1 \documentclass[11pt,a4paper]{article}
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2 \usepackage[latin1]{inputenc}
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4 \usepackage{amsfonts}
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7 \title{Paper1_kahina}
\r
9 \section{Root finding problem}
\r
10 we consider a polynomial of degree \textit{n} having coefficients
\r
11 in the complex \textit{C} and zeros $\alpha
\r
12 _{i},\textit{i=1,...,n}$. \\
\r
15 {\Large p(x)=\sum{a_{i}x^{i}}=a_{n}\prod(x-\alpha_{i}),a_{0} a_{n}\neq 0}
\r
19 the root finding problem consist to find
\r
20 all n root of \textit{p(x)}. the problem of finding a root is
\r
21 equivalent to the problem of finding a fixed-point. To see this
\r
22 consider the fixed-point problem of finding the n-dimensional
\r
27 where $g : C^{n}\longrightarrow C^{n}$. Note that we can easily
\r
28 rewrite this fixed-point problem as a root-finding problem by
\r
29 setting $f (x) = x-g(x)$ and likewise we can recast the
\r
30 root-finding problem into a fixed-point problem by setting
\r
34 Often it will not be possible to solve such nonlinear equation
\r
35 root-finding problems analytically. When this occurs we turn to
\r
36 numerical methods to approximate the solution. Generally speaking,
\r
37 algorithms for solving problems numerically can be divided into
\r
38 two main groups: direct methods and iterative methods.
\r
40 Direct methods exist only for $n \leqslant4$,solved in closed form by G. Cardano
\r
41 in the mid-16th century. However, N.H. Abel in the early 19th
\r
42 century showed that polynomials of degree five or more could not
\r
43 be solved by directs methods. Since then researchers have
\r
44 concentrated on numerical (iterative) methods such as the famous
\r
45 Newton s method, Bernoulli s method of the 18th, and Graeffe s.
\r
46 With the advent of electronic computers, different methods has
\r
47 been developed such as the Jenkins-Traub method, Larkin s method,
\r
48 Muller s method, and several methods for simultaneous
\r
49 approximation of all the roots, starting with the Durand-Kerner
\r
52 \begin{equation} Z_{i}=Z_{i}-\frac{P(Z_{i})}{\prod_{i\neq j}(z_{i}-z_{j})}
\r
56 This formula is mentioned for the first time from
\r
57 Weiestrass~\cite{Weierstrass03} as part of the fundamental theorem
\r
58 of Algebra and is rediscovered from Ilieff~\cite{Ilie50},
\r
59 Docev~\cite{Docev62}, Durand~\cite{Durand60},
\r
60 Kerner~\cite{Kerner66}. Another method discovered from
\r
61 Borsch-Supan~\cite{ Borch-Supan63} and also described and brought
\r
62 in the following form from Ehrlich~\cite{Ehrlich67} and
\r
63 Aberth~\cite{Aberth73}.
\r
66 Z_{i}=Z_{i}-\frac{1}{{\frac {P'(Z_{i})} {P(Z_{i})}}-{\sum_{i\neq j}(z_{i}-z_{j})}}
\r
70 Aberth, Ehrlich and Farmer-Loizou~\cite{Loizon83} have proved that
\r
71 the above method has cubic order of convergence for simple roots.
\r
74 Iterative methods raise several problem when implemented e.g.
\r
75 specific sizes of numbers must be used to deal with this
\r
76 difficulty.Moreover,the convergence time of iterative methods
\r
77 drastically increase like the degrees of high polynomials. The
\r
78 parallelization of these algorithms will improve the convergence
\r
81 Many authors have treated the problem of parallelization of
\r
82 simultaneous methods. Freeman~\cite{Freeman89} has tested the DK
\r
83 method, EA method and another method of the fourth order proposed
\r
84 from Farmer and Loizou~\cite{Loizon83},on a 8- processor linear
\r
85 chain, for polynomial of degree up to 8. The third method often
\r
86 diverges, but the first two methods have speed-up 5.5
\r
87 (speed-up=(Time on one processor)/(Time on p processors)). Later
\r
88 Freeman and Bane~\cite{Freemanall90} consider asynchronous
\r
89 algorithms, in which each processor continues to update its
\r
90 approximations even although the latest values of other $z_i((k))$
\r
91 have not received from the other processors, in difference with
\r
92 the synchronous version where it would wait.
