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Date: Sat, 01 Mar 2008 00:33:32 +0100
From: Willem Jan Withagen <wjw@digiware.nl>
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Cc: =?ISO-8859-1?Q?alves?= <daniel@dgnetwork.com.br>,
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	freebsd-performance@freebsd.org, Kevin Oberman <oberman@es.net>
Subject: Re: FBSD 1GBit router?
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> I have a 1.2Ghz Pentium-M appliance, with 4x 32bit, 33MHz pci intel 
> e1000 cards.
> With maximum tuning I can "route" ~400mbps with big packets and ~80mbps 
> with 64byte packets.
> around 100kpps, whats not bad for a pci architecture.
> 
> To reach higher bandwiths, better busses are needed.
> pci-express cards are currently the best choice.
> one dedicated pci-express lane (1.25gbps) has more bandwith than a whole 
> 32bit, 33mhz pci-bus.

Like you say routing 400 Mb/s is close to the max of the PCI bus, which
has a theoretical max of 33*4*8 ~ 1Gbps. Now routing is 500Mb/s in, 
500Mb/s out. So you are within 80% of the bus-max, not counting 
memory-access and others.

PCI express will give you a bus per PCI-E device into a central hub, 
thus upping the limit to the speed of the FrontSideBus in Intel 
architectures. Which at the moment is a lot higher than what a single 
PCI bus does.

What it does not explain is why you can only get 80Mb/s with 64byte 
packets, which would suggest other bottlenecks than just the bus.

--WjW