The Benchmark Launch: A Pivotal Moment in Phase 11 Memory-Bandwidth Optimization

The Message

[assistant] [bash] FIL_PROOFS_PARAMETER_CACHE=/data/zk/params nohup /home/theuser/curio/extern/cuzk/target/release/cuzk-daemon --config /tmp/cuzk-p11-int12.toml > /home/theuser/cuzk-p11-int123.log 2>&1 &
echo "Daemon PID: $!"
Daemon PID: 677688

At first glance, this appears to be a routine operational command: launch a background daemon process with a specific configuration file and redirect its output to a log. But in the context of the broader optimization campaign, this single message represents a critical inflection point — the moment when three carefully engineered memory-bandwidth interventions converge into an empirical test. The assistant is not merely starting a process; it is committing to an experimental verdict that will determine the trajectory of the entire Phase 11 optimization effort.

The Context: A Deep-Dive into Memory Subsystem Bottlenecks

To understand why this message matters, one must appreciate the journey that led to it. The cuzk SNARK proving engine had been through nine prior optimization phases, each targeting a different bottleneck in the Groth16 proof generation pipeline. Phase 9 had achieved a 14.2% throughput improvement through PCIe transfer optimization, but dual-worker mode revealed a new bottleneck: DDR5 memory bandwidth contention. Phase 10 attempted a two-lock GPU interlock architecture but was abandoned after discovering fundamental CUDA device-global synchronization conflicts — a costly detour that required reverting to the Phase 9 single-lock design.

Phase 11 was born from the ashes of Phase 10. The post-mortem analysis, documented in c2-optimization-proposal-11.md, identified three specific interventions to reduce DDR5 memory bandwidth contention:

Intervention 1: Serialize async_dealloc calls with a static mutex to prevent TLB shootdown storms when multiple GPU workers free host-pinned memory simultaneously.
Intervention 2: Reduce groth16_pool thread count from 192 to 32 via gpu_threads = 32, limiting the number of CPU threads competing for L3 cache and memory bandwidth during the b_g2_msm computation.
Intervention 3: A global atomic throttle flag that the C++ code sets before entering b_g2_msm and clears after, which Rust's SpMV synthesis checks periodically — if the flag is set, the synthesis thread yields its time slice to reduce memory bandwidth contention. Each intervention was carefully designed based on waterfall timing analysis and system-level profiling. The assistant implemented them iteratively, building and testing each one before moving to the next. By message 2803, all three interventions compiled successfully. The stage was set for the definitive benchmark.

The Config File: A Silent Witness to Design Decisions

The config file /tmp/cuzk-p11-int12.toml is itself a artifact of prior decision-making. Its name — "int12" — signals that it enables Interventions 1 and 2. The assistant's log file name cuzk-p11-int123.log reveals the intent to benchmark all three interventions together, implying that Intervention 3's throttle flag was added to the same binary but the config file may not have needed a separate toggle for it (the throttle is a runtime FFI mechanism, not a configuration parameter).

The config was created during the Phase 11 implementation cycle, likely containing settings like gpu_threads = 32 (Intervention 2) and the standard parameters for partition_workers, SRS cache paths, and GPU worker counts. The FIL_PROOFS_PARAMETER_CACHE environment variable points to /data/zk/params, a directory containing the precomputed Structured Reference Strings (SRS) needed for the proof generation — files that total hundreds of gigabytes and must be loaded into GPU memory before any proof can be generated.

The Benchmark Methodology: Why This Launch Matters

The assistant is not casually starting a daemon; it is initiating a controlled experiment. The benchmark protocol, established over many optimization phases, works as follows:

Launch the cuzk-daemon with a specific config that encodes the experimental variables (which interventions are active, thread counts, worker counts).
Wait for the daemon to initialize — load SRS, pre-warm GPU memory, start synthesis dispatchers and GPU workers.
Run cuzk-bench batch with standardized parameters: --count 20 (20 proofs), --concurrency 15 (15 concurrent jobs), measuring throughput in seconds per proof.
Analyze the timing logs to extract per-phase durations and identify remaining bottlenecks. The choice of nohup and backgrounding with & is deliberate: the benchmark run takes several minutes (each proof takes ~37 seconds, times 20 proofs at concurrency 15), and the assistant needs the shell to remain free for subsequent monitoring commands. The PID capture (echo "Daemon PID: $!") enables later process management — killing the daemon after the benchmark completes, or checking if it has crashed.

