Some basic questions about PETSC

Matthew Knepley knepley at
Sun Dec 3 21:49:07 CST 2006

On 12/3/06, Ben Tay <zonexo at> wrote:
> Hi,
> I'm still relatively new to PETSC but I've finally managed to solve my own
> poisson eqn using it in about a week's time.Thank you for writing such a
> good solver.
> However, I have some questions still about PETSC:
> 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).


> 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.

> 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.

> 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.


> Thank you very much and have a nice day
> Regards,
> Ben

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

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