[petsc-users] petsc4py help with parallel execution

[Matthew Knepley]

On Thu, Nov 15, 2018 at 8:07 AM Ivan Voznyuk <ivan.voznyuk.work at gmail.com>
wrote:

> Hi Matthew,
> Thanks for your reply!
>
> Let me precise what I mean by defining few questions:
>
> 1. In order to obtain a parallel execution of simple_code.py, do I need to
> go with mpiexec python3 simple_code.py, or I can just launch python3
> simple_code.py?
>

mpiexec -n 2 python3 simple_code.py


> 2. This simple_code.py consists of 2 parts: a) preparation of matrix b)
> solving the system of linear equations with PETSc. If I launch mpirun (or
> mpiexec) -np 8 python3 simple_code.py, I suppose that I will basically
> obtain 8 matrices and 8 systems to solve. However, I need to prepare only
> one matrix, but launch this code in parallel on 8 processors.
>

When you create the Mat object, you give it a communicator (here
PETSC_COMM_WORLD). That allows us to distribute the data. This is all
covered extensively in the manual and the online tutorials, as well as the
example code.


> In fact, here attached you will find a similar code (scipy_code.py) with
> only one difference: the system of linear equations is solved with scipy.
> So when I solve it, I can clearly see that the solution is obtained in a
> parallel way. However, I do not use the command mpirun (or mpiexec). I just
> go with python3 scipy_code.py.
>

Why do you think its running in parallel?

  Thanks,

     Matt


> In this case, the first part (creation of the sparse matrix) is not
> parallel, whereas the solution of system is found in a parallel way.
> So my question is, Do you think that it s possible to have the same
> behavior with PETSC? And what do I need for this?
>
> I am asking this because for my colleague it worked! It means that he
> launches the simple_code.py on his computer using the command python3
> simple_code.py (and not mpi-smth python3 simple_code.py) and he obtains a
> parallel execution of the same code.
>
> Thanks for your help!
> Ivan
>
>
> On Thu, Nov 15, 2018 at 11:54 AM Matthew Knepley <knepley at gmail.com>
> wrote:
>
>> On Thu, Nov 15, 2018 at 4:53 AM Ivan Voznyuk via petsc-users <
>> petsc-users at mcs.anl.gov> wrote:
>>
>>> Dear PETSC community,
>>>
>>> I have a question regarding the parallel execution of petsc4py.
>>>
>>> I have a simple code (here attached simple_code.py) which solves a
>>> system of linear equations Ax=b using petsc4py. To execute it, I use the
>>> command python3 simple_code.py which yields a sequential performance. With
>>> a colleague of my, we launched this code on his computer, and this time the
>>> execution was in parallel. Although, he used the same command python3
>>> simple_code.py (without mpirun, neither mpiexec).
>>>
>>> I am not sure what you mean. To run MPI programs in parallel, you need a
>> launcher like mpiexec or mpirun. There are Python programs (like nemesis)
>> that use the launcher API directly (called PMI), but that is not part of
>> petsc4py.
>>
>>   Thanks,
>>
>>      Matt
>>
>>> My configuration: Ubuntu x86_64 Ubuntu 16.04, Intel Core i7, PETSc
>>> 3.10.2, PETSC_ARCH=arch-linux2-c-debug, petsc4py 3.10.0 in virtualenv
>>>
>>> In order to parallelize it, I have already tried:
>>> - use 2 different PCs
>>> - use Ubuntu 16.04, 18.04
>>> - use different architectures (arch-linux2-c-debug, linux-gnu-c-debug,
>>> etc)
>>> - ofc use different configurations (my present config can be found in
>>> make.log that I attached here)
>>> - mpi from mpich, openmpi
>>>
>>> Nothing worked.
>>>
>>> Do you have any ideas?
>>>
>>> Thanks and have a good day,
>>> Ivan
>>>
>>> --
>>> Ivan VOZNYUK
>>> PhD in Computational Electromagnetics
>>>
>>
>>
>> --
>> What most experimenters take for granted before they begin their
>> experiments is infinitely more interesting than any results to which their
>> experiments lead.
>> -- Norbert Wiener
>>
>> https://www.cse.buffalo.edu/~knepley/
>> <http://www.cse.buffalo.edu/~knepley/>
>>
>
>
> --
> Ivan VOZNYUK
> PhD in Computational Electromagnetics
> +33 (0)6.95.87.04.55
> My webpage <https://ivanvoznyukwork.wixsite.com/webpage>
> My LinkedIn <http://linkedin.com/in/ivan-voznyuk-b869b8106>
>


-- 
What most experimenters take for granted before they begin their
experiments is infinitely more interesting than any results to which their
experiments lead.
-- Norbert Wiener

https://www.cse.buffalo.edu/~knepley/ <http://www.cse.buffalo.edu/~knepley/>
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