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A multimedia server requires a real-time disk-scheduling algorithm to deliver isochronous data for real-time streams. Traditional disk-scheduling algorithms focus on providing good quality in a best-effort manner. In this paper, we propose a novel real-time disk-scheduling algorithm called WRRSCAN (Weighted-Round-Robin-SCAN) to provide quality guaranty for all in-service streams encoded at variable bit rates. WRR-SCAN uniquely adopts the concept of imprecise computation to divide a real-time stream into mandatory jobs and optional jobs. The admission control admits a stream as long as its mandatory jobs are guaranteed to meet their deadlines. Such a decision is made in O(1) time as WRR-SCAN reserves a fixed weight for each stream. WRR-SCAN incorporates an aggressive policy to dynamically reclaim reserved-but-unused bandwidth during run-time. The reclaimed bandwidth is used to serve optional jobs for better quality or aperiodic jobs for reduced response time. We conducted a set of experiments to compare WRR-SCAN with a couple of representative and referred disk-scheduling algorithms. The evaluations are conducted on a commonly-used disk simulator with traces from a real multimedia server. The experimental results show that WRR-SCAN completes significantly more disk jobs and yields considerably better quality for real-time streams.
Real-time systems, Performance evaluation, Performance, Scheduling, Imprecise computation, real-time disk-scheduling algorithms, weighted round-robin

C. Tsai, Y. Tsai, T. Huang, E. Chu and C. Wei, "An Efficient Real-Time Disk-Scheduling Framework with Adaptive Quality Guarantee," in IEEE Transactions on Computers, vol. 57, no. , pp. 634-657, 2007.
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