RE: Random seed (RANDOMIZE)

Alberto,

thanks for your quick answer on this quite interesting& lively discussion.

I take your point on the matter of philosophy& compatibility of suggestions.
But I would like to stress once again that my proposal does not at all imply
any change of philosophy but only a reversal of what is considered as default/optional and a bit of automatized help for the user!

As I wrote in each previous e-mail the reproducability of pseudo-random numbers is crucial - this is a point that I have never questioned. However, I
stand by my opinion that for a user this is in most cases not of concern. If the code generated the initial random seeds by itself, while outputting
the first one, then the user would not have to worry about correctly initiating FLUKA's way of parallelization himself, while retaining the possibility
to fully reproduce the random sequence which was "chosen" by the machine.
The way that the seed is generated originally is something which can be done via hardware
- like I said with VIA mobos for example - or deterministic master-slave mechanisms
as long as the disjointness of parallel sequences is preserved and the initial
seed is available for later use to reproduce the pseudo-random sequence.

If this approach, for which I do not claim authorship but which has been devised
and discussed in a plethora of mathematical treatments over the last decades,
is naive, is something that might lie in the eye of the beholder.
Yet, it is a popular and widely successful approach implemented in various
Monte Carlo codes, a technique that extends far beyond the domain of radiation transport.
So what I do object against is the statement that it is un-workable.

> Referring to what I have said above, your idea of what a default should be conflicts hardly with the FLUKA philosophy that all jobs should
> be reproducible.

Well, this is a philosophical design question which can be argued about. But
you are absolutely right that in any case it is the developer's decision, no matter if the users like it or not.

However, this discussion hides a probably much more interesting and important aspect regarding parallelization. In times of N-core CPUs and GPGPUs,
found even in cell-phones, the question is rather why a user has to do parallelization& distribution himself? This surely is a philosophical question
which I would be interested to hear your opinion about.

Ciao
Chris

________________________________________
From: Alberto Fasso' [fasso_at_slac.stanford.edu]
Sent: 29 June 2011 19:13
To: fluka-discuss_at_fluka.org
Cc: Chris Theis; Alfredo Ferrari; denis bertini
Subject: RE: Random seed (RANDOMIZE)

Two answers, one to Chris and one to Denis. In both cases, I ask you to keep
in mind that every Monte Carlo code is designed according to a particular
"philosophy", that you may like or not. Suggestions to do modifications
are ok, but not if they are incompatible with that philosophy.

On Wed, 29 Jun 2011, Chris Theis wrote:

> with all due respect but I beg to differ on this point. The approach that=
  I
> describe is nothing new and actually implemented in various Monte Carlo codes,
> admittedly in the CG domain, but this is not of concern for this discussion.
>
> The approach is simply to revert the default behavior that FLUKA currently
> exhibits. So mathematically nothing changes at all. More specifically, it
> would mean deterministic selection of predefined seeds as an option for
> debugging and development, while starting from (pseudo)random seeds as a
> default. However, this does not mean that the user would be prevented from
> selecting a seed himself, but he would not be forced to do so to get
> independent runs. It is true that mathematically it cannot be fully excluded
> that the same seed would be re-used if generated in a pseudo-random manner,
> but given the periodicity of modern RNG or the application of for example a
> VIA mobo the probability can be kept at an almost zero level.

Chris, your proposal would make impossible for anybody to help a user who
has got a problem, or to find a rare programming bug in the code.
Reproducibility of random numbers is as important as their independence, and
probably more. This was realized by Monte Carlo developers since the very
early times: the first generators produced numbers completely independent,
obtained from electronic noise measurements, and they were soon abandoned
because they were not reproducible.
Referring to what I have said above, your idea of what a default should be
conflicts hardly with the FLUKA philosophy that all jobs should be reproducible
(see for instance the care taken about overlapping ORs in the geometry).

> Alfredo Ferrari wrote:

> I believe Denis question is different, if I understand correctly he would like
> to have a way through a user routine to input something equivalent to what is
> now input in what(2) of RANDOMIZe? Am I correct?

It is difficult to understand why making a copy of an input file should be
more complicated than writing, compiling and writing a user routine.
The fact that GEANT does it this way is coherent with the fact that all
GEANT input is programmed by the user. FLUKA has been written with the opposite
philosophy: that the user should program nothing or as little as possible.
All that can be done with an input file shall be done with it.

Alberto
Received on Thu Jun 30 2011 - 18:48:00 CEST