Building a Microscope for Performance: The synth-only Microbenchmark Decision

In the middle of a disciplined performance engineering campaign, a single message can mark the pivot from speculation to measurement. Message 1008 in this opencode session is precisely such a pivot. The assistant writes:

Now I have the full picture. Let me add the synth-only subcommand. I need to: 1. Add cuzk-core as an optional dependency in cuzk-bench/Cargo.toml 2. Add a synth-bench feature 3. Add the SynthOnly command variant and handler

These three bullet points, accompanied by a file read of cuzk-bench/Cargo.toml, represent the culmination of an intensive diagnostic effort spanning dozens of messages. The assistant has been chasing a 5–6 second synthesis regression introduced by the A1 optimization (replacing Vec with SmallVec in the Indexer data structure), and has reached the conclusion that the only way to definitively isolate the cause is to build a dedicated microbenchmark that measures only circuit synthesis — stripping away GPU proving, SRS loading, daemon overhead, and network communication. This article examines why this message was written, the reasoning behind it, the assumptions it makes, and the knowledge it both consumes and produces.

The Regression That Wouldn't Yield

To understand message 1008, one must understand the crisis that precipitated it. The cuzk project had successfully completed Phases 0 through 3, building a pipelined Groth16 proof generation engine for Filecoin's Proof of Replication (PoRep) that achieved a solid baseline of 88.9 seconds for a single 32 GiB proof. Phase 4 was supposed to improve upon this with five targeted optimizations drawn from a detailed proposal document (c2-optimization-proposal-4.md).

Instead, when all five optimizations were applied and tested, the total proof time regressed to 106 seconds — a 17-second slowdown. This triggered a systematic diagnostic process that is a textbook example of performance engineering discipline:

  1. Instrument first: The assistant added CUZK_TIMING printf instrumentation to the CUDA kernel code, enabling precise phase-level timing breakdowns.
  2. Revert suspects: The B1 optimization (cudaHostRegister for pinning ~125 GiB of host memory) was identified as adding 5.7 seconds of overhead and was reverted.
  3. Measure again: With B1 removed, total time dropped to 94.4 seconds, but synthesis remained at 60.3 seconds versus the 54.7-second baseline.
  4. Hypothesize: The only synthesis change was A1 — replacing Vec<(usize, Scalar)> with SmallVec<[(usize, Scalar); 4]> in the Indexer struct used during constraint system synthesis. This was intended to eliminate heap allocations for the common case of 1–3 term linear combinations, but it appeared to be causing a ~10.5% slowdown. The assistant explored several hypotheses for why SmallVec might be slower. The inline capacity of 4 made each Indexer struct approximately 170 bytes instead of 24 bytes for a Vec. With three linear combinations (a, b, c) created per enforce() call, and each linear combination containing two Indexers, the total stack footprint jumped from ~144 bytes to ~1020 bytes. On AMD Zen4 (Threadripper PRO 7995WX), where L1 data cache is 32 KB per core, this extra stack pressure could cause cache evictions in the tight synthesis loop. The user suggested considering cache line alignment and AVX operations, and the assistant experimented with reducing inline capacity to 1 (making each Indexer ~56 bytes, fitting in a single 64-byte cache line). But even with cap=1, the assistant couldn't be certain whether SmallVec was the true culprit without isolating synthesis from the rest of the pipeline. Every full E2E test took 90+ seconds and included GPU proving, SRS loading, and daemon startup overhead — all of which added noise and slowed the iteration cycle.

The User's Prompt and the Assistant's Response

The user's question at message 998 — "Mircobench possible?" — was a catalytic moment. The assistant immediately recognized the value: a microbenchmark of just the synthesis path would enable rapid A/B testing of SmallVec variants without waiting for GPU initialization, SRS parameter loading, and proof serialization. The assistant responded enthusiastically at message 999, then immediately spawned a subagent task (message 1000) to explore the codebase and determine what would be required to call the synthesis function standalone.

The task result (message 1001) returned a detailed analysis of the synthesize_porep_c2_batch function in pipeline.rs, identifying that the core synthesis call was bellperson::synthesize_circuits_batch() and showing how to invoke it with the necessary parameters. The task also identified the key data structures (SynthesizedProof, ProvingAssignment) and confirmed that the synthesis function could be called without GPU dependencies.

Message 1008 is the assistant's response to receiving that analysis. The phrase "Now I have the full picture" is significant — it marks the transition from investigation to implementation. The assistant has verified that:

The Implementation Plan

The three steps the assistant outlines are carefully sequenced:

Step 1: Add cuzk-core as an optional dependency. This is necessary because cuzk-bench currently communicates with the daemon over gRPC and doesn't link against cuzk-core directly. Making it optional with a feature flag ensures that the normal benchmarking workflow (which talks to a running daemon) doesn't require linking the entire synthesis pipeline. The synth-bench feature will enable this dependency only when needed.

