From mboxrd@z Thu Jan 1 00:00:00 1970 From: Daniel Veillard Subject: Re: paravirtualization & cr3-cache feature Date: Mon, 12 Feb 2007 09:03:59 -0500 Message-ID: <20070212140359.GE1879@redhat.com> References: <64F9B87B6B770947A9F8391472E032160A66BE0A@ehost011-8.exch011.intermedia.net> <20070212130644.GF25460@amd.com> <64F9B87B6B770947A9F8391472E032160A66C0A6@ehost011-8.exch011.intermedia.net> Reply-To: veillard-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Cc: kvm-devel-5NWGOfrQmneRv+LV9MX5uipxlwaOVQ5f@public.gmane.org To: Dor Laor Return-path: Content-Disposition: inline In-Reply-To: <64F9B87B6B770947A9F8391472E032160A66C0A6-yEcIvxbTEBqsx+V+t5oei8rau4O3wl8o3fe8/T/H7NteoWH0uzbU5w@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 On Mon, Feb 12, 2007 at 05:21:26AM -0800, Dor Laor wrote: > >> > >> >hi, > >> > Can someone briefly explain what the cr3-cache feature on intel > VT > >> >processors is and how it is used to improve performance in > >> >paravirtualization. > >> > >> In short, this is a unique Intel VT feature that supposed to save > vmexit > >> on cr3 switch. The vmexit operation is quite expensive at the moment > >> (something like 2-5 ms!). PV kernels can use this feature by sharing > > ^^^^^^^^^^^^^^^^^^^^^^^^ > > > >Where do you measured this unbelievable large number? On AMD-V I > >measured around 2400 processor cycles for a VMEXIT/VMENTRY roundtrip. I > >would really really wonder if Intel-VT is that slow... > > Intel is indeed slower than AMD with this one. > I didn't measure such roundtrip specifically. We usually see that the > VMETNRY/VMEXIT cycle limits our performance (for io/mmio,..) > On Intel core duo we cannot get more than 150k-200k VMEXITS per second > for regular guest doing extensive IO. > > Anyway 2ms for a 2.4G processor equals 4800 cycles so it's not that far. Hum, ms means millisecond in the SI system, so on a 2.4 billion cycle/s 2ms would be 4.8 million cycles, hence IMHO the unbelievably large perception. I assume you mean VMEXITS would be around 2-5 microsecond, which would fit quite better in the overall picture :-) Daniel -- Red Hat Virtualization group http://redhat.com/virtualization/ Daniel Veillard | virtualization library http://libvirt.org/ veillard-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org | libxml GNOME XML XSLT toolkit http://xmlsoft.org/ http://veillard.com/ | Rpmfind RPM search engine http://rpmfind.net/ ------------------------------------------------------------------------- Using Tomcat but need to do more? Need to support web services, security? Get stuff done quickly with pre-integrated technology to make your job easier. Download IBM WebSphere Application Server v.1.0.1 based on Apache Geronimo http://sel.as-us.falkag.net/sel?cmd=lnk&kid=120709&bid=263057&dat=121642