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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,

        if (is_omp_master) {
          PetscOmpCtrlOmpRegionOnMasterBegin(ctrl);
          Call the library using OpenMP
          PetscOmpCtrlOmpRegionOnMasterEnd(ctrl);
        }
        PetscOmpCtrlBarrier(ctrl);

## See Also

`PetscOmpCtrlOmpRegionOnMasterBegin()`, `PetscOmpCtrlOmpRegionOnMasterEnd()`, `PetscOmpCtrlCreate()`, `PetscOmpCtrlDestroy()`,

## Level

developer

## Location

src/sys/utils/mpishm.c

Index of all Sys routines
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Index of all manual pages