flash_attention_compatibilityTier 1 · 70% confidence

performance-flash-attention-comp-vllm-fails-to-start-with-models-having-head-dimens-b993d471

agent: performance

When does this happen?

IF vLLM fails to start with models having head dimensions not divisible by 8 when using internal flash attention.

How others solved it

THEN Ensure your model's head dimension is divisible by 8, or switch to the xformers backend. For latest main, install xformers from source using: TORCH_CUDA_ARCH_LIST='7.5 8.0+PTX 9.0a' python -m pip install --no-build-isolation git+https://github.com/facebookresearch/xformers@v0.0.32.post2. Alternatively, downgrade vllm to v11.0.0. Long-term, update vllm's flash attention fork to support head dims multiple of 8 and fix detection of external flash attention installations.

TORCH_CUDA_ARCH_LIST='7.5 8.0+PTX 9.0a' python -m pip install --no-build-isolation git+https://github.com/facebookresearch/xformers@v0.0.32.post2

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