Re: [PATCH 6/6] mm: compaction: Restart compaction from near whereit left off
From: Rik van Riel
Date: Thu Sep 20 2012 - 14:57:17 EST
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@xxxxxxx>
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@xxxxxxxxxx>
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