From mboxrd@z Thu Jan 1 00:00:00 1970 From: Ingo Molnar Subject: Re: [PATCH] raise tsc clocksource rating Date: Tue, 30 Oct 2007 00:17:02 +0100 Message-ID: <20071029231702.GA8506@elte.hu> References: <11936994092607-git-send-email-gcosta@redhat.com> <1193697734.9793.86.camel@bodhitayantram.eng.vmware.com> <20071029224852.GA27547@elte.hu> <1193698505.9793.90.camel@bodhitayantram.eng.vmware.com> <20071029230213.GA1982@elte.hu> <1193699638.9793.97.camel@bodhitayantram.eng.vmware.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Cc: jeremy-TSDbQ3PG+2Y@public.gmane.org, avi-i/T1Zxg06OZWk0Htik3J/w@public.gmane.org, kvm-devel-5NWGOfrQmneRv+LV9MX5uipxlwaOVQ5f@public.gmane.org, Dan Hecht , linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, Glauber de Oliveira Costa , Glauber de Oliveira Costa , --cc-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org, Garrett Smith To: Zachary Amsden Return-path: Content-Disposition: inline In-Reply-To: <1193699638.9793.97.camel-cxY/u30q8FloTgUnLF1by8fTvwmfpRNyZeezCHUQhQ4@public.gmane.org> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: kvm-devel-bounces-5NWGOfrQmneRv+LV9MX5uipxlwaOVQ5f@public.gmane.org Errors-To: kvm-devel-bounces-5NWGOfrQmneRv+LV9MX5uipxlwaOVQ5f@public.gmane.org List-Id: kvm.vger.kernel.org * Zachary Amsden wrote: > On Tue, 2007-10-30 at 00:02 +0100, Ingo Molnar wrote: > > * Zachary Amsden wrote: > > > > Not every guest support paravirt, but for correctness, all guests > > > require TSC, which must be exposed all the way up to userspace, no > > > matter what the efficiency or accuracy may be. > > > > but if there's a perfect TSC available (there is such hardware) then the > > TSC _is_ the best clocksource. Paravirt now turns it off unconditionally > > in essence. > > No, if no paravirt clocksource is detected, nothing can override the > perfect TSC hardware clocksource rating of 400. And if a paravirt > clocksource is detected, it is always better than TSC. > > > anyway, that's at most an optimization issue. No strong feelings here, > > and we can certainly delay this patch until this gets all sorted out. > > This patch should be nacked, since it is just wrong. This is not an > optimization issue. It is an accuracy issue for all virtualization > environments that affects long term kernel clock stability, which is > important to fix, and the best way to do that is to use a paravirt > clocksource. i know it's not an optimization issue. Your current pessimisation of even perfect TSCs _is_ an optimization issue. (and note that if the TSC is unstable the guest will/should notice it and will fall back to the hyper clocksource) Ingo ------------------------------------------------------------------------- This SF.net email is sponsored by: Splunk Inc. Still grepping through log files to find problems? Stop. Now Search log events and configuration files using AJAX and a browser. Download your FREE copy of Splunk now >> http://get.splunk.com/