Re: [Xen-devel] CAP and performance problem

[George Dunlap <george.dunlap@xxxxxxxxxxxxx>]

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

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