Step 2: Add a synth-bench feature. Feature gating is important here. The synth-only subcommand is a diagnostic tool, not part of the normal benchmarking workflow. By hiding it behind a feature flag, the assistant avoids bloating the default build or introducing unnecessary compilation dependencies. This also means the microbenchmark won't accidentally be used in production scripts.

Step 3: Add the SynthOnly command variant and handler. This is the actual subcommand implementation. It will deserialize a C1 output file (the output of Phase 1 of the PoRep proof), call synthesize_porep_c2_batch (or its underlying bellperson::synthesize_circuits_batch), and report the synthesis time. The handler will be feature-gated to only compile when synth-bench is enabled.

The file read of cuzk-bench/Cargo.toml that follows the planning is not incidental — it's the assistant gathering the input knowledge needed to execute step 1. The current dependency list shows cuzk-proto, tonic, tokio, clap, etc., but no cuzk-core. The assistant needs to understand the exact workspace path and version to add the dependency correctly.

Assumptions and Potential Pitfalls

The assistant makes several assumptions in this message:

  1. That the synthesis function is the bottleneck worth isolating. This is well-supported by the timing data: synthesis at 60.3s accounts for ~64% of the 94.4s total, and the 5.6s gap from the 54.7s baseline is the largest remaining regression. However, there's a subtle assumption that the synthesis slowdown is consistent — that measuring it in isolation will reproduce the same 5.6s gap seen in the full pipeline. If the slowdown is somehow an artifact of interactions with GPU memory pressure or SRS loading (e.g., NUMA effects, memory bandwidth contention), the microbenchmark might not reproduce it.
  2. That the synthesis function can be called meaningfully without GPU context. The synthesize_porep_c2_batch function constructs circuits and evaluates constraints but doesn't touch the GPU. The assistant verified this through the task analysis. However, the function does allocate large vectors (the a/b/c constraint evaluations), and the memory allocation patterns might differ when the GPU isn't simultaneously allocating and freeing memory. This is a minor concern — the synthesis phase completes before GPU proving begins in the pipeline, so there shouldn't be contention.
  3. That the cuzk-core default features include cuda-supraseal. The assistant later verifies this (message 1016) by checking cuzk-core/Cargo.toml, confirming that the default features include cuda-supraseal. This is important because the SynthesizedProof struct with full field visibility is only defined under that feature. If the default features didn't include it, the microbenchmark code would fail to compile.
  4. That three iterations are sufficient for statistical significance. The assistant plans to run three iterations per configuration. Given that synthesis times are typically stable (standard deviation of <1% across runs), three iterations should be enough to detect a 5–6 second difference. But if the variance is higher, more iterations might be needed.

The Thinking Process Visible in the Message

Message 1008 is concise, but it reveals a sophisticated thinking process:

First, the assistant has internalized the task analysis and synthesized it into an actionable plan. The phrase "Now I have the full picture" signals that the exploration phase is complete and the implementation phase can begin.

Second, the assistant is thinking in terms of minimal intervention. Rather than modifying the daemon or adding instrumentation to the synthesis path, the assistant chooses to build a standalone tool that calls the same function. This is a classic scientific approach: isolate the variable, control the environment, measure precisely.

Third, the assistant is thinking about build system hygiene. The three-step plan shows awareness of Rust's feature system, optional dependencies, and the importance of not polluting the default build with diagnostic tools. This is engineering maturity — knowing that a quick hack would work but a properly structured feature gate is better.

Fourth, the assistant is thinking about iteration speed. A full E2E test takes 90+ seconds and requires a running daemon with GPU initialized. A synth-only microbenchmark can run in under 60 seconds with no GPU dependency, no daemon, no SRS loading. This 2–3x speedup in iteration time is crucial for the A/B testing that will follow.

Input Knowledge Required

To understand this message, one needs:

Output Knowledge Created

This message creates:

The Broader Significance

Message 1008 represents a methodological commitment. Throughout the Phase 4 regression diagnosis, the assistant has demonstrated a pattern: measure, hypothesize, test, revert. The B1 optimization was reverted based on timing data. The A2 pre-sizing optimization was partially reverted. Now, instead of reverting A1 blindly or accepting the regression, the assistant chooses to build a tool that can provide definitive evidence.

This is the hallmark of disciplined performance engineering: when you can't determine the cause of a regression through reasoning alone, you build instrumentation to measure it. The synth-only microbenchmark is that instrumentation. It will allow the assistant to test SmallVec at various inline capacities (cap=1, cap=2, cap=4, and Vec as control) with minimal overhead, producing clean A/B data that can either exonerate or condemn the optimization.

The message also reveals something about the assistant's working style: it prefers to build tools rather than speculate. When faced with a puzzle — why would eliminating heap allocations cause a slowdown? — the assistant doesn't spend time reasoning about CPU microarchitecture, cache hierarchies, or allocation patterns (though it does some of that in earlier messages). Instead, it builds a tool that can answer the question empirically. The synth-only microbenchmark is that tool, and message 1008 is the moment it is conceived.