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SAP Note

Header Data

Symptom

Your AIX LPAR occupies high amounts of the physical memory (RAM), much more than you expect from your application configuration (SAPinstance, may be Database instance).

Other Terms

High Usage of RAM, Memory Usage, Memory Consumption, File cache

Reason and Prerequisites

AIX Virtual Memory Management (VMM) is designed to exploit all available physical memory in a system. VMM distinguishes betweencomputational memory segments (working storage segments like process data and stack or program text segments) and file memory segments(file cache). As long as the virtual memory demand (processes and Operating System requirements) can be fullfilled by real memory pages (i.e. nopaging space usage required), VMM may utilize remaining available free pages for file cache purposes.

So the free list, i.e. pages of physical memory neither assigned to processes/OS nor to file cache could become quite small, and the impression ofupcoming memory shortage could arise.

The parameters controlling that behaviour are the following, with the default values as from AIX version 6.1 on (check via 'vmo -aF'):

*from AIX 7.1 on it is hard coded, no more seen in vmo

The setting 'lru_file_repage = 0' means that processes (and OS) demands get priority over file cache demands. As soon as process demands canno longer be fullfilled by the physical memory's free list, the file cache will give back pages. This continues until the process demands are fullfilled or

until the file cache size has shrinked below the threshold 'minperm%'. Then further memory demands will be fullfilled by stealing file pages fromcomputational/process pages, depending on LRU logic, which then results in paging. Process pages (of any processes) which haven't been usedfor some time are moved from RAM to paging space, until the freelist has been replenished to its higher threshold ('maxfree'), and until the currentdemands are fulfilled. The actual procedure can be more complex, e.g. the freelist related settings hold per 'memory pool', where larger system canhave more than one (seen under 'vmstat -v' command).

Solution

In case of a memory shortage, meaning the free list has shrinked to near 0, check whether the memory usage is caused by active virtual memory(i.e. processes and OS demands), or alternatively whether significant amount of file cache usage (exceeding the file cache guaranteed minimum of'minperm%', usually 3%) can be seen.

A valuable command to decide about that is 'svmon -G'. Here an example console output (the units shown are 4-KByte pages, which is the default).

1971698 - High Memory Consumption on AIX

Version 4 Validity: 11.02.2014 - active Language English (Master)

Released On 18.02.2014 08:24:38Release Status Released for CustomerComponent BC-OP-AIX IBM AIX

Priority Recommendations / Additional InfoCategory Help for error analysis

minperm% = 3maxperm% = 90maxclient% = 90lru_file_repage* = 0strict_maxclient = 1strict_maxperm = 0

minfree = 960maxfree = 1088

size inuse free pin virtualmemory 1048576 977607 70969 177924 393209pg space 1310720 1460

work pers clnt other pin 114900 0 0 63024use 393209 5 584393

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At first glance the available physical memory (size) of 1048576 pages (4 GByte) is mostly used up, i.e. 977607 pages are assigned (inuse), andonly 70969 pages are free (free). Due to the total virtual memory (virtual) being 393209 pages, only about 40% of total available memory is used.The file cache occupies 584393 pages (clnt); in case of high demands from processes/OS it will give back everything but 3% of 1048576 = 31457pages.

As a side remark, when estimating your expected memory consumption, please take into account that besides the applications/processes also theOS has some demands, which could be seen as at least 10% of RAM.

Validity

This document is not restricted to a software component or software component version

References

This document is referenced by:

SAP Notes (1) 1972803 SAP on AIX: Recommendations