netdev - RE: TCP performance regression

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Date:	Mon, 11 Nov 2013 09:41:53 -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 16:35 +0000, David Laight wrote:
> > > 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.
> 
> 532 is a ridiculously large number especially for a slow interface.
> At a guess you don't want more than 10-20ms of data in the tx ring.
> You might need extra descriptors for badly fragmented packets.

Thats why we invented BQL.

Problem is most driver authors don't care of the problem.

They already have hard time to make bug free drivers.

BQL is adding pressure and expose long standing bugs.

Some drivers have large TX rings to lower race probabilities.



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