From mboxrd@z Thu Jan 1 00:00:00 1970 From: Rik van Riel Subject: Re: [PATCH] emulate accessed bit for EPT Date: Thu, 04 Feb 2010 08:40:43 -0500 Message-ID: <4B6ACE5B.9080601@redhat.com> References: <20100203161103.11e2b572@annuminas.surriel.com> <20100204041212.GI19641@balbir.in.ibm.com> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Cc: jdike@addtoit.com, kvm@vger.kernel.org, linux-kernel@vger.kernel.org, avi@redhat.com, aarcange@redhat.com, mtosatti@redhat.com To: balbir@linux.vnet.ibm.com Return-path: In-Reply-To: <20100204041212.GI19641@balbir.in.ibm.com> Sender: linux-kernel-owner@vger.kernel.org List-Id: kvm.vger.kernel.org On 02/03/2010 11:12 PM, Balbir Singh wrote: > * Rik van Riel [2010-02-03 16:11:03]: > >> Currently KVM pretends that pages with EPT mappings never got >> accessed. This has some side effects in the VM, like swapping >> out actively used guest pages and needlessly breaking up actively >> used hugepages. >> >> We can avoid those very costly side effects by emulating the >> accessed bit for EPT PTEs, which should only be slightly costly >> because pages pass through page_referenced infrequently. > Quite a clever implementation, one side effect is that one would see a > larger number of minor faults with EPT enabled and an increase in > allocation/frees of rmap entries, but that can be easily explained. I suspect it won't be very many. I have been monitoring /proc/meminfo on my system while testing this patch, and it is quite typical that the size of the inactive anon list does not change for minutes at a time. In other words, no pages are moved onto or off of the inactive anon list for several minutes. That corresponds to a very small number of minor faults introduced by my patch. Of course, when the system is swapping, we will have more minor faults. However, minor faults should be less of a performance issue than major faults :) -- All rights reversed.