netdev - Re: [PATCH net-next 0/2] Neighbour table prep for MPLS

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Date:	Thu, 05 Mar 2015 06:35:00 -0600
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	David Miller <davem@...emloft.net>
Cc:	netdev@...r.kernel.org, Andy Gospodarek <gospo@...ulusnetworks.com>
Subject: Re: [PATCH net-next 0/2] Neighbour table prep for MPLS

David Miller <davem@...emloft.net> writes:

> From: ebiederm@...ssion.com (Eric W. Biederman)
> Date: Tue, 03 Mar 2015 23:53:21 -0600
>
>> We could potentially translate the numbers into the enumeration that is
>> NEIGH_ARP_TABLE, NEIGH_ND_TABLE, and NEIGH_DN_TABLE.  Or waste a little
>> bit of memory in have a 30 entry array and looking things up by address
>> protocol number.   The only disadvantage I can see to using AF_NNN as
>> the index is that it might be a little less cache friendly.
>
> Yes, you can just store NEIGH_*_TABLE in your route entries and
> pass that directly into neigh_xmit(), or something like that.

And using the NEIGH_*_TABLE defines doesn't look too bad.  I walked a
little ways down the path of what would it take to remove NEIGH_*_TABLE
altogether and replacing NEIGH_*_TABLE with AF_* but the loops that are
for each possible neighbout table made just seemed to horrible to
convert that way.

So I now have an implementation that changes my routing tables to hold
NEIGH_*_ and it doesn't look bad at all.  Especially given that I
already have to filter the address families who's neighbour tables I can
use.

>> Other issues the hh header cache doesn't work. (How much do we care).
>> 
>> I worry a little that supporting AF_PACKET case might cause problems
>> in the future.
>> 
>> The cumulus folks are probably going to want to use neigh_xmit so they
>> can have ipv6 nexthops on ipv4.  Using this for IPv4 and loosing the
>> header cache worries me a little.
>
> We can have variable hard header caches per neigh entry if we really
> want to.  The only issue is, again, making the demux simple.

This is where things start creeping up on benchmarking that is really
more work than I am ready to take on for this project.

I think it would be really interesting to know if the hardware header
caches are worth it, and if they are is it only because it avoids a
function pointer or because all of our data comes from the same cache
line.

Looking at the code I find it interesting that we reserve less space
for the hardware header than we do for the hardware address itself.

Optimization opportunities clearly abound.

Eric

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