Multilevel solver

[Matthew Knepley]

On Thu, Apr 24, 2008 at 11:07 AM,  <Amit.Itagi at seagate.com> wrote:
>
>              "Matthew Knepley"
>              <knepley at gmail.co
>
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>              04/24/2008 10:19
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>              Please respond to
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> >  3) Since I want to set PC type to lu for field 0, and I want to use
>  MUMPS
>  >  for parallel LU, where do I set the submatrix type to MATAIJMUMPS ? In
>  this
>  >  case, will a second copy of the submatrix be generated - one of type
>  MUMPS
>  >  for the PC and the other of the original MATAIJ type for the KSP ?
>
>  I will have to check. However if we are consistent, then it should be
>
>   -field_split_0_mat_type aijmumps
>
>
>  Can these options be set inside the code, instead of on the command line ?

Yes, however it becomes more complicated. You must pull these objects out of
the FieldSplitPC (not that hard), but you must also be careful to set the values
after options are read in, but before higher level things are
initialized (like the
outer solver), so it can be somewhat delicate. I would not advise hardcoding
things which you are likely to change based upon architecture, problem, etc.
However, if you want to, the easiest way is to use PetscSetOption().

  Matt

>  >  4) How is the PC applied when I do PC_COMPOSITE_SYMMETRIC_MULTIPLICATIVE
>  ?
>
>  It is just the composition of the preconditioners, which is what you want
>  here.
>
>   Matt
>
>  >  Thanks
>  >
>  >  Rgds,
>  >  Amit
>  >
>  >
>  >
>  >
>  >              Barry Smith
>  >              <bsmith at mcs.anl.g
>  >              ov>
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>  >
>  >
>  >              04/22/2008 10:08
>  >              PM
>  >
>  >
>  >              Please respond to
>  >              petsc-users at mcs.a
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>  >
>  >
>  >
>  >
>  >
>  >
>  >   Amit,
>  >
>  >      Using a a PCSHELL should be fine (it can be used with GMRES),
>  >  my guess is there is a memory corruption error somewhere that is
>  >  causing the crash. This could be tracked down with www.valgrind.com
>  >
>  >     Another way to you could implement this is with some very recent
>  >  additions I made to PCFIELDSPLIT that are in petsc-dev
>  >  (http://www-unix.mcs.anl.gov/petsc/petsc-as/developers/index.html)
>  >  With this you would chose
>  >  PCSetType(pc,PCFIELDSPLIT
>  >  PCFieldSplitSetIS(pc,is1
>  >  PCFieldSplitSetIS(pc,is2
>  >  PCFieldSplitSetType(pc,PC_COMPOSITE_SYMMETRIC_MULTIPLICATIVE
>  >  to use LU on A11 use the command line options
>  >  -fieldsplit_0_pc_type lu -fieldsplit_0_ksp_type preonly
>  >  and SOR on A22
>  >  -fieldsplit_1_pc_type sor -fieldsplit_1_ksp_type preonly -
>  >  fieldsplit_1_pc_sor_lits <lits> where
>  >     <its> is the number of iterations you want to use block A22
>  >
>  >  is1 is the IS that contains the indices for all the vector entries in
>  >  the 1 block while is2 is all indices in the
>  >  vector for the 2 block. You can use ISCreateGeneral() to create these.
>  >
>  >    Probably it is easiest just to try this out.
>  >
>  >    Barry
>  >
>  >
>  >  On Apr 22, 2008, at 8:45 PM, Amit.Itagi at seagate.com wrote:
>  >
>  >  >
>  >  > Hi,
>  >  >
>  >  > I am trying to implement a multilevel method for an EM problem. The
>  >  > reference is : "Comparison of hierarchical basis functions for
>  >  > efficient
>  >  > multilevel solvers", P. Ingelstrom, V. Hill and R. Dyczij-Edlinger,
>  >  > IET
>  >  > Sci. Meas. Technol. 2007, 1(1), pp 48-52.
>  >  >
>  >  > Here is the summary:
>  >  >
>  >  > The matrix equation Ax=b is solved using GMRES with a multilevel
>  >  > pre-conditioner. A has a block structure.
>  >  >
>  >  > A11    A12       *         x1  =  b1
>  >  > A21    A22                  x2       b2
>  >  >
>  >  > A11 is mxm and A33 is nxn, where m is not equal to n.
>  >  >
>  >  > Step 1  :      Solve  A11 *  e1   = b1     (parallel LU using
>  >  > superLU or
>  >  > MUMPS)
>  >  >
>  >  > Step 2:        Solve   A22 * e2    =b2-A21*e1    (might either user
>  >  > a SOR
>  >  > solver or a parallel LU)
>  >  >
>  >  > Step 3:        Solve   A11* e1 = b1-A12*e2   (parallel LU)
>  >  >
>  >  > This gives the approximate solution to
>  >  >
>  >  > A11     A12     *      e1   =  b1
>  >  > A21     A22             e2       b2
>  >  >
>  >  > and is used as the pre-conditioner for the GMRES.
>  >  >
>  >  >
>  >  > Which PetSc method can implement this pre-conditioner ? I tried a
>  >  > PCSHELL
>  >  > type PC. With Hong's help, I also got the parallel LU to work
>  >  > withSuperLU/MUMPS. My program runs successfully on multiple
>  >  > processes on a
>  >  > single machine. But when I submit the program over multiple
>  >  > machines, I get
>  >  > a crash in the PCApply routine after several GMRES iterations. I
>  >  > think this
>  >  > has to do with using PCSHELL with GMRES (which is not a good idea). Is
>  >  > there a different way to implement this ? Does this resemble the usage
>  >  > pattern of one of the AMG preconditioners ?
>  >  >
>  >  >
>  >  > Thanks
>  >  >
>  >  > Rgds,
>  >  > Amit
>  >  >
>  >
>  >
>  >
>  >
>
>
>
>  --
>  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
>
>
>
>



-- 
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