On Feb 17, 2009, at 12:04 AM, Ross J. Reedstrom wrote:
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?
Try running tests with ttcp to eliminate any PostgreSQL overhead and find out the real bandwidth between the two machines. If its results are also slow, you know the problem is TCP related and not PostgreSQL related.
