The Art of Measurement: Tracing a Zen4 Performance Anomaly Through Hardware Counters

In the high-stakes world of Filecoin proof generation, every second of synthesis time translates directly into operational cost. The cuzk proving engine, a specialized Groth16 prover for Filecoin's Proof-of-Replication (PoRep) protocol, had already undergone three phases of architectural optimization — from pipelined synthesis to cross-sector batching — and was now deep into Phase 4: compute-level micro-optimizations. But one optimization had gone unexpectedly wrong, and the debugging process had led to a pivotal moment captured in a single, deceptively simple message.

The Message

The subject message ([msg 1054]) reads:

Excellent — AMD Zen4 has rich per-core PMU events. Let me check the synth-only subcommand to understand its options, then run the perf stat comparison.

``bash FIL_PROOFS_PARAMETER_CACHE=/data/zk/params extern/cuzk/target/release/cuzk-bench synth-only --help 2>&1 In-process synthesis microbenchmark (requires synth-bench` feature).

Runs only the CPU synthesis step (circuit construction + R1CS witness generation) without daemon, GPU, or SRS. Useful for A/B testing bellpepper-core / bellperson changes.

Usage: cuzk-bench synth-only [OPTIONS] --c1 <C1>

Options: --c1 <C1> Path to C1 output JSON

--sector-num <SECTOR_NUM> Sector number

[default: 1]

--miner-id <MINER_ID> Miner ID ... ```

On the surface, this is nothing more than a developer reading a help screen. But in context, this message represents a critical inflection point in a deeply technical investigation — the moment when the team shifted from implementing optimizations to understanding why one of them had failed.

The Context: A Performance Regression That Defied Intuition

The story begins with Optimization A1: replacing Rust's standard Vec with SmallVec in the LinearCombination data structure within bellpepper-core. SmallVec is a well-known optimization technique that stores a small number of elements inline (on the stack) rather than heap-allocating, which should reduce allocation pressure and improve cache locality. The change seemed straightforward — set an inline capacity of 2 (the most common number of terms in a linear combination) and let the small vector optimization handle the rest.

But microbenchmarks told a different story. On the AMD Zen4 architecture powering the test system, SmallVec was consistently slower than plain Vec in every configuration tested — cap=1, cap=2, cap=4. This was counterintuitive. SmallVec is widely used in Rust ecosystems precisely to avoid heap allocations for small collections. Why would it regress on Zen4?

The assistant had already formulated a hypothesis in [msg 1050]: "Zen4 cache pressure." The idea was that Zen4's large L2 cache (1 MiB per core) and aggressive prefetchers might handle the Vec heap allocations just fine, while SmallVec's inline storage could be causing subtle cache issues — perhaps by increasing the size of the LinearCombination struct itself, reducing the number of objects that fit in a cache line, or by interacting poorly with Zen4's particular prefetching algorithms.

But a hypothesis is not evidence. To confirm it, the assistant needed hardware performance counter data.

Why This Message Matters

The message captures the precise moment when the investigation transitions from speculation to measurement. The assistant has just spent several messages ([msg 1051] through [msg 1053]) probing the available perf events on the Zen4 system, discovering that AMD provides rich per-core Performance Monitoring Unit (PMU) events — L1 and L2 cache access statistics, TLB miss counters, and fill-source tracking that reveals where cache lines are being fetched from.

Now, in message 1054, the assistant checks the synth-only --help output. This is a preparatory step: before constructing a perf stat command with dozens of hardware event counters, one must know the exact flags and options the benchmark tool accepts. The synth-only subcommand is a custom microbenchmark built specifically for this optimization work — it runs only the CPU synthesis step (circuit construction and R1CS witness generation) without touching the GPU, the daemon process, or loading SRS parameters. This isolation is crucial for A/B testing because it eliminates confounding variables. If you're measuring the effect of a change to LinearCombination's internal storage, you don't want GPU kernel launches or disk I/O polluting your measurements.

The help output confirms the key options: --c1 (path to the circuit parameters), --partition (which partition to synthesize), --sector-num and --miner-id (to construct the correct public inputs). The assistant already knows these from the earlier planning in [msg 1050], where the plan was to use --partition 0 for fast ~6-second runs. But verifying the interface before executing is a mark of disciplined engineering — one wrong flag could invalidate an entire measurement session.

The Reasoning Process Visible in the Message

The assistant's opening line — "Excellent — AMD Zen4 has rich per-core PMU events" — reveals the thinking that led to this moment. The previous messages had been a frustrating search for working perf events. In [msg 1058], the assistant tried l3_cache_accesses and got a syntax error. In [msg 1060], it tried again and got another error. In [msg 1061], it tried the tracepoint syntax l3_cache:l3_cache_accesses and got "unknown tracepoint." By [msg 1062], even the metric expression -M l3_misses failed. Each failure narrowed the set of usable events.

Then in [msg 1063], the assistant discovered ls_dmnd_fills_from_sys and ls_any_fills_from_sys — AMD-specific events that track where cache lines are filled from (local L2, local CCX, DRAM, remote). These events are precisely what's needed to test the "cache pressure" hypothesis. If SmallVec is causing more L2 misses or more DRAM fills compared to Vec, the fill-source counters will reveal it.

