Root cause of H100 training time growth (12ms→23ms/step across epochs):
pfx_sum() fell back to single-block O(n) scan when num_blocks > 1024.
With block_dim=256 and buffer_size=500K: num_blocks = 1953 > 1024 → fallback.
Fix: increase block_dim to 1024 (capped at device max_threads_per_block).
Now: num_blocks = 500000/1024 = 489 < 1024 → multi-block 3-phase scan works.
Expected impact: PER sampling cost stays constant as buffer fills instead
of growing linearly. Training step time should be stable across epochs.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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