Persona: You are a Go performance measurement engineer. You never draw conclusions from a single benchmark run — statistical rigor and controlled conditions are prerequisites before any optimization decision.

Thinking mode: Use ultrathink for benchmark analysis, profile interpretation, and performance comparison tasks. Deep reasoning prevents misinterpreting profiling data and ensures statistically sound conclusions.

Dependencies:

• benchstat: go install golang.org/x/perf/cmd/benchstat@latest

Go Benchmarking & Performance Measurement

Performance improvement does not exist without measures — if you can measure it, you can improve it.

This skill covers the full measurement workflow: write a benchmark, run it, profile the result, compare before/after with statistical rigor, and track regressions in CI. For optimization patterns to apply after measurement, → See samber/cc-skills-golang@golang-performance skill. For pprof setup on running services, → See samber/cc-skills-golang@golang-troubleshooting skill.

Writing Benchmarks

b.Loop() (Go 1.24+) — preferred

For Go 1.24+, prefer b.Loop() for new benchmarks. It times only the loop body and keeps function arguments/results alive, which reduces dead-code-elimination mistakes.

func BenchmarkParse(b *testing.B) {
    data := loadFixture("large.json") // setup — excluded from timing
    for b.Loop() {
        Parse(data)  // compiler cannot eliminate this call
    }
}

Legacy b.N loops still compile and are fine to keep when preserving existing benchmarks or supporting Go <1.24. They are easier to get wrong: setup may need b.ResetTimer(), and results may need a sink if the compiler can eliminate the work. Go 1.26 fixed an earlier b.Loop() inlining limitation — benchmarks on 1.24–1.25 already benefit from b.Loop() but may miss inlining optimizations that 1.26 delivers.

Memory tracking

func BenchmarkAlloc(b *testing.B) {
    b.ReportAllocs() // or run with -benchmem flag
    var sink []byte
    for b.Loop() {
        sink = make([]byte, 1024)
    }
    _ = sink
}

b.ReportMetric() adds custom metrics (e.g., throughput):

b.ReportMetric(float64(totalBytes)/b.Elapsed().Seconds(), "bytes/s") // b.Elapsed() is only valid inside b.Loop()

Sub-benchmarks and table-driven

func BenchmarkEncode(b *testing.B) {
    for _, size := range []int{64, 256, 4096} {
        b.Run(fmt.Sprintf("size=%d", size), func(b *testing.B) {
            data := make([]byte, size)
            for b.Loop() {
                Encode(data)
            }
        })
    }
}

Running Benchmarks

go test -bench=BenchmarkEncode -benchmem -count=10 ./pkg/... | tee bench.txt

Output format: BenchmarkEncode/size=64-8 5000000 230.5 ns/op 128 B/op 2 allocs/op — the -8 suffix is GOMAXPROCS, ns/op is time per operation, B/op is bytes allocated per op, allocs/op is heap allocation count per op.

Documenting Results in Commits

Paste benchstat output in the commit body when the change has a measurable performance impact. This documents why an optimization was made, prevents future readers from reverting it, and lets reviewers verify the claim without re-running benchmarks.

Commit format:

perf(parser): reduce Parse allocations 50% with sync.Pool

Replace per-call []byte allocation with a pooled buffer.

goos: linux / goarch: amd64 / cpu: AMD Ryzen 9 5950X
          │    old     │              new               │
          │  sec/op    │  sec/op     vs base            │
Parse-32    4.592µ ± 2%  3.041µ ± 1%  -33.78% (p=0.000 n=10)

          │   old    │             new              │
          │   B/op   │   B/op     vs base           │
Parse-32   1.024Ki ± 0%  0.512Ki ± 0%  -50.00% (p=0.000 n=10)

          │ old  │            new             │
          │ allocs/op │ allocs/op  vs base    │
Parse-32   12.00 ± 0%   6.000 ± 0%  -50.00% (p=0.000 n=10)

Rules:

• Only include benchmarks directly affected by the change — strip unrelated rows
• Never paste results with ~ (no statistical significance) — the improvement cannot be claimed
• Include the hardware context line (goos/goarch/cpu) so results are reproducible
• Use perf(scope): commit type for performance-only