The disk scheduling algorithms are: First Come-First Serve (FCFS) Shortest Seek
ID: 3770822 • Letter: T
Question
The disk scheduling algorithms are: First Come-First Serve (FCFS)Shortest Seek Time First (SSTF)
Elevator (SCAN)
Circular SCAN (C-SCAN)
LOOK
C-LOOK
Talke about THREE of them has if have a fairness ? Throughput? Starvation? And WHY? The disk scheduling algorithms are: First Come-First Serve (FCFS)
Shortest Seek Time First (SSTF)
Elevator (SCAN)
Circular SCAN (C-SCAN)
LOOK
C-LOOK
Talke about THREE of them has if have a fairness ? Throughput? Starvation? And WHY? First Come-First Serve (FCFS)
Shortest Seek Time First (SSTF)
Elevator (SCAN)
Circular SCAN (C-SCAN)
LOOK
C-LOOK
Talke about THREE of them has if have a fairness ? Throughput? Starvation? And WHY?
Explanation / Answer
FCFS
1. FCFS stands for First Come First Serve. It means first element serviced first.
2. In FCFS, incoming requests are placed at the end of the queue. Whatever number that is next in the queue will be the next number served.
3.Using this algorithm doesn't provide the best results. To determine the number of head movements you would simply find the number of tracks it took to move from one request to the next.
4. For this case it went from 30 to 55 to 100 and so on. From 30 to 55 it moved 25 tracks. If you tally up the total number of tracks you will find how many tracks it had to go through before finishing the entire request. In this example, it had a total head movement of 640 tracks. The disadvantage of this algorithm is noted by the oscillation from track 30 to track 100. and then back to track 11 to 123, this is the worse algorithm that one can use. Its throughput is less.
SSTF
1. SSTF stands for Shortest Seek Time First.
2.In this case request is serviced according to next shortest distance.
3. For example here we have three elements 30, 40 and 60. It is starting at 40, the next shortest distance would be 30 instead of 60 since it is only 10 tracks away from 40 and 20 tracks away from 60.
4. The process would continue until all the process are taken care of.
5.Although this seems to be a better service being that it moved a total of 236 tracks, this is not an optimal one. There is a great chance that starvation would take place. The reason for this is if there were a lot of requests close to eachother the other requests will never be handled since the distance will always be greater.
Elevator (SCAN)
1. This approach works like an elevator operation.
2. It scans down towards the nearest end and then when it hits the bottom it scans up servicing the requests that it didn't get going down.
3. If a request comes in after it has been scanned it will not be serviced until the process comes back down or moves back up. It has less throughput. It is also worst algorithm approach.
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