Chris Wedgwood wrote:
> 
> You can measure this latency; and it's indeed very low (lmbench gives
> 28 usecs on one of my machines).
> 
> If you don't see this I would suspect an application bug -- can you
> use strace or some such and confirm this is not the case?

OK, two new data points (thanks for staying with me here):

1. The problem only occurs when traffic is travelling over DIFFERENT
sockets (i.e. A->B->C->D... or A->B->A but using a separate socket for
traffic in each direction).

2. I wrote a very ugly program (attached) to reproduce the problem. Lest
you think ill of me, most of this isn't actual code I wrote (the actual
program that first reproduced the problem was in C++). Just run

	sockperf localhost 54321 54322
	sockperf localhost 54322 54321 1

to see it in action.

--
Mike Lindner