Chapter

Recent Advances in Parallel Virtual Machine and Message Passing Interface

Volume 5205 of the series Lecture Notes in Computer Science pp 141-150

Constructing MPI Input-output Datatypes for Efficient Transpacking

  • Faisal Ghias MirAffiliated withNEC Laboratories Europe, NEC Europe Ltd.
  • , Jesper Larsson TräffAffiliated withNEC Laboratories Europe, NEC Europe Ltd.

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Abstract

Communication and file I/O buffers in MPI can contain contiguous as well as non-contiguous, structured data. To describe non-consecutive data layouts compactly, MPI provides a powerful concept of derived or user-defined datatypes. Especially for MPI-IO, where data are transferred between file and memory buffers, the need for copying between differently typed MPI buffers arise. A straightforward solution to this typed copy problem consists in packing and unpacking the differently structured data via an intermediate buffer. For a maximally efficient MPI(-IO) implementation, means of copying directly (without intermediate buffers) between differently typed MPI buffers are needed.

We present a new approach to the typed copy problem. For any two given MPI datatypes describing the layout of input and output buffer, respectively, we show how to efficiently construct an input-output type that subsumes both. This type is used to copy directly from input to output buffer by means of a special transpack function. By completely eliminating the need for intermediate buffering, the typed copy performance can in theory be improved by up to a factor of two, with only a modest overhead for constructing the input-output type. An experimental evaluation shows that even more significant improvements can be achieved in practice.