linux-kernel - Re: [PATCH] mm: Fix XFS oops due to dirty pages without buffers on s390

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Date:	Fri, 19 Oct 2012 16:38:14 +0200
From:	Martin Schwidefsky <schwidefsky@...ibm.com>
To:	Hugh Dickins <hughd@...gle.com>
Cc:	Jan Kara <jack@...e.cz>, linux-mm@...ck.org,
	LKML <linux-kernel@...r.kernel.org>, xfs@....sgi.com,
	Mel Gorman <mgorman@...e.de>, linux-s390@...r.kernel.org
Subject: Re: [PATCH] mm: Fix XFS oops due to dirty pages without buffers on
 s390

On Tue, 9 Oct 2012 16:21:24 -0700 (PDT)
Hugh Dickins <hughd@...gle.com> wrote:

> > 
> > I am seriously tempted to switch to pure software dirty bits by using
> > page protection for writable but clean pages. The worry is the number of
> > additional protection faults we would get. But as we do software dirty
> > bit tracking for the most part anyway this might not be as bad as it
> > used to be.  
> 
> That's exactly the same reason why tmpfs opts out of dirty tracking, fear
> of unnecessary extra faults.  Anomalous as s390 is here, tmpfs is being
> anomalous too, and I'd be a hypocrite to push for you to make that change.

I tested the waters with the software dirty bit idea. Using kernel compile
as test case I got these numbers:

disk backing, swdirty: 10,023,870 minor-faults 18 major-faults
disk backing, hwdirty: 10,023,829 minor-faults 21 major-faults                          

tmpfs backing, swdirty: 10,019,552 minor-faults 49 major-faults
tmpfs backing, hwdirty: 10,032,909 minor-faults 81 major-faults

That does not look bad at all. One test I found that shows an effect is
lat_mmap from LMBench:

disk backing, hwdirty: 30,894 minor-faults 0 major-faults
disk backing, swdirty: 30,894 minor-faults 0 major-faults

tmpfs backing, hwdirty: 22,574 minor-faults 0 major-faults
tmpfs backing, swdirty: 36,652 minor-faults 0 major-faults 

The runtime between the hwdirty vs. the swdirty setup is very similar,
encouraging enough for me to ask our performance team to run a larger test.

-- 
blue skies,
   Martin.

"Reality continues to ruin my life." - Calvin.

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