pagereplacement

Pagereplacement is a memory management mechanism used by operating systems that support virtual memory. It decides which pages residing in physical memory should be evicted to make room for a page that is not currently in memory. The goal is to minimize the total number of page faults and the cost of handling them while maintaining acceptable performance for running processes.

When a reference to a page that is not in physical memory occurs, a page fault is raised. The replacement policy selects a candidate page to evict, and the OS writes dirty pages back to disk if necessary before loading the requested page into a free frame. The decision is influenced by workload, access patterns, and the available number of physical frames.

Common page replacement algorithms include first-in, first-out (FIFO), which evicts the oldest loaded page; least recently used (LRU), which targets pages not used for the longest time; and optimizations of LRU such as the CLOCK and second-chance algorithms. The optimal page replacement algorithm (MIN) evicts the page that will be used farthest in the future, but it requires future knowledge and is theoretical. Aging and NRU provide practical approximations.

Replacement decisions interact with hardware features such as the translation lookaside buffer (TLB) and the page table. Disk performance dominates the cost of faults, making thrashing a risk when too few frames are available. Some systems use global replacement (across all processes) while others use local replacement (per process) to limit interference and improve predictability.

Other considerations include working-set models, frame allocation policies, and page pinning in real-time systems. Page replacement remains central to virtual memory, enabling processes to use memory larger than physical RAM while balancing responsiveness and throughput.