[petsc-users] Issue on Block Jacobi Preconditioner Reuse

[王一甲]

Hi!Everyone:

    Glad to join the mailing list.Recently I 've been working on a program using block jacobi preconditioner for a sequence solve of two linear system A_1x_1=b_1,A_2x_2=b_2.

    Since A1 and A2 have same nonzero pattern and their element values are quite close, we hope to reuse the preconditoners constructed when solving A_1x_1=b_1 ,

    however after calling KSPSetReusePreconditioner, though the iteration number of second ksp solve is very small but the time used did not decrease much, these are my code:

             ierr = KSPSetUp(ksp);CHKERRQ(ierr);
             ierr = PCBJacobiGetSubKSP(pc, &num_local, &idx_first_local, &subksp);CHKERRQ(ierr);


                for (i=0; i<num_local; i++)
                {
                        ierr = KSPGetPC(subksp[i], &subpc);CHKERRQ(ierr);
                        if (i==0)
                        {
                                ierr = KSPSetType(subksp[i],"gmres");CHKERRQ(ierr);
                                ierr = PCSetType(subpc, PCLU);CHKERRQ(ierr);
                                ierr = PCFactorSetMatSolverType(subpc, "mkl_pardiso");CHKERRQ(ierr);
                                ierr = KSPSetOptionsPrefix(subksp[i], "Blk0_");CHKERRQ(ierr);
                                ierr = KSPSetReusePreconditioner(subksp[i],PETSC_TRUE);CHKERRQ(ierr);
                                ierr = KSPSetFromOptions(subksp[i]);CHKERRQ(ierr);
                        }
                        if (i==1)
                        {
                                ierr = KSPSetType(subksp[i],"gmres");CHKERRQ(ierr);
                                ierr = PCSetType(subpc, PCHYPRE);CHKERRQ(ierr);
                                ierr = PCHYPRESetType(pc, "boomeramg");CHKERRQ(ierr);
                                ierr = KSPSetReusePreconditioner(subksp[i],PETSC_TRUE);CHKERRQ(ierr);
                                ierr = KSPSetOptionsPrefix(subksp[i], "Blk1_");CHKERRQ(ierr);
                        }
                }//nlocal
        }//isbjacobi


        //Solve the Linear System
        ierr = KSPSetFromOptions(ksp);CHKERRQ(ierr);
        //Solve the Linear System
t0 = MPI_Wtime();
        ierr = KSPSolve(ksp, b1, x1);CHKERRQ(ierr);
t0 = MPI_Wtime()-t0;
        ierr = PetscPrintf(PETSC_COMM_SELF,"First KSP Solve time: %g s\n",t0);CHKERRQ(ierr);
        //Preconditioner Reuse
        ierr = KSPSetReusePreconditioner(ksp, PETSC_TRUE);CHKERRQ(ierr);
        ierr = KSPSetOperators(ksp,A2,A1);CHKERRQ(ierr);
t0 = MPI_Wtime();
        ierr = KSPSolve(ksp,b2,x2);CHKERRQ(ierr);
t0 = MPI_Wtime()-t0;
        ierr = PetscPrintf(PETSC_COMM_SELF,"Second KSP Solve time: %g s\n",t0);CHKERRQ(ierr);




The total block number is 2, and the first block is solved using direct method and the second block is solved using hypre's boomeramg, so I hope to reuse the factor of the first block and the set up phase of boomeramg as preconditioner of the next solve, is there anything wrong with the reuse code?

                                                                                                                                                                                                                                                                                                                                                                                                                        Best Wishes

                                                                                                                                                                                                                                                                                                                                                                                                                        WANG Yijia


                                                                                                                                                                                                                                                                                                                                                                                                                         2021/10/15 


                                                                                                                                                                                                                                                                                                                                                                        
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