Weakly-Hard Real-Time Systems

In a hard real-time system it is assumed that no deadline is missed, whereas in a soft or firm real-time system deadlines can be missed, although, this usually happens in a non-predictable way. However, most hard real-time systems could miss some deadlines provided that it happens in a known and predictable way. Also adding predictability on the pattern of missed deadlines for soft and firm real-time systems is desirable, for instance to guarantee levels of quality of service. We introduce the concept of weakly-hard real-time systems to model real-time systems that can tolerate a clearly specified degree of missed deadlines. For this purpose we define four temporal constraints based on determining a maximum number of deadlines that can be missed during a window of time (a given number of invocations). This paper provides the theoretical analysis of the properties and relationships of these constraints. It also shows the exact conditions under which a constraint is harder to satisfy than another constraint. Finally, results on fixed priority scheduling and response-time schedulability tests for a wide range of process models are presented.