[Users] MPI error in tutorial for new users

[Roland Haas]

Hello Praveer Krishna,

since mpirun was not found initially it would seem that you had not MPI
stack installed when compiling Cactus.

Cactus is best used with an MPI stack installed beforehand. The new
user tutorial

https://nbviewer.jupyter.org/github/nds-org/jupyter-et/blob/master/CactusTutorial.ipynb 

contains instructions on how to install a set up packages that simplify
building Cactus, including MPI. Did you follow those instructions or
started from a vanilla, freshly installed OS? Also, what OS are you
using (OSX, Linux, Linux-Subsystem-on-Windows)?

The easiest way to get a working copy of Cactus, if you did not install
an MPI stack before compiling Cactus, is:

* install an MPI stack (OpenMPI is fine)
* remove the current Cactus build using "rm -rf configs/sim"
* build once more

The solution above is the suggested solution.

If you would like to avoid recompiling (and risk triggering other
issues instead), here's an alternative:  When it detected that no MPI
stack was installed then Cactus built its own copy of OpenMPI. It will
have copied "mpirun" to "exe/sim/mpirun" and you can use that by making
sure that the full path to the directory "exe/sim" appears in your PATH
before you start the simulation. Using an mpirun from a different MPI
stack than the one used to compile Cactus is what gives the error that
Erik descibes in the email you found.

Yours,
Roland

> Dear ET Users,
> 
> I'm a beginner attempting to follow the Jupyter tutorial for new users. I
> have been able to  successfully run the helloworld.par and tov_ET.par files
> with procs set to 1 but I'm having issues when I try to use multiple
> processors on my laptop.
> I initially got this error <https://imgur.com/a/cuLLp79> saying "mpirun:
> not found", following which I installed Open MPI on my system. The error
> persisted, so I removed Open MPI and installed mpich instead. This
> seemingly fixed the issue, but I received another error
> <https://imgur.com/gdXSqCj> saying "CACTUS_NUM_PROCS is set to 4 but there
> are 1 MPI processes"
> 
> I found one
> <https://www.mail-archive.com/search?l=users@einsteintoolkit.org&q=subject:%22%5C%5BUsers%5C%5D+problem+on+running+bbh+expample%22&o=newest&f=1>
> or two
> <https://www.mail-archive.com/users@einsteintoolkit.org/msg01591.html>
> previous queries regarding this exact error, but the suggestions have not
> worked for me so far. Having never worked with MPI before, I am not sure
> what else I should try.
> 
> I'd be grateful for a few words of advice on this issue from anyone in the
> community.
> Thank you in advance for your time.
> 
> Regards,
> Praveer Krishna


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