It’s a pretty simple difference. In a shared memory model, multiple workers all operate on the same data. This opens up a lot of the concurrency issues that are common in parallel programming. Message passing systems make workers communicate through a messaging system. Messages keep everyone seperated, so that workers cannot modify each other’s data. … Read more
Trying again as I just saw the bounty 😉 Basically I think the error message means what it said – multiprocessing shared memory Arrays can’t be passed as arguments (by pickling). It doesn’t make sense to serialise the data – the point is the data is shared memory. So you have to make the shared … Read more
Both methods are viable. mmap method is a little bit more restrictive then shmget, but easier to use. shmget is the old System V shared memory model and has the widest support. mmap/shm_open is the new POSIX way to do shared memory and is easier to use. If your OS permits the use of POSIX … Read more
From the comments above, it seems that this is planned for pandas some time (there’s also an interesting-looking rosetta project which I just noticed). However, until every parallel functionality is incorporated into pandas, I noticed that it’s very easy to write efficient & non-memory-copying parallel augmentations to pandas directly using cython + OpenMP and C++. … Read more
If you are on Linux (or any POSIX-compliant system), you can define this array as a global variable. multiprocessing is using fork() on Linux when it starts a new child process. A newly spawned child process automatically shares the memory with its parent as long as it does not change it (copy-on-write mechanism). Since you … Read more
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 … Read more
If you use an operating system that uses copy-on-write fork() semantics (like any common unix), then as long as you never alter your data structure it will be available to all child processes without taking up additional memory. You will not have to do anything special (except make absolutely sure you don’t alter the object). … Read more
There are two approaches: shmget and mmap. I’ll talk about mmap, since it’s more modern and flexible, but you can take a look at man shmget (or this tutorial) if you’d rather use the old-style tools. The mmap() function can be used to allocate memory buffers with highly customizable parameters to control access and permissions, … Read more