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fix(attention): size the decode KV tile to the shared-memory budget #840

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jason-fxz merged 1 commit into from
Oct 29, 2026
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fix(attention): size the decode KV tile to the shared-memory budget #840

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10 changes: 8 additions & 2 deletions python/freetoken/kernel/triton/attention.py
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Original file line number Diff line number Diff line change
Expand Up @@ -454,9 +454,15 @@ def decode_paged_attention(
# a grid that fits in one wave runs about one program per SM, which then needs wider tiles to keep enough loads in flight
block_n = 64 if grid[0] * grid[1] * grid[2] <= _num_sms(q.device.index) else _MIN_BLOCK_KV
num_stages = 3
# stage 1 keeps one K tile and one V tile in shared memory
if 2 * block_n * block_d * k_cache.element_size() > 0.8 * _optin_smem_bytes(q.device.index):
# stage 1 keeps the Q tile and one K and one V tile in shared memory
smem = _optin_smem_bytes(q.device.index)
elem = k_cache.element_size()
if 2 * block_n * block_d * elem > 0.8 * smem:
block_n = _MIN_BLOCK_KV
# 64 KiB parts (sm_75) cannot fit a head_dim 512 Q tile next to 32-row K/V tiles:
# halve the KV tile down to tl.dot's 16-row minimum until the three tiles fit.
while smem and block_n > 16 and (block_h * block_d + 2 * block_n * block_d) * elem > smem:
block_n //= 2
if sliding_window:
split_len = triton.cdiv(triton.cdiv(sliding_window, max_kv_splits), _MIN_BLOCK_KV) * _MIN_BLOCK_KV
if split_len <= block_n:
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