As a side note, the MWE I provide calls graph_tool.topology.shortest_path without specifying the target., but the logs refer to the target, this is counterintuitive, sorry about that. However, the behavior is as described above when one does specify the target to graph_tool.topology.shortest_path.

François

On Sun, Nov 13, 2016 at 6:03 PM, François Kawala <francois.kawala@gmail.com> wrote:

To be comprehensive, I add here the MWE source. Note that I fixed the vertices so that the output is reproducible. However, one could select randomly the vertices and would end with the same behavior.

Bests,

François.

  

import multiprocessing

import graph_tool as gt 

import graph_tool.topology as gtt

import hashlib

import sys

class MyProcess(multiprocessing.Process):

    """

    A process that computes shortest paths and shortest distances in a graph tool graph.

    """

    def __init__(self, graph, test):

        super(MyProcess, self).__init__()

        self.graph = graph

        self.test = test

    def run(self):

        while True:

            # Operation is repeated so that the bug is cristal clear. 

            source, target = self.test

            source = self.graph.vertex(source)

            target = self.graph.vertex(target)

            

            # We start the work.

            print('{} does shortest_distance from {} to {}'.format(self, source, target))

        

            gtt.shortest_distance(self.graph, 

                                  source=source,

                                  weights=self.graph.ep['weight'],

                                  max_dist=1400,

                                  pred_map=True)

            # We end the work.

            print('{} done.'.format(self))

def hash_graphs(*args):

    """

    Provides an edge based graph digest that can be used to invalidate old cache.

    :type args: tuple of :class:`graph_tool.GraphView`

    :param args: the graphs to be hashed.

    :rtype: str

    :return: a hash digest of the input graph.

    """

    graph_hash = hashlib.md5()

    for graph in args:

        graph_hash.update(gt.edge_endpoint_property(graph, graph.vp['id'], "source").a.tobytes())

        graph_hash.update(gt.edge_endpoint_property(graph, graph.vp['id'], "target").a.tobytes())

    return graph_hash.hexdigest()

if __name__ == '__main__':

    

    # Unserialize the graph.

    graph = gt.load_graph('./mwe/graph.gt.gz')

    

    # Bug switch.

    if sys.argv[-1] == 'DO_HASH':

        graph_hash = hash_graphs(graph)

    

    # Repetable inputs.

    tests = [(452946, 391015), 

             (266188, 207342),

             (514127, 290838),

             (439705, 87897),

             (223098, 440593),

             (279880, 368550),

             (108357, 199593),

             (273888, 275937)]

    # Actual work.

    procs = [MyProcess(graph, tests[i]) for i in range(8)]

    for proc in procs:

        proc.start()

    for proc in procs:

        proc.join() 

On Thu, Nov 10, 2016 at 7:24 PM, François Kawala <francois.kawala@gmail.com> wrote:

Hello,

I observe a quite strange bug that involves python's multiprocessing library. I try to use (read only) one graph instance with several Multithreading.Process. The graph is unserialized in the parent process. Each child receives a reference to the graph. Then each child does simple repetitive calls to graph_tool.topology.shortest_distance. Everything great each child process works as fast as it can. However when the main process executes the hash_graphs function presented below, each child process hangs infinitely. The hash_graphs is executed prior to the children start.

def hash_graphs(*args):

    """

    Provides an edge based graph digest that can be used to invalidate old cache.

    :type args: tuple of :class:`graph_tool.GraphView`

    :param args: the graphs to be hashed.

    :rtype: str

    :return: a hash digest of the input graph.

