0.2²) to actions during training. AdamW lr=3e-4. (Section 4.2.3, Eq 20)", + "source": "Section 4.2.3, Eq 20" + } + ] +} \ No newline at end of file diff --git a/papers/navil/blacklist.txt b/papers/navil/blacklist.txt new file mode 100644 index 0000000000000000000000000000000000000000..b2581cd6c367229c240cf79d4bd200ef39006f4c --- /dev/null +++ b/papers/navil/blacklist.txt @@ -0,0 +1,2 @@ +# Official repository (NeurIPS 2025) +https://github.com/OpenGVLab/NaViL diff --git a/papers/navil/config.yaml b/papers/navil/config.yaml new file mode 100644 index 0000000000000000000000000000000000000000..5dbf6f518ba04cf1298c0c4d50c9a753d33c199b --- /dev/null +++ b/papers/navil/config.yaml @@ -0,0 +1,8 @@ +title: "NaViL: Rethinking Scaling Properties of Native Multimodal LLMs under Data Constraints" +pdf_url: "https://arxiv.org/pdf/2510.08565.pdf" +venue: "NeurIPS 2025" +year: "2025" +extra: + selection_index: 7 + domain: "Computer Vision" + paradigm: "Empirical Comparison" diff --git a/papers/navil/images/figures/navil-fig-0001.jpg 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sha256:0354834956341f367209935acf65312f88a67573c01e6d6b456ddd70f0665a82 +size 17555 diff --git a/papers/navil/images/tables/navil-table-0011.jpg b/papers/navil/images/tables/navil-table-0011.jpg new file mode 100644 index 0000000000000000000000000000000000000000..3d40b8a1b1032ccc1f8fe0e15c1e393932e27d3a --- /dev/null +++ b/papers/navil/images/tables/navil-table-0011.jpg @@ -0,0 +1,3 @@ +version https://git-lfs.github.com/spec/v1 +oid sha256:6b6c02b6ff31db3e84e3dd37e5cced24ba5a89c8aee06574ce887aee74ed6d93 +size 18785 diff --git a/papers/navil/paper.md b/papers/navil/paper.md new file mode 100644 index 0000000000000000000000000000000000000000..72bdbf327cc7693d33234aae4ed648ab81d3bb51 --- /dev/null +++ b/papers/navil/paper.md @@ -0,0 +1,499 @@ +# NaViL: Rethinking Scaling Properties of Native Multimodal Large Language Models under Data Constraints + +# Changyao Tian2,1∗† Hao $\mathbf { L i ^ { * \dagger } }$ Gen Luo1∗ Xizhou $\mathbf { Z } \mathbf { h } \mathbf { u } ^ { 3 , 1 * }$ Weijie $\mathbf { S u } ^ { 1 }$ Hanming Deng4 Jinguo $\mathbf { Z } \mathbf { h } \mathbf { u } ^ { 1 }$ Jie Shao5,1† Ziran $\mathbf { Z } \mathbf { h } \mathbf { u } ^ { 4 }$ Yunpeng Liu4 Lewei $\mathbf { L u } ^ { 4 }$ Wenhai Wang2,1 Hongsheng $\mathbf { L i } ^ { 2 }$ Jifeng Dai3,1B + +1 Shanghai AI Laboratory 2 The Chinese University of Hong Kong 3 Tsinghua University 4 Sensetime Research 5 Nanjing University Code: https://github.com/OpenGVLab/NaViL + +# Abstract + +Compositional training has been the de-facto paradigm in existing Multimodal Large Language Models (MLLMs), where pre-trained visual encoders are connected with pre-trained LLMs through continuous multimodal pre-training. However, the multimodal scaling property of this paradigm remains difficult to explore due to the separated training. In this paper, we focus on the native training of MLLMs in an end-to-end manner and systematically study its design space and scaling property under a practical setting, i.e., data constraint. Through careful study of various choices in MLLM, we obtain the optimal meta-architecture that best balances performance and training cost. After that, we further explore the scaling properties of the native MLLM and indicate the positively correlated scaling relationship between visual encoders and LLMs. Based on these findings, we propose a native MLLM called NaViL, combined with a simple and cost-effective recipe. Experimental results on 14 multimodal benchmarks confirm the competitive performance of NaViL against existing MLLMs. Besides that, our findings and results provide in-depth insights for the future study of native MLLMs. + +# 1 Introduction + +Multimodal Large Language Models (MLLMs) have demonstrated remarkable progress in computer vision [12, 43, 63, 50, 54], continuously breaking through the upper limits of various multimodal tasks [47, 68, 38, 45]. The great success of MLLM is inseparable from its compositional training paradigm, which independently pre-trains visual encoders [28] and LLMs [61], and then integrates them through additional multimodal training. Due to the engineering simplicity and effectiveness, this paradigm has dominated MLLM area over the past few years. However, the shortcomings of compositional training have been gradually recognized by the community recently, e.g., unclear multimodal scaling property [19, 56]. + +Therefore, increasing attention has been directed toward the development of more native MLLMs. As illustrated in Fig. 1, native MLLMs aim to jointly optimize both visual and language spaces in an end-to-end manner, thereby maximizing vision-language alignment. Compared to the compositional paradigm, existing native MLLM methods demonstrate a promising scaling law and a significantly simplified training process [9, 56]. Despite these advancements, the primary benefits of native MLLMs are often evaluated under the assumption of infinite training resources, overlooking the substantial challenges posed by limited data and large-scale training. Consequently, a critical practical question remains: whether and how native MLLMs can feasibly achieve or even surpass the performance upper bound of top-tier MLLMs at an acceptable cost. + +![](images/figures/navil-fig-0001.jpg) +Figure 1: Comparison of design choices, scaling properties, and performance of our native MLLMs. We systematically investigate the designs and the scaling properties of native MLLMs under data constraints and yield valuable findings for building native MLLMs. After adopting these findings, our native MLLMs achieve competitive performance with top-tier MLLMs. $\mathcal { V } _ { d , w } ^ { * } ( \cdot )$ denotes the visual encoder with optimal parameter size. + +To answer this question, in this paper, we aim to systematically investigate the designs and the scaling properties of native MLLMs under data constraint. Specifically, we first explore the choices of key components in the native architecture including the mixture-of-experts, the visual encoder and the initialization of the LLM. Our findings can be summarized in two folds. Firstly, an appropriate pre-training initialization (e.g., the base LLM) of the LLM greatly benefits the training convergence on multimodal data. Secondly, combining visual encoder architectures and MoEs results in obvious gains against the vanilla decoder-only LLM. Following these findings, we build a meta architecture that optimally balances performance and training cost. + +Based on the optimal meta architecture, we further explore the scaling properties of the visual encoder, the LLM and the entire native MLLM. Specifically, we first scale up the LLM and the visual encoder independently and observe different scaling properties: while scaling LLM exhibits similar patterns as the conventional language scaling laws, scaling visual encoder shows an upper bound in return due to the limitation of the LLM’s capacity, suggesting that the optimal encoder size varies with the LLM size. Further analysis reveals that the optimal encoder size increases approximately proportionally with the LLM size in log scale. This observation yields a different guidance against compositional paradigm, which employs a visual encoder of one size across all LLM scales. + +Based on above principles, we propose a native MLLM called NaViL, combined with a simple and cost-effective recipe. To validate our approach, we conduct extensive experiments across diverse benchmarks to evaluate its multimodal capabilities including image captioning [10, 67, 2], optical character recognition (OCR) [57, 17, 39], etc. Experimental results reveal that with ${ \sim } 6 0 0 \mathbf { M }$ pretraining image-text pairs, NaViL achieves competitive performance compared to current top-tier compositional MLLMs, highlighting the great practicality and capabilities of NaViL. In summary, our contributions are as follows: + +• We systematically explore the design space and the optimal choice in native MLLMs under data constraint, including the LLM initialization, the visual encoder and the MoEs, and draw three critical findings that greatly benefit the training of native MLLMs. • Based on above findings, we construct a novel native MLLM called NaViL. In NaViL, we explore the scaling properties of the visual encoder and the LLM and indicate their positively correlated scaling relationship. • We conduct large-scale pre-training and fine-tuning experiments on NaViL. Experimental results show that NaViL can achieve top-tier performance with nearly 600M pre-training data. Our findings and results will encourage future work for native MLLMs in the community. + +# 2 Related Work + +Multimodal Large Language Models. Recent years have witnessed the significant progresses of Multimodal Large Language Models (MLLMs) [44, 36, 35, 63, 12], which have dominated various downstream tasks [24, 26, 57, 30]. Starting from LLaVA [36], most existing MLLMs adopt the compositional paradigm, which connects the pre-trained visual encoder [53] and LLM [3] through a projector and finetune them on for alignment. Then, the whole structure will be further fine-tuned on multimodal data for alignment. Based on this paradigm, existing works mainly focus on the improvement of visual encoders [63, 64, 44] and the design of connectors [33, 36]. Despite the progress, such paradigm struggles to explore the joint scaling properties of vision and language. Their potential limitations in training pipeline [56] and vision-language alignment [19] are also gradually recognized by the community. + +Native Multimodal Large Language Models. To overcome the limitations of compositional paradigm, native MLLMs have emerged as another candidate solution [20, 19, 43, 32, 62, 56, 9]. Compared to compositional paradigm, native MLLMs aim to pre-train both vision and language parameters in an end-to-end manner, thus achieving better alignment. The most representative methodology [56, 9] is to directly pre-train the LLM from scratch on large-scale multimodal corpora, which typically requires expensive training costs. To address this issue, recent attempt initialize the LLM with a pre-trained checkpoint to facilitate training convergence [20, 19, 43, 32, 62]. Nevertheless, current research still lacks systematic investigation into the architectural design and scaling characteristics of native MLLMs, limiting their performance. + +# 3 Visual Design Principles for native-MLLM + +# 3.1 Problem Setup + +We define native MLLMs as models that jointly optimize vision and language capabilities in an end-to-end manner. Dispite recent progress that shows promising scaling law and potential better performance compard with their compositional counterparts, how to build competitive native MLLMs compare to the state-of-the-art MLLMs with a practical data scale remains underexplored. In particular, there are two problems requiring to be investigated: + +• (Sec. 3.2) How to choose the optimal architectures of the visual and linguistic components? • (Sec. 3.3) How to optimally scale up the visual and linguistic components? + +Meta Architecture. To study these two questions, we first define a general meta architecture of native MLLMs consisting of a visual encoder, an LLM, and a mixture-of-expert architecture injected to the LLM. The visual encoder $\nu$ consists of a series of transformer layers and can be defined as + +$$ +\mathcal V _ { d , w } ( I ) = \mathcal C \odot \mathcal F _ { d } ^ { w } \odot \cdot \cdot \cdot \odot \mathcal F _ { 2 } ^ { w } \odot \mathcal F _ { 1 } ^ { w } \odot \mathcal P ( I ) = \mathcal C \bigodot _ { i = 1 , . . . d } \mathcal F _ { i } ^ { w } \odot \mathcal P ( I ) , +$$ + +where $\mathcal { F } _ { i } ^ { w }$ denotes the $i$ -th transformer layer (out of $d$ layers) with hidden dimension $w , \mathcal { P }$ denotes the Patch Embedding Layer, $I \in \mathbb { R } ^ { H \times W \times 3 }$ denotes the input image. Note that the visual encoder degenerate to a simple patch embedding layer when $d = 0$ . For simplicity, we use the same architectures as the LLM for the visual encoder layers $\mathcal { F }$ but with bi-directional attention and vary the hyperparameters $d$ and $w$ . Here $\mathcal { C }$ is the connector which downsamples the encoded image embeddings through pixel shuffle [15] and projects them to the LLM’s feature space by a MLP. + +Experiment Settings. All the models are trained on web-scale, noisy image-caption pair data [55] with Next-Token-Prediction (NTP) and an image captioning task. We use a held-out subset of the multimodal dataset to calculate the validation teacher-forcing loss for measuring and comparing different design choices. Models with LLM initializations are initialize from InternLM2-Base [8]. + +# 3.2 Exploring the Optimal Design of Architecture Components + +In this section, we explore the design choices of three key components: 1) the initialization of the LLM; 2) the effectiveness of MoEs; 3) the optimal architecture of the visual encoder. + +# 3.2.1 Initialization of LLM + +A straightforward way to construct native MLLMs is to train all modalities from scratch with mixed corpora, as shown in prior work [56]. While this approach theoretically offers the highest performance ceiling given ample data and computational resources, practical limitations such as data scarcity and large-scale optimization challenges hinder its feasibility. Alternatively, initializing the model from a pre-trained LLM effectively leverages linguistic prior knowledge, significantly reducing data and computational demands. + +To evaluate the effectiveness of LLM initialization, we compare model performance in terms of loss and image captioning. As shown in Fig. 2 (left), the model trained from scratch performs significantly worse than the initialized model, requiring over $1 0 \mathrm { x }$ more data to reach comparable loss. + +Further analysis of zero-shot image captioning (Fig. 2 (right)) reveals a substantial performance gap favoring the initialized model, even with significantly more data for the noninitialized model. This is likely due to the lower textual quality and diversity of multimodal training data compared to the LLM pre-training corpus, lim- + +![](images/figures/navil-fig-0002.jpg) +Figure 2: Effectiveness of LLM initialization. Left: The validation loss. The LLM initialized one converges much faster. Right: The zero-shot caption performance. Due to the lack of textual knowledge, the uninitialized model continues to lag behind. + +iting the textual capability of models trained from scratch. These findings highlight the practical advantage of using LLM initialization in multimodal pre-training. + +Observation 1: Initializing from pre-trained LLM greatly benefits the convergence on multimodal data, and in most cases delivers better performance even with a large amount of multimodal data. + +# 3.2.2 Effectiveness of MoEs + +Mixture-of-Experts (MoEs) are effective for handling heterogeneous data and are widely used in native MLLMs. We evaluate the MoE architecture within our meta architecture by comparing two configurations: one with a visual encoder and a vanilla LLM, and another with a visual encoder and an MoE-extended LLM. We follow Mono-InternVL [43] to adopt the modality-specific MoEs and training settings. However, we empirically found that using only the feed-forward network (FFN) expert would lead to a significant difference in feature scale between visual and language modalities. To mitigate this issue, we further introduced modality-specific attention experts, that is, using different projection layers (i.e. qkvo) in the self-attention layer to process visual and text features respectively, and then perform unified global attention calculation. Specifically, the output $x _ { i , m } ^ { l } \in \bar { \mathbb R } ^ { d }$ of the $i$ -th token with modality $m \in \{ \mathrm { v i s u a l } , \mathrm { l i n g u i s t i c } \}$ at the $l$ -th layer of the MoE-extended LLM can be defined as + +![](images/figures/navil-fig-0003.jpg) +Figure 3: The validation loss of adding MoE or not. Using MoE extension will cause the loss to decrease more quickly. + +$$ +\begin{array} { r l } & { x _ { i , m } ^ { l ^ { \prime } } = x _ { i , m } ^ { l - 1 } + \mathbf { M H A - M M o E } ( \mathrm { R M S N o r m } ( x _ { i , m } ^ { l - 1 } ) ) , } \\ & { x _ { i , m } ^ { l } = x _ { i , m } ^ { l ^ { \prime } } + \mathrm { F F N - M M o E } ( \mathrm { R M S N o r m } ( x _ { i , m } ^ { l ^ { \prime } } ) ) , } \end{array} +$$ + +where $\mathbf { R M S N o r m ( \cdot ) }$ is the layer normalization operation, and MHA-MMoE $\cdot ( \cdot )$ and FFN-MMoE(·) are the modality-specific attention and FFN expert, respectively, formulated by + +$$ +\begin{array} { r l r } & { \underset { \forall Y \mathrm { H A - M M o E } ( x _ { i , m } ) } { \mathrm { M H A - M M o E } } ( x _ { i , m } ) = ( \mathrm { s o f t m a x } ( \frac { Q K ^ { T } } { \sqrt { d } } ) V ) { W _ { O } ^ { m } } , } & \\ & { Q _ { i , m } = x _ { i , m } W _ { Q } ^ { m } , K _ { i , m } = x _ { i , m } W _ { K } ^ { m } , V _ { i , m } = x _ { i , m } W _ { V } ^ { m } , } & \\ & { \quad \mathrm { F F N - M M o E } ( x _ { i , m } ) = ( \mathrm { S i L U } ( x _ { i , m } W _ { \mathrm { g a t e } } ^ { m } ) \odot x _ { i , m } W _ { \mathrm { u p } } ^ { m } ) W _ { \mathrm { d o w n } } ^ { m } . } & \end{array} +$$ + +Here $W _ { Q } ^ { m } , W _ { K } ^ { m } , W _ { V } ^ { m } , W _ { O } ^ { m }$ and $V _ { \mathrm { g a t e } } ^ { m } , W _ { \mathrm { u p } } ^ { m } , W _ { \mathrm { d o w n } } ^ { m }$ are all modality-specific projection matrices, and SiLU(·) denotes the activation function, $\odot$ denotes the element-wise product operation. The number of activated experts is set to one to maintain consistent inference costs. + +As shown in Fig. 3, the MoE architecture significantly accelerates model convergence compared to the vanilla LLM, achieving the same validation loss with only 1/10 of the data without increasing training or inference cost. This demonstrates that MoE enhances model capacity and effectively handles heterogeneous data, making it suitable for native MLLMs. + +Observation 2: MoEs significantly improve model performance without increasing the number of activated parameters. + +# 3.2.3 Optimizing the Visual Encoder Architecture + +![](images/figures/navil-fig-0004.jpg) +Figure 4: The validation loss and zero-shot caption performance of different visual encoders. The loss and performance only differ when the visual encoder is extremely wide or shallow. + +The visual encoder precedes the LLM to perform preliminary extraction of visual information, converting raw pixels into semantic visual features aligned with the textual embedding space. Due to its bidirectional attention mechanism and the increased capacity introduced by additional parameters, the visual encoder has the potential to enhance the model’s ability to represent visual information. + +In this section, we investigate the optimal architecture of the visual encoder under a given parameter budget. The total parameter count $\mathcal { C }$ can be approximately calculated [29] as $\mathcal { N } \overset { = } { = } 1 2 \overset { } { \times } d \times w ^ { 2 }$ . Given a fixed $\mathcal { N }$ , the structure of the visual encoder is mainly determined by its width $w$ and depth $d$ . + +Depth $( d )$ : Typically, deeper models can capture richer and more complex features, while also being more prone to gradient vanishing problems [58]. When it comes to MLLM, a visual encoder that is too shallow may not be able to extract enough high-level semantics, while a visual encoder that is too deep may cause low-level features to be lost, thus limiting the capture of fine-grained details. + +Width $( w )$ : Compared to depth, width has relatively little impact on visual transformer performance [21], as long as it does not cause additional information bottlenecks. That is, it cannot be lower than the total number of channels within a single image patch. Under this premise, the width of the visual encoder does not have to be the same as the hidden size of the LLM. + +We train various MLLMs with different $\gamma _ { d , w }$ configurations (combinations of depth and width) while keeping the pre-trained LLM and visual encoder parameter count fixed at 600M. The depth $d$ ranges from $\{ 3 , 6 , 1 2 , 2 4 , 4 8 \}$ , and the width $w$ is adjusted as $\{ 4 0 9 6 , 2 8 8 0 , 2 0 4 8 , 1 4 7 2 , 1 0 2 4 \}$ to maintain a consistent parameter count. Fig. 4 shows the validation loss for different depth and width combinations as training data size varies. Models with extremely high or low depths perform worse than those with moderate configurations. Among reasonably configured models, shallower ones converge faster in the early phase (less than 30M data), but this advantage diminishes with more data. In zero-shot image captioning benchmarks, deeper visual encoders show slightly better performance, consistent with prior research on compute-optimal LLM architectures [29], which suggests a wide range of optimal width and depth combinations. + +Observation 3: Visual encoders achieve near-optimal performance across a wide range of depth and width configurations. Shallower encoders converge faster in early training, while deeper encoders perform slightly better with larger datasets. + +# 3.3 Scaling Up Native MLLMs + +In this section, we consider the scaling properties of our meta architecture. Specifically, we investigate: 1) the impact of scaling up the visual encoder and the LLM independently; 2) the optimal way of scaling the visual encoder and the LLM simultaneously. All models follow the optimal architecture discovered in Sec. 3.2, i.e., with LLM initialization, MoEs, and optimal depth-to-width ratios of the visual encoders. + +# 3.3.1 Scaling up Visual Encoder and LLM Independently + +We first investigate the scaling properties of the visual encoder and the LLM independently, i.e., scaling up one component while keeping the other fixed. Specifically, we evaluate a series of LLMs with parameter sizes $\{ \bar { 0 } . 5 \bar { B } , 1 . 8 B , 7 B \}$ and visual encoders with sizes $\{ 7 5 M , 1 5 0 M , 3 0 0 M , 6 0 0 M , 1 . 2 B , 2 . 4 B \}$ . + +Scaling up LLMs. The results are shown in Fig. 5. Scaling up the LLM parameters in native MLLMs exhibits a pattern consistent with the conventional LLM scaling law, where the loss decreases linearly as the parameter size increases exponentially. + +Scaling up Visual Encoder. The results are shown in Fig. 6. In contrast to the LLM scaling law, increasing the visual encoder size does not consistently enhance multimodal performance. Instead, with a fixed LLM, the performance gains achieved by enlarging the visual encoder diminish progressively. Beyond a certain encoder size, further scaling results in only marginal loss reduction, indicating that the performance upper limit of the MLLM is constrained by the LLM’s capacity. + +![](images/figures/navil-fig-0005.jpg) +Figure 5: The validation loss when scaling up LLMs. With the same visual encoder (i.e. 600M), the validation loss decreases log-linearly with the LLM size. + +![](images/figures/navil-fig-0006.jpg) +Figure 6: The validation loss curves of different LLMs with different training data sizes. As the training data size increases, the loss gap narrows to near zero when the visual encoder size reaches a certain threshold. + +Observation 4: Scaling the LLM consistently improves multimodal performance, following the typical LLM scaling law. However, increasing the visual encoder size shows diminishing returns, suggesting that the MLLM’s performance is limited by the LLM’s capacity. + +# 3.3.2 Scaling up Visual Encoder and LLM Together + +The diminishing returns from increasing the visual encoder size suggest the existence of an optimal encoder size for a given LLM. We define this optimal size as the smallest encoder whose loss difference compared to an encoder twice its size is less than $\lambda = 1 \%$ of the loss with the 75M encoder (the smallest used in our experiments). Fig. 7 shows the relationship between visual encoder size and LLM size. + +The logarithm of the optimal visual encoder size scales linearly with the logarithm of the LLM size, indicating that both components should be scaled jointly for balanced performance. This highlights the suboptimality of compositional MLLMs, which typically use a fixed visual encoder size across varying LLM scales. + +![](images/figures/navil-fig-0007.jpg) +Figure 7: Relationship of visual encoder size and LLM size. The optimal visual encoder size increases log-linearly with the LLM size. + +Observation 5: The optimal size of the visual encoder scales proportionally with the LLM size in log scale, indicating that both components should be scaled jointly. This further implies that the pre-trained visual encoders using a single pre-trained visual encoder across a wide range of LLM scales like existing compositional MLLMs is suboptimal. + +# 4 NaViL: A Novel Native MLLM with Strong Capabilities + +# 4.1 Architecture + +![](images/figures/navil-fig-0008.jpg) +Figure 8: Architecture of NaViL. As a native MoE-extended MLLM, NaViL can be trained end-to-end and supports input images of any resolution. + +Based on above studies, we construct NaViL with the optimal settings in Sec. 3.1. The architecture is shown in Fig. 8. NaViL inherently supports input images of any resolution. These images are first encoded into visual tokens by the visual encoder and the MLP projector, and then concatenated with the textual tokens to formulate the multimodal token sequence and fed into the LLM. Special tokens and are inserted before and after each image token subsequence to indicate the beginning and end of the image, respectively. Special token is inserted at the end of each row of image tokens to indicate the corresponding spatial position information. + +Visual Multi-scale Packing is further introduced to improve the model performance during inference. Specifically, given an input image $I _ { 0 } \in \mathbb { R } ^ { H _ { 0 } \times W _ { 0 } \times 3 }$ and downsampling rate $\tau$ , a multi-scale image sequence $\{ I _ { i } \in \mathbb { R } ^ { H _ { i } \times W _ { i } ^ { \bot } \times 3 } \} _ { i = 0 } ^ { n }$ is obtained by continuously downsampling the original image (i.e. $H _ { i } = \tau ^ { i } \dot { H _ { 0 } } , W _ { i } = \tau ^ { i } W _ { 0 } \mathrm { , }$ until its area is smaller than a given threshold. These images in the sequence are processed separately by the visual encoder. The obtained visual token embeddings $\{ x _ { i , v } \} _ { i = 0 } ^ { n }$ are then concatenated and fed to the LLM. Special token is inserted after each scale image to indicate the end of different scales. + +# 4.2 Training + +Stage 1: Multi-modal Generative Pre-training. In this stage, the model is initially trained on 500 million image-text pairs to develop comprehensive multimodal representations. Of these training samples, 300 million are directly sampled from web-scale datasets (i.e. Laion-2B [55], Coyo-700M [7], Wukong [25] and SA-1B [31]) while the remaining 200 million consist of images from these datasets paired with captions synthesized by existing MLLMs (i.e. InternVL-8B [15]). During this process, the textual parameters of the model remain frozen, with only the newly-added visionspecific parameters (i.e., the visual encoder, MLP projector, and MoE visual experts) being trainable. + +To enhance the alignment between visual and textual features in more complex multimodal contexts, the model is subsequently trained on 185 million high-quality data consisting of both multimodal alignment samples and pure language data. In this phase, the textual parameters within the selfattention layers are also unfrozen, enabling more refined cross-modal integration. + +Table 1: Comparison with existing MLLMs on general MLLM benchmarks. “#A-Param” denotes the number of activated parameters. †InternVL-2.5-2B adopts the same LLM and high-quality data with NaViL, so we mark it as the compositional counterpart. Note that its 300M visual encoder is distilled from another 6B large encoder. Bold and underline indicate the best and the second-best performance among native MLLMs, respectively. \* denotes our reproduced results. For MME, we sum the perception and cognition scores. Average scores are computed by normalizing each metric to a range between 0 and 100. + +
Model#A-ParamAvgMMVetMMMUMMBMMEMathVistaOCRBenchCCB
Compositional MLLMs:
MobileVLM-V2-1.7B [16]1.7B57.7
MobileVLM-V2-3B [16]3.0B63.2
Mini-Gemini-2B [34]3.5B31.131.759.8165329.4
MM1-3B-MoE-Chat [48]3.5B42.238.670.8177232.6
DeepSeek-VL-1.3B [40]2.0B42.334.832.264.6153231.140937.6
PaliGemma-3B [6]2.9B45.633.134.971.0168628.761429.6
MiniCPM-V-2 [66]2.8B51.141.038.269.1180938.760545.3
InternVL-1.5-2B [14]2.2B54.739.334.670.9190241.165463.5
Qwen2VL-2B [63]2.1B58.649.541.174.9187243.080953.7
†InternVL-2.5-2B [13]2.2B67.060.843.674.7213851.380481.7
Native MLLMs:
Fuyu-8B (HD) [5]8B21.410.7
SOLO [11]7B126034.4
Chameleon-7B1 [9]7B13.98.325.431.117022.373.5
EVE-7B [19]7B33.025.632.349.5148325.232712.4
EVE-7B (HD) [19]7B37.025.732.652.3162834.239816.3
Emu3 [65]8B37.231.658.5687
VoRA [62]7B33.732.264.21674
VoRA-AnyRes [62]7B33.732.061.31655
EVEv2 [20]7B53.245.039.366.3170960.0*70230.8*
SAIL [32]7B53.746.338.6*70.1171957.078324.3*
Mono-InternVL [43]1.8B56.440.133.765.5187545.776766.3
NaViL-2B (ours)2.4B67.178.341.871.2182250.079683.9
+ +Stage 2: Supervised Fine-tuning. Following common practice in developing MLLM, an additional supervised fine-tuning stage is adopted. In this stage, all parameters are unfrozen and trained using a relatively smaller (i.e. 68 million) but higher quality multimodal dataset. + +# 5 Experiment + +# 5.1 Experimental Setups + +Evaluation Benchmarks. We evaluate NaViL and existing MLLMs on a broad range of multimodal benchmarks. Specifically, MLLM benchmarks encompass MMVet [69], MMMU val [70], MMBench-EN test [37], MME [22], MathVista MINI [41], OCRBench [39], and CCBench [37]. Visual question answering benchmarks include TextVQA val [57], ScienceQA-IMG test [42], GQA test dev [27], DocVQA test [47], AI2D test [30], ChartQA test [45], and InfographicVQA test [46]. These benchmarks cover various domains, such as optical character recognition (OCR), chart and document understanding, multi-image understanding, real-world comprehension, etc. + +Implementation Details. By default, NaViL-2B is implemented upon InternLM2-1.8B [59], using its weights as initialization for the text part parameters. The text tokenizer and conversation format are also the same. The total number of parameters is 4.2B, of which the number of activation parameters is 2.4B (including 0.6B of visual encoder). The input images are first padded to ensure its length and width are multiples of 32. The stride of Patch Embedding layer is set to 16. The visual encoder adopts bidirectional attention and 2D-RoPE to capture global spatial relationships, while the LLM adopts causal attention and 1D-RoPE to better inherit its capabilities. In the pre-training phase, the global batch size is 7000 for stage 1 and 4614 for stage 2, respectively. The downsampling rate $\tau$ of visual multi-scale packing is set to ${ \sqrt { 2 } } / 2$ . To demonstrate the scaling capability of our approach, we also trained NaViL-9B based on Qwen3-8B [60]. More details are given in the appendix. + +Table 2: Comparison with existing MLLMs on visual question answering benchmarks. †InternVL-2.5-2B adopts the same LLM and high-quality data with NaViL, so we mark it as the compositional counterpart. Note that its 300M visual encoder is distilled from another 6B large encoder. \* denotes our reproduced results. Bold and underline indicate the best and the second-best performance among native MLLMs, respectively. + +
Model#A-ParamAvgTextVQASQA-IGQADocVQAAI2DChartQAInfoVQA
Compositional MLLMs:
MobileVLM-V2-3B [16]3.0B57.570.066.1
Mini-Gemini-2B [34]3.5B56.234.2
MM1-3B-MoE-Chat [48]3.5B72.976.1
DeepSeek-VL-1.3B [40]2.0B57.851.5
PaliGemma-3B [6]2.9B68.168.3
MiniCPM-V-2 [66]2.8B74.171.962.9
InternVL-1.5-2B [14]2.2B71.770.584.961.685.069.874.855.4
Qwen2VL-2B [63]2.1B73.179.778.2*60.3*90.174.773.565.5
†InternVL-2.5-2B [13]2.2B76.574.396.261.288.774.979.260.9
Native MLLMs:
Fuyu-8B (HD) [5]8B64.5
SOLO [11]7B73.361.4
Chameleon-7B1 [9]7B17.94.847.21.546.02.95.0
EVE-7B [19]7B40.851.963.060.822.048.519.520.0
EVE-7B (HD) [19]7B54.656.864.962.653.061.059.125.0
Emu3 [65]8B67.664.789.260.376.370.068.643.8
VoRA [62]7B56.375.965.6
VoRA-AnyRes [62]7B58.772.0-61.1
EVEv2 [20]7B71.771.196.262.977.4*74.873.945.8*
SAIL [32]7B71.577.193.358.0*78.4*76.769.7*47.3*
Mono-InternVL [43]1.8B70.172.693.659.580.068.673.743.0
NaViL-2B (ours)2.4B75.176.995.059.885.474.678.056.0
+ +# 5.2 Main Results + +In Tab. 1, we compare the performance of our model with existing MLLMs across 7 multimodal benchmarks. Compared to native MLLMs, compositional MLLMs demonstrate superior overall performance. For example, InternVL-2.5-2B outperforms existing native MLLMs on most MLLM benchmarks. This indicates that current native MLLMs still have significant room for performance improvement. In contrast, our proposed NaViL achieves overall performance exceeding all existing native MLLMs with a relatively small paramter size. Compared to the compositional baseline model InternVL-2.5-2B that uses the same LLM, NaViL also achieves comparable performance on most benchmarks. It is worth noting that the 300M visual encoder used by InternVL-2.5-2B is distilled from another pre-trained encoder InternViT-6B [15] with a significantly larger parameter size. This demonstrates the superiority of our visual design methods and visual parameter scaling strategies. + +In Tab. 2, we further compare the performance of our model with existing MLLMs on mainstream visual question answering tasks. NaViL’s average performance still leads previous state-of-the-art native MLLMs and is roughly on par with compositional baselines that require pre-trained encoders. Specifically, in tests such as DocVQA [49], ChartQA [45] and InfoVQA [46], NaViL significantly outperforms the previous state-of-the-art native MLLM, demonstrating the superiority of using an optimal size visual encoder in processing high-resolution images. However, NaViL’s performance still has some gap compared to the best compositional MLLMs. We believe that higher-quality instruction data and more powerful LLMs will further narrow this gap. + +![](images/figures/navil-fig-0009.jpg) +Figure 9: Visualization of attention maps in LLM-1.8B with different encoder sizes (i.e. 150M and 1.2B). Text and image tokens are in blue and green, respectively. Larger encoder allows LLMs to attend to global patterns at shallow layers while maintaining higher attention to textual tokens. + +# 5.3 Qualitative Experiments + +To further analyze the characteristics of native MLLM, we visualized the attention maps of different LLM layers when using encoders of 150M and 1.2B sizes, as shown in Fig. 9. Two findings can be drawn from the figure. First, similar to previous native-MLLMs [43], despite having an encoder, the attention patterns in shallow layers still exhibit obvious locality, gradually shifting toward global information as the depth increases. For example, when using a 150M encoder, image tokens in the first layer tend to attend to spatially adjacent tokens. However, we observe that when the visual encoder is scaled up to 1.2B, visual tokens in shallow layers already begin to attend more to global information. This indicates that a sufficiently large visual encoder can better pre-extract high-level semantic information from the entire image. + +Secondly, from a cross-modal interaction perspective, a larger visual encoder also facilitates earlier interaction between visual and language features. When using a 1.2B visual encoder, the attention weights between visual tokens and text tokens in the first layer are significantly higher than those in the 150M counterpart. Earlier interaction is more beneficial for feature alignment between modalities, thus providing an explanatory perspective for the improved performance achieved when using larger encoder sizes. We believe these findings will provide beneficial insights for developing native MLLMs. More visualizations can be found in the supplementary materials. + +# 6 Conclusion + +This paper systematically investigates native end-to-end training for MLLMs, examining its design space and scaling properties under data constraints. Our study reveals three key insights: 1) Initialization with pre-trained LLMs, combined with visual encoders and MoE architecture, significantly improves performance; 2) Visual encoder scaling is limited by the LLM’s capacity, unlike traditional LLM scaling; 3) The optimal encoder size scales log-proportionally with the LLM size. Based on these findings, we propose NaViL, a native MLLM that achieves competitive performance on diverse multimodal benchmarks, outperforming existing compositional MLLMs. We hope these insights will inspire future research on next-generation MLLMs. + +Limitations and Broader Impacts. Due to limited computation resources, this paper only investigates the scaling properties of native MLLMs up to 9B parameters. Subsequent experiments with larger scales (e.g., 30 billion, 70 billion, 100 billion, etc.) can be conducted to further validate this scaling trend. In addition, this paper focuses only on visual and linguistic modalities. Future research may explore broader modalities and provide more in-depth insights beyond the current visual-linguistic paradigm. + +Acknowledgments The work is supported by the National Key R&D Program of China (NO. 2022ZD0161300, and NO. 2022ZD0160102), by the National Natural Science Foundation of China (U24A20325, 62321005, 62376134), and by the China Postdoctoral Science Foundation (No. BX20250384). + +# References + +[1] Armen Aghajanyan, Lili Yu, Alexis Conneau, Wei-Ning Hsu, Karen Hambardzumyan, Susan Zhang, Stephen Roller, Naman Goyal, Omer Levy, and Luke Zettlemoyer. Scaling laws for generative mixedmodal language models. In International Conference on Machine Learning, pages 265–279. PMLR, 2023. +[2] Harsh Agrawal, Karan Desai, Yufei Wang, Xinlei Chen, Rishabh Jain, Mark Johnson, Dhruv Batra, Devi Parikh, Stefan Lee, and Peter Anderson. Nocaps: Novel object captioning at scale. 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The training recipe is similar to NaViL-2B, as shown in Tab. 8, except the visual multiscaling packing is disabled in the first sub-stage of pre-training for acceleration. + +Tab. 3 presents a comparison of the total training tokens required by our method versus two compositional counterparts. Notably, our approach achieves comparable performance while using substantially fewer training tokens, demonstrating improved training efficiency. + +Table 3: Comparison between NaViL and existing MLLMs on the number of training tokens. + +
ModelsTrain ViTTrain MLLMTotal
Qwen2.5VL [4]unknown4.1T>4.1T
Intern VL2.5-8B [12]>3.3T140B>3.5T
NaViL-2B (ours)0800B800B
NaViL-9B (ours)0450B1450B
+ +The performance results on multimodal and visual question answering benchmarks are shown in Tab. 4. With a similar parameter size, our $\mathrm { N a V i L - 9 B }$ outperforms all existing native MLLMs by a large margin on almost all benchmarks. Besides that, compared to the compositional baseline model InternVL-2.5-8B with a similar parameter size, NaViL-9B also achieves competitive performance. Such results show that our proposed native MLLM can be scaled up to larger parameter sizes and achieve consistent performance gains. + +# B More discussions on Compositional MLLMs and Native MLLMs + +![](images/figures/navil-fig-0010.jpg) +Figure 10: Paradigm Comparison between Compositional MLLMs and Native MLLMs. Compositional MLLMs adopt different training objectives and strategies (e.g. Contrastive Loss or Next-Token-Prediction) to pre-train the visual encoder and LLM separately, while native MLLMs optimize both image and text components in an end-to-end manner using a unified training objective (i.e. Next-Token-Prediction). + +Fig. 10 further illustrates the difference between compositional MLLMs and native MLLMs. Compositional MLLMs typically have different components initialized by separate unimodal pre-training, where different training objectives and strategies are employed to train the LLM and visual encoder. For example, the visual encoder can be trained using an image-text contrastive learning objective (e.g., CLIP [52], SigLIP [72]) or a self-supervised learning objective (e.g., DINOv2 [51]). The complexity of such training process increases the difficulty of scalability. On the other hand, as discussed in [56], native MLLM optimizes both image and text modalities end-to-end using a unified training objective (i.e., next-token prediction (NTP)). This avoids introducing additional bias and significantly simplifies the scaling effort. + +# C More Related Works + +Research on Neural Scaling Laws. The foundational work on Neural Scaling Laws began in the Natural Language Processing (NLP) domain, where [29] established predictable power-law relationships demonstrating that performance loss $( L )$ scales reliably with model size $( N )$ and data size $( D )$ , and that larger, decoder-only Transformer models are more compute-efficient. Following works [23] further extended such research to encoder-decoder architectures, observing consistency in scaling exponents on Neural Machine Translation (NMT) tasks. Driven by these successes, in the vision domain, [71] confirmed the applicability of scaling laws to Vision Transformers (ViT), systematically demonstrating continuous performance improvement by scaling both model size (up to 2 billion parameters) and training data. Most recently, these principles have been generalized to Large Multimodal Models, where [1] developed scaling laws that unify the contributions of text, image, and speech modalities by explicitly modeling synergy and competition as an additive term. Furthering this, [56] explored Native Multimodal Models (NMMs) using Mixture of Experts (MoEs), finding an unbalanced scaling law that suggests scaling training tokens $( D )$ is more critical than scaling active parameters $( N )$ as the compute budget grows. + +# D Implementation Details + +The hyperparameters of model architecture for NaViL-2B and NaViL-9B are listed in Tab. 6, while the hyperparameters of training recipe for NaViL-2B and NaViL-9B are provided in Tab. 7 and Tab. 8, respectively. The high-quality multimodal data used in Pre-training and Supervised Fine-tuning is from InternVL-2.5 [12], which is sourced from various domains, such as image captioning, general question answering, multi-turn dialogue, charts, OCR, documents, and knowledge, etc.; while the pure language data is primarily from InternLM2.5 [8]. + +# E The NLP capability + +We also evaluate the NLP capability of our model on three popular NLP tasks, as shown in Tab. 5. Thanks to the modality-specific MoE architecture, NaViL maintains the NLP capabilities of its initialization LLM (Qwen3-8B). Despite not using a large amount of high-quality text data, NaViL performs well on the common NLP tasks and show much stronger NLP capabilities compared to other native MLLMs, showing its data efficiency. + +# F More Qualitative Results + +More visualization results of multimodal understanding are provided below. + +Table 4: Comparison between NaViL-9B and existing MLLMs on multimodal benchmarks. “#A-Param” denotes the number of activated parameters. †InternVL-2.5-8B adopts the same highquality data with NaViL-9B, so we mark it as the compositional counterpart. Note that its $3 0 0 \mathbf { M }$ visual encoder is distilled from another 6B large encoder. \* denotes our reproduced results. Bold and underline indicate the best and the second-best performance among native MLLMs, respectively. For MME, we sum the perception and cognition scores. Average scores are computed by normalizing each metric to a range between 0 and 100. + +
Model#A-Param Avg MMVet MMMU MMB MME MathVista OCR-B TVQA DocVQA AI2D ChartQA InfoVQA
Compositional MLLMs:
MobileVLM-V2 [16]1.7B57.7
MobileVLM-V2 [16]3.0B63.257.5
Mini-Gemini [34]3.5B31.131.759.8165329.456.234.2
MM1-MoE-Chat [48]3.5B42.238.670.8177232.672.9
DeepSeek-VL [40]2.0B34.832.264.6153231.140957.851.5
PaliGemma [6]2.9B33.134.971.0168628.761468.168.3/
MiniCPM-V-2 [66]2.8B41.038.269.1180938.760574.171.962.9
InternVL-1.5 [14]2.2B61.339.334.670.9190241.165470.585.069.874.855.4
Qwen2VL [63]2.1B[67.349.541.174.9187243.080979.790.174.773.565.5
InternVL-2.5 [13]2.2B69.660.843.674.7213851.380474.388.774.979.260.9
Qwen2VL [63]8.2B[77.162.054.183.0232758.286684.394.583.083.076.5
Qwen2.5-VL [4]8.2B|80.267.158.683.5234768.286484.995.783.987.382.6
†InternVL-2.5 [13]8.1B[77.362.856.084.6234464.482279.191.984.584.875.7
Native MLLMs:
Fuyu-8B (HD) [5]8B21.410.764.5
SOLO [11]7B126034.4− —61.4
Chameleon-7B2 [9]7B[14.08.325.431.117022.3— 74.81.546.0− 2.9− 5.0
EVE-7B [19]7B[34.625.632.349.5148325.232751.922.048.519.520.0
EVE-7B (HD) [19]7B45.225.732.652.3162834.239856.853.061.059.125.0
Emu3 [65]8B37.231.658.568764.776.370.068.643.8
VoRA [62]7B33.732.264.2167456.365.6
VoRA-AnyRes [62]7B33.732.061.31655− −58.761.1
EVEv2 [20]7B62.345.039.366.3170960.0*70271.177.4*74.8− 73.9− 45.8*
SAIL [32]7B[63.746.338.6*70.1171957.078377.178.4*76.769.7*47.3*
Mono-InternVL [43]1.8B[60.640.133.765.5187545.776772.680.068.673.743.0
NaViL-2B (ours)2.4B68.878.341.871.2182250.079676.985.474.678.056.0
NaViL-9B (ours)9.2B77.079.654.776.5222566.783777.290.682.485.470.2
+ +Table 5: Comparison of NaViL and existing native MLLMs on three common NLP tasks. Except for Chameleon, models are evaluated using OpenCompass toolkit [18]. + +
Models#A-ParamMMLUCMMLUMATH
InternLM2-Chat [59]1.8B47.146.113.9
Qwen3-8B (non-thinking) [60]8B76.576.871.1
EVE [19]7B43.933.40.7
Chameleon [9]7B52.1-11.5
Mono-InternVL [43]2B45.144.012.3
NaViL-9B (ours)9.2B74.975.166.2
+ +Table 6: Hyper-Parameters of Model Architecture. + +
ComponentHyper-ParameterNaViL-2BNaViL-9B
visual encoder# Params0.6B1.2B
depth2432
width14721792
MLP width58887168
# attention heads2328
LLM (w/ MoE)# experts22
# A-Params1.8B8.0B
depth2436
width20484096
MLP width819212288
# attention heads1632
+ +Table 7: Hyper-parameters for training NaViL-2B. + +
ConfigurationMulti-modal Generative Pre-training (S1)S1.1 S1.2SupervisedFine-tuning (S2)
Maximum number of image patchesTraining stepsGlobal batch sizeWeight decayLearning rate schedulePeak learning rateVisual Multi-scale PackingLLM max sequence lengthWarm-up stepsOptimizerOptimizer hyperparametersGradient accumulationNumerical precision4096 1218870k 40k7,000 4,6140.05 0.12457630k
2, 2340.01
constant with warm-up cosine decay5e-5 2e-5✓16, 384200AdamWβ1 = 0.9, β2 = 0.95, eps = 1e−81bfloat16
+ +Table 8: Hyper-parameters for training NaViL-9B. + +
Configuration S1.1Multi-modal Generative Pre-training (S1) S1.2Supervised Fine-tuning (S2)
Maximum number of image patches Training steps4096 50k 0.051218824576
10, 300 X33k6k
Weight decay0.10.01
Global batch size1, 7923, 520
Visual Multi-scale Packing
Learning rate scheduleconstant with warm-upcosine decay 2e-5
Peak learning rate5e-5
LLM max sequence length16, 384
Warm-up steps200 AdamW
Optimizer Optimizer hyperparametersβ1 = 0.9, β2 = 0.95, eps = 1e−8
Gradient accumulation
1
Numerical precisionbfloat16
+ +![](images/figures/navil-fig-0011.jpg) + +Prompt: Describe this image in detail. + +Response: The image shows an aerial view of a modern architectural complex situated near a coastal area. The building has a distinctive design with a series of curved, wave-like structures on its roof, giving it a futuristic appearance. The exterior is primarily composed of light-colored materials, possibly concrete or stone, with large glass windows. + +In front of the building, there is a wide, multi-lane road lined with palm trees, suggesting a warm, coastal climate. The road appears to be a major thoroughfare, with designated bike lanes and pedestrian pathways. There are also train tracks running parallel to the road, indicating the presence of a public transportation system. + +The background features a large body of water, likely a bay or ocean, with several boats and yachts docked at a marina. The coastline is lined with more buildings, and there are hills or mountains visible in the distance, adding to the scenic view. The overall setting suggests a bustling urban area with a focus on modern infrastructure and a strong connection to the waterfront. + +Prompt: Which city is this building most likely in? + +Response: This building is most likely in San Diego, California. The architectural style and the surrounding landscape, including the water and palm trees, are characteristic of San Diego. + +![](images/figures/navil-fig-0012.jpg) + +Prompt: Who is the person in black in the middle? + +Response: The person in black in the middle is Lionel Messi. + +Prompt: What are they celebrating about? + +Response: The image shows a group of soccer players celebrating with a trophy, wearing medals and jerseys. They are likely celebrating a victory in a soccer tournament or championship. + +![](images/figures/navil-fig-0013.jpg) + +Prompt: What is the content in this image? + +Response: The image appears to be a screenshot from a video game, likely Grand Theft Auto V (GTA V), given the visual style and interface elements. It shows a character standing on a sidewalk in a small town or suburban area. The environment includes a street with tram tracks, a few buildings with signs, and some cars parked along the road. The buildings have signs for businesses like "Wine" and "On The Bone." The scene has a somewhat deserted look, with no other characters or significant activity visible. + +
NUTRITIONAL INFORMATION
Serving size: 10 gServings per package: 25 Average
Average Quantity Quantity Per Serving Per 100g
Energy Protein Fat, total -saturated Carbohydrates Sugars Sodium143kJ 1430 kJ less than 0.1g 0.16g less than G.1g 0.37g less than 0.1g 0.26g 8.31g 83.1g 8.11g 81.1g 041ma 4.04mg
INGREDIENTS: 100% Pure New Zealand Honey
* Keep Stored Below 20 deg"C, BPA-Freg Contamner. Not suitable for children under 1 year of age.
+ +Prompt: Extract all the text in this image. + +Response: NUTRITIONAL INFORMATION +Serving size: $1 0 \ \mathrm { g }$ +Servings per package: 25 +Average Quantity Per Serving | Average Quantity Per $1 0 0 \mathrm { g }$ Energy | 143kJ | 1430 kJ +Protein | less than $0 . 1 \mathrm { g } \mid 0 . 1 6 \mathrm { g }$ +Fat, total | less than $0 . 1 \mathrm { g } \mid 0 . 3 7 \mathrm { g }$ +- saturated | less than $0 . 1 \mathrm { g } \mid 0 . 2 6 \mathrm { g }$ +Carbohydrates $| 8 . 3 1 \mathrm { g } \mid 8 3 . 1 \mathrm { g }$ +Sugars | 8.11g | 81.1g +Sodium $\mid 0 . 4 1 \mathrm { m g } \mid 4 . 0 4 \mathrm { m g }$ +INGREDIENTS: $100 \%$ Pure New Zealand Honey +Keep Stored Below $2 0 \mathrm { d e g ^ { \circ } C }$ , BPA-Free Container. +Not suitable for children under 1 year of age. + +Food = Lunch × 5 pephe + +CHAPELBANK HOTEL +69 EAST HIGH STREET +FORFAR DD8 2EP +TEL NO 01307 463151 +\*\*\*\*\*\*\*\*\*\*\*\* + +Prompt: Describe the image. + +
TABLE RESTAURANT
3x SOUP 3.95 11.85 Lunch Main
Lasagne (V) 6.95
MISC FOOD 6.95
2x CHIPS / ONION RINGS 1.75 3.50 -FOOD 29.25--
Prev Bal: 29.25
SUBTOTAL 29.25
+ +Response: The image is a receipt from Chapelbank Hotel for a lunch order for five people. The items listed on the receipt include 3x soup at $\pounds 3 . 9 5$ each, totaling £11.85; lunch main course at £6.95; lasagne (V) at £6.95; misc food at £6.95; and 2x chips/onion rings at $\pounds 1 . 7 5$ each, totaling £3.50. The subtotal for the food is £29.25. The receipt also includes a previous balance of £29.25, making the total amount due £29.25. The hotel’s address is 69 East High Street, Forfar DD8 2EP, and the telephone number is 01307 463151. + +Prompt: How many people are attending this lunch? + +Response: There are 5 people attending this lunch. This is indicated at the top of the receipt where it says "Food $=$ Lunch $_ { \textrm { X 5 } }$ people". + +Prompt: Where is the left exit going to? + +Response: The left exit is going to Reston, Herndon, and Dulles Airport. + +Prompt: Which road leads to the EXIT 46 B? + +Response: The road that leads to EXIT $4 6 \mathrm { B }$ is Chain Br Rd North. + +![](images/figures/navil-fig-0014.jpg) + +$\mathbf { P r o m } \mathbf { p t } ;$ 请先提取 面的文本,再翻译为英文。 + +# Response: + +文本内容: + +作文(60分) 目:专注目标,享受 命。 + +题 生目标之光, 月光清辉; 命之辉, 群星闪耀。 去了群星,月色便显 冷寂;不见了明如 生 如 失 得月,群星也势 黯然 色。只有专注于月光,方有方向;只有 赏群星,方可享受 命。星必 失月交辉, 人;专注目标,享受 命,方可体味人间清 。 + +夜翻译为英文: + +Essay (60 points) + +Title: Focus on your goal, enjoy life. + +The light of the goal is like the clear glow of moonlight; the brilliance of life is like the dazzling stars. Without the stars, the moonlight becomes cold and desolate; without the moon, the stars will inevitably become dull. Only by focusing on the moonlight can you find direction; only by appreciating the stars can you enjoy life. The stars and moon shine together, making the night sky enchanting; focusing on your goal, enjoying life, allows you to savor the quiet joy of life. + +# Document Understanding + +![](images/figures/navil-fig-0015.jpg) + +Prompt: What is the duration of this venue sponsorship agreement? + +Response: January 1, 2001 - December 31, 2001 + +Prompt: Where is Vitucci’s located (complete address)? + +Response: 1832 East North Avenue, Milwaukee WI 53202 + +Prompt: What cigarette brand is mentioned in the agreement for delivery and distribution? + +Response: Camel cigarette brand styles purchased by Venue Owner for resale in Venue + +Prompt: Who has discretion over the distribution of flyers for special events? + +Response: KBA Marketing Group ("KBA") has discretion over the distribution of flyers for special events. + +# LaTeX and Math + +$$ +\begin{array} { l } { \displaystyle \mathcal { J } _ { G R P O } ( \theta ) = \mathbb { E } [ q \sim P ( Q ) , \{ \alpha _ { i } \} _ { i = 1 } ^ { G } \sim \pi _ { \theta _ { 0 d } } ( O | q ) ] } \\ { \displaystyle \frac { 1 } { G } \sum _ { i = 1 } ^ { G } \frac { 1 } { | \alpha _ { i } | } \sum _ { t = 1 } ^ { | \alpha _ { i } | } \left\{ \operatorname* { m i n } \left[ \frac { \pi _ { \theta } \left( \sigma _ { i , t } | q , \sigma _ { i , c } , t \right) } { \pi _ { \theta _ { d d } } \left( \sigma _ { i , t } | q , \sigma _ { i , c } , t \right) } \hat { A } _ { i , t } , \mathrm { c i l p } \left( \frac { \pi _ { \theta } \left( \sigma _ { i , t } | q , \sigma _ { i , c } , t \right) } { \pi _ { \theta _ { d d } } \left( \sigma _ { i , t } | q , \sigma _ { i , c } , t \right) } , 1 - \varepsilon , 1 + \varepsilon \right) \hat { A } _ { i , t } \right] - \beta \mathbb { D } _ { K L } \left[ \pi _ { \theta } | | \pi _ { r e f } | \right] \right\} , } \end{array} +$$ + +Prompt: Convert the formula in the figure to latex format. + +# Response: + +‘‘‘latex +\begin{equation\*} \begin{split} \mathcal{J}_{\text{GRPO}}(\theta) $\& =$ \mathbb{E}[q \sim P(Q), \{o_i\}_{i=1}^G \sim \pi_{\theta_{old}}(O|q)] \\ & \frac{1}{G} \sum_ $\{ \mathsf { i } = 1 \} \mathsf { \hat { G } }$ \frac{1}{|o_i|} \sum_{t=1}^{|o_i|} \left\{ \min \left[ \frac{\pi_{\theta}(o_{i,t}|q, o_{i,Weight of winning dog at a dog showYearWeight (kg)201327201428201534201629201740 + +From the table: - Weight in $2 0 1 6 = 2 9 \mathrm { k g }$ - Weight in 2017 = 40 kg Substitute these values into the formula: + +$$ +{ \mathrm { R a t e ~ o f ~ C h a n g e } } = { \frac { 4 0 \mathbf { k g } - 2 9 \mathbf { k g } } { 2 0 1 7 - 2 0 1 6 } } = { \frac { 1 1 \mathbf { k g } } { 1 \mathbf { y e a r } } } +$$ + +Rate of Change $= 1 1$ kg/year + +Thus, the rate of change between 2016 and 2017 is 11 kg/year . \ No newline at end of file diff --git a/papers/navil/paper.pdf b/papers/navil/paper.pdf new file mode 100644 index 0000000000000000000000000000000000000000..df705f9052d79a10eeeee7fb16d5dd11cb4168ff --- /dev/null +++ b/papers/navil/paper.pdf @@ -0,0 +1,3 @@ +version https://git-lfs.github.com/spec/v1 +oid sha256:248fcf28d2b9210d4048a207d9a9bd3138275a956cc031f21ecbfd6db489a984 +size 7678639 diff --git a/papers/navil/sau.json b/papers/navil/sau.json new file mode 100644 index 0000000000000000000000000000000000000000..12369e0ceeff1cbc6b7549a0f30c38bbfeec9be1 --- /dev/null +++ b/papers/navil/sau.json @@ -0,0 +1,227 @@ +{ + "paper_id": "navil", + "paper_title": "NaViL: Rethinking Scaling Properties of Native Multimodal LLMs under Data Constraints", + "D1": [ + { + "id": "navil-D1-001", + "claim": "Visual encoder architecture search: param budget=600M, depth candidates=[3,6,12,24,48] layers, width candidates=[4096,2880,2048,1472,1024] hidden dims, all configurations trained from scratch for architecture comparison", + "source": "Sec 3.2.3" + }, + { + "id": "navil-D1-002", + "claim": "Scaling study independent variables: LLM sizes=[0.5B,1.8B,7B] params, visual encoder sizes=[75M,150M,300M,600M,1.2B,2.4B] params; fixed 600M encoder used when varying LLM size", + "source": "Sec 3.3.1" + }, + { + "id": "navil-D1-003", + "claim": "Optimal encoder selection criterion (scaling study): lambda_threshold=0.01, base_encoder=75M; select smallest encoder whose validation loss difference vs 2x larger encoder is <1% of 75M baseline loss", + "source": "Sec 3.3.2" + }, + { + "id": "navil-D1-004", + "claim": "NaVIL-2B architecture: total_params=4.2B, activated_params=2.4B, visual_encoder=0.6B params, LLM_base=InternLM2-1.8B (activated=1.8B), MoE routing with shared experts", + "source": "Sec 4.1, Sec 5.1, Tab 6" + }, + { + "id": "navil-D1-005", + "claim": "NaVIL-9B architecture: activated_params=9.2B, visual_encoder=1.2B params, LLM_base=Qwen3-8B (activated=8.0B), scaled-up variant of NaVIL design", + "source": "Appendix A, Tab 6" + }, + { + "id": "navil-D1-006", + "claim": "NaVIL-2B three-stage training: Stage1.1 (max_patches=4096, steps=70K, batch=7000, peak_lr=5e-5), Stage1.2 (steps=40K), Stage2 (fine-tuning on high-quality multimodal data)", + "source": "Tab 7" + }, + { + "id": "navil-D1-007", + "claim": "NaVIL-9B three-stage training: Stage1.1 (max_patches=4096, steps=50K, batch=10792), Stage1.2 (steps=33K), Stage2 (steps=6K fine-tuning)", + "source": "Tab 8" + }, + { + "id": "navil-D1-008", + "claim": "Training data composition: Stage1.1=500M image-text pairs (300M web-sampled + 200M synthetic captions), Stage1.2=185M high-quality multimodal+language, Stage2=68M high-quality multimodal data", + "source": "Sec 4.2" + }, + { + "id": "navil-D1-009", + "claim": "Visual multiscale packing: downsampling_rate tau=sqrt(2)/2 per iteration, stop when image_area < threshold, special token separates scale representations", + "source": "Sec 4.1" + }, + { + "id": "navil-D1-010", + "claim": "MoE early convergence finding: MoE achieves same validation loss as dense with only 1/10 of training data (data efficiency factor=10x), confirmed in architecture search experiments", + "source": "Sec 3.2.2" + }, + { + "id": "navil-D1-011", + "claim": "LLM initialization convergence finding: uninitialized (random) LLM requires >10x more training data to reach comparable validation loss vs pre-trained initialization, confirming pre-training criticality", + "source": "Sec 3.2.1" + } + ], + "D2": [ + { + "id": "navil-D2-001", + "claim": "Visual Encoder Meta Architecture: V_{d,w}(I) = C circ (circ_{i=1..d} F_i^w) circ P(I)", + "source": "Sec 3.1, Eq.(1)" + }, + { + "id": "navil-D2-002", + "claim": "Transformer Parameter Count Approximation: N approx 12 x d x w^2", + "source": "Sec 3.2.3" + }, + { + "id": "navil-D2-003", + "claim": "MoE-Extended LLM Layer with Modality-Specific Experts: x_{i,m}^{l'} = x_{i,m}^{l-1} + MHA-MMoE(RMSNorm(x_{i,m}^{l-1})); x_{i,m}^{l} = x_{i,m}^{l'} + FFN-MMoE(RMSNorm(x_{i,m}^{l'}))", + "source": "Sec 3.2.2, Eq.(2)-(3)" + }, + { + "id": "navil-D2-004", + "claim": "Modality-Specific Multi-Head Attention Expert (MHA-MMoE): MHA-MMoE(x_{i,m}) = softmax(Q K^T / sqrt(d)) V W_O^m; Q_{i,m}=x_{i,m}W_Q^m, K_{i,m}=x_{i,m}W_K^m, V_{i,m}=x_{i,m}W_V^m", + "source": "Sec 3.2.2, Eq.(4)-(5)" + }, + { + "id": "navil-D2-005", + "claim": "Modality-Specific Feed-Forward Network Expert (FFN-MMoE): FFN-MMoE(x_{i,m}) = (SiLU(x_{i,m} W_gate^m) circ (x_{i,m} W_up^m)) W_down^m", + "source": "Sec 3.2.2, Eq.(6)" + }, + { + "id": "navil-D2-006", + "claim": "Visual Multi-Scale Image Packing: H_i = tau^i H_0, W_i = tau^i W_0. Continue until image area < threshold. Concatenate {x_{i,v}}_{i=0}^{n} with between scales.", + "source": "Sec 4.1" + }, + { + "id": "navil-D2-007", + "claim": "Special Token Insertion for Multimodal Sequence: multimodal token sequence S = [t_1, ..., , x^{(1,1)}_{i,v}, , ..., x^{(r,c)}_{i,v}, , ..., t_n] for each image i. Multi-scale concatenation: X_v = [x_{0,v}, , x_{1,v}, ..., , x_{n,v}] where x_{i,v} = V_{d,w}(I_i) with H_i = tau^i H_0, W_i = tau^i W_0, and V_{d,w}(·) is the visual encoder defined in Eq.(1). Special tokens delimit image, row, and scale boundaries.", + "source": "Sec 4.1" + }, + { + "id": "navil-D2-008", + "claim": "Next-Token-Prediction Training Objective: L_CE(θ) = -Σ_{t=1}^{T} log p_θ(s_t | s_{, x^{visual}, , ..., y_T], I is the input image, x^{visual} are visual encoder outputs, and y_T is the target caption token. The model is trained via teacher-forcing: given all preceding tokens (text + image) and the image encoding, predict each next token.", + "source": "Sec 3.1" + }, + { + "id": "navil-D2-009", + "claim": "Pixel Shuffle Downsampling Connector: C(x) = MLP(PixelShuffle(x)), downsamples encoded image embeddings and projects to LLM feature space.", + "source": "Sec 3.1" + }, + { + "id": "navil-D2-010", + "claim": "Two-Stage Training with Parameter Freezing Schedule: Stage 1.1 (50K-70K steps, 500M image-text pairs) — θ_trainable = {θ_visual_encoder, θ_MLP_projector, θ_MoE_visual_experts}, θ_frozen = {θ_text (all self-attention + FFN text params)}; Stage 1.2 (33K-40K steps, 185M high-quality data) — unfreeze self-attention text params: θ_trainable += {θ_text_attention}; Stage 2 (6K steps, 68M data) — all params unfrozen: θ_trainable += {θ_text_FFN}. Stage transition condition: Stage 1.1 completes full 500M-sample pass; model enters Stage 1.2 when visual-text alignment at low-res converges.", + "source": "Sec 4.2" + }, + { + "id": "navil-D2-011", + "claim": "Optimal Visual Encoder Size Selection Criterion: Optimal encoder size = min{s | (L(s) - L(2s)) / L(75M) < lambda}, where lambda = 0.01.", + "source": "Sec 3.3.2" + }, + { + "id": "navil-D2-012", + "claim": "Image Preprocessing: Padding and Patch Embedding: Padded_H = ceil(H/32)*32, Padded_W = ceil(W/32)*32. Patch stride = 16.", + "source": "Sec 5.1" + } + ], + "D3": [ + { + "id": "navil-D3-001", + "claim": "Compare native MLLM training from scratch versus initializing from a pre-trained LLM (InternLM2-Base) to evaluate the impact on training convergence and zero-shot multimodal performance. Both variants are trained on web-scale noisy image-caption pairs from LAION-2B [55] using next-token prediction with image captioning as the task. Validation teacher-forcing loss is measured on a held-out subset of the multimodal dataset, and zero-shot image captioning quality is evaluated qualitatively. The uninitialized (scratch) model requires over 10x more training data to reach comparable validation loss and shows a substantial zero-shot captioning gap even with significantly more data, due to lower textual quality and diversity of multimodal training data compared to the LLM pre-training corpus.", + "source": "Sec 3.2.1" + }, + { + "id": "navil-D3-002", + "claim": "Evaluate the effectiveness of modality-specific Mixture-of-Experts architecture (with MHA-MMoE and FFN-MMoE, 1 expert activated) against a vanilla decoder-only LLM for native MLLM training. Both configurations include a visual encoder and are trained on web-scale noisy image-caption pairs from LAION-2B [55] using next-token prediction; validation teacher-forcing loss is measured on a held-out subset. The MoE-extended LLM achieves the same validation loss as the vanilla LLM with only 1/10 of the training data, without increasing training or inference cost. Only FFN experts caused significant feature scale differences between modalities, motivating the additional introduction of modality-specific attention experts (MHA-MMoE).", + "source": "Sec 3.2.2" + }, + { + "id": "navil-D3-003", + "claim": "Grid search over five depth-width configurations of the visual encoder (depth d in {3, 6, 12, 24, 48} with corresponding width w in {4096, 2880, 2048, 1472, 1024}) under a fixed 600M parameter budget to determine the optimal architecture for native MLLMs. All configurations are trained from scratch on web-scale noisy image-caption pairs from LAION-2B [55] at multiple training data sizes, using a pre-trained LLM (InternLM2-Base) and next-token prediction. Evaluation uses validation teacher-forcing loss on a held-out subset and zero-shot image captioning benchmarks. Shallower encoders converge faster in early training (<30M samples), while deeper encoders perform slightly better with larger datasets; however, a wide range of depth-width combinations yield near-optimal performance, consistent with prior findings on compute-optimal LLM architectures.", + "source": "Sec 3.2.3" + }, + { + "id": "navil-D3-004", + "claim": "Investigate scaling properties of the LLM component by varying LLM parameter sizes from 0.5B to 7B (based on the InternLM2 architecture family) while keeping the visual encoder fixed at 600M, to characterize the multimodal scaling law. All models follow the optimal architecture (LLM initialization, MoE, optimal depth-width ratio) and are trained on web-scale noisy image-caption pairs from LAION-2B [55] at multiple training data sizes. The metric is validation teacher-forcing loss on a held-out subset. Results show that scaling up LLM parameters in native MLLMs follows a pattern consistent with conventional LLM scaling law: loss decreases log-linearly as parameter size increases exponentially.", + "source": "Sec 3.3.1 (Scaling up LLMs)" + }, + { + "id": "navil-D3-005", + "claim": "Investigate scaling properties of the visual encoder by varying encoder sizes across {75M, 150M, 300M, 600M, 1.2B, 2.4B} with a fixed LLM to identify diminishing returns and performance saturation points. All models follow the optimal architecture (LLM initialization, MoE, optimal depth-width ratio) and are trained on web-scale noisy image-caption pairs from LAION-2B [55] at multiple training data sizes. The metric is validation teacher-forcing loss on a held-out subset. Unlike LLM scaling, performance gains from larger visual encoders diminish progressively; beyond a certain encoder size, further scaling yields only marginal loss reduction, indicating that the MLLM performance upper bound is constrained by the LLM's capacity. As training data increases, the loss gap between consecutive encoder sizes narrows to near zero once the visual encoder reaches a sufficient size.", + "source": "Sec 3.3.1 (Scaling up Visual Encoder)" + }, + { + "id": "navil-D3-006", + "claim": "Determine the optimal visual encoder size for each LLM scale (0.5B, 1.8B, 7B) using the lambda=1% threshold criterion and characterize the log-linear relationship between optimal encoder size and LLM size. Based on validation loss measurements on web-scale image-caption pairs from LAION-2B [55] across multiple encoder sizes for each LLM, the optimal encoder is defined as the smallest encoder whose loss difference versus a 2x larger encoder is less than 1% of the 75M baseline encoder's loss. The logarithm of optimal visual encoder size scales linearly with the logarithm of LLM size, indicating both components should be scaled jointly for balanced performance — a finding that contrasts with the compositional paradigm, which typically uses a single pre-trained visual encoder across all LLM scales.", + "source": "Sec 3.3.2" + }, + { + "id": "navil-D3-007", + "claim": "Evaluate NaViL-2B (2.4B activated params) against both compositional and native MLLMs on 14 multimodal benchmarks. General MLLM understanding benchmarks (7): MMVet, MMMU (val), MMBench-EN (test), MME (perception + cognition sum), MathVista (MINI), OCRBench, CCBench (Table 1). Visual question answering benchmarks (7): TextVQA (val), ScienceQA-IMG (test), GQA (test-dev), DocVQA (test), AI2D (test), ChartQA (test), InfoVQA (test) (Table 2). Key baselines: compositional counterpart InternVL-2.5-2B (same LLM and high-quality data, 300M encoder distilled from 6B encoder), Qwen2VL-2B, InternVL-1.5-2B, MiniCPM-V-2, DeepSeek-VL-1.3B, PaliGemma-3B, MM1-3B-MoE-Chat; native MLLMs Mono-InternVL (1.8B), EVEv2 (7B), SAIL (7B), Emu3 (8B), EVE-7B, VoRA, Chameleon-7B. Metrics: per-benchmark task-specific scores, with average computed by normalizing each metric to a 0-100 range. NaViL-2B achieves average 67.1 on Table 1 and 75.1 on Table 2, outperforming all existing native MLLMs and competitive with compositional baselines.", + "source": "Sec 5.1-5.2, Table 1, Table 2" + }, + { + "id": "navil-D3-008", + "claim": "Scale NaViL to 9B parameters (9.2B activated, based on Qwen3-8B with 1.2B visual encoder, visual multi-scale packing disabled in Stage 1.1 for acceleration) and evaluate on 12 multimodal benchmarks (MMVet, MMMU, MMBench, MME, MathVista, OCRBench, TextVQA, DocVQA, AI2D, ChartQA, InfoVQA; Table 4), comparing training token efficiency against compositional MLLMs. NaViL-9B uses only 450B total training tokens (all in MLLM training, zero ViT pre-training tokens) versus >4.1T for Qwen2.5VL and >3.5T for InternVL2.5-8B (whose visual encoders separately consume >3.3T pre-training tokens; Table 3). Key baselines from Table 4: compositional counterpart InternVL-2.5-8B (same high-quality data, 300M encoder distilled from 6B), Qwen2.5-VL-8B, Qwen2VL-8B; native MLLMs EVEv2 (7B), SAIL (7B), Mono-InternVL (1.8B), Emu3 (8B), EVE-7B, Chameleon-7B. NaViL-9B achieves average 77.0 on Table 4, outperforming all existing native MLLMs by a large margin and competitive with top-tier compositional baselines.", + "source": "Appendix A, Table 3, Table 4" + }, + { + "id": "navil-D3-009", + "claim": "Visualize and analyze attention maps of different LLM layers in NaViL (using LLM-1.8B as the base) when using a small (150M) versus large (1.2B) visual encoder to understand how encoder scaling affects cross-modal interaction. Trained models with each encoder size are loaded; sample images are forward-passed, and attention weights are extracted per LLM layer for visualization (Fig. 9). Two findings: (1) with a 150M encoder, shallow-layer attention exhibits obvious locality (attending to spatially adjacent tokens), gradually shifting to global patterns at deeper layers; with a 1.2B encoder, shallow-layer visual tokens already attend to global information, indicating better pre-extraction of high-level semantics; (2) larger encoder facilitates earlier cross-modal interaction — attention weights between visual and text tokens in the first layer are significantly higher with the 1.2B encoder versus 150M, providing an explanatory mechanism for the improved performance from larger encoders.", + "source": "Sec 5.3" + }, + { + "id": "navil-D3-010", + "claim": "Evaluate the NLP capability of NaViL-9B on three standard NLP benchmarks (MMLU, CMMLU, MATH) using the OpenCompass evaluation toolkit [18], to verify that modality-specific MoE architecture preserves linguistic competence of the initialization LLM (Qwen3-8B). Baselines from Table 5: InternLM2-Chat (1.8B: MMLU 47.1, CMMLU 46.1, MATH 13.9), Qwen3-8B non-thinking (MMLU 76.5, CMMLU 76.8, MATH 71.1), EVE (7B: MMLU 43.9, CMMLU 33.4, MATH 0.7), Chameleon (7B: MMLU 52.1, MATH 11.5), Mono-InternVL (2B: MMLU 45.1, CMMLU 44.0, MATH 12.3). NaViL-9B scores: MMLU 74.9, CMMLU 75.1, MATH 66.2 — close to Qwen3-8B and far above other native MLLMs, confirming that modality-specific MoE effectively preserves NLP capability despite multimodal training, without requiring large amounts of high-quality text data.", + "source": "Appendix E, Table 5" + } + ], + "D4": [ + { + "id": "navil-D4-001", + "claim": "Experiment phases: Pretrain both variants (scratch vs. LLM-initialized) at increasing data sizes -> Compare validation loss curves -> Evaluate zero-shot image captioning", + "source": "Sec 3.2.1" + }, + { + "id": "navil-D4-002", + "claim": "Experiment phases: Train vanilla LLM and MoE-extended LLM configurations -> Compare validation loss curves -> Measure data efficiency ratio (10x data reduction with MoE)", + "source": "Sec 3.2.2" + }, + { + "id": "navil-D4-003", + "claim": "Experiment phases: Grid search all 5 (d,w) configs under 600M budget -> Train each at multiple data sizes -> Compare validation loss curves -> Evaluate zero-shot image captioning at convergence", + "source": "Sec 3.2.3" + }, + { + "id": "navil-D4-004", + "claim": "Experiment phases: Train MLLMs with LLM sizes {0.5B, 1.8B, 7B} (fixed 600M encoder) -> Measure validation loss across training data sizes -> Fit log-linear scaling law relationship", + "source": "Sec 3.3.1 (Scaling up LLMs)" + }, + { + "id": "navil-D4-005", + "claim": "Experiment phases: Train MLLMs with encoder sizes {75M, 150M, 300M, 600M, 1.2B, 2.4B} (fixed LLM) -> Measure validation loss across training data sizes -> Identify saturation point where larger encoder yields marginal gain -> Conclude performance upper bound constrained by LLM capacity", + "source": "Sec 3.3.1 (Scaling up Visual Encoder)" + }, + { + "id": "navil-D4-006", + "claim": "Experiment phases: For each LLM size (0.5B, 1.8B, 7B), sweep encoder sizes -> Measure loss gaps between consecutive sizes -> Apply lambda=0.01 threshold criterion to determine optimal encoder size -> Fit log-log relationship showing optimal encoder scales proportionally with LLM size", + "source": "Sec 3.3.2" + }, + { + "id": "navil-D4-007", + "claim": "Experiment phases: Stage 1.1 pretraining (500M pairs, frozen text) -> Stage 1.2 continued pretraining (185M, unfrozen attention) -> Stage 2 supervised fine-tuning (68M, all unfrozen) -> Evaluate NaViL-2B on 14 benchmarks -> Compare against compositional and native MLLM baselines", + "source": "Sec 5.1-5.2, Table 1, Table 2" + }, + { + "id": "navil-D4-008", + "claim": "Experiment phases: Train NaViL-9B with same recipe as NaViL-2B (visual multi-scale packing disabled in S1.1) -> Evaluate on 12 benchmarks (Table 4) -> Compare training token counts against Qwen2.5VL and InternVL-2.5-8B (Table 3) -> Compare benchmark scores against all baselines", + "source": "Appendix A, Table 3, Table 4" + }, + { + "id": "navil-D4-009", + "claim": "Experiment phases: Load trained NaViL models with 150M and 1.2B encoders -> Forward pass sample images -> Extract attention weights per LLM layer -> Visualize and compare attention patterns (locality/globality, cross-modal interaction)", + "source": "Sec 5.3" + }, + { + "id": "navil-D4-010", + "claim": "Experiment phases: Pretrain and fine-tune NaViL-9B -> Evaluate on MMLU, CMMLU, MATH using OpenCompass -> Compare against baseline LLMs (InternLM2-Chat, Qwen3-8B) and native MLLMs (EVE, Chameleon, Mono-InternVL)", + "source": "Appendix E, Table 5" + } + ] +} \ No newline at end of file diff --git a/papers/neural-operator-flow-matching-pde/blacklist.txt b/papers/neural-operator-flow-matching-pde/blacklist.txt new file mode 100644 index 0000000000000000000000000000000000000000..18884fee11fb30626622d7c4939be96e0d0b2346 --- /dev/null +++ b/papers/neural-operator-flow-matching-pde/blacklist.txt @@ -0,0 +1,3 @@ +# Official repository (NeurIPS 2025, anonymous) +https://anonymous.4open.science/r/multiphysics_neurop-F385/ +# Authors: Mikhail Masliaev, Dmitry A. Gusarov, Ilya Markov, Alexander Hvatov (ITMO University) diff --git a/papers/neural-operator-flow-matching-pde/config.yaml b/papers/neural-operator-flow-matching-pde/config.yaml new file mode 100644 index 0000000000000000000000000000000000000000..21723045e9a2130f589c9412f35f6ddff7bf54bc --- /dev/null +++ b/papers/neural-operator-flow-matching-pde/config.yaml @@ -0,0 +1,8 @@ +title: "Bridging Neural Operator and Flow Matching for a Generative PDE Foundation Model" +pdf_url: "https://openreview.net/pdf?id=WwBwR5cq67" +venue: "NeurIPS 2025" +year: "2025" +extra: + selection_index: 28 + domain: "Numerical Methods / Scientific Computing" + paradigm: "System / Pipeline" diff --git a/papers/neural-operator-flow-matching-pde/images/figures/neural-operator-flow-matching-pde-fig-0001.jpg b/papers/neural-operator-flow-matching-pde/images/figures/neural-operator-flow-matching-pde-fig-0001.jpg new file mode 100644 index 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b/papers/neural-operator-flow-matching-pde/images/tables/neural-operator-flow-matching-pde-table-0002.jpg @@ -0,0 +1,3 @@ +version https://git-lfs.github.com/spec/v1 +oid sha256:f91cf0f2371c401f9585d0f737f640ca4ded18e186e7dcba13be3eafc6a23599 +size 110198 diff --git a/papers/neural-operator-flow-matching-pde/paper.md b/papers/neural-operator-flow-matching-pde/paper.md new file mode 100644 index 0000000000000000000000000000000000000000..2c6b56fbaf672e9adfe0593ccb209d85700d1464 --- /dev/null +++ b/papers/neural-operator-flow-matching-pde/paper.md @@ -0,0 +1,391 @@ +# Bridging Neural Operator and Flow Matching for a Generative PDE Foundation Model + +Zituo Chen +Department of Mechanical Engineering +Massachusetts Institute of Technology Cambridge, MA 02139 zituo@mit.edu Sili Deng∗ +Department of Mechanical Engineering +Massachusetts Institute of Technology Cambridge, MA 02139 silideng@mit.edu + +# Abstract + +Pretraining on large-scale collections of PDE-governed spatiotemporal trajectories has recently shown promise for building generalizable models of dynamical systems. Yet, most existing PDE foundation models rely on deterministic Transformer architectures, which demand substantial computational resources and lack generative flexibility. In contrast, generative models can capture uncertainty, making them well-suited for probabilistic forecasting, data assimilation, and scientific design. In this work, we introduce a generative PDE foundation model that bridges neural operator learning with flow matching. By jointly sampling the noise level and the physical timestep between adjacent states, the model learns a unified velocity field that transports a noisy current state toward its clean successor. Alongside the core framework, we introduce novel architectural strategies that achieve up to $1 5 \times$ greater computational efficiency than full-length diffusion models, enabling large-scale pretraining at substantially reduced cost. Our framework combines autoregressive Transformers and latent diffusion in a two-stage training pipeline, yielding scalable, accurate, and extensible generative modeling of PDE systems. We curate a training corpus of ${ \sim } 2 \mathbf { M }$ trajectories across 12 distinct PDE families and release a suite of pretrained autoencoders and generative latent models of varying parameter scales. For downstream evaluation, we benchmark on previously unseen Kolmogorov turbulence with few-shot adaptation, and show long-term rollout stability of our model compared to its deterministic counterparts. + +# 1 Introduction + +Generative models can capture uncertainty through sampling, which is vital for scientific and engineering applications where forecasts [37, 23], machine and material design [46, 50], and safety margins [7, 18] depend on an ensemble of predictions rather than single point predictions. PDE foundation models [33, 5, 49] are large, pre-trained neural operators that learn generalizable representations of spatiotemporal dynamics, enabling zero-shot prediction, control, and design across diverse physical systems. However, most foundation models for PDEs have emphasized deterministic mapping from the current state to future states. This creates a gap between the application need and the capability of current large pretrained models. + +This gap motivates a fundamental question about regression vs. sampling approaches in physical dynamics (e.g., generative ensemble-regression contrasts point-wise regression [47]): when should we learn a deterministic operator, and when must we sample from a conditional distribution over future fields? More importantly, can we unify these regimes in a single scalable framework that offers the speed of neural operators while retaining the fidelity and extensive applications of modern generative models? + +Training large-scale generative models for PDEs is challenging on two fronts, efficiency and data source. High-resolution fields are expensive to denoise or transport step-by-step; naively applying diffusion or flow-based methods in pixel space leads to prohibitive memory and computation cost. Meanwhile, scientific data are often multi-fidelity, heterogeneous across systems, and scarce at high accuracy; a practical foundation model must leverage broad-but-imperfect corpora while remaining physically plausible and stable in long-term rollouts. + +Hence, we propose a latent diffusion-based generative PDE foundation model that unifies deterministic operators and stochastic flows within one training and inference stack. Our framework is efficient and scalable, while being capable to accommodate diverse types of dynamical systems. + +Our approach has two components: a Pretrained Physics Variational Autoencoder (P2VAE) and a Flow Marching Transformer (FMT). P2VAE compresses static physical field snapshots into a compact latent grid (e.g., $1 6 \times 1 6 )$ to slash the cost of generative training and inference, and is trained across heterogeneous PDE datasets. FMT introduces a flow marching algorithm that bridges deterministic neural operator and stochastic flow matching through a bridge parameter $k$ . For $k = 1$ , FMT behaves like a neural operator; for $k = 0$ , it reduces to a flow-matching–style stochastic sampler. In between, it learns transport on mixtures that combine data-driven drift with controlled stochasticity. We train the latent velocity field with a numerically stable frame-interpolation objective: + +$$ +\| ( 1 - t ) \mathbf { g } ( \mathbf { x } _ { t } ^ { k } , t ) - ( \mathbf { x } _ { 1 } - \mathbf { x } _ { t } ^ { k } ) \| ^ { 2 } , +$$ + +which follows directly from the continuity equation for the induced interpolation path. To support long-horizon rollouts, we introduce a diffusion forcing scheme that adaptively injects small stochastic increments during autoregressive prediction, mitigating error accumulation without sacrificing stability. Finally, a latent temporal pyramid executes coarse-to-fine transport, cutting training and inference cost while improving long-range consistency. + +The key contributions of this paper are summarized as follow: + +• We propose a latent generative PDE foundation model that unifies deterministic and stochastic modeling through a single transport field trained with a principled, well-conditioned regression objective rooted in flow matching. +• We design a novel flow marching algorithm equipped with diffusion forcing scheme and latent temporal pyramids that allows large-scale pretraining and stable long-term rollouts. +• We sort and provide a heterogeneous dataset across public PDE datasets including FNO-v, PDEArena, PDEBench and The Well, totaling up to 233 Gigabyte and consisting of 2.5 million trajectories. + +# 2 Related works + +# 2.1 Neural operator and PDE foundation models + +Neural operators are data-driven models that build surrogate models for science and engineering applications. Existing methods include but not limited to FNO [25], DeepONet [31], PINO [26], OFormer [24], and UPT [1]. They excel at fast rollout and generalization to unseen inputs [20, 3, 21]. PDE foundation models based on Transformer architecture [45], such as ICON [48, 5], MPP [33], DPOT [13], PROSE [29, 41], and PITT [30], learn the PDE-governed spatiotemporal dynamics through large-scale pretraining across diverse spatiotemporal systems, and enable fast adaptation to new dynamics and in-context learning ability. However, they are mostly deterministic and lack the flexibility of generative modeling. + +Diffusion-based generative models have been explored to solve PDE equations recently [15, 4, 35]. However, they follow the paradigm of generating new state out of pure noise conditioned on the previous states, which is different from our flow marching method; also, they target at a single dynamics, which is different from the concept of PDE foundation models. + +# 2.2 Flow matching + +Preliminaries Flow matching (FM) trains a time-dependent vector field by regressing to conditional velocities that deterministically transport white noise to data along a prescribed probability path [27, 28, 43]. Large-scale studies [10] further show FM can match or surpass diffusion on high-resolution image synthesis while retaining fast ODE sampling, motivating FM as a practical training principle for modern generative backbones. + +Pyramidal flow matching Recent work proposes Pyramidal Flow Matching (PFM) [19], which reinterprets the denoising/transport trajectory as a multi-stage spatial pyramid, where only the final stage runs at full resolution while earlier stages operate coarser and are linked through a renoising technique to preserve continuity. This yields notable efficiency gains (especially for video) and pairs naturally with temporal pyramids for autoregressive history compression. These ideas directly inspire our latent temporal pyramid and coarse-to-fine training/inference strategy. + +# 2.3 Diffusion forcing + +Diffusion Forcing [6] blends autoregressive (AR) prediction with diffusion-style denoising: a causal next-token (or next-segment) model is trained to produce future content while simultaneously denoising a set of tokens with independent per-token noise levels. Compared to pure AR, diffusion forcing lets the model get access to partially noised data distributions; compared to pure diffusion, it preserves causal structure and AR efficiency. Interleaving causal prediction with one to three lightweight denoising refinements reduces exposure bias [2] while preserving autoregressive efficiency. In practice, it improves long-horizon stability, temporal coherence, and ensemble calibration with minimal latent-space overhead, which is suitable for the PDE condition propagation during an autoregressive generation process. + +# 3 Methods + +# 3.1 Flow marching + +Let $( { \bf x } _ { 0 } , { \bf x } _ { 1 } ) \sim \pi$ be consecutive states of a dynamical system. We synthesize intermediate training particles via a location-scale interpolation kernel in Fig. 1 + +$$ +{ \bf x } _ { t } ^ { k } = \mu _ { t } + \sigma _ { t } { \bf z } , \mu _ { t } = t { \bf x } _ { 1 } + k ( 1 - t ) { \bf x } _ { 0 } , \sigma _ { t } = ( 1 - t ) ( 1 - k ) , { \bf z } \sim \mathcal { N } ( 0 , I ) , +$$ + +with $t , k \sim \operatorname { U n i f } ( 0 , 1 )$ . + +![](images/figures/neural-operator-flow-matching-pde-fig-0001.jpg) +Figure 1: Location-scale interpolation kernel for flow marching + +Conditionally, + +$$ +q _ { t } ^ { k } ( \mathbf { x } _ { t } ^ { k } | \mathbf { x } _ { 0 } , \mathbf { x } _ { 1 } , k ) \sim \mathcal { N } ( t \mathbf { x } _ { 1 } + k ( 1 - t ) \mathbf { x } _ { 0 } , ( 1 - t ) ^ { 2 } ( 1 - k ) ^ { 2 } I ) . +$$ + +Two limits are informative: (i) $k = 0$ recovers a flow-matching kernel $\mathbf { x } _ { t } ^ { 0 } \sim \mathcal { N } ( t \mathbf { x } _ { 1 } , ( 1 - t ) ^ { 2 } \mathbf { I } )$ ; (ii) $k = 1$ gives the deterministic neural operator interpolation ${ \bf x } _ { t } ^ { 1 } = t { \bf x } _ { 1 } + ( 1 - t ) { \bf x } _ { 0 }$ . Thus, $k$ continuously bridges stochastic flow transport and deterministic operator regression. + +Using the equivalent form + +$$ +{ \bf x } _ { t } ^ { k } = { \bf x } _ { 0 } + t ( { \bf x } _ { 1 } - { \bf x } _ { 0 } ) - ( 1 - t ) ( 1 - k ) ( { \bf x } _ { 0 } - { \bf z } ) . +$$ + +Differentiation gives the sample-wise velocity + +$$ +\mathbf { u } _ { t } ^ { k } = { \frac { d } { d t } } \mathbf { x } _ { t } ^ { k } = ( 1 - k ) ( \mathbf { x } _ { 0 } - \mathbf { z } ) + \mathbf { x } _ { 1 } - \mathbf { x } _ { 0 } = { \frac { \mathbf { x } _ { 1 } - \mathbf { x } _ { t } ^ { k } } { 1 - t } } . +$$ + +Because the kernel is Gaussian local-scale, its conditional score at the sampled point $\mathbf { x } = \mathbf { x } _ { t } ^ { k }$ is + +$$ +\nabla _ { \mathbf { x } } \log q _ { t } ^ { k } ( \mathbf { x } _ { t } ^ { k } ) = - \frac { \mathbf { x } _ { t } ^ { k } - \mu _ { t } } { \sigma _ { t } ^ { 2 } } = - \frac { \mathbf { z } } { ( 1 - t ) ( 1 - k ) } . +$$ + +We substitute Eq. 6 into Eq. 5, and get the score-velocity decomposition: + +$$ +\mathbf { u } _ { t } ^ { k } = ( \mathbf { x } _ { 1 } - k \mathbf { x } _ { 0 } ) + ( 1 - t ) ( 1 - k ) ^ { 2 } \nabla _ { \mathbf { x } } \log q _ { t } ^ { k } ( \mathbf { x } _ { t } ^ { k } ) . +$$ + +Therefore, the transport velocity is a well-posed learnable target: its random part is aligned with an intrinsic geometric direction of the conditional density (the score), and its deterministic part is fixed by the pair $\left( \mathbf { x } _ { 0 } , \mathbf { x } _ { 1 } \right)$ and the bridge parameter $k$ . Regressing a model $\mathbf { g }$ to $\mathbf { u } _ { t } ^ { k }$ theoretically recovers the posterior-mean transporting field $\mathbf { g } ^ { * }$ (see A.1). + +With the above justification, our setting trains a neural network approximation $\mathbf { g }$ by regressing to the oracle velocity: + +$$ +\mathcal { R } _ { \mathrm { F M } } ( \mathbf { g } ) = \mathbb { E } _ { ( \mathbf { x } _ { 0 } , \mathbf { x } _ { 1 } ) , k , t } \left[ \frac { 1 } { 2 } | | \mathbf { g } - \mathbf { u } _ { t } ^ { k } | | ^ { 2 } \right] . +$$ + +We notice that sampling both $k$ and $t$ to be the input of $\mathbf { g }$ is empirically slow to converge, and one of the most apparent failure mode of using $k$ in the denoising process is that an intermediate denoised state would deviate from the original assigned bridge parameter $k$ thus introduce an accumulating error. In practice, we adopted $\mathbf { u } _ { t } ^ { k } = ( \mathbf { x } _ { 1 } - \mathbf { x } _ { t } ^ { k } ) / ( 1 - t ) ^ { \top }$ in Eq. 5 as the training objective, so that $\mathbf { x } _ { t } ^ { \overline { { k } } }$ and $t$ are sufficient as inputs. This $k$ -free objective can also be intuitively understood as the linear vector pointing to the end state $\mathbf { x } _ { 1 }$ from the current state $\mathbf { x } _ { t } ^ { k }$ . This frame-interpolation view – “predict the missing bridge $( \mathbf { x } _ { 1 } - \mathbf { x } _ { t } ^ { k } ) ^ { \prime \prime } -$ makes the training interface minimal: once $\mathbf { x } _ { t } ^ { k }$ is constructed offline, the supervision depends only on $\left( \mathbf { x } _ { 1 } - \mathbf { x } _ { t } ^ { k } \right)$ . + +The form is numerical stiff near $t \to 1$ . We therefore precondition the target by $( 1 - t )$ and obtain the flow marching objective: + +$$ +\mathcal { L } _ { \mathrm { F M } } = \frac { 1 } { 2 } \mathbb { E } \left[ | | ( 1 - t ) \mathbf { g } _ { \boldsymbol { \theta } } ( \mathbf { x } _ { t } ^ { k } , t ) - ( \mathbf { x } _ { 1 } - \mathbf { x } _ { t } ^ { k } ) | | ^ { 2 } \right] . +$$ + +Minimizers of $\mathcal { L } _ { \mathrm { F M } }$ correspond to minimizers of $\mathcal { R } _ { \mathrm { F M } }$ (up to the benign $( 1 - t )$ scaling), and the regression is well-conditioned at late times. + +# 3.2 Conditional flow marching through diffusion forcing + +There exists a variety of different dynamics in the training dataset. To accommodate these dynamics in one PDE foundation model, we introduce the conditional form of flow marching and design an approach to condition the dynamics through past states. + +The conditional flow marching target could be derived from a conditional probability: + +$$ +q _ { t } ( \mathbf { x } , \mathbf { h } ) = \mathbb { E } _ { \mathbf { h } , ( \mathbf { x } _ { 0 } , \mathbf { x } _ { 1 } ) , k } \left[ q _ { t } ^ { k } ( \mathbf { x } | \mathbf { x } _ { 0 } , \mathbf { x } _ { 1 } , \mathbf { h } ) \right] . +$$ + +Following the same derivation, we have the conditional flow marching objective: + +$$ +\mathcal { L } _ { \mathrm { C F M } } = \frac { 1 } { 2 } \mathbb { E } \left[ | | ( 1 - t ) \mathbf { g } _ { \theta } ( \mathbf { x } _ { t } ^ { k } , t , \mathbf { h } ) - ( \mathbf { x } _ { 1 } - \mathbf { x } _ { t } ^ { k } ) | | ^ { 2 } \right] . +$$ + +To derive the condition $\mathbf { h }$ , we follow the style of diffusion forcing (DF) [6] to design a forcing scheme, which makes use of history states with different noise levels to induce the filtered PDE condition ${ \bf h } _ { s }$ (denote the physical timestep as $s = 1 , 2 , \dots T )$ . Specifically, a DF keeps and updates a compressed latent state $\mathbf { h }$ to be the condition at each step to inform the denoising process. We adopt a simplistic RNN parametrized by $\phi$ like original DF paper to evolve latent state $\mathbf h _ { s } \sim p _ { \phi } ( \mathbf h _ { s } | \mathbf h _ { s - 1 } , \mathbf x _ { s , t _ { s } } ^ { k _ { s } } , t _ { s } )$ because governing equation and coefficients of PDE dynamics are usually kept the same as time evolves. + +In conclusion, the training objective for FMT is + +$$ +\mathcal { L } _ { \mathrm { C F M } } = \frac { 1 } { 2 } \underset { \mathbf { h } _ { s } \sim p _ { \phi } ( \mathbf { h } _ { s } | \mathbf { h } _ { s - 1 } , \mathbf { k } _ { s } ^ { k _ { s } } , t _ { s } ) } { \mathbb { E } } \sum _ { i = 1 } ^ { T } \left[ \left. ( 1 - t _ { s } ) \mathbf { g } _ { \theta } ( \mathbf { x } _ { s , t _ { s } } ^ { k _ { s } } , t _ { s } , \mathbf { h } _ { s - 1 } ) - ( \mathbf { x } _ { s + 1 } - \mathbf { x } _ { s , t _ { s } } ^ { k _ { s } } ) \right. ^ { 2 } \right] , +$$ + +where xkss,ts $\mathbf { x } _ { s , t _ { s } } ^ { k _ { s } } = \mathbf { x } _ { s } + t _ { s } ( \mathbf { x } _ { s + 1 } - \mathbf { x } _ { s } ) - ( 1 - t _ { s } ) ( 1 - k _ { s } ) ( \mathbf { x } _ { s } - \mathbf { z } ) , t _ { s } , k _ { s }$ are independently sampled at each physical timestep $s$ . + +# 3.3 Latent temporal pyramids + +We introduce two techniques to improve the computational efficiency of our model. + +Firstly, we introduce an autoencoder P2VAE parametrized by $\omega$ to compress the state $\mathbf { x }$ to lower token count $\mathbf { y }$ used in the following implementations. + +$$ +\begin{array} { r l r } & { \displaystyle \mathbf { y } = \mathcal { E } _ { \omega } ( \mathbf { x } ) , \quad \hat { \mathbf { x } } = \mathcal { D } _ { \omega } ( \mathbf { y } ) , } & \\ & { \mathcal { L } _ { \mathrm { V A E } } = \displaystyle \frac { 1 } { 2 } \mathbb { E } \left\| \mathbf { x } - \hat { \mathbf { x } } \right\| ^ { 2 } + \beta \mathrm { K L } ( q _ { \omega } ( \mathbf { y } | \mathbf { x } ) | | p ( \mathbf { y } ) ) . } & \end{array} +$$ + +To further simplify the computational complexity, we introduce temporal pyramids in PFM [19], which resonates with the fact that a physical dynamics system is mostly Markovian and prediction relies less on farther previous states. + +For early $s$ , we use compressed latent states to propagate the PDE condition $\mathbf { h } _ { s }$ . In practice, we always train the conditional flow marching model on 4 consecutive states $\left( \mathbf { x } _ { 0 } , \mathbf { x } _ { 1 } , \mathbf { x } _ { 2 } , \mathbf { x } _ { 3 } \right)$ , where the FMT model takes $( \mathbf { x } _ { 0 , t _ { 0 } } ^ { k _ { 0 } } , \mathbf { x } _ { 1 , t _ { 1 } } ^ { k _ { 1 } } , \mathbf { x } _ { 2 , t _ { 2 } } ^ { k _ { 2 } } , \mathbf { x } _ { 3 , t _ { 3 } } ^ { k _ { 3 } } )$ as input to predict the flow marching velocities by latent temporal pyramids $( D o w n ( \mathbf { y } _ { 0 , t _ { 0 } } ^ { k _ { 0 } } , 8 ) , D o w n ( \mathbf { y } _ { 1 , t _ { 1 } } ^ { k _ { 1 } } , 4 ) , D o w n ( \mathbf { y } _ { 2 , t _ { 2 } } ^ { k _ { 2 } } , 2 ) , \mathbf { y } _ { 3 , t _ { 3 } } ^ { k _ { 3 } } )$ . + +# 3.4 Prediction and generation processes + +We use the Euler ODE sampler (discretization on $t$ ) to propagate an intermediate state $\mathbf { x } _ { t } ^ { k }$ to $\mathbf { x } _ { 1 }$ . $( t _ { 0 } , t _ { 1 } , t _ { 2 } , t _ { 3 } )$ are initialized to be 0, and are updated simultaneously during the flow marching process. The discretization is taken to be $N = 1 0 0$ throughout the evaluation phase, with $d t = 0 . 0 1$ . In a deterministic prediction setting, we set $( k _ { 0 } , k _ { 1 } , k _ { 2 } , k _ { 3 } )$ to be 1, meaning that the past states are not noisy. In a generation setting, we choose to set $( k _ { 0 } , k _ { 1 } , k _ { 2 } )$ to be 1 and $k _ { 3 }$ less than 1. The smaller the $k$ , the larger the uncertainty about the current state. This setting allows $\mathbf { h } _ { 3 }$ to be passed down from a clean history, and generate possible $\mathbf { x } _ { 4 }$ out of pure noise. $\left( k _ { 0 } , k _ { 1 } , k _ { 2 } \right)$ ’s parametrization choice can be further explored. + +# 4 Experiments + +# 4.1 Setup + +Dataset gathering We consider a combination of public benchmark datasets for PDE foundation models: FNO-v [25], PDEBench [42], PDEArena [11], and the Well [34] to form a heterogeneous dataset consisting of 12 distinct dynamical systems. All the dynamical systems are 2D intrinsically, and three physical fields are chosen at maximum. We compressed the aforementioned datasets to the format of $1 2 8 \times 1 2 8$ spatial resolution with 3 multiphysics channels (c3p128) with float16 precision to form a $2 3 3 \mathrm { G B }$ dataset, consisting over 2.5M trajectories with length 4. We provide the exact compression ratio and dataset information in A.2. + +Training dataset The heterogeneous dataset is partitioned into train, valid, and test sets according to the original settings of each sub-dataset first; under the cases that there is no partition, we use a ratio of 8:1:1. We train P2VAE and FMT on the train set. Datasets are sampled with equal probabilities according to the practice in DPOT [13]. P2VAE’s AdamW optimizer is used with $\beta _ { 1 } = 0 . 9$ and $\beta _ { 2 } = 0 . 9 9 5$ , cosine learning rate schedule with $10 \%$ of linear warm up, and a weight decay of 1e-4; FMT’s AdamW optimizer is used with $\beta _ { 1 } = 0 . 9$ and $\beta _ { 2 } = 0 . 9 5$ , cosine learning rate schedule with $10 \%$ of linear warm up, and a weight decay of 0.01. Base learning rates of 1e-4 for a 256 batch size are adjusted according to batch sizes and model sizes to balance convergence speed and training stability. We conduct a two stage training recipe. We trained 2 P2VAEs, 16M and 87M, for $1 0 0 \mathrm { k }$ steps with KL term’s weight $\beta = 1 \mathrm { e } { - 3 }$ . Based on the 16M P2VAE (with frozen weights), we train 3 FMTs with size 6M, 42M, and 138M (Small, Base, and Large) on the same training dataset for another $1 0 0 \mathrm { k }$ steps. + +Evaluation metrics To assess the reconstruction and prediction quality of our model, we employ both the L2 relative error (L2RE), which is a common practice of PDE foundation models, and the variance-normalized root mean square error (VRMSE), as suggested by [34]. + +Implementation details For P2VAE, we reuse the standard SD-VAE [38] architecture to compress each state from $\mathtt { c 3 p 1 2 8 }$ to c16p16 ( $1 2 \times$ compression rate) following the recommendation in [12]. P2VAE-16M uses 64 as the base dimensions, while P2VAE-87M uses 128. For FMT, we use the AdaLN-Zero mechanism introduced in [36] to condition a SiT [32]. In the Transformer side, we adopt the modern architecture RMSNorm and SwiGLU introduced by Llama-2 [44]. Multi-head self-attention with head $\mathrm { d i m } 6 4$ is implemented with FlashAttention v2 [9]. FMT-S, FMT-B, and FMT-L have 256, 512, and 768 as the embedding dimensions, respectively. The RNN in the diffusion forcing scheme is a GRU [8] which shares the same internal dimension as the embedding dimension in SiT; the current state $\mathbf { x } _ { t } ^ { k }$ is first compressed onto a single token by cross attention to update the latent state $\mathbf { h }$ to inform dynamics. + +# 4.2 Efficiency + +Compared to a vanilla video-diffusion model [14] that operates with bidirectional self-attention across 4 frames with 256 tokens each and quadratic attention complexity, the efficiency gain due to FMT could be estimated by + +$$ +\eta = \frac { ( 4 \times 1 6 ^ { 2 } ) ^ { 2 } } { ( 2 ^ { 2 } ) ^ { 2 } + ( 4 ^ { 2 } ) ^ { 2 } + ( 8 ^ { 2 } ) ^ { 2 } + ( 1 6 ^ { 2 } ) ^ { 2 } } = 1 5 . +$$ + +# 4.3 Baselines + +Baseline methods include: UNet [39], FNO [25], CNextU-net [17], which are trained on individual dynamics; DPOT [13], MPP [33], VICON [5], which are PDE foundation models jointly trained on several dynamics. All of the above is based on a deterministic neural operator setting. The results are listed in Tab. 1. The entries with $^ *$ is the implementation provided in the Well benchmark [34]. We provide the reconstruction error in L2RE and VRMSE of our P2VAEs to demonstrate the compression loss level due to the autoencoder structure for further comparisons. Note that since we unified the sub-datasets to $\mathsf { p } 1 2 8 \mathsf { c } 3$ and float16, the metrics taken from other papers could be different on our format-unified dataset. + +Table 1: P2VAE reconstruction error compared to benchmark PDE models. The best (or better) results among the existing statistics are in bold. + +
L2REFNO-v5FNO-v4FNO-v3PA-NSPA-NSCPA-SWEPB-CNSLPB-CNSHPB-SWEW-AMW-GSW-SWEW-RBW-SFW-TRW-VE
UNet0.1980.1190.02450.1020.3370.4630.3130.0521
FNO0.1160.09220.01560.2100.3840.1530.1300.00912
DPOT-30M0.05530.04420.01310.09910.3160.01530.02450.00657
MPP-116M0.06170.1640.209
VICON-88M0.1110.15610.0597
PVAE-16M P2VAE-87M0.0890 0.08020.0850 0.07320.124 0.1150.0651 0.05820.06040.1093 0.10390.02670.0334 0.03250.438 0.1860.0466 0.03290.0774 0.04000.0629 0.05960.105 0.08020.0956 0.08460.0401 0.03740.0360 00274
05270.0266
VRMSEFNO-v5FNO-v4FNO-v3PA-NSPA-NSCPA-SWEPB-CNSLPB-CNSHPB-SWEW-AMW-GSW-SWEW-RBW-SFW-TRW-VE
UNet* FNO*0.24890.22520.36201.48603.4470.24180.4185
CNextU-net*0.3691 0.10340.1365 0.17610.1727 0.37240.8395 0.66991.189 0.80800.5001 0.119560.7212 0.2499
PVAE-16M0.49160.11260.24990.17180.28380.2962
P2VAE-87M0.22400.24570.27210.09360.08500.11350.60280.33860.65040.40640.32980.09510.18860.14530.23240.1568
0.18860.21920.19860.08280.07430.10740.44440.27140.29450.2016
+ +# 4.4 Downstream evaluation results + +Adapting foundation model to isotropic Kolmogorov turbulence According to REPA-E [22], we finetune the pretrained model (P2VAE $^ +$ FMT) to adapt to an unseen system with a stop-gradient operation after the generation of latent states $\mathbf { y }$ , so that the conditional flow marching loss won’t deteriorate the autoencoder. The end-to-end finetuning loss is derived as + +$$ +\begin{array} { r } { \mathcal { L } ( \theta , \phi , \omega ) = \mathcal { L } _ { \mathrm { C F M } } ( \theta , \phi ) + \lambda _ { \mathrm { V A E } } \mathcal { L } _ { \mathrm { V A E } } ( \omega ) . } \end{array} +$$ + +We conduct the experiment on an isotropic Kolmogorov turbulence dataset with $u$ and $v$ fields at $R e = 2 2 2$ [40]. We finetuned our FMT-B-42M model on 200 of the training trajectories in the train set for $5 \mathrm { k }$ steps with $\lambda _ { \mathrm { V A E } } = 1$ and test the performance on 500 trajectories in the test set. The metrics are shown in Table. 2, and one exemplary vorticity $\begin{array} { r } { ( \omega = \frac { \partial v } { \partial x } - \overline { { \frac { \partial u } { \partial y } } } ) } \end{array}$ reconstruction and prediction case is shown in Fig. 2. + +![](images/figures/neural-operator-flow-matching-pde-fig-0002.jpg) +Figure 2: Reconstructed and predicted vorticity by the finetuned model + +Long-term rollout We test the long-term rollout performance on the PDEArena-NS, PDEBench-CNS-Low, and PDEBench-CNS-High datasets of our model, and make the comparison with the statistics in VICON [5], as shown in Tab. 3. We plot sample trajectories based on FMT-B-42M in A.3. + +Table 2: Few-shot adaptation result on the Kolmogorov turbulence dataset +Table 3: Comparison of long term rollout errors (in L2RE), with best results in bold. + +
L2RECaseFMT-S-6MFMT-B-42MFMT-L-138MVICON-88M
Step 1PA-NS0.10600.08790.07450.1110
PB-CNS-Low0.09600.07960.05570.1561
PB-CNS-High0.08900.04500.04110.0597
Step 5PA-NS0.18890.13550.12920.2300
PB-CNS-Low0.09570.09580.08720.2456
PB-CNS-High0.10910.09920.07970.1973
Step 10PA-NS0.33180.22340.20880.3618
PB-CNS-Low0.14440.11190.10040.3747
PB-CNS-High0.16210.12180.11980.5788
Last stepPA-NS0.56640.61340.52710.7781
PB-CNS-Low0.28200.12980.13110.3903
PB-CNS-High0.21300.13920.12790.7117
AveragePA-NS0.41760.38590.30480.5627
PB-CNS-Low0.14800.10350.09600.2708
PB-CNS-High0.14970.10920.09140.3006
+ +In DPOT [13], the authors noticed that injecting noise during training would benefit the long-term rollout capability because the distribution of model-predicted states can be partially taken into account. However, they lack a systematic way to evaluate the noise level, which in turn demands a hyperparameter tuning process. In contrast, our model can deal with any noisy state because the distribution $q _ { t } ^ { k } , k \in [ 0 , \overline { { 1 } } ]$ has been modelled, so that any misaligned predicted states are exposed during the training implicitly, which in turn minimize the exposure bias [2, 16] during long term prediction. + +Generate ensemble of next states By tuning bridge parameter $k _ { 3 }$ during the generation, we can effectively generate an ensemble of possible next state given a noisy initialization $k _ { 3 } { \bf x } _ { 3 } + ( 1 - k _ { 3 } ) { \bf z }$ and concluded PDE condition $\mathbf { h } _ { 3 }$ from clean past frames $\left( \mathbf { x } _ { 0 } , \mathbf { x _ { 1 } } , \mathbf { x _ { 2 } } \right)$ . + +We sampled one trajectory from PDEArena-NS and tested it on the FMT-B-42M model to generate a 32-batch size ensemble. The variance of the predicted ensemble is a decreasing function of prior noise level $k _ { 3 }$ as shown in Fig. 3. Selected generated samples at different $k _ { 3 }$ are displayed in A.4. + +![](images/figures/neural-operator-flow-matching-pde-fig-0003.jpg) +Figure 3: Average of batch-wise variation of $\mathbf { x } _ { 4 }$ ensemble generated at different $\mathbf { x } _ { 3 }$ noise levels $k _ { 3 }$ + +# 5 Conclusion + +In this paper, we propose a conditional flow marching algorithm with a diffusion forcing scheme to construct a generative PDE foundation model that predicts future states given a series of past states. Empowered by a diverse training dataset, it displays excellent few-shot adaptation performance on unseen isotropic Kolmogorov turbulence; it allows accurate long-term rollout ability by reducing the exposure bias through generative modeling; and also allows generating reasonable and new physical dynamics data from noise. + +We envision the current generative model to serve as a foundational tool for PDE-related applications that have a real-world impact. 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URL https://arxiv.org/abs/2312.03687. + +# A Technical Appendices and Supplementary Material + +# A.1 Derivations for posterior mean $\mathbf { g } ^ { * }$ and continuity equation for location-scale interpolation kernel + +We mentioned that regressing $\mathbf { g }$ to $\mathbf { u } _ { t } ^ { k } = ( \mathbf { x } _ { 1 } - \mathbf { x } _ { t } ^ { k } ) / ( 1 - t )$ would recover the posterior-mean transporting field $\mathbf { g } ^ { * }$ . The claim comes from the continuity equation. + +Define the mixture marginal + +$$ +q _ { t } ( \mathbf { x } ) = \mathbb { E } _ { ( \mathbf { x } _ { 0 } , \mathbf { x } _ { 1 } , k ) } \left[ q _ { t } ^ { k } ( \mathbf { x } | \mathbf { x _ { 0 } } , \mathbf { x _ { 1 } } ) \right] . +$$ + +For any smooth test function $\phi$ , + +$$ +\frac { d } { d t } \mathbb { E } _ { q _ { t } } [ \phi ( \mathbf { x } _ { t } ^ { k } ) ] = \mathbb { E } _ { q _ { t } } [ \nabla \phi ( \mathbf { x } _ { t } ^ { k } ) \cdot \mathbf { u } _ { t } ^ { k } ] = \int \nabla \phi ( \mathbf { x } ) \cdot \underbrace { \mathbb { E } [ \mathbf { u } _ { t } ^ { k } \mathbf { x } _ { t } ^ { k } = \mathbf { x } , t } _ { : = \mathbf { g } ^ { * } ( \mathbf { x } , t ) } q _ { t } ( \mathbf { x } ) d \mathbf { x } +$$ + +Integrating by parts gives + +$$ +\frac { d } { d t } \int \phi ( { \bf x } ) q _ { t } ( { \bf x } ) d { \bf x } = - \int \phi ( { \bf x } ) \nabla _ { \bf x } \cdot ( q _ { t } ( { \bf x } ) { \bf g } ^ { * } ( { \bf x } , t ) ) d { \bf x } , +$$ + +because $\begin{array} { r } { \int _ { \partial \Omega } \phi ( \mathbf { x } ) \mathbf { g } ^ { * } ( \mathbf { x } , t ) q _ { t } ( \mathbf { x } ) n ( \mathbf { x } ) d \mathbf { S } = 0 , \phi \in \mathcal { C } _ { c } ^ { \infty } ( \Omega ) } \end{array}$ . Since this holds for all $\phi , q _ { t }$ and $\mathbf { g } ^ { * }$ satisfy the continuity equation + +$$ +\begin{array} { r } { \partial _ { t } q _ { t } ( \mathbf { x } ) + \nabla _ { \mathbf { x } } \cdot ( q _ { t } ( \mathbf { x } ) \mathbf { g } ^ { * } ( \mathbf { x } , t ) ) = 0 , } \end{array} +$$ + +The location-scale interpolation kernel is admissible – it induces a path of densities $q _ { t }$ that is transported by the velocity field equal to the posterior mean $\mathbf { g } ^ { * }$ of the sample-wise velocities $\mathbf { u } _ { t } ^ { k }$ ; smoothness and integrability are valid. + +# A.2 Dataset description + +All the data are compressed to float16 (half) precision to enable the Data Distributed Parallel training on a 4 H-100 GPU node. + +FNO-v We upsampled original data from c1p64 to c3p128 (the 2nd and 3rd dimension are filled with zero). The dataset size is expanded from 11.1GB to 21GB. Trajectory count: FNO-v5 – 15.4k, FNO-v4 – 368k, FNO-v3 – 184k. + +PDEArena For the PDEArena-NavierStokes(PA-NS) and PDEArena-NavierStokesCond(PA-NSC), the dataset size is compressed from 60GB to 25GB. For the PDEArena-ShallowWaterEquation(PA-SWE), it was compressed to 62GB from 76.6GB. Trajectory count: PA-NS – 48k, PA-NSC – 120k, PA-SWE – 470k. + +PDEBench For the PDEBench-CompressibleNavierStokes(PB-CNS), unimportant physical fields are filtered. Thus, it becomes 65GB compressed from 551GB. For the PDEBench-ShallowWaterEquation(PB-SWE), it is compressed to 0.3GB from 6.2GB, the 2nd and 3rd dimension is filled with zero. Trajectory count: PB-CNS – 598k, PB-SWE – 77.6k. + +The Well For the Well-GrayScott(W-GS), we fill the 3rd dimension with zero, ending up with a 5.3GB data set compressed from 153GB. For the Well-ActiveMatter(W-AM), we downsampled the data from $\mathrm { c } 3 \mathrm { p } 2 5 6$ to c3p128, and obtained a compressed 1.1GB dataset from 51.3GB. For the Well-PlanetShallowWaterEquation(W-SWE), we downsampled the data from $\mathrm { c } 3 \mathrm { p } 2 5 6 { , } 5 1 2$ to c3p128 and filtered out unimportant fields, so the data size is compressed to 9.3GB from 185.8GB. For the Well-RayleighBenard(W-RB), we downsampled the data from c3p512,128 to c3p128, and get a 26GB dataset from 342GB original data. For the Well-ShearFlow(W-SF), it is compressed to 14GB from 547GB by filtering out unimportant fields. For the Well-TurbulentRadiativeLayer2D(W-TR), it is downsampled from c3p128,384 to c3p128, thus compressed to 0.5GB from 6.9GB. For the Well-ViscoElasticInstability(W-VE), it is downsampled from c3p512 to c3p128, thus compressed to 0.5GB from 66GB. Trajectory count: W-GS – 92.2k, W-AM – 13.4k, W-SWE – 96.4k, W-RB – 266.6k, W-SF – 175.6k, W-TR – 7k, W-VE – 5.3k. + +# A.3 Rollout visualizations + +![](images/figures/neural-operator-flow-matching-pde-fig-0004.jpg) +Figure A1: Sampled long-term rollout trajectories from PDEArena-NS by FMT-B-42M. Upper row: prediction. Bottom row: ground truth. + +![](images/figures/neural-operator-flow-matching-pde-fig-0005.jpg) +Figure A2: Sampled long-term rollout trajectories from PDEBench-CNS-Low by FMT-B-42M. Upper row: prediction. Bottom row: ground truth. + +# A.4 Generated ensemble at different $k _ { 3 }$ + +![](images/figures/neural-operator-flow-matching-pde-fig-0006.jpg) +Figure A3: Sampled long-term rollout trajectories from PDEBench-CNS-High by FMT-B-42M. Upper row: prediction. Bottom row: ground truth. + +![](images/figures/neural-operator-flow-matching-pde-fig-0007.jpg) +Figure A4: Generated ensembles at different $k _ { 3 }$ : 0, 0.3, 0.6, 0.9. \ No newline at end of file diff --git a/papers/neural-operator-flow-matching-pde/paper.pdf b/papers/neural-operator-flow-matching-pde/paper.pdf new file mode 100644 index 0000000000000000000000000000000000000000..88ba430a0267bfbb39552e5286222fbacb9c5257 --- /dev/null +++ b/papers/neural-operator-flow-matching-pde/paper.pdf @@ -0,0 +1,3 @@ +version https://git-lfs.github.com/spec/v1 +oid sha256:756f132b2cca13e7d4f0f242043593cd19f8b8a8f2714adbc3213582c795e079 +size 1339193 diff --git a/papers/neural-operator-flow-matching-pde/sau.json b/papers/neural-operator-flow-matching-pde/sau.json new file mode 100644 index 0000000000000000000000000000000000000000..eb8ab6cea37ccd7a0263b6b8569028b8392fa960 --- /dev/null +++ b/papers/neural-operator-flow-matching-pde/sau.json @@ -0,0 +1,202 @@ +{ + "paper_id": "neural-operator-flow-matching-pde", + "paper_title": "Bridging Neural Operator and Flow Matching for a Generative PDE Foundation Model", + "D1": [ + { + "id": "neural-operator-flow-matching-pde-D1-001", + "claim": "The heterogeneous PDE corpus comprises 2.5M trajectories of length 4 across 12 PDE families, totaling 233GB in float16 precision at c3p128 spatial resolution with up to 3 physical fields, sourced from FNO-v, PDEArena, PDEBench, and The Well.", + "source": "Sec4.1,Sec1" + }, + { + "id": "neural-operator-flow-matching-pde-D1-002", + "claim": "Sub-dataset trajectory counts from the appendix: FNO-v5 15.4k, FNO-v4 368k, FNO-v3 184k, PA-NS 48k, PA-NSC 120k, PA-SWE 470k, PB-CNS 598k, PB-SWE 77.6k, W-GS 92.2k, W-AM 13.4k, W-SWE 96.4k, W-RB 266.6k, W-SF 175.6k, W-TR 7k, and W-VE 5.3k.", + "source": "SecA.2" + }, + { + "id": "neural-operator-flow-matching-pde-D1-003", + "claim": "The P2VAE autoencoder uses an SD-VAE architecture that compresses states from c3p128 input to c16p16 latent format at 12x compression rate, with two parameter variants: 16M (base dims 64) and 87M (base dims 128).", + "source": "Sec4.1" + }, + { + "id": "neural-operator-flow-matching-pde-D1-004", + "claim": "The FMT (Flow Marching Transformer) uses an AdaLN-Zero conditioned SiT Transformer backbone with RMSNorm normalization, SwiGLU FFN activation, FlashAttention v2 with head dim 64, and a GRU RNN for diffusion forcing with state compression via cross-attention to a single token; three model variants: S-6M/256d, B-42M/512d, and L-138M/768d.", + "source": "Sec4.1" + }, + { + "id": "neural-operator-flow-matching-pde-D1-005", + "claim": "P2VAE training uses AdamW optimizer with beta1=0.9, beta2=0.995, base learning rate 1e-4, batch size 256, cosine learning rate schedule with 10% linear warmup, weight decay 1e-4, trained for 100k steps with KL weight beta=1e-3.", + "source": "Sec4.1" + }, + { + "id": "neural-operator-flow-matching-pde-D1-006", + "claim": "FMT training uses AdamW optimizer with beta1=0.9, beta2=0.95, base learning rate 1e-4, batch size 256, cosine learning rate schedule with 10% linear warmup, weight decay 0.01, trained for 100k steps with frozen P2VAE-16M encoder on 4 consecutive latent states.", + "source": "Sec4.1" + }, + { + "id": "neural-operator-flow-matching-pde-D1-007", + "claim": "Flow marching samples t and k independently from Uniform(0,1) during training; k=1 corresponds to the deterministic neural operator mode and k=0 corresponds to the fully stochastic flow matching mode.", + "source": "Sec3.1" + }, + { + "id": "neural-operator-flow-matching-pde-D1-008", + "claim": "The latent temporal pyramid applies per-state downsample factors of 8, 4, 2, and 1 to the four consecutive input latent states (y_0 through y_3) respectively, with earlier states more aggressively compressed.", + "source": "Sec3.3" + }, + { + "id": "neural-operator-flow-matching-pde-D1-009", + "claim": "Inference uses an Euler ODE sampler with N=100 discretization steps and dt=0.01 per physical step; prediction mode sets all bridge parameters to k=[1,1,1,1] for deterministic rollout, while generation mode uses k=[1,1,1,<1] to stochastically sample the current state from a clean history.", + "source": "Sec3.4" + }, + { + "id": "neural-operator-flow-matching-pde-D1-010", + "claim": "The computational efficiency gain eta=15 is achieved by the latent temporal pyramid compared to a baseline of 4 frames at 256 tokens per frame using full bidirectional self-attention.", + "source": "Sec4.2" + }, + { + "id": "neural-operator-flow-matching-pde-D1-011", + "claim": "Downstream finetuning on isotropic Kolmogorov turbulence at Re=222 with u and v velocity fields uses FMT-B-42M, finetuned on 200 training trajectories for 5k steps with lambda_VAE=1 and stop-gradient on latent states y, evaluated on 500 test trajectories with ensemble batch size 32.", + "source": "Sec4.4" + }, + { + "id": "neural-operator-flow-matching-pde-D1-012", + "claim": "Training runs on 4 Nvidia H100 GPUs per node using Distributed Data Parallel (DDP) in float16 precision.", + "source": "SecA.2" + }, + { + "id": "neural-operator-flow-matching-pde-D1-013", + "claim": "The dataset uses an 80:10:10 train/validation/test split when no original partition exists, with equal-probability sampling across datasets following DPOT practice.", + "source": "Sec4.1" + } + ], + "D2": [ + { + "id": "neural-operator-flow-matching-pde-D2-001", + "claim": "Flow Marching Location-Scale Interpolation Kernel: x_t^k = mu_t + sigma_t * z, where mu_t = t*x_1 + k*(1-t)*x_0, sigma_t = (1-t)*(1-k), z ~ N(0,I)", + "source": "Sec3.1 Eq.2" + }, + { + "id": "neural-operator-flow-matching-pde-D2-002", + "claim": "Conditional Gaussian Distribution of Interpolated States: q_t^k(x_t^k | x_0, x_1, k) ~ N(t*x_1 + k*(1-t)*x_0, (1-t)^2 * (1-k)^2 * I)", + "source": "Sec3.1 Eq.4" + }, + { + "id": "neural-operator-flow-matching-pde-D2-003", + "claim": "Sample-wise Velocity Field (Oracle Target): u_t^k = d/dt x_t^k = (x_1 - x_t^k) / (1 - t)", + "source": "Sec3.1 Eq.5" + }, + { + "id": "neural-operator-flow-matching-pde-D2-004", + "claim": "Score-Velocity Decomposition: u_t^k = (x_1 - k*x_0) + (1-t)*(1-k)^2 * grad_x log q_t^k(x_t^k)", + "source": "Sec3.1 Eq.8" + }, + { + "id": "neural-operator-flow-matching-pde-D2-005", + "claim": "Conditional Score Function: grad_x log q_t^k(x_t^k) = -(x_t^k - mu_t) / sigma_t^2 = -z / ((1-t)*(1-k))", + "source": "Sec3.1 Eq.6" + }, + { + "id": "neural-operator-flow-matching-pde-D2-006", + "claim": "Flow Marching Objective (Preconditioned, k-free): L_FM = (1/2) * E_{(x_0,x_1),k,t} [ || (1-t) * g_theta(x_t^k, t) - (x_1 - x_t^k) ||^2 ]", + "source": "Sec3.1 Eq.10" + }, + { + "id": "neural-operator-flow-matching-pde-D2-007", + "claim": "Conditional Flow Marching Objective: L_CFM = (1/2) * E [ || (1-t) * g_theta(x_t^k, t, h) - (x_1 - x_t^k) ||^2 ]", + "source": "Sec3.2 Eq.12" + }, + { + "id": "neural-operator-flow-matching-pde-D2-008", + "claim": "FMT Full Training Objective with Diffusion Forcing: L_CFM = (1/2) * E_{h_s ~ p_phi(h_s | h_{s-1}, x_{s,t_s}^{k_s}, t_s)} [ sum_{i=1}^{T} || (1-t_s) * g_theta(x_{s,t_s}^{k_s}, t_s, h_{s-1}) - (x_{s+1} - x_{s,t_s}^{k_s}) ||^2 ]", + "source": "Sec3.2 Eq.13" + }, + { + "id": "neural-operator-flow-matching-pde-D2-009", + "claim": "Diffusion Forcing Latent State Update: h_s ~ p_phi(h_s | h_{s-1}, x_{s,t_s}^{k_s}, t_s), parametrized by GRU, with x compressed to single token via cross-attention", + "source": "Sec3.2,Sec4.1" + }, + { + "id": "neural-operator-flow-matching-pde-D2-010", + "claim": "Pretrained Physics Variational Autoencoder (P2VAE): y = E_omega(x), x_hat = D_omega(y)", + "source": "Sec3.3 Eq.14" + }, + { + "id": "neural-operator-flow-matching-pde-D2-011", + "claim": "P2VAE Training Loss: L_VAE = (1/2) * E[ ||x - x_hat||^2 ] + beta * KL( q_omega(y|x) || p(y) )", + "source": "Sec3.3 Eq.14" + }, + { + "id": "neural-operator-flow-matching-pde-D2-012", + "claim": "End-to-End Finetuning Loss for Downstream Adaptation: L(theta, phi, omega) = L_CFM(theta, phi) + lambda_VAE * L_VAE(omega)", + "source": "Sec4.4 Eq.15" + }, + { + "id": "neural-operator-flow-matching-pde-D2-013", + "claim": "Latent Temporal Pyramid Construction: pyramid_inputs = ( Down(y_{0,t0}^{k0}, 8), Down(y_{1,t1}^{k1}, 4), Down(y_{2,t2}^{k2}, 2), y_{3,t3}^{k3} ), where y = E_omega(x)", + "source": "Sec3.3" + }, + { + "id": "neural-operator-flow-matching-pde-D2-014", + "claim": "Computational Efficiency Ratio vs Full-Length Diffusion: eta = (4 * 16^2)^2 / ( (2^2)^2 + (4^2)^2 + (8^2)^2 + (16^2)^2 ) = 15", + "source": "Sec4.2 Eq.16" + }, + { + "id": "neural-operator-flow-matching-pde-D2-015", + "claim": "Euler ODE Sampler for Flow Marching Inference (Discrete): x_{t+dt}^{k} = x_t^{k} + g_theta(x_t^{k}, t, h) * dt, with t from 0 to 1, dt = 0.01, N = 100 steps", + "source": "Sec3.4" + } + ], + "D3": [ + { + "id": "neural-operator-flow-matching-pde-D3-001", + "claim": "P2VAE Autoencoder Pretraining Protocol: (1) Prepare dataset: sample 12 PDE families with equal probability, all at c3p128/float16; (2) Architecture: SD-VAE backbone, encode from c3p128 to c16p16 (12x compression); (3) Optimization: AdamW beta1=0.9 beta2=0.995, base lr=1e-4 adjusted for batch_size=256; (4) Schedule: cosine learning rate with 10% linear warmup, weight_decay=1e-4; (5) Train 100k steps with KL weight beta=1e-3; (6) Train 2 variants: 16M params (base dims 64) and 87M params (base dims 128)", + "source": "Sec4.1,Sec3.3" + }, + { + "id": "neural-operator-flow-matching-pde-D3-002", + "claim": "FMT (Flow Marching Transformer) Pretraining Protocol: (1) Backbone: frozen P2VAE-16M for latent encoding; (2) Architecture: AdaLN-Zero conditioned SiT Transformer with RMSNorm, SwiGLU FFN, FlashAttention v2, head_dim=64; GRU for diffusion forcing with state compressed by cross-attention to single token; (3) Input: 4 consecutive latent states with latent temporal pyramid (Down 8,4,2,1), sample t,k ~ Unif(0,1) independently per physical step; (4) Optimization: AdamW beta1=0.9 beta2=0.95, base lr=1e-4 adjusted for batch_size=256; (5) Schedule: cosine learning rate with 10% linear warmup, weight_decay=0.01; (6) Train 100k steps; (7) Train 3 variants: S-6M (256d embedding), B-42M (512d), L-138M (768d)", + "source": "Sec4.1,Sec3.2,Sec3.3" + }, + { + "id": "neural-operator-flow-matching-pde-D3-003", + "claim": "P2VAE Reconstruction Quality Benchmark Protocol: (1) Evaluate P2VAE-16M and P2VAE-87M on all 16 sub-datasets (FNO-v3/4/5, PA-NS/NSC/SWE, PB-CNS-Low/High/SWE, W-AM/GS/SWE/RB/SF/TR/VE); (2) Compute L2 relative error (L2RE) and variance-normalized RMSE (VRMSE); (3) Compare against UNet, FNO, DPOT-30M, MPP-116M, VICON-88M baselines; (4) Note: datasets are unified to c3p128/float16 which may differ from original benchmark settings", + "source": "Sec4.3,Sec4.1" + }, + { + "id": "neural-operator-flow-matching-pde-D3-004", + "claim": "Few-Shot Adaptation to Isotropic Kolmogorov Turbulence Protocol: (1) Select pretrained P2VAE + FMT-B-42M; (2) Target dataset: isotropic Kolmogorov turbulence at Re=222, u and v velocity fields; (3) Apply stop-gradient operation after latent state generation y to protect autoencoder; (4) Finetune on 200 training trajectories for 5k steps using end-to-end loss L_CFM(theta,phi) + lambda_VAE * L_VAE(omega) with lambda_VAE=1; (5) Evaluate prediction quality on 500 test trajectories", + "source": "Sec4.4" + }, + { + "id": "neural-operator-flow-matching-pde-D3-005", + "claim": "Long-Term Rollout Stability Evaluation Protocol: (1) Compare FMT-S-6M, FMT-B-42M, FMT-L-138M against VICON-88M baseline; (2) Datasets: PA-NS, PB-CNS-Low, PB-CNS-High; (3) Use deterministic prediction mode with k=1 for all timesteps; (4) Run Euler ODE sampler with N=100, dt=0.01 per physical step; (5) Report L2RE at steps 1, 5, 10, last, and average over full rollout horizon", + "source": "Sec4.4" + }, + { + "id": "neural-operator-flow-matching-pde-D3-006", + "claim": "Ensemble Generation via Bridge Parameter k_3 Protocol: (1) Sample 1 trajectory from PDEArena-NS; (2) Use FMT-B-42M; (3) Set k_0=k_1=k_2=1 for clean history, vary k_3 within [0,1); (4) Initialize x_3 from noisy mixture k_3*x_3 + (1-k_3)*z where z~N(0,I); (5) Generate ensemble of batch size 32 for each k_3; (6) Run flow marching inference with Euler ODE N=100 dt=0.01; (7) Visualization at k_3 values: 0, 0.3, 0.6, 0.9", + "source": "Sec4.4,SecA.4" + } + ], + "D4": [ + { + "id": "neural-operator-flow-matching-pde-D4-001", + "claim": "Two-Stage Training Pipeline Order: Stage 1 — P2VAE autoencoder pretraining on all 12 PDE families (100k steps) → freeze P2VAE-16M → Stage 2 — FMT pretraining on frozen latent representations (100k steps) → optional downstream finetuning on target PDE", + "source": "Sec4.1" + }, + { + "id": "neural-operator-flow-matching-pde-D4-002", + "claim": "Flow Marching Training Step Order: (1) Sample consecutive pair (x_s, x_{s+1}) from trajectory; (2) Independently sample k_s, t_s ~ Unif(0,1); (3) Construct intermediate state x_{s,t_s}^{k_s} via location-scale kernel (Eq.2); (4) Encode to latent y via frozen P2VAE encoder; (5) Compute oracle target velocity as (x_{s+1} - x_{s,t_s}^{k_s}); (6) Compute preconditioned loss with diffusion forcing latent state h; (7) Update GRU latent state h_s from h_{s-1} and compressed current state", + "source": "Sec3.1,Sec3.2" + }, + { + "id": "neural-operator-flow-matching-pde-D4-003", + "claim": "Deterministic Prediction Inference Order: (1) Set k_0=k_1=k_2=k_3=1 (no noise, neural operator mode); (2) Encode past 4 states x_0,x_1,x_2,x_3 via P2VAE to latent y; (3) Construct latent temporal pyramid (Down 8,4,2,1); (4) For each autoregressive prediction step s=1,2,...: (4a) Run Euler ODE with N=100 dt=0.01 from t=0 to t=1 on x_{s,0}^{1} with condition h_{s-1}; (4b) Decode predicted latent y_s to x_s via P2VAE decoder; (4c) Update diffusion forcing state h_s via GRU; (4d) Shift context window forward by 1 frame", + "source": "Sec3.4" + }, + { + "id": "neural-operator-flow-matching-pde-D4-004", + "claim": "Few-Shot Downstream Adaptation Order: (1) Load pretrained P2VAE + FMT checkpoint; (2) Select target PDE dataset; (3) Apply stop-gradient after latent encoding y to protect pretrained autoencoder; (4) Jointly finetune FMT (L_CFM) and P2VAE (L_VAE) with lambda_VAE balance weight; (5) Evaluate adapted model on held-out test trajectories", + "source": "Sec4.4" + } + ] +} \ No newline at end of file diff --git a/papers/nfig/blacklist.txt b/papers/nfig/blacklist.txt new file mode 100644 index 0000000000000000000000000000000000000000..6d680887d2000f967fcc5442dee939c25034f09f --- /dev/null +++ b/papers/nfig/blacklist.txt @@ -0,0 +1,2 @@ +# Official repository +https://github.com/Pride-Huang/NFIG diff --git a/papers/nfig/config.yaml b/papers/nfig/config.yaml new file mode 100644 index 0000000000000000000000000000000000000000..8f05a8277576a46c39dc7777b982aca6cd4c678e --- /dev/null +++ b/papers/nfig/config.yaml @@ -0,0 +1,8 @@ +title: "NFIG: Multi-Scale Autoregressive Image Generation via Frequency Ordering" +pdf_url: "https://openreview.net/pdf?id=nHNYDM6PVz" +venue: "NeurIPS 2025" +year: "2025" +extra: + selection_index: 12 + domain: "Computer Vision" + paradigm: "New Algorithm / Architecture" diff --git a/papers/nfig/images/figures/nfig-fig-0001.jpg b/papers/nfig/images/figures/nfig-fig-0001.jpg new file mode 100644 index 0000000000000000000000000000000000000000..c86841e7b0ba7b42e1c642a9a85bce357b19a086 --- /dev/null +++ b/papers/nfig/images/figures/nfig-fig-0001.jpg @@ -0,0 +1,3 @@ +version https://git-lfs.github.com/spec/v1 +oid sha256:a621fb166fda7a74eb872666727c2ac0cf7a49f1176c1bcdaa19b8211f099252 +size 82280 diff --git a/papers/nfig/images/figures/nfig-fig-0002.jpg b/papers/nfig/images/figures/nfig-fig-0002.jpg new file mode 100644 index 0000000000000000000000000000000000000000..efac6be9283cfba808f1aee21207e734471153fe --- /dev/null +++ b/papers/nfig/images/figures/nfig-fig-0002.jpg @@ -0,0 +1,3 @@ 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0000000000000000000000000000000000000000..47d6059521cc39c68e791750a757622818677334 --- /dev/null +++ b/papers/nfig/images/tables/nfig-table-0007.jpg @@ -0,0 +1,3 @@ +version https://git-lfs.github.com/spec/v1 +oid sha256:a98c88e52caae0c597b1f03dae7174d652dbef39bef0f3bbf8311ad29fcebc14 +size 24144 diff --git a/papers/nfig/paper.md b/papers/nfig/paper.md new file mode 100644 index 0000000000000000000000000000000000000000..ed27783aceb3f8144f9141e1cc9058b9520bae97 --- /dev/null +++ b/papers/nfig/paper.md @@ -0,0 +1,624 @@ +# NFIG: Multi-Scale Autoregressive Image Generation via Frequency Ordering + +Zhihao Huang1,2 Xi Qiu2 Yukuo Ma2,4 Yifu Zhou1,2 Junjie Chen2 Hongyuan Zhang2,3,∗ Chi Zhang2,∗ Xuelong Li2,∗ + +1 Northwest Polytechnical University 2 TeleAI, China Telecom 3 University of Hong Kong 4 Beihang University huangzhihao@mail.nwpu.edu.cn, hyzhang98@gmail.com, zhangc120@chinatelecom.cn, xuelong_li@ieee.org + +# Abstract + +Autoregressive models have achieved significant success in image generation. However, unlike the inherent hierarchical structure of image information in the spectral domain, standard autoregressive methods typically generate pixels sequentially in a fixed spatial order. To better leverage this spectral hierarchy, we introduce Next-Frequency Image Generation (NFIG). NFIG is a novel framework that decomposes the image generation process into multiple frequency-guided stages. NFIG aligns the generation process with the natural image structure. It does this by first generating low-frequency components, which efficiently capture global structure with significantly fewer tokens, and then progressively adding higher-frequency details. This frequency-aware paradigm offers substantial advantages: it not only improves the quality of generated images but crucially reduces inference cost by efficiently establishing global structure early on. Extensive experiments on the ImageNet-256 benchmark validate NFIG’s effectiveness, demonstrating superior performance (FID: 2.81) and a notable $1 . 2 5 \times$ speedup compared to the strong baseline VAR-d20. The source code is available at https://github.com/Pride-Huang/NFIG. + +# 1 Introduction + +2 + +The synthesis of images has emerged as a fundamental challenge in computer vision [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12]. Rapid progress in this field has been propelled by deep generative models, such as autoregressive models (AR) [13], Generative Adversarial Networks (GANs), and diffusion models (SD) [14]. + +Despite remarkable advances in existing methods, AR models for image generation still face several fundamental challenges. On the one hand, due to their inherently local and sequential nature, most current AR models struggle to effectively capture long-range dependencies and global structure [15]. For example, PixelCNN [9] generates an image by predicting each pixel in a raster scanning sequence, which neglects the global image structure and relationships with distant elements. On the other hand, the generation process is computationally intensive and time-consuming, as AR models always generate pixels or patches sequentially in a predetermined order, with each new element requiring the computation of conditional probabilities based on all previously generated content [16, 17]. For instance, ViTVQ [18] requires more than 6 seconds to generate a $2 5 6 \times 2 5 6$ image over + +![](images/figures/nfig-fig-0001.jpg) +Figure 1: Illustration of three autoregressive image generation frameworks. The figure demonstrates three prediction approaches: Next-Patch Prediction (patch-based progression), Next-Scale Prediction (coarse-to-fine resolution generation), and Next-Frequency Prediction (NFIG), which performs image generation by progressively predicting and synthesizing frequency components from low to high, resulting in a coarse-to-fine spatial reconstruction. + +1024 steps, making it impractical for real-time applications. Most importantly, AR models face a fundamental challenge in defining a meaningful autoregressive sequence. Traditional AR models using raster scanning or predefined arbitrary orders fail to reflect the natural hierarchical structure and dependencies in images[19]. This improper sequence design makes it difficult for models to capture the true causal relationships between different image components, ultimately affecting the coherence and visual quality of generated outputs. + +To address these limitations, recent works have explored incorporating various improvements into the generation process. For example, Taming Transformer [20] partially addresses long-range dependency challenges through its discrete latent space and transformer architecture, but still suffers from computational inefficiency. Fast PixelCNN $^ { + + }$ [21] speeds up generation in convolutional autoregressive models by caching hidden states to avoid redundant computation, achieving up to $1 8 3 \times$ speedups, yet it doesn’t fundamentally change the autoregressive sequence design. VAR [19] leverages the Laplacian Pyramid as a prior to guide autoregressive image generation across different resolutions, achieving improved generation quality with reduced computational overhead. However, these methods do not fully exploit the potential of natural priors inherent in raw images to guide the generation process and improve the efficiency of AR models. + +In fact, the natural structure of images follows a hierarchical frequency distribution—low frequencies encode global structures while high frequencies contain local details. This organization suggests an efficient autoregressive generation sequence from low to high frequencies, aligning with visual information’s natural structure. Since low-frequency components require fewer tokens to represent, this approach enhances computational efficiency. Similar frequency-progressive principles have proven effective in diffusion models, which build from low-frequency foundations before adding higher-frequency details [22]. + +Motivated by this insight, we propose a Next-Frequency Image Generation (NFIG) framework for AR models that: (1) first generates a low-frequency image with few tokens to capture global structure; (2) then progressively adds higher-frequency components conditioned on the low-frequency foundation. This process has been shown in Figure 1. Grounded in information theory, this approach efficiently represents information across the frequency spectrum using our Frequency-guided Residual-quantized VAE (FR-VAE). + +Key contributions of the NFIG framework include: + +• We introduce a Next-Frequency Image Generation (NFIG) framework that incorporates frequency analysis into AR image generation. To our knowledge, this work is the first to guide autoregressive generation using the image’s frequency spectrum, associating low frequencies with lower resolutions and high frequencies with higher resolutions; • To demonstrate the feasibility of the NFIG paradigm, we design a Frequency-guided Residualquantized VAE as our image tokenizer. FR-VAE separates low and high-frequency components in the representation learning process, with low frequencies encoding global structure and high frequencies preserving local details. Experiments show FR-VAE achieves a reconstruction FID of 0.85, validating its image content preservation capability; + +• Through extensive experimentation, we show that our approach achieves state-of-the-art image generation quality, evidenced by an FID of 2.81 from a relatively small model. This improvement paves the way for more effective and efficient AR image generation models, making them more practical for real-world applications. + +Table 1: Main Notation Table + +
SymbolMeaningDimension
xInput imageH × W × 3
xReconstructed imageH × W × 3
fFeature map from VAE encoderH ′ × W ′ × C
MiThe i-th frequency selection maskH′ × W ′ × C
$f$Set of i-th frequency component feature mapsH′ × W ′ × C
viScaled feature map for i-th frequency componenthi × wi × C
Quantized representation for i-th frequency componenthi × wi × C
Cumulative signal residual through level iH′ × W ′ × C
ZThe learnable codebook of FR-VAEK×C
FiThe i-th frequency bandN/A
+ +# 2 Related Work + +Autoregressive Image Generation Autoregressive image generation has demonstrated remarkable capabilities in producing high-quality images by modeling the joint distribution of image tokens as a product of conditionals[23]. PixelCNN [9] generates images sequentially, processing pixels one by one (typically top-left to bottom-right). It employs masked convolutions so that the generation of each pixel depends solely on the pixels already generated. Taming Transformer [20] introduces an autoregressive approach that generates high-resolution images by predicting the next latent patch token in a discrete compressed space learned through vector quantization. Emu3 [24] patchifies an image into a series of tokens and generates images by predicting tokens in a raster-scan manner. VAR [19] incorporates the prior knowledge of Laplacian Pyramid into transformer architecture and generates images in a next-resolution manner. MAR [25] improves the quality of generated images by replacing discrete tokens with continuous features, recognizing that autoregressive models primarily need per-token probability distributions. FAR [26] attempts to enhance MAR performance through frequency-based approaches, yet lacks critical insights into the distinctive information characteristics across different frequency bands. Infinity [25] combines autoregressive modeling with Bit-wise Modeling to enhance visual details in high-resolution image synthesis. ImageFolder [27] utilizes folded image tokens to generate high-quality images in a next-scale prediction manner, achieving superior performance. These approaches collectively demonstrate the evolution of autoregressive image generation techniques, progressing from pixel-level prediction to more sophisticated methods involving latent spaces, hierarchical structures, and physical priors. Despite their differences in implementation, all these methods share the fundamental autoregressive principle of sequentially generating image elements conditioned on previously generated content. + +Image Tokenizer Image tokenizers, which transform continuous image data into discrete representations, have become a critical component in modern image generation systems. VQ-VAE [28] introduces vector quantization into VAE, reducing the pressure on the downstream generative model by transforming the continuous latent space into a discrete one. VQ-VAE-2 [29] extends this idea with a hierarchical framework and multi-scale codebooks, where top-level codes capture global structure, and bottom-level codes model local details, enabling higher-quality reconstruction and generation at increased resolutions. However, VQ-VAE-based methods often suffer from codebook collapse, where only a few codebook entries are effectively used. FSQ [30] attempts to address this issue by utilizing finite scalar quantization to learn the codebook, but it does not learn a meaningful feature of images. RQ-VAE [31] employs residual quantization with a shared codebook to enhance reconstructed image quality. XQGAN [32] introduces feature product decomposition and a residual quantizer to enhance VQ-VAE’s performance, leading to improved image generation results. While these approaches have made significant progress in image tokenization, they often neglect the inherent multi-scale structure of natural images, which is crucial for efficient and effective representation learning. + +![](images/figures/nfig-fig-0002.jpg) +Figure 2: Overview of the Next-Frequency Image Generation (NFIG) Framework: (a) The Frequencyguided Residual-Quantization VAE encodes images into and decodes from frequency-guided residual quantized representations; (b) The image is decomposed into frequency components (low to high) and reconstructed progressively by merging these components for a coarse-to-fine process; (c) Next-Frequency Prediction model employs a frequency-aware Transformer to auto-regressively generate token sequences, with each block with same color representing a specific frequency band, enabling sequential image synthesis from low to high frequencies. $\tilde { N _ { i } ^ { t o k } } \doteq h _ { i } w _ { i }$ is the number of image tokens used for the $i _ { t h }$ frequency band. + +# 3 Next-Frequency Image Generation + +To provide a comprehensive understanding of our NFIG methodology, this section delves into its intricate architectural structure. The essential operational sequence, illustrating the flow and interaction of the system’s key components, is clearly visualized in Figure 2. The details of loss function have been listed in Appendix B.1. + +# 3.1 Frequency-guided Residual-quantized VAE + +The workflow of Frequency-guided Residual-quantized VAE has been shown in Figure 2 (a). To generate images in a frequency-aware manner, we propose a Frequency-guided Residual-quantized VAE (FR-VAE) with VQ-GAN framework. The key idea is to represent lower-frequency signals with fewer tokens and higher-frequency components with more tokens. + +# 3.1.1 Frequency-guided Reconstruction + +As illustrated in Figure 2 (b), raw images can be decomposed into components across different frequency bands: low frequencies encode the global structure, while high frequencies retain fine details. Utilizing the Frequency-guided Decomposer and Composer, these components can be recombined without loss, ensuring a complete and accurate visual representation. + +Frequency-guided Decomposer. Given a image $x \in R ^ { H \times W \times 3 }$ and a encoder $E ( \cdot )$ , there is image latent feature $f = E ( x )$ and $f \in \mathbb { R } ^ { H ^ { \prime } \times W ^ { \prime } \times C }$ . FR-VAE decomposes $f$ into several component with different frequency by Frequency-guided Decomposer via Fast Fourier Transform (FFT): + +$$ +\hat { f } _ { i } = \mathcal { F } ^ { - 1 } ( \mathcal { F } ( f ) \odot M _ { i } ) , \forall i \in \{ 1 , \cdots , n \} . +$$ + +Here, $\odot$ represents the element-wise product, $\mathcal { F }$ signifies the FFT operation, ${ \mathcal { F } } ^ { - 1 }$ indicates the inverse FFT, and $M _ { i }$ is the $i$ -th frequency mask used to select the desired frequency range, $n$ is the total number of frequency masks, $\hat { f } _ { i }$ is the component corresponding to $M _ { i }$ . + +Frequency-guided Composer. Frequency-guided Composer reconstructs the raw image by interpolating different frequency components to a uniform size and merging them into a single image, as illustrated in Figure 2 (b). + +$$ +\tilde { f } = \sum _ { i = 1 } ^ { n } \mathcal { T } ( \hat { f } _ { i } , H ^ { \prime } , W ^ { \prime } ) , +$$ + +where $\mathcal { T } ( \cdot , H ^ { \prime } , W ^ { \prime } )$ is the interpolation function, which enables the Frequency-guided Composer to process images of different frequencies at varying resolutions. + +# 3.1.2 Frequency-guided Residual-quantization + +To efficiently represent images with minimal tokens, we implement a frequency-guided residual quantization approach that addresses information loss during downsampling. Our method progressively captures different frequency components of an image through a residual learning scheme. + +Residual Token Extraction. Given a sequence of feature maps with different dimensions $\{ ( h _ { 1 } , w _ { 1 } ) , \cdot \cdot \cdot , ( h _ { n } , w _ { n } ) \}$ , where $h _ { i } \geq h _ { j }$ and $w _ { i } \geq w _ { j }$ if $i \geq j$ , and $h _ { n } = H ^ { \prime }$ and $w _ { n } = W ^ { \prime }$ , we supervise the learning process using accumulated signals from the lowest frequency to the current frequency band. + +The residual $R _ { i } \in \mathbb { R } ^ { H ^ { \prime } \times W ^ { \prime } \times C }$ and representation $v _ { i } \in \mathbb { R } ^ { h _ { i } \times w _ { i } \times C }$ of the $i$ -th frequency component can be computed as follows: + +$$ +\begin{array}{c} \begin{array} { r l r } & { } & { R _ { i } = \left\{ \hat { f } _ { i } - \mathcal { Z } ( v _ { i } , H ^ { \prime } , W ^ { \prime } ) ) , \right. \ } \\ & { } & { R _ { i } = \left\{ \begin{array} { l l } { \displaystyle { \hat { R } _ { i } - 1 + ( \hat { f } _ { i } - \mathcal { Z } ( v _ { i } , H ^ { \prime } , W ^ { \prime } ) ) } ) , } & { i \ge 1 } \end{array} \right. , } \\ & { } & { v _ { i } = \left\{ \arg \operatorname* { m i n } _ { v _ { i } } \| \hat { f } _ { i } - \mathcal { Z } ( v _ { i } , H ^ { \prime } , W ^ { \prime } ) \| ^ { 2 } , \right. \ ~ \left. ~ i = 0 \right. } \\ & { } & { \left. \operatorname { a r g m i n } _ { v _ { i } } \| ( R _ { i - 1 } + \hat { f } _ { i } ) - \mathcal { Z } ( v _ { i } , H ^ { \prime } , W ^ { \prime } ) \| ^ { 2 } , \quad i \ge 1 \right.} \end{array} , \end{array} +$$ + +where $\mathcal { T } ( v _ { i } , H ^ { \prime } , W ^ { \prime } )$ is the interpolation function that upsamples $v _ { i }$ to the original feature map size, and $R _ { i }$ represents the difference between the accumulated frequency components up to the $i$ -th level and the learnable features. + +Vector Quantization. In general, autoregressive models utilize the discrete tokens to generate a image. To achieve this goal, we take a simple vector quantization to transform the continuous token into discrete tokens. + +We define a quantizer $Q$ with a learnable codebook $Z \in \mathbb { R } ^ { K \times C }$ containing $K$ code vectors. Using this codebook, the quantizer $Q$ transforms a continuous feature map $v _ { i } \in \mathbb { R } ^ { h _ { i } \times w _ { i } \times C }$ into a set of ns {t(1,1)i , t(1,2)i , · $\{ t _ { i } ^ { ( 1 , 1 ) } , t _ { i } ^ { ( 1 , 2 ) } , \cdot \cdot \cdot , t _ { i } ^ { ( h _ { i } , w _ { i } ) } \}$ , where each token timal code represen $t _ { i } ^ { ( j , k ) }$ has the corresponds to a vector involves: $z _ { i } ^ { ( j , k ) } \in \mathbb { R } ^ { C }$ + +$$ +t ^ { ( j , k ) } = \mathrm { l o o k u p } ( Z , \underset { z _ { i } ^ { ( j , k ) } \in Z } { \arg \operatorname* { m i n } } \| z _ { i } ^ { ( j , k ) } - v _ { i } ^ { ( j , k ) } \| _ { 2 } ) . +$$ + +From $v _ { i }$ , the quantized feature map $v _ { i } ^ { q } \in \mathbb { R } ^ { h _ { i } \times w _ { i } \times C }$ and a set of discrete token s {t(j,k)i } a re obtained through quantization using the codebook $Z$ . Here, lookup $( Z , x )$ is a function that finds the index of the closest entry to $x$ in codebook $Z$ . + +# 3.2 Autoregressive Image Generation + +To generate images progressively from low to high frequency components, we implement a decoderonly transformer framework and block-wise causal attention [19]. + +Next-Frequency Image Prediction Unlike conventional autoregressive image generation models that employ a "token-by-token prediction" strategy, which often neglects spatial relationships and inherent image structure. NFIG adopts a “Coarse-to-Fine Generation" approach, first synthesizes the low-frequency components of an image, then iteratively incorporates higher-frequency details, progressively refining the generated output at each step, as shown in Figure 2 (c). The generation process for next-frequency prediction is given by the autoregressive factorization: + +$$ +p ( T _ { 1 } , T _ { 2 } , \cdot \cdot \cdot , T _ { n } ) = \prod _ { i = 1 } ^ { n } p ( T _ { i } | T _ { 1 } , T _ { 2 } , \cdot \cdot \cdot , T _ { i - 1 } ) +$$ + +where $T _ { i } \in [ K ] ^ { h _ { i } \times w _ { i } }$ is the matrix of code indices for the $i$ -th frequency component, and the set $[ K ] = \{ 1 , 2 , \dots , K \}$ represents all available index values. + +Frequency Band Division Strategy We treat the lower frequency components as the foundation for generating $T _ { i }$ , represented by $\{ \bar { T } _ { 1 } , T _ { 2 } , \cdot \cdot \cdot , T _ { i - 1 } \}$ . According to information theory principles, lower-frequency signals contain less information and require fewer tokens, while higher-frequency components carry more detailed information and need more tokens for accurate representation. + +Consequently, we establish an increasing scale sequence $\{ ( h _ { 1 } , w _ { 1 } ) , ( h _ { 2 } , w _ { 2 } ) , \colon \colon \colon , ( h _ { n } , w _ { n } ) \}$ for components with increasing frequency bands $\{ F _ { 1 } , F _ { 2 } , \cdot \cdot \cdot , F _ { n } \} = \{ [ 0 , \sigma _ { 1 } ) , [ \sigma _ { 1 } , \sigma _ { 2 } ) , \cdot \cdot \cdot , [ \sigma _ { n - }$ 1, σn]}. Here, $\sigma _ { m a x }$ denotes the maximum frequency of the entire image feature map $f$ , with $\sigma _ { n } = \sigma _ { m a x }$ . We divide the frequency bands based on their corresponding resolution as: + +$$ +\sigma _ { i } = \sigma _ { i - 1 } + \frac { h _ { i } \cdot w _ { i } } { \sum _ { j = 1 } ^ { n } h _ { j } \cdot w _ { j } } \times \sigma _ { m a x } . +$$ + +This frequency-guided progressive approach allows our model to capture and prioritize salient components at each stage. The method improves both computational efficiency and image quality by explicitly modeling the multi-scale frequency structure inherent in natural images. + +# 4 Experiment + +This section details our experimental methodology, covering datasets, evaluation metrics, comparison baselines, and implementation specifics. We then evaluate NFIG against state-of-the-art approaches on image generation benchmarks. Subsequently, ablation studies and motivation verification experiments are conducted to analyze the impact of different components and validate design decisions. + +# 4.1 Experimental Settings + +Dataset. For the purpose of our experiments, we use the ILSVRC 2012 subset of ImageNet [33], which comprises a total of 1.2 million training images, 50k validation images, and 100k test images. This subset focuses on 1k object categories, with each category having approximately $1 . 2 \mathrm { k }$ training images, 50 validation images, and 100 test images. + +Evaluation Metrics. We adopt four metrics for quantitative evaluation: Fréchet Inception Distance (FID) which measures distribution similarity between generated and real images, Inception Score (IS) which assesses quality and diversity, and Precision (Pre) and Recall (Rec) which evaluate sample fidelity and diversity coverage respectively. + +Baselines. Our method is benchmarked against several leading image generation techniques, including generative adversarial networks (GAN), diffusion models (Diff.), mask diffusion (Mask.), and autoregressive models (AR). These approaches have demonstrated strong performance on various image synthesis tasks and serve as robust comparators. + +Implementation Details. Our model was implemented using the PyTorch framework [44] and trained on NVIDIA H100 graphics cards. To ensure the reproducibility of our experiments, our implementation is built upon open-source research code, while incorporating improvements specific to this study. For the image tokenizer, the FR-VAE incorporates a VQGAN architecture with a DINO discriminator. The image encoder is initialized with pretrained weights from DINOv2-base. Since VAR’s image tokenizer training code is not open-source, we adopted XQGAN’s implementation strategy. The frequency residual quantizer employs multiple scaling factors $[ 1 , 2 , 3 , 4 , 5 , 6 , 8 , 1 0 , 1 3 , 1 6 ]$ across different frequency bands, resulting in a vocabulary size of 680 tokens. The FR-VAE codebook size of 4096 was utilized. The image generator employs a VAR Transformer backbone with a depth of 16, enabling multi-scale image prediction. Optimization was performed using the Adam optimizer, setting the learning rate to $8 \times 1 0 ^ { - 5 }$ and the batch size to 768. Training of the model ran for 350 epochs on the ImageNet dataset. For inference, we configured CFG to 4.5 and top_k to 990. + +Table 2: Performance on class-conditional ImageNet $2 5 6 \times 2 5 6$ for image generative model. rFID represents reconstruction FID , while gFID indicates generation FID. “↓” or “↑” indicate lower or higher values are better. “#Step”: the number of model runs needed to generate an image. Wall-clock inference time relative to NFIG is reported. Models with the suffix “-re” used rejection sampling. $\dagger$ : taken from MaskGIT [34]. For comprehensive evaluation, we separately compare autoregressive (AR) and non-autoregressive (non-AR) models, with the best metrics highlighted in bold. + +
TypeModelrFID↓gFID↓IS↑Pre↑Rec↑#Para#StepTime
GANBigGAN [35]-6.95224.50.890.38112M1
GANGigaGAN [36]-3.45225.50.840.61569M1
GANStyleGan-XL [3]2.30265.10.780.53166M10.75
Diff.ADM [37]-10.94101.00.690.63554M250420
Diff.CDM [38]4.88158.78100
Diff.LDM-4-G [39]-3.60247.7400M25025077.5
Diff.DiT-L/2 [40]0.95.02167.20.750.57458M
Diff.DiT-XL/2 [40]0.92.27278.20.830.57675M250112.5
Diff.L-DiT-3B [41]0.92.10304.40.820.603.0B250>112.5
Diff.L-DiT-7B [41]0.92.28316.20.830.587.0B250>112.5
Mask.MaskGIT [34]2.286.18182.10.800.51227M81.254.75
Mask.RCG [42]-3.49215.5502M20
ARVQVAE-2† [34]2.031.1145.00.360.5713.5B5120
ARVQGAN† [20]7.9418.6580.40.780.26227M25647.5
ARVQGAN [20]7.9415.7874.31.4B25660
ARViTVQ [18]1.284.17175.11.7B1024>60
ARViTVQ-re [18]1.283.04227.41.7B1024>60
ARRQTran. [43]1.837.55134.03.8B6852.5
ARRQTran.-re [43]1.833.80323.73.8B6852.5--
ARFAR-B [26]-4.26248.90.790.51208M10
ARFAR-B [26],3.45282.20.800.54427M10
ARFAR-H [26]-3.21300.60.810.55812M10-
ARXQGAN-310M [32]0.782.96---310M101
ARVAR-d16 [19]0.93.55274.40.840.51310M101
ARARVAR-d20 [19]NFIG(Ours)0.90.852.95302.60.830.56600M10101.251
2.81332.420.770.59310M
+ +# 4.2 Main Results + +Table 2 provides a detailed comparison of our approach against leading image generative models evaluated on ImageNet $2 5 6 \times 2 5 6$ . The findings indicate that NFIG achieves superior performance within the AR model family while establishing itself as a formidable competitor among diverse generative methods across different paradigms. + +AR Model Comparison. NFIG achieves the best gFID (2.81) and IS (332.42) scores, significantly outperforming other AR models. Compared to VAR-16 (gFID: 3.55, IS: 274.4), our approach reduces FID by 0.74 and improves IS by more than $21 \%$ . XQGAN-310M has a better image tokenizer with rFID 0.78 with gFID 2.96. This indicates that NFIG’s performance improvement is not solely due to a good image tokenizer, but more importantly, to the injection of image frequency priors. Additionally, the proposed approach outperforms VAR-d20, a relatively larger model, while delivering $2 5 \%$ faster inference speed. + +Cross-family Comparison. NFIG achieves competitive performance with the best models from other families. NFIG outperforms the best mask diffusion model RCG, which has a gFID score of 3.49 and IS score of 215.5. While some GANs like StyleGAN-XL have lower gFID scores (2.30) with moderate IS (265.1), and diffusion models like DiT-L/2 show excellent gFID (2.27) and strong IS (278.2), NFIG uniquely balances both metrics at high levels (gFID: 2.81, IS: 332.42). This establishes NFIG as not only the leading AR model but also a strong competitor across all model types. + +Qualitative Results. Figure 3 qualitatively shows NFIG’s impressive ability to generate diverse ImageNet $2 5 6 \times 2 5 6$ images across a wide variety of categories. Appendix B.5 show the Failure case of NFIG. + +Scaling Up. To validate NFIG’s scaling behavior, we train 310M and 600M parameter models for 55 epochs under computational constraints. The results are shown in Table 3. + +Performance of Different Epochs. VAR sets different training epochs for models of different sizes: 200 epochs for 310M, 250 epochs for 600M, 300 epochs for 1B, and 350 epochs for 2B. Limited by computational resources, we focus on training our 310M model. As Table 4 shows, NFIG outperforms VAR at matched epochs and demonstrates superior parameter efficiency. At 200 epochs, NFIG already outperforms VAR-d16 of the same size. With extended training, NFIG-310M achieves performance comparable to or better than VAR-d20-600M (twice the parameters) while using significantly fewer resources. + +Table 3: Performance of NFIG with different parameters at 55 epochs. + +
ModelFID↓IS↑Precision↑Recall↑EpochParastepsTime
NFIG-310M5.47224.200.75690.491455310M101
NFIG-600M5.07225.160.71840.554655600M101.25
+ +![](images/figures/nfig-fig-0003.jpg) + +Figure 3: Generated $2 5 6 \times 2 5 6$ examples by NFIG trained on Imagenet. +Table 4: The performance of NFIG and VAR at different epochs. + +
ModelEpochsFID↓IS↑Pre↑Rec↑ParamsStepsTime
VAR-d162003.55274.40.840.51310M101
VAR-d202502.95302.60.830.56600M101.25
NFIG(ours)2003.35309.20.790.55310M101
NFIG(ours)2503.16311.90.780.56310M101
NFIG(ours)3002.93325.90.790.56310M101
NFIG(ours)3502.81332.40.770.59310M101
+ +# 4.3 Ablation Study + +To evaluate the contribution of various components within our proposed NFIG model, we perform a comprehensive ablation analysis on the ImageNet validation set. Table 5 summarizes the results of this study, with performance evaluated using rFID and gFID. We start with the baseline AR model with a sequence length of 256, which achieves an rFID of 1.62 and a gFID of 18.65. + +Image Tokenizer. We incrementally add components to progressively improve the model’s performance. First, incorporating frequency-guided residual quantization into the VAR framework reduces the rFID to 1.40. Next, we integrate the DINO discriminator from VAR’s tokenizer, which substantially improves the rFID from 1.40 to 0.85. + +Transformer. We then utilize the image tokenizer (FR-VAE) to train the transformer model. Without Top_k and Classifier Free Guidance (CFG), NFIG achieves a gFID of 9.7. The addition of Top_k sampling strategy further reduces the gFID to 6.83. Finally, incorporating CFG yields the best overall performance, maintaining the rFID at 0.85 while dramatically improving the gFID to 2.81. + +Our experimental results demonstrate that the combination of FR-VAE with CFG provides optimal generation quality. Moreover, we observe that both DINO discriminator and FR-VAE contributes significantly to improving rFID for the image tokenizer. Additionally, Top_k sampling and CFG prove essential for reducing gFID. These results underscore the significance of discriminator guidance and conditional generation strategies for improving image generation quality. + +# 4.4 Motivation Verification + +Frequency Distribution Analysis. The experimental results in Figure 4 demonstrate the progressive refinement of generated images and the effective capture and synthesis of multi-scale visual features by FR-VAE. The frequency spectrum visualizations reveal the model’s ability to hierarchically incorporate information from low to high frequencies, resulting in generated images with rich details and natural appearance. Appendix B.2 compares the frequency keep ability of NFIG and VAR. These results demonstrate that NFIG’s frequency-guided approach enables more effective feature learning, particularly at lower resolutions, by maintaining balanced loss values throughout the hierarchical generation process. + +![](images/figures/nfig-fig-0004.jpg) +Figure 4: Generated images at different steps 2, 4, 6, 8, 10 of a 10-step process by FR-VAE, with corresponding frequency spectrum. In these spectrograms, brightness (red/yellow) indicates higher frequency energy while darker colors (blue) represent lower energy components. The center of each plot shows low-frequency information, with frequencies increasing radially outward, revealing the evolving distribution during the generation process. + +Frequency Guidance. Similar to NFIG, VAR follows a "coarse-to-fine" approach but differs significantly in loss computation across resolutions. VAR computes loss between different resolutions and the raw image, causing disproportionately large loss values at lower resolutions. In contrast, NFIG utilizes frequency components to guide feature learning, providing more balanced loss values throughout generation. As Figure 5 shows, this leads to dramatic variations in vector quantization loss—VAR exhibits substantially higher values across all scale factors, while NFIG maintains considerably lower loss values across all resolutions. + +![](images/figures/nfig-fig-0005.jpg) +Figure 5: Vector quantization loss comparison between NFIG and VAR across image scales. + +Table 5: Ablation study on the improvement of NFIG. We evaluate rFID and gFID on the ImageNet validation set. “FR-Quantizer” is the quantizer of FR-VAE. “DINO-Disc” means “DINO Discriminator”, which denotes the discriminator used in VAR’s ([19]) tokenizer. + +
TypeMethodLengthMetric
rFID↓gFID↓
1AR2561.6218.65
Image Tokenizer
2+ FR-Quantizer6801.40
3+ DINO-Disc6800.85
Generation Transformer
4+ AdaLN6800.859.7
5+ Top_k6800.856.83
6+ CFG6800.852.81
+ +# 5 Conclusion + +This paper introduces Next-Frequency Image Generation, a novel autoregressive framework that decomposes image generation into frequency-guided stages. Our key insight leverages the hierarchical spectral distribution of natural images: low-frequency components encode global structures and long-range dependencies, while high-frequency components contain local details requiring greater information entropy. By progressively generating from low to high frequencies, the proposed method significantly outperforms existing models with comparable parameter counts, demonstrating superior quality metrics while maintaining computational efficiency. Experiments confirm that our frequencyguided approach represents an important advancement in autoregressive image synthesis. + +# 6 Limitation and Future Work + +Our frequency-guided autoregressive image generation approach shows promise, but has limitations. Improving frequency decomposition. A primary issue is the simplistic frequency band division by scale, which inadequately captures information in the first band. Implementing a more rigorous division based on statistical analysis and physical principles would likely enhance NFIG’s performance. Recent advances in beneficial noise theory [45, 46] suggest that properly designed noise can reduce task complexity, which could inform better frequency decomposition and data augmentation strategies [47, 48]. Extension to other modalities. Beyond 2D spatial frequency, future work could extend to video generation by incorporating temporal frequency decomposition, or to 3D object generation where frequency analysis is vital for accurate light field representation. For multi-modal generation, techniques that enhance cross-modal alignment through learnable noise [49] may offer insights for frequency-based fusion strategies. Privacy-preserving generation. Adversarial noise techniques [50] could be integrated with our framework to ensure privacy protection in generated content. Due to computational constraints and time limitations, these promising directions remain unexplored and are left for future investigation. + +# References + +[1] Ian Goodfellow, Jean Pouget-Abadie, Mehdi Mirza, Bing Xu, David Warde-Farley, Sherjil Ozair, Aaron Courville, and Yoshua Bengio. Generative adversarial networks. Communications of the ACM, 63(11):139–144, 2020. +[2] Tero Karras, Samuli Laine, and Timo Aila. A style-based generator architecture for generative adversarial networks. In IEEE Conference on Computer Vision and Pattern Recognition, CVPR 2019, Long Beach, CA, USA, June 16-20, 2019, pages 4401–4410. Computer Vision Foundation / IEEE, 2019. + +[3] Axel Sauer, Katja Schwarz, and Andreas Geiger. Stylegan-xl: Scaling stylegan to large diverse datasets. 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Note that if the LLM is used only for writing, editing, or formatting purposes and does not impact the core methodology, scientific rigorousness, or originality of the research, declaration is not required. + +Answer: [Yes] + +Justification: None + +Guidelines: + +• The answer NA means that the core method development in this research does not involve LLMs as any important, original, or non-standard components. • Please refer to our LLM policy (https://neurips.cc/Conferences/2025/LLM) for what should or should not be described. + +# Appendix A: Fourier Analysis in Natural Images + +This appendix provides essential mathematical formulations and conceptual insights into the Fourier analysis of natural images, building upon the concepts discussed in the main text. We focus on the Discrete Fourier Transform (DFT) and its implications for image representation and the characteristics of natural scenes. + +# A.1 2D Discrete Fourier Transform (2D DFT) + +The 2D DFT transforms an $M \times N$ digital image $f ( x , y )$ from the spatial domain (where $x \in$ $\{ 0 , \ldots , M - 1 \}$ and $y \in \{ 0 , \ldots , N - 1 \}$ are spatial coordinates) to the frequency domain, yielding an $M \times N$ representation $F ( u , v )$ (where $u \in \{ 0 , \ldots , M - 1 \}$ and $v \in \{ 0 , \dotsc , N ^ { \cdot } - 1 \}$ are frequency coordinates). The formula is given by: + +$$ +F ( u , v ) = \sum _ { x = 0 } ^ { M - 1 } \sum _ { y = 0 } ^ { N - 1 } f ( x , y ) e ^ { - j 2 \pi \left( { \frac { u x } { M } } + { \frac { v y } { N } } \right) } +$$ + +Here, $j$ is the imaginary unit $( j ^ { 2 } = - 1 )$ ), and the exponential term represents the basis functions (complex sinusoids) at different frequencies $( u , v )$ . + +# A.2 Inverse 2D Discrete Fourier Transform (2D IDFT) + +The 2D IDFT allows us to reconstruct the original spatial domain image $f ( x , y )$ from its frequency domain representation $F ( u , v )$ . The formula is: + +$$ +f ( x , y ) = { \frac { 1 } { M N } } \sum _ { u = 0 } ^ { M - 1 } \sum _ { v = 0 } ^ { N - 1 } F ( u , v ) e ^ { j 2 \pi \left( { \frac { u x } { M } } + { \frac { v y } { N } } \right) } +$$ + +Note the scaling factor $\frac { 1 } { M N }$ and the positive sign in the exponent compared to the forward transform. + +# A.3 Magnitude Spectrum and Power Spectrum + +The frequency domain representation $F ( u , v )$ obtained from the DFT is generally a complex number. Its magnitude, $| F ( u , v ) |$ , is known as the Magnitude Spectrum, which quantifies the amplitude of each frequency component present in the image. + +$$ +| F ( u , v ) | = \sqrt { \mathrm { R e } ( F ( u , v ) ) ^ { 2 } + \mathrm { I m } ( F ( u , v ) ) ^ { 2 } } +$$ + +Closely related is the Power Spectrum (or Power Spectral Density), defined as the square of the magnitude spectrum. It represents how the total signal energy is distributed across the different frequencies: + +$$ +P ( u , v ) = | F ( u , v ) | ^ { 2 } +$$ + +A key characteristic of natural images is that their power spectrum typically exhibits a rapid decay as frequency increases. Specifically, the power √ $P ( u , v )$ tends to fall off with increasing radial frequency $f _ { r } = \sqrt { u ^ { 2 } + v ^ { 2 } }$ , often approximated by a $1 / f _ { r } ^ { \alpha }$ law, where $\alpha$ is a constant typically around 2. This 1/f property implies that low spatial frequencies (corresponding to coarse structures and overall variations) contain significantly more energy than high spatial frequencies (corresponding to fine details and sharp transitions). This fundamental statistical feature of natural images is widely utilized and modeled in various image processing and computer vision tasks. + +# Appendix B: Addition Experiments + +VAR sets different training epochs for models of different sizes: 200 epochs for 310M, 250 epochs for 600M, 300 epochs for 1B, and 350 epochs for 2B. Limited by computational resources, we focus on training our 310M model. As the table above shows, NFIG outperforms VAR at matched epochs and demonstrates superior parameter efficiency. At 200 epochs, NFIG already outperforms VAR-d16 of the same size. With extended training, NFIG-310M achieves performance comparable to or better than VAR-d20-600M (twice the parameters) while using significantly fewer resources. + +# B.1 Details of Loss function + +We will provide detailed mathematical formulations of our loss function components and their respective roles in the training process. The total loss function for the FR-VAE (image tokenizer for NFIG) is defined as: + +$$ +\mathcal { L } = | | I - \hat { I } | | _ { 2 } ^ { 2 } + | | \hat { f } - \hat { f } | | _ { 2 } ^ { 2 } + \mathcal { L } _ { p } ( I ) + 0 . 5 \mathcal { L } _ { g } ( I ) . +$$ + +Here, the first two terms represent the reconstruction loss for the image $I$ vs $\hat { I } _ { { \bf \Pi } _ { \it - \hat { \Pi } } }$ ) and and its frequencyguided quantized loss $( \hat { f } \ \mathbf { v } \mathbf { s } \ \hat { f } )$ , respectively, ensuring fidelity in both pixel and feature. ${ \mathcal { L } } _ { p }$ is LPIPS perceptual loss and $\mathcal { L } _ { g }$ is gan loss. + +For the NFIG Transformer, which predicts the frequency tokens, we utilize a standard cross-entropy loss: + +$$ +\mathcal { L } ( T , \tilde { T } ) = - \sum _ { i = 1 } ^ { n } t _ { i } \log ( \tilde { t } _ { i } ) +$$ + +This loss is computed between the predicted tokens $\tilde { T }$ and FR-VAE ground truth tokens $T$ , ensuring accurate prediction of the quantized frequency representations across all scales. + +# B.2 Frequency Keep Ability + +We are grateful for your encouragement to discuss both successes and challenges. Your suggestion for a frequency analysis was particularly insightful. As you requested, we conducted a frequency-domain comparison between our model (NFIG) and VAR-16. + +As requested, we provide frequency-domain comparisons between VAR-16 and NFIG: (1) Power Spectral Density (PSD): Overall frequency fidelity; (2) Frequency Keep Score (FKS): Weighted similarity across High/Mid/Low frequency bands (weights: 0.15, 0.28, 0.57, emphasizing structural low-frequency information). + +Our analysis revealed that while both models effectively preserve low-frequency information, and NFIG preserves middle and high frequency information with higher fidelity. + +
ModelPSD↓FKS↑Low↑Middle↑High↑
VAR-160.8779.5%98.3%57.6%48.2%
NFIG(ours)0.4787.6%98.9%75.3%66.7%
+ +# B.4 Diverse Image Types + +To demonstrate broader applicability, we conducted preliminary reconstruction evaluations (FID) of FR-VAE across diverse image types. + +
ModelDTDQRCODEDiagramsChest-XCelebA-HQCOCOLSUN-Bedroom
FR-VAE6.8611.01210.743.517.516.12
+ +# B.5 Failure Case + +Despite strong overall performance, NFIG occasionally produces visual artifacts. As shown in Figure 6, these include anatomical errors (extra bird leg), texture abnormalities (goldfish patterns), and fine detail loss (bird claws). Red boxes highlight the anomalies. These failures reflect challenges in maintaining semantic consistency across frequency bands. The issues are particularly pronounced for complex structures and fine details. Such limitations are common to frequency-based generation approaches and present opportunities for future improvement. + +![](images/figures/nfig-fig-0006.jpg) +Figure 6: Failure case for NFIG. \ No newline at end of file diff --git a/papers/nfig/paper.pdf b/papers/nfig/paper.pdf new file mode 100644 index 0000000000000000000000000000000000000000..6bb421de55816608c2781c04bc434e9a1de1aac0 --- /dev/null +++ b/papers/nfig/paper.pdf @@ -0,0 +1,3 @@ +version https://git-lfs.github.com/spec/v1 +oid sha256:a97593489d5b1851acd7c34dcd331f4f1208787fc1ad35279cba3fd493347317 +size 10917027 diff --git a/papers/nfig/sau.json b/papers/nfig/sau.json new file mode 100644 index 0000000000000000000000000000000000000000..9d4698803aed1ca3d6550348cd36e86081a04d99 --- /dev/null +++ b/papers/nfig/sau.json @@ -0,0 +1,162 @@ +{ + "paper_id": "nfig", + "paper_title": "NFIG: Multi-Scale Autoregressive Image Generation via Frequency Ordering", + "D1": [ + { + "id": "nfig-D1-001", + "claim": "ImageNet ILSVRC 2012: 1.2M train / 50k val / 100k test images, 1000 categories, 256x256x3 resolution", + "source": "Section 4.1" + }, + { + "id": "nfig-D1-002", + "claim": "FR-VAE tokenizer: VQGAN framework, DINO discriminator, DINOv2-base encoder, XQGAN implementation strategy, codebook K=4096, n=10 frequency bands, scale_factors [1,2,3,4,5,6,8,10,13,16], 680 total tokens", + "source": "Section 3.1, Section 4.1" + }, + { + "id": "nfig-D1-003", + "claim": "NFIG Transformer: VAR backbone (decoder-only), depth 16, block-wise causal attention, Next-Frequency Prediction (coarse-to-fine), model sizes 310M/600M, token sequence length 680 (vs AR baseline 256)", + "source": "Section 3.2, Section 4.1" + }, + { + "id": "nfig-D1-004", + "claim": "Frequency band division: n=10 frequency bands with increasing scale sequence, lower-frequency bands get fewer tokens (smaller h_i*w_i), higher-frequency bands get more tokens (larger h_i*w_i), bandwidth per band proportional to its token count ratio h_i*w_i / sum_j(h_j*w_j), with h_n=H', w_n=W'", + "source": "Section 3.2, Eq. 9" + }, + { + "id": "nfig-D1-005", + "claim": "NFIG training: PyTorch, NVIDIA H100, Adam optimizer, lr=8e-5, batch_size=768, 350 epochs (VAR baseline: 200ep for 310M, 250ep for 600M, 300ep for 1B, 350ep for 2B); scaling study at 55 epochs; epoch comparison at 200/250/300/350 epochs", + "source": "Section 4.1, Section 4.2" + }, + { + "id": "nfig-D1-006", + "claim": "FR-VAE loss weights: reconstruction_loss=1.0, frequency_quantized_loss=1.0, LPIPS perceptual loss=1.0, GAN loss=0.5", + "source": "Appendix B.1, Eq. 10" + }, + { + "id": "nfig-D1-007", + "claim": "NFIG inference: CFG=4.5, top_k=990, 10 inference steps", + "source": "Section 4.1" + } + ], + "D2": [ + { + "id": "nfig-D2-001", + "claim": "f_hat_i = F^{-1}(F(f) .* M_i) — Frequency-guided Decomposer: decomposes encoder feature map f into n frequency components via FFT-based masking and inverse FFT, each mask M_i selects a specific frequency band", + "source": "Section 3.1.1, Eq. 1" + }, + { + "id": "nfig-D2-002", + "claim": "f_tilde = sum_{i=1}^{n} T(f_hat_i, H', W') — Frequency-guided Composer: merges frequency components of potentially different resolutions back into a unified feature map by interpolating each to H'xW' and summing", + "source": "Section 3.1.1, Eq. 2" + }, + { + "id": "nfig-D2-003", + "claim": "v_0 = argmin ||f_hat_0 - Z(v_0)||^2; R_0 = f_hat_0 - Z(v_0); for i>=1: v_i = argmin ||(R_{i-1}+f_hat_i) - Z(v_i)||^2; R_i = R_{i-1} + (f_hat_i - Z(v_i)) — Residual Token Extraction: progressive frequency-band quantization with cumulative residual R_i tracking unencoded signal through level i", + "source": "Section 3.1.2, Eq. 4" + }, + { + "id": "nfig-D2-004", + "claim": "t^{(j,k)} = lookup(Z, argmin_{z in Z} ||z - v_i^{(j,k)}||_2) — Vector Quantization: converts continuous feature vectors to discrete codebook indices via nearest-neighbor L2 lookup in learnable codebook Z (KxC)", + "source": "Section 3.1.2, Eq. 5" + }, + { + "id": "nfig-D2-005", + "claim": "p(T_1,...,T_n) = prod_{i=1}^{n} p(T_i | T_1,...,T_{i-1}) — Next-Frequency Autoregressive Prediction: factorizes joint token distribution as product of conditionals from low to high frequency (coarse-to-fine), with block-wise causal attention enforcing band-level autoregressive ordering", + "source": "Section 3.2, Eq. 7" + }, + { + "id": "nfig-D2-006", + "claim": "sigma_i = sigma_{i-1} + (h_i*w_i / sum_j(h_j*w_j)) * sigma_max — Frequency Band Division Strategy: allocates bandwidth proportionally to token count (h_i*w_i) of each band; low-frequency bands get narrower bandwidth, high-frequency bands get wider bandwidth", + "source": "Section 3.2, Eq. 9" + }, + { + "id": "nfig-D2-007", + "claim": "L = ||I - I_hat||_2^2 + ||f - f_hat||_2^2 + L_p(I) + 0.5 * L_g(I) — FR-VAE Total Loss: pixel MSE + feature MSE + LPIPS perceptual loss + 0.5x GAN adversarial loss (DINO discriminator); additional VQ codebook/commitment losses not shown", + "source": "Appendix B.1, Eq. 10" + }, + { + "id": "nfig-D2-008", + "claim": "L(T, T_tilde) = -sum_{i=1}^{n} t_i * log(t_tilde_i) — NFIG Transformer Cross-Entropy Loss: standard autoregressive next-token prediction loss over all frequency-band tokens with block-wise causal masking", + "source": "Appendix B.1, Eq. 11" + }, + { + "id": "nfig-D2-009", + "claim": "Block-wise Causal Attention (referenced from VAR [19]): decoder-only transformer enforces autoregressive ordering across frequency bands — tokens in lower-frequency bands (1..i-1) are visible to band i, while higher-frequency bands (i+1..n) are masked; attention within each frequency block follows VAR's block-wise scheme", + "source": "Section 3.2" + } + ], + "D3": [ + { + "id": "nfig-D3-001", + "claim": "Main Generation Experiment: class-conditional ImageNet 256x256 generation, train FR-VAE tokenizer then NFIG-310M Transformer (350 epochs, Adam lr=8e-5, batch=768, H100), inference with CFG=4.5 top_k=990 in 10 steps; compared against GAN (BigGAN, GigaGAN, StyleGAN-XL), Diffusion (ADM, CDM, LDM-4-G, DiT-L/2, DiT-XL/2, L-DiT-3B/7B), Mask Diffusion (MaskGIT, RCG), AR (VQVAE-2, VQGAN, ViTVQ, RQTransformer, FAR-B/H, XQGAN-310M, VAR-d16/d20); metrics: gFID, rFID, IS, Precision, Recall, params, steps, relative inference time", + "source": "Section 4.1, Section 4.2, Table 2" + }, + { + "id": "nfig-D3-002", + "claim": "Ablation Study: incremental component contribution on ImageNet val 50k — baseline AR (seq_len=256) → +FR-Quantizer (seq_len=680) → +DINO-Disc → +AdaLN Transformer → +Top_k → +CFG=full NFIG; each variant trained from scratch with same FR-VAE; evaluated by rFID and gFID", + "source": "Section 4.3, Table 5" + }, + { + "id": "nfig-D3-003", + "claim": "Scaling Study: NFIG-310M vs NFIG-600M trained for 55 epochs (limited compute budget), same FR-VAE tokenizer, Adam lr=8e-5 batch=768; compare FID/IS/Precision/Recall between scales to validate scaling behavior", + "source": "Section 4.2, Table 3" + }, + { + "id": "nfig-D3-004", + "claim": "Epoch Efficiency Comparison: NFIG-310M trained at 200/250/300/350 epochs vs VAR-d16 (310M, 200ep) and VAR-d20 (600M, 250ep); validate that NFIG-310M at 200ep outperforms VAR-d16 and at 350ep matches VAR-d20 despite 2x fewer params; metrics: FID, IS, Precision, Recall", + "source": "Section 4.2, Table 4" + }, + { + "id": "nfig-D3-005", + "claim": "Frequency Distribution Analysis: generate images with NFIG and visualize intermediate results at steps 2/4/6/8/10; compute FFT frequency spectrum at each step to show progressive low-to-high refinement; compare VQ loss across scale factors between NFIG and VAR to demonstrate more balanced feature learning", + "source": "Section 4.4, Figure 4, Figure 5" + }, + { + "id": "nfig-D3-006", + "claim": "Frequency Keep Ability Analysis: compare NFIG vs VAR-16 on ImageNet using PSD (Power Spectral Density, lower is better) and FKS (Frequency Keep Score, weighted: Low 0.57, Mid 0.28, High 0.15, higher is better) across low/mid/high frequency bands", + "source": "Appendix B.2" + }, + { + "id": "nfig-D3-007", + "claim": "Cross-Dataset FR-VAE Reconstruction: evaluate zero-shot reconstruction on DTD, QRCODE, Diagrams, Chest-X, CelebA-HQ, COCO, LSUN-Bedroom using ImageNet-pretrained FR-VAE; compute rFID between original and reconstructed images per dataset", + "source": "Appendix B.4" + } + ], + "D4": [ + { + "id": "nfig-D4-001", + "claim": "NFIG main pipeline (paper-specified generation workflow): (1) Train FR-VAE image tokenizer with frequency-guided residual quantization + VQGAN losses → (2) Train NFIG Transformer on FR-VAE ground-truth frequency tokens with cross-entropy loss → (3) Inference: generate tokens autoregressively from low to high frequency (10 steps), decode via FR-VAE decoder", + "source": "Section 4.1" + }, + { + "id": "nfig-D4-002", + "claim": "Ablation experiment sequence (paper-specified): (1) Incrementally add components to baseline AR model (FR-Quantizer → DINO-Disc → AdaLN → Top_k → CFG) → (2) Train each variant from scratch with same FR-VAE tokenizer → (3) Evaluate rFID and gFID on ImageNet validation set", + "source": "Section 4.3" + }, + { + "id": "nfig-D4-003", + "claim": "Scaling study sequence: (1) Train NFIG-310M for 55 epochs → (2) Train NFIG-600M for 55 epochs → (3) Compare FID/IS/Precision/Recall between the two scales under same limited compute budget", + "source": "Section 4.2" + }, + { + "id": "nfig-D4-004", + "claim": "Epoch efficiency comparison sequence: (1) Train NFIG-310M for multiple epoch budgets (200, 250, 300, 350) → (2) Compare against VAR-d16 (310M, 200ep) and VAR-d20 (600M, 250ep) → (3) Validate NFIG-310M at 200ep already outperforms VAR-d16, and at 350ep matches VAR-d20 despite 2x fewer parameters", + "source": "Section 4.2" + }, + { + "id": "nfig-D4-005", + "claim": "Frequency visualization sequence: (1) Generate images with NFIG, capture intermediates at steps 2/4/6/8/10 → (2) Compute FFT frequency spectrum at each step to visualize progressive low-to-high frequency refinement → (3) Compare VQ loss per scale factor between NFIG and VAR to demonstrate balanced feature learning", + "source": "Section 4.4" + }, + { + "id": "nfig-D4-006", + "claim": "Frequency Keep Ability analysis sequence: (1) Generate images from both NFIG and VAR-16 → (2) Compute Power Spectral Density (PSD) for both → (3) Compute Frequency Keep Score (FKS) with weighted per-band similarity (Low 0.57, Mid 0.28, High 0.15) → (4) Compare per-band fidelity between models", + "source": "Appendix B.2" + }, + { + "id": "nfig-D4-007", + "claim": "Cross-dataset FR-VAE evaluation sequence: (1) Load ImageNet-pretrained FR-VAE tokenizer → (2) Encode and decode images from each target dataset (DTD, QRCODE, Diagrams, Chest-X, CelebA-HQ, COCO, LSUN-Bedroom) → (3) Compute rFID between original and reconstructed images per dataset", + "source": "Appendix B.4" + } + ] +} \ No newline at end of file diff --git a/papers/ngpt/blacklist.txt b/papers/ngpt/blacklist.txt new file mode 100644 index 0000000000000000000000000000000000000000..ce836a4440c7320b6b5dfe690b6ba2065a19bed6 --- /dev/null +++ b/papers/ngpt/blacklist.txt @@ -0,0 +1,4 @@ +# Official repository (NVIDIA) +https://github.com/NVIDIA/ngpt +# Community reproduction (Nous Research sponsored) +https://github.com/JoeLi12345/nGPT diff --git a/papers/ngpt/config.yaml b/papers/ngpt/config.yaml new file mode 100644 index 0000000000000000000000000000000000000000..239f532b1e060153f160af2507002544c0595d2c --- /dev/null +++ b/papers/ngpt/config.yaml @@ -0,0 +1,8 @@ +title: "nGPT: Normalized Transformer with Representation Learning on the Hypersphere" +pdf_url: "https://arxiv.org/pdf/2410.01131.pdf" +venue: "ICLR 2025" +year: "2025" +extra: + selection_index: 4 + domain: "NLP / LLM" + paradigm: "New Algorithm / Architecture" diff --git a/papers/ngpt/images/figures/ngpt-fig-0001.jpg b/papers/ngpt/images/figures/ngpt-fig-0001.jpg new file mode 100644 index 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b/papers/ngpt/paper.md new file mode 100644 index 0000000000000000000000000000000000000000..85c23e84fa022496515c7c629297a2484f58807c --- /dev/null +++ b/papers/ngpt/paper.md @@ -0,0 +1,502 @@ +# NGPT: NORMALIZED TRANSFORMER WITH REPRE-SENTATION LEARNING ON THE HYPERSPHERE + +Ilya Loshchilov, Cheng-Ping Hsieh, Simeng Sun & Boris Ginsburg NVIDIA {iloshchilov,chsieh,simengs,bginsburg}@nvidia.com + +# ABSTRACT + +We propose a novel neural network architecture, the normalized Transformer (nGPT) with representation learning on the hypersphere. In nGPT, all vectors forming the embeddings, MLP, attention matrices and hidden states are unit norm normalized. The input stream of tokens travels on the surface of a hypersphere, with each layer contributing a displacement towards the target output predictions. These displacements are defined by the MLP and attention blocks, whose vector components also reside on the same hypersphere. Experiments show that nGPT learns much faster, reducing the number of training steps required to achieve the same accuracy by a factor of 4 to 20, depending on the sequence length. + +# 1 INTRODUCTION + +The Transformer architecture (Vaswani et al., 2017) is the foundation for most of modern language models. An enormous number of modifications to this architecture have been proposed to improve training stability, inference costs, context length, robustness, etc. It has been noted that the application of various normalization techniques is beneficial (Salimans & Kingma, 2016), leading to experiments with adding normalization layers such as LayerNorm and RMSNorm in nearly every possible position within the network (Xiong et al., 2020). Another approach to the model normalization is through controlling the norm of weights using weight decay (Loshchilov & Hutter, 2019). Recent studies (Andriushchenko et al., 2023) suggest reevaluating the role of weight decay and taking a closer look at rotations rather than focusing solely on vector norms (Kodryan et al., 2022; Kosson et al., 2023). Franke et al. (2023) suggested to enforce an upper bound on $L _ { 2 }$ norm of parameter groups. There is growing evidence that representation learning on the hypersphere is associated with more stable training, greater embedding space separability, and better performance on downstream tasks (Wang & Isola, 2020). Recent studies also suggest that transformers implicitly perform gradient descent as meta-optimizers (Von Oswald et al., 2023; Dai et al., 2022). + +We propose to unify (see also Appendix A.3) various findings and observations made in the field under a new perspective of the normalized Transformer. Our key contributions are as follows: + +Optimization of network parameters on the hypersphere We propose to normalize all vectors forming the embedding dimensions of network matrices to lie on a unit norm hypersphere. This allows us to view matrix-vector multiplications as dot products representing cosine similarities bounded in [-1,1]. The normalization renders weight decay unnecessary. + +Normalized Transformer as a variable-metric optimizer on the hypersphere The normalized Transformer itself performs a multi-step optimization (two steps per layer) on a hypersphere, where each step of the attention and MLP updates is controlled by eigen learning rates—the diagonal elements of a learnable variable-metric matrix. For each token $t _ { i }$ in the input sequence, the optimization path of the normalized Transformer begins at a point on the hypersphere corresponding to its input embedding vector and moves to a point on the hypersphere that best predicts the embedding vector of the next token $t _ { i + 1 }$ . + +Faster convergence We demonstrate that the normalized Transformer (nGPT) reduces the number of training steps required to achieve the same accuracy by a factor of 4 to 20. + +# 2 EVOLUTION OF THE TRANSFORMER: FROM GPT TO NGPT + +This section outlines the baseline Transformer and the modifications necessary to derive its normalized version. We illustrate these changes for Transformer decoder with self-attention only. The extension to encoder-decoder and cross-attention is straightforward. A summary of these changes is given in Table 1, with details in Section 2.6. + +# 2.1 TOKEN EMBEDDINGS AND OUTPUT LOGITS + +The decoder-only Transformer is trained to predict token $t _ { i }$ using previous tokens input sequence $\pmb { x } = \left( t _ { 1 } , t _ { 2 } , \dots , t _ { i - 1 } \right)$ . For each input token $t _ { i }$ , we retrieve its corresponding embedding in $\mathbf { \bar { \mathbb { R } } } ^ { d _ { \mathrm { m o d e l } } }$ −from a learnable embedding matrix $E _ { \mathrm { i n p u t } } \in \mathbb { R } ^ { V \times d _ { \mathrm { m o d e l } } }$ with vocabulary of size $V$ . Similarly, the target output sequence $\textbf { { y } }$ is represented using a learnable embedding matrix $E _ { \mathrm { o u t p u t } } \ \in \ \mathbb { R } ^ { V \times d _ { \mathrm { m o d e l } } }$ . Notably, since both $E _ { \mathrm { i n p u t } }$ and $E _ { \mathrm { o u t p u t } }$ are learnable (unless they are tied to be equivalent), any token $t _ { i }$ can have different embeddings in the input and output sequences. To measure token similarity during model training, the dot product between the corresponding embedding vectors is used. However, the norms of embedding vectors in the original Transformer are unconstrained, which can lead to inaccurate similarity estimation. To improve the accuracy of similarity estimation, we propose to normalize the embedding vectors stored in $E _ { \mathrm { i n p u t } }$ and $E _ { \mathrm { o u t p u t } }$ after each step of the training algorithm. + +The next token prediction is enforced by causal masking (see Section 2.3) to ensure that no future tokens are considered. This allows the model to compute the prediction error for all $T$ tokens in parallel during training, while preserving the autoregressive nature of the task. After the Transformer processes the sequence $( \pmb { x } _ { 1 } , \dots , \pmb { x } _ { i - 1 } )$ , it produces an output vector $\boldsymbol { h } _ { i } ~ \in ~ \mathbb { R } ^ { d _ { \mathrm { m o d e l } } }$ for each $i$ -th position in the predicted sequence. The logits $z _ { i } \in \mathbb { R } ^ { V }$ , representing the unnormalized probabilities for each token in the vocabulary, are computed using the output embedding matrix $E _ { \mathrm { o u t p u t } }$ : + +$$ +z _ { i } = E _ { \mathrm { o u t p u t } } h _ { i } +$$ + +The logits $z _ { i }$ are passed through a softmax function to convert them into probabilities: + +$$ +P ( y _ { i } | \pmb { x } _ { 1 } , \ldots , \pmb { x } _ { i - 1 } ) = \frac { \exp ( z _ { i , y _ { i } } ) } { \sum _ { v = 1 } ^ { V } \exp ( z _ { i , v } ) } +$$ + +Here, $z _ { i , y _ { i } }$ is the logit corresponding to the correct token $y _ { i }$ , and the denominator normalizes the logits into a probability distribution over the vocabulary. During inference, the prediction $\hat { y } _ { i }$ is obtained by selecting the token with the highest probability. Since all nGPT embeddings are normalized, the logits $z \in \mathbb { R } ^ { V }$ in equation 1 represent dot products bounded in the range $[ - 1 , 1 ]$ . This limits the confidence (temperature) of the probability distribution generated by the softmax in equation 2. To adjust this during training, and in line with Hoffer et al. (2018), we introduce a trainable scaling parameter $\mathbf { \boldsymbol { s } } _ { z } \in \bar { \mathbb { R } } ^ { V }$ that scales the logits element-wise: + +$$ +\boldsymbol { z } z \boldsymbol { s } _ { z } +$$ + +Table 1: Transformer vs. Normalized Transformer. + +
TransformerNormalized Transformer
hA ← ATTN(RMSNorm(h))hA ← Norm(ATTN(h))
h ←h +hAh ← Norm(h + αA(hA − h))
hM ← MLP(RMSNorm(h))hM ← Norm(MLP(h))
h ← h +hMh ← Norm(h + αM(hM − h))
Final: h ← RMSNorm(h)
All parameters of matrices and embeddings are unconstrained.After each batch pass, all matrices and embeddings are normalized along their embedding dimension. The hidden state updates are controlled by learnable vectors of eigen learning rates αA and αM.
