testing: benchmark performance misaligned for fast tests #30928
Description
What version of Go are you using (go version)?
$ go version go version go1.12.1 darwin/amd64
Does this issue reproduce with the latest release?
Yes
What operating system and processor architecture are you using (go env)?
go env Output
$ go env GOARCH="amd64" GOBIN="" GOCACHE="/Users/ondrej.kokes/Library/Caches/go-build" GOEXE="" GOFLAGS="" GOHOSTARCH="amd64" GOHOSTOS="darwin" GOOS="darwin" GOPATH="/Users/ondrej.kokes/go" GOPROXY="" GORACE="" GOROOT="/usr/local/Cellar/go/1.12.1/libexec" GOTMPDIR="" GOTOOLDIR="/usr/local/Cellar/go/1.12.1/libexec/pkg/tool/darwin_amd64" GCCGO="gccgo" CC="clang" CXX="clang++" CGO_ENABLED="1" GOMOD="" 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 -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/mk/dqryy0_947936yjsdw8jl6v00000gp/T/go-build420651873=/tmp/go-build -gno-record-gcc-switches -fno-common"
What did you do?
I ran some fast tests, where each iteration took less than a nanosecond. This led to the results table to be misaligned.
Here's a reproducible example:
package foo
import (
"testing"
"time"
)
func BenchmarkFoo(b *testing.B) {
for j := 0; j < b.N; j++ {
time.Sleep(time.Millisecond)
}
}
func BenchmarkBar(b *testing.B) {
for j := 0; j < b.N; j++ {
}
}
What did you expect to see?
goos: darwin
goarch: amd64
BenchmarkFoo-4 1000 1357941 ns/op
BenchmarkBar-4 2000000000 0.32 ns/op
What did you see instead?
goos: darwin
goarch: amd64
BenchmarkFoo-4 1000 1357941 ns/op
BenchmarkBar-4 2000000000 0.32 ns/op
Thoughts
I guess this has to do with performance numbers being printed as %8d, so anything larger than that will get shifted.