Multiprocess + Semaphore Semaphore

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 通过下面一个例子进行理解。 
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 <pre>from multiprocessing import Process
from multiprocessing import Semaphore
import datetime
import time
import multiprocessing
 
def worker(s, i):
 s.acquire()
 print(multiprocessing.current_process().name + " acquire",datetime.datetime.now())
 time.sleep(i)
 print(multiprocessing.current_process().name + " release",datetime.datetime.now(),"\n")
 s.release()
 
if __name__ == "__main__":
 s = multiprocessing.Semaphore(2)
 for i in range(5):
 p = multiprocessing.Process(target = worker, args=(s, i*2))
 p.start()</pre> 
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 运行结果： 
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 <img src="http://images2017.cnblogs.com/blog/1255548/201711/1255548-20171120232934790-2105426550.png" alt="" />
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 分析：
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 p = multiprocessing.Process(……) defines five processes, p.start five processes in parallel, resulting in the result of the semaphore. 
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 s = multiprocessing.Semaphore(2) defines a maximum semaphore of 2, release: +1 acquire: -1

 22:41:30
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 第一步，五个进程并发执行，进程1执行并等待0s，S-1=1
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 In the second step, five processes are executed concurrently, process 2 executes and waits for 2s, S-1=0 ; <div style="white-space: pre-wrap; text-indent: 56px; line-height: 1.75; font-size: 14px">
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 The third step, because process 1 is executed, and the waiting time is 0, and process 2 needs to wait 2s. So this step must be the execution of process 1, and the execution of process 1 is completed, and the semaphore +1 and enters non-blocking. 
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 Step 4, process 2 enters and waits for 2s, so Only three processes 3, 4, and 5 are left in parallel, process 4 executes and waits for 6s, S-1=0
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 Step 5, process 2 waits for 2s to complete, process 2 executes, S+1=1
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 Step 6, process 4 is waiting, only two processes 3 and 5 are left in parallel, process 3 executes and waits for 4s, S- 1=0
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 22:41:36
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 Step 7, process 4 is waiting for 6s, process 3 is 4s, so process 3 executes , S+1=1
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 Step 8, Process 4 Wait, process 5 executes and waits for 8s, S-1=0
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 第九步，进程4执行，S+1=1
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 22:41:44
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 第十步，进程5执行，S-1=0
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Multiprocess + Semaphore Semaphore

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