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解決Golang中goroutine執行速度的問題

突然想到了之前一直沒留意的for循環中開goroutine的執行順序問題，就找了段代碼試了試，試了幾次后發現幾個有意思的地方，我暫時沒有精力往更深處挖掘，希望有golang大神能簡單說一說這幾個地方是怎么回事。

代碼：

package main  
import "fmt" 
func Count(ch chan int) {
	fmt.Println("Count doing")
	ch - 1
	fmt.Println("Counting")
}
 
func main() {
    chs := make([]chan int, 100)
	for i := 0; i  100; i++ {
		chs[i] = make(chan int)
		go Count(chs[i])
		fmt.Println("Count",i)
	}
	for i, ch := range chs {
		-ch
		fmt.Println("Counting ", i)
	}
}

試了幾次之后，反復的想goroutine執行的問題。

根據下面的輸出，我能看到的是：

1. for循環的速度比 for中開出goroutine并執行的速度執行的快

2. 但是開goroutine和執行第一個fmt的速度可能趕上 for循環的速度比如前12個count和count doing

3. 關鍵問題，第二個for循環執行的fmt竟然要比goroutine中的第二個fmt快？？（放入channel很耗時？）

4. main結束時，也就是第二個for循環結束時，還有goroutine中的第二個fmt沒執行

輸出：

Count 0
Count 1
Count 2
Count 3
Count 4
Count 5
Count 6
Count 7
Count 8
Count 9
Count 10
Count 11
Count doing
Count doing
Count doing
Count doing
Count doing
Count 12
Count doing
Count doing
Count doing
Count doing
Count doing
Count doing
Count doing
Count 13
Count 14
Count 15
Count 16
Count 17
Count 18
Count 19
Count 20
Count 21
Count doing
Count doing
Count doing
Count 22
Count doing
Count doing
Count doing
Count 23
Count 24
Count 25
Count 26
Count 27
Count 28
Count 29
Count 30
Count doing
Count 31
Count doing
Count doing
Count 32
Count 33
Count 34
Count 35
Count doing
Count 36
Count doing
Count doing
Count 37
Count 38
Count doing
Count doing
Count doing
Count doing
Count 39
Count 40
Count 41
Count 42
Count 43
Count doing
Count doing
Count 44
Count 45
Count 46
Count 47
Count doing
Count 48
Count 49
Count doing
Count doing
Count 50
Count 51
Count doing
Count doing
Count doing
Count doing
Count doing
Count 52
Count 53
Count doing
Count doing
Count doing
Count doing
Count 54
Count doing
Count 55
Count 56
Count 57
Count 58
Count 59
Count 60
Count 61
Count 62
Count 63
Count 64
Count 65
Count doing
Count doing
Count doing
Count 66
Count 67
Count 68
Count 69
Count doing
Count 70
Count doing
Count 71
Count 72
Count doing
Count 73
Count doing
Count doing
Count 74
Count doing
Count 75
Count 76
Count 77
Count doing
Count doing
Count doing
Count doing
Count 78
Count 79
Count 80
Count 81
Count 82
Count 83
Count 84
Count 85
Count 86
Count 87
Count 88
Count 89
Count 90
Count 91
Count 92
Count 93
Count 94
Count doing
Count doing
Count doing
Count doing
Count doing
Count doing
Count doing
Count doing
Count 95
Count doing
Count 96
Count doing
Count 97
Count 98
Count doing
Count 99
Count doing
Count doing
Counting  0
Counting  1
Counting  2
Counting  3
Counting  4
Counting  5
Counting  6
Count doing
Count doing
Counting  7
Counting  8
Count doing
Counting
Count doing
Counting  9
Counting
Count doing
Count doing
Count doing
Count doing
Count doing
Counting
Count doing
Count doing
Count doing
Counting
Count doing
Counting
Count doing
Counting  10
Counting  11
Counting
Count doing
Count doing
Count doing
Count doing
Count doing
Count doing
Counting
Count doing
Count doing
Counting
Counting
Count doing
Count doing
Count doing
Count doing
Counting
Count doing
Counting
Count doing
Count doing
Counting  12
Counting  13
Counting  14
Counting  15
Counting  16
Counting  17
Counting  18
Counting  19
Counting  20
Counting  21
Counting  22
Counting  23
Counting  24
Counting  25
Counting  26
Counting  27
Counting  28
Counting  29
Counting  30
Counting  31
Counting  32
Counting  33
Counting  34
Counting  35
Counting  36
Counting  37
Counting  38
Counting  39
Counting  40
Counting  41
Counting  42
Counting  43
Counting  44
Counting  45
Counting  46
Counting  47
Counting  48
Counting  49
Counting  50
Counting  51
Counting  52
Counting  53
Counting  54
Counting  55
Counting  56
Counting
Counting
Counting
Counting
Counting
Counting
Count doing
Counting
Count doing
Counting
Counting
Counting  57
Counting  58
Counting  59
Counting  60
Counting  61
Counting  62
Counting  63
Counting  64
Counting  65
Counting  66
Counting  67
Counting  68
Counting  69
Counting  70
Counting  71
Counting  72
Counting  73
Counting  74
Counting  75
Counting  76
Counting
Counting
Counting
Counting
Counting
Counting
Counting
Counting
Counting
Counting
Counting
Counting
Counting
Counting
Counting
Counting
Counting
Counting
Counting
Counting  77
Counting  78
Counting  79
Counting  80
Counting  81
Counting  82
Counting  83
Counting  84
Counting  85
Counting  86
Counting  87
Counting  88
Counting  89
Counting  90
Counting  91
Counting  92
Counting  93
Counting  94
Counting  95
Counting  96
Counting  97
Counting  98
Counting  99

