[petsc-users] How to create a mapping of global to local indices?

[Dongyu Liu - CITG]

Thank you, Mark. My question is if I have some node indices like [0, 1, 3, 8, 9] handled by the current processor, is that necessary to renumber these indices to something like 0 to 4? or is there any alternative?


Dongyu

________________________________
From: Mark Adams <mfadams at lbl.gov>
Sent: Wednesday, July 3, 2019 3:54:49 PM
To: Dongyu Liu - CITG
Cc: petsc-users at mcs.anl.gov
Subject: Re: [petsc-users] How to create a mapping of global to local indices?

PETSc matrices and vectors are created with a local size n or global size N and PETSC_DECIDE instead of n. The global PETSc indices are ordered from 0 to n_0 - 1 where n_0 is the number of equations on process 0. This numbering continues for all processes.

You can use:

PetscErrorCode<https://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/Sys/PetscErrorCode.html#PetscErrorCode> MatGetOwnershipRange<https://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/Mat/MatGetOwnershipRange.html#MatGetOwnershipRange>(Mat<https://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/Mat/Mat.html#Mat> mat,PetscInt<https://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/Sys/PetscInt.html#PetscInt> *m,PetscInt<https://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/Sys/PetscInt.html#PetscInt> *n)

To see what your local global indices are (rows m to n-1 are on this process and so its the local indices are 0 to (n-m-1)).

Mark


On Wed, Jul 3, 2019 at 9:11 AM Dongyu Liu - CITG via petsc-users <petsc-users at mcs.anl.gov<mailto:petsc-users at mcs.anl.gov>> wrote:

Hi,


I am running a FEM program using PETSc. In the beginning,  the mesh is partitioned in gmsh, and we read the partitioned mesh using our own reader.


Now my question is: How can I get a global to local indices mapping? and do I need to renumber the indices after I read the partitioned mesh?


>From my understanding, the rows handled by each processor should be consecutive, but the indices of nodes which I directly read from a gmsh file are not. Is this an issue?


Best,


Dongyu

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