    """

    graph_hash = hashlib.md5()

    for graph in args:

        graph_hash.update(gt.edge_endpoint_property(graph, graph.vp['id'], "source").a.tobytes())

        graph_hash.update(gt.edge_endpoint_property(graph, graph.vp['id'], "target").a.tobytes())

    return graph_hash.hexdigest()

I package a MWE, it is available here : https://drive.google.com/file/d/0B5GhhBKHOKOxVnpfYTBwNDZxODA/view?usp=sharing. To run it simply do :

tar xzf mwe.tar.gz

# run the buggy version

python3 -m mwe DO_HASH

# run as expected

python3 -m mwe

The buggy output looks like : 

<MyProcess(MyProcess-1, started)> does shortest_distance from 452946 to 391015

<MyProcess(MyProcess-2, started)> does shortest_distance from 266188 to 207342

<MyProcess(MyProcess-3, started)> does shortest_distance from 514127 to 290838

<MyProcess(MyProcess-4, started)> does shortest_distance from 439705 to 87897

<MyProcess(MyProcess-5, started)> does shortest_distance from 223098 to 440593

<MyProcess(MyProcess-6, started)> does shortest_distance from 279880 to 368550

<MyProcess(MyProcess-7, started)> does shortest_distance from 108357 to 199593

<MyProcess(MyProcess-8, started)> does shortest_distance from 273888 to 275937

The expected output looks like : 

<MyProcess(MyProcess-1, started)> does shortest_distance from 452946 to 391015

<MyProcess(MyProcess-2, started)> does shortest_distance from 266188 to 207342

<MyProcess(MyProcess-3, started)> does shortest_distance from 514127 to 290838

<MyProcess(MyProcess-5, started)> does shortest_distance from 223098 to 440593

<MyProcess(MyProcess-6, started)> does shortest_distance from 279880 to 368550

<MyProcess(MyProcess-1, started)> done.

<MyProcess(MyProcess-1, started)> does shortest_distance from 452946 to 391015

<MyProcess(MyProcess-2, started)> done.

<MyProcess(MyProcess-2, started)> does shortest_distance from 266188 to 207342

<MyProcess(MyProcess-4, started)> does shortest_distance from 439705 to 87897

<MyProcess(MyProcess-7, started)> does shortest_distance from 108357 to 199593

<MyProcess(MyProcess-3, started)> done.

<MyProcess(MyProcess-1, started)> done.

<MyProcess(MyProcess-3, started)> does shortest_distance from 514127 to 290838

<MyProcess(MyProcess-1, started)> does shortest_distance from 452946 to 391015

<MyProcess(MyProcess-8, started)> does shortest_distance from 273888 to 275937

<MyProcess(MyProcess-2, started)> done.

<MyProcess(MyProcess-2, started)> does shortest_distance from 266188 to 207342

<MyProcess(MyProcess-3, started)> done.

<MyProcess(MyProcess-3, started)> does shortest_distance from 514127 to 290838

<MyProcess(MyProcess-1, started)> done.

<MyProcess(MyProcess-1, started)> does shortest_distance from 452946 to 391015

<MyProcess(MyProcess-6, started)> done.

<MyProcess(MyProcess-6, started)> does shortest_distance from 279880 to 368550

<MyProcess(MyProcess-4, started)> done.

<MyProcess(MyProcess-4, started)> does shortest_distance from 439705 to 87897

<MyProcess(MyProcess-8, started)> done.

<MyProcess(MyProcess-8, started)> does shortest_distance from 273888 to 275937

<MyProcess(MyProcess-1, started)> done.

<MyProcess(MyProcess-1, started)> does shortest_distance from 452946 to 391015

<MyProcess(MyProcess-2, started)> done.

<MyProcess(MyProcess-2, started)> does shortest_distance from 266188 to 207342

<MyProcess(MyProcess-3, started)> done.

<MyProcess(MyProcess-3, started)> does shortest_distance from 514127 to 290838

<MyProcess(MyProcess-5, started)> done.

<MyProcess(MyProcess-5, started)> does shortest_distance from 223098 to 440593

<MyProcess(MyProcess-1, started)> done.

<MyProcess(MyProcess-1, started)> does shortest_distance from 452946 to 391015

<MyProcess(MyProcess-8, started)> done.

<MyProcess(MyProcess-8, started)> does shortest_distance from 273888 to 275937

<MyProcess(MyProcess-7, started)> done.

<MyProcess(MyProcess-7, started)> does shortest_distance from 108357 to 199593

<MyProcess(MyProcess-3, started)> done.

<MyProcess(MyProcess-3, started)> does shortest_distance from 514127 to 290838

...

How could I explain this behavior ? 

Bests,

François.

--

François Kawala

--

François Kawala