jungfrauandClaude Opus 5
c1b85c7e88
fit and boxsum were 80% of GPU time on a crowded crystal - 78 s of it. Neither was bandwidth- or occupancy-bound: both sat at about an eighth of the issue rate the card can sustain, stalled. What stalls them is the block-wide accumulations. Every one has all 128 lanes of the block adding into one shared address, and a shared-memory atomicAdd on a float or a 64-bit integer has no instruction on either Turing or Ada - it compiles to a compare-and-swap retry loop. So those 128 lanes serialise into 128 retries, eighteen times per thread in fit. Summing across the warp first and letting one lane do the atomic leaves four per block instead of 128. That is the whole story: the arithmetic below was worth 2%, the atomics 5.6x. The arithmetic is still worth having, and is what was expected to matter: - compute_shell ran on all 128 threads of a block for a value that belongs to the reflection. It is two software double-precision divisions, on a card whose double throughput is a thirty-second (a sixty-fourth on the production one) of its single. One thread does it now. - The Kabsch inner loop divided by the same weight three times; the compiler emits the whole correctly-rounded sequence each time. One reciprocal now. Likewise the two Gaussian widths and the profile normalisation, which are constant over a reflection's cells and were divided per cell. - boxsum read the pixel before deciding whether it wanted it. The window is the bounding box of an ellipse, so nearly half of it is neither the signal disk nor the background ring, and those slots were fetching a cache line for nothing. Measured: fit 50.8 s -> 9.1 s, boxsum 27.5 s -> 12.1 s. A crowded crystal 2m22s -> 1m58s, a 16M-pixel one 39.5 s -> 37.2 s, the whole battery 12m30s -> 11m35s. Same space group on all 24 crystals, none failed. The integer sums are unchanged - addition is associative. The float ones move in their last bits and become more reproducible, since a fixed shuffle tree replaces whatever order the atomics arrived in. Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Jungfraujoch
Application to receive data from the PSI JUNGFRAU and EIGER detectors.
All documentation is now placed in docs/ subdirectory and for the current version hosted on Jungfraujoch Read The Docs page.