Some basic questions about PETSC

[Matthew Knepley]

I would, but not knowing anything about your application, I can't tell whether
it will make a difference.

  Matt

On 12/4/06, Ben Tay <zonexo at gmail.com> wrote:
> "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
> >
> >
>
>


-- 
"Failure has a thousand explanations. Success doesn't need one" -- Sir
Alec Guiness