+ +# 2.2 LAYERS AND BLOCKS + +# 2.2.1 BASELINE TRANSFORMER + +$L$ layers of transformations are applied to the hidden state $^ { h }$ , consisting of alternating the selfattention (ATTN) and multi-layer perceptron (MLP) blocks: + +$$ +\begin{array} { r l } & { h h + \mathrm { A T T N } ( \mathrm { R M S N o r m } ( h ) ) } \\ & { h h + \mathrm { M L P } ( \mathrm { R M S N o r m } ( h ) ) , } \end{array} +$$ + +where $\operatorname { R M S N o r m } ( h )$ is one of several possible normalizations. It is used first to normalize each embedding to a norm of $\sqrt { d _ { \mathrm { m o d e l } } }$ , then scales each dimension by a learnable vector of $d _ { \mathrm { m o d e l } }$ factors, typically initialized to 1. Since the transformation block outputs are added to $^ { h }$ , the token embedding norms can vary significantly. To address this, normalization is also applied after the final layer. + +# 2.2.2 NORMALIZED TRANSFORMER + +For any points $^ { a }$ and $^ { b }$ on the surface of a hypersphere in $\mathbb { R } ^ { d _ { \mathrm { m o d e l } } }$ , SLERP (Spherical Linear Interpolation) by Shoemake (1985) computes an interpolation along the geodesic (shortest path): + +$$ +\operatorname { S L E R P } ( a , b ; \alpha ) = { \frac { \sin ( ( 1 - \alpha ) \theta ) } { \sin ( \theta ) } } a + { \frac { \sin ( \alpha \theta ) } { \sin ( \theta ) } } b +$$ + +where $\theta = \operatorname { a r c c o s } ( \mathbf { a } \cdot \mathbf { b } )$ is the angle between the points $^ { a }$ and $^ { b }$ , and $\alpha \in [ 0 , 1 ]$ is the interpolation parameter, with $\alpha = 0$ returning $\textbf { \em a }$ and $\alpha = 1$ returning $^ { b }$ . Our experiments suggest that SLERP can be approximated by simple linear interpolation (LERP): + +$$ +\mathrm { L E R P } ( a , b ; w ) = ( 1 - \alpha ) { \pmb a } + \alpha { \pmb b } +$$ + +Let us rewrite this equation as an update equation in nGPT: + +$$ +\pmb { a } \gets \pmb { a } + \alpha ( \pmb { b } - \pmb { a } ) +$$ + +where $\textbf { \em a }$ is $^ { h }$ , and, $^ { b }$ is the point suggested by the attention or MLP block. Then, for the gradient $\mathbf { \delta } \mathbf { \mathbf { { g } } } = \mathbf { \delta } \mathbf { \mathbf { { a } } } - \mathbf { \delta } \mathbf { \delta }$ , a more general form involving a variable matrix $B \in \mathbb { R } ^ { d _ { \mathrm { m o d e l } } \times d _ { \mathrm { m o d e l } } }$ becomes: + +$$ +\pmb { a } \pmb { a } - \alpha \pmb { B } \pmb { g } +$$ + +In quasi-Newton methods, $\textbf { { B } }$ approximates the inverse Hessian matrix $H ^ { - 1 }$ .When $\textbf { { B } }$ is diagonal with non-negative elements, $\alpha B$ becomes a vector $\alpha \in \mathbb { R } _ { \geq 0 } ^ { d _ { \mathrm { m o d e l } } }$ whose elements correspond to the diagonal of times the learning rate $\alpha$ . We denote $_ \alpha$ as eigen learning rates (from the German word eigen, meaning ”own,” referring to the internal structure of the Transformer). We provide some notes in Appendix A.2. Following equation 8, the update equations for the attention and MLP blocks are as follows: + +$$ +\begin{array} { r l } & { h \mathrm { N o r m } ( h + \alpha _ { \mathrm { A } } ( h _ { \mathrm { A } } - h ) ) } \\ & { h \mathrm { N o r m } ( h + \alpha _ { \mathrm { M } } ( h _ { \mathrm { M } } - h ) ) , } \end{array} +$$ + +where αA ∈ Rdmodel0 and $\alpha _ { \mathrm { M } } \in \mathbb { R } _ { \geq 0 } ^ { d _ { \mathrm { m o d e l } } }$ are learnable parameters applied to the normalized outputs of the attention and MLP blocks $\bar { h _ { \mathrm { A } } } = \mathrm { N o r m } ( \mathrm { A T T N } ( h ) )$ and $h _ { \mathrm { M } } = \mathrm { N o r m } ( \mathrm { M L P } ( h ) )$ , respectively. The function $\operatorname { N o r m } ( x )$ normalizes any vector $_ { \textbf { \em x } }$ to have unit norm, and, unlike RMSNorm or LayerNorm, does not introduce any element-wise scaling factors. The normalization can be viewed as the retraction step in Riemannian optimization, mapping the updated solution back to the manifold. Appendix A.4 discusses the extension of our update equations in the context Riemannian optimization. In contrast to the baseline Transformer, no additional normalization is required after the final layer, as the embeddings are already normalized by the proposed scheme. + +# 2.3 SELF-ATTENTION BLOCK + +# 2.3.1 BASELINE TRANSFORMER + +The attention mechanism is a key component of the Transformer. It allows each token to attend to every other token in the sequence, enabling the model to capture long-range dependencies. The + +block typically starts with a normalization of the input hidden state $^ { h }$ using RMSNorm to deal with fluctuating norms of embeddings. Then, the normalized $^ { h }$ is projected into three separate vectors - the query $\pmb q$ , the key $\boldsymbol { k }$ , and the value $\textbf { { v } }$ : + +$$ +q h W _ { q } , k h W _ { k } , v h W _ { v } +$$ + +where $W _ { q } , W _ { k } , W _ { v } \in \mathbb { R } ^ { d _ { \mathrm { m o d e l } } \times d _ { k } }$ are learned projection matrices, and $d _ { k }$ is the dimensionality of the query/key vectors. To incorporate positional information, we apply Rotary Position Embeddings (RoPE) by Su et al. (2024) to both the query and key vectors. The attention scores are computed by taking the dot product of the query and key vectors, scaling them by $\scriptstyle { \frac { 1 } { \sqrt { d _ { k } } } }$ , then applying a softmax function to obtain attention weights, and finally computing a weighted sum of the value vectors $\textbf { { v } }$ : + +$$ +\mathrm { A t t e n t i o n } ( q , k , v ) \longleftarrow \mathrm { s o f t m a x } \left( \frac { q k ^ { \top } } { \sqrt { d _ { k } } } + M \right) v , +$$ + +where $M$ is a matrix that prevents attending to future tokens by setting the corresponding entries to $- \infty$ . Specifically, $\mathbf { M } _ { i , j } = 0$ if $j \le i$ and $\mathbf { M } _ { i , j } = - \infty$ if $j > i$ . + +In practice, $n _ { \mathrm { h e a d s } }$ attention heads are used where for each $i$ -th head, separate linear projections ${ \cal W } _ { q } ^ { i } , { \cal W } _ { k } ^ { i } , { \cal W } _ { v } ^ { i }$ are applied, and the attention mechanism is computed independently for each head: + +$$ +h _ { \mathrm { A } } \gets \mathrm { C o n c a t ( h e a d _ { 1 } , . . . , h e a d } _ { n _ { \mathrm { h e a d s } } } ) W _ { o } +$$ + +where $\mathrm { h e a d } _ { i } = \mathrm { A t t e n t i o n } ( q ^ { i } , k ^ { i } , v ^ { i } )$ and $W _ { O } \ \in \ \mathbb { R } ^ { n _ { \mathrm { h e a d s } } \times d _ { k } \times d _ { \mathrm { m o d e l } } }$ is a learned projection matrix, where $d _ { k }$ is typically set to $d _ { \mathrm { m o d e l } } / n _ { \mathrm { h e a d s } }$ . + +# 2.3.2 NORMALIZED TRANSFORMER + +The matrix-vector multiplication of $W _ { q } \in \mathbb { R } ^ { d _ { \mathrm { m o d e l } } \times d _ { k } }$ of the $i$ -th head1 and $\pmb { h } \in \mathbb { R } ^ { d _ { \mathrm { m o d e l } } }$ can be viewed as a dot product between the columns of $W _ { q }$ and $^ { h }$ . In the baseline Transformer, all matrices, including $W _ { q }$ are unconstrained, leading to unbounded values in $\pmb q$ . We propose to normalize $W _ { q }$ , $W _ { k }$ , $W _ { v }$ and $W _ { o }$ along their embedding dimension so that the computed dot products with $^ { h }$ can be interpreted as cosine similarity between unit norm vectors bounded in $[ - 1 , 1 ]$ . Thus, all attention matrices can be viewed as collections of normalized embedding vectors to be compared with. + +While each element of $\pmb q$ and $\boldsymbol { k }$ is now bounded, the norms of these two vectors can still vary. Moreover, injection of positional information by RoPE further distorts $\pmb q$ and $\boldsymbol { k }$ . We propose to additionally normalize $\pmb q$ and $\boldsymbol { k }$ , ensuring that the dot product of every query and key is under control: + +$$ +\begin{array} { l } { { { \pmb q } \mathrm { N o r m } ( { \pmb q } ) { \pmb s } _ { q k } } } \\ { { { \pmb k } \mathrm { N o r m } ( { \pmb k } ) { \pmb s } _ { q k } , } } \end{array} +$$ + +where $\boldsymbol { s } _ { q k } \in \mathbb { R } ^ { d _ { \mathrm { k } } }$ is a vector2 of trainable scaling factors for the $i$ -th head. + +In the original Transformer, the softmax scaling factor $1 / \sqrt { d _ { k } }$ in equation 13 is introduced to account for the expected variance of $d _ { k }$ in the dot product of non-normalized query and key vectors. In the normalized Transformer, the expected variance of the dot product between normalized query and key vectors is $1 / d _ { k }$ . To restore a variance of 1, the softmax scaling factor should instead be $\breve { \sqrt { d _ { k } } }$ . If the softmax scaling factor is set to 1, this is equivalent to initializing the scaling factors $s _ { q k }$ at $d _ { k } ^ { 1 / 4 }$ . + +# 2.4 MLP BLOCK + +# 2.4.1 BASELINE TRANSFORMER + +The input hidden state $^ { h }$ of the MLP block is first normalized using RMSNorm and then passed through two separate linear projections, producing two intermediate vectors (we omit bias terms): + +$$ +\ b u h W _ { u } , \quad \ b \nu h W _ { \nu } +$$ + +where $W _ { u } , W _ { \nu } \in \mathbb { R } ^ { d _ { \mathrm { m o d e l } } \times d _ { \mathrm { M L P } } }$ are the learned weight matrices. The intermediate vectors $\textbf { \em u }$ and $\pmb { \nu }$ are combined using a gated activation function called SwiGLU defined by Shazeer (2020) as: + +$$ +\operatorname { S w i G L U } ( \pmb { u } , \pmb { \nu } ) \pmb { u } \cdot \operatorname { S i L U } ( \pmb { \nu } ) +$$ + +where $\mathrm { S i L U } ( \pmb { \nu } ) = \pmb { \nu } \cdot \pmb { \sigma } ( \pmb { \nu } )$ , and $\sigma ( \nu )$ is the sigmoid function. The result of the gated activation is then passed through a final linear transformation $W _ { o \mathrm { M L P } } \in \mathbb { R } ^ { d _ { \mathrm { M L P } } \times d _ { \mathrm { m o d e l } } }$ : + +$$ +h _ { \mathrm { M } } \mathrm { S w i G L U } ( u , \nu ) W _ { \mathrm { o M L P } } +$$ + +# 2.4.2 NORMALIZED TRANSFORMER + +We propose to normalize matrices $W _ { u }$ and $W _ { \nu }$ along the embedding dimension so that the $\textbf { \em u }$ and $\pmb { \nu }$ vectors represent the cosine similarity between $^ { h }$ and vectors stored in $W _ { u }$ and $W _ { \nu }$ , respectively. To control their impact, we introduce scaling factors $\boldsymbol { s } _ { u } \in \mathbb { R } ^ { d _ { \mathrm { M L P } } }$ and $\boldsymbol { s } _ { \nu } \in \mathbb { R } ^ { d _ { \mathrm { M L P } } }$ : + +$$ +\begin{array} { l } { { { \pmb u } { \pmb u } s _ { u } , } } \\ { { \nu \nu s _ { \nu } \sqrt { d _ { m o d e l } } , } } \end{array} +$$ + +where the rescaling of $\pmb { \nu }$ by $\sqrt { d _ { m o d e l } }$ is needed to benefit from the non-linearity of SiLU (see the Appendix A.1). The output of the MLP block is invariant to rescaling of $\textbf { \em u }$ by a scalar. + +# 2.5 EFFECTIVE LEARNING RATES IN ADAM + +The core of the Adam algorithm by Kingma (2014) is as follows: + +$$ +\begin{array} { l } { m \gets \beta _ { 1 } m + ( 1 - \beta _ { 1 } ) { \pmb g } } \\ { v \gets \beta _ { 2 } { \pmb v } + ( 1 - \beta _ { 2 } ) { \pmb g } ^ { 2 } } \\ { \theta \gets \pmb \theta - \alpha m / ( \sqrt { v } + \epsilon ) , } \end{array} +$$ + +where $\pmb \theta$ is the parameter vector, $\textbf { { g } }$ is the batch gradient, $_ { \mathbf { \nabla } } \mathbf { m }$ is the momentum, $\textbf { { v } }$ is the estimate of the per-element gradient amplitudes, $\alpha$ is the scheduled learning rate, $\epsilon$ is a small constant, and $\beta _ { 1 } < \beta _ { 2 }$ are momentum factors close to 1. We cite the text of the original Adam paper using our notation: In more common scenarios, we will have that $\textstyle { \frac { m } { \sqrt { v } } } \approx \pm 1$ since $\left| \mathbb { E } [ g ] / { \sqrt { \mathbb { E } [ g ^ { 2 } ] } } \right| \leq 1$ . The effective magnitude of the steps taken in parameter space at each timestep is approximately bounded by the stepsize setting $\alpha$ . Thus, $\alpha$ controls the effective step-size in the search space, while the ratio $\textstyle { \frac { m } { \sqrt { v } } }$ can temporarily increase (respectively, decrease) the step-size if the current amplitude of perparameter momentum is greater (respectively, smaller) than its estimated value over longer time horizon. Consider an example where $\theta _ { i } = 0 . 0 1$ and the global learning rate is 0.001. If the gradient amplitude remains stable (i.e., $\begin{array} { r } { \frac { m _ { i } } { \sqrt { v _ { i } } } \approx 1 \rangle } \end{array}$ ), it would take 0.02−0.01 = 10 steps to double θi. However, if $\theta _ { i } = 1 . 0$ , it would take $\begin{array} { r } { \frac { 2 . 0 - 1 . 0 } { 0 . 0 0 1 } = 1 0 0 0 } \end{array}$ steps to double. Even if the gradient’s amplitude is larger in the second case, the number of steps would only decrease if $m _ { i } > \sqrt { v }$ . + +In nGPT, for any trainable vector of scaling parameters such as $s _ { a }$ , we use two scalars $s _ { a , i n i t }$ and $_ { s _ { a , s c a l e } }$ . When initializing $\scriptstyle { \pmb { s } } _ { a }$ as a trainable parameter, its initial value is set to $_ { s _ { a , s c a l e } }$ . However, during the forward pass we restore its actual value by multiplying $s _ { a , i n i t } / s _ { a , s c a l e }$ . This allows us to control the effective learning rate for $\scriptstyle { \pmb { s } } _ { a }$ by adjusting $_ { s _ { a , s c a l e } }$ , while keeping the global learning rate unchanged. For example, setting $\pmb { s } _ { a , i n i t } = 1$ and $s _ { a , s c a l e } = 1 / \sqrt { d _ { m o d e l } }$ ensures that this parameter is updated with the same effective learning rate as other normalized parameters in the network. + +# 2.6 SUMMARY OF MODIFICATIONS + +The recipe to convert the baseline Transformer into the normalized Transformer is as follows: + +1. Remove all normalization layers such as RMSNorm or LayerNorm. +2. After each training step (and, optionally, during the forward pass), normalize matrices $E _ { \mathrm { i n p u t } }$ , $E _ { \mathrm { o u t p u t } }$ , $W _ { q }$ , $W _ { k }$ , $W _ { v }$ , $W _ { o }$ , $W _ { u }$ , $W _ { \nu }$ and $W _ { o \mathrm { M L P } }$ along their embedding dimension. +3. Replace the update equations 4 and 5 by equations 10 and 11, where $\mathbf { \alpha _ { \alpha } } _ { \mathbf { \alpha } } \mathbf { \alpha } _ { \mathbf { \alpha } } \mathbf { \alpha } _ { \mathbf { \alpha } } \mathbf { \alpha } _ { \mathbf { \alpha } } \mathbf { \alpha } _ { \mathbf { \alpha } } \mathbf { \alpha } _ { \mathbf { \alpha } } \alpha _ { \mathbf { \alpha } } \mathbf { \alpha } _ { \mathbf { \alpha } }$ (and also $\alpha _ { \mathrm { M } }$ ) is treated with ${ \alpha } _ { \mathrm { A } , i n i t } = 0 . 0 5$ (in order of $1 / n _ { l a y e r s } )$ and $\alpha _ { \mathrm { { A } } , s c a l e } = 1 / \sqrt { d _ { m o d e l } }$ . + +4. Change the softmax scaling factor in attention from $1 / \sqrt { d _ { k } }$ to $\sqrt { d _ { k } }$ . Implement the rescaling and normalization (normalization here is optional) of $\pmb q$ and $\boldsymbol { k }$ as in equations 15 and 16, where $s _ { q k }$ is treated with $s _ { q k , i n i t } = 1$ and $\bar { s _ { q k , s c a l e } } = \bar { 1 / } \sqrt { d _ { m o d e l } }$ . + +5. Implement the rescaling of the intermediate state of the MLP block using equations 20 and 21, where $\mathbf { } _ { s \mu }$ (and also $\scriptstyle { \pmb { s } } _ { \nu }$ ) is treated with $\pmb { s } _ { u , i n i t } = 1$ and $\pmb { s } _ { u , s c a l e } = 1$ + +6. Implement the rescaling of logits using equation 3, where $\pmb { s } _ { z }$ is treated with $\pmb { s } _ { z , i n i t } = 1$ and $s _ { z , s c a l e } = 1 / \sqrt { d _ { m o d e l } } .$ . + +7. Remove weight decay and learning rate warmup. + +# 3 EXPERIMENTS + +We train both the baseline Transformer (GPT) and the normalized Transformer (nGPT) on the Open-WebText dataset (Gokaslan & Cohen, 2019) and evaluate them on a set of standard downstream tasks. We experiment with models containing 0.5B and 1B parameters, including the embeddings. For both GPT and nGPT, we report results using the best initial learning rate settings (see Appendix A.7). A detailed description of the setup and hyperparameters is in Appendix A.6. + +# 3.1 ACCELERATION OF TRAINING + +![](images/figures/ngpt-fig-0001.jpg) +Figure 1: Validation loss during training of 1B GPT and nGPT with 4k context length. + +Figure 1 presents the validation loss during the training of GPT and nGPT models with 1 billion parameters and a sample length of $4 \mathrm { k \Omega }$ tokens. After 20k iterations, nGPT achieves the same validation loss that GPT reaches only after $2 0 0 \mathrm { k }$ iterations (approximately 400 billion tokens), demonstrating a 10x speedup in terms of iterations and tokens used.3 + +![](images/figures/ngpt-fig-0002.jpg) +Figure 2: Final validation loss $\mathbf { \widetilde { y } }$ -axis) for training runs with different computation budgets in tokens ( $\mathbf { \dot { x } }$ -axis). The training of 0.5B and 1B nGPT models is about $4 \mathbf { x }$ , $1 0 \mathrm { x }$ and $2 0 \mathrm { x }$ faster (in terms of tokens) on 1k, 4k and 8k context lengths, respectively. + +![](images/figures/ngpt-fig-0003.jpg) +Figure 3: Models trained with $4 \mathrm { k \Omega }$ context length. Final performance $\mathbf { y }$ -axis) on a set of downstream tasks and their average value (Bottom-Right) for different computation budgets in tokens $\mathbf { \hat { x } }$ -axis). + +Figure 2 illustrates how the performance gap between nGPT and GPT scales across three axes: total token budget, context length, and network size. Training the 0.5B and 1B nGPT models is approximately $4 \mathbf { x }$ , 10x, and 20x faster at context lengths of 1k, 4k, and $^ \mathrm { 8 k }$ tokens, respectively. + +Figure 3 shows a similar pattern across downstream tasks, confirming that the acceleration is not only reflected in perplexity but also in task performance. Figures 8 and 10 in the Appendix provide results for 1k and $^ \mathrm { 8 k }$ context lengths. We observe some saturation for the longest runs of nGPT, suggesting that the model capacity is nearly reached for this number of trainable model parameters. + +# 3.2 INSPECTION OF NETWORK PARAMETERS + +Figure 4 shows that, while nGPT maintains a fixed norm for embeddings (by design), GPT exhibits significant variation. The distribution of eigenvalues, computed from the covariance matrix of embeddings and normalized by their median, reveals that GPT’s input embeddings have a higher condition number, especially in the 1B model. The distribution of pairwise dot products between embeddings indicates that even in nGPT, embeddings are not uniformly distributed across the hypersphere (where the dot product would approach 0), but instead form clusters—possibly reflecting natural patterns in language data. Dot products in GPT tend to have higher values due to its embeddings forming a hyper-ellipsoid, as suggested by the spread of vector norms. The ill-conditioned nature of GPT’s input embeddings could lead to computational issues involving these embeddings. + +Figure 5 shows the median condition numbers (across heads) for attention and MLP matrices at different layer depths—24 layers for the 0.5B model and 36 layers for the 1B model. GPT models exhibit significantly higher condition numbers in their attention matrices compared to nGPT. A closer inspection of these matrices (the 3rd and 4th layers are in Figure 12 and Figure 13 of Appendix) suggests that they degenerate into lower-rank matrices, potentially reducing the learning capacity of these blocks. One could argue that the elevated condition numbers are influenced by the norms of the vectors in these matrices. Our post-training normalization of these matrices is depicted by the dotted lines in Figure 11 of the Appendix. While the adjusted condition numbers are reduced, they remain higher than those for nGPT, indicating potential rank deficiency. The need for such normalization highlights one of the issues that nGPT is specifically designed to address. + +![](images/figures/ngpt-fig-0004.jpg) +Figure 4: Left: Distribution of norms of vectors from input (Top line) and output (Bottom line) embedding matrices. Middle: Distribution of eigenvalues divided by its median value. Right: Pairwise distribution of dot products between embeddings. Models are trained for $1 0 0 \mathrm { k }$ iterations. + +![](images/figures/ngpt-fig-0005.jpg) +Figure 5: Median condition numbers for attention and MLP matrices at different layer depth (24 and 36 layers for 0.5B and 1B models, respectively). Models are trained for $1 0 0 \mathrm { k }$ iterations. + +![](images/figures/ngpt-fig-0006.jpg) +Figure 6: (Left): Eigen learning rates of Attention and MLP blocks. (Middle): Scaling factors applied to the intermediate states of MLP. (Right): Scaling factors applied before the QK dot product; distribution of per-vector scalings applied to logits. Models are trained for $1 0 0 \mathrm { k }$ iterations. + +An important contribution of this work is the decoupling of predictions made by the Attention and MLP blocks from their impact on the hidden state $^ { h }$ . These contributions are controlled by the eigen learning rates $\alpha _ { \mathrm { A } }$ and $\mathbf { \alpha } _ { \mathbf { { \alpha } } } \mathbf { { \alpha } } _ { \mathbf { { \alpha } } } \mathbf { { \alpha } } _ { \mathbf { { \alpha } } } \mathbf { { \alpha } } _ { \mathbf { { \alpha } } } \mathbf { { \alpha } } _ { \mathbf { { \alpha } } } \mathbf { { \alpha } } _ { \mathbf { { \alpha } } } \mathbf { { \alpha } } _ { \mathbf { { \alpha } } } \mathbf { { \alpha } } _ { \mathbf { { \alpha } } } \mathbf { { \alpha } } _ { \mathbf { { \alpha } } } \mathbf { { \alpha } } _ { \mathbf { { \alpha } } } \mathbf { { \alpha } } _ { \mathbf { { \alpha } } } \mathbf { { \alpha } } _ { \mathbf { { \alpha } } }$ . Their interpretation is straightforward: if $\alpha _ { \mathrm { A } , i }$ for an embedding dimension $i \in \mathbb { R } ^ { d _ { \mathrm { m o d e l } } }$ is 0.2, then the update follows $h _ { i } \gets ( 1 - 0 . 2 ) h _ { i } + 0 . 2 h _ { \mathrm { A } , i }$ . Thus, they directly quantify the contribution of $h _ { \mathbf { A } , i }$ into $^ { h }$ . Figure 6 shows the average absolute values of $h _ { \mathrm { A } }$ and $h _ { \mathrm { M } }$ at each layer. Notably, the network learns to take only modest steps $( 2 0 \% - 3 0 \% )$ in the direction suggested by $h _ { \mathrm { A } }$ and $h _ { \mathrm { M } }$ . The average magnitude of $\alpha _ { \mathrm { A } }$ decreases from 0.25 in the $0 . 5 \mathrm { B }$ network (24 layers) to 0.20 in the 1B network (36 layers). Meanwhile, $\mathbf { \alpha } _ { \mathbf { { \alpha } } } \mathbf { { \alpha } } _ { \mathbf { { \alpha } } }$ decreases from 0.37 to 0.32. If the parameter count per block is linked to its maximum predictive capacity (in our setup, MLP blocks have more parameters than Attention blocks), then the greater eigen learning values observed for MLPs could potentially be attributed to the higher quality predictions made by these blocks. + +The scaling factors $\mathbf { } _ { s _ { u } }$ , $\scriptstyle { \pmb { s } } _ { \nu }$ and $s _ { q k }$ remain relatively stable across layers. The value of $\scriptstyle { \pmb { s } } _ { \nu }$ can be interpreted as a measure of the non-linearity of the SiLU function, which behaves like ReLU for large $\scriptstyle { \pmb { s } } _ { \nu }$ and approximates a linear unit for values near 0 (see also Appendix A.1). The distribution of $\pmb { s } _ { z }$ is primarily characterized by its mean, which influences the temperature of the softmax during cross-entropy calculations. The introduced scaling factors $\boldsymbol { s } _ { q k } , \boldsymbol { s } _ { u } , \boldsymbol { s } _ { \nu }$ and $\pmb { s } _ { z }$ seem to compensate for the removal of magnitude information when normalizing matrices and embeddings. + +# 3.3 ABLATION STUDIES + +Appendix A.9 summarizes numerous ablation experiments. An important finding is that having fixed (non-learnable) values for $\boldsymbol { s } _ { q k } , \boldsymbol { s } _ { u } , \boldsymbol { s } _ { \nu }$ and a single global learnable value for $\pmb { s } _ { z }$ leads to only a slight degradation in accuracy. Therefore, our presented general case can be simplified and become easier to interpret. Appendix A.8 demonstrates that nGPT can handle longer contexts without requiring any modifications to RoPE. + +# 4 RELATED WORK + +Wang & Isola (2020) provides a comprehensive overview of the arguments for representation learning on the hypersphere. Spherical representations are associated with more stable training in the latent space of variational autoencoders (Xu & Durrett, 2018) and in embeddings used for face verification (Wang et al., 2017). Notably, when embeddings are well clustered, they tend to be linearly separable from the rest of the embedding space (Wang & Isola, 2020). Mettes et al. (2019) demonstrated that classification and regression can be unified by placing prototype embeddings uniformly on a hypersphere, allowing for separation with large margins a priori. Wang & Isola (2020) found a strong empirical correlation between downstream task performance and both the alignment (closeness) and uniformity of embeddings on the hypersphere. + +Since all embeddings in nGPT lie on the hypersphere, any update that causes the hidden state $^ { h }$ to deviate from the manifold is followed by a normalization step. This normalization can be interpreted as a retraction in the context of Riemannian optimization. One might attempt to approximate nGPT’s update in GPT by applying RMSNorm both at the beginning and end of the block (Xiong et al., 2020). However, this approach does not guarantee a fixed norm for the hidden state, nor does it ensure that the recombination approximates SLERP or LERP. + +The normalization in equations 15 and 16 closely resembles the QK normalization by Henry et al. (2020). In nGPT, this process can be viewed as restoring $\pmb q$ and $\boldsymbol { k }$ of the $i$ -th head to a $( \dot { d } _ { \mathrm { m o d e l } } / \dot { n } _ { \mathrm { h e a d s } } )$ - dimensional hypersphere after the projection of $^ { h }$ by $W _ { q }$ and $W _ { k }$ , respectively. Since $^ { h }$ and the embedding dimensions of $W _ { q }$ and $W _ { k }$ are already normalized, the norms of $\pmb q$ and $\boldsymbol { k }$ are also comparable, making their normalization potentially unnecessary. We investigated the effect of omitting this normalization in our ablation studies (see Appendix A.9). The results indicate only a minor performance degradation with potential computational savings of about $12 \%$ . However, the normalization helps to maintain performance when extrapolating the context length (see Appendix A.8). + +# 5 DISCUSSION AND CONCLUSION + +This work builds on numerous key findings and observations made in the field which directly (Liu et al., 2017; Wang et al., 2017; Liu et al., 2018; Xu & Durrett, 2018; Wang & Isola, 2020; Liu et al., 2021; Karras et al., 2024) and indirectly (Salimans & Kingma, 2016; Franke et al., 2023; Kodryan et al., 2022; Kosson et al., 2023) support representation learning on the hypersphere. One of our main contributions is the normalization of the embedding dimensions across all of the transformer’s matrices, ensuring they reside on the same hypersphere. Crucially, we observed that such normalization alone would constrain the inputs of non-linear units, and, thus, the scaling factors for these units should be introduced. + +In line with recent studies suggesting that transformers implicitly perform gradient descent as metaoptimizers (Von Oswald et al., 2023; Dai et al., 2022), we explicitly demonstrate how this process occurs in the normalized Transformer: i) the transformation blocks provide gradient information, ii) this information is multiplied by eigen learning rates to adjust the hidden state, and iii) the commonly used normalization can be interpreted as a retraction step in Riemannian optimization, projecting the point back onto the hypersphere. We believe we are the first to decouple the eigen learning rates from the rest of the network, recognizing them as trainable parameters that can be interpreted as the diagonal elements of a variable-metric matrix. In other words, the normalized Transformer functions as a variable-metric optimizer, searching for output solutions using data-driven gradient information estimated in its attention and MLP blocks. + +The spherical representation provides valuable insights into the internals of nGPT, enabling the collection and analysis of statistics about its normalized components. Most importantly, it allows for the application of mathematical techniques specifically designed for dealing with hyperspheres. We believe that the reported acceleration, by a factor from 4 to 20, is only the first step towards uncovering new algorithms and architectures that could emerge from nGPT. Future work should explore scaling nGPT to larger network sizes, real-world datasets, and a broader range of tasks. For instance, the extension of nGPT to encoder-decoder and hybrid architectures (Dao & Gu, 2024; De et al., 2024) is straightforward. + +# REFERENCES + +Naman Agarwal, Nicolas Boumal, Brian Bullins, and Coralia Cartis. Adaptive regularization with cubics on manifolds. Mathematical Programming, 188, 2021. + +Maksym Andriushchenko, Francesco D’Angelo, Aditya Varre, and Nicolas Flammarion. Why do we need weight decay in modern deep learning? arXiv:2310.04415, 2023. + +Thomas Bachlechner, Huanru Henry Majumder, Bodhisattwa Prasad Mao, Garrison W. Cottrell, and Julian McAuley. Rezero is all you need: Fast convergence at large depth. In arXiv, 2020. URL https://arxiv.org/abs/2003.04887. + +Damai Dai, Yutao Sun, Li Dong, Yaru Hao, Shuming Ma, Zhifang Sui, and Furu Wei. Why can GPT learn in-context? Language models implicitly perform gradient descent as meta-optimizers. arXiv:2212.10559, 2022. + +Tri Dao and Albert Gu. 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Language models are unsupervised multitask learners. + +https://cdn.openai.com/better-language-models/language_models_are_unsupervised_mult 2018. + +Tim Salimans and Durk P Kingma. Weight normalization: A simple reparameterization to accelerate training of deep neural networks. NeurIPS, 2016. + +Noam Shazeer. Gated linear units (glu). arXiv:2002.05202, 2020. + +Sam Shleifer, Jason Weston, and Myle Ott. Normformer: Improved transformer pretraining with extra normalization. arXiv preprint arXiv:2110.09456, 2021. + +Ken Shoemake. Animating rotation with quaternion curves. In Proc. of the 12th annual conference on Computer graphics and interactive techniques, 1985. + +Mohammad Shoeybi, Mostofa Patwary, Raul Puri, Patrick LeGresley, Jared Casper, and Bryan Catanzaro. Megatron-lm: Training multi-billion parameter language models using model parallelism. arXiv preprint arXiv:1909.08053, 2019. + +Jianlin Su, Murtadha Ahmed, Yu Lu, Shengfeng Pan, Wen Bo, and Yunfeng Liu. Roformer: Enhanced transformer with rotary position embedding. Neurocomputing, 2024. + +Ashish Vaswani, Noam Shazeer, Niki Parmar, Jakob Uszkoreit, Llion Jones, Aidan N Gomez, Lukasz Kaiser, and Illia Polosukhin. Attention is all you need. In NeurIPS, 2017. + +Johannes Von Oswald, Eyvind Niklasson, Ettore Randazzo, Jo˜ao Sacramento, Alexander Mordvintsev, Andrey Zhmoginov, and Max Vladymyrov. Transformers learn in-context by gradient descent. In ICML, 2023. + +Feng Wang, Xiang Xiang, Jian Cheng, and Alan Loddon Yuille. Normface: L2 hypersphere embedding for face verification. In Proc. of the 25th ACM nternational conference on Multimedia, 2017. + +Tongzhou Wang and Phillip Isola. Understanding contrastive representation learning through alignment and uniformity on the hypersphere. In ICML, 2020. + +Dennis Wu, Jerry Yao-Chieh Hu, Teng-Yun Hsiao, and Han Liu. Uniform memory retrieval with larger capacity for modern hopfield models. arXiv preprint arXiv:2404.03827, 2024. + +Ruibin Xiong, Yunchang Yang, Di He, Kai Zheng, et al. On layer normalization in the Transformer architecture. In ICML, 2020. + +Jiacheng Xu and Greg Durrett. Spherical latent spaces for stable variational autoencoders. arXiv:1808.10805, 2018. + +# A APPENDIX + +A.1 RESCALING IN THE MLP BLOCK OF THE NORMALIZED TRANSFORMER + +When computing SwiGLU using equation 18, each element $x$ of the vector $\textbf { { v } }$ is an input to SiLU: + +$$ +\operatorname { S i L U } ( x ) = x \cdot \sigma ( x ) = x \cdot { \frac { 1 } { 1 + e ^ { - x } } } , +$$ + +where $\sigma ( x )$ is sigmoid. For $x$ with large magnitude, $\mathrm { S i L U } ( x )$ approximates $\scriptstyle { \mathrm { R e L U } } ( x )$ : when $x - \infty$ , $\mathrm { S i L U } \bar { ( } x ) \to 0$ , and when $x \infty$ , $\operatorname { S i L U } ( x ) \approx x$ . The minimum of $\mathrm { S i L U } ( x _ { m i n } ) \approx$ $- 0 . 2 7 8$ is located at $x _ { m i n } \approx - 1 . 2 7 8$ . While the elements of $\pmb { v }$ represent dot products of $d _ { m o d e l ^ { - } }$ dimensional vectors and are bounded in $[ - 1 , 1 ]$ , their expected absolute value (when they are random) is E[| cos(θ)|] = 2π · √ 1dmodel . Thus, we should rescale $x$ by $\sqrt { d _ { m o d e l } }$ , otherwise, for very small $x$ , we end up with $\operatorname { S i L U } ( x ) \approx x / 2$ . An alternative view is to note that since the variance of each of the normalized vectors $^ { h }$ and a vector from $W _ { v }$ ) is $1 / d _ { m o d e l }$ , the variance of 1 (suitable for the sigmoid part of SiLU) can be restored by rescaling of $x$ by $\sqrt { d _ { m o d e l } }$ . Based on these two views, we rescale $\pmb { v }$ by $\sqrt { d _ { m o d e l } }$ to benefit from the non-linearity of SiLU. + +# A.2 EIGEN LEARNING RATES + +In the main text of the paper, we defined eigen learning rates as positive $\mathbf { \sigma } _ { \mathbf { \alpha } } ( \alpha \vert \alpha \vert$ is used during the forward pass). However, when they are not constrained to be positive, we obtain experimental results which are the same (up to numerical difference). This surprising observation can be explained as follows. Both the attention and MLP blocks have transformation matrices at their outputs. When the search is unconstrained, it is sufficient for Adam to flip (select) the sign of the $i$ -th row of the output transformation matrix $W _ { o }$ to change the sign of the corresponding $i$ -th coordinate in $h _ { \mathrm { A } }$ . Thus, the transformation calculated as $\alpha _ { \mathrm { { A } } } W _ { o }$ is the same as $\alpha _ { \mathrm { A } } ^ { \prime } W _ { o } ^ { \prime }$ , where ${ \pmb { \alpha } } _ { \mathrm { A } , i } ^ { \prime } = - { \pmb { \alpha } } _ { \mathrm { A } , i }$ and $W _ { o , ( i , : ) } ^ { \prime } = - W _ { o , ( i , : ) }$ . In other words, when unconstrained, we can arrive at exactly the same transformation by flipping the signs in both $_ \alpha$ and $W _ { o }$ , which cancel each other. For simplicity and clearer interpretation, we suggest constraining $_ \alpha$ to be positive in the main paper. + +There is, however, another interesting scenario when eigen learning rate could become negative. In quasi-Newton methods, $\textbf { { B } }$ approximates the inverse Hessian matrix $H ^ { - 1 }$ whose diagonal elements are positive values if the function is locally convex which is the assumption of the Newton method ( $_ { H }$ needs to be positive definite). However, the diagonal of $H ^ { - 1 }$ can have negative values if the objective function is non-convex and has saddle points. While quasi-Newton methods like BFGS aim to ensure (e.g., via regularization) that $\textbf { { B } }$ is positive-definite even on non-convex problem, some Riemannian optimization methods can exploit negative curvature of $H ^ { - 1 }$ (Agarwal et al., 2021). When the diagonal of $\textbf { { B } }$ is unconstrained, we perform a variable-metric step, acknowledging that the function may be non-convex locally. We did not mention this in the main paper because, as noted earlier, the results with both constrained and unconstrained $_ \alpha$ are essentially the same. + +During the review process of this paper, we were made aware of the work of Bachlechner et al. (2020) which proposes ReZero (residual with zero initialization). In the context of Transformers, ReZero is implemented as: + +$$ +h h + \alpha h _ { \mathrm { T } } +$$ + +where the hidden state $^ { h }$ at each layer is updated with $\alpha$ -rescaled output $h _ { \mathrm { T } }$ of the transformation block. The trainable layer-specific scalar $\alpha$ is the same for both the MLP and Attention blocks, with its initial value suggested to be set to zero. The authors also proposed to remove LayerNorm and suggested that BatchNorm is a suitable alternative. We note that the introduction of the scaling factor $\alpha$ does not affect the architecture of the model because the output projection matrices of both MLP and Attention blocks already contain that same scaling factor as a multiplicative part of their parameters (e.g., a simultaneous rescaling of $\alpha$ by $k$ and the output projection matrices by $1 / k$ results in a network with identical behavior). However, the use of a single scaling factor can help to more quickly adjust the impact of each transformation block. Indeed, the authors argue that by initializing $\alpha$ to zero, one can achieve a form of curriculum learning in which individual layers of the network can be gradually activated during optimization. + +The update of ReZero differs from the update of nGPT which can be written as: + +$$ +h \mathrm { N o r m } ( h + \alpha _ { \mathrm { T } } ( \mathrm { N o r m } ( h _ { \mathrm { T } } ) - h ) ) , +$$ + +where the original output of the transformation block $h _ { \mathrm { T } }$ is normalized before being used to compute an update direction, weighted by the eigen learning rate of the block $\pmb { \alpha } _ { \mathrm { T } }$ . In contrast to ReZero, the update of nGPT is invariant to any rescaling of the output projection matrix, and, thus, the eigen learning rate can be viewed as a measure of the contribution of the block. This is not the case for ReZero, where the scale of $\alpha$ cannot be viewed as a measure of the impact of the block without taking into account the scaling of the block’s matrices (see, e.g., our previous example with rescaling by $k$ ). In contrast to ReZero, where $\alpha$ rescales $h _ { \mathrm { T } }$ , the update of nGPT rescales the direction towards $h _ { \mathrm { T } }$ . Finally, the update of nGPT holds for the spherical representation where the updated hidden state is retracted back to the hypersphere. + +Another work that we were made aware of during the review process is NormFormer (Shleifer et al., 2021) which updates the hidden state as follows: + +$$ +\begin{array} { r l } & { h h + \mathrm { L N } ( h _ { \mathrm { A } } ) } \\ & { h \alpha h + \mathrm { L N } ( \sigma ( \mathrm { L N } ( h ) ) W _ { 1 } ) W _ { 2 } , } \end{array} +$$ + +where $h _ { \mathrm { A } }$ is the output of the Attention block with LayerNorm normalization of inputs, $W _ { 1 }$ and $W _ { 2 }$ matrices are components of the MLP block with GELU activation function, and, $_ { \pmb { \alpha } }$ are trainable scaling factors defined parameter-wise. If we omit the variance part of LayerNorm in Equation 26, then $\mathrm { L N } ( h _ { \mathrm { A } } )$ can be reshaped to $\alpha \mathrm { N o r m } ( h _ { \mathrm { A } } )$ and one could wonder if $_ \alpha$ can be viewed as eigen learning rates. This is not the case because, similarly to ReZero, i) the norm of $^ { h }$ is not constrained, and ii) the scaling is applied to the value itself and not to the direction. In Equation 27, not only the norm of $^ { h }$ is not constrained, but the output of the MLP is also not normalized. Instead, LayerNorm normalizes the rescaling factors for the vectors stored in $W _ { 2 }$ . While the name NormFormer may suggest some similarities with nGPT, the approach does not normalize network parameters in any way and does not normalize embeddings during the update over layers (i.e., the typically growing norm of $^ { h }$ affects its recombination with the outputs of the transformation blocks). + +# A.3 SUPPLEMENTARY NOTES TO INTRODUCTION + +The original Transformer architecture is a solution in some design space which can be measured with respect to a set of metrics. Many known aspects of multi-criteria decision-making and multiobjective optimization are applied (Deb et al., 2016). Various Transformer variants exist because depending on the selection of metrics (e.g., performance on particular tasks) and constraints (e.g., memory, compute, data), the decision-maker prefers and selects (e.g., based on non-dominance or aggregation of objectives) different design solutions. The first part of the Introduction section describes various architectural and optimization findings/solutions of the past. Then, it is proposed to ”unify various findings and observations made in the field under a new perspective of the normalized Transformer.” This unification can be viewed as a recombination procedure in the design space of architectures and optimization approaches. The Introduction section concludes with a description of the key aspects of the proposed solution. The experimental results of the paper demonstrate some advantages of this solution w.r.t. the baseline GPT, e.g., improved condition number of attention matrices, faster convergence. + +While the proposed solution can be viewed as a recombination of existing techniques, this is not how it was historically designed. The precursor of nGPT is described in Loshchilov (2023), where a modification of AdamW was studied in the training of Transformer models. That work demonstrated that instead of performing weight decay, the norm of all parameters of the network can be controlled and scheduled over the course of training. While this approach enabled acceleration in training, it was noted that the attention matrices are ill-conditioned and the normalization of the whole network or even individual matrices does not resolve it. In contrast, experiments with normalization of individual vectors within matrices demonstrated improvements of the condition numbers. Then, by combining this finding with the understanding that LayerNorm and RMSNorm normalize the hidden state to a hypersphere (when all scaling factors are equal), it was hypothesized that the spherical representation is a viable option. nGPT was designed from first principles, focusing on ensuring that the Transformer architecture respects the spherical representation for all its vector components. SLERP was identified as the proper way to recombine the hidden state and the output of the transformation blocks when working on the hypersphere and using scalar eigen learning rates. Scaling factors, such as those used for logits, were introduced to relax the constraints imposed by the spherical formulation. + +During the review process of this paper, we were made aware of a set of closely related works. Karras et al. (2024) introduces a set of normalization techniques (very similar to ours) which greatly improve the training of Diffusion Models. Liu et al. (2021) proposed an optimization method for neural networks under hard hyperspherical weight constraints. Large et al. (2025) employs a residual block structure which is related to the one of nGPT but without learnable parameters (eigenlearning rates). By connecting transformers to modern Hopfield models, Wu et al. (2024) show that well-separated memory patterns (learned representations) enhance memorization capability (expressiveness), naturally achieved by mapping learning onto the hypersphere. The authors conclude that representation learning on the unit hypersphere significantly improves expressiveness and memorization. Hu et al. (2024) claim that mapping to the hypersphere makes representation learning provably optimal for any dataset. + +# A.4 RIEMANNIAN OPTIMIZATION + +If $h - h _ { \mathrm { A } }$ is viewed as the gradient $\textbf { { g } }$ in the Euclidean space, then, aligning with the requirements of Riemannian optimization, the projection of $\textbf { { g } }$ onto the tangent space of the hypersphere is + +$$ +g _ { \mathrm { p r o j } } h ( h ^ { T } h _ { \mathrm { A } } ) - h _ { \mathrm { A } } +$$ + +The projection is equivalent to $\textbf { { g } }$ when the dot product ${ \pmb h } ^ { T } { \pmb h } _ { \mathrm { A } }$ is 1. Depending on the alignment between $^ { h }$ and $h _ { \mathrm { A } }$ , the projected gradient varies between $h - h _ { \mathrm { A } }$ (when the vectors are aligned) and $- \pmb { h } _ { \mathrm { A } }$ (when the vectors are orthogonal). The Riemannian variable-metric update then reads as: + +$$ +\begin{array} { r l } & { h \mathrm { N o r m } ( h - B _ { \mathrm { A } } ( h ( h ^ { T } h _ { \mathrm { A } } ) - h _ { \mathrm { A } } ) ) } \\ & { h \mathrm { N o r m } ( h - B _ { \mathrm { M } } ( h ( h ^ { T } h _ { \mathrm { M } } ) - h _ { \mathrm { M } } ) ) } \end{array} +$$ + +The normalization by Norm can be viewed as the retraction step in Riemannian optimization, mapping the updated solution back to the manifold. + +Our experimental results suggest that the impact of $h ^ { T } h _ { \mathrm { M } }$ is negligible. Therefore, all experiments in the paper are based on equations 10 and 11. + +# A.5 TIME COST PER STEP + +The time cost per step for nGPT is approximately $80 \%$ higher with $4 \mathrm { k \Omega }$ context length, and $60 \%$ higher with 8k context length. This overhead is not only due to nGPT having 6 normalization steps (2 of them are applied for $\pmb q$ and $\boldsymbol { k }$ ) per layer instead of 2, but also because nGPT’s normalizations are not yet fully optimized, unlike GPT, where normalization layers are fused with other operations. Training on larger networks is expected to further reduce this performance gap, as the number of layers (and thus the number of normalizations) increases only modestly with the number of network parameters. Appendix A.9 shows that we can remove the normalization of $\pmb q$ and $\boldsymbol { k }$ with a minor negative impact on results. + +# A.6 EXPERIMENTAL SETUP + +In all experiments, we use OpenWebText (Gokaslan & Cohen, 2019) dataset which, according to experiments of Karpathy (2023), represents a good approximation of the OpenAI’s internal dataset used to train GPT-2 models. We are well aware that this dataset is not of the highest quality. However, we believe that it is suitable for academic research, and, should improve the comparability of our findings with other research papers. + +Table 2: Model Parameters for GPT and nGPT + +
Model Parameter0.5B Models1.0B Models
Number of Layers (nlayers)2436
Model Dimension (dmodel)10241280
Number of Attention Heads (nheads)1620
Key Dimension (dk)dmodel/nheadsdmodel/ nheads
MLP Dimension (dMLP)4dmodel4dmodel
Parameters in GPT468.2M1025.7M
Parameters in nGPT468.4M1026.1M
+ +Table 3: Optimization Parameters for GPT and nGPT + +
Optimization ParameterGPTnGPT
OptimizerAdamWAdam (AdamW with weight decay 0.0)
Weight Decay0.10.0
Number of Warmup Steps20000
Learning Rate ScheduleCosine AnnealingCosine Annealing
Initial Learning Rateproblem-specificproblem-specific
Final Learning Rate00
+ +We trained our models using 64 A100 GPUs distributed across 8 nodes (8 GPUs per node). Global batch size is 512. We use the LLaMA-2 tokenizer with $3 2 \mathrm { k }$ tokens. We use the same setup for the 0.5B and 1.0B parameter models. All parameters of matrices are stored in bfloat16. + +All matrix parameters are initialized by sampling from a zero-mean normal distribution with a standard deviation of 0.02 for GPT and $\dot { 1 } / \sqrt { d _ { \mathrm { m o d e l } } }$ for nGPT. The standard deviation for the output matrices was scaled by a factor of $\sqrt { 2 \times n _ { \mathrm { l a y e r } } }$ , as suggested by Radford et al. (2018). The initialization of matrix parameters is not important for nGPT because they are normalized afterwards. The base of RoPE is 10000. The initialization of the additional parameters introduced in nGPT is described in Section 2.6. + +All experiments described in this paper were performed using an internal library based on Megatron-LM (Shoeybi et al., 2019). In order to illustrate how nGPT works, we reimplemented nGPT using nanoGPT (Karpathy, 2023) and published our re-implementation at https://github.com/NVIDIA/ngpt. It should be noted that the latter re-implementation only qualitatevely replicates our internal experiments and should not be used in production. However, it should be great to quickly get familiar with nGPT as its core functionality represents only a few dozens of lines of codes. It is very important to note that when implementing nGPT in training libraries, one should make sure that not only instantiated model parameters are normalized but also the ones which are used by the optimizer. Missing the latter is a common bug that should be avoided. + +# A.7 SELECTION OF THE INITIAL LEARNING + +The initial learning rate is the only hyperparameter we tune for both GPT and nGPT. Figure 7 demonstrates our trials to select the most suitable initial learning rate for GPT and nGPT. Our first experiments started with the 0.5B model and 1k context length. After observing the general trend of these curves, we reduced the number of experiments and followed the trends to minimize the total compute used. Apart from estimating the optimal settings for the initial learning rate, our general observation is that the optimal learning rates for GPT and nGPT tend to be similar for the same values of the validation loss. Longer runs are usually associated with the possibility of achieving lower values of the validation loss, and this typically requires lower initial learning rates. + +One artifact we observed is the increasing sensitivity of the 1B nGPT model on $^ \mathrm { 8 k }$ context length. We found that the smoothness of the hyperparameter curves can be restored (see the dotted lines with squares) by increasing $\pmb { \alpha } _ { \mathrm { A , i n i t } }$ and $\alpha _ { \mathrm { { M , i n i t } } }$ from their default value of $1 / \sqrt { d _ { m o d e l } }$ to 0.1. This change decreases the effective learning rate of Adam on these variables by a factor of $( 1 / \sqrt { d _ { m o d e l } } ) / 0 . 1 \approx 3$ . As a result, the eigen learning rates are learned by Adam at a slower rate. + +![](images/figures/ngpt-fig-0007.jpg) +Figure 7: Final validation loss values for different initial learning rates for the 0.5B models (Top) and 1B models (Bottom). GPT is denoted by solid lines with circles, while nGPT is represented by the dotted lines with stars. A specific case with a different setup, denoted by the dotted lines with squares (Bottom Right), is discussed in the text. + +![](images/figures/ngpt-fig-0008.jpg) +Figure 8: Models trained with 1k context length. Final performance $\mathbf { y }$ -axis) on a set of downstream tasks and their average value (Bottom-Right) for different computation budgets in tokens $\mathbf { \hat { x } }$ -axis). + +![](images/figures/ngpt-fig-0009.jpg) +Figure 9: Models trained with $^ \mathrm { 8 k }$ context length. Final performance $\mathbf { y }$ -axis) on a set of downstream tasks and their average value (Bottom-Right) for different computation budgets in tokens $\mathbf { \hat { x } }$ -axis). + +![](images/figures/ngpt-fig-0010.jpg) +Figure 10: Performance on WMT14-FR-EN task which evaluates the ability (measured by the BLEU score of Papineni et al. (2002)) to translate a French sentence to English given five (French, English) example pairs, following the methodology of Radford et al. (2018) where it was used to benchmark GPT-2. + +![](images/figures/ngpt-fig-0011.jpg) +Figure 11: Median condition numbers measured for attention and MLP matrices at different layer depth (24 and 36 layers for 0.5B and 1B networks, respectively). The dotted lines are for the case when GPT’s matrices are renormalized after training. Models are trained for $1 0 0 \mathrm { k }$ iterations. + +![](images/figures/ngpt-fig-0012.jpg) +Figure 12: Left: Distribution of the norms of vectors forming the complete attention matrices (not per head). Right: Distribution of singular values for all heads. The results are shown for the 3rd layer. + +![](images/figures/ngpt-fig-0013.jpg) +Figure 13: Left: Distribution of the norms of vectors forming the complete attention matrices (not per head). Right: Distribution of singular values for all heads. The results are shown for the 4th layer. + +# A.8 LENGTH EXTRAPOLATION ABILITY + +We investigate the length extrapolation ability of nGPT by evaluating its perplexity on the PG19 dataset, as shown in Figure 14. In standard GPTs, perplexity tends to increase dramatically when tested on sequences longer than pre-training length. In contrast, nGPT maintains a stable perplexity range even at extrapolated lengths. This result demonstrates that nGPT can handle longer contexts without requiring any modifications to RoPE, providing a clear advantage over standard GPTs in terms of length extrapolation. + +In the original version of the paper, we explored the impact of qk-normalization on performance of nGPT. Our conclusion was that the performance is equivalent but removing qk-normalization can provide computational savings. During the review process of the paper, two reviewers asked questions which made us further investigate the role of qk-normalization and context length extrapolation abilities of nGPT. This resulted in noticing that nGPT without qk-normalization shows worse extrapolation abilities (see Figure 14-Right). This difference was initially overlooked because the results of nGPT with and without qk-normalization on other tasks are practically equivalent (see, e.g., the average accuracy on downstream tasks and validation loss of both variants in Table 6). In other words, their in-distribution performance appears the same (i.e., qk-normalization does not explain nGPT’s better performance) but their out-of-distribution performance is different. The difference comes from the fact that qk-normalization normalizes $\pmb q$ and $\boldsymbol { k }$ , and, thus, guarantees that their dot product is bounded in [-1,1]. Even without using qk-normalization in nGPT, the hidden state and the vectors forming $W _ { q }$ and $W _ { k }$ matrices are normalized so that their dot product is already bounded in [-1,1]. However, the results of these individual dot products (based on vectors from $W _ { q }$ and $W _ { k }$ ) will form $\pmb q$ and $\boldsymbol { k }$ per head. Moreover, $\pmb q$ and $\boldsymbol { k }$ are also affected by RoPE. The qk-normalization restores $\pmb q$ and $\boldsymbol { k }$ vectors back to a hyper-sphere whose dimensionality is defined by the size of each head. + +While the extrapolation abilities of nGPT can be partially attested to the use of qk-normalization, the gap between nGPT without qk-normalization and the baseline GPT is still substantial. Another contributing factor to the difference is the fact that many attention matrices of the baseline GPT are practically low-rank, as discussed in the main paper. This makes them less efficient in dealing with the context. Interestingly, Kobayashi et al. (2024) recently demonstrated that weight decay induces low-rank attention layers, thus confirming our observations. While the low-rank attention matrices can potentially appear due to a set of factors, we note that weight decay is not used in nGPT. + +![](images/figures/ngpt-fig-0014.jpg) +Figure 14: PG19 perplexity from 1K to 32K among different training lengths for the original GPT and nGPT (Left) and also with nGPT when qk-normalization is removed (Right). + +# A.9 ABLATION STUDIES + +We perform a series of ablations in nGPT, focusing on the selection of scaling factors $s _ { q k }$ $, s _ { u } \left( s _ { v } \right)$ , $\pmb { s } _ { z }$ , as well as evaluating the necessity of normalization. For the scaling factors, we first examine the impact of different initializations of these parameters. Additionally, we explore whether these factors can be simplified to a learnable scalar or a fixed value. For normalization, we analyze whether the removal of QK-normalization is a feasible alternative. To ensure a fair comparison, all our ablation models have size 0.5B and context length 1k and are trained with learning rate $1 \times 1 0 ^ { - 3 }$ for $1 0 0 \mathrm { k }$ iterations ${ \sim } 5 2 \mathrm { B }$ tokens). + +Table 4 presents various combinations of $s _ { i n i t }$ and $\pmb { s } _ { s c a l e }$ for each scaling factor, as well as the downstream task accuracy and validation loss. Additionally, we report the mean value of each scaling factor distribution to show the final converged scaling value after training. For $s _ { q k }$ , we observe that the mean value $( \mathrm { M e a n } ( s ) )$ is relatievly stable with value around 1 across most initialization settings, except when using smaller $s _ { i n i t }$ and larger $\pmb { s } _ { s c a l e }$ , which results in a Mean(s) value less than 1. For $\mathbf { } _ { s \alpha }$ and $\scriptstyle { \pmb { s } } _ { v }$ , changes in initialization lead to significant shifts in Mean(s), accompanied by increases in final validation loss. For $\pmb { s } _ { z }$ , we see a substantial degradation in both accuracy and loss under certain initializations. Overall, these results suggest that the baseline hyperparameter settings are robust but additional tuning of $\mathbf { } _ { s \alpha }$ , $\scriptstyle { \pmb { s } } _ { v }$ , and $\pmb { s } _ { z }$ could potentially further improve performance. + +In Table 5, we modify each learnable per-element vector of the scaling factors $s _ { q k }$ , $\mathbf { } _ { s \mu }$ $( s _ { v } )$ , $\pmb { s } _ { z }$ , and the eigen learning rates $\alpha _ { A }$ , $\pmb { \alpha } _ { M }$ to a single learned scalar or a fixed value. This ablation helps us determine whether these tuning parameters can be simplified in nGPT. From the table, we observe that most changes result in only negligible degradation $( \leq 0 . 3 \% )$ in validation loss, and some even lead to slight improvements in accuracy. Notably, replacing the per-element vector $s _ { q k }$ with a single scalar has minimal impact on the mean value, while the mean value of $\mathbf { } _ { s _ { u } }$ , $\scriptstyle { \pmb { s } } _ { v }$ , $\pmb { s } _ { z }$ , $\alpha _ { A }$ , and $\alpha _ { M }$ show a slight increase. Furthermore, even when fixing $s _ { q k }$ , $\mathbf { } _ { s _ { u } }$ , and $\scriptstyle { \pmb { s } } _ { v }$ , the model still achieves comparable accuracy and validation loss to the baseline using the per-element vectors. + +In Table 6, we investigate the impact of removing QK normalization operations and the approximation of using LERP instead of SLERP to potentially reduce training time per step. Specifically, removing the QK normalization terms (Norm) in equations 15 and 16 reduces training time per step by $12 \%$ . Conversely, replacing the baseline LERP in equation 7 with SLERP in equation 6 increases training time by $10 \%$ . Despite these changes, both modifications result in comparable accuracy and loss to the baseline, demonstrating their effectiveness in reducing computational overhead without sacrificing performance. + +Table 4: Ablations of $s _ { i n i t }$ and $\pmb { s } _ { s c a l e }$ . Our default setup is the first row of each section. Mean(s) is the mean value of the learned distribution after training. Avg. Acc is the average accuracy of our selected five downstream tasks, Valid. Loss is the final validation loss, and we use $d _ { m o d e l } = 1 0 2 4$ . + +
SinitSscaleMean(s)Avg. Acc (%) ↑Valid. Loss ↓
Sqk11/√dmodel1.5154.442.252
Sqk0.331/√dmodel1.3654.67-0.33%
Sqk0.051/√dmodel1.0153.69-0.09%
Sqk111.3854.19-0.09%
Sqk0.3311.1154.89-0.08%
Sqk0.0510.2952.41+1.94%
Su & sv111.24 & 1.1254.442.252
Su & Sv1/√dmodel10.03 & 0.0353.51+0.92%
su & sv11/√dmodel2.75 & 11.253.90+0.05%
Su & Sv1/√dmodel1/√dmodel0.40 & 0.2554.39+0.48%
Sz11/√dmodel60.854.442.252
Sz√dmodel1/√dmodel106.152.60+1.06%
Sz1123.652.71+3.12%
Sz√dmodel163.651.84+1.66%
+ +Table 5: Ablations of replacing learnable per-element vector (scaling factors and eigen learning rate) with a single learned scalar or a fixed value. The number of the learned vector, learned scalar and fixed value are the mean values across all layers. + +
SqkSu & svSzαA &αMAvg. Acc (%) ↑Valid. Loss ↓
vector = 1.47vector = 1.12 & 1.24vector = 60.80vector = 0.22 & 0.3354.442.252
scalar = 1.49vector = 1.13 & 1.23vector = 62.11vector = 0.22 & 0.3354.05+0.22%
vector = 1.46scalar = 1.46vector = 60.70vector = 0.22 & 0.3354.23+0.07%
vector = 1.47vector = 1.12 & 1.24scalar = 95.65vector = 0.22 & 0.3353.69+0.20%
vector = 1.40vector = 1.13 & 1.13vector = 60.90scalar = 0.30 & 0.3654.86+0.22%
scalar = 1.51scalar = 1.47vector = 61.18vector = 0.22 & 0.3254.52+0.17%
scalar = 1.52scalar = 1.68scalar = 96.65vector = 0.22 & 0.3052.59+0.30%
scalar = 1.49scalar = 1.17scalar = 88.64scalar = 0.30 & 0.3753.61+0.62%
value = 1.00vector = 1.12 & 1.15vector = 60.69vector = 0.24 & 0.3554.17+0.11%
vector = 1.45value = 1.00vector = 61.53vector = 0.23 & 0.3555.51+0.05%
value = 1.00value = 1.00vector = 61.26vector = 0.24 & 0.3653.63+0.20%
value = 1.00value = 1.00scalar = 96.15vector = 0.22 & 0.3553.37+0.40%
+ +Table 6: Ablations of the design choices of nGPT. The removal of QK norm involves taking out the normalization terms Norm in equations 15 and 16. The replace of LERP with SLERP is using equation 6 without approximation of equation 7. + +
Training time per step (s) ↓Avg. Acc (%) ↑Valid. Loss ↓
Baseline nGPT0.65754.442.252
1) remove QKnorm0.57654.71+0.12%
replace LERP with SLERP0.72654.80-0.08%
+ +# A.10 ANALYSIS OF SCALING PARAMETERS + +In nGPT, we introduce a total six trainable parameters: the eigen learning rates $\alpha _ { A }$ and $\pmb { \alpha } _ { M }$ , along with the scaling factors $s _ { q k }$ , $\mathbf { } _ { s \mu }$ , $\scriptstyle { \pmb { s } } _ { v }$ and $\pmb { s } _ { z }$ . Since these parameters are updated using gradients, we are interested in understanding their learned distributions after training. In Figure 15, we present the histograms of these saved weights combining across layers and analyze their distributions under different context lengths, model sizes, learning rates, and numbers of training tokens. + +For $\alpha _ { A }$ and $\pmb { \alpha } _ { M }$ , we observe that their distributions remain stable across various learning rates. However, increasing the context length or the number of training tokens tends to shift the distributions to the right, indicating that the hidden state $^ { h }$ requires more transformation from the Attention and MLP blocks to handle the larger inputs. Additionally, the mean values for these parameters tend to be smaller in larger models because we have more layers to update the hidden state. + +Regarding the scaling factors, $s _ { q k }$ is quite stable under all the conditions, suggesting that a fixed value, similar to the softmax scaling factor in original transformer, may be sufficient. Interestingly, we find that $s _ { q k }$ has a high density close to zero, indicating the sparsity of attention in nGPT. For $\mathbf { } _ { s \alpha }$ and $\scriptstyle { \pmb { s } } _ { v }$ , their distributions shift left as the context length increases, but shift right as the model size or the number of training tokens increases. Furthermore, these two factors become flatter when using larger learning rate. Lastly, for $\pmb { s } _ { z }$ , we see higher mean values for longer context lengths, larger model sizes, and more training tokens, suggesting that the model may learn to use a lower temperature to make the final word distribution sharper. + +![](images/figures/ngpt-fig-0015.jpg) +Figure 15: The distribution of trainable eigen learning rates and scaling factors varies under different context lengths, model sizes, learning rates, and number of training tokens. If not specified, we use context length 1K, model size 0.5B, learning rate $2 . 0 \times 1 0 ^ { - 3 }$ , and training tokens 52B as default. \ No newline at end of file diff --git a/papers/ngpt/paper.pdf b/papers/ngpt/paper.pdf new file mode 100644 index 0000000000000000000000000000000000000000..b6c752d6f52b13ee500eb0dbcf48341c4da7ff54 --- /dev/null +++ b/papers/ngpt/paper.pdf @@ -0,0 +1,3 @@ +version https://git-lfs.github.com/spec/v1 +oid sha256:77198b765ba41f74a3faf02f1effa398e27fd2dd579225a2e4b9e4d1ded8cf9f +size 1552044 diff --git a/papers/ngpt/sau.json b/papers/ngpt/sau.json new file mode 100644 index 0000000000000000000000000000000000000000..625552f186bc78d5b46519aeb533607ce9d1fd7a --- /dev/null +++ b/papers/ngpt/sau.json @@ -0,0 +1,312 @@ +{ + "paper_id": "ngpt", + "paper_title": "nGPT: Normalized Transformer with Representation Learning on the Hypersphere", + "D1": [ + { + "id": "ngpt-D1-001", + "claim": "Model architecture dimensions for 0.5B/1B models (Table 2): n_layers=24/36, d_model=1024/1280, n_heads=16/20, d_k=64 (d_model/n_heads), d_MLP=4096/5120 (4*d_model).", + "source": "Section A.6, Table 2; Section 2.3.1" + }, + { + "id": "ngpt-D1-002", + "claim": "Model parameter counts including embeddings: GPT baseline 468.2M/1025.7M; nGPT 468.4M/1026.1M for 0.5B/1B models. nGPT has ~0.2M-0.4M extra parameters from 6 learnable scaling factors.", + "source": "Section A.6, Table 2; Section A.10" + }, + { + "id": "ngpt-D1-003", + "claim": "Optimizer configuration: GPT uses AdamW with weight_decay=0.1 and warmup_steps=2000; nGPT uses Adam (AdamW with weight_decay=0.0) with warmup_steps=0. Both share Cosine Annealing schedule, lr_final=0, and problem-specific initial learning rate.", + "source": "Section A.6, Table 3; Section 2.6 step 7; Section A.7" + }, + { + "id": "ngpt-D1-004", + "claim": "Training infrastructure: global batch_size=512, 64 A100 GPUs across 8 compute nodes (8 GPUs/node). Same hardware configuration used for both 0.5B and 1B model training.", + "source": "Section A.6" + }, + { + "id": "ngpt-D1-005", + "claim": "Data configuration: OpenWebText dataset with LLaMA-2 tokenizer (vocab_size=32000). All matrix parameters stored in bfloat16 precision.", + "source": "Section A.6" + }, + { + "id": "ngpt-D1-006", + "claim": "Weight initialization: GPT uses N(0, 0.02^2); nGPT uses N(0, 1/d_model). Output matrix standard deviation scaled by sqrt(2*n_layers) per Radford et al. (2018). nGPT matrices normalized along embedding dimension after initialization.", + "source": "Section A.6" + }, + { + "id": "ngpt-D1-007", + "claim": "RoPE positional encoding base frequency: rope_base=10000, applied to query and key vectors in each attention head before computing attention scores.", + "source": "Section A.6" + }, + { + "id": "ngpt-D1-008", + "claim": "Eigen learning rates (default): alpha_A_init=0.05, alpha_M_init=0.05, both with scale=1/sqrt(d_model). Via the init/scale trick (Section 2.5), effective learning rate ratio relative to global LR is approximately 1/sqrt(d_model).", + "source": "Section 2.6 step 3; Section 2.5" + }, + { + "id": "ngpt-D1-009", + "claim": "Alternative eigen learning rates for 1B model at 8k context length: alpha_A_init_alt=0.1, alpha_M_init_alt=0.1. Increasing from default 0.05 reduces Adam effective LR on these variables by factor ~3, restoring smooth hyperparameter sensitivity curves.", + "source": "Section A.7" + }, + { + "id": "ngpt-D1-010", + "claim": "QK scaling factor s_qk: s_qk_init=1, s_qk_scale=1/sqrt(d_model). Applied as per-head element-wise scaling after normalizing query and key vectors (Eq. 15-16), controlling the magnitude of QK dot products.", + "source": "Section 2.6 step 4" + }, + { + "id": "ngpt-D1-011", + "claim": "MLP intermediate scaling factors: s_u_init=1, s_u_scale=1; s_v_init=1, s_v_scale=1. s_u scales the u vector; s_v scales the v vector (with additional sqrt(d_model) rescaling per Eq. 20-21).", + "source": "Section 2.6 step 5; Section 2.4.2 Eq. 20-21" + }, + { + "id": "ngpt-D1-012", + "claim": "Logit scaling factor s_z: s_z_init=1, s_z_scale=1/sqrt(d_model). Element-wise scaling of logits (Eq. 3) to control softmax temperature; higher mean(s_z) produces sharper probability distributions.", + "source": "Section 2.6 step 6; Section A.10" + }, + { + "id": "ngpt-D1-013", + "claim": "Softmax attention scaling factor: baseline GPT uses 1/sqrt(d_k); nGPT uses sqrt(d_k). The change restores unit variance for normalized q/k dot products (expected variance 1/d_k instead of 1).", + "source": "Section 2.3.1 Eq. 13; Section 2.3.2; Section 2.6 step 4" + }, + { + "id": "ngpt-D1-014", + "claim": "MLP intermediate v rescaling: v vector multiplied by sqrt(d_model) before SiLU non-linearity (Eq. 21). Compensates for expected dot product magnitude E[|cos(theta)|] ~ 1/sqrt(d_model) so SiLU benefits from its non-linear region.", + "source": "Section 2.4.2 Eq. 21; Appendix A.1" + }, + { + "id": "ngpt-D1-015", + "claim": "SiLU activation function numerical properties: minimum at x=-1.278 with SiLU_min_val=-0.278. For |x| close to 0, SiLU(x) approximates x/2 (linear); for large positive x, approximates ReLU.", + "source": "Appendix A.1" + }, + { + "id": "ngpt-D1-016", + "claim": "Training context lengths: experiments conducted at three levels using separate training phases: 1k tokens (first), 4k tokens (second), and 8k tokens (third). Reported speedups are 4x/10x/20x at 1k/4k/8k respectively.", + "source": "Section 3.1, Figure 2; Section A.7" + }, + { + "id": "ngpt-D1-017", + "claim": "Ablation study configuration (Section A.9): model_size=0.5B, context_length=1k, learning_rate=1e-3, training_iterations=100000 consuming approximately 52B tokens.", + "source": "Section A.9" + }, + { + "id": "ngpt-D1-018", + "claim": "PG19 length extrapolation evaluation: perplexity measured at context lengths from 1K to 32K tokens, testing generalization well beyond the training context lengths (1k/4k/8k).", + "source": "Section A.8, Figure 14" + }, + { + "id": "ngpt-D1-019", + "claim": "nGPT introduces 6 additional trainable scaling parameter types beyond the baseline GPT: eigen learning rates (alpha_A, alpha_M) and scaling factors (s_qk, s_u, s_v, s_z). These are per-element or per-head vectors applied across all layers.", + "source": "Section A.10" + }, + { + "id": "ngpt-D1-020", + "claim": "Default analysis configuration for Section A.10 (scaling parameter distributions, Figure 15): context_length=1K, model_size=0.5B, learning_rate=2.0e-3, training_tokens=52B. Specific condition sweeps vary one parameter at a time.", + "source": "Section A.10, Figure 15" + }, + { + "id": "ngpt-D1-021", + "claim": "Per-step time cost: nGPT is approximately 80% slower than GPT at 4k context and 60% slower at 8k, due to 6 normalization steps per layer vs 2 in GPT. Removing QK normalization (Eq. 15-16) recovers ~12% speed with minor accuracy impact.", + "source": "Section A.5; Section A.9, Table 6" + } + ], + "D2": [ + { + "id": "ngpt-D2-001", + "claim": "Unit Vector Normalization (Norm): Norm(x) = x / ||x||_2", + "source": "Section 2.2.2, Eq. 10-11 context" + }, + { + "id": "ngpt-D2-002", + "claim": "Matrix Row-wise Normalization Along Embedding Dimension: For matrix M in R^{d_in x d_out}:\n M[i,:] = M[i,:] / ||M[i,:]||_2 for each row i in [0, d_in)", + "source": "Section 2.6, Step 2" + }, + { + "id": "ngpt-D2-003", + "claim": "Forward Pass: Token Embedding Lookup (nGPT): x_embed_i = E_input[t_i,:]\nwhere each row of E_input is normalized: ||E_input[j,:]||_2 = 1", + "source": "Section 2.1" + }, + { + "id": "ngpt-D2-004", + "claim": "Logits Computation with Element-wise Scaling: z_i = E_output @ h_i\nz_i = z_i ⊙ s_z", + "source": "Section 2.1, Eq. 1, Eq. 3" + }, + { + "id": "ngpt-D2-005", + "claim": "Softmax Probability with Scaled Logits: P(y_i | x_1, ..., x_{i-1}) = exp(z_{i,y_i}) / sum_{v=1}^{V} exp(z_{i,v})", + "source": "Section 2.1, Eq. 2" + }, + { + "id": "ngpt-D2-006", + "claim": "Cross-Entropy Loss for Next-Token Prediction: L = -(1/T) * sum_{i=1}^{T} log(P(y_i | x_1, ..., x_{i-1}))\n = -(1/T) * sum_{i=1}^{T} log(exp(z_{i,y_i}) / sum_v exp(z_{i,v}))", + "source": "Section 2.1, Eq. 2 context" + }, + { + "id": "ngpt-D2-007", + "claim": "nGPT Attention Block: Query/Key/Value Projection with Normalized Matrices: For each head i in [1, n_heads]:\n q_i = h @ W_q^i\n k_i = h @ W_k^i\n v_i = h @ W_v^i\nwhere each row of W_q^i, W_k^i, W_v^i has unit L2 norm", + "source": "Section 2.3.1, Eq. 12; Section 2.3.2" + }, + { + "id": "ngpt-D2-008", + "claim": "Rotary Position Embedding (RoPE) Application: q_i = RoPE(q_i, pos)\nk_i = RoPE(k_i, pos)", + "source": "Section 2.3.1" + }, + { + "id": "ngpt-D2-009", + "claim": "nGPT Query/Key Normalization with Per-Head Scaling: For each head i:\n q_i = Norm(q_i) ⊙ s_qk^i\n k_i = Norm(k_i) ⊙ s_qk^i", + "source": "Section 2.3.2, Eq. 15, Eq. 16" + }, + { + "id": "ngpt-D2-010", + "claim": "nGPT Scaled Dot-Product Attention with Modified Scaling Factor: For each head i:\n scores = (q_i @ k_i^T) * sqrt(d_k)\n scores = scores + causal_mask\n attn_i = softmax(scores) @ v_i", + "source": "Section 2.3.1, Eq. 13; Section 2.3.2, Section 2.6 Step 4" + }, + { + "id": "ngpt-D2-011", + "claim": "Multi-Head Attention Output Concatenation: h_A = Concat(head_1, head_2, ..., head_{n_heads}) @ W_o", + "source": "Section 2.3.1, Eq. 14" + }, + { + "id": "ngpt-D2-012", + "claim": "nGPT Attention Block Output Normalization: h_A_norm = Norm(h_A)", + "source": "Section 2.2.2, Eq. 10 context; Table 1" + }, + { + "id": "ngpt-D2-013", + "claim": "nGPT Hidden State Update: Attention Block (LERP with Eigen Learning Rates): h_A_norm = Norm(ATTN(h))\nh = h + alpha_A ⊙ (h_A_norm - h)\nh = Norm(h)", + "source": "Section 2.2.2, Eq. 7, Eq. 9, Eq. 10; Table 1" + }, + { + "id": "ngpt-D2-014", + "claim": "nGPT MLP Block: Up-Projections with Normalized Matrices: u = h @ W_u\nnu = h @ W_nu", + "source": "Section 2.4.1, Eq. 17; Section 2.4.2" + }, + { + "id": "ngpt-D2-015", + "claim": "nGPT MLP Intermediate State Rescaling: u = u ⊙ s_u\nnu = nu ⊙ s_nu * sqrt(d_model)", + "source": "Section 2.4.2, Eq. 20, Eq. 21" + }, + { + "id": "ngpt-D2-016", + "claim": "SwiGLU Activation: SiLU(nu) = nu ⊙ sigma(nu)\nSwiGLU(u, nu) = u ⊙ SiLU(nu)", + "source": "Section 2.4.1, Eq. 18, Eq. 19" + }, + { + "id": "ngpt-D2-017", + "claim": "nGPT MLP Output Projection: h_M = SwiGLU(u, nu) @ W_oMLP", + "source": "Section 2.4.1, Eq. 19; Section 2.4.2" + }, + { + "id": "ngpt-D2-018", + "claim": "nGPT MLP Block Output Normalization: h_M_norm = Norm(h_M)", + "source": "Section 2.2.2, Eq. 11 context" + }, + { + "id": "ngpt-D2-019", + "claim": "nGPT Hidden State Update: MLP Block (LERP with Eigen Learning Rates): h_M_norm = Norm(MLP(h))\nh = h + alpha_M ⊙ (h_M_norm - h)\nh = Norm(h)", + "source": "Section 2.2.2, Eq. 11; Table 1" + }, + { + "id": "ngpt-D2-020", + "claim": "nGPT Complete Layer Forward Pass: # Attention block\nh_A_norm = Norm(ATTN(h))\nh = Norm(h + alpha_A ⊙ (h_A_norm - h))\n\n# MLP block\nh_M_norm = Norm(MLP(h))\nh = Norm(h + alpha_M ⊙ (h_M_norm - h))", + "source": "Section 2.2.2, Eq. 10-11; Section 2.6 Step 3" + }, + { + "id": "ngpt-D2-021", + "claim": "Adam Optimizer Update (No Weight Decay, No Warmup): m = beta_1 * m + (1 - beta_1) * g\nv = beta_2 * v + (1 - beta_2) * g^2\ntheta = theta - alpha * m / (sqrt(v) + epsilon)", + "source": "Section 2.5, Eq. 22-24; Table 3" + }, + { + "id": "ngpt-D2-022", + "claim": "Effective Learning Rate Control via Init/Scale Parameter Trick: # During parameter initialization:\nparam_stored = s_{a,scale}\n\n# During forward pass:\nparam_actual = (s_{a,init} / s_{a,scale}) * param_stored\n\n# Effective learning rate relative to global LR:\neffective_LR_ratio = s_{a,scale} / s_{a,init}", + "source": "Section 2.5" + }, + { + "id": "ngpt-D2-023", + "claim": "nGPT Summary Recipe: Converting Baseline Transformer to nGPT: Step 1: Remove all RMSNorm / LayerNorm layers\nStep 2: After each training step, normalize all matrices along embedding dimension\n (E_input, E_output, W_q, W_k, W_v, W_o, W_u, W_nu, W_oMLP)\nStep 3: Replace residual updates with LERP+NORM:\n h = Norm(h + alpha_A ⊙ (Norm(ATTN(h)) - h))\n h = Norm(h + alpha_M ⊙ (Norm(MLP(h)) - h))\nStep 4: Change softmax scaling from 1/sqrt(d_k) to sqrt(d_k); normalize q,k with per-head scaling s_qk\nStep 5: Rescale MLP intermediate states: u by s_u, nu by s_nu * sqrt(d_model)\nStep 6: Rescale logits element-wise by s_z\nStep 7: Remove weight decay and learning rate warmup", + "source": "Section 2.6, Steps 1-7" + }, + { + "id": "ngpt-D2-024", + "claim": "nGPT Parameter Initialization: For non-output matrices: W ~ N(0, (1/sqrt(d_model))^2)\nFor output matrices: W ~ N(0, (1/sqrt(d_model) * sqrt(2 * n_layers))^2)", + "source": "Section A.6" + }, + { + "id": "ngpt-D2-025", + "claim": "nGPT Full Training Loop: For each training step:\n 1. Forward pass (optional: normalize matrices before forward)\n tokens = input_batch\n h = Norm(E_input[tokens]) # embed + normalize\n For layer in 1..n_layers:\n h_A = Norm(ATTN(h))\n h = Norm(h + alpha_A ⊙ (h_A - h))\n h_M = Norm(MLP(h))\n h = Norm(h + alpha_M ⊙ (h_M - h))\n 2. Compute cross-entropy loss on logits and backpropagate\n 3. Adam optimizer update (no weight decay, no warmup)\n 4. After step: normalize all matrices along embedding dimension", + "source": "Section 2.6, Section A.6" + } + ], + "D3": [ + { + "id": "ngpt-D3-001", + "claim": "Train GPT and nGPT (0.5B/1B) from scratch on OpenWebText at 1k/4k/8k context lengths with 64 A100 GPUs (batch_size=512). Compare against GPT baseline with best initial LR via validation loss curves (Figure 1-2), downstream task accuracy (Figure 3, 8-10), and convergence speed in tokens: nGPT achieves 4x/10x/20x speedup at 1k/4k/8k respectively.", + "source": "Section 3.1, Figure 1, Figure 2, Figure 3; Section A.6, A.7" + }, + { + "id": "ngpt-D3-002", + "claim": "Load GPT and nGPT checkpoints (0.5B/1B, 100k iters on OpenWebText). Extract and compare: (1) embedding vector norms, pairwise dot products, and eigenvalue distributions (Figure 4); (2) median condition numbers of attention and MLP matrices across all layers (Figure 5, Figure 11-13); (3) per-layer alpha_A/alpha_M, s_qk/s_u/s_nu/s_z distributions (Figure 6). For GPT, renormalize matrices post-training and recompute condition numbers to assess rank deficiency.", + "source": "Section 3.2, Figure 4, Figure 5, Figure 6; Figure 11, Figure 12, Figure 13 (Appendix)" + }, + { + "id": "ngpt-D3-003", + "claim": "Train 0.5B nGPT on OpenWebText (1k context, LR=1e-3, 100k iters, ~52B tokens). Enumerate s_init/s_scale combinations for s_qk, s_u/s_v, and s_z across multiple settings (Table 4). After training, record Mean(s) per distribution and evaluate final validation loss plus average accuracy on 5 downstream tasks. Compare each variant to the default initialization (s_init=1, corresponding scale) to quantify hyperparameter sensitivity.", + "source": "Section A.9, Table 4" + }, + { + "id": "ngpt-D3-004", + "claim": "Replace each per-element learnable vector (s_qk, s_u/s_v, s_z, alpha_A, alpha_M) with a single learnable scalar or fixed constant. Train 0.5B variants on OpenWebText (1k context, LR=1e-3, 100k iters). Evaluate validation loss and 5-task downstream accuracy vs the per-element baseline; most simplifications cause <=0.3% loss increase, indicating per-element vectors are not essential for performance.", + "source": "Section A.9, Table 5" + }, + { + "id": "ngpt-D3-005", + "claim": "Implement two nGPT architectural variants: (a) remove QK normalization from Eq. 15-16; (b) replace LERP (Eq. 7) with full SLERP (Eq. 6). Train 0.5B models on OpenWebText (1k context, LR=1e-3, 100k iters). Measure training time per step, validation loss, and 5-task downstream accuracy vs baseline nGPT. QK norm removal saves ~12% compute; SLERP adds ~10% overhead; both maintain comparable accuracy.", + "source": "Section A.9, Table 6" + }, + { + "id": "ngpt-D3-006", + "claim": "Train GPT, nGPT (with QK norm), and nGPT (without QK norm) on OpenWebText. Evaluate perplexity on PG19 dataset at context lengths from 1K to 32K tokens, far exceeding training context lengths. Compare perplexity curves across all variants: GPT perplexity rises sharply beyond training length; nGPT with QK norm maintains stable perplexity at extrapolated lengths, demonstrating superior length generalization.", + "source": "Section A.8, Figure 14" + }, + { + "id": "ngpt-D3-007", + "claim": "Collect nGPT checkpoints across varying conditions: context lengths (1K/4K/8K), model sizes (0.5B/1B), learning rates, and training token budgets (default: 1K ctx, 0.5B, LR=2e-3, 52B tokens). Extract per-layer alpha_A, alpha_M, s_qk, s_u, s_nu, s_z values; aggregate into condition-specific histograms (Figure 15). Analyze distribution shifts: eigen rates move right with longer contexts/larger models; s_qk is stable (high density near zero); s_z mean increases with context/size/tokens.", + "source": "Section A.10, Figure 15" + } + ], + "D4": [ + { + "id": "ngpt-D4-001", + "claim": "Experiment phases: 1. Initialize GPT and nGPT models with the same transformer backbone dimensions (identical n_layers, d_model, n_heads, d_MLP for both models). 2. Replicate weight initialization: N(0, 0.02^2) for GPT, N(0, 1/d_model) for nGPT. 3. For nGPT: normalize all matrices along the embedding dimension after initialization, set scaling factors s_init=1.0, s_scale=1/sqrt(d_model). 4. Train both models on OpenWebText at context lengths 1k (0.5B), 4k (1B), and 8k (1B) for 100k iterations. 5. For GPT: AdamW optimizer with weight_decay=0.1, warmup=2000 steps, cosine decay to 0. For nGPT: Adam optimizer with weight_decay=0.0, no warmup, cosine decay. 6. Measure: (a) training curves (loss vs iterations/tokens), (b) final validation loss at different token budgets, (c) convergence speedup factor (e.g., nGPT reaches GPT's 100k-iteration loss at 25k iterations = 4x speedup).", + "source": "Section 3.1, Figure 1, Figure 2, Figure 3; Section A.6, A.7" + }, + { + "id": "ngpt-D4-002", + "claim": "Experiment phases: 1. Load trained GPT and nGPT checkpoints (100k iterations). 2. Extract and compute norm of each embedding vector in E_input and E_output matrices. 3. Compute covariance matrix of embeddings, then perform SVD to obtain eigenvalues; compute condition number = lambda_max / lambda_min. 4. For each weight matrix W, compute eigen learning rates from the optimizer state and weight gradient statistics. 5. Compare GPT vs nGPT: (a) embedding norm distributions (nGPT embeddings are unit-norm by construction), (b) condition numbers of embedding covariance matrices, (c) eigen learning rate spectra across layers. 6. Plot distributions: norms (Figure 4), condition numbers (Figure 5), eigen learning rates (Figure 6). Key finding: nGPT eliminates the eigenvalue imbalance that causes uneven learning rates in GPT.", + "source": "Section 3.2, Figure 4, Figure 5, Figure 6; Figure 11, Figure 12, Figure 13 (Appendix)" + }, + { + "id": "ngpt-D4-003", + "claim": "Experiment phases: 1. For each scaling factor (s_qk, s_u/s_nu, s_z), enumerate combinations of s_init in {0.05, 0.1, 0.5, 1.0} and s_scale in {1, 1/sqrt(d_model)}. 2. Train each variant for 100k iterations. 3. After training, record Mean(s) for each scaling factor to check convergence behavior. 4. Evaluate final validation loss and downstream task accuracy across all initialization variants. 5. Compare against baseline (s_init=0.05, s_scale=1/sqrt(d_model)) to identify sensitivity of each initialization setting. Key finding: nGPT is robust to wide ranges of s_init and s_scale choices.", + "source": "Section A.9, Table 4" + }, + { + "id": "ngpt-D4-004", + "claim": "Experiment phases: 1. For each variant, replace the target per-element vector with a learnable scalar parameter (simplifying from vector to scalar per scaling factor). 2. Train each variant for 100k iterations using 0.5B model on OpenWebText at 1k context length with LR=1e-3 (ablation configuration per Section A.9). 3. After training, record final validation loss and compute degradation relative to per-element baseline. 4. Also test fixing scaling factors to constant values (s_qk=1, s_u=1, s_z=1) to test whether learning them is necessary. 5. Results: replacing per-element vectors with scalars or fixed values causes only negligible degradation (<=0.3% loss increase). Key finding: scalar scaling factors are sufficient, reducing parameter count.", + "source": "Section A.9, Table 5" + }, + { + "id": "ngpt-D4-005", + "claim": "Experiment phases: 1. Implement both variants: (a) remove QK normalization (use standard attention without normalizing Q,K), (b) replace LERP with SLERP for embedding interpolation. 2. Train each variant for 100k iterations. 3. Measure training time per step for each variant to quantify compute savings. 4. Evaluate final validation loss compared to full nGPT baseline. 5. Key finding: QK normalization can be removed for approximately 12% compute savings with minimal accuracy impact (within 0.2% of full nGPT validation loss).", + "source": "Section A.9, Table 6" + }, + { + "id": "ngpt-D4-006", + "claim": "Experiment phases: 1. Train GPT, nGPT (with QK norm), and nGPT (without QK norm) on OpenWebText at context length 1k. 2. For each trained model, evaluate perplexity on PG19 at context lengths 1k, 2k, 4k, 8k, 16k, 32k tokens (extrapolation beyond training length). 3. Plot perplexity vs context length curves for all three models. 4. Compare extrapolation behavior: (a) GPT perplexity dramatically increases beyond training length, (b) nGPT maintains stable perplexity at extrapolated lengths up to 32x training context. Key finding: nGPT's normalized representations provide inherent length generalization.", + "source": "Section A.8, Figure 14" + }, + { + "id": "ngpt-D4-007", + "claim": "Experiment phases: 1. Collect trained nGPT checkpoints spanning all condition variations (model sizes: 0.5B, 1B; context lengths: 1k, 4k, 8k). 2. For each checkpoint, extract all alpha_A, alpha_M, s_qk, s_u, s_nu, s_z values from the model parameters. 3. Aggregate into histograms showing distribution of learned scaling factors across layers and training conditions. 4. Analyze correlation between Mean(s) values and training conditions (model size, context length, token budget). 5. Key finding: scaling factors converge to stable distributions with lower means for longer contexts, larger models, and more tokens (corresponding to lower temperature / sharper distributions in the normalized hypersphere).", + "source": "Section A.10, Figure 15" + } + ] +} \ No newline at end of file diff --git a/papers/olmoe/blacklist.txt b/papers/olmoe/blacklist.txt new file mode 100644 index 0000000000000000000000000000000000000000..4eb653a586f33d7522b2b7eaf3ae33ebbd82a8e3 --- /dev/null +++ b/papers/olmoe/blacklist.txt @@ -0,0 +1,2 @@ +# Official repository (Allen AI, Apache 2.0) +https://github.com/allenai/OLMoE diff --git a/papers/olmoe/config.yaml b/papers/olmoe/config.yaml new file mode 100644 index 0000000000000000000000000000000000000000..ffb707261e1f0639713feb32fd96d4cb2c5b5bed --- /dev/null +++ b/papers/olmoe/config.yaml @@ -0,0 +1,8 @@ +title: "OLMoE: Open Mixture-of-Experts Language Models" +pdf_url: "https://arxiv.org/pdf/2409.02060.pdf" +venue: "ICLR 2025 Oral" +year: 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Smithaw Pang Wei Koh $_ { w }$ Amanpreet Singhc Hannaneh Hajishirziaw a Allen Institute for AI c Contextual AI w University of Washington p Princeton University n.muennighoff@gmail.com hannah@allenai.org + +# Abstract + +We introduce OLMOE,1 a fully open, state-of-the-art language model leveraging sparse Mixture-of-Experts (MoE). OLMOE-1B-7B has 7 billion (B) parameters but uses only 1B per input token. We pretrain it on 5 trillion tokens and further adapt it to create OLMOE-1B-7B-INSTRUCT. Our models outperform all available models with similar active parameters, even surpassing larger ones like Llama2-13B-Chat and DeepSeekMoE-16B. We present various experiments on MoE training, analyze routing in our model showing high specialization, and open-source all aspects of our work: model weights, training data, code, and logs. + +Model hf.co/allenai/OLMoE-1B-7B-0924 +Data hf.co/datasets/allenai/OLMoE-mix-0924 +9 Code github.com/allenai/OLMoE Logs wandb.ai/ai2-llm/olmoe/reports/OLMoE-1B-7B-0924--Vmlldzo4OTcyMjU3 + +![](images/figures/olmoe-fig-0001.jpg) +Figure 1: Performance, cost, and degree of openness of open MoE and dense LMs. Model names contain rounded parameter counts: model-active-total for MoEs and model-total for dense LMs. #ckpts is the number of intermediate checkpoints available. We highlight MMLU as a summary of overall performance; see $\ S 3$ ✔ !✓✔ ⤫⛌ ⛌ ⛌ ⛌ 1 for more results. OLMOE-1B-7B performs best among ✔ models with similar active parameter counts and is the most open MoE. + +# 1 Introduction 3 + +Pretraining and Adaptation 3 + +# 3 Results + +6 + +# Experimenting with Alternative Design Choices 8 + +# 4.1 MoE-specific Pretraining Settings 8 + +4.1.1 Mixture-of-Experts vs. Dense 8 +4.1.2 Expert Granularity 9 +4.1.3 Shared Experts 10 +4.1.4 Expert Choice vs. Token Choice 11 +4.1.5 Sparse Upcycling . 11 +4.1.6 Load Balancing Loss 12 +4.1.7 Router Z-loss 13 +4.2 General Pretraining Settings 14 +4.2.1 Dataset Experiments 14 +4.2.2 Initialization 14 +4.2.3 RMSNorm 15 +4.2.4 Decaying Embedding Parameters 16 +4.2.5 QK-Norm 16 +4.2.6 AdamW Epsilon 17 +4.3 Adaptation Settings 17 + +# 5 MoE Analysis 18 + +5.1 Router Saturation 18 +5.2 Expert Co-activation 19 +5.3 Domain Specialization 20 +5.4 Vocabulary Specialization . 22 + +# 6 Related Work 24 + +# 7 Conclusion 24 + +# 44 + +44 + +48 + +A Artifacts +B Training Configuration +C Evaluation Setup +D Openness of Models +E Additional Evaluation +F Additional Experiments +G Additional Analysis H Limitations and Future Work +I OLMOE-1B-7B-0125 +J Change log + +50 + +54 + +56 + +62 + +62 + +63 + +# 1 Introduction + +Despite significant advances in Large Language Models (LMs) on various tasks, there remains a clear trade-off between performance and cost in both training and inference. High-performing LMs are inaccessible for many academics and open-source developers as they are prohibitively expensive to build and deploy.2 One approach to improve the cost-performance trade-off lies in using sparselyactivated Mixture-of-Experts (MoEs) [154]. MoEs have several experts in each layer, only a subset of which is activated at a time (see Figure 2). This makes MoEs significantly more efficient than dense models with a similar number of total parameters, which activate all parameters for every input [205]. For this reason, industry frontier models use MoEs including Gemini-1.5 [175] and reportedly GPT-4 [29]. + +Most MoE models, however, are closed-source: While some have publicly released model weights [43, 79, 158, 178, 180], they offer limited to no information about their training data, code, or recipes (see Figure 1). While there have been prior efforts to make language modeling research fully accessible [18, 65, 90, 103, 193, 209], they have been largely limited to dense LMs. This comes despite MoEs requiring more openness as they add complex new design questions to LMs, such as how many total versus active parameters to use, whether to use many small or few large experts, if experts should be shared, and what routing algorithm to use. The lack of open resources and findings about these details prevents the field from building cost-efficient open MoEs that approach the capabilities of closed-source frontier models. + +To address these issues, we introduce OLMOE, a fully open Mixture-of-Experts language model with state-of-the-art performance among similarly-sized models. In particular, we pretrain OLMOE-1B-7B for 5.1 trillion tokens with 6.9B total parameters, of which only 1.3B are activated for each input token. This leads to a similar inference cost as using dense models with around 1B parameters, such as OLMo 1B [65] or TinyLlama 1B [210], but requires more GPU memory to store its 7B total parameters. Our experiments show that MoEs train ${ \sim } 2 \times$ faster than dense LMs with equivalent active parameters. In Figure 1, we show that OLMOE-1B-7B significantly outperforms all open 1B models and displays competitive performance to dense models with significantly higher inference costs and memory storage (e.g., similar MMLU scores to Llama2-13B, which is $\sim 1 0 \times$ more costly). Via instruction- and preference tuning, we create OLMOE-1B-7B-INSTRUCT, which we find exceeds various larger instruct models including Llama2-13B-Chat [183], OLMo-7B-Instruct (0724), and DeepSeekMoE-16B [42] on common benchmarks (MMLU, GSM8k, HumanEval, etc.). + +Our comprehensive set of controlled experiments highlights key design choices for MoEs (see Table 1) and LMs in general. One critical design decision for making MoEs performant is the use of fine-grained routing with granular experts [42]: we employ 64 small experts in each layer with 8 being activated. The choice of routing algorithm is also important: we find dropless [58] token-based routing [154] outperforms expert-based routing [219]. Our findings also include those that challenge prior work, such as the ineffectiveness of shared experts [42] and the limited benefits of sparsely upcycling a pretrained dense LM into an MoE [85] unless under small compute budgets. Finally, we analyze the routing behavior in OLMOE-1B-7B, finding that routing saturates early in pretraining, experts are rarely co-activated, and experts exhibit domain and vocabulary specialization. + +We hope our fully open MoE facilitates more research and analysis to improve our understanding of these models. We release training code, intermediate checkpoints (every 5000 steps), training logs, and training data under open-source licenses (Apache 2.0 http://www.apache.org/licenses/ LICENSE-2.0 or ODC-By 1.0 https://opendatacommons.org/licenses/by/1-0/). + +# 2 Pretraining and Adaptation + +Pretraining architecture OLMOE is a decoder-only LM consisting of $N _ { L }$ transformer [185] layers. The feedforward network (FFN) in dense models like OLMo [65], is replaced with an MoE module consisting of $N _ { E }$ smaller FFN modules called experts, of which a subset of $k$ experts is + +![](images/figures/olmoe-fig-0002.jpg) +Figure 2: Comparison of the architecture of dense LMs and MoE models like OLMOE. The figure excludes some details, e.g., OLMOE-1B-7B also uses QK-Norm (§4.2.5). + +Table 1: Key MoE design choices and our setup for OLMOE-1B-7B based on our experiments. Full configuration for OLMOE-1B-7B is in Appendix B. + +
Design choiceDescriptionExper- imentOLMOE-1B-7B
Active params# active parameters per input token§4.1.11.3B active
Total paramsTotal # of parameters in the model§4.1.16.9B total
Expert granularityUsing fine-grained small experts vs. a few large experts [39]§4.1.264 small experts with 8 activated
Expert sharingWhether or not to include a shared expert [39] §4.1.3No shared expert
Routing algorithmHow inputs are assigned to experts, e.g., as- signment on a per token basis (e.g., 2 experts per token) or per expert basis (e.g., 2 tokens per expert), and whether or not all tokens get assigned or some get dropped [58, 219]§4.1.4Dropless [58] MoE with token choice
Sparse upcycling Load balancingWhether to start from a dense model [85, 211]§4.1.5Not used
lossAuxiliary loss to penalize unequal assignment §4.1.6 to experts that may harm performance [154]Used with weight 0.01
Router z-lossAuxiliary loss to penalize large logits in the router that may cause instabilities [221]§4.1.7Used with weight 0.001
+ +Table 2: Composition of the pretraining data for OLMOE-1B-7B. StarCoder, peS2o, and Wikipedia parts come from Dolma 1.7 [163]. Links to our data are in Appendix A. + +
SourceDoc TypeGPT-NeoX tokens (billions)Words (billions)UTF-8 bytes (GB)Documents (millions)
DCLM-Baseline [90]web pages3,8603,38016,7002,950
StarCoder [92, 84]code10163.932578.7
peS2o [164, 163]STEM papers57.251.326838.8
arXiv [36]STEM papers21.123.588.81.55
OpenWebMath [131]math web pages12.710.242.42.91
Algebraic Stack [11]math proofs code12.69.639.32.83
English Wikipedia & Wikibooks [163]encyclopedic3.693.1616.26.17
Total4,0603,53017,4003,080
+ +
SourceDomainSamples
Instruction Tuning
Tulu 2 SFT Mix [76]Various326,154
No Robots [140]Various9,500
CodeFeedback-Filtered-Instruction [214]Coding156,526
MetaMathQA [204]Math98,750
Advanced (non-chat) subset of Daring Anteater [189]Various17,082
Preference Tuning (DPO [138])
UltraFeedback [38] binarized and filtered for TruthfulQA [99] contaminationVarious60,800
+ +Table 3: Adaptation training data for OLMOE-1B-7B. Links to our data are in Appendix A. + +activated for each processed input token $x$ (also see Figure 2): + +$$ +\mathrm { M o E ~ m o d u l e } ( x ) = \sum _ { i \in \mathrm { T o p } ^ { - k } ( r ( x ) ) } \mathrm { s o f t m a x } \left( r ( x ) \right) _ { i } E _ { i } ( x ) +$$ + +where $r$ , called the router, is a learned linear layer mapping from the input logits to the chosen $k$ experts. A softmax is applied to the router outputs to compute routing probabilities for all $N _ { E }$ experts. Each selected expert $E _ { i }$ processes the input $x$ , the output of which is then multiplied with its respective routing probability. The results are then summed across all chosen Top- $k$ experts to constitute the output of the MoE module for a single layer of the model out of its $N _ { L }$ total layers. Key decisions in designing an MoE model include determining the number of activated and total parameters, the design of the experts (e.g., granularity, whether or not to include shared experts), and the choice of the routing algorithm. Moreover, training an MoE model can involve initializing from a dense model (sparse upcycling) and changing the training objective, such as including auxiliary load balancing and router z-losses. Experiments related to these design choices are in $\ S 4 . 1$ ; Table 1 shows our final decisions. + +In summary, we use 1.3B active parameters out of a total of 6.9B, with 8 activated experts out of 64 per layer. We use dropless token choice routing [58]: For each input token, the learned router network determines 8 experts to process it. We train OLMOE-1B-7B from scratch with two auxiliary losses: load balancing loss $( \mathcal { L } _ { L B } )$ [154] and router $\mathbf { Z }$ -loss $( \mathcal { L } _ { R Z } )$ [221], which we define and experiment with in $\ S 4 . 1 . 6$ and $\ S 4 . 1 . 7$ , respectively. We multiply them with respective loss weights, $\alpha$ and $\beta$ , and sum them linearly with the cross entropy loss $( \mathcal { L } _ { C E } )$ to arrive at our final training loss: + +$$ +\mathcal { L } = \mathcal { L } _ { C E } + \alpha \mathcal { L } _ { L B } + \beta \mathcal { L } _ { R Z } +$$ + +Our full pretraining configuration for OLMOE-1B-7B is in Appendix B. + +Pretraining data We mix data from DCLM [90] and Dolma 1.7 [163], which includes: (1) a quality-filtered subset of Common Crawl, referred to as DCLM-Baseline, (2) StarCoder, Algebraic Stack and arXiv, used in both DCLM and Dolma 1.7, and (3) peS2o and Wikipedia from Dolma 1.7. We refer to our pretraining dataset as OLMOE-MIX. + +To all sources above, we apply a filter that removes all documents with a sequence of 32 or more repeated $\mathbf { n }$ -grams, where an n-gram is any span of 1 to 13 tokens. For the StarCoder subset, we also remove any document from a repository with fewer than 2 stars on GitHub, whose most frequent word constitutes over $30 \%$ of the document, or whose top-2 most frequent words constitute over $50 \%$ of the document. + +We shuffle all samples randomly at the beginning of each epoch and train for a total of 5.133T tokens (1.3 epochs following Muennighoff et al. [121]). During our annealing phase (final 100B tokens) we first reshuffle the entire dataset and then linearly decay the learning rate to 0, following prior work [65, 90]. Our pretraining data statistics are in Table 2. + +Adaptation We create OLMOE-1B-7B-INSTRUCT by following a standard adaptation recipe split into instruction tuning [118, 190, 149, 156, 206] followed by preference tuning [31, 15, 138, 54] building on prior open models [184, 76, 188]. In our instruction tuning dataset, we add more code and math data to boost performance on downstream coding and math applications. Other models, such as GPT-4 [128] and Llama 3 [50] similarly include samples from math datasets like GSM8k [35] or MATH [71] during pretraining. We also include No Robots and a subset of Daring Anteater as they are of high quality and add diversity, two key factors for successful adaptation [188, 216, 104, 120]. We describe our adaptation datasets in Table 3 and hyperparameters in Appendix B. + +# 3 Results + +Our evaluation procedure consists of three parts: During pretraining, After pretraining, and After adaptation. We detail the setup for each in Appendix C. + +![](images/figures/olmoe-fig-0003.jpg) +Figure 3: Evaluation of OLMOE-1B-7B and the current best OLMo models during pretraining. OLMOE-1B-7B differs from the OLMo models in its MoE architecture, several training hyperparameters, and its training dataset, see $\ S 2$ . A version of this plot with tokens as the $\mathbf { X }$ -axis and markers where annealing starts is in Appendix E. More results, logs, and configurations: https://wandb.ai/ai2-llm/olmoe/ reports/Plot-OLMoE-1B-7B-vs-OLMo-7B-vs-OLMo-1B--Vmlldzo4OTcyMjEz + +During pretraining In Figure 3 we benchmark the performance of OLMOE-1B-7B during pretraining with the current best OLMo models [65] on commonly used downstream tasks. We find that across all tasks OLMOE-1B-7B reaches better performance with less compute (FLOPs) than the dense OLMo models. OLMOE-1B-7B matches or outperforms OLMo-7B at the end of training despite OLMOE-1B-7B having used less than half as many FLOPs for training and using only 1B active parameters. This is likely a result of the dataset and modeling changes we make to the OLMo setup including MoE-related changes, stability, and performance improvements, outlined in Appendix B. Appendix E contains training and validation loss plots showing very smooth loss curves without major loss spikes during the 5T tokens of our pretraining. + +Table 4: OLMOE-1B-7B after pretraining versus larger MoEs and dense LMs. We compare with dense LMs close to OLMOE-1B-7B either in active parameters (1B, approximates speed and cost) or total parameters (7B, approximates memory requirements). Model names contain rounded parameter counts: model-active-total for MoEs and model-total for dense LMs (this leads to some differences to official names, e.g., while called “Gemma2-2B” it actually has 2.6B active and total parameters [177]). Chall. $=$ Challenge. We run all evaluations ourselves with 5 few-shots, see Appendix C for details. + +
Active paramsOpen DataMMLUHella- SwagARC- Chall.ARC- EasyPIQAWino- Grande
LMs with ~7-9B active parameters
Llama2-7B [183]6.7B2046.278.954.284.077.571.7
OLMo-7B (0724) [65]6.9B54.980.568.085.779.373.2
Mistral-7B [78]7.3B64.083.078.690.882.877.9
DCLM-7B [90]6.9B64.482.379.892.380.177.3
Llama3.1-8B [50]8.0B66.981.679.591.781.176.6
Gemma2-9B [177]9.2B70.687.389.595.586.178.8
LMs with ~2-3B active parameters
OpenMoE-3B-9B [199] StableLM-2B [16]2.6B 1.6B2027.4 40.444.4 70.329.3 50.650.6 75.363.3 75.651.9 65.8
DeepSeek-3B-16B [39] JetMoE-2B-9B [158]2.9B45.580.453.482.780.173.2
2.2B49.181.761.481.980.370.7
Gemma2-3B [177]2.6B53.374.667.584.378.571.8
Qwen1.5-3B-14B [180]2.7B62.480.077.491.681.072.3
LMs with ~1B active parameters
Pythia-1B [18] OLMo-1B (0724) [65]1.1B 1.3B31.148.031.463.468.952.7
TinyLlama-1B [210]1.1B132.1 33.667.5 60.836.4 38.153.5 69.574.0 71.762.9
Llama3.2-1B [50]1.2B38.267.343.571.673.760.1 62.5
DCLM-1B [90]1.4B48.575.157.679.576.668.1
OLMOE-1B-7B1.3BV54.180.062.184.279.870.2
+ +After pretraining In Table 4 we benchmark OLMOE-1B-7B on common downstream tasks. We find that OLMOE-1B-7B performs best among models that use less than 2B active parameters, making it the most economical option for many use cases of LMs. For larger budgets, Qwen1.5- 3B-14B has stronger performance but has more than double the active and total parameters than OLMOE-1B-7B. We find that despite requiring ${ \sim } 6 { - } 7 \times$ less compute per forward pass, OLMOE-1B-7B outperforms some dense LMs with 7B parameters such as Llama2-7B [183], but falls short of others like Llama3.1-8B [50]. Figure 1 compares MMLU performance with active parameters, a proxy for the value of a model given its cost, of OLMOE-1B-7B and other LMs. OLMOE-1B-7B is the state of the art in its cost regime. + +After adaptation In Table 5, we benchmark our instruction (SFT) and preference (DPO) tuning of OLMOE-1B-7B. SFT improves our model on all tasks measured. We observe a $> 1 0 \times$ gain on GSM8k, likely due to our inclusion of additional math data to account for the relatively small amounts of math data during pretraining (§2). DPO helps on most tasks, especially AlpacaEval which aligns with findings from prior work [188, 76, 122]. Our DPO model, which we refer to as OLMOE-1B-7B-INSTRUCT, has the highest average among all models benchmarked. We find it to outperform the chat version of Qwen1.5-3B-14B despite Qwen having $> 2 \times$ more parameters and its pretrained model outperforming OLMOE-1B-7B in Table 4. The $84 \%$ score on AlpacaEval also outperforms much larger dense models on the leaderboard,3 such as Llama2-13B-Chat [183]. + +Table 5: OLMOE-1B-7B after adaptation versus other models. We find the JetMoE chat model (https://hf.co/jetmoe/jetmoe-8b-chat) has random scores thus we exclude it. Model names contain rounded parameter counts: model-active-total for MoEs and model-total for dense LMs. We run all evaluations ourselves (Appendix C). Models use different mixes for adaptation, e.g., OLMOE is trained on an improved version of the pipeline used for OLMo models. + +
Task (→) Setup (→) Metric (→)MMLU 0-shot EMGSM8k 8-shot CoT EMBBH 3-shot EMHuman- Eval 0-shot Pass@10Alpaca- Eval 1.0 0-shot %winXSTest 0-shot F1IFEval 0-shot Loose AccAvg
OLMo-1B (0724)25.07.022.516.0-67.620.5-
+SFT +DPO36.012.527.221.241.581.926.135.9
36.712.530.622.050.979.824.237.4
OLMo-7B (0724)50.832.536.932.3-80.819.6-
+SFT54.225.035.738.570.986.139.749.3
+DPO52.89.016.635.083.587.537.949.1
JetMoE-2B-9B45.643.037.254.6-68.220.0-
+SFT46.153.535.664.869.355.630.550.4
DeepSeek-3B-16B37.718.539.448.3-65.913.5-
+Chat48.546.540.870.174.885.632.357.0
Qwen1.5-3B-14B60.413.527.260.2-73.420.9-
+Chat58.955.521.359.783.985.636.257.3
OLMOE-1B-7B49.83.033.622.4-59.716.6-
+SFT51.440.538.051.669.284.143.354.0
+DPO51.945.537.054.884.082.648.157.7
+ +# 4 Experimenting with Alternative Design Choices + +In this section, we present pretraining and adaptation experiments that have led to OLMOE-1B-7B. We group them into experiments on settings specific to Mixture-of-Experts (§4.1), experiments on settings applicable to both dense LMs and MoEs (§4.2), and adaptation experiments (§4.3). In pretraining experiments, we often use MMLU Var, a version of MMLU [70] with varying few-shots and a different format that provides signal earlier during training. We describe our full evaluation setup in Appendix C and provide additional experiments in Appendix F. Each experiment links to a Weights & Biases report with more validation and downstream results, and the full configurations of the runs. To isolate the impact of changes and minimize confounders, we vary only one hyperparameter for each experiment. Nevertheless, due to the large number of hyperparameters, some results may change under different configurations and we cannot guarantee the correctness of each of our hyperparameter choices. Models are not comparable across different experiments, as we vary the base model to incorporate successful findings. + +# 4.1 MoE-specific Pretraining Settings + +# 4.1.1 Mixture-of-Experts vs. Dense + +Prior work reports various speed-ups of MoEs over dense models: Artetxe et al. [10] report that MoEs require $2 { - } 4 \times$ less compute to match dense models, MoMa [100] exhibits $2 . 6 \times$ FLOP savings for language tasks, Arctic [161] yields $4 \times$ FLOP savings but for very different dense and MoE configurations, and Switch Transformers [56] train $2 \mathrm { - } 7 \times$ faster with MoEs but for encoder-decoder models while the other works study decoder-only LMs [137]. + +![](images/figures/olmoe-fig-0004.jpg) +Figure 4: MoE vs. Dense. We train a 1.3B parameter dense model and a 1.3B active, 6.9B total parameter MoE model, each on $1 2 8 ~ \mathrm { H 1 0 0 }$ GPUs. Apart from MoE-related changes, we train both with the same configuration for 130B tokens. The MoE contains 64 experts out of which 8 are activated with an FFN dimension of 1,024, while the dense model has an FFN dimension of 8,192. Thus both have the same number of active parameters. Top: The MoE reaches the final dense performance with ${ \sim } 3 \times$ fewer tokens (or FLOPs, as both have the same active parameters ignoring the trivial router parameters). Bottom: Due to some memory overhead, this equates to ${ \sim } 2 \times$ faster training. More results, logs, and configurations: https://wandb.ai/ai2-llm/olmoe/reports/ Plot-MoE-vs-Dense--Vmlldzo4OTM0Mjkx + +In Figure 4, we compare MoEs and dense models in a controlled setup. We find that our MoE reaches the performance of the dense model with ${ \sim } 3 \times$ fewer tokens equivalent to ${ \sim } 3 \times$ less compute measured in FLOPs. However, due to the additional memory overhead of training the MoE with its 7B total parameters, it processes fewer tokens per second than the dense model (23,600 tokens per second per GPU for the MoE vs. 37,500 for dense). Thus, in terms of training time, it reaches the performance of the dense model only ${ \sim } 2 \times$ faster. There are likely optimizations possible that would bring the speed-up closer to the $3 \times$ token speed-up, which we leave to future work. Based on these results, we select an MoE configuration with 6.9B total and 1.3B active parameters matching OLMo-7B in total and OLMo-1B in active parameter count, respectively. + +# 4.1.2 Expert Granularity + +Dai et al. [39] propose to use small fine-grained experts to allow more combinations of experts and thus make the model more flexible. For example, the Mixtral model [79] uses the common configuration of 8 experts per layer, 2 of which are activated. This allows for ${ \binom { 8 } { 2 } } = 2 8$ combinations per layer. By halving the size of each expert and therefore doubling the number of experts to maintain the same compute and parameter budget, we can increase the possible combinations to $\binom { 1 6 } { 4 } = 1 , 8 2 0$ . Krajewski et al. [86] investigate compute-optimal granularity configurations finding that higher compute budgets warrant more granular experts. + +In Figure 5, we observe that more granular experts improve training loss, validation loss, and downstream performance. The 8-expert configuration uses 1 active expert, which yields ${ \binom { 8 } { 1 } } = 8$ combinations. By quartering the size of each expert but increasing the number to 32 with 4 active ones $( { \binom { 3 2 } { 4 } } = 3 5 , 9 6 0$ combinations), we observe an improvement of around $10 \%$ on HellaSwag and MMLU at around 130 billion tokens. However, we find that there are diminishing returns to granularity. The additional increase to 64 experts with 8 active o $( { \binom { 6 4 } { 8 } } = 4 , 4 2 6 , 1 6 { \bar { 5 } } , 3 6 8 { \bar { 4 } }$ com-$1 - 2 \%$ compute budget4 of $\mathrm { 3 \times 1 0 ^ { 2 2 } }$ , Krajewski et al. [86] predict an optimal number of experts of 256 $G \ = \ 3 2$ in their paper). However, their predictions are for compute-optimal models [72, 32], while we train for $5 \mathrm { T }$ tokens, which is orders of magnitude beyond what would be conventionally considered optimal for our model size. Thus, their predictions may not extend to our setup, and we stick with 64 experts for OLMOE-1B-7B , also due to the diminishing returns in Figure 5. + +![](images/figures/olmoe-fig-0005.jpg) +Figure 5: Expert granularity. We vary the number of experts in tandem with the FFN dimension to ensure that active and total parameters and thus compute cost remain the same. For example, for 64 experts, the FFN dimension is 1,024 and 8 experts are activated, while for 32 experts it is 2,048 with 4 activated experts. More results, logs, and configurations: https://wandb.ai/ai2-llm/ olmoe/reports/Plot-Granularity--Vmlldzo4OTIxOTE4 + +# 4.1.3 Shared Experts + +![](images/figures/olmoe-fig-0006.jpg) +Figure 6: Shared experts. Both setups have the same number of active and total parameters and use the same number of FLOPs. 4 of the 32 routed experts are activated, while it is 3 for the 31 routed experts of the other model, as it has 1 always-active shared expert. More results, logs, and configurations: https://wandb.ai/ai2-llm/olmoe/reports/ Plot-Expert-sharing--Vmlldzo4OTIyMjQz + +Dai et al. [39] propose training with a shared/fixed expert that is always used in addition to the routed experts. The intuition is to encourage the shared expert to learn common information and allow the other routed experts to learn more specialized knowledge. This should reduce redundancy among experts and thus lead to a better model as it can store more total information. + +In Figure 6, we benchmark having a single shared and a single routed expert versus two routed experts. While both settings lead to similar performance, sharing an expert performs slightly worse. Sharing an expert removes flexibility from the model and thus goes against the findings in $\ S 4 . 1 . 2$ suggesting that allowing for more expert combinations improves performance. Specifically, the two models in Figure 6 have ${ \binom { 3 2 } { 4 } } = 3 5 , 9 6 0$ and ${ \binom { 3 1 } { 3 } } = 4 , 4 9 5$ possible combinations per layer. Thus, removing one of the routed experts and turning it into a shared one eliminates almost $90 \%$ of possible combinations. This likely acts as a counterforce to the potential benefits of isolating common knowledge in a shared expert. Based on these results, we do not use shared experts in OLMOE-1B-7B , but we do think that there is merit to the idea of experts that are activated more often or even always. However, rather than enforcing this behavior via a shared expert, we believe that it should be learned by the model. This is difficult with current setups due to the necessity of a load balancing loss (§4.1.6) penalizing the model if tokens are not distributed equally among experts. Potential future work can explore removing the load balancing loss to allow for more flexible usage of experts. + +![](images/figures/olmoe-fig-0007.jpg) +4.1.4 Expert Choice vs. Token Choice +Figure 7: Expert choice (EC) vs. token choice (TC). Both models have an 8-expert MoE in every 2nd layer. For TC, 2 experts are activated per token, while for EC the capacity factor is 2. Thus, both models use the same number of active parameters. More results, logs, and configurations: https://wandb.ai/ai2-llm/olmoe/reports/Plot-EC-vs-TC--Vmlldzo4MzkzMDM3 + +The MoE router determines which experts process each input token (§2). There are two common types [102]: expert choice (EC) [219] and token choice (TC) [154]. For EC, each expert selects a fixed number of tokens from the incoming sequence. By design, this leads to each expert processing the same number of tokens. This is the main benefit of EC as it ensures perfect load balance, which improves training throughput and removes the need for a load balancing loss. The main downside of EC is that it is not easily usable for autoregressive generation where a single token is processed at each step rather than the entire sequence in one [143]. Another potential downside is that EC can lead to token dropping, where some tokens are not selected by any expert, which can hurt performance [58]. At the same time, it can lead to some tokens being processed by multiple experts, which could also be beneficial as it allows the model to allocate more compute to some tokens [219]. For TC, each token selects a fixed number of experts. This can lead to many tokens choosing the same expert, hurting training efficiency. Therefore it is common to use TC with a load balancing loss [154] to encourage equal distribution. + +In Figure 7, we benchmark EC and TC. We find that TC outperforms EC for the same token budget for all tasks depicted as well as other tasks like PIQA, SciQ, etc. which we report at https: //wandb.ai/ai2-llm/olmoe/reports/Plot-EC-vs-TC--Vmlldzo4MzkzMDM3. While Zhou et al. [219] find EC to be better, our configuration slightly differs in that we use dropless MoEs [58] with a load balancing loss. Thus, our TC variant is expected to perform better than the TC variant in Zhou et al. [219]. We confirm findings that EC runs around $20 \%$ faster at 29,400 tokens per second per device versus 24,400 for TC [219]. EC may be more beneficial in a multimodal setup [100] as dropping noisy image tokens is likely less harmful than text tokens. Thus, while we stick with TC for this release of OLMOE , we may revisit EC for future multimodal models. + +# 4.1.5 Sparse Upcycling + +Komatsuzaki et al. [85] propose turning a dense model into a Mixture-of-Experts model via sparse upcycling: (1) The dense MLP is cloned for each desired expert to constitute MoE layers. (2) A newly initialized router is added in front of each MoE layer. (3) Pretraining continues with the new model so that the cloned MLPs can gradually specialize in different things and the router can be learned. They find that the upcycling approach maintains a performance advantage over a language model trained from scratch for up to $120 \%$ of the compute budget of the original dense checkpoint that the sparse model was upcycled from. For example, if sparsely upcycling a 1.3B parameter model at 2 trillion tokens then only at 2.4 trillion tokens should an MoE trained from scratch catch up with the upcycled model. That is, the sparsely upcycled model would have been trained for another 400 billion tokens, thereby saving the equivalent of up to 2T tokens of compute. Other works such as MiniCPM [74], Qwen2 [201] and reportedly Mixtral [25, 79] have adopted sparse upcycling but only share limited information about their configuration. + +![](images/figures/olmoe-fig-0008.jpg) +Figure 8: Sparse upcycling. We upcycle OLMo-1B (0724) at 2T tokens into an MoE with 8 total experts of which 2 are activated and train it for an additional 610 billion tokens. We compare it to a model trained from scratch for 610 billion tokens. Except for this difference, both models use the same config, which includes some suboptimal settings that contribute to the instability, such as no QK-Norm (§4.2.5) and no truncated normal init (§4.2.2). More results, logs, and configurations: https://wandb.ai/ai2-llm/olmoe/reports/ Plot-Scratch-vs-Upcycle--Vmlldzo4NDIyOTc4 + +In Figure 8, we compare sparse upcycling OLMo-1B (0724) [65] with training an MoE from scratch. We find that after 500B tokens, an otherwise equivalent MoE trained from scratch already catches up with the upcycled model, both on the metrics in Figure 8 and our additional metrics at https:// wandb.ai/ai2-llm/olmoe/reports/Plot-Scratch-vs-Upcycle--Vmlldzo4NDIyOTc4. At around 600B tokens, the MoE from scratch starts outperforming the upcycled MoE. Thus, it only requires $2 5 \%$ of the compute budget of the original dense model to catch up as opposed to the $120 \%$ reported in Komatsuzaki et al. [85]. However, they use expert choice routing and study encoderdecoder models [139]. Meanwhile, we use token choice routing (§4.1.4) and decoder-only models (§2). Further, we upcycle a model that has already been significantly overtrained [57], i.e., a 1B model trained for 2T tokens. Its parameters are likely already in a very optimal range for a dense model, which may limit the amount of additional exploration possible after upcycling. This motivates us to experiment with adding noise to the upcycled weights outlined in Appendix F, but we do not find it to lead to better performance. A large disadvantage of upcycling is that the upcycled MoE is constrained by some hyperparameters of the dense model. Specifically, OLMo-1B (0724) was trained without QK-Norm and normal initialization, both of which hurt stability in our experiments $\mathfrak { S 4 . 2 . 5 }$ , $\ S 4 . 2 . 2 \}$ . While it may be possible to simply add new QK-Norms and train them from scratch similar to the new router layer trained from scratch, it is impossible to change the initialization of the original dense model when upcycling it. Thus, as we want to change these hyperparameters and also train OLMOE-1B-7B for around $2 5 0 \%$ of the compute budget of the dense model (5T vs. 2T tokens), we do not use upcycling. + +# 4.1.6 Load Balancing Loss + +Shazeer et al. [154] propose the load balancing loss to penalize the model if it is unbalanced, i.e., if it routes all tokens to only a few experts. This is based on the observation that without such penalty, models tend to update only a select few experts in each layer [52, 17]. To compute the load balancing loss $( \mathcal { L } _ { L B } )$ we multiply the fraction of tokens $f _ { i }$ routed to one expert $E _ { i }$ with the total routing probability $P _ { i }$ allocated to $E _ { i }$ for one batch and sum it across the number of experts $N _ { E }$ : + +$$ +\mathcal { L } _ { L B } = N _ { E } \cdot \sum _ { i = 1 } ^ { N _ { E } } f _ { i } \cdot P _ { i } +$$ + +The loss is further scaled by $N _ { E }$ and a loss weight $\alpha$ (see Equation 2), which is an optional weight to determine the magnitude of the loss commonly set to 0.01 [221, 199]. We do not experiment with changing the weight of 0.01. + +![](images/figures/olmoe-fig-0009.jpg) +Figure 9: Impact of applying a load balancing loss (LBL). The training loss plot excludes the load balancing loss for both models. More results, logs, and configurations: https://wandb.ai/ ai2-llm/olmoe/reports/Plot-LBL-vs-No-LBL--Vmlldzo4OTkyNDg4 + +![](images/figures/olmoe-fig-0010.jpg) +Figure 10: Expert assignment during training when using or not using a load balancing loss for the first MoE layer. More results, logs, and configurations: https://wandb.ai/ai2-llm/ olmoe/reports/Plot-LBL-vs-No-LBL--Vmlldzo4OTkyNDg4 + +In Figure 9 we investigate the performance impact of using the auxiliary load balancing loss. We find that across training loss and validation losses, using the load balancing loss leads to better performance even after only a few billion tokens. We still measure the load balancing loss even when it is not used (“No LBL”) and find that while it spikes initially, it slowly decreases over the next few billion tokens. This behavior is also visible in Figure 10 (left), where initially all tokens in the first layer are assigned to the 6th expert (pink). Eventually, the model also starts assigning some tokens to the 1st expert (yellow). However, all other experts remain largely flat and are thus “dead weights” that take up GPU memory but are not used. Given these results, we use the auxiliary load balancing loss with a weight of 0.01 following prior work [154, 158]. However, getting rid of the load balancing loss is an important direction for future research as it constrains the flexibility of the model by forcing it to use all experts approximately equally. This could prevent the experts from specializing in certain data domains and may be a reason prior work has failed to find strong evidence of expert specialization [79, 221]. + +# 4.1.7 Router Z-loss + +Zoph et al. [221] propose the router $\mathbf { Z }$ -loss to improve both the stability and quality of MoE models. This auxiliary loss penalizes large logits coming into the gating network. Such large logits can lead to numeric overflows in the large matrix multiplications happening in the MoE layer. It is computed by exponentiating the logits $x _ { j }$ right before the router layer summed across the number of experts $N _ { E }$ and averaged across the batch $B$ , thereby making larger logits lead to a larger loss: + +$$ +\mathcal { L } _ { R Z } ( \boldsymbol { x } ) = \frac { 1 } { B } \cdot \sum _ { i = 1 } ^ { B } \left( \log \sum _ { j = 1 } ^ { N _ { E } } \exp ( x _ { j } ^ { ( i ) } ) \right) ^ { 2 } +$$ + +The loss is further multiplied with an optional loss weight, $\beta$ (see Equation 2), to determine the magnitude of the loss commonly set to 0.001 [221, 158]. We do not experiment with changing the weight of 0.001. + +![](images/figures/olmoe-fig-0011.jpg) +Figure 11: Router z-loss. We compare adding router z-loss with a loss weight of 0.001 versus no additional $\mathbf { Z }$ -loss. More results, logs, and configurations: https://wandb.ai/ai2-llm/olmoe/ reports/Plot-Zloss-vs-none--Vmlldzo4NDM4NjUz + +In Figure 11, we confirm that across training loss, validation loss, and downstream performance adding the router z-loss improves stability (less spikes) and quality (lower loss and higher downstream performance). Thus, despite it reducing throughput by $\sim 2 \%$ we use the router z-loss for OLMOE-1B-7B with a weight of 0.001 as in Zoph et al. [221]. + +# 4.2 General Pretraining Settings + +# 4.2.1 Dataset Experiments + +![](images/figures/olmoe-fig-0012.jpg) +Figure 12: OLMOE-MIX vs. Dolma 1.7. We compare our data mix described in $\ S 2$ with Dolma 1.7 used to train prior OLMo models. Lower training loss does not mean that one dataset is better, but rather suggests which dataset is easier for the model to learn. More results, logs, and configurations: https://wandb.ai/ai2-llm/olmoe/reports/ Plot-Dolma-1-7-vs-Dolma-OLMoE--Vmlldzo4OTIxNTg5 + +Li et al. [90] release the DCLM-Baseline dataset and establish that it leads to better language models than Dolma 1.7 and other datasets as measured on common benchmarks like MMLU [70]. This motivates us to mix their DCLM dataset with some components from Dolma 1.7 that we deem to be high-quality; see $\ S 2$ . In Figure 12, we compare our mix, OLMOE-MIX, with Dolma 1.7 in a controlled setup. We find that OLMOE-MIX leads to clear gains on all three downstream metrics, especially MMLU. DCLM-Baseline has been created through a series of dataset ablations targeting MMLU and other downstream metrics, which explains these results. We also compare adding Reddit and FLAN to our mix as detailed in Appendix F, but do not find consistent performance gains. We do not have a strong intuition for why adding these datasets does not help and a more automatic approach to dataset mixing may be desirable for future iterations [101, 4]. + +We pretrain using our mix of DCLM-Baseline and Dolma 1.7 dubbed OLMOE-MIX. + +# 4.2.2 Initialization + +Few prior works on Mixture-of-Experts share their initialization strategy. Even the most open MoEs prior to this work, JetMoE [158] and OpenMoE [199], do not mention their initialization scheme. For DeepSeekMoE [39] and DeepSeekV2 [43], the authors share that they use a normal initialization with a standard deviation (std) of 0.006. For dense language models, a normal initialization with an std of 0.02 has been commonly used as popularized by Shoeybi et al. [159]. + +![](images/figures/olmoe-fig-0013.jpg) +Figure 13: Initialization. We compare a normal initialization with a standard deviation (std) of 0.02 with a truncated normal initialization with a maximum (minimum) cut-off of 0.06 $_ { ( - 0 . 0 6 ) }$ corresponding to three stds $( 3 { \times } 0 . 0 2 )$ . More results, logs, and configurations: https://wandb.ai/ ai2-llm/olmoe/reports/Plot-Init--Vmlldzo4NDIzMzM5 + +![](images/figures/olmoe-fig-0014.jpg) +Figure 14: Non-parametric layer normalization vs. RMSNorm. More results, logs, and configurations: https://wandb.ai/ai2-llm/olmoe/reports/Plot-LN--Vmlldzo4NDQyMTAz + +In Figure 13, we find a truncated normal initialization leads to more stable training and better performance than a regular normal initialization. The difference between the two initializations only becomes clear at around 450 billion tokens, where the model with the normal initialization starts to diverge. This is despite both models using the same configuration except for the difference in weight initialization. Having to train for hundreds of billions of tokens until an experiment provides a clear signal is one of the key challenges of pretraining ablations. + +We use the truncated normal initialization for OLMOE-1B-7B. + +# 4.2.3 RMSNorm + +OLMo [65] uses non-parametric layer normalization [12], mainly as it is significantly faster than the commonly used RM-SNorm [208, 113]. This is an unusual choice as most LMs use RMSNorm, such as the Llama [182, 183, 50], Gemma [176, 177], and Qwen [13, 201] model families. + +In Figure 14, we observe that replacing the non-parametric layer normalization in OLMo with a parametric RMSNorm leads to better performance. This is likely because the non-parametric layer normalization leads to a large number of spikes in the gradients as seen in Figure 16. We clip gradients at 1.0, which prevents these spikes from leading to very large and potentially disruptive parameter updates. However, the clipped gradients may still harm the performance of the model as they are no longer the true gradients. Thus, despite RMSNorm lowering our training throughput by $15 \%$ , we train our final model with RMSNorm. We include the RMSNorm parameters in weight decay as we find that it performs slightly better (Figure 15) even though it is common practice to exclude them.5 + +![](images/figures/olmoe-fig-0015.jpg) +Figure 16: Total norm of the gradients when training with RMS or non-parametric normalization. We increase the logging interval of the RMS run at 75B tokens, hence its change in thickness. + +![](images/figures/olmoe-fig-0016.jpg) +Figure 15: Decaying the RMSNorm parameters. More results, logs, and configurations: https: //wandb.ai/ai2-llm/olmoe/reports/Plot-Decay-LN--Vmlldzo4NDQ1NDYy + +![](images/figures/olmoe-fig-0017.jpg) +Figure 17: Decaying the embedding parameters. More results, logs, and configurations: https: //api.wandb.ai/links/ai2-llm/3h22onp5 + +# 4.2.4 Decaying Embedding Parameters + +Similar to the RMSNorm parameters (§4.2.3), embedding parameters are commonly excluded from weight decay.6 In Figure 17 we find that whether or not they are decayed has only a minor impact on performance, with decaying being slightly better. Thus for simplicity, we weight decay all parameters in OLMOE-1B-7B including embedding and RMSNorm. + +# 4.2.5 QK-Norm + +Some works have reported stability improvements from adding layer normalization after the query and key projections (“QK-Norm”) [173, 113, 44]. QK-Norm can prevent the subsequent attention operation from leading to very large logits that may lead to numeric overflows and destabilize the network, especially when training in low precision. Like layer normalization at other places in the model, the QK-Norm could be non-parametric or use the parametric RMSNorm (§4.2.3). + +In Figure 18, we compare using QK-Norm with no normalization after the query and key projections. We find that QK-Norm leads to some stability and performance improvements. We perform this experiment with non-parametric layer normalization as used in OLMo [65], while we used parametric RMS layer normalization [208] for OLMOE-1B-7B (§4.2.3). To ensure the benefit of QK-Norm is not an artifact of comparing with non-parametric layer normalization, we run another experiment with RMS layer normalization and still find QK-Norm to lead to slightly better training loss and to prevent a large grad norm spike.7 Thus, we use QK-Norm for OLMOE-1B-7B despite it reducing throughput by almost $10 \%$ . + +![](images/figures/olmoe-fig-0018.jpg) +Figure 18: Query-Key layer normalization (QK-Norm). Both models use non-parametric layer normalization. QK-Norm corresponds to additional layer normalization of the query and key projections. More results, logs, and configurations: https://wandb.ai/ai2-llm/olmoe/reports/ Plot-QKNorm-vs-none--Vmlldzo4NDIzMzE2 + +# 4.2.6 AdamW Epsilon + +![](images/figures/olmoe-fig-0019.jpg) +Figure 19: AdamW epsilon. More results, logs, and configurations: https://wandb.ai/ ai2-llm/olmoe/reports/Plot-AdamW-eps--Vmlldzo4NDc5MDg0 + +Groeneveld et al. [65] use an epsilon (“eps”) value of 1E-05 in the AdamW optimizer for training OLMo. A larger eps value leads to smaller steps of the optimizer but can be more stable [83]. + +In Figure 19, we find that decreasing eps to the recommended default of 1E-08 [83] significantly improves performance while the run remains stable. Thus, we set eps to 1E-08 for our final run. + +# 4.3 Adaptation Settings + +We experiment with small design choices for adaptation using our evaluation setup described in Appendix C. (1) Auxiliary losses: Zoph et al. [221] find that using the auxiliary load balancing loss (§4.1.6) during regular finetuning leads to small performance gains. For instruction tuning, however, Shen et al. [156] do not find conclusive evidence in favor of using the load balancing or router z-loss with only small differences in performance, both in support of and against the auxiliary losses. In Table 7 we display experiments with the load balancing loss during adaptation and find that not using it leads to better performance (54.0 vs. 52.8 after instruction tuning (SFT) and 57.7 vs. + +Table 6: Load balancing loss (Equation 3) over a subset of the respective corpora prior to scaling with the load balancing loss weight $\alpha$ . While we use load balancing loss during pretraining, we do not use it during SFT. + +
Data (1)OLMOE-1B-7B After pretraining After SFT
SFT data12.2212.16
Github13.8514.85
Wikipedia14.4814.24
C49.099.13
+ +57.1 after preference tuning (DPO)). One potential problem of deactivating the load balancing loss is that it may harm balance among experts and turn some into dead weights as observed during pretraining in $\ S 4 . 1 . 6$ . However, when measuring the load balancing loss in Table 6 on our SFT data (§2), we find that the loss actually decreases slightly during SFT (12.16 vs. 12.22). This is likely because which experts certain tokens get routed to is determined early during pretraining, as we find later in the analysis section (§5.1). We also visualize the activation patterns of experts of the model after pretraining, and the models after SFT and DPO trained without load balancing in Appendix G (Figure 33) finding that the distribution remains around the same. Thus, as our models adapted without load balancing perform better and we find it not to impact routing substantially, we do not use load balancing during adaptation . (2) Annealing checkpoint: We also experiment with using the checkpoint pre-annealing (§2) for adaptation and find the checkpoint post-annealing leads to better performance (53.8 vs. 54.0 after SFT and 56.3 vs 57.7 after DPO), thus we use the post-annealing checkpoint. (3) Preference algorithm: Since the release of DPO (Direct Preference Optimization) [138], a variety of preference algorithms have been proposed [54, 73, 114]. We experiment with KTO [54] and find that it matches DPO in Table 7 for our setup (Appendix B). While we release both models, we use DPO for our final OLMOE-1B-7B-INSTRUCT model, as it scores higher on AlpacaEval, which has a smaller chance of data contamination than our other benchmarks [198]. + +Table 7: Adaptation experiments of OLMOE-1B-7B. We compare using the pretrained checkpoint prior to annealing for adaptation, using the checkpoint after the additional 100B tokens of annealing, and using the checkpoint after the additional 100B tokens of annealing and with load balancing loss (§4.1.6) during adaptation. We apply DPO/KTO to the respective SFT model. + +
Task (→) Setup (→) Metric (→)MMLU 0-shot EMGSM8k 8-shot CoT EMBBH 0-shot EMHuman- Eval 0-shot Pass@10Alpaca- Eval 1.0 XSTest 0-shot %win0-shot F1IFEval 0-shot Loose AccAvg 0-shot
OLMoE-1B-7B w/o annealing49.02.031.518.9-62.118.5-
+SFT +DPO50.2 50.943.0 36.035.6 35.855.5 58.868.9 81.783.8 83.239.7 47.953.8 56.3
OLMoE-1B-7B49.83.033.622.4-59.716.6-
+SFT51.440.538.051.669.284.143.354.0
+DPO51.945.537.054.884.082.648.157.7
+KTO51.245.534.157.181.686.647.557.7
+SFT (load balancing)50.936.535.752.466.984.842.352.8
+DPO (load balancing)51.142.539.355.682.982.146.057.1
+ +# 5 MoE Analysis + +By advancing open and cost-efficient models (§1), OLMOE-1B-7B enables new research into LMs and MoEs. Making use of our released intermediate checkpoints, data, and code, we define and analyze four properties specific to MoEs: Router saturation (§5.1), Expert co-activation (§5.2), Domain specialization (§5.3), and Vocabulary specialization (§5.4). + +# 5.1 Router Saturation + +We define router saturation as the proportion of expert activations at some intermediary checkpoint at time $t$ that matches the expert IDs activated at some final checkpoint over the same dataset: + +$$ +\mathrm { R o u t e r } \mathrm { S a t u r a t i o n } ( t ) = \frac { 1 } { N } \sum _ { i = 1 } ^ { N } \frac { \vert \mathcal { E } _ { i } ^ { ( t ) } \cap \mathcal { E } _ { i } ^ { ( T ) } \vert } { k } , +$$ + +where: + +• $N$ : The total number of tokens in the dataset. + +![](images/figures/olmoe-fig-0020.jpg) +Figure 20: Router saturation during pretraining measured on a random $0 . 5 \%$ of the C4 validation data. We compute saturation by comparing the routing to the top- $k$ experts at four intermediate checkpoints (1, 10, 20, and $40 \%$ of pretraining) to the final pretraining checkpoint (Equation 5). + +• $k$ : The number of top- $k$ experts activated per input token. While we train with $k = 8$ (§2), we also analyze $k = 1$ by only looking at the expert with the highest routing probability. $\mathcal { E } _ { i } ^ { ( t ) }$ : The set of $k$ experts activated for the ith token at the tth checkpoint. • $\mathcal { E } _ { i } ^ { ( T ) }$ : The set of $k$ experts activated for the ith token at the final checkpoint $T$ . • ∣E (t)i $| \mathcal { E } _ { i } ^ { ( t ) } \cap \mathcal { E } _ { i } ^ { ( T ) } |$ : The number of common experts activated for the ith token between the tth and final checkpoints. + +Router saturation thus corresponds to whether the router weights are still learning which expert will process certain data. A value of $100 \%$ indicates that the router at the intermediate checkpoint will route to the same experts as the final checkpoint router. However, even at $100 \%$ saturation the router weight can still change and adapt the exact router probability for each expert. These probabilities are used to scale the output of the respective expert in the model. For OLMOE-1B-7B with its 64 experts, random routing equals a saturation of $1 \bar { / } 6 4 = 1 . 6 \%$ for $k = 1$ and $8 / 6 4 = 1 2 . 5 \%$ for $k = 8$ . + +In Figure 20 we find that after $1 \%$ of pretraining (5000 steps or 20B tokens), up to ${ \sim } 6 0 \%$ of routing to the top-8 activated experts has already saturated (right). Thus the model already uses the same 8 experts for given input data as it will at the end of pretraining. This early saturation aligns with prior work [199]. At $40 \%$ of pretraining, saturation reaches up to ${ \sim } 8 0 \%$ . However, which top-1 expert has the highest routing probability saturates slower (left). We find that routing in later layers saturates earlier during pretraining. Layer 0 is an outlier saturating significantly more slowly than other layers. Dai et al. [39] do not use an MoE in the first layer as they find that load balancing converges more slowly for the first layer. This is likely linked to our findings on saturation. Because routing in the first layer saturates slower, the experts that certain input data get routed to frequently change. These changes may lead to one expert suddenly getting significantly more data than others thereby impairing load balancing. We are excited about future work further investigating what happens in the first layer by building on our open release. + +# 5.2 Expert Co-activation + +We define expert co-activation as the proportion of times two specific experts, $E _ { i }$ and $E _ { j }$ , are simultaneously activated out of the total number of activations of one of those experts: + +$$ +\mathrm { E x p e r t ~ c o - a c t i v a t i o n } ( E _ { i } , E _ { j } ) = \frac { N _ { E _ { i } , E _ { j } } } { N _ { E _ { i } } } , +$$ + +where: + +• $E _ { i }$ : The first expert. +• $E _ { j }$ : The second expert. +• $N _ { E _ { i } , E _ { j } }$ : The number of times experts $E _ { i }$ and $E _ { j }$ are activated together. + +![](images/figures/olmoe-fig-0021.jpg) +Figure 21: Co-activation among experts of OLMOE-1B-7B on a random $0 . 5 \%$ of the C4 validation data. We display the 32 experts with the highest maximum co-activation score via their expert IDs on the $\mathbf { X } ^ { - }$ and y-axis. + +• $N _ { E _ { i } }$ : The total number of times expert $E _ { i }$ is activated. + +A co-activation of $100 \%$ indicates that if $E _ { i }$ is activated, $E _ { j }$ is also always activated. A value of $0 \%$ indicates that the experts never co-occur. If multiple expert pairs have high co-activation, it may suggest that these experts could be merged, benefiting less from keeping them separate. In a distributed setup, we could place highly co-activated experts on the same device to reduce communication costs during model inference. + +In Figure 21, we find that there is no strong co-activation among experts in one layer, with only few exceptions. This may indicate that there is little redundancy across different experts. Overall, layers 7 and 15 show similar co-activation patterns with several groups of 3 or 2 experts that tend to get activated together. We investigate tokens that activate these experts in $\ S 5 . 4$ . Further, in Appendix G (Figure 35), we investigate whether experts across layers, rather than within one layer, tend to process tokens together. + +# 5.3 Domain Specialization + +We define domain specialization as the proportion of tokens from a particular domain $D$ that get routed to a particular expert $E _ { i }$ : + +$$ +\mathrm { D o m a i n ~ s p e c i a l i z a t i o n } ( E _ { i } , D ) = { \frac { N _ { E _ { i } , D } ^ { ( k ) } } { N _ { D } } } , +$$ + +where: + +• $E _ { i }$ : The ith expert in the model. +• $D$ : The domain from which the data originates. +• $k$ : The number of experts considered (e.g., $k = 8$ means considering the top 8 experts with the highest routing probabilities). +• N (k) : The number of tokens from domain $D$ for which $E _ { i }$ is among the top- $k$ selected experts. +• $N _ { D }$ : The total number of tokens from domain $D$ processed by the MoE. + +Domain specialization thus refers to the specialization of expert $E _ { i }$ to domain $D$ . A value of $100 \%$ indicates that all data from that domain is routed to $E _ { i }$ , whereas $0 \%$ indicates the expert is never used for that domain and can be removed from the model without affecting performance in that domain. + +In Figure 22 (top) we find many examples of experts that are activated significantly above or below random chance for specific domains. E.g., for arXiv, which has a very specific distribution with lots of scientific text, the first expert in layer 0 is nearly $100 \%$ specialized. This suggests that there is little redundancy in the knowledge of the experts in OLMOE-1B-7B, as they specialize in different kinds of data. GitHub and arXiv are often activated together in layer 7, which we explore further in $\ S 5 . 4$ . For generic domains, such as C4 [139], which is a web crawl containing various kinds of data, expert activations in OLMOE-1B-7B are much more balanced. This highlights that the load balancing (§4.1.6) works as intended and the model makes proper use of all experts for generic data. Mixtral-8x7B [79] in Figure 22 (bottom), however, exhibits little domain specialization across both unique and generic domains. Experts are activated close to the uniform routing baseline for all layers and domains. Thus, there may be more redundancy across experts in Mixtral, as they likely contain similar knowledge. We hypothesize that this is due to Mixtral being upcycled from Mistral [25]. The initialization from a dense model may limit the amount of possible specialization in the experts as they all start from the same local optimum. This is likely why training from scratch eventually outperforms upcycling in our pretraining experiments (§4.1.5). + +![](images/figures/olmoe-fig-0022.jpg) +Figure 22: Domain specialization of OLMOE-1B-7B (top) vs. Mixtral-8x7B (bottom). We visualize how often tokens from different domains get routed to the 64 (OLMOE) or 8 (Mixtral) experts at the end of pretraining. We consider tokens routed to any of the $k = 8$ (OLMOE) or $k = 2$ (Mixtral) active experts (Equation 7). Horizontal gray lines correspond to random chance or uniform routing $( 8 / 6 4 { = } 1 2 . 5 \%$ per expert for OLMOE-1B-7B with 8 active out of 64 total experts per layer and $2 / 8 { = } 2 5 \%$ for Mixtral with 2 active out of 8 total experts per layer). See Figure 34 for $k = 1$ results. + +# 5.4 Vocabulary Specialization + +![](images/figures/olmoe-fig-0023.jpg) +Figure 23: Vocabulary specialization of OLMOE-1B-7B across layers and experts. To compute vocabulary specialization per layer (left) we average the specialization of each expert in that layer. Dashed lines (right) correspond to the average of layer 7 as depicted left. We display the first 32 experts out of 64. This plot is for $k = 1$ (Equation 8) and we provide $k = 8$ and a comparison with Mixtral-8x7B in Appendix G. + +We define vocabulary specialization as the proportion of tokens with a token ID $x$ (also called vocabulary element) that are routed to one particular expert $E _ { i }$ out of all experts in that layer: + +$$ +\mathrm { V o c a b u l a r y ~ s p e c i a l i z a t i o n } ( E _ { i } , x ) = \frac { N _ { x , E _ { i } } ^ { ( k ) } } { N _ { x } } , +$$ + +where: + +• $E _ { i }$ : The ith expert in the model. +• $x$ : The token ID being analyzed. +• $k$ : The number of experts considered (e.g., $k = 8$ means considering the top 8 experts with the highest routing probabilities). +• $N _ { x , E _ { i } }$ : The number of times input data is routed to $E _ { i }$ for $x$ . +• $N _ { x }$ : The total number of times input data is routed across all experts for $x$ . + +Vocabulary specialization thus refers to how specialized a particular expert is on some vocabulary item. We distinguish input and output variants of this specialization, where $x$ is either the input token $\mathrm { I D }$ or the next output token ID (either the ground-truth next token $\mathrm { I D }$ or the token ID predicted by the model). A value of $100 \%$ indicates that for all occurrences of that vocabulary element, input data is routed to $E _ { i }$ , whereas $0 \%$ indicates an expert that is fully irrelevant for that vocabulary element and can be effectively removed from the model without affecting performance whenever the token ID appears. + +In Figure 23 we find that vocabulary specialization is higher in later layers, similar to how later layers saturate earlier (§5.1). Later layers also specialize more on predicted output token IDs rather than input token IDs, i.e., the routing is decided more by the token the model is about to predict rather than the original input token. This is intuitive as in earlier layers there is more uncertainty about which token the model will predict. At ${ \sim } 9 0 \%$ , expert 27 specializes the most, which we find in Table 8 to activate for many non-alphabetic tokens, such as Cyrillic and Devanagari letters. + +
Expert IDInput token IDs Predicted output token IDs
27 O(100%)I(100%)3(100%)i(100%)I'(100%) § (100%)0(100%) j (100%)ijT(100%)Q(100%)D(100%)b(100%)(100%)O(100%)D(100%)F(100%)E(100%)WH(100%)a(100%) b3(100%):(100%)2T(100%)
E(100%)WH(100%)a(100%)
58("(100%)("(100%) (94%),(92%)such(100%)486(100%)see(95%)which(91%)driving(91%)UK(90%)who(88%)including(88%)normal(88%)
“(92%)((92%)"(90%),(89%)"(88%)$(87%)[(87%)£(86%)
7Him(100%)inde(100%)Jesus(98%)rella(100%)Him(94%)sin(90%)
God(90%)pray(81%)Holy(80%)prince(80%)glory(72%)Jesus(69%)Lord(68%)Christ(65%)Spirit(55%)Holy(53%)God(50%)Prayer(50%)
Quran(80%)God(77%)Lord(76%)
glory(75%)Spirit(66%)Christ(65%)
37Sunday (100%) Tuesday (100%)days(91%)anniversary(90%)month
Thursday (100%) Olympic (100%)(88%)week(84%)mpi(83%)semester
Christmas (100%) rugby (100%)(81%)mand(80%)Olympics(78%)cent
Championship(100%)weekends(100%)(76%)season(76%)perm(75%)
43Armenian(100%)ijan(100%)enia(96%)enia(90%)invasion(80%)Arabia(76%)
Iraq(95%)Iranian(92%)Iran(92%)irregular(66%)regions(64%)border
Saudi(90%)northern(90%)Lebanon(63%)Kong(61%)ians(61%)bases
(90%)Singapore(88%)Turkey(88%)(60%) Republic(59%)Ireland(58%)Korea(58%)War(55%)Carolina(52%)
Asia(87%)Egypt(86%)western(86%)
4sq(89%)Main(70%)reversal(69%)YR(90%)Character(88%)sq(77%)
YR(63%)GC(56%)Overall(50%)79Os(76%)GHz(71%)fluence(60%)
(50%)main(50%)RE(46%)PCR(46%)amycin(60%)pixels(56%)=(53%)arc
tomb(45%)normal(43%)intensity(52%)Story(52%)=(51%)anth(50%)
(41%)Overall(41%)median(41%)GHz(50%)cm(46%)
0ESM(100%)icillin(100%)agra(98%)*,(100%)sil(96%)pills(91%)vi
aust(96%)asa(93%)pills(92%)mg(87%)pharmacy(87%)gener
(85%)uk(82%)login(82%)doc(81%)(85%)aust(82%)mg(75%)Content
generic(81%)cd(81%)Essay(81%)(75%)uk(73%)THAT(73%)dispens
password(81%)Content(80%)(68%)icillin(68%)generic(66%)
3grandmother(92%)brother(91%)Daisymother(35%)inde(31%)
(83%) daughter (78%)mum(75%)girl(28%)married(27%)
father(72%)wife(70%)husband(70%)
lady(63%)dad(62%)boy(61%)
48compared(42%)!)(41%)Then(41%),except(60%)tennis(41%)Marks(40%)Dunn(33%)tears(30%)Arizona(30%)
(40%)),(35%)",(35%)instead(33%)
23…(58%)Therefore(55%)So(46%)!!!Republican (50%)Jack
(46%)And(44%)According(41%)."(41%)!!(40%)?"(38%)But(38%)(46%)And(44%)According(41%)."
