1. LOOK and C-LOOK disk scheduling
algorithms
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2. • HOME PREVIOUS TOPIC NEXT
• PREVIOUS QUESTION PAPERS FOR
OS
• CPP TUTORIALS
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3. Recap
In the last class, you have learnt:
The SSTF, SCAN & C-SCAN Scheduling their
Advantages
Disadvantages
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4. Objectives
On Completion of this period, you would be able to
know :
• The LOOK & C-LOOK Scheduling
• Advantages
• Disadvantages
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5. LOOK Scheduling
• SCAN and C-SCAN algorithms move the disk arm
across the full width of the disk
• In practice neither of the algorithm is implemented
this way
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6. LOOK Scheduling
• In LOOK Scheduling the arm goes only as far as final
request in each direction
• Then it reverses direction immediately without going
all the way to the end of the disk
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7. LOOK 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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8. LOOK Scheduling
• If the disk head is initially at cylinder 53,
– it will first move from 53 to 65, (towards right)
– then to 67, 98, 122, 124, 183, 37 and 14 as
shown in the next slides
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9. LOOK Scheduling
Initial Head position
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10. LOOK Scheduling
Head movement from 53 to 65
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11. LOOK Scheduling
Head movement from 65 to 67th track
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12. LOOK Scheduling
Head movement from 67 to 98th track
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13. LOOK Scheduling
Head movement from 98 to 122 track
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14. LOOK Scheduling
Head movement from 122 to 124th track
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15. LOOK Scheduling
Head movement from 124 to 183rd track
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16. LOOK Scheduling
Head movement from 183 to 37th track
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17. LOOK Scheduling
Head movement from 37th track to 14th track
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18. LOOK Scheduling
• Advantages
– Better than SCAN algorithm in terms of head movement
• Disadvantages
– Doesn't provide uniform waiting time
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19. Summary
In this class, you have learnt:
• The LOOK and C-LOOK Scheduling
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20. Frequently Asked Questions
1. List the advantages and disadvantages of LOOK
scheduling
2. With the help of an example, explain LOOK
scheduling
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21. Frequently Asked Questions
3. List C-LOOK advantages
4. Explain C-LOOK scheduling with the help of an
example
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22. Quiz
1. SCAN and C-SCAN algorithms move the disk arm
across the full width of the disk [T/F]
TRUE
2. LOOK Scheduling is better than SCAN algorithm in
terms of head movement [T/F]
TRUE
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23. Quiz
3. C-LOOK algorithm is a version of C-SCAN [T/F]
TRUE
4. C-LOOK algorithm better than the C-SCAN
algorithm in terms of head movement [T/F]
TRUE
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24. Other subject materials
• Web designing
• Micro processors
• C++ tutorials
• java
home
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