Guaranteed on-line weakly-hard real-time systems

A weakly hard real-time system is a system that can tolerate some degree of missed deadlines provided that this number is bounded and guaranteed off-line. In this paper we present an on-line scheduling framework called Bi-Modal Scheduler (BMS) for weakly-hard real-time systems. It is characterised by two modes of operation. In normal mode tasks can be scheduled with a generic scheduler (possibly best-effort). Weakly hard constraints are guaranteed to be satisfied by switching, whenever necessary, to a panic mode for which schedulability tests exist that guarantee that deadlines are met. Due to the sources of pessimism in the analysis (mainly WCET and critical instant assumptions) the worst case situations may never arise, thus almost all the time all deadlines are met, only at peak loads some deadlines may be missed, however the behaviour of the system is predicable and bounded. This allows building systems which maximise resource usage during normal operation and that resort to a guaranteed and predictable performance degradation specified by the weakly hard constraints should a transient overload arise.