Python 中 Ctrl+C 不能终止 Multiprocessing Pool 的解决方案

本文理论上对multiprocessing.dummy的Pool同样有效。

python2.x中multiprocessing提供的基于函数进程池,join后陷入内核态,按下ctrl+c不能停止所有的进程并退出。即必须ctrl+z后找到残留的子进程,把它们干掉。先看一段ctrl+c无效的代码:

#!/usr/bin/env python
import multiprocessing
import os
import time


def do_work(x):
    print 'Work Started: %s' % os.getpid()
    time.sleep(10)
    return x * x


def main():
    pool = multiprocessing.Pool(4)
    try:
        result = pool.map_async(do_work, range(8))
        pool.close()
        pool.join()
        print result
    except KeyboardInterrupt:
        print 'parent received control-c'
        pool.terminate()
        pool.join()
 

if __name__ == "__main__":
    main()

这段代码运行后,按^c一个进程也杀不掉,最后会残留包括主进程在内共5个进程(1+4),kill掉主进程能让其全部退出。很明显,使用进程池时KeyboardInterrupt不能被进程捕捉。解决方法有两种。

方案一

下面这段是python源码里multiprocessing下的pool.py中的一段,ApplyResult就是Pool用来保存函数运行结果的类

class ApplyResult(object):

    def __init__(self, cache, callback):
        self._cond = threading.Condition(threading.Lock())
        self._job = job_counter.next()
        self._cache = cache
        self._ready = False
        self._callback = callback
        cache[self._job] = self

而下面这段代码也是^c无效的代码

if __name__ == '__main__':
    import threading

    cond = threading.Condition(threading.Lock())
    cond.acquire()
    cond.wait()
    print "done"

很明显,threading.Condition(threading.Lock())对象无法接收KeyboardInterrupt,但稍微修改一下,给cond.wait()一个timeout参数即可,这个timeout可以在map_async后用get传递,把

result = pool.map_async(do_work, range(4))

改为

result = pool.map_async(do_work, range(4)).get(1)

就能成功接收^c了,get里面填1填99999还是0xffff都行

方案二

另一种方法当然就是自己写进程池了,需要使用队列,贴一段代码感受下

#!/usr/bin/env python
import multiprocessing, os, signal, time, Queue

def do_work():
    print 'Work Started: %d' % os.getpid()
    time.sleep(2)
    return 'Success'

def manual_function(job_queue, result_queue):
    signal.signal(signal.SIGINT, signal.SIG_IGN)
    while not job_queue.empty():
        try:
            job = job_queue.get(block=False)
            result_queue.put(do_work())
        except Queue.Empty:
            pass
        #except KeyboardInterrupt: pass

def main():
    job_queue = multiprocessing.Queue()
    result_queue = multiprocessing.Queue()

    for i in range(6):
        job_queue.put(None)

    workers = []
    for i in range(3):
        tmp = multiprocessing.Process(target=manual_function,
                                      args=(job_queue, result_queue))
        tmp.start()
        workers.append(tmp)

    try:
        for worker in workers:
            worker.join()
    except KeyboardInterrupt:
        print 'parent received ctrl-c'
        for worker in workers:
            worker.terminate()
            worker.join()

    while not result_queue.empty():
        print result_queue.get(block=False)

if __name__ == "__main__":
    main()

方案三

使用一个全局变量eflag作标识,让SIG_INT信号绑定一个处理函数,在其中对eflag的值更改,线程的函数中以eflag的值判定作为while的条件,把语句写在循环里,老实说这个方案虽然可以用,但是简直太差劲。线程肯定是可行的,进程应该还需要单独共享变量,非常不推荐的方式

常见的错误方案

这个必须要提一下,我发现segmentfault上都有人被误导了

理论上,在Pool初始化时传递一个initializer函数,让子进程忽略SIGINT信号,也就是^c,然后Pool进行terminate处理。代码

#!/usr/bin/env python
import multiprocessing
import os
import signal
import time


def init_worker():
    signal.signal(signal.SIGINT, signal.SIG_IGN)


def run_worker(x):
    print "child: %s" % os.getpid()
    time.sleep(20)
    return x * x


def main():
    pool = multiprocessing.Pool(4, init_worker)
    try:
        results = []
        print "Starting jobs"
        for x in range(8):
            results.append(pool.apply_async(run_worker, args=(x,)))

        time.sleep(5)
        pool.close()
        pool.join()
        print [x.get() for x in results]
    except KeyboardInterrupt:
        print "Caught KeyboardInterrupt, terminating workers"
        pool.terminate()
        pool.join()


if __name__ == "__main__":
    main()

然而这段代码只有在运行在time.sleep(5)处的时候才能用ctrl+c中断,即前5s你按^c有效,一旦pool.join()后则完全无效!

建议

先确认是否真的需要用到多进程,如果是IO多的程序建议用多线程或协程,计算特别多则用多进程。如果非要用多进程,可以利用Python3的concurrent.futures包(python2.x也能装),编写更加简单易用的多线程/多进程代码,其使用和Java的concurrent框架有些相似.
经过亲自验证,ProcessPoolExecutor是没有^c的问题的,要用多进程建议使用它

参考

  1. http://bryceboe.com/2010/08/26/python-multiprocessing-and-keyboardinterrupt/#georges

  2. http://stackoverflow.com/questions/1408356/keyboard-interrupts-with-pythons-multiprocessing-pool#comment12678760_6191991

    原文作者:quietin
    原文地址: https://segmentfault.com/a/1190000004172444
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