[petsc-users] Question about using MatCreateAIJ

[Karsten Lettmann]

Dear all,


I'm quite new to PETSC. So I hope the following questions are not too 
stupid.


1) We have a (Fortran) code, that we want to update from an older PETSC 
version (petsc.2.3.3-p16) to a newer version.

Inside the old code, for creating matrices A, there are function calls 
of the from:
MatCreateMPIAIJ

In the reference page for this old version it says:
When calling this routine with a single process communicator, a matrix 
of type SEQAIJ is returned.

So I assume the following behavior of this old routine:
- for N_proc == 1:
    a matrix of type SEQAIJ is returned.

- for N_proc > 1:
    a matrix of type MPIAIJ is returned



2a) So, in the new code, we want to have a similar behavior.

I found that this function is not present any more in the newer PETSC 
versions.

Instead, one might use: MatCreateAIJ(….)
( https://petsc.org/release/manualpages/Mat/MatCreateAIJ/ )

If I understand the reference page of the function correctly, then, 
actually, a similar behavior should be expected:

- for N_proc == 1:
    a matrix of type SEQAIJ is returned.

- for N_proc > 1:
    a matrix of type MPIAIJ is returned


2b) However, on the reference page, there is the note:

It is recommended that one use the MatCreate(), MatSetType() and/or 
MatSetFromOptions(), MatXXXXSetPreallocation() paradigm instead of this 
routine directly.

So, if I want the behavior above, it is recommended to code it like 
this, isn't it:

If (N_Proc == 1)

     MatCreate(.. ,A ,...)
     MatSetType(…,A, MATSEQAIJ,..)
     MatSetSizes(…,A, ..)
     MatSeqAIJSetPreallocation(,...A,...)

else

     MatCreate(.. ,A ,...)
     MatSetType(…,A, MATMPIAIJ,..)
     MatSetSizes(…,A, ..)
     MatMPIAIJSetPreallocation(,...A,...)

end



3) So my questions are:

- Is my present understanding correct?

If  yes:

- Why might using MatCreateAIJ(….) for my case not be helpful?

- So, why is it recommended to use the way 2b) instead of this 
MatCreateAIJ(….) ?


Best, Karsten




-- 
ICBM
Section: Physical Oceanography
Universitaet Oldenburg
Postfach 5634
D-26046 Oldenburg
Germany

Tel:    +49 (0)441 798 4061
email:  karsten.lettmann at uni-oldenburg.de