# PetscBuildTwoSidedType
algorithm for setting up two-sided communication 
## Synopsis
```
typedef enum {
  PETSC_BUILDTWOSIDED_NOTSET     = -1,
  PETSC_BUILDTWOSIDED_ALLREDUCE  = 0,
  PETSC_BUILDTWOSIDED_IBARRIER   = 1,
  PETSC_BUILDTWOSIDED_REDSCATTER = 2
  /* Updates here must be accompanied by updates in finclude/petscsys.h and the string array in mpits.c */
} PetscBuildTwoSidedType;
```
```none
`PETSC_BUILDTWOSIDED_ALLREDUCE` - classical algorithm using an MPI_Allreduce with
```
```none
a buffer of length equal to the communicator size. Not memory-scalable due to
```
```none
the large reduction size. Requires only MPI-1.
```
```none
`PETSC_BUILDTWOSIDED_IBARRIER` - nonblocking algorithm based on MPI_Issend and MPI_Ibarrier.
```
```none
Proved communication-optimal in Hoefler, Siebert, and Lumsdaine (2010). Requires MPI-3.
```
```none
`PETSC_BUILDTWOSIDED_REDSCATTER` - similar to above, but use more optimized function
```
```none
that only communicates the part of the reduction that is necessary.  Requires MPI-2.
```




## See Also
 `PetscCommBuildTwoSided()`, `PetscCommBuildTwoSidedSetType()`, `PetscCommBuildTwoSidedGetType()`

## Level
developer

## Location
<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/include/petscsystypes.h.html#PetscBuildTwoSidedType">include/petscsystypes.h</A>


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