Re: Parallelization

[Jean-Noël Grad]

Dear Ahmad,

Your espresso script will run in parallel if you call it with `mpiexec 
-n 4 ./pypresso script.py`. No other action is required. You can find 
the number of processors on your workstation with the command `nproc`. 
On clusters you will probably need to load mpiexec with `module load 
openmpi` or similar.

The ESPResSo results are automatically collected on MPI rank 0 for you, 
so matplotlib and other data processing tools will work as usual. The 
box will be divided automatically. If you have good reasons to override 
the default box partition, you can add `system.cell_system.node_grid = 
[i,j,k]` right after the line `system = System(box_l=...)`.

I wouldn't recommend running a Jupyter notebook in parallel if you have 
no prior experience with MPI parallelization. It's quite difficult to do 
and there isn't much information available online to help with that 
setup. You will find it much easier to export your notebook as a Python 
script and run it directly with mpiexec or mpirun. Just make sure to 
write the results of the calculation to a file, so that you can read 
them later and plot them in a Python script or Jupyter notebook.

I'll look into writing something in the user guide about MPI 
parallelization in the next few weeks. Most of the information is 
already in my previous mail. ESPResSo was designed to work with 
mpiexec/mpirun in a completely transparent manner.

Best regards,
JN

On 2021-12-30 09:06, Ahmad Reza Motezakker wrote:
> Dear Rudolf/Dear Jean-Noel,
>
> Thank you very much for your help. I just need parallelization to run
> a simulation.
>
> I really appreciate if you provide a general document on
> parallelization (also in Jupyter) with details. For example, how to
> divide the box, collecting results and etc.
>
> Thank you for your efforts,
>
> Ahmad Reza
> -------------------------
>
> From: Rudolf Weeber <weeber@icp.uni-stuttgart.de>
> Sent: Wednesday, December 29, 2021 7:23:20 PM
> To: Ahmad Reza Motezakker
> Cc: espressomd-users@nongnu.org
> Subject: Re: Parallelization
>
> Hi Ahmad,
> On Wed, Dec 29, 2021 at 05:08:18PM +0000, Ahmad Reza Motezakker wrote:
> > Is there any tutorial or any document on EspressoMD parallelization?
> I really appreciate your help. Any suggestion or experience will be
> really helpful for me.
> Do you just want to run a simulation in parallel, or do you want to
> extend Espresso andneed to write parallel code for that.
> If it is about running, you use
> mpirun -np <n> ./pypresso <script.py> <arguments>
> where <n> is the number of processor cores you want to run on.
> More is not necessarily better, as parallelizaiton requires
> communicaiton, whicht akes time of its own.
> For pure Lennard-Jones-systems parallelizaiton tends not to be worth
> it belwo 10k particles. If you use electrostatics or other
> computaiton-intensive algorithms, the number is lower.
> In practice, just time some 100 integration steps using the Python
> `time` or `timeit`modules and pick the number of cpu cores which
> serves you best.
>
> Hope that helps!
> Regareds, Rudolf