MatGetFactor — PETSc 3.23.0 documentation

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# MatGetFactor

Returns a matrix suitable to calls to MatXXFactorSymbolic,Numeric()

## Synopsis

    #include "petscmat.h"
    PetscErrorCode MatGetFactor(Mat mat, MatSolverType type, MatFactorType ftype, Mat *f)

Collective

## Input Parameters

- mat - the matrix

- type - name of solver type, for example, superlu, petsc (to use PETSc’s solver if it is available), if this is ‘NULL’, then the first result that satisfies the other criteria is returned

- ftype - factor type, `MAT_FACTOR_LU`, `MAT_FACTOR_CHOLESKY`, `MAT_FACTOR_ICC`, `MAT_FACTOR_ILU`, `MAT_FACTOR_QR`

## Output Parameter

- f - the factor matrix used with MatXXFactorSymbolic,Numeric() calls. Can be `NULL` in some cases, see notes below.

## Options Database Keys

- -pc_factor_mat_solver_type - choose the type at run time. When using `KSP` solvers

- -pc_factor_mat_factor_on_host - do mat factorization on host (with device matrices). Default is doing it on device

- -pc_factor_mat_solve_on_host - do mat solve on host (with device matrices). Default is doing it on device

## Notes

The return matrix can be `NULL` if the requested factorization is not available, since some combinations of matrix types and factorization types registered with `MatSolverTypeRegister()` cannot be fully tested if not at runtime.

Users usually access the factorization solvers via `KSP`

Some PETSc matrix formats have alternative solvers available that are contained in alternative packages such as pastix, superlu, mumps etc. PETSc must have been ./configure to use the external solver, using the option –download-package or –with-package-dir

When `type` is `NULL` the available results are searched for based on the order of the calls to `MatSolverTypeRegister()` in `MatInitializePackage()`. Since different PETSc configurations may have different external solvers, seemingly identical runs with different PETSc configurations may use a different solver. For example if one configuration had –download-mumps while a different one had –download-superlu_dist.

Some of the packages have options for controlling the factorization, these are in the form -prefix_mat_packagename_packageoption where prefix is normally obtained from the calling `KSP`/`PC`. If `MatGetFactor()` is called directly one can set call `MatSetOptionsPrefixFactor()` on the originating matrix or `MatSetOptionsPrefix()` on the resulting factor matrix.

## Developer Note

This should actually be called `MatCreateFactor()` since it creates a new factor object

## See Also

Matrices, `Mat`, Matrix Factorization, `KSP`, `MatSolverType`, `MatFactorType`, `MatCopy()`, `MatDuplicate()`, `MatGetFactorAvailable()`, `MatFactorGetCanUseOrdering()`, `MatSolverTypeRegister()`, `MatSolverTypeGet()` `MAT_FACTOR_LU`, `MAT_FACTOR_CHOLESKY`, `MAT_FACTOR_ICC`, `MAT_FACTOR_ILU`, `MAT_FACTOR_QR`, `MatInitializePackage()`

## Level

intermediate

## Location

src/mat/interface/matrix.c

## Implementations

MatGetFactor_pastix() in src/mat/impls/aij/mpi/pastix/pastix.c
MatGetFactor_HT() in src/mat/impls/transpose/htransm.c
MatGetFactor_Transpose() in src/mat/impls/transpose/transm.c

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