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

Tue May 23 11:40:59 CDT 2017

I let this small piece of code to demonstrate the basics.... If you believe (like I do) that this is worth to be added to the available examples: feel free to do it !... 

Franck 

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

> De: "Franck Houssen" <franck.houssen at inria.fr>
> À: "Matthew Knepley" <knepley at gmail.com>
> Cc: "PETSc" <petsc-users at mcs.anl.gov>, "PETSc" <petsc-dev at mcs.anl.gov>
> Envoyé: Mardi 23 Mai 2017 18:28:03
> Objet: Re: [petsc-dev] Using PETSc MatIS, how to matmult a global IS matrix
> and a global vector ?

> 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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