From mboxrd@z Thu Jan 1 00:00:00 1970 From: Konrad Rzeszutek Wilk Subject: Re: [PATCH] docs: Make note for the scheduler "cap" option warning about power management effects Date: Fri, 14 Jun 2013 14:38:10 -0400 Message-ID: <20130614183810.GB21280@phenom.dumpdata.com> References: <1370953898-10278-1-git-send-email-george.dunlap@eu.citrix.com> <20130612135710.GI2918@phenom.dumpdata.com> <1371049188.24512.451.camel@zakaz.uk.xensource.com> <51B88D38.6030601@eu.citrix.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: Content-Disposition: inline In-Reply-To: <51B88D38.6030601@eu.citrix.com> List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Sender: xen-devel-bounces@lists.xen.org Errors-To: xen-devel-bounces@lists.xen.org To: George Dunlap Cc: Ian Jackson , Massimo Canonico , Ian Campbell , xen-devel@lists.xen.org List-Id: xen-devel@lists.xenproject.org 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.