netdev - Re: [PATCH] net: allow netdev_wait

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Date:	Fri, 30 Oct 2009 10:35:27 -0400
From:	Benjamin LaHaise <bcrl@...et.ca>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	Eric Dumazet <eric.dumazet@...il.com>,
	Octavian Purdila <opurdila@...acom.com>,
	netdev@...r.kernel.org, Cosmin Ratiu <cratiu@...acom.com>
Subject: Re: [PATCH] net: allow netdev_wait_allrefs() to run faster

On Thu, Oct 29, 2009 at 06:45:32PM -0700, Eric W. Biederman wrote:
> The reason for the existence of sysfs_dirent is as things grow larger
> we want to keep the amount of RAM consumed down.  So we don't pin
> everything in the dcache.  So we try and keep the amount of memory
> consumed down.

I'm aware of that, but for users running into this sort of scaling issue, 
the amount of RAM required is a non-issue (30,000 interfaces require about 
1GB of RAM at present), making the question more one of how to avoid the 
overhead for users who don't require it.  I'd prefer a config option.  The 
only way I can really see saving memory usage is to somehow tie sysfs dirent 
lookups into the network stack's own tables for looking up device entries.  
The network stack already has to cope with this kind of scaling, and that 
would save the RAM.

> So I would like to see how much we can par down.

> For dealing with seeks in the middle of readdir I expect the best way
> to do that is to be inspired by htrees in extNfs and return a hash of
> the filename as our position, and keep the filename list sorted by
> that hash.  Since we are optimizing for size we don't need to store
> that hash.  Then we can turn that list into a some flavor of sorted
> binary tree.

readdir() generally isn't an issue at present.

> I'm surprised sysfs_count_nlink shows up, as it is not directly on the
> add or remove path.  I think the answer there is to change s_flags
> into a set of bitfields and make link_count one of them, perhaps
> 16bits long.  If we ever overflow our bitfield we can just set link
> count to 0, and userspace (aka find) will know it can't optimized
> based on link count.

It shows up because of the bits of userspace (udev) touching the directory 
from things like the hotplug code path.

> I was expecting someone to run into problems with the linear directory
> of sysfs someday.

Alas, sysfs isn't the only offender.

		-ben
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