python中的线程之semaphore控制每次信号量

semaphore是一个内置的计数器

每当调用acquire()时,内置计数器-1
每当调用release()时,内置计数器+1

计数器不能小于0,当计数器为0时,acquire()将阻塞线程直到其他线程调用release()
来看下面的代码:

import time
import threading
def foo():
	time.sleep(2)	#程序休息2秒
	print("ok",time.ctime())
for i in range(20):
	t1=threading.Thread(target=foo,args=())	#实例化一个线程
	t1.start()	#启动线程

执行结果:

ok Tue Jul 18 20:05:58 2017
ok Tue Jul 18 20:05:58 2017
ok Tue Jul 18 20:05:58 2017
ok Tue Jul 18 20:05:58 2017
ok Tue Jul 18 20:05:58 2017
ok Tue Jul 18 20:05:58 2017
ok Tue Jul 18 20:05:58 2017
ok Tue Jul 18 20:05:58 2017
ok Tue Jul 18 20:05:58 2017
ok Tue Jul 18 20:05:58 2017
ok Tue Jul 18 20:05:58 2017
ok Tue Jul 18 20:05:58 2017
ok Tue Jul 18 20:05:58 2017
ok Tue Jul 18 20:05:58 2017
ok Tue Jul 18 20:05:58 2017
ok Tue Jul 18 20:05:58 2017
ok Tue Jul 18 20:05:58 2017
ok Tue Jul 18 20:05:58 2017
ok Tue Jul 18 20:05:58 2017
ok Tue Jul 18 20:05:58 2017

可以看到,程序会在很短的时间内生成20个线程来打印一句话。

如果在主机执行IO密集型任务的时候再执行这种类型的程序时,计算机就有很大可能会宕机。
这时候就可以为这段程序添加一个计数器功能,来限制一个时间点内的线程数量。

代码如下:

import time
import threading
s1=threading.Semaphore(5)	#添加一个计数器
def foo():
	s1.acquire()	#计数器获得锁
	time.sleep(2)	#程序休眠2秒
	print("ok",time.ctime())
	s1.release()	#计数器释放锁

for i in range(20):
	t1=threading.Thread(target=foo,args=())	#创建线程
	t1.start()	#启动线程	

执行结果:

ok Tue Jul 18 20:04:38 2017
ok Tue Jul 18 20:04:38 2017
ok Tue Jul 18 20:04:38 2017
ok Tue Jul 18 20:04:38 2017
ok Tue Jul 18 20:04:38 2017
ok Tue Jul 18 20:04:40 2017
ok Tue Jul 18 20:04:40 2017
ok Tue Jul 18 20:04:40 2017
ok Tue Jul 18 20:04:40 2017
ok Tue Jul 18 20:04:40 2017
ok Tue Jul 18 20:04:42 2017
ok Tue Jul 18 20:04:42 2017
ok Tue Jul 18 20:04:42 2017
ok Tue Jul 18 20:04:42 2017
ok Tue Jul 18 20:04:42 2017
ok Tue Jul 18 20:04:44 2017
ok Tue Jul 18 20:04:44 2017
ok Tue Jul 18 20:04:44 2017
ok Tue Jul 18 20:04:44 2017
ok Tue Jul 18 20:04:44 2017

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