[petsc-users] MatMult inside a for loop

[Barry Smith]

> 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