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TCP network cost

From: Ross J. Reedstrom
Subject: TCP network cost
Date: ,
Msg-id: 20090217070403.GB2486@cooker
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Responses: Re: TCP network cost  (Rusty Conover)
Re: TCP network cost  (Aaron Turner)
List: pgsql-performance

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TCP network cost  ("Ross J. Reedstrom", )
 Re: TCP network cost  (Rusty Conover, )
  Re: TCP network cost  (, )
  Re: TCP network cost  ("Ross J. Reedstrom", )
   Re: TCP network cost  (Rusty Conover, )
    Re: TCP network cost  ("Ross J. Reedstrom", )
     Re: TCP network cost  ("Ross J. Reedstrom", )
      Re: TCP network cost  (Aaron Turner, )
     Re: TCP network cost  (PFC, )
      Re: TCP network cost  ("Ross J. Reedstrom", )
   Re: TCP network cost  (Gregory Stark, )
    Re: TCP network cost  ("Ross J. Reedstrom", )
     Re: TCP network cost  (Tom Lane, )
      Re: TCP network cost  ("Ross J. Reedstrom", )
       Re: TCP network cost  (Linos, )
        Re: TCP network cost  (Tom Lane, )
         Re: TCP network cost  (Linos, )
          Re: TCP network cost  (Tom Lane, )
           Re: TCP network cost  (Linos, )
            Re: TCP network cost  (Tom Lane, )
             Re: TCP network cost  (Magnus Hagander, )
              Re: TCP network cost  (Linos, )
 Re: TCP network cost  (Aaron Turner, ) 
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
--
Ross Reedstrom, Ph.D.                                 
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
GPG Key fingerprint = F023 82C8 9B0E 2CC6 0D8E  F888 D3AE 810E 88F0 BEDE

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