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 < [email protected]> 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*
