---
license: apache-2.0
language:
- pyt
base_model:
- openai-community/gpt2
---
# MemoryDecoder-GPT2-Small

## Model Description

Memory Decoder is a pretrained, plug-and-play memory component designed for efficient domain adaptation of large language models. This checkpoint contains the GPT2-small Memory Decoder trained on WikiText-103, as described in our NeurIPS 2025 paper.

- **Paper:** [Memory Decoder: A Pretrained, Plug-and-Play Memory for Large Language Models](https://www.arxiv.org/abs/2508.09874)
- **GitHub:** [https://github.com/LUMIA-Group/MemoryDecoder](https://github.com/LUMIA-Group/MemoryDecoder/tree/main)
- **Conference:** NeurIPS 2025 (Poster)
- **Model Size:** 124M parameters
- **Base Architecture:** GPT2-small transformer decoder

## Overview

Memory Decoder bridges the gap between non-parametric retrieval methods and parametric fine-tuning approaches. By pre-training a compact transformer decoder to internalize retrieval patterns, it provides:

- **Plug-and-Play Integration:** Works with any GPT2 model variant without modifying original parameters
- **Efficient Inference:** No retrieval overhead - just parallel forward passes
- **Domain Expertise:** Captures long-tail knowledge like kNN-LM but with parametric efficiency
- **Preserved Capabilities:** Original model remains unchanged

## Quick Start

### Step 1: Import Libraries and Initialize Models

```python
from memDec import MemoryDecoder
import transformers
from transformers import AutoModelForCausalLM
from loguru import logger

# Define paths to your models
base_lm_path = "gpt2-xl"  # or any GPT2 variant
knn_generator_path = "Clover-Hill/MemoryDecoder-gpt2-small"

# Load tokenizer and models
tokenizer = transformers.AutoTokenizer.from_pretrained(base_lm_path)
base_lm = AutoModelForCausalLM.from_pretrained(base_lm_path)
knn_generator = AutoModelForCausalLM.from_pretrained(knn_generator_path)
```

### Step 2: Prepare Models and Create Joint Model

```python
# Resize embeddings and set to evaluation mode
base_lm.eval()
knn_generator.eval()

# Create the joint Memory Decoder model
joint = MemoryDecoder(base_lm, knn_generator, lmbda=0.55, knn_temp=1.0).to("cuda")
```

### Step 3: Generate Text and Compare Results

```python
# Prepare input prompt
prompt = "As with previous Valkyira Chronicles games , Valkyria Chronicles III is"
inputs = tokenizer(prompt, return_tensors="pt").to("cuda")

# Generate with Memory Decoder
out_ids = joint.generate(**inputs, max_new_tokens=20, do_sample=False)
logger.info(f"Memory Decoder output: {tokenizer.decode(out_ids[0], skip_special_tokens=True)}")

# Generate with base model for comparison
out_ids = base_lm.generate(**inputs, max_new_tokens=20, do_sample=False)
logger.info(f"Base Model output: {tokenizer.decode(out_ids[0], skip_special_tokens=True)}")
```

**📊 Generation Results Comparison:**

| Model | Generated Continuation |
|-------|------------------------|
| **Base Model** | *"...is a turn-based strategy game. The player takes control of a squad of Valkyria soldiers..."* |
| **+Memory Decoder** | *"...is a **role-playing** video game developed by Sega and published by Sega for the PlayStation 2."* |

> [!NOTE]
> Memory Decoder correctly identifies Valkyria Chronicles III as a **role-playing game** (factually accurate), while the base model incorrectly predicts it as a strategy game.

## Performance on WikiText-103

| Model Configuration | Perplexity | Improvement |
|:-------------------|:----------:|:-----------:|
| GPT2-small (baseline) | 24.89 | - |
| GPT2-small + MemoryDecoder | **13.36** | -11.53 |
| GPT2-medium (baseline) | 18.29 | - |
| GPT2-medium + MemoryDecoder | **12.25** | -6.04 |
| GPT2-large (baseline) | 15.80 | - |
| GPT2-large + MemoryDecoder | **11.53** | -4.27 |
| GPT2-xl (baseline) | 14.39 | - |
| GPT2-xl + MemoryDecoder | **10.93** | -3.46 |

## Key Features

- **Universal Compatibility:** Works with all GPT2 model sizes (small, medium, large, xl)
- **Parameter Efficient:** Only 124M additional parameters enhance models up to 1.5B
- **Domain Adaptation:** Trained to capture WikiText-103 domain knowledge
- **Inference Speed:** Minimal overhead compared to retrieval-based methods

## Training Details

- **Training Data:** WikiText-103
- **Training Objective:** Hybrid KL divergence and language modeling loss
- **Supervision Signal:** kNN distributions from GPT2-xl, it is suggested to use the finetuned version of GPT2-xl [here](https://huggingface.co/Clover-Hill/gpt2-xl-finetuned-wikitext103).
- **Hyperparameters:** 
  - Learning rate: 1e-3
  - Beta (loss balance): 0.5
  - Training Epoch: 70

## Citation

```bibtex
@article{cao2025memory,
  title={Memory decoder: A pretrained, plug-and-play memory for large language models},
  author={Cao, Jiaqi and Wang, Jiarui and Wei, Rubin and Guo, Qipeng and Chen, Kai and Zhou, Bowen and Lin, Zhouhan},
  journal={arXiv preprint arXiv:2508.09874},
  year={2025}
}
```

## Contact

For questions and support: maximus.cao@outlook.com