補充：【golang】goroutine調度的坑

今天說說我遇到的一個小坑，關于goroutine 調度的問題。

關于goroutine的調度，網上資料已經一大堆了，這里就不再贅述了。

還是簡單的說一下我理解的goroutine的調度。goroutine是語言層面的，它和內核線程是M:N的關系，并且用了分段棧，是相當輕量了。

如果設置runtime.GOMAXPROCS為1，那么會有一個上下文G，在G上會有一個對應的內核線程M，內核線程M上可以對應很多個goroutine記作G，每個上下文都會有一個隊列稱作runqueue，在用go關鍵字開啟一個goroutine的時候，該goroutine就會被裝入runqueue中，然后被M用來執行，如果剛好有兩個goroutine在隊列里，先執行的goroutine因為執行一些耗時操作（系統調用，讀寫 channel，gosched 主動放棄，網絡IO）會被掛起（扔到全局runqueue），然后調度后面的goroutine。

好，重點在這里，看一下下面的一段代碼

func main(){
    runtime.GOMAXPROCS(1)
    waitGroup.Add(1)
    go func(){
        defer waitGroup.Done()
        for i := 0;i  20;i++ {
            fmt.Println("hello")
            f, _ := os.Open("./data")
            f.Write([]byte("hello"))
        }
    }()
    waitGroup.Add(1)
    go func(){
        defer waitGroup.Done()
        for {
        }
    }()
    waitGroup.Wait()
}

這段代碼你運行，你會發現，永遠都會被阻塞住，hello永遠都打印不出來

好，這里出現了兩個問題

1.為什么死循環的goroutine總是先運行？按理說不應該是隨機的嗎？

2.為什么死循環的goroutine會阻塞而沒有被掛起？

先看第二個問題。這里的話，我當時也很苦惱，于是在網上發了問題，得到的回復是，死循環不屬于上述任何一種需要被掛起的狀態，于是死循環的goroutine會一直運行，想象一個高并發的場景，如果其中一個goroutine因為某種原因陷入死循環了，當前執行這個goroutine的OS thread基本就會被一直執行這個goroutine，直到程序結束，這簡直是一場災難。但是，1.12 會修正這個小問題。我們還是默默的等待新版本發布吧。

再看第一個問題。為什么死循環的goroutine總是先運行？按理說不應該是隨機的嗎？測試過很多次，都是第二個goroutine先運行。嗯，其實就算是第二個goroutine先運行也是具有隨機性的，這關于golang的編譯器如何去實現隨機。看一下大佬的回答 :

不是說測試很多遍它就會一直這樣，語言規范沒有說必須是這個順序，那編譯器怎么實現都可以，因為都不違反規范。

所以你要把它看作是隨機的，不能依賴這種未確定的行為，不然很可能新版的編譯器就會破壞你依賴的事實。有些項目不敢升級編譯器版本，就是因為依賴了特定版本的編譯器的行為，一升級就壞了。

不是你自己測試很多遍你就能依賴它，編譯器、操作系統、硬件等等不同，都有可能出現不同的結果。可以依賴的只有語言規范（ https://golang.org/ref/spec ），編譯器實現者是一定會遵守的。

到這里也算是解決了上述的兩個問題了。

來看一下另外一個版本

func main(){
    runtime.GOMAXPROCS(1)
    waitGroup.Add(1)
    go func(){
        defer waitGroup.Done()
        for {
        }
    }()
    waitGroup.Add(1)
    go func(){
        defer waitGroup.Done()
        for i := 0;i  20;i++ {
            fmt.Println("hello")
            f, _ := os.Open("./data")
            f.Write([]byte("hello"))
            http.Get("http://www.baidu.com")
            fmt.Println("request successful")
        }
    }()
    waitGroup.Wait()
}

執行結果是，會先打印一個hello，然后陷入死循環，這也是說明了goroutine在遇到耗時操作或者系統調用的時候，后面的代碼都不會執行了（request successful 沒有被打印），會被拋到全局runqueue里去，然后執行runqueue中等待的goroutine

希望能夠幫助和我一樣正在學習golang的友軍們更好的理解goroutine的調度問題

以上為個人經驗，希望能給大家一個參考，也希望大家多多支持腳本之家。如有錯誤或未考慮完全的地方，望不吝賜教。

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