runtime: Different behavior of go routines in go1.5beta2 on MacOS and Linux

[matyunin]

The example is taken from the "A Tour of Go": https://tour.golang.org/concurrency/1

Obviously, program output should have 10 rows: 5 for "hello" and 5 for "world".

But we have on:

• Linux - 9 rows
• MacOS - 10 rows

Linux output (9 rows):

$ go run 1.go 
hello
world
hello
world
hello
world
world
hello
hello

MacOS X output (10 rows):

$ go run 1.go 
hello
world
world
hello
hello
world
hello
world
hello
world

Can anyone explain - why?

Linux uname -a:

Linux desktop 3.16.0-4-amd64 #1 SMP Debian 3.16.7-ckt11-1 (2015-05-24) x86_64 GNU/Linux

MacOS X uname -a:

Darwin 14.5.0 Darwin Kernel Version 14.5.0: Thu Jul  9 22:56:16 PDT 2015; root:xnu-2782.40.6~1/RELEASE_X86_64 x86_64

Source code from tour:

package main

import (
    "fmt"
    "time"
)

func say(s string) {
    for i := 0; i < 5; i++ {
        time.Sleep(1000 * time.Millisecond)
        fmt.Println(s)
    }
}

func main() {
    go say("world")
    say("hello")
}

[crawshaw]

This program does not necessarily output ten rows. What happens is: it starts say("world") in a background goroutine, executes say("hello"), and then exits. In practice, because the sleep is so long, the two likely scenarios are:

• say("world") completing before say("hello"), in which case you get 10 lines, or
• say("hello") completing before the final loop of say("world"), in which case you get 9 lines.

(On a machine starved for CPU you could see fewer "world" lines, but you would have to construct such a case.)

[matyunin]

@crawshaw thanks :) 👍