# PetscOpenMPMerge

Initializes the PETSc and MPI to work with OpenMP. This is not usually called by the user. One should use -openmp_merge_size <n> to indicate the node size of merged communicator to be.

### Synopsis

    #include "petsc.h"
    PetscErrorCode  PetscOpenMPMerge(PetscMPIInt nodesize)

Collective on MPI_COMM_WORLD or PETSC_COMM_WORLD if it has been set

### Input Parameter

nodesize -size of each compute node that will share processors

### Options Database

-openmp_merge_size <n> -

       Comparison of two approaches for OpenMP usage (MPI started with N processes)

       -openmp_spawn_size <n> requires MPI 2, results in n*N total processes with N directly used by application code

                                              and n-1 worker processes (used by PETSc) for each application node.

                              You MUST launch MPI so that only ONE MPI process is created for each hardware node.

       -openmp_merge_size <n> results in N total processes, N/n used by the application code and the rest worker processes

                               (used by PETSc)

                              You MUST launch MPI so that n MPI processes are created for each hardware node.

       petscmpiexec -np 2 ./ex1 -openmp_spawn_size 3 gives 2 application nodes (and 4 PETSc worker nodes)

       petscmpiexec -np 6 ./ex1 -openmp_merge_size 3 gives the SAME 2 application nodes and 4 PETSc worker nodes

          This is what would use if each of the computers hardware nodes had 3 CPUs.

         These are intended to be used in conjunction with USER OpenMP code. The user will have 1 process per

      computer (hardware) node (where the computer node has p cpus), the user's code will use threads to fully

      utilize all the CPUs on the node. The PETSc code will have p processes to fully use the compute node for

      PETSc calculations. The user THREADS and PETSc PROCESSES will NEVER run at the same time so the p CPUs

      are always working on p task, never more than p.

       See PCOPENMP for a PETSc preconditioner that can use this functionality

### See Also

PetscFinalize(), PetscInitializeFortran(), PetscGetArgs(), PetscOpenMPFinalize(), PetscInitialize()

Level:developer
Location:src/sys/objects/mpinit.c
Index of all Sys routines
Table of Contents for all manual pages
Index of all manual pages