Fcfs scheduling
- 1. FCFS Scheduling • The First Come First Serve Scheduling Advantages – Simplest form of disk scheduling algorithm – Easy to program – Intrinsically fair http://raj-os.blogspot.in/ 1
- 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) http://raj-os.blogspot.in/ 2
- 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 http://raj-os.blogspot.in/ 3
- 4. FCFS Scheduling Initial Position http://raj-os.blogspot.in/ 4
- 5. FCFS Scheduling Head is at 98th track http://raj-os.blogspot.in/ 5
- 6. FCFS Scheduling Head is at 183rd track http://raj-os.blogspot.in/ 6
- 7. FCFS Scheduling Head movement from 183 to 37th track http://raj-os.blogspot.in/ 7
- 8. FCFS Scheduling Head movement from 37 to 122 http://raj-os.blogspot.in/ 8
- 9. FCFS Scheduling Head movement from 122 to 14th track http://raj-os.blogspot.in/ 9
- 10. FCFS Scheduling Head movement from 14th to 124th track http://raj-os.blogspot.in/ 10
- 11. FCFS Scheduling Head movement from 124 to 65 http://raj-os.blogspot.in/ 11
- 12. FCFS Scheduling Head movement from 65 to 67th track http://raj-os.blogspot.in/ 12
- 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 http://raj-os.blogspot.in/ 14
- 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 http://raj-os.blogspot.in/ 15
- 16. Summary In this class , you have learnt Disk scheduling-Need FCFS disk scheduling algorithm http://raj-os.blogspot.in/ 16
- 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 http://raj-os.blogspot.in/ 17