Why do I have more free memory after I upgrade to Solaris 8?


In Solaris 7 and below all spare memory was used for the file system
cache. This resulted in cache pollution with file system data and
meant that file system data often had to be paged out in order start
applications, and it also meant that large amounts of file system
activity could purge critical data from the cache.

Application startup performance was dealt with by adding the
priority_paging tuneable, however, large amounts of file system
activity could still result in cache pollution. Another side effect
of this feature was that machines would rarely show any free memory,
except at initial startup time. This meant that memory shortages had
to be second guessed by studying the output of sar -g to spot
consistently high scan rates, although sometimes this could just be
due to high levels of file system activity.

In Solaris 8 the memory model was changed. A 'cyclical page cache'
model was introduced. The file system free list is dedicated to the
caching of file system data only and all other objects, such as
applications, the kernel, shared memory segments, etc. are maintained
on a separate free list.

This means that many of the problems associated with the pre-Solaris
8 model are overcome as the file system cache essentially pages
against itself and heavy file system activity will not cause
applications or other non-file system data to be paged out. If more
memory is required by non-file system data is required then the size
of the file system cache is reduced.

The major differences with this architecture are that the system no
longer includes the file system cache as used memory resulting in the
three following most noticeable changes:

1) Higher Page Reclaims 
2) Higher Free Memory Values 
3) Zero Scan Rates

This means now that if the amount of free memory displayed by either
vmstat or sar -r is low then you really have a memory shortage and
that the if the output of sar -g show non-zero scan rates then the
machine is really paging system data, as file system data is no
longer included.


-by Ken Robson