PyTorch FSDP Backend

Last updated: 12/01/2025.

We support PyTorch FSDP Backend by implementing various workers for actor, critic, reference, rollout and reward models.

Pros

• Readily support various models.

  • Users only need to implement the corresponding dtensor_weight_loader for weight synchronization between FSDP and vLLM. While for hf_weight_loader, users can directly apply any models supported both in HF and vLLM without any code change.

• Easy to organize the forward and backward computation for each model.

Cons

• Poor scalability when it comes to large-scale models (e.g. Llama 70B and 405B)
• The resharding overhead between actor and rollout could be larger than Megatron-LM backend.

Due to the simplicity, we recommend using FSDP backend for algorithm research and prototyping.

FSDP Workers

ActorRolloutRefWorker ^^^^^^^^^^^^^^^^^^^^^

Actor/Rollout HybridEngine ''''''''''''''''''''''''''

1. HybridEngine, Actor and Rollout initialization API.

.. code:: python

@register(dispatch_mode=Dispatch.ONE_TO_ALL) def init_model(self):

ONE_TO_ALL: when calling the init_model function from the driver process, each worker (on a GPU) will execute the following model initialization process.

The initialization details of HybridEngine, Actor and Rollout are highlighted below:

1. DataParallelPPOActor implements the simple PPO computation logics when the model is built with FSDP, including compute log prob, model update.
2. vLLMRollout support generation with vLLM. We modify the vLLM Engine and make it executed under SPMD to fit into our WorkerGroup design.

See source code <https://github.com/volcengine/verl/blob/main/verl/workers/fsdp_workers.py>_. for more information.

1. Generate sequence and recompute log prob

.. code:: python

@register(dispatch_mode=Dispatch.DP_COMPUTE_PROTO) def generate_sequences(self, prompts: DataProto):

• Dispatch.DP_COMPUTE_PROTO: The data will be dispatched and collected along the DP dimension

• In this function, the rollout model will perform auto-regressive generation and the actor model will recompute the old log prob for the generated response.

3. Update actor model

.. code:: python

@register(dispatch_mode=Dispatch.DP_COMPUTE_PROTO) def update_actor(self, data: DataProto):

• Update the actor model weight using PPO & entropy loss.

ReferenceModel ''''''''''''''

1. Reference model initialization

The reference model is initialized using the same function as the actor model without initializing the HybridEngine and Optimizer. Then the actor model is also wrapped by the DataParallelPPOActor.

2. Compute reference log prob

.. code:: python

@register(dispatch_mode=Dispatch.DP_COMPUTE_PROTO) def compute_ref_log_prob(self, data: DataProto):

• In this function, the reference model will call the compute log prob function in DataParallelPPOActor to compute the reference log prob.

CriticWorker and RewardWorker ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

1. Model initialization

Quite similar to reference model. The CriticWorker will perform additional initialization for the Optimizer.

2. Compute Values for CriticWorker

.. code:: python

@register(dispatch_mode=Dispatch.DP_COMPUTE_PROTO) def compute_values(self, data: DataProto):

3. Update Critic

.. code:: python

@register(dispatch_mode=Dispatch.DP_COMPUTE_PROTO) def update_critic(self, data: DataProto):

4. Compute Reward

.. code:: python

@register(dispatch_mode=Dispatch.DP_COMPUTE_PROTO) def compute_rm_score(self, data: DataProto):

HybridShard

We didn't support FSDP HybridShard. To support this, we may need to construct a 2D device mesh and test the corresponding dtensor_weight_loader and hf_weight_loader for each model.