Scheduling Algorithms in Modern Disk Drives

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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

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.
• Disk traffic is bursty, resulting in long queues of requests. Algorithms can reschedule requests with respect to disk state to improve performance.
Outline
• Disk basics
• Disk Evaluation
• Common Algorithms
• Simulation Issues
• Findings
• Conclusion
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)
• 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.
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
• 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

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

Total: 63

SSTF: Shortest Seek Time

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

Total: 30

SCAN

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

Total: 23

LOOK

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

Total: 21

CLOOK

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

Total: 33

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.
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)
• 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
• 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

Why the difference between the two?

• Synthetic workloads are random, traces are not.
• Algorithms that preserve read sequentiality can take advantage of perfetching cache to perform better!
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
• 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
• 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.