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# PetscSFSetGraphWithPattern
Sets the graph of a `PetscSF` with a specific pattern 
## Synopsis
```
#include "petscsf.h" 
PetscErrorCode PetscSFSetGraphWithPattern(PetscSF sf, PetscLayout map, PetscSFPattern pattern)
```
Collective


## Input Parameters

- ***sf      -*** The `PetscSF`
- ***map     -*** Layout of roots over all processes (insignificant when pattern is `PETSCSF_PATTERN_ALLTOALL`)
- ***pattern -*** One of `PETSCSF_PATTERN_ALLGATHER`, `PETSCSF_PATTERN_GATHER`, `PETSCSF_PATTERN_ALLTOALL`





## Notes
It is easier to explain `PetscSFPattern` using vectors. Suppose we have an MPI vector `x` and its `PetscLayout` is `map`.
`n` and `N` are the local and global sizes of `x` respectively.

With `PETSCSF_PATTERN_ALLGATHER`, the routine creates a graph that if one does `PetscSFBcastBegin()`/`PetscSFBcastEnd()` on it, it will copy `x` to
sequential vectors `y` on all MPI processes.

With `PETSCSF_PATTERN_GATHER`, the routine creates a graph that if one does `PetscSFBcastBegin()`/`PetscSFBcastEnd()` on it, it will copy `x` to a
sequential vector `y` on rank 0.

In above cases, entries of `x` are roots and entries of `y` are leaves.

With `PETSCSF_PATTERN_ALLTOALL`, map is insignificant. Suppose NP is size of `sf`'s communicator. The routine
creates a graph that every rank has NP leaves and NP roots. On rank i, its leaf j is connected to root i
of rank j. Here 0 <=i,j<NP. It is a kind of `MPI_Alltoall()` with sendcount/recvcount being 1. Note that it does
not mean one can not send multiple items. One just needs to create a new MPI datatype for the mulptiple data
items with `MPI_Type_contiguous` and use that as the <unit> argument in SF routines.

In this case, roots and leaves are symmetric.


## See Also
 `PetscSF`, `PetscSFCreate()`, `PetscSFView()`, `PetscSFGetGraph()`

## Level
intermediate

## Location
<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/vec/is/sf/interface/sf.c.html#PetscSFSetGraphWithPattern">src/vec/is/sf/interface/sf.c</A>


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[Edit on GitLab](https://gitlab.com/petsc/petsc/-/edit/release/src/vec/is/sf/interface/sf.c)


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