[FLASH-USERS] Big Problem??

[Seyit Hocuk]

Oh man,

I am using 3 different computers (with different architectures, 
compilers, optimizations, environments). All three start differently 
immediately from the beginning (difference in dt_hydro and refinement).
Should I worry very very very much?
I'm now doing a complete simulation and am going to check if my end 
results also change.

I'm trembling by the thought.....









Paul M. Rich wrote:
> 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