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# Recipe: Fully Async Policy Trainer
**Author:** `https://github.com/meituan-search`
Last updated: 10/18/2025.
This document introduces a fully asynchronous PPO training system that completely decouples the Trainer and Rollouter,
supporting asynchronous sample generation and training.
Under this system, we achieved a 2.35x-2.67x performance improvement when training the Qwen2.5-7B model with 128 GPUs,
without significantly affecting the results.
## Introduction
### Background
The separated rollout and train architecture, compared to the colocate architecture, can allocate resources more
flexibly and design more flexible training logic, thereby addressing issues such as low GPU utilization and training
efficiency caused by long-tail problems.
The one_step_off_policy alleviates the problem of long rollout times and achieves some gains in training efficiency by
designing a separated architecture and performing asynchronous training between rollout and train for one round.
However, it forcibly uses data from one round of asynchronous training, which is not flexible enough and cannot
completely eliminate the impact of long-tail on training efficiency.
In other frameworks such as AReaL, Magistral, StreamRL, and AsyncFlow, asynchronous training and streaming training have
been implemented based on the separated architecture and have achieved gains.
We borrow from their methods and implemented them in VERL. The fully_async_policy supports asynchronous, streaming, and partial
rollout training.
By reasonably setting parameters such as resource allocation and parameter synchronization frequency, fully_async_policy
can significantly improve training efficiency.
> Magistral https://arxiv.org/abs/2506.10910
>
> AReaL: A Large-Scale Asynchronous Reinforcement Learning System for Language
> Reasoning https://arxiv.org/abs/2505.24298
>
> StreamRL: Scalable, Heterogeneous, and Elastic RL for LLMs with Disaggregated Stream
> Generation https://arxiv.org/abs/2504.15930
>
> AsyncFlow: An Asynchronous Streaming RL Framework for Efficient LLM Post-Training https://arxiv.org/abs/2507.01663
>
### Core Contributions
* **Resource Isolation**: Unlike using hybrid_engine, Rollouter and Trainer use separate computing resources and need to
specify the resources they occupy separately.
* **Parallel Generation and Training**: While the Trainer is training, the Rollouter is generating new samples.
* **Multi-step Asynchronous**: Compared to one step off policy, it supports asynchronous settings from 0.x steps to
multiple steps, making the asynchronous solution more flexible.
* **NCCL Parameter Synchronization**: Uses NCCL communication primitives for parameter communication between Rollouter
and Trainer.
* **Stream Inference and Training**: Rollouter generates data sample by sample, and data transmission uses a single
sample as the minimum transmission unit.
* **Asynchronous Training and Freshness Control**: By setting the parameter async_training.staleness_threshold, it
supports training with samples generated by old parameters.
* **PartialRollout**: The Rollouter's inference process supports partial rollout logic. During parameter
synchronization, by adding `sleep() and resume()` logic, it
saves samples from ongoing rollouts and continues using them in the next rollout, reducing the time spent waiting for
ongoing tasks to finish during parameter synchronization.
Currently, the supported usage mode is fsdp+vllm. vllm must use the server mode based on AgentLoop.
## Design
The overall architecture of fully_async_policy is shown in the figure below. fully_async_policy mainly consists of four
parts: Rollouter, MessageQueue, Trainer, and ParameterSynchronizer.
![fully_async_policy_structure](
https://github.com/ArronHZG/verl-community/blob/recipe/async_policy/docs/fully_async_policy_structure.svg?raw=true)
1. Rollouter generates sequences sample by sample and puts the generated samples into the MessageQueue, with the
production speed controlled by freshness.
2. MessageQueue is used to temporarily store samples generated by Rollouter.
3. Trainer fetches samples from MessageQueue sample by sample. After fetching `require_batches*ppo_mini_batch_size`
samples, it will perform training. After training for async_training.trigger_parameter_sync_step rounds, it triggers
a parameter synchronization with Rollouter.
4. ParameterSynchronizer implements the NCCL synchronous parameter synchronization capability.
The source of benefits compared to the base scheme lies in the fact that in the colocate case, using more resources for
rollout cannot solve the idleness caused by long-tail samples.
