The Art of the Hardware Counter: Debugging a perf stat Command in the cuzk Optimization Pipeline

Introduction

In the middle of a deep optimization campaign targeting the SUPRASEAL_C2 Groth16 proof generation pipeline for Filecoin's Proof-of-Replication (PoRep), a seemingly mundane error halted progress. The assistant had just attempted to collect hardware performance counter data using perf stat to compare two implementations of a core data structure — SmallVec versus Vec — in the bellpepper-core library. The command failed with a cryptic error: Bad event name. Message [msg 1056] captures the assistant's immediate response: a quick probe of the system to discover which AMD-specific perf events are actually available. While brief, this message reveals the gritty, iterative reality of performance engineering — where high-level optimization strategies meet the unforgiving specificity of hardware profiling tools.

Why This Message Was Written

The message exists because the previous command (in [msg 1055]) failed. The assistant had constructed an elaborate perf stat invocation targeting a suite of hardware counters: instructions, cycles, cache references, cache misses, branch instructions, branch misses, L2 cache accesses (data and instruction), L2 cache hits, L2 cache misses, and L1 dTLB misses that also miss L2. This was part of a carefully planned investigation to understand why the SmallVec optimization (codenamed A1) — which stores small vectors inline rather than heap-allocating — was performing worse than plain Vec on an AMD Zen4 processor. The hypothesis was that SmallVec's inline storage was causing increased cache pressure, leading to more L1/L2 cache misses and potentially higher TLB pressure.

The error message from perf was unambiguous: l2_cache_req_stat.dc_miss_in_l2 was not a valid event on this PMU (Performance Monitoring Unit). The assistant could have tried guessing alternative event names, consulted documentation, or simply removed the offending event and rerun. Instead, it chose a more systematic approach: query the system directly for the available l2_cache_req_stat events. This decision reflects a disciplined debugging methodology — before proceeding further, establish what the hardware actually supports.

The Context: A Larger Optimization Narrative

To appreciate this message, one must understand its place in the broader optimization campaign. The cuzk project (a fork of the Curio Filecoin mining stack) had been progressively optimizing the Groth16 proof generation pipeline across multiple phases:

• Phase 1: Mapping the call chain and identifying bottlenecks, producing a background reference document with nine structural bottlenecks.
• Phase 2: Implementing a batch-mode pipeline with async overlap between CPU synthesis and GPU proving, achieving 1.27× throughput improvement.
• Phase 3: Cross-sector batching, achieving 1.46× throughput improvement.
• Phase 4 (current): Compute-level micro-optimizations, including SmallVec for the LC Indexer (A1), pre-sizing for ProvingAssignment (A2), parallelizing B_G2 CPU MSMs (A4), pinning a/b/c vectors with cudaHostRegister (B1), and per-MSM window tuning (D4). The SmallVec optimization (A1) was particularly intriguing. SmallVec is a Rust library that stores a small number of elements inline (on the stack) rather than heap-allocating a Vec. For the LC (LinearCombination) Indexer — a data structure used heavily during circuit synthesis — an inline capacity of 2 seemed like a natural fit, since most constraints involve only one or two variables. The expectation was that eliminating heap allocations would reduce overhead and improve performance. Yet microbenchmarks consistently showed SmallVec was slower than Vec on the AMD Zen4 test system. The perf stat comparison was designed to gather hardware-level evidence for why. The assistant needed cache miss rates, instruction counts, and branch prediction statistics to either confirm the cache-pressure hypothesis or reveal a different root cause.

Assumptions Made and Mistakes Encountered

The assistant made a reasonable but incorrect assumption: that the AMD Zen4 PMU events documented in kernel headers or online references would match the exact naming scheme on this particular system. The event l2_cache_req_stat.dc_miss_in_l2 — which would measure L2 data cache misses — seemed like a natural name following the pattern of l2_cache_req_stat.dc_access_in_l2 and l2_cache_req_stat.dc_hit_in_l2. However, the system's PMU simply did not expose this event under that name.

This is a common pitfall in hardware profiling. AMD's PMU events are architecture-specific and sometimes even kernel-version-specific. The Zen4 microarchitecture (codename "Zen 4" or "Ryzen 7000 series") has a rich set of performance events, but not all documented events are available through perf on every kernel configuration. The event l2_cache_req_stat.dc_miss_in_l2 might exist under a different name (e.g., as part of a combined counter or computed from other events), or it might require raw event codes rather than symbolic names.

