Megatron-LM Backend

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 in megatron_vllm.py.

Pros

  • Support 3D parallelism 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

  • Users should implement their own models for Megatron-LM

  • Users should implement the corresponding weight_loader to

    • synchronize the model weight between actor (in Megatron) and rollout (in vLLM).

    • load weights from checkpoints to corresponding model in Megatron-LM

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 retrive 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.

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.

@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. AllGatherPPModel holds memory buffer for both Actor and Rollout and support weight resharding between actor and rollout.

  2. MegatronPPOActor implements the simple PPO computation logics when the model is built with Megatron, including compute log prob, model update.

  3. vLLMRollout support generation with vLLM. We modify the vLLM Engine and make it executed under SPMD to fit into our WorkerGroup design.

  4. MegatronVLLMShardingManager a context manager to perform actual resharding between actor and rollout.

See source code for more information.

# Initialize the 3D HybridEngine
hybrid_engine = AllGatherPPModel(model_provider=megatron_actor_model_provider)
# Fetch the model at current rank
actor_module = hybrid_engine.this_rank_models
...

# 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())
# perform weight resharding between actor and rollout
sharding_manager = MegatronVLLMShardingManager(module=self.hybrid_engine,
                                               inference_engine=rollout.inference_engine,
                                               model_config=self.actor_model_config,
                                               layer_name_mapping=layer_name_mapping)
...

  1. Generate sequence and recompute log prob

@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 retrived 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.

  1. Update actor model

@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.

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.

  1. Compute reference log prob

@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.

  1. Compute Values for CriticWorker

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

  1. Update Critic

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

  1. Compute Reward

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

Context Parallel

Currently we can only use LLaMa and Qwen models implemented in verl, and context parallel is not supported by far.

We are working in progress to support Megatron implementation of GPTModel, with TransformerEngine support. So if the itegration goes well, we can support Ulysses, Ring and AllGather context parallel in the future.

Now we support Megatron checkpointing save/load function with original models. Please check the Config Explanation page to see how to use the APIs.