goroutine放到频道顺序固定吗？

Question

package main

import "fmt"

func sum(a []int, c chan int) {
    sum := 0
    for _, v := range a {
        sum += v
    }
    c <- sum // send sum to c
}

func main() {
    a := []int{7, 2, 8, -9, 4, 0}

    c := make(chan int)
    go sum(a[len(a)/2:], c)
    go sum(a[:len(a)/2], c)

    x, y := <-c, <-c // receive from c

    fmt.Println(x, y, x+y)
}

x, y := <-c, <-c // receive from c

Why does this line always print the same result?

I think it should be 50/50 chance to print 17 -5 12 or -5 17 12

I think the two go routines should be parallel

Thanks in advance!

package main

import "fmt"
import "time"
import "math/rand"

func sum(a []int, c chan int) {
    sum := 0
    for _, v := range a {
        sum += v
    }
    time.Sleep(time.Millisecond * time.Duration(rand.Intn(1000)))
    c <- sum // send sum to c

}

func main() {
    a := []int{7, 2, 8, -9, 4, 0}

    c := make(chan int)
    go sum(a[len(a)/2:], c)
    go sum(a[:len(a)/2], c)

    x, y := <-c, <-c // receive from c

    fmt.Println(x, y, x+y)
}

I make the thread to sleep, but same thing happens. I am still confusing.

Answer 1

The assumption that it should be a 50/50 chance is incorrect in this situation. To be clear, I'm not suggesting it should always be one or the other but rather that you shouldn't expect it to change each time the program runs. The behavior is not necessarily random (or even pseudo-random).

A goroutine's guarantee is not that it will be scheduled at a random future time, but rather just that it will run at some time in the future. If the current implementation of the scheduler decided to put goroutines in a simple queue, it wouldn't automatically mean it's broken. For your particular code's case, if you stick a few time.Sleeps or fmt.Printfs in different places, you'd likely see the order change around sometimes.

I wonder if you're confusing Go's scheduler's selection of the next goroutine with the documented pseudo-random behavior of select when used with channels. The behavior there is defined to be random and it's correct to say the behavior should have a 50/50 chance.

As far as I've read, the choice of which goroutine the runtime will select is not random. That's not the same as saying it can't be random, but moreso that it's not supposed to be random.

The important thing is that your code shouldn't care about the order in which goroutines are scheduled. Whether it's always in order like a queue, or backwards (a stack), or random, or something else (likely the reality).