[nSLUG] 2.6 kernel performance

Bill Davidson bdavidso at supercity.ns.ca
Tue Jun 21 16:56:53 ADT 2005


On Tue, 2005-21-06 at 10:17 -0300, Donald Teed wrote:
> One is the load factor reporting.  The first number in the series
> seems to "start" at 1 rather than 0.  When the machine is
> relatively idle with a 2.4 kernel, I see numbers like : 0.04, 0.03, 0.00
> When I run a 2.6 kernel, the first number is at least 1.
> The top report shows processors that are about 98% idle
> while the load number is like that.

What does "ps aux" show?  Do you see any processes that seem to be using
a lot of cpu time?  How is memory usage?

> The other issue is hdparm performance numbers.  With the same
> software raid devices, the hdparm -Tt numbers come in much
> lower with 2.6 than they do with 2.4 kernel.

I wouldn't worry too much about these numbers.  I think you would need
to see changes in a more application-oriented test before being

Were I you I would be far more concerned about the load average numbers
over 1.  Trying to diagnose low-level performance issues while your load
average is that high is pointless, like trying to tweak your carburetor
for best performance while you have a flat tire.

> I had also tried CONFIG_IOSCHED_DEADLINE , and for some
> reason all of the load factor numbers were always over 1 while
> that was on.

What about your hdparm numbers -- did they change at all when you
changed scheduler?

> I see that I should have used an "elevator=cfq" kernel option
> along with those IO scheduler options.

Yes, I believe that is the "normal" way to select a different scheduler
algorithm, or would be if any of this could be considered "normal".

> I'm not up on the technical bits of how they work.  I see cautions related
> to performance under anticipatory IO scheduler and RAID, and
> I've stayed away from that option.

I haven't researched this but my reading of the documentation suggests
possible problems with *hardware* RAID, not software.

> Does anyone have a tip on the best kernel options for SMP and software RAID
> over IDE?  Perhaps I should just try all of the schedulers off.

Well, there is always some scheduler present.  I suggest you compile
your 2.6 kernel with all the schedulers, and then try booting with each
one selected (noop, deadline, as, cfq), then run some meaningful tests.
Like try copying some files over the network, and see if you can tell
the difference.  Again, my reading suggests that the cfq scheduler is
intended to provide fairness to all processes and is intended for
desktop use.  Under the right circumstances the as scheduler should give
you the best performance, followed by the deadline scheduler.

But again, this is pointless if your load average is unusually high.  I
think you need to diagnose that first.

As usual, I have no informed advice since I don't have either an SMP
machine, nor a machine running 2.6 and RAID.

> --Donald
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Bill Davidson
bdavidso at supercity.ns.ca


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