Weakly hard Real-Time Systems
G. Bernat, A. Burns and A. Llamosi
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 introduce several temporal constraints based on determining a maximum number of deadlines that can be missed during a window of time (a given number of invocations). Weakly hard real-time systems can be guaranteed off-line with an extension of traditional schedulability analysis. Also, new scheduling algorithm can be defined that specifically address weakly hard constraints. This report studies in detail different constraints, their properties and relationships and introduces new scheduling paradigms based on this concept.
BibTex Entry
@techreport{Bernat1999a,
author = {G. Bernat and A. Burns and A. Llamosi},
category = {scheduling},
institution = {Dep. Computer Science. University of York},
keyword = {Scheduling, real-time, missed deadlines},
month = {Dec},
number = {YCS-99-320},
title = {Weakly hard Real-Time Systems},
year = {1999}
}