linux-kernel - Re: [patch v2 1/3] mm: vmscan: fix numa reclaim balance problem in kswapd

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Date:	Wed, 7 Aug 2013 15:15:55 +0100
From:	Mel Gorman <mgorman@...e.de>
To:	Johannes Weiner <hannes@...xchg.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Rik van Riel <riel@...riel.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Zlatko Calusic <zcalusic@...sync.net>,
	Minchan Kim <minchan@...nel.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [patch v2 1/3] mm: vmscan: fix numa reclaim balance problem in
 kswapd

On Fri, Aug 02, 2013 at 11:37:24AM -0400, Johannes Weiner wrote:
> When the page allocator fails to get a page from all zones in its
> given zonelist, it wakes up the per-node kswapds for all zones that
> are at their low watermark.
> 
> However, with a system under load the free pages in a zone can
> fluctuate enough that the allocation fails but the kswapd wakeup is
> also skipped while the zone is still really close to the low
> watermark.
> 
> When one node misses a wakeup like this, it won't be aged before all
> the other node's zones are down to their low watermarks again.  And
> skipping a full aging cycle is an obvious fairness problem.
> 
> Kswapd runs until the high watermarks are restored, so it should also
> be woken when the high watermarks are not met.  This ages nodes more
> equally and creates a safety margin for the page counter fluctuation.
> 
> By using zone_balanced(), it will now check, in addition to the
> watermark, if compaction requires more order-0 pages to create a
> higher order page.
> 
> Signed-off-by: Johannes Weiner <hannes@...xchg.org>
> Reviewed-by: Rik van Riel <riel@...hat.com>

Acked-by: Mel Gorman <mgorman@...e.de>

-- 
Mel Gorman
SUSE Labs
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