A common operation when developing a Go application, or benchmarking it to find ways to improve it, is benchmarking a function execution time. I introduced the topic of CPU and memory profiling in Go already, but this is different and more suited to ad-hoc function execution time measurement.

Determine the time interval between two expressions: use time.Since

time.Since is a function provided by the time standard library package that takes a Time value, computes the difference with the current time, and returns a Duration value, which is an int64 type with a String() method attached, which provides a human friendly notation for the time computed.

import (
    "fmt"
    "time"
)

func main() {
    start := time.Now()

    //... do something

    fmt.Println(time.Since(start))
}

Internally, time.Since is a shorthand for time.Now().Sub(t).

Using defer to measure time inside a function

You can abstract this by using defer:

package main

import (
    "log"
    "time"
)

func runningtime(s string) (string, time.Time) {
    log.Println("Start:	", s)
    return s, time.Now()
}

func track(s string, startTime time.Time) {
    endTime := time.Now()
    log.Println("End:	", s, "took", endTime.Sub(startTime))
}

func execute() {
    defer track(runningtime("execute"))
    time.Sleep(3 * time.Second)
}

func main() {
    execute()
}

Benchmark a function performance by running it multiple times: use the testing package

Here is a simple library I wrote, that counts the number of runes in a string.

runecount.go

package runecount

import "unicode/utf8"

func RuneCount(s string) int {
    return len([]rune(s))
}

func RuneCount2(s string) int {
    return utf8.RuneCountInString(s)
}

Which of the two ways to count the runes in a string is faster? We find out by writing a benchmark. Benchmarks live in the *_test.go file, like regular tests, and can stay in live with testing methods, that start with Test*, but they start with Benchmark* instead:

runecount_test.go

package runecount

import "testing"

func BenchmarkRuneCount(b *testing.B) {
    s := "Gophers are amazing 😁"
    for i := 0; i < b.N; i++ {
        RuneCount(s)
    }
}
func BenchmarkRuneCount2(b *testing.B) {
    s := "Gophers are amazing 😁"
    for i := 0; i < b.N; i++ {
        RuneCount2(s)
    }
}

I run this by executing, in the same folder where the files are located:

go test -bench=.

This causes all the benchmarks to run. I got just a single file, but in case you have many, you can specify which file to run.

Each benchmark runs the loop passing the b.N variable, which is automatically determined by the benchmark runner, until the average is stable enough to determine a result.

This is the output I get on my Mac:

# flavio @ Flavios-MacBook-Pro in ~/go/src/github.com/flaviocopes/snippets/benchmark on git:master x [18:32:26]
$ go test -bench=.
BenchmarkRuneCount-2            20000000               115 ns/op
BenchmarkRuneCount2-2           30000000                44.1 ns/op
PASS
ok      github.com/flaviocopes/snippets/benchmark       3.812s

20000000 and 30000000 are the number of operations run.

The command returns the benchmark results: it ran 20M times RuneCount(), which took on average 163 nanoseconds, and then it ran 30M times RuneCount2(), which took on average 74.2 nanoseconds per each iteration.

So Benchmark1, which runs RuneCount(), is 2x slower than RuneCount2(), that uses the built-in utf8.RuneCountInString function. No surprise here, as utf8.RuneCountInString is highly optimized for this specific task.

Instead of using go test, you can also call testing.Benchmark from a command:

package main

import (
    "fmt"
    "testing"
    "runecount"
)

func BenchmarkRuneCount(b *testing.B) {
    s := "Gophers are amazing 😁"
    for i := 0; i < b.N; i++ {
        RuneCount(s)
    }
}
func BenchmarkRuneCount2(b *testing.B) {
    s := "Gophers are amazing 😁"
    for i := 0; i < b.N; i++ {
        RuneCount2(s)
    }
}

func main() {
    fmt.Println(testing.Benchmark(BenchmarkRuneCount))
    fmt.Println(testing.Benchmark(BenchmarkRuneCount2))
}

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