vllm-omni/docs/design/feature/distributed_layerwise_offload.md

Distributed Layerwise Offload

This document describes distributed layerwise offload (DLO) for diffusion models. DLO keeps only a small number of DiT blocks on the accelerator and streams the remaining blocks from host memory. The distributed backend can either shard those host-side weights across an existing parallel group or keep the standard loader's rank-local weights and avoid an additional collective.

For user-facing commands, see the distributed layerwise offloading guide and the Cosmos3 recipe.

Status

DLO is implemented for multi-device diffusion execution. The default AllGather path is the primary path for DP and SP deployments. The --dlo-no-use-allgather path is a rank-local compatibility mode: it is useful for standard-loader sharding, workstation bring-up, and systems where an additional DLO collective is undesirable, but it does not reduce host weight storage across ranks.

The compatibility matrix below describes the current implementation. The unit-level guards are covered, but not every parallelism combination has a full model-and-hardware end-to-end test.

Design

DLO consumes the existing parallel topology

DLO does not create a new DP, TP, or SP topology. It reads the configured DiffusionParallelConfig and attaches offload hooks to the DiT blocks after the standard distributed groups have been initialized.

The DLO weight-sharding group is selected as follows:

1. Use the existing DP group when data_parallel_size > 1.
2. When DP is one and SP is greater than one, use the SP group.
3. Otherwise, run rank-locally without a DLO process group.

TP is deliberately not used as DLO's AllGather group. HSDP has its own parameter-sharding lifecycle and is not allowed to be sharded a second time by DLO's AllGather path.

AllGather path

With the default dlo_use_allgather=True, each rank stores approximately 1 / group_size of each streamable block in pinned host memory. The next block's shard is copied to a device buffer and reconstructed with all_gather_into_tensor on a communication stream while the current block is executing.

Compute:    [Block N]             [Block N+1]          [Block N+2]
H2D:                      [shard N+1]           [shard N+2]
AllGather:                [full N+1]             [full N+2]
Buffers:    [current slot]       [prefetch slot]       [current slot]

The backend uses two shared device buffers, so accelerator weight residency is bounded by the largest streamed blocks rather than the complete model.

When DP is greater than one, the engine can process one request per DP rank in the same denoising wave. Because AllGather is a collective, all participating requests must take the same execution path at every denoising step.

Rank-local path without DLO AllGather

With --dlo-no-use-allgather, DLO forces its internal offload shard size to one. The regular model loader remains responsible for preparing each rank's weights, including TP-local tensors or HSDP-managed parameters. DLO then streams those rank-local tensors block by block using H2D copies only.

This mode means:

• DP still provides independent replicas, but DLO does not shard weights across DP ranks.
• SP still performs its normal activation/attention collectives, but DLO does not shard weights across SP ranks.
• TP/HSDP/SP collectives, if configured, are not disabled by this flag; only DLO's additional weight AllGather is disabled.
• Pure DP deployments keep a full host-side model copy per rank, subject to shared OS page-cache behavior.
• The scheduler does not require a synchronized DP request wave for DLO.

This path is intentionally not implemented by reusing the AllGather mmap loader. It relies on the standard loader so model-specific TP/HSDP transforms remain intact.

Parallelism compatibility

Parallelism	DLO + AllGather	DLO without AllGather
DP	Supported primary path. DLO shards host weights across the DP group and can run DP multi-concurrency.	Supported rank-local path. DP replicas remain independent; no DLO weight sharding or cross-DP DLO collective.
SP	Supported in the implementation. With DP=1, DLO uses the SP group for host-weight sharding; SP still shards sequence/activation work.	SP remains active, but DLO keeps standard-loader rank-local weights and adds no SP weight collective.
TP > 1	Unsupported in the DLO mmap path. TP-aware loader callbacks are bypassed, so the backend rejects this configuration when it enters that path.	Standard loading is retained and TP-local tensors can be streamed. This is the intended compatibility path, but it still needs broader model and hardware validation.
HSDP	Rejected. HSDP has already sharded parameters, so DLO AllGather would double-shard them.	Accepted by configuration. HSDP owns parameter sharding and its own gathers; DLO only stages rank-local parameters. End-to-end coverage is limited.

Combined dimensions

• DP + SP: DLO uses the DP group for weight sharding when DP is greater than one; SP continues to use its own sequence-parallel group. If DP is one, the SP group becomes DLO's sharding group in AllGather mode.
• DP + TP/SP without AllGather: standard model loading defines the rank-local tensor layout. DLO adds no cross-DP, cross-TP, or cross-SP weight collective.
• HSDP + SP: the general parallel configuration permits HSDP over SP, but DLO must use --dlo-no-use-allgather. HSDP remains responsible for weight materialization and synchronization.
• HSDP + DP or TP: rejected independently by the diffusion parallel configuration.

Request and loading constraints

AllGather DP multi-concurrency requires:

• explicit num_inference_steps;
• the same num_inference_steps for all requests in a wave; and
• identical request arguments that affect the collective execution path.

The no-AllGather path does not impose these DLO-specific synchronized-wave requirements.

The mmap loader is used only by the supported DLO+AllGather path when the model has a compatible checkpoint layout. Online quantization is incompatible with that sharded mmap path; use --dlo-no-use-allgather or disable online quantization. Models that do not meet the mmap requirements use the regular loader path.

Validation coverage

Current source-level validation includes:

• HSDP + DLO + AllGather rejection;
• HSDP + DLO without AllGather acceptance at configuration level;
• TP rejection in the DLO+AllGather mmap path;
• resident-layer requests requiring no-AllGather;
• DP request-wave validation for denoising-step compatibility;
• sharding, double-buffer, AllGather-size, and heterogeneous-block regression tests.

The highest-value missing coverage is end-to-end numerical comparison against ordinary layerwise offload for DP+SP, TP+no-AllGather, and HSDP+SP+no-AllGather on the target CUDA/NCCL or CANN/HCCL hardware.

Recommendations

• Use DP + DLO AllGather for the supported throughput and host-memory scaling path.
• Use SP + DLO AllGather for long-sequence workloads when DP concurrency is not the goal.
• Use no-AllGather to bring up TP or workstation PCIe configurations, with the expectation of higher host-memory use and lower validation confidence.
• Prefer HSDP alone for production HSDP deployments until the combined HSDP + DLO no-AllGather path has broader end-to-end coverage.