Upload
hamid-mozaffari
View
1.007
Download
0
Embed Size (px)
Citation preview
H. Mozaffari, A. Ghanaatpishe, H. Mobasher Information Storage and Retrieval
April 21, 2014
Disk scheduling algorithms
Introduction Various Scheduling algorithms
FCFS SSTF SCAN C-SCAN N-Step-SCAN, FSCAN LOOK C-LOOK
Comparison of Disk Scheduling Algorithms Conclusion
Overview
What is disk scheduling?Servicing the disk I/O requests
Why disk Scheduling?Use hardware efficiently
Disk scheduling includesFast access time (seek time+ rotational latency)
Introduction
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
To read or write, the disk head must be positioned at the desired track and at the beginning of the desired sector
Seek timeTime it takes to position the head at the desired track
Rotational delay or rotational latencyTime its takes for the beginning of the sector to reach the head
Data transfer timeData transfer occurs as the sector moves under the head
Disk Performance Parameters
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
Work Queue: 23, 89, 132, 42, 187 There are 200 cylinders numbered from 0 – 199. The disk head stars at number 100.
Example
Process request sequentially. Fair to all processes, no starvation. Approaches random scheduling in performance if there
are many processes.
First come, first serve(FCFS)
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)
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
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
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
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
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
FCFS SSTF SCAN C-SCAN LOOK C-LOOK
421 263 287 368 241 296Total seek
length
84.2 52.6 57.4 73.6 48.2 59.2Average
seek length
Comparison of Disk Scheduling Algorithms
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
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