python - multiprocessing with large data

Question

I am using multiprocessing.Pool() to parallelize some heavy computations.

The target function returns a lot of data (a huge list). I'm running out of RAM.

Without multiprocessing, I'd just change the target function into a generator, by yielding the resulting elements one after another, as they are computed.

I understand multiprocessing does not support generators -- it waits for the entire output and returns it at once, right? No yielding. Is there a way to make the Pool workers yield data as soon as they become available, without constructing the entire result array in RAM?

Simple example:

def target_fnc(arg):
   result = []
   for i in xrange(1000000):
       result.append('dvsdbdfbngd') # <== would like to just use yield!
   return result

def process_args(some_args):
    pool = Pool(16)
    for result in pool.imap_unordered(target_fnc, some_args):
        for element in result:
            yield element

This is Python 2.7.

Answer 1

This sounds like an ideal use case for a Queue: http://docs.python.org/2/library/multiprocessing.html#exchanging-objects-between-processes

Simply feed your results into the queue from the pooled workers and ingest them in the master.

Note that you still may run into memory pressure issues unless you drain the queue nearly as fast as the workers are populating it. You could limit the queue size (the maximum number of objects that will fit in the queue) in which case the pooled workers would block on the queue.put statements until space is available in the queue. This would put a ceiling on memory usage. But if you're doing this, it may be time to reconsider whether you require pooling at all and/or if it might make sense to use fewer workers.