-%\documentclass[conference]{IEEEtran}
\documentclass[times]{cpeauth}
\usepackage{moreverb}
\algnewcommand\algorithmicoutput{\textbf{Output:}}
\algnewcommand\Output{\item[\algorithmicoutput]}
-\newcommand{\MI}{\mathit{MaxIter}}
+\newcommand{\TOLG}{\mathit{tol_{gmres}}}
+\newcommand{\MIG}{\mathit{maxit_{gmres}}}
+\newcommand{\TOLM}{\mathit{tol_{multi}}}
+\newcommand{\MIM}{\mathit{maxit_{multi}}}
+\newcommand{\TOLC}{\mathit{tol_{cgls}}}
+\newcommand{\MIC}{\mathit{maxit_{cgls}}}
\usepackage{array}
\usepackage{color, colortbl}
Email: \email{{raphael.couturier,arnaud.giersch,david.laiymani,charles.ramamonjisoa}@univ-fcomte.fr}
}
+%% Lilia Ziane Khodja: Department of Aerospace \& Mechanical Engineering\\ Non Linear Computational Mechanics\\ University of Liege\\ Liege, Belgium. Email: l.zianekhodja@ulg.ac.be
+
\begin{abstract}
-ABSTRACT
+ The behavior of multicore applications is always a challenge to predict, especially with a new architecture for which no experiment has been performed. With some applications, it is difficult, if not impossible, to build accurate performance models. That is why another solution is to use a simulation tool which allows us to change many parameters of the architecture (network bandwidth, latency, number of processors) and to simulate the execution of such applications. We have decided to use SimGrid as it enables to benchmark MPI applications.
+
+In this paper, we focus our attention on two parallel iterative algorithms based
+on the Multisplitting algorithm and we compare them to the GMRES algorithm.
+These algorithms are used to solve libear systems. Two different variantsof the Multisplitting are
+studied: one using synchronoous iterations and another one with asynchronous
+iterations. For each algorithm we have tested different parameters to see their
+influence. We strongly recommend people interested by investing into a new
+expensive hardware architecture to benchmark their applications using a
+simulation tool before.
+
+
+
+
\end{abstract}
-\keywords{Algorithm; distributed; iterative; asynchronous; simulation; simgrid; performance}
+%\keywords{Algorithm; distributed; iterative; asynchronous; simulation; simgrid; performance}
+\keywords{Multisplitting algorithms, Synchronous and asynchronous iterations, SimGrid, Simulation}
\maketitle
-\section{Introduction}
+\section{Introduction}
+The use of multi-core architectures for solving large scientific problems seems to become imperative in a lot of cases.
+Whatever the scale of these architectures (distributed clusters, computational grids, embedded multi-core,~\ldots) they are generally
+well adapted to execute complex parallel applications operating on a large amount of data. Unfortunately, users (industrials or scientists),
+who need such computational resources, may not have an easy access to such efficient architectures. The cost of using the platform and/or the cost of
+testing and deploying an application are often very important. So, in this context it is difficult to optimize a given application for a given
+architecture. In this way and in order to reduce the access cost to these computing resources it seems very interesting to use a simulation environment.
+The advantages are numerous: development life cycle, code debugging, ability to obtain results quickly,~\ldots at the condition that the simulation results are in education with the real ones.
+
+In this paper we focus on a class of highly efficient parallel algorithms called \emph{iterative algorithms}. The
+parallel scheme of iterative methods is quite simple. It generally involves the division of the problem
+into several \emph{blocks} that will be solved in parallel on multiple
+processing units. Each processing unit has to
+compute an iteration, to send/receive some data dependencies to/from
+its neighbors and to iterate this process until the convergence of
+the method. Several well-known methods demonstrate the convergence of these algorithms~\cite{BT89,bahi07}.
+In this processing mode a task cannot begin a new iteration while it
+has not received data dependencies from its neighbors. We say that the iteration computation follows a synchronous scheme.
+In the asynchronous scheme a task can compute a new iteration without having to
+wait for the data dependencies coming from its neighbors. Both
+communication and computations are asynchronous inducing that there is
+no more idle times, due to synchronizations, between two
+iterations~\cite{bcvc06:ij}. This model presents some advantages and drawbacks that we detail in section~\ref{sec:asynchro} but even if the number of iterations required to converge is generally greater than for the synchronous case, it appears that the asynchronous iterative scheme can significantly reduce overall execution
+times by suppressing idle times due to synchronizations~(see~\cite{bahi07} for more details).
