Testing download speed using iperf

I am using iperf to test my bandwidth. But I am not getting whether it is showing my upload or download speed. Say, I have two PCs A and B. If I want to test how fast I can download a file to A from B, where should I run the server i.e "iperf -s" ? on A or B ? Thanks in advance.

With the iperf3 you can use the -R option to send the traffic from Server to client.

  -R, Reverse test mode – Server sends, client receives 

Traffic way is from client to server.

From manpage:

To perform an iperf test the user must establish both a server (to discard traffic) and a client (to generate traffic)