After we perform resource isolation, the time for rollout and train may be longer than before (because fewer resources
are used),
but the overlap in their time consumption reduces the end-to-end time consumption.
![fully_async_policy_revenue](
https://github.com/ArronHZG/verl-community/blob/recipe/async_policy/docs/fully_async_policy_revenue.svg?raw=true)
## Usage
### Parameter Description
| super params | implication |
|-----------------------------------------------|------------------------------------------------------------------------------------------------|
| `trainer.nnodes` | Number of nodes for Trainer |
| `trainer.n_gpus_per_node` | Number of GPUs per node for Trainer |
| `rollout.nnodes` | Number of nodes for Rollouter |
| `rollout.n_gpus_per_node` | Number of GPUs per node for Rollouter |
| `data.train_batch_size` | In the fully async strategy, this value is not effective (default is 0) |
| `data.gen_batch_size` | In the fully async strategy, uses streaming sample production logic (default is 1) |
| `rollout.total_rollout_steps` | Total number of rollout samples |
| `rollout.test_freq` | How many times Rollouter updates parameters before performing a validation |
| `actor_rollout_ref.actor.ppo_mini_batch_size` | The ppo_mini_batch_size is a global num across all workers/gpus |
| `async_training.require_batches` | Number of ppo_mini_batch_size that FullyAsyncTrainer fetches at once |
| `async_training.trigger_parameter_sync_step` | Indicates how many local updates FullyAsyncTrainer performs before a parameter synchronization |
| `async_training.staleness_threshold` | Freshness control |
| `async_training.partial_rollout` | Whether to perform partial_rollout |
| `async_training.use_rollout_log_probs` | Use log_probs generated by rollout |
**Further Explanation:**
* `rollout.total_rollout_steps`
Compared to colocate, the quantity can be aligned by multiplying train_batch_size and step:
`rollout.total_rollout_steps = data.train_batch_size * step`.
* `async_training.trigger_parameter_sync_step`
In the fully async strategy, it indicates how many local updates the Trainer performs (i.e., how many times it fetches
`require_batches * ppo_mini_batch_size` samples) before a parameter synchronization with Rollouter.
Between every two parameter synchronizations between Rollouter and Trainer, the Trainer will process
`trigger_parameter_sync_step* require_batches*ppo_mini_batch_size` samples.
To fairly compare speed with colocate, trigger_parameter_sync_step should be set to
`data.train_batch_size / (require_batches * ppo_mini_batch_size)`.
* `async_training.staleness_threshold`
In the fully async strategy, it indicates the maximum proportion of stale samples allowed to be used.
* staleness_threshold=0, indicates synchronous training.
Rollouter will generate a fixed number of samples between two parameter updates, the sample count is:
$$rollout\_num = (trigger\_parameter\_sync\_step*require\_batches*ppo\_mini\_batch\_size)$$
* staleness_threshold>0, indicates asynchronous training, can be set to a decimal for more flexible asynchronous
calls.
Rollouter will generate at most the following number of samples between two parameter updates:
$$rollout\_num = (1+staleness\_threshold)*(trigger\_parameter\_sync\_step*require\_batches*ppo\_mini\_batch\_size) - num\_staleness\_sample $$
num_staleness_sample represents the number of stale samples generated in excess during the last rollout.
Since it's a streaming system, rollout continues to generate and trainer continues to consume. If rollouter is slower,
trainer will trigger parameter synchronization earlier, and rollouter will not actually produce rollout_num samples.
When rollout is fast enough, setting staleness_threshold to 1 is basically equivalent to one_step_off policy.
To avoid too many expired samples affecting training accuracy, it is recommended to set this value to less than 1.
* `async_training.partial_rollout`
partial_rollout only actually takes effect when staleness_threshold>0.
* `async_training.use_rollout_log_probs`
In reinforcement learning algorithms, log_probs have implicit correlations with parameter versions and tokens. Due to
the settings of algorithms like PPO/GRPO/DAPO, when calculating importance sampling,
old_log_prob must use the log_probs corresponding to the rollout parameters and tokens to ensure algorithm
correctness. In the fully
async strategy, we default to old_log_prob being calculated by rollout rather than by trainer.
