NCCL Engine

The NCCL weight transfer engine uses NCCL broadcast operations to transfer weights from the trainer to inference workers. It supports multi-node and multi-GPU setups where the trainer and inference engine run on separate GPUs.

When to Use NCCL

• Training and inference on separate GPUs (possibly across nodes)
• Tensor-parallel inference with multiple workers that all need the updated weights
• You need high-bandwidth, low-latency weight transfer over NVLink or InfiniBand

How It Works

1. The trainer and all inference workers join a shared NCCL process group using StatelessProcessGroup (vLLM's torch.distributed-independent group abstraction). The trainer is rank 0; workers start at rank_offset (1).
2. The trainer broadcasts weights to all workers simultaneously. Each worker receives and loads the weights.
3. Optionally, packed tensor broadcasting batches multiple small tensors into larger buffers with double/triple buffering and CUDA stream overlap for higher throughput. This implementation is based on NeMo-RL's packed tensor.

The workers' update_weights and the trainer's broadcast run at the same time — both sides rendezvous inside the same NCCL calls. The trainer engine owns that concurrency internally.

Inference Side

The inference side takes a plain backend selector. The rendezvous parameters and the packing wire params arrive from the trainer at the init handshake.

from vllm import LLM
from vllm.config import WeightTransferConfig

llm = LLM(model="my-model", weight_transfer_config=WeightTransferConfig(backend="nccl"))

vllm serve my-model --weight-transfer-config '{"backend": "nccl"}'

Nothing else is required: init_weight_transfer_engine, start_weight_update, update_weights, and finish_weight_update are all driven remotely by the trainer engine.

Trainer Side

from vllm.distributed.weight_transfer import (
    ModuleSource,
    HTTPVLLMWeightSyncClient,
    WeightTransferTrainerFactory,
)
from vllm.distributed.weight_transfer.nccl_engine import NCCLTrainerInitInfo

engine = WeightTransferTrainerFactory.trainer_init(
    init_info=NCCLTrainerInitInfo(
        master_address=master_address,
        master_port=master_port,
        world_size=world_size,   # trainer + all inference workers
        rank=0,                  # this trainer rank; rank 0 is the sender
        packed=True,
    ),
    client=HTTPVLLMWeightSyncClient("http://localhost:8000"),  # or RayVLLMWeightSyncClient(llm)
    source=ModuleSource(model),
)

engine.send_weights()   # once per sync

trainer_init drives the whole handshake: it kicks off the inference side's init_weight_transfer_engine (through the client) on a side thread while opening the trainer's own rank-0 endpoint, since both ends must rendezvous together. It builds the worker's init info itself, with rank_offset=1 and the same packed params, so the two sides cannot disagree.

send_weights() then drives start_weight_update, update_weights — run concurrently with the broadcast — and finish_weight_update, and returns only once every transfer has drained.

NCCLTrainerInitInfo

packed defaults to True here. The worker-side default is False, but it only applies when no trainer ships a value — which, on this path, never happens.

Packed Tensor Broadcasting

When packed=True, weight tensors are packed into large contiguous buffers before broadcasting. This cuts the number of NCCL operations and uses double/triple buffering with dedicated CUDA streams to overlap packing, broadcasting, and unpacking.

You set packed, packed_buffer_size_bytes, and packed_num_buffers only on NCCLTrainerInitInfo. The trainer propagates them to the worker inside trainer_init, the worker records them at the handshake, and receive_weights decodes with exactly the values the trainer encoded with. They are not per-round update_weights fields.

!!! note "Memory" The rotating buffers are live for the whole transfer: packed_buffer_size_bytes * packed_num_buffers on each side (2 GiB at the defaults). Lower packed_buffer_size_bytes if that is too much headroom.

The two WeightSource channels must agree

Dense NCCL is the backend that reads both WeightSource channels, so it is the one where a disagreement between them is fatal. The engine builds the per-round update info from metadata() and ships it ahead of the bytes; the worker sizes its receive buffers from that info, and in packed mode cuts its chunk boundaries from it. The bytes themselves come from iterating the source.

If iteration disagrees with what metadata() declared — a reordered, omitted, or re-dtyped parameter — the two sides split the same byte stream differently. The transfer then either hangs in NCCL waiting for a length that never arrives, or loads garbage into the model.

The sender therefore checks each pair against the declared metadata as it goes, one comparison per parameter, and raises naming the first divergent parameter rather than letting it reach the wire. ModuleSource satisfies this by construction. If you write a custom source — a Megatron export, an MoE re-fusing pass — this is the invariant to test first.

!!! note IPC does not read metadata() at all: it derives the update info from iteration as it goes, so it cannot observe a divergence.

Sparse NCCL

Sparse, flat-index weight patches use a separate backend, WeightTransferConfig(backend="sparse_nccl"). It shares only NCCL process-group initialization with the dense engine; patches are applied directly in place to existing parameters, with no layerwise reload. The current sparse MVP requires TP=1 and PP=1, and assumes a single-rank trainer.

Sparse is a delta backend: each round's patches are a fresh set of deltas from the latest optimizer step, not a stable stream of the model's parameters. So the engine takes no WeightSource — passing one raises — and each round's patches go straight to send_weights(patches). A round with no patches is a no-op.

from vllm.distributed.weight_transfer import (
    RayVLLMWeightSyncClient,
    WeightTransferTrainerFactory,
)
from vllm.distributed.weight_transfer.sparse_nccl_engine import (
    SparseNCCLTrainerInitInfo,
    SparseWeightPatch,
)

engine = WeightTransferTrainerFactory.trainer_init(
    init_info=SparseNCCLTrainerInitInfo(
        master_address=master_address,
        master_port=master_port,
        world_size=world_size,
        rank=0,
    ),
    client=RayVLLMWeightSyncClient(llm),
)

patches = [
    SparseWeightPatch(
        name="model.layers.0.mlp.down_proj.weight",
        indices=flat_indices,           # int32, 1-D
        values=new_values,              # same length as indices
        full_shape=tuple(param.shape),  # required when sending via the engine
    )
]
engine.send_weights(patches)

SparseNCCLTrainerInitInfo takes the same rendezvous fields as the dense backend and no packed params — sparse transfers are never packed.

Patches are validated on the trainer before start_weight_update: int32 indices, 1-D, and matching indices/values lengths, with full_shape set. The worker checks the same invariants when applying, but by then the broadcasts are under way, where a size mismatch wedges both sides instead of raising.

A trainer can hold a dense engine and a sparse engine at once — a full resync through the dense engine, cheap sparse deltas in between — as rlhf_sparse_nccl.py demonstrates.

Examples

• RLHF with NCCL weight syncing (vllm serve, HTTP) - Start here. Trainer on one GPU, 2x tensor-parallel fp8 server on two others; HTTP control plane, NCCL data plane. Launches and tears down its own server
• RLHF with NCCL + FSDP2 and expert parallelism - Multi-rank trainer: every FSDP rank builds an engine and joins the full_tensor() gather while only rank 0 touches the wire
• RLHF with sparse NCCL weight syncing (offline, Ray) - Dense-vs-sparse equivalence demo with a real model on a 2-GPU trainer/inference setup; sparse patches use backend="sparse_nccl" and currently require TP=1 and PP=1
• RLHF with async weight syncing (offline, Ray) - Async generation with mid-flight pause, weight sync, resume, and validation against a fresh model; uses an in-process AsyncLLMEngine with RayVLLMWeightSyncClient