Re: [PATCH 1/3] page-allocator: Under memory pressure, wait on pressureto relieve instead of congestion

[Christian Ehrhardt]

Nick Piggin wrote:
> On Tue, Mar 09, 2010 at 02:17:13PM +0000, Mel Gorman wrote:
> > On Wed, Mar 10, 2010 at 12:35:13AM +1100, Nick Piggin wrote:
> > > On Mon, Mar 08, 2010 at 11:48:21AM +0000, Mel Gorman wrote:
> > > > Under heavy memory pressure, the page allocator may call congestion_wait()
> > > > to wait for IO congestion to clear or a timeout. This is not as sensible
> > > > a choice as it first appears. There is no guarantee that BLK_RW_ASYNC is
> > > > even congested as the pressure could have been due to a large number of
> > > > SYNC reads and the allocator waits for the entire timeout, possibly uselessly.
> > > >
> > > > At the point of congestion_wait(), the allocator is struggling to get the
> > > > pages it needs and it should back off. This patch puts the allocator to sleep
> > > > on a zone->pressure_wq for either a timeout or until a direct reclaimer or
> > > > kswapd brings the zone over the low watermark, whichever happens first.
> > > >
> > > > Signed-off-by: Mel Gorman <mel@xxxxxxxxx>
> > > > ---
> > > >  include/linux/mmzone.h |    3 ++
> > > >  mm/internal.h          |    4 +++
> > > >  mm/mmzone.c            |   47 +++++++++++++++++++++++++++++++++++++++++++++
> > > >  mm/page_alloc.c        |   50 +++++++++++++++++++++++++++++++++++++++++++----
> > > >  mm/vmscan.c            |    2 +
> > > >  5 files changed, 101 insertions(+), 5 deletions(-)
> > > >
> > > > diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> > > > index 30fe668..72465c1 100644
> > > > --- a/include/linux/mmzone.h
> > > > +++ b/include/linux/mmzone.h
[...]
> > > > +{
> > > > +	/* If no process is waiting, nothing to do */
> > > > +	if (!waitqueue_active(zone->pressure_wq))
> > > > +		return;
> > > > +
> > > > +	/* Check if the high watermark is ok for order 0 */
> > > > +	if (zone_watermark_ok(zone, 0, low_wmark_pages(zone), 0, 0))
> > > > +		wake_up_interruptible(zone->pressure_wq);
> > > > +}
> > > If you were to do this under the zone lock (in your subsequent patch),
> > > then it could avoid races. I would suggest doing it all as a single
> > > patch and not doing the pressure checks in reclaim at all.
> > That is reasonable. I've already dropped the checks in reclaim because as you
> > say, if the free path check is cheap enough, it's also sufficient. Checking
> > in the reclaim paths as well is redundant.
> >
> > I'll move the call to check_zone_pressure() within the zone lock to avoid
> > races.

Mel, we talked about a thundering herd issue that might come up here in 
very constraint cases.
So wherever you end up putting that wake_up call, how about being extra 
paranoid about a thundering herd flagging them WQ_FLAG_EXCLUSIVE and 
waking them with something like that:

wake_up_interruptible_nr(zone->pressure_wq, #nrofpagesabovewatermark#);

That should be an easy to calculate sane max of waiters to wake up.
On the other hand it might be over-engineered and it implies the need to 
reconsider when it would be best to wake up the rest.

Get me right - I don't really have a hard requirement or need for that, 
I just wanted to mention it early on to hear your opinions about it.

looking forward to test the v2 patch series, adapted to all the good 
stuff already discussed.

--

Grüsse / regards, Christian Ehrhardt
IBM Linux Technology Center, System z Linux Performance
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