GATE CSE 2018 | Question: 53

Question

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 _____

Comments

Why are we not converting sector address to linear address and then applying SSTF ? 

Although for some cases power dissipation is negligible but it does not mean time will also be negligible.

@Hemant Parihar, @Soumya29, @Kushagra Chatterjee, @ankitgupta.1729, @Subarna Das, @VS, @sonam vyas, @surajumang08, @Sambit Kumar, @srestha, @abhishekmehta4u What is your opinion ?

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@ Chhotu 

good point

Actually in disk movement we only consider cylinder number(sector number not needed there)

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The average power dissipation in moving the head over 100 cylinders is 20 milliwatts 

most important line most of us think this line as after crossing cylinder number which are above 100 have power dissipation 20 milliwatts

but this means when move more than 100 cylinder then average power dissipation 20 milliwatts so counted for total head movement

Answer 1

Shortest Seek Time First (SSTF), selects the request with minimum to seek time first from the current head position.

In the given question disk requests are given in the form of $\langle \text{sectorNo, cylinderNo, platterNo}\rangle$. 

Cylinder Queue $: 72, 134, 20, 86, 116, 16$

Head starts at $:80$

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

• Power dissipated in moving $100$ cylinder $= 20\;\text{mW}$
• Power dissipated by $200$ movements $($say $P1) = 0.2\ast 200= 40\;\text{mW}$
• Power dissipated in reversing head direction once $= 15\;\text{mW}$
• Number of times head changes its direction $= 3$
• Power dissipated in reversing head direction $($say $P2)= 3\ast 15= 45\;\text{mW}$

Total Power Consumption is $P1+P2 = 85\;\text{mW}$

Hence, $85\;\text{mW}$ is the correct answer.

Comments

One at 86 and one at 72 and one at 134 so total 3 movement

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That was the easiest question with hard looking language.

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seems very hard but you made it very easy.

Answer 2

I have written the answer in best way possible in this image

As we reverse the direction of the head 3 times so 15mW power will be used thrice i.e. 45mW

20mW per 100 head movements i.e. 0.2 mW or 200kW for 1 head movement.

I hope the answer