You could, but we removed time-slice-based preemption a few versions ago b/c it doesn't make sense for fibers if you also have access to heavyweight threads, so now we only preempt on IO/sleep etc.. The thing about time-slice preemption is that it just doesn't help with lightweight threads anyway, as your machine can only support a tiny number of execution threads that require time-slice preemption while you can have hundreds of thousands, or even millions, of lightweight threads. So runtimes like Erlang that don't give you direct access to kernel threads have no choice but to support time slice preemption, but on the JVM we realized that it serves no purpose, so we took it out.
The IO you are talking about - that is I assume IO methods/libraries that are specifically written/adapted for use in Quasar, yes? If you just use an off-the-shelf JDBC provider, you would block the entire thread when one fiber calls out to the database right?
If so, that is not preemption, that is cooperative multi-tasking.
It's not cooperative because the fibers don't explicitly yield control; they are preempted as soon as they perform an operation that does not require use of the CPU.
As to the choice of libraries, that is an artificial distinction. Runtimes like Erlang and Go also require libraries specifically written/adapted for the runtime; calling an off-the-shelf IO library from Erlang/Go would also block the entire thread. As integrating a library with Quasar does not require changing its interface, the question of whether to trap calls to specific implementations (which is trivial on the JVM) or require the use of fiber-friendly implementation (wrappers) is a question of design. So far, we've opted for the latter simply for the sake of "least surprise", but we may choose to do the former, too. Also, unlike Erlang/Go (I believe), any accidental blocking of the kernel thread is automatically detected by Quasar at run time, and reported with a stack-trace.
Quasar operates in the exact same manner as Erlang/Go, only that we've disabled time-slice preemption once we realized it neither helps nor is it required on the JVM. The only real difference is one the nature of the ecosystem: while all pure-Erlang libraries are designed to work with fibers, most JVM libraries aren't, and so require a thin integration layer that is provided separately. OTOH, thanks to the size of the JVM ecosystem and the standard interface approach, I believe it is the case that today there are more IO libraries that support Quasar fibers (e.g. all servlet servers) than those supporting Erlang processes.
