is twisted incompatible with multiprocessing events and queues?

Question

I am trying to simulate a network of applications that run using twisted. As part of my simulation I would like to synchronize certain events and be able to feed each process large amounts of data. I decided to use multiprocessing Events and Queues. However, my processes are getting hung.

I wrote the example code below to illustrate the problem. Specifically, (about 95% of the time on my sandy bridge machine), the 'run_in_thread' function finishes, however the 'print_done' callback is not called until after I press Ctrl-C.

Additionally, I can change several things in the example code to make this work more reliably such as: reducing the number of spawned processes, calling self.ready.set from reactor_ready, or changing the delay of deferLater.

I am guessing there is a race condition somewhere between the twisted reactor and blocking multiprocessing calls such as Queue.get() or Event.wait()?

What exactly is the problem I am running into? Is there a bug in my code that I am missing? Can I fix this or is twisted incompatible with multiprocessing events/queues?

Secondly, would something like spawnProcess or Ampoule be the recommended alternative? (as suggested in Mix Python Twisted with multiprocessing?)

Edits (as requested):

I've run into problems with all the reactors I've tried glib2reactor selectreactor, pollreactor, and epollreactor. The epollreactor seems to give the best results and seems to work fine for the example given below but still gives me the same (or a similar) problem in my application. I will continue investigating.

I'm running Gentoo Linux kernel 3.3 and 3.4, python 2.7, and I've tried Twisted 10.2.0, 11.0.0, 11.1.0, 12.0.0, and 12.1.0.

In addition to my sandy bridge machine, I see the same issue on my dual core amd machine.

#!/usr/bin/python
# -*- coding: utf-8 *-*

from twisted.internet import reactor
from twisted.internet import threads
from twisted.internet import task

from multiprocessing import Process
from multiprocessing import Event

class TestA(Process):
    def __init__(self):
        super(TestA, self).__init__()
        self.ready = Event()
        self.ready.clear()
        self.start()

    def run(self):
        reactor.callWhenRunning(self.reactor_ready)
        reactor.run()

    def reactor_ready(self, *args):
        task.deferLater(reactor, 1, self.node_ready)
        return args

    def node_ready(self, *args):
        print 'node_ready'
        self.ready.set()
        return args

def reactor_running():
    print 'reactor_running'
    df = threads.deferToThread(run_in_thread)
    df.addCallback(print_done)

def run_in_thread():
    print 'run_in_thread'
    for n in processes:
        n.ready.wait()

def print_done(dfResult=None):
    print 'print_done'
    reactor.stop()

if __name__ == '__main__':
    processes = [TestA() for i in range(8)]
    reactor.callWhenRunning(reactor_running)
    reactor.run()

Answer 1

The short answer is yes, Twisted and multiprocessing are not compatible with each other, and you cannot reliably use them as you are attempting to.

On all POSIX platforms, child process management is closely tied to SIGCHLD handling. POSIX signal handlers are process-global, and there can be only one per signal type.

Twisted and stdlib multiprocessing cannot both have a SIGCHLD handler installed. Only one of them can. That means only one of them can reliably manage child processes. Your example application doesn't control which of them will win that ability, so I would expect there to be some non-determinism in its behavior arising from that fact.

However, the more immediate problem with your example is that you load Twisted in the parent process and then use multiprocessing to fork and not exec all of the child processes. Twisted does not support being used like this. If you fork and then exec, there's no problem. However, the lack of an exec of a new process (perhaps a Python process using Twisted) leads to all kinds of extra shared state which Twisted does not account for. In your particular case, the shared state that causes this problem is the internal "waker fd" which is used to implement deferToThread. With the fd shared between the parent and all the children, when the parent tries to wake up the main thread to deliver the result of the deferToThread call, it most likely wakes up one of the child processes instead. The child process has nothing useful to do, so that's just a waste of time. Meanwhile the main thread in the parent never wakes up and never notices your threaded task is done.

It's possible you can avoid this issue by not loading any of Twisted until you've already created the child processes. This would turn your usage into a single-process use case as far as Twisted is concerned (in each process, it would be initially loaded, and then that process would not go on to fork at all, so there's no question of how fork and Twisted interact anymore). This means not even importing Twisted until after you've created the child processes.

Of course, this only helps you out as far as Twisted goes. Any other libraries you use could run into similar trouble (you mentioned glib2, that's a great example of another library that will totally choke if you try to use it like this).

I highly recommend not using the multiprocessing module at all. Instead, use any multi-process approach that involves fork and exec, not fork alone. Ampoule falls into that category.