ulimit - locked memory limit

There are two ways to control the amount of memory that a user process can lock:

System-wide values

Assuming that the PAM limits module is being used (see full docs for the Linux PAM limits module), the system-level default values are controlled by putting a file in /etc/security/limits.d/ (or directly editing the /etc/security/limits.conf file on older systems). Two limits are configurable:

• Soft: The "soft" value is how much memory is allowed to be locked by user processes by default. Set it by creating a file in /etc/security/limits.d/ (e.g., 95-openfabrics.conf) with the line below (or, if your system doesn't have a /etc/security/limits.d/ directory, add a line directly to /etc/security/limits.conf):

  * soft memlock <number>
  

  where "<number>" is the number of bytes that you want user processes to be allowed to lock by default (presumably rounded down to an integral number of pages). "<number>" can also be "unlimited".

   

• Hard: The "hard" value is the maximum amount of memory that a user process can lock. Similar to the soft lock, add it to the file you added to /etc/security/limits.d/ (or editing /etc/security/limits.conf directly on older systems):

  * hard memlock <number>
  

  where "<number>" is the maximum number of bytes that you want user processes to be allowed to lock (presumably rounded down to an integral number of pages). "<number>" can also be "unlimited".

Per-user default values

Per-user default values are controlled via the ulimit (or limit in csh) command. The default amount of memory allowed to be locked will correspond to the "soft" limit set in /etc/security/limits.d/ (or limits.conf -- see above); users cannot use ulimit (or limit in csh) to set their amount to be more than the hard limit in /etc/security/limits.d (or limits.conf).

Users can increase the default limit by adding the following to their shell startup files for Bourne style shells (sh, bash):

ulimit -l unlimited

Or for C style shells (csh, tcsh):

limit memorylocked unlimited

This effectively sets their limit to the hard limit in /etc/security/limits.d (or limits.conf). Alternatively, users can set a specific number instead of "unlimited," but this has limited usefulness unless a user is aware of exactly how much locked memory will require (which is difficult to know since Open MPI manages locked memory behind the scenes).

It is important to realize that this must be set in all shells where Open MPI processes using OpenFabrics will be run. For example, if you are using rsh or ssh to start parallel jobs, it will be necessary to set the ulimit in your shell startup files so that it is effective on the processes that are started on each node.

More specifically -- it may not be sufficient to simply execute the following, because the ulimit may not be in effect on all nodes where Open MPI processes will be run:

shell$ ulimit -l unlimited
shell$ mpirun -np 2 my_mpi_application

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