linux-kernel - Re: [PATCH 3/3] Do not unconditionally treat zones that fail zone

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Message-ID: <20090609085037.GI18380@csn.ul.ie>
Date:	Tue, 9 Jun 2009 09:50:38 +0100
From:	Mel Gorman <mel@....ul.ie>
To:	Wu Fengguang <fengguang.wu@...el.com>
Cc:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Rik van Riel <riel@...hat.com>,
	Christoph Lameter <cl@...ux-foundation.org>,
	"Zhang, Yanmin" <yanmin.zhang@...el.com>,
	"linuxram@...ibm.com" <linuxram@...ibm.com>,
	linux-mm <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/3] Do not unconditionally treat zones that fail
	zone_reclaim() as full

On Tue, Jun 09, 2009 at 11:11:19AM +0800, Wu Fengguang wrote:
> On Mon, Jun 08, 2009 at 09:01:30PM +0800, Mel Gorman wrote:
> > On NUMA machines, the administrator can configure zone_reclaim_mode that
> > is a more targetted form of direct reclaim. On machines with large NUMA
> > distances for example, a zone_reclaim_mode defaults to 1 meaning that clean
> > unmapped pages will be reclaimed if the zone watermarks are not being
> > met. The problem is that zone_reclaim() failing at all means the zone
> > gets marked full.
> > 
> > This can cause situations where a zone is usable, but is being skipped
> > because it has been considered full. Take a situation where a large tmpfs
> > mount is occuping a large percentage of memory overall. The pages do not
> > get cleaned or reclaimed by zone_reclaim(), but the zone gets marked full
> > and the zonelist cache considers them not worth trying in the future.
> > 
> > This patch makes zone_reclaim() return more fine-grained information about
> > what occured when zone_reclaim() failued. The zone only gets marked full if
> > it really is unreclaimable. If it's a case that the scan did not occur or
> > if enough pages were not reclaimed with the limited reclaim_mode, then the
> > zone is simply skipped.
> > 
> > There is a side-effect to this patch. Currently, if zone_reclaim()
> > successfully reclaimed SWAP_CLUSTER_MAX, an allocation attempt would
> > go ahead. With this patch applied, zone watermarks are rechecked after
> > zone_reclaim() does some work.
> >
> > Signed-off-by: Mel Gorman <mel@....ul.ie>
> 
> Thanks for making the code a lot more readable :)
> 
> Reviewed-by: Wu Fengguang <fengguang.wu@...el.com>
> 

Thanks.

> >  	/*
> >  	 * Do not scan if the allocation should not be delayed.
> >  	 */
> >  	if (!(gfp_mask & __GFP_WAIT) || (current->flags & PF_MEMALLOC))
> > -			return 0;
> > +			return ZONE_RECLAIM_NOSCAN;
> 
> Why not kill the extra tab?
> 

Why not indeed. Tab is now killed.

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab
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