The mistake was not in the choice of events to measure — those were well-justified for the hypothesis — but in assuming the symbolic names would work without verification. The assistant's response demonstrates the correct recovery: rather than continuing to guess, query the system for what is actually available.

Input Knowledge Required

To understand this message, the reader needs familiarity with several domains:

1. Linux perf tool: Understanding that perf stat collects hardware performance counter data, that events are specified with -e, and that symbolic event names must match the PMU's naming scheme.
2. AMD Zen4 microarchitecture: Knowledge that Zen4 has a Performance Monitoring Unit with specific events for L2 cache accesses (data and instruction), cache hits, fills, and various other microarchitectural events. The l2_cache_req_stat group tracks L2 cache request statistics.
3. The optimization context: Understanding that SmallVec vs Vec comparison is about inline storage vs heap allocation, and that cache behavior is a key differentiator between the two approaches.
4. The cuzk project: Awareness that this is a Filecoin mining stack optimizing Groth16 proof generation, and that the LC Indexer is a hot data structure during circuit synthesis.

Output Knowledge Created

The perf list command in this message produces a concrete list of available l2_cache_req_stat events:

• all — Total L2 cache requests
• dc_access_in_l2 — Data cache accesses in L2
• dc_hit_in_l2 — Data cache hits in L2
• ic_access_in_l2 — Instruction cache accesses in L2
• ic_dc_hit_in_l2 — Instruction/data cache hits in L2
• ic_dc_miss_in_l2 — Instruction/data cache misses in L2
• ic_fill_hit_s — Instruction cache fill hits (shared)
• ic_fill_hit_x — Instruction cache fill hits (exclusive)
• ic_fill_miss — Instruction cache fill misses
• ic_hit_in_l2 — Instruction cache hits in L2
• ls_rd_blk_c — Load-store read block code
• ls_rd_blk_cs — Load-store read block code/data
• ls_rd_blk_l_hit_s — Load-store read block L hit shared
• ls_rd_blk_l_hit_x — Load-store read block L hit exclusive Notably, dc_miss_in_l2 is absent from this list. This means the assistant cannot directly measure L2 data cache misses with a single symbolic event. Instead, it would need to compute misses as dc_access_in_l2 - dc_hit_in_l2, or use raw event codes, or rely on the generic cache-misses event (which typically measures last-level cache misses, not L2-specific misses). This output also reveals that the earlier event ls_l1_d_tlb_miss.all_l2_miss was not checked — the assistant would need to verify its availability separately. The output knowledge thus shapes the next iteration of the perf stat command, guiding which events can be used and which must be substituted or computed.

The Thinking Process Visible in the Reasoning

The assistant's reasoning is evident in the structure of the response. It does not panic, does not retry with random guesses, and does not abandon the profiling approach. Instead, it executes a targeted probe: perf list 2>&1 | grep 'l2_cache_req_stat\.'. This command filters the full list of available events to just those in the l2_cache_req_stat group, showing exactly what the PMU supports.

The thinking process follows a clear pattern:

1. Error recognition: The previous command failed because l2_cache_req_stat.dc_miss_in_l2 is invalid.
2. Hypothesis formation: The event name might be wrong, or the event might not exist on this system.
3. Systematic verification: Query the system for the actual event list rather than consulting external documentation or guessing.
4. Result interpretation: Compare the available events against what was attempted, identify the missing event, and plan the next iteration. This is classic debugging methodology — when a tool rejects a command, ask the tool what it accepts, then adjust accordingly.

Broader Significance

While [msg 1056] is only two commands long, it encapsulates a crucial lesson in performance optimization: hardware profiling is never straightforward. The gap between what you want to measure and what the system lets you measure is often bridged only through iterative probing and adaptation. The assistant's disciplined response — query first, guess later — is a model for how to navigate this complexity.

The message also highlights the importance of platform-specific knowledge in optimization work. A SmallVec optimization that works well on Intel's Ice Lake might regress on AMD's Zen4 due to different cache hierarchies, TLB designs, and branch predictor characteristics. Without the ability to measure these microarchitectural differences, optimization becomes guesswork. The perf stat comparison, once the correct events are identified, will provide the evidence needed to either confirm the cache-pressure hypothesis or redirect the investigation toward other factors like instruction count increases or branch mispredictions.

In the end, this small message — a simple grep of perf list output — is a testament to the patience and precision required in systems-level optimization. It is not glamorous, but it is essential.