The document discusses the First Come First Serve (FCFS) disk scheduling algorithm. FCFS is the simplest disk scheduling algorithm as requests are served in the order they arrive. It is easy to program and intrinsically fair, but does not provide optimal disk head movement as requests are not ordered for proximity. The document provides an example of the FCFS algorithm applied to a disk request queue, showing the path the disk head takes to fulfill the requests and the total head movement distance.

1. FCFS Scheduling
• The First Come First Serve Scheduling
Advantages
– Simplest form of disk scheduling algorithm
– Easy to program
– Intrinsically fair
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2. FCFS Scheduling
• Consider for example an ordered disk queue with
requests involving
– Tracks 98, 183, 37, 122, 14, 124, 65, 67
– “Head pointer” 53
• The request queue (0-199)
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3. FCFS Scheduling
• If the disk head is initially at cylinder 53,
– it will first move from 53 to 98,
– then to 183, 37, 122, 14, 124, 65 and 67 as shown in the
next slides
• The total head movement of 640 tracks
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4. FCFS Scheduling
Initial Position
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5. FCFS Scheduling
Head is at 98th track
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6. FCFS Scheduling
Head is at 183rd track
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7. FCFS Scheduling
Head movement from 183 to 37th track
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8. FCFS Scheduling
Head movement from 37 to 122
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9. FCFS Scheduling
Head movement from 122 to 14th track
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10. FCFS Scheduling
Head movement from 14th to 124th track
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11. FCFS Scheduling
Head movement from 124 to 65
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12. FCFS Scheduling
Head movement from 65 to 67th track
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14. FCFS Scheduling
Disadvantages:
• It does not provide best service
• Wild swing from 122 to 14 and then back to 124
• If the requests for tracks 37 and 124 could be serviced
together
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15. FCFS Scheduling
• Before or after the requests at 122 and 124
• The average head movement would decrease total
head movement &
• Thus Improving disk throughput
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16. Summary
In this class , you have learnt
Disk scheduling-Need
FCFS disk scheduling algorithm
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17. Frequently asked questions
1. Define seek time and latency time
2.Explain the need for disk scheduling
3.List different disk scheduling algorithms
4.List the advantages and disadvantages of FCFS disk
scheduling algorithm
5.List the advantages and disadvantages of FCFS
algorithm
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