linux-kernel - Re: [PATCH 8/8] Add VM counters for transparent hugepages

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Date:	Tue, 22 Feb 2011 19:02:34 +0100
From:	Andi Kleen <andi@...stfloor.org>
To:	Andrea Arcangeli <aarcange@...hat.com>
Cc:	Dave Hansen <dave@...ux.vnet.ibm.com>,
	Andi Kleen <andi@...stfloor.org>, akpm@...ux-foundation.org,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	lwoodman@...hat.com, Andi Kleen <ak@...ux.intel.com>
Subject: Re: [PATCH 8/8] Add VM counters for transparent hugepages

On Tue, Feb 22, 2011 at 05:43:31PM +0100, Andrea Arcangeli wrote:
> On Tue, Feb 22, 2011 at 08:36:26AM -0800, Dave Hansen wrote:
> > On Mon, 2011-02-21 at 11:07 -0800, Andi Kleen wrote:
> > > From: Andi Kleen <ak@...ux.intel.com>
> > > 
> > > I found it difficult to make sense of transparent huge pages without
> > > having any counters for its actions. Add some counters to vmstat
> > > for allocation of transparent hugepages and fallback to smaller
> > > pages.
> > > 
> > > Optional patch, but useful for development and understanding the system.
> > 
> > Very nice.  I did the same thing, splits-only.  I also found this stuff
> > a must-have for trying to do any work with transparent hugepages.  It's
> > just impossible otherwise.
> 
> This patch is good too. 1 and 8 I think can go in, patch 1 is high
> priority.
> 
> Patches 2-5 I've an hard time to see how they're not hurting
> performance instead of improving it, especially patch 3 looks dead

Well right now THP destroys memory locality and that is a quite bad
regression. Destroying memory locality hurts performance significantly.

In general the assumption that you can get the full policy from the 
vma only is wrong: for local you always have to look at the node 
of the existing page too.

I haven't had any reports of KSM doing so, but it seems better
to fix it in the same way. Don't feel very strongly about KSM
for this though, i guess these parts could be dropped too. I guess
you're right and KSM is a bit of a lost cause for NUMA anyways.
Also in my experience KSM is very little memory usually anyways so
it shouldn't matter too much. I guess I can drop that part if it's
controversal.

-Andi
-- 
ak@...ux.intel.com -- Speaking for myself only.
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