[petsc-users] How rto use multigrid?

[Matthew Knepley]

On Tue, Oct 30, 2012 at 6:34 PM, TAY wee-beng <zonexo at gmail.com> wrote:

>  On 30/10/2012 1:44 PM, Matthew Knepley wrote:
>
> On Tue, Oct 30, 2012 at 7:41 AM, TAY wee-beng <zonexo at gmail.com> wrote:
>
>>  On 28/10/2012 2:17 PM, Jed Brown wrote:
>>
>> Algebraic multigrid can be used directly, -pc_type gamg
>> -pc_gamg_agg_nsmooths 1. Geometric either required that you use the PCMG
>> interface to set interpolation (and provide a coarse operator for
>> non-Galerkin) or use a DM that provides coarsening capability.
>>
>> What kind of problem are you solving?
>>
>>
>> Hi,
>>
>> May I know if there is an example which explains using the geometric MG,
>> either using PCMG or DM?
>>
>
>  SNES ex5 and ex19 both use MG.
>
>     Matt
>
> Are there examples which uses ksp and geometric MG? I want to solve a
> Poisson eqn which has been discretized to give a sys of linear eqns.
>

This is geometric MG. Did you run any of the tutorial runs? For example,
src/snes/examples/tutorials/makefile has

runex5:
-@${MPIEXEC} -n 1 ./ex5 -pc_type mg -ksp_monitor_short  -snes_view
-pc_mg_levels 3 -pc_mg_galerkin -da_grid_x 17 -da_grid_y 17
-mg_levels_ksp_monitor_short -mg_levels_ksp_norm_type unpreconditioned
-snes_monitor_short -mg_levels_ksp_chebyshev_estimate_eigenvalues 0.5,1.1
-mg_levels_pc_type sor -pc_mg_type full > ex5_1.tmp 2>&1; \
   if (${DIFF} output/ex5_1.out ex5_1.tmp) then true; \
   else  echo  ${PWD} "\nPossible problem with with ex5, diffs above
\n========================================="; fi; \
   ${RM} -f ex5_1.tmp

and many more.

   Matt


> I also found this thread :
>
> http://lists.mcs.anl.gov/pipermail/petsc-users/2012-August/015073.html
>
> Is it using geometric MG to solve a sys of linear eqns?
>
>
>
>>  Yours sincerely,
>>
>> TAY wee-beng
>>
>>
>>  On Oct 28, 2012 6:09 AM, "w_ang_temp" <w_ang_temp at 163.com> wrote:
>>
>>>  Hello,
>>>    I want to use the multigrid as a preconditioner. The introduction
>>> about it in the manual is little.
>>> So are there some typical examples or details about multigrid? Is it
>>> used just like other preconditioners
>>> like jacobi, sor, which can be simply used in the cammand line options?
>>>    Thanks.
>>>
>>> Jim
>>>
>>>
>>>
>>
>
>
>  --
> 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
>
>
>


-- 
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
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