Actual source code: fbcgsr.c

petsc-master 2020-09-26
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  2: /*
  3:     This file implements FBiCGStab-R.
  4:     Only allow right preconditioning.
  5:     FBiCGStab-R is a mathematically equivalent variant of FBiCGStab. Differences are:
  6:       (1) There are fewer MPI_Allreduce calls.
  7:       (2) The convergence occasionally is much faster than that of FBiCGStab.
  8: */
  9: #include <../src/ksp/ksp/impls/bcgs/bcgsimpl.h>
 10: #include <petsc/private/vecimpl.h>

 12: static PetscErrorCode KSPSetUp_FBCGSR(KSP ksp)
 13: {

 17:   KSPSetWorkVecs(ksp,8);
 18:   return(0);
 19: }

 21: static PetscErrorCode  KSPSolve_FBCGSR(KSP ksp)
 22: {
 23:   PetscErrorCode    ierr;
 24:   PetscInt          i,j,N;
 25:   PetscScalar       tau,sigma,alpha,omega,beta;
 26:   PetscReal         rho;
 27:   PetscScalar       xi1,xi2,xi3,xi4;
 28:   Vec               X,B,P,P2,RP,R,V,S,T,S2;
 29:   PetscScalar       *PETSC_RESTRICT rp, *PETSC_RESTRICT r, *PETSC_RESTRICT p;
 30:   PetscScalar       *PETSC_RESTRICT v, *PETSC_RESTRICT s, *PETSC_RESTRICT t, *PETSC_RESTRICT s2;
 31:   PetscScalar       insums[4],outsums[4];
 32:   KSP_BCGS          *bcgs = (KSP_BCGS*)ksp->data;
 33:   PC                pc;
 34:   Mat               mat;

 37:   if (!ksp->vec_rhs->petscnative) SETERRQ(PetscObjectComm((PetscObject)ksp),PETSC_ERR_SUP,"Only coded for PETSc vectors");
 38:   VecGetLocalSize(ksp->vec_sol,&N);

 40:   X  = ksp->vec_sol;
 41:   B  = ksp->vec_rhs;
 42:   P2 = ksp->work[0];

 44:   /* The followings are involved in modified inner product calculations and vector updates */
 45:   RP = ksp->work[1]; VecGetArray(RP,(PetscScalar**)&rp); VecRestoreArray(RP,NULL);
 46:   R  = ksp->work[2]; VecGetArray(R,(PetscScalar**)&r);   VecRestoreArray(R,NULL);
 47:   P  = ksp->work[3]; VecGetArray(P,(PetscScalar**)&p);   VecRestoreArray(P,NULL);
 48:   V  = ksp->work[4]; VecGetArray(V,(PetscScalar**)&v);   VecRestoreArray(V,NULL);
 49:   S  = ksp->work[5]; VecGetArray(S,(PetscScalar**)&s);   VecRestoreArray(S,NULL);
 50:   T  = ksp->work[6]; VecGetArray(T,(PetscScalar**)&t);   VecRestoreArray(T,NULL);
 51:   S2 = ksp->work[7]; VecGetArray(S2,(PetscScalar**)&s2); VecRestoreArray(S2,NULL);

 53:   /* Only supports right preconditioning */
 54:   if (ksp->pc_side != PC_RIGHT) SETERRQ1(PetscObjectComm((PetscObject)ksp),PETSC_ERR_SUP,"KSP fbcgsr does not support %s",PCSides[ksp->pc_side]);
 55:   if (!ksp->guess_zero) {
 56:     if (!bcgs->guess) {
 57:       VecDuplicate(X,&bcgs->guess);
 58:     }
 59:     VecCopy(X,bcgs->guess);
 60:   } else {
 61:     VecSet(X,0.0);
 62:   }

 64:   /* Compute initial residual */
 65:   KSPGetPC(ksp,&pc);
 66:   PCSetUp(pc);
 67:   PCGetOperators(pc,&mat,NULL);
 68:   if (!ksp->guess_zero) {
 69:     KSP_MatMult(ksp,mat,X,P2); /* P2 is used as temporary storage */
 70:     VecCopy(B,R);
 71:     VecAXPY(R,-1.0,P2);
 72:   } else {
 73:     VecCopy(B,R);
 74:   }

