Re: [Xen-devel] CAP and performance problem

[Massimo Canonico <mex@xxxxxxxxxxxx>]

On 06/06/2013 06:27 PM, George Dunlap wrote:
> On 06/06/13 16:54, Dario Faggioli wrote:
> > On gio, 2013-06-06 at 17:42 +0200, Dario Faggioli wrote:
> > > Jokes apart, we may want to add a line there, perhaps on the Wiki, 
> > > but I
> > > honestly don't think this is by any means the cap mechanism's. :-)
> > ^fault
> >
> > What I meant is: I don't think this can be considered cap's fault.
> It's certainly not cap's fault, but it wouldn't hurt to just put a 
> comment saying something like,
> "NB that many systems have features that will scale down the computing 
> power of a cpu that is not 100% utilized.  This can be in the 
> operating system, but can also sometimes be below the operating system 
> in the BIOS.  If you set a cap such that individual cores are running 
> at less than 100%, this may have an impact on the performance of your 
> workload over and above the impact of the cap. For example, a workload 
> that may take 300s with a cap of 100% may take 900s with a cap of 50%, 
> because in addition to being run only 50% of the time, the cpu is 
> running in a lower state and so the 50% you get is less powerful.  If 
> this is a problem, look at performance and cpufreq options in our 
> operating system and your BIOS."
>  -George
Thanks George, this is exactly what I meant and I think it is quite 
important to put this "warning" in evidence.
M


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