多处理启动太多 Python VM 实例 [英] Multiprocessing launching too many instances of Python VM

问题描述

我正在编写一些多处理代码(Python 2.6.4、WinXP)来生成进程以运行后台任务.在玩一些琐碎的例子时，我遇到了一个问题，即我的代码只是不断产生新的进程，即使我只告诉它产生一个固定的数字.

I am writing some multiprocessing code (Python 2.6.4, WinXP) that spawns processes to run background tasks. In playing around with some trivial examples, I am running into an issue where my code just continuously spawns new processes, even though I only tell it to spawn a fixed number.

程序本身运行良好，但如果我查看 Windows 任务管理器，我不断看到新的python.exe"进程出现.随着程序的运行(最终使我的机器挨饿)，它们只会越来越多地产生.

The program itself runs fine, but if I look in Windows TaskManager, I keep seeing new 'python.exe' processes appear. They just keep spawning more and more as the program runs (eventually starving my machine).

例如，
我希望下面的代码能够启动 2 个 python.exe 进程.第一个是程序本身，第二个是它产生的子进程.知道我做错了什么吗?

For example,
I would expect the code below to launch 2 python.exe processes. The first being the program itself, and the second being the child process it spawns. Any idea what I am doing wrong?

import time
import multiprocessing


class Agent(multiprocessing.Process):
    def __init__(self, i):
        multiprocessing.Process.__init__(self)
        self.i = i

    def run(self):
        while True:
            print 'hello from %i' % self.i
            time.sleep(1)


agent = Agent(1)
agent.start()

推荐答案

您似乎没有仔细遵循文档中的指南，特别是 本节 讨论安全导入主模块".

It looks like you didn't carefully follow the guidelines in the documentation, specifically this section where it talks about "Safe importing of main module".

您需要使用 if __name__ == '__main__': 块来保护您的启动代码，否则我相信您会得到您所得到的.

You need to protect your launch code with an if __name__ == '__main__': block or you'll get what you're getting, I believe.

我认为这归结为多处理模块无法像在 Linux 上那样使用 os.fork()，在 Linux 中，已经运行的进程基本上克隆在内存中.在 Windows(没有这样的 fork())上，它必须运行一个新的 Python 解释器并告诉它导入你的主模块，然后在完成后执行 start/run 方法.如果您有模块级别"的代码，不受名称检查的保护，那么在导入过程中它会重新开始整个序列，无穷无尽

I believe it comes down to the multiprocessing module not being able to use os.fork() as it does on Linux, where an already-running process is basically cloned in memory. On Windows (which has no such fork()) it must run a new Python interpreter and tell it to import your main module and then execute the start/run method once that's done. If you have code at "module level", unprotected by the name check, then during the import it starts the whole sequence over again, ad infinitum

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