Consider the following five disk access requests of the form (request id, cylinder number) that are present in the disk scheduler queue at a given time. (P, 155), (Q, 85), (R, 110), (S, 30), (T, 115) Assume the head is positioned at cylinder 100. The scheduler follows Shortest Seek Time First scheduling to service the requests. Which one of the following statements is FALSE?

T is serviced before P.
Q is serviced after S, but before T.
The head reverses its direction of movement between servicing of Q and P.
R is serviced before P.

LohaHridoy

4 views

2025-01-04 04:42

1 Answers

AhmedNazir

Option 2 : Q is serviced after S, but before T.

Concept:

In the Shortest Seek Time First (SSTF) disk scheduling algorithm, the I/O request which requires the least disk arm movement, irrespective of the direction, from the current position is selected.

Explanation:

Given the disk head is currently at cylinder number 100. Using SSTF it would next serve the request id R with cylinder number 110, because it requires least amount of arm movement. After R being served, T would be served and so on, as shown below:

[ src="//storage.googleapis.com/tb-img/production/20/04/F1_Raju.s_24-04-2020_Savita_D1.png" style="width: 376px; height: 179px;">

Statement I is TRUE.

Request T is serviced before request P.

Statement II is FALSE.

Q is serviced before S and after T.

Statement III is TRUE.

The direction of disk arm movement changes between servicing of Q and P, as visible from the diagram.

Statement IV is TRUE.

R is serviced before P.

4 views Answered 2022-09-04 04:29

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