\section{Introduction}\label{sec:intro}

The MLD2P4 library provides ....


\subsection{Programming model}

The MLD2P4 librarary is based on the Single Program Multiple Data
(SPMD) programming model: each process participating in the
computation performs the same actions on a chunk of data. Parallelism
is thus data-driven. 

Because of this structure, many subroutines coordinate their action
across the various processes, thus providing an implicit
synchronization point, and therefore \emph{must} be
called simultaneously by all processes participating in the
computation. 
However there are many cases where no synchronization, and indeed no
communication among processes, is implied. 

Throughout this user's guide each subroutine will be clearly indicated
as:
\begin{description}
\item[Synchronous:] must be called simultaneously by all the
  processes in the relevant communication context;
\item[Asynchronous:] may be called in a totally independent manner.
\end{description}

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