Received 29 September 2015; accepted 24 May 2016; published 27 May 2016
Scheduling is one of the most critical issues in planning and managing of manufacturing activities. Mathematically it is treated as NP-Hard problem. An optimal schedule for a given problem (a manufacturing industry) depends on so many factors like shop floor condition, constraints with which each process is carried out and so on. Job shop scheduling is one of the most difficult problems in this area  . Fisher and Thompson introduced benchmark problems in 1963  and since then many researchers have studied these problems and proposed exact methods and approximate algorithms  -  . Exact methods like branch and bound, linear programming and Lagrangian relaxation are able to solve small instances and will require a large amount of time with increase in problem size. In most of the cases it is reasonable to use a technique which may yield a near optimal solution requiring a lesser amount of time compared to the methods listed above. This has given rise to the use of heuristics, meta-heuristics or hybrid search algorithms (for ex. shifting bottleneck procedure, tabu search, ant colony etc.) by many researchers in the recent past. These algorithms have potential to find high quality solutions in a reasonable computational time. These algorithms have a special quality of adapting themselves to different kinds of scheduling problems and are easy to implement. Genetic algorithms were first successfully applied by Davis in 1985  . Scheduling rules such as Shortest Processing Time (SPT), Most Work Remaining (MWKR) were used by Zhou and Feng  , in his proposed hybrid heuristics GA for JSSP. Extensive use of genetic algorithms to solve job shop scheduling problems can be seen through literature survey  . But it can be observed that no systematic approach has been adopted in modifying the genetic algorithm while using it for JSSP. It may be noted that simple genetic algorithm essentially consists of mainly three aspects and needs critical observation. They are initialization, cross over and mutation operations. According to the authors a systematic approach to modify the simple genetic algorithm would be to initiate the modification in initializing the population itself which helps in lowering the make span and with remaining two operators, it may be further reduced.
If initial population is diverse enough then it is possible to choose best solutions for recombination operations and this may reduce the computational time required. Dispatching Rules (DRs) have been applied consistently to scheduling problems. They are procedures designed to provide good solutions to complex problems in real time. Many authors claim that priority dispatching rules can be successfully used in solving large JSSPs and even other scheduling problems  . Mahanim et al.  used Genetic Algorithm (GA) with some modifications to deal with problem of job shop scheduling which generated an initial population randomly including the result obtained by some well-known priority rules such as shortest processing time and longest processing time. Kuczapski et al.  presented an efficient method of enhancing Genetic Algorithms (GAs) for solving the Job- Shop Scheduling Problem (JSSP), by generating near optimal initial populations.
2. Problem Formulation
Because of its practical importance, Job shop Scheduling Problems have been modeled in different ways based on assumptions as well as situations of the production system. This study considers a single objective of minimizing the make span. The assumptions under which this objective holds well are as follows:
At a time only one job is processed on a machine.
Processing in strict adherence to the precedence constraints.
No set up times.
Mathematically it may be expressed as:
where, Sti is the starting time, Pi is the processing time of ith operation and Stj is the starting time of operation “j”. The second equation resolves the conflict between two jobs to be operated at the same time on the same machine.
So many models were presented in the past  -  and were used either to obtain minimum make span or simply to make representation simpler. Algorithms like immediate selection and shifting bottleneck heuristics were proposed by Carlier  , Adams  , and Lars Monch  . And these algorithms were due to disjunctive graph model.
Table 1. Results of instances as % deviation.
Average deviation is found to be 2.461 across all the instances.
