NonlinearSystemNldd.hpp

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/*
  Copyright 2013, 2015 SINTEF ICT, Applied Mathematics.
  Copyright 2014, 2015 Dr. Blatt - HPC-Simulation-Software & Services
  Copyright 2014, 2015 Statoil ASA.
  Copyright 2015 NTNU
  Copyright 2015, 2016, 2017 IRIS AS
 
  This file is part of the Open Porous Media project (OPM).
 
  OPM is free software: you can redistribute it and/or modify
  it under the terms of the GNU General Public License as published by
  the Free Software Foundation, either version 3 of the License, or
  (at your option) any later version.
 
  OPM is distributed in the hope that it will be useful,
  but WITHOUT ANY WARRANTY; without even the implied warranty of
  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  GNU General Public License for more details.
 
  You should have received a copy of the GNU General Public License
  along with OPM.  If not, see <http://www.gnu.org/licenses/>.
*/
 
#ifndef OPM_NONLINEAR_SYSTEM_NLDD_HEADER_INCLUDED
#define OPM_NONLINEAR_SYSTEM_NLDD_HEADER_INCLUDED
 
#include <dune/common/fvector.hh>
#include <dune/common/timer.hh>
#include <dune/istl/bvector.hh>
#include <dune/istl/istlexception.hh>
 
#include <opm/common/Exceptions.hpp>
 
#include <opm/grid/common/SubGridPart.hpp>
 
#include <opm/simulators/aquifers/AquiferGridUtils.hpp>
 
#include <opm/simulators/flow/countGlobalCells.hpp>
#include <opm/simulators/flow/NlddReporting.hpp>
#include <opm/simulators/flow/NonlinearSolver.hpp>
#include <opm/simulators/flow/partitionCells.hpp>
#include <opm/simulators/flow/priVarsPacking.hpp>
#include <opm/simulators/flow/SubDomain.hpp>
 
#include <opm/simulators/linalg/extractMatrix.hpp>
 
#if COMPILE_GPU_BRIDGE
#include <opm/simulators/linalg/ISTLSolverGpuBridge.hpp>
#else
#include <opm/simulators/linalg/ISTLSolver.hpp>
#endif
#include <opm/simulators/linalg/ISTLSolverRuntimeOptionProxy.hpp>
 
#include <opm/simulators/timestepping/ConvergenceReport.hpp>
#include <opm/simulators/timestepping/SimulatorReport.hpp>
#include <opm/simulators/timestepping/SimulatorTimerInterface.hpp>
 
#include <opm/simulators/utils/ComponentName.hpp>
#include <opm/simulators/utils/DeferredLoggingErrorHelpers.hpp>
 
#include <opm/simulators/wells/BlackoilWellModelNldd.hpp>
 
#include <fmt/format.h>
 
#include <algorithm>
#include <array>
#include <cassert>
#include <cmath>
#include <cstddef>
#include <filesystem>
#include <memory>
#include <numeric>
#include <set>
#include <sstream>
#include <string>
#include <type_traits>
#include <utility>
#include <vector>
 
namespace Opm {
 
template<class TypeTag> class NonlinearSystemBlackOilReservoir;
 
namespace detail {
 
template <class Grid, class ElementMapper, class EntitySeed>
std::vector<int> findOverlapCells(const Grid& grid,
                                  const ElementMapper& elementMapper,
                                  const std::vector<int>& owned_cells,
                                  std::vector<EntitySeed>& seeds,
                                  const int num_layers,
                                  const int mark,
                                  std::vector<int>& in_domain)
{
    const auto& gridView = grid.leafGridView();
    // The owned cells are in the subdomain from the start.
    for (const int cell : owned_cells) {
        in_domain[cell] = mark;
    }
 
    std::vector<int> overlap;
    // seeds[layer_begin, layer_end) are the cells added by the previous layer,
    // initially the owned cells.
    std::size_t layer_begin = 0;
    for (int layer = 0; layer < num_layers; ++layer) {
        // seeds grows inside the loop; the new entries form the next layer.
        const std::size_t layer_end = seeds.size();
        for (std::size_t i = layer_begin; i < layer_end; ++i) {
            const auto element = grid.entity(seeds[i]);
            for (const auto& intersection : intersections(gridView, element)) {
                // Skip faces without a neighbour cell (domain boundary).
                if (!intersection.neighbor()) {
                    continue;
                }
                // Skip cells owned by another MPI rank.
                const auto outside = intersection.outside();
                if (outside.partitionType() != Dune::InteriorEntity) {
                    continue;
                }
                // Skip cells that are already in the subdomain.
                const int cell = elementMapper.index(outside);
                if (in_domain[cell] == mark) {
                    continue;
                }
                // A new overlap cell: it is also a seed for the next layer.
                in_domain[cell] = mark;
                overlap.push_back(cell);
                seeds.push_back(outside.seed());
            }
        }
        layer_begin = layer_end;
    }
    return overlap;
}
 
} // namespace detail
 
template <class TypeTag>
class NonlinearSystemNldd
{
public:
    using ElementContext = GetPropType<TypeTag, Properties::ElementContext>;
    using FluidSystem = GetPropType<TypeTag, Properties::FluidSystem>;
    using Grid = GetPropType<TypeTag, Properties::Grid>;
    using Indices = GetPropType<TypeTag, Properties::Indices>;
    using Scalar = GetPropType<TypeTag, Properties::Scalar>;
    using ModelParameters = typename NonlinearSystemBlackOilReservoir<TypeTag>::ModelParameters;
    using SolutionVector = GetPropType<TypeTag, Properties::SolutionVector>;
 
    using BVector = typename NonlinearSystemBlackOilReservoir<TypeTag>::BVector;
    using Domain = SubDomain<Grid>;
    using ISTLSolverType = ISTLSolver<TypeTag>;
    using Mat = typename NonlinearSystemBlackOilReservoir<TypeTag>::Mat;
 
    static constexpr int numEq = Indices::numEq;
 
    explicit NonlinearSystemNldd(NonlinearSystemBlackOilReservoir<TypeTag>& model)
        : model_(model)
        , wellModel_(model.wellModel())
        , rank_(model_.simulator().vanguard().grid().comm().rank())
    {
        // Create partitions.
        const auto& [partition_vector_initial, num_domains_initial] = this->partitionCells();
 
        int num_domains = num_domains_initial;
        std::vector<int> partition_vector = partition_vector_initial;
 
        // Fix-up for an extreme case: Interior cells who do not have any on-rank
        // neighbours. Move all such cells into a single domain on this rank,
        // and mark the domain for skipping. For what it's worth, we've seen this
        // case occur in practice when testing on field cases.
        bool isolated_cells = false;
        for (auto& domainId : partition_vector) {
            if (domainId < 0) {
                domainId = num_domains;
                isolated_cells = true;
            }
        }
        if (isolated_cells) {
            num_domains++;
        }
 
