floating point reproduceability

From: Thomas Brian (thomasbrianxlii_at_gmail.com)
Date: Wed Apr 17 2013 - 17:08:34 CDT

Question on results reproduceability. Does anyone know if reproduceability
of results on different processors can be improved, for instance, by
changing gcc compilation options, or perhaps by some NAMD options?

I have compiled the mpi version according to the readme file with default
options for linux-x86_32-g++.

I run on a system with some Intel Nehalem E5520 cpus, and some Intel
Westmere X5650 cpus. Results are identical across machines if NAMD is run
on one thread. Results are different if using more than one. This
suggests floating point differences from order of operations? Any way to
get around this, aside from only running one thread?


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