GOMAXPROCS已经为2，但是程序仍然挂起

Question

I already set runtime.GOMAXPROCS(2), but this program still hang when output some numbers. I can see high cpu used by this program(more than 100%), but I can't understand why the for loop goroutine can make my program not work.

go version is 1.4.2 on linux/amd64, and my pc has 4 CPUs.

here is the code:

package main

import "fmt"
import "runtime"
import "time"

func forever() {
    for {
    }   
}

func show() {
    for number := 1; number < 999999; number++ {
        time.Sleep(1000)
        fmt.Println(number)
    }   
}

func main() {
    runtime.GOMAXPROCS(2)
    go show()
    go forever()
    for {
        time.Sleep(1000)
    }   
} 

Answer 1

There's no need to ever have a busy loop that does nothing except burn CPU time. Not only does it consume an entire OS thread, but goroutines are cooperatively scheduled, and it will interfere with the runtime's goroutines. For example, on Go1.5 this will usually block the stop-the-world phase of the GC, (which you can test by setting GOGC=off).

To make this program run, you could insert a scheduling point in the for loop, but it would be better to remove it altogether.

func forever() {
    for {
        runtime.Gosched()
    }
}