Why is the memory usage of a child (python multiprocessing) process so different when sharing a ctypes.Structure with a string vs. only a string?

Question

The following code uses multiprocessing's Array to share a large array of unicode strings across processes. If I use c_wchar_p as the type, the child process' memory usage is about one quarter of memory used in the parent process (the amount changes if I change the amount of entries in the Array).

However, if I use a ctypes.Structure with a single c_wchar_p field the child process' memory usage is constant and very low while the parent process' memory usage doubles.

import ctypes
import multiprocessing
import random
import resource
import time

a = None

class Record(ctypes.Structure):
    _fields_ = [('value', ctypes.c_wchar_p)]
    def __init__(self, value):
        self.value = value

    def __str__(self):
        return '(%s)' % (self.value,)

def child(i):
    while True:
        print "%ik memory used in child %i: %s" % (resource.getrusage(resource.RUSAGE_SELF).ru_maxrss / 1024, i, a[i])
        time.sleep(1)
        for j in xrange(len(a)):
            c = a[j]

def main():
    global a
    # uncomment this line and comment the next to switch
    #a = multiprocessing.Array(ctypes.c_wchar_p, [u'unicode %r!' % i for i in xrange(1000000)], lock=False)
    a = multiprocessing.Array(Record, [Record(u'unicode %r!' % i) for i in xrange(1000000)], lock=False)
    for i in xrange(5):
        p = multiprocessing.Process(target=child, args=(i + 1,))
        p.start()
    while True:
        print "%ik memory used in parent: %s" % (resource.getrusage(resource.RUSAGE_SELF).ru_maxrss / 1024, a[0])
        time.sleep(1)

if __name__ == '__main__':
    main()

Using c_wchar_p results in this output:

363224k memory used in parent: unicode 0!
72560k memory used in child 5: unicode 5!
72556k memory used in child 3: unicode 3!
72536k memory used in child 1: unicode 1!
72568k memory used in child 4: unicode 4!
72576k memory used in child 2: unicode 2!

Using Record results in this output:

712508k memory used in parent: (unicode 0!)
1912k memory used in child 1: (unicode 1!)
1908k memory used in child 2: (unicode 2!)
1904k memory used in child 5: (unicode 5!)
1904k memory used in child 4: (unicode 4!)
1908k memory used in child 3: (unicode 3!)

Why?

Answer 1

I don't know about the increase in memory usage but I don't think it is really doing what you intend to do.

If you modify a[i] in your parent process, the child processes don't get the same value.

It's best not to pass pointers (which is exactly what the _p types are) between processes. As quoted from multiprocessing docs:

Although it is possible to store a pointer in shared memory remember that this will refer to a location in the address space of a specific process. However, the pointer is quite likely to be invalid in the context of a second process and trying to dereference the pointer from the second process may cause a crash.