[webgpu] Optimize FlashAttention for prefill #25395
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This PR enhances unidirectional `FlashAttention` by applying causal masking inside the main loop. This optimization eliminates unnecessary memory loads by avoiding future entries in the KV cache. Testing on Lunar Lake shows up to a 20% performance improvement for `phi-4-mini-accuracy4` (with a prompt of 4096). Similar performance gains were also observed for other models, including `Qwen3-0.6B-accuracy4`. This PR now uses the more readable `unidirectional` attribute instead of `is_gpa`, to control causal masking.
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Lunar Lake, Phi-4-mini-accuracy4:
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@sushraja-msft @qjia7 pls take a look. |
qjia7
reviewed
Jul 15, 2025
qjia7
reviewed
Jul 15, 2025
qjia7
previously approved these changes
Jul 15, 2025
| @@ -337,7 +352,7 @@ Status FlashAttentionProgram::GenerateShaderCode(ShaderHelper& shader) const { | |||
| qk_4 = qk_4 + loadAttentionBias(q_idx_global, k_start+12, head_idx); | |||
| } | |||
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| let seq_causal_length = select(uniforms.total_sequence_length, uniforms.past_sequence_length + q_idx_global + 1, uniforms.is_gqa > 0); | |||
| let seq_causal_length = select(uniforms.total_sequence_length, uniforms.past_sequence_length + q_idx_global + 1, uniforms.is_unidirectional > 0); | |||
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nit: Is it better if we move this assignment out of the for loop?
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Sure. Let me address it in follow-up PRs to keep this one focused.
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/azp run Linux QNN CI Pipeline, Win_TRT_Minimal_CUDA_Test_CI, Windows ARM64 QNN CI Pipeline, Windows GPU Doc Gen CI Pipeline, Windows x64 QNN CI Pipeline |
fs-eire
approved these changes
Jul 22, 2025
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Azure Pipelines successfully started running 5 pipeline(s). |
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Description
This PR enhances unidirectional
FlashAttentionby applying causal masking inside the main loop. This optimization eliminates unnecessary memory loads by avoiding future entries in the KV cache.Testing on Lunar Lake shows up to a 20% performance improvement for
phi-4-mini-accuracy4(with a prompt of 4096). Similar performance gains were also observed for other models, includingQwen3-0.6B-accuracy4.This PR now uses the more readable
unidirectionalattribute instead ofis_gqa, to control causal masking.Motivation and Context
See above.