netdev - Re: what is the expeted performance from a dual port 10G card ?

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Message-ID: <20081129173442.GI32518@solarflare.com>
Date:	Sat, 29 Nov 2008 17:34:43 +0000
From:	Ben Hutchings <bhutchings@...arflare.com>
To:	Raz <raziebe@...il.com>
Cc:	netdev@...r.kernel.org
Subject: Re: what is the expeted performance from a dual port 10G card ?

Raz wrote:
> PCI Encoding overhead is 8 bits /10 bits . so 16Gbps supposed to be
> the practical limit.

The 8b10 encoding is already accounted for in the 2.5 Gbps figure;
the raw bit rate is 3.125 Gbps.

I'm thinking about the transaction layer overhead.  Since PCI Express
has few side-band signals there is a protocol a little like TCP running
over the link, which adds a header to each request or response packet
(TLP).  The maximum TLP size is usually quite small so the header
accounts for a fair proportion of the link bandwidth.

> I still lack 3 Gbps in the Intel case. where are they ?

That's for Intel to answer.

Ben.

-- 
Ben Hutchings, Senior Software Engineer, Solarflare Communications
Not speaking for my employer; that's the marketing department's job.
They asked us to note that Solarflare product names are trademarked.
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