[petsc-users] About Preconditioner and MUMPS

[Matthew Knepley]

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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