Review, for a moment, MPI's basic datatypes, shown below:

• MPI basic predefined datatypes for C

  MPI Datatype	C datatype
  MPI_CHAR	signed char
  MPI_DOUBLE	double
  MPI_FLOAT	float
  MPI_INT	signed int
  MPI_LONG	signed long int
  MPI_LONG_DOUBLE	long double
  MPI_LONG_LONG_INT	signed long long int
  MPI_SHORT	signed short int
  MPI_UNSIGNED	unsigned int
  MPI_UNSIGNED_CHAR	unsigned char
  MPI_UNSIGNED_LONG	unsigned long int
  MPI_UNSIGNED_SHORT	unsigned short int
  MPI_BYTE
  MPI_PACKED

• MPI basic predefined datatypes for FORTRAN

  MPI Datatype	FORTRAN Datatype
  MPI_CHARACTER	CHARACTER(1)
  MPI_COMPLEX	COMPLEX
  MPI_DOUBLE_COMPLEX	DOUBLE COMPLEX
  MPI_DOUBLE_PRECISION	DOUBLE PRECISION
  MPI_INTEGER	INTEGER
  MPI_INTEGER1	INTEGER*1
  MPI_INTEGER2	INTEGER*2
  MPI_INTEGER4	INTEGER*4
  MPI_LOGICAL	LOGICAL
  MPI_REAL	REAL
  MPI_REAL2	REAL*2
  MPI_REAL4	REAL*4
  MPI_REAL8	REAL*8
  MPI_BYTE
  MPI_PACKED

Given these datatypes and a count, you can handle messages of contiguous data of the same type.

Motivation

• non-contiguous data of a single type?
• contiguous data of mixed types?
• non-contiguous data of mixed types?

A few possible solutions suggest themselves:

• You could make multiple MPI calls to send and receive each data element in turn.

• You could copy the data to a buffer before sending it. One of the two MPI-specific basic datatypes, MPI_PACKED, could be used to send data that has been explicitly packed, or receive data that will be explicitly unpacked.

• You could use MPI_BYTE to get around the datatype-matching rules. Like MPI_PACKED, MPI_BYTE (the other MPI-specific basic datatype) can be used to match any byte of storage (on a byte-addressable machine), irrespective of the datatype of the variable that contains this byte.

Generally, however, these solutions are slow, clumsy, and wasteful of memory. Using MPI_BYTE or MPI_PACKED might also result in a program that isn't portable to a heterogeneous system of machines.

The idea of MPI derived datatypes is to provide a portable and efficient way of communicating non-contiguous or mixed types in a message. MPI derived datatypes provide a simpler, cleaner, more elegant and efficient way to handle this type of data, which is common. While you can get along without derived datatypes, you couldn't do so easily.

[ Caution: some implementations of MPI derived datatypes are not as efficient as they should be, particularly in terms of I/O wait times. Once you have your code running correctly, you may want to convert a few of the derived datatypes in your code's hotspots to MPI_BYTE or MPI_PACKED. Compare the timings of your converted code to that of your code using all MPI derived datatypes, and choose the one that gives you optimal performance. ]