Transformation of Sporadic Tasks for Off-line Scheduling with Utilization and Response Time Trade-offs

Many safety critical real-time systems follow the time triggered approach using off-line scheduling methods for reasons of determinism, simple fault tolerance, certifiability, etc. By their very nature, offline schedules cannot handle activities which are not known completely at design time. While some methods have been presented to integrate event-triggered activities at runtime, these require modification of the simple and deterministic table look up scheduling at runtime. In this paper, we provide a proactive method for handling sporadic tasks with standard off-line scheduling in time-triggered systems without changing the schedule at runtime. The method is based on transforming the parameters of sporadic tasks to create periodic reservation tasks, for including in regular task sets for standard off-line schedulers. The resulting scheduling tables can be executed by regular time-triggered systems. The transformation requires more reserved utilization than the sporadic tasks actually need. We use this not needed reserved utilization to reduce the worst case response time of the sporadic tasks. A trade-off between reserved utilization and worst case response time of sporadic tasks is presented in this paper, too. We provide a feasibility analysis also considering the switching overhead for given bounds on the reserved utilization and worst case response time. With this analysis, we determine feasible parameters of the reservation tasks and hence, provide a designer choice for the reserved utilization and the worst case response time.