[mesa-users] problem with different results on different platforms -- mac vs. linux

Sat Jan 12 15:39:22 EST 2013

A few comments on this, as this is always an interesting problem.  I am not
running MESA and don't know the magnitude of the differences, but seeing
differences on different machines is itself not unusual.  As you know,
floating point math is an approximation (if anyone hasn't read the paper
"What Every Computer Scientist Should Know About Floating-Point
Arithmetic", they should), and the system of equations that MESA solves is
highly nonlinear, so any small differences in the roundoff behavior of some
operations (for instance, maybe one chip is doing a multiply-add as a
single instruction while the other does it as two) can get amplified
greatly over the life of the simulation.

Gfortran (and gcc) have a huge number of options that control exactly what
sort of transformations are allowed when optimizing and what hardware
features to use.  In order to understand what is going on better, you want
to make sure that you control for any optimizations that the compilers are
doing on the different machines.

Are they the same chip family?  I am assuming that they are both 64-bit, if
not, that's a big difference in instruction sets.  Within the same
'bitness', there are still big differences between chip versions.  Perhaps
you can try forcing a specific instruction set by using -march (see "man
gcc" to see the list of different architectures just within the Intel
family).

Are there defaults for the compilers hidden somewhere that are different on
the two machines?  The gcc FAQ shows how to see exactly what options are
used at the different optimization levels

http://gcc.gnu.org/wiki/FAQ#What_specific_flags_are_enabled_by_-O1_.28-O2.2C_-O3_or_-Os.29.3F

Do this on each machine and 'diff' the output and account for
any differences there.

It may also be a good idea to scale back on the optimization level
in general and see if the solutions agree then.

Finally, just to reiterate, these are highly nonlinear problems, so if the
excutable does some rounding a little different on one machine vs. the
other, this can propagate and cause a large divergence in the results.
 Knowing this, and knowing that if you ran with a different set of
compilers you'd get different #s, when doing simulations, the
thing you should focus on are the trends and the spectrum of
possible outcomes and variability allowed in the outcomes.  The tracks
should qualitatively look similar, but obsessing on specific numbers is
often futile when solving nonlinear systems.

Just my 2 cents.

On Sat, Jan 12, 2013 at 2:50 PM, Jakub Ostrowski <
ostrowski at astro.uni.wroc.pl> wrote:

> Bill, the differences are huge. You might be right, but it's hard to
> believe that "within the accuracy tolerances" one model at the same spot on
> the HR diagram has Y ~ 0.70 while the other one has Y ~ 0.35.
>
> I don't require perfect agreement between the platforms but I definitely
> need to know which one is closer to reality. It's obvious that at least one
> of these two solutions is totally wrong. And I have observed that changing
> parameters have dramatic effect on the shape of the blue loop. Well, I
> guess I'll have a hard time trying to explaining my supervisor why I
> obtained two results with one code :)
>
> I think we have to take some effort to achieve better compatibility, I
> mean only one solution. I'm going to work with stars with range of masses,
> let's say, 12-25 M_S so I can help I guess.
>
> Thank you for your help. I'll keep you updated about any other tests I'm
> going to make.
>
> Cheers,
> Jakub
>
>
> On 12 sty 2013, at 20:32, Bill Paxton <paxton at kitp.ucsb.edu> wrote:
>
> >
> > On Jan 12, 2013, at 10:55 AM, Jakub Ostrowski wrote:
> >
> >> I use SDK in version 20121106 (the latest one) both on Mac and Linux.
> And yes, I'm sure that I use gfortran from SDK on the supercomputer. So we
> have a situation where I use the same version of compiler on both machines.
> The only difference is an operating system.
> >
> > Excellent!   Now perhaps I understand the problem.
> >
> > Aaron Dotter has recently done comparisons of mesa/star results on Mac
> vs Linux;
> > he found "small" differences even when using the same gfortran and SDK,
> > where small means within the accuracy tolerances of the test case.
> > It is of course distressing that there are any differences at all --
> ideally,
> > the results would be bit-for-bit identical.   But that goal has not been
> reached.
> >
> > So if you require bit-for-bit identical on Mac and Linux, then mesa is
> currently out.
> >
> > If you can tolerate some differences, then it becomes a question of the
> sensitivity
> > of your problem to small perturbations caused by the different platforms.
> > For example, I believe the details of blue-loops are very sensitive so
> they might
> > show large cumulative effects from the many small perturbations that
> happen
> > when moving to a different platform.  You might confirm that by checking
> > their sensitivity to small changes in parameters when running on the
> same platform;
> > changes in numerical parameters (such as mesh resolution) as well as
> > physical ones (such as mixing length alpha) might result in big changes
> > in the shape of the blue loop.
> >
> > If you are trying to study a problem that is very sensitive, then you
> have a challenge ahead.
> >
> > Perhaps other mesa-users will have suggestions.
> >
> > Good luck,
> > Bill
> >
> >
> >
> >
> >
> >
>
>
>
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-- 
Michael Zingale
Associate Professor

Dept. of Physics & Astronomy • Stony Brook University • Stony Brook, NY
11794-3800
*phone*:  631-632-8225
*e-mail*: Michael.Zingale at stonybrook.edu
*web*: http://www.astro.sunysb.edu/mzingale
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