linux-kernel - Re: [PATCH 1/1] mm: thp: Redefine default THP defrag behaviour disable it by default

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Date:	Thu, 25 Feb 2016 13:32:50 -0500
From:	Rik van Riel <riel@...hat.com>
To:	Mel Gorman <mgorman@...hsingularity.net>,
	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Vlastimil Babka <vbabka@...e.cz>,
	Johannes Weiner <hannes@...xchg.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Linux-MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/1] mm: thp: Redefine default THP defrag behaviour
 disable it by default

On Thu, 2016-02-25 at 17:12 +0000, Mel Gorman wrote:

> THP gives impressive gains in some cases but only if they are quickly
> available.  We're not going to reach the point where they are
> completely
> free so lets take the costs out of the fast paths finally and defer
> the
> cost to kswapd, kcompactd and khugepaged where it belongs.
> 
> Signed-off-by: Mel Gorman <mgorman@...hsingularity.net>

I agree with your conclusions, but with the caveat
that if we do not try to defragment memory for THP
at fault time, mlocked programs might not have any
opportunity at all to get transparent huge pages.

I wonder if we should consider mlock one of the slow
paths where we should try to actually take the time
to create THPs.

Also, we might consider doing THP collapse from the
NUMA page migration opportunistically, if there is a
free 2MB page available on the destination host.

Having said all that ...

Acked-by: Rik van Riel <riel@...hat.com>

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