ISRO2007-39

Question

Disk requests are received by a disk drive for cylinder 5, 25, 18, 3, 39, 8 and 35 in that order. A seek takes 5 msec per cylinder moved. How much seek time is needed to serve these requests for a Shortest Seek First (SSF) algorithm? Assume that the arm is at cylinder 20 when the last of these requests is made with none of the requests yet served

• A. 125 msec
• B. 295 msec
• C. 575 msec
• D. 750 msec

Comments

got it thanks and yes it would be 195ms

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? should be replaced by 20.

https://courses.cs.washington.edu/courses/cse451/03sp/sections/DiskScheduling.pdf

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please provide right ans... thanku

Answer 1

Initially cylinder is at 20. So, as per shortest seek time, the requests will be serviced in the order: 18, 25, 35, 39, 8, 5, 3. So, total no. of seeks

$= (20-18)+ (25-18) + (35-25) + (39-35) + (39-8) + (8-5) + (5-3)$

$= 2+ 7 + 10 + 4 + 31 + 3 + 2 = 59$.

A seek takes 5 msec. So, total seek time = $59 \times 5 = 295 msec.$

Comments

Sir u haven't considered 35..can u please explain why ?

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That was a mistake - corrected now.

Answer 2

if at starting at o then order will be 0-3-5-8-18-25-35-39(total movements =3+2+3+10+7+10+4=39

total time 39x5=195 msec

Comments

But there is no answer as 195 in the options

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some misprint in question

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Question says cylinder is at 20. If in question, it was mentioned that cylinder is at 0 , then the answer would be 195.

But the answer is 295.

Answer 3

Solution with minimum calculation:

With shortest seek time, the requests will be in order: 18,25,35,39, 8, 5,3

• First Arm will move backward, = 20-18 =2
• Then forward = 39-18 =21
• Then backward = 39-3= 36

Total seek time =5*(2+21+36)= 295 ms