[petsc-users] How to speed up geometric multigrid

[Michele Rosso]

Barry,

thank you!
I absolutely do not need  such accuracy!!! In my code I set an accuracy 
of 10^(-5) for the rtol instead of atol.
Thanks for pointing that out. I only need to solve up to discretization 
error.
Anyway I will try all the variants (this time by setting the proper 
tolerance!) you suggested and let you know.
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 <mrosso at uci.edu> 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 <mrosso at uci.edu> 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>
>