Re: Is there solution to numerical inaccuracy

From: Peter Freddolino (petefred_at_ks.uiuc.edu)
Date: Tue Nov 20 2007 - 11:48:58 CST

Hi Alok,
could you clarify your particular testing procedure? Also, did you
compare the error of two different split runs with that between two
identical continuous runs?

The reason I ask is that for a parallel setup that does floating point
operations in arbitrary orders one can *never* expect absolutely
identical results between different runs, even if those runs use
identical input, due to various nondeterministic differences in the
ordering of floating point operations (which could also be the reason
for differences during a restart). The errors noted in the thread you
mentioned are on the order of deviations I've observed previously
between different, identical, runs due to such imprecision, and should
not affect results significantly. I'd refer you to Jim's comments on
non-repeatability in the same thread
(http://www.ks.uiuc.edu/Research/namd/mailing_list/namd-l/1317.html) for
more information.

Jim or Dave, do you have any comments on this?

Peter

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