"Higher level section"_Python_head.html - "LAMMPS WWW Site"_lws -
"LAMMPS Documentation"_ld - "LAMMPS Commands"_lc :c

:link(lws,http://lammps.sandia.gov)
:link(ld,Manual.html)
:link(lc,Commands_all.html)

:line

Extending Python to run in parallel :h3

If you wish to run LAMMPS in parallel from Python, you need to extend
your Python with an interface to MPI.  This also allows you to
make MPI calls directly from Python in your script, if you desire.

We recommend use of mpi4py:

"PyPar"_https://github.com/daleroberts/pypar :ul

As of version 2.0.0 it allows passing a custom MPI communicator to
the LAMMPS constructor, which means one can easily run one or more
LAMMPS instances on subsets of the total MPI ranks.

To install mpi4py (version mpi4py-2.0.0 as of Oct 2015), unpack it
and from its main directory, type

python setup.py build
sudo python setup.py install :pre

Again, the "sudo" is only needed if required to copy mpi4py files into
your Python distribution's site-packages directory. To install with
user privilege into the user local directory type

python setup.py install --user :pre

If you have successfully installed mpi4py, you should be able to run
Python and type

from mpi4py import MPI :pre

without error.  You should also be able to run python in parallel
on a simple test script

% mpirun -np 4 python test.py :pre

where test.py contains the lines

from mpi4py import MPI
comm = MPI.COMM_WORLD
print "Proc %d out of %d procs" % (comm.Get_rank(),comm.Get_size()) :pre

and see one line of output for each processor you run on.

NOTE: To use mpi4py and LAMMPS in parallel from Python, you must
insure both are using the same version of MPI.  If you only have one
MPI installed on your system, this is not an issue, but it can be if
you have multiple MPIs.  Your LAMMPS build is explicit about which MPI
it is using, since you specify the details in your lo-level
src/MAKE/Makefile.foo file.  Mpi4py uses the "mpicc" command to find
information about the MPI it uses to build against.  And it tries to
load "libmpi.so" from the LD_LIBRARY_PATH.  This may or may not find
the MPI library that LAMMPS is using.  If you have problems running
both mpi4py and LAMMPS together, this is an issue you may need to
address, e.g. by moving other MPI installations so that mpi4py finds
the right one.


