4.10. Installation location

A common environment to run Open MPI is in a “Beowulf”-class or similar cluster (e.g., a bunch of 1U servers in a bunch of racks). Simply stated, Open MPI can run on a group of servers or workstations connected by a network.

This raises the question for Open MPI system administrators: where to install the Open MPI binaries, header files, etc.? This discussion mainly addresses this question for homogeneous clusters (i.e., where all nodes and operating systems are the same), although elements of this discussion apply to heterogeneous clusters as well.

Important

For simplicity, the Open MPI team strongly recommends that you install Open MPI at the same path location on all nodes in your cluster. This greatly simplifies the user experience of running MPI jobs across multiple nodes in your cluster.

It is possible to install Open MPI in unique path locations in the different nodes in your cluster, but it is not advisable.

4.10.1. Filesystem types

There are two common approaches.

4.10.1.1. Network filesystem

Have a common filesystem, such as NFS, between all the machines to be used. Install Open MPI such that the installation directory is the same value on each node. This will greatly simplify user’s shell startup scripts (e.g., .bashrc, .cshrc, .profile etc.) — the PATH can be set without checking which machine the user is on. It also simplifies the system administrator’s job; when the time comes to patch or otherwise upgrade Open MPI, only one copy needs to be modified.

For example, consider a cluster of four machines: inky, blinky, pinky, and clyde.

  • Install Open MPI on inky’s local hard drive in the directory /opt/openmpi-VERSION. The system administrator then mounts inky:/opt/openmpi-VERSION on the remaining three machines, such that /opt/openmpi-VERSION on all machines is effectively “the same”. That is, the following directories all contain the Open MPI installation:

    inky:/opt/openmpi-VERSION
blinky:/opt/openmpi-VERSION
pinky:/opt/openmpi-VERSION
clyde:/opt/openmpi-VERSION

  • Install Open MPI on inky’s local hard drive in the directory /usr/local/openmpi-VERSION. The system administrator then mounts inky:/usr/local/openmpi-VERSION on all four machines in some other common location, such as /opt/openmpi-VERSION (a symbolic link can be installed on inky instead of a mount point for efficiency). This strategy is typically used for environments where one tree is NFS exported, but another tree is typically used for the location of actual installation. For example, the following directories all contain the Open MPI installation:

    inky:/opt/openmpi-VERSION
blinky:/opt/openmpi-VERSION
pinky:/opt/openmpi-VERSION
 clyde:/opt/openmpi-VERSION

    Notice that there are the same four directories as the previous example, but on inky, the directory is actually located in /usr/local/openmpi-VERSION.

There is a bit of a disadvantage in this approach; each of the remote nodes have to incur NFS (or whatever filesystem is used) delays to access the Open MPI directory tree. However, both the administration ease and low cost (relatively speaking) of using a networked file system usually greatly outweighs the cost. Indeed, once an MPI application is past MPI initialization, it doesn’t use the Open MPI binaries very much.

4.10.1.2. Local filesystem

If you are concerned with networked filesystem costs of accessing the Open MPI binaries, you can install Open MPI on the local hard drive of each node in your system. Again, it is highly advisable to install Open MPI in the same directory on each node so that each user’s PATH can be set to the same value, regardless of the node that a user has logged on to.

This approach will save some network latency of accessing the Open MPI binaries, but is typically only used where users are very concerned about squeezing every single cycle out of their machines, or are running at extreme scale where a networked filesystem may get overwhelmed by filesystem requests for Open MPI binaries when running very large parallel jobs.

4.10.2. Installing over a prior Open MPI installation

Warning

The Open MPI team does not recommend installing a new version of Open MPI over an existing / older installation of Open MPI.

In its default configuration, an Open MPI installation consists of several shared libraries, header files, executables, and plugins (dynamic shared objects — DSOs). These installation files act together as a single entity. The specific filenames and contents of these files are subject to change between different versions of Open MPI.

Important

Installing one version of Open MPI does not uninstall another version.

If you install a new version of Open MPI over an older version, this may not overwrite all the files from the older version. Hence, you may end up with an incompatible muddle of files from two different installations — which can cause problems.

The Open MPI team recommends one of the following methods for upgrading your Open MPI installation:

  • Install newer versions of Open MPI into a different directory. For example, install into /opt/openmpi-a.b.c and /opt/openmpi-x.y.z for versions a.b.c and x.y.z, respectively.

