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regexp: MatchString run time could be independent of the string length #39480

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ghost opened this issue Jun 9, 2020 · 2 comments
Closed

regexp: MatchString run time could be independent of the string length #39480

ghost opened this issue Jun 9, 2020 · 2 comments
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FrozenDueToAge NeedsInvestigation Someone must examine and confirm this is a valid issue and not a duplicate of an existing one. Performance

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@ghost
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ghost commented Jun 9, 2020

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

$ go version
go version go1.14.4 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="/home/xxx/.cache/go-build"
GOENV="/home/xxx/.config/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE=""
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/home/xxx/gocode"
GOPRIVATE=""
GOPROXY="direct"
GOROOT="/home/xxx/go"
GOSUMDB="off"
GOTMPDIR=""
GOTOOLDIR="/home/xxx/go/pkg/tool/linux_amd64"
GCCGO="gccgo"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD="/home/xxx/gocode/src/gitlab.com/opennota/tl/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 -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build900652540=/tmp/go-build -gno-record-gcc-switches"

What did you do?

go test -bench=.

https://play.golang.org/p/VD6DFGlxha-

What did you expect to see?

Both benchmarks show about the same ns/op.

What did you see instead?

BenchmarkLength1-4      13320451                89.7 ns/op
BenchmarkLength1M-4      8138482               143 ns/op

The difference seems to be constant, but it's a bit surprising.
@randall77
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Run the CPU profiler and see where the extra time is going.

@mvdan mvdan added Performance NeedsInvestigation Someone must examine and confirm this is a valid issue and not a duplicate of an existing one. labels Jun 9, 2020
@ghost
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ghost commented Jun 16, 2020 •
edited by ghost

It looks like the regexp package uses backtracking for strings which are short enough, and does not for longer ones. So this difference in speed is probably expected.

@ghost ghost closed this as completed Jun 16, 2020
@golang golang locked and limited conversation to collaborators Jun 16, 2021
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