AIX > Administrator > Performance

Tuning a Perfect Note

A look at performance tuning and new AIX 5.3 commands.

A look at performance tuning and new AIX 5.3 commands.

Parameters

In AIX 5.3, you need to have transitioned from using vmtune and schedtune to the replacements - ioo, vmo and schedo. Before this year, the general recommendation for vmo (or vmtune) tuning was to set minperm to 5, maxperm to somewhere between 30 and 50 and maxclient to something lower than maxperm but higher than minperm. Since maxperm is a soft limit and maxclient is a hard limit, we also recommended in some cases that you should change strict_maxclient to set this to a soft limit. These recommendations have recently changed. Before going into the new recommendations, it's necessary to understand what these parameters do.

  • minperm specifies the value for file pages (%), below which the page stealer steals both file and computational pages.
  • maxperm specifies (for JFS) the value for file pages (%), above which the page stealer should steal only file pages. The default is 80%, but this isn't a hard limit. If you set maxperm to 30%, and the pages are available, then I/O can still use up to 100% of memory. What you're changing is which pages get stolen when a page is needed. Maxperm includes ALL file pages, not just JFS, whereas maxclient is only the JFS2/NFS/GPFS subset of pages.
  • maxclient allows you to further control how much memory is used for caching client (NFS) and JFS2 pages. It can't be set to a value higher than maxperm and it's a hard limit.

When the percentage of file pages falls between minperm and maxperm, then the page stealer steals file pages - unless the number of file repages is higher than the number of computational repages.

A key difference between maxperm and maxclient is how they're limited. When maxperm is set to 30%, it caps maxclient at 30% automatically, as maxclient can't exceed maxperm. However, maxperm is a soft limit (controlled by strict_maxperm, which defaults to 0 or off). This means that file caching can use more memory than 30% if it's available. Maxclient is a hard limit, so setting this low arbitrarily limits memory for JFS2 caching. We used to alter strict_maxclient. When this is set to 0, it alters the maxclient percentage to be a soft limit.

The new recommendations are to leave maxclient and maxperm at their default settings of 80, but to still set minperm to something like 5. We also don't change the strict settings. Instead, we alter other parameters as follows:

vmo -p -o minperm%=5
vmo -p -o lru_file_repage=0
vmo -p -o lru_poll_interval=10

Effectively, what setting the two lru values above does is the following: If filesystem buffer pages exceed minperm (in this case 5%), the LRUD (AIX page-stealing daemon) will show preference to executables, always choosing filesystem pages if possible. Setting lru_file_repage to 0 causes the LRUD to do this. The lru_poll_interval is set to improve the responsiveness of the LRUD process while it's running. The combination of these two parameters should give better and more flexible results than limiting maxperm and maxclient. You're basically setting a simple rule. Lru_file_repage is available on AIX 5.2 as well, so long as the system is at ML5 or higher.

Jaqui Lynch is an independent consultant, focusing on enterprise architecture, performance and delivery on Power Systems with AIX and Linux.


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