[petsc-users] which is the best PC for GMRES Poisson solver?

[Matthew Knepley]

On Sat, Aug 24, 2013 at 12:07 AM, Zhang <zyzhang at nuaa.edu.cn> wrote:

> Hi,
>
> Recently  I wrote a code of projection method for solving the
> incompressile flow. It includes a poisson solution of pressure.
> I transfered the code to be based on Petsc .
>

Use -pc_type gamg

  Matt


> However, in the case of 3D lid-driven flow case, the speed of petsc
> version is not advantageous yet.
>
> I tried different combination of preconditioner with  GMRES solver. Among
> them GMRES+PCILU or GMRES+SOR are both the fastest.
> For a grid 80x80x80, GMRES+SOR serial version used 185.816307 secs.
> However, for case 120x120x120, it diverged. So is GMRES+PCILU.
>
> Then  I tried a parallel comparison, as follows,
>
>
>
> #############################################################################################
> # with Petsc-3.4.2, time comparison (sec)
> # size (80,80,80), 200 steps, dt=0.002
> #debug version  177.695939 sec
> #opt   version  106.694733 sec
> #mpirun -np 8 ./nsproj  ../input/sample_80.zzy.dat -ksp_type gmres -p
> c_type hypre -ksp_rtol 1.e-2
> #debug version  514.718544 sec
> #opt   version  331.114555 sec
> #mpirun -np 12 ./nsproj  ../input/sample_80.zzy.dat -ksp_type gmres
> -pc_type hypre -ksp_rtol 1.e-2
> #debug version  796.765428 sec
> #opt   version  686.151788 sec
> #mpirun -np 16 ./nsproj  ../input/sample_80.zzy.dat -ksp_type gmres
> -pc_type hypre -ksp_rtol 1.e-2
>
> I do know sometimes problem with speed is not due to petsc, but my own
> code, so please you are welcome for any suggestion
> about which combination is the best for such a computation. I know solving
> Poiison and Helmholtz is so common to see in numerical work.
> Thank you first.
>
> BTW, I also tried to use superLU_dist as PC.
> #mpirun -np 16 ./nsproj  ../input/sample_20.zzy.dat -ksp_type gmres
> -pc_type lu -pc_factor_mat_solver_package superlu_dist -ksp_rtol 1.e-2
>
> But with 16 nodes, except case of 20x20x20 grids, all larger grids run e
> xtremly slow.
>  Since I never use the direct PC before, is it true that a good usage of
> direct LU as Preconditioner
> requires that the amount of procedures be much larger so that for each
> node the calculation of direct solver is smalled enough to use it?
>
> Cheers,
>
> Zhenyu
>



-- 
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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