testing: b.StopTimer breaks b.Loop

Go version

go version go1.24.1 linux/amd64

Output of `go env` in your module/workspace:

AR='ar'
CC='gcc'
CGO_CFLAGS='-O2 -g'
CGO_CPPFLAGS=''
CGO_CXXFLAGS='-O2 -g'
CGO_ENABLED='1'
CGO_FFLAGS='-O2 -g'
CGO_LDFLAGS='-O2 -g'
CXX='g++'
GCCGO='gccgo'
GO111MODULE=''
GOAMD64='v1'
GOARCH='amd64'
GOAUTH='netrc'
GOBIN=''
GOCACHE='/home/dominikh/.cache/go-build'
GOCACHEPROG=''
GODEBUG=''
GOENV='/home/dominikh/.config/go/env'
GOEXE=''
GOEXPERIMENT=''
GOFIPS140='off'
GOFLAGS=''
GOGCCFLAGS='-fPIC -m64 -pthread -Wl,--no-gc-sections -fmessage-length=0 -ffile-prefix-map=/tmp/go-build1859758891=/tmp/go-build -gno-record-gcc-switches'
GOHOSTARCH='amd64'
GOHOSTOS='linux'
GOINSECURE=''
GOMOD='/home/dominikh/prj/src/example.com/go.mod'
GOMODCACHE='/home/dominikh/prj/pkg/mod'
GONOPROXY=''
GONOSUMDB=''
GOOS='linux'
GOPATH='/home/dominikh/prj'
GOPRIVATE=''
GOPROXY='https://proxy.golang.org,direct'
GOROOT='/home/dominikh/prj/go'
GOSUMDB='sum.golang.org'
GOTELEMETRY='on'
GOTELEMETRYDIR='/home/dominikh/.config/go/telemetry'
GOTMPDIR=''
GOTOOLCHAIN='auto'
GOTOOLDIR='/home/dominikh/prj/go/pkg/tool/linux_amd64'
GOVCS=''
GOVERSION='go1.24.1'
GOWORK=''
PKG_CONFIG='pkg-config'

What did you do?

Consider the following benchmark function:

func BenchmarkFoo(b *testing.B) {
	for b.Loop() {
		println("iter")
		b.StopTimer()
		// expensive per-iteration setup work
		b.StartTimer()
		time.Sleep(100 * time.Millisecond)
	}
}

Run with go test -bench=. -benchtime=1s

b.Loop never returns false and the benchmark never terminates.

In B.stopOrScaleBLoop we use b.start to compute how long we took to iterate b.N times. However, calling B.StartTimer sets b.start to the current time.

None of the documentation says that using b.StopTimer/b.StartTimer is forbidden when using b.Loop.

@JunyangShao

CC @JunyangShao

Related Issues

Related Code Changes

testing: enable better loop time measurement for benchmarking.

Related Documentation

Package testing > type B ¶ > func (*B) Loop ¶ 1.24

Related Discussions

Bencharking issue with b.StartTimer and b.StopTimer. Was: Benchmark using b.StopTimer and b.StartTimer has unexpected behaviour - to me at least
benchmark and start/stop timer

_{(Emoji vote if this was helpful or unhelpful; more detailed feedback welcome in this discussion.)}

Change https://go.dev/cl/658975 mentions this issue: testing: allow manual timer control in testing.B.Loop

@gopherbot

@gopherbot, please backport to Go 1.24.

Backport issue(s) opened: #72934 (for 1.24).

Remember to create the cherry-pick CL(s) as soon as the patch is submitted to master, according to https://go.dev/wiki/MinorReleases.

testing: allow manual timer control in testing.B.Loop

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testing: b.StopTimer breaks b.Loop #72922

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testing: b.StopTimer breaks b.Loop #72922

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Output of go env in your module/workspace:

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ianlancetaylor commented on Mar 18, 2025

gabyhelp commented on Mar 18, 2025

gopherbot commented on Mar 18, 2025

aclements commented on Mar 18, 2025

gopherbot commented on Mar 18, 2025

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Output of `go env` in your module/workspace: