Editing Cache replacement policies (section)

=== Bélády's algorithm ===
The most efficient caching algorithm would be to discard information which would not be needed for the longest time; this is known as [[László Bélády|Bélády]]'s optimal algorithm, optimal replacement policy, or [[Page replacement algorithm#The theoretically optimal page replacement algorithm|the clairvoyant algorithm]]. Since it is generally impossible to predict how far in the future information will be needed, this is unfeasible in practice. The practical minimum can be calculated after experimentation, and the effectiveness of a chosen cache algorithm can be compared.

[[File:Beladysalgoworking.png|center|alt=Chart of Belady's algorithm]]

When a [[page fault]] occurs, a set of pages is in memory. In the example, the sequence of 5, 0, 1 is accessed by Frame 1, Frame 2, and Frame 3 respectively. When 2 is accessed, it replaces value 5 (which is in frame 1, predicting that value 5 will not be accessed in the near future. Because a general-purpose operating system cannot predict when 5 will be accessed, Bélády's algorithm cannot be implemented there.