polling decreases throughput ~50%

[Victor Semionov]

Hello,

I've noticed something that seems strange to me and I'd like to ask for an 
explanation, because I don't think it should be happening.

I was uploading some large file to my neighbor's windows box. With polling 
disabled, the transfer rate was 8 MB/s and CPU usage was 90%. I turned on 
polling with sysctl and CPU usage decreased to 50%, which I expected, but 
also the transfer rate dropped to 4.5 MB/s, about half. The numbers are 
approximate. The interface is a RealTek (rl driver). No background jobs were 
running. kern.polling.* all defaults.

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.

Thanks