> REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) < 1 An Efficient Population-Merging Execution Model, for Evolutionary Algorithms Javier Arellano-Verdejo, Salvador Godoy-Calderón, Adolfo Guzmán-Arenas, Senior Member, IEEE Abstract—In this paper a coarse-grain execution model for evolutionary algorithms is proposed and used for solving numerical optimization problems. This model does not use migration as the solution dispersion mechanism, in its place a more efficient population-merging mechanism is used that dynamically reduces the population size as well as the total number of parallel evolving populations (i.e. islands). Even more relevant is the fact that the proposed model eliminates the need to set up an interconnection topology among islands. Extensive numerical experimentation, using genetic algorithms over a well-known set of typo problems, shows the proposed model to be faster and more accurate than the traditional one. Index Terms—Evolutionary algorithms, Genetic algorithms, Optimization, Parallel algorithms, Parallel island model, Spatially structured evolutionay algorithms. I. INTRODUCTION During the last few decades several evolutionary algorithms (EAs) have been successfully applied in the solution of many optimization problems [6]-[9]. Nevertheless, as problems become more and more complex, the execution time of these algorithms increases, making them prohibitively slow for single-core execution. Of course, the populationbased nature of evolutionary algorithms, along with the current accelerated development on multi-core hardware technologies, has turned EAs into strong candidates for parallel implementation. Besides the differences in the way each algorithm manages its own heuristic search for optimum values, all EAs share the need to evaluate the fitness of a possibly large population, as well as the need to balance their own mechanisms for global and local search (i.e exploration and exploitation) over the search-space. Thus, several parallel execution models have been proposed for virtually all types of EAs [10]-[12], [14]. J. Arellano-Verdejo, S. Godoy-Calderón are with the Laboratorio de Inteligencia Artificial, Centro de Investigación en Computación (CIC) del Instituto Politécnico Nacional (IPN). Av. Juan de Dios Bátiz, Esq. Miguel Othón de Mendizábal, Col. Nueva Industrial Vallejo Delegación Gustavo A. Madero C.P 07738, México (email: [email protected], [email protected]). Adolfo Guzmán Arenas is with the Laboratorio de Bases de Datos y Tecnología de Software, Centro de Investigación en Computación (CIC) del Instituto Politécnico Nacional (IPN), (email [email protected]). Parallel evolutionary algorithms (PEAs) have been classified into three groups according to their execution model [5], [13], [15] (See figure 1): master-slave PEAs, coarse-grain PEAs (also known as island-model PEAs) and fine-grain PEAs (also known as cellular-diffusion PEAs). Although the master-slave model offers a practical organization and structure for the tasks of the algorithm, it turns to be a terrible waste of the parallel capabilities of the running system. In contrast, the fine-grain model requires too much system capacity and provides no extra mechanisms for controlling the way in which the search space is to be explored and exploited. The island-model appears to be a natural balance between requirements and performance, one that allows leveraging the capabilities of the underlying parallel system and at the same time retain the ability to adjust the exploration/exploitation behavior of the EA. One peculiar characteristic of traditional island-based PEAs is that the population of each island is randomly selected from an initial uniformly distributed random population [16]. Such a selection method implies that an island’s population might not be representative of any particular region of the search space, and that it can theoretically span over almost the same search space than the original population but with significantly less individuals. Since in the island-model the evolution process of each island is executed by a different core, the performance of the underlying parallel system is notoriously wasted, and the algorithm is left dangerously prone to premature convergence. Seemingly, this model uses parallelism only to perform the search in less time, without this implying any improvement in the way in which the problem search space is explored or exploited. As a way of compensating, a migration mechanism is used to disseminate the best fitted solutions among all islands. While this mechanism can be extremely effective in some specific situations when properly configured [1]-[3] it imposes the need to setup an interconnection topology among the islands (i.e star, ring, toroid, etc.). This implies the forcible inclusion of an extra parameter into the execution model, one whose choice has a strong impact on the way the search for the optimal value develops, as well as on the overall performance of the evolutionary algorithm itself. Unfortunately there are no easy to follow or universally accepted criteria to make that choice. > REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) < 2 To reduce the number of parameters that need to be set, T. Hiroyasu et al [19] propose an algorithm called DuDGA (Dual Distributed Genetic Algorithm) which defines islands with only two individuals in each of them. After the only two individuals on each island are used for a one-point crossover operation and their descendants are mutated, only the two best fitted individuals (ancestors or descendants) survive for the next generation. Unfortunately, the final result’s quality, as well as the overall performance of this EA, turns out to be very poor as it still depend on a