Chunk 23.0

In this session, the team conclusively identified and implemented a fix for the persistent GPU underutilization (~50%) in the cuzk proving daemon. The root cause was confirmed through detailed C++ timing instrumentation (`CUZK_TIMING`, `CUZK_NTT_H`): the GPU mutex was held for 1.6–7.0 seconds per partition, but the GPU was only actively computing for ~1.2 seconds. The remaining time was spent in `ntt_kernels` (H2D transfer) copying a/b/c vectors from unpinned Rust heap memory (`Vec<Scalar>`) via `cudaMemcpyAsync`, which forced CUDA to stage through a tiny internal pinned bounce buffer at only 1–4 GB/s instead of the PCIe Gen5 line rate of ~50 GB/s. The chosen solution was to implement a zero-copy pinned memory pool (`PinnedPool`) integrated with the `MemoryBudget` system. The core components (`PinnedPool`, `PinnedBacking` struct, `release_abc()` method, `new_with_pinned()` constructor) were already implemented and compiling cleanly. The critical work accomplished in this chunk was fully wiring the pool into the synthesis and engine paths. The `pinned_pool` reference was threaded from `Engine::new()` through the evictor callback, `dispatch_batch`, `process_batch`, and into `PartitionWorkItem`. The synthesis functions (`synthesize_auto`, `synthesize_with_hint`) now accept an optional `Arc<PinnedPool>`, and when a capacity hint is available, they check out pinned buffers and create `ProvingAssignment` instances with pinned backing via a new `synthesize_circuits_batch_with_prover_factory` function. A fallback path ensures graceful degradation to standard heap allocations if the pool is exhausted. The full implementation compiles cleanly (`cargo check --features cuda-supraseal` passes). A Docker build (`cuzk-rebuild:pinned1`) was successfully created. The immediate next step is to deploy this binary to the remote test machine, replacing the current timing-instrumented binary (`/data/cuzk-timing2`). The primary verification metric will be the `ntt_kernels` time in the CUZK timing logs, which should drop from the current 2–9 second range down to under 100ms, effectively eliminating the H2D bottleneck and raising GPU utilization towards 100%.

The Final Mile: Wiring a Zero-Copy Pinned Memory Pool Through the cuzk Proving Pipeline 2354 words

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