+
+Nevertheless, in both cases (synchronous or asynchronous) it is very time consuming to find optimal configuration and deployment requirements
+for a given application on a given multi-core architecture. Finding good resource allocations policies under varying CPU power, network speeds and
+loads is very challenging and labor intensive~\cite{Calheiros:2011:CTM:1951445.1951450}. This problematic is even more difficult for the asynchronous scheme
+where variations of the parameters of the execution platform can lead to very different number of iterations required to converge and so to very different execution times.
+In this challenging context we think that the use of a simulation tool can greatly leverage the possibility of testing various platform scenarios.
+
+The main contribution of this paper is to show that the use of a simulation tool (i.e. the SimGrid toolkit~\cite{SimGrid}) in the context of real
+parallel applications (i.e. large linear system solvers) can help developers to better tune their application for a given multi-core architecture.
+To show the validity of this approach we first compare the simulated execution of the multisplitting algorithm with the GMRES (Generalized Minimal Residual) solver~\cite{saad86} in synchronous mode. The obtained results on different simulated multi-core architectures confirm the real results previously obtained on non simulated architectures.
+We also confirm the efficiency of the asynchronous multisplitting algorithm comparing to the synchronous GMRES. In this way and with a simple computing architecture (a laptop) SimGrid allows us to run a test campaign of a real parallel iterative applications on different simulated multi-core architectures.
+To our knowledge, there is no related work on the large-scale multi-core simulation of a real synchronous and asynchronous iterative application.
+
+This paper is organized as follows. Section~\ref{sec:asynchro} presents the iteration model we use and more particularly the asynchronous scheme.
+In section~\ref{sec:simgrid} the SimGrid simulation toolkit is presented. Section~\ref{sec:04} details the different solvers that we use.
+Finally our experimental results are presented in section~\ref{sec:expe} followed by some concluding remarks and perspectives.
+
\section{The asynchronous iteration model}
+\label{sec:asynchro}
\section{SimGrid}
+ \label{sec:simgrid}
+
+%%%%%%%%%%%%%%%%%%%%%%%%%
+%%%%%%%%%%%%%%%%%%%%%%%%%
+
+\section{Two-stage multisplitting methods}
+\label{sec:04}
+\subsection{Synchronous and asynchronous two-stage methods for sparse linear systems}
+\label{sec:04.01}
+In this paper we focus on two-stage multisplitting methods in their both versions synchronous and asynchronous~\cite{Frommer92,Szyld92,Bru95}. These iterative methods are based on multisplitting methods~\cite{O'leary85,White86,Alefeld97} and use two nested iterations: the outer iteration and the inner iteration. Let us consider the following sparse linear system of $n$ equations in $\mathbb{R}$
+\begin{equation}
+Ax=b,
+\label{eq:01}
+\end{equation}
+where $A$ is a sparse square and nonsingular matrix, $b$ is the right-hand side and $x$ is the solution of the system. Our work in this paper is restricted to the block Jacobi splitting method. This approach of multisplitting consists in partitioning the matrix $A$ into $L$ horizontal band matrices of order $\frac{n}{L}\times n$ without overlapping (i.e. sub-vectors $\{x_\ell\}_{1\leq\ell\leq L}$ are disjoint). The two-stage multisplitting methods solve the linear system~(\ref{eq:01}) iteratively as follows
+\begin{equation}
+x_\ell^{k+1} = A_{\ell\ell}^{-1}(b_\ell - \displaystyle\sum^{L}_{\substack{m=1\\m\neq\ell}}{A_{\ell m}x^k_m}),\mbox{~for~}\ell=1,\ldots,L\mbox{~and~}k=1,2,3,\ldots
+\label{eq:02}
+\end{equation}
+where $x_\ell$ are sub-vectors of the solution $x$, $b_\ell$ are the sub-vectors of the right-hand side $b$, and $A_{\ell\ell}$ and $A_{\ell m}$ are diagonal and off-diagonal blocks of matrix $A$ respectively. The iterations of these methods can naturally be computed in parallel such that each processor or cluster of processors is responsible for solving one splitting as a linear sub-system
+\begin{equation}
+A_{\ell\ell} x_\ell = c_\ell,\mbox{~for~}\ell=1,\ldots,L,
+\label{eq:03}
+\end{equation}
+where right-hand sides $c_\ell=b_\ell-\sum_{m\neq\ell}A_{\ell m}x_m$ are computed using the shared vectors $x_m$. In this paper, we use the well-known iterative method GMRES ({\it Generalized Minimal RESidual})~\cite{saad86} as an inner iteration to approximate the solutions of the different splittings arising from the block Jacobi multisplitting of matrix $A$. The algorithm in Figure~\ref{alg:01} shows the main key points of our block Jacobi two-stage method executed by a cluster of processors. In line~\ref{solve}, the linear sub-system~(\ref{eq:03}) is solved in parallel using GMRES method where $\MIG$ and $\TOLG$ are the maximum number of inner iterations and the tolerance threshold for GMRES respectively. The convergence of the two-stage multisplitting methods, based on synchronous or asynchronous iterations, is studied by many authors for example~\cite{Bru95,bahi07}.
