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

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Date:	Fri, 03 Oct 2014 15:10:51 +0200
From:	Nicolas Dichtel <nicolas.dichtel@...nd.com>
To:	Stephen Hemminger <stephen@...workplumber.org>
CC:	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	davem@...emloft.net, ebiederm@...ssion.com,
	akpm@...ux-foundation.org, adobriyan@...il.com,
	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 18:46, Stephen Hemminger a écrit :
> On Thu,  2 Oct 2014 17:25:01 +0200
> Nicolas Dichtel <nicolas.dichtel@...nd.com> wrote:
>
>> 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.
>>
>> Here are some numbers:
>>
>> dummy30000.batch contains 30 000 times 'link add type dummy'.
>>
>> Before the patch:
>> time ip -b dummy30000.batch
>> real    2m32.221s
>> user    0m0.380s
>> sys     2m30.610s
>>
>> After the patch:
>> time ip -b dummy30000.batch
>> real    1m57.190s
>> user    0m0.350s
>> sys     1m56.120s
>>
>> The single 'subdir' list head is replaced by a subdir hash table. The subdir
>> hash buckets are only allocated for directories. The number of hash buckets
>> is a compile-time parameter.
>>
>> For all functions which handle directory entries, an additional check on the
>> directory nature of the dir entry ensures that pde_hash_buckets was allocated.
>> This check was not needed as subdir was present for all dir entries, whether
>> actual directories or simple files.
>>
>> Signed-off-by: Thierry Herbelot <thierry.herbelot@...nd.com>
>> Signed-off-by: Nicolas Dichtel <nicolas.dichtel@...nd.com>
>
> I think the speed up is a good idea and makes sense.
> It would be better to use exist hlist macros for hash table rather than
> open coding it.
>
Right, I will rework this after looking at rbtree.
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