Page Replacement
Overview
1. Why we need page replacement?
2. Basic page replacement technique.
3. Different type of page replacement
algorithm and their examples.
Why????
• Limited physical memory --> limited
number of frame --> limited number of
frame allocated to a process.
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.
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.
Replacement Algorithm
1. FIFO page replacement
2. Optimal page replacement
3. LRU page replacement
4. LRU-Approximation page replacement
5. Counting-Based page replacement
example
String
Reference
7
0
1
2
3
0
4
2
3
0
3
2
1
2
0
1
7
0
1
Young page
Frames initially
empty
7
0
7
7
0
0
1
2
3
0
4
2
2
2
3
0
0
0
7
7
7
0
1
1
1
2
3
0
4
3
3
3
3
0
1
1
1
7
7
7
0
2
2
3
0
4
2
2
0
0
0
1
2
2
2
7
7
7
*
*
*
*
*
*
*
*
*
*
*
*
Old Page
Page faults
FIFO Page Replacement
• FIFO is similar to First In First Out CPU scheduling algorithm
wherein the requested page is loaded first and subsequent fetches
are also serviced in order
of request arrival.
• 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.
FIFO
7
0
1
2
0
3
0
4
2
3
0
3
2
1
2
0
1
7
0
1
7
7
7
0
0
1
2
3
0
4
2
2
2
3
0
0
0
7
7
7
0
1
1
1
2
3
0
4
3
3
3
3
0
1
1
1
7
7
7
0
2
2
3
0
4
2
2
0
0
0
1
2
2
2
7
7
7
*
*
*
*
*
*
*
*
*
*
*
*
pf/n
pf = page faults
N = number of reference made
12/20 = 60%
Optimal Page Replacement
(OPT)
• The main idea of OPT is to replace the pages found n the frames that
re not going to be used/ referenced soon.
• An optimal page-replacement algorithm has the lowest page-fault
rate of all algorithms (called OPT or MIN). It is simply this: Replace
the page that will not be used for the longest period of time.
- At the moment of page fault:
• Label each page in memory is labeled with the number of
instructions that will be executed before that page is first referenced
• Replace the page with the highest number: i.e. postpone as much as
possible the next page fault
OPT
7
0
1
2
0
3
0
4
2
3
0
3
2
1
2
0
1
7
0
1
7
7
7
0
0
0
0
4
4
4
0
0
0
2
2
0
0
0
0
0
0
1
1
1
2
3
2
2
2
2
2
2
0
0
1
1
1
1
1
0
2
2
3
0
3
3
3
3
3
3
1
1
2
2
7
7
7
*
pf/n
pf = page faults
N = number of reference made
6/20 = 30%
*
*
*
*
*
Least Recently Used Page
Replacement Algorithm
• This algorithm reposes on the opposite of the
concept of temporal locality, wherein if a page
has not been referenced for quite some time then
it is not going to be referenced in the future.
Least Recently Used Page
Replacement Algorithm
7
0
1
2
0
3
0
4
2
3
0
3
2
1
2
0
1
7
0
1
7
7
7
0
1
2
2
3
2
4
2
2
0
3
3
1
2
0
1
7
0
0
1
2
0
3
0
4
2
3
3
3
2
1
2
0
7
7
0
1
2
0
3
0
4
2
3
0
0
2
1
2
0
1
1
0
1
*
*
*
*
*
•
Pf/N
9 / 20 = 45%
*
*
*
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