From: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
To: Andrea Arcangeli <aarcange@redhat.com>
Subject: Re: [PATCH 55 of 66] select CONFIG_COMPACTION if TRANSPARENT_HUGEPAGE enabled
Cc: kosaki.motohiro@jp.fujitsu.com, linux-mm@kvack.org,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        linux-kernel@vger.kernel.org, Marcelo Tosatti <mtosatti@redhat.com>,
        Adam Litke <agl@us.ibm.com>, Avi Kivity <avi@redhat.com>,
        Hugh Dickins <hugh.dickins@tiscali.co.uk>,
        Rik van Riel <riel@redhat.com>, Mel Gorman <mel@csn.ul.ie>,
        Dave Hansen <dave@linux.vnet.ibm.com>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Ingo Molnar <mingo@elte.hu>, Mike Travis <travis@sgi.com>,
        KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>,
        Christoph Lameter <cl@linux-foundation.org>,
        Chris Wright <chrisw@sous-sol.org>, bpicco@redhat.com,
        Balbir Singh <balbir@linux.vnet.ibm.com>,
        "Michael S. Tsirkin" <mst@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>,
        Chris Mason <chris.mason@oracle.com>, Borislav Petkov <bp@alien8.de>
In-Reply-To: <20101109211145.GB6809@random.random>
References: <20101109151756.BC7B.A69D9226@jp.fujitsu.com> <20101109211145.GB6809@random.random>
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Date: Sun, 14 Nov 2010 14:07:07 +0900 (JST)
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> On Tue, Nov 09, 2010 at 03:20:33PM +0900, KOSAKI Motohiro wrote:
> > > From: Andrea Arcangeli <aarcange@redhat.com>
> > > 
> > > With transparent hugepage support we need compaction for the "defrag" sysfs
> > > controls to be effective.
> > > 
> > > Signed-off-by: Andrea Arcangeli <aarcange@redhat.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
> (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.

ok, I beleive you.
but please add this reason to the description.


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