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Weakly hard real-time systems

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3 Author(s)
Bernat, G. ; Dept. of Comput. Sci., York Univ., UK ; Burns, A. ; Liamosi, A.

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

Published in:

Computers, IEEE Transactions on  (Volume:50 ,  Issue: 4 )