Date: 2025-03-28T21:10:57+00:00

Actual source code: ex4.c

      1: static char help[] = "Applies the 2023 preconditioner of Benzi and Faccio\n\n";

      3: #include <petscmat.h>
      4: #include <petscviewer.h>
      5: #include <petscvec.h>
      6: #include <petscis.h>
      7: #include <petscksp.h>

      9: /*
     10:  * This example reproduces the preconditioner outlined in Benzi's paper
     11:  * https://doi.org/10.1137/22M1505529. The problem considered is:
     12:  *
     13:  * (A + gamma UU^T)x = b
     14:  *
     15:  * whose structure arises from, for example, grad-div stabilization in the
     16:  * Navier-Stokes momentum equation. In the code we will also refer to
     17:  * gamma UU^T as J. The preconditioner developed by Benzi is:
     18:  *
     19:  * P_alpha = (A + alpha I)(alpha I + gamma UU^T)
     20:  *
     21:  * Another variant which may yield better convergence depending on the specific
     22:  * problem is
     23:  *
     24:  * P_alpha = (A + alpha D) D^-1 (alpha D + gamma UU^T)
     25:  *
     26:  * where D = diag(A + gamma UU^T). This is the variant implemented
     27:  * here. Application of the preconditioner involves (approximate) solution of
     28:  * two systems, one with (A + alpha D), and another with (alpha D + gamma
     29:  * UU^T). For small alpha (which generally yields the best overall
     30:  * preconditioner), (alpha D + gamma UU^T) is ill-conditioned. To combat this we
     31:  * solve (alpha D + gamma UU^T) using the Sherman-Morrison-Woodbury (SMW) matrix
     32:  * identity, which effectively converts the grad-div structure to a much nicer
     33:  * div-grad (laplacian) structure.
     34:  *
     35:  * The matrices used as input can be generated by running the matlab/octave
     36:  * program IFISS. The particular matrices checked into the datafiles repository
     37:  * and used in testing of this example correspond to a leaky lid-driven cavity
     38:  * with a stretched grid and Q2-Q1 finite elements. The matrices are taken from
     39:  * the last iteration of a Picard solve with tolerance 1e-8 with a viscosity of
     40:  * 0.1 and a 32x32 grid. We summarize below iteration counts from running this
     41:  * preconditioner for different grids and viscosity with a KSP tolerance of 1e-6.
     42:  *
     43:  *       32x32 64x64 128x128
     44:  * 0.1   28    36    43
     45:  * 0.01  59    75    73
     46:  * 0.002 136   161   167
     47:  *
     48:  * A reader of Benzi's paper will note that the performance shown above with
     49:  * respect to decreasing viscosity is significantly worse than in the
     50:  * paper. This is actually because of the choice of RHS. In Benzi's work, the
     51:  * RHS was generated by multiplying the operator with a vector of 1s whereas
     52:  * here we generate the RHS using a random vector. The iteration counts from the
     53:  * Benzi paper can be reproduced by changing the RHS generation in this example,
     54:  * but we choose to use the more difficult RHS as the resulting performance may
     55:  * more closely match what users experience in "physical" contexts.
     56:  */

     58: PetscErrorCode CreateAndLoadMat(const char *mat_name, Mat *mat)
     59: {
     60:   PetscViewer viewer;
     61:   char        file[PETSC_MAX_PATH_LEN];
     62:   char        flag_name[10] = "-f";
     63:   PetscBool   flg;

     65:   PetscFunctionBeginUser;
     66:   PetscCall(PetscOptionsGetString(NULL, NULL, strcat(flag_name, mat_name), file, sizeof(file), &flg));
     67:   PetscCheck(flg, PETSC_COMM_WORLD, PETSC_ERR_USER, "Must indicate file with the -f<mat_name> option");
     68:   PetscCall(PetscViewerBinaryOpen(PETSC_COMM_WORLD, file, FILE_MODE_READ, &viewer));
     69:   PetscCall(MatCreate(PETSC_COMM_WORLD, mat));
     70:   PetscCall(MatSetType(*mat, MATAIJ));
     71:   PetscCall(PetscObjectSetName((PetscObject)*mat, mat_name));
     72:   PetscCall(MatSetFromOptions(*mat));
     73:   PetscCall(MatLoad(*mat, viewer));
     74:   PetscCall(PetscViewerDestroy(&viewer));
     75:   PetscFunctionReturn(PETSC_SUCCESS);
     76: }

     78: typedef struct {
     79:   Mat       U, UT, D, aD, aDinv, I_plus_gammaUTaDinvU;
     80:   PC        smw_cholesky;
     81:   PetscReal gamma, alpha;
     82:   PetscBool setup_called;
     83: } SmwPCCtx;

