Can someone explain to me how scheduling works between a main thread and an NSThread?

Question

For example is it doing something like SCHED_FIFO or SCHED_RR from pthreads? Can the scheduling policy be changed? Can priorities be assigned to the main thread and an NSThread?

Ultimately I would like the main thread to run at 30 Hz with a CADisplayLink. In addition an NSThread would run at a lower priority. Ideally the main thread would pre-empt the NSThread to maintain the 30 Hz. I have no idea if this is possible.