[petsc-users] How rto use multigrid?

[Mark F. Adams]

You need an updated PETSc. (I thought 3.2 had an early version of gamg … but you need 3.3 or dev)

Mark

On Oct 29, 2012, at 8:49 AM, w_ang_temp <w_ang_temp at 163.com> wrote:

> Hello, Jed
>     I use the command:
>     mpiexec -n 4 ./ex4f -ksp_type cgs -pc_type gamg -pc_gamg_agg_nsmooths 1  -ksp_rtol 1.0e-15 -ksp_converged_reason
>     The error is as follows:
> [0]PETSC ERROR: --------------------- Error Message ------------------------------------
> [0]PETSC ERROR: Unknown type. Check for miss-spelling or missing external package needed for type
>  seehttp://www.mcs.anl.gov/petsc/petsc-as/documentation/installation.html#external!
> [0]PETSC ERROR: Unable to find requested PC type gamg!
> [0]PETSC ERROR: ------------------------------------------------------------------------
> [0]PETSC ERROR: Petsc Release Version 3.2.0, Patch 7, Thu Mar 15 09:30:51 CDT 2012 
> [0]PETSC ERROR: See docs/changes/index.html for recent updates.
> [0]PETSC ERROR: See docs/faq.html for hints about trouble shooting.
> [0]PETSC ERROR: See docs/index.html for manual pages.
> [0]PETSC ERROR: ------------------------------------------------------------------------
> [0]PETSC ERROR: ./ex4f on a arch-linu named ubuntu by geo Mon Oct 29 05:40:13 2012
> [0]PETSC ERROR: Libraries linked from /home /geo/soft/petsc/petsc-3.2-p7/arch-linux2-c-opt/lib
> [0]PETSC ERROR: Configure run at Mon Jul  2 20:33:17 2012
> [0]PETSC ERROR: Configure options --with-mpi-dir=/home/geo/soft/mpich2 --download-f-blas-lapack=1 --with-x=1 --with-debugging=0 --download-parmetis --download-mumps --download-scalapack --download-blacs
> [0]PETSC ERROR: ------------------------------------------------------------------------
> [0]PETSC ERROR: PCSetType() line 67 in src/ksp/pc/interface/pcset.c
> [0]PETSC ERROR: PCSetFromOptions() line 184 in src/ksp/pc/interface/pcset.c
> [0]PETSC ERROR: KSPSetFromOptions() line 286 in src/ksp/ksp/interface/itcl.c
>      When I use 'mpiexec -n 4 ./ex4f -ksp_type cgs -pc_type sor -ksp_rtol 1.0e-15 -ksp_converged_reason', it is ok. So
> what is the possible reason?
>      Thanks.
>                                                     Jim
> 
> 
> 
> 
> 
> At 2012-10-28 22:25:31,"Jed Brown" <jedbrown at mcs.anl.gov> wrote:
> Try the simple option I just sent.
> 
> On Oct 28, 2012 6:38 AM, "w_ang_temp" <w_ang_temp at 163.com> wrote:
> Hello, Jed
>     Thanks for your timely reply. I deal with the soil-water coupled problem in geotechnical engineering, 
> whose stiffness matrix is ill-conditioned. I have did some work about it, mainly finding the effective
> solvers and preconditioners. I used the command line option like this:
> mpiexec -n 4 ./ex4f -ksp_type cgs -pc_type sor -ksp_rtol 1.0e-15 -ksp_converged_reason.
>     So, I also want to use the multigrid just like the simple command. There is only a little introduction
> about multigrid in the manual. Multigrid is complex and not a easy thing for me, so I just need to konw how
> to use it simply in PETSc to solve the Ax=b system.
>     Thanks.
>                                                                Jim
> 
> 
> 
> 
> 
> >At 2012-10-28 21:17:00,"Jed Brown" <jedbrown at mcs.anl.gov> wrote:
> >Algebraic multigrid can be used directly, -pc_type gamg -pc_gamg_agg_nsmooths 1. Geometric either required that you use the PCMG interface to set >interpolation (and provide a coarse operator for non-Galerkin) or use a DM that provides coarsening capability.
> 
> >What kind of problem are you solving?
> 
> >>On Oct 28, 2012 6:09 AM, "w_ang_temp" <w_ang_temp at 163.com> wrote:
> >>Hello,
> >>   I want to use the multigrid as a preconditioner. The introduction about it in the manual is little.
> >>So are there some typical examples or details about multigrid? Is it used just like other preconditioners
> >>like jacobi, sor, which can be simply used in the cammand line options?
> >>   Thanks.
> >>                                                                       Jim
> 
> 
> 
> 
> 
> 

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