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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.

[1] S. Bennett, Real-Time Computer Control: An Introduction. Prentice Hall Int'l, 1994.
[2] C. Houpis, Digital Control Systems. Theory, Hardware and Software. McGraw-Hill, 1985.
[3] C.C. Bissell, Control Engineering, second ed. Chapman&Hall, 1998.
[4] M. Törngren, “Fundamentals of Implementing Real-Time Control Applications in Distributed Computer Systems,” Real-Time Systems, vol. 14, pp. 219-250, 1998.
[5] G. Bernat and A. Burns, "Combining (n, m)-Hard Deadlines and Dual Priority Scheduling," Proc. IEEE Real-Time Systems Symp.,San Francisco, pp. 46-57, Dec. 1997.
[6] G. Bernat, “Specification and Analysis of Weakly Hard Real-Time Systems,” PhD thesis, Departament de Ciències Matemàtiques i Informàtica. Universitat de les Illes Balears. Spain,, Jan. 1998.
[7] M. Hamdaoui and P. Ramanathan, "A Dynamic Priority Assignment Technique for Streams with (m, k)-Firm Deadlines," IEEE Trans. Computers, vol. 44, no. 12, pp. 1,443-1,451, Dec. 1995.
[8] R. West and C. Poellabauer, “Analysis of a Window-Constrained Scheduler for Real-Time and Best-Effort Packet Streams,” Proc. 21st IEEE Real-Time Systems Symp., Nov. 2000.
[9] G. Koren and D. Shasha, “D-Over: An Optimal On-Line Scheduling Algorithm for Overloaded Real-Time Systems,” Proc. IEEE Real-Time Systems Symp., pp. 290-299, Dec. 1992.
[10] G. Koren and D. Shasha, "Skip-Over: Algorithms and Complexity for Overloaded Systems that Allow Skips," Proc. IEEE Real-Time System Symp., pp. 110-117,Pisa, Italy, Dec. 1995.
[11] M. Caccamo and G.C. Buttazzo, "Exploiting Skips In Periodic Tasks For Enhancing Aperiodic Responsiveness," Proc. IEEE Real-Time Systems Symp., pp. 330-339,San Francisco, Dec. 1997.
[12] J.A. Stankovic, M. Spuri, K. Ramamritham, and G. Buttazzo, Deadline Scheduling for Real-Time Systems: EDF and Related Algorithms. Kluwer Academic, 1998.
[13] G. Buttazzo, M. Spuri, and F. Sensini, Value vs. Deadline Scheduling in Overload Conditions Proc. IEEE Real-Time Systems Symp. (RTSS), pp. 90-95. 1995.
[14] M. Spuri, G.C. Buttazzo, and F. Sensini, “Robust Aperiodic Scheduling under Dynamic Priority Systems,” Proc. IEEE Real-Time Systems Symp., Pisa, Italy, Dec. 1995.
[15] H. Wedde and J. Lind, “Building Large, Complex, Distributed Safety-Critical Operating Systems,” Real-Time Systems J., vol. 13, pp. 277-302, 1997.
[16] L. Abeni and G. Buttazzo, “Integrating Multimedia Applications in Hard Real-Time Systems,” Proc. IEEE Real-Time Systems Symp., Dec. 1998.
[17] C.D. Locke, D.R. Vogel, and T.J. Mesler, “Building a Predictable Avionics Platform in Ada: A Case Study,” Proc. IEEE 12th Real-Time Systems Symp., Dec. 1991.
[18] N.C. Audsley, A. Burns, M. Richardson, K. Tindell, and A. Wellings, "Applying New Scheduling Theory to Static Priority Preemptive Scheduling," Software Eng. J. vol. 8, no. 5, pp. 284-292, Sept. 1993.
[19] A. Burns and A.J. Wellings, Real-Time Systems and Their Programming Languages. Addison-Wesley, 1990.
[20] G. Bernat and A. Burns, “Weakly-Hard Temporal Constraints,” Technical Report YCS, Dept. of Computer Science. Univ. of York, 2000.
[21] L. Sha, R. Rajkuma, and J.P. Lehoczky, "Priority Inheritance Protocols: An Approach to Real-Time Synchronization," IEEE Trans. Computers, vol. 39, no. 9, pp. 1,175-1,185, Sept. 1990.

Index Terms:
Real-time systems, specification, temporal constraints, scheduling algorithms, schedulability analysis, transient overload.
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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