8000 OpenMPI does not run on fewer cores than requested · Issue #557 · pism/pism · GitHub
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mankoff opened this issue Mar 13, 2025 · 0 comments
Open

OpenMPI does not run on fewer cores than requested #557

mankoff opened this issue Mar 13, 2025 · 0 comments

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@mankoff
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mankoff commented Mar 13, 2025

pism/doc/sphinx/manual/std-greenland/run-1.rst

Line 49 in cd005ee

4 cores, this example will work with 1 to about 50 processors, with reasonably good

This example does not work with fewer processors than requested. I get the following. I've tagged this issue to one specific line stating it runs on fewer cores than requested, but I believe this statement is repeated elsewhere in the documentation.

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

pism

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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