Go Benchmarking & Performance Measurement

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: Reason as thoroughly as possible for benchmark analysis, profile interpretation, and performance comparison tasks — deep reasoning prevents misinterpreting profiling data and ensures statistically sound conclusions. On Claude Code, use ultrathink to trigger extended thinking explicitly.

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

File and Ordering Conventions

Benchmark functions live in a _bench_test.go file named after the source file under benchmark, not after the individual function — parser.go -> parser_bench_test.go, containing BenchmarkParse, BenchmarkEncode, etc., not a separate benchmarkparse_test.go per function.

• Keeping benchmarks in their own file (instead of mixed into parser_test.go) keeps go test -bench=. ./pkg/parser output free of unrelated Test* noise.
• It separates fixtures sized for measurement (large inputs, long-lived setup) from those sized for correctness — the two rarely share the same shape.
• The file still follows Go's one-test-file-per-source-file convention (→ See samber/cc-skills-golang@golang-testing skill), just with the _bench suffix marking its narrower purpose.

Order Benchmark* functions inside parser_bench_test.go to mirror the order of the functions/methods they measure in parser.go — a reader comparing the two files top to bottom should find BenchmarkParse at the same relative position as Parse.

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.