Re: [Swarm-Modelling] True concurrency?

[Steve Railsback]

Valmir Meneses wrote:
>  
> Hi,
> Is Swarm concurrent (agents run concurrently) or implements threaded 
> schedules like "swarm-alike" Repast?
> A bit more detailed. Repast runs the agent action within a schedule. At 
> some tick, the agent list is scanned and each agent is given attention. 
> But, as far as I know, the agents do not run concurrently. Within each 
> tick, the agents are chosen randomly to determine who will have 
> attention. Repast implements "simulated concurrency".
> Since Repast claims to be based in Swarm, I wonder what is the Swarm 
> behaviour?

Hello,

Swarm, like Repast, lets you control the exact order in which you choose 
events to occur--including choosing to have actions executed in random 
order, or to have one action executed by a list of agents in randomly 
shuffled order. So you get to decide how you want to approximate 
concurrency. Best of all, it's easy to try alternative ways of 
representing concurrency and seeing what the consequences are, if you 
code your schedule tidily. (Look in the activity library at classes like 
ActionGroup and Schedule.)

My limited experience is that "real" concurrency, where the computer 
tells many agents to execute at once, can make it very difficult to 
figure out exactly what's going on and to know whether  your model is 
really doing what you want. We just ran a test model in NetLogo (which 
does claim "real" concurrency) and in Repast, using exactly the same 
algorithm. We got somewhat different results that appear to be an 
artifact of the "real" concurrency. Especially,  I would be cautious if 
different agents need to do different amounts of computation- the agents 
that do more thinking might get executed less often, or always tend to 
finish last. That could seriously bias results.

Steve Railsback

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