[wplug] Understanding Load

Matthew Zwier mczwier at gmail.com
Fri Dec 9 21:26:29 EST 2011


Ditto on gauging the effect of a given load.  We have a NAS box with 8
cores that hovers around load=64 when it's getting slammed by NFS
requests, but the machine remains perfectly functional and usable.
That load is acceptable in that case.  On the other hand, my
workstation's mouse stops moving (even with Zen patches and the BFS
scheduler) under high disk load sometimes, even though the load is
only about 1.5 on a 4-core box.  So there, the load number is
irrelevant with respect to functionality.

MZ

On Fri, Dec 9, 2011 at 10:35 AM, Nicholas A. Schembri
<nschembr at gmail.com> wrote:
> In the wikipedia  artical they site the paper on "Unix load Average".  That
> paper talks about, " when two hot processes are running, the maximum load
> is two (not one) on a single CPU."
>
> They also quote, "
>  ... High load averages usually mean that the system is being used heavily
> and the response
> time is correspondingly slow."
>
> The goal is to gage system load.  A system is fully loaded when the system
> scheduler always has something to run, ie a load of 2 x cores.  I've seen a
> system with a load of 800 on a single core that was responsive. It's just a
> gage you can watch.  Match how the system feels with the task at hand.  I
> now think about power usage more then the cost of another box. I'm now over
> the tipping point. VM are now good enough for my workloads.
>
> http://en.wikipedia.org/wiki/Load_%28computing%29
> Neil J. Gunther <http://en.wikipedia.org/wiki/Neil_J._Gunther>. "UNIX Load
> Average – Part 1: How It
> Works"<http://www.teamquest.com/pdfs/whitepaper/ldavg1.pdf>(pdf).
> TeamQuest.
> Retrieved 2009-08-12.[note
> 1]<http://en.wikipedia.org/wiki/Load_%28computing%29#cite_note-1>
> http://blog.scoutapp.com/articles/2009/07/31/understanding-load-averages
>
>
> Today's systems the disks are the slowest part.  I've noted poor
> performance under low system load and High i0.  The following might help.
> http://www.sysxperts.com/home/announce/vmdirtyratioandvmdirtybackgroundratio
> http://rackerhacker.com/2008/08/07/reduce-disk-io-for-small-reads-using-memory/
>
>
> Nicholas.A.Schembri
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