[petsc-users] About Preconditioner and MUMPS

[김성익]

I want to use METIS for ordering.
I heard the MUMPS has good performance with METIS ordering.

However there are some wonder things.
1. With option   "-mpi_linear_solver_server -ksp_type preonly -pc_type mpi
-mpi_pc_type lu " the MUMPS solving is slower than with option
"-mpi_linear_solver_server -pc_type mpi  -ksp_type preonly".
   Why does this result happen?


2. (MPIRUN case  (actually, mpi_linear_solver_server case)))  In my code,
there is already has "PetscCall(PCSetType(pc,PCLU))" . However, to use
METIS by using "-mpi_mat_mumps_icntl_7 5"  I must append this option
"-mpi_pc_type pu".
If I don't apply "-mpi_pc_type lu", the metis option
("-mpi_mat_mumps_icntl_7 5"). Can I get some information about this?



Thanks,
Hyung Kim

2022년 12월 7일 (수) 오전 12:24, Barry Smith <bsmith at petsc.dev>님이 작성:

>
>
> On Dec 6, 2022, at 5:15 AM, 김성익 <ksi2443 at gmail.com> wrote:
>
> Hello,
>
>
> I have some questions about pc and mumps_icntl.
>
> 1.     What’s the difference between adopt preconditioner by code (for
> example, PetscCall(PCSetType(pc,PCLU)) and option -pc_type lu??
> And also, What’s the priority between code pcsettype and option -pc_type ??
>
> 2.     When I tried to use METIS in MUMPS, I adopted metis by option (for
> example, -mat_mumps_icntl_7 5). In this situation, it is impossible to use
> metis without pc_type lu. However, in my case pc type lu makes the
> performance poor. So I don’t want to use lu preconditioner. How can I do
> this?
>
>    The package MUMPS has an option to use metis in its ordering process
> which can be turned on as indicated while using MUMPS.  Most
> preconditioners that PETSc can use do not use metis for any purpose hence
> there is no option to turn on its use.  For what purpose do you wish to use
> metis? Partitioning, ordering, ?
>
>
>
>
>
>
> Thanks,
>
> Hyung Kim
>
>
>
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