From mboxrd@z Thu Jan 1 00:00:00 1970 From: Michael Tokarev Subject: Re: 64 bit guest much faster ? Date: Mon, 26 Oct 2009 13:42:12 +0300 Message-ID: <4AE57D04.10408@msgid.tls.msk.ru> References: <4AE1D1A4.6050001@pms.ifi.lmu.de> <4AE564C6.3040605@redhat.com> <4AE575FD.6010801@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-15; format=flowed Content-Transfer-Encoding: 7bit Cc: Gerd Hoffmann , Stefan , kvm@vger.kernel.org To: Avi Kivity Return-path: Received: from isrv.corpit.ru ([81.13.33.159]:58191 "EHLO isrv.corpit.ru" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755517AbZJZKmK (ORCPT ); Mon, 26 Oct 2009 06:42:10 -0400 In-Reply-To: <4AE575FD.6010801@redhat.com> Sender: kvm-owner@vger.kernel.org List-ID: Avi Kivity wrote: > On 10/26/2009 10:58 AM, Gerd Hoffmann wrote: >> On 10/23/09 17:54, Stefan wrote: >>> >>> Hello, >>> >>> I have a simple question (sorry I'm a kvm beginner): >>> Is it right that a 64bit guest (8 CPUs, 16GB) is >>> much faster than a 32bit guest (8 CPUs, 16GB PAE). >> ^^^^ >> Yes. With *that* much memory the 32bit guest struggles with address >> space limitations (32bit -> 4G), whereas the 64bit guest doesn't. >> >> With up to 1G you shouldn't see a noticable difference. But the more >> highmem the 32bit guest uses the higher is the penalty. Especially >> without ept/npt as every kmap() of a high page is a roundtrip to the >> hypervisor then. > > Oh yes, without ept/npt the slowdown should indeed be significant with > this much memory. How it is with 4Gb guest/mem without PAE (I mean, with CONFIG_HIGHMEM_4G=y)? Or even 2Gb? In case of npt or without. Can we construct a sort of a table of expected slowdowns (not in numbers but just in terms "significant", "minor" etc) of running <4Gb or >4Gb (and <1Gb and >1Gb if that makes significant diffencece) 32bit guests with and without npt and 64bit guests, please? I guess it's quite interesting to many users. From the above it looks like it's better to run 64bit kernel in the 32bit guest in these situations too. I haven't measured it, just because it never occured to me that there MAY be any difference. But I've only non-npt hardware here at the moment. Thanks! /mjt