new

[kahina_paper2.git] / paper.tex

diff --git a/paper.tex b/paper.tex
index 5df0ef9ccf4d22496e782ee70983c9a04c2a7923..7d4fe8a3046decb03464aefa320d42e0d790f29c 100644 (file)
--- a/paper.tex
+++ b/paper.tex
@@ -7,6 +7,9 @@
 \usepackage[utf8]{inputenc}
 \usepackage[T1]{fontenc}
 \usepackage[textsize=footnotesize]{todonotes}
 \usepackage[utf8]{inputenc}
 \usepackage[T1]{fontenc}
 \usepackage[textsize=footnotesize]{todonotes}

+\usepackage{amsmath}
+\usepackage{amssymb}
+

 \newcommand{\LZK}[2][inline]{%
   \todo[color=red!10,#1]{\sffamily\textbf{LZK:} #2}\xspace}
 \newcommand{\RC}[2][inline]{%
 \newcommand{\LZK}[2][inline]{%
   \todo[color=red!10,#1]{\sffamily\textbf{LZK:} #2}\xspace}
 \newcommand{\RC}[2][inline]{%


@@ -21,7 +24,7 @@
 
 \title{Two parallel implementations of Ehrlich-Aberth algorithm for root-finding of polynomials on multiple GPUs with OpenMP and MPI}
 
 
 \title{Two parallel implementations of Ehrlich-Aberth algorithm for root-finding of polynomials on multiple GPUs with OpenMP and MPI}
 

-\author{\IEEEauthorblockN{Kahina Guidouche, Abderrahmane Sider }
+\author{\IEEEauthorblockN{Kahina Ghidouche, Abderrahmane Sider }

   \IEEEauthorblockA{Laboratoire LIMED\\
     Faculté des sciences exactes\\
     Université de Bejaia, 06000, Algeria\\
   \IEEEauthorblockA{Laboratoire LIMED\\
     Faculté des sciences exactes\\
     Université de Bejaia, 06000, Algeria\\


@@ -49,9 +52,9 @@ million. Moreover, other experiments show it is possible to find roots
 of polynomials of degree up-to 5 millions.
 \end{abstract}
 
 of polynomials of degree up-to 5 millions.
 \end{abstract}
 

-% no keywords
-\LZK{Faut pas mettre des keywords?}
-
+\begin{IEEEkeywords}
+  root finding method, Ehrlich-Aberth method, GPU, MPI, OpenMP
+\end{IEEEkeywords}

 
 \IEEEpeerreviewmaketitle
 
 
 \IEEEpeerreviewmaketitle
 


@@ -61,13 +64,13 @@ of polynomials of degree up-to 5 millions.
 \section{Introduction}
 
 
 \section{Introduction}
 
 

-Finding roots of polynomials of very high degrees arises in many complex problems in various domains such as algebra, biology or physics. A polynomial $p(x)$ in $\mathbb{C}$ in one variable $x$ is an algebraic expression in $x$ of the form:
+Finding roots of polynomials of very high degrees arises in many complex problems of various domains such as algebra, biology or physics. A polynomial $p(x)$ in $\mathbb{C}$ in one variable $x$ is an algebraic expression in $x$ of the form:

 \begin{equation}
 \begin{equation}

-p(x) = \displaystyle\sum^n_{i=0}{a_ix^i},a_n\neq 0, 
+p(x) = \displaystyle\sum^n_{i=0}{\alpha_ix^i},\alpha_n\neq 0, 

 \end{equation}
 \end{equation}

-where $\{a_i\}_{0\leq i\leq n}$ are complex coefficients and $n$ is a high integer number. If $a_n\neq0$ then $n$ is called the degree of the polynomial. The root-finding problem consists in finding the $n$ different values of the unknown variable $x$ for which $p(x)=0$. Such values are called roots of $p(x)$. Let $\{z_i\}_{1\leq i\leq n}$ be the roots of polynomial $p(x)$, then $p(x)$ can be written as :
+where $\{\alpha_i\}_{0\leq i\leq n}$ are complex coefficients and $n$ is a high integer number. If $\alpha_n\neq0$ then $n$ is called the degree of the polynomial. The root-finding problem consists in finding the $n$ different values of the unknown variable $x$ for which $p(x)=0$. Such values are called roots of $p(x)$. Let $\{z_i\}_{1\leq i\leq n}$ be the roots of polynomial $p(x)$, then $p(x)$ can be written as :

 \begin{equation}
 \begin{equation}

- p(x)=a_n\displaystyle\prod_{i=1}^n(x-z_i), a_n\neq 0.
+ p(x)=\alpha_n\displaystyle\prod_{i=1}^n(x-z_i), \alpha_n\neq 0.

 \end{equation}
 
 Most of the numerical methods that deal with the polynomial root-finding problem are simultaneous methods, \textit{i.e.} the iterative methods to find simultaneous approximations of the $n$ polynomial roots. These methods start from the initial approximations of all $n$ polynomial roots and give a sequence of approximations that converge to the roots of the polynomial. Two examples of well-known simultaneous methods for root-finding problem of polynomials are  Durand-Kerner method~\cite{Durand60,Kerner66} and Ehrlich-Aberth method~\cite{Ehrlich67,Aberth73}.
 \end{equation}
 
 Most of the numerical methods that deal with the polynomial root-finding problem are simultaneous methods, \textit{i.e.} the iterative methods to find simultaneous approximations of the $n$ polynomial roots. These methods start from the initial approximations of all $n$ polynomial roots and give a sequence of approximations that converge to the roots of the polynomial. Two examples of well-known simultaneous methods for root-finding problem of polynomials are  Durand-Kerner method~\cite{Durand60,Kerner66} and Ehrlich-Aberth method~\cite{Ehrlich67,Aberth73}.


