RE: [Repast-developer] Question on New Scheduler

[North, M. <no...@an...>]

Gulya:

Insuring that simulation results are reproducible is
clearly an critical issue.  It should be noted that
guaranteeing reproducible results is a concern that
goes well beyond multi-threaded scheduling.  For
example, the introduction of random numbers into
simulations, as is commonly practiced with agent-based
modeling, requires careful attention to insure
reproducibility.  Furthermore, the implications of
reproducibility require thoughtful consideration since
most simulations that use random numbers are valid
only when run in a stochastic mode.  Individual runs
of such simulations may be useful for model debugging
but should not be used to draw scientific conclusions.

That being said, while multi-threaded scheduling in
Java is non-deterministic even within a single virtual
machine, multi-threaded programs still can be
deterministic.  It all depends on the way that the
programs are written.

The new multi-threaded scheduler is being developed
using the "Principle of Least Astonishment."  Thus,
by default, the scheduler will operate in a
deterministic, single-threaded mode.  This will
prevent breakage of existing programs and new
programmers alike.

Multi-threaded scheduling can be activated on a
case-by-case basis by specifying both a start time and
an end time for tasks.  The start time is the
simulation scheduling time you are already familiar
with in RePast.  The end time defines when the
results of a given task are available for other tasks.
These other tasks may be run on separate, concurrent
threads.  Details such as thread pooling and
concurrency control will be automatically handled by
the new scheduler.  Note that the end time is in the
same units as the start time, which is to say units
of simulation time, not wall clock or process time.

Given a set of valid concurrent tasks, a new task can
properly be started if the start time of the new task is
less than or equal to the minimum end time of all of
the currently running tasks.  This is true since the end
time states when the results of a given task are available
for other tasks to use.  The scheduler will use this
rule to select tasks to be run concurrently.  It is up
to model developers to properly specify end times if they
wish tasks to be concurrent.  The details of managing and
using this information will be embedded within the new
scheduler.  The new RePast scheduler will thus make
multi-threading available by simply adding optional
simulation clock end times to the standard scheduler
method calls.

Clearly, proper use of multi-threading will require
model developers to correctly set task end times.
Choosing to use this new feature will thus create a
new responsibility for developers.  However, which
powerful programming feature can be used carelessly
yet effectively?

Michael

-----Original Message-----
From: Laszlo Gulyas [mailto:gu...@la...]
Sent: Sunday, January 06, 2002 9:38 PM
To: rep...@li...
Subject: [Repast-developer] Question on New Scheduler


Hi Everybody and Happy New Year to All, 

I seem to be lagging back on all the issues here, and I am
sorry for that. 

My question here is about the new scheduling mechanism
in the works that has been announced some weeks ago. I
have to tell, it sounds terrific in general. What I am
worried about, however, is replicability of the simulation results,
when using multi-threaded scheduling. As far as I know,
but I might be wrong of course, the exact scheduling of
threads in Java is undeterministic (at least, accross
JVMs). Am I right on this? If yes, can we still somehow
guarantee that the simulation results will be reproducible? 

Well, that's it. Thanks for the answers in advance, and
I am sorry if this what I wrote makes no sense. 

Gulya
 --
Laszlo Gulyas          (617) 384-9216
Government Dept.       Weatherhead Center
602C Coolidge Hall     for International Affairs
1737 Cambridge str.    Cambridge, MA 02138

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