[petsc-users] DMDA Global vs Local indexing

[Mark Lohry]

        Look at the source for DMDAVecGetArray() it calls either VecGetArray1d, VecGetArray2d, VecGetArray3d. You can call the VecGetArrayNd directly setting the local start and stop you want.


Thanks, I'll look into this.

       As a PETSc developer, of course, I would recommend keeping your local/temporary data also in Vecs and using the DMDAVecGetArray() for access to those also and having all code written in the “local patch style” with loops i=gxs; i<gxs+gxm  I think the code is clearer and easier to reason about than having each process from 0 to vxm etc.

Yeah, I definitely see the attraction and I may ultimately go that 
route. As a non-PETSc developer however, it seems preferable to 
absolutely minimize the reliance on PETSc data management for code 
re-use in a non-PETSc application going forward. I'd be eager to hear 
from other devs on how they approach this.




On 03/06/2014 09:23 AM, Barry Smith wrote:
> On Mar 6, 2014, at 8:05 AM, Mark Lohry <mlohry at gmail.com> wrote:
>
>> I'm using DMDAs for managing my DOF data on structured grids, so processes have access to local array chunks from i=gxs; i<gxs+gxm. I have a lot of local/temporary data that I don't want PETSc to manage and I'm trying to keep PETSc as segregated from the rest of code as possible, so I'd prefer for any functions to be ignorant of particular ordering, and just access from i=0; i<gxm so that I'm using 0-indexed arrays locally. What's the appropriate way to go about doing this / getting direct access to the beginning of global-indexed local array?
>     Mark,
>
>     Look at the source for DMDAVecGetArray() it calls either VecGetArray1d, VecGetArray2d, VecGetArray3d. You can call the VecGetArrayNd directly setting the local start and stop you want.
>
>     Barry
>
>    As a PETSc developer, of course, I would recommend keeping your local/temporary data also in Vecs and using the DMDAVecGetArray() for access to those also and having all code written in the “local patch style” with loops i=gxs; i<gxs+gxm  I think the code is clearer and easier to reason about than having each process from 0 to vxm etc.
>

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