On Jan 10, 2012, at 7:43 AM, Jed Brown wrote:
> On Tue, Jan 10, 2012 at 03:50, Alexander Grayver <agrayver at gfz-potsdam.de>
> wrote:
> http://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/KSP/KSPSetNullSpace.html
>
> You set nullspace without any particular information:
>
> ierr = MatNullSpaceCreate(PETSC_COMM_WORLD, PETSC_TRUE, 0, PETSC_NULL,
> &nullsp);
> ierr = KSPSetNullSpace(ksp, nullsp);
>
> What does it project and how works in this case?
>
> This removes the constant null space. The has_const argument is equivalent to
> creating a constant vector (but more efficient).
Alexandar,
You need to learn to use etags or one of the other mechanisms for
searching PETSc source code, see sections
13.8 Emacs Users . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . 174
13.9 Vi and Vim Users . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . 174
13.10EclipseUsers ..........................................175
13.11Qt Creator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . 175 13.12
DevelopersStudioUsers ....................................177 13.13
XCodeUsers(TheAppleGUIDevelopmentSystem . . . . . . . . . . . . . . . . . . . .
. 177
in the users manual: http://www.mcs.anl.gov/petsc/petsc-dev/docs/manual.pdf
This way you can see for yourself exactly how PETSc is doing things. It will be
quicker and more accurate than always asking us (especially since we have to
look at the code to answer your question anyways). Anyways
Here is how the multiplies and preconditioner applies are done in the KSP
solvers
#define KSP_MatMult(ksp,A,x,y) (!ksp->transpose_solve) ?
MatMult(A,x,y) :
MatMultTranspose(A,x,y)
#define KSP_MatMultTranspose(ksp,A,x,y) (!ksp->transpose_solve) ?
MatMultTranspose(A,x,y) :
MatMult(A,x,y)
#define KSP_PCApply(ksp,x,y) (!ksp->transpose_solve) ?
(PCApply(ksp->pc,x,y) || KSP_RemoveNullSpace(ksp,y)) :
PCApplyTranspose(ksp->pc,x,y)
#define KSP_PCApplyTranspose(ksp,x,y) (!ksp->transpose_solve) ?
PCApplyTranspose(ksp->pc,x,y) :
(PCApply(ksp->pc,x,y) || KSP_RemoveNullSpace(ksp,y))
#define KSP_PCApplyBAorAB(ksp,x,y,w) (!ksp->transpose_solve) ?
(PCApplyBAorAB(ksp->pc,ksp->pc_side,x,y,w) || KSP_RemoveNullSpace(ksp,y)) :
PCApplyBAorABTranspose(ksp->pc,ksp->pc_side,x,y,w)
#define KSP_PCApplyBAorABTranspose(ksp,x,y,w) (!ksp->transpose_solve) ?
(PCApplyBAorABTranspose(ksp->pc,ksp->pc_side,x,y,w) ||
KSP_RemoveNullSpace(ksp,y)) : PCApplyBAorAB(ksp->pc,ksp->pc_side,x,y,w)
while
#define KSP_RemoveNullSpace(ksp,y) ((ksp->nullsp && ksp->pc_side == PC_LEFT) ?
MatNullSpaceRemove(ksp->nullsp,y,PETSC_NULL) : 0)
and
/*@C
MatNullSpaceRemove - Removes all the components of a null space from a
vector.
Collective on MatNullSpace
Input Parameters:
+ sp - the null space context
. vec - the vector from which the null space is to be removed
- out - if this is requested (not PETSC_NULL) then this is a vector with the
null space removed otherwise
the removal is done in-place (in vec)
Note: The user is not responsible for the vector returned and should not
destroy it.
Level: advanced
.keywords: PC, null space, remove
.seealso: MatNullSpaceCreate(), MatNullSpaceDestroy(), MatNullSpaceSetFunction()
@*/
PetscErrorCode MatNullSpaceRemove(MatNullSpace sp,Vec vec,Vec *out)
{
PetscScalar sum;
PetscInt i,N;
PetscErrorCode ierr;
PetscFunctionBegin;
PetscValidHeaderSpecific(sp,MAT_NULLSPACE_CLASSID,1);
PetscValidHeaderSpecific(vec,VEC_CLASSID,2);
if (out) {
PetscValidPointer(out,3);
if (!sp->vec) {
ierr = VecDuplicate(vec,&sp->vec);CHKERRQ(ierr);
ierr = PetscLogObjectParent(sp,sp->vec);CHKERRQ(ierr);
}
ierr = VecCopy(vec,sp->vec);CHKERRQ(ierr);
vec = *out = sp->vec;
}
if (sp->has_cnst) {
ierr = VecGetSize(vec,&N);CHKERRQ(ierr);
if (N > 0) {
ierr = VecSum(vec,&sum);CHKERRQ(ierr);
sum = sum/((PetscScalar)(-1.0*N));
ierr = VecShift(vec,sum);CHKERRQ(ierr);
}
}
if (sp->n) {
ierr = VecMDot(vec,sp->n,sp->vecs,sp->alpha);CHKERRQ(ierr);
for (i=0; i<sp->n; i++) sp->alpha[i] = -sp->alpha[i];
ierr = VecMAXPY(vec,sp->n,sp->alpha,sp->vecs);CHKERRQ(ierr);
}
if (sp->remove){
ierr = (*sp->remove)(sp,vec,sp->rmctx);CHKERRQ(ierr);
}
PetscFunctionReturn(0);
}
All I found within seconds using etags. Now you can see exactly where and how
the null space is being removed.
Barry
