On Fri, Feb 10, 2017 at 06:23:40PM +0100, Vlastimil Babka wrote:

The migrate scanner in async compaction is currently limited to MIGRATE_MOVABLE
pageblocks. This is a heuristic intended to reduce latency, based on the
assumption that non-MOVABLE pageblocks are unlikely to contain movable pages.

However, with the exception of THP's, most high-order allocations are not
movable. Should the async compaction succeed, this increases the chance that
the non-MOVABLE allocations will fallback to a MOVABLE pageblock, making the
long-term fragmentation worse.

This patch attempts to help the situation by changing async direct compaction
so that the migrate scanner only scans the pageblocks of the requested
migratetype. If it's a non-MOVABLE type and there are such pageblocks that do
contain movable pages, chances are that the allocation can succeed within one
of such pageblocks, removing the need for a fallback. If that fails, the
subsequent sync attempt will ignore this restriction.

Signed-off-by: Vlastimil Babka <redacted>

Ok, I really like this idea. The thinking of async originally was to
reduce latency but that was also at the time when THP allocations were
stalling for long periods of time. Now that the default has changed,
this idea makes a lot of sense. A few months ago I would have thought
that this will increase the changes that a high-order allocation for the
stack may have a higher chance of failing but with VMAP_STACK, this is
much less of a concern. It would be very nice to know for this patch if
the number of times the extfrag tracepoint is triggered is reduced or
increased by this patch. Do you have that data?

-- 
Mel Gorman
SUSE Labs

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