In Genetic Programming (GP), when island model is used, does it mean that it will split the population size between islands?
For example, if in parameters file we have
pop.subpop.0.size = 4000
and we have 4 islands, does it mean that each island will have a population of size 1000? What if we put this line of code in parameters file of each island? Is it possible to have different population size for each island?
I'm using Java and ECJ package to implement island models in GP.
No, in your example you only have defined one island of 4000 individuals. The number is never automatically splited.
There are two ways to use Islands model in ECJ:
One unique Java process that share variables. The islands are the subpopulations of the Population object. Therefore, you need to set sizes for each subpopulation in the parameter file. In your example, you only have set the island (subpopulation) 0 to 4000 individuals, but you should also set the other sizes. For example, for 10 islands of 4000 individuals each:
exch = ec.exchange.InterPopulationExchange
pop.subpops = 10
pop.subpop.0.size = 4000
pop.subpop.1.size = 4000
pop.subpop.2.size = 4000
...etc
pop.subpop.10.size = 4000
In this case, every island is executed in a different Java process, so, every islandID.params file (one per island/process) needs to set only one population:
exch = ec.exchange.InterPopulationExchange
pop.subpop.0.size = 4000
And the number of islands is set in the server.params file:
exch.num-islands = 10
You can see the rest of parameters and more information on page 223 of the ECJ documentation pdf: https://cs.gmu.edu/~eclab/projects/ecj/docs/manual/manual.pdf