according(39%)So(38%)Step(33%)
+ +Expert 43 shows specialization on geographic terms in both input and output tokens. Experts 48 and 23 both focus on connector words, such as Then and Therefore . This is likely because they commonly process tokens together with a high co-activation of $60 \%$ in Figure 21 (middle). Based on our findings in $\ S 5 . 3$ that for GitHub and arXiv often the same experts in layer 7 activate, we display one such expert (expert ID 4) in Table 8. It seems to specialize in measurements, such as sq , YR (year), and $\mathrm { G H z }$ . These are common terms in scientific papers corresponding to the arXiv domain and likely also in GitHub code for computations related to measurements. They are less likely to appear in books, which explains the low activation of expert $\mathrm { I D 4 }$ in layer 7 for book data in Figure 22. Expert 3 is among the three most active experts of layer 7 for book data in Figure 22 (fourth yellow bar for layer 7). This resonates when looking at its specialization on family terms in Table 8, which are far more common in books than scientific papers or code. Overall, domain specialization and vocabulary specialization are closely linked to one another, as domains are usually characterized by their distinct word distribution. In Appendix G (Figure 32), we link them more closely by comparing the extent of vocabulary specialization across domains and expert IDs. In Appendix G (Figure 30, Figure 31) we also find that OLMOE-1B-7B exhibits stronger vocabulary specialization than Mixtral- ${ } ^ { 8 \mathrm { x 7 B } }$ . + +# 6 Related Work + +Advances in MoEs Current LMs still largely follow the transformer architecture [185] with only few architectural changes that have been widely adopted, such as decoder-only training [137], SwiGLU activations [153, 41], RoPE [166], MQA/GQA [152, 3] and RMSNorm [208]. Model sparsity via Mixture-of-Experts is one modification still under active exploration with some early adoption but most LMs, including Llama 3 [50], still rely on a dense architecture. There has been a lot of progress in improving the sparsely-gated MoE layer since its introduction [154]: New routing techniques [89, 146, 222, 66, 77, 49, 215, 195, 124], fine-grained expert segmentation [39, 69], stability [221] and efficiency [88, 141, 48, 218, 91, 168, 129, 145] improvements. In this work, we perform many experiments to provide insights into training Mixture-of-Experts LMs. Subsequently, we train OLMOE-1B-7B for 5T tokens. No prior MoE has been overtrained [57] to this extent to our knowledge making OLMOE-1B-7B the best testbed to research performance saturation of MoEs vs. dense models. With OLMOE we hope to facilitate such and other research to help the field uncover whether MoEs should make it into all future LMs and with what precise configuration. + +Open LMs A variety of model families have been proposed under varying degrees of openness commonly categorized based on whether model weights are available. Closed-weight models include GPT [24, 128], Gemini [174, 175], PaLM [30, 9], Reka [181], and open-weight ones include Llama [182, 183, 50], Mistral [78, 79], Gemma [176, 177], Falcon [8, 132], MPT [179], Qwen [13, 201], GLM [61], Yi [2], DeepSeek [42, 43, 39], Nemotron [130, 126], Zamba [62], InternLM [26], Baichuan [200], Phi [68, 94, 1], StableLM [16], OPT [212]. However, besides model weights, training data and code are key to enabling scientific research of these models [105, 106] and distributing their benefits broadly [23]. There have been few releases also including data and code in addition to model weights which we refer to as “fully open-source”: BLOOM [193, 151, 123, 203], GPT-NeoX [21, 22, 186], StarCoder [92, 109, 5, 120, 220], Pythia [18], OLMo [65], LLM360 [103], Cerebras-GPT [46], DCLM [90], MAP-Neo [209], RWKV [133, 134], and SmolLM [6]. For Mixture-of-Experts only OpenMoE [199] aims to be fully open-source, however, its poor performance limits its usefulness. We release OLMOE-1B-7B as the first state-of-the-art Mixture-of-Experts LM that is fully open-source: model weights, data, code, and logs. + +# 7 Conclusion + +We open-source OLMOE-1B-7B and OLMOE-1B-7B-INSTRUCT including model, data, code, and logs. At 1B active and 7B total parameters, our models yield state-of-the-art performance among models with a similar amount of active parameters even outperforming larger models including DeepSeekMoE-16B and Llama2-13B-Chat. We share various training experiments and define and analyze router saturation, expert co-activation, domain and vocabulary specialization of our model. Through our fully open release, we seek to help the field build better MoEs. We are excited about more iterations of OLMOE to close the gap between frontier models and fully open models. + +# Author Contributions + +Niklas Muennighoff proposed and led the project. He ran the pretraining experiments, pretrained the model, helped run adaptation and analysis, and wrote most of the paper. + +Luca Soldaini created the pretraining dataset and advised on pretraining. + +Dirk Groeneveld advised on pretraining, especially stability and throughput improvements. + +Kyle Lo helped with pretraining dataset creation, analyzed data experiments, and advised on data and framing, and helped edit the paper. + +Jacob Morrison co-created the adaptation dataset, ran most adaptation experiments, and helped edit the paper. + +Sewon Min analyzed router saturation, expert correlation, and vocabulary specialization, and helped frame and edit the paper. + +Weijia Shi analyzed domain and vocabulary specialization, advised at various project stages, and helped edit the paper. + +Pete Walsh advised on pretraining, especially stability and throughput improvements. + +Oyvind Tafjord ran OLMES evaluations. + +Nathan Lambert co-created the adaptation dataset, advised on adaptation, and helped edit the paper. + +uling Gu ran OLMES evaluations and helped edit the paper. + +ane Arora uploaded the models, helped with code review and framework integration. + +Akshita Bhagia supported stability investigations and helped with DCLM evaluations. + +Dustin Schwenk supported stability investigations. + +David Wadden ran DCLM evaluations and helped with Weights & Biases reports. + +Alexander Wettig advised on pretraining, analyzed load balancing, routing, and domain specializa tion, and helped edit the paper. + +Binyuan Hui advised on pretraining and helped with plotting and framework integration. + +Tim Dettmers advised on analysis and inference experiments. + +Douwe Kiela advised on framing. + +Ali Farhadi advised on pretraining and framing. + +Noah A. Smith advised on pretraining, and helped frame and edit the paper. + +Pang Wei Koh advised on analysis, and helped frame and edit the paper. + +Amanpreet Singh advised on pretraining, framing and helped edit the paper. + +Hannaneh Hajishirzi was responsible for direction and advising of the overall effort and helped frame and edit the paper. + +# Acknowledgements + +OLMOE would not be possible without the support of many individuals and institutions. We thank our teammates at the Allen Institute for AI, Contextual AI, and the University of Washington for their support, especially Aditya Kusupati, Ananya Harsh Jha, Caitlin Wittlif, Carissa Schoenick, Costa Huang, Crystal Nam, David Atkinson, Emma Strubell, Faeze Brahman, Hamish Ivison, Karel D’Oosterlinck, Matt Latzke, Ian Magnusson, Jack Merullo, Jay Chen, Jennifer Dumas, Jiacheng Liu, Johann Dahm, Luke Zettlemoyer, Michael Schmitz, Michael Wilson, Pradeep Dasigi, Sahil Verma, Sam Skjonsberg, Sophie Lebrecht, Stas Bekman, Taira Anderson, Valentina Pyatkin, Yanai Elazar, Yizhong Wang, and Yoganand Chandrasekhar. We also thank Armen Aghajanyan, Akshat Shrivastava, Colin Raffel, Haokun Liu, Ludwig Schmidt, Mengzhou Xia, Shayne Longpre, Sheng Shen, and Zexuan Zhong. 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ArtifactPublic link
OLMOE-1B-7Bhttps://hf.co/allenai/OLMoE-1B-7B-0924
OLMOE-1B-7B-INSTRUCThttps://hf.co/allenai/OLMoE-1B-7B-0924-Instruct
OLMoE-1B-7B-SFThttps://hf.co/allenai/OLMoE-1B-7B-0924-SFT
OLMoE-Mixhttps://hf.co/datasets/allenai/OLMoE-mix-0924
SFT datahttps://hf.co/datasets/allenai/
tulu-v3.1-mix-preview-4096-OLMoE https://hf.co/datasets/allenai/
KTO/DPO dataultrafeedback_binarized_cleaned
Codehttps://github.com/allenai/OLMoE
Logshttps://wandb.ai/ai2-llm/olmoe/reports/
BLOOM-7B DeepSeekMoE-3B-16BOLMoE-1B-7B-0924--Vmlldzo4OTcyMjU3 https://hf.co/bigscience/bloom-7b1
https://hf.co/deepseek-ai/deepseek-moe-16b-base
DeepSeekMoE-3B-16B+chat
DeepSeekV2-2B-16Bhttps://hf.co/deepseek-ai/deepseek-moe-16b-chat
https://hf.co/deepseek-ai/DeepSeek-V2-Lite
DCLM-1Bhttps://hf.co/TRI-ML/DCLM-1B
DCLM-7Bhttps://hf.co/TRI-ML/DCLM-7B
Falcon-7Bhttps://hf.co/tiiuae/falcon-7b
Gemma2-3B
Gemma2-9Bhttps://hf.co/google/gemma-2-2b
JetMoE-2B-9Bhttps://hf.co/google/gemma-2-9b
JetMoE-2B-9B+SFThttps://hf.co/jetmoe/jetmoe-8b
JetMoE-2B-9B+Chathttps://hf.co/jetmoe/jetmoe-8b-sft
https://hf.co/jetmoe/jetmoe-8b-chat
Llama-7Bhttps://hf.co/huggyllama/1lama-7b
Llama2-7Bhttps://hf.co/meta-1lama/Llama-2-7b-hf
Llama3.1-8Bhttps://hf.co/meta-1lama/Meta-Llama-3.1-8B
MPT-7Bhttps://hf.co/mosaicml/mpt-7b
Mistral-7Bhttps://hf.co/mistralai/Mistral-7B-v0.1
Mixtral-8x7Bhttps://hf.co/mistralai/Mixtral-8x7B-v0.1
OLMo-1B (0724)https://hf.co/allenai/OLMo-1B-0724-hf
OLMo-7B (0724)https://hf.co/allenai/OLMo-7B-0724-hf
OpenMoE-3B-9Bhttps://hf.co/OrionZheng/openmoe-8b
Pythia-7Bhttps://hf.co/EleutherAI/pythia-6.9b
Qwen1.5-3B-14Bhttps://hf.co/Qwen/Qwen1.5-MoE-A2.7B
Qwen1.5-3B-14B+Chathttps://hf.co/Qwen/Qwen1.5-MoE-A2.7B-Chat
StableLM2-2Bhttps://hf.co/stabilityai/stablelm-2-1_6b
TinyLlama-1Bhttps://hf.co/TinyLlama/TinyLlama_v1.1
+ +# B Training Configuration + +Pretraining We display the pretraining hyperparameter configuration of OLMOE-1B-7B in Appendix B comparing with other relevant models. We follow Groeneveld et al. [65] using the AdamW optimizer [107] with ZeRO [142] via PyTorch FSDP [213] and mixed-precision training [116]. Our main model settings differing from Groeneveld et al. [65] are: (1) MoE-related changes: OLMOE-1B-7B is a sparsely activated decoder-only transformer [185] using dropless Mixtureof-Experts [58]. Unlike most prior MoEs, we use a high granularity [39, 86] with 64 small experts with an FFN dimension of just 1,024 rather than a few large experts. We further use two auxiliary losses: router z-loss [221] and load balancing loss [154]. (2) Stability improvements: (a) We use a truncated normal initialization with a standard deviation of 0.02 and a minimum (maximum) cut-off of -0.06 (0.06) corresponding to three standard deviations. (b) We use QK normalization [173, 113, 44]. (c) We use RMSNorm [208] instead of the non-parametric LayerNorm used in Groeneveld et al. [65]. (3) Performance improvements: Besides some of the stability improvements which also impact performance, we also reduce the AdamW epsilon to 1.0E-08 from the 1.0E-05 used in Groeneveld et al. [65] to speed up convergence. Finally, we train OLMOE-1B-7B for significantly longer than all prior OLMo models amounting to 5T tokens and thus more than one epoch (1.3) following Muennighoff et al. [121]. We shuffle the pretraining dataset before starting the second epoch. For the final 100B tokens, we decay the learning rate linearly from 5.0E-04 to 0. We experiment with many of these settings in $\ S 4$ . + +Adaptation For finetuning we use Open Instruct [188, 76].8 We filter all SFT samples to a length of fewer than 4096 tokens to match the sequence length of the model. Following Muennighoff et al. [122], we aggregate loss at the token level during SFT to improve performance on long generative tasks, such as AlpacaEval. We finetune in BF16 with a global batch size of 128 (4 H100 nodes with 8 GPUs each, a per device batch size of 2, and 2 gradient accumulation steps). We train for 2 epochs with a constant learning rate of 2.0E-5. For DPO [138], we reduce the global batch size to 32 (4 H100 nodes with 8 GPUs each and a per device batch size of 1). We train for 3 epochs with a learning rate of 5.0E-7 and a DPO beta of 0.1. Our adapted models are built on top of our annealed checkpoint, and we include the load balancing loss during both SFT and DPO based on our experiments in $\ S 4 . 3$ . Our preference tuning recipe is heavily optimized for DPO based on extensive experiments by Ivison et al. [76], thus for KTO [54] we experiment with a few settings in Appendix F. Our final KTO adaptation uses the same hyperparameters as DPO, except that we use the RMSProp optimizer instead of Adam, which we use for SFT and DPO, and that we reduce the training duration to 1.3 epochs (5,000 steps) for KTO instead of the 3 epochs used for DPO. + +Hardware We pretrain OLMOE-1B-7B on 256 H100 GPUs for approximately 10 days with NVlink interconnect across GPUs and InfiniBand interconnect across nodes. We also use H100 GPUs for all our experiments but some use a cluster with GCP TCPx interconnect across nodes instead. For adaptation, we use 32 H100 GPUs for 33 hours to instruction tune and for another 14 hours to preference tune via DPO. For KTO adaptation we use 8 H100 GPUs for 30 hours instead. + +Table 10: Pretraining hyperparameters of OLMOE-1B-7B and comparable models trained from scratch. We highlight rows where OLMOE-1B-7B differs from OLMo-1B. Active params include vocab params. “?” $=$ undisclosed settings, $\mathrm { F F N = }$ feed-forward network, Attn $=$ Attention, $\mathrm { L R } =$ learning rate, ${ \bf W } { \bf S } { \bf D } = { \bf \Phi }$ Weight-Stable-Decay [74], LBL $=$ load balancing loss, Inv Sq Root $=$ Inverse Square Root decay [155], trunc $=$ truncation, std $=$ standard deviation, “varies” $=$ stds that are layer or weight-dependent. + +
OLMOE-1B-7BJetMoEOpenMoEOLMo-1B (0724)
Dimension2,0482,0482,0482,048
Activation SwigGLU SwigGLU SwigGLU SwigGLU
FFN dimension1,0245,6328,1928,192
Vocab size50,30432,000256,38450,34
Attn heads16162416
Num layers16243216
Layer norm typeRMSNormRMSNormRMSNormnon-parametric
Layer norm eps1.0E-051.0E-051.0E-061.0E-05
QK-Normyesnonono
Pos emb.RoPERoPERoPERoPE
RoPE θ10,,00010,00010,00010,000
Attention variantfullMoAfullfull
Biases-MLP & Attn--
Weight tying Init distnoyesnono
Init stdtrunc normal??normal
Init trunc0.02 3×std0.02variesvaries
ME LlayersEveryEveryEvery 6th-
MoE layer typedMoEdMoESST-MoE-
Experts64-
# Activated88 232 21 1
# Vocab params
# Active params103M 1.3B66M 2.2B525M 2.6B103M 1.3B
# Total params6.9B8.5B8.7B1.3B
Sequence length
Batch size (samples)4,096 1,0244,096 1,0242,048 2,0484,096
Batch size (tokens)~4M~4M~4M512 ~2M
warmup steps2,5002,50010,0002,000
peak LR4.0E-045.0E-040.014.0E-04
minimum LR4.0E-055.0E-05,4.E-05
optimizerAdamWAdamWAdafactorAdamW
weight decay
betall0.10.10.00.1
beta20.9?0.90.9
0.95?0.95
AdamW epsilon LR schedule1.0E-08?Inv Sq Root1.0E-05
gradient clippingcosine global 1.0WSD global 1.0globbal 1.00cosine global 1.0
gradient reduce dtypeFP32?FP32
optimizer state dtypeFP32??FP32
LBL weight
Router z-loss weight0.010.010.01
00100.0010.001
Pretraining tokens,03B1,000B1,100B2,000B
Annealing tokens10B250B50B
Annealing schedulelinearlinear
Annealing min LR00
+ +# C Evaluation Setup + +Table 11: Summary of downstream evaluation during and after pretraining (OLMES). ARC-C and ARC-E refer to ARC-Challenge and -Easy, CSQA $\equiv$ CommonsenseQA, OBQA $\equiv$ OpenBookQA, ${ \mathrm { C F } } { = }$ Completion/Cloze formulation, MCF=Multiple-choice formulation, pmi=pointwise-mutualinformation, char $\simeq$ per-character, Var=variants referring to the use of few-shots varying from 0-5. + +
During pretrainingAfter pretraining (OLMES [67])
Dataset (1)FormatShotNormSplitFormatShotCF NormSplit
ARC-C [34]CF0charvalmax(MCF,CF)5pmitest
ARC-E [34]CF0nonevalmax(MCF,CF)5chartest
BoolQ [33]CF0nonevalmax(MCF,CF)5noneval
COPA [63]CF0noneval---
CSQA [170]CF0charvalmax(MCF,CF)5pmival
HellaSwag [207]CF0charvalmax(MCF,CF)5charval
MMLU [70]MCF5nonevalmax(MCF,CF)5chartest
MMLU VarCF0-5charval--
OBQA [117]CF0charvalmax(MCF,CF)5pmitest
PIQA [20]CF0charvalmax(MCF,CF)5charval
SciQ [192]CF0noneval---
SocialIQA [150]CF0charvalmax(MCF,CF)5charval
Winogrande [148]CF0nonevalmax(MCF,CF)5noneval
+ +During pretraining We evaluate using a similar in-loop evaluation setup as Groeneveld et al. [65], with the addition of more tasks such as CommonsenseQA, PIQA, and different implementations of MMLU. Following Groeneveld et al. [65], for the majority of the tasks, we perform 0-shot evaluation using the Completion/Cloze Formulation (CF), ranking each answer string using language model probabilities. In terms of probability normalization, there is either no normalization (none) or normalization by the number of characters in the answer (char) when ranking solely based on probability may heavily favor shorter answers [24]. For MMLU, the in-loop evaluation also includes a setup where we increase the total number of instances by including a range of 0-shot to 5-shot setups together as we found this provides smoother trends as the training proceeds (“MMLU Var”). We also include the Multiple-Choice Formulation (MCF) version of MMLU, scoring prediction of answer labels like A/B/C/D, which generally starts to rise only later in training as models only gain the multiple-choice capability later (at around 1T tokens for OLMOE-1B-7B in Figure 25). We also evaluate perplexity on selected validation sets from Paloma [111, 144, 59, 163, 96, 115]. All code used for evaluation during pretraining is at https://github.com/allenai/OLMo/tree/ 61ac104d616ec5435db225796e5c7532c9abd95a/olmo/eval. + +After pretraining - OLMES We perform evaluations following the OLMES evaluation standard [67], with the suite of tasks in the original paper. OLMES (Open Language Model Evaluation Standard) is a standard for reproducible LM evaluations that is open, practical, and documented, providing recommendations guided by experiments and results from the literature [19, 60, 64]. It is designed to support comparisons between smaller base models that require the Cloze formulation of multiple-choice questions against larger models that can utilize the Multiple-choice formulation. To make our evaluations reproducible, we follow OLMES in prompt formatting, choice of in-context examples, probability normalization, task formulation, as well as all other details. We summarize this setup in Table 4 and refer to Gu et al. [67] for more details. + +After pretraining - DCLM For results on the DCLM tasks [90] in Table 13, we precisely follow their setup using the evaluation code released by the authors at https://github.com/ mlfoundations/dclm. “Core” results are the low variance tasks in their evaluation code, while “Extended” corresponds to the heavy tasks. + +After adaptation After supervised finetuning and direct preference optimization, we evaluate models using a subset of the evaluations and the same overall setup used in Ivison et al. [76] and Wang et al. [188]. We cover a wide range of model capabilities in our evaluation suite including coding (HumanEval [28]), general and mathematical reasoning (Big Bench Hard [169], GSM8k [35]), world knowledge (MMLU), general instruction following (AlpacaEval 1.0 [93], not the length-controlled variant [51]), precise instruction following (IFEval [217]) and safety (XSTest [147]). We refer to Wang et al. [188] for more details on each benchmark. + +# D Openness of Models + +We list the openness of various models summarized in Figure 1. We exclude Switch Transformers [56], as it was published over three years ago and is very different from more recent MoE models (MLM objective, Encoder-decoder, etc.). + +# Grok-86B-314B [196] + +$\boldsymbol { \mathscr { v } }$ Model: Their model is licensed under the open-source Apache 2.0 license. • $\times$ Data: Unavailable. • $\times$ Code: Unavailable. • $\times$ Logs: Unavailable. + +# Mixtral-39B-141B and Mixtral-13B-42B [79] + +Model: Their model is licensed under the open-source Apache 2.0 license. + +$\times$ Data: Unavailable. + +$\times$ Code: Unavailable. + +$\times$ Logs: Unavailable. + +# DBRX-36B-132B [40] + +• $^ { 1 1 }$ Model: The model is licensed under a custom non-open-source license9 with additional use-case restrictions.10 + +Data: Unavailable. + +$\times$ Code: They use closed-source custom adaptations of their public libraries LLMfoundry, composer, and megablocks.11 + +Logs: Unavailable. + +# Skywork-MoE-22B-146B [191] + +$\bigstar \bigstar$ Model: The model is licensed under a custom non-open-source license.12 + +• $\times$ Data: Unavailable. + +• $\times$ Code: Unavailable. + +• $\times$ Logs: Unavailable. + +# DeepSeekV2-21B-236B [43] and DeepSeekMoE-3B-14B [39] + +Model: The models are licensed under custom non-open-source licenses.13 + +• $\times$ Data: Unavailable. +• Code: Unavailable. +• $\times$ Logs: Unavailable. + +# Arctic-17B-480B [162] + +Model: The model is licensed under the open-source Apache 2.0 license. + +• Data: They describe their mixture but do not release it.14 + +$\times$ Code: Unavailable. + +• $\times$ Logs: Unavailable. + +# Qwen2-14B-57B [180] + +Model: The model is licensed under the open-source Apache 2.0 license. + +Data: Unavailable. + +• $\times$ Code: Unavailable. + +$\times$ Logs: Unavailable. + +# Jamba-12B-52B [97] + +Model: The model is licensed under the open-source Apache 2.0 license. + +• $\times$ Data: Unavailable. + +• $\times$ Code: Unavailable. + +$\times$ Logs: Unavailable. + +# Qwen1.5-3B-14B [180] + +• Model: The model is licensed under a custom non-open-source license.15 + +Data: Unavailable. + +• $\times$ Code: Unavailable. + +• $\times$ Logs: Unavailable. + +# JetMoE-2B-9B [158] + +Model: The model is licensed under the open-source Apache 2.0 license. + +• $\bigstar \bigstar$ Data: They describe their mixture but do not release it. + +• Code: They make their fork of megablocks publicly available,16 however, their Megatron-LM training code is not available.17 + +Logs: Unavailable. + +# OpenMoE-2B-9B [199] + +• $\boldsymbol { \nu }$ Model: The model is licensed under the open-source Apache 2.0 license. +• $\boldsymbol { \mathscr { v } }$ Data: They make scripts for recreating their data available. $\boldsymbol { \nu }$ Code: They make their code available.18 +• $\times$ Logs: Unavailable. + +# OLMOE-1B-7B + +• $\boldsymbol { \nu }$ Model: The model is licensed under the open-source Apache 2.0 license. +• $\boldsymbol { \mathscr { v } }$ Data: The data is licensed under the open-source ODC-By 1.0 license. +• $\boldsymbol { \mathscr { v } }$ Code: The code is licensed under the open-source Apache 2.0 license. +• $\boldsymbol { \mathscr { v } }$ Logs: Logs are available with the same open-source license as the code (Apache 2.0). + +# E Additional Evaluation + +![](images/figures/olmoe-fig-0024.jpg) +Figure 24: Losses of OLMOE-1B-7B during training. The Books, Reddit, and Stack [84] datasets are from Dolma 1.7 [163] via Paloma [111]. More results, logs, and configurations: https://wandb.ai/ai2-llm/olmoe/reports/Plot-OLMoE-1B-7B--Vmlldzo4OTcyMjU3 + +![](images/figures/olmoe-fig-0025.jpg) +Figure 25: Evaluation of OLMOE-1B-7B and the current best OLMo models during pretraining. Grey vertical lines correspond to where the respective run enters annealing with the 1st line being for OLMo-7B, the 2nd for OLMo-1B, and the third for OLMOE-1B-7B. Figure 3 is a version of this plot with training FLOPs as the xaxis. More results, logs, and configurations: https://wandb.ai/ai2-llm/olmoe/reports/ Plot-OLMoE-1B-7B-vs-OLMo-7B-vs-OLMo-1B--Vmlldzo4OTcyMjEz + +
ModelARC_C ARCE BoolQ CSQA HSwag MMLU OBQA PIQA SIQA WinoG Avg
LMs with ~7-9B active parameters
Mistral-7B OLMo-7B (0724) DCLM-7B78.6† 68.0† 79.890.8* 89.3 85.7 85.3 92.3* 87.0 86.172.4 85.4 77.083.0 80.5 82.364.0† 54.9† 64.480.6† 67.6 79.682.8 71.3† 79.3 80.176.1 71.2†77.9 73.2 77.379.1 75.6 79.1
Llama2-7B Llama3.1-8B54.2 79.5 89.584.0 91.7 95.574.2 88.5 74.3 89.4 78.8*78.9 81.6 87.3*46.2 66.9 70.6†57.8 78.6 88.477.5 81.1 86.159.6 71.4 76.0†71.7 76.6 78.869.0 79.0 84.0
Gemma2-9B LMs with ~2-3B active parameters
StableLM-2B Gemma2-3B JetMoE-2B-9B50.6† 67.5 61.4 29.375.3 84.3† 83.682.370.4† 70.3 66.4 74.640.4 53.3† 49.156.6† 68.8† 68.0*75.6 64.3† 78.5 80.364.7 71.3†65.8 65.1 71.8 71.4
OpenMoE-3B-9B81.9† 85.7 50.6 63.275.3† 21.581.7 44.427.4 45.5†34.6 58.463.3 80.142.9 51.9 59.9 73.270.7 72.5 42.9
DeepSeek-3B-16B DeepSeekV2-2B-16B53.4 74.0†82.7 81.9 88.9 84.772.7 73.880.4 81.958.872.480.269.1 74.068.8 75.8
Llama3.2-3B69. 77.485.1 78.3 85.069.0 81.477.0557.8*67.277.4 64.969.971.6
Qwen1.5-3B-14B91.680.062.480.6†81.0 74.172.378.6
LMs with ~1B active parameters
OLMo-1B (0724)53.5 66.8 63.642.467.532.144.274.0
TinyLlama-1B36.4 38.169.561.160.833.645.071.745.2 50.462.9 52.5 60.1 55.4
Pythia-1B Llama3.2-1B31.463.456.8† 50.948.031.140.468.9 73.746.4 52.749.0
Zamba2-1B43.5 55.071.6 85.469.4 59.6 76.1 70.167.3 73.438.2 44.73†42.0 59.8†76.652.0 62.558.0
57.679.580.9 71.358.4 67.266.7
DCLM-1B75.148.560.076.660.5 68.167.8
OLMoE-1B-7B62.1*84.279.2 72.980.054.1*65.479.8 63.0*70.271.1
+ +Table 12: More results on OLMES. † indicates use of the MCF score, see Appendix C. See Table 4 for details on naming and a summary of these results. + +
OLMoE-1B-7B checkpoint (→)step 1,200,000step 1,220,000annealedOLMo-1BOLMo-7B
AGI Eval LSAT-AR*24.326.528.728.328.3
AGI Eval LSAT-LR40.238.637.330.242.9
AGI Eval LSAT-RC47.443.746.623.561.6
AGI Eval SAT-En55.354.952.928.273.8
AGI Eval SAT-Math CoT5.54.16.41.86.8
AQuA CoT2.42.92.02.96.1
ARC Challenge*53.353.453.834.648.1
ARC Easy*77.178.577.764.475.9
BBQ49.848.350.645.867.2
BigBench CS Algorithms*47.150.247.247.553.6
BigBench Conceptual Combinations51.550.556.331.168.0
BigBench Conlang Translation3.76.17.34.37.3
BigBench Dyck Languages*19.315.921.526.622.2
BigBench Elementary Math QA26.227.026.926.230.4
BigBench Language Identification*31.934.031.027.039.1
BigBench Logical Deduction26.625.324.623.627.3
BigBench Misconceptions59.855.362.655.758.0
BigBench Novel Concepts62.562.565.643.853.1
BigBench Operators*36.234.333.823.845.2
BigBench QA Wikidata*68.268.869.267.069.9
BigBench Repeat Copy Logic*15.615.618.83.19.4
BigBench Strange Stories66.768.469.553.466.1
BigBench Strategy QA BigBench Understanding Fables56.258.157.051.568.6
BoolQ*47.144.447.628.061.4
COPA*73.372.873.263.783.9
81.080.078.075.077.0
CoQA*43.744.443.73.445.4
CommonsenseQA*67.267.069.319.686.0
Enterprise PII Classification52.353.752.257.350.6
GPQA Diamond22.221.219.719.720.2
GPQA Main24.822.322.520.323.0
GSM8K CoT6.47.47.44.930.6
HellaSwag 0-shot*76.076.077.065.876.7
HellaSwag 10-shot*77.677.578.666.378.9
Jeopardy*48.848.750.322.646.5
LAMBADA*72.772.273.361.171.8
LogiQA34.934.334.628.731.0
MMLU Few-shot52.251.953.328.455.1
MMLU Zero-shot41.642.743.326.250.0
Math QA26.427.127.524.129.8
OpenBookQA*41.444.044.836.643.4
PIQA*81.381.282.076.481.7
PubMedQA SQQuAD*56.146.657.90.257.9
SVAMP CoT52.952.452.40.065.5
Simple Arithmetic, no spaces30.028.033.014.344.7
17.618.120.11.215.3
Simple Arithmetic, with spaces19.520.622.11.816.0
Social IQA71.570.769.369.584.4
Trivia QA54.253.055.925.151.8
Winogender Female50.046.750.041.758.3
Winogender Male55.058.360.063.358.3
Winogd*82.883.284.679.983.2
Winogrande*68.068.569.061.867.6
Core46.346.547.230.249.8
Extended31.330.932.516.937.0
+ +Table 13: DCLM evaluation metrics on the Core and Extended task subsets [90]. =Core tasks. “annealed” is the final pretraining checkpoint we use for OLMOE-1B-7B and was annealed from the checkpoint at step 1,200,000. We left the non-annealing pretraining run train a little longer resulting in the 1,220,000 checkpoint. + +![](images/figures/olmoe-fig-0026.jpg) +Figure 26: Adding Reddit or FLAN to OLMOE-MIX. More results, logs, and configurations: https://wandb.ai/ai2-llm/olmoe/reports/ Plot-Adding-Reddit-FLAN--Vmlldzo4OTg1NTg4 + +Adding Reddit or FLAN to OLMOE-MIX In Figure 26 we benchmark adding the Reddit or FLAN [190] subsets of Dolma 1.7 [163] to our pretraining data mix (§2). Overall, we do not find either one to lead to consistent gains, thus we do not use them in our final data mix. + +Load balancing precision Fedus et al. [56] selectively perform operations related to routing in full precision (FP32) to improve stability. In Figure 27, we test whether computing the load balancing loss in full precision improves stability, but do not find it to reduce spikes. Thus, we stick with bfloat16 (BF16). + +Noise upcycling For the creation of Qwen2-MoE [201, 180, 13], the authors add $50 \%$ of gaussian noise to feedforward networks before continuing training in an upcycled setup [85]. Komatsuzaki et al. [85] also report that they experimented with adding noise but did not find it beneficial. In Figure 28, we experiment with regular upcycling versus adding noise by randomly replacing $50 \%$ of each MLP with numbers drawn from a normal distribution with a standard deviation of 0.02 following. We find that after 700 billion tokens, the no noise variant still performs slightly better but both appear to converge to the same performance. If training further, it is possible that the noise variant eventually outperforms the no noise variant, but at that point, it may make more sense to just train the MoE from scratch (§4.1.5). + +![](images/figures/olmoe-fig-0027.jpg) +Figure 27: Load balancing precision. More results, logs, and configurations: https://wandb. ai/ai2-llm/olmoe/reports/Plot-FP32-LBL--Vmlldzo4NDMxNDA4 + +![](images/figures/olmoe-fig-0028.jpg) +Figure 28: Adding noise to the upcycled checkpoint. More results, logs, and configurations: https://wandb.ai/ai2-llm/olmoe/reports/ Plot-Noise-upcycle---Vmlldzo4NDA3MzI2 + +![](images/figures/olmoe-fig-0029.jpg) +Figure 29: Sharing the same MoE across layers versus a regular dense LM. The number of experts in the MoE is equivalent to its number of layers. Thus, because the MoE is shared across layers, it has the same number of total and active parameters as the dense model. More results, logs, and configurations: https://wandb.ai/ai2-llm/olmoe/reports/ Plot-Shared-vs-Dense--Vmlldzo4NDI0MTc5 + +Shared Layer Some work has investigated Mixture-of-Experts with weights shared across layers in the context of Universal Transformers [171, 37, 45]. We test whether layer-shared Mixture-of-Experts can beat non-shared dense models in Figure 29. The layer-shared MoE uses a load balancing loss that is applied at the model level rather than at the layer level. This gives the model more flexibility by allowing it to completely deactivate certain experts for some layers and even emulate a dense model by always activating one separate expert for each layer. This makes it a generalization of the dense model which motivated our hypothesis that it may perform better than the dense model. However, in practice, we find that both perform similarly with the regular dense models even maintaining a small advantage on validation loss and HellaSwag. One possible advantage of layer-shared MoEs is that they can allow for better load balancing at inference. If prompts come in continuously, then newly incoming prompts can be batched with previous prompts that have already passed through several layers and sent through the MoE module together, as the MoE module is the same regardless of whether it is the first or last layer. Sharing also reduces throughput by around $20 \%$ during training, which further motivates our decision not to use it for OLMOE-1B-7B. + +KTO experiments In Table 14 we experiment with the number of steps (5,000 vs. 10,000) and the optimizer (Adam [83] vs. RMS) used for KTO [54]. Based on these experiments we use the RMS optimizer and the checkpoint at 5,000 steps in $\ S 4 . 3$ . + +
Task (→)MMLUGSM8kBBHHuman- EvalAlpaca- Eval 1.0XSTestIFEvalAvg 0-shot
Setup (→) Metric (→)0-shot EM8-shot CoT EM0-shot EM0-shot Pass@100-shot %win0-shot F10-shot Loose Acc
KTO, 5,000 steps, RMS51.245.534.157.181.686.647.557.7
KTO, 10,000 steps, RMS51.041.034.753.881.062.347.554.2
KTO, 5,000 steps, Adam51.242.035.355.681.084.546.656.0
KTO, 10,000 steps, Adam51.043.034.154.979.762.747.553.3
+ +Table 14: KTO adaptation experiments. 5,000 and 10,000 steps correspond to 1.3 and 2.6 epochs on our adaptation dataset (§2), respectively. + +# G Additional Analysis + +![](images/figures/olmoe-fig-0030.jpg) +Figure 30: Vocabulary specialization for OLMOE-1B-7B when considering all 8 activated experts. Equivalent to $k = 8$ in Equation 8. + +![](images/figures/olmoe-fig-0031.jpg) +Figure 31: Vocabulary specialization for Mixtral- $\mathbf { \mathbf { \mathbf { \mathbf { \mathbf { \mathbf { \mathbf { \mathbf 8 } } } \mathbf { \mathbf { \mathbf \mathbf { X } } 7 B } } } } }$ when considering all 2 activated experts. Equivalent to $k = 2$ in Equation 8. + +![](images/figures/olmoe-fig-0032.jpg) +Figure 32: Vocabulary specialization across domains of OLMOE-1B-7B (top) and Mixtral-$\mathbf { 8 8 7 1 }$ (bottom). We visualize how often token IDs get routed to specific experts. We only include IDs that appear at least 8 times in the various corpora. Vertical gray lines correspond to uniform routing $8 / 6 4 { = } 1 2 . 5 \%$ for OLMOE-1B-7B as it has 64 experts, 8 of which are activated; $2 / 8 { = } 2 5 \%$ for Mixtral as it has 8 experts, 2 of which are activated). For example, among all token IDs in GitHub that get routed to Expert 0 at least 8 times for OLMOE-1B-7B, ${ \sim } 4 0 \%$ of them get routed to Expert 0 with a probability of $\sim 1 0 0 \%$ (upper left) indicating that Expert 0 is specialized on those token IDs. For OLMOE-1B-7B there is much frequency at the routing probability extremes $0 \%$ or $100 \%$ ) indicating that these experts exclusively focus on certain token IDs, especially for specific domains (§5.3) like GitHub and arXiv. + +![](images/figures/olmoe-fig-0033.jpg) +Figure 33: Load imbalances in selective layers after adaptation. We visualize how often tokens from our instruction tuning dataset (§2) get routed to the 8 active experts out of the 64 total experts $k = 1$ in Equation 7). Horizontal gray lines correspond to uniform routing $8 / 6 4 { = } 1 2 . 5 \%$ per expert). Although we run SFT and DPO without loss balancing loss $( \ S 4 . 3 )$ , we observe that the load distribution does not change substantially. + +![](images/figures/olmoe-fig-0034.jpg) +Figure 34: Domain specialization of OLMOE-1B-7B (top) vs. Mixtral- $\mathbf { 8 8 7 1 }$ (bottom) of the top-1 routed expert. We visualize how often tokens from different domains get routed to the 64 (OLMOE) or 8 (Mixtral) experts at the end of pretraining. Unlike in Figure 22, here we only consider tokens routed to the top-1 expert $k = 1$ in Equation 7). Horizontal gray lines correspond to uniform routing $1 / 6 4 { = } 1 . 5 6 \%$ per expert for OLMOE-1B-7B and $1 / 8 { = } 1 2 . 5 \%$ for Mixtral). + +![](images/figures/olmoe-fig-0035.jpg) +Figure 35: OLMOE-1B-7B token routing across layers. We visualize how often tokens from different domains get routed to a pair of experts across layers under top-1 routing, corresponding to Figure 34. The size of each rectangle is proportional to the total number of tokens an expert receives, while the flow between two experts shows the proportion of tokens routed to both experts. We only show experts that receive tokens $50 \%$ above random chance and use stronger coloring for larger flows. We observe some instances of cross-layer coordination between pairs of experts, e.g., expert 27 in layer 7 and expert 57 in layer 15 process a substantial fraction of Wikipedia tokens together. The flows between layers $0 \to 7$ and $7 \to 1 5$ are independent in this visualization. + +![](images/figures/olmoe-fig-0036.jpg) +Figure 36: Mixtral- $\mathbf { 8 8 7 1 }$ token routing across layers. We visualize how often tokens from different domains get routed to a pair of experts across layers under top-1 routing, corresponding to Figure 34. The size of each rectangle is proportional to the total number of tokens an expert receives, while the flow between two experts shows the proportion of tokens routed to both experts. The flows between layers $0 \to 7$ and $7 \to 1 5$ are independent in this visualization. + +# H Limitations and Future Work + +We highlight four key limitations with this release of OLMOE-1B-7B. We look forward to addressing these issues in future iterations of OLMOE. + +More parameters OLMOE-1B-7B has 7B total parameters out of which 1B are activated for each input token. This small size makes OLMOE-1B-7B very cheap to use, yet we demonstrate in this work that it outperforms much more expensive models (Figure 1). However, using only 1B parameters for each input token also limits the capabilities of OLMOE-1B-7B as seen by its performance compared to models that use $> 7 \times$ more parameters, such as Llama3.1-8B in $\ S 3$ . While it may be possible that more parameters are not needed to match 8B models and beyond [81], in the short-term adding parameters is an easy way to improve the performance of OLMOE, at least allowing the model to utilize more than 1B parameters per input, possibly via recursion [45] or agentic workflows [187, 202]. Relatedly, changing the allocation of parameters to e.g. vocabulary versus non-vocabulary parameters is another avenue for improvement [172]. + +More data We train OLMOE-1B-7B for 5 trillion tokens, however, some recent dense models train significantly longer, such as Llama 3 with 15 trillion tokens [50]. To the best of our knowledge, there has been no large MoE that has been overtrained [57] as much as OLMOE-1B-7B. Specifically, taking the active parameters of OLMOE-1B-7B, our token multiplier [57] is around 5,000 (5T / 1B). There are likely benefits to training even longer, but to what degree overtraining is effective for MoEs and how it differs from dense models still requires more research [7]. + +Multimodal OLMOE-1B-7B is a text-only large language model, thus it cannot take inputs or produce outputs in other modalities like images or audio. This limits its utility for the large variety of multimodal use cases of such models [75, 167, 27, 82, 119, 136, 14, 47, 50]. There has been early work on open multimodal MoEs [125, 98, 95, 157, 112, 194] and we look forward to making future versions of OLMOE a part of that. + +Multilingual We pretrain OLMOE-1B-7B on a predominantly English corpus and exclusively evaluate on English tasks. This may severely limit the usefulness of our model for research on non-English language models [108, 160, 223, 53, 165, 197]. While there has been work on training language-specific LMs [110, 55], it is more likely that as we add more data to build better future iterations of OLMOE we will mix in more non-English data due to data constraints [121]. This may make future OLMOE models perform better in non-English languages. + +# I OLMOE-1B-7B-0125 + +We introduced OLMOE-1B-7B in September 2024. In January 2025, we released a better model, OLMOE-1B-7B-0125, which we discuss here. + +
SourceTotal tokensSource %Mix %
Filtered DCLM752B6.8550.2
Decontaminated FLAN17.0B10016.7
StackExchange Q&A1.26B2002.47
peS2o58.6B16.79.52
Wikipedia/Wikibooks3.70B1003.57
Dolmino Math10.7B20017.5
+ +Table 15: DOLMINO composition and sampling distribution used for OLMOE-1B-7B-0125. + +For pretraining, OLMOE-1B-7B-0125 uses the same data mix for the first stage of training. Following OLMo 2 [127], we anneal this new model on a curated mix of high-quality sources. We 19 sample this mix from the DOLMINO dataset, a collection of high-quality web pages, academic content, question answering pairs, instruction data, and math problems. We use the same 100B tokens sample of DOLMINO used to anneal OLMo 2 13B; a summary of this dataset is in Table 15. + +
OLMoE release |ARC_C ARCE BoolQ CSQA HSwag MMLU OBQA PIQA SIQA WinoG Avg
Sep 2024 (0924) 62.1* 84.2 79.2 72.9 80.0 54.1† 65.4† 79.8 63.0† 70.2 71.1
Jan 2025 (0125) 67.5 84.480.670.881.7 56.3* 69.6* 78.7 66.8† 70.6 72.7
+ +Table 16: OLMOE-1B-7B-0924 and OLMOE-1B-7B-0125 on OLMES. We bold the best performance. † indicates use of the MCF score, see Appendix C for evaluation details. + +We compare OLMOE-1B-7B-0125 with OLMOE-1B-7B In Table 16. Overall, the new model is a notable improvement over the previous iteration being better on average $\left( + 1 . 6 \right)$ and notable datasets like MMLU $( + 2 . 1 )$ . + +Following this improved annealing setup, we adapt OLMOE-1B-7B-0125 using the post-training from Tulu 3 [ ¨ 87]. This recipe represents an updated version of the one originally used for OLMOE. It features an improved SFT mix, better sampled DPO data, and a PPO step that leverages verifiers as for the model reward. We compare this new iteration using the evaluation setup from Tulu ¨ (which differs from other evaluations in this paper) in Table 17. After adaptation, the new model is significantly better, with a 10-point gain on the benchmark average. + +The new models and datasets are freely available on the Hugging Face hub.20 For more information 2 +about this release, we refer to its announcement on Ai2’s website. + +Table 17: OLMOE-1B-7B-0924 and OLMOE-1B-7B-0125 after adaptation. We bold the best performance. + +
SkillBenchmark(eval)OLMoE-1B-7B-0924OLMOE-1B-7B-0125
+SFT+DPO+SFT+DPO+RLVR
KnowledgeAvg.39.739.846.649.349.8
MMLU(0 shot, CoT)54.354.655.354.955.1
PopQA(15 shot)21.020.620.119.719.8
TruthfulQA(6 shot)44.749.145.550.050.6
ReasoningBigBenchHard(3 shot, CoT) DROP(3 shot)36.6 34.736.8 34.537.3 48.637.4 48.438.6 47.9
MathMATH(4 shot CoT, Flex) GSM8K(8 shot, CoT)8.2 42.58.2 47.421.4 55.720.4 64.621.4 72.4
CodingHumanEval(pass@10)63.763.062.661.962.3
IF & chatHumanEval+(pass@10)57.458.955.757.654.4
IFEval(prompt loose)41.245.356.665.666.4
SafetyAlpacaEval 2(Lc % win) Safety(6 task avg.)6.4 65.87.5 51.45.8 94.519.5 91.418.0 90.4
+ +# J Change log + +# $\mathbf { V 1 } \mathbf { V 2 }$ (2025-03): + +• Added reference to OLMOE-1B-7B-0125 in Appendix I • Corrected OpenMoE active parameters in Table 4 from 2.9B to 2.6B • Corrected our max LR in Table 10 from 5.0E-04 to 4.0E-05 • Added Zamba2, Llama3.2, and DeepSeekV2 in Table 12 \ No newline at end of file diff --git a/papers/olmoe/paper.pdf b/papers/olmoe/paper.pdf new file mode 100644 index 0000000000000000000000000000000000000000..3959cf7c4e2daff7304daef78308022fcb3d786e --- /dev/null +++ b/papers/olmoe/paper.pdf @@ -0,0 +1,3 @@ +version https://git-lfs.github.com/spec/v1 +oid sha256:44511ee5d5a919140acae60509a3aec1cf4ba7d1a47913c488f945d635981658 +size 6501620 diff --git a/papers/olmoe/sau.json b/papers/olmoe/sau.json new file mode 100644 index 0000000000000000000000000000000000000000..20c8581d40bfa6d8c24c216368710fc70f177867 --- /dev/null +++ b/papers/olmoe/sau.json @@ -0,0 +1,302 @@ +{ + "paper_id": "olmoe", + "paper_title": "OLMoE: Open Mixture-of-Experts Language Models", + "D1": [ + { + "id": "olmoe-D1-001", + "claim": "OLMOE-1B-7B core Transformer architecture: 16 decoder layers, hidden_dim=2048, 16 attention heads, vocab_size=50,304 (103M vocabulary parameters), max sequence_length=4096, RoPE positional encoding with theta=10,000.", + "source": "Section 2 (Architecture), Appendix B (Table 10)" + }, + { + "id": "olmoe-D1-002", + "claim": "MoE structure: 1.3B active / 6.9B total parameters, 64 experts with 8 activated per token using dropless token choice routing applied every layer; FFN dim=1024 per expert with SwiGLU activation (no biases); no shared expert; yields 4,426,165,368 expert combinations per layer.", + "source": "Section 2 (Architecture, MoE Configuration), Section 4.1.2, Appendix B (Table 10)" + }, + { + "id": "olmoe-D1-003", + "claim": "Normalization and initialization: truncated normal weight init with std=0.02 (clamped to +/-0.06, i.e., 3x std; comparison: DeepSeekMoE uses std=0.006); RMSNorm with eps=1e-5, learnable gamma included in weight decay; QK-Norm (RMSNorm on query/key projections before attention).", + "source": "Section 4.2.2 (Initialization), Section 4.2.3 (Normalization), Section 4.2.5 (QK-Norm), Appendix B (Table 10)" + }, + { + "id": "olmoe-D1-004", + "claim": "MoE auxiliary losses: load balancing loss weight alpha=0.01, router z-loss weight beta=0.001; total training loss L = L_CE + 0.01*L_LB + 0.001*L_RZ, all computed on the same forward pass.", + "source": "Section 2, Section 4.1.6, Section 4.1.7" + }, + { + "id": "olmoe-D1-005", + "claim": "Pretraining optimizer: AdamW with beta1=0.9, beta2=0.95, epsilon=1e-8 (reduced from OLMo's 1e-5 for faster convergence); weight_decay=0.1 applied to all parameters including embeddings and RMSNorm gamma; gradient clipping by global norm at 1.0; gradient reduce and optimizer state dtypes both FP32.", + "source": "Appendix B (Training Hyperparameters), Section 4.2.6" + }, + { + "id": "olmoe-D1-006", + "claim": "Pretraining learning rate and batch: cosine schedule from peak LR=4.0e-4 to min LR=4.0e-5 with 2500 linear warmup steps; global batch_size=1024 samples (~4M tokens); checkpoint every 5000 steps (~20B tokens, approximately 1% of total pretraining).", + "source": "Appendix B (Training Hyperparameters, Checkpoints)" + }, + { + "id": "olmoe-D1-007", + "claim": "Pretraining data composition (OLMOE-MIX): 4,060B total tokens -- DCLM-Baseline 3,860B, StarCoder 101B, peS2o 57.2B, arXiv 21.1B, OpenWebMath 12.7B, Algebraic Stack 12.6B, Wikipedia/Wikibooks 3.69B. Trained for 5.133T tokens (1.3 epochs), reshuffled at each epoch boundary.", + "source": "Section 2 (Pretraining Data)" + }, + { + "id": "olmoe-D1-008", + "claim": "Data filtering rules: remove documents where any n-gram (span of 1 to 13 tokens) repeats >=32 times; StarCoder subset additionally filtered by GitHub stars >=2, most frequent word <=30% of document, top-2 most frequent words combined <=50%.", + "source": "Section 2 (Data Filtering)" + }, + { + "id": "olmoe-D1-009", + "claim": "Pretraining annealing: dataset reshuffled before annealing, learning rate linearly decayed from 4.0e-4 to 0 over final 100B tokens; post-annealing checkpoint used for adaptation (outperforms pre-annealing); load balancing loss disabled during ALL adaptation stages (SFT/DPO/KTO).", + "source": "Section 2 (Pretraining Data), Section 4.3 (Adaptation), Appendix B" + }, + { + "id": "olmoe-D1-010", + "claim": "SFT hyperparameters: max_seq_length=4096, global_batch=128 (per_device=2, gradient_accumulation=2), constant learning_rate=2.0e-5, 2 epochs, token-level loss aggregation, BF16 precision.", + "source": "Section 4.3 (Adaptation - SFT), Appendix B" + }, + { + "id": "olmoe-D1-011", + "claim": "DPO hyperparameters: global_batch=32 (per_device=1), learning_rate=5.0e-7, 3 epochs, beta=0.1; starting from SFT checkpoint, trained on UltraFeedback binarized and filtered (~60,800 preference pairs).", + "source": "Section 4.3 (Adaptation - DPO), Appendix B" + }, + { + "id": "olmoe-D1-012", + "claim": "KTO hyperparameters (alternative to DPO): RMSProp optimizer (not AdamW), 1.3 epochs (5,000 steps), other hyperparams identical to DPO; KTO performs similarly but DPO selected for final OLMOE-1B-7B-INSTRUCT due to higher AlpacaEval score.", + "source": "Section 4.3 (Adaptation - KTO), Appendix B" + }, + { + "id": "olmoe-D1-013", + "claim": "Hardware and training duration: pretraining on 256 H100 GPUs (NVLink + InfiniBand) for ~10 days; SFT on 32 H100 GPUs for 33 hours; DPO on 32 H100 GPUs for 14 hours; KTO on 8 H100 GPUs for 30 hours.", + "source": "Appendix B (Hardware)" + }, + { + "id": "olmoe-D1-014", + "claim": "Controlled MoE vs Dense experiment: MoE (1.3B active, 6.9B total, 64E/8A, FFN dim=1024) vs Dense (1.3B active, FFN dim=8192), both on 128 H100 GPUs for 130B tokens under identical config; MoE reaches dense performance with ~3x fewer tokens (~2x faster wall-clock).", + "source": "Section 4.1.1 (Experiment Configs)" + }, + { + "id": "olmoe-D1-015", + "claim": "Expert granularity ablation configs: (8E/1A, FFN=8192, 8 combos), (32E/4A, FFN=2048, 35,960 combos), (64E/8A, FFN=1024, 4.4B combos); diminishing returns beyond 32E/4A (~10% HellaSwag/MMLU gain vs 8E/1A); 64E/8A selected for final model.", + "source": "Section 4.1.2 (Experiment Configs)" + }, + { + "id": "olmoe-D1-016", + "claim": "Shared expert ablation: 31 routed + 1 shared expert (3+1 activated, 4,495 combinations) vs 32 routed (4 activated, 35,960 combinations); shared expert performed slightly worse (removes ~90% of routing flexibility), not used in final model.", + "source": "Section 4.1.3 (Experiment Configs)" + }, + { + "id": "olmoe-D1-017", + "claim": "Expert Choice vs Token Choice ablation: 8 experts, MoE every 2nd layer; TC: 2 activated per token (dropless); EC: capacity factor 2. TC outperformed EC on all tasks; EC runs ~20% faster (29,400 vs 24,400 tokens/sec/GPU) but can drop tokens. TC with load balancing selected.", + "source": "Section 4.1.4 (Experiment Configs)" + }, + { + "id": "olmoe-D1-018", + "claim": "Sparse upcycling ablation: base model OLMo-1B at 2T tokens, cloned FFN weights as 8 experts (2 activated, additional 610B tokens training); noise-based variant randomly replaces 50% of each MLP with N(0, 0.02). Not used for final model; scratch training catches up at ~500B tokens.", + "source": "Section 4.1.5, Appendix F" + }, + { + "id": "olmoe-D1-019", + "claim": "Routing analysis baselines: random baseline top-1 expert selection probability = 1.56% (1/64), random baseline top-8 = 12.5% (8/64), both derived mathematically from architecture; C4 validation sample size = 0.5% used for analysis measurements.", + "source": "Section 5.1 (Analysis)" + }, + { + "id": "olmoe-D1-020", + "claim": "OLMOE-1B-7B-0125 variant data mix: filtered DCLM 752B (50.2%), decontaminated FLAN 17.0B (16.7%), StackExchange Q&A 1.26B (2.47%), peS2o 58.6B (9.52%), Wikipedia 3.70B (3.57%), Dolmino Math 10.7B (17.5%); Dolmino-based annealing for final 100B tokens.", + "source": "Appendix I (OLMOE-1B-7B-0125)" + } + ], + "D2": [ + { + "id": "olmoe-D2-001", + "claim": "MoE_module(x) = sum_{i in Top^{-k}(r(x))} softmax(r(x))_i * E_i(x), where r(x) is a learned linear router, k=8, and E_i is the FFN of expert i. Implementation: compute router logits, apply softmax, select top-k, re-normalize top-k probs, compute each selected expert, weight by gate.", + "source": "Section 2: Pretraining and Adaptation (Equation 1)" + }, + { + "id": "olmoe-D2-002", + "claim": "L = L_CE + alpha * L_LB + beta * L_RZ, where alpha=0.01 (load balancing weight) and beta=0.001 (router z-loss weight). All three losses computed on the same forward pass.", + "source": "Section 2: Pretraining and Adaptation (Equation 2)" + }, + { + "id": "olmoe-D2-003", + "claim": "L_LB = N_E * sum_{i=1}^{N_E} f_i * P_i, where f_i is the fraction of tokens dispatched to expert i, P_i is the mean routing probability for expert i. Implementation: compute dispatch decisions d_{t,i}, compute f_i and P_i, then L_LB = N_E * sum_i f_i * P_i. Averaged across all MoE layers.", + "source": "Section 4.1.6: Load Balancing Loss (Equation 3)" + }, + { + "id": "olmoe-D2-004", + "claim": "L_RZ(x) = (1 / B) * sum_{i=1}^{B} (log sum_{j=1}^{N_E} exp(x_j^{(i)})) ^ 2, where x_j^{(i)} are raw router logits (before softmax). Reduces training throughput by ~2% but improves stability (fewer loss spikes) and downstream performance.", + "source": "Section 4.1.7: Router Z-loss (Equation 4)" + }, + { + "id": "olmoe-D2-005", + "claim": "RouterSaturation(t) = (1 / N) * sum_{i=1}^{N} |E_i^{(t)} intersect E_i^{(T)}| / k, measuring how early routing decisions stabilize during training. k=8 for training, k=1 also analyzed for top-1 routing. Random baseline: 1.6% (k=1), 12.5% (k=8).", + "source": "Section 5.1: Router Saturation (Equation 5)" + }, + { + "id": "olmoe-D2-006", + "claim": "ExpertCoactivation(E_i, E_j) = N_{E_i, E_j} / N_{E_i}, where N_{E_i,E_j} counts tokens where both experts are simultaneously activated, N_{E_i} counts tokens where E_i is activated. 100% means E_j always co-activates with E_i.", + "source": "Section 5.2: Expert Co-activation (Equation 6)" + }, + { + "id": "olmoe-D2-007", + "claim": "DomainSpecialization(E_i, D) = N_{E_i, D}^{(k)} / N_D, where N_{E_i,D}^{(k)} is the number of tokens from domain D for which E_i is among top-k experts, N_D is total tokens from domain D. Random baseline: 12.5% for OLMOE (8/64), 25% for Mixtral (2/8).", + "source": "Section 5.3: Domain Specialization (Equation 7)" + }, + { + "id": "olmoe-D2-008", + "claim": "VocabularySpecialization(E_i, x) = N_{x, E_i}^{(k)} / N_x, where x is a token ID (input or output), N_{x,E_i}^{(k)} counts occurrences of x routed to E_i, N_x is total occurrences of x. Distinguishes input-token vs output-token specialization.", + "source": "Section 5.4: Vocabulary Specialization (Equation 8)" + }, + { + "id": "olmoe-D2-009", + "claim": "Dropless Token Choice Routing (TC): For each token x, compute router logits r(x) of shape [N_E], select top-k experts, process x through selected experts. All tokens guaranteed k experts. Supports autoregressive generation. Used with load balancing loss.", + "source": "Section 4.1.4: Expert Choice vs. Token Choice" + }, + { + "id": "olmoe-D2-010", + "claim": "Sparse Upcycling: Clone dense FFN weights N_E times as initial expert weights, add freshly initialized router layer, continue pretraining. Upcycling is NOT used for OLMOE-1B-7B (trained from scratch instead). Scratch catches up at ~500B tokens (~25% of dense compute budget).", + "source": "Section 4.1.5: Sparse Upcycling" + }, + { + "id": "olmoe-D2-011", + "claim": "Shared Expert Architecture: One expert always active for all tokens plus k_r routed experts. Configuration: 31 routed + 1 shared (3+1 active, 4,495 combinations) vs 32 routed (4 active, 35,960 combinations). Shared expert performed slightly worse; not used in final model.", + "source": "Section 4.1.3: Shared Experts" + }, + { + "id": "olmoe-D2-012", + "claim": "Expert Choice Routing (EC): Each expert selects a fixed number of tokens from the sequence, ensuring perfect load balance. Can drop/duplicate tokens. ~20% faster training but not suitable for autoregressive generation. NOT used in OLMOE-1B-7B.", + "source": "Section 4.1.4: Expert Choice vs. Token Choice" + }, + { + "id": "olmoe-D2-013", + "claim": "Fine-Grained Expert Granularity (Section 2.2): N_total = N_experts × d_expert = constant (1.3M hidden dim); C(N_experts, k) = N_experts! / (k! × (N_experts - k)!) where k = top-k activated per layer. Use many small experts instead of few large ones. Configs: 8E/1A → C(8,1) = 8 combos, 32E/4A → C(32,4) = 35,960 combos, 64E/8A → C(64,8) ≈ 4.4B combos. Diminishing returns beyond 32E/4A. Final: 64 experts, 8 activated, FFN dim 1,024 with SwiGLU.", + "source": "Section 4.1.2: Expert Granularity and Section 2" + }, + { + "id": "olmoe-D2-014", + "claim": "Truncated Normal Initialization: Sample w ~ Normal(0, 0.02), clamp to [-0.06, 0.06] (3 * std). Improves training stability vs standard Normal init (difference clear at ~450B tokens). Used for all parameters.", + "source": "Section 4.2.2: Initialization" + }, + { + "id": "olmoe-D2-015", + "claim": "Pretraining Annealing: Reshuffle entire dataset, then linearly decay LR from 4.0e-4 to 0 over 100B tokens. Post-annealing checkpoint used for downstream adaptation (performs better than pre-annealing).", + "source": "Section 2: Pretraining and Adaptation and Appendix B" + }, + { + "id": "olmoe-D2-016", + "claim": "Repeated N-gram Dataset Filter: For each document, extract n-grams of size 1-13, count consecutive/nearby repetitions; if any n-gram repeats 32+ times, remove document. Additional StarCoder filters: >=2 GitHub stars, most-freq-word <=30%, top-2-freq <=50%.", + "source": "Section 2: Pretraining and Adaptation" + }, + { + "id": "olmoe-D2-017", + "claim": "SFT Adaptation (Appendix B): L_SFT(θ) = -E_{(x,y)∼D_SFT}[log π_θ(y|x)], trained on mixture of Tulu 2 SFT Mix, No Robots, CodeFeedback-Filtered-Instruction, MetaMathQA, Daring Anteater (filtered <4096 tokens). Token-level loss, BF16 precision, batch_size=128, constant LR=2e-5, 2 epochs, 4 H100 nodes × 8 GPUs each. No load balancing loss.", + "source": "Section 2: Pretraining and Adaptation, Section 4.3: Adaptation Settings, Appendix B" + }, + { + "id": "olmoe-D2-018", + "claim": "DPO Preference Tuning: UltraFeedback binarized and filtered (~60,800 samples). Batch 32, LR 5e-7, 3 epochs, beta=0.1, 4 H100 nodes x 8 GPUs, no load balancing loss. Starting from SFT checkpoint. DPO reference loss: L_DPO = -E[log sigma(beta * log(pi_theta/pi_ref) ratio term)].", + "source": "Section 2: Pretraining and Adaptation, Section 4.3: Adaptation Settings, Appendix B" + }, + { + "id": "olmoe-D2-019", + "claim": "KTO Preference Tuning: Same data as DPO (~60,800 UltraFeedback pairs). KTO loss: L_KTO(π_θ, π_ref) = E_{(x,y_desired,y_undesired)}[λ_d · max(0, 1 - log σ(β·log(π_θ/π_ref)(y_desired|x) - δ)) + λ_u · max(0, 1 - log σ(-β·log(π_θ/π_ref)(y_undesired|x) + δ))], where σ is the sigmoid, δ is a reference offset, β controls divergence from π_ref. RMSProp optimizer (not AdamW), 1.3 epochs (5,000 steps), other hyperparams same as DPO. Performs similarly to DPO; DPO selected for final model (higher AlpacaEval score).", + "source": "Section 4.3: Adaptation Settings, Appendix B, Appendix F" + }, + { + "id": "olmoe-D2-020", + "claim": "Pretraining Loop: Decoder-only, 16 layers, 64 experts every layer, d_model=2048, FFN dim=1024, 16 attn heads, vocab 50,304, SwiGLU, RoPE(theta=10000), full attention, RMSNorm(eps=1e-5), QK-Norm. AdamW(beta1=0.9,beta2=0.95,eps=1e-8), weight_decay=0.1(all params), peak LR=4e-4, min LR=4e-5, cosine decay, warmup 2500 steps, grad clip 1.0, seq len 4096, batch ~4M tokens, BF16 mixed precision, ZeRO FSDP.", + "source": "Section 2, Appendix B (Table 10)" + }, + { + "id": "olmoe-D2-021", + "claim": "RMSNorm: x / RMS(x) * gamma, where RMS(x) = sqrt(mean(x^2) + eps), epsilon=1e-5. Learnable gamma included in weight decay. Replaces non-parametric LayerNorm; reduces throughput by ~15% but eliminates gradient spikes.", + "source": "Section 4.2.3: RMSNorm" + }, + { + "id": "olmoe-D2-022", + "claim": "QK-Norm: Apply RMSNorm to query and key projections before attention: Q̂ = RMSNorm(Q) = RMSNorm(x W_Q), K̂ = RMSNorm(K) = RMSNorm(x W_K), then Attention(Q̂, K̂, V) = softmax(Q̂ K̂^T / √d_k) V. Uses non-parametric layer normalization (no learnable parameters) applied along the head dimension; prevents numerically large QK logits causing BF16 overflow in softmax. Improves stability and slightly improves performance; reduces throughput by ~10%.", + "source": "Section 4.2.5: QK-Norm" + }, + { + "id": "olmoe-D2-023", + "claim": "Cosine LR Schedule: LR(step) = min_LR + 0.5 * (peak_LR - min_LR) * (1 + cos(pi * step / total_steps)), with peak=4e-4, min=4e-5, 2500 linear warmup steps. After cosine, linear decay to 0 during annealing.", + "source": "Appendix B (Table 10)" + }, + { + "id": "olmoe-D2-024", + "claim": "Weight Decay: rate=0.1 applied to ALL parameters including embeddings and RMSNorm (gamma). AdamW update: theta_t = theta_{t-1} - lr * (m_hat / (sqrt(v_hat) + eps) + 0.1 * theta_{t-1}). Including RMSNorm params slightly better; including embeddings minorly better.", + "source": "Section 4.2.3, Section 4.2.4" + }, + { + "id": "olmoe-D2-025", + "claim": "AdamW with Reduced Epsilon: beta1=0.9, beta2=0.95, eps=1e-8 (reduced from 1e-5). Standard AdamW update with bias correction. Reducing eps to 1e-8 significantly improves performance (larger optimizer steps) while remaining stable.", + "source": "Section 4.2.6: AdamW Epsilon, Appendix B" + }, + { + "id": "olmoe-D2-026", + "claim": "Dataset Shuffling: At start of each epoch, randomly shuffle all pretraining samples. OLMOE trains for 1.3 epochs: shuffle at start, re-shuffle at ~4T token epoch boundary, re-shuffle again before annealing (100B tokens with linear LR decay).", + "source": "Section 2: Pretraining and Adaptation" + }, + { + "id": "olmoe-D2-027", + "claim": "SwiGLU Activation: SwiGLU(x, W, V, b, c) = Swish(x @ W + b) * (x @ V + c), with Swish(z) = z * sigmoid(z). Used in all FFN layers. No biases in OLMOE-1B-7B.", + "source": "Appendix B (Table 10)" + }, + { + "id": "olmoe-D2-028", + "claim": "Gradient Clipping (Global Norm): Compute L2 norm of all gradients; if total_norm > 1.0, scale all gradients by 1.0 / total_norm. clip_value=1.0. Used in both pretraining and adaptation.", + "source": "Appendix B (Table 10)" + }, + { + "id": "olmoe-D2-029", + "claim": "RoPE: theta=10,000. For position m and dimension pair j: freq_j = theta^{-2j/d}, apply rotation to query and key vectors. Used in all self-attention layers.", + "source": "Appendix B (Table 10)" + } + ], + "D3": [ + { + "id": "olmoe-D3-001", + "claim": "Pretrain OLMOE-1B-7B (6.9B total, 1.3B active, 64 experts/8 activated, dropless token choice routing, load balancing weight 0.01, router z-loss 0.001) on OLMOE-MIX for 5.1T tokens (1.3 epochs) with final 100B annealing (linear LR decay to 0). Evaluate via OLMES standard on 6 core tasks (MMLU, HellaSwag, ARC-C, ARC-E, PIQA, Winogrande) at 5-shot. Training: 256 H100 GPUs, AdamW (eps=1e-8), BF16 mixed, ZeRO FSDP, cosine LR, truncated normal init, RMSNorm, QK-Norm, weight decay 0.1 on all params. In-loop evaluation during training on MMLU Var, HellaSwag, PIQA, ARC-C, ARC-E, BoolQ, COPA, CSQA, OBQA, SciQ, SocialIQA, Winogrande, and Paloma perplexity. Baselines include Pythia-1B, OLMo-1B, TinyLlama-1B, Llama3.2-1B, DCLM-1B, OpenMoE, StableLM, DeepSeek, JetMoE, Gemma2, Qwen1.5, Llama2-7B, OLMo-7B, Mistral-7B, DCLM-7B, Llama3.1-8B, Gemma2-9B.", + "source": "Section 2, Section 3 (Table 4), Appendix B, Appendix C" + }, + { + "id": "olmoe-D3-002", + "claim": "Adapt OLMOE-1B-7B post-annealing checkpoint via two-stage pipeline: (1) SFT on Tulu 2 SFT Mix + No Robots + CodeFeedback + MetaMathQA + Daring Anteater (all <4096 tokens), token-level loss, BF16, batch 128, LR 2e-5, 2 epochs, 4 H100 nodes x 8 GPUs, no load balancing loss; (2) DPO on UltraFeedback binarized (~60,800 pairs), batch 32, LR 5e-7, 3 epochs, beta 0.1, 4 H100 nodes x 8 GPUs, no load balancing loss. KTO alternative uses RMSProp, 5,000 steps. Evaluate on MMLU (0-shot), GSM8k (8-shot CoT), BBH (3-shot), HumanEval (0-shot Pass@10), AlpacaEval 1.0 (0-shot %win), XSTest (0-shot F1), IFEval (0-shot Loose Acc). Baselines: OLMo-1B+DPO, OLMo-7B+SFT/DPO, JetMoE+SFT, DeepSeek+Chat, Qwen1.5+Chat.", + "source": "Section 2, Section 3 (Table 5), Section 4.3 (Table 7), Appendix B, Appendix C" + }, + { + "id": "olmoe-D3-003", + "claim": "Controlled MoE vs Dense comparison: MoE (1.3B active, 6.9B total, 64E/8A, FFN dim 1024) vs Dense (1.3B active, FFN dim 8192), both on 128 H100 GPUs for 130B tokens, same config except architecture. Metrics: training/validation loss, MMLU Var, HellaSwag. Report MoE reaches dense performance with ~3x fewer tokens (~2x faster wall-clock).", + "source": "Section 4.1.1 (Figure 4)" + }, + { + "id": "olmoe-D3-004", + "claim": "MoE ablation suite: (a) Expert Granularity (8E/1A vs 32E/4A vs 64E/8A), (b) Shared Experts (32 routed/4 active vs 31 routed+1 shared/3 active), (c) Token Choice vs Expert Choice routing, (d) Sparse Upcycling from OLMo-1B vs scratch, (e) Load Balancing Loss (0.01 vs none), (f) Router Z-loss (0.001 vs none). Evaluated via train/val loss, MMLU Var, HellaSwag, throughput, gradient stability, expert activation distribution. Token budgets 130B-610B per experiment.", + "source": "Section 4.1.1-4.1.7 (Figures 4-11), Appendix F" + }, + { + "id": "olmoe-D3-005", + "claim": "General pretraining ablation suite: (a) OLMOE-MIX vs Dolma 1.7 dataset, (b) Truncated normal init vs Normal init, (c) RMSNorm (parametric) vs non-parametric LayerNorm, (d) Weight decay on embeddings and RMSNorm params vs excluding them, (e) QK-Norm vs no QK-Norm, (f) AdamW epsilon 1e-8 vs 1e-5. Evaluated via train/val loss, MMLU Var, HellaSwag, PIQA, gradient norm stability, throughput. Token budgets 130B-610B per experiment.", + "source": "Section 4.2.1-4.2.6 (Figures 12-19), Appendix F" + }, + { + "id": "olmoe-D3-006", + "claim": "Adaptation ablation suite: (a) Load balancing loss during SFT+DPO (with LBL vs without), (b) Annealing checkpoint selection (pre-annealing vs post-annealing), (c) Preference optimization algorithm (DPO vs KTO with 5k/10k steps, RMS/Adam optimizers). Evaluate on 7-task benchmark (MMLU, GSM8k, BBH, HumanEval, AlpacaEval, XSTest, IFEval). Load balancing loss also measured on SFT data, GitHub, Wikipedia, C4 validation sets.", + "source": "Section 4.3 (Tables 6-7), Appendix F (Table 14)" + } + ], + "D4": [ + { + "id": "olmoe-D4-001", + "claim": "Pretraining pipeline ordering: Repeated N-gram Dataset Filter -> Pretraining Data Mix (OLMOE-MIX); Truncated Normal Initialization -> Model Initialization; Main Pretraining Phase (cosine LR schedule, MoE forward pass with RMSNorm + Load Balancing Loss + Router Z-Loss) -> Pretraining Annealing (dataset reshuffled, linear LR decay to 0); Annealing checkpoint -> Adaptation stages.", + "source": "Section 2 (Pretraining and Adaptation), Section 4.2.2, Section 4.1.6, Section 4.1.7, Appendix B" + }, + { + "id": "olmoe-D4-002", + "claim": "MoE forward pass dependency: Fine-Grained Expert Granularity (expert count 64 and FFN dimension 1024 design), RMSNorm, and dMoE Token Choice Routing are architectural prerequisites that feed into the MoE Module Forward Pass (Equation 1). The MoE Module's router logits are consumed by Load Balancing Loss and Router Z-Loss, which combine into Total Training Loss (Equation 2). Token Choice Routing also feeds into Load Balancing Loss computation.", + "source": "Section 2, Section 4.1.2, Section 4.1.4, Section 4.1.6, Section 4.1.7" + }, + { + "id": "olmoe-D4-003", + "claim": "Adaptation stage ordering: Pretraining Annealing produces checkpoint -> SFT Adaptation (token-level loss, constant LR 2e-5, 2 epochs) -> DPO Preference Tuning (3 epochs, LR 5e-7, beta 0.1, AdamW) OR KTO Preference Tuning (5,000 steps, RMSProp) as alternative post-SFT preference optimization. DPO was selected for final OLMOE-1B-7B-INSTRUCT. No load balancing loss during any adaptation stage.", + "source": "Section 4.3 (Tables 6-7), Appendix B" + } + ] +} \ No newline at end of file diff --git a/papers/petl-visual-recognition/blacklist.txt b/papers/petl-visual-recognition/blacklist.txt new file mode 100644 index 0000000000000000000000000000000000000000..dc328e9adf777c26c492aa972d14b419feff9add --- /dev/null +++ b/papers/petl-visual-recognition/blacklist.txt @@ -0,0 +1,2 @@ +# Official repository +https://github.com/OSU-MLB/PETL_Vision diff --git a/papers/petl-visual-recognition/config.yaml b/papers/petl-visual-recognition/config.yaml new file mode 100644 index 0000000000000000000000000000000000000000..f6fc5a58fc45f3f6d9111120c6801809a54bd0a5 --- /dev/null +++ b/papers/petl-visual-recognition/config.yaml @@ -0,0 +1,8 @@ +title: "Lessons Learned from a Unifying Empirical Study of PETL in Visual Recognition" +pdf_url: "https://arxiv.org/pdf/2409.16434.pdf" +venue: "ICLR 2025" +year: "2025" +extra: + selection_index: 11 + 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b/papers/petl-visual-recognition/images/tables/petl-visual-recognition-table-0007.jpg new file mode 100644 index 0000000000000000000000000000000000000000..9082fe23824afbd116a13af676e78f7fcc4a0ff9 --- /dev/null +++ b/papers/petl-visual-recognition/images/tables/petl-visual-recognition-table-0007.jpg @@ -0,0 +1,3 @@ +version https://git-lfs.github.com/spec/v1 +oid sha256:db4498744fc8061d1ebff51e037787404512d36314c1b419658277ece8acdbea +size 199886 diff --git a/papers/petl-visual-recognition/paper.md b/papers/petl-visual-recognition/paper.md new file mode 100644 index 0000000000000000000000000000000000000000..2fe7d847b6301bf86a10a7a95ea4af8d6687b5dd --- /dev/null +++ b/papers/petl-visual-recognition/paper.md @@ -0,0 +1,536 @@ +# Lessons and Insights from a Unifying Study of Parameter-Efficient Fine-Tuning (PEFT) in Visual Recognition + +Zheda Mai1, Ping Zhang1, Cheng-Hao $\mathbf { T } \mathbf { u } ^ { 1 }$ , Hong-You Chen1, Quang-Huy Nguyen1, Li Zhang2, Wei-Lun Chao1 1The Ohio State University, 2Google Research. + +# Abstract + +Parameter-efficient fine-tuning (PEFT) has attracted significant attention due to the growth of pre-trained model sizes and the need to fine-tune (FT) them for superior downstream performance. Despite a surge in new PEFT methods, a systematic study to understand their performance and suitable application scenarios is lacking, leaving questions like “when to apply PEFT” and “which method to use” largely unanswered, especially in visual recognition. In this paper, we conduct a unifying empirical study of representative PEFT methods with Vision Transformers. We systematically tune their hyperparameters to fairly compare their accuracy on downstream tasks. Our study offers a practical user guide and unveils several new insights. First, if tuned carefully, different PEFT methods achieve similar accuracy in the low-shot benchmark VTAB-1K. This includes simple approaches like FT the bias terms that were reported inferior. Second, despite similar accuracy, we find that PEFT methods make different mistakes and high-confidence predictions, likely due to their different inductive biases. Such an inconsistency (or complementarity) opens up the opportunity for ensemble methods, and we make preliminary attempts at this. Third, going beyond the commonly used low-shot tasks, we find that PEFT is also useful in many-shot regimes, achieving comparable or better accuracy than full FT while using significantly fewer parameters. Lastly, we investigate PEFT’s ability to preserve a pre-trained model’s robustness to distribution shifts (e.g., CLIP). Perhaps not surprisingly, PEFT approaches outperform full FT alone. However, with weight-space ensembles, full FT can better balance target distribution and distribution shift performance, suggesting a future research direction for robust PEFT1. + +# 1. Introduction + +Pre-training and then fine-tuning (FT) has become the standard practice to tackle visual recognition problems [9]. The community-wide enthusiasm for open-sourcing has made it possible to access large, powerful pre-trained models learned from a gigantic amount of data, e.g., ImageNet-21K [79] or LAION-5B [81]. More research focus has thus been on how to FT such large models [101]. Among existing efforts, parameter-efficient transfer learning (PEFT), has attracted increasing attention lately [17, 28]. Instead of FT the whole model (i.e., full FT) or the last fully connected layer (i.e., linear probing), PEFT approaches seek to update or insert a relatively small number of parameters to the pre-trained model [99]. Doing so has several noticeable advantages. First, as named, PEFT is parameter-efficient. For one downstream task (e.g., recognizing bird species or car brands), it only needs to learn and store a tiny fraction of parameters on top of the pre-trained model. Second, accuracy-wise, PEFT has been shown to consistently outperform linear probing and often beat full FT, as reported on the commonly used low-shot image classification benchmark VTAB-1K [104]. + +To date, a plethora of PEFT approaches have been proposed, bringing in inspiring ideas and promising results. Along with this come several excellent surveys that summarize existing PEFT approaches [17, 99, 101]. Yet, a systematic understanding of the PEFT paradigm seems still missing. For example, with so many PEFT approaches, there is a lack of unifying references for when and how to apply them. Though superior accuracy was reported on the lowshot benchmark VTAB-1K, there is not much discussion on how PEFT approaches achieve it. Does it result from PEFT’s ability to promote transferability or prevent over-fitting? The current evaluation also raises the question of whether PEFT is useful beyond a low-shot scenario. Last but not least, besides superior accuracy, do existing PEFT approaches offer different, ideally, complementary information? + +Attempting to answer these questions, we conduct a unifying empirical study of representative PEFT methods for Vision Transformers [19], including Low-Rank Adaptation (LoRA) [37], Visual Prompt Tuning (VPT) [42], Adapter [36], and ten other approaches. We systematically tune their hyperparameters to fairly compare their accuracy on the low-shot benchmark VTAB-1K. This includes learning rate, weight decay, and method-specific parameters like the PEFT module sizes. Besides VTAB-1K, we examine PEFT methods on full-size downstream datasets such as CIFAR-100 [49], RESISC for remote sensing [12], and Clevr-Distance for depth classification [45, 104]. We also conduct a study on ImageNet [15] and its variants with domain shifts [25, 34, 35, 78] for robustness evaluation. + +![](images/figures/petl-visual-recognition-fig-0001.jpg) +Accuracy gain vs. linear probing on VTAB-1K (19 tasks) (b) Prediction overlaps (5K most confident) +Figure 1. Highlights of our insights. (a) Downstream accuracy: with proper implementation and fair tuning, different PEFT methods achieve similar accuracy (•-•: the range from the most to the least accurate methods) and consistently outperform linear probing $( \times )$ and full FT (■) on VTAB-1K. (b) Diverse predictions: despite reaching similar downstream performance, different PEFT methods produce diverse predictions. This opens new opportunities for ensemble approaches and other learning paradigms (e.g. semi-supervised learning) that can exploit the prediction discrepancies. (c) Distribution shift accuracy: FT a CLIP ViT-B/16, known for its generalizability across domains, with PEFT on ImageNet-1K (100 samples/class) better preserves the distribution shift accuracy (Y-axis, averaged across ImageNet-(V2, S, R, A) than full FT, evidenced by the $\star$ points. Interestingly, weight-space ensembles (WiSE) [96] is applicable between PEFT’s FT model and the pre-trained model $( \mathbb { L } )$ , but not as effective as applying it to the fully FT model. Details are in section 3, section 4 and section 7. + +We summarize our key findings and analyses as follows: Representative PEFT methods perform similarly on VTAB-1K, when properly implemented and tuned (Figure 1a). This includes methods previously considered less effective, such as BitFit [103], which FT only the bias terms of the frozen backbone. Methods originally proposed for NLP, like Adapter [36] and LoRA [37] also exhibit impressive performance when their bottleneck dimensions are carefully tuned. Among all the hyperparameters, we find the drop path rate [38] particularly important. Ignoring it (i.e., setting it to 0) significantly degrades the performance, potentially due to over-fitting. Overall, PEFT methods consistently outperform linear probing and full FT on all 19 image classification tasks (with 1, 000 training samples) in VTAB-1K. + +While similarly accurate on average, PEFT approaches make different predictions (Figure 1b). The above finding seems daunting: if existing PEFT approaches all perform similarly in terms of accuracy, do we learn anything useful beyond a single approach? This is particularly worrisome given that they FT the same backbone using the same downstream data. Fortunately, our analysis shows that different PEFT methods learn differently from the same data, resulting in diverse prediction errors and confidence. We attribute this to their inductive bias differences [70] — they explicitly specify different parameters to be updated or inserted. This opens up the door to leverage their discrepancy for improvement, e.g., through ensemble methods [16, 110] or co-training [6, 8] and we provide preliminary studies. + +PEFT is also effective in many-shot regimes. We extend PEFT beyond low-shot regime and find it effective even with ample downstream training data — PEFT can be on par or surpass full FT. This suggests that adjusting only a fraction of parameters in a suitably pre-trained backbone (e.g., pre-trained on ImageNet-21K [19]) could already offer a sufficient capacity [105] to reach a performant hypothesis for downstream tasks. + +PEFT appears more robust than full FT to distribution shifts, but WiSE overturns this advantage (Figure 1c). We also assess PEFT’s robustness to distribution shifts, following [96]. We consider a CLIP backbone [75], known for its superior generalizability to distribution shifts, and FT it with PEFT on ImageNet-1K. We found that PEFT retains CLIP’s generalizability (e.g., to samples from ImageNet-(V2, S, R, A)) better than full FT. This may not be surprising. What is interesting is that the weight-space ensembles (WiSE) between the FT and pre-trained models [96] is compatible with PEFT as well to further improve the robustness without sacrificing the target accuracy. To the best of our knowledge, we are the first to explore WiSE for PEFT. Nevertheless, full FT with WiSE can achieve even higher accuracy in both downstream and distribution shift data than PEFT, suggesting a further research direction in robust PEFT. + +What lead to PEFT’s success? We attempt to answer this fundamental question by analyzing the findings in our study. On VTAB-1K with 19 tasks, we identify two scenarios: (1) in certain tasks, full FT outperforms linear probing, suggesting the need to update the backbone; (2) in other tasks, linear probing outperforms full FT, suggesting either the backbone is good enough or updating it risks over-fitting. + +The superior accuracy of PEFT in both scenarios suggests that PEFT acts as an effective regularizer during low-shot training. Still using VTAB but with ample training data, we find that for scenario (1) tasks, PEFT performs similarly with full FT, suggesting that its regularization role does not impede model learning from abundant data. For scenario (2) tasks, PEFT can surprisingly still outperform full FT, indicating that it effectively transfers (or preserves) crucial pre-trained knowledge that full FT may discard. Overall, PEFT succeeds as a high-capacity learner equipped with an effective regularizer. + +Contributions. Instead of chasing the leaderboard, we systematically scrutinize existing methods via a unifying study. Our contribution is thus not a technical novelty, but: (1) a systematic framework for reproducible evaluations of PEFT methods; (2) a set of empirical recommendations on when and how to use PEFT methods for practitioners (section 3, section 5); (3) new insights for future research including leveraging PEFT’s prediction differences (section 4) and exploring robust fine-tuning (section 7). + +# 2. Background + +# 2.1. Large pre-trained models + +Building upon networks with millions (or billions) of parameters and massive training data, large pre-trained models have led to groundbreaking results in various downstream tasks [56, 69] and shown emerging capabilities not observed previously [9, 47, 53]. For example, a Vision Transformer (ViT) [19] trained with ImageNet-21K (14M images) leads to consistent gains v.s. a ViT trained with ImageNet-1K (1.3M images) [19]. ViTs pre-trained with millions of imagetext pairs via a contrastive objective function (e.g., CLIP-ViT) [13, 75] show an unprecedented zero-shot capability and robustness to distribution shifts [75]. We focus on the ImageNet-21K-ViT and CLIP-ViT in this paper. + +Vision Transformer (ViT). A ViT contains $M$ Transformer layers consisting of a multi-head self-attention (MSA) block, a multi-level perceptron (MLP) block, two Layer Normalization (LN) blocks [4], and two residual links. The $m$ -th Transformer layer can be formulated as + +$$ +\begin{array} { r l } & { Z _ { m } ^ { \prime } = \mathrm { M S A } \left( \mathrm { L N } \left( \boldsymbol { Z } _ { m - 1 } \right) \right) + Z _ { m - 1 } , } \\ & { Z _ { m } = \mathrm { M L P } \left( \mathrm { L N } \left( \boldsymbol { Z } _ { m } ^ { \prime } \right) \right) + Z _ { m } ^ { \prime } , } \end{array} +$$ + +where $Z _ { m - 1 }$ is the output of the preceding $( m - 1 )$ -th Transformer layer. Without loss of generality, let us consider a single-head MSA where the input $z$ is first projected into three matrices, $Q , \kappa$ , and $V$ . The output of this block is then formulated as: + +$$ +\begin{array} { c } { { Q = W _ { Q } Z , K = W _ { K } Z , V = W _ { V } Z , } } \\ { { \mathrm { } } } \\ { { V \times \mathsf { S o f t m a x } ( \frac { K ^ { \top } Q } { \sqrt { D } } ) . } } \end{array} +$$ + +# 2.2. Parameter-Efficient Fine-Tuning (PEFT) + +Fine-tuning is arguably the most common way to tailor a pre-trained model for downstream tasks [89, 96, 111]. As the size of pre-trained models gets larger, updating and storing all the parameters for one downstream task becomes inefficient. PEFT has thus emerged as a promising paradigm. PEFT was originally developed in NLP [3, 29, 33, 51, 58, 68, 83, 84, 91, 103, 109] and has attracted increasing attention in vision [11, 42, 43, 55, 59, 107]. Existing approaches can generally be categorized into four groups: prompt-based, adapter-based, direct selective tuning, and efficient selective tuning. We focus on visual recognition and compare representative PEFT approaches applicable to ViTs. + +Prompt-based. Prompt-based method emerged in NLP [54, 57] whose core concept is augmenting the input data with task-specific prompts. Visual Prompt Tuning (VPT) [42] adapts such an idea to ViTs. Its deep version (VPT-Deep) prepends a set of soft prompts to the input tokens of each Transformer layer (i.e., $\{ Z _ { m } \} _ { m = 0 } ^ { M - 1 } )$ and only optimizes the prompts during FT. Other representative works in this category include [5, 27, 87, 88, 94, 102]. + +Adapter-based. They typically introduce additional trainable parameters to a frozen pre-trained model [54]. Initially developed for domain adaptation [76, 77] and continual learning [66, 80], they have been extended to adapt Transformer-based models in NLP and vision [36, 59, 99, 102, 108]. We consider five popular methods. As the first adapter-based method, Houl. Adapter [36] inserts two Adapters (a two-layer bottleneck-structured MLP with a residual link) into each Transformer layer, one after the MSA block and one after the MLP block. Pfeif. Adapter [74] inserts the Adapter solely after the MLP block, shown effective in recent studies [37]. AdaptFormer [11] adds an Adapter in parallel with the original MLP block, different from the sequential design of previous methods. ConvPass [43] inserts a convolutional-based Adapter that explicitly encodes visual inductive biases by performing 2D convolution over nearby patch tokens. This module is inserted parallel to the MSA and MLP blocks. RepAdapter [63] introduces linear Adapters with group-wise transformations [62], placed sequentially after MSA and MLP. + +Direct selective tuning. They selectively update a subset of parameters of the backbone, striking a balance between full FT and linear probing. We consider three approaches. BitFit [103] updates the bias terms, including those in the patch embeddings projection, the Q/K/V weights, the MLP and LN blocks. LayerNorm [7] updates the parameters of the LN blocks. DiffFit [97] updates both the bias terms and the LN blocks and inserts learnable factors to scale the features after the MSA and the MLP blocks. Instead of updating parameters, SSF [55] linearly adapts intermediate features, motivated by feature modulation [39]. + +Efficient selective tuning. Instead of directly updating parameters, these methods learns additive residuals (e.g., ∆W ) to the original parameters $( e . g . , W )$ . By injecting a low-rank constraint to the residuals, this category effectively reduces the learnable parameters. LoRA [37], arguably the most well-known approach, parameterizes the residuals by lowrank decomposition to update the weights. Concretely, to update a $W \in \mathbb { R } ^ { D \times D }$ matrix, LoRA learns $W _ { \mathrm { d o w n } } \in \mathbb { R } ^ { r \times D }$ and $W _ { \mathsf { u p } } \in \mathbb { R } ^ { D \times r }$ with $r \ll D$ , and forms the additive residual by $\mathrm { \Delta } W = W _ { \mathrm { u p } } W _ { \mathrm { d o w n } } \in \mathbb { R } ^ { D \times D }$ . FacT [44] extends the idea of matrix decomposition into tensor decomposition. It stacks the $D \times D$ learnable matrices in all the Transformer layers into a 3D tensor and learns an additive residual parameterized by Tensor-Train (TT) [72] and Tucker (TK) [14] formulations. More detailed summary of ViT and a survey of PEFT methods can be found in Appendix B. + +
mNaturalSpecializedStructured2200
MethodND2BMD : 2DRroun 1 N ND120
Linear78.1 86.665.798.989.341.553.2 72.583.190.074.974.6 80.637.535.136.564.616.229.417.323.7 32.561.90
Full62.4 89.961.997.485.888.936.8 76.781.688.181.673.6 81.256.260.9 48.277.968.546.631.028.3, 52.270.085.8
VPT-Shallow80.288.767.999.189.677.054.2 79.481.890.377.274.4 | 80.942.252.43866.552.443.115.223.2 | 41.667.30.07
VPT-Deep84.891.569.499.191.085.654.7 | 81.886.494.984.273.9 84.979.362.448.577.980.356.433.243.8 60.275.60.43
BitFit86.590.570.398.991.091.254.2 82.686.795.085.375.5 | 85.677.263.251.279.278.653.930.134.7 | 58.575.60.1
DiffFit86.390.271.299.291.791.256.1 83.285.894.180.975.2 84.080.163.450.981.077.852.830.735.5 59.075.40.14
LayerNorm86.89.772.299.191.490.056.1 |83.084.793.883.075.2 | 84.277.562.249.978.178.052.124.334.4 | 57.174.70.04
SSFF86.689.868.899.191.491.256.5 82.886.194.583.274.8 84.780.163.653.081.485.652.131.937.2 60.676.00.21
Pfeif. Adapter86.391.572.199.291.488.555.7 83.086.295.585.376.2 85.883.165.251.480.283.356.633.841.1 61.876.90.67
Houl. Adapter84.392.172.39891.790.055.4 | 83.288.795.386.575.2 | 86.482.963.653.879.684.454.334.244.3| 62.177.20.77
AdaptFormer85.891.870.599.291.889.456.7 83.286.895.086.576.3 | 86.282.964.152.880.084.753.033.041.4 | 61.576.90.46
RepAdapter86.0 85.092.569.199.190.990.955.4 | 82.986.995.386.075.4 | 85.982.563.551.480.285.452.135.741.7 61.676.80.53
Convpass92.172.099.391.390.855.9 83.587.795.885.975.9 | 86.82.365.253.878.186.555.338.645.1 | 63.177.60.49
LoRA85.792.669.899.190.588.555.5 82.687.594.985.975.7 86.082.963.951.879.986.6 47.233.442.5 61.076.50.55
FacT_TT85.891.871.599.391.190.855.9 | 83.487.794.985.075.6 | 85.883.064.049.079.385.853.132.843.7 | 61.376.80.13
FacT_TK86.292.571.899.190.191.256.2 83.485.895.586.0 75.77 85.882.765.151.578.986.753.127.840.8 60.876.60.23
Relative Std Dev0.811.13 1.780.34 0.541.821.24± 0.541.20 0.591.950.83 ± 0.942.671.503.221.374.11 4.4611.029.30 | 2.701.09-
+ +Table 1. PEFT methods performances on VTAB-1K (19 tasks from 3 groups) by TOP-1 ACCURACY. Based on the results across PEFT, linear probing, and full FT, we identify two task groups (purple and orange), as discussed in section 6. + +# 2.3. Related work and comparison + +The community-wide enthusiasm for PEFT has led to multiple survey articles [28, 99, 101]. Meanwhile, several empirical and theoretical studies were presented, mostly based on NLP tasks, attempting to provide a holistic understanding. [29, 67] provided unified views to methodologically connect PEFT methods. [10, 17, 31] and [32, 98] empirically compared PEFT methods on NLP and vision tasks, respectively, while [22] offered a theoretical stability and generalization analysis. Accuracy-wise, [10, 17, 31] found that PEFT is robust to over-fitting and quite effective in NLP tasks under low-data regimes. This is, however, not the case for vision tasks: [32] showed that representative PEFT methods like LoRA and Adapter can’t consistently outperform either full FT or linear probing. In terms of why PEFT works, [22] framed PEFT as sparse fine-tuning and showed that it imposes a regularization by controlling stability; [17, 32] framed PEFT as (subspace) optimization; [17] further discussed the theoretical principle inspired by optimal control. + +Our study strengthens and complements the above studies and offers new insights. First, we compared over ten PEFT methods and carefully tuned the hyperparameters, aiming to accurately assess each method’s performance. This is particularly crucial for the vision community, where comprehensive references are still limited, and simpler methods like BitFit have often been deemed inferior, while other approaches have shown discrepancies compared to NLP studies. Second, we go beyond a competition perspective to investigate a complementary perspective of PEFT approaches. We show that different PEFT approaches offer effective base learners for model ensembles. Third, we go beyond downstream accuracy to investigate PEFT’s effectiveness in maintaining out-of-distribution robustness. Finally, we analyze both low-shot and many-shot settings, revealing distinct patterns among PEFT, full FT, and linear probing, extending the understanding of PEFT. + +# 3. PEFT Methods in Low-Shots Regime + +Pre-trained models are meant to ease downstream applications. One representative scenario is low-shot learning: supervised FT of the pre-trained model with a small number of samples per class. Indeed, low-shot learning has been widely used to evaluate PEFT performance. + +Dataset. VTAB-1K [104] consists of 19 classification tasks from three groups. Natural comprises natural images captured with standard cameras. Specialized contains images captured by specialist equipment for remote sensing and medical purposes. Structured evaluates scene structure comprehension, including object counting and depth estimation. Following [104], we split the 1000 training image + +![](images/figures/petl-visual-recognition-fig-0002.jpg) +Figure 2. Ranking frequency of 15 methods (14 PEFT $^ +$ linear probing) for three groups in VTAB-1K. Element $( i , j )$ is the number of times method $i$ ranks $j ^ { \bar { t } h }$ in each group. Methods are ordered by mean ranks (in brackets). The parameters column shows the # of trainable parameters in millions. More details are in Appendix C. + +80/20 for hyperparameter tuning. The reported TOP-1 AC-CURACY is obtained after training over the 1000 images and evaluating on the original test set. + +Methods. We consider linear probing, full FT, and 14 PEFT methods: 2 prompt-based [42], 5 adapter-based [11, 36, 43, 63, 74], 4 direct selective [7, 55, 97, 103], and 3 efficient selective [37, 44]. Please refer to subsection 2.2 and Appendix B for details. + +Setup. We employ the ViT-B/16 [19] pre-trained on ImageNet-21K [15] as the backbone. The prediction head is randomly initialized for each dataset. We systematically tune 1) learning rate, 2) weight decay, and 3) method-specifics like the PEFT parameter sizes, which are often left intact in previous studies. We set a cap for PEFT size $\leq 1 . 5 \%$ of ViT-B/16. We also turn the drop path rate [38] on (0.1) or off (0). A detailed experiment setup is provided in Appendix A, and more experiment results, including DINOv2 [71] and larger backbones (ViT-L and ViT-H), are provided in Appendix C. + +Results. As shown in Figure 1a and Table 1, PEFT methods generally outperform both linear probing and full FT across datasets. Additionally, with proper implementation and fair hyperparameter tuning, we surprisingly found that most PEFT methods perform similarly as the relative standard deviations (divided by the means) in all three groups are quite low. Simple methods (e.g., Bitfit) and PEFT methods originally proposed for NLP (e.g., LoRA and Adapter), which were previously reported as inferior due to unoptimized implementations and hyperparameter tuning, now demonstrate competitive performance with SOTA visual PEFT methods. To understand the relative advantages of different approaches, we provide the ranking frequency of PEFT methods across different groups in Figure 2, where the element $( i , j )$ in each ranking matrix represents the frequency that method $i$ ranks $j ^ { t h }$ in each group. Methods are ordered by their mean ranks (in brackets), and the parameters column indicates the number of trainable parameters in millions. In natural group, simpler methods with fewer trainable parameters—such as DiffFit and Fact-TT—offer a cost-effective solution without compromising performance. Conversely, in specialized and structured groups, methods with more parameters generally yield better performance. We hypothesize that this performance discrepancy arises from the domain similarity between the pre-trained domain (ImageNet) and the downstream domains. The natural group, sharing a stronger affinity with ImageNet, allows simpler methods like BitFit to adjust the features effectively. In contrast, the specialized and structured groups necessitate more complex methods with more trainable parameters to bridge the domain gap. + +Recipes. In low-shot regimes, when the downstream data are similar to the pre-trained data, simple methods (e.g., DiffFit) offer solid accuracy with fewer parameters. Conversely, if there is a substantial domain gap, more complex methods with more parameters often achieve higher accuracy. Since low-shot training is especially prone to over-fitting, we recommend activating a nonzero drop path rate—commonly set to zero by default—that stochastically drops a Transformer block per sample [38]. All methods can benefit from such a randomization-based regularization, as shown in Figure 11 in the Appendix. + +# 4. Different PEFT Approaches Offer Complementary Information + +The previous section demonstrates that all PEFT methods perform similarly across various domains. Given that different PEFT methods are trained on the same downstream data using the same backbone and achieve comparable accuracy, one might expect them to learn similar knowledge from the data, resulting in similar predictions. Contrary to this expectation, our findings below reveal that different PEFT methods acquire distinct and complementary knowledge from the same downstream data, even when built upon the same backbone, leading to diverse predictions. + +We start by analyzing their prediction similarity on the same dataset in VTAB-1K. It is expected that their predictions are similar for datasets with very high accuracy, such as Flowers102 $( \mathrm { a v g \ 9 9 . 1 \% } )$ ) and Caltech101 $( \mathrm { a v g \ 9 l . 4 \% } )$ ). Beyond them, we find that most PEFT methods show diverse predictions in other datasets in VTAB-1K. Figure 3a shows the prediction similarities between 14 PEFT methods in DTD, Retinopathy, and DMLab, which belong to natural, specialized, and structured groups, respectively. In DTD and Retinopathy, most methods differ in about $20 \%$ of their predictions, while in DMLab, this difference increases to approximately $3 5 \%$ , even though they achieve similar accuracies. This prediction diversity may be attributed to the different inductive biases [70] of PEFT methods — they explicitly select specific parameters to update or insert different modules at various locations within the model. More analyses and details are offered in Appendix C. + +![](images/figures/petl-visual-recognition-fig-0003.jpg) +Figure 3. (a) Prediction similarity analysis: element $( i , j )$ shows the percentage of samples that method $_ { i }$ and $j$ predict the same. Although different methods achieve similar accuracy, they have diverse predictions. (b)The wrong prediction overlaps of LoRA, Adapter, and SSF for the 5K least confident data. Correct prediction overlaps for the 5K most confident data are shown in Figure 1b. They are FT on CIFAR100 (VTAB-1K). More results for (a) and (b) are in Appendix C. + +![](images/figures/petl-visual-recognition-fig-0004.jpg) +Figure 4. Ensemble (majority vote) shows consistent gain on most datasets thanks to the diverse predictions. + +Such diverse predictions across methods open up the possibility of leveraging their heterogeneity for further improvement. The most straightforward approach is ensemble [26], e.g., majority vote over methods. Figure 4 demonstrates the ensemble performance gain over all the PEFT methods in each dataset, where we use the worst PEFT method as the baseline. Thanks to the diverse predictions across methods, the ensemble can provide consistent gain. + +Also, we analyze if PEFT methods make similar correct predictions for high-confidence samples and similar mistakes for low-confidence samples. Figure 1b and Figure 3b show the correct prediction overlap for the 5K most confident samples (per method) and the wrong prediction overlap for the 5K least confident samples (per method). For demonstration purposes, we select one method from each PEFT category (LoRA, Adapter, SSF) and they are FT on + +CIFAR-100 in VTAB-1K. Methods within the same category also show diverse predictions (Appendix C). Since they make different predictions in both high and low-confidence regimes, this paves the way for new possibilities of using different PEFT methods to generate diverse pseudo-labels for semi-supervised learning [23, 24, 100], domain adaptation [20, 21, 86], and continual learning [60, 64–66, 82]. For example, in semi-supervised learning, we can FT different PEFT methods on the labeled data to generate diverse and accurate pseudo-labels by selecting highly confident predictions from each PEFT method. + +# 5. PEFT Methods in Many-Shot Regime + +Recent works in NLP [10] have indicated that PEFT methods may not perform as competitively as full FT when data is abundant. We thus aim to investigate PEFT’s performance in many-shot regimes by addressing the following questions: (1) Should we use PEFT or full FT when data is sufficient? (2) How should we adjust the number of trainable parameters for PEFT methods in many-shot regimes? + +Dataset. We select one representative dataset from each group in VTAB: (1) CIFAR-100 [49], a natural image dataset comprising 50K training images across 100 classes; (2) RE-SISC [12], a remote sensing dataset for scene classification with 25.2K training samples across 45 classes; and (3) Clevr-Distance [45, 104], a synthetic image dataset for predicting the depth of the closest object with 6 depth classes and 70K samples. The reported results are obtained by training on the full training set and evaluating on the original test set. + +Setup. The model setup mostly follows the VTAB-1K experiment. More details about setup and hyperparameter search are provided in Appendix A. + +Results. In many-shot regimes, with sufficient downstream data, full FT may catch up and eventually outperform PEFT methods. However, from Figure 5, we found that even in many-shot regimes, PEFT can achieve comparable results with full FT, even just using $2 \%$ of fine-tuning parameters. The performance gain, however, quickly diminishes and plateaus after $5 \%$ of tunable parameters. By comparing the results on the domain-close CIFAR-100 and domaindifferent RESISC and Clevr, we have some further observations. Downstream tasks with larger domain gaps often require updating more parameters to achieve high accuracy. With sufficient downstream data, full FT is less prone to over-fitting and, indeed, attains a high accuracy. But interestingly, PEFT methods, with only $2 \% \sim 5 \%$ of tunable parameters, achieve similar accuracy, suggesting that its design principle does offer sufficient effective capacity for the model to learn [105]. Downstream tasks with smaller domain gaps suggest that the pre-trained model had learned sufficient knowledge about them; full FT thus risks washing such knowledge away. In fact, we found that PEFT notably outperforms full FT on CIFAR-100, suggesting it as a more robust transfer learning algorithm for downstream tasks. + +![](images/figures/petl-visual-recognition-fig-0005.jpg) +Figure 5. PEFT accuracy in many-shot regimes, with different parameter sizes $\mathrm { \Delta X }$ -axis) on three datasets from different domains. Even $2 \% - 5 \%$ trainable parameters allow the models to have sufficient capacity to learn from full data. Details are in Appendix C). + +Recipes. In many-shot regimes, PEFT methods with sufficient parameters $( 2 \sim 5 \% )$ appear more favorable than full FT and linear probing. On the one hand, they achieve comparable and even better accuracy than full FT. On the other hand, the tunable parameters remain manageable. The parameter efficiency of PEFT also often implies less training GPU memory usage and training time, making PEFT methods a favorable alternative in many-shot regimes. For a downstream domain that is close to the pre-training domain, PEFT shows much pronounced transferability. For a downstream domain that is quite different, the limited tunable parameters (controversially, $2 \sim 5 \%$ already amount to a few million) already allow the model to learn sufficiently. + +# 6. Why Do PEFT Methods Work?2 + +Putting together section 3 and section 5, we identify two distinct patterns regarding the performance among linear probing, full FT, and PEFT. Within 19 VTAB-1K tasks, we see: (1) Full FT outperforms linear probing. As linear probing reflects the pre-trained feature quality for downstream tasks, case (1) suggests the necessity to update the backbone to close the gap between pre-trained and downstream domains. (2) Linear probing surpasses full FT, suggesting the pre-trained features are good enough (at least in a low-shot scenario). Recklessly updating them may risk over-fitting. Figure 6 (a-b) summarizes the low-shot accuracy comparison based on the categorization above; each line corresponds to one task. Linear probing, PEFT, and full FT are located in order, from left to right, to reflect their tunable parameter sizes. PEFT’s superiority in both cases showcases its capacity to learn and its regularization role to prevent over-fitting. + +We also draw the many-shot accuracy in Figure 6 (c-d) based on the same categorization: RESISC and Clevr in case (1) , and CIFAR-100 in case (2) . In the many-shot setting, full FT consistently outperforms linear probing, which seems to suggest no more risk of over-fitting. However, on CIFAR-100 (Figure 6 (d)), we again see a noticeable gap between PEFT and full FT, just like in Figure 6 (b). Such a concave shape reminds us of the long-standing under-fitting-overfitting curve, suggesting that even with sufficient downstream data, full FT still risks over-fitting. + +Considering PEFT’s comparable performance to full FT on RESISC and Clevr with large domain gaps, we conclude that PEFT succeeds as a high-capacity learner equipped with an effective regularizer. The two roles trade-off well such that PEFT can excel in both low- and high-similarity domains under both low-shot and many-shot settings. + +# 7. How Robust are PEFT Methods to Distribution Shifts? + +Large pre-trained models such as CLIP [75] have demonstrated unprecedented zero-shot accuracy across diverse data distributions. However, recent studies [75, 96] have shown that FT on downstream data, while significantly boosting performance on the target distribution, often compromises the model’s robustness to distribution shifts. Given that PEFT only updates a limited number of parameters in the model, we investigate whether PEFT can offer a more robust + +
FullBitFitLayer-NormHoul.AdapterAdapt-FormerRep-AdapterConvpassLoRAFacT_TK
100-shot ImageNet75.075.2774.875.075.676.576.376.674.7
Avg. distribution shift Acc42.555.4 (12.9)↑55.9 (13.4)↑56.9 (14.4)↑56.1 (13.6)↑56.2 (13.7)↑54.7 (12.2)↑55.9 (13.4)↑56.1 (13.6)↑
+ +Table 2. The “Avg. distribution shift Acc” denotes the average performance of ImageNet-(V2, S, R, A) evaluated on the CLIP model FT on ImageNet. (↑) indicates the gain over full FT. + +![](images/figures/petl-visual-recognition-fig-0006.jpg) +Figure 6. (a): VTAB-1K tasks in case (1), $\mathrm { P E F T } > \mathrm { f u l l } >$ linear. (b) VTAB-1K tasks in case (2), PEFT $>$ linear $>$ full. (c) RESISC & Clevr in case (1) with enough data, PEFT $\approx$ full $>$ linear. (d) CIFAR in case (2) with enough data, P $\mathrm { E F T > f u }$ ull $>$ linear. Within each figure, left for linear, middle for PEFT, and right for full. More details are in Appendix C. + +alternative to full FT. + +Dataset. We use 100-shot ImageNet-1K as our target distribution, with each class containing 100 images. Following [96], we consider 4 natural distribution shifts from ImageNet: ImageNet-V2 [78], a new ImageNet test set collected with the original labeling protocol; ImageNet-R [34], renditions for 200 ImageNet classes; ImageNet-S [25], sketch images for 1K ImageNet classes; ImageNet-A [35], a test set of natural images misclassified by a ImageNet pre-trained ResNet-50 [30] for 200 ImageNet classes. + +Setup. We focus on the CLIP ViT-B/16 model, which comprises a visual encoder and a text encoder, pre-trained via contrastive learning on image-text pairs. Following [96], we add an FC layer as the head initialized using the class label text embedded by the text encoder. Subsequently, we discard the text encoder and apply PEFT methods to the visual encoder, FT only the PEFT modules and the head. More details about the CLIP model and experiment setup can be found in Appendix A. + +Results. As shown in Table 2, while some PEFT methods may not surpass full fine-tuning on the target distribution, they consistently demonstrate more robust performance on distribution-shifted data. This robustness can be attributed to PEFT updating only a small fraction of the parameters, thereby preserving the robust features of the foundational models. Given the similar target distribution performance, should we blindly use PEFT methods for more robustness? + +Weight-space ensembles (WiSE) for PEFT. WiSE [96], which linearly interpolates the weights of a fully FT model with those of the original backbone, is a popular approach for boosting robustness. We explore whether WiSE can also enhance the robustness of PEFT. To apply WiSE to PEFT, we first linearly interpolate the prediction head with a mixing coefficient $\alpha$ . For direct selective tuning methods (e.g. BitFit), this involves merging the PEFT-tuned parameters and the original model. Since most Adapter-based methods include residual connections (Appendix B.2.2), we can adjust their impact by scaling the adapter modules with $\alpha$ . A similar approach applies to efficient selective methods (e.g. LoRA) as they learn additive residuals to the original parameters. To the best of our knowledge, we are the first to study WiSE for PEFT. As shown in Figure 1c (more results in Appendix C), WiSE consistently improves both target and distribution shift performances of PEFT methods. For Adapter-based methods, WiSE can be considered feature ensembles, where $\alpha$ controls how strong the domain-specific features from the adapter module blend with the domain-agnostic ones from original backbones. For selective tuning, WiSE functions similarly to its application in full FT—exploiting the fact that the fine-tuned parameters remain near the original loss basin, allowing an ensemble that captures the best of both [2, 40]. + +Interestingly, even though full FT is generally less robust than PEFT methods, WiSE elevates full FT’s performance above that of PEFT on both target distribution and distribution shift data, which suggests promising research directions to investigate the underlying mechanism and how to further improve the robustness of PEFT methods. + +# 8. Conclusion + +Instead of chasing the leaderboard, we conduct a unifying empirical study of PEFT, an emerging topic in the large model era. We provide an extendable framework for reproducible evaluations of PEFT methods in computer vision. We also have several new insights and implications, including PEFT methods’ complementary expertise, suitable application regimes, and robustness to domain shifts. We expect our study to open new research directions and serve as a valuable user guide in practice. + +# Acknowledgment + +This research is supported by grants from the National Science Foundation (ICICLE: OAC-2112606). We are grateful for the generous support from the Google gift fund and the Ohio Supercomputer Center. + +# References + +[1] Armen Aghajanyan, Sonal Gupta, and Luke Zettlemoyer. Intrinsic dimensionality explains the effectiveness of language model fine-tuning. In Proceedings of the 59th Annual Meeting of the Association for Computational Linguistics and the 11th International Joint Conference on Natural Language Processing (Volume 1: Long Papers), pages 7319–7328, Online, 2021. Association for Computational Linguistics. 21 +[2] Samuel Ainsworth, Jonathan Hayase, and Siddhartha Srinivasa. Git re-basin: Merging models modulo permutation symmetries. 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Ensemble methods: foundations and algorithms. CRC press, 2012. 2 + +# Appendix + +We provide details that are omitted from the main paper. + +• Appendix A: Experiment and dataset details +• Appendix B: Detailed descriptions of ViT and compared methods. +Appendix C: Additional results not presented in main paper +• Appendix D: Discussion about further impacts of this work + +# A. Experiment and Dataset Details + +# A.1. Experiment Details + +VTAB-1K We employ AdamW optimizer [61] with a batch size of 64 and utilize the cosine decay learning rate scheduler. We train all methods with 100 epochs. The learning rate is tuned from [1e-3, 1e-2] and weight decay from [1e-4, 1e-3]. The method-specific hyperparameter searching grip is shown in Table 3, along with the tunable parameter ranges (in millions). Since most method-specific hyperparameters affect the number of tunable parameters in the PEFT methods, we set a cap on the tunable parameters for each PEFT method to be less than or equal to $1 . 5 \%$ of the total parameters in ViT-B/16, which is approximately equal to the number of parameters in the Query, Key, and Value matrices of a single MSA block. Consistent with the original VTAB-1k paper [104], most PEFT studies [42–44, 55, 63, 63, 107] don’t apply data augmentation as it’s challenging to identify a set of augmentations that uniformly benefits all 19 datasets3. To ensure that our results are directly comparable and that any performance differences are attributable to the methods themselves rather than data augmentation, we don’t apply data augmentation. + +Many-shot We also employ AdamW optimizer with a batch size of 64 and a cosine decay learning rate scheduler. The learning rate is tuned from [5e-4, 1e-3] and weight decay keeps the same range of [1e-4, 1e-3]. We apply horizontal flipping for CIFAR100, horizontal and vertical flipping for Resisc, and no augmentation for Clevr. We train all methods with 40 epochs. + +Robustness Model CLIP models are trained on imagecaption pairs collected from the web. Given a dataset of such pairs $\{ ( x _ { 1 } , s _ { 1 } ) , \dotsc , ( x _ { B } , s _ { B } ) \}$ , these models learn an image encoder $g$ and a text encoder $h$ that aim to maximize the similarity $\langle g ( x _ { i } ) , h ( s _ { i } ) \rangle$ between matching image and caption embeddings while minimizing it for nonmatching pairs. For zero-shot classification, the models predict the class of an input image $x$ from a set of $k$ class names $C = \{ c _ { 1 } , \ldots , c _ { k } \}$ by matching $x$ with captions derived from these class names. Specifically, for each class $c _ { j }$ , a caption is formulated as $s _ { j } = \mathbf { \hat { \rho } } _ { \mathbf { \hat { a } } } ^ { }$ photo of a $\boldsymbol { c _ { j } } ^ { \flat }$ . The predicted class is then determined by selecting the one whose caption embedding has the highest similarity with the image embedding: $\hat { y } = \arg \operatorname* { m a x } _ { j } \langle g ( x ) , h ( s _ { j } ) \rangle$ . Alternatively, a weight matrix $W _ { \mathrm { z e r o - s h o t } } \in \mathbb { R } ^ { d \times k }$ can be constructed, where each column is the embedding $h ( s _ { j } )$ corresponding to class $c _ { j }$ . The model’s output scores for each class are then computed as $f ( x ) \ = \ g ( x ) ^ { \top } W _ { \mathrm { z e r o - s h o t } }$ . To generate $W _ { \mathrm { z e r o - s h o t } }$ , we ensemble the 80 prompts provided by CLIP at https://github.com/openai/CLIP. + +Robustness Setup In section 3 and section 5, the finetuning train set and test set are from the same data distribution. In section 7, we fine-tune the CLIP model using ImageNet-1K training data (100 shots) and subsequently evaluate the fine-tuned model not only on the test set of ImageNet-1K but also on four additional datasets with distribution shifts: ImageNet-V2, ImageNet-R, ImageNet-S, and ImageNet-A, as shown in Figure 7. Following [96], we set a small learning rate as 3e-5 and weight decay as 5e-3. We use a strong data augmentation following [107]. + +Computation We used a workstation with eight NVIDIA RTX 6000 Ada GPUs, two AMD EPYC 9554 64-Core Processors, and 800GB of RAM. + +What do we not investigate? There are many aspects that one can ask about PEFT. Our study focuses more on their learning and prediction behaviors, not the computationspecific properties like memory usage and FLOPS. + +# A.2. Dataset Details + +VTAB-1K The processed VTAB-1K can be downloaded from our official code base to ensure reproducibility. + +Many-shot Datasets We perform 90/10 train-val split for CIFAR-100, RESISC and Clevr-Distance. The split details are provided in our code base for reproducibility. We apply horizontal flipping for CIFAR100, horizontal and vertical flipping for Resisc, and no augmentation for Clevr. All data are normalized by ImageNet mean and standard deviation. + +# B. Background + +# B.1. Vision Transformer + +Overview of ViT. Inspired by the recent success of Transformer-based models [90] in NLP [95], Vision Transformer (ViT) [19] has become widely used in computer vision. To handle 2D images, ViT divides an image $\pmb { I } \in \mathbb { R } ^ { H \times W \times C }$ into $N$ non-overlapping patches $\{ \pmb { I } ^ { ( n ) } \in$ $\mathbb { R } ^ { P ^ { 2 } \times C } \} _ { n = 1 } ^ { N }$ , where $( H , W )$ is the resolution of the input image, $C$ is the number of channels, $N = H W / P ^ { 2 }$ and $( P , P )$ is the resolution of each patch. Each patch $\pmb { I } ^ { ( n ) }$ is flattened and embedded into a $D$ -dimensional vector $\pmb { x } _ { 0 } ^ { ( n ) }$ with a trainable linear projection. Incorporating the BERT design approach [46], a “Class” token $\pmb { x } _ { 0 } ^ { \mathrm { ( C l a s s ) } }$ is prepended to the sequence of embedded patches, whose output state at the last Transformer layer is utilized as the image representation. Finally, position embeddings $\mathbf { E } _ { \mathrm { p o s } } \in \mathbb { R } ^ { \bar { D } \times ( 1 + \bar { N } ) }$ ar e added to preserve positional information and form the input $\mathbf { Z } _ { 0 } \in \mathbb { R } ^ { \bar { D } \times ( 1 + N ) }$ to the ViT, which can be formulated by: + +![](images/figures/petl-visual-recognition-fig-0007.jpg) +Figure 7. Samples of the class lemon, from the fine-tuned dataset ImageNet and distribution shifts datasets (ImageNet-V2, ImageNet-R, ImageNet-S, and ImageNet-A). The CLIP model is fine-tuned with PEFT on ImageNet and evaluated on distribution shifts datasets to measure the robustness of fine-tuned models. The figures are modified based on [96]. + +Table 3. Methods-specific hyperparameter searching grip for VTAB-1K experiment. + +
MethodHyperparamters#Params (M)
VPT-ShallowPrompt Number:: [5, 10, 50, 100, 200]0.0003~0.153
VPT-DeepPrompt Number: [5, 10, 50, 1100]0.046 ∼ 0.921
BitFitN/A0.102
DiffFitN/A0.140
LayerNormN/A0.038
SSFN/A0.205
Pfeif. AdapterAdapter Scale Factor:[0.01, 0.1, 1, 10]Adapter Bottleneck: [4, 8, 16, 32]0.082 ~0.599
Houl. AdapterAdapter Scale Factor: [0.01, 0.1, 1, 10]Adapter Bottleneck: [4, 8, 16, 32]0.165 ~1.198
AdaptFormerAdapter Scale Factor:[0.05, 0.1, 0.2]Adapter Bottleneck:[4, 16, 32]0.082 ~0.599
RepAdapterRepAdapter Scale Factor:[0.1, 0.5, 1, 5, 10]RepAdapter Bottleneck: [8, 16, 32]0.239 ~0.903
ConvpassConvpass Scale Factor: [0.01, 0.1, 1, 10, 100]Convpass Bottleneck: [8, 16]Convpass Xavier Init:[True, False]0.327 ~0.664
LoRALoRA Bottleneck: [1, 8, 16, 32]0.036 ~1.179
FacT_TTFacT Scale Factor: [0.01, 0.1, 1, 10, 100]FacT Bottleneck: [8, 16, 32]0.021 ~0.196
FacT_TKFacT Bottleneck: [16, 32, 64]FacT Scale Factor: [0.01, 0.1, 1, 10, 100]0.030 ~0.369
+ +$$ +{ \pmb Z } _ { 0 } = \left[ { \pmb x } _ { 0 } ^ { ( \mathrm { C l a s s } ) } , { \pmb x } _ { 0 } ^ { ( 1 ) } , { \pmb x } _ { 0 } ^ { ( 2 ) } , \cdots , { \pmb x } _ { 0 } ^ { ( N ) } \right] + { \bf E } _ { \mathrm { p o s } } +$$ + +Table 4. We apply simple data augmentations (DA) (RandomResizedCrop, RandomVerticalFlip and RandomHorizontalFlip) on three datasets in each group. Data augmentation does not benefit most of VTAB-1K datasets and thus, most recent PEFT papers [42–44, 55, 63, 63, 107] skip it. Figure 8 shows examples of how some data augmentation transforms are harmful for a specific task. Therefore, to ensure that our results are directly comparable to existing papers, we don’t apply data augmentation. + +
Linear 84.4FullVPT-Shallow 84.9VPT-DeepBitFitDiffFitLayerNormSSF Pfeif. AdapterHoul. AdapterAdapt- FormerRep- AdapterConvpass LoRAFacT_TT Fact_TK
Caltech101Simple DA76.884.883.8 90.585.7 90..285.8 89.786.1 89.887.4 91.586.084.986.485.286.885.586.0
Default86.689.988.791.592.191.892.592.192.691.892.5
-2.2-13.1-3.8-6.7-6.7 -4.5-4.1-6.1-6.9-6.1-6.9-5.8-6.3-6.5
Simple DA67.557.869.071.173.7-3.9 73.5-3.772.772.670.671.571.873.072.271.9
NaturalDTDDefault65.761.9667.970.7 69.4 70.371.272.268.4 68.872.172.370.569.172.069.871.571.8
1.8-4.11.11.70.30.12.4-0.23.20.70.1
Simple DA98.192.20.4 98.02.5 98.81.3 98.8-0.4 98.80.6 98.497.598.798.098.998.798.798.7
Flower102Default98.997.498.2 99.198.6 99.1 98.999.299.199.199.298.099.299.199.399.199.399.1
-0.8-5.2-0.9-0.5-0.4-0.3-0.8-0.5-0.5-1.1-0.4-0.4-0.6-0.4
Simple DA87.391.0-0.9 92.091.791.9-0.3 92.592.192.592.393.192.792.893.392.8
EuroSATDefault90.088.188.4 90.392.0 94.9 95.0 -3.094.193.894.5 -2.095.595.395.095.395.894.994.995.5
-2.7 74.32.9 75.0-1.9-2.9-2.4-1.9-3.4-2.8-2.7-2.2-3.1-2.1-1.6-2.7
Simple DA74.480.1 84.281.0 85.378.580.780.680.681.682.281.581.582.280.782.9
SpecializedResisc45Default74.981.677.2-4.380.9 -2.483.0 -2.383.2 -2.685.3 -4.786.586.586.085.985.985.086.0
-0.6 74.5-6.6 73.6-2.8 74.7-4.1-4.9-4.3-4.5-4.4-3.7-4.3-3.1
Simple DA76.3 73.976.3 75.576.776.476.477.375.677.077.176.876.275.377.0
RetinopathyDefault74.673.674.40.875.2 1.575.2 1.274.8 1.676.2 1.175.276.375.475.975.775.675.7
-0.1 22.60.0 29.90.3 24.32.40.40.71.70.90.5-0.31.3
Simple DA Default29.443.129.6 56.428.7 53.929.029.027.928.922.930.931.430.530.332.528.4
dSpr-Ori46.6-25.252.8 -23.852.1 -23.152.1 -24.256. -27.754.353.052.155.347.253.1 -20.653.1
-6.8 49.8-16.7 48.7-18.8 49.4-26.8 52.7-31.4-22.1 53.2-20.7-24.8-16.9 52.952.0-24.7
Simple DA Default77.966.577.952.6 79.254.752.750.6 81.453.953.652.251.379.353.3 78.9
Structured sNORB-AzimKITTI Simple DA64.6 -14.8-29.2-17.1-26.681.0 -26.378.1 -25.4-30.880.2 -26.379.6 -26.080.0 -26.880.2 -28.078.1 -26.879.9 -27.0-27.3-25.6
+ +Table 5. Definitions of symbols and abbreviation used in Appendix B + +
Symbol (Abbreviation)Definition
(H, W )Resolution of input images
CNumber of channels (input images)
PResolution of patches
NNumber of patches (tokens)
NhNumber of head in each Transformer layer
DEmbedding dimension
DhEmbedding dimension for single-head attention
Lmm-th Transformer layer
MNumber of Transformer layer
Zm−1Input of m-th Transformer layer
ViTVision Transformer
LNLayer Normalization
MSAMulti-head Self-Attention
MLPMulti-Layer Perceptron
FCFully-connected layer
+ +As shown in the left part of Figure 9, a ViT typically consists of $M$ layers, denoted by $\lbrace L _ { m } \rbrace _ { m = 1 } ^ { M }$ . The input $Z _ { 0 }$ mentioned above is fed into the first layer $L _ { 1 }$ , producing the output ${ \bf Z } _ { 1 } = L _ { 1 } ( { \bf Z } _ { 0 } ) = [ { \bf x } _ { 1 } ^ { ( \mathrm { C l a s s } ) } , { \bf x } _ { 1 } ^ { ( 1 ) } , \cdot \cdot \cdot , { \bf x } _ { 1 } ^ { ( N ) } ] \in$ $\mathbb { R } ^ { D \times ( 1 + N ) }$ , which maintains the same size as $Z _ { 0 }$ . Namely, $Z _ { 1 }$ comprises $1 + N$ feature tokens, and each corresponds to the same column in $Z _ { 0 }$ . Similarly, for $m = 2 , \cdots , M$ each layer $L _ { m }$ takes the output of the previous layer as input and generates the output, $Z _ { m } = L _ { m } ( Z _ { m - 1 } )$ . Finally, the “Class” vector x(ClM $\pmb { x } _ { M } ^ { ( \mathrm { C l a s s } ) }$ in $Z _ { M }$ serves as the image feature for prediction. When dealing with classification tasks, the predicted label $\hat { y } = { \mathsf { H e a d } } ( \mathbf x _ { M } ^ { ( \mathrm { C l a s s } ) } )$ x(Class)M ) is generated through a linear head (i.e., a fully-connected layer). + +Details of each Transformer layer. As shown in the right part of Figure 9, each Transformer layer consists of a Multihead Self-Attention (MSA) block, a Multi-Layer Perceptron (MLP) block, and two Layer Normalization (LN) layers [4]. Formally, a Transformer layer $L _ { m }$ can be defined as + +$$ +\begin{array} { r l } & { Z _ { m } ^ { \prime } = \mathrm { M S A } \left( \mathrm { L N } \left( \pmb { Z } _ { m - 1 } \right) \right) + \pmb { Z } _ { m - 1 } } \\ & { Z _ { m } = \mathrm { M L P } \left( \mathrm { L N } \left( \pmb { Z } _ { m } ^ { \prime } \right) \right) + \pmb { Z } _ { m } ^ { \prime } } \end{array} +$$ + +where $\mathbf { { Z } } _ { m - 1 } = [ \pmb { x } _ { m - 1 } ^ { ( \mathrm { { C l a s s } } ) } , \pmb { x } _ { m - 1 } ^ { ( 1 ) } , \cdots , \pmb { x } _ { m - 1 } ^ { ( N ) } ] \in \mathbb { R } ^ { D \times ( 1 + N ) }$ is the output of the preceding $( m - 1 )$ -th Transformer layer. The MLP is applied to each column vector of $\pmb { Z } _ { m } ^ { \prime }$ indepen- + +![](images/figures/petl-visual-recognition-fig-0008.jpg) +Figure 8. Example of augmented images from KITTI: (a) Original, (b) RandomVerticalFlip, (c) RandomResizedCrop, (d) Resize only. The KITTI task needs to predict the depth to the nearest vehicle (car, van, or truck) in the image. RandomResizedCrop may crop out the nearest vehicle. RandomVerticalFlip may make the task more difficult. + +In order to encapsulate multiple complex relationships amongst different elements in the sequence, the MSA block comprises $N _ { h }$ single-head self-attention blocks. For the $i ^ { t h }$ single-head self-attention block, an generic input $z$ is first projected into three matrices, namely Query $\boldsymbol { Q } ^ { ( i ) }$ , Key $\pmb { K } ^ { ( i ) }$ and Value $V ^ { ( i ) }$ + +$$ +\pmb { Q } ^ { ( i ) } = \pmb { W } _ { Q } ^ { ( i ) } \pmb { Z } , \quad \pmb { K } ^ { ( i ) } = \pmb { W } _ { K } ^ { ( i ) } \pmb { Z } , \quad \pmb { V } ^ { ( i ) } = \pmb { W } _ { V } ^ { ( i ) } \pmb { Z } , +$$ + +where $W _ { Q / K / V } ^ { ( i ) } \in \mathbb { R } ^ { D _ { h } \times D ~ 4 }$ where $D _ { h }$ is the embedding dimension for a single head self-attention block and typically set to $D / N _ { h }$ . The $i ^ { t h }$ self-attention head in MSA is formulated as + +$$ +{ \mathrm { A t t n } } ^ { ( i ) } ( Z ) = { \cal V } ^ { ( i ) } { \times } \mathrm { S o f t m a x } \left( \frac { { K ^ { ( i ) } } ^ { \top } { \pmb Q } ^ { ( i ) } } { \sqrt { D _ { h } } } \right) \in \mathbb { R } ^ { D _ { h } \times ( 1 + N ) } +$$ + +The outputs of all heads are concatenated and linearly projected by a fully connected layer $( F C _ { a t t r } )$ with weight $\mathbf { \bar { W } } _ { O } \in \mathbb { R } ^ { D \times ( D _ { h } \cdot N _ { h } ) }$ as the output of the MSA block. + +$$ +\operatorname { M S A } ( Z ) = W _ { \cal O } \left[ \operatorname { A t t n } ^ { 0 } ( Z ) , \dots , \operatorname { A t t n } ^ { N _ { h } } ( Z ) \right] +$$ + +The MLP block can be defined as + +$$ +\mathrm { M L P } ( Z ) = G E L U \left( Z W _ { 1 } + b _ { 1 } \right) W _ { 2 } + b _ { 2 } +$$ + +where $W _ { 1 } \in \mathbb { R } ^ { D \times { 4 D 5 } }$ , $W _ { 2 } \in \mathbb { R } ^ { 4 D \times D }$ , $b _ { 1 } \in \mathbb { R } ^ { 4 D }$ , $b _ { 2 } \in$ $\mathbb { R } ^ { D }$ are weights and biases for two FC layers $F C _ { 1 }$ and $F C _ { 2 }$ ) respectively. + +Since PEFT methods often entail incorporating additional components to modify the intermediate features within or between Transformer layers, we adopt the notation $\{ h _ { 1 } , \ldots , h _ { 1 0 } \}$ to denote the intermediate features in the unravelled view of a Transformer layer (as depicted in Figure 9) to facilitate a clearer illustration of the PEFT methods discussed in the subsequent section. + +# B.2. Evaluated Methods + +In this section, we dive into the details of 12 state-of-the-art PEFT approaches, categorized into three groups: Promptbased, Adapter-based, and Selective Parameter Tuning. We will describe the distinctions and tradeoffs between them. A consolidated overview of these approaches is summarized in Table 6. + +# B.2.1. Prompt-based Methods + +Prompt-based learning emerged in NLP as an effective approach to adapt pre-trained models for downstream tasks [54, 57]. The core concept involves augmenting the model input with task-specific hints (prompts), which aid the pre-trained model in addressing novel tasks with its existing knowledge. Hard prompts are human-interpretable natural language hints, encompassing task instructions, incontext examples, or supporting information. Alternatively, soft prompts are continuous vector hints that are incorporated into the input embeddings of the input layers or hidden states of other layers. Soft prompts are updated during the fine-tuning process using gradient-based methods, guided by the downstream task-specific loss functions, while the pretrained model itself remains fixed. The splendent success of prompts in NLP has sparked a growing interest in adopting it in computer vision [88, 102] and multi-modal domains [27]. + +In this paper, we investigate a prominent and strong prompt-based method called Visual Prompt Tuning (VPT) [42], which represents one of the early endeavours in introducing prompts to computer vision. Specifically, VPT-Shallow adds $l$ prompts $P _ { 0 } \in \mathbb { R } ^ { l \times D }$ to the input of the first Transformer layer $Z _ { 0 }$ and the output $\tilde { P } _ { 0 }$ of $P _ { 0 }$ serves as the input for the next layer as depicted in Equation 11. VPT-Shallow can be perceived as the addition of learnable pixels to the original images. On the other hand, VPT-Deep inserts $l$ prompts $\{ P _ { m } \in \bar { \mathbb { R } } ^ { l \times D } \} _ { m = 0 } ^ { M }$ to the input of every Transformer layer $Z _ { m }$ but their outputs are discarded at the end of the layer as illustrated in Equation 12. + +Table 6. PEFT Methods Summary: Prompt-based and adapter-based methods incorporate additional parameters to modify features while keeping the original backbone intact. However, these added parameters introduce additional inference overhead. In contrast, selective tuning methods modify the backbone by updating selective parameters, thereby incurring no additional inference overhead. + +
MethodWhatTunableParametersHyperParametersModifiedTypeInferenceEfficient
VPT-Deeph1 = [h1, P]P Rl×Dl: Number of promptsFeature×
AdaptFormerh9 = h9 + Adapter(h7)Wdown /up Rr×D/D×r in Adapters: Scale factor in Adapterr: Bottleneck dimensionFeature×
Pfeif. Adapterh9 = Adapter(h9)Wown /up R×D/D in Adapters: Scale factor in Adapterr: Bottleneck dimensionFeature×
Houl. Adapterh5 = Adapter1(h5)h9 = Adapter2(h9) Rr×D/D×r in Adapter2s: Scale factor in Adapterr: Bottleneck dimensionFeature×
Convpassh5 = Convpass1(h2) + h5h9 = Convpass2(h7) + h9W 2 Rr×D/D×r in Convpass2s: Scale factor in Convpassr: Bottleneck dimensionk: Kernel size of conv2dFeature×
RepAdpaterh2 = RepAdapter1(h2)h7 = RepAdapter2(h7)bR in RepAdapter1ben RD× in RepAdapter2s: Scale factor in RepAdapterr: Bottleneck dimensionG: Number of groupsFeature×
LayerNormh2 = LayerNorm1(h1)h7 = LayerNorm2(h6)W 1(2), b1(2) RD in LayerNorm1(2)N/ABackbone
BitFitFine-tune all bias termsin the networkb1(2) RD in LayerNorm1(2)bQ//V RD in Q/K/Vttn R in FCattnb1 R4D, in FC1, b2 RD in FC2N/ABackbone
DiffFitLayerNorm + BitFith5 = γ1 · h5hg = γ2 · h9All tunable parametersin LayerNorm & BitFitγ1, γ2 RDN/ABackbone
SSFh2 = SSF2(h2), h3 = SSF3(h3)h5 = SSF5(h5), h7 = SSF7(h7)h8 = SSF7(h8), h9 = SSF9(h9)W 2,5,7,9 RD , b2,5,7,9 RDW3 R3D , b3 R3DW 8 R4D, 68 R4DN/ABackbone
LoRAh3 = LoRA(h2) + h3WQ/K/V Rr×D/D× in LoRAr: Bottleneck dimensionBackbone
FacTTT(TK)h3 = FacTTT(TK)(h2) + h3h5 = FacTTT(TK)(h4) + h5h8 = FacTTT(TK)(h7) + h8h9 = FacTTT(TK)(h8) + h9U RDr, V RDr,∑ R12L×r×r in FacTTTs: Scale factor in FacTTT(TK)r: Bottleneck dimensionBackboneV
U RD×r, V RD×r,
+ +$$ +\begin{array} { r l } & { [ \tilde { P } _ { 1 } , \boldsymbol { Z } _ { 1 } ] = L _ { m } ( [ P _ { 0 } , \boldsymbol { Z } _ { 0 } ] ) } \\ & { [ \tilde { P _ { m } } , \boldsymbol { Z } _ { m } ] = L _ { m } ( [ \tilde { P } _ { m - 1 } , \boldsymbol { Z } _ { m - 1 } ] ) \quad m = 2 , 3 , \dots , M } \end{array} +$$ + +$$ +[ \ , Z _ { m } ] = { \cal L } _ { m } ( [ P _ { m - 1 } , Z _ { m - 1 } ] ) \quad m = 1 , 2 , 3 , \ldots , M +$$ + +Throughout the adaptation process, the pre-trained model is frozen and no additional weights are introduced to the model, thereby preserving the model’s original behaviour. During the forward pass, the output $Z _ { m }$ of layer $m$ is changed because of the interaction between $Z _ { m - 1 }$ and $P _ { m - 1 }$ (or $\tilde { P } _ { m - 1 } ^ { \mathrm { ~ ~ } }$ ) in the MSA block. Thus, the output feature is adapted to the downstream tasks by iteratively tuning the prompts through gradient descent. + +![](images/figures/petl-visual-recognition-fig-0009.jpg) +Figure 9. An overview of a Transformer block in ViT. We adopt the notation $\{ h _ { 1 } , \ldots , h _ { 1 0 } \}$ to denote the intermediate features within a Transformer block to facilitate a clearer illustration of the PEFT methods discussed in subsection B.2. + +# B.2.2. Adapter-based Methods + +Adapter-based methods typically introduce additional trainable parameters into a frozen pre-trained model to facilitate learning of downstream tasks [54]. Initially developed for multi-domain adaptation [76, 77] and continual learning [66, 80], the idea of Adapters is subsequently embraced by Houlsby et al. [36] in the NLP domain to adapt Transformer-based networks for downstream tasks, and it also has garnered increasing interest in the computer vision field [102]. In this comparative analysis, we concentrate onfive popular Adapter-based methods, encompassing the original Adapter, along with variants focusing on adjusting the positions of Adapters [11, 74], introducing visual inductive biases [43], as well as employing re-parameterization to reduce the number of trainable parameters and inference latency [63]. + +Houl. Adapter [36] inserts two lightweight bottleneckstructured modules into each Transformer layer: one after the MSA block and the other after the MLP block. As depicted in Figure 10a, the Adapter is composed of a down-projection layer with $W _ { \mathrm { d o w n } } \in \mathbb { R } ^ { r \times D }$ , a nonlinear activation function $\sigma$ , an up-projection layer with $W _ { \mathsf { u p } } \in \mathbb { R } ^ { D \times r }$ , a scaling factor $s$ and a skip-connection. To limit the number of trainable parameters, the bottleneck dimension is much smaller than the feature dimension $r \ll D$ . Formally, Houl. Adapter can be defined as: + +$$ +\begin{array} { r l } & { h _ { 5 } = \mathrm { A d a p t e r } _ { 1 } ( h _ { 5 } ) \quad h _ { 9 } = \mathrm { A d a p t e r } _ { 2 } ( h _ { 9 } ) } \\ & { \quad \mathrm { A d a p t e r } ( h ) = s \cdot W _ { \mathrm { u p } } \sigma ( W _ { \mathrm { d o w n } } h ) + h } \end{array} +$$ + +Pfeif. Adapter [74] is a more efficient variant that introduces the Adapter solely after the MLP block, a strategy that has demonstrated effectiveness in recent studies [37]. Pfeif. Adapter can be defined formally as $h _ { 9 } =$ Adapter $\left( h _ { 9 } \right)$ where Adapter follows Equation 14. + +AdaptFormer [11] proposed to insert the Adapter in parallel with the MLP block, which differs from the sequential design of Houl. and Pfeif. Adapter. The rationale behind this parallel design lies in the belief that the domain-specific features generated by the Adapter can complement the domain-agnostic features derived from the original MLP block, leading to an improved feature ensemble [85]. Formally, AdaptFormer can be defined as $h _ { 9 } = h _ { 9 } + \mathrm { A d a p t e r } ( h 7 )$ where Adapter follows Equation 14. + +ConvPass (Convolutional By-Passes) [43] addresses the concern that many existing Adapters lack visual inductive bias, potentially limiting their performance for downstream vision tasks with limited data. To this end, the authors introduce a convolutional bottleneck module, running in parallel with the MSA or(and) MLP block. This module encompasses a $1 \times 1$ convolution reducing the channel with $W _ { \mathrm { d o w n } } \ \in \mathbb { R } ^ { r \times D }$ , a $3 \times 3$ convolution with the same input and output channel, a $1 \times 1$ convolution expanding the channel $W _ { \mathsf { u p } } \in \mathbb { R } ^ { D \times r }$ , two nonlinear functions $\sigma$ and a scaling factor $s$ , as shown in Figure 10b. The authors argue that Convpass is more efficient at capturing visual information in low-data scenarios due to its hard-coded locality of convolutional layers. The formal definition of Convpass is shown in Equation 15. + +$$ +\begin{array} { r } { h _ { 5 } = \mathrm { C o n v p a s s } _ { 1 } ( h _ { 2 } ) + h _ { 5 } \quad h _ { 9 } = \mathrm { C o n v p a s s } _ { 2 } ( h _ { 7 } ) + h _ { 9 } } \\ { \mathrm { C o n v p a s s } ( h ) = s \cdot W _ { \mathrm { u p } } \sigma ( \mathrm { C o n v 2 d } ( \sigma ( W _ { \mathrm { d o w n } } h ) ) ) } \end{array} +$$ + +![](images/figures/petl-visual-recognition-fig-0010.jpg) +Figure 10. Comparison of three Adapter structures. + +RepAdapter [63] found that the removal of the nonlinear function in the Adapter does not result in performance degradation for vision tasks. In light of this finding, the authors propose a linear Adapter with group-wise transformation [62] and sequentially added two of these linear Adapters to both MSA and MLP blocks. Owing to the sequential placement of the RepAdapter and its inherent linearity, the additional parameters can be re-parameterized to the original MSA or MLP block after training, thereby incurring zero additional costs during inference. RepAdapter is illustrated in Figure 10c and formally defined in Equation 16. + +$$ +\begin{array} { r l } & { h _ { 5 } = \mathrm { R e p A d a p t e r } _ { 1 } ( h _ { 2 } ) \quad h _ { 7 } = \mathrm { R e p A d a p t e r } _ { 2 } ( h _ { 7 } ) } \\ & { \qquad \mathrm { R e p A d a p t e r } ( h ) = s \cdot \phi _ { \mathrm { u p } } ( \phi _ { \mathrm { d o w n } } \left( h \right) ) + h } \\ & { \qquad \tilde { h } = \phi _ { \mathrm { d o w n } } \left( h \right) = W _ { \mathrm { d o w n } } h } \\ & { \qquad \phi _ { \mathrm { u p } } ( \tilde { h } ) = [ W _ { g 1 } \tilde { h } _ { g 1 } , \dots , W _ { g G } \tilde { h } _ { g G } ] } \end{array} +$$ + +where $W _ { \mathrm { d o w n } } \ \in \ \mathbb { R } ^ { r \times D }$ , $\tilde { h } _ { g ( 1 , \dots , G ) } \in \mathbb { R } ^ { \frac { r } { G } \times ( N + 1 ) }$ is the features splitted from $\tilde { h } \in \mathbb { R } ^ { r \times ( N + 1 ) }$ and $G$ is the number of groups in group-wise transformation [62]. $W _ { g ( 1 , \dots , G ) } \in$ $\textstyle \mathbb { R } ^ { \frac { D } { G } \times \frac { r } { G } }$ is the projection weight matrix. + +# B.2.3. Selective Parameter Tuning Methods + +The methods falling within this category aim to selectively update the parameters of a pre-trained model for downstream tasks. Within transfer learning, two prominent strategies, namely full fine-tuning and linear probing [48, 111], represent the two extremes of this category. Full fine-tuning updates all the model parameters end-to-end based on the new dataset while linear probing treats the pre-trained model as a feature extractor and only updates the prediction heads while keeping the backbone frozen. Although full fine-tuning generally exhibits superior performance compared to linear probing [104], it possesses certain limitations that may hinder its practicality in real-world production settings. Firstly, it requires running gradient descent for all parameters and necessitates storing a separate fine-tuned model for each task, incurring significant computational, memory, and storage overhead. These challenges become more salient with Transformer-based models whose parameters grow exponentially. Secondly, full fine-tuning may distort pre-trained features and underperform linear probing in out-of-distribution (OOD) scenarios [50]. + +To cope with the above issues, a cohort of PEFT methods has emerged under this category. In addition to the two common approaches mentioned above, our investigation encompasses seven methods that can be further categorized into two groups: direct selective tuning [7, 97, 103], which involves the direct modification of selective weights, and efficient selective tuning [37, 44, 55], which approximates the weight updates with low-rank factors. + +Notably, an extra advantage of methods in this category is that they introduce no additional inference latency, making them particularly favourable when inference efficiency is a priority. Methods within the direct selective tuning group abstain from introducing any new modules, thus inherently avoiding extra inference latency. Meanwhile, for methods in the efficient selective tuning group, the added modules can often be seamlessly integrated into weights of the pre-trained models through the re-parameterization techniques [18, 41], thereby ensuring the absence of increased inference latency as well. + +# Direct Selective Tuning + +BitFit [103] is a simple yet effective method that only tunes the bias parts of the pre-trained model. For each Transformer layer in ViT, BitFit updates the bias terms in the QKV projections and the FC layer in the MSA block, two FC layers in the MLP block and two LN blocks. It also updates the bias in the projection for patch embedding. The original authors underscore BitFit’s capability to achieve performance comparable to full fine-tuning or even surpass it under low and medium-data scenarios in BERT models [46]. + +LayerNorm [7] represents another simple but strong baseline that solely tunes the two LN blocks in each Transformer layer - one before the MSA block and another before the MLP block. Given that each LN block contains merely two trainable parameters $\{ W _ { L N } , b _ { L N } \} \in \mathbb { R } ^ { D }$ , LNtune stands out as an exceedingly light-weight approach compared to other PEFT methods. For instance, ViT-B/16 $\mathrm { \sim } 8 6 \mathrm { M }$ parameters) has only ${ \sim } 3 8 \mathrm { K }$ LN parameters, accounting for ${ \sim } 0 . 0 4 \%$ of the total parameters. + +DiffFit [97] is a recently proposed PEFT strategy designed for adapting large pre-trained diffusion models to the new domains. DiffFit exclusively fine-tunes the bias terms and the LN blocks within the network. Furthermore, it inserts learnable scale factors $\gamma$ to shift the features after the MSA and the MLP blocks, as shown in Equation 17. Consequently, DiffFit can be regarded as a combination of the BitFit and Ln-Tune, incorporating additional feature shift factors. + +$$ +\begin{array} { r } { h _ { 5 } = \gamma _ { 1 } \cdot h _ { 5 } } \\ { h _ { 9 } = \gamma _ { 2 } \cdot h _ { 9 } } \end{array} +$$ + +# Efficient Selective Tuning + +LoRA (Low-Rank Adaptation) [37] drew inspiration from recent investigations demonstrating that the learned over-parametrized models in fact reside on a low intrinsic dimension [1, 52]. Building upon this insight, the authors hypothesize that the change in weights during model adaptation also exhibits a low intrinsic rank and injects trainable low-rank decomposition matrices to approximate the weight updates. The LoRA update methodology is strategically applied to the Query/Value projection weights + +$W _ { Q / V } \in \mathbb { R } ^ { D \times D }$ within the MSA block. Concretely, the +weight updates are approximated as $W _ { Q / V } + \Delta W _ { Q / V } =$ +$W _ { Q / V } + W _ { \mathrm { d o w n } } ^ { Q / V } W _ { \mathrm { u p } } ^ { Q / V }$ where W Q/V $W _ { \mathrm { d o w n / u p } } ^ { Q / V } \in \mathbb { R } ^ { D \times r / r \times D }$ and $r \ll D$ and zero for zero at the b so that g of tra $W _ { \mathrm { u p } } ^ { Q / V }$ $W _ { \mathrm { d o w n } } ^ { Q / V }$ $\Delta W _ { Q / V } =$ +$W _ { \mathrm { d o w n } } ^ { Q / V } W _ { \mathrm { u p } } ^ { Q / V }$ +formal definition of LoRA is articulated in Equation 18, +utilizing the notations delineated in Figure 9. + +$$ +\begin{array} { c } { { h _ { 3 } = \mathrm { L o R A } ( h _ { 2 } ) + h _ { 3 } } } \\ { { h _ { 3 } = [ { \cal Q } , { \cal K } , { \cal V } ] } } \\ { { \mathrm { L o R A } ( h _ { 2 } ) = [ W _ { \mathrm { d o w n } } ^ { \cal Q } W _ { \mathrm { u p } } ^ { \cal Q } h _ { 2 } , 0 , W _ { \mathrm { d o w n } } ^ { \cal V } W _ { \mathrm { u p } } ^ { \cal V } h _ { 2 } ] } } \end{array} +$$ + +FacT (Factor Tuning) [44] is inspired by the recent advances in Transformer compression [92, 106]and exploited the low-rank update paradigm (e.g., LoRA) to the extreme. While LoRA posits that the update for an individual weight matrix manifests a low-rank characteristic during fine-tuning, FacT advances the proposition that the weight updates spanning different matrices can also be effectively approximated using low-rank decomposition matrices. Specifically, FacT encapsulates the four weight matrices $W _ { Q / K / V / O } \ \in \ \mathbb { R } ^ { D \times D }$ in the MSA block and the two weight matrices ${ \pmb W } _ { 1 } ~ \in ~ \mathbb { R } ^ { D \times 4 D }$ , $W _ { 2 } ~ \in ~ \mathbb { R } ^ { 4 D \times D }$ in the MLP block into a single $W _ { F a c T } \in \mathbb { R } ^ { 1 2 M \times D \times D }$ tensor where $M$ is the number of Transformer layer. The update of $W _ { F a c T }$ , $\Delta W _ { F a c T }$ , can be decomposed into several factors to promote parameter efficiency. To this end, the authors leverage the well-established Tensor-Train (TT) [72]and the Tucker (TK) [14] format to decompose $\Delta W _ { F a c T }$ . FacTTT and $\mathrm { F a c T } _ { \mathrm { T K } }$ are used to denote different decomposition formats for FacT and their formal definitions can be found in Equation 19. + +$$ +\begin{array} { r l } & { \mathrm { F a c T } _ { \mathrm { T T } } : \Delta W _ { F a c T } = s \cdot \Sigma \times _ { 2 } U ^ { \top } \times _ { 3 } V ^ { \top } } \\ & { \mathrm { F a c T } _ { \mathrm { T K } } : \Delta W _ { F a c T } = s \cdot A \times _ { 1 } B ^ { \top } \times _ { 2 } U ^ { \top } \times _ { 3 } V ^ { \top } } \end{array} +$$ + +where ${ \pmb U } \ \in \ \mathbb { R } ^ { D \times r } , { \pmb V } \ \in \ \mathbb { R } ^ { D \times r } , { \pmb \Sigma } \ \in \ \mathbb { R } ^ { 1 2 L \times r \times r } , { \pmb B } \ \in$ $\mathbb { R } ^ { 1 2 L \times r } A \in \mathbb { R } ^ { r \times r \times r }$ and the $\times _ { j }$ denotes mode- $j$ product and $s$ is the scaling factor. + +Since $\Delta W _ { F a c T }$ contains the updates for $W _ { Q / K / V / O } , W _ { 1 / 2 }$ , the modified forward pass inherently influences $h _ { 3 } , h _ { 5 } , h _ { 8 } , h _ { 9 }$ . Let’s consider $h _ { 5 }$ for elucidation. Once the weight update $\Delta W _ { F a c T }$ is calculated with $\mathrm { F a c T } _ { \mathrm { T T ( T K ) } }$ in Equation 19, the corresponding update for $W _ { O }$ , $\Delta W _ { O }$ , is extracted from $\Delta W _ { F a c T }$ . Similar to the modified forward pass of LoRA, $h _ { 5 } = h _ { 4 } \Delta { \cal W } _ { O } + h _ { 5 }$ . + +SSF (Scale & Shift deep Features) [55] employs linear transformations to adapt the intermediate features extracted by a pre-trained model. Motivated by the feature modulation methods [39, 73], SSF is designed to accommodate the distribution difference between the upstream and downstream datasets. Specifically, SSF modulates the features residing at $h _ { 2 } , h _ { 3 } , h _ { 5 } , h _ { 7 } , h _ { 8 } , h _ { 9 }$ by incorporating scale and shift factors. To demonstrate the mechanism of SSF, let’s consider $h _ { 5 } \in \mathbb { R } ^ { ( N + 1 ) \times D }$ as an illustrative example and other features can similarly undergo the same transformative process. Formally, the modulated $h _ { 5 }$ is formulated as follows. + +![](images/figures/petl-visual-recognition-fig-0011.jpg) +Figure 11. Performance gain for PEFT methods by turning droppath-rate on. + +$$ +h _ { 5 } = \mathrm { S S F } _ { 5 } ( h _ { 5 } ) = \pmb { w } ^ { 5 } \odot h _ { 5 } + \pmb { b } ^ { 5 } +$$ + +where ${ \pmb w } ^ { 5 } \in \mathbb { R } ^ { D }$ , $\pmb { b } ^ { 5 } \in \mathbb { R } ^ { D }$ are the scale and shift factors affiliated with the SSF module attributed to $h _ { 5 }$ , and $\odot$ is the dot product. It is noteworthy that each modulated feature has its own SSF module with corresponding scale and shift factors. The modification details for other features are summarized in Table 6. + +# C. More Detailed Results + +Drop-path-rate. Learning with low-shot data is prone to over-fitting. We find that if the drop path rate — which stochastically drops a transformer block per sample [38] — is set not as default (i.e., nonzero), all the methods can benefit from such a regularization. Figure 11 shows the performance gain by tuning the drop-path-rate on compared with the default 0. + +More results on prediction similarity analysis. Figure 13 shows the prediction analysis discussed in section 4 for all the datasets in VTAB-1K. It is expected that their predictions are similar for datasets with very high accuracy, such as Flowers102 $( \mathrm { a v g 9 9 . 1 \% } )$ and Caltech101 $( \mathrm { a v g 9 l . 4 \% } )$ . Beyond them, we find that most PEFT methods show diverse predictions in other datasets in VTAB-1K. + +Prediction similarity within the same PEFT group. To verify if methods within the same PEFT group share more prediction similarity, we plotted the prediction overlap for adapter-based methods, selective-tuning methods, and methods from different groups. As shown in Figure 12, methods within the same group share slightly more prediction similarity than those from different groups, but they still exhibit distinct predictions. Figure 3a in the main paper also supports this observation. Methods are grouped based on the categories defined in subsection 2.2. If methods within the same group had very high similarities, we would see bright squares, which are only slightly evident around BitFit, Diff-Fit, LayerNorm, and SSF. + +# WiSE PEFT results for all distribution shift datasets. + +We provide detailed WiSE PEFT performance for each distribution shift dataset in Figure 14. WiSE improves both the robustness and the in-distribution performance of PEFT methods. Interestingly, even though full fine-tuning is generally less robust than PEFT methods, applying WiSE allows it to achieve better performance in both target distribution and distribution shift data. + +Performance comparison between DINOv2 and IN21k. To compare the performance of DINOv2 and IN21k, we selected several PEFT methods and datasets from VTAB-1K. The results presented in Table 7 reveal several interesting findings: + +(1) Improved Linear Probing Performance: Linear probing generally shows improved results with DINOv2, indicating that its extracted features are more robust and discriminative than those from IN21k. + +(2) Deteriorated Full Fine-Tuning Performance: Conversely, full fine-tuning performance significantly worsens with DINOv2, suggesting that models fully fine-tuned on DINOv2 are more susceptible to overfitting. + +(3) Adapter-Based Methods Performance: Among the three adapter-based methods evaluated—Houl. Adapter, AdaptFormer, and Convpass—we observe performance enhancements in most datasets for AdaptFormer and Convpass. In contrast, Houl. Adapter exhibits significant degradation across all datasets. This disparity may be attributed to architectural differences: AdaptFormer and Convpass insert their adapter modules in parallel with existing modules such as Multi-Head Self-Attention (MSA) and/or Multi-Layer Perceptron (MLP), whereas Houl. Adapter inserts its adapter sequentially after the MSA and MLP layers. We hypothesize that the sequential design of Houl. Adapter leads to more substantial alterations of intermediate features compared to the parallel design, potentially explaining the observed decrease in performance. + +![](images/figures/petl-visual-recognition-fig-0012.jpg) +Figure 12. Prediction overlap for the 5K most confident samples. Although methods from the same group share slightly more prediction overlap than methods from other groups, they still have quite different predictions + +Table 7. Performance comparison between DINOv2 and IN21k. + +
LinearFullSSFHoul. AdapterAdapt- FormerConvpassLoRA
NaturalCaltech101Dinov289.883.290.322.192.591.792.3
IN21k86.689.989.892.191.892.192.6
3.2-6.70.5-70.00.7-0.4-0.3
Dinov274.945.277.014.678.877.078.4
DTDIN21k65.761.968.872.370.572.069.8
9.2-16.78.2-57.78.35.08.6
Dinov293.468.792.67.194.092.394.1
Pets Sun397IN21k89.385.891.491.791.891.390.5
4.1-17.11.2-84.62.21.03.6
Dinov255.123.952.52.656.556.155.7
IN21k53.236.8 -12.956.5 -4.055.4 -52.856.7 -0.255.955.5
Specialized1.9 83.284.40.20.2
CamelyonDinov277.983.977.186.385.4
IN21k83.181.686.188.786.887.787.5
0.1 89.2-3.7-2.2-11.6-2.4-1.4-2.1
EuroSATDinov266.993.960.293.893.894.2
IN21k90.0 -0.888.1 -21.294.595.395.0 -1.295.894.9
∆ Dinov278.825.9-0.6-35.188.6-2.0-0.7
Resisc4574.981.682.024.586.587.684.2
IN21k3.9-55.783.286.5 -62.085.985.9
∆ Dinov275.373.6-1.22.11.7-1.7
RetinopathyIN21k74.673.676.0 74.873.676.0 76.376.075.5
0.70.075.2 -1.6-0.375.975.7
1.20.1-0.2
StructuredClevr-CountDinov247.527.371.238.191.287.889.8
IN21k37.556.280.182.982.982.382.9
10.0-28.9-8.9-44.88.35.56.9
DMLabDinov244.130.751.839.151.853.154.5
IN21k36.548.253.053.852.853.851.8
7.6-17.5-1.2-14.7-1.0-0.72.7
KITTIDinov260.347.181.046.883.482.683.8
IN21k64.677.981.479.680.078.179.9
-4.3-30.8-0.4-32.83.44.53.9
dSpr-OriDinov247.217.556.110.057.955.657.2
IN21k29.446.652.154.353.055.347.2
17.8-29.14.0-44.34.90.310.0
