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.