linux-kernel - Re: [PATCH 0/2] Disable zone_reclaim

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Date:	Tue, 8 Apr 2014 16:26:42 +0200
From:	Andres Freund <andres@...quadrant.com>
To:	Christoph Lameter <cl@...ux.com>
Cc:	Vlastimil Babka <vbabka@...e.cz>, Mel Gorman <mgorman@...e.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Robert Haas <robertmhaas@...il.com>,
	Josh Berkus <josh@...iodbs.com>, Linux-MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>, sivanich@....com
Subject: Re: [PATCH 0/2] Disable zone_reclaim_mode by default

On 2014-04-08 09:17:04 -0500, Christoph Lameter wrote:
> On Tue, 8 Apr 2014, Vlastimil Babka wrote:
> 
> > On 04/08/2014 12:34 AM, Mel Gorman wrote:
> > > When it was introduced, zone_reclaim_mode made sense as NUMA distances
> > > punished and workloads were generally partitioned to fit into a NUMA
> > > node. NUMA machines are now common but few of the workloads are NUMA-aware
> > > and it's routine to see major performance due to zone_reclaim_mode being
> > > disabled but relatively few can identify the problem.
> >     ^ I think you meant "enabled" here?
> >
> > Just in case the cover letter goes to the changelog...
> 
> Correct.
> 
> Another solution here would be to increase the threshhold so that
> 4 socket machines do not enable zone reclaim by default. The larger the
> NUMA system is the more memory is off node from the perspective of a
> processor and the larger the hit from remote memory.

FWIW, I've the problem hit majorly on 8 socket machines. Those are the
largest I have seen so far in postgres scenarios. Everything larger is
far less likely to be used as single node database server, so that's
possibly a sensible cutoff.
But then, I'd think that special many-socket machines are setup by
specialists, that'd know to enable if it makes sense...

Greetings,

Andres Freund

-- 
 Andres Freund	                   http://www.2ndQuadrant.com/
 PostgreSQL Development, 24x7 Support, Training & Services
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