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

[Najeeb Ahmad]

Very good point. Thank you for the clarification.

On Sun, Jun 24, 2018 at 8:46 PM Mark Adams <mfadams at lbl.gov> 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 <nahmad16 at ku.edu.tr> 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 <
>> 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*
>>
>>

-- 
*Najeeb Ahmad*


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

*Computer Science and Engineering*
*Koç University, Istanbul, Turkey*
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.mcs.anl.gov/pipermail/petsc-users/attachments/20180625/5a568b21/attachment.html>