Python3多进程 multiprocessing 模块实例详解

时间:2022-11-25 10:57:45

本文实例讲述了Python3多进程 multiprocessing 模块。分享给大家供大家参考,具体如下:

多进程 Multiprocessing 模块

multiprocessing 模块官方说明文档

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)

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:

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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。

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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。

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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!

Queue 进程间通信

Queue 用来在多个进程间通信。Queue 有两个方法,get 和 put。

put 方法

Put 方法用来插入数据到队列中,有两个可选参数,blocked 和 timeout。
- blocked = True(默认值),timeout 为正

该方法会阻塞 timeout 指定的时间,直到该队列有剩余空间。如果超时,抛出 Queue.Full 异常。

blocked = False

如果 Queue 已满,立刻抛出 Queue.Full 异常

get 方法

get 方法用来从队列中读取并删除一个元素。有两个参数可选,blocked 和 timeout
- blocked = False (默认),timeout 正值

等待时间内,没有取到任何元素,会抛出 Queue.Empty 异常。

blocked = True

Queue 有一个值可用,立刻返回改值;Queue 没有任何元素,

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from multiprocessing import Process, Queue
import os, time, random
# 写数据进程执行的代码:
def proc_write(q,urls):
  print('Process(%s) is writing...' % os.getpid())
  for url in urls:
    q.put(url)
    print('Put %s to queue...' % url)
    time.sleep(random.random())
# 读数据进程执行的代码:
def proc_read(q):
  print('Process(%s) is reading...' % os.getpid())
  while True:
    url = q.get(True)
    print('Get %s from queue.' % url)
if __name__=='__main__':
  # 父进程创建Queue,并传给各个子进程:
  q = Queue()
  proc_writer1 = Process(target=proc_write, args=(q,['url_1', 'url_2', 'url_3']))
  proc_writer2 = Process(target=proc_write, args=(q,['url_4','url_5','url_6']))
  proc_reader = Process(target=proc_read, args=(q,))
  # 启动子进程proc_writer,写入:
  proc_writer1.start()
  proc_writer2.start()
  # 启动子进程proc_reader,读取:
  proc_reader.start()
  # 等待proc_writer结束:
  proc_writer1.join()
  proc_writer2.join()
  # proc_reader进程里是死循环,无法等待其结束,只能强行终止:
  proc_reader.terminate()

结果:

Process(1083) is writing...
Put url_1 to queue...
Process(1084) is writing...
Put url_4 to queue...
Process(1085) is reading...
Get url_1 from queue.
Get url_4 from queue.
Put url_5 to queue...
Get url_5 from queue.
Put url_2 to queue...
Get url_2 from queue.
Put url_6 to queue...
Get url_6 from queue.
Put url_3 to queue...
Get url_3 from queue.

Pipe 进程间通信

常用来在两个进程间通信,两个进程分别位于管道的两端。

multiprocessing.Pipe([duplex])

示例一和示例二,也是网上找的别人的例子,尝试理解并增加了注释而已。网上的例子,大多是例子一和例子二在一起的,这里分开来看,比较容易理解。

示例一:

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from multiprocessing import Process, Pipe
def send(pipe):
  pipe.send(['spam'] + [42, 'egg'])  # send 传输一个列表
  pipe.close()
if __name__ == '__main__':
  (con1, con2) = Pipe()              # 创建两个 Pipe 实例
  sender = Process(target=send, args=(con1, ))   # 函数的参数,args 一定是实例化之后的 Pip 变量,不能直接写 args=(Pip(),)
  sender.start()                  # Process 类启动进程
  print("con2 got: %s" % con2.recv())       # 管道的另一端 con2 从send收到消息
  con2.close()                   # 关闭管道

结果:

con2 got: ['spam', 42, 'egg']

示例二:

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from multiprocessing import Process, Pipe
def talk(pipe):
  pipe.send(dict(name='Bob', spam=42))      # 传输一个字典
  reply = pipe.recv()               # 接收传输的数据
  print('talker got:', reply)
if __name__ == '__main__':
  (parentEnd, childEnd) = Pipe()         # 创建两个 Pipe() 实例,也可以改成 conf1, conf2
  child = Process(target=talk, args=(childEnd,)) # 创建一个 Process 进程,名称为 child
  child.start()                  # 启动进程
  print('parent got:', parentEnd.recv())     # parentEnd 是一个 Pip() 管道,可以接收 child Process 进程传输的数据
  parentEnd.send({x * 2 for x in 'spam'})     # parentEnd 是一个 Pip() 管道,可以使用 send 方法来传输数据
  child.join()                  # 传输的数据被 talk 函数内的 pip 管道接收,并赋值给 reply
  print('parent exit')

结果:

parent got: {'name': 'Bob', 'spam': 42}
talker got: {'ss', 'aa', 'pp', 'mm'}
parent exit

希望本文所述对大家Python程序设计有所帮助。

原文链接:https://blog.csdn.net/cityzenoldwang/article/details/78584175