Update DDS-tuning.rst with hints to try Cyclone

[Pleune]

It may not be clear that the default FastDDS does not work well in all situations by default, and that one easy workaround could be to try CycloneDDS.

See some discussion at ros2/ros2#1289
And some more information about my particular issue at ros2/rmw_fastrtps#739

[kscottz]

I have two suggestions here:

• The default DDS vendor can and will change between releases. I think we might want to mention this as the generic way to change the underlying RMW.
• In the case of FastDDS, a lot of users have found success using the FastDDS Discovery server. We might want to mention this.

[fujitatomoya]

I really do not think changing rmw implementation is easy thing to do once it comes to production. btw, what do you mean by issues here? if you want to mention that user can configure the rmw implementation with environmental variable, we already have https://docs.ros.org/en/rolling/How-To-Guides/Working-with-multiple-RMW-implementations.html?

[fujitatomoya]

Now i see the specific issue for large data.

We have https://github.com/ros2/rmw_fastrtps?tab=readme-ov-file#large-data-transfer-over-lossy-network, but that kind of tuning information is not well described in ros2 documentation. I do agree that we can add more suggested configuration for use case basis, that would be useful for users to save time.

[clalancette]

So we discussed this with the ROS 2 team.

The most basic problem is that depending on your application, different RMW implementation work differently, and in some cases, better. Also, as @kscottz pointed out, the default may change between distributions.

Thus, the advice here should be generic, that switching to a different RMW backend may help. Thus I think the advice should be something like the following:

.. note::

    If you are having issues with the default RMW implementation, sometimes switching to another one may help.  You can find out what your current RMW implementation is by running ``ros2 doctor -r``.  If you want to switch to a different DDS vendor, please see :doc:`<./Working-with-multiple-RMW-implementations>`.

[clalancette]

@Pleune Friendly ping here.

[YossiO-BWR]

Suggested change

	**Note:** If you are seeing issues with the default FastDDS middleware, the easiest thing too try may be to switch to another, such as CycloneDDS. Try this by exporting ``export RMW_IMPLEMENTATION=rmw_cyclonedds_cpp``.
	**Note:** If you are seeing issues with the default FastDDS middleware, the easiest thing to try may be to switch to another, such as CycloneDDS. Try this by exporting ``export RMW_IMPLEMENTATION=rmw_cyclonedds_cpp``.