From mboxrd@z Thu Jan  1 00:00:00 1970
From: George Dunlap <george.dunlap@eu.citrix.com>
Subject: Re: [PATCH] docs: Make note for the scheduler "cap"
 option warning about power management effects
Date: Wed, 12 Jun 2013 16:01:12 +0100
Message-ID: <51B88D38.6030601@eu.citrix.com>
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To: Ian Campbell <Ian.Campbell@citrix.com>
Cc: Ian Jackson <ian.jackson@citrix.com>, xen-devel@lists.xen.org, Massimo Canonico <mex@di.unipmn.it>, Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
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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