[petsc-users] pcsetup

[Alex Fleeter]

If I call MatAssembly for individual sub-matrices,
 MatAssemblyBegin(subA[ii], MAT_FINAL_ASSEMBLY);
 MatAssemblyEnd(subA[ii], MAT_FINAL_ASSEMBLY);

then call KSPSetOperators, the PC setup is not called.

I have to call the assembly for the whole nest matrix
 MatAssemblyBegin(A, MAT_FINAL_ASSEMBLY);
 MatAssemblyEnd(A, MAT_FINAL_ASSEMBLY);
to trigger the setup of PC.

I found some comments in matnest.c
<https://www.mcs.anl.gov/petsc/petsc-current/src/mat/impls/nest/matnest.c>,
line 474-478: "Note: split assembly will fail if the same block appears
more than once (even indirectly through a nested

475:            * sub-block). This could be fixed by adding a flag to
Mat <https://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/Mat/Mat.html#Mat>
so that there was a way to check if a Mat
<https://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/Mat/Mat.html#Mat>
was476:            * already performing an assembly, but the result
would by more complicated and appears to offer less477:            *
potential for diagnostics and correctness checking. Split assembly
should be fixed once there is an478:            * interface for
libraries to make asynchronous progress in "user-defined non-blocking
collectives"."

I guess the comment suggests calling matassembly for the big nest
matrix after having setted values for individual sub-matrices, instead
of calling matassembly for individual submatrices.


Can you confirm that?



On Tue, Jul 21, 2020 at 4:35 PM Mark Adams <mfadams at lbl.gov> wrote:

> KSPSetOperator tells the KSP that the PC should be resetup.
>
> On Tue, Jul 21, 2020 at 3:45 PM Alex Fleeter <luis.saturday at gmail.com>
> wrote:
>
>> Hi:
>>
>> I want to ask under what circumstance will trigger a call for pc setup.
>>
>> I call KSPSolve to solve with the same Mat object with different entry
>> values each time. I can see that the pc setup is only called at the
>> beginning of the first solve.
>>
>> I tried to read the implementation, but quickly get lost...
>>
>
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