Disk Scheduling Full

7/30/2019 Disk Scheduling Full

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BITS 1213

OPERATING SYSTEM

1 BITI

TITLE:

DISK SCHEDULING ALGORITHM

LECTURER NAME:

DR. NURUL AZMA ZAKARIA

NAME &

MATRIK NO

ADLI MUQMIN BIN AZHAR (B031210304)AHMAD KHAIRIL BIN KASIM (B031210013)ANG KUAN KEE (B031210344)

LUA XIN LIN (B031210345)NOOR ALIA BINTI AHMAD (B031210380)

LECTURERDR. NURUL AZMA ZAKARIA


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INTRODUCTION:(CREATED BY LUA XIN LIN)

What is disk-scheduling?

Selecting job/tasks that are to be dispatched in the disk

Why perform disk-scheduling?

To optimize hardware usage by:

o Minimizing seek time

To prioritize I/O requests of certain processes

To allow a large disk bandwidth

To guarantee that certain requests will be issued before a particular deadline

The disk Data Layout of a Magnetic Disk

Disk Performance Parameters

(A hard disk head on an access arm resting on a hard disk platter)


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Seek time - the time taken to move the head to the desired track

o Algorithms that reduce seek time:

SSTF

SCAN

C-SCAN

FIFO

LIFO

RSS

PRI

N-SCAN

F-SCAN

Rotational latency the delay waiting for the rotation of the disk to bring the required disk

sector under the read and write head.

o Factors: rotational speed of disk

o Transfer time , T =

, where

b=no. of bytes to be transferred;

N= no. of bytes on track;

r= rotational speed (r.p.m)

RANDOM SCHEDULING ALGORITHM:(CREATED BY AHMAD KHAIRIL)

RSS (Random Scheduling) For analysis and simulation

The reason for the difference in performance can be traced to seek time. If sector access requests

involve selection of tracks at random, then the performance of the disk I/O system will be poor as

possible. Hence, we need to reduce the average time spent on seeks.

The OS maintains a queue of requests for each I/O device. For a single disk there will be number of I/O

requests from various processes in the queue. If selected items from the queue in random order, then

the tracks to be visited will occur randomly, giving poor performance. This random scheduling is useful

as a benchmark against which to evaluate other techniques.


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PRIORITY ALGORITHM:

PRI (Priority by process) Control outside of disk queue management

With a system based on priority (PRI), the control of the scheduling is outside the control of disk

management software. Such an approach is not intended to optimize disk utilization but to meet other

objectives within the OS. Often short batch jobs and interactive jobs are given higher priority thanlonger jobs that require longer computation. This allows a lot of short jobs to be flushed through the

system quickly and may provide good interactive response time. However, longer jobs may have to wait

excessively long times. Furthermore, such a policy could lead to countermeasures on the part of users,

who split their jobs into smaller pieces to beat the system. This type of policy tends to be poor for

database systems.

FIRST IN FIRST OUT (FIFO)(CREATED BY ADLI MUQMIN)

The simplest form of scheduling in First In First Out (FIFO) scheduling, which processes items

from the queue in sequential order. This strategy has the advantage of being fair, because every

request is honoured and the requests are honoured in order received.

KEYWORD

Process request sequentially

Fair to all processes

Approaches random scheduling in performance if there are many processes

Figure 1 Illustrates the disk arm movement with FIFO


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This graph is generated directly from the data in Table 1. As can be seen, the disk accesses are in

the same order as the requests were originally received.

With FIFO, is there are only a few processes that require access and if many of the requests areto clustered file sectors,then we can hope for good performance. However, this technique will

often approximate random scheduling in performance, if there are many processes competing for

the disk.

Thus, it may be profitable to consider a more sophisticated scheduling policy.

FIFO (starting at track 100)

Next track accessed

5558

3918

90

160150

38

184

Average seek length

Number of tracks traversed

453

1921

72

7010

112

146

55.3

Table 1 Disk scheduling algorithms

LAST IN FIRST OUT (LIFO)

LIFO is an acronym that stands for Last In First Out. By definition, in a LIFO structured linear

list, elements can added or taken off from only one end, called the Top. A LIFO structure can

be illustrated with the example of a stack of trays. The last tray to be placed on top is also the

first to be taken off the top.

LIFO is most used in cases where the last data added to the structure must be the first data to beremoved or evaluated. It need to access the data in limited amounts and in a certain order.

LIFO structure is accessed in opposite order to a queue.


