python - Using multiprocessing.Process with a maximum number of simultaneous processes

Question

I have the Python code:

from multiprocessing import Process

def f(name):
    print 'hello', name

if __name__ == '__main__':
    for i in range(0, MAX_PROCESSES):
        p = Process(target=f, args=(i,))
        p.start()

which runs well. However, MAX_PROCESSES is variable and can be any value between 1 and 512. Since I'm only running this code on a machine with 8 cores, I need to find out if it is possible to limit the number of processes allowed to run at the same time. I've looked into multiprocessing.Queue, but it doesn't look like what I need - or perhaps I'm interpreting the docs incorrectly.

Is there a way to limit the number of simultaneous multiprocessing.Processs running?

Answer 1

It might be most sensible to use multiprocessing.Pool which produces a pool of worker processes based on the max number of cores available on your system, and then basically feeds tasks in as the cores become available.

The example from the standard docs (http://docs.python.org/2/library/multiprocessing.html#using-a-pool-of-workers) shows that you can also manually set the number of cores:

from multiprocessing import Pool

def f(x):
    return x*x

if __name__ == '__main__':
    pool = Pool(processes=4)              # start 4 worker processes
    result = pool.apply_async(f, [10])    # evaluate "f(10)" asynchronously
    print result.get(timeout=1)           # prints "100" unless your computer is *very* slow
    print pool.map(f, range(10))          # prints "[0, 1, 4,..., 81]"

And it's also handy to know that there is the multiprocessing.cpu_count() method to count the number of cores on a given system, if needed in your code.

Edit: Here's some draft code that seems to work for your specific case:

import multiprocessing

def f(name):
    print 'hello', name

if __name__ == '__main__':
    pool = multiprocessing.Pool() #use all available cores, otherwise specify the number you want as an argument
    for i in xrange(0, 512):
        pool.apply_async(f, args=(i,))
    pool.close()
    pool.join()