[petsc-users] How to use multigrid?

[Jed Brown]

Just pass it as a command line option. It gives profiling output in
PetscFinalize().


On Sat, Nov 3, 2012 at 10:52 AM, w_ang_temp <w_ang_temp at 163.com> wrote:

> Is there something that need attention when setting up PETSc? The
> -log_summary
> is no use in my system.
>
>
> >At 2012-11-03 23:31:52,"Jed Brown" <jedbrown at mcs.anl.gov> wrote:
>
> >1. *Always* send -log_summary when asking about performance.
> >2. AMG setup costs more, the solve should be faster, especially for large
> problems.
> >3. 30k degrees of freedom is not large.
>
>
> >>On Sat, Nov 3, 2012 at 10:27 AM, w_ang_temp <w_ang_temp at 163.com> wrote:
>
>>  >>Hello,
>> >>    I have tried AMG, but there are some problems. I use the command:
>> >>    mpiexec -n 4 ./ex4f -ksp_type gmres -pc_type gamg
>> -pc_gamg_agg_nsmooths 1 -ksp_gmres_restart 170 -ksp_rtol 1.0e-15
>> ->>ksp_converged_reason.
>> >>    The matrix has a size of 30000. However, compared with -pc_type asm,
>> >>the amg need more time:asm needs 4.9s, amg needs 13.7s. I did several
>> tests
>> >>and got the same conclusion. When it begins, the screen shows the
>> information:
>> >>[0]PCSetData_AGG bs=1 MM=7601. I do not know the meaning. And if there
>> is some
>> >>parameters that affect the performance of AMG?
>> >>    Besides, I want to confirm a conception. In my view, AMG itself can
>> be a solver
>> >>like gmres. It can also be used as a preconditioner like jacobi and is
>> used by combining
>> >>with other solver. Is it right? If it is right, how use AMG solver?
>> >>    My codes are attached.
>> >>    Thanks.
>> >>                                             Jim
>>
>>
>>
>> >At 2012-11-01 22:00:28,"Jed Brown" <jedbrown at mcs.anl.gov> wrote:
>>
>> >Yes, it's faster to understand this error message than to have
>> "mysteriously slow performance".
>>
>> >** Preallocation routines now automatically set
>> MAT_NEW_NONZERO_ALLOCATION_ERR, if you intentionally preallocate less than
>> necessary then *>*use
>> MatSetOption(mat,MAT_NEW_NONZERO_ALLOCATION_ERR,PETSC_FALSE) to disable the
>> error generation.
>> *
>> >http://www.mcs.anl.gov/petsc/documentation/changes/33.html
>>
>> >On Thu, Nov 1, 2012 at 8:57 AM, w_ang_temp <w_ang_temp at 163.com> wrote:
>>
>>> >Do you mean that the two versions have a different in this point? If I
>>> use the new version, I have to
>>> >make some modifications on my codes?
>>>
>>
>>
>>
>>
>
>
>
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