Possible multiprocessing deadlock when placing too many objects in Queue()

[fhstoica]

BPO	34140
Nosy	@pitrou, @applio, @zhangyangyu, @fhstoica
Files	IssuesWithQueueMultiProcessing.tar.bz

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2018-07-17.16:51:46.885>
created_at = <Date 2018-07-17.14:21:19.056>
labels = ['type-bug', 'invalid']
title = 'Possible multiprocessing deadlock when placing too many objects in Queue()'
updated_at = <Date 2018-07-17.16:51:46.883>
user = 'https://github.com/fhstoica'

bugs.python.org fields:

activity = <Date 2018-07-17.16:51:46.883>
actor = 'pitrou'
assignee = 'none'
closed = True
closed_date = <Date 2018-07-17.16:51:46.885>
closer = 'pitrou'
components = []
creation = <Date 2018-07-17.14:21:19.056>
creator = 'fhstoica'
dependencies = []
files = ['47702']
hgrepos = []
issue_num = 34140
keywords = []
message_count = 3.0
messages = ['321832', '321842', '321843']
nosy_count = 4.0
nosy_names = ['pitrou', 'davin', 'xiang.zhang', 'fhstoica']
pr_nums = []
priority = 'normal'
resolution = 'not a bug'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue34140'
versions = ['Python 3.5', 'Python 3.6']

[fhstoica]

I am trying to use the multiprocessing module for a simulation on a spherical lattice, but the process hangs when the lattice is too large.

In the file IssuesWithQueueMultiProcessing.py, the method createLattice(), use either "return(I4)" for the small lattice or "return(I5)" for the large lattice.

Running the script when using the large lattice causes the process to hang while for the small lattice it works fine. I have tested with Python 3.5.2 and 3.6.1 and the behavior is the same in both versions.

[pitrou]

The problem is you're joining the child processes before draining the queue in the parent.

Generally, instead of building your own kind of synchronization like this, I would recommend you use the higher-level abstractions provided by multiprocessing.Pool or concurrent.futures.ProcessPoolExecutor.

By the way, this issue is mentioned precisely in the documentation:

"""
As mentioned above, if a child process has put items on a queue (and it has not used JoinableQueue.cancel_join_thread), then that process will not terminate until all buffered items have been flushed to the pipe.

This means that if you try joining that process you may get a deadlock unless you are sure that all items which have been put on the queue have been consumed. Similarly, if the child process is non-daemonic then the parent process may hang on exit when it tries to join all its non-daemonic children.
"""

(from https://docs.python.org/3/library/multiprocessing.html#pipes-and-queues)

[pitrou]

Closing as not a bug.