proper choice for the rest the parameters. Fig. 1. a) Master-Slave model, b) Fine-Grain model c) Coarse-Grain model In this paper a new coarse-grain parallel execution model is introduced, which was originally inspired by the traditional island-based model but does not use migration as a solution dispersion mechanism and consequently eliminates the need to setup an interconnection topology among islands. In its place a population merging mechanism is used with the option to use several different criteria for selecting island candidates to merge their populations. This option of the model provides by itself an extra control layer that adds to the exploration/exploitation capabilities of the running EA. The rest of this paper is organized as follows: in Section II related works are presented, in Section III, the proposed new model is described, detailing each of the characteristics that make it different and more versatile compared to a traditional migration-based island-model. Section IV shows a set of comparative experiments run with genetic algorithms over the same populations in a migration-based parallel model and in the proposed population-merging model. Finally, in Section V, conclusions are offered. II. RELATED WORKS Every kind of EA requires multiple parameters to properly function. The correct choice for the value of each parameter provides the algorithm with the right balance of exploration and exploitation capabilities. When implemented in parallel, the resulting PEA works as a high level operational layer, thus needing its own set of parameters. It is quite clear that the larger the number of parameters that must be set for the execution of the algorithm, the greater the difficulty to find a combination of the right values for them. In fact, the selection of parameters has been posed as an optimization problem itself, with multiple solution proposals as in [4], [17], [18]. In [20], H. Sawai et al describe an algorithm called PfGA (Parameter free Genetic Algorithm) in which the population size changes as a function of the fitness value of individuals. From every generation only two parents are selected for multi-point crossover and there is no need to setup any other parameters. Even traditional genetic algorithm’s parameters like crossover and mutation rates are not needed since the selection strategy always keeps a small population and a high performance. Although the PfGA itself requires no parameters, the parallel execution layer does and in fact, in [21] it is shown that migration plays an essential role as a tool to improve global search, yet the authors clearly showed that the optimal migration model is different for each problem. The same type of reduced population (two individuals) is used in [22], where authors introduce an algorithm called VIGA (Variant Island Genetic Algorithm). This algorithm works with a dynamic number of islands and explicit mechanisms called resize, copy and create operations to incrementally take advantage of the convergence status of the search process. Although experimental results show VIGA to be computationally efficient, it relies on global fitness information, which implies high communication overhead when implemented as a distributed parallel algorithm. Also, the fact that a new individual or a mutant is created with low probability means that the parallel execution model dangerously decreases the variance of the population and increases the risk of premature convergence. Parallel execution models have been proposed for many kinds of EAs other than genetic algorithms. In [23], for example, de la Ossa et al, describe a family of island-based EDAs (Estimation of Distribution Algorithms) and use them to solve numerical optimization problems. The authors select star and ring topologies to experiment with and add a new level of information exchange between islands when using both individual and model migration. Also in [23], an empirical comparison of several parallel Particle Swarm Optimization (pPSO) algorithms is shown. > REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) < The alternative execution model proposed in this paper does not use migration nor a pre-defined connection topology among islands, so the dissemination of the solutions is not dependent on the interconnection topology selected. The migration process has been replaced by a population-merging mechanism between islands, which allows a dynamic way of handling the number of islands as well as the size of the global population. All those tunings and new elements have resulted in a dramatic reduction in the number of times the fitness function is evaluated along the development of the execution and endows the proposed model with a better control of the way in which the search space is explored and exploited. III. MERGING POPULATIONS The Population-Merging Island Model (PMIM) proposed herein slightly modifies the above parameter list and operates on the parameters shown in Table 1. It can be seen that in the PMIM the global population size monotonically decreases over the course of the heuristic search. This is achieved by eliminating all the f population g (ni )k times, where total number of generations the algorithm evolves, population size of island Ii and k Parameter Semantics Initial population’s size. Initial number of islands. Number of generations to evolve before searching for merging candidates. Maximum number of evolution/fusion rounds. Island selection criterion for the merger. n k m r fisland Population’s replacement criteria for merged islands. f population g ni fisland The use of criterion 1) Create a uniformly distributed P population 2) Create k with 3) Execute evolutionary algorithm of each island with populations Ii , i 1..k A during on the population m generations. 