Re: [PATCH 4/4] mm: numa: Slow PTE scan rate if migration failures occur

[Mel Gorman]

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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