From mboxrd@z Thu Jan 1 00:00:00 1970 From: Xiao Guangrong Subject: Re: [PATCH v2 19/22] KVM: MMU: lockless walking shadow page table Date: Wed, 29 Jun 2011 20:39:04 +0800 Message-ID: <4E0B1CE8.2060803@cn.fujitsu.com> References: <4E01FBC9.3020009@cn.fujitsu.com> <4E01FDB4.60306@cn.fujitsu.com> <4E0AED86.2060908@redhat.com> <4E0B0997.4090206@cn.fujitsu.com> <4E0B09EA.1040304@redhat.com> <4E0B1191.6040100@cn.fujitsu.com> <4E0B182C.1090402@redhat.com> <4E0B1A75.4070705@cn.fujitsu.com> <4E0B1A4E.2030301@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Cc: Marcelo Tosatti , LKML , KVM To: Avi Kivity Return-path: In-Reply-To: <4E0B1A4E.2030301@redhat.com> Sender: linux-kernel-owner@vger.kernel.org List-Id: kvm.vger.kernel.org On 06/29/2011 08:27 PM, Avi Kivity wrote: > On 06/29/2011 03:28 PM, Xiao Guangrong wrote: >> On 06/29/2011 08:18 PM, Avi Kivity wrote: >> > On 06/29/2011 02:50 PM, Xiao Guangrong wrote: >> >> >> > >> >> >> > I think we should do this unconditionally. The cost of ping-ponging the shared cache line containing reader_counter will increase with large smp counts. On the other hand, zap_page is very rare, so it can be a little slower. Also, less code paths = easier to understand. >> >> >> > >> >> >> >> >> >> On soft mmu, zap_page is very frequently, it can cause performance regression in my test. >> >> > >> >> > Any idea what the cause of the regression is? It seems to me that simply deferring freeing shouldn't have a large impact. >> >> > >> >> >> >> I guess it is because the page is freed too frequently, i have done the test, it shows >> >> about 3219 pages is freed per second >> >> >> >> Kernbench performance comparing: >> >> >> >> the origin way: 3m27.723 >> >> free all shadow page in rcu context: 3m30.519 >> > >> > I don't recall seeing such a high free rate. Who is doing all this zapping? >> > >> > You may be able to find out with the function tracer + call graph. >> > >> >> I looked into it before, it is caused by "write flood" detected, i also noticed >> some pages are zapped and allocation again and again, maybe we need to improve >> the algorithm of detecting "write flood". > > Ok. Let's drop the two paths, and put this improvement on the TODO instead. > Avi, i am sorry, i do not understand it clearly, it means keep the patch as the original way and do the improvement after it merged?