A System Administrator can specify whether a cache is going to be maintained as an MRU/LRU-linked list of pages (strict) or whether relaxed LRU replacement policy can be used. The two replacement policies are:
Strict replacement policy replaces the least recently used page in the pool, linking the newly read page(s) at the beginning (MRU end) of the page chain in the pool.
Relaxed replacement policy attempts to avoid replacing a recently used page, but without the overhead of keeping buffers in LRU/MRU order.
The default cache replacement policy is strict replacement. Relaxed replacement policy should be used only when both of these conditions are true:
There is little or no replacement of buffers in the cache.
The data is not updated or is updated infrequently.
Relaxed LRU policy saves the overhead of maintaining the cache in MRU/LRU order. On SMP systems, where copies of cached pages may reside in hardware caches on the CPUs themselves, relaxed LRU policy can reduce bandwidth on the bus that connects the CPUs.
If you have created a cache to hold all, or most of, certain objects, and the cache hit rate is above 95%, using relaxed cache replacement policy for the cache can improve performance slightly.
See the System Administration Guide for more information.
Log pages are filled with log records and are immediately written to disk. When applications include triggers, deferred updates or transaction rollbacks, some log pages may be read, but usually they are very recently used pages, which are still in the cache.
Since accessing these pages in cache moves them to the MRU end of a strict-replacement policy cache, log caches may perform better with relaxed LRU replacement.
User-defined caches that are sized to hold indexes and frequently used lookup tables are good candidates for relaxed LRU replacement. If a cache is a good candidate, but you find that the cache hit ratio is slightly lower than the performance guideline of 95%, determine whether slightly increasing the size of the cache can provide enough space to completely hold the table or index.