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This paper is a reminder of the danger of allowing ``risk'' when
synchronizing a parallel discrete-event simulation: a simulation code
that runs correctly on a serial machine may, when run in parallel,
fail catastrophically. This can happen when Time Warp presents an
``inconsistent'' message to an LP, a message that makes absolutely no
sense given the LP's state. Failure may result if the simulation
modeler did not anticipate the possibility of this inconsistency.
While the problem is not new, there has been little discussion of how
to deal with it; furthermore the problem may not be evident to new
users or potential users of parallel simulation. This paper shows how
the problem may occur, and the damage it may cause. We show how one
may eliminate inconsistencies due to lagging rollbacks and stale
state, but then show that so long as risk is allowed it is still
possible for an LP to be placed in a state that is inconsistent with
model semantics, again making it vulnerable to failure. We finally
show how simulation code can be tested to ensure safe execution under
a risk-free protocol. Whether risky or risk-free, we conclude that
under current practice the development of correct and safe parallel
simulation code is not transparent to the modeler; certain protections
must be included in model code or model testing that are not
rigorously necessary if the simulation were executed only serially.
Bibliographic citation for this report: [plain text] [BIB] [BibTeX] [Refer]
Or copy and paste:
David M. Nicol and Xiaowen Liu, "The Dark Side of Risk (What your mother never told you about Time Warp) ." Dartmouth Computer Science Technical Report PCS-TR96-298, November 1996.
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