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

On Wed, Jun 12, 2013 at 04:01:12PM +0100, George Dunlap wrote:
> 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?

Perhaps. But since the docs talk about 'your operating system
and your BIOS' I figured it should also mention how to configure Xen
to pretty much ignore any P-states and just sit at P0 all the time.

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