Re: [Xen-devel] [PATCH] docs: Make note for the scheduler "cap" option warning about power management effects

[George Dunlap <george.dunlap@xxxxxxxxxxxxx>]

On 12/06/13 15:59, Ian Campbell wrote:
> On Wed, 2013-06-12 at 09:57 -0400, Konrad Rzeszutek Wilk wrote:
>
> > > +NB: 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, if your
> > > +processor runs at 2GHz, and you cap a vm at 50%, the power management
> > > +system may also reduce the clock speed to 1GHz; the effect will be
> > > +that your VM gets 25% of the available power (50% of 1GHz) rather than
> > > +50% (50% of 2GHz).  If you are not getting the performance you expect,
> > > +look at performance and cpufreq options in your operating system and
> > > +your BIOS.
> > Or .. use 'cpufreq=xen:performance' ?
> >
> > That should set it to the highest P state.
> I committed this already. Assuming this is a good suggestion can we get
> an incremental patch please.
Might that kind of thing be better on the wiki page?

 -George


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