Consider a disk queue with requests for I/O to blocks on cylinders 98, 183,…
2017
Consider a disk queue with requests for I/O to blocks on cylinders 98, 183, 37, 122, 14, 124, 65, 67. Suppose SSTF disk scheduling algorithm implemented to meet the requests then the total number of head movements are __________ if the disk head is initially at 53.
- A.
224
- B.
248
- C.
236
- D.
240
Attempted by 109 students.
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Correct answer: C
Solution (SSTF computation):
Start at head position 53. At each step pick the request with the smallest distance from the current head position.
Move from 53 to 65: distance = 12, cumulative = 12.
Move from 65 to 67: distance = 2, cumulative = 14.
Move from 67 to 37: distance = 30, cumulative = 44.
Move from 37 to 14: distance = 23, cumulative = 67.
Move from 14 to 98: distance = 84, cumulative = 151.
Move from 98 to 122: distance = 24, cumulative = 175.
Move from 122 to 124: distance = 2, cumulative = 177.
Move from 124 to 183: distance = 59, cumulative = 236.
Therefore, the total head movement using SSTF starting at 53 is 236.
Service order produced by SSTF: 65, 67, 37, 14, 98, 122, 124, 183.
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