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

[brian avants]

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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