@@ -88,28 +91,30 @@ which each processor continues to update its approximations even
 though the latest values of other approximations $z^{k}_{i}$ have not
 been received from the other processors, in contrast with synchronous
 algorithms where it would wait those values before making a new
 though the latest values of other approximations $z^{k}_{i}$ have not
 been received from the other processors, in contrast with synchronous
 algorithms where it would wait those values before making a new

-iteration. Couturier et al.~\cite{Raphaelall01} proposed two methods
+iteration. Couturier and al.~\cite{cs01:nj} proposed two methods

 of parallelization for a shared memory architecture with OpenMP and
 for a distributed memory one with MPI. They are able to compute the
 of parallelization for a shared memory architecture with OpenMP and
 for a distributed memory one with MPI. They are able to compute the

-roots of sparse polynomials of degree 10,000 in 116 seconds with
-OpenMP and 135 seconds with MPI only by using 8 personal computers and
-2 communications per iteration. The authors showed an interesting
-speedup comparing to the sequential implementation which takes up-to
-3,300 seconds to obtain same results. 
-\RC{si on donne des temps faut donner le proc, comme c'est vieux à mon avis faut supprimer ca, votre avis?} 
-\LZK{Supprimons ces détails et mettons une référence s'il y en a une}
+roots of sparse polynomials of degree 10,000. The authors showed an interesting
+speedup that is 20 times as fast as the sequential implementation. 
+

 
 

-Very few work had been performed since then until the appearing of the Compute Unified Device Architecture (CUDA)~\cite{CUDA15}, a parallel computing platform and a programming model invented by NVIDIA. The computing power of GPUs (Graphics Processing Units) has exceeded that of traditional processors CPUs. However, CUDA adopts a totally new computing architecture to use the hardware resources provided by the GPU in order to offer a stronger computing ability to the massive data computing. Ghidouche et al.~\cite{Kahinall14} proposed an implementation of the Durand-Kerner method on a single GPU. Their main results showed that a parallel CUDA implementation is about 10 times faster than the sequential implementation on a single CPU for sparse polynomials of degree 48,000.
+Very few work had been performed since then until the appearing of the Compute Unified Device Architecture (CUDA)~\cite{CUDA15}, a parallel computing platform and a programming model invented by NVIDIA. The computing power of GPUs (Graphics Processing Units) has exceeded that of traditional processors CPUs. However, CUDA adopts a totally new computing architecture to use the hardware resources provided by the GPU in order to offer a stronger computing ability to the massive data computing. Ghidouche et al.~\cite{Kahinall14} proposed an implementation of the Durand-Kerner method on a single GPU. Their main results showed that a parallel CUDA implementation is about 10 times faster than the sequential implementation on a single CPU for sparse polynomials of degree 48,000. %\LZK{Y a pas d'autres travaux pour la résolution de polynômes sur GPUs?}

 
 In this paper we propose the parallelization of Ehrlich-Aberth method which has a good convergence and it is suitable to be implemented in parallel computers. We use two parallel programming paradigms OpenMP and MPI on CUDA multi-GPU platforms. Our CUDA-MPI and CUDA-OpenMP codes are the first implementations of Ehrlich-Aberth method with multiple GPUs for finding roots of polynomials. Our major contributions include:
 
 In this paper we propose the parallelization of Ehrlich-Aberth method which has a good convergence and it is suitable to be implemented in parallel computers. We use two parallel programming paradigms OpenMP and MPI on CUDA multi-GPU platforms. Our CUDA-MPI and CUDA-OpenMP codes are the first implementations of Ehrlich-Aberth method with multiple GPUs for finding roots of polynomials. Our major contributions include:

-\LZK{J'ai ajouté une phrase pour justifier notre choix de la méthode Ehrlich-Aberth. A revérifier.}

  \begin{itemize}
 
 \item The parallel implementation of Ehrlich-Aberth algorithm on a multi-GPU platform with a shared memory using OpenMP API. It is based on threads created from the same system process, such that each thread is attached to one GPU. In this case the communications between GPUs are done by OpenMP threads through shared memory.
  \begin{itemize}
 
 \item The parallel implementation of Ehrlich-Aberth algorithm on a multi-GPU platform with a shared memory using OpenMP API. It is based on threads created from the same system process, such that each thread is attached to one GPU. In this case the communications between GPUs are done by OpenMP threads through shared memory.

-\item The parallel implementation of Ehrlich-Aberth algorithm on a multi-GPU platform with a distributed memory using MPI API, such that each GPU is attached and managed by a MPI process. The GPUs exchange their data by message-passing communications. 
+\item The parallel implementation of Ehrlich-Aberth algorithm on a
+  multi-GPU platform with a distributed memory using MPI API, such
+  that each GPU is attached and managed by a MPI process. The GPUs
+  exchange their data by message-passing communications.
+\item
+  Our method is efficient to compute the roots of sparse and full
+  polynomials of degree up to 5 millions.

  \end{itemize}
 This latter approach is more used on clusters to solve very complex problems that are too large for traditional supercomputers, which are very expensive to build and run.
  \end{itemize}
 This latter approach is more used on clusters to solve very complex problems that are too large for traditional supercomputers, which are very expensive to build and run.