+
+\begin{figure}[t]
+%\begin{algorithm}[t]
+%\caption{Block Jacobi two-stage multisplitting method}
+\begin{algorithmic}[1]
+ \Input $A_\ell$ (sparse matrix), $b_\ell$ (right-hand side)
+ \Output $x_\ell$ (solution vector)\vspace{0.2cm}
+ \State Set the initial guess $x^0$
+ \For {$k=1,2,3,\ldots$ until convergence}
+ \State $c_\ell=b_\ell-\sum_{m\neq\ell}A_{\ell m}x_m^{k-1}$
+ \State $x^k_\ell=Solve_{gmres}(A_{\ell\ell},c_\ell,x^{k-1}_\ell,\MIG,\TOLG)$\label{solve}
+ \State Send $x_\ell^k$ to neighboring clusters\label{send}
+ \State Receive $\{x_m^k\}_{m\neq\ell}$ from neighboring clusters\label{recv}
+ \EndFor
+\end{algorithmic}
+\caption{Block Jacobi two-stage multisplitting method}
+\label{alg:01}
+%\end{algorithm}
+\end{figure}
+
+In this paper, we propose two algorithms of two-stage multisplitting methods. The first algorithm is based on asynchronous model which allows the communications to be overlapped by computations and reduces the idle times resulting from the synchronizations. So in the asynchronous mode, our two-stage algorithm uses asynchronous outer iterations and asynchronous communications between clusters. The communications (i.e. lines~\ref{send} and~\ref{recv} in Figure~\ref{alg:01}) are performed by message passing using MPI non-blocking communication routines. The convergence of the asynchronous iterations is detected when all clusters have locally converged
+\begin{equation}
+k\geq\MIM\mbox{~or~}\|x_\ell^{k+1}-x_\ell^k\|_{\infty }\leq\TOLM,
+\label{eq:04}
+\end{equation}
+where $\MIM$ is the maximum number of outer iterations and $\TOLM$ is the tolerance threshold for the two-stage algorithm.
+
+The second two-stage algorithm is based on synchronous outer iterations. We propose to use the Krylov iteration based on residual minimization to improve the slow convergence of the multisplitting methods. In this case, a $n\times s$ matrix $S$ is set using solutions issued from the inner iteration
+\begin{equation}
+S=[x^1,x^2,\ldots,x^s],~s\ll n.
+\label{eq:05}
+\end{equation}
+At each $s$ outer iterations, the algorithm computes a new approximation $\tilde{x}=S\alpha$ which minimizes the residual
+\begin{equation}
+\min_{\alpha\in\mathbb{R}^s}{\|b-AS\alpha\|_2}.
+\label{eq:06}
+\end{equation}
+The algorithm in Figure~\ref{alg:02} includes the procedure of the residual minimization and the outer iteration is restarted with a new approximation $\tilde{x}$ at every $s$ iterations. The least-squares problem~(\ref{eq:06}) is solved in parallel by all clusters using CGLS method~\cite{Hestenes52} such that $\MIC$ is the maximum number of iterations and $\TOLC$ is the tolerance threshold for this method (line~\ref{cgls} in Figure~\ref{alg:02}).
