Rust Keyword Generics Progress Report: February 2023

Rust's async (or JavaScript's, or Python's) is basically isomorphic to segmented stacks. You save your true stack in a linked list of heap-allocated objects, and the system/kernel stack is only borrowed to run coroutines. You only need a handful of real threads, and there is no problem with having millions of coroutines.

The problem is the speed. Go tried essentially this approach earlier in its life, and segmented stacks failed because they can cause unpredictable and horrible slowdowns when a tight loop crosses over the segmentation threshold.

Instead, now Go uses moveable and resizable stacks, which provide the best of both worlds. This is possible because Go can maintain an invariant that no pointer on the heap can point to an object on the stack. So the runtime can just use contiguous stacks, without any penalty for normal functions. At the same time, the minimum stack size can be very small (2kb for Go, it can go down further, but apparently this is the best compromise).

This kind of design is probably the best overall, but it's very hard to do without a rather intrusive runtime support.