[petsc-users] About Preconditioner and MUMPS

[Matthew Knepley]

On Wed, Dec 7, 2022 at 6:15 AM 김성익 <ksi2443 at gmail.com> wrote:

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

This option says to use the PCMPI preconditioner. This allows you to
parallelize the
solver in what is otherwise a serial code.


> -mpi_pc_type lu
>

This tells the underlying solver in PCMPI to use the LU preconditioner.


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

Please use -ksp_view as I asked to look at the entire solver. Send it
anytime you mail about solver questions.

  Thanks

     Matt


> 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/>
>>
>

-- 
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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