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project_6 5ca49d0e7c [docs] GridEvenShare work distribution — BI-V100 attention reduce needs only 9 CTAs
From reading cccl_upstream/cub/cub/grid/grid_even_share.cuh:

Key finding: For Qwen3.6 attention score reduction (100K seq_len),
with tile_items=12288 (512 threads × 24 items), only 9 CTAs are
needed. All fit in one wave on 16 SMs.

This means reduce tuning (items/threads) matters less than the V1/V2
dispatch choice in paged_attn.py. V1 uses a single CTA iterating
sequentially over all KV blocks, completely bypassing GridEvenShare's
parallel distribution. V2 would enable partition-based parallelism.

Also documents: RAKE (scan) vs STRIP_MINE (reduce) strategies,
'big shares' load balancing, and the SingleTile fast path for
short sequences.
2026-08-05 03:36:45 +00:00
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