commons-dev mailing list archives

Site index · List index
Message view « Date » · « Thread »
Top « Date » · « Thread »
From Thomas Neidhart <thomas.neidh...@gmail.com>
Subject Re: [Math] "FixedElapsedTime" in genetic algorithm (Was: svn commit: r1409352 - ...)
Date Thu, 15 Nov 2012 09:09:36 GMT
On 11/15/2012 09:55 AM, Thomas Neidhart wrote:
> On 11/15/2012 09:08 AM, Thomas Neidhart wrote:
>> On 11/15/2012 02:23 AM, Gilles Sadowski wrote:
>>> On Wed, Nov 14, 2012 at 04:38:47PM -0800, Phil Steitz wrote:
>>>> On 11/14/12 4:05 PM, Gilles Sadowski wrote:
>>>>> Hi.
>>>>>
>>>>> On Wed, Nov 14, 2012 at 08:09:52PM -0000, tn@apache.org wrote:
>>>>>> Author: tn
>>>>>> Date: Wed Nov 14 20:09:51 2012
>>>>>> New Revision: 1409352
>>>>>>
>>>>>> URL: http://svn.apache.org/viewvc?rev=1409352&view=rev
>>>>>> Log:
>>>>>> Reduce sleep time in unit test to prevent possible race condition.
>>>>>>
>>>>>> Modified:
>>>>>>     commons/proper/math/trunk/src/test/java/org/apache/commons/math3/genetics/FixedElapsedTimeTest.java
>>>>>>
>>>>>> Modified: commons/proper/math/trunk/src/test/java/org/apache/commons/math3/genetics/FixedElapsedTimeTest.java
>>>>>> URL: http://svn.apache.org/viewvc/commons/proper/math/trunk/src/test/java/org/apache/commons/math3/genetics/FixedElapsedTimeTest.java?rev=1409352&r1=1409351&r2=1409352&view=diff
>>>>>> ==============================================================================
>>>>>> --- commons/proper/math/trunk/src/test/java/org/apache/commons/math3/genetics/FixedElapsedTimeTest.java
(original)
>>>>>> +++ commons/proper/math/trunk/src/test/java/org/apache/commons/math3/genetics/FixedElapsedTimeTest.java
Wed Nov 14 20:09:51 2012
>>>>>> @@ -59,7 +59,7 @@ public class FixedElapsedTimeTest {
>>>>>>  
>>>>>>          while (!tec.isSatisfied(pop)) {
>>>>>>              try {
>>>>>> -                Thread.sleep(500);
>>>>>> +                Thread.sleep(50);
>>>>>>              } catch (InterruptedException e) {
>>>>>>                  // ignore
>>>>>>              }
>>>>>>
>>>>> I have never read this part of CM yet. But just this makes me think that
the
>>>>> "FixedElapsedTime" class should not exist in CM. Indeed, using it makes
the
>>>>> algorithm behave differently at each run!
>>>>> I don't deny the usefulness of allotting some amount of time for a
>>>>> computation, but this has absolutely nothing to do with genetic algorithms
>>>>> (nor any others).
>>>>> CM can help in implementing this high-level requirement, by allowing
>>>>> <some search algorithm> to be restarted with the result of a previous
run
>>>>> but IMHO it is a bad idea to have the various CM codes directly implement
>>>>> that functionality.
>>>>
>>>> I disagree.  This looks like a useful feature to me, contributed by
>>>> a user.  GA algorithms are by nature iterative with run-time
>>>> evaluated StoppingConditions.  FixedElapsedTime is a reasonable
>>>> stopping condition supported by other GA frameworks.  The feature is
>>>> well-documented, so users know exactly what it does.  In particular,
>>>> the number of generations is not fixed, so using this
>>>> StoppingCondition does not guarantee the same results on successive
>>>> runs with the same input parameters.  GAs are often
>>>> non-deterministic for other reasons (randomness in mutation), so
>>>> this is not a practical issue for those choosing to use the feature.
>>>>
>>>
>>> This one is really not worth a fight, but something which you have to adapt
>>> depending on the machine on which you run your program can only be qualified
>>> as a toy feature (IMHO): Unless we talk about real-time processing (which GA
>>> are not really suited for...), some fixed amount of time can be too small
>>> (solution not found on a "slow" machine) or too large (solution found long
>>> before the end of the alloted time slot, but the algo keeps the CPU busy).
>>>
>>> As for "non-deterministic", we had several discussions on this ML about
>>> being able to control the pseudo-random behaviour of the algorithms, by
>>> using a fixed seed. This is the same here, should you use the same seed,
>>> the search should go through the same steps. But in this case, it is
>>> _impossible_ to reproduce the same result since the number of generations
>>> will depend on how busy the machine is with other processes.
>>
>> tbh I do not see the issue here.
>>
>> As Phil pointed out, this is a commonly used stopping condition for GAs
>> so it makes sense to have it supported by default.
>>
>> I do not see the link to a discussion about deterministic behavior of
>> algorithms. It is much more similar to a fixed iteration count (a
>> stopping condition that already exists), whereas in this case it is
>> time-based (if a user prefers it this way).
> 
> I just re-read my post and wanted to clarify:
> 
>  * the algorithm is deterministic
>  * the result of course not, as it depends on the number of generations
> 
> So you are right, getting the same result with such a stopping condition
> is indeed tricky, but sometimes this is not the goal. You just want to
> get the best solution so far, even if it is not the best possible.

ah, I do write stupid things today.

Phil explained it much better, I will be quit on the topic ;-)

Thomas

---------------------------------------------------------------------
To unsubscribe, e-mail: dev-unsubscribe@commons.apache.org
For additional commands, e-mail: dev-help@commons.apache.org


Mime
View raw message