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KEYWORD

Good for transaction processing systems The device is given to the most recent user so there should be little

arm movement

Possibility of starvation since a job may never regain the head of the line

The FIFO and LIFO are listed in Table 2 and differences both of them in Figure 2

Table 2 Description of FIFO and LIFO

Figure 2 Differences of FIFO and LIFO


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o

o

o

o


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o

o And finally

o Which gives us a total of 248 tracks traversed

drawbacks of the SSTF algorithm:

o Starvation:

a disks positioned far from a current area of activity may be starved indefinitely

o switching directions slows down the process

SCAN

(CREATED BY ANG KUAN KEE)

For the FIFO may have some request unfulfilled until the entire queue is emptied. A SCAN

algorithm can prevent this sort of starvation, also known as the elevator algorithm because the

disk arms behaves just like an elevator in a building, first servicing all the requests going up and

then reversing to service requests the other way.

With SCAN, the arm is required to move in one direction only, satisfying all outstanding

requests en route, until it reaches the last track in that direction or until there are no more

requests in that direction, servicing requests as it reaches each cylinder, until it gets to the other

end of the disk.At the other end, the direction of head movement is reversed, and servicing

continues. The head continuously scans back and forth across the disk. So that, it is a better

service distributor.

The service direction is then reversed and the scan proceeds in the opposite direction, again picking up

all requests in order.


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This latter refinement is sometimes referred to as the LOOK policy.

It is no difficult to see that the SCAN policy favors jobs whose requests are for tracks nearest to both

innermost and outermost tracks and favors the latest-arriving jobs.

With SCAN, if the expected time for scan from inner track to outer track is t, then the expected service

interval for sectors at the periphery is 2t.

Figure (e) is showing how SCAN work. Assume it starting scan at tracking 100, it will going by the

direction of increasing track number and when no any higher track, it will decreasing slowly till lower

track number.

If a request arrives in the queue just in front of the head, it will be serviced almost immediately.

If a request arriving just behind the head will have to wait until the arm moves to the end of thedisk, reverses direction, and comes back.

C-SCAN

The C-SCAN (circular SCAN) works just like the elevator to some extent and it policy restricts

scanning to one direction only. Thus, when the last track has been visited in one direction, it

begins its scan toward the nearest end and works it way all the way to the end of the system.

Once it hits the bottom or top it jumps to the other end and moves in the same direction. Keep in

mind that the huge jump doesn't count as a head movement. C-SCAN is a fast one way with fast

return but it is lower service variability.


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This reduces the maximum delay experienced by new requests.

With C-SCAN, the interval is on the order of t + smax, where smax is the maximum seek time.

As the figure (d) for C-SCAN behavior, assume it starting point at 100, the first three requested track

encountered are 150, 160, and 184. Then it will scan from begins starting at the lowest track number

again to the higher track number.

N-STEP-SCAN(CREATED BY NOOR ALIA)

Segments the disk request queue into subqueues of length N.

Subqueue are processed one at a time, using SCAN.

While a queue is being processed, new requests must be added to some other queue.

If fewer then N requests are available at the end of a scan, then all of them are processed with the

next scan.

With large values of N, the performances of N-step-SCAN approaches that of SCAN; with a value of

N = 1, the FIFO policy is adopted.

Example:

Another way to look at N-step SCAN is this: A buffer for N requests is kept.

All the requests in this buffer are serviced in any particular sweep.

All the incoming requests in this period are not added to this buffer but are kept up in a

separate buffer. When these top N requests are serviced.

The IO scheduler chooses the next N requests and this process continues.

This allows for better throughput and avoids starvation.


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FSCAN

It uses two subqueues.

During the scan, all of the requests are in the first queue and all new requests are put into the

second queue.

Thus, service of new requests is deferred until all of the old requests have been processed.

When the scan ends, the arm is taken to the first queue entries and is started all over again.

The conclusion is FSCAN along with N-Step-SCAN prevents "arm stickiness" unlike SSTF,

SCAN, and C-SCAN.

Arm stickiness in those other algorithms occurs when a stream of requests for the same track

causes the disk arm to stop progressing at that track, preferring to satisfy the no-seek requests for

the track it is on.

Because FSCAN separates requests into two queues, with new requests going into a waiting

queue, the arm continues its sweep to the outer track and is therefore not "sticky."

There is an obvious trade-off in that the requests in the waiting queue must wait longer to be

fulfilled, but in exchange FSCAN is more fair to all requests.
http://en.wikipedia.org/wiki/N-Step-SCANhttp://en.wikipedia.org/wiki/Shortest_seek_firsthttp://en.wikipedia.org/wiki/Elevator_algorithmhttp://en.wikipedia.org/wiki/C-SCANhttp://en.wikipedia.org/wiki/C-SCANhttp://en.wikipedia.org/wiki/Elevator_algorithmhttp://en.wikipedia.org/wiki/Shortest_seek_firsthttp://en.wikipedia.org/wiki/N-Step-SCAN

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