X-Git-Url: https://bilbo.iut-bm.univ-fcomte.fr/and/gitweb/rce2015.git/blobdiff_plain/1162f45db29e22394d4564bea71aef816419cba2..a2c6056cb76ee160a5ad4afb5495207c9158a85c:/paper.tex?ds=sidebyside diff --git a/paper.tex b/paper.tex index 1468528..e37e28c 100644 --- a/paper.tex +++ b/paper.tex @@ -1,4 +1,17 @@ -\documentclass[conference]{IEEEtran} +\documentclass[times]{cpeauth} + +\usepackage{moreverb} + +%\usepackage[dvips,colorlinks,bookmarksopen,bookmarksnumbered,citecolor=red,urlcolor=red]{hyperref} + +%\newcommand\BibTeX{{\rmfamily B\kern-.05em \textsc{i\kern-.025em b}\kern-.08em +%T\kern-.1667em\lower.7ex\hbox{E}\kern-.125emX}} + +\def\volumeyear{2015} + +\usepackage{graphicx} +\usepackage{wrapfig} +\usepackage{grffile} \usepackage[T1]{fontenc} \usepackage[utf8]{inputenc} @@ -8,10 +21,11 @@ \usepackage{algpseudocode} %\usepackage{amsthm} \usepackage{graphicx} -\usepackage[american]{babel} % Extension pour les liens intra-documents (tagged PDF) % et l'affichage correct des URL (commande \url{http://example.com}) %\usepackage{hyperref} +\usepackage{multirow} + \usepackage{url} \DeclareUrlCommand\email{\urlstyle{same}} @@ -23,6 +37,7 @@ \usepackage{xspace} \usepackage[textsize=footnotesize]{todonotes} + \newcommand{\AG}[2][inline]{% \todo[color=green!50,#1]{\sffamily\textbf{AG:} #2}\xspace} \newcommand{\RC}[2][inline]{% @@ -31,6 +46,8 @@ \todo[color=blue!10,#1]{\sffamily\textbf{LZK:} #2}\xspace} \newcommand{\RCE}[2][inline]{% \todo[color=yellow!10,#1]{\sffamily\textbf{RCE:} #2}\xspace} +\newcommand{\DL}[2][inline]{% + \todo[color=pink!10,#1]{\sffamily\textbf{DL:} #2}\xspace} \algnewcommand\algorithmicinput{\textbf{Input:}} \algnewcommand\Input{\item[\algorithmicinput]} @@ -38,7 +55,12 @@ \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} @@ -48,70 +70,851 @@ \newcolumntype{g}{>{\columncolor{Gray}}c} \definecolor{Gray}{gray}{0.9} + + \begin{document} -\RCE{Titre a confirmer.} -\title{Comparative performance analysis of simulated grid-enabled numerical iterative algorithms} - -\author{% - \IEEEauthorblockN{% - Charles Emile Ramamonjisoa and - David Laiymani and - Arnaud Giersch and - Lilia Ziane Khodja and - Raphaël Couturier - } - \IEEEauthorblockA{% - Femto-ST Institute - DISC Department\\ - Université de Franche-Comté\\ - Belfort\\ - Email: \email{{raphael.couturier,arnaud.giersch,david.laiymani,charles.ramamonjisoa}@univ-fcomte.fr} - } +\title{Grid-enabled simulation of large-scale linear iterative solvers} +%\itshape{\journalnamelc}\footnotemark[2]} + +\author{Charles Emile Ramamonjisoa\affil{1}, + David Laiymani\affil{1}, + Arnaud Giersch\affil{1}, + Lilia Ziane Khodja\affil{2} and + Raphaël Couturier\affil{1} } -\maketitle +\address{ + \affilnum{1}% + Femto-ST Institute, DISC Department, + University of Franche-Comté, + Belfort, France. + Email:~\email{{charles.ramamonjisoa,david.laiymani,arnaud.giersch,raphael.couturier}@univ-fcomte.fr}\break + \affilnum{2} + Department of Aerospace \& Mechanical Engineering, + Non Linear Computational Mechanics, + University of Liege, Liege, Belgium. + Email:~\email{l.zianekhodja@ulg.ac.be} +} -\begin{abstract} -ABSTRACT +\begin{abstract} %% The behavior of multi-core 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. The main contribution of this paper is to show that the use of a +%% simulation tool (here we have decided to use the SimGrid toolkit) can really +%% help developers to better tune their applications for a given multi-core +%% architecture. +%% In this paper we focus our attention on the simulation of iterative algorithms to solve sparse linear systems on large clusters. We study the behavior of the widely used GMRES algorithm and two different variants of the Multisplitting algorithms: one using synchronous iterations and another one with asynchronous iterations. +%% For each algorithm we have simulated +%% different architecture parameters to evaluate their influence on the overall +%% execution time. +%% The simulations confirm the real results previously obtained on different real multi-core architectures and also confirm the efficiency of the asynchronous Multisplitting algorithm on distant clusters compared to the synchronous GMRES algorithm. -Keywords : Algorithm distributed iterative asynchronous simulation simgrid performance +The behavior of multi-core 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. -\end{abstract} +In this paper we focus on the simulation of iterative algorithms to solve sparse linear systems. We study the behavior of the GMRES algorithm and two different variants of the multisplitting algorithms: using synchronous or asynchronous iterations. For each algorithm we have simulated different architecture parameters to evaluate their influence on the overall execution time. The simulations confirm the real results previously obtained on different real multi-core architectures and also confirm the efficiency of the asynchronous multisplitting algorithm on distant clusters