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

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Date:	Mon, 23 Feb 2015 22:45:21 +0100
From:	Vlastimil Babka <vbabka@...e.cz>
To:	Rik van Riel <riel@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>
CC:	Ebru Akagunduz <ebru.akagunduz@...il.com>, linux-mm@...ck.org,
	kirill@...temov.name, mhocko@...e.cz, mgorman@...e.de,
	rientjes@...gle.com, sasha.levin@...cle.com, hughd@...gle.com,
	hannes@...xchg.org, linux-kernel@...r.kernel.org,
	aarcange@...hat.com, keithr@...m.mit.edu, dvyukov@...gle.com
Subject: Re: [PATCH v2] mm: incorporate zero pages into transparent huge pages

On 23.2.2015 20:43, Rik van Riel wrote:
> -----BEGIN PGP SIGNED MESSAGE-----
> Hash: SHA1
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> On 02/23/2015 02:16 PM, Andrew Morton wrote:
>> On Wed, 18 Feb 2015 19:08:12 -0500 Rik van Riel <riel@...hat.com>
>> wrote:
>>>> If so, this might be rather undesirable behaviour in some
>>>> situations (and ditto the current behaviour for pte_none
>>>> ptes)?
>>>>
>>>> This can be tuned by adjusting khugepaged_max_ptes_none,
>> Here's a live one:
>> https://bugzilla.kernel.org/show_bug.cgi?id=93111
>>
>> 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
> unreasonable.
>
> 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
> THP.
>
> 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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