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arXiv · 2604.00028

Sequence-Aware Split Heuristic to Mitigate SM Underutilization in FlashAttention-3 Low-Head-Count Decoding

Martí Llopart Font·Javier Hernando·Cristina España-Bonet
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Abstract

The standard FlashAttention-3 heuristic exhibits a GPU occupancy bottleneck in low-head-count decoding configurations because it disables sequence splitting based on sequence length alone, underutilizing the Streaming Multiprocessors of Hopper GPUs. Our proposed sequence-aware split policy mitigates this by allowing sequence-level parallelism in low-head-count regimes, improving hardware utilization to deliver roughly a 21 to 24% improvement in decoder kernel efficiency on metadata-enabled inference paths, with no observed regressions.

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Martí Llopart Font, Javier Hernando, Cristina España-Bonet. 2026-03-19. Sequence-Aware Split Heuristic to Mitigate SM Underutilization in FlashAttention-3 Low-Head-Count Decoding. https://arxiv.org/abs/2604.00028

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