From mboxrd@z Thu Jan 1 00:00:00 1970 From: Avi Kivity Subject: Re: [kvm-devel] performance with guests running 2.4 kernels (specifically RHEL3) Date: Mon, 19 May 2008 17:27:48 +0300 Message-ID: <48318E64.8090706@qumranet.com> References: <48054518.3000104@cisco.com> <4805BCF1.6040605@qumranet.com> <4807BD53.6020304@cisco.com> <48085485.3090205@qumranet.com> <480C188F.3020101@cisco.com> <480C5C39.4040300@qumranet.com> <480E492B.3060500@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> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Cc: kvm@vger.kernel.org To: "David S. Ahern" Return-path: Received: from bzq-179-150-194.static.bezeqint.net ([212.179.150.194]:24382 "EHLO il.qumranet.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756682AbYESO1w (ORCPT ); Mon, 19 May 2008 10:27:52 -0400 In-Reply-To: <4830FE8D.6010006@cisco.com> Sender: kvm-owner@vger.kernel.org List-ID: David S. Ahern wrote: >> [dsa] No. I saw the same problem with the flood count at 5. The >> attachment in the last email shows kvm_stat data during a kscand event. >> The data was collected with the patch you posted. With the flood count >> at 3 the mmu cache/flood counters are in the 18,000/sec and pte updates >> at ~50,000/sec and writes at 70,000/sec. With the flood count at 5 >> mmu_cache/flood drops to 0 and pte updates and writes both hit >> 180,000+/second. In both cases these last for 30 seconds or more. I only >> included data for the onset as it's pretty flat during the kscand activity. >> It makes sense. We removed a flooding false positive, and introduced a false negative. The guest access sequence is: - point kmap pte at page table - use the new pte to access the page table Prior to the patch, the mmu didn't see the 'use' part, so it concluded the kmap pte would be better off unshadowed. This shows up as a high flood count. After the patch, this no longer happens, so the sequence can repreat for long periods. However the pte that is the result of the 'use' part is never accessed, so it should be detected as flooded! But our flood detection mechanism looks at one page at a time (per vcpu), while there are two pages involved here. There are (at least) three options available: - detect and special-case this scenario - change the flood detector to be per page table instead of per vcpu - change the flood detector to look at a list of recently used page tables instead of the last page table I'm having a hard time trying to pick between the second and third options. -- error compiling committee.c: too many arguments to function