python2.5 multiprocessing Pool

Question

I have python2.5 and multiprocessoring (get from http://code.google.com/p/python-multiprocessing/)

This simple code (get from docs), works very strange from time to time, sometimes it ok, but sometimes it throw timeout ex or hang my Windows (Vista), only reset helps :) Why this can happen?

from multiprocessing import Pool
def f(x):
 print "fc",x
 return x*x
pool = Pool(processes=4) 
if __name__ == '__main__':
 result = pool.apply_async(f, (10,)) # evaluate "f(10)" asynchronously
 print result.get(timeout=3) # prints "100" unless your computer is *very* slow

Answer 1

This is just a wild guess, but have you tried to move the Pool creation into the if block? I suspect that otherwise it might spawn an unlimited number of new processes, causing the freeze.

• Evg: i was afraid to start this again :) but it works! thanx) really pull was creating over and over and this hangs.
• Eric O. Lebigot: @nikow: Why would processes be spawned indefinitely? I can see the 4 initial children each creating a new pool of 4 children upon importing the program, but I don't see how they would run the main code and call f for each of their children...
• nikow: @EOL: I suspect that in every child the module code is executed. Every time a new Pool is created, spawning 4 new children. I had this happen on my machine, and saw a very large number of Python processes before the machine froze.
• Eric O. Lebigot: @nikow: yes, but why would the 4 new children (for each new pool) create a "very large number of processes"? I would expect the 4 normal children, plus 4*4 new processes, not more. I don't see this as a reason to freeze... I'm still puzzled. :)
• nikow: @EOL: It's recursive. Each child accidentally creates a new pool with 4 new children. Note that the process in the first original children is no different from the one in their children. That's how multiprocessing is implemented.