To expand on this, by allowing preemption on function calls and simply not providing a loop mechanism, you can guarantee an upper bound on how long a process may need to wait until it can be scheduled.
Guarantee is a strong term. If you have an infinite number of processes to running, there aren't guarantees on how long until it is scheduled.
Similarly, with cooperative multi-tasking as in Gevent, you can manipulate scheduling to try to provide better guarantees about wait times. It's just... you can't ignore the problem.
