The Design of verl.single_controller

Last updated: 05/21/2025.

Author: Wang Zhang

Preface

We prepared this document for developers of verl, particularly those interested in understanding or contributing to the verl.single_controller module. It is not intended for end users, but for contributors seeking to understand the architectural rationale and internal mechanics.

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Origin

The single_controller module originated from a request I received — to adapt a toy single-process RLHF script into a distributed system with minimal changes, while maintaining ease of debugging.

Common practice — such as using PyTorch’s Distributed Data Parallel (DDP) — typically involves wrapping nn.Module and launching multiple processes that execute the same function under different ranks. However, this approach presents two main limitations in the context of distributed RLHF: - Difficulty representing multiple DAGs as required by PPO; - Difficulty inspecting intermediate tensors during training.

To maintain debuggability, we opted for a different approach — breaking the training loop into well-defined stages like generate_sequences, compute_advantages, and so on.

We selected Ray as the initial backend for verl due to its ability to expose Python class methods as RPC endpoints. However, Ray’s default model only supports one method call, one RPC, while training LLMs typically requires coordination across multiple processes.

To hide this multi-Ray actors invocation for a single method from users, we introduced the following components:

• WorkerGroup – manages a group of remote workers and provides a unified interface for multi-process distributed computation;

• ResourcePool – binds computational resources to worker processes;

• ClassWithArgs – enables delayed remote instantiation with specified initialization arguments.

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A Running Example: generate_sequences

To illustrate the design, we walk through how the generate_sequences method in the ActorRolloutRefWorker class is registered and invoked across distributed workers.

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Step 1: Register with a Decorator

The first step is to define the generate_sequences and decorate it with @register as it will be called in driver script.

Source: fsdp_workers.py

class ActorRolloutRefWorker(Worker):
    ...
    @register(dispatch_mode=Dispatch.DP_COMPUTE_PROTO)
    def generate_sequences(self, prompts: DataProto):
        prompts = prompts.to(torch.cuda.current_device())
        ...

The @register decorator adds metadata to the generate_sequences method. Currently, it doesn’t alter functionality, but attaches attributes via a magic key (MAGIC_ATTR):

Source: decorator.py

def register(dispatch_mode=Dispatch.ALL_TO_ALL, execute_mode=Execute.ALL, blocking=True, materialize_futures=True):
    ...
    def decorator(func):
        @wraps(func)
        def inner(*args, **kwargs):
            if materialize_futures:
                args, kwargs = _materialize_futures(*args, **kwargs)
            return func(*args, **kwargs)

        attrs = {"dispatch_mode": dispatch_mode, "execute_mode": execute_mode, "blocking": blocking}
        setattr(inner, MAGIC_ATTR, attrs)
        return inner

    return decorator

As the code shows, values of dispatch_mode, execute_mode and blocking is attached the generate_sequences method.

---

Step 2: Binding During Initialization

These attached attributes are extracted and utilized when ActorRolloutRefWorker, wrapped in a RayClassWithArgs, is passed into a RayWorkerGroup.

Source: main_generation.py

ray_cls_with_init = RayClassWithInitArgs(cls=ray.remote(ActorRolloutRefWorker), config=config, role="rollout")
resource_pool = RayResourcePool(process_on_nodes=[config.trainer.n_gpus_per_node] * config.trainer.nnodes)
wg = RayWorkerGroup(resource_pool=resource_pool, ray_cls_with_init=ray_cls_with_init)

During the initialization of RayWorkerGroup, two key steps occur:

1. Worker instances (Ray actors) are created: RayWorkerGroup._init_with_resource_pool

2. Methods decorated with @register are bound to RayWorkerGroup: RayWorkerGroup._bind_worker_method

initialization_and_binding_of_worker_group

The binding procedure is the heart of verl.single_controller.

Key function: WorkerGroup._bind_worker_method

def _bind_worker_method(self, user_defined_cls, func_generator):
    ...
    for method_name in dir(user_defined_cls):
        try:
            method = getattr(user_defined_cls, method_name)
            assert callable(method)
        except Exception:
            continue  # Skip properties
        <<<to be continue 1>>>

When a method has the MAGIC_ATTR, the attributes set by @register are extracted:

<<<continue 1>>>
if hasattr(method, MAGIC_ATTR):
    attribute = getattr(method, MAGIC_ATTR)
    dispatch_mode = attribute["dispatch_mode"]
    execute_mode = attribute["execute_mode"]
    blocking = attribute["blocking"]

