linux-kernel - Re: [PATCH 4/4] zone_reclaim

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Date:	Wed, 13 May 2009 10:22:57 -0500
From:	Robin Holt <holt@....com>
To:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Cc:	LKML <linux-kernel@...r.kernel.org>, linux-mm <linux-mm@...ck.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Rik van Riel <riel@...hat.com>,
	Christoph Lameter <cl@...ux-foundation.org>
Subject: Re: [PATCH 4/4] zone_reclaim_mode is always 0 by default

On Wed, May 13, 2009 at 12:08:12PM +0900, KOSAKI Motohiro wrote:
> Subject: [PATCH] zone_reclaim_mode is always 0 by default
> 
> Current linux policy is, if the machine has large remote node distance,
>  zone_reclaim_mode is enabled by default because we've be able to assume to 
> large distance mean large server until recently.
> 
> Unfrotunately, recent modern x86 CPU (e.g. Core i7, Opeteron) have P2P transport
> memory controller. IOW it's NUMA from software view.
> 
> Some Core i7 machine has large remote node distance and zone_reclaim don't
> fit desktop and small file server. it cause performance degression.
> 
> Thus, zone_reclaim == 0 is better by default. sorry, HPC gusy. 
> you need to turn zone_reclaim_mode on manually now.

I am _VERY_ concerned about this change in behavior as it has been the
default for a considerable period of time.  I realize it is an easily
changed setting, but it is churn in the default behavior.  Are there
any benefits for these small servers to have zone_reclaim turned on?
If you have a large node distance, I would expect they should benefit
_MORE_ than those with small or no node distances.

Are you seeing an impact of the load not distributing pages evenly across
processors instead of a reclaim effect (ie, a single threaded process
faulting in more memory than is node local and expecting those pages
to come from the other node first before doing reclaim)?  Maybe there
is a different issue than the ones I am used to thinking about and I am
completely missing the point, please enlighten me.

If this proceeds forward, I would like to propose we at least leave
it on for SGI SN and UV hardware.  I can provide a quick patch that
may be a bit ugly because it will depend upon arch specific #defines.
I have not investigated this, but any alternative suggestions are
certainly welcome.  Currently, I am envisioning bringing something like
ia64_platform_is("sn2") and is_uv_system into page_alloc.c.

> 
> Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
> Cc: Christoph Lameter <cl@...ux-foundation.org>
> Cc: Rik van Riel <riel@...hat.com>

Please add me:

Cc: Robin Holt <holt@....com>
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