     85: PetscErrorCode SmwSetup(PC pc)
     86: {
     87:   SmwPCCtx *ctx;
     88:   Vec       aDVec;

     90:   PetscFunctionBeginUser;
     91:   PetscCall(PCShellGetContext(pc, &ctx));

     93:   if (ctx->setup_called) PetscFunctionReturn(PETSC_SUCCESS);

     95:   // Create aD
     96:   PetscCall(MatDuplicate(ctx->D, MAT_COPY_VALUES, &ctx->aD));
     97:   PetscCall(MatScale(ctx->aD, ctx->alpha));

     99:   // Create aDinv
    100:   PetscCall(MatDuplicate(ctx->aD, MAT_DO_NOT_COPY_VALUES, &ctx->aDinv));
    101:   PetscCall(MatCreateVecs(ctx->aD, &aDVec, NULL));
    102:   PetscCall(MatGetDiagonal(ctx->aD, aDVec));
    103:   PetscCall(VecReciprocal(aDVec));
    104:   PetscCall(MatDiagonalSet(ctx->aDinv, aDVec, INSERT_VALUES));

    106:   // Create UT
    107:   PetscCall(MatTranspose(ctx->U, MAT_INITIAL_MATRIX, &ctx->UT));

    109:   // Create sum Mat
    110:   PetscCall(MatMatMatMult(ctx->UT, ctx->aDinv, ctx->U, MAT_INITIAL_MATRIX, PETSC_CURRENT, &ctx->I_plus_gammaUTaDinvU));
    111:   PetscCall(MatScale(ctx->I_plus_gammaUTaDinvU, ctx->gamma));
    112:   PetscCall(MatShift(ctx->I_plus_gammaUTaDinvU, 1.));

    114:   PetscCall(PCCreate(PETSC_COMM_WORLD, &ctx->smw_cholesky));
    115:   PetscCall(PCSetType(ctx->smw_cholesky, PCCHOLESKY));
    116:   PetscCall(PCSetOperators(ctx->smw_cholesky, ctx->I_plus_gammaUTaDinvU, ctx->I_plus_gammaUTaDinvU));
    117:   PetscCall(PCSetOptionsPrefix(ctx->smw_cholesky, "smw_"));
    118:   PetscCall(PCSetFromOptions(ctx->smw_cholesky));
    119:   PetscCall(PCSetUp(ctx->smw_cholesky));

    121:   PetscCall(VecDestroy(&aDVec));

    123:   ctx->setup_called = PETSC_TRUE;
    124:   PetscFunctionReturn(PETSC_SUCCESS);
    125: }

    127: PetscErrorCode SmwApply(PC pc, Vec x, Vec y)
    128: {
    129:   SmwPCCtx *ctx;
    130:   Vec       vel0, pressure0, pressure1;

    132:   PetscFunctionBeginUser;
    133:   PetscCall(PCShellGetContext(pc, &ctx));

    135:   PetscCall(MatCreateVecs(ctx->UT, &vel0, &pressure0));
    136:   PetscCall(VecDuplicate(pressure0, &pressure1));

    138:   // First term
    139:   PetscCall(MatMult(ctx->aDinv, x, vel0));
    140:   PetscCall(MatMult(ctx->UT, vel0, pressure0));
    141:   PetscCall(PCApply(ctx->smw_cholesky, pressure0, pressure1));
    142:   PetscCall(MatMult(ctx->U, pressure1, vel0));
    143:   PetscCall(MatMult(ctx->aDinv, vel0, y));
    144:   PetscCall(VecScale(y, -ctx->gamma));

    146:   // Second term
    147:   PetscCall(MatMult(ctx->aDinv, x, vel0));

    149:   PetscCall(VecAXPY(y, 1, vel0));

    151:   PetscCall(VecDestroy(&vel0));
    152:   PetscCall(VecDestroy(&pressure0));
    153:   PetscCall(VecDestroy(&pressure1));
    154:   PetscFunctionReturn(PETSC_SUCCESS);
    155: }

