Python multiprocessing with single worker faster than sequential operation

Question

A brief overview - I wrote some random files with lots of random numbers to disc to test the performance of python multiprocessing vs sequential operations.

Function description

putfiles : write test files to drive

readFile : reads the passed file location and returns result(sum of numbers in the code)

getSequential : reads some files with a for loop

getParallel : read file with multiple processes spawned

Performance results: (Read and process 100 files, with sequential and process pool)

timeit getSequential(numFiles=100) - around 2.85s best

timeit getParallel(numFiles=100, numProcesses=4) -around 960ms best

timeit getParallel(numFiles=100, numProcesses=1) -around 980ms best

Surprisingly single process pool performs better than sequential and at par with 4 process pool. Is this behavior expected or am I doing something wrong here?

import os
import random
from multiprocessing import Pool

os.chdir('/Users/test/Desktop/filewritetest')

def putfiles(numFiles=5, numCount=100):
    #numFiles = int(input("how many files?: "))
    #numCount = int(input('How many random numbers?: '))
    for num in range(numFiles):
        with open('r' + str(num) + '.txt', 'w') as f:
            f.write("\n".join([str(random.randint(1, 100)) for i in range(numCount)]))

def readFile(fileurl):
    with open(fileurl, 'r') as f, open("ans_" + fileurl, 'w') as fw:
        fw.write(str((sum([int(i) for i in f.read().split()]))))

def getSequential(numFiles=5):
    #in1 = int(input("how many files?: "))
    for num in range(numFiles):
        (readFile('r' + str(num) + '.txt'))


def getParallel(numFiles=5, numProcesses=2):
    #numFiles = int(input("how many files?: ")) 
    #numProcesses = int(input('How many processes?: '))
    with Pool(numProcesses) as p:
        p.map(readFile, ['r' + str(num) + '.txt' for num in range(numFiles)])


#putfiles()

putfiles(numFiles=1000, numCount=100000)

timeit getSequential(numFiles=100)
##around 2.85s best

timeit getParallel(numFiles=100, numProcesses=1)
##around 980ms best
timeit getParallel(numFiles=100, numProcesses=4)
##around 960ms best

Update: in a new session of sypder, I don't see this issue. Updated runtime below

##100 files
#around 2.97s best
timeit getSequential(numFiles=100)

#around 2.99s best
timeit getParallel(numFiles=100, numProcesses=1)

#around 1.57s best
timeit getParallel(numFiles=100, numProcesses=2)

#around 942ms best
timeit getParallel(numFiles=100, numProcesses=4)

##1000 files
#around 29.3s best
timeit getSequential(numFiles=1000)

#around 11.8s best
timeit getParallel(numFiles=1000, numProcesses=4)

#around 9.6s best
timeit getParallel(numFiles=1000, numProcesses=16)

#around 9.65s best  #let pool choose best default value
timeit getParallel(numFiles=1000)

Answer 1

please do not consider this as an answer, it is for showing you my code when running the stuff in python 3.x (your timeit usage did not work at all for me, i assumed it is 2.x). Sorry but i dont have the time to look into it deeply now.

[EDIT] on a spinning drive, consider disk cache: do not access the same files in different tests or just switch the order of your tests to see if disk cache is involved

Using the following code, changing manually the numProcesses=X argument, i got these results:

On SSD, 0.31 seconds for 1000 sequential and 0.37 seconds for 1000 paralell with 1 thread, 0.23 1000 paralell using 4 threads

import os
import random
import timeit
from multiprocessing import Pool
from contextlib import closing

os.chdir('c:\\temp\\')

def putfiles(numFiles=5, numCount=1):
    #numFiles = int(input("how many files?: "))
    #numCount = int(input('How many random numbers?: '))
    for num in range(numFiles):
        #print("num: " + str(num))
        with open('r' + str(num) + '.txt', 'w') as f:
            f.write("\n".join([str(random.randint(1, 100)) for i in range( numCount )]))
    #print ("pufiles done")

def readFile(fileurl):
    with open(fileurl, 'r') as f, open("ans_" + fileurl, 'w') as fw:
        fw.write(str((sum([int(i) for i in f.read().split()]))))


def getSequential(numFiles=10000):
   # print ("getSequential, nufile: " + str (numFiles))
    #in1 = int(input("how many files?: "))
    for num in range(numFiles): 
        #print ("getseq for")
        (readFile('r' + str(num) + '.txt'))
    #print ("getSequential done")


def getParallel(numFiles=10000, numProcesses=1):
    #numFiles = int(input("how many files?: ")) 
    #numProcesses = int(input('How many processes?: '))
    #readFile, ['r' + str(num) + '.txt' for num in range(numFiles)]
    #with Pool(10) as p:
    with closing(Pool(processes=1)) as p:
       p.map(readFile, ['r' + str(num) + '.txt' for num in range(numFiles)])

if __name__ == '__main__':
    #putfiles(numFiles=10000, numCount=1)

    print (timeit.timeit ("getSequential()","from __main__ import getSequential",number=1))

    print (timeit.timeit ("getParallel()","from __main__ import getParallel",number=1)) 

#timeit (getParallel(numFiles=100, numProcesses=4)) #-around 960ms best

#timeit (getParallel(numFiles=100, numProcesses=1)) #-around 980ms best