带通道的简单并行示例,导致死锁

Simple parallel example with channel, leading to deadlock

我正在尝试使用通道在 golang 中实现一个简单的并行化示例。该代码尝试实现并行化映射函数。也使通道缓冲以减轻对通道阻塞性质的限制。但是代码仍然会导致死锁。

func pmmap(inp []int, f func(int) int, p int) {

    var wg sync.WaitGroup
    var output []chan int
    tot := len(inp)
    out := make(chan int, tot)
    slice := tot / p

    for i := 0; i < p; i++ {
        temp := make(chan int, slice)
        output = append(output, temp)
        start_ind := slice * i
        end_ind := start_ind + slice
        fmt.Println(start_ind, end_ind)

        wg.Add(1)
        go func(si, ei int, out chan int, wg *sync.WaitGroup) {
            fmt.Println("goroutine started with ", si, ei)
            for ind := si; ind < ei; ind++ {
                out <- f(inp[ind])
            }
            wg.Done()
        }(start_ind, end_ind, output[i], &wg)
    }

    wg.Add(1)
    go func(wg *sync.WaitGroup) {
        for i := 0; i < p; i++ {
            for val := range output[i] {
                out <- val
            }
            close(output[i])
        }
        wg.Done()
    }(&wg)

    wg.Add(1)
    go func(wg *sync.WaitGroup) {
        for i := range out {
            fmt.Println(i)
        }
        wg.Done()
    }(&wg)

    time.Sleep(time.Second * 6)
    wg.Wait()
    close(out)
}

func add4(i int) int {
    return i + 4
}

func main() {
    temp := []int{}
    for i := 1; i <= 20; i++ {
        temp = append(temp, i)
    }
    pmmap(temp, add4, 2)
}

从上面代码的输出中,我发现死锁是因为从未读取过通道输出[1]。但我不确定为什么

0 10
10 20
goroutine started with  0 10
goroutine started with  10 20
5
6
7
8
9
10
11
12
13
14
fatal error: all goroutines are asleep - deadlock!

问题是 range 频道一直在尝试从频道接收,直到频道关闭,但您在完成发送后没有 close 频道。

在您的代码中 wg.Done 之前添加 close(out) 即可解决问题。

游乐场:https://play.golang.org/p/NbKTx6Lke7X

编辑:修复了关闭已关闭频道的错误。