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Re: Scalability issues in the Internet routing system

  • From: Blaine Christian
  • Date: Thu Oct 27 10:32:09 2005 




Neat! So you were thinking you would leave the actual route
selection process monolithic and create separate processes per
peer? I have seen folks doing something similar with separate
MBGP routing instances. Had not heard of anyone attempting this
for a "global" routing table with separate threads per neighbor
(as opposed to per table). What do you do if you have one
neighbor who wants to send you all 2M routes though? I am
thinking of route reflectors specifically but also confederation
EIBGP sessions.




> I think you hit the nail on the head regarding record locking.
This is

> the thing that is going to bite you if anything will. I have heard

> none of the usual suspects speak up so I suspect that either this

> thread is now being ignored or no one has heard of an
implementation

> like the one you just described.



In BGP there is no 'global' route (actually path) selection in BGP.

Everything is done per prefix+path. In the RIB you can just lock
the prefix,

insert the new path and recalculate which one wins. Then issue the
update

to the FIB, if any. Work done. Statistically there is very little

contention on the prefix and the path records. For contention two
updates

for the same prefix would have to arrive at the same time from two
different

peers handled by different CPU's. I'd guess the SMP scaling factor
for BGP

is around 1.98. The 0.02 go lost for locking overhead and negative
caching

effects. Real serialization happens only at the FIB change queue.
However

serializing queues can be handled very efficiently on SMP too.





Hey Andre,



If you are intending to break the BGP process into per neighbor
threads this does not sound like it would have beneficial impact on a
single neighbor with the majority of the routes (thinking
specifically of EIBGP and/or Route Reflectors). Was your idea
specifically related to per neighbor processing or were you thinking
you could break the BGP process itself into chunks?