Can anyone recommend where to look for discussion of what tweaking
various sysctl vars will do for network performance, perhaps also with 
information on what the tradeoffs are?

For example, just by poking around, I figured out that by setting:

sysctl -w net.inet.tcp.recvspace=65536
sysctl -w net.inet.tcp.sendspace=65536

I can improve the max throughput of two 1.5.2/i386 boxes with gigE interfaces
from a max tcp throughput of ~200 Mbps to a max tcp throughput of ~400Mbps.
However, I don't know what all the ramifications are.  I assume
these are per-connection buffers?

I realize that the documentation I'm *really* looking for (a list
of these sysctls followed by a description of why or why not to change
them) doesn't exist, but if someone can point me to a list of
common things to tweak, I'd be happy to massage any info I can get into
better documentation on sysctl settings...

Thanks,
+j

-- 
Jeff Rizzo                                         http://boogers.sf.ca.us/~riz