linux-kernel - Re: [PATCH] memory hotplug disable boot option

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Date:	Mon, 28 Jun 2010 10:37:12 -0500
From:	Nathan Fontenot <nfont@...tin.ibm.com>
To:	Greg KH <gregkh@...e.de>
CC:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Andi Kleen <andi@...stfloor.org>, linux-kernel@...r.kernel.org,
	"Eric W. Biederman" <ebiederm@...ssion.com>
Subject: Re: [PATCH] memory hotplug disable boot option

On 06/28/2010 10:02 AM, Greg KH wrote:
> On Mon, Jun 28, 2010 at 11:20:27AM +0900, KOSAKI Motohiro wrote:
>>> On 06/25/2010 04:19 AM, Andi Kleen wrote:
>>>> KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com> writes:
>>>>
>>>>>> Proposed patch to disable memory hotplug via a boot option,
>>>>>> mem_hotplug=[on|off].  The patch only disables memory hotplug in that it 
>>>>>> prevents the creation of the memory sysfs directories for memory sections.
>>>>>>
>>>>>> This patch is meant to help alleviate very long boot times on systems with
>>>>>> large memory (1+ TB) and many memory sections (10's of thousands).
>>>>>
>>>>> Why making simple /sys file is so slowly? Couldn't we fix such performance
>>>>> problem?
>>>
>>> The issue is the large number of sysfs memory directories that get created.
>>> On a system with 1 TB of memory I am seeing ~63,00 directories.  The long
>>> creation time is due to the string compare check in sysfs code to ensure
>>> we are not creating a directory with a duplicate name.
>>
>> Ah, I see. probably this is sysfs issue. So Let's cc Greg and Eric.
>> Greg, I have dumb question. Why sysfs call strcmp() so heavily? I mean why sysfs
>> don't have hash based name dupliation check?
> 
> Because we have not needed such complexity before.
> 
> You might want to take a step back and asky why you are creating 63
> thousand directories in sysfs for large memory systems.  Are you really
> going to use all of those directories?  What are they for?  Perhaps
> someone created the wrong interface to memory and that needs to be fixed
> instead?
> 

The directories being created are the standard directories, one for each of the memory
sections present at boot.  I think the most used files in each of these directories
is the state and removable file used to do memory hotplug.

-Nathan
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