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perf(sm120): skip split-K when the reduction is slower than the single-token GEMM #77

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perf(sm120): skip split-K when the reduction is slower than the single-token GEMM #77

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12 changes: 12 additions & 0 deletions csrc/jit_kernels/heuristics/sm120.hpp
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Original file line number Diff line number Diff line change
Expand Up @@ -326,6 +326,18 @@ struct SM120ArchSpec {
if (num_mn_blocks >= desc.num_sms / 2)
return 1;

// Single-token decode (M well below block_m) leaves the split-K
// reduction dominated by launch and write-back cost rather than by the
// partial-sum arithmetic it parallelises: the reduction reads
// split_k * M * N floats to produce M * N, and at M = 4 that is a
// separate kernel whose runtime exceeds the GEMM time it saves.
// Measured on SM120 (RTX PRO 6000, DeepSeek-V4-Flash decode, M = 4,
// N = 8192, K = 1024): the split GEMM plus reduction costs about
// 22.4 us against about 13.2 us for the same projection without a
// split. Keep split-K for shapes with a real M tile.
if (desc.get_expected_m() * 4 <= layout.block_m)
return 1;

const int target_blocks = desc.num_sms * 3 / 4;
int split_k = ceil_div(target_blocks, num_mn_blocks);

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