-\LZK{Pas d'autres contributions possibles? J'ai supprimé les deux premiers points proposés précédemment.}
+%\LZK{Pas d'autres contributions possibles? J'ai supprimé les deux premiers points proposés précédemment.
+%\AS{La résolution du problème pour des polynomes pleins de degré 6M est une contribution aussi à mon avis}}

 
 The paper is organized as follows. In Section~\ref{sec2} we present three different parallel programming models OpenMP, MPI and CUDA. In Section~\ref{sec3} we present the implementation of the Ehrlich-Aberth algorithm on a single GPU. In Section~\ref{sec4} we present the parallel implementations of the Ehrlich-Aberth algorithm on multiple GPUs using the OpenMP and MPI approaches. In section~\ref{sec5} we present our experiments and discuss them. Finally, Section~\ref{sec6} concludes this paper and gives some hints for future research directions in this topic. 
 
 
 The paper is organized as follows. In Section~\ref{sec2} we present three different parallel programming models OpenMP, MPI and CUDA. In Section~\ref{sec3} we present the implementation of the Ehrlich-Aberth algorithm on a single GPU. In Section~\ref{sec4} we present the parallel implementations of the Ehrlich-Aberth algorithm on multiple GPUs using the OpenMP and MPI approaches. In section~\ref{sec5} we present our experiments and discuss them. Finally, Section~\ref{sec6} concludes this paper and gives some hints for future research directions in this topic. 
 


@@ -134,7 +139,7 @@ MPI (Message Passing Interface) is a portable message passing style of the paral
 \subsection{CUDA}
 
 
 \subsection{CUDA}
 
 

-CUDA (Compute Unified Device Architecture) is a parallel computing architecture developed by NVIDIA~\cite{CUDA15} for GPUs. It provides a high level GPGPU-based programming model to program GPUs for general purpose computations and non-graphic applications. The GPU is viewed as an accelerator such that data-parallel operations of a CUDA program running on a CPU are off-loaded onto GPU and executed by this later. The data-parallel operations executed by GPUs are called kernels. The same kernel is executed in parallel by a large number of threads organized in grids of thread blocks, such that each GPU multiprocessor executes one or more thread blocks in SIMD fashion (Single Instruction, Multiple Data) and in turn each core of the multiprocessor executes one or more threads within a block. Threads within a block can cooperate by sharing data through a fast shared memory and coordinate their execution through synchronization points. In contrast, within a grid of thread blocks, there is no synchronization at all between blocks. The GPU only works on data filled in the global memory and the final results of the kernel executions must be transferred out of the GPU. In the GPU, the global memory has lower bandwidth than the shared memory associated to each multiprocessor. Thus in the CUDA programming, it is necessary to design carefully the arrangement of the thread blocks in order to ensure low latency and a proper usage of the shared memory, and the global memory accesses should be minimized.
+CUDA (Compute Unified Device Architecture) is a parallel computing architecture developed by NVIDIA~\cite{CUDA15} for GPUs. It provides a high level GPGPU-based programming model to program GPUs for general purpose computations. The GPU is viewed as an accelerator such that data-parallel operations of a CUDA program running on a CPU are off-loaded onto GPU and executed by this later. The data-parallel operations executed by GPUs are called kernels. The same kernel is executed in parallel by a large number of threads organized in grids of thread blocks, such that each GPU multiprocessor executes one or more thread blocks in SIMD fashion (Single Instruction, Multiple Data) and in turn each core of the multiprocessor executes one or more threads within a block. Threads within a block can cooperate by sharing data through a fast shared memory and coordinate their execution through synchronization points. In contrast, within a grid of thread blocks, there is no synchronization at all between blocks. The GPU only works on data filled in the global memory and the final results of the kernel executions must be transferred out of the GPU. In the GPU, the global memory has lower bandwidth than the shared memory associated to each multiprocessor. Thus in the CUDA programming, it is necessary to design carefully the arrangement of the thread blocks in order to ensure low latency and a proper usage of the shared memory, and the global memory accesses should be minimized.

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


@@ -177,7 +182,7 @@ In order to solve this problem, we propose to modify the iterative
 function by using the logarithm and the exponential of a complex and
 we propose a new version of the Ehrlich-Aberth method.  This method
 allows us to exceed the computation of the polynomials of degree
 function by using the logarithm and the exponential of a complex and
 we propose a new version of the Ehrlich-Aberth method.  This method
 allows us to exceed the computation of the polynomials of degree

-100,000 and to reach a degree up to more than 1,000,000. The reformulation of the iteration~(\ref{Eq:EA1}) of the Ehrlich-Aberth method with exponential and logarithm is defined as follows, for $i=1,\dots,n$:
+100,000 and to reach a degree up to more than 1,000,000. The reformulation of the iteration~(\ref{Eq:EA1}) of the Ehrlich-Aberth method with exponential and logarithm operators is defined as follows, for $i=1,\dots,n$:

 
 \begin{equation}
 \label{Log_H2}
 
 \begin{equation}
 \label{Log_H2}


@@ -193,7 +198,16 @@ Q(z^k_i) = \exp(\ln(p(z^k_i)) - \ln(p'(z^k_i)) + \ln(\sum_{i\neq j}^n\frac{1}{z^
 
 
 
 
 
 

-Using the logarithm  and the exponential operators, we can replace any multiplications and divisions with additions and subtractions. Consequently, computations manipulate lower values in absolute values~\cite{Karimall98}. 
+Using the logarithm  and the exponential operators, we can replace any
+multiplications and divisions with additions and
+subtractions. Consequently, computations manipulate lower values in
+absolute values~\cite{Karimall98}. In practice, the  exponential and
+logarithm mode is used a root excepts the circle unit, represented by the radius $R$ evaluated in C language as :
+\begin{equation}
+\label{R.EL}
+R = exp(log(DBL\_MAX)/(2*n) );
+\end{equation}
+where \verb=DBL_MAX= stands for the maximum representable \verb=double= value.

