Figure 6-15. This is an example of Oracle's LRU chains. There are 2 LRU chain latches and 6 LRU chains, protecting 20 buffer headers. Each buffer header is associated with a single free, pinned, or dirty buffer.

Figure 6-15 is a good scaled-down diagram of Oracle LRU chains. Production Oracle systems usually have less than 50 LRU chains; most have between 8 and 16 chains. The hidden instance parameter _db_block_lru_latches shows how many LRU chains the instance is using. As with the CBCs, each LRU chain latch controls serialization for a group of LRU chains. Each of the Figure 6-15 LRU chains is protected by either latch CS 900 or CS 910. Just as with the CBC structure, each LRU chain is entirely made up of buffer headers. In fact, each of the buffer headers in the Figure 6-15 LRU chains is also linked to one of the CBCs! Each buffer header is labeled with the letter F, P, or D to signify it is free, pinned, or dirty. As I mentioned, in most production Oracle systems, each LRU chain links thousands of buffer headers.

The current LRU chain algorithm is called the touch-count algorithm, which uses a count frequency scheme to place a value on each buffer header. But it has taken Oracle many years to arrive at this algorithm. Understanding Oracle's LRU algorithm progression provides an insight into how the LRU chains work, their shortcomings, and how to ensure they perform as needed.