        // Set nldd handler in main well model
        model.wellModel().setNlddAdapter(&wellModel_);
 
        // Scan through partitioning to get correct size for each.
        std::vector<int> sizes(num_domains, 0);
        for (const auto& p : partition_vector) {
            ++sizes[p];
        }
 
        // Set up correctly sized vectors of entity seeds and of indices for each partition.
        using EntitySeed = typename Grid::template Codim<0>::EntitySeed;
        std::vector<std::vector<EntitySeed>> seeds(num_domains);
        std::vector<std::vector<int>> partitions(num_domains);
        for (int domain = 0; domain < num_domains; ++domain) {
            seeds[domain].resize(sizes[domain]);
            partitions[domain].resize(sizes[domain]);
        }
 
        // Iterate through grid once, setting the seeds of all partitions.
        // Note: owned cells only!
        const auto& grid = model_.simulator().vanguard().grid();
 
        std::vector<int> count(num_domains, 0);
        const auto& gridView = grid.leafGridView();
        const auto beg = gridView.template begin<0, Dune::Interior_Partition>();
        const auto end = gridView.template end<0, Dune::Interior_Partition>();
        int cell = 0;
        for (auto it = beg; it != end; ++it, ++cell) {
            const int p = partition_vector[cell];
            seeds[p][count[p]] = it->seed();
            partitions[p][count[p]] = cell;
            ++count[p];
        }
        assert(count == sizes);
 
        // Create the domains.
        const int num_overlap_layers = model_.param().nldd_num_overlap_layers_;
        const auto& elementMapper = model_.simulator().model().elementMapper();
        // Scratch space for detail::findOverlapCells, shared by all domains.
        std::vector<int> in_domain(num_overlap_layers > 0 ? partition_vector.size() : 0, -1);
        for (int index = 0; index < num_domains; ++index) {
            std::vector<bool> interior(partition_vector.size(), false);
            for (int ix : partitions[index]) {
                interior[ix] = true;
            }
 
            // Mark the last domain for skipping if it contains isolated cells
            const bool skip = isolated_cells && (index == num_domains - 1);
 
            std::vector<int> overlap_cells;
            if (num_overlap_layers > 0 && !skip) {
                overlap_cells = detail::findOverlapCells(grid, elementMapper, partitions[index],
                                                         seeds[index], num_overlap_layers,
                                                         index, in_domain);
            }
 
            // The view holds exactly the cells of the local problem, and
            // reports all of them, overlap cells included, as interior since
            // all of them are solved for. The one-layer neighbour overlap
            // that SubGridPart adds by default is not used, so it is left out.
            Dune::SubGridPart<Grid> view{grid, std::move(seeds[index]), /*overlap=*/false};
 
            this->domains_.emplace_back(index,
                                        std::move(partitions[index]),
                                        std::move(overlap_cells),
                                        std::move(interior),
                                        std::move(view),
                                        skip);
        }
 
        // Initialize storage for previous mobilities in a single flat vector
        const auto numCells = grid.size(0);
        previousMobilities_.resize(numCells * FluidSystem::numActivePhases(), 0.0);
        for (const auto& domain : domains_) {
            updateMobilities(domain);
        }
 
        // Initialize domain_needs_solving_ to true for all domains
        domain_needs_solving_.resize(num_domains, true);
 
        // Set up container for the local system matrices.
        domain_matrices_.resize(num_domains);
 
        // Set up container for the local linear solvers.
        for (int index = 0; index < num_domains; ++index) {
            // TODO: The ISTLSolver constructor will make
            // parallel structures appropriate for the full grid
            // only. This must be addressed before going parallel.
            const auto& eclState = model_.simulator().vanguard().eclState();
            FlowLinearSolverParameters loc_param;
            loc_param.is_nldd_local_solver_ = true;
            loc_param.init(eclState.getSimulationConfig().useCPR());
            // Override solver type with umfpack if small domain.
            if (domains_[index].cells.size() < 200) {
                loc_param.linsolver_ = "umfpack";
            }
            loc_param.linear_solver_print_json_definition_ = false;
            const bool force_serial = true;
            domain_linsolvers_.emplace_back(model_.simulator(), loc_param, force_serial);
            domain_linsolvers_.back().setDomainIndex(index);
        }
 
        assert(int(domains_.size()) == num_domains);
 
        domain_reports_accumulated_.resize(num_domains);
 
        // Print domain distribution summary
        ::Opm::printDomainDistributionSummary(
            partition_vector,
            domains_,
            local_reports_accumulated_,
            domain_reports_accumulated_,
            grid,
            wellModel_.numLocalWellsEnd());
    }
 
    void prepareStep()
    {
        // Setup domain->well mapping.
        wellModel_.setupDomains(domains_);
    }
 
    template <class NonlinearSolverType>
    SimulatorReportSingle nonlinearIterationNldd(const SimulatorTimerInterface& timer,
                                                 NonlinearSolverType& nonlinear_solver)
    {
        // -----------   Set up reports and timer   -----------
        SimulatorReportSingle report;
        // When nldd_num_initial_newton_iter_ == 0, NLDD starts immediately but wells are still
        // properly initialized via NewtonIterationContext::needsTimestepInit() which
        // ensures prepareTimeStep() is called during the initial global linearization.
        // If nldd_num_initial_newton_iter_ > 0, run global Newton iterations first.
        // TODO: Investigate whether wells and other subsystems are correctly initialized
        // for the following NLDD iteration.
        if (model_.simulator().problem().iterationContext().iteration() < model_.param().nldd_num_initial_newton_iter_) {
            report = model_.nonlinearIterationNewton(timer, nonlinear_solver);
            return report;
        }
 
        model_.initialLinearization(report, model_.param().newton_min_iter_,
                                    model_.param().newton_max_iter_, timer);
 
        if (report.converged) {
            return report;
        }
 
        // Remove the initial linearization entry; the final Newton step will
        // record its own, keeping one entry per outer iteration.
        model_.popLastConvergenceReport();
 
        // -----------   If not converged, do an NLDD iteration   -----------
        Dune::Timer localSolveTimer;
        Dune::Timer detailTimer;
        localSolveTimer.start();
        detailTimer.start();
        auto& solution = model_.simulator().model().solution(0);
        auto initial_solution = solution;
        auto locally_solved = initial_solution;
 
        // -----------   Decide on an ordering for the domains   -----------
        const auto domain_order = this->getSubdomainOrder();
        local_reports_accumulated_.success.pre_post_time += detailTimer.stop();
 