3. Representation of the Problem in GA and GA Operators
The very objective of using evolutionary algorithms like Genetic Algorithms is to make the search process computationally efficient. This is so because techniques like branch and bound etc. are guaranteed techniques to provide the optimal solution but are computationally inefficient. Random walk or gradient search for example is basically random search or gradient descent techniques which will search one solution at a time. Hence these methods are also computationally inefficient with the problems of larger size. Genetic algorithms are well suited in such cases to find the best possible solution close to optimal solution in a computationally efficient manner. Different mathematical models may lead to different representations for the same problem  . Attempts to explain different available representations and explore the use of a better representation scheme for the job shop scheduling problems while using genetic algorithms was done and the study was conducted over 66 bench mark instances. An attempt was made in  to classify the representations as direct and indirect type which was further classified as model based and algorithm based in  . Giffler and Thompson proposed an algorithm way back in 1960  which makes use of priority rules and have enough potential to provide feasible or good solutions. It was due to Bean  , Beirwirth  -  the representations like random keys representation, permutation with repetition, machine based representation and job based representation respectively were developed and studied for their effectiveness.
With the job based representation  , the previous study had shown favorable results. Therefore, by using the same representation in genetic algorithms, this study aims to establish the effect of initial population scheme on the overall convergence of each benchmark instance. For initial population only random job based selection is used in place of other schemes. The pseudo-codes for the same are given below:
Step1: Obtain an eligible set of jobs from the given instance. Let it be.
Step 2: Choose randomly any one job from the eligible set and place its corresponding operation in the sequence set. Let it be.
Step 4: Update the job status.
Step 5: Delete the sequenced job from eligible set Ø, if all the operations of the job are sequenced and then update it with eligible jobs.
Step 6: Repeat the step 2 until all the operations on all the jobs are sequenced.
Uniform crossover  , One-Point and Two-Point crossover techniques are used in the study as one of the recombination operators or in some proportion. Flipping is used as a mutation operator. For better convergence, a Static Critical Path was also found for each instance and mutation was carried out among the critical operations (i.e. to add the processing times of all the jobs getting processed on all the machines and the job with longest duration is to be considered as critical path for the instance). General flow chart of GA is given below:
Step 1: Generate the initial population using random job selection method. Evaluate the fitness of each individual. Let t = 0.
Step 2: Use crossover reproduction operator to generate the offspring.
Step 3: Carry out mutation on each offspring to generate new individuals. Calculate the fitness value of each offspring.
Step 4: If the stop criterion is satisfied, then stop. Otherwise let t = t + 1 and turn to step 2.
In this study, Random Job Based algorithm is used as a first step to generate the initial population to analyze whether this algorithm has any effect on the overall performance of GA which is presented above.
5. Results and Analysis
With the random job selection for initial population followed by job based representation scheme adopted, the study was conducted with 50 generations and a population size of 1000.Mutation probability varies with 0.1 to 0.9 values dynamically and elite population size is 20%. Reproduction probability used in this study is 0.1 Parents in this study were selected from two groups sorted out based on fitness value (i.e. minimum make span). Each parent is selected from these groups probabilistically. In the study, GA is programmed with different reproduction and mutation operators like PPX, single point and N-point crossover mechanisms. The benchmark problems used in this paper are taken from OR library  available in World Wide Web. All the experiments are conducted with a Pentium-4 single core processor with clock speed of 2.06 GHz and RAM of 512 Mbs. 66 benchmark instances were considered and in the twelve runs, the best values obtained are compared with lower bound or optimum value of the benchmark instances reported in the literature. The percentage deviation for each instance is calculated by the formula shown below:
where BKS = Best Known Solution; Opt. = Optimum Solution and Curr.Solution = Solution obtained by the method used in the study. Results as % deviation are shown in Table 1.
6. Conclusion & Future Scope
In this paper, new method is introduced for initializing population. By using simple job based representation followed by random job based initialization along with reproduction operators like PPX, single point and three points cross over techniques, it is possible to get optimal or near optimal results. The average deviation obtained is much less compared with any other results obtained in literature  , where other advanced GA operators are also used. In continuation with the theme of applying a common algorithm for maximum number of instances available in the literature, authors intend to use a new approximation method to detect critical operations based on the schedule and this will be followed by CPM based mutation operator to improve the solutions further.
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