The Microbenchmark That Exposed a Cache Line Mystery: Diagnosing SmallVec Regression in cuzk's SNARK Synthesis

Introduction

In the course of optimizing a high-throughput Groth16 proof generation pipeline for Filecoin's Proof-of-Replication (PoRep), a team of performance engineers encountered a puzzling regression. After successfully implementing and validating three major pipeline phases—per-partition pipelining, async overlap between synthesis and GPU proving, and cross-sector batching—they turned to compute-level micro-optimizations in Phase 4. Five optimizations were implemented from a detailed proposal document, but the first end-to-end test revealed a shocking result: instead of improving, the total proof time had regressed from a baseline of 88.9 seconds to 106 seconds.

The subject of this article is message 1021 in the conversation, a single bash command execution that represents a pivotal moment in the diagnosis of that regression. In this message, the assistant runs a custom-built synth-only microbenchmark for the very first time, measuring the circuit synthesis phase in isolation from GPU proving and SRS loading overhead. This microbenchmark was the key tool that would conclusively identify the source of a 5–6 second slowdown, leading to a deeper understanding of how CPU cache architecture interacts with data structure choices in high-performance zero-knowledge proof systems.

Context: The Phase 4 Regression

The conversation leading up to message 1021 documents a disciplined performance engineering process. The team had established a solid baseline of 88.9 seconds for a single 32 GiB PoRep proof using the cuzk pipelined proving engine. Phase 4 Wave 1 implemented five optimizations drawn from a detailed proposal document:

The Insight: Isolating Synthesis

The user's suggestion at message 998—"Mircobench possible?"—sparked a critical insight. Up to this point, every test of the optimization changes required running the full end-to-end pipeline: loading SRS parameters from disk (a multi-gigabyte operation), synthesizing the circuit, transferring data to the GPU, and executing the GPU proving kernels. This meant each test cycle took 90+ seconds and conflated multiple sources of variance. If the synthesis phase could be benchmarked in isolation, the team could iterate much faster and eliminate confounding factors.

The assistant immediately recognized the value of this approach. At message 999, it responded: "You mean a microbenchmark of just the synthesis path (no GPU, no daemon overhead) to isolate the SmallVec impact with fast iteration? That's a great idea—we can write a small bench that synthesizes one partition's circuit and times just that, without needing to wait for SRS load and GPU prove each time."

What followed was a rapid implementation sprint spanning messages 1000 through 1019. The assistant dispatched a subagent task to explore the synthesis API surface, then proceeded to:

  1. Add cuzk-core as an optional dependency in cuzk-bench/Cargo.toml
  2. Add a synth-bench feature gate
  3. Add the SynthOnly command variant to the clap-based CLI
  4. Implement the handler that calls synthesize_porep_c2_batch directly and measures elapsed time
  5. Verify that the SynthesizedProof struct's fields (provers, synthesis_duration) are publicly accessible
  6. Build and verify compilation This entire implementation, from concept to compiled binary, took approximately 15 messages—a testament to the assistant's ability to rapidly scaffold tooling when the architecture is well-understood.

The Subject Message: First Run of the Microbenchmark

Message 1021 is the first execution of this newly-built microbenchmark. The assistant issues a bash command:

echo "=== SmallVec cap=1 ==="
FIL_PROOFS_PARAMETER_CACHE=/data/zk/params /home/theuser/curio/extern/cuzk/target/release/cuzk-bench synth-only --c1 /data/32gbench/c1.json --iterations 3 2>&1

Several details in this command reveal the assistant's reasoning and assumptions:

The choice of "cap=1": The assistant had previously changed the INDEXER_INLINE_CAP constant from 4 (the original SmallVec implementation) to 1. This decision was driven by a cache-line alignment hypothesis developed across messages 991–995. The reasoning was meticulous: on AMD Zen4 (the Threadripper PRO 7995WX system under test), each CPU cache line is 64 bytes. A (usize, Scalar) tuple is 40 bytes (8 + 32). With inline capacity 4, each SmallVec occupies 160 bytes of inline storage—three cache lines. With six Indexers created per enforce() call (three linear combinations × two Indexers each), the total inline data was approximately 960 bytes spread across 15 cache lines. The assistant hypothesized that this stack pressure was causing L1 data cache evictions in the tight synthesis loop. Reducing to cap=1 would shrink each Indexer to approximately 56 bytes (40 bytes inline + metadata), fitting in a single cache line, with the six Indexers totaling approximately 336 bytes—roughly 5–6 cache lines.

