PCFieldSplitSetFields#

Sets the fields that define one particular split in PCFIELDSPLIT

Synopsis#

#include "petscpc.h" 
PetscErrorCode PCFieldSplitSetFields(PC pc, const char splitname[], PetscInt n, const PetscInt fields[], const PetscInt fields_col[])

Logically Collective

Input Parameters#

pc - the preconditioner context
splitname - name of this split, if NULL the number of the split is used
n - the number of fields in this split
fields - the fields in this split
fields_col - generally the same as fields, if it does not match fields then the submatrix that is solved for this set of fields comes from an off-diagonal block of the matrix and fields_col provides the column indices for that block

Options Database Key#

-pc_fieldsplit_%d_fields <a,b,..> - indicates the fields to be used in the %d’th split

Notes#

Use PCFieldSplitSetIS() to set a general set of indices as a split.

If the matrix used to construct the preconditioner is MATNEST then field i refers to the is_row[i] IS passed to MatCreateNest().

If the matrix used to construct the preconditioner is not MATNEST then PCFieldSplitSetFields() is for defining fields as strided blocks (based on the block size provided to the matrix with MatSetBlocksize() or to the PC with PCFieldSplitSetBlockSize()). For example, if the block size is three then one can define a split as 0, or 1 or 2 or 0,1 or 0,2 or 1,2 which mean 0xx3xx6xx9xx12 … x1xx4xx7xx … xx2xx5xx8xx.. 01x34x67x… 0x1x3x5x7.. x12x45x78x…. where the numbered entries indicate what is in the split.

This function is called once per split (it creates a new split each time). Solve options for this split will be available under the prefix -fieldsplit_SPLITNAME_.

PCFieldSplitSetIS() does not support having a fields_col different from fields

Developer Notes#

This routine does not actually create the IS representing the split, that is delayed until PCSetUp_FieldSplit(), because information about the vector/matrix layouts may not be available when this routine is called.