dolfin team mailing list archive

["Garth N. Wells" <gnw20@xxxxxxxxx>]

Kent-Andre Mardal wrote:
> On to., 2008-07-03 at 08:03 +0100, Garth N. Wells wrote:
> > DOLFIN wrote:
> > > One or more new changesets pushed to the primary dolfin repository.
> > > A short summary of the last three changesets is included below.
> > >
> > > changeset:   4374:6e1db926186ebfe9d9316b729a7d5747be3c5115
> > > tag:         tip
> > > parent:      4373:8f4accd9eb039d0d13412e8d871e7f1bfce896b8
> > > parent:      4372:3abbccc877d3bb358c76ef36dbbe252155b88183
> > > user:        Kent-Andre Mardal <kent-and@xxxxxxxxx>
> > > date:        Wed Jul 02 16:02:17 2008 +0200
> > > files:       dolfin/fem/Assembler.cpp dolfin/fem/DirichletBC.cpp site-packages/dolfin/assemble.py
> > > description:
> > > merge
> > >
> > >
> > > changeset:   4373:8f4accd9eb039d0d13412e8d871e7f1bfce896b8
> > > parent:      4369:b272b94b43ae789e2c72c90f6a027ce9fb89f6fc
> > > user:        Kent-Andre Mardal <kent-and@xxxxxxxxx>
> > > date:        Wed Jul 02 16:01:09 2008 +0200
> > > files:       demo/pde/sym-dirichlet-bc/demo.py dolfin/fem/Assembler.cpp dolfin/fem/Assembler.h dolfin/fem/DirichletBC.cpp dolfin/fem/DirichletBC.h dolfin/fem/assemble.cpp dolfin/fem/assemble.h dolfin/la/AssemblyMatrix.cpp dolfin/la/AssemblyMatrix.h dolfin/la/EpetraMatrix.cpp dolfin/la/EpetraMatrix.h dolfin/la/EpetraVector.cpp dolfin/la/EpetraVector.h dolfin/la/GenericMatrix.h dolfin/la/GenericTensor.h dolfin/la/GenericVector.h dolfin/la/Matrix.h dolfin/la/PETScMatrix.cpp dolfin/la/PETScMatrix.h dolfin/la/PETScVector.cpp dolfin/la/PETScVector.h dolfin/la/Scalar.h dolfin/la/Vector.h dolfin/la/uBlasMatrix.h dolfin/la/uBlasVector.cpp dolfin/la/uBlasVector.h dolfin/swig/dolfin_fem_pre.i dolfin/swig/dolfin_headers.i site-packages/dolfin/assemble.py
> > > description:
> > > implemented symmetric handling of boundary conditions
> > Great.
> >
> > I looked at the demo but could see how to specify the method for 
> > applying boundary conditions. Could you write a short description on how 
> > to use it and it compares in terms of performance?
> >
> > Garth
>
> Can you explain to me what the different methods do ? I assume you refer
> to the topological, geometric, pointwise switch in computeBC (which
> is used). 

I haven't looked at it in detail for some time, so I have to defer to 
Anders and Kristian for a good explanation.

> What is done is the following: 
> After the global matrix is computed, computeBC is used to compute the
> boundary conditions. 
> Then the BC is enforced as follows: 
>
> for all element: 
>    get element matrix and element vector 
>    enforce Dirichlet BC locally 
>    symmetrice the element matrix (and update the element rhs) by 
>      partial Gaussian elimination on the element linear system 
>    set the element matrix and element vector into the global matrix 

So if I pass the boundary conditions to the assembler it will do this?

Is this method of applying bcs taken into account when computing the 
sparsity pattern?

> Hence the assembly loop is as follows: 
>
>
> for all elements: 
>    compute cell contributions
>
> for all elements: 
>    compute interior facet contributions
>
> for all elements: 
>    compute exterior facet  contributions
>
> for all elements: 
>    enforce symmetric bc 

Isn't this different from what's described above? I thought that the 
Dirichlet bcs were being imposed within after the computation of each 
cell contribution?

> which can be more efficiently done as: 
>
> for all elements: 
>    compute cell contributions
>    compute interior facet contributions
>    compute exterior facet  contributions
>    enforce symmetric bc 
>
>
> Concerning efficiency the PETSc implemententation may be slow since
>
>    MatAssemblyBegin(A, MAT_FINAL_ASSEMBLY);
>    MatAssemblyEnd(A, MAT_FINAL_ASSEMBLY);
>
> must be called between each get and set call. 

Why is a get call needed? Don't you just assemble the reduced system for 
each cell?

Garth

> Kent