Page replacement algorithm
algorithm in an OS that decides which memory pages to page out (swap out, write to disk) when a requested page is not in memory (page fault) and a free page cannot be used to satisfy the allocation (because there are too few or none)
In a computer operating system that uses paging for virtual memory management, page replacement algorithms decide which memory pages to page out, sometimes called swap out, or write to disk, when a page of memory needs to be allocated. Page replacement happens when a requested page is not in memory (page fault) 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.
When the page that was selected for replacement and paged out is referenced again it has to be paged in (read in from disk), and this involves waiting for I/O completion. This determines the quality of the page replacement algorithm: the less time waiting for page-ins, the better the algorithm. A page replacement algorithm looks at the limited information about accesses to the pages provided by hardware, and tries to guess which pages should be replaced to minimize the total number of page misses, while balancing this with the costs (primary storage and processor time) of the algorithm itself.
The page replacing problem is a typical online problem from the competitive analysis perspective in the sense that the optimal deterministic algorithm is known.
History
Page replacement algorithms were a hot topic of research and debate in the 1960s and 1970s.
That mostly ended with the development of sophisticated LRU (least recently used) approximations and working set algorithms. Since then, some basic assumptions made by the traditional page replacement algorithms were invalidated, resulting in a revival of research. In particular, the following trends in the behavior of underlying hardware and user-level software have affected the performance of page replacement algorithms:
Size of primary storage has increased by multiple orders of magnitude.
“Page replacement algorithm” enters the record as algorithm in an OS that decides which memory pages to page out (swap out, write to disk) when a requested page is not in memory (page fault) and a free page cannot be used to satisfy the allocation (because there are too few or none). Crown Archives preserves that source wording while asking what Page, replacement and algorithm can confirm, complicate or overturn.
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