Filling a queue and managing multiprocessing in python

Question

I'm having this problem in python:

• I have a queue of URLs that I need to check from time to time
• if the queue is filled up, I need to process each item in the queue
• Each item in the queue must be processed by a single process (multiprocessing)

So far I managed to achieve this "manually" like this:

while 1:
        self.updateQueue()

        while not self.mainUrlQueue.empty():
            domain = self.mainUrlQueue.get()

            # if we didn't launched any process yet, we need to do so
            if len(self.jobs) < maxprocess:
                self.startJob(domain)
                #time.sleep(1)
            else:
                # If we already have process started we need to clear the old process in our pool and start new ones
                jobdone = 0

                # We circle through each of the process, until we find one free ; only then leave the loop 
                while jobdone == 0:
                    for p in self.jobs :
                        #print "entering loop"
                        # if the process finished
                        if not p.is_alive() and jobdone == 0:
                            #print str(p.pid) + " job dead, starting new one"
                            self.jobs.remove(p)
                            self.startJob(domain)
                            jobdone = 1

However that leads to tons of problems and errors. I wondered if I was not better suited using a Pool of process. What would be the right way to do this?

However, a lot of times my queue is empty, and it can be filled by 300 items in a second, so I'm not too sure how to do things here.

Answer 1

You could use the blocking capabilities of queue to spawn multiple process at startup (using multiprocessing.Pool) and letting them sleep until some data are available on the queue to process. If your not familiar with that, you could try to "play" with that simple program:

import multiprocessing import os import time  the_queue = multiprocessing.Queue()   def worker_main(queue):     print os.getpid(),"working"     while True:         item = queue.get(True)         print os.getpid(), "got", item         time.sleep(1) # simulate a "long" operation  the_pool = multiprocessing.Pool(3, worker_main,(the_queue,)) #                           don't forget the comma here  ^  for i in range(5):     the_queue.put("hello")     the_queue.put("world")   time.sleep(10) 

^{Tested with Python 2.7.3 on Linux}

This will spawn 3 processes (in addition of the parent process). Each child executes the worker_main function. It is a simple loop getting a new item from the queue on each iteration. Workers will block if nothing is ready to process.

At startup all 3 process will sleep until the queue is fed with some data. When a data is available one of the waiting workers get that item and starts to process it. After that, it tries to get an other item from the queue, waiting again if nothing is available...

Answer 2

Added some code (submitting "None" to the queue) to nicely shut down the worker threads, and added code to close and join the_queue and the_pool:

import multiprocessing import os import time  NUM_PROCESSES = 20 NUM_QUEUE_ITEMS = 20  # so really 40, because hello and world are processed separately   def worker_main(queue):     print(os.getpid(),"working")     while True:         item = queue.get(block=True) #block=True means make a blocking call to wait for items in queue         if item is None:             break          print(os.getpid(), "got", item)         time.sleep(1) # simulate a "long" operation   def main():     the_queue = multiprocessing.Queue()     the_pool = multiprocessing.Pool(NUM_PROCESSES, worker_main,(the_queue,))                  for i in range(NUM_QUEUE_ITEMS):         the_queue.put("hello")         the_queue.put("world")          for i in range(NUM_PROCESSES):         the_queue.put(None)      # prevent adding anything more to the queue and wait for queue to empty     the_queue.close()     the_queue.join_thread()      # prevent adding anything more to the process pool and wait for all processes to finish     the_pool.close()     the_pool.join()  if __name__ == '__main__':     main()