> On Dec 18, 2018, at 9:36 AM, Marco Tiberga via petsc-users
> <[email protected]> wrote:
>
> Dear all,
>
> We are using PETSc in our (Fortran) CFD code, which we have recently
> parallelized.
>
> We need to access the values stored in the solution vector to assemble the
> matrix of the linear system. In particular, we need to access the values on
> the local elements, plus the ones on the first neighbors, which might be
> handled by other processes.
> Hence, we defined our solution vector as MPI Vec with ghost values.
>
> At the moment, to access all the required values, at each time step we update
> the ghost values, then call ‘VecGhostGetLocalForm’ and finally
> ‘VecGetArrayReadF90’.
> Since all these procedures are collective, we store the gotten values in a
> local Fortran array (of length n_local + n_ghosts) and then proceed with the
> parallel matrix assembly.
Why do you need to copy the values from the VecGetArrayReadF90() vector to a
local Fortran array? Why can't you just access them directly with the pointer
obtained from VecGetArrayReadF90() as needed.
> The code works perfectly, so I believe we are doing things in the right way.
>
> However, I was wondering whether there is a more efficient way to access the
> local values of a ghosted vector; something not collective, so that we could
> access the values on-demand while assembling the matrix.
> In this way, we could avoid storing the values in Fortran array, thus saving
> memory.
>
> Less important issue, but it puzzles me: Why is VecGetArrayReadF90 collective
> while its C counterpart VecGetArrayRead is not? (same thing for
> VecRestoreArrayReadF90 and VecRestoreArrayRead).
Hmm, the Read versions of these routines are not collective; can you point
to where in the documentation it says they are collective so we can fix the
documentation. Note: the VecGetArray() (non-read only versions) are collective.
Thanks
Barry
>
>
> Thanks a lot for your time and help.
>
>
> Best regards,
>
> Marco Tiberga
>
> PhD candidate
> Delft University of Technology
> Faculty of Applied Sciences
> Radiation Science & Technology Department
> Mekelweg 15, 2629 JB Delft, The Netherlands
> E-Mail: [email protected]
> Website: http://www.nera.rst.tudelft.nl/
