4.16. Advice for System Administrators

Several members of the Open MPI team have strong system administrator backgrounds; we recognize the value of having software that is friendly to system administrators. Here are some of the reasons that Open MPI is attractive for system administrators:

Simple, standards-based installation
Reduction of the number of MPI installations
Ability to set system-level and user-level parameters
Scriptable information sources about the Open MPI installation

See the rest of this section for more details.

4.16.1. Setting Global MCA Parameters

MCA parameters are a way to tweak Open MPI’s behavior at run-time. For example, MCA parameters can specify:

Which interconnect networks to use
Which interconnect networks not to use
The size difference between eager sends and rendezvous protocol sends
How many registered buffers to pre-pin (e.g., for InfiniBand)
The size of the pre-pinned registered buffers
…etc.

It can be quite valuable for a system administrator to play with such values a bit and find an “optimal” setting for a particular operating environment. These values can then be set in a global text file that all users will, by default, inherit when they run Open MPI jobs.

For example, say that you have a cluster with 2 ethernet networks — one for NFS and other system-level operations, and one for MPI jobs. The system administrator can tell Open MPI to not use the NFS TCP network at a system level, such that when users invoke mpirun or mpiexec to launch their jobs, they will automatically only be using the network meant for MPI jobs.

See how to set MCA params for information on how to set global MCA parameters.

4.16.2. Setting MCA Parameters for a Global Open MPI installation

Which global MCA parameters to set depends on both your specific cluster setup and the applications that typically run there.

The best thing to do is to use the ompi_info(1) command to see what parameters are available and relevant to you. Specifically, ompi_info --all can be used to show all the parameters that are available for each plug-in. Two common places that system administrators like to tweak are:

Only allow specific networks: If you have a cluster with a high-speed interconnect (such as InfiniBand) and a low-speed ethernet network (e.g., 1Gpbps). The high-speed network is intended for MPI jobs; the control network is intended for NFS and other administrative-level tasks. In this case, you can simply turn off Open MPI’s TCP support. The btl framework contains Open MPI’s network support; in this case, you want to disable the tcp plug-in. You can do this by adding the following line in the file $prefix/etc/openmpi-mca-params.conf:
```
btl = ^tcp
```
This tells Open MPI to load all BTL components except tcp.

Consider another example: your cluster has two TCP networks, one for NFS and administration-level jobs, and another for MPI jobs. You can tell Open MPI to ignore the TCP network used by NFS by adding the following line in the file $prefix/etc/openmpi-mca-params.conf:
```
btl_tcp_if_exclude = lo,eth0
```
The value of this parameter is the device names to exclude. In this case, we’re excluding lo (localhost, because Open MPI has its own internal loopback device) and eth0.
Tune the parameters for specific networks: Each network plug-in has a variety of different tunable parameters. Use the ompi_info command to see what is available. You show all available parameters with:
```
shell$ ompi_info --param all all
```
Note

Starting with Open MPI v1.8, ompi_info categorizes its parameters in levels, as defined by the MPI_T interface from the MPI standard. You will need to specify --level 9 (or --all) to show all MCA parameters. See this blog entry for further information.
```
shell$ ompi_info --level 9
# or
shell$ ompi_info --all
```
Beware: there are many variables available. You can limit the output by showing all the parameters in a specific framework or in a specific plug-in with the command line parameters:
```
shell$ ompi_info --param btl all --level 9
```
Shows all the parameters of all BTL components, and:
```
shell$ ompi_info --param btl tcp --level 9
```
Shows all the parameters of just the tcp BTL component.

4.16.3. Adding a new plugin to a global Open MPI installation

If you add a new component (such as support for a new network) to a global Open MPI installation, Open MPI will simply open the new plugin at run-time — your applications do not need to be recompiled or re-linked.

See this section for more details.

4.16.4. Upgrading network hardware with a global Open MPI installation

If you upgrade your network hardware and your installation of Open MPI uses shared libraries, you simply need to recompile the Open MPI components that support that network and re-install them.

More specifically, Open MPI shifts the dependency on the underlying network away from the MPI applications and to the Open MPI plug-ins. This is a major advantage over many other MPI implementations. MPI applications will simply open the new plugin when they run.

4.16.5. User customization of a global Open MPI installation

It is typically sufficient for a single Open MPI installation (or perhaps a small number of Open MPI installations, depending on compiler interoperability, see installing multiple copies of Open MPI for more information) to serve an entire parallel operating environment.

However, a system-wide Open MPI installation can be customized on a per-user basis in two important ways:

Per-user MCA parameters: Each user can set their own set of MCA parameters, potentially overriding system-wide defaults.
Per-user plug-ins: Users can install their own Open MPI plug-ins under $HOME/.openmpi/components. In this way, developers can experiment with new components without destabilizing Open MPI for the rest of the users on the system. Or power users can download 3rd party components (perhaps even research-quality components) without affecting other users.