Intel® MPI Library User's Guide for Linux* OS
Environment variable I_MPI_DEBUG provides a very convenient way to get information from an MPI application at runtime. You can set value of this variable from 0 (the default value) to 1000. The higher the value, the more debug information you get.
$ mpirun -genv I_MPI_DEBUG 5 -n 8 ./my_application
High values of I_MPI_DEBUG can output a lot of information and significantly reduce performance of an application. A value of I_MPI_DEBUG=5 is generally a good starting point, which provides sufficient information to find common errors. See the I_MPI_DEBUG description in Intel® MPI Library Reference Manual for Linux* OS for more details.