* `async_training.require_batches`
In streaming training, require_batches should be set to 1, indicating that training is performed after producing
enough ppo_mini_batch_size samples.
In actual testing, we found that if fewer samples are issued at once, due to the order of data distribution, it can
cause training instability and longer response lengths.
Here, we additionally provide require_batches for streaming distribution and control the number of samples
participating in training at once.
### Supported Modes
1. on policy pipeline:
1. **trigger_parameter_sync_step=1, staleness_threshold=0**
2. Rollouter produces `require_batches*ppo_mini_batch_size` samples at once, Trainer fetches these samples for
training, and after training completes, Trainer and Rollouter perform a parameter synchronization;
3. During the rollout phase, if there are long-tail samples but few rollout samples, shorter samples cannot fill
idle resources, causing some resource waste.
4. As shown in figure a;
2. stream off policy pipeline:
1. **trigger_parameter_sync_step>1, staleness_threshold=0**
2. Synchronous streaming training will be performed. Rollouter produces
`require_batches*ppo_mini_batch_size*trigger_parameter_sync_step` samples at once, Trainer performs a local
training every time it fetches `require_batches*ppo_mini_batch_size` samples, and after training
trigger_parameter_sync_step times, Trainer and Rollouter perform a parameter synchronization;
3. Compared to a, since more samples are generated at once, resource idleness will be lower.
4. In one step training, there will be two periods of resource idleness: when fetching the first batch of samples,
train waits for `require_batches*ppo_mini_batch_size` samples to be produced, and during the last parameter
update, rollout waits for training to complete.
5. As shown in figure b;
3. async stream pipeline with stale samples:
1. **trigger_parameter_sync_step>=1, staleness_threshold>0, partial_rollout=False**
2. After each parameter update, Rollouter will plan to produce at most rollout_num samples (in practice, the number
of samples generated may be less than this value depending on rollout speed).
3. If the rollout process is relatively fast, Rollouter will generate some additional samples num_stale_samples
before parameter synchronization for immediate use by Trainer after synchronization.
When triggering parameter synchronization, if Rollouter has ongoing tasks, it will wait for the tasks to complete
and not add new tasks;
4. Compared to b, except for the first step training, subsequent training will not have the time to wait for the
first batch rollout to finish, but will have the time to wait for active tasks to finish.
5. As shown in figure c;
4. async stream pipeline with partial rollout:
1. **trigger_parameter_sync_step>=1, staleness_threshold>0, partial_rollout=True**
2. Compared to c, when triggering parameter synchronization, if Rollouter has samples being produced, it will
interrupt the rollout process and perform parameter synchronization. The interrupted samples will continue to be
generated after synchronization. This reduces the time to wait for active tasks to finish.
3. As shown in figure d;
![fully_async_policy_mode](
https://github.com/ArronHZG/verl-community/blob/recipe/async_policy/docs/fully_async_policy_mode.svg?raw=true)
### Key Metrics
| metrics | implication |
|------------------------------------------------|--------------------------------------------------------------------------------------------------------|
| `trainer/idle_ratio` | Trainer idle rate |
| `rollouter/idle_ratio` | Rollouter idle rate |
| `fully_async/count/stale_samples_processed` | Total number of old samples used in training |
| `fully_async/count/stale_trajectory_processed` | Total number of old trajectories used in training (one sample produces rollout.n trajectories) |
| `fully_async/partial/total_partial_num` | Number of partial samples processed by Trainer between two trigger_parameter_sync_step |
| `fully_async/partial/partial_ratio` | Ratio of partial samples processed by Trainer between two trigger_parameter_sync_step |
| `fully_async/partial/max_partial_span` | Maximum parameter span of partial samples processed by Trainer between two trigger_parameter_sync_step |
### Parameter Tuning Recommendations
* Resource Allocation and Adjustment:
* Reasonable resource allocation is the prerequisite for achieving good training efficiency. The ideal resource
allocation should make the rollout time and train time close, thereby minimizing pipeline bubbles in the entire
training process,
avoiding resource idleness, and ensuring Trainer does not use old samples. In real training scenarios, resource
allocation can be adjusted based on the idle time of rollout and train during actual training,
which can be obtained from rollouter/idle_ratio and trainer/idle_ratio. If rollouter/idle_ratio is high and
trainer/idle_ratio is low,
Trainer resources should be increased and Rollouter resources should be reduced, and vice versa.
