如何正确使用渠道来控制并发？

Question

I'm new to concurrency in Go and I'm trying to figure out how to use channels to control concurrency. What I would like to do have a loop where I can call out to a function using a new go routine and continue looping while that function processes and I would like to limit the number of routines that run to 3. My first attempt to do this was the code below:

func write(val int, ch chan bool) {
    fmt.Println("Processing:", val)
    time.Sleep(2 * time.Second)
    ch <- val % 3 == 0
}

func main() {
    ch := make(chan bool, 3) // limit to 3 routines?
    for i := 0; i< 10; i++ {
        go write(i, ch)
        resp := <- ch
        fmt.Println("Divisible by 3:", resp)
    }
    time.Sleep(20 * time.Second)
}

I was under the impression that this would basically make calls to write 3 at a time and then hold off on processing the next 3 until the first 3 had finished. Based on what is logging it appears to only be processing one at a time. The code can be found and executed here.

What would I need to change in this example to get the functionality that I described above?

Answer 1

The problem here is very simple:

for i := 0; i< 10; i++ {
    go write(i, ch)
    resp := <- ch
    fmt.Println("Divisible by 3:", resp)
}

You spin up a goroutine, then wait for it to respond, before you continue around the loop and spin up the next goroutine. They can't run in parallel because you never run two of them at the same time.

To fix this, you need to spin up all 10 goroutines, then wait on all 10 responses (playground):

for i := 0; i< 10; i++ {
    go write(i, ch)
}
for i := 0; i<10; i++ {
    resp := <- ch
    fmt.Println("Divisible by 3:", resp)
}

Now you do have 7 goroutines blocking on the channel—but it's so brief that you can't see it happening, so the output won't be very interesting. If you try adding a Processed message at the end of the goroutine, and sleeping between each channel read, you'll see that 3 of them finish immediately (well, after waiting 2 seconds), and then the others unblock and finish one by one (playground).