Let's take a close look at Figure 3-4. The lines represent the linked lists, and the arrows represent the latches. The top single LRU is protected by a single LRU latch. It's easy to understand with just a few users needing to acquire the latch, significant latch contention could occur. So, as the bottom picture in Figure 3-4 shows, Oracle divided the single LRU structure into multiple LRU lists, each with its own LRU latch. By default, Oracle has multiple LRU latches, and each LRU latch has an associated LRU list. For example, even for a small 250MB buffer cache in Release 1 of Oracle Database 10g and 11g, by default, Oracle creates eight LRU latches.

However, simply dividing a large single list into smaller lists may not always work. It depends on how the memory structure is accessed. The Oracle kernel architects must ensure deadlock-type situations are unlikely, and if they do occur, they are recoverable. Oracle architects should know (and we really hope they do) that this particular situation was well suited for a simple multiple-latch solution.

The cache buffer chain (CBC) structure is used to quickly answer the question, "Is the Oracle block in the buffer cache?" Thinking about all the Oracle systems in the world today, the number of times this question is asked each day is mind-boggling.