Benchmarking Go Programs

In this lesson, explore benchmarking go programs through one small Go program, and connect its output to the language rules behind it.

Benchmark functions start with BenchmarkXxx(b *testing.B) and run a loop controlled by b.N in a _test.go file. The test runner chooses repetitions to collect timings.

Benchmarks should do representative work, avoid accidentally optimized-away results, and report allocations when useful with -benchmem.

Follow this diagram before executing the example. Check where a value or decision changes what runs next.

sum_test.go

package main
import "testing"
var benchmarkResult int
func BenchmarkSum(b *testing.B) {
    values := []int{1,2,3,4}
    for i := 0; i < b.N; i++ { benchmarkResult = sum(values) }
}

Run go test -run '^$' -bench BenchmarkSum -benchmem.

This example is a complete program. Save it as main.go or paste it into the Go Playground.

Go code

package main

import "fmt"

func sum(values []int) int {
	total := 0
	for _, n := range values {
		total += n
	}
	return total
}
func main() { fmt.Println("Sum:", sum([]int{1, 2, 3, 4})) }

Output

Sum: 10

The small sum function has a predictable result. A benchmark can call it repeatedly under the testing package without hard-coding a wall-clock result.

• Never present a benchmark time as a universal constant.
• Printing within a benchmark loop usually measures I/O rather than the intended operation.

Benchmark sum on a 100-element slice and compare -benchmem with a different algorithm.

Tip: Edit the example above, predict the output, and run the modified code to test your reasoning.

You have practiced Benchmarking Go Programs in a small, self-contained program. The important habit is to predict what runs, inspect the actual output and distinguish the language rule from the particular example.

Before continuing, make sure you can explain the program to someone who has not seen it, including the line that changes its result.