Subject: Re: [PATCH 0/10] af_unix: add multicast and filtering features to AF_UNIX

From: Javier Martinez Canillas <[email-masked]>
Date: Thu, 01 Mar 2012 14:56:11 +0100

On 03/01/2012 01:59 PM, Eric Dumazet wrote:
> Le jeudi 01 mars 2012 à 13:50 +0100, Rodrigo Moya a écrit :
>> the main problem in D-Bus we are trying to solve is the context
>> switches, since right now, there is a daemon, which listens on a UNIX
>> socket, and all traffic in the bus goes through it, and then the daemon
>> has to route the messages it gets on that socket to the corresponding
>> place(s). So, every time someone sends a message to D-Bus, since all
>> traffic goes through the daemon, dbus-daemon gets waked-up, which is one
>> of the biggest bottlenecks we are trying to fix.
>> 
>> That's why we are thinking about using multicast with socket filters, so
>> that the daemon only gets traffic it cares about and thus is not waked
>> up and context switches don't happen when not needed.
>> 
>> Using message queues, AFAICS, we would have the same problem, as the
>> daemon would create the message queue and would get all traffic, right?
>> 
> 
> This is why I mentioned extensions.

> 
> Anyway, if you think multicast sockets is the way to go, then you could
> setup a virtual network just to be able to use AF_INET multicast.
> 
> Thats probably doable without kernel patching.
> 

We could use AF_INET multicast on a local machine but we need some
ordering and control flow requirements that are not guaranteed on UDP
multicast over IP. That's why we thought to add a new address family
AF_MCAST.

To make it a general local multicast solution and not being too specific
we added some flags to control its behavior like
MCAST_MREQ_DROP_WHEN_FULL to decide to either block the sender or drop
the packet when one receiver has its queue full.

Regards,
Javier