[petsc-users] Fwd: Accessing Global Vector Data with

[Daniel Mckinnell]

-----Original Message-----
From: Daniel Mckinnell <d_mckinnell at aol.co.uk>
To: knepley <knepley at gmail.com>
Sent: Thu, 15 Aug 2019 17:05
Subject: Re: [petsc-users] Accessing Global Vector Data with

 Thank you, it seemed to work nicely for the fields associated with cells but I'm having problems with the field on vertices. The code for altering the field is as follows:
for (int i = vStart; i < vEnd; i++)
    {
        PetscScalar *values;
        PetscInt vsize, voff;
        PetscSectionGetDof(globalsection, i, &vsize);
        PetscSectionGetOffset(globalsection, i, &voff);
        VecGetArray(u, &values);
        if (voff >= 0)
        {
            PetscScalar *p;
            DMPlexPointLocalRef(*dm, i, values, &p);
            p[0] = i + 1;
        }
        VecRestoreArray(u, &values);
The Global PetscSection is:
PetscSection Object: 2 MPI processes
  type not yet set
Process 0:
  (   0) dim  2 offset   0
  (   1) dim  2 offset   2
  (   2) dim -3 offset -13
  (   3) dim -3 offset -15
  (   4) dim  1 offset   4
  (   5) dim  1 offset   5
  (   6) dim  1 offset   6
  (   7) dim -2 offset -17
  (   8) dim -2 offset -18
  (   9) dim -2 offset -19
  (  10) dim -2 offset -20
  (  11) dim -2 offset -21
  (  12) dim -2 offset -22
  (  13) dim  1 offset   7
  (  14) dim  1 offset   8
  (  15) dim  1 offset   9
  (  16) dim  1 offset  10
  (  17) dim  1 offset  11
  (  18) dim -2 offset -23
  (  19) dim -2 offset -24
  (  20) dim -2 offset -25
  (  21) dim -2 offset -26
  (  22) dim -2 offset -27
  (  23) dim -2 offset -28
  (  24) dim -2 offset -29
Process 1:
  (   0) dim  2 offset  12
  (   1) dim  2 offset  14
  (   2) dim -3 offset  -1
  (   3) dim -3 offset  -3
  (   4) dim  1 offset  16
  (   5) dim  1 offset  17
  (   6) dim  1 offset  18
  (   7) dim  1 offset  19
  (   8) dim  1 offset  20
  (   9) dim  1 offset  21
  (  10) dim -2 offset  -5
  (  11) dim -2 offset  -6
  (  12) dim -2 offset  -7
  (  13) dim  1 offset  22
  (  14) dim  1 offset  23
  (  15) dim  1 offset  24
  (  16) dim  1 offset  25
  (  17) dim  1 offset  26
  (  18) dim  1 offset  27
  (  19) dim  1 offset  28
  (  20) dim -2 offset  -8
  (  21) dim -2 offset  -9
  (  22) dim -2 offset -10
  (  23) dim -2 offset -11
  (  24) dim -2 offset -12
where points 4-6 on proc 0 and points 4-9 on proc 1 are vertices.
The altered vec is:
Vec Object: 2 MPI processes
  type: mpi
Process [0]
0.
0.
0.
0.
0.
0.
0.
0.
5.
6.
7.
0.
Process [1]
0.
0.
0.
0.
0.
0.
0.
0.
5.
6.
7.
8.
9.
10.
0.
0.
0.

However the altered values do not correspond with the offsets in the PetscSection and I'm not sure what is going wrong.Thanks for all of the help.Daniel
 
 
-----Original Message-----
From: Matthew Knepley via petsc-users <petsc-users at mcs.anl.gov>
To: Daniel Mckinnell <d_mckinnell at aol.co.uk>
CC: PETSc <petsc-users at mcs.anl.gov>
Sent: Thu, 15 Aug 2019 16:11
Subject: Re: [petsc-users] Accessing Global Vector Data with

On Thu, Aug 15, 2019 at 10:59 AM Daniel Mckinnell via petsc-users <petsc-users at mcs.anl.gov> wrote:

Hi,Attached is a way I came up with to access data in a global vector, is this the best way to do this or are there other ways? It would seem intuitive to use the global PetscSection and VecGetValuesSection but this doesn't seem to work on global vectors.
Instead I have used VecGetValues and VecSetValues, however I also have a problem with these when extracting more than one value, I initialise the output of VecGetValues as PetscScalar *values; and then call VecGetValues(Other stuff... , values). This seems to work some times and not others and I can't find any rhyme or reason to it?

I guess I should write something about this. I like to think of it as a sort of decision tree.
1) Get just local values, meaning those owned by this process
    These can be obtained from either a local or global vector.
2) Get ghosted values, meaning those values lying on unowned points that exist in the default PetscSF
    These can only get obtained from a local vector
3) Get arbitrary values
    You must use a VecScatter or custom PetscSF to get these
For 1) and 2), I think the best way to get values normally is to use
  https://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/DMPLEX/DMPlexPointLocalRef.html  https://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/DMPLEX/DMPlexPointGlobalRef.html
These also have Read versions, and Field version to split off a particular field in the Section.
  Does this help?
    Thanks,
      Matt 
Finally I was wondering if there is a good reference code base on Github including DMPlex that would be helpful in viewing applications of the DMPlex functions?Thanks,Daniel Mckinnell



-- 
What most experimenters take for granted before they begin their experiments is infinitely more interesting than any results to which their experiments lead.
-- Norbert Wiener
https://www.cse.buffalo.edu/~knepley/
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