Why Can't I saturate with Iperf

[Frozennova]

I was moving some files around over the network to my FreeNAS server yesterday and noticed slower than normal transfers.

After running Iperf between the FreeNAS Server and my PC I'm seeing 500-600Mbits/sec transfer rates. Along with ~56% NIC Usage on the windows PC in Task manager.

If I run the same iperf command from two seperate windows machines they will push around 450Mbit/sec to the server each, so I assume it is safe to say that the FreeNAS server is more than capable of fully utilizing its gigabit connection. I've tried using jumbo frames which makes no difference, and the windows machine are using onboard Realtek NICs and the FreeNAS server has a Intel PRO/1000 GT.

Is this simply a case of the Realtek NICs being crap?

 

[Frozennova]

Yea, all drivers are the latest. I've tried directly connecting the two machines which makes no bit of difference either along with a fresh install of windows.

 

[Frozennova]

By packet size do you mean the window size? If so 64k.

The Iperf command im using is

iperf -c 192.168.1.250 -w 64k -t 15 -i 1

edit: Two parallel TCP streams using -P 2 flag ups throughput to around 650Mbit sec

Running UDP with the following command

iperf -c -u -b 1000m -t 15 -i 1

Gives me 850Mbits/sec average with a peak NIC usage of ~89%.