python第三方库系列之十七--multiprocessing库

说到并发，我们想到多线程和多进程。

那到底使用多进程还是多线程呢？这得看情况。我们的程序一般分为：

1）耗网络的（有很大一部分时间是在网络交互）；

2）耗CPU的（得充分利用多核）

在第一种情况，时间大部分被网络延时占用，所以使用多线程和多进程都差不多。

在第二种情况，时间的长短由CPU是否被充分利用决定，看《python第三方库系列之十二--多线程threading库》可知，多线程会出现锁机制，所以多进程更胜一筹，充分利用了CPU，节省了时间。

以下是一个多进程例子：

#coding=utf-8
import os
import traceback
import time
from multiprocessing import Process, Manager

CASE_COUNTS = 100
PROCESS_COUNTS = 10

def assign_task_index(index):
    try:
        if CASE_COUNTS % PROCESS_COUNTS == 0:
            section_num = CASE_COUNTS / PROCESS_COUNTS
        else:
            section_num = CASE_COUNTS / PROCESS_COUNTS + 1
        if CASE_COUNTS % section_num == 0:
            process_count = CASE_COUNTS / section_num
        else:
            process_count = CASE_COUNTS / section_num + 1
        start_index = section_num * (index - 1) + 1
        end_index = section_num * index
        if index == process_count:
            end_index = CASE_COUNTS
        #print start_index, end_index
        return start_index, end_index
    except Exception as e:
        print traceback.format_exc()

def do_single_process(index, q):
    start_index, stop_index = assign_task_index(index)
    for i in range(start_index, stop_index+1):
        try:
            is_even_number = i % 2
            if is_even_number == 0:
                q.put(i)
            else:
                #print i
                pass
        except Exception as e:
            print traceback.format_exc()


def do_multi_process():
    try:
        process_queue = []
        m = Manager()
        q = m.Queue()  # 用于进程间通信，记录运行结果
        for index in range(PROCESS_COUNTS) :
            try :
                pid = Process(target=do_single_process, args=(index+1, q))
            except :
                print traceback.format_exc()
                os._exit(-1)
            else :
                process_queue.append(pid)

        time_start = time.time()
        for pid in process_queue :
            pid.start()

        # 等待所有进程都完成
        for pid in process_queue:
            pid.join()

        time_end = time.time()
        time_delta = time_end - time_start
        print "总共个数：%s，偶数个数：%s，奇数个数：%s，耗时（秒）：%s" % \
              (str(CASE_COUNTS), str(q.qsize()), str(CASE_COUNTS - q.qsize()), str(time_delta))
    except Exception as e:
        print traceback.format_exc()


if __name__ == '__main__':
    do_multi_process()