Very good point. Thank you for the clarification. On Sun, Jun 24, 2018 at 8:46 PM Mark Adams <[email protected]> wrote:
> Note, you converged because of the relative tolerance so initial guess > does not really matter except that the initial residual on the second time > step may have more high frequency content then your first initial residual. > > On Sun, Jun 24, 2018 at 6:37 AM Najeeb Ahmad <[email protected]> wrote: > >> 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* >> >> -- *Najeeb Ahmad* *Research and Teaching Assistant* *PARallel and MultiCORE Computing Laboratory (ParCoreLab) * *Computer Science and Engineering* *Koç University, Istanbul, Turkey*
