RE: Combining data from runs on different machines/architectures

Hi Roger,

> I plan to use a condor cluster to enhance the CPU power for my
> simulations. The cluster consists of machines with different
> architectures (Intel, AMD, AMD64, x86_64). I want to start several runs
> with one or more cycles on many machines in parallel using different
> initial seeds and combine the simulated data in the end.

This is exactly the environment in which we're running FLUKA simulations
in the RP group at CERN without any problems. The version of FLUKA which
is publicly available is a 32-bit native and thus, also 64-bit machines
will either run in compatibility or in legacy mode. Therefore, combining
data from different machines & architectures will be no problem.

There is only one thing that you might want to keep in mind and this is
not FLUKA specific but rather due to different CPU architectures. So far
I've seen that in some cases floating point rounding can be slightly
different on Intel & AMD even if the FPU has been set to strict
double-precision instead of the default extended-precision mode.
However, this behavior is not limited to FLUKA but you can also find it
in other programs. As a consequence you should make sure that you re-run
the simulation on the same architecture in case you want to fully
reproduce results starting from the same random seed. Otherwise the
particle histories might diverge at one point which would lead to
slightly different results as it is in the nature of MC simulations.

Cheers
Chris

------------------------------------------------------------------------
Chris Theis
CERN/DG-SCR - European Organization for Nuclear Research
1211 Geneva 23, Switzerland
Phone: +41 22 767 8069 Office: 892-2A-015
e-mail: Christian.Theis@cern.ch www: http://www.cern.ch/theis
------------------------------------------------------------------------

> -----Original Message-----
> From: owner-fluka-discuss_at_mi.infn.it [mailto:owner-fluka-
> discuss_at_mi.infn.it] On Behalf Of Roger Hälg
> Sent: 28 May 2009 14:25
> To: fluka-discuss_at_fluka.org
> Subject: Combining data from runs on different machines/architectures
>
> Dear FLUKA experts
>
> I plan to use a condor cluster to enhance the CPU power for my
> simulations. The cluster consists of machines with different
> architectures (Intel, AMD, AMD64, x86_64). I want to start several runs
> with one or more cycles on many machines in parallel using different
> initial seeds and combine the simulated data in the end. Inevitably the
> runs will be performed on Linux PCs with different architectures.
>
> I have in mind to have read here in the mailinglist something about the
> problems using the data simulated on different machines, mainly
> concerning the usage of the random number generator. Unfortunately I
> can't find the post anymore.
>
> So my question is, if it is possible to combine the data from the
> different machines without adulterate the results? Or do I have to
> account for something in addition?
> Any further explanation of this topic would be greatly appreciated.
>
> Thanks
>
> Roger
Received on Thu May 28 2009 - 18:16:16 CEST