compared to the GMRES algorithm. -\section{Introduction} +\end{abstract} -\section{The asynchronous iteration model} +%\keywords{Algorithm; distributed; iterative; asynchronous; simulation; simgrid; +%performance} +\keywords{ Performance evaluation, Simulation, SimGrid, Synchronous and asynchronous iterations, Multisplitting algorithms} -\section{SimGrid} +\maketitle -\section{Simulation of the multisplitting method} +\section{Introduction} The use of multi-core architectures to solve large +scientific problems seems to become imperative in many situations. +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\dots{} In counterpart, the simulation results need to be consistent 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 studies 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 +\textit{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 communications and computations are \textit{asynchronous} +inducing that there is no more idle time, 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 a small +parameter variation of the execution platform and of the application data can +lead to very different numbers of iterations to reach the convergence 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 {\bf 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 applications for a given multi-core architecture. To show the +validity of this approach we first compare the simulated execution of the Krylov +multisplitting algorithm with the GMRES (Generalized Minimal RESidual) +solver~\cite{saad86} in synchronous mode. The simulation results allow us to +determine which method to choose for a given multi-core architecture. +Moreover the obtained results on different simulated multi-core architectures +confirm the real results previously obtained on non simulated architectures. +More precisely the simulated results are in accordance (i.e. with the same order +of magnitude) with the works presented in~\cite{couturier15}, which show that +the synchronous Krylov multisplitting method is more efficient than GMRES for large +scale clusters. Simulated results also confirm the efficiency of the +asynchronous multisplitting algorithm compared to the synchronous GMRES +especially in case of geographically distant clusters. + +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 and the motivations of our work} +\label{sec:asynchro} + +Asynchronous iterative methods have been studied for many years theoretically and +practically. Many methods have been considered and convergence results have been +proved. These methods can be used to solve, in parallel, fixed point problems +(i.e. problems for which the solution is $x^\star =f(x^\star)$. In practice, +asynchronous iteration methods can be used to solve, for example, linear and +non-linear systems of equations or optimization problems, interested readers are +invited to read~\cite{BT89,bahi07}. + +Before using an asynchronous iterative method, the convergence must be +studied. Otherwise, the application is not ensure to reach the convergence. An +algorithm that supports both the synchronous or the asynchronous iteration model +requires very few modifications to be able to be executed in both variants. In +practice, only the communications and convergence detection are different. In +the synchronous mode, iterations are synchronized whereas in the asynchronous +one, they are not. It should be noticed that non-blocking communications can be +used in both modes. Concerning the convergence detection, synchronous variants +can use a global convergence procedure which acts as a global synchronization +point. In the asynchronous model, the convergence detection is more tricky as +it must not synchronize all the processors. Interested readers can +consult~\cite{myBCCV05c,bahi07,ccl09:ij}. + +The number of iterations required to reach the convergence is generally greater +for the asynchronous scheme (this number depends on the delay of the +messages). Note that, it is not the case in the synchronous mode where the +number of iterations is the same than in the sequential mode. In this way, the +set of the parameters of the platform (number of nodes, power of nodes, +inter and intra clusters bandwidth and latency,~\ldots) and of the +application can drastically change the number of iterations required to get the +convergence. It follows that asynchronous iterative algorithms are difficult to +optimize since the financial and deployment costs on large scale multi-core +architectures are often very important. So, prior to deployment and tests it +seems very promising to be able to simulate the behavior of asynchronous +iterative algorithms. The problematic is then to show that the results produced +by simulation are in accordance with reality i.e. of the same order of +magnitude. To our knowledge, there is no study on this problematic. -\section{Experiments and