Re: [PATCH v2] mm: incorporate zero pages into transparent huge pages

From: Vlastimil Babka
Date: Mon Feb 23 2015 - 16:45:35 EST

On 23.2.2015 20:43, Rik van Riel wrote:
Hash: SHA1

On 02/23/2015 02:16 PM, Andrew Morton wrote:
On Wed, 18 Feb 2015 19:08:12 -0500 Rik van Riel <riel@xxxxxxxxxx>
If so, this might be rather undesirable behaviour in some
situations (and ditto the current behaviour for pte_none

This can be tuned by adjusting khugepaged_max_ptes_none,
Here's a live one:

Application does MADV_DONTNEED to free up a load of memory and
then khugepaged comes along and pages that memory back in again.
It seems a bit silly to do this after userspace has deliberately
discarded those pages!

OK that's a nice example how a more conservative default for
max_ptes_none would make sense even with the current aggressive
THP faulting.

Presumably MADV_NOHUGEPAGE can be used to prevent this, but it's a
bit of a hand-grenade. I guess the MADV_DONTNEED manpage should be
updated to explain all this?

Probably, together with the tunable documentation. Seems like we
didn't add enough details to madvise manpage in the recent round :)

That makes me wonder what a good value for khugepaged_max_ptes_none
would be.

Doubling the amount of memory a program uses seems quite unreasonable.

Increasing the amount of memory a program uses by 512x seems totally

Increasing the amount of memory a program uses by 20% might be
reasonable, if that much memory is available, since that seems to
be about how much performance improvement we have ever seen from

Andrew, Andrea, do you have any ideas on this?

Is this something to just set, or should we ask Ebru to run
a few different tests with this?

If there is a good test for this, sure.

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