Date: Sun, 20 Jul 2008 18:48:43 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: "KOSAKI Motohiro" <kosaki.motohiro@jp.fujitsu.com>
Cc: "Johannes Weiner" <hannes@saeurebad.de>, "Rik van Riel" <riel@redhat.com>,
       "Peter Zijlstra" <peterz@infradead.org>,
       Nossum <vegard.nossum@gmail.com>, linux-mm@kvack.org,
       linux-kernel@vger.kernel.org
Subject: Re: [PATCH -mm] mm: more likely reclaim MADV_SEQUENTIAL mappings
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On Mon, 21 Jul 2008 09:09:26 +0900 "KOSAKI Motohiro" <kosaki.motohiro@jp.fujitsu.com> wrote:

> Hi Johannes,
> 
> > File pages accessed only once through sequential-read mappings between
> > fault and scan time are perfect candidates for reclaim.
> >
> > This patch makes page_referenced() ignore these singular references and
> > the pages stay on the inactive list where they likely fall victim to the
> > next reclaim phase.
> >
> > Already activated pages are still treated normally.  If they were
> > accessed multiple times and therefor promoted to the active list, we
> > probably want to keep them.
> >
> > Benchmarks show that big (relative to the system's memory)
> > MADV_SEQUENTIAL mappings read sequentially cause much less kernel
> > activity.  Especially less LRU moving-around because we never activate
> > read-once pages in the first place just to demote them again.
> >
> > And leaving these perfect reclaim candidates on the inactive list makes
> > it more likely for the real working set to survive the next reclaim
> > scan.
> 
> looks good to me.
> Actually, I made similar patch half year ago.
> 
> in my experience,
>   - page_referenced_one is performance critical point.
>     you should test some benchmark.
>   - its patch improved mmaped-copy performance about 5%.
>     (Of cource, you should test in current -mm. MM code was changed widely)
> 
> So, I'm looking for your test result :)

The change seems logical and I queued it for 2.6.28.

But yes, testing for what-does-this-improve is good and useful, but so
is testing for what-does-this-worsen.  How do we do that in this case?

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