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From: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
To: Andrea Arcangeli <aarcange@redhat.com>
Subject: Re: [PATCH 01 of 66] disable lumpy when compaction is 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>,
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        Hugh Dickins <hugh.dickins@tiscali.co.uk>,
        Rik van Riel <riel@redhat.com>, Mel Gorman <mel@csn.ul.ie>,
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        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>,
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> From: Andrea Arcangeli <aarcange@redhat.com>
> 
> Compaction is more reliable than lumpy, and lumpy makes the system unusable
> when it runs.
> 
> Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
> ---
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -274,6 +274,7 @@ unsigned long shrink_slab(unsigned long 
>  static void set_lumpy_reclaim_mode(int priority, struct scan_control *sc,
>  				   bool sync)
>  {
> +#ifndef CONFIG_COMPACTION
>  	enum lumpy_mode mode = sync ? LUMPY_MODE_SYNC : LUMPY_MODE_ASYNC;
>  
>  	/*
> @@ -294,11 +295,14 @@ static void set_lumpy_reclaim_mode(int p
>  		sc->lumpy_reclaim_mode = mode;
>  	else
>  		sc->lumpy_reclaim_mode = LUMPY_MODE_NONE;
> +#endif
>  }

I'm talking very personal thing now. I'm usually testing both feature.
Then, runtime switching makes my happy :-)
However I don't know what are you and Mel talking and agree about this.
So, If many developer prefer this approach, I don't oppose anymore.

But, I bet almost all distro choose CONFIG_COMPACTION=y. then, lumpy code
will become nearly dead code. So, I like just kill than dead code. however
it is also only my preference. ;)