Re: [PATCH] mm: do not sleep in balance_pgdat if there's no i/o congestion

[Andrew Morton <akpm@linux-foundation.org>]

On Thu, 20 Dec 2012 11:12:08 +0000
Mel Gorman <mgorman@suse.de> wrote:

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