Python code with multiprocessing does not work on Windows

Question

The following naive, absolute-beginner code works 100% well on Ubuntu 14.04 (Python 2.7.6) and Cygwin (Python 2.7.8) but it hangs on Windows 64-bit (Python 2.7.8). I have observed the same with another snippets using the multiprocessing package.

from multiprocessing import Process, Queue
from time import time

def WallisPi(N,out):    # Pi by the (slowly convergent) Wallis method.
    prod = 1.0
    for i in xrange(2,N,2):
        prod = prod*(i**2)/((i+1)**2)
    prod = 2.0e0*prod*(i+1)
    out.put(prod)
    return 0

if __name__ == '__main__':
    
    T = [15000000, 25000000, 30000000, 40000000]
    
    ti = time()
    
    q1 = Queue()
    p1 = Process(target=WallisPi, args=(T[0],q1))
    p1.start()
    
    q2 = Queue()
    p2 = Process(target=WallisPi, args=(T[1],q2))
    p2.start()

    q3 = Queue()
    p3 = Process(target=WallisPi, args=(T[2],q3))
    p3.start()

    q4 = Queue()
    p4 = Process(target=WallisPi, args=(T[3],q4))
    p4.start()
    
    p = [p1, p2, p3, p4]
    
    for item in p:
        item.join()

    q = [q1, q2, q3, q4]
    
    print "\n"
    
    Num = len(q)
    
    for i in range(0,Num):
        print "Pi at ",T[i], "terms = ", q[i].get()

    tf = time()
    print "\nElapsed time: ", round((tf-ti),2), " secs."

I wonder what's wrong with this code?

Many thanks in advance for any help.

Fausto

Ferdinand Beyer · Accepted Answer · 2014-12-03T16:51:16.703

0

Depending on how you run Python, you might need to use freeze_support on Windows:

One needs to call this function straight after the if __name__ == '__main__' line of the main module. For example:
from multiprocessing import Process, freeze_support

def f():
    print 'hello world!'

if __name__ == '__main__':
    freeze_support()
    Process(target=f).start()

See also the programming guidelines on Windows.

edited Dec 03 '14 at 16:51

answered Dec 03 '14 at 16:45

Ferdinand Beyer

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Hi Ferdinand, thanks! In fact, I found out that the code eventually runs on Windows without the freeze_support() call provided I give it time enough (an obscene amount of time, may I say) to finish. It wasn't hanging; it just takes 14 times longer to execute on Windows than it does on Cygwin. The problem isn't Windows; it is I who am impatient. ;-) Thanks, – Fausto Arinos Barbuto Dec 03 '14 at 17:15

Python code with multiprocessing does not work on Windows

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