results} +\section{SimGrid} +\label{sec:simgrid} +SimGrid~\cite{SimGrid,casanova+legrand+quinson.2008.simgrid,casanova+giersch+legrand+al.2014.versatile} is a discrete event simulation framework to study the behavior of large-scale distributed computing platforms as Grids, Peer-to-Peer systems, Clouds and High Performance Computation systems. It is widely used to simulate and evaluate heuristics, prototype applications or even assess legacy MPI applications. It is still actively developed by the scientific community and distributed as an open source software. + +%%%%%%%%%%%%%%%%%%%%%%%%% +% SimGrid~\cite{SimGrid,casanova+legrand+quinson.2008.simgrid,casanova+giersch+legrand+al.2014.versatile} +% is a simulation framework to study the behavior of large-scale distributed +% systems. As its name suggests, it emanates from the grid computing community, +% but is nowadays used to study grids, clouds, HPC or peer-to-peer systems. The +% early versions of SimGrid date back from 1999, but it is still actively +% developed and distributed as an open source software. Today, it is one of the +% major generic tools in the field of simulation for large-scale distributed +% systems. + +SimGrid provides several programming interfaces: MSG to simulate Concurrent +Sequential Processes, SimDAG to simulate DAGs of (parallel) tasks, and SMPI to +run real applications written in MPI~\cite{MPI}. Apart from the native C +interface, SimGrid provides bindings for the C++, Java, Lua and Ruby programming +languages. SMPI is the interface that has been used for the work described in +this paper. The SMPI interface implements about \np[\%]{80} of the MPI 2.0 +standard~\cite{bedaride+degomme+genaud+al.2013.toward}, and supports +applications written in C or Fortran, with little or no modifications (cf Section IV - paragraph B). + +Within SimGrid, the execution of a distributed application is simulated by a +single process. The application code is really executed, but some operations, +like communications, are intercepted, and their running time is computed +according to the characteristics of the simulated execution platform. The +description of this target platform is given as an input for the execution, by +means of an XML file. It describes the properties of the platform, such as +the computing nodes with their computing power, the interconnection links with +their bandwidth and latency, and the routing strategy. The scheduling of the +simulated processes, as well as the simulated running time of the application +are computed according to these properties. + +To compute the durations of the operations in the simulated world, and to take +into account resource sharing (e.g. bandwidth sharing between competing +communications), SimGrid uses a fluid model. This allows users to run relatively fast +simulations, while still keeping accurate +results~\cite{bedaride+degomme+genaud+al.2013.toward, + velho+schnorr+casanova+al.2013.validity}. Moreover, depending on the +simulated application, SimGrid/SMPI allows to skip long lasting computations and +to only take their duration into account. When the real computations cannot be +skipped, but the results are unimportant for the simulation results, it is +also possible to share dynamically allocated data structures between +several simulated processes, and thus to reduce the whole memory consumption. +These two techniques can help to run simulations on a very large scale. + +The validity of simulations with SimGrid has been asserted by several studies. +See, for example, \cite{velho+schnorr+casanova+al.2013.validity} and articles +referenced therein for the validity of the network models. Comparisons between +real execution of MPI applications on the one hand, and their simulation with +SMPI on the other hand, are presented in~\cite{guermouche+renard.2010.first, + clauss+stillwell+genaud+al.2011.single, + bedaride+degomme+genaud+al.2013.toward}. All these works conclude that +SimGrid is able to simulate pretty accurately the real behavior of the +applications. +%%%%%%%%%%%%%%%%%%%%%%%%% + +\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). 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~\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, has been studied by many authors for example~\cite{Bru95,bahi07}. + +\begin{figure}[htpb] +%\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 the asynchronous model which allows 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}[htbp] +%\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 the two-stage methods using SimGrid toolkit} +\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 to ensure the +test reproducibility under the same 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 +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" +(smpi/privatize\_global\_variables). Indeed, global variables can generate side +effects on runtime between the threads running in the same process and generated by +SimGrid to simulate the grid environment. + +\paragraph{Parameters