        // -----------   Solve each domain separately   -----------
        DeferredLogger logger;
        std::vector<SimulatorReportSingle> domain_reports(domains_.size());
 
        OPM_BEGIN_PARALLEL_TRY_CATCH()
        for (const int domain_index : domain_order) {
            const auto& domain = domains_[domain_index];
            SimulatorReportSingle local_report;
            detailTimer.reset();
            detailTimer.start();
 
            domain_needs_solving_[domain_index] = checkIfSubdomainNeedsSolving(domain);
 
            updateMobilities(domain);
 
            if (domain.skip || !domain_needs_solving_[domain_index]) {
                local_report.skipped_domains = true;
                local_report.converged = true;
                domain_reports[domain.index] = local_report;
                continue;
            }
            switch (model_.param().local_solve_approach_) {
            case DomainSolveApproach::Jacobi:
                solveDomainJacobi(solution, locally_solved, local_report, logger,
                                  timer, domain);
                break;
            default:
            case DomainSolveApproach::GaussSeidel:
                solveDomainGaussSeidel(solution, locally_solved, local_report, logger,
                                       timer, domain);
                break;
            }
            // This should have updated the global matrix to be
            // dR_i/du_j evaluated at new local solutions for
            // i == j, at old solution for i != j.
            if (!local_report.converged) {
                // TODO: more proper treatment, including in parallel.
                logger.debug(fmt::format("Convergence failure in domain {} on rank {}." , domain.index, rank_));
            }
            local_report.solver_time += detailTimer.stop();
            domain_reports[domain.index] = local_report;
        }
        OPM_END_PARALLEL_TRY_CATCH("Unexpected exception in local domain solve: ", model_.simulator().vanguard().grid().comm());
 
        detailTimer.reset();
        detailTimer.start();
        // Communicate and log all messages.
        auto global_logger = gatherDeferredLogger(logger, model_.simulator().vanguard().grid().comm());
        global_logger.logMessages();
 
        // Accumulate local solve data.
        // Putting the counts in a single array to avoid multiple
        // comm.sum() calls. Keeping the named vars for readability.
        std::array<int, 5> counts{ 0, 0, 0, static_cast<int>(domain_reports.size()), 0 };
        int& num_converged = counts[0];
        int& num_converged_already = counts[1];
        int& num_local_newtons = counts[2];
        int& num_domains = counts[3];
        int& num_skipped = counts[4];
        {
            auto step_newtons = 0;
            const auto dr_size = domain_reports.size();
            for (auto i = 0*dr_size; i < dr_size; ++i) {
                // Reset the needsSolving flag for the next iteration
                domain_needs_solving_[i] = false;
                const auto& dr = domain_reports[i];
                if (dr.converged) {
                    ++num_converged;
                    if (dr.total_newton_iterations == 0) {
                        ++num_converged_already;
                    }
                    else {
                        // If we needed to solve the domain, we also solve in next iteration
                        domain_needs_solving_[i] = true;
                    }
                }
                if (dr.skipped_domains) {
                    ++num_skipped;
                }
                step_newtons += dr.total_newton_iterations;
                // Accumulate local reports per domain
                domain_reports_accumulated_[i] += dr;
                // Accumulate local reports per rank
                local_reports_accumulated_ += dr;
            }
            num_local_newtons = step_newtons;
        }
 
        if (model_.param().local_solve_approach_ == DomainSolveApproach::Jacobi) {
            solution = locally_solved;
            model_.simulator().model().invalidateAndUpdateIntensiveQuantities(/*timeIdx=*/0);
        }
 
#if HAVE_MPI
        // Communicate solutions:
        // With multiple processes, this process' overlap (i.e. not
        // owned) cells' solution values have been modified by local
        // solves in the owning processes, and remain unchanged
        // here. We must therefore receive the updated solution on the
        // overlap cells and update their intensive quantities before
        // we move on.
        const auto& comm = model_.simulator().vanguard().grid().comm();
        if (comm.size() > 1) {
            const auto* ccomm = model_.simulator().model().newtonMethod().linearSolver().comm();
 
            // Copy numerical values from primary vars.
            ccomm->copyOwnerToAll(solution, solution);
 
            // Copy flags from primary vars, together with whether the cell switched
            // primary variables in this time step. The local solves update that only
            // on owned cells, and the global Newton update must treat a cell and its
            // copies on other processes alike.
            auto& newton = model_.simulator().model().newtonMethod();
            const std::size_t num = solution.size();
            Dune::BlockVector<Dune::FieldVector<std::size_t, 2>> flags(num);
            for (std::size_t ii = 0; ii < num; ++ii) {
                flags[ii][0] = PVUtil::pack(solution[ii]);
                flags[ii][1] = newton.wasSwitched(ii);
            }
            ccomm->copyOwnerToAll(flags, flags);
            for (std::size_t ii = 0; ii < num; ++ii) {
                PVUtil::unPack(solution[ii], flags[ii][0]);
                newton.setWasSwitched(ii, flags[ii][1] != 0);
            }
 
            // Update intensive quantities for our overlap values.
            model_.simulator().model().invalidateAndUpdateIntensiveQuantitiesOverlap(/*timeIdx=*/0);
 
            // Make total counts of domains converged.
            comm.sum(counts.data(), counts.size());
        }
#endif // HAVE_MPI
 
        const bool is_iorank = this->rank_ == 0;
        if (is_iorank) {
            OpmLog::debug(fmt::format(fmt::runtime("Local solves finished. Converged for {}/{} domains. {} domains were skipped. {} domains did no work. {} total local Newton iterations.\n"),
                                      num_converged, num_domains, num_skipped, num_converged_already, num_local_newtons));
        }
        auto total_local_solve_time = localSolveTimer.stop();
        report.local_solve_time += total_local_solve_time;
        local_reports_accumulated_.success.total_time += total_local_solve_time;
        local_reports_accumulated_.success.pre_post_time += detailTimer.stop();
 
        // Finish with a global Newton step.
        auto rep = model_.nonlinearIterationNewton(timer, nonlinear_solver);
        report += rep;
        if (rep.converged) {
            report.converged = true;
        }
        return report;
    }
 
    const SimulatorReport& localAccumulatedReports() const
    {
        return local_reports_accumulated_;
    }
 
    const std::vector<SimulatorReport>& domainAccumulatedReports() const
    {
        // Update the number of wells for each domain that has been added to the well model at this point
        // this is a mutable operation that updates the well counts in the domain reports.
        const auto dr_size = domain_reports_accumulated_.size();
        // Reset well counts before updating
        for (auto i = 0*dr_size; i < dr_size; ++i) {
            domain_reports_accumulated_[i].success.num_wells = 0;
        }
        // Update the number of wells for each domain
        for (const auto& [wname, domain] : wellModel_.well_domain()) {
            domain_reports_accumulated_[domain].success.num_wells++;
        }
        return domain_reports_accumulated_;
    }
 
    void writePartitions(const std::filesystem::path& odir) const
    {
        const auto& grid = this->model_.simulator().vanguard().grid();
        const auto& elementMapper = this->model_.simulator().model().elementMapper();
        const auto& cartMapper = this->model_.simulator().vanguard().cartesianIndexMapper();
 
