Slow performance with Intel X540-T2 10Gb NIC

[Chris Dunbar]

Hello, 

I am new to FreeBSD and recently built a file server out of new components running FreeBSD 10.3. I installed an Intel X540-T2 10 Gb NIC and am experiencing what I consider to be slow transfer speeds. I am using iperf3 to measure the speed and test the results of modifications. So far nothing I have done has made a noticeable difference. If I run iperf3 -s on the FreeBSD server, I see transfer speeds of approximately 1.6 Gb/s. If I run iperf3 in client mode, the speed improves to ~2.75 Gb/s. However, if I replace FreeBSD with CentOS 7 on the same hardware, I see iperf3 speeds surpassing 8 GB/s. The other end of my iperf3 test is a Windows 10 box that also has an Intel X540-T2 installed. 

I did notice that FreeBSD 10.3 (and 11.0 alpha 6 for that matter) includes a slightly older Intel driver (v3.1.13-k). I managed to build a custom kernel that removed the Intel PRO/10GbE PCIE NIC drivers. That allowed me to manually load the latest 3.1.14 driver downloaded from Intel's web site. Unfortunately that did not produce any improvements. I am working my way through man tuning() and some other articles on network performance. So far nothing I tweak makes a noticeable difference. I'm increasingly skeptical that I am going to find a setting or two that more than doubles the speed I am currently experiencing. 

I am open to any and all suggestions at this point. 

Thank you! 
Chris