    157: int main(int argc, char **args)
    158: {
    159:   Mat               A, B, Q, Acondensed, Bcondensed, BT, J, AplusJ, QInv, D, AplusD, JplusD, U;
    160:   Mat               AplusJarray[2];
    161:   Vec               bound, x, b, Qdiag, DVec;
    162:   PetscBool         flg;
    163:   PetscViewer       viewer;
    164:   char              file[PETSC_MAX_PATH_LEN];
    165:   PetscInt         *boundary_indices;
    166:   PetscInt          boundary_indices_size, am, an, bm, bn, condensed_am, astart, aend, Dstart, Dend, num_local_bnd_dofs = 0;
    167:   const PetscScalar zero = 0;
    168:   IS                boundary_is, bulk_is;
    169:   KSP               ksp;
    170:   PC                pc, pcA, pcJ;
    171:   PetscRandom       rctx;
    172:   PetscReal        *boundary_indices_values;
    173:   PetscReal         gamma = 100, alpha = .01;
    174:   PetscMPIInt       rank;
    175:   SmwPCCtx          ctx;

    177:   PetscFunctionBeginUser;
    178:   PetscCall(PetscInitialize(&argc, &args, NULL, help));
    179:   PetscCallMPI(MPI_Comm_rank(PETSC_COMM_WORLD, &rank));

    181:   PetscCall(CreateAndLoadMat("A", &A));
    182:   PetscCall(CreateAndLoadMat("B", &B));
    183:   PetscCall(CreateAndLoadMat("Q", &Q));

    185:   PetscCall(PetscOptionsGetString(NULL, NULL, "-fbound", file, sizeof(file), &flg));
    186:   PetscCheck(flg, PETSC_COMM_WORLD, PETSC_ERR_USER, "Must indicate file with the -fbound option");

    188:   if (rank == 0) {
    189:     PetscCall(PetscViewerBinaryOpen(PETSC_COMM_SELF, file, FILE_MODE_READ, &viewer));
    190:     PetscCall(VecCreate(PETSC_COMM_SELF, &bound));
    191:     PetscCall(PetscObjectSetName((PetscObject)bound, "bound"));
    192:     PetscCall(VecSetType(bound, VECSEQ));
    193:     PetscCall(VecLoad(bound, viewer));
    194:     PetscCall(PetscViewerDestroy(&viewer));
    195:     PetscCall(VecGetLocalSize(bound, &boundary_indices_size));
    196:     PetscCall(VecGetArray(bound, &boundary_indices_values));
    197:   }
    198:   PetscCallMPI(MPI_Bcast(&boundary_indices_size, 1, MPIU_INT, 0, PETSC_COMM_WORLD));
    199:   if (rank != 0) PetscCall(PetscMalloc1(boundary_indices_size, &boundary_indices_values));
    200:   PetscCallMPI(MPI_Bcast(boundary_indices_values, (PetscMPIInt)boundary_indices_size, MPIU_SCALAR, 0, PETSC_COMM_WORLD));

    202:   PetscCall(MatGetSize(A, &am, NULL));
    203:   // The total number of dofs for a given velocity component
    204:   const PetscInt nc = am / 2;
    205:   PetscCall(MatGetOwnershipRange(A, &astart, &aend));

    207:   PetscCall(PetscMalloc1(2 * boundary_indices_size, &boundary_indices));

    209:   //
    210:   // The dof index ordering appears to be all vx dofs and then all vy dofs.
    211:   //

    213:   // First do vx
    214:   for (PetscInt i = 0; i < boundary_indices_size; ++i) {
    215:     // MATLAB uses 1-based indexing
    216:     const PetscInt bnd_dof = (PetscInt)boundary_indices_values[i] - 1;
    217:     if ((bnd_dof >= astart) && (bnd_dof < aend)) boundary_indices[num_local_bnd_dofs++] = bnd_dof;
    218:   }

    220:   // Now vy
    221:   for (PetscInt i = 0; i < boundary_indices_size; ++i) {
    222:     // MATLAB uses 1-based indexing
    223:     const PetscInt bnd_dof = ((PetscInt)boundary_indices_values[i] - 1) + nc;
    224:     if ((bnd_dof >= astart) && (bnd_dof < aend)) boundary_indices[num_local_bnd_dofs++] = bnd_dof;
    225:   }
    226:   if (rank == 0) PetscCall(VecRestoreArray(bound, &boundary_indices_values));
    227:   else PetscCall(PetscFree(boundary_indices_values));

    229:   PetscCall(ISCreateGeneral(PETSC_COMM_WORLD, num_local_bnd_dofs, boundary_indices, PETSC_USE_POINTER, &boundary_is));
    230:   PetscCall(ISComplement(boundary_is, astart, aend, &bulk_is));

