linux-kernel - Re: [PATCH 6/6] mm: compaction: Restart compaction from near where it left off

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Message-ID: <505B6706.1000104@redhat.com>
Date:	Thu, 20 Sep 2012 14:57:10 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Mel Gorman <mgorman@...e.de>
CC:	Richard Davies <richard@...chsys.com>,
	Shaohua Li <shli@...nel.org>, Avi Kivity <avi@...hat.com>,
	QEMU-devel <qemu-devel@...gnu.org>, KVM <kvm@...r.kernel.org>,
	Linux-MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 6/6] mm: compaction: Restart compaction from near where
 it left off

On 09/20/2012 10:04 AM, Mel Gorman wrote:
> This is almost entirely based on Rik's previous patches and discussions
> with him about how this might be implemented.
>
> Order > 0 compaction stops when enough free pages of the correct page
> order have been coalesced.  When doing subsequent higher order allocations,
> it is possible for compaction to be invoked many times.
>
> However, the compaction code always starts out looking for things to compact
> at the start of the zone, and for free pages to compact things to at the
> end of the zone.
>
> This can cause quadratic behaviour, with isolate_freepages starting at
> the end of the zone each time, even though previous invocations of the
> compaction code already filled up all free memory on that end of the zone.
> This can cause isolate_freepages to take enormous amounts of CPU with
> certain workloads on larger memory systems.
>
> This patch caches where the migration and free scanner should start from on
> subsequent compaction invocations using the pageblock-skip information. When
> compaction starts it begins from the cached restart points and will
> update the cached restart points until a page is isolated or a pageblock
> is skipped that would have been scanned by synchronous compaction.
>
> Signed-off-by: Mel Gorman <mgorman@...e.de>

Together with patch 5/6, this has the effect of
skipping compaction in a zone if the free and
isolate markers have met, and it has been less
than 5 seconds since the "skip" information was
reset.

Compaction on zones where we cycle through more
slowly can continue, even when this particular
zone is experiencing problems, so I guess this
is desired behaviour...

Acked-by: Rik van Riel <riel@...hat.com>

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