Iperf tests in reverse direction

I used iperf to measure the max throughput for TCP connection between two ubuntu machines. Both the machines were directly connected be means of a 1Gbps cable. When I performed iperf tests i obtained a bandwidth of 845 Mbps on an average. But when i tried reversing the direction (i.e changed server to client and viceversa) the results went down to 185 Mbps.These results are an average of ten readings and the max i was able to obtain was 370Mbps. I then decided to set the same value for parameters such as rmem_max, wmem_max, tcp_rmem, tcp_wmem but even after setting same parameters i got similiar results. i.e(845Mbps in one direction and 185Mbps in the other direction). I don't know what the possible cause could be. Is there something important that i have missed?

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