Priority-driven, preemptive I/O controllers for real-time systems

The effect of three I/O controller architectures on schedulable utilization, which is the highest attainable resource utilization at or below which all deadlines can be guaranteed, is examined. FIFO (first-in-first-out) request queuing, priority queuing, and priority queuing with preemptable service are simulated for a range of CPU computation to I/O traffic ratios. The results show that, for I/O-bound task sets and zero preemption costs, priority queuing with preemptable service can provide a level of schedulable utilization 35% higher than that attainable with FIFO queuing, and 20% higher than priority queuing and nonpreemptable service. Although the potential gain for priority queuing with preemptable service is large, further simulations that incorporate a time penalty for each preemption show that the gain is very sensitive to preemption cost. With preemption cost represented as a ratio of preemption time to the minimum-task period, the level of schedulable utilization for priority queuing with preemptable service degrades to that of priority queuing with nonpreemptible service, for a preemption cost ratio of 0.04. A high-level design of a preemptable I/O controller is described and the issues determining preemption cost are detailed, along with techniques for its minimization