Towards RTOS support for mixed time-triggered and event-triggered task sets

Many embedded systems have complex timing constraints and, at the same time, have flexibility requirements which prohibit offline planning of the entire system. To support a mixture of time-triggered and event-triggered tasks, some industrial systems deploy a real-time operating system (RTOS) with a table-driven dispatcher complemented with a preemptive scheduler to allocate free time slots to event-driven tasks. Rather than allocating dedicated time-slots to time-triggered tasks, we propose to dynamically re-allocate time-slots of time-triggered tasks within a pre-computed time range to maximize the available processing capacity for event-triggered tasks. Although the concept — called slotshifting — is not new, we are unaware of a commercial RTOS with such support. After identifying the mechanisms for an RTOS implementation of slotshifting, we discuss the run-time overheads for admitting aperiodic requests into the system1.