[petsc-dev] Using PETSc MatIS, how to matmult a global IS matrix and a global vector ?

Tue May 23 11:28:03 CDT 2017

OK, thanks. This is helpfull... But I really think the doc should be more verbose about that: this is really confusing and I didn't find any simple example to begin with which make all this even more confusing (personal opinion). 

Franck 

----- Mail original -----

> De: "Matthew Knepley" <knepley at gmail.com>
> À: "Franck Houssen" <franck.houssen at inria.fr>
> Cc: "Stefano Zampini" <stefano.zampini at gmail.com>, "PETSc"
> <petsc-users at mcs.anl.gov>, "PETSc" <petsc-dev at mcs.anl.gov>
> Envoyé: Mardi 23 Mai 2017 13:21:21
> Objet: Re: [petsc-dev] Using PETSc MatIS, how to matmult a global IS matrix
> and a global vector ?

> On Tue, May 23, 2017 at 4:53 AM, Franck Houssen < franck.houssen at inria.fr >
> wrote:

> > The first thing I did was to put 3, not 4 : I got an error thrown in
> > MatCreateIS (see the git diff + stack below). As the error said I used
> > globalSize = numberOfMPIProcessus * localSize : my understanding is that,
> > when using MatIS, the global size needs to be the sum of all local sizes.
> > Correct ?
> 

> No. MatIS means that the matrix is not assembled. The easiest way (for me) to
> think of this is that processes do not have
> to hold full rows. One process can hold part of row i, and another processes
> can hold another part. However, there are still
> the same number of global rows.

> > I have a 3x3 global matrix made of two overlapping 2x2 local matrix (=
> > diagonal with 1.). Each local matrix correspond to one domain (each domain
> > is delegated to one MPI proc, so, I have 2 MPI procs because I have 2
> > domains).
> 

> So the global size is 3. The local size here is not the size of the local IS
> block, since that is a property only of MatIS. It is the
> size of the local piece of the vector you multiply. This allows PETSc to
> understand the parallel layout of the Vec, and how it
> matched the Mat.

> This is somewhat confusing because FEM people mean something different by
> "local" than we do here, and in fact we use this
> other definition of local when assembling operators.

> Matt

> > This is the simplest possible example: I have two 2x2 (local) diag matrix
> > that overlap so that the global matrix built from them is 1, 2, 1 on the
> > diagonal (local contributions add up in the middle).
> 
> > I need to MatMult this global matrix with a global vector filled with 1.
> 

> > Franck
> 

> > Git diff :
> 

> > --- a/matISLocalMat.cpp
> 
> > +++ b/matISLocalMat.cpp
> 
> > @@ -16,7 +16,7 @@ int main(int argc,char **argv) {
> 
> > int size = 0; MPI_Comm_size(MPI_COMM_WORLD, &size); if (size != 2) return
> > 1;
> 
> > int rank = 0; MPI_Comm_rank(MPI_COMM_WORLD, &rank);
> 

> > - PetscInt localSize = 2, globalSize = localSize*2 /*2 MPI*/;
> 
> > + PetscInt localSize = 2, globalSize = 3;
> 
> > PetscInt localIdx[2] = {0, 0};
> 
> > if (rank == 0) {localIdx[0] = 0; localIdx[1] = 1;}
> 
> > else {localIdx[0] = 1; localIdx[1] = 2;}
> 

> > Stack error:
> 

> > [0]PETSC ERROR: Nonconforming object sizes
> 
> > [0]PETSC ERROR: Sum of local lengths 4 does not equal global length 3, my
> > local length 2
> 
> > [0]PETSC ERROR: [0] ISG2LMapApply line 17
> > /home/fghoussen/Documents/INRIA/petsc-3.7.6/src/vec/is/utils/isltog.c
> 
> > [0]PETSC ERROR: [0] MatSetValues_IS line 692
> > /home/fghoussen/Documents/INRIA/petsc-3.7.6/src/mat/impls/is/matis.c
> 
> > [0]PETSC ERROR: [0] MatSetValues line 1157
> > /home/fghoussen/Documents/INRIA/petsc-3.7.6/src/mat/interface/matrix.c
> 
> > [0]PETSC ERROR: [0] MatISSetPreallocation_IS line 95
> > /home/fghoussen/Documents/INRIA/petsc-3.7.6/src/mat/impls/is/matis.c
> 
> > [0]PETSC ERROR: [0] MatISSetPreallocation line 80
> > /home/fghoussen/Documents/INRIA/petsc-3.7.6/src/mat/impls/is/matis.c
> 
> > [0]PETSC ERROR: [0] PetscSplitOwnership line 80
> > /home/fghoussen/Documents/INRIA/petsc-3.7.6/src/sys/utils/psplit.c
> 
> > [0]PETSC ERROR: [0] PetscLayoutSetUp line 129
> > /home/fghoussen/Documents/INRIA/petsc-3.7.6/src/vec/is/utils/pmap.c
> 
> > [0]PETSC ERROR: [0] MatSetLocalToGlobalMapping_IS line 628
> > /home/fghoussen/Documents/INRIA/petsc-3.7.6/src/mat/impls/is/matis.c
> 
> > [0]PETSC ERROR: [0] MatSetLocalToGlobalMapping line 1899
> > /home/fghoussen/Documents/INRIA/petsc-3.7.6/src/mat/interface/matrix.c
> 
> > [0]PETSC ERROR: [0] MatCreateIS line 986
> > /home/fghoussen/Documents/INRIA/petsc-3.7.6/src/mat/impls/is/matis.c
> 

