Question

I’m having deadlock problems with this piece of code:

def _entropy_split_parallel(data_train, answers_train, weights):
    CPUS = 1 #multiprocessing.cpu_count()
    NUMBER_TASKS = len(data_train[0])
    processes = []

    multi_list = zip(data_train, answers_train, weights)

    task_queue = multiprocessing.Queue()
    done_queue = multiprocessing.Queue()

    for feature_index in xrange(NUMBER_TASKS):
        task_queue.put(feature_index)

    for i in xrange(CPUS):
        process = multiprocessing.Process(target=_worker, 
                args=(multi_list, task_queue, done_queue))
        processes.append(process)
        process.start()

    min_entropy = None
    best_feature = None
    best_split = None
    for i in xrange(NUMBER_TASKS):
        entropy, feature, split = done_queue.get()
        if (entropy < min_entropy or min_entropy == None) and entropy != None:
            best_feature = feature
            best_split = split

    for i in xrange(CPUS):
        task_queue.put('STOP')

    for process in processes:
        process.join()

    return best_feature, best_split


def _worker(multi_list, task_queue, done_queue):
    feature_index = task_queue.get()
    while feature_index != 'STOP':
        result = _entropy_split3(multi_list, feature_index)
        done_queue.put(result)
        feature_index = task_queue.get()

When I run my program, it works fine for several runs through _entropy_split_parallel, but eventually deadlocks. The parent process is blocking on done_queue.get(), and the worker process is blocking on done_queue.put(). Since the queue is always empty when this happens, blocking on get is expected. What I don’t understand is why the worker is blocking on put, since the queue is obviously not full (it’s empty!). I’ve tried the block and timeout keyword arguments, but get the same result.

I’m using the multiprocessing backport, since I’m stuck with Python 2.5.

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EDIT: It looks like I’m also getting deadlock issues with one of the examples provided with the multiprocessing module. It’s the third example from the bottom here. The deadlocking only seems to occur if I call the test method many times. For example, changing the bottom of the script to this:

if __name__ == '__main__':
    freeze_support()
    for x in xrange(1000):
        test()

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EDIT: I know this is an old question, but testing shows that this is no longer a problem on windows with Python 2.7. I will try Linux and report back.

Answer 1

This problem went away with newer versions of Python, so I’m assuming it was a problem with the backport. Anyways, it’s no longer an issue.