[petsc-dev] VecNest

[Pierre Jolivet]

> On 12 Jul 2022, at 2:50 PM, Matthew Knepley <knepley at gmail.com> wrote:
> 
> On Tue, Jul 12, 2022 at 7:41 AM Matthew Knepley <knepley at gmail.com <mailto:knepley at gmail.com>> wrote:
> On Tue, Jul 12, 2022 at 7:39 AM Pierre Jolivet <pierre at joliv.et <mailto:pierre at joliv.et>> wrote:
> 
> 
>> On 12 Jul 2022, at 2:32 PM, Matthew Knepley <knepley at gmail.com <mailto:knepley at gmail.com>> wrote:
>> 
>> On Mon, Jul 11, 2022 at 1:17 PM Pierre Jolivet <pierre at joliv.et <mailto:pierre at joliv.et>> wrote:
>> Hello,
>> Could anyone help me understand what is going on in the following example, please?
>> I have a VecNest.
>> I either: a) initialize all values to 0.0, then set a specific part of the vector to nonzero or b) initialize a part of the vector to 0.0 and set the other part to nonzero.
>> I don’t see why a) and b) produce different results.
>> 
>> $ ./ex1111 -pc_type fieldsplit -ksp_monitor_true_residual -ksp_converged_reason -fieldsplit_pc_type jacobi -ksp_pc_side right -ksp_view_final_residual -nest_subvec true 
>>   0 KSP unpreconditioned resid norm 8.375635517980e-01 true resid norm 8.375635517980e-01 ||r(i)||/||b|| 1.000000000000e+00
>>   1 KSP unpreconditioned resid norm 4.748816884247e-01 true resid norm 4.748816884247e-01 ||r(i)||/||b|| 5.669798875623e-01
>>   2 KSP unpreconditioned resid norm 4.713006778679e-01 true resid norm 4.713006778679e-01 ||r(i)||/||b|| 5.627043784990e-01
>>   3 KSP unpreconditioned resid norm 7.092979927129e-02 true resid norm 7.092979927129e-02 ||r(i)||/||b|| 8.468587144106e-02
>>   4 KSP unpreconditioned resid norm 1.457836310255e-02 true resid norm 1.457836310255e-02 ||r(i)||/||b|| 1.740567992870e-02
>>   5 KSP unpreconditioned resid norm 1.625040500524e-14 true resid norm 1.633468028779e-14 ||r(i)||/||b|| 1.950261595401e-14
>> Linear solve converged due to CONVERGED_RTOL iterations 5
>> KSP final norm of residual 1.63347e-14
>> $ ./ex1111 -pc_type fieldsplit -ksp_monitor_true_residual -ksp_converged_reason -fieldsplit_pc_type jacobi -ksp_pc_side right -ksp_view_final_residual -nest_subvec false
>>   0 KSP unpreconditioned resid norm 0.000000000000e+00 true resid norm 8.375635517980e-01 ||r(i)||/||b||            inf
>> Linear solve converged due to CONVERGED_ATOL iterations 0
>> KSP final norm of residual 0.837564
>> 
>> I find if I assemble the vector, I get the same answers. Will try to figure out what assembly is doing.
> 
> It’s probably reseting all these values https://petsc.org/main/src/vec/vec/interface/rvector.c.html#line511 <https://petsc.org/main/src/vec/vec/interface/rvector.c.html#line511>, which I believe are being used in VecNorm() inside VecNormalize().
> I guess any call to VecNestSubVec() should invalidate those as well, otherwise we get wrong cached norms.
> I will give this a go.
> 
> I believe the bug is the following:
> 
>   You change values in a subvector, which does StateIncrease on the subvector, but not StateIncrease on the nest vector, so it has its cached norm
> 
> I am not sure what to do about this, since how can the parent know you pulled out the subvector? Will think.
> 
> Okay, the error is more subtle. VecSet() automatically computes some norms. VecNestGetSubVec() is broken with respect to caching.
> I think the right fix now is VecNestGetSubVec/s() should increase the state no matter what. If we want to get more precise, we need
> to have Restore() for the subvecs, which checks the state of the subvector when it is restored.
> 
> Do you want to change it?

https://gitlab.com/petsc/petsc/-/merge_requests/5420 <https://gitlab.com/petsc/petsc/-/merge_requests/5420> gets the job done, and is hopefully not too far from what you had in mind.
If anyone is not confortable with the incurred cost (e.g., of recomputing some norms), please let me know.

Thanks,
Pierre

>   Thanks,
> 
>      Matt
>  
>    Matt
>  
> Thanks,
> Pierre
> 
>>   Thanks,
>> 
>>     Matt
>>  
>> Thanks,
>> Pierre
>> 
>> 
>> 
>> -- 
>> 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/>
> 
> 
> 
> -- 
> 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/>
> 
> 
> -- 
> 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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