linux-kernel - Re: [RFC PATCH 2/3] Add tunable to control THP behavior

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Date:	Thu, 12 Dec 2013 16:37:11 -0500
From:	Rik van Riel <riel@...hat.com>
To:	Alex Thorlton <athorlton@....com>, linux-mm@...ck.org
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Wanpeng Li <liwanp@...ux.vnet.ibm.com>,
	Mel Gorman <mgorman@...e.de>,
	Michel Lespinasse <walken@...gle.com>,
	Benjamin LaHaise <bcrl@...ck.org>,
	Oleg Nesterov <oleg@...hat.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Andy Lutomirski <luto@...capital.net>,
	Al Viro <viro@...iv.linux.org.uk>,
	David Rientjes <rientjes@...gle.com>,
	Zhang Yanfei <zhangyanfei@...fujitsu.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Johannes Weiner <hannes@...xchg.org>,
	Michal Hocko <mhocko@...e.cz>,
	Jiang Liu <jiang.liu@...wei.com>,
	Cody P Schafer <cody@...ux.vnet.ibm.com>,
	Glauber Costa <glommer@...allels.com>,
	Kamezawa Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
	linux-kernel@...r.kernel.org,
	Andrea Arcangeli <aarcange@...hat.com>
Subject: Re: [RFC PATCH 2/3] Add tunable to control THP behavior

On 12/12/2013 01:00 PM, Alex Thorlton wrote:
> This part of the patch adds a tunable to
> /sys/kernel/mm/transparent_hugepage called threshold.  This threshold
> determines how many pages a user must fault in from a single node before
> a temporary compound page is turned into a THP.

> +++ b/mm/huge_memory.c
> @@ -44,6 +44,9 @@ unsigned long transparent_hugepage_flags __read_mostly =
>   	(1<<TRANSPARENT_HUGEPAGE_DEFRAG_KHUGEPAGED_FLAG)|
>   	(1<<TRANSPARENT_HUGEPAGE_USE_ZERO_PAGE_FLAG);
>
> +/* default to 1 page threshold for handing out thps; maintains old behavior */
> +static int transparent_hugepage_threshold = 1;

I assume the motivation for writing all this code is that "1"
was not a good value in your tests.

That makes me wonder, why should 1 be the default value with
your patches?

If there is a better value, why should we not use that?

What is the upside of using a better value?

What is the downside?

Is there a value that would to bound the downside, so it
is almost always smaller than the upside?
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