linux-kernel - Re: [RFC PATCH] mm: thp: Add per-mm

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Date:	Fri, 10 Jan 2014 16:01:55 -0600
From:	Alex Thorlton <athorlton@....com>
To:	"Kirill A. Shutemov" <kirill@...temov.name>
Cc:	linux-mm@...ck.org, Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Rik van Riel <riel@...hat.com>,
	Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
	Oleg Nesterov <oleg@...hat.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Andy Lutomirski <luto@...capital.net>,
	Al Viro <viro@...iv.linux.org.uk>,
	Kees Cook <keescook@...omium.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH] mm: thp: Add per-mm_struct flag to control THP

On Fri, Jan 10, 2014 at 10:23:10PM +0200, Kirill A. Shutemov wrote:
> Do you know what cause the difference? I prefer to fix THP instead of
> adding new knob to disable it.

The issue is that when you touch 1 byte of an untouched, contiguous 2MB
chunk, a THP will be handed out, and the THP will be stuck on whatever
node the chunk was originally referenced from.  If many remote nodes
need to do work on that same chunk, they'll be making remote accesses.
With THP disabled, 4K pages can be handed out to separate nodes as
they're needed, greatly reducing the amount of remote accesses to
memory.  I give a bit better description here:

https://lkml.org/lkml/2013/8/27/397

I had been looking into better ways to handle this issues, but after
spinning through a few other ideas:

- Per cpuset flag to control THP:
https://lkml.org/lkml/2013/6/10/331

- Threshold to determine when to hand out THPs:
https://lkml.org/lkml/2013/12/12/394

We've arrived back here.  Andrea seemed to think that this is an
acceptable approach to solve the problem, at least as a starting point:

https://lkml.org/lkml/2013/12/17/397

I agree that we should, ideally, come up with a way to appropriately
handle this problem in the kernel, but as of right now, it appears that
that might be a rather large undertaking.

- Alex
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