python multiprocessing queue not properly parallelizing

Question

I am having a hard time getting my code to properly run in parallel. What I need is the following:

• I have 4 disk codes that I need to evolve for a certain amount of time, in discrete time steps
• I want to evolve each of those codes in parallel

Because of the nature of the codes and of the data that I need to store, after trying processes pools, I've decided my best way to go is to use a Queue. I have tried two different codes. For this example, disk_codes are just numbers, but in my actual code they are individual instances of another code to solve equations of motion in a disk.

Code 1:

import multiprocessing

try:
    import queue
except:
    import Queue as queue

def evolve_single_disk(queue, dt):
    print "Empty queue? ", queue.empty()
    code = queue.get()
    print "Evolving disk {0} in {1}".format(code, multiprocessing.current_process().name)
    queue.task_done()

if __name__ == '__main__':
    ncores = 4
    code_queue = queue.Queue()
    disk_codes = range(ncores)

    for disk in disk_codes:
        code_queue.put(disk)

    dt = 1
    t_end = 10
    t = 0

    # Evolve!
    while t < t_end:
        print "t=", t

        processes = multiprocessing.Process(target=evolve_single_disk, args=(code_queue, dt, ))

        processes.start()
        processes.join()

        disk_codes = code_queue.get()
        print "disk codes: ", disk_codes


        t += dt

This results in:

t= 0
Empty queue?  False
Evolving disk 0 in Process-1
disk codes:  0

t= 1
Empty queue?  False
Evolving disk 1 in Process-2
disk codes:  1

t= 2
Empty queue?  False
Evolving disk 2 in Process-3
disk codes:  2

t= 3
Empty queue?  False
Evolving disk 3 in Process-4
disk codes:  3

t= 4
Empty queue?  True

So, in every time step, one of the disks is "evolved". This is not what I want, since I want all four disks to be evolved in parallel in the same time step.

Then I tried this:

Code 2:

import multiprocessing

try:
    import queue
except:
    import Queue as queue


def evolve_single_disk(queue, dt):
    print "Empty queue? ", queue.empty()
    code = queue.get()  
    print "Evolving disk {0} in {1}".format(code, multiprocessing.current_process().name)
    queue.task_done()


if __name__ == '__main__':
    ncores = 4
    code_queue = queue.Queue()
    disk_codes = range(ncores)

    for disk in disk_codes:
        code_queue.put(disk)

    dt = 1
    t_end = 10
    t = 0

    # Evolve!
    while t < t_end:
        print ""
        print "t=", t

        processes = [multiprocessing.Process(target=evolve_single_disk, args=(code_queue, dt, )) for x in range(ncores)]

        for p in processes:
            p.start()
            p.join()

        disk_codes = [code_queue.get() for p in processes]
        print "disk codes: ", disk_codes

        t += dt

Which results in:

t= 0
Empty queue?  False
Evolving disk 0 in Process-1
Empty queue?  False
Evolving disk 0 in Process-2
Empty queue?  False
Evolving disk 0 in Process-3
Empty queue?  False
Evolving disk 0 in Process-4
disk codes:  [0, 1, 2, 3]

t= 1
Empty queue?  True

...and then the code just hangs. So here I am starting 4 processes in every time step, but each process is receiving the exact same disk.

How can I do this properly, so that on each time step there are 4 processes and each one of them evolves 1 single disk? I have read the documentation and many tutorials and SO questions/answers, but I am still very confused.

Edit:

I tried using multiprocessing queues, but then I get a TypeError when I try to put disk codes into the queue. Sadly the disk codes are also not pickleable. Traceback when using multiprocessing queue with disk codes:

t= 0
<multiprocessing.queues.Queue object at 0x7fee4a2d3950>
Traceback (most recent call last):
  File "/home/fran/anaconda2/lib/python2.7/multiprocessing/queues.py", line 268, in _feed
    send(obj)
TypeError: expected string or Unicode object, NoneType found
Traceback (most recent call last):
  File "/home/fran/anaconda2/lib/python2.7/multiprocessing/queues.py", line 268, in _feed
    send(obj)
TypeError: expected string or Unicode object, NoneType found
Traceback (most recent call last):
  File "/home/fran/anaconda2/lib/python2.7/multiprocessing/queues.py", line 268, in _feed
    send(obj)
TypeError: expected string or Unicode object, NoneType found
Traceback (most recent call last):
  File "/home/fran/anaconda2/lib/python2.7/multiprocessing/queues.py", line 268, in _feed
    send(obj)
TypeError: expected string or Unicode object, NoneType found
Empty queue?  True

Answer 1

Move this inside the while loop:

    for disk in disk_codes:
        code_queue.put(disk)

Here is the complete code:

import multiprocessing

def evolve_single_disk(queue, result, dt):
    print "Empty queue? ", queue.empty()
    code = queue.get()
    print "Evolving disk {0} in {1}".format(code, multiprocessing.current_process().name)
    result.put(code)


if __name__ == '__main__':
    ncores = 4
    code_queue = multiprocessing.Queue()
    result_queue = multiprocessing.Queue()
    disk_codes = range(ncores)

    dt = 1
    t_end = 10
    t = 0

    # Evolve!
    while t < t_end:
        for disk in disk_codes:
            code_queue.put(disk)

        print ""
        print "t=", t

        process_list = list()
        for x in range(ncores):
            process = multiprocessing.Process(target=evolve_single_disk, args=(code_queue, result_queue, dt))
            process_list.append(process)

        for p in process_list:
            p.start()
            p.join()

        disk_codes = [result_queue.get() for p in process_list]
        print "disk codes: ", disk_codes

        t += dt

Output

t= 0
Empty queue?  False
Evolving disk 0 in Process-1
Empty queue?  False
Evolving disk 1 in Process-2
Empty queue?  False
Evolving disk 2 in Process-3
Empty queue?  False
Evolving disk 3 in Process-4
disk codes:  [0, 1, 2, 3]

t= 1
Empty queue?  False
Evolving disk 0 in Process-5
Empty queue?  False
Evolving disk 1 in Process-6
Empty queue?  False
Evolving disk 2 in Process-7
Empty queue?  False
Evolving disk 3 in Process-8
disk codes:  [0, 1, 2, 3]

...

t= 9
Empty queue?  False
Evolving disk 0 in Process-37
Empty queue?  False
Evolving disk 1 in Process-38
Empty queue?  False
Evolving disk 2 in Process-39
Empty queue?  False
Evolving disk 3 in Process-40
disk codes:  [0, 1, 2, 3]