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. 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. When a process incurs a page fault, a local page replacement algorithm selects for replacement some page that belongs to that same process or a group of processes sharing a memory partition.
A global replacement algorithm is free to select any page in memory. Local page replacement assumes some form of memory partitioning that determines how many pages are to be assigned to a given process or a group of processes.
Most popular forms of partitioning are fixed partitioning and balanced set algorithms based on the working set model. The advantage of local page replacement is its scalability: each process can handle its page faults independently, leading to more consistent performance for that process. However global page replacement is more efficient on an overall system basis. Modern general purpose computers and some embedded processors have support for virtual memory. Each process has its own virtual address space.
A page table maps a subset of the process virtual addresses to physical addresses. In addition, in most architectures the page table holds an "access" bit and a "dirty" bit for each page in the page table. The CPU sets the access bit when the process reads or writes memory in that page. The CPU sets the dirty bit when the process writes memory in that page. The operating system can modify the access and dirty bits. The operating system can detect accesses to memory and files through the following means:.
By clearing the access bit in pages present in the process' page table. After some time, the OS scans the page table looking for pages that had the access bit set by the CPU.
This is fast because the access bit it set automatically by the CPU and inaccurate because the OS does not immediately receives notice of the access nor does it have information about the order in which the process accessed these pages. By removing pages from the process' page table without necessarily removing them from physical memory. The next access to that page is detected immediately because it causes a page fault. This is slow because a page fault involves a context switch to the OS, software lookup for the corresponding physical address, modification of the page table and a context switch back to the process and accurate because the access is detected immediately after it occurs.
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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. Optimal Page replacement — 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.
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