linux-kernel - Re: e1000 full-duplex TCP performance well below wire speed

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Date:	Wed, 30 Jan 2008 08:01:46 -0600 (CST)
From:	Bruce Allen <ballen@...vity.phys.uwm.edu>
To:	netdev@...r.kernel.org,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	David Miller <davem@...emloft.net>
cc:	Henning Fehrmann <henning.fehrmann@....mpg.de>,
	Carsten Aulbert <carsten.aulbert@....mpg.de>,
	Bruce Allen <bruce.allen@....mpg.de>
Subject: Re: e1000 full-duplex TCP performance well below wire speed

Hi David,

Thanks for your note.

>> (The performance of a full duplex stream should be close to 1Gb/s in
>> both directions.)
>
> This is not a reasonable expectation.
>
> ACKs take up space on the link in the opposite direction of the
> transfer.
>
> So the link usage in the opposite direction of the transfer is
> very far from zero.

Indeed, we are not asking to see 1000 Mb/s.  We'd be happy to see 900 
Mb/s.

Netperf is trasmitting a large buffer in MTU-sized packets (min 1500 
bytes).  Since the acks are only about 60 bytes in size, they should be 
around 4% of the total traffic.  Hence we would not expect to see more 
than 960 Mb/s.

We have run these same tests on older kernels (with Broadcomm NICS) and 
gotten above 900 Mb/s full duplex.

Cheers,
     Bruce
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