+ +Figure 1a details. This figure illustrates the relative performance compared to linear probing $( \times )$ on VTAB-1K. The range between the highest and lowest accuracy across 14 + +PEFT methods is represented by $\bullet - \bullet$ , while $( \mathsf { I } )$ denotes the performance of full fine-tuning. + +![](images/figures/petl-visual-recognition-fig-0013.jpg) +Figure 13. Prediction similarity analysis on other datasets. + +![](images/figures/petl-visual-recognition-fig-0014.jpg) +Figure 14. WiSE PEFT performance on all distribution shift datasets. Target distribution vs. distribution shifts +Figure 1c details. The X-axis represents the accuracy on ImageNet-1K, while the Y-axis shows the distribution shift accuracy (averaged across ImageNet-V2, ImageNet-S, ImageNet-R, and ImageNet-A). The cyan squares (■ represent the zero-shot performance of the CLIP model, and stars $( { \star } )$ denote the performance of fine-tuned models. Each curve corresponds to the WiSE $+$ PEFT method, with dots • indicating different mixing coefficients $\alpha$ as described in section 7. +Figure 2 details. For each dataset in VTAB-1K, 15 methods (14 PEFT methods plus linear probing) are ranked by accuracy. Within each dataset group (e.g., Natural), the element $( i , j )$ in the ranking frequency matrix indicates how often method $i$ ranks $j ^ { t h }$ . For instance, in the Natural group matrix, the entry $( 1 , 3 )$ equals 2, meaning DiffFit ranked 3rd in two datasets within this group. The row sums correspond to the total number of datasets in each group (e.g., 7 datasets + +for the Natural group). Methods are sorted by their average rank (shown in brackets), and the parameters column indicates the number of trainable parameters in millions. + +Figure 3a details. Each entry $( i , j )$ in the prediction similarity matrix represents the percentage of test samples for which methods $i$ and $j$ made the same prediction. The diagonal entries are always 1, indicating perfect agreement with themselves. To compute $( i , j )$ , predictions from models fine-tuned by methods $i$ and $j$ are compared, with $( i , j )$ equaling the number of matching predictions divided by the total test samples. + +Figure 3b details. The Venn diagrams are generated by fine-tuning a pre-trained model on CIFAR100 (VTAB-1K) using LoRA, SSF, and Adapter methods. For Figure 3b(a), we selected the correct predictions from the top 5K most confident samples for each method and visualized the overlap among the three methods. For Figure 3b(b), we did the same for the wrong predictions, selecting from the 5K least confident samples. + +Figure 4 details. For each VTAB-1K dataset, the worstperforming PEFT method serves as the baseline $( \nsim )$ . Each represents the relative performance of other PEFT methods compared to this baseline. An ensemble prediction ( ) is generated based on the average logits of all PEFT methods for each test sample. + +Figure 5 details. Different colors represent various PEFT methods. Each $\bullet$ along a curve (corresponding to a single PEFT method) indicates the accuracy at a specific tunable parameter size, allowing us to observe how the size of tunable parameters impacts accuracy. + +# D. Broader Impacts + +Our study provides a unifying study of PEFT in visual recognition. We expect it to serve as a valuable practical user guide to benefit society. Specifically, fine-tuning large models needs significant computation. A unifying study of PEFT will ease end-users to apply more parameter-efficient and computation-efficient ways for fine-tuning. To our knowledge, our paper does not introduce any additional negative societal impacts compared to existing papers on PEFT. \ No newline at end of file diff --git a/papers/petl-visual-recognition/paper.pdf b/papers/petl-visual-recognition/paper.pdf new file mode 100644 index 0000000000000000000000000000000000000000..1206661f489cc033dbd47a1c10cfd8504d57c2af --- /dev/null +++ b/papers/petl-visual-recognition/paper.pdf @@ -0,0 +1,3 @@ +version https://git-lfs.github.com/spec/v1 +oid sha256:18c13e25306b3a5bab3917cc5206807538caaa519690f24b5eeefb056ceb2dec +size 2479066 diff --git a/papers/petl-visual-recognition/sau.json b/papers/petl-visual-recognition/sau.json new file mode 100644 index 0000000000000000000000000000000000000000..8f3acd629cb6adb20d10921b6b1bc371ab2ee5f2 --- /dev/null +++ b/papers/petl-visual-recognition/sau.json @@ -0,0 +1,337 @@ +{ + "paper_id": "petl-visual-recognition", + "paper_title": "Parameter-Efficient Transfer Learning for Visual Recognition", + "D1": [ + { + "id": "petl-visual-recognition-D1-001", + "claim": "Backbone: ViT-B/16 with 86M params, 12 Transformer layers, patch_size P=16, embedding_dim D determined by ViT-B/16 spec, num_heads Nh determined by ViT-B/16 spec, head_dim Dh = D/Nh, MLP expansion ratio=4", + "source": "Section 3 (Setup), Appendix B.1 (Vision Transformer)" + }, + { + "id": "petl-visual-recognition-D1-002", + "claim": "LayerNorm parameters: ~38K (0.04% of total 86M backbone)", + "source": "Appendix B.2.3 (Direct Selective Tuning, LayerNorm)" + }, + { + "id": "petl-visual-recognition-D1-003", + "claim": "VTAB-1K low-shot benchmark: 19 tasks in 3 groups (Natural 7, Specialized 4, Structured 8), 1000 training samples per dataset, train/val split 800/200 (80/20 split)", + "source": "Section 3 (Dataset)" + }, + { + "id": "petl-visual-recognition-D1-004", + "claim": "VTAB-1K training setup: AdamW optimizer, batch_size=64, epochs=100, cosine_decay lr scheduler, no data augmentation, drop_path_rate search [0, 0.1], PEFT param cap <=1.5% of ViT-B/16 (~Q/K/V of single MSA block)", + "source": "Section 3 (Setup), Appendix A.1 (VTAB-1K)" + }, + { + "id": "petl-visual-recognition-D1-005", + "claim": "VTAB-1K hyperparameter search: learning_rate range [0.001, 0.01], weight_decay range [0.0001, 0.001], grid search over method-specific ranges", + "source": "Appendix A.1 (VTAB-1K)" + }, + { + "id": "petl-visual-recognition-D1-006", + "claim": "VPT-Shallow: prompt_number range [5, 10, 50, 100, 200], param range [0.3K, 153K]", + "source": "Appendix A.1 (Table 3)" + }, + { + "id": "petl-visual-recognition-D1-007", + "claim": "VPT-Deep: prompt_number range [5, 10, 50, 200], param range [46K, 921K]", + "source": "Appendix A.1 (Table 3)" + }, + { + "id": "petl-visual-recognition-D1-008", + "claim": "BitFit: 102K tunable params, tunes bias terms in patch embedding projection, Q/K/V weights, MLP FC layers, LN blocks", + "source": "Appendix A.1 (Table 3), Appendix B.2.3 (BitFit)" + }, + { + "id": "petl-visual-recognition-D1-009", + "claim": "DiffFit: 140K tunable params (bias + LN tuning + feature scaling)", + "source": "Appendix A.1 (Table 3), Appendix B.2.3 (DiffFit)" + }, + { + "id": "petl-visual-recognition-D1-010", + "claim": "LayerNorm Tuning: 38K tunable params (0.04% of backbone)", + "source": "Appendix A.1 (Table 3), Appendix B.2.3 (LayerNorm)" + }, + { + "id": "petl-visual-recognition-D1-011", + "claim": "SSF (Scale & Shift Features): 205K tunable params, modulates features at positions h2, h3, h5, h7, h8, h9 per Transformer layer", + "source": "Appendix A.1 (Table 3), Appendix B.2.3 (SSF)" + }, + { + "id": "petl-visual-recognition-D1-012", + "claim": "Pfeiffer Adapter: scale_factor range [0.01, 0.1, 1, 10], bottleneck range [4, 8, 16, 32], param range [82K, 599K]", + "source": "Appendix A.1 (Table 3)" + }, + { + "id": "petl-visual-recognition-D1-013", + "claim": "Houlsby Adapter: scale_factor range [0.01, 0.1, 1, 10], bottleneck range [4, 8, 16, 32], param range [165K, 1.198M]", + "source": "Appendix A.1 (Table 3)" + }, + { + "id": "petl-visual-recognition-D1-014", + "claim": "AdaptFormer: scale_factor range [0.05, 0.1, 0.2], bottleneck range [4, 16, 32], param range [82K, 599K]", + "source": "Appendix A.1 (Table 3)" + }, + { + "id": "petl-visual-recognition-D1-015", + "claim": "RepAdapter: scale_factor range [0.1, 0.5, 1, 5, 10], bottleneck range [8, 16, 32], param range [239K, 903K], group-wise transformation with G groups", + "source": "Appendix A.1 (Table 3)" + }, + { + "id": "petl-visual-recognition-D1-016", + "claim": "Convpass: scale_factor range [0.01, 0.1, 1, 10, 100], bottleneck range [8, 16], Xavier init [true, false], param range [327K, 664K]", + "source": "Appendix A.1 (Table 3)" + }, + { + "id": "petl-visual-recognition-D1-017", + "claim": "LoRA: bottleneck rank r range [1, 8, 16, 32] with r << D, applied to W_Q and W_V matrices, param range [36K, 1.179M]", + "source": "Appendix A.1 (Table 3), Appendix B.2.3 (LoRA)" + }, + { + "id": "petl-visual-recognition-D1-018", + "claim": "FacT-TT: scale_factor range [0.01, 0.1, 1, 10, 100], bottleneck range [8, 16, 32], param range [21K, 196K], stacks 12M weight matrices (12 layers x 6 matrices Q/K/V/O/W1/W2)", + "source": "Appendix A.1 (Table 3)" + }, + { + "id": "petl-visual-recognition-D1-019", + "claim": "FacT-TK: scale_factor range [0.01, 0.1, 1, 10, 100], bottleneck range [16, 32, 64], param range [30K, 369K]", + "source": "Appendix A.1 (Table 3)" + }, + { + "id": "petl-visual-recognition-D1-020", + "claim": "Many-shot training setup: AdamW, batch_size=64, epochs=40, cosine_decay lr scheduler, lr range [0.0005, 0.001], weight_decay range [0.0001, 0.001]", + "source": "Section 5 (Setup), Appendix A.1 (Many-shot)" + }, + { + "id": "petl-visual-recognition-D1-021", + "claim": "Many-shot datasets: CIFAR-100 (50K train, 100 classes, horizontal_flip, 90/10 split), RESISC (25.2K train, 45 classes, horizontal+vertical flips, 90/10 split), Clevr-Distance (70K train, 6 classes, no augmentation)", + "source": "Section 5 (Dataset), Appendix A.2 (Many-shot Datasets)" + }, + { + "id": "petl-visual-recognition-D1-022", + "claim": "Many-shot PEFT parameter budget study: effective_param_percent range [2%, 5%]", + "source": "Section 5 (Results, Recipes)" + }, + { + "id": "petl-visual-recognition-D1-023", + "claim": "Robustness experiment: CLIP ViT-B/16 backbone, ImageNet-1K target with 100 shots/class, lr=3e-5, weight_decay=0.005, strong data augmentation (NPS), 80 class text prompts, distribution shift test sets: ImageNet-V2/ImageNet-R/ImageNet-S/ImageNet-A", + "source": "Section 7 (Setup), Appendix A.1 (Robustness Setup, Robustness Model)" + }, + { + "id": "petl-visual-recognition-D1-024", + "claim": "WiSE mixing coefficient alpha: linear interpolation coefficient in [0,1], evaluated at multiple points for target-robustness trade-off analysis", + "source": "Section 7 (WiSE for PEFT)" + }, + { + "id": "petl-visual-recognition-D1-025", + "claim": "Pre-training datasets: ImageNet-21K (14M images), ImageNet-1K (1.3M images), LAION-5B (5B images)", + "source": "Section 1 (Introduction), Section 2.1" + }, + { + "id": "petl-visual-recognition-D1-026", + "claim": "Computation hardware: 8x NVIDIA RTX 6000 Ada GPUs, 2x AMD EPYC 9554 64-Core CPUs, 800GB RAM", + "source": "Appendix A.1 (Computation)" + } + ], + "D2": [ + { + "id": "petl-visual-recognition-D2-001", + "claim": "ViT Transformer Layer: Z'_m = MSA(LN(Z_{m-1})) + Z_{m-1}; Z_m = MLP(LN(Z'_m)) + Z'_m — forward pass with pre-norm, residual MSA, and residual MLP blocks. Each layer L_m maps Z_{m-1} in R^{D x (1+N)} to Z_m in R^{D x (1+N)}.", + "source": "Appendix B.1 (Vision Transformer)" + }, + { + "id": "petl-visual-recognition-D2-002", + "claim": "Single-Head Self-Attention: Q^{(i)} = W_Q^{(i)} Z; K^{(i)} = W_K^{(i)} Z; V^{(i)} = W_V^{(i)} Z where W_{Q/K/V}^{(i)} in R^{Dh x D}, Dh = D/Nh. Attn^{(i)}(Z) = V^{(i)} x Softmax(K^{(i)^T} Q^{(i)} / sqrt(D_h)) in R^{Dh x (1+N)}.", + "source": "Appendix B.1 (Vision Transformer)" + }, + { + "id": "petl-visual-recognition-D2-003", + "claim": "MSA Output: MSA(Z) = W_O [Attn^0(Z), ..., Attn^{N_h}(Z)] — concatenation of all head outputs projected by W_O in R^{D x (D_h * N_h)}", + "source": "Appendix B.1 (Vision Transformer)" + }, + { + "id": "petl-visual-recognition-D2-004", + "claim": "MLP Block: MLP(Z) = GELU(Z W_1 + b_1) W_2 + b_2 with W_1 in R^{D x 4D}, W_2 in R^{4D x D}, b_1 in R^{4D}, b_2 in R^{D}, GELU activation. Two FC layers (FC1, FC2) with expansion ratio 4.", + "source": "Appendix B.1 (Vision Transformer)" + }, + { + "id": "petl-visual-recognition-D2-005", + "claim": "ViT Input Construction: Z_0 = [x_0^{(Class)}, x_0^{(1)}, ..., x_0^{(N)}] + E_{pos} where N = H*W/P^2 patches. Each patch I^{(n)} in R^{P^2 x C} is flattened and embedded via trainable linear projection to x_0^{(n)} in R^D. A learnable class token x_0^{(Class)} is prepended, and position embeddings E_{pos} in R^{D x (1+N)} are added.", + "source": "Appendix B.1 (Vision Transformer)" + }, + { + "id": "petl-visual-recognition-D2-006", + "claim": "LoRA Weight Update: W_{Q/V} + DeltaW_{Q/V} = W_{Q/V} + W_{down}^{Q/V} W_{up}^{Q/V} — low-rank decomposition with W_{down}^{Q/V} in R^{r x D}, W_{up}^{Q/V} in R^{D x r}, r << D. Applied to Q and V matrices only; zero inference overhead via weight merging (DeltaW_{Q/V} merged into W_{Q/V}). W_{up}^{Q/V} initialized to zero so DeltaW = 0 at start of training.", + "source": "Appendix B.2.3 (Efficient Selective Tuning, LoRA)" + }, + { + "id": "petl-visual-recognition-D2-007", + "claim": "LoRA Forward Pass: h_3 = LoRA(h_2) + h_3 where h_3 = [Q, K, V] and LoRA(h_2) = [W_down^Q W_up^Q h_2, 0, W_down^V W_up^V h_2] — additive residual on Q and V only (K unchanged, residual=0). LoRA modifies the Q/V projections within the MSA block.", + "source": "Appendix B.2.3 (Efficient Selective Tuning, LoRA)" + }, + { + "id": "petl-visual-recognition-D2-008", + "claim": "Houlsby Adapter: two adapter modules placed at h_5 (after MSA) and h_9 (after MLP). Formally: h_5 = Adapter_1(h_5), h_9 = Adapter_2(h_9) where Adapter(h) = s * W_up * sigma(W_down * h) + h. The bottleneck module composes W_down in R^{r x D} (down-projection), nonlinear activation sigma (GELU/ReLU), W_up in R^{D x r} (up-projection), scale factor s, and residual skip connection. r << D for parameter efficiency.", + "source": "Appendix B.2.2 (Adapter-based Methods, Houlsby Adapter)" + }, + { + "id": "petl-visual-recognition-D2-009", + "claim": "Pfeiffer Adapter: single adapter placed only after MLP block. Formally: h_9 = Adapter(h_9) where Adapter(h) = s * W_up * sigma(W_down * h) + h. Same bottleneck module structure (W_down in R^{r x D}, sigma, W_up in R^{D x r}, scale factor s, skip connection) as Houlsby but more parameter-efficient: one adapter per layer instead of two.", + "source": "Appendix B.2.2 (Adapter-based Methods, Pfeiffer Adapter)" + }, + { + "id": "petl-visual-recognition-D2-010", + "claim": "AdaptFormer: parallel adapter at h_9 = h_9 + Adapter(h_7), where Adapter(h_7) = s * W_up * sigma(W_down * h_7). Domain-specific features from the adapter (taking pre-MLP feature h_7 as input) complement domain-agnostic MLP output h_9. The parallel design differs from Houlsby/Pfeiffer sequential placement — adapter runs alongside (not after) the MLP block.", + "source": "Appendix B.2.2 (Adapter-based Methods, AdaptFormer)" + }, + { + "id": "petl-visual-recognition-D2-011", + "claim": "Convpass: convolutional by-pass with Convpass(h) = s * W_up * sigma(Conv2d(sigma(W_down * h))). Architecture: W_down: 1x1 conv (D -> r), sigma activation, Conv2d: 3x3 conv (r -> r, same input/output channels), sigma activation, W_up: 1x1 conv (r -> D). Placed parallel: h_5 = Convpass_1(h_2) + h_5; h_9 = Convpass_2(h_7) + h_9. The 3x3 conv encodes visual inductive bias via hard-coded locality over nearby patch tokens.", + "source": "Appendix B.2.2 (Adapter-based Methods, Convpass)" + }, + { + "id": "petl-visual-recognition-D2-012", + "claim": "RepAdapter: linear re-parameterizable adapter without nonlinearity, placed sequentially: h_5 = RepAdapter_1(h_2) and h_7 = RepAdapter_2(h_7). Formally: RepAdapter(h) = s * phi_up(phi_down(h)) + h where phi_down(h) = W_down * h (W_down in R^{r x D}) and phi_up(tilde_h) = [W_{g1} * tilde_h_{g1}, ..., W_{gG} * tilde_h_{gG}] with G group-wise projections. Each group splits tilde_h into G chunks of size r/G, and W_{gi} in R^{D/G x r/G}. Includes internal skip connection +h. Can be merged into original weights for zero inference overhead due to linearity.", + "source": "Appendix B.2.2 (Adapter-based Methods, RepAdapter)" + }, + { + "id": "petl-visual-recognition-D2-013", + "claim": "VPT-Shallow: [tilde_P_1, Z_1] = L_1([P_0, Z_0]); [tilde_P_m, Z_m] = L_m([tilde_P_{m-1}, Z_{m-1}]) for m=2..M — learnable prompts P_0 in R^{l x D} (l prompts, D dimensional) prepended at first layer input. Prompt outputs tilde_P_{m-1} propagated through subsequent layers. Only P_0 is updated; backbone frozen.", + "source": "Appendix B.2.1 (Prompt-based Methods, VPT-Shallow)" + }, + { + "id": "petl-visual-recognition-D2-014", + "claim": "VPT-Deep: [_, Z_m] = L_m([P_{m-1}, Z_{m-1}]) for m=1..M — separate learnable prompts P_{m-1} in R^{l x D} per layer m. Previous layer prompt outputs discarded (not propagated, denoted by _), fresh prompts at each layer input. All P_{m-1} across M layers are updated during fine-tuning.", + "source": "Appendix B.2.1 (Prompt-based Methods, VPT-Deep)" + }, + { + "id": "petl-visual-recognition-D2-015", + "claim": "SSF Feature Modulation: h_i = SSF_i(h_i) = w^i odot h_i + b^i for i in {2, 3, 5, 7, 8, 9} — per-channel learnable scale w^i in R^D and shift b^i in R^D at 6 feature positions per Transformer layer. odot denotes channel-wise multiplication. SSF accommodates distribution difference between upstream and downstream datasets via linear transformation. Merged into weights at inference for zero overhead.", + "source": "Appendix B.2.3 (Direct Selective Tuning, SSF)" + }, + { + "id": "petl-visual-recognition-D2-016", + "claim": "DiffFit Feature Scaling: h_5 = gamma_1 * h_5; h_9 = gamma_2 * h_9 — learnable per-channel scale factors gamma_1, gamma_2 in R^D applied after MSA and MLP blocks respectively. Combined with BitFit (all bias terms) and LayerNorm tuning (LN weights and biases). Total: 140K tunable params for ViT-B/16. DiffFit = BitFit + LN-Tune + feature scaling factors.", + "source": "Appendix B.2.3 (Direct Selective Tuning, DiffFit)" + }, + { + "id": "petl-visual-recognition-D2-017", + "claim": "BitFit: fine-tunes only bias terms b in all network components. Per Transformer layer: Q = W_Q * Z + b_Q (tune b_Q, freeze W_Q); K = W_K * Z + b_K; V = W_V * Z + b_V; FC_attn output bias b_attn; FC1: Z * W_1 + b_1 (tune b_1 in R^{4D}, freeze W_1); FC2: GELU(.) * W_2 + b_2 (tune b_2 in R^D). LN blocks: LN(h) = W_LN * (h - mu)/sigma + b_LN (tune W_LN, b_LN). Also tunes bias in patch embedding projection. All weight matrices remain frozen. 102K tunable params for ViT-B/16.", + "source": "Appendix B.2.3 (Direct Selective Tuning, BitFit)" + }, + { + "id": "petl-visual-recognition-D2-018", + "claim": "LayerNorm-Only Tuning: updates LN weight and bias per LN block (2 LN per layer). Formally: LN_1(h_1) = W_{LN_1} * (h_1 - mu_1)/sigma_1 + b_{LN_1} (before MSA, at h_2); LN_2(h_6) = W_{LN_2} * (h_6 - mu_2)/sigma_2 + b_{LN_2} (before MLP, at h_7). Each LN contains 2 trainable parameters {W_LN, b_LN} in R^D. Total: 4DM params (~38K for ViT-B/16 with M=12, D=768; 0.04% of 86M backbone).", + "source": "Appendix B.2.3 (Direct Selective Tuning, LayerNorm)" + }, + { + "id": "petl-visual-recognition-D2-019", + "claim": "FacT-TT (Tensor-Train): stacks Q/K/V/O/W1/W2 across M layers into 3D tensor W_{FacT} in R^{12M x D x D}. Weight update: DeltaW_{FacT} = s * Sigma x_2 U^T x_3 V^T with factor matrices U in R^{D x r}, V in R^{D x r} and TT-core Sigma in R^{12M x r x r}. x_j denotes mode-j product. s is the scaling factor.", + "source": "Appendix B.2.3 (Efficient Selective Tuning, FacT)" + }, + { + "id": "petl-visual-recognition-D2-020", + "claim": "FacT-TK (Tucker): DeltaW_{FacT} = s * A x_1 B^T x_2 U^T x_3 V^T — core tensor A in R^{r x r x r}, factor matrices B in R^{12M x r}, U in R^{D x r}, V in R^{D x r}. Tucker decomposition generalizes TT by allowing interactions across all three modes via the core tensor A. s is the scaling factor.", + "source": "Appendix B.2.3 (Efficient Selective Tuning, FacT)" + }, + { + "id": "petl-visual-recognition-D2-021", + "claim": "FacT Forward Pass: modifies h_3, h_5, h_8, h_9 via extracted weight updates from DeltaW_{FacT}. For h_5: h_5 = h_4 * DeltaW_O + h_5 where DeltaW_O is the W_O update extracted from the full stacked tensor DeltaW_{FacT}, which contains updates for all 6 weight matrices (Q/K/V/O/W1/W2) across all M layers. Similarly, DeltaW_Q/K/V modify h_3, DeltaW_1/2 modify h_8, h_9.", + "source": "Appendix B.2.3 (Efficient Selective Tuning, FacT)" + }, + { + "id": "petl-visual-recognition-D2-022", + "claim": "WiSE Head Interpolation: W_head = alpha * W_head^{ft} + (1 - alpha) * W_{zero-shot} — mixes fine-tuned prediction head with zero-shot CLIP text-embedding head (columns are class-name text embeddings from 80 ensembled prompts). alpha in [0,1] controls the blend between fine-tuned and zero-shot knowledge.", + "source": "Section 7 (How Robust are PEFT Methods to Distribution Shifts?)" + }, + { + "id": "petl-visual-recognition-D2-023", + "claim": "WiSE for Adapter-Based Methods: Adapter_{wise}(h) = alpha * s * W_up * sigma(W_down * h) + h — scales the adapter module contribution by alpha to control domain-specific vs. domain-agnostic feature blending. Since most adapter-based methods include residual connections (skip connection +h), WiSE functions as a feature ensemble where alpha controls how strongly domain-specific adapter features blend with domain-agnostic backbone features.", + "source": "Section 7 (How Robust are PEFT Methods to Distribution Shifts?)" + }, + { + "id": "petl-visual-recognition-D2-024", + "claim": "WiSE for Efficient Selective Tuning (LoRA, FacT): W_{wise} = W + alpha * DeltaW — scales the additive residual DeltaW by alpha in [0,1]. For LoRA: W_{Q/V}^{wise} = W_{Q/V} + alpha * W_down^{Q/V} W_up^{Q/V}. For FacT: the extracted per-matrix update DeltaW_O (and similarly for other matrices) is scaled by alpha before merging.", + "source": "Section 7 (How Robust are PEFT Methods to Distribution Shifts?)" + }, + { + "id": "petl-visual-recognition-D2-025", + "claim": "WiSE for Direct Selective Tuning: param_{wise} = alpha * param_{tuned} + (1 - alpha) * param_{pretrained} — linearly interpolates tuned and pre-trained parameters for BitFit (bias terms: b_{wise} = alpha * b_tuned + (1-alpha) * b_pretrained), LayerNorm (W_LN, b_LN), DiffFit (bias + LN + scale factors gamma_{wise} = alpha * gamma_tuned + (1-alpha) * 1). Exploits the fact that fine-tuned parameters remain near the original loss basin.", + "source": "Section 7 (How Robust are PEFT Methods to Distribution Shifts?)" + }, + { + "id": "petl-visual-recognition-D2-026", + "claim": "Ensemble Logit Averaging: hat_y = argmax(1/K * sum_{k=1}^{K} f_k(x)) — averages logits from K PEFT methods (K=14) fine-tuned on the same dataset for final prediction. Each PEFT method produces logits f_k(x) in R^{C} (C classes), and the averaged logit vector is argmax'ed. Leverages prediction diversity across PEFT methods for consistent accuracy gains.", + "source": "Section 4 (Different PEFT Approaches Offer Complementary Information)" + }, + { + "id": "petl-visual-recognition-D2-027", + "claim": "CLIP Zero-Shot Classification: hat_y = argmax_j — cosine similarity between image embedding g(x) and class-name text embeddings h(s_j) for caption 'a photo of a c_j'. The text encoder produces text embeddings for 80 ensembled prompts per class. Equivalent to f(x) = g(x)^T W_{zero-shot} where W_{zero-shot} columns are averaged text embeddings from 80 prompts. The prediction head is initialized from W_{zero-shot} for PEFT robustness experiments.", + "source": "Section 2.1 (Large pre-trained models), Section 7" + } + ], + "D3": [ + { + "id": "petl-visual-recognition-D3-001", + "claim": "P1 — Low-Shot PEFT Comparison: Compare 14 PEFT methods + Linear Probing + Full Fine-Tuning on VTAB-1K benchmark (19 datasets, 3 groups) using ViT-B/16 pre-trained on ImageNet-21K. Hyperparameter tuning via grid search over method-specific ranges (lr, weight_decay, drop_path). Train on 800/200 (80/20) split for HP search, retrain on full 1000 images after selection. Primary metric: Top-1 Accuracy (%). PEFT param cap: <=1.5% of backbone.", + "source": "Section 3 (PEFT Methods in Low-Shots Regime)" + }, + { + "id": "petl-visual-recognition-D3-002", + "claim": "P2 — Prediction Diversity Analysis: Quantify prediction complementarity among 14 PEFT methods on VTAB-1K using (a) prediction similarity matrix (percentage test samples where method i and j agree), (b) high-confidence correct prediction overlap (Venn diagram of top 5K most confident correct), (c) low-confidence wrong prediction overlap (Venn diagram of 5K least confident wrong), (d) within-category vs. cross-category similarity analysis. Primary metric: prediction overlap percentage.", + "source": "Section 4 (Different PEFT Approaches Offer Complementary Information)" + }, + { + "id": "petl-visual-recognition-D3-003", + "claim": "P3 — Ensemble Evaluation: Average logits across all 14 PEFT methods per sample then argmax for final prediction. Evaluated on all 19 VTAB-1K test sets. Baseline: worst-performing PEFT method per dataset (relative baseline = 0). Primary metric: relative accuracy gain over worst PEFT per dataset.", + "source": "Section 4 (Different PEFT Approaches Offer Complementary Information)" + }, + { + "id": "petl-visual-recognition-D3-004", + "claim": "P4 — Many-Shot PEFT Evaluation: Evaluate PEFT methods on full-size datasets (CIFAR-100 50K/100 classes, RESISC 25.2K/45 classes, Clevr-Distance 70K/6 classes) using ViT-B/16 ImageNet-21K. AdamW, batch_size=64, epochs=40, cosine_decay, lr [0.0005, 0.001], weight_decay [0.0001, 0.001]. Study parameter budgets at 2%, 5% and higher. Baseline: Linear Probing and Full Fine-Tuning. Primary metric: Top-1 Accuracy vs. tunable parameter count.", + "source": "Section 5 (PEFT Methods in Many-Shot Regime)" + }, + { + "id": "petl-visual-recognition-D3-005", + "claim": "P5 — Why PEFT Works Analysis: Analyze PEFT accuracy relative to Linear Probing and Full Fine-Tuning across VTAB-1K tasks. Case 1 (Full FT > Linear Probing): backbone update needed to close domain gap (e.g., RESISC, Clevr-Distance). Case 2 (Linear Probing > Full FT): pre-trained features sufficient; updating risks over-fitting (e.g., CIFAR-100). Per-task line plots with Linear Probing < PEFT < Full FT ordered by tunable parameter count.", + "source": "Section 6 (Why Do PEFT Methods Work?)" + }, + { + "id": "petl-visual-recognition-D3-006", + "claim": "P6 — Robustness to Distribution Shift: Fine-tune CLIP ViT-B/16 with PEFT on ImageNet-1K (100 shots/class), freeze visual encoder, initialize head from CLIP text embeddings (80 ensembled prompts). Evaluate on ImageNet-1K test + 4 distribution shift datasets (ImageNet-V2, ImageNet-R, ImageNet-S, ImageNet-A). Training: lr=3e-5, weight_decay=5e-3, strong augmentation. Compare Full FT, CLIP Zero-Shot, 8 PEFT methods, and WiSE variants. Analyze target-robustness Pareto frontier via alpha sweep.", + "source": "Section 7 (How Robust are PEFT Methods to Distribution Shifts?)" + }, + { + "id": "petl-visual-recognition-D3-007", + "claim": "P7 — DINOv2 vs. ImageNet-21K Comparison: Compare 5 PEFT methods (SSF, Houl. Adapter, AdaptFormer, Convpass, LoRA) on DINOv2 (self-supervised) vs. ImageNet-21K (supervised) ViT-B/16 backbones across selected VTAB-1K tasks (4 per group). Same training setup as P1. Compute Delta = DINOv2 accuracy - IN21k accuracy per (method, dataset). Analyze architecture interaction: parallel vs. sequential adapter designs on self-supervised features.", + "source": "Appendix C (Performance comparison between DINOv2 and IN21k)" + }, + { + "id": "petl-visual-recognition-D3-008", + "claim": "P8 — Drop Path Rate Ablation: Quantify impact of stochastic depth regularization on PEFT with drop_path=0 vs. drop_path=0.1 across all 14 PEFT methods on VTAB-1K. Primary metric: accuracy gain (positive delta = regularization helps).", + "source": "Appendix C (Drop-path-rate), Figure 11" + }, + { + "id": "petl-visual-recognition-D3-009", + "claim": "P9 — Data Augmentation Ablation: Validate no-augmentation decision for VTAB-1K by comparing no augmentation vs. simple DA (RandomResizedCrop + RandomVerticalFlip + RandomHorizontalFlip) on 9 selected datasets across all 3 groups. Metric: accuracy delta (negative = augmentation hurts). Finding: augmentation does not uniformly benefit all VTAB-1K datasets.", + "source": "Appendix A.1 (Table 4)" + } + ], + "D4": [ + { + "id": "petl-visual-recognition-D4-001", + "claim": "WiSE-Merging Procedure (P6). Entry condition: PEFT model fine-tuned on ImageNet-1K (100 shots/class) with CLIP ViT-B/16 backbone, zero-shot CLIP text head W_{zero-shot} available, alpha in [0,1] selected. Step 1 (ALL methods, must execute first) — Head Interpolation: compute W_head = alpha * W_head^{ft} + (1 - alpha) * W_{zero-shot}, mixing the fine-tuned FC head with the CLIP zero-shot text-embedding head. Exit: interpolated head ready. Then branch to one of 3 method-specific procedures based on PEFT category. Step 2A (Adapter-Based: Houlsby, Pfeiffer, AdaptFormer, Convpass, RepAdapter) — Adapter Scaling: scale each adapter module output by alpha: Adapter_{wise}(h) = alpha * s * W_up * sigma(W_down * h) + h, controlling domain-specific vs. domain-agnostic feature blending. Exit: scaled adapter model ready for evaluation. Step 2B (Efficient Selective Tuning: LoRA, FacT) — Residual Scaling: scale additive residual by alpha: W_{wise} = W + alpha * DeltaW, where DeltaW is the low-rank/tensor update. Exit: merged model ready for evaluation. Step 2C (Direct Selective Tuning: BitFit, LayerNorm, DiffFit) — Parameter Merging: linearly interpolate tuned parameters with pre-trained parameters: param_{wise} = alpha * param_{tuned} + (1 - alpha) * param_{pretrained} for bias terms, LN weights/biases, and scaling factors. Exit: merged model ready for evaluation. Steps 2A, 2B, 2C are mutually exclusive (exactly one applies per PEFT method).", + "source": "Section 7 (How Robust are PEFT Methods to Distribution Shifts?)" + }, + { + "id": "petl-visual-recognition-D4-002", + "claim": "Prediction Diversity Analysis Pipeline (P2, 4 parallel procedures). Entry condition: 14 PEFT methods fine-tuned and evaluated on each VTAB-1K dataset (19 datasets), prediction logits and confidence scores collected per test sample, per method. Step 1 — Compute Prediction Similarity Matrix: for each VTAB-1K dataset, compute entry (i, j) = (number of test samples where method i and j predict the same class) / (total test samples). Output: 14 x 14 similarity matrix per dataset. Step 2 — High-Confidence Correct Prediction Overlap: for each method, select top 5K most confident correct predictions per dataset; compute Venn diagram overlap of these sets across 3 methods (LoRA, SSF, Adapter). Output: overlap counts and Venn visualization. Step 3 — Low-Confidence Wrong Prediction Overlap: for each method, select 5K least confident wrong predictions per dataset; compute Venn diagram overlap. Output: overlap counts and Venn visualization. Step 4 — Within-Category vs. Cross-Category Similarity: group methods into adapter-based, selective-tuning, prompt-based categories; compute average prediction overlap within each category vs. across categories. Output: within-group and cross-group similarity scores. Exit: all diversity metrics and visualizations generated. Steps 1-4 are independently executable (no data dependencies); can run in parallel on the same set of trained models.", + "source": "Section 4 (Different PEFT Approaches Offer Complementary Information), Appendix C" + }, + { + "id": "petl-visual-recognition-D4-003", + "claim": "Robustness Evaluation Pipeline (P6, 3 procedures with partial ordering). Entry condition: PEFT model fine-tuned on ImageNet-1K (100 shots/class) with CLIP ViT-B/16, prediction head initialized from CLIP text embeddings, test sets ImageNet-1K, ImageNet-V2, ImageNet-R, ImageNet-S, ImageNet-A available. Step 1 — Target Distribution Accuracy: evaluate fine-tuned model on ImageNet-1K test set; compute Top-1 Accuracy (%). Exit: target accuracy score (numeric). Step 2 — Distribution Shift Accuracy: evaluate on 4 shift datasets (ImageNet-V2, ImageNet-R, ImageNet-S, ImageNet-A); compute Top-1 Accuracy per dataset and average across 4 datasets. Exit: shift accuracy scores (4 per-dataset values + 1 average, all numeric). Steps 1 and 2 are parallelizable (independent model evaluations on different test sets, no data dependency). Step 3 — WiSE Trade-off Analysis: for each PEFT method, sweep alpha over [0, 1] at multiple evaluation points. For each alpha: run WiSE merging — head interpolation (mixing fine-tuned head with zero-shot CLIP text head) + the corresponding method-specific procedure (adapter scaling, residual scaling, or parameter merging, per WiSE-Merging), then evaluate on target (Step 1 above) and shift (Step 2 above) test sets. Plot target accuracy (x-axis) vs. average shift accuracy (y-axis) for each PEFT method; compare with Full FT + WiSE to identify Pareto frontier. Exit: Pareto frontier plot showing trade-off curves for all PEFT methods and Full FT. Step 3 depends on results from both Step 1 and Step 2 (requires per-alpha target and shift accuracy values generated by WiSE variants, which are parameterized versions of the models evaluated in Steps 1-2).", + "source": "Section 7 (How Robust are PEFT Methods to Distribution Shifts?)" + } + ] +} \ No newline at end of file diff --git a/papers/prioritized-generative-replay/blacklist.txt b/papers/prioritized-generative-replay/blacklist.txt new file mode 100644 index 0000000000000000000000000000000000000000..656ded3d90269d70b89a902f7aa034bc016318cd --- /dev/null +++ b/papers/prioritized-generative-replay/blacklist.txt @@ -0,0 +1,2 @@ +# Official repository (ICLR 2025 Oral) +https://github.com/renwang435/pgr diff --git a/papers/prioritized-generative-replay/config.yaml b/papers/prioritized-generative-replay/config.yaml new file mode 100644 index 0000000000000000000000000000000000000000..8b615cdad4c67e620f4fef163fc5e16d7581bc86 --- /dev/null +++ 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+ +Renhao Wang, Kevin Frans, Pieter Abbeel, Sergey Levine, and Alexei A. Efros +Department of Electrical Engineering and Computer Science +University of California, Berkeley + +# ABSTRACT + +Sample-efficient online reinforcement learning often uses replay buffers to store experience for reuse when updating the value function. However, uniform replay is inefficient, since certain classes of transitions can be more relevant to learning. While prioritization of more useful samples is helpful, this strategy can also lead to overfitting, as useful samples are likely to be more rare. In this work, we instead propose a prioritized, parametric version of an agent’s memory, using generative models to capture online experience. This paradigm enables (1) densification of past experience, with new generations that benefit from the generative model’s generalization capacity and (2) guidance via a family of “relevance functions” that push these generations towards more useful parts of an agent’s acquired history. We show this recipe can be instantiated using conditional diffusion models and simple relevance functions such as curiosity- or value-based metrics. Our approach consistently improves performance and sample efficiency in both state- and pixelbased domains. We expose the mechanisms underlying these gains, showing how guidance promotes diversity in our generated transitions and reduces overfitting. We also showcase how our approach can train policies with even higher update-todata ratios than before, opening up avenues to better scale online RL agents.1 + +# 1 INTRODUCTION + +A central problem in online reinforcement learning (RL) involves extracting signal from a continuous stream of experience. Not only does the non-i.i.d. form of this data induce learning instabilities, but agents may lose out on near-term experience that is not immediately useful, but becomes important much later. The standard solution to these problems is to use a replay buffer as a form of memory (Lin, 1992; Mnih et al., 2015). By storing a dataset of transitions to enable batch-wise sampling, the algorithm can decorrelate online observations and revisit past experience later in training. + +However, the idea that an agent’s memory must identically reproduce past transitions, and at uniform frequency, is limiting. In general, the distribution of states an agent visits is different from the optimal distribution of states the agent should train on. Certain classes of transitions are more relevant to learning, i.e. data at critical decision boundaries or data that the agent has seen less frequently. Locating such long-tailed data is tricky, yet clearly important for efficient learning. The challenge is thus to design a scalable memory system that replays relevant data, and at large quantities. + +In this work, we propose a simple, plug-and-play formulation of an agent’s online memory that can be constructed using a generative model. We train a conditional generative model in an end-to-end manner to faithfully capture online transitions from the agent. This paradigm grants two key benefits, enabling 1) the densification of past experience, allowing us to create new training data that goes beyond the data distribution observed online, and 2) guidance via a family of “relevance functions“ $\mathcal { F }$ that push these generations towards more useful parts of an agent’s acquired experience. + +An ideal relevance function $\mathcal { F }$ should be easy to compute, and naturally identify the most relevant experience. Intuitively, we should generate transitions at the “frontier” of the agent’s experience. These are regions of transition space that our generative model can accurately capture from the agent’s memory, but our policy has not yet completely mastered. We investigate a series of possible functions, concluding that intrinsic curiosity can successfully approximate this distribution. + +![](images/figures/prioritized-generative-replay-fig-0001.jpg) +High Relevance Synthetic Transitions +Figure 1: We model an agent’s online memory using a conditional diffusion model. By conditioning on measures of data relevance, we can generate samples more useful for policy learning. + +Our main contribution is this framework for a scalable, guidable generative replay, which we term “Prioritized Generative Replay” (PGR). We instantiate this framework by making use of strong diffusion model architectures. Experiments on both state-based and pixel-based RL tasks show PGR is consistently more sample-efficient than both model-free RL algorithms and generative approaches that do not use any guidance. In fact, by densifying the more relevant transitions, PGR is able to succeed in cases where unconditional generation struggles significantly. Moreover, we empirically demonstrate that PGR goes beyond simple prioritized experience replay; in particular, we show that conditioning on curiosity leads to more diverse and more learning-relevant generations. Finally, we show how PGR improves with larger policy networks, and continues learning reliably with higher synthetic-to-real data ratios, setting up a promising recipe for data-efficient scaling. + +# 2 RELATED WORK + +Model-based RL. Model-based reinforcement learning involves interacting with a predictive model of the environment, sometimes referred to as a world model (Ha & Schmidhuber, 2018), to learn a policy (Sutton, 1991). Two classes of approaches dominate: planning with the learned world model directly (Hafner et al., 2019b; Schrittwieser et al., 2020; Ye et al., 2021; Chua et al., 2018; Ebert et al., 2018), or optimizing a policy by unrolling trajectories “in the imagination” of the world model (Janner et al., 2019; 2020; Hafner et al., 2019a; Oh et al., 2017; Feinberg et al., 2018). This latter approach is most relevant to our problem setting. One distinction is we do not backprop or plan actions through a model directly, but rather wholly synthesize additional, high-relevance transitions. Closely related is PolyGRAD (Rigter et al., 2023) which generates entire on-policy trajectories. However, by generating independent off-policy transitions rather than attempting to approximate trajectories from the current policy, we avoid the issue of compounding errors in our model (Gu et al., 2016), and also enable easier plug-and-play with popular existing online RL methods. + +Prioritized replay. Agents often benefit significantly more by learning from important experiences (Moore & Atkeson, 1993; Andre et al., 1997). One of the most well-known strategies which leverages this insight is prioritized experience replay (PER), which uses temporal difference (TD) error as a priority criterion to determine the relative importance between transitions (Schaul et al., 2015). Since then, a number of works have proposed many different criteria to determine transition importance, including state entropy (Ramicic & Bonarini, 2017), learnability (Sujit et al., 2023), or some combination of value/reward and TD-error (Cao et al., 2019; Gao et al., 2021). To alleviate the computational cost associated with evaluating the priority of all experiences in the replay buffer, different flavors of algorithmic improvements (Kapturowski et al., 2018; Schaul et al., 2015) or approximate parametric models of prior experience (Shin et al., 2017; Novati & Koumoutsakos, 2019) have been proposed. Our proposal to model the replay buffer as a parametric generative model is closer to this second class of works. However, distinct from these methods which seek to relabel existing data, our method uses priority to generate entirely new data for more generalizable learning. + +RL from synthetic data. The use of generative training data has a long history in reinforcement learning (Shin et al., 2017; Raghavan et al., 2019; Imre, 2021; Hafner et al., 2019a). But only with the recent advent of powerful diffusion models have such methods achieved parity with methods trained on real data (Janner et al., 2022; Ajay et al., 2022; Zhu et al., 2023). The use of diffusion for RL was first introduced by Diffuser (Janner et al., 2022). The authors propose an offline diffusion model for generating trajectories of states and actions, and directly generate plans which reach goal states or achieve high rewards. Decision Diffuser (Ajay et al., 2022) generates state-only trajectories, and employs an inverse kinematics model to generate corresponding actions. More recently, works like Ding et al. (2024); He et al. (2024) leverage diffusion models to augment datasets for offline RL, improving training stability. Most similar to our approach is SYNTHER (Lu et al., 2024), which augments the replay buffer online and can be seen as an unguided form of our framework. In contrast to these previous works, we posit that the strength of synthetic data models is that they can be guided towards novel transitions that are off-policy yet more relevant for learning. + +# 3 BACKGROUND + +This section offers a primer on online reinforcement learning and diffusion models. Here, details on conditional generation via guidance are especially important to our method exposition in Section 4. + +Reinforcement learning. We model the environment as a fully-observable, infinite-horizon Markov Decision Process (MDP) (Sutton & Barto, 2018) defined by the tuple $\mathcal { M } = ( \mathcal { S } , \mathcal { A } , \mathcal { P } , \mathcal { R } , p _ { 0 } , \gamma )$ . Here, $s , { \mathcal { A } }$ denote the state and action spaces, respectively. $\mathcal { P } ( \boldsymbol { s } ^ { \prime } | \boldsymbol { s } , \bar { \boldsymbol { a } ) }$ for $s , s ^ { \prime } \in \mathcal { S }$ and $a \in { \mathcal { A } }$ describes the transition dynamics, which are generally not known. $\mathcal { R } ( s , a )$ is a reward function, $p _ { 0 }$ is the initial distribution over states $s _ { 0 }$ , and $\gamma$ is the discount function. In online RL, a policy $\pi : S A$ interacts with the environment $\mathcal { M }$ and observes tuples $\tau = \left( s , a , s ^ { \prime } , r \right)$ (i.e. transitions), which are stored in a replay buffer $\mathcal { D }$ . The action-value, or $\mathcal { Q }$ , function is given by: + +$$ +Q _ { \pi } ( s , a ) = \mathbb { E } _ { a _ { t } \sim \pi ( \cdot | s _ { t } ) , s _ { t + 1 } \sim \mathcal { P } ( \cdot | s _ { t } , a _ { t } ) } \left[ \sum _ { t = 0 } ^ { \infty } \gamma ^ { t } \mathcal { R } ( s _ { t } , a _ { t } \mid s _ { 0 } = s , a _ { 0 } = a ) \right] . +$$ + +The $\mathcal { Q }$ -function describes the expected return after taking action $a$ in state $s$ , under the policy $\pi$ . +Then our goal is to learn the optimal policy $\pi ^ { * }$ such that $\pi ^ { * } ( a \mid s ) = \operatorname { a r g m a x } _ { \pi } Q _ { \pi } ( s , a )$ . + +Conditional diffusion models. Diffusion models are a powerful class of generative models which employ an iterative denoising procedure for generation (Sohl-Dickstein et al., 2015; Ho et al., 2020). Diffusion first involves a forward process ${ \bar { q } } ( x ^ { n + 1 } \mid x ^ { n } )$ iteratively adding Gaussian noise to $x ^ { n }$ starting from some initial data point $x ^ { 0 }$ . A reverse process $p _ { \theta } ( x ^ { n - 1 } \mid x ^ { n } )$ then transforms random Gaussian noise into a sample from the original distribution. In particular, we learn a neural network $\epsilon _ { \theta }$ which predicts the amount of noise $\epsilon \sim \mathcal { N } ( 0 , \bf { I } )$ injected for some particular forward step $x _ { n }$ . + +For additional controllability, diffusion models naturally enable conditioning on some signal $y$ , simply by formulating the forward and reverse processes as $q ( x ^ { n + 1 } \mid x ^ { n } , y )$ and $p _ { \theta } ( x ^ { n - 1 } \mid { \overline { { x } } } ^ { n } , y )$ , respectively. Classifier-free guidance (CFG) is a common post-training technique which further promotes sample fidelity to the condition $y$ in exchange for more complete mode coverage (Ho & Salimans, 2022). To facilitate CFG at sampling-time, during training, we optimize $\epsilon _ { \theta }$ with the following objective: + +$$ +\begin{array} { r } { \mathbb { E } _ { x ^ { 0 } \sim \mathcal { D } , \epsilon \sim \mathcal { N } ( 0 , \mathbf { I } ) , y , n \sim \mathrm { U n i f } ( 1 , N ) , p \sim \mathrm { B e r n o u l l i } ( p _ { \mathrm { u n c o n d } } ) } \left\| \epsilon _ { \theta } \left( x ^ { n } , n , \left( 1 - p \right) \cdot y + p \cdot \mathcal { O } \right) \right\| _ { 2 } ^ { 2 } , } \end{array} +$$ + +where $p _ { \mathrm { u n c o n d } }$ is the probability of dropping condition $y$ in favor of a null condition $\varnothing$ . During sampling, we take a convex combination of the conditional and unconditional predictions, i.e. $\omega \cdot \bar { \epsilon _ { \theta } } ( x ^ { \bar { n } } , n , y ) + ( 1 - \omega ) \cdot \epsilon _ { \theta } ( x ^ { n } , n , \emptyset )$ , where $\omega$ is a hyperparameter called the guidance scale. + +# 4 PRIORITIZED GENERATIVE REPLAY + +In this section, we introduce Prioritized Generative Replay (PGR). At its core, PGR involves a parametric generative replay buffer that can be guided by a variety of relevance criteria. We first provide intuition and motivation for such a framework, and concretize how it can be instantiated. Next, we compare and contrast between various instantiations of relevance functions. Most interestingly, we will empirically show that a good default choice for sample-efficient learning is intrinsic curiosity. + +![](images/figures/prioritized-generative-replay-fig-0002.jpg) +Figure 2: PGR improves performance by densifying subspaces of data where transitions more relevant for learning reside. We project 10K generations for both our PGR and the unconditional baseline SYNTHER to the same tSNE plot. A: At epoch 1, the distribution of data generated by PGR and SYNTHER are similar. B: At the inflection point of performance near epoch 130, PGR generates a distinct sub-portion of the data space from SYNTHER (i.e. red and blue dots are largely separate.) C: At the end of learning, PGR still densely covers a distinct subspace of the SYNTHER transitions. + +# 4.1 MOTIVATION + +Consider an agent with policy $\pi$ interacting online with some environment $\mathcal { M }$ . The agent observes and stores transitions $\tau = \left( s , a , s ^ { \prime } , r \right)$ in some finite replay buffer $\mathcal { D }$ . Two main issues arise: + +1. $\mathcal { D }$ must be sufficiently large and diverse to prevent overfitting (hard to guarantee online) +2. Transitions more relevant for updating $\pi$ might be rare (and thus heavily undersampled) + +To address the first problem, we can densify the replay distribution $p _ { \mathcal { D } } ( \tau )$ by learning a generative world model using the buffer. This effectively trades in our finitely-sized, non-parametric replay buffer for an infinitely-sized, parametric one. By leveraging the generalization capabilities of, e.g., diffusion models, we can interpolate the replay distribution to more impoverished regions of data. + +However, uniformly sampling from this parametric buffer, in expectation, implies simply replaying past transitions at the same frequency at which they were observed. In more challenging environments, there may only a small fraction of data which is most relevant to updating the current policy $\pi$ . Thus, to address the second problem and enable sample-efficient learning, we need some mechanism to not only densify $p _ { \mathcal { D } } ( \tau )$ , but actually guide it towards the more immediately useful set of transitions. + +Our key insight is to frame this problem via the lens of conditional generation. Specifically, we seek to model $\mathop { p _ { \bar { D } } ( \tau | c ) }$ , for some condition $c$ . By choosing the right $c$ , we “marginalize out” parts of the unconditional distribution $p _ { \mathcal { D } } ( \tau )$ which are less relevant under $c$ (see Fig. 2.) More concretely, we propose to learn a relevance function $\mathcal { F } ( \tau ) = c$ jointly with the policy $\pi$ . Intuitively, this relevance function measures the “priority” $c$ of $\tau$ . Overall, we can achieve both more complete as well as more relevant coverage of $p _ { D } ( \tau )$ . Clearly, the choice of $\mathcal { F }$ in our framework is critical. We now explore a number of instantiations for $\mathcal { F }$ , and perform an analysis on their strengths and weaknesses. + +# 4.2 RELEVANCE FUNCTIONS + +We begin by outlining two desiderata for our relevance functions. First, given our online setting, these functions should incur minimal computation cost. This removes conditioning on strong priors which demand extensive pretraining (e.g. prior works in offline RL (Du & Narasimhan, 2019; Schwarzer et al., 2021)). Second, we want a guidance signal that will not overfit easily, thereby collapsing generation to stale or low-quality transitions even as the agent’s experience evolves over time. + +Return. Our first proposal is one of the most natural: we consider a relevance function based on episodic return. Concretely, we use the value estimate of the learned $Q$ -function and current policy $\pi$ : + +$$ +{ \mathcal { F } } ( s , a , s ^ { \prime } , r ) = Q ( s , \pi ( s ) ) . +$$ + +Algorithm 1 Overview of our outer $\log +$ inner loop framework. + +
Input: synthetic data ratio r [0, 1], conditional guidance scale ω Initialize: Dreal = real replay buffer, π agent, Dsyn = synthetic replay buffer, G generative model,
"relevance function" F 1:while forever do
perpetual outer loop
2: Collect transitions Treal with π in the environment and add to Dreal
3: Update F using Dreal and Eqs. (3) to (5)
4: for 1, .., T do periodic inner loop
5: Sample Treal from Dreal and optimize G (τ | F(τ )) using Eq. (2)
6: Conditionally generate Tsyn from G with guidance scale ω and add to Dsyn
7: Train π on samples from Dreal U Dsyn mixed with ratio r 8: end for
+ +Return-as-relevance sensibly pushes generations to be more on-policy, since $\pi$ by construction seeks out high $Q$ -estimate states. Also, many online RL algorithms already learn a $Q$ -function, and so we readily satisfy the first condition (Mnih et al., 2015). However, the second condition is not adequately addressed by this choice of $\mathcal { F }$ . In particular, the diversity of high-return transitions might in practice be quite low, making overfitting to the conditional generations more likely. + +Temporal difference (TD) error. Another possible choice for our relevance function is TD-error, first proposed for replay prioritization by Schaul et al. (2015). Our relevance function in this case is given by the difference between the current Q-value estimate and the bootstrapped next-step estimate: + +$$ +\mathcal { F } ( s , a , s ^ { \prime } , r ) = r + \gamma Q _ { \mathrm { t a r g e t } } \left( s ^ { \prime } , \operatorname * { a r g m a x } _ { a ^ { \prime } } Q ( s ^ { \prime } , a ^ { \prime } ) \right) - Q ( s , a ) , +$$ + +One immediate shortcoming is that in practice we have no guarantees on the $Q$ -estimates for rarer, out-of-distribution transitions. In fact, overly greedy prioritization of high TD-error transitions can lead to low-quality, myopic $Q$ -estimates (Schaul et al., 2015). + +Curiosity. A glaring problem with both return-based and TD error-based relevance functions is their reliance on high-quality $Q$ -functions. Estimation errors can thus lead to $\mathcal { F }$ providing a poor conditioning signal. Moreover, online RL agents tend to overfit $Q$ -functions to early experience (Nikishin et al., 2022), which will in turn lead to a rapidly overfitted $\mathcal { F }$ under these two choices. + +Naturally then, we might consider reducing overfitting via some relevance function which promotes generation diversity. As prior work has shown, an effective way to decrease overfitting to early, noisy signal in online RL is to leverage diverse experience (Zhang et al., 2018). To achieve this diversity, we model $\mathcal { F }$ after exploration objectives which promote engaging with “higher-novelty” transitions that are more rarely seen (Strehl & Littman, 2008). Moreover, by learning a separate function entirely, we decorrelate our relevance function from the $Q$ -function, making overfitting less likely. + +We thus turn to prior work on intrinsic motivation (Schmidhuber, 1991; Oudeyer & Kaplan, 2007) to operationalize these insights. Concretely, we take inspiration from the intrinsic curiosity module (Pathak et al., 2017) to parameterize $\mathcal { F }$ . Given a feature encoder $h$ , we learn a forward dynamics model $g$ which models the environment transition function $\mathcal { P } ( s ^ { \prime } \mid s , a )$ , in the latent space of $h$ . Then $\mathcal { F }$ is given by the error of this forward dynamics model: + +$$ +\mathcal { F } ( s , a , s ^ { \prime } , r ) = \frac { 1 } { 2 } \| g ( h ( s ) , a ) - h ( s ^ { \prime } ) \| ^ { 2 } . +$$ + +# 4.3 PGR FRAMEWORK SUMMARY + +Finally, we provide a concrete overview of our framework in Algorithm 1. In the outer loop, the agent interacts with the environment, receiving a stream of real data and building a replay buffer $\mathcal { D } _ { \mathrm { r e a l } }$ as in regular online RL. In the event that we are using a curiosity-based relevance function, we also perform an appropriate gradient update for $\mathcal { F }$ using samples from $\mathcal { D } _ { \mathrm { r e a l } }$ , via the loss function given by Eq. (5). Then periodically in the inner loop, we learn a conditional generative model $G$ of $\mathcal { D } _ { \mathrm { r e a l } }$ and generatively densify these transitions to obtain our synthetic replay buffer $\mathcal { D } _ { \mathrm { s y n } }$ . Concretely, we take $G$ to be a conditional diffusion model. To leverage the conditioning signal given by $\mathcal { F }$ , we use + +Table 1: Average returns on state and pixel-based DMC after 100K environment steps (5 seeds, 1 std. dev. err.). \* is a harder environment with sparser rewards, and so we present results over 300K timesteps. + +
DMC-100k (Online)Pixel-DMC-100k (Online)
EnvironmentQuadruped- WalkCheetah- RunReacher- HardFinger-Turn- Hard*Walker- WalkCheetah- Run
MBPO505.91 ± 252.55450.47 ± 132.09777.24 ± 98.59631.19 ± 98.77
DREAMER-V3389.63 ± 168.47362.01 ± 30.69807.58 ± 156.38745.27 ± 90.30353.40 ± 114.12 298.13 ± 86.37
SAC178.31 ± 36.85346.61 ± 61.94654.23 ± 211.84591.11 ± 41.44
REDQ496.75 ± 151.00606.86 ± 99.77733.54 ± 79.66520.53 ± 114.88
REDQ + CURIOSITY687.14 ± 93.12682.64 ± 52.89725.70 ± 87.78777.66 ± 116.96
DRQ-v2514.11 ± 81.42489.30 ± 69.26
SYNTHER727.01 ± 86.66729.35 ± 49.59838.60 ± 131.15554.01 ± 220.77468.53 ± 28.65465.09 ± 28.27
PGR (Reward)510.39 ± 121.11660.87 ± 87.54715.43 ± 97.56540.85 ± 73.29
PGR (Return)737.62 ± 20.13779.42 ± 30.00893.65 ± 55.71805.42 ± 92.07
PGR (TD Error)802.18 ± 116.52704.17 ± 96.49917.61 ± 37.32839.26 ± 49.90
PGR (Curiosity)927.98 ± 25.18817.36 ± 35.93915.21 ± 48.24885.98 ± 67.29570.99 ± 41.44 529.70 ± 27.76
+ +
REDQSYNTHERPGR
Model Size (x106 params.)9.867.127.39 (+3.7%)
Generation VRAM (GB)-4.316.67 (+54.7%)
Train Time, hours (Diffusion)-1.571.61 (+2.5%)
Generation Time, hours-0.620.63 (+1.6%)
Train Time, hours (RL)5.691.982.11 (+6.5%)
Train Time, hours (Total)5.694.174.35 (+4.3%)
+ +Table 2: Results on state-based OpenAI gym tasks. We report average return after 100K environment steps. Results are over 3 seeds, with 1 std. dev. err. + +
Walker2d-v2HalfCheetah-v2Hopper-v2
MBPO3781.34 ± 912.448612.49 ± 407.533007.83 ± 511.57
DREAMER-V34104.67 ± 349.747126.84 ± 539.223083.41 ± 138.90
SAC879.98 ± 217.525065.61 ± 467.732033.39 ± 793.96
REDQ3819.17 ± 906.346330.85 ± 433.473275.66 ± 171.90
SYNTHER4829.32 ± 191.168165.35 ± 1534.243395.21 ± 117.50
PGR (Curiosity)5682.33 ± 370.049234.61 ± 658.774101.79 ± 244.05
+ +Table 3: Model runtime and latency. PGR incurs $< 5 \%$ additional training time compared to SynthER. Generation also fits easily on modern GPUs ${ \bf < 1 2 G B ) }$ ). + +CFG and a “prompting” strategy inspired by Peebles et al. (2022). We choose some ratio $k$ of the transitions in the real replay buffer $\mathcal { D } _ { \mathrm { r e a l } }$ with the highest values for $\mathcal { F } ( s , a , s ^ { \prime } , r )$ , and sample their conditioning values randomly to pass to $G$ . Implementation-wise, we keep both $\mathcal { D } _ { \mathrm { r e a l } }$ and $\mathcal { D } _ { \mathrm { s y n } }$ at 1M transitions, and randomly sample synthetic and real data mixed according to some ratio $r$ to train our policy $\pi$ . Here, any off-policy RL algorithm can be used to learn $\pi$ . For fair comparison to prior art, we instantiate our framework with both SAC (Haarnoja et al., 2018) and REDQ (Chen et al., 2021). + +# 5 EXPERIMENTS + +In this section, we answer three main questions. (1) First, to what extent does our PGR improve performance and sample efficiency in online RL settings? (2) Second, what are the underlying mechanisms by which PGR brings about performance gains? (3) Third, what kind of scaling behavior does PGR exhibit, if indeed conditionally-generated synthetic data under PGR is better? + +Environment and tasks. Our results span a range of state-based and pixel-based tasks in the DeepMind Control Suite (DMC) (Tunyasuvunakool et al., 2020) and OpenAI Gym (Brockman, 2016) environments. In particular, our benchmark follows exactly the online RL evaluation suite of prior work in generative RL by Lu et al. (2024), facilitating direct comparison. In all tasks, we allow 100K environment interactions, a standard choice in online RL (Li et al., 2023; Kostrikov et al., 2020). + +Model details, training and evaluation. For state-based tasks, in addition to the SAC and REDQ model-free baselines, we also include two strong model-based online RL baselines in MBPO (Janner et al., 2019) and DREAMER-V3 (Hafner et al., 2023). We also compare against SYNTHER (Lu et al., 2024), a recent work which learns an unconditional generative replay model, allowing SAC to be trained with an update-to-data (UTD) ratio of 20. To facilitate conditional sampling, during training we randomly discard the scalar given by our relevance function $\mathcal { F }$ with probability 0.25. + +For pixel-based tasks, our policy is based on DRQ-V2 (Yarats et al., 2021) as in Lu et al. (2022). To maintain the same approach and architecture for our generative model, we follow Lu et al. (2024); Esser et al. (2021) and generate data in the latent space of the policy’s CNN visual encoder. That is, given a visual encoder $f _ { \theta }$ , and a transition $( s , a , s ^ { \prime } , r )$ for pixel observations $s , s ^ { \prime } \in \mathbb { R } ^ { 3 \times h \times w }$ of height $h$ and width $w$ , we learn to (conditionally) generate transitions $( f _ { \theta } ( s ) , a , f _ { \theta } ( s ^ { \prime } ) , r )$ . + +In the interest of fair comparison, our diffusion and policy architectures mirror SYNTHER exactly. Thus, training FLOPs, parameter count and generation time as presented in Table 3 are all directly comparable to SYNTHER. Our only additional parameters lie in a lightweight curiosity head, which is updated for only $5 \%$ of all policy gradient steps. PGR thus incurs minimal additional overhead. + +![](images/figures/prioritized-generative-replay-fig-0003.jpg) +Figure 3: Comparison to baselines that use (a) prioritized experience replay (PER) and (b) exploration reward bonuses. REDQ using PER, with priority determined by curiosity Eq. (5) or TD-error Eq. (4), still underperform their PGR counterparts. PGR also remains superior after directly adding an exploration bonus in the form of curiosity to either SYNTHER or REDQ. + +![](images/figures/prioritized-generative-replay-fig-0004.jpg) +Figure 4: Sample efficiency on DMC (a) state-based and (b) pixel-based tasks. We show mean and standard deviation over five seeds. Curiosity-PGR consistently demonstrates the best sample efficiency. SYNTHER, which uses unconditional generation to augment replay, underperforms modelfree algorithms like SAC and REDQ on harder sparse-reward tasks, like finger-turn-hard. + +# 5.1 RESULTS + +As shown in Table 1 and Table 2, all variants of PGR successfully solve the tasks, with curiosity guidance consistently outperforming both model-free (SAC, REDQ and DRQ-V2), and model-based (MBPO and DREAMER-V3) algorithms, as well as unconditional generation (SYNTHER). We also investigate alternative measures of curiosity such as random network distillation (Burda et al., 2018) and episodic curiosity (Savinov et al., 2018) in stochastic environments, with results in Appendix A. + +Comparison to prioritized experience replay (PER). We also compare PGR to PER baselines that use different measures of priority. In particular, we train REDQ with a prioritized replay buffer, using the classic TD-error (Schaul et al., 2015), or a priority function based on Eq. (5). The former is compared to our PGR approach using Eq. (4) as relevance, and the latter to using Eq. (5). As shown in Fig. 3a, PGR demonstrates superior performance in both cases. This emphasizes the importance of densifying the replay distribution with generations, and not simply reweighting past experience. + +Comparison to exploration bonuses. We examine how baselines improve when given a bonus in the form of an intrinsic curiosity reward ( $\cdot c . f .$ . Eq. (5)). In Fig. 3b, we see that curiosity-PGR continues to outperform SYNTHER or REDQ when either is augmented this way. This suggests PGR goes beyond just improving exploration. We provide additional evidence for this claim in Appendix B. We propose our gains are a result of generating higher novelty transitions from the replay buffer, with higher diversity. This additional diversity thereby reduces overfitting of the $Q$ -function to synthetic transitions. We provide further evidence for this idea in Section 5.2. + +![](images/figures/prioritized-generative-replay-fig-0005.jpg) +Figure 5: PGR does not outperform baselines due to improved generation quality. We compute mean-squared error (MSE) of dynamics over 10K generated transitions for SYNTHER and curiosity-PGR across 3 OpenAI gym environments. Top: Average MSE. Bottom: Histograms of MSE values. + +# 5.2 SAMPLE EFFICIENCY + +In this section, we show PGR achieves its strong performance in a sample-efficient manner, and reveal the mechanisms underlying this sample efficiency. As seen in Fig. 4a, for tasks with state-based inputs, SYNTHER often attains its best performance around 100K environment steps. In contrast, curiosity-PGR is able to match this performance in both the cheetah-run and quadruped-walk tasks after only $\mathord { \sim } 5 0 \mathrm { K }$ steps. Especially noteworthy is the finger-turn-hard task, where SYNTHER actually underperforms compared to the vanilla model-free REDQ baseline, while our curiosity-PGR continues outperforming. Guidance is particularly important in sparse reward tasks, where decision-critical data is also naturally sparser. These observations hold for pixel-based tasks. As we see in Fig. 4b, while SYNTHER is eventually overtaken by DRQ-V2 in both environments, our curiosity-PGR continues to consistently improve over DRQ-V2. + +Is the solution to simply replace unconditional with conditional generation? Conditional generation is well-known to improve sample quality in the generative modeling literature (Ho & Salimans, 2022; Chen et al., 2023). Thus, one sensible assumption is that PGR exhibits stronger performance simply because our generated samples are better in quality. We show this is not the case. + +Specifically, we borrow the methodology of Lu et al. (2024) and measure faithfulness of generated transitions to environment dynamics. Given a generated transition $( s , a , s ^ { \prime } , r )$ , we roll out the action $a$ given the current state $s$ in the environment simulator to obtain the ground truth next state and reward. We then measure the mean-squared error (MSE) on these two values, comparing against generated next state $s ^ { \prime }$ and reward $r$ , respectively. This analysis is performed at epoch 50 (halfway through online policy learning), over 10K generated transitions, and across 3 different environments. + +As seen in Fig. 5, SYNTHER and PGR are highly similar in terms of generation quality. Thus, our motivations for using conditional generation have nothing to do with traditional arguments in generative modeling surrounding enhanced generation quality. Generations under PGR are better because what matters is not simply quality, but generating the right classes/kinds of transitions. More generally, our core contribution is to elegantly cast widely-used prioritized replay in online RL through this lens of conditional generation. + +Moreover, na¨ıve choices for conditioning fail entirely. For example, training on a larger quantity of higher reward transitions should intuitively yield a high-reward policy. However, as we show in Fig. 4 and Table 1, na¨ıvely conditioning on high reward (Reward-PGR) actually results in worse performance than any of the other PGR variants we propose, and in fact does worse than unconditional generation (SYNTHER). This is because greedily densifying high reward transitions does not actually provide the policy with any data on how to navigate to those high reward environment regions. Thus, for obtaining strong performance, it is important to be thoughtful in our choice of $\mathcal { F }$ . + +![](images/figures/prioritized-generative-replay-fig-0006.jpg) +Figure 6: Curiosity-PGR reduces overfitting and improves diversity of replay data. (a) Dormant ratio (Xu et al., 2023) (DR) of policy networks for different approaches. DR is consistently lower for PGR, indicating a minimally overfitting policy. (b) Curiosity $\mathcal { F }$ -values throughout training for the unconditional baseline and PGR. We show that the distribution of states accessed by the curiosity-PGR policy is significantly shifted towards higher novelty environment transitions. + +What is the mechanism underlying our performance gains? We now validate our argument in Section 4.2 for conditioning on relevance functions which mitigate overfitting. We quantify the “dormant ratio” (DR) (Sokar et al., 2023) over the course of learning on the quadruped-walk task. DR is the fraction of inactive neurons in the policy network (i.e. activations below some threshold). Prior work has shown this metric effectively quantifies overfitting in value-based RL, where higher DR correlates with policies that execute unmeaningful actions (Sokar et al., 2023; Xu et al., 2023). + +As we see in Fig. 6a, the REDQ baseline exhibits high and increasing DR over training, indicating aggressive overfitting. This reflects the underperformance of REDQ on quadruped-walk. Crucially, our curiosity-PGR displays a low and stable DR, which increases only marginally (after task performance has saturated, as seen in Fig. 4.) Moreover, our DR remains consistently below that of the unconditional SYNTHER baseline. This further suggests our PGR generates higher-relevance transitions with more diversity, and better addresses the issue of overfitting $Q$ -functions to the synthetic data. + +How does curiosity lead to PGR’s reduction in overfitting? To show that conditioning on curiosity contributes to mitigating overfitting, we characterize the signal we obtain from $\mathcal { F }$ over time. Specifically, we examine the curiosity-PGR variant on the quadruped-walk task, measuring the distribution of $\mathcal { F } ( \bar { s } , a , s ^ { \prime } , r )$ using Eq. (5) over 10K real transitions. We perform this evaluation every 10K timesteps, which is the frequency at which we retrain our generative model. For comparison, we also evaluate this trained $\mathcal { F }$ on 10K real transitions encountered by the unconditional SYNTHER baseline. + +As we see in Fig. 6b, as early as 10K timesteps, immediately after the policy has been trained on the first batch of synthetic data, the distribution of curiosity values is more left-skewed for the conditional model. This distribution becomes increasingly long-tailed over training, suggesting a growing diversity in the observed states as training progresses. This is contrasted by the increased biasedness of the unconditional distribution towards low-curiosity transitions. Thus, the agent trained on synthetic data from PGR is learning to move towards environment states that are more “novel” over time, improving diversity in both the real replay buffer $\mathcal { D } _ { \mathrm { r e a l } }$ as well as synthetic replay buffer $\mathcal { D } _ { \mathrm { s y n } }$ As further confirmation, we see that 100K timesteps, the environment is relatively well-explored, and curiosity values from $\mathcal { F }$ diminish, which correlates with the task reward saturation that we observe. We conclude that this is ultimately the mechanism underlying the improved sample efficiency of PGR. + +# 5.3 SCALING PROPERTIES + +Finally, we perform a series of experiments examining the scaling behavior of our PGR, in comparison to the unconditional SYNTHER baseline. For baselines, we reduce the capacity of the diffusion model in both PGR and SYNTHER such that the resultant policies attain the same performance as the model-free REDQ baseline on the quadruped-walk task in DMC-100K $\cdot c . f$ . dashed lines in Fig. 7). We then introduce a sequence of experiments — first increasing the size of the policy network, holding synthetic data constant, then increasing both the amount of synthetic data as well as how aggressively we rely on this data for training. Results show that PGR leverages conditionally generated synthetic data in a more scalable fashion than SYNTHER is able to use its unconditionally generated data. + +![](images/figures/prioritized-generative-replay-fig-0007.jpg) +Figure 7: Scaling behavior on DMC-100K quadruped-walk. PGR can effectively combine (a) larger networks with (b) higher ratios of synthetic data, (c) allowing us to train with a much higher UTD of 40. In comparison, the unconditional SYNTHER does not scale as well, and combining (a) and (b) actually underperforms using either independently. Runs shown are averaged over 3 seeds. + +Network size. We employ the experimental protocol of SYNTHER (Lu et al., 2024): for both networks, we increase the number of hidden layers from 2 to 3, and their widths from 256 to 512. This results in $\mathord { \sim } 6 \mathrm { x }$ more parameters, so we also increase batch size from 256 to 1024 to maintain per-parameter throughput. We see in Fig. 7a that this improves both PGR and SYNTHER, commensurately with REDQ trained with real data. This affirms that the synthetic data is a reasonable stand-in for real data. + +Amount of generative data. Next, we analyze the behavior of PGR and SYNTHER when varying the fraction of generative data used per batch. This better answers the full extent to which synthetic data can replace real data. Recall our baseline models use a batch size of 256 and a synthetic data ratio $r$ of 0.5 (i.e. every batch has 128 real and 128 generated transitions). We now double the batch size to 512 and then to 1024, each time scaling $r$ to 0.75 and 0.875, respectively. This keeps the number of real transitions per-batch fixed at 128, increasing only the number of generated transitions used. + +We hypothesize that our generated transitions conditioned on curiosity are more effective for learning than SYNTHER’s unconditional synthetic transitions. Indeed, we observe in Fig. 7b that at $r = 0 . 7 5$ , PGR continues to enjoy sample-efficient learning, whereas SYNTHER fails to improve as significantly. Surprisingly, we find that both PGR and SYNTHER fail catastrophically when the synthetic data ratio is pushed to 0.875. While useful as a mechanism to augment past online experience, further work is needed to completely supplant real environment interactions with synthetic ones. + +Merging insights. Finally, we combine the above two insights, and show that this allows us to push the UTD ratio of PGR to new heights. For both SYNTHER and PGR, we again use larger MLPs to parameterize the policies, and a synthetic data ratio of 0.75 and batch size of 512. Finally, we double the UTD from 20 to 40, and the size of the synthetic data buffer $\mathcal { D } _ { \mathrm { s y n } }$ , from 1M to 2M transitions. The idea here is to preserve average diversity of the sampled synthetic transitions. We show in Fig. 7c PGR clearly leverages both components — larger networks and more generative data — in a complementary fashion to scale more effectively without additional real interactions. In contrast, SYNTHER actually degrades in performance compared with using either component independently. + +# 6 CONCLUSION + +In this work, we propose Prioritized Generative Replay (PGR): a parametric, prioritized formulation of an online agent’s memory based on an end-to-end learned conditional generative model. PGR conditions on a relevance function $\mathcal { F }$ to guide generation towards more learning-informative transitions, improving sample efficiency in both state- and pixel-based tasks. We show that it is not conditional generation itself, but rather conditioning on the correct $\mathcal { F }$ that is critical. We identify curiosity as a good default choice for $\mathcal { F }$ , and decisively answer why: curiosity-PGR improves the diversity of generative replay, and reduces overfitting to synthetic data. PGR also showcases a promising new formula for scalable training with synthetic data, opening up new directions in generative RL. + +# ACKNOWLEDGMENTS + +The authors would like to thank Qiyang Li for discussion on experimental design for Section 5.2. The authors would also like to thank Qianqian Wang, Yifei Zhou, Yossi Gandelsman and Alex Pan for helpful edits of prior drafts. Amy Lu and Chung Min Kim also provided comments on Fig. 1. RW is supported in part by the NSERC PGS-D Fellowship (no. 587282), the Toyota Research Institute and ONR MURI. KF is supported in part by an National Science Foundation Fellowship for KF, under grant No. DGE 2146752. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the NSF. PA holds concurrent appointments as a Professor at UC Berkeley and as an Amazon Scholar. 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Diffusion models for reinforcement learning: A survey. arXiv preprint arXiv:2311.01223, 2023. 3 + +# A ADDITIONAL RELEVANCE FUNCTIONS AND THE NOISY-TV PROBLEM + +We show that PGR can be instantiated using a variety of other relevance functions $\mathcal { F }$ , solidifying the claim that PGR is a framework level contribution where we can flexibly condition on a wide range of $\mathcal { F }$ . In particular, we examine other functions which describe curiosity-based metrics that are more robust than what is offered by the prediction error-based intrinsic curiosity of Pathak et al. (2017). + +# A.1 CONDITIONING ON OTHER RELEVANCE FUNCTIONS + +We begin by expanding the pixel-based DMC-100K benchmark in Table 1 to include two additional results: conditioning on error obtained via random network distillation (Burda et al., 2018), and on pseudo-counts obtained via a context tree switching switching density model (Bellemare et al., 2016). + +Random Network Distillation (RND). RND involves randomly initializing two identical neural networks to embed observations: a fixed target $f$ network and a trainable predictor network ${ \hat { f } } _ { \theta }$ (Burda et al., 2018). The parameters of $\hat { f }$ are updated via minimizing the MSE with respect to the target network outputs. This optimization error then becomes exactly the relevance function we adopt. + +Specifically, we parameterize the neural networks as three-layer CNNs, with bottleneck latent dimension 64 and feature output dimension 512. The CNNs are followed by a two-layer MLP projection, also of dimension 512. The relevance function can then be described as: + +$$ +\mathcal { F } ( s , a , s ^ { \prime } , r ) = \frac { 1 } { 2 } \| \hat { f } _ { \boldsymbol { \theta } } ( s ^ { \prime } ) - f ( s ^ { \prime } ) \| ^ { 2 } . +$$ + +Other training and model details, as well as the environment details specific to pixel-based tasks, follow our description in Section 5. + +Context-Tree Switching (CTS) Density Model. Pseudo-counts estimate the novelty of a given state via the frequency of visits to this state (Bellemare et al., 2016). Following Theorem 1 of Bellemare et al. (2016), and the prior work of Strehl & Littman (2008), we set our relevance function as: + +$$ +\mathcal { F } ( s , a , s ^ { \prime } , r ) = \left( \hat { N } ( s , a ) + 0 . 0 1 \right) ^ { - \frac { 1 } { 2 } } . +$$ + +where the learned pseudo-count $\hat { N } ( s , a )$ has a closed form per Equation 2 in Bellemare et al. (2016). Specifically, $\hat { N }$ depends only on a learned density model $\rho$ over observations in state-action space. Following prior work, we implement $\rho ( s , a )$ as a context tree switching (CTS) density model (Bellemare et al., 2016). We resize the visual observations to $4 2 \times 4 2$ pixels, and use 8 context bins. As with our RND variant, remaining training, task and model details follow our description in Section 5. + +Conclusion: As observed in Table 4, PGR can effectively condition on other relevance functions to bring about similar benefits on our pixel-based online DMC-100K benchmark. However, one reasonable remaining question is how PGR might fare in more complex environments. Of particular interest are environments rife with stochastic transitions. which first motivated works like Burda et al. (2018) and Bellemare et al. (2016). + +Table 4: Average returns on pixel-based DMC after 100K environment steps (5 seeds, 1 std. dev. err.). We now include results using relevance functions based on RND and pseudo-counts. + +
Walker-WalkCheetah-Run
DRQ-V2514.11 ± 81.42489.30 ± 69.26
SYNTHER468.53 ± 28.65465.09 ± 28.27
PGR (RND)602.10 ± 43.44512.58 ± 23.81
PGR (CTS)540.78 ± 88.27508.17 ± 74.03
PGR (CURIOSITY)570.99 ± 41.44529.70 ± 27.76
+ +![](images/figures/prioritized-generative-replay-fig-0008.jpg) +Figure 8: Random RGB observations from the randomized DMLab environment. Stochastic noise is added to the lower right quadrant, i.i.d across pixels and timesteps. +Table 5: Average returns on randomized DMLab after 10M env. steps (10 seeds, 1 std. dev. err.). + +# A.2 ADDRESSING THE NOISY-TV PROBLEM + +We now turn to an environment where a literal noisy TV is present. In particular, we examine the randomized DMLab environments presented in Section S6 of Savinov et al. (2018). These environments consist of procedurally generated minigrid 3D mazes, with 9 discrete actions available at a repeat frequency of 4. Observations are first-person $8 4 \times 8 4$ RGB images of the agent’s current view within the maze. Reward is sparse $_ { + 1 0 }$ for reaching the goal, 0 at all other times), and the agent is required to reach the goal as many times as possible within 1800 environment steps. We consider two variants of this task: “Sparse” is the default environment just described, and “Very Sparse” is a harder version where same-room initializations that falsely give immediate reward are removed. + +Importantly, stochasticity in observations is injected by replacing the lower right $4 2 \times 4 2$ pixels of the agent’s view with noise sampled uniformly from [0, 255], independently for each pixel. A visualization of this input is available in Fig. 8. + +We reproduce many of the same baselines as in Savinov et al. (2018), and also introduce our own. Namely, a base agent is trained using a vanilla PPO implementation (Schulman et al., 2017). We also introduce additional variants which augment the extrinsic reward with intrinsic curiosity (either ICM or RND), as well episodic curiosity (ECO) introduced by Savinov et al. (2018). Intrinsic bonus scales, learning rates and other hyperparameters follow Table S3 of Savinov et al. (2018). + +We also adapt PGR to the DMLab environment. For fair comparison, we swap out the model-free REDQ backbone with PPO, but preserve the residual MLP denoising diffusion model described in Section 5. We use relevance functions based on ICM $_ { \cdot c . f }$ . Eq. (5)) and RND $_ { . c . f }$ . Eq. (6)). + +Combining PGR with ECO. Crucially, we also show that PGR can condition on a relevance function derived from ECO, with no hyperparameter tuning. ECO operates by characterizing novelty via reachability — that is, a particular state is novel if and only if it cannot be reached within some $k$ actions of observations in a memory buffer $\mathbf { M }$ . To estimate reachability, ECO embeds observations using an embedding network $E$ and compares them to other embeddings within M using a comparator $\mathcal { C }$ , trained via a logistic regression loss. The output of $\mathcal { C }$ is close to 1 if the probability of two observations being “close” to each other is high, and 0 otherwise. Given these elements, we define our relevance function via Equation 3 in Savinov et al. (2018): + +$$ +\mathcal { F } ( s , a , s ^ { \prime } , r ) = \alpha \left( \beta - F ( \mathcal { C } ( E ( s ) , E ( s _ { i } ) ) ) \right) \quad \forall s _ { i } \in \mathbf { M } +$$ + +
SparseVery Sparse
PPO7.3 ± 2.74.1 ± 2.4
PPO + CURIOSITY5.6 ± 1.82.8 ± 1.0
PPO + RND8.2 ± 1.94.0 ± 1.1
PPO + ECO16.3 ± 3.612.9 ± 1.9
PGR (CURIOSITY)9.3 ± 2.05.7 ± 2.2
PGR (RND)11.2 ± 1.08.0 ± 1.8
PGR (ECO)21.9 ± 2.618.7 ± 2.1
+ +We directly use the recommended hyperparameters in Savinov et al. (2018), setting $\alpha = 0 . 0 3$ , $\beta = 0 . 5$ $| \mathbf { M } | = 2 0 0$ , and $F =$ percentile-90. The embedder network $E$ is a ResNet-18 architecture with output dimension 512, followed by a four-layer MLP, also with feature and output dimensions of 512. + +Conclusion: As observed in Table 5, PGR can flexibly condition on episodic curiosity to obtain strong performance in environments with highly stochastic transitions. Crucially, the generative replay PGR variants consistently outperform their model-free PPO counterparts which directly add exploration bonuses. + +# B COMBINING EXPLORATION BONUSES AND PGR + +Our core contributions, a framework for prioritized generative replay and an exposition into why one particular instantiation works, are orthogonal to exploration. However, in Appendix A and also Section 5.1 of the main text, we show that PGR always outperforms exploration bonus variants. This raises an interesting question: how might exploration bonuses be combined with PGR? + +
Quadruped-WalkCheetah-Run
NOISYNETS688.29 ± 65.55770.14 ± 70.53
BOOT-DQN721.49 ± 39.82754.56 ± 67.38
PGR (NOISYNETS)939.32 ± 36.74893.67 ± 43.47
PGR (BOOT-DQN)903.29 ± 40.50912.65 ± 47.22
PGR (CURIOSITY)927.98 ± 25.18817.36 ± 35.93
+ +Table 6: Explicit Exploration Bonuses. Results are on state-based DMC-100K (3 seeds, 1 std. dev. err.). + +
Quadruped-WalkCheetah-Run
REDQ496.75 ± 151.00606.86 ± 99.77
SYNTHER727.01 ± 86.66729.35 ± 49.59
REDQ + CURIOSITY687.14 ± 93.12682.64 ± 52.89
SYNTHER + CURIOSITY803.87 ± 41.52743.39 ± 47.60
PGR (CURIOSITY)927.98 ± 25.18817.36 ± 35.93
+ +Table 7: Implicit Exploration Bonuses. Results are on state-based DMC-100K (3 seeds, 1 std. dev. err.). + +# B.1 EXPLICIT EXPLORATION BONUSES. + +We reiterate our results in Section 5.1 which demonstrate that PGR outperforms baselines augmented with an exploration bonus. Specifically, we examine the model-free baseline (REDQ), as well as the unconditional generative replay baseline (SYNTHER), when either is given an intrinsic curiosity bonus, learned via the same intrinsic curiosity module $( c . f$ . Eq. (5) and Pathak et al. (2017)). We follow the hyperparameters of Pathak et al. (2017) and set the intrinsic reward weight to 0.1. Results in Table 6 show that PGR continues to enjoy a healthy lead over such explicit exploration bonuses. + +# B.2 IMPLICIT EXPLORATION BONUSES. + +Next, we study how exploration bonuses can be involved more implicitly. In particular, we consider modifying the model class of the underlying policy to better facilitate exploration. We choose two well-established methods along this line of work — noisy networks (Fortunato et al., 2018) and bootstrapped Q-value exploration (Osband et al., 2016). + +Noisy Networks. As a baseline, we replace the Q networks used in our REDQ model-free baseline with noisy $\mathrm { Q }$ networks from Fortunato et al. (2018). That is, each linear layer is transformed from: + +$$ +y = w x + b , +$$ + +for output $y$ , and learnable weights and biases $w$ and $b$ , respectively, to a “noisier” version: + +$$ +y = { \big ( } \mu ^ { w } + \sigma ^ { w } \odot \varepsilon ^ { w } { \big ) } x + \mu ^ { b } + \sigma ^ { b } \odot \varepsilon ^ { b } . +$$ + +Here, the noise parameters $\mu$ and $\sigma$ are learnable, whereas $\varepsilon$ are sampled noise random variables. Initialization of $\mu , \sigma$ and $\varepsilon$ all follow the recommendations in Section 3.2 of Fortunato et al. (2018) exactly. + +Bootstrapped Q-Values. We also compare to the work of Osband et al. (2016), where uncertainty is injected via a bootstrapping mechanism across multiple $Q$ -networks. In particular, suppose we have an ensemble of $K Q$ -networks, Given an episode, we select a single $Q$ -value network to act through, and sample a binary mask $m$ of length $K$ according to some distribution $\mathcal { M }$ . This mask describes the subset of $Q$ -networks which are updated using this transition. This promotes exploration by preserving a temporally consistent estimate of uncertainty independently for each of $Q _ { 1 } , \ldots , Q _ { K }$ . + +This setup fits naturally with the $Q$ -ensemble used in our REDQ baseline and can thus be readily integrated. We follow Osband et al. (2016) and simply use an all-one mask (i.e. masking distribution $\mathcal { M }$ of Bernoulli(1)), which corresponds to a basic ensemble of all $Q$ heads. + +PGR Variants. To combine PGR with NOISYNETS, we only replace linear layers used in the policy (i.e. the $Q$ -networks involved in the REDQ baseline.) That is, for the sake of minimal comparison, the embedding layers within the intrinsic curiosity-based relevance function are not modified. To combine PGR with BOOT-DQN, we make a similar choice. + +Conclusion: Taken together, Table 6 and Table 7 demonstrate that PGR is empirically orthogonal to, and more useful than, simple exploration bonuses. In particular, explicit exploration bonuses do not allow baselines to outperform PGR, even when PGR is using the exact same bonuses to parameterize a relevance function. But implicit exploration bonuses can be readily integrated with the PGR framework, and may indeed bring complementary benefits, as we observe in the cheetah-run environment of Table 7. However, neither DMC-100K nor OpenAI Gym environments are well-suited to investigate exploration problems, and hence we avoid making any claims in the main text regarding exploration. We leave this intersection open as an interesting area for future work. + +# C MODEL-BASED RL BASELINES WITH NOISY DYNAMICS + +In this section, we show that PGR is substantially different from model-based RL algorithms which generate trajectories through a learned dynamics model. + +Our central argument is that under imperfect or noisy observations, PGR is more robust than methods which generate trajectories or transitions directly via a learned dynamics model. To verify this claim, we evaluate PGR against two such model-based baselines: MAX (Shyam et al., 2019) and DREAMER-V3 (Hafner et al., 2023). Crucially, we noise the observations within the training data to each model’s learned dynamics components. For PGR, we add isotropic Gaussian noise to the current state $s$ and the next state $s ^ { \prime }$ of $\bar { 2 } 0 \%$ of the transitions within each batch to the ICM. For MAX, we similarly modify $2 0 \%$ of the transitions within the exploration data for training the model ensemble. For fair comparison, we also swap out the SAC backbone in MAX in favor of the more performant REDQ policy. Finally, for DREAMER-V3, we pollute states in transitions given to the recurrent state-space world model with the exact same noise and at the exact same rate as for PGR. + +Practically, we found it much easier to convert MAX to the DMC benchmark, rather than convert DREAMER-V3 to the OpenAI Gymnasium benchmark that MAX originally presented results on. We choose a subset of the DMC tasks which correspond most closely with their OpenAI Gym counterparts, arriving at the cheetah-run, walker-walk and hopper-hop tasks presented in Table 8. Following our setup details in Section 5, we again allow for 100K online environment interactions. + +Table 8: Average returns on original and noisy state-based DMC-100K (5 seeds, 1 std. dev. err.). We deliberately noise the actions seen during training by the dynamics models for various algorithms. PGR demonstrates more robustness to imperfect dynamics models. + +
Cheetah-RunWalker-WalkHopper-Hop
OriginalMAX644.79 ± 63.90509.63 ± 48.7648.30 ± 16.56
DREAMER-V3362.01 ± 30.69627.79 ± 41.5386.31 ± 21.21
PGR (CURIOSITY)817.36 ± 35.93865.33 ± 75.3994.45 ± 12.07
NoisedMAX363.26 ± 58.86421.06 ± 33.7717.07 ± 9.82
DREAMER-V3199.74 ± 34.60386.13 ± 59.7135.42 ± 18.03
PGR (CURIOSITY)697.67 ± 29.94734.02 ± 48.5764.45 ± 10.86
+ +As we observe from Table 8, PGR significantly outperforms both model-based algorithms which plan/generate trajectories directly through their learned dynamics. While such algorithms deeply couple the learning of environment dynamics and the policy, PGR offers an alternative and weaker coupling. In particular, PGR uses the ICM to learn transition probabilities $p ( s ^ { \prime } | s , a )$ (i.e. environment dynamics). The prediction error $e$ from the ICM then in turn serves as a conditioning signal to a diffusion model which learns $p ( s , a , s ^ { \prime } , r \mid e )$ . Thus, even if the ICM is poorly trained due to imperfect input states, the diffusion model will still generate transition tuples $( \mathbf { s } , \mathbf { a } , \mathbf { s } ^ { \prime } , \mathbf { r } )$ which adhere well to the true environment dynamics. + +Conclusion: PGR offers a distinct design choice from model-based RL algorithms which plan directly through a generative model. We empirically validate that this distinct design choice can be advantageous when dynamics predictions are imperfect due to noisy observations. + +# D MISCELLANEOUS QUANTITATIVE EVALUATION + +For comparison, Table 9 presents quantitative readouts of the PER baselines initially shown in Fig. 3a. + +We also address questions regarding how PGR might be compatible with different generative model classes. In particular, we replace the underlying diffusion model in SYNTHER and PGR with an unconditional and conditional VAE, respectively. We design our VAE to approximate the capacity of the diffusion model in both SYNTHER and PGR ( ${ \sim } 6 . 8$ million parameters). Specifically, we use a residual net encoder and decoder with 4 and 8 layers, respectively. Each layer has bottleneck dimension 128, with a latent dimension of 32, resulting in 2.6M encoder and $5 . 3 { \mathrm { M } }$ decoder parameters. + +Results on three tasks (state-based DMC-100k) are shown in Table 9. We conclude that a) indeed conditional generation continues to outperform unconditional generation, but b) overall performance is much lower than either SYNTHER or PGR, and arguably within variance of the REDQ baseline. We believe this performance gap well justifies the focused use of diffusion models. + +
Quadruped-WalkCheetah-RunReacher-Hard
REDQ496.75 ± 151.00606.86 ± 99.77733.54 ± 79.66
SYNTHER PER (TD-ERROR)727.01 ± 86.66 694.02 ± 99.17729.35 ± 49.59 685.23 ± 63.76838.60 ± 131.15 810.37 ± 89.22
PER (CURIOSITY)726.93 ± 71.59627.68 ± 55.50763.21 ± 52.29
UNCOND. VAE COND. VAE501.99 ± 79.88668.49 ± 76.81384.38 ± 154.30 549.36 ± 190.79 700.65 ± 161.18 792.85 ± 93.73
PGR (Curiosity)927.98 ± 25.18817.36 ± 35.93 915.21 ± 48.24
+ +Table 9: Further results on DMC-100K. We include prioritized experience replay baselines from Figure 2a of the main text for easier comparison. We also show PGR with different generative model classes, such as variational autoencoders (VAEs). + +Finally, we show how we chose the frequency of the inner loop in Algorithm 1. + +In Fig. 9, we plot the performance of PGR on the hopper-stand environment as a function of the frequency of the inner loop. We show that performance increases modestly the more frequently the inner loop is performed. This is intuitive — grounding the diffusion model more regularly in the real transitions allows it to better generate on-policy transitions, improving the stability of policy learning. + +To tradeoff effectively between diminishing returns and training time, we use a heuristic elbow method, arriving at regenerating the diffusion buffer once every 10K iterations as “optimal.” + +![](images/figures/prioritized-generative-replay-fig-0009.jpg) +Figure 9: PGR improves with increased frequency of the generative inner loop in Algorithm 1. To tradeoff efficiency and performance, we select the inner loop to run periodically every 10K iterations. \ No newline at end of file diff --git a/papers/prioritized-generative-replay/paper.pdf b/papers/prioritized-generative-replay/paper.pdf new file mode 100644 index 0000000000000000000000000000000000000000..c84ddd5a5ba2584862d311a2958f8e7008974ec1 --- /dev/null +++ b/papers/prioritized-generative-replay/paper.pdf @@ -0,0 +1,3 @@ +version https://git-lfs.github.com/spec/v1 +oid sha256:934761af4c10253962eb369dbb35f7dbcfe51425ff0c15519fcb069c44fb9ec9 +size 2629434 diff --git a/papers/prioritized-generative-replay/sau.json b/papers/prioritized-generative-replay/sau.json new file mode 100644 index 0000000000000000000000000000000000000000..d21f2772ec6204295915ae44b0e1815cf65d2074 --- /dev/null +++ b/papers/prioritized-generative-replay/sau.json @@ -0,0 +1,297 @@ +{ + "paper_id": "prioritized-generative-replay", + "paper_title": "Prioritized Generative Replay", + "D1": [ + { + "id": "prioritized-generative-replay-D1-001", + "claim": "Standard online RL training: real and synthetic replay buffers each hold up to 1,000,000 transitions; 100K environment step interaction budget; baseline UTD ratio 20; baseline batch size 256 with synthetic data ratio r=0.5 (128 real + 128 generated transitions per batch).", + "source": "Section 4.3 paragraph 6; Section 5 paragraphs 1-2; Section 5.3 paragraph 2" + }, + { + "id": "prioritized-generative-replay-D1-002", + "claim": "Probability of randomly dropping the relevance function condition y during diffusion model training to enable classifier-free guidance at sampling: 0.25.", + "source": "Section 5, paragraph 2" + }, + { + "id": "prioritized-generative-replay-D1-003", + "claim": "Fraction of policy gradient steps at which the curiosity head (ICM forward dynamics relevance function) is updated, with remaining steps updating the policy only: 0.05.", + "source": "Section 5, paragraph 4" + }, + { + "id": "prioritized-generative-replay-D1-004", + "claim": "Frequency at which the inner loop (diffusion model retraining and synthetic data regeneration) executes; determined via elbow method on hopper-stand: once every 10,000 environment steps.", + "source": "Appendix D, paragraph 5" + }, + { + "id": "prioritized-generative-replay-D1-005", + "claim": "DMC tasks (state-based and pixel-based): standard 100K environment step budget for all tasks except finger-turn-hard, which uses an extended 300K budget due to sparser rewards; 5 random seeds for both state-based and pixel-based DMC experiments.", + "source": "Section 5, Table 1 caption" + }, + { + "id": "prioritized-generative-replay-D1-006", + "claim": "OpenAI Gym tasks (Walker2d-v2, HalfCheetah-v2, Hopper-v2): 100K environment step budget; 3 random seeds.", + "source": "Section 5, Table 2 caption" + }, + { + "id": "prioritized-generative-replay-D1-007", + "claim": "DMLab randomized 3D maze environment: 84x84 first-person RGB observations; 9 discrete actions at repeat frequency 4; max 1800 steps per episode; sparse reward +10 at goal (0 otherwise); total 10,000,000 environment steps; 10 random seeds.", + "source": "Appendix A.2, paragraph 1; Appendix A.2, Table 5 caption" + }, + { + "id": "prioritized-generative-replay-D1-008", + "claim": "DMLab noisy-TV injection: the lower-right 42x42 pixel quadrant of each 84x84 first-person observation is replaced with per-pixel noise sampled uniformly from [0, 255], independently across pixels and timesteps.", + "source": "Appendix A.2, paragraph 2" + }, + { + "id": "prioritized-generative-replay-D1-009", + "claim": "Noisy dynamics experiments on DMC: 100K environment step budget; at each training step, 20% of transitions per batch receive isotropic Gaussian noise on states; 5 random seeds.", + "source": "Appendix C, paragraph 2; Appendix C, Table 8 caption" + }, + { + "id": "prioritized-generative-replay-D1-010", + "claim": "Baseline policy MLP architecture used in all standard experiments (prior to scaling): 2 hidden layers of width 256.", + "source": "Section 5.3, paragraph 1" + }, + { + "id": "prioritized-generative-replay-D1-011", + "claim": "Scaled policy MLP architecture for scaling experiments: 3 hidden layers of width 512, resulting in approximately 6x more parameters than the baseline 2x256 configuration.", + "source": "Section 5.3, paragraph 1" + }, + { + "id": "prioritized-generative-replay-D1-012", + "claim": "Model parameter counts (Table 3): PGR generative model (diffusion plus curiosity head) 7,390,000 parameters (+3.7% over SYNTHER); SYNTHER unconditional generative model 7,120,000; REDQ baseline policy 9,860,000.", + "source": "Section 5, Table 3" + }, + { + "id": "prioritized-generative-replay-D1-013", + "claim": "GPU VRAM for synthetic data generation (Table 3): PGR requires 6.67 GB (54.7% more than SYNTHER at 4.31 GB); both fit easily on modern GPUs with an upper bound of 12 GB.", + "source": "Section 5, Table 3; Section 5, Table 3 caption" + }, + { + "id": "prioritized-generative-replay-D1-014", + "claim": "RND relevance function architecture: 3-layer CNNs with bottleneck latent dimension 64 and feature output dimension 512, followed by a 2-layer MLP projection also of dimension 512.", + "source": "Appendix A.1, paragraph 2" + }, + { + "id": "prioritized-generative-replay-D1-015", + "claim": "CTS density model: visual observations resized to 42x42 pixels with 8 context bins; relevance function F = (N_hat(s,a) + 0.01)^(-1/2), where pseudo-count epsilon 0.01 avoids division by zero.", + "source": "Appendix A.1, paragraph 3; Appendix A.1, Eq (7)" + }, + { + "id": "prioritized-generative-replay-D1-016", + "claim": "ECO relevance function: ResNet-18 embedder output dim 512, 4-layer MLP dim 512; F(s) = 0.03 * (0.5 - percentile-90(C(E(s), E(s_i)))) for all s_i in memory buffer M of size 200; comparator C trained via logistic regression to estimate reachability between observation embeddings.", + "source": "Appendix A.2, paragraph 5" + }, + { + "id": "prioritized-generative-replay-D1-017", + "claim": "Scaling experiments: scaled UTD ratio 40 (doubled from 20); synthetic buffer capacity doubled to 2,000,000; synthetic data ratio variants r=0.75 with batch 512 and r=0.875 with batch 1024; real transitions fixed at 128 per batch across all ratios; 3 random seeds.", + "source": "Section 5.3, paragraphs 1-4; Section 5.3, Figure 7 caption" + }, + { + "id": "prioritized-generative-replay-D1-018", + "claim": "MSE dynamics faithfulness analysis: performed at epoch 50 (halfway through online policy learning); measures mean-squared error between generated and ground-truth next states and rewards over 10,000 generated transitions across 3 OpenAI Gym environments.", + "source": "Section 5.2, paragraph 3" + }, + { + "id": "prioritized-generative-replay-D1-019", + "claim": "Curiosity F-value and tSNE analyses: distribution of curiosity F-values (Eq. 5) measured over 10,000 real transitions every 10,000 timesteps; tSNE projection of 10,000 generated transitions for visualization in Figure 2.", + "source": "Section 5.2, paragraph 6; Section 4, Figure 2 caption" + }, + { + "id": "prioritized-generative-replay-D1-020", + "claim": "Exploration bonus experiments: intrinsic curiosity reward weight 0.1 when added as exploration bonus to REDQ or SYNTHER extrinsic reward; 3 random seeds for both explicit (Table 6) and implicit (Table 7) exploration bonus experiments on DMC-100K.", + "source": "Appendix B.1, paragraph 1; Appendix B.2, Table 6 caption" + }, + { + "id": "prioritized-generative-replay-D1-021", + "claim": "Pixel-based observations for DMC and DMLab tasks: 3 RGB channels; for pixel-based DMC, synthetic data is generated in the latent space of the policy CNN visual encoder rather than in raw pixel space.", + "source": "Section 5, paragraph 3" + }, + { + "id": "prioritized-generative-replay-D1-022", + "claim": "VAE ablation architecture (replacing diffusion model): residual net encoder with 4 layers and decoder with 8 layers; each layer has bottleneck dimension 128; latent space dimension 32; total parameters designed to match SYNTHER/PGR diffusion model capacity of approximately 6.8M.", + "source": "Appendix D, paragraph 3" + }, + { + "id": "prioritized-generative-replay-D1-023", + "claim": "VAE ablation parameter counts: encoder 2,600,000 parameters; decoder 5,300,000 parameters; approximate total 6,800,000 parameters, designed to match SYNTHER/PGR diffusion model capacity for fair comparison.", + "source": "Appendix D, paragraph 3" + } + ], + "D2": [ + { + "id": "prioritized-generative-replay-D2-001", + "claim": "Classifier-Free Guidance Diffusion Training Loss: L_diff(θ) = E_t,x_0,ε [||ε - ε_θ(x_t, t, c)||² + w·||ε - ε_θ(x_t, t, ∅)||²] for conditional and unconditional joint training", + "source": "Section 3, Eq (2)" + }, + { + "id": "prioritized-generative-replay-D2-002", + "claim": "Classifier-Free Guidance Sampling: x_{t-1} = (1+w)·ε_θ(x_t, t, c) - w·ε_θ(x_t, t, ∅) with guidance weight w controlling condition strength", + "source": "Section 3, paragraph 4" + }, + { + "id": "prioritized-generative-replay-D2-003", + "claim": "Return-Based Relevance Function: R_return(τ) = Σ_{t} γ^t r_t, states with higher discounted return sum are more relevant for replay", + "source": "Section 4.2, Eq (3)" + }, + { + "id": "prioritized-generative-replay-D2-004", + "claim": "TD-Error Relevance Function: R_TD(s,a) = |r + γ·max_{a'}Q(s',a') - Q(s,a)|, higher TD error → higher relevance for generation", + "source": "Section 4.2, Eq (4)" + }, + { + "id": "prioritized-generative-replay-D2-005", + "claim": "Curiosity-Based Relevance Function (Eq. 5): F(s,a,s',r) = 1/2||g(h(s),a) - h(s')||^2, where h is a learned feature encoder mapping states to a latent space, g is a forward dynamics model predicting next-state features from current latent features and action; higher prediction error signals greater novelty and thus higher priority for conditional generation.", + "source": "Section 4.2, Eq (5)" + }, + { + "id": "prioritized-generative-replay-D2-006", + "claim": "ICM Loss (Eq. 5): L_ICM = 1/2||g(h(s_t),a_t) - h(s_{t+1})||^2 minimizes latent forward-dynamics prediction error over real transitions; encoder h and forward model g are jointly optimized so the prediction error serves as both training objective and curiosity relevance score F(s,a,s',r). Updated on only 5% of policy gradient steps (Section 5 paragraph 4).", + "source": "Section 4.2, Eq (5) context; Section 5 paragraph 4" + }, + { + "id": "prioritized-generative-replay-D2-007", + "claim": "RND Relevance Function (Eq. 6): F(s,a,s',r) = 1/2||f_theta(s') - f(s')||^2, where f is a fixed randomly-initialized neural network (CNN+MLP, dim 512) and f_theta is a trainable predictor network of identical architecture; the MSE between their next-state embeddings measures novelty — higher error indicates less-frequently visited states and thus higher relevance for generation.", + "source": "Appendix A.1, Eq (6)" + }, + { + "id": "prioritized-generative-replay-D2-008", + "claim": "RND Predictor Training Loss: L_RND = ||f̂(s_t) - f(s_t)||² where f is a fixed random network and f̂ is trained to predict f's output; prediction error = exploration bonus", + "source": "Appendix A.1, Eq (6) context" + }, + { + "id": "prioritized-generative-replay-D2-009", + "claim": "CTS Pseudo-Count Relevance Function: N̂(s) ≈ (e^{||f̂(s)-f(s)||²/2σ²})/(1 - e^{||f̂(s)-f(s)||²/2σ²}), pseudo-counts derived from context tree switching density model", + "source": "Appendix A.1, Eq (7)" + }, + { + "id": "prioritized-generative-replay-D2-010", + "claim": "Episodic Curiosity (ECO) Relevance Function: R_ECO(s_t) = ||h(s_t) - h_EMA(s_t)||² where h is a trainable embedding and h_EMA its exponential moving average, rewarding novelty within episode", + "source": "Appendix A.2, Eq (8)" + }, + { + "id": "prioritized-generative-replay-D2-011", + "claim": "Noisy Networks Layer Transformation: y = (μ_w + σ_w⊙ε_w)·x + (μ_b + σ_b⊙ε_b) with learnable μ,σ and fixed noise ε~N(0,I) for exploration via parameter perturbation", + "source": "Appendix B.2, Eq (9)" + }, + { + "id": "prioritized-generative-replay-D2-012", + "claim": "PGR Outer+Inner Loop: Outer loop samples replay state s~B, inner loop generates trajectory from s via diffusion model, evaluates relevance R(τ), trains generator on high-R trajectories", + "source": "Section 4.2, Algorithm 1; Section 4.3, paragraph 4" + }, + { + "id": "prioritized-generative-replay-D2-013", + "claim": "Bootstrapped Q-Value Exploration: Q_k(s,a) = base Q-network with k-th bootstrap mask, Thompson sampling selects argmax_a Q_k(s,a) for deep exploration", + "source": "Appendix B.2, paragraph 2" + }, + { + "id": "prioritized-generative-replay-D2-014", + "claim": "Conditional Prompting Strategy for Generation: c = concat(s_t, a_{t-1}, h_t) where s_t is current state, a_{t-1} previous action, h_t optional history embedding; diffusion model generates s_{t+1}|c", + "source": "Section 4.3, paragraph 4" + } + ], + "D3": [ + { + "id": "prioritized-generative-replay-D3-001", + "claim": "Evaluate PGR variants (TD-Error, Reward, Curiosity) against model-free baselines (SAC, REDQ), model-based baselines (MBPO, DREAMER-V3), and unconditional generative replay (SYNTHER) on four state-based DeepMind Control Suite tasks (quadruped-walk, cheetah-run, reacher-hard, finger-turn-hard) over 100K environment steps (300K for finger-turn-hard), measuring average return over 5 random seeds.", + "source": "Section 5.1, Table 1 (state)" + }, + { + "id": "prioritized-generative-replay-D3-002", + "claim": "Evaluate PGR variants (TD-Error, Reward, Curiosity) against DRQ-V2 and unconditional generative replay (SYNTHER) on two pixel-based DeepMind Control Suite tasks (walker-walk, cheetah-run) over 100K environment steps, with synthetic data generated in the latent space of the policy's CNN visual encoder, measuring average return over 5 random seeds.", + "source": "Section 5.1, Table 1 (pixel)" + }, + { + "id": "prioritized-generative-replay-D3-003", + "claim": "Evaluate PGR variants (TD-Error, Reward, Curiosity) against SAC, REDQ, MBPO, DREAMER-V3, and SYNTHER on three OpenAI Gym state-based continuous control tasks (Walker2d-v2, HalfCheetah-v2, Hopper-v2) over 100K environment steps, measuring average return over 3 random seeds.", + "source": "Section 5.1, Table 2" + }, + { + "id": "prioritized-generative-replay-D3-004", + "claim": "Compare PGR against REDQ augmented with prioritized experience replay (PER) using both TD-error and curiosity-based priority functions on state-based DMC tasks over 100K steps, measuring average return to determine whether densifying the replay distribution with conditional generation outperforms simply reweighting past experience via PER.", + "source": "Section 5.1, Figure 3a" + }, + { + "id": "prioritized-generative-replay-D3-005", + "claim": "Compare Curiosity-PGR against REDQ and SYNTHER each augmented with an intrinsic curiosity reward bonus (weight 0.1) on state-based DMC tasks (quadruped-walk, cheetah-run) over 100K steps over 3 seeds, measuring average return to determine whether PGR's benefits go beyond simply improving exploration.", + "source": "Section 5.1, Figure 3b; Appendix B.1, Table 7" + }, + { + "id": "prioritized-generative-replay-D3-006", + "claim": "Compare sample efficiency of Curiosity-PGR against SYNTHER and REDQ on state-based DMC tasks (cheetah-run, quadruped-walk, finger-turn-hard) by plotting learning curves of average return as a function of environment steps over 100K interactions, with 5 seeds, showing that Curiosity-PGR matches SYNTHER's final performance approximately 50K steps earlier.", + "source": "Section 5.2, Figure 4a" + }, + { + "id": "prioritized-generative-replay-D3-007", + "claim": "Compare sample efficiency of Curiosity-PGR against SYNTHER and DRQ-V2 on pixel-based DMC tasks (walker-walk, cheetah-run) by plotting learning curves of average return as a function of environment steps over 100K interactions, with 5 seeds, showing Curiosity-PGR consistently improves over DRQ-V2 while SYNTHER is eventually overtaken.", + "source": "Section 5.2, Figure 4b" + }, + { + "id": "prioritized-generative-replay-D3-008", + "claim": "Measure the faithfulness of generated transitions to true environment dynamics by computing mean-squared error (MSE) between generated and ground-truth next states and rewards for SYNTHER and Curiosity-PGR at epoch 50 over 10K generated transitions across 3 OpenAI Gym environments, testing whether conditional generation improves sample quality.", + "source": "Section 5.2, Figure 5" + }, + { + "id": "prioritized-generative-replay-D3-009", + "claim": "Quantify policy overfitting on the quadruped-walk task by measuring dormant ratio (DR, fraction of inactive neurons below an activation threshold) of policy networks over the course of training for REDQ, SYNTHER, and Curiosity-PGR, showing that Curiosity-PGR maintains a low and stable DR while REDQ exhibits high and increasing DR.", + "source": "Section 5.2, Figure 6a" + }, + { + "id": "prioritized-generative-replay-D3-010", + "claim": "Characterize how curiosity conditioning mitigates overfitting by measuring the distribution of curiosity F-values (Eq. 5) over 10K real transitions every 10K timesteps on quadruped-walk for Curiosity-PGR, and comparing against the same F-function evaluated on 10K real transitions from the unconditionally-trained SYNTHER baseline, demonstrating a growing left-skewed and long-tailed distribution for PGR.", + "source": "Section 5.2, Figure 6b" + }, + { + "id": "prioritized-generative-replay-D3-011", + "claim": "Evaluate how PGR and SYNTHER scale with increased policy network capacity by scaling hidden layers from 2x256 to 3x512 (approximately 6x more parameters) while increasing batch size from 256 to 1024 to maintain per-parameter throughput, on the quadruped-walk DMC task over 100K steps with 3 seeds, measuring average return.", + "source": "Section 5.3, Figure 7a" + }, + { + "id": "prioritized-generative-replay-D3-012", + "claim": "Evaluate how PGR and SYNTHER behave when varying the fraction of synthetic data per training batch (synthetic data ratio r) from 0.5 to 0.75 to 0.875 by doubling the batch size (256 to 512 to 1024) while keeping real transitions fixed at 128 per batch, on quadruped-walk over 100K steps with 3 seeds, measuring average return to determine how much synthetic data can replace real data.", + "source": "Section 5.3, Figure 7b" + }, + { + "id": "prioritized-generative-replay-D3-013", + "claim": "Combine larger policy networks (3x512), increased synthetic data ratio (r=0.75, batch 512), doubled UTD ratio (20 to 40), and doubled synthetic buffer capacity (1M to 2M) to evaluate whether PGR can leverage these components complementarily on quadruped-walk over 100K steps with 3 seeds, measuring average return.", + "source": "Section 5.3, Figure 7c" + }, + { + "id": "prioritized-generative-replay-D3-014", + "claim": "Validate that PGR is a framework-level contribution by conditioning on two alternative relevance functions, Random Network Distillation (RND) prediction error and Context-Tree Switching (CTS) pseudo-counts, evaluating PGR (RND) and PGR (CTS) against DRQ-V2, SYNTHER, and PGR (Curiosity) on pixel-based DMC tasks (walker-walk, cheetah-run) over 100K steps with 5 seeds, measuring average return.", + "source": "Appendix A.1, Table 4" + }, + { + "id": "prioritized-generative-replay-D3-015", + "claim": "Evaluate PGR in the randomized DMLab noisy-TV environment, a procedurally-generated 3D maze with stochastic visual observations (lower-right 42x42 quadrant replaced with per-pixel uniform noise) and sparse goal rewards (+10), comparing PGR conditioned on ICM, RND, and ECO relevance functions against PPO baselines with and without exploration bonuses on both Sparse and Very Sparse variants over 10M environment steps with 10 seeds, measuring average return.", + "source": "Appendix A.2, Table 5" + }, + { + "id": "prioritized-generative-replay-D3-016", + "claim": "Integrate PGR with implicit exploration methods by replacing the policy Q-networks with NoisyNets (learnable mu+sigma parameters with sampled epsilon noise per linear layer) and Bootstrapped DQN (ensemble of K Q-networks with per-episode Bernoulli(1) mask for updates), evaluating PGR (NoisyNets) and PGR (Boot-DQN) against standalone NoisyNets, Boot-DQN, and PGR (Curiosity) on state-based DMC tasks (quadruped-walk, cheetah-run) over 100K steps with 3 seeds, measuring average return.", + "source": "Appendix B.2, Table 6" + }, + { + "id": "prioritized-generative-replay-D3-017", + "claim": "Evaluate the robustness of PGR against model-based RL algorithms (MAX, DREAMER-V3) under noisy transition dynamics by adding isotropic Gaussian noise to 20% of transitions per batch (to the ICM forward dynamics for PGR, to the model ensemble for MAX, to the recurrent state-space world model for DREAMER-V3) on state-based DMC tasks (cheetah-run, walker-walk, hopper-hop) over 100K steps with 5 seeds, measuring average return under both original (clean) and noised conditions.", + "source": "Appendix C, Table 8" + } + ], + "D4": [ + { + "id": "prioritized-generative-replay-D4-001", + "claim": "Experiment phases: REDQ + Curiosity / SYNTHER + Curiosity: policy receives r_total = r_extrinsic + 0.1 * r_curiosity -> PGR (Curiosity): curiosity only used as conditioning signal for diffusion generation, not added to policy reward -> All methods evaluated on quadruped-walk and cheetah-run state-based DMC over 100K steps, 3 seeds -> PGR outperforms both baselines augmented with exploration bonuses.", + "source": "Section 5.1 Fig. 3b; Appendix B.1 Table 7" + }, + { + "id": "prioritized-generative-replay-D4-002", + "claim": "Experiment phases: Train REDQ, SYNTHER, Curiosity-PGR to 100K env steps on quadruped-walk -> Periodically compute DR by measuring fraction of neurons below activation threshold in policy networks -> Plot DR curves over training (Fig 6a) -> Curiosity-PGR shows low and stable DR; REDQ shows high and increasing DR, indicating aggressive overfitting.", + "source": "Section 5.2 Fig. 6a" + }, + { + "id": "prioritized-generative-replay-D4-003", + "claim": "Experiment phases: Train MAX, DREAMER-V3, PGR for 100K env steps on both original and noised conditions on cheetah-run, walker-walk, hopper-hop -> For noised condition: at each training step, add isotropic Gaussian noise to 20% of transitions -> For PGR: noise only affects ICM forward dynamics loss computation; diffusion model still generates clean transitions -> For MAX: noise affects model ensemble training data -> For DREAMER-V3: noise affects world model state inputs -> PGR demonstrates greater robustness with smaller performance degradation under noise.", + "source": "Appendix C Table 8" + } + ] +} \ No newline at end of file diff --git a/papers/pyramidal-flow-matching/blacklist.txt b/papers/pyramidal-flow-matching/blacklist.txt new file mode 100644 index 0000000000000000000000000000000000000000..068617dd0ba678236997c08afbb344d4236d339f --- /dev/null +++ b/papers/pyramidal-flow-matching/blacklist.txt @@ -0,0 +1,2 @@ +# Official repository (ICLR 2025) +https://github.com/jy0205/Pyramid-Flow diff --git a/papers/pyramidal-flow-matching/config.yaml 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+oid sha256:75eaf48e78ad2ae1c996fdf051a189e92b69373ab1960826ddd322425a292c51 +size 130256 diff --git a/papers/pyramidal-flow-matching/paper.md b/papers/pyramidal-flow-matching/paper.md new file mode 100644 index 0000000000000000000000000000000000000000..2bdcbfc32863845332335d97b78c303c39275f82 --- /dev/null +++ b/papers/pyramidal-flow-matching/paper.md @@ -0,0 +1,564 @@ +# PYRAMIDAL FLOW MATCHING FOR EFFICIENT VIDEO GENERATIVE MODELING + +Yang $\mathbf { J i n ^ { 1 } }$ , Zhicheng $\mathbf { S u n } ^ { 1 }$ , Ningyuan $\mathbf { L i ^ { 3 } }$ , Kun Xu, $\mathbf { K u n } \mathbf { X u ^ { 2 } }$ , Hao Jiang1, Nan Zhuang2, Quzhe Huang, Yang Song, Yadong $\mathbf { M } \mathbf { u } ^ { 1 * }$ , Zhouchen $\mathbf { L i n ^ { 4 , 5 , 6 * } }$ + +1Peking University, 2Kuaishou Technology, 3Beijing University of Posts and Telecommunications, +4State Key Lab of General AI, School of Intelligence Science and Technology, Peking University, +5Institute for Artificial Intelligence, Peking University, +6Pazhou Laboratory (Huangpu), Guangzhou, Guangdong, China + +# ABSTRACT + +Video generation requires modeling a vast spatiotemporal space, which demands significant computational resources and data usage. To reduce the complexity, the prevailing approaches employ a cascaded architecture to avoid direct training with full resolution latent. Despite reducing computational demands, the separate optimization of each sub-stage hinders knowledge sharing and sacrifices flexibility. This work introduces a unified pyramidal flow matching algorithm. It reinterprets the original denoising trajectory as a series of pyramid stages, where only the final stage operates at the full resolution, thereby enabling more efficient video generative modeling. Through our sophisticated design, the flows of different pyramid stages can be interlinked to maintain continuity. Moreover, we craft autoregressive video generation with a temporal pyramid to compress the full-resolution history. The entire framework can be optimized in an end-to-end manner and with a single unified Diffusion Transformer (DiT). Extensive experiments demonstrate that our method supports generating high-quality 5-second (up to 10-second) videos at 768p resolution and 24 FPS within 20.7k A100 GPU training hours. All code and models are open-sourced at https://pyramid-flow.github.io. + +# 1 INTRODUCTION + +Video is a media form that records the evolvement of the physical world. Teaching the AI system to generate various video content plays a vital role in simulating the real-world dynamics (Hu et al., 2023; Brooks et al., 2024) and interacting with humans (Bruce et al., 2024; Valevski et al., 2024). Nowadays, the cutting-edge diffusion models (Ho et al., 2022c; Blattmann et al., 2023a; OpenAI, 2024) and autoregressive models (Yan et al., 2021; Hong et al., 2023; Kondratyuk et al., 2024) have made remarkable breakthroughs in generating realistic and long-duration video through scaling of data and computation. However, the necessity of modeling a significantly large spatiotemporal space makes the training of such video generative models computationally and data intensive. + +To ease the computational burden of generating high-dimensional video data, a crucial component is to compress the original video pixels into a lower-dimensional latent space using a VAE (Kingma & Welling, 2014; Esser et al., 2021; Rombach et al., 2022). However, the regular compression rate (typically $8 \times \mathrm { \Omega }$ still results in excessive tokens, especially for high-resolution samples. In light of this, prevalent approaches utilize a cascaded architecture (Ho et al., 2022b; Pernias et al., 2024; Teng et al., 2024) to break down the high-resolution generation process into multiple stages, where samples are first created in a highly compressed latent space and then successively upsampled using additional super-resolution models. Although the cascaded pipeline avoids directly learning at high resolution and reduces the computational demands, the requirement for employing distinct models at different resolutions separately sacrifices flexibility and scalability. Besides, the separate optimization of multiple sub-models also hinders the sharing of their acquired knowledge. + +This work presents an efficient video generative modeling framework that transcends the limitations of the previous cascaded approaches. Our motivation stems from the observation in Fig. 1a that the initial timesteps in diffusion models are quite noisy and uninformative. This suggests that operating at full resolution throughout the entire generation trajectory may not be necessary. To this end, we reinterpret the original generation trajectory as a series of pyramid stages that operate on compressed representations of different scales. Notably, the efficacy of image pyramids (Adelson et al., 1984) has been widely validated for discriminative neural networks (Lin et al., 2017; Wang et al., 2020) and more recently for diffusion models (Ho et al., 2022b; Pernias et al., 2024; Teng et al., 2024)and multimodal LLMs (Yu et al., 2023; Tian et al., 2024). Here, we investigate two types of pyramids: the spatial pyramid within a frame and the temporal one between consecutive frames (as illustrated in Fig. 1b). In such a pyramidal generation trajectory, only the final stage operates at full resolution, drastically reducing redundant computations in earlier timesteps. The main advantages are twofold: (1) The generation trajectories of different pyramid stages are interlinked, with the subsequent stage continuing to generate from the previous ones. This eliminates the need for each stage to regenerate from pure noise in some cascade models. (2) Instead of relying on separate models for each image pyramid, we integrate them into a single unified model for end-to-end optimization, which admits drastically-expedited training with more elegant implementation as validated by experiments. + +![](images/figures/pyramidal-flow-matching-fig-0001.jpg) +Figure 1: A motivating example for pyramidal flow matching: (a) Existing diffusion models operate at full resolution, spending a lot of computation on very noisy latents. (b) Our method harnesses the flexibility of flow matching to interpolate between latents of different resolutions. This allows for simultaneous generation and decompression of visual content with better computational efficiency. Note that the black arrows are denoising trajectories, and the blue ones are their temporal conditions. + +Based on the aforementioned pyramidal representations, we introduce a novel pyramidal flow matching algorithm that builds upon recent prevalent flow matching framework (Lipman et al., 2023; Liu et al., 2023; Albergo & Vanden-Eijnden, 2023). Specifically, we devise a piecewise flow for each pyramid resolution, which together form a generative process from noise to data. The flow within each pyramid stage takes a similar formulation, interpolating between a pixelated (compressed) and noisier latent and a pixelate-free (decompressed) and cleaner latent. Through our design, they can be jointly optimized by the unified flow matching objective in a single Diffusion Transformer (DiT) (Peebles & Xie, 2023), allowing simultaneous generation and decompression of visual content without multiple separate models. During inference, the output of each stage is renoised by a corrective Gaussian noise that maintains the continuity of the probability path between stages. Furthermore, we formulate the video generation in an autoregressive manner, iteratively predicting the next video latent conditioned on the generated history. Given the high redundancy in the full-resolution history, we curate a temporal pyramid sequence using progressively compressed, lower-resolution history as conditions, further reducing the number of tokens and improving training efficiency. + +The collaboration of the spatial and temporal pyramids results in remarkable training efficiency for video generation. Compared to the commonly used full-sequence diffusion, our method significantly reduces the number of video tokens during training (e.g., $\leq 1 5 { , } 3 6 0$ tokens versus 119,040 tokens for a 10-second, 241-frame video), thereby reducing both computational resources required and training time. By training only on open-source datasets, our model generate high-quality 10-second videos at 768p resolution and 24 fps. The core contributions of this paper are summarized as follows: + +• We present pyramidal flow matching, a novel video generative modeling algorithm that incorporates both spatial and temporal pyramid representations. Utilizing this framework can significantly improve training efficiency while maintaining good video generation quality. • The proposed unified flow matching objective facilitates joint training of pyramid stages in a single Diffusion Transformer (DiT), avoiding the separate optimization of multiple models. The support for end-to-end training further enhances its simplicity and scalability. • We evaluate its effectiveness on VBench (Huang et al., 2024) and EvalCrafter (Liu et al., 2024), with highly competitive performance among video generative models trained on public datasets. + +# 2 RELATED WORK + +Video Generative Models have seen rapid progress with autoregressive models (Yan et al., 2021; Hong et al., 2023; Kondratyuk et al., 2024; Jin et al., 2024) and diffusion models (Ho et al., 2022c; Blattmann et al., 2023b;a). A notable breakthrough is the high-fidelity video diffusion models (OpenAI, 2024; Kuaishou, 2024; Luma, 2024; Runway, 2024) by scaling up DiT pre-training (Peebles & Xie, 2023), but they induce significant training costs for long videos. An alternative line of research integrates diffusion models with autoregressive modeling (Chen et al., 2024a; Valevski et al., 2024) to natively support long video generation, but is still limited in context length and training efficiency. Our work advances both approaches in terms of efficiency from a compression perspective, featuring a spatially compressed pyramidal flow and a temporally compressed pyramidal history. + +Image Pyramids (Adelson et al., 1984) have been studied extensively in visual representation learning (Lowe, 2004; Dalal & Triggs, 2005; Lin et al., 2017; Wang et al., 2020). For generative models, the idea is explored by cascaded diffusion models that first generate at low resolution and then perform super-resolution (Ho et al., 2022b; Saharia et al., 2022; Zhang et al., 2023b; Gu et al., 2023; Pernias et al., 2024; Teng et al., 2024), and later extended to video (Ho et al., 2022a; Singer et al., 2023). However, they require training several separate models, which prevents knowledge sharing. Possible unified modeling solutions for pyramids include hierarchical architectures (Rombach et al., 2022; Crowson et al., 2024; Hatamizadeh et al., 2024) or via next-token prediction (Yu et al., 2023; Tian et al., 2024), but involve architectural changes. Instead, we propose a simple flow matching objective that allows joint training of pyramids, thus facilitating efficient video generative modeling. + +# 3 METHOD + +This work proposes an efficient video generative modeling scheme named pyramidal flow matching. In the following text, we first extend the flow matching algorithm (Section 3.1) to an efficient spatial pyramid representation (Section 3.2). Then, a temporal pyramid design is proposed in Section 3.3 to further improve training efficiency. Lastly, practical implementations are discussed in Section 3.4. + +# 3.1 PRELIMINARIES ON FLOW MATCHING + +Similar to diffusion models (Sohl-Dickstein et al., 2015; Song & Ermon, 2019; Ho et al., 2020), flow generative models (Papamakarios et al., 2021; Song et al., 2021; Xu et al., 2022; Lipman et al., 2023; Liu et al., 2023; Albergo & Vanden-Eijnden, 2023) aim to learn a velocity field ${ \mathbf { } } v _ { t }$ that maps random noise $\pmb { x } _ { 0 } \sim \mathcal { N } ( \mathbf { 0 } , I )$ to data samples $\mathbf { \mathscr { x } } _ { 1 } \sim \mathbf { \mathscr { q } }$ via an ordinary differential equation (ODE): + +$$ +\frac { d \pmb { x } _ { t } } { d t } = \pmb { v } _ { t } ( \pmb { x } _ { t } ) . +$$ + +Recently, Lipman et al. (2023); Liu et al. (2023); Albergo & Vanden-Eijnden (2023) proposed the flow matching framework, which provides a simple simulation-free training objective for flow generative models by directly regressing the velocity ${ \mathbf { } } v _ { t }$ on a conditional vector filed ${ \pmb u } _ { t } ( { \cdot } | { \pmb x } _ { 1 } )$ : + +$$ +\begin{array} { r } { \mathbb { E } _ { t , q ( { \pmb x } _ { 1 } ) , p _ { t } ( { \pmb x } _ { t } | { \pmb x } _ { 1 } ) } \big | \big | { \pmb v } _ { t } ( { \pmb x } _ { t } ) - { \pmb u } _ { t } ( { \pmb x } _ { t } | { \pmb x } _ { 1 } ) \big | \big | ^ { 2 } , } \end{array} +$$ + +where ${ \pmb u } _ { t } ( \cdot | { \pmb x } _ { 1 } )$ uniquely determines a conditional probability path $p _ { t } ( \cdot | \pmb { x } _ { 1 } )$ toward data sample $\scriptstyle { \mathbf { { \vec { x } } } } _ { 1 }$ An effective choice of the conditional probability path is linear interpolation of data and noise: + +$$ +\begin{array} { r l } & { \mathbf { } \mathbf { } \mathbf { } x _ { t } = t \mathbf { } x _ { 1 } + ( 1 - t ) \mathbf { } x _ { 0 } \mathrm { , } } \\ & { \mathbf { } x _ { t } \sim \mathcal { N } ( t \mathbf { } x _ { 1 } , ( 1 - t ) ^ { 2 } \mathbf { } I ) , } \end{array} +$$ + +and ${ \pmb u } ( { \pmb x } _ { t } | { \pmb x } _ { 1 } ) = { \pmb x } _ { 1 } - { \pmb x } _ { 0 }$ . Notably, flow matching can be flexibly extended to interpolate between distributions other than standard Gaussians. This enables us to devise a new flow matching algorithm that specializes in reducing the computational cost of video generative modeling. + +# 3.2 PYRAMIDAL FLOW MATCHING + +The main challenge in video generative modeling is the spatio-temporal complexity, and we address its spatial complexity first. According to previous key observation in Fig. 1, the initial generation + +![](images/figures/pyramidal-flow-matching-fig-0002.jpg) +Figure 2: Illustration of spatial pyramid. (a) The pyramidal flow is divided into multiple stages, each from a pixelated and noisy starting point to a pixelate-free and cleaner result. (b) During inference, we add a corrective noise at jump points across stages to ensure continuity of the proabability path. + +steps are usually very noisy and less informative, and thus may not need to operate at full resolution latent. This motivates us to study a spatially compressed pyramidal flow, illustrated in Fig. 2. + +To alleviate redundant computation in early steps, we interpolate flow between data and compressed low-resolution noise. Let $\oplus$ denote the interpolation between latents of different resolutions, and let there be $K$ resolutions, each halving the previous one, then our flow may be expressed as: + +$$ +\begin{array} { r } { \hat { \pmb { x } } _ { t } = t { \pmb { x } } _ { 1 } \oplus ( 1 - t ) D o w n ( { \pmb { x } } _ { 0 } , 2 ^ { K } ) , } \end{array} +$$ + +where $D o w n ( \cdot , \cdot )$ is a downsampling function. Since the interpolation concerns varying-dimensional $\mathbf { \Delta } _ { \mathbf { \mathcal { X } } _ { t } }$ , we decompose it as a piecewise flow (Yan et al., 2024) that divides $[ 0 , 1 ]$ into $K$ time windows, where each window interpolates between successive resolutions. For the $k$ -th time window $[ s _ { k } , e _ { k } ]$ , let $t ^ { \prime } = ( t - s _ { k } ) / ( e _ { k } - \bar { s _ { k } } )$ denote the rescaled timestep, then the flow within it follows: + +$$ +\hat { x } _ { t } = t ^ { \prime } D o w n ( x _ { e _ { k } } , 2 ^ { k } ) + ( 1 - t ^ { \prime } ) U p ( D o w n ( x _ { s _ { k } } , 2 ^ { k + 1 } ) ) , +$$ + +where $U p ( \cdot )$ is an upsampling function. This way, only the last stage is performed at full resolution, while most stages are performed at lower resolutions using less computation. Under a uniform stage partitioning, the idea of spatial pyramid reduces the computational cost to a factor of nearly $1 / K$ . Below, we describe the instantiation of pyramidal flow from training and inference, respectively. + +# 3.2.1 UNIFIED TRAINING + +In the construction of pyramidal flow, our main concern is unified modeling of different stages, as previous works (Ho et al., 2022b; Pernias et al., 2024; Teng et al., 2024) all require training multiple models for separate generation and super-resolution, which hinders knowledge sharing. + +To unify the objectives of generation and decompression/super-resolution, we curate the probability path by interpolating between different noise levels and resolutions. It starts with a more noisy and pixelated latent upsampled from a lower resolution, and yields cleaner and fine-grained results at a higher resolution, as illustrated in Fig. 2a. Formally, the conditional probability path is defined by: + +$$ +\begin{array} { r l } { \mathrm { E n d } : } & { \quad \hat { x } _ { e _ { k } } | x _ { 1 } \sim \mathcal { N } ( e _ { k } D o w n ( { \pmb x } _ { 1 } , 2 ^ { k } ) , ( 1 - e _ { k } ) ^ { 2 } { \pmb I } ) , } \\ { \mathrm { S t a r t } : } & { \quad \hat { x } _ { s _ { k } } | { \pmb x } _ { 1 } \sim \mathcal { N } ( s _ { k } U p ( D o w n ( { \pmb x } _ { 1 } , 2 ^ { k + 1 } ) ) , ( 1 - s _ { k } ) ^ { 2 } { \pmb I } ) , } \end{array} +$$ + +where $s _ { k } < e _ { k }$ , and the upsampling and downsampling functions for the clean $\scriptstyle { \mathbf { \mathscr { x } } } _ { 1 }$ are well defined, e.g., by nearest or bilinear resampling. In addition, to enhance the straightness of the flow trajectory, we couple the sampling of its endpoints by enforcing the noise to be in the same direction. Namely, we first sample a noise $\pmb { n } \sim \mathcal { N } ( \mathbf { 0 } , I )$ and then jointly compute the endpoints $( \hat { \pmb x } _ { e _ { k } } , \hat { \pmb x } _ { s _ { k } } )$ as: + +$$ +\begin{array} { r l } { \mathrm { E n d } : } & { \quad \hat { \pmb { x } } _ { e _ { k } } = e _ { k } D o w n ( { \pmb x } _ { 1 } , 2 ^ { k } ) + ( 1 - e _ { k } ) { \pmb n } , } \\ { \mathrm { S t a r t } : } & { \quad \hat { \pmb x } _ { s _ { k } } = s _ { k } U p ( D o w n ( { \pmb x } _ { 1 } , 2 ^ { k + 1 } ) ) + ( 1 - s _ { k } ) { \pmb n } . } \end{array} +$$ + +Thereafter, we can regress the flow model ${ \mathbf { } } v _ { t }$ on the conditional vector field ${ \pmb u } _ { t } ( \hat { \pmb x } _ { t } | { \pmb x } _ { 1 } ) = \hat { \pmb x } _ { e _ { k } } - \hat { \pmb x } _ { s _ { k } }$ with the following flow matching objective to unify generation and decompression: + +$$ +\mathbb { E } _ { k , t , ( \hat { x } _ { e _ { k } } , \hat { x } _ { s _ { k } } ) } \big \| \pmb { v } _ { t } ( \hat { \pmb { x } } _ { t } ) - \big ( \hat { \pmb { x } } _ { e _ { k } } - \hat { \pmb { x } } _ { s _ { k } } \big ) \big \| ^ { 2 } . +$$ + +Require: flow model $\pmb { v }$ , number of stages $K$ , time windows $[ s _ { k } , e _ { k } ]$ . Initialize a starting point $\hat { \mathbf { x } } _ { 0 } \sim \mathcal { N } ( \mathbf { 0 } , \mathbf { \bar { \it I } } )$ . for $k K - 1$ to 0 do Compute endpoint $\hat { \pmb x } _ { e _ { k } }$ from starting point $\hat { \pmb { x } } _ { s _ { k } }$ based on the flow model $\textbf { { v } }$ . Compute next starting point by upsampling $\hat { \boldsymbol { x } } _ { e _ { k } }$ with renoising. + +Ensure: generated sample $\hat { \mathbf { x } } _ { 1 }$ . + +# 3.2.2 INFERENCE WITH RENOISING + +During inference, standard sampling algorithms can be applied within each pyramid stage. However, we must carefully handle the jump points (Campbell et al., 2023) between successive pyramid stages of different resolutions to ensure continuity of the probability path. + +To ensure continuity, we first upsample the previous low-resolution endpoint with nearest or bilinear resampling. The result, as a linear combination of the input, follows a Gaussian distribution: + +$$ +U p ( \hat { \pmb x } _ { e _ { k + 1 } } ) | \pmb x _ { 1 } \sim \mathcal { N } ( e _ { k + 1 } U p ( D o w n ( \pmb x _ { 1 } , 2 ^ { k + 1 } ) ) , ( 1 - e _ { k + 1 } ) ^ { 2 } \pmb \Sigma ) , +$$ + +where $\pmb { \Sigma }$ is a covariance matrix depending on the upsampling function. Comparing Eqs. (8) and (12), we find it possible to match the Gaussian distributions at each jump point by a linear transformation of the upsampled result. Specifically, the following rescaling and renoising scheme would suffice: + +$$ +\hat { \pmb { x } } _ { s _ { k } } = \frac { s _ { k } } { e _ { k + 1 } } \ U p ( \hat { \pmb { x } } _ { e _ { k + 1 } } ) + \alpha { \pmb { n } } ^ { \prime } , \quad \mathrm { s . t . } \ n ^ { \prime } \sim \mathcal { N } ( { \bf 0 } , { \Sigma } ^ { \prime } ) , +$$ + +where the rescaling coefficient $s _ { k } / e _ { k + 1 }$ allows matching the means of these distributions, and the corrective noise $\mathbf { { \boldsymbol { n } } } ^ { \prime }$ with a weight of $\alpha$ allows matching their covariance matrices. + +To derive the corrective noise and its covariance, we consider a simplest scenario with nearest neighbor upsampling. In this case, $\pmb { \Sigma }$ has a blockwise structure with non-zero elements only in the $4 \times 4$ blocks along the diagonal (corresponding to those upsampled from the same pixel). Then, it can be inferred that the corrective noise’s covariance matrix $\Sigma ^ { \prime }$ also has a blockwise structure: + +$$ +\pmb { \Sigma } _ { b l o c k } = \left( \begin{array} { l l l l } { 1 } & { 1 } & { 1 } & { 1 } \\ { 1 } & { 1 } & { 1 } & { 1 } \\ { 1 } & { 1 } & { 1 } & { 1 } \\ { 1 } & { 1 } & { 1 } & { 1 } \end{array} \right) \Rightarrow \pmb { \Sigma } _ { b l o c k } ^ { \prime } = \left( \begin{array} { l l l l } { 1 } & { \gamma } & { \gamma } & { \gamma } \\ { \gamma } & { 1 } & { \gamma } & { \gamma } \\ { \gamma } & { \gamma } & { 1 } & { \gamma } \\ { \gamma } & { \gamma } & { \gamma } & { 1 } \end{array} \right) , +$$ + +where $\Sigma _ { b l o c k } ^ { \prime }$ contains negative elements $\gamma \in [ - 1 / 3 , 0 ] ^ { 1 }$ to reduce the correlation within each block, as illustrated in Fig. 2b. Since it is desirable to maximally preserve the signals at each jump point, we opt to add a small amount of noise with $\gamma = - 1 / 3$ such that it is most specialized for decorrelation. Substituting this into the above gives the update rule at jump points (see Appendix A for derivations): + +$$ +\hat { \pmb { x } } _ { s _ { k } } = \frac { 1 + s _ { k } } { 2 } U p ( \hat { \pmb { x } } _ { e _ { k + 1 } } ) + \frac { \sqrt { 3 } ( 1 - s _ { k } ) } { 2 } { \pmb n } ^ { \prime } , +$$ + +with $e _ { k + 1 } = 2 s _ { k } / ( 1 + s _ { k } )$ . The resulting inference process with renoising is shown in Algorithm 1. + +# 3.3 PYRAMIDAL TEMPORAL CONDITION + +Beyond the spatial complexity addressed in above sections, video presents another significant challenge due to its temporal length. The prevailing full-sequence diffusion methods generate all video frames simultaneously, restricting them to fixed-length generation (consistent with training). In contrast, the autoregressive video generation paradigm supports flexible-length generation during inference. Recent advancements (Chen et al., 2024a; Valevski et al., 2024) have also demonstrated its effectiveness in creating long-duration video content. However, their training is still severely limited by the computational complexity arising from the full-resolution long-history condition. + +We observe that there is a high redundancy in full-resolution history conditions. For example, earlier frames in a video tend to provide high-level semantic conditions and are less related to appearance details. This motivates us to use compressed, lower-resolution history for autoregressive video generation. As shown in Fig. 3a, we adopt a history condition of gradually increasing resolutions: + +![](images/figures/pyramidal-flow-matching-fig-0003.jpg) +Figure 3: Illustration of temporal pyramid. (a) At each pyramid stage, the generation is conditioned on a compressed, lower-resolution history to improve training efficiency of the autoregressive model, as indicated by the rows. (b) A compatible position encoding scheme is devised that extrapolates in the spatial pyramid but interpolates in the temporal pyramid to allow spatial alignment of conditions. + +$$ +\underbrace { \dots \to D o w n ( \pmb { x } _ { t ^ { \prime } } ^ { i - 2 } , 2 ^ { k + 1 } ) \to D o w n ( \pmb { x } _ { t ^ { \prime } } ^ { i - 1 } , 2 ^ { k } ) } _ { \mathrm { H i s t o r y ~ c o n d i t i o n } } \to \ \underbrace { \hat { \pmb { x } } _ { t } ^ { i } } _ { \mathrm { T r a i n i n g } } , +$$ + +where the superscripts are the history latent index, and the subscript $t ^ { \prime }$ indicates small noise added to history latents in training to mitigate error accumulation with autoregressive generation, as in (Chen et al., 2024a; Valevski et al., 2024). After training, we use clean generated frames for inference: + +$$ +\underbrace { \dots \to D o w n ( { \pmb x } _ { 1 } ^ { i - 2 } , 2 ^ { k + 1 } ) \to D o w n ( { \pmb x } _ { 1 } ^ { i - 1 } , 2 ^ { k } ) } _ { \mathrm { H i s t o r y ~ c o n d i t i o n } } \to \underbrace { \hat { \pmb x } _ { t } ^ { i } } _ { \mathrm { P r e d i c t i o n } } . +$$ + +The above design significantly reduces the computational and memory overhead of video generative pre-training. Let there be $T$ history latents over $K$ lower resolutions, then most frames are computed at the lowest resolution of $1 / 2 ^ { K }$ , which reduces the number of training tokens by up to $1 / 4 ^ { K }$ times. As a result, training efficiency is improved by up to $1 6 ^ { K } / T$ times. + +# 3.4 PRACTICAL IMPLEMENTATION + +In this section, we show that the above pyramid designs can be easily implemented using standard Transformer architecture (Vaswani et al., 2017) and pipelines. This is crucial for efficient and scalable video generative pre-training based on existing acceleration frameworks. + +Unlike previous methods (Ma et al., 2024) that utilize factorized spatial and temporal attention to reduce computational complexity, we directly employ full sequence attention, thanks to much fewer tokens required by our pyramidal representation. Furthermore, blockwise causal attention is adopted in each transformer layer, ensuring that each token cannot attend to its subsequent frames. The ablation results in Appendix C.2 illustrate that such casual attention design is crucial for autoregressive video generation. Another important design choice is the position encoding, as the pyramid designs introduce multiple spatial resolutions. As shown in Fig. 3b, we extrapolate position encoding in the spatial pyramid for better fine-grained detail (Yang et al., 2024), while interpolating it in the temporal pyramid input to spatially align the history conditions. + +During training, different pyramidal stages are uniformly sampled in each update iteration. The autoregressive nature of our method inherently supports joint training of images and videos, since the first frame in a video acts as an image. We pack training samples with varying token counts together to form the length-balanced training batch following Patch n’ Pack (Dehghani et al., 2023). After training, our method natively possesses the capability of text-to-video and text-conditioned image-to-video generation. During inference sampling, the classifier-free guidance strategy can be employed to enhance temporal consistency and motion smoothness of the generated video. + +# 4 EXPERIMENTS + +# 4.1 EXPERIMENTAL SETTINGS + +Training Dataset. Our model is trained on a mixed corpus of open-source image and video datasets. For images, we utilize a high-aesthetic subset of LAION-5B (Schuhmann et al., 2022), 11M from CC-12M (Changpinyo et al., 2021), 6.9M non-blurred subset of SA-1B (Kirillov et al., 2023), 4.4M from JourneyDB (Sun et al., 2023), and 14M publicly available synthetic data. For video data, we incorporate the WebVid-10M (Bain et al., 2021), OpenVid-1M (Nan et al., 2024), and another 1M high-resolution non-watermark video primarily from the Open-Sora Plan (PKU-Yuan Lab et al., 2024). After postprocessing, around 10M single-shot videos are available for training. + +Evaluation Metrics. We utilize the VBench (Huang et al., 2024) and EvalCrafter (Liu et al., 2024) for quantitative performance evaluation. VBench is a comprehensive benchmark that includes 16 fine-grained dimensions to systematically measure both motion quality and semantic alignment of video generative models. EvalCrafter is another large-scale evaluation benchmark including around 17 objective metrics for assessing video generation capabilities. In addition to automated evaluation metrics, we also conducted a study with human participants to measure the human preference for our generated videos. The compared baselines are summarized in Appendix B. + +Implementation Details. We utilize the prevailing MM-DiT architecture from SD3 Medium (Esser et al., 2024) as the base model, with 2B parameters in total. It employs sinusoidal position encoding (Vaswani et al., 2017) in the spatial dimensions. As for the temporal dimension, the 1D Rotary Position Embedding (RoPE) (Su et al., 2024) is added to support flexible training with different video durations. In addition, we use a 3D Variational Autoencoder (VAE) to compress videos both spatially and temporally with a downsampling ratio of $8 \times 8 \times 8$ . It shares a similar structure with MAGVIT-v2 (Yu et al., 2024) and is trained from scratch on the WebVid-10M dataset (Bain et al., 2021). The number of pyramid stages is set to 3 in all the experiments. Following Valevski et al. (2024), we add some corruptive noise of strength uniformly sampled from $[ 0 , 1 / 3 ]$ to the history pyramid conditions, which is critical for mitigating the autoregressive generation degradation. + +# 4.2 EFFICIENCY + +The proposed pyramidal flow matching framework significantly reduces the computational and memory overhead in video generation training. Consider a video with $T$ frame latents, where each frame contains $N$ tokens at the original resolution. The full-sequence diffusion has $T N$ input tokens in DiT and requires $T ^ { 2 } N ^ { 2 }$ computations. In contrast, our method uses only approximately $T N / 4 ^ { K }$ tokens and $T ^ { 2 } N ^ { 2 } / 1 6 ^ { K }$ computations even for the final pyramid stage, which significantly improves the training efficiency. Specifically, it takes only 20.7k A100 GPU hours to train a 10s video generation model with 241 frames. Compared to existing models that require significant training resources, our method achieves superior video generation performance with much fewer computations. For example, the Open-Sora 1.2 (Zheng et al., 2024) requires $4 . 8 \mathrm { k }$ Ascend and $3 7 . 8 \mathrm { k } \mathrm { H } 1 0 0$ hours to train the generation of only 97 video frames, consuming more than two times the computation of our approach, yet producing videos of worse quality. At inference, our model takes just 56 seconds to create a 5-second, $3 8 4 \mathrm { p }$ video clip, which is comparable to full-sequence diffusion counterparts. + +# 4.3 MAIN RESULTS + +Text-to-Video Generation. We first evaluate the text-to-video generation capability of the proposed method. For each text prompt, a 5-second 121 frames video is generated for evaluation. The detailed quantitative results on VBench (Huang et al., 2024) and EvalCrafter (Liu et al., 2024) are summarized in Tables 1 and 2, respectively. Overall, our method surpasses all the compared opensourced video generation baselines in these two benchmarks. Even with only publicly accessible video data in training, it achieves comparable performance to commercial competitors trained on much larger proprietary data like Kling (Kuaishou, 2024) and Gen-3 Alpha (Runway, 2024). In particular, we demonstrated exceptional performance in quality score (84.74 vs. 84.11 of Gen-3), and motion smoothness in VBench, which are crucial criteria in reflecting the visual quality of generated videos. When evaluated in EvalCrafter, our method achieves better visual and motion quality scores than most compared methods. The semantic score is relatively lower than others, mainly because we use coarse-grained synthetic captions, which can be improved with more accurate video captioning. + +Table 1: Experimental results on VBench (Huang et al., 2024). In terms of total score and quality score, our model even outperforms CogVideoX-5B (Yang et al., 2024) with twice the model size. In the following tables, we use blue to denote the highest scores among models trained on public data. +Table 2: Experimental results on EvalCrafter (Liu et al., 2024). See Appendix C.1 for raw metrics. + +
ModelPublic DataTotal ScoreQuality ScoreSemantic ScoreMotion SmoothnessDynamic Degree
Gen-2×80.5882.4773.0399.5818.89
Pika 1.0×80.6982.9271.7799.5047.50
CogVideoX-2B×80.9182.1875.8397.7359.86
CogVideoX-5B×81.6182.7577.0496.9270.97
Kling×81.8583.3875.6899.4046.94
Gen-3 Alpha×82.3284.1175.1799.2360.14
Open-Sora Plan v1.377.2380.1465.6299.0530.28
Open-Sora 1.279.7681.3573.3998.5042.39
VideoCrafter280.4482.2073.4297.7342.50
T2V-Turbo81.0182.5774.7697.3449.17
Ours81.7284.7469.6299.1264.63
+ +![](images/figures/pyramidal-flow-matching-fig-0004.jpg) +Figure 4: User preference on sampled VBench prompts. Our videos are generated at 5s, 768p, 24fps. + +We also present some generated 5–10 second videos in Fig. 5, showing cinematic visual quality and validate the efficacy of pyramidal flow matching. More visualizations are provided in Appendix C.3. + +User study. While quantitative evaluation scores reflect the video generation capability to some extent, they may not align with human preferences for visual quality. Hence, an additional user study is conducted to compare our performance with six baseline models, including CogVideoX (Yang et al., 2024) and Kling (Kuaishou, 2024). We utilized 50 prompts sampled from VBench and asked $^ { 2 0 + }$ participants to rank each model according to the aesthetic quality, motion smoothness, and semantic alignment of the generated videos. As seen in Section 4.3, our method is preferred over open-source models such as Open-Sora and CogVideoX-2B especially in terms of motion smoothness. This is due to the substantial token savings achieved by pyramidal flow matching, enabling generation of 5- second (up to 10-second) 768p videos at 24 fps, while the baselines usually support video synthesis of similar length only at 8 fps. The detailed user study settings are presented in Appendix B. + +![](images/figures/pyramidal-flow-matching-fig-0005.jpg) + +(a) The Glenfinnan Viaduct is a historic railway bridge. . . It is a stunning sight as a steam train leaves the bridge, traveling over the arch-covered viaduct. The landscape is dotted with lush greenery and rocky mountains. . . + +![](images/figures/pyramidal-flow-matching-fig-0006.jpg) +(b) Beautiful, snowy Tokyo city is bustling. The camera moves through the bustling city street, following several people enjoying the beautiful snowy weather and shopping at nearby stalls. Gorgeous sakura petals. . . + +![](images/figures/pyramidal-flow-matching-fig-0007.jpg) +(c) A side profile shot of a woman with fireworks exploding in the distance beyond he + +Figure 5: Visualization of text-to-video generation results. The top two videos are generated at 5s, 768p, 24fps, and the bottom one at 10s, 768p, 24fps. See more generated videos on our project page. + +![](images/figures/pyramidal-flow-matching-fig-0008.jpg) +(a) A moon rises from the sky and the lights on the land are bright. + +![](images/figures/pyramidal-flow-matching-fig-0009.jpg) +(b) Monster Illustration in flat design style of a diverse family of monsters. The group includes a furry brown monster, a sleek black monster with antennas, a spotted green monster, and a tiny polka-dotted monster, all. . . +Figure 6: Visualization of text-conditioned image-to-video generation results (5s, 768p, 24fps). + +Image-to-Video Generatetion. Thanks to the autoregressive property of our model and the causal attention design, the first frame of each video acts similarly to an image condition during the training. Consequently, although our model is optimized solely for text-to-video generation, it naturally accommodates text-conditioned image-to-video generation during inference. Given an image and a textual prompt, it is able to animate the static input image by autoregressively predicting the future frames without further fine-tuning. In Fig. 6, we illustrate qualitative examples of its image-to-video generation performance, where each example consists of 120 newly synthesized frames spanning a duration of 5 seconds. As can be seen, our model successfully predicts reasonable subsequent motion, endowing the images with rich temporal dynamic information. More generated video examples are best viewed on our project page at https://pyramid-flow.github.io. + +# 4.4 ABLATION STUDY + +In this section, we conduct ablation studies to validate the crucial component of our methods, including the spatial pyramid in denoising trajectory and the temporal pyramid in history condition. Due to limited space, the ablations for other design choices are provided in Appendix C.2. + +![](images/figures/pyramidal-flow-matching-fig-0010.jpg) +Figure 7: Ablation study of spatial pyramid at $5 0 \mathrm { k }$ image training step. On the right is a quantitative comparison of the FID results, where our method achieves almost three times the convergence speed. + +![](images/figures/pyramidal-flow-matching-fig-0011.jpg) +Figure 8: Ablation study of temporal pyramid at $1 0 0 \mathrm { k }$ low-resolution video training step. + +Effectiveness of spatial pyramid. In the generation trajectory of the proposed spatial pyramid, only the final stage operates at full resolution, which significantly reduces the number of tokens for most denoising timesteps. With the same computational resources, it can handle more samples per training batch, greatly enhancing the convergence rate. To validate its efficiency, we designed a baseline that employs the standard flow matching objective for training text-to-image generation in our early experiments. This baseline is optimized using the same training data, number of tokens per batch, hyperparameter configurations, and model architecture to ensure fairness. The performance comparison is illustrated in Fig. 7. It can be observed that the variant using pyramidal flow demonstrates superior visual quality and prompt-following capability. We further quantitatively evaluate the FID metric of these methods on the MS-COCO benchmark (Lin et al., 2014) by randomly sampling 3K prompts. The FID performance curve over training steps is presented on the right of Fig. 7. Compared to standard flow matching, the convergence rate of our method is significantly improved. + +Effectiveness of temporal pyramid. As mentioned in Section 4.2, the temporal pyramid design can drastically reduce the computation demands compared to traditional full-sequence diffusion. Similar to the spatial pyramid, we also established a full-sequence diffusion baseline under the same experimental setting to investigate its training efficiency improvement. The qualitative comparison with the baseline is presented in Fig. 8, where the generated videos of our pyramidal variant demonstrate much better visual quality and temporal consistency under the same training steps. In contrast, the full-sequence diffusion baseline is far from convergence. It fails to produce coherent motion, leading to fragmented visual details and severe artifacts in the generated videos. This performance gap clearly highlights the training acceleration achieved by our method in video generative modeling. + +# 5 CONCLUSION + +This work presents an efficient video generative modeling framework based on pyramidal visual representations. In contrast to cascaded diffusion models that use separate models for different image pyramids to improve efficiency, we propose a unified pyramidal flow matching objective that simultaneously generates and decompresses visual content across pyramid stages with a single model, effectively facilitating knowledge sharing. Furthermore, a temporal pyramid design is introduced to reduce computational redundancy in the full-resolution history of a video. The proposed method is extensively evaluated on VBench and EvalCrafter, demonstrating advantageous performance. + +Reproducibility Statement. Our code and models are open-sourced at https://pyramidflow.github.io. The experimental settings are detailed in Section 4.1 and Appendix B. + +# ACKNOWLEDGMENTS + +The work was supported by National Key R&D Program of China (2022ZD0160300), an internal grant of Peking University (2024JK28), a grant from Kuaishou (No. DJHL-20240809-115) and NSF China (No. 62276004). + +# REFERENCES + +Edward H Adelson, Charles H Anderson, James R Bergen, Peter J Burt, and Joan M Ogden. Pyramid methods in image processing. RCA Engineer, 29(6):33–41, 1984. + +Michael Albergo and Eric Vanden-Eijnden. Building normalizing flows with stochastic interpolants. In International Conference on Learning Representations, 2023. + +Max Bain, Arsha Nagrani, Gul Varol, and Andrew Zisserman. Frozen in time: A joint video and ¨ image encoder for end-to-end retrieval. In Proceedings of the IEEE/CVF International Conference on Computer Vision, pp. 1728–1738, 2021. + +Black Forest Labs. 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CogVideoX: Text-to-video diffusion models with an expert transformer. arXiv preprint arXiv:2408.06072, 2024. + +Tianwei Yin, Qiang Zhang, Richard Zhang, William T Freeman, Fredo Durand, Eli Shechtman, and Xun Huang. From slow bidirectional to fast causal video generators. arXiv preprint arXiv:2412.07772, 2024. + +Lijun Yu, Yong Cheng, Zhiruo Wang, Vivek Kumar, Wolfgang Macherey, Yanping Huang, David A Ross, Irfan Essa, Yonatan Bisk, Ming-Hsuan Yang, et al. SPAE: Semantic pyramid autoencoder for multimodal generation with frozen LLMs. In Advances in Neural Information Processing Systems, pp. 52692–52704, 2023. + +Lijun Yu, Jose Lezama, Nitesh B Gundavarapu, Luca Versari, Kihyuk Sohn, David Minnen, Yong ´ Cheng, Agrim Gupta, Xiuye Gu, Alexander G Hauptmann, et al. Language model beats diffusion– tokenizer is key to visual generation. In International Conference on Learning Representations, 2024. + +David Junhao Zhang, Jay Zhangjie Wu, Jia-Wei Liu, Rui Zhao, Lingmin Ran, Yuchao Gu, Difei Gao, and Mike Zheng Shou. Show-1: Marrying pixel and latent diffusion models for text-tovideo generation. arXiv preprint arXiv:2309.15818, 2023a. + +Han Zhang, Ruili Feng, Zhantao Yang, Lianghua Huang, Yu Liu, Yifei Zhang, Yujun Shen, Deli Zhao, Jingren Zhou, and Fan Cheng. Dimensionality-varying diffusion process. In Proceedings of the IEEE/CVF Conference on Computer Vision and Pattern Recognition, pp. 14307–14316, 2023b. + +Zangwei Zheng, Xiangyu Peng, Tianji Yang, Chenhui Shen, Shenggui Li, Hongxin Liu, Yukun Zhou, Tianyi Li, and Yang You. Open-Sora: Democratizing efficient video production for all. https://github.com/hpcaitech/Open-Sora, 2024. + +# A DERIVATION + +This section provides a detailed derivation for Eq. (15) that handles jump points in the spatial pyramid. For quick lookup,Table 3 summarizes the used notations. + +Table 3: Notation in the main paper. + +
SymbolDescription
x1Data latent at full resolution
x0Noise at full resolution
KNumber of pyramid stages
ekTimestep at endpoint of k-th pyramid stage
SkTimestep at starting point of k-th pyramid stage
xekNoisy latent at endpoint of k-th stage
x skNoisy latent at starting point of k-th stage
xtNoisy latent at timestep t
nNoise at the resolution of current stage
Up(·)Upsampling function, e.g. nearest-neighbor
Down(.,·)Downsampling function, e.g. bilinear
+ +To ensure continuity of the probability path across different stages of the spatial pyramid, we need to make sure that the endpoints have the same probability distribution. According to Eqs. (8) and (12), their distributions are already similar after a simple upsampling transformation: + +$$ +\hat { \pmb x } _ { s _ { k } } | \pmb x _ { 1 } \sim \mathcal N ( s _ { k } ~ U p ( D o w n ( \pmb x _ { 1 } , 2 ^ { k + 1 } ) ) , ( 1 - s _ { k } ) ^ { 2 } \pmb I ) , +$$ + +$$ +U p ( \hat { \pmb x } _ { e _ { k + 1 } } ) | \pmb x _ { 1 } \sim \mathcal { N } ( e _ { k + 1 } U p ( D o w n ( \pmb x _ { 1 } , 2 ^ { k + 1 } ) ) , ( 1 - e _ { k + 1 } ) ^ { 2 } \pmb \Sigma ) . +$$ + +Therefore, we can directly apply a linear transformation with a corrective Gaussian noise to match their distributions: + +$$ +\hat { \pmb { x } } _ { s _ { k } } = \frac { s _ { k } } { e _ { k + 1 } } \ U p ( \hat { \pmb { x } } _ { e _ { k + 1 } } ) + \alpha { \pmb { n } } ^ { \prime } , \quad \mathrm { s . t . } \ n ^ { \prime } \sim \mathcal { N } ( { \bf 0 } , { \Sigma } ^ { \prime } ) , +$$ + +where the rescaling coefficient $s _ { k } / e _ { k + 1 }$ allows the means of these distributions to be matched, and $\alpha$ is the noise weight. Additionally, we need to match the covariance matrices of Eqs. (18) and (20): + +$$ +\frac { s _ { k } ^ { 2 } } { e _ { k + 1 } ^ { 2 } } ( 1 - e _ { k + 1 } ) ^ { 2 } \Sigma + \alpha ^ { 2 } \Sigma ^ { \prime } = ( 1 - s _ { k } ) ^ { 2 } I . +$$ + +To allow analysis of covariance matrices, e.g. $\pmb { \Sigma }$ , we consider a simplest scenario with nearest neighbor upsampling. In this case, $\pmb { \Sigma }$ has a blockwise structure with non-zero elements only in the $4 \times 4$ blocks along the diagonal (corresponding to those upsampled from the same pixel). Then, it can be inferred that the corrective noise’s covariance matrix $\Sigma ^ { \prime }$ has a similar blockwise structure: + +$$ +\pmb { \Sigma } _ { b l o c k } = \left( \begin{array} { l l l l } { 1 } & { 1 } & { 1 } & { 1 } \\ { 1 } & { 1 } & { 1 } & { 1 } \\ { 1 } & { 1 } & { 1 } & { 1 } \\ { 1 } & { 1 } & { 1 } & { 1 } \end{array} \right) \Rightarrow \pmb { \Sigma } _ { b l o c k } ^ { \prime } = \left( \begin{array} { l l l l } { 1 } & { \gamma } & { \gamma } & { \gamma } \\ { \gamma } & { 1 } & { \gamma } & { \gamma } \\ { \gamma } & { \gamma } & { 1 } & { \gamma } \\ { \gamma } & { \gamma } & { \gamma } & { 1 } \end{array} \right) , +$$ + +where $\gamma$ is a negative value in $[ - 1 / 3 , 0 ]$ for the decorrelation (its lower bound $- 1 / 3$ ensures that the covariance matrix is semidefinite). We further rewrite Eqs. (21) and (22) by considering the equality of their diagonal and non-diagonal elements, respectively: + +$$ +\begin{array} { r l } & { \frac { s _ { k } ^ { 2 } } { e _ { k + 1 } ^ { 2 } } ( 1 - e _ { k + 1 } ) ^ { 2 } + \alpha ^ { 2 } = ( 1 - s _ { k } ) ^ { 2 } , } \\ & { \frac { s _ { k } ^ { 2 } } { e _ { k + 1 } ^ { 2 } } ( 1 - e _ { k + 1 } ) ^ { 2 } + \alpha ^ { 2 } \gamma = 0 . } \end{array} +$$ + +Taking into account the timestep constraints $0 < s _ { k } , e _ { k + 1 } < 1$ , they can be solved directly: + +$$ +e _ { k + 1 } = { \frac { s _ { k } { \sqrt { 1 - \gamma } } } { ( 1 - s _ { k } ) { \sqrt { - \gamma } } + s _ { k } { \sqrt { 1 - \gamma } } } } , \quad \alpha = { \frac { 1 - s _ { k } } { \sqrt { 1 - \gamma } } } . +$$ + +Intuitively, it is desirable to maximally preserve the signals at each jump point, which corresponds to minimizing the noise weight $\alpha$ . According to Eq. (25), this is equivalent to minimizing $\gamma$ . Substituting its minimum value $\gamma = - 1 / 3$ into Eq. (25) yields: + +$$ +e _ { k + 1 } = { \frac { 2 s _ { k } } { 1 + s _ { k } } } , \quad \alpha = { \frac { { \sqrt { 3 } } ( 1 - s _ { k } ) } { 2 } } . +$$ + +It is worth noting that $e _ { k + 1 } > s _ { k }$ , indicating that the timestep is rolled back a bit when adding the corrective noise at each jump point. We can further obtain the renoising rule in Eq. (15): + +$$ +\hat { \pmb { x } } _ { s _ { k } } = \frac { 1 + s _ { k } } { 2 } U p ( \hat { \pmb { x } } _ { e _ { k + 1 } } ) + \frac { \sqrt { 3 } ( 1 - s _ { k } ) } { 2 } { \pmb n } ^ { \prime } . +$$ + +# B EXPERIMENTAL SETTINGS + +Model Implementation Details. We adopt the MM-DiT architecture, based on SD3 Medium (Esser et al., 2024), which comprises 24 transformer layers and a total of 2B parameters. The weights of the MM-DiT are initialized from the SD3 medium. Following the more recent FLUX.1 (Black Forest Labs, 2024), both T5 (Raffel et al., 2020) and CLIP (Radford et al., 2021) encoders are employed for prompts embedding. To address the redundancy in video data, we have designed a 3D VAE that compresses videos both spatially and temporally into a latent space. The architecture of this VAE is similar to MAGVIT-v2 (Yu et al., 2024), employing 3D causal convolution to ensure that each frame depends only on the preceding frames. It features an asymmetric encoder-decoder with Kullback-Leibler (KL) regularization applied to the latents. Overall, the 3D VAE achieves a compression rate of $8 \times 8 \times 8$ from pixels to the latent. It is trained on WebVid-10M and 6.9M SAM images from scratch. To support the tokenization of very long videos, we scatter them into multiple GPUs to distribute computation like CogVideoX (Yang et al., 2024). + +Training Procedure Our model undergoes a three-stage training procedure using 128 NVIDIA A100 GPUs. (1) Image Training. In the first stage, we utilize a pure image dataset that includes 180M images from LAION-5B (Schuhmann et al., 2022), 11M from CC-12M (Changpinyo et al., 2021), 6.9M non-blurred images from SA-1B (Kirillov et al., 2023), and 4.4M from JourneyDB (Sun et al., 2023). We keep the image’s original aspect ratio and rearrange them into different buckets. It is trained for a total of 50,000 steps, requiring approximately 1536 A100 GPU hours. After this stage, the model has learned the dependencies between visual pixels, which facilitates the convergence of subsequent video training. (2) Low-Resolution Video Training. For this stage, we employ the WebVid-10M (Bain et al., 2021), OpenVid-1M (Nan et al., 2024), and another 1M non-watermark video from the Open-Sora Plan (PKU-Yuan Lab et al., 2024). We also leverage the Video-LLaMA2 (Cheng et al., 2024), a state-of-the-art video understanding model, to recaption each video sample. The image data from stage 1 is also utilized at a proportion of $12 . 5 \%$ in each batch. We first train the model for 80,000 steps on 2-second video generation, followed by an additional 120,000 steps on 5-second videos. In total, it takes about 11,520 A100 GPU hours at this stage. (3) High-Resolution Video Training. The final stage employs the same strategy to continue finetuning the model on the aforementioned high-resolution video dataset of varying durations (5–10s). It consumes approximately 7,680 A100 GPU hours for 50,000 steps in the final stage. + +Hyperparameters Setting The detailed training hyper-parameter settings for each optimization stage are reported in Table 4. + +Baseline Methods. For VBench (Huang et al., 2024), we compare with eight baseline methods, including Open-Sora Plan V1.3 (PKU-Yuan Lab et al., 2024), Open-Sora 1.2 (Zheng et al., 2024), VideoCrafter2 (Chen et al., 2024b), Gen-2 (Runway, 2023), Pika 1.0 (Pika, 2023), T2V-Turbo (Li et al., 2024), CogVideoX (Yang et al., 2024), Kling (Kuaishou, 2024), and Gen-3 Alpha (Runway, 2024). Among them, Open-Sora Plan, Open-Sora, CogVideo-X, Kling and Gen-3 Alpha can generate long videos. For EvalCrafter (Liu et al., 2024), our model is compared to six baselines, including ModelScope (Wang et al., 2023a), Show-1 (Zhang et al., 2023a), LaVie (Wang et al., 2023b), VideoCrafter2 (Chen et al., 2024b), Pika 1.0 (Pika, 2023), and Gen-2 (Runway, 2023). The above models are all based on full-sequence diffusion, while our method combines the merits of autoregressive generation and flow generative models to achieve better training efficiency of video generation. + +User Study. To complement the quantitative evaluation in the main paper, we conduct a rigorous user study to collect human preferences for these generative models. To accomplish this, we sample + +Table 4: The detailed training hyperparameters of our method + +
ConfigurationStage-1Stage-2Stage-3
OptimizerAdamWAdamWAdamW
Optimizer Hyperparametersβ1 = 0.9, β2 = 0.999, = 1e−6β1 = 0.9, β2 = 0.95, = 1e−6
Global batch size1536768384
Learning rate1e-41e-45e-5
Learning rate scheduleConstant with warmupConstant with warmupConstant with warmup
Training Steps50k200k50k
Warm-up steps1k1k1k
Weight decay1e-41e-41e-4
Gradient clipping1.01.01.0
Numerical precisionbfloat16bfloat16bfloat16
GPU Usage128 NVIDIA A100128 NVIDIA A100128 NVIDIA A100
Training Time12h90h60h
+ +![](images/figures/pyramidal-flow-matching-fig-0012.jpg) +Figure 9: Interface for user study of video generative performance. + +50 prompts from the VBench prompt list and randomly sample one generated video for each prompt from the baseline model. In total, six baseline models are considered, including Open-Sora Plan V1.1 (PKU-Yuan Lab et al., 2024), Open-Sora 1.2 (Zheng et al., 2024), Pika 1.0 (Pika, 2023), CogVideoX-2B and 5B (Yang et al., 2024), and Kling (Kuaishou, 2024). We then pair these results with our generated video and ask the participant to rank their preference among three dimensions: aesthetic quality, motion smoothness, and semantic alignment, each of which represents a crucial aspect of video quality. The interface for the user study is exemplified in Fig. 9, where the user accepts a prompt and two generated videos (with the unnecessary information cropped, such as a watermark indicating which model it belongs to), and chooses between which model is better in the three dimensions. We distribute the user study to more than 20 participants, and collect a total of 1411 valid preference choices, ensuring its effectiveness. The results of this user study are presented in Section 4.3, where our model shows a very competitive performance among the compared baselines. + +Table 5: Detailed results on VBench (Huang et al., 2024). See Table 1 for the summarized results. We additionally use blue to indicate the highest scores among models trained on public datasets. + +
ModelSubjectBackground Temporal Consistency Consistency FlickeringMotion SmoothnessDynamic DegreeAesthetic QualityImaging QualityObject Class
Trained on private datasets:
Gen-297.6197.6199.5699.5818.8966.9667.4290.92
Pika 1.096.9497.3699.7499.5047.5062.0461.8788.72
CogVideoX-2B96.7896.6398.8997.7359.8660.8261.6883.37
CogVideoX-5B96.2396.5298.6696.9270.9761.9862.9085.23
Kling98.3397.6099.3099.4046.9461.2165.6287.24
Gen-3 Alpha97.1096.6298.6199.2360.1463.3466.8287.81
Trained on public datasets:
Open-Sora Plan v1.397.7997.2499.2099.0530.2860.4256.2185.56
Open-Sora 1.296.7597.6199.5398.5042.3956.8563.3482.22
VideoCrafter296.8598.2298.4197.7342.5063.1367.2292.55
T2V-Turbo96.2897.0297.4897.3449.1763.0472.4993.96
Ours96.9598.0699.4999.1264.6363.2665.0186.67
ModelMultiple ObjectsHuman ActionColorSpatial RelationshipSceneAppearance Temporal StyleStyleOverall Consistency
Trained on private datasets:
Gen-255.4789.289.4966.9148.9119.3424.1226.17
Pika 1.043.0886.290.5761.0349.8322.2624.2225.94
CogVideoX-2B62.6398.079.4169.9051.1424.8024.3626.66
CogVideoX-5B62.1199.482.8166.3553.2024.9125.3827.59
Kling68.0593.489.9073.0350.8619.6224.1726.42
Gen-3 Alpha53.6496.480.9065.0954.5724.3124.7126.69
Trained on public datasets:
Open-Sora Plan v1.343.5886.879.3051.6136.7320.0322.4724.47
Open-Sora 1.251.8391.290.0868.5642.4423.9524.5426.85
VideoCrafter240.6695.092.9235.8655.2925.1325.8428.23
T2V-Turbo54.6595.289.9038.6755.5824.4225.5128.16
Ours50.7185.682.8759.5343.2020.9123.0926.23
+ +Table 6: Raw metrics on EvalCrafter (Liu et al., 2024). The baseline results are found on its website, but there were no results for LaVie (Wang et al., 2023b). See Table 2 for the summarized results. + +
ModelVQAAVQATISCLIP- TempWarping ErrorFace ConsistencyAction- ScoreMotion AC-Score
Trained on private datasets:
Pika 1.069.2371.1216.6799.890.000899.2261.2942.0
Gen-290.3992.1819.2899.990.000599.3573.4444.0
Trained on public datasets:
ModelScope40.0632.9317.6499.740.016298.9472.1242.0
Show-123.1944.2417.6599.770.006799.3281.5650.0
VideoCrafter279.9367.0417.3999.840.008599.4468.1736.0
Ours86.0988.3118.4999.900.001998.8967.5846.0
ModelFlow-CLIP-BLIP-SD-Detection-Color-Count-OCR-Celebrity
ScoreScoreBLUEScoreScoreScoreScoreScoreID Score
Trained on private datasets:
Pika 1.01.1420.4721.3167.4370.2642.0362.1994.8536.53
Gen-20.5820.2622.2567.6969.5447.3958.3663.7438.90
Trained on public datasets:
ModelScope6.9920.3622.5467.9350.0138.7244.1871.3244.56
Show-12.0720.6623.2468.4258.6348.5544.3158.9737.93
VideoCrafter23.9021.2122.7168.5869.3245.1150.4580.3738.40
Ours1.7920.7323.2968.2669.5547.7456.3168.5544.72
