netdev - RE: [PATCH v1 4/6] net: fec: Increase buffer descriptor entry number

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Date:	Fri, 30 May 2014 06:13:06 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	"fugang.duan@...escale.com" <fugang.duan@...escale.com>
Cc:	David Laight <David.Laight@...LAB.COM>,
	"Frank.Li@...escale.com" <Frank.Li@...escale.com>,
	"davem@...emloft.net" <davem@...emloft.net>,
	"ezequiel.garcia@...e-electrons.com" 
	<ezequiel.garcia@...e-electrons.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"shawn.guo@...aro.org" <shawn.guo@...aro.org>,
	"bhutchings@...arflare.com" <bhutchings@...arflare.com>,
	"stephen@...workplumber.org" <stephen@...workplumber.org>
Subject: RE: [PATCH v1 4/6] net: fec: Increase buffer descriptor entry number

On Fri, 2014-05-30 at 09:42 +0000, fugang.duan@...escale.com wrote:
> From: David Laight <David.Laight@...LAB.COM> Data: Friday, May 30, 2014 5:11 PM
> >
> >Does the driver support BQL (or similar) in order to limit the amount of
> >queued tx traffic?
> >Otherwise you've significantly increased the latency for connections other
> >than one doing bulk tx.
> >
> >	David
> >
> The driver still don't support BQL.
> I will add the feature to support FEC. Thanks for your advise.

Note that a full size TSO packet (44 or 45 MSS) requires about 88 or 90
descriptors.

So I do not think BQL is really needed, because a 512 slots TX ring wont
add a big latency : About 5 ms max.

BQL is generally nice for NIC supporting hardware TSO, where a 64KB TSO
packet consumes 3 or 4 descriptors.

Also note that TCP Small Queues should limit TX ring occupancy of a
single bulk flow anyway.


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