[petsc-users] Arbitrary ownership IS for a matrix

[Nicolás Barnafi]

Hello, sorry to bring back this issue.

I am observing some behavior that I don't understand to try to debug
our code, so my question is: what happens when to set values to a
matrix from dofs that don't belong to the processor? I.e. if processor
0 has [0 1 2] and proc 1 has dofs [3 4 5], if I set the value in
position (3,3) in proc 0, does this not complain during assemble as
long as I preallocated sufficient rows, even if these do not coincide
with the ones from MatSetSizes?

Thanks in advance,
Nicolas

On Thu, Mar 10, 2022 at 2:50 AM Nicolás Barnafi <nabw91 at gmail.com> wrote:
>
> Thank you both very much, it is exactly what I needed.
>
> Best regards
>
> On Wed, Mar 9, 2022, 21:19 Matthew Knepley <knepley at gmail.com> wrote:
>>
>> On Wed, Mar 9, 2022 at 5:13 PM Barry Smith <bsmith at petsc.dev> wrote:
>>>
>>>
>>>   You need to do a mapping of your global numbering to the standard PETSc numbering and use the PETSc numbering for all access to vectors and matrices.
>>>
>>>    https://petsc.org/release/docs/manualpages/AO/AOCreate.html provides one approach to managing the renumbering.
>>
>>
>> You can think of this as the mapping to offsets that you would need in any event to store your values (they could not be directly addressed with your random indices).
>>
>>   Thanks,
>>
>>      Matt
>>
>>>
>>>   Barry
>>>
>>>
>>> On Mar 9, 2022, at 3:42 PM, Nicolás Barnafi <nabw91 at gmail.com> wrote:
>>>
>>> Hi community,
>>>
>>> I have an application with polytopal meshes (elements of arbitrary shape) where the distribution of dofs is not PETSc friendly, meaning that it is not true that cpu0 owns dofs [0,a), then cpu1 owns [a,b) and so on, but instead the distribution is in fact random. Another important detail is that boundary dofs are shared, meaning that if dof 150 is on the boundary, each subdomain vector has dof 150.
>>>
>>> Under this considerations:
>>>
>>> i) Is it possible to give an arbitrary mapping to the matrix structure or is the blocked distribution hard coded?
>>> ii) Are the repeated boundary dofs an issue when computing a Fieldsplit preconditioner in parallel?
>>>
>>> Best regards,
>>> Nicolas
>>>
>>>
>>
>>
>> --
>> 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/



-- 
Nicolás Alejandro Barnafi Wittwer