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Optimize your multi-process Python programs by choosing the right start method with `multiprocessing.set_start_method`.

Using multiprocessing.set_start_method in Python

multiprocessing is a powerful module in Python that allows you to create processes, manage their execution, and share data between them. One useful function within this module is set_start_method, which allows you to specify the method used to start new processes. This can be crucial for ensuring compatibility and optimizing performance on different operating systems.

Introduction to multiprocessing.set_start_method

The multiprocessing.set_start_method function is used to set the method that will be used to start child processes. This must be called at the beginning of the program, before any processes are created. The available start methods are:

  1. ‘fork’: The parent process is forked to create the child process. The child process inherits the memory of the parent.
  2. ‘spawn’: A new Python interpreter is started. Only the necessary resources are inherited by the child process.
  3. ‘forkserver’: A server process is started. Whenever a new process needs to be started, the server forks a new process.

Why Use Different Start Methods?

Each start method has its own set of advantages and drawbacks:

  • ‘fork’: This method is quick and memory-efficient because it doesn’t require starting a new interpreter. However, it may not be safe to use in a multi-threaded program due to the risk of deadlocks.
  • ‘spawn’: This method is more flexible and safer for multi-threaded programs but can be slower and use more memory as it starts a new interpreter.
  • ‘forkserver’: This method offers a compromise between speed and safety. It is safer than ‘fork’ and typically faster than ‘spawn’ but requires additional setup as it starts a server process.

Setting the Start Method

To set the start method, use the set_start_method function at the beginning of your script, before any other multiprocessing functions are called:

import multiprocessing

if __name__ == '__main__':
    # Your code here

Once a start method has been set, it cannot be changed. Attempting to do so will raise an AssertionError.

Example Usage

Here is a simple example demonstrating how to use multiprocessing.set_start_method with the ‘spawn’ method:

import multiprocessing
import time

def worker(num):
    print(f'Worker: {num}')
    print(f'Worker {num} done')

if __name__ == '__main__':
    processes = []

    for i in range(5):
        p = multiprocessing.Process(target=worker, args=(i,))

    for p in processes:

In this example, we set the start method to ‘spawn’ and create five worker processes. Each worker process prints its number, sleeps for two seconds, and then prints a completion message. Finally, we wait for all processes to complete using the join method.

Practical Considerations


Different operating systems may have different default start methods. For example, ‘fork’ is the default on Unix-based systems (like Linux and macOS), while ‘spawn’ is the default on Windows. It is essential to be aware of these differences to write cross-platform compatible code.


The choice of start method can significantly impact the performance of your program. For CPU-bound tasks, ‘fork’ might provide the best performance due to its low overhead. For I/O-bound or multi-threaded tasks, ‘spawn’ or ‘forkserver’ may be more appropriate despite their higher overhead.


In multi-threaded programs, using ‘fork’ can lead to deadlocks. This is because the forked process inherits the state of all threads from the parent process, which can cause issues if those threads hold locks. Using ‘spawn’ or ‘forkserver’ can help avoid these issues by starting new, clean interpreter instances.


Debugging multi-process programs can be challenging. Using set_start_method('spawn') can make debugging easier as it starts a new interpreter for each process, providing a more isolated environment. This can help identify issues more clearly compared to the ‘fork’ method, which shares memory between parent and child processes.


The multiprocessing.set_start_method function is a valuable tool in the Python multiprocessing module. By choosing the appropriate start method for your application, you can optimize performance and ensure compatibility across different operating systems. Whether you need the speed and efficiency of ‘fork’, the safety of ‘spawn’, or the balance of ‘forkserver’, understanding how to use set_start_method effectively is crucial for writing robust, high-performance multi-process Python programs.

import multiprocessing

if __name__ == '__main__':
    # Your code here

This simple line of code can make a significant difference in the behavior and performance of your multi-process applications. Be sure to consider the implications of each start method and choose the one that best fits your needs.