Re: [PATCH 1/2] mm: page allocator: Adjust the per-cpu counter threshold when memory is low

[Andrew Morton <akpm@linux-foundation.org>]

On Fri, 29 Oct 2010 09:58:25 -0500 (CDT)
Christoph Lameter <cl@linux.com> wrote:

> On Thu, 28 Oct 2010, Andrew Morton wrote:
> 
> > > To ensure that kswapd wakes up, a safe version of zone_watermark_ok()
> > > is introduced that takes a more accurate reading of NR_FREE_PAGES when
> > > called from wakeup_kswapd, when deciding whether it is really safe to go
> > > back to sleep in sleeping_prematurely() and when deciding if a zone is
> > > really balanced or not in balance_pgdat(). We are still using an expensive
> > > function but limiting how often it is called.
> >
> > Here I go again.  I have a feeling that I already said this, but I
> > can't find versions 2 or 3 in the archives..
> >
> > Did you evaluate using plain on percpu_counters for this?  They won't
> > solve the performance problem as they're basically the same thing as
> > these open-coded counters.  But they'd reduce the amount of noise and
> > custom-coded boilerplate in mm/.
> 
> The zone counters are done using the ZVCs in vmstat.c to save space

well, they actually waste space because of that threshold thing.

> and to
> be in the same cacheline as other hot data necessary for allocation and
> free.

Yes, that'll save some misses.

>  >
> > > +	threshold = max(1, (int)(watermark_distance / num_online_cpus()));
> > > +
> > > +	/*
> > > +	 * Maximum threshold is 125
> >
> > Reasoning?
> 
> Differentials are stored in 8 bit signed ints.
> 
> > > +	put_online_cpus();
> > > +}
> >
> > Given that ->stat_threshold is the same for each CPU, why store it for
> > each CPU at all?  Why not put it in the zone and eliminate the inner
> > loop?
> 
> Doing that caused cache misses in the past and reduced the performance of
> the ZVCs. This way the threshold is in the same cacheline as the
> differentials.

This sounds wrong.  As long as that threshold isn't stored in a
cacheline which other CPUs are modifying, all CPUs should be able to
happily cache it.  Maybe it needed a bit of padding inside the zone
struct.