How to import OpenMP and MPI to a large CLion CMake project?

Question

I am new to OpenMP and MPI, and I want to import OpenMP and MPI to my existing CMake project. Therefore --

First, I created a very simple MPI project.

main.cpp:

#include "mpi.h"
#include "iostream"

int main(int argc, char** argv){
    MPI_Init(NULL, NULL);

    int world_size;
    MPI_Comm_size(MPI_COMM_WORLD, &world_size);

    int world_rank;
    MPI_Comm_rank(MPI_COMM_WORLD, &world_rank);

    char processor_name[MPI_MAX_PROCESSOR_NAME];
    int name_len;
    MPI_Get_processor_name(processor_name, &name_len);

    printf("Hello world from processor %s, rank %d out of %d processors\n",processor_name, world_rank, world_size);

    MPI_Finalize();
}

I can successfully compile and run the project using the command:

$ mpicxx main.cpp -o main.out
$ mpirun -np 2 main.out

Then I created and configurated a CMake project using CLion. I can also successfully compile and run it using CLion, but it only used one processor.

CMakeLists.txt:

cmake_minimum_required(VERSION 3.5)
project(Test)
set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++11")

find_package(MPI)
include_directories(${MPI_INCLUDE_PATH})

SET(CMAKE_CXX_COMPILER mpicxx)
SET(CMAKE_C_COMPILER  mpicc)

set(SOURCE_FILES main.cpp)
add_executable(Test ${SOURCE_FILES})

Thus my first question is how can I use multiple processors using CLion?

Second, I created a simple OpenMP project and run successfully.

But my second question is how can I include both MPI and OpenMP? It seems that I cannot just merge the two CMakeLists.txt of the MPI project and OpenMP project. Because I got an error about the -fopenmp flag:

clang: error: unsupported option '-fopenmp'

CMakeLists.txt:

cmake_minimum_required(VERSION 3.12)
project(OpenMP_MPI)

set(CMAKE_CXX_STANDARD 14)

OPTION (USE_OpenMP "Use OpenMP to enamble <omp.h>" ON)

if(APPLE)
    set(CMAKE_C_COMPILER "/usr/local/Cellar/llvm/9.0.0_1/bin/clang")
    set(CMAKE_CXX_COMPILER "/usr/local/Cellar/llvm/9.0.0_1/bin/clang++")
    set(OPENMP_LIBRARIES "/usr/local/Cellar/llvm/9.0.0_1/lib")
    set(OPENMP_INCLUDES "/usr/local/Cellar/llvm/9.0.0_1/include")
    if(CMAKE_C_COMPILER_ID MATCHES "Clang")
        set(OpenMP_C "${CMAKE_C_COMPILER}")
        set(OpenMP_C_FLAGS "-fopenmp=libomp -Wno-unused-command-line-argument")
        set(OpenMP_C_LIB_NAMES "libomp" "libgomp" "libiomp5")
        set(OpenMP_libomp_LIBRARY ${OpenMP_C_LIB_NAMES})
        set(OpenMP_libgomp_LIBRARY ${OpenMP_C_LIB_NAMES})
        set(OpenMP_libiomp5_LIBRARY ${OpenMP_C_LIB_NAMES})
    endif()
    if(CMAKE_CXX_COMPILER_ID MATCHES "Clang")
        set(OpenMP_CXX "${CMAKE_CXX_COMPILER}")
        set(OpenMP_CXX_FLAGS "-fopenmp=libomp -Wno-unused-command-line-argument")
        set(OpenMP_CXX_LIB_NAMES "libomp" "libgomp" "libiomp5")
        set(OpenMP_libomp_LIBRARY ${OpenMP_CXX_LIB_NAMES})
        set(OpenMP_libgomp_LIBRARY ${OpenMP_CXX_LIB_NAMES})
        set(OpenMP_libiomp5_LIBRARY ${OpenMP_CXX_LIB_NAMES})
    endif()
endif()

find_package(OpenMP REQUIRED)
if (OPENMP_FOUND)
    if(APPLE)
        include_directories("${OPENMP_INCLUDES}")
        link_directories("${OPENMP_LIBRARIES}")
    endif()
    set (CMAKE_C_FLAGS "${CMAKE_C_FLAGS} ${OpenMP_C_FLAGS}")
    set (CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} ${OpenMP_CXX_FLAGS}")
    # set (CMAKE_EXE_LINKER_FLAGS "${CMAKE_EXE_LINKER_FLAGS} ${OpenMP_EXE_LINKER_FLAGS}")
endif(OPENMP_FOUND)

find_package(MPI REQUIRED)
include_directories(${MPI_INCLUDE_PATH})

SET(CMAKE_CXX_COMPILER mpicxx)
SET(CMAKE_C_COMPILER  mpicc)

add_executable(OpenMP_MPI main.cpp)

Answer 1

Thus my first question is how can I use multiple processors, just like using "mpirun -np 4 xxxx" from terminal?

When you compile an MPI program, the number of processes does not affect your compilation options. So, your sample code will work on multiple processes as mentioned by yourself:

$ mpirun -np 4 xxxx

To do that in CLion, go to Run/Debug configurations, and add mpirun as your executable, and -np 4 xxxx as your program arguments, as shown here.

But there will be some errors about -fopenmp flag:

I know that compiling an openmp program on Mac using clang is not that straightforward. You may take a look at this answer for more information.