polling decreases throughput ~50%

[Victor Semionov]

> >> Why is that? I thought polling should decrease CPU usage by avoiding too
> >> many context switches when a hw irq is generated frequently, but it
> >> shouldn't make the transfer slower if there are no other jobs running.
>
> You have to poll often enough to keep the pipe full, otherwise your max
> throughput can be limited.  Also, rl hardware isn't the greatest and
> probably requires a lot more CPU than a device with working buffer/DMA
> design.

HZ is 1000, which I guess should be more than enough with 
kern.polling.burst_max=150.

Indeed, it was hardware's fault - my other NIC is a fxp and I got much better 
results with it - less CPU, while throughput stayed the same as without 
polling.