python 多进程 Multiprocessing 模块

Process 类

Process 类用来描述一个进程对象。创建子进程的时候，只需要传入一个执行函数和函数的参数即可完成 Process 示例的创建。

• star() 方法启动进程，
• join() 方法实现进程间的同步，等待所有进程退出。
• close() 用来阻止多余的进程涌入进程池 Pool 造成进程阻塞。

multiprocessing.Process(group=None, target=None, name=None, args=(), kwargs={}, *, daemon=None)

• 1

• target 是函数名字，需要调用的函数
• args 函数需要的参数，以 tuple 的形式传入

示例：

import multiprocessing
import os

def run_proc(name):
    print('Child process {0} {1} Running '.format(name, os.getpid()))

if __name__ == '__main__':
    print('Parent process {0} is Running'.format(os.getpid()))
    for i in range(5):
        p = multiprocessing.Process(target=run_proc, args=(str(i),))
        print('process start')
        p.start()
    p.join()
    print('Process close')

结果：

Parent process 809 is Running
process start
process start
process start
process start
process start
Child process 0 810 Running 
Child process 1 811 Running 
Child process 2 812 Running 
Child process 3 813 Running 
Child process 4 814 Running 
Process close

Pool

Pool 可以提供指定数量的进程供用户使用，默认是 CPU 核数。当有新的请求提交到 Poll 的时候，如果池子没有满，会创建一个进程来执行，否则就会让该请求等待。 
- Pool 对象调用 join 方法会等待所有的子进程执行完毕 
- 调用 join 方法之前，必须调用 close 
- 调用 close 之后就不能继续添加新的 Process 了

pool.apply_async

apply_async 方法用来同步执行进程，允许多个进程同时进入池子。

import multiprocessing
import os
import time

def run_task(name):
    print('Task {0} pid {1} is running, parent id is {2}'.format(name, os.getpid(), os.getppid()))
    time.sleep(1)
    print('Task {0} end.'.format(name))

if __name__ == '__main__':
    print('current process {0}'.format(os.getpid()))
    p = multiprocessing.Pool(processes=3)
    for i in range(6):
        p.apply_async(run_task, args=(i,))
    print('Waiting for all subprocesses done...')
    p.close()
    p.join()
    print('All processes done!')

结果：

current process 921
Waiting for all subprocesses done...
Task 0 pid 922 is running, parent id is 921
Task 1 pid 923 is running, parent id is 921
Task 2 pid 924 is running, parent id is 921
Task 0 end.
Task 3 pid 922 is running, parent id is 921
Task 1 end.
Task 4 pid 923 is running, parent id is 921
Task 2 end.
Task 5 pid 924 is running, parent id is 921
Task 3 end.
Task 4 end.
Task 5 end.
All processes done!

pool.apply

apply(func[, args[, kwds]])

该方法只能允许一个进程进入池子，在一个进程结束之后，另外一个进程才可以进入池子。

import multiprocessing
import os
import time

def run_task(name):
    print('Task {0} pid {1} is running, parent id is {2}'.format(name, os.getpid(), os.getppid()))
    time.sleep(1)
    print('Task {0} end.'.format(name))

if __name__ == '__main__':
    print('current process {0}'.format(os.getpid()))
    p = multiprocessing.Pool(processes=3)
    for i in range(6):
        p.apply(run_task, args=(i,))
    print('Waiting for all subprocesses done...')
    p.close()
    p.join()
    print('All processes done!')

结果：

Task 0 pid 928 is running, parent id is 927
Task 0 end.
Task 1 pid 929 is running, parent id is 927
Task 1 end.
Task 2 pid 930 is running, parent id is 927
Task 2 end.
Task 3 pid 928 is running, parent id is 927
Task 3 end.
Task 4 pid 929 is running, parent id is 927
Task 4 end.
Task 5 pid 930 is running, parent id is 927
Task 5 end.
Waiting for all subprocesses done...
All processes done!