netdev - RE: TCP performance regression

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Date:	Mon, 11 Nov 2013 08:17:02 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	David Laight <David.Laight@...LAB.COM>
Cc:	Sujith Manoharan <sujith@...jith.org>, netdev@...r.kernel.org,
	Dave Taht <dave.taht@...il.com>
Subject: RE: TCP performance regression

On Mon, 2013-11-11 at 15:43 +0000, David Laight wrote:
> > > Or, maybe:
> > > 5) call skb_orphan() (I think that is the correct function) when transmit
> > >    packets are given to the hardware.
> > 
> > This is the worth possible solution, as it basically re-enables
>               ^^^^^ worst ?
> > bufferbloat again.
> 
> It should be ok if the mac driver only gives the hardware a small
> number of bytes/packets - or one appropriate for the link speed.

There is some confusion here.

mvneta has a TX ring buffer, which can hold up to 532 TX descriptors.

If this driver used skb_orphan(), a single TCP flow could use the whole
TX ring.

TCP Small Queue would only limit the number of skbs on Qdisc.

Try then to send a ping message, it will have to wait a lot.



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