Chunk 21.1

This chunk shifts focus from synthesis ordering (now fixed) to a performance investigation of GPU utilization. The user reports that despite a large backlog of synthesized partitions waiting for GPU proving, GPU compute utilization hovers around 50% with multi-second idle gaps visible at 0.2s resolution. The assistant traces the GPU worker hot path and identifies two potential bottlenecks: (1) the GPU worker acquires the tracker lock twice between partition proves (to check job status and mark itself busy), while finalizer tasks also hold this same lock to call `malloc_trim(0)` on every partition completion—a heavy operation that walks the entire 400+ GiB heap; (2) the two-phase prove flow (`prove_start`/`prove_finish`) may serialize GPU access via a C++ mutex. Rather than guessing, the assistant adds precise timing instrumentation to the GPU worker loop (tracking each step between prove_start return and next prove_start entry), the `spawn_blocking` GPU prove call (separating mutex wait from actual compute), and the finalizer (timing prove_finish, reservation drop, tracker lock wait, `process_partition_result`, and `malloc_trim`). The instrumentation uses `Instant::now()` with `GPU_TIMING`/`FIN_TIMING` log prefixes for easy grep analysis. The user approves this evidence-gathering approach before any fix is applied, noting that `malloc_trim` is a plausible suspect. The next step will be to deploy the instrumented binary, gather logs from a live workload, and pinpoint the actual cause of the multi-second gaps.

The Evidence Imperative: How Instrumentation Replaced Guessing in the cuzk GPU Utilization Investigation 2137 words

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