runtime: with -test.blockprofile program run even faster.

[zdyj3170101136]

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

$ go version
go version go1.19.3 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="/root/.cache/go-build"
GOENV="/root/.config/go/env"
GOEXE=""
GOEXPERIMENT=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE=""
GOMODCACHE="/usr/local/go/pkg/mod/"
GONOPROXY=""
GONOSUMDB="gopkg.mihoyo.com"
GOOS="linux"
GOPATH="/root/go"
GOPRIVATE=""
GOPROXY="https://goproxy.cn,direct"
GOROOT="/root/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/root/go/pkg/tool/linux_amd64"
GOVCS=""
GOVERSION="go1.19.3"
GCCGO="gccgo"
GOAMD64="v1"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD="/root/profilerbench/go.mod"
GOWORK=""
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 -Wl,--no-gc-sections -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build799047141=/tmp/go-build -gno-record-gcc-switches"GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/root/.cache/go-build"
GOENV="/root/.config/go/env"
GOEXE=""
GOEXPERIMENT=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE=""
GOMODCACHE="/usr/local/go/pkg/mod/"
GONOPROXY=""
GONOSUMDB="gopkg.mihoyo.com"
GOOS="linux"
GOPATH="/root/go"
GOPRIVATE=""
GOPROXY="https://goproxy.cn,direct"
GOROOT="/root/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/root/go/pkg/tool/linux_amd64"
GOVCS=""
GOVERSION="go1.19.3"
GCCGO="gccgo"
GOAMD64="v1"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD="/root/profilerbench/go.mod"
GOWORK=""
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 -Wl,--no-gc-sections -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build799047141=/tmp/go-build -gno-record-gcc-switches"

What did you do?

run benchmark test of src/runtime/chan_test.go.
one with blockprofile, another not.

/root/go/bin/go test -run=^$ -bench=BenchmarkChanProdConsWork0$ -benchtime=10000000x
goos: linux
goarch: amd64
pkg: github.com/zdyj3170101136/profilerbench/sync
cpu: Intel(R) Xeon(R) Platinum 8269CY CPU @ 2.50GHz
BenchmarkChanProdConsWork0-8    10000000               839.0 ns/op
PASS
ok      github.com/zdyj3170101136/profilerbench/sync    8.490s

/root/go/bin/go test -run=^$ -bench=BenchmarkChanProdConsWork0$ -benchtime=10000000x -blockprofile=b
goos: linux
goarch: amd64
pkg: github.com/zdyj3170101136/profilerbench/sync
cpu: Intel(R) Xeon(R) Platinum 8269CY CPU @ 2.50GHz
BenchmarkChanProdConsWork0-8    10000000               793.7 ns/op
PASS
ok      github.com/zdyj3170101136/profilerbench/sync    8.059s

What did you expect to see?

with blockprofile, the program should run slower!

What did you see instead?

with blockprofile, the program run faster !

helpful information

here is the cpu profile diff:

• saveblockevent cost lots of cpu.
• runtime.lock save lots of cpu, which means contention is less.

[thanm]

@golang/runtime

[mknyszek]

Since that benchmark is a microbenchmark, it's possible there's a scalability failure as it hammers on a particular lock. The CPU profile seems to suggest this. By adding in time between lock calls via the block profile operations, it actually eases up barging behaviors causing better performance.

I'm not sure there's anything to do here. In general, scalability failures aren't a good thing to have, but unless they're affecting real-world Go programs, I'm not too worried. They usually require a non-trivial amount of work to resolve, though this may be related to just general channel performance.

Please see #8899 for one possible solution/mitigation. Closing for now.

[mknyszek]

(To be clear, I don't think we shouldn't make channels scale better in general. :) It's just going to be a non-trivial effort, potentially with a long bug tail, and we have to weigh that against everything else.)