[the8472]

Loading 100GB into RAM and then calling fork() is just painting a giant OOM Killer target on your back. It'll work until something breaks the CoWs or the parent gets restarted while some forks still linger or other fun things like that.

Threads make it transparent to the OS that this memory really must be shared between compute tasks.

[zbentley]

While that does sometimes happen, I find the risk to be overstated. Most simple "allocate a large, complex data structure (e.g. dict of vectors of dataclasses) before creating a multiprocessing.Pool/Process/concurrent.futures.ProcessPoolExecutor and then refer to parts of it in the executor's jobs" work that deals in GBs of data does not suffer from copy-on-write-induced OOM issues in my experience. If the data in the shared memory isn't mutated in python, the refcount mutations are rarely enough to dirty more than a fraction of a percent of pages (though there are pathological allocation/reference schemes where that's not true).

If you do have memory issues, calling 'gc.freeze()' right before creating your multiprocessing.Pool/Process/concurrent.futures.ProcessPoolExecutor is sufficient to mitigate refcount-related page dirtying in the vast majority of cases. In the small remaining minority of cases, 'gc.disable()' as suggested by the freeze docs[1] may help. If that still doesn't do it, or if your page-dirtying is due to actual mutations of data (not just refcounts), it may be time to reach for actual shared memory instead[2][3].

1. https://docs.python.org/3/library/gc.html#gc.freeze 2. https://docs.python.org/3/library/multiprocessing.html#share... 3. https://docs.python.org/3/library/multiprocessing.shared_mem...