Re: [PATCH -mm] vmscan: protect a fraction of file backed mappedpages from reclaim

[Wu Fengguang]

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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