+
+\begin{figure}[t]
+%\begin{algorithm}[t]
+%\caption{Krylov two-stage method using block Jacobi multisplitting}
+\begin{algorithmic}[1]
+ \Input $A_\ell$ (sparse matrix), $b_\ell$ (right-hand side)
+ \Output $x_\ell$ (solution vector)\vspace{0.2cm}
+ \State Set the initial guess $x^0$
+ \For {$k=1,2,3,\ldots$ until convergence}
+ \State $c_\ell=b_\ell-\sum_{m\neq\ell}A_{\ell m}x_m^{k-1}$
+ \State $x^k_\ell=Solve_{gmres}(A_{\ell\ell},c_\ell,x^{k-1}_\ell,\MIG,\TOLG)$
+ \State $S_{\ell,k\mod s}=x_\ell^k$
+ \If{$k\mod s = 0$}
+ \State $\alpha = Solve_{cgls}(AS,b,\MIC,\TOLC)$\label{cgls}
+ \State $\tilde{x_\ell}=S_\ell\alpha$
+ \State Send $\tilde{x_\ell}$ to neighboring clusters
+ \Else
+ \State Send $x_\ell^k$ to neighboring clusters
+ \EndIf
+ \State Receive $\{x_m^k\}_{m\neq\ell}$ from neighboring clusters
+ \EndFor
+\end{algorithmic}
+\caption{Krylov two-stage method using block Jacobi multisplitting}
+\label{alg:02}
+%\end{algorithm}
+\end{figure}
+
+\subsection{Simulation of two-stage methods using SimGrid framework}
+\label{sec:04.02}
+
+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 ensure the test reproducibility under the same conditions. According our experience, very few modifications are required to adapt a MPI program to run in SIMGRID simulator using SMPI (Simulator MPI).The first modification is to include SMPI libraries and related header files (smpi.h). The second and important modification is to eliminate all global variables in moving them to local subroutine or using a Simgrid selector called "runtime automatic switching" (smpi/privatize\_global\_variables). Indeed, global variables can generate side effects on runtime between the threads running in the same process, generated by the Simgrid to simulate the grid environment.The last modification on the MPI program pointed out for some cases, the review of the sequence of the MPI\_Isend, MPI\_Irecv and MPI\_Waitall instructions which might cause an infinite loop.
+
+
+\paragraph{SIMGRID Simulator parameters}
+
+\begin{itemize}
+ \item hostfile: Hosts description file.
+ \item plarform: File describing the platform architecture : clusters (CPU power,
+\dots{}), intra cluster network description, inter cluster network (bandwidth bw,
+latency lat, \dots{}).
+ \item archi : Grid computational description (Number of clusters, Number of
+nodes/processors for each cluster).
+\end{itemize}
+
-\section{Simulation of the multisplitting method}
+In addition, the following arguments are given to the programs at runtime:
-\section{Experimental, Results and Comments}
+\begin{itemize}
+ \item Maximum number of inner and outer iterations;
+ \item Inner and outer precisions;
+ \item Matrix size (N$_{x}$, N$_{y}$ and N$_{z}$);
+ \item Matrix diagonal value = 6.0;
+ \item Execution Mode: synchronous or asynchronous.
+\end{itemize}
+At last, note that the two solver algorithms have been executed with the Simgrid selector -cfg=smpi/running\_power which determine the computational power (here 19GFlops) of the simulator host machine.
-\textbf{V.1. Setup study and Methodology}
+%%%%%%%%%%%%%%%%%%%%%%%%%
+%%%%%%%%%%%%%%%%%%%%%%%%%
+
+\section{Experimental Results}
+\label{sec:expe}
+
+
+\subsection{Setup study and Methodology}
To conduct our study, we have put in place the following methodology
-which can be reused with any grid-enabled applications.
+which can be reused for any grid-enabled applications.
\textbf{Step 1} : Choose with the end users the class of algorithms or
the application to be tested. Numerical parallel iterative algorithms
-have been chosen for the study in the paper.
+have been chosen for the study in this paper. \\
\textbf{Step 2} : Collect the software materials needed for the
-experimentation. In our case, we have three variants algorithms for the
-resolution of three 3D-Poisson problem: (1) using the classical GMRES
-\textit{(Generalized Minimal RESidual Method)} alias Algo-1 in this
-paper, (2) using the multisplitting method alias Algo-2 and (3) an
-enhanced version of the multisplitting method as Algo-3. In addition,
-SIMGRID simulator has been chosen to simulate the behaviors of the
-distributed applications. SIMGRID is running on the Mesocentre
-datacenter in Franche-Comte University $[$10$]$ but also in a virtual
-machine on a laptop.
+experimentation. In our case, we have two variants algorithms for the
+resolution of three 3D-Poisson problem: (1) using the classical GMRES (Algo-1)(2) and the multisplitting method (Algo-2). In addition, SIMGRID simulator has been chosen to simulate the behaviors of the
+distributed applications. SIMGRID is running on the Mesocentre datacenter in Franche-Comte University but also in a virtual machine on a laptop. \\
\textbf{Step 3} : Fix the criteria which will be used for the future
results comparison and analysis. In the scope of this study, we retain
in one hand the algorithm execution mode (synchronous and asynchronous)
and in the other hand the execution time and the number of iterations of
-the application before obtaining the convergence.
