Re: Cyclictest usage

[Gregory Haskins <ghaskins@novell.com>]

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Hi Tobias,

  Some more questions (inline)

Tobias Knutsson wrote:
> 2.6.25.8-rt7 and 2.6.25.8
>
> On Mon, Aug 11, 2008 at 14:48, Gregory Haskins <ghaskins@novell.com> wrote:
>   
>> Tobias Knutsson wrote:
>>     
>>> Hello,
>>>
>>> I'm trying to do some initial benchmarking on a rt-kernel versus a
>>> vanilla kernel. On the rt the results are stable in between test runs
>>> using the following command:
>>>
>>> cyclictest -t 1 -p 80 -c 1 -n -i 10000 -l 10000
>>>       

What architecture is this?  And what do you see for ave/max in the 
cyclictest output on -rt (so we have a reference)


>>> However, using the same command while running the vanilla kernel, I
>>> get very varying results. And I would completely understand if it was
>>> due to increased jitter. The thing is, the jitter is very low (0%
>>> load) but the average times varies from 600 us to 6500 us depending on
>>> when the test is initiated.
>>>       

What are you considering "jitter" when you say it is very low?  Are the 
600-6500us values truly from the "ave" output of cyclictest, or are 
these an average of the "max" output?


>>> I imagine this might have something to do with not having high res
>>> timers in the vanilla kernel.

I will defer to Thomas, but my understanding is that 2.6.25 vanilla 
should have HRT as an option (though I am not sure if you enabled it or 
not).

>>>  If that is what's causing my problems.
>>>       

Cyclictest will report that it cannot access the HRT when it cannot find 
it (at least it does on my system, but I might have an older version 
running).  Do you see any such warnings?

>>> What is the best way of comparing the two kernels?
>>>       

This is a pretty good way, as long as you have HRT in the vanilla 
kernel.  Assuming for a second that you do, your results are showing you 
what I would expect to see for vanilla (600-6500us spikes).

HTH

-Greg


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