[petsc-dev] PCFIELDSPLIT with inner -ksp_reuse_preconditioner

[Pierre Jolivet]

> On 13 Jul 2022, at 6:01 PM, Barry Smith <bsmith at petsc.dev> wrote:
> 
> 
>   Some of the ugliness of SetFromOptions in some of the nested solvers :-(

Right, I figured it out slightly after sending the mail by pulling the sub KSP and explicitly calling KSPSetFromOptions on it, which also works.
I’ve provided a fix to my user by telling them to pull the sub KSP, so if you are not satisfied by your patch, rest assured that they can live with this “undocumented feature” for the time being.

Thanks,
Pierre

> * thread #1, queue = 'com.apple.main-thread', stop reason = breakpoint 1.1
>   * frame #0: 0x00000001132795f0 libpetsc.3.017.3.dylib`KSPSetFromOptions(ksp=0x00007f899f149a70) at itcl.c:324:19
>     frame #1: 0x0000000112eddda4 libpetsc.3.017.3.dylib`PCSetUp_FieldSplit(pc=0x00007f899f08d870) at fieldsplit.c:1087:32
>     frame #2: 0x0000000112dfb874 libpetsc.3.017.3.dylib`PCSetUp(pc=0x00007f899f08d870) at precon.c:991:5
>     frame #3: 0x0000000113285834 libpetsc.3.017.3.dylib`KSPSetUp(ksp=0x00007f899e95f670) at itfunc.c:401:3
>     frame #4: 0x000000011328b5e9 libpetsc.3.017.3.dylib`KSPSolve_Private(ksp=0x00007f899e95f670, b=0x00007f899f0dec70, x=0x00007f899f0ce870) at itfunc.c:835:3
>     frame #5: 0x000000011328aa66 libpetsc.3.017.3.dylib`KSPSolve(ksp=0x00007f899e95f670, b=0x00007f899f0dec70, x=0x00007f899f0ce870) at itfunc.c:1066:3
>     frame #6: 0x000000010e7c94e8 ex1111`main(argc=12, args=0x00007ff7b1739328) at ex1111.c:33:3
>     frame #7: 0x000000011703751e dyld`start + 462
> 
> The SetFromOptions doesn't always traverse all the sub-methods suitably calling SetFromOptions on them because they may not have been created by the time that
> the SetFromOptions is first called. In this case, the PCSetUp_FieldSplit() creates the sub solvers and calls the options database on them after the initial call to PCSetFromOptions_FieldSplit() 
> 
> I've attached a patch that should fix this particular issue. 
> <fix-fieldsplit-recall-setfromoptions.patch>
> I ran it with your test code, and it seemed to resolve the issue and ran the test suite, and it did not break anything.
> 
> A redesigned/implemented PETSc would need some thought on how to improve the SetFromOptions model to prevent this kind of difficulty.
> 
> Barry
> 
> 
> 
> 
>> On Jul 13, 2022, at 8:40 AM, Pierre Jolivet <pierre at joliv.et <mailto:pierre at joliv.et>> wrote:
>> 
>> Hello,
>> A user(*) wants to switch back and forth with the option -fieldsplit_1_ksp_reuse_preconditioner, but it’s generating results that I don’t understand.
>> In the following example I solve three successive linear systems.
>> If I run
>> $ ./ex1111 -pc_type fieldsplit -fieldsplit_pc_type lu -log_view -info -options_view -switch false
>> -fieldsplit_1_ksp_reuse_preconditioner is always false, and I properly get:
>> 1) convergence in a single iteration every time 
>> 2) [0] PCSetUp(): Setting up PC with same nonzero pattern for the last solve
>> 3) MatLUFactorNum         3 in -log_view
>> 
>> Now, if I run
>> $ ./ex1111 -pc_type fieldsplit -fieldsplit_pc_type lu -log_view -info -options_view -switch true
>> -fieldsplit_1_ksp_reuse_preconditioner is set to true for the second solve and switched back to false for the last solve.
>> I (wrongfully, IMHO?) get:
>> 1) convergence in 2 iterations for the last solve (instead of 1?)
>> 2) [0] PCSetUp(): Leaving PC with identical preconditioner since reuse preconditioner is set (which should not be set for the last solve?)
>> 3) MatLUFactorNum         1 (instead of 2?)
>> 4) #PETSc Option Table entries:
>> -fieldsplit_1_ksp_reuse_preconditioner false (though from -info, cf. point #2 just above, this option value is true?)
>> 
>> Does it make sense or do you agree that something is off?
>> 
>> Thanks,
>> Pierre
>> 
>> <ex1111.c>
>> 
>> (*) https://community.freefem.org/t/supply-petsc-numbering-for-shared-memory-block-matrix/486/18 <https://community.freefem.org/t/supply-petsc-numbering-for-shared-memory-block-matrix/486/18>

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