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Didu December 2, 2019 04:35

Using several nodes in a simulation (OpenFOAM)
Dear all,
I'm quite new in using OpenFOAM in a cluster, using only one node is a breeze...
the problem starts when I need to use more than one node...
I'm using the command 'mpirun --host node1,node2 -np 48 rhoSimpleFoam -parallel' and I get the following info/error

There are not enough slots available in the system to satisfy the 48
slots that were requested by the application:


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

I'm doing something wrong, please help...
OpenFOAM - v1906

dewey February 19, 2020 13:52

Did you find what was going wrong?

I am trying to simulate in a cluster but I dont have idea how to start

HPE February 19, 2020 17:44

This had nothing to with OF.

I strongly suggest not to rush running big cases in a supercomputer to avoid wasting money (of taxpayers).

Learn 5w1h of the cluster, follow the tutorials, start with the simplest case, and build up from there.

Yann February 20, 2020 03:29

Hello Didu,

As HPE said, your problem is not related to OpenFOAM but to openmpi. It would be useful to know which openmpi version you are using.

I think you just need to specify how many processes you want to run on each node. Try using this syntax:

mpirun --host node1:24,node2:24 -np 48 rhoSimpleFoam -parallel


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