Figure 6-16. A single LRU chain with the important standard LRU algorithm aspects highlighted: the least recently used (LRU) end and the most recently used (MRU) end.

Regardless of Oracle's LRU algorithm, every Oracle LRU chain has a least recently used (LRU) end. It also has a most recently used (MRU) end. In very general terms, the more popular buffer headers will reside near the MRU end, and the less popular buffers will reside near the LRU end.

The standard LRU algorithm is very simple. When a buffer is brought into the cache or accessed (query or DML), it is placed at the MRU end of the session's LRU chain (each session is associated with an LRU chain). The thinking is that a popular buffer will be repeatedly touched and therefore repeatedly moved to the MRU end of the LRU chain. The movement to the MRU of this list is commonly called, buffer promotion. If a buffer is not popular, then as other buffers are promoted or inserted into the LRU, the unpopular buffers will naturally migrate toward the LRU end of the LRU chain.