11-12-2014 v05

[Krylov_multi.git] / krylov_multi.tex

diff --git a/krylov_multi.tex b/krylov_multi.tex
index 5715dc24479cbde273a2150e1acbb36ec339573f..1ab94c6f15f34cce6269b9ba1d8248f0ba8505cd 100644 (file)
--- a/krylov_multi.tex
+++ b/krylov_multi.tex
@@ -6,6 +6,7 @@
 \usepackage{algorithm}
 \usepackage{algpseudocode}
 \usepackage{multirow}
 \usepackage{algorithm}
 \usepackage{algpseudocode}
 \usepackage{multirow}

+\usepackage{authblk}

 
 \algnewcommand\algorithmicinput{\textbf{Input:}}
 \algnewcommand\Input{\item[\algorithmicinput]}
 
 \algnewcommand\algorithmicinput{\textbf{Input:}}
 \algnewcommand\Input{\item[\algorithmicinput]}


@@ -17,18 +18,23 @@
 \newcommand{\Prec}{\mathit{prec}}
 \newcommand{\Ratio}{\mathit{Ratio}}
 
 \newcommand{\Prec}{\mathit{prec}}
 \newcommand{\Ratio}{\mathit{Ratio}}
 

-%\usepackage{xspace}
-%\usepackage[textsize=footnotesize]{todonotes}
-%\newcommand{\LZK}[2][inline]{%
-%\todo[color=green!40,#1]{\sffamily\textbf{LZK:} #2}\xspace}
+\def\changemargin#1#2{\list{}{\rightmargin#2\leftmargin#1}\item[]}
+\let\endchangemargin=\endlist

 
 

-\title{A scalable multisplitting algorithm for solving large sparse linear systems} 
+\title{A scalable multisplitting algorithm to solve large sparse linear systems} 

 \date{}
 
 \date{}
 

-
-
+\author[1]{Raphaël Couturier}
+\author[2]{ Lilia Ziane Khodja}
+\affil[1]{ Femto-ST Institute\\
+    University of Franche Comte\\
+    France\\
+    email: raphael.couturier@univ-fcomte.fr}
+\affil[2]{Inria Bordeaux Sud-Ouest\\
+    France\\
+    email: lilia.ziane@inria.fr}

 \begin{document}
 \begin{document}

-\author{Raphaël Couturier \and Lilia Ziane Khodja}
+

 
 \maketitle
 
 
 \maketitle
 


@@ -45,7 +51,7 @@ parallel GMRES method inside each block and on a parallel Krylov minimization in
 order to improve the convergence. Some large scale experiments with a 3D Poisson
 problem  are  presented  with  up   to  8,192  cores.   They  show  the  obtained
 improvements compared to a classical GMRES both in terms of number of iterations
 order to improve the convergence. Some large scale experiments with a 3D Poisson
 problem  are  presented  with  up   to  8,192  cores.   They  show  the  obtained
 improvements compared to a classical GMRES both in terms of number of iterations

-and execution times.
+and in terms of execution times.

 \end{abstract}
 
 %%%%%%%%%%%%%%%%%%%%%%%%
 \end{abstract}
 
 %%%%%%%%%%%%%%%%%%%%%%%%


@@ -54,9 +60,9 @@ and execution times.
 \section{Introduction}
 Iterative methods are used to solve  large sparse linear systems of equations of
 the form  $Ax=b$ because they are  easier to parallelize than  direct ones. Many
 \section{Introduction}
 Iterative methods are used to solve  large sparse linear systems of equations of
 the form  $Ax=b$ because they are  easier to parallelize than  direct ones. Many

-iterative  methods have  been proposed  and  adapted by  many researchers.   For
+iterative  methods have  been proposed  and  adapted by  different researchers.   For

 example, the GMRES method and the  Conjugate Gradient method are very well known
 example, the GMRES method and the  Conjugate Gradient method are very well known

-and  used by  many researchers~\cite{S96}. Both methods  are based  on the
+and  used~\cite{S96}. Both methods  are based  on the

 Krylov subspace which consists in forming  a basis of a sequence of successive
 matrix powers times the initial residual.
 
