Combining Time-Triggered Plans with Priority Scheduled Task Sets

Abstract

Time-triggered and concurrent priority-based scheduling are the two major approaches in use for real-time and embedded systems. Both approaches have their own advantages and drawbacks. On the one hand, priority-based systems facilitate separation of concerns between functional and timing requirements by relying on an underlying real-time operating system that takes all scheduling decisions at run time. But this is at the cost of indeterminism in the exact timing pattern of execution of activities, namely variable release jitter. On the other hand, time-triggered schedules are more intricate to design since all scheduling decisions must be taken beforehand in the design phase, but their advantage is determinism and more chances for minimisation of release jitter. In this paper we propose a software architecture that enables the combined and controlled execution of time-triggered plans and priority-scheduled tasks. We also describe the implementation of an Ada library supporting it. Our aim is to take advantage of the best of both approaches by providing jitter-controlled execution of time-triggered tasks e.g., control tasks, coexisting with a set of priority-scheduled tasks, with less demanding jitter requirements.