[python] Multiprocessing vs Threading Python

I am trying to understand the advantages of multiprocessing over threading. I know that multiprocessing gets around the Global Interpreter Lock, but what other advantages are there, and can threading not do the same thing?

Threading's job is to enable applications to be responsive. Suppose you have a database connection and you need to respond to user input. Without threading, if the database connection is busy the application will not be able to respond to the user. By splitting off the database connection into a separate thread you can make the application more responsive. Also because both threads are in the same process, they can access the same data structures - good performance, plus a flexible software design.

Note that due to the GIL the app isn't actually doing two things at once, but what we've done is put the resource lock on the database into a separate thread so that CPU time can be switched between it and the user interaction. CPU time gets rationed out between the threads.

Multiprocessing is for times when you really do want more than one thing to be done at any given time. Suppose your application needs to connect to 6 databases and perform a complex matrix transformation on each dataset. Putting each job in a separate thread might help a little because when one connection is idle another one could get some CPU time, but the processing would not be done in parallel because the GIL means that you're only ever using the resources of one CPU. By putting each job in a Multiprocessing process, each can run on it's own CPU and run at full efficiency.

Process may have multiple threads. These threads may share memory and are the units of execution within a process.

Processes run on the CPU, so threads are residing under each process. Processes are individual entities which run independently. If you want to share data or state between each process, you may use a memory-storage tool such as Cache(redis, memcache), Files, or a Database.

Here are some pros/cons I came up with.

Multiprocessing

Pros

• Separate memory space
• Code is usually straightforward
• Takes advantage of multiple CPUs & cores
• Avoids GIL limitations for cPython
• Eliminates most needs for synchronization primitives unless if you use shared memory (instead, it's more of a communication model for IPC)
• Child processes are interruptible/killable
• Python multiprocessing module includes useful abstractions with an interface much like threading.Thread
• A must with cPython for CPU-bound processing

Cons

• IPC a little more complicated with more overhead (communication model vs. shared memory/objects)
• Larger memory footprint

Threading

Pros

• Lightweight - low memory footprint
• Shared memory - makes access to state from another context easier
• Allows you to easily make responsive UIs
• cPython C extension modules that properly release the GIL will run in parallel
• Great option for I/O-bound applications

Cons

• cPython - subject to the GIL
• Not interruptible/killable
• If not following a command queue/message pump model (using the Queue module), then manual use of synchronization primitives become a necessity (decisions are needed for the granularity of locking)
• Code is usually harder to understand and to get right - the potential for race conditions increases dramatically

As I learnd in university most of the answers above are right. In PRACTISE on different platforms (always using python) spawning multiple threads ends up like spawning one process. The difference is the multiple cores share the load instead of only 1 core processing everything at 100%. So if you spawn for example 10 threads on a 4 core pc, you will end up getting only the 25% of the cpus power!! And if u spawn 10 processes u will end up with the cpu processing at 100% (if u dont have other limitations). Im not a expert in all the new technologies. Im answering with own real experience background

Another thing not mentioned is that it depends on what OS you are using where speed is concerned. In Windows processes are costly so threads would be better in windows but in unix processes are faster than their windows variants so using processes in unix is much safer plus quick to spawn.

Python documentation quotes

The canonical version of this answer is now at the dupliquee question: What are the differences between the threading and multiprocessing modules?

I've highlighted the key Python documentation quotes about Process vs Threads and the GIL at: What is the global interpreter lock (GIL) in CPython?

Process vs thread experiments

I did a bit of benchmarking in order to show the difference more concretely.

In the benchmark, I timed CPU and IO bound work for various numbers of threads on an 8 hyperthread CPU. The work supplied per thread is always the same, such that more threads means more total work supplied.

The results were:

Plot data.

Conclusions:

• for CPU bound work, multiprocessing is always faster, presumably due to the GIL

• for IO bound work. both are exactly the same speed

• threads only scale up to about 4x instead of the expected 8x since I'm on an 8 hyperthread machine.

  Contrast that with a C POSIX CPU-bound work which reaches the expected 8x speedup: What do 'real', 'user' and 'sys' mean in the output of time(1)?

  TODO: I don't know the reason for this, there must be other Python inefficiencies coming into play.

