> GC still stops the world, right?

Yes, Ractor local GC is the one feature that didn't make it into 4.0.

> Can't you just have completely separate boxes with their own copies of all classes etc, or does that use too much memory?

Ruby::Box is kinda complicated, and still need a lot of work, so it's unclear how the final implementation will be. Right now there is no CoW or any type of sharing for most classes, except for core classes.

Core classes are the same object (pointer) across all boxes, however they have a constant and method table for each box.

But overall what I meant to say is that Box wouldn't make GC any easier for Ractors.