Pge Replacement Algorithm.pdf
- 2. Table of Contents Introduction to the page replacement algorithms Why we use Page replacement Algorithm ? Working of algorithms Here, we conclude the topic Working Conclusion Introduction Why Page Repleacement
- 3. Introduction ● The page replacement algorithm decides which memory page is to be replaced. ● The process of replacement is sometimes called swap out or write to disk. ● Page replacement is done when the requested page is not found in the main memory (page fault). ● There are two main aspects of virtual memory, Frame allocation and Page Replacement. ● If the Page Found then it is called as Page Hit. ● If the page not found then it is called Page Fault. ● Page Hit/Page fault=Hit Ratio
- 4. Types of Page Replacement Algorithms There are various page replacement algorithms. Each algorithm has a different method by which the pages can be replaced. 1. Optimal Page Replacement algorithm → this algorithms replaces the page which will not be referred for so long in future. Although it can not be practically implementable but it can be used as a benchmark. Other algorithms are compared to this in terms of optimality. 2. Least recent used (LRU) page replacement algorithm → this algorithm replaces the page which has not been referred for a long time. This algorithm is just opposite to the optimal page replacement algorithm. In this, we look at the past instead of staring at future. 3. FIFO → in this algorithm, a queue is maintained. The page which is assigned the frame first will be replaced first. In other words, the page which resides at the rare end of the queue will be replaced on the every page fault. ●
- 5. Page replacement algorithms are an important part of virtual memory management and it helps the OS to decide which memory page can be moved out, making space for the currently needed page. However, the ultimate objective of all page replacement algorithms is to reduce the number of page faults.
- 6. WORKING
- 7. This is the simplest page replacement algorithm. In this algorithm, the operating system keeps track of all pages in the memory in a queue, the oldest page is in the front of the queue. When a page needs to be replaced page in the front of the queue is selected for removal. Example- Consider page reference string 1, 3, 0, 3, 5, 6, 3 with 3 page frames .Find the number of page faults. FIFO
- 8. FIFO example ● Initially, all slots are empty, so when 1, 3, 0 came they are allocated to the empty slots — > 3 PageFaults. ● when 3 comes, it is already in memory so —> 0 Page Faults. ● Then 5 comes, it is not available in memory so it replaces the oldest page slot i.e 1. —>1 Page Fault. ● 6 comes, it is also not available in memory so it replaces the oldest page slot i.e 3 —>1 Page Fault. ● Finally, when 3 come it is not available so it replaces 0 1 page fault
- 9. ● In this algorithm, pages are replaced which would not be used for the longest duration of time in the future. Example - :Consider the page references 7, 0, 1, 2, 0, 3, 0, 4, 2, 3, 0, 3, 2, with 4 page frame. Find number of page fault. Optimal Page Replacement
- 10. Optimal Page Replacement example ● Initially, all slots are empty, so when 7 0 1 2 are allocated to the empty slots —> 4 Page faults 0 is already there so —> 0 Page fault. ● when 3 came it will take the place of 7 because it is not used for the longest duration of time in the future.—>1 Page fault. ● 0 is already there so —> 0 Page fault.. ● 4 will takes place of 1 —> 1 Page Fault. ● Now for the further page reference string —> 0 Page fault because they are already available in the memory. ● Optimal page replacement is perfect, but not possible in practice as the operating system cannot know future requests. The use of Optimal Page replacement is to set up a benchmark so that other replacement algorithms can be analyzed against it.
- 11. In this algorithm, page will be replaced which is least recently used. Example - Consider the page reference string 7, 0, 1, 2, 0, 3, 0, 4, 2, 3, 0, 3, 2 with 4 page frames . Find number of page faults. LRU(Least Recently Used)
- 12. LRU example ● Initially, all slots are empty, so when 7 0 1 2 are allocated to the empty slots —> 4 Page faults ● 0 is already their so —> 0 Page fault. ● when 3 came it will take the place of 7 because it is least recently used —>1 Page fault ● 0 is already in memory so —> 0 Page fault. ● 4 will takes place of 1 —> 1 Page Fault ● Now for the further page reference string —> 0 Page fault because they are already available in the memory.
- 13. Conclusion