This Article 
   
 Share 
   
 Bibliographic References 
   
 Add to: 
 
Digg
Furl
Spurl
Blink
Simpy
Google
Del.icio.us
Y!MyWeb
 
 Search 
   
The Deferrable Server Algorithm for Enhanced Aperiodic Responsiveness in Hard Real-Time Environments
January 1995 (vol. 44 no. 1)
pp. 73-91

Abstract—Most existing scheduling algorithms for hard real-time systems apply either to periodic tasks or aperiodic tasks but not to both. In practice, real-time systems require an integrated, consistent approach to scheduling that is able to simultaneously meet the timing requirements of hard deadline periodic tasks, hard deadline aperiodic (alert-class) tasks, and soft deadline aperiodic tasks. This paper introduces the Deferrable Server (DS) algorithm which will be shown to provide improved aperiodic response time performance over traditional background and polling approaches. Taking advantage of the fact that, typically, there is no benefit in early completion of the periodic tasks, the Deferrable Server (DS) algorithm assigns higher priority to the aperiodic tasks up until the point where the periodic tasks would start to miss their deadlines. Guaranteed alert-class aperiodic service and greatly reduced response times for soft deadline aperiodic tasks are important features of the DS algorithm, and both are obtained with the hard deadlines of the periodic tasks still being guaranteed. The results of a simulation study performed to evaluate the response time performance of the new algorithm against traditional background and polling approaches are presented. In all cases, the response times of aperiodic tasks are significantly reduced (often by an order of magnitude) while still maintaining guaranteed periodic task deadlines.

Index Terms—Aperiodics, hard deadlines, deferrable server, periodics, real-time, response times, schedulability.

[1] J. P. Lehoczky,“Fixed priority scheduling of periodic task sets with arbitrary deadlines,”inProc. 11th IEEE Real-Time Syst. Symp., 1990, pp. 201–209.
[2] C.L. Liu and J.W. Layland, “Scheduling Algorithms for Multiprogramming in a Hard Real-Time Environment,” J. ACM, vol. 20, no. 1, pp. 40-61, 1973.
[3] J. Liu, C. L. Liu, and L. Liestman,“Scheduling with slack time,”Acta Informatica,vol. 17, pp. 31–41, 1982.
[4] J. Y. Leung and M. L. Merrill,“A note on preemptive scheduling of periodic, real-time tasks,”Inform. Processing Lett.,vol. 11, no. 3, pp. 115–118, Nov. 1980.
[5] E. L. Lawler and C. U. Martel,“Scheduling periodically occurring tasks on multiprocessors,”Inform. Processing Lett.,vol. 12, no. 1, pp. 9–12, Feb. 1981.
[6] J. P. Lehoczky, L. Sha, and Y. Ding,“The rate monotonic scheduling algorithm: exact characterization and average case behavior,”inProc. 10th IEEE Real-Time Syst. Symp., 1989, pp. 166–171.
[7] J. P. Lehoczky, L. Sha, J. K. Strosnider, and H. Tokuda,“Fixed priority scheduling theory for hard real-time systems,”inFoundations of Real-Time Computing: Scheduling and Resource Management,A.M. van Tilborg and G. M. Koob, Eds. New York: Kluwer Academic, 1991, pp. 1–30.
[8] A. K. Mok,“Fundamental Design problems of distributed systems for the hard real-time environment,”Ph.D. thesis, M.I.T., 1983.
[9] L. Sha, J. P. Lehoczky, and R. Rajkumar,“Solutions for some practical problems in prioritized preemptive scheduling,”inProc. 7th IEEE Real-Time Syst. Symp.,1986, pp. 181–191.
[10] J. K. Strosnider and T. Marchok,“Responsive, deterministic ieee 802.5 token ring scheduling,”Real-Time Syst. J.,pp. 133–158, Sept. 1989.
[11] B. Sprunt, “Aperiodic Task Scheduling for Real-Time Systems,” PhD dissertation, Dept. of Electrical and Computer Eng., Carnegie Mellon Univ., 1990.
[12] J. K. Stronsnider,“Highly responsive real-time token rings,”Ph.D. thesis, Carnegie Mellon Univ., Aug. 1988.
[13] ——,“Enhanced responsiveness protocol for real-time token rings,”Tech. Rep. CMU-90-1, Carnegie Mellon Univ., Jan. 1990.
[14] M. Joseph and P. Pandya,“Finding response times in a real-time system,”Comput. J., vol. 29, no. 5, pp. 390–394, 1986.

Citation:
Jay K. Strosnider, John P. Lehoczky, Lui Sha, "The Deferrable Server Algorithm for Enhanced Aperiodic Responsiveness in Hard Real-Time Environments," IEEE Transactions on Computers, vol. 44, no. 1, pp. 73-91, Jan. 1995, doi:10.1109/12.368008
Usage of this product signifies your acceptance of the Terms of Use.