Re: [PATCH v3 7/8] mm, compaction: restrict async compaction to pageblocks of same migratetype

[Joonsoo Kim]

On Tue, Mar 07, 2017 at 02:15:44PM +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.

I agree with this idea but have some concerns on this change.

*ASYNC* compaction is designed for reducing latency and this change
doesn't fit it. If everything works fine, there is a few movable pages
in non-MOVABLE pageblocks as you noted above. Moreover, there is quite
less the number of non-MOVABLE pageblock than MOVABLE one so finding
non-MOVABLE pageblock takes long time. These two factors will increase
the latency of *ASYNC* compaction.

And, there is a concern in implementaion side. With this change, there
is much possibilty that compaction scanner's met by ASYNC compaction.
It resets the scanner position and SYNC compaction would start the
scan at the beginning of the zone every time. It would make cached
position useless and inefficient.

Thanks.