pratima page replacement
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Page Replacement AlgorithmsBYPRATIMA SHARMAASSISTANT PROFESSOR
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When ever a page is searched in main memory and it is not found there a page fault occurs, so to bring that page in memory we have to remove some other page(victim). Page replacement algorithms selects which page to remove from the existing pages.
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Page Replacement Policy
Basic Page Replacement If the desired page is not in the memory, then we have to make the space for that page by replacing some other page or allotting it free frame if available.
1. Find the location of the desired page on the disk.
2. Find a free frame
(a) If there is a free frame, use it.
(b) If there is no free frame, use page replacement algorithm to select a victim.
(c) Write the victim frame to the disk and update the page table accordingly.
3. Read the desired page into the new frame and change the page table.
4. Restart the process.
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Page ReplacementDiagram shows how page replacement takes place.
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Types of Page Replacement 1. FIFO Page Replacement
2. Optimal Page Replacement
3. LRU Page Replacement
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First In First Out (FIFO)page replacement
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This is the simplest page replacement algorithm.
1. The page which come first is replaced first.
2. In this we have some frames ,if all the frames are occupied and a request to access new page comes then the oldest page is chosen.
3. Here we are taking 3 frames (3 pages can be in memory at a time).
4. We have created a FIFO Queue to hold all pages in memory.
5. A reference string used is 7 0 1 2 0 3 0 4 2 3 0 3 0 3 2 1 2 0 1 7 0 1
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 70
770
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
Page in green colour is replaced
Total number of Page Fault= 15 (count the number of queues)
In FIFO we have Belady’s anomaly if we increase number of frames page fault also increasesTry it with reference string 1 2 3 4 1 2 5 1 2 3 4 5
Optimal Page Replacement
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1. The Optimal policy selects for replacement the page that will not be used in future for longest period of time. You have to see reference string towards right hand side.
2. This is the best type of Page Replacement Algorithm.
3. But predicting the future is difficult so this is practically not possible.
Reference string
7 0 1 2 0 3 0 4 2 3 0 3 2 1 2 0 1 7 0 17 7
0
7
0
1
2
0
1
2
0
3
2
4
3
2
0
3
2
0
1
7
0
1
The page with green colour is Replaced
Total Number of Page Fault = 9.
Least Recently Used1. Least recently Used algorithm replace the page that has not been used for the longest period of
time.
2. You have to see past string in this(Means Left hand side of the string is considered.)
For example from the string 7 0 1 we will replace 7 as it was least recently used.
A reference string used is 7 0 1 2 0 3 0 4 2 3 0 3 0 3 2 1 2 0 1 7 0 1
Reference string 7 0 1 2 0 3 0 4 2 3 0 3 2 1 2 0 1 7 0 1
7 7
0
7
0
1
2
0
1
2
0
3
4
0
3
4
0
2
4
3
2
0
3
2
1
3
2
1
0
2
1
0
7
Total number of Page Fault= 12
Pratima SharmaAssistant Professor
Advanced Educational Institutions,70 km Milestone,
Delhi-Mathura Road, Dist. Palwal, Haryana-121105
+91–1275–398400, 302222www.advance.edu.in