Add num_splits for mha_varlen_fwd FA2, support batch invariant#110
Merged
LucasWilkinson merged 2 commits intovllm-project:mainfrom Dec 11, 2025
Merged
Add num_splits for mha_varlen_fwd FA2, support batch invariant#110LucasWilkinson merged 2 commits intovllm-project:mainfrom
LucasWilkinson merged 2 commits intovllm-project:mainfrom
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Signed-off-by: quanliu <18646313696@163.com>
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Add
num_splitsparameter formha_varlen_fwdFA2 to support batch-invariant processingBatch-invariant is currently unsupported on FA2 for SM80 GPUs, which can lead to inconsistent outputs for the same input.
This PR adds a num_splits parameter. Setting num_splits=1 ensures consistent outputs when batch-invariant is enabled.
vllm-project/vllm#27433 (comment)
Verified on A800 GPU with Qwen-3 32B model.
Testing done with --disable-cascade-attn disabled, under TP=1 and TP=2 settings.
Outputs are consistent.