 Krylov subspace which consists in forming  a basis of a sequence of successive
 matrix powers times the initial residual.
 


@@ -67,46 +73,68 @@ Preconditioners can be  used in order to increase  the convergence of iterative
 solvers.   However, most  of the  good preconditioners  are not  scalable when
 thousands of cores are used.
 
 solvers.   However, most  of the  good preconditioners  are not  scalable when
 thousands of cores are used.
 

-Traditional iterative  solvers have  global synchronizations that  penalize the
-scalability.   Two  possible solutions  consists  either  in using  asynchronous
-iterative  methods~\cite{ref18} or  to  use multisplitting  algorithms. In  this
-paper, we will  reconsider the use of a multisplitting  method. In opposition to
-traditional  multisplitting  method  that  suffer  from  slow  convergence,  as
-proposed  in~\cite{huang1993krylov},  the  use  of a  minimization  process  can
-drastically improve the convergence.
+%Traditional iterative  solvers have  global synchronizations that  penalize the
+%scalability.   Two  possible solutions  consists  either  in using  asynchronous
+%iterative  methods~\cite{ref18} or  to  use multisplitting  algorithms. In  this
+%paper, we will  reconsider the use of a multisplitting  method. In opposition to
+%traditional  multisplitting  method  that  suffer  from  slow  convergence,  as
+%proposed  in~\cite{huang1993krylov},  the  use  of a  minimization  process  can
+%drastically improve the convergence.
+
+Traditional parallel iterative solvers are based on fine-grain computations that
+frequently  require  data exchanges  between  computing  nodes  and have  global
+synchronizations  that penalize  the  scalability. Particularly,  they are  more
+penalized on large  scale architectures or on distributed  platforms composed of
+distant  clusters interconnected  by  a high-latency  network.  It is  therefore
+imperative to develop coarse-grain based algorithms to reduce the communications
+in the  parallel iterative  solvers. Two possible  solutions consists  either in
+using  asynchronous  iterative  methods~\cite{ref18}  or in  using  multisplitting
+algorithmss.  In this  paper,  we will  reconsider  the use  of a  multisplitting
+method. In opposition to traditional multisplitting method that suffer from slow
+convergence, as  proposed in~\cite{huang1993krylov},  the use of  a minimization
+process can drastically improve the convergence.
+
+The present paper is  organized as follows. First, Section~\ref{sec:02} presents
+some  related  works and  the  principle  of  multisplitting methods.  Then,  in
+Section~\ref{sec:03}  the algorithm  of our  Krylov multisplitting
+method, based  on inner-outer  iterations, is presented. Finally, in  Section~\ref{sec:04}, the
+parallel experiments on Hector architecture  show the performances of the Krylov
+multisplitting algorithm compared to the classical GMRES algorithm to solve a 3D
+Poisson problem.

 
 
 %%%%%%%%%%%%%%%%%%%%%%%%
 %%%%%%%%%%%%%%%%%%%%%%%%
 
 
 
 %%%%%%%%%%%%%%%%%%%%%%%%
 %%%%%%%%%%%%%%%%%%%%%%%%
 

-\section{Related works and presention of the multisplitting method}
-A general framework  for studying parallel multisplitting has  been presented in~\cite{o1985multi}
+\section{Related works and presentation of the multisplitting method}
+\label{sec:02}
+A general framework  to study parallel multisplitting methods has  been presented in~\cite{o1985multi}

 by O'Leary and White. Convergence conditions are given for the
 by O'Leary and White. Convergence conditions are given for the

-most general case.  Many authors improved multisplitting algorithms by proposing
-for  example  an  asynchronous  version~\cite{bru1995parallel}  and  convergence
-conditions~\cite{bai1999block,bahi2000asynchronous}   in  this  case   or  other
+most general cases.  Many authors have improved multisplitting algorithms by proposing,
+for  example,  an  asynchronous  version~\cite{bru1995parallel} or  convergence
+conditions~\cite{bai1999block,bahi2000asynchronous}     or  other

 two-stage algorithms~\cite{frommer1992h,bru1995parallel}.
 
 two-stage algorithms~\cite{frommer1992h,bru1995parallel}.
 

