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Jungfraujoch/image_analysis/spot_finding
leonarski_fandClaude Opus 5 6e805f53c0
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image_analysis: stop paying for work that is thrown away
Three independent costs, each measured, none changing a result. Across the
37-crystal regression set the run time halves (median per crystal 2.0x, total
2.3x) and every crystal's merge statistics are unchanged.

The image copy back from the device moved the whole preprocessed frame - 72 MB
on a large detector, every frame, per worker - to serve a single host consumer
that reads only the strong pixels, at most a few hundred kilobytes of it. Give
the buffer a Gather() so that consumer asks for the values it actually wants (a
host loop on the CPU, a small kernel on the GPU), and copy the frame back only
when a CPU spot finder will genuinely read it. The copy the other way was worse:
it came from an unregistered vector, so the driver staged it through its own
pinned pool with a host-side memcpy on the calling thread, which does not overlap
and collapses under concurrency - 11.6 GB/s at one worker, 1.6 GB/s at eight.
That, not any hardware limit, is why throughput stopped improving past four to
eight workers. Pinning the decompression buffer once per worker fixes it: on a
18 Mpx dataset the image loop goes from 13.6 to 7.9 ms per image at 32 workers,
and 32 workers now beat 8 instead of losing to them.

Ceres was computing seventeen partial derivatives where five are free. The
per-image rotation refinement frees the beam and the orientation and holds
distance, detector angles, rotation axis and cell constant, but the cost
function declared all seven blocks, so every residual evaluated in Jet<17>
arithmetic. A residual exposing only the two free blocks - the same arithmetic,
the constants baked in - halves refinement, and it is exact rather than merely
close: dual coordinates evolve independently, so the residuals and the free
Jacobian columns are unchanged bit for bit.

The merge sorted an index array with a comparator that dereferenced a 1.6 GB
array of 72-byte records, i.e. a random walk over memory, single-threaded, twice
per two-pass run. Sorting a packed key instead is 2.4x. French-Wilson allocated
its integration scratch per reflection and ran serially; it now takes caller-owned
scratch and runs over chunks, 4.2x. The correction surfaces re-tested every
observation for usability and parity on each of ~22 passes and re-allocated their
accumulators each time; bucket the indices once and hoist the buffers.

Also convert std::round to std::rint where the rounded value only ever enters a
squared residual. The tie rules differ - away from zero against to even - so this
is safe exactly where a tie flips the sign but not the magnitude, and unsafe
wherever the value becomes a Miller index; those sites keep std::round. Verified
over all 2^32 float bit patterns: 8388608 exact ties exist, and the squared
residual is bitwise equal for every one of them. Worth little on its own here,
because the rounding that dominates is in candidate refinement, where the value
is an index and the substitution is not available.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
2026-08-01 21:35:28 +02:00
..
2026-06-23 20:29:49 +02:00