4) Pause evolution on all islands and using islandselection criterion, fisland select two or more islands for merging their populations. is the 5) Merge the population of all selected islands into a If In contrast, the PMIM starts with the same population size for each island but keeps it only during m generations. When the merging process takes place the population of all selection criterion is merged and subsequently initial P1,..., Pk randomly selected from P . new island fisland random individuals. I1,..., I k islands islands. islands selected by n is the is the total number of for selecting islands to be merged eliminates the need to setup interconnection topologies while allowing more freedom in the information exchange mechanism. If the islands selected for merging have a similar fitness variance of their populations then exploitation is promoted, but if islands with notorious population fitness variances are selected, then more exploration is induced into the heuristic search. All of the above happens independently of the EA’s own mechanisms for exploration and exploitation. The PMIM model is specified as follows: criterion after the merging process. Consequently, as the global population size decreases, so does the number of times the fitness function is evaluated and, since the total number of islands also decreases, one execution core is freed each round for the parallel system to use for other tasks. In a traditional migration-based island-model both the global population size and the number of islands are kept constant, so the fitness function is evaluated exactly TABLE I PMIM PARAMETERS reduced as specified by the f population criterion. It is a well-known fact that the quality of the solution found by any EA is directly dependent on the value of its parameters. In addition, the traditional island-model requires the following parameters: number of populations to evolve in parallel (i.e. number of islands), size of the population on each island, interconnection topology and migration frequency. There is also the need to establish a migration policy to determine the number and selection of individuals who will migrate, as well as a replacement policy for the newly arrived individuals on each island. individuals that were not selected by the 3 6) Update and reduce its population using f population . k k 1 . > REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) < 7) k 1 go back to step 3 and resume evolution on all islands, else execute algorithm A on the last island’s population during m more generations or TABLE 2 PORTFOLIO OF TEST FUNCTIONS If until the preset termination conditions are met. IV. EXPERIMENTAL RESULTS Labe l Each problem was solved 50 times by the same parallel genetic algorithm running separately in the traditional islandmodel and in the PMIM model. Each run generated exactly the same initial population for both models so that results could be fairly compared. All experiments, a total of 1000 (10 functions, 50 runs and 2 models), were run on an 8-core Intel Xeon system with 32 GB RAM over a 64 bits Linux operating system. In both models the evolution process for each island is run by a different core. Function name and search interval f1 100 xi 100 Schwefel’s problem 1 10 xi 10 f3 f4 f5 100 xi 100 Schwefel’s problem 3 100 xi 100 Generalized Rosenbrock’s 30 xi 30 f6 100 xi 100 f7 1.28 xi 1.28 f9 For the PMIM model the f island criterion was set to f10 However, results yield by each model are surprisingly close to each other so one could be drawn to conclude that both models are more or less equivalent in their precision. When comparing such small quantities it is often best to work in terms of percentage. Thus, second column of table 5 shows the percentage of improvement implied in table 4. The third column of table 5 shows the improvement percentage in number of evaluations of the fitness function (remember that in the traditional island-model the number of evaluations of the fitness function is constant). As a direct consequence of 30 i 1 i 1 min( f 2 ) f 2 (0,...,0) 0 30 i f3 ( x) x j i 1 j 1 min( f3 ) f3 (0,...,0) 0 f 4 ( x) max i | xi |,1 i 30 min( f 4 ) f 4 (0,...,0) 0 29 f5 ( x) |100( xi1 xi2 )2 ( xi 1)2 | i 1 min( f5 ) f5 (1,...,1) 0 f 6 ( x) (| xi 0.5 |) 2 i 1 min( f 6 ) f 6 (0,...,0) 0 30 Quartic with noise Generalized Rastringin’s Table number 4 shows the average best solution found by the same GA running under each model with all test functions. As it can be seen the PMIM model was closer to the known optimum in all cases. 30 f 2 ( x) | xi | | xi | 30 Step f8 criterion always selected the best fitted two thirds of the individuals from a merged island. The rest of the parameters used for both models are shown in table 3. i 1 min( f1 ) f1 (0,...,0) 0 2 Schwefel’s problem 2 In the case of the traditional island-model a ring topology, as is the most common case, was used. Migration is accomplished by copying the eight best fitted individuals to other islands according to the selected interconnection topology. f population f1 ( x) xi2 Sphere model Generalized Schwefel’s problem randomly select two islands each time and the Function 30 f2 To test the performance of the PMIM the complete benchmark set of ten numerical optimization problems proposed by De Jong and extended by Coello et al [24] were used. The complete set of functions, along with a specification of their search interval and their known optimum value is shown in Table 2. 