Integrated dynamic scheduling of hard and QoS degradable real-time tasks in multiprocessor systems Anita Mittal, G. Manimaran 1 , C. Siva Ram Murthy * Department of Computer Science and Engineering, Indian Institute of Technology, Madras 600 036, India Received 16 September 1998; received in revised form 16 June 1999; accepted 9 September 1999 Abstract Many time-critical applications require predictable performance and tasks in these applications have deadlines to be met. For tasks with hard deadlines, a deadline miss can be catastrophic while for Quality of Service (QoS) degradable tasks (soft real-time tasks) timely approximate results of poorer quality or occasional deadline misses are acceptable. Imprecise computation and m; k-®rm guarantee are two workload models that quantify the trade-o between schedulability and result quality. In this paper, we propose dynamic scheduling algorithms for integrated scheduling of real-time tasks, represented by these workload models, in multiprocessor systems. The algorithms aim at improving the schedulability of tasks by exploiting the properties of these models in QoS degradation. We also show how the proposed algorithms can be adapted for integrated scheduling of multimedia streams and hard real-time tasks, and demonstrate their eectiveness in quantifying QoS degradation. Through simulation, we evaluate the performance of these algo- rithms using the metrics ± success ratio (measure of schedulability) and quality. Our simulation results show that one of the proposed algorithms, multilevel degradation algorithm, outperforms the others in terms of both the performance metrics. Ó 2000 Elsevier Science B.V. All rights reserved. Keywords: Multiprocessor; Real-time system; Dynamic scheduling; QoS degradation 1. Introduction Real-time systems are de®ned as those in which the correctness of the system depends not only on the logical result of computation, but also on the time at which the results are produced [1]. The real-time processes impose explicit timing con- straints on the individual tasks which must be met in order to ensure the correctness and safety of real-time systems. For some tasks, a deadline miss can lead to catastrophe, while for some other tasks timely approximate results of poorer quality or occasional deadline misses can be tolerated. Tasks in the former category are hard real-time tasks while tasks in the latter category are soft real-time (QoS degradable) tasks. Multimedia information, which has soft real-time constraints, is being in- creasingly used in real-time applications. Because of the wide range of applications having tasks of the hard and soft deadline category, it is only natural to look at the possibility of a common platform that supports both hard and soft www.elsevier.com/locate/sysarc Journal of Systems Architecture 46 (2000) 793±807 * Corresponding author. Tel.: 91 44 2351365 ext. 3501; fax: 0091 44 2350509. E-mail addresses: naman@hpc.iitm.ernet.in (A. Mittal), gmani@hpc.iitm.ernet.in, gmani@shasta.ee.iastate.edu (G. Manimaran), murthy@iitm.ernet.in (C. Siva Ram Murthy). 1 Present Address: Department of Electrical and Computer Engineering, Iowa State University, Ames, IA 50011, USA. 1383-7621/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 1 3 8 3 - 7 6 2 1 ( 9 9 ) 0 0 0 3 9 - 9