Date: 2025-03-29T05:58:26+00:00

Actual source code: qmrcgs.c

      1: /*
      2:     This file implements QMRCGS (QMRCGStab).
      3: */
      4: #include <../src/ksp/ksp/impls/bcgs/bcgsimpl.h>

      6: static PetscErrorCode KSPSetUp_QMRCGS(KSP ksp)
      7: {
      8:   PetscFunctionBegin;
      9:   PetscCall(KSPSetWorkVecs(ksp, 14));
     10:   PetscFunctionReturn(PETSC_SUCCESS);
     11: }

     13: /* Only need a few hacks from KSPSolve_BCGS */

     15: static PetscErrorCode KSPSolve_QMRCGS(KSP ksp)
     16: {
     17:   PetscInt    i;
     18:   PetscScalar eta, rho1, rho2, alpha, eta2, omega, beta, cf, cf1, uu;
     19:   Vec         X, B, R, P, PH, V, D2, X2, S, SH, T, D, S2, RP, AX, Z;
     20:   PetscReal   dp   = 0.0, final, tau, tau2, theta, theta2, c, F, NV, vv;
     21:   KSP_BCGS   *bcgs = (KSP_BCGS *)ksp->data;
     22:   PC          pc;
     23:   Mat         mat;

     25:   PetscFunctionBegin;
     26:   X  = ksp->vec_sol;
     27:   B  = ksp->vec_rhs;
     28:   R  = ksp->work[0];
     29:   P  = ksp->work[1];
     30:   PH = ksp->work[2];
     31:   V  = ksp->work[3];
     32:   D2 = ksp->work[4];
     33:   X2 = ksp->work[5];
     34:   S  = ksp->work[6];
     35:   SH = ksp->work[7];
     36:   T  = ksp->work[8];
     37:   D  = ksp->work[9];
     38:   S2 = ksp->work[10];
     39:   RP = ksp->work[11];
     40:   AX = ksp->work[12];
     41:   Z  = ksp->work[13];

     43:   /*  Only supports right preconditioning */
     44:   PetscCheck(ksp->pc_side == PC_RIGHT, PetscObjectComm((PetscObject)ksp), PETSC_ERR_SUP, "KSP qmrcgs does not support %s", PCSides[ksp->pc_side]);
     45:   if (!ksp->guess_zero) {
     46:     if (!bcgs->guess) PetscCall(VecDuplicate(X, &bcgs->guess));
     47:     PetscCall(VecCopy(X, bcgs->guess));
     48:   } else {
     49:     PetscCall(VecSet(X, 0.0));
     50:   }

     52:   /* Compute initial residual */
     53:   PetscCall(KSPGetPC(ksp, &pc));
     54:   PetscCall(PCGetOperators(pc, &mat, NULL));
     55:   if (!ksp->guess_zero) {
     56:     PetscCall(KSP_MatMult(ksp, mat, X, S2));
     57:     PetscCall(VecCopy(B, R));
     58:     PetscCall(VecAXPY(R, -1.0, S2));
     59:   } else {
     60:     PetscCall(VecCopy(B, R));
     61:   }

     63:   /* Test for nothing to do */
     64:   if (ksp->normtype != KSP_NORM_NONE) PetscCall(VecNorm(R, NORM_2, &dp));
     65:   PetscCall(PetscObjectSAWsTakeAccess((PetscObject)ksp));
     66:   ksp->its   = 0;
     67:   ksp->rnorm = dp;
     68:   PetscCall(PetscObjectSAWsGrantAccess((PetscObject)ksp));
     69:   PetscCall(KSPLogResidualHistory(ksp, dp));
     70:   PetscCall(KSPMonitor(ksp, 0, dp));
     71:   PetscCall((*ksp->converged)(ksp, 0, dp, &ksp->reason, ksp->cnvP));
     72:   if (ksp->reason) PetscFunctionReturn(PETSC_SUCCESS);

     74:   /* Make the initial Rp == R */
     75:   PetscCall(VecCopy(R, RP));

     77:   eta   = 1.0;
     78:   theta = 1.0;
     79:   if (dp == 0.0) {
     80:     PetscCall(VecNorm(R, NORM_2, &tau));
     81:   } else {
     82:     tau = dp;
     83:   }

     85:   PetscCall(VecDot(RP, RP, &rho1));
     86:   PetscCall(VecCopy(R, P));

     88:   i = 0;
     89:   do {
     90:     PetscCall(KSP_PCApply(ksp, P, PH));      /*  ph <- K p */
     91:     PetscCall(KSP_MatMult(ksp, mat, PH, V)); /* v <- A ph */

     93:     PetscCall(VecDot(V, RP, &rho2)); /* rho2 <- (v,rp) */
     94:     if (rho2 == 0.0) {
     95:       PetscCheck(!ksp->errorifnotconverged, PetscObjectComm((PetscObject)ksp), PETSC_ERR_NOT_CONVERGED, "KSPSolve has not converged due to division by zero");
     96:       ksp->reason = KSP_DIVERGED_NANORINF;
     97:       break;
     98:     }

