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Message-Id: <20121220125802.23e9b22d.akpm@linux-foundation.org> Date: Thu, 20 Dec 2012 12:58:02 -0800 From: Andrew Morton <akpm@...ux-foundation.org> To: Mel Gorman <mgorman@...e.de> Cc: Zlatko Calusic <zlatko.calusic@...on.hr>, Linus Torvalds <torvalds@...ux-foundation.org>, Hugh Dickins <hughd@...gle.com>, linux-mm <linux-mm@...ck.org>, Linux Kernel Mailing List <linux-kernel@...r.kernel.org> Subject: Re: [PATCH] mm: do not sleep in balance_pgdat if there's no i/o congestion On Thu, 20 Dec 2012 11:12:08 +0000 Mel Gorman <mgorman@...e.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@...e.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(...)) ? -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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