* Key Parameters:
* staleness_threshold: Setting it too high will cause more old samples to be used, affecting model performance. It
is recommended to set it to less than 1.
* require_batches: The closer to 1, the closer to a pure streaming process, the smaller the training bubbles, and
the faster the acceleration effect that can be achieved in terms of speed, but it will affect the order of sample
processing;
* trigger_parameter_sync_step: The smaller the setting, the closer to on policy, but it will cause frequent
parameter synchronization. Long-tail samples waste resources that cannot be filled by short samples, resulting in
low resource utilization.
The larger the setting, the higher the computational efficiency, but the accuracy will be affected by off policy.
* rollout.test_freq: It will occupy Rollouter resources and is not recommended to be set too small.
* Mode Selection: By adjusting different parameters, the Fully Async architecture supports optimization acceleration at
different levels, suitable for tasks in different scenarios.
* For small-scale tasks that need to ensure training stability and on-policy nature, and have low speed
requirements, the on policy pipeline mode (Mode 1) can be tried.
* For scenarios that need to improve training throughput but are sensitive to staleness, the stream off policy
pipeline mode can be tried. That is, by
setting trigger_parameter_sync_step>1 to improve training efficiency, but still maintaining the synchronization
mechanism (staleness_threshold=0) (Mode 2).
* For large-scale tasks with high training speed requirements and can tolerate a certain degree of off-policy and
staleness, setting staleness_threshold>
0 and partial_rollout=True can improve training efficiency, using the async stream pipeline mode (Mode 3 or 4).
### Quick Start
```shell
rollout_mode="async"
rollout_name="vllm" # sglang or vllm
if [ "$rollout_mode" = "async" ]; then
export VLLM_USE_V1=1
return_raw_chat="True"
fi
train_prompt_bsz=0
gen_prompt_bsz=1
n_resp_per_prompt=16
train_prompt_mini_bsz=32
total_rollout_steps=$(((512*400)))
test_freq=10
staleness_threshold=0
trigger_parameter_sync_step=16
partial_rollout=False
python -m recipe.fully_async_policy.fully_async_main \
train_batch_size=${train_prompt_bsz} \
data.gen_batch_size=${gen_prompt_bsz} \
data.return_raw_chat=${return_raw_chat} \
actor_rollout_ref.rollout.n=${n_resp_per_prompt} \
actor_rollout_ref.actor.strategy=fsdp2 \
critic.strategy=fsdp2 \
actor_rollout_ref.hybrid_engine=False \
actor_rollout_ref.actor.use_dynamic_bsz=${use_dynamic_bsz} \
actor_rollout_ref.ref.log_prob_use_dynamic_bsz=${use_dynamic_bsz} \
actor_rollout_ref.rollout.log_prob_use_dynamic_bsz=${use_dynamic_bsz} \
actor_rollout_ref.rollout.name=${rollout_name} \
actor_rollout_ref.rollout.mode=${rollout_mode} \
actor_rollout_ref.rollout.calculate_log_probs=True \
trainer.nnodes="${NNODES_TRAIN}" \
trainer.n_gpus_per_node="${NGPUS_PER_NODE}" \
rollout.nnodes="${NNODES_ROLLOUT}" \
rollout.n_gpus_per_node="${NGPUS_PER_NODE}" \
rollout.total_rollout_steps="${total_rollout_steps}" \
rollout.test_freq="${test_freq}" \
async_training.staleness_threshold="${staleness_threshold}" \
async_training.trigger_parameter_sync_step="${trigger_parameter_sync_step}" \
async_training.partial_rollout="${partial_rollout}"
```
## Experiments
### Asynchronous Training on 7B Model
We used Qwen2.5-Math-7B to verify the benefits of the fully async strategy under long candidates and multiple resources.
Using the `async stream pipeline with stale samples` strategy, we achieved about 2x performance improvement on 32 cards,
64 cards, and 128 cards without significantly affecting experimental results.