    100:     if (rho1 == 0) {
    101:       PetscCheck(!ksp->errorifnotconverged, PetscObjectComm((PetscObject)ksp), PETSC_ERR_NOT_CONVERGED, "KSPSolve has stagnated");
    102:       ksp->reason = KSP_DIVERGED_BREAKDOWN; /* Stagnation */
    103:       break;
    104:     }

    106:     alpha = rho1 / rho2;
    107:     PetscCall(VecWAXPY(S, -alpha, V, R)); /* s <- r - alpha v */

    109:     /* First quasi-minimization step */
    110:     PetscCall(VecNorm(S, NORM_2, &F)); /* f <- norm(s) */
    111:     theta2 = F / tau;

    113:     c = 1.0 / PetscSqrtReal(1.0 + theta2 * theta2);

    115:     tau2 = tau * theta2 * c;
    116:     eta2 = c * c * alpha;
    117:     cf   = theta * theta * eta / alpha;
    118:     PetscCall(VecWAXPY(D2, cf, D, PH));   /* d2 <- ph + cf d */
    119:     PetscCall(VecWAXPY(X2, eta2, D2, X)); /* x2 <- x + eta2 d2 */

    121:     /* Apply the right preconditioner again */
    122:     PetscCall(KSP_PCApply(ksp, S, SH));      /*  sh <- K s */
    123:     PetscCall(KSP_MatMult(ksp, mat, SH, T)); /* t <- A sh */

    125:     PetscCall(VecDotNorm2(S, T, &uu, &vv));
    126:     if (vv == 0.0) {
    127:       PetscCall(VecDot(S, S, &uu));
    128:       if (uu != 0.0) {
    129:         PetscCheck(!ksp->errorifnotconverged, PetscObjectComm((PetscObject)ksp), PETSC_ERR_NOT_CONVERGED, "KSPSolve has not converged due to division by zero");
    130:         ksp->reason = KSP_DIVERGED_NANORINF;
    131:         break;
    132:       }
    133:       PetscCall(VecAXPY(X, alpha, SH)); /* x <- x + alpha sh */
    134:       PetscCall(PetscObjectSAWsTakeAccess((PetscObject)ksp));
    135:       ksp->its++;
    136:       ksp->rnorm  = 0.0;
    137:       ksp->reason = KSP_CONVERGED_RTOL;
    138:       PetscCall(PetscObjectSAWsGrantAccess((PetscObject)ksp));
    139:       PetscCall(KSPLogResidualHistory(ksp, dp));
    140:       PetscCall(KSPMonitor(ksp, i + 1, 0.0));
    141:       break;
    142:     }
    143:     PetscCall(VecNorm(V, NORM_2, &NV)); /* nv <- norm(v) */

    145:     if (NV == 0) {
    146:       PetscCheck(!ksp->errorifnotconverged, PetscObjectComm((PetscObject)ksp), PETSC_ERR_NOT_CONVERGED, "KSPSolve has not converged due to singular matrix");
    147:       ksp->reason = KSP_DIVERGED_BREAKDOWN;
    148:       break;
    149:     }

    151:     if (uu == 0) {
    152:       PetscCheck(!ksp->errorifnotconverged, PetscObjectComm((PetscObject)ksp), PETSC_ERR_NOT_CONVERGED, "KSPSolve has stagnated");
    153:       ksp->reason = KSP_DIVERGED_BREAKDOWN; /* Stagnation */
    154:       break;
    155:     }
    156:     omega = uu / vv; /* omega <- uu/vv; */

    158:     /* Computing the residual */
    159:     PetscCall(VecWAXPY(R, -omega, T, S)); /* r <- s - omega t */

    161:     /* Second quasi-minimization step */
    162:     if (ksp->normtype != KSP_NORM_NONE && ksp->chknorm < i + 2) PetscCall(VecNorm(R, NORM_2, &dp));

    164:     if (tau2 == 0) {
    165:       PetscCheck(!ksp->errorifnotconverged, PetscObjectComm((PetscObject)ksp), PETSC_ERR_NOT_CONVERGED, "KSPSolve has not converged due to division by zero");
    166:       ksp->reason = KSP_DIVERGED_NANORINF;
    167:       break;
    168:     }
    169:     theta = dp / tau2;
    170:     c     = 1.0 / PetscSqrtReal(1.0 + theta * theta);
    171:     if (dp == 0.0) {
    172:       PetscCall(VecNorm(R, NORM_2, &tau));
    173:     } else {
    174:       tau = dp;
    175:     }
    176:     tau = tau * c;
    177:     eta = c * c * omega;

    179:     cf1 = theta2 * theta2 * eta2 / omega;
    180:     PetscCall(VecWAXPY(D, cf1, D2, SH)); /* d <- sh + cf1 d2 */
    181:     PetscCall(VecWAXPY(X, eta, D, X2));  /* x <- x2 + eta d */

