If all global state is made thread safe and, then whether threads are subinterpreters or a single interpreter is conceptually irrelevant and probably easier to implement.
It really really depends on what you mean by global state here. If you mean the global state within the interpreter that's one thing. Preserving the global state of your application is another.
But a weird "global state" (really more a global property) is the semantics between concurrent pieces of code and the expectations about things like setting variables, possibly interleavings etc.
The nice part of different interpreters isn't just getting around the gil, and maintaining similar isolation, but it's almost like a Terms and Services agreement: I opened this can of worms and it's my responsibility to learn what the consequences are.
It is not conceptually irrelevant. With threads, you can create a Python object on one, store it into a global (or some shared state), and use that object from a different thread. You can't do that with subinterpreters tho.
I think the last decade or so of programming has taught us that people just plain suck at multithreading. Go, Rust are all languages that solve this problem in different ways. It would be a tragedy if Python went back to the old way and didn't have a better solution.
"Went back" implies that threads and shared state are not the status quo. They definitely are in Python (and, realistically, they also are in general, given the degree of Rust adoption vis a vis other PLs). So Python will have support them, if only so that we don't have to rewrite all the Python code that's already around. A new language has the luxury of not caring about backwards compatibility like that.
Also, Go doesn't really solve the problem - sure, it has channels, but it still allows for mutable shared state, and unlike Rust, it doesn't make it hard to use.
In my career, I would say 95% of parallelism does not require low level threading primitives like locks. A lot of it is solved by queues which can be provided by the runtime. The rest of the 5% usually takes up 25% of the debugging, lol.
It's not a question of having a mechanism to transfer data. Sure, you can easily use a static global in a native module to easily transfer a reference across subinterpreter boundaries. But the moment you try to increment refcount for the referenced object, things already break, because you're going to be using the wrong (subinterpreter-global) lock.
Even a completely empty object will contain a reference to its type, which is itself an object. How will you marshal that? Bear in mind that each subinterpreter has its own copy of type objects, and there isn't even a guarantee that those types match even if their names do.
It sounds it is a problem but I feel it can be solved with engineering and mathematics. Not saying it would be easy though.
Couldn't you separate the storage of the refcounts from the objects and use a map to get at them?
As for the identities between types being different.
To create an subinterpreter that can marshall between subinterpreters without copying the data structures requires a different data structure that is safe to use from any interpreter. We need to decouple the book keeping data structures from the underlying data.
We can regenerate book keeping data structures during a .send or .receive
Maintaining identity equivalence is an interesting problem. I think it's a more fundamental problem that probably has mathematical solutions.
If we think of objects as being a vector in mathematical space. We have pointers between vectors in this memory space.
For a data structure to be position independent. We need some way of intending references to be global. But we don't want to introduce a layer of indirection on the reference of object relationships. That would be slower. Could use an atomic counter to ensure that identifiers are globally unique.
Don't want to serialize the access to global types.
It sounds to me it is a many-to-many to many-to-many problem. Which even databases don't solve very well.
It occurred to me, that I was told about Unison Lang recently and this language uses content addressable functions.
In other words, the code for a function is hashed and that is its identity that never changes while the program is running.
If we use the same approach with Python, each object could have a hash that corresponds to the code only, instead of the data. This is the objects identity even when added to the book keeping data of another subinterpreter.
This requires decoupling of book keeping information from actual object storage. But replaces pointers with lookups which could be inlined to pointer arithmetic.
> conceptually irrelevant and probably easier to implement.
Well, it depends on how it’s implemented.
If “made thread safe” means constantly grabbing locks around large blocks of data then the end result is concurrency (hopefully!) but not parallelism. Meaning you might only have one thread active at a time in practice.
Wrapping the universe in a mutex is thread safe. But it’s not a good solution.
