> On Feb 8, 2015, at 6:14 PM, Ronal Celaya <[email protected]> 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 <[email protected]> wrote: > > > On Feb 8, 2015, at 5:41 PM, Ronal Celaya <[email protected]> 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
