[PATCH 25/25] mm, compaction: Do not direct compact remote memory

[Mel Gorman]

Remote compaction is expensive and possibly counter-productive. Locality
is expected to often have better performance characteristics than remote
high-order pages. For small allocations, it's expected that locality is
generally required or fallbacks are possible. For larger allocations such
as THP, they are forbidden at the time of writing but if __GFP_THISNODE
is ever removed, then it would still be preferable to fallback to small
local base pages over remote THP in the general case. kcompactd is still
woken via kswapd so compaction happens eventually.

While this patch potentially has both positive and negative effects,
it is best to avoid the possibility of remote compaction given the cost
relative to any potential benefit.

Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
---
 mm/compaction.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/mm/compaction.c b/mm/compaction.c
index ae70be023b21..cc17f0c01811 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -2348,6 +2348,16 @@ enum compact_result try_to_compact_pages(gfp_t gfp_mask, unsigned int order,
 			continue;
 		}
 
+		/*
+		 * Do not compact remote memory. It's expensive and high-order
+		 * small allocations are expected to prefer or require local
+		 * memory. Similarly, larger requests such as THP can fallback
+		 * to base pages in preference to remote huge pages if
+		 * __GFP_THISNODE is not specified
+		 */
+		if (zone_to_nid(zone) != zone_to_nid(ac->preferred_zoneref->zone))
+			continue;
+
 		status = compact_zone_order(zone, order, gfp_mask, prio,
 				alloc_flags, ac_classzone_idx(ac), capture);
 		rc = max(status, rc);
-- 
2.16.4