[petsc-users] saving results

[Matthew Knepley]

On Tue, Feb 19, 2019 at 4:12 AM Sal Am via petsc-users <
petsc-users at mcs.anl.gov> wrote:

> It is a finite-element problem of an RF antenna dielectric interaction
> where all the non-zero elements are on the diagonal of the sparse matrix
> (if that is relevant).
>

I am unsure whether this is what you mean. If you matrix is diagonal, you
can invert it exactly in a single step so you
do not need a Krylov solver. What equations are you solving? Maxwell?
Electrostatic? What finite element are you using?
  Thanks,

    Matt


> More about the matrix: 25Mx25M, 2B non-zeros. I checked the KSPMonitor, it
> seems that I have to write my own routine?
>
>> Running for a Krylov method for tens of thousands of iterations is very
>> rarely recommended.
>>
> GAMG and BCGS is the only ones that have actually worked for me so far, I
> increased the max_it because it was not enough with the default one. With
> the default tolerance I got ||r(i)||/||b|| in the order of 10^-3, but I
> need more accuracy so I increased the tol to 10^-7 and that is when it
> crashed after ~51000 iterations.
>
> @Barry
>
>> I'm guessing you mange your own time stepping (and nonlinear solver if
>> there is one)?
>>
>
> It is the default from the solver (attached the short code).
>
>> As Jed says it doesn't make sense to save "partial" solutions within the
>> linear solver.  Just save it every several time-steps.
>>
>
> Yes maybe I worded it wrong. I did mean to save checkpoints basically such
> that the next time it is running it can pick up from where it crashed.
>
>    Also the code should not be "crashing" at seemingly long times (after
>> hours) with a Segmentation fault. Send us the full error message and we'll
>> see if there is some way we can help you fix this problem.
>>
> I have added this before, but seemingly they conclusion was that it is
> memory error...although I am not sure how, but I have added the entire
> error.
>
> On Mon, Feb 18, 2019 at 8:21 PM Smith, Barry F. <bsmith at mcs.anl.gov>
> wrote:
>
>>
>>    I'm guessing you mange your own time stepping (and nonlinear solver if
>> there is one)?
>>
>>    You can save the solution with a call to VecView() and then reload the
>> solution with a VecLoad() but you need to manage any other restart data
>> that you may need, like the value of the current time etc.
>>
>>    As Jed says it doesn't make sense to save "partial" solutions within
>> the linear solver.  Just save it every several time-steps.
>>
>>    Barry
>>
>>    Also the code should not be "crashing" at seemingly long times (after
>> hours) with a Segmentation fault. Send us the full error message and we'll
>> see if there is some way we can help you fix this problem.
>>
>>
>> > On Feb 18, 2019, at 9:47 AM, Sal Am via petsc-users <
>> petsc-users at mcs.anl.gov> wrote:
>> >
>> > Is there a function/command line option to save the solution as it is
>> solving (and read in the file from where it crashed and keep iterating from
>> there perhaps)?
>> > Had a seg fault and all the results until that point seems to have been
>> lost.
>> >
>> >
>> >
>> >
>>
>>

-- 
What most experimenters take for granted before they begin their
experiments is infinitely more interesting than any results to which their
experiments lead.
-- Norbert Wiener

https://www.cse.buffalo.edu/~knepley/ <http://www.cse.buffalo.edu/~knepley/>
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