The --iterations 3 flag: Running three iterations provides a basic statistical sample. The assistant wanted to see consistency across runs, not just a single measurement that could be contaminated by cold caches, scheduling anomalies, or other transient effects.

The FIL_PROOFS_PARAMETER_CACHE environment variable: This points to a directory containing the Filecoin proving parameters (the structured reference string, or SRS). Even though the microbenchmark doesn't load SRS into GPU memory, the synthesis phase still needs access to certain parameter data for circuit construction.

The 2>&1 redirect: This merges stderr into stdout, capturing all log output. The synthesis function emits structured log messages via the log crate (visible in the output as the synthesize_porep_c2_batch INFO lines), and the assistant wanted to capture these alongside any timing output.

The Output and Its Significance

The microbenchmark output reveals the initial moments of the synthesis run:

=== SmallVec cap=1 ===
=== Synthesis Microbenchmark ===
c1:         /data/32gbench/c1.json
sector:     1 (miner 1000)
partition:  all
iterations: 3

c1 loaded:  51510727 bytes
2026-02-17T23:08:57.750628Z  INFO synthesize_porep_c2_batch{job_id="synth-bench-0"}: building circuits for all partitions (parallel) num_partitions=10

The C1 file (the output of the Phase 1 proof computation) is 51.5 MB—a substantial JSON payload containing the intermediate state needed for Phase 2 synthesis. The synthesis processes 10 partitions in parallel, leveraging rayon's work-stealing thread pool. The log timestamp shows the run started at 23:08:57.

The full output is truncated in the conversation data (indicated by ...), but from the chunk summary and subsequent messages, we know the result: SmallVec cap=1 produced synthesis times of approximately 59.5–59.9 seconds, very consistent across the three iterations. This was actually slower than the original Vec baseline of approximately 54.5 seconds, confirming that SmallVec—regardless of inline capacity—was causing a 5–6 second regression.

Assumptions and Their Validity

The assistant operated under several key assumptions in this message:

Assumption 1: The microbenchmark would produce reliable, reproducible timing data. This proved correct—the three iterations showed remarkable consistency (59.5–59.9s), giving confidence that the measurement was stable and the regression was real.

Assumption 2: Cap=1 would mitigate the regression by reducing stack frame size. This assumption was incorrect. While cap=1 did reduce the inline storage per Indexer from 160 bytes to 40 bytes, the synthesis time remained essentially identical to cap=4 (approximately 60 seconds). The cache-line hypothesis, while intellectually elegant, did not explain the regression. The actual cause would require deeper investigation with hardware performance counters.

Assumption 3: The SmallVec optimization would be "pure win" for SHA-256 circuits where most linear combinations have 1–3 terms. This assumption was also incorrect. The Vec baseline was faster despite performing heap allocations for every linear combination. This suggests that on the AMD Zen4 platform, jemalloc's thread-local allocation is extremely fast (the assistant noted 10–15 ns per allocation), and the pointer chase to heap data hits L2 cache (512 KB per core on Zen4) rather than suffering a full memory stall. The Vec path's smaller stack footprint may allow better L1 cache utilization for the synthesis algorithm's working set.

Assumption 4: The build system would correctly propagate the bellpepper-core change through the dependency chain. This assumption was validated—the build log shows bellpepper-core recompiling and cascading through bellperson, storage-proofs-*, filecoin-proofs, cuzk-core, and finally cuzk-bench.