\r
93 in~\cite{Raphaelall01}proposed two methods of parallelization for
\r
94 architecture with shared memory and distributed memory,it able to
\r
95 compute the root of polynomial degree 10000 on 430 s with only 8
\r
96 pc and 2 communications per iteration. Compare to the sequential
\r
97 it take 3300 s to obtain the same results.
\r
99 After this few works discuses this problem until the apparition of
\r
100 the Compute Unified Device Architecture (CUDA)~\cite{CUDA10},a
\r
101 parallel computing platform and a programming model invented by
\r
102 NVIDIA. the computing ability of GPU has exceeded the counterpart
\r
103 of CPU. It is a waste of resource to be just a graphics card for
\r
104 GPU. CUDA adopts a totally new computing architecture to use the
\r
105 hardware resources provided by GPU in order to offer a stronger
\r
106 computing ability to the massive data computing.
\r
109 Indeed,~\cite{Kahinall14}proposed the implementation of the
\r
110 Durand-Kerner method on GPU (Graphics Processing Unit). The main
\r
111 result prove that a parallel implementation is 10 times as fast as
\r
112 the sequential implementation on a single CPU for high degree
\r
113 polynomials that is greater than about 48000.
\r
115 The mean part of our work is to implement the Aberth method on GPU
\r
116 and compare it with the Durand Kerner
\r
117 implementation.................To be continued..................
\r
120 \section{Aberth method and difficulties}
\r
121 A cubically convergent iteration method for finding zeros of
\r
122 polynomials was proposed by O.Aberth~\cite{Aberth73}.The Aberth
\r
123 method is a purely algebraic derivation.To illustrate the
\r
124 derivation, we let $w_{i}(z)$ be the product of linear factor $
\r
125 w_{i}(z)=\prod_{j=1,j \neq i}^{n} (z-x_{j})$
\r
127 and rational function $R_{i}(z)$ be the correction term of
\r
128 Weistrass method~\cite{Weierstrass03}:
\r
131 R_{i}(z)=\dfrac{p(z)}{w_{i}(Z)} , i=1,2,...,n.
\r
134 Differentiating the rational function $R_{i}(z)$ and applying the
\r
135 Newton method, we have
\r
138 \dfrac{R_{i}(z)}{R_{i}^{'}(z)}=
\r
139 \dfrac{p(z)}{p^{'}(z)-p(z)\dfrac{w_{i}(z)}{w_{i}^{'}(z)}}=
\r
140 \dfrac{p(z)}{p^{'}(z)-p(z) \sum _{j=1,j \neq
\r
141 i}^{n}\dfrac{1}{z-x_{i}}}, i=1,2,...,n
\r
144 Substituting $x_{j}$ for z we obtain the Aberth iteration method
\r
146 Let present the means stages of Aberth's method.
\r
148 \subsection{Polynomials Initialization}
\r
149 The initialization of polynomial P(z) with complex coefficients
\r
153 p(z)=\sum{a_{i}z^{n-i}}. where a_{n} \neq 0,a_{0}=1, a_{i}\subset C
\r
157 \subsection{Vector $Z^{0)}$ Initialization}
\r
159 The choice of the initial points $z^{(0)}_{i} , i = 1, . . . , n,$
\r
160 from which starting the iteration (2) or (3), is rather delicate
\r
161 since the number of steps needed by the iterative method to reach
\r
162 a given approximation strongly depends on it.
