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

From: Andrea Arcangeli <aarcange@redhat.com>
To: Nai Xia <nai.xia@gmail.com>
Cc: Rik van Riel <riel@redhat.com>,
	Izik Eidus <izik.eidus@ravellosystems.com>,
	Avi Kivity <avi@redhat.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Hugh Dickins <hughd@google.com>,
	Chris Wright <chrisw@sous-sol.org>, linux-mm <linux-mm@kvack.org>,
	Johannes Weiner <hannes@cmpxchg.org>,
	linux-kernel <linux-kernel@vger.kernel.org>,
	kvm <kvm@vger.kernel.org>
Subject: Re: [PATCH] mmu_notifier, kvm: Introduce dirty bit tracking in spte and mmu notifier to help KSM dirty bit tracking
Date: Thu, 23 Jun 2011 01:59:06 +0200	[thread overview]
Message-ID: <20110622235906.GC20843@redhat.com> (raw)
In-Reply-To: <BANLkTinRYr9Vg==C-qyCaRmO7C_aQqBPzw@mail.gmail.com>

On Thu, Jun 23, 2011 at 07:37:47AM +0800, Nai Xia wrote:
> On 2MB pages, I'd like to remind you and Rik that ksmd currently splits
> huge pages before their sub pages gets really merged to stable tree.
> So when there are many 2MB pages each having a 4kB subpage
> changed for all time, this is already a concern for ksmd to judge
> if it's worthwhile to split 2MB page and get its sub-pages merged.

Hmm not sure to follow. KSM memory density with THP on and off should
be identical. The cksum is computed on subpages so the fact the 4k
subpage is actually mapped by a hugepmd is invisible to KSM up to the
point we get a unstable_tree_search_insert/stable_tree_search lookup
succeeding.

> I think the policy for ksmd in a system should be "If you cannot do sth good,
> at least do nothing evil". So I really don't think we can satisfy _all_ people.
> Get a general method and give users one or two knobs to tune it when they
> are the corner cases. How do  you think of my proposal ?

I'm neutral, but if we get two methods for deciding the unstable tree
candidates, the default probably should prioritize on maximum merging
even if it takes more CPU (if one cares about performance of the core
dedicated to ksmd, KSM is likely going to be off or scanning at low
rate in the first place).

> > On a side note, khugepaged should also be changed to preserve the
> > dirty bit if at least one dirty bit of the ptes is dirty (currently
> > the hugepmd is always created dirty, it can never happen for an
> > hugepmd to be clean today so it wasn't preserved in khugepaged so far).
> >
> 
> Thanks for the point that out. This is what I have overlooked!

No prob. And its default scan rate is very slow compared to ksmd so
it was unlikely to generate too many false positive dirty bits even if
you were splitting hugepages through swap.