        ::Opm::writePartitions(
            odir,
            domains_,
            grid,
            elementMapper,
            cartMapper);
    }
 
    void writeNonlinearIterationsPerCell(const std::filesystem::path& odir) const
    {
        const auto& grid = this->model_.simulator().vanguard().grid();
        const auto& elementMapper = this->model_.simulator().model().elementMapper();
        const auto& cartMapper = this->model_.simulator().vanguard().cartesianIndexMapper();
 
        ::Opm::writeNonlinearIterationsPerCell(
            odir,
            domains_,
            domain_reports_accumulated_,
            grid,
            elementMapper,
            cartMapper);
    }
 
private:
    ConvergenceReport
    solveDomain(const Domain& domain,
                const SimulatorTimerInterface& timer,
                SimulatorReportSingle& local_report,
                DeferredLogger& logger,
                const bool initial_assembly_required)
    {
        auto& modelSimulator = model_.simulator();
        Dune::Timer detailTimer;
 
        // RAII guard: local iteration context for this domain, restored on scope exit.
        LocalContextGuard localCtxGuard(modelSimulator.problem());
        auto& localCtx = localCtxGuard.context();
 
        // When called, if assembly has already been performed
        // with the initial values, we only need to check
        // for local convergence. Otherwise, we must do a local
        // assembly.
        if (initial_assembly_required) {
            detailTimer.start();
            // TODO: we should have a beginIterationLocal function()
            // only handling the well model for now
            wellModel_.assemble(modelSimulator.timeStepSize(),
                                domain);
            const double tt0 = detailTimer.stop();
            local_report.assemble_time += tt0;
            local_report.assemble_time_well += tt0;
            detailTimer.reset();
            detailTimer.start();
            // Assemble reservoir locally.
            this->assembleReservoirDomain(domain);
            local_report.assemble_time += detailTimer.stop();
            local_report.total_linearizations += 1;
        }
        detailTimer.reset();
        detailTimer.start();
        std::vector<Scalar> resnorms;
        auto convreport = this->getDomainConvergence(domain, timer, logger, resnorms);
        local_report.update_time += detailTimer.stop();
        if (convreport.converged()) {
            // TODO: set more info, timing etc.
            local_report.converged = true;
            return convreport;
        }
 
        // We have already assembled for the first iteration,
        // but not done the Schur complement for the wells yet.
        detailTimer.reset();
        detailTimer.start();
        model_.wellModel().linearizeDomain(domain,
                                           modelSimulator.model().linearizer().jacobian(),
                                           modelSimulator.model().linearizer().residual());
        const double tt1 = detailTimer.stop();
        local_report.assemble_time += tt1;
        local_report.assemble_time_well += tt1;
 
        // Local Newton loop.
        const int max_iter = model_.param().max_local_solve_iterations_;
        const auto& grid = modelSimulator.vanguard().grid();
        double damping_factor = 1.0;
        std::vector<std::vector<Scalar>> convergence_history;
        convergence_history.reserve(20);
        convergence_history.push_back(resnorms);
        do {
            // Solve local linear system.
            // Note that x has full size, we expect it to be nonzero only for in-domain cells.
            const int nc = grid.size(0);
            BVector x(nc);
            detailTimer.reset();
            detailTimer.start();
            double setup_time = 0.0;
            try {
                this->solveJacobianSystemDomain(domain, x, setup_time);
            }
            catch (const NumericalProblem& e) {
                // Local linear solve failed - treat as domain-level failure, not global timestep cut.
                logger.debug(fmt::format(
                    "Local linear solver failed in domain {} on rank {}: {}",
                    domain.index, rank_, e.what()));
                // record statistics and return early
                local_report.linear_solve_time += detailTimer.stop();
                local_report.linear_solve_setup_time += setup_time;
                local_report.total_linear_iterations = domain_linsolvers_[domain.index].iterations();
                modelSimulator.problem().endIteration();
                local_report.converged = false;
                local_report.total_newton_iterations = localCtx.iteration();
                local_report.total_linearizations += localCtx.iteration();
                return convreport;
            }
            model_.wellModel().postSolveDomain(x, domain);
            if (damping_factor != 1.0) {
                x *= damping_factor;
            }
            local_report.linear_solve_time += detailTimer.stop();
            local_report.linear_solve_setup_time += setup_time;
            local_report.total_linear_iterations = domain_linsolvers_[domain.index].iterations();
 
            // Update local solution. // TODO: x is still full size, should we optimize it?
            detailTimer.reset();
            detailTimer.start();
            this->updateDomainSolution(domain, x);
            local_report.update_time += detailTimer.stop();
 
            // Assemble well and reservoir.
            detailTimer.reset();
            detailTimer.start();
            localCtx.advanceIteration();
            // TODO: we should have a beginIterationLocal function()
            // only handling the well model for now
            // Assemble reservoir locally.
            wellModel_.assemble(modelSimulator.timeStepSize(),
                                domain);
            const double tt3 = detailTimer.stop();
            local_report.assemble_time += tt3;
            local_report.assemble_time_well += tt3;
            detailTimer.reset();
            detailTimer.start();
            this->assembleReservoirDomain(domain);
            local_report.assemble_time += detailTimer.stop();
 
            // Check for local convergence.
            detailTimer.reset();
            detailTimer.start();
            resnorms.clear();
            convreport = this->getDomainConvergence(domain, timer, logger, resnorms);
            convergence_history.push_back(resnorms);
            local_report.update_time += detailTimer.stop();
 
            // apply the Schur complement of the well model to the
            // reservoir linearized equations
            detailTimer.reset();
            detailTimer.start();
            model_.wellModel().linearizeDomain(domain,
                                               modelSimulator.model().linearizer().jacobian(),
                                               modelSimulator.model().linearizer().residual());
            const double tt2 = detailTimer.stop();
            local_report.assemble_time += tt2;
            local_report.assemble_time_well += tt2;
 
