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testing: b.StopTimer breaks b.Loop #72922

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dominikh opened this issue Mar 18, 2025 · 5 comments
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testing: b.StopTimer breaks b.Loop #72922

dominikh opened this issue Mar 18, 2025 · 5 comments
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@JunyangShao
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BugReport Issues describing a possible bug in the Go implementation. FixPending Issues that have a fix which has not yet been reviewed or submitted.
Milestone
Go1.25

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@dominikh
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dominikh commented Mar 18, 2025 •
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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.
@ianlancetaylor
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CC @JunyangShao

@gabyhelp
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(Emoji vote if this was helpful or unhelpful; more detailed feedback welcome in this discussion.)

@gabyhelp gabyhelp added the BugReport Issues describing a possible bug in the Go implementation. label Mar 18, 2025
@JunyangShao JunyangShao self-assigned this Mar 18, 2025
@gopherbot
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Change https://go.dev/cl/658975 mentions this issue: testing: allow manual timer control in testing.B.Loop

@aclements
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@gopherbot, please backport to Go 1.24.

@gopherbot
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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.

@gopherbot gopherbot mentioned this issue Mar 18, 2025
Closed
@dmitshur dmitshur added this to the Go1.25 milestone Mar 19, 2025
@dmitshur dmitshur added the FixPending Issues that have a fix which has not yet been reviewed or submitted. label Mar 19, 2025
@aclements aclements mentioned this issue Mar 20, 2025
Closed
@gabyhelp gabyhelp mentioned this issue Apr 4, 2025
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@dominikh @dmitshur @aclements @ianlancetaylor @gopherbot @JunyangShao @gabyhelp