Why host transmit rate on 1Gb ethernet is only ~750Mbps ?

[Jason Leschnik]

I would first start by doing a point-to-point link between your two
end nodes to rule out your network gear as being the problem

On Sun, Apr 15, 2012 at 4:20 AM, Yuri <yuri at rawbw.com> wrote:
> I am running some tests with gigabit switch between two 9.0 hosts using
> iperf.
> The best UDP transmit rate I am getting is 753 Mbits/sec with ~3-6% packet
> loss @1500 MTU @ 2.5GHz CPU, even though command 'iperf -c X.X.X.X -u -b
> 1000m' requests the full gigabit.
>
> I am trying to understand what exactly usually limits the speed with one way
> ethernet traffic and also what causes the packet loss.
> Is it the host's CPU or rather the need to make ~83K system calls in order
> to send 1Gb of UDP data at 1500 MTU?
> Is it the NIC or it's driver ('re' on source and 'em' on destination in my
> case)?
> Is it the router which is maybe too old? (Linksys EG008W, with tcpdump I
> made sure there is no ICMP traffic that is coming back from the router
> during the test).
>
> Why can't NIC send say 900 Mbps with MTU 1500 since the only overhead is
> ethernet+UDP headers which are ~120bytes out of 1500?
> Why packet loss occurs in such a simple network with just one switch and
> speeds of 75% of the maximum?
> How can I troubleshoot such situation and understand the reasons?
>
> Yuri
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-- 
Regards,
Jason Leschnik.

[m] 0432 35 4224
[w@] jason dot leschnik <at> ansto dot gov dot au
[U@] jml974 at uow.edu.au