Rate-Monotonic scheduling for hard-real-time systems

Hard-real-time computing systems are widely used in our society, for example, in nuclear and industrial plants, telecommunications, avionics and robotics. In such systems, almost all tasks occur infinitely often and have time deadlines, namely, their correctness relies not only on their logical results, but also on the time at which the results are available. A scheduling algorithm specifies an order in which all the tasks are to be executed, in a way that all the time deadlines are met. This paper provides a review on deterministic scheduling algorithms for hard-real-time systems, focusing mainly on fixed priority, preemptive scheduling of periodic tasks on a single processor and, in particular, on the Rate-Monotonic algorithm. After presenting some basic results, several generalisations, aimed at relaxing some constraints and facing more realistic cases, are described. Issues covered include uniprocessor and multiprocessor systems, periodic and non-periodic tasks, restricted and arbitrary deadlines, fixed and dynamic priorities, independent and synchronised tasks, as well as fault-free and fault-tolerant systems.