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

[Simon Alexander]

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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