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# PCASMSetOverlap
Sets the overlap between a pair of subdomains for the additive Schwarz preconditioner, `PCASM`. 
## Synopsis
```
#include "petscpc.h" 
PetscErrorCode PCASMSetOverlap(PC pc, PetscInt ovl)
```
Logically Collective


## Input Parameters

- ***pc  -*** the preconditioner context
- ***ovl -*** the amount of overlap between subdomains (ovl >= 0, default value = 1)



## Options Database Key

- ***-pc_asm_overlap <ovl> -*** Sets overlap





## Notes
By default the `PCASM` preconditioner uses 1 block per processor.  To use
multiple blocks per perocessor, see `PCASMSetTotalSubdomains()` and
`PCASMSetLocalSubdomains()` (and the option -pc_asm_blocks <blks>).

The overlap defaults to 1, so if one desires that no additional
overlap be computed beyond what may have been set with a call to
`PCASMSetTotalSubdomains()` or `PCASMSetLocalSubdomains()`, then ovl
must be set to be 0.  In particular, if one does not explicitly set
the subdomains an application code, then all overlap would be computed
internally by PETSc, and using an overlap of 0 would result in an `PCASM`
variant that is equivalent to the block Jacobi preconditioner.

The default algorithm used by PETSc to increase overlap is fast, but not scalable,
use the option -mat_increase_overlap_scalable when the problem and number of processes is large.

One can define initial index sets with any overlap via
`PCASMSetLocalSubdomains()`; the routine
`PCASMSetOverlap()` merely allows PETSc to extend that overlap further
if desired.


## See Also
 `PCASM`, `PCASMSetTotalSubdomains()`, `PCASMSetLocalSubdomains()`, `PCASMGetSubKSP()`,
`PCASMCreateSubdomains2D()`, `PCASMGetLocalSubdomains()`, `MatIncreaseOverlap()`, `PCGASM`

## Level
intermediate

## Location
<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/ksp/pc/impls/asm/asm.c.html#PCASMSetOverlap">src/ksp/pc/impls/asm/asm.c</A>

## Examples
<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/ksp/ksp/tutorials/ex8.c.html">src/ksp/ksp/tutorials/ex8.c</A><BR>

## Implementations

<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/ksp/pc/impls/asm/asm.c.html#PCASMSetOverlap_ASM">PCASMSetOverlap_ASM in src/ksp/pc/impls/asm/asm.c</A><BR>


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[Edit on GitLab](https://gitlab.com/petsc/petsc/-/edit/release/src/ksp/pc/impls/asm/asm.c)


[Index of all PC routines](index.md)  
[Table of Contents for all manual pages](/manualpages/index.md)  
[Index of all manual pages](/manualpages/singleindex.md)