vllm-omni/docs/user_guide/diffusion/offloader/distributed_layerwise_offload.md

Distributed Layerwise Offloading

Distributed layerwise offloading (DLO) extends block streaming to multi-device deployments. With AllGather enabled, each rank stores roughly 1 / dp_size of the host weights and reconstructs each layer at runtime. Without AllGather, each rank streams a complete block independently. Compatible TP1 deployments can share checkpoint-backed host pages among processes on the same node; otherwise DLO streams the ordinary loader's rank-local tensors. The optional runtime-weight cache extends node-local sharing to final ordinary-loader layouts, including matching TP coordinates across independent engines.

See the DLO feature design for the implementation contract and compatibility matrix.

Execution model

DLO overlaps three operations with a fixed two-block device buffer:

Compute stream:  [Layer N]          [Layer N+1]        [Layer N+2]
H2D stream:      [H2D shard N+1]    [H2D shard N+2]
AllGather:       [AG N+1]           [AG N+2]
Slots:           slot 0: Layer N    slot 1: Layer N+1

AllGather communicates request-independent weight shards, but every member of the transfer group must request the same next block in the same collective order. A synchronized DP wave may contain different prompts only when those requests follow the same denoising and block-execution path.

Usage

# Four ranks with sharded host weights and AllGather
vllm serve /path/to/model --omni \
  --enable-distributed-layerwise-offload \
  --data-parallel-size 4

# Standard-loader rank-local weights, without DLO AllGather
vllm serve /path/to/model --omni \
  --enable-distributed-layerwise-offload \
  --data-parallel-size 4 \
  --dlo-no-use-allgather

# Independently scheduled replicas sharing final runtime weights on one node
vllm serve /path/to/model --omni \
  --enable-distributed-layerwise-offload \
  --tensor-parallel-size 2 \
  --dlo-no-use-allgather \
  --dlo-enable-runtime-cache \
  --dlo-runtime-cache-dir /var/cache/vllm-omni/dlo-runtime-weights

# Sequence parallel deployment
vllm serve /path/to/model --omni \
  --enable-distributed-layerwise-offload \
  --usp 4

from vllm_omni import Omni

omni = Omni(
    model="/path/to/model",
    enable_distributed_layerwise_offload=True,
    dlo_use_allgather=False,
    dlo_enable_runtime_cache=True,
    dlo_runtime_cache_dir="/var/cache/vllm-omni/dlo-runtime-weights",
)

Flags

Flag	Meaning	Default
--enable-distributed-layerwise-offload	Enable DLO	false
--data-parallel-size N	DP ranks and AllGather weight-sharding group	1
--dlo-use-allgather	Shard host weights and reconstruct with AllGather	true
--dlo-no-use-allgather	Stream complete rank-local blocks without a DLO weight collective	false
--dlo-enable-runtime-cache	Normalize final ordinary-loader DiT weights into a node-local mmap cache; requires no-AllGather	false
--dlo-runtime-cache-dir PATH	Shared local-disk cache root	~/.cache/vllm-omni/dlo-runtime-weights
--dlo-runtime-cache-lock-timeout SECONDS	Maximum wait for another process publishing the same layout	600
--dlo-resident-layers N	Keep N leading main-DiT blocks on device; requires no-AllGather and model-declared resident paths	0

Host-weight loading

The diffusion loader chooses host storage before DLO is enabled. It first attempts to build a complete, validated direct-checkpoint mmap plan. If names, coverage, shape, dtype, topology, or loader-callback compatibility cannot be proven, it runs the ordinary model loader instead. DLO consumes that result and does not make a second checkpoint-compatibility decision.

The shared-mmap optimization in this phase is supported only with TP1. TP greater than one falls back before model mutation to ordinary TP-aware loading. DLO may still consume those TP-local tensors, but this is a compatibility path: it does not share checkpoint-backed runtime weights across DP replicas and provides no shared-mmap host-memory guarantee.

When --dlo-enable-runtime-cache is set with no-AllGather, that fallback gains a second storage opportunity. Every rank still completes ordinary loading, loader callbacks, post-load processing, validation, and calibration. It then hashes the final CPU DiT parameters and persistent buffers. One process per equivalent runtime layout publishes sharded safetensors plus a checksummed manifest; peers validate and mmap the same files. DLO replaces only tensor storage, preserves the final Parameter objects, and releases the private allocator-backed copies before installing its streaming hooks.

The runtime layout identity excludes DP rank and SP rank. It includes TP world size/rank and conservative SP implementation/world-size guards. Therefore two independent TP=2 engines normally create two entries: both TP0 workers map one entry and both TP1 workers map the other. Cache sharing does not create a process group or an inference-time collective.

