[petsc-users] How to speed up geometric multigrid

Matthew Knepley knepley at gmail.com
Mon Sep 23 11:36:39 CDT 2013

On Mon, Sep 23, 2013 at 9:34 AM, Michele Rosso <mrosso at uci.edu> wrote:

> On 09/23/2013 09:24 AM, Mark F. Adams wrote:
>  On Sep 23, 2013, at 11:55 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?
>  Just to add, it is not unusual as structures develop in your simulation
> to see an increase in iterations.  If you plot the material coefficients I
> suspect that you will see sharper structures developing in time.  Can you
> look at this in some way or generate some statistics like (max) gradient of
> material coefficients or even just max & min coefficients?
> Yes, I suspected that this may have been a reason. Initially both phases
> are at rest, so basically no fluid structures are present, thus the
> convergence is very fast(6 iterations maximum). Then vortices start
> developing both inside and outside the droplet and at that point the number
> of iteration increases. I noticed that the convergence rate becomes almost
> steady at a certain point, I guess when the flow is fully developed. I
> would like to speed up the solve for this last scenario.

Mark, is this where we do Bootstrap? :)



>   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>

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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