# MPI BSP Process Group

Contents

- Introduction
- Where Defined
- Reference

# Introduction

The MPI `mpi_process_group` is an implementation of the process group interface using the Message Passing Interface (MPI). It is the primary process group used in the Parallel BGL at this time.

# Where Defined

Header `<boost/graph/distributed/mpi_process_group.hpp>`

# Reference

    namespace boost { namespace graph { namespace distributed {

    class mpi_process_group
    {
    public:
      typedef boost::mpi::communicator communicator_type;

      // Process group constructors
      mpi_process_group(communicator_type comm = communicator_type());
      mpi_process_group(std::size_t num_headers, std::size_t buffer_size,
                        communicator_type comm = communicator_type());

      mpi_process_group();
      mpi_process_group(const mpi_process_group&, boost::parallel::attach_distributed_object);

      // Triggers
      template<typename Type, typename Handler>
        void trigger(int tag, const Handler& handler);

      template<typename Type, typename Handler>
        void trigger_with_reply(int tag, const Handler& handler);

      trigger_receive_context trigger_context() const;

      // Helper operations
      void poll();
      mpi_process_group base() const;
    };

    // Process query
    int process_id(const mpi_process_group&);
    int num_processes(const mpi_process_group&);

    // Message transmission
    template<typename T>
      void send(const mpi_process_group& pg, int dest, int tag, const T& value);

    template<typename T>
      void receive(const mpi_process_group& pg, int source, int tag, T& value);

    optional<std::pair<int, int> > probe(const mpi_process_group& pg);

    // Synchronization
    void synchronize(const mpi_process_group& pg);

    // Out-of-band communication
    template<typename T>
      void send_oob(const mpi_process_group& pg, int dest, int tag, const T& value);

    template<typename T, typename U>
      void
      send_oob_with_reply(const mpi_process_group& pg, int dest, int
                          tag, const T& send_value, U& receive_value);

    template<typename T>
      void receive_oob(const mpi_process_group& pg, int source, int tag, T& value);

    } } }

Since the `mpi_process_group` is an implementation of the process group interface, we omit the description of most of the functions in the prototype. Two constructors need special mentioning:

    mpi_process_group(communicator_type comm = communicator_type());

The constructor can take an optional MPI communicator. As default a communicator constructed from MPI_COMM_WORLD is used.

    mpi_process_group(std::size_t num_headers, std::size_t buffer_size,
                      communicator_type comm = communicator_type());

For performance fine tuning the maximum number of headers in a message batch (num_headers) and the maximum combined size of batched messages (buffer_size) can be specified. The maximum message size of a batch is 16*num_headers+buffer_size. Sensible default values have been found by optimizing a typical application on a cluster with Ethernet network, and are num_header=64k and buffer_size=1MB, for a total maximum batches message size of 2MB.

Copyright (C) 2007 Douglas Gregor

Copyright (C) 2007 Matthias Troyer