Part 2: Multiprocessing

Python has many libraries available to help you parallelise your scripts across the cores of a single multicore computer. The established option is the multiprocessing library. You can import multiprocessing by typing into ipython

import multiprocessing

You can read the documentation for this module by typing

help(multiprocessing)

One of the useful functions in multiprocessing is cpu_count(). This returns the number of CPUs (computer cores) available on your computer to be used for a parallel program. Type into ipython

print(multiprocessing.cpu_count())

to see how many cores you have available.

Nearly all modern computers have several processor cores, so you should see that you have at least 2, and perhaps as many as 40 available on your machine. Each of these cores is available to do work, in parallel, as part of your Python script. For example, if you have two cores in your computer, then your script should ideally be able to do two things at once. Equally, if you have forty cores available, then your script should ideally be able to do forty things at once.

Multiprocessing allows your script to do lots of things at once by actually running multiple copies of your script in parallel, with (normally) one copy per processor core on your computer. One of these copies is known as the master copy, and is the one that is used to control all of worker copies. Because of this, multiprocessing python code has to be written into a text file and executed using the python interpreter. It is not recommended to try to run a multiprocessing python script interactively, e.g. via ipython or ipython notebook. In addition, because multiprocessing achieves parallelism by running multiple copies of your script, it forces you to write it in a particular way. All imports should be at the top of the script, followed by all function and class definitions. This is to ensure that all copies of the script have access to the same modules, functions and classes. Then, you should ensure that only the master copy of the script runs the code by protecting it behind an if __name__ == "__main__" statement.

An example (non-functional) script is shown below;

# all imports should be at the top of your script
import multiprocessing
import sys
import os

# all function and class definitions must be next
def sum( x, y ):
    """Function to return the sum of the two arguments"""
    return x+y

def product( x, y ):
    """Function to return the product of the two arguments"""
    return x*y

if __name__ == "__main__":
    # You must now protect the code being run by
    # the master copy of the script by placing it

    a = [1, 2, 3, 4, 5]
    b = [6, 7, 8, 9, 10]

    # Now write your parallel code...
    etc. etc.

(if you are interested, take a look here for more information about why parallel Python is based on forking multiple processes, rather than splitting multiple threads)


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