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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

Scheduling Algorithms in Modern Disk Drives

B. Worthington,

G. Ganger,

Y. Patt

Presented by: Chiu Tan


Introduction
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.


Outline
Outline

  • Disk basics

  • Disk Evaluation

  • Common Algorithms

  • Simulation Issues

  • Findings

  • Conclusion


Disk basics
Disk Basics

Sector (512 bytes + error correction)

Track (One of many on a surface)

Cylinder(Set of tracks, one from each surface)


Disk basics ii
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.


Disk evaluation
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.


Common algorithms
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


Fcfs first come first serve
FCFS: First come first serve

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

Total: 63


Sstf shortest seek time
SSTF: Shortest Seek Time

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

Total: 30


Scheduling algorithms in modern disk drives
SCAN

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

Total: 23


Scheduling algorithms in modern disk drives
LOOK

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

Total: 21


Clook
CLOOK

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

Total: 33


Sptf shortest positioning
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.


Simulation issues
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.


Simulation issues ii
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.


Findings scheduling via lbn
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.


Insights scheduling via lbn
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!


Findings via full knowledge
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.


Insights via full knowledge
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.


Conclusion
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.