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Slides Ch4 Disk Scheduling

This document discusses disk scheduling algorithms used by operating systems to efficiently service disk I/O requests. It describes several common algorithms like FCFS, SSTF, SCAN, C-SCAN, and C-LOOK and compares their performance in minimizing disk head movement. The optimal algorithm depends on the request load and pattern. SSTF and C-LOOK are often reasonable defaults but SCAN-based algorithms generally perform better under heavy I/O loads.

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Abuky Ye Aman
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518 views13 pages

Slides Ch4 Disk Scheduling

This document discusses disk scheduling algorithms used by operating systems to efficiently service disk I/O requests. It describes several common algorithms like FCFS, SSTF, SCAN, C-SCAN, and C-LOOK and compares their performance in minimizing disk head movement. The optimal algorithm depends on the request load and pattern. SSTF and C-LOOK are often reasonable defaults but SCAN-based algorithms generally perform better under heavy I/O loads.

Uploaded by

Abuky Ye Aman
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
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Download as PDF, TXT or read online on Scribd
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Disk Scheduling

 The operating system is responsible for using hardware efficiently — for the disk
drives, this means having a fast access time and disk bandwidth.

 Access time has two major components

 Seek time is the time for the disk to move the heads to the cylinder
containing the desired sector.

 Rotational latency is the additional time waiting for the disk to rotate the
desired sector to the disk head.

 Minimize seek time

 Seek time  seek distance

 Disk bandwidth is the total number of bytes transferred, divided by the total
time between the first request for service and the completion of the last transfer.
Moving-head Disk Machanism
Disk Scheduling (Cont.)

 Several algorithms exist to schedule the servicing of disk I/O requests.

 We illustrate them with a request queue (0-199).

98, 183, 37, 122, 14, 124, 65, 67

Head pointer 53
FCFS

Illustration shows total head movement of 640 cylinders.


SSTF

 Selects the request with the minimum seek time from the current head

position.

 SSTF scheduling is a form of SJF scheduling; may cause starvation of some

requests.
SSTF (Cont.)
Illustration shows total head movement of 236 cylinders.
SCAN

 The disk arm starts at one end of the disk, and moves toward the other end,

servicing requests until it gets to the other end of the disk, where the head

movement is reversed and servicing continues.

 Sometimes called the elevator algorithm.


SCAN (Cont.)
Illustration shows total head movement of 208 cylinders.
C-SCAN

 Provides a more uniform wait time than SCAN.

 The head moves from one end of the disk to the other. servicing requests

as it goes. When it reaches the other end, however, it immediately returns

to the beginning of the disk, without servicing any requests on the return

trip.

 Treats the cylinders as a circular list that wraps around from the last

cylinder to the first one.


C-SCAN (Cont.)
C-LOOK

 Version of C-SCAN

 Arm only goes as far as the last request in each direction, then reverses

direction immediately, without first going all the way to the end of the disk.
C-LOOK (Cont.)
Selecting a Disk-Scheduling Algorithm

 SSTF is common and has a natural appeal

 SCAN and C-SCAN perform better for systems that place a heavy load on
the disk.

 Performance depends on the number and types of requests.

 Requests for disk service can be influenced by the file-allocation method.

 The disk-scheduling algorithm should be written as a separate module of the


operating system, allowing it to be replaced with a different algorithm if
necessary.

 Either SSTF or LOOK is a reasonable choice for the default algorithm.

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