使用testing.Benchmark不会产生任何输出

Question

I'm using testing.Benchmark to manually run a couple benchmarks but the result object is always empty.

Am I missing something here?

Here's an example:

package main

import "testing"

func main() {

    result := testing.Benchmark(func(parentB *testing.B) {
        parentB.Run("example", func(b *testing.B) {
            for n := 0; n < b.N; n++ {
                println("ok")
            }
        })
    })

    println(result.String())

}

This will print ok a couple times and then 0 0 ns/op but the benchmark clearly did run something.

Answer 1

I think you are doing everything right. Doc of testing.Benchmark() says:

Benchmark benchmarks a single function. Useful for creating custom benchmarks that do not use the "go test" command.

If f calls Run, the result will be an estimate of running all its subbenchmarks that don't call Run in sequence in a single benchmark.

Looking into the implementation (Go 1.7.4):

func Benchmark(f func(b *B)) BenchmarkResult {
    b := &B{
        common: common{
            signal: make(chan bool),
            w:      discard{},
        },
        benchFunc: f,
        benchTime: *benchTime,
    }
    if !b.run1() {
        return BenchmarkResult{}
    }
    return b.run()
}

This line:

    if !b.run1() {
        return BenchmarkResult{}
    }

b.run1() is supposed to run your passed function once, and detect if it has sub-benchmarks. Yours has. It returns a bool whether more runs are needed. Inside run1():

if b.hasSub || b.finished {
    // ...
    return true
}

It properly tells it has sub-benchmark, and Benchmark() –with noble simplicity– just returns an empty BenchmarkResult:

    if !b.run1() {
        return BenchmarkResult{}
    }

I do believe that either this is a bug (or rather "incomplete" feature), or doc is incorrect. I suggest to file an issue here: https://github.com/golang/go/issues