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Weakly Hard Real-Time Systems
April 2001 (vol. 50 no. 4)
pp. 308-321

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.

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Index Terms:
Real-time systems, specification, temporal constraints, scheduling algorithms, schedulability analysis, transient overload.
Citation:
Guillem Bernat, Alan Burns, Albert Llamosí, "Weakly Hard Real-Time Systems," IEEE Transactions on Computers, vol. 50, no. 4, pp. 308-321, April 2001, doi:10.1109/12.919277
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