linux-kernel - Re: bizarre network timing problem

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Message-ID: <472BAB90.8020306@hp.com>
Date:	Fri, 02 Nov 2007 15:58:24 -0700
From:	Rick Jones <rick.jones2@...com>
To:	Felix von Leitner <felix-linuxkernel@...e.de>
Cc:	Chuck Ebbert <cebbert@...hat.com>, linux-kernel@...r.kernel.org,
	Netdev <netdev@...r.kernel.org>
Subject: Re: bizarre network timing problem

Felix von Leitner wrote:
> Thus spake Rick Jones (rick.jones2@...com):
> 
>>>Oh I'm pretty sure it's not my application, because my application performs
>>>well over ethernet, which is after all its purpose.  Also I see the
>>>write, the TCP uncork, then a pause, and then the packet leaving.
>>
>>Well, a wise old engineer tried to teach me that the proper spelling is 
>>ass-u-me :) so just for grins, you might try the TCP_RR test anyway :)  And 
>>even if your application is correct (although I wonder why the receiver 
>>isn't sucking data-out very quickly...) if you can reproduce the problem 
>>with netperf it will be easier for others to do so.
> 
> 
> My application is only the server, the receiver is smbget from Samba, so
> I don't feel responsible for it :-)

Might want to strace it anyway... no good deed (such as reporting a potential 
issue) goes unpunished :)

> Still, when run over Ethernet, it works fine without waiting for
> timeouts to expire.

Past performance is no guarantee of current correctness :)  And over an 
Ethernet, there will be a very different set of both timings and TCP segment 
sizes compared to loopback.

My guess is that you will find setting the lo mtu to 1500 a very interesting 
experiment.

> 
> To reproduce this:
> 
>   - smbget is from samba, you probably already have this
>   - gatling (my server) can be gotten from
>     cvs -d :pserver:cvs@....fefe.de:/cvs -z9 co dietlibc libowfat gatling
> 
> dietlibc is not strictly needed, but it's my environment.
> First built dietlibc, then libowfat, then gatling.
> 
> Felix

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