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Re: TCP network cost

From: david(at)lang(dot)hm
To: Rusty Conover <rconover(at)infogears(dot)com>
Cc: "Ross J(dot) Reedstrom" <reedstrm(at)rice(dot)edu>, pgsql-performance <pgsql-performance(at)postgresql(dot)org>
Subject: Re: TCP network cost
Date: 2009-02-17 08:34:15
Message-ID: alpine.DEB.1.10.0902170032440.25606@asgard.lang.hm (view raw or flat)
Thread:
Lists: pgsql-performance
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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