Re: TCP network cost

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