[petsc-users] How to speed up geometric multigrid

[Mark F. Adams]

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?

> 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>

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