\r
163 In~\cite{Aberth73}the Aberth iteration is started by selecting n
\r
164 equispaced points on a circle of center 0 and radius r, where r is
\r
165 an upper bound to the moduli of the zeros. After,~\cite{Bini96}
\r
166 performs this choice by selecting complex numbers along different
\r
167 circles and relies on the result of~\cite{Ostrowski41}.
\r
170 \sigma_{0}=\frac{u+v}{2};u=\frac{\sum_{i=1}^{n}u_{i}}{n.max_{i=1}^{n}u_{i}};
\r
171 v=\frac{\sum_{i=0}^{n-1}v_{i}}{n.min_{i=0}^{n-1}v_{i}};u_{i}=2.|a_{i}|^{\frac{1}{i}};
\r
172 v_{i}=\frac{|\frac{a_{n}}{a_{i}}|^{\frac{1}{n-i}}}{2}
\r
175 \subsection{Iterative Function Hi}
\r
176 The operator used with Aberth's method is corresponding to the
\r
177 following equation which will enable the convergence towards
\r
178 polynomial solutions, provided all the roots are distinct.
\r
181 H_{i}(z)=z_{i}-\frac{1}{\frac{P^{'}(z_{i})}{P(z_{i})}-\sum_{j\neq
\r
182 i}{\frac{1}{z_{i}-z_{j}}}}
\r
185 \subsection{Convergence condition}
\r
186 determines the success of the termination. It consists in stopping
\r
187 the iterative function $H_{i}(z)$ when the are stable,the method
\r
188 converge sufficiently:
\r
192 [1,n];\frac{z_{i}^{(k)}-z_{i}^{(k-1)}}{z_{i}^{(k)}}<\xi
\r
196 \section{Difficulties and amelioration}
\r
197 the Aberth method implementation suffer of overflow problems. This
\r
198 situation occurs, for instance, in the case where a polynomial
\r
199 having positive coefficients and large degree is computed at a
\r
200 point $\xi$ where $|\xi| > 1$.Indeed the limited number in the
\r
201 mantissa of floating takings the computation of P(z) wrong when z
\r
202 is large. for example $(10^{50}) +1+ (- 10_{50})$ will give result
\r
203 0 instead of 1 in reality.consequently we can't compute the roots
\r
204 for large polynomial's degree. This problem was discuss in
\r
205 ~\cite{Karimall98} for the Durand-Kerner method, the authors
\r
206 propose to use the logratihm and the exponential of a complex:
\r
209 \forall(x,y)\in R^{*2}; \ln (x+i.y)=\ln(x^{2}+y^{2})
\r
210 2+i.\arcsin(y\sqrt{x^{2}+y^{2}})_{\left] -\pi, \pi\right[ }
\r
214 \forall(x,y)\in R^{*2}; \exp(x+i.y)&= \exp(x).\exp(i.y)\\
\r
215 &=\exp(x).\cos(y)+i.\exp(x).\sin(y)
\r
219 The application of logarithm can replace any multiplications and
\r
220 divisions with additions and subtractions; consequently it
\r
221 manipulates lower absolute values and can be compute the roots for
\r
222 large polynomial's degree exceed~\cite{Karimall98}.
