linux-kernel - [PATCH] mm/compaction: restore COMPACT_CLUSTER

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Date:	Thu,  3 Dec 2015 13:11:40 +0900
From:	Joonsoo Kim <js1304@...il.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Vlastimil Babka <vbabka@...e.cz>, Mel Gorman <mgorman@...e.de>,
	David Rientjes <rientjes@...gle.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Joonsoo Kim <iamjoonsoo.kim@....com>
Subject: [PATCH] mm/compaction: restore COMPACT_CLUSTER_MAX to 32

Until now, COMPACT_CLUSTER_MAX is defined as SWAP_CLUSTER_MAX.
Commit ("mm: increase SWAP_CLUSTER_MAX to batch TLB flushes")
changes SWAP_CLUSTER_MAX from 32 to 256 to improve tlb flush performance
so COMPACT_CLUSTER_MAX is also changed to 256. But, it has
no justification on compaction-side and I think that loss is more than
benefit.

One example is that migration scanner would isolates and migrates
too many pages unnecessarily with 256 COMPACT_CLUSTER_MAX. It may be
enough to migrate 4 pages in order to make order-2 page, but, now,
compaction will migrate 256 pages.

To reduce this unneeded overhead, this patch restores
COMPACT_CLUSTER_MAX to 32.

Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@....com>
---
 include/linux/swap.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/linux/swap.h b/include/linux/swap.h
index d08feef..31eb343 100644
--- a/include/linux/swap.h
+++ b/include/linux/swap.h
@@ -155,7 +155,7 @@ enum {
 };

 #define SWAP_CLUSTER_MAX 256UL
-#define COMPACT_CLUSTER_MAX SWAP_CLUSTER_MAX
+#define COMPACT_CLUSTER_MAX 32UL

 /*
  * Ratio between zone->managed_pages and the "gap" that above the per-zone
-- 
1.9.1

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