+the application before obtaining the convergence. \\
\textbf{Step 4 }: Setup up the different grid testbeds environment
which will be simulated in the simulator tool to run the program. The
4x16, 8x8 and 2x50. The network has been designed to operate with a
bandwidth equals to 10Gbits (resp. 1Gbits/s) and a latency of 8E-6
microseconds (resp. 5E-5) for the intra-clusters links (resp.
-inter-clusters backbone links).
+inter-clusters backbone links). \\
-\textbf{Step 5}: Process an extensive and comprehensive testings
+\textbf{Step 5}: Conduct an extensive and comprehensive testings
within these configurations in varying the key parameters, especially
the CPU power capacity, the network parameters and also the size of the
-input matrix. Note that some parameters should be invariant to allow the
-comparison like some program input arguments.
+input matrix. Note that some parameters should be fixed to be invariant to allow the
+comparison like some program input arguments. \\
\textbf{Step 6} : Collect and analyze the output results.
-\textbf{ V.2. Factors impacting distributed applications performance in
+\subsection{Factors impacting distributed applications performance in
a grid environment}
From our previous experience on running distributed application in a
application is the network configuration. Two main network parameters
can modify drastically the program output results : (i) the network
bandwidth (bw=bits/s) also known as "the data-carrying capacity"
-$[$13$]$ of the network is defined as the maximum of data that can pass
+of the network is defined as the maximum of data that can pass
from one point to another in a unit of time. (ii) the network latency
(lat : microsecond) defined as the delay from the start time to send the
data from a source and the final time the destination have finished to
between distant clusters might be a bottleneck for the global
performance of the application.
-\textbf{V.3 Comparing GMRES and Multisplitting algorithms in
+\subsection{Comparing GMRES and Multisplitting algorithms in
synchronous mode}
In the scope of this paper, our first objective is to demonstrate the
Algo-2 (Multisplitting method) shows a better performance in grid
architecture compared with Algo-1 (Classical GMRES) both running in
\textbf{\textit{synchronous mode}}. Better algorithm performance
-should mean a less number of iterations output and a less execution time
+should means a less number of iterations output and a less execution time
before reaching the convergence. For a systematic study, the experiments
should figure out that, for various grid parameters values, the
simulator will confirm the targeted outcomes, particularly for poor and
slow networks, focusing on the impact on the communication performance
-on the chosen class of algorithm $[$12$]$.
+on the chosen class of algorithm.
The following paragraphs present the test conditions, the output results
-and our comments.
+and our comments.\\
\textit{3.a Executing the algorithms on various computational grid
\begin{tabular}{r c }
\hline
Grid & 2x16, 4x8, 4x16 and 8x8\\ %\hline
- Network & N2 : bw=1Gbs-lat=5E-05 \\ %\hline
- Input matrix size & N$_{x}$ =150 x 150 x 150 and\\ %\hline
- - & N$_{x}$ =170 x 170 x 170 \\ \hline
+ Network & N2 : bw=1Gbits/s - lat=5.10$^{-5}$ \\ %\hline
+ Input matrix size & N$_{x}$ x N$_{y}$ x N$_{z}$ =150 x 150 x 150\\ %\hline
+ - & N$_{x}$ x N$_{y}$ x N$_{z}$ =170 x 170 x 170 \\ \hline
\end{tabular}
-\end{footnotesize}
+Table 1 : Clusters x Nodes with N$_{x}$=150 or N$_{x}$=170 \\
+\end{footnotesize}
- Table 1 : Clusters x Nodes with NX=150 or NX=170
-\RCE{J'ai voulu mettre les tableaux des données mais je pense que c'est inutile et ça va surcharger}
+
+%\RCE{J'ai voulu mettre les tableaux des données mais je pense que c'est inutile et ça va surcharger}
-The results in figure 1 show the non-variation of the number of
+The results in figure 3 show the non-variation of the number of
iterations of classical GMRES for a given input matrix size; it is not
the case for the multisplitting method.
-%%\begin{wrapfigure}{l}{60mm}
+%\begin{wrapfigure}{l}{100mm}
\begin{figure} [ht!]
