dolfin team mailing list archive

["Martin Sandve Alnæs" <martinal@xxxxxxxxx>]

2007/8/27, Anders Logg <logg@xxxxxxxxx>:
> jesperc wrote:
> > Hi,
> >
> > I'm taking a summer course in parallel programming and want to
> > parallelize something in Dolfin as a small project for the course. How
> > much is currently sucessfully parallelized in the code when for example
> > solving the Poisson equation? I know that some of you are working with
> > the parallel implementation of the assembly, but what is the status there?
> > Would it be possible for me to make some contribution?
>
> Yes, definitely. Here's what we have now:
>
> 1. Mesh partitioning works:
>
>      uint n = 16;
>      MeshFunction<uint> partitions;
>      mesh.partition(n, partitions);
>
> This returns a MeshFunction which labels all the cells of the mesh with
> the number of the partition to which they belong.

For proper scalability, distributed meshes is necessary, but I guess
you're on top of this.

> 2. Garth has implemented a working (last time I checked) prototype of a
> parallel assembler in src/sandbox/passembly/main.cpp.
>
> 3. It should be relatively easy to extend the current assembler in
> src/kernel/fem/Assembler.cpp to do parallel assembly. It currently knows
> how to assemble over subdomains (defined by some MeshFunctions) and the
> parallel assembly would be similar: skip the cells (if (.. != ... )
> continue;) that don't belong to the current processor.

It should rather iterate over cells that _are_ on the current
processor. Even if the mesh isn't distributed yet, this is probably
important.

> 4. The first thing we need is to be able to communicate Mesh and
> MeshFunction between processes. There is a sketch in
> src/kernel/mesh/MPIMeshCommunicator.cpp (empty). One process reads the
> mesh, partitions it and sends it to all processes. (To begin with, each
> process will have a copy of the mesh.)

Sounds good.

> 5. The tricky part will be how to order degrees of freedom (class
> DofMap). And this relates to ongoing work on merging the linear algebra
> interfaces of DOLFIN and PyCC (Simula in-house code). We need to find
> a design that works well for communicating both with PETSc and Epetra
> (Trilinos).

This problem can probably benefit from someone having the time to
really focus, there are a lot of details to consider...

-- 
Martin