Building the Microbenchmark: Isolating a 5-Second Synthesis Regression in the cuzk Proving Pipeline

In the high-stakes world of Filecoin proof generation, every second counts. When a carefully planned set of Phase 4 optimizations turned a 88.9-second baseline into a 106-second regression, the assistant faced a classic performance engineering challenge: which of the five simultaneous changes caused the damage? Message [msg 1012] represents the decisive moment when the assistant pivoted from guesswork to measurement, implementing a dedicated microbenchmark to isolate the root cause of a stubborn synthesis slowdown.

The Context: A Regression in the Making

The cuzk project had been progressing smoothly through three phases. Phase 0 established the baseline, Phases 1 and 2 introduced pipelining, and Phase 3 added cross-sector batching. Each phase delivered measurable improvements. Then came Phase 4: a suite of five compute-level optimizations (A1 SmallVec, A2 Pre-sizing, A4 Parallel B_G2, B1 cudaHostRegister, D4 Per-MSM window tuning) intended to push throughput even higher.

The result was a disaster: total proof time ballooned from 88.9 seconds to 106 seconds. The assistant methodically eliminated suspects. CUDA timing instrumentation (the CUZK_TIMING printf's) revealed that B1 (cudaHostRegister) was the primary culprit, adding 5.7 seconds of overhead by pinning ~125 GiB of host memory. Reverting B1 brought the total down to 94.4 seconds, but that was still 5.5 seconds above baseline. The synthesis phase was now running at 60.3 seconds versus the baseline 54.7 seconds — a 10.5% regression. The only synthesis change in play was A1: replacing Vec<(usize, Scalar)> with SmallVec<[(usize, Scalar); 4]> in the Indexer struct of bellpepper-core.

The User's Insight: "Microbench Possible?"

The user's question at [msg 998] — "Mircobench possible?" — was the catalyst. Rather than continuing to run full end-to-end proofs through the daemon (each taking ~90 seconds and requiring GPU, SRS loading, and daemon orchestration), the assistant could build a targeted microbenchmark that synthesized just one partition's circuit, measuring only the CPU synthesis time. This would enable rapid A/B testing with iteration times measured in seconds rather than minutes.

The assistant immediately recognized the value. At [msg 999], it confirmed the idea: "a microbenchmark of just the synthesis path (no GPU, no daemon overhead) to isolate the SmallVec impact with fast iteration." It then spawned a task agent ([msg 1000]) to explore the synthesis code path and understand what dependencies and data structures were needed. The task returned a detailed analysis of synthesize_porep_c2_batch, mapping the full call chain from C1 deserialization through circuit construction to the final SynthesizedProof.

The Subject Message: Adding the run_synth_only Implementation

Message [msg 1012] is the culmination of this effort. After reading the existing cuzk-bench/src/main.rs structure (messages [msg 1002] through [msg 1007]), editing Cargo.toml to add cuzk-core as an optional dependency with a synth-bench feature gate ([msg 1009]), adding the SynthOnly command variant ([msg 1010]), and wiring the handler into the command dispatch ([msg 1011]), the assistant now writes the actual function body:

[assistant] Now add the run_synth_only function implementations (feature-gated): [edit] /home/theuser/curio/extern/cuzk/cuzk-bench/src/main.rs Edit applied successfully.

This terse message belies the weight of the design decisions it represents. The run_synth_only function would directly call synthesize_porep_c2_batch from cuzk-core, bypassing the daemon's gRPC server, the GPU proving pipeline, and the SRS parameter loading. It would accept the same C1 output file path as the single command, but instead of submitting a proof job to the daemon, it would run the CPU synthesis inline and report just the synthesis duration. The feature gate (synth-bench) ensured that the dependency on cuzk-core remained optional for normal builds, keeping the binary size and compile time reasonable.

Design Decisions and Tradeoffs

Several design choices are implicit in this message. First, the assistant chose to extend the existing cuzk-bench binary rather than creating a standalone microbenchmark. This was pragmatic: cuzk-bench already had the CLI infrastructure (clap argument parsing, logging setup, C1 file deserialization helpers) and the user was already familiar with its interface. Adding a subcommand was the path of least resistance.

Second, the feature gate approach (synth-bench) reflected careful dependency management. cuzk-core is a heavyweight dependency — it pulls in bellperson, filecoin-proofs, and the entire proof verification stack. Making it optional meant that users who only wanted to submit proofs to a remote daemon wouldn't need to compile all of that. This is particularly important for a benchmarking tool that might be deployed on systems without the full proving stack.

Third, the microbenchmark would run synthesis exactly once per invocation, timing it with std::time::Instant. This is deliberately simple — no warmup iterations, no statistical sampling, no outlier detection. The assumption was that synthesis time is deterministic enough that a single measurement would be sufficient for A/B comparison, and that the speed of iteration (changing a constant, rebuilding, running again) mattered more than statistical rigor.

Assumptions and Their Consequences

The message makes several assumptions that would be tested in the following messages. It assumes that the SynthesizedProof struct exposes a num_constraints field — but as [msg 1013] and [msg 1014] reveal, there are actually two SynthesizedProof structs (one for the cuda-supraseal path and one for the non-CUDA path), and neither has a num_constraints field. The assistant quickly pivots to using provers[0].a.len() instead ([msg 1015]).

It also assumes that provers[0].a is publicly accessible. The ProvingAssignment type comes from bellperson, and the assistant verifies at [msg 1018] that the a field is indeed pub. This kind of API spelunking is typical when building tooling against an evolving codebase — you can't always know the exact interface without checking.

The most significant assumption is that the microbenchmark would actually reveal the cause of the regression. This was not guaranteed. The 5.5-second gap could have been caused by interactions between synthesis and GPU proving (e.g., memory contention, TLB shootdowns) that wouldn't appear in a standalone synthesis run. Fortunately, as the chunk summary reveals, the microbenchmark would conclusively show that SmallVec — in any inline capacity configuration — causes a 5-6 second regression, confirming the hypothesis.

The Broader Significance

Message [msg 1012] exemplifies disciplined performance engineering. When faced with a regression, the assistant did not guess or rely on intuition. It built measurement infrastructure, isolated variables, and tested hypotheses. The synth-only microbenchmark is a scalpel — a targeted instrument that cuts away the complexity of the full proving pipeline to reveal the behavior of a single component.

This approach is particularly valuable in systems like cuzk, where end-to-end tests are expensive (each proof takes ~90 seconds and consumes ~200 GiB of memory). A microbenchmark that runs in 55-60 seconds with minimal memory overhead enables dozens of iterations in the time it would take to run a handful of full proofs. The assistant would go on to benchmark four configurations (Vec, SmallVec cap=1, cap=2, cap=4) across three iterations each, generating conclusive data that SmallVec was the culprit.

The message also demonstrates the importance of tooling in performance work. The assistant didn't just analyze the code and form an opinion about whether SmallVec should be faster or slower. It built a tool that could answer the question empirically. This is the difference between speculation and science.

Conclusion

In the span of a single edit operation, message [msg 1012] completed the construction of a critical diagnostic tool. The synth-only microbenchmark would go on to prove that SmallVec, despite its theoretical advantages (eliminating heap allocations, improving data locality), caused a consistent 5-6 second regression on the AMD Zen4 Threadripper PRO 7995WX system. This finding would lead to further investigation with hardware performance counters (perf stat) to understand the microarchitectural reasons — likely related to increased stack frame size causing L1 cache pressure.

The message is a testament to the power of targeted measurement. When a system is too complex to reason about analytically, build a tool that isolates the variable of interest. Measure, don't guess.