netdev - Re: [PATCH] spidernet: don't use debug flag

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Message-ID: <20070709212617.GM4457@austin.ibm.com>
Date:	Mon, 9 Jul 2007 16:26:17 -0500
From:	linas@...tin.ibm.com (Linas Vepstas)
To:	Ishizaki Kou <kou.ishizaki@...hiba.co.jp>
Cc:	netdev@...r.kernel.org, cbe-oss-dev@...abs.org
Subject: Re: [PATCH] spidernet: don't use debug flag

On Mon, Jul 09, 2007 at 05:45:21PM +0900, Ishizaki Kou wrote:
> GDTDCEIDIS flag is defined that it is for debug and should not be used.

!? Certainly, my spec doesn't say anything like this;
I don't know of any other way of turning off the descriptor 
chain end interrupt; leaving it on hurts performance in a big way.

I get the following TX performance numbers:

pkt sz       rate w/o patch      rate w/patch
(bytes)      (Mbits/sec)         (Mbits/sec)
-------      ----------          ---------
400            503                 353
200            239                  88
100            122                  44
 60             73                  26

That's not quite a 3x performance degradation.

In addition, with your patch, the number of interrupts jumps
from just about zero, to about 55K/second. From what I can tell, 
this huge interrupt rate eats up all the CPU cycles, which is
why the performance drops so drasically.

> We met some troubles on Celleb platform by setting this flag.
>  -network does not recover after ifconfig down, then up operations.

Can you be more specific?  I can't imagine why this flag would
have anything to do with ifdown/ifup. The device open/close 
routines should reset all hardware state; this shouldn't make
any difference. (It doesn't for me, at least).  

--linas
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