testing: benchmark output is not properly aligned on large b.N values

[artyom]

What did you do?

go test -run=NONE -bench=Mutex sync

What did you expect to see?

Middle column aligned.

What did you see instead?

~ ¶ go test -run=NONE -bench=Mutex sync
goos: darwin
goarch: amd64
pkg: sync
BenchmarkMutexUncontended-4      	100000000	        11.3 ns/op
BenchmarkMutex-4                 	10000000	       111 ns/op
BenchmarkMutexSlack-4            	10000000	       207 ns/op
BenchmarkMutexWork-4             	10000000	       139 ns/op
BenchmarkMutexWorkSlack-4        	 5000000	       213 ns/op
BenchmarkMutexNoSpin-4           	 2000000	       777 ns/op
BenchmarkMutexSpin-4             	  500000	      3450 ns/op
BenchmarkRWMutexUncontended-4    	30000000	        43.7 ns/op
BenchmarkRWMutexWrite100-4       	20000000	        75.2 ns/op
BenchmarkRWMutexWrite10-4        	20000000	       116 ns/op
BenchmarkRWMutexWorkWrite100-4   	10000000	       190 ns/op
BenchmarkRWMutexWorkWrite10-4    	 3000000	       416 ns/op
PASS
ok  	sync	20.832s

Note how the first line for BenchmarkMutexUncontended breaks alignment since 100M run count doesn't fit %8d format defined here:

go/src/testing/benchmark.go

Line 351 in 24e4a12

return fmt.Sprintf("%8d\t%s%s", r.N, ns, mb)

8475832 introduced scaling of ns/op reported to 0.01ns, but for such small numbers cycle count may not fit 8 symbols, so maybe it's worth increasing width to 9-10 digits?

System details

go version go1.9.1 darwin/amd64
GOARCH="amd64"
GOBIN=""
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOOS="darwin"
GOPATH="/tmp/go:/Users/artyom/go"
GORACE=""
GOROOT="/Users/artyom/Library/go"
GOTOOLDIR="/Users/artyom/Library/go/pkg/tool/darwin_amd64"
GCCGO="gccgo"
CC="clang"
GOGCCFLAGS="-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/lb/3rk8rqs53czgb4v35w_342xc0000gn/T/go-build603696847=/tmp/go-build -gno-record-gcc-switches -fno-common"
CXX="clang++"
CGO_ENABLED="1"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOROOT/bin/go version: go version go1.9.1 darwin/amd64
GOROOT/bin/go tool compile -V: compile version go1.9.1
uname -v: Darwin Kernel Version 17.0.0: Thu Aug 24 21:48:19 PDT 2017; root:xnu-4570.1.46~2/RELEASE_X86_64
ProductName:	Mac OS X
ProductVersion:	10.13
BuildVersion:	17A365
lldb --version: lldb-900.0.45
  Swift-4.0

[robpike]

Why NeedsFix? It's a minor cosmetic issue whose solution is disproportionate to its severity.

[ianlancetaylor]

It's OK with me if you just want to close it.

[robpike]

OK, closing.

The solution would require implementing a columnator in the testing package, or using something like tabwriter, but that would destroy the dependency tree for the package.

If you care about alignment, I suggest taking https://github.com/robpike/tab/blob/master/main.go plus a little work to turn the spaces into tabs before passing the text to tabwriter.

If you want numeric alignment, it's even more work though.