如何收听N个频道？(动态选择语句)

Question

要开始执行两个goroutine的无限循环,我可以使用下面的代码:

收到消息后,它将启动一个新的goroutine并继续下去.

c1 := make(chan string)
c2 := make(chan string)

go DoStuff(c1, 5)
go DoStuff(c2, 2)

for ; true;  {
    select {
    case msg1 := <-c1:
        fmt.Println("received ", msg1)
        go DoStuff(c1, 1)
    case msg2 := <-c2:
        fmt.Println("received ", msg2)
        go DoStuff(c2, 9)
    }
}

我现在想对N goroutines有相同的行为,但是在这种情况下select语句会如何？

这是我开始使用的代码位,但我很困惑如何编写select语句

numChans := 2

//I keep the channels in this slice, and want to "loop" over them in the select statemnt
var chans = [] chan string{}

for i:=0;i<numChans;i++{
    tmp := make(chan string);
    chans = append(chans, tmp);
    go DoStuff(tmp, i + 1)

//How shall the select statment be coded for this case?  
for ; true;  {
    select {
    case msg1 := <-c1:
        fmt.Println("received ", msg1)
        go DoStuff(c1, 1)
    case msg2 := <-c2:
        fmt.Println("received ", msg2)
        go DoStuff(c2, 9)
    }
}

Answer 1

您可以使用reflect包中的Select函数执行此操作:

func Select(cases []SelectCase) (chosen int, recv Value, recvOK bool)

Select执行由案例列表描述的选择操作.与Go select语句一样,它会阻塞,直到至少有一个案例可以继续,进行统一的伪随机选择,然后执行该案例.它返回所选情况的索引,如果该情况是接收操作,则返回接收的值和指示该值是否对应于通道上的发送的布尔值(与通道关闭时接收的零值相反).

传入一个SelectCase结构数组,用于标识要选择的通道,操作方向以及发送操作时要发送的值.

所以你可以这样做:

cases := make([]reflect.SelectCase, len(chans))
for i, ch := range chans {
    cases[i] = reflect.SelectCase{Dir: reflect.SelectRecv, Chan: reflect.ValueOf(ch)}
}
chosen, value, ok := reflect.Select(cases)
# ok will be true if the channel has not been closed.
ch := chans[chosen]
msg := value.String()

你可以在这里试验一个更加丰富的例子:http://play.golang.org/p/8zwvSk4kjx

Answer 2

您可以通过将每个通道包装在goroutine中来实现此目的,该goroutine将"转发"消息"转发"到共享的"聚合"通道.例如:

agg := make(chan string)
for _, ch := range chans {
  go func(c chan string) {
    for msg := range c {
      agg <- msg
    }
  }(ch)
}

select {
case msg <- agg:
    fmt.Println("received ", msg)
}

如果您需要知道消息来自哪个通道,您可以将其包含在带有任何额外信息的结构中,然后再将其转发到聚合通道.

在我的(有限的)测试中,这个方法使用反射包大大地执行:

$ go test dynamic_select_test.go -test.bench=.
...
BenchmarkReflectSelect         1    5265109013 ns/op
BenchmarkGoSelect             20      81911344 ns/op
ok      command-line-arguments  9.463s

基准代码在这里

Answer 3

扩展对先前答案的一些评论并提供更清晰的比较,这是迄今为止给出的两种方法的示例,给出相同的输入,一段要读取的通道以及调用每个值的函数,这些函数也需要知道哪个渠道价值来自.

这些方法有三个主要区别:

• 复杂.虽然它可能部分是读者偏好,但我发现频道方法更具惯用性,直接性和可读性.

• 性能.在我的Xeon amd64系统上,goroutines + channel out执行反射解决方案大约两个数量级(一般来说Go中的反射通常较慢,只应在绝对需要时使用).当然,如果在处理结果的函数或向输入通道写入值时存在任何显着延迟,则这种性能差异很容易变得无关紧要.

• 阻止/缓冲语义.这一点的重要性取决于用例.大多数情况下它要么无关紧要,要么goroutine合并解决方案中的轻微额外缓冲可能有助于吞吐量.但是,如果希望在任何其他编写器被解除阻塞之前只有单个编写器被解除阻塞并且其值完全处理的语义,那么这只能通过反射解决方案来实现.

注意,如果不需要发送通道的"id"或者源通道永远不会关闭,则可以简化这两种方法.

