# PCMGSetLevels#

Sets the number of levels to use with `PCMG`

. Must be called before any other `PCMG`

routine.

## Synopsis#

```
#include "petscksp.h"
PetscErrorCode PCMGSetLevels(PC pc, PetscInt levels, MPI_Comm *comms)
```

Logically Collective

## Input Parameters#

the preconditioner context**pc -**the number of levels**levels -**optional communicators for each level; this is to allow solving the coarser problems on smaller sets of processes. For processes that are not included in the computation you must pass**comms -**`MPI_COMM_NULL`

. Use comms =`NULL`

to specify that all processes should participate in each level of problem.

## Notes#

If the number of levels is one then the multigrid uses the `-mg_levels`

prefix
for setting the level options rather than the `-mg_coarse`

or `-mg_fine`

prefix.

You can free the information in comms after this routine is called.

The array of MPI communicators must contain `MPI_COMM_NULL`

for those ranks that at each level
are not participating in the coarser solve. For example, with 2 levels and 1 and 2 ranks on
the two levels, rank 0 in the original communicator will pass in an array of 2 communicators
of size 2 and 1, while rank 1 in the original communicator will pass in array of 2 communicators
the first of size 2 and the second of value `MPI_COMM_NULL`

since the rank 1 does not participate
in the coarse grid solve.

Since each coarser level may have a new `MPI_Comm`

with fewer ranks than the previous, one
must take special care in providing the restriction and interpolation operation. We recommend
providing these as two step operations; first perform a standard restriction or interpolation on
the full number of ranks for that level and then use an MPI call to copy the resulting vector
array entries (after calls to VecGetArray()) to the smaller or larger number of ranks, note in both
cases the MPI calls must be made on the larger of the two communicators. Traditional MPI send and
receives or `MPI_AlltoAllv()`

could be used to do the reshuffling of the vector entries.

## Fortran Notes#

Use comms = `PETSC_NULL_MPI_COMM`

as the equivalent of `NULL`

in the C interface. Note `PETSC_NULL_MPI_COMM`

is not `MPI_COMM_NULL`

. It is more like `PETSC_NULL_INTEGER`

, `PETSC_NULL_REAL`

etc.

## See Also#

## Level#

intermediate

## Location#

## Examples#

src/ksp/ksp/tutorials/ex42.c

src/ksp/ksp/tutorials/ex35.cxx

src/dm/impls/stag/tutorials/ex4.c

src/ksp/ksp/tutorials/ex36.cxx

## Implementations#

PCMGSetLevels_MG() in src/ksp/pc/impls/mg/mg.c

Index of all PC routines

Table of Contents for all manual pages

Index of all manual pages