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Date: Mon, 13 Feb 2017 10:57:10 +0000
From: Mel Gorman <mgorman@techsingularity.net>
Subject: Re: [PATCH v2 08/10] mm, compaction: finish whole pageblock to
 reduce fragmentation
Message-ID: <20170213105710.knhhoqe6jfkwoaxo@techsingularity.net>
References: <20170210172343.30283-1-vbabka@suse.cz>
 <20170210172343.30283-9-vbabka@suse.cz>
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In-Reply-To: <20170210172343.30283-9-vbabka@suse.cz>
Sender: owner-linux-mm@kvack.org
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To: Vlastimil Babka <vbabka@suse.cz>
Cc: linux-mm@kvack.org, Johannes Weiner <hannes@cmpxchg.org>, Joonsoo Kim <iamjoonsoo.kim@lge.com>, David Rientjes <rientjes@google.com>, linux-kernel@vger.kernel.org, kernel-team@fb.com

On Fri, Feb 10, 2017 at 06:23:41PM +0100, Vlastimil Babka wrote:
> The main goal of direct compaction is to form a high-order page for allocation,
> but it should also help against long-term fragmentation when possible. Most
> lower-than-pageblock-order compactions are for non-movable allocations, which
> means that if we compact in a movable pageblock and terminate as soon as we
> create the high-order page, it's unlikely that the fallback heuristics will
> claim the whole block. Instead there might be a single unmovable page in a
> pageblock full of movable pages, and the next unmovable allocation might pick
> another pageblock and increase long-term fragmentation.
> 
> To help against such scenarios, this patch changes the termination criteria for
> compaction so that the current pageblock is finished even though the high-order
> page already exists. Note that it might be possible that the high-order page
> formed elsewhere in the zone due to parallel activity, but this patch doesn't
> try to detect that.
> 
> This is only done with sync compaction, because async compaction is limited to
> pageblock of the same migratetype, where it cannot result in a migratetype
> fallback. (Async compaction also eagerly skips order-aligned blocks where
> isolation fails, which is against the goal of migrating away as much of the
> pageblock as possible.)
> 
> As a result of this patch, long-term memory fragmentation should be reduced.
> 
> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>

Acked-by: Mel Gorman <mgorman@techsingularity.net>

-- 
Mel Gorman
SUSE Labs

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