Tutorial 9
Page Replacement Algorithms
In the following examples
• A frame with a value of 0 is empty. A frame
shown with an “=” sign holds the same value
as it held in the previous time period
First in first out (FIFO)
• The reference string of pages is 123472512645
• When a page must be replaced, the oldest page is chosen
1
2
3
4
7
2
5
1
2
6
4
5
1
=
=
=
7
=
=
=
=
6
=
=
0
2
=
=
=
=
5
=
=
=
4
=
0
=
3
=
=
=
=
1
=
=
=
5
0
=
=
4
=
=
=
=
2
=
=
=
– page faults: 11
Optimal
• The reference string of pages is 123472512645
• Replace the page that will not be used for the longest period of time
1
2
3
4
7
2
5
1
2
6
4
5
1
=
=
=
=
=
=
=
=
6
=
=
0
2
=
=
=
=
=
=
=
=
=
=
0
=
3
=
7
=
5
=
=
=
=
=
0
=
=
4
=
=
=
=
=
=
=
=
– page faults: 7
Least recently used (LRU)
•
•
•
The reference string of pages is 123472512645
When a page must be replaced , LRU choses the page that has not been used
for the largest period of time
1
2
3
4
7
2
5
1
2
6
4
5
1
=
=
=
7
=
=
=
=
6
=
=
0
2
=
=
=
=
=
=
=
=
=
=
0
=
3
=
=
=
5
=
=
=
4
=
0
=
=
4
=
=
=
1
=
=
=
5
page faults: 10
Second chance
When a page is used its use bit is set to 1. We also
use a pointer to the next victim which is initialized
to the first page/frame. When a page is loaded, it is
set to point to the next frame. The list of pages is
considered as a circular queue. When a page is
considered for replacement, its use bit is examined.
If it is zero [that page is replaced] otherwise [the
use bit is set to zero, the next victim pointer is
advanced, and the process repeated until a page is
found with a zero use bit].
Second chance
1 Bit 2 Bit 3 Bit
1 1 1 1 1 1
2 1 2 1
3 1
4
1
2
3
4
Bit
1
1
1
1
– 9 page faults.
7
7
2
3
4
Bit
1
0
0
0
2
7
2
3
4
Bit
1
1
0
0
5
7
2
5
4
Bit
1
0
1
0
1
7
2
5
1
Bit
1
0
1
1
2
7
2
5
1
Bit
1
1
1
1
6
6
2
5
1
Bit
1
0
0
0
4
6
4
5
1
Bit
1
1
0
0
5
6
4
5
1
Bit
1
1
1
0