DEVICE_POLLING IF_EM CPU usage

[Daniel Dias Gonçalves]

Hi,

I activated polling in 8 interfaces em0... em7 (ifconfig em* polling) to 
carry through performance tests, when activating, it had a significant 
reduction of CPU usage, load average measured of "0,50, 0,69, 0,52" for 
"0,43, 0,39, 0,21" and the CPU usage (SNMP Graphic) measured of 35% for 
5%. Passed some hours I disactivated polling (ifconfig em* - polling) 
and CPU usage continues low.
Somebody could explain this to me?

Some information:

FreeBSD 6.3-RELEASE

CPU: Intel(R) Xeon(TM) CPU 3.20GHz (3200.13-MHz 686-class CPU)
 Logical CPUs per core: 2
FreeBSD/SMP: Multiprocessor System Detected: 2 CPUs
SMP: AP CPU #1 Launched!

Interfaces if_em:
   vendor     = 'Intel Corporation'
   device     = '82546EB Dual Port Gigabit Ethernet Controller'


Thanks.

Daniel