netdev - Re: Peak TCP performance

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Date:	Thu, 10 Oct 2013 20:21:04 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Kyle Hubert <khubert@...il.com>
Cc:	netdev@...r.kernel.org
Subject: Re: Peak TCP performance

On Thu, 2013-10-10 at 22:34 -0400, Kyle Hubert wrote:
> I'm working on a device, I consistently get 27gbps via netperf-2.6.
> UDP reports 54gbps.
> 
> TCP is maxed out at 100% CPU on the transmit side. On the receive
> side, 40% of the CPU. Thus, I didn't believe I could eek anymore
> performance out of it.
> 
> However, very oddly, if I enabled bridged mode to forward some
> packets, TCP performance goes up to 32gbps. The thing that bothers me
> is that transmit CPU utilization drops to 65%, and receive CPU
> utilization increases to 60%.
> 
> What happens when the device becomes bridged to gain so much
> performance? Also, can I now take advantage of the extra CPU time
> available to drive more traffic? No tunable seems to have any effect..
> (except down)


You do not give what version of linux you use, but my guess is that
using latest trees should help, because of
http://git.kernel.org/cgit/linux/kernel/git/davem/net-next.git/commit/?id=c9eeec26e32e087359160406f96e0949b3cc6f10

Also try to disable tx-nocache-copy

ethtool -K eth0 tx-nocache-copy off



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