yade-dev team mailing list archive

["Eulitz, Alexander" <Alexander.Eulitz@xxxxxxxxxxxxxxxx>]

Hi,

I can confirm the scattering. Some time ago I repeated a simulation multiple times (same initial state) and recorded forces at facets. Here is a graph:

 

The deviation between the red and blue as well as between black and brown is apparent.

Cheers,

Alex

 

 

Von: Yade-dev [mailto:yade-dev-bounces+alexander.eulitz=iwf.tu-berlin.de@xxxxxxxxxxxxxxxxxxx] Im Auftrag von Václav Šmilauer
Gesendet: Donnerstag, 5. März 2015 12:05
An: Yade Development Group
Betreff: [Yade-dev] results scatter with identical initial simulation

 

Hi there,

I'm having an issue with results differing between initially identical simulations. I know we have two stock explanations:

*	the fairytale, authored by myself back then, that (force, ...) summation order is non-deterministic due to OpenMP (threads switching controlled by the kernel) and these small rounding errors propagate in chaos-like fashion, having macroscopic effect at the end;
*	CPU may "randomly" keep some doubles in registers (which have more width, IIRC 80bit?), and keep other ones in cache/RAM, which have less width (64bit); thus some numbers will be randomly more precise than others, which again leads to non-determinism and propagation as above. I heard this from Stefan Luding (IIRC) some 10 years ago at a conference, but I am not sure how valid that is at all; I have not heard about that from other sources.

I am not conviced by either explanation, the scatter seems to be too big (roller screens -- I get something like 2% scatter in resulting PSD curves over a long simulation run multiple times, with completely idetnical initial situation). Even though I use Woo, I assume the problem is still the same as in Yade. So I would like to hear about:

*	Someone else having that problem at all?
*	Someone did a proper (quantitative) in-depth investigation of this?
*	Someone run Yade through valgrind recently, to check for uninitialized memory reads?
*	I know that PFC 5.0 has what they call "deterministic mode" simulations, someone knows the internals of what it does? (Emanuele? If you may disclose that --)

Cheers,

Václav