linux-kernel - Re: [PATCH 1/3] mm: completely disable THP by transparent

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Date:	Mon, 20 Jun 2011 13:28:07 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Cong Wang <amwang@...hat.com>
CC:	Mel Gorman <mgorman@...e.de>, linux-kernel@...r.kernel.org,
	akpm@...ux-foundation.org, Andrea Arcangeli <aarcange@...hat.com>,
	Johannes Weiner <jweiner@...hat.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	linux-mm@...ck.org
Subject: Re: [PATCH 1/3] mm: completely disable THP by transparent_hugepage=never

On 06/20/2011 01:19 PM, Cong Wang wrote:
> 于 2011年06月21日 01:10, Rik van Riel 写道:
>> On 06/20/2011 01:07 PM, Cong Wang wrote:
>>> 于 2011年06月21日 00:58, Mel Gorman 写道:
>>>> On Tue, Jun 21, 2011 at 12:34:28AM +0800, Amerigo Wang wrote:
>>>>> transparent_hugepage=never should mean to disable THP completely,
>>>>> otherwise we don't have a way to disable THP completely.
>>>>> The design is broken.
>>>>>
>>>>
>>>> I don't get why it's broken. Why would the user be prevented from
>>>> enabling it at runtime?
>>>>
>>>
>>> We need to a way to totally disable it, right? Otherwise, when I
>>> configure
>>> THP in .config, I always have THP initialized even when I pass "=never".
>>>
>>> For me, if you don't provide such way to disable it, it is not flexible.
>>>
>>> I meet this problem when I try to disable THP in kdump kernel, there is
>>> no user of THP in kdump kernel, THP is a waste for kdump kernel. This is
>>> why I need to find a way to totally disable it.
>>
>> What you have not explained yet is why having THP
>> halfway initialized (but not used, and without a
>> khugepaged thread) is a problem at all.
>>
>> Why is it a problem for you?
>
> It occupies some memory, memory is valuable in kdump kernel (usually
> only 128M). :) Since I am sure no one will use it, why do I still need
> to initialize it at all?

Lets take a look at how much memory your patches end
up saving.

By bailing out earlier in hugepage_init, you end up
saving 3 sysfs objects, one slab cache and a hash
table with 1024 pointers.  That's a total of maybe
10kB of memory on a 64 bit system.

I'm not convinced that a 10kB memory reduction is
worth the price of never being able to enable
transparent hugepages when a system is booted with
THP disabled...

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