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

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Date:	Fri, 29 Oct 2010 09:58:25 -0500 (CDT)
From:	Christoph Lameter <cl@...ux.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
cc:	Mel Gorman <mel@....ul.ie>, Shaohua Li <shaohua.li@...el.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	David Rientjes <rientjes@...gle.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Linux-MM <linux-mm@...ck.org>
Subject: Re: [PATCH 1/2] mm: page allocator: Adjust the per-cpu counter
 threshold when memory is low

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



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