I have looked into your advice of changing multiprocessing.heap.Arena.__init__, I have removed the code that allocated the file and reverted to the old behaviour.

I have done some brief runs and it seems to bring back the old behaviour which is allocating the space in RAM, rather than with IO. I am not sure what things this might break, and it might make the other usages of multiprocessing unstable! 

Can anyone think of anything this change might break? The Arena.__init__ code is the one from Python 2.7:

    class Arena(object):
        def __init__(self, size, fd=-1):
             self.size = size
             self.fd = fd  # still kept but is not used !
             self.buffer = mmap.mmap(-1, self.size)

There does not seem to be a difference regardless of the start method setting multiprocessing.set_start_method('fork') to be 'fork'.