I believe the core problem to be in my network driver (forcedeth) and/or my BIOS/PCI/whatever settings.
However, after much fiddling, I managed to get TCP transfers to a reasonable point. I'm easily able to sustain some 11.1 MiB/s. However, AoE suffers much more badly, around 150-300 KiB/s. I believe this to be due to a fairly small amount of network loss (a few percent at most), but the differences between AoE's lost-packet handling and TCP are /remarkable/.

Am I barking up the wrong tree or are there knobs I can fiddle that will improve AoE's performance over less-than-perfect Ethernet?

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Jon