more performance

[Robert Morris]

The regular Linux software can forward 47000 minimum-size
packets/second when using a Tulip (rather than a via-rhine).  The
tulip lets our software forward at 25400 p/s.  This on cone, a P6-200.

Here's what I see when I look at cycle counters, running our software
on cone. Config file ipb2/conf/cone-kbm.ipb.

Tulip interrupts cost about 15 microseconds per packet forwarded.
This is receive processing; there aren't really transmit interrupts.

The time per packet from when we enter the IPB's push world to when we
leave is 33 microseconds. In the pull world, 7 microseconds. These
numbers may include interrupt time counted in the tulip's 15
microseconds.

So the total cost is between 33+7=40 and 33+7+15=55 microseconds per
packet. Or between 25000 and 18000 packets/second. If you assume 40,
then our software actually costs 25 microseconds per packet.