启动100个goroutines去抓取URL，这100个抓取完后再继续下一组, v2版本更简洁明了,

fetchv1.go

package main

import (
	"fmt"
	"runtime"
	"sync"
	"time"
)

const MAX = 100

func watching() {
	fmt.Printf("NumGoroutine: %d\n", runtime.NumGoroutine())
}

func doSomething(url int, wg *sync.WaitGroup) {
	// Fetch URL
	// TODO something...
	time.Sleep(100 * time.Microsecond)
	wg.Done()
}

func main() {
	runtime.GOMAXPROCS(runtime.NumCPU())
	urls := make(chan int, MAX)
	total := 10000000
	count := 0
	var wg sync.WaitGroup

	// Get urls
	go func() {
		for i := 0; i < total; i++ {
			urls <- i
		}
	}()

	fmt.Printf("Looking for fetch\n")
	t := time.Tick(1 * time.Second)
	for {
		select {
		case url := <-urls:
			wg.Add(1)
			go func(u int, wg *sync.WaitGroup) {
				doSomething(u, wg)
			}(url, &wg)
			count++
		case <-t:
			watching()
		}

		if count%MAX == 0 {
			wg.Wait()
			//fmt.Printf("N: %d ", count/MAX)
		}
		if count == total {
			break
		}
	}

	fmt.Printf("%d Done\n", count)
}

fetchv2.go

package main

import (
	"fmt"
	"runtime"
	"time"
)

func main() {
	runtime.GOMAXPROCS(runtime.NumCPU())
	c := make(chan bool, 100)
	t := time.Tick(time.Second)

	go func() {
		for {
			select {
			case <-t:
				watching()
			}
		}
	}()

	for i := 0; i < 10000000; i++ {
		c <- true
		go worker(i, c)
	}

	fmt.Println("Done")
}

func watching() {
	fmt.Printf("NumGoroutine: %d\n", runtime.NumGoroutine())
}

func worker(i int, c chan bool) {
	//fmt.Println("worker", i)
	time.Sleep(100 * time.Microsecond)
	<-c
}