From mboxrd@z Thu Jan 1 00:00:00 1970 From: Anthony Liguori Subject: Re: KVM Processor cache size Date: Mon, 02 Aug 2010 18:36:13 -0500 Message-ID: <4C57566D.7000007@codemonkey.ws> References: <4C56BF6F.9040402@amd.com> <4C56CCFB.204@redhat.com> <4C574559.40509@codemonkey.ws> <4C5749C6.8010901@amd.com> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Cc: Ulrich Drepper , Ricardo Martins , "kvm@vger.kernel.org" To: Andre Przywara Return-path: Received: from mail-qy0-f174.google.com ([209.85.216.174]:61665 "EHLO mail-qy0-f174.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751727Ab0HBXgS (ORCPT ); Mon, 2 Aug 2010 19:36:18 -0400 Received: by qyk7 with SMTP id 7so126766qyk.19 for ; Mon, 02 Aug 2010 16:36:18 -0700 (PDT) In-Reply-To: <4C5749C6.8010901@amd.com> Sender: kvm-owner@vger.kernel.org List-ID: On 08/02/2010 05:42 PM, Andre Przywara wrote: > Anthony Liguori wrote: >> On 08/02/2010 08:49 AM, Ulrich Drepper wrote: >>> glibc uses the cache size information returned by cpuid to perform >>> optimizations. For instance, copy operations which would pollute too >>> much of the cache because they are large will use non-temporal >>> instructions. There are real performance benefits. >> >> I imagine that there would be real performance problems from doing >> live migration with -cpu host too if we don't guarantee these values >> remain stable across migration... > Again, -cpu host is not meant to be migrated. Then it needs to prevent migration from happening. Otherwise, it's a bug waiting to happen. > There are other virtualization use cases than cloud-like server > virtualization. Sometimes users don't care about migration (or even > the live version), but want full CPU exposure for performance reasons > (think of virtualizing Windows on a Linux desktop). > I agree that -cpu host and migration should be addressed, but only to > a certain degree. And missing migration experience should not be a > road blocker for -cpu host. When we can reasonably prevent it, we should prevent users from shooting themselves in the foot. Honestly, I think -cpu host is exactly what you would want to use in a cloud. A lot of private clouds and even public clouds are largely based on homogenous hardware. I actually think the case where you want to migrate between heterogenous hardware is grossly overstated. Regards, Anthony Liguori > > Regards, > Andre. >