Introduction

The MPI communication mechanisms introduced in the previous chapter allows one to send or receive a sequence of identical elements that are contiguous in memory. It is often desirable to send data that is not homogeneous, such as a structure, or that is not contiguous in memory, such as an array section. This allows one to amortize the fixed overhead of sending and receiving a message over the transmittal of many elements, even in these more general circumstances. MPI provides two mechanisms to achieve this.

• The user can define derived datatypes, that specify more general data layouts. User-defined datatypes can be used in MPI communication functions, in place of the basic, predefined datatypes. derived datatype
• A sending process can explicitly pack noncontiguous data into a contiguous buffer, and next send it; a receiving process can explicitly unpack data received in a contiguous buffer and store in noncontiguous locations. packunpack

The construction and use of derived datatypes is described in Section -. The use of Pack and Unpack functions is described in Section . It is often possible to achieve the same data transfer using either mechanisms. We discuss the pros and cons of each approach at the end of this chapter.