Segment 27

In this sub-session, the assistant first committed the Phase 9 PCIe optimization code and ran extensive benchmarks with higher concurrency to diagnose jumpy GPU utilization, discovering that the bottleneck had shifted to CPU memory bandwidth contention (prep_msm, b_g2_msm). To address this, the assistant designed a Phase 10 two-lock architecture (mem_mtx/compute_mtx) to better overlap CPU and GPU work, documented in c2-optimization-proposal-10.md. The assistant then implemented Phase 10, changing the FFI interface and setting gpu_workers_per_device to 3, but correctness testing revealed severe OOM failures and performance regressions. Debugging the timing logs showed that cudaDeviceSynchronize and cudaMemPoolTrimTo inside mem_mtx are device-global operations that serialized with compute_mtx, destroying the intended overlap and forcing all subsequent partitions to use the slow fallback path. The assistant ultimately diagnosed the fundamental conflict: memory management operations on a single CUDA device cannot be fully isolated from compute operations, leading to the solution of removing device-wide synchronization from the mem_mtx region.

The Bottleneck That Wouldn't Stay Still: How Phase 10's Two-Lock Architecture Collided with CUDA's Device-Global Reality 3570 words

Chunks

• The Bottleneck Shift: How Phase 9's PCIe Optimization Revealed the CPU Memory Wall and Sparked Phase 10's Two-Lock Architecture
  2293 words
• When Hardware Overrides Abstraction: The Phase 10 Two-Lock Design and the Discovery of CUDA's Device-Global Trap
  2129 words