GATE CSE 1999 | Question: 1.10

Question

Which of the following disk scheduling strategies is likely to give the best throughput?

• A. Farthest cylinder next

• B. Nearest cylinder next

• C. First come first served

• D. Elevator algorithm

Comments

what is Farthest cylinder next algorithm

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Option (B) and (D) both are acceptable. For more details please check comment of @Vicky rix ji.

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As throughput is asked can it be interpreted as at any point of time during disk scheduling which Algo gives max request processing per second. Which implies minimum head movements.

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@tusharp It should be minimum head movements not maximum as overall seek latency is minimum when head movements are minimum thus maximum throughput.

Answer 1

• A. Farthest cylinder next $\rightarrow$ This might be candidate for worst algorithm . This is false.
• B. Nearest cylinder next $\rightarrow$ This is output.
• C. First come first served $\rightarrow$ This will not give best throughput. It is random .
• D. Elevator algorithm $\rightarrow$ This is good but issue is that once direction is fixed we don't come back, until we go all the other way. So it does not give best throughput.

Correct Answer: $B$

Comments

I am confused between nearest cylinder next or elevator?? Because both gives good throughput. what should be the answer?? and why?? why it shouldnt be elevator since it doesnt even have starvation problem??

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How to decide between nearest cylinder and elevator algo.

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Option B is like SSTF ...

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Hi @sushmita, vaishali jhalani and Puja Mishra ji,

First of all thanks for raising good point. But could you please provide some counter example where Elevator algorithm will perform better then Nearest cylinder next ? 

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In nearest cylinder next, we cant fix the direction in which go next

but in Elevator , first we fix the diection then go only that direction and then come back to opposite direction

like if we have 10,50 request and head is on 20

then nearest cylinder next , 20->10->50   total seek time 10+40=50

elevator 20->50->10  total seek time=30+40=70 ( direction is fix toward 50)

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As long as the block request string is given, we cannot decide between B) and D)

B) is not always the best EX:

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throughput put depends on the number of requests served per second  as we always go for the shortest reach SSTF Does this best as it tries to serve the maximum number of processes in less time 

@https://gateoverflow.in/user/sushmita

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Thank you @Vicky rix ji.

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Scan is called elevator algorithm ...

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For more reference go to this question.

https://gateoverflow.in/8227/gate2015-1-30

How SCAN Algo hear move one side head and then return.

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It should be Elevator algorithm since it provides more uniform waiting time than the SSTF or nearest cylinder next algorithm.

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Both B AND D are correct. 

D is also correct(see @Vicky rix comment ) and as it's a GATE 1999 question multiple options are possible as answers. 

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I dont think there can be 2 correct answers for this as it is asking for the BEST, how can two strategies be best simultaneously. What to write if it appears in the exam now…. Other sites like GFG have also given B

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Abhineet Singh see question is asking- likely to give the best throughput hence Nearest cylinder next is better option for exam

Answer 2

Option B

Nearest cylinder next but it can lead to starvation

Answer 3

Nearest cylinder next . in this many pages can be accessed in less time  . So throughput will be high .

Answer 4

The only catch in the question is that they are asking for the most likely to give the best throughput.

Even if for some cases SCAN can have better results but the most likely to result the best throughput is Nearest cylinder Algo ;)