When perf stat Fails: A Micro-Optimization Detective Story

Introduction

In the high-stakes world of zero-knowledge proof generation, every millisecond counts. The SUPRASEAL_C2 Groth16 pipeline for Filecoin's Proof-of-Replication (PoRep) is a computational behemoth, consuming ~200 GiB of peak memory and running for over a minute even on high-end hardware. When the team behind the cuzk proving engine embarked on Phase 4 of their optimization campaign, they turned their attention to CPU synthesis — the circuit construction and R1CS witness generation that runs before GPU proving. This is where message 1055 enters the story: a single, seemingly trivial perf stat command that failed, yet reveals the depth of reasoning, the precision of the optimization methodology, and the unforgiving nature of hardware-level performance analysis.

The Message in Full

The message, authored by the AI assistant in the opencode session, reads as follows:

Good. The plan: run perf stat with SmallVec cap=2 (current), then revert to Vec, rebuild, and run again. Using --partition 0 -i 1 for a single fast ~6s run, focusing on the key counters.

>

Let me first run SmallVec cap=2 under perf stat: `` perf stat -e instructions,cycles,cache-references,cache-misses,branch-instructions,branch-misses,l2_cache_req_stat.dc_access_in_l2,l2_cache_req_stat.dc_hit_in_l2,l2_cache_req_stat.dc_miss_in_l2,ls_l1_d_tlb_miss.all_l2_miss -- env FIL_PROOFS_PARAMETER_CACHE=[REDACTED] [REDACTED]/curio/extern/cuzk/target/release/cuzk-bench synth-only --c1 [REDACTED]/32gbench/c1.json --partition 0 -i 1 2>&1 | tee /tmp/perf-smallvec-cap2-run1.txt ``

>

The command fails with: ``` event syntax error: '.._hit_in_l2,l2_cache_req_stat.dc_miss_in_l2,ls_l1_d_tlb..' \___ Bad event name

>

Unable to find event on a PMU of 'l2_cache_req_stat.dc_miss_in_l2' ```

At first glance, this is just a failed shell command — a typo in a PMU event name. But beneath the surface lies a carefully crafted experiment designed to resolve one of the most stubborn performance mysteries in the entire optimization campaign.

Why This Message Was Written: The Reasoning and Motivation

To understand why the assistant issued this particular perf stat command, we must trace back through the optimization history. The Phase 4 effort had already implemented several promising changes: a SmallVec optimization (A1) that replaced Vec<(usize, T)> with SmallVec<[(usize, T); 2]> in the LinearCombination indexer, pre-sizing for ProvingAssignment (A2), parallelized B_G2 CPU MSMs (A4), pinned a/b/c vectors with cudaHostRegister (B1), and per-MSM window tuning (D4). Yet the microbenchmarks told a troubling story: the SmallVec change (A1), which should have improved cache behavior by reducing pointer chasing and heap allocations, was actually slower than the original Vec implementation on the Zen4 test system.

This was deeply counterintuitive. SmallVec is a well-known optimization in the Rust ecosystem — it stores a small number of elements inline on the stack, avoiding heap allocation for small collections. For the LinearCombination indexer, which typically holds 1–2 entries, SmallVec should have been a clear win. The fact that it regressed demanded an explanation.

The assistant's hypothesis was that Zen4's cache hierarchy was somehow penalizing the SmallVec layout. Perhaps the inline storage was causing more L1 data cache misses, or the branch logic for SmallVec's dynamic dispatch (inline vs. heap) was causing branch mispredictions. Alternatively, the larger struct size of SmallVec (which must carry both inline storage and a discriminant) could be increasing cache pressure in the hot loop. These are the kinds of subtle, hardware-dependent effects that can only be diagnosed with hardware performance counters — hence the perf stat command.

The Decision-Making Process

The message reveals a clear experimental design. The assistant made several deliberate choices:

Choice of workload: --partition 0 -i 1 — running a single iteration of partition 0 only. This was chosen to keep each run short (~6 seconds) while still exercising the hottest synthesis code. A full 10-partition run would take ~55 seconds, making iterative A/B testing impractical.

Choice of counters: The event list was carefully curated to test the cache-pressure hypothesis. The assistant selected:

Assumptions Made

The message rests on several assumptions, some explicit and some implicit:

Assumption 1: Zen4 PMU event names are correct. The assistant assumed that l2_cache_req_stat.dc_miss_in_l2 and ls_l1_d_tlb_miss.all_l2_miss were valid event names on the target Zen4 system. This turned out to be incorrect — the l2_cache_req_stat.dc_miss_in_l2 event does not exist on this PMU, and the ls_l1_d_tlb_miss.all_l2_miss event may have a different naming convention. The perf list output shown in earlier messages (msg 1053) listed many Zen4 events, but the assistant may have extrapolated from documentation or prior experience rather than verifying each event name against the running kernel.

Assumption 2: The synth-only microbenchmark is representative. The assistant assumed that a single-partition, single-iteration run of the synthesis microbenchmark would faithfully reproduce the cache behavior of the full pipeline. This is reasonable but not guaranteed — cache behavior can be sensitive to working set size, and a partial run may not warm the caches in the same way.

Assumption 3: Hardware counters will reveal the root cause. The assistant implicitly assumed that the performance difference between SmallVec and Vec would be visible in cache miss rates, branch misprediction rates, or TLB miss rates. It's possible that the difference stems from a different mechanism entirely — perhaps instruction count differences (SmallVec may require more instructions per access due to its branching), or pipeline stalls from the SmallVec discriminant check.

