C-SCAN and C-LOOk

Question

No. of seek distances with C-SCAN and C-LOOK?

Comments

c scan -4986(dont count the long jump)

C look=3363

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@Red_devil

Why not count the long jump? ans should be 9985 for C-Scan

Answer 1

Look at the difference between Scan, C-Scan, and C-Look. Snapshots are taken from Galvin.

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Note: In C-Scan (in the graph) while coming back we don't process any request, we first come to the start, Then move back and start processing the requests.

Back to question.
Currently, we are in cylinder 143 and moving forward.

In C-Scan, 143 - 913 - 948 - 1022 - 1470 - 1509 - 1750 - 1774 - 4999   $\Rightarrow$ (4999 - 143) = 4856 moves

4999 - 0 $\Rightarrow$ 4999 moves

0 - 86 - 130. $\Rightarrow$ 130 moves

Total moves = 4856 + 4999 + 130 = 9985.

In C-Look, we don't' move to 4999 instead we move to the last request on the right side. Then we come back without servicing any request to the first request.Then we move right side to process request.

C-Look, 143 - 913 - 948 - 1022 - 1470 - 1509 - 1750 - 1774 $\Rightarrow$ (1774 - 143) = 1631 moves

1774 - 86 $\Rightarrow$ 1688 moves.

86 - 130. $\Rightarrow$ 44 moves.

Total = 3363 moves.

Comments

I did like in C-SCAN, (4999-143)+(4999-0)+(130-0)= 9985, why you have taken 4999 to 0 as 5000 moves ??

In C-LOOK, (1774-143)+(130-86)+(1774-86)= 3363.

Apart from this what they have given the solution in Galvin 

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I corrected that 5000 moves to 4999.

While calculating for C Look you take 1744. It is 1774.

For C-SCAN look at the image after 4999 they go to 0 then they go to 86. But in the screenshot you gave, they didn't go to 0. After 4999 they directly go to 86. Why this? I don't know.

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C-LOOK is fine. 

In C-SCAN I have the same doubt, because it is written in book "when it reaches other end of cylinder it immediately return at the beginning of cylinder" but in solution they reaches at the first request like we did in C-LOOK

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@ Hemant Parihar In C-Scan, should'nt we ignore the disk movement while moving from 4999 to 0 as cylinders are treated as a circular list ??