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

[Bill Paxton]

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