Graceful degradation in real-time control applications using (m, k)-firm guarantee

Tasks in a real-time control application are usually periodic and they have deadline constraints by which each instance of a task is expected to complete its computation even in the adverse circumstances caused by component failures. Techniques to recover from processor failures often involve a reconfiguration in which all tasks are assigned to fault-free processors. This reconfiguration may result in processor overload where it is no longer possible to meet the deadlines of all tasks. In this paper, we discuss an overload management technique which discards selected task instances in such a way sheet the performance of the control loops in the system remain satisfactory even after a failure. The technique is based on the rationale that real-time control applications can tolerate occasional misses of the control law updates, especially if the control law is modified to account for these missed updates. The paper devises a scheduling policy which deterministic guarantees when and where the misses will occur and proposes a methodology for modifying the control law to minimize the deterioration in the control system behavior as a result of these missed control law updates.