突然想到了之前一直沒留意的for循環(huán)中開goroutine的執(zhí)行順序問題，就找了段代碼試了試，試了幾次后發(fā)現(xiàn)幾個有意思的地方，我暫時沒有精力往更深處挖掘，希望有g(shù)olang大神能簡單說一說這幾個地方是怎么回事。
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(fù)的想goroutine執(zhí)行的問題。
1. for循環(huán)的速度 比 for中開出goroutine并執(zhí)行的速度 執(zhí)行的快
2. 但是 開goroutine和執(zhí)行第一個fmt的速度可能趕上 for循環(huán)的速度 比如前12個count和count doing
3. 關(guān)鍵問題，第二個for循環(huán)執(zhí)行的fmt竟然要比goroutine中的第二個fmt快？？（放入channel很耗時？）
4. main結(jié)束時，也就是第二個for循環(huán)結(jié)束時， 還有g(shù)oroutine中的第二個fmt沒執(zhí)行
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(diào)度的坑
今天說說我遇到的一個小坑， 關(guān)于goroutine 調(diào)度的問題。
關(guān)于goroutine的調(diào)度，網(wǎng)上資料已經(jīng)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(diào)度。goroutine是語言層面的，它和內(nèi)核線程是M:N的關(guān)系，并且用了分段棧，是相當(dāng)輕量了。
如果設(shè)置runtime.GOMAXPROCS為1，那么會有一個上下文G，在G上會有一個對應(yīng)的內(nèi)核線程M，內(nèi)核線程M上可以對應(yīng)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(guān)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(zhí)行，如果剛好有兩個goroutine在隊列里，先執(zhí)行的goroutine因為執(zhí)行一些耗時操作（系統(tǒng)調(diào)用，讀寫 channel，gosched 主動放棄，網(wǎng)絡(luò)IO）會被掛起（扔到全局runqueue），然后調(diào)度后面的goroutine。
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(fā)現(xiàn)，永遠都會被阻塞住，hello永遠都打印不出來
1.為什么死循環(huán)的goroutine總是先運行？按理說不應(yīng)該是隨機的嗎？
2.為什么死循環(huán)的goroutine會阻塞而沒有被掛起？
先看第二個問題。這里的話，我當(dāng)時也很苦惱，于是在網(wǎng)上發(fā)了問題，得到的回復(fù)是，死循環(huán)不屬于上述任何一種需要被掛起的狀態(tài)，于是死循環(huán)的goroutine會一直運行，想象一個高并發(fā)的場景，如果其中一個goroutine因為某種原因陷入死循環(huán)了，當(dāng)前執(zhí)行這個goroutine的OS thread基本就會被一直執(zhí)行這個goroutine，直到程序結(jié)束，這簡直是一場災(zāi)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(fā)布吧。
再看第一個問題。為什么死循環(huán)的goroutine總是先運行？按理說不應(yīng)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(guān)于golang的編譯器如何去實現(xiàn)隨機?？匆幌麓罄械幕卮?:
不是說測試很多遍它就會一直這樣，語言規(guī)范沒有說必須是這個順序，那編譯器怎么實現(xiàn)都可以，因為都不違反規(guī)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(tǒng)、硬件等等不同，都有可能出現(xiàn)不同的結(jié)果?？梢砸蕾嚨闹挥姓Z言規(guī)范（ https://golang.org/ref/spec ），編譯器實現(xiàn)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。
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(zhí)行結(jié)果是，會先打印一個hello，然后陷入死循環(huán)，這也是說明了goroutine在遇到耗時操作或者系統(tǒng)調(diào)用的時候，后面的代碼都不會執(zhí)行了（request successful 沒有被打?。?，會被拋到全局runqueue里去，然后執(zhí)行runqueue中等待的goroutine
希望能夠幫助和我一樣正在學(xué)習(xí)golang的友軍們更好的理解goroutine的調(diào)度問題
以上為個人經(jīng)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。
標(biāo)簽：梅河口 重慶 雞西 吐魯番 欽州 銅川 汕頭 蘭州
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