For example, if we consider reference string 3, 2, 1, 0, 3, 2, 4, 3, 2, 1, 0, 4 and 3 slots, we get 9 total page faults, but if we increase slots to 4, we get 10 page faults. > 1 Page Fault.Ħ comes, it is also not available in memory so it replaces the oldest page slot i.e 3 -> 1 Page Fault.įinally when 3 come it is not available so it replaces 0 1 page faultīelady’s anomaly – Belady’s anomaly proves that it is possible to have more page faults when increasing the number of page frames while using the First in First Out (FIFO) page replacement algorithm. Then 5 comes, it is not available in memory so it replaces the oldest page slot i.e 1. When 3 comes, it is already in memory so -> 0 Page Faults. Initially all slots are empty, so when 1, 3, 0 came they are allocated to the empty slots -> 3 Page Faults. When a page needs to be replaced page in the front of the queue is selected for removal.Įxample-1Consider page reference string 1, 3, 0, 3, 5, 6 with 3 page frames.Find number of page faults. 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. This is the simplest page replacement algorithm. The target for all algorithms is to reduce the number of page faults. Different page replacement algorithms suggest different ways to decide which page to replace. In case of page fault, Operating System might have to replace one of the existing pages with the newly needed page. Since actual physical memory is much smaller than virtual memory, page faults happen.
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Page Fault – A page fault happens when a running program accesses a memory page that is mapped into the virtual address space, but not loaded in physical memory. In an operating system that uses paging for memory management, a page replacement algorithm is needed to decide which page needs to be replaced when new page comes in.
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