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testing: high start up cost with high -count and no tests #49050

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prattmic opened this issue Oct 18, 2021 · 2 comments
Closed

testing: high start up cost with high -count and no tests #49050

prattmic opened this issue Oct 18, 2021 · 2 comments
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FrozenDueToAge NeedsDecision Feedback is required from experts, contributors, and/or the community before a change can be made. Performance

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@prattmic
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prattmic commented Oct 18, 2021

What version of Go are you using (go version)?

$ go version
go version devel +b7a85e0003 linux/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
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/usr/local/google/home/mpratt/.cache/go-build"
GOENV="/usr/local/google/home/mpratt/.config/go/env"
GOEXE=""
GOEXPERIMENT=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE=""
GOMODCACHE="/usr/local/google/home/mpratt/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/usr/local/google/home/mpratt/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/lib/google-golang"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/lib/google-golang/pkg/tool/linux_amd64"
GOVCS=""
GOVERSION="devel +b7a85e0003"
GCCGO="gccgo"
AR="ar"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD="/usr/local/google/home/mpratt/src/scratch/go_bench/go.mod"
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=/tmp/go-build1003331103=/tmp/go-build -gno-record-gcc-switches"

What did you do?

Run a benchmark many, many times. (The intention here is to run benchmarks for hours)

https://play.golang.org/p/tKSzmTOK_-m

go test -bench=. -count=10000000

What did you expect to see?

Benchmarks start running immediately.

What did you see instead?

Nothing seems to happen for ~30s before the benchmarks start running. The issue appears to be that runTests runs its count loop 10000000 times, even though there are no tests to run.

One quick fix may be for runTests to exit immediately if len(tests) == 0, though I've distilled this issue down from my initial problem of a long time waiting for benchmarks to start in a package that has tests which I skip with -run=none. It would be nice to fix that case too. For that, perhaps the first pass through runTests records if it ran any tests. If not, the next iteration immediately breaks.

@prattmic prattmic added Performance NeedsDecision Feedback is required from experts, contributors, and/or the community before a change can be made. labels Oct 18, 2021
@gopherbot
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gopherbot commented Oct 18, 2021

Change https://golang.org/cl/356669 mentions this issue: testing: skip extra -count iterations if there are no tests

@prattmic
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prattmic commented Oct 18, 2021

Note that my CL would break tests like https://play.golang.org/p/mGVn27Zl2lq when run like go test -count=2 -run=TestDouble/real_test.

I don't think that is a valid use of the testing package, but could certainly be convinced otherwise.

@golang golang locked and limited conversation to collaborators Oct 25, 2022
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