-In~\cite{huang1993krylov},  the  authors  proposed  a  parallel  multisplitting
+In~\cite{huang1993krylov},  the  authors  have proposed  a  parallel  multisplitting

 algorithm in which all the tasks except  one are devoted to solve a sub-block of
 the splitting  and to send their  local solutions to  the first task which  is in
 algorithm in which all the tasks except  one are devoted to solve a sub-block of
 the splitting  and to send their  local solutions to  the first task which  is in

-charge to  combine the vectors at  each iteration.  These vectors  form a Krylov
+charge of  combining the vectors at  each iteration.  These vectors  form a Krylov

 basis for  which the first task minimizes  the error function over  the basis to
 basis for  which the first task minimizes  the error function over  the basis to

-increase the convergence, then the other tasks receive the updated solution until
+increase the convergence, then the other tasks receive the updated solution until the

 convergence of the global system. 
 
 convergence of the global system. 
 

-In~\cite{couturier2008gremlins}, the  authors proposed practical implementations
+In~\cite{couturier2008gremlins}, the  authors have developed practical implementations

 of multisplitting algorithms to solve  large scale linear systems. Inner solvers
 could be  based on sequential direct method  with the LU method  or sequential iterative
 one with GMRES.
 
 of multisplitting algorithms to solve  large scale linear systems. Inner solvers
 could be  based on sequential direct method  with the LU method  or sequential iterative
 one with GMRES.
 

-In~\cite{prace-multi},  the  authors have  proposed a  parallel  multisplitting
+In~\cite{prace-multi},  the  authors have  designed a  parallel  multisplitting

 algorithm in which large blocks are solved using a GMRES solver. The authors have
 performed large scale experiments up-to  32,768 cores and they conclude that
 algorithm in which large blocks are solved using a GMRES solver. The authors have
 performed large scale experiments up-to  32,768 cores and they conclude that

-asynchronous  multisplitting algorithm  could be more  efficient  than traditional
+an asynchronous  multisplitting algorithm  could be more  efficient  than traditional

 solvers on an exascale architecture with hundreds of thousands of cores.
 
 solvers on an exascale architecture with hundreds of thousands of cores.
 

-So compared to these works, we propose in this paper a practical multisplitting method based on parallel iterative blocks and gives better results than classical GMRES method for the 3D Poisson problem we considered.
+So, compared to these works, we propose in this paper a practical multisplitting method based on parallel iterative blocks which gives better results than classical GMRES method for the 3D Poisson problem we considered.

 \\
 
 The key idea of a multisplitting method to solve a large system of linear equations $Ax=b$ is defined as follows. The first step consists in partitioning the matrix $A$ in $L$ several ways 
 \\
 
 The key idea of a multisplitting method to solve a large system of linear equations $Ax=b$ is defined as follows. The first step consists in partitioning the matrix $A$ in $L$ several ways 


@@ -114,7 +142,7 @@ The key idea of a multisplitting method to solve a large system of linear equati
 A = M_\ell - N_\ell,
 \label{eq01}
 \end{equation}
 A = M_\ell - N_\ell,
 \label{eq01}
 \end{equation}