The Assumptions Embedded in This Launch

Every benchmark launch carries assumptions, and this one is no exception. The assistant assumes:

Build correctness: The compiled binary faithfully implements all three interventions. The C++ extern "C" declarations for set_membw_throttle link correctly with the Rust cuzk-pce crate. The static mutex for async_dealloc properly serializes deallocations without deadlocking. The gpu_threads = 32 setting actually reaches the CUDA code path that spawns the prep_msm thread pool.
Config integrity: The config file /tmp/cuzk-p11-int12.toml contains the correct settings and hasn't been corrupted or overwritten by a previous experiment.
System state: The GPU is available, the SRS parameter cache is populated, the previous daemon instance has been killed (as confirmed in message 2800), and no other process is competing for GPU or memory bandwidth.
Measurement validity: The benchmark will produce representative numbers. The assistant assumes that 20 proofs at concurrency 15 provides sufficient statistical significance, and that the first proof's timing (which includes SRS loading and GPU warm-up) can be distinguished from steady-state throughput.

The Outcome: What This Launch Revealed

The subsequent messages tell the story. Message 2806 shows the benchmark results: 36.8 seconds per proof with all three interventions active. This is essentially identical to the 36.7 seconds achieved with only Interventions 1 and 2. Intervention 3 — the global throttle flag — added no measurable benefit.

This negative result is itself informative. The assistant's analysis in message 2808 correctly identifies the reason: with gpu_threads = 32, the b_g2_msm computation already uses fewer threads, so L3 contention during its window is already reduced. The throttle mechanism, which would have been more valuable at higher thread counts, has nothing left to optimize.

The launch also set the stage for the next exploration. The user's prompt in message 2807 — "Try 3/4 gpu workers per dev?" — led the assistant to test higher GPU worker counts, which ultimately showed regression (37.2s for gw=3, worse than gw=2). This negative result was equally valuable, confirming that the sweet spot for GPU workers was already found.

The Deeper Significance: An Engineering Philosophy

This message exemplifies a particular engineering philosophy: measure, don't guess. The assistant could have reasoned theoretically about whether Intervention 3 would help, but instead chose to run the experiment. The cost of a benchmark cycle — a few minutes of compute time — is trivial compared to the cost of shipping unnecessary complexity.

The message also reveals the iterative nature of performance optimization. Each benchmark launch is a hypothesis test. The config file encodes the independent variables. The log file captures the dependent variables. The assistant plays the role of experimenter, analyzing results and designing the next experiment. The daemon is not a production service here; it is a test harness.

This is particularly visible in the naming convention. The config file /tmp/cuzk-p11-int12.toml and the log file cuzk-p11-int123.log are ephemeral artifacts, created for a specific experiment and discarded after analysis. They are not meant to be preserved; they are the equivalent of a scientist's lab notebook — a record of what was tested and what was found.

Conclusion

Message 2804 is, on its surface, a simple bash command to launch a daemon process. But in the context of the Phase 11 optimization campaign, it represents the culmination of days of analysis, design, implementation, and debugging. Three interventions, each targeting a specific memory-bandwidth contention mechanism, are about to be tested together for the first time. The daemon PID 677688 will live for only a few minutes — long enough to generate 20 proof timings, long enough to answer the question of whether Intervention 3 adds value, long enough to guide the next phase of optimization.

The message is a reminder that in systems engineering, the most important work often happens not in the code editor but in the moments between — when a carefully built hypothesis meets the unforgiving reality of benchmark results.