April 2010


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Martin Pokropp <[log in to unmask]>
Reply To:
MASON Multiagent Simulation Toolkit <[log in to unmask]>
Wed, 14 Apr 2010 22:34:30 +0200
text/plain (89 lines)
Dear Mr. Luke, dear "Masonists",

I'm working on a model where agents (consumers) are connected by a  
social network. On every time step they take a new decision, based on  
their individual parameters and the parameters of their direct  
neighbors in the network.
In order to implemet multiple runs with multiple parameter  
specifications for different runs, I wrote a wrapper class from which  
the model ("MainModel") is called. The basic structure of the wrapper  
class, holding the main(String[] args) method, is:

	for # runs
		MainModel model = new MainModel(seed, parameter specifications...);	
		long steps;
			if (!model.schedule.step(model))
			steps = model.schedule.getSteps();
		while(steps < 240);
	... do some file writing etc.

In order to speed up the simulation I decided to let  
ParallelSequence's do the job of scheduling the model agents,  
contained in the Bag networkAgents, to step and do their lookups in  
their neighborhoods and to take new decisions each time step. The  
structure is as follows:

Steppable steppable = new Steppable[numberOfCPUs];
	for(int j = 0; j<numberOfCPUs;j++)
		final int p = j;
		steppable[j] = new Steppable ()
				public void step(SimState state)
					for(int i = p*NUMBER_OF_AGENTS/numberOfCPUs;i<(p+1) 
						Agent a = (Agent)(networkAgents.get(i));
						a does something...
	ParallelSequence update = new ParallelSequence(steppable);
The problem I'am facing now is that after some simulation runs, speed  
drops sharply until it finally breaks down and java throws its  
"Exception in thread "main" java.lang.OutOfMemoryError: Java heap  
space". The problem occurs faster if the number of model agents is  
higher. I also tried it with more space by java -Xmx200m or 400m,  
even 800m, unfortunately the break down is only postponed but still  
occurs. The problem occurs both under linux and Mac Osx (10.4), I am  
using simple personal computers with two or for CPUs.
The Apple Activity Monitor shows for every new sim run an increasing  
number of threads. It also shows that the space the simulation uses  
is constantly growing but at break down is still far below the  
virtual space reserved by java.  The heap space problem only comes up  
with ParallelSequences, not with Sequences.
I'm not an experienced programmer, but this seems to tell me that  
after a sim run is over and the model instance has been finished the  
threads that were created for or by the ParallelSequences are not  
finished and therefore not garbage collected. Obviously the creation  
of the sequence alone is sufficient to evoke these problems, because  
when I commented out "schedule.scheduleRepeating(Schedule.EPOCH, 
0,update,1);", and the simulation made only "empty" steps without any  
real actions of the agents, the problem did not go away. Also the  
idea of putting the Sequences into a OneShotStoppable wrapper (see 
L&P=R236&I=-3) did not do the job.

Do you have an idea, what could be done to avoid these problems? Help  
is really appreciated...

With Best Regards