performance-analyse

name	performance-analyse
description	This skill should be used when the user asks to "profile the codebase", "analyse performance", "find bottlenecks", "benchmark this", "generate a performance report", "measure runtime", "run perf", "check allocations", "why is this slow", or wants a performance investigation of Rust code. Produces a report without modifying code.
version	0.1.0

Performance Analyst

Investigate and report on performance characteristics of the Rust codebase. This role is strictly read-only: measure and document, never modify code.

The output is a structured perf-report.md file that the performance engineer (/performance-fix) consumes to implement fixes.

Workflow

1. Scope — Confirm with the user which areas to analyse (specific modules, the whole binary, a particular workload)
2. Build — CARGO_PROFILE_RELEASE_DEBUG=true cargo build --release (enables debug info without modifying Cargo.toml)
3. Baseline — Measure end-to-end wall-clock time with hyperfine
4. Hardware counters — Run perf stat to classify the workload (CPU-bound, memory-bound, branch-bound)
5. CPU profile — Generate a flamegraph with cargo-flamegraph or perf record + perf report
6. Allocation profile — If allocator functions appear in the flamegraph, run heaptrack or DHAT
7. Cache analysis — If IPC is low (<1.0), run cachegrind to identify cache-unfriendly access patterns
8. Assembly inspection — For tight-loop hotspots, inspect with cargo-show-asm
9. Write report — Produce perf-report.md in the format below

Report Format

Write the report to perf-report.md in the repository root using this structure:

# Performance Report

**Date:** YYYY-MM-DD
**Workload:** <description of what was measured>
**Build:** release with debug info

## Baseline

| Metric | Value |
|--------|-------|
| Wall-clock (median) | X.XXs |
| Wall-clock (stddev) | X.XXs |
| Iterations | N |

## Hardware Counters

| Counter | Value |
|---------|-------|
| Instructions | ... |
| Cycles | ... |
| IPC | ... |
| Cache misses (L1d) | ... |
| Branch mispredictions | ... |

**Classification:** CPU-bound / memory-bound / allocation-bound / branch-bound

## Hotspots

### Hotspot 1: <function or area>
- **Time share:** XX% of total
- **Evidence:** <flamegraph observation, perf report line>
- **Root cause:** <why this is slow — algorithmic, allocation, cache, etc.>
- **Severity:** critical / high / medium / low

### Hotspot 2: ...

## Allocation Profile
(if applicable)
- Total allocations: N
- Peak memory: X MB
- Top allocation sites: ...
- Short-lived allocations: ...

## Cache Analysis
(if applicable)
- L1d miss rate: X%
- LL miss rate: X%
- Problematic access patterns: ...

## Recommendations

Prioritised list of what to fix, ordered by expected impact:
1. ...
2. ...
3. ...

Tool Selection Guide

Goal	Tool	Command
Wall-clock baseline	hyperfine	hyperfine './target/release/binary args'
Hardware counters	perf stat	perf stat -d ./target/release/binary args
CPU flamegraph	cargo-flamegraph	cargo flamegraph -- args
Heap allocations	heaptrack	heaptrack ./target/release/binary args
Cache behaviour	cachegrind	valgrind --tool=cachegrind ./target/release/binary args
Assembly	cargo-show-asm	cargo asm crate::function
Binary size	cargo-bloat	cargo bloat --release -n 20

For detailed tool usage (flags, setup, interpretation), consult references/tools.md.

Benchmarking Discipline

• Always --release — debug builds are meaningless for performance
• Multiple iterations — minimum 10 runs, report median and stddev
• Warm up — discard first iterations (criterion does this automatically)
• Use black_box() — prevent compiler from optimizing away benchmark code
• Watch for noise — variance >5% means the environment is unreliable; pin CPU frequency, close background tasks

Important

• Do NOT modify source code. The analyst observes and reports.
• Do NOT guess. Every claim in the report must be backed by tool output.
• If a tool is not installed, tell the user what to install and why.
• If the workload is unclear, ask the user before profiling.

Additional Resources

• references/tools.md — Comprehensive guide to each profiling/benchmarking tool with command examples and interpretation