问题
Some Genetic Algorithm frameworks, such as http://www.aforgenet.com/ requires many parameters, such as mutation rate, population size, etc
There is universal best numbers for such parameters? I believe that it depends on the problem (fitness function delay, mutation delay, recombination delay, evolution rate, etc). My first thought was to use a GA to configure another GA.
Any better ideas?
回答1:
The one time I programmed a genetic algorithm I included those values in the values to mutate, basically like you said using a GA to configure itself. It worked surprisingly well, especially since I've found it to be beneficial for those values to change over the course of it's computation.
回答2:
I find it helps to think of these problems as a landscape, where you're trying to find the lowest point.
Methods like genetic algorithms are used when the landscape is too large to just test all the points, and the "shape" of the landscape is such that methods like gradient-descent will get you stuck in local minima.
One good example is Rastrigin's function (image ref):
(source: scientific-computing.com)
:
Your choices are:
Generation size:
- Too large: you're going to have a long epoch time, restricting how many chances each individual has to explore it's neighbourhood.
- Too small: you don't get good coverage of the search space.
Mutation rate:
- Too high: you risk the individuals "jumping" over a solution they were close to.
- Too low: they're all going to get stuck in local minima.
So it really does depend on your own particular search space. Experiment with the parameters and try to find the optimal combination. I would agree that using another GA to optimise the parameters is not going to solve the problem.
回答3:
I find it rather disappointing there are so many answers that assume you can't find the best parameters for the Genetic Algorithm automatically. I agree that the parameters does depend on the problem however there are methods where you can find them.
Additionally the No Free Lunch Theorem by no means stops you from finding the best parameters, as there have already been discussion that disputes the fact:
- Practically it does not hold
- Description length may dispute the theorem
- Coevolution can make the theorem irrelevant
There are two types of parameter setting:
- Parameter Tuning (offline parameter search - before the GA is run)
- Parameter Control (online parameter tweaking - during the GA run)
- Adaptive
- Self Adaptive
- Deterministic
There are a lot of literature out there that describe how one can find these optimal parameters, it depends whether you are wanting to do the parameter search offline or online. Popular belief is that offline is better suited for most cases because the online parameter control methods would add too much complexity over offline.
Here are some examples to find the "best" parameters:
Parameter Tuning:
- Simple parameter sweep (try everything)
- Meta-GA ontop of a GA
- Racing strategy to find best parameters
- Meta-GA and Racing together
Parameter Control:
- Adaptive probabilities for mutation and crossover
- Self adaptive popuation size
- Deterministic Genetic Operators
And many more, just search for the literature using keywords used above. There are scientific methods for finding suitable parameters for any given problem!
回答4:
It ain't easy.
Why? Because of the No Free Lunch theorem. This basically states that there is no general search algorithm that works well for all problems.
The best you can do is tailor the search for a specific problem space. You'll have to manually tweak your parameters to fit your solution. Sorry.
Using a GA to find GA parameters gets complicated. How do you find the optimal parameters for your GAGA search? Another GA...?
回答5:
There really isn't an automatic way to do it for a given dataset. If there were, they wouldn't expose those parameters. Using a second GA to tune the parameters of the first GA is perilous -- do you use a third GA to tune the parameters of the second? Even if you did that, it's a recipe for overfitting anyway.
My advice would be to play with the parameters, and see how they affect your population distrubution at each generation, how many generations it takes to get to an acceptable answer, etc. If you have too much mutation your population will never stabilize. Too little and you'll end up with homogeneity.
It's a dirty secret of GAs that tuning them is an art, not a science.
回答6:
As everyone else said, there is no one answer. Although there is some tendency to use crossover rate on level 0.7-0.9 and mutation on 0.1-0.3 it really depends. Depends on problem, may depend on fitness function, and definitely depends on Genetic Algorithm itself. There are many GA variations, optimal parameters for the same problem may vary.
As for using GA to tune parameters of target GA there are approaches like that, but, as it was pointed out, how to tune parameters of first GA? Keep in mind, that maybe mutation rate should be higher at the beginning, and than it should decreasing while cross over rate should be increasing. It is issue of exploration versus exploitation. There are approaches to let GA be more adaptive and let it change its parameters as it looks for solution. Fuzzy controllers are sometimes used to manipulate parameters of GA. There are also other approaches.
If you want to know about it more, buy some books, or look through academic research papers.
If you need to setup your own GA without extensive research, try some values from others work, and experiment with them.
来源：https://stackoverflow.com/questions/1075628/how-to-find-the-best-parameters-for-a-genetic-algorithm