Re: [PATCH 3/5] mm: compaction: Capture a suitable high-order page immediately when it is made available

[Mel Gorman <mgorman@suse.de>]

On Thu, Aug 09, 2012 at 10:33:58AM +0900, Minchan Kim wrote:
> Hi Mel,
> 
> Just one questoin below.
> 

Sure! Your questions usually get me thinking about the right part of the
series, this series in particular :)

> > <SNIP>
> > @@ -708,6 +750,10 @@ static int compact_zone(struct zone *zone, struct compact_control *cc)
> >  				goto out;
> >  			}
> >  		}
> > +
> > +		/* Capture a page now if it is a suitable size */
> 
> Why do we capture only when we migrate MIGRATE_MOVABLE type?
> If you have a reasone, it should have been added as comment.
> 

Good question and there is an answer. However, I also spotted a problem when
thinking about this more where !MIGRATE_MOVABLE allocations are forced to
do a full compaction. The simple solution would be to only set cc->page for
MIGRATE_MOVABLE but there is a better approach that I've implemented in the
patch below. It includes a comment that should answer your question. Does
this make sense to you?

diff --git a/mm/compaction.c b/mm/compaction.c
index 63af8d2..384164e 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -53,13 +53,31 @@ static inline bool migrate_async_suitable(int migratetype)
 static void compact_capture_page(struct compact_control *cc)
 {
 	unsigned long flags;
-	int mtype;
+	int mtype, mtype_low, mtype_high;
 
 	if (!cc->page || *cc->page)
 		return;
 
+	/*
+	 * For MIGRATE_MOVABLE allocations we capture a suitable page ASAP
+	 * regardless of the migratetype of the freelist is is captured from.
+	 * This is fine because the order for a high-order MIGRATE_MOVABLE
+	 * allocation is typically at least a pageblock size and overall
+	 * fragmentation is not impaired. Other allocation types must
+	 * capture pages from their own migratelist because otherwise they
+	 * could pollute other pageblocks like MIGRATE_MOVABLE with
+	 * difficult to move pages and making fragmentation worse overall.
+	 */
+	if (cc->migratetype == MIGRATE_MOVABLE) {
+		mtype_low = 0;
+		mtype_high = MIGRATE_PCPTYPES;
+	} else {
+		mtype_low = cc->migratetype;
+		mtype_high = cc->migratetype + 1;
+	}
+
 	/* Speculatively examine the free lists without zone lock */
-	for (mtype = 0; mtype < MIGRATE_PCPTYPES; mtype++) {
+	for (mtype = mtype_low; mtype < mtype_high; mtype++) {
 		int order;
 		for (order = cc->order; order < MAX_ORDER; order++) {
 			struct page *page;
@@ -752,8 +770,7 @@ static int compact_zone(struct zone *zone, struct compact_control *cc)
 		}
 
 		/* Capture a page now if it is a suitable size */
-		if (cc->migratetype == MIGRATE_MOVABLE)
-			compact_capture_page(cc);
+		compact_capture_page(cc);
 	}
 
 out:

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