Average Page Residency Time is quite easy to calculate on a gross basis by using the following formula both at overall pool and for the objects:

(Getpages - PagesRd)/Getpages * Time

This ignores data hot spots, but provides a general idea how long pages are remaining in the pool. At the same time, if most of the getpages are for the hot pages, this becomes quite relevant. The important point to remember, is that we want a page to be in the pool when the getpage is issued. Technically, for our purposes, a page could live in a pool for ten minutes without being referenced, and be thrown out just before it is needed...this is not of any value and affects the residency time as a ‘miss’, or zero for that page.

Clustering analysis of actual page numbers provides the true reference patterns, and at the same time can provide the ‘range of pages’ in the average and maximum working sets within the pages in the buffer pool.