I am curious about why this is seen only on loopback. Over ether, with client and server on two machines, there is no 40ms. ack penalty. If anything, I would expect to see things the other way around--penalty on ether, no penalty on loopback.
What is the MTU of the loopback interface? The way 2.6.15 maps the byte-based heuristics of the ABC RFC to the stack's packet-based cwnd it requires the exchange of an entire MSS worth of data before an additional packet is added to cwnd.
IIRC the loopback MTU is 16384 bytes, and an Ethernet MTU is 1500, so it will take a _lot_ more small packets sent back and forth over loopback to increase the cwnd by one packet than it would over Ethernet. I suspect that were Gigabit Ethernet involved and the MTU set to 9000 bytes the Ethernet case might look more like the loopback case.
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