Run function in parallel and grab outputs using Queue

user_6435921 0
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I would like to fun a function using different arguments. For each different argument, I would like to run the function in parallel and then get the output of each run. It seems that the multiprocessing module can help here. I am not sure about the right steps to make this work.

Do I start all the processes, then get all the queues and then join all the processes in this order? Or do I get the results after I have joined? Or do I get the ith result after I have joined the ith process?

from numpy.random import uniform
from multiprocessing import Process, Queue

def function(x):
    return uniform(0.0, x)

if __name__ == "__main__":
    queue = Queue()
    processes = []
    x_values = [1.0, 10.0, 100.0]
    
    # Start all processes
    for x in x_values:
        process = Process(target=function, args=(x, queue, ))
        processes.append(process)
        process.start()

    # Grab results of the processes?
    outputs = [queue.get() for _ in range(len(x_values))]
    
    # Not even sure what this does but apparently it's needed
    for process in processes:
        process.join()
parallel-processing python-3.x multiprocessing python
2 Answers

So lets make a simple example for multiprocessing pools with a loaded function that sleeps for 3 seconds and returns the value passed to it(your parameter) and also the result of the function which is just doubling it. IIRC there's some issue with stopping pools cleanly

from multiprocessing import Pool
import time

def time_waster(val):
    try:
        time.sleep(3)
    
        return (val, val*2) #return a tuple here but you can use a dict as well with all your parameters
    except KeyboardInterrupt:
        raise KeyboardInterruptError()
        
if __name__ == '__main__':
    x = list(range(5)) #values to pass to the function
    results = []
    try:
        with Pool(2) as p: #I use 2 but you can use as many as you have cores
            results.append(p.map(time_waster,x))
    except KeyboardInterrupt:
        p.terminate()
    except Exception as e:
        p.terminate()
    finally:
        p.join()
    print(results)

As an extra service added some keyboardinterrupt handlers as IIRC there are some issues interrupting pools.https://stackoverflow.com/questions/1408356/keyboard-interrupts-with-pythons-multiprocessing-pool

user_120857720

proc.join() blocks until the process ended. queue.get() blocks until there is something in the queue. Because your processes don't put anything into the queue (in this example) than this code will never get beyond the queue.get() part... If your processes put something in the queue at the very end, then it doesn't matter if you first join() or get() because they happen at about the same time.

user_167563740
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