I have a Dell PowerEdge 1850 running FreeBSD 5.4-RELEASE that I'm
trying to setup as a router primarily for small UDP packets (~300
bytes).  It has an Intel Pro/1000 MT network card directly connected
via crossover to another system at the moment for testing.  Testing is
being performed with netperf on the other system using "netperf -H
10.0.5.1 -t UDP_STREAM -l 30 -- -m 300" to generate a stream of ~300
byte packets.  There is no other traffic on the interfaces being used.

This network card and the em driver support interrupt moderation, but
I'm not seeing any benefit from it.  Testing using netperf seems to
show virtually as many interrupts being generated as packets.  I would
expect the interrupt rate to be much lower with the moderation.

Netstat statistics (taken per second) of the interface during this test
show:

            input          (em1)           output
   packets  errs      bytes    packets  errs      bytes colls
    132077     0   45170334          0     0          0     0
    132116     0   46242532          0     0          0     0
    132121     0   45185382          0     0          0     0
    132058     0   46217230          0     0          0     0
    132197     0   45211374          0     0          0     0
    132188     0   46267206          0     0          0     0

Vmstat shows the following over 1 second intervals:

 procs      memory      faults              cpu
 r b w     avm    fre    in     sy     cs   us sy id
 0 0 0   43352 969856  150427 132160 40353  4  50 47
 0 0 0   43352 969856  150528 132318 40356  3  48 49
 0 0 0   43352 969856  150574 132351 40342  2  48 49
 0 0 0   43352 969856  150665 132446 40332  2  49 48
 1 0 0   43352 969856  150596 132373 40352  2  53 45
 0 0 0   43352 969856  150662 132435 40351  2  50 48

Idle interrupt rate is about 2300 interrupts/second.

em1 kernel settings are:
hw.em1.rx_int_delay: 0
hw.em1.tx_int_delay: 66
hw.em1.rx_abs_int_delay: 66
hw.em1.tx_abs_int_delay: 66
hw.em1.int_throttle_ceil: 8000

How can I get those interrupts down to a more reasonable level
using interrupt moderation (I'd expect under 20,000 interrupts/sec or so)?  
With moderation I would expect processing 130kpps to generate far less
than the 130,000 interrupts/sec I'm seeing.  I've tried setting the
int_delays to different values between 0-200, but that has had no
effect.  I've also tried enabling polling (and setting different
polling intervals between 1000-32000) but for some reason using
polling just causes both the idle and the loaded interrupt rate to
increase instead of decrease.  Also tried net.inet.ip.fastforwarding
just to see if it might help in this instance, but it had no effect
either.

Full dmesg for the system can be found at http://darklegacies.com/pe1850/dmesg
Full list of sysctl values can be found at
http://darklegacies.com/pe1850/sysctl

Thanks.
Brian