            // Check if we should dampen, also when the wells are not
            // converged. Waiting for the wells would tie the start of the
            // damping to the iteration where the well tolerances are relaxed.
            if (!convreport.converged()) {
                bool oscillate = false;
                bool stagnate = false;
                const auto num_residuals = convergence_history.front().size();
                detail::detectOscillations(convergence_history, localCtx.iteration(), num_residuals,
                                           Scalar{0.2}, 1, oscillate, stagnate);
                if (oscillate) {
                    damping_factor *= 0.85;
                    logger.debug(fmt::format(fmt::runtime("| Damping factor is now {}"), damping_factor));
                }
            }
        } while (!convreport.converged() && localCtx.iteration() <= max_iter);
 
        modelSimulator.problem().endIteration();
 
        local_report.converged = convreport.converged();
        local_report.total_newton_iterations = localCtx.iteration();
        local_report.total_linearizations += localCtx.iteration();
        // TODO: set more info, timing etc.
        return convreport;
    }
 
    void assembleReservoirDomain(const Domain& domain)
    {
        OPM_TIMEBLOCK(assembleReservoirDomain);
        // -------- Mass balance equations --------
        model_.simulator().model().linearizer().linearizeDomain(domain);
    }
 
    void solveJacobianSystemDomain(const Domain& domain, BVector& global_x, double& setup_time)
    {
        const auto& modelSimulator = model_.simulator();
 
        Dune::Timer perfTimer;
        perfTimer.start();
 
        const Mat& main_matrix = modelSimulator.model().linearizer().jacobian().istlMatrix();
        if (domain_matrices_[domain.index]) {
            Details::copySubMatrix(main_matrix, domain.cells, *domain_matrices_[domain.index]);
        } else {
            domain_matrices_[domain.index] = std::make_unique<Mat>(Details::extractMatrix(main_matrix, domain.cells));
        }
        auto& jac = *domain_matrices_[domain.index];
        auto res = Details::extractVector(modelSimulator.model().linearizer().residual(),
                                          domain.cells);
        auto x = res;
 
        // set initial guess
        global_x = 0.0;
        x = 0.0;
 
        auto& linsolver = domain_linsolvers_[domain.index];
 
        linsolver.prepare(jac, res);
        setup_time = perfTimer.stop();
        linsolver.setResidual(res);
        linsolver.solve(x);
 
        Details::setGlobal(x, domain.cells, global_x);
    }
 
    void updateDomainSolution(const Domain& domain, const BVector& dx)
    {
        OPM_TIMEBLOCK(updateDomainSolution);
        auto& simulator = model_.simulator();
        auto& newtonMethod = simulator.model().newtonMethod();
        SolutionVector& solution = simulator.model().solution(/*timeIdx=*/0);
 
        newtonMethod.update_(/*nextSolution=*/solution,
                             /*curSolution=*/solution,
                             /*update=*/dx,
                             /*resid=*/dx,
                             domain.cells); // the update routines of the black
                                            // oil model do not care about the
                                            // residual
 
        // if the solution is updated, the intensive quantities need to be recalculated
        simulator.model().invalidateAndUpdateIntensiveQuantities(/*timeIdx=*/0, domain);
    }
 
    std::pair<Scalar, Scalar> localDomainConvergenceData(const Domain& domain,
                                                         std::vector<Scalar>& R_sum,
                                                         std::vector<Scalar>& maxCoeff,
                                                         std::vector<Scalar>& B_avg,
                                                         std::vector<int>& maxCoeffCell)
    {
        const auto& modelSimulator = model_.simulator();
 
        Scalar pvSumLocal = 0.0;
        Scalar numAquiferPvSumLocal = 0.0;
        const auto& model = modelSimulator.model();
        const auto& problem = modelSimulator.problem();
 
        const auto& modelResid = modelSimulator.model().linearizer().residual();
 
        ElementContext elemCtx(modelSimulator);
        const auto& gridView = domain.view;
        const auto& elemEndIt = gridView.template end</*codim=*/0>();
        IsNumericalAquiferCell isNumericalAquiferCell(gridView.grid());
        const auto& elementMapper = model.elementMapper();
 
        for (auto elemIt = gridView.template begin</*codim=*/0>();
             elemIt != elemEndIt;
             ++elemIt)
        {
            if (elemIt->partitionType() != Dune::InteriorEntity) {
                continue;
            }
            const auto& elem = *elemIt;
            // Convergence is measured on the owned cells only. Overlap cells
            // are owned, and checked, by another subdomain.
            if (domain.hasOverlap() && !domain.interior[elementMapper.index(elem)]) {
                continue;
            }
            elemCtx.updatePrimaryStencil(elem);
            elemCtx.updatePrimaryIntensiveQuantities(/*timeIdx=*/0);
 
            const unsigned cell_idx = elemCtx.globalSpaceIndex(/*spaceIdx=*/0, /*timeIdx=*/0);
            const auto& intQuants = elemCtx.intensiveQuantities(/*spaceIdx=*/0, /*timeIdx=*/0);
            const auto& fs = intQuants.fluidState();
 
            const auto pvValue = problem.referencePorosity(cell_idx, /*timeIdx=*/0) *
                                 model.dofTotalVolume(cell_idx);
            pvSumLocal += pvValue;
 
            if (isNumericalAquiferCell(elem))
            {
                numAquiferPvSumLocal += pvValue;
            }
 
            model_.getMaxCoeff(cell_idx, intQuants, fs, modelResid, pvValue,
                               B_avg, R_sum, maxCoeff, maxCoeffCell);
        }
 
        // compute local average in terms of global number of elements
        const int bSize = B_avg.size();
        for ( int i = 0; i<bSize; ++i )
        {
            B_avg[ i ] /= Scalar(domain.interior_cells.size());
        }
 
        return {pvSumLocal, numAquiferPvSumLocal};
    }
 
    ConvergenceReport getDomainReservoirConvergence(const double reportTime,
                                                    const double dt,
                                                    const Domain& domain,
                                                    DeferredLogger& logger,
                                                    std::vector<Scalar>& B_avg,
                                                    std::vector<Scalar>& residual_norms)
    {
        using Vector = std::vector<Scalar>;
 
        const auto& iterCtx = model_.simulator().problem().iterationContext();
        
        const int numComp = numEq;
        Vector R_sum(numComp, 0.0 );
        Vector maxCoeff(numComp, std::numeric_limits<Scalar>::lowest() );
        std::vector<int> maxCoeffCell(numComp, -1);
        const auto [ pvSum, numAquiferPvSum]
            = this->localDomainConvergenceData(domain, R_sum, maxCoeff, B_avg, maxCoeffCell);
 
        auto cnvErrorPvFraction = computeCnvErrorPvLocal(domain, B_avg, dt);
        cnvErrorPvFraction /= (pvSum - numAquiferPvSum);
 
