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Running testing.Benchmark produces wrong number of iterations #40012

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aslatter opened this issue Jul 3, 2020 · 1 comment
Closed

Running testing.Benchmark produces wrong number of iterations #40012

aslatter opened this issue Jul 3, 2020 · 1 comment

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@aslatter
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@aslatter aslatter commented Jul 3, 2020

What version of Go are you using (go version)?

$ go version
go version go1.13.5 linux/amd64

Does this issue reproduce with the latest release?

This issue reproduces on the Go playground (as of 2020-07-02).

What operating system and processor architecture are you using (go env)?

go env Output
$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/antoine/.cache/go-build"
GOENV="/home/antoine/.config/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/home/antoine/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/home/antoine/local/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/home/antoine/local/go/pkg/tool/linux_amd64"
GCCGO="gccgo"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD="/home/antoine/projects/testing/go-bench/go.mod"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build279526372=/tmp/go-build -gno-record-gcc-switches"

What did you do?

https://play.golang.org/p/CYHoKOYMgTj

package main

import (
	"log"
	"sync/atomic"
	"testing"
	"time"
)

var actualBenchIters uint32 = 0

func main() {
	result := testing.Benchmark(myBench)

	log.Printf("Result Duration: %v", result.T)
	log.Printf("Result Iterations: %v", result.N)
	log.Printf("Actual Iterations: %v (no more than)", actualBenchIters)
	log.Printf("Summary: %v", result)

}

func myBench(b *testing.B) {
	atomic.AddUint32(&actualBenchIters, 1)
	time.Sleep(time.Microsecond * 3)
}

What did you expect to see?

Printed 'actual iteterations' should not be less-than the iterations reported by the return-value of testing.Benchmark.

What did you see instead?

The number of iterations recorded by my counter was '6'. The number of iterations reported by testing.Benchmark was '1000000000'.

Playground output:

2009/11/10 23:00:00 Result Duration: 3µs
2009/11/10 23:00:00 Result Iterations: 1000000000
2009/11/10 23:00:00 Actual Iterations: 6 (no more than)
2009/11/10 23:00:00 Summary: 1000000000	         0.000003 ns/op

This makes the 'ns/op' calculation incorrect.

@davecheney
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@davecheney davecheney commented Jul 3, 2020

Your benchmark does not iterate over b.N. I will not report correct values as the benchmark will appear to run faster the larger the value of b.N supplied.

@davecheney davecheney closed this Jul 3, 2020
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