Am Fri, 9 Nov 2012 08:36:37 +0000
schrieb Mel Gorman [off-list ref]:

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

quoted

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?

It is really high. I've seen with compile-jobs (make -j4 on dual
core) kswapd0 consuming at least 50% CPU most time.

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