[petsc-users] Re-use the matrix in petsc

[Edoardo alinovi]

Dear petsc users,

I am new to petsc, but I am really enjoying it.

I am developing a CFD code in fortran and I have a (newby) question for you.

I have a subroutine that assembles the matrices arising from the finite
volume method. They are sparse and the coefficients change every time step,
but their sparsity remains unchanged (i.e. non-zeros entries are always in
the same position).

In the subroutine, which is called every time step, I am basically
following the manual and I do something like:

-  call MatCreate(PETSC_COMM_WORLD,A,ierr)
-   call MatSetSizes(A,lm,lm,M,M,ierr)
-   call MatSetType(A, myType,ierr)
-    call MatMPIAIJSetPreallocation(A,d_nz,PETSC_NULL_INTEGER, o_nz,
PETSC_NULL_INTEGER,ierr)
           call MatSetUp(A,ierr)

-   call MatSetValues
-   call MatAssemblyBegin(A,MAT_FINAL_ASSEMBLY,ierr)
-   call MatAssemblyEnd(A,MAT_FINAL_ASSEMBLY,ierr)

-   Solve the system

-  call MatDestroy


Here is my question:

As you can note, every time step, I allocate the matrix, fill it, use it
and the destroy it. Is there a way to avoid the matrix preallocation every
time? I just would like to change matrix the entries and avoid the
preallocation.

Thank you for the help that you can give me,

Edoardo
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