        // For each iteration, we need to determine whether to use the relaxed tolerances.
        // To disable the usage of relaxed tolerances, you can set the relaxed tolerances
        // equal to the strict tolerances.
        //
        // If min_strict_mb_iter_ = -1 (default) we use a relaxed tolerance for the mass
        // balance only on the last local iteration. For non-negative values we use the
        // relaxed tolerance after the given number of iterations.
        const int maxLocalIter = model_.param().max_local_solve_iterations_;
 
        const bool relax_final_iteration_mb =
            model_.param().min_strict_mb_iter_ < 0 && iterCtx.iteration() == maxLocalIter;
 
        const bool relax_iter_mb =
            model_.param().min_strict_mb_iter_ >= 0 &&
            iterCtx.shouldRelax(model_.param().min_strict_mb_iter_);
 
        const bool use_relaxed_mb = relax_final_iteration_mb || relax_iter_mb;
 
        const Scalar tol_mb = model_.param().local_tolerance_scaling_mb_
            * (use_relaxed_mb ? model_.param().tolerance_mb_relaxed_ : model_.param().tolerance_mb_);
 
        // If min_strict_cnv_iter_ = -1 (default) we use a relaxed tolerance for the CNV
        // only on the last local iteration. For non-negative values we use the relaxed
        // tolerance after the given number of iterations. We also use relaxed tolerances
        // for cells with total pore-volume less than relaxed_max_pv_fraction_.
        const bool relax_final_iteration_cnv =
            model_.param().min_strict_cnv_iter_ < 0 && iterCtx.iteration() == maxLocalIter;
 
        const bool relax_iter_cnv =
            model_.param().min_strict_cnv_iter_ >= 0 &&
            iterCtx.shouldRelax(model_.param().min_strict_cnv_iter_);
 
        const bool relax_pv_fraction_cnv =
            cnvErrorPvFraction < model_.param().relaxed_max_pv_fraction_;
 
        const bool use_relaxed_cnv = relax_final_iteration_cnv ||
                                     relax_iter_cnv ||
                                     relax_pv_fraction_cnv;
 
        // Tighter bound for local convergence should increase the
        // likelyhood of: local convergence => global convergence
        const Scalar tol_cnv = model_.param().local_tolerance_scaling_cnv_
            * (use_relaxed_cnv ? model_.param().tolerance_cnv_relaxed_
                           : model_.param().tolerance_cnv_);
 
        // TODO: Energy-specific tolerances are not currently applied. A closer analysis
        // is needed to determine if everything in NLDD is compatible with thermal simulations.
 
        // Finish computation
        std::vector<Scalar> CNV(numComp);
        std::vector<Scalar> mass_balance_residual(numComp);
        for (int compIdx = 0; compIdx < numComp; ++compIdx )
        {
            CNV[compIdx]                    = B_avg[compIdx] * dt * maxCoeff[compIdx];
            mass_balance_residual[compIdx]  = std::abs(B_avg[compIdx]*R_sum[compIdx]) * dt / pvSum;
            residual_norms.push_back(CNV[compIdx]);
        }
 
        // Create convergence report.
        ConvergenceReport report{reportTime};
        using CR = ConvergenceReport;
        for (int compIdx = 0; compIdx < numComp; ++compIdx) {
            Scalar res[2] = { mass_balance_residual[compIdx], CNV[compIdx] };
            CR::ReservoirFailure::Type types[2] = { CR::ReservoirFailure::Type::MassBalance,
                                                    CR::ReservoirFailure::Type::Cnv };
            Scalar tol[2] = { tol_mb, tol_cnv };
            model_.addReservoirConvergenceMetrics(
                report,
                compIdx,
                model_.compNames().name(compIdx),
                std::span<const Scalar>{res},
                std::span<const CR::ReservoirFailure::Type>{types},
                std::span<const Scalar>{tol},
                model_.param().max_residual_allowed_,
                [&logger](const std::string& message)
                {
                    logger.debug(message);
                });
        }
 
        // Output of residuals. If converged at initial state, log nothing.
        const bool converged_at_initial_state = (report.converged() && iterCtx.iteration() == 0);
        if (!converged_at_initial_state) {
            if (iterCtx.iteration() == 0) {
                // Log header.
                std::string msg = fmt::format("Domain {} on rank {}, size {} (overlap {}), containing cell {}\n| Iter",
                                              domain.index, this->rank_, domain.interior_cells.size(),
                                              domain.cells.size() - domain.interior_cells.size(),
                                              domain.interior_cells[0]);
                for (int compIdx = 0; compIdx < numComp; ++compIdx) {
                    msg += "    MB(";
                    msg += model_.compNames().name(compIdx)[0];
                    msg += ")  ";
                }
                for (int compIdx = 0; compIdx < numComp; ++compIdx) {
                    msg += "    CNV(";
                    msg += model_.compNames().name(compIdx)[0];
                    msg += ") ";
                }
                logger.debug(msg);
            }
            // Log convergence data.
            std::ostringstream ss;
            ss << "| ";
            const std::streamsize oprec = ss.precision(3);
            const std::ios::fmtflags oflags = ss.setf(std::ios::scientific);
            ss << std::setw(4) << iterCtx.iteration();
            for (int compIdx = 0; compIdx < numComp; ++compIdx) {
                ss << std::setw(11) << mass_balance_residual[compIdx];
            }
            for (int compIdx = 0; compIdx < numComp; ++compIdx) {
                ss << std::setw(11) << CNV[compIdx];
            }
            ss.precision(oprec);
            ss.flags(oflags);
            logger.debug(ss.str());
        }
 
        return report;
    }
 
    ConvergenceReport getDomainConvergence(const Domain& domain,
                                           const SimulatorTimerInterface& timer,
                                           DeferredLogger& logger,
                                           std::vector<Scalar>& residual_norms)
    {
        OPM_TIMEBLOCK(getDomainConvergence);
        std::vector<Scalar> B_avg(numEq, 0.0);
        auto report = this->getDomainReservoirConvergence(timer.simulationTimeElapsed(),
                                                          timer.currentStepLength(),
                                                          domain,
                                                          logger,
                                                          B_avg,
                                                          residual_norms);
        report += wellModel_.getWellConvergence(domain, B_avg, logger);
        return report;
    }
 
    std::vector<int> getSubdomainOrder()
    {
        const auto& modelSimulator = model_.simulator();
        const auto& solution = modelSimulator.model().solution(0);
 
        std::vector<int> domain_order(domains_.size());
        std::iota(domain_order.begin(), domain_order.end(), 0);
 
