[Gllug] Observed speed of gigabit ethernet only about 1/4 or 1/5 of advertised

[Mike Brodbelt]

Richard Jones wrote:
> Hi, I've just got a new gigabit ethernet switch to replace my ancient
> 10 Mbps home switch.  The old switch easily maxed out at 10 Mbps.
> 
> With the other ports on the switch idle and two machines connected
> through gigabit ethernet, I'm seeing 220 Mbps with ssh.  That's about
> a quarter of what I'd expect to be the theoretical maximum.
> Surprisingly with ttcp it's slower -- 170 Mbps, or about one fifth of
> the theoretical max.  With UDP it's little better -- 195 Mbps.
> 
> I wonder if someone can suggest from the description below where the
> weak point is likely to be in this setup.  Crappy on-board ethernet?
> SoHo switch?  PCI bus or processor speeds?  Or does everyone with
> gigabit ethernet only see a fraction of the advertised throughput?

I've had machines running at close to wire speed, but for that level of 
performance you tend to need to tweak things a bit. CPU load on the host 
is significant, and the quality of the network drivers can be variable - 
  I saw CPU use halve between two kernel releases with the Intel e1000 
driver, for example.

Have a look at http://www-didc.lbl.gov/TCP-tuning/linux.html as well, 
many of the sysctl's default to values that aren't suitable for GigE 
connected machines, especially if link latency is and higher than LAN speed.

HTH,

Mike
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