\r
224 Applying this solution for the Aberth method we obtain the
\r
225 iteration function with logarithm:
\r
226 %%$$ \exp \bigl( \ln(p(z)_{k})-ln(\ln(p(z)_{k}^{'}))- \ln(1- \exp(\ln(p(z)_{k})-ln(\ln(p(z)_{k}^{'})+\ln\sum_{i\neq j}^{n}\frac{1}{z_{k}-z_{j}})$$
\r
228 H_{i}(z)=z_{i}^{k}-\exp \left(\ln \left(
\r
229 p(z_{k})\right)-\ln\left(p(z_{k}^{'})\right)- \ln
\r
230 \left(1-Q(z_{k})\right)\right)
\r
235 Q(z_{k})=\exp\left( \ln (p(z_{k}))-\ln(p(z_{k}^{'}))+\ln \left(
\r
236 \sum_{k\neq j}^{n}\frac{1}{z_{k}-z_{j}}\right)\right)
\r
240 this solution is applying when it is necessary
\r
242 \section{The implementation of simultaneous methods in a parallel computer}
\r
243 The main problem of the simultaneous methods is that the necessary
\r
244 time needed for the convergence is increased with the increasing
\r
245 of the degree of the polynomial. The parallelization of these
\r
246 algorithms will improve the convergence time. Researchers usually
\r
247 adopt one of the two following approaches to parallelize root
\r
248 finding algorithms. One approach is to reduce the total number of
\r
249 iterations as implemented by Miranker
\r
250 ~\cite{Mirankar68,Mirankar71}, Schedler~\cite{Schedler72} and
\r
251 Winogard~\cite{Winogard72}. Another approach is to reduce the
\r
252 computation time per iteration, as reported
\r
253 in~\cite{Benall68,Jana06,Janall99,Riceall06}. There are many
\r
254 schemes for simultaneous approximations of all roots of a given
\r
255 polynomial. Several works on different methods and issues of root
\r
256 finding have been reported in~\cite{Azad07,Gemignani07,Kalantari08
\r
257 ,Skachek08,Zhancall08,Zhuall08}. However, Durand-Kerner and
\r
258 Ehrlich methods are the most practical choices among
\r
259 them~\cite{Bini04}. These two methods have been extensively
\r
260 studied for parallelization due to their following advantages. The
\r
261 computation involved in these methods has some inherent
\r
262 parallelism that can be suitably exploited by SIMD machines.
\r
263 Moreover, they have fast rate of convergence (quadratic for the
\r
264 Durand-Kerner method and cubic for the Ehrlich). Various parallel
\r
265 algorithms reported for these methods can be found
\r
266 in~\cite{Cosnard90, Freeman89,Freemanall90,,Jana99,Janall99}.
\r
267 Freeman and Bane~\cite{Freemanall90} presented two parallel
\r
268 algorithms on a local memory MIMD computer with the compute-to
\r
269 communication time ratio O(n). However, their algorithms require
\r
270 each processor to communicate its current approximation to all
\r
271 other processors at the end of each iteration. Therefore they
\r
272 cause a high degree of memory conflict. Recently the author
\r
273 in~\cite{Mirankar71} proposed two versions of parallel algorithm
\r
274 for the Durand-Kerner method, and Aberth method on an on model of
\r
275 Optoelectronic Transpose Interconnection System (OTIS).The
\r
276 algorithms are mapped on an OTIS-2D torus using N processors. This
\r
277 solution need N processors to compute N roots, that it is not
\r
278 practical (is not suitable to compute large polynomial's degrees).
