How to enable full- and half- handshake mode?

[wwwzrb]

It seems that this demo works in half-handshake mode by default.

However, it's reported that half-handshake my have some issues: "RDMA write may experience performance degradation when working with Adaptive Routing and DCT half-handshake mode."
https://docs.nvidia.com/networking/display/ConnectX6Firmwarev20321010/Known%20Issues

The workaroud is "Set a full-handshake mode using PRM API or UCX flags".

The question is how to switch between full- and half- handshake mode using PRM API in this example?

By the way, intuitively the full-handshake mode should cause performance degradation due to the extra RTT for handshake.
What are the salient problems of full-/half-handshake mode?

Thanks for any suggestions!