[petsc-users] MatMult inside a for loop

[Ronal Celaya]

I know that. I want to have all the vector x replicated in all processes
and update it in each iteration, so I don't need to communicate the vector
x each time MatMult() is called.
I'm not sure I'm making myself clear, sorry

On Sun, Feb 8, 2015 at 7:52 PM, Barry Smith <bsmith at mcs.anl.gov> wrote:

>
> > On Feb 8, 2015, at 6:14 PM, Ronal Celaya <ronalcelayavzla at gmail.com>
> wrote:
> >
> > Thank you Barry.
> > Is there a way to reuse the vector x? I don't want to gather the vector
> in each iteration, I'd rather replicate the vector x in each process.
>
>   I don't understand. With each new matrix vector product there are new
> values in x (which are updated from other parts of the CG algorithm), these
> new values need to be communicated in the MatMult() to where they are
> needed; you can't just reuse the old values in x.
>
>   Barry
>
> >
> > Thanks in advance.
> >
> > On Sun, Feb 8, 2015 at 7:17 PM, Barry Smith <bsmith at mcs.anl.gov> wrote:
> >
> > > On Feb 8, 2015, at 5:41 PM, Ronal Celaya <ronalcelayavzla at gmail.com>
> wrote:
> > >
> > > Hello
> > > If I have a MatMult operation inside a for loop (e. g. CG algorithm),
> and the matrix A is MPIAIJ, vector x is gathered to local process in every
> loop?
> >
> >   Yes, internal to MatMult() it calls MatMult_MPIAIJ() which is in
> src/mat/impls/aij/mpi/mpiaij,c which has the following code:
> >
> > PetscErrorCode MatMult_MPIAIJ(Mat A,Vec xx,Vec yy)
> > {
> >   Mat_MPIAIJ     *a = (Mat_MPIAIJ*)A->data;
> >   PetscErrorCode ierr;
> >   PetscInt       nt;
> >
> >   PetscFunctionBegin;
> >   ierr = VecGetLocalSize(xx,&nt);CHKERRQ(ierr);
> >   if (nt != A->cmap->n)
> SETERRQ2(PETSC_COMM_SELF,PETSC_ERR_ARG_SIZ,"Incompatible partition of A
> (%D) and xx (%D)",A->cmap->n,nt);
> >   ierr =
> VecScatterBegin(a->Mvctx,xx,a->lvec,INSERT_VALUES,SCATTER_FORWARD);CHKERRQ(ierr);
> >   ierr = (*a->A->ops->mult)(a->A,xx,yy);CHKERRQ(ierr);
> >   ierr =
> VecScatterEnd(a->Mvctx,xx,a->lvec,INSERT_VALUES,SCATTER_FORWARD);CHKERRQ(ierr);
> >   ierr = (*a->B->ops->multadd)(a->B,a->lvec,yy,yy);CHKERRQ(ierr);
> >   PetscFunctionReturn(0);
> >
> > The needed values of x are communicated in the VecScatterBegin() to
> VecScatterEnd(). Note only exactly those values needed by each process are
> communicated in the scatter so not all values are communicated to all
> processes. Since the matrix is very sparse (normally) only a small
> percentage of the values need to be communicated.
> >
> >   Barry
> >
> > >
> > > I'm sorry for my English.
> > >
> > > Regards,
> > >
> > > --
> > > Ronal Celaya
> >
> >
> >
> >
> > --
> > Ronal Celaya
>
>


-- 
Ronal Celaya
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