11.3. Building Open MPI



11.3.2. Why do I get errors about hwloc or libevent not found?

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: * hwloc and/or Libevent * gcc and gfortran

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 ...
    

Be sure to see this section of the Installation guide for more information about the bundled hwloc and/or Libevent vs. system-installed versions.