Chunk 21.0

This sub-session focused on diagnosing and addressing the GPU idle gap identified in the previous benchmarks. The user first implemented a detailed waterfall timeline instrumentation in the proving engine, confirming a ~12s structural GPU idle gap per proof cycle caused by strictly sequential synthesis (38s) exceeding GPU time (27s). To close this gap, the user implemented parallel synthesis by refactoring the engine's synthesis task loop to use a `tokio::sync::Semaphore`, allowing multiple proofs to be synthesized concurrently (controlled by a new `synthesis_concurrency` config parameter). Benchmarking the parallel synthesis pipeline yielded nuanced results. With `synthesis_concurrency=2` and client concurrency `-j 3`, GPU utilization jumped to 99.3%, effectively eliminating the idle gap. However, the overall throughput improvement was modest (~5-7%, from ~45.3s to ~42.2s per proof). The root cause was CPU resource contention: running two full 10-partition syntheses simultaneously competed with the GPU prover's CPU-intensive `b_g2_msm` step, inflating both synthesis and GPU times. Over-aggressive client concurrency (`-j 4`) led to catastrophic contention (60.2s/proof), while insufficient depth (`-j 2`) left the pipeline starved. The key theme of this chunk was the law of diminishing returns in pipeline optimization. While parallel synthesis successfully saturated the GPU, it merely shifted the bottleneck from the GPU to the CPU, revealing that the system's 96 cores are a shared resource that cannot be fully dedicated to synthesis without impacting the GPU's CPU-bound work. The waterfall instrumentation proved critical for diagnosing this shift. The analysis concluded that further throughput gains require reducing the absolute CPU time of synthesis (e.g., via the proposed Phase 5 Wave 2/3 optimizations like specialized MatVec and pre-sorted SRS) or decoupling the CPU workloads, rather than simply adding more parallelism to the existing architecture.

The Law of Diminishing Returns: When Saturating the GPU Reveals the CPU Bottleneck 2212 words

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