is intra-process communication meta-message transfered via DDS?

[Aerotic]

According to the following text fromhttps://github.com/ros2/design/blob/gh-pages/articles/intraprocess_communication.md:

In order to extract a message from the IntraProcessManager two pieces of information are needed: the id of the Publisher (in order to select the correct ring buffer) and the position of the message within its ring buffer. A meta-message with this information is created and sent through the ROS 2 middleware to all the Subscriptions, which can then retrieve the original message from the IntraProcessManager

It seems meaning that when using intra-process communication, a meta-message must be sent to IntraProcessManager every publish via FastRTPS(or another DDS middleware). But I didn't detect any activity of DDS after initialization phase. Besides, I find that the arguments like publisher_id transfered among functions may be the so called meta-message.

In addition, the following description seems to disaccord the code (/src/ros2/rclcpp/rclcpp/include/rclcpp/experimental/subscription_intra_process.hpp), There is an instance of BufferUniquePtr in SubscriptionIntraProcess and I think BufferUniquePtr is the "ring buffer".

Here the message will be stored in the ring buffer associated with the Publisher.

[mauropasse]

Before the IntraProcessManager were created, there was a proper meta-message being sent through the DDS with some info needed to retreive the intra-process message. The CPU performances were good for really big messages, but when small messages were used (10b) the need for a meta-message resulted in poor performances, because sending a message through the DDS is not cheap. Now everything is handled by the IntraProcessManager, no message is sent through the DDS. But, there's still some DDS activity. The publisher publishes an intra-process message an triggers the subscription DDS guard condition. This happens and goes through the DDS, so the DDS is not really bypassed when using intra-process.

[Aerotic]

But, there's still some DDS activity. The publisher publishes an intra-process message an triggers the subscription DDS guard condition. This happens and goes through the DDS, so the DDS is not really bypassed when using intra-process.

Sorry, I've found something different from the above statement. At src/ros2/rmw_fastrtps/rmw_fastrtps_shared_cpp/src/types/guard_condition.hpp:46, it seems that what the guard condition triggering process mainly does is conditionVariable_->notify_one(). So this may be not doing with DDS like eProsima FastRTPS

[mauropasse]

Yes, you're correct. Is the RMW guard condition, not DDS. What I meant was that there is some RMW specific work after all, not all communication stays in rclcpp. The goal with IPC (rclcpp, not DDS) was totally by-pass the specific RMW's for passing messages.

[clalancette]

Since this is a question, and there is no further activity, I'm going to close this out.