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

On mar, 2013-06-11 at 13:31 +0100, George Dunlap wrote:
> Suggested-by: Massimo Canonico <mex@xxxxxxxxxxxx>
> Signed-off-by: George Dunlap <george.dunlap@xxxxxxxxxxxxx>
> CC: Ian Campbell <ian.campbell@xxxxxxxxxx>
> CC: Ian Jackson <ian.jackson@xxxxxxxxxx>
> CC: Massimo Canonico <mex@xxxxxxxxxxxx>
Reviewed-by: Dario Faggioli <dario.faggioli@xxxxxxxxxx>

> ---
>  docs/man/xl.cfg.pod.5 |   13 +++++++++++++
>  docs/man/xl.pod.1     |   13 +++++++++++++
>  docs/man/xm.pod.1     |   13 +++++++++++++
>  3 files changed, 39 insertions(+)
> diff --git a/docs/man/xl.cfg.pod.5 b/docs/man/xl.cfg.pod.5
> index b7d64a6..069b73f 100644
> --- a/docs/man/xl.cfg.pod.5
> +++ b/docs/man/xl.cfg.pod.5
> @@ -153,6 +153,19 @@ The cap is expressed in percentage of one physical CPU:
>  The default, 0, means there is no upper cap.
>  Honoured by the credit and credit2 schedulers.
> +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.
> +
I'd have been less 'politically correct' toward BIOSes, but I guess
that's me! :-P


<<This happens because I choose it to happen!>> (Raistlin Majere)
Dario Faggioli, Ph.D, http://about.me/dario.faggioli
Senior Software Engineer, Citrix Systems R&D Ltd., Cambridge (UK)

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