Input Knowledge Required

To fully understand this message, a reader needs knowledge of:

  1. The cuzk project architecture: The pipelined SNARK proving engine with separate synthesis (CPU) and proving (GPU) phases, the cuzk-bench testing utility, and the cuzk-core library containing the synthesis pipeline.
  2. Groth16 proof generation: The two-phase proving protocol (Phase 1: circuit-specific computation, Phase 2: generic proof generation), the role of the C1 intermediate output, and the structure of constraint systems with linear combinations (a, b, c vectors).
  3. SmallVec semantics: The smallvec crate provides a SmallVec type that stores up to N elements inline and spills to heap allocation only when the collection exceeds the inline capacity. This avoids heap allocations for small collections but increases the struct's stack size.
  4. CPU cache architecture: Cache line size (64 bytes on x86-64), L1/L2/L3 cache hierarchy, and how stack frame size affects cache utilization in tight loops.
  5. AMD Zen4 characteristics: The Threadripper PRO 7995WX has 32 KB L1d cache per core, 512 KB L2 per core, and large shared L3. Its memory subsystem includes aggressive prefetchers and a fast jemalloc implementation.
  6. The Filecoin PoRep circuit: SHA-256-based proof-of-replication circuits where linear combinations typically have 1–3 terms, making them candidates for SmallVec optimization.

Output Knowledge Created

This message produced several valuable pieces of knowledge:

  1. A validated microbenchmark tool: The synth-only subcommand in cuzk-bench now exists as a reusable tool for rapid A/B testing of synthesis optimizations. It eliminates the 30+ second overhead of SRS loading and GPU proving from each measurement cycle.
  2. The first data point in the SmallVec investigation: SmallVec cap=1 yields ~59.5–59.9s synthesis time, establishing that the regression is not sensitive to inline capacity.
  3. Confirmation that SmallVec is the culprit: Combined with the Vec baseline test that followed (message 1025, showing ~54.5s), this message proves that SmallVec causes a 5–6 second regression regardless of capacity setting.
  4. A refined research question: The regression is not about stack frame size or cache line alignment. The next investigation would need to examine hardware-level metrics—cache misses, branch mispredictions, instruction-level parallelism—to understand why a data structure that eliminates heap allocations is slower.

The Thinking Process

The assistant's reasoning throughout this sequence demonstrates several hallmarks of expert performance engineering:

Hypothesis-driven experimentation: Rather than randomly tweaking parameters, the assistant formed a specific hypothesis (cache line alignment → L1 evictions → regression) and designed an experiment to test it (cap=1 reduces inline storage to one cache line per SmallVec).

Building the right measurement tool first: Before running any more end-to-end tests, the assistant invested effort in building a microbenchmark that isolates the variable of interest. This is a classic optimization principle: measure the right thing, eliminate confounding factors, iterate fast.

Layered diagnosis: The regression was first observed at the system level (106s vs 88.9s). Instrumentation narrowed it to the synthesis phase (60.3s vs 54.7s). The microbenchmark then isolated it to the SmallVec change specifically. Each layer of diagnosis reduced the search space.

Awareness of platform characteristics: The assistant explicitly considered AMD Zen4's cache hierarchy, jemalloc performance, and prefetching capabilities. The note that "Vec on Zen3+ may not be as slow as the proposal assumed" shows an understanding that optimization advice must be contextualized to the target hardware.

Intellectual honesty: When the cache-line hypothesis proved insufficient (cap=1 didn't help), the assistant didn't double down. Instead, it prepared to gather perf stat hardware counters to understand the actual mechanism. This willingness to let data override theory is essential in performance work.

Conclusion

Message 1021 represents a pivotal moment in a disciplined performance engineering investigation. It is the first execution of a purpose-built microbenchmark that isolates circuit synthesis from the rest of the proof generation pipeline. The data produced—that SmallVec causes a 5–6 second regression regardless of inline capacity—overturned the elegant cache-line hypothesis and pointed toward a more subtle performance interaction. The message embodies the principle that in optimization work, the most important tool is not a clever idea but a precise measurement. By building the right instrument and using it to test a specific hypothesis, the assistant transformed a vague "synthesis is slow" observation into a concrete, actionable finding: SmallVec itself is the problem, and the solution lies not in tuning its capacity but in understanding why it underperforms Vec on this particular architecture.