1. Storage Management
Page Replacement – LRU, Optimal
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3. Recap
In the last class, you have learnt
• Concepts of page replacement
• Page replacement algorithm
• FIFO
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4. Objective
On completion of this class, you would be able
to know
• LRU algorithm
• Optimal algorithm
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5. Least Recently Used (LRU) Algorithm
• LRU algorithm chooses that page that has not
been used for the longest period of time
• LRU algorithm looks at the pages which are
already page faulted
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6. LRU Page Replacement
The LRU algorithm produces 12 page faults for
the reference string
7,0,1,2,0,3,0,4,2,3,0,3,2,1,2,0,1,7,0,1
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7. Least Recently Used (LRU) Algorithm
Advantages
 LRU policy is often used as a page replacement
algorithm
 It is quite good algorithm
 It is easy to choose that has already page faulted
and not in use for long period
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8. Least Recently Used (LRU) Algorithm
Disadvantages
 Problem is how to implement LRU replacement
 Require substantial hardware assistance
 Problem in determining the frame order by the
time of last use
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10. Optimal Algorithm
 Replace page that will not be used for longest
period of time
 4 frames example
 Reference string : 1, 2, 3, 4, 1, 2, 5, 1, 2, 3, 4, 5
1 4
2 6 page faults
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4 5
How do you know this?
 Used for measuring how well your algorithm
performs 10

11. Optimal Page Replacement
Reference string : 7 0 1 2 0 3 0 4 2 3 0 3 2 1 2 0
1701
The above optimal replacement algorithm
causes only 9 page faults
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12. Optimal Example
12 references, 7
faults
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14. Optimal Page Replacement
Advantages
 Has the lowest page fault rate
 It never suffers from Belady’s anomaly
 Used for comparison studies
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15. Optimal Page Replacement
Disadvantages
 Difficult to implement
 It requires future reference string
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16. Summary
In this class, you have learnt
• LRU page replacement algorithm
- advantages and disadvantages
• Optimal page replacement algorithm
- advantages and disadvantages
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17. Frequently Asked Questions
1. What is meant by Page replacement?
2.Explain the LRU algorithms
3. What are the advantages and disadvantages of LRU?
4. Explain the concept of optimal Page replacement
algorithm
5. Write the advantages and disadvantages of Optimal page
replacement algorithm
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18. Quiz
1. LRU algorithm chooses that page that has not been
used for the ________period of time
a) Shortest
b) Longest
c) Widest
d) None
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19. Quiz
2. __________has the lowest page fault rate
a) FIFO
b) LRU
c) Optimal
d) None
3. ___________ requires future reference string
a) FIFO
b) LRU
c) Optimal
d) None
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20. Quiz
4. __________never suffers from Belady’s anomaly
a) Optimal replacement
b) LRU
c) FIFO
d) None
5. Algorithm Replace page that will not be used for longest
period of time
a) LRU
b) Optimal replacement
c) FIFO
d) None
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