**-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

Index of all PC routines

Table of Contents for all manual pages

Index of all manual pages

### Examples

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