Open-falcon semaphore的源码实现

semaphore即信号量，Open-falcon在将queue中的数据发送给graph时，用semaphore控制并发的goroutine数量，避免产生大量的发送goroutine。

func forward2GraphTask(Q *list.SafeListLimited, node string, addr string, concurrent int) {
    sema := nsema.NewSemaphore(concurrent)
    for {
        items := Q.PopBackBy(batch)
        sema.Acquire()
        go func(addr string, graphItems []*cmodel.GraphItem, count int) {
            defer sema.Release()
            err = GraphConnPools.Call(addr, "Graph.Send", graphItems, resp)
            ....
        }
        ....
    }
}

semaphore使用channel实现：

• 定义一个len的channel；
• 当申请semaphore时，入队channel一个元素，当超过len时，阻塞住，也就是申请失败；
• 当释放semaphore是，出队channel一个元素；

type Semaphore struct {
   bufSize int
   channel chan int8
}

func NewSemaphore(concurrencyNum int) *Semaphore {
   return &Semaphore{channel: make(chan int8, concurrencyNum), bufSize: concurrencyNum}
}

申请semaphore:

//channel入队成功，返回true；否则返回false；
func (this *Semaphore) TryAcquire() bool {
   select {
   case this.channel <- int8(0):
      return true
   default:
      return false
   }
}

//若channel未满，入队返回；否则，阻塞
func (this *Semaphore) Acquire() {
   this.channel <- int8(0)
}

释放semaphore: channel出队一个元素

func (this *Semaphore) Release() {
   <-this.channel
}

查询可用的semaphore:

func (this *Semaphore) AvailablePermits() int {
   return this.bufSize - len(this.channel)
}