I know, but that's a set of libraries. Nothing which can't be done, because the foundation is there. In most basic terms, you need to be able to freeze apps and be able to tell when they have nothing to do, so they can be scheduled with minimal time slices to run a single, lightweight function to check whether they need to do anything at all.

Of course it's more complicated than that, but it's not impossible, since the foundation is already there.