Virtual Memory Management. Rab Nawaz Jadoon. Assistant Professor DCS. Pakistan. COMSATS, Lahore. Department of Computer Science

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1 Virtual Memory Management DCS COMSTS Institute of Information Technology Rab Nawaz Jadoon ssistant Professor COMSTS, Lahore Pakistan Operating System Concepts

fetch nticipatory fetch In Paging (placement) In

2 VM Management Strategies VM Strategies Fetch strategies Placement strategies Replacement strategies Demand fetch nticipatory fetch In Paging (placement) In Segmentation (placement) Many lgo s (details in further slides) 2

3 Replacement strategies Determines which page or segment to replace to provide space for an incoming page or segment 3

4 Locality process tends to reference memory in highly localized patterns Temporal locality Locality over time Spatial locality Locality over space (nearby items) 4

5 Page Replacement Strategies Random page replacement FIFO Least recently used (LRU) Least frequently used (LFU) Not used recently (NUR) There are many algorithm but some important are mentioned above, 5

6 Random page replacement It is very simple and easy to implement, with low overhead page replacement Each page in memory has an equal likelihood of being selected for replacement The main problem of this strategy is that it may accidently select as the next page to replace the page that will be referenced next Which is of course the worst page to replace The main benefit is that it replace pages very quickly and fairly 6

7 FIFO Replace the page that has been in the system the longest Under this scheme, the system keeps track of the order in which pages enter main memory It can replace heavy used pages For example, on large time sharing systems it is common for many users to share a copy of the text editor as they enter and correct programs Under FIFO, it replaces the editor page This would be a poor choice, bc the page would be recalled to main memory almost immediately, resulting more page fault 7

8 FIFO FIFO page Replacement With three pages available Page references Result C C C is replaced No fault D C is replaced D D C 8

9 FIFO nomaly Page references Result FIFO page Replacement With three pages available FIFO page Replacement With four pages available C C C - D D C D C D C D C D D C E E E D C No E E D C No E E D C C E C E D D C E D C E No D C E E D C Three NO s Two NO s 9

10 FIFO nomaly When the process executes with four pages in memory it actually experience one more page fault than when it executes with only three pages 10

11 Least Recently Used (LRU) LRU relies on the locality heuristic that a process s recent past behaviors is a good indicator of the process s near future behavior (temporal locality) It replaces the page that has spent the longest time in memory without being referenced LRU can provide better performance than FIFO 11

12 Least Recently Used (LRU) Page references C D F Result No No No No No No LRU page Replacement With three pages available C C C C D D D F F 12

13 Least Frequently Used (LFU) It makes replacement decision based on how intensively each page is being used Under LFU, the system replaces the page that is least frequently used or least intensively referenced page that is not being intensively referenced is not as likely to be referenced in the future This is implemented through a counter that is updated each time its corresponding page is referenced, but this can incur substantial overhead 13

14 Not Used Recently (NUR) popular scheme for approximating LRU with little overhead is NUR replacement strategy NUR is based on the idea that a page that has not been used recently is not likely to be used in the near future The NUR is implemented using two hardware bits per page table entry Referenced it: 0 not referenced, 1 referenced Modified it: 0 not modified, 1 modified 14

15 Not Used Recently (NUR) Initially the system set the referenced bit of all pages to zero The modified bits are also set to zero When a process references a particular page, the system sets the referenced bit to one (1), and whenever a page is modified, the system set the modified bit to one (1) NUR first find the page that has not been referenced If no such page exists, the system must replace a referenced page In this case NUR check the modified bit If not modified the system select it for replacement 15

16 Page types under NUR Group Referenced Modified Remarks Group est choice to replace Group [Seams unrealistic] Group Group Worst choice to replace 16

17 17

SMD149 - Operating Systems - VM Management

SMD149 - Operating Systems - VM Management Roland Parviainen November 17, 2005 1 / 35 Outline Overview Virtual memory management Fetch, placement and replacement strategies Placement strategies Paging,