[c-nsp] High interrupt load on 7507

[David Coulson]

I'm trying to figure out a few issues with our 7507s running 12.4(8),
where the CPU usage for interrupting will hit >95%. For example, we had
a 4Gb file transfer which occurred between a VLAN on a GEIP and a plain
old 100meg Ethernet interface on a PA-FE-TX (on a VIP2). We got up to
about 60mbit in data rate transfer before it peaked out and the RSP was
spending all it's time interrupting. We didn't seem to be doing that
many packets/sec (maybe 7-8k), so I guess I'm a little confused as to
why interrupting was so high on the box. Is interrupt rate a factor of
packets/sec (like you'd think it is), or do other variables come into play.

Does using dot1q cause the RSP4 to interrupt rather than dCEF switch the
packet? I'd like to try to get CPU down on the boxes, and it doesn't
appear like the VIP processors are loaded (They run 30% tops, if that)
during these times when the RSP is going nuts. I need to figure out what
the best way to keep the RSP out of the way of packets is to avoid these
periods of excessive interrupting, but I can't get rid of the VLANs and
the router is already running dCEF on whatever it can. Since CEF
requires an interrupt hit, am I just hitting my limit on what my RSP4
can do?

Is it time to look at something else to do my dot1q L3, or am I just
missing something with my config?

David