I have designed a simple patch that fixes the bug for simple situations
that were previously failing with deal.II 9.6.1's FETools::extrapolate. I
will continue the testing and propose a PR together with a bug report on
github once I have time. Given the lack of answers here, I guess this is
not the right channel to discuss all this?
Best
Guilhem
On Wednesday, 16 April 2025 at 18:27:15 UTC+2 Guilhem Poy wrote:
> Dear all,
>
> I think I have pinpointed the problem, which comes from l. 724-733 in
> deal.II/fe/fe_tools_extrapolate.templates.h: when a cell which has a more
> refined ghost neighbour owns a DoF that lives at the interface between this
> cell and this refined neighbour, the associated DoF value will never be set
> and therefore stays at 0. However, on the neighbouring process, the
> corresponding ghost DoF value will be set, which explains the
> compress("insert") exception later thrown due to the inconsistency between
> a nonzero ghost value and zero "owned" value. *So really, the triggering
> condition for my bug is not that there are ghost hanging nodes, but that
> one of the entry in dofs_on_refined_neighbors is associated with a locally
> owned DoF on a given process.*
>
> I guess the idea behind these lines is that we should favour extrapolation
> on the side that has more information, i.e. with the refined cell. But for
> this to work, I think one need to communicate data between process if the
> relevant DoFs on the refined cell are owned by a different process than the
> one that owns this refined cell. Or maybe I completely misunderstood the
> thinking behind this dofs_on_refined_neighbors map?
>
> Best regards,
>
> Guilhem
>
>
> Le 15/04/2025 à 09:04, Guilhem Poy a écrit :
>
> Hi everyone,
>
> I have continued trying to fix the issue below on my own. First, I
> discovered that the extrapolate function is thoroughly tested inside the
> testsuite, so I went there to see how it is used to fix my MWE, but I still
> get the exception. This led me to conclude that some triangulations with
> certain refinement pattern triggers the compress exception, while other do
> not, including the triangulation that is created in the make_tria function
> of the file "tests/mpi/fe_tools_extrapolate_common.h" of the deal.II
> repository. To convince you of this I have attached a patch that changes
> the refinement pattern in the aforementioned make_tria function. With the
> patch applied on the v9.6.1 repository, running again the tests
> "mpi/fe_tools_extrapolate_03***" will throw the same exception mentioned in
> my previous mails for three mpi ranks.
>
> *To summarize, I think there is a real bug involved since one can trigger
> the exception just by changing the triangulation involved in the
> "mpi/fe_tools_extrapolate*" tests*, touching nothing about the
> interpolation and extrapolation code*.* Now, the problem is that I still
> don't understand why some triangulations works and others don't. Clearly,
> my affirmation on the triggering condition from my previous emails is
> imperfect, ghost hanging nodes seems to be a sufficient but not necessary
> condition to throw the exception. For example, the below distributed
> triangulation on the left triggers the exception, while the one on the
> right don't:
>
> I will continue to investigate by diving into the internal functions of
> extrapolate, but I would welcome some help on this since I am not really
> familiar with the p4est code.
>
> Best regards,
>
> Guilhem
>
>
> On 10/04/2025 14:31, Guilhem Poy wrote:
>
> As a complement (I realized I should have put this in the first email),
> here is (one of) the exception that is thrown with the attached MWE from my
> first mail:
>
>
>
>
>
>
>
>
>
> *An error occurred in line <738> of file
> </home/gpoy/.local/share/deal-ii-candi/tmp/unpack/deal.II-v9.6.1/include/deal.II/base/partitioner.templates.h>
>
> in function void
> dealii::Utilities::MPI::Partitioner::import_from_ghosted_array_finish(dealii::VectorOperation::values,
>
> const dealii::ArrayView<const ElementType, MemorySpaceType>&, const
> dealii::ArrayView<ElementType, MemorySpace>&, const
> dealii::ArrayView<ElementType, MemorySpace>&,
> std::vector<ompi_request_t*>&) const [with Number = double; MemorySpaceType
> = dealii::MemorySpace::Host] The violated condition was: *read_position
> == Number() || internal::get_abs(locally_owned_array[j] - *read_position)
> <= internal::get_abs(locally_owned_array[j] + *read_position) * 100000. *
> std::numeric_limits<typename numbers::NumberTraits<
> Number>::real_type>::epsilon() Additional information: Called
> compress(VectorOperation::insert), but the element received from a
> remote processor, value 0.7071067811865476, does not match with the
> value 0 on the owner processor 0 *
>
> I also realized I was wrong below for the actual vector that fails to
> compress when there are ghost hanging nodes: I think it corresponds to the
> output vector "u2" in ExtrapolateImplementation<dim, spacedim,
> OutVector>::extrapolate_parallel.
>
> Best
>
> Guilhem
> On 10/04/2025 10:13, Guilhem Poy wrote:
>
> Dear deal-ii users and developers,
>
> I am trying to use the FETools::extrapolate method on a distributed
> triangulation that is adaptively refined. However, I get an exception
> inside this function every time my triangulation contains hanging nodes at
> ghost interfaces. I have attached a minimal working example that
> illustrates this on deal.II version 9.6.1. It should be compiled in debug
> mode as for usual dealii example codes, and then run with "mpirun -np 2
> main" with two mpi ranks to generate the exception. The macro
> NO_GHOST_HANGING_NODES can be commented out to check that
> FETools::extrapolate works correctly when there are no hanging nodes at
> ghost interfaces.
>
> Before opening a bug, I would like to check with you that I am not doing
> something wrong in this test program. I was careful to set up my
> distributed vectors as large as possible (i.e. with all relevant dofs) to
> see if the problem was coming from there, and of course I updated the ghost
> values of the "coarse" vector before calling FETools::extrapolate. If I am
> not mistaken, the exception is thrown when an internal vector with all
> relevant dofs is compressed line 1455 of fe_tools_extrapolate.templates.h,
> but of course this internal vector depends on the input vector I give to
> extrapolate so the problem could very much come from my code. I am ready to
> provide any further information that may be useful! Thanks in advance.
>
> Best regards,
>
> Guilhem
>
>
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