 76:   /* Test for nothing to do */
 77:   VecNorm(R,NORM_2,&rho);
 78:   PetscObjectSAWsTakeAccess((PetscObject)ksp);
 79:   ksp->its = 0;
 80:   if (ksp->normtype != KSP_NORM_NONE) ksp->rnorm = rho;
 81:   else ksp->rnorm = 0;
 82:   PetscObjectSAWsGrantAccess((PetscObject)ksp);
 83:   KSPLogResidualHistory(ksp,ksp->rnorm);
 84:   KSPMonitor(ksp,0,ksp->rnorm);
 85:   (*ksp->converged)(ksp,0,ksp->rnorm,&ksp->reason,ksp->cnvP);
 86:   if (ksp->reason) return(0);

 88:   /* Initialize iterates */
 89:   VecCopy(R,RP); /* rp <- r */
 90:   VecCopy(R,P); /* p <- r */

 92:   /* Big loop */
 93:   for (i=0; i<ksp->max_it; i++) {

 95:     /* matmult and pc */
 96:     KSP_PCApply(ksp,P,P2); /* p2 <- K p */
 97:     KSP_MatMult(ksp,mat,P2,V); /* v <- A p2 */

 99:     /* inner prodcuts */
100:     if (i==0) {
101:       tau  = rho*rho;
102:       VecDot(V,RP,&sigma); /* sigma <- (v,rp) */
103:     } else {
104:       PetscLogEventBegin(VEC_ReduceArithmetic,0,0,0,0);
105:       tau  = sigma = 0.0;
106:       for (j=0; j<N; j++) {
107:         tau   += r[j]*rp[j]; /* tau <- (r,rp) */
108:         sigma += v[j]*rp[j]; /* sigma <- (v,rp) */
109:       }
110:       PetscLogFlops(4.0*N);
111:       PetscLogEventEnd(VEC_ReduceArithmetic,0,0,0,0);
112:       insums[0] = tau;
113:       insums[1] = sigma;
114:       PetscLogEventBegin(VEC_ReduceCommunication,0,0,0,0);
115:       MPIU_Allreduce(insums,outsums,2,MPIU_SCALAR,MPIU_SUM,PetscObjectComm((PetscObject)ksp));
116:       PetscLogEventEnd(VEC_ReduceCommunication,0,0,0,0);
117:       tau       = outsums[0];
118:       sigma     = outsums[1];
119:     }

121:     /* scalar update */
122:     alpha = tau / sigma;

124:     /* vector update */
125:     VecWAXPY(S,-alpha,V,R);  /* s <- r - alpha v */

127:     /* matmult and pc */
128:     KSP_PCApply(ksp,S,S2); /* s2 <- K s */
129:     KSP_MatMult(ksp,mat,S2,T); /* t <- A s2 */

131:     /* inner prodcuts */
132:     PetscLogEventBegin(VEC_ReduceArithmetic,0,0,0,0);
133:     xi1  = xi2 = xi3 = xi4 = 0.0;
134:     for (j=0; j<N; j++) {
135:       xi1 += s[j]*s[j]; /* xi1 <- (s,s) */
136:       xi2 += t[j]*s[j]; /* xi2 <- (t,s) */
137:       xi3 += t[j]*t[j]; /* xi3 <- (t,t) */
138:       xi4 += t[j]*rp[j]; /* xi4 <- (t,rp) */
139:     }
140:     PetscLogFlops(8.0*N);
141:     PetscLogEventEnd(VEC_ReduceArithmetic,0,0,0,0);

143:     insums[0] = xi1;
144:     insums[1] = xi2;
145:     insums[2] = xi3;
146:     insums[3] = xi4;