of the simulation in SimGrid} +\ \\ \noindent Before running a SimGrid benchmark, many parameters for the +computation platform must be defined. For our experiments, we consider platforms +in which several clusters are geographically distant, so there are intra and +inter-cluster communications. In the following, these parameters are described: + +\begin{itemize} + \item hostfile: hosts description file, + \item platform: 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 in each cluster). +\end{itemize} +\noindent +In addition, the following arguments are given to the programs at runtime: + +\begin{itemize} + \item maximum number of inner iterations $\MIG$ and outer iterations $\MIM$, + \item inner precision $\TOLG$ and outer precision $\TOLM$, + \item matrix sizes of the problem: N$_{x}$, N$_{y}$ and N$_{z}$ on axis $x$, $y$ and $z$ respectively (in our experiments, we solve 3D problem, see Section~\ref{3dpoisson}), + \item matrix diagonal value is fixed to $6.0$ for synchronous experiments and $6.2$ for asynchronous ones, + \item matrix off-diagonal value is fixed to $-1.0$, + \item number of vectors in matrix $S$ (i.e. value of $s$), + \item maximum number of iterations $\MIC$ and precision $\TOLC$ for CGLS method, + \item maximum number of iterations and precision for the classical GMRES method, + \item maximum number of restarts for the Arnorldi process in GMRES method, + \item execution mode: synchronous or asynchronous. +\end{itemize} + +It should also be noticed that both solvers have been executed with the SimGrid selector \texttt{-cfg=smpi/running\_power} which determines the computational power (here 19GFlops) of the simulator host machine. + +%%%%%%%%%%%%%%%%%%%%%%%%% +%%%%%%%%%%%%%%%%%%%%%%%%% + +\section{Experimental results} +\label{sec:expe} + +In this section, experiments for both multisplitting algorithms are reported. First the 3D Poisson problem used in our experiments is described. + +\subsection{The 3D Poisson problem} +\label{3dpoisson} +We use our two-stage algorithms to solve the well-known Poisson problem $\nabla^2\phi=f$~\cite{Polyanin01}. In three-dimensional Cartesian coordinates in $\mathbb{R}^3$, the problem takes the following form: +\begin{equation} +\frac{\partial^2}{\partial x^2}\phi(x,y,z)+\frac{\partial^2}{\partial y^2}\phi(x,y,z)+\frac{\partial^2}{\partial z^2}\phi(x,y,z)=f(x,y,z)\mbox{~in the domain~}\Omega +\label{eq:07} +\end{equation} +such that: +\begin{equation*} +\phi(x,y,z)=0\mbox{~on the boundary~}\partial\Omega +\end{equation*} +where the real-valued function $\phi(x,y,z)$ is the solution sought, $f(x,y,z)$ is a known function and $\Omega=[0,1]^3$. The 3D discretization of the Laplace operator $\nabla^2$ with the finite difference scheme includes 7 points stencil on the computational grid. The numerical approximation of the Poisson problem on three-dimensional grid is repeatedly computed as $\phi=\phi^\star$ such that: +\begin{equation} +\begin{array}{ll} +\phi^\star(x,y,z)=&\frac{1}{6}(\phi(x-h,y,z)+\phi(x,y-h,z)+\phi(x,y,z-h)\\&+\phi(x+h,y,z)+\phi(x,y+h,z)+\phi(x,y,z+h)\\&-h^2f(x,y,z)) +\end{array} +\label{eq:08} +\end{equation} +until convergence where $h$ is the grid spacing between two adjacent elements in the 3D computational grid. + +In the parallel context, the 3D Poisson problem is partitioned into $L\times p$ sub-problems such that $L$ is the number of clusters and $p$ is the number of processors in each cluster. We apply the three-dimensional partitioning instead of the row-by-row one in order to reduce the size of the data shared at the sub-problems boundaries. In this case, each processor is in charge of parallelepipedic block of the problem and has at most six neighbors in the same cluster or in distant clusters with which it shares data at boundaries. + +\subsection{Study setup and simulation methodology} + +First, to conduct our study, we propose the following methodology +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 this paper. \\ + +\textbf{Step 2}: Collect the software materials needed for the experimentation. +In our case, we have two variants algorithms for the resolution of the +3D-Poisson problem: (1) using the classical GMRES; (2) and the multisplitting +method. In addition, the SimGrid simulator has been chosen to simulate the +behaviors of the distributed applications. SimGrid is running in a virtual +machine on a simple 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 +on the one hand the algorithm execution mode (synchronous and asynchronous) +and on the other hand the execution time and the number of iterations to reach the convergence. \\ + +\textbf{Step 4}: Set up the different grid testbed environments that will be +simulated in the simulator tool to run the program. The following architectures +have been configured in SimGrid : 2$\times$16, 4$\times$8, 4$\times$16, 8$\times$8 and 2$\times$50. The first number +represents the number of clusters in the grid and