[FLASH-USERS] Problem with Compiling Flash3.0

[Paul M. Rich]

Seyit Hocuk wrote:
> There is one huge problem that I noticed though. Normally I am always 
> doing simulations on my own computer or at the cluster available to 
> me. However, now that I am doing on this new computer with this new 
> compiler, my initial results are different and the difference seems to 
> grow bigger during the simulation. I use exactly! the same initial 
> conditions. But I immediately notice that refinement is different 
> (refines less quicker) and dtHydro is slightly different and growing 
> more different compared to my computer.
>
> Could this be due to the fact that the new machine is 64 bit and mine 
> is 32 bit and thus more precise? Although of-course the compiler is 
> also different as are the libraries (Hdf5 and Mpich) as is the 
> operating system.

Yes, this most certainly can cause appreciable differences between 
runs.  Different computer architectures and compilers can and will 
result in different optimizations being used, and will pretty much 
ensure that the order of floating point operations will be changed.  
Depending on the problem, this can give very different-appearing results 
when output files are compared with sfocu, particularly if the problem 
has a lot of very sharp discontinuities or other small, rapidly 
changing, features.  If the initial differences are small, at about 
machine precision for double values, it is probably due to this.

It also may be the case that one compiler was automatically initializing 
data differently, or that uninitialized data is being used in the 
problem at initialization time unintentionally.  If you're getting large 
errors at initialization, I'd check this.

Hope this clarifies things for you,

Paul Rich