From mboxrd@z Thu Jan 1 00:00:00 1970 From: "David S. Ahern" Subject: Re: [kvm-devel] performance with guests running 2.4 kernels (specifically RHEL3) Date: Mon, 23 Jun 2008 08:09:24 -0600 Message-ID: <485FAE94.4040001@cisco.com> References: <48054518.3000104@cisco.com> <480EEDA0.3080209@qumranet.com> <480F546C.2030608@cisco.com> <481215DE.3000302@cisco.com> <20080428181550.GA3965@dmt> <4816617F.3080403@cisco.com> <4817F30C.6050308@cisco.com> <48184228.2020701@qumranet.com> <481876A9.1010806@cisco.com> <48187903.2070409@qumranet.com> <4826E744.1080107@qumranet.com> <4826F668.6030305@qumranet.com> <48290FC2.4070505@cisco.com> <48294272.5020801@qumranet.com> <482B4D29.7010202@cisco.com> <482C1633.5070302@qumranet.com> <482E5F9C.6000207@cisco.com> <482FCEE1.5040306@qumranet.com> <4830F90A.1020809@cisco.com> <4830FE8D.6010006@cisco.com> <48318E64.8090706@qumranet.com> <4832DDEB.4000100@qumranet.com> <4835EEF5.9010600@cisco.com> <483D391F.7050007@qumranet.com> <483EDCEE.6070307@cisco.com> <4841094A.8090507@qumranet.com> <484422EE.5090501@cisco.com> <4847A5B8.6020503@qumranet.com> <48481250.6060005@cisco.com> <4849686E.8000402@qumranet.com> <4859DEA6.5010503@cisco.com> <485DF25E.6060400@qumranet.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Cc: kvm@vger.kernel.org To: Avi Kivity Return-path: Received: from rtp-iport-2.cisco.com ([64.102.122.149]:7190 "EHLO rtp-iport-2.cisco.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751583AbYFWOTM (ORCPT ); Mon, 23 Jun 2008 10:19:12 -0400 In-Reply-To: <485DF25E.6060400@qumranet.com> Sender: kvm-owner@vger.kernel.org List-ID: Avi Kivity wrote: > David S. Ahern wrote: >> Avi: >> >> We did not get a chance to do this at the Forum. I'd be interested in >> whatever options you have for reducing the scan time further (e.g., try >> to get scan time down to 1-2 seconds). >> >> > > I'm unlikely to get time to do this properly for at least a week, as > this will be quite difficult and I'm already horribly backlogged. > However there's an alternative option, modifying the source and getting > it upstreamed, as I think RHEL 3 is still maintained. > > The attached patch (untested) should give a 3X boost for kmap_atomics, > by folding the two accesses to set the pte into one, and by dropping the > access that clears the pte. Unfortunately it breaks the ABI, since > external modules will inline the original kmap_atomic() which expects > the pte to be cleared. > > This can be worked around by allocating new fixmap slots for kmap_atomic > with the new behavior, and keeping the old slots with the old behavior, > but we should first see if the maintainers are open to performance > optimizations targeting kvm. > RHEL3 is in Maintenance mode (for an explanation see http://www.redhat.com/security/updates/errata/) which means performance enhancement patches will not make it in. Also, I'm going to be out of the office for a couple of weeks in July, so I will need to put this aside until mid-August or so. I'll reevaluate options then. david