How to use a Software Module hierarchyΒΆ

Libraries built with one compiler need to be linked with applications with the same compiler version. For High Performance Computing there are libraries called Message Passing Interface (MPI) that allow for efficient communicating between tasks on a distributed memory computer with many processors. Parallel libraries and applications must be built with a matching MPI library and compiler. To make this discussion clearer, suppose we have the intel compiler version 15.0.1 and the gnu compiler collection version 4.9.2. Also we have two MPI libraries: mpich version 3.1.2 and openmpi version 1.8.2. Finally we have a parallel library HDF5 version 1.8.13 (phdf5).

Of the many possible ways of specifying a module layout, this flat layout of modules is a reasonable way to do this:

$ module avail

--------------- /opt/apps/modulefiles ----------------------
gcc/4.9.2                        phdf5/gcc-4.9-mpich-3.1-1.8.13
intel/15.0.2                     phdf5/gcc-4.9-openmpi-15.0-1.8.13
mpich/gcc-4.9-3.1.2              phdf5/intel-15.0-mpich-3.1-1.8.13
mpich/intel-15.0-3.1.2           phdf5/intel-15.0-openmpi-15.0-1.8.13

In order for users to load the matching set of modules, they will have to load the matching set of modules. For example this would be correct:

$ module load gcc/4.9.2 openmpi/gcc-4.9-1.8.2  phdf5/gcc-4.9-openmpi-15.0-1.8.13

It is quite easy to load an incompatible set of modules. Now it is possible that the system administrators at your site might have setup conflict s to avoid loading mismatched modules. However using conflicts can be fragile. What happens if a site adds a new compiler such as clang or pgi or a new mpi stack. All those module file conflict statements will have to be updated.

A different strategy is to use a software hierarchy. In this approach a user loads a compiler which extends the MODULEPATH to make available the modules that are built with the currently loaded compiler (similarly for the mpi stack).

Our modulefile hierarchy is stored under /opt/apps/modulefiles/{Core,Compiler,MPI}. The Core directory is for modules that are not dependent on Compiler or MPI implementations. The Compiler directory is for packages which are only Compiler dependent. Lastly, the MPI directory is packages which dependent on MPI-Compiler pairing. The modulefiles for the compilers are placed in the Core directory. For example the gcc version 4.9.2 file is in Core/gcc/4.9.2.lua and contains:

-- Setup Modulepath for packages built by this compiler
local mroot = os.getenv("MODULEPATH_ROOT")
local mdir  = pathJoin(mroot,"Compiler/gcc", "4.9")
prepend_path("MODULEPATH", mdir)

This code asks the environment for MODULEPATH_ROOT which is /opt/apps/modulefiles and the last two lines prepend /opt/apps/modulefiles/Compiler/gcc/4.9 to the MODULEPATH .

The modulefiles for the MPI implementations are placed under the Compiler directory because they only depend on a compiler. The openmpi module file for the gcc-4.9.2 compiler is then stored at /opt/apps/modulefiles/Compilers/gcc/4.9/openmpi/1.8.2.lua and it contains:

-- Setup Modulepath for packages built by this MPI stack
local mroot = os.getenv("MODULEPATH_ROOT")
local mdir = pathJoin(mroot,"MPI/gcc", "4.9","openmpi","1.8")
prepend_path("MODULEPATH", mdir)

The above code will prepend /opt/apps/modulefiles/MPI/gcc/4.9/openmpi/1.8 to the MODULEPATH.

We store packages as follows:

  1. Core packages: /opt/apps/pkgName/version
  2. Compiler dependent packages: /opt/apps/compilerName-version/pkgName/version
  3. MPI-Compiler dependent packages: /opt/apps/compilerName-version/mpiName-version/pkgName/version

When MODULEPATH changes, Lmod unloads any modules which are not currently in the MODULEPATH and then tries to reload all the previously loaded modules. Any modules which are not available are marked as inactive. Those inactive modules become active if found with new MODULEPATH changes.


In all of the example above, We have used just the first two version numbers. In other words, we have use 4.9 and not 4.9.2 and similarly 1.8 instead of 1.8.2. It is our view that for at least compilers and MPI stacks that the third digit is typically a bug fix and doesn’t require rebuilding all the dependent packages. Y.M.M.V.