<P> Page faults, by their very nature, degrade the performance of a program or operating system and in the degenerate case can cause thrashing . Optimization of programs and the operating system that reduce the number of page faults improve the performance of the program or even the entire system . The two primary focuses of the optimization effort are reducing overall memory usage and improving memory locality . To reduce the page faults in the system, programmers must make use of an appropriate page replacement algorithm that suits the current requirements and maximizes the page hits . Many have been proposed, such as implementing heuristic algorithms to reduce the incidence of page faults . Generally, making more physical memory available also reduces page faults . </P> <P> Major page faults on conventional computers (which use hard disk drives for storage) can have a significant impact on performance . An average hard disk drive has an average rotational latency of 3 ms, a seek time of 5 ms, and a transfer time of 0.05 ms / page . Therefore, the total time for paging is near 8 ms (= 8,000 μs). If the memory access time is 0.2 μs, then the page fault would make the operation about 40,000 times slower . </P>

Describe the action taken by the os when a page fault occurs