Goroutine合并渠道:

// Process1 calls `fn` for each value received from any of the `chans`
// channels. The arguments to `fn` are the index of the channel the
// value came from and the string value. Process1 returns once all the
// channels are closed.
func Process1(chans []<-chan string, fn func(int, string)) {
    // Setup
    type item struct {
        int    // index of which channel this came from
        string // the actual string item
    }
    merged := make(chan item)
    var wg sync.WaitGroup
    wg.Add(len(chans))
    for i, c := range chans {
        go func(i int, c <-chan string) {
            // Reads and buffers a single item from `c` before
            // we even know if we can write to `merged`.
            //
            // Go doesn't provide a way to do something like:
            //     merged <- (<-c)
            // atomically, where we delay the read from `c`
            // until we can write to `merged`. The read from
            // `c` will always happen first (blocking as
            // required) and then we block on `merged` (with
            // either the above or the below syntax making
            // no difference).
            for s := range c {
                merged <- item{i, s}
            }
            // If/when this input channel is closed we just stop
            // writing to the merged channel and via the WaitGroup
            // let it be known there is one fewer channel active.
            wg.Done()
        }(i, c)
    }
    // One extra goroutine to watch for all the merging goroutines to
    // be finished and then close the merged channel.
    go func() {
        wg.Wait()
        close(merged)
    }()

    // "select-like" loop
    for i := range merged {
        // Process each value
        fn(i.int, i.string)
    }
}

反思选择:

// Process2 is identical to Process1 except that it uses the reflect
// package to select and read from the input channels which guarantees
// there is only one value "in-flight" (i.e. when `fn` is called only
// a single send on a single channel will have succeeded, the rest will
// be blocked). It is approximately two orders of magnitude slower than
// Process1 (which is still insignificant if their is a significant
// delay between incoming values or if `fn` runs for a significant
// time).
func Process2(chans []<-chan string, fn func(int, string)) {
    // Setup
    cases := make([]reflect.SelectCase, len(chans))
    // `ids` maps the index within cases to the original `chans` index.
    ids := make([]int, len(chans))
    for i, c := range chans {
        cases[i] = reflect.SelectCase{
            Dir:  reflect.SelectRecv,
            Chan: reflect.ValueOf(c),
        }
        ids[i] = i
    }

    // Select loop
    for len(cases) > 0 {
        // A difference here from the merging goroutines is
        // that `v` is the only value "in-flight" that any of
        // the workers have sent. All other workers are blocked
        // trying to send the single value they have calculated
        // where-as the goroutine version reads/buffers a single
        // extra value from each worker.
        i, v, ok := reflect.Select(cases)
        if !ok {
            // Channel cases[i] has been closed, remove it
            // from our slice of cases and update our ids
            // mapping as well.
            cases = append(cases[:i], cases[i+1:]...)
            ids = append(ids[:i], ids[i+1:]...)
            continue
        }

        // Process each value
        fn(ids[i], v.String())
    }
}

[ Go操场上的完整代码.]

Answer 4

我们实际上对此主题进行了一些研究并找到了最佳解决方案。我们使用了reflect.Select一段时间，它很好地解决了这个问题。它比每个通道一个 goroutine 轻得多，并且操作简单。但不幸的是，它并没有真正支持大量的频道，这就是我们的情况，所以我们发现了一些有趣的东西并写了一篇关于它的博客文章： https: //cyolo.io/blog/how-we-enabled-dynamic-channel -在进行中按规模选择/

我将总结那里写的内容：我们为 32 以内的 2 次幂的每个结果静态创建了一批 select..case 语句，以及一个路由到不同情况并通过聚合通道聚合结果的函数。

此类批次的示例：

func select4(ctx context.Context, chanz []chan interface{}, res chan *r, r *r, i int) {
    select {
    case r.v, r.ok = <-chanz[0]:
        r.i = i + 0
        res <- r
    case r.v, r.ok = <-chanz[1]:
        r.i = i + 1
        res <- r
    case r.v, r.ok = <-chanz[2]:
        r.i = i + 2
        res <- r
    case r.v, r.ok = <-chanz[3]:
        r.i = i + 3
        res <- r
    case <-ctx.Done():
        break
    }
}

以及使用这些类型的批次聚合来自任意数量通道的第一个结果的逻辑select..case：

    for i < len(channels) {
        l = len(channels) - i
        switch {
        case l > 31 && maxBatchSize >= 32:
            go select32(ctx, channels[i:i+32], agg, rPool.Get().(*r), i)
            i += 32
        case l > 15 && maxBatchSize >= 16:
            go select16(ctx, channels[i:i+16], agg, rPool.Get().(*r), i)
            i += 16
        case l > 7 && maxBatchSize >= 8:
            go select8(ctx, channels[i:i+8], agg, rPool.Get().(*r), i)
            i += 8
        case l > 3 && maxBatchSize >= 4:
            go select4(ctx, channels[i:i+4], agg, rPool.Get().(*r), i)
            i += 4
        case l > 1 && maxBatchSize >= 2:
            go select2(ctx, channels[i:i+2], agg, rPool.Get().(*r), i)
            i += 2
        case l > 0:
            go select1(ctx, channels[i], agg, rPool.Get().(*r), i)
            i += 1
        }
    }