Multiport NICs - VLAN and Polling Support?

[Drew Linsalata]

Kurt Jaeger writes: 

> Hi! 
> 
>> polling is almost never a performance advantage
>> for ethernet, as virtually all modern controllers
>> have some sort of interrupt moderation built-in.
> 
> We had the case on fbsd 5.4p7 with SMP where bge drivers produced
> much slower throughput if we did not use polling. We measured using
> ttcp and the difference was from 2-3Mbyte/sec to 11 Mbyte/sec.

What Danial is saying is that polling is whacking your CPU, while 
manipulating interrupt frequency at the NIC hardware level should make for a 
"free" (read: no cpu overhead) performance enhancement.    If he's correct, 
you should see the same performance increase if you skip polling and go 
right to the NIC hardware interrupt tweak (assuming your BGE card supports 
it), AND you should get it without adding CPU load. 

This is interesting.  I'm going to set up testbed when I have a little time 
and run both ways to see what happens. 

Question is, what mechanism do we have available to manipulate interrupt 
frequency on the NIC itself.  The em driver does not appear to offer any 
options in that area, nor is anything like that configurable via ifconfig.  
Are we looking at a sysctl variable, or are we tweaking the driver source 
and rebuilding each time we want to try a new combination of values? 


Drew Linsalata
The Gotham Bus Company, Inc.
Dedicated Servers and Colocation
Long Island, New York
http://www.gothambus.com