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

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Date:	Tue, 21 Jun 2011 01:34:51 +0800
From:	Cong Wang <amwang@...hat.com>
To:	Rik van Riel <riel@...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

于 2011年06月21日 01:28, Rik van Riel 写道:
> 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...
>

Even if it is really 10K, why not save it since it doesn't
much effort to make this. ;) Not only memory, but also time,
this could also save a little time to initialize the kernel.

For me, the more serious thing is the logic, there is
no way to totally disable it as long as I have THP in .config
currently. This is why I said the design is broken.

Thanks.
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