Using diffusion transformers for media generation may require evaluating attention over extremely long sequences, with attention layers accounting for the majority of generation latency. Exploiting sparsity in attention maps offers a promising opportunity to reduce this cost. In this work, we show that attention maps in diffusion transformers exhibit significant fine-grained sparsity in video generation models. Existing sparse attention methods, however, are too coarse-grained, leaving a large fraction of redundant computation unaddressed, or incur high overheads at finer granularity. We propose FG-Attn, a novel, low-overhead fine-grained sparse attention mechanism that skips score computations at the granularity of a MxN tile, where N>=1 and M>=16, and where each block is the result of query-key dot products between M queries and N keys. FG-Attn addresses the key challenge of hardware underutilization in sparse attention kernels on GPUs, without incurring the overheads of irregular memory access and redundant operations. FG-Attn can fully supersede existing sparse attention methods and extend block sparse attention methods to finer granularities on modern GPUs. At 70% sparsity, FG-Attn is up to 2.45X faster than the state-of-art FlashInfer, and reduces attention kernel time by 14.7% on average. FG-Attn speeds up end-to-end video generation times by up to 1.40X (1.18X on average) over Flash Attention 3.

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