May 2010


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Steven Saul <[log in to unmask]>
Reply To:
MASON Multiagent Simulation Toolkit <[log in to unmask]>
Thu, 13 May 2010 12:21:42 -0400
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Hi Sean,

Thanks for your reply!  From the reading I have recently done on
concurrent programming, that actually makes a lot of sense and avoiding
race conditions are my primary concern.  It seems like I don't have much
of a choice but to parallelize my own simulation because it involves
modeling millions of individual fish that are fished by a fishing fleet. 
It sounds like I can process in parallel all of the things that each fish
does itself without dependency on anything else i.e. (things like growth,
maturity, etc.). From what you said, it also sounds like the fish can go
through their movement algorithm in parallel (which tends to take some
processing juice), store their new x and y location, then within that time
step, after all fish went through the parallel sequence, they can go
serial again at which point the fish are assigned to their new grid
location.  I wonder if this would be computationally efficient as you
would kind of need to loop through the objects twice, right, once in
parallel and then once serially?


> On May 12, 2010, at 3:34 PM, Steven Saul wrote:
>> After looking at HeatBugs and the ThreadedDiffuser class, I was
>> wondering
>> if there was a way to use ParallelSequence to split the processing of
>> agents (in this case the actual heat bugs) in each time step.  For
>> example, if you have 800 heatbugs in any given time step, a computer
>> with
>> 8 cores would process 100 bugs on each core simultaneously in that
>> time
>> step.
> Sort of.  You just schedule a ParallelSequence on the schedule.  In
> the ParallelSequence you put, say, eight RandomSequences.  Each of the
> RandomSequences
> holds 1/8 of all your bugs.
> However you have to be careful there.  Each HeatBug reads information,
> moves, and writes information. This presents opportunities for race
> conditions.  First, if one HeatBug is reading a location while another
> is writing the same location, you'll get messed up results.  Second,
> if two HeatBugs access the SparseGrid2D at the same time to move,
> they'll break the hash table.  This means that realistically you can
> only do the read portion of these operations in parallel.  So you'd
> have the parallel sequence call step() methods which do the reads and
> internal computation for each bug in parallel; but after that you'd
> have to schedule the bugs serially (as before, maybe in a later
> priority) to move themselves and then write to the heat array.
> This is advanced threading stuff for people who are really familiar
> with the perils of threaded coding.  If the above paragraph causes you
> to say "huh?", then the answer is NO, you SHOULD NOT parallelize your
> agents.  :-)
> Sean

Steven Saul, M.A.
Graduate Assistant, Marine Biology and Fisheries
Cooperative Institute for Marine and Atmospheric Studies
Cooperative Unit for Fisheries Education and Research
University of Miami - RSMAS
4600 Rickenbacker Cswy.
Miami, Florida  33149
+ 1 305-421-4831