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

From: Mel Gorman
Date: Thu Dec 20 2012 - 06:12:58 EST

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@xxxxxxx

Mel Gorman
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