Actual source code: mfnkrylov.c

slepc-3.18.3 2023-03-24
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  1: /*
  2:    - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
  3:    SLEPc - Scalable Library for Eigenvalue Problem Computations
  4:    Copyright (c) 2002-, Universitat Politecnica de Valencia, Spain

  6:    This file is part of SLEPc.
  7:    SLEPc is distributed under a 2-clause BSD license (see LICENSE).
  8:    - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
  9: */
 10: /*
 11:    SLEPc matrix function solver: "krylov"

 13:    Method: Arnoldi with Eiermann-Ernst restart

 15:    Algorithm:

 17:        Build Arnoldi approximations using f(H) for the Hessenberg matrix H,
 18:        restart by discarding the Krylov basis but keeping H.

 20:    References:

 22:        [1] M. Eiermann and O. Ernst, "A restarted Krylov subspace method
 23:            for the evaluation of matrix functions", SIAM J. Numer. Anal.
 24:            44(6):2481-2504, 2006.
 25: */

 27: #include <slepc/private/mfnimpl.h>
 28: #include <slepcblaslapack.h>

 30: PetscErrorCode MFNSetUp_Krylov(MFN mfn)
 31: {
 32:   PetscInt       N;

 34:   MatGetSize(mfn->A,&N,NULL);
 35:   if (mfn->ncv==PETSC_DEFAULT) mfn->ncv = PetscMin(30,N);
 36:   if (mfn->max_it==PETSC_DEFAULT) mfn->max_it = 100;
 37:   MFNAllocateSolution(mfn,1);
 38:   return 0;
 39: }

 41: PetscErrorCode MFNSolve_Krylov(MFN mfn,Vec b,Vec x)
 42: {
 43:   PetscInt          n=0,m,ld,ldh,j;
 44:   PetscBLASInt      m_,inc=1;
 45:   Mat               M,G=NULL,H=NULL;
 46:   Vec               F=NULL;
 47:   PetscScalar       *marray,*farray,*harray;
 48:   const PetscScalar *garray;
 49:   PetscReal         beta,betaold=0.0,nrm=1.0;
 50:   PetscBool         breakdown;

 52:   m  = mfn->ncv;
 53:   ld = m+1;
 54:   MatCreateSeqDense(PETSC_COMM_SELF,ld,m,NULL,&M);
 55:   MatDenseGetArray(M,&marray);

 57:   /* set initial vector to b/||b|| */
 58:   BVInsertVec(mfn->V,0,b);
 59:   BVScaleColumn(mfn->V,0,1.0/mfn->bnorm);
 60:   VecSet(x,0.0);

 62:   /* Restart loop */
 63:   while (mfn->reason == MFN_CONVERGED_ITERATING) {
 64:     mfn->its++;

 66:     /* compute Arnoldi factorization */
 67:     BVMatArnoldi(mfn->V,mfn->transpose_solve?mfn->AT:mfn->A,M,0,&m,&beta,&breakdown);

 69:     /* save previous Hessenberg matrix in G; allocate new storage for H and f(H) */
 70:     if (mfn->its>1) { G = H; H = NULL; }
 71:     ldh = n+m;
 72:     MFN_CreateVec(ldh,&F);
 73:     MFN_CreateDenseMat(ldh,&H);

 75:     /* glue together the previous H and the new H obtained with Arnoldi */
 76:     MatDenseGetArray(H,&harray);
 77:     for (j=0;j<m;j++) PetscArraycpy(harray+n+(j+n)*ldh,marray+j*ld,m);
 78:     if (mfn->its>1) {
 79:       MatDenseGetArrayRead(G,&garray);
 80:       for (j=0;j<n;j++) PetscArraycpy(harray+j*ldh,garray+j*n,n);
 81:       MatDenseRestoreArrayRead(G,&garray);
 82:       MatDestroy(&G);
 83:       harray[n+(n-1)*ldh] = betaold;
 84:     }
 85:     MatDenseRestoreArray(H,&harray);

 87:     if (mfn->its==1) {
 88:       /* set symmetry flag of H from A */
 89:       MatPropagateSymmetryOptions(mfn->A,H);
 90:     }

 92:     /* evaluate f(H) */
 93:     FNEvaluateFunctionMatVec(mfn->fn,H,F);

 95:     /* x += ||b||*V*f(H)*e_1 */
 96:     VecGetArray(F,&farray);
 97:     PetscBLASIntCast(m,&m_);
 98:     nrm = BLASnrm2_(&m_,farray+n,&inc);   /* relative norm of the update ||u||/||b|| */
 99:     MFNMonitor(mfn,mfn->its,nrm);
100:     for (j=0;j<m;j++) farray[j+n] *= mfn->bnorm;
101:     BVSetActiveColumns(mfn->V,0,m);
102:     BVMultVec(mfn->V,1.0,1.0,x,farray+n);
103:     VecRestoreArray(F,&farray);

105:     /* check convergence */
106:     if (mfn->its >= mfn->max_it) mfn->reason = MFN_DIVERGED_ITS;
107:     if (mfn->its>1) {
108:       if (m<mfn->ncv || breakdown || beta==0.0 || nrm<mfn->tol) mfn->reason = MFN_CONVERGED_TOL;
109:     }

111:     /* restart with vector v_{m+1} */
112:     if (mfn->reason == MFN_CONVERGED_ITERATING) {
113:       BVCopyColumn(mfn->V,m,0);
114:       n += m;
115:       betaold = beta;
116:     }
117:   }

119:   MatDestroy(&H);
120:   MatDestroy(&G);
121:   VecDestroy(&F);
122:   MatDenseRestoreArray(M,&marray);
123:   MatDestroy(&M);
124:   return 0;
125: }

127: SLEPC_EXTERN PetscErrorCode MFNCreate_Krylov(MFN mfn)
128: {
129:   mfn->ops->solve          = MFNSolve_Krylov;
130:   mfn->ops->setup          = MFNSetUp_Krylov;
131:   return 0;
132: }