Go 语言笔试面试题(并发编程)

func main() {
	st := time.Now()
	ch := make(chan bool)
	go func ()  {
		time.Sleep(time.Second * 2)
		<-ch
	}()
	ch <- true  // 无缓冲，发送方阻塞直到接收方接收到数据。
	fmt.Printf("cost %.1f s\n", time.Now().Sub(st).Seconds())
	time.Sleep(time.Second * 5)
}

func main() {
	st := time.Now()
	ch := make(chan bool, 2)
	go func ()  {
		time.Sleep(time.Second * 2)
		<-ch
	}()
	ch <- true
	ch <- true // 缓冲区为 2，发送方不阻塞，继续往下执行
	fmt.Printf("cost %.1f s\n", time.Now().Sub(st).Seconds()) // cost 0.0 s
	ch <- true // 缓冲区使用完，发送方阻塞，2s 后接收方接收到数据，释放一个插槽，继续往下执行
	fmt.Printf("cost %.1f s\n", time.Now().Sub(st).Seconds()) // cost 2.0 s
	time.Sleep(time.Second * 5)
}

func query() int {
	ch := make(chan int)
	for i := 0; i < 1000; i++ {
		go func() { ch <- 0 }()
	}
	return <-ch
}

func main() {
	for i := 0; i < 4; i++ {
		query()
		fmt.Printf("goroutines: %d\n", runtime.NumGoroutine())
	}
}
// goroutines: 1001
// goroutines: 2000
// goroutines: 2999
// goroutines: 3998

func request(url string, wg *sync.WaitGroup) {
	i := 0
	for {
		if _, err := http.Get(url); err == nil {
			// write to db
			break
		}
		i++
		if i >= 3 {
			break
		}
		time.Sleep(time.Second)
	}
	wg.Done()
}

func main() {
	var wg sync.WaitGroup
	for i := 0; i < 1000; i++ {
		wg.Add(1)
		go request(fmt.Sprintf("https://127.0.0.1:8080/%d", i), &wg)
	}
	wg.Wait()
}

runtime.GOMAXPROCS(1) // 限制同时执行Go代码的操作系统线程数为 1