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Cominations of elementary datatypes into a derived user defined datatype allows clean communication of collections of disparate types in a single MPI call.
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Elimination of length (in bytes) in favor of count (of items of a given type) is clearer
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Specifying application-oriented layouts allows maximal use of special hardware and optimized memory use
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However this wonderful technology is problematical in Java where layout of data structures in memory is not defined in most cases
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Java's serialization subsumes user defined datatypes as a general way of packing a class of disparate types into a message that can be sent between heterogeneous computers
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