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

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Message-ID: <alpine.DEB.2.00.1010291425360.26312@router.home>
Date:	Fri, 29 Oct 2010 14:33:02 -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 Fri, 29 Oct 2010, Andrew Morton wrote:

> > 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.

High speed cpu caches are a very scarce resource. The differentials are
not in the zone struct. Tried to put it onto a single cacheline. Even that
did not do the trick for the large configurations.

The same optimizations are done in the slab allocators by the way.

Use of the percpu_counter() would at least quadruple the cache footprint
in use for the counters. percpu_counters() is using s32 and not s8.

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