\section{SimGrid}
\label{sec:simgrid}
++<<<<<<< HEAD
+In the scope of this paper, we have chosen the SimGrid toolkit~\cite{SimGrid,casanova+giersch+legrand+al.2014.versatile} to simulate the behavior of parallel iterative linear solvers on different computational grid configurations. Contrary to most simulators which remain very application-oriented, the SimGrid framework is designed to study the behavior of many large-scale distributed computing platforms as Grids, Peer-to-Peer systems, Clouds or High Performance Computation systems. It is still actively developed by the scientific community and distributed as an open source software.
+
+SimGrid provides four user interfaces which can be convenient for different distributed applications~\cite{casanova+legrand+quinson.2008.simgrid}. In this paper we are interested in the SMPI user interface (Simulator MPI) which implements about \np[\%]{80} of the MPI 2.0 standard and allows minor modifications of the initial code~\cite{bedaride+degomme+genaud+al.2013.toward} (see Section~\ref{sec:04.02}). SMPI enables the direct simulation of the execution, as in real life, of an unmodified MPI distributed application, and gets accurate results with the detailed resources consumption.
+
+The SimGrid simulator uses at least three XML input files describing the computational grid resources: the number of clusters in the grid, the number of processors/cores in each cluster, the detailed description of the intra and inter networks and the list of the hosts in each cluster (see the details in Section~\ref{sec:expe}). SimGrid uses a fluid model to simulate the program execution. It allows several simulation modes which produce accurate results~\cite{bedaride+degomme+genaud+al.2013.toward,velho+schnorr+casanova+al.2013.validity}. For instance, the "in vivo" mode really executes the computation but "intercepts" the communications (the execution time is then evaluated according to the parameters of the simulated platform). It is also possible for SimGrid/SMPI to only keep the duration of large computations by skipping them. Moreover the application can be run in an "in vitro" mode by sharing some structures between the simulated processes and thus allowing the use of very large-scale data.
+
+The choice of SimGrid/SMPI as a simulator tool in this study has been emphasized by the results obtained by several studies to validate, in real environments, the behavior of different network models simulated in SimGrid~\cite{velho+schnorr+casanova+al.2013.validity}. Other studies underline the comparison between the real MPI application executions and the SimGrid/SMPI ones~\cite{guermouche+renard.2010.first,clauss+stillwell+genaud+al.2011.single,bedaride+degomme+genaud+al.2013.toward}. These works show the accuracy of SimGrid simulations compared to the executions on real physical architectures.
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++=======
++>>>>>>> e2082ae796c90fe550e1ad8e44f43f9d5bfedf95
+ In the scope of this paper, we have chosen the SimGrid
+ toolkit~\cite{SimGrid,casanova+legrand+quinson.2008.simgrid,casanova+giersch+legrand+al.2014.versatile}
+ to simulate the behavior of parallel iterative linear solvers on different
+ computational grid configurations. In opposite to most of the simulators which
+ are stayed very application-oriented, the SimGrid framework is designed to study
+ the behavior of many large-scale distributed computing platforms as Grids,
+ Peer-to-Peer systems, Clouds or High Performance Computation systems. It is
+ still actively developed by the scientific community and distributed as an open
+ source software.
+
+ SimGrid provides four user interfaces which can be convenient for different
+ distributed applications. In this paper we are interested on the SMPI
+ (Simulated MPI) user interface which implements about \np[\%]{80} of the MPI 2.0
+ standard~\cite{bedaride+degomme+genaud+al.2013.toward}, and allows minor
+ modifications of the initial code (see Section~\ref{sec:04.02}). SMPI enables
+ the direct simulation of the execution, as in the real life, of an unmodified
+ MPI distributed application, and gets accurate results with the detailed
+ resources consumption.
+
+ SimGrid simulator uses an XML input file describing the computational grid
+ resources: the number of clusters in the grid, the number of processors/cores in
+ each cluster, the detailed description of the intra and inter networks and the
+ list of the hosts in each cluster (see the details in
+ Section~\ref{sec:expe}). SimGrid employs a fluid model to simulate the use of
+ these resources along the program execution. This model produces accurate
+ results while still running relatively
+ fast~\cite{bedaride+degomme+genaud+al.2013.toward,velho+schnorr+casanova+al.2013.validity}.
+ During the simulation, the computation is really executed, but the commuications
+ are intercepted and their execution time evaluated according to the parameters
+ of the simulated platform. It is also possible for SimGrid/SMPI to only keep the
+ duration of large computations by skipping them. Moreover, when applicable, the
+ application can be run by sharing some in-memory structures between the
+ simulated processes and thus allowing the use of very large-scale data.
+
+ The choice of SimGrid/SMPI as a simulator tool in this study has been emphasized
+ by the results obtained by several studies to validate, in the real
+ environments, the behavior of different network models simulated in
+ SimGrid~\cite{velho+schnorr+casanova+al.2013.validity}. Other studies underline
+ the comparison between the real MPI application executions and the SimGrid/SMPI
+ ones~\cite{guermouche+renard.2010.first,clauss+stillwell+genaud+al.2011.single,bedaride+degomme+genaud+al.2013.toward}. These
+ works show the accuracy of SimGrid simulations compared to the executions on
+ real physical architectures.
%% In the scope of this paper, the SimGrid toolkit~\cite{SimGrid,casanova+legrand+quinson.2008.simgrid,casanova+giersch+legrand+al.2014.versatile},
%% an open source framework actively developed by its scientific community, has been chosen to simulate the behavior of iterative linear solvers in different computational grid configurations. SimGrid pretends to be non-specialized in opposite to some other simulators which stayed to be very specific oriented-application. One of the well-known SimGrid advantage is its SMPI (Simulated MPI) user interface. SMPI purpose is to execute by simulation in a similar way as in real life, an MPI distributed application and to get accurate results with the detailed resources
One of our objectives when simulating the application in SimGrid is, as in real
life, to get accurate results (solutions of the problem) but also to ensure the
-test reproducibility under the same conditions. According to our experience,
+test reproducibility under similar conditions. According to our experience,
very few modifications are required to adapt a MPI program for the SimGrid
- simulator using SMPI (Simulator MPI). The first modification is to include SMPI
+ simulator using SMPI (Simulated MPI). The first modification is to include SMPI
libraries and related header files (\verb+smpi.h+). The second modification is to
suppress all global variables by replacing them with local variables or using a
SimGrid selector called "runtime automatic switching"
