2 An Overview of MPI

  MPI is intended to be a standard message passing interface for applications and libraries running on concurrent computers with a logically distributed memory. MPI is not specifically designed for use by parallelizing compilers. MPI provides no explicit support for multithreading, although one of the design goals of MPI was to ensure that it can be implemented efficiently in a multithreaded environment. The MPI standard does not mandate that an implementation should be interoperable with other MPI implementations. However, MPI does provide to message passing routines all the information needed to allow a single MPI implementation to operate in a heterogeneous environment.

The main features of MPI are as follows:

• A set of routines that support point-to-point communication between pairs of processes. Blocking and nonblocking versions of the routines are provided which may be used in four different communication modes. As discussed in Section 4.1, these modes correspond to different communication protocols. Message selectivity in point-to-point communication is by source process and message tag, each of which may be wildcarded to indicate that any valid value is acceptable.
• The communicator abstraction that provides support for the design of safe, modular parallel software libraries.
• General, or derived, datatypes, that permit the specification of messages of noncontiguous data of differing datatypes.
• Application topologies that specify the logical layout of processes. A common example is a Cartesian grid which is often used in two and three-dimensional problems.
• A rich set of collective communication routines that perform coordinated communication among a set of processes.

In MPI there is no mechanism for creating processes, and an MPI program is parallel ab initio, i.e., there is a fixed number of processes from the start to the end of an application program. All processes are members of at least one process group. Initially all processes are members of the same group, and a number of routines are provided that allow an application to create (and destroy) new subgroups. Within a group each process is assigned a unique rank in the range 0 to n-1, where n is the number of processes in the group. This rank is used to identify a process, and, in particular, is used to specify the source and destination processes in a point-to-point communication operation, and the root process in certain collective communication operations.

MPI was designed as a message passing interface rather than a complete parallel programming environment, and in its current form intentionally omits many desirable features. For example, MPI lacks

• mechanisms for process creation and control;
• one-sided communication operations that would permit put and get messages, and active messages;
• nonblocking collective communication operations, and the ability for a collective communication operation to involve more than one group;
• language bindings for Fortran 90 and C++.

These issues, and other possible extensions to MPI, are currently being considered in the MPI-2 effort. Extensions to MPI for performing parallel I/O are also under consideration.