Shared memory in multiprocessing

Question

I have three large lists. First contains bitarrays (module bitarray 0.8.0) and the other two contain arrays of integers.

l1=[bitarray 1, bitarray 2, ... ,bitarray n]
l2=[array 1, array 2, ... , array n]
l3=[array 1, array 2, ... , array n]

These data structures take quite a bit of RAM (~16GB total).

If i start 12 sub-processes using:

multiprocessing.Process(target=someFunction, args=(l1,l2,l3))

Does this mean that l1, l2 and l3 will be copied for each sub-process or will the sub-processes share these lists? Or to be more direct, will I use 16GB or 192GB of RAM?

someFunction will read some values from these lists and then performs some calculations based on the values read. The results will be returned to the parent-process. The lists l1, l2 and l3 will not be modified by someFunction.

Therefore i would assume that the sub-processes do not need and would not copy these huge lists but would instead just share them with the parent. Meaning that the program would take 16GB of RAM (regardless of how many sub-processes i start) due to the copy-on-write approach under linux? Am i correct or am i missing something that would cause the lists to be copied?

EDIT: I am still confused, after reading a bit more on the subject. On the one hand Linux uses copy-on-write, which should mean that no data is copied. On the other hand, accessing the object will change its ref-count (i am still unsure why and what does that mean). Even so, will the entire object be copied?

For example if i define someFunction as follows:

def someFunction(list1, list2, list3):
    i=random.randint(0,99999)
    print list1[i], list2[i], list3[i]

Would using this function mean that l1, l2 and l3 will be copied entirely for each sub-process?

Is there a way to check for this?

EDIT2 After reading a bit more and monitoring total memory usage of the system while sub-processes are running, it seems that entire objects are indeed copied for each sub-process. And it seems to be because reference counting.

The reference counting for l1, l2 and l3 is actually unneeded in my program. This is because l1, l2 and l3 will be kept in memory (unchanged) until the parent-process exits. There is no need to free the memory used by these lists until then. In fact i know for sure that the reference count will remain above 0 (for these lists and every object in these lists) until the program exits.

So now the question becomes, how can i make sure that the objects will not be copied to each sub-process? Can i perhaps disable reference counting for these lists and each object in these lists?

EDIT3 Just an additional note. Sub-processes do not need to modify l1, l2 and l3 or any objects in these lists. The sub-processes only need to be able to reference some of these objects without causing the memory to be copied for each sub-process.

Answer 1

Because this is still a very high result on google and no one else has mentioned it yet, I thought I would mention the new possibility of 'true' shared memory which was introduced in python version 3.8.0: https://docs.python.org/3/library/multiprocessing.shared_memory.html

I have here included a small contrived example (tested on linux) where numpy arrays are used, which is likely a very common use case:

# one dimension of the 2d array which is shared
dim = 5000

import numpy as np
from multiprocessing import shared_memory, Process, Lock
from multiprocessing import cpu_count, current_process
import time

lock = Lock()

def add_one(shr_name):

    existing_shm = shared_memory.SharedMemory(name=shr_name)
    np_array = np.ndarray((dim, dim,), dtype=np.int64, buffer=existing_shm.buf)
    lock.acquire()
    np_array[:] = np_array[0] + 1
    lock.release()
    time.sleep(10) # pause, to see the memory usage in top
    print('added one')
    existing_shm.close()

def create_shared_block():

    a = np.ones(shape=(dim, dim), dtype=np.int64)  # Start with an existing NumPy array

    shm = shared_memory.SharedMemory(create=True, size=a.nbytes)
    # # Now create a NumPy array backed by shared memory
    np_array = np.ndarray(a.shape, dtype=np.int64, buffer=shm.buf)
    np_array[:] = a[:]  # Copy the original data into shared memory
    return shm, np_array

if current_process().name == "MainProcess":
    print("creating shared block")
    shr, np_array = create_shared_block()

    processes = []
    for i in range(cpu_count()):
        _process = Process(target=add_one, args=(shr.name,))
        processes.append(_process)
        _process.start()

    for _process in processes:
        _process.join()

    print("Final array")
    print(np_array[:10])
    print(np_array[10:])

    shr.close()
    shr.unlink()

Note that because of the 64 bit ints this code can take about 1gb of ram to run, so make sure that you won't freeze your system using it. ^_^

Answer 2

Generally speaking, there are two ways to share the same data:

• Multithreading
• Shared memory

Python's multithreading is not suitable for CPU-bound tasks (because of the GIL), so the usual solution in that case is to go on multiprocessing. However, with this solution you need to explicitly share the data, using multiprocessing.Value and multiprocessing.Array.

Note that usually sharing data between processes may not be the best choice, because of all the synchronization issues; an approach involving actors exchanging messages is usually seen as a better choice. See also Python documentation:

As mentioned above, when doing concurrent programming it is usually best to avoid using shared state as far as possible. This is particularly true when using multiple processes.

However, if you really do need to use some shared data then multiprocessing provides a couple of ways of doing so.

In your case, you need to wrap l1, l2 and l3 in some way understandable by multiprocessing (e.g. by using a multiprocessing.Array), and then pass them as parameters.
Note also that, as you said you do not need write access, then you should pass lock=False while creating the objects, or all access will be still serialized.

Answer 3

For those interested in using Python3.8 's shared_memory module, it still has a bug (github issue link here) which hasn't been fixed and is affecting Python3.8/3.9/3.10 by now (2021-01-15). The bug affects posix systems and is about resource tracker destroys shared memory segments when other processes should still have valid access. So take care if you use it in your code.

Answer 4

If you want to make use of copy-on-write feature and your data is static(unchanged in child processes) - you should make python don't mess with memory blocks where your data lies. You can easily do this by using C or C++ structures (stl for instance) as containers and provide your own python wrappers that will use pointers to data memory (or possibly copy data mem) when python-level object will be created if any at all. All this can be done very easy with almost python simplicity and syntax with cython.

# pseudo cython
cdef class FooContainer:
   cdef char * data
   def __cinit__(self, char * foo_value):
       self.data = malloc(1024, sizeof(char))
       memcpy(self.data, foo_value, min(1024, len(foo_value)))
   
   def get(self):
       return self.data

# python part
from foo import FooContainer

f = FooContainer("hello world")
pid = fork()
if not pid:
   f.get() # this call will read same memory page to where
           # parent process wrote 1024 chars of self.data
           # and cython will automatically create a new python string
           # object from it and return to caller

The above pseudo-code is badly written. Dont use it. In place of self.data should be C or C++ container in your case.

Answer 5

You can use memcached or redis and set each as a key value pair {'l1'...