13.5. Building Open MPI

13.5.1. General

Once you have run autogen.pl successfully, you can configure and build Open MPI just like end users do with official distribution Open MPI tarballs.

See the general “Install Open MPI” documentation for more details.

13.5.2. Building Against External OpenPMIx / PRRTE

One thing that developers and/or packagers may need to do is to build Open MPI against an external OpenPMIx installation, and/or configure Open MPI’s mpirun / mpiexec launchers to use an external PRRTE installation (i.e., installations that were not built from the embedded copies inside the Open MPI source tree / Git submodules).

With regards to Open MPI’s required dependent libraries (Hwloc, Libevent, OpenPMIx, and PRRTE), it generally is simplest to build Open MPI in one of two ways:

  1. Build and use all the internal copies of Open MPI’s required dependent libraries.

    • Specifically: use the Hwloc, Libevent, OpenPMIx, and PRRTE source trees that are bundled in with Open MPI’s source code.

  2. Build and use all external copies of Open MPI’s required dependent libraries.

    • Specifically: ignore the Hwloc, Libevent, OpenPMIx, and PRRTE source trees that are bundled in with Open MPI’s source code, and, instead, build Open MPI against already-installed Hwloc, Libevent, and OpenPMIx libraries, and configure Open MPI’s launchers to use an already-installed PRRTE.

Other variations are possible, but can get tricky and complicated because Open MPI, the OpenPMIx library that it uses, Hwloc, and Libevent can be loaded into the same process. They are therefore not recommended unless you understand the run-time linker consequences.

Some facts that are relevant to know when building against an external OpenPMIx / PRRTE:

  1. Open MPI and the OpenPMIx library that Open MPI links against must be built against the same installation of Hwloc and Libevent. Meaning:

    • Assumedly the external OpenPMIx was built against external Hwloc and Libevent. Open MPI must compile and link against the same Hwloc and Libevent that the external OpenPMIx was built against.

      Critical

      Open MPI and the OpenPMIx library that it links against must use the same Hwloc and Libevent libraries at run time (e.g., they must resolve to the same run-time loadable libraries at run time).

      Important

      This statement applies regardless of whether Open MPI – and/or the other libraries – are built as static or dynamically-loadable libraries.

    • Unless you really know what you are doing, this usually means building and installing Open MPI against the same installation tree(s) of Hwloc and Libevent that OpenPMIx used to build itself.

      For example, consider an environment where you install Hwloc, Libevent, OpenPMIx, and PRRTE via the operating system’s package manager. Assuming that the package-manager install of OpenPMIx was built against the package-manager-provided Hwloc and Libevent, then Open MPI will also need to be built against the package-manager-provided Hwloc and Libevent. To build Open MPI this way, you may need to install the package manager’s “developer” Hwloc, Libevent, and OpenPMIx packages.

  1. PRRTE and the OpenPMIx library that PRRTE uses must be built against the same installation of Hwloc and Libevent.

    This is a separate requirement from Open MPI’s requirement above: Open MPI does not link against PRRTE, and MPI applications do not load libprrte. Therefore, PRRTE’s Hwloc, Libevent, and OpenPMIx dependencies do not have to match Open MPI’s dependencies merely because Open MPI uses PRRTE as a launcher.

  1. Open MPI and PRRTE do not have to use the same OpenPMIx installation.

    PMIx supports cross-version operations, so Open MPI and PRRTE can use different OpenPMIx installations, and those installations do not need to be the same OpenPMIx version.

    If Open MPI and PRRTE do use the same OpenPMIx installation, then the requirements above mean that Open MPI, PRRTE, OpenPMIx, Hwloc, and Libevent will all use the same Hwloc and Libevent installations. However, this is a consequence of sharing one OpenPMIx installation; it is not a requirement that Open MPI and PRRTE share one OpenPMIx installation.