the second number represents +the number of hosts (processors/cores) in each cluster. \\ + +\textbf{Step 5}: Conduct an extensive and comprehensive testings +within these configurations by varying the key parameters, especially +the CPU power capacity, the network parameters and also the size of the +input data. \\ + +\textbf{Step 6} : Collect and analyze the output results. + +\subsection{Factors impacting distributed applications performance in a grid environment} + +When running a distributed application in a computational grid, many factors may +have a strong impact on the performance. First of all, the architecture of the +grid itself can obviously influence the performance results of the program. The +performance gain might be important theoretically when the number of clusters +and/or the number of nodes (processors/cores) in each individual cluster +increase. + +Another important factor impacting the overall performance of the application +is the network configuration. Two main network parameters can modify drastically +the program output results: +\begin{enumerate} +\item the network bandwidth ($bw$ in bits/s) also known as "the data-carrying + capacity" of the network is defined as the maximum of data that can transit + from one point to another in a unit of time. +\item the network latency ($lat$ in microseconds) defined as the delay from the + start time to send a simple data from a source to a destination. +\end{enumerate} +Upon the network characteristics, another impacting factor is the volume of data exchanged between the nodes in the cluster +and between distant clusters. This parameter is application dependent. + + In a grid environment, it is common to distinguish, on the one hand, the + "intra-network" which refers to the links between nodes within a cluster and + on the other hand, the "inter-network" which is the backbone link between + clusters. In practice, these two networks have different speeds. + The intra-network generally works like a high speed local network with a + high bandwidth and very low latency. In opposite, the inter-network connects + clusters sometime via heterogeneous networks components through internet with + a lower speed. The network between distant clusters might be a bottleneck + for the global performance of the application. + + +\subsection{Comparison between GMRES and two-stage multisplitting algorithms in synchronous mode} +In the scope of this paper, our first objective is to analyze when the synchronous Krylov two-stage method has better performance than the classical GMRES method. With a synchronous iterative method, better performance means a smaller number of iterations and execution time before reaching the convergence. + +Table~\ref{tab:01} summarizes the parameters used in the different simulations: the grid architectures, the network of inter-clusters backbone links and the matrix sizes of the 3D Poisson problem. However, for all simulations we fix the network parameters of the intra-clusters links: the bandwidth $bw$=10Gbs and the latency $lat$=8$\times$10$^{-6}$. In what follows, we will present the test conditions, the output results and our comments. + +\begin{table} [ht!] +\begin{center} +\begin{tabular}{ll} +\hline +Grid architecture & 2$\times$16, 4$\times$8, 4$\times$16 and 8$\times$8\\ +\multirow{2}{*}{Network inter-clusters} & $N1$: $bw$=1Gbs, $lat$=5$\times$10$^{-5}$ \\ + & $N2$: $bw$=10Gbs, $lat$=8$\times$10$^{-6}$ \\ +\multirow{2}{*}{Matrix size} & $Mat1$: N$_{x}\times$N$_{y}\times$N$_{z}$=150$\times$150$\times$150\\ + & $Mat2$: N$_{x}\times$N$_{y}\times$N$_{z}$=170$\times$170$\times$170 \\ \hline +\end{tabular} +\caption{Parameters for the different simulations} +\label{tab:01} +\end{center} +\end{table} + + + + +\subsubsection{Simulations for various grid architectures and scaling-up matrix sizes} +\ \\ +% environment + +Table~\ref{tab:01} summarizes the different parameters used in the simulations: the grid architectures, the network of inter-cluster backbone links and the matrix sizes of the 3D Poisson problem. + + + + + + + + + + + +In this section, we analyze the simulations conducted on various grid +configurations presented in Table~\ref{tab:01}. It should be noticed that two +networks are considered: N1 is the network between clusters (inter-cluster) and +N2 is the network inside a cluster (intra-cluster). Figure~\ref{fig:01} shows, +for all grid configurations and a given matrix size, a non-variation in the +number of iterations for the classical GMRES algorithm, which is not the case of +the Krylov two-stage algorithm. +%% First, the results in Figure~\ref{fig:01} +%% show for all grid configurations the non-variation of the number of iterations of +%% classical GMRES for a given input matrix size; it is not the case for the +%% multisplitting method. +%\RC{CE attention tu n'as pas mis de label dans tes figures, donc c'est le bordel, j'en mets mais vérifie...} +%\RC{Les légendes ne sont pas explicites...