Re: [petsc-users] GAMG speed

[Michele Rosso]

Matt,

thank you! I will try to reduce the number of levels and see how it goes.
I asked about the speed since CG + Block Jacobi with ICC in each block 
runs faster then CG + MG, so I thought I was missing something.
Could you please tell me how to get rid of Chebichev?
Best,
Michele

On 08/13/2013 05:51 M, Matthew Knepley wrote:
On Tue, Aug 13, 2013 at 7:05 PM, Michele Rosso <mro...@uci.edu 
<mailto:mro...@uci.edu>> wrote:

    Hi Matt,

    I attached the output of the commands you suggested.
    The options I used are:

    -log_summary -ksp_monitor -ksp_view -ksp_converged_reason -pc_type
    mg  -pc_mg_galerkin -pc_mg_levels 5 -options_left


The convergence is great. I notice that your coarse solve takes no 
time. You could probably use fewer levels for
this problem. For this problem there is no easy things left I think. 
We are currently debating how you can squeeze
something extra out of the smoother. Here you could probably get rid 
of Chebychev and use only SOR.

    Matt

    and here are the lines of codes where I setup the solution process:

        call DMDACreate3d( PETSC_COMM_WORLD
    ,                                  &
                        & DMDA_BOUNDARY_PERIODIC ,
    DMDA_BOUNDARY_PERIODIC,     &
                        & DMDA_BOUNDARY_PERIODIC ,
    DMDA_STENCIL_STAR,          &
                        & N_Z , N_Y , N_X , N_B3 , N_B2 , 1_ip,  1_ip
    , 1_ip , &
                        & NNZ ,NNY , NNX, da , ierr)


        ! Create Global Vectors
        call DMCreateGlobalVector(da,b,ierr)
        call VecDuplicate(b,x,ierr)

        ! Set initial guess for first use of the module to 0
        call VecSet(x,0.0_rp,ierr)

        ! Create matrix
        call DMCreateMatrix(da,MATAIJ,A,ierr)

        ! Create solver
        call KSPCreate(PETSC_COMM_WORLD,ksp,ierr)
        call KSPSetDM(ksp,da,ierr)
        call KSPSetDMActive(ksp,PETSC_FALSE,ierr)
        call KSPSetOperators(ksp,A,A,SAME_NONZERO_PATTERN,ierr)
        call KSPSetType(ksp,KSPCG,ierr)
        call KSPSetNormType(ksp,KSP_NORM_UNPRECONDITIONED,ierr)
        call KSPSetInitialGuessNonzero(ksp,PETSC_TRUE,ierr)
        call KSPSetTolerances(ksp, tol ,PETSC_DEFAULT_DOUBLE_PRECISION,&
            & PETSC_DEFAULT_DOUBLE_PRECISION,PETSC_DEFAULT_INTEGER,ierr)

        ! Nullspace removal
        call MatNullSpaceCreate(
    PETSC_COMM_WORLD,PETSC_TRUE,PETSC_NULL_INTEGER,&
                               & PETSC_NULL_INTEGER,nullspace,ierr)
        call KSPSetNullspace(ksp,nullspace,ierr)
        call MatNullSpaceDestroy(nullspace,ierr)

        ! To allow using option from command line
        call KSPSetFromOptions(ksp,ierr)


    Hope I did not omit anything useful.
    Thank you for your time.

    Best,
    Michele




    On 08/13/2013 04:26 PM, Matthew Knepley wrote:
    On Tue, Aug 13, 2013 at 6:09 PM, Michele Rosso <mro...@uci.edu
    <mailto:mro...@uci.edu>> wrote:

        Hi Karli,

        thank you for your hint: now it works.
        Now I would like to speed up the solution: I was counting on
        increasing the number of levels/the number of processors
        used, but now I see I cannot do that.
        Do you have any hint to achieve better speed?
        Thanks!


    "Better speed" is not very helpful for us, and thus we cannot
    offer much help. You could

     1) Send the output of -log_summary -ksp_monitor -ksp_view

     2) Describe the operator succintly

        Matt

        Best,
        Michele





--
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