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Relax SM90 shared-memory estimate for wide values #16

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Merged
ivandobskygithub merged 2 commits into from
Nov 25, 2025
Merged

Relax SM90 shared-memory estimate for wide values #16

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cdadd02
Adjust buffering estimate for wide value dims
ivandobskygithub Nov 25, 2025
d058f84
Merge branch 'main' into codex/fix-flashattention-shared-memory-error…
ivandobskygithub Nov 25, 2025
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6 changes: 3 additions & 3 deletions hopper/tile_size.h
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Original file line number Diff line number Diff line change
Expand Up @@ -8,9 +8,9 @@

constexpr int smem_estimate_bytes(int block_m, int block_n, int headdim, int headdim_v, int element_size) {
// Double-buffer the residency for Q/K/V and the accumulators when the head/value dims are modest.
// For very large dimensions the single-stage SM120 pipeline only needs a single residency footprint, so
// avoid over-clamping by scaling the estimate down to one buffer in that regime.
int const buffering = (headdim + headdim_v >= 512) ? 1 : 2;
// Value dimensions 256+ already drive the shared-memory footprint high, so treat them like the large
// combined-head/value case and drop to a single buffer in that regime to avoid over-clamping.
int const buffering = (headdim_v >= 256 || headdim + headdim_v >= 512) ? 1 : 2;
return buffering * (block_m + block_n) * (headdim + headdim_v) * element_size;
}

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2 changes: 1 addition & 1 deletion tests/hopper/test_tile_size_shared_memory.py
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Expand Up @@ -53,7 +53,7 @@ def _load_bridge() -> ctypes.CDLL:

def estimate_smem_bytes(block_m: int, block_n: int, headdim: int, headdim_v: int, element_size: int) -> int:
# Mirror the buffering-aware estimate used in hopper/tile_size.h.
buffering = 1 if headdim + headdim_v >= 512 else 2
buffering = 1 if (headdim_v >= 256 or headdim + headdim_v >= 512) else 2
return buffering * (block_m + block_n) * (headdim + headdim_v) * element_size


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