multiprocessing.Pool example

Question

I'm trying to learn how to use multiprocessing, and found the following example.

I want to sum values as follows:

from multiprocessing import Pool from time import time  N = 10 K = 50 w = 0  def CostlyFunction(z):     r = 0     for k in xrange(1, K+2):         r += z ** (1 / k**1.5)     print r     w += r     return r  currtime = time()  po = Pool()  for i in xrange(N):     po.apply_async(CostlyFunction,(i,)) po.close() po.join()  print w print '2: parallel: time elapsed:', time() - currtime 

I can't get the sum of all r values.

Answer 1

If you're going to use apply_async like that, then you have to use some sort of shared memory. Also, you need to put the part that starts the multiprocessing so that it is only done when called by the initial script, not the pooled processes. Here's a way to do it with map.

from multiprocessing import Pool from time import time  K = 50 def CostlyFunction((z,)):     r = 0     for k in xrange(1, K+2):         r += z ** (1 / k**1.5)     return r  if __name__ == "__main__":     currtime = time()     N = 10     po = Pool()     res = po.map_async(CostlyFunction,((i,) for i in xrange(N)))     w = sum(res.get())     print w     print '2: parallel: time elapsed:', time() - currtime