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// ------------------------------------------------------------------------
//
// SPDX-License-Identifier: LGPL-2.1-or-later
// Copyright (C) 2021 - 2023 by the deal.II authors
//
// This file is part of the deal.II library.
//
// Part of the source code is dual licensed under Apache-2.0 WITH
// LLVM-exception OR LGPL-2.1-or-later. Detailed license information
// governing the source code and code contributions can be found in
// LICENSE.md and CONTRIBUTING.md at the top level directory of deal.II.
//
// ------------------------------------------------------------------------
#ifndef dealii_base_communication_pattern_base_h
#define dealii_base_communication_pattern_base_h
#include <deal.II/base/config.h>
#include <deal.II/base/mpi_stub.h>
DEAL_II_NAMESPACE_OPEN
// Forward declaration
#ifndef DOXYGEN
class IndexSet;
#endif
namespace Utilities
{
namespace MPI
{
/**
* CommunicationPattern is an abstract class that is used to define a
* communication plan that can be called repeatedly to efficiently obtain
* off-processor elements. The idea is to decouple the communication pattern
* from the data that needs to be communicated. The goal is to reuse the
* same communication pattern for different containers. This is similar to
* the way SparseMatrix and SparsityPattern works.
*
* Conceptually, this class operates under the assumption that data
* are stored in one linearly indexed array of which every MPI process
* stores some elements (or possibly none). In practice it does of course
* not matter whether the elements are stored in contiguous arrays; the
* point is simply that a single integer index can be used to access a
* specific element. The elements of this logical array are (or at least
* may be) stored on different processes in a parallel MPI universe.
*
* In this world view, every process has a set of elements and their
* indices in the array form the "locally owned indices". Next, every
* process will as part of executing an algorithm require access to some
* of the elements stored elsewhere; we call the indices of these elements
* the "ghost indices", in analogy to how vectors and triangulations
* partition vector elements and mesh cells into locally
* owned ones and ghost ones (along, of course, with cells and ghosts stored
* on other processes that the current process simply does not care about
* and consequently needs not know anything about).
*
* The point of this class (and its implementations in derived classes) is
* to set up communication infrastructure whereby one process can inquire
* efficiently about the "ghost elements" stored on other processes, and
* to send those locally owned elements to those other processes that
* require knowledge of their value because they list these elements among
* their respective "ghost indices".
*/
class CommunicationPatternBase
{
public:
/**
* Destructor.
*/
virtual ~CommunicationPatternBase() = default;
/**
* Reinitialize the communication pattern.
*
* @param[in] locally_owned_indices The set of indices of elements
* in the array mentioned in the class documentation that are
* stored on the current process.
* @param[in] ghost_indices The set of indices of elements in the
* array mentioned in the class documentation that the current
* process will need to be able to import.
* @param[in] communicator The MPI communicator used to describe the
* entire set of processes that participate in the storage and
* access to elements of the array.
*/
virtual void
reinit(const IndexSet &locally_owned_indices,
const IndexSet &ghost_indices,
const MPI_Comm communicator) = 0;
/**
* Return the underlying MPI communicator.
*/
virtual MPI_Comm
get_mpi_communicator() const = 0;
};
} // namespace MPI
} // namespace Utilities
DEAL_II_NAMESPACE_CLOSE
#endif
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