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## Getting started

Getting started with Boost.MPI requires a working MPI implementation, a recent version of Boost, and some configuration information.

### MPI Implementation

To get started with Boost.MPI, you will first need a working MPI implementation. There are many conforming MPI implementations available. Boost.MPI should work with any of the implementations, although it has only been tested extensively with:

- Open MPI
- MPICH2
- Intel MPI

You can test your implementation using the following simple program, which passes a message from one processor to another. Each processor prints a message to standard output.

    #include <mpi.h>
    #include <iostream>

    int main(int argc, char* argv[])
    {
      MPI_Init(&argc, &argv);

      int rank;
      MPI_Comm_rank(MPI_COMM_WORLD, &rank);
      if (rank == 0) {
        int value = 17;
        int result = MPI_Send(&value, 1, MPI_INT, 1, 0, MPI_COMM_WORLD);
        if (result == MPI_SUCCESS)
          std::cout << "Rank 0 OK!" << std::endl;
      } else if (rank == 1) {
        int value;
        int result = MPI_Recv(&value, 1, MPI_INT, 0, 0, MPI_COMM_WORLD,
    			  MPI_STATUS_IGNORE);
        if (result == MPI_SUCCESS && value == 17)
          std::cout << "Rank 1 OK!" << std::endl;
      }
      MPI_Finalize();
      return 0;
    }

You should compile and run this program on two processors. To do this, consult the documentation for your MPI implementation. With OpenMPI, for instance, you compile with the `mpiCC` or `mpic++` compiler, boot the LAM/MPI daemon, and run your program via `mpirun`. For instance, if your program is called `mpi-test.cpp`, use the following commands:

    mpiCC -o mpi-test mpi-test.cpp
    lamboot
    mpirun -np 2 ./mpi-test
    lamhalt

When you run this program, you will see both `Rank 0 OK!` and `Rank 1 OK!` printed to the screen. However, they may be printed in any order and may even overlap each other. The following output is perfectly legitimate for this MPI program:

    Rank Rank 1 OK!
    0 OK!

If your output looks something like the above, your MPI implementation appears to be working with a C++ compiler and we're ready to move on.

### Configure and Build

As the rest of Boost, Boost.MPI uses version 2 of the Boost.Build system for configuring and building the library binary.

Please refer to the general Boost installation instructions for Unix Variant (including Unix, Linux and MacOS) or Windows. The simplified build instructions should apply on most platforms with a few specific modifications described below.

#### Bootstrap

As explained in the boost installation instructions, running the bootstrap (`./bootstrap.sh` for unix variants or `bootstrap.bat` for Windows) from the boost root directory will produce a 'project-config.jam` file. You need to edit that file and add the following line:

    using mpi ;

Alternatively, you can explicitly provide the list of Boost libraries you want to build. Please refer to the `--help` option of the `bootstrap` script.

#### Setting up your MPI Implementation

First, you need to scan the `include/boost/mpi/config.hpp` file and check if some settings need to be modified for your MPI implementation or preferences.

In particular, the `BOOST_MPI_HOMOGENEOUS` macro, that you will need to comment out if you plan to run on a heterogeneous set of machines. See the optimization notes below.

Most MPI implementations require specific compilation and link options. In order to mask theses details to the user, most MPI implementations provide wrappers which silently pass those options to the compiler.

Depending on your MPI implementation, some work might be needed to tell Boost which specific MPI option to use. This is done through the `using mpi ;` directive in the `project-config.jam` file those general form is (do not forget to leave spaces around : and before ;):

    using mpi
       : [<MPI compiler wrapper>]
       : [<compilation and link options>]
       : [<mpi runner>] ;

Depending on your installation and MPI distribution, the build system might be able to find all the required informations and you just need to specify:

    using mpi ;

##### Trouble shooting

Most of the time, specially with production HPC clusters, some work will need to be done.

Here is a list of the most common issues and suggestions on how to fix those.

- Your wrapper is not in your path or does ot have a standard name

You will need to tell the build system how to call it using the first parameter:

    using mpi : /opt/mpi/bullxmpi/1.2.8.3/bin/mpicc ;

| Warning
Boost.MPI only uses the C interface, so specifying the C wrapper should be enough. But some implementations will insist on importing the C++ bindings.

- Your wrapper is really eccentric or does not exist

With some implementations, or with some specific integration[9] you will need to provide the compilation and link options through de second parameter using 'jam' directives. The following type configuration used to be required for some specific Intel MPI implementation (in such a case, the name of the wrapper can be left blank):

    using mpi : mpiicc :
          <library-path>/softs/intel/impi/5.0.1.035/intel64/lib
          <library-path>/softs/intel/impi/5.0.1.035/intel64/lib/release_mt
          <include>/softs/intel/impi/5.0.1.035/intel64/include
          <find-shared-library>mpifort
          <find-shared-library>mpi_mt
          <find-shared-library>mpigi
          <find-shared-library>dl
          <find-shared-library>rt ;

As a convenience, MPI wrappers usually have an option that provides the required informations, which usually starts with `--show`. You can use those to find out the requested jam directive:

    $ mpiicc -show
    icc -I/softs/.../include ... -L/softs/.../lib ... -Xlinker -rpath -Xlinker /softs/.../lib .... -lmpi -ldl -lrt -lpthread
    $

    $ mpicc --showme
    icc -I/opt/.../include -pthread -L/opt/.../lib -lmpi -ldl -lm -lnuma -Wl,--export-dynamic -lrt -lnsl -lutil -lm -ldl
    $ mpicc --showme:compile
    -I/opt/mpi/bullxmpi/1.2.8.3/include -pthread
    $ mpicc --showme:link
    -pthread -L/opt/.../lib -lmpi -ldl -lm -lnuma -Wl,--export-dynamic -lrt -lnsl -lutil -lm -ldl
    $

To see the results of MPI auto-detection, pass `--debug-configuration` on the bjam command line.

- The launch syntax cannot be detected

| Note
This is only used when running the tests.

If you need to use a special command to launch an MPI program, you will need to specify it through the third parameter of the `using mpi` directive.

So, assuming you launch the `all_gather_test` program with:

    $mpiexec.hydra -np 4 all_gather_test

The directive will look like:

    using mpi : mpiicc :
         [<compilation and link options>]
     : mpiexec.hydra -n  ;

#### Build

To build the whole Boost distribution:

    $cd <boost distribution>
    $./b2

To build the Boost.MPI library and dependancies:

    $cd <boost distribution>/lib/mpi/build
    $../../../b2

#### Tests

You can run the regression tests with:

    $cd <boost distribution>/lib/mpi/test
    $../../../b2

#### Installation

To install the whole Boost distribution:

    $cd <boost distribution>
    $./b2 install

### Using Boost.MPI

To build applications based on Boost.MPI, compile and link them as you normally would for MPI programs, but remember to link against the `boost_mpi` and `boost_serialization` libraries, e.g.,

    mpic++ -I/path/to/boost/mpi my_application.cpp -Llibdir \
      -lboost_mpi -lboost_serialization

If you plan to use the Python bindings for Boost.MPI in conjunction with the C++ Boost.MPI, you will also need to link against the boost_mpi_python library, e.g., by adding `-lboost_mpi_python-gcc` to your link command. This step will only be necessary if you intend to register C++ types or use the skeleton/content mechanism from within Python.

[9] Some HPC cluster will insist that the users uss theirs own in house interface to the MPI system.