Some basic questions about PETSC

[Ben Tay]

"Its normally better to change the values in place"

So you mean I should use VecSetValue/s instead?

Thank you.

Regards.

On 12/4/06, Matthew Knepley <knepley at gmail.com> wrote:
>
> On 12/4/06, Ben Tay <zonexo at gmail.com> wrote:
> >
> > Hi,
> >
> > regarding qn 4 ie  the requirement of
> > VecAssemblyBegin,VecAssemblyEnd, I used VecPlaceArray,
> > VecResetArray to change the values. I did not use Assembly after the 1st
> > time step and it works ok. So that is fine too, right?
>
> Its alright, but probably not optimal. It could result in slower code
> due to all the
> allocation. Its normally better to change the values in place.
>
>   Matt
>
> > Thank you!
> >
> > On 12/4/06, Satish Balay <balay at mcs.anl.gov> wrote:
> > > On Sun, 3 Dec 2006, Matthew Knepley wrote:
> > >
> > > > > 1. Are petscscaler the same as real nos.? I am writing in double
> > precision
> > > > > or real(8). So are these 2 interchangeable? (same as petscint and
> > integer).
> > > >
> > > > Yes.
> > >
> > > You can check these defines in include/finclude/petscdef.h
> > >
> > > PetscInt -> integer*4
> > > PetscScalar -> real*8
> > >
> > > >
> > > > > 2. while linking in compaq visual fortran, I get the warning
> message
> > about
> > > > > conflict library with LIBCMT. Is it important or can I just ignore
> it?
> > > >
> > > > This is determined by your configure. Satish will know better how to
> > control
> > > > it.
> > >
> > > There are some warnings because of the way compqaq f90 tries to
> > > workarround some bugs in Visual Studio 6 version of the libraries. It
> > > will be fine as long as all code [PETSc & application] is compiled
> > > with the same set of compiler options,
> > >
> > > >
> > > > > 3. I am writing in f90 free form format. When I modify from fixed
> f77
> > format
> > > > > to f90, it gives me a lot of error, which seems to be the include
> file
> > > > > error. So, is it possible to write in f90 format?
> > > >
> > > > Depending on which compiler you use, you might have to give a flag.
> For
> > g95
> > > > it is -ffree-form.
> > >
> > > We'll have to see the error messgaes to determine what the problem is
> > > - however - if you check the PETSc fortran examples - they comply with
> > > both fixed and free from syntax.
> > >
> > > > > 4. I am writing a Navier stokes solver and I'm using PETSC to
> solve
> > the
> > > > > poisson eqn. If only the RHS changes with each time step, do I
> need to
> > call
> > > > >
> > > > > VecCreateSeq
> > > > > <insert values in RHS b>
> > > > > VecAssemblyBegin
> > > > > VecAssemblyEnd
> > > > > VecDestroy
> > > > >
> > > > > at each time step? I don't think I can (or I should) change values
> of
> > b
> > > > > after VecAssemblyBegin,VecAssemblyEnd, or can I?
> > > >
> > > > You can just change the values. Call AssemblyBegin/End() after each
> > group
> > > > of changes.
> > >
> > > Another note: - If you change values with VecSetValues() - you'll need
> > > to call VecAssemblyBegin,VecAssemblyEnd. However if you
> > do
> > > VecGetArray() [or VecGetArrayF0()] - and change values - then you
> > > don't need the calls to VecAssembly...
> > >
> > > Satish
> > >
> > >
> >
> >
>
>
> --
> "Failure has a thousand explanations. Success doesn't need one" -- Sir
> Alec Guiness
>
>
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