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Scheduling Algorithms in Modern Disk Drives

Scheduling Algorithms in Modern Disk Drives. B. Worthington, G. Ganger, Y. Patt. Presented by: Chiu Tan. Introduction. Why study scheduling algorithms ? Latency gap between disk and memory is high. Good algorithms can narrow this gap.

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Scheduling Algorithms in Modern Disk Drives

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  1. Scheduling Algorithms in Modern Disk Drives B. Worthington, G. Ganger, Y. Patt Presented by: Chiu Tan

  2. Introduction • Why study scheduling algorithms? • Latency gap between disk and memory is high. • Good algorithms can narrow this gap. • Disk traffic is bursty, resulting in long queues of requests. Algorithms can reschedule requests with respect to disk state to improve performance.

  3. Outline • Disk basics • Disk Evaluation • Common Algorithms • Simulation Issues • Findings • Conclusion

  4. Disk Basics Sector (512 bytes + error correction) Track (One of many on a surface) Cylinder(Set of tracks, one from each surface)

  5. Disk Basics (II) • How does a disk service a request? • Read/Write performed by a disk arm. • Disk arm has to first find the correct cylinder. This is known as the seek time. • The disk is spinning. The arm waits until the correct sector reaches the read/write head of the disk arm. This is the rotational latency. • Generally, multiple surfaces, but only 1 read/wrote head is active.

  6. Disk Evaluation • Disk performance primarily measured using access time. • Access time comprises of seek time and rotational latency. • Seek time: Time needed to move disk head to correct cylinder. • Rotational latency: Time needed for disk to rotate to correct sector.

  7. Common Algorithms • To improve response time, we can reduce seek time or reduce rotational latency. • Simplest algorithm: FCFS • Seek Time: SSTF. SCAN, LOOK, CLOOK • Seek + Rotational Latency: SPTF • To illustrate: 5,9,18,3,12,1,13,6,7

  8. FCFS: First come first serve 5, 9, 18, 3, 12, 1, 13, 6, 7 Total: 63

  9. SSTF: Shortest Seek Time 5, 9, 18, 3, 12, 1, 13, 6, 7 Total: 30

  10. SCAN 5, 9, 18, 3, 12, 1, 13, 6, 7 Total: 23

  11. LOOK 5, 9, 18, 3, 12, 1, 13, 6, 7 Total: 21

  12. CLOOK 5, 9, 18, 3, 12, 1, 13, 6, 7 Total: 33

  13. SPTF: Shortest Positioning • Reducing seek delay needs only relative seek distances. Easy to estimate. • SPTF is like SSTF, but need to minimize seek time and rotational latency. • Need actual LBN-to-PBN number, accurate position of read/write head.

  14. Simulation Issues • Synthetic workload and generated traces used. • Synthetic workloads are easier to generate, but unrealistic. • Traces are more realistic, but adjustments needed. • Different storage capacity. • Different service rates.

  15. Simulation Issues (II) • 2 metrics are used: average response time and squared coefficient of variance • A balance between the two is most desirable. Faster performance with lesser starvation.

  16. Findings: Scheduling via LBN • For synthetic workloads, CLOOK average response time is 5% worst than LOOK, SSTF, VSCAN. • But CLOOK outperformed the rest in terms of starvation resistance. • For traces, CLOOK on average outperforms the rest, and on starvation resistance as well.

  17. Insights: Scheduling via LBN Why the difference between the two? • Synthetic workloads are random, traces are not. • Random read/writes fail to take advantage of prefetching cache. • Algorithms that preserve read sequentiality can take advantage of perfetching cache to perform better!

  18. Findings: via Full Knowledge • With full knowledge, SPTF considered. Aging component is added to prevent starvation. • For synthetic workloads, all SPTFs perform better than CLOOK. Appropriate aging results in SPFT with CLOOK starvation performance. • For traces, clear wins for SPTFs over CLOOK for some traces, ambiguous for others.

  19. Insights: via Full Knowledge • SPTF-based algorithms perform well compared to CLOOK, but clearer wins occur when on-board cache is exploited! • Assign positioning time to zero if found inside cache. ASPCTF and SPCTF. • Good choice of aging weights have potential to have superior performance and starvation resistance.

  20. Conclusion • CLOOK performs best under real world situations under limited knowledge. • SPTF-based algorithms result in better performance, but results not that much better than CLOOK. • Careful aging and exploitation of cache needed for superior performance. • Algorithms have to exploit perfetching cache to improve performance regardless.

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