netdev - Re: TCP performance regression

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Date:	Tue, 12 Nov 2013 08:42:43 +0100
From:	Willy Tarreau <w@....eu>
To:	David Laight <David.Laight@...LAB.COM>
Cc:	Eric Dumazet <eric.dumazet@...il.com>,
	Sujith Manoharan <sujith@...jith.org>, netdev@...r.kernel.org,
	Dave Taht <dave.taht@...il.com>
Subject: Re: TCP performance regression

On Mon, Nov 11, 2013 at 04:35:30PM -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.

Well, it's not *that* large, 532 descriptors is 800 kB or 6.4 ms with
1500-bytes packets, and 273 microseconds for 64-byte packets. In fact
it's not a slow interface, it's the systems it runs on which are
generally not that fast. For example it is possible to saturate two
gig ports at once on a single-core Armada370. But you need buffers
large enough to compensate for the context switch time if you use
multiple threads to send.

Regards,
Willy

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