Wolfgang,

Upon further investigation you are correct. If I go beyond 4 processors I 
get the same error even with the default zero subdivisions. 

Thank you for your response.  
- Justin

On Tuesday, December 13, 2016 at 12:00:18 PM UTC-5, Wolfgang Bangerth wrote:
>
> Justin
> Is it really working with zero subdivisions? Step 40 only uses the DataOut 
> class, not the DataOutFaces class, so that may not be the best comparison. 
> I don't recall whether the latter class works in parallel at all, 
> regardless of subdivision. 
> Best
> Wolfgang
>
>
>
> Sent from Samsung tablet.
>
>
> -------- Original message --------
> From: Justin Kauffman <jak...@gmail.com <javascript:>> 
> Date: 12/13/2016 07:46 (GMT-07:00) 
> To: "deal.II User Group" <dea...@googlegroups.com <javascript:>> 
> Subject: [deal.II] build_patches(degree) for higher order dg elements in 
> parallel 
>
> Hi all:
>
> I am writing an HDG code and trying to parallelize it in a fashion similar 
> to step-40. I am having a problem with data_out.build_patches(degree). In 
> step-40 the argument is the default value of zero, but I eventually want to 
> run this code for higher order dg elements. I believe that the problem in 
> parallel is that build_patches is trying to subdivide all cells for each 
> processor (even the ghost_cells) instead of just the locally_owned_cells. I 
> get the following error output:
>
> An error occurred in line <934> of file 
> </include/deal.II/grid/tria_iterator.h> in function
>     Accessor& dealii::TriaRawIterator<Accessor>::operator*() [with 
> Accessor = dealii::CellAccessor<2, 2>]
> The violated condition was:
>     Accessor::structure_dimension!=Accessor::dimension || state() == 
> IteratorState::valid
> Additional information:
>     You tried to dereference a cell iterator for which this is not 
> possible. More information on this iterator: level=-2, index=-2, 
> state=invalid
>
> Stacktrace:
> -----------
> #0  dealii::TriaRawIterator<dealii::CellAccessor<2, 2> >::operator*()
> #1   dealii::DataOutFaces<2, dealii::DoFHandler<2, 2> 
> >::next_face(std::pair<dealii::TriaIterator<dealii::CellAccessor<2, 2> >, 
> unsigned int> const&)
> #2   dealii::DataOutFaces<2, dealii::DoFHandler<2, 2> 
> >::build_patches(dealii::Mapping<2, 2> const&, unsigned int)
> #3   dealii::DataOutFaces<2, dealii::DoFHandler<2, 2> 
> >::build_patches(unsigned int)
>
> I am using deal.ii version 8.5.0-pre. In step-51 build_patches takes the 
> dg element degree as the argument and this is true when running on a single 
> processor, but breaks down when running on multiple processors. Any help 
> would be greatly appreciated. 
>
> Thank you,
> - Justin
>
> -- 
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