To permit easy and efficient access to non-contiguous regions in memory for communication and I/O the message passing interface offers nested datatypes. Since nested datatypes can be very complicated, the understanding of non-contiguous access patterns and the debugging of wrongly accessed memory regions is hard for the developer.

HDTrace is an environment which allows to trace the behavior of MPI programs and to simulate them for arbitrary virtual cluster configuration. It is designed to record all MPI parameters including MPI datatypes. In this paper we present the capabilities to visualize usage of derived datatypes for communication and I/O accesses – a simple hierarchical view is introduced which presents them in a compact form and allows to dig into the nested datatypes. File regions accessed in noncontiguous I/O calls can be visualized in terms of the original datatype.

The presented feature assists developers in understanding the datatype layout and spatial I/O access patterns of their application.