[petsc-users] accessing DMDA Vec ghost values

[Dave May]

Matt beat me to the punch... :D
Anyway, here is my more detailed answer.


> Thanks!  Somehow I missed DM{Get,Create}LocalVector().  BTW what is the
> difference between the Get and Create versions?  It is not obvious from the
> documentation.
>

The DMDA contains a pool of vectors (both local and global) which can be
re-used by the user. This avoids the need to continually allocate and
deallocate memory. Thus, the Get methods are simply an optimization.

The Get methods retrieve from the pool, a vector which isn't currently in
use. In this case, You can think of Restore as returning the vector back to
the pool to be used somewhere else.
If all vectors in the pool are in use, a new one will be allocated for you.
In this case, Restore will actually deallocate memory.

Since Get methods may return vectors which have been used else where in the
code, you should always call VecZeroEntries() on them.

The Create methods ALWAYS allocate new memory and thus you ALWAYS need to
call Destroy on them. Vectors obtained via VecCreate() will always be
initialized with 0's.


Thanks,
  Dave


>
> Also, can you explain the difference between DMDAVecGetArrayDOF and
> DMDAVecGetArrayDOFRead?
>
> Thanks again,
> Sean
>
>
>
> Thanks,
>   Dave
>
>
>
>>
>> Thanks very much!
>>
>> Sean Dettrick
>>
>>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.mcs.anl.gov/pipermail/petsc-users/attachments/20160512/c416e671/attachment.html>