Test code:

#!/usr/bin/env python3

import multiprocessing
import threading
import time
import sys

def cpu_func(result, niters):
    '''
    A useless CPU bound function.
    '''
    for i in range(niters):
        result = (result * result * i + 2 * result * i * i + 3) % 10000000
    return result

class CpuThread(threading.Thread):
    def __init__(self, niters):
        super().__init__()
        self.niters = niters
        self.result = 1
    def run(self):
        self.result = cpu_func(self.result, self.niters)

class CpuProcess(multiprocessing.Process):
    def __init__(self, niters):
        super().__init__()
        self.niters = niters
        self.result = 1
    def run(self):
        self.result = cpu_func(self.result, self.niters)

class IoThread(threading.Thread):
    def __init__(self, sleep):
        super().__init__()
        self.sleep = sleep
        self.result = self.sleep
    def run(self):
        time.sleep(self.sleep)

class IoProcess(multiprocessing.Process):
    def __init__(self, sleep):
        super().__init__()
        self.sleep = sleep
        self.result = self.sleep
    def run(self):
        time.sleep(self.sleep)

if __name__ == '__main__':
    cpu_n_iters = int(sys.argv[1])
    sleep = 1
    cpu_count = multiprocessing.cpu_count()
    input_params = [
        (CpuThread, cpu_n_iters),
        (CpuProcess, cpu_n_iters),
        (IoThread, sleep),
        (IoProcess, sleep),
    ]
    header = ['nthreads']
    for thread_class, _ in input_params:
        header.append(thread_class.__name__)
    print(' '.join(header))
    for nthreads in range(1, 2 * cpu_count):
        results = [nthreads]
        for thread_class, work_size in input_params:
            start_time = time.time()
            threads = []
            for i in range(nthreads):
                thread = thread_class(work_size)
                threads.append(thread)
                thread.start()
            for i, thread in enumerate(threads):
                thread.join()
            results.append(time.time() - start_time)
        print(' '.join('{:.6e}'.format(result) for result in results))

GitHub upstream + plotting code on same directory.

Tested on Ubuntu 18.10, Python 3.6.7, in a Lenovo ThinkPad P51 laptop with CPU: Intel Core i7-7820HQ CPU (4 cores / 8 threads), RAM: 2x Samsung M471A2K43BB1-CRC (2x 16GiB), SSD: Samsung MZVLB512HAJQ-000L7 (3,000 MB/s).

Visualize which threads are running at a given time

This post https://rohanvarma.me/GIL/ taught me that you can run a callback whenever a thread is scheduled with the target= argument of threading.Thread and the same for multiprocessing.Process.

This allows us to view exactly which thread runs at each time. When this is done, we would see something like (I made this particular graph up):

            +--------------------------------------+
            + Active threads / processes           +
+-----------+--------------------------------------+
|Thread   1 |********     ************             |
|         2 |        *****            *************|
+-----------+--------------------------------------+
|Process  1 |***  ************** ******  ****      |
|         2 |** **** ****** ** ********* **********|
+-----------+--------------------------------------+
            + Time -->                             +
            +--------------------------------------+

which would show that:

• threads are fully serialized by the GIL
• processes can run in parallel

MULTIPROCESSING

• Multiprocessing adds CPUs to increase computing power.
• Multiple processes are executed concurrently.
• Creation of a process is time-consuming and resource intensive.
• Multiprocessing can be symmetric or asymmetric.

• The multiprocessing library in Python uses separate memory space, multiple CPU cores, bypasses GIL limitations in CPython, child processes are killable (ex. function calls in program) and is much easier to use.
• Some caveats of the module are a larger memory footprint and IPC’s a little more complicated with more overhead.

MULTITHREADING

• Multithreading creates multiple threads of a single process to increase computing power.
• Multiple threads of a single process are executed concurrently.
• Creation of a thread is economical in both sense time and resource.

• The multithreading library is lightweight, shares memory, responsible for responsive UI and is used well for I/O bound applications.
• The module isn’t killable and is subject to the GIL.
• Multiple threads live in the same process in the same space, each thread will do a specific task, have its own code, own stack memory, instruction pointer, and share heap memory.
• If a thread has a memory leak it can damage the other threads and parent process.

Example of Multi-threading and Multiprocessing using Python

Python 3 has the facility of Launching parallel tasks. This makes our work easier.

It has for thread pooling and Process pooling.

