petsc-3.10.5 2019-03-28
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# PCASMSetOverlap

Sets the overlap between a pair of subdomains for the additive Schwarz preconditioner. Either all or no processors in the PC communicator must call this routine.

### Synopsis

    #include "petscpc.h"
    PetscErrorCode  PCASMSetOverlap(PC pc,PetscInt ovl)

Logically Collective on PC

### 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 ASM 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 ASM 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.

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

### Keywords

PC, ASM, set, overlap

### See Also

PCASMSetTotalSubdomains(), PCASMSetLocalSubdomains(), PCASMGetSubKSP(),
PCASMCreateSubdomains2D(), PCASMGetLocalSubdomains(), MatIncreaseOverlap()

### Level

intermediate

### Location

src/ksp/pc/impls/asm/asm.c

### Examples

src/ksp/ksp/examples/tutorials/ex8.c.html

### Implementations

PCASMSetOverlap_ASM in src/ksp/pc/impls/asm/asm.c
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