On Tue, 17 Feb 2009, Rusty Conover wrote:
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.
note that he saw problems even on localhost.
in the last couple of months I've seen a lot of discussin on the
linux-kernel list about the performance of localhost. unfortunantly those
fixes are only in the 2.6.27.x and 2.6.28.x -stable kernels.
David Lang
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