INFODOC ID: 17946 
SYNOPSIS: New Kernel Tunables in 2.5.1+ for priority paging 
DETAIL DESCRIPTION: 

Solaris 2.6 kernel patch 105181-09, the equivalent x86 patch 105182-09,
Solaris 2.5.1 kernel patch 103640-25 and the equivalent x86 patch 103641-25 
introduced 2 new tunables:

        cachefree
        priority_paging

These tunables provide a mechanism to allow priority paging, whereby
filesystem pages will be paged out before application, executable, and/or
shared library pages.  This can provide a significant performance benefit
when using applications which perform frequent, random access to filesystem
data which is used once and then no longer required, by flushing filesystem
pages in preference to other types of pages.

By default, priority paging is disabled (priority_paging = 0).  It is
enabled by adding the following line to the /etc/system file and rebooting:

        set priority_paging=1

The default value of cachefree depends on the value of the priority_paging
tunable.  If priority_paging is 0, then cachefree is set equal to lotsfree.
If priority_paging=1, then cachefree is set to 2 times lotsfree.

When priority paging is enabled and free memory drops below cachefree, the
page scanner will start to run, but will only mark pages used for
filesystem data for pageout.  If freemem drops below lotsfree, the
scanning algorithm performs just as it did before this patch, regardless of
the setting of priority_paging.

cachefree can be adjusted by setting it in the /etc/system file;  if the
value specified in /etc/system is less than lotsfree, it will be set to
lotsfree instead.  Setting cachefree equal to lotsfree is equivalent to
not having priority paging enabled.

Priority paging, as described here, is implemented in Solaris 7 FCS.      

Priority paging will not help systems running applications which perform
sequential access to filesystem data, since sequential access performs page
free-behind by default.

Enabling priority paging may cause scan rates well in excess of those which
would previously have considered a problem (300+); the new algorithm causes
the page scanner to start sooner, and when freemem is in the range between
lotsfree and cachefree, it has to scan more pages because some are not
eligible for removal.      

APPLIES TO: Hardware, Operating Systems/Solaris/Solaris 2.x, AFO Vertical Team Docs, AFO Vertical Team Docs/Kernel