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

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Message-ID: <20100204153013.GK19641@balbir.in.ibm.com>
Date:	Thu, 4 Feb 2010 21:00:13 +0530
From:	Balbir Singh <balbir@...ux.vnet.ibm.com>
To:	Rik van Riel <riel@...hat.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

* 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. 

-- 
	Balbir
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