The GATE Grind

GATE 2020 CS – Question 45

Operating System · File Systems and Disk Performance · 2 marks · Multiple choice

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?

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

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Show answer and explanation

Correct answer: (B) Q is serviced after S, but before T.

Explanation

From 100, SSTF gives the order R(110), T(115), P(155), Q(85), S(30). Q is serviced before S, so the statement 'Q after S but before T' is false.