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

From: Stephen Hemminger <[email-masked]>
Date: Thu, 2 Oct 2014 09:46:57 -0700

On Thu,  2 Oct 2014 17:25:01 +0200
Nicolas Dichtel <[email-masked]> wrote:

> From: Thierry Herbelot <[email-masked]>
> 
> 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 <[email-masked]>
> Signed-off-by: Nicolas Dichtel <[email-masked]>

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.