testing: RunParallel benchmark runs too many iterations #75488
Description
Go version
go version go1.25.1 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/radu_cockroachlabs_com/.cache/go-build'
GOCACHEPROG=''
GODEBUG=''
GOENV='/home/radu_cockroachlabs_com/.config/go/env'
GOEXE=''
GOEXPERIMENT=''
GOFIPS140='off'
GOFLAGS=''
GOGCCFLAGS='-fPIC -m64 -pthread -Wl,--no-gc-sections -fmessage-length=0 -ffile-prefix-map=/tmp/go-build4207328522=/tmp/go-build -gno-record-gcc-switches'
GOHOSTARCH='amd64'
GOHOSTOS='linux'
GOINSECURE=''
GOMOD='/dev/null'
GOMODCACHE='/home/radu_cockroachlabs_com/go/pkg/mod'
GONOPROXY=''
GONOSUMDB=''
GOOS='linux'
GOPATH='/home/radu_cockroachlabs_com/go'
GOPRIVATE=''
GOPROXY='https://proxy.golang.org,direct'
GOROOT='/usr/local/go'
GOSUMDB='sum.golang.org'
GOTELEMETRY='local'
GOTELEMETRYDIR='/home/radu_cockroachlabs_com/.config/go/telemetry'
GOTMPDIR=''
GOTOOLCHAIN='auto'
GOTOOLDIR='/usr/local/go/pkg/tool/linux_amd64'
GOVCS=''
GOVERSION='go1.25.1'
GOWORK=''
PKG_CONFIG='pkg-config'What did you do?
I have a standard parallel benchmark:
func BenchmarkFoo(b *testing.B) {
// <some setup work>
b.ResetTimer()
b.RunParallel(func (pb *testing.PB) {
for pb.Next() {
// very small amount of work
}
}
}I ran: `go test -run - -bench Foo --benchtime=30s -v .'
What did you see happen?
It does not complete, even after several minutes.
I added a log during the setup work that prints b.N. The verbose output is like this:
goos: linux
goarch: amd64
pkg: github.com/cockroachdb/pebble/internal/cache
cpu: Intel(R) Xeon(R) CPU @ 2.80GHz
BenchmarkCacheGet
cache_test.go:261: N: 1
cache_test.go:261: N: 69
cache_test.go:261: N: 4462
cache_test.go:261: N: 309830
cache_test.go:261: N: 20830386
cache_test.go:261: N: 521723665
cache_test.go:261: N: 521723666
cache_test.go:261: N: 521723667
cache_test.go:261: N: 521723668
cache_test.go:261: N: 521723669
cache_test.go:261: N: 521723670
cache_test.go:261: N: 521723671
cache_test.go:261: N: 521723672
cache_test.go:261: N: 521723673
cache_test.go:261: N: 521723674
cache_test.go:261: N: 521723675
cache_test.go:261: N: 521723676
cache_test.go:261: N: 521723677
cache_test.go:261: N: 521723678
cache_test.go:261: N: 521723679
cache_test.go:261: N: 521723680
cache_test.go:261: N: 521723681
cache_test.go:261: N: 521723682
...
Clearly there is something wrong with the algorithm that iterates toward finding the "right" N. Which, by the way, shouldn't even be necessary when using pb.Next() (it could in principle behave like b.Loop()).
The machine has 24 CPUs. I can't reproduce this on a different (smaller) machine.
What did you expect to see?
Expected this to take between 30-60s.