CODEWITHSUNDEEP
python
user2569466
user2569466

Reputation: 31

Python multiprocessing doesn't work as predicted

I found this code online and the predicted result was supposed to show the process names currently being processed, but when I run the code it only gives me the result and not the names. Im running Windows 7 Python 2.7.3.

Code:

import multiprocessing

def do_calculation(data):
    return data * 2

def start_process():
    print 'Starting', multiprocessing.current_process().name

if __name__ == '__main__':
    inputs = list(range(10))
    print 'Input   :', inputs

    builtin_outputs = map(do_calculation, inputs)
    print 'Built-in:', builtin_outputs

    pool_size = multiprocessing.cpu_count() * 2
    pool = multiprocessing.Pool(processes=pool_size,
                                initializer=start_process,
                                )
    pool_outputs = pool.map(do_calculation, inputs)
    pool.close() # no more tasks
    pool.join()  # wrap up current tasks

    print 'Pool    :', pool_outputs

Expected Result:

$ python multiprocessing_pool.py

Input   : [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
Built-in: [0, 2, 4, 6, 8, 10, 12, 14, 16, 18]
Starting PoolWorker-11
Starting PoolWorker-12
Starting PoolWorker-13
Starting PoolWorker-14
Starting PoolWorker-15
Starting PoolWorker-16
Starting PoolWorker-1
Starting PoolWorker-2
Starting PoolWorker-3
Starting PoolWorker-4
Starting PoolWorker-5
Starting PoolWorker-8
Starting PoolWorker-9
Starting PoolWorker-6
Starting PoolWorker-10
Starting PoolWorker-7
Pool    : [0, 2, 4, 6, 8, 10, 12, 14, 16, 18]

Upvotes: 3

Views: 687

Answers (1)

Noctis Skytower
Noctis Skytower

Reputation: 21991

If I run the following code in a console and not IDLE (which I keep going back to after using PyCharm):

from multiprocessing import *

def main():
    data = list(range(10))
    print('Data:', data)

    result = list(map(calculate, data))
    print('Map: ', result)

    pool = Pool(cpu_count() * 2, initialize)
    result = pool.map(calculate, data, 1)
    pool.close()
    pool.join()

    print('Pool:', result)

def calculate(number):
    return number * 2

def initialize():
    print('Starting', current_process().name)

if __name__ == '__main__':
    main()

The following is printed to the console screen as you expected (with a random order for PoolWorkers):

C:\Users\schappell\Desktop>stack_overflow.py
Data: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
Map:  [0, 2, 4, 6, 8, 10, 12, 14, 16, 18]
Starting PoolWorker-1
Starting PoolWorker-13
Starting PoolWorker-2
Starting PoolWorker-3
Starting PoolWorker-8
Starting PoolWorker-5
Starting PoolWorker-14
Starting PoolWorker-16
Starting PoolWorker-15
Starting PoolWorker-12
Starting PoolWorker-6
Starting PoolWorker-9
Starting PoolWorker-4
Starting PoolWorker-10
Starting PoolWorker-7
Starting PoolWorker-11
Pool: [0, 2, 4, 6, 8, 10, 12, 14, 16, 18]

C:\Users\schappell\Desktop>

As a side note, try running the following code instead (with and without time.sleep commented out):

from multiprocessing import *
import time

def main():
    data = list(range(10))
    print('Data:', data)
    print('Map: ', list(map(calculate, data)))
    pool = Pool(cpu_count() * 2, initialize)
##    time.sleep(1)
    print('Pool:', pool.map(calculate, data, 1))
    pool.close()
    pool.join()

def calculate(number):
    return number * 2

def initialize():
    print('Starting', current_process().name)

if __name__ == '__main__':
    main()

Without the sleep, you may get something like this (showing your pool has not been fully created yet):

C:\Users\schappell\Desktop>stack_overflow.py
Data: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
Map:  [0, 2, 4, 6, 8, 10, 12, 14, 16, 18]
Starting PoolWorker-2
Pool: [0, 2, 4, 6, 8, 10, 12, 14, 16, 18]
Starting PoolWorker-1
Starting PoolWorker-10
Starting PoolWorker-9
Starting PoolWorker-6
Starting PoolWorker-14
Starting PoolWorker-13
Starting PoolWorker-5
Starting PoolWorker-4
Starting PoolWorker-3
Starting PoolWorker-8
Starting PoolWorker-11
Starting PoolWorker-12
Starting PoolWorker-16
Starting PoolWorker-7
Starting PoolWorker-15

C:\Users\schappell\Desktop>

Upvotes: 1

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