LIRS caching algorithm
LIRS (Low Inter-reference Recency Set) is a page replacement algorithm with an improved performance over LRU (Least Recently Used) and many other newer replacement algorithms. This is achieved by using "reuse distance" as the locality metric for dynamically ranking accessed pages to make a replacement decision. Summary. Quantifying the locality. While all page replacement algorithms rely on existence of reference locality to function, a major difference among different replacement algorithms is on how this locality is quantified. LIRS uses reuse distance of a page, or the number of distinct pages accessed between two consecutive references of  the page, to quantify locality. Specifically, LIRS uses last and second-to-last references (if any) for this purpose. If a page is accessed for the first time, its reuse distance is infinite. In contrast, LRU uses recency of a page, which is the number of distinctive pages accessed after the reference of the page, to quantify locality. To take into account of up-to-date access history, the implementation of LIRS actually uses the larger of reuse distance and recency of a page as the metric to quantify its locality, denoted as RD-R. Assuming the cache has a  capacity of C pages, the LIRS algorithm is to rank recently accessed pages according to their RD-R values and retain the C most highly ranked pages in the cache. The concepts of reuse distance and recency can be visualized as below, in which T1 and T2 are page B’s second-to-last and last reference times, respectively, and T3 is the current time. Selecting the replacement victim. LIRS organizes metadata of cached pages and some uncached pages and conducts its replacement operations described as below, which are also illustrated with an example in the graph. Deployment. LIRS has  been deployed in MySQL since version 5.1. It is also adopted in Infinispan data grid platform. An approximation of LIRS, CLOCK-Pro, is adopted in NetBSD.