\centering
-\includegraphics[width=60mm]{"Cluster x Nodes NX=150 and NX=170".JPG}
-\caption{Cluster x Nodes NX=150 and NX=170 \label{overflow}}
+\includegraphics[width=100mm]{cluster_x_nodes_nx_150_and_nx_170.pdf}
+\caption{Cluster x Nodes N$_{x}$=150 and N$_{x}$=170}
+%\label{overflow}}
\end{figure}
-%%\end{wrapfigure}
-
+%\end{wrapfigure}
Unless the 8x8 cluster, the time
execution difference between the two algorithms is important when
(compared with the classical GMRES) when scaling up to higher input
matrix size.
-\textit{3.b Running on various computational grid architecture}
+\textit{\\3.b Running on various computational grid architecture\\}
% environment
\begin{footnotesize}
\begin{tabular}{r c }
\hline
Grid & 2x16, 4x8\\ %\hline
- Network & N1 : bw=10Gbs-lat=8E-06 \\ %\hline
- - & N2 : bw=1Gbs-lat=5E-05 \\
- Input matrix size & N$_{x}$ =150 x 150 x 150\\ \hline \\
+ Network & N1 : bw=10Gbs-lat=8.10$^{-6}$ \\ %\hline
+ - & N2 : bw=1Gbs-lat=5.10$^{-5}$ \\
+ Input matrix size & N$_{x}$ x N$_{y}$ x N$_{z}$ =150 x 150 x 150\\ \hline \\
\end{tabular}
-\end{footnotesize}
+Table 2 : Clusters x Nodes - Networks N1 x N2 \\
-%Table 2 : Clusters x Nodes - Networks N1 x N2
-%\RCE{idem pour tous les tableaux de donnees}
+ \end{footnotesize}
+
+%\begin{wrapfigure}{l}{100mm}
\begin{figure} [ht!]
\centering
-\includegraphics[width=60mm]{"Cluster x Nodes N1 x N2".JPG}
-\caption{Cluster x Nodes N1 x N2\label{overflow}}
+\includegraphics[width=100mm]{cluster_x_nodes_n1_x_n2.pdf}
+\caption{Cluster x Nodes N1 x N2}
+%\label{overflow}}
\end{figure}
+%\end{wrapfigure}
The experiments compare the behavior of the algorithms running first on
-speed inter- cluster network (N1) and a less performant network (N2).
-The figure 2 shows that end users will gain to reduce the execution time
+a speed inter- cluster network (N1) and a less performant network (N2).
+Figure 4 shows that end users will gain to reduce the execution time
for both algorithms in using a grid architecture like 4x16 or 8x8: the
performance was increased in a factor of 2. The results depict also that
when the network speed drops down, the difference between the execution
times can reach more than 25\%.
-\textit{\\\\\\\\\\\\\\\\\\3.c Network latency impacts on performance}
+\textit{\\3.c Network latency impacts on performance\\}
% environment
\begin{footnotesize}
\hline
Grid & 2x16\\ %\hline
Network & N1 : bw=1Gbs \\ %\hline
- Input matrix size & N$_{x}$ =150 x 150 x 150\\ \hline\\
+ Input matrix size & N$_{x}$ x N$_{y}$ x N$_{z}$ =150 x 150 x 150\\ \hline\\
\end{tabular}
+Table 3 : Network latency impact \\
+
\end{footnotesize}
-Table 3 : Network latency impact
\begin{figure} [ht!]
\centering
-\includegraphics[width=60mm]{"Network latency impact on execution time".JPG}
-\caption{Network latency impact on execution time\label{overflow}}
+\includegraphics[width=100mm]{network_latency_impact_on_execution_time.pdf}
+\caption{Network latency impact on execution time}
+%\label{overflow}}
\end{figure}
-According the results in table and figure 3, degradation of the network
+According the results in figure 5, degradation of the network
latency from 8.10$^{-6}$ to 6.10$^{-5}$ implies an absolute time
increase more than 75\% (resp. 82\%) of the execution for the classical
GMRES (resp. multisplitting) algorithm. In addition, it appears that the
multisplitting method tolerates more the network latency variation with
-a less rate increase. Consequently, in the worst case (lat=6.10$^{-5
+a less rate increase of the execution time. Consequently, in the worst case (lat=6.10$^{-5
}$), the execution time for GMRES is almost the double of the time for
the multisplitting, even though, the performance was on the same order
of magnitude with a latency of 8.10$^{-6}$.
