netdev - Re: [PATCH 1/2] ipv4: Improve the scaling of the ARP cache for multicast destinations.

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Message-ID: <504581EA.8040807@aristanetworks.com>
Date:	Mon, 03 Sep 2012 21:22:02 -0700
From:	Bob Gilligan <gilligan@...stanetworks.com>
To:	Nicolas de Pesloüan 
	<nicolas.2p.debian@...il.com>
CC:	David Miller <davem@...emloft.net>, netdev@...r.kernel.org
Subject: Re: [PATCH 1/2] ipv4: Improve the scaling of the ARP cache for multicast
 destinations.

On 9/2/12 6:26 AM, Nicolas de Pesloüan wrote:
> Le 31/08/2012 21:21, Bob Gilligan a écrit :
>> On 8/30/12 6:06 PM, David Miller wrote:
>>> From: Bob Gilligan<gilligan@...stanetworks.com>
>>> Date: Thu, 30 Aug 2012 17:55:04 -0700
>>>
>>>> The mapping from multicast IPv4 address to MAC address can just as
>>>> easily be done at the time a packet is to be sent.  With this change,
>>>> we maintain one ARP cache entry for each interface that has at least
>>>> one multicast group member.  All routes to IPv4 multicast destinations
>>>> via a particular interface use the same ARP cache entry.  This entry
>>>> does not store the MAC address to use.  Instead, packets for multicast
>>>> destinations go to a new output function that maps the destination
>>>> IPv4 multicast address into the MAC address and forms the MAC header.
>>>
>>> Doing an ARP MC mapping on every packet is much more expensive than
>>> doing a copy of the hard header cache.
>>>
>>> I do not believe the memory consumption issue you use to justify this
>>> change is a real issue.
> 
> My two cents:
> 
> Why do we need a per interface neighbor cache for multicast? Isn't the
> target MAC expected to be the same for a give target multicast IP
> address, whatever the egress interface?
> 
> Can't we store the target MAC for multicast address on a neighbor cache
> entry with a fake dev value meaning "multicast"?
> 
> Can't we detect the fact that the IP destination address is multicast
> and search only the neighbor cache entries that have this fake dev
> value, instead of the egress interface?
> 
> This should reduce memory consumption (not N-squared anymore) without
> significantly increasing the CPU usage.
> 
> Do I miss something?

Sounds like you are suggesting the converse of my proposed patch:  One
"special" Neighbor Cache Entry per multicast group instead of one per
interface.  Unfortunately, I don't think this will work.  Different MAC
types have different frame formats and different multicast address
mappings, so can't share a common neighbor cache entry.

Bob.
> 
>     Nicolas.

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