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ISRO CSE 2007 | Question: 39

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8 8 votes

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

  1. 125 msec
  2. 295 msec
  3. 575 msec
  4. 750 msec

3 Answers

Best answer
21 21 votes
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 = $59 \times 5 = 295 msec.$
• edited by
2 2 votes
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
2 2 votes

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

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