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SubjectRe: 2.6.23-rc1-mm2 (vm-dont-run-touch_buffer-during-buffercache-lookups.patch)
On Wed, 01 Aug 2007 03:36:30 -0400 Eric St-Laurent <> wrote:

> On Tue, 2007-31-07 at 23:09 -0700, Andrew Morton wrote:
> > +vm-dont-run-touch_buffer-during-buffercache-lookups.patch
> >
> > A little VM experiment. See changelog for details.
> > We don't have any tests to determine the effects of this, and nobody will
> > bother setting one up, so ho hum, this remains in -mm for ever.
> > I don't think there's any point in doing this until we have some decent
> > testcases.
> Hi Andrew,
> For which problem this patch was coded?

Good question. I think the current behaviour is just wrong. What the
effect of changing it will be is hard to predict - probably little.

> Is it a potential fix to the
> updatedb problem?

That's one workload which is particularly susceptible to the problemn which
that patch addresses, yes. But in my (brief) testing it didn't make musch

> Is the patch effective without the filesystem dependant change you talk
> about? (I use reiserfs)

Yes, it'll work as designed with reiserfs.

> I've been thinking about a test case for the updatedb problem:
> 1. Script or program that create a large number of directories and zero
> sized files. Same setup for everyone to have reproducible results.
> 2. Run updatedb on those.
> 3. Observe the effects (with vmstat, slabinfo and meminfo) before,
> during and after the updatedb run.
> 4. Do something to trigger some reclaim like copying a large file.
> 5. See the effects.
> What do you think? What would be the ideal test case for the problem in
> your opinion?

Sounds good, yes.

Or you could do something more real-worldly like start up OO, firefox and
friends, then run /etc/cron.daily/everything and see what the
before-and-after effects are. The aggregate info we're looking for is
captured in /proc/meminfo: swapped, Mapped, Cached, Buffers.

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