linux-kernel - Re: [PATCH] emulate accessed bit for EPT

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Message-ID: <4B6ACE5B.9080601@redhat.com>
Date:	Thu, 04 Feb 2010 08:40:43 -0500
From:	Rik van Riel <riel@...hat.com>
To:	balbir@...ux.vnet.ibm.com
CC:	jdike@...toit.com, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org, avi@...hat.com, aarcange@...hat.com,
	mtosatti@...hat.com
Subject: Re: [PATCH] emulate accessed bit for EPT

On 02/03/2010 11:12 PM, Balbir Singh wrote:
> * Rik van Riel<riel@...hat.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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