 
 
 \subsection{The Ehrlich-Aberth parallel implementation on CUDA}
 
 
 \subsection{The Ehrlich-Aberth parallel implementation on CUDA}


@@ -204,20 +218,20 @@ Using the logarithm  and the exponential operators, we can replace any multiplic
 The code is organized as kernels which are parts of code that are run
 on GPU devices. Algorithm~\ref{alg1-cuda} describes the CUDA
 implementation of the Ehrlich-Aberth on a GPU.  This algorithms starts
 The code is organized as kernels which are parts of code that are run
 on GPU devices. Algorithm~\ref{alg1-cuda} describes the CUDA
 implementation of the Ehrlich-Aberth on a GPU.  This algorithms starts

-by initializinf the polynomial and its derivative (line 1). The
-initialization of the roots is performed (line 2). Data are transfered
+by initializing the polynomial and its derivative (line 1). The
+initialization of the roots is performed (line 2). Data are transferred

 from the CPU to the GPU (after allocation of the required memory on
 the GPU) (line 3). Then at each iteration, if the error is greater
 than a threshold, the following operations are performed. The previous
 from the CPU to the GPU (after allocation of the required memory on
 the GPU) (line 3). Then at each iteration, if the error is greater
 than a threshold, the following operations are performed. The previous

-roots are saved using a kernel (line 5). Then the new root with the
-new iterations are computed using the EA method with a Gauss Seidel
-iteration modes in order to use the lastest roots updated (line
-6). This improves the convergence. This kernel is, in pratice, very
+roots are saved using a kernel (line 5). Then the new roots with the
+new iterations are computed using the EA method with a Gauss-Seidel
+iteration mode in order to use the lastest updated roots (line
+6). This improves the convergence. This kernel is, in practice, very

 long since it performs all the operations with complex numbers with
 the normal mode of the EA method but also with the
 logarithm-exponential one. Then the error is computed with a final
 kernel (line 7). Finally when the EA method has converged, the roots
 long since it performs all the operations with complex numbers with
 the normal mode of the EA method but also with the
 logarithm-exponential one. Then the error is computed with a final
 kernel (line 7). Finally when the EA method has converged, the roots

-are transferred from the GPU to the CPU.
+are transferred from the GPU to the CPU.%\LZK{Quand est ce qu'on utilise un normal mode ou logarithm-exponential mode?}

 
 \begin{algorithm}[htpb]
 \label{alg1-cuda}
 
 \begin{algorithm}[htpb]
 \label{alg1-cuda}


@@ -249,32 +263,35 @@ OpenMP and MPI is presented.
 
 
  
 
 
  

-\section{The EA algorithm on multiple GPUs}
+\section{The Ehrlich-Aberth algorithm on multiple GPUs}

 \label{sec4}
 \label{sec4}

-\subsection{an OpenMP-CUDA approach}
+\subsection{An OpenMP-CUDA approach}

 Our OpenMP-CUDA implementation of EA algorithm is based on the hybrid
 Our OpenMP-CUDA implementation of EA algorithm is based on the hybrid

-OpenMP and CUDA programming model.  All the data are shared with
-OpenMP amoung all the OpenMP threads. The shared data are the solution
-vector $Z$, the polynomial to solve $P$, and the error vector $\Delta
-z$. The number of OpenMP threads is equal to the number of GPUs, each
-OpenMP thread binds to one GPU, and it controls a part of the shared
-memory. More precisely each OpenMP thread will be responsible to
-update its owns part of the vector Z. This part is call $Z_{loc}$ in
-the following. Then all GPUs will have a grid of computation organized
-according to the device performance and the size of data on which it
-runs the computation kernels.
+OpenMP and CUDA programming model. This algorithm is presented in
+Algorithm~\ref{alg2-cuda-openmp}.  All the data are shared with OpenMP
+among all the OpenMP threads. The shared data are the solution vector
+$Z$, the polynomial to solve $P$, its derivative $P'$, and the error
+vector $error$. The number of OpenMP threads is equal to the number of
+GPUs, each OpenMP thread binds to one GPU, and it controls a part of
+the shared memory. More precisely each OpenMP thread will be
+responsible to update its owns part of the vector $Z$. This part is
+called $Z_{loc}$ in the following. Then all GPUs will have a grid of
+computation organized according to the device performance and the size
+of data on which it runs the computation kernels.

 
 To compute one iteration of the EA method each GPU performs the
 followings steps. First roots are shared with OpenMP and the
 
 To compute one iteration of the EA method each GPU performs the
 followings steps. First roots are shared with OpenMP and the

-computation of the local size for each GPU is performed (lines 5-7 in
-Algorithm \ref{alg2-cuda-openmp}). Each thread starts by copying all the
-previous roots inside its GPU (line 9). Then each GPU will copy the
-previous roots (line 10) and it will compute an iteration of the EA
-method on its own roots (line 11).  For that all the other roots are
-used. The convergence is checked on the new roots (line 12). At the end
-of an iteration, the updated roots are copied from the GPU to the
-CPU (line 14) by direcly updating its own roots in the shared memory
-arrays containing all the roots.
+computation of the local size for each GPU is performed (line 4). Each
+thread starts by copying all the previous roots inside its GPU (line
+5). At each iteration, the following operations are performed. First
+the vector $Z$ is transferred from the CPU to the GPU (line 7).  Each
+GPU copies the previous roots (line 8) and it computes an iteration of
+the EA method on its own roots (line 9).  For that all the other roots
+are used. The local error is computed on the new roots (line 10) and
+the max of the local errors is computed on all OpenMP threads (lien 11). At
+the end of an iteration, the updated roots are copied from the GPU to
+the CPU (line 12) by directly updating its own roots in the shared
+memory arrays containing all the roots.

