How do you write and interpret Go benchmarks?

Go's testing package has built-in benchmark support. Benchmarks are functions with signature func BenchmarkXxx(b *testing.B) and run with go test -bench=.. The framework automatically calibrates the number of iterations.

// Benchmark function
func BenchmarkJSONMarshal(b *testing.B) {
    user := User{ID: 1, Name: "Alice", Email: "alice@example.com"}

    b.ResetTimer() // exclude setup time from measurement
    for i := 0; i < b.N; i++ { // b.N is auto-calibrated
        _, err := json.Marshal(user)
        if err != nil {
            b.Fatal(err)
        }
    }
}

// Memory allocations benchmark
func BenchmarkStringBuilder(b *testing.B) {
    words := []string{"hello", "world", "foo", "bar"}

    b.ReportAllocs() // show allocations in output
    b.ResetTimer()
    for i := 0; i < b.N; i++ {
        var sb strings.Builder
        for _, w := range words {
            sb.WriteString(w)
            sb.WriteByte(' ')
        }
        _ = sb.String()
    }
}

// Run benchmarks:
// go test -bench=. -benchmem ./...
// -benchmem: show memory allocations
// -benchtime=5s: run for 5 seconds instead of default 1s
// -count=5: run 5 times for stable results

// Output:
// BenchmarkJSONMarshal-8   1234567   987 ns/op   256 B/op   3 allocs/op
// Columns: name, iterations, ns per op, bytes per op, allocs per op

// Compare benchmarks with benchstat:
// go test -bench=. -count=5 > before.txt
// # make changes
// go test -bench=. -count=5 > after.txt
// benchstat before.txt after.txt  # shows % improvement and p-value

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