Interrupts + Polling mode (similar to Linux's NAPI)

[Luigi Rizzo]

On Fri, Mar 27, 2009 at 11:05:00AM +0000, Andrew Brampton wrote:
> 2009/3/27 Luigi Rizzo <rizzo at iet.unipi.it>:
> > The load of polling is pretty low (within 1% or so) even with
> > polling. The advantage of having interrupts is faster response
> > to incoming traffic, not CPU load.
> 
> oh, I was under the impression that polling spun in a tight loop, thus
> using 100% of the processor. After a quick test I see this is not the
> case. I assume it will get to 100% CPU load if I saturate my network.

Well the motivation for the original polling code in FreeBSD was
to keep the CPU usage under strict control -- you could set the
max CPU fraction that you wanted to dedicate to packet handling,
and you were guaranteed not to exceed that fraction.

> > There is nothing difficult in having both active, except figuring
> > out a good logic for when to disable polling on an interface
> > that has been quiet for a while.
> 
> Looking at Linux's logic, it appears to poll until there are no more
> packets, and thus re-enables interrupts.

the complete definition should be "no more packets for X seconds".
Enabling and disabling interrupts is slightly expensive so you
don't want to do it too often.

cheers
luigi