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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