[ROCm][Perf] Expose AITER MoE sorting dispatch policy via env var

[nholmber]

Thread moe_sorting_dispatch_policy through the vLLM → AITER fused MoE call chain, controlled by VLLM_ROCM_AITER_MOE_DISPATCH_POLICY env var (default: 0, matching current behavior).

Setting VLLM_ROCM_AITER_MOE_DISPATCH_POLICY=2 enables an alternative dispatch policy. The best value (1 or 2) is model/workload-dependent.

Requires the companion AITER fix (ROCm/aiter#2639, fixes ROCm/aiter#2576) that corrects the moe_sorting_dispatch_policy type annotation from bool to int, without which values >1 are silently cast to 1.

Purpose

Expose AITER's moe_sorting_dispatch_policy parameter to vLLM users via environment variable. Currently there is no way to set this parameter from vLLM, leaving performance on the table for ROCm users.

3 files changed, 16 lines added:

• vllm/envs.py: New VLLM_ROCM_AITER_MOE_DISPATCH_POLICY: int = 0
• vllm/_aiter_ops.py: Add moe_sorting_dispatch_policy to impl/fake/static method signatures and pass through to torch.ops.vllm.rocm_aiter_fused_moe
• vllm/model_executor/layers/fused_moe/rocm_aiter_fused_moe.py: Add parameter to rocm_aiter_fused_experts(), read env var in AiterExperts.apply()

Test Plan

Tested on Qwen3-Next-80B-A3B-Instruct-FP8 (MI355X, TP1):

# Baseline (default, no behavior change)
VLLM_ROCM_AITER_MOE_DISPATCH_POLICY=0 vllm serve ...

# Alternative dispatch
VLLM_ROCM_AITER_MOE_DISPATCH_POLICY=2 vllm serve ...

• Accuracy: lm_eval --tasks gsm8k (5-shot)
• Throughput: vllm bench serve --dataset-name random --random-input-len 1024 --random-output-len 1024 --max-concurrency 16 --num-prompts 64

Test Result

Metric	dp=0 (baseline)	dp=2
gsm8k flex_extract	0.8560	0.8628
gsm8k strict_match	0.8143	0.8196
Output throughput (tok/s)	1527.83	1551.84 (+1.6%)
Mean TPOT (ms)	10.12	10.06 (-0.6%)

No accuracy regression. Default value 0 preserves existing behavior.

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[nholmber]

cc: @ChuanLi1101 @tpopp

[gemini-code-assist]

Code Review

This pull request introduces a new configuration parameter, moe_sorting_dispatch_policy, to the ROCm AITER fused MoE kernels. This includes adding a corresponding environment variable VLLM_ROCM_AITER_MOE_DISPATCH_POLICY to allow users to select alternative dispatch policies for performance tuning. The parameter is propagated through the AITER operations and the model executor layers. I have no feedback to provide.

[tjtanaa]

currently aiter v0.1.10.post3 does not have this changes. So I will wait till the upgrade PR is in.

[tpopp]

@tjtanaa an aiter update has occurred last week, so AITER should no longer be a blocker.

[gshtras]

Should something like #41159 (a draft proposal) be a better way than adding ever more aiter envs?

[tpopp]

Should something like #41159 (a draft proposal) be a better way than adding ever more aiter envs?

Certainly better. The only thing I would object to is blocking changes if the reason is a refactor that will not land in less than 24 hours. That is closer to what I had with LLVM and I think it's more fair than prolonged and open-ended blocks, so a refactor doesn't have to do a final update before merge.

[nholmber]

Rebased. The required AITER version was merged last week. Can we move forward with this PR @tjtanaa ?

[ChuanLi1101]

@tjtanaa heads-up — the AITER version blocker you flagged on 5/3 is resolved (rc5 merged last week via #42113), and @nholmber rebased onto latest main today. PR is +16/-0, default-off (VLLM_ROCM_AITER_MOE_DISPATCH_POLICY=0 matches current behavior). Mind taking another look so we can get this landed?