Re: [PATCH] emulate accessed bit for EPT

[Balbir Singh <balbir@linux.vnet.ibm.com>]

* Rik van Riel <riel@redhat.com> [2010-02-04 08:40:43]:

> 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 :)
>

I do agree with you. 

-- 
	Balbir