    232:   PetscCall(MatCreateSubMatrix(A, bulk_is, bulk_is, MAT_INITIAL_MATRIX, &Acondensed));
    233:   // Can't pass null for row index set :-(
    234:   PetscCall(MatTranspose(B, MAT_INPLACE_MATRIX, &B));
    235:   PetscCall(MatCreateSubMatrix(B, bulk_is, NULL, MAT_INITIAL_MATRIX, &Bcondensed));
    236:   PetscCall(MatGetLocalSize(Acondensed, &am, &an));
    237:   PetscCall(MatGetLocalSize(Bcondensed, &bm, &bn));

    239:   // Create QInv
    240:   PetscCall(MatCreateVecs(Q, &Qdiag, NULL));
    241:   PetscCall(MatGetDiagonal(Q, Qdiag));
    242:   PetscCall(VecReciprocal(Qdiag));
    243:   PetscCall(MatDuplicate(Q, MAT_DO_NOT_COPY_VALUES, &QInv));
    244:   PetscCall(MatDiagonalSet(QInv, Qdiag, INSERT_VALUES));
    245:   PetscCall(MatAssemblyBegin(QInv, MAT_FINAL_ASSEMBLY));
    246:   PetscCall(MatAssemblyEnd(QInv, MAT_FINAL_ASSEMBLY));

    248:   // Create J
    249:   PetscCall(MatTranspose(Bcondensed, MAT_INITIAL_MATRIX, &BT));
    250:   PetscCall(MatMatMatMult(Bcondensed, QInv, BT, MAT_INITIAL_MATRIX, PETSC_CURRENT, &J));
    251:   PetscCall(MatScale(J, gamma));

    253:   // Create sum of A + J
    254:   AplusJarray[0] = Acondensed;
    255:   AplusJarray[1] = J;
    256:   PetscCall(MatCreateComposite(PETSC_COMM_WORLD, 2, AplusJarray, &AplusJ));

    258:   // Create decomposition matrices
    259:   // We've already used Qdiag, which currently represents Q^-1,  for its necessary purposes. Let's
    260:   // convert it to represent Q^(-1/2)
    261:   PetscCall(VecSqrtAbs(Qdiag));
    262:   // We can similarly reuse Qinv
    263:   PetscCall(MatDiagonalSet(QInv, Qdiag, INSERT_VALUES));
    264:   PetscCall(MatAssemblyBegin(QInv, MAT_FINAL_ASSEMBLY));
    265:   PetscCall(MatAssemblyEnd(QInv, MAT_FINAL_ASSEMBLY));
    266:   // Create U
    267:   PetscCall(MatMatMult(Bcondensed, QInv, MAT_INITIAL_MATRIX, PETSC_CURRENT, &U));

    269:   // Create x and b
    270:   PetscCall(MatCreateVecs(AplusJ, &x, &b));
    271:   PetscCall(PetscRandomCreate(PETSC_COMM_WORLD, &rctx));
    272:   PetscCall(VecSetRandom(x, rctx));
    273:   PetscCall(PetscRandomDestroy(&rctx));
    274:   PetscCall(MatMult(AplusJ, x, b));

    276:   // Compute preconditioner operators
    277:   PetscCall(MatGetLocalSize(Acondensed, &condensed_am, NULL));
    278:   PetscCall(MatCreate(PETSC_COMM_WORLD, &D));
    279:   PetscCall(MatSetType(D, MATAIJ));
    280:   PetscCall(MatSetSizes(D, condensed_am, condensed_am, PETSC_DETERMINE, PETSC_DETERMINE));
    281:   PetscCall(MatGetOwnershipRange(D, &Dstart, &Dend));
    282:   for (PetscInt i = Dstart; i < Dend; ++i) PetscCall(MatSetValues(D, 1, &i, 1, &i, &zero, INSERT_VALUES));
    283:   PetscCall(MatAssemblyBegin(D, MAT_FINAL_ASSEMBLY));
    284:   PetscCall(MatAssemblyEnd(D, MAT_FINAL_ASSEMBLY));
    285:   PetscCall(MatCreateVecs(D, &DVec, NULL));
    286:   PetscCall(MatGetDiagonal(AplusJ, DVec));
    287:   PetscCall(MatDiagonalSet(D, DVec, INSERT_VALUES));
    288:   PetscCall(MatDuplicate(Acondensed, MAT_COPY_VALUES, &AplusD));
    289:   PetscCall(MatAXPY(AplusD, alpha, D, SUBSET_NONZERO_PATTERN));
    290:   PetscCall(MatDuplicate(J, MAT_COPY_VALUES, &JplusD));
    291:   PetscCall(MatAXPY(JplusD, alpha, D, SUBSET_NONZERO_PATTERN));

    293:   // Initialize our SMW context
    294:   ctx.U            = U;
    295:   ctx.D            = D;
    296:   ctx.gamma        = gamma;
    297:   ctx.alpha        = alpha;
    298:   ctx.setup_called = PETSC_FALSE;