-where for all $\ell\in\{1,\ldots,L\}$ $M_\ell$ are non-singular matrices. Then the linear system is solved by iteration based on the obtained splittings as follows
+where for all $\ell\in\{1,\ldots,L\}$ $M_\ell$ are non-singular matrices. Then the linear system is solved by an iteration based on the obtained splittings as follows

 \begin{equation}
 x^{k+1}=\displaystyle\sum^L_{\ell=1} E_\ell M^{-1}_\ell (N_\ell x^k + b),~k=1,2,3,\ldots
 \label{eq02}
 \begin{equation}
 x^{k+1}=\displaystyle\sum^L_{\ell=1} E_\ell M^{-1}_\ell (N_\ell x^k + b),~k=1,2,3,\ldots
 \label{eq02}


@@ -142,6 +170,7 @@ In the case where the diagonal weighting matrices $E_\ell$ have only zero and on
 %%%%%%%%%%%%%%%%%%%%%%%%
 
 \section{A two-stage method with a minimization}
 %%%%%%%%%%%%%%%%%%%%%%%%
 
 \section{A two-stage method with a minimization}

+\label{sec:03}

 Let $Ax=b$ be a given large and sparse linear system of $n$ equations to solve in parallel on $L$ clusters of processors, physically adjacent or geographically distant, where $A\in\mathbb{R}^{n\times n}$ is a square and non-singular matrix, $x\in\mathbb{R}^{n}$ is the solution vector and $b\in\mathbb{R}^{n}$ is the right-hand side vector. The multisplitting of this linear system is defined as follows
 \begin{equation}
 \left\{
 Let $Ax=b$ be a given large and sparse linear system of $n$ equations to solve in parallel on $L$ clusters of processors, physically adjacent or geographically distant, where $A\in\mathbb{R}^{n\times n}$ is a square and non-singular matrix, $x\in\mathbb{R}^{n}$ is the solution vector and $b\in\mathbb{R}^{n}$ is the right-hand side vector. The multisplitting of this linear system is defined as follows
 \begin{equation}
 \left\{


@@ -179,8 +208,8 @@ of       the       global      sparse       linear       system      to       be
 solved~\cite{o1985multi,ref18}. Furthermore, the  splitting of the linear system
 among  several clusters  of  processors  increases the  spectral  radius of  the
 iteration  matrix, thereby  slowing the  convergence.  In  fact, the  larger the
 solved~\cite{o1985multi,ref18}. Furthermore, the  splitting of the linear system
 among  several clusters  of  processors  increases the  spectral  radius of  the
 iteration  matrix, thereby  slowing the  convergence.  In  fact, the  larger the

-number of  splitting is, the larger the  spectral radius is.  In  this paper, we
-based  on   the  work   presented  in~\cite{huang1993krylov}  to   increase  the
+number of  splitting is, the larger the  spectral radius is.  In  this paper, our
+work is based  on   the  work   presented  in~\cite{huang1993krylov}  to   increase  the

 convergence and improve the scalability of the multisplitting methods.
 
 In order to accelerate the convergence, we implemented the outer iteration of the multisplitting solver as a Krylov iterative method which minimizes some error function over a Krylov subspace~\cite{S96}. The Krylov subspace that we used is spanned by a basis composed of successive solutions issued from solving the $L$ splittings~(\ref{sec03:eq03})
 convergence and improve the scalability of the multisplitting methods.
 
 In order to accelerate the convergence, we implemented the outer iteration of the multisplitting solver as a Krylov iterative method which minimizes some error function over a Krylov subspace~\cite{S96}. The Krylov subspace that we used is spanned by a basis composed of successive solutions issued from solving the $L$ splittings~(\ref{sec03:eq03})


@@ -188,7 +217,7 @@ In order to accelerate the convergence, we implemented the outer iteration of th
 S=\{x^1,x^2,\ldots,x^s\},~s\leq n,
 \label{sec03:eq04}
 \end{equation}
 S=\{x^1,x^2,\ldots,x^s\},~s\leq n,
 \label{sec03:eq04}
 \end{equation}

-where for $j\in\{1,\ldots,s\}$, $x^j=[X_1^j,\ldots,X_L^j]$ is a solution of the global linear system. The advantage of such a Krylov subspace is that we need neither an orthogonal basis nor synchronizations between clusters to generate this basis.
+where for $j\in\{1,\ldots,s\}$, $x^j=[X_1^j,\ldots,X_L^j]$ is a solution of the global linear system. The advantage of such a Krylov subspace is that we neither need an orthogonal basis nor any synchronization between clusters to generate this basis.

