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Apply heuristic for DeepSeek MLA prefill splitting #126

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Merged
tlrmchlsmth merged 1 commit into from
Mar 11, 2026
Merged

Apply heuristic for DeepSeek MLA prefill splitting #126

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24 changes: 9 additions & 15 deletions flash_attn/cute/interface.py
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Original file line number Diff line number Diff line change
Expand Up @@ -300,26 +300,20 @@ def _flash_attn_fwd(
seqlen_k_loaded = max_seqlen_k if not local else max(0, min(max_seqlen_k, window_size_right + window_size_left + 1 + m_block_size))
num_m_blocks = (seqlen_q_packgqa + m_block_size_effective - 1) // m_block_size_effective
total_mblocks = batch_size * num_head_kv * num_m_blocks
num_n_blocks = (seqlen_k_loaded + n_block_size - 1) // n_block_size
num_SMs = torch.cuda.get_device_properties(device).multi_processor_count
if num_splits < 1:
num_n_blocks = (seqlen_k_loaded + n_block_size - 1) // n_block_size
num_splits = num_splits_heuristic(
total_mblocks,
torch.cuda.get_device_properties(device).multi_processor_count,
num_n_blocks,
128,
)
num_splits = num_splits_heuristic(total_mblocks, num_SMs, num_n_blocks, 128)

# SplitKV uses float32 partial output, which doubles the O buffer size
# in shared memory, causing OOM for diff-headdim (192, 128)
if compute_capability in [10, 11] and head_dim != head_dim_v and num_splits > 1:
n_block_size = 64
num_n_blocks = (seqlen_k_loaded + n_block_size - 1) // n_block_size
num_splits = num_splits_heuristic(
total_mblocks,
torch.cuda.get_device_properties(device).multi_processor_count,
num_n_blocks,
128,
)
if num_n_blocks >= 64:
n_block_size = 64
num_n_blocks = (seqlen_k_loaded + n_block_size - 1) // n_block_size
num_splits = num_splits_heuristic(total_mblocks, num_SMs, num_n_blocks, 128)
else:
num_splits = 1

is_split_kv = num_splits > 1
if is_split_kv:
Expand Down