-\textit{3.d Network bandwidth impacts on performance}
+\textit{\\3.d Network bandwidth impacts on performance\\}
% environment
\begin{footnotesize}
\begin{tabular}{r c }
\hline
Grid & 2x16\\ %\hline
- Network & N1 : bw=1Gbs - lat=5E-05 \\ %\hline
- Input matrix size & N$_{x}$ =150 x 150 x 150\\ \hline
+ Network & N1 : bw=1Gbs - lat=5.10$^{-5}$ \\ %\hline
+ Input matrix size & N$_{x}$ x N$_{y}$ x N$_{z}$ =150 x 150 x 150\\ \hline \\
\end{tabular}
+Table 4 : Network bandwidth impact \\
+
\end{footnotesize}
-Table 4 : Network bandwidth impact
\begin{figure} [ht!]
\centering
-\includegraphics[width=60mm]{"Network bandwith impact on execution time".JPG}
-\caption{Network bandwith impact on execution time\label{overflow}}
+\includegraphics[width=100mm]{network_bandwith_impact_on_execution_time.pdf}
+\caption{Network bandwith impact on execution time}
+%\label{overflow}
\end{figure}
The results of increasing the network bandwidth depict the improvement
of the performance by reducing the execution time for both of the two
-algorithms. However, and again in this case, the multisplitting method
+algorithms (Figure 6). However, and again in this case, the multisplitting method
presents a better performance in the considered bandwidth interval with
a gain of 40\% which is only around 24\% for classical GMRES.
-\textit{3.e Input matrix size impacts on performance}
+\textit{\\3.e Input matrix size impacts on performance\\}
% environment
\begin{footnotesize}
\begin{tabular}{r c }
\hline
Grid & 4x8\\ %\hline
- Network & N2 : bw=1Gbs - lat=5E-05 \\ %\hline
- Input matrix size & N$_{x}$ = From 40 to 200\\ \hline
+ Network & N2 : bw=1Gbs - lat=5.10$^{-5}$ \\ %\hline
+ Input matrix size & N$_{x}$ = From 40 to 200\\ \hline \\
\end{tabular}
+Table 5 : Input matrix size impact\\
+
\end{footnotesize}
-Table 5 : Input matrix size impact
\begin{figure} [ht!]
\centering
-\includegraphics[width=60mm]{"Pb size impact on execution time".JPG}
-\caption{Pb size impact on execution time\label{overflow}}
+\includegraphics[width=100mm]{pb_size_impact_on_execution_time.pdf}
+\caption{Pb size impact on execution time}
+%\label{overflow}}
\end{figure}
In this experimentation, the input matrix size has been set from
-Nx=Ny=Nz=40 to 200 side elements that is from 40$^{3}$ = 64.000 to
-200$^{3}$ = 8.000.000 points. Obviously, as shown in the figure 5,
-the execution time for the algorithms convergence increases with the
-input matrix size. But the interesting result here direct on (i) the
+N$_{x}$ = N$_{y}$ = N$_{z}$ = 40 to 200 side elements that is from 40$^{3}$ = 64.000 to
+200$^{3}$ = 8.000.000 points. Obviously, as shown in the figure 7,
+the execution time for the two algorithms convergence increases with the
+input matrix size. But the interesting results here direct on (i) the
drastic increase (300 times) of the number of iterations needed before
the convergence for the classical GMRES algorithm when the matrix size
-go beyond Nx=150; (ii) the classical GMRES execution time also almost
-the double from Nx=140 compared with the convergence time of the
+go beyond N$_{x}$=150; (ii) the classical GMRES execution time also almost
+the double from N$_{x}$=140 compared with the convergence time of the
multisplitting method. These findings may help a lot end users to setup
the best and the optimal targeted environment for the application
deployment when focusing on the problem size scale up. Note that the
same test has been done with the grid 2x16 getting the same conclusion.
-\textit{3.f CPU Power impact on performance}
+\textit{\\3.f CPU Power impact on performance\\}
% environment
\begin{footnotesize}
Network & N2 : bw=1Gbs - lat=5E-05 \\ %\hline
Input matrix size & N$_{x}$ = 150 x 150 x 150\\ \hline
\end{tabular}
+Table 6 : CPU Power impact \\
+
\end{footnotesize}
-Table 6 : CPU Power impact
\begin{figure} [ht!]
