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% *======================================================================*
% Cactus Thorn template for ThornGuide documentation
% Author: Ian Kelley
% Date: Sun Jun 02, 2002
% $Header$
%
% Thorn documentation in the latex file doc/documentation.tex
% will be included in ThornGuides built with the Cactus make system.
% The scripts employed by the make system automatically include
% pages about variables, parameters and scheduling parsed from the
% relevant thorn CCL files.
%
% This template contains guidelines which help to assure that your
% documentation will be correctly added to ThornGuides. More
% information is available in the Cactus UsersGuide.
%
% Guidelines:
% - Do not change anything before the line
% % START CACTUS THORNGUIDE",
% except for filling in the title, author, date, etc. fields.
% - Each of these fields should only be on ONE line.
% - Author names should be separated with a \\ or a comma.
% - You can define your own macros, but they must appear after
% the START CACTUS THORNGUIDE line, and must not redefine standard
% latex commands.
% - To avoid name clashes with other thorns, 'labels', 'citations',
% 'references', and 'image' names should conform to the following
% convention:
% ARRANGEMENT_THORN_LABEL
% For example, an image wave.eps in the arrangement CactusWave and
% thorn WaveToyC should be renamed to CactusWave_WaveToyC_wave.eps
% - Graphics should only be included using the graphicx package.
% More specifically, with the "\includegraphics" command. Do
% not specify any graphic file extensions in your .tex file. This
% will allow us to create a PDF version of the ThornGuide
% via pdflatex.
% - References should be included with the latex "\bibitem" command.
% - Use \begin{abstract}...\end{abstract} instead of \abstract{...}
% - Do not use \appendix, instead include any appendices you need as
% standard sections.
% - For the benefit of our Perl scripts, and for future extensions,
% please use simple latex.
%
% *======================================================================*
%
% Example of including a graphic image:
% \begin{figure}[ht]
% \begin{center}
% \includegraphics[width=6cm]{MyArrangement_MyThorn_MyFigure}
% \end{center}
% \caption{Illustration of this and that}
% \label{MyArrangement_MyThorn_MyLabel}
% \end{figure}
%
% Example of using a label:
% \label{MyArrangement_MyThorn_MyLabel}
%
% Example of a citation:
% \cite{MyArrangement_MyThorn_Author99}
%
% Example of including a reference
% \bibitem{MyArrangement_MyThorn_Author99}
% {J. Author, {\em The Title of the Book, Journal, or periodical}, 1 (1999),
% 1--16. {\tt http://www.nowhere.com/}}
%
% *======================================================================*
% If you are using CVS use this line to give version information
% $Header$
\documentclass{article}
% Use the Cactus ThornGuide style file
% (Automatically used from Cactus distribution, if you have a
% thorn without the Cactus Flesh download this from the Cactus
% homepage at www.cactuscode.org)
\usepackage{../../../../doc/latex/cactus}
\begin{document}
% The author of the documentation
\author{Erik Schnetter \textless eschnetter@perimeterinstitute.ca\textgreater}
% The title of the document (not necessarily the name of the Thorn)
\title{OpenCL}
% the date your document was last changed, if your document is in CVS,
% please use:
% \date{$ $Date: 2004-01-07 14:12:39 -0600 (Wed, 07 Jan 2004) $ $}
\date{May 9 2012}
\maketitle
% Do not delete next line
% START CACTUS THORNGUIDE
% Add all definitions used in this documentation here
% \def\mydef etc
% Add an abstract for this thorn's documentation
\begin{abstract}
\emph{OpenCL} is a programming standard for heterogeneous systems,
i.e.\ for programming CPUs, GPUs, and other types of accelerators.
OpenCL is implemented as a library, and OpenCL codes are compiled at
run time by passing OpenCL routines, as strings, to the OpenCL
library. This is different e.g.\ from \texttt{CUDA}, which is
implemented as a language such as C or C++.
This thorn \texttt{OpenCL} provides the configuration bits that
ensure that Cactus applications can use OpenCL libraries.
\end{abstract}
\section{OpenCL Programming}
OpenCL is very similar to C\@. However, it differs from C in several
key aspects:
\begin{itemize}
\item much smaller run-time library, consisting mostly of mathematical
functions (such as sqrt) and printf;
\item built-in support for fine-grained and coarse-grainded
multi-threading;
\item built-in support for vectorisation.
\end{itemize}
Given this, it is not possible to write a whole application in
OpenCL\@. Instead, only the expensive parts (so-called \emph{compute
kernels}) are written in OpenCL, and are launched e.g.\ from C or
C++.
In addition, the hardware architecture of GPUs and other accelerators
differs from CPUs in one key aspect:
\begin{itemize}
\item memory is separate from the host (regular CPU) memory.
\end{itemize}
That means that one has to explicitly copy data between the host
memory and the device memory before and/or after calling compute
kernels.
\section{OpenCL Programming in Cactus}
Cactus supports OpenCL programming at several levels. At the lowest
level, one can use this thorn \texttt{OpenCL} directly. While this
works fine, it is somewhat tedious because one has to write a certain
amount of boilerplate code to detect and initialise the device, to
copy data between host and device, and to build and run compute
kernels.
Since OpenCL is implemented as a library, the flesh knows only little
about OpenCL\@. For example, there are no configuration options to
spedify an OpenCL compiler, since code is compiled at run time via a
library call to which the source code is passed as string. There is,
however, one way in which the flesh supports OpenCL: Files with a
\texttt{.cl} suffix are converted into a string and placed into the
executable. These strings have the type \texttt{char~const~*} in C,
and can be accessed at run time under a (globally visible) name
\texttt{OpenCL\_source\_THORN\_FILE}, where \texttt{THORN} and
\texttt{FILE} and are the thorn name and file name, respectively.
(This is also explained in the users' guide.)
\begin{thebibliography}{9}
\end{thebibliography}
% Do not delete next line
% END CACTUS THORNGUIDE
\end{document}
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