At inference time, cached tensors remain pageable mmap views. DLO packs each streamed block into one of two bounded pinned host slots before the asynchronous H2D copy. The private ordinary-loader path instead creates model-sized pinned storage once during initialization. The cache therefore exchanges recurrent host-to-host copies and DDR bandwidth for lower steady-state PSS; competing TP ranks or engines can turn staging skew into collective wait time. Treat this as a host-capacity mode and benchmark it on the target CPU/memory topology.

The mmap plan skips only dedicated DiT weight sources. Other component sources, such as a text encoder loaded through the shared diffusion loader, continue to use their ordinary component loader. A checkpoint source that mixes DiT and non-DiT weights falls back completely rather than leaving an unplanned component uninitialized.

With direct checkpoint mmap, the loader:

1. saves non-persistent buffers such as RoPE frequencies;
2. moves the normally created transformer to the meta device;
3. loads checkpoint tensors as mmap views backed by the shared OS page cache;
4. applies any loader-owned bounded layout adapters while packing blocks;
5. restores saved non-persistent buffers; and
6. preserves post_load_weights() and validate_loaded_weights() lifecycle hooks.

For AllGather with a group larger than one, each process copies only its persistent shard and then releases the source mapping. For no-AllGather, each process keeps the mapping open and packs complete blocks through two bounded pinned staging slots. Processes mapping the same files on one node share the immutable pages; no-AllGather still performs a complete-block H2D copy in each process.

When the effective DLO group size is one, dlo_use_allgather=True does not perform a collective and uses the same rank-local transfer behavior.

Runtime-cache lifecycle

Runtime-cache publication uses a per-layout POSIX file lock, a temporary directory on the same filesystem, file and final-tensor SHA-256 validation, fsync, and an atomic directory rename. A crashed writer releases its kernel lock; the next writer removes only that layout's stale temporary directories while holding the lock. Lock timeout, unsupported layout, insufficient disk, or validation failure keeps the already-valid ordinary tensors.

The cache root is deliberately independent of VLLM_CACHE_ROOT, because vLLM-Omni may isolate that variable between stage replicas. All consumers that should share must receive the same --dlo-runtime-cache-dir on a local, disk-backed filesystem. Do not place the cache on tmpfs when the goal is lower host-memory PSS, and do not use a cross-node directory in this version.

There is no automatic eviction. Budget roughly one final DiT copy per distinct TP coordinate, dtype/layout, and model version, plus temporary space while an entry is published. Operators may remove entries or the complete cache root only after all workers using its mmap files have stopped. The feature is opt-in for this reason.

Declarative topology

Models may declare an OffloadPlan instead of embedding offload logic:

from vllm_omni.diffusion.offloader import OffloadPlan


class MyPipeline(nn.Module):
    _dit_modules = ["transformer"]
    _offload_plan = OffloadPlan(
        block_attrs={"transformer": ("blocks",)},
        offload_submodules={"context_encoder": "layers"},
    )

When no plan exists, discovery falls back to _layerwise_offload_blocks_attrs and then heuristic attribute lookup.

Data-parallel concurrency

With data_parallel_size > 1 and AllGather enabled, the scheduler can process up to dp_size requests per denoising step. Every concurrent request must set the same explicit num_inference_steps; None is rejected because every rank must enter each collective.

Limitations

• Direct checkpoint mmap currently requires TP1. TP greater than one falls back to the ordinary TP-aware loader. With the optional runtime cache, equivalent processes at the same TP coordinate can then share that final rank-local layout in no-AllGather mode.
• HSDP plus AllGather is rejected to avoid double sharding. HSDP without AllGather has limited end-to-end validation.
• Online quantization uses the ordinary loader with no-AllGather. It remains incompatible with DLO AllGather and runtime-cache v1. Runtime-cache v1 also rejects other quantized, non-contiguous, aliased/tied, device-only, HSDP, expert-parallel, CFG-parallel, and PP layouts.
• Resident leading layers require --dlo-no-use-allgather and a model OffloadPlan that declares eligible resident_dit_paths.
• DP concurrency requires an explicit, identical inference-step count.

The runtime cache improves steady-state host PSS, not startup peak: every rank loads private weights before remapping, and validation adds full content-hash passes. Skip-load-on-cache-hit and cache eviction remain follow-up work in RFC #6195.

It is also not latency-neutral. In the four-L20X MiniMax-H3 smoke documented in the L20X runtime-cache matrix in the feature design, two independent TP2 engines reduced request-time host PSS by 36.6% with the cache, but recurrent mmap-to-pinned staging reduced combined throughput by 69.3%. HBM and GPU compute were unchanged. Use the cache only when that measured memory/latency tradeoff is appropriate for the deployment.

See the Cosmos3 DistOffload recipe for an end-to-end example.