[petsc-users] Guidance on setting up BDDC

[Manav Bhatia]

Hi, 

   I am currently accessing PETSc through libMesh where the matrices of type MPIAIJ are being initialized using the MatSetSizes, MatSetFromOptions, and MatMPIAISetPreallocation calls. 

   I am interested in accessing the BDDC solver and have looked through the PDF manual that talks about BDDC setup through MATIS. However, it is not clear what would be the most efficient route to accessing BDDC for my unstructured FE code. 

   Example 72 (https://www.mcs.anl.gov/petsc/petsc-3.10/src/ksp/ksp/examples/tutorials/ex72.c.html) seems to be calling MatConvert to make the conversion to MATIS. However, the documentation of MATIS says that MatSetLocalToGlobalMapping <https://www.mcs.anl.gov/petsc/petsc-3.10/docs/manualpages/Mat/MatSetLocalToGlobalMapping.html#MatSetLocalToGlobalMapping> should be called before initializing MATIS, which is not done in Example 72. I am also not sure if I need to setup any DM/DA constructs to aid in this process. 

   I would appreciate if someone could provide an outline of steps required to make the conversion of MatMPIAIJ in order to use BDDC/FETI preconditioners. 

Thanks,
Manav

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