> > > De: "Stefano Zampini" < stefano.zampini at gmail.com >
> > 
> 
> > > À: "Matthew Knepley" < knepley at gmail.com >
> > 
> 
> > > Cc: "Franck Houssen" < franck.houssen at inria.fr >, "PETSc" <
> > > petsc-users at mcs.anl.gov >, "PETSc" < petsc-dev at mcs.anl.gov >
> > 
> 
> > > Envoyé: Dimanche 21 Mai 2017 23:02:37
> > 
> 
> > > Objet: Re: [petsc-dev] Using PETSc MatIS, how to matmult a global IS
> > > matrix
> > > and a global vector ?
> > 
> 

> > > Franck,
> > 
> 

> > > PETSc takes care of doing the matrix-vector multiplication properly using
> > > MatIS. As Matt said, the layout of the vectors is the usual parallel
> > > layout.
> > 
> 
> > > The local sizes of the MatIS matrix (i.e. the local size of the left and
> > > right vectors used in MatMult) are not the sizes of the local subdomain
> > > matrices in MatIS.
> > 
> 

> > > > On May 21, 2017, at 6:47 PM, Matthew Knepley < knepley at gmail.com >
> > > > wrote:
> > > 
> > 
> 

> > > > On Sun, May 21, 2017 at 11:26 AM, Franck Houssen <
> > > > franck.houssen at inria.fr
> > > > >
> > > > wrote:
> > > 
> > 
> 

> > > > > Using PETSc MatIS, how to matmult a global IS matrix and a global
> > > > > vector
> > > > > ?
> > > > > Example is attached : I don't get what I expect that is a vector such
> > > > > that
> > > > > proc0 = [1, 2] and proc1 = [2, 1]
> > > > 
> > > 
> > 
> 

> > > > 1) I think the global size of your matrix is wrong. You seem to want 3,
> > > > not
> > > > 4
> > > 
> > 
> 

> > > > 2) Global vectors have a non-overlapping row partition. You might be
> > > > thinking
> > > > of local vectors
> > > 
> > 
> 

> > > > Thanks,
> > > 
> > 
> 

> > > > Matt
> > > 
> > 
> 

> > > > --
> > > 
> > 
> 
> > > > 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
> > > 
> > 
> 

> > > > http://www.caam.rice.edu/~mk51/
> > > 
> > 
> 

> > > De: "Stefano Zampini" < stefano.zampini at gmail.com >
> > 
> 
> > > À: "Matthew Knepley" < knepley at gmail.com >
> > 
> 
> > > Cc: "Franck Houssen" < franck.houssen at inria.fr >, "PETSc" <
> > > petsc-users at mcs.anl.gov >, "PETSc" < petsc-dev at mcs.anl.gov >
> > 
> 
> > > Envoyé: Dimanche 21 Mai 2017 23:02:37
> > 
> 
> > > Objet: Re: [petsc-dev] Using PETSc MatIS, how to matmult a global IS
> > > matrix
> > > and a global vector ?
> > 
> 

> > > Franck,
> > 
> 

> > > PETSc takes care of doing the matrix-vector multiplication properly using
> > > MatIS. As Matt said, the layout of the vectors is the usual parallel
> > > layout.
> > 
> 
> > > The local sizes of the MatIS matrix (i.e. the local size of the left and
> > > right vectors used in MatMult) are not the sizes of the local subdomain
> > > matrices in MatIS.
> > 
> 

> > > > On May 21, 2017, at 6:47 PM, Matthew Knepley < knepley at gmail.com >
> > > > wrote:
> > > 
> > 
> 

> > > > On Sun, May 21, 2017 at 11:26 AM, Franck Houssen <
> > > > franck.houssen at inria.fr
> > > > >
> > > > wrote:
> > > 
> > 
> 

> > > > > Using PETSc MatIS, how to matmult a global IS matrix and a global
> > > > > vector
> > > > > ?
> > > > > Example is attached : I don't get what I expect that is a vector such
> > > > > that
> > > > > proc0 = [1, 2] and proc1 = [2, 1]
> > > > 
> > > 
> > 
> 

> > > > 1) I think the global size of your matrix is wrong. You seem to want 3,
> > > > not
> > > > 4
> > > 
> > 
> 

> > > > 2) Global vectors have a non-overlapping row partition. You might be
> > > > thinking
> > > > of local vectors
> > > 
> > 
> 

> > > > Thanks,
> > > 
> > 
> 

> > > > Matt
> > > 
> > 
> 

> > > > > Franck
> > > > 
> > > 
> > 
> 

> > > > --
> > > 
> > 
> 
> > > > 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
> > > 
> > 
> 

> > > > http://www.caam.rice.edu/~mk51/
> > > 
> > 
> 

> --
> 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

> http://www.caam.rice.edu/~mk51/
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