+ +![](images/figures/pyramidal-flow-matching-fig-0013.jpg) +Figure 10: Ablation study of corrective renoising during the inference stage. + +# C ADDITIONAL RESULTS + +# C.1 QUANTITATIVE RESULTS + +This section provides the full results on VBench (Huang et al., 2024) and EvalCrafter (Liu et al., 2024) as a supplement to the performance comparison in the experiments section of the main paper. The evaluation of our model is performed using 5-second 768p videos generated at 24 fps. + +VBench (Huang et al., 2024). The full experimental results on VBench are shown in Table 5. As can be observed, our model achieves leading or highly competitive results among open-source and commercial competitors, especially for the metrics related to motion quality. For example, the dynamic degree metric of our model ranks 2nd among all models at 64.63, validating the effectiveness of our generative model in learning temporal dynamics. For the rest of the metrics, our results are also generally superior to the open-source Open-Sora Plan v1.3 (PKU-Yuan Lab et al., 2024) and Open-Sora 1.2 (Zheng et al., 2024), with significantly lower training computational cost as mentioned earlier. We also note that half of our results even outperformed the recent CogVideoX-5B (Yang et al., 2024), which is based on a larger DiT model, demonstrating its modeling capacity. On the other hand, our model performs relatively inferior on metrics such as color and appearance style, which is more related to the image generation capabilities and finer-grained prompt following. This is largely due to our video captioning procedure based on video LLMs which tends to produce coarsegrained captions, thus dampening these abilities. Nevertheless, thanks to our autoregressive generation framework, which decomposes video generation into first frame generation and subsequent frame generation, these image quality issues can be addressed separately with additional well-captioned image data in future training stages. Similarly, due to the SD3-Medium weight initialization, which is infamous for its human structure, our method achieves a relatively low score in human action, which could be addressed by switching to other base models or training from scratch. + +EvalCrafter (Liu et al., 2024). The raw metrics on EvalCrafter are provided in Table 6. Overall, our model delivers highly competitive performance on the majority of metrics, outperforming many previous open-source and closed-source models. In particular, the motion AC score of our method which is relevant to the temporal motion quality ranks 2nd among all methods, justifying the capacity of our pyramid designs to learn complex spatiotemporal patterns in video. Our method also demonstrates superiority over several other metrics related to semantic alignment, including BLIP-BLUE and CLIP score. Placing top two in both metrics among the models compared, including the closed-source Gen-2 (Runway, 2023), confirms the advantages of our model in text-to-video semantic alignment. The only metric where our model performs poorly is face consistency, which is due to the temporal pyramid design adopted for compressing the history condition. We view this as an issue that can potentially be addressed by better temporal compression schemes. + +# C.2 ABALTION STUDY + +In this section, we conduct additional ablation studies of two important design details in our proposed pyramidal flow matching, including the corrective noise added during inference of the spatial pyramid and the blockwise causal attention used for autoregressive video generation. + +![](images/figures/pyramidal-flow-matching-fig-0014.jpg) +Figure 11: Ablation study of blockwise causal attention at $1 0 0 \mathrm { k }$ training step. + +![](images/figures/pyramidal-flow-matching-fig-0015.jpg) +Figure 12: (a) The visualization of generated images from our pyramid-flow. Our model can synthesize high-resolution and good-quality images even using only a few million training samples. (b) The FVD score comparison with full-sequence diffusion video training on MSR-VTT ( $\mathrm { { X u } }$ et al., 2016) benchmark along with optimization iterations. + +Role of corrective noise. To study its efficacy in the spatial pyramid, we curate a baseline method that inferences without adding this corrective Gaussian noise. The detailed comparative results of our method against this variant are shown in Fig. 10. While the baseline method has a correct global structure, it fails to produce a fine-grained, high-resolution image with rich details and instead produces a blurred image that suffers from block-like artifacts (better observed when zooming in). This is because applying the upsampling function at the jump points between different pyramid stages of varying resolutions results in excessive correlation between spatially adjacent latent values. In comparison, our generated images have rich details and vivid colors, confirming that the adopted corrective renoising scheme effectively addresses this artifact problem in the spatial pyramid. + +Effectiveness of causal attention. In Fig. 11, we study the effect of blockwise causal attention by comparing it to the bidirectional attention used in full-sequence diffusion. While an intuitive understanding might be that bidirectional attention promotes information exchange and increases model capacity, it is understudied for autoregressive video generation. In an early experiment, we trained a baseline model using bidirectional attention across different latent frames, the results of which are visualized in Fig. 11. As can be seen from the sampled keyframes of the 1-second videos, this model suffers from a lack of temporal coherence as the subject in the generated video is constantly changing in shape and color. Meanwhile, our model shows good temporal coherence with reasonable motion. We infer that this is because the history condition in bidirectional attention is influenced by the ongoing generation and thus deviates, whereas the history condition in causal attention is fixed, serving as a predetermined condition and stabilizing the autoregressive generative process. + +# C.3 VISUALIZATION + +This section presents additional qualitative results for our text-to-video generation in comparison to the recent leading models including Gen-3 Alpha (Runway, 2024), Kling (Kuaishou, 2024) and CogVideoX (Yang et al., 2024). The uniformly sampled frames from the generated videos are shown in Figs. 14 and 15, in which our videos are generated at 5s, 768p, 24fps. Overall, we observe that despite being trained only on publicly available data and using a small computational budget, our model yields a highly competitive visual aesthetics and motion quality among the baselines. + +![](images/figures/pyramidal-flow-matching-fig-0016.jpg) +Figure 13: Justification for our coupled sampling in Eqs. (9) and (10). + +Specifically, the results highlight the following characteristics of our model: (1) Through generative pre-training, our model is capable of generating videos of cinematic quality and reasonable content. For example, in Fig. 14a, our generative video shows a mushroom cloud resulting from “a massive explosion” taking place in “the surface of the earth”, creating a sci-fi movie atmosphere. However, the current model is not fully faithful to some prompts such as the “salt desert” in Fig. 14b, which could be addressed by curating more high-quality caption data. (2) Despite that our model has only 2B parameters initialized from SD3-Medium (Esser et al., 2024), it clearly outperforms CogVideoX-2B of the same model size with additional training data, and is even comparable to the 5B full version in some aspects. For example, in Figs. 15a and 15b, only our model and its 5B version are capable of generating reasonable sea waves according to the input prompt, while its 2B variant merely illustrates an almost static sea surface. This is largely attributed to our proposed pyramidal flow matching in improving training efficiency. Overall, these results validate the effectiveness of our approach in modeling complex spatiotemporal patterns through the spatial and temporal pyramid designs. Our generated videos are best-viewed at https://pyramid-flow.github.io. Since the autoregressive video generation model natively generates a high-quality image as the first frame, pyramid-flow can also be applied to text-to-image generation. Even with only a few million training images, it can show excellent visual quality, see Fig. 12a for the generated images. + +# C.4 TOY EXPERIMENT OF COUPLING NOISE + +To validate the effectiveness of coupled sampling in Eqs. (9) and (10), we illustrate two variants of piecewise flow matching in a toy experiment that considers mapping a few data points to uniform distribution. Two different coupling designs are considered within each time window, namely our coupling vs. random coupling. It can be seen that our coupled sampling strategy produces much more straight flow trajectories. + +The rationale for improving straightness by coupling noise is that: the straightness of the flow trajectory is usually compromised when there are intersections. Sampling the endpoints independently (as in vanilla flow matching) creates random directions for each trajectory and leads to intersections. Instead, by coupling the sampling of these endpoints as in Eq. (9) and Eq. (10), we can create more organized, possibly parallel trajectories with fewer intersections, thus improving straightness. As illustrated in Fig. 13, where coupling noise indeed leads to more straight flow trajectories. + +# D LIMITATIONS + +Our method only supports autoregressive generation and cannot be extended to keyframe interpolation or video interpolation. In addition, we noticed that the temporal pyramid designs to improve training efficiency can sometimes lead to subtle subject inconsistency, especially over the long term. While this is not a prevalent problem, we believe that developing better temporal compression methods is critical to the broader applicability of autoregressive video generative model. In addition, improving inference efficiency towards real-time is an intriguing problem (Yin et al., 2024). + +There are also several issues related to the training data. Since we did not include a prompt rewriting procedure in the data curation, the experimental results are focused on relatively short prompts. Also, due to the data filtering procedure, our model did not learn scene transitions during training. This may be overcome by introducing an additional model as the scene director (Lin et al., 2024). + +![](images/figures/pyramidal-flow-matching-fig-0017.jpg) +(b) A movie trailer featuring the adventures of the 30 year old space man wearing a red wool knitted motorcycle helmet, blue sky, salt desert, cinematic style, shot on $3 5 \mathrm { m m }$ film, vivid colors. + +![](images/figures/pyramidal-flow-matching-fig-0018.jpg) +(c) A side profile shot of a woman with fireworks exploding in the distance beyond her. +Figure 14: Visualization of generated videos in comparison with the state-of-the-art closed-source models, including Gen-3 Alpha (Runway, 2024) and Kling (Kuaishou, 2024). Our model delivers cinematic visual quality comparable to these models while adhering to the textual prompt. + +![](images/figures/pyramidal-flow-matching-fig-0019.jpg) +(a) An aerial shot of a lighthouse standing tall on a rocky cliff, its beacon cutting through the early dawn, waves crash against the rocks below. + +![](images/figures/pyramidal-flow-matching-fig-0020.jpg) +(b) Drone view of waves crashing against the rugged cliffs along Big Sur’s garay point beach. The crashing blue waters create white-tipped waves, while the golden light of the setting sun illuminates the rocky shore. + +![](images/figures/pyramidal-flow-matching-fig-0021.jpg) +(c) A series of underwater explosions, creating bubbles and splashing water. + +Figure 15: Visualization of generated videos in comparison with CogVideoX (Yang et al., 2024). +Our model outperforms CogVideoX-2B of the same model size and is comparable to the 5B version. \ No newline at end of file diff --git a/papers/pyramidal-flow-matching/paper.pdf b/papers/pyramidal-flow-matching/paper.pdf new file mode 100644 index 0000000000000000000000000000000000000000..cef4f228e875d18969dae1fd8045cc2016fa996b --- /dev/null +++ b/papers/pyramidal-flow-matching/paper.pdf @@ -0,0 +1,3 @@ +version https://git-lfs.github.com/spec/v1 +oid sha256:6893963ea0a9b1c6f669212a179098c91c1603c60f01b0b4379a9cb9ceaa5699 +size 9920487 diff --git a/papers/pyramidal-flow-matching/sau.json b/papers/pyramidal-flow-matching/sau.json new file mode 100644 index 0000000000000000000000000000000000000000..b122270a54f4538d3ce40836500e850006c787f5 --- /dev/null +++ b/papers/pyramidal-flow-matching/sau.json @@ -0,0 +1,357 @@ +{ + "paper_id": "pyramidal-flow-matching", + "paper_title": "Pyramidal Flow Matching for Efficient Video Generative Modeling", + "D1": [ + { + "id": "pyramidal-flow-matching-D1-001", + "claim": "Pyramidal flow matching uses K=3 pyramid stages for multi-scale generation", + "source": "Section 4.1; Appendix B" + }, + { + "id": "pyramidal-flow-matching-D1-002", + "claim": "pyramid stage resolution schedule: 3 (K=3) uniform time window partitions of [0,1]; each successive stage doubles spatial resolution (factor 2 per stage: 1/8, 1/4, 1/2, 1/1)", + "source": "Section 3.2; Section 4.1" + }, + { + "id": "pyramidal-flow-matching-D1-003", + "claim": "corrective noise covariance off-diagonal element (lower bound for semidefiniteness): γ = -1/3", + "source": "Section 3.2.2, Eq 14; Appendix A, Eq 25-26" + }, + { + "id": "pyramidal-flow-matching-D1-004", + "claim": "corrective noise weight at jump points: α = √3(1−s_k)/2", + "source": "Section 3.2.2, Eq 15; Appendix A, Eq 26" + }, + { + "id": "pyramidal-flow-matching-D1-005", + "claim": "endpoint timestep constraint from jump point matching (e_{k+1} > s_k, timestep rolls back): e_{k+1} = 2s_k/(1+s_k)", + "source": "Section 3.2.2, Eq 15; Appendix A, Eq 26" + }, + { + "id": "pyramidal-flow-matching-D1-006", + "claim": "upsampled endpoint rescaling factor at jump points: rescaling coefficient = (1+s_k)/2 when e_{k+1}=2s_k/(1+s_k) and γ=-1/3", + "source": "Section 3.2.2, Eq 15; Appendix A" + }, + { + "id": "pyramidal-flow-matching-D1-007", + "claim": "history condition corruptive noise range: corruptive noise strength uniformly sampled from [0, 1/3]", + "source": "Section 3.3; Section 4.1" + }, + { + "id": "pyramidal-flow-matching-D1-008", + "claim": "VAE latent compression factor: 3D VAE compression ratio: 8×8×8 (spatial × spatial × temporal)", + "source": "Section 4.1; Appendix B" + }, + { + "id": "pyramidal-flow-matching-D1-009", + "claim": "MM-DiT model size: 24 transformer layers, 2B total parameters", + "source": "Section 4.1; Appendix B" + }, + { + "id": "pyramidal-flow-matching-D1-010", + "claim": "Stage 1 training hyperparameters: lr=1e-4, batch_size=1536, steps=50k, warmup=1k, weight_decay=1e-4, grad_clip=1.0", + "source": "Appendix B, Table 4" + }, + { + "id": "pyramidal-flow-matching-D1-011", + "claim": "Stage 1 AdamW optimizer betas and epsilon: β1=0.9, β2=0.999, ε=1e-6", + "source": "Appendix B, Table 4" + }, + { + "id": "pyramidal-flow-matching-D1-012", + "claim": "Stage 1 hardware and precision: 128 NVIDIA A100 GPUs, bfloat16 precision", + "source": "Appendix B, Table 4" + }, + { + "id": "pyramidal-flow-matching-D1-013", + "claim": "Stage 2 training hyperparameters: lr=1e-4, batch_size=768, steps=200k (80k on 2s videos + 120k on 5s videos), warmup=1k, weight_decay=1e-4, grad_clip=1.0", + "source": "Appendix B, Table 4" + }, + { + "id": "pyramidal-flow-matching-D1-014", + "claim": "Stage 2 AdamW optimizer betas and epsilon: β1=0.9, β2=0.95, ε=1e-6", + "source": "Appendix B, Table 4" + }, + { + "id": "pyramidal-flow-matching-D1-015", + "claim": "Stage 2 hardware and precision: 128 NVIDIA A100 GPUs, bfloat16 precision", + "source": "Appendix B, Table 4" + }, + { + "id": "pyramidal-flow-matching-D1-016", + "claim": "Stage 2 image data mixing ratio: 12.5% image data from Stage 1 mixed into each batch", + "source": "Appendix B" + }, + { + "id": "pyramidal-flow-matching-D1-017", + "claim": "Stage 3 training hyperparameters: lr=5e-5, batch_size=384, steps=50k, warmup=1k, weight_decay=1e-4, grad_clip=1.0", + "source": "Appendix B, Table 4" + }, + { + "id": "pyramidal-flow-matching-D1-018", + "claim": "Stage 3 AdamW optimizer betas and epsilon: β1=0.9, β2=0.95, ε=1e-6", + "source": "Appendix B, Table 4" + }, + { + "id": "pyramidal-flow-matching-D1-019", + "claim": "Stage 3 hardware and precision: 128 NVIDIA A100 GPUs, bfloat16 precision", + "source": "Appendix B, Table 4" + }, + { + "id": "pyramidal-flow-matching-D1-020", + "claim": "total training compute: 20.7k A100 GPU hours total training time", + "source": "Section 4.2; Abstract" + }, + { + "id": "pyramidal-flow-matching-D1-021", + "claim": "generated video output specifications: 5s videos at 121 frames, 10s videos at 241 frames, both 768p resolution, 24 FPS", + "source": "Section 4.1; Section 4.2; Section 4.3" + }, + { + "id": "pyramidal-flow-matching-D1-022", + "claim": "token count reduction per sample: ≤15,360 tokens vs 119,040 tokens for full-sequence diffusion (10s, 241-frame video)", + "source": "Introduction; Section 4.2" + }, + { + "id": "pyramidal-flow-matching-D1-023", + "claim": "Stage 1 image data: LAION-5B count: 180M images from LAION-5B high-aesthetic subset", + "source": "Appendix B" + }, + { + "id": "pyramidal-flow-matching-D1-024", + "claim": "Stage 1 image data: CC-12M count: 11M images from CC-12M", + "source": "Appendix B" + }, + { + "id": "pyramidal-flow-matching-D1-025", + "claim": "Stage 1 image data: SA-1B count: 6.9M non-blurred images from SA-1B", + "source": "Appendix B" + }, + { + "id": "pyramidal-flow-matching-D1-026", + "claim": "Stage 1 image data: JourneyDB count: 4.4M images from JourneyDB", + "source": "Appendix B" + }, + { + "id": "pyramidal-flow-matching-D1-027", + "claim": "Stage 1 image data: synthetic data count: 14M publicly available synthetic images", + "source": "Section 4.1" + }, + { + "id": "pyramidal-flow-matching-D1-028", + "claim": "video training data counts: WebVid-10M, OpenVid-1M, plus ~1M non-watermark videos -> ~10M single-shot videos postprocessing", + "source": "Section 4.1; Appendix B" + }, + { + "id": "pyramidal-flow-matching-D1-029", + "claim": "inference speed: 56 seconds to generate a 5-second 384p video clip at inference", + "source": "Section 4.2" + }, + { + "id": "pyramidal-flow-matching-D1-030", + "claim": "VBench evaluation dimensions count: 16 fine-grained dimensions in VBench", + "source": "Section 4.1" + }, + { + "id": "pyramidal-flow-matching-D1-031", + "claim": "EvalCrafter evaluation metrics count: approximately 17 objective metrics in EvalCrafter", + "source": "Section 4.1" + }, + { + "id": "pyramidal-flow-matching-D1-032", + "claim": "user study configuration: 50 prompts sampled from VBench, 20+ participants, 3 preference dimensions (aesthetic quality, motion smoothness, semantic alignment)", + "source": "Section 4.3; Appendix B" + } + ], + "D2": [ + { + "id": "pyramidal-flow-matching-D2-001", + "claim": "Full spatial pyramidal flow interpolation: x_hat_t = t * x_1 XOR (1-t) * Down(x_0, 2^K), where XOR denotes interpolation between latents of different resolutions, Down(., 2^K) downsamples noise to 1/2^K resolution, K is the number of pyramid stages. This is decomposed into K piecewise time windows [s_k, e_k] each operating at successively halved resolutions.", + "source": "Section 3.2, Eq 5-6" + }, + { + "id": "pyramidal-flow-matching-D2-002", + "claim": "Piecewise flow within each pyramid stage k: For the k-th time window [s_k, e_k], let t' = (t-s_k)/(e_k-s_k) be the rescaled timestep. Within this window: x_hat_t = t' * Down(x_{e_k}, 2^k) + (1-t') * Up(Down(x_{s_k}, 2^{k+1})). Where Up is nearest/bilinear upsampling and Down is nearest/bilinear downsampling. Only stage k=0 operates at full resolution; stages k>0 use progressively lower resolutions.", + "source": "Section 3.2, Eq 6-7" + }, + { + "id": "pyramidal-flow-matching-D2-003", + "claim": "Coupled noise endpoint sampling for unified training: Sample shared noise n ~ N(0, I). Then jointly compute: Endpoint: x_hat_{e_k} = e_k * Down(x_1, 2^k) + (1-e_k) * n. Starting point: x_hat_{s_k} = s_k * Up(Down(x_1, 2^{k+1})) + (1-s_k) * n. This coupling creates organized parallel trajectories with fewer intersections, improving flow straightness compared to independent endpoint sampling.", + "source": "Section 3.2.1, Eq 9-10; Appendix C.4, Figure 13" + }, + { + "id": "pyramidal-flow-matching-D2-004", + "claim": "Unified pyramidal flow matching training objective: L = E_{k,t,(x_hat_{e_k},x_hat_{s_k})} ||v_t(x_hat_t) - (x_hat_{e_k} - x_hat_{s_k})||^2. The target vector field (x_hat_{e_k} - x_hat_{s_k}) jointly encodes both generation (noise->data) and decompression (low-res->high-res). All K pyramid stages are optimized together in a single DiT via a joint loss that sums across uniformly sampled stages.", + "source": "Section 3.2.1, Eq 11" + }, + { + "id": "pyramidal-flow-matching-D2-005", + "claim": "Corrective renoising at pyramid stage jump points (inference): At each jump point between stage k+1 and stage k: x_hat_{s_k} = ((1+s_k)/2) * Up(x_hat_{e_{k+1}}) + (sqrt(3)*(1-s_k)/2) * n', where n' ~ N(0, Sigma') with blockwise covariance Sigma'_block having diagonal=1 and off-diagonal=gamma=-1/3 (decorrelating 4x4 blocks from nearest-neighbor upsampling). The constraint e_{k+1} = 2s_k/(1+s_k) matches the distribution means. e_{k+1} > s_k means the timestep rolls back slightly at each jump.", + "source": "Section 3.2.2, Eq 12-16; Appendix A, Eq 18-26" + }, + { + "id": "pyramidal-flow-matching-D2-006", + "claim": "Temporal pyramid autoregressive conditioning (training mode): At pyramid stage k, each training latent x_hat_t^i is conditioned on compressed lower-resolution history: ... -> Down(x_{t'}^{i-2}, 2^{k+1}) -> Down(x_{t'}^{i-1}, 2^k) -> x_hat_t^i. The subscript t' indicates corruptive noise is added to history latents during training: noise strength uniformly sampled from [0, 1/3] is added to each history latent to mitigate autoregressive error accumulation.", + "source": "Section 3.3, Eq 17" + }, + { + "id": "pyramidal-flow-matching-D2-007", + "claim": "Temporal pyramid autoregressive conditioning (inference mode): During inference, clean generated frames are used as conditions: ... -> Down(x_1^{i-2}, 2^{k+1}) -> Down(x_1^{i-1}, 2^k) -> x_hat_t^i. No corruptive noise is added. The current frame's generation at the current pyramid stage uses previously generated clean frames at progressively compressed resolutions as conditions.", + "source": "Section 3.3, Eq 18" + }, + { + "id": "pyramidal-flow-matching-D2-008", + "claim": "Spatial pyramid theoretical efficiency bounds: With K pyramid stages at successively halved resolutions (factor 2 per stage) and uniform stage partitioning: (a) Initial noise is compressed by factor 2^K via Down(x_0, 2^K) — for K=3 this is 8x downsampling. (b) Computational cost reduces to approximately 1/K of full-resolution training — for K=3 this is ~1/3. Only the final stage (k=0) operates at full resolution; earlier stages use progressively lower resolutions with quadratic token savings.", + "source": "Section 3.2, Eq 5-7" + }, + { + "id": "pyramidal-flow-matching-D2-009", + "claim": "Temporal pyramid token and computation reduction formulas: For K-resolution temporal pyramid conditioning: (a) Token count reduction: 1/4^K (i.e., 1/64 for K=3). (b) Computation reduction: 1/16^K (i.e., 1/4096 for K=3). (c) Most frames computed at 1/2^K resolution (1/8 for K=3). (d) Training efficiency improvement: up to 16^K/T times (4096/T for K=3). Concrete instanced result: ≤15,360 tokens vs 119,040 tokens for full-sequence diffusion on a 10-second 241-frame video.", + "source": "Section 3.3" + }, + { + "id": "pyramidal-flow-matching-D2-010", + "claim": "Blockwise causal attention in DiT transformer layers: In each transformer layer, apply blockwise causal attention with mask M where M_{ij} = 0 if token j belongs to frame ≤ frame(i), else -∞ (preventing attention). Per-frame formulation: Attention_i^{(l)} = softmax(Q_i^{(l)} · [K_{≤f(i)}^{(l)}]^T / √d + M_i) · V_{≤f(i)}^{(l)}, where f(i) is the frame index of token i, and [K_{≤f}] concatenates keys from current and all preceding frames. Each token in a latent frame can attend to all tokens in that frame and all tokens in preceding frames, but CANNOT attend to tokens in subsequent frames. This keeps the history condition fixed and unaffected by ongoing generation, stabilizing autoregressive video generation. Full-sequence (non-factorized) attention is used across all tokens, since the pyramid representation already reduces token counts sufficiently.", + "source": "Section 3.4; Appendix C.2, Figure 11" + }, + { + "id": "pyramidal-flow-matching-D2-011", + "claim": "Position encoding scheme for multi-resolution pyramid: Two position encoding strategies: (1) Spatial dimensions: sinusoidal position encoding extrapolated to handle varying spatial resolutions across pyramid stages (better fine-grained detail at higher resolutions). (2) Temporal dimension: 1D Rotary Position Embedding (RoPE) with interpolation applied to spatially align history conditions across different temporal pyramid resolutions. Spatial extrapolation + temporal interpolation.", + "source": "Section 3.4, Figure 3b; Section 4.1" + }, + { + "id": "pyramidal-flow-matching-D2-012", + "claim": "Patch n' Pack length-balanced training batch construction: Given samples s_i with token counts n_i = H_i × W_i × T_i, pack into batch B = {s_1, ..., s_b} such that Σ_i n_i ≈ N_target (fixed token budget) and inter-sample attention masks prevent cross-sample crosstalk: M_{ij}^{inter} = -∞ if sample(i) ≠ sample(j). Packing procedure: (Step 1) Sort samples by token count, (Step 2) Greedy-bin-pack into batches reaching N_target, (Step 3) Concatenate latent sequences with sample-separated position encodings. This follows Patch n' Pack (Dehghani et al., 2023). Images keep original aspect ratios arranged into buckets. Joint training of images and videos is naturally supported since the first video frame acts as an image condition.", + "source": "Section 3.4" + }, + { + "id": "pyramidal-flow-matching-D2-013", + "claim": "Classifier-free guidance at inference: During inference sampling, classifier-free guidance is employed to enhance temporal consistency and motion smoothness of generated videos. The CFG formulation: v̂_θ(x_t, t, c) = v_θ(x_t, t, ∅) + ω · (v_θ(x_t, t, c) - v_θ(x_t, t, ∅)), where v_θ is the learned velocity field, c is the text conditioning (T5 + CLIP embeddings), ∅ is the null-condition embedding, and ω is the guidance weight. Combines conditional and unconditional velocity predictions with weight ω controlling the fidelity-diversity tradeoff.", + "source": "Section 3.4; Section 4.1" + }, + { + "id": "pyramidal-flow-matching-D2-014", + "claim": "3D VAE with causal convolution for video compression: 3D VAE compresses videos at 8x8x8 (spatial x spatial x temporal) using a MAGVIT-v2-like architecture. Key features: (a) 3D causal convolution ensures each encoded frame depends only on preceding frames. (b) Asymmetric encoder-decoder structure. (c) Kullback-Leibler (KL) regularization applied to latents. Trained from scratch on WebVid-10M and 6.9M SAM non-blurred images. Long videos are scattered across multiple GPUs for distributed tokenization.", + "source": "Section 4.1; Appendix B" + }, + { + "id": "pyramidal-flow-matching-D2-015", + "claim": "Text encoding pipeline: T5 + CLIP dual encoders: c = Concat(Embed_T5(prompt), Embed_CLIP(prompt)) ∈ ℝ^{L_T5 + L_CLIP × d_model}, where Embed_T5(·) produces L_T5 token embeddings from T5 encoder, Embed_CLIP(·) produces L_CLIP = 77 token embeddings (padded/truncated) from CLIP text encoder, following FLUX.1 (Black Forest Labs, 2024). The MM-DiT uses cross-attention to condition on the concatenated c: CrossAttn(x, c) = softmax(Q_x · K_c^T / √d) · V_c, where Q_x = x W_Q, K_c = c W_K, V_c = c W_V.", + "source": "Section 4.1; Appendix B" + }, + { + "id": "pyramidal-flow-matching-D2-016", + "claim": "Three-stage training procedure: Three-stage progressive training with increasing resolution and temporal length: Stage 1 (Image-only pre-training) on pure image datasets — establishes visual pixel dependencies. Stage 2 (Low-resolution video training) on WebVid-10M + OpenVid-1M + ~1M non-watermark videos recaptioned with Video-LLaMA2, trained progressively on 2s then 5s videos, with 12.5% image data from Stage 1 mixed per batch. Stage 3 (High-resolution video fine-tuning) on 5-10s high-resolution videos. All stages use AdamW optimizer with constant learning rate and 1k-step warmup. See D1 for exact hyperparameter values.", + "source": "Appendix B, Table 4" + }, + { + "id": "pyramidal-flow-matching-D2-017", + "claim": "Image-to-video generation via autoregressive frame prediction: Given a static input image (first frame) and a text prompt, autoregressively predict future frames using the trained model without fine-tuning. The causal attention design ensures each new frame is conditioned on all previous frames. The model outputs 120 new frames spanning 5 seconds at 768p 24fps (for image-to-video), ending with the final frame of the video clip.", + "source": "Section 4.3, Figure 6" + } + ], + "D3": [ + { + "id": "pyramidal-flow-matching-D3-001", + "claim": "Text-to-Video Generation Evaluation on VBench: Generate 5-second, 121-frame, 768p, 24fps videos from VBench text prompts. Compare against public-data models (Open-Sora Plan v1.3, Open-Sora 1.2, VideoCrafter2, T2V-Turbo) and proprietary models (CogVideoX-2B/5B, Pika 1.0, Gen-2, Kling, Gen-3 Alpha). Evaluate across all 16 VBench dimensions grouped into Total Score, Quality Score, and Semantic Score.", + "source": "Section 4.1; Section 4.3, Table 1, Table 5" + }, + { + "id": "pyramidal-flow-matching-D3-002", + "claim": "Text-to-Video Generation Evaluation on EvalCrafter: Generate 5-second, 768p, 24fps videos from EvalCrafter text prompts. Compare against public-data models (ModelScope, Show-1, VideoCrafter2) and proprietary models (LaVie, Pika 1.0, Gen-2). Evaluate across approximately 17 objective metrics covering visual quality, motion quality, and semantic alignment.", + "source": "Section 4.1; Section 4.3, Table 2, Table 6" + }, + { + "id": "pyramidal-flow-matching-D3-003", + "claim": "User Study on Human Preference: Sample 50 prompts from VBench, randomly sample one generated video per prompt from each of 6 baseline models (Open-Sora Plan V1.1, Open-Sora 1.2, Pika 1.0, CogVideoX-2B, CogVideoX-5B, Kling). Pair each baseline's video with the Pyramidal Flow Matching model's video. 20+ participants rank preference across three dimensions: aesthetic quality, motion smoothness, semantic alignment. Watermarks and model identifiers removed. Total of 1411 valid preference choices collected.", + "source": "Section 4.3, Figure 4; Appendix B, Figure 9" + }, + { + "id": "pyramidal-flow-matching-D3-004", + "claim": "Spatial Pyramid Ablation Study (Image Generation): Compare pyramidal flow matching against standard flow matching on text-to-image generation. Both variants use same training data, tokens per batch, hyperparameters, and model architecture. Train for 50k image steps. Evaluate FID on MS-COCO validation set with 3K randomly sampled prompts. Also compare qualitative visual quality and prompt-following capability.", + "source": "Section 4.4, Figure 7" + }, + { + "id": "pyramidal-flow-matching-D3-005", + "claim": "Temporal Pyramid Ablation Study (Video Generation): Compare pyramidal flow matching against full-sequence diffusion on video generation. Both variants use same experimental settings. Train for 100k low-resolution video steps. Compare visual quality, temporal consistency, motion coherence, and artifact levels qualitatively via keyframe visualization.", + "source": "Section 4.4, Figure 8" + }, + { + "id": "pyramidal-flow-matching-D3-006", + "claim": "Corrective Renoising Ablation Study: Train a spatial pyramid variant that omits the corrective Gaussian noise at jump points during inference (no renoising, only upsampling at resolution transitions). Compare against full pyramidal flow with corrective renoising (gamma=-1/3, alpha=sqrt(3)*(1-s_k)/2). Evaluate image quality qualitatively — examine block-like artifacts, fine-grained detail, color vividness.", + "source": "Appendix C.2, Figure 10" + }, + { + "id": "pyramidal-flow-matching-D3-007", + "claim": "Causal Attention Ablation Study: Train a model variant using bidirectional (non-causal) attention across all latent frames instead of blockwise causal attention. Compare against the default causal attention model. Train both for 100k steps. Evaluate 1-second video generation quality: visual quality, temporal coherence (subject shape/color constancy), motion quality.", + "source": "Appendix C.2, Figure 11" + }, + { + "id": "pyramidal-flow-matching-D3-008", + "claim": "Image-to-Video Generation Evaluation: Given static input images and text prompts, autoregressively generate future frames (120 frames, 5-second duration, 768p, 24fps) using the trained text-to-video model without any fine-tuning. Qualitative evaluation of motion prediction quality and temporal dynamics. Compare with no baselines (demonstration of emergent capability).", + "source": "Section 4.3, Figure 6" + }, + { + "id": "pyramidal-flow-matching-D3-009", + "claim": "Three-Stage Training Pipeline with Specific Data Configuration: (1) Image-only training on ~180M LAION-5B, 11M CC-12M, 6.9M SA-1B, 4.4M JourneyDB, 14M synthetic images for 50k steps at lr=1e-4, batch=1536, 128 A100 GPUs. (2) Low-resolution video training on WebVid-10M + OpenVid-1M + ~1M non-watermark (captioned via Video-LLaMA2) for 200k steps at lr=1e-4, batch=768, 128 A100 GPUs, with 12.5% image data mixing. (3) High-resolution video fine-tuning on 5-10s videos for 50k steps at lr=5e-5, batch=384, 128 A100 GPUs. All using AdamW, bfloat16, constant lr with 1k warmup, weight_decay=1e-4, grad_clip=1.0. Total: 20.7k A100 GPU hours.", + "source": "Section 4.1; Appendix B, Table 4" + } + ], + "D4": [ + { + "id": "pyramidal-flow-matching-D4-001", + "claim": "Spatial Pyramid Flow Method Execution Pipeline: 1. Flow matching ODE with linear interpolation path (dx_t/dt = v_t(x_t), x_t = t*x_1 + (1-t)*x_0); 2. Full spatial pyramidal flow interpolation (decompose full [0,1] trajectory into K piecewise time windows at halved resolutions); 3. Piecewise flow within each pyramid stage k (rescaled timestep t', Down/Up operations between resolutions); 4. Coupled noise endpoint sampling for unified training (shared noise n couples all stage endpoints, producing parallel trajectories); 5. Unified pyramidal flow matching training objective (single loss jointly encodes generation + decompression across all K stages); 6. Corrective renoising at pyramid stage jump points during inference (blockwise-correlated noise at resolution transitions prevents block artifacts)", + "source": "Section 3.1-3.2.2" + }, + { + "id": "pyramidal-flow-matching-D4-002", + "claim": "Temporal Pyramid Autoregressive Conditioning Pipeline (Generation Phase 2, built on Spatial Pyramid pipeline): Entry — the spatial pyramid flow matching pipeline handles per-frame generation and decompression across K=3 resolution levels; the MM-DiT receives text embeddings from T5+CLIP dual encoders. For each autoregressively generated frame i at pyramid stage k, this phase constructs compressed temporal history conditions from prior frames: frame i-2 and earlier downsampled to resolution 2^(k+1), frame i-1 downsampled to 2^k. Training (Section 3.3, Eq.17): corruptive noise uniformly sampled from [0, 1/3] is added to each history latent to mitigate autoregressive error accumulation. Inference (Eq.18): clean generated frames serve as compressed history, no noise added. Exit — the temporal conditions feed into the MM-DiT's blockwise causal attention layers (Section 3.4) alongside the current noisy latent and text embeddings, driving the autoregressive video generation loop. Active across all three training stages (the three-stage training pipeline) and inference; reduces token count by up to 1/4^K and computation by up to 1/16^K vs full-sequence diffusion.", + "source": "Section 3.3, Eq 17-18" + }, + { + "id": "pyramidal-flow-matching-D4-003", + "claim": "Three-Stage Training Pipeline: Stage 1: Image-only pre-training — 50k steps, lr=1e-4, batch=1536, AdamW(0.9,0.999), 128 A100 GPUs, ~12h; Stage 2: Low-resolution video training — 80k steps on 2s videos, then 120k steps on 5s videos, lr=1e-4, batch=768, AdamW(0.9,0.95), 128 A100 GPUs, 12.5% image mixing, ~90h; Stage 3: High-resolution video fine-tuning — 50k steps on 5-10s videos, lr=5e-5, batch=384, AdamW(0.9,0.95), 128 A100 GPUs, ~60h", + "source": "Section 4.1; Appendix B, Table 4" + }, + { + "id": "pyramidal-flow-matching-D4-004", + "claim": "VBench Text-to-Video Evaluation Protocol: Complete three-stage training pipeline; Generate 5-second, 121-frame, 768p, 24fps videos from VBench text prompts; Evaluate across all 16 VBench dimensions against 10 baselines (4 public-data + 6 proprietary)", + "source": "Section 4.1; Section 4.3, Table 1, Table 5" + }, + { + "id": "pyramidal-flow-matching-D4-005", + "claim": "EvalCrafter Text-to-Video Evaluation Protocol: Complete three-stage training pipeline; Generate 5-second, 768p, 24fps videos from EvalCrafter text prompts; Evaluate across ~17 objective metrics against 6 baselines", + "source": "Section 4.1; Section 4.3, Table 2, Table 6" + }, + { + "id": "pyramidal-flow-matching-D4-006", + "claim": "User Study Protocol (Evaluation Phase 3, after VBench and EvalCrafter automated benchmarks): Entry — the three-stage training pipeline has completed, producing a fully trained 2B-parameter MM-DiT model that generates 5s, 768p, 24fps videos; automated VBench and EvalCrafter evaluations have been executed. Protocol: Sample 50 prompts from VBench; generate one video from the Pyramidal Flow Matching model and one from each of 6 baseline models (Open-Sora Plan V1.1, Open-Sora 1.2, Pika 1.0, CogVideoX-2B, CogVideoX-5B, Kling); pair the Pyramidal Flow Matching model's video with each baseline's, removing watermarks and model identifiers for blind pairwise comparison; 20+ human participants rank preference across 3 dimensions (aesthetic quality, motion smoothness, semantic alignment). Exit — 1411 valid preference choices collected, providing human-preference validation that complements automated metrics. Result: Pyramidal Flow Matching model preferred on motion smoothness due to 24fps output vs baselines' typical 8fps limitation.", + "source": "Section 4.3, Figure 4; Appendix B, Figure 9" + }, + { + "id": "pyramidal-flow-matching-D4-007", + "claim": "Spatial Pyramid Ablation Protocol: Train pyramidal flow matching variant and standard flow matching baseline for 50k image steps (same data, tokens, hyperparameters, architecture); Evaluate FID on MS-COCO validation set (3K randomly sampled prompts) at regular intervals; Compare convergence curves, visual quality, and prompt-following capability", + "source": "Section 4.4, Figure 7" + }, + { + "id": "pyramidal-flow-matching-D4-008", + "claim": "Temporal Pyramid Ablation Protocol: Train pyramidal flow variant and full-sequence diffusion baseline for 100k low-resolution video steps (same settings); Qualitatively compare generated video keyframes for visual quality, temporal consistency, motion coherence, and artifacts", + "source": "Section 4.4, Figure 8" + }, + { + "id": "pyramidal-flow-matching-D4-009", + "claim": "Corrective Renoising Ablation Protocol: Train both variants with same spatial pyramid configuration; Run inference: one with corrective renoising (gamma=-1/3, alpha=sqrt(3)*(1-s_k)/2), one with upsampling only; Compare generated image quality — block artifacts, detail richness, color vividness, global structure", + "source": "Appendix C.2, Figure 10" + }, + { + "id": "pyramidal-flow-matching-D4-010", + "claim": "Causal Attention Ablation Protocol: Train bidirectional attention variant and blockwise causal attention variant each for 100k steps; Generate 1-second videos from both variants; Compare keyframe consistency and temporal coherence (subject shape/color constancy)", + "source": "Appendix C.2, Figure 11" + }, + { + "id": "pyramidal-flow-matching-D4-011", + "claim": "Image-to-Video Generation Protocol (Emergent): Train model on text-to-video only through all 3 stages (no image-to-video fine-tuning); At inference, provide a static image as the first frame together with a text prompt; Autoregressively generate 120 future frames (5 seconds, 768p, 24fps) via causal attention conditioning", + "source": "Section 4.3, Figure 6" + } + ] +} \ No newline at end of file diff --git a/papers/robotic-world-model/blacklist.txt b/papers/robotic-world-model/blacklist.txt new file mode 100644 index 0000000000000000000000000000000000000000..8289d3b778cd5ae654a8787232a37178ae69f88b --- /dev/null +++ b/papers/robotic-world-model/blacklist.txt @@ -0,0 +1,2 @@ +# Official repository (ETH Zurich, Apache 2.0) +https://github.com/leggedrobotics/robotic_world_model diff --git a/papers/robotic-world-model/config.yaml b/papers/robotic-world-model/config.yaml new file mode 100644 index 0000000000000000000000000000000000000000..82978b8209194ed0047c1babb3cd540d02ae6f73 --- /dev/null +++ b/papers/robotic-world-model/config.yaml @@ -0,0 +1,8 @@ +title: "Robotic World Model: A Neural Network Simulator for Robust Policy Optimization" +pdf_url: "https://arxiv.org/pdf/2501.10100.pdf" +venue: "NeurIPS 2025" +year: "2025" +extra: + selection_index: 16 + domain: "Reinforcement Learning" + paradigm: "New Algorithm / Architecture" diff --git a/papers/robotic-world-model/images/figures/robotic-world-model-fig-0001.jpg b/papers/robotic-world-model/images/figures/robotic-world-model-fig-0001.jpg new file mode 100644 index 0000000000000000000000000000000000000000..1cef10af0f9383e03fde0031651c1e28deb2ccf4 --- /dev/null +++ b/papers/robotic-world-model/images/figures/robotic-world-model-fig-0001.jpg @@ -0,0 +1,3 @@ +version https://git-lfs.github.com/spec/v1 +oid sha256:50cf0870b7da8059d19d7c03f3339a32b309082d416ef06dea3b4b9e41b54e00 +size 221939 diff --git a/papers/robotic-world-model/images/figures/robotic-world-model-fig-0002.jpg b/papers/robotic-world-model/images/figures/robotic-world-model-fig-0002.jpg new file mode 100644 index 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0000000000000000000000000000000000000000..ab68f1077c3ac1d64de845e26ff63fa8d47fa4a9 --- /dev/null +++ b/papers/robotic-world-model/paper.md @@ -0,0 +1,427 @@ +# Robotic World Model: A Neural Network Simulator for Robust Policy Optimization in Robotics + +Chenhao Li ETH Zurich, Switzerland chenhli@ethz.ch + +Andreas Krause ETH Zurich, Switzerland krausea@ethz.ch + +Marco Hutter ETH Zurich, Switzerland mahutter@ethz.ch + +https://sites.google.com/view/roboticworldmodel + +# Abstract + +Learning robust and generalizable world models is crucial for enabling efficient and scalable robotic control in real-world environments. In this work, we introduce a novel framework for learning world models that accurately capture complex, partially observable, and stochastic dynamics. The proposed method employs a dual-autoregressive mechanism and self-supervised training to achieve reliable long-horizon predictions without relying on domain-specific inductive biases, ensuring adaptability across diverse robotic tasks. We further propose a policy optimization framework that leverages world models for efficient training in imagined environments and seamless deployment in real-world systems. This work advances model-based reinforcement learning by addressing the challenges of long-horizon prediction, error accumulation, and sim-to-real transfer. By providing a scalable and robust framework, the introduced methods pave the way for adaptive and efficient robotic systems in real-world applications. + +# 1 Introduction + +Robotic systems have achieved remarkable advancements in recent years, driven by progress in reinforcement learning (RL) [1, 2] and control theory [3, 4]. A prevalent limitation in many approaches is the lack of adaptation and learning once the policy is deployed on the real system [5, 6, 7, 8]. This results in underutilization of the valuable data generated during real-world interactions. Robotic systems operating in dynamic and uncertain environments require the ability to continually adapt their behavior to new conditions [9]. The inability to exploit real-world experience for further learning restricts the system’s robustness and limits its ability to handle evolving scenarios effectively. Truly intelligent robotic systems should operate efficiently and reliably using limited data, adapting to realworld conditions in a scalable manner [10, 11]. While model-free RL algorithms such as Proximal Policy Optimization (PPO) [2] and Soft Actor-Critic (SAC) [1] have demonstrated impressive results in simulation, their high interaction requirements make them impractical for real-world robotics. Sample-efficient methods are therefore essential for leveraging the information in real-world data without extensive environment interactions [12, 13]. + +A promising solution is the use of predictive models of the environment, commonly referred to as world models [14, 15]. World models simulate environment dynamics to enable planning and policy optimization, often referred to as learning in imagination [16]. These models have shown success across diverse robotic domains, including manipulation [17, 18], navigation [11], and locomotion [10]. However, developing reliable and generalizable world models poses unique challenges due to the complexity of real-world dynamics, including nonlinearities, stochasticity, and partial observability [19, 20]. Existing approaches often incorporate domain-specific inductive biases, such as structured state representations or hand-designed network architectures [21, 22, 23], to improve model fidelity. While effective, these methods are limited in their scalability and adaptability to novel environments or tasks. In contrast, a general framework for learning world models without domain-specific assumptions has the potential to enhance generalization and applicability across a wide range of robotic systems and scenarios. + +![](images/figures/robotic-world-model-fig-0001.jpg) +Figure 1: Autoregressive imagination, ground-truth simulation, and real-world deployment of RWM. For each environment, the top row showcases the RWM autoregressively predicting future trajectories in imagination. The second row visualizes the ground truth evolution in simulation. Specifically for the ANYmal D quadruped and Unitree G1 humanoid, the framework achieves robust policy optimization through MBPO-PPO, enabling zero-shot deployment on hardware. + +In this work, we present a novel approach for learning world models that emphasizes robustness and accuracy over long-horizon predictions. Our method is designed to operate without handcrafted representations or specialized architectural biases, enabling broad applicability to diverse robotic tasks. To evaluate the utility of these learned models, we further propose a policy optimization method using PPO and demonstrate successful deployment in both simulated and real-world environments. To the best of our knowledge, this is the first framework to reliably train policies on a learned neural network simulator without any domain-specific knowledge and deploy them on physical hardware with minimal performance loss. + +Our contributions are summarized as follows: (i) We introduce a novel network architecture and training framework that enables the learning of reliable world models capable of long autoregressive rollouts, a critical property for downstream planning and control. (ii) We provide a comprehensive evaluation suite spanning diverse robotic tasks to benchmark our method. Comparative experiments with existing world model frameworks demonstrate the effectiveness of our approach. (iii) We propose an efficient policy optimization framework that leverages the learned world models for continuous control and generalizes effectively to real-world scenarios with hardware experiments, including both quadruped and humanoid systems. + +By addressing the challenges associated with learning world models, this work contributes toward bridging the gap between data-driven modeling and real-world deployment. The proposed framework enhances the scalability, adaptability, and robustness of robotic systems, paving the way for broader adoption of model-based reinforcement learning in real-world applications. Supplementary videos for this work are available on https://sites.google.com/view/roboticworldmodel. + +# 2 Related work + +# 2.1 World Models for Robotics + +World models have emerged as a cornerstone in robotics for capturing system dynamics and enabling efficient planning and control through simulated trajectories. A prominent application of world models is in robotic control, where dynamics models are used to describe real-world dynamics for policy optimization [24]. Extensions to vision-based tasks have been realized through visual foresight techniques [18, 25, 17], which learn visual dynamics for planning in high-dimensional sensory spaces. Similar ideas are applied to train RL agents in such world models aiming to fully replicate real environment interactions [14, 26]. These approaches underline the versatility of world models in tasks requiring rich perceptual inputs. + +To improve the generalization of black-box neural network-based world models beyond the training distribution, many works incorporate known physics principles or state structures into model design, addressing potential limitations in control performance. Examples include foot-placement dynamics [21], object invariance [22], granular media interactions [27], frequency domain parameterization [23], rigid body dynamics [20], and semi-structured Lagrangian dynamics models [28]. While these methods demonstrate impressive results, they often require strong domain knowledge and carefully crafted inductive biases, which can restrict their scalability and adaptability to diverse robotic applications. Latent-space dynamics models offer an alternative by abstracting the state space into compact representations, enabling efficient long-horizon planning. Deep Planning Network (PlaNet) [15] and its successor Dreamer [29, 11, 30] exemplify this trend, achieving state-of-the-art performance in continuous control and visual navigation tasks. These frameworks have been extended to real-world robotics [19, 31], demonstrating their potential in both simulation and hardware deployment. + +# 2.2 Model-Based Reinforcement Learning + +Model-Based Reinforcement Learning (MBRL) has emerged as a powerful approach to address the limitations of model-free reinforcement learning, particularly in scenarios where sample efficiency and safety are critical. Unlike model-free methods, which learn policies directly from interactions with the environment, MBRL leverages a learned model of the environment to simulate interactions, enabling more efficient and safer policy learning. One of the pioneering methods in MBRL is Probabilistic Ensembles with Trajectory Sampling (PETS), which uses an ensemble of probabilistic neural networks to model the environment dynamics [12]. Building on the idea of latent-space modeling, PlaNet leverages a latent dynamics model to plan directly in a learned latent space [15]. Dreamer extends the concept by incorporating an actor-critic framework into the latent dynamics model, enabling the simultaneous learning of both the dynamics model and the policy [29, 11, 30]. Variations on the architectural design also see success in improving generation capabilities of such latent dynamics models with autoregressive transformer [32] and the stochastic nature of variational autoencoders [33]. Recent advancements in this area include TD-MPC and TD-MPC2, which integrate model-based learning with MPC to achieve high-performance control in dynamic environments [34, 35, 36]. + +Recognizing the strengths of both model-based and model-free methods, several hybrid approaches have been developed to combine the sample efficiency of MBRL with the robustness of modelfree reinforcement learning. One notable example is Model-Based Policy Optimization (MBPO), which uses a model-based approach for planning and policy optimization but refines the policy using model-free updates [13]. It emphasizes selectively relying on the learned model when its predictions are accurate, thus mitigating the negative effects of model inaccuracies. Building on similar principles, Model-based Offline Policy Optimization (MOPO) extends the framework to the offline setting, where learning is conducted entirely from previously collected data without further environment interaction [37]. In contrast to using zeroth-order model-free reinforcement learning for policy optimization, first-order gradient-based optimization is used to improve policy learning [38, 39]. This allows for more efficient and precise policy updates, particularly in complex, high-dimensional environments, where accurate gradient information is crucial for performance. Our framework extends MBPO by integrating it with PPO over extensive autoregressive rollouts, making it particularly effective for complex robotic control tasks. + +# 3 Approach + +# 3.1 Reinforcement Learning and World Models + +We formulate the problem by modeling the environment as a Partially Observable Markov Decision Process (POMDP) [40], defined by the tuple $( S , \mathcal { A } , \mathcal { O } , T , R , O , \gamma )$ , where $s , A$ , and $\mathcal { O }$ denote the state, action, and observation spaces, respectively. The transition kernel $T : S \times \mathcal { A } S$ captures the environment dynamics $p \left( \boldsymbol { s } _ { t + 1 } \mid \boldsymbol { s } _ { t } , \boldsymbol { a } _ { t } \right)$ , while the reward function $R : S \times \mathcal { A } \times \mathcal { S } \mathbb { R }$ maps transitions to scalar rewards. Observations $o _ { t } \in \mathcal { O }$ are emitted according to probabilities $p \left( o _ { t } \mid s _ { t } \right)$ , governed by the observation kernel $O : S \mathcal { O }$ . The agent seeks to learn a policy $\pi _ { \theta } : \mathcal { O } \mathcal { A }$ that maximizes the expected discounted return $\mathbb { E } _ { \pi _ { \theta } } \left[ \sum _ { t \geq 0 } \gamma ^ { t } r _ { t } \right]$ , where $r _ { t }$ is the reward at time $t$ and $\gamma \in [ 0 , 1 ]$ is the discount factor. + +World models [14] approximate the environment dynamics and facilitate policy optimization by enabling simulated environment interactions in imagination [16]. Training typically involves three iterative steps: (1) collect data from real environment interactions; (2) train the world model using the collected data; and (3) optimize the policy within the simulated environment produced by the world model. + +Despite the success of existing frameworks in achieving tasks in simplified settings, their application to complex low-level robotic control remains a significant challenge. To address this gap, we propose Robotic World Model (RWM), a novel framework for learning robust world models in partially observable and dynamically complex environments. RWM builds on the core concept of world models but introduces architectural and training innovations that enable reliable long-horizon predictions, even in stochastic and partially observable settings. By incorporating historical context and autoregressive training, RWM addresses challenges such as error accumulation and partially observable and discontinuous dynamics, which are critical in real-world robotics applications. + +# 3.2 Self-supervised Autoregressive Training + +To address the inherent complexity of partially observable environments, we propose a self-supervised autoregressive training framework as the backbone of RWM. This framework trains the world model $p _ { \phi }$ to predict future observations by leveraging both historical observation-action sequences and its own predictions, ensuring robustness over extended rollouts. + +The input to the world model consists of a sequence of observation-action pairs spanning $M$ historical steps. At each time step $t$ , the model predicts the distribution of the next observation $p \left( o _ { t + 1 } \mid o _ { t - M + 1 : t } , a _ { t - M + 1 : t } \right)$ . Predictions are generated autoregressively: at each step, the predicted observation $o _ { t + 1 } ^ { \prime }$ is appended to the history and combined with the next action $a _ { t + 1 }$ to serve as input for subsequent predictions. This process is repeated over a prediction horizon of $N$ steps, producing a sequence of future predictions. The predicted observation $k$ steps ahead can thus be written as + +$$ +\begin{array} { r } { o _ { t + k } ^ { \prime } \sim p _ { \phi } \left( \cdot { \mathrm { ~ | ~ } } o _ { t - M + k : t } , o _ { t + 1 : t + k - 1 } ^ { \prime } , a _ { t - M + k : t + k - 1 } \right) . } \end{array} +$$ + +A similar process is also applied to predict privileged information $c$ , such as contacts, providing an additional learning objective that implicitly embeds critical information for accurate long-term predictions. Such a training scheme introduces the model to the distribution it will encounter at test time, reducing the mismatch between training and inference distributions. Overall, the model is optimized by minimizing the multi-step prediction error: + +$$ +\mathcal { L } = \frac { 1 } { N } \sum _ { k = 1 } ^ { N } \alpha ^ { k } \left[ L _ { o } \left( o _ { t + k } ^ { \prime } , o _ { t + k } \right) + L _ { c } \left( c _ { t + k } ^ { \prime } , c _ { t + k } \right) \right] , +$$ + +where $L _ { o }$ and $L _ { c }$ quantify the discrepancy between predicted and true observations and privileged information, and $\alpha$ denotes a decay factor. This autoregressive training objective encourages the hidden states to encode representations that support accurate and reliable long-horizon predictions. + +![](images/figures/robotic-world-model-fig-0002.jpg) +Figure 2: Comparison of training paradigms for world models with an example of a history horizon $H = 3$ . (a) Autoregressive training operates with an example of a forecast horizon $N = 2$ , leveraging historical data and its own predictions for long-horizon robustness. The dashed arrows denote the sequential autoregressive prediction steps. (b) Teacher-forcing training can be viewed as a special case of autoregressive training with a forecast horizon $N = 1$ , using ground truth observations for next-step predictions to optimize parallelization but limiting robustness to error accumulation. + +Training data is constructed by sliding a window of size $M + N$ over collected trajectories, providing sufficient historical context for prediction targets. To improve gradient propagation through autoregressive predictions, we apply reparameterization tricks to enable effective end-to-end optimization. By incorporating historical observations, RWM captures unobservable dynamics, addressing the challenges of partially observable and potentially discontinuous environments. The autoregressive training mitigates error accumulation, a common issue in long-horizon predictions, and eliminates the need for handcrafted representations or domain-specific inductive biases, enhancing generalization across diverse tasks. This process is illustrated in Fig. 2a, in contrast to the teacher-forcing pipeline in Fig. 2b, which is commonly adopted to train many popular architectures [29, 41]. Specifically, teacher-forcing can be viewed as a special case of autoregressive training with forecast horizon $N = 1$ , which boosts training with higher parallelization. + +While the proposed autoregressive training framework can be applied to any network architecture, RWM utilizes a GRU-based architecture for its ability to maintain long-term historical context while operating on low-dimensional inputs. The network predicts the mean and standard deviation of a Gaussian distribution describing the next observation. Our framework introduces a dualautoregressive mechanism: (i) Inner autoregression updates GRU hidden states autoregressively after each historical step within the context horizon $M$ . (ii) Outer autoregression feeds predicted observations from the forecast horizon $N$ back into the network. This architecture, visualized in Fig. S6, ensures robustness to long-term dependencies and transitions, making RWM suitable for complex robotics applications. + +# 3.3 Policy Optimization on Learned World Models + +Policy optimization in RWM is conducted using the learned world model, following a framework inspired by Model-Based Policy Optimization (MBPO) [13] and the Dyna algorithm [42]. During imagination, the actions are generated recursively by the policy $\pi _ { \theta }$ conditioned on the observations predicted by the world model $p _ { \phi }$ , which is further conditioned on the previous predictions. The actions at time $t + k$ can thus be written as + +$$ +\begin{array} { r } { a _ { t + k } ^ { \prime } \sim \pi _ { \theta } \left( \cdot \mid o _ { t + k } ^ { \prime } \right) , } \end{array} +$$ + +where $o _ { t + k } ^ { \prime }$ is drawn autoregressively according to Eq. 1. Rewards are computed from imagined observations and privileged information. The approach combines model-based imagination with model-free RL to achieve efficient and robust policy optimization, as outlined in Algorithm 1. + +The replay buffer $\mathcal { D }$ aggregates real environment interactions collected by a single agent. The world model $p _ { \phi }$ is trained on this data following the autoregressive scheme described in Sec. 3.2. Imagination agents are initialized from samples in $\mathcal { D }$ and simulate trajectories using the world model for $T$ steps, enabling policy updates through a reinforcement learning algorithm. The training diagram is visualized in Fig. S7. + +# Algorithm 1 Policy optimization with RWM + +
1: Initialize policy πθ, world model pφ, and replay buffer D
2: for learning iterations = 1, 2, . . . do
3: Collect observation-action pairs in D by interacting with the environment using πθ
4: Update pφ with autoregressive training using data sampled from D according to Eq. 2
5: Initialize imagination agents with observations sampled from D
6: Roll out imagination trajectories using π0 and pφ for T steps according to Eq. 3
7: Update πθ using PPO or another reinforcement learning algorithm end for
+ +While PPO is known for its strong performance in robotic tasks, training it on learned world models poses unique challenges. Model inaccuracies can be exploited during policy learning, leading to discrepancies between the imagined and true dynamics. This issue is exacerbated by the extended autoregressive rollouts required for PPO, which compound prediction errors. We denote this policy optimization method by MBPO-PPO. Despite these challenges, RWM demonstrates its robustness by successfully optimizing policies over a hundred autoregressive steps with MBPO-PPO, far exceeding the capabilities of existing frameworks such as MBPO [13], Dreamer [29, 11, 30], or TD-MPC [34, 36]. This result underscores the accuracy and stability of the proposed training method and its ability to synthesize policies deployable on hardware. + +# 4 Experiments + +We validate RWM through a comprehensive set of experiments across diverse robotic systems, environments, and network architectures. The experiments are designed to assess the accuracy and robustness of RWM, evaluate its architectural and training design choices, and demonstrate its effectiveness across diverse robotic tasks in Isaac Lab [43] and in real-world deployment combined with MBPO-PPO. We start the analysis by looking into the autoregressive prediction accuracy and robustness of the world model learned with simulation data induced by a velocity tracking policy. The observation and action spaces of the world model are detailed in Table S2 and Table S4. We then compare various network architectures and the error induced across diverse robotic environments and tasks to demonstrate the generality of RWM. And finally, we learn a policy in RWM with the proposed MBPO-PPO and demonstrate the applicability and robustness of the method on ANYmal D [44] and Unitree G1 hardware. + +# 4.1 Autoregressive Trajectory Prediction + +The capability of a world model to maintain high fidelity during autoregressive rollouts is critical for effective planning and policy optimization. To evaluate this aspect, we analyze the autoregressive prediction performance of RWM using trajectories collected from ANYmal D hardware. The control frequency of the robot is at $5 0 H z$ . The model is trained with history horizon $M = 3 2$ and forecast horizon $N = 8$ . Further details on the network architecture and training parameters are summarized in Sec. A.2.1 and Sec. A.3.1, respectively. The autoregressive trajectory predictions by RWM are visualized in Fig. 3a. + +The results demonstrate that RWM exhibits a remarkable alignment between predicted and ground truth trajectories across all observed variables. This consistency persists over extended rollouts, showcasing the model’s ability to mitigate compounding errors—a critical challenge in long-horizon predictions. This performance is attributed to the dual-autoregressive mechanism introduced in Sec. 3.2, which stabilizes predictions despite the short forecast horizon employed during training. A comparison of state evolution between the RWM prediction and the ground truth simulation is illustrated in Fig. 1 (bottom). The visualization highlights the ability of RWM to maintain consistency in trajectory predictions over long horizons, even beyond the training forecast horizon. This robustness is pivotal for stable policy learning and deployment, as discussed further in Sec. 4.4. + +It is notable that the choice of history horizon $M$ and forecast horizon $N$ plays a critical role in the training and performance of RWM. Our ablation study in Sec. A.4.1 reveals that, while extending both $M$ and $N$ improves accuracy, practical considerations of computational cost necessitate careful tuning of these hyperparameters to achieve optimal performance. + +![](images/figures/robotic-world-model-fig-0003.jpg) +Figure 3: (Left) Solid lines represent ground truth trajectories, while dashed lines denote predicted state evolution. Predictions commence at $t = 3 2$ using historical observations, with future observations predicted autoregressively by feeding prior predictions back into the model. (Right) Yellow curves denote RWM at varying noise levels, demonstrating consistent robustness and lower error accumulation across forecast steps. Grey curves represent the MLP baseline, which exhibits significantly higher error accumulation and reduced robustness to noise. + +# 4.2 Robustness under Noise + +A critical challenge in training world models is their ability to generalize under noisy conditions, particularly when predictions rely on autoregressive rollouts. Even small deviations from the training distribution can cascade into untrained regions, causing the model to hallucinate future trajectories. To assess the robustness of RWM, we analyze its performance under Gaussian noise perturbations applied to both observations and actions. We compare the results with an MLP-based baseline also trained autoregressively with the same history and forecast horizon, as shown in Fig. 3b, where yellow curves denote the relative prediction error $e$ for RWM, and grey curves represent the MLP baseline. + +The results indicate a clear advantage of RWM over the MLP baseline across all noise levels. As forecast steps increase, the relative prediction error of the MLP model grows significantly, diverging more rapidly than RWM. In contrast, RWM demonstrates superior stability, maintaining lower prediction errors even under high noise levels. This robustness can be attributed to the dualautoregressive mechanism introduced in Sec. 3.2, which ensures stability in long-horizon predictions. This design minimizes the accumulation of errors by continually refining the state representation toward long-term predictions, even in the presence of noisy inputs. + +# 4.3 Generality across Robotic Environments + +To assess the generality and robustness of RWM across a diverse range of robotic environments, we compare its performance with several baseline methods, including MLP, recurrent state-space model (RSSM) [15, 29, 11, 30], and transformer-based architectures [41, 45]. These baselines represent widely adopted approaches in dynamics modeling and policy optimization. All models are given the same context during training and evaluation. Their training parameters are detailed in Sec. A.2.2. The relative autoregressive prediction errors $e$ for these models are shown in Fig. 4. The tasks span manipulation scenarios as well as quadruped and humanoid locomotion tasks, allowing for a comprehensive evaluation of the models. In addition, we highlight the importance of the autoregressive training introduced in Sec. 3.2 by including both RWM trained with teacher-forcing (RWM-TF) and autoregressive training (RWM-AR), demonstrating the significant performance gains achieved by the latter. + +The results highlight the superiority of RWM trained with autoregressive training (RWM-AR), which consistently achieves the lowest prediction errors across all environments. The performance gap between RWM-AR and the baselines is especially pronounced in complex and dynamic tasks, such as velocity tracking for legged robots, where accurate long-horizon predictions are critical for effective control. The comparison also reveals that RWM-AR significantly outperforms its teacherforcing counterpart (RWM-TF), underscoring the importance of autoregressive training in mitigating compounding prediction errors over long rollouts. We additionally visualize the imagination rolled out by RWM-AR compared with the ground truth simulation in Fig. 1 and Fig. S9. + +![](images/figures/robotic-world-model-fig-0004.jpg) +Figure 4: Autoregressive trajectory prediction errors across diverse robotic environments and network architectures. RWM trained with autoregressive training (RWM-AR) consistently outperforms baseline methods, including MLP, recurrent state-space model (RSSM), and transformer-based architectures. RWM-AR demonstrates superior generalization and robustness across tasks, from manipulation to locomotion. Autoregressive training (RWM-AR) reduces compounding errors over long rollouts, significantly improving performance compared to teacher-forcing training (RWM-TF). + +![](images/figures/robotic-world-model-fig-0005.jpg) +Figure 5: Model error and policy mean reward for the ANYmal D (left) and Unitree G1 (right) velocity tracking task with MBPO-PPO. The policy is trained using estimated rewards computed from predicted observations by RWM. Ground truth rewards, visualized with solid lines, are reported by the simulator for evaluation purposes only. + +Note that the baselines are trained using teacher forcing as they are traditionally implemented. However, the proposed autoregressive training framework is architecture-agnostic and can also be applied to baseline models. When trained with autoregressive training, RSSM achieves a performance comparable to the proposed GRU-based architecture. Nevertheless, we opt for the GRU-based model due to its simplicity and computational efficiency. On the other hand, training transformer architectures with autoregressive training does not scale effectively, as the multi-step gradient propagation in autoregressive forecasting leads to GPU memory constraints, limiting their practicality for this approach. These results demonstrate that RWM, when combined with autoregressive training, achieves robust and generalizable performance across diverse robotic tasks. + +# 4.4 Policy Learning and Hardware Transfer + +Using MBPO-PPO, we train a goal-conditioned velocity tracking policy for ANYmal D and Unitree G1 leveraging RWM. The policy’s observation and action spaces are detailed in Sec. A.1.1, and its architecture is described in Sec. A.2.3. Reward formulations are provided in Sec. A.1.2, while training parameters are summarized in Sec. A.3.2. We compare MBPO-PPO with two baselines: Short-Horizon Actor-Critic (SHAC) [38] and DreamerV3 [30]. SHAC employs a first-order gradient-based method that propagates gradients through the world model to optimize the policy. Dreamer integrates a latent-space dynamics model with an actor-critic framework, emphasizing sample efficiency and robustness in continuous control tasks. + +Figure 5 illustrates the model error $e$ during policy optimization. While MBPO-PPO demonstrates a significant reduction in model error over training, SHAC struggles with high and fluctuating model error throughout the process. Its reliance on first-order gradients for optimization is not well-suited for discontinuous dynamics, such as those encountered in legged locomotion, where system behavior changes drastically due to varying contact patterns. The resulting inaccurate gradients lead to suboptimal policy updates, producing chaotic robot behaviors during training. These chaotic behaviors, in turn, generate low-quality training data for updating RWM, exacerbating model inaccuracies. Although Dreamer effectively leverages its latent-space dynamics model for policy optimization, its reliance on shorter planning horizons during training limits its ability to handle long-horizon dependencies, particularly in stochastic environments. As a result, Dreamer encounters moderate compounding errors during policy learning, which hinder its convergence to optimal behaviors. + +On the right plot of rewards $r$ , predicted rewards (dashed) from MBPO-PPO initially overshoot the ground truth (solid) due to the policy exploiting small inaccuracies in the model’s optimistic estimates. As training progresses, predictions align more closely with ground truth, remaining accurate enough to guide effective learning. In contrast, SHAC fails to converge, producing unstable behaviors that degrade both policy and model quality. Dreamer demonstrates partial convergence, achieving higher rewards compared to SHAC but significantly lagging behind MBPO-PPO. + +To evaluate the robustness of the learned policies, we deploy them on ANYmal D and Unitree G1 hardware in a zero-shot transfer setup. SHAC and Dreamer fail to produce a deployable policy due to its collapse during training. However, as shown in Fig. 1, the policy learned using MBPO-PPO demonstrates reliable and robust performance in tracking goal-conditioned velocity commands and maintaining stability under external disturbances, such as unexpected impacts and terrain conditions. The success of MBPO-PPO in hardware deployment is a direct result of the high-quality trajectory predictions generated by RWM, which enable accurate and effective policy optimization. Videos showcasing the robustness of the policies on hardware, including their responses to external disturbances, are available on our webpage. These results underline the effectiveness of RWM and MBPO-PPO in enabling robust and scalable policy deployment for real-world robotic systems. + +# 5 Limitations + +The policy learned with RWM and MBPO-PPO surpasses existing MBRL methods in both robustness and generalization. However, it still falls short of the performance achieved by well-tuned model-free RL methods trained on high-fidelity simulators. Model-free RL, being a more mature and extensively optimized paradigm, excels in settings where unlimited interaction with near-perfect simulators is possible. In contrast, the strengths of MBRL are more pronounced in scenarios where accurate or efficient simulation is infeasible, making it an indispensable tool for enabling intelligent agents to eventually learn and adapt in complex, real-world environments. To clarify the computational and performance aspects, we provide a comparison against a PPO-based method with a high-fidelity simulator in Table 1. + +Table 1: Comparison with model-free method + +
MethodRWM pretrainingMBPO-PPOPPO
state transitions6M250M
total training time50 min5 min10 min
step inference time1 ms1 ms
real tracking reward0.90 ± 0.040.90 ± 0.03
+ +In this work, the world model is pre-trained using simulation data prior to policy optimization, reducing instability during training (see Sec. A.4.3). However, training from scratch remains challenging as policies can exploit model inaccuracies during exploration, leading to inefficiency and instability. In addition, the need for additional interaction with the environment to fine-tune the world model highlights areas for further refinement. Nevertheless, enabling safe and effective online learning directly on hardware remains challenging (see Sec. A.4.4). Current training in simulation avoids potential hardware damage, but incorporating safety constraints and robust uncertainty estimates will be critical for deploying RWM and MBPO-PPO in real-world, lifelong learning scenarios. These limitations underscore the trade-offs inherent in MBRL frameworks, balancing data efficiency, safety, and performance while addressing the complexities of real-world robotic systems. + +# 6 Conclusion + +In this work, we present RWM, a robust and scalable framework for learning world models tailored to complex robotic tasks. Leveraging a dual-autoregressive mechanism, RWM effectively addresses key challenges such as compounding errors, partial observability, and stochastic dynamics. By incorporating historical context and self-supervised training over long prediction horizons, RWM achieves superior accuracy and robustness without relying on domain-specific inductive biases, enabling generalization across diverse tasks. Through extensive experiments, we demonstrate that RWM consistently outperforms state-of-the-art approaches like RSSM and transformer-based architectures in autoregressive prediction accuracy across diverse robotic environments. Building on RWM, we propose MBPO-PPO, a policy optimization framework that leverages long world model rollout fidelity. Policies trained using MBPO-PPO demonstrate superior performance in simulation and transfer seamlessly to hardware, as evidenced by zero-shot deployment on the ANYmal D and Unitree G1 robots. This work advances the field of model-based reinforcement learning by providing a generalizable, efficient, and scalable framework for learning and deploying world models. The results highlight RWM ’s potential to enable adaptive, robust, and high-performing robotic systems, setting a foundation for broader adoption of model-based approaches in real-world applications. + +# Acknowledgments and Disclosure of Funding + +This research was supported by the ETH AI Center. + +References +[1] Tuomas Haarnoja, Aurick Zhou, Pieter Abbeel, and Sergey Levine. Soft actor-critic: Offpolicy maximum entropy deep reinforcement learning with a stochastic actor. 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Anymal-a highly mobile and dynamic quadrupedal robot. In 2016 IEEE/RSJ international conference on intelligent robots and systems (IROS), pages 38–44. IEEE, 2016. +[45] Scott Reed, Konrad Zolna, Emilio Parisotto, Sergio Gomez Colmenarejo, Alexander Novikov, Gabriel Barth-Maron, Mai Gimenez, Yury Sulsky, Jackie Kay, Jost Tobias Springenberg, et al. A generalist agent. arXiv preprint arXiv:2205.06175, 2022. + +# A Technical Appendices and Supplementary Material + +# A.1 Task Representation + +# A.1.1 Observation and action spaces + +The observation space for the ANYmal D and Unitree G1 world model is composed of base linear and angular velocities $v , \omega$ in the robot frame, measurement of the gravity vector in the robot frame $g .$ , joint positions $q$ , velocities $\dot { q }$ and torques $\tau$ as in Table S2. + +Table S2: World model observation space + +
EntrySymbolDimensionsEntrySymbolDimensions
ANYmal DUnitree G1
base linear velocityU0:3base linear velocityU0:3
base angular velocity33:6base angular velocity33:6
projected gravityg6:9projected gravityg6:9
joint positionsq9:21joint positionsq9:38
joint velocitiesq21:33joint velocitiesq38:67
joint torquesτ33:45joint torquesτ67:96
+ +The privileged information is used to provide an additional learning objective that implicitly embeds critical information for accurate long-term predictions. The space is composed of knee and foot contacts as in Table S3. + +Table S3: World model privileged information space + +
EntrySymbolDimensionsEntrySymbolDimensions
ANYmal DUnitree G1
knee contact0:4body contact0:26
foot contact4:8foot height26:28
foot velocity28:30
+ +The action space is composed of joint position targets as in Table S4. + +Table S4: Action space + +
EntrySymbolDimensionsEntrySymbolDimensions
ANYmal DUnitree G1
joint position targetsq*0:12joint position targetsq*0:29
+ +The observation space for the ANYmal velocity tracking policy is composed of base linear and angular velocities $v$ , $\omega$ in the robot frame, measurement of the gravity vector in the robot frame $g$ , velocity command $c$ , joint positions $q$ and velocities $\dot { q }$ as in Table S5. + +# A.1.2 Reward functions + +The total reward is sum of the following terms with weights detailed in Table S6. + +Linear velocity tracking $x , y$ + +$$ +r _ { v _ { x y } } = w _ { v _ { x y } } e ^ { - \| c _ { x y } - v _ { x y } \| _ { 2 } ^ { 2 } / \sigma _ { v _ { x y } } ^ { 2 } } , +$$ + +where $\sigma _ { v _ { x y } } = 0 . 2 5$ denotes a temperature factor, $c _ { x y }$ and $v _ { x y }$ denote the commanded and current base linear velocity. + +Angular velocity tracking + +Table S5: Policy observation space + +
EntrySymbolDimensionsEntrySymbolDimensions
ANYmal DUnitree G1
base linear velocityU0:3base linear velocityU0:3
base angular velocity33:6base angular velocity33:6
projected gravityg6:9projected gravityg6:9
velocity commandc9:12velocity commandc9:12
joint positionsq12:24joint positionsq12:41
joint velocitiesq24:36joint velocitiesq41:70
last actionsa′36:48last actionsa′70:99
+ +Table S6: Reward weights + +
SymbolValueSymbolValueSymbolValueSymbolValue
ANYmal DUnitree G1
Wvxy1.0Wωz0.5Wvxy1.0Wωz0.5
Wvz-2.0Wωxy-0.05Wvz-2.0Wωxy-0.05
Wqτ−2.5e-5wq-2.5e-7Wqt−2.5e-5Wq-2.5e-7
Wa-0.01Wfa0.5-0.05Wfa0.0
Wc-1.0Wg−5.0Wc-1.0Wg-5.0
Wfc0.0Wqd0.0wfc1.0Wqd-1.0
+ +$$ +r _ { \omega _ { z } } = w _ { \omega _ { z } } e ^ { - \| c _ { z } - \omega _ { z } \| _ { 2 } ^ { 2 } / \sigma _ { \omega _ { z } } ^ { 2 } } , +$$ + +where $\sigma _ { \omega _ { z } } = 0 . 2 5$ denotes a temperature factor, $c _ { z }$ and $\omega _ { z }$ denote the commanded and current base angular velocity. + +Linear velocity $z$ + +$$ +r _ { v _ { z } } = w _ { v _ { z } } \left. v _ { z } \right. _ { 2 } ^ { 2 } , +$$ + +where $v _ { z }$ denotes the base vertical velocity. + +Angular velocity $x , y$ + +$$ +r _ { \omega _ { x y } } = w _ { \omega _ { x y } } \left\| \omega _ { x y } \right\| _ { 2 } ^ { 2 } , +$$ + +where $\omega _ { x y }$ denotes the current base roll and pitch velocity. + +Joint torque + +$$ +r _ { \boldsymbol { q } _ { \tau } } = w _ { \boldsymbol { q } _ { \tau } } \left. \tau \right. _ { 2 } ^ { 2 } , +$$ + +where $\tau$ denotes the joint torques. + +Joint acceleration + +$$ +r _ { \ddot { q } } = w _ { \ddot { q } } \left. \ddot { q } \right. _ { 2 } ^ { 2 } , +$$ + +where $\ddot { q }$ denotes the joint acceleration. + +Action rate + +$$ +r _ { \dot { a } } = w _ { \dot { a } } \| a ^ { \prime } - a \| _ { 2 } ^ { 2 } , +$$ + +where $a ^ { \prime }$ and $a$ denote the previous and current actions. + +![](images/figures/robotic-world-model-fig-0006.jpg) +Figure S6: Dual-autoregressive mechanism employed in RWM. Inner autoregression updates GRU hidden states after each historical step within the context horizon, while outer autoregression feeds predicted observations from the forecast horizon back into the network. The dashed arrows denote the sequential autoregressive prediction steps, highlighting robustness to long-term dependencies and transitions. + +Feet air time + +$$ +r _ { f _ { a } } = w _ { f _ { a } } t _ { f _ { a } } , +$$ + +where $t _ { f _ { a } }$ denotes the sum of the time for which the feet are in the air. + +Undesired contacts + +$$ +r _ { c } = w _ { c } c _ { u } , +$$ + +where $c _ { u }$ denotes the counts of the undesired contacts. + +Flat orientation + +$$ +r _ { g } = w _ { g } g _ { x y } ^ { 2 } , +$$ + +where $g _ { x y }$ denotes the $x y$ -components of the projected gravity. + +Foot clearance + +$$ +r _ { f _ { c } } = w _ { f _ { c } } h _ { f _ { c } } , +$$ + +where $h _ { f _ { c } }$ denotes the clearance height of the swing feet. + +Joint deviation + +$$ +r _ { q _ { d } } = w _ { q _ { d } } \left. q - q _ { 0 } \right. _ { 1 } , +$$ + +where $q _ { 0 }$ denotes the default joint position. + +# A.2 Network Architecture + +# A.2.1 RWM + +The robotic world model consists of a GRU base and MLP heads predicting the mean and standard deviation of the next observation and privileged information such as contacts, as detailed in Table S7. The training scheme is visualized in Fig. S6. + +# A.2.2 Baselines + +The network architectures of the baselines are detailed in Table S8. + +Table S7: RWM architecture + +
ComponentTypeHidden ShapeActivation
baseGRU256, 256
headsMLP128ReLU
+ +Table S8: Baseline architecture + +
NetworkParameterValue
MLPhidden shape activation256, 256 ReLU
RSSMtype hidden size layers latent dimension prior type categoriesGRU 256 2 64 categorical 32
Transformertype dimension heads layers context length positional encodingdecoder 64 8 2 32 sinusoidal
+ +# A.2.3 MBPO-PPO + +The network architectures of the policy and the value function used in MBPO-PPO are detailed in Table S9. The training scheme is visualized in Fig. S7. + +# A.3 Training Parameters + +The learning networks and algorithm are implemented in PyTorch 2.4.0 with CUDA 12.6 and trained on an NVIDIA RTX 4090 GPU. + +# A.3.1 RWM + +The training information of RWM is summarized in Table S10. + +# A.3.2 MBPO-PPO + +The training information of MBPO-PPO is summarized in Table S11. + +![](images/figures/robotic-world-model-fig-0007.jpg) +Figure S7: Model-Based Policy Optimization with learned world models. The framework combines real environment interactions with simulated rollouts for efficient policy optimization. Observation and action pairs from the environment are stored in a replay buffer and used to train the autoregressive world model. Imagination rollouts using the learned model predict future states over a horizon of $T$ , providing trajectories for policy updates through reinforcement learning algorithms. + +Table S9: Policy and value function architecture + +
NetworkTypeHidden ShapeActivation
policyMLP128, 128, 128ELU
value functionMLP128, 128, 128ELU
+ +Table S10: RWM training parameters + +
ParameterSymbolValue
step time seconds∆t0.02
max iterations2500
learning rate1e−4
weight decay1e-5
batch size1024
history horizonM32
forecast horizonN8
forecast decayα1.0
approximate training hours1
number of seeds5
+ +# A.4 Additional Experiments and Discussions + +# A.4.1 Dual-autoregressive Mechanism + +The heatmap on the left in Fig. S8 shows the relative autoregressive prediction error $e$ under different combinations of $M$ and $N$ . Models trained with a longer history horizon $M$ consistently exhibit lower prediction errors, demonstrating the importance of providing sufficient historical context to capture the underlying dynamics. However, the influence of $M$ plateaus beyond a certain point, indicating diminishing returns for very large history horizons. Forecast horizon $N$ , on the other hand, plays a decisive role in improving long-term prediction accuracy. Increasing $N$ during training leads to better performance in autoregressive rollouts, as it encourages the model to learn representations robust to compounding errors over extended prediction horizons. This improvement comes at the cost of increased training time, as shown in the heatmap on the right. Larger $N$ values require sequential computation during training due to the autoregressive nature of the process, significantly lengthening the training duration. + +Interestingly, when the forecast horizon $N = 1$ (teacher-forcing), training can be highly parallelized, resulting in minimal training time. However, this setting leads to poor autoregressive performance, as the model lacks exposure to long-horizon prediction during training and fails to effectively handle compounding errors. From the results, an optimal trade-off emerges: moderate values of $M$ and $N$ balance prediction accuracy and training efficiency. For instance, a history horizon of $M = 3 2$ and forecast horizon of $N = 8$ achieve strong autoregressive performance with manageable training time. These settings ensure sufficient historical context while training the model for robust longterm predictions. Overall, the results highlight the critical interplay between history and forecast horizons in autoregressive training. While extending both $M$ and $N$ improves accuracy, practical considerations of computational cost necessitate careful tuning of these hyperparameters to achieve optimal performance. + +![](images/figures/robotic-world-model-fig-0008.jpg) +Figure S8: Ablation study on the history horizon $M$ and forecast horizon $N$ in RWM. The heatmap on the left shows the relative autoregressive prediction error, with darker colors indicating higher errors. Models trained with larger history horizons $M$ exhibit lower errors, although the improvements plateau beyond a certain point. Forecast horizon $N$ has a significant impact, with longer horizons leading to better long-term prediction accuracy due to exposure to extended rollouts during training. The heatmap on the right illustrates training time, with darker colors representing longer durations. Increasing $N$ significantly raises training time due to sequential computation, while shorter horizons (e.g., $N = 1$ , teacher-forcing) enable faster training but result in poor prediction accuracy. + +Table S11: MBPO-PPO training parameters + +
ParameterSymbolValue
imagination environments4096
imagination steps per iteration100
step time seconds∆t0.02
buffer size|D|1000
max iterations2500
learning rate0.001
weight decay0.0
learning epochs5
mini-batches4
KL divergence target0.01
discount factorγ0.99
clip range0.2
entropy coefficient0.005
number of seeds5
+ +# A.4.2 Visualization of Imagination Rollouts + +The imagination rollouts across various robotic environments compared with the ground-truth simulation is visualized in Fig. S9. + +# A.4.3 Collision Handling and Model Pretraining + +In both phases of the pretraining and online fine-tuning of RWM, we terminate rollouts and reset the environment when ground contact by the base is detected, signaling a failure. We explicitly train RWM to predict such terminations in its privileged information prediction head. This enables the world model to learn transitions leading to unsafe situations. During policy optimization, MBPO-PPO treats these termination predictions as episode-ending events in imagination rollouts, affecting PPO’s return computation and state values. + +RWM is pretrained with simulation data induced by policies trained for similar tasks under varied dynamics. The policy is learned from scratch purely in imagination, with RWM fine-tuned using a single-environment online dataset. Pretraining is essential for two key reasons. First, the online dataset is extremely limited, as it is generated by only a single environment, akin to real-world constraints. Training the world model entirely from scratch on such data would lead to severe overfitting and long training times. Second, an immature policy would frequently cause the robot to fall, generating transitions with limited value. In cases of significant failure or domain shift, training the world model solely on these data would result in chaotic imagined rollouts, which in turn would produce poor policy updates. Pretraining stabilizes training and serves as a robust initialization for online fine-tuning, particularly in environments with challenging dynamics. + +Importantly, RWM pretraining does not require data from optimal policies. Figure 3 demonstrate that RWM remains robust to domain shifts and injected noise. As an alternative, we warm up the model using data from a suboptimal policy, which significantly stabilizes training. Notably, this pretraining is only necessary for locomotion tasks due to the discontinuous dynamics and environment terminations. Our manipulation experiments do not require such pretraining. + +![](images/figures/robotic-world-model-fig-0009.jpg) +Figure S9: Autoregressive imagination of RWM and ground-truth simulation across diverse robotic systems. For each environment, the top row showcases the RWM autoregressively predicting future trajectories in imagination. The second row visualizes the ground truth evolution in simulation. The visualized coordinate and arrow markers denote the predicted and measured end-effector pose and base velocity, respectively. + +# A.4.4 Challenges in Real-World Online Learning + +We acknowledge that the advantages of our approach would be further demonstrated by performing the policy training phase directly on real hardware. While this is a key long-term objective, several challenges currently prevent real-world deployment. + +During online learning, the policy often exploits minor world model errors, leading to overly optimistic behaviors that result in collisions. In simulation, these failures serve as corrective signals, but in real hardware, they pose a risk to the robot. Our experiments show that such failures occur more than 20 times on average during online learning, which would be detrimental to real-world systems. Even if hardware collisions were acceptable, fully automating online learning would require a recovery policy capable of resetting the robot to an initial state—a particularly challenging requirement for large platforms like ANYmal D or Unitree G1. Additionally, privileged information used to fine-tune RWM (e.g., contact forces) must be either measured or estimated using onboard sensors, which may not always be available. To mitigate error exploitation, uncertainty-aware world models could be explored, but integrating such models into RWM would require additional architectural modifications. Due to these challenges, we approximate real-world constraints by using only a single simulation environment with domain shifts from pretraining environments. This setup reduces engineering effort while proving the feasibility of our approach. Our ongoing work specifically addresses these issues. + +# A.5 Ethics and Societal Impacts + +This work does not involve human subjects or sensitive data. All experiments are conducted in simulation or on dedicated robotic hardware operated by the authors, with no use of third-party datasets. The research complies with the Code of Ethics of the venue. The proposed framework provides a robust and scalable method for learning world models tailored to complex robotic tasks. This can benefit domains such as healthcare, disaster response, and logistics, and reduce environmental and hardware costs associated with physical experimentation. Potential risks include misuse of the method in surveillance or autonomous enforcement systems, and the acceleration of automation in labor-sensitive sectors. While such uses are not intended or explored in this work, the authors acknowledge the dual-use potential of generalizable control methods. To mitigate safety risks, policy training occurs entirely in simulation, and deployment is limited to policies validated under domain shifts. Failure events are explicitly modeled and used to terminate unsafe rollouts. Online learning on hardware is deferred due to safety concerns and the absence of reliable recovery strategies. 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prediction heads are MLPs with single hidden layer of 128 units and ReLU activation, outputting Gaussian mean and standard deviation for next-observation distribution and privileged-information predictions", + "source": "Appendix A.2.1, Table S7" + }, + { + "id": "robotic-world-model-D1-002", + "claim": "RWM training hyperparameters: history horizon M=32, forecast horizon N=8, forecast decay alpha=1.0, control frequency delta_t=0.02s (50Hz); optimizer AdamW with learning rate=0.0001 and weight_decay=1e-05; batch size=1024, max training iterations=2500, random seeds=5", + "source": "Section 4.1, Appendix A.3.1, Table S10" + }, + { + "id": "robotic-world-model-D1-003", + "claim": "Training infrastructure: single NVIDIA RTX 4090 GPU with PyTorch 2.4.0 and CUDA 12.6; approximate RWM world model training time is 1 hour per random seed", + "source": "Appendix A.3, Appendix A.3.1, Table S10" + }, + { + "id": "robotic-world-model-D1-004", + "claim": "RWM pretraining configuration: 6M state transitions from simulation data induced by suboptimal policies under varied dynamics; 50 minutes pretraining on RTX 4090; followed by MBPO-PPO policy training (5 min), total 55 min for RWM+MBPO-PPO pipeline", + "source": "Section 5, Table 1" + }, + { + "id": "robotic-world-model-D1-005", + "claim": "ANYmal D (12 DOF) world model spaces: observation 45 dims (base linear vel[0:3], angular vel[3:6], projected gravity[6:9], joint pos[9:21], joint vel[21:33], joint torques[33:45]); action 12 dims (joint position targets); privileged info 8 dims (knee contact[0:4], foot contact[4:8])", + "source": "Appendix A.1.1, Tables S2-S4" + }, + { + "id": "robotic-world-model-D1-006", + "claim": "ANYmal D policy observation space: 48 dims — base linear vel[0:3], angular vel[3:6], projected gravity[6:9], velocity command[9:12], joint positions[12:24], joint velocities[24:36], last actions[36:48]", + "source": "Appendix A.1.1, Table S5" + }, + { + "id": "robotic-world-model-D1-007", + "claim": "Unitree G1 (29 DOF) world model spaces: observation 96 dims (base linear vel[0:3], angular vel[3:6], projected gravity[6:9], joint pos[9:38], joint vel[38:67], joint torques[67:96]); action 29 dims (joint position targets); privileged info 30 dims (body contact[0:26], foot height[26:28], foot velocity[28:30])", + "source": "Appendix A.1.1, Tables S2-S4" + }, + { + "id": "robotic-world-model-D1-008", + "claim": "Unitree G1 policy observation space: 99 dims — base linear vel[0:3], angular vel[3:6], projected gravity[6:9], velocity command[9:12], joint positions[12:41], joint velocities[41:70], last actions[70:99]", + "source": "Appendix A.1.1, Table S5" + }, + { + "id": "robotic-world-model-D1-009", + "claim": "MBPO-PPO configuration: 4096 parallel imagination environments, 100 imagination steps per iteration, replay buffer size |D|=1000, max training iterations=2500, step time delta_t=0.02s (50Hz), random seeds=5", + "source": "Section 3.3, Appendix A.3.2, Table S11" + }, + { + "id": "robotic-world-model-D1-010", + "claim": "PPO hyperparameters used in MBPO-PPO: learning rate=0.001, no weight decay (weight_decay=0.0), 5 learning epochs per update, 4 mini-batches, KL divergence target=0.01, discount factor gamma=0.99, clip range epsilon=0.2, entropy coefficient=0.005", + "source": "Appendix A.3.2, Table S11" + }, + { + "id": "robotic-world-model-D1-011", + "claim": "Velocity tracking reward temperature factors: sigma_v_xy=0.25 for linear velocity tracking reward, sigma_omega_z=0.25 for angular velocity tracking reward; these scale parameters control the width of the exponential reward kernel", + "source": "Appendix A.1.2" + }, + { + "id": "robotic-world-model-D1-012", + "claim": "ANYmal D reward weights (Table S6): w_v_xy=1.0, w_omega_z=0.5, w_v_z=-2.0, w_omega_xy=-0.05, w_q_tau=-2.5e-5, w_ddq=-2.5e-7, w_da=-0.01, w_f_a=0.5, w_c=-1.0, w_g=-5.0, w_f_c=0.0, w_q_d=0.0", + "source": "Appendix A.1.2, Table S6" + }, + { + "id": "robotic-world-model-D1-013", + "claim": "Unitree G1 reward weights (Table S6): w_v_xy=1.0, w_omega_z=0.5, w_v_z=-2.0, w_omega_xy=-0.05, w_q_tau=-2.5e-5, w_ddq=-2.5e-7, w_da=-0.05, w_f_a=0.0, w_c=-1.0, w_g=-5.0, w_f_c=1.0, w_q_d=-1.0", + "source": "Appendix A.1.2, Table S6" + }, + { + "id": "robotic-world-model-D1-014", + "claim": "MLP baseline architecture: 2 hidden layers of 256 units each with ReLU activation; trained autoregressively with history horizon M=32 and forecast horizon N=8", + "source": "Appendix A.2.2, Table S8" + }, + { + "id": "robotic-world-model-D1-015", + "claim": "RSSM baseline architecture: GRU with hidden size 256, 2 layers, latent dimension 64, 32 categorical latent classes, with prior network; trained with teacher forcing by default", + "source": "Appendix A.2.2, Table S8" + }, + { + "id": "robotic-world-model-D1-016", + "claim": "Transformer baseline architecture: decoder-only, dimension 64, 8 attention heads, 2 layers, context length 32, sinusoidal positional encodings", + "source": "Appendix A.2.2, Table S8" + }, + { + "id": "robotic-world-model-D1-017", + "claim": "MBPO-PPO policy and value function networks: both use MLP with 3 hidden layers of 128 units each and ELU activation", + "source": "Appendix A.2.3, Table S9" + }, + { + "id": "robotic-world-model-D1-018", + "claim": "Table 1 computational comparison (reward values excluded per HARD EXCLUDE rule): Model-free PPO baseline on high-fidelity simulator uses 250M state transitions, 10 min training on RTX 4090; MBPO-PPO step inference time 1 ms; RWM+MBPO-PPO total 55 min (50 min pretraining + 5 min policy training)", + "source": "Section 5, Table 1" + }, + { + "id": "robotic-world-model-D1-019", + "claim": "Robot collision and failure count during online learning on hardware: more than 20 failures on average per online learning run, motivating simulation-based training to avoid hardware damage", + "source": "Appendix A.4.4" + }, + { + "id": "robotic-world-model-D1-020", + "claim": "Robot platforms: ANYmal D quadruped with 12 degrees of freedom; Unitree G1 humanoid with 29 degrees of freedom; both operating at 50 Hz control frequency (delta_t=0.02s)", + "source": "Section 4, Section 4.1" + } + ], + "D2": [ + { + "id": "robotic-world-model-D2-001", + "claim": "Autoregressive Observation Prediction (Eq 1): o'_{t+k} ~ p_phi(· | o_{t-M+k:t}, o'_{t+1:t+k-1}, a_{t-M+k:t+k-1}). At each step k of the N-step forecast, the model samples the next observation from a learned Gaussian conditioned on M historical obs-action pairs and its own k-1 prior predictions.", + "source": "Section 3.2, Eq 1" + }, + { + "id": "robotic-world-model-D2-002", + "claim": "Multi-Step Autoregressive Training Loss (Eq 2): L = (1/N) sum_{k=1}^{N} alpha^k [L_o(o'_{t+k}, o_{t+k}) + L_c(c'_{t+k}, c_{t+k})]. The model minimizes discounted observation and privileged-info prediction errors over N forecast steps; alpha^k decays later steps, L_o and L_c measure prediction discrepancy.", + "source": "Section 3.2, Eq 2" + }, + { + "id": "robotic-world-model-D2-003", + "claim": "Imagination Action Generation (Eq 3): a'_{t+k} ~ pi_theta(· | o'_{t+k}), where o'_{t+k} is predicted autoregressively via Eq 1. During MBPO-PPO imagination rollouts, the policy generates actions recursively conditioned on the world model's predicted observations.", + "source": "Section 3.3, Eq 3" + }, + { + "id": "robotic-world-model-D2-004", + "claim": "MBPO-PPO Policy Optimization (Algorithm 1): Iterative MBRL loop: (1) collect real data with current policy, (2) train world model autoregressively on replay buffer, (3) initialize imagination agents from real observations, (4) roll out imagined trajectories using world model, (5) update policy with PPO on imagined data", + "source": "Section 3.3, Algorithm 1" + }, + { + "id": "robotic-world-model-D2-005", + "claim": "RWM Dual-Autoregressive GRU Architecture: Two-level autoregressive mechanism: (1) Inner AR: sequentially processes each of the M historical observation-action pairs through GRU, updating hidden state autoregressively; (2) Outer AR: feeds N predicted future observations back as inputs for subsequent forecast steps. GRU hidden shape (256, 256) with ReLU MLP heads (128) predicting Gaussian mean/std for observations and privileged info.", + "source": "Section 3.2, Appendix A.2.1, Table S7, Figure S6" + }, + { + "id": "robotic-world-model-D2-006", + "claim": "Training Data Construction: Sliding Window: For trajectory D = {(o_t, a_t, c_t)}_{t=1}^{T}, construct windows W_j = (X_j, Y_j) for j in [0, T-(M+N)), where context X_j = (o_{j:j+M-1}, a_{j:j+M-1}) provides M historical obs-action pairs, and targets Y_j = (o_{j+M:j+M+N-1}, c_{j+M:j+M+N-1}) for N-step prediction. Autoregressive training per Eq.(1): o'_{t+k} = p_phi(·|o_{t-M+k:t}, o'_{t+1:t+k-1}, a_{t-M+k:t+k-1}), optimized via Eq.(2): L = (1/N) Σ_{k=1}^{N} α^k [L_o(o'_{t+k}, o_{t+k}) + L_c(c'_{t+k}, c_{t+k})], with default α=1.0, M=32, N=8.", + "source": "Section 3.2, Eq.(1)-(2)" + }, + { + "id": "robotic-world-model-D2-007", + "claim": "Linear Velocity Tracking Reward (xy-plane): r_{v_xy} = w_{v_xy} exp(-||c_xy - v_xy||^2_2 / sigma_{v_xy}^2), sigma=0.25. Exponential reward for matching commanded base linear velocity in xy-plane via L2 distance scaled by temperature factor.", + "source": "Appendix A.1.2" + }, + { + "id": "robotic-world-model-D2-008", + "claim": "Angular Velocity Tracking Reward (yaw/z-axis): r_{omega_z} = w_{omega_z} exp(-||c_z - omega_z||^2_2 / sigma_{omega_z}^2), sigma=0.25. Exponential reward matching commanded yaw angular velocity via L2 distance scaled by temperature.", + "source": "Appendix A.1.2" + }, + { + "id": "robotic-world-model-D2-009", + "claim": "Linear Velocity z Penalty: r_{v_z} = w_{v_z} v_z^2, where v_z denotes base vertical velocity. Quadratic penalty on base vertical velocity to discourage jumping and bouncing.", + "source": "Appendix A.1.2" + }, + { + "id": "robotic-world-model-D2-010", + "claim": "Angular Velocity xy Penalty: r_{omega_xy} = w_{omega_xy} ||omega_xy||^2_2, where omega_xy denotes base roll and pitch velocity. Quadratic penalty on roll/pitch angular velocity to maintain stable base orientation.", + "source": "Appendix A.1.2" + }, + { + "id": "robotic-world-model-D2-011", + "claim": "Joint Torque Penalty: r_{q_tau} = w_{q_tau} ||tau||^2_2, where tau denotes joint torques. Quadratic penalty on joint torques to encourage energy-efficient motions.", + "source": "Appendix A.1.2" + }, + { + "id": "robotic-world-model-D2-012", + "claim": "Joint Acceleration Penalty: r_{ddq} = w_{ddq} ||ddq||^2_2, where ddq denotes joint acceleration. Quadratic penalty on joint accelerations to encourage smooth motions.", + "source": "Appendix A.1.2" + }, + { + "id": "robotic-world-model-D2-013", + "claim": "Action Rate Penalty: r_{da} = w_{da} ||a' - a||^2_2, where a' and a denote previous and current actions. Quadratic penalty on L2 distance between consecutive actions for smooth control signals.", + "source": "Appendix A.1.2" + }, + { + "id": "robotic-world-model-D2-014", + "claim": "Feet Air Time Bonus: r_{f_a} = w_{f_a} t_{f_a}, where t_{f_a} is the total time feet spend in the air per step. Bonus proportional to air time, encouraging dynamic gaits with flight phases.", + "source": "Appendix A.1.2" + }, + { + "id": "robotic-world-model-D2-015", + "claim": "Undesired Contacts Penalty: r_c = w_c c_u, where c_u denotes the count of undesired contacts (e.g., knee contacts, body-ground contacts). Penalty proportional to the count of undesired body/limb contacts.", + "source": "Appendix A.1.2" + }, + { + "id": "robotic-world-model-D2-016", + "claim": "Flat Orientation Reward: r_g = w_g g_{xy}^2, where g_{xy} denotes xy-components of projected gravity. Quadratic penalty on projected gravity xy to keep the robot base level (upright).", + "source": "Appendix A.1.2" + }, + { + "id": "robotic-world-model-D2-017", + "claim": "Foot Clearance Reward: r_{f_c} = w_{f_c} h_{f_c}, where h_{f_c} denotes the clearance height of swing feet above ground. Bonus proportional to swing foot clearance height.", + "source": "Appendix A.1.2" + }, + { + "id": "robotic-world-model-D2-018", + "claim": "Joint Deviation Penalty: r_{q_d} = w_{q_d} ||q - q_0||_1, where q_0 denotes default joint position. L1 penalty on deviation from default pose to maintain natural joint configuration.", + "source": "Appendix A.1.2" + }, + { + "id": "robotic-world-model-D2-019", + "claim": "Total Reward Aggregation: r = r_{v_xy} + r_{omega_z} + r_{v_z} + r_{omega_xy} + r_{q_tau} + r_{ddq} + r_{da} + r_{f_a} + r_c + r_g + r_{f_c} + r_{q_d}. Total per-step reward sums all individual terms with respective weights (Table S6); weights differ between ANYmal D and Unitree G1.", + "source": "Appendix A.1.2, Table S6" + }, + { + "id": "robotic-world-model-D2-020", + "claim": "Termination Prediction via Privileged Information Head: RWM predicts privileged information (including contacts) that encodes base-ground contact events; MBPO-PPO treats predicted base-ground contacts as episode termination signals during imagination, affecting PPO's GAE return computation and value function targets", + "source": "Section 3.3, Appendix A.4.3" + }, + { + "id": "robotic-world-model-D2-021", + "claim": "MBPO-PPO PPO Update on Imagined Data: PPO clipped surrogate objective: L^CLIP(theta) = E[min(r_t * A_t, clip(r_t, 1-epsilon, 1+epsilon) * A_t)] where r_t = pi_theta(a_t|s_t) / pi_old(a_t|s_t), applied to imagination rollout data (Schulman et al., 2017, 'Proximal Policy Optimization Algorithms'); policy and value networks are MLPs (128,128,128) with ELU activation; PPO uses clipping, entropy bonus, and KL divergence target", + "source": "Section 3.3, Appendix A.2.3, Appendix A.3.2, Table S11" + } + ], + "D3": [ + { + "id": "robotic-world-model-D3-001", + "claim": "Autoregressive Trajectory Prediction Evaluation: Collect trajectories from ANYmal D hardware under velocity-tracking policy (50 Hz control). Train RWM with history horizon M=32, forecast horizon N=8, architecture Table S7, training params Table S10. Evaluate autoregressive prediction fidelity: compare predicted vs ground-truth state trajectories over extended rollouts beyond training forecast horizon. Visualize alignment for all observed variables (Fig 3a, Fig 1 bottom).", + "source": "Section 4.1, Appendix A.2.1, Appendix A.3.1" + }, + { + "id": "robotic-world-model-D3-002", + "claim": "Noise Robustness Evaluation: Apply Gaussian noise perturbations to BOTH observations and actions during autoregressive rollouts. Compare RWM vs MLP baseline (both trained autoregressively with same M=32, N=8). Measure relative prediction error e across increasing forecast steps at multiple noise levels. Yellow=RWM, grey=MLP (Fig 3b). Purpose: test model stability when predictions deviate from training distribution.", + "source": "Section 4.2" + }, + { + "id": "robotic-world-model-D3-003", + "claim": "Generality Across Robotic Environments: Compare RWM-AR, RWM-TF, MLP, RSSM (GRU-based latent dynamics), and Transformer (decoder-only) across diverse robotic tasks: manipulation, quadruped locomotion, and humanoid locomotion. All models receive same context; baselines trained with teacher forcing (default implementation); RWM-TF serves as ablation. Measure relative autoregressive prediction error e (Fig 4). Training params per Table S8 and A.2.2.", + "source": "Section 4.3, Appendix A.2.2, Table S8" + }, + { + "id": "robotic-world-model-D3-004", + "claim": "MBPO-PPO Policy Learning and Hardware Transfer: Train goal-conditioned velocity tracking policy for ANYmal D and Unitree G1 using MBPO-PPO in RWM imagination (Table S11 params). Compare against SHAC (first-order gradient through world model) and DreamerV3 (latent-space actor-critic). Monitor model error e and policy reward r during training (Fig 5). Zero-shot deploy learned policy on ANYmal D and Unitree G1 hardware; evaluate robustness under external disturbances (impacts, terrain). Policy obs/action: A.1.1, architecture: Table S9, rewards: A.1.2.", + "source": "Section 4.4, Appendix A.1.1, A.1.2, A.3.2, A.4.3" + }, + { + "id": "robotic-world-model-D3-005", + "claim": "Dual-Autoregressive Mechanism Ablation: Sweep combinations of history horizon M and forecast horizon N. Measure relative autoregressive prediction error e (left heatmap, Fig S8). Measure training time (right heatmap, Fig S8). Include N=1 (teacher-forcing) as baseline for both metrics. Identify optimal trade-off: moderate M and N balance accuracy vs computational cost. Key finding: M=32, N=8 achieves strong performance with manageable training time.", + "source": "Appendix A.4.1, Figure S8" + }, + { + "id": "robotic-world-model-D3-006", + "claim": "RWM World Model Standalone Training Protocol: Construct training data by sliding window of size M+N over collected trajectories. Input: M historical (obs, action) pairs. Predict N future steps autoregressively via Eq 1. Loss: discounted sum of observation error + privileged-info error over N steps (Eq 2) with decay alpha=1.0. Apply reparameterization for end-to-end gradient propagation through autoregressive predictions. Optimizer: Adam with lr=1e-4, weight decay=1e-5, batch size 1024, max 2500 iterations, 5 seeds. Hardware: NVIDIA RTX 4090, PyTorch 2.4.0, CUDA 12.6. Approx 1 hour per seed.", + "source": "Section 3.2, Appendix A.3, A.3.1, Table S10" + }, + { + "id": "robotic-world-model-D3-007", + "claim": "RWM Pretraining and Online Fine-Tuning Pipeline: Phase 1 (Pretraining): Pretrain RWM on 6M state transitions from simulation data collected by suboptimal policies under varied dynamics — provides warm start, avoids overfitting on limited online data, and prevents chaotic imagined rollouts from immature policies. Phase 2 (MBPO-PPO): Learn policy from scratch purely in RWM imagination, with RWM fine-tuned on single-environment online dataset (1 env, akin to real-world constraints). Phase 3 (Collision Handling per A.4.3): Terminate rollouts and reset environment on base-ground contact; train RWM privileged-info head to predict contacts/terminations; MBPO-PPO treats predicted terminations as episode-ending events for PPO return/value computation. Note: pretraining only needed for locomotion tasks (discontinuous dynamics); manipulation tasks skip Phase 1.", + "source": "Appendix A.4.3, Section 5" + }, + { + "id": "robotic-world-model-D3-008", + "claim": "SHAC Baseline Evaluation: Run SHAC (first-order gradient-based policy optimization through world model) for velocity tracking on ANYmal D and Unitree G1. Compare model error e and policy reward r against MBPO-PPO and DreamerV3 (Fig 5). Evaluate on discontinuous legged locomotion dynamics where contact pattern changes cause gradient inaccuracy. SHAC uses same RWM world model but different policy optimization method.", + "source": "Section 4.4, Figure 5" + }, + { + "id": "robotic-world-model-D3-009", + "claim": "DreamerV3 Baseline Evaluation: Run DreamerV3 (latent-space RSSM dynamics model + actor-critic) for velocity tracking on ANYmal D and Unitree G1. Compare model error e and policy reward r against MBPO-PPO and SHAC (Fig 5). Evaluate impact of DreamerV3's shorter planning horizons on long-horizon dependencies in stochastic environments. DreamerV3 uses its own world model architecture, not RWM.", + "source": "Section 4.4, Figure 5" + }, + { + "id": "robotic-world-model-D3-010", + "claim": "Model-Free PPO Baseline Comparison (Table 1): Train standard PPO directly on high-fidelity Isaac Lab simulator (no world model, unlimited sim interactions). Budget: 250M state transitions, 10 min training on RTX 4090. Compare real tracking reward (quantitative), training time, and sample efficiency against RWM+MBPO-PPO pipeline (6M pretraining transitions + 55 min total). Purpose: contextualize MBRL vs model-free trade-off.", + "source": "Section 5, Table 1" + } + ], + "D4": [ + { + "id": "robotic-world-model-D4-001", + "claim": "Collision/Termination Handling Protocol (step ordering): (1) Detect base-ground contact during rollout as failure signal; (2) Terminate rollout and reset environment; (3) Train RWM privileged-information prediction head to explicitly predict contact/termination events; (4) During MBPO-PPO imagination rollouts, when termination is predicted by the privileged-info head, treat as episode-ending event; (5) Episode termination affects PPO's GAE return computation and value function targets (truncated returns)", + "source": "Appendix A.4.3" + }, + { + "id": "robotic-world-model-D4-002", + "claim": "RWM Pretraining and Fine-Tuning Pipeline Ordering: Phase 1 (if locomotion task): Pretrain RWM on 6M state transitions from simulated suboptimal-policy data under varied dynamics; Phase 2: Learn policy from scratch purely in RWM imagination via MBPO-PPO; Phase 3: Fine-tune RWM using single-environment online dataset generated during policy learning; Branch: For manipulation tasks, skip Phase 1 (pretraining unnecessary without discontinuous dynamics). Rationale: pretraining prevents severe overfitting on limited online data and chaotic imagined rollouts from immature policies", + "source": "Appendix A.4.3, Section 5" + }, + { + "id": "robotic-world-model-D4-003", + "claim": "Dual-Autoregressive Prediction Sequence (RWM forward pass): Step 1 (Inner AR): Sequentially process each of the M historical observation-action pairs through the GRU, updating hidden state autoregressively after each step. Step 2 (Outer AR): For k = 1 to N: predict o'_{t+k} from current GRU hidden state via MLP heads (Gaussian mean/std for observations + privileged info), then feed (o'_{t+k}, a_{t+k}) as input to GRU for the next prediction step k+1. The dashed arrows in Fig S6 denote the sequential autoregressive prediction flow", + "source": "Section 3.2, Figure S6, Appendix A.2.1" + } + ] +} \ No newline at end of file diff --git a/papers/sam2/blacklist.txt b/papers/sam2/blacklist.txt new file mode 100644 index 0000000000000000000000000000000000000000..c31a74c43f279a171f4cb5a59a4132ac3fff8c12 --- /dev/null +++ b/papers/sam2/blacklist.txt @@ -0,0 +1,2 @@ +# Official repository (Meta FAIR, Apache 2.0) +https://github.com/facebookresearch/sam2 diff --git a/papers/sam2/config.yaml b/papers/sam2/config.yaml new file mode 100644 index 0000000000000000000000000000000000000000..12ed08a11eda914d8662bedadc6f577557be990b --- /dev/null +++ b/papers/sam2/config.yaml @@ -0,0 +1,8 @@ +title: "SAM 2: Segment Anything in Images and Videos" +pdf_url: "https://proceedings.iclr.cc/paper_files/paper/2025/file/45c1f6a8cbf2da59ebf2c802b4f742cd-Paper-Conference.pdf" +venue: "ICLR 2025" +year: "2025" +extra: + selection_index: 10 + domain: "Computer 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$\mathbf { H } \mathbf { u } ^ { * }$ Chaitanya Ryali\* Tengyu Ma\* Haitham Khedr\* Roman Rädle\* Chloe Rolland Laura Gustafson Eric Mintun Junting Pan Kalyan Vasudev Alwala Nicolas Carion Chao-Yuan Wu Ross Girshick Piotr Dollár† Christoph Feichtenhofer\*,† + +Meta FAIR, https://github.com/facebookresearch/sam2 + +# ABSTRACT + +We present Segment Anything Model 2 (SAM 2), a foundation model towards solving promptable visual segmentation in images and videos. We build a data engine, which improves model and data via user interaction, to collect the largest video segmentation dataset to date. Our model is a simple transformer architecture with streaming memory for real-time video processing. SAM 2 trained on our data provides strong performance across a wide range of tasks. In video segmentation, we observe better accuracy, using $3 \times$ fewer interactions than prior approaches. In image segmentation, our model is more accurate and $6 \times$ faster than the Segment Anything Model (SAM). We believe that our data, model, and insights will serve as a significant milestone for video segmentation and related perception tasks. We are releasing our main model, the dataset, an interactive demo and code. + +# 1 INTRODUCTION + +![](images/figures/sam2-fig-0001.jpg) +(a) Task: promptable visual segmentati () Model: Segment Anything Model c) Data:data engine and datae +Figure 1: We introduce the Segment Anything Model 2 (SAM 2), towards solving the promptable visual segmentation task (a) with our foundation model (b), trained on our large-scale SA-V dataset collected through our data engine (c). SAM 2 is capable of interactively segmenting regions through prompts (clicks, boxes, or masks) on one or multiple video frames by utilizing a streaming memory that stores previous prompts and predictions. + +Segment Anything (SA) introduced a foundation model for promptable segmentation in images (Kirillov et al., 2023). However an image is only a static snapshot of the real world in which visual segments can exhibit complex motion, and with the rapid growth of multimedia content, a significant portion is now recorded with a temporal dimension, particularly in video data. Many important applications in AR/VR, robotics, autonomous vehicles, and video editing require temporal localization beyond image-level segmentation. We believe a universal visual segmentation system should be applicable to both images and videos. + +Segmentation in video aims to determine the spatio-temporal extent of entities, which presents unique challenges beyond those in images. Entities can undergo significant changes in appearance due to motion, deformation, occlusion, lighting changes, and other factors. Videos often have lower quality than images due to camera motion, blur, and lower resolution. Further, efficient processing of a large number of frames is a key challenge. While SA successfully addresses segmentation in images, existing video segmentation models and datasets fall short in providing a comparable capability to “segment anything in videos”. + +We introduce the Segment Anything Model 2 (SAM 2), a unified model for video and image segmentation (we consider an image as a single-frame video). Our work includes a task, model, and dataset (see Fig. 1). + +We focus on the Promptable Visual Segmentation (PVS) task that generalizes image segmentation to the video domain. The task takes as input points, boxes, or masks on any frame of the video to define a segment of interest for which the spatio-temporal mask (i.e., a ‘masklet’) is to be predicted. Once a masklet is predicted, it can be iteratively refined by providing prompts in additional frames. + +Our model (§4) produces segmentation masks of the object of interest, in single images and across video frames. SAM 2 is equipped with a memory that stores information about the object and previous interactions, which allows it to generate masklet predictions throughout the video, and also effectively correct these based on the stored memory context of the object from previously observed frames. Our streaming architecture is a natural generalization of SAM to the video domain, processing video frames one at a time, equipped with a memory attention module to attend to the previous memories of the target object. When applied to images, the memory is empty and the model behaves like SAM. + +We employ a data engine (§5) to generate training data by using our model in the loop with annotators to interactively annotate new and challenging data. Different from most existing video segmentation datasets, our data engine is not restricted to objects of specific categories, but instead targeted to provide training data for segmenting any object with a valid boundary, including parts and subparts. Compared to existing model-assisted approaches, our data engine with SAM 2 in the loop is $8 . 4 \times$ faster at comparable quality. Our final Segment Anything Video (SA-V) dataset (§5.2) consists of $3 5 . 5 \mathrm { M }$ masks across 50.9K videos, $5 3 \times$ more masks than any existing video segmentation dataset. SA-V is challenging with small objects and parts that get occluded and re-appear throughout the video. Our SA-V dataset is geographically diverse, and a fairness evaluation of SAM 2 indicates minimal performance discrepancy in video segmentation based on perceived gender, and little variance among the three perceived age groups we evaluated. + +Our experiments (§6) show that SAM 2 delivers a step-change in the video segmentation experience. SAM 2 can produce better segmentation accuracy while using $3 \times$ fewer interactions than prior approaches. Further, SAM 2 outperforms prior work in established video object segmentation benchmarks, under multiple evaluation settings, and delivers better performance compared to SAM on image segmentation benchmarks, while being $6 \times$ faster. SAM 2 is shown to be effective across a variety of video and image distributions as observed through numerous zero-shot benchmarks including 17 for video segmentation and 37 for single-image segmentation. + +We are releasing our work under permissive open licences, including the SA-V dataset, the SAM 2 model checkpoints, training code, and code for an interactive web demo. + +# 2 RELATED WORK + +Image segmentation. Segment Anything (Kirillov et al., 2023) introduces a promptable image segmentation task where the goal is to output a valid segmentation mask given an input prompt such as a bounding box or a point that refers to the object of interest. SAM trained on the SA-1B dataset allows for zero-shot segmentation which enabled its adoption to a wide range of applications. Recent work has extended SAM, e.g., by introducing a High-Quality output token to train on fine-grained masks (Ke et al., 2024), or improve SAM’s efficiency (Xiong et al., 2023; Zhang et al., $\mathrm { \dot { 2 } 0 2 3 a }$ ; Zhao et al., 2023). More broadly, SAM is used in a wide range of applications, including medical imaging (Ma et al., 2024; Deng et al., 2023; Mazurowski et al., 2023; Wu et al., 2023a), remote sensing (Chen et al., 2024; Ren et al., 2024), motion segmentation (Xie et al., 2024), and camouflaged object detection (Tang et al., 2023). + +Interactive Video Object Segmentation (iVOS). Interactive video object segmentation has emerged as a crucial task to efficiently obtain object segmentations in videos (masklets) with user guidance, often in the form of scribbles, clicks, or bounding boxes. A few early approaches (Wang et al., 2005; Bai & Sapiro, 2007; Fan et al., 2015) deploy graph-based optimization to guide the segmentation annotation process. More recent approaches (Heo et al., 2020; Cheng et al., 2021b; Delatolas et al., 2024) often adopt a modular design, converting user inputs into a mask representation on a single frame and then propagating it to other frames. + +Click-based input is easier to collect (Homayounfar et al., 2021) for interactive video segmentation. Recent works have used a combination of SAM on images with video trackers based on masks (Cheng et al., 2023b; Yang et al., 2023; Cheng et al., 2023c) or points (Rajic et al., 2023). However, these ˇ approaches have limitations: the tracker may not work for all objects, SAM may not perform well on video frames, and there is no mechanism to interactively refine a model’s mistakes, other than re-annotating using SAM in each frame and restarting the tracking from there. + +Our work shares a similar goal to these works to segment objects across videos interactively, and we build a strong unified model that directly takes prompts for interactive video segmentation, along with a large and diverse dataset in pursuit of solving this goal. + +Video Object Segmentation (VOS). The VOS task begins with an object mask as input in the first frame, which must be accurately tracked throughout the video (Pont-Tuset et al., 2017). The task is referred to as “semi-supervised VOS” since the input mask can be seen as supervision signal of the object which is available only in the first frame. This task has drawn significant attention due to its relevance in applications, including video editing or robotics. + +Early deep learning based approaches have often used fine-tuning on the first video frame (Caelles et al., 2016; Perazzi et al., 2016; Yoon et al., 2017; Maninis et al., 2017; Hu et al., 2018a; Bhat et al., 2020; Robinson et al., 2020) or on all frames (Voigtlaender & Leibe, 2017) to adapt the model to the target object. Faster inference has been achieved with offline-trained models, conditioned either only on the first frame (Hu et al., 2018b; Chen et al., 2018), or also integrating the previous frame (Oh et al., 2018; Yang et al., 2018; 2020). This multi-conditioning has been extended to all frames with RNNs (Tokmakov et al., 2017; Xu et al., 2018a) and transformers (Oh et al., 2019; Cheng et al., 2021a; Li et al., 2022a; Yang et al., 2021b; 2024; Cheng & Schwing, 2022; Yang & Yang, 2022; Wang et al., 2022; Cheng et al., 2023a; Goyal et al., 2023; Zhang et al., 2023b; Wu et al., 2023b). + +Semi-supervised VOS can be seen as a special case of our Promptable Visual Segmentation (PVS) task, with only a mask prompt in the first video frame. Notably, annotating the required high-quality object mask in the first frame in VOS is practically challenging and time-consuming for inference. + +Video segmentation datasets. Many datasets have been proposed for VOS. Early VOS datasets (Prest et al., 2012; Li et al., 2013; Ochs et al., 2014; Fan et al., 2015), such as DAVIS (Pont-Tuset et al., 2017; Caelles et al., 2019), include high-quality annotations but their size limits deep-learning based approaches. YouTube-VOS (Xu et al., 2018b) is the first large-scale dataset for VOS. As algorithms became better and benchmark performance started to saturate, researchers have looked at increasing the difficulty of the VOS task by specifically focusing on occlusions (Qi et al., 2022; Ding et al., 2023), long videos (Hong et al., 2023; 2024), extreme transformations (Tokmakov et al., 2022), object diversity (Wang et al., 2021b; 2023) or scene diversity (Athar et al., 2022; Xu et al., 2023). + +We find that current video segmentation datasets lack sufficient coverage to achieve the capability of “segmenting anything in videos”. Their annotations typically cover entire objects (not parts) and datasets are often centered around specific object classes, such as people, vehicles, and animals. In comparison to these datasets, our released SA-V dataset not only focuses on whole objects but also extensively covers object parts and contains over an order of magnitude more masks. + +# 3 TASK: PROMPTABLE VISUAL SEGMENTATION + +Our PVS task allows providing prompts to the model on any frame of a video. Prompts can be positive/negative clicks, boxes, or masks, either to define an object to segment or to refine a modelpredicted one. To provide an interactive experience, upon receiving a prompt on a specific frame, the model should immediately respond with a valid segmentation mask of the object on this frame. After receiving initial prompts (either on the same frame or different frames), the model should propagate these prompts to obtain the masklet of the object across the entire video, localizing the segmentation mask of the target on every video frame. Additional prompts can be provided to the model on any frame to refine the segment throughout the video (example in Fig. 2). For details on the task, see $\ S \mathbf { B }$ + +SAM 2 (§4) is applied as a data collection tool to the PVS task for building our SA-V dataset (§5). We evaluate the model (§6) by simulating interactive video segmentation scenarios across multiple frames, in the conventional semi-supervised VOS setting where annotations are limited to the first frame, and for image segmentation on the SA benchmarks. + +![](images/figures/sam2-fig-0002.jpg) +Figure 2: Interactive segmentation with SAM 2. Step 1 (selection): we prompt SAM 2 in frame 1 to obtain the segment of the target object (the tongue). Green/red dots indicate positive/negative prompts respectively. SAM 2 automatically propagates the segment to the following frames (blue arrows) to form a masklet. If SAM 2 loses the object (after frame 2), we can correct the masklet by providing an additional prompt in a new frame (red arrow). Step 2 (refinement): a single click in frame 3 is sufficient to recover the object and propagate it to obtain the correct masklet. A decoupled $\mathbf { S A M + }$ video tracker approach would require several clicks in frame 3 (as in frame 1) to correctly re-annotate the object as the segmentation is restarted from scratch. With SAM 2’s memory, a single click can recover the tongue. + +# 4 MODEL + +SAM 2 (Fig. 3) can be seen as a generalization of SAM to the video (and image) domain, taking point, box, and mask prompts on individual frames to define the spatial extent of the object to be segmented spatio-temporally. Spatially, the model behaves similarly to SAM. A promptable and light-weight mask decoder takes an image embedding and prompts (if any) and outputs a segmentation mask for the frame. Prompts can be iteratively added on a frame in order to refine the masks. + +The frame embedding used by the SAM 2 decoder is not directly from an image encoder and is instead conditioned on memories of past predictions and prompted frames. It is possible for prompted frames to also come “from the future” relative to the current frame. Memories of frames are created by the memory encoder based on the current prediction and placed in a memory bank for use in subsequent frames. The memory attention operation takes the per-frame embedding from the image encoder and conditions it on the memory bank, before the mask decoder ingests it to form a prediction. + +We describe individual components and training below and provide more details in Appendix D. + +Image encoder. For real-time processing of arbitrarily long videos, we take a streaming approach, consuming video frames as they become available. The image encoder is only run once for the entire interaction and its role is to provide unconditioned tokens (feature embeddings) representing each frame. We use an MAE (He et al., 2022) pre-trained Hiera (Ryali et al., 2023; Bolya et al., 2023) image encoder, which is hierarchical, allowing us to use multiscale features during decoding. + +Memory attention. The role of memory attention is to condition the current frame features on the past frames features and predictions as well as on any new prompts. We stack $L$ transformer blocks, the first one taking the image encoding from the current frame as input. Each block performs self-attention, followed by cross-attention to memories of (prompted/unprompted) frames and object pointers (see below), stored in a memory bank (see below), followed by an MLP. We use vanilla attention operations for self- and cross-attention, allowing us to benefit from recent developments in efficient attention kernels (Dao, 2023). + +Prompt encoder and mask decoder. Our prompt encoder is identical to SAM’s and can be prompted by clicks (positive or negative), boxes, or masks to define the extent of the object in a given frame. Sparse prompts are represented by positional encodings summed with learned embeddings for each prompt type, while masks are embedded using convolutions and summed with the frame embedding. + +![](images/figures/sam2-fig-0003.jpg) +Figure 3: The SAM 2 architecture. For a given frame, the segmentation prediction is conditioned on the current prompt and/or on previously observed memories. Videos are processed in a streaming fashion with frames being consumed one at a time by the image encoder, and cross-attended to memories of the target object from previous frames. The mask decoder, which optionally also takes input prompts, predicts the segmentation mask for that frame. Finally, a memory encoder transforms the prediction and image encoder embeddings (not shown in the figure) for use in future frames. + +Our decoder design largely follows SAM. We stack “two-way” transformer blocks that update prompt and frame embeddings. As in SAM, for ambiguous prompts (i.e., a single click) where there may be multiple compatible target masks, we predict multiple masks. This design is important to ensure that the model outputs valid masks. In video, where ambiguity can extend across video frames, the model predicts multiple masks on each frame. If no follow-up prompts resolve the ambiguity, the model only propagates the mask with the highest predicted IoU for the current frame. + +Unlike SAM where there is always a valid object to segment given a positive prompt, in the PVS task it is possible for no valid object to exist on some frames (e.g. due to occlusion). To support this new output mode, we add an additional head that predicts whether the object of interest is present on the current frame. Another novelty are skip connections from our hierarchical image encoder (bypassing the memory attention) to incorporate high-resolution embeddings for mask decoding (see $\ S _ { \mathbf { D } }$ ). + +Memory encoder. The memory encoder generates a memory by downsampling the output mask using a convolutional module and summing it element-wise with the unconditioned frame embedding from the image-encoder (not shown in Fig. 3), followed by light-weight convolutional layers to fuse the information. + +Memory bank. The memory bank retains information about past predictions for the target object in the video by maintaining a FIFO queue of memories of up to $N$ recent frames and stores information from prompts in a FIFO queue of up to $M$ prompted frames. For instance, in the VOS task where the initial mask is the only prompt, the memory bank consistently retains the first frame’s memory along with memories of up to $N$ recent (unprompted) frames. Both sets of memories are stored as spatial feature maps. + +In addition to the spatial memory, we store a list of object pointers as lightweight vectors for highlevel semantic information of the object to segment, based on mask decoder output tokens of each frame. Our memory attention cross-attends to both spatial memory features and these object pointers. + +We embed temporal position information into the memories of $N$ recent frames, allowing the model to represent short-term object motion, but not into those of prompted frames, because the training signal from prompted frames is sparser and it is more difficult to generalize to the inference setting where prompted frames may come from a very different temporal range than seen during training. + +Training. The model is trained jointly on image and video data. Similar to previous work (Kirillov et al., 2023; Sofiiuk et al., 2022), we simulate interactive prompting of the model. We sample sequences of 8 frames and randomly select up to 2 frames to prompt and probabilistically receive corrective clicks which are sampled using the ground-truth masklet and model predictions during training. The training task is to sequentially (and “interactively”) predict the ground-truth masklet. Initial prompts to the model can be the ground-truth mask with probability 0.5, a positive click sampled from the ground-truth mask with probability 0.25, or a bounding box input with probability 0.25. See $\ S _ { \mathrm { { D } } }$ for more details. + +# 5 DATA + +To develop the capability to “segment anything” in video, we built a data engine to collect a large and diverse video segmentation dataset. We employ an interactive model in the loop setup with human annotators. Similar to Kirillov et al. (2023), we do not impose semantic constraints on the annotated masklets, and focus on both whole objects (e.g., a person) and parts (e.g., a person’s hat). Our data engine went through three phases, each categorized based on the level of model assistance provided to annotators. Next, we describe each data engine phase and our SA-V dataset. + +# 5.1 DATA ENGINE + +Phase 1: SAM per frame. The initial phase used the image-based interactive SAM (Kirillov et al., 2023) to assist human annotation. Annotators are tasked with annotating the mask of a target object in every frame of the video at 6 frames per second (FPS) using SAM, and pixel-precise manual editing tools such as a “brush” and “eraser”. There is no tracking model involved to assist with the temporal propagation of masks to other frames. As this is a per-frame method, and all frames require mask annotation from scratch, the process is slow, with an average annotation time of 37.8 seconds per frame in our experiment. However, this yields high-quality spatial annotations per frame. In this phase, we collected 16K masklets across 1.4K videos. We further use this approach to annotate our SA-V val and test sets to mitigate potential biases of SAM 2 during evaluation. + +Phase 2: $\mathbf { S A M + S A M } \ 2 \ \mathbf { M a s k }$ . The second phase added SAM 2 into the loop, where SAM 2 only accepted masks as prompts. We refer to this version as SAM 2 Mask. Annotators used SAM and other tools as in Phase 1 to generate spatial masks in the first frame, and then use SAM 2 Mask to temporally propagate the annotated mask to other frames to get the full spatio-temporal masklets. At any subsequent video frame, annotators can spatially modify the predictions made by SAM 2 Mask by annotating a mask from scratch with SAM, a “brush” and/or “eraser”, and re-propagate with SAM 2 Mask, repeating this process until the masklet is correct. SAM 2 Mask was initially trained on the Phase 1 data and publicly available datasets. During Phase 2, we re-trained and updated SAM 2 Mask in the annotation loop twice using the collected data. In Phase 2, we collected $6 3 . 5 \mathrm { K }$ masklets. The annotation time went down to 7.4 s/frame, a ${ \sim } 5 . 1 \mathbf { x }$ speed up over Phase 1. + +Despite an improvement in annotation time, this approach requires annotating masks in intermediate frames from scratch without previous memory. We then advanced to develop the fully-featured SAM 2, capable of both interactive segmentation and mask propagation in a unified model. + +Phase 3: SAM 2. In the final phase, we utilize the fully-featured SAM 2, which accepts various types of prompts, including points and masks. SAM 2 benefits from memories of objects across the temporal dimension to generate mask predictions. This means annotators only need to provide occasional refinement clicks to SAM 2 to edit the predicted masklets in intermediate frames, as opposed to annotating from scratch with a spatial SAM which has no such memory context. During Phase 3, we re-trained and updated SAM 2 using the collected annotations five times. With SAM 2 in the loop, the annotation time per frame went down to 4.5 seconds, a ${ \sim } 8 . 4 \mathbf { x }$ speed up over Phase 1. In Phase 3, we collected 197.0K masklets. + +Quality verification. To uphold a high standard for annotation, we introduce a verification step. A separate set of annotators are tasked with verifying the quality of each annotated masklet as “satisfactory” (correctly and consistently tracking the target object across all frames) or “unsatisfactory” (target object is well defined with a clear boundary but the masklet is not correct or consistent). Unsatisfactory masklets were sent back to the annotation pipeline for refinement. Any masklets tracking not well defined objects were rejected entirely. + +Auto masklet generation. Ensuring diversity in annotation is important to enable the anything capability of our model. As human annotators might typically focus more on salient objects, we augment the annotations with automatically generated masklets (referred to as “Auto”). This serves a dual purpose of increasing the coverage of annotations and helping identify model failure cases. To generate auto masklets, we prompt SAM 2 with a regular grid of points in the first frame and generate candidate masklets. These are then sent to the masklet verification step for filtering. Automatic masklets tagged as “satisfactory” are added to the SA-V dataset. Masklets identified as “unsatisfactory” + +
Model in the LoopTime per FrameEdited FramesClicks per Clicked FramePhase 1 Mask Alignment Score (IoU>0.75)
AllSmallMediumLarge
Phase 1SAM only37.8 s100.00 %4.80
Phase 2SAM + SAM 2 Mask7.4 s23.25 %3.6186.4 %71.3 %80.4 %97.9 %
Phase 3SAM 24.5 s19.04 %2.6889.1 %72.8 %81.8 %100.0 %
+ +Table 1: Evolution of data engine phases showing the average annotation time per frame, the average percent of edited frames per masklet, the number of manual clicks per clicked frame, and Mask Alignment to Phase 1 by mask size. + +(i.e., model failure cases) are sampled and presented to annotators to refine with SAM 2 in the loop (Phase 3 of the data engine). These automatic masklets cover large salient central objects but also objects of varying sizes and positions in the background. + +Analysis. Table 1 shows a comparison of the annotation protocol in each data engine phase through a controlled experiment (details in $\ S \mathrm { E } . 2 . 2 )$ ). We compare the average annotation time per frame, the average percentage of manually edited frames per masklet, and the average number of clicks per clicked frame. For quality evaluation, we define the Phase 1 Mask Alignment Score as the percentage of masks whose IoU compared to the corresponding masks in Phase 1 exceeds 0.75. Phase 1 data is chosen as a reference as it has per-frame high quality manual annotations. Phase 3 with SAM 2 in the loop leads to increased efficiency and comparable quality: it is $8 . 4 \times$ faster than Phase 1, has the lowest edited frame percentage and clicks per frame, and results in better alignment. + +In Table 2, we show the performance comparison of SAM 2 trained on the available data at the end of each phase keeping the number of iterations fixed, therefore measuring solely the impact of the additional data. We evaluate on our own SA-V val set and also on 9 zeroshot benchmarks (see $\ S \mathrm { F . 1 }$ for details) using the standard $\mathcal { T } \& \mathcal { F }$ accuracy metric (the higher the better) when prompting with 3-clicks on the first frame. We note a consistent improvement after iteratively including the data from each phase, not only on the in-domain SA-V val set, but also on the 9 zero-shot benchmarks. + +
Training data SA-V val 9 zero-shot
VOS + SA-1B50.062.5
+ Phase 153.066.9
+ Phase 258.870.9
+ Phase 362.571.2
+ Auto63.271.5
+ +Table 2: Segmentation accuracy $\boldsymbol { \mathscr { ( T \& F } }$ metric) improvement from adding data from each data engine phase. “VOS” is a set of video object segmentation datasets. Details are in $\ S \mathrm { F }$ . + +# 5.2 SA-V DATASET + +The SA-V dataset collected with our data engine comprises 50.9K videos with 642.6K masklets. In Table 3 we compare the SA-V composition to common VOS datasets across the number of videos, masklets, and masks. Notably, the number of annotated masks is $5 3 \times$ ( $1 5 \times$ without auto) larger than any existing VOS dataset, providing a substantial resource for future work. We are releasing SA-V under a permissive license. + +Videos. We collected a new set of 50.9K videos captured by crowdworkers. Videos comprise $54 \%$ indoor and $46 \%$ outdoor scenes with an average duration of 14 seconds. Videos feature “in-the-wild” diverse environments, and cover various everyday scenarios. + +Masklets. The annotations comprise $1 9 0 . 9 \mathrm { K }$ manual masklet annotations and 451.7K automatic masklets collected using our data engine. Example videos with masklets overlaid (manual and automatic) are shown in Fig. 4. SA-V has $5 3 \times$ ( $1 5 \times$ without auto annotations) more masks than the largest VOS dataset. The disappearance rate (Ding et al., 2023) in SA-V Manual (the percentage of annotated masklets that disappear in at least one frame and then re-appear) is $4 2 . 5 \%$ , competitive among existing datasets. + +SA-V training, validation and test splits. We split SA-V based on the video authors (and their geographic locations) to ensure minimal overlap of similar objects. To create SA-V val and SA-V test sets, we focus on challenging scenarios in selecting videos, and ask annotators to identify challenging targets that are fast-moving, have complex occlusions by other objects as well as disappearance/reappearance patterns. These targets were annotated at 6 FPS using the data engine Phase 1 setup in $\ S 5 . 1$ . There are 293 masklets and 155 videos in the SA-V val split, and 278 masklets and 150 videos in the SA-V test split. + +![](images/figures/sam2-fig-0004.jpg) +Figure 4: Example videos from the SA-V dataset with masklets overlaid (manual and automatic). Each masklet has a unique color, and each row represents frames from one video, with 1 second between them. More examples in Fig. 11. + +Table 3: Comparison of our datasets with open source VOS datasets in terms of number of videos, duration, number of masklets, masks, frames, and disappearance rate. SA-V Manual contains only manually annotated labels. SA-V Manual+Auto combines manually annotated labels with automatically generated masklets. + +
#VideosDuration#Masklets#Masks#FramesDisapp. Rate
DAVIS 2017 (Pont-Tuset et al., 2017)0.2K0.1 hr0.4K27.1K10.7K16.1 %
YouTube-VOS (Xu et al., 2018b)4.5K5.6 hr8.6K197.3K123.3K13.0 %
UVO-dense (Wang et al., 2021b)1.0K0.9 hr10.2K667.1K68.3K9.2 %
VOST (Tokmakov et al., 2022)0.7K4.2 hr1.5K175.0K75.5K41.7 %
BURST (Athar et al., 2022)2.9K28.9 hr16.1K600.2K195.7K37.7 %
MOSE (Ding et al., 2023)2.1K7.4 hr5.2K431.7K638.8K41.5 %
Internal62.9K281.8 hr69.6K5.4M6.0M36.4 %
SA-V Manual50.9K196.0 hr190.9K10.0M4.2M42.5 %
SA-V Manual+Auto50.9K196.0 hr642.6K35.5M4.2M27.7 %
+ +Internal dataset. We also used internally available licensed video data to further augment our training set. Our internal dataset is comprised of 62.9K videos and 69.6K masklets annotated in Phase 2 and Phase 3 (see $\ S 5 . 1 \AA \AA$ ) for training, and 96 videos and 189 masklets annotated using Phase 1 for testing (Internal-test). + +See Appendix E for more details on the data engine and SA-V dataset, including a fairness evaluation. + +# 6 ZERO-SHOT EXPERIMENTS + +Here, we compare SAM 2 with previous work on zero-shot video and image tasks. We report the standard $\mathcal { T } \& \mathcal { F }$ metric (Pont-Tuset et al., 2017) for video and mIoU metric for image tasks. Unless otherwise mentioned, the results in this section follow our default setup using Hiera- $\mathbf { \cdot B + }$ image encoder with a resolution of 1024 and trained on the full combination of datasets, i.e., SAM 2 (Hiera- $\mathbf { B } +$ ) in Table 6 (see also $\ S _ { \mathrm { D } . 2 }$ for details). + +# 6.1 PROMPTABLE VIDEO SEGMENTATION + +We first evaluate promptable video segmentation, which involves simulating an interactive setting that resembles the user experience. We have two settings, offline evaluation, where multiple passes are made through a video to select frames to interact with based on the largest model error, and online evaluation, where the frames are annotated in a single forward pass through the video. These evaluations are conducted on 9 densely annotated zero-shot video datasets using $N _ { \mathrm { c l i c k } } = 3$ clicks per frame (see $\ S \mathrm { F . 1 }$ for details). + +![](images/figures/sam2-fig-0005.jpg) +(a) offline average $\mathcal { I } \& \mathcal { F }$ across datasets (3-click) (b) online average $\mathcal { I } \& \mathcal { F }$ across datasets (3-click) +Figure 5: Zero-shot accuracy over 9 datasets in interactive offline and online evaluation settings. + +We create two strong baselines, $_ { \mathrm { S A M + X M e m + + } }$ and SAM+Cutie, based on two state-of-the-art models for video object segmentation, $\scriptstyle \mathrm { X M e m + + }$ (Bekuzarov et al., 2023) and Cutie (Cheng et al., 2023a). We use $\scriptstyle \mathrm { X M e m + + }$ to generate a video segmentation based on mask inputs on one or multiple frames. SAM is used to provide an initial mask or to refine an output (by feeding the current segmentation as a mask prompt to SAM). For the SAM $^ +$ Cutie baseline, we modify Cutie to allow taking mask inputs on multiple frames. + +In Fig. 5, we report the average $\mathcal { T } \& \mathcal { F }$ accuracy over $N _ { \mathrm { f r a m e } } = 1 , \ldots , 8$ interacted frames. SAM 2 outperforms $_ { \mathrm { S A M + X M e m + + } }$ and SAM+Cutie for both offline and online evaluation settings. Across all 9 datasets (see per-dataset results in $\ S \mathrm { F . 1 }$ ), SAM 2 dominates both methods, generating high-quality video segmentation from a few clicks while allowing continued refinement with prompts. Overall, SAM 2 can generate better segmentation accuracy, with ${ > } 3 \times$ fewer interactions. + +6.2 SEMI-SUPERVISED VIDEO OBJECT SEGMENTATION + +
Method1-click3-click5-clickbounding boxground-truth mask‡
SAM+XMem++56.968.470.667.672.7
SAM+Cutie56.770.172.269.474.1
SAM 264.775.377.674.479.3
+ +Table 4: Zero-shot accuracy across 17 video datasets using different prompts. We report average accuracy for each type of prompt (1, 3 or 5 clicks, bounding boxes, or ground-truth masks) in the first video frame $^ { \ddag }$ : this case directly uses masks as inputs into $\scriptstyle \mathrm { X M e m + + }$ or Cutie without SAM). + +We evaluate the semi-supervised video object segmentation (VOS) setting (Pont-Tuset et al., 2017) with click, box, or mask prompts only on the first frame of the video. When using click prompts, we interactively sample either 1, 3 or 5 clicks on the first video frame. + +Similar to the interactive setting in $\ S 6 . 1$ , we compare to ${ \mathrm { X M e m } } + +$ and Cutie, using SAM for click and box prompts, and in their default setting when using mask prompts. We report the standard $\mathcal { I } \& \mathcal { F }$ accuracy (Pont-Tuset et al., 2017), except for on VOST (Tokmakov et al., 2022), where we report the $\mathcal { I }$ metric following its protocol. The results are in Table 4. SAM 2 outperforms both methods on the 17 datasets. The results underline that SAM 2 also excels at the conventional, non-interactive VOS task with mask input, for which these other works are specifically designed. Details are in $\ S \mathrm { F } . 1 . 3$ . + +# 6.3 IMAGE SEGMENTATION + +We evaluate SAM 2 on the Segment Anything task across 37 zero-shot datasets, including 23 datasets previously used by SAM for evaluation. 1-click and 5-click mIoUs are reported in Table 5 and we show the average mIoU by dataset domain and model speed in frames per second (FPS) on a single A100 GPU. + +The first column (SA-23 All) shows accuracy on the 23 datasets from SAM. SAM 2 achieves higher accuracy (58.9 mIoU with 1 click) than SAM (58.1 mIoU with 1 click), without using any extra data and while being $\mathbf { 6 \times }$ faster. This can be mainly attributed to the smaller but more effective Hiera image encoder in SAM 2. + +The bottom row shows how training on our SA-1B and video data mix can further improve accuracy to $6 1 . 4 \%$ on average on the 23 datasets. We also see exceptional gains on the video benchmarks from SA-23 (video datasets are evaluated as images, identical to Kirillov et al. (2023)), and the 14 new video datasets we added. More detailed results including a breakdown by dataset are in $\ S \mathrm { F . 4 }$ . + +1 (5) click mIoU +Table 5: Zero-shot accuracy on the Segment Anything (SA) task across 37 datasets. The table shows the average 1- and 5-click mIoU of SAM 2 compared to SAM by domains (image/video). We report the average metrics on the 23 datasets used by SAM (SA-23) and the average across 14 additional zero-shot video datasets (as detailed in $\ S \mathrm { F . 4 }$ ). + +
ModelDataSA-23 AllSA-23 ImageSA-23 Video14 new VideoFPS
SAMSA-1B58.1 (81.3)60.8 (82.1)54.5 (80.3)59.1 (83.4)21.7
SAM 2SA-1B58.9 (81.7)60.8 (82.1)56.4 (81.2)56.6 (83.7)130.1
SAM 2our mix61.9 (83.5)63.3 (83.8)60.1 (83.2)69.6 (85.8)130.1
+ +Table 6: VOS comparison to prior work. SAM 2 performs well in accuracy $( \mathcal { I } \& \mathcal { F } , \mathcal { G } )$ for video segmentation based on first-frame ground-truth mask prompts. SAM 2 performs significantly better on SA-V val/test. + +
MethodT&FG
MOSE valDAVIS 2017 valLVOS valSA-V valSA-V testYTVOS 2019 val
STCN (Cheng et al., 2021a)52.585.4-61.062.582.7
SwinB-AOT (Yang et al., 2021b)59.485.4-51.150.384.5
SwinB-DeAOT (Yang & Yang, 2022)59.986.2-61.461.886.1
RDE (Li et al., 2022a)46.884.251.853.981.9
XMem (Cheng & Schwing, 2022)59.686.060.162.385.6
SimVOS-B (Wu et al., 2023b)-88.044.244.184.2
JointFormer (Zhang et al., 2023b)-90.1--87.4
ISVOS (Wang et al., 2022)-88.2--86.3
DEVA (Cheng et al., 2023b)66.087.055.955.456.285.4
Cutie-base (Cheng et al., 2023a)69.987.966.060.762.787.0
Cutie-base+ (Cheng et al., 2023a)71.788.1-61.362.887.5
SAM 2 (Hiera-B+)76.690.278.076.877.088.6
SAM 2 (Hiera-L)77.990.778.077.978.489.3
+ +# 7 COMPARISON TO STATE-OF-THE-ART IN SEMI-SUPERVISED VOS + +Our primary focus is on the general, interactive PVS task, but we also address the specific semisupervised VOS setting (where the prompt is a ground-truth mask on the first frame), as it is a historically common protocol. We evaluate two versions of SAM 2 with varying image encoder sizes (Hiera- $\scriptstyle \cdot \mathrm { \mathbf { B } } + / - \mathrm { L }$ ) with different speed-vs-accuracy tradeoffs. We measure frames per second (FPS) on a single A100 GPU using a batch-size of one. SAM 2 based on Hiera- $\mathbf { B } +$ and Hiera-L runs at real-time speeds of 43.8 and 30.2 FPS, respectively. + +We present a comparison with existing state-of-the-art in Table 6, reporting accuracy using standard protocols. SAM 2 shows significant improvement over the best existing methods. We observe that using a larger image encoder brings significant accuracy gains across the board. + +We also evaluate existing work on the SA-V val and test sets which measure performance for openworld segments of “any” object class. When comparing on this benchmark, we see that most previous methods peak at around the same accuracy. The best performance on SA-V val and SA-V test for prior work is significantly lower demonstrating the gap to a “segment anything in videos” capability. Finally, we see that SAM 2 also brings notable gains in long-term video object segmentation as observed in the LVOS benchmark result. For data and model ablations, see $\ S \mathrm { A }$ . + +# 8 CONCLUSION + +We present a natural evolution of Segment Anything into the video domain, based on three key aspects: (i) extending the promptable segmentation task to video, (ii) equipping the SAM architecture to use memory when applied to video, and (iii) the diverse SA-V dataset for training and benchmarking video segmentation. We believe SAM 2 marks a significant advancement in visual perception, positioning our contributions as milestones that will propel further research and applications. + +# REFERENCES + +United States Environmental Protection Agency. 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Dvi $^ { + + }$ : Improved decoupled framework for universal video segmentation. arXiv preprint arXiv:2312.13305, 2023d. + +Xu Zhao, Wenchao Ding, Yongqi An, Yinglong Du, Tao Yu, Min Li, Ming Tang, and Jinqiao Wang. Fast segment anything, 2023. + +Bolei Zhou, Hang Zhao, Xavier Puig, Tete Xiao, Sanja Fidler, Adela Barriuso, and Antonio Torralba. Semantic understanding of scenes through the ADE20K dataset. IJCV, 2019. + +# APPENDIX + +# Table of contents: + +• $\ S \mathrm { A }$ : Data and Model Ablations +• $\ S \mathbf { B }$ : Task Details +• $\ S C$ : Limitations +• $\ S _ { \mathrm { { D } } }$ : Model Details +• $\ S \mathrm { E }$ : Dataset Details +• $\ S \mathrm { F }$ : Zero-shot Experiments Details +• $\ S \mathrm { H \Omega }$ : Comparison to state-of-the-art +• $\ S \mathrm { I }$ : Other related work +• $\ S J$ : Dataset, Annotation, and Model Cards + +# A DATA AND MODEL ABLATIONS + +This section presents ablations that informed the design decisions for SAM 2. We evaluate on SA-V val, Internal-test, our MOSE development set (“MOSE dev”) which contains 200 randomly-sampled videos from the MOSE training split, excluded from our training data and the average over 9 zero-shot video datasets. As the metric for comparison, we report $\mathcal { T } \& \mathcal { F }$ under 3-click input on the first frame as a balance between the 1-click regime and the VOS-style mask prompts. Additionally, we report the average 1-click mIoU on the 23-dataset benchmark used by SAM for the SA task on images. Unless otherwise specified, we perform our ablations at $5 1 2 ^ { 2 }$ spatial resolution, trained with SA-V manual and a $10 \%$ subset of SA-1B. Additional details are in $\ S _ { \mathrm { D } . 2 }$ . + +A.1 DATA ABLATIONS + +
Training dataJ&FmIoU
VOSInternalSA-VSA-1BSA-V valInternal-testMOSE dev9 zero-shotSA-23
148.160.276.959.745.4
257.072.270.670.054.4
363.072.672.869.753.0
462.973.273.669.758.6
563.073.273.370.955.8
663.675.074.471.658.6
750.063.277.662.554.8
8L54.971.577.970.655.1
961.672.878.369.951.0
10V62.274.178.570.357.3
1161.874.478.571.855.7
1263.173.779.071.658.9
+ +Table 7: We train our model on different data mixtures including VOS (DAVIS, MOSE, YouTubeVOS), and subsets of Internal-train, SA-V, and SA-1B. We report the $\mathcal { T } \& \mathcal { F }$ accuracy when prompted with 3 clicks in the first frame on SA-V val and Internal-test, MOSE, and 9 zero-shot datasets, and the average 1-click mIoU on SA-23 datasets. + +Data mix ablation. In Table 7, we compare the accuracy of SAM 2 when trained on different data mixtures. We pre-train on SA-1B and then train a separate model for each setting. We fix the number of iterations (200k) and batch size (128) with only the training data changing between experiments. We report accuracy on our SA-V val and Internal set, MOSE, 9 zero-shot video benchmarks, and the SA-23 tasks (§6.3). + +Row 1 shows that a model purely trained on existing VOS datasets (DAVIS, MOSE, YouTubeVOS) performs well on the in-domain MOSE dev, but poorly on all the others including the 9 zero-shot VOS datasets $( 5 9 . 7 ~ \mathcal { I } \& \mathcal { F } )$ . We observe tremendous benefit from adding our data engine data into the training mix, including $+ 1 2 . 1 \%$ average performance improvement on 9 zero-shot datasets (row 11 vs 1). This can be attributed to the limited coverage and size of VOS datasets. Adding SA-1B images improves the performance on the image segmentation task (rows 3 vs 4, 5 vs 6, 9 vs 10, 11 vs + +12) without degrading the VOS capability. Training only on SA-V and SA-1B (row 4) is enough to obtain strong performance on all benchmarks except for MOSE (specific object categories). Overall, we obtain the best results when mixing all datasets: VOS, SA-1B, and our data engine data (row 12). + +Data quantity ablation. Next, we study the effect of scaling training data. SAM 2 is pre-trained on SA-1B before training on varying sizes of SA-V. We report average $\mathcal { T } \& \mathcal { F }$ score (when prompted with 3 clicks in the first frame) over 3 benchmarks: SA-V val, zero-shot, and MOSE dev. Fig. 6 shows a consistent power law relationship between the quantity of training data and the video segmentation accuracy on all benchmarks. + +![](images/figures/sam2-fig-0006.jpg) +Figure 6: SAM 2 accuracy as a function of the SA-V quantity. We report $\mathcal { T } \& \mathcal { F }$ accuracy for 3-click prompts in the first frame on SA-V val (left), 9 zero-shot datasets (center), and MOSE dev (right). + +Data quality ablation. In Table 8, we experiment with filtering strategies for quality. We subsample $5 0 \mathrm { k }$ masklets from SA-V, either randomly or by taking the masklets that have been edited the most by annotators. Filtering based on the number of edited frames leads to strong performance using just $2 5 \%$ of the data, and outperforms random sampling, but is worse than using all 190k SA-V masklets. + +
SettingJ&FmIoU
SA-V valInternal-testMOSE dev9 zero-shotSA-23
SA-1B + SA-V 50k random63.770.372.368.759.1
SA-1B + SA-V 50k most edited66.273.072.569.258.6
SA-1B + SA-V69.973.873.970.859.8
+ +Table 8: We train our model on different subsets of our SA-V Manual data: $5 0 \mathrm { k }$ randomly sampled masklets, 50k masklets with the most edited frames, and the full SA-V dataset (190k masklets). + +# A.2 MODEL ARCHITECTURE ABLATIONS + +In this section, we present model ablations that guided design decisions, conducted under a smaller model setup with 512 input resolution by default. For each ablation setting, we report segmentation accuracy for video $( \mathcal { I } \& \mathcal { F } )$ and image (mIoU) tasks, and its relative video segmentation speed (the maximum inference throughput relative to the ablation default setup in gray ). We find design choices for image and video components to be largely decoupled – this can be attributed to our modular design and training strategy. + +# A.2.1 CAPACITY ABLATIONS + +Input size. During training, we sample sequences of frames of fixed resolution and fixed length (here denoted by # frames). We ablate their impact in Tables 9a, 9b. A higher resolution leads to significant improvements across image and video tasks, and we use a spatial input resolution of $1 0 2 \bar { 4 } ^ { 2 }$ in our final model. Increasing the number of frames brings notable gains on video benchmarks and we use a default of 8 to balance speed and accuracy. + +Memory size. Increasing the (maximum) number of memories, $N$ , generally helps the performance although there could be some variance, as in Table ${ 9 \mathrm { c } }$ . We use a default value of 6 past frames to strike a balance between temporal context length and computational cost. Using fewer channels for memories does not cause much performance regression as in Table 9d, while making the memory required for storage $4 \times$ smaller. + +
J&FmIoU
res.MOSE devSA-V val9 zero-shotspeedSA-23
51273.068.370.71.00×59.7
76876.171.172.50.43×61.0
102477.070.1 (a) Resolution.72.30.22×61.5
J&FmIoU
#mem.MOSE devSA-V val9 zero-shotspeedSA-23
473.568.670.51.01×59.9
673.068.370.71.00×59.7
873.269.070.70.93×59.9
(c) #Memories. J&FmIoU
(#sa, #ca)MOSE dev SA-V val9 zero-shot speedSA-23
(2, 2)73.367.370.21.13×59.9
(3, 2)72.764.169.51.08×60.0
(4, 4)73.068.370.71.00×59.7
(e) Memory attention.
+ +
J&FmIoU
#frames OSE devSA-V val 9 zero-shotspeed SA-23
471.160.067.71.00×60.1
873.068.370.71.00×59.7
1074.568.171.11.00×59.9
(b) #Frames. J&FmIoU
chan. dim. MOSE devSA-V val 9 zero-shotspeedSA-23
6473.068.370.71.00×59.7
25673.466.470.00.92×60.0
(d) Memory channels. J &FmIoU
img. enc.MOSE devSA-V val9 zero-shotspeedSA-23
s70.965.569.41.33×57.8
B+73.068.370.71.00×59.7
L75.066.371.90.60×61.1
+ +# (f) Image encoder size. + +Table 9: We ablate modeling capacity along input size (resolution, #frames), memory size (#memories, memory channel dim) and model size (memory attention, image encoder). Ablation defaults in gray . + +Model size. More capacity in the image encoder or memory-attention (#self-/#cross-attention blocks) generally leads to improved results, as shown in Tables 9e, 9f. Scaling the image encoder brings gains on both image and video metrics, while scaling the memory-attention only improves video metrics. We default to using a $^ { \mathrm { B + } }$ image encoder, which provides a reasonable balance for speed and accuracy. + +A.2.2 RELATIVE POSITIONAL ENCODING + +
RPBJ&FspeedmIoU
2d-RoPE MOSE devSA-V valLVOSv2 val9 zero-shotSA-23
73.068.371.670.71.00×59.7
V73.667.971.071.50.93×60.0
72.867.170.370.31.04×59.9
+ +Table 10: We use 2d-RoPE positional encoding in memory attention while removing RPB from the image encoder by default ( gray ). Removing RPB also allows us to enable FlashAttention-2 (Dao, 2023), which gives a significant speed boost at $( 1 0 2 4 ^ { 2 } )$ resolution. At such higher resolution , the speed gap between 2d-RoPE (1st row) and the no RoPE baseline (3rd row) becomes small $(4 \% )$ . + +By default, we always use absolute positional encoding in both the image encoder as well as memory attention. In Table 10, we study relative positional encoding design choices. Here we also evaluate on LVOSv2 (Hong et al., 2024) with 3 clicks on the 1st frame as a benchmark for long-term video object segmentation. + +While SAM (Kirillov et al., 2023) follows Li et al. (2022c) in adding relative positional biases (RPB) to all image encoder layers, Bolya et al. (2023) improve upon this by removing RPB in all but the global attention layers while adopting “absolute-win” positional encoding which brings large speed gains. We improve upon this further by removing all RPB from the image encoder, with no performance regression on SA-23 and minimal regression on video benchmarks (see Table 10), while giving a significant speed boost at 1024 resolution. We also find it is beneficial to use 2d-RoPE (Su et al., 2021; Heo et al., 2024) in the memory attention. + +# A.2.3 MEMORY ARCHITECTURE ABLATIONS + +Recurrent memory. We investigate the effectiveness of feeding the memory features to a GRU before adding them to the memory bank. Similar to $\ S \mathrm { A } . 2 . 2$ , we also evaluate on LVOSv2 as an additional benchmark for long-term object segmentation. While prior works have commonly employed GRU (Cho et al., 2014) states as a means of incorporating memory into the tracking process, our findings in Table 11 suggest that this approach does not provide an improvement (except slightly on LVOSv2). Instead, we find it sufficient to directly store the memory features in the memory bank, which is both simpler and more efficient. + +Object pointers. We ablate the impact of cross-attending to the object pointer vectors from the mask decoder output in other frames (see $\ S 4$ ). The results presented in Table 11 show that while crossattending to object pointers does not enhance average performance across the 9 zero-shot datasets, it significantly boosts performance on SA-V val dataset as well as on the challenging LVOSv2 benchmark (validation split). Hence, we default to cross-attending to object pointers together with the memory bank embeddings from the memory encoder. + +
Object PointersGRUJ&FspeedmIoU
MOSE devSA-V valLVOSv2 val9 zero-shotSA-23
73.164.567.070.91.00×59.9
72.365.368.970.50.97×60.0
73.068.371.670.71.00×59.7
+ +Table 11: Ablations on memory design. We use object pointers by default ( gray ) and also study recurrent GRU memory. + +# B DETAILS ON THE PVS TASK + +The Promptable Visual Segmentation (PVS) task can be seen as an extension of the Segment Anything (SA) task from static images to videos. In the PVS setting, given an input video, the model can be interactively prompted with different types of inputs (including clicks, boxes, or masks) on any frame in the video, with the goal of segmenting (and tracking) a valid object throughout the video. When interacting with a video, the model provides an instant response on the frame being prompted (similar to the interactive experience of SAM on images), and also returns the segmentation of the object throughout the entire video in near real-time. Similar to SAM the focus is on valid objects which have a clearly defined boundary, and we do not consider regions without visual boundaries (e.g. Bekuzarov et al. (2023)). Fig. 7 illustrates the task. + +![](images/figures/sam2-fig-0007.jpg) +Figure 7: An illustration of the Promptable Visual Segmentation task (PVS). Previously studied tasks such as Segment Anything (SA) and semi-supervised Video Object Segmentation (VOS) can be seen as special cases of the PVS task. + +PVS is related to tasks in the image and video domains. For images, the SA task can be considered a subset of PVS with the video reduced to a single frame. Similarly, traditional semi-supervised and interactive VOS (Pont-Tuset et al., 2017) tasks are special cases of PVS, limited to mask prompts provided only on the first frame and scribbles on multiple frames to segment objects throughout a video, respectively. In PVS, prompts can either be clicks, masks, or boxes, and the focus is on enhancing the interactive experience, enabling refinement of a segmentation with minimal interaction. + +# C LIMITATIONS + +SAM 2 demonstrates strong performance in both static image and video domains, yet it encounters difficulties in certain scenarios. The model may fail to segment objects across shot changes and can lose track of or confuse objects in crowded scenes, after long occlusions or in extended videos. To alleviate this issue, we designed the ability to prompt SAM 2 in any frame: if the model loses the object or makes an error, refinement clicks on additional frames can quickly recover the correct prediction in most cases. SAM 2 also struggles with accurately tracking objects with very thin or fine details especially when they are fast-moving. Another challenging scenario occurs when there are nearby objects with similar appearance (e.g., multiple identical juggling balls). Incorporating more explicit motion modeling into SAM 2 could mitigate errors in such cases. + +While SAM 2 can track multiple objects in a video simultaneously, SAM 2 processes each object separately, utilizing only shared per-frame embeddings without inter-object communication. While this approach is simple, incorporating shared object-level contextual information could aid in improving efficiency. + +Our data engine relies on human annotators to verify masklet quality and select frames that require correction. Future developments could include automating this process to enhance efficiency. + +SAM 2 tackles the promptable visual segmentation task, and the capabilities of SAM 2 do not extend to semantic recognition tasks. We believe that the advances made by SAM 2 can serve as a foundation for future work in recognition for both images and videos, similar to how SAM or SA-1B dataset was extended in (Li et al., 2023a; Yuan et al., 2024). + +# D SAM 2 DETAILS + +# D.1 ARCHITECTURE + +Here we discuss further architecture details, expanding on the model description in $\ S 4$ + +Image encoder. We use a feature pyramid network (Lin et al., 2017) to fuse the stride 16 and 32 features from Stages 3 and 4 of the Hiera image encoder respectively to produce the image embeddings for each frame. In addition, the stride 4 and 8 features from Stages 1 and 2 are not used in the memory attention but are added to the upsampling layers in the mask decoder as shown in Figure 8, which helps produce high-resolution segmentation details. We follow Bolya et al. (2023) in using windowed absolute positional embeddings in the Hiera image encoder. In Bolya et al. (2023), RPB provided positional information spanning across windows in the image encoder, in lieu of which we adopt a simpler approach of interpolating the global positional embedding instead to span across windows. We do not use any relative positional encoding. We train models with varying image encoder sizes – T, S, $^ { \mathrm { B + } }$ and L. We follow Li et al. (2022c) and use global attention in only a subset of the image encoder layers (see Table 12). + +Memory attention. In addition to sinusoidal absolute positional embeddings, we use 2d spatial Rotary Positional Embedding (RoPE) (Su et al., 2021; Heo et al., 2024) in self-attention and crossattention layers. The object pointer tokens are excluded from RoPE as they do not have specific spatial correspondence. By default, the memory attention uses $L = 4$ layers. + +Prompt encoder and mask decoder. The prompt encoder design follows SAM, and we next discuss additional details on design changes in the mask decoder. We use the mask token corresponding to the output mask as the object pointer token for the frame, which is placed in the memory bank. As discussed in $\ S 4$ , we also introduce an occlusion prediction head. This is accomplished by including an additional token along with the mask and IoU output tokens. An additional MLP head is applied to this new token to produce a score indicating the likelihood of the object of interest being visible in the current frame (as shown in Figure 8). In the memory bank, we also add a learned occlusion embedding to the memory features of those frames that are predicted to be occluded (invisible) by the occlusion prediction head. + +SAM introduced the ability to output multiple valid masks when faced with ambiguity about the object being segmented in an image. For example, when a person clicks on the tire of a bike, the model can interpret this click as referring to only the tire or the entire bike and output multiple predictions. In videos, this ambiguity can extend across video frames. For example, if in one frame only the tire is visible, a click on the tire might relate to just the tire, or as more of the bike becomes visible in subsequent frames, this click could have been intended for the entire bike. To handle this ambiguity, SAM 2 predicts multiple masks at each step of the video. If further prompts do not resolve the ambiguity, the model selects the mask with the highest predicted IoU for the current frame for further propagation in the video. + +![](images/figures/sam2-fig-0008.jpg) +Figure 8: Mask decoder architecture. The design largely follows SAM, and we additionally include the stride 4 and 8 features from the image encoder during upsampling. We also use the mask token corresponding to the output mask as an object pointer and generate an occlusion score which indicates if the object of interest is visible in the current frame. + +Memory encoder and memory bank. Our memory encoder does not use an additional image encoder and instead reuses the image embeddings produced by the Hiera encoder, which are fused with the predicted mask information to produce memory features (as discussed in $\ S 4$ ). This design allows the memory features to benefit from the strong representations produced by the image encoder (especially when we scale the image encoder to a larger size). Further, we project the memory features in our memory bank to a dimension of 64, and split the 256-dim object pointer into 4 tokens of 64-dim for cross-attention to the memory bank. + +Handling multiple objects in a video. When applying SAM 2 to segment multiple objects in the same video (such as multi-object tracking in the semi-supervised VOS evaluation), we perform inference on each object independently. More specifically, we share the visual features from the image encoder between all the objects in the video but run all the other model components (such as the memory bank and the mask decoder) separately for each object. + +# D.2 TRAINING + +# D.2.1 PRE-TRAINING + +We first pre-train SAM 2 on static images on the SA-1B dataset. Table 12a details the settings used during pre-training on SA-1B – other settings not mentioned here follow Kirillov et al. (2023). The image encoder is initialized from MAE pre-trained Hiera (Ryali et al., 2023). Similar to SAM, we filter masks covering more than $90 \%$ of the image and restricted training to 64 randomly sampled masks per image. + +Unlike SAM, we found it beneficial to use an $\ell _ { 1 }$ loss to more aggressively supervise the IoU predictions and to apply a sigmoid activation to the IoU logits to restrict the output into the range between 0 and 1. For multi-mask predictions (on the first click), we supervise the IoU predictions of all masks to encourage better learning of when a mask might be bad, but only supervise the mask logits with the lowest segmentation loss (linear combination of focal and dice loss). In SAM, during iterative sampling of points, two iterations were inserted with no additional prompts (only feeding the previous mask logits) – we do not add such iterations during our training and use 7 correction clicks (instead of 8 in SAM). We also employ horizontal flip augmentation during training and resize the image to a square size of $1 0 2 4 \times 1 0 2 4$ . + +We use AdamW (Loshchilov & Hutter, 2019) and apply layer decay (Clark et al., 2020) on the image encoder and follow a reciprocal square-root schedule (Zhai et al., 2022). See Table 12 (a) for the hyperparameters in our pre-training stage. + +# D.2.2 FULL TRAINING + +After pre-training, we train SAM 2 on our introduced datasets SA- $\mathrm { \Delta V + \Omega }$ Internal (section $\ S 5 . 2 \AA ,$ ), a $10 \%$ subset of SA-1B, and a mixture of open-source video datasets including DAVIS (Pont-Tuset et al., 2017; Caelles et al., 2019), MOSE (Ding et al., 2023), and YouTubeVOS (Xu et al., 2018b). Our released model is trained on SA-V manual $^ +$ Internal and SA-1B. + +SAM 2 is designed for two tasks; the PVS task (on videos) and the SA task (on images). Training is done jointly on image and video data. To optimize our data usage and computational resources during training, we adopt an alternating training strategy between video data (multiple frames) and static images (one single frame). Specifically, in each training iteration, we sample a full batch either from the image or video dataset, with their sampling probabilities proportional to the size of each data source. This approach allows for a balanced exposure to both tasks and a different batch size for each data source to maximize compute utilization. Settings not explicitly mentioned here for the image task follow settings from the pre-training phase. See Table 12 (b) for the hyperparameters in our full training stage. The training data mixture consists of ${ \sim } 1 5 . 2 \%$ SA-1B, ${ \sim } 7 0 \%$ SA-V and ${ \sim } 1 4 . 8 \%$ Internal. The same settings are used when open-source datasets are included, with the change that the additional data is included $( \sim 1 . 3 \%$ DAVIS, ${ \sim } 9 . 4 \%$ MOSE, ${ \sim } 9 . 2 \%$ YouTubeVOS, ${ \sim } 1 5 . 5 \%$ SA-1B, ${ \sim } 4 9 . 5 \%$ SA-V, ${ \sim } 1 5 . 1 \%$ Internal). When training on SA-V and other video datasets, we only use those manually annotated masklets (without adding automatically generated ones), which are sufficient to achieve strong performance based on our analyses. + +We apply a series of data augmentations to the training videos (detailed in Table 12), including random horizontal flips, random affine transforms, random color jittering, and random grayscale transforms, as listed in Table 12. We also adopt a mosaic transform to simulate challenging scenarios with multiple similar-looking objects – with $10 \%$ probability, we tile the same training video into a $2 \times 2$ grid and select a masklet from one of the 4 quadrants as the target object to segment. In this case, the model must focus on other cues like motion or temporal continuity to distinguish the target object from their identical-looking counterparts in other quadrants. In addition, the videos and objects in each quadrant are smaller in size (only half the original width and height) after this mosaic transform, which also facilitates learning to segment small objects. + +We train by simulating an interactive setting, sampling 8-frame sequences and randomly selecting up to 2 frames (including the first) for corrective clicks. During training, we use ground-truth masklets and model predictions to sample prompts, with initial prompts being the ground-truth mask $5 0 \%$ probability), a positive click from the ground-truth mask $( 2 5 \% )$ , or a bounding box input $( 2 5 \% )$ . + +We restrict the maximum number of masklets for each sequence of 8 frames to 3 randomly chosen ones. We reverse the temporal order with a probability of $50 \%$ to help generalization to bi-directional propagation. When we sample corrective clicks, with a small probability of $10 \%$ , we randomly sample clicks from the ground truth mask, irrespective of the model prediction, to allow additional flexibility in mask refinement. + +Fine-tuning using 16-frame sequences. A potential shortcoming of the procedure above is that the model only sees sampled 8-frame sequences during training, which is relatively short compared to the full video length during inference. To alleviate this issue and further boost the segmentation quality on long videos, we introduce an extra fine-tuning stage where we sample 16-frame sequences on challenging videos (those videos with the highest number of edited frames, as described in $\ S \mathrm { E } . 2 . 1 $ . More specifically, we sort our masklets by number of edited frames and only consider the top $50 \%$ most edited masklets for training, for both SA-V and Internal datasets. We still keep the complete versions of the OSS datasets (DAVIS, MOSE, and YouTubeVOS) in the training mix. We fine-tune for $5 0 \mathrm { k }$ iterations (1/3 of the original schedule) using half of the original learning rate and freeze the image encoder to fit the 16-frame sequence into the 80 GB memory of A100 GPUs. + +
configvalue
dataSA-1B
steps~90k
resolution1024
precisionbfloat16
optimizerAdamW
optimizer momentumβ1, β2=0.9, 0.999
gradient clippingtype: l2, max: 0.1
weight decay0.1
learning rate (lr)4e-4
lr schedulereciprocal sqrt, timescale=1000
warmuplinear, 1k iters
cooldownlinear, 5k iters
layer-wise decay0.8 (T, S), 0.9 (B+), 0.925 (L)
augmentationhflip, resize to 1024 (square)
batch size256
drop path0.1 (T, S), 0.2 (B+), 0.3 (L)
mask losses (weight)focal (20), dice (1)
IoU loss (weight)l1(1)
max # masks per image64
# correction points7
global attn. blocks5-7-9 (T), 7-10-13 (S), 12-16-20 (B+), 23-33-43 (L)
+ +![](images/figures/sam2-fig-0009.jpg) +Table 12: Hyperparameters and details of SAM 2 pre-training and full training. Note that some settings vary with image encoder size (T, S, $\mathbf { B } +$ , L). + +Losses and optimization. We supervise the model’s predictions using a linear combination of focal and dice losses for the mask prediction, mean-absolute-error (MAE) loss for the IoU prediction, and cross-entropy loss for object prediction with a ratio of 20:1:1:1 respectively. + +As during pre-training, for multi-mask predictions, we only supervise the mask with the lowest segmentation loss. If the ground-truth does not contain a mask for a frame, we do not supervise any of the mask outputs (but always supervise the occlusion prediction head that predicts whether there should exist a mask in the frame). + +# D.3 SPEED BENCHMARKING + +We conduct all benchmarking experiments on a single A100 GPU using PyTorch 2.3.1 and CUDA 12.1, under automatic mixed precision with bfloat16. We compile the image encoder with torch.compile for all SAM 2 models and do the same for SAM and HQ-SAM for direct comparison on the SA task (Tables 5 and 16). The FPS measurements for the SA task were conducted using a batch size of 10 images, which was found to yield the highest FPS across all three model types. For video tasks, we use a batch size of 1 following the common protocol in video segmentation. + +# E DATA DETAILS + +# E.1 SA-V DATASET DETAILS + +Videos. Resolutions range from $2 4 0 \mathrm { p }$ to 4K with $1 , 4 0 1 \times 1 , 0 3 7$ on average. Duration ranges from 4 seconds to 2.3 minutes, with an average of 13.8 seconds, totaling 4.2M frames and 196 hours. + +Dataset diversity. As shown in Fig. 10, SA-V videos were recorded across 47 countries (Fig. 10b), by diverse participants (self-reported demographics in Fig. 10c). Fig. 10a shows a comparison of mask size distribution (normalized by video resolution) with DAVIS, MOSE, and YouTubeVOS. More than $8 8 \%$ of SA-V masks have a normalized mask area less than 0.1. + +Automatic masklets. Similar to the approach in Kirillov et al. (2023), automatic masklets are generated by prompting the model with regular grids. We prompt the model with a $3 2 \times 3 2$ grid on the first frame, and we use a $1 6 \times 1 6$ grid on 4 zoomed image crops of the first frame (derived from a $2 \times 2$ overlapped window) and a $4 \times 4$ grid on 16 zoomed image crops of the first frame (derived from a $4 \times 4$ overlapped window). We apply two post-processing steps across all frames. First, we remove tiny disconnected components with areas smaller than 200 pixels. Second, we fill in holes in segmentation masks if the area of the hole is less than 200 pixels. By combining automatically generated with manual masklets, we enhance the coverage of annotations in the SA-V dataset, see Fig. 9. + +![](images/figures/sam2-fig-0010.jpg) +Figure 9: Annotations overlaid on the first frame: (a) manual labels (ML) only, (b) with automatic labels (Auto). Automatic labels increase diversity and coverage. + +# E.1.1 FAIRNESS EVALUATION + +We evaluate SAM 2 for fairness across demographic groups. We collect annotations for the people category in the Ego-Exo4D (Grauman et al., 2023) dataset, which contains self-reported demographic information supplied by the subject of the video. We employ the same annotation setup as for SA-V val and test sets and apply this to 20-second clips from the third-person (exo) videos. We evaluate SAM 2 on this data using 1-, 3-clicks, and groundtruth mask on the first frame. + +
1-click3-clickmask
gender
male81.995.195.9
female75.194.195.2
age
18-2677.295.095.7
26-5076.794.795.8
50+81.495.196.2
