# PCILU

Incomplete factorization preconditioners.

### Options Database Keys

| -pc_ilu_levels <k> | - number of levels of fill for ILU(k)
| -pc_ilu_in_place | - only for ILU(0) with natural ordering, reuses the space of the matrix for its factorization (overwrites original matrix)
| -pc_ilu_diagonal_fill | - fill in a zero diagonal even if levels of fill indicate it wouldn't be fill
| -pc_ilu_reuse_ordering | - reuse ordering of factorized matrix from previous factorization
| -pc_ilu_use_drop_tolerance <dt,dtcol,maxrowcount> | - use Saad's drop tolerance ILUdt
| -pc_ilu_fill <nfill> | - expected amount of fill in factored matrix compared to original matrix, nfill > 1
| -pc_ilu_nonzeros_along_diagonal | - reorder the matrix before factorization to remove zeros from the diagonal, this decreases the chance of getting a zero pivot
| -pc_ilu_mat_ordering_type <natural,nd,1wd,rcm,qmd> | - set the row/column ordering of the factored matrix
| -pc_ilu_pivot_in_blocks | - for block ILU(k) factorization, i.e. with BAIJ matrices with block size larger than 1 the diagonal blocks are factored with partial pivoting (this increases the stability of the ILU factorization
| -pc_factor_shift_nonzero <shift> | - Sets shift amount or PETSC_DECIDE for the default
| -pc_factor_shift_positive_definite [PETSC_TRUE/PETSC_FALSE] | - Activate/Deactivate PCFactorSetShiftPd(); the value is optional with PETSC_TRUE being the default

Notes: Only implemented for some matrix formats. Not implemented in parallel

For BAIJ matrices this implements a point block ILU

### See Also

PCCreate(), PCSetType(), PCType (for list of available types), PC, PCSOR, MatOrderingType,
PCFactorSetZeroPivot(), PCFactorSetShiftNonzero(), PCFactorSetShiftPd(), PCILUSetUseDropTolerance(), PCILUSetFill(), PCILUSetMatOrdering(), PCILUSetReuseOrdering(), PCILUDTSetReuseFill(), PCILUSetLevels(), PCILUSetUseInPlace(), PCILUSetAllowDiagonalFill(), PCILUSetPivotInBlocks(), PCFactorSetShiftNonzero(),PCFactorSetShiftPd()

Level:beginner
Location:src/ksp/pc/impls/factor/ilu/ilu.c
Index of all PC routines
Table of Contents for all manual pages
Index of all manual pages

### Examples

src/ksp/pc/examples/tutorials/ex1.c.html
src/ksp/examples/tutorials/ex7.c.html
src/ksp/examples/tutorials/ex8.c.html