[petsc-users] Issue on Block Jacobi Preconditioner Reuse

[Mark Adams]

You seem to be trying to use the setup of one solver (lu) for a different
solver (hypre).
You can't do that in general and  PCSetType(subpc, PCHYPRE); will delete
the old solvers data.

On Thu, Oct 14, 2021 at 11:48 PM 王一甲 via petsc-users <
petsc-users at mcs.anl.gov> wrote:

> 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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