linux-kernel - Re: Idea for reducing sysfs memory usage

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Date:	Wed, 17 Feb 2016 00:37:38 +0000
From:	Edward Cree <ec429@...tab.net>
To:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:	linux-kernel@...r.kernel.org, David Ahern <dsa@...ulusnetworks.com>
Subject: Re: Idea for reducing sysfs memory usage

On 16/02/16 23:55, Greg Kroah-Hartman wrote:
> On Tue, Feb 16, 2016 at 11:46:49PM +0000, Edward Cree wrote:
>> Sorry if this has been suggested before, but if so I couldn't find it.
>> Short version: could a sysfs dir reference a list of default attributes
>> rather than having to instantiate them all?
> Shorter version, why do you think it is?  :)
>
> Have you done some testing of the amount of memory that sysfs entries
> consume and found any problems with it?
Two reasons:
a) in his netdev1.1 talk "Scaling the Number of Network Interfaces on 
Linux",
    David Ahern claimed a memory overhead of (iirc) about 45kB per 
netdevice,
    of which he attributed (again, iirc) about 20kB to sysfs entries.  
He also
    indicated that this was a problem for his use case.  (My apologies to
    David if I've misrepresented him.  CCed him so he can correct me.)
b) my reading of the code suggested it was allocating stuff for every 
call to
    sysfs_create_file() in the loop in populate_dir().
Having re-read __kernfs_new_node() and struct kernfs_node, I now realise I
misinterpreted them - the name isn't being allocated at all 
(kstrdup_const())
and the struct kernfs_node consists chiefly (if not entirely) of fields
specific to the individual file rather than shareable between multiple
instances.  So there isn't any memory we can save here.

Sorry for the noise.

--
-ed