Re: [PATCH] mmu_notifier, kvm: Introduce dirty bit tracking in spteand mmu notifier to help KSM dirty bit tracking

From: Nai Xia
Date: Wed Jun 22 2011 - 19:13:59 EST

On Wed, Jun 22, 2011 at 11:39 PM, Rik van Riel <riel@xxxxxxxxxx> wrote:
> On 06/22/2011 07:19 AM, Izik Eidus wrote:
>> So what we say here is: it is better to have little junk in the unstable
>> tree that get flushed eventualy anyway, instead of make the guest
>> slower....
>> this race is something that does not reflect accurate of ksm anyway due
>> to the full memcmp that we will eventualy perform...
> With 2MB pages, I am not convinced they will get "flushed eventually",
> because there is a good chance at least one of the 4kB pages inside
> a 2MB page is in active use at all times.
> I worry that the proposed changes may end up effectively preventing
> KSM from scanning inside 2MB pages, when even one 4kB page inside
> is in active use.  This could mean increased swapping on systems
> that run low on memory, which can be a much larger performance penalty
> than ksmd CPU use.
> We need to scan inside 2MB pages when memory runs low, regardless
> of the accessed or dirty bits.

I agree on this point. Dirty bit , young bit, is by no means accurate. Even
on 4kB pages, there is always a chance that the pte are dirty but the contents
are actually the same. Yeah, the whole optimization contains trade-offs and
trades-offs always have the possibilities to annoy someone. Just like
page-bit-relying LRU approximations none of them is perfect too. But I think
it can benefit some people. So maybe we could just provide a generic balanced
solution but provide fine tuning interfaces to make sure tha when it really gets
in the way of someone, he has a way to walk around.
Do you agree on my argument? :-)

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