How does the LRU algorithm work in cache management?

The Least Recently Used (LRU) algorithm in cache management works by discarding the least recently used items first.

In more detail, the LRU algorithm is a cache replacement policy that is used to manage memory within a computer. It operates on the principle that the data which has been most recently used is likely to be used again in the near future. Therefore, when the cache is full and a new item needs to be loaded into the cache, the LRU algorithm will discard the item that has not been used for the longest time.

The LRU algorithm keeps track of what was used when, which is expensive if one wants to make sure the algorithm always discards the least recently used item. General implementations of this technique require keeping "age bits" for cache-lines and track the "Least Recently Used" cache-line based on age-bits. In such an implementation, every time a cache-line is used, the age of all other cache-lines changes.

LRU is actually a family of caching algorithms with members including 2Q by Theodore Johnson and Dennis Shasha, and LRU/K by Pat O'Neil, Betty O'Neil and Gerhard Weikum. These algorithms try to 'intelligently' determine which data to keep in cache and which to discard to make room for new data, based on the access patterns of the data.

The LRU algorithm is easy to understand and implement, but it is not always the best choice for all caching scenarios. For example, in situations where older items are more likely to be accessed than newer ones, a Most Recently Used (MRU) algorithm might be a better choice. However, for most general-purpose cache systems, LRU provides a good compromise between complexity and performance.