linux-kernel - Re: [PATCH 30/34] mm: vmscan: Do not force kswapd to scan small targets

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Message-ID: <20120725195948.GB5444@kroah.com>
Date:	Wed, 25 Jul 2012 12:59:48 -0700
From:	Greg KH <gregkh@...uxfoundation.org>
To:	Mel Gorman <mgorman@...e.de>
Cc:	Stable <stable@...r.kernel.org>, Linux-MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 30/34] mm: vmscan: Do not force kswapd to scan small
 targets

On Mon, Jul 23, 2012 at 02:38:43PM +0100, Mel Gorman wrote:
> commit ad2b8e601099a23dffffb53f91c18d874fe98854 upstream - WARNING: this is a substitute patch.
> 
> Stable note: Not tracked in Bugzilla. This is a substitute for an
> 	upstream commit addressing a completely different issue that
> 	accidentally contained an important fix. The workload this patch
> 	helps was memcached when IO is started in the background. memcached
> 	should stay resident but without this patch it gets swapped more
> 	than it should. Sometimes this manifests as a drop in throughput
> 	but mostly it was observed through /proc/vmstat.
> 
> Commit [246e87a9: memcg: fix get_scan_count() for small targets] was
> meant to fix a problem whereby small scan targets on memcg were ignored
> causing priority to raise too sharply. It forced scanning to take place
> if the target was small, memcg or kswapd.
> 
> >From the time it was introduced it cause excessive reclaim by kswapd
> with workloads being pushed to swap that previously would have stayed
> resident. This was accidentally fixed by commit [ad2b8e60: mm: memcg:
> remove optimization of keeping the root_mem_cgroup LRU lists empty] but
> that patchset is not suitable for backporting.
> 
> The original patch came with no information on what workloads it benefits
> but the cost of it is obvious in that it forces scanning to take place
> on lists that would otherwise have been ignored such as small anonymous
> inactive lists. This patch partially reverts 246e87a9 so that small lists
> are not force scanned which means that IO-intensive workloads with small
> amounts of anonymous memory will not be swapped.
> 
> Signed-off-by: Mel Gorman <mgorman@...e.de>
> ---
>  mm/vmscan.c |    3 ---
>  1 file changed, 3 deletions(-)

I don't understand this patch.  The original
ad2b8e601099a23dffffb53f91c18d874fe98854 commit touched the file
mm/memcontrol.c and seemed to do something quite different from what you
have done below.

I'm all for fixing things in a different way than what was done in
Linus's tree, IF there is a reason to, but the comparison between these
two patches (yours and upstream) are not making any sense at all.

confused,

greg k-h
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