Preemptive Scheduling
Given event-driven scheduling, the OS must choose how to react to such an event. One choice is to postpone a thread switch until the end of the current timer period (this is a choice made in a time-sharing system). Another option is to react immediately to an event and to perform a thread switch if the event wakes up a thread with a higher priority than the running thread; in that case it is preemptive scheduling.