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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

  • 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
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:


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


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

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.
  • 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.