linux-kernel - Re: [PATCH 1/3] Reintroduce zone_reclaim_interval for when zone_reclaim() scans and fails to avoid CPU spinning at 100% on NUMA

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Date:	Mon, 8 Jun 2009 16:11:51 +0100
From:	Mel Gorman <mel@....ul.ie>
To:	Christoph Lameter <cl@...ux-foundation.org>
Cc:	Rik van Riel <riel@...hat.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	yanmin.zhang@...el.com, Wu Fengguang <fengguang.wu@...el.com>,
	linuxram@...ibm.com, linux-mm <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/3] Reintroduce zone_reclaim_interval for when
	zone_reclaim() scans and fails to avoid CPU spinning at 100% on NUMA

On Mon, Jun 08, 2009 at 10:55:55AM -0400, Christoph Lameter wrote:
> On Mon, 8 Jun 2009, Mel Gorman wrote:
> 
> > > The tmpfs pages are unreclaimable and therefore should not be on the anon
> > > lru.
> > >
> >
> > tmpfs pages can be swap-backed so can be reclaimable. Regardless of what
> > list they are on, we still need to know how many of them there are if
> > this patch is to be avoided.
> 
> If they are reclaimable then why does it matter? They can be pushed out if
> you configure zone reclaim to be that aggressive.
> 

Because they are reclaimable by kswapd or normal direct reclaim but *not*
reclaimable by zone_reclaim() if the zone_reclaim_mode is not configured
appropriately. I briefly considered setting zone_reclaim_mode to 7 instead of
1 by default for large NUMA distances but that has other serious consequences
such as paging in preference to going off-node as a default out-of-box
behaviour.

The point of the patch is that the heuristics that avoid the scan are not
perfect. In the event they are wrong and a useless scan occurs, the response
of the kernel after a useless scan should not be to uselessly scan a load
more times around the LRU lists making no progress.

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