Scan Rates

Negm · March 20, 2005, 4:15am

Dear Gurus,
Can any one advice about the normal limits for the Page scanning rates on the AIX platforms, i am having enormous values for the scan rate along the hour it may reache 3000 pages/sec.

Regards, Negm

seacros · April 16, 2009, 10:00am

I would like to know this also. Mine have not reached that high but I would like to know so i can tune my monitoring agents for my AIX servers.

SeaCros

zaxxon · April 16, 2009, 10:29am

3k pages as scan rate per sec is no big value. I sometimes see bigger ones. Usually it's the ratio/relation between scanned pages and freed pages that matters. Optimal to ok should be ratio fr/sr of ~ 1:1 - 1:3 afaik; can be but must not.

From some IBM AIX Performance Management Guide, note the last line in bold letters:

# fr

Number of pages that were freed per second by the page-replacement algorithm during the interval. As the VMM page-replacement routine scans the Page Frame Table (PFT), it uses criteria to select which pages are to be stolen to replenish the free list of available memory frames. The criteria include both kinds of pages, working (computational) and file (persistent) pages. Just because a page has been freed, it does not mean that any I/O has taken place. For example, if a persistent storage (file) page has not been modified, it will not be written back to the disk. If I/O is not necessary, minimal system resources are required to free a page.
# sr

Number of pages that were examined per second by the page-replacement algorithm during the interval. The VMM page-replacement code scans the PFT and steals pages until the number of frames on the free list is at least the maxfree value. The page-replacement code might have to scan many entries in the PFT before it can steal enough to satisfy the free list requirements. With stable, unfragmented memory, the scan rate and free rate might be nearly equal. On systems with multiple processes using many different pages, the pages are more volatile and disjoint. In this scenario, the scan rate might greatly exceed the free rate.

Memory is over committed when the ratio of fr to sr (fr:sr) is high.

An fr:sr ratio of 1:4 means that for every page freed, four pages had to be examined. It is difficult to determine a memory constraint based on this ratio alone, and what constitutes a high ratio is workload/application dependent.

For tuning in general and to get a first impression, you should look at the output from "vmstat 1 20" for example at busy times. Just looking at "sr" will show/mean nothing.