[petsc-users] Fortran interface -- low level access to petsc data

[Hossein Talebi]

1. You really need to profile.  Copying a vector is cheap.

>>>You are right. Maybe I make a big thing for this copy in/out and it is
not that problematic or slow even for the whole 2D matrix. I have to test
it with a large one to see.



> 2. If you make your functions (to evaluate residuals, etc) take their
>    inputs as explicit arguments, you can just pass the PETSc objects (or

   get the arrays from PETSc).
>

>>>I did not understand this completely.

>
> 3. Parallel matrix assembly is almost certainly not something for which
>    you want to hard-code the data structure.  PETSc provides generic
>    interfaces for _incrementally_ assembling matrices, so that there is
>    no extra storage and so the resulting data structure can be chosen at
>    run-time.  I challenge you to find evidence that you can do this
>    better.
>
> >>>Well, no wonder petsc is a very general and mature solution. I have
coded this already and it works fine with couple of other solvers. I cannot
say it is faster/better than petsc and I even don't think the comparison
would be correct. Though I would accept the challenge if I had time :-). I
have the single code/single language strategy so my code should be able to
compile and run without any external library. I attach many external
libraries for additional tasks. Anyhow, my primary aim is to use petsc for
its nice linear equation solvers.



> 4. If you really want to use _your_ storage for a Vec or Mat, you can
>    use VecCreateMPIWithArray, MatCreateMPIAIJWithSplitArrays, and the
>    like.
>
This looks to be a solution. Thank you.



> All of the above are available through _public_ interfaces that are
> already available to Fortran callers.  Please don't use F2003 to expose
> the _private_ struct definitions that we do not even offer to C callers.
>
> >>>I agree.


> > 3) easy definitions of petsc namespace through the Fortran modules, so I
> > can choose what to import (using the Fortran 'only' statement).
>
> PETSc uses prefixes for namespaces.  You can use "only" with the normal
> modules, but I can't tell if you are suggesting changing the names to
> avoid the prefix (which I don't think is a good idea and will lead to
> some collisions).
>
>>>Not to change the namespace but to limit it. When I put C-type include
statements it brings all mostly unnecessary namespace which might collide
with others, as you mentioned.
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