Multiprocessing causes Python to crash and gives an error may have been in progress in another thread when fork() was called

Question

I am relatively new to Python and trying to implement a Multiprocessing module for my for loop.

I have an array of Image url\'s stored in img_urls which I need to do

Answer 1

The solution that works for me without OBJC_DISABLE_INITIALIZE_FORK_SAFETY flag in the environment involves initializing the multiprocessing.Pool class right after the main() program starts.

This is most likely not the fastest solution possible and I am not sure if it works in all situations, however, pre-heating the worker processes early enough before my programs starts does not result in any ... may have been in progress in another thread when fork() was called errors and I do get a significant performance boost compared to what I get with non-parallelized code.

I have created a convenience class Parallelizer which I am starting very early and then using throughout the lifecycle of my program.

# entry point to my program
def main():
    parallelizer = Parallelizer()
    ...

Then whenever you want to have parallelization:

# this function is parallelized. it is run by each child process.
def processing_function(input):
    ...
    return output

...
inputs = [...]
results = parallelizer.map(
    inputs,
    processing_function
)

And the parallelizer class:

class Parallelizer:
    def __init__(self):
        self.input_queue = multiprocessing.Queue()
        self.output_queue = multiprocessing.Queue()
        self.pool = multiprocessing.Pool(multiprocessing.cpu_count(),
                                         Parallelizer._run,
                                         (self.input_queue, self.output_queue,))

    def map(self, contents, processing_func):
        size = 0
        for content in contents:
            self.input_queue.put((content, processing_func))
            size += 1
        results = []
        while size > 0:
            result = self.output_queue.get(block=True)
            results.append(result)
            size -= 1
        return results

    @staticmethod
    def _run(input_queue, output_queue):
        while True:
            content, processing_func = input_queue.get(block=True)
            result = processing_func(content)
            output_queue.put(result)

One caveat: the parallelized code might be difficult to debug so I have also prepared a non-parallelizing version of my class which I enable when something goes wrong in the child processes:

class NullParallelizer:
    @staticmethod
    def map(contents, processing_func):
        results = []
        for content in contents:
            results.append(processing_func(content))
        return results

Answer 2

This error occurs because of added security to restrict multithreading in Mac OS High Sierra. I know this answer is a bit late, but I solved the problem using the following method:

Set an environment variable .bash_profile to allow multithreading applications or scripts under the new Mac OS High Sierra security rules.

Open a terminal:

$ nano .bash_profile

Add the following line to the end of the file:

export OBJC_DISABLE_INITIALIZE_FORK_SAFETY=YES

Save, exit, close terminal and re-open the terminal. Check to see that the environment variable is now set:

$ env

You will see output similar to:

TERM_PROGRAM=Apple_Terminal
SHELL=/bin/bash
TERM=xterm-256color
TMPDIR=/var/folders/pn/vasdlj3ojO#OOas4dasdffJq/T/
Apple_PubSub_Socket_Render=/private/tmp/com.apple.launchd.E7qLFJDSo/Render
TERM_PROGRAM_VERSION=404
TERM_SESSION_ID=NONE
OBJC_DISABLE_INITIALIZE_FORK_SAFETY=YES

You should now be able to run your python script with multithreading.