Python, using multiprocess is slower than not using it

Question

After spending a lot of time trying to wrap my head around multiprocessing I came up with this code which is a benchmark test:

Example 1:

Answer 1

Multiprocessing could be useful for what you're doing, but not in the way you're thinking about using it. As you're basically doing some computation on every member of a list, you could do it using the multiprocessing.Pool.map method, to do the computation on the list members in parallel.

Here is an example that shows your code's performance using a single process and using multiprocessing.Pool.map:

from multiprocessing import Pool
from random import choice
from string import printable
from time import time

def build_test_list():
    # Builds a test list consisting of 5 sublists of 10000 strings each.
    # each string is 20 characters long
    testlist = [[], [], [], [], []]
    for sublist in testlist:
        for _ in xrange(10000):
            sublist.append(''.join(choice(printable) for _ in xrange(20)))
    return testlist

def process_list(l):
    # the time-consuming code
    result = []
    tmp = []
    for n in range(len(l)):
        if l[n] not in tmp:
            result.insert(n, l[n]+' ('+str(l.count(l[n]))+')')
            tmp.insert(0, l[n])
    return result

def single(l):
    # process the test list elements using a single process
    results = []
    for sublist in l:
        results.append(process_list(sublist))
    return results

def multi(l):
    # process the test list elements in parallel
    pool = Pool()
    results = pool.map(process_list, l)
    return results

print "Building the test list..."
testlist = build_test_list()

print "Processing the test list using a single process..."
starttime = time()
singleresults = single(testlist)
singletime = time() - starttime

print "Processing the test list using multiple processes..."
starttime = time()
multiresults = multi(testlist)
multitime = time() - starttime

# make sure they both return the same thing
assert singleresults == multiresults

print "Single process: {0:.2f}sec".format(singletime)
print "Multiple processes: {0:.2f}sec".format(multitime)

Output:

Building the test list...
Processing the test list using a single process...
Processing the test list using multiple processes...
Single process: 34.73sec
Multiple processes: 24.97sec