Native 512 SSB frequency pairing

Question and workload

Can the paired-frequency prototype remove duplicate work while retaining the previous GPU result? The earlier instruction-reduction experiment was faster but failed numerical acceptance. This follow-up preserves its frozen reference and enables a corrected implementation only for native 512-square, higher-order coherent reconstruction with real-FFT half-plane storage.

The real acquisition is 512 × 512 × 192 × 192, with all 9,275 active BF pixels, the same recorded hot-pixel preparation, fifth-order optics, tilt and depth. Neither input data nor precision, tolerances or summation order change. These are fixed-calibration redraws, not search/refinement timings.

Hardware: RTX PRO 6000 Blackwell Workstation Edition, CUDA GPU 0. Additional numerical checks use the Blackwell Max-Q device. The checkout is e8750f3e3652 plus the previously tested coefficient-struct and launch-layout changes. Final source SHA256:

• cuda/kernels/engine.py: 0ea3ad99f6d7395c1d945b4c0a0d840746da9588f343e3a63b8417cd3695e59f

• cuda/thick.py: 22f23a6e4ba3990597d575c6503b96297c8d3f16da3fb35c3f23212af8d9c504

Private evidence: 20261023-ssb-pair-parity. Frozen sources, parameter cases, timing samples, hashes and profiler captures are retained there.

Implementation

At opposite spatial frequencies, the shifted probe evaluations exchange roles. One shared correction helper reuses their polynomial, geometry and depth intermediates, while evaluating and normalizing both complex outputs. It does not assume Hermitian symmetry of the corrected higher-order reconstruction. The BF order, compensated sums, complex128 reduction and inverse FFT remain unchanged. The existing 128 MiB default partial workspace is retained; there is no acquisition-sized cache.

Explicit rounding of the depth shift and gamma normalization preserves the reference arithmetic. This fixes the earlier mismatch: the corrected version matches all 109 real-data parameter cases exactly. The two exceptional columns are scheduled together so normal warps avoid mixed edge/pair branches; the Nyquist row still receives direct corrections. This remains one shared kernel source, with a guarded launch variant. Other shapes, fine grids and MPS retain their existing paths.

Matched warm comparison

Alternating reference/candidate observations use eight C10 settings in the same prepared session. GPU synchronization bounds each reconstruction; output copies and comparisons are outside timing. The first two rounds are excluded from these medians.

Implementation	Median reconstruction
Frozen full-plane reference	21.93 ms
Paired corrections with original frequency layout	15.95 ms
Paired corrections with grouped edge frequencies	14.36 ms

The accepted layout reduces time by about 35%, or 1.53× throughput. A separate 32-round launch-size comparison measured 14.31 ms with 256 threads. The 512 thread result was effectively tied, so the existing 256-thread configuration was retained. This is approximately 70 backend reconstructions per second, not measured notebook frames per second.

Standard preview runner

quantem.gpu.benchmark.run_ssb_preview also ran against the frozen reference and the final checkout, with 11 changed-C10 observations per factor. The following candidate results separate reconstruction and host delivery. First observation does not mean cold disk or cold compilation. Sequential runner passes are descriptive; the alternating experiment above isolates the speedup.

Upsampling	First reconstruction	Warm median	Warm p90	Median download	Median phase delivery
1×	15.12 ms	13.42 ms	14.61 ms	0.14 ms	13.56 ms
2×	81.43 ms	83.76 ms	84.29 ms	0.39 ms	84.15 ms
3×	165.23 ms	164.83 ms	166.08 ms	0.77 ms	165.58 ms
4×	229.05 ms	223.13 ms	223.62 ms	3.21 ms	226.33 ms
8×	279.50 ms	274.86 ms	275.62 ms	13.52 ms	288.12 ms

The 8× host copy had outliers, including 244 ms; its delivery p90 was 357 ms. Fine-grid paths were not optimized here. All 55 complete output hashes match the frozen reference. Changing upsampling changes the evaluated frequency coverage, so these rows are different workloads, not quality-equivalent speedups.

Hardware counters and verification

A quiet Nsight Compute capture of the middle correction launch reports:

Counter	Previous reference	Accepted implementation
Executed warp instructions	6.689 billion	3.998 billion
Profiled launch duration	4.27 ms	2.83 ms
Instruction issue slots busy	about 92%	83.10%
DRAM throughput	about 32%	48.31%
Achieved occupancy	about 65%	64.92%

The accepted kernel uses 64 registers per thread and has no local-memory spilling. Instructions fall by about 40%; profiler replay duration is separate from normal warm reconstruction. Clocks and caches were not forcibly controlled. These counters do not establish an algorithmic or hardware lower bound.

• 109 real-data joint-parameter comparisons match the frozen GPU phase exactly.

• 24 synthetic calibration combinations cover rotation, depth and unequal detector sampling, with full-plane GPU comparison in regression tests.

• Three full-data GPU 1 checks compare both complex spectra and final phase byte for byte, including zero depth and a large defocus/depth change.

• The final CUDA wave-average, upsampling, higher-order and thick-sample suite passes all 45 tests. An initial run passed the scientific assertions but failed one stale error-message expectation (num_trials=0 versus the current trials=0); the expectation was corrected and the complete suite rerun.

The native CUDA path is enabled in the working checkout. No commit, release or push is part of this experiment. MPS and notebook/browser image rate have not been requalified by these measurements.