[petsc-users] Understanding MatCreate bsize parameter

[Matthew Knepley]

On Fri, Mar 27, 2015 at 4:28 AM, Florian Lindner <mailinglists at xgm.de>
wrote:

> Am Donnerstag, 26. März 2015, 07:34:27 schrieb Jed Brown:
> > Florian Lindner <mailinglists at xgm.de> writes:
> >
> > > Hello,
> > >
> > > I'm using petsc with petsc4py.
> > >
> > > A matrix is created like that
> > >
> > >     MPIrank = MPI.COMM_WORLD.Get_rank()
> > >     MPIsize = MPI.COMM_WORLD.Get_size()
> > >     print("MPI Rank = ", MPIrank)
> > >     print("MPI Size = ", MPIsize)
> > >     parts = partitions()
> > >
> > >     print("Dimension= ", nSupport + dimension, "bsize = ",
> len(parts[MPIrank]))
> > >
> > >     MPI.COMM_WORLD.Barrier() # Just to keep the output together
> > >     A = PETSc.Mat(); A.createDense( (nSupport + dimension, nSupport +
> dimension), bsize = len(parts[MPIrank]) ) # <-- crash here
> >
> > bsize is collective (must be the same on all processes).  It is used for
> > vector-valued problems (like elasticity -- bs=3 in 3 dimensions).
>
> It seems I'm still misunderstanding the bsize parameter.
>
> If I distribute a 10x10 matrix on three ranks I need to have a
> non-homogenous distribution, and thats what petsc does itself:
>

blockSize really means the uniform block size of the matrix, thus is HAS to
divide the global size. If it does not,
you do not have a uniform block size, you have a bunch of different sized
blocks.

  Thanks,

     Matt


> A.createDense( (n, n) )
>
> print("Rank = ", rank, "Range    = ", A.owner_range, "Size = ",
> A.owner_range[1] - A.owner_range[0])
> print("Rank = ", rank, "ColRange = ", A.getOwnershipRangeColumn(), "Size =
> ", A.getOwnershipRangeColumn()[1] - A.getOwnershipRangeColumn()[0])
>
> gives:
>
> Rank =  2 Range    =  (7, 10) Size =  3
> Rank =  2 ColRange =  (7, 10) Size =  3
> Rank =  0 Range    =  (0, 4)  Size =  4
> Rank =  0 ColRange =  (0, 4)  Size =  4
> Rank =  1 Range    =  (4, 7)  Size =  3
> Rank =  1 ColRange =  (4, 7)  Size =  3
>
>
> How can I manually set a distribution of rows like above? My approach was
> to call create with bsize = [3,3,4][rank] but that obviously is not the
> way...
>
> Thanks,
> Florian
>



-- 
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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