Minimum of dealii:Vector in MPI parallel application

[maurice rohracker]

Dear all,

In a parallel distributed code, similar to step-42, we use a dealii::Vector of n_active_cells() to compute the error indicators of the locally owned cells. (This can easily be used in e.g. dealii::parallel::distributed::GridRefinement::
refine_and_coarsen_fixed_number())
As we want to add an additional criterion based on the minimum and maximum error indicator to mark cells, we need the global minimum and maximum value.
Is there a smart way to get the global minimum based on the minimum of all processes of all locally owned cells? Using std::min_element does not care about locally owned cells and so the results are wrong.

Thanks and best regards,
Maurice

[Wolfgang Bangerth]

On 7/26/23 02:59, 'maurice rohracker' via deal.II User Group wrote:
>
> In a parallel distributed code, similar to step-42, we use a/dealii::Vector/
> of /n_active_cells()/ to compute the error indicators of the locally owned

> cells. (This can easily be used in e.g.

> /dealii::parallel::distributed::GridRefinement::
> refine_and_coarsen_fixed_number()/)

> As we want to add an additional criterion based on the minimum and maximum
> error indicator to mark cells, we need the global minimum and maximum value.
> Is there a smart way to get the global minimum based on the minimum of all

> processes of all locally owned cells? Using /std::min_element/ does not care

> about locally owned cells and so the results are wrong.

You can get the local min/max via a loop of the form

double min=std::numeric_limits<double>::max(),
max=std::numeric_limits<double>::smallest(),
for (auto &cell : triangulation.active_cell_iterators())
if (cell->is_locally_owned())
min = std::min (min, vec(cell->active_cell_index()));
...same for max...

and then compute the global min and max via Utilities::min()/max().

Best
W.

--
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Wolfgang Bangerth email: bang...@colostate.edu
www: http://www.math.colostate.edu/~bangerth/