Pagereplacement algorithm(computional concept)

2. Overview
1. Why we need page replacement?
2. Basic page replacement technique.
3. Different type of page replacement algorithm and
their examples.

3. Why????
Limited physical memory --> limited number of
frame --> limited number of frame allocated to a
process.

4. Basic Page Replacement
1. Find the location of the desired page on the
disk.
2. Find a free frame
 If there is a free frame use it.
 No free frame – use page replacement algorithm to
select a victim frame.
 Write the victim frame to disk, change the frame and
page tables accordingly.

5. Basic Page Replacement (Contd)
3. Read the desired page into the newly freed frame,
change the page and frame tables.
4. Restart the user process.

6. Page Replacement
Victim
Swap desire
page in
Reset page
table for new
page
Change to
invaid
Page table
Frame
Valid
invalid bit
2
4
3
0
F
I
V
Swap out
victim page
1
Physical memory

7. Overhead
If no free frames - 2 page transfers.
Solution : Modify bit or Dirty bit.

8. Replacement Policy
Which page to be replaced?
Page removed should be the page least likely to be
referenced in the near future.
Most policies predict the future behavior on the basis
of past behavior.

9. Page faults versus number of frames
Number of frames
Number of page
faults

10. Replacement Algorithm
1. FIFO page replacement
2. Optimal page replacement
3. LRU page replacement

11. FIFO Page Replacement
Easy to understand and program.
Performance is not always good.
Belady’s Anomaly

12. Drawbacks - FIFO
A page which is being accessed quite often may also
get replaced because it arrived earlier than those
present
Ignores locality of reference. A page which was
referenced last may also get replaced, although there
is high probability that the same page may be needed
again.

13. FIFO – An Example
7 0 1 2 0 3 0 4 2 3 0 3 2 1 2 0 1 7 0 1
7
0
7 7
0
1
2
0
1
2
3
1
2
3
0
4
3
0
4
2
0
4
2
3
0
2
3
0
1
3
0
1
2
7
1
2
7
0
2
7
0
1

14. Optimal Page Replacement
Lowest page fault rate of all algorithms
Free from Belady’s anomaly

15. Optimal Page Replacement (contd)
“Replace the page that will not be used for the longest
period of time.”
Requires future knowledge of the reference string.
Used for comparison studies.

16. OPR – An Example
7 0 1 2 0 3 0 4 2 3 0 3 2 1 2 0 1 7 0 1
7
0
7 7
0
1
2
0
1
2
0
3
2
4
3
2
0
3
7
0
1
2
0
1

17. LRU Page Replacement
Free from Belady’s Anomaly.
Problem: How to order the frame defined by the time
of last use.
Solution:
 Counters
 Queue

18. LRU – An Example
7 0 1 2 0 3 0 4 2 3 0 3 2 1 2 0 1 7 0 1
7
0
7 7
0
1
2
0
1
2
0
3
4
3
4
2
3
2
0
3
7
0
1
0
4
2
0
2
3
1
2
0
1

19. Ans: C
1. Dirty bit is used to show the
a. page with corrupted data
b. the wrong page in the memory
c. page that is modified after being loaded
into cache memory
d. page that is less frequently accessed.

20. Ans: C
2. What replacement policy is used by Windows NT
a. LRU
b. NFU
c. FIFO
d. Clock Replacement
e. None of the above

21. Reference String
4. The string of memory references is called
___________.

22. Belady’s Anomaly
5. FIFO page replacement suffers from
________________anomaly.

23. Page fault increases with increase in number of
frames.
6. What is Belady’s anomaly??

24. TRUE
7. Optimal Page Replacement algorithm has the
lowest page fault rate.
TRUE/FALSE