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

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Date:	Fri, 30 May 2014 07:54:02 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	"ezequiel.garcia@...e-electrons.com" 
	<ezequiel.garcia@...e-electrons.com>
Cc:	"fugang.duan@...escale.com" <fugang.duan@...escale.com>,
	David Laight <David.Laight@...LAB.COM>,
	"Frank.Li@...escale.com" <Frank.Li@...escale.com>,
	"davem@...emloft.net" <davem@...emloft.net>,
	"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 11:01 -0300, ezequiel.garcia@...e-electrons.com
wrote:
> Hello Eric,
> 
> On 30 May 06:13 AM, Eric Dumazet wrote:
> > 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.
> > 
> 
> What's the rationale behing those numbers?
> 

64KB TSO packet, with MSS=1460 -> 44 segments  (44*1460 = 64240)
with MSS=1448 (TCP timestamps) -> 45 segments  (45*1448 = 65160)

This software TSO emulation uses at least 2 descriptors per MSS

one descriptor to hold the headers (ethernet + ip + tcp)
one descriptor (or two) to hold the payload for this MSS


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