device polling takes more CPU hits??

[Don Bowman]

From: Luigi Rizzo [mailto:rizzo at icir.org]
> On Mon, Jul 26, 2004 at 01:18:46PM -0700, Kelly Yancey wrote:
> ...
> >   Out of curiousity, what sort of testing did you do to 
> arrive at these
> > settings?  I did some testing a while back with a SmartBits 
> box pumping
> > packets through a FreeBSD 2.8Ghz box configured to route 
> between two em
> > gigabit interfaces; I found that changing the burst_max and 
> each_burst
> > parameters had almost no effect on throughput (maximum 1% 
> difference).
> 
> fast boxes are pci-bus limited, not CPU limited(*) so 
> changing the burst
> size (which basically amortizes some CPU costs) has little if any
> effect.

The PCI-X bus will probably be 64-bit 133MHz in this case,
the limit moves up to the P64H2 hub for large packets,
to the CPU for small packets. Polling becomes quite
critical to prevent livelock.

--don