        if (model_.param().local_solve_approach_ == DomainSolveApproach::Jacobi) {
            // Do nothing, 0..n-1 order is fine.
            return domain_order;
        } else if (model_.param().local_solve_approach_ == DomainSolveApproach::GaussSeidel) {
            // Calculate the measure used to order the domains.
            std::vector<Scalar> measure_per_domain(domains_.size());
            switch (model_.param().local_domains_ordering_) {
            case DomainOrderingMeasure::AveragePressure: {
                // Use average pressures to order domains.
                for (const auto& domain : domains_) {
                    const Scalar press_sum =
                        std::accumulate(domain.interior_cells.begin(), domain.interior_cells.end(), Scalar{0},
                                        [&solution](const auto acc, const auto c)
                                        { return acc + solution[c][Indices::pressureSwitchIdx]; });
                    const Scalar avgpress = press_sum / domain.interior_cells.size();
                    measure_per_domain[domain.index] = avgpress;
                }
                break;
            }
            case DomainOrderingMeasure::MaxPressure: {
                // Use max pressures to order domains.
                for (const auto& domain : domains_) {
                    measure_per_domain[domain.index] =
                        std::accumulate(domain.interior_cells.begin(), domain.interior_cells.end(), Scalar{0},
                                        [&solution](const auto acc, const auto c)
                                        { return std::max(acc, solution[c][Indices::pressureSwitchIdx]); });
                }
                break;
            }
            case DomainOrderingMeasure::Residual: {
                // Use maximum residual to order domains.
                const auto& residual = modelSimulator.model().linearizer().residual();
                const int num_vars = residual[0].size();
                for (const auto& domain : domains_) {
                    Scalar maxres = 0.0;
                    for (const int c : domain.interior_cells) {
                        for (int ii = 0; ii < num_vars; ++ii) {
                            maxres = std::max(maxres, std::fabs(residual[c][ii]));
                        }
                    }
                    measure_per_domain[domain.index] = maxres;
                }
                break;
            }
            } // end of switch (model_.param().local_domains_ordering_)
 
            // Sort by largest measure, keeping index order if equal.
            const auto& m = measure_per_domain;
            std::stable_sort(domain_order.begin(), domain_order.end(),
                             [&m](const int i1, const int i2){ return m[i1] > m[i2]; });
            return domain_order;
        } else {
            throw std::logic_error("Domain solve approach must be Jacobi or Gauss-Seidel");
        }
    }
 
    template<class GlobalEqVector>
    void solveDomainJacobi(GlobalEqVector& solution,
                           GlobalEqVector& locally_solved,
                           SimulatorReportSingle& local_report,
                           DeferredLogger& logger,
                           const SimulatorTimerInterface& timer,
                           const Domain& domain)
    {
        auto initial_local_well_primary_vars = wellModel_.getPrimaryVarsDomain(domain.index);
        auto initial_local_solution = Details::extractVector(solution, domain.cells);
        auto convrep = solveDomain(domain, timer, local_report, logger, false);
        if (local_report.converged) {
            // Each cell takes its value from the subdomain that owns it.
            auto local_solution = Details::extractVector(solution, domain.interior_cells);
            Details::setGlobal(local_solution, domain.interior_cells, locally_solved);
            Details::setGlobal(initial_local_solution, domain.cells, solution);
            model_.simulator().model().invalidateAndUpdateIntensiveQuantities(/*timeIdx=*/0, domain);
        } else {
            wellModel_.setPrimaryVarsDomain(domain.index, initial_local_well_primary_vars);
            Details::setGlobal(initial_local_solution, domain.cells, solution);
            model_.simulator().model().invalidateAndUpdateIntensiveQuantities(/*timeIdx=*/0, domain);
        }
    }
 
    template<class GlobalEqVector>
    void solveDomainGaussSeidel(GlobalEqVector& solution,
                                GlobalEqVector& locally_solved,
                                SimulatorReportSingle& local_report,
                                DeferredLogger& logger,
                                const SimulatorTimerInterface& timer,
                                const Domain& domain)
    {
        auto initial_local_well_primary_vars = wellModel_.getPrimaryVarsDomain(domain.index);
        auto initial_local_solution = Details::extractVector(solution, domain.cells);
        auto convrep = solveDomain(domain, timer, local_report, logger, true);
        if (!local_report.converged) {
            // We look at the detailed convergence report to evaluate
            // if we should accept the unconverged solution.
            // We do not accept a solution if the wells are unconverged.
            if (!convrep.wellFailed()) {
                // Calculare the sums of the mb and cnv failures.
                Scalar mb_sum = 0.0;
                Scalar cnv_sum = 0.0;
                for (const auto& rc : convrep.reservoirConvergence()) {
                    if (rc.type() == ConvergenceReport::ReservoirFailure::Type::MassBalance) {
                        mb_sum += rc.value();
                    } else if (rc.type() == ConvergenceReport::ReservoirFailure::Type::Cnv) {
                        cnv_sum += rc.value();
                    }
                }
                // If not too high, we overrule the convergence failure.
                const Scalar acceptable_local_mb_sum = 1e-3;
                const Scalar acceptable_local_cnv_sum = 1.0;
                if (mb_sum < acceptable_local_mb_sum && cnv_sum < acceptable_local_cnv_sum) {
                    local_report.converged = true;
                    local_report.accepted_unconverged_domains += 1;
                    logger.debug(fmt::format("Accepting solution in unconverged domain {} on rank {}.", domain.index, rank_));
                    logger.debug(fmt::format("Value of mb_sum: {}  cnv_sum: {}", mb_sum, cnv_sum));
                } else {
                    logger.debug("Unconverged local solution.");
                }
            } else {
                logger.debug("Unconverged local solution with well convergence failures:");
                for (const auto& wf : convrep.wellFailures()) {
                    logger.debug(to_string(wf));
                }
            }
        }
        if (local_report.converged) {
            local_report.converged_domains += 1;
            auto local_solution = Details::extractVector(solution, domain.interior_cells);
            Details::setGlobal(local_solution, domain.interior_cells, locally_solved);
            // The updated overlap values stay in the global solution vector,
            // so subdomains solved later in the sweep start from them. An
            // overlap cell ends the sweep with the value from the last
            // subdomain that solved for it, which need not be its owner.
        } else {
            local_report.unconverged_domains += 1;
            wellModel_.setPrimaryVarsDomain(domain.index, initial_local_well_primary_vars);
            Details::setGlobal(initial_local_solution, domain.cells, solution);
            model_.simulator().model().invalidateAndUpdateIntensiveQuantities(/*timeIdx=*/0, domain);
        }
    }
 
