GATE CS 2020 Set 1 — Question 45
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Computer Organization & Architecture → I/O Organization → Disk Scheduling Algorithms
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Question
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?
Correct answer
(B) Q is serviced after S, but before T.
Solution
Using Shortest Seek Time First (SSTF), we select the request closest to the current head position.Initial Head Position: 100
Requests: P(155), Q(85), R(110), S(30), T(115)
(A) T is serviced before P. (True: T is 2nd, P is 5th)
(B) Q is serviced after S, but before T. (False: Q is 3rd, S is 4th, T is 2nd. Q is serviced before S and after T.)
(C) The head reverses direction between Q and P. (True: Q is at 85 [moving down from 115], then S at 30 [down], then P at 155 [up]. Reversal happens at 30, which is between Q and P.)
(D) R is serviced before P. (True: R is 1st, P is 5th)The FALSE statement is (B).
Requests: P(155), Q(85), R(110), S(30), T(115)
1.Current: 100
- Distances: P(55), Q(15), R(10), S(70), T(15)
- Closest: R(110) (Distance 10)
- Move to 110. Serviced: R.
- Remaining: P(155), Q(85), S(30), T(115)
- Distances: P(45), Q(25), S(80), T(5)
- Closest: T(115) (Distance 5)
- Move to 115. Serviced: T.
- Remaining: P(155), Q(85), S(30)
- Distances: P(40), Q(30), S(85)
- Closest: Q(85) (Distance 30)
- Move to 85. Serviced: Q.
- Remaining: P(155), S(30)
- Distances: P(70), S(55)
- Closest: S(30) (Distance 55)
- Move to 30. Serviced: S.
- Remaining: P(155)
- Closest: P(155) (Distance 125)
- Move to 155. Serviced: P.
(A) T is serviced before P. (True: T is 2nd, P is 5th)
(B) Q is serviced after S, but before T. (False: Q is 3rd, S is 4th, T is 2nd. Q is serviced before S and after T.)
(C) The head reverses direction between Q and P. (True: Q is at 85 [moving down from 115], then S at 30 [down], then P at 155 [up]. Reversal happens at 30, which is between Q and P.)
(D) R is serviced before P. (True: R is 1st, P is 5th)The FALSE statement is (B).
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