= Offloading computational tasks of hybrid MPI + OpenMP/OmpSs-2 applications to GPUs =

Table of contents:
* [#QuickOverview Quick Overview]
* Examples:
  * [#NBodyBenchmark NBody Benchmark]

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= Quick Overview =

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= NBody Benchmark =

Users can clone or download this examples from the https://pm.bsc.es/gitlab/DEEP-EST/apps/NBody 
repository and transfer it to a DEEP working directory.


== Description
An NBody simulation numerically approximates the evolution of a system of
bodies in which each body continuously interacts with every other body.  A
familiar example is an astrophysical simulation in which each body represents a
galaxy or an individual star, and the bodies attract each other through the
gravitational force.

N-body simulation arises in many other computational science problems as well.
For example, protein folding is studied using N-body simulation to calculate
electrostatic and ''Van der Waals'' forces. Turbulent fluid flow simulation and
global illumination computation in computer graphics are other examples of
problems that use NBody simulation.

== Requirements
The requirements of this application are shown in the following lists. The main requirements are:

  * '''GNU Compiler Collection'''.
  * '''!OmpSs-2''': !OmpSs-2 is the second generation of the '''!OmpSs''' programming model. It is a task-based
    programming model originated from the ideas of the OpenMP and !StarSs programming models. The
    specification and user-guide are available at https://pm.bsc.es/ompss-2-docs/spec/ and
    https://pm.bsc.es/ompss-2-docs/user-guide/, respectively. !OmpSs-2 requires both '''Mercurium''' and
    '''Nanos6''' tools. Mercurium is a source-to-source compiler which provides the necessary support for
    transforming the high-level directives into a parallelized version of the application. The Nanos6
    runtime system library provides the services to manage all the parallelism in the application
    (e.g., task creation, synchronization, scheduling, etc). Downloads at https://github.com/bsc-pm.
  * '''Clang + LLVM OpenMP''' (derived):
  * '''MPI''': This application requires an MPI library supporting the multi-threading level of
    thread support.

In addition, there are some optional tools which enable the building of other application versions:

  * '''CUDA''' and NVIDIA '''Unified Memory''' devices: This application has CUDA variants in which some of
    the N-body kernels are executed on the available GPU devices.
  * '''Task-Aware MPI (TAMPI)''': The Task-Aware MPI library provides the interoperability mechanism
    for MPI and OpenMP/!OmpSs-2. Downloads and more information at https://github.com/bsc-pm/tampi.