GATE | GATE CS 2018 | Question 36

Consider a storage disk with 4 platters (numbered as 0, 1, 2 and 3), 200 cylinders (numbered as 0, 1, … , 199), and 256 sectors per track (numbered as 0, 1, … 255). The following 6 disk requests of the form [sector number, cylinder number, platter number] are received by the disk controller at the same time:

[120, 72, 2], [180, 134, 1], [60, 20, 0], [212, 86, 3], [56, 116, 2], [118, 16, 1]

Currently head is positioned at sector number 100 of cylinder 80, and is moving towards higher cylinder numbers. The average power dissipation in moving the head over 100 cylinders is 20 milliwatts and for reversing the direction of the head movement once is 15 milliwatts. Power dissipation associated with rotational latency and switching of head between different platters is negligible.



The total power consumption in milliwatts to satisfy all of the above disk requests using the Shortest Seek Time First disk scheduling algorithm is ______ .

Note –This was Numerical Type question.
(A) 45
(B) 80
(C) 85
(D) None of these


Answer: (C)

Explanation: Head starts at 80.
gate_cs_2018_36(1)

Total Head movements in SSTF = (86-80) + (86-72) + (134-72) + (134-16) = 200

Power dissipated by 200 movements : P1 = 0.2 * 200 = 40 mW
Power dissipated in reversing head direction once = 15 mW
Number of time Head changes its direction = 3

Power dissipated in reversing head direction: P2 = 3 * 15 = 45 mW
Total power consumption (in mW) is P1 + P2 = 40 mW + 45 mW = 85 mW

So, answer is 85.


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