Add comments on MPI ports vs sockets

pages/docs/configuration/configuration-communication.md

@@ -44,11 +44,11 @@ The alternative to TCP/IP sockets is MPI ports (an MPI 2.0 feature):
 <m2n:mpi .../>
 ```
 
-As the ports functionality is not a highly used feature of MPI, it has robustness issues for several MPI implementations ([for OpenMPI, for example](https://github.com/precice/precice/issues/746)). In principle, MPI gives you faster communication roughly by a factor of 10, but, for most applications, you will not feel any difference as both are very fast. We recommend using `sockets`.
+In preCICE, we always start simulations in separated MPI communicators (remember: we start solvers in different terminals, with their own `mpirun` commands), a feature that highly improves flexibility (solvers do not need to be in the same MPI communicator at any time). As the MPI ports functionality is not a highly used feature of MPI (at least not with separated `MPI_COMM_WORLD` communicators), it has robustness issues for several MPI implementations ([for OpenMPI, for example](https://github.com/precice/precice/issues/746)). In principle, MPI gives you faster communication roughly by a factor of 10 (see [Benjamin Uekermann's dissertation](https://mediatum.ub.tum.de/doc/1320661/document.pdf), section 4.2.3), but, for most applications, you will not feel any difference as both are very fast. We recommend using `sockets` by default, unless you are performing large performance-critical simulations with very large coupling meshes.
 
 Which participant is `from` and which one is `to` makes almost no difference and cannot lead to deadlock. Only for massively parallel runs, it can make a performance difference at initialization. For such cases, [ask us for advice](https://precice.discourse.group/new-topic).
 
-The `exchange-directory` should point to the same location for both participants. We use this location to exchange hidden files with initial connection information. It defaults to `"."`, i.e. both participants need to be started in the same folder. We give some best practices on how to arrange your folder structure and start the coupled solvers [here](TODO).  
+The `exchange-directory` should point to the same location for both participants. We use this location to exchange hidden files with initial connection information. It defaults to `"."`, i.e. both participants need to be started in the same folder. We give some best practices on how to arrange your folder structure and start the coupled solvers in the page [running on a local machine](running-simple.html).  
 
 {% important %}
 If you face any problems with establishing the communication, have a look [at this Discourse post](https://precice.discourse.group/t/help-the-participants-are-not-finding-each-other/646/2).