Sets the matrix associated with the linear system and a (possibly) different one from which the preconditioner will be built
ksp - the
Amat - the matrix that defines the linear system
Pmat - the matrix to be used in constructing the preconditioner, usually the same as Amat.
If you know the operator Amat has a null space you can use
MatSetTransposeNullSpace() to supply the null
Amat and the
KSP solvers will automatically use that null space as needed during the solution process.
All future calls to
KSPSetOperators() must use the same size matrices!
Pmat removes the matrix that is currently used.
If you wish to replace either
Pmat but leave the other one untouched then
KSPGetOperators() to get the one you wish to keep, call
on it and then pass it back in in your call to
If the operators have NOT been set with
KSPSetOperators() then the operators
are created in the
PC and returned to the user. In this case, if both operators
mat and pmat are requested, two DIFFERENT operators will be returned. If
only one is requested both operators in the
PC will be the same (i.e. as
if one had called
KSPSetOperators() with the same argument for both
The user must set the sizes of the returned matrices and their type etc just
as if the user created them with
MatCreate(). For example,
KSPGetOperators(ksp/pc,&mat,NULL); is equivalent to set size, type, etc of mat MatCreate(comm,&mat); KSP/PCSetOperators(ksp/pc,mat,mat); PetscObjectDereference((PetscObject)mat); set size, type, etc of mat and KSP/PCGetOperators(ksp/pc,&mat,&pmat); is equivalent to set size, type, etc of mat and pmat MatCreate(comm,&mat); MatCreate(comm,&pmat); KSP/PCSetOperators(ksp/pc,mat,pmat); PetscObjectDereference((PetscObject)mat); PetscObjectDereference((PetscObject)pmat); set size, type, etc of mat and pmat
The rationale for this support is so that when creating a
KSP the hierarchy
of underlying objects (i.e.
Mat) and their lifespans can be completely
managed by the top most level object (i.e. the
KSP). Another way to look
at this is when you create a
SNES you do not NEED to create a
KSP and attach it to
SNES object (the
SNES object manages it for you). Similarly when you create a
you do not need to attach a
PC to it (the
KSP object manages the
PC object for you).
Thus, why should YOU have to create the
Mat and attach it to the
it can be created for you?