multiprocessing - execute external command and wait before proceeding

Question

I am using Linux. I have an external executable called "combine" and a loop of 20 iterations. Per each iteration, "combine" needs to be called with an argument that depends on the i-th iteration. Example:

arguments = " "

for i in range(1,20):
    arguments += str(i) + "_image.jpg "
    # begin of pseudo-code 
    execute: "./combine" + arguments  # in parallel using all cores

# pseudo-code continues
wait_for_all_previous_process_to_terminate
execute: "./merge_resized_images"  # use all cores - possible for one single command?

How do I achieve this using the multiprocessing module in Python?

Answer 1

You can use subprocess.Popen to launch the external commands asynchronously, and store each Popen object returned in a list. Once you've launched all the processes, just iterate over them and wait for each to finish using popen_object.wait.

from subprocess import Popen

processes = []
for i in range(1,20):
    arguments += str(i) + "_image.jpg "
    processes.append(subprocess.Popen(shlex.split("./combine" + arguments)))

for p in processes:
    p.wait()
subprocess.call("./merge_resized_images")

However, this will launch twenty concurrent processes, which is probably going to hurt performance.

To avoid that, you can use a ThreadPool to limit yourself to some lower number of concurrent processes (multiprocessing.cpu_count is a good number), and then use pool.join to wait for them all to finish.

import multiprocessing
import subprocess
import shlex

from multiprocessing.pool import ThreadPool

def call_proc(cmd):
    """ This runs in a separate thread. """
    #subprocess.call(shlex.split(cmd))  # This will block until cmd finishes
    p = subprocess.Popen(shlex.split(cmd), stdout=subprocess.PIPE, stderr=subprocess.PIPE)
    out, err = p.communicate()
    return (out, err)


pool = ThreadPool(multiprocessing.cpu_count())
results = []
for i in range(1,20):
    arguments += str(i) + "_image.jpg "
    results.append(pool.apply_async(call_proc, ("./combine" + arguments,)))

# Close the pool and wait for each running task to complete
pool.close()
pool.join()
for result in results:
    out, err = result.get()
    print("out: {} err: {}".format(out, err))
subprocess.call("./merge_resized_images")

Each thread will release the GIL while waiting for the subprocess to complete, so they'll all run in parallel.

Answer 2

My solution to this problem is to create and manage a list of subprocesses. Pay special attention to startencoder and manageprocs. That is where the actual work is being started and managed.

#!/usr/bin/env python2
# -*- coding: utf-8 -*-
#
# Author: R.F. Smith
# $Date: 2014-02-15 14:44:31 +0100 $
#
# To the extent possible under law, Roland Smith has waived all copyright and
# related or neighboring rights to vid2mkv.py. This work is published from the
# Netherlands. See http://creativecommons.org/publicdomain/zero/1.0/

"""Convert all video files given on the command line to Theora/Vorbis streams
in a Matroska container."""

from __future__ import print_function, division

__version__ = '$Revision: a42ef58 $'[11:-2]

import os
import sys
import subprocess
from multiprocessing import cpu_count
from time import sleep


def warn(s):
    """Print a warning message.

    :param s: Message string
    """
    s = ' '.join(['Warning:', s])
    print(s, file=sys.stderr)


def checkfor(args, rv=0):
    """Make sure that a program necessary for using this script is
    available.

    :param args: String or list of strings of commands. A single string may
    not contain spaces.
    :param rv: Expected return value from evoking the command.
    """
    if isinstance(args, str):
        if ' ' in args:
            raise ValueError('no spaces in single command allowed')
        args = [args]
    try:
        with open(os.devnull, 'w') as bb:
            rc = subprocess.call(args, stdout=bb, stderr=bb)
        if rc != rv:
            raise OSError
    except OSError as oops:
        outs = "Required program '{}' not found: {}."
        print(outs.format(args[0], oops.strerror))
        sys.exit(1)


def startencoder(fname):
    """Use ffmpeg to convert a video file to Theora/Vorbis
    streams in a Matroska container.

    :param fname: Name of the file to convert.
    :returns: a 3-tuple of a Process, input path and output path
    """
    basename, ext = os.path.splitext(fname)
    known = ['.mp4', '.avi', '.wmv', '.flv', '.mpg', '.mpeg', '.mov', '.ogv']
    if ext.lower() not in known:
        warn("File {} has unknown extension, ignoring it.".format(fname))
        return (None, fname, None)
    ofn = basename + '.mkv'
    args = ['ffmpeg', '-i', fname, '-c:v', 'libtheora', '-q:v', '6', '-c:a',
            'libvorbis', '-q:a', '3', '-sn', ofn]
    with open(os.devnull, 'w') as bitbucket:
        try:
            p = subprocess.Popen(args, stdout=bitbucket, stderr=bitbucket)
            print("Conversion of {} to {} started.".format(fname, ofn))
        except:
            warn("Starting conversion of {} failed.".format(fname))
    return (p, fname, ofn)


def manageprocs(proclist):
    """Check a list of subprocesses tuples for processes that have ended and
    remove them from the list.

    :param proclist: a list of (process, input filename, output filename)
    tuples.
    """
    print('# of conversions running: {}\r'.format(len(proclist)), end='')
    sys.stdout.flush()
    for p in proclist:
        pr, ifn, ofn = p
        if pr is None:
            proclist.remove(p)
        elif pr.poll() is not None:
            print('Conversion of {} to {} finished.'.format(ifn, ofn))
            proclist.remove(p)
    sleep(0.5)


def main(argv):
    """Main program.

    :param argv: command line arguments
    """
    if len(argv) == 1:
        binary = os.path.basename(argv[0])
        print("{} version {}".format(binary, __version__), file=sys.stderr)
        print("Usage: {} [file ...]".format(binary), file=sys.stderr)
        sys.exit(0)
    checkfor(['ffmpeg', '-version'])
    avis = argv[1:]
    procs = []
    maxprocs = cpu_count()
    for ifile in avis:
        while len(procs) == maxprocs:
            manageprocs(procs)
        procs.append(startencoder(ifile))
    while len(procs) > 0:
        manageprocs(procs)


if __name__ == '__main__':
    main(sys.argv)