  • Completely uninstall the old version of Open MPI before installing the new version. The make uninstall process from Open MPI a.b.c build tree should completely uninstall that version from the installation tree, making it safe to install a new version (e.g., version x.y.z) into the same installation tree.

  • Remove the old installation directory entirely and then install the new version. For example rm -rf /opt/openmpi (assuming that there is nothing else of value in this tree!) The installation of Open MPI x.y.z will safely re-create the /opt/openmpi tree. This method is preferable if you no longer have the source and build trees to Open MPI a.b.c available from which to make uninstall.

  • Go into the Open MPI a.b.c installation directory and manually remove all old Open MPI files. Then install Open MPI x.y.z into the same installation directory. This can be a somewhat painful, annoying, and error-prone process. We do not recommend it. Indeed, if you no longer have access to the original Open MPI a.b.c source and build trees, it may be far simpler to download Open MPI version a.b.c again from the Open MPI web site, configure it with the same installation prefix, and then run make uninstall. Or use one of the other methods, above.

4.10.3. Relocating an Open MPI installation

It can be desirable to initially install Open MPI to one location (e.g., /path/to/openmpi) and then later move it to another location (e.g., /opt/myproduct/bundled-openmpi-a.b.c).

Note

Open MPI hard-codes some directory paths in its executables based on installation paths specified by the configure script. For example, if you configure with an installation prefix of /opt/openmpi/, Open MPI encodes in its executables that it should be able to find its help files in /opt/openmpi/share/openmpi.

The “installdirs” functionality in Open MPI lets you change any of these hard-coded directory paths at run time (assuming that you have already adjusted your PATH and/or LD_LIBRARY_PATH environment variables to the new location where Open MPI now resides).

There are three methods.

4.10.3.1. Move an existing Open MPI installation to a new prefix

Set the OPAL_PREFIX environment variable before launching Open MPI. For example, if Open MPI had initially been installed to /opt/openmpi and the entire openmpi tree was later moved to /home/openmpi, setting OPAL_PREFIX to /home/openmpi will enable Open MPI to function properly.

4.10.3.2. “Stage” an Open MPI installation in a temporary location

When creating self-contained installation packages, systems such as RPM install Open MPI into temporary locations. The package system then bundles up everything under the temporary location into a package that can be installed into its real location later. For example, when creating an RPM that will be installed to /opt/openmpi, the RPM system will transparently prepend a “destination directory” (or “destdir”) to the installation directory. As such, Open MPI will think that it is installed in /opt/openmpi, but it is actually temporarily installed in (for example) /var/rpm/build.1234/opt/openmpi. If it is necessary to use Open MPI while it is installed in this staging area, the OPAL_DESTDIR environment variable can be used; setting OPAL_DESTDIR to /var/rpm/build.1234 will automatically prefix every directory such that Open MPI can function properly.

4.10.3.3. Overriding individual directories

Open MPI uses the GNU-specified directories (per Autoconf/Automake), and can be overridden by setting environment variables directly related to their common names. The list of environment variables that can be used is:

  • OPAL_PREFIX

  • OPAL_EXEC_PREFIX

  • OPAL_BINDIR

  • OPAL_SBINDIR

  • OPAL_LIBEXECDIR

  • OPAL_DATAROOTDIR

  • OPAL_DATADIR

  • OPAL_SYSCONFDIR

  • OPAL_SHAREDSTATEDIR

  • OPAL_LOCALSTATEDIR

  • OPAL_LIBDIR

  • OPAL_INCLUDEDIR

  • OPAL_INFODIR

  • OPAL_MANDIR

  • OPAL_PKGDATADIR

  • OPAL_PKGLIBDIR

  • OPAL_PKGINCLUDEDIR

Note that not all of the directories listed above are used by Open MPI; they are listed here in entirety for completeness.

Also note that several directories listed above are defined in terms of other directories. For example, the $bindir is defined by default as $prefix/bin. Hence, overriding the $prefix (via OPAL_PREFIX) will automatically change the first part of the $bindir (which is how method 1 described above works). Alternatively, OPAL_BINDIR can be set to an absolute value that ignores $prefix altogether.