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PERF: Optimize compare image

Luc Hermitte requested to merge lhermitte/otb:optimize_compare_image into develop

Summary

Speed-up the performances on compare-image.

On a test case, a rough x3 speed-up has been observed.

Rationale

Comparing the images produced against their baseline is not as efficient as it could be at the end of automated tests.

Implementation Details

The improvements have been done on two axes

Hidden costs of vector images

The DifferenceImageFilter is often used on VectorImages, in particular when doing compare-image.

Creating new VariableLengthVector pixels at each iteration, or casting them will automatically end up in construction and destruction of VLV pixels, and thus allocation and liberation of heap memory. This is not efficient.

The first refactoring aims at minimizing the number of allocation by factorizing out everything that can be (pixel size, typical max value, typical zero value). Resetting value is done by assignment, which has the good property of not inducing any allocation on VLV variables.

Non-localized reductions

The reduce computation done in DifferenceImageFilter was done on some m_ThreadAccumulatedCounter[threadId] member. This has two drawbacks:

  • first the compiler cannot know whether we are the only one function to work on that value, it cannot isolate it in a register nor anything similar
  • moreover false sharing happens and prevent further optimizations.

As a consequence the counters are now local variables in the ThreadedGenerateData() function, and they are commited into the shared member variables at the end of the execution.

Note: this anti-pattern appears in other places like for instance StreamingCompareImageFilter. This performance related refactoring should be extended to these other cases.

Copyright

The copyright owner is CNES and has signed the ORFEO ToolBox Contributor License Agreement.

Check before merging:

  • All discussions are resolved
  • At least 2 👍 votes from core developers, no 👎 vote.
  • The feature branch is (reasonably) up-to-date with the base branch
  • Dashboard is green
  • Copyright owner has signed the ORFEO ToolBox Contributor License Agreement

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