An Introduction to MPI-3 Shared Memory Programming

The Message Passing Interface (MPI) standard is a widely used programming interface for distributed memory systems. Hybrid parallel programming on many-core systems most often combines MPI with OpenMP*. This MPI/OpenMP approach uses an MPI model for communicating between nodes while utilizing groups of threads running on each computing node in order to take advantage of multicore/many-core architectures such as Intel® Xeon® processors and Intel® Xeon Phi™ processors.

The MPI-3 standard introduces another approach to hybrid programming that uses the new MPI Shared Memory (SHM) model.1 The MPI SHM model, supported by the Intel® MPI Library since version 5.0.2, enables changes to existing MPI codes incrementally in order to accelerate communication between processes on the shared-memory nodes.

In this article, we present a tutorial on how to start using MPI SHM on multinode systems using Intel Xeon with Intel Xeon Phi. The article uses a 1-D ring application as an example and includes code snippets to describe how to transform common MPI send/receive patterns to utilize the MPI SHM interface. The MPI functions that are necessary for internode and intranode communications will be described. A modified MPPTEST benchmark has been used to illustrate performance of the MPI SHM model with different synchronization mechanisms on Intel Xeon and Intel Xeon Phi based clusters. With the help of the Intel MPI Library, which implements the MPI-3 standard since version 5.0, we show that the shared memory approach produces significant performance advantages compared to the MPI send/receive model.

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