 
 
 
 
 
 


@@ -304,37 +321,37 @@ Copy $P$, $P'$ from CPU to GPU\;
 
 
 
 
 
 

-\subsection{a MPI-CUDA approach}
+\subsection{A MPI-CUDA approach}

 
 

-Our parallel implementation of EA to find root of polynomials using a
-CUDA-MPI approach follows a similar computing approach to the one used
-in CUDA-OpenMP. Each process is responsible to compute its own part of
+Our parallel implementation of EA to find roots of polynomials using a
+CUDA-MPI approach follows a similar approach to the one used in
+CUDA-OpenMP. Each process is responsible to compute its own part of

 roots using all the roots computed by other processors at the previous
 iteration. The difference between both approaches lies in the way
 roots using all the roots computed by other processors at the previous
 iteration. The difference between both approaches lies in the way

-processes communicate and exchange data. With MPI processors need to
-send and receive data explicitely. So in
+processes communicate and exchange data. With MPI, processors need to
+send and receive data explicitly. So in

 Algorithm~\ref{alg2-cuda-mpi}, after the initialization all the
 processors have the same $Z$ vector. Then they need to compute the
 Algorithm~\ref{alg2-cuda-mpi}, after the initialization all the
 processors have the same $Z$ vector. Then they need to compute the

-parameters used by the $MPI\_AlltoAll$ routines (line 4). In practise,
-each processor needs to compute its offset and its local size. Then
-processors need to allocate memory on their GPU (line 5). At the
-beginning of each iteration, a processor starts by transfering the
-whole vector Z from the CPU to the GPU (line 7). Then only the local
-part of $Z^{prev}$ is saved (line 8). After that, a processor is able
-to compute its own roots (line 9). Next, the local error can be
-computed (ligne 10) and the global error (line 11). Then the local
-roots are transfered from the GPU memory to the CPU memory (line 12)
-before being exchanged between all processors (linge 13) in order to
-give to all processors the last version of the roots. If the
-convergence is not statisfied, an new iteration is executed.
-
-
+parameters used by the $MPI\_AlltoAll$ routines (line 4). In practice,
+each processor needs to compute its offset and its local
+size. Processors need to allocate memory on their GPU and need to copy
+their data on the GPU (line 5). At the beginning of each iteration, a
+processor starts by transferring the whole vector $Z$ from the CPU to the
+GPU (line 7). Only the local part of $Z^{prev}$ is saved (line
+8). After that, a processor is able to compute an updated version of
+its own roots (line 9) with the EA method. The local error is computed
+(line 10) and the global error using $MPI\_Reduce$ (line 11). Then
+the local roots are transferred from the GPU memory to the CPU memory
+(line 12) before being exchanged between all processors (line 13) in
+order to give to all processors the last version of the roots (with
+the MPI\_AlltoAll routine). If the convergence is not satisfied, an
+new iteration is executed.

 
 \begin{algorithm}[htpb]
 \label{alg2-cuda-mpi}
 \LinesNumbered
 \SetAlgoNoLine
 
 \begin{algorithm}[htpb]
 \label{alg2-cuda-mpi}
 \LinesNumbered
 \SetAlgoNoLine

-\caption{CUDA-MPI Algorithm to find roots with the Ehrlich-Aberth method}
+\caption{Finding roots of polynomials with the Ehrlich-Aberth method on multiple GPUs using MPI}

 
 \KwIn{  $\epsilon$ (tolerance threshold)}
 
 
 \KwIn{  $\epsilon$ (tolerance threshold)}
 


@@ -363,201 +380,111 @@ Copy $P$, $P'$ from CPU to GPU\;
 We study two categories of polynomials: sparse polynomials and full polynomials.\\
 {\it A sparse polynomial} is a polynomial for which only some coefficients are not null. In this paper, we consider sparse polynomials for which the roots are distributed on 2 distinct circles:
 \begin{equation}
 We study two categories of polynomials: sparse polynomials and full polynomials.\\
 {\it A sparse polynomial} is a polynomial for which only some coefficients are not null. In this paper, we consider sparse polynomials for which the roots are distributed on 2 distinct circles:
 \begin{equation}

-       \forall \alpha_{1} \alpha_{2} \in C,\forall n_{1},n_{2} \in N^{*}; P(z)= (z^{n_{1}}-\alpha_{1})(z^{n_{2}}-\alpha_{2})
+       \forall \alpha_{1} \alpha_{2} \in \mathbb{C},\forall n_{1},n_{2} \in \mathbb{N}^{*}; p(z)= (z^{n_{1}}-\alpha_{1})(z^{n_{2}}-\alpha_{2})

 \end{equation}\noindent
 {\it A full polynomial} is, in contrast, a polynomial for which all the coefficients are not null. A full polynomial is defined by:
 \end{equation}\noindent
 {\it A full polynomial} is, in contrast, a polynomial for which all the coefficients are not null. A full polynomial is defined by:

-%%\begin{equation}
-       %%\forall \alpha_{i} \in C,\forall n_{i}\in N^{*}; P(z)= \sum^{n}_{i=1}(z^{n^{i}}.a_{i})
-%%\end{equation}

 
 \begin{equation}
 
 \begin{equation}

-     {\Large \forall a_{i} \in C, i\in N;  p(x)=\sum^{n}_{i=0} a_{i}.x^{i}} 
+     {\Large \forall \alpha_{i} \in \mathbb{C}, i\in \mathbb{N};  p(x)=\sum^{n}_{i=0} \alpha_{i}.x^{i}} 

