# 4.8. Required support libraries

While Open MPI can be built with support for a wide variety of systems, a small set of support libraries are required in order to build Open MPI in any environment. Several of these packages are both fundamental to Open MPI’s operation and not universally available in all environments. As such, these “fundamental” packages are both embedded in Open MPI’s distribution tarballs and also directly incorporated into Open MPI’s configure, build, and installation process.

See below for a description of how Open MPI chooses whether to use the embedded versions of these packages or versions already installed on your system.

- Hardware Locality

  - This library is required; Open MPI will not build without it.

  - Minimum version required: 1.11.0

    Note

    While the minimum version of Hwloc will work, it may still have bugs and/or have less functionality as compared to later versions.

    Other than the Hwloc restriction about v3.0.0 and beyond (see below), the Open MPI community generally recomends using the latest available version of Hwloc unless there is a specific reason not to.

  - Version embedded in Open MPI distribution: 2.7.1

  Danger

  As of v5.0.6, Open MPI does not yet support the Hwloc v3.x series (which may not even be available at the time of Open MPI v5.0.6’s release). Hwloc v3.x is anticipated to break API and/or ABI compared to the Hwloc v2.x series.

  Open MPI will refuse to build if it finds an external Hwloc installation that is >= v3.0.0 on the assumption that other HPC applications and/or libraries may be using it. Such a configuration could lead to obscure and potentially confusing run-time failures of Open MPI applications.

  If Open MPI’s `configure` script aborts because it finds an Hwloc installation that is >= v3.0.0, you can either ensure that Open MPI finds a < v3.0.0 Hwloc installation (e.g., by changing the order of paths in `LD_LIBRARY_PATH`), or force the use of Open MPI’s bundled Hwloc via:

      shell$ ./configure --with-hwloc=internal ...

  Regardless, it is critically important that if an MPI application — or any of its dependencies — uses Hwloc, it uses the same Hwloc with which Open MPI was compiled.

- Libevent

  - This library is required; Open MPI will not build without it.

  - Minimum version required: 2.0.21

  - Version embedded in Open MPI distribution: 2.1.12

    Note

    The Open MPI community has heavily tested Libevent 2.1.12. Other versions should work, but — unlike Hwloc, OpenPMIx, and PRRTE — there is not much reason to upgrade to use a later version of Libevent.

- OpenPMIx

  - This library is required; Open MPI will not build without it.

  - Minimum version required when building without PRRTE: 4.2.0

    Note

    While the minimum version of OpenPMIx will work, it may still have bugs and/or have less functionality as compared to later versions.

    The Open MPI community generally recomends using the latest available version of OpenPMIx unless there is a specific reason not to.

    Note

    While OpenPMIx is the formal name of the software that implements the PMIx standard, the term “PMIx” is used extensively throughout this documentation to refer to the OpenPMIx software package.

  - Minimum version required when building with PRRTE: See the PRRTE project documentation.

  - Version embedded in Open MPI distribution: v5.0.4

- PRRTE

  - This library is optional in some environments. See below.

  - Minimum version required: 3.0.0

    Note

    While the minimum version of PRRTE will work, it may still have bugs and/or have less functionality as compared to later versions. For example, if you build and run with 3.0.0, you will not get a fully-populated `mpirun(1)` man page.

    The Open MPI community generally recomends using the latest available version of PRRTE unless there is a specific reason not to.

  - Version embedded in Open MPI distribution: v3.0.7

  PRRTE provides Open MPI’s full-featured `mpirun` / `mpiexec` MPI application launchers (the two commands are identical; they are symbolic links to the same executable).

  Warning

  If you are building the PRRTE that is embedded in the Open MPI v5.0.6 distribution:

  - If you are also building the PMIx that is embedded in the Open MPI v5.0.6 distribution, that combination of packages is supported.

  - If you are building against an external PMIx installation (i.e., a version of PMIx that is not embedded in the Open MPI v5.0.6 distribution), you should check the PRRTE project documentation to see what minimum version of PMIx is required.

  - If your environment uses another MPI application launcher (e.g., Slurm users can use the `srun` launcher to “direct launch” Open MPI applications), then the use of PRRTE is optional.

  - If your environment has no other MPI application launcher, then you need to install PRRTE and build Open MPI with PRRTE support.

  - Open MPI can use the copy of PRRTE embedded in its source code tree, or compile/link against an external PRRTE installation. See this section for details about how to specify each method.

Note

In general, the Open MPI community recommends using the most recent versions of both the required support libraries and any other optional support libraries. This is because more recent versions typically tend to include bug fixes, sometimes affecting Open MPI functionality. As a specific example, there is a known issue with Hardware Locality releases older than v2.8.0 on systems with Intel Ponte Vecchio accelerators. If you run Open MPI on such systems, you need to use Hwloc v2.8.0 or newer, or you will experience undefined behavior. This effect is not unique to the Hardware Locality library; this is why the Open MPI community recommends using as recent as possible versions of all support libraries.

## 4.8.1. Library dependencies

These support libraries have dependencies upon each other:

Open MPI required support library dependency graph.

