GlobalLinSysIterative.cpp

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///////////////////////////////////////////////////////////////////////////////
//
// File: GlobalLinSysIterative.cpp
//
// For more information, please see: http://www.nektar.info
//
// The MIT License
//
// Copyright (c) 2006 Division of Applied Mathematics, Brown University (USA),
// Department of Aeronautics, Imperial College London (UK), and Scientific
// Computing and Imaging Institute, University of Utah (USA).
//
// Permission is hereby granted, free of charge, to any person obtaining a
// copy of this software and associated documentation files (the "Software"),
// to deal in the Software without restriction, including without limitation
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// OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
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//
// Description: GlobalLinSysIterative definition
//
///////////////////////////////////////////////////////////////////////////////
 
#include <MultiRegions/GlobalLinSysIterative.h>
 
using namespace std;
 
namespace Nektar::MultiRegions
{
std::string GlobalLinSysIterative::IteratSolverlookupIds[4] = {
    LibUtilities::SessionReader::RegisterEnumValue(
        "LinSysIterSolver", "ConjugateGradient",
        MultiRegions::eConjugateGradient),
    LibUtilities::SessionReader::RegisterEnumValue(
        "LinSysIterSolver", "ConjugateGradientLoc",
        MultiRegions::eConjugateGradient),
    LibUtilities::SessionReader::RegisterEnumValue("LinSysIterSolver", "GMRES",
                                                   MultiRegions::eGMRES),
    LibUtilities::SessionReader::RegisterEnumValue(
        "LinSysIterSolver", "GMRESLoc", MultiRegions::eGMRESLoc),
};
 
std::string GlobalLinSysIterative::IteratSolverdef =
    LibUtilities::SessionReader::RegisterDefaultSolverInfo("LinSysIterSolver",
                                                           "ConjugateGradient");
 
/**
 * @class GlobalLinSysIterative
 *
 * Solves a linear system using iterative methods.
 */
 
/// Constructor for full iterative matrix solve.
GlobalLinSysIterative::GlobalLinSysIterative(
    const GlobalLinSysKey &pKey, const std::weak_ptr<ExpList> &pExpList,
    const std::shared_ptr<AssemblyMap> &pLocToGloMap)
    : GlobalLinSys(pKey, pExpList, pLocToGloMap),
      m_rhs_magnitude(NekConstants::kNekUnsetDouble), m_rhs_mag_sm(0.9),
      m_precon(NullPreconditionerSharedPtr), m_totalIterations(0),
      m_useProjection(false), m_numPrevSols(0)
{
    m_isAbsoluteTolerance = pLocToGloMap->IsAbsoluteTolerance();
    m_linSysIterSolver    = pLocToGloMap->GetLinSysIterSolver();
 
    LibUtilities::CommSharedPtr vComm =
        m_expList.lock()->GetComm()->GetRowComm();
    m_root = (vComm->GetRank()) ? false : true;
 
    m_numSuccessiveRHS = pLocToGloMap->GetSuccessiveRHS();
    m_isAconjugate     = m_numSuccessiveRHS > 0;
    m_numSuccessiveRHS = std::abs(m_numSuccessiveRHS);
    m_useProjection    = m_numSuccessiveRHS > 0;
 
    // Check for advection matrix and switch to GMRES, if not already used
    m_matrixType           = StdRegions::MatrixTypeMap[pKey.GetMatrixType()];
    m_isNonSymmetricLinSys = m_matrixType.find("Advection") != string::npos;
 
    if (m_isNonSymmetricLinSys &&
        !m_linSysIterSolver.compare("ConjugateGradient"))
    {
        m_linSysIterSolver = "GMRES";
        WARNINGL0(
            false,
            "Detected ConjugateGradient solver and a "
            "Advection-type matrix. "
            "Switchted to a GMRES solver for this non-symmetric matrix type. "
            "Change SolverInfo 'LinSysIterSolver' to GMRES in the session file "
            "to suppress this warning.");
    }
 