 
 The multisplitting method is periodically restarted every $s$ iterations with a new initial guess $\tilde{x}=S\alpha$ which minimizes the error function $\|b-Ax\|_2$ over the Krylov subspace spanned by vectors of $S$. So $\alpha$ is defined as the solution of the large overdetermined linear system
 \begin{equation}
 
 The multisplitting method is periodically restarted every $s$ iterations with a new initial guess $\tilde{x}=S\alpha$ which minimizes the error function $\|b-Ax\|_2$ over the Krylov subspace spanned by vectors of $S$. So $\alpha$ is defined as the solution of the large overdetermined linear system
 \begin{equation}


@@ -247,14 +276,15 @@ where $R^T$ denotes the transpose of matrix $R$. Since $R$ (i.e. $AS$) and $b$ a
 \label{algo:01}
 \end{algorithm}
 
 \label{algo:01}
 \end{algorithm}
 

-The main key points of our Krylov multisplitting method to solve a large sparse linear system are given in Algorithm~\ref{algo:01}. This algorithm is based on a two-stage method with a minimization using restarted GMRES iterative method as an inner solver. It is executed in parallel by each cluster of processors. Matrices and vectors with the subscript $\ell$ represent the local data for cluster $\ell$, where $\ell\in\{1,\ldots,L\}$. The two-stage solver uses two different parallel iterative algorithms: GMRES method to solve each splitting~(\ref{sec03:eq03}) on a cluster of processors, and CGNR method executed in parallel by all clusters to minimize the function error~(\ref{sec03:eq07}) over the Krylov subspace spanned by $S$. The algorithm requires two global synchronizations between $L$ clusters. The first one is performed at line~\ref{line12} in Algorithm~\ref{algo:01} to exchange local values of vector solution $x$ (i.e. the minimizer $\tilde{x}$) required to restart the multisplitting solver. The second one is needed to construct the matrix $R$. We chose to perform this latter synchronization $s$ times in every outer iteration $k$ (line~\ref{line7} in Algorithm~\ref{algo:01}). This is a straightforward way to compute the sparse matrix-dense matrix multiplication $R=AS$. We implemented all synchronizations by using message passing collective communications of MPI library.
+The main key points of our Krylov multisplitting method to solve a large sparse linear system are given in Algorithm~\ref{algo:01}. This algorithm is based on a two-stage method with a minimization using restarted GMRES iterative method as an inner solver. It is executed in parallel by each cluster of processors. Matrices and vectors with the subscript $\ell$ represent the local data for cluster $\ell$, where $\ell\in\{1,\ldots,L\}$. The two-stage solver uses two different parallel iterative algorithms: the GMRES method to solve each splitting~(\ref{sec03:eq03}) on a cluster of processors, and the CGNR method, executed in parallel by all clusters, to minimize the function error~(\ref{sec03:eq07}) over the Krylov subspace spanned by $S$. The algorithm requires two global synchronizations between $L$ clusters. The first one is performed  line~\ref{line12} in Algorithm~\ref{algo:01} to exchange local values of vector solution $x$ (i.e. the minimizer $\tilde{x}$) required to restart the multisplitting solver. The second one is needed to construct the matrix $R$. We chose to perform this latter synchronization $s$ times in every outer iteration $k$ (line~\ref{line7} in Algorithm~\ref{algo:01}). This is a straightforward way to compute the sparse matrix-dense matrix multiplication $R=AS$. We implemented all synchronizations by using message passing collective communications of MPI library.

