tigon3: 2466 and bad performance at 32bits/33mhz ?

[Robert Latham <robl@mcs.anl.gov>]

I've got a 3com 3C996B-T (copper) and a tyan 2466 motherboard.  The
kernel is 2.4.19-rc1-ac2, and i'm using the tg3 v0.99 (Jun 11, 2002).

When i run the 3C996B-T in the 64-bit/66-MHz slot, i get great
performance.  When i run the card in the 32-bit/66-MHz slot, i get
pretty bad performance.  Here's a netpipe graph to demonstrate:

http://www-unix.mcs.anl.gov/~robl/storage_box/tg3-32_33.2.png

The fast curve is with the 3c996B-T in the 64/66 slot.  It peaks out
around 850 Mbps.  The slow curve is the same card in the 32/33 slot.
It peaks around 180 Mbps.

Is a maximum throughput of ~180 MBits/sec all that can be hoped for?  I
don't have the hardware to verify, but 
http://www.cs.uni.edu/~gray/gig-over-copper/#3Com%203c996BT:|outline
offers the datapoint of 436 MBits/sec in an oldish dell optiplex ( old
being whenever p3 500 mhz machines were still made)

So, am i looking at a hardware limitation?  driver quirk?  I'm open to
any suggestions.  

I'll keep an eye on the web archives, but please CC me on any replies.

thanks
==rob

-- 
Rob Latham
Mathematics and Computer Science Division    A215 0178 EA2D B059 8CDF
Argonne National Labs, IL USA                B29D F333 664A 4280 315B