IPC on separate executors

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Yes, you are right. I used the term IPC quite loosely, i really meant IPC as in using a shared memory space which can take advantage of zero-copy. So to have zero copy I really need to put my nodes in a composable container and have them run in the same thread? I think this is the case from looking at the intra-comms demos.

Correct, to have intraprocess zero-copy you have to put your nodes in one program, but not necessarily in the same thread (since one process can have multiple threads and they share the same memory space), as well as follow the instructions you linked to in your original question.

As a side note, the term shared memory, usually means something else. (it is a mechanism to share a block of memory between more than one process). There is recent work being done for this in ROS2, but it requires allocating chunks of memory and is quite a bit harder to get working.

Yes a process can have multiple threads, but i need to expect a single thread when it's only used with a single thread executor. For example main() of the example i previously linked. Yes shared memory is used, with mmap to achieve zero-copy, for example with https://github.com/eclipse-iceoryx/ic..., which cyclonedds uses. is it triggered by 'use_intra_process_comms'?

Eclipse iceoryx tries to achieve true zero-copy inter-process communication, and isn't related to the use_intra_process_comms flag. Iceoryx tries to deal with the zero-copy at the DDS level.

Okey, so intra_comms have nothing todo with DDS, good to know. But how do I let CycloneDDS know that I want zero-copy between two nodes? or is it default?

At this point, it may be better to start a new question, but make sure you understand that Eclipse iceoryx and Eclipse cyclonedds are different.

Eclipse iceoryx is the one that has zero-copy interprocess. See the README of https://github.com/ros2/rmw_iceoryx to run their demos. Once you run those demos, consider if the zero-copy interprocess is absolutely necessary for what you're trying to achieve as there are draw backs, like not being able to use dynamic message types like vectors.