[CP2K:2211] Re: determinism of CP2K runs

Noam Bernstein noam.be... at gmail.com
Fri Aug 7 22:36:27 CEST 2009


Hi Teo - I thought of all the things you mentioned, but I doubt that
they are the cause.  I'll explain why briefly now (and also why
I need it to be deterministic, unfortunately :), and I'll have a more
complete explanation and hopefully a better (smaller, maybe usable
in serial) test case.

First the reason I need it to be deterministic:  I'm running MD, and
it's chaotic (in the technical sense), so unless I have deterministic
runs I can't reproduce a trajectory (for example with different output
options), even from the same input file.

As to why I doubt that it's differences in machines, I've run many
electronic structure codes on this cluster, and I've never seen
non-determinism (for a fixed _number_ of processes) except for uses
of random variables or uninitialized variables (i.e. inadvertently
random variables).  To me this means that the machines are
reasonably reliable, and that the MPI implementation is deterministic
(although since floating-point math isn't associative, changing
the _number_ of processes does change the answer at the level
of roundoff).  I also don't think it's a memory error/cosmic ray
for the same reason - CP2K is never the same twice (for my test
input), while other (MPI+ScaLAPACK electronic structure) codes
always are.

Anyway, I'll work on reproducing the issue either in serial or at least
in parallel on the same set of nodes, so feel free to ignore me until then.

     thanks,
     Noam



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