linux-kernel - Re: [PATCH 10/11] mm: vmscan: Check if reclaim should really abort even if compaction

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Date:	Fri, 16 Dec 2011 11:29:54 +0000
From:	Mel Gorman <mgorman@...e.de>
To:	Rik van Riel <riel@...hat.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Minchan Kim <minchan.kim@...il.com>,
	Dave Jones <davej@...hat.com>, Jan Kara <jack@...e.cz>,
	Andy Isaacson <adi@...apodia.org>,
	Johannes Weiner <jweiner@...hat.com>,
	Nai Xia <nai.xia@...il.com>, Linux-MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 10/11] mm: vmscan: Check if reclaim should really abort
 even if compaction_ready() is true for one zone

On Thu, Dec 15, 2011 at 11:38:43PM -0500, Rik van Riel wrote:
> On 12/14/2011 10:41 AM, Mel Gorman wrote:
> >If compaction can proceed for a given zone, shrink_zones() does not
> >reclaim any more pages from it. After commit [e0c2327: vmscan: abort
> >reclaim/compaction if compaction can proceed], do_try_to_free_pages()
> >tries to finish as soon as possible once one zone can compact.
> >
> >This was intended to prevent slabs being shrunk unnecessarily but
> >there are side-effects. One is that a small zone that is ready for
> >compaction will abort reclaim even if the chances of successfully
> >allocating a THP from that zone is small. It also means that reclaim
> >can return too early even though sc->nr_to_reclaim pages were not
> >reclaimed.
> 
> Having slabs shrunk "too much" might actually be good,
> because it does result in more memory blocks where
> compaction can be successful.
> 
> If we end up frequently evicting frequently accessed
> data from the slab cache, chances are the buffer cache
> will cache that data (since we reload it often).
> 
> If we end up evicting infrequently used data, chances
> are it won't really matter for performance.
> 

True, but I was being mindful of Dave Chinners recent work on
preventing slab cache being dumped entirely. There still may be an
impact to metadata-intensive workloads although I did not spot any
problems myself.

> >This partially reverts the commit until it is proven that slabs are
> >really being shrunk unnecessarily but preserves the check to return
> >1 to avoid OOM if reclaim was aborted prematurely.
> >
> >[aarcange@...hat.com: This patch replaces a revert from Andrea]
> >Signed-off-by: Mel Gorman<mgorman@...e.de>
> 
> Reviewed-by: Rik van Riel<riel@...hat.com>
> 

Thanks.

-- 
Mel Gorman
SUSE Labs
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