python 64式: 第15式、多进程，信号量与进程间通信

#!/usr/bin/env python
# -*- coding: utf-8 -*-

import multiprocessing
import time

'''
关键
1 信号量
multiprocessing.Semaphore(n):
n表示资源总数
作用: 控制对共享资源的访问数量
适用: 池的最大连接数
本质: 相当于N把锁，获取其中一把就可以执行
信号量总数N在构造时传入
s = Semaphore(N)
如果信号量为0:进程堵塞，直到信号量大于0
适用:控制对共享资源的访问数量，例如池的最大连接数，访问服务器，文件
信号量基于计数器，每调用一次acquire(),计数器可减去1；
每调用一次release(),计数器加1，当计数器为0，acquire()被阻塞。

2 事件
multiprocessing.Event:
作用:实现进程之间同步通信
默认值为false:
set(): 表示这是为True
wait():等待

ref:
https://www.cnblogs.com/lidagen/p/7252247.html
https://www.cnblogs.com/kaituorensheng/p/4445418.html
'''

def first(event):
    time.sleep(3)
    print "begin set flag"
    event.set()

def second(event):
    print "wait first process"
    event.wait()
    print "end wait first process"

def processEvent():
    myEvent = multiprocessing.Event()
    firstProc = multiprocessing.Process(target=first, args=(myEvent,))
    secondProc = multiprocessing.Process(target=second, args=(myEvent,))
    secondProc.start()
    firstProc.start()
    firstProc.join()
    secondProc.join()

def smaphoreTask(semaphore, msg):
    semaphore.acquire()
    # print "hello: %s" % (msg)
    print "{name} acquire".format(name=multiprocessing.current_process().name)
    time.sleep(msg)
    semaphore.release()
    print "{name} release".format(name=multiprocessing.current_process().name)


def processSemaphore():
    s = multiprocessing.Semaphore(multiprocessing.cpu_count())
    processes = []
    for x in range(8):
        p = multiprocessing.Process(target=smaphoreTask, args=(s,x,))
        processes.append(p)
    start = time.time()
    for p in processes:
        p.start()
    for p in processes:
        p.join()
    end = time.time()
    print "8 process cost: %s seconds" % (end - start)

def process():
    processSemaphore()
    processEvent()

if __name__ == "__main__":
    process()