linux-kernel - Re: [RFC PATCH linux 2/2] fs/proc: use a hash table for the directory entries

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Message-ID: <542E501B.1010109@6wind.com>
Date:	Fri, 03 Oct 2014 09:28:27 +0200
From:	Nicolas Dichtel <nicolas.dichtel@...nd.com>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Alexey Dobriyan <adobriyan@...il.com>
CC:	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	davem@...emloft.net, akpm@...ux-foundation.org,
	rui.xiang@...wei.com, viro@...iv.linux.org.uk, oleg@...hat.com,
	gorcunov@...nvz.org, kirill.shutemov@...ux.intel.com,
	grant.likely@...retlab.ca, tytso@....edu,
	Thierry Herbelot <thierry.herbelot@...nd.com>
Subject: Re: [RFC PATCH linux 2/2] fs/proc: use a hash table for the directory
 entries

Le 02/10/2014 23:07, Eric W. Biederman a écrit :
> Alexey Dobriyan <adobriyan@...il.com> writes:
>
>> On Thu, Oct 02, 2014 at 11:01:50AM -0700, Eric W. Biederman wrote:
>>> Nicolas Dichtel <nicolas.dichtel@...nd.com> writes:
>>>
>>>> From: Thierry Herbelot <thierry.herbelot@...nd.com>
>>>>
>>>> The current implementation for the directories in /proc is using a single
>>>> linked list. This is slow when handling directories with large numbers of
>>>> entries (eg netdevice-related entries when lots of tunnels are opened).
>>>>
>>>> This patch enables multiple linked lists. A hash based on the entry name is
>>>> used to select the linked list for one given entry.
>>>>
>>>> The speed creation of netdevices is faster as shorter linked lists must be
>>>> scanned when adding a new netdevice.
>>>
>>> Is the directory of primary concern /proc/net/dev/snmp6 ?
>>>
>>> Unless I have configured my networking stack weird by mistake that
>>> is the only directory under /proc/net that grows when we add an
>>> interface.
>>>
>>> I just want to make certain I am seeing the same things that you are
>>> seeing.
>>>
>>> I feel silly for overlooking this directory when the rest of the
>>> scalability work was done.
>>
>> Slowdown comes from "duplicate name" check:
>>
>>          for (tmp = dir->subdir; tmp; tmp = tmp->next)
>>                  if (strcmp(tmp->name, dp->name) == 0) {
>>                          WARN(1, "proc_dir_entry '%s/%s' already registered\n",
>>                                  dir->name, dp->name);
>>                          break;
>>                  }
>>
>> Removal can be made O(1) after switching to doubly-linked list.
>
> Yes.  There is the however unfortunate fact that proc directories exist
> to be used.  If we don't switch to a better data structure than a linked
> list the actual use will then opening of the files under
> /proc/net/dev/snmp6/ will become O(N^2).  Which doesn't help much
> (assuming those files are good for something).
>
> If those files aren't actually useful we should just make registering
> them a config option.  Deprecate them strongly and let only people who
> need extreme backwards compatibility enable them.
>
> Alexey do you know that those files aren't useful?  Unless we know
> otherwise we should make those files useful.
This was proposed and nacked in a first version:
http://thread.gmane.org/gmane.linux.network/285840/


Regards,
Nicolas
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