\centering
-\includegraphics[width=60mm]{"CPU Power impact on execution time".JPG}
-\caption{CPU Power impact on execution time\label{overflow}}
+\includegraphics[width=100mm]{cpu_power_impact_on_execution_time.pdf}
+\caption{CPU Power impact on execution time}
+%\label{overflow}}
\end{figure}
Using the SIMGRID simulator flexibility, we have tried to determine the
after adding more powerful CPU. Note that the execution time axis in the
figure is in logarithmic scale.
- \textbf{V.4 Comparing GMRES in native synchronous mode and
+\subsection{Comparing GMRES in native synchronous mode and
Multisplitting algorithms in asynchronous mode}
The previous paragraphs put in evidence the interests to simulate the
classical GMRES time.
-The test conditions are summarized in the table below :
+The test conditions are summarized in the table below : \\
% environment
\begin{footnotesize}
Intra-Network & bw=1.25 Gbits - lat=5E-05 \\ %\hline
Inter-Network & bw=5 Mbits - lat=2E-02\\
Input matrix size & N$_{x}$ = From 62 to 150\\ %\hline
- Residual error precision: 10$^{-5}$ to 10$^{-9}$\\ \hline
+ Residual error precision: 10$^{-5}$ to 10$^{-9}$\\ \hline \\
\end{tabular}
\end{footnotesize}
\centering
\caption{Relative gain of the multisplitting algorithm compared with
the classical GMRES}
- \label{tab.cluster.2x50}
+ \label{"Table 7"}
\begin{mytable}{6}
\hline
- bw
- & 5 & 5 & 5 & 5 & 5 & 50 \\
+ bandwidth (Mbit/s)
+ & 5 & 5 & 5 & 5 & 5 \\
\hline
- lat
- & 0.02 & 0.02 & 0.02 & 0.02 & 0.02 & 0.02 \\
+ latency (ms)
+ & 20 & 20 & 20 & 20 & 20 \\
\hline
- power
- & 1 & 1 & 1 & 1.5 & 1.5 & 1.5 \\
+ power (GFlops)
+ & 1 & 1 & 1 & 1.5 & 1.5 \\
\hline
- size
- & 62 & 62 & 62 & 100 & 100 & 110 \\
+ size (N)
+ & 62 & 62 & 62 & 100 & 100 \\
\hline
- Prec/Eprec
- & \np{E-5} & \np{E-8} & \np{E-9} & \np{E-11} & \np{E-11} & \np{E-11} \\
+ Precision
+ & \np{E-5} & \np{E-8} & \np{E-9} & \np{E-11} & \np{E-11} \\
\hline
- speedup
- & 0.396 & 0.392 & 0.396 & 0.391 & 0.393 & 0.395 \\
+ Relative gain
+ & 2.52 & 2.55 & 2.52 & 2.57 & 2.54 \\
\hline
\end{mytable}
\begin{mytable}{6}
\hline
- bw
- & 50 & 50 & 50 & 50 & 10 & 10 \\
+ bandwidth (Mbit/s)
+ & 50 & 50 & 50 & 50 & 50 \\
\hline
- lat
- & 0.02 & 0.02 & 0.02 & 0.02 & 0.03 & 0.01 \\
+ latency (ms)
+ & 20 & 20 & 20 & 20 & 20 \\
\hline
- power
- & 1.5 & 1.5 & 1.5 & 1.5 & 1 & 1.5 \\
+ power (GFlops)
+ & 1.5 & 1.5 & 1 & 1.5 & 1.5 \\
\hline
- size
- & 120 & 130 & 140 & 150 & 171 & 171 \\
+ size (N)
+ & 110 & 120 & 130 & 140 & 150 \\
\hline
- Prec/Eprec
- & \np{E-11} & \np{E-11} & \np{E-11} & \np{E-11} & \np{E-5} & \np{E-5} \\
+ Precision
+ & \np{E-11} & \np{E-11} & \np{E-11} & \np{E-11} & \np{E-11}\\
\hline
- speedup
- & 0.398 & 0.388 & 0.393 & 0.394 & 0.63 & 0.778 \\
+ Relative gain
+ & 2.53 & 2.51 & 2.58 & 2.55 & 2.54 \\
\hline
\end{mytable}
\end{table}
The authors would like to thank\dots{}
-% trigger a \newpage just before the given reference
-% number - used to balance the columns on the last page
-% adjust value as needed - may need to be readjusted if
-% the document is modified later
-\bibliographystyle{IEEEtran}
-\bibliography{hpccBib}
+\bibliographystyle{wileyj}
+\bibliography{biblio}
\end{document}
