[petsc-users] About Preconditioner and MUMPS

[김성익]

I think I don't understand the meaning of
-pc_type mpi
-mpi_pc_type lu

What's the exact meaning of -pc_type mpi and -mpi_pc_type lu??
Is this difference coming from 'mpi_linear_solver_server' option??

Thanks,
Hyung Kim

2022년 12월 7일 (수) 오후 8:05, Matthew Knepley <knepley at gmail.com>님이 작성:

> On Wed, Dec 7, 2022 at 5:13 AM 김성익 <ksi2443 at gmail.com> wrote:
>
>> 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?
>>
>
> You are probably not using MUMPS. Always always always use -ksp_view to
> see exactly what solver you are using.
>
>
>> 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?
>>
>
> Again, it seems like the solver configuration is not what you think it is.
>
>   Thanks,
>
>      Matt
>
>
>> 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
>>>
>>>
>>>
>
> --
> 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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