How do you write and run benchmarks in Go?

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

// benchmark_test.go
func BenchmarkStringConcat(b *testing.B) {
    // b.N is set by the framework — run the measured code exactly b.N times
    for i := 0; i < b.N; i++ {
        s := ""
        for j := 0; j < 100; j++ {
            s += "x" // inefficient — baseline
        }
        _ = s
    }
}

func BenchmarkStringBuilder(b *testing.B) {
    for i := 0; i < b.N; i++ {
        var sb strings.Builder
        for j := 0; j < 100; j++ {
            sb.WriteString("x")
        }
        _ = sb.String()
    }
}

// Run benchmarks:
// go test -bench=. -benchmem ./...
// BenchmarkStringConcat-8    123456   9876 ns/op   5264 B/op  99 allocs/op
// BenchmarkStringBuilder-8   456789   2345 ns/op    128 B/op   2 allocs/op

// -benchmem: shows memory allocations per op
// -benchtime=5s: run each benchmark for 5 seconds
// -count=5: run each benchmark 5 times for statistical stability

// Reset timer to exclude setup time
func BenchmarkWithSetup(b *testing.B) {
    data := make([]byte, 1<<20) // setup — excluded from timing
    b.ResetTimer()              // start timing from here
    for i := 0; i < b.N; i++ {
        process(data)
    }
}

// Run with pprof to get a flame graph:
// go test -bench=BenchmarkStringConcat -cpuprofile=cpu.out
// go tool pprof cpu.out

The benchstat tool (part of golang.org/x/perf) compares two sets of benchmark results statistically, accounting for variance — essential for determining whether an optimisation is genuinely significant or within noise.

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