} +%\RCE{Corrige} + +\begin{figure} [htbp] + \begin{center} + \includegraphics[width=100mm]{cluster_x_nodes_nx_150_and_nx_170.pdf} + \end{center} + \caption{Various grid configurations with the matrix sizes 150$^3$ and 170$^3$} +%\AG{Utiliser le point comme séparateur décimal et non la virgule. Idem dans les autres figures.} +%\LZK{Pour quelle taille du problème sont calculés les nombres d'itérations? Que représente le 2 Clusters x 16 Nodes with Nx=150 and Nx=170 en haut de la figure?} + %\RCE {Corrige} + \RC{Idéalement dans la légende il faudrait insiquer Pb size=$150^3$ ou $170^3$ car pour l'instant Nx=150 ca n'indique rien concernant Ny et Nz} + \label{fig:01} +\end{figure} + + + +The execution times between the two algorithms is significant with different +grid architectures, even with the same number of processors (for example, 2 $\times$ 16 +and 4 $\times 8$). We can observe a better sensitivity of the Krylov multisplitting method +(compared with the classical GMRES) when scaling up the number of the processors +in the grid: in average, the GMRES (resp. Multisplitting) algorithm performs +$40\%$ better (resp. $48\%$) when running from 32 (grid 2 $\times$ 16) to 64 processors/cores (grid 8 $\times$ 8). Note that even with a grid 8 $\times$ 8 having the maximum number of clusters, the execution time of the multisplitting method is in average 32\% less compared to GMRES. +\RC{pas très clair, c'est pas précis de dire qu'un algo perform mieux qu'un autre, selon quel critère?} +\LZK{A revoir toute cette analyse... Le multi est plus performant que GMRES. Les temps d'exécution de multi sont sensibles au nombre de CLUSTERS. Il est moins performant pour un nombre grand de cluster. Avez vous d'autres remarques?} +\RCE{Remarquez que meme avec une grille 8x8, le multi est toujours plus performant} + +\subsubsection{Simulations for two different inter-clusters network speeds \\} + +\begin{table} [ht!] +\begin{center} +\begin{tabular}{ll} + \hline + Grid architecture & 2$\times$16, 4$\times$8\\ %\hline + \multirow{2}{*}{Inter Network} & N1: $bw$=1Gbs, $lat$=5$\times$10$^{-5}$ \\ %\hline + & N2: $bw$=10Gbs, $lat$=8$\times$10$^{-6}$ \\ + Matrix size & $N_{x} \times N_{y} \times N_{z} =150 \times 150 \times 150$\\ \hline + \end{tabular} +\caption{Test conditions: grid configurations 2$\times$16 and 4$\times$8 with networks N1 vs. N2} +\label{tab:02} +\end{center} +\end{table} + +In this section, the experiments compare the behavior of the algorithms running on a +speeder inter-cluster network (N2) and also on a less performant network (N1) respectively defined in the test conditions Table~\ref{tab:02}. +%\RC{Il faut définir cela avant...} +Figure~\ref{fig:02} shows that end users will reduce the execution time +for both algorithms when using a grid architecture like 4 $\times$ 16 or 8 $\times$ 8: the reduction factor is around $2$. The results depict also that when +the network speed drops down (variation of 12.5\%), the difference between the two Multisplitting algorithms execution times can reach more than 25\%. + + + +%\begin{wrapfigure}{l}{100mm} +\begin{figure} [htbp] +\centering +\includegraphics[width=100mm]{cluster_x_nodes_n1_x_n2.pdf} +\caption{Various grid configurations with networks N1 vs N2} +%\AG{\np{8E-6}, \np{5E-6} au lieu de 8E-6, 5E-6}} +%\RCE{Corrige} +\label{fig:02} +\end{figure} +%\end{wrapfigure} + + +\subsubsection{Network latency impacts on performance} +\ \\ +\begin{table} [ht!] +\centering +\begin{tabular}{r c } + \hline + Grid Architecture & 2 $\times$ 16\\ %\hline + \multirow{2}{*}{Inter Network N1} & $bw$=1Gbs, \\ %\hline + & $lat$= From 8$\times$10$^{-6}$ to $6.10^{-5}$ second \\ + Input matrix size & $N_{x} \times N_{y} \times N_{z} = 150 \times 150 \times 150$\\ \hline + \end{tabular} +\caption{Test conditions: network latency impacts} +\label{tab:03} +\end{table} + +\begin{figure} [htbp] +\centering +\includegraphics[width=100mm]{network_latency_impact_on_execution_time.pdf} +\caption{Network latency impacts on execution time} +%\AG{\np{E-6}}} +\label{fig:03} +\end{figure} + +In Table~\ref{tab:03}, parameters for the influence of the network latency are +reported. According to the results of Figure~\ref{fig:03}, a degradation of the +network latency from $8.10^{-6}$ to $6.10^{-5}$ implies an absolute time +increase of more than $75\%$ (resp. $82\%$) of the execution for the classical +GMRES (resp. Krylov multisplitting) algorithm. The execution time factor +between the two algorithms varies from 2.2 to 1.5 times with a network latency +decreasing from $8.10^{-6}$ to $6.10^{-5}$. + + +\subsubsection{Network bandwidth impacts on performance} +\ \\ +\begin{table} [ht!] +\centering +\begin{tabular}{r c } + \hline + Grid Architecture & 2 $\times$ 16\\ %\hline +\multirow{2}{*}{Inter Network N1} & $bw$=From 1Gbs to 10 Gbs \\ %\hline + & $lat$= 5.10$^{-5}$ second \\ + Input matrix size & $N_{x} \times N_{y} \times N_{z} =150 \times 150 \times 150$\\ \hline \\ + \end{tabular} +\caption{Test conditions: Network bandwidth impacts} +% \RC{Qu'est ce qui varie ici? Il n'y a pas de variation dans le tableau} +%\RCE{C est le bw} +\label{tab:04} +\end{table} + + +\begin{figure} [htbp] +\centering +\includegraphics[width=100mm]{network_bandwith_impact_on_execution_time.pdf} +\caption{Network bandwith impacts on execution time} +%\AG{``Execution time'' avec un 't' minuscule}. Idem autres figures.