How to use golang Benchmark RunParallel?

this is my code

func BenchmarkParallel(b *testing.B) {
	var mu sync.Mutex
	var cnt int
	b.ResetTimer()
	b.SetParallelism(100)

	fmt.Println("cnt", cnt)
	b.RunParallel(func(pb *testing.PB) {
		mu.Lock()
		defer mu.Unlock()
		cnt++
		for pb.Next() {
			time.Sleep(time.Nanosecond)
			// time.Sleep(time.Second)
		}
	})
	fmt.Println("cnt", cnt)
}

$ go test -bench BenchmarkParallel -benchmem -test.count=1

Received the following output below

cnt 0
cnt 800
BenchmarkParallel-8     cnt 0
cnt 800
cnt 0
cnt 800
cnt 0
cnt 800
cnt 0
cnt 800
 3987668               320.3 ns/op             0 B/op          0 allocs/op
PASS

Why does fmt.Println execute so many times,
but if sleep is changed to 1 second, cnt is only output once?

I expected to see fmt.Println only once

BenchmarkParallel
cnt 0
cnt 800
BenchmarkParallel-8   	       1	1001106668 ns/op
PASS

go test -bench is designed to run for 1 second by default, and run the benchmark as many times as it can without exceeding that time! So the reason there are more prints than you expect is that bench is actually running your function more than once to get a better sample size.

When you change the sleep time to 1 second, then it only has time to run one sample of the benchmark, so it only prints once.

You can read more about changing this timeout on this page.