[POLLING] strange interrupt/system load

[rihad]

The box experiences ~230 mbit/s traffic flow through it. I've doubled 
some sysctls after reading polling(4):
kern.polling.each_burst=10 # was: 5
kern.polling.burst_max=350 # was: 150

FreeBSD 7.2-RELEASE-p3 amd64
HZ=1000

Now for the fun part.

With kern.polling.idle_poll = 1 top shows:
CPU:  0.0% user,  0.0% nice, 26.9% system,  3.1% interrupt, 70.0% idle
~8000 interrupts/s total according to systat -vmstat:
1999 cpu0: time
2000 cpu1: time
1999 cpu2: time
1999 cpu3: time

With kern.polling.idle_poll = 0 top shows:
CPU:  0.0% user,  0.0% nice,  0.0% system, 13.9% interrupt, 86.0% idle
Still the same ~8000 clock interrupts/s.

Under both scenarios polling is enabled on both em0 and em1 through 
ifconfig.


1) Why is the interrupt load relatively high with polling enabled?
2) How come 13.9% interrupts are not also in the first scenario if their 
total rate is the same (~8000)?

Thanks.