ROS 2 zero copy

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Hi, On the Efficient intra-process communication It say about intra-process zero copy data transfer.

How about inter-process? How can I get ROS2 to do zero-copy inter-process communication using shared memory if no other host node is subscribed? Does the use_intra_process_comms flag affect inter-process as well?

and 1 more question, why not setting use_intra_process_comms default on?

Does it comes with a memory allocation cost?

Thanks