Always have x number of goroutines running at any time

Question

I see lots of tutorials and examples on how to make Go wait for x number of goroutines to finish, but what I\'m trying to do is have ensure there are always x number running

Answer 1

Thanks to everyone for helping me out with this. However, I don't feel that anyone really provided something that both worked and was simple/understandable, although you did all help me understand the technique.

What I have done in the end is I think much more understandable and practical as an answer to my specific question, so I will post it here in case anyone else has the same question.

Somehow this ended up looking a lot like what OneOfOne posted, which is great because now I understand that. But OneOfOne's code I found very difficult to understand at first because of the passing functions to functions made it quite confusing to understand what bit was for what. I think this way makes a lot more sense:

package main

import (
"fmt"
"sync"
)

const xthreads = 5 // Total number of threads to use, excluding the main() thread

func doSomething(a int) {
    fmt.Println("My job is",a)
    return
}

func main() {
    var ch = make(chan int, 50) // This number 50 can be anything as long as it's larger than xthreads
    var wg sync.WaitGroup

    // This starts xthreads number of goroutines that wait for something to do
    wg.Add(xthreads)
    for i:=0; i<xthreads; i++ {
        go func() {
            for {
                a, ok := <-ch
                if !ok { // if there is nothing to do and the channel has been closed then end the goroutine
                    wg.Done()
                    return
                }
                doSomething(a) // do the thing
            }
        }()
    }

    // Now the jobs can be added to the channel, which is used as a queue
    for i:=0; i<50; i++ {
        ch <- i // add i to the queue
    }

    close(ch) // This tells the goroutines there's nothing else to do
    wg.Wait() // Wait for the threads to finish
}

Answer 2

Grzegorz Żur's answer is the most efficient way to do it, but for a newcomer it could be hard to implement without reading code, so here's a very simple implementation:

type idProcessor func(id uint)

func SpawnStuff(limit uint, proc idProcessor) chan<- uint {
    ch := make(chan uint)
    for i := uint(0); i < limit; i++ {
        go func() {
            for {
                id, ok := <-ch
                if !ok {
                    return
                }
                proc(id)
            }
        }()
    }
    return ch
}

func main() {
    runtime.GOMAXPROCS(4)
    var wg sync.WaitGroup //this is just for the demo, otherwise main will return
    fn := func(id uint) {
        fmt.Println(id)
        wg.Done()
    }
    wg.Add(1000)
    ch := SpawnStuff(10, fn)
    for i := uint(0); i < 1000; i++ {
        ch <- i
    }
    close(ch) //should do this to make all the goroutines exit gracefully
    wg.Wait()
}

playground

Answer 3

1. Create channel for passing data to goroutines.
2. Start 20 goroutines that processes the data from channel in a loop.
3. Send the data to the channel instead of starting a new goroutine.

Answer 4

I've wrote a simple package to handle concurrency for Golang. This package will help you limit the number of goroutines that are allowed to run concurrently: https://github.com/zenthangplus/goccm

Example:

package main

import (
    "fmt"
    "goccm"
    "time"
)

func main()  {
    // Limit 3 goroutines to run concurrently.
    c := goccm.New(3)

    for i := 1; i <= 10; i++ {

        // This function have to call before any goroutine
        c.Wait()

        go func(i int) {
            fmt.Printf("Job %d is running\n", i)
            time.Sleep(2 * time.Second)

            // This function have to when a goroutine has finished
            // Or you can use `defer c.Done()` at the top of goroutine.
            c.Done()
        }(i)
    }

    // This function have to call to ensure all goroutines have finished 
    // after close the main program.
    c.WaitAllDone()
}

Answer 5

You may find Go Concurrency Patterns article interesting, especially Bounded parallelism section, it explains the exact pattern you need.

You can use channel of empty structs as a limiting guard to control number of concurrent worker goroutines:

package main

import "fmt"

func main() {
    maxGoroutines := 10
    guard := make(chan struct{}, maxGoroutines)

    for i := 0; i < 30; i++ {
        guard <- struct{}{} // would block if guard channel is already filled
        go func(n int) {
            worker(n)
            <-guard
        }(i)
    }
}

func worker(i int) { fmt.Println("doing work on", i) }

Answer 6

This is a simple producer-consumer problem, which in Go can be easily solved using channels to buffer the paquets.

To put it simple: create a channel that accept your IDs. Run a number of routines which will read from the channel in a loop then process the ID. Then run your loop that will feed IDs to the channel.

Example:

func producer() {
    var buffer = make(chan uint)

    for i := 0; i < 20; i++ {
        go consumer(buffer)
    }

    for _, id :=  range IDs {
        buffer <- id
    }
}

func consumer(buffer chan uint) {
    for {
        id := <- buffer
        // Do your things here
    }
}

Things to know:

• Unbuffered channels are blocking: if the item wrote into the channel isn't accepted, the routine feeding the item will block until it is
• My example lack a closing mechanism: you must find a way to make the producer to wait for all consumers to end their loop before returning. The simplest way to do this is with another channel. I let you think about it.