python multiprocessing process.terminate code example

Example: python multiprocessing terminate

You might run the child processes as daemons in the background.

process.daemon = True
Any errors and hangs (or an infinite loop) in a daemon process will not affect the main process, and it will only be terminated once the main process exits.

This will work for simple problems until you run into a lot of child daemon processes which will keep reaping memories from the parent process without any explicit control.

Best way is to set up a Queue to have all the child processes communicate to the parent process so that we can join them and clean up nicely. Here is some simple code that will check if a child processing is hanging (aka time.sleep(1000)), and send a message to the queue for the main process to take action on it:

import multiprocessing as mp
import time
import queue

running_flag = mp.Value("i", 1)

def worker(running_flag, q):
    count = 1
    while True:
        if running_flag.value:
            print "working {0} ...".format(count)
            count += 1
            q.put(count)
            time.sleep(1)
            if count > 3:
                # Simulate hanging with sleep
                print "hanging..."
                time.sleep(1000)

def watchdog(q):
    """
    This check the queue for updates and send a signal to it
    when the child process isn't sending anything for too long
    """
    while True:
        try:
            msg = q.get(timeout=10.0)
        except queue.Empty as e:
            print "[WATCHDOG]: Maybe WORKER is slacking"
            q.put("KILL WORKER")

def main():
    """The main process"""
    q = mp.Queue()

    workr = mp.Process(target=worker, args=(running_flag, q))
    wdog = mp.Process(target=watchdog, args=(q,))

    # run the watchdog as daemon so it terminates with the main process
    wdog.daemon = True

    workr.start()
    print "[MAIN]: starting process P1"
    wdog.start()

    # Poll the queue
    while True:
        msg = q.get()
        if msg == "KILL WATCHDOG":
            print "[MAIN]: Terminating slacking WORKER"
            workr.terminate()
            time.sleep(0.1)
            if not workr.is_alive():
                print "[MAIN]: WORKER is a goner"
                workr.join(timeout=1.0)
                print "[MAIN]: Joined WORKER successfully!"
                q.close()
                break # watchdog process daemon gets terminated

if __name__ == '__main__':
    main()
Without terminating worker, attempt to join() it to the main process would have blocked forever since worker has never finished.