 
 %%%%%%%%%%%%%%%%%%%%%%%%
 %%%%%%%%%%%%%%%%%%%%%%%%
 
 \section{Experiments}
 
 %%%%%%%%%%%%%%%%%%%%%%%%
 %%%%%%%%%%%%%%%%%%%%%%%%
 
 \section{Experiments}

-In order to illustrate  the interest  of our algorithm. We have  compared our
-algorithm  with  the  GMRES  method  which  is a very  well  used  method  in  many
+\label{sec:04}
+In order to illustrate  the interest  of our algorithm, we have  compared our
+algorithm  with  the  GMRES  method  which  is a commonly  used  method  in  many

 situations.  We have chosen to focus on only one problem which is very simple to
 implement: a 3 dimension Poisson problem.
 
 situations.  We have chosen to focus on only one problem which is very simple to
 implement: a 3 dimension Poisson problem.
 


@@ -276,19 +306,21 @@ preconditioner  it  is   possible  to  reduce  the  number   of  iterations  but
 preconditioners are not scalable when using many cores.
 
 %Doing many experiments  with many cores is  not easy and requires to  access to a supercomputer  with several  hours for  developing  a code  and then  improving it. 
 preconditioners are not scalable when using many cores.
 
 %Doing many experiments  with many cores is  not easy and requires to  access to a supercomputer  with several  hours for  developing  a code  and then  improving it. 

-In the following we present  some experiments we could achieved out on the
-Hector architecture,  the previous UK's  high-end computing resource,  funded by
-the UK Research Councils, which has been stopped in the early 2014.
+In the following we present some experiments we could achieve out on the Hector
+architecture,  a UK's  high-end computing  resource, funded  by the  UK Research
+Councils~\cite{hector}.  This is  a Cray  XE6 supercomputer,  equipped  with two
+16-core AMD  Opteron 2.3 Ghz  and 32 GB  of memory. Machines  are interconnected
+with a 3D torus.

 
 Table~\ref{tab1} shows  the result of  the experiments.  The first  column shows
 the  size of  the  3D Poisson  problem.  The size  is chosen  in  order to  have
 approximately  50,000 components  per core.   The second  column  represents the
 
 Table~\ref{tab1} shows  the result of  the experiments.  The first  column shows
 the  size of  the  3D Poisson  problem.  The size  is chosen  in  order to  have
 approximately  50,000 components  per core.   The second  column  represents the

-number of  cores used. In parenthesis,  there is the decomposition  used for the
+number of  cores used. In brackets,  one can find the decomposition  used for the

 Krylov multisplitting. The  third column and the sixth  column respectively show
 the execution time for the GMRES  and the Krylov multisplitting codes. The fourth
 Krylov multisplitting. The  third column and the sixth  column respectively show
 the execution time for the GMRES  and the Krylov multisplitting codes. The fourth

-and  the   seventh  column  describes   the  number  of  iterations.    For  the
+and  the   seventh  column  describe   the  number  of  iterations.    For  the

 multisplitting  code, the  total number  of inner  iterations is  represented in
 multisplitting  code, the  total number  of inner  iterations is  represented in

-parenthesis. For  the GMRES code (alone  and in the  multisplitting version) the
+brackets. For  the GMRES code (alone  and in the  multisplitting version) the

 restart parameter is fixed to 16. The precision of the GMRES version is fixed to
 1e-6. For  the multisplitting,  there are two  precisions, one for  the external
 solver which is fixed to 1e-6 and another one for the inner solver (GMRES) which
 restart parameter is fixed to 16. The precision of the GMRES version is fixed to
 1e-6. For  the multisplitting,  there are two  precisions, one for  the external
 solver which is fixed to 1e-6 and another one for the inner solver (GMRES) which


