What does Fast-DDS maxInitialPeersRange restrict?

edit

This is a follow up question on https://answers.ros.org/question/3249....

What does the maxInitialPeersRange that can be configured on the TransportDescriptor really do? As mentioned in the linked question / answer, does it limit the number of nodes a node can communicate with?

I tried using the following as the contents of my DEFAULT_FASTRTPS_PROFILES.xml:

<?xml version="1.0" ?>
<profiles>
    <transport_descriptors>
        <transport_descriptor>
            <transport_id>lotsOfPeers</transport_id>
            <type>UDPv4</type>
            <maxInitialPeersRange>100</maxInitialPeersRange>
        </transport_descriptor>
    </transport_descriptors>
    <participant profile_name="participant_profile" is_default_profile="true">
        <rtps>
            <builtin>
                <metatrafficUnicastLocatorList>
                    <locator/>
                </metatrafficUnicastLocatorList>
                <initialPeersList>
                    <locator>
                        <udpv4>
                            <address>127.0.0.1</address>
                        </udpv4>
                    </locator>
                </initialPeersList>
            </builtin>
            <userTransports>
                <transport_id>lotsOfPeers</transport_id>
            </userTransports>
        </rtps>
    </participant>
</profiles>

Now I noticed that using this setup, if I run the service server and client demo, the latencies are quite high.

Terminal 1:

$ ros2 run demo_nodes_cpp add_two_ints_server

Terminal 2:

$ while true; do time ros2 run demo_nodes_cpp add_two_ints_client; sleep 1; done;                                                                                                  
[INFO] [1600895510.402647349] [add_two_ints_client]: service not available, waiting again...                                                                                                                                                  
[INFO] [1600895511.403126903] [add_two_ints_client]: service not available, waiting again...                                                                                                                                                  
[INFO] [1600895511.839904424] [add_two_ints_client]: Result of add_two_ints: 5                                                                                                                                                                

real    0m2.669s                                                                                                                                                                                                                              
user    0m0.215s                                                                                                                                                                                                                              
sys     0m0.049s                                                                                                                                                                                                                              
[INFO] [1600895514.082000200] [add_two_ints_client]: service not available, waiting again...                                                                                                                                                  
[INFO] [1600895514.919007937] [add_two_ints_client]: Result of add_two_ints: 5                                                                                                                                                                

real    0m2.075s                                                                                                                                                                                                                              
user    0m0.212s                                                                                                                                                                                                                              
sys     0m0.060s                                                                                                                                                                                                                              
[INFO] [1600895517.155697003] [add_two_ints_client]: service not available, waiting again...                                                                                                                                                  
[INFO] [1600895517.918913346] [add_two_ints_client]: Result of add_two_ints: 5                                                                                                                                                                

real    0m2.001s                                                                                                                                                                                                                              
user    0m0.231s                                                                                                                                                                                                                              
sys     0m0.039s

When I remove the DEFAULT_FASTRTPS_PROFILES.xml file, the latency goes down consistently to something between 0.2 and 0.4 seconds.

I found out that the main trigger for this is the maxInitialPeersRange, anything below 36 seems to be good in my current setup.

Based on the answer I mentioned earlier, a low maxInitialPeersRange would result in nodes not being able to communicate with each other, however I can set maxInitialPeersRange to 1 and run ros2 run demo_nodes_cpp talker with 20 instances of ros2 run demo_nodes_cpp listener and eventually all 20 listeners will be receiving the data.

I haven't been able to figure out how maxInitialPeersRange influences the communication except for the latency at values beyond 36.