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# Scheduling Algorithms in Modern Disk Drives - PowerPoint PPT Presentation

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

B. Worthington,

G. Ganger,

Y. Patt

Presented by: Chiu Tan

• 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.

• Disk basics

• Disk Evaluation

• Common Algorithms

• Simulation Issues

• Findings

• Conclusion

Sector (512 bytes + error correction)

Track (One of many on a surface)

Cylinder(Set of tracks, one from each surface)

• 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.

• 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.

• 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

5, 9, 18, 3, 12, 1, 13, 6, 7

Total: 63

5, 9, 18, 3, 12, 1, 13, 6, 7

Total: 30

5, 9, 18, 3, 12, 1, 13, 6, 7

Total: 23

5, 9, 18, 3, 12, 1, 13, 6, 7

Total: 21

5, 9, 18, 3, 12, 1, 13, 6, 7

Total: 33

• 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.

• 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.

• 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.

• 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.

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!

• 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.

• 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.

• 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.