linux-kernel - Re: [PATCH] mmu_notifier, kvm: Introduce dirty bit tracking in spte and mmu notifier to help KSM dirty bit tracking

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Date:	Thu, 23 Jun 2011 09:30:33 +0800
From:	Nai Xia <nai.xia@...il.com>
To:	Andrea Arcangeli <aarcange@...hat.com>
Cc:	Rik van Riel <riel@...hat.com>,
	Izik Eidus <izik.eidus@...ellosystems.com>,
	Avi Kivity <avi@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Hugh Dickins <hughd@...gle.com>,
	Chris Wright <chrisw@...s-sol.org>,
	linux-mm <linux-mm@...ck.org>,
	Johannes Weiner <hannes@...xchg.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	kvm <kvm@...r.kernel.org>
Subject: Re: [PATCH] mmu_notifier, kvm: Introduce dirty bit tracking in spte
 and mmu notifier to help KSM dirty bit tracking

On Thu, Jun 23, 2011 at 7:25 AM, Andrea Arcangeli <aarcange@...hat.com> wrote:
> On Thu, Jun 23, 2011 at 07:13:54AM +0800, Nai Xia wrote:
>> 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
>
> Just a side note: the fact the dirty bit would be set even when the
> data is the same is actually a pros, not a cons. If the content is the
> same but the page was written to, it'd trigger a copy on write short
> after merging the page rendering the whole exercise wasteful. The
> cksum plays a double role, it both "stabilizes" the unstable tree, so
> there's less chance of bad lookups, but it also avoids us to merge
> stuff that is written to frequently triggering copy on writes, and the
> dirty bit would also catch overwrites with the same data, something
> the cksum can't do.

Good point. I actually have myself another version of ksm(off topic, but
if you want to take a glance: http://code.google.com/p/uksm/ :-) )
that did do statistics of the ratio of the pages in a VMA that really got COWed.
due to KSM merging on each scan round basis.

It's  complicated to deduce a precise  information only
from the dirty and cksum.


Thanks,
Nai
>
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