The group reduction reaches every observation through the group permutation, and so fetched one
32-byte sector per field for I, sigma, partiality and corr. I, sigma, partiality and prescaling_corr
now sit together in one aligned record, so it fetches two sectors per observation; the grouping
kernels' finiteness test and the combine read the same record. The per-observation fit coefficient,
its flag and 1/sigma are no longer stored: the per-frame fit forms them as it reads each observation
(the same expressions, so the same bits), which removes PrepScaleObsKernel and 13 bytes per
observation of device memory. DeviceBytes' per-observation figure drops by those 13 bytes.
Every sum keeps its order: p.hkl md5 identical to the base on cytc, 8a1a and 8qaw.
RTX 5080, nsys, merge-engine kernels: 8a1a 2459 -> 1924 ms, 8qaw 3624 -> 3025 ms, cytc 501 -> 476 ms
(ReduceGroupMeans 1.5-1.6x faster, PrepScaleObs gone, FitPerFrameG absorbs it).
Measured and not taken: storing all per-observation arrays in group order makes the reduction 3-4x
faster on its own, but the per-frame fit then has to gather, and moving 15 arrays at every grouping
costs as much as the reduction saves (net 0 on the 5080); a record for the combine-only fields
did not speed the combine.
Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EizBKhTqrAYago9KJG3dA3
leonarski_fandClaude Opus 5.5
7d656c13de