* Machine: H20
* Model: Qwen2.5-Math-7B
* Rollout length: max_response_length FSDP2: 28K tokens;
* Algorithm: DAPO
* Dataset: TRAIN_FILE: dapo-math-17k.parquet TEST_FILE: aime-2024.parquet
* Engine: vllm+FSDP2
* rollout.n: 16
* ppo_mini_batch_size: 32
* test_freq: 20
* colocate sync:
* step: 400
* train_batch_size: 512
* fully_async_policy
* total_rollout_steps: 512*400
* require_batches: 4
* trigger_parameter_sync_step: 4
* staleness_threshold: 0.5
* partial_rollout: True
| training mode | resource allocation | step | gen | old_log_prob | update_actor | total time<br>100 step | total time<br>200 step | total time<br>300 step | total time<br>400 step | acc/mean@1 |
|:--------------------:|:---------------------:|:--------:|:--------:|:--------------:|:--------------:|:------------------------:|:------------------------:|:------------------------:|:------------------------:|:------------------------------:|
| colocate sync | 32 | 790.10 | 357.41 | 107.71 | 313.81 | 13h 44m | 1d 3h 43m | 2d 9h 22m | 3d 17h 5m | max: 0.3313<br>last: 0.2448 |
| fully_async_policy | 16:16 | | | \ | | | | | | max: <br>last: |
| colocate sync | 64 | 365.28 | 150.72 | 70.26 | 133.41 | 10h 22m | 20h 45m | 1d 7h 6m | 1d 17h 32m | max: 0.3365<br>last: 0.2333 |
| fully_async_policy | 32:32 | 189.26 | 28.46 | \ | 156.98 | 4h 57m<br>(2.09x) | 10h 14m<br>(2.03x) | 16h 58m<br>(1.83x) | 21h 40m<br>(1.92x) | max: 0.3677<br>last: 0.3406 |
| colocate sync | 128 | 356.30 | 177.85 | 53.92 | 113.81 | 8h 36m | 17h 56m | 1d 5h 6m | 1d 16h 48m | max: 0.3573<br>last: 0.2958 |
| fully_async_policy | 64:64 | 150.63 | 33.14 | \ | 113.16 | 3h 13m<br>(2.67x) | 6h 46m<br>(2.65x) | 10h 53m<br>(2.67x) | 17h 22m<br>(2.35x) | max: 0.3521<br>last: 0.3094 |
> source data: https://wandb.ai/hou-zg-meituan/fully-async-policy-colocate_async?nw=nwuserhouzg
### 128-card 7B Asynchronous Mode Experiment
We used Qwen2.5-Math-7B to verify the effects of various modes supported by fully async.
We can see that the benefit brought by streaming is approximately 0.6x, and after combining staleness and
partial_rollout, the benefit reaches 2.35x.
| mode | step | gen | old_log_prob | update_actor | total time<br>100 step | total time<br>200 step | total time<br>300 step | total time<br>400 step | acc/mean@1 |
|:-------------------------------------------------------------------------------------------------------:|:---------------------:|:--------:|:--------------:|:--------------:|:------------------------:|:------------------------:|:------------------------:|:------------------------:|:-----------------------------:|
| colocate sync | 128 | 356.30 | 177.85 | 53.92 | 113.81 | 8h 36m | 17h 56m | 1d 5h 6m | 1d 16h 48m | max: 0.3573<br>last: 0.2958 |
| `stream off policy pipeline`<br>(+fully async: trigger_parameter_sync_step= 4,<br>require_batches= 4) | 231.34 | 128.47 | \ | 98.77 | 4h 25m | 9h 41m | 15h 2m | 1d 1h 53m | max: 0.2844<br>last: 0.2604 |
| `async stream pipeline with stale samples`<br>(+staleness_threshold=0.5) | | | | | | | | | |
| `async stream pipeline with partial rollout`<br>(+partial_rollout=True) | 150.63 | 33.14 | \ | 113.16 | 3h 13m | 6h 46m | 10h 53m | 17h 22m | max: 0.3521<br>last: 0.3094 |
> source data: https://wandb.ai/hou-zg-meituan/fully-async-policy-stream_stale_partial?nw=nwuserhouzg
### 128-card Stale Ablation Experiment
Under the `async stream pipeline with partial rollout` mode, we verified the impact of staleness settings on training
efficiency.