    if (m_isAconjugate && m_linSysIterSolver.compare("GMRES") == 0 &&
        m_linSysIterSolver.compare("GMRESLoc") == 0)
    {
        WARNINGL0(false, "To use A-conjugate projection, the matrix "
                         "should be symmetric positive definite.");
    }
 
    m_NekSysOp.DefineNekSysLhsEval(&GlobalLinSysIterative::DoMatrixMultiplyFlag,
                                   this);
    m_NekSysOp.DefineNekSysPrecon(&GlobalLinSysIterative::DoPreconditionerFlag,
                                  this);
    m_NekSysOp.DefineAssembleLoc(&GlobalLinSysIterative::DoAssembleLocFlag,
                                 this);
}
 
GlobalLinSysIterative::~GlobalLinSysIterative()
{
}
 
/**
 * This method implements projection techniques
 * in order to speed up successive linear solves with
 * right-hand sides arising from time-dependent discretisations.
 * (P.F.Fischer, Comput. Methods Appl. Mech. Engrg. 163, 1998)
 */
void GlobalLinSysIterative::DoProjection(
    const int nGlobal, const Array<OneD, const NekDouble> &pInput,
    Array<OneD, NekDouble> &pOutput, const int nDir, const bool isAconjugate)
{
    int numIterations = 0;
    if (m_numPrevSols == 0)
    {
        // no previous solutions found
        numIterations = m_linsol->SolveSystem(nGlobal, pInput, pOutput, nDir);
    }
    else
    {
        // Get the communicator for performing data exchanges
        LibUtilities::CommSharedPtr vComm =
            m_expList.lock()->GetComm()->GetRowComm();
 
        // Get vector sizes
        int nNonDir = nGlobal - nDir;
 
        // check the input vector (rhs) is not zero
        Array<OneD, NekDouble> tmp;
 
        NekDouble rhsNorm =
            Vmath::Dot2(nNonDir, pInput + nDir, pInput + nDir, m_map + nDir);
 
        vComm->AllReduce(rhsNorm, Nektar::LibUtilities::ReduceSum);
 
        NekDouble tol = m_linsol->GetNekLinSysTolerance();
        if (rhsNorm < tol * tol * m_rhs_magnitude)
        {
            Vmath::Zero(nNonDir, tmp = pOutput + nDir, 1);
            if (m_verbose && m_root)
            {
                cout << "No iterations made"
                     << " using tolerance of " << tol
                     << " (error = " << sqrt(rhsNorm / m_rhs_magnitude)
                     << ", rhs_mag = " << sqrt(m_rhs_magnitude) << ")" << endl;
            }
            return;
        }
 
        // Create NekVector wrappers for linear algebra operations
        NekVector<NekDouble> b(nNonDir, pInput + nDir, eWrapper);
        NekVector<NekDouble> x(nNonDir, tmp = pOutput + nDir, eWrapper);
        // Allocate array storage
        Array<OneD, NekDouble> px_s(nGlobal, 0.0);
        Array<OneD, NekDouble> pb_s(nGlobal, 0.0);
        Array<OneD, NekDouble> tmpAx_s(nGlobal, 0.0);
        Array<OneD, NekDouble> tmpx_s(nGlobal, 0.0);
 
        NekVector<NekDouble> pb(nNonDir, tmp = pb_s + nDir, eWrapper);
        NekVector<NekDouble> px(nNonDir, tmp = px_s + nDir, eWrapper);
        NekVector<NekDouble> tmpAx(nNonDir, tmp = tmpAx_s + nDir, eWrapper);
        NekVector<NekDouble> tmpx(nNonDir, tmp = tmpx_s + nDir, eWrapper);
 
        // notation follows the paper cited:
        // \alpha_i = \tilda{x_i}^T b^n
        // projected x, px = \sum \alpha_i \tilda{x_i}
 