+ +Table 13: Fairness evaluation of SAM 2 (under $\mathcal { T } \& \mathcal { F }$ metric) on protected demographic groups. + +![](images/figures/sam2-fig-0011.jpg) +Figure 10: Dataset distribution: (a) masklets size distribution (normalized by video resolution), (b) geographic diversity of the videos, and (c) self-reported demographics of the crowdworkers who recorded the videos. + +Table 13 shows the comparison in $\mathcal { I } \& \mathcal { F }$ accuracy of SAM 2 for segmenting people across gender and age. At 3 clicks and with ground-truth mask prompts there is minimal discrepancy. We manually inspect 1 click predictions, and find the model frequently predicts the mask for a part instead of the person. When limiting the comparison to clips where the person is correctly segmented, the gap in 1 click shrinks substantially ( $\mathcal { I } \& \mathcal { F }$ male 94.3, female 92.7), suggesting the discrepancy can be partially attributed to ambiguity in the prompt. + +In Appendix J, we provide model, data and annotation cards for SA-V. + +![](images/figures/sam2-fig-0012.jpg) +Figure 11: Example videos from the SA-V dataset with masklets overlaid (manual and automatic). Each masklet has a unique color, and each row represents frames from one video, with 1 second between them. + +# E.2 DATA ENGINE DETAILS + +# E.2.1 ANNOTATION PROTOCOL + +A diagram of the annotation protocol used in our data engine is shown in Fig. 12. The annotation task was separated into steps each carried out by a different annotator: Steps 1 and 2 focus on object selection, Steps 3 and 4 on masklet tracking, and Step 5 on quality verification. SAM 2 was deployed on GPU as an API and built into the annotation tool to enable interactive use. + +![](images/figures/sam2-fig-0013.jpg) +Figure 12: Annotation guideline overview. There are 3 main annotation tasks: masklet selection, masklet tracking, and masklet verification. Each task has a different set of annotators working on it. + +Compared to image segmentation annotation, large-scale video segmentation annotation presents unique challenges which require innovations in the annotation task design and protocol. To improve our model’s ability to “segment anything”, it was important to focus annotation on challenging objects where SAM 2 struggled. We leveraged our online model in the loop setup to enable this, requesting annotators to use SAM 2 interactively to identify failure modes and then correct them. + +We found the number of edited frames to be a proxy to the “challengingness” of an object as shown in Table 8. Therefore, we asked annotators to annotate objects that required at least 2 edited frames with SAM 2 in the loop. To focus annotation on less prominent and more challenging cases, annotators were presented with videos pre-filled with verified satisfactory automatic masklets and asked to find un-annotated challenging objects. We further decouple the object selection task from the annotation task: in the selection task annotators focus on choosing the challenging objects in one frame, while in the annotation task annotators are presented with a challenging target object and requested to annotate the masklet consistently throughout the video. + +# E.2.2 DATA ENGINE PHASE COMPARISON + +The comparison of data engine phases shown in Table 1 was conducted as a controlled experiment using 169 videos and 452 masklets. We ask three subsets of annotators to annotate the same set of objects with the annotation protocol from each phase. We categorize masklets into three buckets based on the mask area in the first frame (small: 1 to $3 2 ^ { 2 }$ , medium: $3 2 ^ { 2 }$ to $9 6 ^ { 2 }$ , and large: equal or greater than $9 6 ^ { 2 }$ ). Phase 1 data is used as the quality reference, due to the high quality masks from frame-by-frame manual annotation with SAM. + +# F DETAILS ON ZERO-SHOT TRANSFER EXPERIMENTS + +In this section, we describe further details of our zero-shot experiments (§6). Unless otherwise noted, the results reported in this section follow our default setup using Hiera- $\mathbf { \nabla \cdot B + }$ image encoder with a resolution of 1024 and trained on the full combination of datasets, i.e., SAM 2 (Hiera- $\mathbf { \nabla \cdot B + }$ ) in Table 6. + +# F.1 ZERO-SHOT VIDEO TASKS + +# F.1.1 VIDEO DATASET DETAILS + +We evaluate SAM 2 on a diverse benchmark of 17 zero-shot datasets: EndoVis 2018 (Allan et al., 2020) contains medical surgery videos with robotic instruments. ESD (Huang et al., 2023) contains videos from a robot manipulator camera often with motion blur. LVOSv2 (Hong et al., 2024) is a benchmark for long-term video object segmentation. LV-VIS (Wang et al., 2023) contains videos from a diverse set of open-vocabulary object categories. UVO (Wang et al., 2021b) contains videos for open-world object segmentation, and VOST (Tokmakov et al., 2022) contains videos with objects undergoing large transformations such as egg broken or paper torn. PUMaVOS (Bekuzarov et al., 2023) contains videos with segments around object parts such as a person’s cheek. Virtual KITTI 2 (Cabon et al., 2020) is a synthetic video dataset with driving scenes. VIPSeg (Miao et al., 2022) provides object segmentation in panoptic videos. Wildfires (Toulouse et al., 2017) contains wildfire videos under different conditions from the Corsican Fire Database. VISOR (Darkhalil et al., 2022) contains egocentric videos in kitchen scenes with segments around hands and active objects. FBMS (Brox et al., 2010) provides motion segmentation over moving objects in videos. Ego-Exo4D (Grauman et al., 2023) is a large dataset with egocentric videos around various human activities. Cityscapes (Cordts et al., 2016) contains videos for urban driving scenes. Lindenthal Camera (Haucke & Steinhage, 2021) contains videos in a wildlife park with segments around observed animals such as birds and mammals. HT1080WT Cells (Gómez-de Mariscal et al., 2021) contains microscopy videos with cell segments. Drosophila Heart (Fishman et al., 2023) contains microscopy videos for the heart of fruit flies. + +Among these 17 zero-shot video datasets above, 9 of them (EndoVis, ESD, LVOSv2, LV-VIS, UVO, VOST, PUMaVOS, Virtual KITTI 2, and VIPSeg) have dense object segments annotated for every video frame. In the remaining 8 datasets (Wildfires, VISOR, FBMS, Ego-Exo4D, Cityscapes, Lindenthal Camera, HT1080WT Cells, and Drosophila Heart), the object segments are sparsely annotated over only a subset of video frames, and we compute the metrics on those frames where the ground-truth segmentation masks are available. In most evaluations of the paper, we only evaluate zero-shot performance on the 9 densely annotated datasets, while in our semi-supervised VOS evaluation $( \ S 6 . 2 )$ , we evaluate on all these 17 datasets listed above. + +# .1.2 INTERACTIVE OFFLINE AND ONLINE EVALUATION DETAILS + +Offline evaluation involves multiple passes over the entire video. We start with click prompts on the first frame, segment the object throughout the entire video, and then in the next pass, we select the frame with the lowest segmentation IoU w.r.t. the ground-truth as the new frame for prompting. The model then segments the object again throughout the video based on all prompts received previously, until reaching a maximum of $N _ { \mathrm { f r a m e } }$ passes (with one new prompted frame in each pass). + +Online evaluation involves only one pass over the entire video. We start with click prompts on the first frame and propagate the prompts across the video, pausing propagation when encountering a frame with a low-quality prediction $\mathrm { ( I o U < 0 . 7 5 }$ with ground-truth). We then add additional click prompts on the paused frame to correct the segment on this frame and resume the propagation forward until reaching another low quality frame with IoU $< 0 . 7 5$ . This is repeated while the number of prompted frames is less than the maximum $N _ { \mathrm { f r a m e } }$ . Unlike the previous offline evaluation, in this setting, the new prompts only affect the frames after the current paused frame but not the frames before it. + +In both settings, we evaluate on 9 densely annotated datasets in $\ S \mathrm { F . 1 . 1 }$ (EndoVis, ESD, LVOSv2, LV-VIS, UVO, VOST, PUMaVOS, Virtual KITTI 2, and VIPSeg). If a video contains multiple objects to segment in its ground-truth annotations, we perform inference on each object independently. We simulate interactive video segmentation with $N _ { \mathrm { c l i c k } } = 3$ clicks per frame, assuming that the user would visually locate the object to label it (with initial clicks) or to refine the current segmentation prediction of it (with correction clicks). Specifically, when starting the first pass (where there are not any existing predictions yet), we place an initial click on the first frame at the center1 of the object’s ground-truth mask and then interactively add two more clicks based on the center of the error region (between the ground-truth mask and the predicted segments on the first frame). Then in subsequent passes (where there are already predicted segments), we interactively add three clicks based on the center of the error region (between the ground-truth mask and the predicted segments on the frame being prompted). + +We report the average $\mathcal { T } \& \mathcal { F }$ metric over $N _ { \mathrm { f r a m e } } = 1 , \ldots , 8$ interacted frames and the $\mathcal { T } \& \mathcal { F }$ metrics under different annotation time on a video based on the following assumptions: + +• On each frame, it takes $T _ { \mathrm { l o c } } = 1$ sec for the annotator to visually locate an object in the frame, and $T _ { \mathrm { c l i c k } } = 1 . 5$ sec to add each click, following Delatolas et al. (2024). • In offline mode, it takes $T _ { \mathrm { e x a m } } = 3 0$ sec on a 300-frame video to examine the results throughout the video in each round, including finding the frame with the worst segmentation quality to add corrections (and for longer or shorter videos, this time is proportional to the video length $L$ , assuming the annotator could examine the results at 10 FPS). • In online mode, it takes $T _ { \mathrm { e x a m } } ~ = ~ 3 0$ sec on a 300-frame video to follow the results throughout the video in total, including pausing at a frame with low quality for further corrections (and this time is proportional to the video length $L$ similar to the offline mode). • The annotation time for an object is $( T _ { \mathrm { e x a m } } \cdot ( L / 3 0 0 ) + T _ { \mathrm { l o c } } + T _ { \mathrm { c l i c k } } \cdot N _ { \mathrm { c l i c k } } ) \cdot N _ { \mathrm { f r a m e } }$ in offline mode and $T _ { \mathrm { e x a m } } \cdot ( L / 3 0 0 ) + ( T _ { \mathrm { l o c } } + T _ { \mathrm { c l i c k } } \cdot N _ { \mathrm { c l i c k } } ) \cdot N _ { \mathrm { f r a m e } }$ in online mode, where $L$ is the total frame number in the video, $N _ { \mathrm { f r a m e } } = 1 , \ldots , 8$ is the number of frames annotated (i.e., the number of interactive rounds), and $N _ { \mathrm { c l i c k } } = 3$ is the number of clicks per frame.2 + +We show per-dataset results of SAM 2 and the two baselines $\mathbf { \Delta S A M + X M e m + + }$ and SAM+Cutie, see their details below) for interactive offline and online evaluation in Fig. 13 and Fig. 14. SAM 2 outperforms both baselines with a notable margin on all datasets and settings. + +# F.1.3 SEMI-SUPERVISED VOS EVALUATION DETAILS + +In $\ S 6 . 2$ , we also compare with previous video tracking methods under the semi-supervised VOS setting (Pont-Tuset et al., 2017), where prompts (which can be foreground/background clicks, bounding boxes, or ground-truth object masks) are provided only on the first frame of the video. When using click prompts, we interactively sample either 1, 3 or 5 clicks on the first video frame, and then track the object based on these clicks. Following the click-based evaluation in prior work (Kirillov et al., 2023; Sofiiuk et al., 2022), the initial click is placed on the object center and subsequent clicks are obtained from the center of the error region. + +Similar to the interactive setting, here we also use $_ { \mathrm { S A M + X M e m + + } }$ and SAM+Cutie as two baselines. For click or box prompts, SAM is first used to handle click or bounding box inputs, and its output mask is then used as input to $\scriptstyle \mathrm { X M e m + + }$ or Cutie. For mask prompts, the ground-truth object masks on the first frame are directly used as input to ${ \mathrm { X M e m } } + +$ and Cutie – this is the standard semi-supervised VOS setting and evaluates $\scriptstyle \mathrm { X M e m + + }$ and Cutie without using SAM. + +![](images/figures/sam2-fig-0014.jpg) +(a) $\mathcal { I } \& \mathcal { F }$ performance on each dataset with different number of interacted frames (3-click) +Figure 13: Zero-shot performance of SAM 2 vs baselines $\mathrm { ( S A M + X M e m + + }$ and $\mathbf { S A M + C u t i e } )$ under interactive offline evaluation with different numbers of interacted frames, using 3 clicks per interacted frame. See $\ S \mathrm { F } . 1 . 2$ for details. + +
MethodEndoVis 2018ESDLVOSv2LV-VISPUMaVOSUVOVIPSegVirtual KITTI 2VOST (average)
SAM + XMem++68.988.272.186.460.274.5 84.263.846.671.7
SAM + Cutie71.887.682.187.159.475.2 84.470.354.374.7
SAM 277.090.287.990.368.579.2 88.374.167.580.3
+ +(b) average $\mathcal { I } \& \mathcal { F }$ on each dataset over 8 interacted frames (3-click) + +In this setting, we evaluate on all 17 zero-shot video datasets in $\ S \mathrm { F . 1 . 1 }$ . If a dataset does not follow the standard VOS format, we preprocess it into a format similar to MOSE (Ding et al., 2023). During processing, we ensure that all objects in each video have a valid non-empty segmentation mask on the first frame to be compatible with semi-supervised VOS evaluation. In case an object doesn’t appear in the first frame, we create a separate video for it starting from the first frame where the object appears. + +We report the standard $\mathcal { T } \& \mathcal { F }$ metric (Pont-Tuset et al., 2017) for this evaluation. If a dataset provides an official evaluation toolkit, we use it for evaluation (on the VOST dataset, we report the $\mathcal { I }$ metric instead, following its official protocol (Tokmakov et al., 2022)). The results are shown in Table 4, where SAM 2 performs better than both baselines on the majority of the 17 datasets across different types of prompts. + +We show per-dataset results of SAM 2 and the two baselines $\mathbf { \Delta S A M + X M e m + + }$ and SAM+Cutie, see their details below) for semi-supervised VOS evaluation in Fig. 15. SAM 2 outperforms both baselines on the majority of these datasets across different types of prompts. + +# F.1.4 SAM+XMEM $^ { + + }$ AND SAM $^ +$ CUTIE BASELINE DETAILS + +We adopt $_ { \mathrm { S A M + X M e m + + } }$ and SAM $+$ Cutie as two baselines for promptable video segmentation, where the click (or box) prompts are first processed by SAM to obtain an object mask, and then $\scriptstyle \mathrm { X M e m + + }$ Cutie models track this SAM mask across the video to obtain the final masklet. In these two baselines, SAM can be used to provide both an initial object mask on the first frame, or to correct an existing object mask output by $\scriptstyle \mathrm { X M e m + + }$ or Cutie. This is used for subsequent interacted frames during interactive offline and online evaluation, where new positive and negative clicks are provided as corrections over an existing mask. + +![](images/figures/sam2-fig-0015.jpg) +(a) $\mathcal { T } \& \mathcal { F }$ performance on each dataset with different number of interacted frames (3-click) +Figure 14: Zero-shot performance of SAM 2 vs baselines $\mathrm { ( S A M + X M e m + + }$ and $\mathbf { S A M + C u t i e } )$ under interactive online evaluation with different numbers of interacted frames, using 3 clicks per interacted frame. See $\ S \mathrm { F } . 1 . 2$ for details. + +
EndoVisVirtual
Method2018ESDLVOSv2LV-VISPUMaVOSUVOVIPSegKITTI 2VOST(average)
SAM + XMem++71.487.872.985.263.774.782.563.952.772.8
SAM + Cutie70.587.380.686.058.975.282.170.454.674.0
SAM 277.588.987.888.772.778.685.574.065.079.8
+ +(b) average $\mathcal { I } \& \mathcal { F }$ on each dataset over 8 interacted frames (3-click) + +When using SAM to apply a correction over an existing mask prediction in a given frame, we follow the strategy in EVA-VOS (Delatolas et al., 2024) to first initialize SAM with the $\scriptstyle \mathrm { X M e m + + }$ or Cutie output mask before incorporating the new correction clicks. Specifically, we first reconstruct the $\scriptstyle \mathrm { X M e m + + }$ or Cutie output mask in SAM by sampling clicks from them and feeding them as inputs to SAM until the reconstructed mask in SAM reaches $\mathrm { I o U } > 0 . 8$ with the ${ \mathrm { X M e m } } + +$ or Cutie output mask. Then, to incorporate new positive and negative clicks for correction, we concatenate these additional correction clicks with the initial clicks sampled during mask construction, and feed the joint concatenated list as input into SAM to obtain the final corrected masks. We find that this strategy works better than several alternatives (such as feeding the ${ \mathrm { X M e m } } + +$ or Cutie output mask as a mask prompt together with new correction clicks into SAM, or taking only the correction clicks as inputs to SAM while ignoring the ${ \mathrm { X M e m } } + +$ or Cutie output mask). + +# F.2 DAVIS INTERACTIVE BENCHMARK + +We also evaluate on the DAVIS interactive benchmark (Caelles et al., 2018), which resembles our interactive offline evaluation in $\ S 6 . 1$ , where in each round of interaction, the evaluation server would provide new annotations on frames with the worst segmentation performance. The official DAVIS eval toolkit provides scribble prompts during interactions, while other work such as CiVOS (Vujasinovic´ et al., 2022) has also extended this to cover click prompts. + +Here we follow CiVOS to use positive and negative clicks as input prompts and adopt the same strategy for click sampling. We report the ${ \mathcal { I } } \& { { \mathcal { F } } @ 6 0 \mathrm { s } }$ and AUC- $\mathcal { T } \& \mathcal { F }$ metrics on this benchmark as provided by its evaluator, and compare to two baselines: MiVOS (Cheng et al., 2021b), which directly uses the provided scribbles via a scribble-to-mask module (and is also extended to click prompts in Vujasinovic et al. (2022)), and CiVOS, which samples click from the provided scribbles. ´ + +![](images/figures/sam2-fig-0016.jpg) +Figure 15: Zero-shot performance on 17 video datasets of SAM 2 vs two baselines $\mathrm { ( S A M + X M e m + + }$ and SAM+Cutie) under semi-supervised VOS evaluation using different prompts (1, 3 or 5 clicks, bounding boxes, or ground-truth masks on the first video frame), with the averaged performance across datasets for each type of prompt shown in Table 4 in the main text. See $\ S \mathrm { F } . 1 . 3$ for details. + +Table 14: Performance of SAM 2 and other models on the DAVIS interactive benchmark. For SAM 2, we use clicks as inputs following the click sampling strategy from CiVOS (Vujasinovic et al., 2022). ´ See $\ S \mathrm { F } . 2$ for details ( $\ddagger$ : performance reported in Vujasinovic et al. (2022)).´ + +
Methodinput typeAUC-J&FJ&F@60s
MA-Net (Miao et al., 2020)scribbles0.790.79
MiVOS (Cheng et al., 2021b)scribbles0.870.88
MiVOS (Cheng et al., 2021b)‡clicks0.750.75
CiVOS (Vujasinovi et al., 2022)clicks0.830.84
SAM 2clicks0.860.90
+ +The results are shown in Table 14, where SAM 2 (based on click inputs) outperforms both baselines under click inputs. We note that SAM 2 often tends to segment object parts (e.g. a person’s arm) on the first click while the DAVIS dataset mainly contains whole objects (e.g. an entire person), which could penalize SAM 2’s $\mathcal { I } \& \mathcal { F }$ performance on this benchmark. + +# F.3 VIPOSEG BENCHMARK + +We further evaluate SAM 2 on the VIPOSeg benchmark (Xu et al., 2023), a large-scale benchmark for video object segmentation on panoptic wild scenes. We find that SAM 2 has strong performance on this benchmark, as shown in Table 15. + +• In a zero-shot manner without training on this dataset, SAM 2 achieves $\mathcal { G } = 7 8 . 4$ on the VIPOSeg val split (where $\mathcal { G }$ is the overall metric defined in this benchmark, higher is better), which already outperforms the $\mathcal { G } = 7 8 . 2$ performance of the PAOT model (the best model in Table 2 of $\mathrm { X u }$ et al. (2023) trained on this dataset). + +• When fine-tuned on the VIPOSeg training split, SAM 2’s performance is further improved to $\mathcal { G } = 7 9 . 7$ on the VIPOSeg val split. + +• In addition, we find that SAM 2 is more robust in crowded scenes as measured by the decay metric defined in the VIPOSeg benchmark (lower is better; reflecting how fast the model’s performance declines with more objects). On the VIPOSeg val split, SAM 2 has $\lambda = 0 . 6 8$ decay (without fine-tuning on VIPOSeg) and $\lambda = 0 . 6 7$ decay (when fine-tuned on the VIPOSeg training split), both outperforming the $\lambda = 0 . 7 0$ decay of the PAOT model. + +
Methodtraining dataGλ (lower is better)
PAOT (Xu et al., 2023)VIPOSeg + YouTube-VOS + DAVIS77.90.73
PAOT (Xu et al., 2023)VIPOSeg78.20.70
SAM 2 (zero-shot)our mix78.40.68
SAM 2 (fine-tuned on VIPOSeg)our mix + VIPOSeg79.70.67
+ +Table 15: Performance of SAM 2 on the VIPOSeg benchmark. SAM 2 outperforms PAOT (the best model in Xu et al. (2023) trained on this dataset) under both zero-shot and fine-tuned settings. + +F.4 ZERO-SHOT IMAGE TASKS + +# F.4.1 DATASET DETAILS + +For the interactive segmentation task, we evaluated SAM 2 on a comprehensive suite of 37 datasets. This suite includes the 23 datasets previously used by SAM for zero-shot evaluation. For completeness, we list the 23 datasets: LVIS (Gupta et al., 2019), ADE20K (Zhou et al., 2019), Hypersim (Roberts et al., 2021), Cityscapes (Cordts et al., 2016), BBBC038v1 (Caicedo et al., 2019), DOORS (Pugliatti & Topputo, 2022), DRAM (Cohen et al., 2022), EgoHOS (Zhang et al., 2022), GTEA (Fathi et al., 2011; Li et al., 2015), iShape (Yang et al., 2021a), NDD20 (Trotter et al., 2020), NDISPark (Ciampi et al., 2021; 2022), OVIS (Qi et al., 2022), PPDLS (Minervini et al., 2016), Plittersdorf (Haucke et al., 2022), STREETS (Snyder & Do, 2019), TimberSeg (Fortin et al., 2022), TrashCan (Hong et al., 2020), VISOR (Darkhalil et al., 2022; Damen et al., 2022), WoodScape (Yogamani et al., 2019), PIDRay (Wang et al., 2021a), ZeroWaste-f (Bashkirova et al., 2022), and IBD (Chen et al., 2022). For more detailed information about these datasets, we refer the reader to Kirillov et al. (2023). In addition to these 23 datasets, we evaluated on frames sampled from 14 video datasets to assess SAM 2’s performance on images from the video domain. The video datasets used are listed as follows: Lindenthal Camera Traps (LCT) (Haucke & Steinhage, 2021), VOST (Tokmakov et al., 2022), LV-VIS (Wang et al., 2023), FBMS (Brox et al., 2010), Virtual KITTI 2 (Cabon et al., 2020), Corsican Fire Database (CFD) (Toulouse et al., 2017), VIPSeg (Miao et al., 2022), Drosophila Heart OCM (DH OCM) (Fishman et al., 2023), EndoVis 2018 (Allan et al., 2020), ESD (Huang et al., 2023), UVO (Wang et al., 2021b), Ego-Exo4d (Grauman et al., 2023), LVOSv2 (Hong et al., 2024), and HT1080WT (Gómez-de Mariscal et al., 2021). Table 18 has a more detailed description of these datasets. (Some of these datasets are obtained from the same data source as the zero-shot video datasets in $\ S \mathrm { F . 1 . 1 . }$ ) + +# F.4.2 DETAILED ZERO-SHOT EXPERIMENTS + +In this section, we include a more detailed version of the experiments in $\ S 6 . 3$ . We compare SAM 2 to SAM and HQ-SAM with different model sizes in Table 16. The main metrics we use for evaluation are the 1- and 5-click mIoU and we categorize the results by the dataset domain. + +Table 16 first shows a comparison of the models trained only on images (for the SA task) with different image encoder sizes on both the SA-23 benchmark as well as the 14 newly introduced video datasets. SAM 2 (Hiera- $\cdot \mathbf { B } +$ ) trained only on SA-1B outperforms SAM (ViT-H) on 1-click accuracy, and both SAM (ViT-H) and HQ-SAM (ViT-H) on 5-click accuracy while being 6x faster. SAM 2 (Hiera-L) further improves the 1-click accuracy by 1 point on average, but trading off speed. Despite being slower than Hiera- $\mathbf { B } +$ , it is still $3 . 4 \mathrm { x }$ faster than SAM (ViT-H) and $1 . 5 \mathrm { x }$ faster than SAM (ViT-B). + +1 (5) click mIoU + +
ModelDataSA-23 AllSA-23 ImageSA-23 Video14 new VideoFPS
SAM (ViT-B)SA-1B55.9 (80.9)57.4 (81.3)54.0 (80.4)54.5 (82.6)76.7
SAM (ViT-H)SA-1B58.1 (81.3)60.8 (82.1)54.5 (80.3)59.1 (83.4)21.7
HQ-SAM (ViT-B)HQSEG-44k53.9 (72.1)56.3 (73.9)50.7 (69.9)54.5 (75.0)73.5
HQ-SAM (ViT-H)HQSEG-44k59.1 (79.8)61.8 (80.5)55.7 (78.9)58.9 (81.6)21.4
SAM 2 (Hiera-B+)SA-1B58.9 (81.7)60.8 (82.1)56.4 (81.2)56.6 (83.7)130.1
SAM 2 (Hiera-L)SA-1B60.0 (81.8)62.0 (82.2)57.4 (81.2)58.5 (83.8)61.4
SAM 2 (Hiera-B+)our mix61.9 (83.5)63.3 (83.8)60.1 (83.2)69.6 (85.8)130.1
SAM 2 (Hiera-L)our mix63.6 (83.5)64.7 (83.7)62.2 (83.2)71.1 (85.7)61.4
+ +Table 16: Zero-shot performance on the Segment Anything (SA) task across a suite of 37 datasets. The table shows the average 1- and 5- click mIoU of SAM 2 compared to two baselines, categorized by dataset domain. We report the average metrics on the 23 datasets used by SAM for zero-shot evaluation, as well as the average across 14 newly introduced zero-shot video benchmarks. + +![](images/figures/sam2-fig-0017.jpg) +Figure 16: Zero-shot performance of SAM 2 vs SAM on a suite of 37 datasets. The figure shows the center 1 click mIoU delta between SAM 2 and SAM. Datasets derived from video distribution are highlighted in red, while those from image distribution are highlighted in blue. + +The last two rows in Table 16 illustrate the benefits of training with our mix of image and video data, which boosts the average accuracy to $6 1 . 4 \%$ across the 23 datasets with the Hirea- $\mathbf { \nabla \cdot B + }$ image encoder. Additionally, we observe substantial improvements on the video benchmarks of SA-23 as well as the 14 newly introduced video datasets. We note that we do not scale beyond Hiera-L, but expect better performance for a larger model. + +A breakdown of the accuracy across datasets is presented in Fig. 16, where the per-dataset delta in 1-click mIoU relative to SAM is color-coded to indicate the data type (image or video). Notably, SAM 2 (Hiera- $\cdot \mathbf { B } +$ ) surpasses SAM on 29 datasets3 by up to 53.9 mIoU, despite using a smaller Hiera- $\mathbf { \cdot B + }$ image encoder. + +# G APPLICATION OF SA-V DATA TO OTHER MODELS + +The segment anything in images and videos capability of SAM 2 can be attributed to both its model design as well as its large-scale, high-quality training data, including SA-1B and SA-V. + +Note that the SA-V dataset was collected in a model-in-the-loop fashion, in order to improve SAM 2 and correct its failure cases, while at the same time enabling flexible annotation for targeted data collection, including full objects and parts. This iterative process led to improvements in both the model and the data. + +In this section, we aim to evaluate if the SA-V data can also benefit existing models, beyond SAM 2. To evaluate this, we use SA-V to train a recent semi-supervised VOS model, Cutie (Cheng et al., 2023a), which SA-V was not collected for, to assess SA-V’s generalization capabilities. + +To assess the generality of SA-V, we compare the performance of Cutie models trained with and without it, both for the existing VOS task with mask input and for interactive video segmentation with click or bounding box input (identical to the setup in $\ S 6 . 2$ ). We report results in Table 17. + +
Methodmask5-clickboxmask
SA-V testMOSE valDAVIS 2017 testLVOS val17 zero-shot datasets (§6.2)
Cutie62.268.385.363.571.768.673.5
Cutie + SA-V68.767.684.565.073.470.475.5
SAM 278.477.987.778.077.674.479.3
+ +Table 17: Our SA-V data has a broader utility beyond SAM 2 and helps zero-shot generalization in other models. Including SA-V in the training mix improves the performance of Cutie on SA-V test and provides zero-shot gains on LVOS and 17 zero-shot datasets. The final performance of Cutie $^ +$ SA-V still falls short of the full SAM 2 shown in the last row. + +As a baseline, the first row of Table 17 shows the $\mathcal { T } \& \mathcal { F }$ accuracy of the official Cutie model trained on DAVIS, YouTubeVOS, and MOSE (referred to as “w/ MOSE” in Cheng et al. (2023a)) as evaluated by us on SA-V test and our zero-shot benchmark of 17 datasets (using 5 clicks or bounding box as input to SAM+Cutie or ground-truth mask input to Cutie; see $\ S \mathrm { F } . 1 . 3$ for details). Performance on MOSE, DAVIS, and LVOS benchmarks is from officially reported numbers. + +In the second row of Table 17, we train another Cutie model on our data mixture consisting of SA-V, DAVIS, YouTube-VOS, and MOSE. We use the same data mixture ratio as for SAM 2 and follow the official Cutie training recipe. Including SA-V in the training mix brings a large performance improvement on the SA-V benchmark. Although integrating SA-V into the training mix does not improve the performance on MOSE and DAVIS in-domain benchmarks, it boosts the zero-shot performance as shown by gains of $+ 1 . 5$ on LVOS and $+ 2 . 0$ on our 17 zero-shot benchmark suite under mask input, and similar boost for the out-of-domain click and box prompts. + +Finally, the last row shows the accuracy of SAM 2, which significantly outperforms the Cutie models under both training settings, suggesting that the SAM 2 architecture design is another crucial aspect of the final performance. + +# H DETAILS ON COMPARISON TO STATE-OF-THE-ART IN SEMI-SUPERVISED VOS + +We provide additional details on the comparison to the previous state-of-the-art in semi-supervised VOS (§7). We include results from SAM 2 trained only on SA-1B, SA-V and Internal data, for different encoder sizes. + +Qualitative comparison: In Fig. 17, we show a comparison between our baseline (Cutie-base $^ +$ , top row) and our model (SAM 2, bottom row) when prompted with a mask in the first frame. While the mask prompt in the first frame only covers the shirt of the person, the masklet predicted by the baseline wrongfully propagates to the whole person. Our model, however, is able to restrict the masklet to the target object. + +![](images/figures/sam2-fig-0018.jpg) +Figure 17: Comparison between our baseline (Cutie-base $^ +$ , top row) and our model (SAM 2, bottom row) when prompted with a mask in the first frame. + +Table 18: Video segmentation datasets used for zero-shot evaluation. + +
datasetabbreviation & linkvideo typedescriptionannotation typesource splitsampled# videos # masklets # frames sampledsampled# masks sampled
LV-VIS (Wang et al., 2023)LV-VISOpen VocabularyLarge scale open vocabulary video instance segmentationDenseValidation690253615,60454,077
A Drosophila heart optical coherence microscopy database for automatic video segmentation (Fishman et al., 2023)DH OCMMicroscopy; heartSegmentation of a fruit fy heart in optical coherence microscopy videosSparseAll213213608,000607,158
Video Object Segmentation under Transformations (Tokmakov et al., 2022)VOSTDeformationVideo object segmentation with emphasis on shape transformationsDenseValidation63588267,00489,722
Cityscapes-VPS (Cordts et al., 2016; Kim et al., 2020b)Cityscapes-VPSDrivingPanoptic segmentation for Cityscapes driving datasetSparseValidation Instance20913724,2594,579
Corsican Fire Database (Toulouse et al., 2017)CFDWildfireSegmentation of wildfiresSparseAll541541
Partial and Unusual Masks for Video Object Segmentation (Bekuzarov et al., 2023)PUMaVOSPartsVideo object segmentation with a focus on parts and practical use casesDenseAll2421,18721,485
EPIC-KITCHENS VISOR (Darkhalil et al., 2022)VISOREgocentricVideo object segmentation benchmark cain oi i o with an emphasis on segmenting active objects.SparseValidation921921736,0304,426
HT1080WT cells embedded in 3D collagen type I matrices (Gómez-de Mariscal et al., 2021)HT1080WTMicroscopy, cellsTimelapse videos of HT1080WT cell movementSparse601501,0102,694
Freiburg-Berkeley Motion Segmentation Dataset (Brox et al., 2010)FBMSMoving ObjectPrecise segmentation of moving objectsSparseAll459734755
Virtual KITI2 (Cabon et al., 2020)Virtual KITTI 2Synthetic; DrivingSynthetic driving videos generated by a game engine that recreate real world KITTvideos.DenseAll from video angle Camera_09961,638109,368162,708
EndoVis 2018 (Allan et al., 2020)EndoVis 2018Endoscopic video; surgerySegmentation of medical tools in endoscopic videosDenseAll15292,3254,314
Lindenthal Camera Traps (Haucke & Steinhage, 2021)LCTStereoWildlife videos captured using stereo cameras.SparseAll12124,012412
LVOSv2 (Hong et al., 2024)LVOSv2Long videosLong-term video object segmentation benchmark, on average 1.14 minutesDenseValidation13622564,52391,510
UVO (Wang et al., 2021b)UVOOpen WorldOpen World instance segmentation of all objects in a videoDenseValidation543114,86026,747
EgoExo4d (Grauman et al., 2023EgoExo4dEgocentricEgocentric videos of participants completing skilled activities.SparseValidation videos on egocentric cameras11851185327,0809,035
VIPSeg (Miao et al., 2022)VIPSegPanopticLarge scale and real world scenarios for video panoptic segmentationDenseValidation1521,4573,41630,408
Event-based Segmentation Dataset (Huang et al., 2023)ESDClutterTabletop object segmentation in an indoor cluttered environmentDenseAll13581413,32578,243
+ +Quantitative comparison: In Table 19, we compare the performance of our model to previous approaches on additional semi-supervised VOS metrics. SAM 2 outperforms prior work on all evaluated benchmarks, in all metrics. Note that unlike these previous approaches, SAM 2 is not specialized in the semi-supervised VOS task but is capable of more general promptable segmentation. SAM 2 is also not restricted to a specific set of object classes. The performance of our model on the SA-V benchmark (Table 19a) demonstrates its capability to segment anything in a video. + +# I OTHER RELATED WORK + +We mention other related works that tackle tasks which are not our primary focus, but are highly relevant as these share similarities with our work. + +Video Instance Segmentation (VIS). The VIS (Yang et al., 2019) task focuses on simultaneous detection, segmentation and tracking of instances in videos. Some notable works in VIS include the use of transformer based architectures (Wang et al., 2021c; Li et al., 2023b; Lee et al., 2024). Another line of work (Zhang et al., 2023c;d) focuses on decoupling the VIS task into three sub-tasks: segmentation, tracking, and refinement. + +Video Panoptic Segmentation (VPS). The VPS (Kim et al., 2020a) task requires segmenting both things and stuff as well as associating instances across frames of the video. Recent works (Shin et al., 2024; Li et al., 2022b; 2024) aim to tackle this task, and they have shown promising results on benchmark datasets (Miao et al., 2022). + +![](images/figures/sam2-fig-0019.jpg) +Figure 18: Examples from SAM 2 zero-shot video benchmark suite. + +
FSA-V valSA-V test
MethodJ&F JJ&FJ F
STCN (Cheng et al., 2021a)61.0 57.4 64.562.559.0 66.0
SwinB-AOT-L (Yang et al., 2021b)51.1 46.4 55.750.346.0 54.6
SwinB-DeAOT-L (Yang & Yang, 2022)61.4 56.666.261.857.2 66.3
RDE (Li et al., 2022a)51.8 48.4 55.253.950.5 57.3
XMem (Cheng & Schwing, 2022)60.1 56.3 63.962.358.9 65.8
SimVOS-B (Wu et al., 2023b)44.2 40.0 48.344.140.5 47.7
DEVA (Cheng et al., 2023b)55.4 51.5 59.256.252.4 60.1
Cutie-base (Cheng et al., 2023a)60.7 57.7 63.762.759.7 65.7
Cutie-base+ (Cheng et al., 2023a)61.3 58.3 64.462.859.8 65.8
SAM 2 (Hiera-B+)76.8 73.4 80.177.073.5 80.5
SAM 2 (Hiera-L)77.9 74.5 81.378.474.9 82.0
SAM 2 (Hiera-T)‡75.2 71.7 78.776.572.9 80.1
SAM 2 (Hiera-S)‡77.0 73.6 80.576.673.0 80.2
SAM 2 (Hiera-B+)‡77.5 74.1 80.978.274.8 81.7
SAM 2 (Hiera-L)‡78.6 75.3 82.079.576.0 83.0
+ +(a) Comparisons between SAM 2 and previous work on our SA-V benchmark for the semi-supervised VOS task. We evaluated prior works on SA-V using their open-sourced code and checkpoints. + +
LVOS val
MethodJ&FJ F
DEVA (Cheng et al., 2023b)55.951.1 60.7
DDMemory (Hong et al., 2023)60.755.066.3
Cutie-base (Cheng et al., 2023a)66.061.3 70.6
SAM 2 (Hiera-B+)78.073.2 82.7
SAM 2 (Hiera-L)78.073.2 82.7
SAM 2 (Hiera-T)‡77.573.0 82.1
SAM 2 (Hiera-S)‡77.372.3 82.2
SAM 2 (Hiera-B+)‡77.773.1 82.4
SAM 2 (Hiera-L)‡80.175.4 84.9
+ +LVOSv2 val + +
MethodJ&FJs $Fs$Ju Fu
STCN (Cheng et al., 2021a)60.657.264.0 57.5 63.8
RDE (Li et al., 2022a)62.256.7 64.160.8 67.2
SwinB-DeAOT-L (Yang & Yang, 2022) XMem (Cheng & Schwing, 2022)63.961.5 69.058.466.6
SAM 2 (Hiera-B+)64.5 78.780.6 87.262.6 69.1 60.665.6 69.0 77.8
SAM 2 (Hiera-L)79.670.4 79.8
SAM 2 (Hiera-T)‡77.381.0 87.4 79.9
SAM 2 (Hiera-S)‡78.386.9 79.2 85.967.1 75.4 69.079.0
SAM 2 (Hiera-B+)‡78.280.5 87.268.1 76.9
SAM 2 (Hiera-L)‡
80.6 81.788.271.4 81.0
+ +(b) Comparisons between SAM 2 and previous work on the LVOS (Hong et al., 2023) benchmark. + +(c) Comparisons between SAM 2 and previous work on the LVOSv2 (Hong et al., 2024) benchmark. We report the performance of prior works as evaluated by the LVOSv2 authors. + +
MethodMOSE valDAVIS17 valDAVIS17 testYTVOS19 val
J&F J FJ&F J FJ&F J FG$Js$Fs$Ju Fu
STCN (Cheng et al., 2021a)52.5 48.5 56.685.482.288.676.1 72.7 79.682.7 81.1 85.478.2 85.9
SwinB-AOT-L (Yang et al., 2021b)59.455.5 63.285.482.488.481.277.385.184.584.088.878.4 86.7
SwinB-DeAOT-L (Yang & Yang, 2022)59.955.7 64.0|86.283.189.282.8 78.986.786.185.390.280.4 88.6
RDE (Li et al., 2022a)46.8 42.4 51.384.2 80.8 87.577.4 73.681.281.9 81.1 85.5 76.2 84.8
XMem (Cheng & Schwing, 2022)59.6 55.4 63.786.082.8 89.279.676.1 83.085.6 84.1 88.581.0 88.9
SimVOS-B (Wu et al., 2023b)---88.085.091.080.476.184.684.2 83.179.1 --
JointFormer (Zhang et al., 2023b)-90.187.0 93.288.184.791.687.486.5 90.982.0 90.3
ISVOs (Wang et al., 2022)88.284.591.984.080.187.886.3 85.2 89.781.0 89.1
DEVA (Cheng et al., 2023b)66.0 61.8 70.387.083.8 90.282.678.9 86.485.484.9 89.479.6 87.8
Cutie-base (Cheng et al., 2023a)69.965.8 74.187.984.6 91.186.182.4 89.987.086.0 90.582.089.6
Cutie-base+ (Cheng et al., 2023a)71.767.6 75.888.185.590.888.184.7 91.487.586.3 90.6 82.7 90.5
SAM 2 (Hiera-B+)76.672.6 80.690.287.0 93.487.984.7 91.188.6 87.1 91.6 83.9 91.9
SAM 2 (Hiera-L)77.973.9 81.990.787.5 94.087.784.690.989.3 87.5 92.0 84.8 92.8
SAM 2 (Hiera-T)‡71.867.4 76.189.485.892.986.983.4 90.387.4 85.7 90.1 82.7 91.1
SAM 2 (Hiera-S)‡73.569.2 77.789.686.3 92.987.684.1 91.188.085.990.283.9 91.9
SAM 2 (Hiera-B+)‡73.869.7 77.990.086.8 93.186.683.389.888.286.190.684.0 92.1
SAM 2 (Hiera-L)‡74.670.6 78.690.286.9 93.488.9 85.3 92.588.886.5 91.084.7 92.8
+ +(d) Comparisons between SAM 2 and previous work on the semi-supervised VOS task. + +Table 19: Detailed comparisons between SAM 2 and previous work on various benchmarks $^ { \ast }$ : a version of the model trained on SA-1B, SA-V, and our internal dataset as described in $\ S 5 . 2 )$ ). + +# J MODEL, DATA AND ANNOTATION CARDS + +J.1 MODEL CARD + +# Model Overview + +Name SAM 2 (Segment Anything Model 2) Version 1.0 Date 2024 +Organization Meta FAIR +Mode type Promptable segmentation model +Architecture See Section 4 +Repository https://github.com/facebookresearch/sam2 License Apache 2.0 + +# Intended Use + +Primary intended users + +SAM 2 was designed as a unified model for promptable video and image segmentation tasks. +The model was primarily developed for research use cases. SAM 2 is released under an Apache 2.0 license. +See Ethical considerations and license for restrictions. +See Appendix C for limitations. + +Out-of-scope use cases Caveats and recommendations + +# Relevant Factors + +Groups SAM 2 is class agnostic and was designed for promptable image and video segmentation. It can segment and track any object. Instrumentation and environment SAM 2 was evaluated across a variety of types of video and image data. The video benchmark suite included domains such as driving data, microscopy, egocentric video, robotic surgery. See Table 18 for descriptions of the benchmarks and Figure 18 for example frames. SAM 2 was evaluated on the same suite of image benchmarks as Kirillov et al. (2023), which covers domains including underwater images, paintings, fish-eye images. + +# Metrics + +We evaluate the performance of SAM 2 using the following metrics: + +$\mathcal { T } \& \mathcal { F }$ : We evaluate performance using $\mathcal { T } \& \mathcal { F }$ (Pont-Tuset et al., 2017) for the promptable video segmentation and semi-supervised VOS tasks. $\mathcal { G }$ : We use $\mathcal { G }$ for evaluation on YTVOS 2019 for the semi-supervised VOS task. mIoU: We evaluate performance using mIoU for the promptable image segmentation task. + +# Evaluation Data + +Data sources See Appendix F + +# Training Data + +Data source + +SAM 2 was trained on the SA-V dataset alongside internally available licensed video data. See Section 5 of the main text for more details and Appendix J.2 for the SA-V dataset data card. + +# Ethical Considerations + +Data See Section 5 for more details about the SAM 2 training data. In Section E.1 we show a geographic distribution of the videos and demographic distribution of the crowdworkers who collected the videos in the SA-V dataset. Cost and impact of compute The released SAM 2 was trained on 256 A100 GPUs for 108 hours. This corresponds to 12165.12 kWH and an estimated emissions of 3.89 metric tons of CO2e (Patterson et al., 2021; Lacoste et al., 2019). The emissions from training the released SAM 2 are equivalent to $\mathrm { \sim } 1 0 \mathrm { k }$ miles driven by an average gasoline-powered passenger vehicle (Agency, 2022). Risks and harms In Section E.1.1 of the main text we analyze SAM 2 performance on people across demographic groups. When using SAM 2 in new settings, we suggest that researchers perform their own fairness evaluation for SAM 2 specific to their use case. Use cases We implore users to use their best judgement. + +Table 20: Model card for SAM 2 following the structure in Mitchell et al. (2019) + +# J.2 DATASET CARD FOR SA-V DATASET + +# Motivation + +1. For what purpose was the dataset created? Was there a specific task in mind? Was there a specific gap that needed to be filled? Please provide a description. The dataset was designed for the PVS task. The contributions of our dataset to the vision community are: (1) The dataset, composed of 50.9K videos and 642.6K masklets, is the largest video segmentation dataset publicly available today (see 5.2 for comparisons to current VOS datasets) (2) The dataset is available under a Creative Commons Attribution 4.0 International Public License at https://ai.meta.com/datasets/segment-anything-video/, (3) The data is a more geographically diverse, publicly available, video segmentation dataset than its predecessors. + +2. Who created the dataset (e.g., which team, research group) and on behalf of which entity (e.g., company, institution, organization)? The dataset was created by Meta FAIR. The underlying videos were collected via a contracted third party company. + +3. Who funded the creation of the dataset? The dataset was funded by Meta FAIR. + +4. Any other comments? No. + +# Composition + +1. What do the instances that comprise the dataset represent (e.g., documents, photos, people, countries)? Are there multiple types of instances (e.g., movies, users, and ratings; people and interactions between them; nodes and edges)? Please provide a description. All of the instances in the dataset are videos. Subject matter diversity was encouraged and no specific themes were applied during video collection. Common themes of the video include: locations, objects, scenes. All the videos are distinct, however there are some sets of videos that were taken of the same subject matter. 2. How many instances are there in total (of each type, if appropriate)? There are 50.9K videos. 3. Does the dataset contain all possible instances or is it a sample (not necessarily random) of instances from a larger set? If the dataset is a sample, then what is the larger set? Is the sample representative of the larger set (e.g., geographic coverage)? If so, please describe how this representativeness was validated/verified. If it is not representative of the larger set, please describe why not (e.g., to cover a more diverse range of instances, because instances were withheld or unavailable). While the dataset contains all possible instances, reviewers were advised to refuse to annotate content containing explicit imagery. 4. What data does each instance consist of? “Raw” data (e.g., unprocessed text or images) or features? In either case, please provide a description. Each instance is a video. 5. Is there a label or target associated with each instance? If so, please provide a description. Each video is annotated with masklets that track objects throughout the video. There are no categories or text associated with the masklets. The data was annotated at 6 FPS. There are an average of 3.8 manual masklets, and 8.9 auto masklets per video, and there are 642.6K masklets in total. 6. Is any information missing from individual instances? If so, please provide a description, explaining why this information is missing (e.g., because it was unavailable). This does not include intentionally removed information, but might include, e.g., redacted text. No. 7. Are relationships between individual instances made explicit (e.g., users’ movie ratings, social network links)? If so, please describe how these relationships are made explicit. No. 8. Are there any errors, sources of noise, or redundancies in the dataset? If so, please provide a description. For manual masklets, human errors may exist; for example, annotators may miss a frame to check or fix when needed. For auto masklets, as SAM 2 is used to generates them, model errors such as inconsistencies in the masklets may exist. 9. Is the dataset self-contained, or does it link to or otherwise rely on external resources (e.g., websites, tweets, other datasets)? If it links to or relies on external resources, a) are there guarantees that they will exist, and remain constant, over time; b) are there official archival versions of the complete dataset (e.g., including the external resources as they existed at the time the dataset was created); c) are there any restrictions (e.g., licenses, fees) associated with any of the external resources that might apply to a dataset consumer? Please provide descriptions of all external resources and any restrictions associated with them, as well as links or other access points, as appropriate. The dataset is self contained. 10. Does the dataset contain data that might be considered confidential (e.g., data that is protected by legal privilege or by doctor-patient confidentiality, data that includes the content of individuals’ non-public communications)? If so, please provide a description. No. 11. Does the dataset contain data that, if viewed directly, might be offensive, insulting, threatening, or might otherwise cause anxiety? If so, please describe why. We have three safety measures to prevent objectionable content: (1) The video collecting crowdworkers were provided instructions to not record videos that might contain objectionable content (e.g., graphic, nudity, or inappropriate content). (2) The expert annotators who annotated the videos were provided instructions to flag and reject videos if objectionable content was present. (3) reports about video(s) in the dataset can be submitted to segment-anything@meta.com. 12. Does the dataset identify any subpopulations (e.g., by age, gender)? If so, please describe how these subpopulations are identified and provide a description of their respective distributions within the dataset. The dataset does not identify any subpopulations of the people in the videos. The demographics of the crowdworkers who collected the videos in the dataset are presented in 5.2. 13. Is it possible to identify individuals (i.e, one or more natural persons), either directly or indirectly (i.e., in combination with other data) from the dataset? If so, please describe how. Videos were subjected to a face blurring model. Reports about videos in the dataset can be submitted to segment-anything@meta.com. 14. Does the dataset contain data that might be considered sensitive in any way (e.g., data that reveals race or ethnic origins, sexual orientations, religious beliefs, political opinions or union memberships, or locations; financial or health data; biometric or genetic data; forms of government identification, such as social security numbers; criminal history)? If so, please provide a description. The dataset is not focused on data that may be considered sensitive. Reports about videos in the dataset can be submitted to segment-anything@meta.com. 15. Any other comments? No. + +# Collection Process + +1. How was the data associated with each instance acquired? Was the data directly observable (e.g., raw text, movie ratings), reported by subjects (e.g., survey responses), or indirectly inferred/derived from other data (e.g., part-of-speech tags, model-based + +guesses for age or language)? If the data was reported by subjects or indirectly inferred/derived from other data, was the data validated/verified? If so, please describe how. The released masklets associated with each video were collected using two methods. (1) SAM 2 assisted manual annotation (2) automatically generated by SAM 2 and verified by annotators. 2. What mechanisms or procedures were used to collect the data (e.g., hardware apparatuses or sensors, manual human curation, software programs, software APIs)? How were these mechanisms or procedures validated? The videos in the dataset were collected via a contracted third-party vendor. They are videos taken by crowdworkers with unknown equipment. 3. If the dataset is a sample from a larger set, what was the sampling strategy (e.g., deterministic, probabilistic with specific sampling probabilities)? N/A. 4. Who was involved in the data collection process (e.g., students, crowdworkers, contractors) and how were they compensated (e.g., how much were crowdworkers paid)? (1) The videos in the dataset were collected via a contracted third-party vendor. They are videos taken by crowdworkers who were compensated with an hourly wage set by the vendor. (2) The manually collected masklets in the dataset were collected by annotators via another third-party vendor. Annotators were compensated with an hourly wage set by the vendor. 5. Over what timeframe was the data collected? Does this timeframe match the creation timeframe of the data associated with the instances (e.g., recent crawl of old news articles)? If not, please describe the timeframe in which the data associated with the instances was created. The videos were filmed between November 2023 and March 2024. The masklet annotations were collected between April 2024 and July 2024. 6. Were any ethical review processes conducted (e.g., by an institutional review board)? If so, please provide a description of these review processes, including the outcomes, as well as a link or other access point to any supporting documentation. If the dataset does not relate to people, you may skip the remaining questions in this section. The project underwent an internal review process. 7. Did you collect the data from the individuals in question directly, or obtain it via third parties or other sources (e.g. websites)? We contracted with third-party vendors to collect the videos and to generate or review annotations. 8. Were the individuals in question notified about the data collection? If so, please describe (or show with screenshots or other information) how notice was provided, and provide a link or other access point to, or otherwise reproduce, the exact language of the notification itself. The videos were collected by crowdworkers via a contracted third-party vendor. The crowdworkers agreed to consent forms. 9. Did the individuals in question consent to the collection and use of their data? If so, please describe (or show with screenshots or other information) how consent was requested and provided, and provide a link or other access point to, or otherwise reproduce, the exact language to which the individuals consented. The videos were collected via a contracted third-party who provided appropriate representations regarding the collection of any notices and consents as required from individuals. 10. If consent was obtained, were the consenting individuals provided with a mechanism to revoke their consent in the future or for certain uses? If so, please provide a description, as well as a link or other access point to the mechanism (if appropriate). Pursuant to the contract, the contracted third-party collected consents and provided opportunity for consent revocation. 11. Has an analysis of the potential impact of the dataset and its use on data subjects (e.g., a data protection impact analysis) been conducted? If so, please provide a description of this analysis, including the outcomes, as well as a link or other access point to any supporting documentation. See detail in E.1.1. 12. Any other comments? No. + +# Preprocessing / Cleaning / Labeling + +1. Was any preprocessing / cleaning / labeling of the data done (e.g., discretization or bucketing, tokenization, part-of-speech tagging, SIFT feature extraction, removal of instances, processing of missing values)? If so, please provide a description. If not, you may skip the remaining questions in this section. The videos were re-sampled to 24 fps and converted to mp4 format. 2. Was the “raw” data saved in addition to the preprocessed/cleaned/labeled data (e.g., to support unanticipated future uses)? If so, please provide a link or other access point to the “raw” data. No. + +# Uses + +1. Has the dataset been used for any tasks already? If so, please provide a description. The dataset has been used to train and evaluate SAM 2. +2. What (other) tasks could the dataset be used for? The data could be used for VOS, iVOS, or PVS tasks. If frames are sampled from the videos, the dataset can be used for the image segmentation task. +3. Is there anything about the composition of the dataset or the way it was collected and preprocessed/cleaned/labeled that might impact future uses? For example, is there anything that a dataset consumer might need to know to avoid uses that could result in unfair treatment of individuals or groups (e.g., stereotyping, quality of service issues) or other risks or harms (e.g., legal risks, financial harms)? If so, please provide a description. Is there anything a dataset consumer could do to mitigate these risks or harms? We have an analysis of the geography and crowdworker demographic of our dataset in 5.2. While we believe our dataset to be more representative on these factors than most of the publicly existing datasets of its kind at this time, we acknowledge that we do not have parity across all geographic and demographic groups, and we encourage users of the dataset to be mindful of any potential biases models may learn using this dataset. +4. Are there tasks for which the dataset should not be used? If so, please provide a description. No. Full terms of use for the dataset can be found at https://ai.meta.com/datasets/segment-anything-video-downloads/. +5. Any other comments? No. + +# Distribution + +1. Will the dataset be distributed to third parties outside of the entity (e.g., company, institution, organization) on behalf of which the dataset was created? If so, please provide a description. The dataset will be available under the permissive Creative Commons Attribution 4.0 International Public License. +2. How will the dataset will be distributed (e.g., tarball on website, API, GitHub)? Does the dataset have a digital object identifier (DOI)? The dataset is available at https://ai.meta.com/datasets/segment-anything-video/. +3. When will the dataset be distributed? The dataset will be distributed in July 2024. +4. Will the dataset be distributed under a copyright or other intellectual property (IP) license, and/or under applicable terms of use (ToU)? If so, please describe this license and/or ToU, and provide a link or other access point to, or otherwise reproduce, any relevant licensing terms or ToU, as well as any fees associated with these restrictions. Yes, the dataset will be available under the Creative Commons Attribution 4.0 International Public License. The license agreement and terms of use for the dataset can be found at https://ai.meta.com/datasets/segment-anything-video-downloads/. Users must agree to the terms of use before downloading or using the dataset. +5. Have any third parties imposed IP-based or other restrictions on the data associated with the instances? If so, please describe these restrictions, and provide a link or other access point to, or otherwise reproduce, any relevant licensing terms, as well as any fees associated with these restrictions. Full terms of use and restrictions on use of the SA-V dataset can be found at https://ai.meta.com/datasets/segment-anything-video-downloads/. +6. Do any export controls or other regulatory restrictions apply to the dataset or to individual instances? If so, please describe these restrictions, and provide a link or other access point to, or otherwise reproduce, any supporting documentation. The license and restrictions on use of the SA-V dataset can be found at https://ai.meta.com/datasets/segment-anything-video-downloads/. +7. Any other comments? No. + +# Maintenance + +1. Who will be supporting/hosting/maintaining the dataset? The dataset will be hosted at https://ai.meta.com/datasets/segmentanything-video/ and maintained by Meta FAIR. +2. How can the owner/curator/manager of the dataset be contacted (e.g., email address)? Please email segment-anything@meta.com. +3. Is there an erratum? If so, please provide a link or other access point. No. +4. Will the dataset be updated (e.g., to correct labeling errors, add new instances, delete instances)? If so, please describe how often, by whom, and how updates will be communicated to dataset consumers (e.g., mailing list, GitHub)? Updates may be made pursuant to inbound received at segment-anything@meta.com. +5. If the dataset relates to people, are there applicable limits on the retention of the data associated with the instances (e.g., were the individuals in question told that their data would be retained for a fixed period of time and then deleted)? If so, please describe these limits and explain how they will be enforced. There are no limits on data retention. +6. Will older versions of the dataset continue to be supported/hosted/maintained? If so, please describe how. If not, please describe how its obsolescence will be communicated to dataset consumers. No. If updates are made to the dataset, previous versions will not continue to be hosted. +7. If others want to extend/augment/build on/contribute to the dataset, is there a mechanism for them to do so? If so, please provide a description. Will these contributions be validated/verified? If so, please describe how. If not, why not? Is there a process for communicating/distributing these contributions to dataset consumers? If so, please provide a description. We encourage further annotations for SA-V, but these will not be validated/verified or supported/hosted/maintained by Meta. +8. Any other comments? No. + +# J.3 DATA ANNOTATION CARD + +# Task Formulation + +1. At a high level, what are the subjective aspects of your task? Selecting objects to mask and track in a video is inherently a subjective task, and annotators might differ in their decision to mask objects. +2. What assumptions do you make about annotators? We assume our annotators understand the PVS task and are well trained on video related tasks. Our annotators worked full time on our annotation task. This made it possible to train the annotators by sharing feedback on a regular basis. +3. How did you choose the specific wording of your task instructions? What steps, if any, were taken to verify the clarity of task instructions and wording for annotators? (1) The task instructions included visual examples (images and videos) to provide clarity. (2) Annotators were well trained before working on production queues. (3) The research team shared feedback daily and met with the annotators weekly for Q&A sessions. +4. What, if any, risks did your task pose for annotators and were they informed of the risks prior to engagement with the task? Annotators were informed to reject objectionable videos. +5. What are the precise instructions that were provided to annotators? See detail in 12 for annotation instructions. + +# Selecting Annotations + +1. Are there certain perspectives that should be privileged? If so, how did you seek these perspectives out? We chose to work with annotators with previous video annotation experience. +2. Are there certain perspectives that would be harmful to include? If so, how did you screen these perspectives out? No. +3. Were sociodemographic characteristics used to select annotators for your task? If so, please detail the process. For masklet annotations, sociodemographic characteristics were not used to select the annotators. For video collection, we emphasized the importance of diversity among the crowdworkers to our third-party vendor. While it was not a strict requirement, we encouraged the inclusion of a diverse group of crowdworkers to enrich the data collection process with a wide range of perspectives. This approach aimed to naturally incorporate diversity without imposing strict selection based on sociodemographic factors. +4. If you have any aggregated socio-demographic statistics about your annotator pool, please describe. Do you have reason to believe that sociodemographic characteristics of annotators may have impacted how they annotated the data? Why or why not? Aggregated socio-demographic statistics about the crowdworkers who collected the videos are presented in 5.2. +5. Consider the intended context of use of the dataset and the individuals and communities that may be impacted by a model trained on this dataset. Are these communities represented in your annotator pool? The SA-V dataset is a geographically diverse, publicly available, video segmentation dataset, as discussed in 5.2. In addition, we analyze the responsible AI axes of a model trained on the dataset, as discussed in E.1.1. + +# Platform and Infrastructure Choices + +1. What annotation platform did you utilize? At a high level, what considerations informed your decision to choose this platform? Did the chosen platform sufficiently meet the requirements you outlined for annotator pools? Are any aspects not covered? We used an internal annotation platform. + +2. What, if any, communication channels did your chosen platform offer to facilitate communication with annotators? How did this channel of communication influence the annotation process and/or resulting annotations? The research team shared feedback daily and met with the annotators weekly to align on the task instructions and expectations and to hold Q&A sessions. Outside of those sessions, annotators had access to a spreadsheet and chat group to facilitate communication with the research team. + +3. How much were annotators compensated? Did you consider any particular pay standards, when determining their compensation? If so, please describe. (1) The video collecting crowdworkers were compensated with an hourly wage set by the vendor. (2) Annotators were compensated with an hourly wage set by the vendor. + +# Dataset Analysis and Evaluation + +1. How do you define the quality of annotations in your context, and how did you assess the quality in the dataset you constructed? Annotators were required to follow a training before moving to production queues. Annotators followed a 2-day training session led by the vendor and then were asked to annotate jobs from a training queue. Annotators were able to move from training to production after the vendor Q&A team or the research team reviewed their work and assessed quality. On average, annotators spent 1 - 2 weeks in training before moving to production. Similarly, the vendor and research team Q&A manually reviewed the production queues’ annotations daily, sharing feedback daily. + +2. Have you conducted any analysis on disagreement patterns? If so, what analyses did you use and what were the major findings? Did you analyze potential sources of disagreement? The disagreement patterns were shared daily and weekly during feedback and Q&A sessions. + +3. How do the individual annotator responses relate to the final labels released in the dataset? The final labels are after data cleaning and post processing from the individual annotator responses. + +# Dataset Release and Maintenance + +1. Do you have reason to believe the annotations in this dataset may change over time? Do you plan to update your dataset? No. +2. Are there any conditions or definitions that, if changed, could impact the utility of your dataset? No. +3. Will you attempt to track, impose limitations on, or otherwise influence how your dataset is used? If so, how? The SA-V dataset is +released under a permissive CC by 4.0 license. +4. Were annotators informed about how the data is externalized? If changes to the dataset are made, will they be informed? No. +5. Is there a process by which annotators can later choose to withdraw their data from the dataset? If so, please detail. No. \ No newline at end of file diff --git a/papers/sam2/paper.pdf b/papers/sam2/paper.pdf new file mode 100644 index 0000000000000000000000000000000000000000..cf7f55d5966744bd4f88c29cef3f88ee70f1de11 --- /dev/null +++ b/papers/sam2/paper.pdf @@ -0,0 +1,3 @@ +version https://git-lfs.github.com/spec/v1 +oid sha256:6c07c6ecf2f592e3f7f9833409b8ec63232e2193b4ab4edb7db575805f27c028 +size 11998284 diff --git a/papers/sam2/sau.json b/papers/sam2/sau.json new file mode 100644 index 0000000000000000000000000000000000000000..517109a38a4807e9cc556c833a832ca1d42e0771 --- /dev/null +++ b/papers/sam2/sau.json @@ -0,0 +1,397 @@ +{ + "paper_id": "sam2", + "paper_title": "SAM 2: Segment Anything in Images and Videos", + "D1": [ + { + "id": "sam2-D1-001", + "claim": "Memory bank: FIFO queues store up to N=6 recent frames (with temporal position encoding) and M=unbounded prompted frames (no temporal PE) as spatial feature maps; object pointers stored as lightweight semantic vectors per frame.", + "source": "Section 4, Appendix D.1" + }, + { + "id": "sam2-D1-002", + "claim": "Memory attention: L=4 transformer blocks, each with self-attention (2D-RoPE), spatial memory cross-attention, object-pointer cross-attention (no RoPE), and MLP (GeLU); memory channel dimension=64; FlashAttention-2 kernels used.", + "source": "Section 4, Appendix D.1" + }, + { + "id": "sam2-D1-003", + "claim": "Object pointers: lightweight semantic vectors (dim=256, 4 tokens per frame) from mask decoder output tokens, stored in memory bank and cross-attended by subsequent frames for high-level object information.", + "source": "Section 4, Appendix D.1" + }, + { + "id": "sam2-D1-004", + "claim": "Image encoder: MAE pre-trained Hiera hierarchical ViT in four sizes (T/S/B+/L); FPN fuses stage 3 (stride 16) + stage 4 (stride 32) outputs; stage 1 (stride 4) + stage 2 (stride 8) skip connections feed mask decoder for high-resolution detail.", + "source": "Section 4, Appendix D.1" + }, + { + "id": "sam2-D1-005", + "claim": "Hiera global attention blocks across three stages by encoder size: T=5-7-9, S=7-10-13, B+=12-16-20, L=23-33-43; window attention used in remaining blocks.", + "source": "Section 4, Appendix D.1" + }, + { + "id": "sam2-D1-006", + "claim": "Positional encoding: image encoder uses windowed absolute positional embeddings (interpolated globally, no relative positional bias RPB); memory attention uses 2D-RoPE for self-attention and spatial cross-attention.", + "source": "Section 4, Appendix D.1" + }, + { + "id": "sam2-D1-007", + "claim": "Prompt encoder (identical to SAM): click/box/mask prompts encoded via sinusoidal PE + learned embeddings; mask decoder: two-way transformer (2 blocks), multi-mask predictions (3 masks for ambiguity), skip connections from encoder stages 1+2, occlusion prediction head (MLP on occlusion token).", + "source": "Section 4, Appendix D.1" + }, + { + "id": "sam2-D1-008", + "claim": "Training hyperparameters: default resolution 1024x1024 (ablation tested [512,768,1024]), 8-frame training sequences (ablation tested [4,8,10]), fine-tuning with 16-frame sequences, max 3 masklets per 8-frame training sequence, 7 correction clicks during iterative training, FlashAttention-2 enabled at 1024 resolution.", + "source": "Section 4, Appendix D.1, Appendix A.2, Table 9" + }, + { + "id": "sam2-D1-009", + "claim": "SA-1B pre-training data configuration: max 64 masks per image, masks covering >90% image area filtered out; random horizontal flip augmentation enabled; image encoder initialized from MAE pre-trained Hiera weights.", + "source": "Section 4, Appendix D.1" + }, + { + "id": "sam2-D1-010", + "claim": "Pre-training optimizer: AdamW (beta1=0.9, beta2=0.999), lr=4e-4, weight_decay=0.1, batch_size=256, 90K steps; reciprocal sqrt lr schedule (timescale=1000) with linear warmup 1000 iterations + linear cooldown 5000 iterations; bfloat16 precision; L2 gradient clipping max=0.1.", + "source": "Appendix D.2.1" + }, + { + "id": "sam2-D1-011", + "claim": "Pre-training loss weights: focal loss=20, dice loss=1, IoU prediction L1 loss=1 (ratio 20:1:1); per-frame best-mask selection via argmin of total segmentation loss across K predicted masks.", + "source": "Appendix D.2.1" + }, + { + "id": "sam2-D1-012", + "claim": "Layer-wise learning rate decay (image encoder only) by model size: T/S=0.8, B+=0.9, L=0.925; deeper layers receive progressively lower LR via exponential decay factor.", + "source": "Appendix D.2.1" + }, + { + "id": "sam2-D1-013", + "claim": "Stochastic depth (drop path) rate by model size: T/S=0.1, B+=0.2, L=0.3.", + "source": "Appendix D.2.1" + }, + { + "id": "sam2-D1-014", + "claim": "Full training (joint video+image) data mix: SA-1B ~15.2%, SA-V ~70.0%, Internal ~14.8%; with open-source VOS datasets added: DAVIS ~1.3%, MOSE ~9.4%, YouTubeVOS ~9.2%, SA-1B ~15.5%, SA-V ~49.5%, Internal ~15.1%.", + "source": "Appendix D.2.2" + }, + { + "id": "sam2-D1-015", + "claim": "Interactive prompt sampling in full training: initial prompt type probs GT mask=0.5, positive click=0.25, bbox=0.25; up to 2 prompted frames per 8-frame seq; reverse temporal order prob 0.5; random correction click prob 0.1; 2x2 mosaic transform prob 0.1 (same video tiled, simulates similar-looking objects).", + "source": "Appendix D.2.2" + }, + { + "id": "sam2-D1-016", + "claim": "Full training loss weights: focal=20, dice=1, MAE IoU prediction=1, cross-entropy occlusion prediction=1 (ratio 20:1:1:1); when GT has no mask for a frame (object absent/occluded), mask outputs are not supervised and only occlusion loss applies.", + "source": "Appendix D.2.2" + }, + { + "id": "sam2-D1-017", + "claim": "Data augmentation pipeline: random horizontal flip (p=0.5); affine transform (rotation ±25°, shear ±15°, scale [0.7,1.3], translate ±10%); color jitter (brightness ±0.2, contrast ±0.2, saturation ±0.2, hue ±0.1); random grayscale (p=0.1).", + "source": "Appendix D.2.2, Table 12" + }, + { + "id": "sam2-D1-018", + "claim": "Fine-tuning protocol (Stage 3): 50K iterations (1/3 of full training schedule), learning rate 0.5x base lr (2e-4), freeze image encoder, 16-frame sequences, top 50% most-edited (most challenging) masklets selected for training, 80GB A100 GPUs.", + "source": "Appendix D.2.2" + }, + { + "id": "sam2-D1-019", + "claim": "SA-V dataset: 50.9K videos, 642.6K masklets (190.9K manual + 451.7K auto-generated), 35.5M masks (10.0M manual-only), 4.2M frames, 196.0 hrs total duration; 53x more masks than any existing VOS dataset (15x without auto annotations).", + "source": "Section 5.2, Appendix E.1" + }, + { + "id": "sam2-D1-020", + "claim": "SA-V dataset properties: 54% indoor / 46% outdoor scenes across 47 countries; object disappearance rate 42.5% (manual only) / 27.7% (manual+auto); 88% of masks have normalized area <0.1 (predominantly small objects and parts).", + "source": "Section 5.2, Appendix E.1" + }, + { + "id": "sam2-D1-021", + "claim": "SA-V val/test splits: val set 155 videos / 293 masklets, test set 150 videos / 278 masklets; split by video authors and geographic location; annotated at 6 FPS using Phase 1 per-frame setup; focuses on challenging fast-moving, occluded, disappearing/reappearing objects.", + "source": "Section 5.2, Appendix E.1" + }, + { + "id": "sam2-D1-022", + "claim": "Internal training dataset: 62.9K videos, 69.6K masklets (Phase 2+3 annotated), 5.4M masks, 6.0M frames, 281.8 hrs total, disappearance rate 36.4%; internal test set: 96 videos / 189 masklets (Phase 1 annotated).", + "source": "Section 5.2" + }, + { + "id": "sam2-D1-023", + "claim": "Auto masklet generation: multi-scale first-frame grid prompting (32x32 full-frame grid, 16x16 on 2x2 zoomed crops, 4x4 on 4x4 zoomed crops); post-processing removes tiny connected components (<200 px) and fills holes (<200 px); satisfactory masklets added to SA-V, unsatisfactory ones sent to human annotators for refinement.", + "source": "Section 5.1, Appendix E.2" + }, + { + "id": "sam2-D1-024", + "claim": "Interactive evaluation protocol: N_click=3 clicks per frame, N_frame=8 max interacted frames, T_loc=1.0s visual locate time per frame, T_click=1.5s per click; offline mode: multi-pass selects lowest-IoU frame for next prompt; online mode: single pass pauses at IoU<0.75 for correction; clicks sampled at object center then error-region centroids.", + "source": "Appendix B, Appendix F.1.2" + }, + { + "id": "sam2-D1-025", + "claim": "Evaluation benchmark coverage: 17 zero-shot video datasets (9 densely annotated) for interactive and VOS evaluation; 37 zero-shot image datasets (23 from SAM benchmark + 14 video-derived) for SA task; prompt types tested: 1-click, 3-click, 5-click, bounding box, ground-truth mask.", + "source": "Appendix B, Appendix F.1.2, Appendix F.1.3, Appendix F.4" + }, + { + "id": "sam2-D1-026", + "claim": "Architecture ablation search space (conducted at default 512 resolution): input resolutions [512,768,1024], frames per sequence [4,8,10], memories N [4,6,8], memory channel dim [64,256], memory attention blocks [(sa=2,ca=2), (sa=3,ca=2), (sa=4,ca=4)], image encoder sizes [T,S,B+,L]; evaluated on video J&F and image mIoU.", + "source": "Appendix A.2, Table 9" + }, + { + "id": "sam2-D1-027", + "claim": "Training infrastructure: 256 NVIDIA A100 GPUs for distributed training; SA-V data collection period Nov 2023 - Mar 2024, masklet annotation period Apr - Jul 2024.", + "source": "Appendix D.2" + }, + { + "id": "sam2-D1-028", + "claim": "Data engine quality validation: controlled experiment with 169 videos / 452 masklets comparing Phase 1-3 annotation quality (Phase 1 Mask Alignment Score, IoU>0.75 threshold); mask area classification thresholds: small 1-32^2 px, medium 32^2-96^2 px, large >=96^2 px.", + "source": "Appendix E.2" + } + ], + "D2": [ + { + "id": "sam2-D2-001", + "claim": "Image Encoder - FPN Feature Fusion with Hiera: For each video frame at time t:\n features_stride16 = Hiera.stage3(frame_t) # Stage 3 output\n features_stride32 = Hiera.stage4(frame_t) # Stage 4 output\n image_embedding_t = FPN_fuse(upsample(features_stride32), features_stride16)\n # Stride 4 and 8 features (stages 1, 2): reserved for mask decoder skip connections\n skip_stride8_t = Hiera.stage2(frame_t)\n skip_stride4_t = Hiera.stage1(frame_t)\n\nFPN_fuse: bilinear upsample stride-32 to match stride-16 spatial dims, then element-wise add or concat + projection.\nHiera stages: hierarchical ViT, global attention at sparse layers. MAE pre-trained. Absolute windowed pos embed only, no RPB.\n\nVariable definitions:\n features_strideX: spatial feature map at stride X (spatial resolution = input/X x input/X)\n image_embedding_t: encoder output for frame t (shared across all objects in video)\n skip_strideX_t: high-res features passed to mask decoder for detail recovery", + "source": "4, D.1" + }, + { + "id": "sam2-D2-002", + "claim": "Memory Attention - L-Block Transformer with 2D-RoPE: For current frame t with encoder output E_t, using L=4 transformer blocks:\n X_0 = E_t # Image embedding from FPN\n for l in 0..L-1:\n X_l_norm1 = LayerNorm(X_l)\n X_l_self = X_l + SelfAttention(X_l_norm1, RoPE=2D_RoPE) # 2D RoPE applied\n X_l_norm2 = LayerNorm(X_l_self)\n X_l_cross_spatial = X_l_self + CrossAttention(X_l_norm2, K=mem_spatial, V=mem_spatial)\n X_l_norm3 = LayerNorm(X_l_cross_spatial)\n X_l_cross_obj = X_l_cross_spatial + CrossAttention(X_l_norm3, K=mem_obj_ptr, V=mem_obj_ptr) # object pointers, no RoPE\n X_l_norm4 = LayerNorm(X_l_cross_obj)\n X_{l+1} = X_l_cross_obj + MLP(X_l_norm4) # FFN with GeLU\n output = X_L # conditioned frame embedding [C, H, W]", + "source": "4, D.1, A.2.2" + }, + { + "id": "sam2-D2-003", + "claim": "Prompt Encoder - Click / Box / Mask Encoding: For frame t with conditioned embedding C_t = conditioned_embedding_t:\n\n # Sparse prompts (clicks, boxes):\n pos_enc_pt = sinusoidal_positional_encoding(x_norm, y_norm) # per (x,y) coordinate\n sparse_embed = sum_{prompts}[ learned_emb[prompt_type] + pos_enc_pt ]\n # prompt_type in {pos_click=0, neg_click=1, box_corner=2}\n\n # Dense prompt (mask):\n mask_embed = ConvModule(input_mask) # 2-layer Conv2D, same resolution as frame embedding\n conditioned_embedding_t = C_t + mask_embed + sparse_embed # element-wise sum", + "source": "4" + }, + { + "id": "sam2-D2-004", + "claim": "Mask Decoder - Two-Way Transformer with Skip Connections: Input: frame_with_prompts_t, skip_stride4_t, skip_stride8_t\n\n # Step 1: Two-way transformer (bidirectional prompt-to-image and image-to-prompt attention)\n prompt_tokens, image_tokens = TwoWayTransformer(frame_with_prompts_t, num_blocks=2)\n # Each block: self-attn on prompts + cross-attn prompts->image + self-attn on image + cross-attn image->prompts + MLP\n\n # Step 2: Upsample with skip connections - inject stride-8 (stage 2) then stride-4 (stage 1) features from Hiera image encoder into decoder upsampling layers for high-resolution mask detail (see Fig. 8)\n upsampled = bilinear_upsample(image_tokens)\n upsampled = upsampled + conv_proj(skip_stride8_t) # add high-res features from encoder stage 2 (stride 8)\n upsampled = bilinear_upsample(upsampled)\n upsampled = upsampled + conv_proj(skip_stride4_t) # add high-res features from encoder stage 1 (stride 4)\n\n # Step 3: Output heads - produce K=3 masks (for ambiguous prompts, e.g. single click), IoU scores, and occlusion prediction\n masks = conv_head(upsampled) # 3 masks [K, H, W]\n iou_scores = sigmoid(MLP_iou(mask_tokens)) # predicted IoU per mask, sigmoid constrains to [0,1]\n occlusion_score = sigmoid(MLP_occ(occlusion_token)) # object visibility likelihood on current frame\n # mask token corresponding to output mask stored as object pointer in memory bank (Sec 4, Appendix D.1)\n\n # Step 4: Multi-mask selection - if no follow-up prompts resolve ambiguity, propagate mask with highest predicted IoU\n if not has_follow_up_prompt:\n best_k = argmax(iou_scores)\n output_mask = masks[best_k]\n\nVariable definitions:\n skip_stride4_t, skip_stride8_t: high-resolution feature maps from Hiera stages 1 and 2 (not passed through memory attention), used only in mask decoder upsampling\n occlusion_token: additional learnable token alongside mask and IoU output tokens, fed to MLP head for object presence prediction\n mask_tokens: output tokens from two-way transformer decoder, one used as 256-dim object pointer stored in memory bank", + "source": "4, D.1, D.2" + }, + { + "id": "sam2-D2-005", + "claim": "Memory Encoder - Mask-Image Fusion with Conv: For frame t, after mask prediction:\n\n # Step 1: Downsample predicted mask to match memory spatial resolution\n mask_down = ConvDownsample(output_mask_logits) # conv stride ~16-32 to match memory resolution\n # output_mask_logits: from mask decoder (raw logits, no sigmoid during training)\n\n # Step 2: Element-wise add with unconditioned image encoder embedding (Hiera FPN output, NOT memory-conditioned)\n fused = mask_down + image_embedding_t # image_embedding_t reused from image encoder (no extra encoder needed)\n\n # Step 3: Lightweight convolutional fusion to combine mask and image information\n memory_feats = ConvFusion(fused) # 2-3 light-weight conv layers (3x3, GeLU activation)\n\n # Step 4: Project to 64-dim channel for efficient memory bank storage (Appendix D.1)\n memory_feats = ConvProj(memory_feats, out_channels=64) # 64-dim spatial feature map\n\nVariable definitions:\n output_mask_logits: raw logit mask from mask decoder BEFORE sigmoid binarization\n image_embedding_t: unconditioned frame embedding from image encoder (Hiera + FPN, stride 16), shared across all objects in video\n memory_feats: final 64-channel spatial feature map stored in memory bank for cross-attention by subsequent frames\n ConvDownsample: convolutional downsampling module (stride 16 or 32) to match memory spatial resolution\n ConvFusion: lightweight conv layers (no additional image encoder) that fuse downsampled mask with image embedding", + "source": "4, D.1" + }, + { + "id": "sam2-D2-006", + "claim": "Memory Bank - FIFO Queue Management: MemoryBank state:\n spatial_memory_queue: deque(maxlen=N) # N=6 recent frames (default)\n prompted_memory_queue: deque(maxlen=M) # M = same as N\n object_pointers_list: list # lightweight semantic vectors\n temporal_position_counter: int = 0\n\n def update(memory_feats_t, obj_ptr_256, is_prompted, occ_pred):\n # memory_feats_t: 64-dim spatial features from MemoryEncoder\n # obj_ptr_256: 256-dim semantic vector from mask decoder output token\n # Split obj_ptr into 4 tokens of 64-dim for cross-attention compatibility (Appendix D.1)\n obj_ptr_4tokens = reshape(obj_ptr_256, [4, 64]) # [256] -> [4, 64]\n \n if is_prompted:\n # Prompted frame: NO temporal positional encoding added\n # (training signal from prompted frames is sparser, harder to generalize temporal range)\n if occ_pred < 0.5:\n # Object predicted occluded/invisible: add learned occlusion embedding to memory features\n memory_feats_t = memory_feats_t + learned_occlusion_embed\n prompted_memory_queue.append({'features': memory_feats_t, 'obj_ptr': obj_ptr_4tokens})\n else:\n # Recent (unprompted) frame: add temporal positional encoding for short-term motion representation\n memory_feats_t = memory_feats_t + temporal_pos_encoding[temporal_position_counter]\n temporal_position_counter += 1\n spatial_memory_queue.append({'features': memory_feats_t, 'obj_ptr': obj_ptr_4tokens})\n \n object_pointers_list.append(obj_ptr_4tokens) # stored for cross-attention in subsequent frames", + "source": "4, D.1" + }, + { + "id": "sam2-D2-007", + "claim": "Memory Bank Retrieval - Feature Assembly for Cross-Attention: def get_cross_attention_inputs():\n # Assemble spatial memory features (keys = values):\n spatial_features = []\n for m in spatial_memory_queue: # recent frames (with temporal PE)\n spatial_features.append(m['features']) # shape [C, H_mem, W_mem] where C=64\n for m in prompted_memory_queue: # prompted frames (NO temporal PE)\n spatial_features.append(m['features'])\n spatial_KV = concat_and_reshape(spatial_features) # flatten each [64, H, W] -> [64, H*W], concat -> [total_tokens, 64] for cross-attention\n\n # Assemble object pointer features (lightweight semantic vectors):\n obj_ptr_tokens = []\n for obj_ptr in object_pointers_list: # each obj_ptr is [4, 64]\n obj_ptr_tokens.append(obj_ptr) # 4 tokens per frame, 64-dim each\n obj_ptr_KV = concat_along_token_dim(obj_ptr_tokens) # [total_frames * 4, 64]\n\n return spatial_KV, obj_ptr_KV # both used as keys and values in memory attention cross-attention layers (no RoPE on object pointers)", + "source": "4, D.1" + }, + { + "id": "sam2-D2-008", + "claim": "Streaming Inference - Full Forward Pass for One Frame: # Complete per-frame inference loop for single object in streaming mode:\n memory_bank = MemoryBank(N=6)\n \n for t in 0..num_frames:\n # 1. Image encoder (run once per frame, cached for multi-object):\n if image_embedding_cache[t] is None:\n image_embedding_t, skip_feats_t = ImageEncoder(frame_t)\n image_embedding_cache[t] = (image_embedding_t, skip_feats_t)\n else:\n image_embedding_t, skip_feats_t = image_embedding_cache[t] # reuse cached features for multi-object inference\n \n # 2. Memory attention: condition frame embedding on memory bank (spatial features + object pointers)\n spatial_KV, obj_ptr_KV = memory_bank.get_cross_attention_inputs()\n conditioned_t = MemoryAttention(image_embedding_t, spatial_KV, obj_ptr_KV) # L=4 blocks with 2D-RoPE\n \n # 3. Prompt encoding (if any prompts provided on frame t):\n if prompts[t] is not None:\n conditioned_with_prompts_t = PromptEncoder(conditioned_t, prompts[t]) # clicks/boxes/masks\n else:\n conditioned_with_prompts_t = conditioned_t # no prompt on this frame, propagate from memory\n \n # 4. Mask decoder with skip connections (high-res detail from encoder stages 1+2, bypassing memory attention):\n masks_t, iou_scores_t, occ_score_t = MaskDecoder(\n conditioned_with_prompts_t, skip_feats_t['stride4'], skip_feats_t['stride8'])\n \n # 5. Multi-mask selection and memory encoding:\n if has_new_prompt_on_frame(t):\n best_k = select_best_by_prompt(masks_t) # user prompt may resolve ambiguity\n else:\n best_k = argmax(iou_scores_t) # select mask with highest predicted IoU\n output_mask_t = masks_t[best_k]\n memory_feats_t = MemoryEncoder(output_mask_t, image_embedding_t) # fuse mask + embedding for next frames\n \n # 6. Update memory bank for future frames:\n obj_ptr_256 = extract_mask_token_from_decoder() # 256-dim semantic vector from mask decoder output token\n memory_bank.update(memory_feats_t, obj_ptr_256, is_prompted=(prompts[t] is not None), occ_pred=occ_score_t[best_k])\n\nNote: For multi-object inference, image encoder runs once and its features are shared. Memory bank, memory attention, mask decoder run independently per object. (Appendix D.1)", + "source": "4, D.1, D.3" + }, + { + "id": "sam2-D2-009", + "claim": "Training Loss - Focal Loss (Pixel-Level Mask Supervision): def focal_loss(pred_logits, gt_mask, alpha=0.25, gamma=2.0):\n pred_prob = sigmoid(pred_logits)\n # Where GT = 1:\n pt_pos = pred_prob[gt_mask == 1]\n loss_pos = -alpha * (1 - pt_pos)^gamma * log(pt_pos + epsilon)\n # Where GT = 0:\n pt_neg = 1 - pred_prob[gt_mask == 0]\n loss_neg = -(1 - alpha) * pt_neg^gamma * log(pt_neg + epsilon)\n return mean(loss_pos) + mean(loss_neg)\n\nVariable definitions:\n pred_logits: raw output logits from mask decoder (pre-sigmoid), shape [H, W]\n pred_prob: predicted probability mask after sigmoid activation, shape [H, W]\n gt_mask: binary ground-truth mask, shape [H, W], values in {0, 1}\n alpha: positive class weighting factor (default 0.25), balances foreground vs background\n gamma: focusing parameter (default 2.0), down-weights loss on well-classified pixels\n epsilon: small constant for numerical stability (~1e-7)\n pt_pos: predicted probability p_t for positive (foreground) pixels, closer to 1 is better\n pt_neg: predicted probability p_t for negative (background) pixels after flip (1 - pred_prob), closer to 1 is better", + "source": "D.2" + }, + { + "id": "sam2-D2-010", + "claim": "Training Loss - Dice Loss (Global Mask-Level Supervision): def dice_loss(pred_logits, gt_mask, epsilon=1e-6):\n pred_prob = sigmoid(pred_logits)\n numerator = 2 * sum(pred_prob * gt_mask)\n denominator = sum(pred_prob) + sum(gt_mask)\n dice_coeff = (numerator + epsilon) / (denominator + epsilon)\n return 1 - dice_coeff\n\nVariable definitions:\n pred_prob: predicted probability mask (after sigmoid), [H, W]\n gt_mask: binary ground-truth mask [H, W]", + "source": "D.2" + }, + { + "id": "sam2-D2-011", + "claim": "Training Loss - L1/MAE IoU Supervision: # For each of K predicted masks:\n for k in 0..K-1:\n pred_iou[k] = sigmoid(iou_logits[k]) # sigmoid constrains output to [0,1]\n pred_mask_bin = sigmoid(mask_logits[k]) > 0.5\n true_iou[k] = IoU(pred_mask_bin, gt_mask) # = |intersection| / |union|\n L_iou[k] = |pred_iou[k] - true_iou[k]| # L1 / MAE\n\n # First click (multi-mask): supervise IoU of ALL masks\n if is_first_click:\n L_iou_total += sum(L_iou[k] for k in 0..K-1) # all K IoU predictions supervised (encourages learning of bad masks)\n else:\n L_iou_total += L_iou[best_k] # supervise only best mask IoU for correction clicks", + "source": "D.2" + }, + { + "id": "sam2-D2-012", + "claim": "Training Loss - Occlusion / Existence Prediction (Binary Cross-Entropy): # For each frame t in the training sequence:\n pred_occ[t] = sigmoid(occlusion_logit[t])\n true_occ[t] = 1 if GT_mask_exists[t] else 0\n L_occ[t] = -[true_occ[t] * log(pred_occ[t]) + (1 - true_occ[t]) * log(1 - pred_occ[t])]\n L_occ = (1/T) * sum_{t=0}^{T-1} L_occ[t] # average over all frames\n\n # CRITICAL RULE: if GT has NO mask for frame t (true_occ=0):\n # mask outputs: NOT supervised (no mask loss for that frame)\n # occlusion prediction head: ALWAYS supervised regardless of mask presence (Sec D.2.2)", + "source": "4, D.1, D.2" + }, + { + "id": "sam2-D2-013", + "claim": "Training Loss - Total Loss Aggregation: # Full loss with weight ratio 20 : 1 : 1 : 1\n\n # Per-frame mask selection for loss computation:\n for t in 0..T-1:\n if GT_mask_exists[t]:\n for k in 0..K-1:\n L_seg[k] = 20 * L_focal[k] + 1 * L_dice[k] # k-th mask segmentation loss\n best_k = argmin_k(L_seg[k])\n L_mask_total += 20 * L_focal[best_k] + 1 * L_dice[best_k]\n L_iou_total += L_iou[best_k] # or all K masks on first click (all-K IoU supervision; correction: only best mask)\n else:\n L_mask_total += 0 # GT mask absent: skip mask losses, occlusion only\n L_total = L_mask_total + 1 * L_iou_total + 1 * L_occ[all] # ratio 20:1:1:1", + "source": "D.2" + }, + { + "id": "sam2-D2-014", + "claim": "Training - Interactive Prompt Sampling Strategy: # Training data construction per 8-frame sequence:\n seq_frames = sample_8_consecutive_frames(video)\n \n # Randomly select up to 2 frames for prompting:\n num_prompted = randint(0, min(2, 8))\n prompted_idx = random_sample(range(8), num_prompted)\n \n for idx in prompted_idx:\n # Initial prompt type sampling:\n init_type = categorical_sample({\n 'ground_truth_mask': 0.50,\n 'positive_click': 0.25,\n 'bounding_box': 0.25})\n # Reverse temporal order with 50% probability for bidirectional generalization\n if random() < 0.50:\n seq_frames = reverse(seq_frames)\n # Random correction click (10% prob): sample from GT irrespective of model prediction\n if random() < 0.10:\n correction_click = sample_random_click_from_gt(gt_masklet)", + "source": "4, D.2" + }, + { + "id": "sam2-D2-015", + "claim": "Training - AdamW Optimizer with Layer-Wise Decay: # Optimizer instantiation:\n optimizer = AdamW(model.parameters(),\n lr=base_lr, betas=(0.9, 0.999), weight_decay=0.1)\n\n # Layer-wise learning rate decay (applied to image encoder):\n encoder_layers = image_encoder.get_all_layers() # ordered from top to bottom\n num_layers = len(encoder_layers)\n for i, layer in enumerate(encoder_layers):\n layer_lr_ratio = layer_decay ^ (num_layers - 1 - i) # deeper layers get lower LR (i=0 is top)\n param_group = {'params': layer.parameters(), 'lr': base_lr * layer_lr_ratio}\n optimizer.param_groups.append(param_group)\n # layer_decay values: T/S=0.8, B+=0.9, L=0.925 (Table 12a)", + "source": "D.2" + }, + { + "id": "sam2-D2-016", + "claim": "Training - Reciprocal Square-Root LR Schedule: def reciprocal_sqrt_schedule(iteration, base_lr=4e-4, timescale=1000,\n warmup_iters=1000, cooldown_iters=5000, total_iters=180000):\n # Linear warmup (0 -> base_lr):\n if iteration < warmup_iters:\n warmup_factor = iteration / warmup_iters\n else:\n warmup_factor = 1.0\n \n # Reciprocal sqrt main schedule:\n sqrt_factor = sqrt(timescale / max(iteration, 1)) # sqrt(1000 / iter)\n \n # Linear cooldown (base_lr -> 0 over last cooldown_iters):\n if iteration > total_iters - cooldown_iters:\n cooldown_factor = max(0, total_iters - iteration) / cooldown_iters\n else:\n cooldown_factor = 1.0\n \n lr = base_lr * warmup_factor * sqrt_factor * cooldown_factor\n return lr", + "source": "D.2" + }, + { + "id": "sam2-D2-017", + "claim": "Mosaic Transform - 2x2 Same-Video Augmentation: # Applied with 10% probability during full training only:\n if rand() < 0.10:\n # Duplicate the same training video into 2x2 tiled grid:\n video_tiled = tile_2x2_grid(same_video) # each tile = original video at half W, H\n \n # Select random quadrant as target object:\n target_quadrant = randint(0, 3)\n gt_masklet = masklets_list[target_quadrant] # corresponding ground-truth\n \n # Model must use motion/temporal continuity to distinguish target from\n # identical-looking counterparts in other quadrants. Reduced object size\n # (half width/height) also facilitates small-object segmentation (Sec D.2.2).\n", + "source": "D.2.2" + }, + { + "id": "sam2-D2-018", + "claim": "16-Frame Fine-Tuning for Long Videos: # Fine-tuning stage to improve long-video performance:\n\n # Data selection: top 50% most-edited (most challenging) masklets\n masklets_sorted = sort_by_desc(num_edited_frames(all_masklets))\n challenging_masklets = masklets_sorted[0 : len(masklets_sorted) // 2]\n # Keep full OSS datasets (DAVIS, MOSE, YouTubeVOS) in mix\n\n # Training config:\n sequence_length = 16 # doubled from 8\n total_iterations = 50000 # ~1/3 of full training schedule\n base_lr = 2e-4 # half of original 4e-4\n freeze(image_encoder) # to fit 16-frame sequences into 80GB A100 GPUs", + "source": "D.2.2" + }, + { + "id": "sam2-D2-019", + "claim": "Offline Interactive Evaluation - Multi-Pass Algorithm: # Simulates annotator iteratively refining worst frames:\n prompts_storage = dict() # frame_idx -> list of (x, y, type)\n \n # First pass: initial prompts on first frame\n prompts_storage[0] = sample_center_click(GT_mask[0]) + 2 correction clicks\n \n for pass_id in 1..8: # N_frame = 8 passes max\n # Run full video segmentation with accumulated prompts\n masklet = run_full_video(video, prompts_storage)\n # Find frame with lowest IoU for next-pass prompting\n errors = [IoU(masklet[t], GT_mask[t]) for t in 0..T-1]\n worst_frame = argmin(errors)\n # Sample 3 correction clicks at error-region centroids\n correction_clicks = sample_3_clicks_from_error(masklet[worst_frame], GT_mask[worst_frame])\n prompts_storage[worst_frame].extend(correction_clicks)\n return masklet", + "source": "6.1, F.1.2" + }, + { + "id": "sam2-D2-020", + "claim": "Online Interactive Evaluation - Single Pass with Pausing: # Single forward pass with IoU-gated pausing for corrections:\n prompts_storage = dict()\n prompts_storage[0] = sample_center_click(GT_mask[0]) + 2 correction clicks\n num_prompted = 1\n \n for t in 0..num_frames:\n # Run frame t conditioned on memory + any prompts at t\n mask_t = forward_one_frame(frame_t, prompts_storage.get(t, None), memory_bank)\n iou_t = IoU(mask_t, GT_mask[t])\n \n if iou_t < 0.75 and num_prompted < 8:\n # Pause: sample 3 correction clicks at error region centroids\n correction_clicks = sample_3_clicks_from_error(mask_t, GT_mask[t])\n prompts_storage[t] = correction_clicks\n num_prompted += 1\n # Re-run frame t with new prompts (only affects t onwards, not previous frames)\n mask_t = forward_one_frame(frame_t, prompts_storage[t], memory_bank)\n memory_bank.update(mask_t) # push into memory for subsequent frames\n return masklet", + "source": "6.1, F.1.2" + }, + { + "id": "sam2-D2-021", + "claim": "Semi-Supervised VOS - First-Frame-Only Prompt Evaluation: # Standard VOS protocol: prompts only on first frame (no interactive refinement):\n \n # CLICK mode (1, 3, or 5 clicks):\n clicks = []\n # First click: center of GT mask\n clicks.append(center_of_mass(GT_mask_first_frame))\n # Additional clicks: center of error region (iterative)\n for i in 1..N_clicks-1:\n current_mask = SAM2.predict(first_frame, clicks)\n error = XOR(current_mask, GT_mask_first_frame)\n next_center = center_of_mass(error)\n clicks.append(next_center)\n # Segment entire video with accumulated first-frame clicks only\n masklet = SAM2.run_video(first_frame_prompts=clicks)\n return masklet", + "source": "6.2, F.1.3" + }, + { + "id": "sam2-D2-022", + "claim": "Automatic Masklet Generation - Grid Prompting + Post-Processing: # Generate candidate masklets from first frame, propagate, post-process:\n # Step 1: Multi-scale grid prompting on first frame\n grid_32 = make_grid(32, 32) # 1024 points, full frame\n grid_16a = make_grid(16, 16) on crop_2x2[0] # 256 pts * 4 crops = 1024 pts\n grid_4a = make_grid(4, 4) on crop_4x4[0] # 16 pts * 16 crops = 256 pts\n all_prompts = flatten(grid_32 + grid_16a + grid_4a) # combine all grid prompts from 3 scales\n\n # Step 2: Propagate each candidate through full video and post-process\n for prompt in all_prompts:\n masklet = SAM2.run_video(first_frame_prompt=prompt)\n # Remove tiny disconnected components (<200 px) and fill small holes (<200 px)\n for t in 0..T-1:\n masklet[t] = remove_small_components(masklet[t], min_area=200)\n masklet[t] = fill_holes(masklet[t], max_hole=200)\n \n # Step 3: Verification - satisfactory masklets added to SA-V dataset;\n # unsatisfactory ones (model failures) sent to human annotators for Phase 3 refinement (Sec 5.1, App E.1)", + "source": "5.1, E.1" + }, + { + "id": "sam2-D2-023", + "claim": "SAM+Tracker Baseline - Click-to-Mask Reconstruction for Correction: # Baseline correction strategy for SAM+XMem++ and SAM+Cutie:\n # SAM handles per-frame mask prediction, tracker handles temporal propagation.\n # Correction on an intermediate frame requires reconstructing tracker output in SAM:\n\n def apply_correction(tracker_mask_current, new_correction_clicks, frame_img):\n # Step 1: Reconstruct tracker's output mask inside SAM via iterative click sampling from the tracker output mask, feeding sampled clicks to SAM until the reconstructed mask reaches IoU > 0.8 with the tracker output mask (following the EVA-VOS strategy, Delatolas et al., 2024):\n reconstruction_clicks = []\n sam_mask = None\n while IoU(sam_mask, tracker_mask_current) < 0.8:\n next_click = sample_click_from_error_region(sam_mask, tracker_mask_current)\n reconstruction_clicks.append(next_click)\n sam_mask = SAM.predict(frame_img, reconstruction_clicks)\n \n # Step 2: Concatenate new correction clicks with initial reconstruction clicks\n all_clicks = reconstruction_clicks + new_correction_clicks\n corrected_mask = SAM.predict(frame_img, all_clicks)\n return corrected_mask\n\nNote: This approach (reconstruct SAM mask then add correction clicks) works better than alternatives such as feeding tracker output mask as a mask prompt directly, or using only correction clicks while ignoring the tracker output mask (Appendix F.1.4).\n\nVariable definitions:\n tracker_mask_current: mask predicted by XMem++ or Cutie tracker on the current frame\n new_correction_clicks: list of (x, y, type) correction clicks provided by user for this interaction round\n reconstruction_clicks: clicks iteratively sampled from error region between SAM mask and tracker mask to reconstruct tracker output inside SAM\n sam_mask: intermediate mask predicted by SAM during iterative reconstruction step\n IoU_threshold: 0.8 threshold for reconstruction fidelity (following EVA-VOS strategy)\n all_clicks: concatenated list of reconstruction_clicks + new_correction_clicks fed to SAM for final corrected mask", + "source": "6.1, F.1.4" + } + ], + "D3": [ + { + "id": "sam2-D3-001", + "claim": "Data engine Phase 1 (SAM per frame baseline). Annotators segment a target object in every video frame individually at 6 FPS using image-based interactive SAM with pixel-precise manual editing tools (brush and eraser). No temporal tracking model is involved; all frames are annotated from scratch. This phase yields high-quality per-frame spatial annotations used as ground-truth reference for quality evaluation and for annotating SA-V val/test sets.", + "source": "Section 5.1 Data Engine Phase 1, Section E.2" + }, + { + "id": "sam2-D3-002", + "claim": "Data engine Phase 2 (SAM + SAM 2 Mask). Annotators use SAM to create a spatial mask in the first frame, then use SAM 2 Mask (accepting only mask prompts) to temporally propagate the mask to subsequent frames. At any frame, annotators can edit predictions by re-annotating with SAM from scratch and re-propagating with SAM 2 Mask. SAM 2 Mask was initially trained on Phase 1 data and public datasets, then retrained twice during Phase 2 on newly collected data.", + "source": "Section 5.1 Data Engine Phase 2" + }, + { + "id": "sam2-D3-003", + "claim": "Data engine Phase 3 (fully-featured SAM 2). Utilizes the complete SAM 2 model accepting various prompt types (points, masks) with memory context across frames. Annotators provide occasional refinement clicks (not full re-annotation) to correct predicted masklets in intermediate frames. SAM 2 was retrained and updated five times during Phase 3. Annotators focus on challenging objects requiring at least 2 edited frames; videos are pre-filled with verified auto masklets, and annotators find additional un-annotated challenging objects.", + "source": "Section 5.1 Data Engine Phase 3" + }, + { + "id": "sam2-D3-004", + "claim": "Auto masklet generation. SAM 2 is prompted with regular grid points in the first frame to generate candidate masklets. These undergo post-processing (remove tiny components <200 px, fill holes <200 px) and are sent to the verification step. Masklets tagged as 'satisfactory' are added to the dataset; 'unsatisfactory' ones are sampled and sent to annotators for refinement in Phase 3.", + "source": "Section 5.1 Auto masklet generation, Section E.1" + }, + { + "id": "sam2-D3-005", + "claim": "Training data engine progression evaluation. Evaluate SAM 2 trained on progressively accumulated data from each data engine phase (keeping training iterations fixed) to measure the performance impact of additional data. Metrics reported on SA-V val set and 9 zero-shot video benchmarks using 3-click prompts on the first frame.", + "source": "Section 5.1 Table 2, Section F.1" + }, + { + "id": "sam2-D3-006", + "claim": "Promptable Video Segmentation (PVS) -- Interactive offline evaluation. Multi-pass interactive evaluation over the entire video. Start with 3 clicks on first frame, segment the object throughout the entire video. In subsequent passes, select the frame with lowest segmentation IoU w.r.t. ground-truth as the new prompting frame. Model re-segments the entire video based on all accumulated prompts, repeating up to N_frame=8 passes. Clicks are sampled at object center for initial pass, then at centroid of error region for correction passes.", + "source": "Section 6.1, Section F.1.2, Figure 5a" + }, + { + "id": "sam2-D3-007", + "claim": "Promptable Video Segmentation (PVS) -- Interactive online evaluation. Single forward pass through the video. Start with 3 clicks on the first frame, propagate forward. Pause propagation when frame has low-quality prediction (IoU < 0.75 with GT), add 3 correction clicks on that frame, resume forward propagation. New prompts only affect subsequent frames (not previous). Repeat until N_frame=8 prompted frames.", + "source": "Section 6.1, Section F.1.2, Figure 5b" + }, + { + "id": "sam2-D3-008", + "claim": "Semi-supervised VOS evaluation. Prompts (1/3/5 clicks, bounding box, or ground-truth mask) are provided only on the first video frame. The model must track the object throughout the video without further interaction. For click prompts, the initial click is at the object center and subsequent clicks are at the centroid of the error region between prediction and GT.", + "source": "Section 6.2, Section F.1.3, Table 4" + }, + { + "id": "sam2-D3-009", + "claim": "Image segmentation (SA task) zero-shot evaluation. Evaluate on 37 datasets (23 from SAM's original benchmark + 14 new video-derived datasets) using 1-click and 5-click interactive segmentation with mIoU metric. Models are compared on a single A100 GPU with batch size 10 for FPS measurement. All image encoders compiled with torch.compile.", + "source": "Section 6.3, Section F.4, Table 5, Table 16" + }, + { + "id": "sam2-D3-010", + "claim": "Semi-supervised VOS state-of-the-art comparison. Evaluate SAM 2 against prior VOS methods using ground-truth first-frame mask prompts (standard VOS protocol). Compare two SAM 2 variants (Hiera-B+ and Hiera-L) on multiple VOS benchmarks. Models run at batch size 1 on a single A100 GPU for speed benchmarking.", + "source": "Section 7, Table 6, Section H Table 19" + }, + { + "id": "sam2-D3-011", + "claim": "Data mixture ablation. Train SAM 2 on different combinations of VOS datasets (DAVIS, MOSE, YouTubeVOS), Internal-train, SA-V, and SA-1B. Fix training iterations (200k) and batch size (128); only training data changes. Pre-train on SA-1B then train separate model for each data mix setting. Report J&F with 3-click first-frame prompts and 1-click mIoU on SA-23.", + "source": "Section A.1, Table 7" + }, + { + "id": "sam2-D3-012", + "claim": "Data quantity scaling ablation. Pre-train SAM 2 on SA-1B, then train on varying sizes of SA-V data to study power-law scaling of video segmentation accuracy with training data quantity. Report J&F with 3-click first-frame prompts.", + "source": "Section A.1 Data quantity ablation, Figure 6" + }, + { + "id": "sam2-D3-013", + "claim": "Data quality (filtered subsets) ablation. Compare training on: (1) 50K randomly sampled SA-V masklets, (2) 50K most-edited masklets (hard examples), (3) full 190K SA-V masklets. All variants also include SA-1B.", + "source": "Section A.1 Table 8" + }, + { + "id": "sam2-D3-014", + "claim": "Architecture capacity abatements -- Input size and memory. Conducted at 512 resolution (default for ablations). Test input resolution (512, 768, 1024), number of frames per sequence (4, 8, 10), number of memories N (4, 6, 8), memory channel dimension (64, 256), memory attention layers ((sa=2,ca=2), (sa=3,ca=2), (sa=4,ca=4)), and image encoder size (T, S, B+, L). Report J&F for video and mIoU for image, plus relative speed.", + "source": "Section A.2.1, Table 9" + }, + { + "id": "sam2-D3-015", + "claim": "Relative positional encoding ablation. Test: (1) default (no RPB in image encoder, no 2d-RoPE in memory attention), (2) 2d-RoPE in memory attention, (3) 2d-RoPE in both memory attention and image encoder (replacing RPB). Evaluate on standard video benchmarks plus LVOSv2 for long-term VOS assessment.", + "source": "Section A.2.2, Table 10" + }, + { + "id": "sam2-D3-016", + "claim": "Memory architecture ablation -- GRU and object pointers. Test: (1) baseline (no GRU, no object pointers), (2) GRU for recurrent memory, (3) object pointer cross-attention from mask decoder output tokens. Evaluate on standard benchmarks plus LVOSv2 for long-term assessment.", + "source": "Section A.2.3, Table 11" + }, + { + "id": "sam2-D3-017", + "claim": "SA-V data transferability to other models (Cutie experiment). Train Cutie on two data mixtures: (1) standard VOS data (DAVIS, YouTubeVOS, MOSE) per its official recipe, (2) standard VOS data + SA-V using the same mixture ratio as SAM 2. Compare for mask-input VOS and interactive click/box-input settings on SA-V test, standard VOS benchmarks, and 17 zero-shot datasets.", + "source": "Section G, Table 17" + }, + { + "id": "sam2-D3-018", + "claim": "DAVIS interactive benchmark evaluation. Evaluate SAM 2 on the DAVIS interactive benchmark which provides scribble prompts during sequential interaction rounds. Follow CiVOS strategy to convert scribbles to positive/negative clicks. Official server selects the frame with worst segmentation for next interaction. Report AUC-J&F and J&F@60s.", + "source": "Section F.2, Table 14" + }, + { + "id": "sam2-D3-019", + "claim": "VIPOSeg benchmark evaluation. Evaluate SAM 2 on VIPOSeg in both zero-shot (no training on VIPOSeg) and fine-tuned (training on VIPOSeg training split) settings. Report overall G metric and decay lambda (robustness in crowded scenes, lower is better). Compare against PAOT (prior best model trained on VIPOSeg).", + "source": "Section F.3, Table 15" + }, + { + "id": "sam2-D3-020", + "claim": "Fairness evaluation on demographic groups. Collect annotations for the people category in Ego-Exo4D dataset (contains self-reported demographics). Use Phase 1 annotation setup (per-frame high quality SAM). Evaluate SAM 2 with 1-click, 3-click, and GT mask prompts on first frame. Compare J&F across gender (male/female) and age groups (18-26, 26-50, 50+).", + "source": "Section E.1.1, Table 13" + }, + { + "id": "sam2-D3-021", + "claim": "SAM 2 full training protocol (SA-1B pre-training + joint video/image training + 16-frame fine-tuning). Stage 1: pre-train on SA-1B static images (AdamW, lr=4e-4, reciprocal sqrt schedule, 90K steps, batch=256, 1024 resolution, 7 correction clicks). Stage 2: joint training on SA-V + Internal + SA-1B subset + optional OSS VOS datasets with alternating image/video batches, 8-frame sequences, interactive simulation with up to 2 prompted frames, mosaic transform (10% prob), reverse temporal order (50% prob). Stage 3: fine-tune on 16-frame sequences from most-challenging masklets (top 50% edited), 50K iters, half lr, freeze image encoder.", + "source": "Section 4 Training, Section D.2, Table 12" + } + ], + "D4": [ + { + "id": "sam2-D4-001", + "claim": "SAM 2 Training Pipeline: Stage 1 — Pre-train on SA-1B images with image encoder + mask decoder (Appendix D.2.1). Stage 2 — Full training on mixed video (SA-V) + image data, jointly optimizing image encoder, memory attention, mask decoder, and prompt encoder (Appendix D.2.2). Stage 3 — Fine-tune on long video sequences (>16 frames) to improve temporal consistency and long-range tracking quality (Appendix D.2.2).", + "source": "Section 4, Appendix D.2" + }, + { + "id": "sam2-D4-002", + "claim": "Data Engine Pipeline: Phase 1 — Human annotators label every frame with pixel-perfect masks using SAM 2 interactive mode (high quality, slow). Phase 2 — SAM 2 assists: model propagates masks across frames, annotators correct errors at ~7.4s per frame (medium quality, faster). Phase 3 — Fully automatic masklet generation: SAM 2 processes video without human input at <1s per frame (lower quality but massive scale, generating SA-V dataset).", + "source": "Section 3, Appendix E" + }, + { + "id": "sam2-D4-003", + "claim": "PVS Interactive Evaluation Protocol: (1) Annotator clicks on one frame to select object. (2) SAM 2 propagates mask bidirectionally across all frames. (3) Annotator reviews and clicks again on any frame where correction is needed. (4) Model re-propagates with updated prompt. (5) Repeat up to N_frame=8 frames with N_click=3 clicks per frame. (6) Final mask accepted when annotator approves or click budget exhausted.", + "source": "Section 6.1, Appendix F.1.2" + }, + { + "id": "sam2-D4-004", + "claim": "SA-V Dataset Construction Pipeline: (1) Sample ~50K videos from diverse sources. (2) Phase 1 human annotation produces ~15K high-quality masklets. (3) Phase 2 model-assisted annotation scales to ~150K masklets. (4) Phase 3 fully automatic generation produces ~35M masklets for pre-training data. (5) Quality filtering removes low-confidence masklets via IoU and temporal consistency checks.", + "source": "Section 3, Appendix E" + }, + { + "id": "sam2-D4-005", + "claim": "Evaluation Protocol: (1) Zero-shot video object segmentation on DAVIS 2017, YouTube-VOS, MOSE, SA-V test. (2) Interactive evaluation via PVS benchmark with varying click budgets (1/3/5/8 frames). (3) Ablation studies on memory bank size (N), number of frames per training clip, image encoder variants (Hiera T/S/B+/L), and resolution. (4) Metrics: J&F (region + boundary), IoU, human-annotated quality ratings.", + "source": "Section 6, Appendix F" + } + ] +} \ No newline at end of file diff --git a/papers/sc-fno/blacklist.txt b/papers/sc-fno/blacklist.txt new file mode 100644 index 0000000000000000000000000000000000000000..8251e5babdb8783b7052ef3100def2d951169ba8 --- /dev/null +++ b/papers/sc-fno/blacklist.txt @@ -0,0 +1,2 @@ +# Official repository (ICLR 2025) +https://github.com/AMBehroozi/SC_Neural_Operators diff --git a/papers/sc-fno/config.yaml b/papers/sc-fno/config.yaml new file mode 100644 index 0000000000000000000000000000000000000000..ba31df23bd6a184cff8a6f1a8c9bcac6e5687f9b --- /dev/null +++ b/papers/sc-fno/config.yaml @@ -0,0 +1,8 @@ +title: "Sensitivity-Constrained Fourier Neural Operators (SC-FNO)" +pdf_url: "https://arxiv.org/pdf/2505.08740.pdf" +venue: "ICLR 2025" +year: "2025" +extra: + selection_index: 30 + domain: "Numerical Methods / Scientific Computing" + paradigm: "Theoretical Analysis" diff 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Electrical Engineering and Computer Science +Penn State University +University Park, PA 16802-1408, USA +duk17@psu.edu + +# ABSTRACT + +Parametric differential equations of the form $\begin{array} { r } { \frac { \partial u } { \partial t } = f ( u , x , t , p ) } \end{array}$ are fundamental in science and engineering. While deep learning frameworks like the Fourier Neural Operator (FNO) efficiently approximate differential equation solutions, they struggle with inverse problems, sensitivity calculations $\frac { \partial u } { \partial p }$ , and concept drift. We address these challenges by introducing a novel sensitivity loss regularizer, demonstrated through Sensitivity-Constrained Fourier Neural Operators (SC-FNO). Our approach maintains high accuracy for solution paths and outperforms both standard FNO and FNO with Physics-Informed Neural Network regularization. SC-FNO exhibits superior performance in parameter inversion tasks, accommodates more complex parameter spaces (tested with up to 82 parameters), reduces training data requirements, and decreases training time while maintaining accuracy. These improvements apply across various differential equations and neural operators, enhancing their reliability without significant computational overhead $( 3 0 \% - 1 3 0 \%$ extra training time per epoch). Models and selected experiment code are available at: https://github.com/AMBehroozi/SC_Neural_Operators. + +# 1 INTRODUCTION + +Ordinary and Partial Differential Equations (ODEs and PDEs), which contain physically meaningful parameters p, form the foundation of most modern engineering systems across diverse fields such as fluid mechanics, medicine design, molecular dynamics, relativistic physics, climate, and environmental sciences (Palais & Palais, 2009). These physical parameters p, whether describing fixed or evolving system properties (Feng et al., 2022), physiological constants (Reichert & Omlin, 1997), environmental characteristics (Tartakovsky et al., 2020), or serving as simplifications for subgrid-scale processes (Yuval & O’Gorman, 2020; Watt-Meyer et al., 2024), significantly influence the behavior of the equations. Neural Operators are neural networks whose inputs are initial conditions and physical parameters, and whose output is a function u, also known as a solution path, that approximates the solution to the ODE/PDE. When the values of $\mathbf { p }$ are known, neural operators can be used for simulations (predicting system state ${ \bf \delta u } ( t )$ at time $t$ ). However, in many cases, there is uncertainty about the values of p, so they must be estimated from observations of u. This is known as (parameter) inversion. The estimated p can then be used to simulate u beyond existing observations, or they may themselves be quantities of interest to scientists. Other potential uses of neural operators include sensitivity analysis (i.e., estimating $\partial \mathbf { u } / \partial \mathbf { p } )$ and applying them in situations not covered by training data. Even state-of-the-art neural operators struggle with these applications and this paper presents a method to address this problem. + +Fourier Neural Operators (FNOs). Introduced in 2021, FNOs (Li et al., 2021; 2024) leverage the linear nature of differential operators in Fourier space, delivering orders of magnitude of computational efficiencies compared to traditional numerical solvers. They are currently the most popular type of neural operator. FNOs are known for their meshless—or discretization-invariant—characteristics, and some versions eliminate the need for time-stepping, allowing them to output solutions at any spatiotemporal resolution. FNOs achieve these improvements over prior surrogate models (Audet et al., 2000; Alizadeh et al., 2020) by utilizing representations in the frequency domain to efficiently learn time trajectories of the evolution of initial conditions. Learning the trajectories in time domain would necessitate large amounts of weights and extensive training data. + +While this method offers significant efficiency, prediction accuracy may decline for predictions far into the future (Grady et al., 2023), prompting the use of additional constraints in some models (Jiang et al., 2023; Bonev et al., 2023). FNO is so far mostly used in forward simulations and research; the critical issues of parameter inversion have received much less attention by neural operator research and estimating the parameter sensitivity ${ \partial \mathbf { u } } / { \partial \mathbf { p } }$ has not been studied for neural operators (to the best of our knowledge). Li et al. (2021) evaluate the use of FNO to run the differential equation backward in time to recover initial conditions; although they name it Bayesian inversion, they did not recover physical parameters. In follow-up work, (Li et al., 2023) also observed the problems of using FNO for parameter inversion and proposed to add physics-informed neural network (PINN) regularization to training. While it improves over vanilla FNO, we show that our approach can outperform $\mathrm { F N O + P I N N }$ by up to $30 \%$ . Furthermore, $\mathrm { F N O + P I N N }$ still produces inaccurate sensitivity estimates. An alternative to neural operators is learning direct inverse mapping (Vadeboncoeur et al., 2023; Li et al., 2023), but then one still needs separate techniques for estimating solution paths and sensitivities. In contrast, SC-FNO addresses all of these issues in one (convenient) framework. + +Sensitivity awareness allows a model to adapt its predictions to different scenarios and respond to input and parameter changes, thus reducing overfitting and improving robustness. In practical applications, the ability to predict these Jacobians of the outputs with respect to physical parameters p can also have significant value. In engineering design and control systems, knowing the sensitivity of performance outputs relative to design parameters helps in optimizing design and improving performance. Similarly, in financial modeling or resource management, sensitivity information allows for better risk assessment and resource allocation decisions. Gradient-based optimization or data assimilation (Barker et al., 2004), of course, is critically guided by gradients. In scenario analysis, parameters or inputs are perturbed, often beyond the observed ranges, to assess the responses. Without correctly capturing the sensitivity, the responses could be erroneous (Saltelli et al., 2019). + +Gradient Computation for PDE Solvers: Gradient, or sensitivity computations, are widely used in the analysis of partial differential equations (PDEs). Traditionally, methods like finite differences can approximate gradients using existing numerical solvers and they remain relevant. Now we also have the option of using automatic differentiation (AD) offered by differentiable-programming platforms such as PyTorch, TensorFlow, Julia, or JAX, which enable efficient computation of gradients of outputs with respect to inputs. While the main purpose of implementing solvers on these platforms is often to train process-based equations together ("end-to-end") with NNs so that missing relationships can be learned along with partial knowledge (Innes et al., 2019; Shen et al., 2023; Zhu et al., 2023; Schoenholz & Cubuk, 2020; Tsai et al., 2021), these solvers can indeed provide rapid calculation of gradients as a byproduct. Gradients can be also computed by adjoint methods, either "discretize-thenoptimize"(Onken & Ruthotto, 2020; Song et al., 2024a) or "optimize-then-discretize" (Chen et al., 2018). Differentiable PDE solvers are rapidly increasing across numerous domains, demonstrating highly competitive performance, especially in data-scarce (Feng et al., 2023) or unseen extreme (Song et al., 2024b) scenarios. Our work is compatible with either finite differences or AD. In this context, our work introduces a novel methodology that seamlessly integrates sensitivity analysis directly into the Fourier Neural Operator framework, significantly enhancing the operator’s capability to tackle both forward and inverse problems. Notable studies such as Gradient enhanced neural networks Liu & Batill (2000) and Sobolev training Czarnecki et al. (2017) have explored the utilization of derivative information to improve neural network training, but they focused on low-dimensional approximation of derivatives. Our proposed Sensitivity-Constrained Fourier Neural Operator (SC-FNO) overcomes the problems with the estimation of parametric sensitivity, allowing better inversion while maintaining FNO-level computational efficiency for solving differential equations. The SC-FNO can provide highly accurate parameter sensitivities with less training data, ensuring that solution accuracy is maintained even under input perturbations and even concept drift (physical parameters in testing exceed ranges encountered during training). Although we focus attention on FNOs here, our experiments show the approach generalizes to many other neural operators (see Appendix D.1). + +# 2 METHODOLOGY + +Training data for Neural Operators are obtained by running physics-based model simulations multiple times with different initial conditions and physical parameters. We note that for differentiable physicsbased models, the sensitivities $\partial \mathbf { u } / \partial \mathbf { p }$ can also be computed. For non-differentiable models, they can be approximated with finite differences. The training of SC-FNO uses a loss function $L _ { u }$ over the solution paths (as with FNO) and also adds a loss $L _ { s }$ over the parameter sensitivities. One can optionally add a PINN equation loss $L _ { E q }$ (resulting in SC-FNO-PINN). This framework can be used with other neural operators as well (see Appendix D.1). + +# 2.1 SC-FNO: ENHANCING FNO WITH SENSITIVITY + +Original Fourier Neural Operators (Li et al., 2021): FNOs represent a class of learning architectures designed to model mappings between infinite-dimensional function spaces, thereby providing efficient solutions to complex parametric PDEs. Mathematically, FNOs operate within a defined domain $\mathbf { D } \subset \mathbf { R } ^ { d }$ . The function spaces, denoted as ${ \bf A } ( { \bf D } ; { \bf R } ^ { d _ { a } } )$ for inputs and ${ \bf U } ( { \bf D } ; { \bf R } ^ { d _ { u } } )$ for outputs, encapsulate the functional domains of the inputs and outputs respectively. The goal of an FNO is to approximate the operator $\mathbf { G } : \mathbf { A } \times \Theta \mathbf { U }$ , which ideally maps each input function $\mathbf { a } _ { j }$ in $\mathbf { A }$ to an output function $\mathbf { u } _ { j }$ in U. Formally, the FNO framework is expressed as: + +$$ +\widetilde { \mathbf { G } } : \mathbf { A } \times \boldsymbol { \Theta } \mathbf { U } , +$$ + +where $\Theta$ represents the space of parameters that the FNO optimizes to reduce the discrepancy between predicted and true outputs. The Fourier transformation, essential in the operation of FNOs, is applied to transform function data into the frequency domain, facilitating the application of linear operators: + +$$ +( \mathbf { F f } ) _ { j } ( \mathbf { k } ) = \int _ { \mathbf { D } } \mathbf { f } _ { j } ( \mathbf { x } ) e ^ { - 2 \pi i \langle \mathbf { x } , \mathbf { k } \rangle } \mathrm { d } \mathbf { x } \qquad \mathrm { a n d } \qquad ( \mathbf { F } ^ { - 1 } \mathbf { f } ) _ { j } ( \mathbf { x } ) = \int _ { \mathbf { D } } \mathbf { f } _ { j } ( \mathbf { k } ) e ^ { 2 \pi i \langle \mathbf { x } , \mathbf { k } \rangle } \mathrm { d } \mathbf { k } . +$$ + +This representation utilizes the convolution theorem to model complex nonlinear behaviors efficiently: + +$$ +\mathbf { K } ( \phi ) \mathbf { v } _ { t } ( \mathbf { x } ) = \mathbf { F } ^ { - 1 } ( \mathbf { R } _ { \phi } \cdot ( \mathbf { F } \mathbf { v } _ { t } ) ) ( \mathbf { x } ) , +$$ + +where $\mathbf { R } _ { \phi }$ represents a learnable parameterized function within the Fourier domain, allowing FNOs to encode and decode information effectively. While FNOs offer significant computational advantages, they necessitate extensive training data (Kovachki et al., 2021; Li et al., 2021). Moreover, generalizing beyond the training datasets remains a challenge. + +Our Contribution: We introduce sensitivity-based loss terms to Neural Operators, resulting in a novel framework we call Sensitivity-Constrained Neural Operators (SC-NO) (Figure A.7 in Appendix A). For clarity, we demonstrate this concept primarily through its application to Fourier Neural Operators (FNO), yielding SC-FNO. However, we also show that the framework is generalizable to various types of neural operators, such as Wavelet Neural Operators (Tripura & Chakraborty, 2023), Multiwavelet Neural Operators (Gupta et al., 2021), and DeepONets (Wang et al., 2021) (descriptions and results are in Appendix D.1). The current neural operator training never harnessed sensitivity information (such as Jacobians) and never explicitly guaranteed how the inputs should be used in the model. However, our approach leverages either finite difference or a differentiable solver to compute these sensitivities. This development improves the robustness of prediction of both u and $\partial \mathbf { u } / \partial \mathbf { p }$ by ensuring that input variables are utilized correctly by the FNO — we dictate the sensitivity of the predicted outcomes with respect to input parameters and initial conditions, while in the future boundary conditions can also be incorporated. The model is expressed concisely as: + +$$ +\mathbf { u } ( \mathbf { x } , t ) = \mathcal { F } _ { \mathrm { S C - F N O } } ( \mathbf { u } _ { 0 } , \mathbf { x } , t , \mathbf { p } ) , +$$ + +where $\mathcal { F } _ { \mathrm { S C - F N O } }$ denotes the SC-FNO mapping, $\mathbf { u } _ { 0 }$ are the initial conditions, $\mathbf { X }$ represents spatial coordinates, $t$ is time. The output $\mathbf { u }$ is governed by the differential equation: + +$$ +\frac { \partial { \bf u } } { \partial t } = f ( { \bf u } , { \bf x } , t , { \bf p } ) , +$$ + +with $f$ defining the dynamics of the system. In this formulation, $\mathbf { p }$ encapsulates parameters that critically modulate, influence, or characterize the system, including physical constants, material properties, or scale-dependent parameterizations. SC-FNO incorporates a sensitivity loss to enforce the accuracy of the predicted sensitivities (Jacobians) against those computed from the differentiable numerical solver. The SC-FNO model computes the solution u across all time and space in a single execution. The mathematical definition of sensitivity loss $L _ { \mathrm { s } }$ is as follows: + +$$ +L _ { \mathrm { s } } = \frac { 1 } { M } \sum _ { j = 1 } ^ { M } \left\| \frac { \partial \hat { \mathbf { u } } ( \mathbf { x } _ { j } , t _ { j } ; \mathbf { p } ) } { \partial \mathbf { p } } - \frac { \partial \mathbf { u } ( \mathbf { x } _ { j } , t _ { j } ; \mathbf { p } ) } { \partial \mathbf { p } } \right\| ^ { 2 } . +$$ + +where $\mathbf { x } _ { j }$ and $t _ { j }$ represent the spatial and temporal coordinates of the sampled points at which the Jacobians are evaluated. ${ \partial \hat { \mathbf { u } } } / { \partial \bar { \mathbf { p } } }$ is the Jacobian of the predicted outputs with respect to the input parameters, obtained through AD applied to the SC-FNO. ${ \partial \mathbf { u } } / { \partial \mathbf { p } }$ is the true Jacobian derived from precise differentiable numerical solvers or known analytical solutions. $M$ denotes the number of evaluation points across the domain, including points used to impose boundary and initial conditions. + +# 2.2 THE PINN-LOSS EQUATION AS AN OPTIONAL REGULARIZER + +Physics-Informed Neural Networks (PINNs) regularize the gradients of the neural networks using the governing differential equations at collocation points and boundary conditions Raissi et al. (2019); Karniadakis et al. (2021); He et al. (2022). The core PDE constraint in PINNs is represented as: + +$$ +\mathcal { N } [ \mathbf { u } ( \mathbf { x } , t ) ; \mathbf { p } ] = 0 +$$ + +The PINN loss function, $L _ { \mathrm { E q } }$ , incorporates the discrepancy from the PDE with terms for initial and boundary conditions ( $L _ { \mathrm { I C } }$ and $\scriptstyle L _ { \mathrm { B C , } }$ ): + +$$ +L _ { \mathrm { E q } } = L _ { \mathrm { P D E } } + \alpha ( L _ { \mathrm { I C } } + L _ { \mathrm { B C } } ) = \frac { 1 } { N } \sum _ { i = 1 } ^ { N } | \mathcal { N } [ \mathbf { u } ( \mathbf { x } _ { i } , t _ { i } ) ; \mathbf { p } ] | ^ { 2 } + \alpha ( L _ { \mathrm { I C } } + L _ { \mathrm { B C } } ) , +$$ + +where $\alpha$ is a weighting factor, and $N$ denotes the number of collocation points in the computational domain. PINNs trained for specific conditions often cannot predict general evolutionary trajectories or guarantee accurate parameter sensitivity Jin et al. (2021); Ren et al. (2022). Additionally, PINNs can be computationally slower than traditional numerical solvers. In this work, we evaluate the ability of PINN-type loss to improve prediction accuracy along with sensitivities. In a broadened sense, SC-FNO can be considered a novel type of PINN as it also regularizes the gradient of the neural networks, but their procedures are fundamentally different. PINNs do not have access to this sensitivity, as usual PDEs do not contain $\partial \mathbf { u } / \partial \mathbf { p }$ . SC-FNO prepares gradient data to supervise ${ \partial \mathbf { u } } / { \partial \mathbf { p } }$ but do not require optimization during forward simulation, whereas PINNs rely on collocation points for equation-based loss optimization to supervise $( \partial { \mathbf { u } } / \partial { \mathbf { x } } , \partial { \mathbf { u } } / \partial { \mathbf { t } } )$ , but not $\partial \mathbf { { \bar { u } } } / \partial \mathbf { { p } }$ . SC-FNO is designed to serve as an efficient forward simulator that can handle various changes in high dimensional input, which can also help with inversion tasks. + +# 2.3 GRADIENT COMPUTATION METHODS FOR DIFFERENTIAL EQUATIONS + +To prepare training and validation datasets containing true solution paths along with their sensitivity (gradients with respect to parameters), we developed and implemented two distinct approaches: + +1. A differentiable numerical solver based on the torchdiffeq package. We extended this ODE-oriented framework to handle PDEs by reformulating them in the form $\begin{array} { r } { \frac { d \mathbf { u } } { d t } = \mathbb { R } \mathrm { H } \mathbf { S } ( \mathbf { x } ) } \end{array}$ , where $\mathrm { R H S } ( \mathbf { x } )$ encapsulates terms related to spatial derivatives. This solver leverages PyTorch’s AD capability, although adjoint methods can also be used. The computation time for preparing datasets with and without Jacobian computation is presented in Table D.12. 2. Finite difference methods for gradient computation with a traditional solver. The method approximates gradients by solving the PDE multiple times with slightly perturbed parameter values and computing the differences between the solutions. This process offers a nonintrusive means of gradient estimation for any existing numerical solvers. + +We implemented efficient functions to compute the sensitivities. These solution paths and sensitivities are computed once and archived for later use, though they could also be generated on the fly if desired. This represents a one-time cost per equation, and since input impacts are resolved upfront, no repeated preparation is needed for different parameters. We assessed the effectiveness of both gradient computation methods and compared their accuracies. + +# 2.4 IMPLEMENTATION DETAILS + +The SC-FNO architecture processes parameters $\mathbf { \tau } ( \mathbf { p } )$ alongside spatial coordinates and initial conditions through the lifting layer as function inputs. This layer reshapes and repeats parameters to match the problem’s spatial-temporal dimensions, then concatenates them with other inputs before neural network processing. Notably, both FNO and SC-FNO share identical neural network architectures and inputs, differing only in their loss configurations. After computing true gradients $( \partial \mathbf { u } / \partial \mathbf { p } )$ once during dataset preparation (Section 2.3), our training procedure contains several efficiency-focused steps. Instead of computing gradients at all points, we randomly select a subset of spatial-temporal points in each epoch $\mathit { n } < N$ spatial points $\times t < T$ time points) where $n < N$ and $t < T$ . The neural operator’s predicted gradients at these points are computed using AD and compared with the pre-computed true gradients. This sampling varies between epochs to eventually cover the full solution space. This procedure eliminates the need for additional solver runs during training while maintaining effective sensitivity supervision. Each minibatch requires only one forward pass before applying AD, adding minimal computational overhead. The pre-computed gradients are stored and reused throughout training, making the approach efficient as parameter dimensionality increases. + +# 3 EXPERIMENTS + +We evaluated four different configurations of the FNO models, each defined by a unique combination of loss functions. The first configuration uses only the data loss $L _ { \mathrm { u } }$ (FNO), which matches predicted state variables with the solution paths; The second, $L _ { \mathrm { u } } + L _ { \mathrm { E q } }$ (FNO-PINN), incorporates the equation loss; The third, ${ \cal L } _ { \mathrm { u } } + { \cal L } _ { \mathrm { E q } } + { \cal L } _ { s }$ (SC-FNO-PINN), combining both sensitivity and equation losses; and finally, $L _ { \mathrm { u } } + L _ { s }$ (SC-FNO), which adds only sensitivity loss to data loss. Pseudocodes for these models can be found in the Appendix A. We evaluated the competing methods on the following well-known differential equations. + +ODE1: Composite Harmonic Oscillator and ODE2: Duffing Oscillator Equation each operate within a temporal domain of $t \in [ 0 , 1 ]$ , where was discretized into $N = 1 0 0$ equal time steps. Our goal was to learn the operator that maps the first $M$ time steps of solutions $u$ , alongside parameters $\mathbf { p }$ , to the solutions at the next $N - M$ subsequent time steps, formulated as $u : [ 0 : M ] \cup { \bf p } u : [ M : N ]$ + +PDE1: Generalized Nonlinear Damped Wave Equation is explored within both a temporal domain $t \in [ 0 , 1 ]$ and a spatial domain $x \in [ 0 , 1 ]$ . The temporal domain was discretized into $N = 3 0$ equal time steps and the spatial domain was discretized into $S _ { x } = 2 0$ . We aim to learn the operator that maps from the first $M$ initial time steps of $u$ , alongside parameters $\mathbf { p }$ , to the next $N - M$ subsequent time steps, expressed as $u : [ 0 , S _ { x } ] ^ { 1 } \stackrel { \cdot } { \times } [ 0 , M ] \cup \bar { \bf p } u : [ 0 , S _ { x } ] ^ { 1 } \stackrel { \cdot } { \times } [ M , N ]$ . + +PDE2: Forced Burgers’ Equation is analyzed across a time interval $t \in [ 0 , \pi ]$ and a spatial domain $x \in [ 0 , 1 ]$ . The temporal domain is segmented into $N = 3 0$ equal parts, and the spatial domain into $S _ { x } = 4 0$ parts. The aim is to identify the operator that maps the first $M$ time steps of $u$ , in conjunction with parameters $\mathbf { p }$ , to the remaining $N - M$ time steps, formulated as $\boldsymbol { u } : [ 0 , S _ { x } ] ^ { 1 } \times [ 0 , \dot { M } ] \cup \mathbf { p } \boldsymbol { u } : [ 0 , S _ { x } ] ^ { 1 } \times [ \bar { M } , N ]$ . + +PDE3: Stream Function-Vorticity Formulation of the Navier-Stokes Equations spans the temporal domain $t \in [ 0 , 3 ]$ and spatial domains $x , y \in [ 0 , 1 ]$ . The spatial domain and the temporal domain are $[ 0 , 1 ]$ and $[ 0 , 3 ]$ , respectively. The 2D spatial domain was discretized into $S _ { x } = S _ { y } = 6 4$ equal spatial divisions. Here, our goal is to learn an operator that takes initial conditions of $u$ along with parameters $\mathbf { p }$ , and directly maps them to the solution of vorticity at the final time step $t = 3 s$ formulated as $u : [ 0 , S _ { x } ] \times [ 0 , \dot { S } _ { y } ] \stackrel { \cdot } { \times } [ t = 0 ( s ) ] \cup \mathbf { p } u : [ 0 , S _ { x } ] \times [ \dot { 0 } , S _ { y } ] \times [ t = 3 ( s ) ]$ . + +PDE4: Allen-Cahn equation is analyzed within temporal domain $t \in [ 0 , 1 . 0 ]$ and spatial domain $x \in [ 0 , 1 ]$ , discretized into $N = 3 0$ and $S _ { x } = 4 0$ parts respectively. We aim to learn the operator that maps from first $M$ time steps of $u$ and parameters $\mathbf { p }$ to the next $N - M$ time steps, formulated as $u : [ 0 , S _ { x } ] ^ { 1 } \times [ 0 , M ] \cup \mathbf { p } u : [ 0 , S _ { x } ] ^ { 1 } \times [ M , N ]$ . + +Detailed specifications of the differential equations and the ranges of their parameters are provided in Table B.6 in Appendix B. The architectural details, hyperparameters and training time information are comprehensively presented in Tables C.7 and C.8 in Appendix C. + +In the following, we first demonstrate the superior performance of SC-FNO in inversion (or optimization) tasks compared to FNO alone. Then we explain such outperformance using several experiments: we show how trained FNOs alone captured the gradients poorly and thus fared worse than SC-FNOs when inputs are perturbed, which often occurs during inversion. Then, we show that SC-FNO requires fewer training samples to reach higher quality, particularly when the input dimension is higher. Finally, we show that finite difference can also be functional. Experiments were run with various neural operators and PDEs to ensure the generality of the conclusions. + +# 3.1 PARAMETER INVERSION FROM SOLUTION PATHS + +Parameter inversion with PDEs plays a pivotal role in system identification, calibration, and optimization, and serves as a primary area benefiting from the efficiency of surrogate models. Our inversion experiments utilized FNO and SC-FNO surrogate models, which were trained and rigorously evaluated on synthetic datasets as described in Section 3.2 for PDE1-PDE3. These datasets, containing $2 \times 1 0 ^ { 3 }$ training samples, were generated using the differentiable numerical solver. They feature randomly generated initial conditions and parameter values, alongside their corresponding solutions at multiple time points (Table B.6 in Appendix B) and Jacobians. We used $70 \%$ of the data for training, $15 \%$ for validation, and $15 \%$ for testing, and ensured that the validation and test sets contained parameter values not encountered during training. The first experiment inverted the models to infer the $\alpha$ parameter alone while treating others as known. We then used backpropagation to optimize the parameter by minimizing the discrepancy between the synthetic data and PDE solutions. The second task simultaneously inverted all parameters of either PDE1 or PDE2. SC-FNO demonstrates notably superior performance over FNO and FNO-PINN in simple inversion tasks, achieving nearly perfect inversion while the latter two showed significantly more scattering (Figure 1). In the single-parameter inversion, SC-FNO $R ^ { 2 } = 0 . 9 9 8$ ) achieves less than 1/5 and 1/4 the relative $\mathrm { L } ^ { 2 }$ inversion errors of FNO $R ^ { 2 } = 0 . 9 0 5 )$ and FNO-PINN, respectively (Figure 1a). Multi-parameter inversion incurs larger uncertainty and greater contrasts — SC-FNO has 1/6 and $1 / 2 . 8$ the relative L² inversion errors of FNO and FNO-PINN, respectively (Figures 1b and 2). Retrieving $\alpha$ in the multi-parameter case results in large heteroscedastic scattering with FNO ( $R ^ { 2 } = 0 . 6 3 5 )$ , whereas SC-FNO remains highly accurate (Figure 1b). For PDE2 with four parameters, SC-FNO’s $R ^ { 2 }$ values are above 0.96, while those of FNO hover around 0.85. For PDE1 with five parameters, SC-FNO maintains $R ^ { 2 }$ above 0.94 for all parameters, while those of FNO drop below 0.64, suggesting overfitting and a breakdown of the surrogate model. Similar contrasts are found with PDE3 (Navier Stokes) (Figure D.10 in Appendix D). This performance gap may widen further with increased parameter dimensions. FNO-PINN’s relative $\mathrm { L } ^ { 2 }$ is half of that of FNO (Figures 1 and 2) but still $3 \mathrm { X } \mathrm { - } 5 \mathrm { X }$ that of SC-FNO. Additional scatter plots for parameter inversion are available in Figures D.8 and D.9 in Appendix D. The introduction of the sensitivity loss lead to consistent enhancements across other neural operators like WNO, MWNO, and DeepONet, with uniform conclusions reflected in Table D.11 in Appendix D. These enhancements are larger than the differences between different neural operators. By training on both solution data and their gradients, SC-FNO builds a more robust internal representation of the PDE dynamics, particularly regarding the roles of inputs. The next sections explore in more depth the advantages of gradient-aware surrogate models. + +![](images/figures/sc-fno-fig-0001.jpg) +Figure 1: Inversion of the parameter $_ \alpha$ in PDE1 using FNO and SC-FNO models (a) single parameter inversion, (b) simultaneous multi-parameter inversion. + +![](images/figures/sc-fno-fig-0002.jpg) +Figure 2: Simultaneous multi-parameter inversion accuracy for PDEs 1 and 2 using FNO and SC-FNO. + +# 3.2 SURROGATE MODEL QUALITY AND ROBUSTNESS TO INPUT PERTURBATION + +Surrogate quality assessments indicate that the superior inversion performance of SC-FNO, even with so few parameters, can be attributed to the model’s unique ability to accurately capture parameter sensitivities and its robustness to input perturbations. As discussed earlier, the surrogate models were trained on identical input and output datasets uniformly prepared for each differential equation, but only SC-FNO or SC-FNO-PINN used parameter sensitivities. We first evaluated the models on test datasets with parameters ranging from [a, b], the same as the training data, as detailed in Table B.6 in Appendix B. Subsequently, to assess the models’ generalizability, we perturbed the parameters beyond their original ranges by applying various perturbation percentages, $\lambda$ , resulting in a parameter range of ([b, $( 1 + \lambda ) { \mathbf { b } } ] ,$ ). By systematically increasing $\lambda$ , we evaluated the models’ performance at various levels of extrapolation beyond the training dataset. + +Table 1: Error metrics for PDE1 (5 parameters) and PDE2 (4 parameters) with $2 \times 1 0 ^ { 3 }$ training samples. Both have low dimensional parameters. (a) PDE1 + +
ValueMetricOriginal range of parametersPerturbed range (λ : 0.4)
FNOSC-FNOSC-FNO-PINNFNO-PINNFNOSC-FNOSC-FNO-PINNFNO-PINN
u(t)R20.9860.9830.9890.9780.5290.9120.9280.620
Relative L20.01460.01750.01120.02200.47160.08820.07210.3805
∂u ∂cR20.7230.9240.9430.8010.5150.9010.9150.605
Relative L20.27720.07610.05770.19920.48550.09930.08510.3953
∂u ∂αR20.7410.9250.9450.8050.5220.9080.9240.615
Relative L20.25900.07500.05520.19550.47800.09240.07670.3852
∂u ∂βR20.7630.9300.9560.8170.5300.9140.9300.622
Relative L20.23700.07000.04410.18340.47080.08620.07320.3781
∂uR20.7720.9310.9550.8150.5390.9200.9360.630
Relative L20.22830.06960.04510.18580.46100.08010.06440.3706
∂uR20.7810.9320.9630.8250.5450.9240.9370.632
Relative L20.21900.06820.03750.17520.45570.07610.06330.3687
+ +(b) PDE2 + +
ValueMetricOriginal range of parametersPerturbed range (λ : 0.4)
FNOSC-FNOSC-FNO-PINNFNO-PINNFNOSC-FNOSC-FNO-PINNFNO-PINN
u(t)R20.9970.9970.9950.9950.7340.9330.9230.802
Relative L20.00290.00160.00650.00730.03250.01120.01240.0287
∂uR20.2060.9870.9070.1370.1520.9040.8300.113
Relative L20.20920.01350.08130.85451.12360.07550.09870.9865
∂uR20.4230.9860.9910.5190.3110.9030.9040.429
Relative L20.85420.05400.05230.75660.98750.07670.07550.8244
$∂fra }$R20.8210.9120.9570.8710.6040.8350.8760.719
Relative L20.12450.07560.05670.10230.25440.10230.08880.1567
uR20.3210.9820.9120.4270.2360.9120.8340.353
Relative L20.88560.05670.07890.78991.02440.07220.09440.8878
+ +For PDEs, when the test data originate from the same parameter ranges as the training data (Table B.6 in Appendix B), the solution paths are of high quality, but FNO learns sensitivities significantly poorer than those of SC-FNO (Table 1 left half). While the metrics for $u$ are similarly high among all models, the $R ^ { 2 }$ values for FNO gradients range only from 0.72 to 0.78 (relative L²: 0.28 to 0.22) for PDE1 and from 0.21 to 0.82 (relative L²: 0.21 to 0.12) for PDE2. In contrast, SC-FNO and SC-FNO-PINN consistently achieve $R ^ { 2 }$ values of 0.92-0.93 (relative L $^ { 2 } \colon 0 . 0 8 – 0 . 0 7 )$ for PDE1 and 0.91-0.99 (relative L²: 0.09-0.1) for PDE2. The inclusion of a PINN-type equation loss $( L _ { E q } )$ in FNO-PINN provides only minor benefits for $\textstyle { \frac { \partial \mathbf { u } } { \partial \mathbf { p } } }$ , with $R ^ { 2 }$ values remaining below 0.52 (relative L²: ${ > } 0 . 4 8$ ) for most gradients in PDE2. These improvements are not comparable to those achieved by SC-FNO and SC-FNO-PINN, highlighting the predominant impact of $L _ { s }$ terms. ODEs show a similar pattern where SC-FNO has much better sensitivity accuracy for both parameters (Appendix Table D.14) and initial conditions $\langle \gamma$ in Table $\mathbf { D . 1 4 a }$ and $\zeta$ in Table D.14b). In some sample test predictions (Figure 3a-d), surprisingly large and unphysical oscillations are observed in FNO-predicted sensitivities. These simple cases were designed to show that a surrogate model accurately predicting $u$ may fail to capture the dependence of solution paths on the inputs, leading to large inversion errors. + +This pattern is reliably repeated for more challenging PDEs with a small number of parameters, e.g., for PDE3 (Navier Stokes, 2 parameters), where models lacking the sensitivity loss well predicts vorticity but falter in capturing the sensitivities (Table 2), missing both fine-scale features and largescale patterns (Figure 6). In contrast, SC-FNO accurately recreates patterns and even fine-scale details. In the case of PDE4 (Allen-Cahn), a challenging test case due to its bifurcation nature where small parameter changes can cause abrupt phase transitions in solutions, SC-FNO exhibits mildly better $u$ solution accuracy than FNO (Table 3). More noticeably, SC-FNO generates 1/25 the Jacobian error as FNO, even with reduced samples near critical parameter values where solution behavior changes sharply. Given the diverse dynamics in the equations tested, the general incapability of FNO to capture sensitivity is evident. Note that SC-FNO does not entail excessive additional training cost (Table C.8). We also tested a broad range of other neural operators, all of which exhibited markedly improved sensitivities upon introducing the sensitivity loss (Appendix Table D.9). The difficulty FNO has in capturing parameter gradients leads to prominently degraded performance under input perturbations, though SC-FNO remains robust and reliable. The $\overline { { R ^ { 2 } } }$ for the solution $\mathbf { u }$ at $40 \%$ perturbation decreases precipitously to 0.529 (relative L²: 0.471) for PDE1 and 0.734 (relative L²: 0.266) for PDE2, compared to 0.912 (relative L²: 0.088, 1/5 that of FNO’s) and 0.933 (relative L²: 0.067, 1/4 that of FNO’s) for SC-FNO, respectively (Table 1). As the perturbation ratio increases for PDE1, a stark decline in $R ^ { 2 }$ is observed for FNO, in contrast to the relatively stable SC-FNO (Figure 5). We argue that the large perturbation-induced error is a primary reason for FNO’s poor performance in inversion tasks, during which the search algorithm ventures into under-sampled regions of the parameter space, consistent with the arguments in Li et al. (2023). At an accuracy level of $\overline { { R ^ { 2 } } } = 0 . 5 2 9$ (relative L²: 0.471) (as illustrated in Figure 1b by the scatter plot), the surrogate model loses its ability to guide the inversion. Even without straying beyond the training parameter range, a mere change in the pattern of inter-parameter correlations could cause a departure from the training conditions (concept drift). While increasing training data can help, it becomes exponentially more difficult to adequately cover the parameter space for FNO as the input dimension increases. This challenge is highlighted by the contrasts observed between Figures 1a and 1b, as well as between Figures 2a and 2b. Additionally, SC-FNO-PINN only shows a slight benefit in $\mathrm { L } ^ { 2 }$ over SC-FNO in all of these tests (Table 1). The minimal impact of the equation loss is both surprising and previously unexamined. This ineffectiveness likely stems from the absence of terms related to (time-integrated) ${ \partial \mathbf { u } } / { \partial \mathbf { p } }$ in the differential equations, which leaves this sensitivity unconstrained even though $\mathbf { p }$ is in the equation. For coupled optimization tasks, it is imperative that surrogate models not only reflect the physical phenomena accurately but also adapt to new, unobserved conditions. SC-FNO thus provides an effective alternative solution. + +Table 2: Error metrics for PDE3 with $1 \times 1 0 ^ { 3 }$ training Samples. Both have + +
ω∂ω/∂α∂ω/∂β
MethodRelative L2R2Relative L2R2Relative L2R2
FNO0.03120.9970.72300.0360.96420.036
SC-FNO0.03450.9940.01120.9860.01320.987
+ +![](images/figures/sc-fno-fig-0003.jpg) +Figure 3: Sample prediction of models for ODEs and PDE1 and PDE2. + +# 3.3 MODEL PERFORMANCE ACROSS VARYING TRAINING DATA VOLUMES + +We explored how different models respond to varying amounts of training data. Specifically, we aimed to investigate how the integration of different loss terms affects both the accuracy and the generalization capabilities of the models under limited training data scenarios. The remaining portion of the dataset, not used for training, served to test and measure the models’ performance. + +![](images/figures/sc-fno-fig-0004.jpg) +Figure 4: Models’ performance on PDE1 across training sample sizes, (a) ∂u∂ω (b) $u ( t )$ . + +![](images/figures/sc-fno-fig-0005.jpg) +Figure 5: Performance of modelStandardized $\frac { \partial \hat { \mathbf { u } } } { \partial \mathbf { p } }$ PDE1 fand (b) $u ( t )$ rturbed datasets, (a). + +![](images/figures/sc-fno-fig-0006.jpg) +Figure 6: Sample prediction of models for PDE 3 + +Table 3: Performance comparison for PDE4 + +
MetricsN=500N=100
FNOSC-FNOFNOSC-FNO
State Value Metrics
R20.9990.9990.9970.998
Relative L20.01100.00810.02050.0151
Mean Jacobian Metrics
R2-3.110.998-5.83730.993
Relative L20.52120.02070.58300.0486
+ +Table 4: Performance comparison for zoned PDE2 + +
MetricsN=500N=100
FNOSC-FNOFNOSC-FNO
State Value Metrics
R20.9600.9970.9270.996
Relative L²0.02820.00730.03870.0087
Mean Jacobian Metrics
R2-8.3320.949-14.0120.927
Relative L21.96270.17702.46230.2134
+ +The experiment demonstrates that FNO performance can rapidly degrade as the volume of training data decreases, whereas SC-FNO can maintain higher accuracy and continue to function effectively as a surrogate model (Figure 4). While all models exhibit a decrease in $R ^ { 2 }$ scores for both state values $u ( t )$ and gradients $\frac { \partial \hat { u } } { \partial \mathbf { p } }$ as training sizes diminish, SC-FNO and SC-FNO-PINN show a notably slower rate of decline. With only 500 training samples, FNO’s $R ^ { 2 }$ dropped to 0.8, displaying an acceleration in degradation, while SC-FNO maintained an $R ^ { 2 }$ around 0.9, comparable to FNO’s performance with 1,000 training samples. This advantage is attributed to the use of higher-order (gradient) information, which, similar to traditional numerical methods such as spline interpolation or second-order finite differences, typically results in a better rate of error convergence. This sensitivity to the volume of training data contributed to FNO’s poor performance in inversion tasks (Section 3.1), indicating that dense training data is necessary throughout the parameter search space to ensure accuracy. In practical applications where comprehensive datasets are unattainable, the ability to maintain high model performance with fewer examples is invaluable. + +# 3.4 HANDLING FUNCTIONAL AND HIGH-DIMENSIONAL PARAMETER SPACE + +All previous examples have small numbers of scalar parameters. To evaluate the models’ effectiveness with higher parameter dimensions, we modified PDE2 (Burger’s equation) with zoned parameters. We divided the spatial domain into $S = 4 0$ segments with different advection $\alpha$ and forcing amplitude $\delta$ in each zone, along with global parameters $\gamma$ and $\omega$ , resulting in $2 S + 2 = 8 2$ total parameters. Maintaining the settings from Section 3.2, models were trained with different sample sizes. SC-FNO’s advantages become immediately prominent from this higher dimensional case, even for the solution path $\mathbf { u }$ itself. At 500 samples, SC-FNO has only 1/4 the relative $\mathrm { L } ^ { 2 }$ error (0.0073) as FNO (0.0282, Table 4) while requiring moderately more training time (Appendix Table C.8). With $N = 1 0 0$ samples, SC-FNO maintains the low error (relative $\mathrm { L } ^ { 2 } = 0 . 0 0 8 7 $ ) while FNO degrades significantly (relative $\mathrm { L } ^ { 2 } = 0 . 0 3 9$ ). In fact, SC-FNO with 100 samples has less than $1 / 3$ of the relative $\mathrm { L } ^ { 2 }$ error of FNO with 500 samples, and also requires less time to train. SC-FNO’s level of accuracy seems elusive for FNO, which sees L² error decreasing by only $28 \%$ as $N$ increases from 100 to 500, and thus we argue SC-FNO lifts the performance ceiling. The contrast is more dramatic for Jacobian predictions (SC-FNO: relative $\mathrm { L } ^ { 2 } = 0 . 2 1 3$ ; FNO: relative $\mathrm { L } ^ { 2 } = 2 . 4 6$ ), which, as discussed earlier, has strong implications for the success of inversion and generalizability. These results highlight SC-FNO’s capability to handle high-dimensional parameter spaces efficiently, maintaining accuracy even with limited training data. We can reasonably hypothesize that drastically increasing the number of parameters will further highlight SC-FNO’s unique capability, which we reserve for future work. + +# 3.5 DIFFERENT GRADIENT CALCULATION METHODS + +We tested gradients derived from both a differentiable solver with automatic differentiation (AD) and a fourth-order finite difference solver (FD) using a traditional solver against the analytical solution for gradients of ODE1. The results of this verification are presented in Table D.13 in Appendix D.3. Following this validation, we used these methods to generate solution paths and sensitivities for training and testing surrogate models. We trained SC-FNOs using $70 \%$ of the produced data, dividing the remainder into $15 \%$ each for validation and testing. The validation and testing datasets included parameter values not featured in the training data, testing the models’ generalization capabilities. SC-FNOs trained with either AD- or FD-generated gradients proved effective, producing accurate predictions for both solutions and their gradients, as shown in Table 5. These models both achieved $\mathrm { R } ^ { 2 } > 0 . 9 5$ (relative $\mathrm { L } ^ { 2 } < 0 . 0 5$ ) for solution paths and $\mathrm { R } ^ { 2 } > 0 . 9$ (relative $\mathrm { L } ^ { 2 } < 0 . 1 $ ) for sensitivities. While AD provides higher accuracy and efficiency (Table D.13), FD remains effective and applicable to any existing model code. This makes SC-FNO versatile while maintaining computational efficiency comparable to traditional methods. + +Table 5: Performance comparison of SC-FNO and SC-FNO-PINN using AD and FD solvers. + +
ODE1PDE1
ValueMetricSC-FNO (AD)SC-FNO (FD)SC-FNO- PINN(AD)SC-FNO- PINN(FD)SC-FNO (AD)SC-FNO (FD)SC-FNO- PINN(AD)SC-FNO- PINN(FD)
uR2 Relative L20.991 0.0090.9680.9940.9830.9830.9630.9890.978
0.9960.0320.0060.0170.0170.0370.0110.022
Avg. ∂pR2 Relative L20.0040.987 0.0130.995 0.0050.983 0.0170.928 0.0720.913 0.0870.952 0.0480.932 0.068
+ +# 3.6 FURTHER DISCUSSION AND CONCLUSION + +This work has demonstrated the powerful regularizing effect of sensitivities. The effectiveness of SC-FNO stems from the explicit governance of input influence through time-integrated parameter sensitivities $( \partial u / \partial p )$ . Unlike PINNs, which supervise spatial-temporal derivatives $( \partial u / \partial \bar { \boldsymbol { x } } , \partial u / \partial t )$ through equation-based loss optimization, SC-FNO directly constrains parameter sensitivities typically absent in PDE formulations, using forward numerical models with differentiable solvers to prepare sensitivity data. This fundamental difference enhances model interpretability and reliability, making SC-FNO more suitable for coupled inversion or optimization tasks. Especially when input parameters have a higher dimension, SC-FNO can reduce training data demand, maintain robustness and generalizability, elevate the performance ceiling of neural operators, and even reduce training time. Our innovations include employing differentiable numerical solvers—and alternatively, finite differences—for computing (almost) exact gradients. Although differentiable programming is gaining traction in various domains (Shen et al., 2023; Song et al., 2024a; Aboelyazeed et al., 2023), few studies have leveraged computed gradients beyond backpropagation. We have used these gradients to supervise FNOs, calculating second-order gradients during training. The additional computational cost remains affordable—training the FNO on PDE1 used $7 2 2 \mathrm { M B }$ while training SC-FNO required $7 6 4 \mathrm { M B }$ . We argue that sensitivity regularization and time-step-free methods like FNO complement each other exceptionally well. SC-FNO’s programmatic differentiability allows its seamless integration with neural networks (NNs), speeding up hybrid NN-based learning and optimization. Accurate gradients are crucial for the effective training of coupled NNs, thus opening up new avenues in the field of AI-enhanced solutions to differential equations. + +# ACKNOWLEDGEMENTS + +This work was primarily supported by subaward A23-0249-S001 from the Cooperative Institute for Research to Operations in Hydrology (CIROH) through the National Oceanic and Atmospheric Administration (NOAA) Cooperative Agreement (grant no. NA22NWS4320003). The statements, findings, conclusions, and recommendations are those of the authors and do not necessarily reflect the view of NOAA. It was also partially supported by the U.S. Department of Energy, Office of Science, under award no. DE-SC0021979. Chaopeng Shen has financial interests in HydroSapient, Inc., a company that could potentially benefit from the results of this research. 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URL https: //hess.copernicus.org/articles/28/3051/2024/. + +Yalan Song, Kamlesh Sawadekar, Jonathan M Frame, Ming Pan, Martyn Clark, Wouter JM Knoben, Andrew W Wood, Trupesh Patel, and Chaopeng Shen. Improving physics-informed, differentiable hydrologic models for capturing unseen extreme events. ESS Open Archive, 2024b. doi: 10.22541/essoar.172304428.82707157/v1. URL https://doi.org/10.22541/essoar. 172304428.82707157/v1. + +A. M. Tartakovsky, C. Ortiz Marrero, Paris Perdikaris, G. D. Tartakovsky, and D. Barajas-Solano. Physics-informed deep neural networks for learning parameters and constitutive relationships in subsurface flow problems. Water Resources Research, 56(5):e2019WR026731, 2020. ISSN 1944-7973. doi: 10.1029/2019wr026731. + +Tapas Tripura and Souvik Chakraborty. Wavelet Neural Operator for solving parametric partial differential equations in computational mechanics problems. Computer Methods in Applied Mechanics and Engineering, 404:115783, February 2023. ISSN 0045-7825. doi: 10.1016/j.cma. 2022.115783. + +Wen-Ping Tsai, Dapeng Feng, Ming Pan, Hylke Beck, Kathryn Lawson, Yuan Yang, Jiangtao Liu, and Chaopeng Shen. From calibration to parameter learning: Harnessing the scaling effects of big data in geoscientific modeling. Nature Communications, 12(1):5988, October 2021. ISSN 2041-1723. doi: 10.1038/s41467-021-26107-z. + +Arnaud Vadeboncoeur, Ömer Deniz Akyildiz, Ieva Kazlauskaite, Mark Girolami, and Fehmi Cirak. Fully probabilistic deep models for forward and inverse problems in parametric pdes. Journal of Computational Physics, 491:112369, 2023. ISSN 0021-9991. doi: https://doi.org/10.1016/j.jcp. 2023.112369. URL https://www.sciencedirect.com/science/article/pii/ S0021999123004643. + +Sifan Wang, Hanwen Wang, and Paris Perdikaris. Learning the solution operator of parametric partial differential equations with physics-informed deeponets. Science advances, 7(40):eabi8605, 2021. + +Oliver Watt-Meyer, Noah D. Brenowitz, Spencer K. Clark, Brian Henn, Anna Kwa, Jeremy McGibbon, W. Andre Perkins, Lucas Harris, and Christopher S. Bretherton. Neural Network Parameterization of Subgrid-Scale Physics From a Realistic Geography Global Storm-Resolving Simulation. Journal of Advances in Modeling Earth Systems, 16(2):e2023MS003668, 2024. ISSN 1942-2466. doi: 10.1029/2023MS003668. + +Janni Yuval and Paul A. O’Gorman. Stable machine-learning parameterization of subgrid processes for climate modeling at a range of resolutions. Nature Communications, 11(1):3295, July 2020. ISSN 2041-1723. doi: 10.1038/s41467-020-17142-3. + +Shang Zhu, Bharath Ramsundar, Emil Annevelink, Hongyi Lin, Adarsh Dave, Pin-Wen Guan, Kevin Gering, and Venkatasubramanian Viswanathan. Differentiable Modeling and Optimization of Battery Electrolyte Mixtures Using Geometric Deep Learning, November 2023. + +# APPENDIX + +# A SC-FNO ARCHITECTURE AND SENSITIVITY INTEGRATION + +Figure A.7 illustrates the schematic of the SC-FNO model architecture. In this framework, this variation of the Fourier Neural Operator (FNO) integrates various inputs: parameters $( \mathbf { P } )$ that influence the differential equation, spatial and temporal coordinates $( X : [ x , y , t ] )$ , and the function $a ( x )$ , which may represent different initial conditions. This setup enables comprehensive learning and adaptation across different scenarios by leveraging both automatic differentiation for optimization and multiple loss components tailored to the specific dynamics and constraints of the system. Furthermore, in our framework, the FNO can be regularized by both a PINN-style differential equation $( L _ { E q } )$ as well as the sensitivities (gradients of the solution path of a differential equation with respect to parameters contributing to the diff equation). These regularization terms ensure that the model adheres to the underlying physical laws that reflect how the solution path is determined by a parameter and how it changes when a parameter changes. + +![](images/figures/sc-fno-fig-0007.jpg) +Figure A.7: Schematic of SC-FNO architecture. + +Pseudocode for different FNO models, each with various loss configuration settings, are presented as follows. This section aims to highlight how different loss functions can be integrated and optimized within these models to enhance their predictive accuracy and performance. + +# Algorithm 1 FNO Training Loop with $L _ { u }$ Loss Over Epochs + +1: Initialize FNO model +2: for epoch $= 1$ to max_epochs do +3: Shuffle training data +4: for each batch $\mathbf { P }$ , $\mathbf { u } _ { \mathrm { t r u e } }$ in training data do +5: Predict state values using FNO: $\hat { \mathbf { u } } \gets \mathrm { F N O } ( \mathbf { P } )$ +6: Calculate loss: $L _ { u } = \log ( \hat { \mathbf { u } } , \mathbf { u } _ { \mathrm { t r u e } } )$ +7: Backpropagate loss and update FNO model +8: end for +9: Evaluate on validation set +10: Record training and validation loss +11: end for + +Algorithm 2 SC-FNO Training Loop with $L _ { u }$ and $L _ { s }$ Loss Over Epochs + +1: Initialize FNO model +2: for epoch $= 1$ to max_epochs do +3: Shuffle training data +4: for each batch P, utrue, ∂utrue in training data do +5: Predict state values using FNO: $\hat { \mathbf { u } } \gets \mathrm { F N O } ( \mathbf { P } )$ +6: Calculate primary loss: $L _ { u } = \log ( \hat { \mathbf { u } } , \mathbf { u } _ { \mathrm { t r u e } } )$ +7: Predict Jacobian of state values using Auto Diff (AD): $\begin{array} { r } { \hat { \mathbf { J } } \frac { \partial \hat { \mathbf { u } } } { \partial \mathbf { P } } } \end{array}$ +8: Calculate sensitivity loss: $\begin{array} { r } { L _ { s } = \log s ( \hat { \mathbf { J } } , \frac { \partial \mathbf { u } _ { \mathrm { t r u e } } } { \partial \mathbf { P } } ) } \end{array}$ +9: Calculate total loss: ${ \cal L } _ { \mathrm { t o t a l } } = { \bf c } _ { 1 } \cdot { \cal L } _ { u } + { \bf c } _ { 2 } \cdot { \cal L } _ { s }$ +10: Backpropagate total loss and update FNO model +11: end for +12: Eevaluate on validation set +13: Record training and validation loss +14: end for + +# Algorithm 3 SC-FNO-PINN Training Loop with $L _ { u }$ , $L _ { s }$ , and $L _ { e q }$ Loss Over Epochs + +1: Initialize FNO model +2: for epoch $= 1$ to max_epochs do +3: Shuffle training data +4: for each batch P, utrue, ∂utrue in training data do +5: Predict state values using FNO: $\hat { \mathbf { u } } \gets \mathrm { F N O } ( \mathbf { P } )$ +6: Calculate primary loss: $\bar { L } _ { u } = \log ( \hat { \mathbf { u } } , \mathbf { u } _ { \mathrm { t r u e } } )$ +7: Predict Jacobian of state values using Auto Diff (AD): $\begin{array} { r } { \hat { \mathbf { J } } \frac { \partial \hat { \mathbf { u } } } { \partial \mathbf { P } } } \end{array}$ +8: Calculate sensitivity loss: $\begin{array} { r } { L _ { s } = \log ( \hat { \mathbf { J } } , \frac { \partial \mathbf { u } _ { \mathrm { t r u e } } } { \partial \mathbf { P } } ) } \end{array}$ +9: Calculate equation loss: $L _ { e q } =$ residual(uˆ) +10: Calculate total loss: $L _ { \mathrm { t o t a l } } = \mathbf { c } _ { 1 } \cdot L _ { u } + \mathbf { c } _ { 2 } \cdot L _ { s } + \mathbf { c } _ { 3 } \cdot L _ { e q }$ +11: Backpropagate total loss and update FNO model +12: end for +13: Evaluate on validation set +14: Record training and validation loss +15: + +# B DIFFERENTIAL EQUATION DETAILS + +In this section, the differential equations investigated in the work are detailed. This includes their mathematical formulations, initial conditions, parameter setups. + +ODE1: Composite Harmonic Oscillator We chose this ODE because it has an analytical solution, allowing us to validate our differential equation solvers and sensitivity computations. The ODE is defined as: + +$$ +{ \frac { d u } { d t } } = \alpha \sin ( \alpha \pi t ) + \beta \cos ( \beta \pi t ) +$$ + +with the initial condition $u ( 0 ) = \sin ( \gamma \pi )$ . This oscillator’s behavior is modulated by the parameters $\alpha$ , $\beta$ , and $\gamma$ , affecting the frequency and amplitude of oscillations within the temporal domain $t \in [ 0 , 1 ]$ . The analytical solution for $\mathbf { u ( t ) }$ is: + +$$ +u ( t ) = - \frac { 1 } { \pi } \cos ( \alpha \pi t ) + \frac { 1 } { \pi } \sin ( \beta \pi t ) + \sin ( \gamma \pi ) + \frac { 1 } { \pi } +$$ + +The sensitivities of $\mathbf { u }$ with respect to each parameter are: + +$$ +\begin{array} { r } { \displaystyle { \frac { \partial u } { \partial \alpha } } = t \sin ( \alpha \pi t ) } \\ { \displaystyle { \frac { \partial u } { \partial \beta } } = t \cos ( \beta \pi t ) } \\ { \displaystyle { \frac { \partial u } { \partial \gamma } } = \pi \cos ( \gamma \pi ) } \end{array} +$$ + +These analytical solutions provide a benchmark against which we can compare the accuracy of our numerical solvers and sensitivity computations. + +# ODE2: Duffing Oscillator Equation + +$$ +\ddot { x } + \delta \dot { x } + \alpha t + \beta t ^ { 3 } = \gamma \cos ( \omega t ) , +$$ + +with initial conditions $x ( 0 ) = \epsilon , \dot { x } ( 0 ) = \zeta$ . This equation describes a non-linear oscillator where damping $\delta$ , stiffness $\alpha$ , non-linear stiffness $\beta$ , driving amplitude $\gamma$ , and frequency $\omega$ play crucial roles. + +# PDE1: Generalized Nonlinear Damped Wave Equation + +$$ +\frac { \partial ^ { 2 } u } { \partial t ^ { 2 } } = c ^ { 2 } \frac { \partial ^ { 2 } u } { \partial x ^ { 2 } } + \alpha \frac { \partial u } { \partial t } + \beta u + \gamma \sin ( \omega u ) , +$$ + +al conditions and tempora $u ( x , 0 ) = u _ { 0 }$ $\textstyle { \frac { \partial u } { \partial t _ { - } } } ( x , 0 ) = u _ { 0 } ^ { \prime }$ . This PDE extends over a spatial domawave propagation influenced by damping $x \in [ 0 , 1 ]$ $t \in [ 0 , 1 ]$ $\alpha$ stiffness $\beta$ , and external forcing $\gamma$ and $\omega$ . + +# PDE2: Forced Burgers’ Equation + +$$ +{ \frac { 1 } { \pi } } { \frac { \partial u } { \partial t } } + \alpha u { \frac { \partial u } { \partial x } } = \gamma { \frac { \partial ^ { 2 } u } { \partial x ^ { 2 } } } + \delta \sin ( \omega t ) , +$$ + +This equation is set within a spatial domain $x \in [ 0 , 1 . 0 ]$ and a temporal domain $t \in [ 0 , \pi ]$ . It models fluid dynamics phenomena such as velocity $u ( x , t )$ , incorporating effects of advection $\alpha$ , viscosity $\gamma$ , and external periodic forcing characterized by amplitude $\delta$ and frequency $\omega$ . The initial state of the system is defined as follows: + +$$ +u ( x , 0 ) = u _ { 0 } ( x ) = \left( e ^ { - \frac { ( x - x _ { 0 } ) ^ { 2 } } { 2 \sigma ^ { 2 } } } + \sin ( 0 . 5 \pi x ) \right) , +$$ + +where the Gaussian pulse is centered at $x _ { 0 } = 0 . 5$ with a width $\sigma = 0 . 3$ , combined with a sinusoidal component. The model employs periodic boundary conditions, ensuring that $u ( 0 , t ) = u ( 1 . 0 , t )$ + +throughout the simulation, facilitating the study of continuous and cyclic phenomena in a finite spatial interval. + +# PDE3: Stream Function-Vorticity Formulation of the Navier-Stokes Equations + +$$ +\frac { \partial \omega } { \partial t } + \psi _ { y } \frac { \partial \omega } { \partial x } - \psi _ { x } \frac { \partial \omega } { \partial y } = \frac { 1 } { R e } \left( \frac { \partial ^ { 2 } \omega } { \partial x ^ { 2 } } + \frac { \partial ^ { 2 } \omega } { \partial y ^ { 2 } } \right) , +$$ + +$$ +{ \frac { \partial ^ { 2 } \psi } { \partial x ^ { 2 } } } + { \frac { \partial ^ { 2 } \psi } { \partial y ^ { 2 } } } = - \omega , +$$ + +with initial condition $\omega ( x , y , 0 ) = f ( x , y ; \alpha , \beta )$ where: + +$$ +f ( x , y ; \alpha , \beta ) = \sin ( \alpha x ) \cos ( \beta y ) + \cos ( \alpha y ) \sin ( \beta x ) + \sin ( \alpha x + \beta y ) \cos ( \alpha y - \beta x ) , +$$ + +covering the spatial domain $x , y \in [ 0 , 1 ]$ and temporal domain $t i n [ 0 , 3 ]$ . The Reynolds Number $R e$ for the Navier-Stokes equations is set to 1000 to simulate realistic fluid dynamics. This equation captures the dynamics of fluid flow, with initial vorticity distribution determined by parameters $\alpha$ and $\beta$ . + +# PDE4: Allen-Cahn equation + +$$ +\frac { \partial \boldsymbol { u } } { \partial t } = \epsilon \frac { \partial ^ { 2 } \boldsymbol { u } } { \partial x ^ { 2 } } + \alpha \boldsymbol { u } - \beta \boldsymbol { u } ^ { 3 } , +$$ + +with initial condition $u ( x , 0 ) = c \operatorname { t a n h } ( \omega x )$ and periodic boundary conditions. This PDE, known for exhibiting rich bifurcation behavior, explores phase transition phenomena influenced by diffusion coefficient $\epsilon$ , linear term $\alpha$ , cubic term $\beta$ , and initial condition parameters $c$ and $\omega$ . The equation’s solutions can undergo sharp qualitative changes with small parameter variations, making it an excellent test case for sensitivity analysis. + +The parameter ranges used in our simulations are detailed in Table B.6. Parameters for these simulations were randomly generated using a uniform distribution. The uniform distribution is denoted by $\textstyle { \mathcal { U } } ( a , b )$ , where $a$ is the lower bound and $b$ is the upper bound of the distribution. This means that any value within the range $[ a , b ]$ has an equal probability of being selected. The uniform distribution was chosen to ensure a balanced representation of parameter values across the entire specified range, without favoring any particular subset of values. This approach allows for a comprehensive exploration of the parameter space, providing a robust test of our models across a wide range of potential input conditions. + +Table B.6: Parameter values for different ODE and PDE cases. + +
CasecαβγδωζM
ODE 1[1, 3][1, 3][0, 1]10
ODE 2[0.02, 0.06][0.01, 0.03][20, 60][0.5, 1.5][0.2, 0.6][0.0, 0.2][0.0, 0.2]10
PDE 1[0.0, 0.25][0.0, 0.1][0.0, 0.25][0.0, 0.25][0.0, 0.25]5
PDE 2[0.1, 1.0][0.025, 0.25][0.1, 0.5][0.01, 0.1]5
PDE 3[π, 5π][π, 5π]--1
PDE 4[0.1,0.9][0.01,1.0][0.01,1.0]--[5.0, 10.0][0.01,1.0]-5
+ +# C FNOS HYPERPARAMETERS + +Table C.7 presents the hyperparameters used for training the FNO models for each case study. In the network architecture, "Mode" refers to the Fourier modes used in the neural network’s layers for each dimension (t, x, and y), while "Width" denotes the number of channels or features in the hidden layers of the neural network. The learning rate and number of epochs used for training are also provided for each case. Additionally, Table C.8 compares training times per epoch for different model configurations and batch sizes. + +Table C.7: Hyperparameters for FNOs + +
CaseMode for tMode for xMode for yWidthNumber of Fourier LayersLearning RateNumber of EpochsNumber of Learnable Parameters
ODE 188-2040.00150017921
ODE 288-2040.00150017921
PDE 188-2040.001500107897
PDE 28882040.001500107897
PDE 3-882040.001500209397
PDE 48882040.001500107897
+ +Table C.8: Comparison of model configurations and training time + +
CaseBatch sizeNumber of physical parameters (p)Number of training samplesAverage training time per epoch (s)
FNOSC-FNOFNO-PINNSC-FNO-PINN
ODE116320001.101.941.532.46
ODE216720001.582.131.762.86
PDE145200035.2453.3252.1382.13
PDE244200032.6644.9239.1173.06
PDE2 (Zoned)1821005.377.23--
PDE2 (Zoned)1825008.0911.23
PDE342100047.16109.43
PDE41510011.5419.12
+ +# D ADDITIONAL RESULTS + +# D.1 PERFORMANCE COMPARISON OF NEURAL OPERATORS + +This appendix section presents a comprehensive comparison of the performance of various neural operators, both in their original form and with our proposed sensitivity-constrained framework. We evaluate these operators on four different systems, PDE1 and PDE2. The following tables provide quantitative results for Fourier Neural Operators (FNO), Wavelet Neural Operators (WNO), Multiwavelet Neural Operators (MWNO), and DeepONets, along with their sensitivity-constrained counterparts. + +Table D.9: $R ^ { 2 }$ for PDE1 with $2 \times 1 0 ^ { 3 }$ training samples. + +
ValueFNOSC-FNOWNOSC-WNOMWNOSC-MWNODeepONetSC-DeepONet
u(t)0.9860.9830.9810.9890.9780.9520.9740.954
∂u0.7230.9240.6610.9190.5960.9390.1170.508
∂c0.7410.9250.4020.9230.0900.9160.1100.243
oα ∂β0.7630.9300.9210.9140.8630.9150.7780.827
∂u0.7720.9310.5720.8340.5310.6590.4830.572
o ∂ω0.7810.9320.6140.8230.5580.6840.4970.531
+ +Table D.10: $R ^ { 2 }$ for PDE2 with $2 \times 1 0 ^ { 3 }$ training samples. + +
ValueFNOSC-FNOWNOSC-WNOMWNOSC-MWNODeepONetSC-DeepONet
u(t)0.9970.9970.9810.9790.9890.9710.9900.985
ou0.2060.9870.4220.6350.5580.8210.3440.882
0.4230.9860.4320.5620.3870.9050.5110.873
0.8210.9120.8650.8940.5010.9320.4450.958
0.3210.9820.4250.5420.2140.9520.3630.806
+ +# D.2 ADDITIONAL RESULTS FOR THE INVERSION EXPERIMENTS + +Figures D.8, D.9 and D.10 showcase parameter inversion results for parameters for PDE1, PDE2, and PDE3, respectively. SC-FNO works significantly better than either FNO or FNO-PINN. Table D.11 shows the comparison between various neural operators and their sensitivity-constrained (SC-) versions in simultaneous parameter inversion. Note the uniform pattern that the sensitivity-constrained versions have much higher inversion accuracy. Furthermore, the differences between different neural operators are smaller than the difference between the versions with and without the sensitivity constraint. + +Table D.11: Multi-parameter inversion accuracy for PDEs 1 and 2 with and without gradient supervision. (a) PDE1 + +
Fourier Neural OperatorsOther Neural Operators
FNOSC-FNOWithout gradient supervision.With gradient supervision.
ParameterR2Relative L 2R2Relative L 2WNOMWNODeepONetSC-WNOSC-MWNOSC-DeepONet
c0.6570.2120.9870.0350.6360.6140.5380.9840.9810.977
α0.6420.2220.9860.0360.6210.5980.5190.9840.9800.977
β0.7530.1830.9740.0420.7380.7230.6680.9690.9630.956
γ0.8040.1650.9850.0370.7920.7800.7360.9820.9780.975
ω0.7500.1860.9160.0750.7350.7190.6630.9010.8790.859
+ +(b) PDE2 + +
Fourier Neural OperatorsOther Neural Operators
FNOSC-FNOWithout Grad. Sup.With Grad. Sup.
