12. Parallel processing

12.1. Motivation and plan

Parallel processing is key to high performance computing, but it is important for every hacker to know how programs do this kind of processing.

12.2. Concepts

CPU: A physical CPU chip.
core: One of several logical CPUs inside a single physical CPU.
process: An separate flow of execution with its own memory and instructions. Separate processes appear to execute at the same time, and sometimes they do (when you have multiple cores).
thread: A light-weight process which shares data with other threads in the same process.
multiprocessing: …
multithreading: …
race condition: With multithreading and multiprocessing you could have one execution threads trying to access memory while another is modifying it.
synchronization: The collection of techniques that help resolve race conditions.
mutex: A “mutual exclusion” object which allows programs to decide who is currently using a resource.

12.3. Python multithreading library

Listing 12.3.1 Simplest multithreading program.

#! /usr/bin/env python3

import threading


def main():
    threads = []
    for i in range(5):
        t = threading.Thread(target=worker)
        threads.append(t)
        t.start()

def worker():
    """thread worker function"""
    print('Worker')
    return

main()

Listing 12.3.2 Multithreading program with a bit more.

#! /usr/bin/env python3
import threading

def main():
    threads = []
    for i in range(5):
        t = threading.Thread(target=worker, args=(i,))
        threads.append(t)
        t.start()

def worker(num):
    """thread worker function"""
    print('Worker: %s' % num)
    return

main()

Listing 12.3.3 Multithreading program with actual work.

#! /usr/bin/env python3

import threading
import time
import random

def main():
    service_thread = threading.Thread(name='my_service', target=my_service)
    worker_list = []
    print('creating the threads')
    for i in range(5):
        w = threading.Thread(name='worker_' + str(i), target=worker)
        worker_list.append(w)

    print('starting the service and worker threads')
    service_thread.start()
    for i in range(5):
        worker_list[i].start()

    for i in range(5):
        worker_list[i].join()
    service_thread.join()
    print('** main program is at an end **')

def worker():
    """sleep a random amount of time and exit"""
    myname = threading.currentThread().getName()
    sleep_time = random.randint(0, 10)
    print(myname, 'Starting')
    time.sleep(sleep_time)
    print(myname, 'Exiting after', sleep_time, 'seconds')

def my_service():
    """long running service"""
    myname = threading.currentThread().getName()
    sleep_time = 20
    print(myname, 'Starting')
    time.sleep(sleep_time)
    print(myname, 'Exiting after', sleep_time, 'seconds')

main()