vmtune Parameters


About this document
    Related documentation
About vmtune

About this document

This document discusses the vmtune command used to modify the Virtual Memory Manager (VMM) parameters that control the behavior of the memory management subsystem. This information applies to AIX Versions 4.x.

Related documentation

You can obtain more information in the AIX Performance Management Guide, available online:

The product documentation library is also available:

About vmtune

The command and source for vmtune are found in the /usr/samples/kernel directory. It is installed with the bos.adt.samples fileset.

There may be times when the default VMM parameters prove unsatisfactory. For instance, consider the situation where the aggregate size of the working pages is very large and file system pages are only referenced infrequently. In this case, the pages from the file system are read once and never used again. With the default parameters, the system would try to maintain up to ((RAMSIZE-4MB)*.8), or approximately 80%, of real memory with persistent pages, leaving only ((RAMSIZE-4MB)*.2), approximately 20%, for working pages. This imbalance would cause more working pages than necessary to be paged to and from paging space.

In this example, you can choose to reduce the maxperm value to 65% in an attempt to maintain a larger set of working pages in memory. This setting could lead to less paging to page space with no detrimental effect on the persistent storage. That will decrease, though, as the system brings in working pages and replaces some file pages.

If your system frequently requires quick access to many frames from the free list, you may want to increase the minfree and maxfree values. A larger minfree ensures that more frames are readily available when they are needed. The difference between minfree and maxfree determines how many pages are to be stolen for each call to the page replacement code (PRC). A larger difference usually means longer intervals between calls to the PRC but each call takes longer to complete.

You can achieve significant differences in read times by using various minpgahead and maxpgahead values. It is very dependent upon disk activity and access to the individual files. The VMM read-ahead algorithms, which these values affect, are designed to improve sequential access read performance.

It takes experience and know-how to set vmtune parameters properly. Therefore, it is highly recommended that you set vmtune values only after consulting the AIX Performance Team. Contact your AIX support center and ask for a performance analysis through consult line.

Here are all of the parameters available for AIX Version 4.1.3 and later versions. Earlier versions of AIX contain some, but not all, of the following options:

The current values are displayed when vmtune is executed without any parameters. Changes to the values go into effect immediately but do not exist after you restart the system.

[ Doc Ref: 90605227614710     Publish Date: Jan. 18, 2001     4FAX Ref: 1777 ]