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

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Date:	Mon, 18 May 2009 11:49:07 +0800
From:	Wu Fengguang <fengguang.wu@...el.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>,
	"Zhang, Yanmin" <yanmin.zhang@...el.com>
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.

I can confirm this, Yanmin recently ran into exactly such a
regression, which was fixed by manually disabling the zone reclaim
mode. So I guess you can safely add an

Tested-by: "Zhang, Yanmin" <yanmin.zhang@...el.com>

> Thus, zone_reclaim == 0 is better by default. sorry, HPC gusy. 
> you need to turn zone_reclaim_mode on manually now.
 
I guess the borderline will continue to blur up. It will be more
dependent on workloads instead of physical NUMA capabilities. So

Acked-by: Wu Fengguang <fengguang.wu@...el.com> 

> Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
> Cc: Christoph Lameter <cl@...ux-foundation.org>
> Cc: Rik van Riel <riel@...hat.com>
> ---
>  mm/page_alloc.c |    7 -------
>  1 file changed, 7 deletions(-)
> 
> Index: b/mm/page_alloc.c
> ===================================================================
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -2494,13 +2494,6 @@ static void build_zonelists(pg_data_t *p
>  		int distance = node_distance(local_node, node);
>  
>  		/*
> -		 * If another node is sufficiently far away then it is better
> -		 * to reclaim pages in a zone before going off node.
> -		 */
> -		if (distance > RECLAIM_DISTANCE)
> -			zone_reclaim_mode = 1;
> -
> -		/*
>  		 * We don't want to pressure a particular node.
>  		 * So adding penalty to the first node in same
>  		 * distance group to make it round-robin.
> 
> 
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