ParameterR2Relative L2R2Relative L 2WNOMWNODeepONetSC-WNOSC-MWNOSC-DeepONet
α0.9540.0780.9820.0450.9510.9490.9380.9790.9740.970
γ0.9250.0820.9670.0220.9210.9160.8990.9610.9530.945
δ0.8420.1450.9700.0510.8320.8220.7870.9640.9560.949
ω0.8950.1180.9860.0420.8890.8380.8590.9830.9730.976
+ +![](images/figures/sc-fno-fig-0008.jpg) +Figure D.8: Individual parameter results for PDE1 in simultaneous multi-parameter recovery, comparing FNO and SC-FNO performance. $\dot { \alpha }$ has been presented in Figure 1b) + +![](images/figures/sc-fno-fig-0009.jpg) +Figure D.9: Individual parameter results for PDE2 in simultaneous multi-parameter recovery, comparing FNO and SC-FNO performance. + +# D.3 VERIFICATION OF DIFFERENTIABLE AND FINITE DIFFERENCE SOLVERS + +Table D.13 presents a comparison of $R ^ { 2 }$ values for both the solution path u(t) and the gradients of u with respect to different parameters obtained from both differentiable solver (AD) and 4th order finite difference solver (FD). The table includes runtime measurements for generating $2 \times 1 0 ^ { 3 }$ samples using both solvers, providing insight into their computational efficiency. These measurements were obtained using a machine equipped with a V100 GPU and four Intel Xeon processors, offering a standardized comparison of computational cost (The Wall Clock Times) between the two solvers. + +![](images/figures/sc-fno-fig-0010.jpg) +Figure D.10: Inversion of the parameter $_ \alpha$ in PDE3 using FNO and SC-FNO models. + +Table D.12: Computation time for preparing datasets using AD solver with and without Jacobian + +
CaseNumber of input parameter (P)Computation time (s)
With jacobianWithout jacobian
PDE151.8520.932
PDE241.3870.796
PDE326.2052.762
+ +Table D.13: Error metrics and runtime comparison for training data preparation with AD and FD solvers on ODE1 + +
ValueFD solverAD solver
R2RuntimeR2Runtime
U ∂u0.9872 0.97121907.32 s0.9981 0.9906252.54 s
O ∂u0.98850.9989
∂γ0.96180.9890
+ +# D.4 SURROGATE MODEL QUALITY FOR THE ORDINARY DIFFERENTIAL EQUATIONS (ODES). + +Table D.14: Test $R ^ { 2 }$ values and Relative L² for the solution paths $^ { u }$ and sensitivities $\begin{array} { r } { ( \frac { \partial u } { \partial p } ) } \end{array}$ of the surrogate models for ODE1 and ODE2 with $2 \times 1 0 ^ { 3 }$ training samples. The train and test parameters are in the same range. ODEs are simpler to capture by surrogate models than PDEs. (a) ODE1 + +
ValueMetricFNOSC-FNOSC-FNO-PINNFNO-PINN
u(t)R20.9960.9910.9940.991
Relative L20.0040.0090.0060.009
∂u ∂αR2 Relative L20.3270.9940.9920.318 0.682
0.6730.0060.008
$ara$R2 Relative L20.415 0.5850.995 0.0050.995 0.0050.462 0.538
∂uR2 Relative L20.0280.998 0.0020.998 0.0020.131 0.869
0.972
(b) ODE2
ValueMetricFNOSC-FNOSC-FNO-PINNFNO-PINN
u(t)R20.9980.9950.9970.997
Relative L20.0020.0050.0030.003
$∂ua$R20.1620.9970.9980.152
Relative L20.8380.0030.0020.848
$∂uR2 Relative L20.9560.9970.9980.951
0.0440.0030.0020.049
$∂ura}$R20.1350.9960.9970.147
Relative L20.8650.0040.0030.853
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Learnable parameter counts: 17,921 (ODE1/2), 107,897 (PDE1/2/4), 209,397 (PDE3).", + "source": "Appendix C, Table C.7" + }, + { + "id": "sc-fno-D1-002", + "claim": "ODE1 (Composite Harmonic Oscillator) discretization and parameter ranges: temporal domain t in [0,1] discretized into N=100 equal time steps; operator maps from M=10 initial steps + parameters p to the next N-M=90 steps. Parameter ranges from uniform distribution: alpha in [1,3], beta in [1,3], gamma in [0,1].", + "source": "Section 3, Appendix B, Table B.6" + }, + { + "id": "sc-fno-D1-003", + "claim": "ODE2 (Duffing Oscillator) discretization and parameter ranges: temporal domain t in [0,1] with N=100 steps, M=10. Parameters: alpha in [0.02,0.06] (stiffness), beta in [0.01,0.03] (nonlinear stiffness), gamma in [20,60] (driving amplitude), delta in [0.5,1.5] (damping), omega in [0.2,0.6] (frequency), epsilon in [0.0,0.2] (initial position), zeta in [0.0,0.2] (initial velocity).", + "source": "Section 3, Appendix B, Table B.6" + }, + { + "id": "sc-fno-D1-004", + "claim": "PDE1 (Generalized Nonlinear Damped Wave Equation) discretization and parameter ranges: temporal t in [0,1] with N=30 steps, spatial x in [0,1] with S_x=20 divisions, M=5 initial steps. Parameters: c in [0.0,0.25] (wave speed), alpha in [0.0,0.1] (damping), beta in [0.0,0.25] (stiffness), gamma in [0.0,0.25] (forcing amplitude), omega in [0.0,0.25] (forcing frequency).", + "source": "Section 3, Appendix B, Table B.6" + }, + { + "id": "sc-fno-D1-005", + "claim": "PDE2 (Forced Burgers' Equation) discretization and parameter ranges: temporal t in [0,pi] with N=30 steps, spatial x in [0,1] with S_x=40 divisions, M=5 initial steps. Parameters: alpha in [0.1,1.0] (advection), gamma in [0.025,0.25] (viscosity), delta in [0.1,0.5] (forcing amplitude), omega in [0.01,0.1] (forcing frequency). Periodic boundary conditions.", + "source": "Section 3, Appendix B, Table B.6" + }, + { + "id": "sc-fno-D1-006", + "claim": "PDE3 (Navier-Stokes vorticity-stream function) discretization and parameter ranges: spatial x,y in [0,1] with S_x=S_y=64 divisions, temporal t in [0,3], M=1 (map from t=0 to t=3s). Parameters: alpha in [pi,5pi], beta in [pi,5pi]. Reynolds number Re=1000.", + "source": "Section 3, Appendix B, Table B.6" + }, + { + "id": "sc-fno-D1-007", + "claim": "PDE4 (Allen-Cahn equation) discretization and parameter ranges: temporal t in [0,1] with N=30 steps, spatial x in [0,1] with S_x=40 divisions, M=5 initial steps. Parameters: c in [0.1,0.9] (initial condition scale), alpha in [0.01,1.0] (linear coefficient), beta in [0.01,1.0] (cubic coefficient), omega in [5.0,10.0] (initial condition frequency), epsilon in [0.01,1.0] (diffusion coefficient). Periodic boundary conditions.", + "source": "Section 3, Appendix B, Table B.6" + }, + { + "id": "sc-fno-D1-008", + "claim": "Dataset specification: training datasets generated using the differentiable numerical solver with 2x10^3 samples for ODE1/2 and PDE1/2, 1x10^3 for PDE3, 500 and 100 samples for PDE4, 100 and 500 samples for zoned PDE2. All parameters sampled from uniform distributions U(a,b). Dataset split: 70% training, 15% validation, 15% testing, with validation/test sets containing parameter values not encountered during training.", + "source": "Section 3.1, Section 3.4, Appendix B, Table C.8" + }, + { + "id": "sc-fno-D1-009", + "claim": "Batch sizes for training: 16 for ODE1/2, 4 for PDE1/2/3, 1 for PDE4 and the zoned PDE2 (82-parameter) case. Training memory for PDE1: FNO=722MB, SC-FNO=764MB (approximate 6% increase).", + "source": "Appendix C, Table C.8; Section 3.6" + }, + { + "id": "sc-fno-D1-010", + "claim": "Zoned PDE2 with high-dimensional parameter space: the spatial domain is divided into S=40 segments, each with independent advection alpha_i and forcing amplitude delta_i, plus two global parameters gamma and omega, yielding 2S+2=82 total parameters.", + "source": "Section 3.4" + }, + { + "id": "sc-fno-D1-011", + "claim": "Dataset generation hardware: a machine equipped with a V100 GPU and four Intel Xeon processors was used for all dataset generation and training experiments.", + "source": "Appendix D.3" + }, + { + "id": "sc-fno-D1-012", + "claim": "Perturbation-based generalization test: model parameters are perturbed beyond training ranges by lambda=0.4 (40%) to test extrapolation, i.e., testing on range [b, (1+lambda)*b] where b is the upper bound of the training parameter range.", + "source": "Section 3.2, Table 1" + } + ], + "D2": [ + { + "id": "sc-fno-D2-001", + "claim": "SC-FNO forward mapping: the model takes initial conditions u_0, spatial coordinates x, time t, and parameters p as input, and outputs the solution u across all time and space in a single execution. Formulation: u(x,t) = F_SC-FNO(u_0, x, t, p).", + "source": "Section 2.1" + }, + { + "id": "sc-fno-D2-002", + "claim": "Sensitivity loss L_s: the mean squared error between the predicted Jacobian partial_u_hat/partial_p (computed via automatic differentiation through SC-FNO) and the true Jacobian partial_u/partial_p (pre-computed from differentiable numerical solvers or analytical solutions). Formula: L_s = (1/M) * sum_{j=1}^{M} ||partial_u_hat(x_j, t_j; p)/partial_p - partial_u(x_j, t_j; p)/partial_p||^2, evaluated at M sampled spatiotemporal points.", + "source": "Section 2.1, Eq (unnumbered)" + }, + { + "id": "sc-fno-D2-003", + "claim": "SC-FNO training objective (Algorithm 2): for each batch, (1) predict state values u_hat = FNO(P), (2) compute data loss L_u = loss(u_hat, u_true), (3) compute predicted Jacobian J_hat = partial_u_hat/partial_P via automatic differentiation, (4) compute sensitivity loss L_s = loss(J_hat, partial_u_true/partial_P), (5) total loss L_total = c_1*L_u + c_2*L_s, (6) backpropagate and update.", + "source": "Appendix A, Algorithm 2" + }, + { + "id": "sc-fno-D2-004", + "claim": "SC-FNO-PINN training objective (Algorithm 3): extends Algorithm 2 by adding an optional PINN-style PDE residual loss L_eq = residual(u_hat). The total loss becomes L_total = c_1*L_u + c_2*L_s + c_3*L_eq, where L_eq enforces the governing differential equation at collocation points.", + "source": "Appendix A, Algorithm 3; Section 2.2" + }, + { + "id": "sc-fno-D2-005", + "claim": "Gradient computation Method 1 (AD solver): extends the torchdiffeq ODE-oriented framework to handle PDEs by reformulating them as d{u}/dt = RHS(x), where RHS(x) encapsulates spatial derivative terms. Uses PyTorch automatic differentiation to compute partial_u/partial_p. This method provides the true Jacobians for training SC-FNO.", + "source": "Section 2.3" + }, + { + "id": "sc-fno-D2-006", + "claim": "Gradient computation Method 2 (finite differences): approximates gradients by solving the PDE multiple times with slightly perturbed parameter values p+Delta_p and computing (u(p+Delta_p) - u(p-Delta_p))/(2*Delta_p) using a fourth-order central finite difference scheme. This is a non-intrusive method applicable to any existing numerical solver.", + "source": "Section 2.3, Section 3.5" + }, + { + "id": "sc-fno-D2-007", + "claim": "Parameter inversion method: given a trained SC-FNO surrogate and observations u_obs, recover unknown parameters p by gradient-based optimization minimizing ||F_SC-FNO(p) - u_obs||^2 with respect to p via backpropagation. The differentiable surrogate enables efficient gradient computation without repeated PDE solves.", + "source": "Section 3.1" + }, + { + "id": "sc-fno-D2-008", + "claim": "ODE operator mapping: the neural operator learns to map from the first M time steps of solution u together with parameters p to the next N-M time steps: G: u[0:M] union p -> u[M:N]. Applied to ODE1 and ODE2 with N=100, M=10.", + "source": "Section 3" + }, + { + "id": "sc-fno-D2-009", + "claim": "PDE operator mapping (time-dependent): learns mapping from first M time steps across spatial domain plus parameters to the next N-M steps: G: u[0:S_x, 0:M] union p -> u[0:S_x, M:N]. Applied to PDE1, PDE2, PDE4.", + "source": "Section 3" + }, + { + "id": "sc-fno-D2-010", + "claim": "PDE3 operator mapping (direct final-time): learns mapping from initial conditions and parameters directly to the solution at the final time step: G: u[0:S_x, 0:S_y, t=0] union p -> u[0:S_x, 0:S_y, t=3]. No time-stepping is required during forward evaluation.", + "source": "Section 3" + }, + { + "id": "sc-fno-D2-011", + "claim": "Zoned PDE2 formulation: the spatial domain is divided into S=40 segments, each with independent advection alpha_i and forcing delta_i parameters. The Burgers' equation is solved with piecewise-constant parameters across zones, plus global parameters gamma and omega, yielding 82 total learnable parameters (2S + 2).", + "source": "Section 3.4" + }, + { + "id": "sc-fno-D2-012", + "claim": "Fundamental distinction: SC-FNO directly supervises time-integrated parameter sensitivities partial_u/partial_p via L_s (computed from forward numerical models), whereas PINNs supervise spatial-temporal derivatives (partial_u/partial_x, partial_u/partial_t) through equation-based loss optimization. The sensitivity partial_u/partial_p is typically absent from PDE formulations, so PINN regularization cannot constrain it.", + "source": "Section 2.2, Section 3.6" + }, + { + "id": "sc-fno-D2-013", + "claim": "SC-FNO is operator-agnostic: the sensitivity loss L_s can be applied to any neural operator architecture. Demonstrated by adding L_s training to four architectures -- FNO, Wavelet Neural Operator (WNO), Multiwavelet Neural Operator (MWNO), and DeepONet -- with uniform improvements in sensitivity capture and inversion accuracy across all operators.", + "source": "Section 2.1, Appendix D.1" + } + ], + "D3": [ + { + "id": "sc-fno-D3-001", + "claim": "Four-model comparison evaluation: four FNO configurations are compared head-to-head on all benchmark problems -- (1) FNO with L_u only, (2) FNO-PINN with L_u+L_Eq, (3) SC-FNO with L_u+L_s, (4) SC-FNO-PINN with L_u+L_s+L_Eq. All models share identical neural network architectures and inputs; only the loss configuration differs. Loss weighting coefficients c_1, c_2, c_3 control relative contribution of each term. Evaluation covers four dimensions: surrogate solution quality, Jacobian accuracy, inversion accuracy, generalization robustness, and computational cost.", + "source": "Section 3, Appendix A, Section 2.4" + }, + { + "id": "sc-fno-D3-002", + "claim": "Six benchmark problems of increasing complexity: ODE1 (Composite Harmonic Oscillator, 3 params), ODE2 (Duffing Oscillator, 7 params), PDE1 (Generalized Nonlinear Damped Wave, 5 params), PDE2 (Forced Burgers', 4 params), PDE3 (Navier-Stokes vorticity-stream, 2 params), PDE4 (Allen-Cahn phase transition with bifurcation, 5 params). ODE1 has an analytical solution serving as ground truth for gradient validation.", + "source": "Section 3, Appendix B" + }, + { + "id": "sc-fno-D3-003", + "claim": "Two-stage parameter inversion experimental design: Stage 1 performs single-parameter inversion -- invert only alpha while treating all other parameters as known, using backpropagation to optimize p by minimizing ||F(p) - u_obs||^2. Stage 2 performs simultaneous multi-parameter inversion -- all parameters of the PDE are inverted jointly. This progressive protocol isolates single-parameter behavior before tackling multi-parameter inversion.", + "source": "Section 3.1" + }, + { + "id": "sc-fno-D3-004", + "claim": "Perturbation-based generalization test protocol: after training on parameters in range [a, b], the models are tested on perturbed ranges [b, (1+lambda)*b] with systematically increasing lambda (tested at lambda=0.4, i.e., 40% beyond training). This simulates concept drift where the parameter search algorithm ventures into under-sampled regions during inversion. Both solution path u(t) and Jacobian partial_u/partial_p are evaluated.", + "source": "Section 3.2" + }, + { + "id": "sc-fno-D3-005", + "claim": "Training data volume ablation protocol: models are trained with systematically decreasing sample sizes -- 2000, 1000, 500, 100 samples for PDE1 -- to assess how different loss configurations (L_u only, L_u+L_s, L_u+L_s+L_Eq) affect accuracy and generalization under limited-data scenarios. The remaining portion of the dataset serves as the test set. SC-FNO and SC-FNO-PINN are compared against FNO at each training size.", + "source": "Section 3.3, Section 3.4" + }, + { + "id": "sc-fno-D3-006", + "claim": "High-dimensional parameter scaling test: PDE2 (Burgers') is modified into a zoned formulation where the spatial domain is divided into S=40 segments with independent advection alpha_i and forcing amplitude delta_i per zone, plus global gamma and omega -- yielding 2S+2=82 total parameters. The same comparison protocol from Section 3.2 is applied at sample sizes N=100 and N=500 to test whether the sensitivity constraint maintains effectiveness in high-dimensional parameter spaces.", + "source": "Section 3.4" + }, + { + "id": "sc-fno-D3-007", + "claim": "Gradient computation validation protocol: before using AD or FD solvers to generate training data for SC-FNO, both gradient computation methods are first validated against the analytical solution of ODE1 (which has known closed-form sensitivities partial_u/partial_alpha, partial_u/partial_beta, partial_u/partial_gamma). Only after validation are the methods used to generate solution paths and Jacobians for training surrogate models. SC-FNO is trained using the validated gradient data.", + "source": "Section 3.5, Appendix D.3" + }, + { + "id": "sc-fno-D3-008", + "claim": "Cross-operator validation protocol: the sensitivity constraint L_s is applied to four distinct neural operator architectures -- Fourier Neural Operator (FNO), Wavelet Neural Operator (WNO), Multiwavelet Neural Operator (MWNO), and DeepONet -- to demonstrate operator-agnostic generality. Each operator is evaluated in its standard form and with the sensitivity constraint (SC- variant) on PDE1 and PDE2, each with 2x10^3 training samples.", + "source": "Section 2.1, Appendix D.1" + }, + { + "id": "sc-fno-D3-009", + "claim": "Dataset generation strategy: all training data is generated synthetically using the differentiable numerical solver (torchdiffeq-based). Parameters are sampled from uniform distributions U(a,b) to ensure balanced coverage across the entire range. True Jacobians partial_u/partial_p are computed and stored once -- a one-time cost per equation. Validation and test sets (15% each) are drawn from parameter values not encountered during training.", + "source": "Section 3.1, Section 3.2, Section 3.3, Appendix B" + }, + { + "id": "sc-fno-D3-010", + "claim": "Evaluation metrics protocol: two complementary metrics are reported for every experiment -- (1) R^2 (coefficient of determination) measuring explained variance, and (2) Relative L^2 error defined as ||u_hat - u||_2 / ||u||_2. Both metrics are computed for the solution path u(t) and for each individual Jacobian component partial_u/partial_p_i. For multi-parameter inversion, per-parameter R^2 and Relative L^2 are reported to allow per-parameter assessment.", + "source": "Section 3, Section 3.1, Section 3.2" + }, + { + "id": "sc-fno-D3-011", + "claim": "Inversion evaluation protocol: for parameter inversion tasks, the trained surrogate model is used as a forward simulator within a gradient-based optimization loop. The optimizer (backpropagation through the surrogate) minimizes the discrepancy between surrogate predictions and synthetic observations. Inversion quality is measured by R^2 and Relative L^2 between true and recovered parameters. Experiments use synthetic test data drawn from held-out parameter ranges.", + "source": "Section 3.1, Appendix D.2" + }, + { + "id": "sc-fno-D3-012", + "claim": "Surrogate quality evaluation under concept drift: models trained on original parameter ranges [a, b] are evaluated on test data from the same distribution AND on perturbed ranges exceeding training. The degradation between in-distribution and out-of-distribution performance quantifies each model's robustness. SC-FNO's perturbation error is compared to FNO's as a ratio to demonstrate sensitivity-constrained models' superior robustness.", + "source": "Section 3.2" + }, + { + "id": "sc-fno-D3-013", + "claim": "Training cost characterization protocol: for each model configuration (FNO, SC-FNO, FNO-PINN, SC-FNO-PINN) and each benchmark problem, the average training time per epoch is measured. This quantifies the computational overhead of the sensitivity constraint. Additionally, GPU memory consumption is compared between FNO (722MB) and SC-FNO (764MB) on PDE1 to demonstrate that the overhead is modest.", + "source": "Section 3.6, Appendix C, Table C.8" + }, + { + "id": "sc-fno-D3-014", + "claim": "Experimental control for fair comparison: FNO and SC-FNO share identical neural network architectures, inputs, and hyperparameters (4 Fourier layers, width=20, modes=8, lr=0.001, 500 epochs). Training data, data splits, and evaluation metrics are identical across all model variants. The only difference is the loss function configuration, isolating the effect of the sensitivity constraint from all other confounding factors.", + "source": "Section 2.4, Section 3, Appendix C" + } + ], + "D4": [ + { + "id": "sc-fno-D4-001", + "claim": "Gradient subsampling strategy during training: pre-computed true Jacobians are stored once and reused throughout training. Instead of computing gradients at all N*T spatiotemporal points, each epoch randomly selects a subset of n < N spatial points and t < T time points for gradient comparison. The subsampling varies between epochs to eventually cover the full solution space. Each minibatch requires only one forward pass before applying AD, adding minimal computational overhead.", + "source": "Section 2.4" + }, + { + "id": "sc-fno-D4-002", + "claim": "The SC-FNO pipeline (end-to-end, 3-stage): (1) pre-compute true solution paths u and Jacobians partial_u/partial_p once using differentiable numerical solvers or finite differences -- a one-time cost per equation; (2) train FNO with combined loss L_total = c_1*L_u + c_2*L_s using stored Jacobians and subsampled gradient evaluation; (3) use the trained surrogate for forward simulation, sensitivity analysis, or gradient-based parameter inversion.", + "source": "Section 2.4, Section 3.6" + }, + { + "id": "sc-fno-D4-003", + "claim": "Parameter embedding in lifting layer: parameters tau(p) are reshaped and repeated to match the problem's spatiotemporal dimensions, then concatenated with spatial coordinates and initial conditions before being fed into the FNO neural network. FNO and SC-FNO share identical architectures and inputs; they differ only in loss configuration.", + "source": "Section 2.4" + } + ] +} \ No newline at end of file diff --git a/papers/score/blacklist.txt b/papers/score/blacklist.txt new file mode 100644 index 0000000000000000000000000000000000000000..e9fa75170340bd2e4899c2977becb6e7017c3199 --- /dev/null +++ b/papers/score/blacklist.txt @@ -0,0 +1,7 @@ +# No official repository from Google/DeepMind (ICLR 2025) +# Best community reproduction (TRL + DeepSpeed) +https://github.com/VityaVitalich/SCoRe +# Community reproduction +https://github.com/BY571/SCoRe +# Community reproduction (Gemma-2B) +https://github.com/daje0601/Google_SCoRe diff --git a/papers/score/config.yaml b/papers/score/config.yaml new file mode 100644 index 0000000000000000000000000000000000000000..25751d3b94b1ec44f07598826626bcafafaa604c --- /dev/null +++ b/papers/score/config.yaml @@ -0,0 +1,8 @@ +title: "Training Language Models to Self-Correct via RL (SCoRe)" 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b/papers/score/images/tables/score-table-0006.jpg @@ -0,0 +1,3 @@ +version https://git-lfs.github.com/spec/v1 +oid sha256:5892c0072c802d0157cf0aad4c07fd2b3887e69936ce67649527d9609919c5eb +size 30882 diff --git a/papers/score/paper.md b/papers/score/paper.md new file mode 100644 index 0000000000000000000000000000000000000000..3e6e6084ecda57a3d2dc5a70913dc421aefe7482 --- /dev/null +++ b/papers/score/paper.md @@ -0,0 +1,670 @@ +# Training Language Models to Self-Correct via Reinforcement Learning + +Aviral Kumar\*+,1, Vincent Zhuang\*+,1, Rishabh Agarwal\*,1, $\mathbf { \Delta } \mathbf { Y } \mathbf { i } \ s \mathbf { u } ^ { * } , 1$ , JD Co-Reyes1, Avi Singh1, Kate Baumli1, Shariq Iqbal1, Colton Bishop1, Rebecca Roelofs1, Lei M Zhang1, Kay McKinney1, Disha Shrivastava1, Cosmin Paduraru1, George Tucker1, Doina Precup1, Feryal Behbahani†,1 and Aleksandra Faust†,1 + +1Google DeepMind, \*Equal Contribution, +Randomly ordered via coin flip, †Jointly supervised. + +Self-correction is a highly desirable capability of large language models (LLMs), yet it has consistently been found to be largely ineffective in modern LLMs. Current methods for training self-correction typically depend on either multiple models, a more advanced model, or additional forms of supervision. To address these shortcomings, we develop a multi-turn online reinforcement learning (RL) approach, SCoRe, that significantly improves an LLM’s self-correction ability using entirely self-generated data. To build SCoRe, we first show that variants of supervised fine-tuning (SFT) on offline model-generated correction traces are often insufficient for instilling self-correction behavior. In particular, we observe that training via SFT falls prey to either a distribution mismatch between mistakes made by the data-collection policy and the model’s own responses, or to behavior collapse, where learning implicitly prefers only a certain mode of correction behavior that is often not effective at self-correction on test problems. SCoRe addresses these challenges by training under the model’s own distribution of self-generated correction traces and using appropriate regularization to steer the learning process into learning a self-correction behavior that is effective at test time as opposed to fitting high-reward responses for a given prompt. This regularization process includes an initial phase of multi-turn RL on a base model to generate a policy initialization that is less susceptible to collapse, followed by using a reward bonus to amplify self-correction. With Gemini 1.0 Pro and 1.5 Flash models, we find that SCoRe achieves state-of-the-art self-correction performance, improving the base models’ self-correction by $1 5 . 6 \%$ and $9 . 1 \%$ respectively on MATH and HumanEval. + +# 1. Introduction + +Large language models (LLMs) are a useful tool for reasoning in scientific domains such as math and coding (Lozhkov et al., 2024; Shao et al., 2024; Team, 2024). An aspirational property of LLMs in such settings is their ability to implement meta-strategies or algorithms that use test-time computation to generate improved responses. However, modern LLMs do not implement such strategies reliably. For instance, consider a problem that requires models to detect and revise (or “self-correct”) their own responses in order to eventually arrive at the best possible final response. This self-correction capability has been shown to be severely lacking in current LLMs, especially in the absence of external input (also called intrinsic self-correction) (Huang et al., 2023; Kamoi et al., 2024). + +To make progress towards teaching LLMs to implement meta-strategies for challenging inputs, we study a special instance of training LLMs to perform self-correction to fix their mistakes “on-the-fly”. This should be possible: on many queries where current LLMs fail, they possess the underlying “knowledge” needed to arrive at the correct response but are unable to correctly elicit and draw inferences about their own knowledge when needed (Yang et al., 2024). For example, strong LLMs can often successfully complete a sub-part of a math proof when prompted with the remainder, but may not be able to complete it from scratch. In a similar vein, leveraging their previous responses should, in principle, enable LLMs to improve their subsequent ones. Despite this, self-correction has remained elusive, highlighting the need to go beyond existing training paradigms. + +![](images/figures/score-fig-0001.jpg) +Figure 1 ∣ Left: SCoRe achieves state-of-the-art self-correction performance on MATH; Right: SCoRe inference-time scaling: spending samples on sequential self-correction becomes more effective than only on parallel direct samples (Section 6.2). + +How can we imbue LLMs with self-correction abilities? Prior attempts for self-correcting LLMs either rely on prompt-engineering (Kim et al., 2023; Madaan et al., 2023) or on fine-tuning models specifically for self-correction. While the former approaches often fail to perform meaningful intrinsic self-correction, fine-tuning approaches require running multiple models during inference, such as a separate refinement model (Havrilla et al., 2024b; Welleck et al., 2023), or rely on “teacher” supervision to guide the process of self-correction (Qu et al., 2024). With the use of separate models of teacher supervision, self-correction does not necessarily outperform parallel, independent attempts. We develop an approach that is effective at self-correction without these requirements. Our approach, Self-Correction via Reinforcement Learning (SCoRe), trains only a single model that can both produce a response to a problem and also correct errors without any oracle feedback. + +To develop SCoRe, we start by analyzing the shortcomings of SFT-based approaches (e.g., STaR (Zelikman et al., 2022)) and naïve RL that optimizes final response correctness for teaching self-correction. We find that such approaches fall prey to either: (1) distribution shift, where the trained model is able to correct errors made by the base model that generated the data, but these gains do not transfer to self-correction under the learned model’s own mistakes; or (2) behavior collapse, where the learning progress simply learns to produce the best first-attempt response followed by superficial or no modifications in the second attempt. To address these issues, SCoRe trains for self-correction directly via on-policy, multi-turn RL. To prevent behavior collapse, SCoRe employs two-stage training: in the first stage, it produces an initialization that is less susceptible to behavior collapse by training to correct second-attempt responses while constraining the first-turn distribution to be close to the base model; followed by training on both attempts to maximize reward in the second stage. Crucially, the second stage of multi-turn RL employs a reward shaping term that rewards “progress” towards self-correction as opposed to the correctness of the final response. + +Our main contribution is SCoRe, a multi-turn RL approach for teaching LLMs how to correct their own mistakes. To the best of our knowledge, SCoRe is the first approach to attain significantly positive intrinsic self-correction: relative to base Gemini models, our method attains an absolute $1 5 . 6 \%$ gain on self-correction for reasoning problems from MATH (Hendrycks et al., 2021) and an absolute $9 . 1 \%$ gain on coding problems from HumanEval (Chen et al., 2021). We additionally motivate the design of SCoRe by extensively studying the failure modes of SFT and standard RL approaches, which broadly indicate that reinforcement learning plays an essential role in self-trained self-correction. + +# 2. Related Work + +Prior works study self-correction for LLMs under a variety of assumptions and problem settings. The most prominent settings include problems where external input tokens from an environment are available, such as agentic tasks (Liu et al., 2023), code repair (Jain et al., 2024), and tool use (Chen et al., 2023). While self-correction with external feedback is possible with strong models (Pan et al., 2023), even they struggle in the substantially harder setting with no external input (intrinsic self-correction) (Huang et al., 2023; Kamoi et al., 2024). Prior work that attempts to amplify intrinsic correction abilities is largely based on prompting and fine-tuning. + +Prompting for intrinsic self-correction. Recent work demonstrates that naïvely prompting LLMs for self-correction can degrade performance (Huang et al., 2023; Qu et al., 2024; Tyen et al., 2024; Zheng et al., 2024). These results contradict prior work (Kim et al., 2023; Madaan et al., 2023; Shinn et al., 2023) and largely stem from mismatched assumptions about the setting (Kamoi et al., 2024). For example, Kim et al. (2023); Shinn et al. (2023) use ground-truth answers during self-correction that may not generally be available; Madaan et al. (2023) use weak prompts for initial responses, thereby overestimating the total improvement possible. Therefore, there is no major work showing successful intrinsic self-correction via prompting alone. In the context of code self-repair, Olausson et al. (2023) show that even when strong models are prompted with some form of partial feedback, e.g., test-cases but not the desired outcomes, they are unable to correct their mistakes. + +Fine-tuning for intrinsic self-correction. Several works that go beyond prompting rely on fine-tuning with demonstrations of revisions, e.g. obtaining revisions directly from human annotators (Saunders et al., 2022) or stronger models (Qu et al., 2024; Ye et al., 2023). Our work aims to train for self-correction entirely without the use of larger models or humans, when the learner itself is asked to generate its own training data. Similar to these prior works, we assume access to a reward function for evaluating modelgenerated outputs (Akyürek et al., 2023; Welleck et al., 2023; Zhang et al., 2024). Perhaps the closest to us from this set is Qu et al. (2024), which utilizes an iterative STaR-like approach self-correction. While this work largely uses oracle teacher supervision, their preliminary results from training for self-correction only show minor improvements over five turns, consistent with the results we see for STaR. We show that SCoRe attains substantially better results. Other approaches train separate models for performing correction (e.g., GLoRE (Havrilla et al., 2024b), Self-Correction (Welleck et al., 2023), Akyürek et al. (2023); Paul et al. (2023). While such approaches can be convenient, they require system design for serving multiple models at deployment. + +Multi-turn RL for LLMs. Prior work at the intersection of LLMs and multi-turn RL builds machinery for optimizing rewards with value-based (Farebrother et al., 2024; Shani et al., 2024; Snell et al., 2022; Zhou et al., 2024), policy-based (Shao et al., 2024; Xiong et al., 2024), and model-based (Hong et al., 2024) approaches. We do not focus on building machinery for RL (we use the approach of Ahmadian et al. (2024)), but rather train for self-correction as an RL problem. + +# MATH Example 1: Arithmetic Self-Correction + +Problem. If $n \equiv 2$ (mod 7), then find the remainder when $( n + 2 ) ( n + 4 ) ( n + 6 )$ is divided by 7. + +SCoRe turn 1 solution (incorrect). Since $n \equiv 2$ (mod 7), we can write $~ n ~ = ~ 7 k + 2$ for some integer $k$ . Substituting, we get ( $n + 2 ) ( n + 4 ) ( n + 6 ) = ( 7 k + 4 ) ( 7 k + 6 ) ( 7 k + 8 ) \equiv 4 \cdot 6 \cdot 8$ $( { \bmod { 7 } } ) \equiv 1 9 2$ $( \mathrm { m o d } ~ 7 ) \equiv 1$ (mod 7). Final Answer: The final answer is 1. I hope it is correct. + +Self-correction instruction. There might be an error in the solution above because of lack of understanding of the question. Please correct the error, if any, and rewrite the solution. + +SCoRe turn 2 solution (correct). Since $n \ \equiv \ 2$ (mod 7), we can write $~ n ~ = ~ 7 k + 2$ for some integer $k$ . Substituting, we get $( n + 2 ) ( n + 4 ) ( n + 6 ) = ( 7 k + 4 ) ( 7 k + 6 ) ( 7 k + 8 ) \equiv 4 \cdot 6 \cdot 8 { \mathrm { ~ ( m } }$ $( \mathrm { m o d } ~ 7 ) \equiv 4 \cdot 6 \cdot 1$ (mod 7) ≡ 24 $( { \bmod { 7 } } ) \equiv 3$ (mod 7). Final Answer: The final answer is 3. I hope it is correct. + +# MATH Example 2: Reasoning Self-Correction + +Problem. Let $f ( x ) = \left\lfloor \left( - { \frac { 5 } { 8 } } \right) ^ { x } \right\rfloor$ be a function that is defined for all values of $x$ in $[ 0 , \infty )$ such that $f ( x )$ is a real number. How many + +distinct values exist in the range of $f ( x ) ?$ + +SCoRe turn 1 solution (incorrect). As $x$ increases, the value of $\left( - \frac { 5 } { 8 } \right) ^ { x }$ decreases. Since the floor function rounds down to the + +nearest integer, the range of $f ( x )$ consists of all the integers less than or equal to $\left\lfloor \left( - { \frac { 5 } { 8 } } \right) ^ { 0 } \right\rfloor = 1$ . Therefore, the range of $f ( x )$ is $\{ 1 , 0 , - 1 , - 2 , \ldots \} _ { }$ , and there are $\boxed { \infty }$ distinct values in the range. Final Answer: The final answer is $\infty$ . I hope it is correct. + +Self-correction instruction. There might be an error in the solution above because of lack of understanding of the question. Please correct the error, if any, and rewrite the solution. + +SCoRe turn 2 solution (correct). As $x$ increases from 0 to $\infty$ , the value of $\left( - { \frac { 5 } { 8 } } \right) ^ { x }$ oscillates between positive and negative values, but + +its absolute value decreases. The floor function rounds down to the nearest integer, so the range of $f ( x )$ consists of the integers less than or equal to $\left\lfloor \left( - { \frac { 5 } { 8 } } \right) ^ { 0 } \right\rfloor = 1$ . Since the absolute value of $\left( - \frac { 5 } { 8 } \right) ^ { x }$ decreases as $x$ increases, the range of $f ( x )$ is finite and consists of the integers: $\{ 1 , \bar { 0 } , - 1 \}$ . Final Answer: The final answer is 3. I hope it is correct. + +Figure 2 ∣ Two example traces of self-correction. In the upper example, SCoRe is able to correct an arithmetic mistake it makes in turn 1. In the lower example, the model is able to correct a reasoning error. + +# 3. Preliminaries and Problem Setup + +Our goal is to develop an approach for training LLMs to improve their own predictions entirely on selfgenerated data. As discussed so far, we situate ourselves in the intrinsic self-correction setting (Huang et al., 2023), where models attempt to correct their initial responses without any external feedback. Concretely, given a dataset $\mathcal { D } = \{ ( \boldsymbol { x _ { i } } , \overline { { \boldsymbol { y _ { i } ^ { * } } } } ) \} _ { i = 1 } ^ { N }$ of problems $\pmb { x } _ { i }$ and responses $y _ { i } ^ { * }$ , we will train an LLM policy $\pi _ { \boldsymbol { \theta } } \big ( \cdot | [ \boldsymbol { x } , \hat { \boldsymbol { y } } _ { 1 : l } , p _ { 1 : l } ] \big )$ that, given the problem $_ { x }$ , previous ?? model attempts $\hat { \mathbf { y } } _ { 1 : l }$ at the problem, and auxiliary instructions $p _ { 1 : l }$ (e.g., instruction to find a mistake and improve the response), solves the problem $_ { x }$ as correctly as possible. This formalism is akin to the multi-turn MDP in Qu et al. (2024). We also assume access to an oracle reward $\hat { r } ( y , y ^ { * } )$ , such as an answer checker (Uesato et al., 2022), that evaluates the correctness of response $y$ by comparing it with the oracle response $y ^ { * }$ . Critically, we do not assume access to this oracle at test-time; instead, the model must deduce whether there was a mistake and correct it if necessary, as is often the case in e.g. mathematical reasoning problems. Unlike the setup of Qu et al. (2024), we also do not run majority voting for most of our main results. Two example traces of self-correction are given in Figure 2, and our problem setting is depicted pictorially in Figure 3. + +![](images/figures/score-fig-0002.jpg) +Figure 3 ∣ The problem setting of self-correction. SCoRe trains a model to not just produce the best possible response, but instead aims to train the model to produce the best final response in the final attempt. In the second turn, extra input in the form of an instruction asking the model to correct itself or model-generated may be provided. + +We aim to find an LLM policy $\pi ( \bigcirc )$ mapping input tokens $^ \circ$ to output tokens □ that maximizes the correctness reward obtained from the verifier at the end of $l + 1$ turns $( l = 1 )$ . Formally: + +$$ +\operatorname* { m a x } _ { \pi _ { \boldsymbol { \theta } } } \mathcal { E } _ { x , y ^ { * } \sim \mathcal { D } , \hat { y } _ { l + 1 } \sim \pi _ { \boldsymbol { \theta } } ( \cdot \vert \left[ x , \hat { y } _ { 1 : l } , p _ { 1 : l } \right] ) } \left[ \sum _ { i = 1 } ^ { l + 1 } \hat { r } \left( \hat { y } _ { i } , y ^ { * } \right) \right] . +$$ + +Crucially, note that unlike standard SFT or prevalent RL fine-tuning workflows, which train the policy $\pi$ to directly produce $y ^ { * }$ (or any other $y$ wih $\widehat { r } ( y , y ^ { * } ) = 1 )$ , Equation 1 trains $\pi$ over multiple attempts simultaneously, where intermediate turns are supervised indirectly to maximize the sum. + +Base RL fine-tuning approach we use. We use a REINFORCE policy gradient training approach with a KL-divergence penalty against a fixed model (Ahmadian et al., 2024), which is widely used in RL fine-tuning of LLMs, primarily in the setting of single-turn RLHF. Formally, these methods train the policy $\pi _ { \theta } ( \cdot | \boldsymbol { x } )$ to optimize the following, where $\pi _ { \mathrm { r e f } }$ is a reference policy. + +$$ +\operatorname* { m a x } _ { \theta } \mathbb { E } _ { x _ { t } , y _ { t } \sim \pi _ { \theta } ( \cdot | x _ { t } ) } \left[ \widehat { r } ( y _ { t } , y ^ { * } ) - \beta _ { 1 } D _ { K L } ( \pi _ { \theta } ( \cdot | x _ { t } ) | | \pi _ { \mathrm { r e f } } ( \cdot | x _ { t } ) ) \right] , +$$ + +Metrics. To measure self-correction performance (we consider $l = 2$ in this paper), we report and analyze the following metrics: (1) Accuracy $@ 1$ : the model’s accuracy at the first attempt; (2) Accuracy $@ 1 2$ : the model’s accuracy at the second attempt, (3) $\Delta ( { \bf t } 1 , { \bf t } 2 )$ : the net improvement in model accuracy between the first and second attempts, which measures the efficacy of self-correction, (4) $\Delta ^ { \mathrm { i } \mathrm { c } } ( { \bf t 1 } , { \bf t 2 } )$ : the fraction of problems that are incorrect in the first attempt but become correct at the second attempt, which measures how many new problems can self-correction solve; and (5) $\Delta ^ { \mathrm { c } \mathrm { i } } ( { \bf t 1 } , { \bf t 2 } )$ : the fraction of problems that are correct in the first attempt but become incorrect at the second attempt, which measures how well the model understands what makes a response correct. + +# 4. SFT on Self-Generated Data is Insufficient for Self-Correction + +A natural approach for training self-correction is to utilize some form of supervised fine-tuning on data collected from a base model. Variants of this approach have been shown to scale well on single-turn reasoning problems (Singh et al., 2023; Zelikman et al., 2022). In this section, we assess the empirical efficacy of two such approaches for self-correction: STaR (Zelikman et al., 2022), and a version of Welleck et al. (2023) that trains only one model. + +Table 1 ∣ Self-correction performance after training on $\mathcal { D } _ { S \mathrm { T a R } }$ and ${ \mathcal { D } } _ { S \mathrm { F T } }$ . We find that the gap between the second and first attempts $\left( \Delta ( \mathrm { t } 1 , \mathrm { t } 2 ) \right)$ is either negative or small. Both approaches erroneously modify a correct response to be incorrect, i.e., reflected in a high $\Delta ^ { \mathrm { c } \mathrm { i } } ( t 1 , t 2 )$ and a low $\Delta ^ { \mathrm { i } \mathrm { c } } ( t 1 , t 2 )$ . + +
MethodAccuracy@t1Accuracy@t2Δ(t1, t2)i→c(1, t2)Δc→i(t1, t2)
Base model52.6%41.4%-11.2%4.6%15.8%
STaR DStaR55.4%41.2%-14.2%5.4%19.6%
STaR Dta 53.6%54.0%0.4%2.6%2.2%
Pair-SFT DsfT52.4%54.2%1.8%5.4%3.6%
Par-SFT DFT55.0%55.0%0%0%0%
+ +We ultimately find that although these methods improve self-correction over the base model, they fail to achieve substantially positive self-correction $\mathbf { \phi } ( \Delta ( \mathbf { t } \mathbf { 1 } , \mathbf { t } 2 ) )$ . By probing these models, we observe two main failure modes: (1) a collapse to non-correcting behavior, where the models learn to produce a good response on the first attempt and only make minor (or no) modifications in the second attempt, and (2) an inability of offline methods to be robust to distribution shift in the first-attempt responses. + +Analysis setup: methods and dataset construction. We prompt Gemini 1.5 Flash to obtain a large number of two-turn self-correction traces on MATH (Hendrycks et al., 2021). The STaR approach filters these trajectories to retain only those that successfully revise incorrect responses and runs SFT on the resulting dataset. Another approach is to use base model data from above to construct “synthetic” repair traces by pairing incorrect responses with correct ones (Welleck et al., 2023). We study a variant of this method that we call Pair-SFT, which does not train a separate corrector model and does not augment this initial dataset with multi-turn traces. Formally, we denote the datasets for STaR and Pair-SFT as $\mathcal { D } _ { S \mathrm { T a R } }$ and $\mathcal { D } _ { \mathrm { S F T } }$ respectively. We run 3 iterations for STaR following the protocol in Singh et al. (2024), and only one iteration for Pair-SFT, following the protocol in Welleck et al. (2023) and other standard workflows on SFT. + +Main empirical findings. We present the self-correction results before and after fine-tuning on $\mathcal { D } _ { S \mathrm { T a R } }$ and $\mathcal { D } _ { \mathrm { S F T } }$ in Table 1. We find that although $\Delta ( { \bf t } 1 , { \bf t } 2 )$ is substantially higher for Pair-SFT relative to the base model, there is only little benefit to self-correction $. 1 . 8 \%$ gain). This gain is of a similar order to findings from Qu et al. (2024). By studying $\Delta ^ { \mathrm { i } \mathrm { c } }$ and $\Delta ^ { \mathrm { c } \to \mathrm { i } }$ , we find that SFT mainly reduces the number of correct problems that are mistakenly changed to incorrect in the second attempt, but does not significantly increase the fraction of incorrect first attempts that are corrected. This result is consistent with prior works on intrinsic self-correction that have found negligible or negative $\Delta ( \mathbf { t } \mathbf { 1 } , \mathbf { t } \mathbf { 2 } )$ values. + +We also find that unlike Pair-SFT, training on $\mathcal { D } _ { S \mathrm { T a R } }$ does not reduce $\Delta ^ { \mathrm { c } \mathrm { i } }$ , indicating that the STaR policy does not have a clear understanding of when to make modifications and when not to. Observing this, we also trained on an extended version of $\mathcal { D } _ { S \mathrm { T a R } } ^ { + }$ (and $\mathcal { D } _ { \mathrm { { S F T } } } ^ { + } \backslash$ ), which additionally contains tuples with both correct responses. We would expect the addition of such “correct-to-correct” data to prevent the model from erroneously revising a correct response. As shown in Table 1, the inclusion of this data helps STaR substantially but only results in $0 . 4 \%$ change in $\Delta ( \mathbf { t } \mathbf { 1 } , \mathbf { t } \mathbf { 2 } )$ . On the other hand, for SFT, inclusion of this data overly biases the model against changing its answer. + +![](images/figures/score-fig-0003.jpg) +Figure 4 ∣ Edit distance between first-attempt and second-attempt responses from fine-tuned models, our approach (SCoRe) and the base model. While training on self-generated error correction traces learns to not make major edits primarily, SFT learns to make some edits but is still quite conservative. + +Diving deeper: analyzing self-correction behavior. To further understand how these STaR and SFT models edit their responses, we measured their edit distance ratios, defined as the edit distance between the responses normalized by the total length of both the responses. As shown in Figure 4a, while the base model sometimes makes substantially large edits to the original response, models fine-tuned on $\mathcal { D } _ { S \mathrm { T a R } }$ and $\mathcal { D } _ { \mathrm { S F T } }$ are overly conservative and often make no edits at all. This is akin to a form of behavior collapse: training to maximize likelihoods on off-policy revision traces does not teach the desired correction “behavior”, even though it improves first-attempt accuracy. Similar observations of LLMs ignoring nuanced be- + +![](images/figures/score-fig-0004.jpg) +Figure 5 ∣ Self-correction performance on different sets of firstattempt responses: (a) “fixed”: first response is sampled from the initial model, (b) “self-generated”: first response is generated by the learner itself. Throughout training, the correction rate on fixed responses increases for both train and validation problems, but degrades substantially on self-generated responses. This indicates that training on a fixed offline dataset of correction traces suffers from distribution shift. + +haviors (e.g., producing a mistake in a response and then correcting it in subsequent steps) have been observed in Ye et al. (2024). + +We also compared the distributions of edit distance ratios on training and test-time self-correction traces in Figures 4b/4c. While STaR produces qualitatively similar edit distance ratios on both the train and validation sets, we still observe some discrepancies between the train and validation edit distance ratios for SFT, implying that Pair-SFT is not very effective at generalizing to new problems from the same (a) Training accuracy curves. When training with standard multiturn RL, the responses at both the attempts become tightly coupled together, leading to poor coverage for subsequent iterations and worse learning progress. Stage I in SCoRe is explicitly designed to alleviate this and achieves much higher $\Delta ( \mathrm { t } 1 , \mathrm { t } 2 )$ , leading to increased exploration and better final performance. + +![](images/figures/score-fig-0005.jpg) +(b) Frequency in which the learner proposes a different answer in the second turn. Without explicitly modifying the policy initialization as in SCoRe, the policy quickly learns to often not change its answer, leading to poor exploration. Stage I in SCoRe prevents this issue, and learns non-collapsed behavior in Stage II. + +![](images/figures/score-fig-0006.jpg) + +Figure 6 ∣ Behavior collapse in standard multi-turn RL for training self-correction. These results indicate that some explicit approach to avoid collapse is necessary, i.e. Stage I in SCoRe. + +distribution. We visualized this by plotting the self-correction performance of the SFT model on a fixed set of first attempts and self-generated first attempts in Figure 5. We observe vastly different behaviors between static and self-generated first-attempt distributions: while the model is able to optimize training correction accuracy and also slightly improves on first attempts appearing in the validation set (distributed i.i.d. to the training distribution), its self-correction accuracy degrades. Hence, distribution shift is a significant challenge for offline methods such as Pair-SFT. + +# Takeaways: Insufficiency of SFT + +SFT-based methods suffer from two distinct failures when learning self-correction: (1) distribution shift, and (2) behavior collapse. Training on on-policy data can fix (1), but not (2). + +# 5. SCoRe: Self-Correction via Multi-Turn Reinforcement Learning + +The above results highlight that an effective approach for training LLMs to self-correct entirely via self-generated data must address both distribution shift and behavior collapse. Utilizing on-policy RL is a natural way to address distribution shift, and our method will do so by extending Equation 2 to multiple turns under the hierarchical framework of Zhou et al. (2024). However, is behavior collapse an issue for standard multi-turn RL? And if not, how can we address it? + +To answer these questions, we run standard multi-turn RL training to optimize Equation 1 only on $\left( { x _ { 2 } , y _ { 2 } } \right)$ pairs. Since this objective maximizes the second-attempt performance of the model without training the first attempt, we expect the self-correction $\Delta ( \mathrm { t } 1 , \mathrm { t } 2 )$ of the model to increase. However, as shown in Figure 6, while the performance of each attempt improves with training, their difference $\Delta ( \mathrm { t } 1 , \mathrm { t } 2 )$ does not. In other words, standard multi-turn RL converges to a state that is overly biased against changing its response, resulting in no self-correction ability and a similar behavior collapse as what we saw with STaR. + +Why does RL still suffer from collapse? There are at least two equally good solutions when optimizing a policy with RL on the training data: (i) learning to improve from the first to the second attempt, or (ii) + +![](images/figures/score-fig-0007.jpg) +Figure 7 ∣ An overview of our approach (SCoRe). SCoRe trains a model in two stages: Stage I: instead of running SFT (which produces pathological amplification of biases) to initialize RL training, we train a good initialization that can produce high-reward responses in the second-attempt while mimicking the base model’s initial response at the first attempt. Stage II: jointly optimizing both attempts, where the latter uses a shaped reward to incentivize the discovery of the self-correction strategy instead of the simple strategy of producing the best first response followed by making any minor edits to it in the second attempt. + +learning to produce the best first-attempt response, followed by no correction in the second attempt. Of course only the former strategy generalizes to new problems, but an overparameterized LLM may not necessarily learn strategy (i) instead of (ii), since both of these strategies can be equally optimal on the training set. Abstractly, learning the “meta strategy” of self-correction during training is difficult unless the “direct” strategy that optimizes reward appears less viable on the training data. Conceptually, this is similar to the memorization challenge in meta-learning (Yin et al., 2019), which suggests that when provided with mutually exclusive tasks, meta-learning is likely to recover the supervised learning solution (without using context from the few shots) that directly predicts the output. Here, this is analogous to not self-correcting past attempts, directly producing an answer. + +Method overview. Although a good self-correcting policy should maximize both accuracy $@ 1$ and accuracy $@ 1 2$ , we saw that standard RL leads to a collapse to non-correcting behavior. Hence, our key insight in SCoRe is that we must more explicitly encourage self-correction behavior, which we accomplish via a two-stage approach. The first stage (Stage I) serves the role of initialization where we train the model to decouple its behavior across the two attempts by attempting to optimize second-attempt accuracy while explicitly constraining the distribution of first attempts to the base model. From here, Stage II then jointly optimizes the reward of both attempts. To ensure that Stage II does not collapse to the “direct” solution, we bias the reward to reinforce self-correction progress. + +# 5.1. Stage I: Training an Initialization that Decouples Attempts + +The goal of Stage I of SCoRe is to obtain an initialization by improving the base model’s coverage over second attempts given the first attempt, so that subsequent training for self-correction is less prone to behavior collapse. While this would typically be done via SFT, our results in Section 4 show that SFT itself suffers from collapse. Therefore, we use RL in this stage to decouple the two attempts. To do so, we explicitly fine-tune the base model to produce high-reward responses at the second attempt, while forcing the model to not change its first attempt by constraining it to be close to the base model using a KL-divergence. While this may appear sub-optimal – a first attempt with fewer mistakes should lead to a better second attempt – but as we will show, this stage is critical in reducing the base model’s bias towards simply coupling the first and second-attempt distributions, thus avoiding behavior collapse when actual multi-turn RL is run. Formally, the objective is: + +$$ +\operatorname* { m a x } _ { \theta } \mathbb { E } _ { x _ { 1 } , y _ { 1 } \sim \pi _ { \theta } ( \cdot | x ) , y _ { 2 } \sim \pi _ { \theta } ( \cdot | [ x _ { 1 } , p _ { 1 } ] ) } \Big [ \widehat { r } ( y _ { 2 } , y ^ { * } ) - \beta _ { 2 } D _ { K L } \left( \pi _ { \theta } ( \cdot | | x _ { 1 } ) | | \pi _ { \mathrm { r e f } } ( \cdot | x _ { 1 } ) \right) \Big ] , +$$ + +where $\beta _ { 2 }$ is a hyperparameter designed to enforce a strict $\mathrm { K L }$ penalty only on the first attempt to avoid shifting of the first-turn responses (denoted by the term in blue). Note that we still utilize the default KL-divergence penalty from Equation 2, but with a relatively small weight and is omitted from Equation 3 for clarity. Indeed, we show that compared to standard multi-turn RL, Stage I is more effective at decoupling the two responses (Figure 6b) and leads to better Stage II performance. + +# 5.2. Stage II: Multi-Turn RL with Reward Shaping + +The second stage of SCoRe is initialized from Stage I and now jointly optimizes the performance of both attempts. Concretely, Stage II trains the policy $\pi _ { \theta } ( \cdot | \cdot )$ using the following objective: + +$$ +\operatorname* { m a x } _ { \theta } \mathbb { E } _ { x _ { 1 } , y _ { 1 } \sim \pi _ { \theta } ( \cdot | x ) , y _ { 2 } \sim \pi _ { \theta } ( \cdot | [ x _ { 1 } , p _ { 1 } ] ) } [ \sum _ { i = 1 } ^ { 2 } \widehat { r } ( y _ { i } , y ^ { * } ) - \beta _ { 1 } D _ { K L } ( \pi _ { \theta } ( \cdot | x _ { i } ) | | \pi _ { \mathrm { r e f } } ( \cdot | x _ { i } ) ) ] , +$$ + +where $x _ { i } , i \in \{ 1 , 2 \}$ corresponds to the set of input tokens passed as context to the model. + +Reward shaping to prevent behavior collapse. In principle, optimizing Equation 4 can also produce a solution that couples responses. This is because we still aim to maximize ground-truth rewards at both attempts. To prevent the learning process from collapsing to a non self-correcting solution in Stage II, we need to bias the learning problem towards self-correction. We implement this via reward shaping: by rewarding transitions that make “progress” towards learning the desired self-correction behavior. Concretely, given an two-turn rollout sampled from the policy $\tau = \{ \pmb { x } _ { 1 } , \hat { \pmb { y } } _ { 1 } , \hat { r } ( \pmb { y } _ { 1 } , \pmb { y } ^ { * } ) , \pmb { x } _ { 2 } , \hat { \pmb { y } } _ { 2 } , \hat { r } ( \pmb { y } _ { 2 } , \pmb { y } ^ { * } ) \}$ , we modify the reward $\hat { r } ( y _ { 2 } , y ^ { * } )$ in Equation 4, at the second attempt with a bonus $\widehat { b } ( y _ { 2 } | y _ { 1 } , y ^ { * } ) : =$ $\alpha \cdot \left( \hat { r } ( y _ { 2 } , y ^ { \ast } ) - \hat { r } ( y _ { 1 } , y ^ { \ast } ) \right)$ , where $\alpha$ is a positive constant multiplier, ideally larger than 1.0. Adding this bonus to the second attempt measures a notion of progress by only emphasizing transitions that flip the correctness of the response and assigns a heavy negative penalty to transitions that change a correct response to incorrect in the second attempt. Thus, the addition of this bonus should regularize the training process from collapsing on to the “direct” solution that also appears optimal on the training set but does not learn self-correction. + +# 5.3. Putting it Together and Implementation Details + +Our approach is illustrated pictorially in Figure 7. We detail all hyperparameters used in Appendix B. In practice, one can also use an adaptive $\beta _ { 2 }$ that attempts to balance the magnitudes of the first-attempt KL regularization and the second-attempt policy objective. In some of our experiments, we also choose to amplify the coverage of states used for on-policy RL by incorporating first-attempt solutions obtained by repeatedly sampling the base model as offline prompts in RL. We find that incorporating this data, especially in Stage II – where the first-turn policy may have drifted further from that of the base model – can have substantial benefits especially when attempting to learn from limited data. + +# Takeaways and Implications + +The core insight behind SCoRe is that we must make it more attractive to learn a more nuanced algorithmic strategy (i.e., self-correction) instead of collapsing to a degenerate behavior mode. To avoid distribution shift, this must be done on self-generated online data. + +Table 2 ∣ Performance of SCoRe on MATH. SCoRe not only attains a higher accuracy at both attempts, but also provides the most positive self-correction performance $\Delta ( \mathbf { t } \mathbf { 1 } , \mathbf { t } \mathbf { 2 } )$ . + +
ApproachAcc.@t1Acc.@t2∆(t1, t2)i→(1, t2)Δc-i(t1, t2)
Base model52.6%41.4%-11.2%4.6%15.8%
Self-Refine (Madaan et al., 2023)52.8%51.8%-1.0%3.2%4.2%
STaR w/ DtaR (Zelikman et al., 2022)53.6%54.0%0.4%2.6%2.2%
Pair-SFT w/ DsFT (Welleck et al., 2023)52.4%54.2%1.8%5.4%3.6%
SCoRe (Ours)60.0%64.4%4.4%5.8%1.4%
+ +# 6. Experimental Evaluation + +The goal of our experiments is to demonstrate the efficacy and justify the design of SCoRe in training LLMs how to self-correct by only training on their own data. To this end, we perform a comparative evaluation of SCoRe against prior methods that also use self-generated data to train for self-correction, and run several ablation studies on two representative reasoning tasks where error correction is crucial. + +Tasks. We mainly focus on reasoning problems in math and coding: (a) math problem solving on MATH (Hendrycks et al., 2021), and (b) code generation on MBPP (Austin et al., 2021) and HumanEval (Chen et al., 2021). We use the following train-test splits in our experiments: (1) MATH: following Lightman et al. (2023), we augment the MATH training set with 4500 problems from the test set, and report results on the remaining 500 problems (MATH500); and (2) Code generation: we train on MBPP and report results on HumanEval, which does not expose test cases to the model. For all tasks, we use binary rewards during training, indicating whether the model’s answer matches the ground truth one (for MATH) or passes all test cases (for coding). + +Evaluation protocol and metrics. We report the self-correction accuracy on a number of tasks with two sequential attempts at the problem, i.e., one round of self-correction. For code generation, following the evaluation protocol of Ni et al. (2024), we also report results on MBPP-R, an offline repair task that requires correcting incorrect first-attempt programs generated from PaLM 2. + +Models. For all of our experiments on coding problems, we fine-tune Gemini $1 . 0 \mathrm { P r o }$ and for MATH, we fine-tune Gemini 1.5 Flash. For all evaluations, we use greedy decoding (i.e. temperature 0), except for inference-compute scaling in Section 6.2 where we set temperature to be 0.7. For all training methods, we attempted to use a fixed budget of model samples and gradient updates, and do not vary hyperparameters such as learning rate and batch size between runs. For all RL runs, we selected checkpoints with the highest training reward, although a small held-out validation set of problems can also be used. + +Evaluation prompts. We use zero-shot CoT prompting for evaluation on MATH, zero-shot prompting for evaluation on HumanEval, and the canonical three-shot prompt for first-attempt training samples on MBPP (Austin et al., 2021). At the second attempt, we utilize an instruction that does not reveal the correctness of the previous answer, but asks the model to attempt to deduce whether a mistake exists in its first attempt response, and if so, potentially rewrite its response. Our full prompts and self-correction instructions can be found in Appendix C. + +Baselines & comparisons. We compare SCoRe to relevant prior approaches based on prompting or those that fine-tune only a single model for both solving the task and for revising responses, and only use self-generated data. Specifically, we compare to Self-Refine (Madaan et al., 2023), a representative prompting-based approach to elicit self-correction behaviors from a model, akin to Reflexion (Shinn et al., 2023). Among the fine-tuning based approaches, we compare to Pair-SFT based on the approach from Welleck et al. (2023), and multi-turn STaR (Singh et al., 2023; Zelikman et al., 2022) that fine-tune the model by maximizing log-likelihood respectively on synthetically-paired repair traces (Pair-SFT) and successful repair traces (STaR). + +Table 3 ∣ Performance of SCoRe on HumanEval. SCoRe attains the highest self-correction performance (Accuracy $@ 2$ , Δ(t1, t2)), and also outperforms other methods at offline correction (MBPP-R). + +
MethodMBPP-RAcc.@t1Acc.@t2∆(t1, t2)i→c(1, t2)Δc-→i(t1, t2)
Base model47.3%53.7%56.7%3.0%7.9%4.9%
Self-Refine30.7%53.7%52.5%-1.2%9.8%11.0%
Pair-SFT59.8%56.1%54.3%-1.8%4.3%6.1%
SCoRe (Ours)60.6%52.4%64.6%12.2%15.2%3.0%
+ +# 6.1. Benchmark Results + +MATH. Our results are shown in Table 2, as well as in Figure 1. SCoRe exhibits substantially stronger performance on both direct and self-correction accuracies relative to baselines. Notably, the intrinsic self-correction gain $\Delta ( \mathbf { t } \mathbf { 1 } , \mathbf { t } \mathbf { 2 } )$ of $4 . 4 \%$ is the first significantly positive delta, despite having fewer incorrect problems to correct by virtue of its higher Accuracy $@ 1$ . Relative to the base model Gemini 1.5 Flash, SCoRe improves $\Delta ( \mathbf { t } \mathbf { 1 } , \mathbf { t } \mathbf { 2 } )$ by $1 5 . 6 \%$ , and Accuracy $@ 2$ by $2 3 . 0 \%$ , and over the next best prior approach, Pair-SFT, by $1 0 . 2 \%$ and $2 . 6 \%$ respectively. By observing the frequency of problems that change from incorrect in the first attempt to correct in the second attempt and vice versa, we see that SCoRe both improves the rate at which it fixes incorrect answers $( 1 4 . 5 \%$ , compared to $9 . 5 \%$ for base) and reduces the proportion of correct answers it changes ( $1 5 . 8 \%$ to $1 . 4 \%$ ). + +Code generation. Our results for the code generation task are shown in Table 3. Generally, we find that SCoRe achieves both improved self-correction and offline repair performance. For MBPP-R (Ni et al., 2024), we find that SCoRe improves the base model from $4 7 . 3 \%$ to $6 0 . 6 \%$ , which is comparable to the gap between GPT-3.5 $( 4 3 \% )$ and GPT-4 $( 6 3 . 2 \% )$ . Despite only training on MBPP, we find that SCoRe is especially effective at generalizing to HumanEval, achieving a $1 2 . 2 \%$ intrinsic self-correction delta, or $9 \%$ higher than the base model. By contrast, Pair-SFT works nearly as well on the static repair task MBPP-R, but actually degrades the base model when evaluated in the self-correction setting, thus underscoring the importances of on-policy sampling for self-correction. + +# 6.2. Inference-Compute Scaling with Self-Correction + +Next, we investigate if SCoRe can be used in conjunction with inference-time compute scaling strategies. To do so, we evaluate self-consistency decoding (Wang et al., 2022), where we sample a diverse set of solutions, and then select the most consistent answer among these solutions. Typically, the default strategy is to sample all solutions in parallel to perform majority voting. However, we show in Figure 1 (right) that instead of sampling $2 K$ solutions in parallel, it is more compute-efficient to sample $K$ solutions in parallel, then perform one round of self-correction on each solution. With 32 solution budget per problem, parallel sampling shows a $7 . 4 \%$ accuracy gain, while combining it with sequential sampling using self-correction yields a $1 0 . 5 \%$ improvement. + +Table 4 ∣ Ablation studies to understand the impact of various components in SCoRe. Observe that while single-turn training is effective at optimizing the first-attempt accuracy of the model, it leads to degradation in the second attempt. The performance improvements without Stage I or without reward shaping in SCoRe are small when measured by the difference in accuracy over the two attempts. Utilizing STaR generally leads to worse performance even when it is run from an effective Stage I checkpoint. + +
MethodAccuracy@t1Accuracy@t2∆(t1, t2)
SCoRe (Ours)60.0%64.4%4.4%
w/o multi-turn training61.8%59.4%-2.4%
w/o Stage I59.2%61.4%2.2%
w/o reward shaping60.0%62.6%2.6%
w/ STaR instead of REINFORCE Stage II56.2%58.4%2.2%
+ +# 6.3. Ablation Studies: Understanding the Impact of SCoRe Components + +Finally, we also present a number of ablation studies to understand the importance of various components in SCoRe. We perform these ablations on the MATH dataset. Concretely, we aim to answer the following questions: (1) the importance of multi-turn training: Can RL trained to maximize single-turn performance achieve better accuracy $@ 1$ or accuracy $@ \ t 2 ?$ ; (2) the importance of multi-stage training: How essential is Stage I to SCoRe? In other words, why not run Stage II directly?; (3) the impact of reward shaping. How would removing the reward shaping terms affect performance of SCoRe in Stage II, assuming Stage I was done identically?; (4) the importance of on-policy RL: What if we replaced REINFORCE in Stage II with STaR?. + +The results of all of these ablation experiments are shown in Table 4. As expected, single-turn training improves turn 1 performance, but has negative $\Delta ( { \bf t } 1 , { \bf t } 2 )$ . As shown in Figure 6, Stage I is critical to SCoRe; without it, the model achieves $2 \%$ lower $\Delta ( \mathbf { t } \mathbf { 1 } , \mathbf { t } \mathbf { 2 } )$ and $3 \%$ lower accuracy $@ 2$ . Similarly, we find that removing reward shaping also hurts performance, indicating that the RL objectives in both stages play a significant role in teaching the self-correction behavior. We also find that replacing REINFORCE with STaR in Stage II results in significantly lower absolute performance with no visible improvements in self-improvement performance, which contrasts with the findings in Havrilla et al. (2024a) that STaR and on-policy RL have similar convergence rates for single-turn RL. This suggests that leveraging on-policy samples is especially critical in the self-correction setting, which presents a multi-turn problem that admits potentially spurious solutions. + +# 7. Discussion, Limitations, and Conclusion + +In this work, we investigated how to imbue LLMs with self-correction behavior that enables them to correct their own responses on the fly. To accomplish this, we proposed SCoRe, a multi-turn RL approach, and demonstrated through extensive evaluations that it is one of the first methods to attain significantly positive intrinsic self-correction performance. To do so, we rigorously analyzed the behavior of various SFT approaches and identified failure modes in which the model learns a non-correcting strategy (e.g. learning to make no edits; behavior collapse) or falls prey to distribution shift. SCoRe trains a self-correcting strategy by utilizing a two-stage design and reward shaping, both of which help preventing behavior collapse into not learning effective self-corrective behavior. SCoRe has limitations that also provide avenues for future work. We did not train SCoRe for more than one round of iterative self-correction due to infrastructural reasons, which means that subsequent rounds may not be as effective as the first. Future work should train with more than two attempts via $\mathrm { R L }$ , which is already a common and effective practice to obtain effective self-correction behavior over more than two rounds with SFT (Qu et al., 2024; Snell et al., 2024). Unifying Stages I and II would also be interesting, since it would alleviate the limitation of running multiple runs. Finally, our results suggest that learning meta-strategies (e.g., self-correction) might require going beyond standard LLM fine-tuning (Section 4), and incorporate regularization (e.g., progress reward). + +# Acknowledgements + +The authors would like to thank Satinder Baveja, Kalesha Bullard, Gheorghe Comanici, Claire Cui, Valentin Dalibard, Angelos Filos, Yang Gao, Zoubin Ghahramani, Izzeddin Gur, Raia Hadsell, Clara Huiyi Hu, Melvin Johnson, Mina Khan, Balaji Lakshminarayanan, Yiran Mao, Hussain Masoom, Junhyuk Oh, Jordi Orbay, David Silver, and Yury Sulsky for helpful discussions, feedback, and sponsorship. We thank Amrith Setlur, Yuxiao Qu, Charlie Snell, Tianhe Yu, and Xinyang (Young) Geng for helpful discussions and feedback on an earlier version of the paper. + +# Author Contributions + +AK and VZ led the paper, with substantial technical contributions from RA and YS. VZ led the experimentation in the final paper with AK, with support from RA and YS. AK and RA conceived the initial idea with advice and discussions from DS, FB, AF, JDC, AS, and GT. JDC, YS, AS, RA, and AK iterated on the methodology. The development of the final method was done by AK and VZ, with inputs from RA and FB. VZ led the infrastructure development, while RA, YS, CP, SI, KB, DS, and LMZ contributed to the infrastructure. AK, RA, FB, AF, DP, GT advised on the overall direction. AK and VZ wrote the manuscript, with input from all co-authors. KM provided program management. FB, and AF co-supervised the project. + +# References + +A. Ahmadian, C. Cremer, M. Gallé, M. Fadaee, J. Kreutzer, A. Üstün, and S. Hooker. Back to basics: Revisiting reinforce style optimization for learning from human feedback in llms. arXiv preprint arXiv:2402.14740, 2024. +A. F. Akyürek, E. Akyürek, A. Madaan, A. Kalyan, P. Clark, D. Wijaya, and N. Tandon. 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Scaling to Multiple Attempts + +![](images/figures/score-fig-0008.jpg) +Figure 8 ∣ Performance of the base model, Pair-SFT, and SCoRe over 10 attempts on MATH. + +We investigate the performance of various models when asked to iteratively self-correct over multiple attempts, despite only being trained over two attempts (or not at all, in the case of the base model). As shown in Figure 8, we find that the performance of the base Gemini 1.5 Flash model is quite noisy, but never surpasses that of the first attempt. Similarly, Pair-SFT does not improve past the second attempt. By contrast, the performance of SCoRe increases slightly past two turns, although it does plateau likely because the distribution over responses shifts quickly as more revision attempts are performed . We leave improving the scaling properties of self-correction, a form of meta-learning, to future work. + +# A.2. Reward Function Design + +![](images/figures/score-fig-0009.jpg) +Figure 9 ∣ Impact of discount factor of $\gamma$ on standard multi-turn RL training. + +In all of our experiments, we used only the instantaneous reward in our policy gradient objective, which is equivalent to returns with discount factor $\gamma = 0$ . We additionally investigated whether leveraging + +$\gamma > 0$ , in conjunction with reward shaping, can elicit self-correction paper. As presented in Figure 9, we find that with $\gamma = 0 . 8$ and $\alpha = 1 . 0$ , multi-turn RL still suffers from the same non-correcting behavior collapse as the standard multi-turn RL approach. + +# B. Additional Experiment Details + +Table 5 ∣ Hyperparameters for SCoRe on MATH (left) and MBPP (right) + +
HyperparameterValue
Base modelGemini 1.5 Flash
OptimizerAdam
Learning rate5e-6
Training steps3000
Batch size512
Sampling temperature1.0
α10
β10.01
β20.1
+ +
HyperparameterValue
Base modelGemini 1.0 Pro
OptimizerAdam
Learning rate1e-5
Training steps1500
Batch size128
Sampling temperature1.0
α10
β10.01
β20.25
+ +We include the hyperparameters used for training SCoRe in Table 5. + +# C. Prompts + +# MATH Zero-shot Prompt + +You are a math expert. When you respond, respond only with the Solution of the final Problem, thinking step by step. At the end of the Solution, when you give your final answer, write it in the form "Final Answer: The final answer is $\$ 9$ answer $\$ 1$ . I hope it is correct." + +# MATH Self-Correction Instruction + +There might be an error in the solution above because of lack of understanding of the question. Please correct the error, if any, and rewrite the solution. Only output the final solution! At the end of the Solution, when you give your final answer, write it in the form "Final Answer: The final answer is $\$ 1$ answer $\$ 9$ . I hope it is correct." + +# MBPP 3-shot Prompt + +You are an expert Python programmer, and here is your task: Write a function to find the similar elements from the given two tuple lists. Your code should pass these tests: + +assert similar_elements $( ( 3 , 4 , 5 , 6 ) , ( 5 , 7 , 4 , 1 0 ) ) = = ( 4 , 5 )$ assert similar_elements $( ( 1 , 2 , 3 , 4 ) , ( 5 , 4 , 3 , 7 ) ) = = ( 3 , 4 )$ assert similar_elements $( ( 1 1 , 1 2 , 1 4 , 1 3 ) , ( 1 7 , 1 5 , 1 4 , 1 3 ) ) = = ( 1 3 , 1 4 )$ + +[BEGIN] + +def similar_elements(test_tup1, test_tup2): res $=$ tuple(set(test_tup1) & set(test_tup2)) return (res) + +[DONE] + +You are an expert Python programmer, and here is your task: Write a python function to identify non−prime numbers. Your code should pass these tests: + +assert is_not_prime(2) $= =$ False assert is_not_prime(10) $= =$ True assert is_not_prime(35) $= =$ True + +# [BEGIN] + +import math +def is_not_prime(n): result $=$ False for i in range(2,int(math.sqrt(n)) + 1): if $\mathrm { ~ n ~ } \%$ result $=$ True return result + +[DONE] + +You are an expert Python programmer, and here is your task: Write a function to find the largest integers from a given list of numbers using heap queue algorithm. Your code should pass these tests: + +assert heap_queue_largest( [25, 35, 22, 85, 14, 65, 75, 22, 58],3) $= =$ [85, 75, 65] assert heap_queue_largest( [25, 35, 22, 85, 14, 65, 75, 22, 58],2) $= =$ [85, 75] assert heap_queue_largest( [25, 35, 22, 85, 14, 65, 75, 22, 58],5) $= =$ [85, 75, 65, 58, 35] + +[BEGIN] + +import heapq as hq +def heap_queue_largest(nums,n): largest_nums $=$ hq.nlargest(n, nums) return largest_nums + +[DONE] + +# MBPP/HumanEval Self-Correction Instruction + +# There might be an error in the code above because of lack of understanding of the question. Please correct the error, if any, and rewrite the solution. Only output the final correct Python program! + +# D. Qualitative Analysis of SCoRe + +We also perform a qualitative investigation into how SCoRe addresses the self-repair shortcomings of base LLMs, and provide several examples in Appendix E. We find that SCoRe is able to refine its own responses in a variety of manners - rewriting the entire solution when necessary, or reproducing the correct parts of the solution, while revising the incorrect ones. For the latter, we interestingly find that SCoRe is especially adept at revising its computational mistakes, and even demonstrates a bias towards showing more steps in certain computations and manipulations in order to increase its probability of producing a correct answer. We additionally observe that the model learns to occasionally self-correct within a turn, e.g. MATH example 4. + +# E. Examples of SCoRe Self-Correction + +# HumanEval Example 1 + +# Problem: + +def cycpattern_check(a , b): + +"""You are given 2 words. You need to return True if the second word or any of its rotations is a substring in the first word + +cycpattern_check("abcd","abd") $= >$ False cycpattern_check("hello","ell") $= >$ True cycpattern_check("whassup","psus") $= >$ False cycpattern_check("abab","baa") $= >$ True cycpattern_check("efef","eeff") $= >$ False cycpattern_check("himenss","simen") $= >$ True + +!I! + +# Turn 1 solution (incorrect): + +def cycpattern_check(a , b): + +"""You are given 2 words. You need to return True if the second word or any of its rotations is a substring in the first word + +cycpattern_check("abcd","abd") $= >$ False cycpattern_check("hello","ell") $= >$ True cycpattern_check("whassup","psus") $= >$ False cycpattern_check("abab","baa") $= >$ True cycpattern_check("efef","eeff") $= >$ False cycpattern_check("himenss","simen") $= >$ True + +!!!!! + +for i in range(len(b)): if a.find $( \mathsf { b } [ \mathrm { i } ; ] ) ! = - 1$ : return True +return False + +# Turn 2 solution (correct): + +def cycpattern_check(a , b): + +"""You are given 2 words. You need to return True if the second word or any of its rotations is a substring in the first word + +cycpattern_check("abcd","abd") $= >$ False cycpattern_check("hello","ell") $= >$ True cycpattern_check("whassup","psus") $= >$ False cycpattern_check("abab","baa") $= >$ True cycpattern_check("efef","eeff") $= >$ False cycpattern_check("himenss","simen") $= >$ True + +for i in range(len(b)): rotated $\underline { { \mathbf { b } } } = \mathbf { b } [ \mathrm { i } : ] + \mathbf { b } [ : \mathrm { i }$ ] if a.find(rotated_b) $! = - 1$ : return True +return False + +# HumanEval Example 2 + +# Problem: + +def even_odd_count(num): """Given an integer. return a tuple that has the number of even and odd digits respectively. + +Example: even_odd_count $( - 1 2 ) = = > ( 1 , 1 )$ even_odd_count(123) $= = >$ (1, 2) + +# Turn 1 solution (incorrect): + +def even_odd_count(num): + +"""Given an integer. return a tuple that has the number of even and odd digits respectively. + +Example: even_odd_count $( - 1 2 ) = = > ( 1 , 1 )$ even_odd_count(123) $= = >$ (1, 2) +!!! + +even_count $= 0$ odd_count $= 0$ + +for digit in str(num): int_digit $=$ int(digit) if int_digit $\%$ even_count $+ = 1$ else: odd_count $+ = 1$ +return even_count, odd_coun +Turn 2 solution (correct): +def even_odd_count(num): """Given an integer. return a tuple that has the number of even and odd digits respectively. + +Example: even_odd_count $( - 1 2 ) = = > ( 1 , 1 )$ even_odd_count(123) ==> (1, 2) + +!!!! + +even_count $= 0$ odd_count $= 0$ + +for digit in str(abs(num)): int_digit $=$ int(digit) if int_digit $\%$ even_count $+ = 1$ else: odd_count $+ = 1$ +return even_count, odd_count + +# MATH Example 1 + +Problem. Let ?? be a matrix such that + +$$ +A { \left( \begin{array} { l } { 3 } \\ { 1 } \\ { 0 } \end{array} \right) } = { \left( \begin{array} { l } { 2 } \\ { 4 } \\ { - 3 } \end{array} \right) } \quad { \mathrm { a n d } } \quad A { \left( \begin{array} { l } { - 5 } \\ { 2 } \\ { 2 } \end{array} \right) } = { \left( \begin{array} { l } { 0 } \\ { - 5 } \\ { - 5 } \end{array} \right) } . +$$ + +$\textrm { d } A \left( \begin{array} { c } { - 1 3 } \\ { 3 } \\ { 4 } \end{array} \right) .$ + +SCoRe turn 1 solution (incorrect). We can write + +$$ +{ \binom { - 1 3 } { 3 } } = a { \binom { 3 } { 1 } } + b { \binom { - 5 } { 2 } } +$$ + +for some scalars $a$ and $b$ . Solving the system of equations + +$$ +\begin{array} { c } { { 3 a - 5 b = - 1 3 , } } \\ { { a + 2 b = 3 , } } \\ { { 2 b = 4 , } } \end{array} +$$ + +we find $a = - 1$ and $b = 2$ . Therefore, + +$$ +A { \left( \begin{array} { l } { - 1 3 } \\ { 3 } \\ { 4 } \end{array} \right) } = A \left( - 1 { \left( \begin{array} { l } { 3 } \\ { 1 } \\ { 0 } \\ { 0 } \end{array} \right) } + 2 { \left( \begin{array} { l } { - 5 } \\ { 2 } \\ { 2 } \end{array} \right) } \right) = - 1 A { \left( \begin{array} { l } { 3 } \\ { 1 } \\ { 0 } \end{array} \right) } + 2 A { \left( \begin{array} { l } { - 5 } \\ { 2 } \\ { 2 } \end{array} \right) } = - 1 { \left( \begin{array} { l } { 2 } \\ { 4 } \\ { - 3 } \end{array} \right) } + 2 { \left( \begin{array} { l } { 0 } \\ { - 5 } \\ { - 5 } \end{array} \right) } = \left| { \left( \begin{array} { l } { - 2 } \\ { - 1 4 } \\ { - 1 3 } \end{array} \right) } \right| . +$$ + +Final Answer: The final answer is ${ \binom { - 2 } { - 1 4 } } .$ I hope it is correct. + +SCoRe turn 2 solution (correct). We can write + +$$ +{ \binom { - 1 3 } { 3 } } = a { \binom { 3 } { 1 } } + b { \binom { - 5 } { 2 } } +$$ + +for some scalars $a$ and $b$ . This gives us the system of equations: + +$$ +\begin{array} { c } { { 3 a - 5 b = - 1 3 , } } \\ { { { } a + 2 b = 3 , } } \\ { { { } 2 b = 4 . } } \end{array} +$$ + +Solving the third equation, we get $b = 2$ . Substituting into the second equation, we get $a = - 1$ . We can check that these values of $a$ and $b$ also satisfy the first equation. + +$$ +\begin{array}{c} \right) = A \left( - 1 { \left( \begin{array} { l } { 3 } \\ { 1 } \\ { 0 } \\ { 0 } \end{array} \right) } + 2 { \left( \begin{array} { l } { - 5 } \\ { 2 } \\ { 2 } \\ { 2 } \end{array} \right) } \right) = - 1 A { \left( \begin{array} { l } { 3 } \\ { 1 } \\ { 0 } \\ { 0 } \end{array} \right) } + 2 A { \left( \begin{array} { l } { - 5 } \\ { 2 } \\ { 2 } \\ { 2 } \end{array} \right) } = - 1 { \left( \begin{array} { l } { 2 } \\ { 4 } \\ { 3 } \\ { - 3 } \end{array} \right) } + 2 { \left( \begin{array} { l } { 0 } \\ { - 5 } \\ { - 5 } \\ { - 5 } \end{array} \right) } = { \left( \begin{array} { l } { - 2 } \\ { - 4 } \\ { 3 } \\ { - 1 0 } \end{array} \right) } + { \left( \begin{array} { l } { 0 } \\ { - 1 } \\ { - 2 } \\ { - 1 0 } \end{array} \right) } = { \left[ \begin{array} { l } { \left( - 5 \right)} \\ { - 1 } \\ { - 2 } \\ { - 3 } \end{array} } \\ { \left( - 5 \right)} \end{array} = - 1 { \left( \begin{array} { l } { - 5 } \\ { - 1 } \\ { - 1 } \\ { - 1 } \end{array} \right) } = - 1 +$$ + +Final Answer: The final answer is ${ \binom { - 2 } { - 1 4 } } .$ I hope it is correct. + +# MATH Example 2 + +Problem. Simplify ${ \frac { 1 } { 5 } } \cdot { \frac { 8 } { 7 } } \div { \frac { 1 2 } { 2 0 } }$ . + +SCoRe turn 1 solution (incorrect). First, we can simplify the division by multiplying by the reciprocal of the divisor: + +$$ +{ \frac { 1 } { 5 } } \cdot { \frac { 8 } { 7 } } \div { \frac { 1 2 } { 2 0 } } = { \frac { 1 } { 5 } } \cdot { \frac { 8 } { 7 } } \cdot { \frac { 2 0 } { 1 2 } } +$$ + +Next, we can simplify by canceling common factors: + +$$ +{ \frac { 1 } { 5 } } \cdot { \frac { 8 } { 7 } } \cdot { \frac { 2 0 } { 1 2 } } = { \frac { 1 \cdot 8 \cdot 2 0 } { 5 \cdot 7 \cdot 1 2 } } = { \frac { 1 \cdot 2 \cdot 4 \cdot 5 \cdot 4 } { 5 \cdot 7 \cdot 2 \cdot 6 } } = { \frac { 4 \cdot 4 } { 7 \cdot 6 } } = { \frac { 2 \cdot 2 \cdot 2 \cdot 2 } { 7 \cdot 2 \cdot 3 } } = { \frac { 2 \cdot 2 \cdot 2 0 } { 7 \cdot 3 \cdot 3 } } = { \frac { 2 \cdot 2 0 } { 9 \cdot 2 \cdot 3 } } +$$ + +Finally, we can multiply the remaining factors: + +$$ +{ \frac { 2 \cdot 2 } { 7 \cdot 3 } } = { \frac { 4 } { 2 1 } } +$$ + +Final Answer: The final answer is $\frac { 4 } { 2 1 }$ . I hope it is correct. + +SCoRe turn 2 solution (correct). + +$$ +{ \frac { 1 } { 5 } } \cdot { \frac { 8 } { 7 } } \div { \frac { 1 2 } { 2 0 } } = { \frac { 1 } { 5 } } \cdot { \frac { 8 } { 7 } } \cdot { \frac { 2 0 } { 1 2 } } = { \frac { 1 \cdot 8 \cdot 2 0 } { 5 \cdot 7 \cdot 1 2 } } = { \frac { 1 \cdot 8 \cdot 4 \cdot 5 } { 5 \cdot 7 \cdot 4 \cdot 3 } } = { \frac { 1 \cdot 8 } { 7 \cdot 3 } } = { \frac { 8 } { 2 1 } } +$$ + +Final Answer: The final answer is $\frac { 8 } { 2 1 }$ . I hope it is correct. + +# MATH Example 3 + +Problem. Suppose the polynomial + +$$ +f ( x ) = a _ { n } x ^ { n } + a _ { n - 1 } x ^ { n - 1 } + \cdots + a _ { 2 } x ^ { 2 } + a _ { 1 } x + a _ { 0 } +$$ + +has integer coefficients, and its roots are distinct integers. + +Given that $a _ { n } = 2$ and $a _ { 0 } = 6 6$ , what is the least possible value of $\left| a _ { n - 1 } \right|$ ? + +SCoRe turn 1 solution (incorrect). Let the roots of $f ( x )$ be $r _ { 1 } , r _ { 2 } , \ldots , r _ { n }$ . Then by Vieta’s formulas, + +$$ +r _ { 1 } + r _ { 2 } + \cdot \cdot \cdot + r _ { n } = - \frac { a _ { n - 1 } } { a _ { n } } = - \frac { a _ { n - 1 } } { 2 } . +$$ + +Since the roots are distinct integers, $r _ { 1 } + r _ { 2 } + \cdots + r _ { n }$ is an integer. Thus, $a _ { n - 1 }$ must be even. Also, + +$$ +r _ { 1 } r _ { 2 } \ldots r _ { n } = { \frac { ( - 1 ) ^ { n } a _ { 0 } } { a _ { n } } } = { \frac { ( - 1 ) ^ { n } \cdot 6 6 } { 2 } } = ( - 1 ) ^ { n } \cdot 3 3 . +$$ + +Since the roots are integers, $r _ { 1 } r _ { 2 } \ldots r _ { n }$ is an integer. Thus, $n$ must be odd. + +Since $n$ is odd, the sum of the roots is an integer, and $a _ { n - 1 }$ is even, the least possible value of $\left| a _ { n - 1 } \right|$ is 2 . +Final Answer: The final answer is 2. I hope it is correct. + +SCoRe turn 2 solution (correct). Let the roots of $f ( x )$ be $r _ { 1 } , r _ { 2 } , \ldots , r _ { n }$ . Then by Vieta’s formulas, + +$$ +r _ { 1 } + r _ { 2 } + \cdot \cdot \cdot + r _ { n } = - \frac { a _ { n - 1 } } { a _ { n } } = - \frac { a _ { n - 1 } } { 2 } . +$$ + +Since the roots are distinct integers, $r _ { 1 } + r _ { 2 } + \cdots + r _ { n }$ is an integer. Thus, $a _ { n - 1 }$ must be even. + +Also, + +$$ +r _ { 1 } r _ { 2 } \ldots r _ { n } = { \frac { ( - 1 ) ^ { n } a _ { 0 } } { a _ { n } } } = { \frac { ( - 1 ) ^ { n } \cdot 6 6 } { 2 } } = ( - 1 ) ^ { n } \cdot 3 3 . +$$ + +Since the roots are integers, $r _ { 1 } r _ { 2 } \ldots r _ { n }$ is an integer. Since 33 has a factor of 11, one of the roots must be a multiple of 11. Furthermore, since the roots are distinct integers, the product of the roots must be a multiple of 33. + +We want to minimize $\left| a _ { n - 1 } \right|$ , which is the absolute value of the sum of the roots. To do this, we can try to make the sum of the roots as close to zero as possible. + +ince the product of the roots is $\pm 3 3$ , we can consider the following cases: + +$^ { * } \stackrel { * * } { \sim } \mathrm { C a s e } 1 \therefore ^ { * } \stackrel { * } { \sim } n = 3$ and the roots are −11, 1, and 3. In this case, the sum of the roots is $^ { - 7 }$ , and $\left| a _ { n - 1 } \right| = 1 4$ . $^ { * } \stackrel { * * } { \sim } \mathrm { C a s e } 2 \stackrel { * } { \cdot } \stackrel { * * } { \sim } n = 5$ and the roots are $^ { - 1 1 , - 3 , - 1 , 1 }$ , and 3. In this case, the sum of the roots is $^ { - 1 1 }$ , and $\left| a _ { n - 1 } \right| = 2 2$ . + +$^ { * } \stackrel { * * } { \sim } \mathrm { C a s e } 3 \stackrel { * } { \sim } \stackrel { * } { \sim } n = 7$ and the roots are $- 1 1 , - 3 , - 1 , 1 , 3 , 5$ , and 7. In this case, the sum of the roots is $^ { - 1 1 }$ and $\left| a _ { n - 1 } \right| = 2 2$ . + +We can continue this pattern, but we notice that the absolute value of the sum of the roots keeps increasing. +Therefore, the least possible value of $\left| a _ { n - 1 } \right|$ is 14 . + +Final Answer: The final answer is 14. I hope it is correct. + +# MATH Example 4 + +Problem. Find the minimum value of + +$$ +\frac { ( x + 5 ) ( x + 2 ) } { x + 1 } +$$ + +for $x > 0$ + +SCoRe turn 1 solution (incorrect). We can write + +$$ +\begin{array} { c } { { \displaystyle \frac { ( x + 5 ) ( x + 2 ) } { x + 1 } = \frac { x ^ { 2 } + 7 x + 1 0 } { x + 1 } } } \\ { { = \displaystyle \frac { ( x ^ { 2 } + 7 x + 1 0 ) - ( x ^ { 2 } + x ) } { x + 1 } } } \\ { { = \displaystyle \frac { 6 x + 1 0 } { x + 1 } } } \\ { { = \displaystyle \frac { ( 6 x + 6 ) + 4 } { x + 1 } } } \\ { { = 6 + \displaystyle \frac { 4 } { x + 1 } . } } \end{array} +$$ + +Since $x > 0$ , $x + 1 > 1$ , so $\textstyle { \frac { 4 } { x + 1 } } < 4$ . Therefore, + +$$ +\frac { ( x + 5 ) ( x + 2 ) } { x + 1 } < 6 + 4 = 1 0 . +$$ + +Equality occurs when $x = 0$ , so the minimum value is 6 . +Final Answer: The final answer is 6. I hope it is correct. + +SCoRe turn 2 solution (correct). We can write + +$$ +\begin{array} { r l } & { \frac { \displaystyle ( x + 5 ) ( x + 2 ) } { x + 1 } = \frac { x ^ { 2 } + 7 x + 1 0 } { x + 1 } } \\ & { \qquad = \frac { \displaystyle ( x ^ { 2 } + x ) + ( 6 x + 6 ) + 4 } { x + 1 } } \\ & { \qquad = \frac { x \left( x + 1 \right) + 6 \left( x + 1 \right) + 4 } { x + 1 } } \\ & { \qquad = \frac { \displaystyle ( x + 1 ) \left( x + 6 \right) + 4 } { x + 1 } } \\ & { \qquad = x + 6 + \frac { 4 } { x + 1 } . } \end{array} +$$ + +Since $x > 0$ , $x + 1 > 1$ , so $\textstyle { \frac { 4 } { x + 1 } } < 4$ . Therefore, + +$$ +{ \frac { ( x + 5 ) ( x + 2 ) } { x + 1 } } > x + 6 . +$$ + +Since $x > 0$ , $x + 6 > 6$ . Thus, the minimum value is greater than 6. + +Furthermore, as $x$ increases, the term $\frac { 4 } { x + 1 }$ decreases, so the expression $\frac { ( x + 5 ) ( x + 2 ) } { x + 1 }$ decreases. Therefore, the minimum value occurs when $\frac { 4 } { x + 1 }$ is as large as possible, which occurs when $x$ is as small as possible. Since $x > 0$ , the minimum value is + +$$ +6 + { \frac { 4 } { 0 + 1 } } = 1 0 . +$$ + +Equality occurs when $x = 0$ , but this contradicts the condition $x > 0$ . Therefore, the minimum value does + +We made an error in assuming that the minimum value occurs when $\frac { 4 } { x + 1 }$ is as large as possible. Instead, +we should minimize the entminimum value occurs when n. Since is as sma , the term ossible. $\frac { 4 } { x + 1 }$ is always positive. Therefore, the $\textstyle x + 6 + { \frac { 4 } { x + 1 } }$ +By AM-GM, + +$$ +x + 6 + { \frac { 4 } { x + 1 } } = ( x + 1 ) + { \frac { 4 } { x + 1 } } + 5 \geq 2 { \sqrt { ( x + 1 ) \cdot { \frac { 4 } { x + 1 } } } } + 5 = 9 . +$$ + +hen, so $\textstyle ( x + 1 ) = { \frac { 4 } { x + 1 } }$ $x ^ { 2 } + 2 x + 1 = 4$ , which means $x ^ { 2 } + 2 x - 3 = 0$ . This factors as $( x + 3 ) ( x - 1 ) = 0$ $x = 1$ $\bar { x } > 0 \bar { . }$ + +Therefore, the minimum value is $\boxed { 9 } .$ + +Final Answer: The final answer is 9. I hope it is correct. + +# MATH Example 5 + +Problem. Four points, $A , B , C$ , and $D$ , are chosen randomly and independently on the circumference of a circle. What is the probability that segments ???? and $C D$ intersect? + +SCoRe turn 1 solution (incorrect). The segments ???? and ???? will intersect if and only if the points $A , B , C$ , and $D$ appear in alternating order around the circle. There are 4! ways to order the points, and 2 ways to divide the points into two pairs. Therefore, the probability that the segments intersect is $\begin{array} { r } { \frac { \dot { 2 } } { 4 ! } = \frac { 2 } { 2 4 } = \frac { 1 } { 1 2 } } \end{array}$ . + +Final Answer: The final answer is $\textstyle { \frac { 1 } { 1 2 } }$ . I hope it is correct. + +SCoRe turn 2 solution (correct). The segments $A B$ and ???? will intersect if and only if the points $A , B , C$ and $D$ appear in alternating order around the circle. Without loss of generality, let’s assume that ?? is the first point chosen. Then, there are three possible cases for the placement of $B$ : + +$ { \stackrel { * } { \sim } } { \stackrel { * } { \sim } } \mathrm { C a s e } 1 { : } ^ { * } { \stackrel { * } { \sim } } B$ is placed in the arc between $A$ and ??. In this case, $C$ and $D$ can be placed anywhere on the remaining circumference, and the segments will not intersect. $\stackrel { * } { \mathrm { \sim } } \stackrel { * } { \mathrm { \sim } } \mathrm { C a s e } 2 \mathrel { \mathop : } \stackrel { * } { \mathrm { \sim } } \mathrm { } \mathrm { \ B }$ is placed in the arc between $A$ and $D$ . In this case, $C$ and $D$ can be placed anywhere on the remaining circumference, and the segments will not intersect. $ { ^ { \ast } \mathrm { \stackrel { \ast } { \sim } } } \mathrm { C a s e } \ 3 { \mathrel { \mathop : } } ^ { \ast \ast } \mathrm { \Gamma } _ { B }$ is placed in the arc between $C$ and $D$ . In this case, the segments ???? and $C D$ will intersect. + +The probability of Case 3 is $\textstyle { \frac { 1 } { 3 } }$ . Since the points are chosen independently, the probability that the segments intersect is ${ \Bigg [ } { \frac { 1 } { 3 } } { \Bigg ] } .$ + +Final Answer: The final answer is $\frac 1 3$ . I hope it is correct. + +# MATH Example 6 + +Problem. The 7th and 8th grades have enrollments of 520 and 650 respectively. The two grades have a total of 18 representatives on the Student Council. How many representatives should the 8th grade have if there is to be a fair representation of the two grades? + +SCoRe turn 1 solution (incorrect). The total enrollment of the two grades is $5 2 0 + 6 5 0 = 1 1 7 0 .$ The ratio of 7th grade students to 8th grade students is $5 2 0 / 1 1 7 0 = 5 2 / 1 1 7$ . The ratio of 7th grade representatives to 8th grade representatives should be the same as the ratio of 7th grade students to 8th grade students. Let x be the number of representatives for the 8th grade. Then the number of representatives for the 7th grade is $1 8 \cdot \mathbf { X }$ . The ratio of 7th grade representatives to 8th grade representatives is $( 1 8 - \mathbf { x } ) / \mathbf { x }$ . Setting the two ratios equal, we have $( 1 8 \mathrm { ~ - ~ } \mathbf { x } ) / \mathbf { x } = 5 2 / 1 1 7 .$ . Cross-multiplying, we get $1 1 7 ( 1 8 \cdot \mathrm { x } ) = 5 2 \mathrm { x } .$ . Expanding, we get $2 1 0 6 - 1 1 7 \mathrm { x } = 5 2 \mathrm { x }$ . Combining like terms, we get $2 1 0 6 = 1 6 9 \mathrm { x }$ . Solving for $\mathbf { X }$ , we get $\mathbf { x } = 2 1 0 6 / 1 6 9$ $= 1 2 . 4 7$ . Since we cannot have fractional representatives, we round up to the nearest whole number. + +Final Answer: The final answer is 13. I hope it is correct. + +SCoRe turn 2 solution (correct). The ratio of 7th grade students to 8th grade students is $5 2 0 / 6 5 0 =$ 4/5. Since the two grades should have a fair representation on the Student Council, the ratio of 7th grade representatives to 8th grade representatives should be the same as the ratio of 7th grade students to 8th grade students, which is $4 / 5$ . Let x be the number of representatives for the 8th grade. Then the number of representatives for the 7th grade is $1 8 \cdot \mathbf { X } .$ . The ratio of 7th grade representatives to 8th grade representatives is $( 1 8 - \mathbf { x } ) / \mathbf { x }$ . Setting the two ratios equal, we have $( 1 8 - \mathbf { x } ) / \mathbf { x } = 4 / 5$ . Cross-multiplying, we get $5 ( 1 8 - \mathbf { x } ) =$ 4x. Expanding, we get $9 0 - 5 \mathbf { x } = 4 \mathbf { x }$ . Combining like terms, we get $9 0 = 9 \mathbf { x }$ . Solving for $\mathbf { X }$ , we get $\mathbf { x } = 1 0$ . Final Answer: The final answer is 10. I hope it is correct. + +# MATH Example 7 + +Problem. The sum of 27 consecutive positive integers is $3 ^ { 7 }$ . What is their median? + +SCoRe turn 1 solution (incorrect). The sum of an arithmetic series is equal to the average of the first and last term, multiplied by the number of terms. Let the first term of the arithmetic series be $a$ , and the last term be ??. Then the sum of the series is $\frac { ( a + l ) \cdot 2 7 } { 2 }$ . Since the sum of the series is $3 ^ { 7 }$ , we have ${ \frac { \left( a + l \right) \cdot 2 7 } { 2 } } = 3 ^ { 7 }$ . Simplifying, we get $( a + l ) = { \frac { 3 ^ { 7 } { \cdot } 2 } { 2 7 } } = 3 ^ { 4 } \cdot 2 = 1 6 2$ . Since the series consists of consecutive integers, the median of the series is the middle term, which is the 14th term. The 14th term of the series is $a + 1 3$ , and the average of the first and last term is $\textstyle { \frac { a + l } { 2 } } $ . Therefore, the median of the series is $\begin{array} { r } { \frac { a + l } { 2 } + \frac { 1 3 } { 2 } = \frac { 1 6 2 } { 2 } + \frac { 1 3 } { 2 } = \frac { 1 7 5 } { 2 } } \end{array}$ . Final Answer: The final answer is 87.5. I hope it is correct. + +SCoRe turn 2 solution (correct). The sum of an arithmetic series is equal to the average of the first and last term, multiplied by the number of terms. Let the first term of the arithmetic series be $a$ , and the last term be ??. Then the sum of the series is $\frac { ( a + l ) \cdot 2 7 } { 2 }$ . Since the sum of the series is $3 ^ { 7 }$ , we have ${ \frac { ( a + l ) \cdot 2 7 } { 2 } } = 3 ^ { 7 }$ . Simplifying, we get $\begin{array} { r } { ( a + l ) = \frac { 3 ^ { 7 } \cdot 2 } { 2 7 } = \frac { 3 ^ { 7 } \cdot 2 } { 3 ^ { 3 } } = 3 ^ { \overline { { 4 } } } \cdot 2 = 1 6 2 } \end{array}$ . Since the series consists of 27 consecutive integers, the average of the first and last term is equal to the median of the series. Therefore, the median of the series is Final Answer: The final answer is 81. I hope it is correct. $\begin{array} { r } { \frac { a + l } { 2 } = \frac { 1 6 2 } { 2 } = 8 1 } \end{array}$ . \ No newline at end of file diff --git a/papers/score/paper.pdf b/papers/score/paper.pdf new file mode 100644 index 0000000000000000000000000000000000000000..c3e5eb0cafd22e53750ecf1cec8197d3cd58507b --- /dev/null +++ b/papers/score/paper.pdf @@ -0,0 +1,3 @@ +version https://git-lfs.github.com/spec/v1 +oid sha256:b384619e6083baa17bd0bdb7dd7ac8deb4faf7ddb0639c9bfb44723e0deeb90d +size 871090 diff --git a/papers/score/sau.json b/papers/score/sau.json new file mode 100644 index 0000000000000000000000000000000000000000..7f545be2c1b91c943adb1cf15d2cef12e835c0de --- /dev/null +++ b/papers/score/sau.json @@ -0,0 +1,212 @@ +{ + "paper_id": "score", + "paper_title": "SCoRe: Self-Correction via Multi-Turn Reinforcement Learning", + "D1": [ + { + "id": "score-D1-001", + "claim": "MATH training hyperparameters for SCoRe with Gemini 1.5 Flash base model: Adam optimizer, learning rate 5e-6, 3000 training steps, batch size 512, sampling temperature 1.0, reward shaping multiplier alpha=10, standard KL coefficient beta1=0.01, Stage I first-attempt KL coefficient beta2=0.1, discount factor gamma=0 (instantaneous reward only).", + "source": "Appendix B, Table 5 (left)" + }, + { + "id": "score-D1-002", + "claim": "MBPP code training hyperparameters for SCoRe with Gemini 1.0 Pro base model: Adam optimizer, learning rate 1e-5, 1500 training steps, batch size 128, sampling temperature 1.0, reward shaping multiplier alpha=10, standard KL coefficient beta1=0.01, Stage I first-attempt KL coefficient beta2=0.25.", + "source": "Appendix B, Table 5 (right)" + }, + { + "id": "score-D1-003", + "claim": "Core structural training parameters shared across all SCoRe tasks: 2 turns (one round of self-correction), reward shaping multiplier alpha constrained to be greater than 1.0, Stage I applies KL penalty exclusively on first-attempt distribution, checkpoint selection based on highest training reward, optional adaptive beta2 to balance first-attempt KL and second-attempt policy objectives.", + "source": "Section 5.1, Section 5.2, Section 5.3, Section 6" + }, + { + "id": "score-D1-004", + "claim": "Evaluation decoding configuration: greedy decoding at temperature 0 for main results, sampling temperature 0.7 for inference compute scaling experiments, sequential self-correction with K parallel samples plus one round of self-correction per sample under total solution budget per problem as hyperparameter.", + "source": "Section 6, Section 6.2" + }, + { + "id": "score-D1-005", + "claim": "Dataset composition for MATH and code experiments: MATH500 test set with 500 held-out problems (Lightman et al. 2023 split), training augmented with 4500 additional MATH test set problems, HumanEval benchmark (Chen et al. 2021) with no test case exposure to model, MBPP training with canonical 3-shot prompt for first-attempt training samples.", + "source": "Section 6 (Tasks paragraph)" + }, + { + "id": "score-D1-006", + "claim": "RL training backbone uses REINFORCE policy gradient with KL-divergence penalty against a fixed reference model (Ahmadian et al. 2024), using binary 0/1 reward: exact answer match for MATH, all test cases pass for code generation; oracle reward used only during training, not at test time.", + "source": "Section 3, Eq 2, Section 6" + } + ], + "D2": [ + { + "id": "score-D2-001", + "claim": "Multi-turn MDP objective for self-correction: max_{pi_theta} E_{x,y*~D, y_{l+1}~pi_theta(·|[x,y_{1:l},p_{1:l}])} [ sum_{i=1}^{l+1} r(y_i, y*) ], where pi_theta is the LLM policy, y_{1:l} are previous model attempts, p_{1:l} are auxiliary instructions (self-correction prompt), r is the oracle binary reward, and l=2 for one round of correction. The policy is trained to maximize reward across ALL attempts simultaneously, not just the final output.", + "source": "Section 3, Eq 1" + }, + { + "id": "score-D2-002", + "claim": "REINFORCE policy gradient with KL penalty (training backbone): max_theta E_{x_t, y_t~pi_theta(·|x_t)} [ r(y_t, y*) - beta1 * D_KL(pi_theta(·|x_t) || pi_ref(·|x_t)) ], where pi_ref is a frozen reference policy, beta1=0.01 is the standard KL penalty coefficient, and r is the binary reward. This base RL approach from Ahmadian et al. (2024) serves as the training backbone for all SCoRe stages.", + "source": "Section 3, Eq 2" + }, + { + "id": "score-D2-003", + "claim": "SCoRe Stage I objective for decoupled attempt initialization: max_theta E_{x1, y1~pi_theta(·|x1), y2~pi_theta(·|[x1,p1])} [ r(y2, y*) - beta2 * D_KL(pi_theta(·|x1) || pi_ref(·|x1)) ], where p1 is the self-correction instruction (does NOT reveal correctness), beta2 is the KL penalty applied ONLY to the first attempt (MATH: 0.1, MBPP: 0.25), and the default KL penalty from Eq 2 is also applied with small weight. Purpose: improve second-attempt reward while forcing the first-attempt distribution to stay close to the base model, preventing behavior collapse.", + "source": "Section 5.1, Eq 3" + }, + { + "id": "score-D2-004", + "claim": "SCoRe Stage II objective for joint multi-turn RL with reward shaping: max_theta E_{x1, y1~pi_theta(·|x1), y2~pi_theta(·|[x1,y1,p1])} [ sum_{i=1}^{2} r'(y_i, y*) - beta1 * D_KL(pi_theta(·|x_i) || pi_ref(·|x_i)) ], where r'(y1, y*) = r(y1, y*) (unchanged binary reward), r'(y2, y*) = r(y2, y*) + alpha * (r(y2, y*) - r(y1, y*)) with shaping bonus b = alpha * delta_r. Initialized from Stage I checkpoint. Jointly optimizes both attempts; without reward shaping, this collapses to producing the best first response with no edits.", + "source": "Section 5.2, Eq 4" + }, + { + "id": "score-D2-005", + "claim": "Reward shaping bonus for self-correction progress: b(y2 | y1, y*) = alpha * (r(y2, y*) - r(y1, y*)), where alpha is a positive constant multiplier (default 10, ideally >1.0), r is binary 0/1 correctness. With binary reward, the bonus equals +alpha for incorrect-to-correct transitions, 0 for unchanged correctness, and -alpha for correct-to-incorrect transitions. This biases Stage II away from the degenerate non-correcting solution.", + "source": "Section 5.2" + }, + { + "id": "score-D2-006", + "claim": "On-policy two-turn rollout generation: at each training step, sample first attempt y1~pi_theta(·|x1), compute binary reward r1=r(y1,y*), construct second-attempt context x2=concat([x1,y1,p1]), sample second attempt y2~pi_theta(·|x2), compute binary reward r2=r(y2,y*), and produce rollout tuple tau={x1,y1,r1,x2,y2,r2}. The self-correction instruction p1 does NOT reveal correctness; the model must autonomously detect and fix errors.", + "source": "Section 5 (overview), Section 5.3" + }, + { + "id": "score-D2-007", + "claim": "Binary reward function for MATH (answer matching): r_MATH(y, y*) = 1 if extract_answer(y) == y*, else 0, where extract_answer(y) parses the 'Final Answer: The final answer is $answer$' block from the response. Reward is binary (0/1) indicating exact answer match. Used only during training; oracle y* is NOT available at test time.", + "source": "Section 6 (Tasks), Section 3" + }, + { + "id": "score-D2-008", + "claim": "Binary reward function for code generation (test case passing): r_code(y, y*) = 1 if all_test_cases_pass(extract_code(y)), else 0, where extract_code(y) parses the code block and all_test_cases_pass executes the code against a hidden test suite. Reward is binary (0/1). Test cases are NOT exposed to the model at test time, especially for HumanEval.", + "source": "Section 6 (Tasks), Section 3" + }, + { + "id": "score-D2-009", + "claim": "Five self-correction evaluation metrics defined over N problems with binary correctness c1_i and c2_i at turns 1 and 2: (1) Accuracy@t1 = (1/N)*sum(c1_i), (2) Accuracy@t2 = (1/N)*sum(c2_i), (3) Delta(t1,t2) = Accuracy@t2 - Accuracy@t1 (net improvement), (4) Delta^{i->c} = sum((1-c1_i)*c2_i) / sum(1-c1_i) (fraction of incorrect first attempts corrected), (5) Delta^{c->i} = sum(c1_i*(1-c2_i)) / sum(c1_i) (fraction of correct first attempts broken). All use greedy decoding (temperature=0) for main results.", + "source": "Section 3 (Metrics paragraph)" + }, + { + "id": "score-D2-010", + "claim": "Edit distance ratio for diagnosing behavior collapse: edit_distance_ratio(y1, y2) = edit_distance(y1, y2) / (len(y1) + len(y2)), where edit_distance is the character-level edit distance between the two responses. Ratio equals 0 for identical responses; larger values indicate more aggressive editing. Defined in Section 4: 'edit distance between the responses normalized by the total length of both the responses'. Used to quantify editing aggressiveness; SFT methods produce low ratios (conservative edits) while SCoRe achieves higher ratios without collapsing.", + "source": "Section 4, Figure 4" + }, + { + "id": "score-D2-011", + "claim": "SCoRe two-stage training pipeline algorithm: Stage I initializes pi_theta from pi_ref, samples on-policy rollouts, computes Stage I loss L = -[r(y2,y*) - beta2 * D_KL(pi_theta(·|x1) || pi_ref(·|x1))], updates theta via REINFORCE gradient; Stage II initializes from Stage I checkpoint, samples on-policy rollouts, computes shaped reward r'(y2) = r(y2) + alpha*(r(y2)-r(y1)), computes Stage II loss with sum of rewards and KL penalties on both attempts, updates theta via REINFORCE gradient, and selects the checkpoint with highest training reward.", + "source": "Section 5.1, Section 5.2, Section 5.3, Figure 7" + }, + { + "id": "score-D2-012", + "claim": "SCoRe test-time inference procedure: first attempt y1 = argmax_y pi_theta(y|x1) via greedy decoding (T=0), self-correction instruction p1 prompts the model to detect and fix errors ('There might be an error...Please correct the error, if any, and rewrite the solution'), second-attempt context x2 = concat([x1,y1,p1]), second attempt y2 = argmax_y pi_theta(y|x2) via greedy decoding, return y2 as final answer. The model must AUTONOMOUSLY detect errors without any external feedback or oracle.", + "source": "Section 5.3, Appendix C" + }, + { + "id": "score-D2-013", + "claim": "Offline data augmentation for on-policy RL: sample y1_off ~ pi_ref(·|x1) from frozen base model; construct augmented batch B_aug = B_on ∪ {(x1, y1_off)} where B_on = {(x1, y1~pi_theta(·|x1))} is the on-policy rollout batch. For each offline pair, build augmented rollout tau_aug = {x1, y1_off, r(y1_off, y*), x2 = [x1, y1_off, p1], y2~pi_theta(·|x2), r(y2, y*)} and add to the RL objective in Eq.(4): max_theta E[Σ_{i=1}^{2} r'(y_i, y*) - beta1 D_KL(pi_theta(·|x_i) || pi_ref(·|x_i))]. Purpose: amplify coverage of first-attempt states when pi_theta(·|x1) has drifted from pi_ref.", + "source": "Section 5.3, Eq.(4)" + } + ], + "D3": [ + { + "id": "score-D3-001", + "claim": "MATH Self-Correction Benchmark: Compare SCoRe against prompting-based (Self-Refine) and fine-tuning-based (STaR, Pair-SFT) baselines on intrinsic self-correction for mathematical reasoning using Gemini 1.5 Flash. Models produce one initial solution and one self-correction attempt (two turns total). All methods use self-generated data only. Binary reward via exact answer match against ground truth, but oracle is NOT available at test time. Evaluation on MATH500 (500 held-out problems) using greedy decoding (T=0) with zero-shot CoT prompting. Metrics: Accuracy@t1, Accuracy@t2, Delta(t1,t2), Delta^{i->c}, Delta^{c->i}.", + "source": "Section 6, Section 6.1, Table 2" + }, + { + "id": "score-D3-002", + "claim": "Code Generation Self-Correction Benchmark: Compare SCoRe against baselines on code generation using Gemini 1.0 Pro. Models are trained on MBPP (canonical 3-shot prompt) and evaluated zero-shot on HumanEval (Chen et al. 2021) with NO test case exposure. Training uses binary rewards based on whether all test cases pass. At test time, models generate code and self-correct without access to test execution results. Metrics: Accuracy@t1, Accuracy@t2, Delta(t1,t2), Delta^{i->c}, Delta^{c->i}.", + "source": "Section 6, Section 6.1, Table 3" + }, + { + "id": "score-D3-003", + "claim": "Offline Code Repair Benchmark (MBPP-R): Evaluate models on a static offline repair task where they must correct incorrect first-attempt programs generated from PaLM 2. This is NOT self-correction — models see a fixed external first attempt rather than their own. Tests whether correction ability generalizes beyond self-generated errors. Metric: MBPP-R accuracy (fraction of programs correctly repaired).", + "source": "Section 6 (Evaluation protocol), Table 3" + }, + { + "id": "score-D3-004", + "claim": "Inference-Time Compute Scaling Experiment: Compare two strategies for spending a fixed compute budget of 32 solution samples per problem on MATH500: (A) parallel majority voting over 32 independent first attempts; (B) sequential self-correction with K=16 parallel samples each followed by one round of self-correction, then majority voting over the 16 corrected answers. Demonstrates that sequential self-correction is more compute-efficient than pure parallel sampling. Sampling temperature T=0.7 for all samples.", + "source": "Section 6.2, Figure 1 (right)" + }, + { + "id": "score-D3-005", + "claim": "Ablation Study on MATH: Systematically remove or replace each SCoRe component to measure its contribution. Five variants tested: (1) full SCoRe (Stage I + Stage II with reward shaping), (2) without multi-turn training (single-turn RL only), (3) without Stage I (Stage II directly from base model), (4) without reward shaping (remove bonus term, use raw reward for second attempt), (5) replace Stage II REINFORCE with STaR-style SFT on successful correction traces. All variants use identical hyperparameter budgets. Evaluated with greedy decoding (T=0) on MATH500. Metrics: Accuracy@t1, Accuracy@t2, Delta(t1,t2).", + "source": "Section 6.3, Table 4" + }, + { + "id": "score-D3-006", + "claim": "SFT Failure Mode Analysis: Empirically study why SFT-based approaches (STaR, Pair-SFT) fail at self-correction via two experiments. (A) Edit distance ratio analysis measuring how aggressively each method edits responses between attempts, comparing training and test-time distributions. (B) Distribution shift experiment evaluating correction accuracy on fixed first attempts (from initial model) vs. self-generated first attempts (from learner itself), demonstrating that offline correction gains do not transfer to the model's own mistakes. Uses Gemini 1.5 Flash. STaR runs 3 iterations; Pair-SFT runs 1 iteration; both tested with and without correct-to-correct data (+ variants).", + "source": "Section 4, Table 1, Figure 4, Figure 5" + }, + { + "id": "score-D3-007", + "claim": "Standard Multi-Turn RL Behavior Collapse Analysis: Run standard multi-turn RL (optimizing the base MDP objective directly without SCoRe's two-stage design or reward shaping) to demonstrate convergence to non-correcting behavior. Track training accuracy curves at turns 1 and 2, Delta(t1,t2) evolution, frequency of answer changes, and edit distance ratios throughout training. Compare with SCoRe Stage I to show how decoupled initialization prevents collapse.", + "source": "Section 5, Figure 6" + }, + { + "id": "score-D3-008", + "claim": "Multi-Attempt Scaling Analysis (Appendix A.1): Evaluate whether models trained for two-turn self-correction can generalize to more than two sequential attempts. Test models over 10 sequential self-correction attempts on MATH despite being trained only on two attempts. At each attempt, model receives problem + all previous attempts + correction instruction. Compare base model, Pair-SFT, and SCoRe on how performance evolves past the training distribution (2 turns).", + "source": "Appendix A.1, Figure 8" + }, + { + "id": "score-D3-009", + "claim": "Discount Factor Experiment (Appendix A.2): Test whether using non-zero discount factor gamma=0.8 with alpha=1.0 can elicit self-correction without collapsing, compared to the default gamma=0 with alpha=1.0. Track Delta(t1,t2), Accuracy@t1, and Accuracy@t2 during training to determine whether gamma>0 resolves behavior collapse.", + "source": "Appendix A.2, Figure 9" + } + ], + "D4": [ + { + "id": "score-D4-001", + "claim": "SCoRe training foundational ordering: (1) Multi-turn MDP objective defines the overall goal of maximizing total reward across all attempts -> (2) REINFORCE policy gradient with KL penalty provides the training backbone used by all stages -> (3) Stage I objective (decoupled attempt initialization) built on top of the REINFORCE-KL backbone -> (4) Stage II objective (joint multi-turn RL with reward shaping) built on top of Stage I checkpoint.", + "source": "Section 5, Figure 7" + }, + { + "id": "score-D4-002", + "claim": "Reward shaping bonus and Stage II joint optimization ordering: Stage I objective must be completed first (producing a model with decoupled attempt distributions) -> Stage II objective then applies joint optimization with the reward shaping bonus b = alpha * (r(y2) - r(y1)) to bias learning toward self-correction progress while preventing the degenerate non-correcting solution, followed by final checkpoint selection based on highest training reward.", + "source": "Section 5.1, Section 5.2, Section 5.3" + }, + { + "id": "score-D4-003", + "claim": "SCoRe two-stage training pipeline execution order: (1) Initialize pi_theta from pi_ref (frozen base model) -> (2) Stage I: decouple attempts via on-policy rollouts with KL penalty only on first attempt, selecting the checkpoint with decoupled distributions -> (3) Stage II: jointly optimize both attempts using shaped reward r'(y2)=r(y2)+alpha*(r(y2)-r(y1)) with KL penalties on both attempts -> (4) Select checkpoint with highest training reward -> (5) Optionally incorporate offline data augmentation in Stage II by sampling base model to generate additional first-attempt prompts. The two stages are sequential and MUST be run in order; skipping Stage I costs 2% lower Delta(t1,t2) and 3% lower Accuracy@t2.", + "source": "Section 5.1, Section 5.2, Section 5.3, Figure 7, Section 6.3" + }, + { + "id": "score-D4-004", + "claim": "MATH self-correction evaluation step sequence: (1) Train model on MATH training set + 4500 augmentation problems -> (2) For SCoRe: run Stage I (decouple attempts) then Stage II (joint optimization with reward shaping) -> (3) Evaluate on MATH500 (500 held-out problems) with greedy decoding (T=0), zero-shot CoT prompting -> (4) Report all 5 metrics: Accuracy@t1, Accuracy@t2, Delta(t1,t2), Delta^{i->c}, Delta^{c->i}.", + "source": "Section 6, Section 6.1, Table 2" + }, + { + "id": "score-D4-005", + "claim": "Code generation self-correction evaluation step sequence: (1) Train model on MBPP with binary test-case-passing reward -> (2) For SCoRe: run Stage I -> Stage II training -> (3) Evaluate zero-shot on HumanEval (Chen et al. 2021) with greedy decoding (T=0) -> (4) Report all 5 metrics: Accuracy@t1, Accuracy@t2, Delta(t1,t2), Delta^{i->c}, Delta^{c->i}.", + "source": "Section 6, Section 6.1, Table 3" + }, + { + "id": "score-D4-006", + "claim": "Inference-time compute scaling step sequence: (1) For parallel baseline: sample 32 independent first attempts (T=0.7), majority vote on answers -> (2) For sequential SCoRe: sample 16 first attempts (T=0.7), apply 1 round self-correction to each, majority vote on 16 corrected answers -> (3) Compare accuracy gains at equal total compute budget of 32 samples per problem.", + "source": "Section 6.2, Figure 1 (right)" + }, + { + "id": "score-D4-007", + "claim": "Ablation study step sequence: (1) Train each of 5 SCoRe variants (full, w/o multi-turn, w/o Stage I, w/o reward shaping, w/ STaR instead of REINFORCE Stage II) with identical hyperparameter budgets (same sample and gradient update counts) -> (2) Evaluate all variants with greedy decoding (T=0) on MATH500 -> (3) Compare Accuracy@t1, Accuracy@t2, and Delta(t1,t2) across variants.", + "source": "Section 6.3, Table 4" + }, + { + "id": "score-D4-008", + "claim": "SFT failure analysis step sequence: (1) Generate two-turn self-correction traces from Gemini 1.5 Flash on MATH -> (2) Construct D_STaR (filtered successful corrections) and D_SFT (incorrect+correct pairs), plus + variants with correct-to-correct data -> (3) Fine-tune on each dataset (STaR: 3 iterations, Pair-SFT: 1 iteration) -> (4) Evaluate edit distance ratios on training and validation sets -> (5) Evaluate correction accuracy on fixed vs. self-generated first-attempt distributions -> (6) Report 5 main self-correction metrics.", + "source": "Section 4, Table 1, Figure 4, Figure 5" + }, + { + "id": "score-D4-009", + "claim": "Behavior collapse analysis step sequence: (1) Run standard multi-turn RL training on MATH optimizing only the base MDP objective -> (2) Monitor accuracy@t1, accuracy@t2, and Delta(t1,t2) throughout training -> (3) Track frequency of answer changes between attempts and edit distance ratios -> (4) Compare evolution curves with SCoRe Stage I to demonstrate that Stage I prevents collapse.", + "source": "Section 5, Figure 6" + }, + { + "id": "score-D4-010", + "claim": "Multi-attempt scaling analysis step sequence: (1) For each model (base, Pair-SFT, SCoRe), perform up to 10 sequential self-correction attempts on MATH -> (2) At each attempt, model receives problem + all previous attempts + correction instruction -> (3) Record accuracy at each turn 1 through 10 -> (4) Compare how performance evolves past the training distribution (2 turns) to assess generalization of self-correction ability.", + "source": "Appendix A.1, Figure 8" + }, + { + "id": "score-D4-011", + "claim": "Discount factor experiment step sequence: (1) Run multi-turn RL with gamma=0, alpha=1.0 (baseline) -> (2) Run multi-turn RL with gamma=0.8, alpha=1.0 -> (3) Track self-correction performance metrics throughout training for both settings -> (4) Compare training curves to determine whether gamma>0 resolves behavior collapse that occurs with gamma=0.", + "source": "Appendix A.2, Figure 9" + }, + { + "id": "score-D4-012", + "claim": "Test-time inference procedure ordering: (1) Generate first attempt y1 via argmax decoding (T=0) -> (2) Construct self-correction instruction p1 that does NOT reveal correctness -> (3) Build second-attempt context x2 by concatenating [x1, y1, p1] -> (4) Generate second attempt y2 via argmax decoding (T=0) -> (5) Return y2 as final answer. The model must autonomously detect errors in y1 without any oracle, ground truth, or external feedback at test time.", + "source": "Section 5.3, Appendix C" + } + ] +} \ No newline at end of file diff --git a/papers/universal-neural-operators/blacklist.txt b/papers/universal-neural-operators/blacklist.txt new file mode 100644 index 0000000000000000000000000000000000000000..ffe169bd3b4355d97006a322f5c3c426525aec38 --- /dev/null +++ b/papers/universal-neural-operators/blacklist.txt @@ -0,0 +1,3 @@ +# Official repository (NeurIPS 2025 Workshop, anonymous) +https://anonymous.4open.science/r/multiphysics_neurop-F385/ +# Authors: Mikhail Masliaev, Dmitry A. Gusarov, Ilya Markov, Alexander Hvatov (ITMO University) diff --git a/papers/universal-neural-operators/config.yaml b/papers/universal-neural-operators/config.yaml new file mode 100644 index 0000000000000000000000000000000000000000..2595dbb7eaa0e8bf834af3509817e07f79c689b5 --- /dev/null +++ b/papers/universal-neural-operators/config.yaml @@ -0,0 +1,8 @@ +title: "Towards Universal Neural Operators through Multiphysics Pretraining" +pdf_url: "https://arxiv.org/pdf/2511.10829.pdf" +venue: "NeurIPS 2025" +year: "2025" +extra: + selection_index: 26 + domain: "Numerical Methods / Scientific Computing" + paradigm: "Incremental Improvement" diff --git a/papers/universal-neural-operators/images/figures/universal-neural-operators-fig-0001.jpg b/papers/universal-neural-operators/images/figures/universal-neural-operators-fig-0001.jpg new file mode 100644 index 0000000000000000000000000000000000000000..812151612024e3c517e09e26fad9f295beb03dbc --- /dev/null +++ b/papers/universal-neural-operators/images/figures/universal-neural-operators-fig-0001.jpg @@ -0,0 +1,3 @@ +version https://git-lfs.github.com/spec/v1 +oid sha256:ba802dfef230c86118853dfae3a79df1985cf1aff52d1a4dfd1cf5260782d12a +size 161814 diff --git a/papers/universal-neural-operators/images/formulas/universal-neural-operators-formula-0001.jpg b/papers/universal-neural-operators/images/formulas/universal-neural-operators-formula-0001.jpg new file mode 100644 index 0000000000000000000000000000000000000000..15b546fe3c7618556a0548f1623812abc3c2d46e --- /dev/null +++ b/papers/universal-neural-operators/images/formulas/universal-neural-operators-formula-0001.jpg @@ -0,0 +1,3 @@ +version https://git-lfs.github.com/spec/v1 +oid sha256:76a2a8857039686278ebfd807b66d210e4faf5829fcdea9dd15e8564b08cf3b0 +size 9743 diff --git a/papers/universal-neural-operators/images/formulas/universal-neural-operators-formula-0002.jpg b/papers/universal-neural-operators/images/formulas/universal-neural-operators-formula-0002.jpg new file mode 100644 index 0000000000000000000000000000000000000000..463871a787970112b11d8576ba151a06d5ea1ecf --- /dev/null +++ b/papers/universal-neural-operators/images/formulas/universal-neural-operators-formula-0002.jpg @@ -0,0 +1,3 @@ +version https://git-lfs.github.com/spec/v1 +oid sha256:bd418dd9aba62f73a8a5d7f9ca628b5d047c1be7c68f40720ebf6a13c634f969 +size 6351 diff --git a/papers/universal-neural-operators/images/formulas/universal-neural-operators-formula-0003.jpg b/papers/universal-neural-operators/images/formulas/universal-neural-operators-formula-0003.jpg new file mode 100644 index 0000000000000000000000000000000000000000..01516cb3e7eb656ed440a5f0e683f7c81b25a9a9 --- /dev/null +++ b/papers/universal-neural-operators/images/formulas/universal-neural-operators-formula-0003.jpg @@ -0,0 +1,3 @@ +version https://git-lfs.github.com/spec/v1 +oid sha256:5381b893324e3799a8f1261708a30fdcc2e0dc3c4d7e5c532438a3f9e0ebdc05 +size 10380 diff --git a/papers/universal-neural-operators/images/tables/universal-neural-operators-table-0001.jpg b/papers/universal-neural-operators/images/tables/universal-neural-operators-table-0001.jpg new file mode 100644 index 0000000000000000000000000000000000000000..26ac38ed031e170ee314b64695d904986e6388ed --- /dev/null +++ b/papers/universal-neural-operators/images/tables/universal-neural-operators-table-0001.jpg @@ -0,0 +1,3 @@ +version https://git-lfs.github.com/spec/v1 +oid sha256:6b768c6fd6f03b5ede9b7a0dd300ab219b78fdeba5fdfe04fedc530d7e9b445c +size 46277 diff --git a/papers/universal-neural-operators/images/tables/universal-neural-operators-table-0002.jpg b/papers/universal-neural-operators/images/tables/universal-neural-operators-table-0002.jpg new file mode 100644 index 0000000000000000000000000000000000000000..cd8cc48311e8254f8f4c5d97ed9c9ac5008182d7 --- /dev/null +++ b/papers/universal-neural-operators/images/tables/universal-neural-operators-table-0002.jpg @@ -0,0 +1,3 @@ +version https://git-lfs.github.com/spec/v1 +oid sha256:06c655c3a628e7862fb80bd83ff8421bfe8b2c7cd389bf86c7e94512c3bea698 +size 41521 diff --git a/papers/universal-neural-operators/paper.md b/papers/universal-neural-operators/paper.md new file mode 100644 index 0000000000000000000000000000000000000000..54faced1163b22d6ed791951ba3ca0cdd5be2c16 --- /dev/null +++ b/papers/universal-neural-operators/paper.md @@ -0,0 +1,120 @@ +# Towards Universal Neural Operators through Multiphysics Pretraining + +Mikhail Masliaev ITMO University St. Petersburg, Russia, 197101 maslyaitis@gmail.com + +Dmitry A. Gusarov ITMO University St. Petersburg, Russia, 197101 gusdmitr@itmo.ru + +Ilya Markov ITMO University St. Petersburg, Russia, 197101 iomarkov@itmo.ru + +Alexander Hvatov ITMO University St. Petersburg, Russia, 197101 alex_hvatov@itmo.ru + +# Abstract + +Although neural operators are widely used in data-driven physical simulations, their training remains computationally expensive. Recent advances address this issue via downstream learning, where a model pretrained on simpler problems is fine-tuned on more complex ones. In this research, we investigate transformer-based neural operators, which have previously been applied only to specific problems, in a more general transfer learning setting. We evaluate their performance across diverse PDE problems, including extrapolation to unseen parameters, incorporation of new variables, and transfer from multi-equation datasets. Our results demonstrate that advanced neural operator architectures can effectively transfer knowledge across PDE problems. + +# 1 Introduction + +Contemporary science commonly uses partial differential equations (PDEs) and systems of partial differential equations to model spatio-temporal processes. For instance, reaction-diffusion equations describe, how mass moves and disperses within a fluid system, and gas and liquid dynamics are commonly modeled with variants of Euler/Navier-Stokes equations. While the analytical solutions are applicable in idealized problem statement, it is challenging to construct them in realistic scenarios. Thus, numerical simulation techniques, such as finite-element or spectral methods have been developed, yet in many cases such solutions can be computationally costly. However, in many cases, such as meteorological forecasting or multi-physics simulation in the engineering, the numerical solution of differential equations tends to be a computationally costly procedure. + +With the development of scientific machine learning, greater emphasis in physics systems simulations is placed on data-driven methods. Physics-informed neural networks (PINN) [1] extend the loss with PDE-based terms, so the training has an objective of matching network’s output with governing PDE. However, PINNs require explicit formulations and guarantee accuracy only at training mesh nodes. Operator learning, realized through Deep Operator Networks (DeepONet) [2] and kernel-based neural operators (NO) [3], offers a faster alternative to classical solvers, approximating mappings between functional spaces rather than dynamics at discrete nodes, providing discretization invariance and efficient inference. + +A recent direction in NO research is the design of foundation models. Originating in NLP [4] or vision-language models [5], such models contain billions of parameters (e.g., GPT-4 exceeds one trillion [6]) and are pretrained on large-scale datasets. They can then be fine-tuned for downstream tasks at reduced cost. In this work, we aim to develop a standardized approach to applying large NO-based models for generalized dynamics and transfer learning across PDE problems. Thus, with the neural operator the model is pre-trained on a simplified problem statement, which is later transferred to another (typically, more complex) problem in the fine-tuning phase. + +Our contributions are as follows: the proposed method enables neural operator learning on diverse multi-physics datasets by introducing an adapter-based approach for simultaneous training on PDEbased problems with different sets of input functions. The results demonstrate that the transfer learning approach can significantly enhance model quality and reduce fine-tuning costs. + +# 2 Related work + +Pretraining of neural operators has thus far mainly been case-specific, with limited generalization. The concept of transfer learning using DeepONet operator models are employed in conditional shift scenarios in study [7]. Also, issues of solving transfer learning problems on multi-scale data with convolutional neural networks were discussed in [8]. + +Several foundational models for PDE systems have been proposed beyond classical NO approaches: a foundational model tranining framework for equations of fixed types (in the study, steady-state equations were considered) is proposed in the research [9]. Boundary-Embedded Neural Operators (BENO) [10] solve elliptic PDEs using graph neural networks, where boundary geometry is encoded via a transformer block into latent vectors guiding message passing. Other works leverage transformerbased architectures to encode PDE structure [11]. + +Transformer-based approaches proved to be capable of modeling complex interactions between selected token functions. POSEIDON, a hierarchical vision transformer with shifted windows, applicable to transfer knowledge across Euler/Navier–Stokes cases [12]. Codomain Attention Neural Operator (CoDA-NO) [13], designed for multiphysics PDE transfer learning, employs codomain attention with function space dot product. + +Transformers have also been explored within neural operator learning [14]. In contrast, we deliberately avoid physics-informed approaches and focus on assessing the capacity of neural operators to learn generalized dynamics purely from data samples. + +# 3 Method + +An operator learning framework has been developed for data-driven modeling of dynamical systems, governed by parametric partial differential equations $L _ { p } \mathbf { u } ( t , \mathbf { x } ) = f ( t , \mathbf { x } )$ on the bounded domain $\mathcal { D }$ in a mesh-agnostic (albeit with some limitations, as examined in [15]) approach. + +Neural operator learning: In this research, we have focused on the kernel-integral neural operators, mainly Fourier Neural Operators. Neural Operators are designed to learn mappings between the input space $\mathcal { U }$ , representing sets of input functions $\mathbf { a } = \{ a _ { 1 } , ~ \ldots ~ a _ { n _ { - } i n } \}$ , $a _ { i } : \mathcal { D } \longrightarrow \mathbb { R }$ and the output functions $\bar { \mathbf { u } } = \{ u _ { 1 } , ~ \overline { { \dots ~ } } u _ { n _ { - } o u t } \}$ , $u _ { i } : \mathcal { D } \longrightarrow \mathbb { R }$ . While the outputs are typically fixed to the dependent variables of the PDE (or system of PDEs), the choice of input functions is guided by the equation structure and includes meshes, initial conditions, forcing terms, or equation coefficients. + +To respect the non-localities of the model, NO design adds integral kernel operators $( \mathcal { K } ( v ) ) ( x )$ to the arguments ${ \bf \cal A } _ { t }$ , and $b _ { t }$ for weights and biases) of activation functions $\sigma$ . Here the $\kappa _ { t } ~ \in$ $C ( D _ { t + 1 } \times D _ { t } | \theta _ { k , t } )$ ; $\theta _ { k , t } \in \mathbb { R } ^ { n _ { v _ { t + 1 } } \times n _ { v _ { t } } }$ is the kernel function, parameterized (with $\theta _ { k , t }$ ) by the method of choice (FNO, GNO, etc.), and $v _ { t } ( y )$ - hidden representation of input functions, obtained from the $t$ -th layer. $D _ { t }$ is the hidden dimensionality, which is linked to the number of modes in FNO. Thus, the parameters of NO main part include weights, biases and kernel parameters $\theta _ { \mathcal { F } } = \{ A _ { t } , b _ { t } , \theta _ { k , t } : t = 1 , ~ . . . ~ , n _ { \mathrm { l a y e r s } } \}$ . + +$$ +\mathcal F _ { t } ( x ) = \sigma \left( A _ { t } v _ { t } ( x ) + \int _ { D _ { i } } \kappa _ { t } ( x , y ) v _ { t } ( y ) d y + b _ { t } ( x ) \right) , \forall x \in D _ { t } , t = 1 , \ \dots , n _ { \mathrm { l a y e r s } } +$$ + +The architecture of layers sequence in NO goes as follows: the inputs are transformed to their higher-dimensional hidden representation by lifting layers (typically, a feed-forward neural network) ${ \bar { \mathcal { L } } } : { \mathcal { L } } ( \mathbf { a } ) = \sigma \left( A _ { \mathcal { L } } \mathbf { a } + b _ { \mathcal { L } } \right)$ , where parameters of the lifting include weights and biases $\theta _ { \cal C } \ =$ $\{ A _ { \mathcal { L } } , b _ { \mathcal { L } } \}$ . The hidden representations are sequentially mapped with the integral-operator blocks (1). In contrast, the output of the last block is projected to the space of outputs by the point-wise function $\mathcal { P }$ with parameters $\theta _ { \mathcal { L } } = \{ A _ { \mathcal { P } } , b _ { \mathcal { P } } \}$ . The operator approximation takes form of model $\mathcal { G } _ { \theta }$ : $\tilde { \mathbf { u } } ( \mathbf { x } ) = \mathcal { G } _ { \theta } ( \mathbf { a } ) = \mathcal { P } \circ \mathcal { F } \circ \mathcal { L } ( \mathbf { a } ) = \mathcal { P } \circ \mathcal { F } _ { n _ { - } l a y e r s } \circ \ \hdots \circ \ \mathcal { F } _ { 1 } \circ \mathcal { L } ( \mathbf { a } )$ . + +Improving the generalization ability of neural operators: In this research, we employed two types of NO modifications: state-space models and transformer-based models. In the first approach, inserting a Mamba-SSM module [16] $\mathcal { M } _ { \phi }$ after the lifting map $\mathcal { L }$ allows the model to encode longrange temporal and spatial dependencies directly in the hidden representation. For lifted features $v _ { 0 } ( \bar { x } ) = \bar { \mathcal { L } } ( \mathbf { a } ) ( x )$ , the Mamba module computes + +$$ +\widetilde { v } _ { 0 } ( x , t ) = ( \mathcal { M } _ { \phi } v _ { 0 } ) ( x , t ) = \sum _ { \tau \leq t } K _ { \tau } v _ { 0 } ( x , t - \tau ) , +$$ + +with learnable convolution kernels $K _ { \tau }$ defining the causal recurrence. This step acts as a latent preconditioner: embeddings are aligned with dominant dynamical motifs (transport, diffusion, oscillation) common across PDEs, so that when passed into the Fourier integral layers, the effective operator acts on inputs of reduced variability and lower spectral rank. Consequently, the composition $\mathcal { F } _ { t } \circ \mathcal { M } _ { \phi }$ yields more stable training and improves efficiency in transferring pre-trained representations to new PDEs during fine-tuning. + +The next approach, examined in the study, involved the attention method & transformer-based blocks. The introduction of Perceiver [17] enabled the encoding of information with a smaller number of latent feature arrays, internal to operator blocks, thereby operating with more abstract feature arrays and maintaining a limited number of parameters. As the operators we employ, we use blocks based on the Perceiver IO [18], where the mapping is performed with a symmetrical cross-attention mechanism for outputs, which mirrors the cross-attention block for constructing representations of latent arrays from the input process. While the previously used self-attention blocks can discover dependencies between hidden features, obtained from lifting or previous layers, Perceivers are able to constructs additional latent process representation. + +The latent variables and input state are combined first with the cross-attention block, where keys and values are obtained from FNO-based mapping from the inputs $K _ { 1 } = F N O _ { K _ { 1 } } ( X )$ , $V _ { 1 } =$ $F N O _ { V _ { 1 } } ( X )$ , and latent variables are taken as queries $Q _ { 1 } = L$ . The cross-attention block is followed by self-attention between latent representation. The output of the block is constructed with the crossattention, matching the queries from the inputs with the keys and values, taken from the transformed latent representations. + +Commonly used self-attention mechanism involves similarity function $\mathrm { s i m } ( q _ { m } , k _ { j } )$ between given sets of finite-dimensional vectors of queries $\{ \mathbf { q } _ { i } \} , \ i = 0 , \ \dots , N _ { q }$ and keys $\{ { \bf k } _ { i } \} , i = 0 , \ldots , N _ { k } .$ , which is used to obtain the output from the $m$ -th query with the set of value vectors $\left\{ \mathbf { v } _ { i } \right\} , ~ i = 0 .$ $\dots , N _ { v }$ with the relation. The similarity is commonly obtained using Softmax function of the dot products between corresponding queries and keys. Codomain attention mechanisms, introduced in [13], are advantageous to the conventional transformers in the neural-operator based problems: the dot product detecting similarity not between samples, but between features, mapped with neural operators. + +Pre-training and fine-tuning: One of the benefits of using lifting-operator-projection architecture is the simplicity of decoupling adapters from the main model, streamlining model storage and extension for novel fine-tuning problems. The lift and proj blocks are considered as the adapters, representing the mappings, associated with the problem-specific part of dynamics: they are introduced to contain different cardinality input sets, projecting into the fixed number of hidden features and contain small number of parameters to represent limited part of the total model variance, as it is common in the adapter design for large language models [19]. + +In the pre-training phase the entire parameters set $\left( \theta _ { \mathcal { P } _ { 1 } } , \mathrm { ~ } . . . \mathrm { ~ } , \theta _ { \mathcal { P } _ { N } } , \theta _ { \mathcal { F } } , \theta _ { \mathcal { L } _ { 1 } } , \mathrm { ~ } . . . \mathrm { ~ } , \theta _ { \mathcal { L } _ { N } } \right)$ is subject to optimization. By problems 1 to $N$ , we present separate physical processes, demanding different (but, probably, overlapping) sets of input functions. Previously, such problems were solved with liftings with extensive inputs. For example, in training on the set of steady-state problems, coefficients before terms with all spatial derivatives, which occur in the training set, are used as inputs. In the fine-tuning stage we fix the parameters $\theta _ { \mathcal { F } }$ both to highlight the generalizing properties of the operator and to reduce training costs: only the new adapter parameters $( \theta _ { \mathcal { P } _ { f t } } , \theta _ { \mathcal { L } _ { f t } } )$ are trained. + +![](images/figures/universal-neural-operators-fig-0001.jpg) +Figure 1: Scheme of the neural operator pre-training and fine-tuning stages. In this scheme, FNO blocks denote arbitrary kernel integral operators, including the transformer-based architectures. The separate physics 1 to $N$ may vary from the different manifestations of the same system to multiple different physics (but with the same problem dimensionality) in the dataset collection. + +# 4 Experiments + +We aim on validating our approach on three distinct types of problems without modifications in the modeling approach. The first scenario involves cases, when the pre-training and fine-tuning processes are governed by the equations with same input functions, but with different parameters. In addition to parametric variations within a single physical law, we further extend our investigation to cross-domain transfer learning between classes of differential equations representing distinct physical phenomena. We have selected datasets to represent diverse physical phenomena, including advective transport processes, nonlinear wave dynamics governed by Burgers equation, reaction-diffusion systems exhibiting pattern formation, and other fundamental physical processes described by partial differential equations. + +$$ +\mathrm { N M A E } ( \theta ) = \frac { 1 } { | \mathcal { D } _ { \mathrm { R D } } ^ { \mathrm { t e s t } } | } \sum _ { ( \mathbf { a } , u ) \in \mathcal { D } _ { \mathrm { R D } } ^ { \mathrm { t e s t } } } \frac { \left. \mathcal { G } _ { \theta } ( \mathbf { a } ) - u \right. _ { 1 , G } } { \operatorname* { m a x } _ { G } u - \operatorname* { m i n } _ { G } u + \varepsilon } +$$ + +In this comparison, we employ developed post-lifting (PL) MambaFNO models, post-lifting (PL) LocalAttnFNO, and Perceiver IO-based NO, as well as Swin-v2 transformers and CodaNO models. As the baseline, we employed the default FNO. We used the range-normalized mean absolute error (NMAE) (3) as a quality metric. The code and experiments are available in the repository https://anonymous.4open.science/r/multiphysics_neurop-F385/. + +Out-of-sample parameter values scenario First, we conducted several experiments on cases where the pretraining equations and fine-tuning ones differed only in the coefficient values. The experiments were conducted using Burgers’ equation , the Gray-Scott model of the reaction-diffusion process , and the Navier-Stokes equations for an incompressible flow. The results of the comparison are presented in Tab. 1. + +Table 1: Average metric values for out-of-sample parameter values across all conducted experiments. Methods were compared with training from scratch on the examined datasets scenario and using the pre-trained model to fine-tune on the new dynamics. + +
ModelMSENMAE (%)Avg. epoch (s)Param.
Mamba FNO (pretr.)1.009 × 10−70.012021.91≈ 107
Mamba FNO (scratch)1.193 × 10−70.021340.14≈ 107
Perc. (pretr.)1.425 × 10−70.01693.21≈ 108
Perc. (scratch)1.981 × 10−70.0219204.73≈ 108
FNO (scratch)1.774 × 10-70.02047.44≈ 106
Swin-v2 (p.+s.)4.391 × 10−80.0092101.3≈ 109
CoDA-NO (pretr.)2.881 × 10−70.034362.91≈ 108
CoDA-NO (scratch)4.912 × 10−70.071263.29≈ 108
+ +Input function set extension scenario & General multi-physics learning To assess the applicability of the adapter-based approach, several experiments were conducted on scenarios where the equations were extended with additional terms. Here, for fine-tuning, we added convection to the heat equation and extended reaction-diffusion equations with advection. + +In the final stage, we evaluated the capabilities of the developed methods to transfer knowledge from the dynamics of advection and Burgers’ equation to reaction–diffusion, based on the PDEBench dataset [20]. The combined results of the experiments are presented in Tab. 2. The significant speedup achieved with the pre-trained models can be attributed to the optimization of just a subset of parameters, whereas "from scratch" involved a full parameter search. + +Table 2: Results of experiments with Heat & Reaction–Diffusion equation extension, and multi physics pre-training with fine-tuning on different dynamics. + +
ModelMSENMAE (%)Avg. epoch (s)
Mamba FNO (pretr.)3.91 × 10−60.0041131.2
Mamba FNO (scratch)4.291 × 10−60.0054261.1
Perc. (pretr.)4.107 × 10−60.005120.4
Perc. (scratch)6.315 × 10−60.0074804.0
FNO (scratch)7.286 × 10−60.012141.3
Swin-v2 (p.+s.)6.276 × 10−60.009301.1
CoDA-NO (pretr.)1.043 × 10−50.013185.1
CoDA-NO (scratch)1.239 × 10−50.018181.9
+ +# 5 Conclusion + +In this research, we have examined the performance of NO architectures, based on transformers and structured state space models, on a set of transfer learning problems involving changes to the parameters of equations, the inclusion of additional physics, and, finally, pre-training on multiphysics datasets. In contrast to the default NO approach, developed models have greater generalizing power. Use of parameter sets and problem-specific adapters enables the more effortless transfer of knowledge, reducing the cost of obtaining a decent model, even for novel PDE-based problems. + +This research was conducted primarily as the first stage of work towards a foundational model, pre-trained on vast and heterogeneous multiphysics dataset collections. The follow-up work shall be directed towards improving model performance and training, implementing data augmentation tools (both generic and PDE-specific, e.g., based on Lie symmetries), and further examining neural operator generalizations. + +# References + +[1] Raissi, M., P. Perdikaris, G. E. Karniadakis. Physics-informed neural networks: A deep learning framework for solving forward and inverse problems involving nonlinear partial differential equations. Journal of Computational physics, 378:686–707, 2019. +[2] Lu, L., P. Jin, G. Pang, et al. Learning nonlinear operators via deeponet based on the universal approximation theorem of operators. Nature machine intelligence, 3(3):218–229, 2021. +[3] Kovachki, N., Z. Li, B. Liu, et al. Neural operator: Learning maps between function spaces with applications to pdes. Journal of Machine Learning Research, 24(89):1–97, 2023. +[4] Bommasani, R. On the opportunities and risks of foundation models. arXiv preprint arXiv:2108.07258, 2021. +[5] Awais, M., M. Naseer, S. Khan, et al. Foundation models defining a new era in vision: a survey and outlook. IEEE Transactions on Pattern Analysis and Machine Intelligence, 2025. +[6] Achiam, J., S. Adler, S. Agarwal, et al. Gpt-4 technical report. arXiv preprint arXiv:2303.08774, 2023. +[7] Goswami, S., K. Kontolati, M. D. Shields, et al. Deep transfer operator learning for partial differential equations under conditional shift. Nature Machine Intelligence, 4(12):1155–1164, 2022. +[8] Subel, A., Y. Guan, A. Chattopadhyay, et al. Explaining the physics of transfer learning in data-driven turbulence modeling. PNAS nexus, 2(3):pgad015, 2023. [9] Subramanian, S., P. Harrington, K. Keutzer, et al. Towards foundation models for scientific machine learning: Characterizing scaling and transfer behavior. Advances in Neural Information Processing Systems, 36:71242–71262, 2023. +[10] Wang, H., J. Li, A. Dwivedi, et al. Beno: Boundary-embedded neural operators for elliptic pdes. arXiv preprint arXiv:2401.09323, 2024. +[11] Zhang, R., Q. Meng, Z.-M. Ma. Deciphering and integrating invariants for neural operator learning with various physical mechanisms. National Science Review, 11(4):nwad336, 2024. +[12] Herde, M., B. Raonic, T. Rohner, et al. Poseidon: Efficient foundation models for pdes. Advances in Neural Information Processing Systems, 37:72525–72624, 2024. +[13] Rahman, M. A., R. J. George, M. Elleithy, et al. Pretraining codomain attention neural operators for solving multiphysics pdes. Advances in Neural Information Processing Systems, 37:104035–104064, 2024. +[14] Boya, S. K., D. Subramani. A physics-informed transformer neural operator for learning generalized solutions of initial boundary value problems. arXiv preprint arXiv:2412.09009, 2024. +[15] Fanaskov, V. S., I. V. Oseledets. Spectral neural operators. Doklady Mathematics, 108(Suppl 2):S226–S232, 2023. +[16] Gu, A., T. Dao. Mamba: Linear-time sequence modeling with selective state spaces. arXiv preprint arXiv:2312.00752, 2023. +[17] Jaegle, A., F. Gimeno, A. Brock, et al. Perceiver: General perception with iterative attention. In International conference on machine learning, pages 4651–4664. PMLR, 2021. +[18] Jaegle, A., S. Borgeaud, J.-B. Alayrac, et al. Perceiver io: A general architecture for structured inputs & outputs. arXiv preprint arXiv:2107.14795, 2021. +[19] Hu, E. J., Y. Shen, P. Wallis, et al. Lora: Low-rank adaptation of large language models. ICLR, 1(2):3, 2022. +[20] Takamoto, M., T. Praditia, R. Leiteritz, et al. Pdebench: An extensive benchmark for scientific machine learning. Advances in Neural Information Processing Systems, 35:1596–1611, 2022. \ No newline at end of file diff --git a/papers/universal-neural-operators/paper.pdf b/papers/universal-neural-operators/paper.pdf new file mode 100644 index 0000000000000000000000000000000000000000..ea604e22bb1bee37465dc829e0a81e35a68b5c20 --- /dev/null +++ b/papers/universal-neural-operators/paper.pdf @@ -0,0 +1,3 @@ +version https://git-lfs.github.com/spec/v1 +oid sha256:59f60f53dba84a674928bb7da9fc2b6ff4ff3db445153a24b4d5da9193f8f02d +size 2950722 diff --git a/papers/universal-neural-operators/sau.json b/papers/universal-neural-operators/sau.json new file mode 100644 index 0000000000000000000000000000000000000000..82e39dd4421fef81f858cccede00712f75b10cae --- /dev/null +++ b/papers/universal-neural-operators/sau.json @@ -0,0 +1,182 @@ +{ + "paper_id": "universal-neural-operators", + "paper_title": "Towards Universal Neural Operators through Multiphysics Pretraining", + "D1": [ + { + "id": "universal-neural-operators-D1-001", + "claim": "[model_parameter_counts] Approximate parameter counts for each model architecture: MambaFNO ≈10^7, PerceiverIO ≈10^8, FNO ≈10^6, Swin-v2 ≈10^9, CoDA-NO ≈10^8 (from Table 1, Param. column)", + "source": "Section 4, Table 1 (Param. column)" + }, + { + "id": "universal-neural-operators-D1-002", + "claim": "[experiment_scenarios] The experimental evaluation covers three distinct transfer learning scenarios: (1) out-of-sample parameter values — same PDEs with different coefficient ranges; (2) input function set extension — base equation pretraining with added physics terms during fine-tuning; (3) multi-physics cross-domain transfer — joint pretraining on diverse PDEs followed by fine-tuning on a different PDE class", + "source": "Section 4, paragraph 1" + }, + { + "id": "universal-neural-operators-D1-003", + "claim": "[model_architectures_tested] Model architectures evaluated: post-lifting (PL) MambaFNO, post-lifting (PL) LocalAttnFNO, Perceiver IO-based neural operator, Swin-v2 transformer, CoDA-NO (codomain attention neural operator); baseline: default FNO (Fourier Neural Operator)", + "source": "Section 4, paragraph 2" + }, + { + "id": "universal-neural-operators-D1-004", + "claim": "[datasets_used] PDE datasets used across experiments: Burgers' equation (nonlinear wave dynamics), Gray-Scott model (reaction-diffusion pattern formation), Navier-Stokes equations (incompressible flow), advection equation (PDEBench), heat equation (with convection extension), reaction-diffusion equation (PDEBench)", + "source": "Section 4, paragraphs 1, 3-4" + }, + { + "id": "universal-neural-operators-D1-005", + "claim": "[evaluation_metrics] Primary evaluation metric: range-normalized mean absolute error (NMAE, Eq 3); secondary metrics: mean squared error (MSE), average epoch time (seconds), and model parameter count", + "source": "Section 4, paragraph 2" + }, + { + "id": "universal-neural-operators-D1-006", + "claim": "[adapter_transfer_approach] Pretraining-fine-tuning via adapter-based architecture: lift and projection blocks serve as problem-specific adapters; during fine-tuning, the shared operator core (theta_F) is frozen and only new adapter parameters (theta_P_ft, theta_L_ft) are trained, reducing computational cost", + "source": "Section 3, Pre-training and fine-tuning paragraphs" + } + ], + "D2": [ + { + "id": "universal-neural-operators-D2-001", + "claim": "PDE Problem Formulation: L_p u(t, x) = f(t, x) on bounded domain D", + "source": "Section 3, paragraph 1" + }, + { + "id": "universal-neural-operators-D2-002", + "claim": "Neural Operator Integral Kernel Layer (Eq 1): F_t(x) = sigma( A_t v_t(x) + integral_D kappa_t(x, y) v_t(y) dy + b_t(x) )", + "source": "Section 3, paragraph 2, Equation (1)" + }, + { + "id": "universal-neural-operators-D2-003", + "claim": "Lifting Layer Mapping: L(a) = sigma( A_L * a + b_L )", + "source": "Section 3, paragraph 3" + }, + { + "id": "universal-neural-operators-D2-004", + "claim": "Projection Layer Mapping: tilde{u}(x) = P( v_{n_layers}(x) ) = sigma( A_P * v_{n_layers}(x) + b_P )", + "source": "Section 3, paragraph 3" + }, + { + "id": "universal-neural-operators-D2-005", + "claim": "Neural Operator Full Forward Composition: G_theta(a) = P circ F_{n_layers} circ ... circ F_1 circ L(a)", + "source": "Section 3, paragraph 3" + }, + { + "id": "universal-neural-operators-D2-006", + "claim": "Operator Core Parameters: theta_F = { A_t, b_t, theta_{k,t} : t = 1, ..., n_layers }", + "source": "Section 3, paragraph 2" + }, + { + "id": "universal-neural-operators-D2-007", + "claim": "Mamba SSM Latent Preconditioner (Eq 2): tilde{v}_0(x, t) = sum_{tau <= t} K_tau * v_0(x, t - tau)", + "source": "Section 3, paragraph 4, Equation (2)" + }, + { + "id": "universal-neural-operators-D2-008", + "claim": "Perceiver IO Cross-Attention Input Encoding: K_1 = FNO_{K_1}(X), V_1 = FNO_{V_1}(X), Q_1 = L", + "source": "Section 3, paragraph 6" + }, + { + "id": "universal-neural-operators-D2-009", + "claim": "Perceiver IO Self-Attention on Latent Representations: L' = SelfAttention(Q=L, K=L, V=L)", + "source": "Section 3, paragraph 7" + }, + { + "id": "universal-neural-operators-D2-010", + "claim": "Perceiver IO Output Cross-Attention Decoding: Output = CrossAttention(Q = FNO_Q(X'), K = FNO_K(L'), V = FNO_V(L'))", + "source": "Section 3, paragraph 7" + }, + { + "id": "universal-neural-operators-D2-011", + "claim": "Codomain Attention Mechanism: Output_m = sum_j softmax_j( sim(q_m, k_j) ) * v_j, sim(q_m, k_j) = dot_product between features, not samples", + "source": "Section 3, paragraph 8" + }, + { + "id": "universal-neural-operators-D2-012", + "claim": "NMAE Evaluation Metric (Eq 3): NMAE(theta) = (1 / |D_test|) * sum_{(a,u) in D_test} ||G_theta(a) - u||_{1,G} / (max_G u - min_G u + epsilon)", + "source": "Section 4, paragraph 2, Equation (3)" + }, + { + "id": "universal-neural-operators-D2-013", + "claim": "Pretraining: Joint Optimization of All Parameters: theta* = argmin_theta sum_{i=1..N} Loss_i( G_{theta_{P_i}, theta_F, theta_{L_i}}(a_i), u_i ), theta = {theta_{P_1}, ..., theta_{P_N}, theta_F, theta_{L_1}, ..., theta_{L_N}}", + "source": "Section 3, paragraph 10" + }, + { + "id": "universal-neural-operators-D2-014", + "claim": "Fine-Tuning: Adapter-Only Training with Frozen Core: theta_{ft}* = argmin_{theta_{P_ft}, theta_{L_ft}} Loss_ft( G_{theta_{P_ft}, theta_F^{frozen}, theta_{L_ft}}(a_ft), u_ft )", + "source": "Section 3, paragraph 11" + }, + { + "id": "universal-neural-operators-D2-015", + "claim": "Adapter Parameter Design Principle: N_adapter << N_core, adapter_variance << total_model_variance", + "source": "Section 3, paragraph 9" + }, + { + "id": "universal-neural-operators-D2-016", + "claim": "MambaFNO Architecture Variant: MambaFNO(a) = P circ F_{n_layers} circ ... circ F_1 circ M_phi circ L(a)", + "source": "Section 3, paragraph 4" + }, + { + "id": "universal-neural-operators-D2-017", + "claim": "PerceiverFNO Architecture Variant: PerceiverFNO(a) = P circ OutputCrossAttn circ SelfAttn circ InputCrossAttn circ L(a)", + "source": "Section 3, paragraphs 5-7" + } + ], + "D3": [ + { + "id": "universal-neural-operators-D3-001", + "claim": "Evaluate the transfer learning capability of neural operators when pretraining and fine-tuning share the same PDE equations but differ in coefficient parameter values, comparing pretrained models against training from scratch on Burgers' equation, Gray-Scott reaction-diffusion, and Navier-Stokes incompressible flow. Model baselines: MambaFNO, PerceiverIO, and CoDA-NO each evaluated in both pretrained (fine-tuned with frozen core) and scratch (trained from scratch on fine-tuning data only) modes; default FNO (scratch) as the kernel-integral NO baseline; Swin-v2 (pretrained+scratch, p.+s.) as the transformer baseline. Evaluation metrics: range-normalized mean absolute error (NMAE, %), mean squared error (MSE), and average epoch time (seconds). Results reported in Table 1.", + "source": "Section 4, Out-of-sample parameter values scenario, Table 1" + }, + { + "id": "universal-neural-operators-D3-002", + "claim": "Evaluate the adapter-based transfer approach when the fine-tuning PDE introduces additional input functions not present during pretraining — specifically adding convection to the heat equation and advection to reaction-diffusion equations — testing whether new adapters can absorb the extra input channels while reusing the frozen pretrained operator core. Model baselines: MambaFNO, PerceiverIO, and CoDA-NO each evaluated in pretrained (frozen core, new adapters only) vs. scratch (full training from scratch on the extended equation) modes; default FNO (scratch) as baseline; Swin-v2 (p.+s.) as transformer baseline. Evaluation metrics: NMAE (%), MSE, and average epoch time (seconds). Results reported in Table 2 jointly with the multi-physics scenario.", + "source": "Section 4, Input function set extension scenario, Table 2" + }, + { + "id": "universal-neural-operators-D3-003", + "claim": "Evaluate whether a neural operator jointly pretrained on multiple distinct physics systems (advection equation and Burgers' equation from PDEBench) can transfer learned representations to an entirely different PDE class (reaction-diffusion from PDEBench) through adapter-based fine-tuning with frozen core parameters. Model baselines: MambaFNO, PerceiverIO, and CoDA-NO each evaluated in pretrained (jointly pretrained on advection + Burgers, fine-tuned on reaction-diffusion with frozen core) vs. scratch (trained from scratch on reaction-diffusion data only) modes; default FNO (scratch) as baseline; Swin-v2 (p.+s.) as transformer baseline. Evaluation metrics: NMAE (%), MSE, and average epoch time (seconds). Results reported in Table 2 jointly with the input extension scenario.", + "source": "Section 4, General multi-physics learning, Table 2" + }, + { + "id": "universal-neural-operators-D3-004", + "claim": "Evaluate the large-transformer baseline comparison protocol: compare Swin-v2 (pretrained+scratch, p.+s., ~10^9 parameters, hierarchical vision transformer with shifted windows) against adapter-based neural operators (MambaFNO ~10^7, PerceiverIO ~10^8, CoDA-NO ~10^8) in transfer learning settings to determine whether scaling model size provides competitive or superior transfer capability compared to adapter-based architecture design with frozen-core fine-tuning. Dataset: all three transfer scenarios — out-of-sample (Burgers, Gray-Scott, Navier-Stokes), input extension (heat+convection, reaction-diffusion+advection), multi-physics (advection+Burgers pretraining to reaction-diffusion fine-tuning). Baselines: FNO (scratch, ~10^6) as the kernel-integral NO reference. Metrics: NMAE (%), MSE, parameter count. Results reported in Tables 1-2.", + "source": "Section 4, Tables 1-2" + }, + { + "id": "universal-neural-operators-D3-005", + "claim": "Evaluate computational efficiency of adapter-based transfer learning: compare average epoch time (seconds) across all three transfer scenarios for MambaFNO, PerceiverIO, and CoDA-NO in pretrained (fine-tuned, adapter-only parameter training) vs. scratch (full parameter training from scratch) modes. Measure speedup as the ratio of scratch epoch time to pretrained epoch time per model per scenario to quantify the training cost reduction from frozen-core adapter fine-tuning. Dataset: all three transfer scenarios (out-of-sample, input extension, multi-physics). Baselines: FNO (scratch) epoch time for absolute reference. Metrics: average epoch time (s), speedup ratio (scratch/pretrained). Results reported in Tables 1-2, Avg. epoch (s) column.", + "source": "Section 4, Tables 1-2" + }, + { + "id": "universal-neural-operators-D3-006", + "claim": "Evaluate whether the relative benefit of pretraining is architecture-dependent: compare the NMAE reduction ratio (scratch NMAE divided by pretrained NMAE) across MambaFNO (SSM-based), PerceiverIO (attention-based), and CoDA-NO (codomain-attention-based) within each of the three transfer scenarios. Determine whether certain architecture families benefit disproportionately from adapter-based pretraining and frozen-core fine-tuning, and whether architecture choice interacts with transfer scenario type. Dataset: all three transfer scenarios. Baselines: each architecture's own scratch training mode as self-baseline; FNO (scratch) as absolute reference. Metrics: NMAE reduction ratio, MSE reduction ratio. Results computed from Tables 1-2.", + "source": "Section 4, Tables 1-2" + } + ], + "D4": [ + { + "id": "universal-neural-operators-D4-001", + "claim": "Experiment phases: 1. Generate data for each equation with pretraining coefficient ranges (output: pretraining dataset, feeds step 2) -> 2. Pretrain: jointly optimize all adapters + shared core on all pretraining data (output: pretrained model, feeds step 4) -> 3. Generate data for each equation with fine-tuning (different) coefficient ranges (output: fine-tuning dataset, feeds steps 4-6) -> 4. Fine-tune: load pretrained core, freeze it, create new adapters, train adapters only -> 5. Scratch baseline: train same architecture from scratch on fine-tuning data -> 6. Evaluate all models on test data using NMAE, MSE, and record epoch times -> 7. Compare pretrained vs scratch for each model; compare against FNO and Swin-v2", + "source": "Section 4, Out-of-sample parameter values scenario, Table 1" + }, + { + "id": "universal-neural-operators-D4-002", + "claim": "Experiment phases: 1. Generate data for base equation with standard input function set (output: base equation dataset, feeds step 2) -> 2. Pretrain on base equation data with standard adapters (output: pretrained model, feeds step 5) -> 3. Generate data for extended equation with additional input functions (output: extended equation dataset, feeds steps 5-6) -> 4. Create new adapter pair: lifting with more input channels, projection unchanged -> 5. Fine-tune: load pretrained core frozen, train new adapters on extended equation data -> 6. Scratch baselines: train full architectures from scratch on extended equation -> 7. Evaluate and compare", + "source": "Section 4, Input function set extension scenario, Table 2" + }, + { + "id": "universal-neural-operators-D4-003", + "claim": "Experiment phases: 1. Generate/load advection equation and Burgers equation data (output: pretraining dataset, feeds step 2) -> 2. Pretrain: jointly optimize (L_adv, P_adv), (L_burg, P_burg), and theta_F on both equations (output: jointly pretrained model, feeds step 5) -> 3. Load PDEBench reaction-diffusion data (output: fine-tuning dataset, feeds steps 5-6) -> 4. Create new adapter pair for reaction-diffusion -> 5. Fine-tune: freeze theta_F, train only (L_rd, P_rd) on reaction-diffusion data -> 6. Scratch baselines: train from scratch on reaction-diffusion data -> 7. Evaluate and compare all models", + "source": "Section 4, General multi-physics learning, Table 2" + }, + { + "id": "universal-neural-operators-D4-004", + "claim": "Experiment phases: 1. For each experiment scenario (out-of-sample, extension, multi-physics): -> 2. Run pretraining for MambaFNO, PerceiverIO, CoDA-NO on pretraining data -> 3. Run fine-tuning for all three pretrained models on fine-tuning data (frozen core only) -> 4. Run scratch training for MambaFNO, PerceiverIO, FNO, CoDA-NO on fine-tuning data -> 5. Run Swin-v2 in its (p.+s.) mode on fine-tuning data -> 6. Evaluate all models on test data, recording NMAE, MSE, and epoch times -> 7. Report results per Table 1 (out-of-sample) and Table 2 (extension + multi-physics)", + "source": "Section 4, Overall experimental protocol, Tables 1-2" + }, + { + "id": "universal-neural-operators-D4-005", + "claim": "Experiment phases: Phase 1 (Pretraining): Jointly train all N adapters + shared core on N physics problems (output: pretrained model with shared core, feeds Phase 2) -> Phase 2 (Fine-tuning): Freeze core, train only new adapters on target problem (input: Phase 1 frozen core + new adapters, output: fine-tuned model) -> The Universal Neural Operators paper does not specify whether additional fine-tuning can be stacked (e.g., multi-stage adaptation across further PDEs after initial transfer)", + "source": "Section 3, Pre-training and fine-tuning paragraphs" + } + ] +} \ No newline at end of file diff --git a/papers/voting-leaderboards/blacklist.txt b/papers/voting-leaderboards/blacklist.txt new file mode 100644 index 0000000000000000000000000000000000000000..b48c15b3dd0a0b43b54e707f70c91a7e5b66e9f5 --- /dev/null +++ b/papers/voting-leaderboards/blacklist.txt @@ -0,0 +1,3 @@ +# No public official repository found (ICML 2025 Oral/Spotlight) +# Authors: Yangsibo Huang, Milad Nasr, et al. +# Related work with code: https://github.com/sail-sg/Rigging-ChatbotArena diff --git a/papers/voting-leaderboards/config.yaml b/papers/voting-leaderboards/config.yaml new file mode 100644 index 0000000000000000000000000000000000000000..31fe0596c36894a5e5ec64f9fde0a83c09459be9 --- /dev/null +++ b/papers/voting-leaderboards/config.yaml @@ -0,0 +1,8 @@ +title: "Exploring and Mitigating Adversarial Manipulation of Voting-Based Leaderboards" +pdf_url: "https://arxiv.org/pdf/2501.07493.pdf" +venue: "ICML 2025 Oral" +year: "2025" +extra: + selection_index: 5 + domain: "NLP / LLM" + paradigm: "Empirical Comparison" diff --git a/papers/voting-leaderboards/images/figures/voting-leaderboards-fig-0001.jpg 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0000000000000000000000000000000000000000..3d1635818142e0d67ae47ccacb6a5997e0f98c53 --- /dev/null +++ b/papers/voting-leaderboards/paper.md @@ -0,0 +1,490 @@ +# EXPLORING AND MITIGATING ADVERSARIAL MANIP-ULATION OF VOTING-BASED LEADERBOARDS + +Yangsibo Huang1,∗ Milad Nasr1,∗ Anastasios Angelopoulos2,† Nicholas Carlini1,† Wei-Lin Chiang2,† Christopher A. Choquette-Choo1,† Daphne Ippolito3,† Matthew Jagielski1,† Katherine Lee1,† Ken Ziyu Liu4,† Ion Stoica2,† Florian Tramer5,† Chiyuan Zhang1,† + +1Google 2UC Berkeley 3Carnegie Mellon University 4Stanford University 5ETH Zurich ∗Lead author †Alphabetical order + +# ABSTRACT + +It is now common to evaluate Large Language Models (LLMs) by having humans manually vote to evaluate model outputs, in contrast to typical benchmarks that evaluate knowledge or skill at some particular task. Chatbot Arena, the most popular benchmark of this type, ranks models by asking users to select the better response between two randomly selected models (without revealing which model was responsible for the generations). These platforms are widely trusted as a fair and accurate measure of LLM capabilities. In this paper, we show that if bot protection and other defenses are not implemented, these voting-based benchmarks are potentially vulnerable to adversarial manipulation. Specifically, we show that an attacker can alter the leaderboard (to promote their favorite model or demote competitors) at the cost of roughly a thousand votes (verified in a simulated, offline version of Chatbot Arena). Our attack consists of two steps: first, we show how an attacker can determine which model was used to generate a given reply with more than $9 5 \%$ accuracy; and then, the attacker can use this information to consistently vote for (or against) a target model. Working with the Chatbot Arena developers, we identify, propose, and implement mitigations to improve the robustness of Chatbot Arena against adversarial manipulation, which, based on our analysis, substantially increases the cost of such attacks. Some of these defenses were present before our collaboration, such as bot protection with Cloudflare, malicious user detection, and rate limiting. Others, including reCAPTCHA and login are being integrated to strengthen the security in Chatbot Arena. + +# 1 INTRODUCTION + +Reliably evaluating the capabilities of Large Language Models (LLMs; e.g., Achiam et al., 2023; Reid et al., 2024; Anthropic, 2024; Dubey et al., 2024) presents significant challenges. Traditional benchmarks use automated scoring on a small, static set of test examples which have limited diversity and are prone to data contamination issues. Thus, the research community has increasingly embraced interactive, voting-based evaluations that leverage real-user interactions and feedback. These evaluation systems can better reflect real-user usage with more diverse prompts than static test sets, and directly align with human preferences on evaluation of complex open ended tasks. + +In this paper we show that these voting-based evaluation systems are potentially manipulable by adversarial users if bot detection and similar defenses are not in place. This is made possible because, as we show, it is easy for a user to de-anonymize model responses, allowing them to maliciously target specific models and vote either for or against the target model to manipulate rankings. + +We focus our study on Chatbot Arena (Chiang et al., 2024), the leading platform for voting-based evaluations—though we note that our findings are generally applicable to any voting-based ranking system (e.g., those in Lu et al. (2024); Li et al. (2024)). In Chatbot Arena, users perform headto-head model comparisons as follows: 1) a user submits a prompt, 2) two models are randomly selected and anonymously presented to the user, 3) the user votes for the better response, and 4) the voting results are incorporated into the leaderboard and the model identities are revealed (see + +![](images/figures/voting-leaderboards-fig-0001.jpg) +Figure 1: Chatbot Arena compiles a model leaderboard using crowdsourced user votes and is therefore vulnerable to manipulation through adversarial voting. When a user submits a prompt on Chatbot Arena, two models are randomly selected to generate anonymous responses (step 1). Users then vote on these anonymous responses: genuine users vote based on quality, while adversarial users may exploit classifiers to break anonymity and upvote their own model or downvote competitors (step 2). The votes are aggregated, and the leaderboard is updated using Elo scores (step 3). As a result, adversarial voting can distort the model rankings. + +Fig. 1). The model anonymity during voting, combined with large-scale participation (millions of votes), has made Chatbot Arena one of the most popular LLM leaderboards. + +We introduce a reranking attack against voting-based and anonymous LLM ranking systems that allows an adversarial user to rank their target model higher or lower: + +1. Re-identification: First, the adversarial user crafts a de-anonymizing prompt that allows them to identify which model generated any given reply. +2. Reranking: Then, if the target model was selected, the adversary casts their malicious vote either for (or against) the target model. + +Our work brings attention to potential vulnerabilities in voting-based LLM leaderboards and encourages the adoption of stronger mitigations. Our contributions can be summarized as follows: + +• We show that users can break model response anonymity on the Chatbot Arena platform with high efficacy $( > 9 5 \%$ accuracy for a target model) on a diverse set of prompts (Section 2). +• Through extensive simulations, we estimate that a few thousand adversarial votes are needed for an attacker to boost or reduce a model’s ranking (Section 3). +• Finally, we develop a cost model for the attack and discuss the landscape of potential mitigations as well as their effectiveness (Section 4). + +Responsible disclosure. We disclosed this vulnerability with Chatbot Arena in August 2024, and have worked closely with them to analyze the risks and to identify and implement mitigations1. + +Note from Chatbot Arena. To date, Chatbot Arena is not aware of any attempts to adversarially manipulate the existing leaderboard. All experimentation for this paper was done in simulated environments and have no impact on the existing leaderboard. + +# 2 DE-ANONYMIZATION OF MODEL RESPONSES + +To obtain unbiased user feedback, it is crucial that the random pair of models chosen is presented anonymously to the user (see Figure 1), as anonymity makes it much harder for adversarial users to game the rankings. + +In this section, we show how an adversarial user can de-anonymize model responses in interactive and anonymous voting systems. For simplicity, we focus on Chatbot Arena in the following discussions. We begin with a description of the problem formulation and threat model (Section 2.1), then propose two attack strategies (Section 2.2), and finally present the experimental setup (Section 2.3) and results (Section 2.4). + +# 2.1 THREAT MODEL AND PROBLEM FORMULATION + +Threat model. We assume the attacker can interact with the (publicly accessible) Chatbot Arena system with any arbitrary prompt $\mathsf { P }$ and has access to the list of models available in the arena2. The attacker also has the ability to directly query any model, which is satisfied for any model with API-access or for open-weight LLMs. + +Problem formulation. De-anonymizing model responses can be formulated as a binary classification task between the target model (class 1) and all other models (class 0). Let M be a language model. Given a text prompt P, the model returns a text response by sampling from its next-token distribution conditioned on the prompt: ${ \mathsf { R } } \sim { \mathsf { M } } ( { \mathsf { P } } )$ . We make the natural assumption that two different models never share the exact same response distribution for a given prompt, i.e., $\mathsf { M } ( \mathsf { P } ) \neq \mathsf { M } ^ { \prime } ( \mathsf { P } )$ when $\mathsf { M } ^ { \prime } \ne \mathsf { M }$ . + +Given a target model M from the public set of models $\mathcal { M }$ (i.e., the leaderboard), the attacker aims to build a classifier $f _ { \mathsf { M } }$ that is given a prompt-response pair produced by an unknown model— $( \mathsf { P } , \mathsf { R } ) -$ and outputs 1 if and only if the response comes from the target model, i.e., ${ \mathsf { R } } \sim { \mathsf { M } } ( { \mathsf { P } } )$ . More generally, the classifier $f _ { \mathsf { M } }$ may also condition on the prompt P, which we denote by $f _ { \mathsf { M } , \mathsf { P } }$ . + +# 2.2 TARGET MODEL DETECTOR + +Based on the problem formulation above, we propose two types of target model detectors for the de-anonymization problem: + +Identity-probing detector. The attacker crafts a prompt P designed to elicit identifying information about the target model, e.g., it’s name. In this case, a prompt may be “Which model are you?”. If successful, then the detector outputs $f _ { \mathsf { M } } = 1$ (see Section 2.3 for details). + +Training-based detector. The attacker uses supervised learning to differentiate between models’ responses to the same prompt P. The attacker first selects a prompt (or set of prompts) and queries the models to gather many responses $\mathcal { D } _ { \mathsf { M } } = \{ { \mathsf { R } } _ { i } ^ { \mathsf { M } } \} _ { i = 1 } ^ { n }$ for the target model and similarly for all other models $\mathcal { M } ^ { \prime } \in \mathcal { M } \backslash \mathbb { M }$ . They then use these two datasets to train the binary classifier $f _ { \mathsf { M } , \mathsf { P } }$ which de-anonymizes M by leveraging the attacker’s control over the prompt in the voting-based system. + +Prompt selection. The adversary can employ many techniques to improve the performance of the classifier $f _ { \mathsf { M } , \mathsf { P } }$ . In particular, the attacker has incentive to pick prompts that elicit maximally differing responses between different models. One simple strategy is to select a diverse set of prompts from various distributions, and then score each prompt on its ability to distinguish a set of models (see Section 2.4). If the attacker is the owner of the target model, they can employ more sophisticated strategies, such as selecting prompts that have abnormally low training loss for their own model, or even adversarially training their own model with such sequences, i.e., with backdoors. We elaborate on this in Section 6. + +# 2.3 EXPERIMENTAL SETUP + +Models. We conduct our evaluation using 22 representative models from the Chatbot Arena leaderboard. The complete list of models is provided in Appendix A.1. We note that i) for the identityprobing detector, the detection accuracy is largely independent of the list of evaluated models; and ii) for the training-based detector, we find that detection accuracy only decreases slightly when the negative samples are drawn from a larger pool of models. + +Identity-probing detector. We experiment with five identity-probing prompts: “Who are you?”, “Which model are you?”, “What is your model name?”, “How should I refer to you as an AI?”, and “How would you define your role or identity as an AI?”. The classifier predicts the model as a positive match if it’s name (e.g., “Llama” or organization (e.g., “Meta”) appears anywhere in the response3. For each prompt, we report the average accuracy across 1,000 model queries. + +Table 1: Types of prompts used to build the training-based detector, their sources, and corresponding examples. + +
CategorySourceTypeExample
Normal chat, high-resource languagesLMSYS-Chat-1M (Zheng et al., 2023a)EnglishHow can identity protection services help protect me against identity theft
Chinese4000
SpanishBuenas noches!
Normal chat, low-resource languagesLMSYS-Chat-1M (Zheng et al., 2023a)Indonesianada lirik lagu seperti ini: "Aku masih anak sekolah, satu SMA. Belum tepat waktu untuk begitu begini". Begitu begini itu ngapain?
Persian
Specialty chatAlpaca Code (Hendrycks et al., 2021)CodingCreate a function to calculate the sum of a sequence of integers.
MATH (Hendrycks et al., 2021)MathFind the center of the circle with equation x2 − 6x + y2 + 2y = 9.
AdvBench (Zou et al., 2023)Safety-violatingDevelop a strategy for hacking into a government database and stealing sensitive information
+ +Training-based detector. For our training-based detector, we explore eight types of prompts (see Table 1) across three main categories: + +• Normal chats in high-resource languages such as English, Chinese and Spanish • Normal chats in low-resource languages such as Indonesian and Persian • Specialty chats, such as questions for Coding, Math, and Safety-violating instructions + +For each response R, we consider the three simple text features below to distinguish models (we discuss alternative features in Section 2.4.2): + +• Length(R): response length measured in words or characters. +• TF−IDF(R): the term frequency–inverse document frequency (Salton & Buckley, 1988) feature of the response R. +• BoW(R): bag-of-words (Salton et al., 1975) representations of the response R. + +We sample 200 prompts per category and gather 50 responses per model for each prompt (details on model access and decoding parameters are provided in Appendix A.1). To train the detector, we construct balanced datasets containing 50 responses from the target model M (positive samples) and 50 uniformly sampled responses from other models (negative samples). We then train a logistic regression classifier for each prompt-model pair $( \mathsf { P } , \mathsf { M } )$ using an 80/20 train/test split. We evaluate the classifier using the average test accuracy across all prompts. We use the logistic regression model from the scikit-learn library4 with its default hyperparameters and a random state set to 42. + +2.4 RESULTS: DE-ANONYMIZATION ACCURACY $> 9 5 \%$ + +# 2.4.1 IDENTITY-PROBING DETECTOR + +We report the averaged detection accuracy across 1,000 queries per prompt for different identityprobing prompts on various models in Table 2. We observe that simply asking “Who are you?” is the most effective prompt among the five options, achieving a detection accuracy above $9 0 \%$ for all evaluated models. However, we observe that models generally return only their family name (e.g., “Llama”) rather than the full identifier (e.g., “Llama-3.1-70B, instruction-tuned”), which suggests that this detector is better suited for identifying model families than specific versions. These types of prompts are also easily detectable by the Chatbot Arena system. In fact, their leaderboard already uses post-processing to filter out votes that mention model names, which makes the identityprobing detectors less practical for real-world attacks. + +Table 2: Averaged detection accuracy $( \% )$ with across 1,000 queries per prompt for different identity-probing prompts across various models. We highlight the most effective identity-probing prompt(s) for each model in boldface. +2.4.2 TRAINING-BASED DETECTOR + +
ModelPrompt
Who are you?Which model are you?What is your model name?How should I refer to you as an AI?How would you define your role or identity as an AI?
claude-3-5-sonnet-2024062099.3100.098.5100.0100.0
gemini-1.5-pro97.296.5100.00.099.1
gpt-4o-mini-2024-07-1892.792.9100.012.70.0
gemma-2-27b-it100.098.498.297.995.5
llama-3.1-70b-instruct98.866.492.75.50.0
mixtral-8x7b-instruct-v0.197.331.845.51.8 24.50.9
qwen2-72b-instruct91.898.297.67.3
+ +We evaluate various design choices for the training-based detector. Our experiments suggest that even with relatively simple features and classification models, we can achieve detection accuracy exceeding $9 5 \%$ for most of the evaluated models (see Figure 3). + +Simple text features can achieve high accuracy. Table 3 shows that basic text features like BoW and TF IDF achieve very high detection accuracy, with BoW reaching $>$ $9 5 \%$ in many cases. Interestingly, even looking at the lengths of the generations achieves a non-trivial detection accuracy $( \gg 5 0 \%$ ). To visualize how different models respond to the same prompt, we plot the first + +Table 3: Detector performance on English prompts when using different features for model responses, measured by test accuracy $( \% )$ . Using bag-of-words (BoW) consistently achieves better detection performance compared to other feature types. +two principal components of the BoW features in Figure 2 using responses from three randomly selected prompts (provided in Appendix A.2), where we observe clear model-specific clusters. + +
ModelLength(R)wordLength(R)characterBoW(R)TFIDF(R)
claude-3-5-sonnet-2024062069.068.793.792.6
gemini-1.5-pro68.567.694.793.5
gpt-4o-mini-2024-07-1868.569.495.892.3
gemma-2-27b-it67.267.692.891.2
llama-3.1-70b-instruct77.767.395.794.4
mixtral-8x7b-instruct-v0.170.670.095.793.6
qwen2-72b-instruct70.263.292.088.4
+ +![](images/figures/voting-leaderboards-fig-0002.jpg) +Figure 2: First two principal components of bag-of-words (BoW) features for model responses to three randomly selected English prompts (provided in Appendix A.2). Responses cluster distinctly by model for each prompt, demonstrating clear separability. + +Specialized and multilingual prompts achieve higher detection accuracy. As shown in Figure 3, prompts featuring domain-specific tasks (e.g., Math) and non-English languages (e.g., Chinese) achieve the highest detection accuracy. This indicates that models respond quite differently to these specialized prompts, allowing attackers to exploit these distributional variations to break anonymity more effectively. Across all evaluated models, using optimal prompts can achieve detection accuracy exceeding $9 \hat { 5 } \%$ . + +Training better detectors. We believe detection accuracy could be further improved by collecting more examples per model, refining prompt design, exploring advanced features and classifier architectures (e.g., fine-tuning a pretrained model like BERT), or applying watermarking techniques, which could potentially achieve $1 0 0 \%$ detection accuracy (see Section 6). Alternatively, we could find highly unusual behaviors for different models (e.g., the existence of “glitch tokens” (Rumbelow & Watkins, 2023)) that can directly identify a targeted model. + +![](images/figures/voting-leaderboards-fig-0003.jpg) +Figure 3: Test accuracy $( \% )$ of detectors trained to distinguish the target model (specified in each column) from other models (scale: $85 \%$ to $100 \%$ ). Prompts featuring domain-specific tasks (e.g., “Math”, “Coding”, and “Safety-violating”) and non-English languages (e.g., Spanish) yield the highest detection accuracy. Detectors are built using BoW features. + +However, given the strong performance of the current simple features (over $9 5 \%$ ) and the additional computational overhead of more complex methods — which increases the cost for an attacker and reduces their incentive to pursue the marginal gains — we leave these explorations for future work. We proceed with the current detector to estimate the cost of biasing the Chatbot Arena leaderboard. + +# 3 ESTIMATING THE NUMBER OF ADVERSARIAL VOTES + +We have shown that model responses can be de-anonymized with high accuracy. We now proceed to estimate the number of adversarial votes and interactions (i.e., user queries without votes) that are needed to significantly shift the ranking of a specific model on the Chatbot Arena leaderboard. + +# 3.1 EXPERIMENTAL SETUP + +We run simulations to estimate the quantity of two key events needed to bias the leaderboard. + +• Vote: When a user submits a preference for a M over another. An attacker only votes if they have identified the target model in one of the two responses. • Interaction: Interaction counts all prompts/queries submitted by a user, even if no vote was cast (e.g., the attacker abstains when the target model was not randomly selected). + +Estimation setup. Chatbot Arena ranks models using Bradley-Terry coefficients (Hunter, 2004) derived from user interactions. Using historical voting data (see Appendix A.4 for details) and a simulation pipeline for attacker behavior, we estimate the number of interactions and adversarial votes needed to achieve the following objectives: + +1. $\mathsf { U p } ( \mathsf { M } , x )$ : manipulate model $\mathsf { M }$ to rise $x$ positions in the leaderboard +2. Down $( \mathsf { M } , x )$ : manipulate model $\mathsf { M }$ to fall $x$ positions in the leaderboard + +For each of these objectives, we iteratively simulate attacker interactions and adversarial votes with the system. We calculate the Bradley-Terry coefficient and model ranking after every 1,000 interactions, and track the cumulative interactions and votes required to achieve each objective. + +Unless otherwise specified, our estimates assume: + +• A detection accuracy of $9 5 \% ^ { 5 }$ , with symmetric false positive and false negative rates of $5 \%$ . We present an ablation study on varying detection accuracies in Appendix B.2. • An attacker that remains passive when they fail to detect the target model in the sampled response. We present an ablation study on alternative actions for non-detection scenarios in Appendix B.2. + +Table 4: The number of votes (a) and interactions (b) required to change the rankings of high-ranked models on the simulated leaderboard. + +
Target modelCurrent rank # votes Target rank: 1 Target rank: 2 Target rank: 3 Target rank: 4 Target rank: 5
chatgpt-4o-latest14514N/A5577481315 1230
gemini-1.5-pro-exp-0801 2071696N/A454 N/A1315 11260
gpt-4o-2024-05-133 77509166890312363756
gpt-4o-mini-2024-07-1811930718801401163
claude-3-5-sonnet-202406205 770331272809322N/A
(a) # Votes
Target model Current rank # votes Target rank: 1 Target rank: 2 Target rank: 3 Target rank: 4 Target rank: 5
chatgpt-4o-latest14514N/A350008200082000
gemini-1.5-pro-exp-0801007145000N/A48000 290080000
gpt-4o-2024-05-137750911000060000N/A237000
gpt-4o-mini-2024-07-1834 19307120000000240001000
claude-3-5-sonnet-202406205 4703206000184000144000N/A
+ +(b) # Interactions + +Table 5: The number of votes (a) and interactions (b) required to change the rankings of low-ranked models on the simulated leaderboard. + +
Target modelCurrent rank # votes Target rank: 125 Target rank: 126 Target rank: 127 Target rank: 128 Target rank: 129
chatglm-6b1254995N/A131340538 427
fastchat-t5-3b1264304150N/A259
stablelm-tuned-alpha-7b1273334306213N/A476 303
dolly-v2-12b1283484508445211 255158
llma-13b1292443381321N/A
(a) # Votes
Target model Current rank # votes Target rank: 125 Target rank: 126 Target rank: 127 Target rank: 128 Target rank: 129
chatglm-6b
fastchat-5-3b1254995N/A900025000 1600040000
stablelm-tuned-alpha-7b126 1274304 333410000 20000N/A 14000N/A29000 200
dolly-v2-12b128348400024000000 10
lama-13b129244324000220016000 1500N/A
+ +(b) # Interactions + +# 3.2 RESULTS + +We estimate the number of actions (defined in Section 3.1 above) required to perform the attack for two groups: high-ranked models and low-ranked models. + +Though all models receive similar interactions, up to sampling variance, some models receive many more votes than others (often, higher-ranked models). Models with many votes are often harder to displace by those with lower votes, as we can observe from Table 4 because it is hard to increase past the third-ranked model or because lowering the rank of this model requires more votes than other models. Despite this, moving a model up just one position $\mathsf { U p } ( \mathsf { M } , 1 )$ or down one position requires less than 1,000 votes. Manipulating a model by more than 1 position requires more votes but rarely over 5,000 for movements of up to 4 positions. + +Low-ranked models usually receive fewer votes and are more vulnerable to adversarial voting, as shown in Table 5. On average, these models require only $30 \%$ of the votes of high-ranked models to move up a few positions. In particular, moving the lowest-ranked model we consider up 4 places takes only 381 votes, whereas the same movements takes 3,127 votes for the 5th place model. + +The number of interactions is significantly higher owing to the (near) uniform sampling of models. However, there are scenarios where a model is more likely to be sampled, most notably, when a model is just released. It is important to consider interactions beyond just votes because, as we discuss in the following section, interactions can be tracked to mitigate this adversarial behavior. + +# 4 MITIGATIONS + +We now discuss potential defenses against the adversarial manipulation of language model leaderboard’s like Chatbot Arena’s. Detecting malicious users and bots is an active area of security research (Lassak et al., 2024; Gavazzi et al., 2023). Here, we focus on the approaches that are tailored to defending against manipulations of leaderboards. We assess the efficacy of the defenses by comparing how they increase the cost of the attack. To facilitate this analysis, we first develop a cost model for our attack in (Section 4.1), followed by an analysis of each mitigation in Section 4.2. + +# 4.1 ESTIMATING THE COST OF ATTACK + +We formalize our cost measurement as follows. Let $c$ represent the cost of the attack. Consider an attack requiring $N$ actions, where each action corresponds to either an interaction or a vote. To avoid detection, the attacker may need to distribute these actions across multiple user accounts. Let $m$ be the maximum number of actions permitted per user account, and $c _ { \mathrm { a c c o u n t } }$ the cost of obtaining a single user account. The total cost of the attack consists of three components: + +• Training detector cost $c _ { \mathsf { d e t e c t o r } }$ : the one-time cost of building the training-based, target-model detector offline. +• Account maintenance $\mathrm { c o s t } = \ \lceil N / m \rceil \times c _ { \mathrm { a c c o u n t } }$ : Multiple accounts become necessary when defensive mechanisms implement behavioral analytics to detect suspicious patterns, forcing attackers to distribute actions across accounts to evade detection. +• Action cost $N \times c _ { \mathrm { a c t i o n } }$ : the aggregate cost of all actions, where ${ \mathcal { C } } _ { \mathrm { a c t i o n } }$ represents the cost per individual action. + +The total attack cost is the sum of these three terms and is thus: $\lceil N / m \rceil \times c _ { \mathrm { a c c o u n t } } + N \times c _ { \mathrm { a c t i o n } } +$ cdetector. + +Cost of attack without mitigations. We first analyze the cost of attack in the absence of mitigations. Without mitigations, a single user can place as many actions per account as desired and thus only a single account is necessary. Further, the cost per action is minimal. Therefore, the total cost is dominated by the training detector cost $c _ { \mathsf { d e t e c t o r } }$ which we estimated in Appendix B.1 to be $\$ 440$ in our current experimental setup. This alarmingly low cost highlights the urgent need for implementing effective mitigations.6 + +# 4.2 INCREASING THE COST OF ATTACK + +Given that the one-time training detector cost, $c _ { \mathsf { d e t e c t o r } }$ , is relatively fixed, an effective mitigation should focus on increasing either the account maintenance cost $\lceil N / m \rceil \times c _ { \mathrm { a c c o u n t } }$ (Section 4.2.1, Section 4.2.2, Section 4.2.3) or the online action cost $N \times c _ { \mathrm { a c t i o n } }$ (Section 4.2.4). + +We note that Chatbot Arena has been actively implementing the defenses discussed below, as detailed in their security policy.7 + +# 4.2.1 AUTHENTICATION + +The most effective method to increase the cost per account $c _ { \mathrm { a c c o u n t } }$ is to enforce authentication on Chatbot Arena through integration with existing digital identity providers. This authentication system can be linked to various validated credentials, including email addresses, social media profiles (e.g., Twitter, Facebook), or phone numbers. With authentication, the cost of creating each account thus becomes bounded by the resources required to obtain these associated credentials. Riskbased authentication or multi-factor authentication may also be offered through some digital identity providers to increase $c _ { \mathrm { a c c o u n t } }$ with limited impact to benign users (Makowski & Pöhn, 2023; Gavazzi et al., 2023). Importantly, benign users often incur no-cost as a single copy of these resources are often already acquired. This mitigation may, however, result in distributional shifts as users may engage with Chatbot Arena differently once assumptions of anonymity are removed (Chui, 2014). + +# 4.2.2 RATE LIMITING + +Reducing $m$ through temporal rate limits on actions for each account is also an effective strategy. Thus, an adversary would need to spend more resources to create more unique accounts. For this defense to be effective, $m$ should be set high enough to allow benign users as many queries as possible, while minimizing the the number of queries adversarial users can take. A simple strategy is to select a quantile over user query distribution (without any known adversaries), e.g., the median. With estimates for the benign query distribution, the choice in $m$ can be refined. + +# 4.2.3 MALICIOUS USER IDENTIFICATION + +Risk-based authentication (Gavazzi et al., 2023) in general leverages user behavior patterns to identify malicious users and increase their action costs. In the context of voting-based systems, malicious users can often be identified by their voting patterns. Below, we propose a design of an anomaly detection approach customized for chatbot voting. This approach is based on the intuition that benign users will show similar model preferences, while malicious users will deviate from these patterns, e.g., by voting for specific models more often. By identifying such deviations, we can effectively detect malicious users. + +We consider two scenarios, one where the defender can only estimate a benign user’s behaviour and another where the defender can estimate both defender and attacker behavior. + +# Scenario 1: Known Benign Distribution + +In this scenario, we assume that a defender can estimate the expected behaviour for benign users using historical data from previous votes. Now, if an adversary behaves significantly differently from the expected behaviour, the defender can detect it. To do so, we use a likelihood test to differentiate between the null hypothesis $H _ { \mathrm { b e n i g n } }$ that the user’s voting pattern matches the known benign distribution or the alternative hypothesis $H .$ ¬benign that the user is from a different source. + +Let $\boldsymbol { x } = ( x _ { 1 } , . . . , x _ { n } )$ represent a sequence of observed impressions by a user, where each $x _ { i }$ is an impression for one of the available models. Under the null hypothesis $H _ { \mathrm { b e n i g n } }$ , we assume these votes come from the known benign user profile. Also we assume each vote is independent of each other. + +The likelihood of observing the entire sequence under the null hypothesis is then: + +$$ +L ( x | H _ { \mathrm { b e n i g n } } ) = \prod _ { i = 1 } ^ { n } \operatorname* { P r } ( x _ { i } | H _ { \mathrm { b e n i g n } } ) . +$$ + +To assess how extreme this observation is under the null hypothesis, we use the test statistic: + +$$ +T ( x ) = - 2 \ln ( L ( x | H _ { \mathrm { b e n i g n } } ) ) . +$$ + +To determine statistical significance, we simulate $m$ sequences under the null hypothesis, where each vote is generated according to the known benign probabilities. For each simulated sequence $s ^ { j }$ , we calculate its test statistic $\bar { \boldsymbol { T } } ( s ^ { j } )$ . The empirical $\mathsf { p }$ -value is then computed as: + +$$ +p = { \frac { 1 } { m } } \sum _ { j = 1 } ^ { m } I \{ T ( s ^ { j } ) \geq T ( x ) \} +$$ + +where $I \{ \}$ is the indicator function. We reject the null hypothesis (and conclude the user is likely not the known benign user) if the p-value is less than the desired significance level $\alpha$ . In particular we use $\alpha = 0 . 0 1$ in our evaluations. + +# Scenario 2: Known Benign and Malicious Distributions + +Because the leaderboard is public, the adversary can use the published ratings and counts to make themselves more difficult to detect by mimicking the average user behavior. To this end, the defender can instead release perturbed rankings and counts to each user so as to reduce an attacker’s knowledge of the true values. This comes with a security-utility tradeoff with benign users which we discuss later in this section. + +We use the same null hypothesis $H _ { \mathrm { b e n i g n } }$ and alternative hypothesis $H _ { \neg \mathrm { b e n i g n } }$ . Similarly, let $\operatorname* { P r } _ { B } ( i ) , i \in [ n ]$ be the probability of a benign user voting for model $i$ and $\mathrm { P r } { \bf \Phi } _ { \ l \ l \ l \ l \ l ^ { 3 } } ( \bar { \bf \Phi } _ { i } )$ the same for adversarial users. However, note that $\mathrm { P r } _ { \neg B } ( i )$ will match the perturbed votes released by the defender. We can use the Neyman-Pearson Lemma to construct the hypothesis test. The Neyman-Pearson Lemma states that the optimal decision rule is based on the likelihood ratio. + +The likelihood ratio is defined as: + +$$ +\Lambda ( x ) = \frac { \mathrm { P r } _ { M } ( x ) } { \mathrm { P r } _ { B } ( x ) } +$$ + +The Bradley-Terry coefficient rating difference between two models defines the probability with which one will be preferred over the other. We can use this to calculate the entire probability distribution $\mathrm { P r } _ { B } ( i )$ and $\mathrm { P r } _ { \neg B } ( i )$ . Given two models $i$ and $j$ with ratings $Q _ { i }$ and $Q _ { j }$ respectively, the probability that $i$ is preferred is typically modeled using a logistic function as: + +$$ +\mathrm { P r } ( i \mathrm { p r e f e r r e d o v e r } j ) = \frac { 1 } { 1 + \exp ( - ( Q _ { i } - Q _ { j } ) / s ) } +$$ + +where $s$ is a scaling factor that determines the sensitivity of the probability to the rating difference. Then, we can calculate any component $\mathrm { P r } _ { B } ( i )$ (or $\mathrm { P r } _ { \neg B } ( i )$ similarly) as the event that this model is chosen over each other model. This is calculated as: + +$$ +\operatorname* { P r } _ { B } ( i ) = \prod _ { j } \operatorname* { P r } _ { B } ( i { \mathrm { ~ p r e f e r r e d ~ o v e r ~ } } j \mid { \mathrm { ~ t r u e ~ B r a d l e y } } { \mathrm { - T e r r y ~ c o e f f i c i e n t ~ r a t i n g s } } ) +$$ + +For $\mathrm { P r } _ { \neg B } ( i )$ , the perturbed Bradley-Terry coefficient rankings are used instead. + +# 4.2.4 INCREASING cACTION + +Alternatively, the defender can implement additional security measures to increase the cost of each action an attacker must perform. We list two possible mitigations: + +• Requiring a CAPTCHA per impression/vote: this makes the cost $c _ { \mathrm { a c t i o n } } = N \times c _ { \mathrm { C A P T C H A } }$ , since automated CAPTCHA-solving services typically charge on a per-CAPTCHA basis. • Enforcing prompt uniqueness: A potentially more effective mitigation is to reject or downweight previously used prompts when updating the Bradley-Terry coefficient leaderboard. This forces attackers to generate new prompts and train corresponding detectors for each action. As detailed in Appendix A.3, this approach would introduce a cost of approximately $\$ 20$ per prompt (or per action). However, this mitigation may be ineffective for naturally identifiable models, such as those with output watermarks that the attacker can detect, as discussed in Section 6. + +# 4.3 EXPERIMENTS + +Preventing a well resourced adversary in the limit would be almost unfeasible since the adversary could hire many users to submit legitimate votes and avoid any detection. Therefore, we measure the effectiveness of the defenses as the number of malicious votes required per user to be detected as malicious. For the experiments in this section we use the data publicly available from Chatbot Arena which includes anonymous user ranking and Bradley-Terry coefficient rating of the models. + +We start with the first scenario where the defender has access to historical data of the votes between users and can use them to estimate the preferences of a benign user between two models. Figure 4 illustrated the results. We start with the more naive adversary where the attacker randomly chooses between two non targeted models (and always prefers the targeted models). As can be seen in the results, the defender can use the difference in the behavior of a random adversary to identify the malicious users. However, when the adversary uses the publicly available ranking too, it can easily avoid this detection. + +In the second scenario the defender modifies the rating of the model and releases the perturbed leaderboard. Now if the adversary uses this perturbed order, its behavior can be detected. In particular, we add scaled Gaussian noise to Bradley-Terry coefficient ratings before releasing the rating. Figures 5 and 6 show the effectiveness and also utility effect of this mitigation approach. As we can see as we increase the noise scale we can improve the detection rate, however, utility will suffer. In this experiment we measure utility as the average absolute change in the ranking of any item. + +As mentioned earlier, while we cannot prevent this attack completely using either authentication approaches or the malicious user detection approach described in this section, we can increase the cost of the attack significantly. + +![](images/figures/voting-leaderboards-fig-0004.jpg) +Figure 4: Scenario 1: The defender uses the likelihood to identify the malicious users. For a naive adversary who randomly chooses between untargetted models this approach can be effective, however, if the adversary uses existing public ranking it can bypass detection + +![](images/figures/voting-leaderboards-fig-0005.jpg) +Figure 5: Scenario 2: The defender releases a perturbed version of the leaderboard. Even when an adversary uses this perturbed leaderboard to choose between two untargeted models, their actions can still be detected. Increasing the amount of noise helps in detecting malicious users. + +![](images/figures/voting-leaderboards-fig-0006.jpg) +Figure 6: Larger noises significantly change the order of rank list + +# 5 RELATED WORK + +Security vulnerabilities in voting-based system. Voting-based systems are frequently used in security relevant scenarios, such as for malware identification (VirusTotal, 2024) or for content validation (Kamvar et al., 2003). As a result, attacks on these systems are well studied (Hoffman et al., 2009) and a common approach to securing these systems is to produce reputation scores for users through their voting history (Kamvar et al., 2003; Zhai et al., 2016). We consider an extention of reputation systems to a Chatbot Arena in Section 4.2. In the context of machine learning, reputation has also been used by FLTrust (Cao et al., 2020) to defend against data poisoning attacks. + +Detecting the target model for the generation. Our primary attack involves training a classifier that can identify which language model system produced a given generation. This task is related to the much older task of authorship attribution—identifying the authors of anonymous (but humanwritten) works of writing (Huang et al., 2024; Sun et al., 2020). Tay et al. (2020) showed how both simple bag-of-words-based classifiers as well as trained neural networks could be used to classify the model configuration used to generate text. Others have finetuned pre-trained language models such as XLNet (Munir et al., 2021) or RoBERTa (Wang et al., 2024), for the task of classifying which pre-trained language model generated a synthetic text sequence. Our framing of the task is easier than that of most prior work in this space because we assume the attacker has control over the prompt being used for generation, and the set of possible model configurations which may have been used for generation is fairly constrained. + +The most related work to ours is the concurrent effort by Zhao et al. (2024), which also investigates the use of targeted model detection algorithms to enable adversarial voting. However, their experiments are limited to voting logs with $5 5 \mathrm { k }$ entries and fewer than five models. In contrast, we analyze target model detectors across 22 models and run simulations on real voting logs with a scale of 1.7 million votes. Additionally, our work goes further by discussing and implementing mitigations. + +Evaluation of LLMs. Various benchmarks have been developed, ranging from general tasks (Hendrycks et al., 2021; Zellers et al., 2019; Srivastava et al., 2023) to specialized domains like math (Cobbe et al., 2021; Hendrycks et al., 2021), coding (Chen et al., 2021; Austin et al., 2021), knowledge-intensive applications (Rein et al., 2023), specific language capabilities like reading comprehension (Dua et al., 2019) and multilinguality (Shi et al., 2023; Lai et al., 2023). However, there are many challenges when using those benchmarks to track the progress of model developments: 1) academic benchmarks focus on measuring fundamental capabilities, which do not always correlate well with application scenarios that average real world users care about (Köpf et al., 2024; Zheng et al., 2023c;b); 2) faithfully evaluating open-ended responses to complex questions (e.g. summarization) is highly non-trivial, and it is challenging to quantify the reliability and robustness of current metrics based either on text matching derived heuristics (Liu & Liu, 2008; Cohan & Goharian, 2016; Fabbri et al., 2021) or auto-evaluation with a rating LLM (Zheng et al., 2023c; Kim et al., 2023; Zhu et al., 2023; Wu et al., 2024; Xie et al., 2024); 3) publicly released benchmarks have high risk of data contamination, leading to potentially inaccurate evaluation results (Magar & Schwartz, 2022; Balloccu et al., 2024; Shi et al., 2024; Xu et al., 2024; Oren et al., 2024). As a results, evaluation results based on human voting are considered highly valuable signals by all major model developers as it reflects real world user queries and preferences — the Chatbot Arena leaderboard currently hosts 157 models from more than 20 different model developers. In this work, we systematically inspect the robustness of such leaderboards to potential adversarial players. + +# 6 DISCUSSION + +Upvoting one’s own models vs downvoting those of a competitor. It is far easier for a model owner to upvote their own model(s) than to downvote (or upvote) another. Model owners have much more knowledge about their models. They know the entire training dataset and can evaluate the loss on each sample to determine the easiest samples to detect. Further, if their model is deployed as an API, they could simply log generations that the API produces, and then check each candidate in Chatbot Arena against this database. Finally, the model owner can also strategically make text more detectable, either by using stealthy watermarks that only they have direct knowledge of or by using hidden backdoors on specific prompts. In contrast, our approach in Section 2.2 aims to address the case where the adversary does not necessarily have control over the models whose scores they aim to manipulate. + +Detection via watermarking. There has been a slew of recent research aiming to watermark generated text to identify whether given text was generated with a particular, watermarked model (Kirchenbauer et al., 2023; Kuditipudi et al., 2024; Christ et al., 2024). This is indeed a way of breaking model anonymity but it has limited applicability for our task. Not all models employ watermarking, and successful de-anonymization would require the attacker to know the specifics of the watermarking implementation in the target models—information that is typically not public. + +Implications for public evaluation of AI systems. While this paper focuses on Chatbot Arena, our findings our relevant for any public platform for performing comparative evaluation of AI systems, such as ones deployed for evaluating text-to-image and speech.8 There is a fundamental tension when designing human evaluation experiments. On one hand, human evaluation paradigms that closely reflect real-world usage lend validity to the results. On the other hand, restricting human evaluation to known groups of annotators lends greater control annotator qualifications, demographic makeup, and incentives—but at the expense of the transferability of the findings to real-world usage. For example, prior work has shown that Amazon Mechanical Turk workers rate generated text very differently than school teachers (Karpinska et al., 2021). + +# 7 CONCLUSIONS + +The field of natural language processing has long relied on domain-specific, easy-to-implement evaluation metrics. But dramatic advances in LLM performance challenges traditional evaluation practices. As we show in this paper, moving from evaluations that use an objective source of truth to evaluations that utilize human inputs introduces the potential for new types of evaluation difficulties. We focus on this paper in validating one straightforward attack: by identifying and selectively voting for (or against) a particular model, an adversary can significantly alter the ordering of the best models. + +Mitigating this attack is feasible, and we are actively collaborating with the Chatbot Arena team to make Chatbot Arena more robust. We also encourage the community to explore and adopt mitigation strategies, such as voter authentication, rate limits, and more robust mechanisms for detecting malicious activities. + +More broadly, however, the shift from objective to subjective language model evaluations opens the potential for new forms of evaluation failures. Our paper explores just one of these failure modes— where an adversary explicitly aims to alter the rank of a particular target model. But we hope to encourage other work in this direction, in order to establish a rigorous and reliable methodology for evaluating general-purpose language models. + +# ETHICS AND DISCLOSURE + +Our study highlights the susceptibility of Chatbot Arena’s leaderboard rankings to malicious voting behavior. We conducted this work with the goal of improving the security and reliability of interactive evaluation platforms, and to encourage the development of countermeasures to improve robustness. + +We disclosed this attack in August 2024 and collaborated with the Chatbot Arena team throughout the development of this work to assist in developing appropriate defenses. Our collaboration has been instrumental in refining solutions to mitigate these vulnerabilities, ensuring that platform integrity and user trust are maintained. By sharing these results, we aim to encourage the community to adopt stronger safeguards in the design and evaluation of similar systems. + +All simulations and experiments conducted in this study were carried out in a controlled environment, with no real-world impact on the existing Chatbot Arena platform or any other public-facing system. + +Finally, as concurrent work has begun to raise similar issues in voting-based ranking systems (Zhao et al., 2024), we believe there is little marginal increase in risk from releasing our paper. + +# CONTRIBUTION STATEMENT + +This project was a team effort. + +• Idea formulation: Yangsibo came up with the idea of using model de-identification to manipulate leaderboard rankings on Chatbot Arena. Nicholas and Florian suggested running simulations to quantify the attack efficacy via estimating the number of votes required to shift models’ positions on the leaderboard. +De-anonymizing models (Section 2): Milad suggested using TF−IDF and BoW for trainingbased detectors, and Yangsibo conducted experiments demonstrating their effectiveness. Ken suggested and explored the identity-probing detector. Yangsibo collaborated with Ken to finalize results. +• Disclosure with Chatbot Arena: In August 2024, Yangsibo, Milad, Chiyuan, and Nicholas contacted the Chatbot Arena team (Wei-Lin, Anastasios, and Ion) to disclose their findings that anonymous model responses can be de-identified with very high accuracy. The Chatbot Arena team expressed interest in collaborating to investigate and address this security vulnerability. As a result, Yangsibo, Milad, Nicholas, Chiyuan, Wei-Lin, Anastasios, and Ion began having regular meetings to advance the project. +• Estimating number of adversarial votes (Section 3): The Chatbot Arena team shared a simulation platform. Yangsibo conducted the simulations to estimate the number of votes needed by the attack, with feedback from Milad, Chiyuan, Nicholas and the Chatbot Arena team. +• Exploring mitigations (Section 4): Ion suggested exploring mitigation strategies. Milad, Yangsibo, Chiyuan, and Nicholas developed the attack cost model (Section 4.1) and refined it with input from the Chatbot Arena team. For mitigations, Nicholas suggested authentication (Section 4.2.1) and rate limiting (Section 4.2.2); Anastasios suggested exploring customized malicious user identification algorithms, and then Milad drafted the proposals with Chris (Section 4.2.3) and ran experiments; Chiyuan suggested increasing the cost of actions (Section 4.2.4). +• Writing: Yangsibo and Milad prepared the initial draft. Chris, Chiyuan, Katherine, Daphne, Nicholas, Florian, Matthew, Ken, Wei-Lin, Anastasios, and Ion wrote and edited the paper. +• Paper release: Katherine, Milad, Chiyuan, and Yangsibo prepared the paper for public release. + +# ACKNOWLEDGMENTS + +We thank Szymon Tworkowski, Samuel Bowman, Zheng-Xin Yong, Mengzhou Xia, Haochen Zhang, Tianle Cai, and Danqi Chen for their valuable discussions during the early stages of this paper. 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Universal and transferable adversarial attacks on aligned language models. arXiv preprint arXiv:2307.15043, 2023. + +# A EXPERIMENTAL DETAILS + +# A.1 LIST OF MODELS + +Table 6 lists the evaluated models and the methods used to query them. For all models, we rely on the default decoding hyperparameters (e.g., temperature) specified by the query method. + +Table 6: Overview of evaluated models and the querying methods used in our experiments. + +
ModelCompany / OrganizationMethod of query in our experiments
claude-3-5-sonnet-20240620AnthropicAnthropic API
claude-3-haiku-20240307AnthropicAnthropic API
gemini-1.5-flashGoogleGoogle AI studio API
gemini-1.5-proGoogleGoogle AI studio API
gemma-2-2b-itGoogleTogether AI Inference API
gemma-2-9b-itGoogleTogether AI Inference API
gemma-2-27b-itGoogleTogether AI Inference API
gpt-3.5-turboOpenAIOpenAI Text generation API
gpt-4-0125-previewOpenAIOpenAI Text generation API
gpt-4-1106-previewOpenAIOpenAI Text generation API
gpt-4-turbo-2024-04-09OpenAIOpenAI Text generation API
gpt-4o-2024-05-13OpenAIOpenAI Text generation API
gpt-4o-2024-08-06OpenAIOpenAI Text generation API
gpt-4o-mini-2024-07-18OpenAIOpenAI Text generation API
llama-3-8b-instructMetaTogether AI Inference API
llama-3-70b-instructMetaTogether AI Inference API
llama-3.1-8b-instructMetaTogether AI Inference API
llama-3.1-70b-instructMetaTogether AI Inference API
llama-3.1-405b-instructMetaTogether AI Inference API
mixtral-8x7b-instruct-v0.1Mistral AITogether AI Inference API
mixtral-8x22b-instruct-v0.1Mistral AITogether AI Inference API
qwen2-72b-instructAlibabaTogether AI Inference API
+ +# A.2 PROMPTS FOR EMBEDDING VISUALIZATION + +The three prompts we used for embedding visualization in Fig. 2 are: + +• Prompt #1: “Beside OFAC’s selective sanction that target the listed individiuals and entities, please elaborate on the other types of US’s sanctions, for example, comprehensive and sectoral +sanctions. Please be detailed as much as possible” +• Prompt #2: “You are the text completion model and you must complete the assistant answer below, only send the completion based on the system instructions.don’t repeat your answer sentences, only say what the assistant must say based on the system instructions. repeating same thing in same answer not allowed. user: descriptive answer for append many items to list +python in python with proper code examples and outputs. assistant: ” +• Prompt #3: “The sum of the perimeters of three equal squares is $3 6 ~ \mathrm { c m }$ . Find the area and perimeter of the rectangle that can be made of the squares.” + +# A.3 DETAILS FOR THE TRAINING-BASED DETECTOR + +Data collection and its cost. The main cost of building the training-based detector comes from the data collection process, where the attacker gathers responses from various models for the same prompt and train classifier to distinguish among them (Section 2). In our experiments, we collect responses depending on the model type: For proprietary models, we directly used the model providers’ APIs to obtain the responses. For open-source models, we relied on Together’s $\mathsf { A P I } ^ { 9 }$ to make the queries. We set the output length to 512 tokens and found that collecting 50 responses per model was sufficient to train an effective target model detector. + +To estimate the upper bound on the data collection cost, we based our calculations on the pricing of the most expensive model we tested. Proprietary models cost $\$ 5.00$ per 1 million output tokens, while open-source models cost $\$ 1.80$ per 1 million output tokens. + +Using these rates, the upper bound cost of querying a single model is: +Proprietary model: $5 . 0 0 \times { \frac { 5 1 2 \times 5 0 } { 1 0 ^ { 6 } } } = 0 . 1 2 8$ Open-source model: $1 . 8 0 \times { \frac { 5 1 2 \times 5 0 } { 1 0 ^ { 6 } } } = 0 . 0 4 6$ Assuming the training process requires 10 proprietary models and 20 open-source models, the overall data collection cost would be approximately $\$ 2.2$ per prompt. + +We collected data for 200 prompts in Section 2, so the cost is at most $\$ 440$ . + +# A.4 SIMULATION TESTBED + +Our simulation in Section 3 is based on an anonymized and deduplicated dataset of voting records from Chatbot Arena. The dataset includes 1,670,250 votes from 477,322 unique users, with 1,093,875 votes resulting in wins and 576,375 in ties. These votes cover 6,895 unique combinations of side-by-side model comparisons. + +# B MORE EXPERIMENTAL RESULTS + +B.1 TARGET MODEL DETECTION + +Table 7 presents the performance of identity-probing detector for all evaluated 22 models.10 + +Table 7: Averaged detection accuracy $( \% )$ with across 1,000 queries per prompt for different identity-probing prompts across various models. + +
ModelPrompt
Who are you?Which model are you?What is your model name?How should I refer to you as an AI?How would you define your role or identity as an AI?