    300:   // Set preconditioner operators
    301:   PetscCall(KSPCreate(PETSC_COMM_WORLD, &ksp));
    302:   PetscCall(KSPSetType(ksp, KSPFGMRES));
    303:   PetscCall(KSPSetOperators(ksp, AplusJ, AplusJ));
    304:   PetscCall(KSPSetNormType(ksp, KSP_NORM_UNPRECONDITIONED));
    305:   PetscCall(KSPGMRESSetRestart(ksp, 300));
    306:   PetscCall(KSPGetPC(ksp, &pc));
    307:   PetscCall(PCSetType(pc, PCCOMPOSITE));
    308:   PetscCall(PCCompositeSetType(pc, PC_COMPOSITE_SPECIAL));
    309:   PetscCall(PCCompositeAddPCType(pc, PCILU));
    310:   PetscCall(PCCompositeAddPCType(pc, PCSHELL));
    311:   PetscCall(PCCompositeGetPC(pc, 0, &pcA));
    312:   PetscCall(PCCompositeGetPC(pc, 1, &pcJ));
    313:   PetscCall(PCSetOperators(pcA, AplusD, AplusD));
    314:   PetscCall(PCSetOperators(pcJ, JplusD, JplusD));
    315:   PetscCall(PCShellSetContext(pcJ, &ctx));
    316:   PetscCall(PCShellSetApply(pcJ, SmwApply));
    317:   PetscCall(PCShellSetSetUp(pcJ, SmwSetup));
    318:   PetscCall(PCCompositeSpecialSetAlphaMat(pc, D));

    320:   // Solve
    321:   PetscCall(KSPSetFromOptions(ksp));
    322:   PetscCall(KSPSolve(ksp, b, x));

    324:   PetscCall(MatDestroy(&A));
    325:   PetscCall(MatDestroy(&B));
    326:   PetscCall(MatDestroy(&Q));
    327:   PetscCall(MatDestroy(&Acondensed));
    328:   PetscCall(MatDestroy(&Bcondensed));
    329:   PetscCall(MatDestroy(&BT));
    330:   PetscCall(MatDestroy(&J));
    331:   PetscCall(MatDestroy(&AplusJ));
    332:   PetscCall(MatDestroy(&QInv));
    333:   PetscCall(MatDestroy(&D));
    334:   PetscCall(MatDestroy(&AplusD));
    335:   PetscCall(MatDestroy(&JplusD));
    336:   PetscCall(MatDestroy(&U));
    337:   if (rank == 0) PetscCall(VecDestroy(&bound));
    338:   PetscCall(VecDestroy(&x));
    339:   PetscCall(VecDestroy(&b));
    340:   PetscCall(VecDestroy(&Qdiag));
    341:   PetscCall(VecDestroy(&DVec));
    342:   PetscCall(ISDestroy(&boundary_is));
    343:   PetscCall(ISDestroy(&bulk_is));
    344:   PetscCall(KSPDestroy(&ksp));
    345:   PetscCall(PetscFree(boundary_indices));
    346:   PetscCall(MatDestroy(&ctx.UT));
    347:   PetscCall(MatDestroy(&ctx.I_plus_gammaUTaDinvU));
    348:   PetscCall(MatDestroy(&ctx.aD));
    349:   PetscCall(MatDestroy(&ctx.aDinv));
    350:   PetscCall(PCDestroy(&ctx.smw_cholesky));

    352:   PetscCall(PetscFinalize());
    353:   return 0;
    354: }

    356: /*TEST

    358:    build:
    359:       requires: !complex

    361:    test:
    362:       args: -fA ${DATAFILESPATH}/matrices/ifiss/A -fB ${DATAFILESPATH}/matrices/ifiss/B -fQ ${DATAFILESPATH}/matrices/ifiss/Q -fbound ${DATAFILESPATH}/is/ifiss/bound -ksp_monitor
    363:       requires: datafilespath defined(PETSC_USE_64BIT_INDICES) !complex double

    365:    test:
    366:       suffix: 2
    367:       nsize: 2
    368:       args: -fA ${DATAFILESPATH}/matrices/ifiss/A -fB ${DATAFILESPATH}/matrices/ifiss/B -fQ ${DATAFILESPATH}/matrices/ifiss/Q -fbound ${DATAFILESPATH}/is/ifiss/bound -ksp_monitor
    369:       requires: datafilespath defined(PETSC_USE_64BIT_INDICES) !complex double strumpack

    371: TEST*/