GATE CS 2014 Set 1 — Question 29
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Computer Organization & Architecture → I/O Organization → Disk Scheduling Algorithms
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Question
Suppose a disk has 201 cylinders, numbered from 0 to 200. At some time the disk arm is at cylinder 100, and there is a queue of disk access requests for cylinders 30, 85, 90, 100, 105, 110, 135 and 145. If Shortest-Seek Time First (SSTF) is being used for scheduling the disk access, the request for cylinder 90 is serviced after servicing ____________ number of requests.
Correct answer
3 to 3
Solution
The current position of the disk arm is at cylinder 100.
The queue of requests is: .Using SSTF (Shortest Seek Time First), we select the request with the minimum seek time (distance) from the current head position.
Number of requests serviced before 90 is 3 (100, 105, 110).
The queue of requests is: .Using SSTF (Shortest Seek Time First), we select the request with the minimum seek time (distance) from the current head position.
1.Current Head: 100
- Distances: , , , etc.
- Closest is 100. Service 100.
- Serviced so far: 1 (Request 100)
- Remaining Queue:
- Distances: , , , , etc.
- Closest is 105. Service 105.
- Serviced so far: 2 (Requests 100, 105)
- Remaining Queue:
- Distances: , , , , etc.
- Closest is 110. Service 110.
- Serviced so far: 3 (Requests 100, 105, 110)
- Remaining Queue:
- Distances: , , , , .
- Closest is 90. Service 90.
Number of requests serviced before 90 is 3 (100, 105, 110).
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