[petsc-users] Matrix assembly problem of overlapped DMPlex

[Matthew Knepley]

On Sun, Jul 23, 2023 at 11:54 AM 袁煕 <yuanxi at advancesoft.jp> wrote:

> 袁煕 <yuanxi at advancesoft.jp>
> 0:50 (0 分前)
> To PETSc
> 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 a
> 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));
> ```
>
> 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?
>
>
Do you want to ignore off-process entries? Usually in FEM we share part of
the boundary with other processes, and thus ignoring the off-process
entries will not give the correct result.

  Thanks,

     Matt


>
>
> Thanks for your help.
>
> ------
>
> X. Yuan, Ph.D. of solid mechanics
>
> Advancesoft, Japan
>
> ---------
>
>

-- 
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/ <http://www.cse.buffalo.edu/~knepley/>
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