Google's fiber implementation is a take on that actually. Each fiber iirc is an actual thread, but there are some special syscalls that allow context switching

Google's fiber implementation is a take on that actually. Each fiber iirc is an actual thread, but there are some special syscalls that allow context switching to another specific thread and bypassing slow scheduling logic. This can save a lot of cpu scheduling threads, but they are still ordinary threads and stacks that take up the normal amount of memory.