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# PetscOmpCtrlBarrier
Do barrier on MPI ranks in omp_comm contained by the PETSc OMP controller (to let slave ranks free their CPU) 
## Synopsis
```
#include "petscsys.h"  
PetscErrorCode PetscOmpCtrlBarrier(PetscOmpCtrl ctrl)
```

## Input Parameter

- ***ctrl -*** a PETSc OMP controller



## Notes
this is a pthread barrier on MPI ranks. Using `MPI_Barrier()` instead is conceptually correct. But MPI standard does not
require processes blocked by `MPI_Barrier()` free their CPUs to let other processes progress. In practice, to minilize latency,
MPI ranks stuck in `MPI_Barrier()` keep polling and do not free CPUs. In contrast, pthread_barrier has this requirement.

A code using `PetscOmpCtrlBarrier()` would be like this,
```none
    if (is_omp_master) {
      PetscOmpCtrlOmpRegionOnMasterBegin(ctrl);
      Call the library using OpenMP
      PetscOmpCtrlOmpRegionOnMasterEnd(ctrl);
    }
    PetscOmpCtrlBarrier(ctrl);
```





## See Also
 `PetscOmpCtrlOmpRegionOnMasterBegin()`, `PetscOmpCtrlOmpRegionOnMasterEnd()`, `PetscOmpCtrlCreate()`, `PetscOmpCtrlDestroy()`,

## Level
developer

## Location
<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/sys/utils/mpishm.c.html#PetscOmpCtrlBarrier">src/sys/utils/mpishm.c</A>


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[Edit on GitLab](https://gitlab.com/petsc/petsc/-/edit/release/src/sys/utils/mpishm.c)


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