On Thu, Jun 28, 2012 at 01:55:20PM -0400, Rik van Riel wrote:

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.

The obvious solution is to have isolate_freepages remember where
it left off last time, and continue at that point the next time
it gets invoked for an order > 0 compaction. This could cause
compaction to fail if cc->free_pfn and cc->migrate_pfn are close
together initially, in that case we restart from the end of the
zone and try once more.

Forced full (order == -1) compactions are left alone.

Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Mel Gorman <redacted>
Reported-by: Jim Schutt <redacted>
Signed-off-by: Rik van Riel <redacted>
---
v2: implement Mel's suggestions, handling wrap-around etc

I have not tested it myself but it looks correct! Thanks very much.

Acked-by: Mel Gorman <redacted>

-- 
Mel Gorman
SUSE Labs

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