Page Replacement Algorithms
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The document discusses various page replacement algorithms used in computer operating systems. It describes paging, which is a memory management technique that moves data between main memory and secondary storage. When a program requests a page that is not currently in memory, it causes a page fault. Page replacement algorithms determine which memory page to remove to allocate space for the new page. Common algorithms discussed include clock, LRU, NRU, and ARC. The ARC algorithm improves on LRU by maintaining two lists (T1 and T2) to track recently and frequently used pages, along with ghost lists (B1 and B2) of recently evicted pages.
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2. Paging divides memory into pages and processes into page tables to map virtual to physical addresses, reducing fragmentation. It uses translation lookaside buffers (TLBs) to speed address translation.
3. Segmentation divides processes into logical segments and uses segment tables to map segments to physical addresses. It provides a modular view of memory but external fragmentation remains an issue.OS_Ch9
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This document discusses different memory management techniques including:
1. Contiguous allocation allocates processes to contiguous regions of memory but can lead to fragmentation. Paging and segmentation address this by allowing non-contiguous allocation.
2. Paging maps logical addresses to physical frames through a page table. It supports non-contiguous allocation but has translation overhead that is reduced using translation lookaside buffers.
3. Segmentation divides memory into logical segments and uses a segment table to map logical to physical addresses. It matches the user's view of memory but external fragmentation remained an issue until combined with paging.OSCh9
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This document discusses several memory management techniques:
1. Contiguous allocation allocates processes to contiguous regions of memory but can lead to fragmentation.
2. Paging divides memory into pages and processes into page tables to map virtual to physical addresses, reducing fragmentation. It uses translation lookaside buffers (TLBs) to speed address translation.
3. Segmentation divides processes into logical segments and uses segment tables to map segments to physical addresses. It provides a modular view of memory but external fragmentation remains an issue.virtual memory
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Virtual memory is a memory management technique that allows programs to access memory addresses beyond their actual physical RAM size. It maps virtual addresses to physical addresses stored in RAM or on a hard disk using page tables and a translation process. When a program requests a page not in RAM, a page fault occurs and the OS moves a page from disk to RAM, suspending the program until the page is loaded. Page replacement algorithms like LRU then select pages to remove from RAM and write to disk when RAM is full to make space for new pages. This allows for larger memory sizes, more efficient memory usage, and multitasking.call for papers, research paper publishing, where to publish research paper, ...
call for papers, research paper publishing, where to publish research paper, ...International Journal of Engineering Inventions www.ijeijournal.com
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- 1. Page replacement algorithms Ubaidullah alias kashif MSCCN-III Sukkur IBA.
- 2. Paging:- In computer operating systems, paging is one of the memory-management schemes by which a computer can store and retrieve data from secondary storage for use in main memory. Page fault The main functions of paging are performed when a program tries to access pages that are not currently mapped to physical memory (RAM). This situation is known as a page fault.
- 3. Page replacement algorithm Page replacement algorithms decide which memory pages to page out (swap out, write to disk) when a page of memory needs to be allocated. Paging happens when a page fault occurs and a free page cannot be used to satisfy the allocation, either because there are none, or because the number of free pages is lower than some threshold.
- 4. Clock The clock algorithm keeps a circular list of pages in memory, with the "hand" (iterator) pointing to the last examined page frame in the list.
- 5. When a page fault occurs and no empty frames exist, then the R (referenced) bit is inspected at the hand's location. If R is 0, the new page is put in place of the page the "hand" points to, otherwise the R bit is cleared. Then, the clock hand is incremented and this process is repeated until a page is found with R = 0
- 6. LRU & NRU LRU keeps track of page usage over a short period of time, while NRU just looks at the usage in the last clock interval.
- 7. Adaptive replacement cache ARC improves the basic LRU strategy by splitting the cache directory into two lists, T1 and T2, for recently and frequently referenced entries. In turn, each of these is extended with a ghost list (B1 or B2), which is attached to the bottom of the two lists. These ghost lists act as scorecards by keeping track of the history of recently evicted (expelled) cache entries, and the algorithm uses ghost hits to adapt to recent change in resource usage.
- 8. T1, for recent cache entries. T2, for frequent entries, referenced at least twice. B1, ghost entries recently evicted from the T1 cache, but are still tracked. B2, similar ghost entries, but evicted from T2. T1 and B1 together are referred to as L1, a combined history of recent single references. Similarly, L2 is the combination of T2 and B2
- 9. ...[ B1 <-[ T1 <-!-> T2 ]-> B2 ] . . [ . . . . [ . . . . . . ! . .^. ...... ] . . . ] [ fixed cache size (c) ]
Editor's Notes
- #10: New entries enter T1, to the left of !, and are gradually pushed to the left, eventually being evicted from T1 into B1, and finally dropped out altogether.Any entry in L1 that gets referenced once more, gets another chance, and enters L2, just to the right of the central ! marker. From there, it is again pushed outward, from T2 into B2. Entries in L2 that get another hit can repeat this indefinitely, until they finally drop out on the far right of B2.