} +%\RCE{Corrige} +\label{fig:04} +\end{figure} + +The results of increasing the network bandwidth show the improvement of the +performance for both algorithms by reducing the execution time (see +Figure~\ref{fig:04}). However, in this case, the Krylov multisplitting method +presents a better performance in the considered bandwidth interval with a gain +of $40\%$ which is only around $24\%$ for the classical GMRES. + +\subsubsection{Input matrix size impacts on performance} +\ \\ +\begin{table} [ht!] +\centering +\begin{tabular}{r c } + \hline + Grid Architecture & 4 $\times$ 8\\ %\hline + Inter Network & $bw$=1Gbs - $lat$=5.10$^{-5}$ \\ + Input matrix size & $N_{x} \times N_{y} \times N_{z}$ = From 50$^{3}$ to 190$^{3}$\\ \hline + \end{tabular} +\caption{Test conditions: Input matrix size impacts} +\label{tab:05} +\end{table} + + +\begin{figure} [htbp] +\centering +\includegraphics[width=100mm]{pb_size_impact_on_execution_time.pdf} +\caption{Problem size impacts on execution time} +\label{fig:05} +\end{figure} + +In these experiments, the input matrix size has been set from $50^3$ to +$190^3$. Obviously, as shown in Figure~\ref{fig:05}, the execution time for both +algorithms increases when the input matrix size also increases. For all problem +sizes, GMRES is always slower than the Krylov multisplitting. Moreover, for this +benchmark, it seems that the greater the problem size is, the bigger the ratio +between both algorithm execution times is. We can also observ that for some +problem sizes, the Krylov multisplitting convergence varies quite a +lot. Consequently the execution times in that cases also varies. + + +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. It should be noticed that the same test has been done with the +grid 2 $\times$ 16 leading to the same conclusion. + +\subsubsection{CPU Power impacts on performance} + +\begin{table} [htbp] +\centering +\begin{tabular}{r c } + \hline + Grid architecture & 2 $\times$ 16\\ %\hline + Inter Network & N2 : $bw$=1Gbs - $lat$=5.10$^{-5}$ \\ %\hline + Input matrix size & $N_{x} = 150 \times 150 \times 150$\\ + CPU Power & From 3 to 19 GFlops \\ \hline + \end{tabular} +\caption{Test conditions: CPU Power impacts} +\label{tab:06} +\end{table} + +\begin{figure} [ht!] +\centering +\includegraphics[width=100mm]{cpu_power_impact_on_execution_time.pdf} +\caption{CPU Power impacts on execution time} +\label{fig:06} +\end{figure} + +Using the Simgrid simulator flexibility, we have tried to determine the impact +on the algorithms performance in varying the CPU power of the clusters nodes +from $1$ to $19$ GFlops. The outputs depicted in Figure~\ref{fig:06} confirm the +performance gain, around $95\%$ for both of the two methods, after adding more +powerful CPU. +\ \\ +%\DL{il faut une conclusion sur ces tests : ils confirment les résultats déjà +%obtenus en grandeur réelle. Donc c'est une aide précieuse pour les dev. Pas +%besoin de déployer sur une archi réelle} + +To conclude these series of experiments, with SimGrid we have been able to make +many simulations with many parameters variations. Doing all these experiments +with a real platform is most of the time not possible. Moreover the behavior of +both GMRES and Krylov multisplitting methods is in accordance with larger real +executions on large scale supercomputer~\cite{couturier15}. + + +\subsection{Comparing GMRES in native synchronous mode and the multisplitting algorithm in asynchronous mode} + +The previous paragraphs put in evidence the interests to simulate the behavior +of the application before any deployment in a real environment. In this +section, following the same previous methodology, our goal is to compare the +efficiency of the multisplitting method in \textit{ asynchronous mode} compared with the +classical GMRES in \textit{synchronous mode}. + +The interest of using an asynchronous algorithm is that there is no more +synchronization. With geographically distant clusters, this may be essential. +In this case, each processor can compute its iteration freely without any +synchronization with the other processors. Thus, the asynchronous may +theoretically reduce the overall execution time and can improve the algorithm +performance. + +In this section, the Simgrid simulator is used to compare the behavior of the +multisplitting in asynchronous mode with GMRES in synchronous mode. Several +benchmarks have been performed with various combination of the grid resources +(CPU, Network, input matrix