Exploiting skips in periodic tasks for enhancing aperiodic responsiveness

In certain real-time applications, ranging from multimedia to telecommunication systems, timing constraints can be more flexible than scheduling theory usually permits. For example, in video reception, missing a deadline is acceptable, provided that most deadlines are met. In this paper, we deal with the problem of scheduling hybrid sets of tasks, consisting of firm periodic tasks (i.e., tasks with deadlines which can occasionally skip one instance) and soft aperiodic requests, which have to be served as soon as possible to minimize their average response time. We propose and analyze an algorithm, based on a variant of Earliest Deadline First scheduling, which exploits skips to enhance the response time of aperiodic requests. Schedulability bounds are also derived to perform off-line analysis.