linux-kernel - Re: [PATCH 55 of 66] select CONFIG_COMPACTION if TRANSPARENT

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Date:	Thu, 18 Nov 2010 16:22:45 +0000
From:	Mel Gorman <mel@....ul.ie>
To:	Andrea Arcangeli <aarcange@...hat.com>
Cc:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	linux-mm@...ck.org, Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org,
	Marcelo Tosatti <mtosatti@...hat.com>,
	Adam Litke <agl@...ibm.com>, Avi Kivity <avi@...hat.com>,
	Hugh Dickins <hugh.dickins@...cali.co.uk>,
	Rik van Riel <riel@...hat.com>,
	Dave Hansen <dave@...ux.vnet.ibm.com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Ingo Molnar <mingo@...e.hu>, Mike Travis <travis@....com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Chris Wright <chrisw@...s-sol.org>, bpicco@...hat.com,
	Balbir Singh <balbir@...ux.vnet.ibm.com>,
	"Michael S. Tsirkin" <mst@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Johannes Weiner <hannes@...xchg.org>,
	Daisuke Nishimura <nishimura@....nes.nec.co.jp>,
	Chris Mason <chris.mason@...cle.com>,
	Borislav Petkov <bp@...en8.de>
Subject: Re: [PATCH 55 of 66] select CONFIG_COMPACTION if
	TRANSPARENT_HUGEPAGE enabled

On Tue, Nov 09, 2010 at 10:11:45PM +0100, Andrea Arcangeli wrote:
> On Tue, Nov 09, 2010 at 03:20:33PM +0900, KOSAKI Motohiro wrote:
> > > From: Andrea Arcangeli <aarcange@...hat.com>
> > > 
> > > With transparent hugepage support we need compaction for the "defrag" sysfs
> > > controls to be effective.
> > > 
> > > Signed-off-by: Andrea Arcangeli <aarcange@...hat.com>
> > > ---
> > > 
> > > diff --git a/mm/Kconfig b/mm/Kconfig
> > > --- a/mm/Kconfig
> > > +++ b/mm/Kconfig
> > > @@ -305,6 +305,7 @@ config NOMMU_INITIAL_TRIM_EXCESS
> > >  config TRANSPARENT_HUGEPAGE
> > >  	bool "Transparent Hugepage Support"
> > >  	depends on X86 && MMU
> > > +	select COMPACTION
> > >  	help
> > >  	  Transparent Hugepages allows the kernel to use huge pages and
> > >  	  huge tlb transparently to the applications whenever possible.
> > 
> > I dislike this. THP and compaction are completely orthogonal. I think 
> > you are talking only your performance recommendation. I mean I dislike
> > Kconfig 'select' hell and I hope every developers try to avoid it as 
> > far as possible.
> 
> At the moment THP hangs the system if COMPACTION isn't selected

Just to confirm - by hang, you mean grinds to a slow pace as opposed to
coming to a complete stop and having to restart?

If so then

Acked-by: Mel Gorman <mel@....ul.ie>

> (please try yourself if you don't believe), as without COMPACTION
> lumpy reclaim wouldn't be entirely disabled. So at the moment it's not
> orthogonal. When lumpy will be removed from the VM (like I tried
> multiple times to achieve) I can remove the select COMPACTION in
> theory, but then 99% of THP users would be still doing a mistake in
> disabling compaction, even if the mistake won't return in fatal
> runtime but just slightly degraded performance.
> 

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab
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