[petsc4py] What should be the 'default' communicator?

[Matthew Knepley]

On Mon, Aug 25, 2008 at 1:22 PM, Lisandro Dalcin <dalcinl at gmail.com> wrote:
> On Mon, Aug 25, 2008 at 1:08 PM, Matthew Knepley <knepley at gmail.com> wrote:
>> I would still maintain that PETSC_COMM_WORLD is the correct default. There
>> are better paradigms for embarassingly parallel operation, like Condor. PETSc
>> is intended for parallel, domain decomposition runs.
>
> Yes, you are completelly right. But I believe that many people still
> use PETSc in a sequential way just because PETSc is full featured,
> well designed, easy to learn, etc. So, despite PETSc being intended
> for parallel, domain decomposition applications, many people are going
> to use it for sequential apps and embarassingly parallel operations.

I agree that people will do this, I just don't agree that it should be
the default.

> To be honest, I've never looked too much at paradigms like Condor. But
> using them implies to learn yet another framework. Another anecdote: a
> guy sent me a mail with questions about mpi4py for solving a
> embarassingly parallel problems. I asked why he was trying to use such
> a "heavy weight" approach. And then he answered he was tired of the
> complications and performance of using a Grid-based approach, and that
> 'mpiexec' a Python script with some coordinating MPI calls was far
> easier to setup, extend, and maintain and had better overall running
> times than submitting jobs to "The Grid".

That is my experience with grid software as well. However, in the
particular case
of Condor, I disagree. It is fairly easy to setup and has great
features like fault
tolerance, automatic migration and balancing, that make it much more useful
that just MPI.

   Matt

>> On Mon, Aug 25, 2008 at 10:54 AM, Lisandro Dalcin <dalcinl at gmail.com> wrote:
>>> After working hard on mpi4py, this week I'll spend my time cleaning-up
>>> and adding features to the new Cython-based petsc4py. Then, I'll be
>>> asking questions to this list requesting for advise.
>>>
>>> In all calls that create new PETSc objects, I've decided to make the
>>> 'comm' argument optional. If the communicator is not passed,
>>> PETSC_COMM_WORLD is currently used. This is the approach PETSc uses in
>>> some C++ calls implemented through PetscPolymorphicFunction().
>>>
>>> But now I believe that is wrong, and that PETSC_COMM_SELF should be
>>> the default. Or perhaps even better, I should let users set the
>>> default communicator used by petsc4py to create new (parallel)
>>> objects.
>>>
>>> An anecdote: some time ago, a petsc4py user wrote a sequential code
>>> and created objects without passing communicator arguments, next he
>>> wanted to solve many of those problems in different worker processes
>>> in a "ambarrasingly parallel" fashion and collect results at the
>>> master process. Of course, he run into trouble. Then I asked him to
>>> initialize PETSc in such a way that PETSC_COMM_WORLD was actually
>>> PETSC_COMM_SELF (by setting the world comm before PetscInitalize()).
>>> This mostly works, but has a problem: we have lost the actual
>>> PETSC_COMM_WORLD, so we are not able to create a parallel object after
>>> PetscInitialize().
>>>
>>> Any thoughts?
>>>
>>>
>>> --
>>> Lisandro Dalcín
>>> ---------------
>>> Centro Internacional de Métodos Computacionales en Ingeniería (CIMEC)
>>> Instituto de Desarrollo Tecnológico para la Industria Química (INTEC)
>>> Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET)
>>> PTLC - Güemes 3450, (3000) Santa Fe, Argentina
>>> Tel/Fax: +54-(0)342-451.1594
>>>
>>>
>>
>>
>>
>> --
>> 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
>>
>>
>
>
>
> --
> Lisandro Dalcín
> ---------------
> Centro Internacional de Métodos Computacionales en Ingeniería (CIMEC)
> Instituto de Desarrollo Tecnológico para la Industria Química (INTEC)
> Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET)
> PTLC - Güemes 3450, (3000) Santa Fe, Argentina
> Tel/Fax: +54-(0)342-451.1594
>
>



-- 
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