Is there a single thread limit on 40Gbps networking?

Hi there,

I just wanne quickly pick your brain guys. I recently started doing some 40Gbps testing with some HP QSFP+ 544 (based on the ConnectX3 chip from Mellanox).

Is there a reason or limitation on a single TCP thread or am I doing something wrong (maybe some optimitation missing), since I can not get more than 20-22Gbps on a single iperf2 tcp thread. I am able to achive 39.8 Gbps on 4 threads. But on a single thread it maxes out at 20-22Gbps.

I saw that (https://community.mellanox.com/servlet/rtaImage?eid=ka21T000000k9yq&feoid=00N5000000AYucA&refid=0EM1T000000uNJf) on the Mellanox site, which seams that Mellanox itself says something about 25Gbps per second in a single thread

Thanks for your advice guys

Regards Yves