The higher-level boxes depend on the lower-level boxes. Specifically:

- Open MPI depends on PRRTE, PMIx, Hwloc, and Libevent (i.e., everything).

- PRRTE depends on PMIx, Hwloc, and Libevent (i.e., everything except Open MPI).

- PMIx depends on Hwloc and Libevent.

- Hwloc does not depend on anything.

- Libevent does not depend on anything.

At run time, it is critical that the run-time linker loads exactly one copy of each of these libraries.

Note

The required support libraries can have other dependencies, but for simplicity and relevance to building Open MPI, those other dependencies are not discussed here.

## 4.8.2. Potential problems

Problems can (will) arise if multiple different copies of the above shared libraries are loaded into a single process. For example, consider if:

- Loading the Open MPI shared library causes the loading of Libevent shared library vA.B.C.

- But then the subsequent loading of the PMIx shared library causes the loading of Libevent shared library vX.Y.Z.

Since there are now two different versions of the Libevent shared library loaded into the same process (yes, this can happen!), unpredictable behavior can (will) occur.

Many variations on this same basic erroneous scenario are possible. All of them are bad, and can be extremely difficult to diagnose.

## 4.8.3. Avoiding the problems

A simple way to avoid these problems is to configure your system such that it has exactly one copy of each of the required support libraries.

Important

If possible, use your OS / environment’s package manager to install as many of these support libraries — including their development headers — as possible before invoking Open MPI’s `configure` script.

Not all package managers provide all of the required support libraries. But even if your package manager installs — for example — only Libevent and Hwloc, that somewhat simplifies the final Open MPI configuration, and therefore avoids some potentially erroneous configurations.

## 4.8.4. How `configure` finds the required libraries

In an attempt to strike a balance between end-user convenience and flexibility, Open MPI bundles these four required support libraries in its official distribution tarball.

Generally, if Open MPI cannot find a required support library, it will automatically configure, build, install, and use its bundled version as part of the main Open MPI configure, build, and installation process.

Put differently: Open MPI’s `configure` script will examine the build machine and see if it can find each of the required support header files and libraries. If it cannot find them, it will attempt to fall back and use the corresponding bundled support library instead.

Important

Note, however, that `configure` is smart enough to understand the dependencies between the required support libraries.

Specifically: If `configure` finds the development headers and libraries for a given support library already installed on the system, then it will ignore both the corresponding bundled support library, and it will also ignore all bundled support libraries that are below it in the dependency graph shown above.

### 4.8.4.1. Build example 1

`configure` finds the PRRTE development headers and libraries in `/usr/local`. This will cause the following to occur:

1. `configure` will ignore the PRRTE library that is bundled in the Open MPI source tree and will use the PRRTE that is already installed in `/usr/local`.

2. `configure` will also ignore the bundled PMIx, Hwloc, and Libevent libraries in the Open MPI source tree.

   - If `configure` is unable to find header files and libraries for PMIx, Hwloc, and Libevent elsewhere on the build machine (i.e., assumedly the same PMIx, Hwloc, and Libevent than the PRRTE in `/usr/local` is using), this is an error: `configure` will abort, and therefore refuse to build Open MPI.

### 4.8.4.2. Build example 2

`configure` does not find PRRTE on the build machine, but does find PMIx development headers and libraries in `/opt/local`. This will cause the following to occur:

1. `configure` will set up to build the PRRTE library that is bundled in the Open MPI source tree.

2. `configure` will ignore the PMIx library that is bundled in the Open MPI source tree and will use the PMIx that is already installed in `/opt/local`.

3. `configure` will also ignore the bundled Hwloc and Libevent libraries in the Open MPI source tree.

   - If `configure` is unable to find header files and libraries for Hwloc and Libevent elsewhere on the build machine (i.e., assumedly the same Hwloc and Libevent than the PMIx in `/opt/local` is using), this is an error: `configure` will abort, and therefore refuse to build Open MPI.

### 4.8.4.3. Build example 3

`configure` only finds the development headers and libraries for Libevent on the build machine. This will cause the following to occur:

1. `configure` will set up to build the PRRTE, PMIx, and Hwloc libraries that are bundled in the Open MPI source tree.

2. `configure` will ignore the Libevent library that is bundled in the Open MPI source tree and will use the Libevent that is already installed.

## 4.8.5. Difficulties with C and Fortran

Sometimes you may see errors similar to the following when attempting to build Open MPI:

    ...
    PPFC     profile/pwin_unlock_f08.lo
    PPFC     profile/pwin_unlock_all_f08.lo
    PPFC     profile/pwin_wait_f08.lo
    FCLD     libmpi_usempif08.la
    ld: library not found for -lhwloc
    collect2: error: ld returned 1 exit status
    make``2``: *** ``libmpi_usempif08.la`` Error 1

This error can happen when a number of factors occur together:

1. If Open MPI’s `configure` script chooses to use an “external” installation of hwloc and/or Libevent (i.e., outside of Open MPI’s source tree).

