OpenMPI Error in Ubuntu 20.04

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senartcon
Posts: 37
Joined: Wed May 08, 2013 6:46 pm

OpenMPI Error in Ubuntu 20.04

Post by senartcon »

Hello All,

I running into an error as the subject title says it, since the upgrade from Ubuntu 18.04 to 20.04 (CS ver.6.0).
The error message is as follows and the attached files shall describe my system configuration.

Any help is appreciated.

Code: Select all

There are not enough slots available in the system to satisfy the 8
slots that were requested by the application:

  ./cs_solver

Either request fewer slots for your application, or make more slots
available for use.

A "slot" is the Open MPI term for an allocatable unit where we can
launch a process.  The number of slots available are defined by the
environment in which Open MPI processes are run:

  1. Hostfile, via "slots=N" clauses (N defaults to number of
     processor cores if not provided)
  2. The --host command line parameter, via a ":N" suffix on the
     hostname (N defaults to 1 if not provided)
  3. Resource manager (e.g., SLURM, PBS/Torque, LSF, etc.)
  4. If none of a hostfile, the --host command line parameter, or an
     RM is present, Open MPI defaults to the number of processor cores

In all the above cases, if you want Open MPI to default to the number
of hardware threads instead of the number of processor cores, use the
--use-hwthread-cpus option.

Alternatively, you can use the --oversubscribe option to ignore the
number of available slots when deciding the number of processes to
launch.
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Yvan Fournier
Posts: 4077
Joined: Mon Feb 20, 2012 3:25 pm

Re: OpenMPI Error in Ubuntu 20.04

Post by Yvan Fournier »

Hello,

Yes, this is normal, as OpenMPI now checks what hardware is actually available, and asking for more will usually slow you down.

You simply need to reduce the number of requested procs. If you really need to run on more processes (not for performance but for debugging for example), you can add OpenMPI's oversubscription option (check mpiexec manual for exact syntax) to the code_saturne.cfg configuration file.

Best regards,

Yvan
senartcon
Posts: 37
Joined: Wed May 08, 2013 6:46 pm

Re: OpenMPI Error in Ubuntu 20.04

Post by senartcon »

Hello Yvan,

thanks for your help!
I have set 'localhost slots=8' in the hostfile of openmpi. Now I am able to run using nproc = 8 (as against 4 earlier) and I am seeing an increase in the simulation speed.

Regards
Anand
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