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

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Date:	Wed, 9 Apr 2014 14:08:19 +0100
From:	Mel Gorman <mgorman@...e.de>
To:	Josh Berkus <josh@...iodbs.com>
Cc:	Robert Haas <robertmhaas@...il.com>,
	Christoph Lameter <cl@...ux.com>,
	Vlastimil Babka <vbabka@...e.cz>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andres Freund <andres@...quadrant.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 Tue, Apr 08, 2014 at 03:56:49PM -0400, Josh Berkus wrote:
> On 04/08/2014 03:53 PM, Robert Haas wrote:
> > In an ideal world, the kernel would put the hottest pages on the local
> > node and the less-hot pages on remote nodes, moving pages around as
> > the workload shifts.  In practice, that's probably pretty hard.
> > Fortunately, it's not nearly as important as making sure we don't
> > unnecessarily hit the disk, which is infinitely slower than any memory
> > bank.
> 
> Even if the kernel could do this, we would *still* have to disable it
> for PostgreSQL, since our double-buffering makes our pages look "cold"
> to the kernel ... as discussed.
> 

If it's the shared mapping that is being used then automatic NUMA
balancing should migrate those pages to a node local to the CPU
accessing it but how well it works will partially depend on how much
those accesses move around. It's independent of the zone_reclaim_mode
issue.

-- 
Mel Gorman
SUSE Labs
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