Chunk 13.2

This chunk continued the systematic diagnosis of the Phase 4 performance regression. After confirming that the CUDA timing instrumentation (`CUZK_TIMING` printf's) was compiled into the daemon, the assistant discovered that the `printf` output was lost due to full buffering when stdout was redirected to a file. The fix was to add `fflush(stderr)` after each timing print, enabling the first successful collection of a phase-level GPU breakdown. The data immediately identified **B1 (`cudaHostRegister`)** as the primary culprit: pinning ~125 GiB of host memory added **5.7 seconds** of overhead, far exceeding the estimated 150–300 ms. Reverting B1 brought the total proof time from 101.3 s down to **94.4 s**, but this was still 5.5 s above the 88.9 s baseline, with synthesis (60.3 s) now the remaining regression. To isolate the synthesis slowdown without the overhead of GPU proving and SRS loading, the assistant built a **`synth-only` microbenchmark** subcommand in `cuzk-bench`. This enabled rapid A/B testing of the **A1 (SmallVec)** change. Four configurations were benchmarked (three iterations each): **Vec (original)** at **54.5 s**, **SmallVec cap=1** at **59.6 s**, **cap=2** at **60.0 s**, and **cap=4** at **60.2 s**. The results conclusively showed that SmallVec, regardless of inline capacity, causes a **~5–6 s regression** in synthesis time. The chunk concluded with the assistant preparing to gather **low-level `perf stat` hardware counters** (L1/L2/L3 cache misses, branch mispredicts, IPC) on a single-partition run to understand why SmallVec—which was intended to eliminate heap allocations—is slower on this AMD Zen4 Threadripper PRO 7995WX system. The overarching theme remains disciplined performance engineering: building precise instrumentation, reverting harmful changes (B1), and using microbenchmarks with hardware counters to diagnose and fix the remaining synthesis regression (A1).

The Anatomy of a Counterintuitive Regression: How Systematic Diagnosis Uncovered SmallVec's Hidden Cost on AMD Zen4 2420 words

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