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Issue No. 04 - April (2001 vol. 50)
ISSN: 0018-9340
pp: 308-321
<p><b>Abstract</b>—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 <it>harder</it> 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.</p>
Real-time systems, specification, temporal constraints, scheduling algorithms, schedulability analysis, transient overload.

A. Llamosí, A. Burns and G. Bernat, "Weakly Hard Real-Time Systems," in IEEE Transactions on Computers, vol. 50, no. , pp. 308-321, 2001.
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