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 different between eager sends and rendezvous protocol sends
• How many registered buffers to pre-pin (e.g., for GM or mVAPI)
• 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 then 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  the run-time tuning FAQ category for information how to set global MCA parameters.

How do I know what MCA parameters are available?

The ompi_info command can list the parameters for a given component, all the parameters for a specific framework, or all parameters. Most parameters contain a description of the parameter; all will show the parameter's current value.

For example:

shell$ ompi_info --param all all

Shows all the MCA parameters for all components that ompi_info finds, whereas:

shell$ ompi_info --param btl all

Shows all the MCA parameters for all BTL components that ompi_info finds. Finally:

shell$ ompi_info --param btl tcp

Shows all the MCA parameters for the TCP BTL component.

9. How do I set the value of MCA parameters?

There are three main ways to set MCA parameters, each of which are searched in order.

1. Command line: The highest-precedence method is setting MCA parameters on the command line. For example:

   shell$ mpirun --mca mpi_show_handle_leaks 1 -np 4 a.out
   

   This sets the MCA parameter mpi_show_handle_leaks to the value of 1 before running a.out with four processes. In general, the format used on the command line is "--mca <param_name>< value>".

   Note that when senting multi-word values, you need to use quotes to ensure that the shell and Open MPI understand that they are a single value. For example:

   shell$ mpirun --mca param "value with multiple words" ...
   

2. Environment variable: Next, environment variables are searched. Any environment variable named OMPI_MCA_<param_name> will be used. For example, the following has the same effect as the previous example (for sh-flavored shells):

   shell$ OMPI_MCA_mpi_show_handle_leaks=1
   shell$ export OMPI_MCA_mpi_show_handle_leaks
   shell$ mpirun -np 4 a.out
   

   Or, for csh-flavored shells:

   shell% setenv OMPI_MCA_mpi_show_handle_leaks 1
   shell% mpirun -np 4 a.out
   

   Note that setting environment variables to values with multiple words requires quoting, such as:

   # sh-flavored shells
   shell$ OMPI_MCA_param="value with multiple words"
   
   # csh-flavored shells
   shell% setenv OMPI_MCA_param "value with multiple words"
   

    

3. Aggregate MCA parameter files: Simple text files can be used to set MCA parameter values for a specific application. See this FAQ entry (Open MPI version 1.3 and higher).

    

4. Files: Finally, simple text files can be used to set MCA parameter values. Parameters are set one per line (comments are permitted). For example: >

   # This is a comment
   # Set the same MCA parameter as in previous examples
   mpi_show_handle_leaks = 1
   

   Note that quotes are not necessary for setting multi-word values in MCA parameter files. Indeed, if you use quotes in the MCA parameter file, they will be used as part of the value itself. For example:

   # The following two values are different:
   param1 = value with multiple words
   param2 = "value with multiple words"
   

   By default, two files are searched (in order):

    

   1. $HOME/.openmpi/mca-params.conf: The user-supplied set of values takes the highest precedence.
   2. $prefix/etc/openmpi-mca-params.conf: The system-supplied set of values has a lower precedence.

   More specifically, the MCA parameter mca_param_files specifies a colon-delimited path of files to search for MCA parameters. Files to the left have lower precedence; files to the right are higher precedence.

   Keep in mind that, just like components, these parameter files are only relevant where they are "visible" (see this FAQ entry). Specifically, Open MPI does not read all the values from these files during startup and then send them to all nodes in the job -- the files are read on each node during each process' startup. This is intended behavior: it allows for per-node customization, which is especially relevant in heterogeneous environments.

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Last modified: March 12, 2019