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

[Phil Tooley]

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)

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
> <mailto:knepley at gmail.com>> wrote:
>
>     On Tue, Sep 25, 2018 at 7:03 AM Dave May <dave.mayhem23 at gmail.com
>     <mailto:dave.mayhem23 at gmail.com>> wrote:
>
>         On Tue, 25 Sep 2018 at 11:49, Phil Tooley
>         <phil.tooley at sheffield.ac.uk
>         <mailto: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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