So the assistant's "Excellent" is not just enthusiasm — it's the relief of finding the right tools after a search. The check of synth-only --help is the final preparatory step before assembling the full perf stat command with those newly discovered events.

Assumptions and Decisions

Several assumptions underpin this message:

Assumption 1: The synth-only microbenchmark is a valid proxy for full proof generation. The assistant assumes that synthesis time in isolation correlates with overall proof generation time. This is reasonable — synthesis is the dominant CPU cost in Groth16 proving — but it's worth noting that the microbenchmark excludes GPU time, SRS loading, and cross-sector batching overhead. Any optimization that improves synthesis at the expense of these other phases could still be a net loss.

Assumption 2: Hardware performance counters will reveal the root cause. The assistant is betting that the SmallVec regression has a measurable hardware-level signature — more cache misses, higher TLB pressure, or worse branch prediction. If the regression is caused by something else (e.g., a compiler optimization difference, or a subtle change in memory ordering), the perf counters might not tell the full story.

Assumption 3: The Zen4 PMU events are reliable. AMD's PMU events, especially the more exotic ones like ls_dmnd_fills_from_sys.local_ccx, are not always perfectly documented or stable across microarchitectures. The assistant is trusting that these events measure what they claim to measure.

Decision: Use --partition 0 for fast runs. The assistant chooses a single partition (out of potentially many) to keep each run under 10 seconds. This enables multiple runs for statistical confidence but assumes that partition 0 is representative of all partitions. If partition 0 has a different computational profile (e.g., fewer SHA-256 gadgets, or a different ratio of constraints to variables), the results might not generalize.

Input Knowledge Required

To understand this message, one needs:

Knowledge of the cuzk project's architecture: That cuzk-bench is a custom benchmarking tool, that synth-only is a subcommand that runs CPU synthesis in isolation, and that synthesis is the process of building the R1CS circuit and computing witness values.
Knowledge of the optimization taxonomy: That "A1" refers to the SmallVec optimization in bellpepper-core/src/lc.rs, that it was found to be slower than Vec, and that the team is now investigating why.
Knowledge of AMD Zen4 microarchitecture: That Zen4 has a 1 MiB L2 cache per core, aggressive prefetchers, and a particular cache hierarchy that might interact differently with stack-allocated vs. heap-allocated data.
Knowledge of perf and PMU events: That perf stat -e can measure hardware events, that AMD-specific events like ls_dmnd_fills_from_sys track cache fill sources, and that these counters can diagnose cache pressure.
Knowledge of the Filecoin PoRep context: That C2 proof generation is the second stage of a two-stage proving process, that it involves Groth16 proofs over large circuits (~200 GiB peak memory), and that synthesis is the CPU-intensive phase of building the constraint system.

Output Knowledge Created

This message produces several forms of knowledge:

Confirmation of the tool interface: The help output confirms that --c1, --partition, --sector-num, and --miner-id are the relevant flags, and that the tool requires the synth-bench feature flag at compile time.
Documentation of the microbenchmark's purpose: The help text explicitly states that this is "Useful for A/B testing bellpepper-core / bellperson changes," which validates the assistant's approach of using it for the SmallVec investigation.
A record of the investigation methodology: The message, combined with its surrounding context, documents a reproducible process for diagnosing performance regressions: form a hypothesis, build an isolated microbenchmark, identify relevant hardware counters, and measure systematically.
A boundary marker: This message sits at the boundary between planning and execution. After this, the assistant will run the actual perf stat comparisons ([msg 1064] and [msg 1065] for SmallVec, then [msg 1075] for Vec after reverting). The help check is the last planning step before the measurement campaign begins.

The Broader Significance

What makes this message noteworthy is not its content but its position in the workflow. It exemplifies a fundamental principle of performance engineering: measure before you optimize, and measure again when optimization surprises you.

The SmallVec regression was a surprise. Conventional wisdom says small-vector optimizations are almost always beneficial, especially for a data structure like LinearCombination that typically holds 1–3 terms. But conventional wisdom is architecture-dependent. Zen4's particular cache hierarchy, prefetching behavior, and memory subsystem might make heap-allocated Vec the better choice — or the regression might be caused by something entirely different, like increased struct size causing fewer objects per cache line, or a compiler optimization that fires for Vec but not for SmallVec.

The assistant's response to this surprise is textbook good practice: don't just revert and move on; investigate. Collect evidence. Understand why the expected behavior didn't materialize. This knowledge is valuable even if the final decision is to revert — it might inform future optimization choices, or it might reveal a fixable issue with how SmallVec is being used (perhaps a different inline capacity, or a different allocation strategy).

In the end, the perf stat data would show that SmallVec actually reduced instruction count slightly but hurt instruction-level parallelism (IPC), and that the interleaved eval optimization introduced in the same round was the main culprit for the IPC regression. The SmallVec change itself would be reverted, but not before the team gained a deeper understanding of how their code interacts with the Zen4 microarchitecture.

This message, for all its apparent simplicity, is the hinge point of that entire investigation — the moment when speculation gave way to measurement, and the team committed to understanding their hardware as deeply as they understood their software.