[CentOS] Swap: typical rehash. Why?

Les Mikesell lesmikesell at gmail.com
Mon Jun 5 23:40:25 UTC 2006


On Mon, 2006-06-05 at 17:37 -0400, William L. Maltby wrote:
> > > 
> > > Given a "general purpose" system, tunability is a must. UNIX, as
> > > delivered by USL in such examples as Sys V, had tunables that let admins
> > > tune to their needs. A single "swappiness" value is woefully inadequate.
> > 
> > Actually, having these computed dynamically is much better than
> > having to manually tune them every time your mix of programs
> > change or you add memory except in some very unusual circumstances
> > like a server that does a single job forever.  In the general
> > case consider whether you'd rather hire an expert admin to
> > keep your system tuned or buy an extra gig of ram and let the
> > OS figure out how to use it.
> 
> I agree, sort of. The problem occurs in that the OS is only somewhat
> heuristic. It learns, but only a small amount. The admin can run SAR
> reports and use other tools that profile activities and he can use that
> information to "pre-bias" the VM so that it better achieves *long-term*
> performance and responsiveness goals and is less sensitive to "spikes"
> in one factor or another.

If you remember those tunables, you might also remember that there was
a whole fairly large (and boring) book that told an administrator how
to interpret various permutations of SAR reports and which way to adjust
the tunable values.  Even when Linux had more hard-coded tunables I was
never able to find any equivalent reference to use them correctly.

> For many business applications, there is a list of performance
> requirements that can be prioritized. Priorities can vary, even within a
> node, based on many things, including time of day. An admin with access
> to the means can help ensure that his installation is meeting the goals
> necessary for the business.
> 
> When the best of the code is combined with the best of an admin's effort
> and data analysis, a good outcome is likely. Code only or admin with
> tools/means both produce less optimal results.

Yes, but if you can define the solution, the computer could probably
implement it better and certainly faster by itself.  I think these days
the developers would rather write the code to do it instead of the
documentation to tell you what needs to be done.

-- 
  Les Mikesell
   lesmikesell at gmail.com




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