[ITK] [ITK-dev] [ITK Community] [Insight-developers] non-deterministic v4 registrations in 4.5.x

[brian avants]

http://software.intel.com/en-us/articles/consistency-of-floating-point-results-using-the-intel-compiler

just as an example of the generality of this problem


brian




On Wed, Mar 19, 2014 at 11:22 AM, Simon Alexander <skalexander at gmail.com>wrote:

> Brian, Luis,
>
> Thanks.  I have been using Mattes as you suspect.
>
> I don't quite understand how precision is specifically the issue with # of
> cores.  There are all kinds of issues with precision and order of
> operations in numerical analysis, but often data partitioning (i.e. for
> concurrency) schemes can be set up so that the actual sums are done the
> same way regardless of number of workers, which keeps your final results
> identical.  Is there some reason this can't be done for the Matte's metric?
>   I really should look at the implementation to answer that, of course.
>
> Do you have a pointer to earlier discussions?  If I can find the time I'd
> like to dig into this a bit, but I'm not sure when I'll have the bandwidth.
>  I've "solved" this currently by constraining the core count.
>
> Perhaps interestingly, my earlier experiments were confounded a bit by a
> precision issue, but that had to do with intrinsics generation on my
> compiler behaving differently on systems with AVX2 (even though only AVX
> intrinsics were being generated).  So that made things confusing at first
> until I separated the issues.
>
>
> On Wed, Mar 19, 2014 at 9:49 AM, brian avants <stnava at gmail.com> wrote:
>
>> yes - we had several discussions about this during v4 development.
>>
>> experiments showed that differences are due to precision.
>>
>> one solution was to truncate precision to the point that is reliable.
>>
>> but there are problems with that too.   last i checked, this was an
>>
>> open problem, in general, in computer science.
>>
>>
>> brian
>>
>>
>>
>>
>> On Wed, Mar 19, 2014 at 9:16 AM, Luis Ibanez <luis.ibanez at kitware.com>wrote:
>>
>>> Hi Simon,
>>>
>>> We are aware of some multi-threading related issues in
>>> the registration process that result in metric values changing
>>> depending on the number of cores used.
>>>
>>> Are you using the MattesMutualInformationMetric ?
>>>
>>> At some point it was suspected that the problem was the
>>> result of accumulative rounding, in the contributions that
>>> each pixel makes to the metric value.... this may or may
>>> not be related to what you are observing.
>>>
>>>
>>>    Thanks
>>>
>>>        Luis
>>>
>>>
>>>
>>> On Thu, Feb 20, 2014 at 3:27 PM, Simon Alexander <skalexander at gmail.com>wrote:
>>>
>>>> I've been finding some regressions in registration results when using
>>>> systems with different numbers of cores (so the thread count is different).
>>>>  This is resolved by fixing the global max.
>>>>
>>>> It's difficult for me to run the identical code on against 4.4.2, but
>>>> similar experiments were run in that timeframe without these regressions.
>>>>
>>>> I recall that there were changes affecting multhreading in the v4
>>>> registration in 4.5.0 release, so I thought this might be a side effect.
>>>>
>>>> So a few questions:
>>>>
>>>> Is this behaviour expected?
>>>>
>>>> Am I correct that this was not the behaviour in 4.4.x ?
>>>>
>>>> Does anyone who has a feel for  the recent changes 4.4.2 -> 4.5.[0,1]
>>>>  have a good idea where to start looking?  I haven't yet dug into the
>>>> multithreading architecture, but this "smells" like a data partitioning
>>>> issue to me.
>>>>
>>>> Any other thoughts?
>>>>
>>>> cheers,
>>>> Simon
>>>>
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