Data conversion
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One of the goals of MPI is to support parallel computations across heterogeneous environments. Communication in a heterogeneous environment may require data conversions. We use the following terminology.
- type conversion
- changes the datatype of a value, e.g., by rounding a REAL to an INTEGER.
- representation conversion
- changes the binary representation of a value, e.g., from Hex floating point to IEEE floating point.
The type matching rules imply that MPI communication never entails type conversion. On the other hand, MPI requires that a representation conversion be performed when a typed value is transferred across environments that use different representations for the datatype of this value. MPI does not specify rules for representation conversion. Such conversion is expected to preserve integer, logical or character values, and to convert a floating point value to the nearest value that can be represented on the target system.
Overflow and underflow exceptions may occur during floating point conversions. Conversion of integers or characters may also lead to exceptions when a value that can be represented in one system cannot be represented in the other system. An exception occurring during representation conversion results in a failure of the communication. An error occurs either in the send operation, or the receive operation, or both.
If a value sent in a message is untyped (i.e., of type MPI_BYTE), then the binary representation of the byte stored at the receiver is identical to the binary representation of the byte loaded at the sender. This holds true, whether sender and receiver run in the same or in distinct environments. No representation conversion is required. (Note that representation conversion may occur when values of type MPI_CHARACTER or MPI_CHAR are transferred, for example, from an EBCDIC encoding to an ASCII encoding.)
No conversion need occur when an MPI program executes in a homogeneous system, where all processes run in the same environment.
Consider the three examples, Type matching rules
--Type matching rules
.
The first program is correct, assuming that a and b are
REAL arrays of size 
.
If the sender and receiver execute in different environments,
then the ten real values that are fetched from the send buffer will
be converted to the representation for reals on the receiver site
before they are stored in the receive buffer. While the
number of real elements fetched from the send buffer equal the
number of real elements stored in the receive buffer, the number of
bytes stored need not equal the number of bytes loaded. For example, the
sender may use a four byte representation and the receiver an eight
byte representation for reals.
The second program is erroneous, and its behavior is undefined.
The third program is correct. The exact same sequence of forty bytes that were loaded from the send buffer will be stored in the receive buffer, even if sender and receiver run in a different environment. The message sent has exactly the same length (in bytes) and the same binary representation as the message received. If a and b are of different types, or if they are of the same type but different data representations are used, then the bits stored in the receive buffer may encode values that are different from the values they encoded in the send buffer.
Data representation conversion also applies to the envelope of a message: source, destination and tag are all integers that may need to be converted.
[] Advice
to implementors.
The current definition does not require messages to carry data type information. Both sender and receiver provide complete data type information. In a heterogeneous environment, one can either use a machine independent encoding such as XDR, or have the receiver convert from the sender representation to its own, or even have the sender do the conversion.
Additional type information might be added to messages
in order to allow the system to detect mismatches between
datatype at sender and receiver.
This might be particularly useful in a slower but safer debug mode.
( End of advice to implementors.)
MPI
does not require support for inter-language
communication. The behavior of a program is undefined if messages are sent
by a C process and received by a Fortran process, or vice-versa.
[] Rationale.
MPI does not handle inter-language communication because there are no
agreed standards for the correspondence between C types and
Fortran types. Therefore, MPI programs that mix languages would not port.
( End of rationale.)
[] Advice
to implementors.
MPI implementors may want to support inter-language communication by
allowing Fortran programs to use
``C MPI types,'' such as MPI_INT, MPI_CHAR,
etc., and allowing C programs to use Fortran types.
( End of advice to implementors.)
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