ROS2 - Enable data sharing/ zero copy

[Nir-Az]

Hi,

In order to use data sharing and loan messages on ROS2 with FastDDS MW do I need to set some configuration parameters?
What is the recommended flow to configure ROS2 to use FastDDS IPC working with zero copy ?

Thanks

[EduPonz]

Hi @Nir-Az ,

1. Data sharing is enabled and configured as OFF by default in Fast DDS when using it through ROS 2, so to use it in ROS 2:
   1. These constrains must be met.
   2. You'd need to enable it using XML (see How to enable LoanedMessages ros2/rmw_fastrtps#579).
2. As for Zero-copy, which is the combination of data sharing and sample loaning in both sender and receiver side, the ROS 2 subscriber uses loans whenever it's possible, i.e. whenever these constrains are met. As for the publication side, ROS 2 does not provide documentation about the feature just yet, but you can see an example here.

Hope that helps!

[Nir-Az]

@EduPonz Helps a lot thanks,

1. I must use the borrow_loaned_message() API in order for zero copy to be used right? cause I will need to constructe my data direcly on the shared memory?

2. Do you know of an implementation that can transport sensor_msgs/Image topic at zero copy? I know it's size is dynamic allocated so it cannot be used and I will need a dedicated topic for the fixed image size?

[EduPonz]

Hi @Nir-Az ,

We are not aware of such implementation at the moment. However, I think the ROS 2 community would very much appreciate such contribution (if it does not exist yet).

[Nir-Az]

Thanks

[Ryanf55]

Given that statement, is it actually true that the V4L2 package here is zero copy?
https://gitlab.com/boldhearts/ros2_v4l2_camera/-/tree/rolling#features

[sirawats]

I think that V4L2 use intra-process communication which is zero copy by default