+Lab 0: Hello World
+------------------
+
+It is time to simulate your first MPI program. Use the simplistic
+example `roundtrip.c
+<https://framagit.org/simgrid/simgrid-template-smpi/raw/master/roundtrip.c?inline=false>`_
+that comes with the template.
+
+.. literalinclude:: /tuto_smpi/roundtrip.c
+ :language: c
+
+Compiling and Executing
+.......................
+
+Compiling the program is straightforward (double check your
+:ref:`SimGrid installation <install>` if you get an error message):
+
+
+.. code-block:: shell
+
+ $ smpicc -O3 roundtrip.c -o roundtrip
+
+
+Once compiled, you can simulate the execution of this program on 16
+nodes from the ``cluster_crossbar.xml`` platform as follows:
+
+.. code-block:: shell
+
+ $ smpirun -np 16 -platform cluster_crossbar.xml -hostfile cluster_hostfile.txt ./roundtrip
+
+- The ``-np 16`` option, just like in regular MPI, specifies the
+ number of MPI processes to use.
+- The ``-hostfile cluster_hostfile.txt`` option, just like in regular
+ MPI, specifies the host file. If you omit this option, ``smpirun``
+ will deploy the application on the first machines of your platform.
+- The ``-platform cluster_crossbar.xml`` option, **which doesn't exist
+ in regular MPI**, specifies the platform configuration to be
+ simulated.
+- At the end of the line, one finds the executable name and
+ command-line arguments (if any -- roundtrip does not expect any arguments).
+
+
+
+We will use following simple MPI program, roundtrip.c, in which the processes pass around a message and print the elpased time: