#21 Wrong throughput shown?

[Nils]

I have 2 Windows-machines connected with 1x10Gb fiber each.
These are connected back-to-back (also tried with an 10Gb-switch between the servers)

When I run iperf i get really strange results depending on what "window size" i choose.
I think the default Window-size is 56KBytes?

If both servers have MTU 9014:
Default window size (56k): 1.5Mbit
1Mb window size : 9.87Gbps

If i run the tests with MTU 1514 i get these results:
Default window size (56k): 2.85Gbps
1Mb window size : 9.36Gbps

If I try different window-size (between 128-512) the throughput is in line with the increasing window-size.

Why do I get this results?