[ODE] Re: Different simulation results with different versions ofVisual C++?

[Keith Johnston]

I've already been through the fire on getting the
simulation to repeat exactly from one run to the next
on the same machine.  That is working fine.  However,
I haven't tried running two copies of the same program
simultaneously.  That would be an interesting
experiment.

So, yes, I'm using a fixed timestep - I've basically
followed all the procedures on the wiki to the letter
to get the simulation to run identically from one
invocation of the same program to the next.

This final hurdle is really just a cross-platform
issue I believe.  Is there any way to make ODE less
sensitive to compiler differences?

Has anyone tried comparing runs between PC and Mac to
see if they can be made to be identical?




--- John Donovan <JohnD at magentasoftware.com> wrote:

> > Have others found a way to work around this
> problem?  I need *exactly*
> the same results from one simulation to another.
> 
> I haven't found a workaround, but I did notice the
> same problem when
> running the same simulation twice at the same time.
> It all started
> because I was gettinga diverging simulation with two
> slightly different
> algorithms. So to see where it was diverging I
> created two worlds,
> loaded the same data and rendered them
> simultainously. My problem turned
> out to be a contact parameter issue and I fixed it
> quickly. But I then
> noticed that running the two worlds _still_ diverged
> when run
> simultainiously, but were identical when run
> individually. Because it
> didn't really affect me I didn't bother following it
> up. But the only
> conclusion I could come to was the random number
> generator was working
> twice as hard when run in tandem, so generating
> slightly different
> contact results.
> So my advice would be to look at the RNG (there was
> some discussion on
> it a while back on this list), and see if it is
> producing the same
> results for each build.
> Also, are you're running with a fixed timestep? With
> the different
> optimisations from each compilier, you could well
> get a different
> framerate every time.
> 
> -J
> 
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