 \end{equation}
 
 \end{equation}
 

-For our test, 4 cards GPU tesla Kepler K40 are used.  In order to
-evaluate both the GPU and Multi-GPU approaches, we performed a set of
-experiments on a single GPU and multiple GPUs using OpenMP or MPI with
-the EA algorithm, for both sparse and full polynomials of different
-sizes.  All experimental results obtained are perfomed with double
-precision float data and the convergence threshold of the EA method is
-set to $10^{-7}$.  The initialization values of the vector solution of
-the methods are given by Guggenheimer method~\cite{Gugg86}.
+For our tests, 4 cards GPU Tesla Kepler K40 are used.  In order to evaluate both the GPU and Multi-GPU approaches, we performed a set of experiments on a single GPU and multiple GPUs using OpenMP or MPI with the EA algorithm, for both sparse and full polynomials of different sizes.  All experimental results obtained are performed with double precision float data and the convergence threshold of the EA method is set to $10^{-7}$.  The initialization values of the vector solution of the methods are given by Guggenheimer method~\cite{Gugg86}.

 
 

+\subsection{Evaluation of the multi-GPUs approaches}
+Here we evaluate the performances of the CUDA-OpenMP and CUDA-MPI approaches of the EA algorithm on different GPU platforms composed each of 1, 2, 3 or 4 GPUs. In this experiments we report the experimental results of the EA algorithms to find roots of different sparse and full polynomials of high degrees ranging from 100,000 to 1,400,000. Figures~\ref{fig:01} and~\ref{fig:02} show the execution times to solve, respectively, sparse and full polynomials with the CUDA-OpenMP algorithm, and Figures~\ref{fig:03} and~\ref{fig:04} show those to solve, respectively, sparse and full polynomials with the CUDA-MPI algorithm.

 
 

-\subsection{Evaluation of the CUDA-OpenMP approach}
-
-Here we report some experiments witt full and sparse polynomials of
-different degrees with multiple GPUs.
-\subsubsection{Execution times of the EA method to solve sparse polynomials on multiple GPUs}
- 
-In this experiments we report the execution time of the EA algorithm, on single GPU and multi-GPUs with (2,3,4) GPUs, for different sparse polynomial degrees ranging from 100,000 to 1,400,000.
+All these figures show that the CUDA-OpenMP and CUDA-MPI approaches of the EA algorithm, compared to the single GPU version, are efficient and scale well with multiple GPUs. Both approaches allow us to solve sparse and full polynomials of very high degrees. Using 4 GPUs allows us to achieve a quasi-linear speedup.

 
 \begin{figure}[htbp]
 \centering
 
 \begin{figure}[htbp]
 \centering

-  \includegraphics[angle=-90,width=0.5\textwidth]{Sparse_omp}
-\caption{Execution time in seconds of the Ehrlich-Aberth method to
-  solve sparse polynomials on multiple GPUs with CUDA-OpenMP.}
+\includegraphics[angle=-90,width=0.5\textwidth]{Sparse_omp}
+\caption{Execution times in seconds of the Ehrlich-Aberth method to solve sparse polynomials on multiple GPUs with CUDA-OpenMP.}

 \label{fig:01}
 \end{figure}
 
 \label{fig:01}
 \end{figure}
 

-Figure~\ref{fig:01} shows that the CUDA-OpenMP approach scales well
-with multiple GPUs. This version allows us to solve sparse polynomials
-of very high degrees.
-
-\subsubsection{Execution times of the EA method to solve full polynomials on multiple GPUs}
-
-These experiments show the execution times of the EA algorithm, on a single GPU and on multiple GPUs using the CUDA OpenMP approach for full polynomials of degrees ranging from 100,000 to 1,400,000.
-

 \begin{figure}[htbp]
 \centering
 \begin{figure}[htbp]
 \centering

-  \includegraphics[angle=-90,width=0.5\textwidth]{Full_omp}
-\caption{Execution time in seconds of the Ehrlich-Aberth method to
-  solve full polynomials on multiple GPUs with CUDA-OpenMP.}
+\includegraphics[angle=-90,width=0.5\textwidth]{Full_omp}
+\caption{Execution times in seconds of the Ehrlich-Aberth method to solve full polynomials on multiple GPUs with CUDA-OpenMP.}

 \label{fig:02}
 \end{figure}
 
 \label{fig:02}
 \end{figure}
 

-In Figure~\ref{fig:02}, we can observe that with full polynomials the EA version with
-CUDA-OpenMP scales also well. Using 4 GPUs allows us to achieve a
-quasi-linear speedup.
-
-\subsection{Evaluation of the CUDA-MPI approach}
-In this part we perform some experiments to evaluate the CUDA-MPI
-approach to solve full and sparse polynomials of degrees ranging from
-100,000 to 1,400,000.
-
-\subsubsection{Execution times of the EA method to solve sparse polynomials on multiple GPUs}
-

 \begin{figure}[htbp]
 \centering
   \includegraphics[angle=-90,width=0.5\textwidth]{Sparse_mpi}
 \begin{figure}[htbp]
 \centering
   \includegraphics[angle=-90,width=0.5\textwidth]{Sparse_mpi}

-\caption{Execution time in seconds of the Ehrlich-Aberth method to
-  solve sparse polynomials on multiple GPUs with CUDA-MPI.}
+ \caption{Execution times in seconds of the Ehrlich-Aberth method to solve sparse polynomials on multiple GPUs with CUDA-MPI.}

 \label{fig:03}
 \label{fig:03}

-\end{figure}
-Figure~\ref{fig:03} shows the execution times of te EA algorithm,
-for a single GPU, and multiple GPUs (2, 3, 4) with the CUDA-MPI approach.
-
-\subsubsection{Execution time of the Ehrlich-Aberth method for solving full polynomials on multiple GPUs using the Multi-GPU appraoch}
+ \end{figure}

 
 \begin{figure}[htbp]
 
 \begin{figure}[htbp]

