Building a Synthesis-Only Microbenchmark: Isolating the Phase 4 Regression

In the midst of a disciplined performance engineering investigation, a single brief message from the assistant reads:

Now add the SynthOnly subcommand and handler to main.rs: [edit] /home/theuser/curio/extern/cuzk/cuzk-bench/src/main.rs Edit applied successfully.

This terse edit message ([msg 1010]) belies a critical strategic pivot in the Phase 4 regression diagnosis. To understand why this message matters, we must trace the investigation that led to it.

The Context: A 5.5-Second Mystery

The cuzk project had successfully implemented a pipelined SNARK proving engine for Filecoin PoRep proofs, achieving a strong baseline of 88.9 seconds for a single 32 GiB proof. Phase 4 introduced five optimizations (A1 SmallVec, A2 Pre-sizing, A4 Parallel B_G2, B1 cudaHostRegister, D4 Per-MSM window tuning) intended to improve throughput. Instead, the first full test regressed to 106 seconds — a 19% slowdown.

The assistant systematically eliminated suspects. CUDA timing instrumentation (CUZK_TIMING printf's) revealed that B1 (cudaHostRegister), which pinned ~125 GiB of host memory, was adding 5.7 seconds of overhead — far exceeding the estimated 150–300 ms. Reverting B1 brought the total down to 94.4 seconds, but synthesis remained at 60.3 seconds versus the 54.7-second baseline. This 5.5-second synthesis regression was the remaining obstacle.

The prime suspect was A1: replacing Vec<(usize, Scalar)> with SmallVec<[(usize, Scalar); 4]> in the Indexer struct used during constraint synthesis. SmallVec was intended to eliminate heap allocations for the common case of 1–3 term linear combinations, but on the AMD Zen4 Threadripper PRO 7995WX system, it appeared to be causing a slowdown.

The Strategic Pivot: Why a Microbenchmark Was Needed

Testing synthesis changes required running the full daemon pipeline: starting the daemon, waiting for SRS loading, submitting a proof request, waiting for GPU proving, and collecting results. Each E2E test took ~90–100 seconds and involved many moving parts. To rapidly A/B test SmallVec variants — Vec, cap=1, cap=2, cap=4 — the assistant needed a faster feedback loop.

The decision to build a synth-only subcommand within the existing cuzk-bench tool was a deliberate architectural choice. Rather than writing a standalone binary from scratch, the assistant reused the existing CLI framework (clap subcommands), the C1 JSON loading logic, and the synthesize_porep_c2_batch function from cuzk-core. The new subcommand was feature-gated behind synth-bench to avoid pulling cuzk-core and its heavy dependencies (bellperson, filecoin-proofs) into the main binary's dependency tree when not needed.

The Edit Itself: What the Message Accomplishes

The message records a single edit to cuzk-bench/src/main.rs. While the full diff is not shown in the message text, the surrounding conversation reveals what was added:

  1. A new SynthOnly variant in the clap command enum, with parameters for the C1 file path, sector parameters, and iteration count.
  2. A handler branch in the match cli.command block that calls run_synth_only().
  3. The run_synth_only function itself, feature-gated behind #[cfg(feature = "synth-bench")], which loads the C1 output, calls synthesize_porep_c2_batch for each partition, and reports timing breakdowns. The Cargo.toml was also modified in the preceding message ([msg 1009]) to add cuzk-core as an optional dependency and define the synth-bench feature.

Assumptions and Potential Pitfalls

The assistant made several assumptions in this edit. First, that cuzk-core's default features (including cuda-supraseal) would propagate through the optional dependency, making the full SynthesizedProof struct available. This assumption was validated in subsequent messages (<msg id=1016-1017>).

Second, that the SynthesizedProof struct would expose the fields needed for detailed reporting. This turned out to be partially incorrect: the struct lacked a num_constraints field, requiring the assistant to derive it from provers[0].a.len() instead (<msg id=1014-1015>).

Third, that the synthesis path could be cleanly invoked without GPU or SRS dependencies. The synthesize_porep_c2_batch function in pipeline.rs handles circuit construction and constraint synthesis but does not touch GPU resources or load SRS parameters, making it suitable for standalone benchmarking.

Input and Output Knowledge

To understand this message, a reader needs knowledge of: the cuzk project's architecture (daemon + bench tools), the Phase 4 optimization suite and its regression, the distinction between synthesis (CPU) and proving (GPU) phases, the role of SmallVec in the Indexer struct, and the existing cuzk-bench CLI structure with its single, batch, status, and other subcommands.

The message creates new knowledge: a dedicated microbenchmarking tool that can iterate on synthesis changes in ~55 seconds per test instead of ~95 seconds, without the noise of GPU variability, SRS loading, or daemon orchestration overhead. This tool would prove essential in the subsequent investigation, enabling the assistant to conclusively demonstrate that SmallVec — regardless of inline capacity — causes a 5–6 second synthesis regression compared to plain Vec.

The Thinking Process Behind the Message

The assistant's reasoning is visible in the messages leading up to this edit. In [msg 1001], after receiving the subagent task's analysis of the synthesis code path, the assistant wrote: "That's exactly what we need. Let me build a quick synth-only subcommand for fast A/B iteration." The todo list in the same message shows the progression: having completed the A2 reversion and the instrumented GPU test, the assistant was now pivoting to isolate the synthesis regression.

The user's suggestion at [msg 998] — "Mircobench possible?" — likely prompted this direction. The assistant responded enthusiastically ([msg 999]), killed the daemon, and immediately dispatched a subagent task to explore the synthesis code structure. Within three messages, the Cargo.toml was modified and the edit to main.rs was applied.

This rapid execution reflects a key principle of the investigation: when faced with a performance regression, the fastest path to diagnosis is to eliminate variables. By extracting the synthesis phase into a standalone benchmark, the assistant removed GPU variability, SRS loading, daemon IPC overhead, and memory pinning effects — leaving only the SmallVec change as the independent variable.

Conclusion

Message [msg 1010] represents a methodological turning point in the Phase 4 regression diagnosis. It transforms the investigation from slow, noisy E2E tests into a precise, repeatable microbenchmark. This tool would ultimately prove that SmallVec, despite its theoretical advantages, causes a consistent 5–6 second regression on the target AMD Zen4 architecture — a finding that would have been difficult to isolate without the dedicated benchmark. The message exemplifies disciplined performance engineering: when the system is too complex to debug directly, build a simpler system that isolates the variable of interest.