I appreciate not wanting to do things until you can do them right, but equally if you advertise this as a preemptive runtime, people are going to expect that they can use it to throw 32 CPU-spinning threads onto 8 cores and have it behave gracefully. It sounds like from a user's point of view on day 1 this runtime will be the worst of both worlds - you need to take care to not do big chunks of CPU work without yielding, but you don't get the full control that a traditional "userspace" cooperative multitasking framework would give you.