netdev - Re: [BUG,REGRESSION?] 3.11.6+,3.12: GbE iface rate drops to few KB/s

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Date:	Mon, 18 Nov 2013 11:52:58 +0100
From:	Willy Tarreau <w@....eu>
To:	David Laight <David.Laight@...LAB.COM>
Cc:	Arnaud Ebalard <arno@...isbad.org>,
	Cong Wang <xiyou.wangcong@...il.com>, edumazet@...gle.com,
	linux-arm-kernel@...ts.infradead.org, netdev@...r.kernel.org,
	Thomas Petazzoni <thomas.petazzoni@...e-electrons.com>
Subject: Re: [BUG,REGRESSION?] 3.11.6+,3.12: GbE iface rate drops to few KB/s

On Mon, Nov 18, 2013 at 10:09:53AM -0000, David Laight wrote:
> > So it is fairly possible that in your case you can't fill the link if you
> > consume too many descriptors. For example, if your server uses TCP_NODELAY
> > and sends incomplete segments (which is quite common), it's very easy to
> > run out of descriptors before the link is full.
> 
> Or you have a significant number of active tcp connections.
> 
> Even if there were no requirement to free the skb quickly you still
> need to take a 'tx done' interrupt when the link is transmit rate limited.
> There are scenarios when there is no receive traffic - eg asymmetric
> routing, but testable with netperf UDP transmits.

Yes absolutely, but I was talking about the current situation that Arnaud
is facing and which I could reproduce with a large window.

Willy

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