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The '''Least Recently Used algorithm''' (usually abbreviated as '''LRU''') is an algorithm for choosing which entity to deal with first; be it the entry to 'recycle' in an [[array]] of resources, when a need arises for a free resource (and one has to be replaced); or for some other situation.

In replacement situations it says, as the name implies, that when the entries are all already in use, one should recycle the entry which was most recently used the longest time ago, since the ones used since then are more likely to be used again soon.

The reasoning behind minimizing the number of times entries are freed is that freeing a resource likely involves some effort (e.g. writing a modified [[disk]] block from a [[buffer]] back onto [[secondary storage]]), so that effort should be expended are rarely as possible.

Providing the information needed to determine which entry is the least recently used entry may require some effort, in both [[hardware]] and [[software]] systems. There are thus algorithms which approximate 'true' LRU, but use less overhead.

LRU replacement algorithms are used in many circumstances: disk buffers (as above); [[page frame]]s (in [[virtual memory]] systems); and [[cache]]s of all kinds.

In other situations (such as [[arbitration]] for use of a [[bus]]), a similar [[algorithm]] is used - pick whichever was most recently picked the longest time ago.

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[[Category: Theory]]