\r
279 Until then, the related works are not able to compute the root of
\r
280 the large polynomial's degrees (higher then 1000) and with small
\r
283 Finding polynomial roots rapidly and accurately it is our
\r
284 objective, with the apparition of the CUDA(Compute Unified Device
\r
285 Architecture), finding the roots of polynomials becomes rewarding
\r
286 and very interesting, CUDA adopts a totally new computing
\r
287 architecture to use the hardware resources provided by GPU in
\r
288 order to offer a stronger computing ability to the massive data
\r
289 computing.in~\cite{Kahinall14} we proposed the first implantation
\r
290 of the root finding polynomials method on GPU (Graphics Processing
\r
291 Unit),which is the Durand-Kerner method. The main result prove
\r
292 that a parallel implementation is 10 times as fast as the
\r
293 sequential implementation on a single CPU for high degree
\r
294 polynomials that is greater than about 48000. Indeed, in this
\r
295 paper we present a parallel implementation of Aberth's method on
\r
296 GPU, more details are discussed in the following of this paper.
\r
298 \section {A parallel implementation of Aberth's method}
\r
299 \subsection{Background on the GPU architecture}
\r
300 A GPU is viewed as an accelerator for the data-parallel and
\r
301 intensive arithmetic computations. It draws its computing power
\r
302 from the parallel nature of its hardware and software
\r
303 architectures. A GPU is composed of hundreds of Streaming
\r
304 Processors (SPs) organized in several blocks called Streaming
\r
305 Multiprocessors (SMs). It also has a memory hierarchy. It has a
\r
306 private read-write local memory per SP, fast shared memory and
\r
307 read-only constant and texture caches per SM and a read-write
\r
308 global memory shared by all its SPs~\cite{NVIDIA10}
\r
310 On a CPU equipped with a GPU, all the data-parallel and intensive
\r
311 functions of an application running on the CPU are off-loaded onto
\r
312 the GPU in order to accelerate their computations. A similar
\r
313 data-parallel function is executed on a GPU as a kernel by
\r
314 thousands or even millions of parallel threads, grouped together
\r
315 as a grid of thread blocks. Therefore, each SM of the GPU executes
\r
316 one or more thread blocks in SIMD fashion (Single Instruction,
\r
317 Multiple Data) and in turn each SP of a GPU SM runs one or more
\r
318 threads within a block in SIMT fashion (Single Instruction,
\r
319 Multiple threads). Indeed at any given clock cycle, the threads
\r
320 execute the same instruction of a kernel, but each of them
\r
321 operates on different data.
\r
322 GPUs only work on data filled in their
\r
323 global memories and the final results of their kernel executions
\r
324 must be communicated to their CPUs. Hence, the data must be
\r
325 transferred in and out of the GPU. However, the speed of memory
\r
326 copy between the GPU and the CPU is slower than the memory
\r
327 bandwidths of the GPU memories and, thus, it dramatically affects
\r
328 the performances of GPU computations. Accordingly, it is necessary
\r
329 to limit data transfers between the GPU and its CPU during the
\r
331 \subsection{Background on the CUDA Programming Model}
\r
333 The CUDA programming model is similar in style to a single program
\r
334 multiple-data (SPMD) softwaremodel. The GPU is treated as a
\r
335 coprocessor that executes data-parallel kernel functions. CUDA
\r
336 provides three key abstractions, a hierarchy of thread groups,
\r
337 shared memories, and barrier synchronization. Threads have a three
\r
338 level hierarchy. A grid is a set of thread blocks that execute a
\r
339 kernel function. Each grid consists of blocks of threads. Each
\r
340 block is composed of hundreds of threads. Threads within one block
\r
341 can share data using shared memory and can be synchronized at a
\r
342 barrier. All threads within a block are executed concurrently on a
\r
343 multithreaded architecture.The programmer specifies the number of
\r
344 threads per block, and the number of blocks per grid. A thread in
\r
345 the CUDA programming language is much lighter weight than a thread
\r
346 in traditional operating systems. A thread in CUDA typically
\r
347 processes one data element at a time. The CUDA programming model
\r
348 has two shared read-write memory spaces, the shared memory space
\r
349 and the global memory space. The shared memory is local to a block
\r
350 and the global memory space is accessible by all blocks. CUDA also
\r
351 provides two read-only memory spaces, the constant space and the
\r
352 texture space, which reside in external DRAM, and are accessed via
\r
355 \subsection{A parallel implementation of the Aberth's method }
\r
356 \subsection{A CUDA implementation of the Aberth's method }
\r
357 \subsection{A GPU implementation of the Aberth's method }
\r
358 \subsubsection{the step to parallelize}
\r
359 \subsubsection{the kernel corresponding }
\r
360 \subsubsection{Comparison between sequential algorithm and GPU algorithm }
\r
361 \bibliographystyle{plain}
\r
362 \bibliography{biblio}
\r
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\r