[loqi]

How so? Sharing arbitrary objects via memory requires some form of coordinated memory management. So you still need a process-wide GC, meaning you're probably stuck with that 30% hit from thread-safe refcounting.

And all the other locking issues are still in play. How would you share a list with another interpreter such that they don't both have access to the same list items? Any solution that looks like a deep copy isn't going to be much better than just serializing across a process boundary.

[sametmax]

Right now multi-processing use a very expensive serialization to send to one worker to another. Shared memory is much cheaper.

Even if you didn't have the GIL, you would still need locks with threads to manipulate any mutable datastructure. The usual concurreny safety advice always apply.