Why Am I Getting a RuntimeError on Windows When Using Multiprocessing?

RuntimeError on Windows When Using Multiprocessing

Encountering a "RuntimeError" while attempting to use multiprocessing on Windows indicates a potential issue with the program's bootstrapping phase. This error typically occurs when the main module initiates subprocesses without using the proper idiom.

To resolve this problem, ensure that the main module includes the following line before launching any subprocesses:

if __name__ == '__main__':
    freeze_support()

Code Example

Consider the following simplified code snippet that attempts to utilize multiprocessing in separate modules on Windows:

testMain.py (Main Module)

import parallelTestModule

extractor = parallelTestModule.ParallelExtractor()
extractor.runInParallel(numProcesses=2, numThreads=4)

parallelTestModule.py (Separate Module)

import multiprocessing
from multiprocessing import Process
import threading

class ThreadRunner(threading.Thread):
    def __init__(self, name):
        threading.Thread.__init__(self)
        self.name = name
    def run(self):
        print(self.name, '\n')

class ProcessRunner:
    def runp(self, pid, numThreads):
        mythreads = []
        for tid in range(numThreads):
            name = "Proc-"+str(pid)+"-Thread-"+str(tid)
            th = ThreadRunner(name)
            mythreads.append(th) 
        for i in mythreads:
            i.start()
        for i in mythreads:
            i.join()

class ParallelExtractor:    
    def runInParallel(self, numProcesses, numThreads):
        myprocs = []
        prunner = ProcessRunner()
        for pid in range(numProcesses):
            pr = Process(target=prunner.runp, args=(pid, numThreads)) 
            myprocs.append(pr) 
        for i in myprocs:
            i.start()

        for i in myprocs:
            i.join()

Applied Solution

To rectify the RuntimeError on Windows, add the if name == '__main__': guard to the main module:

import parallelTestModule

if __name__ == '__main__':
    extractor = parallelTestModule.ParallelExtractor()
    extractor.runInParallel(numProcesses=2, numThreads=4)

By completing this change, the program will correctly handle subprocesses as intended without triggering the RuntimeError.

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