A. L. N. Reddy
Abstract
This article provides a retrospective of our original paper by the same title in the Proceedings of the First ACM Conference on Multimedia, published in 1993. This article examines the problem of disk scheduling in a multimedia I/O system. In a multimedia server, the disk requests may have constant data rate requirements and need guaranteed service. We propose a new scheduling algorithm, SCAN-EDF, that combines the features of SCAN type of seek optimizing algorithm with an Earliest Deadline First (EDF) type of real-time scheduling algorithm. We compare SCAN-EDF with other scheduling strategies and show that SCAN-EDF combines the best features of both SCAN and EDF. We also investigate the impact of buffer space on the maximum number of video streams that can be supported.We show that by making the deadlines larger than the request periods, a larger number of streams can be supported.We also describe how we extended the SCAN-EDF algorithm in the PRISM multimedia architecture. PRISM is an integrated multimedia server, designed to satisfy the QOS requirements of multiple classes of requests. Our experience in implementing the extended SCAN-EDF algorithm in a generic operating system is discussed and performance metrics and results are presented to illustrate how the SCAN-EDF extensions and implementation strategies have succeeded in meeting the QOS requirements of different classes of requests.
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Index Terms
- Disk scheduling in a multimedia I/O system
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Reviews
Eno Thereska
The progress made over a ten-year period on scheduling policies for multimedia systems is summarized in this paper, which also analyzes in detail a scheduling algorithm called SCAN-EDF, which the authors first developed in 1993. The original 1993 paper [1] described the algorithm and basic performance evaluations; this paper provides a retrospective look, based on measurements taken of the algorithm on a real multimedia server (called PRISM). Whereas resources like the central processing unit (CPU) and the network can be partitioned among several request streams using the earliest deadline first (EDF) algorithm, a disk's bandwidth cannot be partitioned efficiently using this scheduling algorithm. The reason is that mechanical delays induced when moving the disk head are significant (measured in milliseconds in 2005), and the time wasted when positioning from one request stream to the next can be significantly larger than the time required to read the blocks that belong to the request stream. Such waste exists for other resources too (such as CPU context switch times), but it is especially large for disks. Traditional bandwidth-maximizing scheduling algorithms, like shortest seek time first (SSTF) and shortest positioning time first (SPTF), do not work for multimedia streams either, since they are not fair, and may induce starvation. Hence, these algorithms are not considered. The authors instead propose extending a commonly used fair-scheduling policy, SCAN, which provides fairness at the expense of throughput, with EDF. SCAN is not as efficient as SSTF, but approximates SSTF fairly well. Combined with EDF, the amount of disk bandwidth is maximized while still meeting request stream deadlines. The authors compare this new algorithm with other ones (such as EDF, SSTF, and SCAN in isolation), and show it to be efficient. This paper summarizes other related work well. The main deficiency with this paper is that the evaluation considers out-of-date disks, which are more representative of disks ten years ago than of disks today. For example, the disk's capacity is 4.3 gigabytes, the average seek time is 11 milliseconds, and the number of sectors per track is just 63. These are nontrivial parameters that should be updated. In modern disks, the seek time is five milliseconds, the capacity is much larger, and the rotational speeds are very fast. The video and audio rate requirements have been constant during the last ten years (for example, 30 frames per second), so any evaluation done in 2005 should consider the new technology. Another possible addition to this work would be to consider disks with preemptible requests. Recent work by Dimitrijevic and others [2] at the 2003 File and Storage Technologies (FAST) conference illustrates storage systems that can use preemption. Such preemption would perhaps make scheduling in a multimedia environment easier. Online Computing Reviews Service
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