    Scalar computeCnvErrorPvLocal(const Domain& domain,
                                  const std::vector<Scalar>& B_avg, double dt) const
    {
        Scalar errorPV{};
        const auto& simulator = model_.simulator();
        const auto& model = simulator.model();
        const auto& problem = simulator.problem();
        const auto& residual = simulator.model().linearizer().residual();
 
        for (const int cell_idx : domain.interior_cells) {
            const Scalar pvValue = problem.referencePorosity(cell_idx, /*timeIdx=*/0) *
                                   model.dofTotalVolume(cell_idx);
            const auto& cellResidual = residual[cell_idx];
            bool cnvViolated = false;
 
            for (unsigned eqIdx = 0; eqIdx < cellResidual.size(); ++eqIdx) {
                using std::fabs;
                Scalar CNV = cellResidual[eqIdx] * dt * B_avg[eqIdx] / pvValue;
                cnvViolated = cnvViolated || (fabs(CNV) > model_.param().tolerance_cnv_);
            }
 
            if (cnvViolated) {
                errorPV += pvValue;
            }
        }
        return errorPV;
    }
 
    decltype(auto) partitionCells() const
    {
        const auto& grid = this->model_.simulator().vanguard().grid();
 
        using GridView = std::remove_cv_t<std::remove_reference_t<
            decltype(grid.leafGridView())>>;
 
        using Element = std::remove_cv_t<std::remove_reference_t<
            typename GridView::template Codim<0>::Entity>>;
 
        const auto& param = this->model_.param();
 
        auto zoltan_ctrl = ZoltanPartitioningControl<Element>{};
 
        zoltan_ctrl.domain_imbalance = param.local_domains_partition_imbalance_;
 
        zoltan_ctrl.index =
            [elementMapper = &this->model_.simulator().model().elementMapper()]
            (const Element& element)
        {
            return elementMapper->index(element);
        };
 
        zoltan_ctrl.local_to_global =
            [cartMapper = &this->model_.simulator().vanguard().cartesianIndexMapper()]
            (const int elemIdx)
        {
            return cartMapper->cartesianIndex(elemIdx);
        };
 
        // Forming the list of wells is expensive, so do this only if needed.
        const auto need_wells = param.local_domains_partition_method_ == "zoltan";
 
        const auto wells = need_wells
            ? this->model_.simulator().vanguard().schedule().getWellsatEnd()
            : std::vector<Well>{};
 
        const auto& possibleFutureConnectionSet = need_wells
            ? this->model_.simulator().vanguard().schedule().getPossibleFutureConnections()
            : std::unordered_map<std::string, std::set<int>> {};
 
        // If defaulted parameter for number of domains, choose a reasonable default.
        constexpr int default_cells_per_domain = 1000;
        const int num_domains = (param.num_local_domains_ > 0)
            ? param.num_local_domains_
            : detail::countGlobalCells(grid) / default_cells_per_domain;
        return ::Opm::partitionCells(param.local_domains_partition_method_,
                                     num_domains, grid.leafGridView(), wells,
                                     possibleFutureConnectionSet, zoltan_ctrl,
                                     param.local_domains_partition_well_neighbor_levels_);
    }
 
    void updateMobilities(const Domain& domain)
    {
        if (domain.skip || model_.param().nldd_relative_mobility_change_tol_ == 0.0) {
            return;
        }
        const auto numActivePhases = FluidSystem::numActivePhases();
        for (const auto globalDofIdx : domain.interior_cells) {
            const auto& intQuants = model_.simulator().model().intensiveQuantities(globalDofIdx, /* time_idx = */ 0);
 
            for (unsigned activePhaseIdx = 0; activePhaseIdx < numActivePhases; ++activePhaseIdx) {
                const auto phaseIdx = FluidSystem::activeToCanonicalPhaseIdx(activePhaseIdx);
                const auto mobIdx = globalDofIdx * numActivePhases + activePhaseIdx;
                previousMobilities_[mobIdx] = getValue(intQuants.mobility(phaseIdx));
            }
        }
    }
 
    bool checkIfSubdomainNeedsSolving(const Domain& domain)
    {
        if (domain.skip) {
            return false;
        }
 
        // If we domain was marked as needing solving in previous iterations,
        // we do not need to check again
        if (domain_needs_solving_[domain.index]) {
            return true;
        }
 
        // If we do not check for mobility changes, we need to solve the domain
        if (model_.param().nldd_relative_mobility_change_tol_ == 0.0) {
            return true;
        }
 
        // Skip mobility check on first global iteration (no previous mobilities to compare)
        if (model_.simulator().problem().iterationContext().isFirstGlobalIteration()) {
            return true;
        }
 
        return checkSubdomainChangeRelative(domain);
    }
 
    bool checkSubdomainChangeRelative(const Domain& domain)
    {
        const auto numActivePhases = FluidSystem::numActivePhases();
 
        // Check mobility changes for the owned cells. Overlap cells change
        // whenever a neighbouring subdomain is solved, which on its own is no
        // reason to solve this subdomain again.
        for (const auto globalDofIdx : domain.interior_cells) {
            const auto& intQuants = model_.simulator().model().intensiveQuantities(globalDofIdx, /* time_idx = */ 0);
 
            // Calculate average previous mobility for normalization
            Scalar cellMob = 0.0;
            for (unsigned activePhaseIdx = 0; activePhaseIdx < numActivePhases; ++activePhaseIdx) {
                const auto mobIdx = globalDofIdx * numActivePhases + activePhaseIdx;
                cellMob += previousMobilities_[mobIdx] / numActivePhases;
            }
 
            // Check relative changes for each phase
            for (unsigned activePhaseIdx = 0; activePhaseIdx < numActivePhases; ++activePhaseIdx) {
                const auto phaseIdx = FluidSystem::activeToCanonicalPhaseIdx(activePhaseIdx);
                const auto mobIdx = globalDofIdx * numActivePhases + activePhaseIdx;
                const auto mobility = getValue(intQuants.mobility(phaseIdx));
                const auto relDiff = std::abs(mobility - previousMobilities_[mobIdx]) / cellMob;
                if (relDiff > model_.param().nldd_relative_mobility_change_tol_) {
                    return true;
                }
            }
        }
        return false;
    }
 
    NonlinearSystemBlackOilReservoir<TypeTag>& model_; 
    BlackoilWellModelNldd<TypeTag> wellModel_; 
    std::vector<Domain> domains_; 
    std::vector<std::unique_ptr<Mat>> domain_matrices_; 
    std::vector<ISTLSolverType> domain_linsolvers_; 
    SimulatorReport local_reports_accumulated_; 
    // mutable because we need to update the number of wells for each domain in getDomainAccumulatedReports()
    mutable std::vector<SimulatorReport> domain_reports_accumulated_; 
    int rank_ = 0; 
    // Store previous mobilities to check for changes - single flat vector indexed by (globalCellIdx * numActivePhases + activePhaseIdx)
    std::vector<Scalar> previousMobilities_;
    // Flag indicating if this domain should be solved in the next iteration
    std::vector<bool> domain_needs_solving_;
};
 
} // namespace Opm
 
#endif // OPM_NONLINEAR_SYSTEM_NLDD_HEADER_INCLUDED