size, \ldots ). The test conditions are summarized +in Table~\ref{tab:07}. In order to compare the execution times, this table +reports the relative gain between both algorithms. It is defined by the ratio +between the execution time of GMRES and the execution time of the +multisplitting. The ration is greater than one because the asynchronous +multisplitting version is faster than GMRES. + + + +\begin{table} [htbp] +\centering +\begin{tabular}{r c } + \hline + Grid Architecture & 2 $\times$ 50 totaling 100 processors\\ %\hline + Processors Power & 1 GFlops to 1.5 GFlops\\ + Intra-Network & bw=1.25 Gbits - lat=5.10$^{-5}$ \\ %\hline + Inter-Network & bw=5 Mbits - lat=2.10$^{-2}$\\ + Input matrix size & $N_{x}$ = From 62 to 150\\ %\hline + Residual error precision & 10$^{-5}$ to 10$^{-9}$\\ \hline \\ + \end{tabular} +\caption{Test conditions: GMRES in synchronous mode vs Krylov Multisplitting in asynchronous mode} +\label{tab:07} +\end{table} + +Again, comprehensive and extensive tests have been conducted with different +parameters as the CPU power, the network parameters (bandwidth and latency) +and with different problem size. The relative gains greater than $1$ between the +two algorithms have been captured after each step of the test. In +Table~\ref{tab:08} are reported the best grid configurations allowing +the multisplitting method to be more than $2.5$ times faster than the +classical GMRES. These experiments also show the relative tolerance of the +multisplitting algorithm when using a low speed network as usually observed with +geographically distant clusters through the internet. + +% use the same column width for the following three tables +\newlength{\mytablew}\settowidth{\mytablew}{\footnotesize\np{E-11}} +\newenvironment{mytable}[1]{% #1: number of columns for data + \renewcommand{\arraystretch}{1.3}% + \begin{tabular}{|>{\bfseries}r% + |*{#1}{>{\centering\arraybackslash}p{\mytablew}|}}}{% + \end{tabular}} + + +\begin{table}[!t] +\centering +%\begin{table} +% \caption{Relative gain of the multisplitting algorithm compared with the classical GMRES} +% \label{"Table 7"} + \begin{mytable}{11} + \hline + bandwidth (Mbit/s) + & 5 & 5 & 5 & 5 & 5 & 50 & 50 & 50 & 50 & 50 \\ + \hline + latency (ms) + & 20 & 20 & 20 & 20 & 20 & 20 & 20 & 20 & 20 & 20 \\ + \hline + power (GFlops) + & 1 & 1 & 1 & 1.5 & 1.5 & 1.5 & 1.5 & 1 & 1.5 & 1.5 \\ + \hline + size ($N^3$) + & 62 & 62 & 62 & 100 & 100 & 110 & 120 & 130 & 140 & 150 \\ + \hline + Precision + & \np{E-5} & \np{E-8} & \np{E-9} & \np{E-11} & \np{E-11} & \np{E-11} & \np{E-11} & \np{E-11} & \np{E-11} & \np{E-11}\\ + \hline + Relative gain + & 2.52 & 2.55 & 2.52 & 2.57 & 2.54 & 2.53 & 2.51 & 2.58 & 2.55 & 2.54 \\ + \hline + \end{mytable} +%\end{table} + \caption{Relative gain of the multisplitting algorithm compared with the classical GMRES} + \label{tab:08} +\end{table} -\paragraph*{1. Study setup and methodology} -\paragraph*{2. Factors impacting distributed applications performance in a grid environment} +\section{Conclusion} -\paragraph*{3. Comparing GMRES and Multisplitting algorithms in synchronous mode} +In this paper we have presented the simulation of the execution of three +different parallel solvers on some multi-core architectures. We have show that +the SimGrid toolkit is an interesting simulation tool that has allowed us to +determine which method to choose given a specified multi-core architecture. +Moreover the simulated results are in accordance (i.e. with the same order of +magnitude) with the works presented in~\cite{couturier15}. Simulated results +also confirm the efficiency of the asynchronous multisplitting +algorithm compared to the synchronous GMRES especially in case of +geographically distant clusters. -\paragraph*{4. Comparing GMRES in synchronous mode and Multisplitting algorithms in asynchronous mode} +These results are important since 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. This problematic is even more difficult for the asynchronous +scheme where a small parameter variation of the execution platform and of the +application data can lead to very different numbers of iterations to reach the +converge and so to very different execution times. -\section{Conclusion} -CONCLUSION +In future works, we plan to investigate how to simulate the behavior of really +large scale applications. For example, if we are interested to simulate the +execution of the solvers of this paper with thousand or even dozens of thousands +or core, it is not possible to do that with SimGrid. In fact, this tool will +make the real computation. So we plan to focus our research on that problematic. -\section*{Acknowledgment} -The authors would like to thank\dots{} +%\section*{Acknowledgment} +\ack +This work is partially funded by the Labex ACTION program (contract ANR-11-LABX-01-01). +\bibliographystyle{wileyj} +\bibliography{biblio} -% 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} \end{document}