The International Journal of Time-Critical Computing Systems, 19, 149–168 (2000) c  2000 Kluwer Academic Publishers, Boston. Manufactured in The Netherlands. Real-Time Disk Scheduling for Multimedia Applications with Deadline-Modification-Scan Scheme RAY-I CHANG william@iis.sinica.edu.tw Institute of Information Science, Academia Sinica, Taipei, Taiwan, ROC WEI-KUAN SHIH wshih@cs.nthu.edu.tw Department of Computer Science, National Tsing Hua University, Hsin-chu, Taiwan, ROC RUEI-CHUAN CHANG rc@cc.nctu.edu.tw Department of Computer & Information Science, National Chiao Tung University, Hsin-chu, Taiwan, ROC Abstract. Real-time disk scheduling (RTDS) is important for time-critical multimedia applications. In con- ventional approaches of real-time disk scheduling, such as SCAN-EDF, the seek-optimizing SCAN scheme is applied to reduce the disk service time of the real-time EDF schedule. Since only tasks with the same deadline are seek-optimized, the obtained improvement of SCAN-EDF is limited. In this paper, based on the maximum- scannable-group (MSG) concept, a deadline-modification-scan (DMS or DM-SCAN) algorithm is proposed. Our algorithm uses MSG to automatically decide the suitable task groups for seek-optimizing. Based on the MSG concept, we repeatedly apply DMS to further increase disk throughput to support more tasks. We have imple- mented the proposed algorithm on UnixWare 2.01. The appropriate problem behaviors and parameter values to yield good solutions are investigated. Experiments show that DMS is better than conventional approaches in both the obtained disk throughput and the supported tasks. Moreover, our proposed approach can schedule task sets that are not schedulable by EDF and SCAN-EDF. Keywords: RTDS (real-time disk scheduling), multimedia applications, SCAN, EDF (earliest-deadline-first), operating systems, maximum-scannable-group (MSG), deadline-modification-scan (DMS) 1. Introduction Multimedia applications have become increasingly popular. To support these time-critical applications, the scheduler serves disk tasks in real-time manners (Terry and Swinehart, 1988; Anderson et al., 1992). In a real-time disk scheduler, we need to consider two problem parameters (release time r i and deadline d i ) for each task T i to decide the scheduled start-time e i and fulfill-time f i . • Release time: the earliest time at which the task can be started. • Deadline: the latest time at which the task must be completed. • Start-time: the time at which the task is actually started. • Fulfill-time: the time at which the task is actually completed. To satisfy the real-time constraints, we must guarantee r i ≤ e i and f i ≤ d i . A simple example to demonstrate the terminology in a real-time disk task is shown in Fig. 1. Notice