Estoy haciendo un recuento de palabras de frecuencia usando python, la versión de proceso único:

#coding=utf-8
import string
import time
from collections import Counter
starttime = time.clock()
origin = open("document.txt", 'r').read().lower()
for_split = [',','\n','\t','\'','.','\"','!','?','-', '~']

#the words below will be ignoered when counting
ignored = ['the', 'and', 'i', 'to', 'of', 'a', 'in', 'was', 'that', 'had',
       'he', 'you', 'his','my', 'it', 'as', 'with', 'her', 'for', 'on']
i=0
for ch in for_split:
    origin = string.replace(origin, ch, ' ')
words = string.split(origin)
result = Counter(words).most_common(40)
for word, frequency in result:
    if not word in ignored and i < 10:
        print "%s : %d" % (word, frequency)
        i = i+1
print time.clock() - starttime

entonces la versión de multiprocesamiento se ve como:

#coding=utf-8
import time
import multiprocessing
from collections import Counter
for_split = [',','\n','\t','\'','.','\"','!','?','-', '~']
ignored = ['the', 'and', 'i', 'to', 'of', 'a', 'in', 'was', 'that', 'had',
       'he', 'you', 'his','my', 'it', 'as', 'with', 'her', 'for', 'on']
result_list = []

def worker(substr):
    result = Counter(substr)
    return result

def log_result(result):
    result_list.append(result)

def main():
    pool = multiprocessing.Pool(processes=5)
    origin = open("document.txt", 'r').read().lower()
 for ch in for_split:
         origin = origin.replace(ch, ' ')
    words = origin.split()
    step = len(words)/4
        substrs = [words[pos : pos+step] for pos in range(0, len(words), step)]
    result = Counter()
    for substr in substrs:
        pool.apply_async(worker, args=(substr,), callback = log_result)
    pool.close()
    pool.join()
    result = Counter()
    for item in result_list:
        result = result + item
    result = result.most_common(40)
    i=0
    for word, frequency in result:
        if not word in ignored and i < 10:
            print "%s : %d" % (word, frequency)
            i = i+1

if __name__ == "__main__":
        starttime = time.clock()
        main()
        print time.clock() - starttime

el "document.txt" es aproximadamente 22M, mi computadora portátil tiene núcleos, memoria 2G, el resultado de la primera versión es 3.27s, y la segunda es 8.15s, he cambiado el número de procesos (pool = multiprocessing.Pool (procesos = 5)), de 2 a 10, los resultados siguen siendo casi iguales, ¿por qué es eso, cómo puedo hacer que este programa funcione más rápido que la versión de proceso único?