The Last Sweep Point: Precision Benchmarking in the cuzk SNARK Proving Engine

The Message

[assistant] [bash] cd /home/theuser/curio/extern/cuzk && nohup ./target/release/cuzk-daemon --config /tmp/cuzk-sweep.toml > /tmp/cuzk-sweep-pw20.log 2>&1 &
echo "PID=$!"
PID=3615659

At first glance, this is a mundane command: start a background daemon process, redirect its output to a log file, print its process ID. But in the context of a months-long engineering effort to optimize the Filecoin PoRep Groth16 proof generation pipeline, this message represents the culmination of a systematic parameter sweep that would determine the optimal configuration for a 96-core production machine. It is the fifth and final data point in a carefully designed experiment—the last stone turned in a search for throughput.

The Broader Context: Phase 8 and the Dual-Worker GPU Interlock

To understand why this message exists, one must understand the engineering journey that preceded it. The cuzk SNARK proving engine is a high-performance system for generating Groth16 proofs for Filecoin's Proof-of-Replication (PoRep) protocol. These proofs are computationally intensive, requiring roughly 200 GiB of peak memory and involving a complex pipeline spanning Go, Rust, C++, and CUDA code.

The assistant had just implemented Phase 8: Dual-Worker GPU Interlock ([msg 2247]), a significant architectural refinement. The core insight was that the C++ static mutex in generate_groth16_proofs_c was causing GPU idle gaps: one worker would hold the lock while doing CPU preprocessing, preventing another worker from submitting CUDA kernels. By narrowing the mutex to cover only the CUDA kernel region (NTT+MSM, batch additions, tail MSMs), the assistant enabled two GPU workers per device to interleave—one does CPU work while the other runs CUDA kernels. This eliminated GPU idle entirely (100.0% GPU efficiency) and improved throughput by 13–17% over Phase 7.

But a critical question remained: what value of partition_workers (the number of parallel synthesis workers) would yield the best throughput? The Phase 7 sweet spot had been pw=20 on this 96-core machine, but Phase 8's dual-worker architecture changed the CPU contention dynamics. A test with pw=30 had regressed badly (60.4s/proof vs 44.0s at pw=20), confirming that too many synthesis workers could starve the GPU preprocessing threads. The optimal setting needed to be re-established.

The User's Request and the Assistant's Plan

The user's request was terse but precise ([msg 2248]): "sweep 10,12,15,18,20". Five values, one command. The assistant understood immediately what was needed: a systematic benchmark across these partition_workers settings, using the standard 5-proof batch benchmark with c=5 j=3 (5 concurrent proofs, 3 jobs in flight). Each sweep point required restarting the daemon with a modified configuration, waiting for the SRS (Structured Reference String) to preload from disk, running the benchmark, recording the result, and moving to the next value.

The assistant's response ([msg 2249]) laid out the plan with a todo list of five items, each marked with priority "high" and initially "pending". The first, pw=10, was set to "in_progress" immediately. This structured approach—systematic, reproducible, empirically driven—is characteristic of performance engineering at this scale.

The Sweep in Progress

By the time we reach the subject message ([msg 2291]), four of the five sweep points have been completed:

| Sweep Point | Result | |---|---| | pw=10 | 43.5s/proof | | pw=12 | 43.5s/proof | | pw=15 | 44.8s/proof | | pw=18 | 43.8s/proof |

The data already reveals a pattern: pw=10 and pw=12 are tied for best, pw=18 is close behind, and pw=15 shows a slight regression. The final data point—pw=20—will complete the picture.

What the Subject Message Actually Does

The message executes a single bash command with four operations chained together:

1. cd /home/theuser/curio/extern/cuzk — Navigate to the project directory containing the compiled release binary.
2. nohup ./target/release/cuzk-daemon --config /tmp/cuzk-sweep.toml > /tmp/cuzk-sweep-pw20.log 2>&1 & — Launch the daemon in the background using nohup (immune to hangup signals), redirecting both stdout and stderr to a log file specific to this sweep point. The --config flag points to a shared config file that has been updated to set partition_workers = 20.
3. echo "PID=$!" — Print the process ID of the just-launched background job, providing a way to monitor or kill it later.
4. Output: PID=3615659 — The daemon started successfully with process ID 3615659. This is the last daemon start of the sweep. The subsequent messages ([msg 2292], [msg 2293]) show the assistant waiting for the "cuzk-daemon ready" signal, then running the benchmark, which yields 44.9s/proof—confirming that pw=20 is slightly worse than pw=10–12. The sweep is complete.

