[petsc-users] PETSc Solver: Large iteration count for first solution

[Najeeb Ahmad]

Hi Massimiliano,

Thanks for your feedback. Indeed i am using previous solution for the
subsequent ones and probably that is the reason for the faster convergence
for the subsequent iterations. As Matt suggested, i need to improve my
preconditioning to make first iteration converge faster.

Thanks

On Sun, Jun 24, 2018 at 3:26 PM Massimiliano Leoni <
leoni.massimiliano1 at gmail.com> wrote:

> Hi, are the various solves a time stepping procedure? Could it be that it
> is re-using the solution at a previous iteration as an initial guess for
> the subsequent ones, and for this reason it converges sooner?
>
>
>
> Massimiliano
>
>
>
> On sabato 23 giugno 2018 13:36:59 CEST Najeeb Ahmad wrote:
>
> Hi,
>
>
> I am trying to solve a  64008000 x  64008000 system using PETSc. The
> problem I am facing is that I am getting a very large iteration count for
> the first solution (order of thousands). For the subsequent soluitons, the
> iteration count is less. For instance, here is the iteration count for the
> first solution when my code is run on 32 processes.
>
>
> ...
>
> 2772 KSP Residual norm 1.198554448053e-07
>
> 2773 KSP Residual norm 1.190214006391e-07
>
> 2774 KSP Residual norm 1.181931531360e-07
>
> 2775 KSP Residual norm 1.173706620648e-07
>
> 2776 KSP Residual norm 1.165538874733e-07
>
> 2777 KSP Residual norm 1.157427896857e-07
>
> 2778 KSP Residual norm 1.149373293013e-07
>
> 2779 KSP Residual norm 1.141374671925e-07
>
> 2780 KSP Residual norm 1.133431645025e-07
>
> Linear solve converged due to CONVERGED_RTOL iterations 2780
>
>
> For the subsequent solutions, the iteration count is as follows:
>
> ...
>
> 116 KSP Residual norm 2.657138978992e-04
>
> 117 KSP Residual norm 2.767485246885e-04
>
> 118 KSP Residual norm 2.071574619161e-04
>
> 119 KSP Residual norm 2.273562103015e-04
>
> 120 KSP Residual norm 1.693847273193e-04
>
> 121 KSP Residual norm 1.803833437167e-04
>
> 122 KSP Residual norm 1.443434809819e-04
>
> 123 KSP Residual norm 1.390878278231e-04
>
> 124 KSP Residual norm 1.247521972621e-04
>
> 125 KSP Residual norm 1.059224678494e-04
>
> Linear solve converged due to CONVERGED_RTOL iterations 125
>
>
> The solver setup for my code is as follows:
>
>
>  ierr = KSPCreate(PETSC_COMM_WORLD,&ksp);CHKERRQ(ierr);
>
>   ierr = KSPSetOperators(ksp,A,A);CHKERRQ(ierr);
>
>   ierr = KSPSetType(ksp, KSPCG);CHKERRQ(ierr);
>
>   ierr = KSPGetPC(ksp,&pc);CHKERRQ(ierr);
>
>   ierr = PCSetType(pc, PCJACOBI);
>
>   ierr = KSPSetFromOptions(ksp);CHKERRQ(ierr);
>
>   ierr = KSPSetUp(ksp);CHKERRQ(ierr);
>
>
> and the command line for running the code is:
>
> mpiexec -n 32 ./main -mat_type mpiaij -ksp_atol 1E-7 -ksp_rtol 1E-7
> -ksp_converged_reason -ksp_monitor
>
>
> Any help is highly appreciated.
>
>
>
> --
>
> Najeeb Ahmad
>
>
> Research and Teaching Assistant
>
> PARallel and MultiCORE Computing Laboratory (ParCoreLab)
>
> Computer Science and Engineering
>
>
> Koç University, Istanbul, Turkey
>
>
>
>
>

-- 
*Najeeb Ahmad*


*Research and Teaching Assistant*
*PARallel and MultiCORE Computing Laboratory (ParCoreLab) *

*Computer Science and Engineering*
*Koç University, Istanbul, Turkey*
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