Scan Rates Post: 302307784

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Operating Systems AIX Scan Rates Post 302307784 by zaxxon on Thursday 16th of April 2009 10:29:18 AM

04-16-2009

Registered User

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:

Quote:

# 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.

zaxxon

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LEARN ABOUT OSX

vm_stat

VM_STAT(1)						    BSD General Commands Manual 						VM_STAT(1)

NAME

     vm_stat -- show Mach virtual memory statistics

SYNOPSIS

     vm_stat [[-c count] interval]

DESCRIPTION

     vm_stat displays Mach virtual memory statistics.  If the optional interval is specified, then vm_stat will display the statistics every
     interval seconds.	In this case, each line of output displays the change in each statistic (an interval count of 1 displays the values per
     second).  However, the first line of output following each banner displays the system-wide totals for each statistic.  If a count is pro-
     vided, the command will terminate after count intervals.  The following values are displayed:

     Pages free
	     the total number of free pages in the system.

     Pages active
	     the total number of pages currently in use and pageable.

     Pages inactive
	     the total number of pages on the inactive list.

     Pages speculative
	     the total number of pages on the speculative list.

     Pages throttled
	     the total number of pages on the throttled list (not wired but not pageable).

     Pages wired down
	     the total number of pages wired down.  That is, pages that cannot be paged out.

     Pages purgeable
	     the total number of purgeable pages.

     Translation faults
	     the number of times the "vm_fault" routine has been called.

     Pages copy-on-write
	     the number of faults that caused a page to be copied (generally caused by copy-on-write faults).

     Pages zero filled
	     the total number of pages that have been zero-filled on demand.

     Pages reactivated
	     the total number of pages that have been moved from the inactive list to the active list (reactivated).

     Pages purged
	     the total number of pages that have been purged.

     File-backed pages
	     the total number of pages that are file-backed (non-swap)

     Anonymous pages
	     the total number of pages that are anonymous

     Uncompressed pages
	     the total number of pages (uncompressed) held within the compressor

     Pages used by VM compressor:
	     the number of pages used to store compressed VM pages.

     Pages decompressed
	     the total number of pages that have been decompressed by the VM compressor.

     Pages compressed
	     the total number of pages that have been compressed by the VM compressor.

     Pageins
	     the total number of requests for pages from a pager (such as the inode pager).

     Pageouts
	     the total number of pages that have been paged out.

     Swapins
	     the total number of compressed pages that have been swapped out to disk.

     Swapouts
	     the total number of compressed pages that have been swapped back in from disk.

     If interval is not specified, then vm_stat displays all accumulated statistics along with the page size.

Mac OS X							  August 13, 1997							  Mac OS X