GATE CSE 2016 Set 1 | Question: 48

Question

Cylinder a disk queue with requests for $I/O$ to blocks on cylinders $47, 38, 121, 191, 87, 11, 92, 10.$ The C-LOOK scheduling algorithm is used. The head is initially at cylinder number $63$, moving towards larger cylinder numbers on its servicing pass. The cylinders are numbered from $0$ to $199$. The total head movement (in number of cylinders) incurred while servicing these requests is__________.

Comments

Its mentioned in the question that we have to move towards larger value

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Total number of head movements $= (191-63)+(191-10)+(47-10)=128+181+37=346$

Answer 1

$63 \rightarrow 191 = 128$
$191 \rightarrow 10 = 181$
$10 \rightarrow 47 = 37$
Total $= 346$

Comments

@Sourajit25 @Hirak
"The head is initially at cylinder number 63, moving towards larger cylinder numbers on its servicing pass. "

hope that helps

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In CSCAN head traverses the disk and till the final end of the disk and without reversing back(R/W) (just like in SCAN)the head goes to the beginning of the disk without servicing requests in the return round.Thats how it works.

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12.2 (e) C-SCAN answer should be 9985.(And not given 9813)

143 → 4999 = 4856

4999 → 0 = 4999

0 → 130 = 130

so, 4858 + 4999 + 130 = 9985.

Anyone please check and confirm.

Answer 2

Answer is 346 as already calculated in answers here. Those having some doubt regarding long jump can check this image.

In the question Total Head Movements are asked. When Head reaches any End, there is no mechanism for head to jump directly to some arbitrary track. It has to Move. So it has to move along the tracks to reach Track Request on other side. Therefore head will move and we must count it.

Since the purpose of disk scheduling algorithms is to reduce such Head movements by finding an Optimal algorithm. If we ignore the move which is actually happening in disk, that doesn't serve the purpose of analyzing the algorithms.

Comments

much needed.. support..

Answer 3

Answer is 346.

63 -> 87 = 24

87 -> 92 = 5

92 -> 121 = 29

121 -> 191 = 70

191 -> 10 = 181

10 -> 11 = 1

11 -> 38 = 27

38 -> 47 = 9

Total: 24+5+29+70+181+1+27+9 = 346

Comments

Why? that is not considered?? and i think you have implemented LOOK and not CLOOK..

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@abhilash

Sorry. I was wrong. I had modified my answer with CLOOK algorithm.

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191->199

199->0

why these two are eliminated?

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Because this is c-Look not C-scan

Answer 4

We can use this method