@@ -300,41 +332,76 @@ is reached. The precision and the maximum number of iterations of CGNR method ar
 
 \begin{table}[htbp]
 \begin{center}
 
 \begin{table}[htbp]
 \begin{center}

+\begin{changemargin}{-1.8cm}{0cm}
+\begin{small}

 \begin{tabular}{|c|c||c|c|c||c|c|c||c|} 
 \hline
 \multirow{2}{*}{Pb size}&\multirow{2}{*}{Nb. cores} &  \multicolumn{3}{c||}{GMRES} &  \multicolumn{3}{c||}{Krylov Multisplitting} & \multirow{2}{*}{Ratio}\\
  \cline{3-8}
            &                   &  Time (s) & nb Iter. & $\Delta$  &   Time (s)& nb Iter. & $\Delta$ & \\
 \hline
 \begin{tabular}{|c|c||c|c|c||c|c|c||c|} 
 \hline
 \multirow{2}{*}{Pb size}&\multirow{2}{*}{Nb. cores} &  \multicolumn{3}{c||}{GMRES} &  \multicolumn{3}{c||}{Krylov Multisplitting} & \multirow{2}{*}{Ratio}\\
  \cline{3-8}
            &                   &  Time (s) & nb Iter. & $\Delta$  &   Time (s)& nb Iter. & $\Delta$ & \\
 \hline

-$468^3$ & 2048 (2x1024)        &  299.7    & 41,028    & 5.02e-8  &  48.4    & 691(6,146) & 8.24e-08  & 6.19   \\
+$468^3$ & 2,048 (2x1,024)        &  299.7    & 41,028    & 5.02e-8  &  48.4    & 691(6,146) & 8.24e-08  & 6.19   \\

 \hline
 \hline

-$590^3$ & 4096 (2x2048)        &  433.1    & 55,494    & 4.92e-7  &  74.1    & 1,101(8,211) & 6.62e-08  & 5.84   \\
+$590^3$ & 4,096 (2x2,048)        &  433.1    & 55,494    & 4.92e-7  &  74.1    & 1,101(8,211) & 6.62e-08  & 5.84   \\

 \hline
 \hline

-$743^3$ & 8192 (2x4096)        & 704.4     & 87,822    & 4.80e-07 &  151.2   & 3,061(14,914) & 5.87e-08 & 4.65    \\
+$743^3$ & 8,192 (2x4,096)        & 704.4     & 87,822    & 4.80e-07 &  151.2   & 3,061(14,914) & 5.87e-08 & 4.65    \\

 \hline
 \hline

-$743^3$ & 8192 (4x2048)        & 704.4     & 87,822    & 4.80e-07 &  110.3   & 1,531(12,721) & 1.47e-07& 6.39  \\
+$743^3$ & 8,192 (4x2,048)        & 704.4     & 87,822    & 4.80e-07 &  110.3   & 1,531(12,721) & 1.47e-07& 6.39  \\

 \hline
 
 \end{tabular}
 \caption{Results}
 \label{tab1}
 \hline
 
 \end{tabular}
 \caption{Results}
 \label{tab1}

+\end{small}
+\end{changemargin}

 \end{center}
 \end{table}
 
 
 From these  experiments, it can be  observed that the  multisplitting version is
 always  faster   than  the  GMRES   version.   The  acceleration  gain   of  the
 \end{center}
 \end{table}
 
 
 From these  experiments, it can be  observed that the  multisplitting version is
 always  faster   than  the  GMRES   version.   The  acceleration  gain   of  the

-multisplitting version is between 4 and 6.  It can be noticed that the number of
-iterations is drastically reduced with  the multisplitting version even it is not
-neglectable.
+multisplitting version ranges between 4 and 6.  It can be noticed that the number of
+iterations is drastically reduced with the multisplitting version even it is not
+negligible. Moreover, with 8,192 cores, we  can see that using 4 clusters gives a
+better performance than simply using 2 clusters. In fact, we can notice that the
+precision with 2 clusters is slightly  better but in both cases the precision is
+under the specified threshold.

