为什么Benchmark运行六（？）次

Question

This code (playground link):

package main

import (
    "fmt"
    "testing"
)

var test = make([]int, 0)

func main() {
    fmt.Println(testing.Benchmark(testThis))
}

func testThis(b *testing.B) {
    fmt.Println(test)
}

has next output:

[]
[]
[]
[]
[]
[]
2000000000           0.00 ns/op

Program exited.

Why there are six [] inside output?

This code (playground link):

package main

import (
    "fmt"
)

var test = make([]int, 0)

func main() {
    fmt.Println(test)
}

has single output (and it's clear for me):

[]

Program exited.

Answer 1

You are using a benchmark function. This need to execute code multiple times to get a result which is kinda meaningful.

Also your Benchmark is not implemented like the benchmark should be programmed:

The benchmark function must run the target code b.N times. During benchmark execution, b.N is adjusted until the benchmark function lasts long enough to be timed reliably. -- https://golang.org/pkg/testing/

So Benchmark will check the run time and will adjust the b.N to get a good and useful benchmark.

When you just print b.N you get an output like this:

1
100
10000
1000000
100000000
2000000000
2000000000  

So in each of the 6 iterations the benchmark is telling you to run a foor loop b.N times.

Sadly you can not use a correct example in the playground because they take to long. But correct would be:

func testThis(b *testing.B) {
    for i := 0; i < b.N; i++ {
        fmt.Println(test)
    }
}