Signature Datatypes for Type Correct Collective Operations, Revisited

In order to provide for type correct implementations of applications in MPI that use derived datatypes to describe complex and possibly heterogeneous data layouts, signature datatypes describing the sequence of basic datatypes comprising the complex data layout in a compact manner have often been proposed and used to communicate and store such data in a type correct way. Signature datatypes are particularly useful in implementations of algorithms for collective communication employing pipelining and/or message-combining. We (re)examine the properties that signature datatypes must fulfill, and the properties of the MPI collective interfaces that guarantee the existence of proper signature datatypes. The analysis reveals that MPI_Alltoallw does not have the property, and thus that certain non-trivial, type correct implementations of this operation are not easily possible within MPI itself. We observe that the signature datatype for any derived datatype can be computed in O(n) operations in the number of elements n described by the derived datatype. While this improves on certain earlier approaches, this is still not a satisfactory solution for the cases where large layouts are described by small, derived datatypes. We explain how signature type computation is implemented in a library for advanced datatype programming.