The document discusses various disk scheduling algorithms used by operating systems to optimize disk access time and efficiency. It describes common algorithms like First Come First Serve (FCFS), Shortest Seek Time First (SSTF), SCAN, C-SCAN, LOOK, and C-LOOK. For each algorithm, it provides an example to calculate the total seek length for a sample request queue. It then compares the performance of the different algorithms based on total and average seek lengths. In conclusion, it notes that SCAN and C-SCAN work best under heavy disk loads while SSTF and LOOK are commonly used default algorithms.

1. A. Ghanaatpishe, H. Mobasher, H. Mozaffari
Information Storage and Retrieval
April 21, 2014
Disk scheduling algorithms

2.  Introduction
 Various Scheduling algorithms
 FCFS
 SSTF
 SCAN
 C-SCAN
 N-Step-SCAN, FSCAN
 LOOK
 C-LOOK
 Comparison of Disk Scheduling Algorithms
 Conclusion
Overview

3.  What is disk scheduling?
Servicing the disk I/O requests
 Why disk Scheduling?
Use hardware efficiently
 Disk scheduling includes
Fast access time (seek time+ rotational latency)
Introduction

4.  If desired disk drive is available, request is served
immediately.
 If busy, new request for service will be placed in the queue
of pending requests. When one request is completed, the
OS has to choose which pending request to service next.
I/O requests, OS handling

5.  To read or write, the disk head must be positioned at the
desired track and at the beginning of the desired sector
 Seek time
Time it takes to position the head at the desired track
 Rotational delay or rotational latency
Time its takes for the beginning of the sector to reach the head
 Data transfer time
Data transfer occurs as the sector moves under the head
Disk Performance Parameters

6.  Seek time is the reason for differences in performance
 For a single disk there will be a number of I/O requests
 If requests are selected randomly, we will poor
performance
Disk Scheduling Policies

7.  Work Queue: 23, 89, 132, 42, 187
 There are 200 cylinders numbered from 0 – 199.
 The disk head stars at number 100.
Example

8.  Process request sequentially.
 Fair to all processes, no starvation.
 Approaches random scheduling in performance if there
are many processes.
First come, first serve(FCFS)

9. FCFS Solution
Work Queue: 23, 89, 132, 42, 187
Total seek length:
23 − 100 + 89 − 23 + 132 − 89 + 42 − 132 + 187 − 42 = 421

10.  Select the disk I/O request that requires the least
movement of the disk arm from its current position.
 Always choose the minimum Seek time.
 Starvation is possible; stay in one area of the disk.
 If very busy switching directions, slows things down.
 Not the most optimal.
Shortest Service Time First (SSTF)

11. SSTF Solution
Work Queue: 23, 89, 132, 42, 187
Total seek length:
89 − 100 + 132 − 89 + 187 − 132 + 42 − 187 + 23 − 42 = 263

12.  Arm moves in one direction only, satisfying all outstanding
requests until it reaches the last track in that direction
 Direction is reversed
 Sometimes called the elevator algorithm.
SCAN

13. SCAN Solution
Work Queue: 23, 89, 132, 42, 187
Total seek length:
89 − 100 + 42 − 89 + 23 − 42 + 0 − 23 + 132 − 0 + 187 − 132 = 287

14.  Restricts scanning to one direction only
 When the last track has been visited in one direction, the
arm is returned to the opposite end of the disk and the
scan begins again
C-SCAN

15. C-SCAN Solution
Work Queue: 23, 89, 132, 42, 187
Total seek length:
89 − 100 + 42 − 89 + 23 − 42 + 0 − 23 + 199 − 0 + 187 − 199 + 132 − 189 = 368

16.  Segments the disk request queue into sub queues of
length N.
 Sub queues are processed one at a time, using SCAN.
 New requests added to other queue when queue is
processed.
N-step-SCAN

17.  Two queues
 One queue is empty for new requests
FSCAN

18.  The LOOK algorithm is the same as the SCAN algorithm in
that it also honors requests on both sweep direction of the
disk head.
 The arm goes only as far as final requests in each direction
and then reverses direction without going all the way to
the end.
LOOK

19. LOOK Solution
Work Queue: 23, 89, 132, 42, 187
Total seek length:
89 − 100 + 42 − 89 + 23 − 42 + 132 − 23 + 187 − 132 = 241

20.  This is just an enhanced version of C-SCAN.
 In this, the scanning doesn't go past the last request in the
direction that it is moving. It too jumps to the other end
but not all the way to the end, Just to the furthest request.
C-LOOK

21. C-LOOK Solution
Work Queue: 23, 89, 132, 42, 187
Total seek length:
89 − 100 + 42 − 89 + 23 − 42 + 187 − 23 + 132 − 187 = 296

22. FCFS SSTF SCAN C-SCAN LOOK C-LOOK
421 263 287 368 241 296
Total seek
length
84.2 52.6 57.4 73.6 48.2 59.2
Average
seek
length
Comparison of Disk Scheduling Algorithms

23.  Performance depends on number of requests.
 SCAN, C-SCAN for systems that place a heavy load on the
disk, as they are less likely to cause starvation.
 Default algorithms, SSTF or LOOK
Conclusion

24.  Operating Systems. William Stalling, 7th edition.
 Example of Disk Scheduling Algorithms:
http://www2.cs.uregina.ca/~hamilton/courses/330/notes/io/node8.html
 Disk Scheduling, Vaibhav Kumar Gupta
http://www4.comp.polyu.edu.hk/~csajaykr/myhome/teaching/eel358/ds.pdf
 Operating System Concepts. Greg Gagne, Peter Baer Galvin, and
Abraham Silberschatz, 8th edition.
References

25. Questions & Discussion