Update Running libEnsemble docs

Author: shuds13

docs/running_libE.rst

@@ -24,8 +24,12 @@ generator and simulator functions. Many :doc:`examples<examples/examples_index>`
 are available.
 
 There are currently three communication options for libEnsemble (determining how
-the Manager and Workers communicate). These are ``mpi``, ``local``, ``tcp``.
-The default is ``mpi``.
+the Manager and Workers communicate). These are ``local``, ``mpi``, ``tcp``.
+The default is ``local`` if ``nworkers`` is specified, otherwise ``mpi``.
+
+Note that ``local`` comms can be used on multi-node systems, where
+the ``MPI executor`` is used to distribute MPI applications across the nodes.
+Indeed, this is the most commonly used option, even on large supercomputers.
 
 .. note::
     You do not need the ``mpi`` communication mode to use the
@@ -34,38 +38,12 @@ The default is ``mpi``.
 
 .. tab-set::
 
-    .. tab-item:: MPI Comms
-
-        This option uses mpi4py_ for the Manager/Worker communication. It is used automatically if
-        you run your libEnsemble calling script with an MPI runner such as::
-
-            mpirun -np N python myscript.py
-
-        where ``N`` is the number of processes. This will launch one manager and
-        ``N-1`` workers.
-
-        This option requires ``mpi4py`` to be installed to interface with the MPI on your system.
-        It works on a standalone system, and with both
-        :doc:`central and distributed modes<platforms/platforms_index>` of running libEnsemble on
-        multi-node systems.
-
-        It also potentially scales the best when running with many workers on HPC systems.
-
-        **Limitations of MPI mode**
-
-        If launching MPI applications from workers, then MPI is nested. **This is not
-        supported with Open MPI**. This can be overcome by using a proxy launcher
-        (see :doc:`Balsam<executor/balsam_2_executor>`). This nesting does work
-        with MPICH_ and its derivative MPI implementations.
-
-        It is also unsuitable to use this mode when running on the **launch** nodes of
-        three-tier systems (e.g., Summit). In that case ``local`` mode is recommended.
-
     .. tab-item:: Local Comms
 
         Uses Python's built-in multiprocessing_ module.
         The ``comms`` type ``local`` and number of workers ``nworkers`` may
         be provided in :ref:`libE_specs<datastruct-libe-specs>`.
+
         Then run::
 
             python myscript.py
@@ -78,6 +56,10 @@ The default is ``mpi``.
 
         This will launch one manager and ``N`` workers.
 
+        The following abbreviated line is equivalent to the above::
+
+            python myscript.py -n N
+
         libEnsemble will run on **one node** in this scenario. To
         :doc:`disallow this node<platforms/platforms_index>`
         from app-launches (if running libEnsemble on a compute node),
@@ -97,6 +79,33 @@ The default is ``mpi``.
         - In some scenarios, any import of ``mpi4py`` will cause this to break.
         - Does not have the potential scaling of MPI mode, but is sufficient for most users.
 
+    .. tab-item:: MPI Comms
+
+        This option uses mpi4py_ for the Manager/Worker communication. It is used automatically if
+        you run your libEnsemble calling script with an MPI runner such as::
+
+            mpirun -np N python myscript.py
+
+        where ``N`` is the number of processes. This will launch one manager and
+        ``N-1`` workers.
+
+        This option requires ``mpi4py`` to be installed to interface with the MPI on your system.
+        It works on a standalone system, and with both
+        :doc:`central and distributed modes<platforms/platforms_index>` of running libEnsemble on
+        multi-node systems.
+
+        It also potentially scales the best when running with many workers on HPC systems.
+
+        **Limitations of MPI mode**
+
+        If launching MPI applications from workers, then MPI is nested. **This is not
+        supported with Open MPI**. This can be overcome by using a proxy launcher
+        (see :doc:`Balsam<executor/balsam_2_executor>`). This nesting does work
+        with MPICH_ and its derivative MPI implementations.
+
+        It is also unsuitable to use this mode when running on the **launch** nodes of
+        three-tier systems (e.g., Summit). In that case ``local`` mode is recommended.
+
     .. tab-item:: TCP Comms
 
         Run the Manager on one system and launch workers to remote