[petsc-users] How to use multigrid?

[w_ang_temp]

Hello,
    I have tried AMG, but there are some problems. I use the command:
    mpiexec -n 4 ./ex4f -ksp_type gmres -pc_type gamg -pc_gamg_agg_nsmooths 1 -ksp_gmres_restart 170 -ksp_rtol 1.0e-15 -ksp_converged_reason.
    The matrix has a size of 30000. However, compared with -pc_type asm,
the amg need more time:asm needs 4.9s, amg needs 13.7s. I did several tests
and got the same conclusion. When it begins, the screen shows the information:
[0]PCSetData_AGG bs=1 MM=7601. I do not know the meaning. And if there is some
parameters that affect the performance of AMG?
    Besides, I want to confirm a conception. In my view, AMG itself can be a solver
like gmres. It can also be used as a preconditioner like jacobi and is used by combining
with other solver. Is it right? If it is right, how use AMG solver?
    My codes are attached.
    Thanks.
                                            Jim

-----------codes------------------------------------------
      call MatCreate(PETSC_COMM_WORLD,A,ierr)
      call MatSetSizes(A,PETSC_DECIDE,PETSC_DECIDE,m,n,ierr)
      call MatSetType(A, MATMPIAIJ,ierr)
      call MatSetFromOptions(A,ierr)
!premalloc
      !find the max non-zero numbers of all rows
       maxnonzero=0
       do 19,II=1,m
        !no-zero numbers of this row
        maxnonzeroII=NROWIN(II+1)-NROWIN(II)
        if (maxnonzeroII>maxnonzero) then
        maxnonzero=maxnonzeroII
        endif
  19  continue
       call MatMPIAIJSetPreallocation(A,maxnonzero,PETSC_NULL_INTEGER,
     &  maxnonzero,PETSC_NULL_INTEGER,ierr)
      call MatGetOwnershipRange(A,Istart,Iend,ierr)
!set values per row
       do 10,II=Istart+1,Iend
        !no-zero numbers of this row
        rowNum=NROWIN(II+1)-NROWIN(II)
       
        allocate(nColPerRow(rowNum))
        allocate(valuePerRow(rowNum))
       
        kValStart=NROWIN(II)+1-1
        kValEnd=NROWIN(II)+rowNum-1
       
        !column index
        nColPerRow=NNZIJ(kValStart:kValEnd)-1
        valuePerRow=VALUE(kValStart:kValEnd)
        nRow=II-1
        call MatSetValues(A,ione,nRow,rowNum,nColPerRow,valuePerRow,
     &       INSERT_VALUES,ierr)
       deallocate(nColPerRow)
       deallocate(valuePerRow)
  10  continue
      call MatAssemblyBegin(A,MAT_FINAL_ASSEMBLY,ierr)
      call MatAssemblyEnd(A,MAT_FINAL_ASSEMBLY,ierr)
      call VecCreateMPI(PETSC_COMM_WORLD,PETSC_DECIDE,m,b,ierr)
      call VecSetFromOptions(b,ierr)
      call VecDuplicate(b,u,ierr)
      call VecDuplicate(b,x,ierr)
      call KSPCreate(PETSC_COMM_WORLD,ksp,ierr)
      call KSPSetOperators(ksp,A,A,DIFFERENT_NONZERO_PATTERN,ierr)
      call PetscOptionsHasName(PETSC_NULL_CHARACTER,'-my_ksp_monitor',  &
     &                    flg,ierr)
      if (flg) then
        call KSPMonitorSet(ksp,MyKSPMonitor,PETSC_NULL_OBJECT,          &
     &                     PETSC_NULL_FUNCTION,ierr)
      endif
      call KSPSetFromOptions(ksp,ierr)
      call PetscOptionsHasName(PETSC_NULL_CHARACTER,                    &
     &     '-my_ksp_convergence',flg,ierr)
      if (flg) then
        call KSPSetConvergenceTest(ksp,MyKSPConverged,                  &
     &          PETSC_NULL_OBJECT,PETSC_NULL_FUNCTION,ierr)
      endif
      !Assing values to 'b'
      bTemp=Iend-Istart
      ioneb=1
     
      do 12,II=Istart,Iend-1
            voneb=F(II+1)
         call VecSetValues(b,ioneb,II,voneb,INSERT_VALUES,ierr)
  12    continue
      call VecAssemblyBegin(b,ierr)
      call VecAssemblyEnd(b,ierr)
      call KSPSolve(ksp,b,x,ierr)
------codes-------------------------------------------------
 






 


>At 2012-11-01 22:00:28,"Jed Brown" <jedbrown at mcs.anl.gov> wrote:

>Yes, it's faster to understand this error message than to have "mysteriously slow performance".


>* Preallocation routines now automatically set MAT_NEW_NONZERO_ALLOCATION_ERR, if you intentionally preallocate less than necessary then >use MatSetOption(mat,MAT_NEW_NONZERO_ALLOCATION_ERR,PETSC_FALSE) to disable the error generation.

>http://www.mcs.anl.gov/petsc/documentation/changes/33.html


>On Thu, Nov 1, 2012 at 8:57 AM, w_ang_temp <w_ang_temp at 163.com> wrote:

>Do you mean that the two versions have a different in this point? If I use the new version, I have to
>make some modifications on my codes?

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