We found that the larger the staleness, the more obvious the final gains.
We also noticed that the times for staleness values of 0.3 and 0.5 are quite close, because as the training steps
increase, the response length changes significantly, causing training instability.
Further analysis and optimization are needed for this issue.
| staleness_threshold | step | gen | old_log_prob | update_actor | total time<br>100 step | total time<br>200 step | total time<br>300 step | total time<br>400 step | acc/mean@1 |
|:---------------------:|:--------:|:--------:|:--------------:|:--------------:|:------------------------:|:------------------------:|:------------------------:|:------------------------:|:-----------------------------:|
| 0 | 231.34 | 128.47 | \ | 98.77 | 4h 25m | 9h 41m | 15h 2m | 1d 1h 53m | max: 0.2844<br>last: 0.2604 |
| 0.1 | 171.30 | 58.17 | \ | 109.12 | 3h 53m | 8h 37m | 14h 25m | 19h 59m | max: 0.3542<br>last: 0.2979 |
| 0.3 | 146.11 | 38.88 | \ | 103.22 | 3h 18m | 6h 49m | 11h 40m | 17h 20m | max: 0.3469<br>last: 0.2865 |
| 0.5 | 150.63 | 33.14 | \ | 113.16 | 3h 13m | 6h 46m | 10h 53m | 17h 22m | max: 0.3521<br>last: 0.3094 |
> source data: https://wandb.ai/hou-zg-meituan/fully-async-policy-stream_stale_partial?nw=nwuserhouzg
### 128-card 7B require_batches Ablation Experiment
In multiple tests, we found that the number of samples issued each time in streaming affects the response length during
training, which in turn affects training time. We verified the impact on results by modifying
`async_training.require_batches`.
| require_batches | step | gen | old_log_prob | update_actor | total time<br>100 step | total time<br>200 step | total time<br>300 step | acc/mean@1 |
|:-----------------:|:--------:|:-------:|:--------------:|:--------------:|:------------------------:|:------------------------:|:------------------------:|:-----------------------------:|
| 1 | 203.47 | 30.88 | \ | 181.08 | 3h 31m | 8h 29m | 17h 36m | max: 0.349<br>last: 0.326 |
| 2 | 158.72 | 26.32 | \ | 128.08 | 3h 35m | 7h 38m | 13h 57m | max: 0.351<br>last: 0.3406 |
| 4 | 124.64 | 25.62 | \ | 95.06 | 3h 13m | 6h 46m | 10h 53m | max: 0.3521<br>last: 0.3521 |
> source data: https://wandb.ai/hou-zg-meituan/fully-async-policy-ablation_require_batches?nw=nwuserhouzg
### 30B Model Mode Experiment
TODO: The 30B experiment is still in progress.
* Machine: H20
* Model: Qwen2.5-32B
* Rollout length: max_response_length FSDP2: 20K tokens;
* Algorithm: DAPO
* Engine: vllm+FSDP2
* rollout.n: 16
* ppo_mini_batch_size: 32
* test_freq: 20
* colocate sync:
* step:200
* train_batch_size: 512
* fully_async_policy
* total_rollout_steps: 512*200
* trigger_parameter_sync_step: 512/32 = 16
* staleness_threshold: 0
* partial_rollout: False
| training mode | Resource allocation | mode | step | generate_sequences | old_log_prob | update_actor | total time | acc/best@32/mean |
|--------------------|---------------------|--------------------------------------------|------|--------------------|--------------|--------------|------------|------------------|
| colocate sync | 128 | | | | | | | |
| fully_async_policy | 64:64 | stream off policy pipeline | | | | | | |
| fully_async_policy | 64:64 | async stream pipeline with stale samples | | | | | | |
| fully_async_policy | 64:64 | async stream pipeline with partial rollout | | | | | | |
## Future Plans
* GRPO experiments
* Megatron adaptation
* SGLang integration
* Transfer queue integration
* Asynchronous parameter synchronization
* AReaL asynchronous algorithm implementation
* TPPO algorithm implementation
* Multi-turn and Tool support
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