linux-kernel - Re: bizarre network timing problem

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Date:	Thu, 18 Oct 2007 10:22:18 -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:
>>the packet trace was a bit too cooked perhaps, but there were indications 
>>that at times the TCP window was going to zero - perhaps something with 
>>window updates or persist timers?
> 
> 
> Does TCP use different window sizes on loopback?  Why is this not
> happening on ethernet?

I don't think it uses different window sizes on loopback, but with the 
autotuning it can be difficult to say a priori what the window size will be. 
What one can say with confidence is that the MTU and thus the MSS will be 
different between loopback and ethernet.

> 
> How could I test this theory?

Can you take another trace that isn't so "cooked?"  One that just sticks with 
TCP-level and below stuff?

If SMB is a one-request-at-a-time protocol (I can never remember), you could 
simulate it with a netperf TCP_RR test by passing suitable values to the 
test-specific -r option:

netperf -H <remote> -t TCP_RR -- -r <req>,<rsp>

If that shows similar behaviour then you can ass-u-me it isn't your application. 
  One caveat though is that TCP_CORK mode in netperf is very primitive and may 
not match what you are doing, however, that may be a good thing.

http://www.netperf.org/svn/netperf2/tags/netperf-2.4.4/  or
ftp://ftp.netperf.org/netperf/

to get the current netperf bits.  It is also possible to get multiple 
transactions in flight at one time if you configure netperf with --enable-burst, 
which will then enable a test-specific -b option.  With the latest netperf you 
cna also switch the output of a TCP_RR test to bits or bytes per second a la the 
_STREAM tests.

rick jones

> 
> My initial idea was that it has something todo with the different MTU on
> loopback.  My initial block size was 16k, but the problem stayed when I
> changed it to 64k.
> 
> Felix

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