vllm-omni/docs/design/feature/offloader/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 complete rank-local block sources and avoid an additional collective.

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

Feature compatibility

Host-storage optimization and runtime compatibility are separate decisions. When direct checkpoint mmap is unavailable, DLO can still use tensors produced by the ordinary loader. "Compatibility path" below means that fallback is implemented but has less end-to-end coverage than the primary path.

Legend: ✅ supported, ⚠️ compatibility path or limited validation, ❌ unsupported.

Feature	DLO + AllGather	DLO without AllGather
DP	✅ Primary path; host weights are sharded across the DP group.	✅ Each DP rank streams complete rank-local blocks.
SP	✅ When DP=1, DLO uses the SP group for weight sharding.	✅ SP remains active without a DLO weight collective.
TP > 1	⚠️ Ordinary TP-aware loader only; no direct checkpoint mmap.	⚠️ Ordinary TP-aware loader only; no direct checkpoint mmap.
HSDP	❌ Rejected to avoid double-sharding parameters.	⚠️ Limited end-to-end coverage.
Per-tensor online FP8 linears	✅ Ordinary loader finalizes weights and scales before DLO sharding.	✅ Ordinary loader retains complete rank-local tensors.
Other online quantization methods	❌ Rejected until runtime packing and scale layouts are validated.	⚠️ Allowed through the ordinary loader; validation is method-specific.
Model-level or standard layerwise CPU offload	❌ Disabled because DLO takes priority.	❌ Disabled because DLO takes priority.
Resident leading layers	❌ Rejected.	✅ Requires eligible resident paths in the model's OffloadPlan.

See Parallelism compatibility and Request and loading constraints for the detailed contracts and validation boundaries.

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 streams complete blocks independently and adds no DLO weight collective.

Host storage is selected separately from the transfer protocol. The loader can produce a direct-checkpoint mmap plan for a proven-compatible runtime layout; otherwise it uses the ordinary loader. Consequently, no-AllGather replicas on the same node can share immutable checkpoint pages when direct mmap is selected, while the ordinary-loader fallback still keeps a private runtime copy per process.

The Phase A shared-mmap support boundary is TP1. TP greater than one is an ordinary-loader compatibility path: DLO can consume the resulting TP-local tensors, but those configurations do not use checkpoint mmap and must not be used to claim shared-mmap host-memory savings.

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.

The loader owns host-weight planning

Before it decides whether ordinary weight materialization can be skipped, the diffusion loader builds one HostWeightPlan. A direct-checkpoint mmap plan is accepted only when preflight proves all of the following:

• every required DiT parameter and persistent buffer has exactly one source;
• runtime names, checkpoint keys, shapes, and dtypes match;
• the runtime topology is TP1 without HSDP or online quantization; and
• every custom loader operation is represented by a loader-owned checkpoint adapter.

The exact plan object is handed to DLO. The backend does not rescan checkpoint files, repeat the capability decision, or reconstruct names from its block topology. If preflight fails, the loader materializes weights normally and DLO consumes those runtime tensors.

The plan owns only dedicated DiT component sources. If a pipeline also exposes ordinary sources for a text encoder or another non-DiT component, the loader still consumes those sources and includes their loaded names in its strict coverage check. Only the source prefixes covered by the plan skip ordinary materialization. A source that mixes DiT and non-DiT weights fails closed to the complete ordinary-loader path because it cannot be skipped safely as a unit.

This boundary keeps checkpoint semantics out of DLO and avoids model-pipeline flags such as _supports_mmap_loading or parameter attributes for mmap-only transforms. Model-specific direct-layout knowledge, when required, lives in a checkpoint adapter beside the ordinary loader.

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 direct checkpoint mmap is selected, the checkpoint mappings are only the source used to prepare each rank's persistent shard. They can be closed after shard preparation. Across the AllGather group, those private shards total approximately one runtime model copy.

An effective DLO group size of one performs no collective, even when dlo_use_allgather=True; it follows the rank-local transfer path described below.

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 and streams complete blocks using H2D copies only. The host backing may be either a loader-approved checkpoint mapping or ordinary runtime tensors.

For direct mmap, each process retains immutable safetensors views and uses two bounded pinned host staging slots. Processes on the same node that map the same files share physical checkpoint pages through the OS page cache. This removes the persistent private full-model copy per pure-DP process, but each process still packs and transfers every complete block. Sharing is node-local; each node has its own page cache.

When direct mmap preflight fails, the regular model loader remains responsible for preparing each rank's weights, including TP-local tensors or HSDP-managed parameters. In that fallback, each pure-DP process keeps a private full runtime copy.

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 share one checkpoint-backed copy per node when direct mmap is selected; the ordinary-loader fallback keeps one private runtime copy per rank.
• The scheduler does not require a synchronized DP request wave for DLO.

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. Compatible TP1 replicas can share checkpoint pages on each node; fallback runtime tensors remain private.
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	Outside the Phase A shared-mmap support scope. The loader falls back before mutation, preserves TP-local layouts, and DLO may apply DP/SP host sharding to those ordinary runtime tensors.	Outside the Phase A shared-mmap support scope. The ordinary TP-aware loader produces rank-local tensors, which DLO streams without an additional weight collective; DP replicas retain private runtime storage.
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.

