Even if you do have hardware that supports half-duplex gigabit ethernet
on both ends, the need to do carrier extension for any frame shorter
than 512 bytes so that CSMA/CD actually works on a reasonable sized
cable, does horrible things to your throughput if you've got lots of
small frames. (In other words, at gigabit speeds, frames smaller than
512 bytes zip down the wire so quickly that you can no longer reliably
detect collisions, so the frames all get padded.) I'm having trouble wrapping my head around any circumstances other than horribly, horribly broken hardware or software where half-duplex would increase your performance over full-duplex.

actually there are no gigabit devices incapable of full-duplex.
ethernet hardware I've ever touched has been incapable of doing
half-duplex when it's being used at gigabit speeds.  The specs for doing
it exist more for theoretical completeness than out of practical
utility.  See, for example


at 10Gbit/s specs dropped half-duplex and collision detection at all.
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