linux-kernel - Re: [PATCH 2/3] mm: make the threshold of enabling THP configurable

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Date:	Tue, 21 Jun 2011 01:23:19 +0800
From:	Cong Wang <amwang@...hat.com>
To:	Dave Hansen <dave@...ux.vnet.ibm.com>
CC:	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
	Andrea Arcangeli <aarcange@...hat.com>,
	Mel Gorman <mel@....ul.ie>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Rik van Riel <riel@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	linux-mm@...ck.org
Subject: Re: [PATCH 2/3] mm: make the threshold of enabling THP configurable

于 2011年06月21日 00:59, Dave Hansen 写道:
> On Tue, 2011-06-21 at 00:34 +0800, Amerigo Wang wrote:
>> +config TRANSPARENT_HUGEPAGE_THRESHOLD
>> +       depends on TRANSPARENT_HUGEPAGE
>> +       int "The minimal threshold of enabling Transparent Hugepage"
>> +       range 512 8192
>> +       default "512"
>> +       help
>> +         The threshold of enabling Transparent Huagepage automatically,
>> +         in Mbytes, below this value, Transparent Hugepage will be disabled
>> +         by default during boot.
>
> It makes some sense to me that there would _be_ a threshold, simply
> because you need some space to defragment things.  But, I can't imagine
> any kind of user having *ANY* kind of idea what to set this to.  Could
> we add some text to this?  Maybe:
>
>          Transparent hugepages are created by moving other pages out of
>          the way to create large, contiguous swaths of free memory.
>          However, some memory on a system can not be easily moved.  It is
>          likely on small systems that this unmovable memory will occupy a
>          large portion of total memory, which makes even attempting to
>          create transparent hugepages very expensive.
>
>          If you are unsure, set this to the smallest possible value.
>
>          To override this at boot, use the $FOO boot command-line option.
>

Yeah, I totally agree to improve the help message as you said,
please forgive a non-English speaker. ;)

> I'm also not sure putting a ceiling on this makes a lot of sense.
> What's the logic behind that?  I know it would be a mess to expose it to
> users, but shouldn't this be a per-zone limit, logically?  Seems like a
> 8GB system would have similar issues to a two-numa-node 16GB system.
>

I am not sure about this, since I am new to THP, I just replaced
the hard-code 512 with a Kconfig var. But I am certainly open
to improve this as you said if Andrea agrees.

Thanks.
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