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An empty for{} will block large slice allocation in another goroutine, even with GOMAXPROCS > 1 ? #17174

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yaofu1986 opened this issue Sep 21, 2016 · 5 comments

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@yaofu1986
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@yaofu1986 yaofu1986 commented Sep 21, 2016

Please answer these questions before submitting your issue. Thanks!

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

go version go1.6.1 darwin/amd64

What operating system and processor architecture are you using (go env)?

GOARCH="amd64"
GOBIN=""
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOOS="darwin"
GOPATH="/Users/yaofu/go"
GORACE=""
GOROOT="/usr/local/go"
GOTOOLDIR="/usr/local/go/pkg/tool/darwin_amd64"
GO15VENDOREXPERIMENT="1"
CC="clang"
GOGCCFLAGS="-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fno-common"
CXX="clang++"
CGO_ENABLED="1"

What did you do?

package main

import (
    "runtime"
)

func work() {
    println("before")
    s := make([]int, 6000000)
    println("after")
    s[0] = 1
}

func main() {
    runtime.GOMAXPROCS(4)

    //work()   // This is OK. Very quick.
    go work()

    for {
    }
}

PS: I know for{} busy wait is no good. But I'm curious to know the underlying reason.

What did you expect to see?

following print:

before
after

What did you see instead?

following printed:

before
@randall77
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@randall77 randall77 commented Sep 21, 2016

Go has a cooperative, not preemptive, scheduler. The for{} loop never reaches a preemption point, so a garbage collection cannot complete.
Dup of #10958

@randall77 randall77 closed this Sep 21, 2016
@ianlancetaylor
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@ianlancetaylor ianlancetaylor commented Sep 21, 2016

If you want to block forever, use select {}.

@quentinmit
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@quentinmit quentinmit commented Sep 21, 2016

Can/should the compiler automatically turn for {} into select {}?

On Sep 21, 2016 09:11, "Ian Lance Taylor" notifications@github.com wrote:

If you want to block forever, use select {}.


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@ianlancetaylor
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@ianlancetaylor ianlancetaylor commented Sep 21, 2016

It could but I don't think it's worth it. Nobody writes for {} in real code.

@minux
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@minux minux commented Sep 22, 2016

@golang golang locked and limited conversation to collaborators Sep 22, 2017
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