Scheduling periodic and aperiodic tasks in hard real-time computing systems

Scheduling periodic and aperiodic tasks to meet their time constraints has been an important issue in the design of real-time computing systems. Usually, the task scheduling algorithms in such systems must satisfy the deadlines of periodic tasks and provide fast response times for aperiodic tasks. A simple and efficient approach to scheduling real-time tasks is the use of a periodic server in a static preemptive scheduling algorithm. Periodic tasks, including the server, are scheduled at priori to meet their deadlines according to the knowledge of their periods and computation times. The scheduling of aperiodic tasks is then managed by the periodic server during its service time. In this paper, a new scheduling algorithm is proposed. The new algorithm creates a periodic server which will have the highest priority but not necessarily the shortest period. The server is suspended to reduce the overhead if there are no aperiodic tasks waiting, and is activated immediately upon the arrival of the next aperiodic task. After activated, the server performs its duty periodically until all waiting aperiodic tasks are completed. For a set of tasks scheduled by this algorithm, the deadlines of periodic tasks are guaranteed by a deterministic feasibility check, and the mean response time of aperiodic tasks are estimated using a queueing model. Based on the analytical results, we can determine the period and service time of the server producing the minimum mean response time for aperiodic tasks. The analytical results are compared with simulation results to demonstrate the correctness of our model.