On Tue, Nov 06, 2012 at 11:15:54AM +0100, Johannes Hirte wrote:

Am Mon, 5 Nov 2012 14:24:49 +0000
schrieb Mel Gorman [off-list ref]:

quoted

Jiri Slaby reported the following:

	(It's an effective revert of "mm: vmscan: scale number of
pages reclaimed by reclaim/compaction based on failures".) Given
kswapd had hours of runtime in ps/top output yesterday in the morning
	and after the revert it's now 2 minutes in sum for the last
24h, I would say, it's gone.

The intention of the patch in question was to compensate for the loss
of lumpy reclaim. Part of the reason lumpy reclaim worked is because
it aggressively reclaimed pages and this patch was meant to be a sane
compromise.

When compaction fails, it gets deferred and both compaction and
reclaim/compaction is deferred avoid excessive reclaim. However, since
commit c6543459 (mm: remove __GFP_NO_KSWAPD), kswapd is woken up each
time and continues reclaiming which was not taken into account when
the patch was developed.

Attempts to address the problem ended up just changing the shape of
the problem instead of fixing it. The release window gets closer and
while a THP allocation failing is not a major problem, kswapd chewing
up a lot of CPU is. This patch reverts "mm: vmscan: scale number of
pages reclaimed by reclaim/compaction based on failures" and will be
revisited in the future.

Signed-off-by: Mel Gorman <mgorman@suse.de>
---
 mm/vmscan.c |   25 -------------------------
 1 file changed, 25 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 2624edc..e081ee8 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c

@@ -1760,28 +1760,6 @@ static bool in_reclaim_compaction(struct

scan_control *sc) return false;
 }
 
-#ifdef CONFIG_COMPACTION
-/*
- * If compaction is deferred for sc->order then scale the number of
pages
- * reclaimed based on the number of consecutive allocation failures
- */
-static unsigned long scale_for_compaction(unsigned long
pages_for_compaction,
-			struct lruvec *lruvec, struct scan_control
*sc) -{
-	struct zone *zone = lruvec_zone(lruvec);
-
-	if (zone->compact_order_failed <= sc->order)
-		pages_for_compaction <<= zone->compact_defer_shift;
-	return pages_for_compaction;
-}
-#else
-static unsigned long scale_for_compaction(unsigned long
pages_for_compaction,
-			struct lruvec *lruvec, struct scan_control
*sc) -{
-	return pages_for_compaction;
-}
-#endif
-
 /*
  * Reclaim/compaction is used for high-order allocation requests. It
reclaims
  * order-0 pages before compacting the zone.
should_continue_reclaim() returns @@ -1829,9 +1807,6 @@ static inline
bool should_continue_reclaim(struct lruvec *lruvec,
 	 * inactive lists are large enough, continue reclaiming
 	 */
 	pages_for_compaction = (2UL << sc->order);
-
-	pages_for_compaction =
scale_for_compaction(pages_for_compaction,
-						    lruvec, sc);
 	inactive_lru_pages = get_lru_size(lruvec, LRU_INACTIVE_FILE);
 	if (nr_swap_pages > 0)
 		inactive_lru_pages += get_lru_size(lruvec,
LRU_INACTIVE_ANON); --

Even with this patch I see kswapd0 very often on top. Much more than
with kernel 3.6.

How severe is the CPU usage? The higher usage can be explained by "mm:
remove __GFP_NO_KSWAPD" which allows kswapd to compact memory to reduce
the amount of time processes spend in compaction but will result in the
CPU cost being incurred by kswapd.

Is it really high like the bug was reporting with high usage over long
periods of time or do you just see it using 2-6% of CPU for short
periods?

Thanks.

-- 
Mel Gorman
SUSE Labs

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