Assumption 4: The perf stat tool is available and functional. The assistant assumed that perf stat could run unprivileged with these hardware events. On many systems, hardware PMU counters require CAP_PERFMON or root privileges, or at least the kernel.perf_event_paranoid sysctl to be set appropriately. The error message, however, indicates an event name parsing issue rather than a permission problem.

The Mistake: A Single Typo

The immediate mistake is straightforward: l2_cache_req_stat.dc_miss_in_l2 is not a valid event name on this PMU. The error message points to the exact location: dc_miss_in_l2. Looking at the perf list output from msg 1053, we can see that the available L2 events include names like l2_cache_req_stat.dc_access_in_l2 and l2_cache_req_stat.dc_hit_in_l2, but there is no dc_miss_in_l2 variant. The miss event may be named differently (e.g., l2_cache_req_stat.dc_miss without the _in_l2 suffix), or it may not be exposed as a named event at all.

This is a classic performance analysis pitfall: PMU event naming varies between microarchitectures, kernel versions, and even hardware revisions. The Zen4 core (likely a Ryzen 7000-series or EPYC 9004-series) uses a different event encoding than Zen3 or Intel systems. The assistant had consulted perf list earlier but apparently did not verify every event name before constructing the command.

The ls_l1_d_tlb_miss.all_l2_miss event also appears problematic — the error message truncates the event list at that point, suggesting the parser stopped after the first error. It's possible that event would also have failed.

Input Knowledge Required

To understand this message, a reader needs:

  1. Knowledge of the optimization campaign: The message is the latest step in Phase 4 of the cuzk optimization effort. Previous phases established the baseline pipeline (Phase 2) and cross-sector batching (Phase 3). Phase 4 focuses on compute-level micro-optimizations, with A1 (SmallVec) being the most controversial.
  2. Understanding of SmallVec vs Vec tradeoffs: SmallVec is a Rust crate that provides a small-vector optimization — elements are stored inline on the stack up to a capacity threshold, then spill to the heap. This reduces allocation overhead but increases struct size and adds a branch to every access. On cache-constrained hot loops, the tradeoff can go either way.
  3. Familiarity with perf stat and PMU counters: The Linux perf subsystem provides access to hardware performance monitoring units. Each CPU microarchitecture exposes different event names. The -e flag accepts comma-separated event names or raw event codes.
  4. Context about the synthesis pipeline: The synth-only subcommand of cuzk-bench runs only the CPU synthesis step — circuit construction and R1CS witness generation — without the GPU prover, SRS loading, or daemon infrastructure. This isolates the CPU hotpath for A/B testing.
  5. Knowledge of the Zen4 microarchitecture: AMD's Zen4 core has a 32KB L1 data cache, 1MB L2 cache per core, and a shared L3 cache. Its PMU exposes events under names like l2_cache_req_stat.* and ls_l1_d_tlb_miss.*.

Output Knowledge Created

Even in failure, this message produces valuable knowledge:

  1. Negative result: The event l2_cache_req_stat.dc_miss_in_l2 does not exist on this Zen4 PMU. Future attempts must use a different event name or raw event codes.
  2. Experimental design documented: The plan for a paired SmallVec vs Vec comparison under perf stat is established. The specific workload (--partition 0 -i 1) and counter selection are recorded, making the experiment reproducible.
  3. Validation of the methodology: The assistant's approach — formulate a hypothesis, design a measurement, execute, analyze — is sound. The failure is in the execution (event name), not the reasoning.
  4. Trace of the optimization narrative: This message captures the moment where the optimization effort pivots from "implement changes and measure wall time" to "understand why a change regresses using hardware counters." This is a maturation of the optimization methodology.

The Thinking Process Visible in the Message

The message reveals the assistant's reasoning in several ways:

The plan is stated before execution: "The plan: run perf stat with SmallVec cap=2 (current), then revert to Vec, rebuild, and run again." This shows deliberate experimental design — the assistant is not just trying random commands but following a structured investigation.

The workload choice is justified: "Using --partition 0 -i 1 for a single fast ~6s run." The assistant explicitly considers the tradeoff between measurement accuracy (longer runs give more stable counters) and iteration speed (shorter runs enable faster A/B cycles).

The counter selection targets specific hypotheses: The inclusion of L2 cache events and DTLB miss events reveals the assistant's theory that SmallVec's larger struct size is causing cache or TLB pressure. The branch miss counters test the alternative theory that SmallVec's dynamic dispatch is causing branch mispredictions.

The error is accepted and reported: The assistant does not attempt to retry with different event names in the same message. The error is captured via 2>&1 | tee and presented as-is. This suggests the assistant intends to diagnose the event name issue and retry in a subsequent message — which is exactly what happens in the following messages (msg 1056+).

Conclusion

Message 1055 is a perfect microcosm of performance engineering at the hardware level. A single command, a single typo, and yet the message is dense with meaning: the experimental design, the hypotheses being tested, the assumptions about the hardware, and the methodology for resolving a stubborn performance regression. The failed perf stat command is not a mistake — it is a data point. It tells us that Zen4 PMU event naming is tricky, that the assistant is working methodically through a hypothesis tree, and that the optimization of the SUPRASEAL_C2 synthesis pipeline is proceeding with scientific rigor.

In the broader narrative of the cuzk optimization campaign, this message marks the transition from "what" (which changes improve performance) to "why" (what hardware-level mechanism causes the improvement or regression). This deeper understanding is what separates superficial optimization from genuine performance engineering. The typo in the event name is a minor setback; the methodology is sound, and the investigation will continue in the next message with corrected event names or raw event codes.