-\centering
- \includegraphics[angle=-90,width=0.5\textwidth]{Full_mpi}
-\caption{Execution times in seconds of the Ehrlich-Aberth method for
-  full polynomials on  multiple GPUs with CUDA-MPI.}
-\label{fig:04}
-\end{figure}
-
-In Figure~\ref{fig:04}, we can also observe that the CUDA-MPI approach
-is also efficient to solve full polynimails on multiple GPUs.
+ \centering
+  \includegraphics[angle=-90,width=0.5\textwidth]{Full_mpi}
+ \caption{Execution times in seconds of the Ehrlich-Aberth method for full polynomials on  multiple GPUs with CUDA-MPI.}
+ \label{fig:04}
+ \end{figure}

 
 

-\subsection{Comparison of  the CUDA-OpenMP and the CUDA-MPI approaches}

 
 

-In the previuos section we saw that both approches are very effecient
-to  reduce the execution times the  sparse and full polynomials. In
-this section we try to compare these two approaches.
+\subsection{Comparison between the CUDA-OpenMP and the CUDA-MPI approaches}
+In the previous section we saw that both approaches are very efficient to reduce the execution times to solve sparse and full polynomials. In this section we try to compare these two approaches. In this experiment three sparse polynomials and three full polynomials of degrees 200,000, 800,000 and 1,400,000 are investigated. Figures~\ref{fig:05} and~\ref{fig:06} show the comparison between CUDA-OpenMP and CUDA-MPI algorithms of the EA method to solve sparse and full polynomials, respectively. 

 
 

-\subsubsection{Solving sparse polynomials}
-In this experiment three sparse polynomials of size 200K, 800K and 1,4M are investigated.

 \begin{figure}[htbp]
 \centering
  \includegraphics[angle=-90,width=0.5\textwidth]{Sparse}
 \begin{figure}[htbp]
 \centering
  \includegraphics[angle=-90,width=0.5\textwidth]{Sparse}

-\caption{Execution times  to solvs sparse polynomials of three
-  distinct sizes on multiple GPUs using MPI and OpenMP with the
-  Ehrlich-Aberth method}
+\caption{Execution times  to solve sparse polynomials of three distinct degrees on multiple GPUs using OpenMP and MPI with the Ehrlich-Aberth method}

 \label{fig:05}
 \end{figure}
 \label{fig:05}
 \end{figure}

-In Figure~\ref{fig:05} there is one curve for CUDA-MPI and another one
-for CUDA-OpenMP. We can see that the results are quite similar between
-OpenMP and MPI for the polynomials size of 200K. For the size of 800K,
-the MPI version is a little bit slower than the OpenMP approach but for
-the 1,4 millions size, there is a slight advantage for the MPI
-version.
-
-\subsubsection{Solving full polynomials}
+

 \begin{figure}[htbp]
 \centering
  \includegraphics[angle=-90,width=0.5\textwidth]{Full}
 \begin{figure}[htbp]
 \centering
  \includegraphics[angle=-90,width=0.5\textwidth]{Full}

-\caption{Execution time for solving full polynomials of three distinct sizes on multiple GPUs using MPI and OpenMP approaches using Ehrlich-Aberth}
+\caption{Execution times  to solve full polynomials of three distinct degrees on multiple GPUs using OpenMP and MPI with the Ehrlich-Aberth method}

 \label{fig:06}
 \end{figure}
 \label{fig:06}
 \end{figure}

-In Figure~\ref{fig:06}, we can see that when it comes to full polynomials, both approaches are almost equivalent.

 
 

-\subsubsection{Solving sparse and full polynomials of the same size with CUDA-MPI}
+In Figure~\ref{fig:05} there is one curve for CUDA-OpenMP and another one for CUDA-MPI. We can see that the results are quite similar between OpenMP and MPI for the polynomial degree of 200K. For the degree of 800K, the MPI version is a little bit slower than the OpenMP version but for the degree of 1,4 millions, there is a slight advantage for the MPI version. In Figure~\ref{fig:06}, we can see that when it comes to full polynomials, both approaches are almost equivalent.
+
+
+\subsection{Solving sparse and full polynomials of the same degree on multiple GPUs}
+In this experiment we compare the execution times of the EA algorithm according to the number of GPUs to solve sparse and full polynomials on multiple GPUs using OpenMP or MPI approaches. We chose three sparse and three full polynomials of degrees 200,000, 800,000 and 1,400,000. Figures~\ref{fig:07} and~\ref{fig:08} show the execution times to solve sparse and full polynomials of the same degrees with CUDA-OpenMP version and CUDA-MPI version, respectively.

 
 

-In this experiment we compare the execution time of the EA algorithm
-according to the number of GPUs to solve sparse and full
-polynomials on multiples GPUs using MPI. We chose three sparse and full
-polynomials of size 200K, 800K and 1,4M.

 \begin{figure}[htbp]
 \centering
 \begin{figure}[htbp]
 \centering

- \includegraphics[angle=-90,width=0.5\textwidth]{MPI}
-\caption{Execution times to solve sparse and full polynomials of three distinct sizes on multiple GPUs using MPI.}
+ \includegraphics[angle=-90,width=0.5\textwidth]{OMP}
+\caption{Execution times to solve sparse and full polynomials of three distinct degrees on multiple GPUs using OpenMP.}

