linux-kernel - Re: [PATCH -mm] vmscan: protect a fraction of file backed mapped pages from reclaim

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Message-ID: <20090516085416.GA10221@localhost>
Date:	Sat, 16 May 2009 16:54:16 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	Christoph Lameter <cl@...ux-foundation.org>
Cc:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"hannes@...xchg.org" <hannes@...xchg.org>,
	"peterz@...radead.org" <peterz@...radead.org>,
	"riel@...hat.com" <riel@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"tytso@....edu" <tytso@....edu>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"elladan@...imo.com" <elladan@...imo.com>,
	"npiggin@...e.de" <npiggin@...e.de>,
	"minchan.kim@...il.com" <minchan.kim@...il.com>
Subject: Re: [PATCH -mm] vmscan: protect a fraction of file backed mapped
	pages from reclaim

Hi Christoph,

On Fri, May 15, 2009 at 04:14:31AM +0800, Christoph Lameter wrote:
> On Wed, 13 May 2009, KOSAKI Motohiro wrote:
> 
> > > All these expiration modifications do not take into account that a desktop
> > > may sit idle for hours while some other things run in the background (like
> > > backups at night or updatedb and other maintenance things). This still
> > > means that the desktop will be usuable in the morning.
> >
> > Have you seen this phenomenom?
> > I always use linux desktop for development. but I haven't seen it.
> > perhaps I have no luck. I really want to know reproduce way.
> >
> > Please let me know reproduce way.
> 
> Run a backup (or rsync) over a few hundred GB.

Simple experiments show that rsync is use-once workload:

1) fresh run(full backup): the source file pages in the logo/ dir are cached and
   referenced *once*:

        rsync -a logo localhost:/tmp/

2) second run(incremental backup): only the updated files are read and
   read only once:

        rsync -a logo localhost:/tmp/

> > > The percentage of file backed pages protected is set via
> > > /proc/sys/vm/file_mapped_ratio. This defaults to 20%.
> >
> > Why do you think typical mapped ratio is less than 20% on desktop machine?
> 
> Observation of the typical mapped size of Firefox under KDE.

Since the explicit PROT_EXEC targeted mmap page protection plus Rik's
use-once patch works just OK for rsync - a typical backup scenario,
and it works without an extra sysctl tunable, I tend to continue
pushing the PROT_EXEC approach :-)

Thanks,
Fengguang

> > key point is access-once vs access-many.
> 
> Nothing against it if it works.
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