Chunk 28.0

In this chunk, the assistant abandoned the Phase 10 two-lock GPU interlock design after discovering fundamental flaws: 16 GB VRAM cannot accommodate pre-staged buffers from multiple workers simultaneously, and CUDA memory management APIs (cudaDeviceSynchronize, cudaMemPoolTrimTo) are device-global, defeating the lock split's purpose. The code was reverted to Phase 9's proven single-lock approach. Comprehensive benchmarks were then run across concurrency levels (c=5 j=5 through c=20 j=15), and a detailed waterfall timing analysis was performed by extracting TIMELINE events from the daemon logs. This revealed 90.8% GPU utilization at high concurrency, with throughput plateauing at ~38s/proof due to DDR5 memory bandwidth contention—synthesis and prep_msm both inflating under load as they compete for the same memory channels. Building on this analysis, the assistant researched the memory-bandwidth-heavy code paths and proposed Phase 11 with three interventions to reduce contention. The first is bounding async_dealloc to a single thread to eliminate TLB shootdown storms from concurrent munmap() calls. The second is reducing the groth16_pool thread count (default 192) to limit b_g2_msm's memory footprint and L3 cache competition with synthesis. The third is adding a lightweight semaphore interlock to briefly pause some synthesis workers during b_g2_msm's 0.4s window, when all 192 groth16_pool threads run Pippenger simultaneously with 192 rayon synthesis threads. The plan was refined after discovering that prep_msm with num_circuits=1 is actually single-threaded (par_map with 1 item runs on 1 thread), making the original full semaphore interlock between prep_msm and synthesis overkill. The revised three-intervention approach targets the actual contention points: TLB shootdowns from deallocation storms, thread pool oversubscription in b_g2_msm, and brief memory-phase overlap between b_g2_msm and synthesis. The user approved this revised plan, and the assistant is now positioned to implement Phase 11 with careful benchmarking of each intervention independently.

From Failed Locks to Memory Bandwidth: The Phase 10 Post-Mortem and the Birth of Phase 11 3164 words

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