Operational Challenges and Debugging

The path to this message was not smooth. The assistant encountered several operational hiccups during the sweep that reveal important details about the tool execution environment:

The sed substitution problem ([msg 2255]–[msg 2262]): When transitioning from pw=10 to pw=12, the assistant attempted to modify the config file in a single chained command with pkill. The command timed out (120s limit), and the sed substitution never executed. The assistant discovered this when checking the config file and finding it still set to partition_workers = 10. The root cause was that pkill -f cuzk-daemon terminated the daemon but also consumed the rest of the command pipeline in a way that left the sed unexecuted. The assistant had to manually rewrite the entire config file using a heredoc.

The missing log file ([msg 2269]–[msg 2272]): When transitioning from pw=12 to pw=15, the daemon start command again timed out. The assistant checked for the log file and found it didn't exist—the nohup command had never completed. This required a fresh start: kill any remaining daemon, verify the config, update it, and restart in separate steps.

The pw=18 daemon failure ([msg 2280]–[msg 2285]): A similar pattern repeated. The assistant learned to separate the pkill, sed, and daemon start into distinct bash calls, each with its own timeout budget. This incremental debugging—from chained commands to isolated operations—demonstrates adaptive problem-solving under the constraints of a tool environment where each invocation has a finite execution window.

Knowledge Required and Knowledge Created

To fully understand this message, one needs:

• Knowledge of the cuzk project: That it's a Groth16 proving engine for Filecoin PoRep, with a daemon process that preloads SRS data on startup.
• Understanding of partition_workers: That this parameter controls the number of parallel synthesis threads, and that its optimal value depends on CPU core count and the GPU worker architecture.
• Familiarity with the benchmark methodology: That c=5 j=3 means 5 proofs with concurrency 3, and that throughput is measured as total time divided by 5.
• Awareness of the Phase 8 architecture: That dual GPU workers share a per-GPU mutex, and that CPU contention between synthesis workers and GPU preprocessing threads is the key bottleneck. The message creates new knowledge: it records the exact command used to start the daemon for the pw=20 sweep point, including the PID, the config file path, and the log file destination. This is an audit trail—a reproducible record of what was done. Combined with the subsequent benchmark result ([msg 2293]), it establishes that pw=20 yields 44.9s/proof on this hardware, confirming the optimal range of pw=10–12.

Assumptions and Potential Mistakes

The assistant operates under several assumptions:

1. That the config file is correctly set to partition_workers = 20 — This assumption was validated in the preceding message ([msg 2290]) where sed confirmed the change.
2. That the daemon will start successfully — The assistant does not verify the daemon's health in this message; it defers that to the next message where it polls for the "ready" signal.
3. That the benchmark environment is stable — The assistant assumes no external factors (disk I/O, thermal throttling, other processes) will affect the result. This is a reasonable assumption for a controlled benchmark but worth noting.
4. That the PID is meaningful — The assistant captures the PID but never uses it directly; it relies on pkill -f cuzk-daemon later, which matches by process name pattern. One could argue that the assistant should have verified the config file content after starting the daemon, to ensure no race condition. But in practice, the config file is written to disk before the daemon reads it, so this is a minor concern.

The Broader Significance

This message, for all its apparent simplicity, represents a commitment to empirical optimization. Rather than guessing at the optimal partition_workers value or relying on theoretical models, the assistant executed a systematic sweep across five values, each requiring a full daemon restart and SRS preload (a multi-minute operation). The total sweep took approximately 30 minutes of wall-clock time, including the operational hiccups.

The result—that pw=10–12 is optimal, with higher values showing slight regression—validates the Phase 8 architecture's CPU contention model. With two GPU workers per device, the synthesis threads compete with GPU preprocessing for CPU cores. At pw=20, the contention becomes measurable (44.9s vs 43.5s). At pw=30, it becomes severe (60.4s). The narrow performance band (43.5–44.9s across five values) also suggests that the system is relatively robust to this parameter within a reasonable range.

In the larger arc of the cuzk project, this sweep is the final validation step before the Phase 8 architecture could be recommended for production deployment. The assistant had already proven that Phase 8 improves throughput by 13–17% over Phase 7; now it had proven that the optimal partition_workers setting for this architecture is 10–12 on a 96-core machine. The engineering is complete, the data is clear, and the configuration can be codified in the production deployment.

Conclusion

Message [msg 2291] is a single bash command that starts a daemon process. But it is also the last act of a methodical, empirically-driven optimization campaign spanning multiple architectural phases, dozens of benchmark runs, and hundreds of lines of C++/Rust/Go code. It represents the moment when a performance engineer, having eliminated GPU idle gaps, narrowed mutex contention, and validated dual-worker interleaving, finally asks: "What is the best setting for production?" and answers with data rather than intuition. The PID 3615659 is the final witness to that answer.