4 500 xi 500 5.12 xi 5.12 Generalized Griewank’s 600 xi 600 f 7 ( x) ixi4 random[0,1] i 1 min( f 7 ) f 7 (0,...,0) 0 30 f8 ( x) xi sin( | xi |) i 1 min( f8 ) f8 (420.9687,...,420.9687) 12569.5 30 f9 ( x) xi2 10cos(2 xi ) 10 i 1 min( f9 ) f9 (0,...,0) 0 1 30 2 30 x cos i 1 xi 4000 i1 i i 1 min( f10 ) f10 (0,...,0) 0 f10 ( x) the fewest number of fitness function evaluations, runtime also decreases significantly. The last column of table 5 shows the improvement of the PMIM model over traditional migration-based island model in execution time. > REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) < TABLE 3 COMPARISON OF PARAMETERS USED BY THE TWO MODELS TABLE 5 IMPROVEMENT OF THE PMIM MODEL OVER TRADITIONAL ISLAND-MODEL Traditional island-model PMIM model Size of global population ( n ) Size of each island’s population ( ni ) 1000 125 1000 dynamic Number of generations to evolve before migrating or merging (m) Maximum number of evolution/migration or evolution/fusion rounds ( r ) Initial number of islands Island selection criterion for migration or fusion ( f island ) 100 100 Parameter Replacement criterion for emigrant individuals/merged islands ( f population ) 8 8 8 ring topology 8 random 8 newly arrived individuals replace the 8 less fitted ones. best fitted two thirds of merged populati on º TABLE 4 AVERAGE BEST SOLUTION FOUND BY EACH MODEL Label f1 f2 f3 f4 f5 f6 f7 f8 f9 f10 5 Known optimum Best individual in PMIM Best individual in traditional island-model 0 0.0000025 0.000019 0 0.0000075 0.000017 0 0.0000036 0.00001 0 0.0000072 0.00001 0 0.000005 0.000008 0 0.0000049 0.000008 0 0.0000000080 0.000000013 -12569.5 -12568.8972 -12554.2173 0 0.00000162 0.0000045 0 0.0000027 0.0000064 From table 5 it can be seen that the same algorithm, runs faster and gets closer to the global optimum on the PMIM model than on the traditional island-model. Those effects can be attributed to the smaller number of evaluations of the fitness function and the faster dispersion of solutions generated by the population merging process in contrast to the migration process. Table 6 sums up the PMIM’s average percentage of improvement observed during the numerical experiments. Label Improvement in fitness of best individual (%) Improvement in number of evaluations of fitness function (%) Improvement in execution time (%) 76 41.2 10.91 22.66 41.33 8.86 27.22 41.27 11.67 13.88 41.12 11.18 16.6 41.17 9.95 15.30 41.29 11.55 16.25 41.47 11.88 9.99 41.16 9.83 27.77 41.3 14.12 23.7 41.11 10.34 f1 f2 f3 f4 f5 f6 f7 f8 f9 f10 TABLE 6 AVERAGE IMPROVEMENT OF THE PMIM MODEL Average improvement Average improvement in number of Average improvement in fitness of best evaluations of fitness in execution time individual function 76% 41.24% 10.91% V. CONCLUSIONS A new coarse-grain (parallel) execution model for evolutionary algorithms was presented. The new model, called Population Merging Island Model (PMIM) replaces migration with a population merging system as the solution dispersion mechanism used by the traditional island-model. In doing so, the PMIM model eliminates the need for choosing an interconnection topology among islands, which is one of the parameters with the greatest impact on the accuracy and performance of the evolutionary algorithm. When the population of two or more islands is merged and then reduced, the global population size decreases, as well as the number of evolving islands. The immediate effect of this is a reduction in the number of evaluations of the fitness function with the consecuent decrease in execution time. Also the PMIM model offers an extra layer to help on the exploration and exploitation control capabilities of the heuristic search which results in a higher precision on the quality of the found solution. > REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) < Extensive numerical experimentation, using the benchmark function suite proposed by De Joung and a GA as the inner EA shows the PMIM model to be faster and more precise than the traditional island-model (see Table 6). Acknowledgements The authors express their gratitude to the COFAA, SIP, and CIC units of the National Polytechnic Institute IPN, as well as CONACyT and SNI, for their economic support to carry out this work. 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In Proceedings of the 8th annual conference on Genetic and evolutionary computation (GECCO '06). ACM, New York, NY, USA, 485-492 Javier Arellano-Verdejo Recibed the Masters degree in science from Tecnológico de Estudios Superiores de Ecatepec Mexico in 2008. He is currentrly pursing the Ph.D. degree in Computation Science from Centro de Investigación en Computación Instituto Politécnico Nacional Mexico City. His current research interests include parallel bioinspired heuristics , Estimation of Distribution Algorithms and Pattern recognition. > REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) < Salvador Godoy Calderón Ph.D. in Computer Science at CIC-IPN and Master in C.S. by CINVESTAV-IPN. Full time researcher and Head of the Artificial Intelligence Laboratory at CICIPN with research interests including: Pattern Recognition, Evolutionary Computation, Strategic and Tactical Knowledge Systems and Formal Logics. Adolfo Guzmán-Arenas is a Computer Science professor at Centro de Investigación en Computación, Instituto Politécnico Nacional, Mexico City, of which he was Founding Director. He holds a B. Sc. in Electronics from ESIME-IPN, and a Ph. D. from MIT. He is an ACM Fellow, IEEE Life Fellow Member, member of MIT Educational Council, member of the Academia de Ingeniería and the Academia Nacional de Ciencias (Mexico). From the President of Mexico, he has received (1996) the National Prize in Science and Technology and (2006) the Premio Nacional a la Excelencia “Jaime Torres Bodet.” He works in semantic information processing and AI techniques, often mixed with distributed information systems. More at http://alum.mit.edu/www/aguzman 7
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