Why is my python multiprocess program running only in one core?

Question

I'm trying to make a multi-process class objects run in all cpu cores in my python program. The code works, but it only runs in one core.

I wonder if the child processes are running sequentially, but I cant't find an easy way to test this or to avoid it.

My code looks something like this

#child processes

import multiprocessing as mp
import time
import random
class child(mp.Process):
    def __init__(self,comm):
        mp.Process.__init__(self)
        self.comm = comm
    def run(self):
        self.score = self.doWork()
        self.comm.put([self.score])
    def doWork(self):
        k = 0
        for x in range(9999):
            for y in range(9999):
                k = k + 1
        return random.randint(1,1000)
#main process
def runSubProcess():
     list = []
     queue = mp.Queue()
     for p in range(4):
         p = child(queue)
         p.start()
         p.join()
         list.append(p)
     stillRunning = True
     while stillRunning:
         stillRunning = False
         for p in list:
             if p.is_alive():
                 stillRunning = True
         time.sleep(0.1)
     while not queue.empty():
         item = queue.get()
         print (item)

if __name__ == "__main__":
    runSubProcess()

I'm running python 3.8 64 bits in a 4-core environment using Windows 10

Version string:

Python 3.8.0a1 (tags/v3.8.0a1:e75eeb00b5, Feb 3 2019, 19:46:54) [MSC v.1916 32 bit (Intel)] on win32

Answer 1

Because you immediately join() it after starting. That makes the main program wait for the child to finish.

Start all of them first, then join all of them in another loop, like this:

def runSubProcess():
     list = []
     queue = mp.Queue()
     for p in range(4):
         p = child(queue)
         p.start()
         list.append(p)
     for p in list:
         p.join()
     while not queue.empty():
         item = queue.get()
         print (item)

Note that there's no need for sleeping, since join() will "sleep" by itself, if needed - even better: without using any CPU.