Fused Attention

Fused attention is the combined-kernel execution of key attention substeps such as score computation, masking, softmax, and value aggregation - it minimizes intermediate tensor materialization and improves sequence processing efficiency.

What Is Fused attention?

• Definition: Attention implementation that merges multiple stages of scaled dot-product attention into fewer GPU kernels.
• Pipeline Scope: Commonly fuses QK matmul scaling, mask application, softmax normalization, and weighted value accumulation.
• Memory Objective: Keeps blocks on-chip where possible instead of writing full score matrices to HBM.
• Algorithm Family: Includes FlashAttention-like methods and framework-specific fused kernels.

Why Fused attention Matters

• Long-Sequence Performance: Attention dominates runtime and memory at larger context lengths.
• Bandwidth Reduction: Avoiding score-matrix writes removes major memory bottlenecks.
• Higher Throughput: Fewer launches and improved locality increase tokens-per-second.
• Better Scaling: Enables larger batch or context settings under the same memory budget.
• Serving Benefits: Reduces latency and memory overhead in autoregressive decoding paths.

How It Is Used in Practice

• Kernel Selection: Dispatch fused kernels based on head dimension, causal mode, and precision.
• Profile Comparison: Benchmark fused versus unfused attention under representative sequence lengths.
• Stability Checks: Validate numerical parity and masking correctness across edge cases.

Fused attention is one of the most important optimizations in modern transformer systems - combining attention stages into efficient kernels is essential for high-context performance.