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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,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()

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

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

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