Iperf3 process is single-threaded. This means all the parallel streams for one test use the same CPU core.
To run multiple iperf3 processes for a testing a high-speed host, do the following:
Start Multiple Session on iperf3 Server
iperf3 -s -p 5101&; iperf3 -s -p 5102&; iperf3 -s -p 5103 &
Run multiple clients, using the “-T” flag to label the output:
iperf3 -c hostname -T s1 -p 5101 &;
iperf3 -c hostname -T s2 -p 5102 &;
iperf3 -c hostname -T s3 -p 5103 &;