    <<<to be continue 2>>>

As show in the flow chart above, these attributes are fed into func_generator. However, func_generator takes method_name, dispatch_fn, collect_fn, execute_fn, blocking. We need to find the corresponding dispatch_fn and collect_fn associated with the dispatch_mode (DP_COMPUTE_PROTO) from DISPATCH_MODE_FN_REGISTRY:

DISPATCH_MODE_FN_REGISTRY = {
    Dispatch.ONE_TO_ALL: {
        "dispatch_fn": dispatch_one_to_all,
        "collect_fn": collect_all_to_all,
    },
    ...
    Dispatch.DP_COMPUTE_PROTO: {
        "dispatch_fn": dispatch_dp_compute_data_proto,
        "collect_fn": collect_dp_compute_data_proto,
    },
    ...
}

Similarly, the execute_fn is selected by execute_mode and extracted by:

<<<continue 2>>>
# get execute_fn_name
execute_mode = get_predefined_execute_fn(execute_mode=execute_mode)
wg_execute_fn_name = execute_mode["execute_fn_name"]

# get execute_fn from string
try:
    execute_fn = getattr(self, wg_execute_fn_name)
    assert callable(execute_fn), "execute_fn must be callable"
except Exception:
    print(f"execute_fn {wg_execute_fn_name} is invalid")
    raise
<<<to be continue 3>>>

In this generate_sequences cases: - dispatch_mode = Dispatch.DP_COMPUTE_PROTO - dispatch_fn = dispatch_dp_compute_data_proto - collect_fn = collect_dp_compute_data_proto - execute_fn = RayWorkerGroup.execute_all

ONE_TO_ALL v.s. DP_COMPUTE_PROTO

dispatch_mode is associated with a dispatch_fn and a collect_fn. As the name implies, dispatch_fn processes the input arguments in WorkerGroup and generate a batch (list) of input arguments, each of which will be fed into a worker attached to the WorkerGroup.

dispatch_fn of ONE_TO_ALL is dispatch_one_to_all, which just duplicates all the input arguments into N replicas, where N equals the number of Workers attached to the worker_group:

def dispatch_one_to_all(worker_group, *args, **kwargs):
    args = tuple([arg] * worker_group.world_size for arg in args)
    kwargs = {k: [v] * worker_group.world_size for k, v in kwargs.items()}
    return args, kwargs

dispatch_fn of DP_COMPUTE_PROTO is dispatch_dp_compute_data_proto, which uses DataProto.chunk to split a large DataProto into N smaller DataProto, where N equals the world_size (number of the workers) of the worker_group:

def dispatch_dp_compute_data_proto(worker_group, *args, **kwargs):
    from verl.single_controller.base.worker_group import WorkerGroup

    assert isinstance(worker_group, WorkerGroup)
    # Note: enable auto padding for dp compute DatapProto
    splitted_args, splitted_kwargs = _split_args_kwargs_data_proto_with_auto_padding(
        worker_group.world_size,
        *args,
        **kwargs,
    )
    return splitted_args, splitted_kwargs

The collect_fn follows the same pattern and process a batch (list) of returned value from all workers of a WorkerGroup and merge it into a list as collect_all_to_all does or a large DataProto as collect_dp_compute_data_proto does.

Finally, a new method is dynamically generated using func_generator and added to the WorkerGroup instance:

<<<continue 3>>>
# bind a new method to the RayWorkerGroup
func = func_generator(
    self,
    method_name,
    dispatch_fn=dispatch_fn,
    collect_fn=collect_fn,
    execute_fn=execute_fn,
    blocking=blocking,
)

try:
    setattr(self, method_name, func)
    method_names.append(method_name)
except Exception as e:
    raise ValueError(f"Fail to set method_name {method_name}") from e

This makes the method invocable via the WorkerGroup interface.

---

Step 3: Call Chain

All the machinery above ensures that distributed calls feel identical to single-process ones. In the original single-process script, the code looks like:

rollout = Rollout()
rollout.generate_sequences(batch)

With verl, the multiprocess program becomes:

rollout = RayWorkerGroup(resource_pool=[4], RayClassWithArgs(Rollout))
rollout.generate_sequences(batch)

call_chain_of_generate_sequences

Behind this simple call: - dispatch_fn splits input across workers - execute_fn performs the actual remote invocation - collect_fn gathers the results

All of this is abstracted away, enabling developers to write distributed code with minimal changes to their existing logic.

---

Beyond RL Post-Training: Generalizing verl.single_controller

The verl.single_controller module generalizes well beyond reinforcement learning. It provides a clean abstraction to batch-process remote method calls, with automatic input/output handling.

By minimizing the gap between single-process and multi-process scripts, verl.single_controller opens the door to distributed computing in broader domains — not limited to RL post-training.

We hope this design inspires more examples and extensions from the community.