Python multiprocessing, using pool multiple times in a loop gets stuck after first iteration

Question

I have the following situation, where I create a pool in a for loop as follows (I know it's not very elegant, but I have to do this for pickling reasons). Assume that the pathos.multiprocessing is equivalent to python's multiprocessing library (as it is up to some details, that are not relevant for this problem). I have the following code I want to execute:

self.pool = pathos.multiprocessing.ProcessingPool(number_processes)


for i in range(5):


    all_responses = self.pool.map(wrapper_singlerun, range(self.no_of_restarts))

    pool._clear()

Now my problem: The loop successfully runs the first iteration. However, at the second iteration, the algorithm suddenly stops (Does not finish the pool.map operation. I suspected that zombie processes are generated, or that the process was somehow switched. Below you will find everything I have tried so far.

for i in range(5):

    pool = pathos.multiprocessing.ProcessingPool(number_processes)

    all_responses = self.pool.map(wrapper_singlerun, range(self.no_of_restarts))

    pool._clear()

    gc.collect()

    for p in multiprocessing.active_children():
        p.terminate()
        gc.collect()

    print("We have so many active children: ", multiprocessing.active_children()) # Returns []

The above code works perfectly well on my mac. However, when I upload it on the cluster that has the following specs, I get the error that it gets stuck after the first iteration:

DISTRIB_ID=Ubuntu
DISTRIB_RELEASE=18.04
DISTRIB_CODENAME=bionic
DISTRIB_DESCRIPTION="Ubuntu 18.04 LTS"

This is the link to the pathos' multiprocessing library file is

Answer 1

I am assuming that you are trying to call this via some function which is not the correct way to use this.

You need to wrap it around with :

if __name__ == '__main__':
    for i in range(5):

         pool = pathos.multiprocessing.Pool(number_processes)

         all_responses = pool.map(wrapper_singlerun, 

range(self.no_of_restarts))

If you don't do it will keep on creating a copy of itself and will start putting it into stack which will ultimately fill the stack and block everything. The reason it works on mac is that it has fork while windows does not have it.