netdev - Re: bizarre network timing problem

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Message-ID: <4730D8AD.5060401@hp.com>
Date:	Tue, 06 Nov 2007 13:12:13 -0800
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):
> 
>>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.
> 
> Setting the MTU on lo to 1500 eliminates the problem and gives me double
> digit MB/sec throughput.

I'm not in a position at the moment to test it as my IPoIB systems are offline, 
and not sure you are either, but I will note that with IPoIB bits circa OFED1.2 
the default MTU for IPoIB goes up to 65520 bytes.  If indeed the problem you 
were seeing was related to sub-mss sends and window probing and such, it might 
appear on IPoIB in addition to loopback.

rick jones
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