[ROCm] Pass moe_buf to AITER to eliminate MoE output copy#40368
[ROCm] Pass moe_buf to AITER to eliminate MoE output copy#40368nholmber wants to merge 1 commit into
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Plumb `moe_buf` through the vLLM AITER fused MoE interface so the kernel writes directly into the caller's pre-allocated output buffer. This avoids a device-to-device copy of the full MoE output on every forward pass. Requires AITER with ROCm/aiter#2687 merged. When `moe_buf` is `None` (older AITER), the existing allocation + copy behavior is preserved. Co-authored-by: Tres Popp <tres.popp@amd.com> Signed-off-by: nholmber <nholmber@users.noreply.github.com>
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Code Review
This pull request updates the ROCm AITER fused MoE implementation to support in-place mutation of the output buffer (moe_buf). Key changes include updating the operator registration to mark moe_buf as a mutated argument and refactoring the apply method to pass the output tensor directly to the expert computation. Feedback was provided to ensure the fake implementation of the custom operator returns the mutated buffer itself instead of a new tensor, which is necessary for proper functionalization and torch.compile support.
| if moe_buf is not None: | ||
| return torch.empty_like(moe_buf) |
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In the fake implementation of a mutating operation, it is better to return the mutated tensor itself (moe_buf) rather than a new tensor (torch.empty_like(moe_buf)). This ensures that the fake implementation correctly reflects the in-place nature of the operation and maintains tensor identity, which is crucial for torch.compile and functionalization to track the state of the buffer correctly.
| if moe_buf is not None: | |
| return torch.empty_like(moe_buf) | |
| if moe_buf is not None: | |
| return moe_buf |
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Summary
moe_bufthrough the vLLM AITER fused MoE custom op so the kernel writes directly into the caller's pre-allocated output bufferoutput.copy_(result)) on every forward passmoe_buf=None(older AITER without Allow preallocated moe sorting buffer ROCm/aiter#2687), the existing internal allocation behavior is preservedChanges
vllm/_aiter_ops.py: Addmoe_bufparameter to impl, fake, op registration (mutates_args), and static methodvllm/model_executor/layers/fused_moe/rocm_aiter_fused_moe.py: Threadmoe_bufthroughrocm_aiter_fused_experts()and passoutputdirectly inAiterExperts.apply()Test plan
Depends on: ROCm/aiter#2687
Co-authored-by: Tres Popp tres.popp@amd.com