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);

Level#

developer

Location#

src/sys/utils/openmp/mpmpishm.c

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