[petsc-users] FEM Matrix Assembly of Overlapped Mesh

[袁煕]

Hi,

I used PETSc to assemble a FEM stiff matrix of an overlapped (overlap=2)
DMPlex and used the MUMPS solver to solve it. But I got different solution
by using 1 CPU and MPI parallel computation. I am wondering if I missed
some necessary step or setting during my implementation.

    My calling process like follows

1. Generate matrix pkij from DMPlex dm_mesh

```
      PetscCallA( DMCreateMatrix(dm_mesh, pkij, ierr) )
      PetscCallA( MatSetBlockSize(pkij, 1, ierr))
      if( overlap>0 ) call
MatSetOption(pkij,MAT_IGNORE_OFF_PROC_ENTRIES,PETSC_TRUE,ierr)
      PetscCallA( MatSetFromOptions(pkij, ierr))

      PetscCallA(MatGetLocalSize(pkij, m, n, ierr));
      PetscCallA(MatGetSize(pkij, gm, gn, ierr));
      PetscCallA(MatGetBlockSize(pkij, bs, ierr));

      !
      ! Create a preallocator matrix with sizes (local and global)
matching the jacobian A.
      !
      PetscCallA(MatCreate(PETSC_COMM_WORLD, preallocator, ierr));
      PetscCallA(MatSetType(preallocator, MATPREALLOCATOR, ierr));
      PetscCallA(MatSetSizes(preallocator, m, n, gm, gn, ierr));
      PetscCallA(MatSetBlockSize(preallocator, bs, ierr));
      PetscCallA(MatSetUp(preallocator, ierr));
```

2. Do Matrix preallocation

```
      PetscCallA(MatCreate(PETSC_COMM_WORLD, preallocator, ierr));
      PetscCallA(MatSetType(preallocator, MATPREALLOCATOR, ierr));
      PetscCallA(MatSetSizes(preallocator, m, n, gm, gn, ierr));
      PetscCallA(MatSetBlockSize(preallocator, bs, ierr));
      PetscCallA(MatSetUp(preallocator, ierr));

      .......

      PetscCallA( MatPreallocatorPreallocate(preallocator, PETSC_TRUE,
pkij, ierr) )
      PetscCallA( MatDestroy(preallocator,ierr) )
```

3. Assemble matrix pkij by calling MatSetValue of all overlapped elements.

In my above implementation, I used
MatSetOption(pkij,MAT_IGNORE_OFF_PROC_ENTRIES,PETSC_TRUE,ierr). Is
that correct? Or even other options are needed?

Thanks for
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