Online Handling of Firm Aperiodic Tasks in Time Triggered Systems

A number of industrial applications advocate the use of time triggered approaches for reasons of predictability, cost, product reuse, and maintenance. The rigid offline scheduling schemes used for time triggered systems, however, do not provide for flexibility. At runtime, aperiodic tasks can only be included into the unused resources of the offline schedule, supporting neither guarantees nor fast response times. In this paper we present an algorithm for flexible hard aperiodic task handling in offline scheduled systems: it provides an O(N) acceptance test to determine if a set of aperiodics can be feasibly included into the offline scheduled tasks, and does not require runtime handling of resource reservation for guaranteed tasks. Thus, it supports flexible schemes for rejection and removal of aperiodic tasks, overload handling, and simple reclaiming of resources. As a result, our algorithm provides for a combination of offline scheduling and online hard aperiodic task handling.