semaphore是一個内置的計數器
- 每當調用acquire()時,内置計數器-1
- 每當調用release()時,内置計數器+1
計數器不能小于0,當計數器為0時,acquire()将阻塞線程直到其他線程調用release()。
來看下面的代碼:
import time
import threading
def foo():
time.sleep(2) #程式休息2秒
print("ok",time.ctime())
for i in range(20):
t1=threading.Thread(target=foo,args=()) #執行個體化一個線程
t1.start() #啟動線程
執行結果:
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,程式會在很短的時間内生成20個線程來列印一句話。
如果在主機執行IO密集型任務的時候再執行這種類型的程式時,計算機就有很大可能會當機。
這時候就可以為這段程式添加一個計數器功能,來限制一個時間點内的線程數量。
代碼如下:
import time
import threading
s1=threading.Semaphore(5) #添加一個計數器
def foo():
s1.acquire() #計數器獲得鎖
time.sleep(2) #程式休眠2秒
print("ok",time.ctime())
s1.release() #計數器釋放鎖
for i in range(20):
t1=threading.Thread(target=foo,args=()) #建立線程
t1.start() #啟動線程
執行結果:
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