multiprocessing: sharing a large read-only object between processes?

Question

Do child processes spawned via multiprocessing share objects created earlier in the program?

I have the following setup:

do_some_processing(filename):
    for line in file(filename):
        if line.split(',')[0] in big_lookup_object:
            # something here

if __name__ == '__main__':
    big_lookup_object = marshal.load('file.bin')
    pool = Pool(processes=4)
    print pool.map(do_some_processing, glob.glob('*.data'))

I'm loading some big object into memory, then creating a pool of workers that need to make use of that big object. The big object is accessed read-only, I don't need to pass modifications of it between processes.

My question is: is the big object loaded into shared memory, as it would be if I spawned a process in unix/c, or does each process load its own copy of the big object?

Update: to clarify further - big_lookup_object is a shared lookup object. I don't need to split that up and process it separately. I need to keep a single copy of it. The work that I need to split it is reading lots of other large files and looking up the items in those large files against the lookup object.

Further update: database is a fine solution, memcached might be a better solution, and file on disk (shelve or dbm) might be even better. In this question I was particularly interested in an in memory solution. For the final solution I'll be using hadoop, but I wanted to see if I can have a local in-memory version as well.

Answer 1

Do child processes spawned via multiprocessing share objects created earlier in the program?

No (python before 3.8), and Yes in 3.8

Processes have independent memory space.

Solution 1

To make best use of a large structure with lots of workers, do this.

1. Write each worker as a "filter" – reads intermediate results from stdin, does work, writes intermediate results on stdout.

2. Connect all the workers as a pipeline:

   process1 <source | process2 | process3 | ... | processn >result
   

Each process reads, does work and writes.

This is remarkably efficient since all processes are running concurrently. The writes and reads pass directly through shared buffers between the processes.

---

Solution 2

In some cases, you have a more complex structure – often a fan-out structure. In this case you have a parent with multiple children.

1. Parent opens source data. Parent forks a number of children.

2. Parent reads source, farms parts of the source out to each concurrently running child.

3. When parent reaches the end, close the pipe. Child gets end of file and finishes normally.

The child parts are pleasant to write because each child simply reads sys.stdin.

The parent has a little bit of fancy footwork in spawning all the children and retaining the pipes properly, but it's not too bad.

Fan-in is the opposite structure. A number of independently running processes need to interleave their inputs into a common process. The collector is not as easy to write, since it has to read from many sources.

Reading from many named pipes is often done using the select module to see which pipes have pending input.

---

Solution 3

Shared lookup is the definition of a database.

Solution 3A – load a database. Let the workers process the data in the database.

Solution 3B – create a very simple server using werkzeug (or similar) to provide WSGI applications that respond to HTTP GET so the workers can query the server.

---

Solution 4

Shared filesystem object. Unix OS offers shared memory objects. These are just files that are mapped to memory so that swapping I/O is done instead of more convention buffered reads.

You can do this from a Python context in several ways

1. Write a startup program that (1) breaks your original gigantic object into smaller objects, and (2) starts workers, each with a smaller object. The smaller objects could be pickled Python objects to save a tiny bit of file reading time.

2. Write a startup program that (1) reads your original gigantic object and writes a page-structured, byte-coded file using seek operations to assure that individual sections are easy to find with simple seeks. This is what a database engine does – break the data into pages, make each page easy to locate via a seek.

Spawn workers with access this this large page-structured file. Each worker can seek to the relevant parts and do their work there.