 
 \section{Conclusion and perspectives}
 
 \section{Conclusion and perspectives}

-We have implemented a Krylov multisplitting method to solve sparse linear systems on large-scale computing platforms. We have developed a synchronous two-stage method based on the block Jacobi multisplitting and uses GMRES iterative method as an inner iteration. Our contribution in this paper is twofold. First we have constituted a virtual multi-cluster environment based on processors of the computing platform on which each linear sub-system issued from the splitting is solved in parallel by a cluster of processors. Second, we have implemented the outer iteration of the multisplitting method as a Krylov subspace method which minimizes some error function. This increases the convergence and improves the scalability of the multisplitting method.
-
-We have tested our multisplitting method to solve the sparse linear system issued from the discretization of a 3D Poisson problem. We have compared its performances to the classical GMRES method on a supercomputer composed of 2048 to 8192 cores. The experimental results showed that the multisplitting method is about 4 to 6 times faster than the GMRES method for different sizes of the problem split into 2 or 4 blocks when using multisplitting method. Indeed, the GMRES method has difficulties to scale with many cores while the Krylov multisplitting method allows to hide latency and reduce the inter-cluster communications.
-
-In future works, we plan to conduct experiments on larger number of cores and test the scalability of our Krylov multisplitting method. It would be interesting to validate its performances to solve other linear/nonlinear and symmetric/nonsymmetric problems. Moreover, we intend to develop multisplitting methods based on asynchronous iteration in which communications are overlapped by computations. These methods would be interesting for platforms composed of distant clusters interconnected by a high-latency network. In addition, we intend to investigate the convergence improvements of our method by using preconditioning techniques for Krylov iterative methods and multisplitting methods with overlapping blocks.    
-
+We  have implemented  a  Krylov  multisplitting method  to  solve sparse  linear
+systems  on large-scale computing  platforms.  We  have developed  a synchronous
+two-stage  method based  on the  block Jacobi  multisaplitting which  uses GMRES
+iterative  method as  an inner  iteration.  Our  contribution in  this  paper is
+twofold. First we provide a multi cluster decomposition that allows us to choose
+the  appropriate size  of  the clusters  according  to the  architecures of  the
+supercomputer.  Second,   we  have  implemented  the  outer   iteration  of  the
+multisplitting method  as a  Krylov subspace method  which minimizes  some error
+function.  This  increases the convergence  and improves the scalability  of the
+multisplitting method.
+
+We  have tested  our multisplitting  method to  solve the  sparse  linear system
+issued from  the discretization of  a 3D Poisson  problem. We have  compared its
+performances to the  classical GMRES method on a  supercomputer composed of 2,048
+to 8,192 cores. The experimental results showed that the multisplitting method is
+about 4  to 6  times faster  than the GMRES  method for  different sizes  of the
+problem split into  2 or 4 blocks when using the  multisplitting method. Indeed, the
+GMRES  method  has  difficulties to  scale  with  many  cores while  the  Krylov
+multisplitting  method  allows to  hide  latency  and  reduce the  inter-cluster
+communications.
+
+In future  works, we plan to conduct  experiments on larger numbers  of cores and
+test  the  scalability  of  our   Krylov  multisplitting  method.  It  would  be
+interesting  to validate its  performances to  solve other  linear/nonlinear and
+symmetric/nonsymmetric problems.  Moreover, we intend  to develop multisplitting
+methods based  on asynchronous iterations in which  communications are overlapped
+by computations.  These methods would  be interesting for platforms  composed of
+distant  clusters interconnected  by  a high-latency  network.  In addition,  we
+intend  to investigate  the  convergence  improvements of  our  method by  using
+preconditioning  techniques  for  Krylov  iterative methods  and  multisplitting
+methods with overlapping blocks.
+
+\section{Acknowledgement}
+The authors would like to thank Mark Bull of the EPCC his fruitful remarks and the facilities of HECToR.

 
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