Weakly hard real-time systems

TLDR

Real‑time systems are classified as hard, where no deadlines may be missed, or soft/firm, where misses are allowed but typically unpredictable, yet many hard systems can tolerate predictable deadline misses. This work introduces weakly hard real‑time systems, defining a framework that tolerates a specified degree of missed deadlines and analyzes the properties of the resulting constraints. Four temporal constraints are defined, each limiting the maximum number of missed deadlines allowed within a specified window of invocations. The authors determine precise conditions under which one constraint is stricter than another and provide fixed‑priority scheduling and response‑time schedulability results for a broad set of process models.

Abstract

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 nonpredictable 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.