python多线程threading使用Semaphore或BoundedSemaphore实现并发限制

Myth丶恋晨 2022-06-15 08:43 658阅读 0赞

信号量：

class threading.Semaphore(\[value\])  
values是一个内部计数，values默认是1，如果小于0，则会抛出 ValueError 异常，可以用于控制线程数并发数

1.acquire(\[blocking\])  
2.release()

例如：  
可用用来限制数据库的连接数

maxconnections = 5
    ...
    pool_sema = BoundedSemaphore(value=maxconnections)
    
    pool_sema.acquire()
    conn = connectdb()
    ... use connection ...
    conn.close()
    pool_sema.release()

例如：

# encoding: UTF-8
    import threading
    import time
    
    def showfun(n):
        print "%s start -- %d"%(time.ctime(),n)
        print "working"
        time.sleep(2)
        print "%s end -- %d" % (time.ctime(), n)
        semlock.release()
    
    if __name__ == '__main__':
        maxconnections = 5
        semlock = threading.BoundedSemaphore(maxconnections)
        list=[]
        for i in range(8):
            semlock.acquire()
            t=threading.Thread(target=showfun, args=(i,))
            list.append(t)
            t.start()

结果：

Sat May 27 02:43:00 2017 start -- 0
    working
    Sat May 27 02:43:00 2017 start -- 1
    working
    Sat May 27 02:43:00 2017 start -- 2
    working
    Sat May 27 02:43:00 2017 start -- 3
    working
    Sat May 27 02:43:00 2017 start -- 4
    working
    Sat May 27 02:43:02 2017 end -- 0
    Sat May 27 02:43:02 2017 end -- 1
    Sat May 27 02:43:02 2017 end -- 2
    Sat May 27 02:43:02 2017 end -- 3
    Sat May 27 02:43:02 2017 end -- 4
    Sat May 27 02:43:02 2017 start -- 5
    working
    Sat May 27 02:43:02 2017 start -- 6
    working
    Sat May 27 02:43:02 2017 start -- 7
    working
    Sat May 27 02:43:04 2017 end -- 5
    Sat May 27 02:43:04 2017 end -- 6
    Sat May 27 02:43:04 2017 end -- 7

线程数限制到5个，因此等待0-4完毕之后5-7才能请求到资源进行执行