2. If Open MPI’s `configure` script chooses C and Fortran compilers from different suites/installations.

Put simply: if the default search library search paths differ between the C and Fortran compiler suites, the C linker may find a system-installed `libhwloc` and/or `libevent`, but the Fortran linker may not.

This may tend to happen more frequently starting with Open MPI v4.0.0 on Mac OS because:

1. In v4.0.0, Open MPI’s `configure` script was changed to “prefer” system-installed versions of hwloc and Libevent (vs. preferring the hwloc and Libevent that are bundled in the Open MPI distribution tarballs).

2. In MacOS, it is common for Homebrew or MacPorts to install:

   - Hardware Locality

   - Libevent

For example, as of July 2019, Homebrew:

- Installs hwloc v2.0.4 under `/usr/local`

- Installs the Gnu C and Fortran compiler suites v9.1.0 under `/usr/local`. However, the C compiler executable is named `gcc-9` (not `gcc`!), whereas the Fortran compiler executable is named `gfortran`.

These factors, taken together, result in Open MPI’s `configure` script deciding the following:

- The C compiler is `gcc` (which is the MacOS-installed C compiler).

- The Fortran compiler is `gfortran` (which is the Homebrew-installed Fortran compiler).

- There is a suitable system-installed hwloc in `/usr/local`, which can be found — by the C compiler/linker — without specifying any additional linker search paths.

The careful reader will realize that the C and Fortran compilers are from two entirely different installations. Indeed, their default library search paths are different:

- The MacOS-installed `gcc` will search `/usr/local/lib` by default.

- The Homebrew-installed `gfortran` will not search `/usr/local/lib` by default.

Hence, since the majority of Open MPI’s source code base is in C, it compiles/links against hwloc successfully. But when Open MPI’s Fortran code for the `mpi_f08` module is compiled and linked, the Homebrew-installed `gfortran` — which does not search `/usr/local/lib` by default — cannot find `libhwloc`, and the link fails.

There are a few different possible solutions to this issue:

1. The best solution is to always ensure that Open MPI uses a C and Fortran compiler from the same suite/installation. This will ensure that both compilers/linkers will use the same default library search paths, and all behavior should be consistent. For example, the following instructs Open MPI’s `configure` script to use `gcc-9` for the C compiler, which (as of July 2019) is the Homebrew executable name for its installed C compiler:

       shell$ ./configure CC=gcc-9 ...

       # You can be precise and specify an absolute path for the C
       # compiler, and/or also specify the Fortran compiler:
       shell$ ./configure CC=/usr/local/bin/gcc-9 FC=/usr/local/bin/gfortran ...

   Note that this will likely cause `configure` to not find the Homebrew-installed hwloc, and instead fall back to using the bundled hwloc in the Open MPI source tree.

2. Alternatively, you can simply force `configure` to select the bundled versions of hwloc and libevent, which avoids the issue altogether:

       shell$ ./configure --with-hwloc=internal --with-libevent=internal ...

3. Finally, you can tell `configure` exactly where to find the external hwloc library. This can have some unintended consequences, however, because it will prefix both the C and Fortran linker’s default search paths with `/usr/local/lib`:

       shell$ ./configure --with-hwloc-libdir=/usr/local/lib ...

## 4.8.6. Overriding `configure` behavior

If `configure`’s default searching behavior is not sufficient for your environment, you can use command line options to override its default behavior.

For example, if PMIx and/or PRRTE are installed such that the default header file and linker search paths will not find them, you can provide command line options telling Open MPI’s `configure` where to search. Here’s an example `configure` invocation where PMIx and PRRTE have both been installed to `/opt/open-mpi-stuff`:

    ./configure --prefix=$HOME/openmpi-install \
        --with-pmix=/opt/open-mpi-stuff \
        --with-prrte=/opt/open-mpi-stuff ...

As another example, if you do not have root-level privileges to use the OS / environment package manager, and if you have a simple MPI application (e.g., that has no external library dependencies), you may wish to configure Open MPI something like this:

    ./configure --prefix=$HOME/openmpi-install \
        --with-libevent=internal --with-hwloc=internal \
        --with-pmix=internal --with-prrte=internal ...

The `internal` keywords force `configure` to use all four bundled versions of the required libraries.

Danger

Be very, very careful when overriding `configure`’s default search behavior for these libraries. Remember the critical requirement: that Open MPI infrastructure and applications load exactly one copy of each support library. For simplicity, it may be desirable to ensure to use exactly the support libraries that Open MPI was compiled and built against.

For example, using the Open MPI installed from the sample `configure` line (above), you may want to prefix your run-time linker search path (e.g., `LD_LIBRARY_PATH` on Linux) with `$HOME/openmpi-install/lib`. This will ensure that linker finds the four support libraries from your Open MPI installation tree, even if other copies of the same support libraries are present elsewhere on your system.

## 4.8.7. (Strong) Advice for packagers

If you are an Open MPI packager, we strongly suggest that your Open MPI package should not include Hwloc, Libevent, PMIx, or PRRTE. Instead, it should depend on external, independently-built versions of these packages.

See the Advice for packagers section for more details.