linux-kernel - Re: [PATCH] mm: do not sleep in balance

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Message-ID: <20121220111208.GD10819@suse.de>
Date:	Thu, 20 Dec 2012 11:12:08 +0000
From:	Mel Gorman <mgorman@...e.de>
To:	Zlatko Calusic <zlatko.calusic@...on.hr>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...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, 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

-- 
Mel Gorman
SUSE Labs
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