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29 lines
2.0 KiB
ReStructuredText
29 lines
2.0 KiB
ReStructuredText
==============================
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Worklet Error Handling
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.. index::
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double: worklet; error handling
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double: execution environment; errors
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It is sometimes the case during the execution of an algorithm that an error condition can occur that causes the computation to become invalid.
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At such a time, it is important to raise an error to alert the calling code of the problem.
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Since |VTKm| uses an exception mechanism to raise errors, we want an error in the execution environment to throw an exception.
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However, throwing exceptions in a parallel algorithm is problematic.
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Some accelerator architectures, like CUDA, do not even support throwing exceptions.
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Even on architectures that do support exceptions, throwing them in a thread block can cause problems.
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An exception raised in one thread may or may not be thrown in another, which increases the potential for deadlocks, and it is unclear how uncaught exceptions progress through thread blocks.
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|VTKm| handles this problem by using a flag and check mechanism.
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When a worklet (or other subclass of :class:`vtkm::exec::FunctorBase`) encounters an error, it can call its :func:`vtkm::exec::FunctorBase::RaiseError` method to flag the problem and record a message for the error.
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Once all the threads terminate, the scheduler checks for the error, and if one exists it throws a \vtkmcont{ErrorExecution} exception in the control environment.
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Thus, calling :func:`vtkm::exec::FunctorBase::RaiseError` looks like an exception was thrown from the perspective of the control environment code that invoked it.
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.. load-example:: ExecutionErrors
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:file: GuideExampleErrorHandling.cxx
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:caption: Raising an error in the execution environment.
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It is also worth noting that the :c:macro:`VTKM_ASSERT` macro described in :secref:`error-handling:Asserting Conditions` also works within worklets and other code running in the execution environment.
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Of course, a failed assert will terminate execution rather than just raise an error so is best for checking invalid conditions for debugging purposes.
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