linux-kernel - Re: The performance and behaviour of the anti-fragmentation related patches

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Message-Id: <20070302142256.0127f5ac.akpm@linux-foundation.org>
Date:	Fri, 2 Mar 2007 14:22:56 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Rik van Riel <riel@...hat.com>
Cc:	Bill Irwin <bill.irwin@...cle.com>,
	Christoph Lameter <clameter@...r.sgi.com>,
	Mel Gorman <mel@...net.ie>, npiggin@...e.de, mingo@...e.hu,
	jschopp@...tin.ibm.com, arjan@...radead.org,
	torvalds@...ux-foundation.org, mbligh@...igh.org,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: The performance and behaviour of the anti-fragmentation related
 patches

On Fri, 02 Mar 2007 17:03:10 -0500
Rik van Riel <riel@...hat.com> wrote:

> Andrew Morton wrote:
> > On Fri, 02 Mar 2007 16:19:19 -0500
> > Rik van Riel <riel@...hat.com> wrote:
> >> Bill Irwin wrote:
> >>> On Fri, Mar 02, 2007 at 01:23:28PM -0500, Rik van Riel wrote:
> >>>> With 32 CPUs diving into the page reclaim simultaneously,
> >>>> each trying to scan a fraction of memory, this is disastrous
> >>>> for performance.  A 256GB system should be even worse.
> >>> Thundering herds of a sort pounding the LRU locks from direct reclaim
> >>> have set off the NMI oopser for users here.
> >> Ditto here.
> > 
> > Opterons?
> 
> It's happened on IA64, too.  Probably on Intel x86-64 as well.

Opterons seem to be particularly prone to lock starvation where a cacheline
gets captured in a single package for ever.

> >> The main reason they end up pounding the LRU locks is the
> >> swappiness heuristic.  They scan too much before deciding
> >> that it would be a good idea to actually swap something
> >> out, and with 32 CPUs doing such scanning simultaneously...
> > 
> > What kernel version?
> 
> Customers are on the 2.6.9 based RHEL4 kernel, but I believe
> we have reproduced the problem on 2.6.18 too during stress
> tests.

The prev_priority fixes were post-2.6.18

> I have no reason to believe we should stick our heads in the
> sand and pretend it no longer exists on 2.6.21.

I have no reason to believe anything.  All I see is handwaviness,
speculation and grand plans to rewrite vast amounts of stuff without even a
testcase to demonstrate that said rewrite improved anything.

None of this is going anywhere, is it?
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