157 lines
7.5 KiB
Markdown
157 lines
7.5 KiB
Markdown
# Distributed Layerwise Offload
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This document describes distributed layerwise offload (DLO) for diffusion
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models. DLO keeps only a small number of DiT blocks on the accelerator and
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streams the remaining blocks from host memory. The distributed backend can
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either shard those host-side weights across an existing parallel group or keep
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the standard loader's rank-local weights and avoid an additional collective.
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For user-facing commands, see the
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[distributed layerwise offloading guide](../../../user_guide/diffusion/offloader/distributed_layerwise_offload.md)
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and the [Cosmos3 recipe](https://github.com/vllm-project/vllm-omni/blob/main/recipes/cosmos3/Cosmos3-DistOffload.md).
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## Status
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DLO is implemented for multi-device diffusion execution. The default
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AllGather path is the primary path for DP and SP deployments. The
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`--dlo-no-use-allgather` path is a rank-local compatibility mode: it is useful
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for standard-loader sharding, workstation bring-up, and systems where an
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additional DLO collective is undesirable, but it does not reduce host weight
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storage across ranks.
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The compatibility matrix below describes the current implementation. The
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unit-level guards are covered, but not every parallelism combination has a
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full model-and-hardware end-to-end test.
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## Design
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### DLO consumes the existing parallel topology
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DLO does not create a new DP, TP, or SP topology. It reads the configured
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`DiffusionParallelConfig` and attaches offload hooks to the DiT blocks after the
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standard distributed groups have been initialized.
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The DLO weight-sharding group is selected as follows:
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1. Use the existing DP group when `data_parallel_size > 1`.
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2. When DP is one and SP is greater than one, use the SP group.
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3. Otherwise, run rank-locally without a DLO process group.
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TP is deliberately not used as DLO's AllGather group. HSDP has its own
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parameter-sharding lifecycle and is not allowed to be sharded a second time by
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DLO's AllGather path.
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### AllGather path
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With the default `dlo_use_allgather=True`, each rank stores approximately
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`1 / group_size` of each streamable block in pinned host memory. The next
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block's shard is copied to a device buffer and reconstructed with
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`all_gather_into_tensor` on a communication stream while the current block is
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executing.
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```text
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Compute: [Block N] [Block N+1] [Block N+2]
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H2D: [shard N+1] [shard N+2]
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AllGather: [full N+1] [full N+2]
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Buffers: [current slot] [prefetch slot] [current slot]
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```
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The backend uses two shared device buffers, so accelerator weight residency is
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bounded by the largest streamed blocks rather than the complete model.
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When DP is greater than one, the engine can process one request per DP rank in
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the same denoising wave. Because AllGather is a collective, all participating
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requests must take the same execution path at every denoising step.
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### Rank-local path without DLO AllGather
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With `--dlo-no-use-allgather`, DLO forces its internal offload shard size to
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one. The regular model loader remains responsible for preparing each rank's
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weights, including TP-local tensors or HSDP-managed parameters. DLO then
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streams those rank-local tensors block by block using H2D copies only.
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This mode means:
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- DP still provides independent replicas, but DLO does not shard weights
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across DP ranks.
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- SP still performs its normal activation/attention collectives, but DLO does
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not shard weights across SP ranks.
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- TP/HSDP/SP collectives, if configured, are not disabled by this flag; only
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DLO's additional weight AllGather is disabled.
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- Pure DP deployments keep a full host-side model copy per rank, subject to
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shared OS page-cache behavior.
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- The scheduler does not require a synchronized DP request wave for DLO.
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This path is intentionally not implemented by reusing the AllGather mmap
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loader. It relies on the standard loader so model-specific TP/HSDP transforms
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remain intact.
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## Parallelism compatibility
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| Parallelism | DLO + AllGather | DLO without AllGather |
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|---|---|---|
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| **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. |
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| **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. |
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| **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. |
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| **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. |
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### Combined dimensions
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- **DP + SP:** DLO uses the DP group for weight sharding when DP is greater
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than one; SP continues to use its own sequence-parallel group. If DP is one,
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the SP group becomes DLO's sharding group in AllGather mode.
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- **DP + TP/SP without AllGather:** standard model loading defines the
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rank-local tensor layout. DLO adds no cross-DP, cross-TP, or cross-SP weight
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collective.
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- **HSDP + SP:** the general parallel configuration permits HSDP over SP, but
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DLO must use `--dlo-no-use-allgather`. HSDP remains responsible for weight
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materialization and synchronization.
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- **HSDP + DP or TP:** rejected independently by the diffusion parallel
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configuration.
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## Request and loading constraints
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AllGather DP multi-concurrency requires:
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- explicit `num_inference_steps`;
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- the same `num_inference_steps` for all requests in a wave; and
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- identical request arguments that affect the collective execution path.
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The no-AllGather path does not impose these DLO-specific synchronized-wave
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requirements.
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The mmap loader is used only by the supported DLO+AllGather path when the
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model has a compatible checkpoint layout. Online quantization is incompatible
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with that sharded mmap path; use `--dlo-no-use-allgather` or disable online
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quantization. Models that do not meet the mmap requirements use the regular
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loader path.
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## Validation coverage
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Current source-level validation includes:
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- HSDP + DLO + AllGather rejection;
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- HSDP + DLO without AllGather acceptance at configuration level;
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- TP rejection in the DLO+AllGather mmap path;
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- resident-layer requests requiring no-AllGather;
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- DP request-wave validation for denoising-step compatibility;
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- sharding, double-buffer, AllGather-size, and heterogeneous-block regression
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tests.
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The highest-value missing coverage is end-to-end numerical comparison against
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ordinary layerwise offload for DP+SP, TP+no-AllGather, and HSDP+SP+no-AllGather
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on the target CUDA/NCCL or CANN/HCCL hardware.
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## Recommendations
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- Use **DP + DLO AllGather** for the supported throughput and host-memory
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scaling path.
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- Use **SP + DLO AllGather** for long-sequence workloads when DP concurrency is
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not the goal.
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- Use **no-AllGather** to bring up TP or workstation PCIe configurations, with
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the expectation of higher host-memory use and lower validation confidence.
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- Prefer **HSDP alone** for production HSDP deployments until the combined
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HSDP + DLO no-AllGather path has broader end-to-end coverage.
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