Megatron-LM Backend

Last updated: 12/01/2025.

We support Megatron Backend by implementing various workers for actor, critic, reference, rollout and reward models. We also implement the 3DHybridEngine using Megatron-LM and vLLM/SGLang in megatron_vllm.py <https://github.com/volcengine/verl/blob/main/verl/workers/sharding_manager/megatron_vllm.py>_ and megatron_sglang.py <https://github.com/volcengine/verl/blob/main/verl/workers/sharding_manager/megatron_sglang.py>_.

Pros

• Support 5D parallelism (TP, EP, CP, DP, PP) and sequence parallelism for best scalablility and throughput.
• 3D HybridEngine can significantly reduce peak memory usage and reduce weight synchronize overhead between actor and rollout.

Cons

• Huggingface Models and Megatron checkpoints need tools for conversion.

Development Progress

Note that [Deprecated] means that the feature is not supported in the latest version of verl. [To-Optimize] means that the feature is implemented but not optimized yet. [WIP] means that the feature is working in progress. [In-Release] means that the feature is ready and in review process, coming at any time.

+---------------+-----------------------------------------------------------+ | [Deprecated] | Megatron 3D Parallelism with custom models | +---------------+-----------------------------------------------------------+ | [Done] | Megatron 0.11.0 GPTModel support | +---------------+-----------------------------------------------------------+ | [Done] | Megatron GRPO support | +---------------+-----------------------------------------------------------+ | [Done] | Megatron with vLLM 0.8.2, with per-tensor weights loading | +---------------+-----------------------------------------------------------+ | [Done] | Megatron with Context Parallel | +---------------+-----------------------------------------------------------+ | [Done] | Qwen2MoE model support | +---------------+-----------------------------------------------------------+ | [To-Optimize] | Megatron dist Checkpoint | +---------------+-----------------------------------------------------------+ | [To-Optimize] | Huggingface and Megatron Checkpoint Converter | +---------------+-----------------------------------------------------------+ | [To-Optimize] | Efficient fused linear, entropy and cross entropy | +---------------+-----------------------------------------------------------+ | [Done] | Megatron offload(param, grad, optimizer) | +---------------+-----------------------------------------------------------+ | [Done] | Megatron Profiler | +---------------+-----------------------------------------------------------+ | [In-Release] | Megatron 0.12.0, TE 2.2 with vLLM 0.8.3 and Fused Attn | +---------------+-----------------------------------------------------------+ | [WIP] | Moonlight/DeepSeek-V3 model support | +---------------+-----------------------------------------------------------+ | [WIP] | Expert Parallel support | +---------------+-----------------------------------------------------------+ | [WIP] | Megatron support dynamic batch size | +---------------+-----------------------------------------------------------+ | [To-Do] | Performance tuning | +---------------+-----------------------------------------------------------+ | [MileStone] | Runnable with DeepSeek-V3 671B post-training | +---------------+-----------------------------------------------------------+

Utils of Megatron Workers

MegatronWorker ^^^^^^^^^^^^^^

MegatronWorker is the base class of different megatron worker classes. In this class, get_megatron_global_info and get_megatron_rank_info function to retrieve the 3D parallel world size and rank of each Worker running on specific GPU. These information will be used in transfer protocol for Megatron Backend.

The following Worker class for different models will be utilized to construct the WorkerGroup .

We implement various of APIs for each Worker class decorated by the @register(dispatch_mode=) . These APIs can be called by the ray driver process. The data can be correctly collect and dispatch following the dispatch_mode on each function. The supported dispatch_model (i.e., transfer protocols) can be found in decorator.py <https://github.com/volcengine/verl/blob/main/verl/single_controller/base/decorator.py>_.

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

This class is implemented for Actor/Rollout HybridEngine or for the reference model to initialize their model and perform computation.

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. MegatronPPOActor implements the simple PPO computation logics when the model is built with Megatron, 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/megatron_workers.py#L63>_ for more information.

.. code:: python

build actor model

self.actor = MegatronPPOActor(config=self.config.actor, model_config=self.actor_model_config, megatron_config=megatron_config, actor_module=self.actor_module, actor_optimizer=self.actor_optimizer, actor_optimizer_config=self.actor_optim_config)

build rollout

rollout initialization

rollout = vLLMRollout(actor_module=params, config=self.config.rollout, tokenizer=self.tokenizer, model_hf_config=self.actor_model_config, train_tp=mpu.get_tensor_model_parallel_world_size()) ...

1. Generate sequence and recompute log prob

.. code:: python

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

• Dispatch.MEGATRON_PP_AS_DP_PROTO: The PP dimension of the actor model will be regarded as DP dimension. Then the driver process will dispatch and collect the data according to this reorganization. This is because, in HybridEngine, the actor weight, which usually applied larger 3D parallel sizes, will be gathered along the PP dimension and TP dimension. Therefore, the corresponding data should be dispatched and collected through the 3D parallel group of the rollout model, rather than the actor model. However, the world_size and rank information can only be retrieved from get_megatron_global_info and get_megatron_rank_info, which records the 3D information for the actor model. Moreover, the data resharding inside TP dimension will be processed within the HybridEngine.

• 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.MEGATRON_COMPUTE_PROTO) def update_actor(self, data: DataProto):

• Dispatch.MEGATRON_COMPUTE_PROTO: User passes the data partitioned by DP dimension. The data is dispatched to all tp/pp ranks within the same dp group, and ultimately only collects output data from tp=0 and the last pp.
• Update the actor model weight using PPO & entropy loss.

..note::

Currently, training Tensor Parallel Size can be different from inference Tensor Parallel Size.

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 MegatronPPOActor.

2. Compute reference log prob

.. code:: python

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

• In this function, the reference model will call the compute log prob function in MegatronPPOActor 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.MEGATRON_COMPUTE_PROTO) def compute_values(self, data: DataProto):

3. Update Critic

.. code:: python

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

4. Compute Reward

.. code:: python

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

Utils of Train Optimization

Offload ^^^^^^^ When resources are tight, the offload method can lower GPU memory usage, helping training and inference frameworks work well under verl. It moves parameters, gradients, and optimizers to CPU memory and only loads them back to the GPU when needed.

If you want to use the offload, you can add the following parameters for the actor and ref separately.

.. code:: python

For the actor

actor_rollout_ref.actor.megatron.param_offload=True
actor_rollout_ref.actor.megatron.grad_offload=True
actor_rollout_ref.actor.megatron.optimizer_offload=True \

For the ref w/o grad and optimizer

actor_rollout_ref.ref.megatron.param_offload=True \

For the critic, you can include these parameters.

.. code:: python

For the critic

critic.megatron.param_offload=True
critic.megatron.grad_offload=True
critic.megatron.optimizer_offload=True \

Related MCore Document

There is also a detailed document of using MCore to train different kinds of models, please refer to MCore Document <https://github.com/volcengine/verl/blob/main/verl/models/mcore/readme.md>_.