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POPython multiprocessing.Queue deadlocks on put and get
 

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    <p>I'm having deadlock problems with this piece of code:</p> <pre><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 &lt; 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() </code></pre> <p>When I run my program, it works fine for several runs through <code>_entropy_split_parallel</code>, but eventually deadlocks. The parent process is blocking on <code>done_queue.get()</code>, and the worker process is blocking on <code>done_queue.put()</code>. Since the queue is always empty when this happens, blocking on <code>get</code> is expected. What I don't understand is why the worker is blocking on <code>put</code>, since the queue is obviously not full (it's empty!). I've tried the <code>block</code> and <code>timeout</code> keyword arguments, but get the same result.</p> <p>I'm using the multiprocessing backport, since I'm stuck with Python 2.5.</p> <hr> <p>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 <a href="http://docs.python.org/library/multiprocessing.html" rel="noreferrer">here.</a> The deadlocking only seems to occur if I call the test method many times. For example, changing the bottom of the script to this:</p> <pre><code> if __name__ == '__main__': freeze_support() for x in xrange(1000): test() </code></pre> <hr> <p>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.</p>
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