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

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Message-ID: <54EB82D0.9080606@redhat.com>
Date:	Mon, 23 Feb 2015 14:43:12 -0500
From:	Rik van Riel <riel@...hat.com>
To:	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, vbabka@...e.cz, 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

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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!
> 
> 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?

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?

- -- 
All rights reversed
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