Routing SMP benefit

Tiffany Snyder tiffany.snyder at
Fri Dec 28 16:17:30 PST 2007

Hi Andre,
        are those numbers for small (64 bytes) packets? Good job on pushing
the base numbers higher on the same HW.

        What piqued my attention was the note that our forwarding
performance doesn't scale with multiple CPUs. Which means there's a lot of
work to be done :-) Have we taken a look at OpenSolaris' Surya
project? They allow multiple readers/single writer on the radix_node_head
(and not a mutex as we do) and we may be able to do the same to gain some
parallelism. There are other things in Surya that exploit multiple CPUs.
It's definitely worth a read. DragonFlyBSD seems to achieve parallelism by
classifying packet as flows and then redirecting the flows to different
CPUs. OpenSolaris also does something similar. We can definitely think along
those lines.

1) I said  multiple instead of dual CPUs on purpose.
2) I mentioned OpenSolaris and DragonFlyBSD as examples and to acknowledge
the work they are doing and to show that FreeBSD is far behind and is losing
it's lustre on continuing to be the networking platform  of choice.



On 12/29/05, Andre Oppermann <andre at > wrote:

> Markus Oestreicher wrote:
> >
> > Currently running a few routers on 5-STABLE I have read the
> > recent changes in the network stack with interest.
> You should run 6.0R.  It contains many improvements over 5-STABLE.
> > A few questions come to my mind:
> >
> > - Can a machine that mainly routes packets between two em(4)
> > interfaces benefit from a second CPU and SMP kernel? Can both
> > CPUs process packets from the same interface in parallel?
> My testing has shown that a machine can benefit from it but not
> much in the forwarding performance.  The main benefit is the
> prevention of lifelock if you have very high packet loads.  The
> second CPU on SMP keeps on doing all userland tasks and running
> routing protocols.  Otherwise your BGP sessions or OSPF hellos
> would stop and remove you from the routing cloud.
> > - From reading the lists it appears that
> > and net.ip.fastforwarding are doing similar things. Should
> > they be used together or rather not?
> net.inet.ip.fastforwarding has precedence over and
> enabling both at the same doesn't gain you anything.  Fastforwarding
> is about 30% faster than all other methods available, including
> polling.  On my test machine with two em(4) and an AMD Opteron 852
> (2.6GHz) I can route 580'000 pps with zero packet loss on -CURRENT.
> An upcoming optimization that will go into -CURRENT in the next
> few days pushes that to 714'000 pps.  Futher optimizations are
> underway to make a stock kernel do close to or above 1'000'000 pps
> on the same hardware.
> --
> Andre
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