netdev - RE: TCP performance regression

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Date:	Mon, 11 Nov 2013 16:35:30 -0000
From:	"David Laight" <David.Laight@...LAB.COM>
To:	"Eric Dumazet" <eric.dumazet@...il.com>
Cc:	"Sujith Manoharan" <sujith@...jith.org>, <netdev@...r.kernel.org>,
	"Dave Taht" <dave.taht@...il.com>
Subject: RE: TCP performance regression

> > 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.

	David