[petsc-users] Confusion with MatGetLocalSubMatrix

[Jed Brown]

Lawrence Mitchell <lawrence.mitchell at imperial.ac.uk> writes:
> What is ISSetBlockSize for then?  Just hanging information on the IS for use elsewhere?

The index set would need to be contiguous:

static PetscErrorCode ISSetBlockSize_Stride(IS is,PetscInt bs)
{
  IS_Stride     *sub = (IS_Stride*)is->data;
  PetscErrorCode ierr;

  PetscFunctionBegin;
  if (sub->step != 1 && bs != 1) SETERRQ2(PetscObjectComm((PetscObject)is),PETSC_ERR_ARG_SIZ,"ISSTRIDE has stride %D, cannot be blocked of size %D",sub->step,bs);
  ierr = PetscLayoutSetBlockSize(is->map, bs);CHKERRQ(ierr);
  PetscFunctionReturn(0);
}
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