about network interrupt

[zjusomwu at sina.com]

hi,all:
   I'm testing network performance of nginx using freebsd,and I find something strange, that is :while sending huge amount http request to nginx and use top to monitor this machine, for some machines a lot of cpu is consumed by interrupt but the others not! And I use one machine for test using different version of freebsd,then I find Freebsd 8 not using interrupt but 10 does!    In order to explain this I first guess it is the polling make it diff,but after I using "ifconfig em0 -polling" to disable polling of the network interface it does no effect! Now I can't explain it. Who can give me some advice?
       somwu