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

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Date:	Sat, 29 Nov 2008 18:55:50 -0800 (PST)
From:	Trent Piepho <tpiepho@...escale.com>
To:	Roland Dreier <rdreier@...co.com>
cc:	Ben Hutchings <bhutchings@...arflare.com>, Raz <raziebe@...il.com>,
	netdev@...r.kernel.org
Subject: Re: what is the expeted performance from a dual port 10G card ?

On Sat, 29 Nov 2008, Roland Dreier wrote:
> > The 8b10 encoding is already accounted for in the 2.5 Gbps figure;
> > the raw bit rate is 3.125 Gbps.
>
> In addition, PCI Express transfers are broken up into packets, usually
> with very small payloads (128 or 256 bytes are common).  So the packet
> header overhead reduces throughput further, and then the transaction
> layer adds further overhead.  Then transferring NIC control structures
> over the link adds even more overhead.  So achieving 13.something Gb/sec
> of real throughput on a PCIe link theoretically capable of 16 Gb/sec
> seems pretty good to me.

For transfers with only 4 byte payloads, I get about 6.5 MB/sec over a PCIe
4x link.  For small transfers, the overhead is huge.
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