        Array<OneD, NekDouble> alpha(m_prevBasis.size(), 0.0);
        Array<OneD, NekDouble> alphaback(m_prevBasis.size(), 0.0);
        for (int i = 0; i < m_prevBasis.size(); i++)
        {
            alpha[i] = Vmath::Dot2(nNonDir, m_prevBasis[i], pInput + nDir,
                                   m_map + nDir);
        }
        vComm->AllReduce(alpha, Nektar::LibUtilities::ReduceSum);
        int n = m_prevBasis.size(), info = -1;
        Vmath::Vcopy(m_prevBasis.size(), alpha, 1, alphaback, 1);
        Lapack::Dsptrs('U', n, 1, m_coeffMatrixFactor.data(), m_ipivot.data(),
                       alpha.data(), n, info);
        if (info != 0)
        {
            // Dsptrs fails, only keep the latest solution
            int latest = ResetKnownSolutionsToLatestOne();
            alpha[0]   = alphaback[latest];
        }
        for (int i = 0; i < m_prevBasis.size(); ++i)
        {
            NekVector<NekDouble> xi(nNonDir, m_prevLinSol[i], eWrapper);
            px += alpha[i] * xi;
        }
 
        // pb = b^n - A px
        Vmath::Vcopy(nNonDir, pInput.data() + nDir, 1, pb_s.data() + nDir, 1);
 
        DoMatrixMultiplyFlag(px_s, tmpAx_s, false);
 
        pb -= tmpAx;
 
        if (m_verbose)
        {
            if (m_root)
            {
                cout << "SuccessiveRHS: " << m_prevBasis.size()
                     << "-bases projection reduces L2-norm of RHS from "
                     << std::sqrt(rhsNorm) << " to ";
            }
            NekDouble tmprhsNorm =
                Vmath::Dot2(nNonDir, pb_s + nDir, pb_s + nDir, m_map + nDir);
            vComm->AllReduce(tmprhsNorm, Nektar::LibUtilities::ReduceSum);
            if (m_root)
            {
                cout << std::sqrt(tmprhsNorm) << endl;
            }
        }
 
        // solve the system with projected rhs
        numIterations = m_linsol->SolveSystem(nGlobal, pb_s, tmpx_s, nDir);
 
        // remainder solution + projection of previous solutions
        x = tmpx + px;
    }
    // save the auxiliary solution to prev. known solutions
    if (numIterations)
    {
        UpdateKnownSolutions(nGlobal, pOutput, nDir, isAconjugate);
    }
}
 
int GlobalLinSysIterative::ResetKnownSolutionsToLatestOne()
{
    if (m_numPrevSols == 0)
    {
        return -1;
    }
    int latest = (m_numPrevSols - 1 + m_numSuccessiveRHS) % m_numSuccessiveRHS;
    Array<OneD, NekDouble> b = m_prevBasis[latest];
    Array<OneD, NekDouble> x = m_prevLinSol[latest];
    m_prevBasis.clear();
    m_prevLinSol.clear();
    m_prevBasis.push_back(b);
    m_prevLinSol.push_back(x);
    m_numPrevSols = 1;
    return latest;
}
 
/**
 * Updates the storage of previously known solutions.
 * Performs normalisation of input vector wrt A-norm.
 */
void GlobalLinSysIterative::UpdateKnownSolutions(
    const int nGlobal, const Array<OneD, const NekDouble> &newX, const int nDir,
    const bool isAconjugate)
{
    // Get vector sizes
    int nNonDir        = nGlobal - nDir;
    int insertLocation = m_numPrevSols % m_numSuccessiveRHS;
    int fullbuffer     = (m_prevBasis.size() == m_numSuccessiveRHS);
 
    // Get the communicator for performing data exchanges
    LibUtilities::CommSharedPtr vComm =
        m_expList.lock()->GetComm()->GetRowComm();
 
    Array<OneD, NekDouble> tmpAx_s(nGlobal, 0.0);
    Array<OneD, NekDouble> y_s(m_prevBasis.size() - fullbuffer, 0.0);
    Array<OneD, NekDouble> invMy_s(y_s.size(), 0.0);
    Array<OneD, int> ipivot(m_numSuccessiveRHS);
    Array<OneD, NekDouble> tmp, newBasis;
 
    DoMatrixMultiplyFlag(newX, tmpAx_s, false);
 
    if (isAconjugate)
    {
        newBasis = newX + nDir;
    }
    else
    {
        newBasis = tmpAx_s + nDir;
    }
 
    // Check the solution is non-zero
    NekDouble solNorm =
        Vmath::Dot2(nNonDir, newBasis, tmpAx_s + nDir, m_map + nDir);
    vComm->AllReduce(solNorm, Nektar::LibUtilities::ReduceSum);
 
    if (solNorm < NekConstants::kNekSparseNonZeroTol)
    {
        return;
    }
 
    // normalisation of A x
    Vmath::Smul(nNonDir, 1.0 / sqrt(solNorm), tmpAx_s + nDir, 1,
                tmp = tmpAx_s + nDir, 1);
 
    for (int i = 0; i < m_prevBasis.size(); ++i)
    {
        if (i == insertLocation)
        {
            continue;
        }
        int skip = i > insertLocation;
        y_s[i - skip] =
            Vmath::Dot2(nNonDir, m_prevBasis[i], tmpAx_s + nDir, m_map + nDir);
    }
    vComm->AllReduce(y_s, Nektar::LibUtilities::ReduceSum);
 
    // check if linearly dependent
    DNekMatSharedPtr tilCoeffMatrix;
    if (fullbuffer && m_numSuccessiveRHS > 1)
    {
        tilCoeffMatrix = MemoryManager<DNekMat>::AllocateSharedPtr(
            m_numSuccessiveRHS - 1, m_numSuccessiveRHS - 1, 0.0, eSYMMETRIC);
        for (int i = 0; i < m_numSuccessiveRHS; ++i)
        {
            if (i == insertLocation)
            {
                continue;
            }
            int iskip = i > insertLocation;
            for (int j = i; j < m_numSuccessiveRHS; ++j)
            {
                if (j == insertLocation)
                {
                    continue;
                }
                int jskip = j > insertLocation;
                tilCoeffMatrix->SetValue(i - iskip, j - jskip,
                                         m_coeffMatrix->GetValue(i, j));
            }
        }
    }
    else if (!fullbuffer && m_prevBasis.size())
    {
        tilCoeffMatrix = MemoryManager<DNekMat>::AllocateSharedPtr(
            m_prevBasis.size(), m_prevBasis.size(), 0.0, eSYMMETRIC);
        Vmath::Vcopy(tilCoeffMatrix->GetStorageSize(), m_coeffMatrix->GetPtr(),
                     1, tilCoeffMatrix->GetPtr(), 1);
    }
 
    int n, info1 = 0, info2 = 0, info3 = 0;
    if (y_s.size())
    {
        n = tilCoeffMatrix->GetRows();
        Array<OneD, NekDouble> tilCoeffMatrixFactor(
            tilCoeffMatrix->GetStorageSize());
        Vmath::Vcopy(tilCoeffMatrix->GetStorageSize(), tilCoeffMatrix->GetPtr(),
                     1, tilCoeffMatrixFactor, 1);
        Lapack::Dsptrf('U', n, tilCoeffMatrixFactor.data(), ipivot.data(),
                       info1);
        if (info1 == 0)
        {
            Vmath::Vcopy(n, y_s, 1, invMy_s, 1);
            Lapack::Dsptrs('U', n, 1, tilCoeffMatrixFactor.data(),
                           ipivot.data(), invMy_s.data(), n, info2);
        }
    }
    if (info1 || info2)
    {
        int latest     = ResetKnownSolutionsToLatestOne();
        y_s[0]         = y_s[latest - (latest > insertLocation)];
        invMy_s[0]     = y_s[0];
        insertLocation = m_numPrevSols % m_numSuccessiveRHS;
        fullbuffer     = (m_prevBasis.size() == m_numSuccessiveRHS);
    }
    NekDouble residual = 1.;
    NekDouble epsilon  = 10. * NekConstants::kNekZeroTol;
    for (int i = 0; i < m_prevBasis.size() - fullbuffer; i++)
    {
        residual -= y_s[i] * invMy_s[i];
    }
    if (m_verbose && m_root)
    {
        cout << "SuccessiveRHS: residual " << residual;
    }
    if (residual < epsilon)
    {
        if (m_verbose && m_root)
        {
            cout << " < " << epsilon << ", reject" << endl;
        }
        return;
    }
 
