[petsc-users] Checking if a vector is a localvector of a given DMDA

[Dave May]

On Tue, 25 Sep 2018 at 13:31, Phil Tooley <phil.tooley at sheffield.ac.uk>
wrote:

> Thanks both,
>
> I now have what I need.  For now I am checking that the vector I am passed
> has the same local size, global size, and Comm as the vector provided by
> DMGetLocalVector, mostly because I already have a compatibility check
> function written.  (I assume this requires a malloc and free behind the
> scenes)
>

Not necessarily. The Get/Restore strategy will re-use internally cached
vectors.


> At some point I will likely change to explicitly checking for comm size of
> one and appropriate global and local sizes based on the DMDA properties
> instead, for now I want to get to an alpha version I can let people play
> with.
>
> Phil
>
> On 25/09/18 13:07, Dave May wrote:
>
>
>
> On Tue, 25 Sep 2018 at 13:20, Matthew Knepley <knepley at gmail.com> wrote:
>
>> On Tue, Sep 25, 2018 at 7:03 AM Dave May <dave.mayhem23 at gmail.com> wrote:
>>
>>> On Tue, 25 Sep 2018 at 11:49, Phil Tooley <phil.tooley at sheffield.ac.uk>
>>> wrote:
>>>
>>>> Hi all,
>>>>
>>>> Given a vector I know I can get an associated DM (if there is one) by
>>>> calling VecGetDM, but I need to also be able to check that
>>>>
>>>> a) the vector is the localvector of that DM rather than the global
>>>>
>>>
>>> Given the vector, you can check the communicator size via
>>> PetscObjectGetComm()
>>>
>>>
>>> https://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/Sys/PetscObjectGetComm.html
>>> and then MPI_Comm_size()
>>> If the comm size 1, it is local vector.
>>>
>>
>> In serial, both vectors have comm size 1.
>>
>
> Right - and the local and global sizes are the same.
>
>  My point was to check the comm size first. If it's 1 then you have a
> candidate for a local vector. Then you'd check the vec global size matches
> the dmda local size. If the commsize is anything other than 1 then it
> cannot be a local vector
>
>
>>    Matt
>>
>>
>>> You can check the size matches your local DMDA space by using
>>> DMDAGetGhostCorners()
>>>
>>>
>>> https://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/DMDA/DMDAGetGhostCorners.html
>>>
>>> and return the quantities m, n, and p.
>>>
>>> You also need to use  DMDAGetInfo()
>>>
>>>
>>> https://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/DMDA/DMDAGetInfo.html
>>>
>>> The important quantity you want returned is "dof"
>>>
>>> If m x n x p x dof matches the number returned by VecGetSize() (assuming
>>> you know the vector is sequential) then you know the local space will fit
>>> within your vector.
>>>
>>>
>>>
>>>>
>>>> b) the DM is a DMDA rather than some other subclass
>>>>
>>>
>>> See Matt's answer
>>>
>>>
>>>>
>>>> I can't immediately see routines that do what I need, but I am likely
>>>> missing something obvious. Is there a way to achieve the above?
>>>>
>>>> Thanks
>>>>
>>>> Phil
>>>>
>>>> --
>>>> Phil Tooley
>>>> Research Software Engineering
>>>> University of Sheffield
>>>>
>>>>
>>
>> --
>> 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/%7Eknepley/>
>>
>
> --
> Phil Tooley
> Research Software Engineering
> University of Sheffield
>
>
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