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

[Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>]

On Tue, Jun 11, 2013 at 01:31:38PM +0100, George Dunlap wrote:
> Suggested-by: Massimo Canonico <mex@di.unipmn.it>
> Signed-off-by: George Dunlap <george.dunlap@eu.citrix.com>
> CC: Ian Campbell <ian.campbell@citrix.com>
> CC: Ian Jackson <ian.jackson@citrix.com>
> CC: Massimo Canonico <mex@di.unipmn.it>
> ---
>  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.

Or .. use 'cpufreq=xen:performance' ?

That should set it to the highest P state.

> +
>  =item B<period=NANOSECONDS>
>  
>  The normal EDF scheduling usage in nanoseconds. This means every period
> diff --git a/docs/man/xl.pod.1 b/docs/man/xl.pod.1
> index 57c6a79..0e2fe65 100644
> --- a/docs/man/xl.pod.1
> +++ b/docs/man/xl.pod.1
> @@ -848,6 +848,19 @@ is expressed in percentage of one physical CPU: 100 is 1 physical CPU,
>  50 is half a CPU, 400 is 4 CPUs, etc. The default, 0, means there is
>  no upper cap.
>  
> +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.
> +
>  =item B<-p CPUPOOL>, B<--cpupool=CPUPOOL>
>  
>  Restrict output to domains in the specified cpupool.
> diff --git a/docs/man/xm.pod.1 b/docs/man/xm.pod.1
> index 7c4ef85..4d47388 100644
> --- a/docs/man/xm.pod.1
> +++ b/docs/man/xm.pod.1
> @@ -767,6 +767,19 @@ is expressed in percentage of one physical CPU: 100 is 1 physical CPU,
>  50 is half a CPU, 400 is 4 CPUs, etc. The default, 0, means there is
>  no upper cap.
>  
> +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.
> +
>  =back
>  
>  =item B<sched-sedf> I<period> I<slice> I<latency-hint> I<extratime> I<weight>
> -- 
> 1.7.9.5
> 
> 
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