From: couturie Date: Wed, 6 May 2015 14:22:23 +0000 (+0200) Subject: Merge branch 'master' of ssh://bilbo.iut-bm.univ-fcomte.fr/rce2015 X-Git-Url: https://bilbo.iut-bm.univ-fcomte.fr/and/gitweb/rce2015.git/commitdiff_plain/1202c4ef616b826822fb0f5997da04e599a3c20f?ds=sidebyside;hp=-c Merge branch 'master' of ssh://bilbo.iut-bm.univ-fcomte.fr/rce2015 --- 1202c4ef616b826822fb0f5997da04e599a3c20f diff --combined paper.tex index 82777e3,e60e242..1f9b49c --- a/paper.tex +++ b/paper.tex @@@ -96,20 -96,21 +96,21 @@@ performed. With some applications, it i 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. + 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 developpers to better tune their applications for a given multi-core + architecture. - In this paper, we focus our attention on two parallel iterative algorithms based + In particular 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 variants of + These algorithms are used to solve linear systems. Two different variants of 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. - - - + one with asynchronous iterations. For each algorithm we have simulated + different architecture parameters to evaluate their influence on the overall + execution time. The obtain simulated results confirm the real results + previously obtained on different real multi-core architectures and also confirm + the efficiency of the asynchronous multisplitting algorithm compared to the + synchronous GMRES method. \end{abstract} @@@ -690,30 -691,32 +691,30 @@@ powerful CPU \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. We have focused -the study on analyzing the performance in varying the key factors impacting the -results. The study compares the performance of the two proposed algorithms both -in \textit{synchronous mode }. In this section, following the same previous -methodology, the goal is to demonstrate the efficiency of the multisplitting -method in \textit{ asynchronous mode} compared with the classical GMRES staying -in \textit{synchronous mode}. - -Note that the interest of using the asynchronous mode for data exchange -is mainly, in opposite of the synchronous mode, the non-wait aspects of -the current computation after a communication operation like sending -some data between nodes. Each processor can continue their local -calculation without waiting for the end of the communication. Thus, the -asynchronous may theoretically reduce the overall execution time and can -improve the algorithm performance. - -As stated supra, Simgrid simulator tool has been used to prove the -efficiency of the multisplitting in asynchronous mode and to find the -best combination of the grid resources (CPU, Network, input matrix size, -\ldots ) to get the highest \textit{"relative gain"} (exec\_time$_{GMRES}$ / exec\_time$_{multisplitting}$) in comparison with the classical GMRES time. +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} 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. + +\RC{la phrase suivante est bizarre, je ne comprends pas pourquoi elle vient ici} +As stated before, the Simgrid simulator tool has been successfully used to show +the efficiency of the multisplitting in asynchronous mode and to find the best +combination of the grid resources (CPU, Network, input matrix size, \ldots ) to +get the highest \textit{"relative gain"} (exec\_time$_{GMRES}$ / +exec\_time$_{multisplitting}$) in comparison with the classical GMRES time. The test conditions are summarized in the table below : \\ -% environment -\begin{footnotesize} +\begin{figure} [ht!] +\centering \begin{tabular}{r c } \hline Grid & 2x50 totaling 100 processors\\ %\hline @@@ -723,17 -726,15 +724,17 @@@ Input matrix size & N$_{x}$ = From 62 to 150\\ %\hline Residual error precision & 10$^{-5}$ to 10$^{-9}$\\ \hline \\ \end{tabular} -\end{footnotesize} +\end{figure} -Again, comprehensive and extensive tests have been conducted varying the -CPU power and the network parameters (bandwidth and latency) in the -simulator tool with different problem size. The relative gains greater -than 1 between the two algorithms have been captured after each step of -the test. Table 7 below has recorded the best grid configurations -allowing the multisplitting method execution time more performant 2.5 times than -the classical GMRES execution and convergence time. The experimentation has demonstrated the relative multisplitting algorithm tolerance when using a low speed network that we encounter usually with distant clusters thru the internet. +Again, comprehensive and extensive tests have been conducted with different +parametes as the CPU power, the network parameters (bandwidth and latency) in +the simulator tool 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 Figure~\ref{table:01} 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 throuth the internet. % use the same column width for the following three tables \newlength{\mytablew}\settowidth{\mytablew}{\footnotesize\np{E-11}} @@@ -744,12 -745,14 +745,12 @@@ \end{tabular}} -\begin{table}[!t] - \centering +\begin{figure}[!t] +\centering +%\begin{table} % \caption{Relative gain of the multisplitting algorithm compared with the classical GMRES} % \label{"Table 7"} -Table 7. Relative gain of the multisplitting algorithm compared with -the classical GMRES \\ - - \begin{mytable}{11} + \begin{mytable}{11} \hline bandwidth (Mbit/s) & 5 & 5 & 5 & 5 & 5 & 50 & 50 & 50 & 50 & 50 \\ @@@ -770,11 -773,7 +771,11 @@@ & 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} +%\end{table} + \caption{Relative gain of the multisplitting algorithm compared with the classical GMRES} + \label{table:01} +\end{figure} + \section{Conclusion} CONCLUSION