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Date: Fri, 20 Mar 2015 10:12:46 +0000
From: Mel Gorman <mgorman@suse.de>
Subject: Re: [PATCH 4/4] mm: numa: Slow PTE scan rate if migration failures
 occur
Message-ID: <20150320101246.GF3087@suse.de>
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To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Dave Chinner <david@fromorbit.com>, Ingo Molnar <mingo@kernel.org>, Andrew Morton <akpm@linux-foundation.org>, Aneesh Kumar <aneesh.kumar@linux.vnet.ibm.com>, Linux Kernel Mailing List <linux-kernel@vger.kernel.org>, Linux-MM <linux-mm@kvack.org>, xfs@oss.sgi.com, ppc-dev <linuxppc-dev@lists.ozlabs.org>

On Thu, Mar 19, 2015 at 06:29:47PM -0700, Linus Torvalds wrote:
> And the VM_WRITE test should be stable and not have any subtle
> interaction with the other changes that the numa pte things
> introduced. It would be good to see if the profiles then pop something
> *else* up as the performance difference (which I'm sure will remain,
> since the 7m50s was so far off).
> 

As a side-note, I did test a patch that checked pte_write and preserved
it across both faults and setting the protections. It did not alter
migration activity much but there was a  drop in minor faults - 20% drop in
autonumabench, 58% drop in xfsrepair workload. I'm assuming this is due to
refaults to mark pages writable.  The patch looks and is hacky so I won't
post it to save people bleaching their eyes. I'll spend some time soon
(hopefully today) at a smooth way of falling through to WP checks after
trapping a NUMA fault.

-- 
Mel Gorman
SUSE Labs

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