148:     PetscLogEventBegin(VEC_ReduceCommunication,0,0,0,0);
149:     MPIU_Allreduce(insums,outsums,4,MPIU_SCALAR,MPIU_SUM,PetscObjectComm((PetscObject)ksp));
150:     PetscLogEventEnd(VEC_ReduceCommunication,0,0,0,0);
151:     xi1  = outsums[0];
152:     xi2  = outsums[1];
153:     xi3  = outsums[2];
154:     xi4  = outsums[3];

156:     /* test denominator */
157:     if (xi3 == 0.0) SETERRQ(PetscObjectComm((PetscObject)ksp),PETSC_ERR_PLIB,"Divide by zero");
158:     if (sigma == 0.0) SETERRQ(PetscObjectComm((PetscObject)ksp),PETSC_ERR_PLIB,"Divide by zero");

160:     /* scalar updates */
161:     omega = xi2 / xi3;
162:     beta  = -xi4 / sigma;
163:     rho   = PetscSqrtReal(PetscAbsScalar(xi1 - omega * xi2)); /* residual norm */

165:     /* vector updates */
166:     VecAXPBYPCZ(X,alpha,omega,1.0,P2,S2); /* x <- alpha * p2 + omega * s2 + x */

168:     /* convergence test */
169:     PetscObjectSAWsTakeAccess((PetscObject)ksp);
170:     ksp->its++;
171:     if (ksp->normtype != KSP_NORM_NONE) ksp->rnorm = rho;
172:     else ksp->rnorm = 0;
173:     PetscObjectSAWsGrantAccess((PetscObject)ksp);
174:     KSPLogResidualHistory(ksp,ksp->rnorm);
175:     KSPMonitor(ksp,i+1,ksp->rnorm);
176:     (*ksp->converged)(ksp,i+1,ksp->rnorm,&ksp->reason,ksp->cnvP);
177:     if (ksp->reason) break;

179:     /* vector updates */
180:     PetscLogEventBegin(VEC_Ops,0,0,0,0);
181:     for (j=0; j<N; j++) {
182:       r[j] = s[j] - omega * t[j]; /* r <- s - omega t */
183:       p[j] = r[j] + beta * (p[j] - omega * v[j]); /* p <- r + beta * (p - omega v) */
184:     }
185:     PetscLogFlops(6.0*N);
186:     PetscLogEventEnd(VEC_Ops,0,0,0,0);

188:   }

190:   if (i >= ksp->max_it) ksp->reason = KSP_DIVERGED_ITS;
191:   return(0);
192: }

194: /*MC
195:      KSPFBCGSR - Implements a mathematically equivalent variant of FBiCGSTab.

197:    Options Database Keys:
198: .   see KSPSolve()

200:    Level: beginner

202:    Notes:
203:     Only allow right preconditioning

205: .seealso:  KSPCreate(), KSPSetType(), KSPType (for list of available types), KSP, KSPBICG, KSPFBCGSL, KSPSetPCSide()
206: M*/
207: PETSC_EXTERN PetscErrorCode KSPCreate_FBCGSR(KSP ksp)
208: {
210:   KSP_BCGS       *bcgs;

213:   PetscNewLog(ksp,&bcgs);

215:   ksp->data                = bcgs;
216:   ksp->ops->setup          = KSPSetUp_FBCGSR;
217:   ksp->ops->solve          = KSPSolve_FBCGSR;
218:   ksp->ops->destroy        = KSPDestroy_BCGS;
219:   ksp->ops->reset          = KSPReset_BCGS;
220:   ksp->ops->buildsolution  = KSPBuildSolution_BCGS;
221:   ksp->ops->buildresidual  = KSPBuildResidualDefault;
222:   ksp->ops->setfromoptions = KSPSetFromOptions_BCGS;
223:   ksp->pc_side             = PC_RIGHT; /* set default PC side */

225:   KSPSetSupportedNorm(ksp,KSP_NORM_PRECONDITIONED,PC_LEFT,3);
226:   KSPSetSupportedNorm(ksp,KSP_NORM_UNPRECONDITIONED,PC_RIGHT,2);
227:   KSPSetSupportedNorm(ksp,KSP_NORM_NONE,PC_RIGHT,1);
228:   return(0);
229: }