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Preload modules with `multiprocessing.set_forkserver_preload` to speed up worker process initialization and enhance performance in Python applications.

Using multiprocessing.set_forkserver_preload in Python

The multiprocessing module in Python provides a powerful framework for concurrent execution using processes. One of the advanced features provided by this module is the ability to preload modules when using the forkserver start method. This can be particularly beneficial for optimizing the initialization time of processes by ensuring that certain modules are preloaded before any child processes are forked.

Overview of Forkserver Start Method

The forkserver start method is one of the start methods available in the multiprocessing module. It is useful in scenarios where you want to avoid issues related to the Global Interpreter Lock (GIL) and for improving the start-up time of processes. The forkserver start method works by starting a separate server process that forks new worker processes on demand. This method is generally more robust and efficient compared to the default fork method, especially in multi-threaded environments.

Using multiprocessing.set_forkserver_preload

The multiprocessing.set_forkserver_preload function allows you to specify a list of modules to preload in the forkserver process before any worker processes are forked. This can significantly reduce the time it takes to start new worker processes, as the specified modules are already loaded in memory and do not need to be imported again.


  • modulenames: A list of strings representing the names of the modules to preload.

Example Usage

Below is an example demonstrating how to use multiprocessing.set_forkserver_preload in a Python script:

import multiprocessing
import time

# Function to be executed in a worker process
def worker_function():
    print("Worker process started")
    print("Worker process finished")

if __name__ == "__main__":
    # Preload the 'time' module in the forkserver process

    # Set the start method to 'forkserver'

    # Create a pool of worker processes
    with multiprocessing.Pool(4) as pool:
        pool.map(worker_function, range(4))

In this example, the time module is preloaded in the forkserver process. When the worker processes are created, they do not need to import the time module again, resulting in a faster start-up time.

Benefits of Preloading Modules

  1. Reduced Initialization Time: By preloading modules, you can significantly reduce the time it takes to start new worker processes, as the modules are already loaded in memory.
  2. Consistent Environment: Preloading ensures that all worker processes have a consistent environment with the necessary modules already available.
  3. Improved Performance: In scenarios where worker processes are frequently created and destroyed, preloading can lead to improved overall performance by reducing the overhead associated with importing modules repeatedly.

Important Considerations

  1. Memory Usage: Preloading modules will increase the memory usage of the forkserver process, as the preloaded modules are kept in memory. Ensure that you have sufficient memory available to accommodate this.
  2. Module Dependencies: Ensure that the preloaded modules do not have dependencies that may cause issues when shared across multiple worker processes. For example, some modules may rely on global state that can lead to conflicts.
  3. Thread Safety: The forkserver start method is particularly useful in multi-threaded environments, but you should still be mindful of thread safety when designing your application.


The multiprocessing.set_forkserver_preload function is a powerful tool for optimizing the initialization time of worker processes in Python’s multiprocessing module. By preloading necessary modules in the forkserver process, you can achieve faster start-up times, consistent environments, and improved performance in multi-process applications.

By carefully selecting the modules to preload and considering the associated memory and dependency implications, you can leverage this feature to enhance the efficiency and robustness of your concurrent Python applications.