Chunk 30.1

In this chunk, the assistant investigated whether the `PendingProofHandle` held memory unnecessarily after `prove_start`. By tracing the C++ `prep_msm_thread`, it confirmed that the massive `a`, `b`, `c` NTT evaluation vectors (~12 GiB per partition) were only needed by the GPU kernel region, which completed before the function returned. An early deallocation of these vectors was implemented in `prove_start`. To diagnose persistent OOMs at `pw=12`, the assistant built a global buffer tracker with atomic counters (`buf_synth_start`, `buf_abc_freed`, `buf_dealloc_done`) and integrated it into the pipeline and engine, enabling real-time visibility into every large buffer class in flight. The buffer counters revealed the true bottleneck: the `provers` counter peaked at 28, meaning 28 synthesized partitions were queued holding their full ~16 GiB datasets. This occurred because the partition semaphore (`pw=12`) released immediately after synthesis, allowing tasks to pile up while blocking on the single-slot GPU channel. The assistant fixed this by holding the semaphore permit until the synthesized job was fully delivered to the GPU channel, dramatically reducing peak RSS from 668 GiB to 294.7 GiB and enabling `pw=12` to run without OOM for the first time. However, this fix introduced a throughput regression (39.9s vs 37.1s) by serializing synthesis and channel delivery. The assistant reverted the semaphore change and instead increased the channel capacity from 1 to `partition_workers`, allowing a natural buffer of completed jobs without blocking the semaphore. The core theme of this chunk is the intricate trade-off between memory pressure and pipeline throughput in a high-performance GPU proving system. The assistant successfully diagnosed a severe memory buildup issue through targeted instrumentation, applied structural fixes to resolve it, and iteratively balanced the competing constraints of memory usage and performance.

The Memory Wall and the Backpressure Fix: How Instrumentation Unlocked the GPU Proving Pipeline 3027 words

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