IPC Engine¶

The IPC weight transfer engine uses CUDA IPC (Inter-Process Communication) handles to share GPU memory directly between the trainer and inference workers on the same GPU. This avoids any data copying, making it the most efficient option when colocating training and inference. Multi-GPU setups are supported — weights are all gathered by each GPU and are extracted by the correct colocated process.

When to Use IPC¶

• Training and inference share the same GPU(s) (colocated)

How It Works¶

1. The trainer creates CUDA tensors for each weight and generates IPC handles using torch.multiprocessing.reductions.reduce_tensor. In multi-GPU setups (e.g. FSDP), each trainer rank materializes the full tensor for each parameter onto its own GPU before generating the handle — which ModuleSource does for you.
2. Every trainer rank contributes its handles to an all-gather; the sender merges them so each payload maps every GPU UUID to its args, and ships the merged handles to the inference engine through the client. Each worker reads only the handle for its own GPU.
3. The inference worker reconstructs the tensors from the handles using rebuild_cuda_tensor, reading directly from the trainer's GPU memory.

Unlike NCCL, IPC transfer is straight-line: update_weights is the transfer, and it rides the client, so there is no concurrent broadcast to overlap with.

Inference Side¶

from vllm import LLM
from vllm.config import WeightTransferConfig

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

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

IPC needs no data-plane rendezvous, so init_transfer_engine opens no channel — it only records the packed flag the trainer ships at the handshake, which receive_weights then reads. Whether a transfer is packed is therefore never something you configure on the inference side.

Trainer Side¶

from vllm.distributed.weight_transfer import (
    ModuleSource,
    HTTPVLLMWeightSyncClient,
    WeightTransferTrainerFactory,
)
from vllm.distributed.weight_transfer.ipc_engine import IPCTrainerInitInfo

engine = WeightTransferTrainerFactory.trainer_init(
    init_info=IPCTrainerInitInfo(rank=0, packed=False),   # rank 0 is the sender
    client=HTTPVLLMWeightSyncClient("http://localhost:8000"),
    source=ModuleSource(model),
)

engine.send_weights()   # once per sync

send_weights() drives start_weight_update, the transfer itself, and finish_weight_update, and holds strong references to the IPC-shared copies until after the post-send barrier — the consumer's views would otherwise dangle.

Any VLLMWeightSyncClient works here — the built-in HTTP and Ray clients, or an adapter for your own stack. The engine is identical either way.

IPCTrainerInitInfo¶

packed is a must-agree wire param: trainer_init ships it to the worker, which records it and decodes accordingly. It is not a WeightTransferConfig field and not a per-round update_weights field. packed_buffer_size_bytes is producer-only — the consumer rebuilds from the IPC handle plus the per-chunk tensor_sizes, so it never needs the buffer size.

Packed (Chunked) Transfer¶

By default, all weights are sent in a single update_weights call. For large models, this requires the full model to reside in GPU memory on both sides simultaneously. Setting packed=True enables chunked transfer with bounded GPU memory:

• Weights are concatenated into fixed-size packed buffers (packed_buffer_size_bytes).
• Each chunk is sent as a separate update_weights call within a single start_weight_update / finish_weight_update bracket, so the layerwise reload pass is initialized once at the start and finalized once at the end regardless of chunk count.
• After each chunk is consumed, the GPU memory for that chunk can be reclaimed.

engine = WeightTransferTrainerFactory.trainer_init(
    init_info=IPCTrainerInitInfo(
        rank=0,
        packed=True,
        packed_buffer_size_bytes=256 * 1024 * 1024,  # 256 MB chunks
    ),
    client=client,
    source=ModuleSource(model),
)

On a multi-rank trainer the producer reuses one buffer across chunks, so packed mode carries a per-chunk barrier across ranks: without it, a rank could overwrite its buffer while its colocated worker is still reading the current chunk. This is handled inside send_weights().

Examples¶

• RLHF with IPC weight syncing (vllm serve, HTTP) - Start here. Server and training model share a single GPU; HTTP control plane, CUDA IPC data plane. Launches and tears down its own server
• RLHF with IPC + FSDP2 and expert parallelism - Multi-rank trainer colocated with a --data-parallel-size 4 server on the same 4 GPUs: every FSDP rank builds an engine and joins the handle all-gather, with packed chunking and sleep/wake around the transfer