Message-ID: <4B6ACE5B.9080601@redhat.com>
Date: Thu, 04 Feb 2010 08:40:43 -0500
From: Rik van Riel <riel@redhat.com>
Organization: Red Hat, Inc
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To: balbir@linux.vnet.ibm.com
CC: jdike@addtoit.com, kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
       avi@redhat.com, aarcange@redhat.com, mtosatti@redhat.com
Subject: Re: [PATCH] emulate accessed bit for EPT
References: <20100203161103.11e2b572@annuminas.surriel.com> <20100204041212.GI19641@balbir.in.ibm.com>
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On 02/03/2010 11:12 PM, Balbir Singh wrote:
> * Rik van Riel<riel@redhat.com>  [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 :)

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