Repository: go
Issue: testing:  Inconsistent benchmark data when  GOMAXPROCS=1
Labels: NeedsInvestigation

<!-- Please answer these questions before submitting your issue. Thanks! -->

### What version of Go are you using (`go version`)?

<pre>
$ go version
go version go1.12.4 darwin/amd64
</pre>

### Does this issue reproduce with the latest release?

Yes.

### What operating system and processor architecture are you using (`go env`)?

<details><summary><code>go env</code> Output</summary><br><pre>
$ go env
GOARCH="amd64"
GOBIN=""
GOCACHE="/Users/xuanjiazhen/Library/Caches/go-build"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOOS="darwin"
GOPATH="/Users/xuanjiazhen/work"
GOPROXY=""
GORACE=""
GOROOT="/Users/xuanjiazhen/Desktop/go"
GOTMPDIR=""
GOTOOLDIR="/Users/xuanjiazhen/Desktop/go/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/w3/64tyfz3n4cs2jw5lrl7mc5c80000gn/T/go-build723171853=/tmp/go-build -gno-record-gcc-switches -fno-common"

</pre></details>

### What did you do?

<!--
If possible, provide a recipe for reproducing the error.
A complete runnable program is good.
A link on play.golang.org is best.
-->
<pre>
$GOMAXPROCS=1 go test runtime -v -run none -bench  ^BenchmarkConvT2ILarge$ -count 10
goos: darwin
goarch: amd64
pkg: runtime
BenchmarkConvT2ILarge   50000000                46.8 ns/op
BenchmarkConvT2ILarge   30000000                46.9 ns/op
BenchmarkConvT2ILarge   50000000                39.5 ns/op
BenchmarkConvT2ILarge   30000000                39.1 ns/op
BenchmarkConvT2ILarge   30000000                47.6 ns/op
BenchmarkConvT2ILarge   30000000                46.8 ns/op
BenchmarkConvT2ILarge   30000000                49.6 ns/op
BenchmarkConvT2ILarge   30000000                43.6 ns/op
BenchmarkConvT2ILarge   30000000                52.2 ns/op
BenchmarkConvT2ILarge   50000000                46.9 ns/op
PASS
ok      runtime 16.874s
$GOMAXPROCS=1 go test runtime -v -run none -bench  ^BenchmarkConvT2ILarge -count 10
goos: darwin
goarch: amd64
pkg: runtime
BenchmarkConvT2ILarge   100000000               19.6 ns/op
BenchmarkConvT2ILarge   100000000               19.8 ns/op
BenchmarkConvT2ILarge   100000000               20.3 ns/op
BenchmarkConvT2ILarge   100000000               21.2 ns/op
BenchmarkConvT2ILarge   100000000               21.0 ns/op
BenchmarkConvT2ILarge   100000000               20.6 ns/op
BenchmarkConvT2ILarge   100000000               21.1 ns/op
BenchmarkConvT2ILarge   100000000               20.6 ns/op
BenchmarkConvT2ILarge   100000000               20.6 ns/op
BenchmarkConvT2ILarge   100000000               20.8 ns/op
PASS
ok      runtime 21.803s
</pre>


### What did you expect to see?

The test results of the two types of writing should be consistent.


### What did you see instead?
Test results with $ are much worse than test results without it.
<pre>
$GODEBUG=gctrace=1 GOMAXPROCS=1 go test runtime -v -run none -bench  ^BenchmarkConvT2ILarge -count 10
...
gc 3984 @22.195s 2%: 0.001+0.12+0.001 ms clock, 0.001+0.12/0/0+0.001 ms cpu, 4->4->0 MB, 5 MB goal, 1 P
gc 3985 @22.202s 2%: 0.003+0.13+0.001 ms clock, 0.003+0.12/0/0+0.001 ms cpu, 4->4->0 MB, 5 MB goal, 1 P
gc 3986 @22.207s 2%: 0.001+0.11+0.001 ms clock, 0.001+0.11/0/0+0.001 ms cpu, 4->4->0 MB, 5 MB goal, 1 P
gc 3987 @22.213s 2%: 0.001+0.11+0 ms clock, 0.001+0.11/0/0+0 ms cpu, 4->4->0 MB, 5 MB goal, 1 P
gc 3988 @22.218s 2%: 0.001+0.12+0.001 ms clock, 0.001+0.12/0/0+0.001 ms cpu, 4->4->0 MB, 5 MB goal, 1 P
gc 3989 @22.224s 2%: 0.001+0.12+0 ms clock, 0.001+0.12/0/0+0 ms cpu, 4->4->0 MB, 5 MB goal, 1 P
gc 3990 @22.230s 2%: 0+0.12+0 ms clock, 0+0.12/0/0+0 ms cpu, 4->4->0 MB, 5 MB goal, 1 P
gc 3991 @22.236s 2%: 0.001+0.14+0.001 ms clock, 0.001+0.14/0/0+0.001 ms cpu, 4->4->0 MB, 5 MB goal, 1 P
100000000               22.8 ns/op
PASS
</pre>
<pre>
$GODEBUG=gctrace=1 GOMAXPROCS=1 go test runtime -v -run none -bench  ^BenchmarkConvT2ILarge$ -count 10
...
gc 1052 @18.124s 0%: 0.001+20+0.002 ms clock, 0.001+0.13/0.021/0+0.002 ms cpu, 6->11->5 MB, 10 MB goal, 1 P
gc 1053 @18.146s 0%: 0.001+21+0.001 ms clock, 0.001+0.11/0.013/0+0.001 ms cpu, 6->12->6 MB, 11 MB goal, 1 P
gc 1054 @18.169s 0%: 0.001+19+0.001 ms clock, 0.001+0.11/0.014/0+0.001 ms cpu, 7->12->5 MB, 12 MB goal, 1 P
gc 1055 @18.190s 0%: 0.001+23+0.001 ms clock, 0.001+0.11/0.015/0+0.001 ms cpu, 6->12->6 MB, 10 MB goal, 1 P
gc 1056 @18.216s 0%: 0.002+19+0.001 ms clock, 0.002+0.13/0.013/0+0.001 ms cpu, 7->12->5 MB, 12 MB goal, 1 P
gc 1057 @18.237s 0%: 0.002+19+0.001 ms clock, 0.002+0.13/0.017/0+0.001 ms cpu, 6->11->5 MB, 10 MB goal, 1 P
gc 1058 @18.258s 0%: 0.001+20+0.002 ms clock, 0.001+0.11/0.020/0+0.002 ms cpu, 6->12->6 MB, 11 MB goal, 1 P
gc 1059 @18.281s 0%: 0.001+19+0.001 ms clock, 0.001+0.11/0.016/0+0.001 ms cpu, 7->12->5 MB, 12 MB goal, 1 P
gc 1060 @18.302s 0%: 0.001+19+0.002 ms clock, 0.001+0.11/0.017/0+0.002 ms cpu, 6->11->5 MB, 11 MB goal, 1 P
gc 1061 @18.323s 0%: 0.002+11+0.002 ms clock, 0.002+0.13/0.023/0+0.002 ms cpu, 6->8->2 MB, 10 MB goal, 1 P
50000000                51.7 ns/op
PASS
ok      runtime 18.342s
gc 21 @19.662s 0%: 0.015+1.6+0.002 ms clock, 0.015+0.099/0.52/0.90+0.002 ms cpu, 4->4->1 MB, 5 MB goal, 1 P
</pre>