On Oct 1, 2013, at 7:02 PM, Michele Rosso <[email protected]> wrote:
> Barry,
>
> I repeated the previous runs since I noticed that it was not using the options
>
> -mg_levels_ksp_max_it 3
>
> They are faster then before now but still slower than my initial test and
> anyway the solution time increases considerably in time.
> I attached the diagnostics (run1.txt and run2.txt, please see the files for
> the list of the options I used).
> I also run a case by using your last proposed options (run3.txt): there is a
> divergence condition since 30 iterations seem not be enough to low the error
> below the need tolerance and thus after some time steps my solution blows up.
run3.txt uses:
-ksp_type richardson
This is bad and I doubt anyone recommended it intentionally. You also have, in
this file,
-mg_levels_ksp_type gmres
did you or the recommenders mean
-mg_levels_ksp_type richardson ???
you are using gmres here, which forces you to use fgmres in the outer solver.
This is a safe thing to use you if you apply your BCa symmetrically with a low
order discretization then
-ksp_type cg
-mg_levels_ksp_type richardson
-mg_levels_pc_type sor
is what I'd recommend.
Your run2.txt looks like the best. If I understand you you have better results
with some other parameters ("initial test").
From what I can see you are solving a Laplacia with constant coefficients?
I would also add Barry's question to what we want to know.
Mark
> Please let me know what you think about it.
> In the mean time I will try run my initial test with the option
> -mg_levels_ksp_max_it 3 instead of -mg_levels_ksp_max_it 1.
> As usual, thank you very much.
>
> Michele
>
> On 09/30/2013 07:17 PM, Barry Smith wrote:
>> I wasn't expecting this. Try
>>
>> -pc_mg_type full -ksp_type richardson -mg_levels_pc_type bjacobi
>> -mg_levels_ksp_type gmres -mg_levels_ksp_max_it 3
>> -mg_coarse_pc_factor_mat_solver_package superlu_dist -mg_coarse_pc_type lu
>> -pc_mg_galerkin -pc_mg_levels 5 -pc_type mg
>> -log_summary -pc_mg_log -ksp_monitor_true_residual -options_left
>> -ksp_view -ksp_max_it 30
>>
>>
>>
>>
>> On Sep 30, 2013, at 8:00 PM, Michele Rosso <[email protected]> wrote:
>>
>>> Barry,
>>>
>>> sorry again for the very late answer. I tried all the variations you
>>> proposed: all of them converge very slow , except the last one (CG instead
>>> of fgmres) diverges.
>>> I attached the diagnostics for the two options that convergence: each of
>>> the attached files starts with a list of the options I used for the run.
>>> As you pointed out earlier, the residual I used to require was too small,
>>> therefore I increased atol to e-9.
>>> After some tests, I noticed that any further increase of the absolute
>>> tolerance changes significantly the solution.
>>> What would you suggest to try next?
>>> Thank you very much,
>>>
>>> Michele
>>>
>>>
>>>
>>>
>>>
>>>
>>> On 09/24/2013 05:08 PM, Barry Smith wrote:
>>>> Thanks. The balance of work on the different levels and across processes
>>>> looks ok. So it is is a matter of improving the convergence rate.
>>>>
>>>> The initial residual norm is very small. Are you sure you need to
>>>> decrease it to 10^-12 ????
>>>>
>>>> Start with a really robust multigrid smoother use
>>>>
>>>> -pc_mg_type full -ksp_type fgmres -mg_levels_pc_type bjacobi
>>>> -mg_levels_ksp_type gmres -mg_levels_ksp_max_it 3 PLUS
>>>> -mg_coarse_pc_factor_mat_solver_package superlu_dist
>>>> -mg_coarse_pc_type lu -pc_mg_galerkin -pc_mg_levels 5 -pc_mg_log -pc_type
>>>> mg
>>>>
>>>> run with the -log_summary and -pc_mg_log
>>>>
>>>> Now back off a little on the smoother and use -mg_levels_pc_type sor
>>>> instead how does that change the convergence and time.
>>>>
>>>> Back off even more an replace the -ksp_type fgmres with -ksp_type cg and
>>>> the -mg_levels_ksp_type gmres with -mg_levels_ksp_type richardson how
>>>> does that change the convergence and the time?
>>>>
>>>> There are some additional variants we can try based on the results from
>>>> above.
>>>>
>>>> Barry
>>>>
>>>>
>>>>
>>>> On Sep 24, 2013, at 4:29 PM, Michele Rosso <[email protected]> wrote:
>>>>
>>>>> Barry,
>>>>>
>>>>> I re-rerun the test case with the option -pc_mg_log as you suggested.
>>>>> I attached the new output ("final_new.txt').
>>>>> Thanks for your help.
>>>>>
>>>>> Michele
>>>>>
>>>>> On 09/23/2013 09:35 AM, Barry Smith wrote:
>>>>>> Run with the additional option -pc_mg_log and send us the log file.
>>>>>>
>>>>>> Barry
>>>>>>
>>>>>> Maybe we should make this the default somehow.
>>>>>>
>>>>>>
>>>>>> On Sep 23, 2013, at 10:55 AM, Michele Rosso <[email protected]> wrote:
>>>>>>
>>>>>>> Hi,
>>>>>>>
>>>>>>> I am successfully using PETSc to solve a 3D Poisson's equation with CG
>>>>>>> + MG . Such equation arises from a projection algorithm for a
>>>>>>> multiphase incompressible flow simulation.
>>>>>>> I set up the solver as I was suggested to do in a previous thread
>>>>>>> (title: "GAMG speed") and run a test case (liquid droplet with surface
>>>>>>> tension falling under the effect of gravity in a quiescent fluid).
>>>>>>> The solution of the Poisson Equation via multigrid is correct but it
>>>>>>> becomes progressively slower and slower as the simulation progresses (I
>>>>>>> am performing successive solves) due to an increase in the number of
>>>>>>> iterations.
>>>>>>> Since the solution of the Poisson equation is mission-critical, I need
>>>>>>> to speed it up as much as I can.
>>>>>>> Could you please help me out with this?
>>>>>>>
>>>>>>> I run the test case with the following options:
>>>>>>>
>>>>>>> -pc_type mg -pc_mg_galerkin -pc_mg_levels 5 -mg_levels_ksp_type
>>>>>>> richardson -mg_levels_ksp_max_it 1
>>>>>>> -mg_coarse_pc_type lu -mg_coarse_pc_factor_mat_solver_package
>>>>>>> superlu_dist
>>>>>>> -log_summary -ksp_view -ksp_monitor_true_residual -options_left
>>>>>>>
>>>>>>> Please find the diagnostic for the final solve in the attached file
>>>>>>> "final.txt'.
>>>>>>> Thank you,
>>>>>>>
>>>>>>> Michele
>>>>>>> <final.txt>
>>>>> <final_new.txt>
>>> <final1.txt><final2.txt>
>>
>
> <run1.txt><run2.txt><run3.txt>