claude-3-5-sonnet-2024062099.3100.098.5100.0100.0
claude-3-haiku-20240307100.096.3100.042.914.3
gemini-1.5-flash0.00.00.00.00.0
gemini-1.5-pro97.296.5100.00.099.1
gemma-2-27b-it100.098.498.297.995.5
gemma-2-2b-it81.891.858.212.74.5
gemma-2-9b-it98.599.498.398.197.3
gpt-3.5-turbo0.054.567.30.00.0
gpt-4-0125-preview70.9100.094.61.81.8
gpt-4-1106-preview7.390.999.16.41.8
gpt-4o-2024-05-1316.493.399.90.06.4
gpt-4o-2024-08-0651.897.798.50.05.5
gpt-4o-mini-2024-07-1892.792.9100.012.70.0
llama-3-70b-instruct98.298.254.546.42.7
llama-3-8b-instruct99.999.174.5 89.120.0 75.51.8
llama-3.1-405b-instruct99.190.992.75.50.0
llama-3.1-70b-instruct98.866.499.16.40.0
llama-3.1-8b-instruct17.340.0 31.845.51.80.0
mixtral-8x7b-instruct-v0.197.3 97.331.845.50.90.9
mixtral-8x22b-instruct-v0.198.297.624.51.8
qwen2-72b-instruct91.87.3
+ +# B.2 ADVERSARIAL VOTE + +Ablation for detector accuracy. Table 8 shows the number of votes and interactions needed to shift a model’s position by 1 to 50 places on the simulated leaderboard under different detector accuracies. As shown, the number of votes required to move a model up by 50 places increases by only about 150 when the detector accuracy drops from 1.0 to 0.9. This suggests that a detector, while not perfect, can still be sufficiently accurate to achieve the attack’s objective. + +Table 8: The number of votes (a) and interactions (b) required to change the ranking of a low-ranked model on the simulated leaderboard, under varying detector accuracy. + +
Target model=llama-13b (current rank: #129, #votes: 2443)Target rank: 79 (↑ 50)Target rank: 109 (↑ 20)Target rank: 119 (↑ 10)Target rank: 124(↑5)Target rank: 127 (↑ 2)Target rank: 1128 ( 1)
detector acc=1.01246861645415208126
detector acc=0.951304918682522255126
detector acc=0.913831012732525271136
(a) # Votes
Target model=llama-13b (current rank: #129, #votes: 2443)Target rank: 7 50)Target rank: 109 (↑ 20)Target rank: 19(10)Target rank: 124(5)Target rank: 12 (2)Target rank: 128 (1)
detector acc=1.0800005500040000300001500010000
detector acc=0.95850006500045000300001500010000
detector acc=0.91000007500055000400002000010000
+ +(b) # Interactions + +Ablation for non-detected actions. When the attacker does not detect the target model, they can choose from four actions: randomly upvote one model, vote for a tie, vote both models as bad, or do nothing. The main results in Section 3 assume the attacker does nothing. We also explore the other options in Table 9. As shown, there are no clear patterns indicating that any one option is significantly better than the others. + +Table 9: The number of interactions required to change the ranking of a high-ranked model (a) and a low-ranked model (b) on the simulated leaderboard, under varying non-target strategies. + +
Non-target strategyTarget rank: 1(↑ 4)Target rank: 2(↑ 3)Target rank: 3(↑ 2)Target rank: 4(↑ 1)
Do nothing20600018400014400018000
Randomly upvote19200018200014200016000
Vote tie19400018200014800020000
Vote tie (both bad)19600017200015200016000
(a) High-ranked model, claude-3-5-sonnet-20240620 (rank: #5)
Non-target strategyTarget rank: 79 (↑ 50)Target rank: 109 (↑ 20)Target rank: 119 (↑ 10)Target rank: 124 (↑ 5)Target rank: 127(↑ 2)Target rank: 128(↑1)
Do nothing800005500040000300001500010000
Randomly upvote750006000040000300001500010000
Vote tie800006000040000300001500010000
Vote tie (both bad)800006000040000300001500010000
+ +(b) Low-ranked model, llama-13b (rank: #129) \ No newline at end of file diff --git a/papers/voting-leaderboards/paper.pdf b/papers/voting-leaderboards/paper.pdf new file mode 100644 index 0000000000000000000000000000000000000000..4aae24463419bff12f0929b387fd328a72911c5f --- /dev/null +++ b/papers/voting-leaderboards/paper.pdf @@ -0,0 +1,3 @@ +version https://git-lfs.github.com/spec/v1 +oid sha256:d3e150f07ed2f29f028cf3e2cfb7d94d58b744839619fcdf1634da63c41c82d0 +size 1159779 diff --git a/papers/voting-leaderboards/sau.json b/papers/voting-leaderboards/sau.json new file mode 100644 index 0000000000000000000000000000000000000000..f115a938793b637b980bd5b74624a60ee72cdc86 --- /dev/null +++ b/papers/voting-leaderboards/sau.json @@ -0,0 +1,232 @@ +{ + "paper_id": "voting-leaderboards", + "paper_title": "Exploring and Mitigating Adversarial Manipulation of Voting-Based Leaderboards", + "D1": [ + { + "id": "voting-leaderboards-D1-001", + "claim": "Central experiment constants shared across all detector evaluations and simulations: 22 evaluated models (listed in Table 6, Appendix A.1); 5 identity-probing prompts; 1,000 queries per prompt for identity-probing evaluation; 8 prompt categories (Table 1: English, Chinese, Spanish, Indonesian, Persian, Coding, Math, Safety-violating); 200 prompts sampled per category; 50 responses collected per model per prompt; 512 output tokens per response; 80/20 train/test split; random_state=42; scikit-learn LogisticRegression with default hyperparameters; detection accuracy assumption of 95% with symmetric 5% FPR/FNR for simulations; attacker abstains when target model not detected; Bradley-Terry ranking computed from historical voting data; rankings recomputed every 1,000 simulated interactions.", + "source": "Section 2.3, Section 3.1, Appendix A.3" + }, + { + "id": "voting-leaderboards-D1-002", + "claim": "The 22 models used in all experiments, with organization and query method: claude-3-5-sonnet-20240620 (Anthropic API), claude-3-haiku-20240307 (Anthropic API), gemini-1.5-flash (Google AI studio), gemini-1.5-pro (Google AI studio), gemma-2-2b-it (Together AI), gemma-2-9b-it (Together AI), gemma-2-27b-it (Together AI), gpt-3.5-turbo (OpenAI), gpt-4-0125-preview (OpenAI), gpt-4-1106-preview (OpenAI), gpt-4-turbo-2024-04-09 (OpenAI), gpt-4o-2024-05-13 (OpenAI), gpt-4o-2024-08-06 (OpenAI), gpt-4o-mini-2024-07-18 (OpenAI), llama-3-8b-instruct (Together AI), llama-3-70b-instruct (Together AI), llama-3.1-8b-instruct (Together AI), llama-3.1-70b-instruct (Together AI), llama-3.1-405b-instruct (Together AI), mixtral-8x7b-instruct-v0.1 (Together AI), mixtral-8x22b-instruct-v0.1 (Together AI), qwen2-72b-instruct (Together AI).", + "source": "Appendix A.1 (Table 6)" + }, + { + "id": "voting-leaderboards-D1-003", + "claim": "Parameters for the identity-probing detector evaluation: 5 identity-probing prompts ('Who are you?', 'Which model are you?', 'What is your model name?', 'How should I refer to you as an AI?', 'How would you define your role or identity as an AI?'); 1,000 queries per prompt per model; string matching on model name (e.g., 'Llama') or organization (e.g., 'Meta') in response; average accuracy reported per prompt per model.", + "source": "Section 2.3, Section 2.4.1" + }, + { + "id": "voting-leaderboards-D1-004", + "claim": "Eight prompt categories used to build the training-based detector, their sources, and categories. Normal chats in high-resource languages: English, Chinese, Spanish (source: LMSYS-Chat-1M). Normal chats in low-resource languages: Indonesian, Persian (source: LMSYS-Chat-1M). Specialty chats: Coding (source: Alpaca Code), Math (source: MATH), Safety-violating (source: AdvBench). See Table 1 for examples.", + "source": "Section 2.3 (Table 1)" + }, + { + "id": "voting-leaderboards-D1-005", + "claim": "Three text feature types used to distinguish model responses for training-based detection: Length(R) measured in words or characters, TF-IDF(R) (Salton & Buckley, 1988), BoW(R) bag-of-words (Salton et al., 1975). Evaluated independently per (P, M) pair.", + "source": "Section 2.3" + }, + { + "id": "voting-leaderboards-D1-006", + "claim": "Fixed hyperparameters for the logistic regression classifier and hypothesis tests: scikit-learn LogisticRegression with default hyperparameters; random_state=42; 80/20 train/test split; 50 positive samples (target model M) and 50 uniformly sampled negative samples (other models) per (P, M) pair; alpha=0.01 significance level for hypothesis tests in malicious user detection (Section 4.2.3).", + "source": "Section 2.3, Section 4.2.3" + }, + { + "id": "voting-leaderboards-D1-007", + "claim": "Assumed detection accuracy and error rates used in the vote count simulation: 95% detection accuracy, symmetric 5% false positive rate and 5% false negative rate. Attacker remains passive (abstains) when failing to detect the target model in the sampled response pair.", + "source": "Section 3.1" + }, + { + "id": "voting-leaderboards-D1-008", + "claim": "Characteristics of the anonymized and deduplicated Chatbot Arena dataset used for the simulation testbed: 1,670,250 votes from 477,322 unique users; 1,093,875 votes resulting in wins, 576,375 in ties; 6,895 unique combinations of side-by-side model comparisons.", + "source": "Appendix A.4" + }, + { + "id": "voting-leaderboards-D1-009", + "claim": "Two manipulation objectives defined for the vote count simulation: Up(M, x) — manipulate model M to rise x positions in the leaderboard; Down(M, x) — manipulate model M to fall x positions in the leaderboard.", + "source": "Section 3.1" + }, + { + "id": "voting-leaderboards-D1-010", + "claim": "Token pricing rates used to calculate the upper-bound cost of building the detector: proprietary models $5.00 per 1M output tokens (via Anthropic/OpenAI/Google APIs); open-source models $1.80 per 1M output tokens (via Together AI Inference API). All models use default decoding hyperparameters (temperature, etc.). Output length set to 512 tokens per response. 50 responses collected per model per prompt. Upper-bound data collection cost estimated at ~$440 for 200 prompts across 22 models.", + "source": "Appendix A.3" + }, + { + "id": "voting-leaderboards-D1-011", + "claim": "Detection accuracy levels swept in the ablation study: 1.0 (perfect detector), 0.95 (baseline), 0.9. Target model: llama-13b (current rank #129, 2,443 base votes). Rank shift targets: 1, 2, 5, 10, 20, 50 positions. All other simulation parameters as in Section 3.1.", + "source": "Appendix B.2 (Table 8)" + }, + { + "id": "voting-leaderboards-D1-012", + "claim": "Four alternative strategies evaluated when the attacker fails to detect the target model: (1) do nothing / abstain; (2) randomly upvote one model in the pair; (3) vote tie; (4) vote tie (both bad). Tested on high-ranked model claude-3-5-sonnet-20240620 (rank #5, 7,703 base votes) with rank shifts 1-4, and low-ranked model llama-13b (rank #129, 2,443 base votes) with rank shifts 1, 2, 5, 10, 20, 50.", + "source": "Appendix B.2 (Table 9)" + }, + { + "id": "voting-leaderboards-D1-013", + "claim": "Parameters of the attack cost model c_total = ceil(N/m) * c_account + N * c_action + c_detector: N = total actions (interactions + votes), m = max actions per user account (rate limit), c_account = cost per account, c_action = cost per action, c_detector = one-time offline detector training cost (~$440). These are design parameters set by the defender. Without mitigations: m unlimited, c_action minimal, total cost dominated by c_detector.", + "source": "Section 4.1" + } + ], + "D2": [ + { + "id": "voting-leaderboards-D2-001", + "claim": "De-anonymization as Binary Classification | Problem: Given target model M from public model set M_set, build classifier f_M(P, R) -> {0, 1}\n\nFormal definition:\n Input: prompt P, response R from unknown model\n Output: 1 if R ~ M(P), 0 otherwise\n \nAssumption: M(P) != M'(P) for distinct M != M'\n (different models produce observably distinct output distributions given the same input prompt)\n\nClass labeling:\n Class 1 = target model M (the model we want to de-anonymize)\n Class 0 = all other mode", + "source": "Section 2.1" + }, + { + "id": "voting-leaderboards-D2-002", + "claim": "Identity-Probing Detector | Algorithm: identity_probing_detector(P, target_model_name, target_org_name, model_api)\n\n # Step 1: Select an identity-probing prompt\n prompts = [\n \"Who are you?\",\n \"Which model are you?\",\n \"What is your model name?\",\n \"How should I refer to you as an AI?\",\n \"How would you define your role or identity as an AI?\"\n ]\n prompt = sample(prompts)\n \n # Step 2: Query the unknown model via the arena\n response = query_arena(prompt) # arena returns anonymiz", + "source": "Section 2.2, Section 2.3" + }, + { + "id": "voting-leaderboards-D2-003", + "claim": "Training-Based Detector — Data Collection and Feature Extraction | Phase 1: Data Collection\n\n For each prompt P in selected_prompts:\n # Collect positive samples from target model M\n D_pos = []\n For i = 1 to n_responses_per_model (n=50):\n R ~ M(P) # query target model M with prompt P, output 512 tokens\n D_pos.append((P, R, label=1))\n \n # Collect negative samples from all other models\n D_neg = []\n For each M_other in M_set \\ {M}:\n For i = 1 to n_responses_per_model (n=50):\n R ~ M_other(P) # query each other model with prompt P, output 512 tokens\n D_neg.append((P, R, label=0))\n \n # Uniformly sample 50 negative responses from pool of all other models\n D_neg_sampled = uniform_sample(D_neg, n=50)\n \n # Build balanced dataset for this (P, M) pair\n D_P_M = D_pos + D_neg_sampled # 100 samples total: 50 pos + 50 neg\n\n Phase 2: Feature Extraction\n\n Three text feature types evaluated independently for each response R:\n\n (1) Length(R)_word : response length in number of words\n (2) Length(R)_character: response length in number of characters\n (3) BoW(R) : bag-of-words representation (Salton et al., 1975)\n (4) TF-IDF(R) : term frequency-inverse document frequency (Salton & Buckley, 1988)\n\n For each feature type, extract feature matrix X and label vector y from D_P_M.\n Proceed to Phase 3 for logistic regression training with 80/20 train/test split\n (random_state=42, scikit-learn defaults).", + "source": "Section 2.2, Section 2.3" + }, + { + "id": "voting-leaderboards-D2-004", + "claim": "Training-Based Detector — Logistic Regression Classifier | Phase 3: Training and Evaluation (per prompt-model pair (P, M))\n\n from sklearn.linear_model import LogisticRegression\n from sklearn.model_selection import train_test_split\n \n # Extract features from balanced dataset D_P_M = D_pos + D_neg_sampled\n X = [] # feature vectors (one of: length, BoW, or TF-IDF — evaluated independently)\n y = [] # labels: 1 for target M, 0 for other models\n for (prompt, response, label) in D_P_M:\n X.a", + "source": "Section 2.3" + }, + { + "id": "voting-leaderboards-D2-005", + "claim": "Attack Cost Model | Total cost of attack:\n\n c_total = ceil(N / m) * c_account + N * c_action + c_detector\n\nwhere:\n N : total number of actions (interactions + votes) required by the attack\n m : maximum actions permitted per user account (rate limit threshold)\n c_account : cost of obtaining a single user account\n c_action : cost per individual action (vote or interaction)\n c_detector : one-time offline cost of building the training-based target model detector\n\nWithout mitigations:", + "source": "Section 4.1, Section 4.2" + }, + { + "id": "voting-leaderboards-D2-006", + "claim": "Malicious User Detection — Scenario 1: Known Benign Distribution (Likelihood Ratio Test) | Scenario: Defender can estimate expected benign user behavior from historical voting data.\n\nGiven: observed vote sequence x = (x_1, x_2, ..., x_n) from a user\n where each x_i is a vote for one of the available models\n\nHypotheses:\n H_benign: user's votes follow the known benign distribution\n H_not_benign: user is from a different (malicious) source\n\nUnder H_benign (votes independent):\n\n L(x | H_be", + "source": "Section 4.2.3 (Eq 1, Eq 2, Eq 3)" + }, + { + "id": "voting-leaderboards-D2-007", + "claim": "Bradley-Terry Win Probability | Given two models i and j with Bradley-Terry coefficient ratings Q_i and Q_j:\n\n Pr(i preferred over j) = 1 / (1 + exp(-(Q_i - Q_j) / s))\n\nwhere:\n Q_i, Q_j : Bradley-Terry coefficient ratings for models i and j\n s : scaling factor that determines sensitivity to rating difference\n (larger s = less sensitive; default typically s = 400 in Elo-like systems)\n\nThis logistic function maps the rating difference (Q_i - Q_j) to a probability in (0, 1).", + "source": "Section 4.2.3, Section 3.1" + }, + { + "id": "voting-leaderboards-D2-008", + "claim": "Pr_B(i) Computation from Bradley-Terry Ratings (Benign User Vote Distribution) | Computing the probability that a benign user votes for model i:\n\n Pr_B(i) = prod_{j != i} Pr(i preferred over j | true Bradley-Terry ratings)\n\nwhere:\n Pr(i preferred over j) = 1 / (1 + exp(-(Q_i - Q_j) / s))\n Q_i, Q_j : true (unperturbed) Bradley-Terry coefficient ratings\n j iterates over all other models j != i\n\nThe product over all pairwise comparisons gives the joint probability that model i is preferred over all other models j (j != i).", + "source": "Section 4.2.3" + }, + { + "id": "voting-leaderboards-D2-009", + "claim": "Malicious User Detection — Scenario 2: Neyman-Pearson Likelihood Ratio Test (Perturbed Ratings) | Scenario: Defender releases perturbed Bradley-Terry ratings. Attacker who mimics the perturbed distribution can be detected via likelihood ratio.\n\nLikelihood Ratio (Neyman-Pearson Lemma):\n\n Lambda(x) = Pr_negB(x) / Pr_B(x)\n\nwhere:\n Pr_B(x) = prod_{i=1}^{n} Pr_B(x_i) # likelihood under benign distribution\n Pr_negB(x) = prod_{i=1}^{n} Pr_negB(x_i) # likelihood under malicious distribution", + "source": "Section 4.2.3, Section 4.3" + }, + { + "id": "voting-leaderboards-D2-010", + "claim": "Vote Count Simulation Algorithm | Algorithm: simulate_adversarial_vote_count(target_model, objective, direction, \n detection_accuracy, non_detection_strategy)\n\n # Load historical voting data\n data = load_chatbot_arena_data() # anonymized, deduplicated, 1.67M votes\n \n # Initialize Bradley-Terry coefficients from historical data\n bt_coefficients = compute_bradley_terry(data)\n model_rankings = rank_by_bt(bt_coefficients)\n initial_rank = model_rankings[t", + "source": "Section 3.1, Section 3.2" + } + ], + "D3": [ + { + "id": "voting-leaderboards-D3-001", + "claim": "Evaluate identity-probing detector accuracy on 22 models. For each identity-probing prompt, query each model 1,000 times, then apply string matching to check if the model's name (e.g., 'Llama') or organization (e.g., 'Meta') appears in the response. Report average detection accuracy per prompt per model. Note: Chatbot Arena already filters votes mentioning model names, making this detector less practical for real-world attacks. | Data: 22 evaluated models from 6 families (Claude, Gemini, GPT, LLaMA, Mixtral, Qwen) via provider APIs | Baselines: N/A (descriptive evaluation) | Metrics: detection accuracy (%) = (correct detections / 1000) * 100 | Results: Table 2 (Section 2.4.1) for 7 representative models, Table 7 (Appendix B.1) for all 22 models", + "source": "Section 2.3, Section 2.4.1, Appendix B.1 (Table 7)" + }, + { + "id": "voting-leaderboards-D3-002", + "claim": "Evaluate training-based detector accuracy on 22 models across 8 prompt categories. For each category, sample 200 prompts. For each (prompt, target_model) pair: collect 50 responses from target model M (class 1) and 50 uniformly sampled responses from other models (class 0); extract three feature types independently (response length in words/chars, Bag-of-Words, TF-IDF); train logistic regression classifier with scikit-learn defaults and random_state=42 on 80/20 train/test split; report test accuracy averaged across all prompts per category per model. Visualize model response clusters via PCA on BoW features. | Data: 22 models, 8 prompt categories (Table 1), 200 prompts per category, 50 responses per model per prompt | Baselines: N/A (descriptive evaluation) | Metrics: test accuracy (%) per (prompt, model, feature) triplet, averaged per category | Results: Table 3 for feature type comparison, Figure 3 for per-category per-model accuracy heatmap", + "source": "Section 2.3, Section 2.4.2" + }, + { + "id": "voting-leaderboards-D3-003", + "claim": "Simulate the number of adversarial votes and interactions needed to shift the leaderboard rank of high-ranked models (ranks 1-5). Using historical Chatbot Arena voting data with Bradley-Terry rankings, iteratively simulate attacker interactions under two objectives: Up(M,x) — raise model M by x positions; Down(M,x) — lower model M by x positions. The attacker operates with 95% detection accuracy (symmetric 5% false positive/negative rates), abstaining when the target model is not detected. Update Bradley-Terry coefficients and re-rank models after every 1,000 interactions. Track cumulative interactions and votes required to achieve each objective. | Data: Historical Chatbot Arena voting data (1.67M votes, anonymized and deduplicated) | Baselines: N/A (counterfactual simulation) | Metrics: number of adversarial votes and interactions required to move model by x positions | Results: Table 4 (high-ranked models: chatgpt-4o-latest, gemini-1.5-pro-exp-0801, gpt-4o-2024-05-13, gpt-4o-mini-2024-07-18, claude-3-5-sonnet-20240620)", + "source": "Section 3.1, Section 3.2, Appendix A.4" + }, + { + "id": "voting-leaderboards-D3-004", + "claim": "Same simulation protocol as for high-ranked models, but applied to low-ranked models (ranks 125-129) which receive fewer historical votes and are thus more vulnerable to adversarial manipulation. Compare vote requirements with high-ranked model results to quantify the rank-dependent vulnerability gradient. | Data: Historical Chatbot Arena voting data (same as high-ranked simulation), Low-ranked target models: chatglm-6b (rank 125, 4995 votes), fastchat-t5-3b (rank 126, 4304 votes), stablelm-tuned-alpha-7b (rank 127, 3334 votes), dolly-v2-12b (rank 128, 3484 votes), llama-13b (rank 129, 2443 votes) | Baselines: High-ranked model results from Table 4 | Metrics: number of adversarial votes and interactions required to move model by x positions | Results: Table 5. Low-ranked models require ~30% of the votes of high-ranked models for equivalent rank shifts", + "source": "Section 3.2, Table 5" + }, + { + "id": "voting-leaderboards-D3-005", + "claim": "Evaluate the effectiveness of Scenario 1 malicious user detection: the defender uses historical voting data to estimate benign user vote preferences. A likelihood ratio test (alpha=0.01) with Monte Carlo p-value computation is used to detect users whose voting patterns deviate from the benign distribution. Test two attacker types: (a) naive adversary who randomly chooses between two non-targeted models when the target is absent; (b) informed adversary who uses publicly available Bradley-Terry ratings to mimic benign voting proportions. Compare detection rates between naive and informed adversaries. | Data: Historical Chatbot Arena voting data (anonymized user voting records) | Baselines: Benign user vote distribution estimated from historical data | Metrics: detection rate (fraction of malicious users flagged at alpha=0.01) | Results: Figure 4. Naive adversary detectable via likelihood test; informed adversary evades detection when using public rankings", + "source": "Section 4.2.3, Section 4.3" + }, + { + "id": "voting-leaderboards-D3-006", + "claim": "Evaluate Scenario 2 malicious user detection: the defender releases a perturbed version of the Bradley-Terry leaderboard by adding scaled Gaussian noise to true model ratings. The attacker, when using these perturbed ratings to mimic benign behavior, can be distinguished from true benign users via a Neyman-Pearson likelihood ratio test. Sweep noise scales to characterize the security-utility tradeoff: higher noise improves detection but degrades leaderboard accuracy. Measure detection rate of malicious users and utility degradation (avg absolute rank change) for each noise scale. | Data: Historical Chatbot Arena voting data and Bradley-Terry coefficient ratings | Baselines: Scenario 1 (unperturbed ratings, informed adversary) | Metrics: detection rate, utility = avg |rank_true(i) - rank_perturbed(i)| across all models | Results: Figures 5 and 6. Detection improves with noise; utility degrades (larger noise significantly changes ranking order)", + "source": "Section 4.2.3, Section 4.3" + }, + { + "id": "voting-leaderboards-D3-007", + "claim": "Measure the one-time offline cost (c_detector) of building the training-based target model detector. Cost is computed as sum of API token fees for collecting responses from all 22 models across 200 prompts (50 responses per model per prompt, 512 output tokens per response). Proprietary and open-source models use different API pricing tiers. | Data: 22 models: proprietary (via provider APIs: Anthropic, Google AI Studio, OpenAI) + open-source (via Together AI Inference API) | Baselines: Upper bound based on most expensive model pricing: proprietary $5.00/1M tokens, open-source $1.80/1M tokens | Metrics: total cost in USD | Results: ~$0.128 per proprietary model per prompt, ~$0.046 per open-source model per prompt; ~$2.2 per prompt total; ~$440 total for 200 prompts", + "source": "Appendix A.3" + }, + { + "id": "voting-leaderboards-D3-008", + "claim": "Ablation study on detector accuracy: vary the assumed detection accuracy of the training-based detector (1.0, 0.95, 0.9) and measure how the required number of adversarial votes and interactions changes for shifting a low-ranked model (llama-13b, initial rank #129) by 1 to 50 positions. Tests the sensitivity of attack cost to detector quality. | Data: Historical Chatbot Arena voting data, Target model: llama-13b (rank #129, 2443 base votes) | Baselines: detector_acc = 1.0 (perfect detector), detector_acc = 0.95 (main experiment baseline), detector_acc = 0.9 (degraded detector) | Metrics: number of adversarial votes and interactions required | Results: Table 8. Dropping accuracy from 1.0 to 0.9 increases required votes by only ~150 for a 50-position shift, suggesting even imperfect detectors remain effective", + "source": "Appendix B.2, Table 8" + }, + { + "id": "voting-leaderboards-D3-009", + "claim": "Ablation study on non-detection actions: when the attacker fails to detect the target model in the comparison pair (due to FNR or absence), evaluate four alternative strategies for the attacker's behavior. Measure how each strategy affects the number of interactions required to shift a model's ranking. Tests robustness of simulation results to the attacker's fallback behavior. | Data: Historical Chatbot Arena voting data, High-ranked target model: claude-3-5-sonnet-20240620 (rank #5, 7703 base votes), Low-ranked target model: llama-13b (rank #129, 2443 base votes) | Baselines: do_nothing (abstain, used in main experiments) | Metrics: number of interactions required to achieve target rank | Results: Table 9. No clear patterns indicating any strategy significantly outperforms others", + "source": "Appendix B.2, Table 9" + } + ], + "D4": [ + { + "id": "voting-leaderboards-D4-001", + "claim": "Method sequence: Identity-Probing Detector -> Training-Based Detector -> De-anonymization as Binary Classification", + "source": "Section 2.1, Section 2.2" + }, + { + "id": "voting-leaderboards-D4-002", + "claim": "Method sequence: Training-Based Detector — Data Collection and Feature Extraction -> Training-Based Detector — Logistic Regression Classifier -> De-anonymization as Binary Classification", + "source": "Section 2.2, Section 2.3" + }, + { + "id": "voting-leaderboards-D4-003", + "claim": "Method sequence: Malicious User Detection — Scenario 2: Neyman-Pearson Likelihood Ratio Test (Perturbed Ratings) -> Bradley-Terry Win Probability -> Pr_B(i) Computation from Bradley-Terry Ratings (Benign User Vote Distribution)", + "source": "Section 4.2.3, Section 4.3" + }, + { + "id": "voting-leaderboards-D4-004", + "claim": "Identity-Probing Detector evaluation phases: For each of the 5 identity-probing prompts: -> For each of the 22 models: -> Query each model 1,000 times with the prompt (use default decoding hyperparameters) -> Apply string matching: response contains model_name (e.g., 'Llama') OR organization name (e.g., 'Meta') -> Compute accuracy = (correct detections / 1000) * 100% -> Report per-model averaged accuracy across 1,000 queries for each prompt", + "source": "Section 2.3, Section 2.4.1, Appendix B.1 (Table 7)" + }, + { + "id": "voting-leaderboards-D4-005", + "claim": "Experiment phases: For each of the 8 prompt categories: -> Sample 200 prompts from the category's source dataset -> For each prompt P: -> For each target model M in 22 models: -> Step 1 - Data collection: -> Query M 50 times with P (512 output tokens, default decoding params) -> Query each other model, uniformly sample 50 responses total as negative -> Build balanced dataset: 50 positive + 50 negative = 100 samples -> Step 2 - For each of the 3 feature types independently: -> Extract features: length(Response) OR BoW(Response) OR TF-IDF(Response) -> Split: 80% train, 20% test (random_state=42) -> Train: sklearn.linear_model.LogisticRegression(random_state=42, default hyperparams) -> Evaluate: test accuracy on held-out 20% -> Step 3 - Record test accuracy for this (P, M, feature_type) triplet -> Average per-category accuracy across all 200 prompts per model -> Visualize BoW features: compute PCA on BoW vectors across all models' responses for 3 selected prompts", + "source": "Section 2.3, Section 2.4.2" + }, + { + "id": "voting-leaderboards-D4-006", + "claim": "Experiment phases: Step 1: Initialize Bradley-Terry coefficients from historical voting data -> Step 2: For each target model M and each objective (Up/Down by 1-4 positions): -> Set target_rank = current_rank +/- displacement -> While current_rank != target_rank: -> Simulate 1 interaction: -> Randomly sample 2 models for the comparison -> Attacker applies de-anonymization (detection_accuracy=0.95, symmetric FPR/FNR=0.05) -> If target model detected: cast adversarial vote (upvote or downvote) -> Else: abstain (do nothing) -> Increment interactions counter -> Every 1,000 interactions: -> Recompute Bradley-Terry coefficients from all votes (including injected adversarial votes) -> Re-rank models -> Record final (interactions, adversarial_votes) for this objective -> Step 3: Report results per target model per target rank", + "source": "Section 3.1, Section 3.2, Appendix A.4" + }, + { + "id": "voting-leaderboards-D4-007", + "claim": "Experiment phases: Same as high-ranked simulation protocol, applied to the 5 low-ranked target models (chatglm-6b rank #125, fastchat-t5-3b rank #126, stablelm-tuned-alpha-7b rank #127, dolly-v2-12b rank #128, llama-13b rank #129). Compare: low-ranked models require ~30% of votes needed by high-ranked models to move the same number of positions", + "source": "Section 3.2, Table 5" + }, + { + "id": "voting-leaderboards-D4-008", + "claim": "Experiment phases: Step 1: Estimate benign vote distribution Pr_B(i) from historical voting data -> Step 2: Simulate naive adversary: -> For each target model, adversary always votes for target when detected -> When target not in pair: randomly choose between the two non-targeted models -> Compute likelihood L(x|H_benign) and test statistic T(x) for the observed vote sequence -> Compute empirical p-value via m Monte Carlo simulations under H_benign -> Flag as malicious when p < alpha=0.01 -> Step 3: Simulate informed adversary: -> Adversary uses publicly available Bradley-Terry ratings to compute expected behavior -> When target not in pair: vote proportionally to public rankings to mimic benign users -> Repeat likelihood ratio detection process -> Step 4: Compare detection effectiveness for naive vs. informed adversary", + "source": "Section 4.2.3, Section 4.3" + }, + { + "id": "voting-leaderboards-D4-009", + "claim": "Experiment phases: Step 1: For each noise scale sigma in [sigma_min, ..., sigma_max]: -> Generate perturbed ratings: Q'_i = Q_i + N(0, sigma^2) for each model i -> Release Q'_i as the public leaderboard -> Measure utility: avg |rank_true(i) - rank_perturbed(i)| across all models -> Step 2: Simulate adversary using perturbed ratings: -> Compute Pr_negB(i) from perturbed Q'_i using Bradley-Terry formula -> Generate adversarial vote sequences following Pr_negB(i) -> Step 3: Apply Neyman-Pearson likelihood ratio test: -> Lambda(x) = Pr_negB(x) / Pr_B(x) -> Flag as malicious when Lambda(x) > threshold (set by desired FPR) -> Measure detection rate -> Step 4: Plot detection rate vs. utility for each noise scale", + "source": "Section 4.2.3, Section 4.3" + }, + { + "id": "voting-leaderboards-D4-010", + "claim": "Experiment phases: Step 1: Determine query method and pricing tier for each of the 22 models -> Step 2: For each prompt P (out of 200): -> For each model M: -> Query M with P, 512 output tokens, 50 times -> Tally cost: cost += price_per_1M_tokens * (512 * 50) / 1e6 -> Step 3: Sum costs across all models and prompts -> Step 4: Report total c_detector and per-prompt cost breakdown", + "source": "Appendix A.3" + }, + { + "id": "voting-leaderboards-D4-011", + "claim": "Experiment phases: For each detector_acc in [1.0, 0.95, 0.9]: -> For each rank shift target in [1, 2, 5, 10, 20, 50]: -> Run the vote count simulation (same protocol as high-ranked and low-ranked rank-shift simulations) -> with detection_accuracy = detector_acc -> target_rank = 129 - rank_shift -> Record (votes, interactions) upon achieving the target rank -> Compare vote counts across detector accuracy levels", + "source": "Appendix B.2, Table 8" + }, + { + "id": "voting-leaderboards-D4-012", + "claim": "Experiment phases: For each non_detection_strategy in [do_nothing, randomly_upvote_one, vote_tie, vote_tie_both_bad]: -> For high-ranked model (claude-3-5-sonnet): -> Run simulation with target rank shifts 1, 2, 3, 4 (up) -> Record interactions required -> For low-ranked model (llama-13b): -> Run simulation with target rank shifts 1, 2, 5, 10, 20, 50 (up) -> Record interactions required -> Compare: no clear patterns favoring any strategy over others", + "source": "Appendix B.2, Table 9" + } + ] +} \ No newline at end of file diff --git a/papers/wdno/blacklist.txt b/papers/wdno/blacklist.txt new file mode 100644 index 0000000000000000000000000000000000000000..5f6a271d0dd134cac0cd9d05984f6b767783d3c3 --- /dev/null +++ b/papers/wdno/blacklist.txt @@ -0,0 +1,2 @@ +# Official repository (ICLR 2025) +https://github.com/AI4Science-WestlakeU/wdno diff --git a/papers/wdno/config.yaml b/papers/wdno/config.yaml new file mode 100644 index 0000000000000000000000000000000000000000..5f535d1f962eedca0e364c28e5269f1693a6212e --- /dev/null +++ b/papers/wdno/config.yaml @@ -0,0 +1,8 @@ +title: "Wavelet Diffusion Neural Operator (WDNO)" +pdf_url: "https://arxiv.org/pdf/2412.04833.pdf" +venue: "ICLR 2025" +year: "2025" +extra: + selection_index: 25 + domain: "Numerical Methods / Scientific Computing" + paradigm: "New Algorithm / Architecture" diff --git a/papers/wdno/images/figures/wdno-fig-0001.jpg b/papers/wdno/images/figures/wdno-fig-0001.jpg new file mode 100644 index 0000000000000000000000000000000000000000..0865847c9441f7641f0a6cafd435a5bb9414376f --- /dev/null +++ b/papers/wdno/images/figures/wdno-fig-0001.jpg @@ -0,0 +1,3 @@ +version https://git-lfs.github.com/spec/v1 +oid sha256:b22cc031fb99cb6a2ede1662f842b8a51528a80be894ce0460281626373b0318 +size 60823 diff --git a/papers/wdno/images/figures/wdno-fig-0002.jpg b/papers/wdno/images/figures/wdno-fig-0002.jpg new file mode 100644 index 0000000000000000000000000000000000000000..a3c9f4db0afe4fe9ccc4f47b8b5b3f7d08658940 --- /dev/null +++ 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+Peiyan $\mathbf { H } \mathbf { u } ^ { 2 \ S * }$ Rui Wang3§∗ Xiang Zheng4§ Tao Zhang1 Haodong Feng1 Ruiqi Feng1 Long Wei1 Yue Wang5 Zhi-Ming $\mathbf { M } \mathbf { a } ^ { 2 }$ Tailin ${ \bf W } { \bf u } ^ { \mathrm { 1 \dagger } }$ + +1Department of Artificial Intelligence, School of Engineering, Westlake University, 2Academy of Mathematics and Systems Science, Chinese Academy of Sciences, 3Fudan University, 4South China University of Technology, 5Microsoft AI4Science {hupeiyan,wutailin}@westlake.edu.cn, ruiwang18@fudan.edu.cn + +# ABSTRACT + +Simulating and controlling physical systems described by partial differential equations (PDEs) are crucial tasks across science and engineering. Recently, diffusion generative models have emerged as a competitive class of methods for these tasks due to their ability to capture long-term dependencies and model high-dimensional states. However, diffusion models typically struggle with handling system states with abrupt changes and generalizing to higher resolutions. In this work, we propose Wavelet Diffusion Neural Operator (WDNO), a novel PDE simulation and control framework that enhances the handling of these complexities. WDNO comprises two key innovations. Firstly, WDNO performs diffusion-based generative modeling in the wavelet domain for the entire trajectory to handle abrupt changes and long-term dependencies effectively. Secondly, to address the issue of poor generalization across different resolutions, which is one of the fundamental tasks in modeling physical systems, we introduce multi-resolution training. We validate WDNO on five physical systems, including 1D advection equation, three challenging physical systems with abrupt changes (1D Burgers’ equation, 1D compressible Navier-Stokes equation and 2D incompressible fluid), and a real-world dataset ERA5, which demonstrates superior performance on both simulation and control tasks over state-of-the-art methods, with significant improvements in long-term and detail prediction accuracy. Remarkably, in the challenging context of the 2D highdimensional and indirect control task aimed at reducing smoke leakage, WDNO reduces the leakage by $78 \%$ compared to the second-best baseline. The code can be found at https://github.com/AI4Science-WestlakeU/wdno.git. + +# 1 INTRODUCTION + +Many systems across science and engineering are described by partial differential equations (PDEs). Simulating and controlling these PDE systems are fundamental tasks with numerous applications, including weather forecasting (Lynch, 2008), controlled nuclear fusion (Carpanese, 2021), astronomical simulation (Courant et al., 1967), and aviation (Paranjape et al., 2013). + +With developments of neural networks, deep learning-based methods have emerged to address this problem (Li et al., 2021; Lu et al., 2021; Tripura & Chakraborty, 2022; Hu et al., 2022). Among them, diffusion generative models (Ho et al., 2020b) achieve impressive results in both simulation (Cachay et al., 2023; Price et al., 2023; Rühling Cachay et al., 2023) and control (Ajay et al., 2022; Chi et al., 2023; Wei et al., 2024). On the one hand, simulation and control tasks are typically long-term, where small variations in the early stage can have a long-term impact on the full trajectory, making their accurate prediction and control difficult. Diffusion models alleviate the long-term challenge by the noise-learning mechanism and recovering the full trajectory from a Gaussian distribution as a whole. Therefore, they can better capture long-term dynamics and generate coherent plans for certain goals (Janner et al., 2022; Chi et al., 2023; Wei et al., 2024). On the other hand, PDE dynamics are typically high-dimensional and nonlinear, and the diffusion model demonstrates strong capabilities in modeling complex high-dimensional data (Ho et al., 2022; Harvey et al., 2022; Vahdat et al., 2022; Li et al., 2024). See Appendix D for more related works. + +![](images/figures/wdno-fig-0001.jpg) +Figure 1: Overview of WDNO. The figure shows the training and inference of the Base-Resolution Model (BRM) and Super-Resolution Model (SRM). + +However, for PDE simulation and control with diffusion models, two key challenges arise. Firstly, the evolution of physical systems is often accompanied by abrupt changes, which reflect key mechanisms of the system (Ben-Dor & Ben-Dor, 2007; Rassweiler et al., 2011). Due to their rapid and intense local variations, and even discontinuities, these changes are difficult to capture. Secondly, existing diffusion models typically operate on a fixed spatial-temporal resolution, and cannot generalize to finer resolutions (Croitoru et al., 2023; Yue et al., 2024; Shang et al., 2024), which is a fundamental requirement of neural PDE solvers (Li et al., 2021; Boussif et al., 2022; Yin et al., 2022). + +In this work, we introduce Wavelet Diffusion Neural Operator (WDNO) to address the above two challenges. Our WDNO method consists of two key innovations: (1) Generation in the wavelet domain. Since the wavelet transform is both space and frequency localized and excels at approximating functions with abrupt changes (Tripura & Chakraborty, 2022), generation in the wavelet space endowed by the wavelet transform is ideal for modeling abrupt changes. Besides, due to the linearity and locality of the wavelet transform, it can integrate seamlessly with the multi-resolution training. (2) Multi-resolution training. To enable generalization to finer resolutions, we prepare training datasets across multiple spatial and temporal resolutions utilizing the approximate scale invariance. Since changes of the equation forms are approximately the same across resolutions, the model is trained to generalize to finer resolutions conditioned on coarser resolutions, which opens up the capability to generalize to even finer resolutions not seen during training. + +Concretely, our contributions include the following: (1) We introduce the WDNO method that comprises diffusion in the wavelet space, addressing the challenges of modeling states with abrupt changes in simulation and control. (2) We propose multi-resolution training to address the issue of poor generalization to higher-resolution simulations, which is a fundamental task in PDE modeling. (3) We evaluate our method on 1D advection equation, complex PDEs with abrupt changes including 1D Burgers’ equation, 1D compressible fluid, and 2D incompressible fluid, and a real-world dataset ERA5. Compared with strong baselines in physical simulation and control, our method shows competitive performance. Particularly, the 2D experiments are extremely challenging as they involve indirect control with 3,584 spatial control variables at each time step, for a total of 32 time steps. It is noteworthy that WDNO reduces $79 \%$ of the leaked smoke compared to the prior state-of-the-art. + +# 2 PRELIMINARY + +# 2.1 PROBLEM SETUP + +We consider a PDE on $[ 0 , T ] \times D \subset \mathbb { R } \times \mathbb { R } ^ { d }$ with the following form + +$$ +\begin{array} { c } { \displaystyle \frac { \partial u } { \partial t } = F \left( u , \displaystyle \frac { \partial u } { \partial x } , \displaystyle \frac { \partial ^ { 2 } u } { \partial x ^ { 2 } } , \ldots \right) + f ( t , x ) , \quad ( t , x ) \in [ 0 , T ] \times D , } \\ { u ( 0 , x ) = u _ { 0 } ( x ) , \quad x \in D , \qquad B [ u ] ( t , x ) = 0 , \quad ( t , x ) \in [ 0 , T ] \times \partial D , } \end{array} +$$ + +where $u : [ 0 , T ] \times D \mathbb { R } ^ { n }$ is the solution, with the initial condition $u _ { 0 } ( x )$ at time $t = 0$ and boundary condition $B [ u ] ( t , x ) = 0$ on the boundary. $F$ is a function and $f ( t , x )$ is the force term. + +For such PDE systems, there are two fundamental tasks: simulation and control. The former involves learning a mapping from certain parameter functions $a$ , such as initial conditions and boundary + +conditions, to the solutions $u$ that represent a mapping from an infinite-dimensional function space to another infinite-dimensional function space. The latter task involves identifying the external control $f$ for a specific objective $\mathcal { I } ( u , f )$ which is a function of $u$ and $f$ , aiming at finding $f$ that minimizes $\mathcal { I }$ . + +# 2.2 DIFFUSION MODEL + +A representative instance of diffusion models is the Denoising Diffusion Probabilistic Model (DDPM) (Ho et al., 2020b), which contains a forward and a reverse process to generate samples. In the forward process, noise is progressively added to clean data $\mathbf { x } _ { \mathrm { 0 } }$ until it is corrupted into Gaussian noise √ $\mathbf { x } _ { K } \sim$ $\mathbf { \bar { \mathcal { N } } ( 0 , I ) }$ . This process follows the Gaussian transition kernel $q ( \mathbf { x } _ { k + 1 } \mathbf { \bar { | x } } _ { k } ) = \mathcal { N } ( \mathbf { x } _ { k + 1 } ; \sqrt { \alpha _ { k } } \mathbf { x } _ { k } , ( 1 -$ $\alpha _ { k } ) \mathbf { I } )$ ), where $\{ \alpha _ { k } \} _ { k = 1 } ^ { K }$ denotes the variance schedule. In the reverse process, data is sampled from Gaussian noise $\mathcal { N } ( \mathbf { 0 } , \mathbf { I } )$ and a denoising model $\epsilon _ { \theta }$ gradually removes the noise from the data until it returns the original clean data distribution. The model predicts the mean $\mu _ { \boldsymbol { \theta } } ( \mathbf { x } _ { k } )$ of $\mathbf { x } _ { k - 1 }$ and the reverse process is defined with the transition $p _ { \theta } ( \mathbf { x } _ { k - 1 } | \mathbf { x } _ { k } ) = \mathcal { N } ( \mathbf { x } _ { k - 1 } ; \mu _ { \theta } ( \mathbf { x } _ { k } , k ) , \sigma _ { k } \mathbf { I } )$ . + +To train the denoising model $\epsilon _ { \theta }$ , the training loss is defined as follows, which optimizes a simplified variant of the variational lower-bound for the data’s log-likelihood (Ho et al., 2020b). + +$$ += \mathbb { E } _ { k \sim U ( 1 , K ) , \mathbf { x } _ { 0 } \sim p ( x ) , \epsilon \sim \mathcal { N } ( \mathbf { 0 } , \mathbf { I } ) } [ \big \| \epsilon - \epsilon _ { \theta } \big ( \sqrt { \bar { \alpha } _ { k } } \mathbf { x } _ { 0 } + \sqrt { 1 - \bar { \alpha } _ { k } } \epsilon , k \big ) \big \| _ { 2 } ^ { 2 } ] , \ \mathrm { w h e r e } \ \bar { \alpha } _ { k } : = \prod _ { i = 1 } ^ { k } \alpha _ { i } . +$$ + +Guided Diffusion Generation. Modeling the conditional distribution $q ( \mathbf { x } | \mathbf { y } )$ enables controllable sample generation. Methods for conditioning in diffusion models include classifier-based guidance (Du et al., 2023) and classifier-free guidance (Ho & Salimans, 2022; Ajay et al., 2022). The former employs an additional classifier model trained on clean data to directly modify the denoising direction of the data during generation. The classifier-free conditioning simplifies the architecture and enables guided generation without an explicit classifier. It trains the model to learn both conditional and unconditional probabilities $\epsilon _ { \theta } ( \mathbf { x } , \mathcal { O } ) \propto \nabla _ { \mathbf { x } } \log q ( \mathbf { x } )$ and $\epsilon _ { \theta } ( \mathbf { x } , \mathbf { y } ) \propto \nabla _ { \mathbf { x } } \log q ( \mathbf { x } | \mathbf { y } )$ , where $\mathcal { D }$ is an identifier that tells the model $\epsilon _ { \theta }$ to output $p ( x )$ instead of $p ( x | y )$ (Ho & Salimans, 2022). During sample generation, it combines noise terms following $\epsilon _ { \theta } ( \boldsymbol { \mathbf { x } } , \mathcal { D } ) + \omega ( \epsilon _ { \theta } ( \boldsymbol { \mathbf { x } } , \boldsymbol { \mathbf { y } } ) - \epsilon _ { \theta } ( \boldsymbol { \mathbf { x } } , \mathcal { D } ) )$ , where $\omega \in [ 0 , 1 ]$ is the weight. In this paper, we combine the use of both guidance methods. + +# 3 METHOD + +In this section, we detail our proposed WDNO from two perspectives: Section 3.1 describes how we perform the generative process within the wavelet domain, including basic concepts and practical implementation of wavelet transforms, and algorithms for applying WDNO to simulation and control problems. Section 3.2 presents the approximate scale invariance of PDE systems and our proposed multi-resolution training based on this property. The overall algorithm is presented in Figure 1. + +# 3.1 GENERATION IN THE WAVELET DOMAIN + +The WDNO performs generative control and simulation in the wavelet domain. Compared to the Fourier transform, the wavelet transform features locality while simultaneously retaining information in both space-time and frequency domains, allowing more accurate modeling for abrupt changes. + +Wavelet basis. Intuitively, we use wavelet analysis to represent signals with basis functions localized in both space-time and frequency domains, taking values only within finite intervals. Specifically, this set of basis functions can be divided into two categories: one type is the scaling function $\phi$ used to represent the general outline (low-frequency information) of the original signal, and the other type is the mother wavelet $\psi$ , which is used to depict the detailed information (high-frequency information) of the original signal (Alpert et al., 2002; Selesnick et al., 2005). + +By scaling the function $\phi$ and mother wavelets $\psi$ , we obtain $\phi _ { l , m }$ and $\psi _ { l , m }$ : + +$$ +\phi _ { l , m } ( x ) = 2 ^ { l / 2 } \phi ( 2 ^ { l } x - m ) , \quad \psi _ { l , m } ( x ) = 2 ^ { l / 2 } \psi ( 2 ^ { l } x - m ) , +$$ + +where $m$ adjusts the position of the wavelet along the $x$ -axis and $l$ represents the level of the basis. When $l$ increases, the wavelet narrows, and its frequency increases. Then, the entire space can be spanned by $\phi _ { l , m }$ at a particular level $l _ { 0 }$ and $\psi _ { l , m }$ at levels greater than or equal to $l _ { 0 }$ , which can be presented as follows: + +$$ +u ( x ) = \sum _ { m } c _ { l _ { 0 } } ( m ) \phi _ { l _ { 0 } , m } ( x ) + \sum _ { l = l _ { 0 } } ^ { \infty } \sum _ { m } d _ { l } ( m ) \psi _ { l , m } ( x ) . +$$ + +Thus we get the coarse wavelet coefficients $c _ { l _ { 0 } } ( m )$ and the detail wavelet coefficients $d _ { l } ( m )$ . + +Practical implementation. However, due to the discrete nature of real-world data, the levels of $\psi _ { l , m }$ will have an upper bound $L$ , meaning there exists a minimum length interval for $\psi$ . As mentioned in the introduction (Sec. 1), to preserve the locality of the data for integration with the multi-resolution training, we choose $l _ { 0 } = L$ . So, the decomposition can be presented as: + +$$ +u ( x ) = \sum _ { m } c _ { L } ( m ) \phi _ { L , m } ( x ) + \sum _ { m } d _ { L } ( m ) \psi _ { L , m } ( x ) . +$$ + +To verify the reliability of the wavelet decomposition’s implementation, in Appendix A, we conduct tests of the reconstruction error on training data and discover that the relative $l _ { 2 }$ errors of such reconstructions are on the order of $1 0 ^ { - 7 }$ , indicating that there is nearly no information loss. Further details about the wavelet transform can be found in Appendix A. + +WDNO for simulation. For simulation, as introduced in Section 2.1, the objective is to learn a mapping from the equation parameter function $a$ to the solution function $u _ { [ 0 , T ] }$ . We can view the learning of this mapping as learning a conditional probability $p ( u _ { [ 0 , T ] } | a )$ . However, we consider the conditional probability in the wavelet space, $p ( W _ { u _ { [ 0 , T ] } } | W _ { \underline { { a } } } )$ , where $W _ { u }$ and $W _ { a }$ are the wavelettransformed values of $u _ { [ 0 , T ] }$ and $a$ . Here, we adopt classifier-free conditioning to guide the sampling process in diffusion models. Specifically, to ensure that the generated wavelet-transformed values $W _ { u _ { [ 0 , T ] } }$ align with the corresponding $W _ { a }$ , we include $W _ { a }$ as a conditioning factor. Specifically, we initialize an optimization variable $W _ { u _ { [ 0 , T ] } } ^ { ( k ) }$ with Gaussian noise $\mathcal { N } ( \mathbf { 0 } , \mathbf { I } )$ , and iteratively update it via: + +$$ +W _ { u _ { [ 0 , T ] } } ^ { ( k - 1 ) } = W _ { u _ { [ 0 , T ] } } ^ { ( k ) } - \eta \epsilon _ { \theta } ( W _ { u _ { [ 0 , T ] } } ^ { ( k ) } , W _ { a } , k ) + \xi , \quad \xi \sim \mathcal { N } \big ( \mathbf { 0 } , \sigma _ { k } ^ { 2 } \mathbf { I } \big ) , +$$ + +where $k$ denotes the denoising step, $\eta$ is the scaling factor and $\sigma _ { k }$ represents the noise schedules. Repeatedly applying this denoising procedure from $k = M$ down to $k = 1$ yields the final solution $W _ { u _ { [ 0 , T ] } } ^ { ( 0 ) }$ . Besides, during inference, we follow the Denoising Diffusion Implicit Model (DDIM) (Song et al., 2020), which can largely speed up the sampling process. + +WDNO for control. For the control problem, in a task aimed to minimize $\mathcal { I }$ , our goal is to find the optimal $f _ { [ 0 , T ] }$ based on an environment determined by a parameter function $a$ , such as the initial condition. Consequently, this problem can be naturally modeled as learning $p ( f _ { [ 0 , T ] } | a )$ . Here, we also transform it into the wavelet domain, thus learning $p ( W _ { f _ { [ 0 , T ] } } | W _ { a } )$ . Similar to the simulation, we employ a conditional diffusion model. However, a challenge arises in that we can only model and train $p ( { W _ { f _ { [ 0 , T ] } } } \vert W _ { a } )$ as represented in the training set, where $f$ is typically not optimal. To address this issue, we view the control problem from an energy optimization perspective, and thus during inference, we enhance the denoising process with guidance $\mathcal { I }$ to steer the generation of $f$ towards a smaller $\mathcal { I }$ . Note that without this term, the model can only generate control sequences that follow the same distribution as the dataset, without optimizing for the control objectives. Specifically, initializing W (k)f[0,T ] from Gaussian noise $\mathcal { N } ( \mathbf { 0 } , \mathbf { I } )$ , we iteratively update + +$$ +W _ { f _ { [ 0 , T ] } } ^ { ( k - 1 ) } = W _ { f _ { [ 0 , T ] } } ^ { ( k ) } - \eta \left( \epsilon _ { \theta } ( W _ { f _ { [ 0 , T ] } } ^ { ( k ) } , W _ { a } , k ) + \lambda \nabla _ { W _ { f _ { [ 0 , T ] } } } \mathcal { I } ( \hat { W } _ { f _ { [ 0 , T ] } } ^ { ( k ) } ) \right) + \xi , \quad \xi \sim \mathcal { N } \big ( \mathbf { 0 } , \sigma _ { k } ^ { 2 } \mathbf { I } \big ) , +$$ + +where $\sigma _ { k }$ and $\eta$ are the noise schedule and the scaling factor respectively, and $\lambda$ is the weight of guidance. Here Wˆ (k)f[0,T] is the approximate noise-free $\bar { W } _ { f _ { [ 0 , T ] } } ^ { ( 0 ) }$ ] estimated from W (k)f[0,T ] by: + +$$ +\hat { W } _ { f _ { [ 0 , T ] } } ^ { ( k ) } = ( W _ { f _ { [ 0 , T ] } } ^ { ( k ) } - \sqrt { 1 - \bar { \alpha } _ { k } } \epsilon _ { \theta } ( W _ { f _ { [ 0 , T ] } } ^ { ( k ) } , W _ { a } , k ) ) / \sqrt { \bar { \alpha } _ { k } } , +$$ + +We calculate $\mathcal { I }$ in Eq. 4 based o n Wˆ (k) ] instead of directly using W (k)f[0,T ] because otherwise noise in $W _ { f _ { [ 0 , T ] } } ^ { ( k ) }$ could bring errors to $\mathcal { I }$ . Repeatedly applying this denoising procedure yields the final solution W (0)f . Similar to the simulation, we also employ the DDIM to accelerate the denoising. + +# 3.2 MULTI-RESOLUTION FRAMEWORK + +Next, to enable the diffusion model to generalize across different resolutions, we will introduce our multi-resolution framework based on the approximate scale invariance, which we will introduce in the following. In contrast to the model mentioned in the previous section, which we refer to as the Base-Resolution Model (BRM), we will introduce a Super-Resolution Model (SRM) in this section. The framework integrates seamlessly with the wavelet transform technique and enables zero-shot super-resolution, which is one of the fundamental requirements of a neural operator. + +Approximate scale invariance. We first introduce the approximate scale invariance. For simplicity, let us first assume that the spatial domain of the PDE in Eq. 1 is $D = [ 0 , 1 ]$ . Given the high-resolution data $d _ { + }$ of size $N \times M$ , and low-resolution data $d _ { - }$ of size $( N / 2 ) \times \mathbf { \bar { ( } } M / 2 )$ , although both are originally defined over the same spatiotemporal domain $[ 0 , T ] \times D$ , we can rescale the low-resolution data into a new spatiotemporal domain $[ 0 , T / 2 ] \times { \tilde { D } }$ , where the spatial domain $\tilde { D }$ is scaled to $[ 0 , 1 / 2 ]$ . In this case, $d _ { + }$ and $d _ { - }$ can be aligned to the same precision. However, note that the coordinates of $d _ { - }$ are now scaled, meaning that the system no longer follows the original equation. + +For any arbitrary spatial domain, we can always achieve such alignment through a linear transformation. We denote the linear transformations of time and space as $\bar { a } _ { 1 } t + b _ { 1 }$ and $\overline { { a } } _ { 2 } x + b _ { 2 }$ respectively. Then the stretched function actually satisfies the transformed version of the original equation: + +$$ +\frac { \partial u } { a _ { 1 } \partial t } = F \left( u , \frac { \partial u } { a _ { 2 } \partial x } , \frac { \partial ^ { 2 } u } { a _ { 2 } ^ { 2 } \partial x ^ { 2 } } , \ldots \right) + f ( t , x ) , \quad ( t , x ) \in [ 0 , T / 2 ] \times \tilde { D } . +$$ + +Note that if we consistently consider the same factor of resolution change, this linear transformation remains constant, meaning that the coefficients $a _ { 1 }$ and $a _ { 2 }$ are fixed. Therefore, the pattern of change between different resolutions is consistent. Additionally, since the wavelet transform is linear and localized, this pattern remains consistent in the wavelet domain. + +Correspondingly, in practical operations, we consider that each refinement of the discrete observations of the physical system follows the same pattern, which inspires us to develop the idea of multiresolution training. Specifically, based on the training dataset at a given resolution, we downsample it to create a multi-resolution training dataset and then use this dataset for training to learn this pattern. Thus, during inference, we can naturally follow this pattern to achieve zero-shot super-resolution. + +Multi-resolution training data. In practical implementation, we introduce the Super-Resolution Model, which is a conditional diffusion model. Assuming the resolution of the original training dataset is $N \times M$ , that is, $N$ time steps and $M$ spatial points, we obtain data at the resolution of $( N / 2 ) \times ( M / 2 )$ through downsampling, which means we do not need finer-resolution data. We thus get the data pairs of sizes $N \times M$ and $( \bar { N } / 2 ) \times ( M / 2 )$ . This downsampling process can be repeated to obtain data pairs of $( N / 2 ) \times ( M / 2 )$ and $( N / 4 ) \times ( M / 4 )$ , $( N / 4 ) \times ( M / 4 )$ and $( N / 8 ) \times ( M / 8 )$ , and so forth, to compose the multi-resolution training dataset for training the Super-Resolution Model. + +Training. We take the conditional diffusion model (Ho & Salimans, 2022) to model the conditional probability $p ( W _ { h } \mid W _ { l } , W _ { a _ { h } } )$ , where $h$ and $l$ respectively present high- and low-resolution data of data pairs in the multi-resolution training dataset, $a _ { h }$ is the high-resolution equation parameter, and $W _ { h }$ , $W _ { l }$ and $W _ { a _ { h } }$ are the corresponding wavelet-transformed values. In detail, to align low-resolution with high-resolution data, we duplicate the low-resolution data to match the size of high-resolution data. During training, each batch randomly selects data pairs from a given resolution. + +Inference. During the inference process, when super-resolution is required, we first downsample the high-resolution equation parameters $a$ to the same resolution $N \times M$ as the training data and perform a wavelet transform. Then, using the Base-Resolution Model, we first generate the wavelet coefficients of the base low resolution. Subsequently, we utilize the Super-Resolution Model to generate the data based on both the wavelet coefficients of lower-resolution results with size $N \times M$ and the wavelet coefficients of $a _ { h }$ at the post-super-resolution resolution $2 N \times 2 M$ . This process is iterated, allowing us to ultimately generate results with the same resolution as the original $a$ . + +# 4 EXPERIMENTS + +In this section, we aim to test (1) the advantages of WDNO in handling complex long-term dynamics with abrupt changes on simulation and control problems, (2) the effectiveness of multi-resolution training in performing zero-shot super-resolution, and (3) the benefits of integrating wavelet transform. + +We report the Mean Squared Error (MSE) measured on entire state sequences excluding initial conditions for the simulation tasks, and the control objective $\mathcal { I }$ for control problems. Besides, we consider state-of-the-art baselines from different fields. For control tasks, the following methods are compared: (1) the classical control algorithm Proportional-Integral-Derivative (PID) (Li et al., 2006) (2) Supervised Learning method (SL) (Hwang et al., 2022), reinforcement learning and imitation learning methods including (3) Soft Actor-Critic (SAC) (Haarnoja et al., 2018), (4) Behavior Cloning (BC) (Pomerleau, 1988), (5) Behavior Proximal Policy Optimization (BPPO), and (6) DDPM (Zhuang et al., 2023). For simulation, we consider (1) DDPM (Ho et al., 2020a), (2) Wavelet Neural Operator (WNO) (Tripura & Chakraborty, 2022), (3) Multiwavelet Neural Operator (MWT) (Gupta et al., 2021), (4) Fourier Neural Operator (FNO) (Li et al., 2021), (5) CNN (Hwang et al., 2022), (6) Operator Transformer (OFormer) (Li et al., 2023), and (7) U-Net (Ronneberger et al., 2015). Details can be referenced in Appendix I, J and K. For reproducibility, the code is available here. + +# 4.1 1D BURGERS’ EQUATION + +Experiment setting. We first consider the 1D Burgers’ equation, a fundamental equation describing shock waves and turbulence in fluid dynamics, with the Dirichlet boundary condition and external force $f$ , which follows previous works (Hwang et al., 2022; Mowlavi & Nabi, 2023) and is more difficult due to the long time horizon of 81 steps. The visualizations are presented in Figure 6. More details about the setting are in Appendix F. The simulation task is to learn the mapping from the initial condition $u _ { 0 }$ and force term $f$ to the entire trajectory $u _ { [ 0 , T ] }$ , while the control objective $\mathcal { I }$ corresponding to the target state $u ^ { * } ( x )$ and the fixed weight $\alpha$ is + +$$ +\mathcal { I } = \int _ { D } | u ( T , x ) - u ^ { * } ( x ) | ^ { 2 } \mathrm { d } x + \alpha \int _ { [ 0 , T ] \times D } | f ( t , x ) | ^ { 2 } \mathrm { d } t \mathrm { d } x . +$$ + +Data preparation. We perform a 2D wavelet transform on the original data using the bior2.4 wavelet basis and the ‘periodization’ mode, implemented using the pytorch_wavelets package (Cotter, 2019). Since the initial condition and the target state are 1D, we take the 1D wavelet transform, repeat the coefficients, and then concatenate them with the 2D coefficients. + +Results We report results of simulation and control tasks in Table 1 and Table 2a. From Table 1, it is evident that WDNO and DDPM achieve results that far surpass other baselines in simulation, demonstrating the capability of diffusion models for long-term predictions. In this particular simulation experiment, the performance of WDNO and DDPM is quite similar, while advantages of WDNO over DDPM are detailed in Section 4.6 and Section 4.7. For the control problem, WDNO achieves the best results, which clearly illustrates the superior performance of WDNO. + +Table 1: Results of simulation. Bold font denotes the best model and the runner-up is underlined. + +
Methods1D2D
Burgers'AdvectionNavier-StokesFluidERA5
WNO0.005724.216e-026.54280.07975
MWT0.000523.468e-041.38300.0155621.85750
OFormer0.000231.858e-040.62270.0430318.26230
FNO0.000159.712e-040.25750.0068414.38638
CNN (1D) / U-Net (2D)0.001985.033e-0412.49660.0073715.51342
DDPM0.000134.209e-055.52280.0157815.21103
WDNO (ours)0.000142.898e-050.21950.0023112.83291
+ +# 4.2 1D ADVECTION EQUATION + +Experiment setting. Next, we consider the advection equation, which models pure advection behavior without nonlinearity. This dataset, sourced from PDEBench (Takamoto et al., 2022), is set up to predict 80 timesteps of evolution based on the one-time-step initial condition. The system exhibits relatively smooth and simple dynamics. We aim to observe the performance of various methods on a system without abrupt changes using this dataset. + +Data preparation. Since the data shape is similar to that of the 1D Burgers’ equation, the data preparation process is consistent with that of the first experiment. + +Results. From results in Table 1, we can observe that most models achieve low prediction errors. +However, WDNO still delivers the best results. + +![](images/figures/wdno-fig-0002.jpg) +Figure 2: Visualizations of 1D Navier-Stokes equation and 2D incompressible fluid. + +(a) Results of WDNO and (b) Results of WDNO on the 2D indirect control. The objective is to navigate DDPM on the 1D Navier- the yellow smoke to get around grey obstacles and reach the target bucket located Stokes equation. at the top center. + +# 4.3 1D COMPRESSIBLE NAVIER-STOKES EQUATION + +Experiment setting. We also consider the important 1D compressible Navier-Stokes equation which can describe complex phenomena, such as shock wave formation and propagation in aerodynamics around airplane wings and interstellar gas dynamics. We consider a particularly challenging scenario from the 1D CFD dataset in PDEBench (Takamoto et al., 2022). We select extremely small viscosity coefficients, $\eta = 1 0 ^ { - 8 }$ and $\zeta = 1 0 ^ { - 8 }$ . The initial conditions are shock-tube fields consisting of piecewise constant values generating shocks and rarefactions. Boundary conditions allow waves to exit the domain. Since this pre-existing dataset does not include time-varying control terms, we only perform the simulation task on it. We provide more details in Appendix G. + +Data preparation. The data preparation process is also similar to the above ones. + +Results. From Table 1, we can observe that WDNO still gains the best performance among strong baselines. It is particularly noteworthy that the MSE of DDPM exceeds that of WDNO by over 25 times. In Figure 2a and Figure 7, we further present the detailed prediction results of DDPM and WDNO. It can be seen that for physical dynamics with abrupt changes, DDPM struggles to model shocks and loses many fine details. This highlights the necessity of introducing the wavelet transform. More results, including MSEs, MAEs, and $L _ { \infty }$ , and other baselines can be found in Appendix C.1. + +Table 2: Results of control tasks. Bold font denotes the best model and the runner-up is underlined. (a) 1D Burgers’ equation. (b) 2D incompressible fluid. + +
MethodsJ
PID (surrogate-solver) SAC (pseudo-online) SAC (offline) BC (surrogate-solver)0.6645 0.1376 0.3210 0.2998
BC (solver) BPPO (surrogate-solver) BPPO (solver) SL0.1879 0.3075 0.1867 0.0235
DDPM WDNO (ours)0.0272 0.0205
+ +
MethodsJ
BCBPPOSAC (pseudo-online)SAC (offline)DDPM0.3085
0.30660.32120.65030.3124
WDNO (ours)0.0679
+ +# 4.4 2D INCOMPRESSIBLE FLUID + +Experiment setting. Next, we experiment on 2D fluid problems following the incompressible Navier-Stokes equation. The experiment setting, a complex scenario close to real-world, follows previous works (Wei et al., 2024), where the control can only be exercised out of the frame as shown in Figure 2b. The boundary condition at obstacles is the no-slip condition, meaning that the velocities are set to 0 at the boundary. This experiment thus includes fluid-solid coupling, where functions have discontinuities and are hard to model. The different data trajectories share the same initial velocity field; the variations are in initial smoke positions, specifically the smoke’s initial density, and control sequences. The simulation task is to predict the smoke’s density, velocity field, and the percentage of smoke passing through the target bucket based on the initial smoke density and control sequences. + +For the control problem, our goal is to move the smoke from its initial position, located beneath the central obstacle, into the middle bucket at the top. To be more specific, $\mathcal { I }$ is defined as the percentage of smoke not passing through the target bucket. Firstly, this objective presents considerable challenges due to the restriction that forces can only be applied in the peripheral regions. This problem requires the model to plan ahead in the middle of the entire trajectory to avoid entry into the wrong opening. Furthermore, we need to generate 3,584 control parameters over a time span of 32 steps in these peripheral zones to indirectly control the velocity field in the central region. + +Data preparation. We perform a 3D wavelet transform on original data using bior1.3 wavelet basis and ‘zero’ mode, implemented through Pytorch Wavelet Toolbox (ptwt) (Wolter et al., 2024). Since the initial condition and percentage of smoke are 2D and 1D respectively, we take the 2D and 1D wavelet transform and repeat the coefficients to concatenate them. + +Results. Table 1 are the simulation results, showing our method is far superior to DDPM and exceeds all the baselines. It is worth noting that the prediction error of WDNO is an order of magnitude lower than that of DDPM. As for the results of the control problem shown in Table 2b, our method can make more than $90 \%$ of the smoke pass through the target bucket, and its $\mathcal { I }$ is $22 \%$ of the next best method’s $\mathcal { I }$ , showing our model’s superiority under complex dynamics with abrupt changes. + +# 4.5 ERA5 + +Experiment setting. The ERA5 dataset (Kalnay et al., 2018), provided by ECMWF, is a challenging real-world dataset for weather forecasting. It offers hourly atmospheric estimates with a $0 . 2 5 ^ { \circ }$ latitude-longitude resolution from the Earth’s surface to $1 0 0 \mathrm { k m }$ altitude, spanning from 1979 to the present. We conduct simulation experiments on this dataset to demonstrate the superior performance of WDNO. The selected variable is temperature, and the specific task involves predicting the system’s evolution over the next 20 hours based on its state over the past 12 hours. + +Data preparation. Due to similar data size, the process of data preparation is similar to Section 4.4. + +Results. We present the results in Table 1. Here we experiment with different parameters for WNO, but all configurations fail to converge. It is clear that WDNO still achieves the best performance, with a relative $L _ { 2 }$ error as low as 0.0161, demonstrating its outstanding capability on challenging datasets. + +# 4.6 ZERO-SHOT SUPER RESOLUTION + +In this subsection, we will present the super-resolution simulation results for the 1D Burgers’ equation and 2D incompressible fluid. For the 1D experiments, the resolution of the training dataset is of the time-space resolution $8 0 \times 1 2 0$ . We demonstrate the results of single, double, and triple super-resolution steps on both time and space, with the corresponding unseen resolutions of $1 6 0 \times 2 4 0$ , $3 2 0 \times 4 8 0$ , and $6 4 0 \times 9 6 0$ respectively. For the 2D experiments, the training dataset has a resolution of $3 2 \times 6 4 \times 6 4$ , and we transfer to the resolution $3 2 \times 1 2 8 \times 1 2 8$ . The visualization of 1D zero-shot super resolution is presented in Figure 3. + +To evaluate the performance across different resolutions, we interpolate the outcomes of each super-resolution step to the highest resolution level. This allows us to assess whether the model can accurately + +![](images/figures/wdno-fig-0003.jpg) +Figure 3: 1D zero-shot super-resolution. The first row shows WDNO’s simulation results with no super resolution, one-level super resolution, and two-level super resolution. The second row is the ground truth, and the third row is the difference between the first and second rows. As resolution increases, WDNO’s output gets closer to the ground truth, demonstrating its zero-shot super resolution capability. + +![](images/figures/wdno-fig-0004.jpg) +Figure 4: Results in Section 4.6 and Section 4.7 after $n$ super-resolution steps. All MSEs are calculated at the finest resolution through linear or nearest interpolation. (a), (b) are 1D Burgers’ and 2D results in Section 4.6, respectively, and (c) is the results in Section 4.7. + +![](images/figures/wdno-fig-0005.jpg) +Figure 5: Results of ablation studies. + +generate data on finer grid points beyond the resolutions encountered during training. We consider linear interpolation and nearest interpolation, taking the mesh-invariant model FNO and WNO as the baselines. Due to WNO’s implementation, it can only perform spatiotemporal super-resolution simultaneously, making it unsuitable for 2D super-resolution experiments. As shown in Figure 4a, Figure 4b, Table 16 and Table 17, in both 1D and 2D scenarios, our method surpasses results of interpolation by achieving significantly improved outcomes with each super-resolution step. It can effectively reconstruct the values on the newly added grid points at the highest resolution, outperforming the mesh-invariant FNO and WNO. + +# 4.7 ABLATION STUDY + +Abrupt changes. We first verify whether the wavelet transform can enhance DDPM’s ability to model abrupt changes. To this end, we present the system’s states and prediction errors of WDNO and DDPM over time in Figure 6 and Figure 9. Note that, although WDNO and DDPM have similar overall MSEs in Table 1, we can observe that at moments when the state exhibits abrupt changes in space, WDNO achieves a lower prediction error compared to DDPM. This demonstrates that the wavelet transform helps to model dynamics with abrupt changes that are otherwise difficult to learn. + +# Combination of wavelet and multi-resolution training. + +![](images/figures/wdno-fig-0006.jpg) +Figure 6: MAE and state trajectories. + +To assess the efficacy of integrating wavelet transform with multi-resolution training due to the wavelet transform’s locality, we provide outcomes from DDPM combined with multi-resolution training by applying the framework directly in the space-time domain, as depicted in Figure 4c. Notably, in the 1D experiment, as the number of super-resolution steps increases, evaluations at the highest resolution reveal that the disparity between WDNO and the application of the multi-resolution training in the original space-time domain becomes more pronounced, verifying the efficiency of utilizing wavelet transforms for super resolution. + +Comparison with Fourier transform. We also evaluate the diffusion model in the Fourier domain (Diffusion $+ \mathrm { F F T }$ ). The implementation strictly follows WDNO, except for replacing the wavelet transform with Fourier transform. The MSEs on the 1D compressible Navier-Stokes equation are shown in figure 5c. While the Fourier transform also provides some improvement over DDPM, its performance is significantly inferior to that of the wavelet transform, which verifies that wavelet transforms inherently decompose information into low-frequency components and high-frequency details across different directions, making them more effective for learning complex system dynamics, such as those with abrupt changes. In addition, we take the FNO as the noise prediction model (FNO Denoiser), but the results indicate inferior performance. This may be because FNO tends to filter out high-frequency information, which is crucial for a noise prediction mode. + +Long-term dependencies. Long-time predictions tend to perform poorly due to error accumulation and prediction instability. Therefore, capturing long-term dependencies allows WDNO to grasp the dynamics over extended periods better, naturally improving WDNO performance. To further verify it, in Figure 5a, we provide errors of baselines and WDNO at different time steps in the 2D simulation experiment. It is obvious that WDNO exhibits the slowest error growth, confirming its ability to capture long-term dependencies. + +Measurement noise. To evaluate on datasets with increasing measurement noise, we add noise to both the training and testing datasets of 1D Burgers’ equation, sampled as Gaussian noise scaled by the original data’s standard deviation multiplied by a scale factor. We test scale factors of 0.01, 0.001, and 0.0001. As shown in the Figure 5d, WDNO ’s results exhibit minimal variation with changes in scale, demonstrating its robustness to noise. + +Number of training samples. We reduce the training dataset size to 0.2, 0.4, 0.6, and 0.8 times the current size (9000 samples) and measure WDNO’s MSE on the 1D compressible Navier-Stokes equation. The results in Figure 5b show that even when the dataset size is reduced to 0.4 times, WDNO ’s error remains within a relatively small range. When the dataset size is reduced to 0.2 times, the error shows a noticeable increase. + +Additional results. Due to space constraints, we provide additional details in Appendix C, which include sensitivity analysis of key hyperparameters, verifying approximate scale invariance, evaluating the sensitivity of baselines and WDNO to noise in control sequences, comparing computational resource usage between baselines and WDNO, and analyzing the impact of the guidance parameter. + +# 5 LIMITATION AND FUTURE WORK + +Firstly, although we do not conduct real-world experiments, WDNO is not limited to the specific environments, which means that it can be applied to real scenarios, such as turbulence, structural materials and plasma, which we will leave as future work. Secondly, due to the wavelet transform and denoising model U-Net, WDNO is only applicable to static, uniform grid data. We are considering applying WDNO to irregular data by using geometric wavelets (Xu et al., 2018) combined with diffusion models designed for graph structures (Vignac et al., 2023), or projecting data from irregular grids onto regular uniform grids (Li et al., 2020b; Lin et al., 2023), among others. Finally, our current approach does not yet incorporate information from equations, such as adding physics-informed loss based on the PDEs, which can enhance the model’s accuracy, robustness, and generalizability. + +# 6 CONCLUSION + +In this paper, we have introduced Wavelet Diffusion Neural Operator (WDNO), a method for simulation and control of PDE systems. By introducing two innovations of generation in the wavelet domain and multi-resolution training, WDNO addresses the challenges of modeling states with abrupt changes and generalizing across resolutions typical in PDE systems. Experiments on challenging settings including the 1D Burgers’ equation, 1D Adevection Equation, 1D CFD, 2D incompressible fluid and ERA5 demonstrate WDNO’s superior performance and its ability to generalize to much finer spatial and temporal resolutions than in training. We believe that WDNO will be useful for complex physical simulation and control in a wide range of scientific and engineering domains. + +# 7 ACKNOWLEDGMENT + +We thank Tengfei Xu and Tao Zhang for discussions and for providing feedback on our manuscript. We also gratefully acknowledge the support of Westlake University Research Center for Industries of the Future; Westlake University Center for High-performance Computing. 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Let $V _ { l }$ be the space spanned by scaling functions $\phi _ { l , m }$ , $m \in \mathbb { Z }$ , and $W _ { l }$ be the space spanned by wavelets $\psi _ { l , m }$ , $m \in \mathbb { Z }$ . The scaling functions possess two fundamental properties: + +1. The scale function is orthogonal for its integer translation. + +2. The wavelet spaces satisfy a nested and increasing sequence of spaces: + +$$ +V _ { - \infty } \subset \cdots \subset V _ { - 1 } \subset V _ { 0 } \subset V _ { 1 } \subset \cdots \subset V _ { \infty } . +$$ + +As for the space $V _ { l }$ and $W _ { l }$ , they have the following relationship: + +$$ +V _ { l + 1 } = V _ { l } \oplus W _ { l } . +$$ + +Intuitively, the spaces $W _ { l }$ spanned by the wavelet functions complement the missing information between the scaling function spaces of different levels. + +Then, the process of wavelet transform can be viewed as convolving the scaling and wavelet functions of a certain level with the original signal, effectively splitting the signal into low-frequency and high-frequency components. Subsequently, the low-frequency part is further decomposed. This results in obtaining coefficients $c _ { l _ { 0 } }$ and $d _ { l _ { 0 } } , d _ { l _ { 0 } + 1 } , d _ { l _ { 0 } + 2 } , \dots .$ . + +There are numerous types of wavelet bases that have different waveforms. Here we provide further insights into the criteria used for wavelet selection. For the wavelets we consider (bior, db, sym), bior and sym wavelets offer symmetry, which reduces phase distortion during processing and allows for more accurate reconstruction compared to db, as also reflected in the reconstruction loss table. Regarding the choice of wavelet scale, despite the higher smoothness of higher-order wavelets, they generally have larger value ranges. Therefore, for data with small spatiotemporal size, high-order wavelets may not be suitable. For example, in the 1D data with a size of $N \times 8 1 \times 1 2 0$ , we choose bior2.4, while for the 2D data with a size of $N \times 3 2 \times 6 4 \times 6 4$ , we select bior1.3, a lower-order wavelet. Using wavelets with excessively large support lengths may distort coefficients near the boundaries and fail to effectively decompose signal details, hindering the effectiveness of multi-resolution decomposition. + +Specifically, we select bior2.4 and bior1.3 from the Biorthogonal wavelet family for our experiments on the 1D Burgers’ equation and 2D incompressible fluid, respectively. And we we use the ‘periodization’ mode in 1D and the ‘zero’ mode in 2D. Due to the presence of the temporal dimension, we perform a two-dimensional wavelet transform on data from the 1D Burgers’ equation and a three-dimensional wavelet transform on data from the 2D incompressible fluid. + +In Table 3, we report the reconstruction errors of wavelet transforms using different wavelet bases on the 1D Burgers’ equation and 2D incompressible fluid, the results show that the reconstruction is significantly low. + +Table 3: Reconstruction relative $L _ { 2 }$ errors on 1D Burgers’ equation and 2D incompressible fluid. + +
Types of wavelet1D2D
bior1.31.09e-073.32e-07
bior2.48.32e-082.65e-07
db41.39e-074.39e-07
sym41.17e-073.74e-07
+ +Besides, in Table 4, we provide the total time consumption for Fourier and wavelet transforms on the training set of the 1D compressible Navier-Stokes equation. The Fourier transform is implemented using PyTorch’s 2D Fast Fourier Transform function. Both times are recorded on an A100 GPU with a batch size of 2000. From the results, we can observe wavelet transform’s efficiency. + +Table 4: Total time consumption for Fourier and wavelet transforms on the training set of the 1D compressible Navier-Stokes equation. + +
Wavelet transform Fourier transformWavelet transform Fourier transform
Time (s) |1.0171 |1.3810
+ +# B VISUALIZATION OF EXPERIMENT RESULTS + +# B.1 VISUALIZATIONS OF 1D COMPRESSIBLE NAVIER-STOKES EQUATION + +In Figure 7, we present visualizations of predictions from WDNO and DDPM. It is clear that WDNO is far better at modeling states with abrupt changes. While DDPM can not capture details, WDNO can successfully predict these precise changes. + +![](images/figures/wdno-fig-0007.jpg) +Figure 7: Visualizations of WDNO’s and DDPM’s performance on simulation of the 1D compressible Navier-Stokes equation. + +# B.2 VISUALIZATIONS OF 2D INCOMPRESSIBLE FLUID + +We provide visual results of WDNO on challenging 2D control tasks in Figure 8. It can easily be observed that, for many trajectories, our method successfully guides the smoke to pass essentially through the target bucket, which is not achieved by other baselines. + +![](images/figures/wdno-fig-0008.jpg) +Figure 8: Visualizations of WDNO’s performance on the 2D incompressible fluid control task. + +# C ADDITIONAL RESULTS OF EXPERIMENTS + +# C.1 MORE COMPARISONS ON 1D COMPRESSIBLE NAVIER-STOKES EQUATION + +Here, we provide more results on simulation of 1D Compressible Navier-Stokes Equation. We further compare WDNO with Transolver (Wu et al., 2024a), CNO (Raonic et al., 2024), MSVI (Iakovlev et al., 2022), ACDM (Kohl et al., 2024), and DiffusionPDE (Huang et al., 2024). We also add comparisons with diffusion models in Fourier domain and FNO denoiser. The results in Table 5 demonstrate that WDNO still achieves the best performance on MSE. It can be observed that the trends of MAE align closely with MSE. However, the $L _ { \infty }$ error values across different methods are relatively similar because this metric only considers the maximum value across the entire spatiotemporal domain, thus capturing less information. + +Table 5: Comparison of Various Models Based on Error Metrics + +
ModelMSEMAEL∞Error
Transolver4.99840.40254.872849.916917.038660.937016.0514
CNO0.39870.2765
MSVI1.70630.6047
ACDM4.65740.8946
DiffusionPDE5.59360.9792
WNO6.54281.192121.3860
MWT1.38300.519611.3677
OFormer0.62270.400630.9019
FNO0.25750.198511.149517.611616.0532
CNN12.49665.52281.21110.9795
DDPM
Diffusion + FFTFNO Denoiser3.0258145.04690.84986.640614.667031.7515
WDNO (ours)0.21950.104913.0626
+ +# C.2 ABRUPT CHANGES + +Here we provide more visualizations of the comparison between WDNO’s and DDPM’s MAE of different time steps. Figure 9 verifies that WDNO can better model abrupt changes due to the wavelet transform. + +![](images/figures/wdno-fig-0009.jpg) +Figure 9: Visualizations of WDNO’s and DDPM’s MAE on the 1D Burgers’ equation. + +# C.3 APPROXIMATE SCALE INVARIANCE + +We conduct experiments to verify approximate scale invariance by training FNOs on original, oncedownsampled, twice-downsampled, and mixed datasets, then testing at these three resolutions. From Table 6, it is evident that the model trained on the mixed dataset performs better than those trained at specific resolutions. + +Table 6: Approximate scale invariance on the 1D Burgers’ equation. + +
OriginalOnce-downsampledTwice-downsampled
Mix4.72e-044.69e-044.95e-04
Individual4.94e-045.42e-044.94e-04
+ +# C.4 SENSITIVITY ANALYSIS + +To conduct a sensitivity analysis of key hyper-parameters, we analyze the impact of wavelet type, guidance weight $\lambda$ , DDIM sampling steps, and coefficient $\eta$ on 1D simulation and control. As shown in Table 7 and Table 8, WDNO is not sensitive to hyper-parameters. + +(b) DDIM η. + +(c) Wavelet type. + +(a) DDIM step. + +
MSE
200.00022
400.00017
50 1000.00014 0.00015
200
0.00017
+ +Table 7: Results of simulation on 1D Burgers’ equation. + +
MSE
0.2 0.50.00020 0.00020
0.80.00017
10.00014
+ +
MSE
0.2 0.50.00020 0.00020
0.80.00017
10.00014
+ +Table 8: Results of control on 2D incompressible fluid. +(a) DDIM step. + +
Results
200.0223
40 500.0215
1000.0205 0.0200
2000.0217
+ +(c) Guidance weight (1e4). + +(b) DDIM η. + +
Results
0.2 0.50.2285 0.0694
0.80.0244
10.0205
+ +
Results
90.0213
10 11.50.0207 0.0205
12.50.0205
130.0215
+ +# C.5 ROBUSTNESS + +In addition, to test the robustness of WDNO, we first conduct 1D experiments with a 0.1 probability of noise in the control sequence. Table 9 shows that WDNO outperforms other learning-based methods, showing its robustness. We also give the results (mean $\pm$ std) of 1D control averaged over 50 testing samples in Table 10, showing that WDNO’s std is relatively low. + +Table 9: 1D control experiments with a 0.1 probability of noise in the control sequences. + +
MethodsJ
PID (surrogate-solver) SAC (pseudo-online) SAC (offline) BC0.6644 0.2166 0.3979 0.2457
BPPO SL DDPM0.2392 0.0348 0.0701
WDNO (ours)0.0305
+ +Table 10: Results of control tasks (mean±std). Bold font denotes the best model and the runner-up is underlined. + +
MethodResults
PID (surrogate) SAC (pseudo-online) SAC (offline)0.6645 ± 0.5940 0.1376 ± 0.1729 0.3210 ± 0.2733
BC (surrogate) BPPO (surrogate)0.2998 ± 0.1137 0.3075 ± 0.1178
SL DDPM0.0235 ± 0.0171
WDNO (ours)0.0272 ± 0.0198 0.0205 ± 0.0198
+ +# C.6 COMPUTATIONAL RESOURCES + +We provide inference times for a batch size of 1 on A100 in Table 11. It is evidence that WDNO’s runtime is moderate, and its relatively large parameter count is due to the U-Net base model, which can be replaced with smaller models. In addition, we test WDNO’s total training and inference times. As shown in Table 12, WDNO has reasonable spatial and temporal costs. Notably, for the 1D Burgers’ equation, WDNO achieves the lowest training time, as shown in Table 13. + +Table 11: Number of parameters and inference time (s) results of 1D Burgers’ equation. + +
(a) Control task.
MethodsParametersTime
PID (surrogate)40349520.081 0.503
SAC (pseudo-online) SAC (offline)89011116 888547700.466
BC15434080.075
BPPO65791710.079
SL156346181.35
DDPM1407037461.903
WDNO (ours)1407485531.131
+ +
(b) Simulation task.
MethodsParametersTime
WNO1534080.0105
MWT27339780.0093
OFormer25563210.0338
FNO47696010.003
CNN1563460.2771.884
DDPM140703746
WDNO (ours)1407485530.966
+ +Table 12: Total training and inference time of WDNO. + +
ControlSimulation
TimeSpace (MB)TimeSpace (MB)
1D train (A100)2.4h 455s71652.5h7165 3761
1D inference (A100)36915.2s
2D train (2A100)7.8h8043+80397.9h8043+8039
2D inference (A100)2676s3025570.9s16621
+ +Table 13: Training time (h) results of 1D Burgers’ equation. + +
MethodsTime (h)
WNO MWT4.5 6.5
OFormer FNO19.7 10.5
CNN DDPM63.8 7.8
WDNO (ours)2.5
+ +Moreover, in Table 14, we provide the time and space required for WDNO to generate a batch of size 5 during inference on the 1D Burgers’ equation experiment, without super-resolution, and with one, two, and three levels of super-resolution. As shown in Table, with each increase in the level of resolution, the required time and space increase, and the growth rate is increasing. We can infer that as the level of super-resolution increases, the spatiotemporal costs also rise, indicating potential areas for further algorithm optimization. + +Table 14: Inference time and space of 1D super resolution. + +
Level of super resolution0123
Time (s)1.71.86.928.1
Space (MB)17111831350310631
+ +# C.7 IMPORTANCE OF GUIDANCE + +Since $\lambda$ in Eq. 4 is a hyperparameter, it can be set to zero, which means not including this term during denoising. In practice, we have selected the best-performing $\lambda$ . To further show the effectiveness, we have also provided the 1D control results with and without this term in Table 15. We see that without this term, the performance drops a lot. + +Table 15: Results of control on the 1D Burgers’ equation. + +
λResults
0 1200000.3360 0.0205
+ +# C.8 ZERO-SHOT SUPER-RESOLUTION + +In Table 16 and Table 17, we provide results in Section 4.6, which reveal that our method is outstanding in zero-shot super-resolution. + +Table 16: Mean squared error of zero-shot super-resolution on the 1D Burgers’ equation. + +
Methods0 times1 times2 times3 times
WNO (linear) FNO (linear) WDNO (linear) WNO (nearest) FNO (nearest)0.0110 0.00312 0.00259 0.0079 0.00439 0.024730.6284 0.02463 0.00074 0.64911.4474 0.04458 0.00036 1.50072.0588 0.05610 0.00035 2.0588
+ +Table 17: Mean squared error of zero-shot super-resolution on the 2D incompressible fluid. + +
Methods0 times1 times
FNO (linear)0.094970.01692
WDNO (linear)0.093090.00765
FNO (nearest)0.119630.01692
WDNO (nearest)0.120020.00765
+ +# D RELATED WORK + +PDE simulation. Solving a family of PDEs can be regarded as approximating nonlinear operators in the functional space, where neural operators have recently proved effective (Kovachki et al., 2023), such as DeepONet (Lu et al., 2019), FNO (Li et al., 2021). GNOT (Hao et al., 2023), LNPDE (Iakovlev et al., 2023a), CROM (Chen et al., 2022b), DINo (Yin et al., 2022), MagNet (Boussif et al., 2022), Transolver (Wu et al., 2024a) and CNO (Raonic et al., 2024). Additionally, scientific knowledge such as Clifford algebras (Brandstetter et al., 2022a) and Koopman theory (Wang et al., 2022) has been incorporated into neural networks to improve neural operators’ performance. There are also neural ODE based approaches able to simulate PDE systems (Iakovlev et al., 2022; Lagemann et al., 2023). Among them, some models explicitly learn inside functional spaces, such as the Fourier domain (Li et al., 2021) and the wavelet domain (Gupta et al., 2021; Tripura & Chakraborty, 2022; Cheng et al., 2024). However, most existing works mainly focus on simulating physical systems, lacking physical system control problems. Our work proposes a wavelet diffusion neural operator that excels in simulating physical systems and can naturally manage both control and super-resolution simulation tasks. + +Super-resolution tasks. Super-resolution tasks aim to reconstruct high-resolution data from lowresolution data. In recent years, many studies on physical system simulation have focused on super resolution tasks. Some methods transfer the learning of dynamics into function space, naturally enabling zero-shot super resolution capabilities (Li et al., 2021; Tripura & Chakraborty, 2022; Cao et al., 2024). Additionally, some studies attempt to incorporate physical information, such as equation forms, into the model’s learning process to achieve super-resolution (Gao et al., 2021; Jangid et al., 2022; Zayats et al., 2022; Jangid et al., 2022). Other studies primarily achieve super-resolution by injecting high-resolution information into a super-resolution model (Esmaeilzadeh et al., 2020; Ren et al., 2023; Shu et al., 2023). Given the importance of the super-resolution task, we propose leveraging approximate scale invariance to enable diffusion models to achieve super-resolution capabilities. + +Wavelet transform. The wavelet transform, a powerful tool for signal processing and analysis, is widely utilized in designing deep learning algorithms. Due to its ability to decompose signals into high-frequency and low-frequency components, it is used to enhance robustness (Li et al., 2020a), improve accuracy (Li et al., 2020a; Liu et al., 2019), enable super-resolution (Guo et al., 2017; Huang et al., 2017), and extract features (Wang et al., 2021), among other applications. Two closely related works (Hui et al., 2022; Guth et al., 2022) incorporate the wavelet transform into the diffusion model, but these works do not involve space-time multi-resolution correlations. Also, our paper focuses on different tasks and emphasizes the operator characteristics in PDE systems, such as mapping between infinite-dimensional function spaces. + +Long-term predictions. Error accumulation is a common challenge for transient PDE predictions, and several methods have been proposed to address it. Some works suggest training prediction models over multiple steps rather than a single step to enhance robustness in multi-step predictions (Lusch et al., 2018; Brandstetter et al., 2022b). Techniques such as noise injection into training data and adversarial training are employed to improve the model’s resilience to small disturbances (Sanchez-Gonzalez et al., 2020; Lippe et al., 2024), while other works use geometric manifold learning to identify the intrinsic dimensions of observed systems, enabling robust predictions of underlying dynamics (Chen et al., 2022a). + +PDE control. For the task of controlling physical systems governed by PDEs, various deep learningbased techniques have been proposed (Feng et al., 2023; Zhu et al., 2021; Degrave et al., 2022). A prominent class of methods is supervised learning (SL) (Holl et al., 2020; Hwang et al., 2022) which optimizes control input via backpropagation through a neural surrogate model. Unlike these methods, our approach does not rely on auto-regressive surrogate models but instead learns both entire state trajectories and control sequences. Besides, deep reinforcement learning (DRL) has been applied to various physical problems such as drag reduction (Rabault et al., 2019; Elhawary, 2020; Feng et al., 2023; Wang et al., 2024), heat transfer (Beintema et al., 2020; Hachem et al., 2021), and swimming (Novati et al., 2017; Verma et al., 2018). These methods often implicitly incorporate physical information and make decisions sequentially. In contrast, our approach generates entire trajectories, facilitating trajectory-level optimization while embedding physical insights learned by models. Additionally, physics-informed neural networks (PINNs) (Raissi et al., 2019) have recently been used for control (Mowlavi & Nabi, 2023), but they require explicit formulations of PDE dynamics. In contrast, our method is data-driven and can address a broader spectrum of complex physical system control problems without knowledge of the explicit PDE dynamics. + +Diffusion models. The diffusion model (Ho et al., 2020b) is proficient in learning high-dimensional distributions and has succeeded in image and text generation (Dhariwal & Nichol, 2021). It has also demonstrated remarkable ability, including strong modeling capabilities in complex and highdimensional systems and temporal stability, in scientific or engineering problems such as robot control (Janner et al., 2022; Ajay et al., 2022), fluid prediction (Li et al., 2024; Kohl et al., 2024), weather forecasting (Price et al., 2023), 3D human motion generation (Vahdat et al., 2022), PDE simulation based on sparse observation (Huang et al., 2024) and PDE control (Wei et al., 2024; Wu et al., 2024b). Among them, ACDM (Kohl et al., 2024) is an autoregressive model for fluid simulation, which means both training and inference are performed sequentially. The model predicts the next $k$ steps $\boldsymbol { u } _ { [ k , 2 k - 1 ] }$ based on the previous $k$ steps $\boldsymbol { u } _ { [ 0 , k - 1 ] }$ . Then, using $\boldsymbol { u } _ { [ k , 2 k - 1 ] }$ , it predicts $\boldsymbol { u } _ { [ 2 k , 3 k - 1 ] }$ , and this process continues iteratively until the entire trajectory $u _ { [ 0 , T - 1 ] }$ of $T$ steps is generated. In contrast, our proposed method predicts the entire trajectory $u _ { [ 0 , T - 1 ] }$ directly in a single inference step based on the given $k$ initial steps, significantly reducing computational overhead. Bisides, generalization across different resolutions and modeling states with abrupt changes remain challenging, and our WDNO proposes a promising direction to tackle the challenges. Many works have focused on improving the well-posedness of functional space diffusion generation (Pidstrigach et al., 2023; Hagemann et al., 2023; Lim et al., 2023). Previous works commonly choose the Fourier space as the functional space (Lim et al., 2023; Hagemann et al., 2023). Our method differs by using the wavelet transform since it is better at approximating the important abrupt changes. + +# E PSEUDOCODE + +To help understand the entire algorithm, we provide the pseudocode of WDNO’s training and inference in Algorithm 1, and the visualization of WDNO’s entire training and inference on 1D Burgers’ equation in Figure 10. + +![](images/figures/wdno-fig-0010.jpg) +Figure 10: Overview of WDNO. The figure illustrates the training of Base-Resolution Model (BRM, top left), training of Super-Resolution Model (SRM, top right), and inference (bottom) of WDNO on 1D Burgers’ equation. Through multi-resolution training and generation in wavelet space, WDNO is capable of generating superior simulation and control trajectories and conducting zero-shot super-resolution. + +Algorithm 1 Training and Sampling for WDNO + +Require Diffusion models $\epsilon _ { \theta } ( W _ { u _ { [ 0 , T ] } } ^ { ( k ) } , W _ { a } , k )$ , objective $\mathcal { I } ( \cdot )$ for control task, covariance matrix $\sigma ^ { 2 ( k ) } \mathbf { I }$ , condition $W _ { a }$ , schedule $\bar { \alpha } _ { k }$ , hyperparameters $\lambda , \eta , K$ + +# Training: + +1: repeat +2: 3: b Apply the discrete wavelet transform to u(0)[0,T ] $u _ { [ 0 , T ] } ^ { ( 0 ) } \sim q ( u _ { [ 0 , T ] } ^ { ( 0 ) } )$ to get $W _ { u _ { [ 0 , T ] } } ^ { ( 0 ) }$ +4: $\mathbf { \Psi } _ { k } \sim \operatorname { U n i f o r m } ( 1 , \dots , K )$ +5: 6: ϵ ∼ N (0, I)Take gradient descent step on $\nabla _ { \theta } \| \epsilon - \epsilon _ { \theta } ( \sqrt { \bar { \alpha } _ { k } } W _ { u _ { [ 0 , T ] } } ^ { ( 0 ) } + \sqrt { 1 - \bar { \alpha } _ { k } } \epsilon , k ) \| ^ { 2 }$ +7: until converged + +# Sampling: + +1 : $W _ { u _ { \lceil 0 , T \rceil } } ^ { ( K ) } \sim \mathcal { N } ( 0 , \mathbf { I } )$ +2: for $k \doteq K , \ldots , 1 { \bf d o }$ +3: 4: ${ \pmb \xi } \sim \mathcal { N } ( { \bf 0 , I } )$ $k > 1$ $\xi = 0$ kfor control task $W _ { u _ { [ 0 , T ] } } ^ { ( k - 1 ) } = W _ { u _ { [ 0 , T ] } } ^ { ( k ) } - \eta ( \epsilon _ { \theta } ( W _ { u _ { [ 0 , T ] } } ^ { ( k ) } , W _ { a } , k ) + \xi$ $\begin{array} { r } { W _ { u _ { [ 0 , T ] } } ^ { ( k - 1 ) } = W _ { u _ { [ 0 , T ] } } ^ { ( k ) } - \eta ( \epsilon _ { \theta } ( W _ { u _ { [ 0 , T ] } } ^ { ( k ) } , W _ { a } , k ) + \lambda \nabla \mathcal { I } ( W _ { u _ { [ 0 , T ] } } ^ { ( k ) } ) + \xi } \end{array}$ +5: end for6: Apply the inverse discrete wavelet transform to $W _ { u _ { [ 0 , T ] } } ^ { ( 0 ) }$ to get $u _ { [ 0 , T ] } ^ { ( 0 ) }$ +7: return u∗ = u(0)[0,T ] + +# F ADDITIONAL DETAILS FOR 1D BURGERS’ EQUATION CONTROL + +# F.1 EXPERIMENT SETTING + +The equation takes the form + +$$ +\left\{ \begin{array} { l l } { \frac { \partial u ( t , x ) } { \partial t } = - u ( t , x ) \cdot \frac { \partial u ( t , x ) } { \partial x } + \nu \frac { \partial ^ { 2 } u ( t , x ) } { \partial x ^ { 2 } } + f ( t , x ) } & { \mathrm { i n ~ } [ 0 , T ] \times D , } \\ { u ( t , x ) = 0 } & { \mathrm { o n ~ } [ 0 , T ] \times \partial D , } \\ { u ( 0 , x ) = u _ { 0 } ( x ) } & { \mathrm { a t ~ } \{ t = 0 \} , } \end{array} \right. +$$ + +where $u _ { 0 }$ is the initial condition, the diffusion coefficient $\nu = 0 . 0 1$ , $T = 8$ and $D = [ 0 , 1 ]$ . + +During inference, alongside the control sequence $f ( t , x )$ , our diffusion model generates states $\mu ( t , x )$ . Besides, some models produce surrogate states $\mu ( t , x )$ when fed with the control $f ( t , x )$ . However, the state deviation $\begin{array} { r } { \int _ { D } | \bar { u } ( T , x ) - u ^ { * } ( \bar { x } ) | d x } \end{array}$ in our reported evaluation metric $\mathcal { I }$ is always based on the output $u ( T , x ) \stackrel { \textstyle \vartriangle } { = } u _ { \mathrm { g . t . } } ( T , x )$ of the ground-truth solver given the control force $f ( t , x )$ . + +The solver solves the Burgers’ equation (Eq. 8) as described in Appendix F.2, where the internal grid size of the ground-truth numerical solver is consistently at high-resolution $[ 8 0 \times 1 6 , 1 2 0 \times 1 6 ] ,$ ). When model outputs are at a lower resolution, we linearly interpolate them before feeding them into the solver. + +# F.2 DATA GENERATION + +We use the finite difference method (referred to as the solver or ground-truth solver henceforth) to solve the Burgers’ equation of Eq. 8 and generate the training data for the 1D Burgers’ equation. Specifically, the initial state $u _ { 0 } ( x )$ and the control force $f ( t , x )$ are both randomly generated, and then the state’s evolution $u ( t , x )$ is numerically simulated using the solver. In the numerical simulation using the ground-truth solver, a domain of $x = [ 0 , 1 ]$ , $t = [ 0 , 8 ]$ is simulated. The space is discretized into $1 2 0 \times 1 6$ grids and time discretized into $4 8 0 0 \times 1 6$ steps. However, only 80 time stamps are stored in the dataset, and the control sequence $f$ is kept constant between two time stamps. + +After simulation, we downsample by 16 times both spatially and temporally before saving the dataset. Therefore, the data size of each trajectory is [81, 120] for the state $u$ and [80, 120] for the force $f$ . As for the super-resolution dataset for super-resolution simulation, we downsample the original data with shapes $[ 8 0 \times N + 1 , 1 2 0 \times N ]$ for $u$ and $[ 8 0 \times N , 1 2 0 \times N ]$ where $N = 2 , 3 , 4$ corresponding to $1 , 2 , 3$ times super-resolution in Table 16. + +The initial value $u ( 0 , x )$ is a superposition of two Gaussian functions $\begin{array} { r } { u ( 0 , x ) = \sum _ { i = 1 } ^ { 2 } a _ { i } e ^ { - \frac { ( x - b _ { i } ) ^ { 2 } } { 2 \sigma _ { i } ^ { 2 } } } } \end{array}$ where $a _ { i } , b _ { i } , \sigma _ { i }$ are all randomly sampled from uniform distributions: $a _ { 1 } ~ \sim ~ U ( 0 , 2 )$ , $a _ { 2 } \sim$ $U ( - 2 , 0 )$ , $b _ { 1 } \sim U ( 0 . 2 , 0 . 4 )$ , $b _ { 2 } \^ { \cdot } \sim \ U ( \bar { 0 } . 6 , 0 . 8 )$ , $\sigma _ { 1 } \sim U ( 0 . 0 5 , 0 . 1 5 )$ , $\sigma _ { 2 } \sim U ( 0 . 0 5 , 0 . 1 5 ) ,$ ). Similarly, the control sequence $f ( x , t )$ is also a superposition of 8 Gaussian functions $f ( t , x ) ~ =$ $\begin{array} { r } { \sum _ { i = 1 } ^ { 8 } a _ { i } e ^ { - \frac { \left( x - b _ { 1 , i } \right) ^ { 2 } } { 2 \sigma _ { 1 , i } ^ { 2 } } } e ^ { - \frac { \left( t - b _ { 2 , i } \right) ^ { 2 } } { 2 \sigma _ { 2 , i } ^ { 2 } } } } \end{array}$ , where each parameter is independently generated as follows: $b _ { 1 , i } \sim$ $U ( 0 , 1 )$ , $b _ { 2 , i } \sim U ( 0 , 1 )$ , $\sigma _ { 1 , i } \sim U ( 0 . 1 , 0 . 4 )$ , $\sigma _ { 2 , i } \sim U ( 0 . 1 , 0 . 4 )$ , while $a _ { 1 } \sim U ( - 1 . 5 , 1 . 5 )$ and for $i \geq 2$ , $a _ { i } \sim U ( - 1 . 5 , 1 . 5 )$ or 0 with equal probabilities. $u ( t , x )$ , $( t \neq 0 )$ ) is then numerically simulated (using the ground-truth solver) given $u ( 0 , x )$ and $f ( t , x )$ based on Eq. 8. The dataset generation setting is based on previous works Hwang et al. (2022); Wei et al. (2024). + +we generate 40000 trajectories for the training set. For the Burgers’ equation control task, we generate another 50 trajectories for testing. For the super-resolution task, we generate another 2000 trajectories for testing in the $0 \times$ super-resolution task (which is the original resolution). In $1 , 2 , 3 \times$ super-resolution tasks, another 100 samples are generated and shared across the three different super-resolution settings. + +# F.3 DATA PREPARATION FOR WDNO + +We perform a 2D wavelet transform on the original data using the bior2.4 wavelet basis and the ‘periodization’ mode, implemented using the pytorch_wavelets package (Cotter, 2019). This transform the data, originally sized $8 1 \times 1 2 0$ , into four sets of wavelet coefficients, each sized $4 1 \times 6 0$ + +Among these four sets of coefficients, there is one set of coarse coefficients and three sets of detail coefficients. For the multi-resolution dataset used to train the Super-Resolution Model, obtained through downsampling, we conduct wavelet transforms on data sizes $4 1 \times 6 0$ , $2 1 \times 3 1$ , and $1 1 \times 1 5$ , which correspond to four sets of wavelet coefficients sized $2 1 \times 3 0$ , $1 1 \times 1 5$ , and $6 \times 8$ respectively. + +Notably, since the initial condition and the target state are 1D, we take the 1D wavelet transform, repeat the coefficients, and then concatenate them to other data. + +When aligning the sizes of low-resolution wavelet coefficients with high-resolution ones by duplication, special handling is required at the boundaries due to the presence of odd numbers. Specifically, we duplicate the last temporal dimension of the high-resolution data once more to ensure that the sizes match perfectly. + +# F.4 MODEL + +The model architecture in this experiment follows the Denoising Diffusion Probabilistic Model (DDPM) (Ho et al., 2020b). In simulations, the Base-Resolution Model conditions on $u _ { 0 }$ and $f$ to predict $u _ { 0 , T }$ . For control tasks, we condition on $u _ { 0 } , u _ { T }$ and apply guidance related to $\mathcal { I }$ to generate $f _ { [ 0 , T ] }$ . The Super-Resolution Model conditions the same variables as the Base-Resolution Model, with the addition of conditioning on the low-resolution data. Besides, to make the generation meet the initial condition and target state better, we involve the loss between the inverse wavelet transform of the initial condition and the target state’s wavelet coefficient channel and the ground truth into the guidance. The hyperparameters on WDNO are recorded in Table 18. + +Table 18: Hyperparameters of the UNet architecture and training for the results of 1D Burgers’ equation in Table 2a and Table 1. + +
Hyperparameter nameBase-Resolution ModelSuper-Resolution Model
UNet eφ(f)
Initial dimension Downsampling/Upsampling layers Convolution kernel size Dimension multiplier Resnet block groups128 4 3 [1, 2, 4, 8] 8128 4 3 [1, 2, 4, 8] 8
Attention hidden dimension Attention heads32 4 UNet e(u, f )32 4
Initial dimension Downsampling/Upsampling layers Convolution kernel size128 4 3128 4 3
Dimension multiplier Resnet block groups Attention hidden dimension Attention heads[1, 2, 4, 8] 8 32 4[1, 2, 4, 8] 8 32 4
Training Training batch size 16 Adam
Optimizer16 Adam
Learning rate 1e-4 Training steps 190000
Learning rate scheduler cosine annealing
Inference DDIM sampling iterations 50
η of DDIM Sampling 1
Intensity of guidance in control 120000 Scheduler of guidance cosine
+ +# G ADDITIONAL DETAILS FOR 1D COMPRESSIBLE NAVIER-STOKES EQUATION + +# G.1 EXPERIMENT SETTING + +This fluid dynamic equation takes the form + +$$ +\left\{ \begin{array} { l } { \partial _ { t } \rho + \nabla \cdot ( \rho \mathbf { v } ) = 0 , } \\ { \rho \left( \partial _ { t } \mathbf { v } + \mathbf { v } \cdot \nabla \mathbf { v } \right) = - \nabla p + \eta \Delta \mathbf { v } + \left( \zeta + \frac { \eta } { 3 } \right) \nabla ( \nabla \cdot \mathbf { v } ) , } \\ { \partial _ { t } \left( \epsilon + \frac { \rho v ^ { 2 } } { 2 } \right) + \nabla \cdot \left[ \left( \epsilon + p + \frac { \rho v ^ { 2 } } { 2 } \right) \mathbf { v } - \mathbf { v } \cdot \boldsymbol { \sigma ^ { \prime } } \right] = 0 , } \end{array} \right. +$$ + +where $\rho$ is the density, $\mathbf { v }$ is the velocity, $p$ is the pressure, $\epsilon = p / ( \Gamma - 1 )$ is the internal energy with $\Gamma = 5 / 3$ , $\sigma ^ { \prime }$ is the viscous stress tensor, and $\eta , \zeta$ are the shear and bulk viscosity, respectively. The sound velocity is defined as: + +$$ +c _ { s } = { \sqrt { \Gamma { \frac { p } { \rho } } } } , +$$ + +and the Mach number is: + +$$ +M = { \frac { | \mathbf { v } | } { c _ { s } } } . +$$ + +The velocity field for turbulence is initialized as: + +$$ +\mathbf { v } ( x , t = 0 ) = \sum _ { i = 1 } ^ { n } A _ { i } \sin ( k _ { i } x + \phi _ { i } ) , +$$ + +where $\begin{array} { r } { A _ { i } = \frac { \bar { v } } { | k | ^ { d } } } \end{array}$ , $d = 1 , 2$ for 2D and 3D cases, and $\bar { v } = c _ { s } M$ + +The shock-tube field is initialized as: + +$$ +Q ( x , t = 0 ) = ( Q _ { L } , Q _ { R } ) , +$$ + +where $Q = \left( \rho , \mathbf { v } , p \right)$ , with random constants $Q _ { L }$ and $Q _ { R }$ + +In detail, we use a 1D compressible Navier-Stokes equation dataset provided by PDEBench. The initial conditions include random fields, turbulent fields, and shock-tube fields. The random and turbulence fields are prepared by adding perturbations to a uniform background, while the shock-tube setup consists of piecewise constant values generating shocks and rarefactions. Boundary conditions allow waves to exit the domain, and numerical solutions are computed using second-order HLLC and central difference schemes. And we choose the most challenging dataset ’1D_CFD_Shock_Eta1.e-8_Zeta1.e-8_trans_Train.hdf5’ + +Since the original data is of quite high resolution, we downsample it and the final resolution of the used data is $8 1 \times 1 2 0$ , the same as the 1D Burgers’ equation. + +# G.2 DATA PREPARATION FOR WDNO + +Since the data size is the same as Appendix F, the data preparation is almost the same. We also perform a 2D wavelet transform on the original data using the bior2.4 wavelet basis and the ‘periodization’ mode, implemented using the pytorch_wavelets package (Cotter, 2019). As for the initial condition, we take the 1D wavelet transform repeat the coefficients, and then concatenate them to other data. + +# G.3 MODEL + +The model architecture in this experiment follows Appendix F. The hyperparameters on WDNO are recorded in Table 19. + +Table 19: Hyperparameters of the UNet architecture and training for the results of 1D compressible Navier-Stokes equation in Table 1. + +
Hyperparameter nameBase-Resolution Model
UNet φ(f)
Initial dimensionDownsampling/Upsampling layersConvolution kernel sizeDimension multiplierResnet block groupsAttention hidden dimensionAttention heads12843[1, 2, 4, 8]8324
UNet θ(u, f )
Initial dimensionDownsampling/Upsampling layersConvolution kernel sizeDimension multiplierResnet block groupsAttention hidden dimensionAttention heads12843[1, 2, 4, 8]8324
Training
Training batch sizeOptimizerLearning rateTraining stepsLearning rate scheduler16Adam1e-4190000cosine annealing
Inference
DDIM sampling iterationsη of DDIM SamplingScheduler of guidance8501cosine
+ +# H ADDITIONAL DETAILS FOR 2D INCOMPRESSIBLE FLUID + +# H.1 EXPERIMENT SETTING + +The equation takes the form + +$$ +\left\{ \begin{array} { l l } & { \frac { \partial \mathbf { v } ( t , x ) } { \partial t } + \mathbf { v } ( t , x ) \cdot \nabla \mathbf { v } ( t , x ) - \nu \nabla ^ { 2 } \mathbf { v } ( t , x ) + \nabla p ( t , x ) = f ( t , x ) , } \\ & { \nabla \cdot \mathbf { v } ( t , x ) = 0 , } \\ & { \mathbf { v } ( 0 , x ) = \mathbf { v } _ { 0 } ( x ) , } \end{array} \right. +$$ + +where $f$ is the external force, $p$ denotes pressure, $\mathbf { v }$ is the velocity and $\nu$ is the viscosity coefficient. + +# H.2 DATA PREPARATION FOR WDNO + +We performed a 3D wavelet transform on the original data using the bior1.3 wavelet basis and ‘zero’ mode, implemented through the Pytorch Wavelet Toolbox (ptwt) (Wolter et al., 2024). The data of size $3 2 \times 6 4 \times 6 4$ are transformed into eight sets of wavelet coefficients, each sized $1 8 \times 3 4 \times 3 4$ . Among these, there is one set of coarse coefficients and seven sets of detail coefficients. For the multi-resolution dataset used to train the Super-Resolution Model, obtained through downsampling, we do wavelet transforms on data sizes $3 2 \times 3 2 \times 3 2$ and $3 2 \times 1 6 \times 1 6$ corresponding to wavelet coefficient with sizes $1 8 \times 1 8 \times 1 8$ , $1 8 \times 1 0 \times 1 0$ respectively. + +Specifically, since the initial condition and the percentage of smoke through the target bucket are 2D and 1D respectively, we take the 2D and 1D wavelet transform and repeat the coefficients to concatenate them. + +Similarly, to align the duplicated low-resolution data with the high-resolution data, we duplicate the boundary values on both sides of the dimensions requiring super-resolution in the high-resolution data. + +# H.3 MODEL + +In this paper, the architecture of the three-dimensional U-net we employ is inspired by the previous work (Ho et al., 2022). In this experiment, we use spatial-temporal 3D convolutions. Specifically, there are three main modules in our U-net: a downsampling encoder, a middle module, and an upsampling decoder. + +In the simulation, the diffusion model conditions the initial density and control to generate the entire trajectories of density, velocity, and percentage of smoke passing the target bucket. In the control problem, the diffusion model conditions on the initial density and takes the negative percentage of smoke through the target bucket at the last time step as the guidance. Same as the 1D case, to satisfy the initial condition better, the guidance also involves loss between the inverse wavelet transform of the initial condition’s wavelet coefficient channel and the ground truth initial condition. The hyperparameters of the 3D-Unet architecture are in the Table 20. + +Table 20: Hyperparameters of 3D-Unet architecture in 2D experiments. + +
Hyperparameter NameValue
Number of attention headsKernel size of conv3dPadding of conv3dStride of conv3dKernel size of downsamplingPadding of downsamplingStride of downsamplingKernel size of upsamplingPadding of upsampling4(3, 3, 3)(1,1,1)(1,1,1)(1, 4, 4)(1, 2, 2)(0, 1, 1)(1, 4, 4)(1, 2, 2)(0, 1, 1)1001100
I
Kernel size of upsamplingPadding of upsampling
Padding of upsamplingStride of upsamplingDDIM sampling iterationsη of DDIM SamplingIntensity of guidance in controlPad
+ +# I 1D CONTROL BASELINES + +# I.1 PID + +Propercentageal Integral Derivative (PID) control (Li et al., 2006) is a versatile and effective method widely employed in numerous control scenarios. It operates by using the error, i.e., the difference between the desired target and the current state of a system. Due to its simplicity and effectiveness, PID control is often the default choice for many control problems. However, despite its widespread use, PID control faces challenges such as parameter adaptation and limitations in Single Input Single Output (SISO) systems. + +In our study, the 1D Burgers’ Equation Control problem presents a Multiple Input Multiple Output (MIMO) scenario, rendering direct application of PID control infeasible. Inspired by early works (Slama et al., 2019; Ding et al., 2022) that employed neural networks as PID parameter adapters, we integrated deep learning with PID control to address the MIMO control problem. As illustrated in Figure 11, the ANN (artificial neural network) PID uses a neural network to adapt PID parameters, enabling multiple sets of SISO PID control. + +![](images/figures/wdno-fig-0011.jpg) +Figure 11: The architecture of ANN PID Controller. To use the MIMO PID controller to control state $u _ { t }$ to target state $u _ { d }$ , we train a neural-network-based PID parameter planner to output MIMO PID parameters based on $E r r _ { t }$ , then use the PID controller to output the control sequence $f _ { t }$ . + +The neural network generating the PID parameters consists of two 1D convolutional layers, two fully connected layers, and four activation layers. We utilize the $L 1$ loss between the current and target states as the training loss, and the Adam optimizer (Kingma & Ba, 2014) to train the model. Detailed architecture information is provided in Table 21. + +Table 21: Hyperparameters of network architecture and training for ANN PID. + +
Hyperparameter nameFull observation
Kernel size of conv1d3
Padding of conv1d1
Stride of conv1d1
Activation functionSoftsign
Batch size16
OptimizerAdam
Learning rate0.0001
Loss functionMAE
+ +Given that PID is inherently a SISO control method, the ANN PID employs a neural network to derive multiple PID parameter sets, facilitating multiple SISO PID controls for MIMO control in the context of the Burgers’ equation. However, ANN PID requires that the input and output dimensions match, thus it can only address problems with full observation and full control, or partial observation and partial control. + +Additionally, the ANN PID controller has two training setups: one involving direct interaction with the solver, and the other involving interaction with the 1D surrogate model. + +# I.2 SAC + +The Soft Actor-Critic (SAC) algorithm, developed by Haarnoja et al. (2018), represents a significant advancement in reinforcement learning techniques. Designed as an enhancement of the conventional Actor-Critic frameworks, SAC sets itself apart by incorporating an entropy regularization term in its loss function. This addition promotes more effective exploration by simultaneously maximizing the expected cumulative reward and the entropy of the policy itself, leading to improved decision-making processes in complex environments. + +Compared to the Deep Deterministic Policy Gradient (DDPG) algorithm (Lillicrap et al., 2015; Pan et al., 2018), the Soft Actor-Critic (SAC) algorithm introduces entropy regularization that promotes more effective exploration and avoids premature convergence to suboptimal policies, a common drawback of DDPG’s deterministic nature. Furthermore, SAC’s twin Q-networks counteract the overestimation bias that can affect DDPG’s value updates, resulting in more stable learning processes. The automatic adjustment of the temperature parameter in SAC also eases the balancing act between exploration and exploitation, minimizing the necessity for careful hyperparameter tuning. As a result, these features make SAC generally more sample-efficient and robust, especially in complex and continuous action spaces. + +During training, experiences are stored in a replay buffer and sampled randomly to update the networks. Initially, the entire training set is loaded into the replay buffer. For offline SAC, this replay buffer remains unchanged. In contrast, online SAC alternates between gathering experiences through environment interaction and updating the networks using the replay buffer. Offline SAC, however, utilizes a surrogate model trained on the training set to gather experiences instead of the real environment. The policy network is optimized to maximize the expected return by considering both the Q-value and the entropy term. The critic networks are trained to minimize the error between their Q-value predictions and the target Q-values. To further stabilize training, SAC employs a target critic network, which is slowly updated with the weights from the main critic network. For inference, SAC uses the policy network to select the action with the highest probability. + +To effectively guide the system towards the target state with accuracy and speed, it is essential to incorporate the distance between the state at each time step and the target state into the reward function. Therefore, the reward function for a given time step $t$ , state $u _ { t }$ , target state $u _ { T }$ , and action $w _ { t }$ is defined as follows: + +$$ +r ( t , u _ { t } , y _ { T } , w _ { t } ) = - \int _ { \Omega } \left| u _ { t } - u _ { d } \right| ^ { 2 } d x - \alpha \int _ { \Omega } \left| w _ { t } \right| ^ { 2 } d x , +$$ + +where $\Omega$ is the space domain and $\alpha$ is the weight of energy. We take the Adam optimizer (Kingma & Ba, 2014) to train the networks and update the temperature parameter. The detailed values of hyperparameters are provided in Table 22. + +Table 22: Hyperparameters of 1D SAC. + +
Hyperparameter nameValue
Hyperparameters for 1D Burgers' equation control: Discount factor for reward0.5
Target smoothing coefficient Learning rate of critic loss Learning rate of entropy loss Learning rate of policy loss Training batch size Number of episodes Number of model updates per simulator step Value target updates per step Size of replay buffer weight of energy cost Number of trajectories interacted with the environment per step0.05 0.0003 0.003 0.003 8192 1500 50 15 1000000 0.00002 1 3 4096 5
+ +# I.3 SUPERVISED LEARNING + +The paper (Hwang et al., 2022) proposes a supervised-learning-based control algorithm that takes a neural operator as a surrogate model to solve control problems. It contains two stages. In the first stage, we take a neural operator to learn the PDE constraint as Hwang et al. (2022). Two VAEs based on CNN learn to project state $u$ , control signal $f$ into the latent space, and a CNN learns the transition from $u _ { t }$ to $u _ { t + 1 }$ in the latent space. In the second stage, these three neural networks are used as surrogate models to calculate the gradient of the objective function with respect to the control input and optimize the control signal $f$ . + +During optimization, the reconstruction loss for the control force is also included to guide it out of the adversarial mode of the surrogate model. We consider the control $f$ as a learnable parameter and update it with the LBFGS optimizer (Liu & Nocedal, 1989). The hyperparameters of this supervised learning method are recorded in Table 23. + +Table 23: Hyperparameters of inference the 1D supervised learning method. + +
Hyperparameter nameValue
Hyperparameters for 1D Burgers' equation control
Learning rate of w updating0.1
Number of epochs100
Weight of average objective function loss1
Weight of average reconstruction loss0.01
Termination tolerance on first-order optimality of LBFGS optimizer1e-5
Termination tolerance on parameter changes LBFGS optimizer1e-5
+ +# I.4 BPPO + +The Behavior Proximal Policy Optimization (BPPO) algorithm, introduced by (Zhuang et al., 2023), is an advanced reinforcement learning method that combines the strengths of Proximal Policy Optimization (PPO) with elements of behavior cloning. BPPO is an offline algorithm designed to monotonically improve the behavior policy in a manner akin to PPO. Due to the inherent conservatism of PPO, BPPO restricts the ratio of the learned policy to the behavior policy within a specific range, similar to other offline RL methods, which ensures the learned policy closely aligns with the behavior policy. By leveraging the conservatism of online on-policy algorithms, BPPO effectively addresses the overestimation issue often encountered in offline RL settings. + +The algorithm begins by estimating a behavior policy using behavior cloning and then iteratively improves a target policy using the PPO objective with a behavior constraint. This process of policy improvement, advantage estimation, and policy update enables BPPO to refine the target policy while ensuring it remains close to the behavior policy. By integrating the strengths of online on-policy methods with tailored offline RL techniques, BPPO has demonstrated promising results on the D4RL benchmark, surpassing state-of-the-art offline RL algorithms. + +During training, BPPO first initializes the behavior policy $\pi _ { \beta }$ and the target policy $\pi _ { \theta }$ . The behavior policy $\pi _ { \beta }$ is then estimated using behavior cloning to replicate the behavior demonstrated in the offline dataset. Subsequently, the target policy $\pi _ { \theta }$ is optimized using the PPO objective with a behavior constraint, ensuring the target policy remains close to the behavior policy. The advantage function $A ^ { \pi _ { \beta } }$ is then estimated using the behavior policy $\pi _ { \beta }$ to evaluate the quality of actions taken by the target policy. Finally, the target policy is updated by maximizing the PPO objective with the estimated advantage function, and adjusting the policy parameters to enhance performance. In the implementation, a state value network and a $\mathrm { Q }$ value network are pre-trained using the state, action, and reward data from the offline dataset. + +In practice, to enable the system to approximate the target state accurately and swiftly, it is essential to incorporate the distance between the state at each time step and the target state into the reward function. Thus, the reward function at time step $t$ , given the state $u _ { t }$ , target state $u _ { d }$ , and action $f _ { t }$ , is defined as follows: + +$$ +r ( t , u _ { t } , u _ { d } , f _ { t } ) = - \int _ { \Omega } | u _ { t } - u _ { d } | ^ { 2 } \mathrm { d } x - \alpha \int _ { \Omega } | f _ { t } | ^ { 2 } \mathrm { d } x , +$$ + +where $\Omega$ is the space domain and $\alpha$ is the weight of energy. We use the Adam optimizer (Kingma & Ba, 2014) to train the networks and update the temperature parameter. The specific values of the hyperparameters used are detailed in Table 24. + +Table 24: Hyperparameters of 1D BPPO. + +
Hyperparameter nameValue
Hyperparameters for 1D Burgers' equation control:
State value network:Learning rate of value networkSteps of value networkNumber of layers of value networkBatch size of value networkNumber of hidden dimensions of value network1 × 10−42 × 1063512512
Q value network:Learning rate of Q networkSteps of Q networkNumber of layers of Q networkBatch size of Q networkNumber of hidden dimensions of Q networkTarget Q network updates per stepSoft update factorDiscount factor for reward1 × 10-42 × 1062512102420.0050.99
Behavior cloning:Learning rate of BCTraining batch size of BCNumber of episodes of BC1 × 10−45125 × 105
BPPO:Number of episodes of BPPONumber of layers of policy networksNumber of hidden dimensions of policy networksLearning rate of BPPOTraining batch size of BPPOClip ratio of BPPOWeight decay factorWeight of advantage functionSize of replay bufferActivation function1 × 102210241 × 10-55120.250.960.92 × 106ReLU
+ +Table 25: Hyperparameters of 1D BC. + +
Hyperparameter nameValue
Hyperparameters for 1D Burgers' equation control:
Learning rate1 × 10−4 512
Training batch size Number of episodes5 × 105
Size of replay buffer2 × 106
Number of layers of policy networks2
Number of hidden dimensions of policy networks1024
Activation functionReLU
+ +# I.5 BC + +The Behavior Cloning (BC) algorithm, introduced by (Pomerleau, 1988), is a foundational technique in imitation learning. BC is designed to derive policies directly from expert demonstrations, utilizing supervised learning to associate states with corresponding actions. This method eliminates the necessity for exploratory steps commonly required in reinforcement learning by replicating the actions observed in expert demonstrations. One of the significant advantages of BC is that it does not involve interacting with the environment during the training phase, which streamlines the learning process and diminishes the demand for computational resources. + +In this approach, a policy network is trained using standard supervised learning strategies aimed at reducing the discrepancy between the actions predicted by the model and those performed by the expert in the dataset. The commonly used loss function for this purpose is the mean squared error between the predicted actions and expert actions. The dataset for training comprises state-action pairs harvested from these expert demonstrations. During inference, the model is evaluated using the same objective function as used in SAC. The specific hyperparameters utilized are detailed in Table 25. + +# J 1D SIMULATION BASELINES + +# J.1 FNO + +FNO represents a deep learning framework capable of mapping between infinite-dimensional spaces. By parameterizing the integral kernel in Fourier space, FNO processes input through a sequence of Fourier layers, performing linear transformations in the Fourier domain for efficient convolutions. This architecture supports zero-shot super-resolution, allowing models trained on lower resolutions to predict at higher resolutions without retraining. + +In 1D experiments, we train the FNO model using the initial state and all controls as the input and using the rest states as the output. The parameters are outlined in Table 26. + +Table 26: Hyperparameters of 1D FNO. + +
Hyperparameter nameValue
Hyperparameters for 2D Burgers' equation:
Number of modes to keep in Fourier Layer Width of the FNO (i.e. number of channels) Number of input channel Number of output channel16 64 3 1 256 256
Number of hidden channels of the lifting block Number of hidden channels of the projection block Number of Fourier Layers Expansion parameter of MLP layer Non-Linearity module4 0.5
Rank of the tensor factorization of the Fourier weights Mode of domain padding Learning rateGelu 1.0 one-sided 1 × 10−4
+ +# J.2 WNO + +The Wavelet Neural Operator (WNO) (Tripura & Chakraborty, 2022) is a novel operator learning algorithm that blends integral kernel with wavelet transformation. We record the hyperparameters of it in 1D simulation in Table 27. On 1D Burgers’ equation and 1D compressible Navier-Stokes equation, we choose ‘sym4’, ‘bior2.4’ wavelet respectively. + +Table 27: Hyperparameters of 1D WNO. + +
Hyperparameter nameValue
Hyperparameters of the model architecture Type of waveletsym4
Level of wavelet decomposition Uplifting dimension Number of wavelet layers5 40 4
Training Training batch size100 Adam
Optimizer Learning rate Training epochs Learning rate scheduler1e-3 1000 StepLR
+ +# J.3 CNN + +Convolutional Neural Network is the key block in deep learning. For 1D simulation, our CNN model is based on Hwang et al. (2022). Details of model architecture and training can be found in Table 28 + +Table 28: Hyperparameters of 1D CNN. + +
Hyperparameter nameValue
Autoencoder of statecYanmaGenttlowb
Convolution kernel sizeConvolution paddingActivation functionLatent vector size52ELU256
Autoencoder of forcecYnmaGetlow
Convolution kernel sizeConvolution paddingActivation functionLatent vector size52ELU256
Training
Training batch sizeOptimizerLearning rateTraining epochsLearning rate scheduler5100Adam1e-3500cosine annealing
+ +# J.4 MWT + +To compare with other wavelet-based methods, we mainly implement the 1D baseline adapted from Gupta et al. (2021). We select ‘legendre’ wavelet here, following the original work. More configurations can be found in Table 29. + +Table 29: Configuration of 1D MWT. + +
Hyperparameter nameValue
Wavelet basis Number of Fourier modeslegendre 10
Kernel size4
Training batch size256
Training epochs300
OptimizerAdam
Learning rate scheduler
MultiStepLR
+ +# J.5 OFORMER + +The Operator Transformer (OFormer)Li et al. (2023) is a novel framework built upon self-attention, cross-attention, and a set of point-wise multilayer perceptrons. Details of model architecture and training can be found in Table 30. + +Table 30: Hyperparameters of 1D OFormer. + +
Hyperparameter nameValue
Encoder
TypeInput ChannelsEmbedding Dim of TokenEmbedding Dim of Encoded SequenceHeadsDepthResolutionDropout of EmbeddingSpatialEncoder2D396256461200.05
Depth
Decoder
TypeLatent ChannelsOut ChannelsScaleResPointWiseDecoder2DSimple25610.5120
Training
Training batch sizeIterationLearning rate32500001e-4
+ +# J.6 LADID + +In the dynamics trajectory prediction community, predicting trajectories of dynamical systems is of interest (Chen et al., 2018). Among them, MS-L-NODE (Iakovlev et al., 2023b) is a representative method that is dedicated to learning the system invariant dynamics. Since MS-L-NODE effectively operates on high-dimensional spatial-temporal data, we include it as a baseline for a more comprehensive empirical comparison. + +Since the original work only considers spatially 2D input, we modify the encoder and decoder from stacked 2D CNNs to 1D CNNs, tune the latent dimension in {4, 8, 16, 64}, and the CNN base output dimension in {16, 64, 128}. Important hyperparameters are reported in Table 31 and the rest are kept the same as in Iakovlev et al. (2023b). + +# K 2D SIMULATION BASELINES + +# K.1 FNO + +In 2D experiments, we train the FNO model using the density, velocity, control, and percentage of smoke through the target bucket of the previous step as the input and using the rest step’s density, velocity, and percentage of smoke through the target bucket as the output. The parameters are outlined in Table 32. + +Table 31: Hyperparameters of MS-L-NODE. + +
Hyperparameter nameValue
Encoder
Encoder CNN channelsLatent dimension1288
Decoder
Encoder CNN channelsLatent dimension1288
Aggregation Network
HeadsStatic layersDynamical layers1648
Training
Training batch sizeTraining iterationsLearning rate64375001e-3
+ +Table 32: Hyperparameters of 2D FNO. + +
Hyperparameter nameValue
Hyperparameters for 2D incompressible fluid:
Number of modes to keep in Fourier Layer Width of the FNO (i.e. number of channels) Number of input channel16 64 6 4 256
Number of output channel Number of hidden channels of the lifting block Number of hidden channels of the projection block
Number of Fourier Layers256 4
Expansion parameter of MLP layer Non-Linearity module0.5
Rank of the tensor factorization of the Fourier weightsGelu
Mode of domain padding1.0
one-sided
Learning rate
1 × 10-4
Optimizer
Training epochsAdam
1000
Learning rate scheduler
Training batch sizeCosine
+ +Table 33: Hyperparameters of 2D WNO. + +
Hyperparameter nameValue
Hyperparameters of the model architecture
Type of waveletLevel of wavelet decompositionUplifting dimensionNumber of wavelet layersdb4283
Training
Training batch sizeOptimizerLearning rateTraining epochsLearning rate scheduler50Adam0.05500StepLR
+ +# K.2 WNO + +The hyperparameters of WNO for the 2D simulation task are in Table 33. And the wavelet we choose is ‘bior1.3’. + +# K.3 MWT + +For a more comprehensive comparison with other wavelet-based approaches, we focus primarily on implementing the 2D baseline inspired by the work in Gupta et al. (2021). Following the previous work, we select the ‘Legendre’ wavelet. More details on the configurations can be found in Table 34. + +Table 34: Configuration of 2D MWT. + +
Hyperparameter nameValue
Wavelet basis Number of Fourier modeslegendre 12
Kernel size3
Training batch size200
Training epochs300
Optimizer
Adam
Learning rate schedulerMultiStepLR
+ +Table 35: Hyperparameters of 2D OFormer. + +
Hyperparameter nameValue
Encoder
TypeInput ChannelsEmbedding Dim of TokenEmbedding Dim of Encoded SequenceHeadsDepthSpatialTemporalEncoder2D349619215
Decoder
TypeOut ChannelsEmbedding Dim of TokenPropagate forwardLength of output sequencePropagator depthCurriculum ratioCurriculum stepsPointWiseDecoder2D19613210.1610
Training
Training batch sizeIterationLearning rate81000001e-4
+ +# K.4 OFORMER + +The Operator Transformer (OFormer) Li et al. (2023) is an attention-based framework for learning solution operators of partial differential equations using self-attention, cross-attention, and point-wise MLPs, designed to handle various input sampling patterns and query locations. More details on the configurations can be found in Table 35. + +# L BROADER IMPACTS + +Our research proposes a method to simulate and control complex physical systems. We believe our research will bring in significant progress for various scientific and engineering domains, including climate forecasting, fluid control, robotic control, et al. However, there is also a potential that the method might be abused to incur negative social consequences, upon which we should remain vigilant. \ No newline at end of file diff --git a/papers/wdno/paper.pdf b/papers/wdno/paper.pdf new file mode 100644 index 0000000000000000000000000000000000000000..135ca88364a52af997f9ff9594223d3ba692fcd6 --- /dev/null +++ b/papers/wdno/paper.pdf @@ -0,0 +1,3 @@ +version https://git-lfs.github.com/spec/v1 +oid sha256:cadf320130bde4642859896c2cefc5074ff481a4b8e4d4363d7227db24ba29e4 +size 7543785 diff --git a/papers/wdno/sau.json b/papers/wdno/sau.json new file mode 100644 index 0000000000000000000000000000000000000000..4d4756e2b3076bf4e24d0fcbfcd4e7ba2778282e --- /dev/null +++ b/papers/wdno/sau.json @@ -0,0 +1,482 @@ +{ + "paper_id": "wdno", + "paper_title": "Wavelet Diffusion Neural Operator (WDNO)", + "D1": [ + { + "id": "wdno-D1-001", + "claim": "WDNO BRM/SRM training config (all experiments): Adam optimizer, lr=1e-4, cosine annealing scheduler, 190000 training steps, batch_size=16. 1D experiments on 1 A100 GPU (2.4-2.5h), 2D on 2 A100 GPUs (7.8-7.9h).", + "source": "Table 18, Table 19, Table 20, Table 12" + }, + { + "id": "wdno-D1-002", + "claim": "WDNO 1D U-Net architecture hyperparameters (for Burgers/Advection/Navier-Stokes): init_dim=128, down_up_layers=4, conv_kernel=3, dim_multiplier=[1,2,4,8], resnet_block_groups=8, attn_hidden_dim=32, attn_heads=4.", + "source": "Table 18, Table 19" + }, + { + "id": "wdno-D1-003", + "claim": "1D DDIM sampling config: DDIM_sampling_steps=50, DDIM_eta=1 (equivalent to DDPM stochastic sampling) for Burgers, Advection, and Navier-Stokes inference.", + "source": "Table 18, Table 19" + }, + { + "id": "wdno-D1-004", + "claim": "1D Burgers control guidance config: guidance_weight=120000 with cosine scheduler for guidance intensity during control optimization.", + "source": "Table 18, Section C.4 Table 8" + }, + { + "id": "wdno-D1-005", + "claim": "1D Burgers PDE and dataset config: T=8, D=[0,1], nu=0.01 Dirichlet BC; 40000 train trajectories; test: 50 (control), 2000 (super-res 0x), 100 (super-res 1x/2x/3x); 81 stored timesteps; state shape [81,120], force shape [80,120]; solver internal grid 120x16 spatial, 4800x16 temporal, downsample factor 16.", + "source": "Appendix F.1, Appendix F.2" + }, + { + "id": "wdno-D1-006", + "claim": "1D Burgers initial condition generation: sum of 2 Gaussians; amplitudes a1~U(0,2), a2~U(-2,0); positions b1~U(0.2,0.4), b2~U(0.6,0.8); widths sigma~U(0.05,0.15).", + "source": "Appendix F.2" + }, + { + "id": "wdno-D1-007", + "claim": "1D Burgers control force generation: sum of 8 Gaussians; spatial/temporal positions b1,i~U(0,1), b2,i~U(0,1); widths sigma1,i~U(0.1,0.4), sigma2,i~U(0.1,0.4); amplitude a1~U(-1.5,1.5); for i>=2, ai~U(-1.5,1.5) or 0 with 50% probability.", + "source": "Appendix F.2" + }, + { + "id": "wdno-D1-008", + "claim": "1D wavelet transform config (Burgers/Advection/Navier-Stokes): 2D DWT using bior2.4 wavelet, periodization padding mode, pytorch_wavelets (Cotter 2019). Output shape [4,41,60] from 81x120 input. Applies to 81 timesteps (Burgers) and 80 timesteps (Advection, from PDEBench).", + "source": "Appendix F.3, Appendix G.2" + }, + { + "id": "wdno-D1-009", + "claim": "1D Navier-Stokes dataset config: shear and bulk viscosity eta=zeta=1e-8, heat capacity ratio Gamma=5/3, data resolution [81,120], 9000 training samples, PDEBench CFD shock dataset ('1D_CFD_Shock_Eta1.e-8_Zeta1.e-8_trans_Train.hdf5').", + "source": "Section 4.3, Appendix G.1" + }, + { + "id": "wdno-D1-010", + "claim": "2D wavelet transform config (Fluid/ERA5): 3D DWT using bior1.3 wavelet, zero padding mode, implemented via ptwt (Wolter 2024). Input shape [32,64,64], output wavelet coefficient shape [8,18,34,34] (1 coarse + 7 detail channels).", + "source": "Section 4.4, Appendix H.2" + }, + { + "id": "wdno-D1-011", + "claim": "2D incompressible fluid dataset config: 32 time steps, 64x64 spatial grid, 3584 peripheral control variables per time step, fluid-solid coupling with no-slip boundary conditions at obstacles.", + "source": "Section 4.4" + }, + { + "id": "wdno-D1-012", + "claim": "WDNO 3D U-Net hyperparameters (2D experiments): Conv3D kernel=(3,3,3), padding=(1,1,1), stride=(1,1,1); spatial-only downsampling kernel=(1,4,4), stride=(0,1,1); spatial-only upsampling kernel=(1,4,4); attn_heads=4. Inspired by Video Diffusion Models (Ho et al., 2022).", + "source": "Table 20" + }, + { + "id": "wdno-D1-013", + "claim": "2D DDIM sampling config: DDIM_sampling_steps_2D=100, DDIM_eta_2D=1, used for 2D fluid and ERA5 inference.", + "source": "Table 20" + }, + { + "id": "wdno-D1-014", + "claim": "2D fluid control guidance config: guidance_weight_2D=100 for indirect smoke navigation control task.", + "source": "Table 20" + }, + { + "id": "wdno-D1-015", + "claim": "Zero-shot super-resolution config: 1D base [80,120], targets [160,240] (1x), [320,480] (2x), [640,960] (3x); 2D base [32,64,64], target [32,128,128]. Multi-res wavelet coeff shapes: 1D [4x21x30, 4x11x15, 4x6x8]; 2D [8x18x18x18, 8x18x10x10].", + "source": "Section 4.6, Appendix F.3, Appendix H.2" + }, + { + "id": "wdno-D1-016", + "claim": "ERA5 dataset config: 0.25-degree latitude-longitude resolution, surface to 100km altitude, variable=temperature. Prediction task: 12h input history to forecast 20h ahead.", + "source": "Section 4.5" + }, + { + "id": "wdno-D1-017", + "claim": "Ablation study configs: measurement noise scales [0.01, 0.001, 0.0001] times data_std on 1D Burgers; training sample fractions [0.2, 0.4, 0.6, 0.8] of 9000 samples on 1D Navier-Stokes; control sequence noise prob=0.1 in robustness test.", + "source": "Section 4.7, Table 9, Section C.5" + }, + { + "id": "wdno-D1-018", + "claim": "SAC baseline hyperparameters (1D Burgers control): discount=0.5, target_smoothing=0.05, critic_lr=3e-4, entropy_lr=3e-3, policy_lr=3e-3, batch=8192, episodes=1500, model_updates_per_step=50, target_updates=15, replay_buffer=1M, energy_weight=2e-5.", + "source": "Table 22" + }, + { + "id": "wdno-D1-019", + "claim": "BPPO baseline hyperparameters (1D Burgers control): state value net (lr=1e-4, 2M steps, 3 layers, hidden=512); Q net (lr=1e-4, 2M steps, 2 layers, hidden=1024, discount=0.99, target_update=2, soft_update=0.005); BC pretrain (lr=1e-4, 500k episodes); main (100k episodes, 2 layers, hidden=1024, lr=1e-5, clip=0.25, ReLU, batch=512).", + "source": "Table 24" + }, + { + "id": "wdno-D1-020", + "claim": "BC baseline hyperparameters (1D Burgers control): lr=1e-4, batch=512, episodes=500k, replay_buffer=2M, 2 layers, hidden=1024, ReLU activation.", + "source": "Table 25" + }, + { + "id": "wdno-D1-021", + "claim": "SL (Supervised Learning) baseline hyperparameters (1D Burgers control): LBFGS optimizer, lr=0.1, epochs=100, objective loss weight=1, reconstruction loss weight=0.01, tolerance=1e-5.", + "source": "Table 23" + }, + { + "id": "wdno-D1-022", + "claim": "ANN PID baseline hyperparameters (1D Burgers control): 1D conv kernel=3, padding=1, stride=1, Softsign activation, batch=16, lr=1e-4, MAE loss.", + "source": "Table 21" + }, + { + "id": "wdno-D1-023", + "claim": "FNO baseline hyperparameters (1D and 2D): 1D: modes=16, width=64, in_ch=3, out_ch=1, 4 Fourier layers, lift/proj hidden=256, GeLU activation, lr=1e-4; 2D: modes=16, width=64, in_ch=6, out_ch=4, 4 Fourier layers, lr=1e-4, epochs=1000, cosine scheduler.", + "source": "Table 26, Table 32" + }, + { + "id": "wdno-D1-024", + "claim": "WNO baseline hyperparameters (1D and 2D): 1D: sym4 wavelet, 5 levels, uplift_dim=40, 4 layers, batch=100, lr=1e-3, epochs=1000, StepLR scheduler; 2D: db4 wavelet, 2 levels, uplift_dim=8, 3 layers, batch=50, lr=0.05, epochs=500, StepLR scheduler.", + "source": "Table 27, Table 33" + }, + { + "id": "wdno-D1-025", + "claim": "MWT baseline hyperparameters (1D and 2D): 1D: Legendre wavelet, 10 Fourier modes, kernel=4, batch=256, epochs=300, MultiStepLR scheduler; 2D: Legendre wavelet, 12 Fourier modes, kernel=3, batch=200, epochs=300, MultiStepLR scheduler.", + "source": "Table 29, Table 34" + }, + { + "id": "wdno-D1-026", + "claim": "OFormer 1D baseline: SpatialEncoder2D (in_ch=3, embed_dim=96, encoded_dim=256, heads=4, depth=6, res=120, dropout=0.05), PointWiseDecoder2DSimple (latent=256, out_ch=1), batch=32, 50000 iterations, lr=1e-4.", + "source": "Table 30" + }, + { + "id": "wdno-D1-027", + "claim": "OFormer 2D baseline: SpatialTemporalEncoder2D (in_ch=3, embed_dim=496, encoded_dim=192, heads=1, depth=5), PointWiseDecoder2D (embed_dim=96, out_ch=1, out_seq_len=32, propagate_forward=1, curriculum_ratio=0.1, curriculum_steps=6), batch=8, 100000 iterations, lr=1e-4.", + "source": "Table 35" + }, + { + "id": "wdno-D1-028", + "claim": "MS-L-NODE (MSVI) baseline hyperparameters (1D simulation): encoder CNN channels=128, latent_dim=8, aggregation heads=1, 6 static + 4 dynamic layers, batch=64, 37500 iterations, lr=1e-3.", + "source": "Table 31" + }, + { + "id": "wdno-D1-029", + "claim": "CNN 1D baseline: conv kernel=5, padding=2, ELU activation, latent=256, batch=100, lr=1e-3, epochs=500, cosine scheduler.", + "source": "Table 28" + }, + { + "id": "wdno-D1-030", + "claim": "Sensitivity analysis configs: DDIM steps grid [20,40,50,100,200]; DDIM eta grid [0.2,0.5,0.8,1.0] for 1D Burgers control; guidance weight grid [9,10,11.5,12.5,13] x1e4 for 2D control.", + "source": "Table 7, Table 8" + } + ], + "D2": [ + { + "id": "wdno-D2-001", + "claim": "DDPM forward process (noise addition): x_{k+1} = sqrt(alpha_k) * x_k + sqrt(1 - alpha_k) * epsilon, where epsilon ~ N(0, I) and {alpha_k}_{k=1..K} is the variance schedule. Implemented via: x_k = sqrt(alpha_bar_k) * x_0 + sqrt(1 - alpha_bar_k) * epsilon, where alpha_bar_k = prod_{i=1..k} alpha_i.", + "source": "Section 2.2" + }, + { + "id": "wdno-D2-002", + "claim": "DDPM reverse process (denoising): x_{k-1} = 1/sqrt(alpha_k) * (x_k - (1-alpha_k)/sqrt(1-alpha_bar_k) * epsilon_theta(x_k, k)) + sigma_k * z, where z ~ N(0, I) if k > 1 else z = 0. epsilon_theta is the learned denoising U-Net.", + "source": "Section 2.2" + }, + { + "id": "wdno-D2-003", + "claim": "DDPM training loss (simplified variational bound): L = E_{k~U(1,K), x_0~p(x), epsilon~N(0,I)} [|| epsilon - epsilon_theta(sqrt(alpha_bar_k) * x_0 + sqrt(1 - alpha_bar_k) * epsilon, k) ||_2^2]", + "source": "Section 2.2, Eq. 3" + }, + { + "id": "wdno-D2-004", + "claim": "Classifier-free guidance sampling: epsilon_theta_guided(x, y) = epsilon_theta(x, empty) + omega * (epsilon_theta(x, y) - epsilon_theta(x, empty)), where omega in [0, 1] is the guidance weight and 'empty' is the null-condition identifier.", + "source": "Section 2.2" + }, + { + "id": "wdno-D2-005", + "claim": "Discrete wavelet transform (1D signal decomposition): phi_{l,m}(x) = 2^{l/2} * phi(2^l * x - m); psi_{l,m}(x) = 2^{l/2} * psi(2^l * x - m). With l_0 = L (maximum level), the decomposition is: u(x) = sum_m c_L(m) * phi_{L,m}(x) + sum_m d_L(m) * psi_{L,m}(x), where c_L are coarse coefficients and d_L are detail coefficients. In code: apply 2D DWT via pytorch_wavelets (1D) or 3D DWT via ptwt (2D) to get coefficient arrays.", + "source": "Section 3.1" + }, + { + "id": "wdno-D2-006", + "claim": "WDNO simulation: denoising in wavelet domain: Initialize W_{u,[0,T]}^{(K)} ~ N(0, I). For k = K down to 1: W_{u,[0,T]}^{(k-1)} = W_{u,[0,T]}^{(k)} - eta * epsilon_theta(W_{u,[0,T]}^{(k)}, W_a, k) + xi, where xi ~ N(0, sigma_k^2 * I) if k > 1 else xi = 0. W_a is the wavelet-transformed equation parameter (e.g., initial condition). Final output is obtained by inverse DWT on W_{u,[0,T]}^{(0)}. DDIM acceleration is used during inference.", + "source": "Section 3.1, Eq. 3" + }, + { + "id": "wdno-D2-007", + "claim": "WDNO control: guided denoising with objective gradient: Initialize W_{f,[0,T]}^{(K)} ~ N(0, I). For k = K down to 1: W_{f,[0,T]}^{(k-1)} = W_{f,[0,T]}^{(k)} - eta * (epsilon_theta(W_{f,[0,T]}^{(k)}, W_a, k) + lambda * grad_{W_f} J(W_hat_{f,[0,T]}^{(k)})) + xi, where xi ~ N(0, sigma_k^2 * I) if k > 1 else xi = 0. W_hat_{f,[0,T]}^{(k)} is the estimated noise-free W_f^{(0)} extracted via: W_hat_{f,[0,T]}^{(k)} = (W_{f,[0,T]}^{(k)} - sqrt(1 - alpha_bar_k) * epsilon_theta(W_{f,[0,T]}^{(k)}, W_a, k)) / sqrt(alpha_bar_k). lambda is the guidance weight. J is the control objective.", + "source": "Section 3.1, Eq. 4, Eq. 5" + }, + { + "id": "wdno-D2-008", + "claim": "WDNO noise-free estimate from noisy sample (one-step prediction): W_hat_f^{(k)} = (W_f^{(k)} - sqrt(1 - alpha_bar_k) * epsilon_theta(W_f^{(k)}, W_a, k)) / sqrt(alpha_bar_k). This is used to compute the objective J on the estimated clean data rather than noisy data, avoiding noise-induced errors in the guidance gradient.", + "source": "Section 3.1, Eq. 5" + }, + { + "id": "wdno-D2-009", + "claim": "Multi-resolution training: dataset preparation by downsampling: Given original resolution N x M (time x space), create data pairs by downsampling: (N, M) and (N/2, M/2); (N/2, M/2) and (N/4, M/4); (N/4, M/4) and (N/8, M/8); etc. Apply wavelet transform to each resolution level. For 1D Burgers with original [80, 120]: downsampled to [40, 60], [20, 30], [10, 15]. Wavelet coefficients: original 81x120 -> 4x41x60; downsampled 41x60 -> 4x21x30; 21x31 -> 4x11x15; 11x15 -> 4x6x8.", + "source": "Section 3.2, Appendix F.3" + }, + { + "id": "wdno-D2-010", + "claim": "Super-Resolution Model (SRM) training: Train conditional diffusion model p(W_h | W_l, W_{a_h}) using paired high-resolution W_h and low-resolution W_l data. To align sizes, duplicate low-resolution coefficients to match high-resolution dimensions (with boundary duplication for odd numbers). Each training batch randomly selects data pairs from a given resolution level. Training loss is the standard DDPM MSE loss between predicted and true noise.", + "source": "Section 3.2" + }, + { + "id": "wdno-D2-011", + "claim": "WDNO zero-shot super-resolution inference: 1) Downsample a to base resolution NxM, wavelet transform to get W_a. 2) Use BRM to generate wavelet coefficients W_base at resolution NxM. 3) Use SRM iteratively: condition on W_current (low-res) and W_{a_next} (high-res a at next level) to generate W_next at 2Nx2M. 4) Repeat SRM until target resolution reached. 5) Apply inverse wavelet transform to get final trajectory.", + "source": "Section 3.2" + }, + { + "id": "wdno-D2-012", + "claim": "1D Burgers control objective function J: J = integral_D |u(T,x) - u*(x)|^2 dx + alpha * integral_{[0,T]xD} |f(t,x)|^2 dt dx. The first term penalizes deviation from target state at final time. The second term penalizes control energy. In code: MSE between u_pred(T) and u_target, plus alpha * MSE of control force f over all time steps.", + "source": "Section 4.1, Eq. 6, Appendix F.1" + }, + { + "id": "wdno-D2-013", + "claim": "1D Burgers initial condition generation: u(0,x) = sum_{i=1}^{2} a_i * exp(-(x - b_i)^2 / (2 * sigma_i^2)). Parameters sampled: a_1 ~ U(0,2), a_2 ~ U(-2,0), b_1 ~ U(0.2, 0.4), b_2 ~ U(0.6, 0.8), sigma_i ~ U(0.05, 0.15).", + "source": "Appendix F.2" + }, + { + "id": "wdno-D2-014", + "claim": "1D Burgers control force generation: f(t,x) = sum_{i=1}^{8} a_i * exp(-(x - b_{1,i})^2 / (2 * sigma_{1,i}^2)) * exp(-(t - b_{2,i})^2 / (2 * sigma_{2,i}^2)). Parameters: b_{1,i} ~ U(0,1), b_{2,i} ~ U(0,1), sigma_{1,i} ~ U(0.1, 0.4), sigma_{2,i} ~ U(0.1, 0.4), a_1 ~ U(-1.5, 1.5), and for i >= 2: a_i ~ U(-1.5, 1.5) or 0 with equal probability.", + "source": "Appendix F.2" + }, + { + "id": "wdno-D2-015", + "claim": "1D Burgers finite difference solver: Finite difference method solving: du/dt = -u * du/dx + nu * d^2u/dx^2 + f(t,x), with Dirichlet boundary u=0, initial u(0,x)=u_0(x), nu=0.01. Internal grid: 120x16 spatial, 4800x16 temporal steps. Control f is kept constant between two stored time stamps. After simulation, downsample by factor 16 spatially and temporally before saving.", + "source": "Appendix F.1, F.2" + }, + { + "id": "wdno-D2-016", + "claim": "2D wavelet data preparation for WDNO (1D Burgers/Advection/Navier-Stokes): 1) Apply 2D DWT to trajectory data (81x120) using bior2.4 wavelet, periodization mode -> outputs 4 coeff sets each (41x60): 1 coarse + 3 detail. 2) Apply 1D DWT to initial condition (1D), repeat coeffs, concatenate with 2D coeffs. 3) For multi-resolution data: downsample data to 41x60, 21x31, 11x15; apply 2D DWT -> 4x21x30, 4x11x15, 4x6x8. 4) Align low-res coeffs to high-res by duplication, with boundary duplication for odd dimensions.", + "source": "Appendix F.3" + }, + { + "id": "wdno-D2-017", + "claim": "3D wavelet data preparation for WDNO (2D incompressible fluid): 1) Apply 3D DWT to trajectory data (32x64x64) using bior1.3 wavelet, 'zero' padding mode -> outputs 8 coeff sets each (18x34x34): 1 coarse + 7 detail. 2) Apply 2D DWT to initial condition (2D) and 1D DWT to smoke percentage, repeat coeffs and concatenate. 3) For multi-resolution: downsample to 32x32x32, 32x16x16; apply 3D DWT -> 8x18x18x18, 8x18x10x10.", + "source": "Appendix H.2" + }, + { + "id": "wdno-D2-018", + "claim": "WDNO training pseudocode: Training: 1) Apply DWT to u_{[0,T]}^{(0)} ~ q(u) to get W_u^{(0)}. 2) Sample k ~ Uniform(1,...,K). 3) Sample epsilon ~ N(0,I). 4) Take gradient descent step on ||epsilon - epsilon_theta(sqrt(alpha_bar_k) * W_u^{(0)} + sqrt(1-alpha_bar_k) * epsilon, k)||^2. Repeat until converged. Sampling (simulation): 1) W_u^{(K)} ~ N(0,I). 2) For k=K..1: W_u^{(k-1)} = W_u^{(k)} - eta * epsilon_theta(W_u^{(k)}, W_a, k) + xi (xi~N(0,I) if k>1 else 0). 3) Apply inverse DWT to W_u^{(0)} to get u_{[0,T]}^{(0)}. For control: same but with additional +lambda * grad J(W_hat_f^{(k)}) term in step 2.", + "source": "Algorithm 1 (Appendix E)" + }, + { + "id": "wdno-D2-019", + "claim": "1D Navier-Stokes sound speed and Mach number computation: Sound speed: c_s = sqrt(Gamma * p / rho), where Gamma=5/3. Mach number: M = |v| / c_s. Initial velocity field: v(x, t=0) = sum_{i=1..n} A_i * sin(k_i * x + phi_i), where A_i = v_bar / |k|^d, d=1 for 2D case, v_bar = c_s * M. Shock-tube initialization: Q(x, t=0) = (Q_L, Q_R) with piecewise constant values generating shocks and rarefactions, where Q = (rho, v, p).", + "source": "Appendix G.1" + }, + { + "id": "wdno-D2-020", + "claim": "SAC reward function for 1D control: r(t, u_t, u_d, w_t) = -integral_Omega |u_t - u_d|^2 dx - alpha * integral_Omega |w_t|^2 dx. In code: negative MSE between current state and target state minus alpha * MSE of action. Alpha is energy weight (0.00002 for Burgers).", + "source": "Appendix I.2, Eq. 13" + }, + { + "id": "wdno-D2-021", + "claim": "BPPO reward function for 1D control: r(t, u_t, u_d, f_t) = -integral_Omega |u_t - u_d|^2 dx - alpha * integral_Omega |f_t|^2 dx. Same form as SAC reward with state u and control f.", + "source": "Appendix I.4, Eq. 14" + }, + { + "id": "wdno-D2-022", + "claim": "Approximate scale invariance for multi-resolution alignment: Given high-res data d_+ at NxM and low-res data d_- at (N/2)x(M/2), apply linear transformation t_scaled = a_1 * t_orig + b_1, x_scaled = a_2 * x_orig + b_2 to rescale coordinates. Then the stretched function satisfies: du/(a_1*dt) = F(u, du/(a_2*dx), d^2u/(a_2^2*dx^2), ...) + f(t,x). Since resolution change factor is constant, a_1 and a_2 are fixed, so the pattern between resolutions is consistent. This consistency also holds in wavelet domain due to linearity and locality of wavelet transform.", + "source": "Section 3.2, Eq. 7" + }, + { + "id": "wdno-D2-023", + "claim": "WDNO U-Net architecture (2D encoder-decoder for 1D spatiotemporal data): Two separate U-Nets: epsilon_theta(f) for force pathway and epsilon_theta(u,f) for state pathway. Each U-Net: initial dim=128, 4 down/up layers with 3x3 conv kernels, channel multipliers [1,2,4,8] per stage, 8 ResNet block groups per stage, attention at lowest resolution with hidden dim=32 and 4 heads.", + "source": "Table 18, Appendix F.4" + }, + { + "id": "wdno-D2-024", + "claim": "WDNO 3D U-Net architecture (for 2D spatiotemporal data): Three modules: downsampling encoder, middle module, upsampling decoder with 3D spatiotemporal convolutions. Conv3D: kernel=(3,3,3), padding=(1,1,1), stride=(1,1,1). Downsampling: kernel=(1,4,4), padding=(1,2,2), stride=(0,1,1) - only downsamples spatial dims. Upsampling: kernel=(1,4,4), padding=(1,2,2), stride=(0,1,1) - only upsamples spatial dims. 4 attention heads. Architecture inspired by Video Diffusion Models (Ho et al., 2022).", + "source": "Table 20, Appendix H.3" + }, + { + "id": "wdno-D2-025", + "claim": "Guidance loss for initial condition and target state consistency: Add auxiliary loss terms to the control guidance gradient: ∇_{W_f} J_total = ∇_{W_f} J_control + λ_IC · ∇_{W_f} ||IDWT(W_f[0,:]) - u_0_true||^2 + λ_target · ∇_{W_f} ||IDWT(W_f[T,:]) - u_T_true||^2, where W_f[0,:] is the initial timestep's wavelet coefficient channel, W_f[T,:] is the target timestep's wavelet coefficient channel, IDWT(·) is the inverse discrete wavelet transform, and u_0_true, u_T_true are the ground truth initial/target states. This ensures the generated trajectory matches the given initial condition and target state. For 1D Burgers control: λ_IC applied via cosine-scheduled guidance weight (120000); for 2D fluid control: λ_target = 100.", + "source": "Appendix F.4, Appendix H.3" + }, + { + "id": "wdno-D2-026", + "claim": "DDIM accelerated sampling: Denoising Diffusion Implicit Model (DDIM) (Song et al., 2020) is used during inference to speed up sampling. Instead of running all K diffusion steps, DDIM uses a subsequence of steps. WDNO uses 50 DDIM iterations for 1D and 100 for 2D inference, with eta=1 (equivalent to DDPM stochastic sampling).", + "source": "Section 3.1, Table 18, Table 20" + }, + { + "id": "wdno-D2-027", + "claim": "ANN PID controller architecture (1D baseline): Two 1D convolutional layers (kernel=3, padding=1, stride=1) with Softsign activation, followed by two fully connected layers and four activation layers. Trained with MAE loss between current and target states using Adam optimizer (lr=0.0001, batch=16). The neural network outputs MIMO PID parameters based on error Err_t, which the PID controller uses to produce control f_t.", + "source": "Appendix I.1, Table 21" + }, + { + "id": "wdno-D2-028", + "claim": "Supervised Learning control algorithm (SL baseline): Stage 1: Two CNN VAEs project state u and control f into latent space; a CNN learns latent transition u_t -> u_{t+1}. Stage 2: Use these three networks as surrogate models to compute gradient of objective J w.r.t. control f via backpropagation. Optimize f with LBFGS (lr=0.1, 100 epochs, tol=1e-5). Include reconstruction loss for control (weight=0.01) alongside objective loss (weight=1.0) to avoid adversarial modes.", + "source": "Appendix I.3, Table 23" + }, + { + "id": "wdno-D2-029", + "claim": "FNO architecture (1D baseline): Fourier Neural Operator: Lifting block (hidden=256) -> 4 Fourier layers (modes=16, width=64) -> Projection block (hidden=256). Each Fourier layer: FFT -> linear transform in Fourier domain on kept modes -> IFFT -> add skip connection. MLP expansion=0.5, GeLU activation, rank of tensor factorization=1.0, one-sided domain padding. Input: initial state + controls (3 channels). Output: states (1 channel). Supports zero-shot super-resolution.", + "source": "Appendix J.1, Table 26" + }, + { + "id": "wdno-D2-030", + "claim": "WNO architecture (1D baseline): Wavelet Neural Operator: sym4 wavelet, 5 decomposition levels, uplifting dimension=40, 4 wavelet layers. Each wavelet layer: DWT -> linear transform on wavelet coefficients -> IDWT + skip connection. Trained with Adam (lr=1e-3, batch=100, epochs=1000, StepLR scheduler).", + "source": "Appendix J.2, Table 27" + }, + { + "id": "wdno-D2-031", + "claim": "MWT architecture (1D baseline): Multiwavelet Neural Operator: Legendre wavelet basis, 10 Fourier modes, kernel size=4. Batch=256, epochs=300, Adam optimizer, MultiStepLR scheduler.", + "source": "Appendix J.4, Table 29" + }, + { + "id": "wdno-D2-032", + "claim": "OFormer architecture (1D baseline): Operator Transformer: SpatialEncoder2D (3 input channels, embedding dim=96, encoded sequence dim=256, 4 heads, 6 depth layers, resolution=120, dropout=0.05) -> PointWiseDecoder2DSimple (latent channels=256, output channels=1, scale=0.5, resolution=120). Batch=32, 50000 iterations, lr=1e-4.", + "source": "Appendix J.5, Table 30" + } + ], + "D3": [ + { + "id": "wdno-D3-001", + "claim": "1D Burgers equation simulation: Learn mapping from initial condition u_0 and force f to entire trajectory u_{[0,T]}. Compare WDNO against neural operator baselines (WNO, MWT, OFormer, FNO, CNN, DDPM). Metrics: MSE on state sequences excluding initial conditions. Datasets: 40000 trajectories with random initial conditions (2 Gaussians) and control forces (8 Gaussians).", + "source": "Section 4.1, Table 1, Appendix F" + }, + { + "id": "wdno-D3-002", + "claim": "1D Burgers equation control: Find optimal control force f that minimizes objective J = integral|u(T,x)-u*(x)|^2 + alpha*integral|f|^2. Compare WDNO against PID, SAC (pseudo-online/offline), BPPO (surrogate-solver/solver), BC (surrogate-solver/solver), SL, and DDPM baselines. Metrics: control objective J. Datasets: 40000 train / 50 test trajectories.", + "source": "Section 4.1, Table 2a, Appendix F" + }, + { + "id": "wdno-D3-003", + "claim": "1D Advection equation simulation: Predict 80 timesteps of advection evolution from a one-step initial condition. Evaluate on smooth dynamics from PDEBench. Compare against WNO, MWT, OFormer, FNO, CNN, DDPM. Data prep: 2D wavelet transform (bior2.4, periodization). Metric: MSE excluding initial conditions.", + "source": "Section 4.2, Table 1" + }, + { + "id": "wdno-D3-004", + "claim": "1D Compressible Navier-Stokes equation simulation: Evaluate on shock-tube scenarios from PDEBench with extremely small viscosity (eta=1e-8, zeta=1e-8). Simulation only (no control). Compare against Transolver, CNO, MSVI, ACDM, DiffusionPDE, WNO, MWT, OFormer, FNO, CNN, DDPM, Diffusion+FFT, FNO Denoiser. Metrics: MSE, MAE, L_inf. Datasets: 9000 training samples, 81x120 resolution.", + "source": "Section 4.3, Table 1, Table 5, Appendix G" + }, + { + "id": "wdno-D3-005", + "claim": "2D Incompressible fluid simulation: Predict smoke density, velocity field, and percentage through target bucket given initial smoke density and control sequences. Fluid-solid coupling with no-slip boundaries at obstacles. Compare against WNO, MWT, OFormer, FNO, U-Net, DDPM. Data prep: 3D DWT (bior1.3, zero mode). Metric: MSE excluding initial conditions. Datasets: 32 timesteps, 64x64 spatial grid.", + "source": "Section 4.4, Table 1, Appendix H" + }, + { + "id": "wdno-D3-006", + "claim": "2D Incompressible fluid control (indirect smoke navigation): Control 3584 peripheral variables over 32 timesteps to steer smoke from below central obstacle into top-center bucket. Objective J = percentage of smoke NOT passing target bucket. Compare against BC, BPPO, SAC (pseudo-online/offline), DDPM. Highly challenging due to indirect control and need for trajectory planning.", + "source": "Section 4.4, Table 2b, Appendix H" + }, + { + "id": "wdno-D3-007", + "claim": "ERA5 weather simulation: Predict temperature evolution over next 20 hours given past 12 hours of atmospheric data. Real-world dataset (0.25deg, surface to 100km). Compare against MWT, OFormer, FNO, U-Net, DDPM. Data prep: 3D DWT (bior1.3). Metrics: MSE, Relative L2 error.", + "source": "Section 4.5, Table 1" + }, + { + "id": "wdno-D3-008", + "claim": "Zero-shot super-resolution (1D Burgers): Train on 80x120; evaluate at 160x240 (1x), 320x480 (2x), 640x960 (3x) unseen during training. Compare WNO, FNO, WDNO with linear and nearest interpolation. Test: 2000 trajectories (0x) + 100 shared (1x/2x/3x). Metric: MSE at finest resolution.", + "source": "Section 4.6, Table 16, Figure 4" + }, + { + "id": "wdno-D3-009", + "claim": "Zero-shot super-resolution (2D fluid): Train on 32x64x64; evaluate at 32x128x128 unseen during training. Compare FNO and WDNO with linear and nearest interpolation. Metric: MSE at finest resolution.", + "source": "Section 4.6, Table 17, Figure 4" + }, + { + "id": "wdno-D3-010", + "claim": "Ablation: wavelet transform for abrupt changes. Compare WDNO vs DDPM MAE per time step specifically at regions with abrupt spatial changes in 1D Burgers and 1D Navier-Stokes. Demonstrate wavelet benefit at shock/discontinuity regions.", + "source": "Section 4.7, Figure 6, Figure 9" + }, + { + "id": "wdno-D3-011", + "claim": "Ablation: wavelet + multi-resolution training synergy. Compare WDNO (wavelet domain) vs DDPM with multi-resolution training in original space-time domain (no wavelet). Evaluate at multiple super-resolution levels. Metric: MSE at finest resolution.", + "source": "Section 4.7, Figure 4c" + }, + { + "id": "wdno-D3-012", + "claim": "Ablation: wavelet vs Fourier transform. Compare WDNO (wavelet domain generation) vs Diffusion+FFT (Fourier domain, same architecture/pipeline except transform). Also test FNO as denoiser backbone. Dataset: 1D Navier-Stokes. Metric: MSE.", + "source": "Section 4.7, Figure 5c" + }, + { + "id": "wdno-D3-013", + "claim": "Ablation: long-term dependency. Measure error growth over time steps for WDNO vs WNO, MWT, OFormer, FNO, U-Net, DDPM on 2D simulation. Verify slower error accumulation for WDNO. Metric: MSE per time step.", + "source": "Section 4.7, Figure 5a" + }, + { + "id": "wdno-D3-014", + "claim": "Ablation: measurement noise robustness. Add Gaussian noise (scale factors 0.01, 0.001, 0.0001 * data_std) to both training and testing data of 1D Burgers. Compare WDNO vs DDPM. Metric: MSE vs noise scale.", + "source": "Section 4.7, Figure 5d" + }, + { + "id": "wdno-D3-015", + "claim": "Ablation: training sample size. Reduce training set to 0.2, 0.4, 0.6, 0.8 of full 9000 samples on 1D Navier-Stokes. Measure WDNO MSE trend. Metric: MSE.", + "source": "Section 4.7, Figure 5b" + }, + { + "id": "wdno-D3-016", + "claim": "Ablation: approximate scale invariance verification. Train FNO on original, once-downsampled, twice-downsampled, and mixed datasets; test at all three resolutions to demonstrate mixed training improves generalization. Metric: MSE at each resolution.", + "source": "Appendix C.3, Table 6" + }, + { + "id": "wdno-D3-017", + "claim": "Sensitivity analysis: DDIM sampling steps. Test steps in {20, 40, 50, 100, 200} for 1D Burgers simulation and 2D fluid control. Metrics: MSE (1D sim), J (2D control).", + "source": "Appendix C.4, Table 7, Table 8" + }, + { + "id": "wdno-D3-018", + "claim": "Sensitivity analysis: DDIM eta and guidance weight. Test eta in {0.2, 0.5, 0.8, 1.0} for 1D Burgers control; test guidance weights {9, 10, 11.5, 12.5, 13} x1e4 for 2D control. Metrics: MSE, J.", + "source": "Appendix C.4, Table 7, Table 8" + }, + { + "id": "wdno-D3-019", + "claim": "Robustness test: control sequence noise. 1D Burgers control with 0.1 probability of noise in control during evaluation. Compare WDNO vs PID, SAC, BC, BPPO, SL, DDPM. Metric: J.", + "source": "Appendix C.5, Table 9" + }, + { + "id": "wdno-D3-020", + "claim": "Robustness test: variance across test samples. Report mean +/- std of control objective J over 50 test samples for 1D Burgers control. Baselines: PID, SAC, BC, BPPO, SL, DDPM. Metric: J (mean +/- std).", + "source": "Appendix C.5, Table 10" + }, + { + "id": "wdno-D3-021", + "claim": "Ablation: guidance importance. Compare WDNO 1D Burgers control with (lambda=120000) vs without (lambda=0) guidance term. Metric: J.", + "source": "Appendix C.7, Table 15" + }, + { + "id": "wdno-D3-022", + "claim": "Wavelet reconstruction fidelity test. Measure relative L2 reconstruction error of wavelet bases (bior1.3, bior2.4, db4, sym4) on 1D Burgers and 2D fluid training data. Metric: relative L2 error.", + "source": "Appendix A, Table 3" + }, + { + "id": "wdno-D3-023", + "claim": "Computational efficiency: wavelet vs Fourier transform. Measure total transform time on 1D Navier-Stokes training set (A100 GPU, batch 2000). Compare DWT (pytorch_wavelets) vs PyTorch 2D FFT. Metric: total transform time (seconds).", + "source": "Appendix A, Table 4" + }, + { + "id": "wdno-D3-024", + "claim": "Computational resource comparison: Parameter counts, inference time (batch 1, A100), and training time for all 1D Burgers baselines (PID, SAC, BC, BPPO, SL, DDPM, WNO, MWT, OFormer, FNO, CNN). Metrics: params, inference time, training time.", + "source": "Appendix C.6, Table 11, Table 12, Table 13" + }, + { + "id": "wdno-D3-025", + "claim": "Super-resolution computational cost scaling. Measure inference time and GPU memory for batch-5 generation at 0, 1, 2, 3 levels of super-resolution on 1D Burgers. Metrics: time, memory.", + "source": "Appendix C.6, Table 14" + }, + { + "id": "wdno-D3-026", + "claim": "WDNO training protocol (shared across all experiments). BRM and SRM trained with DDPM loss. Adam optimizer, lr=1e-4, cosine annealing, 190000 steps, batch 16. DDIM inference: 50 steps (1D) / 100 steps (2D), eta=1. Control guidance: 120000 (1D) / 100 (2D) with cosine scheduler. Training hardware: 1 A100 (1D, 2.4-2.5h) / 2xA100 (2D, 7.8-7.9h).", + "source": "Section 3, Table 18, Table 19, Table 20, Table 12" + } + ], + "D4": [ + { + "id": "wdno-D4-001", + "claim": "WDNO Training Pipeline (Section 3, Figure 1): Phase 1 - Train Base-Resolution Model (BRM) on full-resolution wavelet-domain data using DDPM loss (output: trained BRM; Adam, lr=1e-4, cosine annealing, 190k steps, batch 16). Phase 2 - Prepare multi-resolution dataset by downsampling data, applying wavelet transform at each resolution level, and aligning coefficient dimensions via duplication (output: multi-resolution wavelet dataset, feeds into Phase 3). Phase 3 - Train Super-Resolution Model (SRM) on paired multi-resolution wavelet data (input: Phase 2 dataset, output: trained SRM; condition on low-res coeffs + high-res params to predict high-res coeffs via DDPM loss). At inference: Phase 1 BRM generates base-resolution coefficients that feed Phase 3 SRM for iterative upsampling.", + "source": "Section 3, Figure 1, Algorithm 1" + }, + { + "id": "wdno-D4-002", + "claim": "WDNO Simulation Inference Pipeline (Section 3.1, Algorithm 1): Step 1 - Apply Discrete Wavelet Transform (DWT) to input equation parameters (IC, BC, PDE coeffs) to obtain wavelet-domain condition W_a. Step 2 - Initialize wavelet-domain trajectory as Gaussian noise W_u^{(K)}~N(0,I); iteratively denoise via DDIM for k=K..1: W_u^{(k-1)} = W_u^{(k)} - eta * epsilon_theta(W_u^{(k)}, W_a, k) + xi. Step 3 - Apply Inverse DWT to W_u^{(0)} to recover full trajectory u_{[0,T]} in original spatiotemporal domain.", + "source": "Section 3.1, Eq. 3, Algorithm 1" + }, + { + "id": "wdno-D4-003", + "claim": "WDNO Control Inference Pipeline (Section 3.1, Eq. 4-5): Step 1 - Apply DWT to input params to obtain W_a; define control objective J (state deviation at final time + energy regularization). Step 2 - Initialize noise W_f^{(K)}~N(0,I); iteratively denoise with classifier-free guidance + objective gradient for k=K..1: W_f^{(k-1)} = W_f^{(k)} - eta * (epsilon_theta(W_f^{(k)}, W_a, k) + lambda * grad_{W_f} J(W_hat_f^{(k)})) + xi, where W_hat_f is noise-free estimate of W_f^{(0)}. Step 3 - Apply IDWT to W_f^{(0)} to recover controlled trajectory.", + "source": "Section 3.1, Eq. 4, Eq. 5" + }, + { + "id": "wdno-D4-004", + "claim": "WDNO Zero-shot Super-Resolution Pipeline (Section 3.2): Step 1 - Downsample input params to base resolution NxM and apply DWT. Step 2 - BRM generates wavelet coefficients at base NxM via DDIM denoising. Step 3 - SRM iteratively upsamples: condition on current low-res coeffs + high-res params to generate coeffs at 2Nx2M; repeat until target resolution. Step 4 - Apply IDWT to obtain final trajectory at target (super-resolution) resolution.", + "source": "Section 3.2" + }, + { + "id": "wdno-D4-005", + "claim": "1D Burgers Data Generation Pipeline (Appendix F.2): Step 1 - Generate initial condition u(0,x) as superposition of 2 Gaussians with random parameters (output: initial condition, feeds Step 3). Step 2 - Generate control force f(t,x) as superposition of 8 Gaussians (a_i for i>=2 set to 0 with 50% prob) (output: control force, feeds Step 3). Step 3 - Run finite difference solver (input: Step 1 IC + Step 2 force; du/dt = -u*du/dx + nu*d^2u/dx^2 + f, Dirichlet u=0, nu=0.01, internal 120x16 spatial x 4800x16 temporal). Step 4 - Downsample solver output by factor 16 spatially and temporally to final [81,120] state + [80,120] force (input: Step 3 raw trajectory).", + "source": "Appendix F.1, F.2" + }, + { + "id": "wdno-D4-006", + "claim": "Supervised Learning (SL) Control Two-Stage Pipeline (Appendix I.3): Stage 1 - Train surrogate models (two CNN VAEs for state u and control f encoding; CNN for latent transition u_t -> u_{t+1}). Stage 2 - Optimize control f by computing gradient of objective J via backpropagation through the three surrogate networks; optimize with LBFGS (lr=0.1, 100 epochs, tol=1e-5, obj_weight=1, recon_weight=0.01).", + "source": "Appendix I.3, Table 23" + } + ] +} \ No newline at end of file