[gmx-developers] Adding computing kernels in shared linked library

[Mark Abraham]

On 25/08/2011 7:11 PM, Sebastian Jodłowski wrote:
> But how could it be a non-reproducible calculation if every run of
> non-library version gives literally the same results and every run of
> library version gives also the same results (but different from from
> non-library version) - it is a deterministic algorithm and all
> calculations are done in the same order.

Each calculation on its own can be reproducible, but they do not 
necessarily reproduce each other. For example different compilation 
and/or linking optimization might lead to different orders of operations 
and thus results. See 
http://www.gromacs.org/Documentation/Terminology/Reproducibility

>   Adding -reprod doesn't change
> anything.

OK. Can you compile both versions without optimizations, and link 
statically, and see if they can reproduce each other? If not, then you 
will have to get out a debugger and find the point they diverge.

Mark

> Sebastian
>
> On 25 August 2011 01:43, Mark Abraham<Mark.Abraham at anu.edu.au>  wrote:
>> On 25/08/2011 12:52 AM, Sebastian Jodłowski wrote:
>>> I'm trying to add some features (e.g. interactions with membrane) into
>>> nb_generic_kernel. I've written all my stuff, tested it hardly and
>>> everything seems to work perfectly - all results are correct. Next
>>> step, which I would like to performe, is to extract my kernels and
>>> move them to the external library. So I added my lib(for now empty
>>> lib) to gromacs makefiles(all of them), and inlcuded my lib header in
>>> the nb_generic.c. In that place gromacs compiled and linked properly.
>>> I cut my kernel code from the nb_generic_kernel() body and above it I
>>> created function my_kernel() which consist body of my kernel. All
>>> parametres are passed via pointers and kernel is called in its earlier
>>> body's place in nb_generic_kernel(). Then I tested it again -
>>> everything works great. So I finally moved my_kernel into the library.
>>> Gromacs compiled and linked properly so I started testing. The results
>>> are already wrong at the stage of minimization. I performed 100 steps
>>> of minimization and version without external library calls 78144 times
>>> my_kernel wherease with external library my_kernel is called 69696.
>>> (What the hell?)The results start to differ at about 60% (45000th
>>> calls of my_kernel).
>> Sounds like you're managing to do a non-reproducible calculation
>> non-reproducibly. Perhaps mdrun -reprod will help.
>>
>> Mark
>>
>>>   I have no idead what's going on and I spent
>>> several days looking for the bug and couldn't find it. I would like to
>>> notice that i don't know how gromacs minimization works and have no to
>>> time for studing these code(but I pretty good analised how computing
>>> of forces works).
>>> Has anybody ever met something similar? Or has any experience with
>>> writing libraries for gromacs?
>>> Greetings
>>> Sebastian Jodlowski
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