    183:     PetscCall(VecDot(R, RP, &rho2));
    184:     PetscCall(PetscObjectSAWsTakeAccess((PetscObject)ksp));
    185:     ksp->its++;
    186:     ksp->rnorm = dp;
    187:     PetscCall(PetscObjectSAWsGrantAccess((PetscObject)ksp));
    188:     PetscCall(KSPLogResidualHistory(ksp, dp));
    189:     PetscCall(KSPMonitor(ksp, i + 1, dp));

    191:     beta = (alpha * rho2) / (omega * rho1);
    192:     PetscCall(VecAXPBYPCZ(P, 1.0, -omega * beta, beta, R, V)); /* p <- r - omega * beta* v + beta * p */
    193:     rho1 = rho2;
    194:     PetscCall(KSP_MatMult(ksp, mat, X, AX)); /* Ax <- A x */
    195:     PetscCall(VecWAXPY(Z, -1.0, AX, B));     /* r <- b - Ax */
    196:     PetscCall(VecNorm(Z, NORM_2, &final));
    197:     PetscCall((*ksp->converged)(ksp, i + 1, dp, &ksp->reason, ksp->cnvP));
    198:     if (ksp->reason) break;
    199:     i++;
    200:   } while (i < ksp->max_it);

    202:   /* mark lack of convergence */
    203:   if (ksp->its >= ksp->max_it && !ksp->reason) ksp->reason = KSP_DIVERGED_ITS;
    204:   PetscFunctionReturn(PETSC_SUCCESS);
    205: }

    207: /*MC
    208:    KSPQMRCGS - Implements the QMRCGStab method {cite}`chan1994qmrcgs` and Ghai, Lu, and Jiao (NLAA 2019).

    210:    Level: beginner

    212:    Note:
    213:    Only right preconditioning is supported.

    215:    Contributed by:
    216:    Xiangmin Jiao (xiangmin.jiao@stonybrook.edu)

    218: .seealso: [](ch_ksp), `KSPCreate()`, `KSPSetType()`, `KSPType`, `KSP`, `KSPBICG`, `KSPFBICGS`, `KSPBCGSL`, `KSPSetPCSide()`
    219: M*/
    220: PETSC_EXTERN PetscErrorCode KSPCreate_QMRCGS(KSP ksp)
    221: {
    222:   KSP_BCGS         *bcgs;
    223:   static const char citations[] = "@article{chan1994qmrcgs,\n"
    224:                                   "  title={A quasi-minimal residual variant of the {Bi-CGSTAB} algorithm for nonsymmetric systems},\n"
    225:                                   "  author={Chan, Tony F and Gallopoulos, Efstratios and Simoncini, Valeria and Szeto, Tedd and Tong, Charles H},\n"
    226:                                   "  journal={SIAM Journal on Scientific Computing},\n"
    227:                                   "  volume={15},\n"
    228:                                   "  number={2},\n"
    229:                                   "  pages={338--347},\n"
    230:                                   "  year={1994},\n"
    231:                                   "  publisher={SIAM}\n"
    232:                                   "}\n"
    233:                                   "@article{ghai2019comparison,\n"
    234:                                   "  title={A comparison of preconditioned {K}rylov subspace methods for large-scale nonsymmetric linear systems},\n"
    235:                                   "  author={Ghai, Aditi and Lu, Cao and Jiao, Xiangmin},\n"
    236:                                   "  journal={Numerical Linear Algebra with Applications},\n"
    237:                                   "  volume={26},\n"
    238:                                   "  number={1},\n"
    239:                                   "  pages={e2215},\n"
    240:                                   "  year={2019},\n"
    241:                                   "  publisher={Wiley Online Library}\n"
    242:                                   "}\n";
    243:   PetscBool         cite        = PETSC_FALSE;

    245:   PetscFunctionBegin;
    246:   PetscCall(PetscCitationsRegister(citations, &cite));
    247:   PetscCall(PetscNew(&bcgs));

    249:   ksp->data                = bcgs;
    250:   ksp->ops->setup          = KSPSetUp_QMRCGS;
    251:   ksp->ops->solve          = KSPSolve_QMRCGS;
    252:   ksp->ops->destroy        = KSPDestroy_BCGS;
    253:   ksp->ops->reset          = KSPReset_BCGS;
    254:   ksp->ops->buildresidual  = KSPBuildResidualDefault;
    255:   ksp->ops->setfromoptions = KSPSetFromOptions_BCGS;
    256:   ksp->pc_side             = PC_RIGHT; /* set default PC side */

    258:   PetscCall(KSPSetSupportedNorm(ksp, KSP_NORM_UNPRECONDITIONED, PC_RIGHT, 2));
    259:   PetscCall(KSPSetSupportedNorm(ksp, KSP_NORM_NONE, PC_RIGHT, 1));
    260:   PetscFunctionReturn(PETSC_SUCCESS);
    261: }