igb(4) Raising IGB_MAX_TXD ??

[Sean Bruno]

We're running a service with a 82576 configured with 4 queues and a
maxed rxd/txd configuration:

http://people.freebsd.org/~sbruno/igb_stats.txt

We still see, under higher load spikes, a tendency to drop packets (I
suspect an application issue at this point, but want to attempt to
alleviate some congestion).

I note that IGB_MAX_RXD is set to 4k.  Reviewing the Intel spec shell on
the 82576 I see that the maximum value for the Descriptor Ring Length
(8.10.8) is 32k descriptors.

Does that mean I should be able to go higher that 4k?  I suspect that
even if I can, that would merely make the traffic fill more buffer
space, but maybe its enough to make it work a bit better.

Sean