linux-kernel - Re: [PATCH v2 2/4] mm: make the threshold of enabling THP configurable

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Date:	Thu, 23 Jun 2011 14:51:47 +0800
From:	Cong Wang <amwang@...hat.com>
To:	Andrea Arcangeli <aarcange@...hat.com>
CC:	David Rientjes <rientjes@...gle.com>, linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	dave@...ux.vnet.ibm.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 v2 2/4] mm: make the threshold of enabling THP configurable

于 2011年06月22日 22:40, Andrea Arcangeli 写道:
> On Wed, Jun 22, 2011 at 07:07:25PM +0800, Cong Wang wrote:
>> Actually, if we move this out of kernel, to user-space, everything
>> you worried will be solved by just changing the user-space code.
>> Just add the following pseudo code into your init script,
>>
>> if [ $total_memory -lt 512 ]
>> then
>> 	echo never>  /sys/kernel/mm/transparent_hugepage/enabled
>> fi
>
> By the time this script runs some app may have allocated hugepages
> already potentially wasting mbytes of ram and undoing the
> min_free_kbytes isn't possible from userland using the kernel
> algorithm (it is possible actually but it's not nearly as simple as
> the above).

I remember there is a way to tell init-scripts to run it as early as
possible. :)

>
> There's no reason to complicate things and involve userland here when
> a simple kernel check can get the default right without userland
> dependency. Plus if this user really wants THP on 512m of ram he can
> still enable it and run hugeadm to enable antifrag too, without the
> need of =force. And forcing when PSE is enabled sounds impossible to be
> useful (maybe with the except of nopentium being passed to the kernel ;).
>
> There is no bug here, just send that printk cleanup and if you really
> want to save 8k the patch to change the number of hash heads structs
> at boot, like for dcache/icache. No other change required.


I never said this is a bug, I just don't think it is flexible. ;)

>
> After you do the above, you can go ahead picking one kernel crashing
> bug and fix it, that is more useful than making this 512m thing a
> .config variable or anything like that, .config is a nightmare already
> so it's probably better not to add anything there.

Well, I can't agree with this point.

Since we can't persuade each other, I give up.

Thanks.
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