netdev - Re: [PATCH 7/7 net-next v2] tg3: Disable multiple TX rings by default due to hardware flaw

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Message-Id: <20120930.021251.1431834423231117398.davem@davemloft.net>
Date:	Sun, 30 Sep 2012 02:12:51 -0400 (EDT)
From:	David Miller <davem@...emloft.net>
To:	mchan@...adcom.com
Cc:	netdev@...r.kernel.org
Subject: Re: [PATCH 7/7 net-next v2] tg3: Disable multiple TX rings by
 default due to hardware flaw

From: "Michael Chan" <mchan@...adcom.com>
Date: Fri, 28 Sep 2012 10:12:43 -0700

> Simple round-robin hardware TX scheduling can cause starvation of TX rings
> with small packets when other TX rings have large TSO or jumbo packets.
> 
> In the simplest case, consider 2 TCP streams running in opposite
> directions.  The TSO TX traffic will hash to one ring and the ACKs for the
> incoming data on a different TCP connection will hash to a different TX
> ring.  The hardware fetches one complete TSO packet (up to 64K data)
> before servicing the other TX ring.  When it gets to the other TX ring, it
> will only fetch one packet (64-byte ACK packet in this case).  After that,
> it will switch back to the 1st ring filled with more TSO packets.  Because
> only one ACK can go out roughly every 500 usec in this case, the incoming
> data rate becomes very low.
> 
> Update version to 3.125.
> 
> Signed-off-by: Michael Chan <mchan@...adcom.com>

Applied.
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