docs/source/async_grpo_trainer.md

# Asynchronous GRPO

> [!IMPORTANT]
> This trainer requires `vllm>=0.17.1` and `transformers>=5.2.0`. For distributed training, only FSDP2 is supported (DeepSpeed ZeRO is not).
>
> Currently, `vllm` and `transformers` have conflicting dependency constraints. To work around this, install vLLM first and then force-install transformers:
>
> ```bash
> pip install 'vllm>=0.17.1'
> pip install 'transformers>=5.2.0' --no-deps
> ```

## Overview

[`AsyncGRPOTrainer`] implements the same [GRPO](grpo_trainer) algorithm but decouples rollout generation from training. A background worker continuously streams completions from a vLLM server while the training loop consumes them, so generation and gradient updates overlap instead of alternating. The API mirrors [`GRPOTrainer`] — for full details on the GRPO method itself (advantage computation, KL estimation, loss formulation, reward functions, etc.), see the [GRPO Trainer](grpo_trainer) documentation. Not all features from [`GRPOTrainer`] are available; refer to [`AsyncGRPOConfig`] for the supported parameters.

This trainer was contributed by [Quentin Gallouédec](https://huggingface.co/qgallouedec) and [Amine Dirhoussi](https://huggingface.co/aminediroHF).

## How it differs from [`GRPOTrainer`]

In the standard [`GRPOTrainer`], generation and training are sequential: generate a batch, compute the loss, update weights, repeat. Even in [vLLM colocate mode](grpo_trainer#speed-up-training-with-vllm), where generation runs on the same GPUs, one phase must finish before the other begins.

[`AsyncGRPOTrainer`] separates these two concerns:

- **Rollout worker** (background thread) — sends prompts to a vLLM server, scores completions with reward functions, computes advantages, and pushes ready-to-train samples into a queue.
- **Training loop** (main process) — pulls samples from the queue, computes the clipped surrogate loss, and updates the model weights.

After every `weight_sync_steps` training steps, the updated weights are transferred to the vLLM server via NCCL so that subsequent generations reflect the latest policy.

Because generation and training run concurrently, the training samples may have been generated by a slightly older version of the model. The `max_staleness` parameter controls how many weight updates a sample can lag behind before being discarded.

The number of concurrent requests sent to the vLLM server is controlled by `max_inflight_tasks`. By default it is set automatically to `max_staleness × per_device_train_batch_size × gradient_accumulation_steps × num_processes` — the maximum number of samples the trainer can consume before they become stale. Generating more than this is wasteful since the excess samples will be discarded.

## Quick start

```python
# train_async_grpo.py
from datasets import load_dataset
from trl.experimental.async_grpo import AsyncGRPOTrainer
from trl.rewards import accuracy_reward

dataset = load_dataset("trl-lib/DeepMath-103K", split="train")

trainer = AsyncGRPOTrainer(
    model="Qwen/Qwen3-4B",
    reward_funcs=accuracy_reward,
    train_dataset=dataset,
)
trainer.train()
```

The vLLM server and the trainer must run on **separate GPUs**. Use `CUDA_VISIBLE_DEVICES` to partition your GPUs. For example, with 2 GPUs, you can run the vLLM server on GPU 0 and the trainer on GPU 1 as follows:

```bash
# Terminal 1: vLLM server on GPU 0 (dev mode + NCCL weight transfer are required)
CUDA_VISIBLE_DEVICES=0 VLLM_SERVER_DEV_MODE=1 vllm serve Qwen/Qwen3-4B \
    --max-model-len 4096 \
    --logprobs-mode processed_logprobs \
    --weight-transfer-config '{"backend":"nccl"}'
```

> [!TIP]
> Set `--max-model-len` to the maximum total sequence length (prompt + completion) you expect. A lower value reduces GPU memory usage on the server, freeing more memory for the KV cache and increasing throughput. A good starting point is the prompt length plus `max_completion_length` from your config.

```bash
# Terminal 2: training on GPU 1
CUDA_VISIBLE_DEVICES=1 accelerate launch train_async_grpo.py
```

## Design philosophy

This trainer is intentionally kept minimal and is not meant to grow into a general-purpose solution. If you need a feature that is not supported, we recommend cloning the repository and adapting the trainer to your needs directly. New features will only be considered when there is significant community demand.

## AsyncGRPOConfig

[[autodoc]] trl.experimental.async_grpo.AsyncGRPOConfig

## AsyncGRPOTrainer

[[autodoc]] trl.experimental.async_grpo.AsyncGRPOTrainer