在Golang中，Goroutine虽然很好，但是数量太多了，往往会带来很多麻烦，比如耗尽系统资源导致程序崩溃，或者CPU使用率过高导致系统忙不过来。所以我们可以限制下Goroutine的数量,这样就需要在每一次执行go之前判断goroutine的数量，如果数量超了，就要阻塞go的执行。第一时间想到的就是使用通道。每次执行的go之前向通道写入值，直到通道满的时候就阻塞了， ``` package main import "fmt" var ch chan int func elegance(){ <-ch fmt.Println("the ch value receive",ch) } func main(){ ch = make(chan int,5) for i:=0;i<10;i++{ ch <-1 fmt.Println("the ch value send",ch) go elegance() fmt.Println("the result i",i) } } ``` 运行: ``` > go run goroutine.go the ch value send 0xc00009c000 the result i 0 the ch value send 0xc00009c000 the result i 1 the ch value send 0xc00009c000 the result i 2 the ch value send 0xc00009c000 the result i 3 the ch value send 0xc00009c000 the result i 4 the ch value send 0xc00009c000 the result i 5 the ch value send 0xc00009c000 the ch value receive 0xc00009c000 the result i 6 the ch value receive 0xc00009c000 the ch value send 0xc00009c000 the result i 7 the ch value send 0xc00009c000 the result i 8 the ch value send 0xc00009c000 the result i 9 the ch value send 0xc00009c000 the ch value receive 0xc00009c000 the ch value receive 0xc00009c000 the ch value receive 0xc00009c000 the result i 10 the ch value send 0xc00009c000 the result i 11 the ch value send 0xc00009c000 the result i 12 the ch value send 0xc00009c000 the result i 13 the ch value send 0xc00009c000 the ch value receive 0xc00009c000 the ch value receive 0xc00009c000 the ch value receive 0xc00009c000 the ch value receive 0xc00009c000 the result i 14 the ch value receive 0xc00009c000 ``` ``` > go run goroutine.go the ch value send 0xc00007e000 the result i 0 the ch value send 0xc00007e000 the result i 1 the ch value send 0xc00007e000 the result i 2 the ch value send 0xc00007e000 the result i 3 the ch value send 0xc00007e000 the ch value receive 0xc00007e000 the result i 4 the ch value send 0xc00007e000 the ch value receive 0xc00007e000 the result i 5 the ch value send 0xc00007e000 the ch value receive 0xc00007e000 the result i 6 the ch value send 0xc00007e000 the result i 7 the ch value send 0xc00007e000 the ch value receive 0xc00007e000 the ch value receive 0xc00007e000 the ch value receive 0xc00007e000 the result i 8 the ch value send 0xc00007e000 the result i 9 ``` 这样每次同时运行的goroutine就被限制为5个了。但是新的问题于是就出现了，因为并不是所有的goroutine都执行完了，在main函数退出之后，还有一些goroutine没有执行完就被强制结束了。这个时候我们就需要用到sync.WaitGroup。使用WaitGroup等待所有的goroutine退出。 ``` package main import ( "fmt" "runtime" "sync" "time" ) // Pool Goroutine Pool type Pool struct { queue chan int wg *sync.WaitGroup } // New 新建一个协程池 func NewPool(size int) *Pool{ if size <=0{ size = 1 } return &Pool{ queue:make(chan int,size), wg:&sync.WaitGroup{}, } } // Add 新增一个执行 func (p *Pool)Add(delta int){ // delta为正数就添加 for i :=0;i<delta;i++{ p.queue <-1 } // delta为负数就减少 for i:=0;i>delta;i--{ <-p.queue } p.wg.Add(delta) } // Done 执行完成减一 func (p *Pool) Done(){ <-p.queue p.wg.Done() } // Wait 等待Goroutine执行完毕 func (p *Pool) Wait(){ p.wg.Wait() } func main(){ // 这里限制5个并发 pool := NewPool(5) fmt.Println("the NumGoroutine begin is:",runtime.NumGoroutine()) for i:=0;i<20;i++{ pool.Add(1) go func(i int) { time.Sleep(time.Second) fmt.Println("the NumGoroutine continue is:",runtime.NumGoroutine()) pool.Done() }(i) } pool.Wait() fmt.Println("the NumGoroutine done is:",runtime.NumGoroutine()) } ``` 运行: ``` the NumGoroutine begin is: 1 the NumGoroutine continue is: 6 the NumGoroutine continue is: 7 the NumGoroutine continue is: 6 the NumGoroutine continue is: 6 the NumGoroutine continue is: 6 the NumGoroutine continue is: 6 the NumGoroutine continue is: 6 the NumGoroutine continue is: 6 the NumGoroutine continue is: 6 the NumGoroutine continue is: 6 the NumGoroutine continue is: 6 the NumGoroutine continue is: 6 the NumGoroutine continue is: 6 the NumGoroutine continue is: 6 the NumGoroutine continue is: 6 the NumGoroutine continue is: 6 the NumGoroutine continue is: 6 the NumGoroutine continue is: 6 the NumGoroutine continue is: 3 the NumGoroutine continue is: 2 the NumGoroutine done is: 1 ``` 其中，Go的GOMAXPROCS默认值已经设置为CPU的核数， 这里允许我们的Go程序充分使用机器的每一个CPU,最大程度的提高我们程序的并发性能。runtime.NumGoroutine函数在被调用后，会返回系统中的处于特定状态的Goroutine的数量。这里的特指是指Grunnable\Gruning\Gsyscall\Gwaition。处于这些状态的Groutine即被看做是活跃的或者说正在被调度。 这里需要注意下：垃圾回收所在Groutine的状态也处于这个范围内的话，也会被纳入该计数器。