On Thu, 20 Dec 2012 11:12:08 +0000
Mel Gorman [off-list ref] wrote:

On Thu, Dec 20, 2012 at 12:17:07AM +0100, Zlatko Calusic wrote:

quoted

On a 4GB RAM machine, where Normal zone is much smaller than
DMA32 zone, the Normal zone gets fragmented in time. This requires
relatively more pressure in balance_pgdat to get the zone above the
required watermark. Unfortunately, the congestion_wait() call in there
slows it down for a completely wrong reason, expecting that there's
a lot of writeback/swapout, even when there's none (much more common).
After a few days, when fragmentation progresses, this flawed logic
translates to a very high CPU iowait times, even though there's no
I/O congestion at all. If THP is enabled, the problem occurs sooner,
but I was able to see it even on !THP kernels, just by giving it a bit
more time to occur.

The proper way to deal with this is to not wait, unless there's
congestion. Thanks to Mel Gorman, we already have the function that
perfectly fits the job. The patch was tested on a machine which
nicely revealed the problem after only 1 day of uptime, and it's been
working great.
---
 mm/vmscan.c |   12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

Acked-by: Mel Gorman <mgorman@suse.de

There seems to be some complexity/duplication here between the new
unbalanced_zone() and pgdat_balanced().

Can we modify pgdat_balanced() so that it also handles order=0, then do

-		if (!unbalanced_zone || (order && pgdat_balanced(pgdat, balanced, *classzone_idx)))
+		if (!pgdat_balanced(...))

?

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