    // calculate new coefficient matrix and its factor
    DNekMatSharedPtr newCoeffMatrix;
    if (fullbuffer)
    {
        newCoeffMatrix = MemoryManager<DNekMat>::AllocateSharedPtr(
            m_numSuccessiveRHS, m_numSuccessiveRHS, 0.0, eSYMMETRIC);
        Vmath::Vcopy(m_coeffMatrix->GetStorageSize(), m_coeffMatrix->GetPtr(),
                     1, newCoeffMatrix->GetPtr(), 1);
        newCoeffMatrix->SetValue(insertLocation, insertLocation, 1.);
        for (int i = 0; i < m_numSuccessiveRHS; ++i)
        {
            if (i == insertLocation)
            {
                continue;
            }
            int iskip = i > insertLocation;
            newCoeffMatrix->SetValue(insertLocation, i, y_s[i - iskip]);
        }
    }
    else
    {
        newCoeffMatrix = MemoryManager<DNekMat>::AllocateSharedPtr(
            m_prevBasis.size() + 1, m_prevBasis.size() + 1, 0.0, eSYMMETRIC);
        newCoeffMatrix->SetValue(insertLocation, insertLocation, 1.);
        for (int i = 0; i < m_prevBasis.size(); ++i)
        {
            newCoeffMatrix->SetValue(insertLocation, i, y_s[i]);
            for (int j = i; j < m_prevBasis.size(); ++j)
            {
                newCoeffMatrix->SetValue(i, j, m_coeffMatrix->GetValue(i, j));
            }
        }
    }
    n = newCoeffMatrix->GetRows();
    Array<OneD, NekDouble> coeffMatrixFactor(newCoeffMatrix->GetStorageSize());
    Vmath::Vcopy(newCoeffMatrix->GetStorageSize(), newCoeffMatrix->GetPtr(), 1,
                 coeffMatrixFactor, 1);
    Lapack::Dsptrf('U', n, coeffMatrixFactor.data(), ipivot.data(), info3);
    if (info3)
    {
        if (m_verbose && m_root)
        {
            cout << " >= " << epsilon << ", reject (Dsptrf fails)" << endl;
        }
        return;
    }
    if (m_verbose && m_root)
    {
        cout << " >= " << epsilon << ", accept" << endl;
    }
 
    // if success, update basis, rhs, coefficient matrix, and its factor
    if (m_prevBasis.size() < m_numSuccessiveRHS)
    {
        m_prevBasis.push_back(tmp = tmpAx_s + nDir);
        if (isAconjugate)
        {
            m_prevLinSol.push_back(tmp);
        }
        else
        {
            Array<OneD, NekDouble> solution(nNonDir, 0.0);
            m_prevLinSol.push_back(solution);
        }
    }
    Vmath::Smul(nNonDir, 1. / sqrt(solNorm), tmp = newX + nDir, 1,
                m_prevLinSol[insertLocation], 1);
    if (!isAconjugate)
    {
        m_prevBasis[insertLocation] = tmpAx_s + nDir;
    }
    m_coeffMatrix       = newCoeffMatrix;
    m_coeffMatrixFactor = coeffMatrixFactor;
    m_ipivot            = ipivot;
    ++m_numPrevSols;
}
 
} // namespace Nektar::MultiRegions