The following gives an insight:

ThreadPoolExecutor Example

import concurrent.futures
import urllib.request

URLS = ['http://www.foxnews.com/',
        'http://www.cnn.com/',
        'http://europe.wsj.com/',
        'http://www.bbc.co.uk/',
        'http://some-made-up-domain.com/']

# Retrieve a single page and report the URL and contents
def load_url(url, timeout):
    with urllib.request.urlopen(url, timeout=timeout) as conn:
        return conn.read()

# We can use a with statement to ensure threads are cleaned up promptly
with concurrent.futures.ThreadPoolExecutor(max_workers=5) as executor:
    # Start the load operations and mark each future with its URL
    future_to_url = {executor.submit(load_url, url, 60): url for url in URLS}
    for future in concurrent.futures.as_completed(future_to_url):
        url = future_to_url[future]
        try:
            data = future.result()
        except Exception as exc:
            print('%r generated an exception: %s' % (url, exc))
        else:
            print('%r page is %d bytes' % (url, len(data)))

ProcessPoolExecutor

import concurrent.futures
import math

PRIMES = [
    112272535095293,
    112582705942171,
    112272535095293,
    115280095190773,
    115797848077099,
    1099726899285419]

def is_prime(n):
    if n % 2 == 0:
        return False

    sqrt_n = int(math.floor(math.sqrt(n)))
    for i in range(3, sqrt_n + 1, 2):
        if n % i == 0:
            return False
    return True

def main():
    with concurrent.futures.ProcessPoolExecutor() as executor:
        for number, prime in zip(PRIMES, executor.map(is_prime, PRIMES)):
            print('%d is prime: %s' % (number, prime))

if __name__ == '__main__':
    main()

As mentioned in the question, Multiprocessing in Python is the only real way to achieve true parallelism. Multithreading cannot achieve this because the GIL prevents threads from running in parallel.

As a consequence, threading may not always be useful in Python, and in fact, may even result in worse performance depending on what you are trying to achieve. For example, if you are performing a CPU-bound task such as decompressing gzip files or 3D-rendering (anything CPU intensive) then threading may actually hinder your performance rather than help. In such a case, you would want to use Multiprocessing as only this method actually runs in parallel and will help distribute the weight of the task at hand. There could be some overhead to this since Multiprocessing involves copying the memory of a script into each subprocess which may cause issues for larger-sized applications.

However, Multithreading becomes useful when your task is IO-bound. For example, if most of your task involves waiting on API-calls, you would use Multithreading because why not start up another request in another thread while you wait, rather than have your CPU sit idly by.

TL;DR

• Multithreading is concurrent and is used for IO-bound tasks
• Multiprocessing achieves true parallelism and is used for CPU-bound tasks

The key advantage is isolation. A crashing process won't bring down other processes, whereas a crashing thread will probably wreak havoc with other threads.

Threads share the same memory space to guarantee that two threads don't share the same memory location so special precautions must be taken the CPython interpreter handles this using a mechanism called GIL, or the Global Interpreter Lock

what is GIL(Just I want to Clarify GIL it's repeated above)?

In CPython, the global interpreter lock, or GIL, is a mutex that protects access to Python objects, preventing multiple threads from executing Python bytecodes at once. This lock is necessary mainly because CPython's memory management is not thread-safe.

For the main question, we can compare using Use Cases, How?

1-Use Cases for Threading: in case of GUI programs threading can be used to make the application responsive For example, in a text editing program, one thread can take care of recording the user inputs, another can be responsible for displaying the text, a third can do spell-checking, and so on. Here, the program has to wait for user interaction. which is the biggest bottleneck. Another use case for threading is programs that are IO bound or network bound, such as web-scrapers.

2-Use Cases for Multiprocessing: Multiprocessing outshines threading in cases where the program is CPU intensive and doesn’t have to do any IO or user interaction.

For More Details visit this link and link or you need in-depth knowledge for threading visit here for Multiprocessing visit here

Other answers have focused more on the multithreading vs multiprocessing aspect, but in python Global Interpreter Lock (GIL) has to be taken into account. When more number (say k) of threads are created, generally they will not increase the performance by k times, as it will still be running as a single threaded application. GIL is a global lock which locks everything out and allows only single thread execution utilizing only a single core. The performance does increase in places where C extensions like numpy, Network, I/O are being used, where a lot of background work is done and GIL is released.
So when threading is used, there is only a single operating system level thread while python creates pseudo-threads which are completely managed by threading itself but are essentially running as a single process. Preemption takes place between these pseudo threads. If the CPU runs at maximum capacity, you may want to switch to multiprocessing.
Now in case of self-contained instances of execution, you can instead opt for pool. But in case of overlapping data, where you may want processes communicating you should use multiprocessing.Process.