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

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Message-ID: <4B6AEA9A.90704@redhat.com>
Date:	Thu, 04 Feb 2010 10:41:14 -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

Balbir Singh wrote:
> * Rik van Riel <riel@...hat.com> [2010-02-04 08:40:43]:
> 
>> 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 :)
>>
> 
> I do agree with you. 

After 20 hours of uptime, it appears that this patch has
resolved the "KVM guests get swapped while buffer and page
cache stay in memory" problem my home system was experiencing.
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