 \label{fig:07}
 \end{figure}
 \label{fig:07}
 \end{figure}

-In Figure~\ref{fig:07} we can see that CUDA-MPI can solve sparse and
-full polynomials of high degrees, the execution times with sparse
-polynomial are very low compared to full polynomials. With sparse
-polynomials the number of monomials is reduced, consequently the number
-of operations is reduced and the execution time decreases.
-
-\subsubsection{Solving sparse and full polynomials of the same size with CUDA-OpenMP}

 
 \begin{figure}[htbp]
 \centering
 
 \begin{figure}[htbp]
 \centering

- \includegraphics[angle=-90,width=0.5\textwidth]{OMP}
-\caption{Execution time for solving sparse and full polynomials of three distinct sizes on multiple GPUs using OpenMP}
+ \includegraphics[angle=-90,width=0.5\textwidth]{MPI}
+\caption{Execution times to solve sparse and full polynomials of three distinct degrees on multiple GPUs using MPI.}

 \label{fig:08}
 \end{figure}
 
 \label{fig:08}
 \end{figure}
 

-Figure ~\ref{fig:08} shows the impact of sparsity on the effectiveness of the CUDA-OpenMP approach. We can see that the impact follows the same pattern, a difference in execution time in favor of the sparse polynomials. 
+In Figure ~\ref{fig:07} the execution times of the CUDA-OpenMP version to solve sparse polynomials are very low compared to those to solve full polynomials. With sparse polynomials the number of monomials is reduced, consequently the number of operations is reduced and the execution time decreases. Figure~\ref{fig:08} shows the impact of sparsity on the effectiveness of the CUDA-MPI approach. We can see that the impact follows the same pattern, a difference in execution times in favor of the sparse polynomials.
+

 
 \subsection{Scalability of the EA method on multiple GPUs to solve very high degree polynomials}
 
 \subsection{Scalability of the EA method on multiple GPUs to solve very high degree polynomials}

-These experiments report the execution times of the EA method for
-sparse and full polynomials ranging from 1,000,000 to 5,000,000.
+These experiments report the execution times of the EA method for sparse and full polynomials of high degrees ranging from 1,000,000 to 5,000,000. In Figure~\ref{fig:09} we can see that both approaches (CUDA-OpenMP and CUDA-MPI) are scalable and can solve very high degree polynomials. In addition, with full polynomial as well as sparse ones, both approaches give very similar results.
+

 \begin{figure}[htbp]
 \centering
  \includegraphics[angle=-90,width=0.5\textwidth]{big}
 \begin{figure}[htbp]
 \centering
  \includegraphics[angle=-90,width=0.5\textwidth]{big}

- \caption{Execution times in seconds of the Ehrlich-Aberth method for solving full polynomials of high degree on 4 GPUs for sizes ranging from 1M to 5M}
+ \caption{Execution times in seconds of the Ehrlich-Aberth method to solve sparse and full polynomials of high degree on 4 GPUs for degrees ranging from 1M to 5M}

 \label{fig:09}
 \end{figure}
 \label{fig:09}
 \end{figure}

-In Figure~\ref{fig:09} we can see that both approaches are scalable
-and can solve very high degree polynomials. In addition, with full polynomial as well as sparse ones, both
-approaches give very similar results.
-
-%SIDER JE viens de virer \c ca For sparse polynomials here are a noticeable difference in favour of MPI when the degree is
-%above 4 millions. Between 1  and 3 millions, OpenMP is more effecient.
-%Under 1 million, OpenMPI and MPI are almost equivalent.
-
-%SIDER : il faut une explication sur les différences ici aussi.

  
 
 \section{Conclusion}
 \label{sec6}
  
 
 \section{Conclusion}
 \label{sec6}

-In this paper, we have presented a parallel implementation of
-Ehrlich-Aberth algorithm to solve full and sparse polynomials, on
-single GPU with CUDA and on multiple GPUs using two parallel
-paradigms: shared memory with OpenMP and distributed memory with
-MPI. These architectures were addressed by a CUDA-OpenMP approach and
-CUDA-MPI approach, respectively.  Experiments show that, using
-parallel programming model like (OpenMP, MPI). We can efficiently
-manage multiple graphics cards to solve the same
-problem and accelerate the parallel execution with 4 GPUs and solve a
-polynomial of degree up to 5,000,000, four times faster than on single
-GPU. 
-
+In this paper, we have presented parallel implementations of the Ehrlich-Aberth algorithm to solve full and sparse polynomials, on a single GPU with CUDA and on multiple GPUs using two parallel paradigms: shared memory with OpenMP and distributed memory with MPI. These architectures were addressed by a CUDA-OpenMP approach and CUDA-MPI approach, respectively. Experiments show that, using parallel programming model like (OpenMP or MPI), we can efficiently manage multiple graphics cards to solve the same problem and accelerate the parallel execution with 4 GPUs and solve a polynomial of degree up-to 5,000,000 four times faster than on single GPU. 

 
 

-Our next objective is to extend the model presented here with clusters
-of GPU nodes, with a three-level scheme: inter-node communication via
-MPI processes (distributed memory), management of multi-GPU node by
-OpenMP threads (shared memory).
+Our next objective is to extend the model presented here with clusters of GPU nodes, with a three-level scheme: inter-node communications via MPI processes (distributed memory), management of multi-GPU nodes by OpenMP threads (shared memory).

 
 
 \section*{Acknowledgment}
 
 
 \section*{Acknowledgment}

-
-Computations have been performed on the supercomputer facilities of
-the Mésocentre de calcul de Franche-Comté. We also would like to thank
-Nvidia for hardware donation under CUDA Research Center 2014.
-
+This  paper  is   partially  funded  by  the  Labex   ACTION  program  (contract
+ANR-11-LABX-01-01). Computations have been performed on the supercomputer facilities of the Mésocentre de calcul de Franche-Comté. We also would like to thank Nvidia for hardware donation under CUDA Research Center 2014.

 
 
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