[petsc-users] accessing DMDA Vec ghost values

[Sean Dettrick]

From: <petsc-users-bounces at mcs.anl.gov<mailto:petsc-users-bounces at mcs.anl.gov>> on behalf of Dave May <dave.mayhem23 at gmail.com<mailto:dave.mayhem23 at gmail.com>>
Date: Thursday, May 12, 2016 at 2:48 AM
To: Sean Dettrick <sdettrick at trialphaenergy.com<mailto:sdettrick at trialphaenergy.com>>
Cc: "petsc-users at mcs.anl.gov<mailto:petsc-users at mcs.anl.gov>" <petsc-users at mcs.anl.gov<mailto:petsc-users at mcs.anl.gov>>
Subject: Re: [petsc-users] accessing DMDA Vec ghost values



On 12 May 2016 at 10:42, Sean Dettrick <sdettrick at trialphaenergy.com<mailto:sdettrick at trialphaenergy.com>> wrote:
Hi,

When discussing DMDAVecGetArrayDOF etc in section 2.4.4,  the PETSc 3.7 manual says "The array is accessed using the usual global indexing on the entire grid, but the user may only refer to the local and ghost entries of this array as all other entries are undefined”.

OK so far.  But how to access the ghost entries?

With a 2D DMDA, I can do this OK:


        PetscInt    xs,xm,ys,ym;

        ierr=DMDAGetCorners(da,&xs,&ys,0,&xm,&ym,0);CHKERRQ(ierr);

        PetscScalar ***es;

        ierr=DMDAVecGetArrayDOF(da,Es,&es);CHKERRQ(ierr);


        for (int j=ys; j < ys+ym; j++) {

            for (int i=xs; i < xs+xm;i++) {

es[j][i][0]=1.;

                es[j][i][1]=1.;

            }

        }

        ierr=DMDAVecRestoreArrayDOF(da,Es,&es);CHKERRQ(ierr);

But if I replace DMDAGetCorners with DMDAGetGhostCorners, then the code crashes with a seg fault, presumably due to out of bounds memory access.

Is that supposed to happen?

If you created the vector Es using the function DM{Get,Create}GlobalVector(), then the answer is yes.

What’s the remedy?

If you want to access the ghost entries, you need to create the vector using the function DM{Get,Create}LocalVector().



Thanks!  Somehow I missed DM{Get,Create}LocalVector().  BTW what is the difference between the Get and Create versions?  It is not obvious from the documentation.

Also, can you explain the difference between DMDAVecGetArrayDOF and DMDAVecGetArrayDOFRead?

Thanks again,
Sean



Thanks,
  Dave



Thanks very much!

Sean Dettrick


-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.mcs.anl.gov/pipermail/petsc-users/attachments/20160512/e065afa8/attachment.html>