[CUDA graphs] Enable full cuda graphs with FA3 AoT scheduling #20301
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Signed-off-by: Woosuk Kwon <[email protected]>
Signed-off-by: Woosuk Kwon <[email protected]>
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Summary of Changes
Hello @WoosukKwon, I'm Gemini Code Assist1! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!
This pull request significantly enhances the FlashAttention backend by enabling full CUDA graph support when using FlashAttention v3's Ahead-of-Time (AoT) scheduling. The core problem addressed was an illegal memory access occurring when the runtime split factor exceeded the capture-time num_splits heuristic. This is resolved by explicitly managing and passing a max_num_splits parameter, ensuring consistent memory allocation and preventing runtime issues, thereby allowing for more efficient execution with CUDA graphs.
Highlights
- CUDA Graph Compatibility: Enabled full CUDA graph support for FlashAttention v3 (FA3) with Ahead-of-Time (AoT) scheduling, which was previously unsupported due to memory access issues.
- Memory Access Fix: Resolved an illegal memory access issue that occurred when the runtime split factor exceeded the
num_splitsset at capture-time. This is addressed by explicitly setting and consistently usingmax_num_splitsas an upper bound. - Dependency Update: Updated the
flash-attentionexternal project dependency to a newer commit (1c2624e) to incorporate necessary changes for FA3 and CUDA graph compatibility. max_num_splitsManagement: Introducedmax_num_splitsinFlashAttentionMetadataandFlashAttentionBackendto explicitly control the number of splits for intermediate buffer pre-allocation when using CUDA graphs, ensuring proper memory handling.- CUDA Graph Constraints: Added validation checks for CUDA graph capture sizes, enforcing that AoT scheduling is required and that capture sizes do not exceed 992 tokens, aligning with current FA3 internal heuristics.
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Code Review
This pull request enables full CUDA graph support with FlashAttention-3 AoT scheduling by explicitly managing the num_splits parameter. The changes look solid and correctly plumb the max_num_splits value from initialization down to the attention kernel. My main feedback is to replace a magic number with a named constant for better code clarity and maintainability.
Signed-off-by: Woosuk Kwon <[email protected]>
Signed-off-by: Woosuk Kwon <[email protected]>
Signed-off-by: Woosuk Kwon <[email protected]>
| "cudagraph_capture_sizes should not be None when " | ||
| "full_cuda_graph is True.") | ||
| self.max_cudagraph_size = max(capture_sizes) | ||
| if self.max_cudagraph_size > 992: |
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were you hitting an assert/IMA here? if the batch is >992 FA3 "should" silently fall back to no dynamic split (AoT scheduling) granted if we fallback on no dynamic split; setting the split manually may be high since it will try to split to that instead of using it as an upper bound. (so perf may be bad so I think it still makes sense to limit this)
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@LucasWilkinson I didn't test bs > 992. I just wanted to add a safety check just because I'm not sure what will happen in the case.
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Makes sense; I agree with this approach. We can try to add support in a later PR
…roject#20301) Signed-off-by: Woosuk Kwon <[email protected]>
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I in main branch run but get this error: TypeError: flash_attn_varlen_func() got an unexpected keyword argument 'num_splits' @WoosukKwon How should I solve it? |
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@lengrongfu Could you please re-install vllm? This happens because we updated the |
Thanks, this |
…roject#20301) Signed-off-by: Woosuk Kwon <[email protected]> Signed-off-by: avigny <[email protected]>
This PR enables the full cuda graph with FA3 AoT scheduling.
Previously, AoT scheduling caused illegal memory access when the run-time split factor is larger than
num_splitsset by internal heuristics at capture-time.This case can be prevented by explicitly setting
num_splits(the upper bound) at both capture and run time.