TCP network cost

Recently I've been working on improving the performance of a system that
delivers files stored in postgresql as bytea data. I was surprised at
just how much a penalty I find moving from a domain socket connection to
a TCP connection, even localhost. For one particular 40MB file (nothing
outragous) I see ~ 2.5 sec. to download w/ the domain socket, but ~ 45 sec
for a TCP connection (either localhost, name of localhost, or from
another machine 5 hops away (on campus - gigabit LAN) Similar numbers
for 8.2.3 or 8.3.6 (on Linux/Debian etch + backports)

So, why the 20 fold penalty for using TCP? Any clues on how to trace
what's up in the network IO stack?

Ross
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Ross Reedstrom, Ph.D. reedstrm(at)rice(dot)edu
Systems Engineer & Admin, Research Scientist phone: 713-348-6166
The Connexions Project http://cnx.org fax: 713-348-3665
Rice University MS-375, Houston, TX 77005
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