[section:point_to_point Point-to-Point communication]

[section:blocking Blocking communication]

As a message passing library, MPI's primary purpose is to routine
messages from one process to another, i.e., point-to-point. MPI
contains routines that can send messages, receive messages, and query
whether messages are available. Each message has a source process, a
target process, a tag, and a payload containing arbitrary data. The
source and target processes are the ranks of the sender and receiver
of the message, respectively. Tags are integers that allow the
receiver to distinguish between different messages coming from the
same sender. 

The following program uses two MPI processes to write "Hello, world!"
to the screen (`hello_world.cpp`):

  #include <boost/mpi.hpp>
  #include <iostream>
  #include <string>
  #include <boost/serialization/string.hpp>
  namespace mpi = boost::mpi;

  int main() 
  {
    mpi::environment env;
    mpi::communicator world;

    if (world.rank() == 0) {
      world.send(1, 0, std::string("Hello"));
      std::string msg;
      world.recv(1, 1, msg);
      std::cout << msg << "!" << std::endl;
    } else {
      std::string msg;
      world.recv(0, 0, msg);
      std::cout << msg << ", ";
      std::cout.flush();
      world.send(0, 1, std::string("world"));
    }

    return 0;
  }

The first processor (rank 0) passes the message "Hello" to the second
processor (rank 1) using tag 0. The second processor prints the string
it receives, along with a comma, then passes the message "world" back
to processor 0 with a different tag. The first processor then writes
this message with the "!" and exits. All sends are accomplished with
the [memberref boost::mpi::communicator::send
communicator::send] method and all receives use a corresponding
[memberref boost::mpi::communicator::recv
communicator::recv] call.

[endsect:blocking]

[section:nonblocking Non-blocking communication]

The default MPI communication operations--`send` and `recv`--may have
to wait until the entire transmission is completed before they can
return. Sometimes this *blocking* behavior has a negative impact on
performance, because the sender could be performing useful computation
while it is waiting for the transmission to occur. More important,
however, are the cases where several communication operations must
occur simultaneously, e.g., a process will both send and receive at
the same time.

Let's revisit our "Hello, world!" program from the previous
[link mpi.tutorial.point_to_point.blocking section].
The core of this program transmits two messages:

    if (world.rank() == 0) {
      world.send(1, 0, std::string("Hello"));
      std::string msg;
      world.recv(1, 1, msg);
      std::cout << msg << "!" << std::endl;
    } else {
      std::string msg;
      world.recv(0, 0, msg);
      std::cout << msg << ", ";
      std::cout.flush();
      world.send(0, 1, std::string("world"));
    }

The first process passes a message to the second process, then
prepares to receive a message. The second process does the send and
receive in the opposite order. However, this sequence of events is
just that--a *sequence*--meaning that there is essentially no
parallelism. We can use non-blocking communication to ensure that the
two messages are transmitted simultaneously
(`hello_world_nonblocking.cpp`): 

  #include <boost/mpi.hpp>
  #include <iostream>
  #include <string>
  #include <boost/serialization/string.hpp>
  namespace mpi = boost::mpi;

  int main() 
  {
    mpi::environment env;
    mpi::communicator world;

    if (world.rank() == 0) {
      mpi::request reqs[2];
      std::string msg, out_msg = "Hello";
      reqs[0] = world.isend(1, 0, out_msg);
      reqs[1] = world.irecv(1, 1, msg);
      mpi::wait_all(reqs, reqs + 2);
      std::cout << msg << "!" << std::endl;
    } else {
      mpi::request reqs[2];
      std::string msg, out_msg = "world";
      reqs[0] = world.isend(0, 1, out_msg);
      reqs[1] = world.irecv(0, 0, msg);
      mpi::wait_all(reqs, reqs + 2);
      std::cout << msg << ", ";
    }

    return 0;
  }

We have replaced calls to the [memberref
boost::mpi::communicator::send communicator::send] and
[memberref boost::mpi::communicator::recv
communicator::recv] members with similar calls to their non-blocking
counterparts, [memberref boost::mpi::communicator::isend
communicator::isend] and [memberref
boost::mpi::communicator::irecv communicator::irecv]. The
prefix *i* indicates that the operations return immediately with a
[classref boost::mpi::request mpi::request] object, which
allows one to query the status of a communication request (see the
[memberref boost::mpi::request::test test] method) or wait
until it has completed (see the [memberref
boost::mpi::request::wait wait] method). Multiple requests
can be completed at the same time with the [funcref
boost::mpi::wait_all wait_all] operation. 

[important Regarding communication completion/progress:
The MPI standard requires users to keep the request
handle for a non-blocking communication, and to call the "wait"
operation (or successfully test for completion) to complete the send
or receive.
Unlike most C MPI implementations, which allow the user to
discard the request for a non-blocking send, Boost.MPI requires the
user to call "wait" or "test", since the request object might contain
temporary buffers that have to be kept until the send is
completed.
Moreover, the MPI standard does not guarantee that the
receive makes any progress before a call to "wait" or "test", although
most implementations of the C MPI do allow receives to progress before
the call to "wait" or "test".
Boost.MPI, on the other hand, generally
requires "test" or "wait" calls to make progress.
More specifically, Boost.MPI  guarantee that calling "test" multiple time will
eventually complete the communication (this is due to the fact that serialized communication are potentially a multi step operation.). ]

If you run this program multiple times, you may see some strange
results: namely, some runs will produce:

  Hello, world!

while others will produce:

  world!
  Hello,

or even some garbled version of the letters in "Hello" and
"world". This indicates that there is some parallelism in the program,
because after both messages are (simultaneously) transmitted, both
processes will concurrent execute their print statements. For both
performance and correctness, non-blocking communication operations are
critical to many parallel applications using MPI.

[endsect:nonblocking]
[endsect:point_to_point]