Direct checkpoint mmap can back either transfer path. It is currently limited to proven TP1, non-HSDP, non-online-quantized layouts. Other layouts use the ordinary loader. Per-tensor online FP8 linears can use DLO AllGather after the ordinary loader finalizes their runtime weights and scales; DLO then shards and reconstructs those tensors with their recorded layouts. Other online methods must use --dlo-no-use-allgather or disable online quantization until their runtime layouts are validated.

A normalized runtime mmap cache, built through the ordinary loader, is the proposed general mechanism for sharing transformed TP or quantized layouts. That cache and its publication/lifecycle protocol are intentionally outside this phase; see RFC #6195.

Validation coverage

Current source-level validation includes:

• HSDP + DLO + AllGather rejection;
• HSDP + DLO without AllGather acceptance at configuration level;
• loader preflight fallback for TP, HSDP, online quantization, unknown custom loaders, missing keys, and shape/dtype mismatches;
• ordinary-loader fallback for per-tensor online FP8 linears followed by DLO sharding of finalized weights and scales;
• exact loader-to-backend plan transfer and ordinary-loader fallback;
• rank-local mmap source retention, bounded two-slot staging, and adapter transforms without parameter-side flags;
• resident-layer requests requiring no-AllGather;
• DP request-wave validation for denoising-step compatibility;
• sharding, double-buffer, AllGather-size, and heterogeneous-block regression tests.

B300 parallel-topology smoke matrix

A four-GPU B300 smoke test covered MiniMax-H3 FL2VA with the same prompt, seed, CUDNN attention backend, 256x256 output, two denoising steps, and dlo_resident_layers=0. The TP2 rows used DiT DP2xTP2 with the text encoder and VAEs at TP1. They validate the ordinary-loader fallback only, not direct mmap or shared-mmap host-memory savings.

Configuration	Result	Warm E2E	Peak device memory	Host PSS
DP4xTP1 AllGather	Passed, 4 concurrent requests	2.87 s / 4 requests	13.84 GiB	211.99 GiB
DP4xTP1 no-AllGather	Passed, 1 request	15.02 s	13.23 GiB	187.77 GiB
DP2xTP2 AllGather	Passed, 2 concurrent requests	4.16 s / 2 requests	12.50 GiB	211.97 GiB
DP2xTP2 no-AllGather	Passed, 1 request	3.51 s	11.88 GiB	314.01 GiB

Within each topology, the AllGather and no-AllGather video and audio outputs were byte-identical. All four runs completed without an ERROR or traceback and released their device allocations. For DP4xTP1, no-AllGather direct mmap reduced total PSS by 24.22 GiB (11.4%) and Private_Dirty from 211.33 to 125.32 GiB (40.7%) relative to AllGather. For DP2xTP2, preflight selected the ordinary loader as designed; no-AllGather PSS was 314.01 GiB, about 48% above AllGather, because DP replicas did not share checkpoint-backed runtime weights. This is a functional and memory smoke test, not a production-quality performance or output-quality benchmark.

Host-memory measurement

A two-worker MiniMax-H3 FL2VA measurement on one L20X node compared the ordinary-loader fallback with direct mmap. Both runs used DP=2, TP=1, no DLO AllGather, BF16 weights, two denoising steps, and a 256x256 four-second request. The ordinary-loader workers were sampled after initialization. The mmap workers were sampled after one completed request, so the checkpoint working set had been faulted into the page cache; this is the more conservative point for mmap.

The values below come from /proc/<worker>/smaps_rollup and include the whole worker, not only the DiT. The stable rank-to-rank difference comes from other pipeline components, so each worker should be compared with the same worker in the other storage mode.

Worker	Ordinary RSS	mmap RSS	Ordinary PSS	mmap PSS	PSS reduction
DP worker 0	168.27 GiB	132.76 GiB	167.84 GiB	101.43 GiB	66.40 GiB
DP worker 1	116.19 GiB	79.97 GiB	115.73 GiB	48.64 GiB	67.09 GiB
Two-worker total	—	—	283.56 GiB	150.08 GiB	133.48 GiB (47.1%)

The direct-mmap workers each reported 62.45 GiB Shared_Clean but only 31.20 GiB Pss_File, which is the proportional charge expected when the same resident checkpoint pages are mapped by two workers. Private_Dirty also fell from 167.53 to 70.24 GiB for worker 0 and from 115.40 to 17.44 GiB for worker 1, a reduction of about 97–98 GiB per worker. RSS understates this benefit because it counts a shared physical page in every process that maps it; summed PSS is the appropriate node-memory comparison.

The highest-value missing coverage is broader end-to-end numerical and lifecycle comparison against ordinary layerwise offload for DP+SP, HSDP+SP+no-AllGather, and TP greater than one across additional models and target CUDA/NCCL or CANN/HCCL hardware. That broader TP coverage does not change the Phase A direct-mmap TP1 support boundary.

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 when independent replica execution is required. TP1 direct-mmap deployments can share checkpoint pages per node; other layouts retain the ordinary loader's private host memory behavior and are outside the Phase A shared-mmap support scope.
• Prefer HSDP alone for production HSDP deployments until the combined HSDP + DLO no-AllGather path has broader end-to-end coverage.