>> Executor runs tasklets, [...] that execute a processing step in a finite amount of time.
> lack of preemption seems hard to reconcile with real time guarantees?
Depends on the requirements and the bounds of the finite time allowed and the enforcement of it. For example, Erlang claims to be soft real-time; it doesn't have preemption, but any function call can result in yielding, and as a functional language there are no loops without calling functions.
And that works because it is a VM that is executing the instructions. The higher level of the VM can switch threads at will. The measure used is the number of reductions, once a set limit is used or something more urgent crops up a context switch will occur. This functionally indistinguishable from a hardware interrupt for the task that was executing but because it is all done in software you don't actually have that interrupt. It's more as though every reduction has the potential to interrupt a task just like you can expect an interrupt everywhere in a regular stream of machine code for some processor.
The nice benefit of the Erlang method is that you can graft this onto a user process without a need for special privilege or exception handling, nothing ever really has to deal with interrupts or the messy aftermath of it, it's much cleaner than an interrupt driven system.
> lack of preemption seems hard to reconcile with real time guarantees?
Depends on the requirements and the bounds of the finite time allowed and the enforcement of it. For example, Erlang claims to be soft real-time; it doesn't have preemption, but any function call can result in yielding, and as a functional language there are no loops without calling functions.